Theoretical Study on the geometrical effect to TADF processes

【Abstract】 In order to investigate what are the most important factors for thermally activated delayed fluorescent (TADF) processes, the molecules, o-, m-, and pcarbazole benzonitrile (CzBN), were used as simplest models. The present study employs the theoretical methods developed for the radiative and non-radiative transitions in single molecules. Based on the displaced harmonic approximation, Franck-Condon overlap integrals were calculated for the estimation of fluorescence intensity and ISC rate constants. The reverse ISC (RISC) rate constants were also estimated under the condition of the strong coupling limit. The calculated results indicate that large geometrical displacements make the fluorescence spectral peaks broadened and largely shifted to the region of longer wavelength. These suggest that effective emission can be obtained only in o-CzBN. The results also suggest that fast ISC/RISC can be observed only in o-CzBN because of a small singlet-triplet gap. Thus, it can be concluded that the geometrical arrangement of a CN group to the ortho site of a Cz group plays important roles in effective TADF processes. These results reasonably explains why the molecules proposed by Adachi et al. are effective for TADF processes.