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Solvent-Shift Effects on Electronic Spectra and Excited-State Dipole Moments and Polarizabilities
Abstract
The spectrum of a substance in a solution is changed compared to its spectrum in the vapor state. Solvent-shift effect is caused by a weak interaction between the solute and surrounding solvent molecules. The change in energy of the system because of this interaction will differ depending on if the solute molecule is in its ground or excited state and consequently there will be the change in excitation energy observed experimentally. The solvent–solute interaction, being weak, is best treated using quantum–mechanical perturbation theory that allows it to be divided into electrostatic type interactions and dispersion interactions. The electrostatic type interactions relate the solvent shift to the change in dipole moment and polarizability of the solute molecule on excitation from its ground state to its excited state and thus, can give experimental values-albeit once removed-of excited-state dipole moments and polarizabilities. The order-of-magnitude values for excited-state dipole moments can be obtained from solvent shift data. All solvent-shift theories are based on the assumption that solvent and solute molecules are sufficiently well separated that overlap of electronic distribution can be neglected. The chapter proves with the help of calculations that Abe's theory and the reaction-field method differ by very little in the final analysis. The solvent-shift data can also be used to obtain the order of magnitude estimates of excited-state dipole moments.
... The properties of organic molecules in the ground and excited states, in particular their polarizability and/or dipole moments, are commonly studied by solvatochromic absorption and emission [5][6][7][8][9][10][11][12][13][14]. Such studies are performed in solvents, characterized by different polarizability and polarity values under atmospheric pressure. ...
... This choice of the solution ensured the reliable and quantitative determination of the effect of repulsive interactions on the ν max A . So far, it has been commonly assumed that the effect of repulsive interactions is small, so that the ν max A and its dependence on f ( n 2 ) in the solvatochromic as well as in the barochromic studies solely originates from dispersive interactions [5,7,9,12,21,22]. However, the unexpected hypsochromic shift of the ν max A of the S 0 → S 1 ( 1 L b ) band for benzene in perfluoro-n-hexane [34] with an increasing pressure up to 0.45 GPa, as well as a practically constant value of ν max A of the S 0 → S 1 ( 1 L b ) band of naphthalene in perfluoro-n-hexane in a wide range of pressures [34], showed clearly that the effect of repulsive interactions on ν max A , at least for the S 0 → S 1 ( 1 L b ) type transitions, must be taken into account. ...
... The solvatochromic shift related to the solvent polarity function, f ( n 2 ) , is caused by dispersive and repulsive solute-solvent interactions. However, it is commonly assumed in solvatochromic and barochromic studies that the effect of repulsive interactions on the ν max A and on the relation between ν max A and f ( n 2 ) is small and can be neglected [7,9,29,32]. ...
The band positions in the UV-VIS absorption spectra of compressed solution of anthracene in n-hexane significantly depend not only on the dispersive but also on the repulsive solute-solvent interactions, what has so far been omitted. Their strength is determined not only by the solvent polarity but also by Onsager cavity radius changing with pressure. The results obtained for anthracene show that repulsive interactions should be included in the interpretation of barochromic and solvatochromic results of aromatic compounds. We show that the barochromic studies in the liquid solvent can be an alternative to solvatochromic studies, e.g. to determine the polarizability of organic molecules in the electronic excited state. The pressure-induced polarity change in n-hexane exceeds that induced by the exchange of n-alkane solvents between n-pentane and n-hexadecane.
... It is well known that photophysical properties of a given molecular system can be strongly influenced, and are often predicated, by phase-sensitive interactions [7,8]. For this reason, considerable resources have been dedicated to the study of environmental influence in order to gain insight into, and control over, different excited state processes which can give rise to potentially useful photophysical traits to be applied within optoelectronic devices. ...
... It is well known that photophysical properties of a given molecular system can be strongly influenced, and are often predicated, by interactions with the surrounding environment, whether it be surrounding solvent [7] or indeed other surrounding molecules within a molecular crystal. [8] For this reason, considerable resources have been dedicated to the study of environmental influence in order to gain insight into, and control over, different excited state processes which can give rise to potentially useful photophysical traits to be applied within optoelectronic devices. ...
To develop a theoretical methodology to study the influence of external stimuli on fluorescent crystals. We are particularly interested in excited state reactivity, aggregation-enhanced fluorescence and responses to mechanical stimuli. Such phenomena are often not yet understood at a molecular level, a problem exacerbated by the absence of adequate and cost-efficient computational models. This thesis counters this problem by developing a method capable of describing the solid state environment in the context of excited-state calculations at the quantum level.
... Some of the earliest theoretical work on solvatochromatic shifts was concerned not just with changes in the chromophore's dipole moment, as in the Onsager-style treatment leading to the Lippert-Mataga equation, but also with the role of dispersion effects. [470][471][472][473] In modern formulations of continuum theory, however, there have been only preliminary efforts to incorporate nonelectrostatic interactions (as described in Section 4) into excited-state calculations. 316,317,474,475 This is an interesting problem insofar as solute-solvent dispersion is likely more attractive in an excited state, as the excited-state wave function is probably more polarizable than the ground state, but at the same time Pauli repulsion probably increases in the excited state due to its larger size. ...
This review describes the theory and implementation of implicit solvation models based on continuum electrostatics. Within quantum chemistry this formalism is sometimes synonymous with the polarizable continuum model, a particular boundary-element approach to the problem defined by the Poisson or Poisson-Boltzmann equation, but that moniker belies the diversity of available methods. This work reviews the current state-of-the art, with emphasis on theory and methods rather than applications. The basics of continuum electrostatics are described, including the nonequilibrium polarization response upon excitation or ionization of the solute. Nonelectrostatic interactions, which must be included in the model in order to obtain accurate solvation energies, are described as well. Numerical techniques for implementing the equations are discussed, including linear-scaling algorithms that can be used in classical or mixed quantum/classical biomolecular electrostatics calculations. Anisotropic models that can describe interfacial solvation are briefly described.
