Donor Bridge Acceptor Molecules Research Articles

Toward quantum sensing of chiral induced spin selectivity: Probing donor-bridge-acceptor molecules with NV centers in diamond.

Photoexcitable donor-bridge-acceptor (D-B-A) molecules that support intramolecular charge transfer are ideal platforms to probe the influence of chiral induced spin selectivity (CISS) in electron transfer and resulting radical pairs. In particular, the extent to which CISS influences spin polarization or spin coherence in the initial state of spin-correlated radical pairs following charge transfer through a chiral bridge remains an open question. Here, we introduce a quantum sensing scheme to measure directly the hypothesized spin polarization in radical pairs using shallow nitrogen-vacancy (NV) centers in diamond at the single- to few-molecule level. Importantly, we highlight the perturbative nature of the electron spin-spin dipolar coupling within the radical pair and demonstrate how Lee-Goldburg decoupling can preserve spin polarization in D-B-A molecules for enantioselective detection by a single NV center. The proposed measurements will provide fresh insight into spin selectivity in electron transfer reactions.

Acceleration of Nonradiative Charge Recombination Reactions at Larger Distances in Kinked Donor-Bridge-Acceptor Molecules.

Photoinduced electron transfer in donor-bridge-acceptor (D-B-A) molecular systems can occur via tunneling over long distances (rDA) of well over 10 Å. We commonly observe decreasing rates of electron transfer with increasing distances, a result of a decrease in the electronic coupling of the donor and acceptor moiety. In the study of D-B-A molecules with Ru(bpy)32+ as a bridge/core, Kuss-Petermann and Wenger observed the opposite trend (J. Am. Chem. Soc.2016, 138, 1349); a maximum rate constant of electron transfer was observed at an intermediate electron transfer distance. Within the high-temperature limit of the classical Marcus equation, their observation was qualitatively explained by a sharp distance dependence of outer sphere (or solvent) reorganization energy, as predicted by Sutin and co-workers (J. Am. Chem. Soc.1984, 106, 6858), and almost distance-independent electronic couplings. Here, we report another example of such an underexplored behavior with three kinked D-B-A systems of rDA ∼ 10-19 Å, showing increasing rates of nonradiative charge recombination with increasing rDA. The three D-B-A systems are based on boron dipyrromethene and triphenylamine as electron acceptor and donor groups, respectively, with aryl bridges where the donor and acceptor moieties are connected at meso-positions. These D-B-A molecules exhibit radiative electron transfer reactions (or charge-transfer emission), which enables us to experimentally determine the solvent reorganization energy and the electronic couplings. The analysis of charge-transfer emission that explicitly considers electron-vibration coupling, in conjunction with the temperature-dependent analysis and computational method, revealed that the solvent reorganization energy indeed increases with distance, and at the same time, the electronic coupling decreases with distance expectedly. Therefore, under the right conditions for solvent reorganization energy and electronic coupling values, our results show that we can observe the acceleration of electron transfer reactions with increasing distance, even when we have the expected distance dependence of electronic coupling. This work indicates that the acceleration of electron transfer with increasing distance may be achieved with a fine-tuning of molecular design.

Impact of π-Conjugation Length on the Excited-State Dynamics of Star-Shaped Carbazole-π-Triazine Organic Chromophores.

Correlating star-shaped donor-bridge-acceptor (DBA) molecular structures with intramolecular charge transfer (ICT) and intersystem crossing (ISC) is essential to their application in photocatalysis, photovoltaics, and organic light-emitting diodes (OLEDs). In this work, we report a systematic photophysical study on a series of star-shaped triazine-phenylene-carbazole DBA molecules with 0, 1, and 2 bridging phenylene units (pTCT-0P, pTCT-1P, pTCT-2P). Using a combination of steady-state and time-resolved spectroscopy with time-dependent density functional theory (TDDFT), we find that the bridge length can strongly impact the structural conformation, ICT, and ISC. Global target analysis of the time-resolved spectroscopy reveals that pTCT-0P has the most favorable ISC rate of 1.96 × 10-4 ps-1, which is competitive with a singlet relaxation rate of 1.92 × 10-4 ps-1. TDDFT aligns with spectroscopic results within an order of magnitude, predicting an ISC rate of 2.1 × 10-5 ps-1 and revealing that the donor/acceptor orthogonalization concomitantly suppresses singlet exciton recombination and lowers the singlet-triplet energy gap. The new fundamental insights gained from this work will help design the next generation of star-shaped DBA-type molecules for photocatalytic and photoelectronic applications.

