Ultrafast Photoinduced Electron Transfer Research Articles

Fluorescent metal nanoclusters: prospects for photoinduced electron transfer and energy harvesting.

Research on noble metal nanoclusters (MNCs) (elements with filled electron d-bands) is progressing forward because of the extensive and extraordinary chemical, optical, and physical properties of these materials. Because of the ultrasmall size of the MNCs (typically within 1-3 nm), they can be applied in areas of nearly all possible scientific domains. The greatest advantage of MNCs is the tunability that can be imposed, not only on their structures, but also on their chemical, physical, and biological properties. Nowadays, MNCs are very effectively used as energy donors and acceptors under suitable conditions and hence act as energy harvesters in solar cells, semiconductors, and biomarkers. In addition, ultrafast photoinduced electron transfer (PET) can be practised using MNCs under various circumstances. Herein, we have focused on the energy harvesting phenomena of Au-, Ag-, and Cu-based MNCs and elaborated on different ways to apply them.

Impact of the unrelaxed vibrational modes on hot-electron transfer.

The ultrafast photoinduced electron or exciton transfer was investigated theoretically. The charge separation on the ultrafast time scale results in the unrelaxed vibrational modes that appear in the initial terms of the generalized master equations. Here, the impact of these initial terms on the electron transfer directionality in the open system was evaluated. Moreover, the role of unrelaxed vibrational modes in electron-hole separation was also examined. It was shown that the unrelaxed vibrational modes significantly increase the efficiency of electron-hole separation. This could play a crucial role in the remarkable efficiency of charge separation in biological systems.

Photoenzymatic Enantioselective Intermolecular Radical Hydroamination.

Since the discovery of Hofmann-Löffler-Freytag reaction more than 130 years ago, nitrogen-centered radicals have been widely studied in both structures and reactivities1-2. Nevertheless, catalytic enantioselective intermolecular radical hydroamination remains a challenge due to the existence of side reactions, short lifetime of nitrogen-centered radicals, and lack of understanding of the fundamental catalytic steps. In chemistry, nitrogen-centered radicals are produced with radical initiators, photocatalysts, or electrocatalysts. On the other hand, the generation and reaction of nitrogen-centered radicals are unknown in nature. Here we report a pure biocatalytic system by successfully repurposing an ene-reductase through directed evolution for the photoenzymatic production of nitrogen-centered radicals and enantioselective intermolecular radical hydroaminations. These reactions progress efficiently at room temperature under visible light without any external photocatalysts and exhibit excellent enantioselectivities. Detailed mechanistic study reveals that the enantioselectivity originates from the radical-addition step while the reactivity originates from the ultrafast photoinduced electron transfer (ET) from reduced flavin mononucleotide (FMNH-) to nitrogen-containing substrates.

Ultrafast Photoinduced Dynamics of Quaterthiophene-Squaraine Hybrid Aggregates

Photoinduced energy transfer, charge transfer, and generation of sufficiently long-lived charge-separated (CS) states in the aggregated oligothiophenes are always challenging. Herein, we have coupled quaterthiophene (QTH) with a squaraine (SQR) dye in hybrid aggregates. A detailed time-resolved investigation using transient absorption spectroscopy and global target analyses revealed ultrafast photoinduced energy and electron transfer from QTH aggregates to SQR aggregates and the formation of a long-lived CS state in these hybrids. The pulse width limited energy transfer (<120 fs) occurred from the S1(Agg.)Ex. state, and the electron transfer occurred from both the S1(Agg.)Ex. state and the S1(Agg.)Rlx. state of QTH aggregates to the SQR aggregates at the timescales 131.5 ps and 3.3 ns, respectively. The resulting CS state lived for more than 8 ns. Moreover, the electrons of the QTH aggregates were prohibited to relax via the dark state and were extracted to the SQR aggregates which would lead to their successful utilization toward light-harvesting applications.

Critical assessment of selenourea as an efficient small molecule fluorescence quenching probe to monitor protein dynamics

This work unravels that reduced ΔE and higher −ve ΔG of the electron transfer reaction are crucial for an ultrafast intrinsic photoinduced electron transfer (PET) process that enables selenourea (SeU) as an efficient fluorescence quenching probe.

Solvent-Modulated Self-Assembly of Naphthalenediimide-Based Cd(II) Complexes and the Controllable Photochromism via Conformational Isomerization.

