Spin？Vibronic Model for Quantitative Prediction of Reverse Inter-System Crossing Rate in Thermally Activated Delayed Fluorescence Systems
Journal
JCTC (Journal of Chemical Theory and Computation)
Date
2020.01.14
Abstract
Computationally predicting reverse inter-system crossing (RISC) rates is important for designing new thermally
activated delayed fluorescence (TADF) materials. We report a method that can quantitatively predict
RISC rates by explicitly considering the spin？vibronic coupling mechanism. The coupling element of the spin？
vibronic Hamiltonian is obtained by expanding the spin？orbit and the non-Born？Oppenheimer terms to the
second-order and is then brought into the Golden Rule rate under the Condon approximation. The rate equation
is solved directly in the time domain using a correlation function approach. The contributions of the first-order
direct spin？orbit coupling and the second-order spin？vibronic coupling to an RISC rate can be quantitatively
analyzed in a separate manner. We demonstrate the utility of the method by applying it to a representative TADF
system, where we observe that the spin？vibronic portion is substantial but not dominant especially with relatively
small triplet？singlet energy gap. Likewise, our method may elucidate the physical background of ecient
nonradiative transitions from the lowest triplet to a higher lying singlet in other purely organic TADF systems,
and will be of great utility toward designing new such molecules.