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 pi-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.

C40H30N4S4, triclinic, P1̅ (no. 2), a = 6.5126(7) Å, b = 10.499(1) Å, c = 12.635(1) Å, α = 87.818(2)°, β = 79.141(2)°, γ = 86.737(2)°, V = 846.7 Å3, Z = 1, Rgt(F) = 0.057, wRref(F2) = 0.150, T = 100 K.

Key indicatorsSingle-crystal X-ray study T = 100 K Mean '(C±C) = 0.002 A Ê R factor = 0.045 wR factor = 0.148 Data-to-parameter ratio = 16.5For details of how these key indicators were automatically derived from the article, see A view of the molecule of (I) (Farrugia, 1997). Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted.

As a minor by‐product from the reaction of copper(II) perchlorate, hydrazine, and 2,3‐dithien‐2′‐yl­quinoxaline, the title compound, C16H11N2S2+·ClO4−, was synthesized. Its crystal structure has been determined at 100 K. The two independent molecules in the asymmetric unit both display hydrogen bonding from a protonated quinoxaline N atom on the cation to a neighboring O atom on a perchlorate anion.

The title compound, C8H9NO3, was synthesized from 4‐nitro­phenol and ethyl iodide via the Williamson ether synthesis. The crystal structure has been determined at 100 K.