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 attentions in recent studies. Corresponding theoretical investigations to reveal the inner mechanisms are highly desired. Herein, the photophysical properties of TSCT based TADF molecule (S-CNDF-S-tCz) are theoretically studied both in solvent and solid environments by using 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 thermal vibration correlation function method, the adiabatic singlet-triplet energy gap, natural transition orbitals properties, TSCT ratio, reorganization energies, intermolecular interactions as well as the rate constants of intersystem crossing and reverse intersystem crossing processes are analyzed, excited state dynamics and energy consumption process are discussed in detail. Results indicate that the geometric changes on accepter unit are restricted from chloroform to solid phase with decreased reorganization energy contributed by bond angle. While for remarkable geometric changes on donor unit from chloroform to solid phase, the increased reorganization energy is mainly contributed by bond length and dihedral angle. Moreover, different triplet exciton conversion processes are illustrated for molecule in chloroform (T3S1) and solid phase (T2S1 and T2S2S1). In addition, the reduced non-radiative consumption is revealed and high fluorescence efficiency (29.2%) is achieved in solid phase, which is corresponding well with the experimental results (31%). Furthermore, the distance between donor and acceptor is suppressed by intermolecular interactions and enhanced TSCT feature is determined in solid phase. Thus, a wise design strategy is confirmed that TSCT feature can light up multi-channel reverse intersystem crossing process to increase the triplet exciton utilizations for realizing highly efficient TADF emission. Our calculations give reasonable explanation for previous experimental measurements and provide theoretical perspective for structure-property relationship, which could facilitate the development of new efficient TSCT based TADF emitters.