Enhancing the Photochemical Stability of N,C-Chelate Boryl Compounds: C−C Bond Formation versus C═C Bond cis,trans-Isomerization

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Abstract
N,C-Chelate boron compounds such as B(ppy)Mes2 (ppy = 2-phenylpyridyl, Mes = mesityl) have been recently shown to undergo a facile and reversible C−C/C−B bond rearrangement upon irradiation with UV-light, quenching the emission of the sample and limiting their use in optoelectronic devices. To address this problem, four molecules have been synthesized in which the π-conjugation is extended using either vinyl or acetylene linkers. These compounds, (ph-C≡C-ppy)BMes2 (B1A), (ph-CH═CH-ppy)BMes2 (B1), [p-bis(ppy-CH═CH)benzene](BMes2)2 (B2), and [1,3,5-tris(ppy-CH═CH)benzene](BMes2)3 (B3) have been fully characterized by NMR and single-crystal X-ray diffraction analyses. All four compounds are light yellow and emit blue or blue-green light upon UV irradiation. The acetylene compound B1A has been found to exhibit photochemical instability the same as that of the parent chromophore B(ppy)Mes2. In contrast, all of the olefin-substituted compounds are photochemically stable, instead undergoing cis−trans isomerization exclusively upon exposure to UV light. Experimental and TD-DFT computational results establish that the presence of the olefinic bond in B1−B3 provides an alternate energy dissipation pathway for the B(ppy)Mes2 chromophore, stabilizing the molecule toward photochromic switching via cis−trans isomerization. Furthermore, the incorporation of a cis−trans isomerization pathway may prove to be a useful strategy for the stabilization of photochemically unstable chromophores in other π-systems as well.

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