Steric and Electronic Influence on Photochromic Switching of N,C‐Chelate Four‐Coordinate Organoboron Compounds

Abstract

A four-coordinate organoboron compound B(ppy)Mes(2) (1, ppy=2-phenylpyridyl, Mes=mesityl) was previously found to undergo reversible photochromic switching through the formation/breaking of a C-C bond, accompanied by a dramatic color change from colorless to dark blue. To understand this unusual phenomenon, a series of new four-coordinate boron compounds based on the ppy-chelate ligand and its derivatives have been synthesized. In addition, new N,C-chelate ligands based on benzo[b]thiophenylpyridine and indolylpyridine have also been synthesized and their boron compounds were investigated. The crystal structures of most of the new compounds were determined by X-ray diffraction analysis. UV/Vis, NMR, and electrochemical methods were used to monitor the photoisomerization process. DFT calculations were performed for all compounds to understand the photophysical and electronic properties of this class of molecules. The results of our study showed that the bulky mesityl group is necessary for photochromic switching. Electron-donating and electron-withdrawing groups on the ppy chelate have a distinct impact on the photoisomerization rate and the photochemical stability of the molecule. Furthermore, we have found that the non-ppy-based N,C-chelate ligands such as benzo[b]thiophenepyridyl can also promote photoisomerization of the boron chromophore in the same manner as the ppy chelate, but the product is thermally unstable.

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

“…[33] This route provides a versatile access to arylbromides that can be converted into the target compounds by a bromide-lithium exchange at 78 C, followed by trapping of the lithiated species with (Mes)2BF (Scheme 2). [23] Dye 3 contains a terminal alkyne group that was introduced as potential handle for click reactions [34] in biomolecule conjugation protocols. [35] Starting from bromide 8 this dye was successfully synthesized by applying a protecting group strategy as outlined in Scheme 3.…”

“…They were fully characterized by 1 2 hybridized carbons. [36] In contrast to the coplanarity of the two aromatic units in most four-coordinate organoboron N,C-chelates (typical dihedral angles of about 4-7°), [15,17,18,23] the high steric repulsion in 1 forces the isoquinolinyl and naphthyl rings to be twisted around the C(1)-C(10) bond…”

“…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.…”

“…[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.…”

“…[25][26][27] The herein investigated N,C chelates (see structures in Scheme 1) feature a higher -conjugation than the abovementioned photochromic B(ppy)Mes2 compounds. [23] In addition they contain electrondonating substituents and the electron-accepting isoquinolinyl moiety. We anticipated that the resulting excited-state intramolecular charge transfer (ICT) would serve as an energy sink and thereby protect the chromophore from unwanted photoreactions.…”