Ultrafast photoisomerization and its single-shot pump pulse efficiency of trans-azobenzene derivative: Compound for photosensitive DNA

The femtosecond photoisomerization processes of trans (T) 4-carboxy-2′,6′-dimethylazobenzen, which has been employed recently as an efficient photoregulator of DNA hybridization, were clarified by the rate equation analysis of measured transient absorbance changes with (350 nm) and without (380 nm) ground-state absorption of both the reactant (T) and photoproduct (cis: C) isomers under S2T-band excitation (360 nm, 150 fs pump): after excitation to the S2T state with a 450-fs lifetime, ~ 1.5% of the T-molecules in the S2T state are isomerized to the C-form within ~ 6 ps through the intermediate state (so called bottleneck state), but most of those return back to the T ground-state S2T via the internal conversion processes with an ultrafast kinetic rate of 2.2 × 1012 s− 1. Moreover, the rate equation analysis enables us to determine the T-to-C photoisomerization rate ηT,C per pump pulse to be 0.0011 at the pump energy of 80 nJ from the amplitude A3,350 of the offset component in the 350-nm probe signal, and to obtain the photoisomerization quantum yield ΦT,C = 0.094. The latter value is slightly lower than that of T-azobenzene, and well agrees with that (ΦT,C = 0.097) measured by the conventional CW irradiation method using a photostationary state.