... These examples have been reported for the dipole-dipole interactions of dye behaviour in different solvents. [9][10][11][12][13][14][15][16][17][18][19][20][21][22] Solvation dynamics of organic dye molecules have also been described macroscopically using spectral properties of the dyes in the ground and excited states. [23][24][25] The ground state is the lowest unoccupied molecular orbital (LUMO) of a molecule or an atom. ...
Solvatochromic behaviours of triazine substituted dyes were evaluated using a novel approach derived from the red shift index (RsI) and associated solvation energy (ASE). These parameters were used to describe the solvation trends of the dye-solvent interactions based on their polarity changes. The concept demonstrates the effect of substituent changes on the triazine scaffold and the induced solvent polarity changes as solvated dyes go through the HOMO-LUMO (highest occupied molecular orbital-lowest uncopied molecular orbital) phases. Primarily, these phases were characterised by evaluating the wavelength of the absorption and emission spectra in different solvents, which in conjunction with recently reported computational approaches provides a well-adjusted model for predicting spectra polarity changes between the dye (solute) and the solvent. Based on the results from this study, predictive polarity changes on the triazine scaffold in different solvents can be empirically monitored both in the ground and excited states. Moreover, the solvatochromic parameters can be extended to evaluating the predictive behaviors of different spectra dyes.
... The influence of solvent on the dipole moment is very great and it's different in different solvents. [69][70][71] The molecular dipole moments of the Uranyl-BFHADAs with the atoms N, S, Se, As and Te in vacuum, water, benzene and methanol were calculated by using B3LYP and M062X, respectively. The results were shown in Table 6. ...
To find new adsorbents for uranyl ions, the density functional theory (DFT) was adopted to design a series of new ligands containing an anthracene and two five‐membered heterocycles with nitrogen family nonmetal elements (N, P, As) or oxygen family nonmetal elements (O, S, Se, Te), for example, ligands N,N′‐bis(2‐five‐membered heterocyclidene)‐1,8‐anthradiamines (BFHADAs). Then the uranyl ions were coordinated with BFHADAs to generate five new coordination complexes (Uranyl‐BFHADAs) with heteroatoms N, S, As, Se and Te, respectively. The five‐membered heterocyclic rings of Uranyl‐BFHADA with oxygen atoms were broken under the structural optimization and Uranyl‐BFHADA with heterocyclic atoms P was not obtained. Several structures and property parameters of the ligands BFHADAs (containing heteroatoms N, S, As, Se and Te) and their uranyl complexes Uranyl‐BFHADAs were theoretically investigated and analyzed. The results showed that uranyl ions could form stable coordination complexes with these five BFHADAs. The formed bonds between uranyl ions and the heteroatoms in BFHADAs were coordination bonds rather than other types of bonds. These results could provide insightful information and theoretical guidance for the coordination of uranyl with the atoms N, S, Se, As and Te in other ligands.
... Solvent-effects, in a broad sense, have been explained by several investigators [12][13][14][15], specifically with the purpose of addressing the complexity of chemical phenomena in solution. Commonly this has necessitated the development of models, application of computational techniques, and sometimes of mathematical concepts, to explain observations about the chemical system of interest. ...
Solvent effects on the UV/vis spectra of metallopthalocyanines (MPcs) have been interpreted using the red-shift index concept (RsI). The concept connects empirically, direct, experimental, easily accessible optical spectral data, which are explained by considering the differential behavior of the solute–solvent interactions at the ground state and excited state using the spectral values of MPcs along with the derived concept, called the associated solvation energy (ASE). RsI is formulated from three fundamental parameters, which are: ground state electronic absorption spectrum, polarization red-shift and a scaling factor of MPc (Ndye) in the respective solvents. The RsIis a reflection of the index value of the chromophore substituent of MPc in the solvent; thus, the concept can be used as a solvatochromic parameter to study a wide range of supramolecular and heterocyclic compounds that can be modified at their periphery or ‘handles’. Particularly, in this study, the concept has been used to rank MPc candidates by using the statistical mean performance of the solvatochromic parameters, which are red shift index, polarizability efficiency and ASE. We hereby review the solvent effects on
the UV/vis spectra of substituted and unsubstituted MPcs.
... In fact, the high degree of sensitivity in fluorescence is primarily due to interactions that occur in the local environment during the excited state lifetime. The general description for environmental effects is given by dielectric continuum theory [17,18]. This theory mainly assumes that a point dipole solute interacts with the solvent by virtue of the change in solute dipole moment and described by the Lippert-Mataga equation [19,20]. ...
In this report, we show interesting correlation between solvent polarity of alcohol solutions and fluores-cence lifetime (τ av), estimated from time-resolved fluores-cence spectroscopy (TRFS), and macroscopic viscosity (η). Non-functionalized carbon nanoparticles (NCNP) were successfully used as flurophores in these measurements. The solvent polarity, described through polarizability (Δf) of dielectric continuum theory, could universally describe both τ av /τ max and η/η max through the relation, X ðΔf Þ ¼ a OH þ bðΔf À Δf OH Þ þ cðΔf À Δf OH Þ 2 with X 0 τ av /τ max or η/ η max , (subscript OH represents corresponding values for alcohols) for alcohol solutions of methanol, ethanol and 1-propanol at room temperature. We show that fluorescence lifetime and solvent viscosity are universal functions of solvent polarity for alcohol solutions.
... These assumptions are not always satisfied, particularly when easily polarizable chromophores (such as pushepull chromophores) are being studied. In such cases, it is convenient to analyze the solvent effects on n A , n F andDn according Equations (3)e(5): [38,39] ...
Description
Discusses recent advances in fluorescence and phosphorescence research, including instrumentation, fundamental photo physical phenomena, and analytical applications. Focuses on probes of the chemical microenvironment, coupled phenomena, and computer-assisted luminescence measurements.
The polarizable density embedding model is combined with the multiconfigurational self-consistent field and MC-srDFT electronic structure methods to calculate solvatochromic shifts of the n-π* absorption of acrolein and the π-π* absorption of the para-nitrophenolate anion in aqueous solution. Differences between linear-response (LR) and state-specific (SS) solvent shifts are analyzed by assessing the contributions of different terms in the solvent potential. This comparison shows that the differences are not only due to the intrinsically different response of LR and SS excitation energies to the polarizability of the environment but also due to a different response to the static part of the environment potential. These observations show that even in nonpolarizable environments, LR and SS calculations based on SCF (orbital optimization) methods do not necessarily agree on the spectral shift. The difference can be as large as, or even dominate, the difference due to dynamical polarization.
