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

Abstract

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.

Axial Imidazole Binding Strengths in Porphyrinoid Cobalt(III) Complexes as Studied by Tandem Mass Spectrometry

“…This parallels the findings for the loss of the first imidazole from the octahedral complexes. It is well established that thienyl groups play a much stronger role in the electronic structure of the porphyrin than the phenyl rings [36]. Our gas-phase findings, in fact, correlate well with the solution data.…”

“…It is possible that this particular distinction arises from the presence of multiple conformations of the thienyl substituents in the gas phase experiments that are not reflected in the computations. It has been shown that such issues affect the photophysics of free [11] and [12] [36].…”

“…The Co(II) complexes [1]Co through [12]Co were synthesized using classic metal ion insertions into the corresponding known free bases that were all prepared along literature procedures: The free base porphyrins 1 and 9 were made according to the method of Adler [9], as were the free base thienylporphyrins 10 through 12 [36]. Free base diolchlorins 4 and 8 were prepared by OsO 4 -mediated dihydroxylation of the corresponding porphyrins [37,40].…”

“…The optical properties of [1]Co and [9]Co are only very little different, highlighting the small interaction between the two essentially perpendicular π-systems on the meso-aryl rings and the porphyrinoid chromophore (all UV-vis spectra are provided as electronic supplementary material). The smaller meso-2-thienyl substituents (in [10]Co and [11]Co) can assume a much more co-planar conformation, enabling a much stronger interaction with the porphyrinic chromophore, while the sterically more demanding 3-thienyl substituent takes up an intermediate position between [10]Co and [1]Co [35,36]. Presumably the trend seen in the optical properties will also be reflected in the coordination properties of the central metal, a hypothesis as yet untested (Figure 2).…”

Introduction of carboxylic ester and acid functionalities to meso-tetrakis(pentafluorophenyl)porphyrin and their limited electronic effects on the chromophore

“…Alas, we would caution against translating this result to porphyrinoid pigments for which a large coplanarity of the meso-aryl substituents is known to take place, and for which a larger electronic influence of the aryl groups has to be assumed. [46,47] 4. Experimental…”