Slower processes of the ultrafast photo-isomerization of an azobenzene observed by IR spectroscopy

The photo-induced trans–cis isomerization of the azobenzene derivative 4-nitro-4'-(dimethylamino)azobenzene in polar solution was studied by femtosecond UV/Vis and IR spectroscopy. The UV/Vis experiment reveals two excited state processes; the slower one (1 ps) is the internal conversion to the ground state. The ensuing spectral changes point to vibrational cooling of the nascent cis product and the recovered trans isomer. Judging from the UV/Vis experiment this ~5 ps process seems to terminate the isomerization. The internal conversion and the cooling also find their manifestation in the IR experiment. In addition slower spectral changes lasting until ~50 ps are detected. These slow IR responses are compared with the behavior of a non-isomerizing analogue, para-N,N-dimethyl-nitroaniline. An origin for this discrepancy is suggested and potential molecular processes causing the slow IR response are discussed