Ludovic Troian-Gautier

Optical Intramolecular Electron Transfer in Opposite Directions through the Same Bridge That Follows Different Pathways

The electrochemical and spectroscopic properties of eight bis(tridentate) cyclometalated RuII compounds covalently linked by a phenyl- or xylyl-thiophene bridge to a pendant triphenylamine (TPA) were characterized in fluid solution and immobilized on metal oxide surfaces. Upon surface immobilization, the TPA+/0 reduction potentials of the phenyl-bridged compounds exhibited large changes, ±100 mV, relative to solution-based values, yet those observed for the xylyl-bridged compounds were relatively unchanged. The highest occupied molecular orbital of the surface-immobilized compounds was associated with either TPA or RuII, enabling the study of the electron transfer in opposite directions. Electron transfer in the mixed-valent states of the compounds was found to proceed by different optical pathways for RuII → TPA+ relative to TPA → RuIII. Mulliken-Hush analysis of intervalence charge transfer bands for the phenyl-bridged compounds revealed that the electronic coupling matrix element, HDA, was ∼950 cm-1 for RuII → TPA+, while HDA for TPA → RuIII appeared to be 2500 cm-1. In contrast, the xylyl-bridged compounds were weakly coupled. A superexchange analysis, where unoccupied bridge orbitals were taken directly into account, led to a very different conclusion: HDA did not depend on the charge-transfer direction or path. The results imply that the electron-transfer direction can alter optical charge transfer pathways without influencing the electronic coupling.

Kinetics teach that electronic coupling lowers the free-energy change that accompanies electron transfer

Significance Nature’s use of electronic coupling (HDA) and free-energy (ΔGo) gradients to vectorially control electron transport provides inspiration for artificial photosynthesis. Theoretical predictions indicate that HDA and ΔGo are not independent parameters, and are instead linked. Reported here is a broadly applicable kinetic approach that was utilized to demonstrate such behavior for four acceptor–bridge–donor compounds. When the electronic coupling was large and electron transfer was adiabatic, the free energy of the reaction |ΔGo| was less than that for nonadiabatic transfer. This finding should be taken into account in the design of hybrid materials for solar energy conversion and has broad implications to the many classes of electron-transfer reactions in biology and chemistry. Electron-transfer theories predict that an increase in the quantum-mechanical mixing (HDA) of electron donor and acceptor wavefunctions at the instant of electron transfer drives equilibrium constants toward unity. Kinetic and equilibrium studies of four acceptor–bridge–donor (A-B-D) compounds reported herein provide experimental validation of this prediction. The compounds have two redox-active groups that differ only by the orientation of the aromatic bridge: a phenyl–thiophene bridge (p) that supports strong electronic coupling of HDA > 1,000 cm−1; and a xylyl–thiophene bridge (x) that prevents planarization and decreases HDA < 100 cm−1 without a significant change in distance. Pulsed-light excitation allowed kinetic determination of the equilibrium constant, Keq. In agreement with theory, Keq(p) were closer to unity compared to Keq(x). A van’t Hoff analysis provided clear evidence of an adiabatic electron-transfer pathway for p-series and a nonadiabatic pathway for x-series. Collectively, the data show that the absolute magnitude of the thermodynamic driving force for electron transfers are decreased when adiabatic pathways are operative, a finding that should be taken into account in the design of hybrid materials for solar energy conversion.

Completing a Charge Transport Chain for Artificial Photosynthesis

A ruthenium polypyridyl chromophore with electronically isolated triarylamine substituents has been synthesized that models the role of tyrosine in the electron transport chain in photosystem II. When bound to the surface of a TiO2 electrode, electron injection from a Ru(II) Metal-to-Ligand Charge Transfer (MLCT) excited state occurs from the complex to the electrode to give Ru(III). Subsequent rapid electron transfer from the pendant triarylamine to Ru(III) occurs with an observed rate constant of ∼1010 s-1, which is limited by the rate of electron injection into the semiconductor. Transfer of the oxidative equivalent away from the semiconductor surface results in dramatically reduced rates of back electron transfer, and a long-lived (τ = ∼165 μs) triarylamine radical cation that has been used to oxidize hydroquinone to quinone in solution.

Ligand Control of Supramolecular Chloride Photorelease

Supramolecular assembly is shown to provide control over excited-state chloride release. Two dicationic chromophores were designed with a ligand that recognizes halide ions in CH2Cl2 and a luminescent excited state whose dipole was directed toward, 12+, or away, 22+, from an associated chloride ion. The dipole orientation had little influence on the ground-state equilibrium constant, Keq ∼ 4 × 106 M-1, but induced a profound change in the excited-state equilibrium. Light excitation of [12+,Cl-]+ resulted in time-dependent shifts in the photoluminescence spectra with the appearance of biexponential kinetics consistent with the photorelease of Cl-. Remarkably, the excited-state equilibrium constant was lowered by a factor of 20 and resulted in nearly 45% dissociation of chloride. In contrast, light excitation of [22+,Cl-]+ revealed a 45-fold increase in the excited-state equilibrium constant. The data show that rational design and supramolecular assembly enables the detection and photorelease of chloride ions with the potential for future applications in biology and chemistry.