Absorption Spectrum Of Acceptor Research Articles

Theory for proton-coupled energy transfer.

In the recently discovered proton-coupled energy transfer (PCEnT) mechanism, the transfer of electronic excitation energy between donor and acceptor chromophores is coupled to a proton transfer reaction. Herein, we develop a general theory for PCEnT and derive an analytical expression for the nonadiabatic PCEnT rate constant. This theory treats the transferring hydrogen nucleus quantum mechanically and describes the PCEnT process in terms of nonadiabatic transitions between reactant and product electron-proton vibronic states. The rate constant is expressed as a summation over these vibronic states, and the contribution of each pair of vibronic states depends on the square of the vibronic coupling as well as the spectral convolution integral, which can be viewed as a generalization of the Förster-type spectral overlap integral for vibronic rather than electronic states. The convolution integral also accounts for the common vibrational modes shared by the donor and acceptor chromophores for intramolecular PCEnT. We apply this theory to model systems to investigate the key features of PCEnT processes. The excited vibronic states can contribute significantly to the total PCEnT rate constant, and the common modes can either slow down or speed up the process. Because the pairs of vibronic states that contribute the most to the PCEnT rate constant may correspond to spectroscopically dark states, PCEnT could occur even when there is no apparent overlap between the donor emission and acceptor absorption spectra. This theory will assist in the interpretation of experimental data and will guide the design of additional PCEnT systems.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Modulating the spectroscopic overlap between the emission bands of donors and the absorption spectra of acceptors by various simulations, it is possible to systematically investigate the emission behaviors of lanthanide complexes under different conditions. To establish the relationships between emission behaviors and various external simulations, it is necessary to study the energy transfer rate and efficiency between the donor and acceptor under different conditions to clarify the luminescent mechanism of the complexes, providing a theoretical basis for high-performance smart materials. This review focuses on the recent progress of luminescence performance of lanthanide complexes, including energy transfer mechanisms, emission color modulation, the strategies for optimizing lanthanide luminescence, and finally, various applications based on luminescence performance of lanthanide complexes and lanthanide metal-organic frameworks.

An ab initio method on large sized molecular aggregate system: Predicting absorption spectra of crystalline organic semiconducting films.

Theoretical description of electronically excited states of molecular aggregates at an ab initio level is computationally demanding. To reduce the computational cost, we propose a model Hamiltonian approach that approximates the electronically excited state wavefunction of the molecular aggregate. We benchmark our approach on a thiophene hexamer, as well as calculate the absorption spectra of several crystalline non-fullerene acceptors, including Y6 and ITIC, which are known for their high power conversion efficiency in organic solar cells. The method qualitatively predicts the experimentally measured spectral shape, which can be further linked to the molecular arrangement in the unit cell.

Implication toward a simple strategy to generate pH tunable FRET-based biosensing

The present contribution depicts a unique approach to generate tunable Förster resonance energy transfer (FRET) emission with variation of pH of the medium. The pH sensitive absorption of Doxorubicin leads to modification of spectral overlap between emission spectra of donor (Pyrazoline) and absorption spectra of acceptor (Doxorubicin) thereby sensing maximum FRET efficiency in an optimum pH (near pKa of Doxorubicin). This drug molecule exhibits an instantaneous conformation change at a particular pH, which consequences on abrupt ON-and-OFF FRET efficiency. At elevated pH, both the drug molecules exhibit conformational change and form stable fluorescent exciplex, switching off the FRET emission. Confocal fluorescence images of live HepG2 cells imply that the sensor can proficiently go through the cell membrane and can be applied in the controlled delivery of drug to the tumor cell lines.

PyFREC 2.0: Software for excitation energy transfer modeling.

Excitation energy transfer is a ubiquitous process of fundamental importance for understanding natural phenomena, such as photosynthesis, as well as advancing technologies ranging from photovoltaics to development of photosensitizers and fluorescent probes used to explore molecular interactions inside living cells. The current version of PyFREC 2.0 is an advancement of the previously reported software (D. Kosenkov, J. Comput. Chem. 2016, 37, 1847-1854). The current update is primarily focused on providing a computational tool based on Förster theory for bridging a gap between theoretically calculated molecular properties (e.g., electronic couplings, orientation factors, etc.) and experimentally measured emission and absorption spectra of molecules. The software is aimed to facilitate deeper understanding of photochemical mechanisms of fluorescence resonance energy transfer (FRET) in donor-acceptor pairs. Specific updates of the software include implementations of overlap integrals between donor emission and acceptor absorption spectra of FRET pairs, estimation of Strickler-Berg fluorescence lifetimes, calculation of Förster radii, energy transfer efficiency, and radiation zones that, in particular, determine applicability of the Förster theory.

