Syntheses of Diverse Donor-Substituted Bisbenzofuro[2,3-<i>b</i>:3′,2′-<i>e</i>]pyridines (BBZFPys) via Pd Catalysis, and Their Photophysical Properties

Abstract

A series of bisbenzofuro[2,3-b:3′,2′-e]pyridines (BBZFPys) bearing a chlorine functionality have been efficiently synthesized through a Pd-catalyzed double oxidative intramolecular C–H/C–H coupling of monochlorinated 2,6-diaryloxypyridines. The subsequent Buchwald–Hartwig amination of the chlorinated BBZFPys allowed for the access to a new class of donor–acceptor (D–A) π-conjugated compounds that comprise BBZFPy as an electron acceptor (A) and diarylamines as a donor (D) unit. The investigation of the steady-state photophysical properties of the prepared D–A compounds revealed that they are emissive in both solution and solid states in the blue-to-green color region. The singlet–triplet energy splitting (ΔEST) was found to be much smaller than that of substituent-free BBZFPy (0.70 eV), ranging from 0.01 to 0.56 eV. The time-resolved spectroscopy revealed that the D–A compounds, comprising a bis(tert-butyl)carbazole as the D and CF3-attached BBZFPy as the A, showed delayed fluorescence (DF) in nonpolar matrix host material (Zeonex), while in a polar matrix (DPEPO), room-temperature phosphorescence (RTP) was faintly observed. Furthermore, organic light-emitting diodes (OLEDs) fabricated with the D–A compounds as a blue emitter showed a moderate external quantum efficiencies (EQEs) up to 1.5%.