CHARGE-TRANSFER PROBES FOR MOLECULAR RECOGNITION VIA STERIC HINDRANCEIN DONOR-ACCEPTOR PAIRS

Molecular association of various aromatic hydrocarbons (D, including s terically hindered donors) with a representative group of diverse acce pters (A = quinone, trinitrobenzene, tetracyanoethylene, tropylium, te tranitromethane, and nitrosonium) is visually apparent in solution by the spontaneous appearance of distinctive colors. Spectral (UV-vis) an alyses of the colored solutions reveal their charge-transfer origin (l ambda(CT)), and they provide quantitative information of the intermole cular association in the form of che K-DA and (epsilon)(CT) values for the formation and visualization, respectively, of different [D,A] com plexes. Importantly, such measurements establish charge-transfer absor ption to be a sensitive analytical tool for evaluating the steric inhi bition of donor-acceptor association. For example, the steric differen ces among various hindered aromatic donors in their association with q uinone are readily dramatized in their distinctive charge-transfer (co lor) absorptions and verified by X-ray crystallography of the charge-t ransfer crystals and/or QUANTA molecular modeling calculations of opti mum intermolecular separations allowed by van der Waals contacts.