New Core-Shell Nanostructures for FRET Studies: Synthesis, Characterization, and Quantitative Analysis.

This work describes the synthesis and characterization of new core-shell material designed for Förster resonance energy transfer (FRET) studies. Synthesis, structural and optical properties of core-shell nanostructures with a large number of two kinds of fluorophores bound to the shell are presented. As fluorophores, strongly fluorescent rhodamine 101 and rhodamine 110 chloride were selected. The dyes exhibit significant spectral overlap between acceptor absorption and donor emission spectra, which enables effective FRET. Core-shell nanoparticles strongly differing in the ratio of donors to acceptor numbers were prepared. This leads to two different interesting cases: typical single-step FRET or multistep energy migration preceding FRET. The single-step FRET model that was designed and presented by some of us recently for core-shell nanoparticles is herein experimentally verified. Very good agreement between the analytical expression for donor fluorescence intensity decay and experimental data was obtained, which confirmed the correctness of the model. Multistep energy migration between donors preceding the final transfer to the acceptor can also be successfully described. In this case, however, experimental data are compared with the results of Monte Carlo simulations, as there is no respective analytical expression. Excellent agreement in this more general case evidences the usefulness of this numerical method in the design and prediction of the properties of the synthesized core-shell nanoparticles labelled with multiple and chemically different fluorophores.

Assessing Protein Interactions in Live-Cells with FRET-Sensitized Emission.

Förster Resonance Energy Transfer (FRET) is the radiationless transfer of energy from an excited donor to an acceptor molecule and depends upon the distance and orientation of the molecules as well as the extent of overlap between the donor emission and acceptor absorption spectra. FRET permits to study the interaction of proteins in the living cell over time and in different subcellular compartments. Different intensity-based algorithms to measure FRET using microscopy have been described in the literature. Here, a protocol and an algorithm are provided to quantify FRET efficiency based on measuring both the sensitized emission of the acceptor and quenching of the donor molecule. The quantification of ratiometric FRET in the living cell not only requires the determination of the crosstalk (spectral spill-over, or bleed-through) of the fluorescent proteins but also the detection efficiency of the microscopic setup. The protocol provided here details how to assess these critical parameters.

An exact solution in the theory of fluorescence resonance energy transfer with vibrational relaxation

The elegant expression of Förster that predicts the well-known 1/R6 distance (R) dependence of the rate of energy transfer, although widely used, was derived using several approximations. Notable among them is the neglect of the vibrational relaxation in the reactant (donor) and product (acceptor) manifolds. Vibrational relaxation can play an important role when the energy transfer rate is faster than the vibrational relaxation rate. Under such conditions, donor to acceptor energy transfer can occur from the excited vibrational states. This phenomenon is not captured by the usual formulation based on the overlap of donor emission and acceptor absorption spectra. Here, we develop a Green's function-based generalized formalism and obtain an exact solution for the excited state population relaxation and the rate of energy transfer in the presence of vibrational relaxation. We find that the application of the well-known Förster's expression might lead to overestimation of R.

Refined standards for simulating UV–vis absorption spectra of acceptors in organic solar cells by TD-DFT

Photoexcitation property of acceptor plays a vital role to understand fundamental physical process of acceptor excitation in organic solar cells (OSCs), which should be described at a reliable level and characterized by ultraviolet-visible (UV–vis) absorption spectra. Generally, adiabatic linear-response time-dependent density functional theory (LR-TD-DFT) as a suitable framework, which requires to select exchange correlation functionals (XCFs) and basis sets to concentrate the approximation of models, is widely recognized. However, selecting reliable theoretical methods tends to consume computational cost and time, and optimal selections have not yet reached general standards. Herein, a systematic investigation including XCF, basis set, calculation density, solvation effect, and full width at half maximum (FWHM) was undertaken to quantify the photoexcitation properties of typical fullerene acceptors (FAs) and non-fullerene acceptors (NFAs). The results reveal that maximum absorption wavelength (λmax) of NFAs is strongly correlated with the fraction of “exact” Hartree-Fock exchange (EEX), thus establishing selection standards. Moreover, calculation density is the primary factor to ensure the integrity of spectra. This work provides reasonable schemes for characterization conditions, which further lays foundation for the reliability of theoretical calculations that will help to advance the theoretical characterization of optical properties of organic materials.

