Photochemical Generation of Strong One-Electron Reductants via Light-Induced Electron Transfer with Reversible Donors Followed by Cross Reaction with Sacrificial Donors

This work illustrates a modified approach for employing photoinduced electron transfer reactions coupled to secondary irreversible electron transfer processes for the generation of strongly reducing equivalents in solution. Through irradiation of [Ru­(LL)₃]²⁺ (LL= diimine ligands) with tritolylamine (TTA) as quencher and various alkyl amines as sacrificial electron donors, yields in excess of 50% can be achieved for generation of reductants with E⁰(2+/1+) values between −1.0 and −1.2 V vs NHE. The key to the system is the fact that the TTA cation radical, formed in high yield in reaction with the photoexcited [Ru­(LL)₃]²⁺ complex, reacts irreversibly with various sacrificial electron donating amines <i>that are kinetically unable to directly react with the photoexcited complex</i>. The electron transfer between the TTA⁺ and the sacrificial amine is an energetically uphill process. Kinetic analysis of these parallel competing reactions, consisting of bimolecular and pseudo first-order reactions, allows determination of electron transfer rate constants for the cross electron transfer reaction between the sacrificial donor and the TTA⁺. A variety of amines were examined as potential sacrificial electron donors, and it was found that tertiary 1,2-diamines are most efficient among these amines for trapping the intermediate TTA⁺. This electron-donating combination is capable of supplying a persistent reducing flux of electrons to catalysts used for hydrogen production

Platinum Chromophore-Based Systems for Photoinduced Charge Separation: A Molecular Design Approach for Artificial Photosynthesis

Photoinduced charge separation is a fundamental step in photochemical energy conversion. In the design of molecularly based systems for light-to-chemical energy conversion, this step is studied through the construction of two- and three-component systems (dyads and triads) having suitable electron donor and acceptor moieties placed at specific positions on a charge-transfer chromophore. The most extensively studied chromophores in this regard are ruthenium(II) tris(diimine) systems with a common 3MLCT excited state, as well as related ruthenium(II) bis(terpyridyl) systems. This Forum contribution focuses on dyads and triads of an alternative chromophore, namely, platinum(II) di- and triimine systems having acetylide ligands. These d8 chromophores all possess a 3MLCT excited state in which the lowest unoccupied molecular orbital is a π* orbital on the heterocyclic aromatic ligand. The excited-state energies of these Pt(II) chromophores are generally higher than those found for the ruthenium(II) tris(diimine) systems, and the directionality of the charge transfer is more certain. The first platinum diimine bis(arylacetylide) triad, constructed by attaching phenothiazene donors to the arylacetylide ligands and a nitrophenyl acceptor to 5-ethynylphenanthroline of the chromophore, exhibited a charge-separated state of 75-ns duration. The first Pt(tpy)(arylacetylide)+-based triad contains a trimethoxybenzamide donor and a pyridinium acceptor and has been structurally characterized. The triad has an edge-to-edge separation between donor and acceptor fragments of 27.95 Å. However, while quenching of the emission is complete for this system, transient absorption (TA) studies reveal that charge transfer does not move onto the pyridinium acceptor. A new set of triads described in detail here and having the formula [Pt(NO2phtpy)(p-C⋮C−C6H4CH2(PTZ-R)](PF6), where NO2phtpy = 4‘-{4-[2-(4-nitrophenyl)vinyl]phenyl}-2,2‘;6‘,2‘ ‘-terpyridine and PTZ = phenothiazine with R = H, OMe, possess an unsaturated linkage between the chromophore and a nitrophenyl acceptor. While the parent chromophore [Pt(ttpy)(C⋮CC6H5)]PF6 is brightly luminescent in a fluid solution at 298 K, the triads exhibit complete quenching of the emission, as do the related donor−chromophore (D−C) dyads. Electrochemically, the triads and D−C dyads exhibit a quasi-reversible oxidation wave corresponding to the PTZ ligand, while the RH triad and related C−A dyad display a facile quasi-reversible reduction assignable to the acceptor. TA spectroscopy shows that one of the triads possesses a long-lived charge-separated state of ∼230 ns

