Protonation and aggregation of the A3B-type porphyrin in floating layers and thin films

Abstract

The formation features and spectral properties of Langmuir floating layers of 5-(4-hydroxyphenyl)-10,15,20-tris(4-hexadecyloxyphenyl) porphyrin on an aqueous subphase with different content of orthophosphoric acid (H₃PO₄) have been studied. The acid concentrations were 0, 10^–3, 10^–2, 10^–1, 1 M, which corresponded to subphase pH values of 5.3, 3.1, 2.3, 1.6, 1.1, respectively. At low acid concentrations (C_H3PO4 = 0 M and 10^–3 M), only one condensed phase with edge-on orientation of porphyrin molecules as formed. At high concentrations (C_H3PO4 = 10^–1 M and 1 M), two phases with face-on and edge-on molecular orientations were detected. The protonation of porphyrin at high acid concentrations (C_H3PO4 = 10^–1, 1 М) proceeds more efficiently, which is confirmed by high-intensity absorption peaks in the range of 683–700 nm and the presence of the shoulder of Soret band at 465 nm. The protonation leads to a stronger interaction of porphyrin macrocycle with the subphase, maintaining the face-on orientation not only in the gas phase, but also partially in the condensed phase. Spectral characteristics of thin films transferred onto solid substrates by the Langmuir-Schaefer method retain the features inherent to the previous floating layers. The signs of porphyrin protonation decrease at strong compression of floating layers, when they are in the condensed phase, or when LS-films undergo heat treatment at a temperature of 110 °C. Obtaining the protonated forms of porphyrins as an electron acceptor is a step towards the creation of donor-acceptor systems, effective in the development of photovoltaic devices based on organic macroheterocyclic compounds.