Development of squaraine dyes for photodynamic therapeutical applications: synthesis and study of electronic factors in the dye formation reaction

Abstract

With a view to developing sensitizers for applications in photodynamic therapy (PDT), a series of quinaldine based squaraine dyes have been synthesized and the role of electronic factors in the dye formation have been investigated through absorption spectroscopy, product analysis and theoretical calculations. It has been observed that semisquaraine intermediates were formed as the sole products in the reaction of quinaldinium salts containing electron-donating substituents with squaric acid. However, with salts possessing electronegative and electron-withdrawing groups, the squaraine dyes were isolated in quantitative yields. The semisquaraine intermediates that have been isolated as the butyl adducts, could be converted to the squaraine dyes by reacting with the quinaldinium salts having electronegative and /or electron-withdrawing groups. These dyes exhibit absorption in the range 700-800 nm with significant molar extinction coefficients (e = 1-3×10⁵ M^-1 cm^-1) and hence can have potential PDT applications.