Excited-State-Selective Ultrafast Relaxation Dynamics and Photoisomerization of trans -4,4'-Azopyridine.

Excited-state dynamics of trans -4,4'-azopyridine in ethanol is studied using femtosecond transient absorption with 30 fs temporal resolution. Exciting the system at three different wavelengths, 460 and 290 (275) nm, to access the S 1 nπ * and S 2 ππ * electronic states, respectively, reveals a 195 cm -1 vibrational coherence, which suggests that the same mode is active in both nπ * and ππ * relaxation channels. Following S 1 -excitation, relaxation proceeds via a nonrotational pathway, where a fraction of the nπ * population is trapped in a planar minimum (lifetime, 2.1 ps), while the remaining population travels further to a second shallow minimum (lifetime, 300 fs) prior to decay into the ground state. Population of the S 2 state leads to 30 fs nonrotational relaxation with a concurrent buildup of nπ * population and nearly simultaneous formation of hot ground-state species. An increase in the cis -isomer quantum yield upon ππ * versus nπ * excitation is observed, which is opposite to trans -azobenzene.