Increasing the Dynamic Range of Metal Ion Affinity Changes in Zn2+ Photocages Using Multiple Nitrobenzyl Groups

Two generations of DiCast photocages that exhibit light-induced decreases in metal ion affinity have been prepared and characterized. Expansion of the common Zn2+ chelator of N,N-dipicolylaniline (DPA) to include additional aniline ligand provides N,N′-diphenyl-N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-diamine, a tetradentate ligand that was functionalized with two photolabile groups to afford DiCast-1. Uncaging of the nitrobenzhydrol reduces the electron density on two metal-bound aniline ligands, which decreases the Zn2+ affinity 190-fold. The analogous MonoCast photocage with a single nitrobenzhydrol group only undergoes a 14-fold reduction in affinity after an identical photochemical transformation. A second series of DiCast photocages based on a N,N′-(pyridine-2,6-diylbis(methylene))dianiline scaffold, which allows the introduction of two additional Zn2+-binding ligands into a preorganized chelator, expand on the multi-photolabile group strategy. DiCast-2 includes two pyridine ligands while DiCast-3 adds two carboxylate groups. Addition of bridging pyridine to the second generation photocages leads to more stable Zn2+ complexes, and photolysis of two photolabile groups increases the Zn2+ affinity changes to 480-fold. The Zn2+, Cu2+, and Cd2+ binding properties were examined in all the DiCast photocages and the corresponding photoproducts using UV–vis spectroscopy. Further insight into the photocage Zn2+-binding motifs was obtained by X-ray analysis of DiCast-2 and DiCast-3 model ligands.