Solubilization of Charged Porphyrins in Interpolyelectrolyte Complexes: A Computer Study.

Karel Šindelka,Zuzana Limpouchová,Karel Procházka

doi:10.3390/polym13040502

Abstract

Using coarse-grained dissipative particle dynamics (DPD) with explicit electrostatics, we performed (i) an extensive series of simulations of the electrostatic co-assembly of asymmetric oppositely charged copolymers composed of one (either positively or negatively charged) polyelectrolyte (PE) block A and one water-soluble block B and (ii) studied the solubilization of positively charged porphyrin derivatives (P) in the interpolyelectrolyte complex (IPEC) cores of co-assembled nanoparticles. We studied the stoichiometric mixtures of 137 AB and 137 AB chains with moderately hydrophobic A blocks (DPD interaction parameter ) and hydrophilic B blocks () with 10 to 120 P added (). The P interactions with other components were set to match literature information on their limited solubility and aggregation behavior. The study shows that the moderately soluble P molecules easily solubilize in IPEC cores, where they partly replace PE and electrostatically crosslink PE blocks. As the large P rings are apt to aggregate, P molecules aggregate in IPEC cores. The aggregation, which starts at very low loadings, is promoted by increasing the number of P in the mixture. The positively charged copolymers repelled from the central part of IPEC core partially concentrate at the core-shell interface and partially escape into bulk solvent depending on the amount of P in the mixture and on their association number, . If is lower than the ensemble average , the copolymer chains released from IPEC preferentially concentrate at the core-shell interface, thus increasing , which approaches . If , they escape into the bulk solvent.

Highlights

Porphyrins, matalloporphyrins, and various porphyrin-based complex molecules belong to the most ubiquitous heterocyclic compounds found in nature
We studied the electrostatic co-assembly of oppositely charged block polyelectrolytes by coarse-grained simulations for almost ten years [49,51,52,54,71]
We investigated the solubilization of various amounts of charged porphyrins into interpolyelectrolyte complex (IPEC) cores in a system composed of 137 A1+0B25 and 137 A1−0B25 copolymer chains with aAS = 35

Summary

Introduction

Porphyrins, matalloporphyrins, and various porphyrin-based complex molecules belong to the most ubiquitous heterocyclic compounds found in nature. When exited below 400 nm, porphyrins exhibit S2 → S0 (relatively weak) emission in the region 400–550 nm and, violate the Kasha rule [17] Their S1 → S0 emission in the region 550–800 nm is strongly affected by the ground-state association due to π–π interaction of large rigid aromatic rings. The principles of formation of weakly bound dimers and higher aggregates, and their fluorescent properties were studied and explained by Kasha in 1965 [18]. He classified the aggregates according to the angle θ between the absorption transition dipole moment and the line connecting the gravity centers of monomers. Thorough experimental and theoretical studies of porphyrin, photophysics revealed that the highly symmetric D4h aromatic rings form four types of structurally different aggregates: two J- and two H-dimers, which form under different conditions and can be experimentally discerned on the basis of their photophysical properties [15,16,19,20]

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Conclusion