Naphthalimide and naphthalic anhydride fluorescent probes for water detection: Molecular structure and photophysics

Herein, a comprehensive investigation combining photophysics and theoretical analysis of molecular structures of four naphthalimide and naphthalic anhydride derivatives has been reported. These compounds present a strong emission band in the green range, which depends on the solvent polarity. Upon the water addition, a pronounced decrease in the green emission intensity was observed accompanied by a red shift of the emission band as the water level increased. The sensitivity to water presence for samples under study in the range of 0–40 % water in DMF varied between 45 and 60 % relating to the decrease of the green emission intensity. Also, the limit of detection for water was estimated based on the emission intensity changes. The photophysical behavior was analyzed in different organic solvent/water mixtures. The optimized molecular geometries, electronic structures, polarizabilities, atomic charges, electrostatic potentials, and frontier molecular orbitals were explored using density functional theory (DFT). Theoretical UV–Vis spectra of naphthalimide and naphthalic anhydride compounds were computed using time-dependent density functional theory (TD-DFT). Computational results revealed that the vertical electronic transition from the ground state to the first excited state (S₀➔S₁) was of π-π* type transition assigned to the HOMO➔LUMO orbital configuration.