Unconventional Fluorescence Quenching in Naphthalimide-Capped CdSe/ZnS Nanoparticles

Core-shell (CS) CdSe/ZnS quantum dots (QD) capped with ligands that possess a mercapto or an amino group and a naphthalimide (NI) as chromophore unit, linked by a short ethylene chain (CS@S-NI and CS@H2N-NI, respectively), have been synthesized and fully characterized by infrared and nuclear magnetic resonance spectroscopies, high-resolution transmission electron microscopy, and voltammetry as well as by steady-state absorption and emission spectroscopies. The organic ligands HS-NI and H2N-NI act as bidentate ligands, thereby causing a drastic decrease in the QD emission. This was particularly evident in the case of CS@S-NI. This behavior has been compared with that of commercially available QDs with octadecylamine as the surface ligand and a QD capped with decanethiol ligands (CS@S-D). The interaction between the anchor groups and the QD surface brings about different consequences for the radiative and nonradiative kinetics, depending on the nature of the anchor group. Our results suggest that the naphthalimide group "stabilizes" empty deep trap states due to the carbonyl group capacity to act as both a sigma-donor and a pi-acceptor toward cations. In addition, the thiolate group can induce the location of electron density at shallow trap states close to the conduction band edge due to the alteration of the QD surface provoked by the thiolate binding.