Development of Nitro-Substituted Pyrido[1,2‑a]indole Fluorophores for Environment-Sensitive Bioimaging

We report the design and synthesis of a novel pyrido[1,2-a]indole fluorophore that exhibits deep-red emission (up
to 660 nm), a large Stokes shift (6118 cm–1) in
methanol, and excellent biocompatibility. Positional nitration enables
systematic modulation of the photophysical properties across a series
of regioisomers. Spectroscopic analyses and calculations revealed
the intramolecular charge transfer (ICT) and rotation-induced quenching
behavior. The fluorophores showed exceptional lipid droplet (LD) labeling
capability in live-cell imaging without washing steps, indicating
their potential for biomedical applications. This study demonstrates
that simple nitro-group relocation on a compact heteroaromatic scaffold
provide tunable, environment-sensitive fluorophores for probe development.