This review describes the theory and implementation of implicit solvation models based on continuum electrostatics. Within quantum chemistry this formalism is sometimes synonymous with the polarizable continuum model, a particular boundary‐element approach to the problem defined by the Poisson or Poisson–Boltzmann equation, but that moniker belies the diversity of available methods. This work reviews the current state‐of‐the art, with emphasis on theory and methods rather than applications. The basics of continuum electrostatics are described, including the nonequilibrium polarization response upon excitation or ionization of the solute. Nonelectrostatic interactions, which must be included in the model in order to obtain accurate solvation energies, are also described. Numerical techniques for implementing the equations are discussed, including linear‐scaling algorithms that can be used in classical or mixed quantum/classical biomolecular electrostatics calculations. Anisotropic models that can describe interfacial solvation are briefly described.
This article is categorized under:
• Electronic Structure Theory > Ab Initio Electronic Structure Methods
• Molecular and Statistical Mechanics > Free Energy Methods
Abstract
Implicit solvation models based on continuum electrostatics provide conceptually simple and computationally convenient boundary conditions to model solution‐phase quantum chemistry.
The Lippert-Mataga equation is used widely to describe the solvatochromic effects of fluorescent molecules through the evaluation of solute–solvent interactions on the basis of the point-dipole model. A large dipole deviation of molecules in the ground-state and the lowest excited-state is a basic requirement for the design of a polarity-sensitive fluorescent probe. Some recently synthesized probes with center-symmetry have near zero dipole deviation while undergoing notably solvatochromic behaviors. Thus, it is necessary to find a new method beyond the Lippert-Mataga model to qualitatively estimate the molecular solvent shifts. To this end, a state-specific descriptor (SSD) based on molecular surface electrostatic potentials (ESP) is proposed to explain the solvatochromic behaviors of the well-studied coumarin C153 and center-symmetric DCB-1d. In contrast to the experimental solvent shifts and state-specific TD-DFT calculations, the SSD successfully explains the solvatochromic effect of C153 and DCB-1d molecules. In addition, the SSD was tested by using eight selected polarity-sensitive fluorescent molecules. The SSD was found to provide a good linear relationship with solvatochromism.
Raman spectral changes of CO2 induced by the absorption were evaluated for the Fermi dyad continuously up to 15 MPa of equilibrium CO2 pressure. Little pressure dependence was observed for both the band positions and the bandwidth of the absorbed CO2, compared to those for neat supercritical CO2. Furthermore, the band positions lay at much lower values than those for neat CO2. The bandwidth broadening was observed for the absorbed CO2 even in the lower pressure region. The absorption state of CO2 is discussed from the viewpoints of structural fluctuations and interaction forces induced by the surrounding ionic liquid.
This chapter reviews characterization techniques with emphasis on their introduction and principle of operation for the experiments to achieve the goal of this thesis. Lights scattering techniques like dynamic light scattering, static light scattering, depolarized light scattering; deformation technique like rheology, microscopic technique such as transmission electron microscopy were used to probe the samples. Other techniques such as Raman spectroscopy, viscometry and tensiometry were also used. In addition, we also give brief accounts of materials such as clays and their characterization and solvents used in this thesis work.
This chapter deals with hydrophobic interaction mediated aggregation of clay particles in monohydric alcohol solutions. Nanoclays of different aspect ratio, [Laponite (aspect ratio = 30) (L) and Na-Montmorillonite (aspect ratio = 250) (MMT)] were used to pursue their physical properties in hydrophobic alcohol solutions.
The importance of the solvent in chemical reactivity has been long recognized,1 albeit the quantification of the various roles that the solvent can play was not possible. These roles include: supplying or dissipating energy by acting as a heat bath, confining reactants, and solvation of ions. Solvation will be important for any reactions in which charge is created, destroyed or transferred, including intra-molecular charge redistribution, which, for example, leads to giant dipole formation in excited states.
Absorption spectra of formaldehyde (FA), acetaldehyde (AA), and acetone are compared in the vapour phase, nonpolar and polar solutions at 295 K. The vibronic n-π* transition of carbonyl chromophore is mainly composed of the overtones of >C=O stretching vibration. A new phenomenon is observed in liquid solutions, consisting of a relative increase of Franck-Condon factors for the 2nd and 3rd harmonics in FA, and the 2nd to 4th replica in AA, with respect to the gas phase. In AA and acetone with poorly resolved vibronic structure the redistribution of intensities produces a false "solvent shift" of band maximum between the vapor and nonpolar liquid phase by -250 ± 50 cm-1. Modification in vibronic coupling can also explain unusual narrowing of band contour in the solution, reported earlier for acetone (Renge, I. J. Phys. Chem. A 2009, 113, 10678). No detectable shift occurs as a function of solvent polarizability (refractive index function (n2 - 1)/(n2 + 2)) in n-alkanes for FA, AA, and acetone, as well as for cyclopentanone and camphor. Incidentally, the bathochromic dispersive shift is almost exactly compensated by hypsochromic induction shift. The latter is due to diminishing of dipole moment in the excited state of carbonyl chromophore. Differences in polarizability α and dipole moments μ were estimated for FA (Δα = 0.33 ± 0.1 Å3), AA (Δμ = 1.05 ± 0.2 D, Δα= 0.5 ± 0.2 Å3) and acetone (Δμ = 1.3 ± 0.2 D, Δα= 0.65 ± 0.2 Å3). The increase of Δαby ~10% upon excitation is plausible for a weak n-π* transition. By contrast, near doubling of Δα in the upper state has been reported recently for several ketones, with Δα reaching 10 Å3 (Catalán, J.; Catalán, J. P. Phys. Chem. Chem. Phys. 2011, 13, 4072). Empirical partitioning of solvent shifts into repulsive-dispersive, induction, dipole-dipole, and hydrogen bonding contributions was proposed to serve as a benchmark in computer chemical calculations.
