Back Electron Transfer Research Articles

Unveiling a Three Phase Mixed Heterojunction via Phase‐Selective Anchoring of Polymer for Efficient Photocatalysis

AbstractThe ligand‐to‐metal charge transfer (LMCT) facilitated activation of TiO2 has noteworthy potential for solar energy harvesting. However, the fast back electron transfer from TiO2 to an oxidized sensitizer is a key limiting factor causing low photocatalyst efficiency. Herein, a new catalyst design to both increase LMCT efficiency and minimize the back electron transfer is presented. A phase‐selective modification of mixed‐phase TiO2 (anatase: rutile interface) with poly‐salophen organic polymer is developed. The salophen and salen family organic monomers are selectively bound and polymerized on the anatase phase but not the rutile phase, which results in the formation of a three‐phase system. Such a three‐phase system converts an unfavorable polymer TiO2 core‐shell structure to an intimately mixed blend morphology, consisting of interfaced crystalline rutile TiO2 and an amorphous polymer‐covered anatase‐phase TiO2. The developed mixed‐blend morphology poly‐S@P25 can produce H2 of 37 410 µmol h–1 g–1 of polymer, which is ≈3.4 times higher than core‐shell poly‐S@anatase TiO2. This approach overcomes the drawback of the traditional core‐shell structured system for efficient electron harvesting from the LMCT process.

An Ab Initio Molecular Dynamics Study of the Mechanism and Rate of Dye Regeneration by Iodide Ions in Dye-Sensitized Solar Cells

In the ambition to improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs), it will be essential to understand the mechanisms and rates of dye regeneration. Although the mechanism of dye regeneration has been studied by static density functional theory (DFT) and classical molecular dynamics (CMD) simulations, ab initio molecular dynamics simulation (aiMD) has the potential to combine the insights from both methods for a deeper understanding. In this work, a series of aiMD simulations has been performed to study the interaction between an oxidized organic model dye, LEG4, and an electrolyte containing iodide ions as reducing agents. Dynamic Mulliken and natural spin population analyses show that two iodide ions, I–···I–, are required for dye regeneration. It was found that a distance between I–···I– of less than 6.5 Å at site 1 benefits from the electrostatic environment of the triphenylamine group of the LEG4 dye, and a corresponding distance of 4.8 Å at site 2 is essential for the dye regeneration process to take place. The rate constants of the LEG4 regeneration by two iodine ions range from 105 to 1012 s–1, spanning a window in which results from both experimental and static theoretical calculations fall. It is also verified that the probability of electron transfer from a radical I2– to the oxidized LEG4 dye is extremely low due to the rapid electron back-transfer. However, it has been found that the addition of an additional iodide ion at a distance of 5 Å with respect to the radical I2– opens the pathway for the reduction of the oxidized LEG4 dye with an associated formation of I3–. The current results highlight the necessity for a dynamical approach for a full understanding of the regeneration process.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad.

Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with MnII present as a proxy for a catalytic site, a strongly accelerated decay of the imidazole radical together with a decreased rate of back electron transfer from the external electron acceptor to the oxidized complex was observed. This transient formation of an imidazolyl radical is clear evidence for the function of the imidazole group as an electron relay. The implication of the imidazole proton and the external base for the kinetics and energetics of the electron trafficking is discussed.

Exciton decay mechanism in DNA single strands: back-electron transfer and ultrafast base motions.

The photochemistry of DNA systems is characterized by the ultraviolet (UV) absorption of π-stacked nucleobases, resulting in exciton states delocalized over several bases. As their relaxation sensitively depends on local stacking conformations, disentangling the ensuing electronic and structural dynamics has remained an experimental challenge, despite their fundamental role in protecting the genome from potentially harmful UV radiation. Here we use transient absorption and transient absorption anisotropy spectroscopy with broadband femtosecond deep-UV pulses (250–360 nm) to resolve the exciton dynamics of UV-excited adenosine single strands under physiological conditions. Due to the exceptional deep-UV bandwidth and polarization sensitivity of our experimental approach, we simultaneously resolve the population dynamics, charge-transfer (CT) character and conformational changes encoded in the UV transition dipoles of the π-stacked nucleotides. Whilst UV excitation forms fully charge-separated CT excitons in less than 0.3 ps, we find that most decay back to the ground state via a back-electron transfer. Based on the anisotropy measurements, we propose that this mechanism is accompanied by a structural relaxation of the photoexcited base-stack, involving an inter-base rotation of the nucleotides. Our results finally complete the exciton relaxation mechanism for adenosine single strands and offer a direct view into the coupling of electronic and structural dynamics in aggregated photochemical systems.

