Halogenated squaraine dyes as potential photochemotherapeutic agents. Synthesis and study of photophysical properties and quantum efficiencies of singlet oxygen generation

Abstract

We describe the synthesis and photophysical studies, including measurements of quantum yields of triplet excited states and singlet oxygen generation of bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (2) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (3). These dyes exist in solution in the protonated, neutral, single and double depro-tonated forms, depending on pH. The pK_a values of these dyes were found to be relatively lower than those of the parent bis(2,4,6-trihydroxyphenyl)squaraine (1). Only the single deprotonated forms (Sq) of 2 and 3 showed measurable fluorescence. In microheterogeneous media such as in the presence of β-cyclodextrin, cetyltrunethylammonium bromide and polyvinylpyrrolidone), bathochromic shifts in the absorption and emission spectra of Sq were observed with a substantial enhancement in their fluorescence yields. Triplet excited states are the main transient intermediates obtained upon 532 nm laser excitation of the various forms of 2 and 3 in methanol. These triplets have lifetimes in the range from 0.061 to 132 μs. The triplet quantum yields of double deprotonated forms are low (φ_T = <0.01), whereas the neutral and Sq^-forms of 2 (φ_r = 0.12 and 0.22) and 3 (φ_T= 0.24 and 0.5), respectively, exhibited significant triplet yields. Quantum yields of singlet oxygen generation by Sq^-forms of 2 and 3 were determined in methanol and were found to be 0.13 and 0.47, respectively, which are in good agreement with the triplet yields obtained in these systems.