New 9H-pyrido[2,3-b]Indoles, Synthesis, Photophysical/Theoretical Studies, and Application as the Fluorescence Probes

Using the “1,2,4-triazine” methodology, we have designed a series of new 9H-pyrido[2,3-b]indole-based fluorophores/probes 4a-e, 7a,b, and 8a,b. Photophysical studies of the push-pull fluorophores 8a and 8b revealed a positive solvatochromic effect with Δμ > 15D and Stokes shifts of up to 270 nm. Experimental data and theoretical calculations of the ground and excited states of the compounds demonstrated that pyridoindoles 4a-e and 7a,b exhibit a planarized intramolecular charge-transfer (PLICT) state, which is accompanied by a gradual increase in luminescence quantum yield for samples 4d and 7b with increasing solvent polarity. Probe 4b, characterized by an acid dissociation constant (pKa) of 5.5, showed high sensitivity to local pH changes in the microenvironment. A “turn-off” acidochromic response was observed for 4d and 8a with Stern–Volmer quenching constants of 1.6 × 103 and 5.5 × 103 M−1, respectively, while a “turn-off-on” response was detected for 4b and 8b during titration with TFA. Fluorescence and UV–vis absorption spectroscopy studies of the interaction between probe 4b and calf thymus DNA (ctDNA) confirmed a static quenching mechanism with the formation of 4b-ctDNA complex, indicating that the probe binds to the DNA helix via a groove binding mode. These findings highlight the potential of this pyridoindoles for visual detection and cellular imaging applications.