Large Increase of the Lifetime of a Charge-Separated State in a Molecular Triad Induced by Hydrogen-Bonding Solvent

Life of triad: The lifetime of a charge-separated state (see figure) increases from about 50 to 2000 ns when the solvent is changed from aprotic CH2Cl2 to the strong hydrogen-bond donor hexafluoroisopropanol

Hydrogen-Bond Strengthening upon Photoinduced Electron Transfer in Ruthenium-Anthraquinone Dyads Interacting with Hexafluoroisopropanol or Water

Quinones play a key role as primary electron acceptors in natural photosynthesis, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, the influence of hydrogen-bond donating solvents on the thermodynamics and kinetics of intramolecular electron transfer between Ru(bpy)32+ (bpy = 2,2′-bipyridine) and 9,10-anthraquinone redox partners linked together via one up to three p-xylene units was investigated. Addition of relatively small amounts of hexafluoroisopropanol to dichloromethane solutions of these rigid rodlike donor–bridge–acceptor molecules is found to accelerate intramolecular Ru(bpy)32+-to-anthraquinone electron transfer substantially because anthraquinone reduction occurs more easily in the presence of the strong hydrogen-bond donor. Similarly, the rates for intramolecular electron transfer are significantly higher in acetonitrile/water mixtures than in dry acetonitrile. In dichloromethane, an increase in the association constant between hexafluoroisopropanol and anthraquinone by more than 1 order of magnitude following quinone reduction points to a significant strengthening of the hydrogen bonds between the hydroxyl group of hexafluoroisopropanol and the anthraquinone carbonyl functions. The photoinduced intramolecular long-range electron transfer process thus appears to be followed by proton motion; hence the overall photoinduced reaction may be considered a variant of stepwise proton-coupled electron transfer (PCET) in which substantial proton density (rather than a full proton) is transferred after the electron transfer has occurred

Photoinduced Electron Transfer in Linear Triarylamine-Photosensitizer-Anthraquinone Triads with Ruthenium(II), Osmium(II), and Iridium(III)

A rigid rod-like organic molecular ensemble comprised of a triarylamine electron donor, a 2,2′-bipyridine (bpy) ligand, and a 9,10-anthraquinone acceptor was synthesized and reacted with suitable metal precursors to yield triads with Ru(bpy)32+, Os(bpy)32+, and [Ir(2-(p-tolyl)pyridine)2(bpy)]+ photosensitizers. Photoexcitation of these triads leads to long-lived charge-separated states (τ = 80–1300 ns) containing a triarylamine cation and an anthraquinone anion, as observed by transient absorption spectroscopy. From a combined electrochemical and optical spectroscopic study, the thermodynamics and kinetics for the individual photoinduced charge-separation and thermal charge-recombination events were determined; in some cases, measurements on suitable donor–sensitizer or sensitizer–acceptor dyads were necessary. In the case of the ruthenium and iridium triads, the fully charge-separated state is formed in nearly quantitative yield

Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d6 metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium(II) and osmium(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ∼750 ns in dichloromethane to ∼3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ∼50 to ∼2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET

Photoinduzierter Elektronentransfer in Dyaden und Triaden mit d6 Metallkomplexen und Antrachinon

In den letzten Jahren hat man der künstlichen Photosynthese insbesondere im Bereich der Chemie viel Aufmerksamkeit geschenkt. Vor allem auf der Synthese molekularer Strukturen, welche die natürlichen Prozesse nachempfinden, lag ein wichtiger Fokus. Viele dieser künstlichen Systeme wurden zum Studium des lichtinduzierten intramolekularen Elektronentransfers herangezogen. Geignete Moleküle bestehen normalerweise aus einem Photosensibilisator, einem Elektrone

Hydrogen-Bond Strengthening upon Photoinduced Electron Transfer in Ruthenium–Anthraquinone Dyads Interacting with Hexafluoroisopropanol or Water

