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

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 (D₀) and the first excited states (D₁) 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 (μ₁₂) and the energy gaps between D₀ and D₁ (ΔE), 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 (ΔG⁰), 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 ΔG⁰ 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.