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XVIth IUPAC Symposium on Photochemistry, Helsinki, Finland, July 21-26, 1996, Book of Abstracts, p. 346.
ABSTRACT.
The exciplexes formed when weak electron donors or acceptors are used as
quenchers (DGet > -0.2 eV) possess the medium extent of electron transfer.
They exist even in polar solvents. Their dipole moments are supposed to
depend on the solvent polarity.
We considered the simplified model of the solvent effect on the electronic
structure of exciplexes. Because of this effect Mataga’s plot can not be used
for determination of the dipole moments of exciplexes.
The combination of three parameters should be used for characterization of
the electronic structure and polarity of such exciplexes: energy gap between
locally excited and CT states in vacuum (H22o-H11o), electronic coupling
energy H12, and parameter (mo2/a3) which is referred to the dipole moment of
contact radical ion pair with complete electron transfer and the size of the
solvent cavity for such radical ion pair. The considerable change of the
exciplex polarity and dipole moment was shown to occur at (H22o-H11o) >
-0.2 eV when solvent polarity increases.
We measured the emission spectra for exciplexes of pyrene (Py) and
1,12-benzperylene (BP) with 1,2,4-trimethoxybenzene (TMB) and determined the
average emission frequencies for this exciplexes in solvents of various polarity
(from hexane to acetonitrile). By fitting of experimental data we obtained the
values of (H22o-H11o), H12 and (mo2/a3) using our model. They are -0.035, 0.35,
0.70 eV for exciplexes of Py with TMB and -0.05, 0.35, 0.70 eV for BP - TMB
correspondingly.
These parameters were obtained also for exciplexes of TCA with various
methyl substituted benzenes (from p-xylene to pentamethylbenzene) from the
published spectral data in solvents of various polarity [I.R. Gould et al.,
J.Am.Chem.Soc. 116 (1994) 8188]. The values of H12 and (mo2/a3) are 0.26 and
0.78 eV correspondingly.
This work was supported by INTAS, RFBR and RU grants.