Increasing Electron-Transfer Rates with Increasing Donor-Acceptor Distance

Intersystem Crossing, Photo-Induced Charge Separation and Regioisomer-Specific Excited State Dynamics in Fully Rigid Spiro Rhodammine-Naphthalene/Anthraquinone Electron Donor-Acceptor Dyads

We prepared a series fully rigid spiro electron donor-acceptor orthogonal dyads, with closed form of rhodamine (Rho) as electron donor and naphthalene (Np)/anthraquinone (AQ) as electron acceptor, to access the long-lived triplet charge separation (3CS) state, via the electron spin control method. We found strong dependency of the photophysical property of the dyads on the amino substitution positions of the Np chromophores in the dyads 1,8-DaNp-Rho and 2,3-DaNp-Rho. Nanosecond transient absorption (ns-TA) spectra show the population of the 3LE state (lifetime: 47 μs) for 2,3-DaNp-Rho, however, long-lived 3CS state was observed (τCS=0.62 μs) for AQ-Rho, with a CS quantum yield of ΦCS=58 %. Based on femtosecond transient absorption (fs-TA) spectra, spin orbit charge transfer ISC (SOCT-ISC) is proposed to be responsible for the formation of the triplet states. Time-resolved electron paramagnetic resonance (TREPR) spectra of AQ-Rho indicate the presence of two states, a 3LE state with zero field splitting (ZFS) D parameter of 1400 MHz and E parameter of -410 MHz, formed via radical pair ISC (RP-ISC) and SOCT-ISC mechanism; and a 3CS state with the electron spin-spin interaction in the regime of spin-correlated radical pair (SCRP).

The Photophysics of Naphthalimide-Phenoselenazine Electron Donor-Acceptor Dyads: Revisiting the Heavy-Atom Effect in Thermally Activated Delayed Fluorescence

We prepared thermally activated delayed fluorescence (TADF) emitter dyads, NI-PTZ, NI-PTZ-2Br and NI-PSeZ, with naphthalimide (NI) as electron acceptor and 10H-phenothiazine (PTZ) or 10H-phenoselenazine (PSeZ) as electron donor to study the heavy-atom effect on the intersystem crossing (ISC) and reverse ISC (rISC) in the TADF emitters. The delayed fluorescence lifetimes of the dyads containing heavy atoms (τ D F ${{\tau }_{{\rm D}{\rm F}}}$ =5.9 μs for NI-PSeZ andτ D F ${{\tau }_{{\rm D}{\rm F}}}$ =16.5 μs for NI-PTZ-2Br, respectively) are longer than the heavy atom-free counterpart NI-PTZ (τ D F ${{\tau }_{{\rm D}{\rm F}}}$ =2.0 μs). Nanosecond transient absorption (ns-TA) spectral study and the time-resolved electron paramagnetic resonance (TREPR) spectra show the presence of both 3LE and 3CS states. These findings represent solid experimental evidences for the spin-vibronic coupling mechanism of TADF. Moreover, the ns-TA spectra show that the heavy atoms don't have a significant effect since the lifetime of the triplet transient species (1.3 μs for NI-PTZ) is not shortened in their presence (4.5 μs for NI-PSeZ and 5.3 μs for NI-PTZ-2Br). These results show that the previously claimed heavy-atom effect on rISC and TADF is not a universal principle. The femtosecond transient absorption (fs-TA) spectra of the compounds indicate the occurrence of fast charge separation within 1-2 ps, and the charge recombination is slow (>4 ns).

