Femtosecond Time-Resolved Fluorescence Study of Photoisomerization of trans-Azobenzene

The electronic relaxation and isomerization mechanism of trans -azobenzene after the S 2 (ππ*) ← S 0 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 ∼390 nm (∼25 750 cm -1 ) and ∼665 nm (∼15 000 cm -1 ). These fluorescence bands showed good mirror images of the S 2 (ππ*) ← S 0 and S 1 (nπ*) ← S 0 absorption bands, so that they were assigned to the fluorescence from the S 2 (ππ*) and S 1 (nπ*) states having “planar” structures. The lifetimes of the S 2 and S 1 states were determined as ∼110 fs (S 2 ) and ∼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 (ππ*) state are relaxed to the “planar” S 1 (nπ*) state. The present fluorescence data clarified that the isomerization following S 2 (ππ*) photoexcitation takes place after the S 2 → planar S 1 electronic relaxation and that the rotational isomerization pathway starting directly from the S 2 (ππ*) 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.