Construction of full-color-tunable and strongly emissive materials by functionalizing a boron-chelate four-ring-fused π-conjugated core

2-(2'-Hydroxyphenyl)benzoxazole (HBO) and 2-(2'-hydroxyphenyl)benzothiazole (HBT) reacted with triphenylborane produced two rigid pi-conjugated fluorescent cores 1 (BPh2(BOZ), BOZ 2-(benzo[d]oxazol-2-yl)phenol) and 2 (BPh2(BTZ), BTZ 2-(benzo[d]thiazol-2-yl)phenol). Comparisons of photophysical properties and calculations between para-and meta-diphenylamine-substituted derivatives 5 (BPh2(para-NPh2-BTZ)) and 7 (BPh2(meta-NPh2-BTZ)) demonstrated that functionalization at the para-position of the rigid core is effective in tuning the electronic structure and hence the photophysical properties of this type of boron-chelate complex. Simple modification of these frameworks by introducing various amine groups at the para-position allows the synthesis of strongly fluorescent materials 3 (BPh2(para-Cz-BTZ), Cz = 9H-carbazol-9-yl), 4 (BPh2(para-NPh2-BOZ), NPh2 = diphenylamino), 5, and 6 (BPh2(para-NMe2-BTZ), NMe2 = dimethylamino). The emission colors of these newly synthesized complexes together with the parent complexes 1 and 2 covered a wide range from deep blue to saturated red in both solution and the solid state. Crystal structure analysis discloses that two phenyl groups attached to the boron atom effectively keep the luminescent ring-fused pi-conjugated skeletons apart, making these fluorophores highly emissive in solid forms (Phi(F) = 0.36-0.71). Organic light-emitting diodes employing these boron complexes as emitters not only keep the full-color tunable emission feature but also show high electroluminescent (EL) performance; for instance, the greenish-blue device based on 2 showed the highest efficiency of 7.8 cd A(-1) and the yellow light-emitting device based on 4 exhibited the highest brightness (31 220 cd m(-2)) among the boron-containing emitters reported so far.