Abstract
The electronic relaxation and isomerization mechanism of trans-azobenzene after the S2(ππ*) ← S0 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 S2(ππ*) ← S0 and S1(nπ*) ← S0 absorption bands, so that they were assigned to the fluorescence from the S2(ππ*) and S1(nπ*) states having “planar” structures. The lifetimes of the S2 and S1 states were determined as ∼110 fs (S2) and ∼500 fs (S1) by time-resolved measurements. The quantum yield of the S2 → S1 electronic relaxation was evaluated by comparing the intensity of the S2 and S1 fluorescence, and it was found to be almost unity. This implies that almost all molecules photoexcited to the S2(ππ*) state are relaxed to the “planar” S1(nπ*) state. The present fluorescence data clarified that the is...
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