Flat-band breathing kagome lattices host strong Coulomb interactions that can fundamentally alter the nature of their low-energy degrees of freedom. Here we show, using first-principles Density Functional Theory-GW–Bethe-Salpeter theory, that single-layer ${\mathrm{Nb}}_3{\mathrm{Cl}}_8$ realizes a spin-triplet Frenkel exciton with exceptionally large binding energy, leading to a dark excitonic Mott insulating phase. The lowest dark exciton lies 0.14 eV below the quasiparticle gap, while bright excitons appear at 0.94 and 1.21 eV with binding energies of 2.05 and 1.77 eV. Because these excitons are localized and possess a strong intrinsic out-of-plane electric dipole, the system hosts two independent excitonic degrees of freedom: a spin-1 moment and an out-of-plane electric dipolar orientation. Their collective behavior is described by an effective Hamiltonian on the triangular lattice, which naturally supports antiferromagnetic and ferroelectric ordering tendencies. Our results thus establish a correlation-driven route to multiferroicity in flat-band frustrated lattices and identify single-layer ${\mathrm{Nb}}_3{\mathrm{Cl}}_8$ as a candidate multiferroic dark excitonic Mott insulator.

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• Kagome lattice
• Mott insulators
• Multiferroics