Donor Bridge Acceptor Systems Research Articles

Electronic dynamics of donor–bridge–acceptor system with a bridge impurity incorporated in dissipative environments

Electronic dynamics of Donor–Bridge–Acceptor system with a bridge impurity incorporated in dissipative environments is studied taking advantage of the quasi-adiabatic propagator path integral approach in this article. An impurity site is introduced in the center of bridge chain of the system. The numerical calculations indicate the electron transfer rates of the system are changed when the energy gap between the center site (impurity site) and other bridge sites increased from −2.5 to 2.5 eV, the relaxation rates decrease and the dephasing rates increase as the energy gap increases, respectively under the same environment factors (e.g. temperature and system–environment coupling). However, the electron transfer mechanism of the system is hardly affected when the bridge impurity energy is increased in the range of ±21% relative to the other bridge energy, and the occupation populations of bridge chain are not more than 10%, which means the superexchange mechanism also works well within the context of this study.

Control and Modulation of Photoinduced Charge Transfer in a Rigid Donor-Bridge-Acceptor System by Electric Fields.

This article theoretically studies the photoinduced charge transfer (CT) of rigid D-B-A molecules in two-photon absorption (TPA) adjusted by the external electric fields. Using a visualization method, the dynamic changes of light-induced CT in different channels of TPA are presented through a two-dimensional (2D) transition density matrix and a three-dimensional (3D) charge different density. Here, we prove the controllability of TPA on CT under the induction of a strong electric field. Adjusting the field direction and intensity significantly affects the position of the strong absorption peak in the TPA spectra, thereby further changing the electron-hole coherence length and the degree of dispersion. Our results can promote the recognition of the optical properties of the D-B-A system in synthetic molecules and provide an idea for increasing the proportion of excited states for CT in the molecule.

The Role of Bridge-State Intermediates in Singlet Fission for Donor-Bridge-Acceptor Systems: A Semianalytical Approach to Bridge-Tuning of the Donor-Acceptor Fission Coupling.

We describe a semianalytical/computational framework to explore structure-function relationships for singlet fission in Donor (D)-Bridge (B)-Acceptor (A) molecular architectures. The aim of introducing a bridging linker between the D and A molecules is to tune, by modifying the bridge structure, the electronic pathways that lead to fission and to D-A-separated correlated triplets. We identify different bridge-mediation regimes for the effective singlet-fission coupling in the coherent tunneling limit and show how to derive the dominant fission pathways in each regime. We describe the dependence of these regimes on D-B-A many-electron state energetics and on D-B (A-B) one-electron and two-electron matrix elements. This semianalytical approach can be used to guide computational and experimental searches for D-B-A systems with tuned singlet fission rates. We use this approach to interpret the bridge-resonance effect of singlet fission that has been observed in recent experiments.

Tuning the UV/Vis Absorption Spectra of Electrochromic Small Molecular Radicals Through Bridge Modulation.

Studies on the optical properties of donor-bridge-acceptor (DBA) materials in their radical anion state are rare but important. Such investigations can help to extend the application of DBA materials to opto-electrochemical devices and no longer limit them to optical physics research. In this work, a series of new DBA materials, TACzs, for overcoming this limitation are reported. All TACzs show strong intramolecular charge transfer (ICT) in their photoexcited states, leading to noticeable solvatochromism. Besides, the electronic structures of their radical anions show great variability, displaying different absorption spectra and diverse colors. Moreover, the potential application of TACzs as electrochromic and electro-fluorochromic materials are discussed. This work demonstrates that manipulating the π bridge between the donor and acceptor in the DBA system is an effective pathway not only to tailor the ICT properties of materials in their neutral state, but also to tune the absorption characteristics of their radical anion state, which makes them very promising for applications in electroluminescent and electrochemical devices.

