Luminescence and structure of the protonated forms of meso-tetraarylporphyrins in solution.
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 S1 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 S1-->S0 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 S1-->T1 intersystem crossing becomes the main channel of deexcitation. The influence of heavy atoms (the Cl- and Br- 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- and Br- on fluorescence is realized according to the external heavy-atom effect mechanism. For nonaqueous o-TTP solutions with admixtures of HCl and CF3COOH the short-wavelength fluorescence from the S3 (B1) level has been detected whose unusual peculiarity is its enhancement in passing from 293 K to 77 K.
Knyukshto
V N
VN
Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, Minsk.
Solovyov
K N
KN
Egorova
G D
GD
D008163:Luminescent Measurements; D008652:Mesoporphyrins / Q000737:chemistry*; D008745:Methylation; D008956:Models, Chemical; D010777:Photochemistry; D012996:Solutions; D013050:Spectrometry, Fluorescence; D013054:Spectrophotometry, Atomic
D008652:Mesoporphyrins; D012996:Solutions