PNNL

Abstract

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 S1 np* and S2 pp* electronic states, respectively, reveals a 195 cm-1 vibrational coherence, that suggests that the same mode is active in both np* and pp* relaxation channels. Following S1-excitation, relaxation proceeds via a nonrotational pathway, where a fraction of the np* 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 S2 state leads to 30-fs non-rotational relaxation with a concurrent buildup of np* population and nearly simultaneous formation of hot ground state species. An increase in the cis-isomer quantum yield upon pp* vs. np* excitation is observed, which is opposite to trans-azobenzene.