Enhancement of Metal−Metal Coupling at a Considerable Distance by Using 4-Pyridinealdazine as a Bridging Ligand in Polynuclear Complexes of Rhenium and Ruthenium

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF6- salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me2bpy)(CO)3(PCA)]+ (Me2bpy = 4,4‘-dimethyl-2,2‘-bipyridine), does not emit at room temperature in CH3CN, and the transient spectrum found by flash photolysis at λexc = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me2bpy)(CO)3ReII(PCA-)]+, with λmax = 460 nm and τ 2bpy)(CO)3}2(PCA)]2+, [Re(CO)3(PCA)2Cl], and [Re(CO)3Cl]3(PCA)4 confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me2bpy)(CO)3ReI(PCA)RuII(NH3)5]3+, presents an intense absorption in the visible spectrum that can be assigned to a MLCT dπ(Ru) → π*(PCA); in CH3CN, the value of λ max = 560 nm is intermediate between those determined for [Ru(NH3)5(PCA)]2+ (λmax = 536 nm) and [(NH3)5Ru(PCA)Ru(NH3)5]4+ (λmax = 574 nm), indicating a significant decrease in the energy of the π*-orbital of PCA. The mixed-valent species, of formula [(Me2bpy)(CO)3ReI(PCA)RuIII(NH3)5]4+, was obtained in CH3CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [ReI,RuII] precursor; the band at λmax = 560 nm disappears completely, and a new band appears at λmax = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) ReI → RuIII. By using the Marcus−Hush formalism, both the electronic coupling (HAB) and the reorganization energy (λ) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (∼15.0 Å), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH3)5Ru(PCA)Ru(NH3)5]5+ (Kc ∼ 102, in CH3CN), puts into evidence an unusual enhancement of the metal−metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of “metal−ligand interface”, as shown by DFT calculations on free PCA. Moreover, λ is lower than the driving force −ΔG° for the recombination charge reaction [ReII,RuII] → [ReI,RuIII] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [ReI,RuIII] complex a promising model for controlling the efficiency of charge-separation processes

Synthesis, Structure, Characterization, and Photophysical Studies of a New Platinum Terpyridyl-Based Triad with Covalently Linked Donor and Acceptor Groups

A new terpyridyl-containing Pt triad [Pt(pytpy)(p-C⋮C−C6H4−NH−CO−C6H2(OMe)3)](PF6)2 (4), where pytpy = 4‘-(4-pyridin-1-ylmethylphenyl)-[2,2‘;6‘,2‘ ‘]terpyridine and p-C⋮C−C6H4−NH−CO−C6H2(OMe)3 = N-(4-ethynylphenyl)-3,4,5-trimethoxybenzamide, has been synthesized and structurally characterized. The related donor−chromophore dyad [Pt(ttpy)(p-C⋮C−C6H4−NH−CO−C6H2(OMe)3)]PF6 (2), where ttpy = 4‘-p-tolyl-[2,2‘;6‘,2‘ ‘]terpyridine, and the chromophore−acceptor dyad [Pt(pytpy)(C⋮CC6H5)](PF6)2 (3), where C⋮CC6H5 = ethynylbenzene, have also been studied. The multistep syntheses culminate with a CuI-catalyzed coupling reaction of the respective acetylene with either [Pt(ttpy)Cl]PF6 or [Pt(pytpy)Cl](PF6)2. X-ray and spectroscopic studies support assignment of a distorted square planar environment around the Pt(II) ion with three of its coordination sites occupied by the terpyridyl N-donors and the fourth coordination site occupied by the acetylenic carbon. Although the parent compound [Pt(ttpy)(C⋮CC6H5)]PF6 (1) is brightly luminescent in fluid solution at 298 K, dyad 2 as well as triad 4 exhibit complete quenching of the emission. The chromophore−acceptor (C−A) dyad 3 displays weak solution luminescence at room temperature with a φrelem of 0.011 (using Ru(bpy)32+ as a standard with φrelem = 0.062). Electrochemically, the donor−chromophore (D−C) dyad and the donor−chromophore−acceptor (D−C−A) triad exhibit both metal-based and donor ligand-based oxidations, whereas the triad and the C−A dyad show the expected pyridinium- and terpyridine-based reductions. Transient absorption studies of the dyad and triad systems indicate that although the trimethoxybenzene group acts as a reductive donor, in the present system, the pyridinium group fails to act as an acceptor

