The molecular structural volume change (as determined by laser-induced optoacoustics), ΔVR −18 Å3, accompanying triplet state formation of free base 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphine (TSPP4-) in aqueous solutions of pH > 5.5, was markedly decreased to −5 Å3 for the dimer, to −3.5 Å3 for ZnTSPP2-, and to −4 Å3 for the nonionic micelle-included free base tetraphenyl porphyrin. For the free bases of other meso-substituted porphyrins with cationic side-groups such as 5,10,15,20-tetrakis-(4-methylaminophenyl)-porphine (TMAPP4+) and 5,10,15,20-tetrakis-(4-methylpyridyl)-porphine, similar large contractions as for monomeric TSPP4- were determined upon triplet formation, that decreased also on lowering pH to −5 Å3 for H2TSPP2- (pH 4) and to −4 Å3 for H2TMAPP6+ (pH 2.2). The triplet state quantum yield was not markedly affected by pH changes, oligomerization (mostly as a dimer), or complexation. The value of pKa = 5.1−5.2 derived from the titration of ΔVR for formation (and decay) of triplet TSPP4- is identical to that for the equilibrium of free base and monoprotonated forms, derived from fittings of the pH-dependent absorption and fluorescence data. For TMAPP4+ the pKa = 3.5 from titration of the contraction (triplet formation) coincided with that for the transition mono-/diprotonated species, whereas upon triplet decay the expansion showed a pKa = 4.0 similar to the value for monoprotonated/free-base equilibrium. The contraction upon triplet formation in the free base porphyrins mainly originates in the rearrangement of water around the excited macrocycle nitrogen atoms and to a minor extent in a contraction due to bonds shortening upon excitation. The contribution of the chromophore-solvent interactions is thus reduced upon impairment of hydrogen bridges between the nitrogen lone electron pairs and water by dimer formation, metal complexation, and protonation.
Structural volume changes upon photoexcitation of porphyrins: Role of the nitrogen - Water interactions / GENSCH T.; VIAPPIANI C.; BRASLAVSKY S.. - In: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. - ISSN 0002-7863. - 121(1999), pp. 10573-10582. [10.1021/ja9913885]
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