Structural effects of the β-vinyl linker in pyridinium porphyrins: spectroscopic studies in organic solvents and AOT reverse micelles.

Two isomeric β-vinylpyridinium porphyrins, 2-[2-(2-methylpyridinium)vinyl]-5,10,15,20-tetraphenylporphyrin (1, ortho isomer) and 2-[2-(4-methylpyridinium)vinyl]-5,10,15,20-tetraphenylporphyrin (2, para isomer), which have shown different photodynamic behavior were investigated in organic solvents and sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. In organic systems, the absorption spectra present a red-shifted band that is more intense in the para isomer, in addition to the usual Soret band. This new band presents interesting solvatochromic effects which obey the multiparametric Kamlet-Taft equation. In AOT reverse micelles, the ortho isomer exhibits a strong dependence with the parameter ω0 = [H2O]/[AOT] which indicates that the molecule resides at the interface toward the organic phase. By contrast, no evidence was detected for the encapsulation of para isomer 2 in AOT reverse micelles. The hypothesis of two ground state isomers with different contributions of trans and quinoid structures is advanced on the basis of the overall data collected from electronic absorption, steady-state, and transient-state fluorescence emission. A charge transfer state in which an electron is fully transferred from the porphyrin to the pyridinium moiety is associated to a quinoid structure in isomer 2. The trans/quinoid relative proportions may be accounted for by the orientation of the ortho-/para-pyridinium isomers relatively to the porphyrin core.