Rational regulation of the properties of photochromic materials is a challenging and meaningful work. In the present work, NDI-based complexes, namely, [Cd0.5(NDI)(HBDC)]·H2O (1) and a series of conformational isomers of {[Cd(NDI)0.5(BDC)]·MeCN}n (2), were synthesized by varying the solvent conditions (H2BDC = terephthalic acid, NDI = N,N'-bis(3-pyridylcarbonylhydrazine)-1,4,5,8-naphthalene diimide). Complex 1 exhibits a 0D mononuclear structure without photochromic behavior due to the bad conjugation of the naphthalene diimide moiety. The conformational isomers of complex 2 manifest a 3D network, showing ultra-fast photo-induced intermolecular electron transfer photochromic behavior under X-ray, UV, and visible light. However, they show different photochromic rates and coloring contrast upon photoirradiation, which originates from their difference in the distances of lone pair(COO)···π(NDI). This was realized via controlling the solvent ratio in the reaction system. In addition, compared to UV/X-ray light, 2 exhibits greater sensitivity to visible light and is an organic-inorganic hybrid material with photomodulated luminescence. Based on the excellent performance, complex 2 can be applied to filter paper, showing potential applications as an inkless printing medium and selective perception of ammonia and amine vapors in the solid state via different visual color changes.

Ultrafast charge transfer in a nonfullerene all-small-molecule organic solar cell: a nonadiabatic dynamics simulation with optimally tuned range-separated functional.

Herein, we have employed linear-response time dependent density functional theory (LR-TDDFT)-based nonadiabatic dynamics simulations to investigate the ultrafast charge transfer in a nonfullerene all-small-molecule donor-acceptor (D-A) system formed by a porphyrin small-molecule donor ZnP and a recently developed nonfullerene small-molecule acceptor 6TIC, during which the optimally tuned range-separated hybrid (OT-RSH) functional was adopted. In combination with static electronic structure calculations, several important conclusions were drawn. Firstly, the ZnP and 6TIC are more likely combined together non-covalently in parallel rather than in perpendicular to form ZnP-6TIC due to the much larger adsorption energies, i.e. -44.6 kcal mol-1vs. -25.2 kcal mol-1. Secondly, the excited state properties obtained by OT-RSH functionals seem more consistent with the experimental results compared to their untuned versions. Specifically, the energy of the lowest charge transfer (CT) state was predicted to be smaller than the lowest lying local excitation (LE) states using the OT-RSH functional-based LR-TDDFT calculations, which is beneficial for the charge transfer process that might be crucial for the high power conversion efficiency (PCE) achieved experimentally. In contrast, the untuned RS functionals all predict higher CT state energies, which is contradictory to the high PCE obtained in the experiment. Moreover, strong hybridization upon excitation between these states was revealed, which might be one of the reasons responsible for the high PCE observed in the experiment. Finally, ultrafast excited state relaxation can be completed within 500 fs due to the small energy gaps and the strong nonadiabatic couplings between these states, which is accompanied by ultrafast photoinduced electron transfer from ZnP to 6TIC and photoinduced hole transfer the other way around. The efficient charge transfer processes and the involvement of two charge generation channels might be another cause resulting in the excellent photovoltaic performance of ZnP-6TIC based OSCs. Our present work not only provides solid evidence for elucidating the underlying mechanism observed in previous experiments, but also suggests that the combination of OT-RSH functionals and LR-TDDFT-based nonadiabatic dynamics simulations might be a powerful tool for investigating the excited state dynamics of organic D-A systems, which is crucial for the theoretical design of novel OSCs with better performances in the future.

Development of an experimental apparatus to observe ultrafast phenomena by tender X-ray absorption spectroscopy at PAL-XFEL.

Understanding the ultrafast dynamics of molecules is of fundamental importance. Time-resolved X-ray absorption spectroscopy (TR-XAS) is a powerful spectroscopic technique for unveiling the time-dependent structural and electronic information of molecules that has been widely applied in various fields. Herein, the design and technical achievement of a newly developed experimental apparatus for TR-XAS measurements in the tender X-ray range with X-ray free-electron lasers (XFELs) at the Pohang Accelerator Laboratory XFEL (PAL-XFEL) are described. Femtosecond TR-XAS measurements were conducted at the Ru L3-edge of well known photosensitizer tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) in water. The results indicate ultrafast photoinduced electron transfer from the Ru center to the ligand, which demonstrates that the newly designed setup is applicable for monitoring ultrafast reactions in the femtosecond domain.

Electron transfer between carbon dots and tetranuclear Dawson-derived sandwich polyanions.

