Evidence for diffusion-controlled electron transfer in exciplex formation reactions. Medium reorganisation stimulated by strong electronic coupling

Kuz’min, M. G.; Соболева, И. В.; Dolotova, E. V.; Dogadkin, D. N.

doi:10.1039/b303203e

Cited by 22 publications

(35 citation statements)

References 41 publications

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“…[2][3][4][5] They have indeed been invoked in intra- [6][7][8][9][10] and intermolecular electron transfer in solution, [11][12][13][14][15][16][17] in organic semiconductors for photovoltaics, [18][19][20][21][22][23] and organic light emitting diodes, [24][25][26][27][28] and in biological systems like DNA [29][30][31][32] and photosynthetic reaction centers, 33 some time under different names, such as charge-transfer excitons or excited chargetransfer complexes. All these monikers designate a species with an electronic structure between those of the reactants and products.…”

Section: Introductionmentioning

confidence: 99%

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption

2015

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The dynamics of a moderately exergonic photoinduced charge separation has been investigated by ultrafast time-resolved infrared absorption with the dimethylanthracene/phthalonitrile donor/acceptor pair in solvents covering a broad range of polarity. A distinct spectral signature of an exciplex could be identified in the -C≡N stretching region. On the basis of quantum chemistry calculations, the 4-5 times larger width of this band compared to those of the ions and of the locally excited donor bands is explained by a dynamic distribution of exciplex geometry with different mutual orientations and distances of the constituents and, thus, with varying charge-transfer character. Although spectrally similar, two types of exciplexes could be distinguished by their dynamics: short-lived, "tight", exciplexes generated upon static quenching and longer-lived, "loose", exciplexes formed upon dynamic quenching in parallel with ion pairs. Tight exciplexes were observed in all solvents, except in the least polar diethyl ether where quenching is slower than diffusion. The product distribution of the dynamic quenching depends strongly on the solvent polarity: whereas no significant loose exciplex population could be detected in acetonitrile, both exciplex and ion pair are generated in less polar solvents, with the relative population of exciplex increasing with decreasing solvent polarity. These results are compared with those reported previously with donor/acceptor pairs in different driving force regimes to obtain a comprehensive picture of the role of the exciplexes in bimolecular photoinduced charge separation.

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Section: Introductionmentioning

confidence: 99%

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption

2015

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“…More and more data to suggest that many types of electron transfer processes, in particular photoinduced electron transfer between organic molecules in the kinetic region (at a free energy of excited-state electron transfer of ∆ > -0.4 eV), occur via the mechanism of intermediate formation of exciplexes [1][2][3][4][5][6], rather than the generally accepted mechanism of the preceding thermally activated solvent reorganization [7][8][9][10], are appearing in the literature. According to the mechanism of intermediate formation of exciplexes, the apparent rate constant of quenching of excited molecules is equal to k Q = 1/[1/ k 1 + exp ( ∆ / RT )] , where k 1 is the rate constant of exciplex formation, which is close to the diffusion rate constant; = 1/( + + + ) is the exciplex lifetime; ∆ is the Gibbs energy of exciplex formation; and , ,…”

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confidence: 99%

The nature of internal conversion and intersystem crossing in exciplexes

et al. 2005

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1 More and more data to suggest that many types of electron transfer processes, in particular photoinduced electron transfer between organic molecules in the kinetic region (at a free energy of excited-state electron transfer of ∆ > -0.4 eV), occur via the mechanism of intermediate formation of exciplexes [1][2][3][4][5][6], rather than the generally accepted mechanism of the preceding thermally activated solvent reorganization [7][8][9][10], are appearing in the literature. According to the mechanism of intermediate formation of exciplexes, the apparent rate constant of quenching of excited molecules is equal to k Q = 1/[1/ k 1 + exp ( ∆ / RT )] , where k 1 is the rate constant of exciplex formation, which is close to the diffusion rate constant; = 1/( + + + ) is the exciplex lifetime; ∆ is the Gibbs energy of exciplex formation; and , ,, and are the rate constants of exciplex emission, internal conversion and intersystem crossing of exciplexes, and exciplex dissociation into radical ions, respectively. Competition between different processes of exciplex decay determines not only the quantum yields of primary products of electron transfer but also the magnitude of the rate constants of quenching of excited molecules. Internal conversion and intersystem crossing, which involve partial back charge transfer and lead to either the initial singlet ground state or the triplet state of reactants, are the major processes of the exciplex decay. The back electron transfer in the radical ion pairs has been extensively investigated by many researchers [11][12][13][14][15][16][17][18][19][20][21], especially in view of the fact that it is for these exciplexes that a decrease in the rate of electron transfer with an increase in the reaction exoergicity (free energy of back electron transfer ∆ G BET = electron transfer) predicted by the Marcus theory (inverted region) [11][12][13][14][15][16][17][18][19][20][21] was observed.In order to reveal most important factors governing the rate of internal conversion and intersystem crossing in exciplexes, we will consider two alternative mechanisms, the thermally activated electron transfer and radiationless quantum transition. An analysis of experimental data shows that it is more sound to consider both internal conversion and intersystem crossing (back electron transfer) in exciplexes as radiationless electronic-vibrational diabatic quantum transitions rather than thermally activated electron transfer.In the previous works [22,23], we measured the quantum yields of fluorescence ( ) of exciplexes of 9-cyanophenanthrene (CP) with 1,2,3-and 1,3,5-trimethoxybenzenes (123TMB and 135TMB), the quantum yields of formation from these of triplet states ( ) and of radical ions ( ), and the rate constants of internal conversion and intersystem crossing of these exciplexes in solvents of different polarities. The experimental methods and the procedures of the determination of the quantum yields of formation of triplets, radical ions, and internal conversion were described elsewhere [22,23]. The d...

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“…The units of x are the same as in Figure . For similar plots displaying the effect of varying λ s , see reference .…”

Section: Resultsmentioning

confidence: 99%

Transition from Charge‐Transfer to Largely Locally Excited Exciplexes, from Structureless to Vibrationally Structured Emissions

Young

Feinberg

Dinnocenzo

et al. 2014

Photochem & Photobiology

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Exciplexes of 9,10-dicyanoanthracene (DCA) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge-transfer (CT) character, when the donor has a relatively low oxidation potential (Eox ), e.g. hexamethylbenzene (HMB). With increasing Eox and stronger mixing with a locally excited (LE) state, vibrational structure begins to appear with 1,2,3,5-tetramethylbenzene and becomes prominent with p-xylene (p-Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g., p-Xy(•+) , are stabilized more in acetonitrile than the heavily substituted HMB(•+) . Remarkably, ion pairing with DCA(•-) in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low-Eox donors are almost identical except for displacements that approximately equal the differences in Eox , even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.

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Evidence for diffusion-controlled electron transfer in exciplex formation reactions. Medium reorganisation stimulated by strong electronic coupling

Cited by 22 publications

References 41 publications

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption

The nature of internal conversion and intersystem crossing in exciplexes

Transition from Charge‐Transfer to Largely Locally Excited Exciplexes, from Structureless to Vibrationally Structured Emissions

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