The disordered condensed media (liquids, solutions, polymers, glasses etc.) are most widespread objects in chemistry and physics. As a rule, the quantum-chemical methods give a description of isolated individual molecules that corresponds to the infinitely rare gas at OK, i.e. to the conditions close to the intergalaxial space. Therefore, it is of large theoretical and practical importance to extend the quantum description of atomic and molecular systems to the disordered condensed media, including pure liquids, solutions, polymeric and glassy materials. The quantum theory of solid matter with ordered crystal lattices has been extensively developed on the basis of use of the translational symmetry in such systems and will not be reviewed in the present lecture notes. The reader is referred to excellent reviews on this subject [1, 2]. However, we will consider the theory which is applicable to low symmetry molecular impurities in regular crystals. In the following, we wish to review theoretical methods used to approach different chemical and physical aspects of the solvation of molecular systems in disordered condensed media.
In the previous chapter [1], the quantum chemical approach to the theoretical calculation of molecular properties in solution was reviewed. In this lecture series, we consider the recipes for the calculation of individual physical and chemical properties of the molecules in the disordered condensed media, i.e. in the pure liquids and solutions. Both the recipes for the calculation of the time-independent properties (such as the solute charge distribution, equilibrium constants of chemical reactions, and molecular free energy of solvation) and the time-dependent properties (such as the spectroscopic transitions of molecules and rates of chemical reactions) will be discussed.
Nonlinear optics refers to any light-induced change in the optical properties of a material. The most commonly used nonlinear optical effects are of second order: frequency doubling and linear electro-optics. For these second-order nonlinear optical effects, which also include optical sum and difference frequency generation, optical parametric amplification, and in most cases also photorefractive effects, noncentrosymmetry is required. Otherwise they are absent.
Abstract Solvent effects on the UV/vis spectra of metallopthalocyanines (MPcs) have been
interpreted using the red-shift index concept (RsI). The concept connects empirically, direct,
experimental, easily accessible optical spectral data, which are explained by considering the
differential behavior of the solute–solvent interactions at the ground state and excited state
using the spectral values of MPcs along with the derived concept, called the associated solvation
energy (ASE). RsI is formulated from three fundamental parameters, which are: ground
state electronic absorption spectrum, polarization red-shift and a scaling factor ofMPc (Ndye) in
the respective solvents. The RsI is a reflection of the index value of the chromophore substituent
of MPc in the solvent; thus, the concept can be used as a solvatochromic parameter to study a
wide range of supramolecular and heterocyclic compounds that can be modified at their
periphery or ‘handles’. Particularly, in this study, the concept has been used to rank MPc
candidates by using the statistical mean performance of the solvatochromic parameters, which
are red shift index, polarizability efficiency and ASE.We hereby review the solvent effects on
the UV/vis spectra of substituted and unsubstituted MPcs.
Keywords Aggregation behavior _ Solvation _ Polarization red-shift _ Solvent polarity _
Metallophthalocyanine
Tuning of Non-Linear Optical (NLO) properties of organic chromophores (OCs) by stereo-electronic and environmental effects has been widely documented by different experimental techniques and theoretical studies. Disentanglement and analysis of the different contributions requires, however, the availability of effective yet accurate quantum mechanical approaches for medium to large size systems in their natural environment. As a first step, we have shortly reviewed the phenomenological models still used by experimentalists to interpret their results and shown that a quantum mechanical approach based on the density functional theory (DFT) and the polarisable continuum model (PCM) should be able to overcome most theoretical limitations allowing, at the same time, the study of large systems with reasonable computational resources and the analysis of the results in terms of well-defined physical-chemical effects. After validation of the most suitable density functional/basis set in conjunction with the PCM description of bulk solvent effects, we have performed a systematic study of representative OCs, especially cationic ones, with special reference to their first order polarisability. The internal consistency of the results and their good agreement with experiment paves the route toward integrated experimental / computational studies of NLO properties taking together physical soundness, feasibility, reliability and ease of interpretation.
In this paper we report on molecular dynamics simulations of the lineshapes of the absorption spectra of perylene Arn heteroclusters (n = 2–22), which rest on the spectral density method in conjunction with the excited‐state potential modelling scheme. Inhomogeneous semiclassical absorption lineshapes were calculated by averaging of microcanonical spectra over the accessible phase space region. The size dependence and isomer specificity of the spectral shifts and spectral linewidths were elucidated, with the spectral shifts providing a powerful tool for the identification of structural isomers. Some information on isomerization dynamics was inferred from the temperature dependence of the linewidths, which mark the onset of correlated surface motion for (5|0) and (5|5) heteroclusters, as well as the onset of 2D 3D isomerization and side crossing for (22|0) heteroclusters.
Following previous studies on α and β polarizabilities of ketocyanines, a subgroup of D-π-A-π-D quadrupolar chromophores with moderately V-shaped structure, the present work analyses the effects of modifying the π-bridges connecting the D (NMe2) and A (CO) groups. This aim is pursued through a detailed comparison between the previously studied ketocyanines (KC2, KC3) and a Michler's ketone analogue (KM1) bearing styrenic (in the place of polyenic) π-bridges. First, we report a spectroscopic study, including absorption and fluorescence anisotropy spectra, aimed to probe the electronic peculiarities of KM1 as well as to derive consistent three-state model (TSM) parameters for the three compounds. The paper goes on with an extensive theoretical study, carried out in the framework of the density functional theory (DFT), encompassing the structure, the electronic spectrum, α and β polarizabilities and two-photon absorption (TPA) cross-sections (σTP). Calculations performed according to the sum-over-states (SOS) approach are discussed with reference to the performances of few-state descriptions, it is shown that such descriptions (including TSM), which have been proved to be quite reliable in the case of KC2 and KC3, lose their effectiveness with KM1 because of the electronic characteristics related to the styrenic π-bridges. As to the TPA cross-sections, the results of TSM and SOS approaches concerning the TSM g → c and g → e transitions are supplemented by those obtained using the quadratic response theory. A common qualitative conclusion, traceable to the degree of bending of the V-shaped structure, is that in the case of KM1 the allowed (g → e) and the "forbidden" (g → c) transitions both should be observable in the TPA spectrum, as confirmed by experiment.
