Structural volume changes upon photoexcitation of porphyrins: Role of the nitrogen - Water interactions

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.