Structural Effects of the β‑Vinyl Linker in Pyridinium Porphyrins: Spectroscopic Studies in Organic Solvents and AOT Reverse Micelles

Abstract

Two isomeric β-vinylpyridinium porphyrins, 2-[2-(2-methylpyridinium)­vinyl]-5,10,15,20-tetraphenylporphyrin (<b>1</b>, <i>ortho</i> isomer) and 2-[2-(4-methylpyridinium)­vinyl]-5,10,15,20-tetraphenylporphyrin (<b>2</b>, <i>para</i> 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 <i>para</i> 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 <i>ortho</i> 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 <i>para</i> isomer <b>2</b> in AOT reverse micelles. The hypothesis of two ground state isomers with different contributions of <i>trans</i> and <i>quinoid</i> 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 <i>quinoid</i> structure in isomer <b>2</b>. The <i>trans/quinoid</i> relative proportions may be accounted for by the orientation of the <i>ortho-</i>/<i>para-</i>pyridinium isomers relatively to the porphyrin core