Figure 6 - available via license: Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International
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Schematic and graphic representation of the thermal isomerization pathway for the neutral geometries 1-O to 1-F and 2-O to 2-F based on DFT calculations (B3LYP/6-31G**). Hydrogens are omitted for clarity.
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A multistable molecular switching system based on an anthracene-extended bis-thiaxanthylidene with three individually addressable states that can be interconverted by electrochemical, thermal, and photochemical reactions is reported. Besides reversible switching between an open-shell diradical- and a closed-shell electronic configuration, our findi...
Context in source publication
Context 1
... of 1 2+ leads to the diradical state 1-O without significant conformational change. 1-O is a local minimum (ΔE = 0.7 kcal mol −1 relative to 1-F, Figure 6) kinetically trapped with a high barrier (ΔE ⧧ = 30.6 kcal mol −1 ) for isomerization at room temperature toward the global minimum 1-F. ...
Citations
... While the closed-shell quinoid form (A) and the open-shell biradical form (B) are formulated as resonance pairs in singlet biradicaloids [Type-I] for II-IV, anthraquinodimethane (V-A) and anthracene-9,10-diyl (V-B) are completely different species based on a drastic change in the structure, and thus they are shown to be in equilibrium while interconverting with a certain energy barrier for changes in their geometrical and electronic structures. [13] A few previous papers [14,15] have reported successful isolation of both the closed-shell quinoid form (V-A) and the open-shell biradical form (V-B), in which light irradiation or heating at high temperature was used to overcome the large energy barrier to trigger interconversion [Type-III (ΔG � > 25 kcal mol À 1 )] (Scheme 2). ...
... In contrast, in Type-III, the two forms have quite different geometrical and electronic structures. A bulky and rigid tricyclic subunit, such as fluorenyl, [17,18] dimethyldihydroanthryl, [14] or dialkoxythioxanthenyl, [15] on each end of the exocyclic bonds provides a high enough energy barrier (ΔG � > 25 kcal mol À 1 ) to suppress mutual interconversion under ambient conditions. These features of the subunit also fulfill the requirement to stabilize the biradical forms (B) by steric protection with suppressing the side reactions of the open-shell species in Type-III molecules. ...
... This outcome is quite different from that with the Type-III compounds, for which, upon reduction of the twisted dications, the twisted biradical forms (B) were generated and kinetically trapped without converting to the corresponding quinoid forms (A) because of the suitably high energy barrier (ΔG � ). [15] Thus, the newly prepared 1-6 were shown to not belong to the Type-III category, which prompted us to further study the present molecules to investigate if they are Type-II compounds that undergo rapid equilibrium with partial conversion of the quinoid form (A) to the open-shell biradical form (B). ...
Upon dibenzo annulation on Thiele's hydrocarbon (tetraphenyl‐p‐quinodimethane), the quinoid form and the biradical form adopt quite different geometries, and thus are no longer resonance structures. When these two forms can interconvert rapidly due to the small energy barrier (ΔG≠), the equilibrated mixture contains both forms in a ratio that is determined by the energy difference (ΔGo) between the two forms. For a series of tetrakis[5‐(4‐methoxyphenyl)‐2‐thienyl]‐substituted derivatives, the more stable quinoid form and the metastable biradical form coexist in solution as an equilibrated mixture due to small ΔG≠ (<15 kcal mol⁻¹) and ΔGo (1–4 kcal mol⁻¹), in which the proportion of the two forms can be regulated by temperature. Since the biradical form can undergo easy two‐electron (2e) oxidation to the corresponding dications as well as easy 2e‐reduction to the dianions, it exhibits very high electrochemical amphotericity. This character with a record‐small span for not only the first oxidation and reduction potentials but also the second those, [E1sum≈E2sum=E2ox−E2red=ca. 1.4 V], is attained through thermally enhanced conversion to the biradical form from the corresponding quinoid form, the latter of which is less amphoteric due to higher Eox and lower Ered values.
... 6 The distinct electronic structure of diradicaloids have attracted enduring attention both in terms of their fundamental understanding 17 as well as their potential as functional materials. 18,19 p-Extended diradicaloids have been applied as organic conductors, 20,21,22 near-IR chromophores, 23 electrochromics, 6,8 singlet fission materials, 24,25 non-linear optical materials, 26,27 and magnetic materials such as spin-switches. 28 , 29 , 30 Most applications require considerable chemical and photostability, which remains a challenge despite significant synthetic efforts to design thermodynamic and kinetic stabilization in such molecules. ...