Deducing the conformational space for an octa-proline helix

A molecular dyad, PY-P8-PER, comprising a proline octamer sandwiched between pyrene and perylene terminals has been synthesized in order to address the dynamics of electronic energy transfer (EET) along the oligo-proline chain. A simple pyrene-based control compound equipped with a bis-proline attachment serves as a reference for spectroscopic studies. The N-H NMR signal at the terminal pyrene allows distinction between cis and trans amides and, although the crystal structure for the control has the trans conformation, temperature-dependent NMR studies provide clear evidence for trans/cis isomerisation in D6-DMSO. Polar solvents tend to stabilise the trans structure for the pyrene amide group, even for longer oligo-proline units. Circular dichroism shows that the proline spacer for PY-P8-PER exists mainly in the all-trans geometry in methanol. Preferential excitation of the pyrene chromophore is possible at wavelengths in the 320-350 nm range and, for the dyad, is followed by efficacious EET to the perylene emitter. The probability for intramolecular EET, obtained from analysis of steady-state spectroscopic data, is ca. 80-90% in solvents of disparate polarity. Comparison with the Förster critical distance suggests the terminals are ca. 18 Å apart. Time-resolved fluorescence spectroscopy, in conjunction with DFT calculations, indicates the dyad exists as a handful of conformers displaying a narrow range of EET rates. Optimisation of a distributive model allows accurate simulation of the EET dynamics in terms of reasonable structures based on isomerisation of certain amide groups.

The effect of thionation of the carbonyl group on the photophysics of compact spiro rhodamine-naphthalimide electron donor-acceptor dyads: intersystem crossing, charge separation, and electron spin dynamics

Herein, a spiro rhodamine (Rho)-thionated naphthalimide (NIS) electron donor-acceptor orthogonal dyad (Rho-NIS) was prepared to study the formation of a long-lived charge separation (CS) state via the electron spin control approach. The transient absorption (TA) spectra of Rho-NIS indicated that the intersystem crossing (ISC) occurs within 7-42 ps to produce the 3NIS state via the spin orbit coupling ISC (SOC-ISC). The energy order of 3CS (2.01 eV in n-hexane, HEX) and 3LE states (1.68 eV in HEX) depended on the solvent polarity. The 3NIS state having n-π* character and a lifetime of 0.38 μs was observed for Rho-NIS in toluene (TOL). Alternatively, in acetonitrile (ACN), the long-lived 3CS state (0.21 μs) with a high CS state quantum yield (ΦCS, 97%) was produced with the 3NIS state as the precursor and the CS took 134 ps. On the contrary, in the case of the reference Rho-naphthalimide (NI) Rho-NI dyad without thionation of its carbonyl group, a long-lived CS state (0.94 μs) with a high energy level (ECS = 2.12 eV) was generated even in HEX with a lower ΦCS (49%). In the presence of an acid, the Rho unit in the Rho-NIS adopted an open form (Rho-o) and the 3NIS state was produced within 24-47 ps with the 1Rho-o state as the precursor. Subsequently, slow intramolecular triplet-triplet energy transfer (TTET, 0.11-0.60 μs) produced the 3Rho-o state (9.4-13.6 μs). According to the time-resolved electron paramagnetic resonance (TREPR) spectra of NIS-NH2, the zero-field splitting (ZFS) parameter |D| and E of the triplet state were determined to be 6165 MHz and -1233 MHz, respectively, indicating that its triplet state has significant nπ* character, which was supported by its short triplet state lifetime (6.1 μs).

Fast Charge Separation in Distant Donor-Acceptor Dyads Driven by Relaxation of a Hot Excited State

A series of three perylenemonoimide-p-oligophenylene-dimethylaniline molecular dyads undergo photoinduced charge separation (CS) with anomalous distance dependence as a function of increasing donor-acceptor (DA) distances. A comprehensive experimental and computational investigation of the photodynamics in the donor-bridge-acceptor (DBA) chromophores reveals a clear demarcation concerning the nature of the CS accessed at shorter (bridgeless) and longer DA distances. At the shortest distance, a strong DA interaction and ground-state charge delocalization populate a hot excited state (ES) with prominent charge transfer (CT) character, via Franck-Condon vertical excitation. The presence of such a CT-polarized hot ES enables a subpicosecond CS in the bridgeless dyad. The incorporation of the p-oligophenylene bridge effectively decouples the donor and the acceptor units in the ground state and consequentially suppresses the CT polarization in the hot ES. Theoretically, this should render a slower CS at longer distances. However, the transient absorption measurement reveals a fast CS process at the longer distance, contrary to the anticipated exponential distance dependence of the CS rates. A closer look into the excited-state dynamics suggests that the hot ES undergoes ultrafast geometry relaxation (τ < 1 ps) to create a relaxed ES. As compared to a decoupled, twisted geometry in the hot ES, the geometry of the relaxed ES exhibits a more planar conformation of the p-oligophenylene bridges. Planarization of the bridge endorses an increased charge delocalization and a prominent CT character in the relaxed ES and forms the origin for the evident fast CS at the longest distance. Thus, the relaxation of the hot ES and the concomitantly enhanced charge delocalization adds a new caveat to the classic nature of distance-dependent CS in artificial DBA chromophores and recommends a cautious treatment of the attenuation factor (β) while discussing anomalous CS trends.

