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

Ramaiah, Danaboyina ; Joy, Abraham ; Chandrasekhar, Nese ; Eldho, Nadukkudy V. ; Das, Suresh ; George, Manapurathu V. (1997) Halogenated squaraine dyes as potential photochemotherapeutic agents. Synthesis and study of photophysical properties and quantum efficiencies of singlet oxygen generation Photochemistry and Photobiology, 65 (5). pp. 783-790. ISSN 0031-8655

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Official URL: http://onlinelibrary.wiley.com/doi/10.1111/j.1751-...

Related URL: http://dx.doi.org/10.1111/j.1751-1097.1997.tb01925.x

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 pKa 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 (φT = <0.01), whereas the neutral and Sq-forms of 2 (φr = 0.12 and 0.22) and 3 (φT= 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.

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