title: Synthesis and photochemical properties of a photochromic iron(II) complex of hexaarylbiimidazole.
abstract: The photochromic ligand bis(terpyridyl)hexaarylbiimidazole (bistpy-HABI) and the Fe(II) complex of bistpy-HABI with formula [{Fe(tpy)}2.bistpy-HABI](PF6)4.4H2O were synthesized and characterized. Bistpy-HABI is readily cleaved into a pair of terpyridyltriphenylimidazolyl radicals (tpy-TPI*) on irradiation with UV light. This photochemical reaction is completely reversible, and the light-induced radicals can thermally recombine to form bistpy-HABI in the dark. [{Fe(tpy)}2.bistpy-HABI]4+ is the first example of a transition-metal complex of an HABI derivative and was found to show photochromic reaction in solution. The spin state of the light-induced radical pair in a frozen matrix was investigated by ESR spectroscopy. The triplet state of the light-induced radical pair from [{Fe(tpy)}2.bistpy-HABI]4+, as well as that from bistpy-HABI, was confirmed to be a ground state or nearly degenerated with a singlet state. Kinetic studies on the radical recombination reaction in solution elucidated the decrease in the activation energy by forming the Fe(II) complex. This is the first observation of a decrease in the activation energy of the radical recombination reaction by the formation of a metal-coordinated radical complex. The syntheses, photochemical properties, and spin states of bistpy-HABI and [{Fe(tpy)}2.bistpy-HABI](PF6)4 are discussed.
authors.affiliation: Department of Chemistry and The 21st Century COE Program, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Miyamoto
authors.forename: Azusa
authors.initials: A
authors.lastname: Kikuchi
authors.forename: Fumiyasu
authors.initials: F
authors.lastname: Iwahori
authors.forename: Jiro
authors.initials: J
authors.lastname: Abe
mesh_terms: D005296:Ferrous Compounds / Q000138:chemical synthesis* / Q000737:chemistry / Q000528:radiation effects*; D007093:Imidazoles / Q000737:chemistry* / Q000528:radiation effects*; D008024:Ligands; D015394:Molecular Structure; D010777:Photochemistry; D013696:Temperature; D014466:Ultraviolet Rays
chemical_list: D005296:Ferrous Compounds; D007093:Imidazoles; D008024:Ligands