Quinones play a key role as primary electron acceptors in natural photosynthesis, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, the influence of hydrogen-bond donating solvents on the thermodynamics and kinetics of intramolecular electron transfer between Ru­(bpy)₃²⁺ (bpy = 2,2′-bipyridine) and 9,10-anthraquinone redox partners linked together via one up to three <i>p</i>-xylene units was investigated. Addition of relatively small amounts of hexafluoroisopropanol to dichloromethane solutions of these rigid rodlike donor–bridge–acceptor molecules is found to accelerate intramolecular Ru­(bpy)₃²⁺-to-anthraquinone electron transfer substantially because anthraquinone reduction occurs more easily in the presence of the strong hydrogen-bond donor. Similarly, the rates for intramolecular electron transfer are significantly higher in acetonitrile/water mixtures than in dry acetonitrile. In dichloromethane, an increase in the association constant between hexafluoroisopropanol and anthraquinone by more than 1 order of magnitude following quinone reduction points to a significant strengthening of the hydrogen bonds between the hydroxyl group of hexafluoroisopropanol and the anthraquinone carbonyl functions. The photoinduced intramolecular long-range electron transfer process thus appears to be followed by proton motion; hence the overall photoinduced reaction may be considered a variant of stepwise proton-coupled electron transfer (PCET) in which substantial proton density (rather than a full proton) is transferred after the electron transfer has occurred

Influence of Experimental Parameters on the Synthesis of Gold Nanoparticles by Electroless Deposition

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Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d⁶ metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium­(II) and osmium­(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ∼750 ns in dichloromethane to ∼3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ∼50 to ∼2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET

Bis(Triphenylamine)Benzodifuran Chromophores: Synthesis, Electronic Properties and Application in Organic Light-Emitting Diodes

A series of bis(triphenylamine)benzodifuran chromophores have been synthesized and fully characterised. Starting from suitably functionalized benzodifuran (BDF) precursors, two triphenylamine (TPA) moieties are symmetrically coupled to a central BDF unit either at 4,8-positions through double bonds (1) and single bonds (2) respectively, or at 2,6-positions through double bonds (3). Their electronic absorption and photoluminescence properties as well as redox behaviour have been investigated in detail, indicating that the π-extended conjugation via vinyl linkers in 1 and 3 leads to comparatively strong electronic interactions between the relevant redox moieties TPA and BDF. Due to intriguing electronic properties and structural planarity, 3a has been applied as a dopant emitter in organic light-emitting diodes. A yellowish-green OLED exhibits a high external quantum efficiency (EQE) of 6.2%, thus exceeding the theoretical upper limit most likely due to energy transfer from an interface exciplex to an emissive layer and/or favorable horizontal orientation

On the role of ligand-field states for the photophysical properties of ruthenium(II) polypyridyl complexes

The role of ligand-field states for the photophysical properties of d6 systems has been discussed in a large number of publications over the past decades. Since the seminal paper by Houten and Watts, for instance, the quenching of the 3MLCT luminescence in ruthenium(II) polypyridyl complexes is attributed to the presence of the first excited ligand-field state, namely a component of the 3T1(t2g5eg1) state, at similar energies. If this state lies above the 3MLCT state, the luminescence is quenched via thermal population at elevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenic temperatures. In this contribution we present transient absorption spectra on non-luminescent ruthenium polypyridyl complexes such as [Ru(m-bpy)3]2+, m-bpy = 6-methyl-2,2’-bipyridine, in acetonitrile at room temperature, which reveal an ultra-rapid depopulation of the 3MLCT state but a much slower ground state recovery. We propose that in this and related complexes the methyl groups force longer metal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the 3dd state drops to below the 3MLCT state, and that furthermore the population of this state from the 3MLCT state occurs faster than its decay to the ground state. In addition we demonstrate that in this complex the luminescence can be switched on by external pressure, which we attribute to a destabilisation of the ligand-field state by the pressure due to its larger molecular volume compared to the ground state as well as the 3MLCT state