Further time-resolved spectroscopic investigations on the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) and its derivatives, 4-diethylaminobenzonitrile (DEABN) and 4-dimethylamino-3,5-dimethylbenzonitrile (TMABN)

Abstract

A wide range of time-resolved spectroscopic methods (time-resolved infrared (TRIR), picosecond Kerr gated time-resolved resonance Raman (ps-K-TR 3) and transient absorption (TA)) have been utilized to investigate the charge transfer reaction of photoexcited 4-dimethylaminobenzonitrile (DMABN), 4-diethylaminobenzonitrile (DEABN) and 4-dimethylamino-3,5-dimethylbenzonitrile (TMABN) in polar aprotic solvents. By monitoring directly the temporal changes of the ICT state TRIR C≡N band, the charge transfer rate for each compound has been measured. The ICT reaction is fastest for TMABN, with the ICT state forming promptly after electronic excitation and DEABN is found to be faster than DMABN. The ps-K-TR 3 spectra of the ICT state show similar resonance enhancement patterns and spectra are dominated by phenyl ring related modes for all three molecules when using a 400 nm probe wavelength. Frequencies of the TRIR C≡N band and the main ps-K-TR 3 mode resemble their counterparts in the benzonitrile radical anion. Close analogy has also been found between the ICT state absorption spectra of the three molecules, being again similar to the absorption spectrum of the benzonitrile radical anion. The results are consistent with the electronically decoupled TICT model.