Straightforward Self-Assembly of Porphyrin Nanowires in Water: Harnessing Adamantane/β-Cyclodextrin Interactions

A convenient approach for the self-assembly of well-defined porphyrin nanowires in water, wherein the individual monomers do not aggregate via π−π interactions, is disclosed. These unidirectional and heteromeric assemblies are instead composed of robust β-CD/adamantane host/guest interactions. A combination of surface microscopies and fluorescence energy transfer experiments were conducted on the nanowires demonstrating their stability and resistance to disassembly

Reversible, Electrochemically Controlled Binding of Phosphine to Iron and Cobalt Bis(dithiolene) Complexes

The homoleptic bis(dithiolene) complexes [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl) undergo two successive reductions to form anions that display [M(S2C2R2)2]22- ↔ 2[M(S2C2R2)2]1- solution equilibria. The neutral dimers react with Ph3P to form square pyramidal [M(Ph3P)(S2C2R2)2]0. Voltammetric measurements upon [M(Ph3P)(S2C2R2)2]0 in CH2Cl2 reveal only irreversible features at negative potentials, consistent with Ph3P dissociation upon reduction. Dissociation and reassociation of Ph3P from and to [Fe(Ph3P)(S2C2R2)2]0 is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of reversible, electrochemically controlled binding of Ph3P to [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl). All members of the cycle ([M(S2C2R2)2]20, [M(S2C2R2)2]21-, [M(S2C2R2)2]22-, [M(S2C2R2)2]1-, [M(Ph3P)(S2C2R2)2]) for M = Fe, Co have been characterized by crystallography. Square planar [Fe(S2C2R2)2]1- is the first such iron dithiolene species to be structurally identified and reveals Fe−S bond distances of 2.172(1) and 2.179(1) Å, which are appreciably shorter than those in corresponding square planar dianions

Reversible, Electrochemically Controlled Binding of Phosphine to Iron and Cobalt Bis(dithiolene) Complexes

The homoleptic bis(dithiolene) complexes [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl) undergo two successive reductions to form anions that display [M(S2C2R2)2]22- ↔ 2[M(S2C2R2)2]1- solution equilibria. The neutral dimers react with Ph3P to form square pyramidal [M(Ph3P)(S2C2R2)2]0. Voltammetric measurements upon [M(Ph3P)(S2C2R2)2]0 in CH2Cl2 reveal only irreversible features at negative potentials, consistent with Ph3P dissociation upon reduction. Dissociation and reassociation of Ph3P from and to [Fe(Ph3P)(S2C2R2)2]0 is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of reversible, electrochemically controlled binding of Ph3P to [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl). All members of the cycle ([M(S2C2R2)2]20, [M(S2C2R2)2]21-, [M(S2C2R2)2]22-, [M(S2C2R2)2]1-, [M(Ph3P)(S2C2R2)2]) for M = Fe, Co have been characterized by crystallography. Square planar [Fe(S2C2R2)2]1- is the first such iron dithiolene species to be structurally identified and reveals Fe−S bond distances of 2.172(1) and 2.179(1) Å, which are appreciably shorter than those in corresponding square planar dianions

