Time-resolved spectroscopy of DMABN and its cage derivatives 6-cyanobenzquinuclidine (CBQ) and benzquinuclidine (BQ)

Picosecond transient absorption spectra of 4-(N,N-dimethylamino)-benzonitri le (DMABN), 6-cyanobenzquinuclidine (CBQ), and benzquinuclidine (BQ) in pol ar and nonpolar solvents were measured for the first time at wavelengths < 400 nm. This was accomplished by the application of a xenon-filled high-pre ssure gas cell to generate a picosecond continuum between 300 and 800 nm. T he picosecond transient spectra of DMABN and CBQ in acetonitrile, attribute d to S-n <-- S-1 excitation, are different from those found in hexane solut ion; they are similar to the absorption spectrum of the benzonitrile radica l anion. This finding supports the intramolecular charge transfer character of the relaxed singlet excited state in polar solvents. The S-n <-- S-1 sp ectra develop into T-n <-- T-1 spectra within the excited state lifetime. A n extremely weak fluorescence emission was also observed for BQ and can mos t probably be ascribed to a charge transfer state, too. The S-n <-- S-1 spe ctrum of BQ could not be recorded due to its very short lifetime, and only T-n <-- T-1 absorption was found. (C) 1999 Elsevier Science B.V. All rights reserved.