Recent Advances in Light Energy Conversion with Biomimetic Vesicle Membranes

Lipid bilayer membranes are ubiquitous in natural chemical conversions. They enable self-assembly and compartmentalization of reaction partners and it becomes increasingly evident that a thorough fundamental understanding of these concepts is highly desirable for chemical reactions and solar energy conversion with artificial systems. This minireview focusses on selected case studies from recent years, most of which were inspired by either membrane-facilitated light harvesting or respective charge transfer. The main focus is on highly biomimetic liposomes with artificial chromophores, and some cases for polymer-membranes will be made. Furthermore, we categorized these studies into energy transfer and electron transfer, with phospholipid vesicles, and polymer membranes for light-driven reactions.

The Carbene Cannibal: Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. Here, we present the first case of direct observation of bimolecular SB-CS in a transition metal complex, [Fe^IIIL₂](PF₆) (L = [phenyl(tris(3-methylimidazol-1-ylidene))borate]⁻). Photoexcitation of the complex in the visible region results in the formation of a doublet ligand-to-metal charge transfer (²LMCT) excited state (E_0–0 = 2.13 eV), which readily reacts with the doublet ground state to generate charge separated products, [Fe^IIL₂] and [Fe^IVL₂]²⁺, with a measurable cage escape yield. Known spectral signatures allow for unambiguous identification of the products, whose formation and recombination are monitored with transient absorption spectroscopy. The unusual energetic landscape of [Fe^IIIL₂]⁺, as reflected in its ground and excited state reduction potentials, results in SB-CS being intrinsically exergonic (ΔG_CS° ∼ −0.7 eV). This is in contrast to most systems investigated in the literature, where ΔG_CS° is close to zero, and the charge transfer driven primarily by solvation effects. The study is therefore illustrative for the utilization of the rich redox chemistry accessible in transition metal complexes for the realization of SB-CS.

Preservation of the donor-acceptor character of a carbazole-phenalenone dyad upon adsorption on Pt(111)

Donor-acceptor molecules are a subject of great attention due to their immense potential in molecular electronics and photovoltaics. Despite numerous extensive studies demonstrating their functionality in solution, the donor-acceptor character is usually lost upon adsorption on a conducting substrate. Here the concept of breaking the conjugation between the donor and acceptor unit by insertion of a bridge is used. Furthermore, the bridge introduces a kink into the dyad and thus, reduces the possibility of hybridization with the substrate. A donor-bridge-acceptor dyad composed of carbazole and phenalenone units joined through a flexible bridge is synthesized and deposited on a Pt(111) surface. Its electronic properties are investigated with a combination of low temperature scanning tunneling microscope measurements and density functional theory simulations. Two preferential adsorption configurations are identified, in which individual molecules form strong bonds to the substrate and to a Pt adatom. Differential conductance measurements and atomistic simulations evidence the preservation of a reduced donor-acceptor character upon adsorption of the molecule, where this reduction is ascribed to the strong molecule-metal hybridization. Our results highlight the changes in donor-acceptor character of the dyad induced by the substrate and provide guidelines for the use of donor-bridge-acceptor molecules as functional units in solid-state devices.

Resonance-Enhanced Charge Delocalization in Carbazole-Oligoyne-Oxadiazole Conjugates

There are notably few literature reports of electron donor-acceptor oligoynes, even though they offer unique opportunities for studying charge transport through "all-carbon" molecular bridges. In this context, the current study focuses on a series of carbazole-(C≡C)_n-2,5-diphenyl-1,3,4-oxadiazoles (n = 1-4) as conjugated π-systems in general and explores their photophysical properties in particular. Contrary to the behavior of typical electron donor-acceptor systems, for these oligoynes, the rates of charge recombination after photoexcitation increase with increasing electron donor-acceptor distance. To elucidate this unusual performance, we conducted detailed photophysical and time-dependent density functional theory investigations. Significant delocalization of the molecular orbitals along the bridge indicates that the bridging states come into resonance with either the electron donor or acceptor, thereby accelerating the charge transfer. Moreover, the calculated bond lengths reveal a reduction in bond-length alternation upon photoexcitation, indicating significant cumulenic character of the bridge in the excited state. In short, strong vibronic coupling between the electron-donating N-arylcarbazoles and the electron-accepting 1,3,4-oxadiazoles accelerates the charge recombination as the oligoyne becomes longer.

