TIME-RESOLVED ION-PAIR CHARGE-TRANSFER FLUORESCENCE OF BIPYRIDINIUM SALTS IN VARIOUS MICROENVIRONMENTS

The temperature dependence of ion-pair charge transfer (IPCT) fluoresc ence was studied by time-resolved spectroscopy in 4,4'-bipyridinium an d 2,2'-bipyridinium salts with tetrakis [3,5-bis (trifluoromethyl)phen yl]berate in microcrystals and polymer films at 10-300 K. Time-resolve d fluorescence, peaking at 400-460 nm and 500 nm, appeared on excitati on of IPCT absorption. A shorter wavelength peak in microcrystals was observed at 440 nm for the 4,4'-bipyridinium salt and at 460 nm for th e 2,2'-bipyridinium salt; this peak was independent of temperature. A shorter wavelength peak in polymer films containing the 4,4'-bipyridin ium salt was observed to shift from 432 nm --> 420 nm --> 402 nm with decreasing temperature from 300 K to 10 K, corresponding to the molecu lar motion of polymer chains. Similarly, a shorter wavelength peak in polymer films containing the 2,2'-bipyridinium salt was observed to sh ift from 460 nm to 440 nm with decreasing temperature from 300 K to 10 K. The fluorescence decay curves were composed of two fast components (tau = 0.3-7 ns) and a slow component. (tau = 12-34 ns). The former c orresponds to the shorter wavelength peak and the latter to the longer wavelength peak in the IPCT fluorescence spectra of the 4,4'-bipyridi nium and 2,2'-bipyridinium salts. A reaction mechanism is proposed on the basis of these results.