Approximate invariance of average Q transition energy in visible spectra of porphyrins, chlorin tautomers, and metal complexes

High D4h symmetry of porphine dianion causes degeneracy of electronic energy levels in metal complexes. The Q states are split, if a pair of hydrogens is attached either to central nitrogens in a free base, or, alternatively, to the outer periphery of a pyrrole ring, the latter resulting in a chlorin (Chl). Average S1 and S2 transition energies in a porphyrin, the respective chlorin, and Zn complex of chlorin coincide (within 5–10% of the S1–S2 gap), approaching the degenerate value in a Mg or Zn porphyrin. This principle applies also to metastable tautomers of chlorin (Chl⁎), trans-octaethylchlorin, and meso-tetraphenylchlorin, accumulated in glassy host matrices at 8 K. Average S1 and S2 wave numbers of both tautomeric forms and the Zn complex remain constant within 200 cm−1. Such invariance helps to locate weak and poorly resolved transitions, such as S2 in Chl, or S1* and S2* origins in Chl*. The Chl* absorption maximum is assigned to vibronic S11* transition overlapping with S2*. Laser excitation in this region yields very broad, V-shaped spectral holes. Usual sharp holes are created in the S1* band of meso-tetraphenylchlorin phototautomer.