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

Abstract

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[triple bond]C-ppy)BMes(2) (B1A), (ph-CH=CH-ppy)BMes(2) (B1), [p-bis(ppy-CH=CH)benzene](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 B1A 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.

Strongly Emissive and Photostable Four‐Coordinate Organoboron N,C Chelates and Their Use in Fluorescence Microscopy

“…Recently, the photochromic properties of structurally related B(ppy)Mes2 N,C-chelates were reported by the Wang group. [21][22][23][24] In oxygen-free solutions these compounds can be transformed by light irradiation into borabicyclo[4.1.0]hepta-2,4-diene derivatives. These are then re-converted into the starting material by a thermally activated process.…”

“…These are then re-converted into the starting material by a thermally activated process. Among the various factors that were shown to influence the photochromic processes of ppybased organoboron N,C-chelates is the retardation by a) competitive cis-trans photoisomerization of attached C=C bond-containing substituents [22] and b) extended -conjugation of the chelate ligand. [23] We tested the photochemical reactivity of our compounds in oxygen-free toluene solution under long-term irradiation for 3-4 hours (150 W Xe lamp with a 395 nm or a 420 nm cut-off filter).…”

“…[20] Interestingly, some of the latter were shown to feature reversible photochromic behaviour. [21][22][23][24] On the one hand, this switching characteristic is interesting by itself for applications that require the light-induced control of optical properties. [22] On the other hand, such photoreactivity constitutes a severe drawback for applications [22] which require strong light sources or prolonged irradiation times and where photodecomposition should be dimished.…”

“…Additional molecular design can help avoiding such undesired processes. [22] Scheme 1. Structures of the dyes 1-6.…”

Das Pentaphenylborol‐2,6‐Lutidin‐Addukt: ein System mit ungewöhnlichen thermo‐ und photochromen Eigenschaften

“…[4] Kürzlich berichteten Wang und Mitarbeiter über Bestrahlungsexperimente an Addukten des Typs A, die zur Bildung von Spezies des Typs B führen, die eine Boracyclopropaneinheit mit intakter B-N-Bindung enthalten (Schema 1). [5] Verwandte Umlagerungsreaktionen, die vermutlich über radikalische Zwischenstufen verlaufen, wurden zudem im Rahmen der Photolyse von Tetraphenylboraten beobachtet (Schema 1).…”