Femtosecond Time-Resolved UV−Visible Absorption Spectroscopy of trans -Azobenzene in Solution

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 S 2 (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 S 2 or S 1 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.