Excited Electron Transfer Research Articles

New Ligand Design Provides Delocalization and Promotes Strong Absorption throughout the Visible Region in a Ru(II) Complex.

The new Ru(II)-anthraquinone complex [Ru(bpy)2(qdpq)](PF6)2 (Ru-qdpq; bpy = 2,2'-bipyridine; qdpq = 2,3-di(2-pyridyl)naphtho[2,3-f]quinoxaline-7,12-quinone) possesses a strong 1MLCT Ru → qdpq absorption with a maximum at 546 nm that tails into the near-IR and is significantly red-shifted relative to that of the related complex [Ru(bpy)2(qdppz)](PF6)2 (Ru-qdppz; qdppz = naphtho[2,3-a]dipyrido[3,2-h:2',3'-f]phenazine-5,18-dione), with λmax = 450 nm. Ru-qdppz possesses electronically isolated proximal and distal qdppz-based excited states; the former is initially generated and decays to the latter, which repopulates the ground state with τ = 362 ps. In contrast, excitation of Ru-qdpq results in the population of a relatively long-lived (τ = 19 ns) Ru(dπ) → qdpq(π*) 3MLCT excited state where the promoted electron is delocalized throughout the qdpq ligand. Ultrafast spectroscopy, used together with steady-state absorption, electrochemistry, and DFT calculations, indicates that the unique coordination modes of the qdpq and qdppz ligands impart substantially different electronic communication throughout the quinone-containing ligand, affecting the excited state and electron transfer properties of these molecules. These observations create a pathway to synthesize complexes with red-shifted absorptions that possess long-lived, redox-active excited states that are useful for various applications, including solar energy conversion and photochemotherapy.

Enhancing the electrical properties of dye-sensitized solar cells by carbon-free titanium gel via a non-hydrolytic method

Abstract In this study, a carbon free titanium gel was synthesized following a non-hydrolytic method, and was used as a binder for the TiO 2 electrodes of dye-sensitized solar cells (DSSCs). The concentration of the binding anode paste was optimized by using varying levels of titanium gel (10, 13, and 16 wt%) to fabricate the anode paste of DSSCs. The TiO 2 layers printed on transparent conducting oxide (TCO) glass substrates had good shape-retention without an organic binder after doctor blade printing, and the amorphous titanium gel crystallized into a pure anatase form from annealing at 450 °C for 1 h. The SEM and HR-TEM analyses revealed a mesoporous TiO 2 layer, and a uniform TiO 2 coating of ∼1.5 nm P-25-TiO 2 nanoparticles. This coating enhanced excited electron transfer and electrical contact between particles. The open-circuit voltage of DSSCs increased by about 50 mV more than that of the DSSCs with an organic-based binder. Furthermore, the dark saturation current of DSSCs fabricated with 10 wt% titanium gel decreased by 1/6 than organic binder-based DSSCs. In summary, the power conversion efficiency of DSSCs fabricated with a titanium gel binder was enhanced by 15.4%, compared to that of organic binder-based DSSCs.

Tungsten-doped TiO2/reduced Graphene Oxide nano-composite photocatalyst for degradation of phenol: A system to reduce surface and bulk electron-hole recombination

Recombination of photogenerated charges is the main factor affecting the photocatalytic activity of TiO2. Here, we report a combined strategy of suppressing both the bulk as well as the surface recombination processes by doping TiO2 with tungsten and forming a nanocomposite with reduced graphene oxide (rGO), respectively. Sol-gel method was used to dope and optimize the concentration of W in TiO2 powder. UV-Vis, XPS, PL and time resolved PL spectra along with DFT calculations indicate that W6+ in TiO2 lattice creates an impurity level just below the conduction band of TiO2 to act as a trapping site of electrons, which causes to improve the lifetime of the photo-generated charges. Maximum reduction in the PL intensity and the improvement in charge carrier lifetime was observed for TiO2 doped with 1 at.% W (1W-TiO2), which also displayed the highest photo-activity for the degradation of p-nitro phenol pollutant in water. Tuning of rGO/TiO2 ratio (weight) disclosed that the highest activity can be achieved with the composite formed by taking equal amounts of TiO2 and rGO (1:1), in which the strong interaction between TiO2 and rGO causes an effective charge transfer via bonds formed near the interface as indicated by XPS. Both these optimized concentrations were utilized to form the composite rGO/1W-TiO2, which showed the highest activity in photo-degradation of p-nitro phenol (87%) as compared to rGO/TiO2 (42%), 1W-TiO2 (62%) and pure TiO2 (29%) in 180 min. XPS and PL results revealed that in the present nanocomposite, tungsten species traps the excited electron to reduce the interband recombination in the bulk, while the interaction between TiO2 and rGO creates a channel for fast transfer of excited electrons towards the latter before being recombined on the surface defect sites.

