I.V. Soboleva, D.N. Dogadkin, M.G. Kuzmin
XVIIIth International Conference on Photochemistry, Book of Abstracts, (Warsaw,Poland August 3-8,1997), p.1P73
Exciplexes with medium extent of electron transfer formed at the interaction of excited molecules with weak donors and acceptors (DGet> -0.2 eV) can be observed even in polar solvents contrary to the well investigated exciplexes with complete extent of electron transfer. Their emission spectra show essentially lower solvent shift and their dipole moments evaluated by Mataga-Lippert equation were in the range 2-6 D. Substantial change in the electronic structure with solvent polarity was observed for the exciplexes with the medium extent of electron transfer [1-4]. To derive semiempiric relationships between the emission spectra of exciplexes, their electronic structure and the solvent polarity we considered a simplified model which took into account in an explicit form the effect of the solvent polarity on the energy levels. According to this model the spectral shift Dn of the exciplex emission relative to the parent excited molecules can be expressed as:
hDn = H12/(1/ c12 - 1)1/2 - c14 (m02/a3) f(n2)
where H12 is electron coupling energy; m0 and a are dipole moment of contact radical ion pair with complete electron transfer and radius of the solvent cavity of the exciplex; f(n2) = (n2-1)/(2n2+1); and the contribution of the CT state into the exciplex structure (the extent of electron transfer) c12 is determined from the equation:
H220-H110 = H12 [ 1/(1/c12-1)1/2 - (1/c12-1)1/2 ] - c12(1+c12) (m02/a3) f(e)
where (H220-H110) is the energy gap between the local excited and CT states in vacuum and f(e) = (e-1)/(2e+1). Parameters (H220-H110), H12, and (m02/a3) for each exciplex can be found by the fitting of the experimental dependence of hDn on f(e). The exciplexes of pyrene, 1,12-benzanthracene, 1,2-benzperylene and fluoranthene with 1,2,4-trimethoxybenzene and 1,4-dymethoxybenzene in the solvents of various polarity from hexane to acetonitrile were investigated in order to test the validity of this model and to determine the equilibrium constants and enthalpy of the exciplex formation. Obtained dependences of hDn on f(e) are essentially non-linear and their slope rises with the rise of the solvent polarity. The deviation from the linearity becomes more pronounced with the growth of DGet. Sigmoidal plots of hDn vs. f(e) with sharp increase of hDn within narrow range of f(e) = 0.4-0.5 indicates the strong change of the exciplex polarity (c12) in this range of f(e). Therefore the considerable change of the contribution of CT state c12 and of the exciplex dipole moment with the solvent polarity renders the use of Mataga-Lippert plot for the determination of the exciplex dipole moments impossible. Parameters (H220-H110), H12, and (m02/a3) should be used as characteristics of the exciplex structure rather than their dipole moments.
This work was supported by INTAS grant 93-0751 and RFBR grant 95-03-09775.
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