External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs

Abstract

Photoswitchable catalysis involves changes in properties of a catalyst based on difference in electronic and steric factors. These changes can selectively turn "ON" and "OFF" the catalytic activity by an external stimulus, light. Herein, we report dithienylethene based photoswitchable frustrated Lewis pairs for facile hydrogen recombination. The diarylethene moiety is not serving as template to alter the catalytic activity rather it is the core part of the catalyst. The rational design principle involves study of proton and hydride affinities of Lewis acid and base functionalities installed on the diarylethene photoswitch pair. Proton and hydride affinities differ significantly between open and closed isomers that help in designing an active molecule for hydrogen recombination. The proton and hydride affinities are then utilized to formulate best relative positions of Lewis acid and base on a single molecule in order to liberate one H-2 molecule per photochrome. Energy barriers for hydrogen recombination are calculated for open and closed isomers. The closed isomer releases the hydrogen with a low activation barrier of 1.9 kcal mol(-1) whereas the open isomer requires relatively high barrier of 4.7 kcal mol(-1). The pronounced differences in energy barrier illustrate the potential of photoswitchable hydrogen recombination with diarylethene photochrome. The chemically stored hydrogen in frustrated Lewis pairs can be liberated in a controlled fashion through external stimulus for catalytic hydrogenation reactions. The present study will provide new dimensions to the scientific community for exploration of other systems with even better selectivities. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.