Promotion of the excited electron transfer over Ni- and Co -sulfide co-doped g-C3N4 photocatalyst (g-C3N4/NixCo1\u2212xS2) for hydrogen Production under visible light irradiation

A Ni- and Co- sulfide co-doped g-C3N4 photocatalyst (g-C3N4/NixCo1−xS2) was prepared by hydrothermal method and this photocatalyst, namely, g-C3N4/NixCo1−xS2 shown excellent photocatalytic properties due to the special structure of Ni-Co-S with boundary different exposure to active site of transition metal-metal (Ni-Co) active planes. With the introduction of Co atoms, the H2 production amount reached the maximum about 400.81 μmol under continuous visible light irradiation for 4 hours based on the efficiently charge separation and greatly improved electron transfer resulted from the presence of sufficient active exposure at the boundary. The serial studies shown that the existence of Ni-Co-S structure over g-C3N4 active surface is the key factor of activity affections by means of several characterizations such as SEM, XRD, XPS diffuse reflectance etc. and the results of which were in good agreement with each other. A possible reaction mechanism over eosin Y-sensitized g-C3N4/NixCo1−xS2 photocatalyst under visible light irradiation was proposed.

Real-Time Quantum Dynamics of Long-Range Electronic Excitation Transfer in Plasmonic Nanoantennas.

Using large-scale, real-time, quantum dynamics calculations, we present a detailed analysis of electronic excitation transfer (EET) mechanisms in a multiparticle plasmonic nanoantenna system. Specifically, we utilize real-time, time-dependent, density functional tight binding (RT-TDDFTB) to provide a quantum-mechanical description (at an electronic/atomistic level of detail) for characterizing and analyzing these systems, without recourse to classical approximations. We also demonstrate highly long-range electronic couplings in these complex systems and find that the range of these couplings is more than twice the conventional cutoff limit considered by Förster resonance energy transfer (FRET)-based approaches. Furthermore, we attribute these unusually long-ranged electronic couplings to the coherent oscillations of conduction electrons in plasmonic nanoparticles. This long-range nature of plasmonic interactions has important ramifications for EET; in particular, we show that the commonly used "nearest-neighbor" FRET model is inadequate for accurately characterizing EET even in simple plasmonic antenna systems. These findings provide a real-time, quantum-mechanical perspective for understanding EET mechanisms and provide guidance in enhancing plasmonic properties in artificial light-harvesting systems.

First-Principles Models for Biological Light-Harvesting: Phycobiliprotein Complexes from Cryptophyte Algae.

There have been numerous efforts, both experimental and theoretical, that have attempted to parametrize model Hamiltonians to describe excited state energy transfer in photosynthetic light harvesting systems. The Frenkel exciton model, with its set of electronically coupled two level chromophores that are each linearly coupled to dissipative baths of harmonic oscillators, has become the workhorse of this field. The challenges to parametrizing such Hamiltonians have been their uniqueness, and physical interpretation. Here we present a computational approach that uses accurate first-principles electronic structure methods to compute unique model parameters for a collection of local minima that are sampled with molecular dynamics and QM geometry optimization enabling the construction of an ensemble of local models that captures fluctuations as these systems move between local basins of inherent structure. The accuracy, robustness, and reliability of the approach is demonstrated in an application to the phycobiliprotein light harvesting complexes from cryptophyte algae. Our computed Hamiltonian ensemble provides a first-principles description of inhomogeneous broadening processes, and a standard approximate non-Markovian reduced density matrix dynamics description is used to estimate lifetime broadening contributions to the spectral line shape arising from electronic-vibrational coupling. Despite the overbroadening arising from this approximate line shape theory, we demonstrate that our model Hamiltonian ensemble approach is able to provide a reliable fully first-principles method for computation of spectra and can distinguish the influence of different chromophore protonation states in experimental results. A key feature in the dynamics of these systems is the excitation of intrachromophore vibrations upon electronic excitation and energy transfer. We demonstrate that the Hamiltonian ensemble approach provides a reliable first-principles description of these contributions that have been detailed in recent broad-band pump-probe and two-dimensional electronic spectroscopy experiments.

