Abstract— The values of the absolute quantum yields of fluorescence and of intersystem crossing have been obtained for porphin and its derivatives by methods based on relative measurements involving flash photolysis. For a given compound the sum of these quantum yields is unity within experimental error which shows that the energetics of the molecules investigated is determined by competition of only two processes, viz., fluorescence and intersystem crossing, while internal conversion plays a negligible role. The values of the corresponding transition probabilities have been determined with the help of phase‐fluorometric measurements. The dependence of the transition probabilities on molecular structure is discussed.

A comparative study of the spectral-luminescent properties (at 293 K and 77 K) of meso-tetrakis (o-tolyl)porphyrin (o-TTP) and meso-tetraphenylporphyrin (TPP ), and also meso-tetrakis(n-propyl)porphyrin (TPrP) in nonaqueous acid media has been carried out with the aim to reveal effects associated with the influence of methyl groups which hinder sterically the twist of the phenyl rings enhancing their conjugation with the macrocycle in acid medium. On the basis of the spectral data it is concluded that although the introduction of the methyl groups diminishes this twist of phenyls it does not eliminate it completely (the absorption spectra of protonated o-TTP are intermediate between those of TTP and TPrP). The fluorescence spectra of the protonated forms of o-TTP and TPP at 77 K are approximately mirror-symmetrical to the Q(0-0) and Q(0-1) absorption bands; on the elevation of the temperature up to 293 K the fluorescence and absorption bands broaden, the Stokes shift largely increases, and the mirror symmetry of the spectra disappears, which is indicative of changes in solute-solvent interactions in the S 1 state. The analysis of the obtained data on the energetics of photophysical processes leads to the conclusion that the significant role of radiationless deexcitation in the S 1 r S 0 channel, characteristic of the protonated form of TPP, is retained at 77 K. The introduction of the methyl groups at the ortho positions sharply decreases the corresponding rate constant, so that the S 1 r T 1 intersystem crossing becomes the main channel of deexcitation. The influence of heavy atoms (the Cl 0 and Br 0 anions) on the luminescence of the protonated forms has been investigated. It is shown that, analogous to the case of octaethylporphyrin investigated earlier, in nonaqueous media the protonated species of o-TTP form bis adducts of the dihydrochloride type. Contrary to this, the experiments with meso-tetrakis(p-sulfophenyl)porphyrin (TPPS) in aqueous solutions have shown that the TPPS dications do not add counterions, whereas the influence of Cl 0 and Br 0 on fluorescence is realized according to the external heavy-atom effect mechanism. For nonaqueous o-TTP solutions with admixtures of HCl and CF 3 COOH the short-wavelength fluorescence from the S 3 (B 1 ) level has been detected whose unusual peculiarity is its enhancement in passing from 293 K to 77 K.

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