Back electron transfer rates determine the photoreactivity of donor-acceptor stilbene complexes in a macrocyclic host.

Host-guest 2 : 1 complexation of photoreactive alkene guests improves the selectivity of [2 + 2] photodimerizations by templating alkene orientation prior to irradiation. Host-guest chemistry can also provide 1 : 1 : 1 complexes through the inclusion of electronically complementary donor and acceptor guests, but the photoreactivity of such complexes has not been investigated. We imagined that such complexes could enable selective cross-[2 + 2] photocycloadditions between donor and acceptor stilbenes. In pursuit of this strategy, we investigated a series of stilbenes and found 1 : 1 : 1 complexes with cucurbit[8]uril that exhibited charge-transfer (CT) absorption bands in the visible and near-IR regions. Irradiation of the CT band of an azastilbene, 4,4'-stilbenedicarboxylate, and cucurbit[8]uril ternary complex led to a selective cross-[2 + 2] photocycloaddition, while other substrate pairs exhibited no productive chemistry upon CT excitation. Using transient absorption spectroscopy, we were able to understand the variable photoreactivity of different stilbene donor-acceptor complexes. We found that back electron transfer following CT excitation of the photoreactive complex is positioned deep in the Marcus inverted region due to electrostatic stabilization of the ground state, allowing [2 + 2] to effectively compete with this relaxation pathway. Control reactions revealed that the cucurbit[8]uril host not only serves to template the reaction from the ground state, but also protects the long-lived radical ions formed by CT from side reactions. This protective role of the host suggests that donor-acceptor host-guest ternary complexes could be used to improve existing CT-initiated photochemistry or access new reactivity.

Solvatofluorochromic Contrast with Supramolecular Stereoisomers Using Linked Rotaxane Structures to Investigate Local Solvation in Excited Donor-Bridge-Acceptor Systems

Abstract To optimize the photophysical properties of donor-bridge-acceptor (D-B-A) systems, the elucidation of the excited state of each component (i.e., D, B, and A) plays a crucial role in controlling the forward and back electron transfer rates. However, the direct evaluation of solvation spheres and their rearrangement on the excited bridge is still challenging. In this work, we report a method that exploits a linked rotaxane structure to uncover the solvation effects on an excited bridge by leveraging solvatofluorochromic contrast. We synthesize a D-insB-A system, in which a diphenylacetylene-based bridging moiety is insulated by a covalently linked permethylated α-cyclodextrin (PM α-CD), as a structural isomer of the uninsulated counterpart (D-uninsB-A). In the D-insB-A system, PM α-CD selectively covers the bridge while the donor and acceptor remain exposed. In this way, a comparison of the fluorescence wavelengths of D-insB-A and D-uninsB-A can reveal the local solvation effects on the excited bridge moiety. The investigation of the fluorescence properties in various solvents highlights a weak solvent interaction with the excited bridge in protic solvents and strong one in chlorinated polar solvents. These results provide crucial insight into the properties of excited D-B-A systems from the perspective of external environments, including solvent interactions.

