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).

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► 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.