Signatures of Quantum Interference in the Time Dependence of Charge Transfer in Donor–Bridge–Acceptor Molecules

Numerical solution of the time-dependent Schrödinger equation is combined with a statistical procedure for analyzing the time-dependent probability density to look for signatures of quantum phase interference in charge transfer across two donor–bridge–acceptor molecules. The results show a strong dependence of transfer time on relative phase in an initially localized state. Additionally, the transfer time shows a stronger dependence on molecular symmetry for asymmetric initial localizations than symmetric initial localizations.

Impact of Temperature and Non-Gaussian Statistics on Electron Transfer in Donor-Bridge-Acceptor Molecules.

A combination of experimental data and theoretical analysis provides evidence of a bell-shaped kinetics of electron transfer in the Arrhenius coordinates ln k vs 1/T. This kinetic law is a temperature analogue of the familiar Marcus bell-shaped dependence based on ln k vs the reaction free energy. These results were obtained for reactions of intramolecular charge shift between the donor and acceptor separated by a rigid spacer studied experimentally by Miller and co-workers. The non-Arrhenius kinetic law is a direct consequence of the solvent reorganization energy and reaction driving force changing approximately as hyperbolic functions with temperature. The reorganization energy decreases and the driving force increases when temperature is increased. The point of equality between them marks the maximum of the activationless reaction rate. Reaching the consistency between the kinetic and thermodynamic experimental data requires the non-Gaussian statistics of the donor-acceptor energy gap described by the Q-model of electron transfer. The theoretical formalism combines the vibrational envelope of quantum vibronic transitions with the Q-model describing the classical component of the Franck-Condon factor and a microscopic solvation model of the solvent reorganization energy and the reaction free energy.

Through-Solvent Tunneling in Donor-Bridge-Acceptor Molecules Containing a Molecular Cleft.

Photoinduced electron transfer is used to investigate the solvent-mediated electron tunneling between electron donor and acceptor groups in polar solvents. Bis-peptide scaffolds are used to control the spatial positioning of electron donor and acceptor groups and create a molecular cleft. The photoinduced electron transfer is studied for two different cleft sizes, and the electronic coupling is found to be controlled by the nature of the solvent and the ability of the molecular cleft to accommodate it, as well as interact directly with it. These studies demonstrate the importance of electron tunneling through nonbonded contacts and reveal a strategy for examining such tunneling pathways in polar solvents.

Tetramethoxybenzene is a Good Building Block for Molecular Wires: Insights from Photoinduced Electron Transfer.

Two donor bridge-acceptor molecules with terminal triarylamine and Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) redox partners were synthesized and investigated by cyclic voltammetry, optical absorption, luminescence, and transient absorption spectroscopy. The two dyads differ only by the central bridging unit, which was tetramethoxybenzene (tmb) in one case and unsubstituted phenylene (ph) in the other case. Photoirradiation of the Ru(bpy)3(2+) complex of the two dyads triggers intramolecular electron transfer from the triarylamine to the (3)MLCT-excited metal complex, and this process occurs with time constants of 1.5 and 6.8 ns for the tmb- and ph-bridged dyads, respectively. Thermal electron transfer in the reverse direction then leads to disappearance of the photoproduct with a time constant of 10 ns in both dyads. The faster rate of photoinduced charge transfer in the tmb-bridged dyad can be understood in the framework of a hole-tunneling model in which the electron-rich tmb bridge imposes a more shallow barrier than the less electron-rich ph spacer. Until now tmb-based molecular wires have received very little attention, and alkoxy substituents have been mostly used for improving the solubility of oligo-p-phenylene vinylene (OPV) and oligo-p-phenylene ethynylene (OPE) wires. Our study illustrates how four alkoxy-substituents on a phenylene backbone can have a significant influence on the charge-transfer properties of a molecular wire, and this is relevant in the greater context of a future molecular electronics technology.

