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

A high-resolution absorption spectrum of the S-1-S-0 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 S-1 state to B-1(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 S-1 state, which is often smaller than that in the ground state for typical molecules, was found to be slightly larger than that in the S-0 (1)A(g) state. This suggests a change in the character of pi bonds with the electronic excitation.

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Yuki Miyamoto, Ayami Hiramoto, Kana Iwakuni, Susumu Kuma, Katsunari Enomoto, Naofumi Nakayama, Masaaki Baba; Analysis on high-resolution spectrum of the S1–S0 transition of free-base phthalocyanine. J. Chem. Phys. 14 April 2024; 160 (14): 144304. https://doi.org/10.1063/5.0191810 and may be found at https://doi.org/10.1063/5.0191810.

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