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 degrees 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(pi pi*) 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 stare, S-dagger, 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 hut t-AB molecules in the ground state, formed by internal conversion from S-dagger, may also contribute to this longer-time-scale process.