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

Abstract

Employing a 2-phenylfuro[2,3-b]quinoxaline (FQ) scaffold as a fluorophore, and phenyl, biphenyl, anthracene, and benzothiadiazole units as π-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° to up to 80.0° due to their steric interaction. Owing to the large bulk and π-conjugation length of the anthracene linker, the torsion angle of 80.0° 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 S₁ state and weaken its transition probabilities. Furthermore, despite the weak electron-donating capability of the phenyl, biphenyl and anthracene π-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 (∼496 nm) from the exciplex.