Synthesis of TiO2/g-C3N4 nanocomposites with phosphate–oxygen functional bridges for improved photocatalytic activity

One of the most general methods to enhance the separation of photogenerated carriers for g-C3N4 is to construct a suitable heterojunctional composite, according to the principle of matching energy levels. The interface contact in the fabricated nanocomposite greatly influences the charge transfer and separation so as to determine the final photocatalytic activities. However, the role of interface contact is often neglected, and is rarely reported to date. Hence, it is possible to further enhance the photocatalytic activity of g-C3N4-based nanocomposite by improving the interfacial connection. Herein, phosphate–oxygen (P–O) bridged TiO2/g-C3N4 nanocomposites were successfully synthesized using a simple wet chemical method, and the effects of the P–O functional bridges on the photogenerated charge separation and photocatalytic activity for pollutant degradation and CO2 reduction were investigated. The photocatalytic activity of g-C3N4 was greatly improved upon coupling with an appropriate amount of nanocrystalline TiO2, especially with P–O bridged TiO2. Atmosphere-controlled steady-state surface photovoltage spectroscopy and photoluminescence spectroscopy analyses revealed clearly the enhancement of photogenerated charge separation of g-C3N4 upon coupling with the P–O bridged TiO2, resulting from the built P–O bridges between TiO2 and g-C3N4 so as to promote effective transfer of excited electrons from g-C3N4 to TiO2. This enhancement was responsible for the improved photoactivity of the P–O bridged TiO2/g-C3N4 nanocomposite, which exhibited three-time photocatalytic activity enhancement for 2,4-dichlorophenol degradation and CO2 reduction compared with bare g-C3N4. Furthermore, radical-trapping experiments revealed that the OH species formed as hole-modulated direct intermediates dominated the photocatalytic degradation of 2,4-dichlorophenol. This work provides a feasible strategy for the design and synthesis of high-performance g-C3N4-based nanocomposite photocatalysts for pollutant degradation and CO2 reduction.

Electronic excitation transfer from an organic matrix to CdS nanocrystals produced by the Langmuir–Blodgett method

The absorption, photoluminescence, and photoluminescence excitation spectra of CdS nanocrystals formed by the Langmuir–Blodgett method are explored. Features of the absorption and photoluminescence excitation spectra defined by optical transitions in the matrix and nanocrystals are identified. The efficiency of electronic excitation transfer from an organic matrix to nanocrystals is studied. It is shown that charge carriers efficiently transfer from the matrix to electron and hole size-quantization levels in nanocrystals and to acceptor defect levels in the band gap of nanocrystals. A large Stokes shift defined by fine exciton structure (bright and dark excitons) is observed. The shift is in the range 140–220 meV for nanocrystals 2.4 and 2.0 nm in radius.

Plasmon Processes of Electronic Energy Transfer to Adsorbed Rhodamine 6G During Clustering of Silver Nanoparticles on the Surface of Macroporous Silica

Spectral and kinetic features of the optical conversion of electronic energy were studied during dipole–dipole transfer of electronic excitation between silver (Ag) sol nanoparticles (NPs) of small and medium sizes and rhodamine 6G (R6G) molecules on the surface of a macroporous silica (C-80) with an irregular pore structure. It was found that Ag NPs were adsorbed unevenly to the C-80 surface with clusters of differently sized Ag NPs distributed as islets. Threshold gain of the light scattered by surface plasmons was detected at a certain concentration of Ag NPs on the silica. Local plasmon excitations in Ag NP clusters on the silica were evaluated quantitatively.

