Photodriven spin change of Fe(II) benzimidazole compounds anchored to nanocrystalline TiO2 thin films

Hai Long Xia, Shane Ardo, Amy A.Narducci Sarjeant, Sunxiang Huang, Gerald J. Meyer

Research output: Contribution to journalArticle

20 Scopus citations

Abstract

Ferrous tris-chelate compounds based on 2-(2'-pyridyl)benzimidazole (pybzim) have been prepared and characterized for studies of spin equilibria in fluid solution and when anchored to the surface of mesoporous nanocrystalline (anatase) TiO2 and colloidal ZrO2 thin films. The solid state structure of Fe(pybzim)3(ClO4)2 CH 3CN.H2O was determined by singlecrystalX-ray diffraction at 110 K to be triclinic, P-1, a=11.6873(18), b=12.2318(12), c=14.723(4) A°, R=89.864(13)°, β= 71.430(17)°,.=73.788(11)°, V=1907.1(6) Å 3, Z=2, and R=0.0491. The iron compound has ameridional FeN6 distorted octahedral geometry with bond lengths expected for a low-spin iron center at 110 K. The visible absorption spectra of Fe(pybzim) 3 2+ and Fe(pymbA)3 2+, where pymbA is 4-(2-pyridin-2-yl-benzimidazol-1-ylmethyl)-benzoic acid, in methanol solution were dominated by metal-to-ligand charge-transfer (MLCT) bands. Variable-temperature UV-visible absorption spectroscopy revealed dramatic changes in the extinction coefficient consistent with a high-spin ( 1A) low-spin (5T) equilibrium. Thermodynamic parameters for the temperature-dependent spin equilibrium of Fe(pymbA)3 2+ in methanol were determined to be.HHL = 3270 ± 210 cm -1 and.SHL = 13.3 ± 0.8 cm-1 K-1. The corresponding values for Fe(pybzimEE)3 2+, where pybzimEE is (2-pyridin-2-yl-benzimidazol-1-yl)-acetic acid ethyl ester, in acetonitrile solution were determined to be 3072 ± 34 cm-1and 10.5 ± 0.1 cm-1 K-1. The temperature-dependent effective magnetic moments of Fe(pybzimEE)3 2+ in acetonitrile solution were also quantified by the Evans method. Pulsed 532 nm light excitation of Fe(pybzim)3 2+ or Fe(pymbA)3 2+ in solution resulted in an immediate bleach of the MLCT absorption bands. Relaxation back to the equilibrium state followed a first-order reaction mechanism. Arrhenius analysis of the 5T → 1A rate constant yielded an activation energy, Ea, of 1090 ± 20 cm-1 and 710 ± 10 cm-1 for Fe(pybzim)3 2+ and Fe(pymbA)3 2+ in methanol, respectively. The compound Fe(pymbA)3 2+ was found to bind to colloidal TiO2 and ZrO2 thin films. The absorption spectra of the surface-attached compounds were quantified from 295 to 193 K. Pulsed light excitation of Fe(pymbA)3/TiO2 and Fe(pymbA) 3/ZrO2 resulted in the immediate bleach of theMLCTabsorption bands. Relaxationwas nonexponential but was well described by kinetic models based on a Gaussian distribution of activation energies or Lévy distribution of lifetimes.An Arrhenius analysis of the Gaussian data yielded average activation energies of 660 ± 80 cm-1 and 730 ± 40 cm-1 for Fe(pymbA)3(ClO4) 2 on TiO2 and ZrO2 surfaces, respectively. The Lévy distribution analysis did not adequately fit the Arrhenius model.

Original languageEnglish (US)
Pages (from-to)13641-13652
Number of pages12
JournalLangmuir
Volume25
Issue number23
DOIs
StatePublished - Dec 1 2009
Externally publishedYes

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Spectroscopy
  • Electrochemistry

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