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Thermochemical Profiles on Hydrogen Atom Transfer from Triplet Naphthol and Proton-Induced Electron Transfer from Triplet Methoxynaphthalene to Benzophenone via Triplet Exciplexes Studied by Laser Flash Photolysis
Abstract
Temperature effects on hydrogen atom transfer (HT) and proton-induced electron transfer (p-ET) via triplet exciplexes in the l-naphthol-benzophenone (ROH-BP (or >CO)) and methoxynaphthalene-BP (ROMe-BP) systems in acetonitrile-H2O (ACN-H2O; 4:1 v/v) were studied by 355-nm laser flash photolysis. The HT rate in the ROH-BP system increased with an increase of temperature. The thermodynamic parameters for HT via the triplet exciplex (3)(ROH-->CO)* were obtained: the enthalpy change, Delta H-1 = -2.0 kcal.mol(-1), and the entropy change, Delta S-1 = -2.4 eu for formation of 3(ROH-->CO)*; the frequency factor, A(ex) = 7.7 x 10(9) s(-1) and activation energy, Delta E(ex) = 3.5 kcal.mol(-1), for intraexciplex HT. The negatively small values of Delta H-1 and Delta S-1 suggest that (3)(ROH-->CO)* has a loose sandwichlike structure. The corresponding parameters in the ROMe-BP system were obtained to be Delta H-1 = -2.2 kcal.mol(-1) and Delta S-1 = -2.8 eu for formation of the triplet exciplex (3)(ROMe-->CO)*. In the presence of protons in the ROH-BP system, proton-enhanced HT (p-HT, electron transfer followed by proton transfer) occurs efficiently due to the protonated triplet exciplex, (3)(ROH-->C+OH)*. The p-HT rate increased with an increase of temperature. In the ROMe-BP system with protons, formation of the protonated triplet exciplex 3(ROMe-->C+OH)* results in p-ET. The p-ET rate increased with an increase of temperature. The energy state diagrams for HT and p-ET are illustrated.
... The details of the detection system for the time profiles of the transient absorp- tion have been reported elsewhere. 16 Transient absorption spectra were obtained using a Unisoku USP-T1000-MLT system, which provides a transient absorption spectrum with one laser pulse. The obtained transient spectral data were analyzed using the least-squares best-fitting method. ...
We prepared amide-heterocycle (HC) compounds having various aromatic -electron systems (Ar), such as phenyl, naphthyl, furyl, thienyl and phenanthryl moieties, and converted them as ligands to difluoroboronated complexes, Ar@HC. Blue fluorescence from Ar@HC was observed in solution and the solid state, and the fluorescence quantum yields (Φf) and lifetimes (τf) were determined. The Φf values in CHCl3 were as small as 0.1 except for the phenanthrene derivatives (0.4 - 0.6). Observation of the triplet-triplet absorption upon laser flash photolysis of Ar@HC in solution indicated that the fluorescence process competes with intersystem crossing to the triplet state. Blue fluorescence in the microcrystals of Ar@HC was observed with the Φf values of 0.3 - 0.7. Based on the crystallographic data, relationship between the crystal structures and emission features of Ar@HC in the solid state is discussed.
... The number of laser pulses in each sample was less than four to avoid excess exposure. The details of the detection system for the time profiles of the transient absorption have been reported elsewhere [27]. The transient absorption spectra were obtained using a USP-554 system from Unisoku, which provides a transient absorption spectrum with one laser pulse. ...
Pyrenes substituted with tert-butylethynyl, trimethylsilylethynyl, and trimethylsilylbutadiynyl groups were prepared, and the fluorescence yields (), lifetimes, and triplet-triplet absorption were measured in cyclohexane. Upon introduction of the groups possessing triple bond(s) to the pyrene skeleton, the fluorescence rate () increased. The variation of the terminating groups did not appreciably affect the and values. Increasing the number of the triple bond, the values increased by the magnitude of order whereas the were not varied. The effect of the ethynyl groups on the values was rationalized by the Strickler-Berg equation considering an increase of the 1 transition moment. Triplet-triplet absorption spectra of pyrene derivatives were obtained. The intersystem crossing rates () increased upon increasing the number of triple bonds terminated with the trimethylsilyl group whereas those between ethynylpyrenes were independent of the terminating groups. Heavy atom effect failed to rationalize the enhancement of the values upon adding the triple bond to the pyrene moiety.
The effects of magnetic fields ranging from 0 T to 0.13 T on the photochemical reactions between benzophenone and isopropanol in the complete reaction cycle were explored. The results showed that with the increase of the applied magnetic field strength, the production rate of the product benzopinacol increased, and the induction period of the reaction was shortened. A new theoretical model suitable for the excitation process was used to explain the magnetic phenomenon in the initiation process of radical reaction, in which the magnetic field was believed to change the lifetime distribution of the excited reactant BP*, thereby affecting the rate of product generation. Specific parameters were used to calculate the lifetime probability distribution of excited state reactant molecules, which provides support for the quantitative analysis of the magnetic field effects of the reaction.
We prepared various compounds having the N-methylaminoanthranyl (MANT) moiety tethered with oligomethylene chains, and investigated the photophysical features in solution and the solid state based on the fluorescence quantum yields and lifetimes, and quantum chemical calculation. The molar absorption coefficient and fluorescence quantum yield were enhanced by an increase of the number of the MANT chromophore, resulting in an increase of the brightness as emission probes. Solid-state blue emission at room temperature was observed with moderate fluorescence quantum yields. The fluorescence rates in the solid state were found to be similar those in solution. From the results of the transient absorption measurements, the photophysical deactivation pathways in the solution were discussed. The studied multi-MANT compounds could be applicable to emissive photonic crystals and an exciton generating source for blue-color OLED.
The effects of silyl groups on the structural, absorption, fluorescence, and photoisomerization properties of stilbenes were investigated. In comparison to that of the parent stilbene (1), fluorescence quantum yields (Φf) of Me3Si‐substituted stilbenes 2–4 and 6 in solution were higher. Derivative 5, in which Me3Si groups are present at all ortho positions of the arene moieties, did not fluoresce at room temperature. The absorption and fluorescence wavelength maxima of Me3SiMe2Si‐substituted stilbene 7 occurred at longer wavelengths compared to those of the Me3Si analog 2. The results of theoretical calculations showed that this difference is a consequence of an increase in the HOMO energy of 7 caused by orbital interaction between π‐system and the σ(Si–Si) orbital. Stilbene 14, with two Me3Si‐C≡C groups at both para positions, had a high Φf (0.95). The calculated transition dipole moment (µ) was well correlated with Φf. Derivative 21, which contains Ph2N and Me3SiC≡C groups exhibited solvatofluorochromism because it possesses a twisted intramolecular charge transfer (TICT) excited state in which the Ph2N group and the aromatic ring are orthogonal.
2′-Hydroxychalcone (HC) analogues 1 and 2 having a diene part tethering the phenyl and naphthyl chromophores, respectively, were prepared, and their photochemical and photophysical properties were studied. Fluorescence from these compounds was absent in solution and the solid state. Based on the results obtained upon steady state and laser flash photolyses, compound 2 was found to be substantially stable on photoirradiation without undergoing intersystem crossing to the triplet state whereas compounds 1 showed transient absorption due to the triplet tautomer. The deactivation processes in the excited states were discussed by considering energetic reaction diagrams for the corresponding tautomers and isomers.
Small molecules having intense luminescence properties are required to promote biological and organic material applications. We prepared five types of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine rings and successfully converted them into three types of the difluoroboronated complexes, Py@BAs, as novel blue fluorophores. Py@BA having a pyridine moiety (2-Py@BA) showed no fluorescence in solution, whereas Py@BAs of pyridazine and pyrazine moieties (2,3-Py@BA and 2,5-Py@BA, respectively) emitted blue fluorescence with quantum yields of ca. 0.1. Transient absorption measurements using laser flash photolysis of the Py@BAs revealed the triplet formation of 2,3- and 2,5-Py@BAs, while little transient signal was observed for 2-Py@BA. Therefore, the deactivation processes from the lowest excited singlet state of fluorescent 2,3- and 2,5-Py@BAs consist of fluorescence and intersystem crossing to the triplet state while that of the nonfluorescent Py@BA is governed almost entirely by internal conversion to the ground state. Conversely, in the solid state, 2-Py@BA emitted intense fluorescence with a fluorescence quantum yield as high as 0.66, whereas 2,3- and 2,5-Py@BAs showed fluorescence with quantum yields of ca. 0.2. The crystal structure of 2-Py@BA took a herringbone packing motif, whereas those for 2,3- and 2,5-Py@BAs were two-dimensional sheetlike. On the basis of the difference in crystal structures, the emission mechanism in the solid state was discussed.
Excited-state relaxation processes of five types of 2′-hydroxychalcone (HC) analogues having naphthol, pyrrole and indole chromophores were studied based on emission and transient absorption measurements. Fluorescence with large Stokes-shifts was observed from these compounds in solution and the solid state, indicating occurrence of excited-state intramolecular hydrogen atom transfer (ESIHT) as observed for HC in solution. These fluorescent species, were, therefore, considered to be the corresponding tautomer (cis-keto form) in the excited singlet (S1) states formed via ESIHT. The fluorescence quantum yields were larger by the magnitude of one order than that of HC. Upon 400 nm laser pulsing in benzene solution of these HC analogues, no transient absorption spectra were obtained for the analogues having the phenolic moiety (Ph@Py and Ph@Ind) whereas rise and decay profiles of transient spectra were recorded independently of the amount of the dissolved oxygen for HC analogues substituted with the naphthol moiety (Np@Py, Np@Ind and Np@Np). On the analogy of the deactivation processes of HC, the obtained transient species were suggested to be the corresponding isomer of the tautomer (trans-keto form) in the ground state due to the isomerization of the excited state cis-keto form. The deactivation processes in the excited states of these compounds were discussed by considering energetic reaction diagrams for the tautomers and isomers. Replacement of the phenol moiety of HC with the naphthol was demonstrated to affect the excited state properties of the tautomer.
We have prepared three types of carbonyl compounds, benzoylethynylmethyl phenyl sulfide (2@SPh), (p-benzoyl)phenylethynylmethyl phenyl sulfide (3@SPh) and p-(benzoylethynyl)benzyl phenyl sulfide (4@SPh) having the benzoyl and phenylthiylmethyl groups which are interconnected with the C-C triple bond and the phenyl ring. Laser flash photolysis of 3@SPh and 4@SPh in acetonitrile provided transient absorption spectra of the corresponding triplet states where no chemical reactions were recognized. Upon laser flash photolysis of 2@SPh, the absorption band due to the phenylthiyl radical (PTR) was obtained, indicating that the C-S bond cleaved in the excited state. Triplet sensitization of these carbonyl compounds using acetone and xanthone was conducted using laser photolysis techniques. Formation of triplet 3@SPh was seen in the transient absorption whereas the PTR formation was observed for 2@SPh and 4@SPh, indicating that the triplet states were reactive for the C-S bond dissociation. The C-S bond dissociation mechanism for 4@SPh upon triplet sensitization was discussed in comparison with those for 2@SPh and 3@SPh.
