Computational insights into the synergistic modulation of ΔEST, spin–orbit coupling, and reorganization energy in phthalimide-based TADF emitters

Abstract

Designing high-efficiency TADF emitters requires more than simply minimizing the singlet–triplet energy gap (ΔE_ST). It also demands careful control over spin–orbit coupling matrix elements (SOCME) and excited-state structural relaxation, which are often overlooked yet critically important. To uncover how these factors interplay, we have computationally investigated and engineered a series of phthalimide-based molecules by tuning three structural elements: the additional chromophore on the acceptor, incorporation of the phenyl spacer and the strength of the donor unit. Introduction of auxiliary chromophores and phenyl spacers (Group 1) enhances through-space charge transfer (TSCT), reducing ΔE_ST to <0.1 eV (0.004–0.074 eV) while maintaining moderate SOCME (0.20–0.50 cm⁻¹). However, these molecules exhibit high reorganization energies (λ = 1.3–4.5 eV), which severely suppress reverse intersystem crossing (rISC). In contrast, replacing carbazole with stronger donors (Group 3) further strengthens TSCT (>80%), rigidifies the molecular framework, and dramatically lowers λ (0.01–0.73 eV), while preserving very small ΔE_ST (0.001–0.085 eV) and finite SOCME. Consequently, Group 3 molecules achieve markedly enhanced rISC rates, reaching 7.2 × 10⁵ s⁻¹. Among all candidates, compounds with acridine and phenoxazine donors exhibit the most favorable balance of ΔE_ST, SOC, TSCT, and λ, identifying them as the most promising TADF emitters. Overall, this study establishes reorganization energy as a decisive design parameter, alongside ΔE_ST and SOC, for the rational development of high-performance phthalimide-based TADF materials.