Photophysics and photochemistry of squaraine dyes. 3. Excited-state properties and poly(4-vinylpyridine)-induced fluorescence enhancement of bis(2,4,6-trihydroxyphenyl)squaraine

Abstract

Bis(2,4,6-trihydroxyphenyl)squaraine (Sq) has been shown to exist in three distinct protonation equilibria (pK_a, = 3.5,7.0, and 9.5) in methanol/water solutions. The spectral properties of these different ionic species have been studied, and it has been observed that only the singly deprotonated Sq^- has a measurable fluorescence yield (Φ = 0.02 in methanol). Addition of poly(4-vinylpyridine) to methanolic solutions of Sq^- brings about a red shift in the absorption band, which is accompanied by a substantial enhancement in the fluorescence yield. These effects have been attributed to specific hydrogen bonding interactions of the singly deprotonated Sq^- with poly(4-vinylpyridine).