We studied photoproducts of 1-(n-phenanthryl)-2-(m-phenanthryl)ethenes (nEm; n, m = 1, 3 and 9) for understanding the photocyclization patterns based on NMR spectroscopies. The crystal structures of the photoproducts were analyzed by X-ray crystallography, and the photophysical features of the photocyclized molecules were investigated based on emission and transient absorption measurements. Phenanthrene derivatives substituted at the 1- and 3-positions were prepared for synthesizing nEm by photocyclization of stilbene derivatives. We obtained four types of primary photoproducts (n@m) from the corresponding nEm. Two of them were found to have racemic molecular structures in the single crystal determined by X-ray crystallography. Besides the primary photoproducts, two types of secondary photoproducts (n@mPP) were isolated. Fluorescence quantum yields and lifetimes of the obtained photoproducts were determined in solution whereas the definite fluorescence quantum yields were obtained in the powder. Observation of the triplet-triplet absorption spectra in solution by laser photolysis techniques showed that intersystem crossing to the triplet state competes with the fluorescence process.
We prepared a variety of coumarin derivatives having expanded π-electron systems along the direction crossing the C3-C4 bond of the coumarin skeleton via a photochemical cyclization process and investigated their photophysical features as a function of the number (n) of the added benzene rings based on emission and transient absorption measurements. Upon increasing n, the fluorescence quantum yields of the π-extended coumarins increased. Expanding the π-electron system on the C3-C4 bond of the coumarin skeleton was found to be efficient for increasing the fluorescence ability more than that on the C7-C8 bond. Introducing the methoxy group at the 7-position was also efficient for enhancing the fluorescence quantum yield and rate of the expanded coumarins. The non-radiative process from the fluorescence state was not substantially influenced by the expanded π-electron system. The competitive process with the fluorescence was found to be intersystem crossing to the triplet state based on the observations of the triplet-triplet absorption. The effects of the expanded π-electron systems on the fluorescence ability were investigated with the aid of TD-DFT calculations.
Phenanthrenes substituted with trimethylsilylethynyl and phenylethynyl groups were photochemically prepared, and their photophysical properties were systematically investigated based on measurements of fluorescence quantum yields, lifetimes, and transient absorption. Introducing ethynyl groups into the phenanthrene skeleton caused an increase in the fluorescence quantum yields compared to that of phenanthrene. The quantum yields and rates of fluorescence were dependent on the substituting position(s) and the terminating group for the C-C triple bond. The observation of the triplet-triplet absorption of the substituted phenanthrenes was evident for the nonradiative process being intersystem crossing competitive with the fluorescence process. The mechanism of increasing the fluorescence abilities by substituting with the ethynyl group(s) was discussed with the aid of TD-DFT calculations.
We investigated the photophysical properties of difluoroboronated β-diketones (BF2DK) with chrysene and pyrene skeletons (ChB and PyB, respectively) in solution and in the solid state. Acetylchrysenes, as the key precursors to ChBs, were photochemically prepared from the corresponding (acetylphenyl)naphthylethenes by means of a modified photocyclization method. The absorption and emission spectra of the BF2DKs were obtained in chloroform and acetonitrile, and the quantum yields and lifetimes of the fluorescence were determined. Excimeric fluorescence from PyB was absent even in highly concentrated solution. Based on the Lippert-Mataga analysis of the absorption and fluorescence features, the photophysical properties of the ChBs were discussed in comparison with those of PyB. The fluorescence states of the studied BF2DKs are shown to be of a charge-transfer character. The fluorescence quantum yields decrease with increasing the solvent polarity due to the enhanced internal conversion process. The fluorescence quantum yields in the solid state of the studied BF2DKs were determined, and it was found that PyB is fluorescent, whereas the fluorescence quantum yields of the ChBs depend on the substituted position of the chrysene moiety. This journal is
A series of amino-2,3-naphthalimide derivatives having the amino functionality at 1-, 5- and 6-positions (1ANI, 5ANI and 6ANI, respectively) were prepared, and their photophysical properties were systematically investigated based on the measurements of steady-state absorption and fluorescence spectra, fluorescence lifetimes as well as transient absorption spectra. The ANIs efficiently fluoresced in solution, and the emission spectra appreciably shifted depending on the solvent polarity. 1ANI displayed only a slight fluorescence red-shift upon increasing the solvent polarity. In contrast, 5ANI and 6ANI showed marked positive solvatofluorochromism with large Stokes shifts displaying multicolour fluorescence; the fluorescence colours of 5ANI and 6ANI varied from violet-blue in hexane to orange-red in methanol. 5ANI and 6ANI, thus, serve as micro-environment responding fluorophores. In methanol, the intensity of the fluorescence emission band of 5ANI and 6ANI significantly reduced. Based on the fluorescence quantum yields and lifetimes, and transient absorption measurements, it has been revealed that internal conversion from the S1 state of ANIs to the ground state was accelerated by the protic medium, resulting in a reduction in their fluorescence efficiency, while intersystem crossing from the S1 state to a triplet state was not responsible for the decrease of fluorescence intensity. This journal is
Six difluoroboronated β-diketones having the phenanthrene skeleton (Phe@Ar) are prepared. Based on the measurements of the fluorescence quantum yields, lifetimes and transient absorption, the photophysical features of Phe@Ar are studied in comparing with those of difluoroboronated β-diketones having the phenyl, naphthyl and anthryl moieties. β-Diketones having 1-, 2-, 3- and 9-phenanthryl moieties (PheDKAr) were prepared as the precursor to Phe@Ar. 1-Acetylphenanthrene was synthesized by photocyclization method as the key building block of PheDKAr having the 1-phenanthryl moiety. The counter aromatic moieties (Ar) of the prepared PheDKAr are varied with phenyl, furyl and thienyl rings (Ar = Ph, F and T, respectively) to investigate the effects of the π-conjugation on the fluorescence properties. The prepared Phe@Ars are fluorescent with appreciable fluorescence quantum yields which depend on the substitution position of the phenanthrene moiety. 3-Phe@Ph having 3-phenanthryl moiety provides the largest fluorescence quantum yield (0.81) in acetonitrile among the Phe@Ars whereas 2-Phe@Ph having 2-phenanthryl moiety shows the smallest fluorescence quantum yield (0.07) in acetonitrile. All the Phe@Ars show fluorescence also in the solid state, and the fluorescence spectra and quantum yields were determined. Transient absorption measurement using laser flash photolysis of the Phe@Ars revealed the triplet formation. DFT and TD-DFT calculations of Phe@Ars rationalize the dependency of the fluorescence quantum yields on the substitution position of the phenanthrene skeleton in terms of difference in the oscillator strength for the HOMO-LUMO transition.
Keto-enol tautomerization in 2-alkyl-1,3-diketons having five-membered rings was studied using steady state and laser flash photolysis techniques in solution. The alkyl keto-diketones undergo photoinduced tautomerization mainly in the triplet state to the enol in acetonitrile. The alkyl enol-diketones, thus formed, undergo thermal tautomerization to the original keto forms in a few days. The alkyl enol-diketones show fast internal conversion from the excited singlet state to the ground state without yielding the corresponding isomeric forms (rotamer). Based on the computation for state energies of the keto, enol and isomer forms, a schematic energy diagram for the tautomerization was drawn.
Photochemical processes of 4-tert-butyl-4’-methoxydibenzoylmethane (avobenzone, AB), 4-phenylbenzoylbenzoyl-, 4-phenylbenzoyl-2'-furanyl- and 4-phenylbenzoyl-2'-thenoylmethanes (PB@Ph, PB@F and PB@T, respectively) substituted with Br and Cl at the C2 position were studied by stationary and laser flash photolyses in solution. The absorption spectral features showed that the molecular structures of the halogenated diketones are in the keto forms while those of halogen-free diketones are in the enol forms. The excited singlet and triplet state energies were determined from the absorption and emission spectra. From the absorption spectral changes upon steady state photolysis of brominated diketones in ethanol, the corresponding halogen-free diketone due to Br elimination was found to be the major photochemical process. The determined quantum yields for the formation of the halogen-free diketones were independent of the amount of the dissolved oxygen, indicating that the elimination process is the event in the excited singlet (S1) states. In contrast, from the observed absorption spectra obtained upon photolysis of chlorinated AB and PB@Ph, it was inferred that Norrish type I is the major photochemical reaction in the S1 states in acetonitrile. Chlorinated PB@F and PB@T were found to undergo Cl elimination in the S1 states in cyclohexane to form the corresponding halogen-free diketones. Laser photolysis studies of brominated AB in acetonitrile and ethanol provided a transient absorption spectrum ascribable to the avobenzone radical (ABR) produced by debromination as the initial intermediate, followed by the AB formation in ethanol. The quenching rate constant of ABR by ethanol and the quantum yield of the AB formation via ABR were determined. These observations provided evidence that H-atom abstraction of ABR from ethanol is responsible for the AB formation. Conversely, laser flash photolysis of brominated and chlorinated PB@Ph, PB@F and PB@T demonstrated the formation of the triplet-triplet absorption. No chemical reactions were found to occur in the triplet (T1) states. Two-color two-laser photolysis studies were carried out on the T1 state of chlorinated PB@Ph, PB@F and PB@T, resulting in the formation of the corresponding halogen-free diketones. These observations convinced occurrence of Cl elimination in the highly excited triplet (Tn, n ≥ 2) states. Based on the computated bond dissociation energies for the C-halogen and C-C bonds, switching mechanisms of dehalogenation and α-cleavage were discussed.
Photophysical and photochemical features of [3n]cyclophanes (3nCPs) (n = 2-6) in solution were investigated by emission and transient absorption measurements. The studied 3nCPs show excimer fluorescence without locally excited fluorescence while some of them emit excimer phosphorescence in rigid glass at 77 K. Probability of excimeric phosphorescence from transannular π-electron systems was shown to strictly depend on the symmetric molecular structures. A feature of intersystem crossing from an excimeric fluorescence state to the excimeric triplet state was observed. Transient absorption spectra obtained upon laser flash photolysis of 3nCP revealed formation of the triplet excimer states. Triplet sensitization of 33CP using xanthone as the sensitizer demonstrated formation of triplet 33CP via triplet energy transfer while from the xanthone ketyl radical formation, it was inferred that triplet xanthone undergoes H-atom abstraction from 32CP, providing a benzylic 32CP radical as the counter species. Based on kinetic and spectroscopic data obtained upon laser flash photolysis, differences in photochemical reactions of triplet xanthone between 32CP and 33CP were discussed.
Based on spectroscopic measurements and DFT calculations, fluorescence properties of thiazolo[4,5-b]pyrazine (TPy) derivatives having the phenyl group at the C2 position were studied. TPys were readily prepared from the corresponding amidopyrazines, which have a similar fluorescent core to a bioluminescence light emitter, Cypridina oxyluciferin. It was found that introduction of electron-donating (methoxy and dimethylamino) groups onto the 2-phenyl moiety of the TPy derivatives as well as the phenyl and 4-(dimethylamino)phenyl groups at C2 and C6, respectively, is operative for increasing the fluorescence yield and appearance of solvatochromic character. The mechanism of increasing the fluorescence yield depending on the substituents is discussed. These findings provide useful information on designing new TPy fluorophores.
