Acid–base properties of 3-[2-(pyridyl)benzoxazol-5-yl]alanine derivatives in the ground and excited state. Experimental and theoretical studies

•Acid–base properties of 3-[2-(pyridyl)benzoxazol-5-yl]alanine derivatives in the ground and excited state.•Equilibrium in the excited state is not established.•Two monocation for derivatives containing 4-pyridyne exist in the ground state contrary to that of 2-pyridyne derivative.•Contrary to the 2-(2-pyridyl)benzimidazole first protonation place is pyridyl nitrogen atom and no tautomerization in the excited state is detected.

Acid–base properties of 3-[2-(2-pyridyl)benzoxazol-5-yl]alanine and 3-[2-(4-pyridyl)benzoxazol-5-yl]alanine as well as a model compound without heterocyclic substituent (3-[2-(4-methylphenyl)benzoxazol-5-yl]alanine) were studied by means of absorption and fluorescence spectroscopy as well as theoretical calculations. It was found that basicity of heterocyclic derivatives in the ground and excited state depends on the position of the heteroatom in the substituent. Contrary to 2-(pyridyl)-benzimidazole, dual fluorescence and formation of dication were not detected for both compounds studied. Moreover, experimental as well as theoretical calculations revealed that pyridyl nitrogen atom is the first protonation center.

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