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 seru m albumin (BSA) and examined how self-association and the formation of host -guest complexes involving biopolymers and photosensitizers affect the comp etition between the photosensitization type I and type II mechanisms. Our r esults have clearly indicated that tri-para-substituted triarylmethane dyes bind efficiently to albumin as monomers and dimers and, interestingly, tha t the formation of dye aggregates in aqueous solutions is actually assisted by the protein. Protein-assisted dye aggregation takes place under conditi ons of high biopolymer loading (high [dye]/[protein] ratios), as attested b y the appearance of a hypsochromically shifted absorption band (H-band) tha t overlaps with the spectral shoulder of the respective dye monomer. As pre dicted by the molecular exciton theory, the intersystem crossing efficiency in H-type dimers is expected to be higher than in the respective dye monom eres, and photoinduced electron transfer events are intrinsically favored i n dye aggregates as a result of the physical contact between donor and acce ptor. We have found that when triarylmethanes are noncovalently bound to BS A their photoreactivity undergoes a remarkable enhancement, and that the ph otooxidation mechanism type I is particularly favored in the macromolecular environment. A comparative examination of the behavior of triarylmethane d yes with that of methylene blue have shown that in the case of methylene bl ue the binding phenomenon also favor the type I mechanism.