FRET-dependent single/two-channel switch endowing a dual detection for sulfite and its organelle targeting applications

Förster resonance energy transfer (FRET) is a relatively new strategy for constructing ratiometric fluorescent probes and increasing the Stokes shift. Fluorescent probes based on FRET require spectral overlap between the energy donor emission spectrum and the acceptor absorption spectrum, while providing high energy transfer efficiency. In addition, systems exhibiting FRET typically exhibit intense dual fluorescence, thereby improving the resolution of biological imaging and detection. Herein, on the union of a benzopyrylium salt with a coumarin carboxylic acid, a novel near-infrared fluorescent probe CM-P-DP with FRET regulation strategy was designed and synthesized for the detection of sulfite (SO32−). CM-P-DP showed large Stokes shift (230 nm) and fast response to SO32− in pure PBS buffer (15 s). When SO32− was added, the fluorescence intensity at 455 nm gradually increased, and the fluorescence intensity at 635 nm decreased rapidly. Moreover, CM-P-DP could effectively target mitochondria and imaging SO32− in biological systems.

Heuristics from Modeling of Spectral Overlap in Förster Resonance Energy Transfer (FRET).

Among the photophysical parameters that underpin Förster resonance energy transfer (FRET), perhaps the least explored is the spectral overlap term ( J). While by definition J increases linearly with acceptor molar absorption coefficient (ε(A) in M-1 cm-1), is proportional to wavelength (λ4), and depends on the degree of overlap of the donor fluorescence and acceptor absorption spectra, the question arose as to the value of J for the case of perfect spectral overlap versus that for representative fluorophores with incomplete spectral overlap. Here, Gaussian distributions of absorption and fluorescent spectra have been modeled that encompass varying degrees of overlap, full-width-at-half-maximum (fwhm), and Stokes shift. For ε(A) = 105 M-1 cm-1 and perfect overlap, the J value (in M-1 cm-1 nm4) ranges from 1.15 × 1014 (200 nm) to 7.07 × 1016 (1000 nm), is almost linear with λ4 (average of λabs and λflu), and is nearly independent of fwhm. For visible-region fluorophores with perfectly overlapped Gaussian spectra, the resulting value of J ( JG-0) is ∼0.71 ε(A)λ4 (M-1 cm-1 nm4). The experimental J values for homotransfer, as occurs in light-harvesting antennas, were calculated with spectra from a static database of 60 representative compounds (12 groups, 5 compounds each) and found to range from 4.2 × 1010 ( o-xylene) to 5.3 × 1016 M-1 cm-1 nm4 (a naphthalocyanine). The degree of overlap, defined by the ratio of the experimental J to the model JG-0 for perfectly overlapped spectra, ranges from ∼0.5% (coumarin 151) to 77% (bacteriochlorophyll a). The results provide insights into how a variety of factors affect the resulting J values. The high degree of spectral overlap for (bacterio)chlorophylls prompts brief conjecture concerning the relevance of energy transfer to the question "why chlorophyll".

Concentration dependence of energy transfer Ce3+→Er3+ in YAG host

In the doubly doped YAG:Ce,Er single crystal we show the effect of the nonradiative energy transfer from the donor (Ce3+) to the acceptor (Er3+) in terms of the Förster-Dexter model. Such an energy transfer process is enabled by the overlap of the donor and acceptor emission and absorption spectra, respectively, and makes the donor to de-excite faster. This phenomenon can be used to tailor the luminescence decay of a phosphor in order to increase the rate at which it converts incoming radiation, if the decrease of its quantum efficiency can be tolerated. Quantitative comparison of experimental data obtained at the sample set with large concentration interval of Er3+ acceptor (up to the concentration Y2.835Al5O12: Ce0.006Er0.159) with the mentioned theoretical model allows the determination of the microscopic parameters of the energy transfer process, such as the critical concentration of the Er3+ ions. The Er concentration higher than 1% has proven to shorten the 1/e decay time from 62 ns below 30 ns. The concentration dependence of the absorption lines of the Er3+ is also discussed.

