Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

Abstract

Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the photochemistry of trans-azobenzene in n-hexane. Excitation to the S₁(nπ*) state results in transient absorption bands at ca. 400 nm (strong) and 550 nm (weaker) which decay with a lifetime 2.5± 0.2 ps on excitation at 503 nm, close to the S₁ origin, and with an additional fast component of ca. 0.6 ps on excitation at 390 or 420 nm, both well above the S₁ origin. Excitation to the S₂(ππ*) state results in transient absorption at 400 nm which decays with a dominant component of ca. 0.9 ps and a weaker component of ca. 15 ps; this 400 nm band itself is observed to rise synchronously as a transient band at 475 nm decays with a lifetime of <200 fs. These results are discussed in terms of the dual mechanism proposed for azobenzene photoisomerization.