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)center dot CH3CN center dot H2O was determined by single-crystal X-ray diffraction at 110 K to be triclinic, P-1, a = 11.6873(18), b = 12.2318(12), c = 14.723(4) angstrom, alpha = 89.864(13)degrees, beta = 71.430(17)degrees, gamma = 73.788(11)degrees, V = 1907.1(6) angstrom(3), Z = 2, and R = 0.0491. The iron compound has a meridional 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 ((1)A) reversible arrow low-spin (T-5) equilibrium. Thermodynamic parameters for the temperature-dependent spin equilibrium of Fe(pymbA)(3)(2+) in methanol were determined to be Delta H-HL = 3270 +/- 210 cm(-1) and Delta S-HL = 13.3 +/- 0.8 cm(-1) K-1. The corresponding values for Fe(pybzimEE)(3)(2+), where pyridin EE is (2-pyridin-2-yl-benzimidazol-1-yl)-acetic acid ethyl ester, in acetonitrile solution were determined to be 3072 +/- 34 cm(-1) and 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 T-5 -> (1)A rate constant yielded an activation energy, E-a, 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 the MLCT absorption bands. Relaxation was nonexponential but was well described by kinetic models based oil a Gaussian distribution of activation energies or a Levy 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 Levy distribution analysis did not adequately fit the Arrhenius model.