title: Dynamic properties of molecular tweezers with a bis(2-hydroxyphenyl)pyrimidine backbone.
abstract: 4,6-Bis(2-hydroxyphenyl)-2-alkylpyrimidines with two anthryl or 9-ethylnylanthryl substituents at the positions para to the OH groups prefer a U-shaped conformation supported by two intramolecular OH⋅⋅⋅N hydrogen bonds in the solid state and in CDCl3 solution. The compound with a hexyl substituent on the pyrimidine group and two 9-ethynylanthryl arms at the hydroxyphenyl groups forms a 1:1 complex with 2,4,7-trinitrofluorenone. Its association constant K(a) was estimated to be 2100 M(-1) at 298 K, which is larger than those of other molecular tweezers (K(a) < 1000 M(-1)). DFT calculations suggested that the complex adopts a stable conformation supported by intramolecular hydrogen bonds among the OH groups and the pyrimidine ring as well as by intermolecular π-π interaction between the anthryl groups and 2,4,7-trinitrofluorenone. Addition of nBu4NF to a solution of the molecular tweezers or their complexes causes the cleavage of one or two OH⋅⋅⋅N hydrogen bonds, formation of new O⋅⋅⋅HF hydrogen bonds, and changes in the molecular conformation. The resulting structure of the molecular tweezers contains nonparallel anthryl groups, which do not bind the guest molecule. Photochemical measurements on 4,6-bis(2-hydroxyphenyl)-2-methylpyrimidine with two anthryl substituents showed negligible luminescence (quantum yield ϕ<0.01), owing to photoinduced electron transfer of the molecule with a U-shaped structure. However, the O-hexylated compound exhibits emission from the anthryl groups with ϕ=0.39.
authors.affiliation: Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 R2-3, Nagastuta, Midori-ku, Yokohama 226-8503 (Japan), Fax: (+81) 45-924-5224.
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Tsuchido
authors.forename: Yuji
authors.initials: Y
authors.lastname: Suzaki
authors.forename: Tomohito
authors.initials: T
authors.lastname: Ide
authors.forename: Kohtaro
authors.initials: K
authors.lastname: Osakada
mesh_terms: D018360:Crystallography, X-Ray; D005449:Fluorenes / Q000737:chemistry; D006860:Hydrogen Bonding; D008968:Molecular Conformation; D011743:Pyrimidines / Q000737:chemistry*
chemical_list: D005449:Fluorenes; D011743:Pyrimidines; C005644:2,4,7-trinitrofluorenone; C030986:pyrimidine
keywords: host-guest systems; hydrogen bonds; molecular tweezers; pi interactions; supramolecular chemistry