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 of 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 1 R-( -) or 1 S-( +) 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.
Highlights
Stimuli-responsive supramolecular systems raise significant research interest since they allow the understanding and mimicking of dynamic biological structures, realize molecular machines, and lead to new materials.[1]
When we examined the degree of chiral induction caused by addition of R/S-CSA to the mixed m-C3:1/o-C sample on the contained o-L chromophores, we observed that only the fraction of contained o-C (∼32%) contributes to the characteristic circular dichroism (CD) signal at 330 nm, while the major species m-C3:1 does not (Supporting Information Figure S60)
Our study revealed the temporal relationship between the stepwise photocyclization events and guest ejection of a light-triggered host composed of four DTE photoswitches
Summary
Stimuli-responsive supramolecular systems raise significant research interest since they allow the understanding and mimicking of dynamic biological structures, realize molecular machines, and lead to new materials.[1]. The CD spectra showed a negative Cotton effect corresponding to the closed-form DTE chromophore for the sample containing R-CSA and a positive one for S-CSA, albeit of low intensity (Figure 5b and d and Supporting Information Figure S42) We explain this observation with a photochemical fixation of an enantiomeric excess in the form of the configurationally stable closed-form DTE isomers, induced by the presence of the chiral guest. On the basis of the observed degree of chiral induction as well as experiments described below, we assumed that the guest is ejected from the host early on, i.e., after the first DTE switch has been closed, during the stepwise ligand isomerization sequence (Figure 1b) Article ing the guest (Figure 6f), indicating that both m-C and c-C show the same outside-aggregation behavior
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