Tuning the Optoelectronic Properties of F8BT for X-Ray Scintillation: A DFT/TD-DFT Study on Halogen-Substituted Derivatives

Organic scintillators present a promising path toward low-cost, flexible, and eco-friendly X-ray detection technologies. Their primary limitation, however, lies in the low atomic number of their constituent elements, which restricts X-ray absorption efficiency. This computational study investigates a strategy to enhance this property by incorporating heavier atoms through halogenation. Using density functional theory (DFT) and time-dependent DFT (TD-DFT), we systematically explore the effects of substituting fluorine, chlorine, bromine, and iodine atoms onto the benzothiadiazole unit of the conjugated polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole), F8BT. Our calculations reveal that halogen substitution systematically modulates the electronic structure by stabilizing the highest occupied molecular orbital (HOMO) and destabilizing the lowest unoccupied molecular orbital (LUMO), leading to a widened optical gap. While the HOMO remains delocalized across the polymer backbone, the LUMO becomes increasingly localized on the substituted benzothiadiazole unit. This localization reduces the transition dipole moment and oscillator strength, consequently lowering the radiative decay rate. A critical finding is that the non-radiative decay rate decreases more significantly than the radiative rate for several derivatives. This favorable balance results in a marginal increase in the calculated photoluminescence quantum yield (PLQY) for the fluorinated and chlorinated systems. The work establishes that halogenation, particularly with lighter halogens, can enhance the heavy-atom content for improved X-ray interaction without sacrificing luminescence efficiency, providing valuable design principles for next-generation organic scintillators.