Theoretical study on photophysical properties of 2,1,3-benzothiadiazole-based star-shaped molecules

Abstract

Star-shaped molecules with tailoring functional groups in the core and the arms have great potential application in organic light-emitting devices, because it can be designed to realize low band gap, broad absorption, and excellent solubility for low-cost solution process. To gain an insight into the structure–property relationships, a set of four-arm star-shaped molecules with 2,1,3-benzothiadiazole as the core, different π-conjugated groups as the arm, and triphenylamine or 2-(pyridin-2-yl) pyridine as the end-group were designed. In this study, a systematic investigation into them was carried out using the density functional theory and time-dependent density functional theory methods. The calculated ionization potentials, electron affinities, and reorganization energies (λ) show that the properties of the π-conjugated bridge and the end-group significantly affect the carrier injection and transport characteristics of these molecules, especially for S-BTDP and S-EBTD. Among these molecules, S-BTDP exhibits better electron injection ability due to the introduction of 2-(pyridin-2-yl) pyridine as the end-group. However, S-EBTD, with ethylene as π-conjugated bridge, has excellent hole injection and carrier transport behaviors. We also calculated the singlet-to-triplet exciton-formation cross-section ratio (σ_S/σ_T), the exciton-formation fractions (χ_S), and the absorption and emission spectra of these molecules. We calculated that σ_S/σ_T ranges from 1.78 to 2.76 and that χ_S is ca. 0.37–0.48. These molecules have two absorption bands in the range of 340–410 nm and 500–613 nm, respectively. The calculated emission spectra range from 619 to 706 nm. It can be deduced that the studied 2,1,3-benzothiadiazole-based star-shaped molecules can serve as efficient red light-emitting electroluminescent materials.