Origin of the bathochromically shifted optical spectra of meso-tetrathien-2′- and 3′-ylporphyrins as compared to meso-tetraphenylporphyrin

The UV-Vis and fluorescence spectra of free base and diprotonated meso-tetrathien-2′-ylporphyrins are, when compared to the spectra of meso-tetra-phenyl- or even -thien-3′-ylporphyrins, characterized by surprisingly large red-shifts. A comparison of the optical spectra and the computed rotational barriers for these meso-aryl-substituted porphyrins and a detailed conformational analysis of the single crystal X-ray structure of a diprotonated meso-tetrathien-2′-ylporphyrin suggest that the origin of the altered electronic properties of meso-tetrathien-2′-ylporphyrins are mainly due to the contribution of conformations in which the thienyl groups adopt idealized co-planar arrangements with the porphyrin ring. These conformations allow an efficient extension of the porphyrinic π-system through conjugation. We synthesized a meso-tetrathien-2′-ylporphyrin with methyl groups in the o-position, thus preventing the formation of conformers with co-planar thienyl groups and a corresponding thien-2′-ylporphyrin with methyl substituents in a distal position that possesses the same steric requirements for thienyl group rotation as the parent compound, to conclusively deduce the influence of the conformers on the electronic structure. A MNDO-PSDCI computation of their optical spectra further supports our key hypothesis. DFT computations of the total energies of the hypothetical diprotonated thien-2′-ylporphyrin conformer with perpendicular thienyl groups and the conformer containing near-co-planar thienyl groups quantify the resonance stabilization energy. Our results support and complement recent photophysical and theoretical studies by Gupta and Ravikanth and Friedlein et al. on thien-2′-yl-substituted core-modified porphyrins and [meso-tetra(5′-bromothien-2′-yl)porphyrinato]Zn(II), respectively.