C3-symmetric triarylboron building blocks: Synthesis, structures, and photophysical properties

Synthesis of C₃-symmetric triarylboranes, tris(4-bromo-2,6-dimethylphenyl)-borane (1), N,N',N''-(boranetriyltris(3,5-diisopropyl-3′,5′-dimethyl-[1,1′-biphenyl]-4′,4-diyl))tris(1,1-diphenylmethanimine) (2), N,N',N''-(boranetriyltris(3′,5′-dimethyl-[1,1′-biphenyl]-4′,4-diyl))tris-(1,1-diphenylmethanimine) (3), 4′,4′'',4′''''-boranetriyltris(3′,5′-dimethyl-[1,1′-biphenyl]-4-carbaldehyde) (4), 4,4′,4′'-boranetriyltris(3,5-dimethylaniline) (5), 4′,4′'',4′''''-borane-triyltris(3′,5′-dimethyl-[1,1′-biphenyl]-4-carbonitrile) (6), 4′,4′'',4′''''-boranetriyltris(3′,5′-dimethyl-[1,1′-biphenyl]-4-amine) (7), and 4,4′,4′'-boranetriyltris(3,5-dimethylbenzaldehyde) (8) has been accomplished using well-known synthetic methodologies. These new potential building blocks have been characterized by various spectroscopic and analytical techniques and their photophysical properties have been examined. It is observed that photophysical properties of these compounds are enhanced with π conjugation expansions along the branches when donor moieties such as -NH₂ groups are present at the para-position of the aryl rings. The highest quantum yield of 0.293 is observed for 4′,4′'',4′''''-boranetriyltris(3′,5′-dimethyl-[1,1′-biphenyl]-4-amine) (7). Interestingly, red-shifted fluorescence spectra follow the same trend as absorption spectra. The fluorescence lifetime increases with the number of donor moieties. The present findings demonstrate the possible use of these C₃-symmetric boron-containing building blocks in developing new polymeric materials with improved photophysical properties for various applications.