1-Naphthyltropylium Ions Having Condensed Aromatic Rings at the 8-Position: Dependence of the Intramolecular Charge-Transfer Interaction upon Gemetry of the Donor

Article abstract of DOI:10.1021/jo00028a043

A series of 1-aryl-8-tropylionaphthalene cations (aryl = 1-naphthyl (8a), 2-naphthyl (8b), 1-pyrenyl (8c), and 2-pyrenyl (8d) have been synthesized, in which the condensed aromatic system and a tropylium ring are placed at a face-to-face arrangement.The 1H NMR spectra of these cations exhibit remarkable upfield shift for the tropylium ring protons in accord with such molecular geometry; this geometry is further supported by theoretical calculations both by MMP2 and by AM1.The cations 8a-d show the intramolecular charge-transfer (CT) band at 510 (sh) (8a), 525 (sh) (8b), 585 (8c), and 600 (sh) nm (8d) in dichloromethane, indiacting a correlation of the CT transition energy with the donor's ionzation potential regardless of the relative geometry with the acceptor.In contrast, the indicies of cation's thermodynamic stability, pK_R+ (6.27 for 8a, 5.45 for 8b, 6.28 for 8c, and 5.75 for 8d), and reduction potential (-0.649 V vs Ag/Ag⁺ for 8a, -0.601 for 8b, -0.665 for 8c, and -0.625 V for 8d) are highly dependent on the mutual geometry of donor and acceptor, reflecting the effectiveness in overlap of donor's HOMO and acceptor's LUMO.