Using fluorescence detected two-dimensional spectroscopy to investigate initial exciton delocalization between coupled chromophores.

Förster theory describes electronic exciton energy migration in molecular assemblies as an incoherent hopping process between donor and acceptor molecules. The rate is expressed in terms of the overlap integral between donor fluorescence and acceptor absorption spectra. Typical time scales for systems like photosynthetic antennae are on the order of a few picoseconds. Prior to transfer, it is assumed that the initially excited donor molecule has equilibrated with respect to the local environment. However, upon excitation and during the equilibration phase, the state of the system needs to be described by the full density matrix, including coherences between donor and acceptor states. While being intuitively clear, addressing this regime experimentally has been a challenge until the recently reported advances in fluorescence detected two-dimensional spectroscopy. Here, we demonstrate using fourth order perturbation theory the conditions for the presence of donor-acceptor coherence induced cross-peaks at zero waiting time between the first and the second pair of pulses. The approach is illustrated for a heterodimer model which facilitates an analytical solution.

Plasmon-Coupled Resonance Energy Transfer II: Exploring the Peaks and Dips in the Electromagnetic Coupling Factor

We study resonance energy transfer between a donor–acceptor pair located on opposite sides of a spherical silver nanoparticle and explore the dependence of energy-transfer rate on nanoparticle size using a quantum electrodynamics theory we developed previously. This theory indicates that the rate is determined by the product of donor emission spectra, acceptor absorption spectra, and an electronic coupling factor (CF) that is determined by electrodynamics associated with the donor as a dipole emitter near the nanoparticle. We find that the CF spectra show peaks that are associated with localized surface plasmon resonances, but the locations of the most significant peaks are less correlated to the size of the nanoparticle than is found for extinction spectra for the same particle. For small nanoparticles (≲30 nm), where dipole plasmon excitation dominates, a quasi-static analysis leads to an analytical formula, in which the CF peaks and dips involve interference between donor electric field and the scattered dipolar field of the nanoparticle. For larger nanoparticles (60–210 nm), the CF maximizes at a wavelength near 355 nm independent of particle size that is determined by the highest multipole plasmon that contributes significantly to the extinction spectrum, with only small contributions arising from lower multipole plasmons, such as the dipole plasmon. Also, for wavelengths near 325 nm where the bulk plasmon resonance of silver can be excited, surface plasmons cannot be excited, so excitation from the donor cannot be transmitted by surface plasmons to the acceptor, leading to a pronounced dip in the CF. This work provides new concepts concerning plasmon-mediated energy transfer that are quite different from conventional (Förster) theory, but which should dominate energy-transfer behavior when donor and acceptor are sufficiently separated.

(Invited) Light Absorption Enhancement Using Graphene Quantum Dots in Dye-Sensitized Solar Cells and Photoelectrochemical Water Splitting

