Prediction of Oscillator Strength and Transition Dipole Moments with the Nuclear Ensemble Approach for Thermally Activated Delayed Fluorescence Emitters

This work incorporates the nuclear ensemble approach into emission simulation of thermally activated delayed fluorescence (TADF) emitters with strong charge transfer (CT). The vibrational distribution of the excited state is described by an ensemble of nuclear geometries with vertical transition properties computed for each point in the ensemble through the time-dependent density functional theory (TDDFT) method. Compared to TDDFT calculation at stationary geometry, this method provides a better estimate of oscillator strength and distribution of transition dipole moments (TDMs). Four different types of CT states are explored. For the twisted intramolecular CT state, the oscillator strength is promoted strongly and direction distribution of TDMs is concentrated, while it increases less with dispersed TDMs for the through-space CT state. The present work provides a feasible calculation method for TADF emitters and will compensate the flaws of the traditional stationary point TDDFT method which hamper their application in understanding and predicting the photophysical properties of emitters with strong CT characteristics.