A multi-responsive system based on intramolecular B ← O = P Lewis acid-base pairs

A series of amino-triarylborane based flexible Lewis pairs with P(O)R₂ (R = phenyl, methylphenyl) and BR₂ (R = 2,4,6-trimethylphenyl (Mes), 2,6-dimethyl-4-methoxyphenyl (Mes(OMe)) were synthesized and fully characterized. These compounds display distinct responses toward multiple external stimuli including solvent, temperature and pressure. A detailed study shows that the ability of reversible switching between the open and closed structures based on the intramolecular B ← O bond plays a key role in their solution state fluorescence response to external stimuli, which can be further tuned by adjusting the electronic/steric properties of the substituents on the BR₂/P(O)R₂ unit. All these compounds exhibit remarkable mechanochromism. Furthermore, upon grinding, pressure-induced blue-shifted and red-shifted emissions were observed for B2/B3 and B1/B4, respectively. This solid state emission change is fully reversible for B2-B4 upon exposure to vapours of several common solvents.