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

The molecular structural volume change (as determined by laser-induced optoacoustics), Delta V-R similar to-18 Angstrom(3), 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 Angstrom(3) for the dimer, to -3.5 Angstrom(3) for ZnTSPP2- and to -4 Angstrom(3) 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 (TMAPP(4+)) and 5,10,15,20-tetrakis-(4-methylpyridyl)-porphine, similar large contractions as for monomeric TSPP4- were determined upon tripler formation, that decreased also on lowering pH to similar to-5 Angstrom(3) for H2TSPP2- (pH 4) and to similar to-4 Angstrom(3) for H(2)TMAPP(6+) (pH 2.2). The tripler state quantum yield was not markedly affected by pH changes, oligomerization (mostly as a dimer), or complexation. The value of pK(a) = 5.1-5.2 derived from the titration of Delta V-R 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 TMAPP(4+) the pK(a) = 3.5 from titration of the contraction (triplet formation) coincided with that for the transition mono-/diprotonated species, whereas upon tripler decay the expansion showed a pK(a) = 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.