Time-resolved ion-pair charge transfer fluorescence of bipyridinium salts in various microenvironments

Abstract

The temperature dependence of ion-pair charge transfer (IPCT) fluorescence was studied by time-resolved spectroscopy in 4,4′-bipyridinium and 2,2′-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in microcrystals and polymer films at 10–300 K. Time-resolved fluorescence, peaking at 400–460 nm and 500 nm, appeared on excitation 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 the 2,2′-bipyridinium salt; this peak was independent of temperature. A shorter wavelength peak in polymer films containing the 4,4′-bipyridinium salt was observed to shift from 432 nm å 420 nm å 402 nm with decreasing temperature from 300 K to 10 K, corresponding to the molecular motion of polymer chains. Similarly, a shorter wavelength peak in polymer films containing the 2,2′-bipyridinium salt was observed to shift 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 ( τ = 0.3−7 ns) and a slow component ( τ = 12–34 ns). The former corresponds to the shorter wavelength peak and the latter to the longer wavelength peak in the IPCT fluorescence spectra of the 4,4′-bipyridinium and 2,2′-bipyridinium salts. A reaction mechanism is proposed on the basis of these results.