Synthesis and Solvatochromic Behavior of Zwitterionic Donor–Bridge–Acceptor Systems with Oligo(p-phenylene) Spacers

Oligo(p-phenylene)s with a donor phenol group and an acceptor pyridinium moiety separated by one and two p-phenylene units were synthesized by the linear iterative Suzuki–Miyaura coupling method using aryl nonaflates as effective coupling reagents. Zwitterionic forms of these push–pull molecules were generated upon deprotonation of the phenol leading to large redshifts in absorbance maxima. UV-vis absorbance studies also revealed strong dependence of the band position on solvent polarity: a smooth bathochromic shift can be observed with the decrease of the solvent polarity. The molecule with one p-phenylene bridging unit showed the strongest solvatochromic characteristics in the series, spanning the range of 167 nm while moving from polar water to less polar N,N-dimethylformamide. The magnitude of this shift was close to Reichardt's dye — one of the most solvatochromic organic dyes known.

Magnetic Control of Recombination Fluorescence and Tunability by Modulation of Radical Pair Energies in Rigid Donor-Bridge-Acceptor Systems.

Magnetic control of molecular emission holds the promise of developing new magneto-optical technologies. Spin dynamics of radical pairs can serve as a basis of control of chemical reactions by weak magnetic fields (<1 T) orders of magnitude smaller than the thermal energy kBT at room temperature. Here we demonstrate control of recombination fluorescence, produced by charge recombination of photogenerated radical pairs, by weak magnetic fields in rigid donor-bridge-acceptor molecules excited with visible light. We can tune the field response range by chemically modulating the energies of the radical pairs affecting exchange interactions. Our results present a new strategy for designing magneto-optical probes for imaging and other molecular spin technology applications.

Interplay between Förster and Dexter Energy Transfer Rates in Isomeric Donor-Bridge-Acceptor Systems.

The ability to direct the flow of excitons enable molecular systems to perform highly advanced functions. Intramolecular energy transfer in donor-bridge-acceptor systems can occur by different mechanisms, and the ability to control the excited state energy pathways depends on the capacity to favor one process over another. Here, we show an anticorrelation between the rates of Förster and Dexter types of energy transfer in two isomeric donor-bridge-acceptor systems. Both dyads display intramolecular Förster triplet-to-singlet and Dexter triplet-to-triplet energy transfers. However, as the bridge-acceptor connection point changes, the rate of one energy transfer process increases at the same time as the other one decreases, allowing us to control the energy flow direction. This work shows how rational design can be used to tune excited state energy pathways in molecular dyads, which is of importance for advanced functions such as multiplicity conversion in future molecular materials.

Interplay of Direct and Indirect Charge-Transfer Pathways in Donor-Bridge-Acceptor Systems.

Charge transfer in donor-bridge-acceptor (DBA) structures typically takes place through the combination of donor-bridge and bridge-acceptor overlap integrals forming an effective, indirect electronic coupling between the donor (D) and acceptor (A) moieties. Here we examine the effects of an additional direct DA electronic coupling of charge-transfer processes in DBA systems with local interaction with thermal baths. First, using the exact Nakajima-Zwanzig master equation (NZME) for the reduced density matrix, we rigorously define probability currents as the coherent part of the NZME, thereby allowing us to quantify the contribution of the different electronic pathways (direct and indirect) to the charge-transfer dynamics. Focusing on two minimal DBA systems of three sites (V and Λ models) and adopting well-developed methods, we find that the interplay between different transfer pathways can be assessed by the McConnell formula in the weak system-bath coupling regime. We then demonstrate that the combination of indirect and direct donor-acceptor coupling either enhances or leads to a destructive quantum interference effect on charge-transport processes, depending on the energy landscape of the DBA system.

Intramolecular Long-Range Charge-Transfer Emission in Donor-Bridge-Acceptor Systems.