Thiele’s hydrocarbon was the first synthesized diradicaloid in the search for stable open-shell structures but itself remains sensitive to oxygen and light. We here report the synthesis of Thiele’s fluorocarbon (TFC) and its derivatives exhibiting exceptional thermal, oxidative, and photostability. TFCs have remarkable luminescent properties with yellow to NIR fluorescence and up to 100% quantum yields. X-ray crystallography and ESR spectroscopy confirm their closed-shell quinoidal ground state. As expected from their uniform HOMO/LUMO distributions, TFCs’ absorption spectra show no solvent effect, but their emission reveals an extraordinarily large Stokes shift which increases with solvent polarity (from 0.9 eV in cyclohexane to 1.5 eV in acetonitrile). We show that this behavior is a result of sudden polarization leading to a zwitterionic excited state.
... 6,[28][29][30] In contrast, the current research is largely focused on the properties of extended diradicaloids, with efforts being made to suppress 17,[30][31][32] their reactivity in order to obtain stable or persistent systems. A few recent reports indicate, however, that the reactivity of "unchained" diradicaloids, often regarded as an undesired or a decomposition feature, can be utilized to create function 25,[33][34][35][36][37] , develop new methods 38,39 , and deepen our chemical concepts 40,41 . Scheme 1. π-Radical Reactivity Overview a a (Top) Decomposition pathway of phenalenyl (PLY) to peropyrene (PP) via 6π EC of biphenalenylidene (BPLY). ...
A dihydro precursor of helical diradicaloid nonacethrene undergoes a reaction cascade triggered by an oxidant to a chiral contorted polycyclic aromatic hydrocarbon named hypercethrene. In this ten-electron-oxidation process, four σ bonds, one π bond, and three six-membered rings are formed in a reaction sequence of up to nine steps to yield a 72-carbon-atom warped framework, comprising two configurationally locked [7]helicene units, fluorescent peropyrene unit, and two precisely installed sp3-defects.The key intermediate in this cascade is a closed nonacethrene derivative with one quaternary center, presumably formed via an electrocyclic ring closure of nonacethrene, which— when activated by oxidation—undergoes an oxidative dimerization of phenalenyl to peropyrene. By controlling the amount oxidant used, two intermediates and one side product could be isolated and fully characterized, including single-crystal X-ray diffraction analysis, and one intermediate was detected by electron paramagnetic resonance spectroscopy. In concert with density functional theory calculations, these intermediates support the proposed reaction mechanism. Compared to peropyrene, the absorption and emission of hypercethrene are slightly red-shifted on account of extended conjugation and the fluorescence quantum yield of 0.45 is decreased by a factor of ~2. Enantiomerically enriched hypercethrene displays circularly polarized luminescence with a CPL brightness value of 8.3 M–1cm–1. This unexpected reaction cascade demonstrates that the reactivity of “unchained”diradicaloid compounds, which is typically considered an undesired feature, can be well-defined and employed as a useful, step-economic synthetic tool toward novel carbon nanostructures.
... [9][10][11][12][13][14][15] Successful switching of spin states in all-organic molecules is rare. [16][17][18][19][20] It can be realized based on several molecular mechanisms, 20 but only two concepts that rely either on 1) a photoconformational trap (Figure 1, top) or on 2) a photochemical reaction (Figure 1, bottom) have the potential of achieving true magnetic bistability. An early example of a photo-induced paramagnetic diradicaloid form with simultaneous conformational change that gave rise to a paramagnetic state was reported by Toda and Tanaka, but no magnetic bistability was achieved ( Figure 1a). ...