Shallow Distance Dependence for Proton-Coupled Tyrosine Oxidation in Oligoproline Peptides

We have explored the kinetic effect of increasing electron transfer (ET) distance in a biomimetic, proton-coupled electron-transfer (PCET) system. Biological ET often occurs simultaneously with proton transfer (PT) in order to avoid the high-energy, charged intermediates resulting from the stepwise transfer of protons and electrons. These concerted proton-electron-transfer (CPET) reactions are implicated in numerous biological ET pathways. In many cases, PT is coupled to long-range ET. While many studies have shown that the rate of ET is sensitive to the distance between the electron donor and acceptor, extensions to biological CPET reactions are sparse. The possibility of a unique ET distance dependence for CPET reactions deserves further exploration, as this could have implications for how we understand biological ET. We therefore explored the ET distance dependence for the CPET oxidation of tyrosine in a model system. We prepared a series of metallopeptides with a tyrosine separated from a Ru(bpy)32+ complex by an oligoproline bridge of increasing length. Rate constants for intramolecular tyrosine oxidation were measured using the flash-quench transient absorption technique in aqueous solutions. The rate constants for tyrosine oxidation decreased by 125-fold with three added proline residues between tyrosine and the oxidant. By comparison, related intramolecular ET rate constants in very similar constructs were reported to decrease by 4-5 orders of magnitude over the same number of prolines. The observed shallow distance dependence for tyrosine oxidation is proposed to originate in part from the requirement for stronger oxidants, leading to a smaller hole-transfer effective tunneling barrier height. The shallow distance dependence observed here and extensions to distance-dependent CPET reactions have potential implications for long-range charge transfers.

Excited-state proton-coupled electron transfer within ion pairs

The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.

Quantitative insights into charge-separated states from one- and two-pulse laser experiments relevant for artificial photosynthesis

Charge-separated states (CSSs) are key intermediates in photosynthesis and solar energy conversion. However, the factors governing the formation efficiencies of CSSs are still poorly understood, and light-induced electron-hole recombinations as deactivation pathways competing with desired charge accumulations are largely unexplored. This greatly limits the possibility to perform efficient multi-electron transfer, which is essential for artificial photosynthesis. We present a systematic investigation of two donor-sensitizer-acceptor triads (with different donor-acceptor distances) capable of storing as much as 2.0 eV in their CSSs upon the absorption of a visible photon. Using quantitative one- and two-pulse laser flash photolysis, we provide deep insights into both the CSS formation quantum yield, which can reach up to 80%, and the fate of the CSS upon further (secondary) excitation with green photons. The triad with shorter intramolecular distances shows a remarkable excitation wavelength dependence of the CSS formation quantum yield, and the CSS of this triad undergoes more efficient light-induced charge recombination than the longer equivalent by about one order of magnitude, whilst thermal charge recombination shows the exact opposite behavior. The unexpected results of our detailed photophysical study can be rationalized by detrimental singlet charge transfer states or structural considerations, and could significantly contribute to the future design of CSS precursors for accumulative multi-electron transfer and artificial photosynthesis.

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

Photoinduced PCET meets catalysis, and the accumulation of multiple redox equivalents is of key importance.

Remote control of electronic coupling – modification of excited-state electron-transfer rates in Ru(tpy)2-based donor–acceptor systems by remote ligand design

Electronic coupling (H_DA) underlying the electron transfer (ET) can be tuned by the remote substituents R.

