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

N,C-Chelate boron compounds such as B(ppy)Mes(2) (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 pi-conjugation is extended using either vinyl or acetylene linkers. These compounds, (ph-C C-ppy)BMes(2) (B1A), (ph-CH=CH-ppy)BMes(2) (B1), [p-bis(ppy-CH=CH)benzone](BMes(2))(2) (B2), and [1,3,5-tris(ppy-CH=CH)benzene](BMes(2))(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 B1 A has been found to exhibit photochemical instability the same as that of the parent chromophore B(ppy)Mes(2). 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)Mes(2) 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 pi-systems as well.