Charge recombination to the electronic ground state typically occurs nonradiatively. We report a rational design of donor-bridge-acceptor molecules that exhibit charge-transfer (CT) emission through conjugated bridges over distances of up to 24 Å. The emission is enhanced by intensity borrowing and extends into the near-IR region. Efficient charge recombination to the initial excited state results in recombination fluorescence. We have established the identity of CT emission by solvent dependence, sensitivity to temperature, femtosecond transient absorption spectroscopy, and unique emission polarization patterns. Large excited-state electronic couplings and small energy gaps enable the observation of intramolecular long-range CT emission over the unprecedented long distance. These results open new possibilities of using intramolecular long-range CT emission in molecular electronic and biomedical imaging probe applications.

Diphenylamine-Substituted Osmanaphthalyne Complexes: Structural, Bonding, and Redox Properties of Unusual Donor-Bridge-Acceptor Systems.

Diarylamine-substituted osmanaphthalyne complexes that feature two redox centers linked by the rigid skeleton of the metallacycle (C^C+ ), specifically, [OsCl2 (PPh3 )2 {(C^C+ )NAr2 }][BF4 - ] (Ar=Ph (1 a), p-MeOPh (1 b)) and their open-ring precursors [OsHCl2 (PPh3 )2 {(≡C-C(PPh3 + )=CHPh)NR2 }][BF4 - ] (Ar=Ph (2 a), p-MeOPh (2 b)), were successfully synthesized and characterized by 1 H, 13 C, and 31 P NMR spectroscopy, ESI-MS, and elemental analysis. The solid-state molecular structures of complexes 1 a and 2 a were ascertained by single-crystal X-ray diffraction. The Os≡C bond length in both complexes 1 a and 2 a fell within the range reported for similar osmanaphthalynes and osmium carbyne complexes, respectively. The structural parameters determined for complex 1 a, which were successfully reproduced by theoretical calculations, point to a π-delocalized metallacycle structure. The purple color of compounds 1 a and b was explained by the diarylamine→Os(metallacycle) charge-transfer absorption in the visible region. The neutral, one-electron-oxidized and one-electron-reduced states of compounds 1 a, b, and a reference complex that lacked the diarylamine substituent, [OsCl2 (PPh3 )2 {(C^C+ )}][BF4 - ] (1'), were investigated by cyclic and square-wave voltammetry, UV/Vis/NIR spectroelectrochemistry, and DFT calculations. The spin density in singly oxidized complexes [1 a]+ and [1 b]+ predominantly resided on the aminyl segment, with osmium involvement controlled by the diphenylamine substitution. Spin density in stable, singly-reduced [1']- was distributed mainly over the osmanaphthalyne metallacycle.

A Benchmark Study of Electronic Couplings in Donor-Bridge-Acceptor Systems with the FMR-B Method.

We present a benchmarking study for the evaluation of electronic couplings in donor-bridge-acceptor systems with the Fock matrix reconstruction-bridge (FMR-B) method. We compile a data set for the benchmark that contains 29 molecules for which reliable experimental coupling values are available: DBA29. This data set is general and includes different types of donor, acceptor, and bridge units as well as different bridge lengths, and it spans a range of couplings from 0.1 to 0.8 eV. We use DBA29 to test FMR-B with 11 density functionals belonging to different classes (pure, global hybrid, and range-separated) and the Hartree-Fock (HF) method. We also test a subset of these methods with nine basis sets from the Pople and Dunning families, which include a varying number of polarization and diffuse functions. We find that the best accuracy and lowest computational cost is obtained with range-separated functionals and compact basis sets. Global hybrids with a large amount of HF exchange also work well because of error cancellation between the approximate exchange-correlation kernel and the HF part. Pure functionals, although less accurate, still provide reasonable results with a consistent underestimation of the experimental values, and they can be used for larger and more computationally demanding systems.

Bridge-driven aggregation control in dibenzofulvene–naphthalimide based donor–bridge–acceptor systems: enabling fluorescence enhancement, blue to red emission and solvatochromism

The photophysical and aggregation behavior of four novel donor–acceptor based luminogens was precisely controlled by the π-electron bridge in the donor–bridge–acceptor core structures.

