Effect of poly(methacrylic acid) on photoinduced processes with cyanine dyes in aqueous solutions

The cyanine dyes thiamonomethinecyanine (I), thiacarbocyanine (II), and thiadicarbocyanine (III) in an aqueous poly(methacrylic acid) (PMA) solution form different molecular species: monomers anchored to PMA, dimers, and H aggregates, in proportions determined by the [dye]/[PMA] concentration ratio. Each molecular species is characterized by its own probabilities of radiative and nonradiative transitions: the absence of photoisomerization for I-III dimers and H aggregates, the absence of fluorescence for H aggregates of II and III, the capability of transition to a triplet state for PMA-anchored monomers I-III, and considerable enhancement of intersystem crossing to the triplet state for the dimers of II.