Among the photocatalysts which could be used for converting solar energy, polyoxometalates are often regarded as ideal candidates because of their remarkable performances in photocatalytic water splitting and photodegradation of pollutants. Nonetheless, these polyanions are only capable of absorbing UV light, unless coupled to a visible-light photosensitizer. Carbon nanodots are especially promising for this purpose because of their strong visible-light absorption, photostability, non-toxicity, and very low production costs. In this work we demonstrate the possibility of coupling carbon dots to polyoxometalates with different structures, by a simple self-assembly approach based on electrostatic interactions in solution phase. Our studies highlight an extremely efficient interaction between the two compounds, resulting in ultrafast photoinduced electron or energy transfer from carbon dots to the coupled polyoxometalates, depending on the structure of the latter, as revealed by a detailed study based on ultrafast transient absorption spectroscopy. The evidence herein provided shows how nanohybrids based on polyoxometalates photosensitized by carbon dots could find their place in photocatalytic applications, thanks to their remarkable efficiency and huge versatility.

Giant Shape-Persistent Tetrahedral Porphyrin System: Light-Induced Charge Separation.

Tetraphenylmethane appended with four pyridylpyridinium units works as a scaffold to self‐assemble four ruthenium porphyrins in a tetrahedral shape‐persistent giant architecture. The resulting supramolecular structure has been characterised in the solid state by X‐ray single crystal analysis and in solution by various techniques. Multinuclear NMR spectroscopy confirms the 1 : 4 stoichiometry with the formation of a highly symmetric structure. The self‐assembly process can be monitored by changes of the redox potentials, as well as by modifications in the visible absorption spectrum of the ruthenium porphyrin and by a complete quenching of both the bright fluorescence of the tetracationic scaffold and the weak phosphorescence of the ruthenium porphyrin. An ultrafast photoinduced electron transfer is responsible for this quenching process. The lifetime of the resulting charge separated state (800 ps) is about four times longer in the giant supramolecular structure compared to the model 1 : 1 complex formed by the ruthenium porphyrin and a single pyridylpyridinium unit. Electron delocalization over the tetrameric pyridinium structure is likely to be responsible for this effect.

Unblocked intramolecular charge transfer for enhanced CO2 photoreduction enabled by an imidazolium-based ionic conjugated microporous polymer

Efficient solar energy-driven conversion of CO2 to valuable chemicals is challenging. Here we design an imidazolium-based ionic conjugated microporous polymer (ImI-CMP), which unblocks readily intramolecular charge transfer and triggers CO2 photoreduction. Under visible light irradiation, ImI-CMP incorporated with Co (II) species exhibits a high CO production rate of 2953 μmol g−1 h−1 and a turnover frequency of 30.8 h−1; these numbers are competitive to that of the best porous organic polymers. The mechanism studies reveal that two factors play key roles in the outstanding photocatalytic performance. First, the imidazolium motifs on the ImI-CMP enhance the activation of CO2. Second, π-conjugation structure and the built-in electric field allow ultrafast intramolecular photoinduced electron transfer in the ImI-CMP. This work provides a new strategy for designing high-performance organic photocatalysts for CO2 reduction by combining cationic imidazolium motifs and π-conjugation structures.

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%.

Synergistic dynamics of photoionization and photoinduced electron transfer probed by laser flash photolysis and ultrafast fluorescence spectroscopy.

Photoionization (PI) and photoinduced electron transfer (PET) dynamics of coumarin 450 (C450) in micelles were investigated in the time domains of micro to femtoseconds using steady-state and time-resolved absorption and fluorescence spectroscopy. The PI of C450 occurs inside the micelles leads to the formation of C450 cation radical (CR) and hydrated electron, which is characterized by the respective transient absorption. The PI of C450 is monophotonic in nature and the yield is dependent on the charge of the micelles. The observation of amine CR in the transient absorption confirms the PET from amine to the excited state of C450 in micelles, which results in the quenching of both fluorescence intensity and lifetime. The decrease in femtosecond fluorescent decay of C450 and the absence of transient C450 radical anion in the presence of amine implies that the concerted ultrafast PET promoted PI and PET to the C450 CR-electron pair. The decrease in the time constant for the formation of relaxed state in the presence of amines is due to the ultrafast PET to the C450 CR-electron pair, which prevents the formation of a relaxed state through recombination at a longer time scale. In the present investigation, the recombination dynamics of the CR-electron pair is justified as one of the origins of the slow solvation in micelles. The influence of amine concentration on the decay of C450 CR indicates ET reaction between C450 CR and amine, which is further confirmed by the bleach recovery of C450 ground state in the presence of amine.

