Conformational changes of intramolecular charge-transfer (CT) species were studied by means of time-resolved fluorescence spectroscopy.From a time-resolved fluorescence spectrum the parameters which describe a kinetic, compartmental model as well as the fluorescence spectral-shape parameters were estimated.The systems (Chart 1) differed in donor and acceptor strengths and possessed two different kinds of bridges: a flexible trimethylene chain and a semirigid piperidine ring.In moderately polar solvents (6 > 3.5) only fluorescence originating from an extended dipolar species (extended charge transfer (ECT) state) was found, possessing a dipole moment of about 27 D.In low dielectric media the systems with a strong cyanonaphthalene acceptor showed "harpooning": the ECT species was converted to a folded dipolar species (contact charge transfer (CCT) state), similar to a tight polar exciplex, on a time scale of less than 1 ns (flexible bridge) or 7 ns (semirigid bridge).No indications were found for an ECT in the systems with a weak naphthalene acceptor in nonpolar solvents.Semirigidly bridged systems displayed only local fluorescence, whereas in the flexibly bridged systems C T fluorescence originating from two types of CCT was found instead.
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Stokkum et al. (1994) studied this question.