Experimental and Computational Studies of a Multi-Electron Donor-Acceptor Ligand Containing the Thiazolo[5,4-d] thiazole Core and its Incorporation into a Metal-Organic Framework

A ligand containing the thiazolo[5,4-d] thiazole (TzTz) core (acceptor) with terminal triarylamine moieties (donors), N,N'-(thiazolo[5,4-d] thiazole-2,5-diylbis(4,1-phenylene)) bis(N-(pyridine-4-yl) pyridin-4-amine (1),was designed as a donor-acceptor system for incorporation into electronically active metal-organic frameworks (MOFs). The capacity for the ligand to undergo multiple sequential oxidation and reduction processes was examined using UV/Vis-near-infrared spectroelectrochemistry (UV/Vis-NIR SEC) in combination with DFT calculations. The delocalized nature of the highest occupied molecular orbital (HOMO) was found to inhibit charge-transfer interactions between the terminal triarylamine moieties upon oxidation,whereas radical species localized on the TzTz core were formed upon reduction. Conversion of 1 to diamagnetic 2+ and 4+ species resulted in marked changes in the emission spectra. Incorporation of this highly delocalized multi-electron donor-acceptor ligand into a new two-dimensional MOF,[Zn(NO3)(2)(1)](2),resulted in an inhibition of the oxidation processes,but retention of the reduction capability of 1. Changes in the electrochemistry of 1 upon integration into 2 are broadly consistent with the geometric and electronic constraints enforced by ligation.