Since laser action was first achieved two decades ago, technological developments have improved the versatility of laser devices so that today we are able to design spectroscopic experiments largely without concern for the limitations of the irradiation source. Further, the development of laser techniques has presented new opportunities for spectroscopic research because of the new and unique properties of laser radiation. Much of the progress has been made possible through the development of tunable dye lasers and the new science of nonlinear optics. Lasers currently provide the purest, most intense sources of radiation while still offering tunability over a wide spectral range, in particular the region 150 to 11, 000 nm. Also, the special conditions of laser operation have allowed the generation of the shortest pulses known from any source of energy down to Fourier-transform-limited single pulses of 10−13 sec duration.
The influences of solvent polarity and substituent on the electronic transition of six different N-butyl-α-styrylpyridinium dyes have been investigated in 21 solvents. Reichardt's ET(30) scale has been used to propose a quantitative approach towards the relative stability of the electronic ground and excited state species. The solvents have been classified into three types and the dyes have been classified into four groups based on the contribution of field and inductive effects of the substituents towards the change in their absorption maxima values. Instead of a steady solvatochromism, all the dyes except p-nitro substituted one, suffer reversals in solvatochromism at ET(30) values of ∼37 and ∼48. The extents of contribution of non-polar and polar protic solvents towards the solvation of the excited states of the dye molecules have been determined to be 30-40% more than that towards the stability of their ground states by the dipolar aprotic solvents. The ortho effect shown by this class of dyes in contrast to their corresponding γ-isomers might have been responsible for their better solvent polarity sensing capability.
We carry out the characterization of a mineral oil during its thermal degradation process by means of spectroscopic methods. If the temperature reached during the thermal treatment is bigger than a critical value Tc, the polarity of the oil increases. The results obtained are based in the changes observed in fluorescence spectra of some organic molecules (indoles) in solution versus their surround environment (solvent polarity). The oil polarity is the parameter that characterizes its degradation level.
The absorption and emission spectroscopic properties of 6-propionyl-2-(dimethylamino)naphthalene (PRODAN) have been studied in a large number of protogenic, nonprotogenic, and amphiprotic solvents. The data obtained can be explained by the inclussion of a new term in the Lippert equation which takes into account the acidity of the solvent. This finding indicates that some precaution should be taken when using PRODAN as an indicator of the polarity of protein cavities if the environments involved include acid sites.
An expression for the solvent dependence of the radiative rate constant of a forbidden transition in the presence of vibronic mixing is derived from perturbation theory. The case of nonpolar solvents and nonpolar fluorophores is considered. It is shown that the radiative rate constant, corrected for local field effects, depends on solvent–fluorophore dispersion interactions whose effect is to modulate the energy separation between the vibronically coupled borrowing and lending states. Analysis of radiative rate data for the linear polyenes diphenylhexatriene and trans‐retinol gives quantitative agreement with theory. Other linear polyenes with larger separations between their first two excited singlet states are predicted to have radiative rates which do not vary appreciably with solvent polarizability as is observed. The procedure used to derive an expression for the solvent dependence of the radiative rate is not restricted to the linear polyenes but is generally valid.
We report solvent dependent spectroscopic study of unique non-functionalized fluorescent carbon nanoparticles (NCNPs) dispersed in 15 organic solvents: aromatic (three), hydrogen bonded (five) and aprotic (seven). Absorption spectra were found to be independent of the solvent nature, with absorption bands located around 430, 405 and 385 nm whereas photoluminescence (PL) spectra exhibited considerable solvent dependence with PL emission peaks lying in the region 405 to 500 nm. Emission life time measured by time resolved fluorescence spectroscopy revealed that in aromatic solvents the average lifetime (τav), did not change significantly with solvent polarity, which was, typically 4–5 ns under the excitation of 405 nm. In hydrogen bonded solvents, τav was observed to decrease with solvent polarity, but in case of aprotic solvents, τav was observed to increase with solvent polarity for a particular excitation. Emission data in hand revealed possible quantum confinement of these nanoparticles inside the cavity of rings of THF, p-xylene, benzene and toluene molecules.
A definition of photoexcitation free energy of solvated molecules, ΔAexc, is proposed by exploiting an analogy between optical transitions driven by the electromagnetic radiation and steered transformations in classical systems. We postulate the applicability of a ‘spectroscopic version’ of the Jarzynski equality (which was originally derived in the classical context) with a likely work distribution function obtainable from the experimental UV–vis spectrum, so to yield ΔAexc on empirical basis. Motivations that support such a postulate are provided, and tests of internal consistency are made on some model cases.
The preferential solvation approach and the dielectric enrichment model have been applied to explain the solvatochromic behavior of o-, m- and p-nitroaniline (oNA, mNA and pNA) in several binary solvent mixtures. Cyclohexane was used as the “inert” nonpolar cosolvent in every mixture. The other solvents were chosen trying to vary their polarity as much as possible as well as their hydrogen bond donor or acceptor capabilities. Preferential solvation is detected in every solvent mixture studied. These global interactions were quantified by calculating the preferential solvation constant, K. Also, by using the previously developed model, we calculated for each pair of solvent mixtures a theoretical curve and the corresponding KD due to dielectric enrichment. Non hydrogen bond acceptor solvents (β=0), give values of K quite similar to those of KD, indicating that the preferential interaction is practically dielectric in nature. When the interacting solvent is a hydrogen bond acceptor, the values of K are higher than KD according to the acidity of the H in the amino group in the solutes. The values of K as well as of KD for any solvent mixtures in general follow the order pNA > mNA > oNA as expected, considering the values of μg and μg-μex. Studies in pure solvent support previous conclusions.
The electronic absorption spectra of some derivatives of pterin and N5-deazapterin are analysed using the CNDO/S-CI method, including allowance for solvent shifts. These calculations give good agreement with the spectra, which may be assigned to a group of π*←π transitions. There are some differences between calculated values for gas-phase and solution models but their general level of conformity with experiment is similar. Band shifts caused by methyl substitution, protonation and replacement of ring nitrogens are investigated, and a number of structural and spectroscopic problems are addressed. The spectral predictions agree well with experimental assignments for tautomeric forms and protonation sites for known compounds, and predictions are made for the spectra of N8-deazapterin and N5,N8-dideazapterin which have not been reported.
