Exciplexes with partial electron transfer. Spectral and thermodynamic properties of exciplexes of 9-cyanophenanthrene with electron donors in solvents of various polarity

E.V. Dolotova, I.V. Soboleva

XX ICP International Conference on Photochemistry, Moscow, Russia, July 30 - August 4, 2001, Book of Abstracts, p. 226.

ABSTRACT. Exciplexes with partial electron transfer formed by 9-cyanophenanthrene with polymethoxybenzenes were studied in solvents of various polarity. Phot.)Э.З.electron transfer Gibbs energy DGet* for investigated systems varies in the range from .0.09 to .0.02 eV. Dependencies of shifts of exciplex emission maximum relative to fluorescer emission maximum (hDv) on solvent polarity are non linear for exciplexes studied. The strong growth of hDv values with solvent polarity increase is the evidence of exciplex charge transfer change. The obtained dependencies of ln{(f'/f)/[D]} and ln{(f0/f - 1)/t0[D]} on reciprocal temperature proved to be bell shaped for most investigated exciplexes as typical for reversible complex formation reactions in excited state. Here f' is the exciplex emission quantum yield, f and f0 are emission quantum yields of fluorescer in the presence and absence of quencher, respectively. For most of studied exciplexes therm.Ь.jh.and activation parameters of exciplex formation were determined from these temperature dependencies. Experimental values of exciplex formation enthalpies DHEx* vary in the range -(15-35) kJ/mol and agree closely with enthalpy values for weak charge-tran.ч.complexes. Obtained values of exciplex formation enthropies DSEx* vary in the range -(7-90) J/mol K. Calculated values of exciplex formation Gibbs energies DGEx* (-(7-15) kJ/mol) are close to literature data for exciplexes [1]. However we have not revealed the regular change of DGEx* with increasing DGet* in any solvent as well as with changing solvent polarity for common exciplex, this may be attributed to compensation effect in DHEx* and DSEx*. Spectral experimental data (hDv) are well described by proposed model of self correlated media and exciplex polarization [2]. However experimental values of exciplex formation enthalpies are essentially less than calculated under this model. (on the base of parameters of exciplex electron structure determined from spectral data). Such deviations result from ignoring of repulsion energy between components of exciplex. It was revealed that this repulsion energy must be considered for more accurate description of experimental data under proposed model of self correlated media and exciplex polarization. The estimated values of repulsion energies for studied exciplexes are in the range from 0.1 to 0.6 eV. This work is supported by grant of Russian Foundation for Basic Research (99-03-32337). References: 1. Weller A., The Exciplex. Ed. By M.Gordon, W.R.Ware. N.Y.Acad.Press. 1975, p.23. 2. Dogadkin D.N., Soboleva I.V., Kuzmin M.G., High Energy Chemistry. 2001, vol.35, .2, p.130.

Laboratory of Photochemistry