The lowest triplet state of Zn porphin

Phosphorescence microwave double resonance experiments are reported on Zn porphin at 1·2 K. In glassy solution very broad resonance transitions are observed. However, for Zn porphin in a crystalline n-octane matrix—a system known for its sharp optical spectra (Shpolskii effect)—three pairs of microwave transitions with widths of a few MHz are found, all of them corresponding to a decrease in phosphorescence intensity. By studying the behaviour of the signals for various methods of preparation of the sample and as a function of the optical bandwidth of excitation and detection, one pair of transitions could be assigned to monomeric solute molecules. The corresponding zero-field splittings are |X - Z| = 1355, |Y - Z| = 806 MHz. It was further established that by ‘pumping’ either of these transitions a third one can be detected at the difference frequency, so that the order of the levels must be X> Y>Z (or reverse). The results indicate that the molecule no longer possesses a four-fold axis in the excited st...