Photophysical and photochemical properties of naphthalenes substituted with trimethylsilylethynyl, tert-butylethynyl, and trimethylsilylbutadiynyl groups were investigated by measurement of fluorescence yields, lifetimes, and triplet absorption. Introducing trimethylsilylethynyl and tert-butylethynyl groups to the 1-position of the naphthalene skeleton substantially enhanced fluorescence and intersystem crossing (ISC). The rates of fluorescence of 2-substituted naphthalenes were low. The effect of ethynyl groups on the 1-substituted naphthalenes was rationalized in terms of an increase of the transition moment along the short axis of the naphthalene skeleton. Substitution of the trimethylsilylbutadiynyl group at the 1 or 2-position of the naphthalene skeleton caused a considerable decrease in the fluorescence yield (approximately 0.01) and an increase in the ISC yield (0.99).
Facile synthesis of fulminene ([6]phenacene) was achieved through the Mallory reaction of 1-(1-naphthyl)-2-(1-phenanthryl)ethene or the 9-fluorenone-sensitized photo-ring-closure of 1-(1-naphthyl)-2-(1-phenanthryl)ethane. The electronic spectral properties of fulminene were investigated for the first time using photoluminescence as well as transient absorption spectroscopy. The spectral features were compared with those of a series of lower phenacene homologs such as phenanthrene ([3]phenacene), chrysene ([4]phenacene), and picene ([5]phenacene). For the [n]phenacene series, both the fluorescence and phosphorescence bands linearly red-shifted with an increase in the number of the benzene rings (n). Trends in the energy levels of the excited singlet (E
S) and the triplet (E
T) states were expressed as E
s = −2.6n + 89.1 (kcal mol−1) and E
T = −1.8n + 66.2 (kcal mol−1), respectively. In the case of fulminene, laser flash photolysis displayed a transient spectrum with an absorption maximum (λ
maxT–T) at 675 nm, which was assigned as the triplet fulminene excited state. The λ
maxT–T values for the [n]phenacene series showed a linear correlation as a function of the ring number n, given by an equation, λ
maxT–T = 60n + 318 (nm).
Photodissociation of the carbonsulfur bond in p-mercaptomethylbenzophenone (MMBP) in acetonitrile has been investigated by means of steady-state photolysis, time-resolved EPR and laser photolysis techniques. MMBP undergoes photodecomposition to yield p-methylbenzophenone in acetonitrile at 295 K. The initial intermediate due to the photodecomposition of MMBP is revealed to be the p-benzoylbenzyl radical (BBR) from the transient absorption and CIDEP measurements. Based on the molar absorption coefficient of BBR, the quantum yield (Φrad) of the BBR formation upon direct photoexcitation was determined to be 0.49±0.03. Triplet sensitization of MMBP by acetone is performed to study the CS bond dissociation in the triplet state of MMBP. Based on the quantum yields and rates of the BBR formation upon sensitization of MMBP, the efficiency (αdis) of the CS bond fission in the triplet state is determined to be 0.51±0.03. The agreement between the Φrad and αdis values indicates that the CS bond dissociation occurs only in the triplet state which is produced with a triplet yield of unity due the rapid intersystem crossing from the lowest singlet state to the triplet state. The lifetime of triplet MMBP at 295 K was determined to be 1.8 ns by using triplet energy transfer from triplet MMBP to 1-methylnaphthalene. The apparent activation energy for the photodecomposition of MMBP was determined to be 0.4 kcal mol−1 whereas the enthalpy of the CS bond of MMBP was estimated to be 60.9 kcal mol−1 that is smaller than the triplet energy (68.5 kcal mol−1) of MMBP. The energy diagram of the excited states of MMBP is shown including the thermodynamic mechanism for the CS bond dissociation of MMBP.
Molecular structures of acetyltetralone (AT) in the enol form were investigated based on the photochemical features in comparison with that of benzoylacetone. Laser flash photolysis of AT in fluid media showed the formation of the triplet state which was characterized by time-resolved electron paramagnetic resonance measurements. From these results, the molecular structure of AT in the enol form was concluded to be endocyclic with the C–C double bond in the tetralone moiety.
Perylenes substituted with trimethylsilylethynyl, tert-butylethynyl and trimethylsilylbutadiynyl groups were prepared, and their photophysical and photochemical properties were investigated through measurements of fluorescence yields, lifetimes, and triplet–triplet absorption. Introduction of these groups caused little change in the rates of fluorescence, but decreased the fluorescence yields and substantially enhanced the rates of non-radiative processes. Observation of the triplet absorption was evident for the origin of the enhanced non-radiative process by the substitution with the triple bond(s).
The decay process of the triplet exciplexes formed between tripler naphthalene derivatives and benzophenone was found to be promoted by their charge transfer character since the rate constant of the nonradiative deactivation process increased with decreasing of the oxidation potentials of naphthalene derivatives.
Photochemical and photophysical aspects of (1) triplet energy transfer (TET) from triplet benzophenone (BP) to naphthalene derivatives (NpD) and (2) formation and decay processes of triplet exciplexes between (NpD)-Np-3* and BP have been studied by 355-nm laser photolysis techniques in the liquid phase. As an initial event, TET occurs from (BP)-B-3* to NpD competing with H-atom transfer (HT), electron transfer (ET), and induced quenching (IQ) on the nanosecond time scale. The TET rate constant increases with an increase of solvent polarity while that of HT decreases, indicating that the (3)(n, pi*) state of BP is slightly mixed with (1)(pi, pi*) in polar media. After formation of 3NpD* by TET from 3BP*, chemical reactions via triplet exciplexes between (NpD)-Np-3* and BP having loose sandwich-like structures with weak charge transfer character take place in the microsecond time scale, depending on the substituent groups of NpD. (1) HT from triplet 2-naphthylammoniun ion and triplet naphthol to BP occurs efficiently to yield the 2-naphthylamine cation radical and naphthoxy radical plus the benzophenone ketyl radical, respectively. The more protic H-atom in NpD is the more reactive in HT. (2) The proton-induced ET from triplet methoxynaphthalene to BP takes place effectively via the protonated triplet exciplex to give the methoxynaphthalene cation and benzophenone ketyl radical. (3) Hydrogen bonding-induced ET from triplet N, N-dialkyl-1-naphthylamine to BP occurs to yield the N-dialkyl-1-naphthylamine cation radical and benzophenone anion radical in the presence of protic solvents.
Photochemical decomposition of amidobiphenyls (R–AN; R=Me, Et and PhCH2) in highly excited triplet states (Tn with n over 2) was investigated in solution by using stepwise two-color laser photolysis techniques. Upon direct excitation, R–ANs underwent photodecomposition in excited singlet states (S1) with a quantum yield (Φ254) of ca. 0.015. Triplet sensitization using acetone (Ac) was employed for efficient formation of the lowest triplet states (T1) of R–AN and for determining molar absorption coefficients of the Tn←T1 absorption. No photochemical reactions were found in the T1 state. Upon 355-nm laser flash photolysis of the T1 states of R–ANs, they underwent decomposition. The quantum yields (Φdec) for the decomposition of the molecules in the T1 states were determined to be ca. 0.08. The photodecomposition of the T1 state was proposed to be responsible for CO–N bond cleavage in the Tn state.
The heats of formation of radicals in solution were measured by using transient absorption and time-resolved thermal lensing methods. The heat of formation of benzophenone ketyl radical (BPK) in benzene solution was determined to be 21kcal/mol. Using this value, the heats of formation of several radicals, which were generated as the counter radicals of BPK in the hydrogen abstraction reaction of triplet benzophenone, were also determined. The solvation energies of radicals are compared with those of their parent molecules and discussed in light of solvation mechanism.
The recovery mechanism of the reaction intermediate (non-chelated enol form) produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane was studied in various solvents by nanosecond laser flash photolysis. The recovery rate and mechanism depend strongly on the nature of the solvent. Unimolecular recovery of the intermediate to the chelated enol form takes place in acetonitrile, diethyl ether and dimethylsulphoxide with extremely small rate constants (1.1, 1.5 and 6.6 s−1 respectively) despite the small activation energy (3.6 kcal mol−1 in 3-methylpentane). The slow unimolecular recovery rate can be ascribed to the small frequency factor (7.0 × 105 s−1), i.e. the large negative entropy change for the formation of the chelated enol form. In non-polar aliphatic hydrocarbon solvents, a bimolecular recovery process via hydrogen-bond interactions between two intermediate molecules is included in addition to unimolecular recovery. In alcohols, a solvent-assisted recovery process by mutual hydrogen exchange between the intermediate and alcohol molecule(s) accelerates the recovery rate. Basic catalysts, e.g. KOH in ethanol and triethylamine in acetonitrile, increase the recovery rate considerably by an additional process through the enolate anion.
Photochemical reactions of 1,4-anthraquinone (1,4-AQ) whose lowest triplet state (T1) is of the π, π∗ type have been investigated in solution by means of steady-state and nanosecond laser flash photolyses. The absorption spectrum of triplet 1,4-AQ in the non-polar solvent is determined for the first time. The triplet state 1,4-AQ is quenched by the ground-state 1,4-AQ in CCl4 to generate a dimerized product. In cycloalkanes, the final products obtained upon photoirradiation are adducts of the solvents to 1,4-AQ initiated with the 1,4-AQ ketyl radical. The formation of the 1,4-AQ ketyl radical originates from H-atom abstraction from cycloalkanes via the T2 (n, π∗) state which is in thermodynamic equilibrium with the T1 (π, π∗) state. In cycloalkenes and styrene, triplet 1,4-AQ gives the [2+2] cycloadducts forming the four-membered rings without H-atom abstraction.
Photophysical and photochemical properties of corannulene (CA) and tetrabromocorannulene (TBCA) in solution were investigated as follows. Spectra, quantum yields and lifetimes of fluorescence in solution at 295K, and spectra and lifetimes of phosphorescence in a rigid matrix at 77K were determined. Phosphorescence at 295K was observed from TBCA. Intersystem crossing yields were determined to be as large as 0.9 by optoacoustic techniques. Triplet–triplet absorption spectra were obtained by laser flash photolysis. Absorption spectra of the radical cation and anion of CA were obtained by pulse radiolysis techniques.
We report on the resonance Raman spectrum of the triplet excited state of 2-methoxy-naphthalene (3ROMe) generated by benzophenone (BP) triplet sensitization. A comparison of the time resolved resonance Raman (TR3) spectra of 3ROMe obtained by energy transfer with that of the spectrum obtained in the absence of BP reveals no change in vibrational frequencies due to weak charge transfer interaction, as expected for a triplet exciplex. It is observed from our computational studies and the experimental data that the unpaired electron in the * orbital of triplet state is more localized on the aromatic ring attached to the methoxy group.
By using syn-[3.4](1,5)naphthalenophane (44NPP) whose naphthalene rings are in a parallel form, photophysical and photochemical properties of naphthalene triplet excimer are reported. 34NPP emits only excimer fluorescence at 295 and 77 K, and phosphorescence was absent even at 77 K. Based on the transient absorption spectra of 34NPP, it is shown that the excimeric triplet state of 34NPP is formed without the locally excited triplet at 295 and 77 K. The triplet excimer of 34NPP is produced via intersystem crossing from the singlet excimer state with a rate of 1.4×107s−1 at 295 K. The triplet excimer in a parallel configuration is shown to be non-phosphorescent.
