A charge transfer interface was created within a type II heterojunction between monolayer MoS 2 and Ru(DPPZ) 2 L-Pro. Based on a combination of UV-Vis absorption, PL, and cyclic voltammetry measurements, (see supplemental Figure and Figure ), the HOMO of Ru(DPPZ) 2 L-Pro lies above the MoS 2 valance band maximum (VBM) while the LUMO is higher than the conduction band minimum (CBM) of MoS 2 , as described in Figure1, facilitating the hole transfer from monolayer MoS 2 to the molecule upon photoexcitation. The efficient interfacial charge transfer between monolayer MoS 2 and Ru(DPPZ) 2 L-Pro was confirmed by the quenching of Ru-MoS 2 PL signals (Figure ). Optical microscope shows an image for a pristine MoS 2 flake containing a large area of monolayer MoS 2 and minor part for bilayer MoS 2 and bulk MoS 2 (Figure ). For pristine monolayer MoS 2 , the PL mapping (Figure ) at 690 nm presents a uniform signal intensity, indicating a highly homogeneous material. The PL spectrum of the pristine sample (Figure ) can be decomposed into three gaussian peaks: the radiative recombination of B-exciton, A-exciton and negative trion, at 2.00, 1.87, and 1.80 eV, respectively. Exciton A and B are formed by strong spin orbital coupling (SOC) that leads to a split in highest valence band of ~160meV. The trion, with an larger coulomb binding energy, is identified at a lower energy, in good agreement with the literatures . Upon functionalizing the monolayer MoS 2 by Ru(DPPZ) 2 L-Pro, the PL emission was quenched by 95% uniformly across the entire monolayer MoS2 surface, as shown in Figure and. All three of the spectra components are reduced in Figure , suggesting all three quasiparticles are subject to interfacial charge transfer. We note that the signal reduction is unlikely to be attributed to the absorbance of the excitation lights by the surface adsorbed Ru(DPPZ) 2 L-Pro layer, as evidenced by the absence of absorption within the excitation wavelength range from a pure Ru(DPPZ) 2 L-Pro surface layer (see supplemental Figure ). Thus, an interfacial charge transfer occurs at the molecule-TMD interface.