New unsymmetrical squaraine dyes derived from imidazo[1,5-a]pyridine

The reaction of squaric acid with excess 3-methylimidazo[1,5-a]pyridine afforded almost exclusively the corresponding semisquaraine. The later underwent condensation with several heterocyclic compounds possessing active methyl groups, forming a novel class of unsymmetrical squaraines. The spectral properties of the obtained dyes were investigated and quantum-chemical calculations were performed to examine the dependence of the electronic structure and electron transitions on the molecular constitution of the dyes. The oxygen-generation ability of the new dyes was accessed by the 1,3-diphenylisobenzofuran (DPBF) quenching method, envisioning their potential use as sensitizers for photodynamic therapy (PDT). [Display omitted] ► A semisquaraine derived from 3-methylimidazo[1,5-a]pyridine nuclei was obtained. ► New unsymmetrical squaraine dyes containing the imidazo[1,5-a]pyridine ring were synthesized. ► Quantum-chemical calculations were performed for the dyes prepared. ► The photostability of the dyes was assessed by simple fading tests. ► All dyes possess inherent singlet oxygen generation ability.