Benzophenone–benzene and benzophenone–1,4-cyclohexadiene clusters are studied by using one-color multiphoton ionization (MPI) method with time-of-flight (TOF) mass spectroscopy. Many kinds of benzophenone–benzene clusters are generated in the benzophenone–benzene expansion. TOF mass spectrum in the benzophenone–1,4-cyclohexadiene system was obtained with 270 nm laser light. The spectrum shows the ions composed of one benzophenone ketyl radical and several 1,4-cyclohexadiene molecules. It is revealed that hydrogen abstraction reaction occurs in benzophenone–1,4-cyclohexadiene clusters and benzophenone ketyl radical is what REMPI process proceeds through.
The formation and decay processes of triplet polyphenyl compounds (3PPC∗: p-terphenyl, trimethylbenzene and triphenylene) sensitized by triplet benzophenone (3BP∗) were investigated by laser photolysis techniques at 295 K. The net rate constant and efficiency for the formation of 3PPC∗ were obtained. The deactivation pathway of 3PPC∗ was found to proceed via triplet exciplexes since the decay rate of 3PPC∗ as a function of [BP] gave a negative curve. The kinetic parameters for the triplet exciplex were determined.
Naproxen [(+)-(S)- or (−)-(R)-2-(6-methoxynaphthalen-2-yl)propanoic acid, NPX] is a photoactive naphthalene derivative, widely prescribed as nonsteroidal anti-inflammatory drug. A variety of NPX-derived dyads have been synthesized, and their photobehavior has been investigated. In addition to the NPX unit, these dyads contain different types of photo- and/or electroactive moieties, such as naphthalenes (NAP), diaryl ketones (KPF), tertiary amines (PYR), oxetanes (OXT) or thymidine (THY). The excited state intramolecular interactions occurring in the dyads have been examined by both steady-state and time-resolved techniques. As a general observation, dynamic quenching of the NPX singlet-excited state is observed, as indicated by the reduced lifetimes in comparison to the isolated NPX chromophore. This is the result of energy transfer (NPX–NAP, NPX–KPF), electron transfer (NPX–PYR, NPX–OXT), excimer (NPX–NPX) or exciplex (NPX–NAP, NPX–KPF) formation, radiationless decay (NPX–THY), and/or chemical reaction (NPX–OXT, NPX–THY). Thus, the discussed dyads constitute interesting case studies for the photophysical behavior of naproxen in various mechanistic scenarios. For the dyads synthesized as diastereomeric pairs, a significant stereodifferentiation in the photophysical/photochemical properties is observed. Due to the delicate balance between the competing excited state deactivation pathways and the multiple signaling possibilities, these dyads can also be used as probes for the study of specific microenvironments of biological interest.
Photoinduced β-bond dissociation of α-phenoxylacetophenones (PAs) substituted in the phenoxyl moiety has been investigated in polar media by means of laser flash photolysis and triplet sensitization techniques to clarify the spin multiplicity of the reactive states, excited lowest singlet (S1) or triplet (T1) state. The electronic character of the T1 state of PAs was found to be of π,π* from transient absorption spectra at 77K. Any photochemical intermediates were absent in transient absorption upon direct photolysis and triplet xanthone sensitization of PAs having electron withdrawing substituent groups at 295K. Hydroxy-PAs and methoxy-PAs undergo β-cleavage upon both direct excitation and triplet sensitization, providing the corresponding phenoxyl radicals. From disagreement between the quantum yields of the radical formation upon direct excitation and the efficiency of β-cleavage in the triplet state determined by triplet sensitization, it was concluded that the S1 and T1 states are both reactive for β-bond dissociation in PAs. The non-radiative deactivation process of triplet PAs was suggested to be due to efficient quenching of the T2(n,π*) state by the β-phenyl ring. The deactivation profiles of excited PAs are discussed.
Photochemical properties of photoinduced ω-bond dissociation in p-phenylbenzoylbenzyl phenyl sulfide (PPS) having the lowest triplet state (T1) of π,π* character in solution were investigated by time-resolved EPR and laser flash photolysis techniques. PPS was found to undergo photoinduced ω-bond cleavage in the excited lowest singlet state (S1(n,π*)) with a quantum yield (Φrad) of 0.15 for the radical formation, which was independent of excitation wavelengths. Based on the facts of the observation of the absorption spectrum of triplet PPS upon triplet sensitization of xanthone, and absence of CIDEP signal, ω-cleavage was shown to be absent in the T1(π,π*) state of PPS. Considering the electronic character of the excited and dissociative states of PPS, a schematic energy diagram for the ω-bond dissociation of PPS was shown.
Triplet state characteristics of 2,2′- and 4,4′-biphenyldiols have been investigated in different organic solvents using 248 nm nanosecond laser flash photolysis technique. While for 2,2′-biphenyldiol, the triplet state of the molecule is produced as the only transient, for 4,4′-biphenyldiol, some phenoxyl radicals are also formed along with the triplet state following the laser excitation. Triplet quantum yields of 2,2′-biphenyldiol in different solvents are seen to be much lower than those of 4,4′-biphenyldiol. The differences in the laser flash photolysis results of the two diols have been explained on the basis of the presence and the absence of intramolecular hydrogen bonding in the two molecules.
By means of nanosecond laser flash photolysis it has been found that the interaction between triplet benzophenone (3BP∗) and naphtholate anions (NpO−) is not triplet energy transfer but electron transfer to yield the benzophenone anion (BP−·) and naphthoxy (NpO·) radicals in acetonitrile-H2O (4:1 v/v) at 295 K. From the kinetic parameters for quenching of 3BP∗ by NpO− and quantum yields for the formation of BP−· and NpO·, it was found that the efficiency for electron transfer between 3BP∗ and NpO− is unity due to the triplet spin-multiplicity of radical pairs.
[3.3](4,4′)Biphenylophane (BPP) is synthesized, and the photophysical and photochemical properties are studied by means of emission and transient absorption measurements. BPP emits excimer fluorescence at 295 and 77K, and phosphorescence from the locally-excited (LE) triplet state at 77K. Based on the transient absorption spectra of BPP, it is found that the excimeric triplet state of BPP is produced along with the LE triplet at 295 and 77K. The triplet excimer of BPP is shown to be formed via intersystem crossing from the singlet excimer state, and concluded to be non-phosphorescent.
A bimolecular rate constant (kq) and an efficiency (αTET) of an endoergonic triplet energy transfer from triplet α-naphthyl phenyl ketone (NPK) to α-naphthylmethyl phenyl sulfide (NMPS) in solution, where the energy difference (ΔET) is 1.7kcalmol−1, have been determined by means of nanosecond laser flash photolysis. Base on the Arrhenius plots of the kq value for the NPK–NMPS system, an activation energy (ΔEa) for the endoergonic triplet energy transfer is found to be 2.6kcalmol−1, which is larger than the endothermic energy gap (ΔET=1.7kcalmol−1). The endoergonic triplet energy transfer in solution is demonstrated to proceed with the aid of thermal activation for the first time.
Formation of a triplet-equilibrium between benzophenone (BP) and dibenzothiophene (DBT) was studied in acetonitrile by 355 nm laser flash photolysis. The triplet-equilibrium constant (Keq = 1.7) was determined at 295 K as well as the absolute rate constants for triplet energy transfer from BP to DBT (kf = 3.1 × 109 M−1 s−1) and back triplet-energy transfer (kb = 2.0 × 109 M−1 s−1) whose ratio (kf/kb) was in agreement with the Keq value. It was revealed that the triplet-equilibrium formation in the BP-DBT system was promoted by the entropy change (ΔS = 2.0 cal mol−1 K−1) rather than the enthalpy change (ΔH = 0.3 kcal/mol).
By means of 355 nm laser flash photolysis in the benzophenone-hydroxyanilinium ion system in acetonitrile-H2O (1 : 1, v/v) with [H2SO4] = 0.18 M, it is found that the products of H-atom abstraction by triplet benzophenone from hydroxyanilinium ion are the benzophenone ketyl and hydroxyaniline cation radicals, i.e. the NH3+ group is more reactive to H-atom abstraction by triplet benzophenone than the OH. The more protic the hydrogen atom is, the more reactive it is found to be for H-atom abstraction by triplet benzophenone in a collision complex with charge-transfer character in the liquid phase.
Photo-induced β-bond dissociation of phenacyl phenyl sulfide (PPS) has been investigated in acetonitrile by laser photolysis techniques.
Direct excitation of PPS at 295 K provided the acetylmethyl and phenylthiyl radicals with a quantum yield (Φrad) of 0.18, whereas triplet sensitization using xanthone revealed an efficiency for β-cleavage of triplet PPS (α
rad) of ≥0.64. From disagreement between the Φrad and α
rad values, it was concluded that both the lowest excited single and triplet states are reactive for β-bond dissociation in PPS. The photochemical processes of excited PPS, including β-cleavage, are discussed in detail.
The interaction of a triphenyl methane (TPM) dye: crystal violet (CV) with the triplet state (S-3*) Of the aromatic molecules biphenyl, p-terphenyl or anthracene was studied by pulse radiolytic kinetic spectrophotometry. in each case, studies of time-resolved spectra and kinetics showed that an exciplex between sensitiser triplet and ground state of CV is formed as an intermediate before the former dissociates to give triplet state of CV, [S-3* + CV (kb)reversible arrow(kf) (3)(S..CV)* -->(k3) S + (CV)-C-3*]. A detailed kinetic analysis has been carried out to evaluate various kinetic kb parameters. The equilibrium constant, K (k(f)/k(b)), was found to be 1.8 X 10(4) dm(3) mol(-1), 1.53 X 10(4) dm(3) mol(-1) and 1.32 X 10(4) dm(3) mol(-1) in the case of biphenyl, p-terphenyl and anthracene sensitiser triplet states, respectively. (C) 1998 Elsevier Science S.A.
The photothermal tautomerization processes between enol and keto forms of 4-tert-butyl-4'-methoxydibenzoylmethane (trade name, Avobenzone) in acetonitrile have been studied by steady-state and laser flash photolysis. The keto form is produced upon photolysis of the enol in only acetonitrile with a quantum yield of 0.014. The molar absorptivity of the keto form was determined. Phototautomerization from the keto to the enol form was not seen. Laser flash photolysis of the keto form recognized the formation of the triplet state. In the dark, the keto form underwent thermal tautomerization to the enol with a lifetime of 5.1 h at 295 K. The enolization rate in acetonitrile was not accelerated by the presence of alcohols and/or water but increased with increasing temperature and followed the Arrhenius expression. The activation energy and the frequency factor were determined for the enolization process from the keto to the enol form. On the basis of the energy states of the tautomers and isomers as estimated by DFT calculations, a schematic energy diagram was determined for the photothermal tautomerization processes in acetonitrile.
The rate constants (kHA and kIQ) for the hydrogen atom abstraction (HA) and induced-quenching (IQ) processes between triplet benzophenone (3BP∗) and halogenated toluenes (XTL) were determined by means of nanosecond laser flash photolysis techniques. It was found that the kIQ value increased proportionally with increasing the squared summation of the spin-orbit coupling constant (ζ) of atoms involved in XTL while the kHA's were almost indifferent. It was demonstrated that the IQ process was the intersystem crossing process which was induced mainly by the spin-orbit interaction between 3BP∗ and XTL.
