Ultrafast Charge Transfer and Long-Lived Charge Separation in Covalently Linked MoS2-Pyrene-Phenothiazine Heterojunction Monolayers.

Covalent functionalization of two-dimensional (2D) transition-metal dichalcogenides is emerging as a powerful route to tailor interfacial photophysics for next-generation energy conversion schemes. Here we present monolayer molybdenum disulfide (MoS2) covalently linked to pyrene (Py) and phenothiazine (PTz) units through phenylene (ph) and xylene (xy) bridges, affording the molecularly well-defined triads MoS2-Py-ph-PTz and MoS2-Py-xy-PTz. Ultrafast transient absorption spectroscopy, complemented by time-resolved electron paramagnetic resonance measurements, reveals that photoexcitation initiates interfacial charge-transfer (CT) excited states that subsequently dissociate to yield a long-lived MoS2•-/PTz•+ charge-separated pair. Direct comparison of the phenylene and xylene bridge architectures shows that the rigidity and conformational freedom of the linker exert decisive control over electronic coupling and the kinetics of both CT and charge dissociation. These results establish a rare platform in which donor-acceptor distance, coupling strength, and CT dynamics at a 2D semiconductor interface can be tuned with molecular precision. Beyond elucidating the governing principles of interfacial CT in 2D-organic hybrids, this work demonstrates that covalent MoS2-donor junctions can sustain long-lived charge separation reminiscent of processes central to organic photovoltaics. The modular design presented here opens clear avenues for constructing function-tailored 2D semiconductor architectures capable of directional charge flow, photoredox activity, and energy transduction, offering foundational insight for artificial photosynthesis, solar-fuel generation, and hybrid optoelectronic devices.