Single Donor-acceptor Research Articles

Excited-State Symmetry Breaking and Localization in a Noncentrosymmetric Electron Donor-Acceptor-Donor Molecule.

Electronic excitation in quadrupolar conjugated molecules rapidly localizes on a single electron donor-acceptor (DA) branch when in polar environments. The loss of center of inversion upon this excited-state symmetry breaking (ES-SB) can be monitored by exploiting the relaxation of the exclusion rules for IR and Raman vibrational transitions. Here, we compare ES-SB in a right-angled (1) and a centrosymmetric (2) DAD dyes using time-resolved IR spectroscopy. We show that the localization of the excitation can also be identified with the bent molecule 1. We find that contrary to dye 2, subpopulations with localized and delocalized excitation coexist for 1 in weak to medium polar solvents. This difference originates from the torsional disorder present in the excited state of 1 but not of 2. Additionally, irreversible localization in a bent molecule is shown to require higher solvent polarity than in a centrosymmetric one.

Chiral Diselenophosphoric Acids for Ion Pair Catalysis: A Novel Approach to Enhance Both Proton Donating and Proton Accepting Properties.

The activation of poorly reactive substrates via strong chiral acids is a central topic in asymmetric ion pair catalysis these days. Despite highly successful scaffolds such as N-triflylphosphoramides, these catalysts either lack C2-symmetry or provide multiple H-bond acceptor sites, leading to lower ee values for certain reactions. We present BINOL-based diselenophosphoric acids (DSA) as an extremely promising alternative. Using an intertwined approach of synthesis and NMR studies, we developed a synthetic approach to DSA with up to 98 % NMR yield. The obtained acids provide both very high proton donor and proton acceptor properties, a bifunctionality, which is key to catalytic applications. Indeed, first reactivity test proved the much higher acidity of DSA and its ability to initiate Mukaiyama-Mannich reaction and protodesilylation of silyl ethers. Together with their C2-symmetry, the single donor and single acceptor situation, the decreased tendency of self-association, and the straightforward synthesis with potential 3,3'-substitution, the DSA provide all features ideal for the further development of ion pair catalysis.

Origin of discrete donor–acceptor pair transitions in 2D Ruddlesden–Popper perovskites

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light–matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by μ-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths (≃120μeV) and long decay times (5–8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden–Popper perovskites, (PEA)2(MA)n−1PbnI3n+1, particularly in phases with n≥2, has been identified as a mechanism of donor–acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of ≃10 nm. The analysis of the spectral and electronic characteristics of these single donor–acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions.

Comparative Investigation of the Microstructure of MgCl2 Aqueous Solutions Using Different X-ray Scattering Sources, Raman Spectroscopy, and Atomistic Simulations.

Aqueous solutions of magnesium chloride (MgCl2(aq)) are often used to test advances in the theory of electrolyte solutions because they are considered an ideal strong 2:1 electrolyte. However, there is evidence that some ion association occurs in these solutions, even at low concentrations. Even a small ion-pairing constant can have a significant impact on the chemical speciation of ions, so it is important to determine whether ion pairing actually occurs. In this study, MgCl2(aq) with concentrations ranging from 1 to 35% was studied using three methods: X-ray scattering (XRS) with the Shanghai Synchrotron Radiation Facility (SSRF) and silver-anode laboratory sources, Raman spectroscopy, and molecular dynamics (MD) simulations with the COMPASS-II and Madrid force fields. XRS results were analyzed in the framework of PDF theory to obtain the reduced structure function F(Q) and the reduced pair distribution function G(r). The F(Q) values from synchrotron radiation and laboratory sources both showed that the tetrahedral hydrogen bonds in bulk water were destroyed with the increased MgCl2 concentration. The results of G(r) indicated that the main peaks centered at 2.05 and 2.80 Å can be ascribed to the interactions of Mg-O and O-O, respectively. The peak at 3.10 Å is attributed to the combined effect of O-O and Cl-O. By comparing the structural information on MgCl2 solution obtained from the two light sources, it was found that both SSRF and silver-anode laboratory sources can reflect the above-mentioned structural information on MgCl2 solution. The radial distribution function (RDF) obtained from MD simulations of MgCl2 solutions assigned the peaks at 2.0, 2.8, and 3.2 Å to the Mg-O, O-O, and Cl-O interatomic pairs, respectively. The decrease in the O-O coordination number confirms that the hydrogen-bonding network of water is disrupted by increasing MgCl2 observed by X-ray scattering. The proportion of Mg-Cl contact ion pairs gradually increases with MgCl2 concentration as does the coordination number. Raman spectroscopy results show that the bond type changes from double donor double acceptor (DDAA) to single donor-single acceptor (DA) with increasing concentration, providing explicit details of the hydrogen-bond evolution in the aqueous solution.

