Photophysical Evidence of Charge-Transfer-Complex Pairs in Mixed-Linker 5-Amino/5-Nitroisophthalate CAU-10

Abstract

The photochemistry of two isostructural metal-organic frameworks based on 5-amino/5-formamidoisophthalate (CAU-10-NH2/NHCHO) or mixed-linker 5-amino/5-formamido- and 5-nitroisophthalate (CAU-10-NO2/NH2/NHCHO) has been studied using laser flash photolysis. 355 nm excitation of CAU-10-NH2/NHCHO leads to a transient absorption spectrum characterized by a broad continuous absorption from 380 to 800 nm that was attributed to the presence of holes (440 nm) and electrons (600 nm) based on iPrOH and N2O quenching, respectively. In contrast, no transients were observed for the isostructural mixed-linker CAU-10-NO2/NH2/NHCHO, data that is compatible with the uniform distribution of linkers 5-amino/5-formamido/5-nitroisophthalate as charge-transfer complex pairs. The same effect of quenching of 5-aminoisophthalate transients by 5-nitroisophthalate was also observed in aqueous solution (pH 9) but with much lower strength. Using a simple Stern-Volmer formalism allowed the estimation of the interaction of 5-aminoisophthalate with 5-nitroisophthalate in MOF to be 5.2x10(4) times stronger than in the aqueous phase.