Unlocking Brightness in Inverted Singlet-Triplet Systems via J-Aggregate Formation

Inverted singlet-triplet (INVEST) emitters are emerging as attractive alternatives to conventional TADF systems, yet their practical use is hindered by poor radiative efficiency. A key limitation arises from the vanishing oscillator strength (f_osc) of the lowest excited singlet state (S₁), where the S₀ → S₁ transition is optically inactive due to strong charge-transfer character and negligible HOMO-LUMO overlap. Here, we investigate a series of cyclazine derivatives, including two newly designed chromophores, and demonstrate that dimerization provides an effective rule to overcome the intrinsic oscillator strength bottleneck. Electronic structure calculations at the double-hybrid functional-based LR-TDDFT and domain-based local pair natural orbital (DLPNO) similarity-transformed EOM-CCSD (STEOM-CCSD) level of theory reveal that J-aggregate dimers substantially enhance f_osc compared to H-aggregate dimers and isolated monomers. We have systematically computed the excited-state properties of the J-aggregate, H-aggregate, and monomer of the studied systems. It is found that the S₁ state of the J-aggregate dimer has a mixed locally excited state, which is absent in the case of the H-aggregate. This aggregation-induced enhancement of the oscillator strength establishes a general design principle for bright INVEST emitters, opening pathways toward efficient organic optoelectronic materials.