Kinetic and thermodynamic properties of exciplexes with partial charge transfer.

D.N. Dogadkin, M.G. Kuzmin

XX ICP International Conference on Photochemistry, Moscow, Russia, July 30 - August 4, 2001, Book of Abstracts, p. 223.

ABSTRACT. Exciplexes with partial charge transfer are key transients in photoinduced electron transfer reactions with Gibbs energy DGet* > .0.2 eV both in nonpolar and polar solvents. Such exciplexes have some substantial differencies from exciplexes with complete charge transfer. Their stabiliza.*'even in polar solvents occurs predominantly due to electronic coupling matrix element between locally excited (LE) and charge transfer (CT) states H12 ra..than electrostatic attraction. The extent of charge transfer z is substantijYraffected by solvent polarity and DGet* [1]. The influence of solvent polarity, DGet* and exciplex electronic structure on activation and thermodynamic parameters of exciplex formation and decay in donor-acceptor systems with DGet* > -0.1 eV is studied. Temperature dependencies of emission quantum yields of exciplexes (f') and fluorophores in the presence (f) and in the absence (f0=) of quencher are measured in the solvents of various polarity (from toluene to acetonitrile). These dependencies follow equations resulting from simplified scheme of exciplex formation and decay: ln{(f0/f - 1)/t0[D]} = k1/(1 + k-1t0') = A + C/T - ln[1 + B exp(D/T)] (1) ln{(f'/f)/[D]} = (kf'/kf)(k1/k-1)/(1 + 1/k-1t0') = A' + C'/T - - ln[1 + B'exp(D'/T)] (2) Here A = ln(k10), B = 1/B' = (k-10/k'0), C = -DH1./R, D = -D' = (DHEx. - DH-1.)/R and A' = ln(kf'/kf) + DSEx*/R, C' = -DHEx*/R, t0', t0, kf' and kf are l.В..g.and emission rate constants of exciplex and fluorophore, respectively. k1 and k-1 are exciplex formation and dissociation rate constants and [D] is the quencher concentration. t0' can be approximated as t0' = k'0exp(-DHEx./RT), where DHEx. is the apparent activation enthalpy of exciplex transformations. Using equations (1,2) one can easily determine all activation and thermodynamic parameters ..describe exciplex formation and decay. Enthalpy (DмEx*) and Gibbs energy (DGEx*) of exciplex formation tend to decrease with increasing of solvent polarity due to exciplex energy UEx* fall caused by z and dielectric constant increase. But DмEx* dependence on DGet* in given solvent seems to be substantially affected by H12 values determined earlier from spectral data [2]. Indeed, the increase of H12 which is in the range 0.2-0.45 eV for studied exciplexes leads to DмEx* decrease due to stronger exciplex stabilisation. Exciplex formation entropy (DSEx*) varies in the range from .90 to .5 J mol-1 K-1 in studied exciplexes and grades the exciplex electron structure influence on DGEx*. Activation enthalpies of exciplex formation DH1. decrease as the solvent ..Z.+rincrease. In nonpolar solvents *H1. achieve 15-17 kJ/mol and in polar solvents especially in acetonitrile tend to DHdiff. (about 8 kJ/mol) in studied range of DGet* (from .0.1 to +0.1 eV). Exciplex lifetimes t0' in our systems estimated by equations (1,2) fall down with increase of solvent polarity in agreement with a wide set of literature date. The calculated value of *0* for 9-cyanoanthracene . 1,6-dimethylnaphtalene exciplex in acetonitrile is close to experimentally determined [3] and relatively large (20 ns). We guess it is directly connected with high value of H12 (about 0.4 eV) providing electron coupling stabilization of the exci.W.even in acetonitrile. The results obtained indicate that exciplex formation enthalpy is controlled by solvent polarity, DGet* and exciplex electronic structure (z and H12). On the contrast, the activation enthalpies of exciplex formation are sensible to solvent polarity only and tend to be diffusion controlled in polar solvents independently on DGet* value. Relatively large exciplex lifetimes in acetonitrile occur due to large values of H12 for exciplexes studied. This work is supported by grant of Russian Foundation for Basic Research (99-03-32337). REFERENCES 1. Kuzmin, M.G., Russian Journal Physical Chemistry, 1999, vol. 73, .10, p. 1625. 2. Dogadkin, D.N., Soboleva, I.V., Kuzmin, M.G., High Energy Chemistry. 2001, vol.35, .2, p.130. 3. Sadovskii, N.A., Kutsenok, O.I., Vainstein, Yu.A., Kuzmin, M.G., Russian Journal Physical Chemistry, 1996, vol. 70, .12, p. 2029.

Laboratory of Photochemistry