Graphene quantum dots (GQDs) have received great interest in the past few years due to its unique properties such as the quantum confinement and edge effects when their sizes are down to 10 nm. These new physical properties can induce size-dependent bandgap, and unique optical and electronic properties, which make them excellent materials for photovoltaics and photoelectochemical (PEC) devices. In a dye-sensitized solar cell (DSSC), light harvesting capability of dyes could be enhanced by Förster resonance energy transfer (FRET) phenomena, wherein energy is transferred from an excited donor fluorophore to a suitable acceptor dye molecule. The strict requirement that the overlap of the emission spectrum of donor and absorption spectrum of acceptor, which also requires proper band edge alignment between the them, significantly limits the variety of donors and acceptors suitable for the FRET-based DSSCs. In our DSSC work, graphene quantum dots were co-sensitized with N719 dyes and we successfully explored that the GQDs is a suitable donor material for the N719 dyes. From the emission and time decay spectra analysis, the FRET efficiency was measured to be 27 %. The power conversion efficiency (ƞ) of the standard DSSC is 6.12 % with a short-circuit current density (Jsc ) of 12.74 mAcm-2 and open-circuit voltage (Voc ) of 760 mV. The co-sensitization of GQD greatly enhances the ƞ up to 7.96 % with a Jsc of 16.54 mAcm-2 and Voc of 770 mV. The co-sensitization of GQDs improved both the Jsc and ƞvalues by ~ 30 %. The FRET enhanced the overall light absorption capacity of the N719 dye and hence photocurrent generation of the DSSC. Moreover, the co-sensitization of GQDs increase the TiO2/dye/electrolyte interface resistance, which suppresses the charge recombination and leads to increased Voc value. For PEC water splitting, higher light absorption can also be achieved by utilizing light-matter interplaymechanisms, such as, surface plasmon resonance (SPR), plasmonicresonance energy transfer (PRET) and exciton−plasmon interactions (EPI). The optical properties of the semiconductor and metal nanoparticles (SNPs-MNPs) are strongly modified viaEPI comparing to single NPs. Therefore, we have utilized the unique combination of GQDs and AuNPs (SNPs-MNPs) to effectively enhance PEC performance of TiO2 nanorods (TNRs). The AuNP-GQD decorated TNR photoelectrode has achieved a superior PEC performance with a photocurrent density of 1.75 mAcm-2 at 1.23 V vs RHE, which is one of higher value obtained in PEC measurement using TiO2 nanorods due to an efficient utilization of solar light. The experimental analysis has indicated that the AuNPs has contributed to the PRET / SPR-mediated hot electron injection into TiO2 and the GQDs contributed to the improved electron injection into the TiO2. Moreover, the significantly improved PEC water splitting efficiency is attributed to the synergistic effect due to EPI and/or FRET between AuNPs and GQDs in addition to the good catalytic property of the AuNPs and the GQDs.

Small-molecule acceptors based on 4H-cyclopenta[1,2-b:5,4-b′]dithiophene units with near-infrared absorption for nonfullerene polymer solar cells

Two small-molecule acceptors 1CIC and 2CIC based on 4H-cyclopenta[1,2-b:5,4-b′]dithiophene units and 1,1-dicyanomethylene-3-indanone were developed. Compared with 1CIC, 2CIC possesses red-shift absorption, higher HOMO and LUMO energy levels, and higher electron mobility. Nonfullerene polymer solar cells based on PTB7-Th:2CIC gave a power conversion efficiency (PCE) of 3.53%, with an open-circuit voltage (Voc) of 0.75 V, a short-circuit current (Jsc) of 10.97 mA/cm2. As contrast, PTB7-Th:1CIC cell gave a PCE of 0.26%. Our results indicated that the absorption spectra of small-molecule acceptors containing indaceno[1,2-b:5,6-b′]dithiophene (IDT) core can be efficiently extended to near-infrared region by replacing the aromatic benzene ring of with 2,2′-bithiophene.

Förster Energy Transfer in the Vicinity of Two Metallic Nanospheres (Dimer)

We present a detailed theoretical analysis of the Forster energy transfer process when a pair of molecules (donor and acceptor) is located nearby a cluster of two metallic nanospheres (dimer). We consider the case in which plasmonic resonances are within the overlap between the donor emission and acceptor absorption spectra, as well as the case that excludes such resonances from the aforementioned spectral overlap. Moreover, we explore the dependence of the Forster energy transfer rate on different dimer configurations (size and separation of nanospheres) and several dipole orientations of molecules. The dimer perturbs strongly the Forster energy transfer rate when plasmons are excited, donor dipole is oriented along the longitudinal axis of the dimer, and the radii of nanospheres and the sphere-gap distance are on the order of a few nanometers. In case of plasmonic excitation, the Forster energy transfer rate is degraded as the sphere-gap distance and size of the nanoparticles increase due to the dephasing of electronic motion arising from ohmic losses of metal. Also, we study the Forster efficiency influenced by the dimer, finding that the high efficiency region (delimited by the Forster radius curve) is reduced as a consequence of significant enhancement of the direct donor decay rate. Our study could impact applications that involve Forster energy transfer.

