Kinetics of low-polar exciplexes formation

Oleg I. Kutsenok, Irina V. Soboleva, Michael G. Kuzmin, Klaas A. Zachariasse

XVIIIth International Conference on Photochemistry, Book of Abstracts, (Warsaw, Poland August 3-8, 1997), p.1P48

Exciplexes with low extent of electron transfer formed when Gibbs energy of electron transfer DGet > -0.2 eV and exist even in polar solvents [1]. They are supposed to be formed as transients in excited-state electron transfer reactions [2]. We studied the kinetics of exciplex formation and decay by single photon counting in several systems (1,12-benzoperylene and 1,2-benzantracene with 1,2,4-trimethoxytoluene, 3,4,5-trimethoxybenzene and 1,2-dimethylnaphtalene) where -0.1 eV < DGet < 0.2 eV at various temperatures in acetonitrile (DGet = EOx(D) - ERed(A) - E* , where EOx(D) and ERed(A) are the oxidation and reduction potentials of the electron donor and acceptor, respectively, and E* is the excitation energy of the acceptor). Even for DGet > 0 the emission of exciplexes was observed and negative values of Gibbs energy of exciplex formation (DGEx = -RT lnKEx , where equilibrium constant of an exciplex formation KEx = k1/k-1 is the ratio of exciplex formation and dissociation rate constants found from the data on kinetics of exciplex formation and decay) were obtained.

Specific difficulties arise in measurements of rate constants in the case of low equilibrium constants of exciplex formation typical for these systems because of very small relative amplitude (< 0.1) of the fast exponent in the kinetics of excited molecules decay. Using high quality equipment for fluorescence kinetics (MPI fuer biophysikalische Chemie, Goettingen) and reliable deconvolution procedure we were able to find rate constants of exciplex formation and its dissociation into parent molecules and exciplex lifetimes.

Rate constants and activation energies of exciplex formation were found to be close to diffusion ones when DGet < 0.1 eV, which confirms minor reorganization of the medium and reactants necessary for the exciplex formation. But for greater DGet when DGEx > -0.1 eV and the extent of electron transfer in the exciplex d < 0.5 (in acetonitrile, and even smaller in less polar solvents), the distinct differences are observed - greater activation energy and smaller magnitude of rate constant than diffusion ones. A reasonable correlation between DGEx and DGet observed points to the considerable contribution of electron coupling into the stabilization energy of these exciplexes. Obtained lifetimes of exciplexes in acetonitrile are in the range 3-50 ns contrary to much shorter lifetimes of radical ion pairs (<0.1 ns). This suggests relatively low rate constant of their dissociation into radical ions (< 0.02-0.3 ns-1) even in so polar solvent as acetonitrile which can be attributed to the considerable stabilization of these exciplexes by electron coupling (> 0.1-0.2 eV) rather than by electrostatic attraction. This work was supported by INTAS grant 93-0751 and RFBR grant 95-03-09775.

References:

  1. N.Sadovskii, O.Kutsenok, J.Weinstein and M.Kuzmin, Zhurnal Fizicheskoi Khimii, 70 (11), 2008 and (12), 2194 (1996); English edition: Russian J. Phys. Chem., 70, 1861 and 2194 (1996).
  2. M.Kuzmin, N.Sadovskii, J.Weinstein and O.Kutsenok, Proc. Indian Acad. Sci. (Chem. Sci.) 105, 637 (1993).

Laboratory of Photochemistry