Correlation of photophysical parameters with macrocycle distortion in porphyrins with graded degree of saddle distortion

Abstract

Different porphyrin conformations are believed to play a role in controlling the cofactor properties in natural tetrapyrrole-protein complexes. In order to study the correlation between macrocycle nonplanarity and physicochemical properties in detail, a series of six porphyrins with graded degree of macrocycle distortion was investigated. These conformationally designed porphyrins are based on the successive introduction of b-ethyl groups into the tetraphenylporphyrin parent macrocycle and the degree of nonplanarity is dependent on the number and localization of the b-ethyl meso-phenyl interactions. The electronic properties of the complete series of porphyrins were investigated in solution. It was found that the singlet and triplet properties depend not only on the out-of-plane distortion parameter but also on the type of central metal. Moreover, it was found that macrocycle distortion affects the singlet state properties significantly stronger than the triplet properties. In addition, the efficiency of energy transfer to molecular oxygen was investigated. It was shown that the singlet oxygen quantum yield depends strongly on the triplet state lifetime of the porphyrins, resulting in differences between the decrease of intersystem crossing and singlet oxygen quantum yield. The observed gradual change of electronic parameters of base free tetraphenylporphyrins with increasing deformation of the macrocycle indicates the validity of using conformationally designed porphyrins to fine-tune photophysical properties.

Conformational control of cofactors in nature—functional tetrapyrrole conformations in the photosynthetic reaction centers of purple bacteria

“…The relevant results from two such studies concerning the effect of a gradual sterically induced, ruffle distortion of nickel porphyrins on redox and absorption properties, [10] and that of a graded saddle distortion of free-base and zinc porphyrins on their photophysical behaviours [11] have been extracted and are illustrated below (Figure 12 Figure 13). These studies also demonstrate that the potential physicochemical effects of macrocycle distortion of the extent observed in the RC are significant in the context of the other protein modulated influences.…”

“…The relevance of this result to the BChl redox processes (macrocycle centred) is only apparent when one considers that reduction potentials are related to electron density; applying a pseudo-conservation principle coupled with the knowledge that the distortion of the wild-type H-NOX cofactor was confirmed to affect a measured decrease in electron density at the iron-centre, [12] renders the proposal acceptable that potentials of macrocycle and metal-centred redox process, when affected predominantly by conformational effects, are inversely proportional. As another example, the decreasing singlet oxygen yields affected by a gradually increasing saddle distortion [11] ( Figure 13) as a consequence of the distortions' effects on numerous photo-physical parameters [11] lends weight to the idea of a photo-protective role of the distortion of the BB cofactor in R. sphaeroides. .…”

“…Note that we have previously described such effects in small molecule bischlorin crystal structures. 11 In light of the fact that increasing nonplanarity of tetrapyrroles has been linked with easier oxidation and more difficult reduction 12 of the molecule, a point which has recently 145 been exploited for heme redox modification via protein induced distortion in vitro, 13 and that photosynthetic electron transfer is subject to the Marcus theory, the relative conformations of the BChl cofactors (above; in particular that the special-pairs are always the most distorted and BA is always relatively planar) 150 would seem to assist in charge-separation on the whole. The conformations may also contribute to the preference for L-branch electron transfer in both species.…”

Structural Dynamics and Relaxation Processes with Participation of Excited Singlet and Triplet States in Sterically Hindered Porphyrins and Their Chemical Dimers

“…[86][87][88] This second step has been also presented in literature by other groups in 2000-2010. [29,39,40,48,49] Finally, theoretical quantumchemical calculations have been used in order to explain the conformational lability of meso-phenyl-substituted octaarylporphyrins leading to the enhancement of the nonradiative deactivation channels of various nature. [44,48,89] This paper is dedicated to the 70 th anniversary of the worldknown expert in tetrapyrrole chemistry and photochemistry as well as of my friend Oscar Koifman.…”

“…[14,[19][20][21][26][27][28][29] Analysis of numerous X-ray structural data, [30][31][32][33] results of spectral-kinetic investigations, [34][35][36][37][38][39][40][41] and quantum-chemical calculations [42][43][44][45][46][47][48][49][50][51][52][53][54] indicate that simultaneous introduction of bulky peripheral substituents in β-and meso-positions of molecules of tetrapyrrole compounds leads to spatial distortions of the macrocycle, considerably changing its geometry and electronic structure as well as the dynamics of relaxation processes. To date, there are a lot of additional publications being devoted to the study of sterically hindered tetrapyrrole macrocycles by methods of electronic, [55][56][57][58][59][60][61][62][63][64][65] vibrational, [66][67]…”