Triphenylene analogues with B2N2C2 cores:: Synthesis, structure, redox behavior, and photophysical properties

A series of alkyl (1-3), aryl (6), and benzo-annulated (4, 5) heteroaromatic triphenylene analogues with B2N2C2 cores have been synthesized via chelation of pyridazine derivatives using difunctional Lewis acidic diborabiphenyl precursors. In contrast to triphenylene, NICS(1) calculations on 1 suggested high aromaticities for the central (-11.3 ppm) and outer borabenzene rings (-7.7 ppm), along with nonaromatic behavior for the pyridazine ring (-0.7 ppm). Crystal structure analyses supported this analysis. When the a- and c-faces of the pyridazine moiety were free of substitution ( 1, 3), planar structures resulted, but upon substitution, a twisted B2N2C2 core was observed due to steric repulsion of neighboring hydrogen atoms ( e. g., 5). The increase of steric bulk from H (1) to Pr-i(3) in the planar species was found to result in a dimeric, head-to-tail herringbone packing motif, held together by close intermolecular (BN)-N-... interactions of 3.39 angstrom. One-electron reduction by Cp*Co-2 was found to afford the radical anions of 3 and 5, which were characterized by broad, featureless singlets in the EPR spectra; [3](.)-[Cp*Co-2](+) was characterized by X-ray crystallography. While the planar structures (1-4) were observed to possess weak fluorescence ( F) 0.02-0.08) with either yellow-orange (ca. 555 nm) or green emission ( 521 nm), the twisted structures ( 5, 6) were found to be nonfluorescent.