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

Automated summary

The excited-state dynamics of trans-4,4'-azopyridine in ethanol were investigated using femtosecond transient absorption. Vibrational coherence was observed in both S1 n pi* and S2 pi pi* electronic states. The relaxation pathway showed nonrotational behavior, with population being trapped in planar and shallow minima before decaying into the ground state. Nonrotational relaxation and the formation of hot ground-state species were observed upon excitation of the S2 state. The cis-isomer quantum yield increased with pi pi* excitation, in contrast to trans-azobenzene.