Structural
Effects of the β‑Vinyl Linker
in Pyridinium Porphyrins: Spectroscopic Studies in Organic Solvents
and AOT Reverse Micelles
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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 ω<sub>0</sub> = [H<sub>2</sub>O]/[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