Applications of Photogenerated “Dark Isomer” Boriranes: Dearomatization and Thermal Energy Storage

Abstract

The works described herein investigate the bimolecular ground state reactivity of boron-containing photochromic systems. Despite a remarkable development in the synthesis and characterization of N,C- and C,C-chelate organoboron species over the last decade, only their photoreactivity and photophysical properties have been well documented. Their ground state reactivity is largely unknown, and apart from luminescence, no applications exploiting their photochromic properties have been reported. To complement their photoreactive profiles, N,C- and C,C-chelate organoboron species will be reacted with conventional organic reagents to target specifically the boron center and boron-chelated ligands to elucidate structural changes in thermal conditions. The first study describes the one-pot, light-assisted dearomative functionalization of N,C and C,C-chelate four-coordinate boron compounds via [4+2] cycloaddition (Diels-Alder reaction) at a boron-anchored mesityl ring. The synthesis progressed rapidly under ambient conditions and all of the four products reported in this work contain a borirane ring that is stable towards oxygen, unlike its starting precursors which undergo oxidative deborylation. The simplicity of this post synthetic modification strategy prompts further investigation into boron-assisted dearomatization of simple aromatic rings. The second study uses a N,C-chelate four coordinate boron photoswitch for solar energy storage. The compound of interest photoisomerizes from a four-coordinate organoboron to a borirane-containing species with 365 nm light and was found to liberate 14 kcal/mol of energy upon its thermally triggered reversion back to the original organoboron in the solid state. We find that the reverse reaction from borirane to N,C-chelate boron compound can be catalyzed by three salts containing the −B(C6F5) counter ion. Radical species were discovered as key intermediates in the reversal process and were detected via NMR and EPR spectroscopy. The facile reversal of the photoswitch and detection of heat generated in the reversal process make this class of compounds suitable candidates for solar energy storage applications.