Photocatalyst assemblies with two halide ions

A series of ruthenium polypyridyl photocatalysts bearing amide functional groups were designed that successfully promoted halide assembly in CH2Cl2 and CH3CN solution. In CH2Cl2, halide assembly was accompanied by a visible color change, and the spectral changes presented clear evidence for two halide binding events, yielding a 1:2 ruthenium:halide assembly. In the more polar solvent CH3CN, 1:2 assembly structures were also observed with chloride, bromide, and iodide, and large equilibrium constants were measured for association of the first and second halide (K11 = 0.04 – 2 × 106 M−1, K12 = 0.01 – 3 × 105 M−1). Varying the functional groups on the ancillary ligands tuned the excited-state reduction potentials (Ru2+⁎/+), resulting in a photocatalyst capable of performing iodide oxidation. Quenching of the photocatalyst excited state resulted in static and dynamic quenching, and a Stern-Volmer analysis yielded two linear regions at low and high iodide concentrations. The dynamic quenching rate constants (kq = 6.8 and 4.0 × 1010 M−1 s−1) and static quenching constants (KS = 2.4 and 0.13 × 104 M−1) at low and high iodide concentrations, respectively, were consistent with dynamic quenching of Ru2+ and [Ru2+,I−]+, and static quenching of [Ru2+,I−]+and [Ru2+,2I−]. Transient absorption spectroscopy revealed that the quenching reaction yielded a reduced ruthenium (Ru+) as the primary photoproduct and diiodide (I2•−) as a secondary photoproduct. The second-order rate constant for I2•− formation was measured to be 2.5 × 1010 M−1 s−1, a value consistent with the diffusion limited reaction. The transient absorption data indicates that oxidized halide photoproducts only result from the diffusional quenching reactions, and not from static quenching with an associated iodide ion. Fast back-electron transfer rates and low cage-escape yields in the ruthenium:iodide assemblies are invoked to explain why the static quenching pathway does not lead to measurable photoproduct yields.

Holistically modulating charge recombination via trisiloxane surface treatment for efficient dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) with TiO2 as electron transport layer (ETL) demonstrate great potential in the field of photoelectric devices featuring low cost and high power conversion efficiency (PCE). However, the photovoltaic performance of DSSCs based on TiO2 ETL is still confined by the undesirable charge recombination occurring in TiO2 layer (intrinsic defects), dye layer (self-quenching) and at the interface between TiO2 and electrolyte (back electron transfer). Herein, we firstly adopted a trisiloxane molecule (denoted as TSi) to holistically reduce the abovementioned charge recombination of DSSCs via passivating surface defects of TiO2, fine-tuning dye molecules arrangement and forming interfacial energy barrier. As a result, the Z907-based DSSCs with TiO2/TSi ETL deliver a high PCE of 9.43%, increasing over 16% than that of DSSCs without TSi treatment. Meanwhile, the DSSCs based on TiO2/TSi ETL present stronger endurance to high humidity and better long-term stability versus that of the DSSCs without TSi treatment. Our work provides a simple yet effective strategy to holistically reduce charge recombination of devices for improving the photovoltaic performance and stability of DSSCs.

Ruthenium (II) Complexes Bearing Heteroleptic Terpyridine Ligands: Synthesis, Photophysics and Solar Energy Conversion

AbstractHeteroleptic ruthenium (II) complexes featuring donor functionalized phenyl‐terpyridine (ph‐tpy) and a monocarboxylic‐(ph‐tpy)/(tpy) are synthesized and characterized. Reactions of ruthenium (II) precursors at 80 °C favored heteroleptic complexes formation over the homoleptic side products. Visible light excitation of these complexes resulted in the metal‐to‐ligand charge transfer (MLCT) transitions. The inter‐planar torsional angle between the atoms of donor functionalized phenyl ring and the central pyridine (py) of the tpy core strongly influences visible light absorption and photovoltaic properties. The lower inter‐ring py‐ph torsion in the acceptor end of the MLCT structures and its increase in the oxidized doublets could prevent the back electron transfer. The ruthenium atom and the acceptor functionalized tpy host the triplet‐MLCT spin density. Ambient temperature excited‐state decay followed the energy gap law and occurred in the order of a few nanoseconds. Herein, we evaluate the photosensitizing ability of these complexes via a combined experimental and computational approach.

The Concept of Photozymes: Short Peptides with Photoredox Catalytic Activity for Nucleophilic Additions to α‐Phenyl Styrenes

AbstractConventional photoredox catalytic additions of alcohols to olefins require additives, like thiophenol, to promote back electron transfer. The concept of “photozymes” assumes that forward and backward electron transfer steps in a photoredox catalytic cycle are controllable by substrate binding to photocatalytically active peptides. Accordingly, we synthesized a short tripeptide modified with 1,7‐dicyano‐perylene‐3,4 : 9,10‐tetracarboxylic acid bisimide as photoredox catalyst. This peptide undergoes an unconventional photoredox catalytic cycle with the radical anion and dianion of the perylene bisimide‐peptide as intermediates. The photoredox catalytic reactions with α‐phenyl styrenes as substrates require remarkably low catalyst loadings (0.5 mol%) and give the methoxylation products in high yields. The concept of “photozymes” for photoredox catalysis has significant potential for other photocatalytic reactions, in particular with respect to enantioselective photocatalysis.

