Journal of Photochemistry and Photobiology A: Chemistry
Photophysical, electro- and spectroelectro-chemical properties of the nonplanar porphyrin [ZnOEP(Py)44+,4Cl−] in aqueous media
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−].
Section snippets
Materials
ZnOEP (Zn-2,3,7,8,12,13,17,18-octaethyl porphyrin), [ZnTMPyP4+,4Cl−] (zinc 5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin chloride) and pyridine compounds were of reagent grade quality, purchased from Sigma Aldrich and used without further purification.
The zinc 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(N-pyridiniumyl)porphyrin chloride, [ZnOEP(Py)44+,4Cl−], was synthesized following previous reports [35].
H2SO4 solutions, solid Na2SO4 and NaOH were commercial products from Prolabo.
Pure
Photophysical properties
The photophysical data of [ZnOEP(Py)44+,4Cl−] and some reference porphyrins are gathered in Table 1.
The ground state absorption spectrum of [ZnOEP(Py)44+,4Cl−] in water, illustrated in Fig. 2, resembles that of the parent porphyrin [(Py)ZnOEP(Py)44+,4PF6−] in acetonitrile [34]. The Soret and Q bands are red-shifted and characterized by a reduced peak intensity and a significant broadening in comparison to the spectrum of the precursor ZnOEP [37] (Table 1). Such a behavior is partially due to
Summary
Regarding the photophysical properties, ZnOEP(Py)44+ must be classified as a distorted porphyrin. Its UV–vis absorption spectrum displays red-shifted and broader Soret and Q bands with reduced extinction coefficients compared to planar analogues. The properties of its S1 state (short lifetime of 170 ps and low fluorescence quantum yield of 0.0025) are comparable with those reported for saddle-shaped porphyrins. Indeed, internal conversion and intersystem crossing to the triplet state T1 are
Acknowledgments
This work was supported by CNRS, Université Paris-Sud 11 (Orsay, France) and Université de Strasbourg (Strasbourg, France). The ANR agency is acknowledged for providing financial support, (Project no. JC05_52437, NCPPOM, N.K. post-doctoral grant). The authors would also like to thank M. Erard for the time-resolved fluorescence measurements.
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