Performance study of the electronic and optical parameters of thermally activated delayed fluorescence nanosized emitters (CCX-I and CCX-II) via DFT, SCC-DFTB and B97-3c approaches

Abstract

Conventional density functional theory (DFT) is the common choice of researchers. However, recently the density functional tight binding (DFTB) methods and composite B97-3c methods have been used as a faster and accurate DFT alternative. Moreover, the B97-3c based time-dependent DFT, simplified Tamm–Dancoff (sTDA) and simplified TDDFT (sTDDFT) calculations have expanded the domain of state of art excited state calculations. Here, the recently developed organic nanosized emitters 3-(9H-[3,9′-bicarbazol]-9-yl)-9H-xanthen-9-one (CCX-I) and 3-(9′H-[9,3′:6′,9″-tercarbazol]-9′-yl)-9H-xanthen-9-one (CCX-II) have been used as samples to study the performance of the above mentioned methods at the ground and excited levels in gas and solvent conditions, both with and without the dispersion corrections respectively. Bond lengths calculated by DFTB and B97-3c approaches are in excellent agreement with the DFT results. Root mean square deviation (RMSD) values suggest that the DFTB approach provides better band gap values than B97-3c. But, B97-3c results in better torsion angles. Optical properties are found to be closest to the DFT values when calculated by time-dependent DFTB (TD-DFTB) approach with the lowest RMSDs for excitation energies (0.75 eV) and oscillator strengths (~0.08). The computational speed boosts of the B97-3c based sTDDFT and sTDA approaches are ~ 4 and ~33 times faster than the TDDFT counterpart.