Dependence of reaction rates for bidirectional PCET on the electron donor-electron acceptor distance in phenol-Ru(2,2'-bipyridine)₃²⁺ dyads.

A homologous series of three donor-bridge-acceptor molecules in which a phenolic unit is attached covalently to a Ru(bpy)3(2+) (bpy =2,2'-bipyridine) complex via rigid rod-like p-xylene spacers was investigated. Photoexcitation at 532 nm in the presence of a large excess of methyl viologen leads to rapid (<10 ns) formation of Ru(bpy)3(3+). When imidazole base is present in CH3CN solution, intramolecular electron transfer from the phenol to Ru(bpy)3(3+) occurs, and this is coupled to proton transfer from the phenol to imidazole. All mechanistic possibilities for this proton-coupled electron transfer (PCET) process are considered, and based on a combination of kinetic and thermodynamic data, one arrives at the conclusion that electron and proton release by the phenol occur in concert. By varying the number of p-xylene bridging units, it then becomes possible to investigate the dependence of the reaction rates for concerted proton-electron transfer (CPET) on the phenol-Ru(bpy)3(3+) distance. A distance decay constant of 0.87 ± 0.09 Å(-1) is obtained. This is one of the largest β values reported for electron transfer across oligo-p-phenylene-based molecular bridges, but it is still relatively close to what was determined for "simple" (i. e., not proton-coupled) electron transfer across oligo-p-xylenes. Bidirectional CPET plays a key role in photosystem II. Understanding the distance dependence of such reactions is of interest, for example, in the context of separating protons and electrons across artificial membranes in order to build up charge gradients for light-to-chemical energy conversion.

Photoinduced electron tunneling between randomly dispersed donors and acceptors in frozen glasses and other rigid matrices

In fluid solution un-tethered donors and acceptors can diffuse freely, and consequently the donor-acceptor distance is usually not fixed on the timescale of an electron transfer event. When attempting to investigate the influence of driving-force changes or donor-acceptor distance variations on electron transfer rates this can be a problem. In rigid matrices diffusion is suppressed, and it becomes possible to investigate fixed-distance electron transfer. This method represents an attractive alternative to investigate rigid rod-like donor-bridge-acceptor molecules which have to be made in elaborate syntheses. This perspective focuses specifically on the distance dependence of photoinduced electron transfer which occurs via tunneling of charge carriers through rigid matrices over distances between 1 and 33 Å. Some key aspects of the theoretical models commonly used for analyzing kinetic data of electron tunneling through rigid matrices are recapitulated. New findings from this rather mature field of research are emphasized.

Phenyl Groups versus tert-Butyl Groups as Solubilizing Substituents for Some [5]Phenacenes and [7]Phenacenes

In recent years, we have used the photocyclizations of diarylethylenes to synthesize a number of [n]phenacenes in the hope that they might be useful as the bridging groups for electron transfer processes in donor-bridge-acceptor molecules. Because [n]phenacenes with n > 5 are very insoluble, their synthesis and characterization has required the attachment of solubilizing substituents such as tert-butyl. The studies of Pascal and co-workers of some large polynuclear aromatic compounds having multiple phenyl substituents prompted us to explore the use of phenyls as alternative solubilizing groups for [n]phenacenes. Although phenyl groups turned out to provide significantly less solubilization than tert-butyl groups in these compounds, we found some interesting structural comparisons of the phenyl-substituted and tert-butyl-substituted [n]phenacenes.

