Dynamics and Equilibrium of Heme Axial Ligation in Mesoporous Nanocrystalline TiO₂ Thin Films

Abstract

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.

Ligand Coordination and Spin Crossover in a Nickel Porphyrin Anchored to Mesoporous TiO2 Thin Films

“…13 On the other hand, hemes and iron polypyridyl compounds generally display very low quantum yields for excited-state injection, if they display any at all. 14,15 A spin change also accompanies ligand, L, coordination to nickel(II) porphyrins (Scheme 1). 16 At high temperatures, the dominant species is low-spin 4-coordinate NiP, while at lower temperatures, high-spin 6-coordinate NiP with two axial ligands is dominant.…”

“…Recent studies of an isoelectronic (d 8 ) Co I porphyrin demonstrated high excited-state injection yields while free base and Zn II porphyrins are known to rapidly inject electrons into TiO 2 from their singlet excited states, suggesting that the same could be true for Ni porphyrins . On the other hand, hemes and iron polypyridyl compounds generally display very low quantum yields for excited-state injection, if they display any at all. , …”

A Quantitative Description of the Binding Equilibria of para-Substituted Aniline Ligands and CdSe Quantum Dots

“…Organic ligands serve many functions with respect to QDs: they prevent aggregation of the QDs, control their dispersibility, and donate electron density to or accept electron density from incompletely coordinated metal ions on their surfaces. [1][2][3][4][5][6][7][8][9] When unpassivated, these defect sites trap excitonic charge carriers and thereby provide nonradiative decay pathways for the excitonic state of the QD. 5,[10][11][12] The quantum yield of photoluminescence (PL) of an ensemble of QDs, therefore, depends on the degree of coverage of the surface by organic ligands and on the electronic properties of the ligands.…”

“…This paper describes the use of 1 H NMR spectroscopy to measure equilibrium constants for the solution-phase binding of two para-substituted aniline molecules (R-An), p -methoxyaniline (MeO-An) and p -bromoaniline (Br-An), to colloidal CdSe quantum dots (QDs). Organic ligands serve many functions with respect to QDs: they prevent aggregation of the QDs, control their dispersibility, and donate electron density to or accept electron density from incompletely coordinated metal ions on their surfaces. − When unpassivated, these defect sites trap excitonic charge carriers and thereby provide nonradiative decay pathways for the excitonic state of the QD. ,− The quantum yield of photoluminescence (PL) of an ensemble of QDs, therefore, depends on the degree of coverage of the surface by organic ligands and on the electronic properties of the ligands. By controlling the relative populations of certain ligands on the surfaces of QDsfor instance, PL-quenching aromatic amines, such as the anilines studied here, and PL-enhancing alkylamineswe can determine the decay pathways available to the excitonic state and thereby engineer the optical and electronic properties of the QD.…”

Excited state electron transfer after visible light absorption by the Co(i) state of vitamin B12

“…We note that the pp* excited states of other open-shell macrocycles, such as hemes, do not undergo excited state electron transfer reactions. 27 The excited state reduction potential of cob(II/I*)alamin can be estimated from the measured Co(II/I) reduction potential and the free energy stored in the excited state. 28 The E 0 (Co II/I ) was measured by spectroelectrochemistry, Table 1.…”