Novel Fullerene Platinum Alkynyl Complexes with High Second-Order Nonlinear Optical Properties as a Springboard for NLO-Active Polymer Films

The synthesis and second-order nonlinear optical (NLO) properties of seven novel fullerene-C60 derivatives are reported. In these donor–bridge–acceptor systems, the fullerene unit (acceptor) is connected through a cyclopropane ring to an ethynyl thienyl fragment (bridge) which binds a trimethylsilyl or platinum alkynyl fragment (donor). All derivatives are NLO-active, as determined by the electric field induced second harmonic generation technique, but the fullerene platinum alkynyl complexes are characterized by a particularly large second-order NLO response. Substitution of the thienyl fragment by a terthiophene leads to an increase of the NLO performance by using trimethylsilyl as a donor group, whereas no effect is observed in the case of platinum derivatives. Remarkably, fullerene platinum alkynyl complexes can be dispersed in poly(methyl methacrylate) or polystyrene, affording NLO-active polymer films.

Influence of Coherent Tunneling and Incoherent Hopping on the Charge Transfer Mechanism in Linear Donor-Bridge-Acceptor Systems.

The mechanism of charge transfer has been observed to change from tunneling to hopping with increasing numbers of DNA base pairs in polynucleotides and with the length of molecular wires. The aim of this paper is to investigate this transition by examining the population dynamics using a tight-binding Hamiltonian with model parameters to describe a linear donor-bridge-acceptor (D-B-A) system. The model includes a primary vibration and an electron-vibration coupling at each site. A further coupling of the primary vibration with a secondary phonon bath allows the system to dissipate energy to the environment and reach a steady state. We apply the quantum master equation (QME) approach, based on second-order perturbation theory in a quantum dissipative system, to examine the dynamical processes involved in charge-transfer and follow the population transfer rate at the acceptor, ka, to shed light on the transition from tunneling to hopping. With a small tunneling parameter, V, the on-site population tends to localize and form polarons, and the hopping mechanism dominates the transfer process. With increasing V, the population tends to be delocalized and the tunneling mechanism dominates. The competition between incoherent hopping and coherent tunneling governs the mechanism of charge transfer. By varying V and the total number of sites, we also examine the onset of the transition from tunneling to hopping with increasing length.

Watching the dynamics of electrons and atoms at work in solar energy conversion.

The photochemical reactions performed by transition metal complexes have been proposed as viable routes towards solar energy conversion and storage into other forms that can be conveniently used in our everyday applications. In order to develop efficient materials, it is necessary to identify, characterize and optimize the elementary steps of the entire process on the atomic scale. To this end, we have studied the photoinduced electronic and structural dynamics in two heterobimetallic ruthenium-cobalt dyads, which belong to the large family of donor-bridge-acceptor systems. Using a combination of ultrafast optical and X-ray absorption spectroscopies, we can clock the light-driven electron transfer processes with element and spin sensitivity. In addition, the changes in local structure around the two metal centers are monitored. These experiments show that the nature of the connecting bridge is decisive for controlling the forward and the backward electron transfer rates, a result supported by quantum chemistry calculations. More generally, this work illustrates how ultrafast optical and X-ray techniques can disentangle the influence of spin, electronic and nuclear factors on the intramolecular electron transfer process. Finally, some implications for further improving the design of bridged sensitizer-catalysts utilizing the presented methodology are outlined.

Intramolecular Charge- and Energy-Transfer Rates with Reduced Modes: Comparison to Marcus Theory for Donor–Bridge–Acceptor Systems

We present a new, fully ab initio approach for computing intramolecular charge- and energy-transfer rates. Using a time-convolutionless master equation approach and parametrizing using couplings obtained using an accurate quantum chemical approach, we benchmark the approach against experimental results and Marcus theory rates for triplet energy transfer for a series of donor-bridge-acceptor systems. An important component of our analysis is the use of a projection operator scheme that parses out specific internal nuclear motions that accompany the electronic transition. Using an iterative Lanczos approach, we concentrate the coupling between the electronic and nuclear degrees of freedom into a small number of reduced harmonic modes. We find that by using only a single reduced mode termed the "primary mode", one obtains an accurate evaluation of the golden rule rate constant and insight into the nuclear motions responsible for coupling the initial and final electronic states. In particular, the primary mode reflects the irreducible representation of the donor and acceptor excited states.

