Investigating the influence of substituent groups in TTM based radicals for the excitation process: a theoretical study.

Tri-(2,4,6-trichlorophenyl)methyl (TTM) based radicals can be promising in providing relatively high fluorescence quantum efficiency. In this study, we have evaluated the photoluminescence properties of a series of TTM-based radicals by means of DFT and TD-DFT methods. The optimized structures of the ground states (D 0 ) and the first excited states (D 1 ) of all the radicals are calculated and the computed emission bands are comparable with previous experimental results. k nr is determined from transition dipole moments ( μ 12 ) and the energy gaps between D 0 and D 1 (Δ E ), both of which can be regulated by the conjugated structures from the substituent groups. k nr was derived from the mode-averaging method and is consistent with the experimental results. Factors influencing k r and k nr , including the potential energy differences (Δ G 0 ), the vibrational reorganization energies ( λ ) and the electron coupling term ( H ab ), are discussed. By comparing k r and k nr in solvents with different polarities (cyclohexane, toluene, and chloroform), TTM based radicals in cyclohexane exhibit the most promising fluorescence efficiencies. Besides, two substituted radicals, namely 2Br-TTM-3PCz and 2F-TTM-3PCz, have been fabricated. The results show that fluorine atoms are able to increase Δ G 0 and a considerably small k nr has been predicted. We expect that our calculation can benefit the design of light-emitting molecules in further experiments.