Symmetry-breaking strategy for enhanced oscillator strength in inverted singlet–triplet (INVEST) emitters for OLED applications

Inverted singlet-triplet (INVEST) emitters have emerged as promising candidates for next-generation organic light-emitting diodes (OLEDs) owing to their ability to harvest triplet excitons efficiently through fast reverse intersystem crossing (RISC) without the need for thermal activation. However, most INVEST emitters suffer from low oscillator strengths (f_osc) and limited emission efficiencies. Here, we explore a heptaazaphenalene (Hz) core as a platform to design high-performance INVEST emitters via a symmetry-breaking strategy employing conjugated donor-acceptor (D-A) substitution. This approach enables precise control of the singlet-triplet energy gap (ΔE_ST), simultaneously achieving inversion and enhancing f_osc. Eight D-A-substituted INVEST derivatives were computationally evaluated using time-dependent density functional theory (TDDFT), where spin-scaled long-range-corrected double-hybrid functionals reliably predicted inverted ΔE_ST values as low as -0.305 eV and f_osc values up to 0.175. The designed molecules also exhibit rapid RISC (k_RISC ≈ 10⁶ s^-1) and radiative fluorescence rates (k_r ≈ 10⁷ s^-1), supported by static electron-correlation analysis using finite-temperature DFT. Among these, HzDA₁ shows the highest figure of merit (FM), highlighting the effectiveness of symmetry breaking for boosting INVEST performance. Overall, this work demonstrates a rational molecular design strategy to overcome the intrinsic limitations of INVEST emitters, providing a pathway to efficient OLEDs with enhanced triplet harvesting.