On the effect of the nature of the bridge on oxidative or reductive photoinduced electron transfer in donor–bridge–acceptor systems

Photoinduced electron transfer is a topical issue in chemistry. In multicomponent donor-bridge-acceptor systems, electron transfer is usually discussed within the frame of superexchange theory, which takes into account electronic coupling mediated by virtual states involving bridge orbitals. However, the schematization used for superexchange in thermal electron transfer processes is not suitable to immediately understand some intriguing aspects of photoinduced charge separation and recombination processes, which are only uncovered by analyzing the virtual states involved in forward and backward excited-state electron transfer. In particular, for oxidative photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward charge separation can efficiently mediate charge recombination via the hole-transfer superexchange route, whereas for reductive photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward process can efficiently mediate charge recombination via electron-transfer superexchange. As a consequence, to obtain long-lived charge-separated states upon oxidative photoinduced electron transfer in donor-bridge-acceptor systems it is preferable to avoid easy-to-oxidize bridges, whereas easy-to-reduce bridges should better be avoided in reductive photoinduced charge separation. These considerations, exemplified by the analysis of some literature cases, can be useful hints for the design of long-lived charge-separated states.

Multistate Treatments of the Electronic Coupling in Donor–Bridge–Acceptor Systems: Insights and Caveats from a Simple Model

We use a simple one-dimensional delta function electronic structure model (dfm) to investigate the results of a pair of multistate diabatization techniques (i.e., based on n states, with n ≥ 2) for linear DBA and DBBA (donor-bridge-acceptor) electron-transfer systems. In particular, we focus on the physical meaning of the couplings obtained from multistate methods and their relationship to two-state (n = 2) coupling elements. On the basis of the simple dfm approach, which allows exact as well as finite basis set treatment and has no many-electron effects, we conclude that for orthogonal diabatic states, it is difficult to assign clear physical significance to multistate matrix elements for coupling beyond nearest-neighbor contacts. The implications of these results for more complex multistate many-electron treatments are discussed. It is emphasized that physically meaningful coupling elements must involve states that are orthogonal, either explicitly or implicitly.

Coherent quantum dynamics in donor–bridge–acceptor systems: beyond the hopping and super-exchange mechanisms

The population transfer dynamics of model donor–bridge–acceptor systems is studied by comparing a recently developed polaron-transformed quantum master equation (PQME) with the well-known Redfield and Förster theories of quantum transport. We show that the PQME approach reduces to these two theories in their respective limits of validity and naturally interpolates between them as a function of the system–bath coupling strength. By exploring the parameter space of our model problem, we identify novel regimes of transport dynamics in bridged systems like those encountered in biological and organic energy transfer problems. Furthermore, we demonstrate that three-level systems like the ones studied herein represent ideal minimal models for the identification of quantum coherent transport as embodied in super-exchange phenomena that cannot be captured by Förster-like hopping approaches.

Electron dynamics across molecular wires: A time-dependent configuration interaction study

In this study we present methodological developments of the time-dependent configuration interaction (TDCI) method for ab initio electron dynamics in donor–bridge–acceptor systems. Especially, we investigate the role of valence electron correlation, the scheme for selecting the determinantal basis, and the computational effort. Our test systems are molecules of the type Li–(C2)n–CN, Li–(C2H2)n–CN, and Na9–(C2H2)2–CN. In this way, this study is intended as a step towards rigorous description of charge transfer in molecular wires attached to metal surfaces or nanoparticles using a many-electron wavefunction. Also, a multi-reference configuration interaction singles approach is suggested as a good compromise between computational effort and accuracy.