Picosecond infrared spectrum of 4-(pyrrol-1-yl)benzonitrile: Structure of the excited charge-transfer states of donor-acceptor systems

The picosecond transient infrared spectrum of the excited charge-transfer (CT) state of 4-(pyrrol-l-yl)benzonitrile (PBN) has been recorded in the frequency range of 1700-920 cm(-1). The CT state of PBN gives several quite strong infrared bands. The band at 1219 cm(-1) shifts toward the high-frequency side with increasing delay time between pump and probe pulses. This frequency shift is attributed to vibrational relaxation. Some of the transient bands (1429, similar to1290, 1219, and 964 cm(-1)) are very close in frequencies to those observed for the benzonitrile moiety of the excited CT state of 4-(dimethylamino)benzonitrile (DMABN). This finding implies that the benzonitrile groups of PBN and DMABN are decoupled from the electron-donating (pyrrole or dimethylamino) groups, and it can be explained by the twisted intramolecular charge-transfer model. To make this point clear, however, further discussion is necessary on the band assignments and on correlation between structure and spectral pattern.