Origin of Kinetic Dispersion in Eosin-Sensitized TiO2: Insights from Single-Molecule Spectroscopy

By removing the effects of ensemble averaging and molecular aggregation, we untangle the factors that govern the dispersive electron transfer kinetics of eosin-sensitized TiO2, focusing on the impact of environmental heterogeneity versus injection from multiple excited states. The blinking dynamics of single eosin Y chromophores on nanocrystalline TiO2 films are analyzed using a change point detection algorithm for binned data. Robust statistical analysis based on maximum likelihood estimation, Kolmogorov–Smirnov tests, and log likelihood ratio tests is used to determine the functional form that best fits the resulting on- and off-time distributions and to distinguish between mechanisms for dispersive electron transfer. Using this approach, we find that the on- and off-time distributions for eosin Y on TiO2 are best fit to lognormal distributions corresponding to μon = −0.64 ± 0.04, σon = 1.52 ± 0.02, μoff = −0.23 ± 0.04, and σoff = 1.96 ± 0.03, respectively. Monte Carlo simulations based on the Albery model for dispersive electron transfer (i.e., where the median rate constant κ is modified by the exponential of a parameter, x, that is normally distributed, k = κ e–γx) successfully reproduce this behavior using a median rate constant for injection and back electron transfer of ∼1010 and ∼104 s–1, respectively, and a corresponding energetic dispersion, γ, of ∼200–350 meV. To examine how injection from both the singlet and triplet excited states contributes to this dispersion, we studied two rhodamine sensitizers, R123 and 5ROX, that inject only from their singlet excited state. Surprisingly, when access to the T1 state is minimized in going from EY to R123 and 5ROX, kinetic dispersion actually increases. Collectively, these observations support the interpretation that static and dynamic inhomogeneities at the EY–TiO2 interface govern kinetic dispersion, with dynamic fluctuations in binding configuration and/or vibrational motion playing a decisive role.

Ultrafast Interface Charge Separation in Carbon Nanodot-Nanotube Hybrids.

Carbon dots are an emerging family of zero-dimensional nanocarbons behaving as tunable light harvesters and photoactivated charge donors. Coupling them to carbon nanotubes, which are well-known electron acceptors with excellent charge transport capabilities, is very promising for several applications. Here, we first devised a route to achieve the stable electrostatic binding of carbon dots to multi- or single-walled carbon nanotubes, as confirmed by several experimental observations. The photoluminescence of carbon dots is strongly quenched when they contact either semiconductive or conductive nanotubes, indicating a strong electronic coupling to both. Theoretical simulations predict a favorable energy level alignment within these complexes, suggesting a photoinduced electron transfer from dots to nanotubes, which is a process of high functional interest. Femtosecond transient absorption confirms indeed an ultrafast (<100 fs) electron transfer independent of nanotubes being conductive or semiconductive in nature, followed by a much slower back electron transfer (≈60 ps) from the nanotube to the carbon dots. The high degree of charge separation and delocalization achieved in these nanohybrids entails significant photocatalytic properties, as we demonstrate by the reduction of silver ions in solution. The results are very promising in view of using these “all-carbon” nanohybrids as efficient light harvesters for applications in artificial photocatalysis and photosynthesis.

Effect of binding geometry on interfacial charge transfer in a hybrid dye sensitizer based on a cadmium selenium quantum dot

The separation of photoexcited charge carriers at the quantum dot (QD)-dye interface is determined by the direction of the charge transfer and electronic coupling. We carried out DFT-based studies to analyze the variation of energy level alignment and the associated QD-dye electronic couplings with respect to the surface orientation of Ru505 dye adsorbed on a CdSe QD. Our calculations show that the structures with two carboxylate anchors have the most stable adsorption on the QD surface. These structures have comparable electronic couplings for electron transfer and back electron transfer, and also possess large electronic couplings for hole extraction from the QD, reducing the chance of photoexcited electrons returning to the QD ground state. Using a cyano group to couple a dye to a QD surface stabilizes the dye’s occupied orbitals, making hole extraction thermodynamically unfavorable. In addition, these structures show no improvement in their electronic couplings when the cyano group is introduced as anchors. Overall, our computational work advances our understanding of the surface chemistry of a CdSe QD functionalized by Ru505 dye, which may help design QD-based materials for photovoltaic and photocatalytic applications by manipulating the morphologies of QD-dye composites.

