Cyanine Borate Penetrated Ion Pair Structures in Solution and the Solid State:  Induced Circular Dichroism

An induced circular dichroism (CD) spectrum is observed for 1,1‘,3,3,3‘,3‘-hexamethyl-9-phenyl-indocarbocyanine, 1,1‘-diethyl-2,2‘-cyanine, 1,1‘-di-(3,5-di-tert-butyl)benzyl-2,2‘-cyanine, or 1,1‘-di-(4-tert-butyl)benzyl-2,2‘-cyanine when penetrated into a chiral cavity of either spirobi[(2-methyl)borataxanthene] or spirobi[(3-methyl)borataxanthene]. Within the ion pair, the cyanine dye exists in two (or more) interconverting conformations of unequal energy. Solid-state structural evidence suggests that a nitrogen-forward mode for penetration by the cyanine dominates the solution behavior. A critical feature for the observation of induced circular dichroism is that the dye is twisted in the ground state. The free energy difference between the diastereomers with right- and left-handed twist in the chiral cavity of the borate is less than 1 kcal/mol, but this is sufficient to induce the CD spectrum. The magnitude of the observed CD spectrum is dependent on the structure of both the cyanine and borate.