Tetrafluoroquinoxaline based polymers for non-fullerene polymer solar cells with efficiency over 9%

In order to increase the power conversion efficiency (PCE) of non -fullerene polymer solar cells (PSCs), the absorption spectra of donor and acceptor should be complementary for efficient light harvesting. Here, we designed and synthesized two types of medium bandgap polymers in which two dimension-conjugated benzodithiophene (BDT) (BDTT and BDTPS) was used as the electron donating units and tetrafluoroquinoxaline (ffQx) was used as electron accepting unit, namely PffQx-T and PffQx-PS, for the application as donor in the PSCs with a popular non-fullerene small molecule ITIC as acceptor. Their optical, electrochemical and photovoltaic properties were well investigated. Under AM 1.5G, 100mW/cm−2, all the PSCs with the ffQx-based polymers as donor exhibited PCEs over 8% (8.47% for PffQx-T and 9.12% for PffQx-PS). To the best of our knowledge, the PCE of 9.12% is the highest value for BDT-Qx fullerene-free PSCs to date, which is even higher than that of fullerene-based devices.

NaEuF4/Au@Ag2S nanoparticles-based fluorescence resonant transfer DNA sensor for ultrasensitive detection of DNA energy

The work investigates a new fluorescence resonance energy transfer (FRET) system using NaEuF4 nanoparticles (NPs) and Au@Ag2S NPs as the energy donor–acceptor pair for the first time. The NaEuF4/Au@Ag2S NPs-based FRET DNA sensor was constructed with NaEuF4 NPs as the fluorescence (FL) donor and Au@Ag2S core−shell NPs as FL acceptor. In order to find the matching energy acceptor, the amount of AgNO3 and Na2S were controlled in the synthesis process to overlap the absorption spectrum of energy acceptor with the emission spectrum of energy donors. The sensitivity of FRET-based DNA sensor can be enhanced and the self-absorption of ligand as well as the background of signals can be decreased because of Eu3+ which owns large Stokes shifts and narrow emission bands due to f–f electronic transitions of 4f shell. We obtained the efficient FRET system by studying suitable distance between the donor and acceptor. Then the FRET-based DNA sensor was used for the design of specific and sensitive detection of target DNA and the quenching efficiency (ΔFL/F0, ΔFL=F−F0) of FL was logarithmically related to the concentration of the target DNA, ranging from 100aM to 100pM. We can realize an ultrasensitive detection of target DNA with a detection limit of 32 aM. This proposed method was feasible to analyse target DNA in real samples with satisfactory results.

A theoretical investigation of the influence of gold nanosphere size on the decay and energy transfer rates and efficiencies of quantum emitters.

We present in this contribution a comprehensive investigation of the effect of the size of gold nanospheres on the decay and energy transfer rates of quantum systems placed close to these nanospheres. These phenomena have been investigated before, theoretically and experimentally, but no comprehensive study of the influence of the nanoparticle size on important dependences of the decay and energy transfer rates, such as the dependence on the donor-acceptor spectral overlap and the relative positions of the donor, acceptor, and nanoparticle, exists. As such, different accounts of the energy transfer mechanism have been presented in the literature. We perform an investigation of the energy transfer mechanisms between emitters and gold nanospheres and between donor-acceptor pairs in the presence of the gold nanospheres using a Green's tensor formalism, experimentally verified in our lab. We find that the energy transfer rate to small nanospheres is greatly enhanced, leading to a strong quenching of the emission of the emitter. When the nanosphere size is increased, it acts as an antenna, increasing the emission of the emitter. We also investigate the emission wavelength and intrinsic quantum yield dependence of the energy transfer to the nanosphere. As evidenced from the literature, the energy transfer process between the quantum system and the nanosphere can have a complicated distance dependence, with a r(-6) regime, characteristic of the Förster energy transfer mechanism, but also exhibiting other distance dependences. In the case of a donor-acceptor pair of quantum systems in the presence of a gold nanosphere, when the donor couples strongly to the nanosphere, acting as an enhanced dipole; the donor-acceptor energy transfer rate then follows a Förster trend, with an increased Förster radius. The coupling of the acceptor to the nanosphere has a different distance dependence. The angular dependence of the energy transfer efficiency between donor and acceptor exhibits a strong focusing effect and the same enhanced donor-dipole character in different angular arrangements. The spectral overlap of the donor emission and acceptor absorption spectra shows that the energy transfer follows the near-field scattering efficiency, with a red-shift from the localized surface plasmon peak for small sphere sizes.