A Facile Synthesis of Chiral Luminescent Organoboron Polymers by Hydroboration Polymerization Utilizing Chiral Borane

Abstract

Hydroboration polycondensations of three aromatic dicyano compounds 1, 1,4-dicyanobenzene (1a), 1,3-dicyanobenzene (1b), and 1,4-didodecyloxy-2,5-dicyanobenzene (1c), with N,N,N′,N′-tetramethylethylenediamine-bis(monoisopinocampheylborane) (S-IpcBH2·TMED) were carried out to obtain optically active poly(cyclodiborazane)s (2a, 2b, and 2c) with Mns in the range from 2400 to 6800 in 61−76% yields. The hydroboration polycondensation efficiently proceeded through the complete formation of boron−nitrogen four-membered ring. The polymer structures were characterized by 1H and 11B NMR, UV−vis, photoluminescence (PL), and CD spectroscopies. Only 2c exhibited intense blue-green light emission from charge-transfer interaction by incorporation of donor and acceptor system into the polymer backbone. The emission behavior of model compounds (A and B), corresponding to polymer units of 2a and 2c, was effectively examined by theoretical calculations using the DFT method. As a result, the intense blue-green emission of 2c was observed by the bathochromic shift to a visible region. All the polymers showed intense CD signals, indicating that they formed chiral boron−nitrogen four-membered ring structure. Further, 3b and 3c were prepared by hydroboration polycondensation of 1b or 1c with R-IpcBH2·TMED. The specific rotations of 2a, 2b, and 2c showed positive values in contrast to those of 3b and 3c. The Cotton effects of the cyanobenzene moieties in 2a, 2b, and 2c at around 300−350 nm were positive, whereas the Cotton effects of the cyanobenzene moieties in 3b and 3c were negative, as were the specific rotations as well.