Watching the conformational changes of maleonitriledithiolate chromophores inside the inclusion complexes with cyclodextrins: probed by ICD spectra and DFT calculations.

A series of inclusion complexes between cyclodextrins (alpha-, beta-, gamma-cyclodextrin and HP-beta-cyclodextrin, HP-beta-cyclodextrin = 2-hydroxypropyl-beta-cyclodextrin) and sodium maleonitriledithiolate (Na(2)mnt) were investigated by electronic spectra, induced circular dichroism (ICD) spectra, and quantum chemical studies. The inclusion complexes Na(2)mnt@alpha-cyclodextrin and Na(2)mnt@gamma-cyclodextrin did not show any ICD signals, whereas Na(2)mnt@HP-beta-cyclodextrin displayed two signs of splitting Cotton effects (CEs), with one positive CE couplet at 376 nm in the 365-410 nm region and the other negative at 277 nm in the 265-306 nm region. In addition, a dimeric host inclusion pattern of Na(2)mnt@HP-beta-cyclodextrin in solution was determined by the method of continuous variation. Density functional theory (DFT) was used to assist assignment of the ICD signals in inclusion complexes Na(2)mnt@beta-cyclodextrin and Na(2)mnt@HP-beta-cyclodextrin in combination with the well-known Harata's rule. The orientation of p --> pi* transition in Na(2)mnt chromophore was predicted by TD-DFT (time-dependent DFT) calculations to be along the C=C double bond instead of being perpendicular. Upon titrations with Zn(2+) solutions, reversals of the p --> pi* transition-relevant ICD peak and splitting CE were experimentally observed in the cases of Na(2)mnt@beta-cyclodextrin and Na(2)mnt@HP-beta-cyclodextrin, respectively, which strongly supported our hypotheses on their coconformations and the subsequent conformational changes of mnt chromophores occurring during the titration procedures. Therefore, on the basis of both the experimental data and TD-DFT calculations, the HP-beta-cyclodextrin dimer host in inclusion complex Na(2)mnt@HP-beta-cyclodextrin was disclosed, which in turn generated the exciton coupling between the two individually included guests and produced the splitting CEs as well as the reversals of CEs.