Ultrafast Back Electron Transfer Research Articles

Impact of Solvent-Solute Hydrogen Bonding on Ultrafast Electron Transfer in a Trimetallic Iron-Ruthenium Complex

Transition metal complexes (TMCs) are widely used as light absorbers and catalysts for photochemical energy conversion processes such as photocatalytic reactions in liquid environments. Of particular interest are TMCs which incorporate multiple bridged metal centres because they represent a step towards more complex supramolecular assemblies that can ultimately enable control of the charge flow in solution-based photo-redox processes. However, the development of design guidelines for such molecular assemblies is impeded by an incomplete understanding of the coupling between their electronic and nuclear dynamics, including the interplay of TMCs with their liquid environment upon photoexcitation. Probing the involved processes requires high spatial sensitivity towards electronic motion and the ability to capture atomic motion on ultrafast timescales – a combination which is not readily available for optical transient spectroscopic techniques.X-ray free electron lasers (XFELs) have revolutionized the field of ultrafast science in recent years. XFELs generate tuneable and extremely bright X-ray pulses with a typical pulse duration of <30 fs, which are well-suited as an element-specific probe of ultrafast electronic and atomic motion in molecules and materials.1,2 Applied to TMCs, XFEL probes are sensitive to the local electronic structure in such molecular systems as well as to intra- and inter-molecular structural reorganization at an atomic level.3 In this talk, I will present work on the interaction of a linear trimetallic cyanide-bridged iron-ruthenium complex (FeRuFe) with its solvent environment, carried out using the LCLS XFEL facility at SLAC National Accelerator Laboratory. The focus of this study lies on elucidating how photoinduced intramolecular electron transfer processes in FeRuFe are affected by hydrogen bonding interactions between its cyanide ligands and the surrounding solvent molecules. In water, such solvent-solute hydrogen bonding interactions are stronger for Fe(II) than Fe(III), leading to substantial changes in solute-solvent interactions upon photoinduced electron transfer between metal centres.The experiment is performed in a pump-probe geometry on a liquid jet, where a visible/near-infrared pump induces a metal-to-metal charge transfer (MMCT) between Ru and Fe which is followed by an ultrafast back-electron transfer (BET). The excited state dynamics are monitored using hard X-ray pulses as a function of time delay between the laser pump and the X-ray probe pulses. To resolve and correlate electronic and structural dynamics upon MMCT excitation, we simultaneously perform ultrafast Fe Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) measurements, where XES is sensitive to the oxidation and spin state of the Fe centres and XSS provides information on atomic motion in solute and solvent, as demonstrated in previous work on a similar system.4 To evaluate the effect of solute-solvent hydrogen bonding interactions on the electron transfer dynamics, we perform measurements in a series of solvents with different hydrogen bonding properties, including water, methanol, and acetonitrile. We find that the BET increases as a function of decreasing hydrogen bonding ability of the solvent, and that distinct signatures of solvation dynamics are present in the scattering data. The resulting molecular level understanding of solvent reorganization coupled to electron transfer demonstrates that the strength and type of solute-solvent interactions are a central factor in determining the outcome of photoinduced charge transfer processes in TMCs.References Bergmann, U. et al. Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes. Nat. Rev. Phys. 3, 264–282 (2021).Schoenlein, R. et al. Recent advances in ultrafast X-ray sources. Philos. Trans. R. Soc. Math. Phys. Eng. Sci. 377, 20180384 (2019).Gaffney, K. J. Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering. Chem. Sci. 12, 8010–8025 (2021).Biasin, E. et al. Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nat. Chem. 13, 343–349 (2021).

Generation of Long-Lived Photoinduced Charge Separation in a Supramolecular Toroidal Assembly.

Efficiencies of artificial photosynthetic and photocatalytic systems depend on their ability to generate long-lived charge-separated (CS) states in photoinduced electron transfer (PET) reactions. PET, in most cases, is followed by an ultrafast back electron transfer, which severely reduces lifetime and quantum yield of CS states. Generation of a long-lived CS state is an important goal in the study of PET reactions. Herein, we report that this goal is achieved using a hierarchically self-assembled anthracene-methyl viologen donor-acceptor system. Anthracene linked to two β-cyclodextrin molecules (CD-AN-CD) and methyl viologen linked to two adamantane units (AD-MV2+-AD) form an inclusion complex in water, which further self-assembled into well-defined toroidal nanostructures. The fluorescence of anthracene is highly quenched in the self-assembled system because of PET from anthracene to methyl viologen. Irradiation of the aqueous toroidal solution led to formation of a long-lived CS state. Rational mechanisms for the formation of the toroidal nanostructures and long-lived photoinduced charge separation are presented in the paper.

Femtosecond excited state dynamics of stilbene-viologen complexes with a weakly pronounced charge transfer.