Ultrafast photoinduced electron transfer in o-aminobenzoate – d-Amino acid oxidase complex

Absorption spectrum of ortho-amino benzoate (oAB) – D-amino acid oxidase complex (ODC) displays a charge transfer (CT) band around 570 nm. The fluorescence decays of the second lowest electronic state of ODC were measured using a fluorescence up-conversion technique. The lifetimes were 100 fs and 130 fs at 500 nm and 514 nm fluorescence emission wavelengths, respectively. These ultrafast lifetimes are ascribed to the photoinduced electron transfer (PET) from the oAB to the excited isoalloxazine (Iso*) in the ODC. The PET mechanism in ODC was studied with Marcus-Kakitani-Mataga (MKM) theory. The protein structures were obtained by molecular dynamics simulation (MDS). Mean distances between Iso and oAB over 12,500 MDS snapshots were 0.60 in subunit A (Sub A) and 0.58 nm in Sub B. The calculated lifetimes at 514 nm-emission were 97 and 179 fs in Sub A and Sub B, while at 500 nm there were 97 and 101 fs, respectively. Solvent reorganization energy was quite different between Sub A and Sub B, which is main reason for difference in the calculated lifetimes. Electrostatic (ES) energies between the photoproducts of donor -acceptor, and net ES energies between the photoproducts and ionic groups in the protein were calculated with MDS structures. These ES energies were evaluated with charge densities of neutral radical of oAB produced after PET and anion radical of Iso obtained by semi-empirical molecular orbital (MO) method. The logarithmic PET rate did not display any clear dependence on Rc, despite the ultrafast scale of the PET rates. Most of the logarithmic PET rates in Sub A displayed in the inverted region of the standard free energy gap in the energy gap law. The transition dynamics among local excited states of Iso, CT state and the ion-pair state were discussed.

Ultrafast Photoinduced Electron Transfer in a Photosensitizer Protein

Photosystem II (PSII) is a large membrane protein (∼700 kDa) complex, harboring P680+, the strongest oxidant known in biological systems, which is responsible for driving tyrosine oxidation and ult...

Ultrafast Interfacial Electron Transfer from Graphene Quantum Dot to 2,4-Dinitrotoluene

Ultrafast photoinduced electron transfer (PET) from a photoexcited graphene quantum dot (GQD*) to an electron-deficient molecule 2,4-dinitrotoluene (DNT) is studied in a water–methanol mixture (1:1 by volume) at different temperatures (5 °C–60 °C). The temperature-dependent study reveals that quenching of GQD emission by DNT is a complex process, where use of collisional or static quenching alone in the fitting model cannot fit the entire time regime of the kinetics. Irrespective of temperature, the collisional quenching rate obtained from the TCSPC lifetime quenching study appears at the upper limit of the bimolecular diffusion-controlled rate of the medium. The weak solubility of DNT in the polar solvents leads to GQD–DNT complex formation through hydrophobic interactions, allowing one to obtain a diffusion-free ultrafast PET timescale of the complex using a femtosecond upconversion setup. GQD–DNT complex formation is manifested by the heat change in the isothermal titration calorimetry (ITC) study upon mixing of DNT with GQD. Findings of our work would reinforce the understanding of the interfacial charge transfer process of GQD and thereby expand the promises to its real applications, especially in sensing and photovoltaics where materials with ultrafast PET are highly desirable.

Efficient Photoinduced Energy and Electron Transfers in a Tetraphenylethene-Based Octacationic Cage Through Host-Guest Complexation.

Artificial photofunctional systems with energy and electron transfer functions, inspired from photosynthesis in nature, have been developed for many promising applications including solar cell, biolabeling, photoelectric materials, and photodriven catalysis. Supramolecular hosts including macrocycles and cages have been explored for simulating photosynthesis based on a host-guest strategy. Herein, we report a host-guest approach by using a tetraphenylethene-based octacationic cage and fluorescent dyes to construct artificial photofunctional systems with energy and electron transfer functions. The cage traps various dyes within its hydrophobic cavity to form 1:1 host-guest complexes via CH-π, π-π, and/or electrostatic interactions in solution. The efficient energy transfer and ultrafast photoinduced electron transfer between the cage and dyes are competitive processes with each other in artificial photofunctional systems. Spectroscopic techniques that confirm energy transfer from the fluorescent cage to dyes (e.g., NiR, R700, and R800) are efficient, which induce the red shift of fluorescence. On the other hand, ultrafast photoinduced electron transfer from dyes (e.g., ICG, AG, and AV) to the fluorescent cage can induce fluorescence quenching. This study provides an insight into the construction of artificial photofunctional systems with energy and electron transfer functions via a host-guest approach in solution.

