Mechanistic interplay between light switching and guest binding in photochromic [Pd2Dithienylethene4] coordination cages

Abstract

Photochromic [Pd2L4] coordination cages based on dithienylethene (DTE) ligands L allow triggering guest uptake and release by irradiation with light of different wavelengths. The process involves four consecutive electrocyclic reactions to convert all chromophores between their open and closed photoisomeric forms. So far, guest affinity of the fully switched species was elucidated, but mechanistic details concerning the intermediate steps remained elusive. Now, a new member of the DTE cage family allows unprecedented insight into the interplay between photoisomerization steps and guest location inside/outside the cavity. Therefore, the intrinsic chirality of the DTE backbones was used as reporter for monitoring the fate af a chiral guest. In its "open" photoisomeric form (o-L, [Pd2(o-L)4] =o-C), the C2-symmetric DTE chromophore quickly converts between energetically degenerate P and M helical conformations. After binding homochiral 1R-(-) or 1S-(+) camphor sulfonate (R-CSA or S-CSA), guest-to-host chirality transfer was observed via a circular dichroism (CD) signal for the cage-centered absorption. Irradiating the R/S-CSA@o-C host-guest complexes at 313 nm produced configurationally stable "closed" photoisomers, thus locking the induced chirality with an enantiomeric excess close to 25%. This value (corresponding to chiral induction for one out of four ligands), together with DOSY NMR, ion mobility mass spectrometry, and X-ray structure results, shows that closure of the first photoswitch is sufficient to expel the guest from the cavity.