In this paper, we apply the theory of solvent effect on electronic spectra developed by Longuet-Higgins and Pople to derive the spectral shift expressi
Previous theories of spectral solvent shifts are briefly discussed. The basic dipole approximation is analyzed both theoretically and in relation to experimental information. On account of the restricted validity of this approximation — especially for molecules consisting of several polar groups — another, electrostatic model, originating from the Born charging, is investigated. This model is applied to semi-empirical PPP-calculations of spectral solvent shifts of some quinones. Among other things, this model predicts that not only n → π8 transitions may be blue shifted but also certain π → π* transitions. When the electrostatic effect is small, other terms may be responsible for the solvent shift, e.g. polarizability. The cavity effect is not expected to be important in the present context.
An analysis of the polarizabilities in the 1La excited electronic state of some aromatic hydrocarbons and 9,10-derivatives of anthracene is done in terms of three solvatochromic models: Suppan, Abe and a modified version of Mc Rae's model. Polarizabilities in excited electronic states obtained from electrochromic data were utilized in order to find out the best interaction radii of the solute molecule with their solvation shell.The theoretical expression derived by Abe gave the best polarizability value in excited state when van der Waals volume was employed for determining of the molecular cavity of the solute in solution.
A series of donor-acceptor derivatives of naphthalene have been studied theoretically by the INDO/S CI method and compared to experimental data. The dimethylamino and the pyrrolo groups in 1- and 2-positions served as donors while the cyano group in position 4 was used as an acceptor. Transition energies, transition moments and electric dipole moments of the lowest 10 singlet states have been calculated for two conformations of the donor group - planar and perpendicular to the naphthalene moiety. Two groups of highly polar biradicaloid charge transfer states have been observed. It is shown that energetic factors alone are not sufficient to explain the observed differences in the fluorescence spectra of 1- and 2-derivatives. The role of the ground state geometry in the formation of twisted intramolecular charge transfer states is also discussed.
Mechanism and dynamics of the excited state relaxation in compounds able to form TICT states were investigated from a theoretical point of view with special reference to the dielectric solvent effects. The study included 4-dimethylamino-4'-cyanostilbene (DCS) and Michler's Hydrol Blue (MHB), which are thought to involve emitting and non-emitting TICT states, respectively. It was found that in DCS the formation of TICT states, by twisting either the dimethy lamino or the dimethylanilino groups, may in principle be induced by the solvent polarity but requires extreme conditions. In MHB, both calculations and new fluorescence quantum-yield measurements in a variety of solvents indicated that the TICT-state formation is essentially controlled by the solvent viscosity.
The energies of the low lying electronic states of p-cyano-N,N-dimethylaniline (CDMA) in its planar and in 90° twisted conformation have been calculated using the modified INDO method. Solvent effects have been taken into account. The results agree reasonably well with the experimental data and support the hypothesis of the dual fluorescence of CDMA as being related to the intramolecular rotation of the N(CH3)2 group, with a decisive role of the solvent polarity.
Laser flash photolysis has been used to investigate the triplet-triplet annihilation (TTA) process of benzophenone (Ph[sub 2]C=O) and the self-termination reaction of benzyl radical (PhCH[sub 2]) in supercritical CO[sub 2] and ethane. Kinetic measurements were performed at various pressures above the critical pressure along two isotherms, one close to the critical temperature of the solutions (35[degrees]C) and one further removed (50[degrees]C). The second-order rate constants obtained indicate that each reaction occurs at the diffusion limit when spin statistical factors are considered. No evidence of enhanced cage effects due to supercritical solvent clustering about diffusive encounter pairs or enhanced solute/solute interactions were observed in these experiments. Additionally, the photocleavage of dibenzyl ketone and the rate constants for decarbonylation of phenylacetyl radical (PhCH[sub 2]CO) have been examined under the above conditions and do not show any anomalous behavior or cage effects. 37 refs., 8 figs.
The theoretical formulation developed in the preceding article [H. J. Kim, J. Chem. Phys. 105, 6818 (1996)] is analyzed via a second-order perturbation method and applied to the static electronic spectra of polarizable solutes in solution. In the Born–Oppenheimer (BO) framework of the solvent electronic polarization P&vec;el, the solute electronic wave functions, together with their (free) energy levels and associated Franck–Condon (FC) energies, are examined in the presence of a spherical cavity of arbitrary size and a nonequilibrium solvent orientational polarization configuration P&vec;or. It is found that the solute electronic structure and its free energetics vary strongly with both P&vec;or and the cavity size. The solute dipole enhancement due to solvation decreases with increasing cavity size. Comparison with the self-consistent (SC) reaction field theory predictions shows that classical P&vec;el is more effective in polarizing the solute than quantum P&vec;el couched in the BO description. This is due to the dispersion stabilization mechanism present in the latter. The static electronic spectroscopy is studied to linear order in the solute polarizability and in the cavity size difference between the lower and upper electronic states involved in the FC transition. In the case of the vanishing cavity size difference, our analytic results for the solvent spectral and Stokes shifts are compared with various existing theories and the sources of the discrepancies are briefly discussed. The effects of the cavity size variation on the electronic spectra are illustrated by using a simple two-state model description for the solute. It is found that even in a nonpolar solvent, there can be a significant Stokes shift arising from the cavity size relaxation subsequent to the FC transition. Also the cavity size fluctuations can make a non-negligible contribution to the spectral line broadening.
Absorption and emission spectra of all-trans 1, 3, 5, 7-octatetraene are presented along with fluorescence quantum yields and lifetimes. In solution, a gap of about 3000 cm−1 is found between the first band of the 1 1Ag↠11Bu transition and the onset of the emission spectrum. Excitation spectra of concentrated solutions at 77 K show low-lying bands in this gap, the lowest energy band being almost coincident with the highest energy fluorescence band. On the other hand, gas phase fluorescence spectra show no gap between the lowest energy 11Ag↠11Bu absorption band and the first fluorescence band. The radiative lifetime in hexane is 220 ns at room temperature and 190 ns at 77 K. The radiative lifetime for the gas phase fluorescence is estimated to be longer than 150 ns. The solvent dependence of the absorption and emission spectra, the fluorescence lifetimes, and the vibrational frequencies observed in solution imply support for the conjecture of Karplus etal. that the lowest excited singlet state is of 1Ag symmetry. The solution data imply that the low-lying state is about 6400 cm−1 below the 11Bu level. On the other hand, the lack of a gap between absorption and emission and the long lifetime found for the gas phase are not compatible with this model.
