Theoretical perspective for carbazole fusion effect on multiple resonance thermally activated delayed fluorescent properties

Abstract

Multiple resonance (MR) type thermally activated delayed fluorescence (TADF) molecules are attracting attention for their narrowband emission, high luminescence efficiency, and high exciton utilization rate. Designing and studying these molecules has become an important direction for developing organic electroluminescent devices. Despite the significant potential in practical applications, developing long-wavelength emitting MR-TADF molecules remains a major challenge. This paper employs quantum mechanics/molecular mechanics (QM/MM) methods and the thermal vibration correlation function (TVCF) theory to study the luminescent properties of two MR-TADF molecules, asymmetric donor BNIP-tBuCz and symmetric donor BNDIP, in doped thin films. The calculations reveal that both molecules, while maintaining MR characteristics, possess small HOMO and LUMO energy gaps, enabling long-wavelength emission. Compared to BNIP-tBuCz, BNDIP has a slightly higher degree of conjugation and a greater proportion of π-π∗ transitions. Additionally, the transition dipole moment density and transition dipole moment in BNDIP are significantly enhanced, increasing the radiative rate. Both BNIP-tBuCz and BNDIP maintain similar porosity in thin films; however, BNIP-tBuCz is more sensitive to the aggregated environment. Stronger intermolecular interactions in BNDIP effectively suppress out-of-plane methyl wagging vibrations in low-frequency modes and C=C stretching vibrations in high-frequency modes, reducing reorganization energy and inhibiting the loss of non-radiative energy in the excited state. Furthermore, BNDIP's smaller energy gap between T₁ and S₁ (ΔE_ST) and its reorganization energy facilitate a faster reverse intersystem crossing rate (k_RISC). Energy transfer characteristics are also studied using the semi-empirical Marcus theory. The calculated energy transfer rates of singlet and triplet states in BNIP-tBuCz are greater than those in BNDIP, due to the larger Gibbs free energy and exciton coupling in BNIP-tBuCz. The calculations reveal the relationship between the molecular structure and luminescent performance of these two novel MR-TADF materials, providing a theoretical basis for designing and developing highly efficient, long-wavelength MR-TADF devices.