Charge-Transfer Probes for Molecular Recognition Via Steric Hindrance in Donor-Acceptor Pairs

Molecular association of various aromatic hydrocarbons (D, including sterically hindered donors) with a representative group of diverse acceptors (A ) quinone, trinitrobenzene, tetracyanoethylene, tropylium, tetranitromethane, and nitrosonium) is visually apparent in solution by the spontaneous appearance of distinctive colors. Spectral (UV-vis) analyses of the colored solutions reveal their charge-transfer origin (λCT), and they provide quantitative information of the intermolecular association in the form of the KDA and !CT values for the formation and visualization, respectively, of different [D,A] complexes. Importantly, such measurements establish chargetransfer absorption to be a sensitive analytical tool for evaluating the steric inhibition of donor-acceptor association. For example, the steric differences among various hindered aromatic donors in their association with quinone are readily dramatized in their distinctive charge-transfer (color) absorptions and verified by X-ray crystallography of the charge-transfer crystals and/or QUANTA molecular modeling calculations of optimum intermolecular separations allowed by van der Waals contacts.