The absorption and emission solvatochromism of five indole derivatives was studied by referring the shifts to the corresponding vapor-phase wave numbers. The emission results clearly demonstrate, in three cases, the change in the nature of the emitting state when the solvent polarity is increased. In absorption, a large red shift in the 1La band is observed in all polar solvents, and this cannot be explained by classical solvent–solute interaction forces. The temperature dependence of the emission shift in a viscous solvent (glycerol) confirms that most of the emission shift is due to solvent relaxation. The 1La–1Lb level inversion model applied to ground-state polar solvent–solute complex, can account for most of the solvatochromism features observed.
Transient absorption spectra (triplet-triplet) of anthracene are observed in the vapor phase, polar and non-polar solvents, and polymethylmethacrylate matrices at room temperature. The results are compared with those of previous observations (singlet-singlet), after estimating the change in polarizability. When the width of the bands studied is plotted versus the viscosity and the dipole moments of the solvents used, straight lines are obtained.
A continuum theory to describe equilibrium and nonequilibrium solvation in polarizable, nondipolar, quadrupolar solvents is developed. By employing the densities of the solvent quadrupole and induced dipole moments as primary field variables, a reaction field theory formulation for quadrupolar solvents is constructed with account of their electronic polarizability. Nonequilibrium solvation aspects are effected via the solvent coordinate description for the quadrupole moment density. It is found that the theory is consistent with the macroscopic Maxwell equations and satisfies the continuity of the electric potential across the cavity boundaries. Solvation stabilization arising from the solvent quadrupoles is captured via novel reaction field factors analogous to those for dipolar solvents. Comparison is made with the dielectric continuum description of the polarizable, dipolar solvents as well as with previous theories of the quadrupolar solvents. Extensions and applications of the current theoretical formulation to study free energetics and dynamics of reactive and spectroscopic processes in the quadrupolar solvents are reported in the following paper [J. Jeon and H. J. Kim, J. Chem. Phys. 119, 8626 (2003)].
The relative cross section for the gas phase photodetachment of electrons has been determined for phenoxide ions in the wavelength region 300−530 nm (3.10−2.34 eV). An upper limit to the electron affinity of C6H5O⋅ has been determined to be 2.35±0.06 eV. Evidence is presented for the existence of an autodetaching state in C6H5O−.
The optical properties of molecules may he affected by an electric field. With suitable molecules, these effects allow to determine the electric dipole moments and certain components of the polarizability tensors in the ground state and in excited electronic states, the directions of transition moments and certain components of the transition polarizability tensors. The magnitude of the electrooptical effects depends on the effective electric field acting on the molecule. In preceding papers the representation of the effective field was based on the Onsager model. More recent experimental investigations have shown that this approximation is not sufficient when using polar solvents. Here, local fluctuations of the electric field have to be taken into account. Basing on previous theories, an extension including these effects is developed which agrees with the experimental results, as will be shown in the following paper⁷. Also, an approximate expression is derived for the mean square of the effective electric field
Auf Grund der quantenmechanischen Störungsrechnung und des ONSAGER-Modells für flüssige Medien wird eine Gleichung abgeleitet, die den elektrostatischen Beitrag zur Verschiebung der 0—0-Banden im Absorptions- und Emissionsspektrum des Moleküls mit dem Brechungsindex und mit der Dielektrizitätskonstante der Lösung verbindet. Die Gleichung ermöglicht unter gewissen Voraussetzungen die Bestimmung des Dipolmomentes des ersten angeregten Singulettzustandes der fluoreszierenden Moleküle. Im Sonderfall, wenn die Polarisierbarkeit des gelösten Moleküls zu vernachlässigen ist, erhält man die von LIPPERT hergeleitete Formel. Die mit der von uns erhaltenen Gleichung aus den Messungen von LIPPERT bestimmten Dipolmomente liegen denen von CZEKALLA bedeutend näher.
Components of the electric polarizability tensor are calculated for a number of conjugated hydrocarbons. The methods of calculation used were the configuration interaction perturbation theory and the single configuration perturbation theory introduced in paper 1. The results obtained are compared with experiment and with Hückel calculations. It is found that there is some ambiguity in the experimental evidence. The σ-π separability approximations are discussed and the relationship between Hückel theory and the single configuration method is examined.
Experimental observations about solvent shifts of absorption bands are reviewed, and correlations with various solvent properties are discussed. The theory is treated according to the Onsager model of dielectrics which shows how data like dipole moments and polarizabilities of excited states can be obtained from solvent-shift experiments. Shifts resulting from dispersion interaction and from specific solvent–solute interaction, in particular hydrogen bonding, are considered. Possible improvements in the theory of solvent shifts are briefly discussed.
A theory of the red shift of low-lying electronic bands of nonpolar molecules in nonpolar solvents is presented. The difference between the dispersion forces in the ground and excited states is investigated by perturbation theory and an approximate formula is derived according to which (in dilute solution) Red shift=16αBzR¯−6{14EαA+M2}, where αA and αB are the molecular polarizabilities of the solute and solvent, respectively, M and E are the dipole moment and energy of the transition, and each solute molecule is supposed to be surrounded by z solvent molecules at a mean distance R¯.
From an extension of the theory of solvent shifts based on the Onsager model of dielectrics, equations are derived which include the change of polarizability Δα from the ground state to an excited state. The result in the case of a few aromatic molecules is that Δα in the lowest excited states is smaller than 50 × 10−25 cm−3, i.e. appreciably smaller than the ground-state polarizabilities. The reasons for the failure to detect large Δα by solvent shift studies are discussed.
