In this work, we implemented transient electronic sum frequency generation (tr-ESFG) to investigate the ultrafast charge dynamics at a molecule-TMD heterojunction. Tr-ESFG is particularly suitable for this study because of its broad-spectrum coverage and surface sensitivity. As a second order nonlinear optics, ESFG only occurs for samples with non-centrosymmetric geometries, such as surfaces and interfaces. Thus, ESFG has been applied on studying interfacial molecule alignment and charge dynamics at surfaces/interfaces . Using this unique method, we report ultrafast dynamics of photo-induced charge transfer between the heterojunction of monolayer molybdenum disulfide (MoS 2 ) and ruthenium(ii) polypyridyl complexes (Ru(DPPZ) 2 L-Pro), referred as Ru-MoS 2 hereafter. We measure that, in contrast to TA, the ground state bleach (GSB) and stimulated emission (SE) of tr-ESFG are out of phase and thereby cancel. This phase cancellation removes the larger ground state signals, promoting the characterization of trion formation timescales. Furthermore, the spectral signal strongly indicates the formation of CT trions at heterojunction. The identification of a CT-trion in molecule-TMD heterojunctions offers the potential for transferring spin or valley degrees of freedom of TMD to molecules for molecular-based spintronics, spin-specific chemistry, and engineering of TMD properties through molecular dopants.