Synthesis of Azadioxa-Planar Triphenylboranes Bridged by Aryl- and Alkylimino Groups and Their Photophysical Properties

Heteroatom-bridged planar triphenylboranes, in which the three phenyl groups are bridged at the ortho positions by heteroatoms, are attracting growing attention as one of the heteroatom-containing π-conjugated molecules. Herein, we developed the synthetic method of planar triphenylboranes bridged by two oxygen atoms and one nitrogen atom, and the substituent on the nitrogen atom is derived into various aryl and alkyl groups. A key intermediate bearing an imino group (-NH-) was synthesized from a bis-triflate precursor bridged by two oxo groups via a nucleophilic aromatic substitution reaction of benzyl amine and following debenzylation. The X-ray crystallographic analysis revealed that the compound exhibits a planar molecular structure which can form a one-dimensionally π-stacked structure. The photophysical and density functional theory studies revealed that their highest occupied molecular orbitals and lowest unoccupied molecular orbitals (LUMOs) are originated from the triphenylborane moiety, while introducing strong electron-withdrawing groups such as the 4-cyanophenyl group on the nitrogen atom can induce the localization of the LUMO at the aryl groups instead of the triphenylborane moiety.