Dynamics and equilibrium of heme axial ligation in mesoporous nanocrystalline TiO(2) thin films.

Comparative studies of axial CO and solvent coordination to iron(II) protoporphyrin IX (FeIIPPIX) anchored to the surface of mesoporous nanocrystalline (anatase) TiO2 thin films (FeIIPPIX/TiO2) immersed in dimethyl sulfoxide (DMSO), pyridine (py), and methanol (MeOH) and to FeIIPPIX in fluid DMSO and py solution are reported. The equilibrium constants, KeqCOS, for CO coordination to FeIIPPIX/TiO2 immersed in py (2.4 x 10(3) M-1) < DMSO (6 x 10(4) M-1) < MeOH (2.3 x 10(5) M-1) were quantified. The corresponding values in fluid py or DMSO solution were 2 times larger (4.5 x 10(3) and 1 x 10(5) M-1, respectively). The observed ligand exchange rates (kobs) measured after pulsed 532 nm laser excitation (5-6 ns fwhm, 1-3 mJ/pulse) of (S)(CO)FeIIPPIX/TiO2, where S is solvent, in saturated CO solutions were measured: py (2.2 s-1), DMSO (460 s-1), MeOH (2.09 x 10(5) s-1). The corresponding values in fluid solution were 2.0 s-1 (py) and 230 s-1 (DMSO). The observed ligand exchange rate varied linearly with [CO], and second-order rate constants were determined for FeIIPPIX/TiO2 immersed in DMSO (3.1 x 10(5) M-1 s-1) and MeOH (1.5 x 10(7) M-1 s-1). The observed rate for CO addition to (py)2FeIIPPIX/TiO2 immersed in py did not vary linearly with [CO]. The relevance of the measured kinetics and thermodynamics to a dissociative mechanism for ligand exchange is discussed.

Theoretical solar-to-electrical energy-conversion efficiencies of perylene-porphyrin light-harvesting arrays.

The efficiencies of organic solar cells that incorporate light-harvesting arrays of organic pigments were calculated under 1 sun of air mass 1.5 solar irradiation. In one set of calculations, photocurrent efficiencies were evaluated for porphyrin, phthalocyanine, chlorin, bacteriochlorin, and porphyrin-bis(perylene) pigment arrays of different length and packing densities under the assumption that each solar photon absorbed quantitatively yielded one electron in the external circuit. In another more realistic set of calculations, solar conversion efficiencies were evaluated for arrays comprising porphyrins or porphyrin-(perylene)2 units taking into account competitive excited-state relaxation pathways. A system of coupled differential equations for all reactions in the arrays was solved on the basis of previously published rate constants for (1) energy transfer between the perylene and porphyrin pigments, (2) excited-state relaxation of the perylene and porphyrin pigments, and (3) excited-state electron injection into the semiconductor. This formal analysis enables determination of the optimal number of pigments in an array for solar-to-electrical energy conversion. The optimal number of pigments depends on the molar absorption coefficient and the density at which the arrays can be packed on an electrode surface. Taken together, the ability to employ fundamental photophysical, kinetic, and structural parameters of modular molecular architectures in assessments of the efficiency of solar-to-electrical energy conversion should facilitate the design of molecular-based solar cells.

Multielectron transfer at heme-functionalized nanocrystalline TiO2: reductive dechlorination of DDT and CCl4 forms stable carbene compounds.

Hemin (chloro(protoporhyrinato)iron(III)) was found to bind to mesoporous nanocrystalline (anatase) TiO2 thin films from dimethyl sulfoxide solution, Keq=10(5) M-1 at 298 K. Band gap illumination in methanol reduced hemin to heme and led to the appearance of TiO2 electrons, heme/TiO2(e-). Reactions of heme/TiO2(e-) with CCl4 or 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) led to the formation of stable carbene products in greater than 60% yield. The spectroscopic data are fully consistent with a dissociative two-electron organohalide reduction mechanism of CCl4 and DDT to yield (protoporhyrinato)FeIICCl2 and (protoporhyrinato)FeIIC=C(p-Cl-phenyl)2 respectively.