Luminescence of pyrazolic 1,3-diketone [formula omitted] complex with 1,10-phenanthroline

We studied luminescent properties of Pr3+ complex with pyrazole and 1, 10-phenanthroline ligand environment. It was demonstrated that the complex exhibits so-called “antenna” effect which involves the electronic excitation transfer from the ligands to the rare-earth ion. The observed complex structure of the luminescence spectrum specific to Pr3+ indicates that in the investigated compounds the luminescence originates from the rare-earth ion exclusively. The obtained data provided a means to identify the electronic transitions in the ion and to develop the corresponding energy diagram. Fluorescence lifetime imaging revealed uniform distribution of luminescence decay times for the Pr3+ complex crystalline phase. Consequently, the Pr3+ complex under study has one type of emission sites.

Active control of probability amplitudes in a mesoscale system via feedback-induced suppression of dissipation and noise

We demonstrate that a three-terminal potentiostat circuit reduces the coupling between an electronic excitation transfer (EET) system and its environment, by applying a low-noise voltage to its electrical terminals. Inter-state interference is preserved in the EET system by attenuating the dissipation in the quantum system arising from coupling to the surrounding thermodynamic bath. A classical equivalent circuit is introduced to model the environment-coupled excitation transfer for a simplified, two-state system. This model provides a qualitative insight into how the electronic feedback affects the transition probabilities and selectively reduces dissipative coupling for one of the participant energy levels EET system. Furthermore, we show that the negative feedback also constrains r.m.s. fluctuations of the energy of environmental vibrational states, resulting in persistent spectral coherence between the decoupled state and vibronic levels of the complementary state. The decoupled vibronic channel therefore can serve as a probe for characterizing the vibronic structure of the complementary channel of the EET system.

Ultrafast Dynamics of Metal Plasmons Induced by 2D Semiconductor Excitons in Hybrid Nanostructure Arrays

With the advanced progress achieved in the field of nanotechnology, localized surface plasmon resonances are actively considered to improve the efficiency of metal-based photocatalysis, photodetection, and photovoltaics. Here, we report on the exchange of energy and electric charges in a hybrid composed of a two-dimensional tungsten disulfide (2D-WS2) monolayer and an array of aluminum (Al) nanodisks. Femtosecond pump–probe spectroscopy results indicate that within ∼830 fs after photoexcitation of the 2D-WS2 semiconductor energy transfer from the 2D-WS2 excitons excites the plasmons of the Al array. Then, upon the radiative and/or nonradiative damping of these excited plasmons, energy and/or electron transfer back to the 2D-WS2 semiconductor takes place as indicated by an increase in the reflected probe at the 2D-exciton transition energies at later time delays. This simultaneous exchange of energy and charges between the metal and the 2D-WS2 semiconductor resulted in an extension of the average lifetime ...

Fabrication and characterization of zinc oxide nanoflower-based electrochemical luminescence cells

In this paper, we report characteristics of electrochemical luminescence (ECL) cells having zinc oxide nanoflower (ZnO NF) layer as an electrode. The characteristics of ZnO NF can be used in electron excitation transfer on Ru(II) complex [Ru(bpy)32+]. The ZnO NF-based ECL cell has a simple structure, which is composed of a ZnO NF layer and Ru(II) complex between fluorine doped tin oxide (FTO) glasses. In order to characterize the structural properties of ZnO NF, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used. The effect of thickness ZnO NF on emissive and opto-electrical properties of ECL cells was investigated. Thicknesses of ZnO NF ranging from 1 μm to 7.5 μm were used in this work. The luminescence spectrum of the emission from ECL cell with different thickness of ZnO NF was measured and the central wavelength of luminescence was located at 620 nm. For characterization of optical and electrical properties of ECL cells with ZnO NF, luminance and driving current was measured and ECL efficiency was calculated at different operating voltages. The optimum thickness of the ZnO NF was found at 5 μm where it improves opto-electrical characteristics of ECL cell. At an operating voltage of 3 V and a frequency of 60 Hz, the highest luminance of 154 cd/m2 and ECL efficiency of 0.456 lm/W were obtained from ECL cell with ZnO NF thickness of 5 μm.

