PHOTOISOMERIZATION OF A CAPPED AZOBENZENE IN SOLUTION PROBED BY ULTRAFAST TIME-RESOLVED ELECTRONIC ABSORPTION-SPECTROSCOPY

Ultrafast time-resolved electronic absorption spectroscopy has been us ed to study the photochemistry of transazobenzene and trans-1, a deriv ative in which azobenzene is capped by an azacrown ether, on UV excita tion to the S-2(pi pi) state. Excitation of trans-1 results in transi ent absorption which decays with a dominant component of lifetime ca. 2.6 ps and in bleaching of the ground-state UV absorption band which r ecovers on a similar time scale. In contrast, excitation of trans-azob enzene results in transient absorption which decays with a dominant co mponent with a shorter lifetime of ca. 1 ps, and in bleaching which re covers on a much longer time scale of ca. 18 ps. The recovery of the g round-state UV absorption band is not complete in either case, and the ultrafast data indicate that the quantum yield of trans-to-cis photoi somerization of 1 is approximately twice that of azobenzene. These obs ervations demonstrate that the restricted rotational freedom of the ph enyl groups in trans-1 has a significant effect on the excited-state d ynamics and decay mechanism. The differences in lifetime and quantum y ield of photoisomerization are attributed to rapid internal conversion from the S-2 to S-1 excited states of trans-1, which results in photo isomerization by an inversion mechanism in the S-1 state, whereas fast rotation in the S-2 State of trans-azobenzene populates a ''bottlenec k'' state which delays the recovery of the ground state and which redu ces the yield of photoisomerization; this ''bottleneck'' state is not accessible by trans-1. The results support the proposal that rotation is the dominant pathway for decay of the first-formed S-2 State of tra ns-azobenzene but that inversion is the dominant pathway for decay of the S-1 state.