Photophysical Properties of a Red-Shift Cu(II) Ratiometric Fluorescent Chemosensor

Density functional theory (DFT) and time-dependent DFT (TDDFT) were used to study the photophysical properties of N-butyl-4,5-di(2-(phenylamino)ethylamino)-1,8-naphthalimide, a ratiometric fluorescent sensor for Cu(II). The geometric structures of the compounds at the ground state were optimized by DFT. Combined with natural bond orbital (NBO) analysis, the binding characteristics of the chemosensor molecule coordinated with a Cu(II) ion were identified. The excitation states of the compounds were investigated and the internal charge transfer (ICT) mechanism was elucidated by theoretical calculations. The results indicated that the coordinated Cu(II) ion induced the dehydrogenation of naphthylamine. The negatively-charged amino N atom then formed a C=N double bond with the naphthalene ring, extending the conjugation of the system. The nonbonding electron of N was transferred to the unoccupied d orbital of Cu(II), preventing fluorescence quenching by paramagnetic Cu(II). It was proposed that in the excited state, n→π* electron transfer from the amino N to the naphthalene ring led to internal charge transfer and resulted in the red shift of fluorescence.