Laser-induced fluorescence resonance energy transfer for analysis of the quality of a DNA double helix

The goal of this work is to use the method of the laser-induced fluorescence resonance energy transfer (FRET) of electronic excitation in a donor–acceptor pair of intercalators, (acridine orange (AO) as a donor and ethidium bromide (EB) as an acceptor), for the quantitative analysis of the quality of a DNA double helix. This approach obtains a visual picture of the defects of the genetic apparatus of tissue cells, particularly those of skin cells in real time and it can be used for the diagnosis of skin diseases and also in cosmetology.Transition metal (TM) ions such as Cu(II), Cu(I), Ag(I), silver nanoparticles (AgNPs), photo- and thermo effects were used to cause double helix defects in DNA.The concentration of DNA sites after exposure to Cu(II), Cu(I), Ag(I) ions, AgNPs impact, as well as laser irradiation (λ = 457 nm) and temperature, which are applicable for intercalation, were estimated in relative units.The nanoscale FRET method enables the estimation of the concentration of double helix areas with high stability, applicable for intercalation in DNA after it was subjected to stress effect. It provides the opportunity to compare DNA-s of (1) different origin; (2) with various degrees of damage; (3) being in various functional states.

High-Capacity and Photoregenerable Composite Material for Efficient Adsorption and Degradation of Phenanthrene in Water.

We report a novel composite material, referred to as activated charcoal supported titanate nanotubes (TNTs@AC), for highly efficient adsorption and photodegradation of a representative polycyclic aromatic hydrocarbon (PAH), phenanthrene. TNTs@AC was prepared through a one-step hydrothermal method, and is composed of an activated charcoal core and a shell of carbon-coated titanate nanotubes. TNTs@AC offered a maximum Langmuir adsorption capacity of 12.1 mg/g for phenanthrene (a model PAH), which is ∼11 times higher than the parent activated charcoal. Phenanthrene was rapidly concentrated onto TNTs@AC, and subsequently completely photodegraded under UV light within 2 h. The photoregenerated TNTs@AC can then be reused for another adsorption-photodegradation cycle without significant capacity or activity loss. TNTs@AC performed well over a wide range of pH, ionic strength, and dissolved organic matter. Mechanistically, the enhanced adsorption capacity is attributed to the formation of carbon-coated ink-bottle pores of the titanate nanotubes, which are conducive to capillary condensation; in addition, the modified microcarbon facilitates transfer of excited electrons, thereby inhibiting recombination of the electron-hole pairs, resulting in high photocatalytic activity. The combined high adsorption capacity, photocatalytic activity, and regenerability/reusability merit TNTs@AC a very attractive material for concentrating and degrading a host of micropollutants in the environment.

Normal Incident Long Wave Infrared Quantum Dash Quantum Cascade Photodetector.

We demonstrate a quantum dash quantum cascade photodetector (QDash-QCD) by incorporating self-assembled InAs quantum dashes into the active region of a long wave infrared QCD. Sensitive photoresponse to normal incident light at 10 μm was observed, which is attributed to the intersubband (ISB) transitions in the quantum well/quantum dash (QW/QDash) hybrid absorption region and the following transfer of excited electrons on the extraction stair-like quantum levels separated by LO-phonon energy. The high density InAs quantum dashes were formed in the Stranski-Krastanow mode and stair-like levels were formed by a lattice matched InGaAs/InAlAs superlattice. A stable responsivity from 5 mA/W at 77 K to 3 mA/W at as high as 190 K was observed, which makes the QDash-QCD promising in high temperature operation.