A theory of absorption spectra of dyes in solution is presented. The wave-number shift of the absorption maximum is calculated by using the perturbation theory, where the distribution of solvent molecules is assumed to be uniform, and the shift is represented by experimentally convenient quantities, such as the dielectric constant, the refractive index, and the absorption wave-length of the solvent. The solvent effects are classified into several groups, and the theory can explain the various experimental facts successfully. Previous theories are criticized.
This chapter provides a detailed quantum mechanical discussion of the problem in terms of second-order perturbation theory. The frequency of a molecular electronic absorption band is generally displaced when a molecule is immersed in a solvent medium such as a liquid or a foreign gas. These shifts are usually towards longer wavelengths although the opposite is sometimes true. In the recent years, considerable progress is made in understanding the effect of solvent on various spectral characteristics of a solute using quantum mechanical methods. When a molecule absorbs or emits light undergoing an electronic transition from one state to another, it is important to realize that both the energy and the electronic charge distribution in the molecule change. The calculation of stabilization energy of an electronic state due to solvent-solute interaction was first made by Ooshika using quantum mechanical perturbation theory. The same quantum mechanical formulation was used by Longuet–Higgins and Pople and by McRae.
Regarding the solvent as a continuous dielectric medium, it is shown that its effect on the Franck‐Condon absorption of light by solute molecules must be expressed in terms of the electronic polarization part of its dielectric constant, K = n2. Using methods based both on quantum theory and on classical dispersion theory, it is shown that the red shift of absorption in solution depends directly on f, the oscillator strength, and inversely either on a3 (a is the radius of the spherical solute molecule) or the polarizability α. The expression Δν(cm−1)=const. (f/νa3)[(n2−1)/(2n2+1)] with two possible values of the constant, and alternatively with the substitution of α for a3, is tested on experimental data for isoprene, benzene, bromine, and iodine. Good quantitative agreement is obtained for the (V, N) transitions of isoprene and benzene. If the strong ultraviolet absorption of bromine and iodine solutions is regarded as the displaced (V, N) transition, the quantitative agreement is poor, although qualitatively in accordance with the theory. The weak λ2600 system of benzene, and the visible continua of bromine and iodine, show the expected smaller Δν with smaller f, although quantitative comparison with theory is prevented by the superposition of other solvent effects which become important in weak absorption bands.
An excited‐state solute—solvent complex (exciplex) has been shown to be responsible for a large red shift and loss of vibrational structure in the fluorescence spectra of indole and indole derivatives in polar solvents. Solute—solvent stoichiometry of 1:2 and 1:1 is observed with associating and nonassociating solvents, respectively. Hydrogen bonding between the indole >N☒H group and solvent is shown not to be responsible for the interaction. It is suggested that the exciplex state is a charge‐transfer state and is an intermediate in the process of electron transfer from the solute to the solvent.
All organic electronic spectra in solution are subject to a generalized polarization red shift which is due to solvent polarization by the transition dipole and which depends on the solvent refractive index. This can be obscured by the effect of dipole-dipole and dipole-polarization forces if the solute is polar, when the application of the Franck-Condon principle shows that the solvent cage around the excited solute molecule is momentarily strained. Orientation strain and packing strain are defined, of which the former is more important. The absorption frequencies of polar solutes are shifted to the red in solution if the dipole moment increases during the transition; they may be shifted to the blue (relative to the gas) if the dipole moment decreases. Four cases are discussed according to whether solute and solvent are polar or non-polar. The place of π* ↔ n transitions is discussed.
The solvent displacement of the molecular absorption maximum of representative cyanine dyes was found to be linear in the refractive index of the solvent or in various refractive index functions. The dielectric constant, hydrogen-bonding capacity, or surface tension of the solvent had little influence on the displacement, which is determined essentially by dispersion interactions and can be interpreted as the "universal red shift" of Bayliss. In binary solvent mixtures of varying composition, deviations from linearity of the frequency of the absorption maximum with the refractive index, when the components differ considerably in polarity, point to selective solvation of the dye cations in the mixed solvents. At low coverages of an adsorbed cyanine dye on solid substrates, the structure of the absorption spectrum reduplicated that of the molecular spectrum in solution, and the position of the maximum was mainly governed by the refractive index of the substrate. The displacement coefficient, the frequency shift per unit change in refractive index or in refractive index function, for adsorbed dye was little more than half that for the dye in solution, probably because of the one-sided interaction of the medium in adsorption. The spectrum of the adsorbed molecular dye on silver chloride and silver bromide fell, for the most part, in line with those on other substrates, but silver iodide induced a uniquely great tendency to dye-dye aggregation, even at very low coverages. Anomalously small displacements of the molecular spectrum of the dye on some hydrated surfaces can be attributed to interaction between adsorbed dye and adsorbed water molecules. On unfired samples of zinc sulfide prepared under acid conditions by precipitation of zinc ion by hydrogen sulfide, proton transfer from active acidic sites in the surface of the substrate caused the dye to be adsorbed in a colorless form from solvents of acidic nature.
The n → π* absorption bands of acetone and of a number of substituted ketones have been measured in a variety of solvents. The blue shifts usually observed in polar solvents are attributed mainly to hydrogen bonding with smaller contributions from a general dielectric effect. Some α-hydroxyketones are anomalous in showing red shifts in many polar solvents. Because of specific solvent—solute interactions no single property of the solvent itself serves as a general criterion for measuring solvent polarity. Information on these interactions for substituted ketones has been obtained by comparing the solvent shifts of the substituted ketones in various solvents with the corresponding shifts of acetone.
Two methods for the study of changes of electron distributions in excited states are evaluated here: the solvent shifts of absorption bands and the changes of pK. The electron distributions in a variety of states of aromatic molecules are considered: CT states of amino- and hydroxy-compounds, n-π* states of carbonyl functions, π-π* states of naphthols. The role of intra- and intermolecular hydrogen-bonding is discussed. When possible, the electron distributions obtained are compared with other methods of determination, and with theoretical expectations.
Theory of Intermolecular Forces
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- F London
- H C Longuet-Higgins
- J A Pople
- E G Mcrae
- H Margenau
- N R Kestner
- Y Ooshika
- P Suppan
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