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

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.