Tuning Electron Delocalization and Transfer Rates in Mixed-Valent Ru3O Complexes through “Push–Pull” Effects

Solvatochromism and solution π-stacking of N-(4-pyridyl)-1,8-naphthalimide and its corresponding triruthenium coordination complex

Due to the scarcity of spectroscopic studies on metal-coordinated naphthalimides, and aiming to investigate fundamental spectroscopic aspects, we have described here the aggregates of N-(4-pyridyl)-1,8-naphthalimide (NI-py) in solution as well as solvatochromism displayed by it and by the coordination compounds [Ru₃O(CH₃COO)₆(NI-py)₃]ⁿ, n = +1 or 0. Based both on theoretical calculations and luminescence spectra, we demonstrated that in aqueous media, the NI-py π-stacking is thermodynamically favored, suggesting a preferable conformation where the pyridine and naphthalene moieties of two NI-py molecules are parallel to each other, but are not co-planar within an individual molecule, due to steric hindrance. The NI-py ππ* band displayed positive solvatochromism, to which the major contribution was the Catalan's SP parameter (solvent polarizability). This observation is fully consistent with the extended π-electron cloud of the NI-py naphthalene ring. However, a secondary contribution of the SA (solvent acidity) was also observed, owing to the electron pairs available at the N-heteroatom of the pyridine rings and at the carbonyl-group oxygen atoms. Finally, the multiparametric solvent effect analysis indicated that the electronic coupling between coordinated NI-py and the metallic core is modulated by the charge of the [Ru₃O(CH₃COO)₆] core, being higher for the reduced species [Ru₃O(CH₃COO)₆(NI-py)]⁰. In addition, in this reduced species, there is no overlap between NI-py ππ* and the [Ru₃O(CH₃COO)₆] charge transfer (CT) transitions, leading to the observation of the dependence of the CT energy with the SdP parameter (solvent dipolarity) since the CT transition implies in a charge-separation state.

Improving Photocatalysis for the Reduction of CO2 through Non-covalent Supramolecular Assembly

We report the enhancement of photocatalytic performance by introduction of hydrogen-bonding interactions to a Re bipyridine catalyst and Ru photosensitizer system (ReDAC/RuDAC) by the addition of amide substituents, with carbon monoxide (CO) and carbonate/bicarbonate as products. This system demonstrates a more-than-3-fold increase in turnover number (TON_CO = 100 ± 4) and quantum yield (Φ_CO = 23.3 ± 0.8%) for CO formation compared to the control system using unsubstituted Ru photosensitizer (RuBPY) and ReDAC (TON_CO = 28 ± 4 and Φ_CO = 7 ± 1%) in acetonitrile (MeCN) with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial reductant. In dimethylformamide (DMF), a solvent that disrupts hydrogen bonds, the ReDAC/RuDAC system showed a decrease in catalytic performance while the control system exhibited an increase, indicating the role of hydrogen bonding in enhancing the photocatalysis for CO₂ reduction through supramolecular assembly. The similar properties of RuDAC and RuBPY demonstrated in lifetime measurements, spectroscopic analysis, and electrochemical and spectroelectrochemical studies revealed that the enhancement in photocatalysis is due not to differences in intrinsic properties of the catalyst or photosensitizer, but to hydrogen-bonding interactions between them.

Photooxidative Generation of Dodecaborate-Based Weakly Coordinating Anions

Redox-active proanions of the type B₁₂(OCH₂Ar)₁₂ [Ar = C₆F₅ (1), 4-CF₃C₆H₄ (2), 3,5-(CF₃)₂C₆H₃ (3)] are introduced in the context of an experimental and computational study of the visible-light-initiated polymerization of a family of styrenes. Neutral, air-stable proanions 1-3 were found to initiate styrene polymerization through single-electron oxidation under blue-light irradiation, resulting in polymers with number-average molecular weights (M_n) ranging from ∼6 to 100 kDa. Shorter polymer products were observed in the majority of experiments, except in the case of monomers containing 4-X (X = F, Cl, Br) substituents on the styrene monomer when polymerized in the presence of 1 in CH₂Cl₂. Only under these specific conditions are longer polymers (>100 kDa) observed, strongly supporting the formulation that reaction conditions significantly modulate the degree of ion pairing between the dodecaborate anion and cationic chain end. This also suggests that 1-3 behave as weakly coordinating anions (WCA) upon one-electron reduction because no incorporation of the cluster-based photoinitiators is observed in the polymeric products analyzed. Overall, this work is a conceptual realization of a single reagent that can serve as a strong photooxidant, subsequently forming a WCA.

