Femtosecond Time-Resolved UV−Visible Absorption Spectroscopy of <i>trans</i>-Azobenzene in Solution

Abstract

Femtosecond time-resolved UV−visible absorption spectroscopy has been used to study the UV photochemistry of trans-azobenzene (t-AB) in solution at 30 °C. Photolysis of t-AB at 303 nm results in transient absorption at 370−450 nm, the decay of which can be fitted by a sum of two exponential components. The shorter-lived component has a lifetime of 0.9 ± 0.2 ps in hexane, cyclohexane, and hexadecane and 1.2 ± 0.2 ps in acetonitrile; this is attributed to the S2(ππ*) excited state of t-AB. The longer-lived component has a lifetime which is similar to the recovery time of the ground-state absorption of t-AB at 303 nm, found to be 13 ± 1 ps in hexane, cyclohexane, and hexadecane and 16 ± 1 ps in acetonitrile. This longer-time-scale process is attributed to the internal conversion of an intermediate excited state, S†, into ground state t-AB, and this intermediate is tentatively assigned as a twisted conformer of excited t-AB on the S2 or S1 potential energy surface. The vibrational relaxation of hot t-AB molecules in the ground state, formed by internal conversion from S†, may also contribute to this longer-time-scale process.