A dual donor–acceptor fluorescent probe with viscosity response and lipid droplets targeting to initiate oxidative stress for tumor elimination

A dual donor–acceptor photosensitizer TCN-2 prepared based on single donor–acceptor could fulfil lipid droplets targeting to trigger apoptosis and tumor growth arrest. Meanwhile, all of experiments both in phosphate buffer solution and intracellular surroundings have demonstrated that TCN-2 catalyzed the production of type I as well as type II reactive oxygen species, forming a hybrid reactive oxygen species pattern, indicating that TCN-2 could be applied to initiate a series of biological responses triggered by oxidative stress within most high-viscosity solid tumors. In addition, TCN-2 also has the capability of fluorescence imaging, which could perfectly combine therapeutic imaging to achieve therapeutic effects while identifying cancerous lesions. Due to the structural design of double electron-absorbing groups, TCN-2 retained excellent lipophilicity while enhancing solubility in the biological environment. Terrific biocompatibility, minimal phototoxic damage to normal cells and tissues, and specific driving to prescriptive organelles to maximize therapeutic effects were used to enhance the therapeutic effect of photodynamic therapy to cease disease progression.

Phase behavior of sulfolane: Potential implications for transport in groundwater

Sulfolane is a toxic pollutant freely miscible in pure water. However, above benchmark concentrations of sulfate or thiosulfate ions, it behaves as a light non-aqueous phase liquid and separates into either bulk phases or kinetically stable emulsions. Sulfolane droplets bear a negative electrostatic charge, as revealed by electrophoretic measurements. With high sulfate salt concentrations, the counterions screen the negative charge of sulfolane droplets. This promotes droplet coalescence and bulk phase separation, as well as droplet adhesion onto negatively charged mineral substrates such as clay and sand. NaCl does not separate sulfolane from water but favors its partitioning into toluene, as demonstrated by attenuated total reflectance-Fourier transform infrared spectroscopy. Water activity measurements show that sodium sulfate interacts with water more strongly than sulfolane, justifying its ability to displace sulfolane and induce its separation. Both sulfolane and sulfate ions interact with water through hydrogen (H) bonds. Water is comprised of water species that donate and accept a different number of H bonds, depending on other molecules in water. Sulfolane is an H bond acceptor. Therefore, it mainly interacts with double H bond donors that either do not accept H or accept only one H (double donor-single acceptor, DD-SA). ATR-FTIR data indicate that sulfate ions compete with sulfolane more effectively than chlorides for interactions with DD-SA, thereby promoting better separation. Sulfolane phase separation can retard its migration in polluted aquifers. This study reveals the complex behaviour of sulfolane in water, with potential implications for sulfolane transport in groundwater.

Chiral perturbation on single benzene-based fluorophores: A structure/(chir)optical activity relationship study.

In this paper, we describe the synthesis and the (chir)optical properties of a novel series of circularly polarized luminescent emitters. These molecules involve a compact single benzene-based donor-acceptor fluorophore composed of two cyclic alkylamines as electron donors and a phthalonitrile moiety as electron acceptor linked to a configurationally stable BINOL acting as a chiral perturbation unit. These new compounds display fair quantum yields (up to 66%) with emission maxima around 500 nm in toluene solutions, and the study of their chiroptical properties has shown that the cyclic alkylamine's ring size affects significantly the luminescence dissymmetry factors, reaching 2.2×10-3 for the larger cyclic alkylamine moieties.