Several high-level quantum chemical calculations have highlighted the prominent weight of nonadditivity within the total stabilization energy of multiply hydrogen-bonded complexes, such as exemplified by water oligomers (reviewed in ref 1). We have evaluated the extent to which the SIBFA (sum of interactions between fragments ab initio computed) molecular mechanics procedure2-4 could account for nonadditivity in such complexes. This takes advantage of the separability of the energy expression into five distinct components, each of which have inherent anisotropic features. For that purpose, we have considered several representative water oligomers encompassing from n = 3 to n = 20 molecules, in cyclic and acyclic, as well as, for n = 6 and beyond, tridimensional cubic arrangements. Single-point ab initio SCF and MP2 supermolecular energy computations were performed in the energy-minimized structures. A decomposition of the SCF intermolecular interaction energy was done, for n = 3−6, using the restricted variational space approximation (RVS) due to Stevens and Fink.5 This enabled the quantification of the relative weights of each of the two individual ab initio second-order terms, polarization and charge-transfer (Epol and Ect, respectively) to nonadditivity. The SIBFA procedure was found to faithfully reproduce the ab initio results, both in terms of the total ΔE's and in terms of the separate nonadditive behaviors of its own Epol and Ect terms. It was also able to match very closely the results of the recent density functional theory computations of Lee et al.6 on cubic arrangements of water as occurring in ice. Thus, upon increasing n from 8 to 20, ΔE(SIBFA) was found to converge asymptotically toward a value of −11.5 kcal per molecule of water, close to the experimental binding energy of ice of −11.4. For n = 20 in this structure, the average dipole moment per water molecule was computed to be 2.74 D, itself very close to the value of 2.70 D in ice.
Laser flash photolysis studies at 355-nm on the photoreactions of the benzophenone (BP) and N,N-dialkyl-1-naphthylamine, DANA (N,N-dimethyl-1-naphthylamine, DMNA, and N,N-diethyl-1-naphthylamine, DENA) system have been carried out with and without H2O and methanol in acetonitrile (ACN) at 295 K. In the nanosecond time scale, triplet energy transfer from triplet BP (3BP*) to DANA occurs with the efficiency φTET (0.74 for DMNA and 0.61 for DENA) regardless of the presence of H2O and methanol. After the formation of triplet DANA (3DANA*), the triplet exciplex 3(DANA···BP)* with weak charge-transfer character is produced with the equilibrium constant K1 (10 M-1 for DMNA and 9 M-1 for DENA) between 3DANA* and BP. The mechanism for the formation of 3(DANA···BP)* is shown in Scheme 1. In the presence of H2O and methanol, it is found that the intraexciplex electron transfer takes place to give the BP anion (BP•-) and DANA cation (DANA•+) radicals in the hydrogen-bonded triplet exciplex 3(DANA···BP)*HB by H2O or methanol. The mechanism for the production of DANA•+ and BP•- is proposed in Scheme 2. The equilibrium constants K2 for the formation of 3(DANA···BP)*HB with H2O and methanol obtained are 0.55 and 0.45 M-1 for DMNA, 0.50 and 0.40 M-1 for DENA. The rate constants ket for the intraexciplex electron transfer induced by hydrogen bonding are determined to be 2.5 × 107 s-1 for DMNA and 1.4 × 107 s-1 for DENA. It was revealed that the driving force for intraexciplex electron transfer is the negatively enlarged reduction potential of BP in 3(DANA···BP)*HB due to the hydrogen bonding to the carbonyl group of BP in 3(DANA··· BP)*.
Photoreduction processes of excited benzophenone (BP) by N,N-dimethylaniline (DMA) in acetonitrile solution were studied by means of picosecond-femtosecond laser photolysis and time resolved transient absorption spectroscopy as well as transient photoconductivity measurement. Proton transfer process in the triplet ion pair formed by electron transfer (ET) at encounter between 3BP* and DMA, competing with the ionic dissociation process was observed. It was clearly demonstrated that, in addition to the triplet ion pair, the ion pair produced by the excitation of the CT complex between BP and DMA formed in the ground state and that produced by ET reaction between 1BP* and DMA played important roles in the reaction processes of excited benzophenone. The behaviors of the three kinds of ion pairs were investigated in detail, leading to the elucidation of reaction mechanism of each ion pair. The reactivity characteristic of each kind of the ion pair and its relation to the structure of the ion pair were discussed.
The mechanism of photoreduction of benzophenone (BP) by diphenylamine (DPA) in isooctane as well as acetonitrile and other polar solvents has been investigated by means of picosecond laser photolysis and time-resolved transient absorption spectral measurements. The results of measurements have demonstrated clearly that the hydrogen abstraction and charge transfer (CT) or ion pair (IP) state formation by electron transfer are competing at encounter between triplet benzophenone (3BP*) and DPA both in nonpolar and polar solvent, and the CT or IP state relaxed with respect to the donor acceptor configurations and solvation does not contribute to the ketyl radical formation. It has been concluded that the very short-lived CT state at encounter between 3BP* and DPA plays a crucial role in the hydrogen abstraction reaction, i.e. the mutual orientation of 3BP* and DPA in this very short-lived CT state at encounter will determine the successive process, the formation of the ketyl radical or relaxed CT or IP state.
A classical intersecting-state-model of harmonic oscillators was applied to the study of proton-transfer reactions. The activation free-energy barriers were found to be strongly dependent on the bond order of the transition state,n†, with this parameter ranging between 0.5 and 1.0. The carbon acids are closer to the first limit, HF is near to the second, and the nitrogen and oxygen acids are somewhere between. Such differences can be attributed to the polarity of the XH bonds. For a considerable number of reactions the distance between the minima of the harmonic oscillators is virtually independent of the reaction free energies, ΔG°, but for others such distance increases with an increase in |ΔG°|. The observed increase depends on the mixing entropy, λ. Non-linear Brönsted relationships and anomalous exponents could be interpreted by the model. The exponents are found to be related with the extent of proton transfer when the stretching force constants of the reactive bonds in reactants and products are the same, n† is constant and λ is high. Isotope effects depend on ΔG°, but their maximum values seem to be related to n†, being higher when n†= 0.5 and close to 1 when n†= 1.0. Proton-transfer reactions in acid–base catalysis are also found to conform with the general picture provided by the model. Suggestions for the molecular parameters which dominated solvent effects are discussed.
Intermolecular and intramolecular hydrogen abstractions of ketones have been studied by a tunnel-effect theory of radiationless transitions. The effects of ketone substituents on reaction rates can be accounted for the change in the reduction potential of the ketones, by the nature and energy of the excited states and by the CH bond strengths. It is shown that π, π* states have an intrinsic reactivity for hydrogen abstraction ≈ 10–2-10–4 times lower than n, π* states, but when both levels are energetically close, the observed reactivity is caused by the thermal equilibrium population of the two states. Substrates with n electron orbitals react generally with ketones via a charge-transfer mechanism, which is controlled by the reduction potential of the ketones, the ionization energy of the substrates, the electronic energy of excitation and the CH bond strengths. Such a mechanism should be viewed as a mechanistic continuum between a radical-like reaction and an electron transfer process and can lead to an increase or to a decrease in the reaction rates, when compared with the radical-like mechanism. Reactions of 1(n, π*), 3(n, π*) and 3(π, π*) states are found to be adiabatic, but photoabstractions by 1(π, π*) levels have an electronic forbidden factor 10–2. Such findings are rationalized in terms of Salem's state correlation diagrams.
The hydrogen abstraction reactions of triplet states of thioketones, quinones, aza-aromatic compounds, olefins and azobenzenes have been studied theoretically as radiationless transitions in terms of the tunnel-effect theory. Good agreement with experiment is found and rates of reaction are predicted for several systems which have not yet been studied experimentally. Although in general n,π* states are more reactive than π,π* states, the theory reveals cases where π,π* reactivity, which is strongly dependent on bond length changes between reactants and products, can be higher than n,π* reactivity.
Hydrogen abstraction reactions of electronically excited carbonyl compounds are treated as radiationless transitions using the tunnel-effect theory. The rates of reaction are controlled by the Franck–Condon factors of the vibrational stretching motions of the carbonyl group in the excited carbonyl compounds, and the CH oscillator of the substrate with the CO and OH vibrations in the ground state of the final products. The rates are dependent on electronic energy, vibrational frequencies, the reduced mass of the oscillators, the CH bond strength of the reactive bonds of the substrates and bond distances. In cases where a charge-transfer mechanism is involved, the rates are also a function of the carbonyl compound reduction potential and substrate ionization energy. The model gives rates in good agreement with experiment and explains singlet and triplet reactivity of n, π* and π, π* states, inter- and intra-molecular processes, normal and inverse deuterium isotope effects and the variation of the rates with temperature. Rates are predicted for several systems which have not yet been studied experimentally. Results from the present model are compared with those from other theories of photochemical reactivity.
Laser flash photolysis studies on hydrogen-atom transfer (HT) from the triplet hydroxynaphthylammonium ion (the protonated form of 5-aminonaphth-1-ol, HORNH+3) to benzophenone (BP or CO) in methanol–water (9 : 1 v/v) at 295 K have been carried out in order to elucidate which hydrogen atom of the substituent groups is more reactive for HT, and what is the mechanism. It was found that HT from triplet HORNH+3(3HORNH+*3), produced by triplet energy transfer from 3BP*, occurs to BP to yield the hydroxynaphthylamine radical cation (HORNH˙+2) and the benzophenone ketyl radical (ĊOH) with efficiencies of 0.92 and 0.48 in the presence of 0.015 and 0.5 mol dm–3 H2SO4, respectively. The decay rate of 3HORNH+*3(kobs) decreases with increasing acid concentration, approaching a constant value at higher acid concentrations. This behaviour of kobs at higher acid concentrations cannot be explained by the HT mechanism alone for the naphthylammonium ion (RNH+3)–BP system previously reported (S. Kohno, M. Hoshino and H. Shizuka, J. Phys. Chem., 1991, 86, 1297). With an increase of [BP], kobs increases showing a levelling off at higher [BP], which indicates the formation of a triplet exciplex between 3HORNH+*3 and BP. The HT mechanism for the HORNH+3–BP system was interpreted as a composite mechanism. The best-fit kinetic parameters were found to be ko= 5.0 × 105 s–1, kHT=ke1= 107 s–1, K1= 5 × 102 dm3 mol–1, K′2= 3 dm3 mol–1 and k′d/kb= 6 × 102 dm3 mol–1. It is concluded that (i) the hydrogen atom of the NH+3 group (which is more protic than that of OH) is the more reactive for the HT reaction in the present system and (ii) the proposed composite mechanism including two different conformations of the protonated triplet exciplexes, 3(HORNH+3[graphic omitted]OH)* and 3(H+3NROH[graphic omitted]OH)*, is involved in the present HT.
Proton-assisted photoionization of methoxynaphthalenes sensitized by triplet aromatic ketones occurs effectively to give the cation radical and the ketyl radical.
Results of our ps-fs laser photolysis studies on interactions between unlinked D(donor)-A(acceptor) systems in luminescence quenching reactions and reaction dynamics of produced transient CT (charge transfer) or IP(ion pair) states and also on some D-A systems linked by flexible chains or directly can be interpreted satisfactorily by considering the formation of multiple CT and/or IP states with different structures depending on solvation, strength of D-A interaction and energy gap for ET (electron transfer) and change of the distribution of those states in the course of reaction. Also, the most fundamental factors regulating the photoinduced ET have been examined by using fixed distance dyads and some new critical aspects concerning the energy gap dependences of ET reactions have been demonstrated.
