Macroscopic symmetry breaking governs many-body excitonic interactions and nonadiabatic carrier dynamics in the Janus ${\mathrm{OMoSiN}}_2$ monolayer. Many-body perturbation theory ($G_0{W}_0$–Bethe-Salpeter equation) and nonadiabatic molecular dynamics characterize this regime. A vacuum-level energy offset of approximately 2.5 eV reflects the strong built-in polarity and accompanies an orbital-specific quantum-confined Stark effect. The band-edge electron and hole centroids show an out-of-plane separation of Δ$z$ = 0.19 Å, and the static in-plane screening length is $r_0$ ≈ 34.8 Å. The direct-gap-referenced exciton binding energy reaches 2.24 eV and reflects the combined influence of reduced dimensionality and nonlocal screening, valley-dependent orbital character, and Janus-induced spatial asymmetry. For the relevant band-edge transitions, out-of-plane optical transitions are symmetry forbidden at the Γ and $K$ extrema. Away from these extrema, the out-of-plane response is finite but strongly suppressed, producing pronounced optical anisotropy. Time-domain carrier kinetics resolve a dynamic bottleneck. Polarity-driven spatial separation formally quenches the nonadiabatic coupling to 0.073 meV. This geometric blockade acts together with ultrafast quantum phase breaking. The resulting carrier lifetime is 409.5 ns. Highly polar two-dimensional architectures may therefore provide a platform for strongly bound excitons and decoherence-suppressed carrier dynamics.

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• Carrier dynamics
• Electronic structure
• Excitons
• 2-dimensional systems
• First-principles calculations
• GW method