Circularly Polarized Luminescence from an Inverted Singlet-Triplet Chiral Dye

Molecules with an inverted singlet-triplet gap (ΔE_ST = E_S1 - E_T1 < 0) hold potential for optoelectronic applications as OLEDs and photocatalysis. Despite growing interest, no single-molecule emission from a chiral dye with an inverted gap has been reported, and only one case has shown such emission from supramolecular aggregates. Here, we present the first circularly polarized light emission (CPL) from a chiral molecule exhibiting an inverted singlet-triplet gap. Our design is based on the achiral heptazine core, functionalized with chiral substituents. Computational screening confirmed that the inversion of the singlet-triplet gap is retained upon derivatization. Photophysical characterization, including temperature-dependent optical spectra and photoluminescence decay profiles, supported by computational results, confirms the inverted gap in both achiral and chiral derivatives. Chiroptical properties show dissymmetry factor g_lum values (≈ 10^-3) comparable to other chromophores functionalized with chiral groups. Combined with promising photoluminescence quantum yields, these findings highlight the potential of such materials for circularly polarized OLED devices.