Fundamental aspects of the photoinduced charge transfer and charge separation processes are discussed on the basis of the picosecond laser photolysis studies of some typical intra- and intermolecular exciplex systems, excited singlet state of EDA complexes and monophotonic electron ejection of some low ionization potential diamines in polar solvents. Picosecond laser studies have been extended to the biologically important porphyrin-quinone systems. Short-lived porphyrin-quinone exciplex can be observed in nonpolar solvent, but solvation-induced ultra-fast deactivation of the electron transfer state takes place in strongly polar solvents.
A new aspects of the role of the solvent mode in the photoinduced electron-transfer process of electron donor and acceptor system in polar solvents has been exploited. Taking into account the important fact that the vibrational frequency of the solvent mode in the initial neutral state of the reactants is considerably smaller than that in the final ionic state, we have derived a new formula for the energy-gap dependence of the electron-transfer rate. In this formulation, the activation energy is greatly reduced and the electron-transfer rate is almost independent of the energy gap over a wide down-hill energy region. This qualitative feature explains the experimental results for the relation between the bimolecular quenching rate constant kw and the standard free-energy change ΔG° associated with electron transfer in the "anomalous region".
Laser flash photolyses have been carried out on solutions of the 2-naphthylammonium ion (RNH+3−) benzophenone (BP) [or acetophenone (AP)] system. It is found that the hydrogen-atom transfer reaction from 3RNH+3 (produced by triplet sensitization of the ketones) to the ground BP (or AP) occurs effectively to give RNH+3 and > COH.
Time-resolved absorption spectra of pyrene-aromatic amine systems have been measured by picosecond laser spectroscopy. It has been confirmed that the heteroexcimer is a real reaction intermediate in the photoinduced hydrogen-atom transfer in the pyrene-diphenylamine system in toluene. The lifetime of the heteroexcimer is ≈ 200 ps, in a good agreement with the rise time of 1-hydro-1-pyrenyl radical.
The dynamic bahavior of the excited singlet of naphthylamines involving proton-induced quenching in a sulfulic acid-water mixture has been studied by means of fluorometry and nanosecond time resolved spectroscopy. A significant fluorescence quenching of neutral α- and β-naphthylamines induced by protons was observed: 1kq = 8.9 (±0.3) × 109 M−1 s−1 and 1kq = 3.3 (±0.2) × 108 M−1 s−1 at 300 K, respectively. On the basis of kinetic analyses, the corrected values of pK*a were determined to be −1.0 (±0.2) and −0.8 (±0.1) for α- and β-amines, respectively. The isotope effect on the quenching was also examined.
Since the late 1940s, the field of electron transfer processes
has grown enormously, both in chemistry and biology.
The development of the field, experimentally and
theoretically, as well as its relation to the study of other
kinds of chemical reactions, represents to us an intriguing
history, one in which many threads have been
brought together. In this lecture, some history, recent
trends, and my own involvement in this research are described.
Laser photolysis studies of benzophenone in a 2-methyltetrahydrofuran (MTHF) solution containing 1.7 M diethylaniline have been carried out in the temperature range 300-80 K. Only the ketyl radical is detected in the temperature range 300-180 K. The transient spectrum having absorption bands at 740 and 470 nm is observed in the range 160-100 K: the former band is very similar to that of the benzophenone anion radical and the latter to that of the cation radical of diethylaniline. The transient decays according to first-order kinetics. The rate constant, kd, is expressed as a function of temperature: kd = k0 + 1.12 × 109 exp(-2000/RT) s-1, where k0 < 104 s-1. The slow formation of the ketyl radical is detected in the range 160-140 K. The rate for the formation of the ketyl radical at 160 K is found to be identical with that for the decay of the transient having absorption bands at 740 and 470 nm, indicating that the ketyl radical is produced from the transient. No formation of the ketyl radical is observed below 100 K. At 80 K, the transients are the triplet benzophenone and the triplet molecular complex between benzophenone and diethylaniline. Phosphorescence spectra are measured for the MTHF solution of benzophenone in the presence of 1.7 M diethylaniline in the temperature range 300-80 K. Broad phosphorescence centered around 520-580 nm observed in the range 140-100 K is considered to be due to the triplet exciplex formed between the triplet benzophenone and diethylaniline. On the basis of phosphorescence and laser photolysis studies, the transient having the absorption bands at 740 and 470 nm is ascribed to the triplet exciplex between the triplet benzophenone and diethylaniline with a strong charge-transfer interaction.
The excited-state proton-transfer reactions of 4-(9-anthryl)-N,N-dimethylaniline (A) in EtOH-H2O (4:1 by weight) mixtures at 300 K have been studied by fluorimetry and the single-photon-counting method. It is found that there is no excited-state prototropic equilibrium of A. The excited acid and base forms of A decay independently with single exponential functions. The pKa* value (2.4 ± 0.2) of A obtained from the usual fluorescence titration curves is, therefore, an apparent value, reflecting the pKa value (2.5 ± 0.2) in the ground state. This is mainly caused by the extremely slow proton-dissociation rate of 1A+H* compared to its decay rate (2 × 108 s-1). No proton-induced quenching of A is observed, though the intramolecular CT state 1ACT* is produced very rapidly via 1A* under the experimental conditions.
Laser photolysis studies of benzophenone in both sec-butylamine and triethylamines were carried out in the temperature range 300-77 K. For the sec-butylamine solution of benzophenone, the transients observed after laser pulsing are found to be the ketyl and anion radicals of benzophenone. The ratio of the yield for the formation of ketyl radical to that of the anion radical is markedly dependent on temperature: the ketyl radical is the major product at high temperatures while the anion radical becomes predominant at low temperatures. On the other hand, the triethylamine solution of benzophenone gives solely the ketyl radical as a photoproduct in the temperature range studied. The photochemical reaction of benzophenone in both sec-butylamine and triethylamine is markedly suppressed on going from high to low temperatures. These results are discussed in detail on the basis of the photochemical reaction mechanism involving the formation of the triplet charge-transfer complex (i.e., triplet exciplex) between triplet benzophenone and an amine molecule. The solvent effects on the photoreaction are also discussed.
Measurements of the stabilization constants Kg of the 1:1 phenanthrylammonium ion-18-crown-6 complexes in MeOH-water (9:1) mixtures have been carried out by means of the fluorimetric titration method. Complex formation of the phenanthrylammonium ions (RN+H3) with 18-crown-6 decreases markedly the fluorescence quantum yields of the excited neutral amine species produced by deprotonation of the excited RN+H3-18-crown-6 complexes. In contrast, the fluorescence quantum yields for the protonated species increase significantly with increasing concentration of 18-crown-6. For 2- or 3-RNH2 having a large Kg value, the emission from the neutral amine species could not be detected. The Kg values for the RN+H3-18-crown-6 complex in the ground state are in the order 3RNH2 > 2RNH2 ≫ 1RNH2 > 9RNH2 ≫ 4RNH2, which reflect well steric hindrance between the phenanthryl group (R) and 18-crown-6 of a Corey-Pauling-Kolton molecular model. The thermodynamic parameters for the complex formation were obtained from temperature effects upon Kg values.
A new method for determining these coefficients has been developed. The method employs pulse radiolysis for triplet excitation and, by means of triplet-triplet (T-T) energy transfer, leads to a comparison between the extinction coefficients of the benzophenone ketyl radical (phi_2COH) and the triplet states of several aromatic compounds whose triplet levels are lower than that of benzophenone. Since the ketyl extinction coefficient can be determined independently, the actual T-T extinction coefficients can be obtained. Values of the extinction coefficient at the absorption maximum were obtained for anthracene (57 200 M-1 cm-1), naphthalene (22 600 M-1 cm-1), phenanthrene (21 000 M-1 cm-1) and 1,2-benzanthracene (25 100 M-1 cm-1), all relative to an extinction coefficient for the benzophenone ketyl maximum around 540 nm determined to be 3220 M-1 cm-1. For several other compounds the above method for various reasons could not be applied. In such cases estimates were obtained by energy transfer experiments, leading to comparisons between their triplet absorptions and those of the compounds already determined. Estimates were thus obtained for the maximum T-T extinction coefficients of benzophenone (10 300 M-1 cm-1), biacetyl (6400 M-1 cm-1), diphenyl (35 400 M-1 cm-1), and acridine (28 800 M-1 cm-1). During the course of these experiments estimates were also obtained for the rates of the following reactions: e^-aq. + benzophenone (kII = 3.0 x 1010 M-1 s-1), OH^. + benzophenone (kII = 8.7 x 10^9 M-1 s-1) and hydrogen abstraction by triplet benzophenone from cyclohexane, to give phi_2COH (kII~ 8 x 10^5 M-1 s-1).
Photoreduction processes of benzophenone (BP)-N,N-diethylaniline (DEA) system in acetonitrile solution were studied by means of femtosecond-pecosecond laser photolysis and time resolved transient absorption spectroscopy. The reaction processes including the formation of geminate ion pairs (IP) by electron transfer (ET) between BP* and DEA at encounter followed by intra-IP proton transfer giving the ketyl radical (BPH.) were clearly observed in both triplet and singlet excited state, while the IP produced by excitation of the CT complex between BP and amine formed easily in more or less concentrated solutions did not give BPH.. In addition to the detailed investigations on the BP-DEA system, we made comparative studies on BP-tertiary aromatic amine systems including N,N-diethyl-p-toluidine, N,N-dimethylaniline, and N-methyldiphenylamine. We observed clearly the characteristic tendency that the proton transfer rate in the 3IP decreased with decrease of the oxidation potential of the amine. This result was interpreted as due to the increase of the inter-ionic distance in the 3IP with increase of the free energy gap for the charge separation at encounter.
The rate constants, activation energies, and A factors for the interaction of triplet benzophenone, acetophenone, and benzaldehyde with a series of hydrocarbons have been determined and the data compared with the corresponding reactions of alkyl and alkoxy radicals. The difference in reactivities for hydrogen abstraction from alkanes is attributed mainly to variations in the activation energy. The A factors are remarkably constant (10 7.8 M -1 sec -1). While triplet benzaldehyde is about six times more reactive than triplet benzophenone for hydrogen abstraction from an aliphatic hydrocarbon such as cyclohexane, its reactivity toward an aromatic hydrocarbon such as benzene is 800 times greater than that of triplet benzophenone. The negative activation energies found in the case of aromatic hydrocarbon interactions with triplet benzaldehyde suggest that there is an equilibrium between benzaldehyde triplets and a complex.
Studies were initiated utilizing picosecond (ps) absorption spectroscopy, to directly monitor the dynamics of electron transfer from 1,4-diazabicyclo(2.2.2)octane (Dabco) to the excited states of benzophenone and fluorenone. These two systems were chosen because of their contrasting photochemistry. The quantum yield for photoreduction of benzophenone in polar solvents is generally greater than 0.1, while that of fluorenone is zero. In polar solvents, the proposed mechanism dictates that an electron is transferred to the excited singlet state fluorenone, which then back-transfers the electron, regenerating ground-state fluorenone and amine. Photolysis of benzophenone in the presence of an amine transfers an electron to an excited triplet state, forming an ion pair that is stable relative to diffusional separation. The results of this study verify this proposal.
Solvent effects on the primary processes in the photoreduction of benzophenone by N,N-dimethylaniline and N,N-diethylaniline have been studied by using picosecond absorption spectroscopy. It has been found that the photoreduction proceeds by rapid electron transfer to form the solvent-separated ion pair. This is followed by diffusion to form a contact ion pair. Subsequent to contact ion-pair formation, proton transfer is observed. Solvent effects on the ion-pair intermediate indicated a dielectric dependent equilibrium between the contact ion pair and the ketyl radical.
