Effect of Subphase pH and Metal Ion on the Molecular Aggregates of Amphiphilic Ru Complexes Containing 2,2‘:6‘,2‘ ‘-Terpyridine-4‘-phosphonic Acid at the Air−Water Interface

Abstract

A novel amphiphilic Ru(II) complex, Ru(L18)(tpy-PO 3 H)PF 6, where L18 = 2,6-bis( N -octadecylbenzimidazol-2-yl)pyridine and tpy-PO 3 H = 2,2‘;6‘,2‘ ‘-terpyridine-4‘-phosphonic acid, was synthesized. The complex, Ru(L18)(tpy-PO 3 H)PF 6, at the air−water interface exhibits a molecular aggregation, which is strongly affected by the solution pH or metal coordination. Surface pressure−area isotherms of Ru(L18)(tpy-PO 3 H)PF 6 show a smaller molecular occupied area at lower pH. Furthermore, in situ UV−vis absorption spectra of the Ru complex at the air−water interface also depend on the subphase pH, which is distinct from the absorption spectra for CH 3 CN/buffer or chloroform solution. As the subphase becomes acidic, the absorption band at 363 nm for the intraligand π−π* transitions is sharpened with a little red shift, which strongly reveals the formation of molecular aggregates at the air−water interface. The monolayers formed on different subphases can be successfully transferred onto hydrophilic and hydrophobic glass substrates. Low-angle X-ray diffractions and UV−vis and X-ray photoelectron spectroscopy measurements indicate a regular layered structure in the Langmuir−Blodgett (LB) films transferred on the solid substrate. A tape-shaped morphology from the atomic force microscopy measurement was observed for the LB monolayer on mica. When the subphase containing metal sulfate MSO 4 (M = Zn, Cd or Mn) is used, metal coordination at the air−water interface was observed. The X-ray photoelectron spectroscopy measurement demonstrates a 1:1 ratio of the Ru complex/metal, M (M = Zn, Cd, and Mn), for the LB composite films, which is isostructural with well-characterized metal phosphonate layered solids such as Mn(O 3 PC 6 H 5 )H 2 O. The Ru(II/III) oxidative electrochemical response was observed at +0.92 V vs SCE for the monolayer LB film on the indium−tin oxide electrode, which is not varied on the metal coordination to the phosphonate group. Therefore, Ru(L18)(tpy-PO 3 H)PF 6 is a good template at the air−water interface for the synthesis of redox-active layered inorganic composites.