Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins.

A Gorski,A Starukhin,T Roliński,M Kijak,J Waluk,V Knyukshto,T Lyubimova,A Semeikin,E Zenkevich

doi:10.1021/acs.jpca.0c06669

Abstract

Absorption and magnetic circular dichroism (MCD) spectra have been measured and theoretically simulated for a series of palladium octaethylporphyrins substituted at the meso positions with phenyl groups (n = 0–4). Analysis of the spectra included the perimeter model and time-dependent density functional theory (TDDFT) calculations. With the increasing number of phenyl substituents, the molecule is transformed from a positive hard (ΔHOMO > ΔLUMO) to a soft (ΔHOMO ≈ ΔLUMO) chromophore. This is manifested by a drastic decrease of the absorption intensity in the 0–0 region of the Q-band and by the strongly altered ratio of MCD intensities in the Q and Soret regions. Such behavior can be readily predicted using perimeter model, by analyzing frontier orbital shifts caused by various perturbations: alkyl and aryl substitution, insertion of a metal, and deviations from planarity. TDDFT calculations confirm the trends predicted by the perimeter model, but they fail in cases of less symmetrical derivatives to properly reproduce the MCD spectra in the Soret region. Our results confirm the power of the perimeter model in predicting absorption and MCD spectra of large organic molecules, porphyrins in particular. We also postulate, contrary to previous works, that the isolated porphyrin dianion is not a soft chromophore, but rather a strongly positive-hard one.

Highlights

Tetrapyrroles are ubiquitous in nature and function in a wide range of biological processes, including respiration, electron transfer, oxidation catalysis, and signaling in photosynthesis, among others.[1]
In addition to their basic macrocycle properties, i.e., coordination ability and the presence of specific functional groups, their conformation may have a significant relevance for the biological function.[2]. This is based on the hypothesis that fine-tuning of the macrocycle conformation by the protein scaffold is a way by which nature might control the physicochemical properties of the tetrapyrrole−protein complexes. It means that the structural organization of tetrapyrrole complexes in vivo, in which chromophore molecules are in nonplanar labile conformations, is a tool for subtle control of enzymatic and photocatalytic properties of natural porphyrins.[2−7] The variations observed in spectral-luminescent properties, as well as in redox characteristics of interacting subunits, can be caused by purely electronic effects in natural nanoassemblies and by changes in the spatial structure of the tetrapyrrole macrocycle itself.[8]
It has been well established that alkyl and aryl substituents can strongly affect absorption and, in particular, magnetic circular dichroism (MCD) spectra of porphyrins.[13,31−34] substituents of the same type can act in different directions, depending on whether they are located at the meso or β positions

Summary

Introduction

Tetrapyrroles (mostly heme and chlorophyll derivatives) are ubiquitous in nature and function in a wide range of biological processes, including respiration, electron transfer, oxidation catalysis, and signaling in photosynthesis, among others.[1]. 2,3,7,8,12,13,17,18-octaethylporphyrin (PdOEP) and Pdmeso-tetraphenyl-tetrabenzo[2,3]porphyrin with bulky side substituents are attractive for many applications, such as photocatalytic cells and photovoltaic devices.[9−11] It cannot be excluded that possible steric hindrance effects should be taken into account in order to optimize the functionality of such nanodevices or to improve photostability In this respect, we recently succeeded in showing the manifestation of steric interactions in PdOEP derivatives with a sequential increase of the number of meso-phenyl substituents (n = 1−4) based on spectral, kinetic, pump−probe, and phosphorescence measurements in the temperature range of 80−293 K (solutions and glassy rigid matrices), as well as on quantum-chemical calculations.[12] It was demonstrated that a sequential transition from a planar PdOEP molecule to the set of sterically hindered derivatives, that is, PdOEP → PdOEP1 → PdOEP2t →

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