Effect of self-association and protein binding on the photochemical reactivity of triarylmethanes. Implications of noncovalent interactions on the competition between photosensitization mechanisms type I and type II

We have explored the photochemical behavior of cationic triarylmethane dye monomers and dimers free in solution and noncovalently bound to bovine serum albumin (BSA) and examined how self-association and the formation of host-guest complexes involving biopolymers and photosensitizers affect the competition between the photosensitization type I and type II mechanisms. Our results have clearly indicated that tri-para-substituted triarylmethane dyes bind efficiently to albumin as monomers and dimers and, interestingly, that the formation of dye aggregates in aqueous solutions is actually assisted by the protein. Protein-assisted dye aggregation takes place under conditions of high biopolymer loading (high [dye]/[protein] ratios), as attested by the appearance of a hypsochromically shifted absorption band (H-band) that overlaps with the spectral shoulder of the respective dye monomer. As predicted by the molecular exciton theory, the intersystem crossing efficiency in H-type dimers is expected to be higher than in the respective dye monomeres, and photoinduced electron transfer events are intrinsically favored in dye aggregates as a result of the physical contact between donor and acceptor. We have found that when triarylmethanes are noncovalently bound to BSA their photoreactivity undergoes a remarkable enhancement, and that the photooxidation mechanism type I is particularly favored in the macromolecular environment. A comparative examination of the behavior of triarylmethane dyes with that of methylene blue have shown that in the case of methylene blue the binding phenomenon also favor the type I mechanism.