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D.N. Dogadkin
XVIIth IUPAC Symposium on Photochemistry, Dresden, German, July 22-27, 2000, Book of Abstracts, p. 218.
ABSTRACT.Exciplexes with partial charge transfer formed by interaction of excited molecules with weak electron donors and acceptors (DGet > -0.2eV) are supposed to be transients in electron transfer reactions. The extent of charge transfer in such exciplexes is substantially affected by solvent polarity e and Gibbs energy of electron transfer DGet. The thorough investigation of electron transfer reactions taking place through exciplex formation requires a detailed study of exciplex formation and decay. In present work we studied solvent and Gibbs energy dependence of exciplex formation enthalpy and activation energy. For this purpose we studied exciplexes of pyrene with 1,4-dimethoxybenzene (DGet = 0.10 eV) and 9-cyanoanthracene with 1,3,5-trimethoxybenzene (DGet = 0.09 eV), 1,6-dimethylnaphtalene (DGet = -0.06 eV) and 1,8-dimethylnaphtalene (DGet = -0.10 eV) in solvents of various polarity (from toluene to acetonitrile). The exciplex formation enthalpy and activation energy were calculated from temperature dependence of exciplex emission using equation: ln(f'/f) = A0 - EA/RT - ln{1 + A1exp[(DHEx - EA)/RT]} where f'/f is the ratio of exciplex and fluorescer quantum yields in the presence of the quencher, DHEx and EA are exciplex formation enthalpy and activation energy, respectively.
The values of -DHEx obtained are in the range 10-40 kJ/mol and increase with the increase of solvent polarity and decrease of DGet. But in contrast to the exciplexes with complete charge transfer (DGet < -0.4 eV) studied earlier by Weller [1] the enthalpy dependence on DGet and solvent polarity for exciplexes with partial charge transfer have much smaller slope and are nonlinear. The obtained dependencies may be explained by substantial changes of the extent of charge transfer (z) with the change of solvent polarity (f'(e)) and DGet and follow the equations derived in [2]:
-DHEx = H12/(1/z-1)^1/2 - z^2(m0^2/r^3)f'(e)
hDv = H12/(1/z-1)^1/2 - z^2(m0^2/r^3)f'(n^2) where H12 is the electronic matrix element coupling between locally excited (LE) and charge transfer (CT) states, r is the equivalent sphere radius according to the Kirkwood-Onsager model,f'(e) is the solvent polarity function expressed as f'(e) = (e - 1)/(2e + 4) and f'(n^2) is the solvent refraction function expressed as f'(n^2) = (n^2 - 1)/(2n^2 + 4) with a mean value close to 0.12. The extent of charge transfer may be expressed in implicit form by the following equation:
(H22o - H11o) = 2z(m0^2/r^3)f'(e) - H12[1/(1/z - 1)1/2 - (1/z - 1)1/2]
where (H22o - H11o) is the difference between CT and LE states in vacuum which is closely related with DGet as (H22o - H11o) = DGet + const. These equations suggest substantial growth of z with the increase of solvent polarity which is in agreement with direct measurements of z obtained earlier by Gould et al. [3,4]. Experimental values of -DHEx are slightly smaller than calculated from parameters (H22o - H11o), H12, and (m0^2/r^3) obtained from the dependence of hDv on the solvent polarity. The reasons of this difference are discussed.
Activation energy of exciplex formation EA decreases from 20 kJ/mol in nonpolar solvents (e = 2, DHEx > -20 kJ/mol) to 10 kJ/mol in solvents of medium polarity (e > 10, DHEx < -25 kJ/mol). The latter values of EA are close to diffusion controlled processes in corresponded solvents.
This work is supported by grants of Russian Foundation for Basic Research (99-03-32337) and "Russian University - Basic Research".
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