Spectroscopic Study of the Dimerization of 5,10,15,20-Tetrakis(6- N -Methylquinolinyl)Porphyrin in Aqueous Solutions
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Journal of Applied Spectroscopy, Vol. 87, No. 5, November, 2020 (Russian Original Vol. 87, No. 5, September–October, 2020)
SPECTROSCOPIC STUDY OF THE DIMERIZATION OF 5,10,15,20-TETRAKIS(6-N-METHYLQUINOLINYL)PORPHYRIN IN AQUEOUS SOLUTIONS D. V. Klenitsky, I. V. Vershilovskaya, and M. M. Kruk*
UDC 535.37+539.19
Dimerization of 5,10,15,20-tetrakis(6-N-methylquinolinyl)porphyrin in aqueous solutions was studied in the range 274–343 K using absorption and fluorescence spectroscopy. The dimerization equilibrium constant KD was shown to vary over a wide range as the temperature changed. The enthalpy ΔH ‡ and entropy ΔS ‡ of dimerization in solutions of different ionic strengths were measured. The dimerization was found to be enthalpy driven in all cases. The proposed explanation for the large negative enthalpies of dimerization ΔH ‡ was the optimal overlap of porphyrin macrocycles due to minimization of Coulombic repulsion of ionized peripheral substituents located at large distances from the macrocycle. Keywords: porphyrin, dimerization, enthalpy, entropy. Introduction. Tetrapyrrole dimers have intrigued researchers for over half of a century, mainly because the chlorophyll dimer, the so-called special pair, plays a pivotal role in the plant photosynthetic apparatus by inducing the primary charge separation after photoexcitation [1]. Many in vitro and in vivo studies have focused on the electron-transfer mechanism and dynamics in the special pair and on the design of artificial systems capable of fulfilling this function [2, 3]. Tetrapyrrole dimers are also interesting because they are relatively easily formed by hydrophilic derivatives in aqueous solutions [4]. The solubility of synthetic hydrophilic tetrapyrroles is limited because of the hydrophobicity of the tetrapyrrole macrocycle itself despite the considerably increased solubility in aqueous solutions due to ionizable groups on the macrocycle periphery. Therefore, tetrapyrroles tend to aggregate in aqueous solutions by forming H-bonds and through van-der-Waals, hydrophobic, and π–π- and π–σ-interactions if their concentration is increased [5 and references therein]. Increasing the concentration of tetrapyrroles causes first the formation of dimers and then higher-order aggregates [5]. As a rule, tetrapyrrole dimers form misaligned stacks with 0.35–0.40 nm between the macrocycle planes. The misalignment can occur either along an axis passing through the meso-carbon atoms of the macrocycle or through the pyrrolenine ring N atoms depending on the nature of the peripheral substituents [5, 6]. The second arrangement is preferred with symmetric peripheral substitution because in this instance the pyrrole rings with an excess of electron density are situated over the macrocycle core where the electron density is reduced. Furthermore, the macrocycles can rotate relative to each other by 45o upon forming dimers. This minimizes steric hindrance for aryl substituents that are not coplanar with the macrocycle. The dimerization equilibrium constant KD was shown to vary over
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