Study of the Energy\nCrossing Between Excited States\nAffected by the Electronegativity of Substituents for Three 4‑Azido-1,8-naphthalimide\nDerivatives

Rapid detection of H<sub>2</sub>S is crucial for human\nphysiological\nhealth and natural ecosystems. In this study, the fluorescent sensing\nmechanisms of three 4-azido-1,8-naphthalimide-based fluorescent probes\nto monitor H<sub>2</sub>S were theoretically investigated by density\nfunctional theory and time-dependent density functional theory. The\npotential energy curve of the charge transfer (CT) state has a crossover\nwith that of the locally excited (LE) state proved by the constructed\nlinear interpolating internal coordinate pathway. Thus, the transform\ntakes place from the LE state to the CT state causing the fluorescence\nquenching of the probes from a nonradiative transition process of\nthe CT state. The distance between the Franck–Condon point\nand the minimal energy conical intersection becomes larger with the\nincrease of the electronegativity of substituents on the 1,8-naphthalimide\nfluorophore. In addition, the degree of charge separation is closely\nrelated to the energy difference between the CT and the LE states\nwhich are also essentially affected by the electronegativity of the\nsubstituents. Since the electronegativity of the substituents has\nproved important for the probes, our work lays a certain theoretical\nfoundation for the design and synthesis of more sensitive 4-azido-1,8-naphthalimide-based\nfluorescent probes.