The effect of -linker bulk on the photophysical properties of 2-phenylfuro[2,3-b]quinoxaline-based FQ--FQ-type compounds

Employing a 2-phenylfuro[2,3-b]quinoxaline (FQ) scaffold as a fluorophore, and phenyl, biphenyl, anthracene, and benzothiadiazole units as pi-linkers, the four compounds Ph-dFQ, bPh-dFQ, ADN-dFQ and Bth-dFQ were respectively constructed, along with BIZ-FQ with a benzimidazole unit, to screen suitable blue emitters. When the linker was changed from phenyl to biphenyl, benzothiadiazole, or anthracene, the torsion angle between the FQ and linker increased from 29.7 degrees to up to 80.0 degrees due to their steric interaction. Owing to the large bulk and pi-conjugation length of the anthracene linker, the torsion angle of 80.0 degrees was predicted from theoretical calculations for ADN-dFQ, and as a result, multiple-wavelength emission is possible. ADN-dFQ and Bth-dFQ provided green emission due to clear charge-transfer emission but low fluorescence efficiency because the large torsion angles would prompt more energy dissipation from the S-1 state and weaken its transition probabilities. Furthermore, despite the weak electron-donating capability of the phenyl, biphenyl and anthracene pi-linkers, a charge-transfer state could be clearly formed with the electron-deficient FQ unit, and as a result, sky-blue to green emissions were observed for the three compounds Ph-dFQ, bPh-dFQ and ADN-dFQ. In addition, it was found that the five compounds could form an exciplex with 2-methyl-9,10-di(2-naphthyl)anthracene (MADN), and thus, when the five compounds were doped into a MADN host, the corresponding OLEDs mainly presented a green emission (similar to 496 nm) from the exciplex.