DFT Insights on the Modulation of Different Bridging Groups for the Optical Properties of Charge-Transfer Cocrystal

The functional group differences in molecular structure and the self-assembly of cocrystals of multicomponent molecules are two important factors affecting the optical properties of solid-state organic luminescent materials. In this work, three electron-donor anthracene-based molecules with different bridging groups and their charge-transfer cocrystals with electron-acceptor 1,2,4,5-benzenetetracarbonitrile molecule were synthesized and cultured, aiming to explore the influence of donors’ conjugate length and steric hindrance effect on cocrystals’ optical properties. Crystal structure analysis confirmed that the donor’s own crystal structure was a dispersed arrangement, while cocrystals had a unified π···π stacking. Solid-state optical testing indicated that increasing the conjugation length and introducing TCNB could lead to significant red-shifts in the UV absorption band and fluorescence emission peak position of three donors, while significantly increasing the conjugate length of a donor’s bridging group would only lead to slight red-shifts in that of the cocrystals, and the steric hindrance effect of donors had no significant impact on it. The difference was that the fluorescence quantum yields and fluorescence lifetime of three donors and cocrystals all increased significantly with the increase of the steric hindrance effect. Theoretical calculations confirmed that the first excited state processes of the three donors and cocrystals were mainly HOMO→LUMO transitions. Therefore, the LUMO–HOMO gap value determined the energy level difference in the first excited state. Both increasing the conjugation length and introducing TCNB would lead to a reduction in energy level difference, thereby causing a red-shift in the spectra.