Excitation-Wavelength-Dependent Photoinduced Electron Transfer in a π-Conjugated Diblock Oligomer.

Control of photoinduced electron transfer through selective excitation of a π-conjugated diblock oligomeric system featuring tetrathiophene (T4) and tetra(phenylene ethynylene) (PE4) donor blocks capped with a naphthalene diimide (NDI) acceptor (T4PE4NDI) is demonstrated. Each π-conjugated oligomeric segment has its own discrete ionization potential, electron affinity, and optical band gap which provides an absorption profile that has specific wavelengths that offer selective excitation of the PE4 and T4 blocks. Therefore, T4PE4NDI can be selectively excited to form a charge-separated state via ultrafast photoinduced electron transfer from the PE4 segment to NDI when excited at 370 nm, but it does not produce a charge-separated state when excited at 420 nm (T4). Picosecond transient absorption techniques were performed to probe the excited-state dynamics, revealing ultrafast charge separation (∼4 ps) occurring from the PE4 segment to NDI when excited at 370 nm, followed by delocalization of the hole over the T4 segment. On the contrary, electron transfer is suppressed with excitation at longer wavelengths (≥420 nm), where the spectrum is dominated by the T4 unit. The rate of electron transfer and charge recombination was investigated versus the length of the PE bridge unit in oligomers featuring zero and two PE units (T4NDI and T4PE2NDI). The rate of charge recombination decreases from 1.2 × 1011 to 1.0 × 109 s-1 with increasing bridge length between the T4 and NDI components (T4NDI to T4PE4NDI). Furthermore, wavelength-dependent photoinduced electron transfer was not observed in either T4NDI or T4PE2NDI due to an insufficient PEn bridge length. This work demonstrates the ability to use optical wavelength to control photoinduced electron transfer in a fully π-conjugated oligomer.

Competing ultrafast photoinduced electron transfer and intersystem crossing of [Re(CO)_3(Dmp)(His124)(Trp122)]^+ in Pseudomonas aeruginosa azurin: a nonadiabatic dynamics study

We present a computational study of sub-picosecond nonadiabatic dynamics in a rhenium complex coupled electronically to a tryptophan (Trp) side chain of Pseudomonas aeruginosa azurin, a prototypical protein used in the study of electron transfer in proteins. To gain a comprehensive understanding of the photoinduced processes in this system, we have carried out vertical excitation calculations at the TDDFT level of theory as well as nonadiabatic dynamics simulations using the surface hopping including arbitrary couplings (SHARC) method coupled to potential energy surfaces represented with a linear vibronic coupling model. The results show that the initial photoexcitation populates both singlet metal-to-ligand charge transfer (MLCT) and singlet charge-separated (CS) states, where in the latter an electron was transferred from the Trp amino acid to the complex. Subsequently, a complex mechanism of simultaneous intersystem crossing and electron transfer leads to the sub-picosecond population of triplet MLCT and triplet CS states. These results confirm the assignment of the sub-ps time constants of previous experimental studies and constitute the first computational evidence for the ultrafast formation of the charge-separated states in Re-sensitized azurin.

Integration of Enzymes and Photosensitizers in a Hierarchical Mesoporous Metal-Organic Framework for Light-Driven CO2 Reduction.

Protection of enzymes with synthetic materials is a viable strategy to stabilize, and hence to retain, the reactivity of these highly active biomolecules in non-native environments. Active synthetic supports, coupled to encapsulated enzymes, can enable efficient cascade reactions which are necessary for processes like light-driven CO2 reduction, providing a promising pathway for alternative energy generation. Herein, a semi-artificial system-containing an immobilized enzyme, formate dehydrogenase, in a light harvesting scaffold-is reported for the conversion of CO2 to formic acid using white light. The electron-mediator Cp*Rh(2,2'-bipyridyl-5,5'-dicarboxylic acid)Clwas anchored to the nodes of the metal-organic framework NU-1006 to facilitate ultrafast photo-induced electron transfer when irradiated, leading to the reduction of the coenzyme nicotinamide adenine dinucleotide at a rate of about 28 mM·h-1. Most importantly, the immobilized enzyme utilizes the reduced coenzyme to generate formic acid selectively from CO2 at a high turnover frequency of about 865 h-1 in 24 h. The outcome of this research is the demonstration of a feasible pathway for solar-driven carbon fixation.