Exceptional Visible‐Light‐Driven Cocatalyst‐Free Photocatalytic Activity of g‐C3N4 by Well Designed Nanocomposites with Plasmonic Au and SnO2

In this work, plasmonic Au/SnO2/g‐C3N4 (Au/SO/CN) nanocomposites have been successfully synthesized and applied in the H2 evolution as photocatalysts, which exhibit superior photocatalytic activities and favorable stability without any cocatalyst under visible‐light irradiation. The amount‐optimized 2Au/6SO/CN nanocomposite capable of producing approximately 770 μmol g−1 h−1 H2 gas under λ > 400 nm light illumination far surpasses the H2 gas output of SO/CN (130 μmol g−1), Au/CN (112 μmol g−1 h−1), and CN (11 μmol g−1 h−1) as a contrast. In addition, the photocatalytic activity of 2Au/6SO/CN maintains unchanged for 5 runs in 5 h. The enhanced photoactivity for H2 evolution is attributed to the prominently promoted photogenerated charge separation via the excited electron transfer from plasmonic Au (≈520 nm) and CN (470 nm > λ > 400 nm) to SO, as indicated by the surface photovoltage spectra, photoelectrochemical I–V curves, electrochemical impedance spectra, examination of formed hydroxyl radicals, and photocurrent action spectra. Moreover, the Kelvin probe test indicates that the newly aligned conduction band of SO in the fabricated 2Au/6SO/CN is indispensable to assist developing a proper energy platform for the photocatalytic H2 evolution. This work distinctly provides a feasible strategy to synthesize highly efficient plasmonic‐assisted CN‐based photocatalysts utilized for solar fuel production.

Electronic excitation energy transport in a DNA-Ag cluster complex

We study electronic excitation transfer in the complexes of Ag clusters with oligonucleotides. Steady-state fluorescence excitation spectra show that the excitation energy is transferred to Ag cluster from all the 30 nucleobases of a 15-mer DNA duplex. This is in contrast to DNA-dyes complexes, where the energy transfer to the dye occurs from neighboring DNA bases only. Fluorescence decay curve for the DNA duplex shows that the process of energy transfer occurs within <100 fs. The obtained results suggest coherent excitonic type of the transfer rather than trivial Forster mechanism.

Determination of Total Phenolic Compounds in Common Beverages Using CdTe Quantum Dots

In this study, we describe a method of total phenolic compounds determination based on CdTe quantum dot (QD) fluorescence recovery in the presence of analyte. Polyphenols present in the solution were reducing agents, acting as electron donors to the CdTe–sodium periodate system, thus leading to perturbation of the transfer of excited electrons from QD to the acceptor molecules occurring in the absence of polyphenols. Using the developed technique the polyphenol content in green tea, black tea, Pu-Erh tea, lemon balm, peppermint, lime, chamomile, and coffee infusions was successfully determined. The limit of detection of the method amounted to 0.63 nM of (+)-catechin equivalent, which makes this technique nearly one order of magnitude more sensitive than the commonly used Folin–Ciocalteu method. Additionally, lack of impact of interference from proteins and reducing sugars on the results makes this technique an advantageous alternative to the Folin–Ciocalteu assay. Practical Applications The developed method for total phenolic compound determination, thanks to higher specificity and a low detection limit may be a beneficial alternative to the popular Folin–Ciocalteu assay. The applications of the described technique include plant raw material and product nutritional quality assessment as well as studying the effects of a polyphenol-rich diet on human health. The assay is easy to use both in industry laboratories as well as in research and development.

Quenching of chlorophyll fluorescence induced by silver nanoparticles

The interaction between chlorophyll (Chl) and silver nanoparticles (AgNPs) was evaluated by analyzing the optical behavior of Chl molecules surrounded by different concentrations of AgNPs (10, 60, and 100nm of diameter). UV–Vis absorption, steady state and time-resolved fluorescence measurements were performed for Chl in the presence and absence of these nanoparticles. AgNPs strongly suppressed the Chl fluorescence intensity at 678nm. The Stern-Volmer constant (KSV) showed that fluorescence suppression is driven by the dynamic quenching process. In particular, KSV was nanoparticle size-dependent with an exponential decrease as a function of the nanoparticle diameter. Finally, changes in the Chl fluorescence lifetime in the presence of nanoparticles demonstrated that the fluorescence quenching may be induced by the excited electron transfer from the Chl molecules to the metal nanoparticles.