Analysis on high-resolution spectrum of the S₁-S₀ transition of free-base phthalocyanine

A high-resolution absorption spectrum of the S₁-S₀ transition of free-base phthalocyanine was observed and analyzed with improved reliability. The spectrum, with a partially resolved rotational structure, was obtained by using the buffer-gas cooling technique and a single-mode tunable laser. Our new analysis reveals that the S 1 ← S 0 0 0 0 band belongs to the a-type transition, where the electronic transition moment aligns parallel to the NH-HN direction, allowing the assignment of the S₁ state to ¹B_3u. These results agree with a prior study using supersonic expansion and are well supported by theoretical calculations. Interestingly, the rotational constant B in the S₁ state, which is often smaller than that in the ground state for typical molecules, was found to be slightly larger than that in the S₀¹A_g state. This suggests a change in the character of π bonds with the electronic excitation.