Ultrafast Relaxation Dynamics of Luminescent Copper Nanoclusters (Cu7L3) and Efficient Electron Transfer to Functionalized Reduced Graphene Oxide

Luminescent copper nanoclusters (Cu NCs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Here, we demonstrate the synthesis of L-cysteine-capped Cu NCs in aqueous medium having a bright cyan emission (489 nm) with a quantum yield of 6.2%. The structure of the Cu NCs (Cu7L3) is investigated by using density functional theory (DFT) calculation and mass spectrometric study. Further, time-dependent density functional theory (TD-DFT) calculations provide the insights of electronic transitions, and it is correlated with experimental data. With near-HOMO-LUMO gap excitation, Cu NCs are excited to the S-4 state and subsequently relaxed to the S-i state through an internal conversion process with a time scale in the ultrafast region (326.8 +/- 6.5 fs). Furthermore, the structural relaxation in S-i takes place at a picosecond time scale, and the radiative relaxation occurs from S-i to S-0. Finally, Cu NCs are attached with imidazole-functionalized partially reduced graphene oxide (ImRGO) via electrostatic attraction. A dramatic photoluminescence (PL) quenching and shortening of the decay time of the Cu cluster in the presence of ImRGO indicate the photoinduced electron transfer process, which is confirmed from ultrafast spectroscopic study. The photoinduced electron transfer in a Cu NC-ImRGO nanocomposite should pave the way for potential applications in light harvesting.