The binding of pseudoisocyanine (PIC+) to the polyelectrolyte poly(methacrylic acid) (PMAA) has profound effects on the photophysical and photochemical properties of this prototypical cyanine dye. The hydrophobic dye was bound in the microdomain of the compact conformation of the polymer in its (uncharged, "hypercoiled") acid form at pH < 4.0 in water. Under these conditions, the fluorescence quantum yield for PIC+ was increased 600-fold and its lifetime is extended to 2.7 ns. The dye triplet state observed by flash photolysis provided a very long-lived phototransient (lambda(max) = 640 nm, 50-100-mu s decay time). Electron-transfer quenching was investigated using the oxidant tetranitromethane (TNM) which provided the semioxidized dye radical intermediate (440-nm transient) on cobinding within PMAA hypercoils. The dye was also bound to a covalently modified form of PMAA in which polymer chains were end-labeled with 9-methylanthracene moieties. Electron transfer between anthracene chromophores and PIC+ within the polymer domain was observed.