Electron Transfer across o-Phenylene Wires

Photoinduced electron transfer across rigid rod-like oligo- p-phenylenes has been thoroughly investigated in the past, but their o-connected counterparts are yet entirely unexplored in this regard. We report on three molecular dyads comprised of a triarylamine donor and a Ru(bpy)₃²⁺ (bpy =2,2'-bipyridine) acceptor connected covalently by 2 to 6 o-phenylene units. Pulsed excitation of the Ru(II) sensitizer at 532 nm leads to the rapid formation of oxidized triarylamine and reduced ruthenium complex via intramolecular electron transfer. The subsequent thermal reverse charge-shift reaction to reinstate the electronic ground-state occurs on a time scale of 120-220 ns in deaerated CH₃CN at 25 °C. The conformational flexibility of the o-phenylene bridges causes multiexponential transient absorption kinetics for the photoinduced forward process, but the thermal reverse reaction produces single-exponential transient absorption decays. The key finding is that the flexible o-phenylene bridges permit rapid formation of photoproducts storing ca. 1.7 eV of energy with lifetimes on the order of hundreds of nanoseconds, similar to what is possible with rigid rod-like donor-acceptor compounds. Thus, the conformational flexibility of the o-phenylenes represents no disadvantage with regard to the photoproduct lifetimes, and this is relevant in the greater context of light-to-chemical energy conversion.

Resolving orbital pathways for intermolecular electron transfer

Over 60 years have passed since Taube deduced an orbital-mediated electron transfer mechanism between distinct metal complexes. This concept of an orbital pathway has been thoroughly explored for donor-acceptor pairs bridged by covalently bonded chemical residues, but an analogous pathway has not yet been conclusively demonstrated for formally outer-sphere systems that lack an intervening bridge. In our present study, we experimentally resolve at an atomic level the orbital interactions necessary for electron transfer through an explicit intermolecular bond. This finding was achieved using a homologous series of surface-immobilized ruthenium catalysts that bear different terminal substituents poised for reaction with redox active species in solution. This arrangement enabled the discovery that intermolecular chalcogen⋯iodide interactions can mediate electron transfer only when these interactions bring the donor and acceptor orbitals into direct contact. This result offers the most direct observation to date of an intermolecular orbital pathway for electron transfer.

Light-driven electron injection from a biotinylated triarylamine donor to [Ru(diimine)3](2+)-labeled streptavidin

Electron transfer from a biotinylated electron donor to photochemically generated Ru(iii) complexes covalently anchored to streptavidin is demonstrated by means of time-resolved laser spectroscopy. Through site-selective mutagenesis, a single cysteine residue was engineered at four different positions on streptavidin, and a Ru(ii) tris-diimine complex was then bioconjugated to the exposed cysteines. A biotinylated triarylamine electron donor was added to the Ru(ii)-modified streptavidins to afford dyads localized within a streptavidin host. The resulting systems were subjected to electron transfer studies. In some of the explored mutants, the phototriggered electron transfer between triarylamine and Ru(iii) is complete within 10 ns, thus highlighting the potential of such artificial metalloenzymes to perform photoredox catalysis.

Tuning the Intrinsic Properties of Carbon Nitride for High Quantum Yield Photocatalytic Hydrogen Production

The low quantum yield of photocatalytic hydrogen production in carbon nitride (CN) has been improved upon via the modulation of both the extrinsic and intrinsic properties of the material. Although the modification of extrinsic properties has been widely investigated in the past, recently there has been growing interest in the alteration of intrinsic properties. Refining the intrinsic properties of CN provides flexibility in controlling the charge transport and selectivity in photoredox reactions, and therefore makes available a pathway toward superior photocatalytic performance. An analysis of recent progress in tuning the intrinsic photophysical properties of CN facilitates an assessment of the goals, achievements, and gaps. This article is intended to serve this purpose. Therefore, selected techniques and mechanisms of the tuning of intrinsic properties of CN are critically discussed here. This article concludes with a recommendation of the issues that need to be considered for the further enhancement in the quantum efficiency of CN photocatalysts.