Dodecaborane-Based Dopants Designed to Shield Anion Electrostatics Lead to Increased Carrier Mobility in a Doped Conjugated Polymer

One of the most effective ways to tune the electronic properties of conjugated polymers is to dope them with small-molecule oxidizing agents, creating holes on the polymer and molecular anions. Undesirably, strong electrostatic attraction from the anions of most dopants localizes the holes created on the polymer, reducing their mobility. Here, a new strategy utilizing a substituted boron cluster as a molecular dopant for conjugated polymers is employed. By designing the cluster to have a high redox potential and steric protection of the core-localized electron density, highly delocalized polarons with mobilities equivalent to films doped with no anions present are obtained. AC Hall effect measurements show that P3HT films doped with these boron clusters have conductivities and polaron mobilities roughly an order of magnitude higher than films doped with F₄ TCNQ, even though the boron-cluster-doped films have poor crystallinity. Moreover, the number of free carriers approximately matches the number of boron clusters, yielding a doping efficiency of ≈100%. These results suggest that shielding the polaron from the anion is a critically important aspect for producing high carrier mobility, and that the high polymer crystallinity required with dopants such as F₄ TCNQ is primarily to keep the counterions far from the polymer backbone.

Effects of electron transfer on the stability of hydrogen bonds

The measurement of the dimerization constants of hydrogen-bonded ruthenium complexes (1₂, 2₂, 3₂) linked by a self-complementary pair of 4-pyridylcarboxylic acid ligands in different redox states is reported.

The measurement of the dimerization constants of hydrogen-bonded ruthenium complexes (1₂, 2₂, 3₂) linked by a self-complementary pair of 4-pyridylcarboxylic acid ligands in different redox states is reported. Using a combination of FTIR and UV/vis/NIR spectroscopies, the dimerization constants (K_D) of the isovalent, neutral states, 1₂, 2₂, 3₂, were found to range from 75 to 130 M^–1 (ΔG⁰ = –2.56 to –2.88 kcal mol^–1), while the dimerization constants (K_2–) of the isovalent, doubly-reduced states, (1₂)^2–, (2₂)^2–, (3₂)^2–, were found to range from 2000 to 2500 M^–1 (ΔG⁰ = –4.5 to –4.63 kcal mol^–1). From the aforementioned values and the comproportionation constant for the mixed-valent dimers, the dimerization constants (K_MV) of the mixed-valent, hydrogen-bonded dimers, (1₂)^–, (2₂)^–, (3₂)^–, were found to range from 0.5 × 10⁶ to 1.2 × 10⁶ M^–1 (ΔG⁰ = –7.78 to –8.31 kcal mol^–1). On average, the hydrogen-bonded, mixed-valent states are stabilized by –5.27 (0.04) kcal mol^–1 relative to the isovalent, neutral, hydrogen-bonded dimers and –3.47 (0.06) kcal mol^–1 relative to the isovalent, dianionic hydrogen bonded dimers. Electron exchange in the mixed valence states imparts significant stability to hydrogen bonding. This is the first quantitative measurement of the strength of hydrogen bonds in the presence and absence of electronic exchange.

Correlating Bridging Ligand with Properties of Ligand-Templated [MnII3X3]3+ Clusters (X = Br-, Cl-, H-, MeO-)

Polynuclear manganese compounds have garnered interest as mimics and models of the water oxidizing complex (WOC) in photosystem II and as single molecule magnets. Molecular systems in which composition can be correlated to physical phenomena, such as magnetic exchange interactions, remain few primarily because of synthetic limitations. Here, we report the synthesis of a family of trimanganese(II) complexes of the type Mn₃X₃L (X = Cl^-, H^-, and MeO^-) where L^3- is a tris(β-diketiminate) cyclophane. The tri(chloride) complex (2) is structurally similar to the reported tri(bromide) complex (1) with the Mn₃X₃ core having a ladder-like arrangement of alternating M-X rungs, whereas the tri(μ-hydride) (3) and tri(μ-methoxide) (4) complexes contain planar hexagonal cores. The hydride and methoxide complexes are synthesized in good yield (48% and 56%) starting with the bromide complex employing a metathesis-like strategy. Compounds 2-4 were characterized by combustion analysis, X-ray crystallography, X-band EPR spectroscopy, SQUID magnetometry, and infrared and UV-visible spectroscopy. Magnetic susceptibility measurements indicate that the Mn₃ clusters in 2-4 are antiferromagnetically coupled, and the spin ground state of the compounds (S = 3/2 (1, 2) or S = 1/2 (3, 4)) is correlated to the identity of the bridging ligand and structural arrangement of the Mn₃X₃ core (X = Br, Cl, H, OCH₃). Electrochemical experiments on isobutyronitrile solutions of 3 and 4 display broad irreversible oxidations centered at 0.30 V.