Photodriven Spin Change of Fe(II) Benzimidazole Compounds Anchored to Nanocrystalline TiO₂ Thin Films

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) TiO(2) and colloidal ZrO(2) thin films. The solid state structure of Fe(pybzim)(3)(ClO(4))(2).CH(3)CN.H(2)O 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) A, alpha = 89.864(13) degrees , beta = 71.430(17) degrees , gamma = 73.788(11) degrees , V = 1907.1(6) A(3), Z = 2, and R = 0.0491. The iron compound has a meridional FeN(6) 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) left harpoon over right harpoon low-spin ((5)T) equilibrium. Thermodynamic parameters for the temperature-dependent spin equilibrium of Fe(pymbA)(3)(2+) in methanol were determined to be DeltaH(HL) = 3270 +/- 210 cm(-1) and DeltaS(HL) = 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(-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 (5)T --> (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 TiO(2) and ZrO(2) thin films. The absorption spectra of the surface-attached compounds were quantified from 295 to 193 K. Pulsed light excitation of Fe(pymbA)(3)/TiO(2) and Fe(pymbA)(3)/ZrO(2) resulted in the immediate bleach of the MLCT absorption bands. Relaxation was nonexponential but was well described by kinetic models based on 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)(ClO(4))(2) on TiO(2) and ZrO(2) surfaces, respectively. The Levy distribution analysis did not adequately fit the Arrhenius model.

The reactivity of macrocyclic Fe(II) paraCEST MRI contrast agents towards biologically relevant anions, cations, oxygen or peroxide

“…The high spin [Fe(L2)] 2+ complex has a corresponding peak that is tentatively assigned as a metal-to-ligand charge-transfer (MLCT) band at λ = 368 nm (ε = 1030 M − 1 cm − 1 ). The low extinction coefficient of the MLCT band for high spin [Fe(L2)] 2+ relative to the low spin analog [Fe(L3)] 2+ is consistent with previous observations of the effect of spin state on the MLCT band [31]. These UV-vis peaks were used to further study dissociation of the Fe(II) complexes in the presence of Cu(NO 3 ) 2 .…”

On the interplay between charge, spin and structural dynamics in transition metal complexes

“…In addition, for a large class of metal complexes, the initial trigger is a charge transfer process. Although there has been an increase in experimental [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] and theoretical [33][34][35][36][37][38][39][40] studies aimed at describing the interplay between the charge, spin and structural dynamics, many aspects are still unclear to this day. Furthermore, the origin and the parameters that govern the spin dynamics in metal complexes represent a wholly unexplored field.…”

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films

“…The success of ruthenium-based sensitizers and observation of ultrafast injection has motivated its replacement by iron. Due to their very short MLCT excited-state lifetimes, Fe II diimine complexes typically display small injection yields with spectroscopic features characteristic of high spin ligand field states; injection is kinetically slow relative to intersystem crossing and/or internal conversion. − A recent breakthrough was the discovery that N-heterocyclic Fe II complexes provide greatly enhanced injection yields, φ inj > 0.9. − This exciting advance appears to emanate from much longer-lived MLCT excited states and may soon enable efficient energy conversion with a first row transition metal sensitizer. Excited-state injection yields near unity have also been reported for Co I complexes, including Vitamin B 12 .…”