Characterizing excited states of single donor-acceptor molecule by high-resolution Raman images

The excited states of charge transfer (CT), local donor (LD) and local acceptor (LA) for donor-acceptor molecules play an important role in many applications. However, these excited quantum states are still impossible to be characterized by conventional experimental techniques. Optical spectroscopy with the supplement of density functional theory calculations is widely adopted to investigate excited states, but is often inefficient for the donor-acceptor molecule owing to substantial errors of functionals in describing CT states. Our calculations show that the resonance tip-enhanced Raman scattering images that utilizes the confined plasmonic field for CT, LD and LA states possess their exclusive imaging characterizations, giving unambiguous identification of different excited-state properties. In addition, the resonance Raman images provide a benchmark to reasonably estimate the accuracy of different functionals in describing excited states of the charge-transfer system. These findings extend the emerging Raman images to the characterization and improvement of functional donor-acceptor molecular materials.

Fine analysis of the component effect on the microstructure of LiCl solution

Lithium resources are abundant in Chinese salt lakes, but the high Mg/Li ratio in such aqueous natural sources has brought difficulties to the separation and purification of lithium. Crystallization processes are highly affected by the solution microstructure, which, in turn, depends on its composition. Synchrotron X-ray scattering, Raman spectroscopy, and molecular dynamics simulation methods were used to study the microstructure of aqueous LiCl solutions with different concentrations. In the reduced structure function, F(Q), the double peak around the scattering vector Q = 2.5 Å−1 becomes a single peak, which indicates the addition of LiCl changes the tetrahedral hydrogen bond structure in the aqueous solution. The deconvolution fitting of the Raman spectra show that the DDAA-type (double donor-double acceptor) hydrogen bonds tend to be transformed into DA-type (single donor–single acceptor) hydrogen bonds, confirming that the addition of LiCl breaks the tetrahedral hydrogen bond structure. This hydrogen-bond transformation was confirmed by pair distribution function analysis from molecular dynamics simulations of aqueous KCl solutions. In addition, considering the large number of magnesium ions coexisting with LiCl in the Salt Lake, the microstructure of mixed LiCl-MgCl2 solution was also studied. According to the X-ray scattering results, the peak in the reduced pair distribution function G(r) near r = 3.25 Å shifted from 3.25 Å to 3.10 Å with increased Mg/Li ratio. This change is likely to result from the Cl-O and OO interactions in the mixed aqueous solution. The deconvolution fitting results of Raman spectra show that the DDAA type hydrogen bond increases with the Mg/Li ratio, while the DA + DDA type hydrogen bond decreases. The molecular dynamics results showed that chlorine ions have a higher tendency to form contact ion pairs with Mg2+ than Li+. We propose this to be one of the processes retarding the separation of pure LiCl crystal from mixed LiCl-MgCl2 systems.

Characterizing Excited States of Single Donor-Acceptor Molecule by High-Resolution Raman Images

Characterizing Excited States of Single Donor-Acceptor Molecule by High-Resolution Raman Images

Lateral Fully Organic P-N Diodes Created in a Single Donor-Acceptor Copolymer.

P-N junctions exist in many solid-state organic devices, such as light-emitting diodes, solar cells, and thermoelectric devices. Creating P-N junctions by bulk chemical doping in a single organic material (like silicon doped by boron and phosphorus) may capitalize the vast scientific and technological groundwork established in the inorganic semiconducting field. However, high-performance single-organic-material P-N junctions are seldom reported, because the diffusion of the dopant counterions often leads to transient rectification properties. Herein, a new type of lateral fully organic diodes created in single donor-acceptor (D-A) copolymer films with only one P-type dopant is reported. The achieved lateral devices exhibit high current densities of ≈3.83 A cm-2 and a high rectification ratio of ≈2100, which are beyond the requirements for high-frequency identification tags. The P- to N-type polarity switching mechanism is proposed after spectroscopic and structural tests. Decent stability of the organic diode is obtained, which is due to the long channel length and low diffusion speed of the large size of dopants. This work opens the opportunities to create P-N junctions in ways of silicon-based inorganic semiconductors and promises new opportunities for integrating organic materials for flexible and printable organic devices.

Understanding Förster Resonance Energy Transfer in the Sheet Regime with DNA Brick-Based Dye Networks.

