Internal Charge Transfer (ICT) Mechanism Of The Ratiometric1,8-naphtalimide Fluorescent Probes For Cu(Ⅱ)

1,8-naphtalimide is a ratiometric fluorescent probe for Cu(II) ion which is own InternalCharge Transfer (ICT) character, This kind of molecular probe has a strong electronic"push-pull" system. Fluorescent emission spectra of the probe in the presence of Cu(II) ionshow a red-shift or blue-shift, so it is usually used as a exclusively selective and sensitivefluorescent probe for Cu(II) ion. In order to explore the microcosmic mechanism of ICTprocess, Density functional theory (DFT) and time-dependent DFT (TDDFT) were taken outto study the photophysical properties of the probes.Geometric structures in the ground state of N-butyl-4,5-di[2-(phenylamino) ethylamino]-1,8-aphthalimide (probe B),4,5-di[N-(phenyl)-2-(amino)-acetamiamino]-1,8-naphthalimide(probe C) and the complex of Cu(II) ion have been optimized by DFT method. Natural bondorbital (NBO) analysis was used for characterization of the bonding features and electronicstructures. Based on the ground state geometries, the excite states of the probes werecalculated by TDDFT method, and based on the optimized excited state the fluorescentemission properties were investigated. The results revealed the reorganizing mechanism ofprobes on Cu(II) ion and the ICT mechanism of fluorescent emission.It was indicated that the probe B bonding with Cu(II) ion induced the dehydrogenation ofnaphthylamine, and formed C=N bond with adjacent naphthalene ring, extending theconjugated system and increasing the dipole moment, which favored the p electron of aminoN atom to transfer to the π*orbital of the fluorophor. It resulted in the absorption wavelengthand emission wavelength red-shift. In excite state, the hybridization state of naphthylamine Natom of probe C has changed. One of N atom form C=N bond with adjacent naphthalene ring,the p electron of amino N atom transfers to the π*orbital the fluorophor, forming an ICT state,resulting in the red shift of fluorescence. After bonding with Cu(II) ion, the rotation of C-Nbond connected to the naphthalene ring is restricted and the coplanar configuration in theexcited state cannot be formed. Therefore, the complex C-Cu(NO3)2showed no ICTfluorescent spectra but only the LE (locally excited state) emission, leading the wavelengthblue-shift compared to that of probe C. In addition, in excited state, the naphthylamine Natom transferred one electron to the empty d orbital of Cu(II) ion, inhibiting the fluorescentquenching caused by the paramagnetic effect of Cu(II) ion.