The objective of this study was to design and evaluate π-extended 1,8-naphthalimide derivatives as photoinduced electron transfer (PET) optical sensors for protons and metal cations, with emphasis on the role of heterocyclic annulation and receptor–chromophore electronic matching. Benzofuran- and benzodioxin-annulated naphthalimides bearing either a dimethylaminoethyl receptor or a non-donating alkyl substituent at the imide nitrogen were synthesized using tailored synthetic strategies. Their photophysical properties were investigated by absorption and fluorescence spectroscopy, while sensing performance was evaluated by fluorescence titrations. Quantum chemistry calculations were employed to rationalize experimental observations. Benzofuran-annulated derivatives exhibit structured π–π* absorption bands and strong fluorescence, whereas introduction of the receptor induces efficient fluorescence quenching via reductive PET. Protonation or metal ion coordination suppresses PET and leads to pronounced fluorescence enhancement, particularly in the presence of Cu(II) and Sn(II). In contrast, benzodioxin-annulated derivatives display intramolecular charge-transfer absorption bands, large Stokes shifts, and low fluorescence quantum yields in polar media, resulting in a negligible sensing response. Computational results attribute this behavior to an unfavorable energy arrangement of the donor–acceptor orbitals. Overall, the study demonstrates that heterocyclic annulation critically governs the electronic structure and sensing performance of naphthalimide fluorophores, providing guidelines for the rational design of PET-based optical sensors.

In this work, we comparatively assess the utility of electrostatic potential at nuclei (V_n) and several frequently used atomic partial charge schemes for describing hydrogen-bond basicity. This is accomplished with a total of 114 examples across four classes of molecules: ketones, ethers, nitriles, and substituted pyridine derivatives. Density functional theory calculations at the CPCM//PBE0-D3/def2-TZVPP level were employed to evaluate reactivity descriptors for the most basic site in each molecule. The effectiveness of the descriptors was assessed through their correlation with experimental basicity data. Three widely used partial charge models—Hirshfeld, CM5, and NPA—were considered. Strong correlations between both V_n and partial charges with basicity were observed across all four molecular classes. Notably, the electrostatic potential at the nucleus (V_n) consistently exhibits stronger correlations with the experimental pK_HB values than the partial charge models, highlighting its robustness as a quantitatively reliable descriptor of basicity across diverse organic systems. Both descriptors display inherent system dependence, and meaningful correlations with basicity are observed primarily within individual classes of compounds.