Luminescence and structure of the protonated forms of meso‐tetraarylporphyrins in solution

Abstract

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.

Influence of Macrocycle Protonation on the Photophysical Properties of Porphyrins

“…[6][7][8][9] In this framework, essential progress has been achieved in the elucidation of the molecular conformation of diprotonated forms, [4][5][6][7][8][9][10][11][12][13][14][15] and the type and extent of macrocycle nonplanar distortions have been extensively discussed in terms of molecular flexibility, [7,9,15] the peripheral substitution pattern, [6,7,9,14,15] and the strength of intermolecular interactions with acid residues and other anionic species in solution. [14,16,17] The relationship between the molecular structure and macrocycle core acid-base equilibrium has been studied. [16,[18][19][20][21][22] Studies of the excited states properties of diprotonated porphyrins have revealed dramatic changes in the rates and channels of the excitation energy deactivation as compared with those for the free bases, and different mechanisms involved in these changes have been proposed.…”

“…[14,16,17] The relationship between the molecular structure and macrocycle core acid-base equilibrium has been studied. [16,[18][19][20][21][22] Studies of the excited states properties of diprotonated porphyrins have revealed dramatic changes in the rates and channels of the excitation energy deactivation as compared with those for the free bases, and different mechanisms involved in these changes have been proposed. [6,15,16,[23][24][25][26] Much less is known about the molecular conformation and optical properties of monoprotonated forms of porphyrins.…”

“…[16,21] Confident formation of monoprotonated forms of β-alkyl-substituted porphyrins was reported and their electronic and IR absorption spectra have been measured and analyzed, [27][28][29] while the monoprotonated forms of meso-aryl-substituted derivatives have not been described for a long time. In several cases, [16][17][18][19][20][21] the spectral features of the monoprotonated forms of mesoarylsubstituted compounds have been derived from global analysis of the electronic absorption and fluorescence spectra and time-resolved fluorescence measurements, taken during the titration experiments. Monoprotonated forms of mesoaryl-substituted porphyrins were also obtained by means of structural modification with hydrocarbon-capped moieties facing the macrocyclic plane, [31] and by complexation of the diprotonated form with bulky and poorly coordinating anions followed by dilution to decrease the solution acidity.…”

Spectral-Luminescent Properties of meso-Tetraarylporphyrins Revisited: The Role of Aryl Type, Substitution Pattern and Macrocycle Core Protonation

“…Fluorescence quantum yields were determined using the standard sample method, using free base 5,10,15,20-tetraphenylporphyrin as a standard (Φ 0 fl =0.09 in air equilibrated toluene solution). [30] The sample concentrations were of the order of 1•10 -6 M and were determined spectrophotometrically based on known extinction coefficients. All the samples were prepared by dissolving the porphyrin powder in dichloromethane, and the mono-and diprotonated forms were obtained by titration of the free base porphyrin solutions with perchloric acid (HClO 4 ).…”

“…The closed symbols indicate the corresponding data for the three species of Mes 4 measured in THF-TFA solutions. [28] The fluorescence quantum yields change due to the attachment of additional protons in the macrocyclic core, the formation of the saddle type macrocycle conformation, which is expected to show an enhanced rate of radiationless deactivation of the excited S 1 state, and the possibility of fluorescence quenching upon association of positively charged macrocyclic core and negatively charged acid residues. [10,34,36,37,53] in the case under consideration, the fluorescence quantum yields increase, indicating that quenching by the perchlorate counter ions is negligibly small (if any).…”