The mechanism of charge transfer followed by proton transfer in the hydrogen atom transfer reaction of excited pyrene-primary and -secondary amine systems has been directly demonstrated by means of a picosecond laser photolysis method. In the case of the pyrene-N-ethylaniline heteroexcimer system in hexane, for example, it has been observed that the 1-hydro-1-pyrenyl radical and the pyrene triplet state are produced simultaneously in the time region of subnanosecond to nanosecond. The deuteration of the NH group of the amine affects considerably the rate of formation of the 1-hydro-1-pyrenyl radical but not the rate of intersystem crossing from the heteroexcimer. On the basis of the results obtained for various pyrene-primary and -secondary aromatic amine heteroexcimer systems, the nature of the interaction between donor and acceptor in these heteroexcimers and their conformations in relation to the mechanism of the hydrogen atom transfer reaction via heteroexcimer have been elucidated.
Proton-induced quenching, photochemical, and thermal isotope-exchange reactions of methoxynaphthalenes in H/sub 2/O (or D/sub 2/O)-CH/sub 2/CN mixtures at moderate acid concentrations were studied by means of emission, /sup 1/H NMR, and mass spectroscopy and measurements of reaction quantum yields. It is demonstrated that the proton-induced fluorescence quenching of 1-methoxynaphthalene (..cap alpha..-RH) proceeds via electrophilic protonation at the proper carbon atom of the aromatic ring in the lowest excited singlet state (/sup 1/L/sub a/) in polar media, leading to proton exchange or isotope exchange mainly at position 5 (slightly at position 8) of the naphthalene ring. The rate constant /sup 1/k/sub R/ for electrophilic protonation to the carbon atom of the aromatic ring in the excited state is almost equal to that of /sup 1/k/sub q/ (the rate constant for proton-induced quenching). The isotope-exchange reaction via the triplet state is negligibly small (about 5% of that via the excited singlet state). For 2-methoxynaphthalene (..beta..-RH) both proton-induced quenching and isotope exchange in the excited state (/sup 1/L/sub b/) scarcely occur. The intramolecular CT structure in the excited state is responsible for the quenching. At higher temperatures (greater than or equal to318 K), the thermal isotope-exchange reactions of ..cap alpha..- and ..beta..-RH takemore » place at positions 2 and 1, respectively; the exchange rate for the latter is faster than that for the former. It is shown that the reaction rate for protonation in the excited state is extremely fast compared with that in the ground state. Activation parameters for the reactions in the excited and ground states are determined. An H-D isotope-exchange reaction mechanism is proposed on the basis of the experimental results with the aid of the usual MO calculations.« less
Laser flash photolyses at 337 nm have been carried out on methanol solutions of the 1-naphthol and benzophenone system. It is found that hydrogen atom transfer reaction from the triplet 1-naphthol 3ROH* (produced by triplet sensitization of benzophenone) to the ground benzophenone BP occurs effectively through the triplet complex 3ROH*⋯OC< to give the 1-naphthoxy radical RO· and the ketyl radical >ĊOH. The triplet-triplet energy transfer kET (4.1 × 109 M-1 s-1) from 3BP* to ROH is competitive with both the usual hydrogen atom abstraction kHA′ (7.4 × 108 M-1 s-1) of 3BP* from ROH and the quenching kq′ (2.2 × 109 M-1 s-1) of 3BP* induced by ROH at 290 K. These primary processes of 3BP* are completed in 300 ns, and the equilibrium 3ROH* + >CO ⇄ 3ROH*⋯OC< is established very quickly. The equilibrium constant K* for the 3ROH*⋯OC< formation was obtained to be 16.7 M-1 at 290 K (ΔH* = -2.4 kcal mol-1 and ΔS* = -2.7 eu). Then, the hydrogen atom transfer reaction takes place via the triplet complex with the rate constant kHT (1.3 × 106 s-1 at 290 K, AHT = 3.7 × 109 s-1; ΔEHT = 4.6 kcal mol-1). Both RO· and >ĊOH produced by laser flash photolysis decay mainly via the radical reaction kR between them with the rate constant 1.7 × 109 M-1 s-1 at 290 K. The reaction mechanism on the hydrogen atom transfer reaction from 3ROH* (produced by triplet sensitization of BP) to yield RO· and >ĊOH is shown in detail.
The dynamics of proton transfer within an acid-base pair, which is created by a photoinduced electron transfer from an amine to a ketone, is investigated through picosecond absorption spectroscopy. Questions relating to molecular motion within the encounter complex and the distance dependence of proton transfer are addressed. Specifically, for substituted benzophenones and diphenylmethylamine, isotope and solvent dependence studies reveal that, although an acid-base pair is formed as an encounter complex, prior reorientation within the complex must take place before proton transfer. Other systems investigated include the transfer of a proton from the radical cation of N-methylacridan to the radical anions of benzophenone and anthraquinone as well as an intramolecular proton transfer for a molecule containing dimethylaniline linked to benzophenone by a methylene bridge.
Proton-transfer reactions in the excited singlet state of naphthylammonium ion-18-crown-6 complexes in MeOH-H2O (9:1) mixtures have been studied by means of the single photon counting method with fluorimetry. It is found that the complex formation of naphthylammonium ions with 18-crown-6 decreases markedly the proton-transfer rate (k1) in the excited state, resulting in an increase of its lifetime. The back protonation rate in the excited state is negligibly small compared with those of the other decay processes; there is no excited-state prototropic equilibrium in the naphthylammonium ion-crown complexes (RN+H3-crown). The one-way proton-transfer reaction is elucidated by the presence of the excited neutral amine-crown complex (RNH2-crown)* produced by deprotonation of (RN+H3-crown)*, where protonation to the amino group is structurally blocked by 18-crown-6 and the naphthyl group (R) of the complex. However, proton-induced quenching (kq′) occurs effectively especially in the 1-naphthylammonium ion-crown complex. The ground-state association constants (Kg) of the complexes can be determined easily by the fluorescence titration method. Temperature effects upon the excited-state proton-transfer reactions of the complexes have been also carried out in order to study their thermodynamic properties. A Corey-Pauling-Kolton model of the anilinium ion-18-crown-6 complex proposed by Izatt et al.2 is strongly supported by the present work.
The fluorescence quenching of tyrptophan (Trp) in the absence and presence of 18-crown-6 in CHâOH-Hâ (9:1), vv) mixtures has been studied by means of nanosecond time-resolved single-photon counting, fluorimetry, and photochemical H-D isotope exchange. The fluorescence intensity increases markedly with increasing concentration of 18-crown-6. The quenching can be estimated from the equation k/sub q/ = tauâ/sup 1 -/ - (tau â/sup max/)â»Â¹, where tauâ and tauâ/sup max/ denote the fluorescence lifetimes for free Trp and the 1:1 Trp-18-crown-6 complex, respectively. The internal quenching originates from the electrophilic protonation of the /sup +/NHâ group of Trp at the C-4 position of the excited indole ring plus the charge-transfer interaction between the excited indole ring and the ammonium group. The stabilization constant K/sub g/ for the 1:1 complex of Trp with 18-crown-6 has been determined by means of fluorimetry.
The excited state behavior of tryptamine and 1,2,3,4-tetrahydrocarbazoles possessing alkylamino side chains in the absence and presence of 18-crown-6 in MeOH-HâO (9:1) mixtures has been studied by means of nanosecond single-photon counting, fluorimetry, and photochemical H-D isotope exchange. The fluorescence intensity of these indoles increases significantly with increasing concentration of 18-crown-6. The relatively short lifetime of the tryptamine ammonium ion 1 is not ascribed to external quenching but rather to internal quenching. The rate constant k/sub q/ for internal quenching can be estimated from the equation k/sub q/ = tauââ»Â¹ - tau/sub max/â»Â¹, where tauâ and tau/sub max/ represent the fluorescence lifetimes for free 1 and the 1:1 1-crown ether complex, respectively. Internal quenching originates from electrophilic proton attack by the -N/sup +/Hâ (or -N/sup +/Dâ) group of 1 at the C-4 position of the excited indole ring. For 3 (the tetrahydrocarbazole derivative R(CHâ)âN/sup +/Hâ) the k/sub q/ value comprises the electrophilic proton attack at the C-8 position plus other quenching (probably charge-transfer quenching) between the excited indole moiety (R*) and the -N/sup +/Hâ (or -N/sup +/Dâ) group. The stabilization constant K/sub g/ for the corresponding ammonium ion and 18-crown-6 can be determined by fluorimetry. The kinetic and thermodynamic parameters for the internal quenching and the complex formation, respectively, have been described.
The mechanisms of photoinduced electron transfer in 1-aminopyrene (AP, D-H)-pyridine (Pyr, A) and -methylpyridine (MePyr, A) hydrogen-bonded complexes in nonpolar hexane solutions have been investigated by means of femtosecond-picosecond laser photolysis methods. Rapid electron transfer from the S1 state of AP to the hydrogen-bonded Pyr or MePyr converging to the equilibrium state, (D*-H...A) reversible (D+-H...A-), has been directly observed by time-resolved transient absorption spectral measurements. The mechanism of the photoinduced electron transfer in nonpolar solvents greatly assisted energetically by specific hydrogen-bonding interactions and also dynamically by a little movement of the proton of D-H toward A in the hydrogen bond has been concluded. The charge recombination deactivation rate from the (D+-H...A-) state to the ground state has been examined for AP-Pyr and various -MePyr systems by time-resolved measurements, and it has been demonstrated that the rate constant decreases exponentially with increase of the energy gap.
The photochemistry of triarylmethanes (TAM) are studied by picosecond absorption spectroscopy. Two series of TAMs are investigated, (Ia-f) triphenylmethane (Ph3CX, X = H, OH, OCH3, Cl, Br, and SCH3 for Ia-f, respectively) and (IIa-c) malachite green ((Me2NPh)2PhCX, X = H, OH, and OCH3 for IIa-c, respectively). The TAMs are studied in polar and nonpolar solvents. The amount of heterolytic (TAM+X-) and homolytic (TAM·X·) cleavage after 266-nm excitation is determined for the various conditions studied. As expected, the amount of heterolytic bond cleavage increases in polar solvents. Furthermore, the amount of cleavage is more dependent on the electron affinity (EA(X·)) than on the bond dissociation energy (BDE(TAM-X)) of the leaving group. The importance of EA(X·) is noted even in nonpolar solvents where only homolytic cleavage is observed, suggesting the homolytic cleavage occurs via initial TAM-X bond heterolysis to give (TAM+X-) with subsequent electron transfer to yield (TAM·X·).
The electronic structure of some excited hydrogen-bonding systems and their electronic structure changes associated with proton movement are investigated by ab initie MO-CI calculations. The present calculation shows that when both the proton donor and the proton acceptor are π-electronic conjugated systems, e.g., phenol-pyridine or aniline-pyridine systems, slight movement of the proton in the neighborhood of the energy barrier brings about a large amount of charge transfer from proton donor to proton acceptor. Furthermore, hydrogen atom transfer might be expected via charge transfer followed by proton transfer in the case of donor excitation. On the other hand, neither charge transfer nor proton transfer occur when one of hydrogen-bonding system does not have conjugated π-electrons.