The femtosecond dynamics of photoinduced electron transfers in supramolecular donor-acceptor complexes between (E)-bis(18-crown-6)stilbene (D) and tetraperchlorates of 2,7-di(2-ammonioethyl)(2,7-diazapyrenium) (A1), 3,3'-(E)-ethene-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A2) and 4,4'-ethane-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A3) was studied. The acceptors A2 and A3 are weak electron acceptors whose first reduction potentials are equal to -1.0 and -1.2 V (Ag), respectively, while A1 is a strong acceptor with a reduction potential of -0.42 V. It was shown that the back electron transfer time in CT-states of the complexes D·A2 and D·A3 is 30-40 ps, which is approximately 50 times greater than the analogous time for the charge transfer complexes studied earlier. The complex D·A1 is characterized by ultrafast back electron transfer (770 fs). The relaxation pathway of excited states of D·A1 depends on the wavelength of the excitation light. When excited at 356 nm, the accumulation of a transient locally excited (LE) state with a 250 fs lifetime was observed. But when excited at 425 nm, the formation of the LE-state was not observed.

Computational study on the removal of photolabile protecting groups by photochemical reactions

The application and versatility of photolabile protecting groups (PPGs) have continued to expand since their inception almost 50 years ago. In the present work we employ density functional theory (DFT) to characterize ground and excited state properties of several compounds with PPGs and to explore possible, controllable photodissociation reactions that remove these PPGs. The reaction mechanism is examined by analyzing the DFT results as well as dynamical simulations of nonadiabatic electron transfer reactions using multilayer multiconfiguration time-dependent Hartree theory. The photoexcited compounds appear to undergo ultrafast back electron transfer to the ground electronic state via internal conversion. For some compounds, the electronic-nuclear couplings facilitate sufficient photo energy transfer to the nuclear degrees of freedom that dominate the reaction coordinate, leading to the dissociation reaction that removes the PPGs.

Effect of geometrical parameters of dyad D–A and triad D–A1–A2 on the efficiency of ultrafast intramolecular charge separation from the second excited state

Ultrafast intramolecular charge transfer induced by the Soret-band excitation of molecular dyads, donor–acceptor, and triads, donor–acceptor1–acceptor2, has been explored within the stochastic point-transition model. The influence of the effective radii of the donor, acceptors, distances between them, and the angle between the directions of two sequential electron transfers on the quantum yield of thermalized charge separated states has been studied. A bell-shaped dependence of the quantum yield of the charge separated state on the center-to-center distance between the donor and acceptor is revealed for dyads. This dependence is a result of competition between two processes suppressing formation of the charge separated state. At short distances the electronic coupling is strong and the non-equilibrium charge recombination into the first excited state is very effective. At large distances the charge separation is slow and can not compete with internal conversion. This is the reason why the quantum yield is expected to be small in both limits. In triads the non-equilibrium electron transfer from the primary to the secondary acceptor can effectively compete with ultrafast back electron transfer into the first excited state of the donor and considerably increase the quantum yield of thermalized charge separated state. The strategy of maximizing the quantum yield is discussed. The regions of spatial parameters of triads and dyads maximizing the quantum yield of the charge separated states have been estimated for typical charge transfer parameters.

Photochemistry of P3HT and PC 60 BM in toluene solution: Evidence of T–T energy transfer

Abstract Photochemical properties of regioregular poly(3-hexylthiophene) (P3HT), fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PC 60 BM) and their 1:1 mixture in argon-saturated toluene solution were studied by optical spectroscopy and nanosecond laser flash photolysis ( λ ex = 532 nm). Separate excitation of components leads to formation of corresponding triplet state of fullerene ( 3 PC 60 BM*, λ max = 710 nm, ϵ max = 22400 M −1 cm −1 , φ T = 0.29) and polymer ( 3 P3HT*, λ max = 850 nm, ϵ 750 nm ≤ 10 5 M −1 cm −1 , φ T ≥ 0.19). Excitation of P3HT/PC 60 BM mixture leads to effective energy transfer from 3 P3HT* to the ground state of fullerene with a formation of 3 PC 60 BM*. No evidence of ion-radical P3HT + and PC 60 BM − formation was obtained though electron transfer between 3 P3HT* and 1 PC 60 BM is energetically favorable in studied system. Presumably ultrafast back electron transfer takes place in geminate P3HT + /PC 60 BM − pair with regeneration of the ground states of P3HT and PC 60 BM.

Efficiency of Intramolecular Charge Separation from the Second Excited State: Suppression of the Hot Charge Recombination by Electron Transfer to the Secondary Acceptor

Ultrafast intramolecular charge transfer induced by the Soret-band excitation of the donor-acceptor1-acceptor2 molecular triads has been explored within the stochastic point-transition model. It is shown that nonthermal (hot) charge transfer from the primary to the secondary acceptor, assisted by relaxation of solvent polarization, can effectively screen ultrafast back electron transfer into the first excited state of the donor. Ways to increase the quantum yield of the charge-separated states are discussed. The dependencies of the quantum yield of the charge-separated states on the main electron transfer parameters: the free energy gaps, the reorganization energy of the solvent and intramolecular vibrational modes, the electronic couplings, and the solvent relaxation timescale are revealed. The important role of the geometry of the donor-acceptor1-acceptor2 triad in charge separation effectiveness is emphasized. For the zinc-porphyrin-imide1-imide2 triad, the charge-transfer parameters maximizing the quantum yield of the charge separated states are estimated.

