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 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.
Brückner
Christian
C
Department of Chemistry, University of Connecticut, Storrs, 06269-3060, USA. c.bruckner@uconn.edu
Foss
Paul C D
PC
Sullivan
James O
JO
Pelto
Ryan
R
Zeller
Matthias
M
Birge
Robert R
RR
Crundwell
Guy
G
D003198:Computer Simulation; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008958:Models, Molecular; D011166:Porphyrins / Q000032:analysis* / Q000737:chemistry*
D011166:Porphyrins; C409084:tetraphenylporphyrin