A QM/MM study on through space charge transfer-based thermally activated delayed fluorescence molecules in the solid state

Through space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) molecules with sky-blue emission have drawn great attention in recent studies. Corresponding theoretical investigations to reveal the inner mechanisms are highly desired. Herein, the photophysical properties of a TSCT-based TADF molecule (S-CNDF-S-tCz) are theoretically studied both in solvent and solid environments by using the polarizable continuum model and the combined quantum mechanics and molecular mechanics method, respectively. Based on density functional theory and time-dependent density functional theory coupled with the thermal vibration correlation function method, the adiabatic singlet-triplet energy gap, natural transition orbital properties, TSCT ratio, reorganization energies, intermolecular interactions and rate constants of the intersystem crossing and reverse intersystem crossing processes are analyzed, and the excited state dynamics and energy consumption process are discussed in detail. The results indicate that the geometric changes on the acceptor unit are restricted from chloroform to the solid phase with decreased reorganization energy contributed by the bond angle. Meanwhile, for the remarkable geometric changes on the donor unit from chloroform to the solid phase, the increased reorganization energy is mainly contributed by the bond length and dihedral angle. Moreover, different triplet exciton conversion processes are illustrated for the molecule in chloroform (T-3 -> S-1) and the solid phase (T-2 -> S-1 and T-2 -> S-2 -> S-1). In addition, the reduced non-radiative consumption is revealed and a high fluorescence efficiency (29.2%) is achieved in the solid phase, which corresponds well with the experimental results (31%). Furthermore, the distance between the donor and the acceptor is suppressed by intermolecular interactions and an enhanced TSCT feature is observed in the solid phase. Thus, a wise design strategy is confirmed where the TSCT feature can light up a multi-channel reverse intersystem crossing process to increase the triplet exciton utilization for realizing highly efficient TADF emission. Our calculations give a reasonable explanation for the previous experimental measurements and provide a theoretical perspective for the structure-property relationship, which could facilitate the development of new efficient TSCT-based TADF emitters.