The lowest-energy exciton in some two-dimensional (2D) materials can be optically dark owing to spin, momentum, or spatial-symmetry selection rules, which suppresses its direct radiative recombination. Controlling the relative energy ordering of bright and dark states in the exciton spectrum is therefore important for engineering the direct optical response of 2D materials. In this work, using the α/β structural difference and Janus Si/Ge substitution as two theoretical symmetry-breaking design dimensions, we construct 2D α- and β-phase CSiN and their Janus derivative CSiGeN. Their electronic structures and excitonic properties are then systematically studied using the $\mathit{GW}$-BSE method within many-body perturbation theory. The results reveal that the lowest singlet exciton in α-CSiN is optically dark because of spatial-symmetry selection rules. The built-in electric field induced by the Janus configuration modifies the momentum-space distribution and relative energies of low-energy singlet excitons. The structural change from the α to the β phase alters the relative energy ordering of low-energy dark and bright singlet excitons, making an optically allowed state the lowest-energy state in the singlet exciton spectrum. The two mechanisms act synergistically in β-CSiGeN, making the lowest singlet exciton, dominated by transitions near the $M$ point, optically bright. This singlet exciton has an exciton binding energy of 1.03 eV and an intrinsic radiative lifetime of 0.50 ps. This study clarifies the independent and synergistic roles of the structural phase transition and the built-in electric field induced by the Janus configuration in controlling the energy-level ordering of bright and dark singlet excitons, providing a theoretical basis for tuning radiative transitions and luminescent properties in low-dimensional materials through the relative energy reordering of bright and dark excitons.

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• Electronic structure
• Excitons
• 2-dimensional systems
• Janus materials
• First-principles calculations
• GW method