The quenching behaviors of triplet benzophenone by some amines in solution have been directly investigated using the nanosecond laser photolysis technique. It has been confirmed in systems of benzophenone and tertiary aromatic amines, such as N,N-diethylaniline and N,N-dimethyl-p-toluidine, that the ionic photodissociation to benzophenone anion and amine cation takes place completely in acetonitrile, while the photoreduction of benzophenone occurs efficiently in benzene. These processes were found to compete with each other depending largely upon solvent polarity. The quenching of the triplet benzophenone by primary and secondary aromatic amines as well as aliphatic amines leads to the formation of a ketyl radical in all solvents used. The marked distinction observed in the quenching processes of triplet benzophenone by various amines has been explained by the difference in ionization potential and the change of molecular structure of the amine in addition to the solvent effect. The solvent effect was discussed on the basis of free-energy calculations regarding the formation of ion pairs and free ions involved in the quenching path. In the case of the benzophenone-tertiary amine system it is concluded that an electron transfer followed by proton transfer results in: rapid hydrogen abstraction. The relaxation time for solvent orientation may be a useful guide to the present conclusion. In the case of primary and secondary amines the importance of a hydrogen atom bonded to the N atom of the donor was suggested.
The dynamic behavior in the excited singlet state of 1:1 phenanthrylammonium ion-18-crown-6 complexes in MeOH-water (9:1) mixtures at various temperatures has been studied by means of the single photon counting method with fluorimetry. Complex formation of phenanthrylammonium ions (RN+H3) with 18-crown-6 decreases markedly the proton dissociation rate in the excited state, resulting in an increase of its lifetime or fluorescence quantum yield. 2- and 3RN+H3-crown complexes are especially stable and do not dissociate into the excited neutral amine species plus proton. The hydrogen-bonded exciplex (RNH2-crown)* is produced by deprotonation of (RN+H3-crown)* for 1-, 4-, and 9RN+H3-crown complexes. The excited-state proton-transfer reaction in the 1-, 4-, and 9RN+H3-crown systems is a one-way process since the proton association rate is negligibly small compared to those of the other competitive decay processes. That is, there is no excited-state prototropic equilibrium in the RN+H3-crown systems. There is a large steric effect on protonation to the amino group of the excited neutral complex. In contrast, proton-induced quenching occurs effectively in (RNH2-crown)* complexes.
Triplet quenching of some aromatic ketones by anions has been studied by nanosecond laser photolysis in H2O:CH3CN (4:1) mixtures. A linear relationship between log 3kq (the quenching rate constant) and ΔG (the free-energy change for electron transfer) was obtained, indicating that the triplet quenching is caused by electron transfer from the donor anion 1X- to the acceptor (the triplet state of aromatic ketones 3A*). Electron transfer (or charge transfer) from 1X- to 3A* is a key step in the quenching. There was no appreciable difference in the quenching efficiency between the 3(π,π)* and 3(n,π) states. Estimations of 3kq values, using ΔG‡ (the potential barrier for electron transfer) calculated by the Marcus, Rehm-Weller, Scandola-Balzani, and Polanyi equations, were carried out. The Polanyi equation was found to fit the experimental data fairly well.
The transient that is produced in the quenching of triplet benzophenone by 1,4-diazabicyclo(2.2.2)octane (DABCO) has been examined by use of nano- and picosecond laser photolysis. The initial step in all solvents, both polar and nonpolar, is electron transfer to form a triplet contact ion pair. In nonpolar solvents, the ion pair remains in this form until it decays. For polar solvents, the spectra change somewhat over the first 100 ps showing that the solvation changes and the ion pair becomes solvent separated. The lifetime of the ion pair varies greatly with the solvent. In saturated hydrocarbons it is about 80 ps. Nonpolar solvents with either Ï electrons or a lone pair of electrons stabilize the ion pair on the nanosecond to microsecond time scale. A small amount of alcohol in benzene also stabilizes the ion pair by hydrogen bonding. A shift in the peak position with time toward the blue accompanies the formation of hydrogen bonds in this case.
Triplet benzophenone (â´K) abstracts a hydrogen from acetonitrile with a rate constant of 130 +/- 30 Mâ»Â¹ sâ»Â¹. Despite this low rate constant acetonitrile is not really an inert solvent and at low light fluxes, where T-T annihilation is not a major fate of the triplet, a major decay path for â´K is hydrogen abstraction with resulting pinacol (KâHâ) formation (phi/sub KâHâ approx. = 0.1 at t = 0). Both KâHâ formation and ³K lifetime rapidly decrease with irradiation due to the light absorbing transients (LAT's) which are formed along with KâHâ from ketyl radicals (KH). The rate constants per hydrogen (k/sub H/) for abstraction from R-H by the electrophilic ³K correlate well with the ionization potentials (IP) of the corresponding radicals (R).
Taking into account the fact that the motion of polar solvent molecules around the charged solute molecule is much more restricted than that around the neutral solute molecule, we have obtained theoretically three different energy gap laws for the photoinduced charge separation (CS), A*⋯D or A⋯D* → Ȧ-⋯Ḋ+, charge shift (CSH), A⋯Ḋ- → Ȧ-⋯D or Ȧ+⋯D → A⋯Ḋ-, and charge recombination (CR), Ȧ-⋯Ḋ+ → A⋯D, reactions. The three energy gap laws differ from each other especially in the strongly downhill region; the rates of CS, CSH, and CR decrease scarcely, moderately, and drastically, respectively. This theoretical prediction is consistent with the experimental results obtained so far for CS, CSH, and CR reactions.
By using a new aspect of the polar solvent mode, that is, that its vibrational frequency is much smaller in the neutral state of the reactants than in the charged state, the authors formulated the charge-recombination rate. Numerical calculations show that in strongly polar solvents, the solvent mode apparently refuses to contribute to the Franck-Condon overlap. As a result, the charge-recombination rate is determined almost exclusively by the intramolecular quantum mode, which varies considerably from molecule to molecule, and by the magnitude of the electronic interaction between the electron donor and acceptor. The result that the solvent mode scarcely contributes to the Franck-Condon overlap in strongly polar solvent is in a marked contrast to that of the charge-separation reaction investigated previously. The condition for the high yield and high stability of the charge-separated state in a polar environment is investigated in detail. As a result, it has been found that the weak and strong dependences of the charge-separation and the charge-recombination rates, respectively, upon the energy gap, which have been demonstrated for the first time by introducing the new aspect of the solvent mode, can afford a favorable condition for an efficient photoinduced charge-separation process. 13 references, 8 figures.
Taking into consideration that the phonon frequency of the solvent surrounding a neutral solute molecule is much smaller than that surrounding a charged species, we have formulated a rate expression for an electron transfer reaction in polar solvents. Our theory can explain satisfactorily the insensitiveness of the rate constant of the fluorescence quenching reaction, A* + B → A∓ + B±, in polar solvents upon the energy gap in the "inverted region" and the strong dependence of the rate of the recombination reaction, A∓ + B± → A + B, upon the energy gap and the nature of the individual electron donor and acceptor.
A laser flash photolysis study at 355 nm has been carried out on acetonitrile-water (4:1 v/v) mixtures of the methoxynaphthalene-benzophenone-H2SO4 system. It is found that the proton-assisted photoionization reaction of triplet 1- and 2-methoxynaphthalenes (ROMe) produced by triplet sensitization of benzophenone (BP) effectively occurs to produce the corresponding methoxynaphthalene cation radical (ROMe.+) and benzophenone ketyl radical (>COH). The triplet energy-transfer reaction from triplet benzophenone ((BP*)-B-3) to ROMe takes place as the primary event to produce triplet methoxynaphthalene (3ROMe*). The 1:1 triplet exciplex 3(ROMe...>CO)* having a weak charge-transfer structure is readily formed between 3ROMe* and BP with the equilibrium constants K1 (10.1 M-1 for 1ROMe; 4.0 M-1 for 2ROMe at 290 K). In the presence of protons, the triplet protonated exciplex 3(ROMe...>C+OH)* is also produced by protonation to 3(ROMe...>CO)*. Subsequently, the intraexciplex electron-transfer reaction in 3(ROMe...>C+OH)* takes place effectively, resulting in the formation of ROMe.+ and >COH. The presence of protons in the ROMe-BP system assists the photoionization of ROMe with the much lower energy (triplet energy of ROMe) than its ionization potential. The reaction mechanism is discussed in detail.
Photoreduction processes of benzophenone (BP)-N-methyldiphenylamine (MDPA) system in acetonitrile solution were studied by means of femtosecond-picosecond laser photolysis and time-resolved transient absorption spectroscopy. The reaction processes including the formation of geminate ion pairs (IP) by electron transfer (ET) between BP* and MDPA at encounter followed by intra-IP proton transfer giving the ketyl radical (BPH.) were clearly observed in both the triplet and singlet excited states, while the IP produced by excitation of the CT complex between BP and MDPA formed in the ground state did not give BPH.. The reactivity of the IP depending on the mode of its production and on the energy pp for its formation and recombination reactions were discussed by integrating the present results with previous ones on BP-tertiary aromatic. amines and with recent experimental and theoretical studies of the ET reactions.
The laser flash photolysis study at 355 nm in the 1-naphthol (ROH) and benzophenone (BP) system with and without H2SO4 has been carried out in acetonitrile-water (4:1 v/v) or methanol at 290 K. For the ROH (3.0 x 10(-3) M)-BP (6.7 X 10(-3) M) system containing [H2SO4] = 0 or 0.5 M in acetonitrile-water (4:1 v/v), triplet benzophenone (3BP*) produced by fast intersystem crossing of singlet benzophenone (1BP*) excited at 355 nm is quenched by ROH with almost diffusion processes (8.4 x 10(9) or 9.3 x 10(9) M-1 s-1, respectively). Triplet naphthol (3ROH*) is produced in the nanosecond region by triplet energy transfer from 3BP* to ROH with efficiencies of 0.73 for [H2SO4] = 0 M and 0.39 for [H2SO4] = 0.5 M. Subsequently, in the microsecond region, the hydrogen atom transfer (HT) reaction from 3ROH* to BP occurs to produce the 1-naphthoxy radical (RO) and the benzophenone kethyl radical (>COH) with efficiencies 0.73 for [H2SO4] = 0 M and 0.85 for [H2SO4] = 0.5 M. The rate constant for the decay of 3ROH* (k(obsd)) linearly increases with an increase of H2SO4 concentration. As for an increase of BP concentration, the k(obsd) value increases not linearly, which shows a negative curve in both systems with and without H2SO4. Especially for the methanol solution with [H2SO4] = 0 M, a leveling off is clearly observed at higher BP concentrations ([BP] greater-than-or-equal-to 0.5 M). In the absence of H2SO4, the mechanism for the HT reaction from 3ROH* to BP can be explained by the intraexciplex HT reaction of the triplet exciplex 3(ROH...>CO)* established with an equilibrium constant of K1 = 6.7 M-1 in acetonitrile-water (4:1 v/v) or 4.2 M-1 in methanol. The mechanism of the proton-enhanced HT reaction is suggested that the protonated triplet exciplex 3(ROH ... >+COH)* formed in a prototropic equilibrium with 3(ROH...>CO)* undergoes the intraexciplex electron-transfer reaction to give the triplet radical pair 3(ROH.+...>COH) which rapidly decomposes into RO + >COH + H+.