Controlling excitonic energy transfer at the molecular level is a key requirement for transitioning nanophotonics research to viable devices with the main inspiration coming from biological light-harvesting antennas that collect and direct light energy with near-unity efficiency using Förster resonance energy transfer (FRET). Among putative FRET processes, point-to-plane FRET between donors and acceptors arrayed in two-dimensional sheets is predicted to be particularly efficient with a theoretical 1/r4 energy transfer distance (r) dependency versus the 1/r6 dependency seen for a single donor-acceptor interaction. However, quantitative validation has been confounded by a lack of robust experimental approaches that can rigidly place dyes in the required nanoscale arrangements. To create such assemblies, we utilize a DNA brick scaffold, referred to as a DNA block, which incorporates up to five two-dimensional planes with each displaying from 1 to 12 copies of five different donor, acceptor, or intermediary relay dyes. Nanostructure characterization along with steady-state and time-resolved spectroscopic data were combined with molecular dynamics modeling and detailed numerical simulations to compare the energy transfer efficiencies observed in the experimental DNA block assemblies to theoretical expectations. Overall, we demonstrate clear signatures of sheet regime FRET, and from this we provide a better understanding of what is needed to realize the benefits of such energy transfer in artificial dye networks along with FRET-based sensing and imaging.

Simultaneous Improvement of Efficiency and Stability of Non‐Fullerene‐Based Organic Solar Cells Via Sequential Deposition of Single Donor and Binary Acceptor

Non‐fullerene acceptor (NFA)‐based organic solar cells often exhibit significant cell degradation in power conversion efficiency (PCE) in the early stages of operation, called “burn‐in.” Generally, to fabricate NFA‐based solar cells, binary blend solution deposition (binary BSD) of a conjugated polymer and an NFA is utilized. Herein, the reasons for burn‐in are investigated by aging organic photovoltaic cells with independent control of temperature and light. The results reveal that burn‐in is mainly due to a rapid increase in the interfacial resistance (R int) rather than photo‐oxidation of the components or oxidation of the electrode. This R int is effectively suppressed by constructing a ternary photoactive layer through the sequential deposition of a polymer solution and a binary acceptor solution consisting of an NFA and a fullerene acceptor (ternary sequential deposition [ternary SqD]). Under the illumination of 1 sun and thermal annealing at 80 °C for 500 h, the binary BSD exhibits a reduction in efficiency of 63% and 59%, respectively, whereas the ternary SqD demonstrates a reduction of only 32% and 35%, respectively. In addition, the ternary SqD improves the PCE on using fullerene acceptors to enhance light harvesting at short wavelengths.

Can Nanocavities Significantly Enhance Resonance Energy Transfer in a Single Donor–Acceptor Pair?

Can Nanocavities Significantly Enhance Resonance Energy Transfer in a Single Donor–Acceptor Pair?

The Effects of Dissolved Hydrophobic and Hydrophilic Groups on Water Structure

In this work, Raman spectroscopy is employed to investigate the effects of hydrophobic and hydrophilic groups on water structure. In aqueous methanol, ethanol, 1-propanol solutions, the addition of methylic groups (-CH3) raises the intensity ratio of the 3430 cm−1 sub-band to the 3220 cm−1 peak, and also increases the intensity of free OH vibrations. From our recent studies, this means that the increase of hydrophobic groups raises the ratio of DA (single donor-single acceptor) to DDAA (double donor-double acceptor) hydrogen bonding. Additionally, increasing the hydrophilic groups (–OH) as 1-propanol, 1,2-propanediol and 1,2,3-propanetriol solutions, also increases the ratio of DA to DDAA structural motifs, but decreases the free OH vibrations. The results of this study do not support the “iceberg” model of hydrophobic effects. Additionally, the dissolved hydrophobic and hydrophilic groups may have different effects on free OH vibrations.

Local structure of NH4Cl solution and its correlation with osmotic coefficient by molecular dynamics simulation and Raman spectroscopy

The NH4Cl aqueous solution ranging from 1.00 wt.% to 22.00 wt.% have been studied by molecular dynamic simulation, Raman spectroscopy and polarized Raman spectroscopy. Radial distribution function (RDF), diffusion property, structure of hydrogen bond network, excess Raman spectroscopy and non-coincidence effect (NCE) were obtained. The contact ion pairs of cation and anion appeared in NH4Cl solution of 15.00 wt.%, leading to fundamental changes in hydrogen bonds and the hydrated shell. The evolution of hydrogen bond with mass fraction in solution was described. The osmotic coefficient of NH4Cl aqueous solution was obtained by thermodynamic calculation, and its linear relationship with double donor single acceptor hydrogen bond (DDA-OH) was preliminarily summarized. Moreover, the experimental results are in good agreement with the simulation results.

