Photophysical, electro- and spectroelectro-chemical properties of the nonplanar porphyrin [ZnOEP(Py)₄⁴⁺,4Cl⁻] in aqueous media

Abstract

The photophysical and electrochemical properties of the tetracationic zinc 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(N-pyridiniumyl) porphyrin chloride (ZnOEP(Py)₄⁴⁺,4Cl⁻) were studied in aqueous solutions. The steady state and time-resolved absorption and emission measurements indicate that the porphyrin skeleton adopts a severely nonplanar conformation which minimizes steric crowding between the 12 peripheral substituents. The absorption spectrum of [ZnOEP(Py)₄⁴⁺,4Cl⁻] in water exhibits significant red shifts of the visible Q and Soret bands as well as considerable broadening and decrease in intensity of the latter compared to the spectrum recorded for the planar [ZnTMPyP⁴⁺,4Cl⁻] porphyrin. The S₂ → S₁ internal conversion is faster than the experimental resolution (<90 fs) while the S₁ excited state has a lifetime of 170 ps. The electrochemical properties of [ZnOEP(Py)₄⁴⁺,4Cl⁻] were investigated in water at pH 6.5 and 3.0 by cyclic and differential pulse voltammetry as well as spectroelectrochemistry. Reductions take place initially at the pyridinium sites with four successive one-electron steps at pH 6.5 or a one-electron step followed by a three-electron process at pH 3.0. Both oxidation and reduction processes undergone by the porphyrin are irreversible.

Introduction

Porphyrins and their derivatives constitute a major class of chemical compounds due to their various optical, physicochemical and redox properties. Naturally occurring porphyrins play essential roles in photosynthesis, cellular respiration and biological electron transfer reactions [1]. Consequently, much attention has been devoted to these compounds with respect to their potential biological and medical applications such as DNA cleavage catalysts [2], [3] or photosensitizers in photodynamic therapy [4], [5], [6], [7], [8], [9]. Interactions between porphyrin derivatives and DNA have also been the subject of several works [10], [11], [12], [13], [14], [15], [16]. But, only a few planar meso-substituted water soluble porphyrins, such as 5,10,15,20-tetra(N-methyl-4-pyridyl)porphyrins (TMPyP⁴⁺), have been investigated.

Studies of the crystal structures of protein complexes formed by porphyrinic chromophores and prosthetic groups have revealed skeletal distortions of the porphyrin macrocycle [17], [18], [19]. These findings have consequently stimulated efforts devoted to synthesis of nonplanar model porphyrins, for instance by substituting bulky groups at the peripheral positions of the macrocycle and/or changing the central metal [20], [21]. Indeed, it has been shown that the conformational distortion of the skeleton minimizes the steric interactions between its substituents. Most of the dodecasubstituted porphyrins which have been synthesized, exhibit nonplanar, highly distorted structures [22]. That was the case for the first reported dodecasubstituted tetracationic metalloporphyrins that were derived from β-octabromo-meso-tetra(N-methyl-4-pyridiniumyl)porphyrin [23]. Even if the origin of the changes is controversial [24], [25], [26], it is known that macrocyclic deformations can profoundly affect the optical, redox, magnetic, radical and excited-state properties of the porphyrins [27], [28], [29], [30], [31], [32], [33].

Substituted metalloporphyrins bearing multiple charges should be of particular interest because they would combine solubility in water and possible interactions with various biological targets such as proteins and nucleic acids. While there are numerous studies in non-aqueous media for conformationally perturbed porphyrins, similar studies in water are rather scarce. The dodecasubstituted and tetracationic zinc porphyrin [(Py)ZnOEP(Py)₄⁴⁺,4PF₆⁻] with four pyridinium groups bound at the meso position through their nitrogen atom and one axially ligated pyridine, was the first representative of a new class of nonplanar metalloporphyrins bearing four positive charges at a distance shorter than 5 Å from the metal centre [34]. The structure has been resolved in the solid state by crystallography and corresponds to a saddle conformation of the cationic macrocycle with the pyrrole rings (and their alkyl substituents) displaced up and down alternately [34]. By passing this porphyrin through a Cl⁻ exchange resin column, the counterion is exchanged, the axial pyridine is removed and the water soluble derivative, [ZnOEP(Py)₄⁴⁺,4Cl⁻] is obtained (Fig. 1) [35].

The present study deals with the photophysical and redox properties of this distorted porphyrin and their relations with the conformation of the macrocycle. In this context, the behavior of [ZnOEP(Py)₄⁴⁺,4Cl⁻] is compared to that of the planar tetracationic porphyrin [ZnTMPyP⁴⁺,4Cl⁻].