Controlling the electronic spin state in single-molecules through an external stimulus is of interest in developing devices for information technology, such as data storage and quantum computing. We report the synthesis and operation mode of two all-organic molecular spin state switches that can be photochemically switched from a diamagnetic (EPR silent) to a paramagnetic (EPR active) form at cryogenic temperatures due to a reversible electrocyclic reaction of its carbon skeleton. Facile synthetic substitution of a configurationally stable 1,14-dimethyl-[5]helicene with radical stabilizing groups at the 4,11-positions afforded two spin state switches as 4,11-dioxo or 4,11-bis(dicyanomethylidenyl) derivatives in a closed diamagnetic form. After irradiation with an LED light source at cryogenic temperatures a stable paramagnetic state is readily obtained, rendering this system a bistable magnetic switch that can reversibly react back to its diamagnetic form through a thermal stimulus. The switching can be monitored with UV/Vis spectroscopy, EPR spectroscopy, or induced by electrochemical reduction and reoxidation. Variable-temperature EPR spectroscopy of the paramagnetic species revealed an open-shell triplet ground state with an experimentally determined triplet–singlet energy gap of < 0.1 kcal/mol. The inherent chirality and the ability to separate the enantiomers turns this helical motif into a potential chiroptical spin state switch. The herein developed 4,11-substitution pattern on the dimethyl[5]helicene introduces a platform for designing future generations of organic molecular photomagnetic switches that might find applications in spintronics and related fields.
This annual review provides critical analysis of the literature on photochemistry and its applications for anyone wanting to keep up to date with the field. Combining reviews on the latest advances in photochemical research with specific topical highlights, this book is the primary resource for anyone wanting succinct and rich information.
The volume starts with periodical reports of the recent literature on physical and inorganic aspects, including the molecules of colour, light induced reactions in cryogenic matrices, photobiological systems studied by time-resolved infrared spectroscopy, photophysics and photochemistry of transition metal complexes, recent advances in photocatalytic water splitting, and finally a chapter on time-resolved spectroscopy application of LFP to heterogeneous photocatalysis. Coverage continues in the second part with highlighted topics including, among others, transition metal complexes-based photochemotherapy, advances in polaritonic photochemistry, synthetic strategies based on halogen atom transfer processes and photochemical water oxidation using metal-based chromospheres. This volume will again include a third section entitled SPR Lectures on Photochemistry, providing examples introducing academic readers to a photochemistry topic and precious help for students in photochemistry.
To mimic biological systems' healing mechanism and sensory motion, the combination of self‐healing, perception, and actuation in a singular soft artificial material is required, which would be enormously valuable for soft robotics that commit to obtaining multifunction and local sensing capacities approaching living organisms. However, most existing soft somatosensory actuators lack self‐healing capability to injuries, and suffer from insufficient actuating performance and sensitivity, and complicated manufacturing operations. Herein, a bioinspired conductive photo‐responsive architecture is reported. Rapidly photo‐responsive anthracene, self‐healing matrix with dynamic interactions, and high‐conductivity slideable silver nanowires chemically integrated with matrix are respectively utilized to mimic the neuromuscular system and effectors, biological tissue systems, and nerve cords and receptors of octopus tentacles. Such a soft somatosensory actuator exhibits rapid actuation (light‐driven bending velocity, 10o s⁻¹), distinctive intrinsic strain sensitivity (gauge factor, 90.88), and decent self‐healing efficiency (92.2%). As a proof of concept, octopi tentacles‐inspired smart grippers are fabricated for various photo‐responsive motions including bending, weightlifting and object grasping that can contemporaneously detect actions by real‐time resistance changes and provide information feedback. This work is anticipated to bring fresh horizons in the design of multifunctional sensory actuating materials and establish a pathway for the advancement of self‐diagnostic soft robots.
A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m‐phenylene bridged electron‐rich bis‐triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half‐field signal. A computational analysis confirms the triplet state to be the ground state. As a proof‐of‐principle for the modular methodology, the 4,6‐dimethyl‐m‐phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6‐dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m‐phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, after 108 years of Schlenk’s m‐phenylene‐bis(diphenylmethyl) synthesis as diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m‐phenylene, between two radical centers in a crystalline form.
A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m‐phenylene bridged electron‐rich bis‐triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half‐field signal. A computational analysis confirms the triplet state to be the ground state. As a proof‐of‐principle for the modular methodology, the 4,6‐dimethyl‐m‐phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6‐dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m‐phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, 108 years after Schlenk's m‐phenylene‐bis(diphenylmethyl) synthesis as a diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m‐phenylene, between two radical centers in a crystalline form.
