Femtosecond time-resolved fluorescence study of photoisomerization of trans-azobenzene

The electronic relaxation and isomerization mechanism of trans-azobenzene after the S-2(pi pi*) <-- SO photoexcitation were investigated in solution by steady-state and femtosecond time-resolved fluorescence spectroscopy. In the steady-state fluorescence spectrum, two bands were observed with their peaks at similar to 390 nm (similar to 25 750 cm(-1)) and similar to 665 nm (similar to 15 000 cm-1,). These fluorescence bands showed good mirror images of the S-2(pi pi*) <-- S-0 and S-1(n pi*) - S-0 absorption bands, so that they were assigned to the fluorescence from the S-2(pi pi*) and S-1(n pi*) states having planar structures. The lifetimes of the S-2 and S-1 states were determined as similar to 110 fs (S-2) and similar to 500 fs (S-1) by time-resolved measurements. The quantum yield of the S-2 --> S-1 electronic relaxation was evaluated by comparing the intensity of the S-2 and S-1 fluorescence, and it was found to be almost unity. This implies that almost all molecules photoexcited to the S-2(pi pi*) state are relaxed to the planar S-1(n pi*) state. The present fluorescence data clarified that the isomerization following S-2(pi pi*) photoexcitation takes place after the S-2 --> planar S-1 electronic relaxation and that the rotational isomerization pathway starting directly from the S-2(pi pi*) state does not exist. It was thus indicated that the isomerization mechanism of azobenzene is the inversion isomerization occurring in the S-1 state, regardless of difference in initial photoexcitation. The relaxation pathways in the S-1 state were also discussed on the basis of spectroscopic and photochemical data.