Electronic Energy Transfer in a Single Donor–Acceptor Pair with Triplet–Triplet Absorption

Forster resonance energy transfer (FRET) is examined in a single donor (D)–acceptor (A) pair whose molecules have both singlet and triplet states. Two cases are considered: (i) the excitation of the D–А pair occurs with the participation of a singlet band of the donor, and (ii) the excitation also involves the participation of an additional triplet–triplet absorption. The triplet state of the donor is shown to have no effect on the expression for the FRET efficiency, but it generates off intervals in the fluorescence track of both the donor and the acceptor. The triplet state of the acceptor strongly affects the expression for the FRET efficiency, making the complete energy transfer impossible and generating off intervals in the acceptor fluorescence track. It is shown that, if the light can excite not only the singlet transition in the donor, but also the triplet–triplet absorption band, then a weak luminescence appears in off intervals, which is orders of magnitude weaker than that in on intervals.

Observation of the Chemical Structure of Water up to the Critical Point by Raman Spectroscopic Analysis of Fluid Inclusions.

Raman spectra were obtained simultaneously from the liquid and vapor phases of pure water trapped at the critical density (322 kg·m-3) within synthetic inclusions in quartz. As these inclusions are heated up to the critical temperature (373.946 °C), the liquid phase decreases in density and the maximum of the Raman OH-stretching band increases in wavenumber. Conversely, as the vapor phase increases in density, the maximum of the Raman OH-stretching band decreases in wavenumber. The Raman bands of the liquid and vapor phases converge to a single band at the critical point of water, where the fluid exists as a single phase. A comparison of the band centroids for the vapor and liquid phases of water indicates respective increases and decreases in the amount of hydrogen bonding in these phases as a function of increasing and decreasing density. These effects were further quantified by peak-fitting the Raman OH-stretching peak with five Gaussian components. All the Gaussian components of the liquid phase decrease in amplitude with increasing temperature with the exception of the double donor-single acceptor (H2O)4 cluster, which increases in amplitude and becomes the most intense component at temperatures above 300 °C. The Raman spectra of the vapor phase are dominated by the free OH component at temperatures below 300 °C, but, above this temperature, the double donor-single acceptor (H2O)4 cluster is again the most intense band. The results indicate that a significant quantity of water clusters is present in both liquid and vapor water at high temperatures and that supercritical water can be considered as a mixture of small water clusters [(H2O)n, n = 1-4] dominated by the double donor-single acceptor (H2O)4 cluster.

Influence of triplet states on single donor-acceptor pair fluorescence

Single donor-acceptor (D-A) pairs whose molecules have both singlet and triplet states are considered. Fluorescence of such D-A pair has two types of fluctuations: fluctuations due to singlet-triplet transitions and fluctuations because of energy transfer in D-A pair. Both types of fluctuations manifest themselves in the duration of on/off intervals in fluorescence trajectories. Triplet state in the donor molecule creates off-intervals in D- and A-fluorescence. However, it does not influence FRET efficiency E. Another picture we will observe if we take into account triplet state in the acceptor molecule. This state influences FRET efficiency distribution S(E) considerable. 1) Acceptor triplet state hampers energy transfer in D-A pair and changes expression for S(E). 2) Acceptor triplet state is responsible for appearance off-intervals in A-fluorescence and dual d-fluorescence: bright and moderate. Fluctuating D- and A-fluorescence are simulated with the help of Monte Carlo method. Dual d-fluorescence of single D-A pair creates FRET efficiency distribution S(E) with two maxima.

Separation of Quantum Emitters Produced by Single Donor–Acceptor Pairs Under Laser Excitation

We study features of point emitters in ZnSe-based quantum wells using laser-based spectroscopy with a high spatial resolution. We determine the experimental conditions under which quantum emitters formed by donor-acceptor pairs are observed. We discuss the possibility of studying individual defects forming a donor–acceptor pair using the spectrum of the corresponding quantum emitters.