Photophysical evidence of charge-transfer-complex pairs in mixed-linker 5-amino/5-nitroisophthalate CAU-10.

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 N2 O 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 5.2×10(4) times stronger than in the aqueous phase.