Ultrafast Electron Transfer in a Self-Assembling Sulfonated Aluminum Corrole-Methylviologen Complex.

Photoinduced electron transfer systems can mimic certain features of natural photosynthetic reaction centers, which are crucial for solar energy production. Among other tetra-pyrroles, the versatile chemical and photophysical properties of corroles make them very promising donors applicable in donor-acceptor complexes. Here, we present a first comprehensive study of ultrafast photoinduced electron transfer in a self-assembling sulfonated aluminum corrole-methylviologen complex combining visible and mid-IR transient absorption spectroscopy. The noncovalent D-A association of the corrole-methylviologen complex has the great advantage that photoinduced charge separation becomes possible even though the back electron transfer (BET) rate is large. Initial forward electron transfer from corrole to methylviologen is observed on an ∼130 fs time scale. Subsequent back electron transfer takes place with τBET = (1.8 ± 0.5) ps, revealing very complex relaxation dynamics. Direct probing in the mid-IR allows us to unravel the back electron transfer and cooling dynamics/electronic reorganization. Upon tracing the dynamics of the methylviologen-radical marker band at 1640 cm-1 and the C═C stretching of corrole at around 1500 cm-1, we observe that large amounts of excess energy survive the back transfer, leading to the formation of hot ground state absorption. A closer examination of the signal after 300 ps, surviving the back transfer, exhibits a charge-separation yield of 10-15%.

Boosting Photoredox Catalysis Using a Two-Dimensional Electride as a Persistent Electron Donor.

Electrides, which have excess anionic electrons, are solid-state sources of solvated electrons that can be used as powerful reducing agents for organic syntheses. However, the abrupt decomposition of electrides in organic solvents makes controlling the transfer inefficient, thereby limiting the utilization of their superior electron-donating ability. Here, we demonstrate the efficient reductive transformation strategy which combines the stable two-dimensional [Gd2C]2+·2e- electride electron donor and cyclometalated Pt(II) complex photocatalysts. Strongly localized anionic electrons at the interlayer space in the [Gd2C]2+·2e- electride are released via moderate alcoholysis in 2,2,2-trifluoroethanol, enabling persistent electron donation. The Pt(II) complexes are adsorbed onto the surface of the [Gd2C]2+·2e- electride and rapidly capture the released electrons at a rate of 107 s-1 upon photoexcitation. The one-electron-reduced Pt complex is electrochemically stable enough to deliver the electron to substrates in the bulk, which completes the photoredox cycle. The key benefit of this system is the suppression of undesirable charge recombination because back electron transfer is prohibited due to the irreversible disruption of the electride after the electron transfer. These desirable properties collectively serve as the photoredox catalysis principle for the reductive generation of the benzyl radical from benzyl halide, which is the key intermediate for dehalogenated or homocoupled products.

Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy.

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO4) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO4 with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO4 and BiVO4/NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole (kh+0) to electron (ke-0) via the photocatalyst of BiVO4/NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO4 under illumination. In this case, the ratio of the rate constants kh+0/ke-0 stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuOx onto BiVO4 further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.

PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst.

Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfer (PCET) process in Photosystem II, a benzimidazole-phenol (BIP) has been covalently attached to the 2,2'-bipyridyl ligand of [Ir(dF(CF3)ppy)2(bpy)][PF6] (dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = 2,2'-bipyridyl). Excitation of the [Ir(dF(CF3)ppy)2(BIP-bpy)][PF6] photocatalyst results in intramolecular PCET to form a charge-separated state with oxidized BIP. Subsequent reduction of methyl viologen dication (MV2+), a substrate surrogate, by the reducing moiety of the charge separated species demonstrates that the inclusion of BIP significantly slows the charge recombination rate. The effect of ∼24-fold slower charge recombination in a photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.

