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

Picosecond transient absorption spectra of 4-(N,N-dimethylamino)-benzonitrile (DMABN), 6-cyanobenzquinuclidine (CBQ), and benzquinuclidine (BQ) in polar and nonpolar solvents were measured for the first time at wavelengths < 400 nm. This was accomplished by the application of a xenon-filled high-pressure gas cell to generate a picosecond continuum between 300 and 800 nm. The picosecond transient spectra of DMABN and CBQ in acetonitrile, attributed to S(n) <-- S(1) excitation, are different from those found in hexane solution; they are similar to the absorption spectrum of the benzonitrile radical anion. This finding supports the intramolecular charge transfer character of the relaxed singlet excited state in polar solvents. The S(n) <-- S(1) spectra develop into T(n) <-- T(1) spectra within the excited state lifetime. An extremely weak fluorescence emission was also observed for BQ and can most probably be ascribed to a charge transfer state, too. The S(n) <-- S(1) spectrum 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.