Fused pyrazole-phenanthridine based dyads: synthesis, photo-physical and theoretical studies, and live cell pH imaging

The arrangement of small sized molecules with a scaffold structure plays an active role in the fields of sensors and health care. An efficient molecular design strategy for four pyrazole-phenanthridine based D–π–A luminophores, denoted as 2a, 2b, 2c and 2d was developed to investigate the effect of acid on the photo-physical properties of these dyes. Photo-physical studies of the synthesized probes showed distinct absorption and emission under various pH conditions. Theoretical calculations using density functional methods were carried out for understanding the mechanistic aspects of the proton induced fluorescence. The experimentally observed photo-physical properties correlated well with theoretical results. Moreover, probes 2 and 2a can be used to monitor the fluorescence changes in E. coli cells under different pH conditions.