Reversible, Electrochemically Controlled Binding of Phosphine to Iron and Cobalt Bis(dithiolene) Complexes

The homoleptic bis(dithiolene) complexes [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl) undergo two successive reductions to form anions that display [M(S2C2R2)2]22- ↔ 2[M(S2C2R2)2]1- solution equilibria. The neutral dimers react with Ph3P to form square pyramidal [M(Ph3P)(S2C2R2)2]0. Voltammetric measurements upon [M(Ph3P)(S2C2R2)2]0 in CH2Cl2 reveal only irreversible features at negative potentials, consistent with Ph3P dissociation upon reduction. Dissociation and reassociation of Ph3P from and to [Fe(Ph3P)(S2C2R2)2]0 is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of reversible, electrochemically controlled binding of Ph3P to [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl). All members of the cycle ([M(S2C2R2)2]20, [M(S2C2R2)2]21-, [M(S2C2R2)2]22-, [M(S2C2R2)2]1-, [M(Ph3P)(S2C2R2)2]) for M = Fe, Co have been characterized by crystallography. Square planar [Fe(S2C2R2)2]1- is the first such iron dithiolene species to be structurally identified and reveals Fe−S bond distances of 2.172(1) and 2.179(1) Å, which are appreciably shorter than those in corresponding square planar dianions

Reversible, Electrochemically Controlled Binding of Phosphine to Iron and Cobalt Bis(dithiolene) Complexes

The homoleptic bis(dithiolene) complexes [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl) undergo two successive reductions to form anions that display [M(S2C2R2)2]22- ↔ 2[M(S2C2R2)2]1- solution equilibria. The neutral dimers react with Ph3P to form square pyramidal [M(Ph3P)(S2C2R2)2]0. Voltammetric measurements upon [M(Ph3P)(S2C2R2)2]0 in CH2Cl2 reveal only irreversible features at negative potentials, consistent with Ph3P dissociation upon reduction. Dissociation and reassociation of Ph3P from and to [Fe(Ph3P)(S2C2R2)2]0 is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of reversible, electrochemically controlled binding of Ph3P to [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl). All members of the cycle ([M(S2C2R2)2]20, [M(S2C2R2)2]21-, [M(S2C2R2)2]22-, [M(S2C2R2)2]1-, [M(Ph3P)(S2C2R2)2]) for M = Fe, Co have been characterized by crystallography. Square planar [Fe(S2C2R2)2]1- is the first such iron dithiolene species to be structurally identified and reveals Fe−S bond distances of 2.172(1) and 2.179(1) Å, which are appreciably shorter than those in corresponding square planar dianions

Reversible, Electrochemically Controlled Binding of Phosphine to Iron and Cobalt Bis(dithiolene) Complexes

The homoleptic bis(dithiolene) complexes [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl) undergo two successive reductions to form anions that display [M(S2C2R2)2]22- ↔ 2[M(S2C2R2)2]1- solution equilibria. The neutral dimers react with Ph3P to form square pyramidal [M(Ph3P)(S2C2R2)2]0. Voltammetric measurements upon [M(Ph3P)(S2C2R2)2]0 in CH2Cl2 reveal only irreversible features at negative potentials, consistent with Ph3P dissociation upon reduction. Dissociation and reassociation of Ph3P from and to [Fe(Ph3P)(S2C2R2)2]0 is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of reversible, electrochemically controlled binding of Ph3P to [M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl). All members of the cycle ([M(S2C2R2)2]20, [M(S2C2R2)2]21-, [M(S2C2R2)2]22-, [M(S2C2R2)2]1-, [M(Ph3P)(S2C2R2)2]) for M = Fe, Co have been characterized by crystallography. Square planar [Fe(S2C2R2)2]1- is the first such iron dithiolene species to be structurally identified and reveals Fe−S bond distances of 2.172(1) and 2.179(1) Å, which are appreciably shorter than those in corresponding square planar dianions