Charge Transfer Complexes, Based on 2,4-Diethyl-9-oxo-10-(4-heptyloxyphenyl)-9H-thioxanthenium Hexafluorophosphate and Thiazole Derivatives, as Photoinitiators of Holographic Free Radical Photopolymerization

A photoinitiating system has been developed based on a charge transfer complex (CTC) between a cationic sulfonium salt derivative synthesized from thioxanthene-9-one and heterocyclic nitrogen- and sulfur-containing donor compounds of thiazole derivatives. It has been found that the absorption bands of the formed CTCs lie in the blue region of the visible spectrum and the presence of a phenolic ring conjugated with the thiazole moiety leads to a hyperchromic effect in the absorption spectrum of the complexes. The molecular composition of 1 : 1 CTC was confirmed using the isomolar series method. The modified Benesi–Hildebrand equation was used to calculate the complexation constant (Kas (278 K) = 48.1 L/mol). the thermodynamic parameters enthalpy (ΔH = −11.5 kJ/mol), entropy (ΔS° = −9.3 J/(mol K)), and Gibbs energy (ΔG° = −8.95 kJ/mol) have been calculated by means of the van’t Hoff equation. According to the negative change in enthalpy, the CTC formation reaction is an exothermic process. The formed complexes have photosensitivity in the spectral region of the charge transfer band (400–500 nm), allowing their use as sensitizers of holographic photopolymer materials for laser hologram recording at λ = 457 nm with a high diffraction efficiency of ~75%.