Influence of pH on radical reactions between kynurenic acid and amino acids tryptophan and tyrosine. Part II. Amino acids within the protein globule of lysozyme

An acidosis, a decrease of pH within a living tissue, may alter yields of radical reactions if participating radicals undergo partial or complete protonation. One of photosensitizers found in the human eye lens, kynurenic acid (KNA−), possesses pKa 5.5 for its radical form that is close to physiological pH 6.89 for a healthy lens. In this work we studied the influence of pH on mechanisms and products of photoinduced radical reactions between KNA− and amino acids tryptophan (Trp) and tyrosine (Tyr) within a globule of model protein, Hen White Egg Lysozyme (HEWL). Our results show that the rate constant of back electron transfer from kynurenyl to HEWL• radicals with the restoration of initial reagents – the major decay pathway for these radicals – does not change in the pH 3–7. The quantum yield of HEWL degradation is also pH independent, however a shift of pH from 7 to 5 completely changes the outcome of photoinduced damage to HEWL from intermolecular cross-linking to oxygenation. HPLC-MS analysis has shown that four of six Trp and all Tyr residues of HEWL are modified in different extents at all pH, but the lowering of pH from 7 to 5 significantly changes the direction of main photodamage from Trp62 to Trp108 located at the entrance and bottom of enzymatic center, respectively. A decrease of intermolecular cross-links via Trp62 is followed by an increase in quantities of intramolecular cross-links Tyr20-Tyr23 and Tyr23-Tyr53. The obtained results point out the competence of cross-linking and oxygenation reactions for Trp and Tyr radicals within a protein globule and significant increase of oxygenation to the total damage of protein in the case of cross-linking deceleration by coulombic repulsion of positively charged protein globules.

Self-Assembly and Photochemistry of a Pyrene-Methyl Viologen Supramolecular Fiber System.

This paper reports the self-assembly of a donor-acceptor system into nanoscopic structures and the photo processes taking place within these structures. The donor employed is pyrene linked to two β-cyclodextrin molecules (CD-PY-CD), and adamantane-linked methyl viologen attached to the three arms of mesitylene (Ms-(MV2+-AD)3) is the acceptor. CD-PY-CD and Ms-(MV2+-AD)3 when dissolved in water self-assembled into vesicles, which joined together to give long fibers. The self-assembly was studied using spectroscopic and microscopic techniques. Fluorescence of the pyrene chromophore was quenched within the self-assembled system due to efficient photoinduced electron transfer to methyl viologen. Photoinduced electron transfer within the assembly is confirmed through identification of product radical ions in flash photolysis experiments. Steady-state irradiation of the self-assembled system in an optical bench led to the formation of methyl viologen radical cation, which was stable for a few hours. Longevity of the radical cation was attributed to the fast reaction of pyrene radical cation with adjacent pyrene to give an unstable adduct, which slows down the back electron transfer process.

Intramolecular Photoinduced Charge Transfer and Recombination Dynamics in Vinylarene Terminated Organosilanes.

We report on charge-transfer dynamics of newly designed acceptor-donor-acceptor organosilanes, with a specific focus on how donor-acceptor combination and local chemical environment can be used to control the lifetime for intramolecular charge-separation between silane electron donors and organic acceptors. In this work linear oligosilanes were capped with arene-vinyl end groups of variable electron-accepting strength: weak (diester vinyl), intermediate (ester,cyano vinyl), and strong (dicyanovinyl). Ultrafast transient absorption spectroscopy was used to characterize their structure-dependent charge-transfer and recombination behaviors. All structures exhibit similar photoinduced ultrafast spectral dynamics that we ascribe to relaxation of the nascent charge-separated excited state followed by a return to the ground state via charge recombination. We find that relaxation of the nascent "hot" charge-separated excited state scales with the strength of dipole-dipole interactions between solvent molecules and the polar arene-vinyl acceptor. Furthermore, electron-accepting strength governs whether electronic coupling dictates charge recombination rate: weak acceptors produce charge-separated states that exhibit relatively large electronic coupling for back-electron transfer (approaching the adiabatic limit) that result in fast recombination, whereas the strong and moderate-strength acceptors support more stable charge-separated states with weaker coupling and longer lifetimes. We find that recombination rates increase substantially for structures with weak and moderate-strength acceptors in cyclohexane (i.e., negligible solvent reorganization energy), which we attribute to an increased electronic coupling in a nonpolar solvent environment where charge pairs are weakly screened. In contrast, for structures with strong electron acceptors, the very low reorganization energy of cyclohexane places back-electron transfer even further into the Marcus inverted regime, with a resultant increase in charge-separation lifetime. Together these results provide critical insights on how to tune photoinduced charge-transfer behavior in organic-inorganic hybrids that have potential material applications in molecular electronics and optoelectronics.