Jahn-Teller instability of the first excited singlet state of zinc porphin: a study by Zeeman spectroscopy in somen-alkane host crystals at 4·2 K

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Abstract
Experiments in a strong magnetic field (7·5 T) have been performed on the S 1 ←S 0 absorption spectrum of zinc porphin (ZnP) at 4·2 K in n-alkane hosts ranging from n-heptane to n-decane, in order to determine the orbital angular momentum Λ, the frequency v, and the distortion parameter α of the active mode of the Jahn-Teller unstable state S 1. Although the crystal-field splitting δ varies strongly for the sites considered (δ = 5–109 cm-1), nearly-constant values for the other three parameters are obtained: Λ = 3.9–4.2, α = 1·1–1·3, and v = 171–180 cm-1. The results prove the validity of the model previously proposed for the Jahn-Teller instability of a metalloporphyrin in a crystal field. A low-frequency mode of 17 cm-1 is observed in the S 1 and S 0 states of ZnP in n-heptane. In the Zeeman spectra it leads to a level anticrossing, and in fluorescence and absorption it shows up in hot bands.