Exploiting Potential Inversion for Photoinduced Multielectron Transfer and Accumulation of Redox Equivalents in a Molecular Heptad

Photoinduced multielectron transfer and reversible accumulation of redox equivalents is accomplished in a fully integrated molecular heptad composed of four donors, two photosensitizers, and one acceptor. The second reduction of the dibenzo[1,2]dithiin acceptor occurs more easily than the first by 1.3 V, and this potential inversion facilitates the light-driven formation of a two-electron reduced state with a lifetime of 66 ns in deaerated CH₃CN. The quantum yield for formation of this doubly charge-separated photoproduct is 0.5%. In acidic oxygen-free solution, the reduction product is a stable dithiol. Under steady-state photoirradiation, our heptad catalyzes the two-electron reduction of an aliphatic disulfide via thiolate-disulfide interchange. Exploitation of potential inversion for the reversible light-driven accumulation of redox equivalents in artificial systems is unprecedented and the use of such a charge-accumulated state for multielectron photoredox catalysis represents an important proof-of-concept.

Reassessment of the Four-Point Approach to the Electron-Transfer Marcus-Hush Theory

The Marcus-Hush theory has been successfully applied to describe and predict the activation barriers and hence the electron-transfer (ET) rates in several physicochemical and biological systems. This theory assumes that in the ET reaction, the geometry of the free Gibbs energy landscape is parabolic, with equal curvature near the local minimum for both reactants and products. In spite of its achievements, more realistic models have included the assumption of the two parabolas having not the same curvature. This situation is analyzed by the Nelsen's four-point method. As a benchmark to compare the Marcus-Hush approximation to a precise calculation of the excitation energy, we studied the non-ET process of the electronic excitation of the aluminum dimer that has two local minima (³∑_g ^- and ³∏_u electronic states) and allows to obtain analytically the Marcus-Hush nonsymmetric parameters. We appraise the ability of the Marcus-Hush formula to approximate the analytical results by using several averages of the two reorganization energies associated with the forward and backward transitions and analyze the error. It is observed that the geometric average minimizes the relative error and that the analytical case is recovered. The main results of this paper are obtained by the application of the Nelsen's four-point method to compute the reorganization energies of a large set of potential π-conjugated molecules proposed for organic photovoltaic devices using the above-mentioned averages for the Marcus-Hush formula. The activation energies obtained with the geometric average are significantly larger for some donor-acceptor pairs in comparison with the previously employed arithmetic average, their differences being suitable for experimental testing.

Electron and excitation energy transfers in covalently linked donor–acceptor dyads: mechanisms and dynamics revealed using quantum chemistry

A quantum chemical strategy is developed to study photoinduced electron and energy transfer processes in covalently linked dyads. The strategy is applied to two zinc and free-based porphyrin–naphthalenediimide dyads in solution.

Photoinduced Electron Transfer Coupled to Donor Deprotonation and Acceptor Protonation in a Molecular Triad Mimicking Photosystem II

The first artificial donor-sensitizer-acceptor compound in which photoinduced long-range electron transfer is coupled to donor deprotonation and acceptor protonation is reported. The long-lived photoproduct stores energy in the form of a radical pair state in which the charges of the donor and the acceptor remain unchanged, much in contrast to previously investigated systems that exhibit charge-separated states comprised of electron-hole pairs. This finding is relevant for light-driven accumulation of redox equivalents, because it exemplifies how the buildup of charge can be avoided yet light energy can be stored. Proton-coupled electron transfer (PCET) reactions at a phenol donor and a monoquat acceptor triggered by excitation of a Ru(II) sensitizer enable this form of photochemical energy storage. Our triad emulates photosystem II more closely than previously investigated systems, because tyrosine Z is oxidized and deprotonated, whereas plastoquinone B is reduced and protonated.

Effect of Donor Strength and Bulk on Thieno[3,4-b]-pyrazine-Based Panchromatic Dyes in Dye-Sensitized Solar Cells

Near-infrared-absorbing organic dyes are critically needed in dye-sensitized solar cells (DSCs). Thieno[3,4-b]pyrazine (TPz) based dyes can access the NIR spectral region and show power conversion efficiencies (PCEs) of up to 8.1 % with sunlight being converted at wavelengths up to 800 nm for 17.6 mA cm^-2 of photocurrent in a co-sensitized DSC device. Precisely controlling dye excited-state energies is critical for good performances in NIR DSCs. Strategies to control TPz dye energetics with stronger donor groups and TPz substituent choice are evaluated here. Additionally, donor size influence versus dye loading on TPz dyes is analyzed with respect to the TiO₂ surface protection designed to prevent recombination of electrons in TiO₂ with the redox shuttle. Importantly, the dyes evaluated were demonstrated to work well with low Li⁺ concentration electrolytes, with iodine and cobalt redox shuttle systems, and efficiently as part of co-sensitized devices.

Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)₃²⁺ photosensitizers and two peripheral triarylamine (TAA) donors was investigated by transient IR and UV-vis spectroscopies in the presence of 0.2 M p-toluenesulfonic acid (TsOH) in deaerated acetonitrile. In ∼15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH₂) via reversible intramolecular electron transfer from the two TAA units (τ = 65 ps), followed by intermolecular proton transfer from TsOH (τ ≈ 3 ns for the first step). Although the light-driven accumulation of reduction equivalents occurs through a sequence of electron and proton transfer steps, the resulting photoproduct decays via concerted PCET (τ = 4.7 μs) with an H/D kinetic isotope effect of 1.4 ± 0.2. Moreover, the reoxidation of AQH₂ seems to take place via a double electron transfer step involving both TAA⁺ units rather than sequential single electron transfer events. Thus, the overall charge-recombination reaction seems to involve a concerted proton-coupled two-electron oxidation of AQH₂. The comparison of experimental data obtained in neat acetonitrile with data from acidic solutions suggests that the inverted driving-force effect can play a crucial role for obtaining long-lived photoproducts resulting from multiphoton, multielectron processes. Our pentad provides the first example of light-driven accumulation of reduction equivalents stabilized by PCET in artificial molecular systems without sacrificial reagents. Our study provides fundamental insight into how light-driven multielectron redox chemistry, for example the reduction of CO₂ or the oxidation of H₂O, can potentially be performed without sacrificial reagents.

Energy versus Electron Transfer: Controlling the Excitation Transfer in Molecular Triads

The photochemistry of Ru^II coordination compounds is generally discussed to originate from the lowest lying triplet metal-to-ligand charge-transfer state (³ MLCT). However, when heteroleptic complexes are considered, for example, in the design of molecular triads for efficient photoinduced charge separation, a complex structure of ¹ MLCT states, which can be populated in a rather narrow spectral window (typically around 450 nm) is to be considered. In this contribution we show that the localization of MLCT excited states on different ligands can affect the following ps to ns decay pathways to an extent that by tuning the excitation wavelength, intermolecular energy transfer from a Ru^II -terpyridine unit to a fullerene acceptor can be favored over electron transfer within the molecular triad. These results might have important implications for the design of molecular dyads, triads, pentads and so forth with respect to a specifically targeted response of these complexes to photoexcitation.

Covalent Photosensitizer-Polyoxometalate-Catalyst Dyads for Visible-Light-Driven Hydrogen Evolution

A general concept for the covalent linkage of coordination compounds to bipyridine-functionalized polyoxometalates is presented. The new route is used to link an iridium photosensitizer to an Anderson-type hydrogen-evolution catalyst. This covalent dyad catalyzes the visible-light-driven hydrogen evolution reaction (HER) and shows superior HER activity compared with the non-covalent reference. Hydrogen evolution is observed over periods >1 week. Spectroscopic, photophysical, and electrochemical analyses give initial insight into the stability, electronic structure, and reactivity of the dyad. The results demonstrate that the proposed linkage concept allows synergistic covalent interactions between functional coordination compounds and reactive molecular metal oxides.

Light-Driven Electron Accumulation in a Molecular Pentad

Accumulation and temporary storage of redox equivalents with visible light as an energy input is of pivotal importance for artificial photosynthesis because key reactions, such as CO2 reduction or water oxidation, require the transfer of multiple redox equivalents. We report on the first purely molecular system, in which a long-lived charge-separated state (τ≈870 ns) with two electrons accumulated on a suitable acceptor unit can be observed after excitation with visible light. Importantly, no sacrificial reagents were employed.

Electron Transfer Rate Maxima at Large Donor-Acceptor Distances

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor-acceptor distance. We report here on electron transfer rate maxima at donor-acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor-photosensitizer-acceptor triads with p-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor-acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion.

Unusual distance dependences of electron transfer rates

There are regimes in which electron transfer rates increase with increasing donor–acceptor distance, leading to rate maxima at large donor–acceptor separations.