Meta-amino substituted naphthalimides exhibit large charge transfer and strong N-H vibrations enabling use as ratiometric fluorescent probe

Fluorescent probes based on intramolecular charge transfer (ICT) have obvious advantages for accurate quantitative analysis. To obtain high-performance ratiometric probes requires distinct photophysical properties during recognition reaction process, which is closely related to their ICT characteristics. 1,8-Naphthalimide is known as a typical fluorophore with desirable ICT property when functionalized with an electron-donating moiety at the para-position of the naphthalene chromophore. Although the photophysical properties of para-substituted 1,8-naphthalimide have been well studied, its meta-substituted counterpart has not been fully evaluated since the meta-position is conventionally thought to be weakly conjugated. Herein, combined experimental and theoretical studies are performed which consistently indicate that stronger charge transfer (CT) is exhibited by the meta-amino substituted 1,8-naphthalimide (m-NH₂) compared to the para-amino substituted one (p-NH₂). The ratiometric response of fluorescence with significant changes in wavelength and intensity upon acetylation (m-NAc and p-NAc) can be attributed to the larger ICT and stronger -NH₂ vibrations. This observation is further demonstrated by deuterium oxide experiments, viscosity experiments and quantum chemical calculations. The practical application of meta-amino-1,8-naphthalimide ICT-based probes is also confirmed. This research is expected to bring an in-depth understanding of π-conjugated systems with ICT characteristics, and facilitates the design of sensitive ICT fluorescent probes with meta-amino substitution.