Ultrafast Dynamics of Nonequilibrium Electron Transfer in Photoinduced Redox Cycle: Solvent Mediation and Conformation Flexibility

We report here our systematic characterization of a photoinduced electron-transfer (ET) redox cycle in a covalently linked donor-spacer-acceptor flexible system, consisting of N-acetyl-tryptophan methylester as an electron donor and thymine as an electron acceptor in three distinct solvents of water, acetonitrile, and dioxane. With femtosecond resolution, we determined all the ET time scales, forward and backward, by following the complete reaction evolution from reactants to intermediates and finally to products. Surprisingly, we observed two distinct ET dynamics in water, corresponding to a stacked configuration with ultrafast ET in 0.7 ps and back ET in 4.5 ps and a partially folded C-clamp conformation with ET in 322 ps but back ET in 17 ps. In acetonitrile and dioxane, only the C-clamp conformations were observed with ET in 470 and 1068 ps and back ET in 110 and 94 ps, respectively. These relatively slow ET dynamics in hundreds of picoseconds all showed significant conformation heterogeneity and followed a stretched decay behavior. With both forward and back ET rates determined, we derived solvent reorganization energies and coupling constants. Significantly, we found that solvent molecules intercalated in the cleft of the C-clamp structure mediate electron transfer with a tunneling parameter (β) of 1.0-1.4 Å(-1) and the high-frequency vibration modes in the product(s) couple with the back ET process, leading to the ultrafast back ET dynamics in tens of picoseconds. These findings provide mechanistic insights of nonequilibrium ET dynamics modulated by conformation flexibility, mediated by unique solvent configuration, and accelerated by vibrational coupling.

Communication: Probing non-equilibrium vibrational relaxation pathways of highly excited C≡N stretching modes following ultrafast back-electron transfer

Fifth-order nonlinear visible-infrared spectroscopy is used to probe coherent and incoherent vibrational energy relaxation dynamics of highly excited vibrational modes indirectly populated via ultrafast photoinduced back-electron transfer in a trinuclear cyano-bridged mixed-valence complex. The flow of excess energy deposited into four C≡N stretching (ν(CN)) modes of the molecule is monitored by performing an IR pump-probe experiment as a function of the photochemical reaction (τ(vis)). Our results provide experimental evidence that the nuclear motions of the molecule are both coherently and incoherently coupled to the electronic charge transfer process. We observe that intramolecular vibrational relaxation dynamics among the highly excited ν(CN) modes change significantly en route to equilibrium. The experiment also measures a 7 cm(-1) shift in the frequency of a ∼57 cm(-1) oscillation reflecting a modulation of the coupling between the probed high-frequency ν(CN) modes for τ(vis) < 500 fs.

On the Role of High-Frequency Intramolecular Vibrations in Ultrafast Back-Electron Transfer Reactions

Femtosecond infrared spectroscopy is used to study photoinduced metal-to-metal charge transfer in the mixed-valence complex [(NC)5FeII–CN–PtIV(NH3)4–NC–FeII(CN)5]4– dissolved in D2O. Four intramolecular cyanide stretching (νCN) vibrations create a multidimensional probe of vibrational excitation, redistribution, and relaxation dynamics following ultrafast back-electron transfer (BET). We find that BET to the electronic ground state occurs in 110 ± 10 fs, during which greater than 6 quanta (n > 6) of vibrational energy are directed into the bridging νCN mode (νbridge). Intramolecular vibrational energy redistribution from the νbridge mode excites a solvent-accessible νCN mode on a 630 ± 50 fs time scale. Vibrational cooling to n = 1 and vibrational relaxation ensue on time scales of 1.3 ± 0.1 and 15–20 ps, respectively. These results highlight the important role played by a coupled network of high-frequency vibrations in ultrafast charge transfer processes in solution.

Ultrafast back electron transfer processes in the photoexcited methylviologen-iodide charge transfer complexes

The ultrafast back electron transfer in the excited charge transfer complexes of the methylviologen with iodide ions has been investigated using femtosecond transient absorption spectroscopy. Methylviologen and iodide form two types of charge transfer complexes each characterized by a charge transfer band in the same spectral region. At low I- concentrations mainly a 1:1 complex MV2+(I-) is present while at high I- concentrations both 1:1 and 1:2 complexes MV2+(I-)2 can be observed. Ultrashort laser pulses at 400 nm are used to excite both complexes in their charge transfer band. The observed transient absorption can be represented by a biexponential function with 1 ps and 20 ps time constants and attributed to the decay of the MV+./I. and MV+./I2.- radical pair respectively. The excitation of the 1:1 complex leads to the formation of the MV+./I. radical pair while the excitation of the 1:2 complex leads to the formation of the MV+./I. and MV+./I2.- radical pairs.