2‑(2-Hydroxyphenyl)benzimidazole-Based Four-Coordinate Boron-Containing Materials with Highly Efficient Deep-Blue Photoluminescence and Electroluminescence

Two
novel four-coordinate boron-containing emitters 1 and 2 with deep-blue emissions were synthesized by refluxing a
2-(2-hydroxyphenyl)­benzimidazole ligand with triphenylborane
or bromodibenzoborole. The boron chelation produced a new π-conjugated
skeleton, which rendered the synthesized boron materials with intense
fluorescence, good thermal stability, and high carrier mobility. Both
compounds displayed deep-blue emissions in solutions with very high
fluorescence quantum yields (over 0.70). More importantly, the samples
showed identical fluorescence in the solution and solid states, and
the efficiency was maintained at a high level (approximately 0.50)
because of the bulky substituents between the boron atom and the benzimidazole
unit, which can effectively separate the flat luminescent units. In
addition, neat thin films composed of 1 or 2 exhibited high electron and hole mobility in the same order of magnitude
10–4, as determined by time-of-flight. The fabricated
electroluminescent devices that employed 1 or 2 as emitting materials showed high-performance deep-blue emissions
with Commission Internationale de L’Eclairage (CIE) coordinates
of (X = 0.15, Y = 0.09) and (X = 0.16, Y = 0.08), respectively. Thus,
the synthesized boron-containing materials are ideal candidates for
fabricating high-performance deep-blue organic light-emitting diodes.