Pyrido[1,2-a]quinoxalines: synthesis, crystal structure determination and pH-dependent fluorescence

A series of novel, fluorescent pyrido[1,2-a]quinoxalines have been synthesized with crystal structures obtained for several analogues. These heterocycles exhibit characteristic pH-dependent UV-visible absorption and fluorescence properties that may be utilized for in-situ pH-sensing in biological experiments. The pKa values for this series of compounds, ranging from 4.5 to 7.5, can be systematically varied by manipulating the substitutions on the quinoxaline ring. These novel fluorescent pH indicators are readily available in one synthetic step from commercially available starting materials therefore being significantly more straightforward and less expensive to synthesize than the current standard fluorescent indicators.