CONFORMATIONAL DYNAMICS OF FLEXIBLY AND SEMIRIGIDLY BRIDGED ELECTRON DONOR-ACCEPTOR SYSTEMS AS REVEALED BY SPECTROTEMPORAL PARAMETERIZATIONOF FLUORESCENCE

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-sha pe 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 moder ately polar solvents (epsilon > 3.5) only fluorescence originating fro m an extended dipolar species (extended charge transfer (ECT) state) w as found, possessing a dipole moment of about 27 D. In low dielectric media the systems with a Strong cyanonaphthalene acceptor showed ''har pooning'': the ECT species was converted to a folded dipolar species ( contact charge transfer (CCT) state), similar to a tight polar exciple x, on a time scale of less than 1 ns (flexible bridge) or 7 ns (semiri gid 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 bri dged systems CT fluorescence originating from two types of CCT was fou nd-instead.