Hydrogen-Bond Strengthening upon Photoinduced Electron Transfer in Ruthenium–Anthraquinone Dyads Interacting with Hexafluoroisopropanol or Water

Quinones play a key role as primary electron acceptors in natural photosynthesis, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, the influence of hydrogen-bond donating solvents on the thermodynamics and kinetics of intramolecular electron transfer between Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and 9,10-anthraquinone redox partners linked together via one up to three p-xylene units was investigated. Addition of relatively small amounts of hexafluoroisopropanol to dichloromethane solutions of these rigid rodlike donor-bridge-acceptor molecules is found to accelerate intramolecular Ru(bpy)(3)(2+)-to-anthraquinone electron transfer substantially because anthraquinone reduction occurs more easily in the presence of the strong hydrogen-bond donor. Similarly, the rates for intramolecular electron transfer are significantly higher in acetonitrile/water mixtures than in dry acetonitrile. In dichloromethane, an increase in the association constant between hexafluoroisopropanol and anthraquinone by more than 1 order of magnitude following quinone reduction points to a significant strengthening of the hydrogen bonds between the hydroxyl group of hexafluoroisopropanol and the anthraquinone carbonyl functions. The photoinduced intramolecular long-range electron transfer process thus appears to be followed by proton motion; hence the overall photoinduced reaction may be considered a variant of stepwise proton-coupled electron transfer (PCET) in which substantial proton density (rather than a full proton) is transferred after the electron transfer has occurred.

Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor–Bridge–Acceptor Molecules: Implications for Wirelike Behavior

Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN(n)) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ(+•)-An(-•), and An(-•) acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ(+•)-An(-•)-FN(n)-NI → DMJ(+•)-An-FN(n)(-•)-NI → DMJ(+•)-An-FN(n)-NI(-•). The charge-shift reaction from An(-•) to NI(-•) exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å(-1), which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN(n)(-•) radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D(+•)-B(-•)-A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.

Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor–bridge–Acceptor Molecules

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+•)-PI(-•) or DMJ(+•)-NI(-•) quantitatively in 1-4 (τ(CS) ≤ 10 ps), and that charge recombination occurs with τ(CR) = 268 and 158 ps for 1 and 3, respectively, and with τ(CR) = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ(+•) and PI(-•) or NI(-•), giving 2J > 600 mT for 1-3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of (3)*An upon charge recombination occurs by spin-orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor-acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.

Photoinduced Processes in Fluorene‐Bridged Rhenium–Phenothiazine Dyads – Comparison of Electron Transfer Across Fluorene, Phenylene, and Xylene Bridges

AbstractThe photoinduced processes occurring after pulsed laser excitation of a series of donor–bridge–acceptor molecules comprising a phenothiazine electron donor, variable‐length fluorene bridges, and a rhenium(I) electron acceptor were investigated. A dyad with a single fluorene bridge unit exhibits electron transfer from phenothiazine to the rhenium(I) complex upon photoexcitation, whereas in dyads with fluorene oligomers bridge‐localized triplet excited states are formed rather than electron transfer products. In the monofluorene‐bridged system with a donor–acceptor distance of ca. 15 Å, electron transfer occurs with a time constant of 1.9 ns. The equidistant electron transfer between the same donor and acceptor is considerably slower across a biphenyl bridge (3.9 ns) or a bi‐p‐xylene spacer (20 ns). This finding is interpreted in terms of different tunneling barrier heights associated with the charge transfer across the three different types of molecular bridges.

Comparing Spin‐Selective Charge Transport through Donor–Bridge–Acceptor Molecules with Different Oligomeric Aromatic Bridges

Burning bridges: The exponential distance dependence (β value) of singlet and triplet charge recombination (CR) pathways is determined for three donor-bridge-acceptor (DBA) molecules. p-Phenylethynylene and fluorenone bridges have similar β values, which differ significantly from those of p-phenylene bridges, thus implying that β for both singlet and triplet CR is system-dependent, not bridge-specific.

Cyclometalated Iridium(III) Complexes as Photosensitizers for Long‐Range Electron Transfer: Occurrence of a Coulomb Barrier

AbstractSix cyclometalated iridium(III) complexes were investigated to assess their potential as photosensitizers for long‐range electron transfer, and two of them were incorporated directly into covalent donor–bridge–acceptor molecules. The influence of ligand substitutions on the excited‐state properties and the photoredox behavior of the iridium complexes was explored by optical absorption, steady‐state and time‐resolved luminescence spectroscopy, as well as by electrochemical methods. Bimolecular electron transfer between the photoexcited complexes and 10‐methylphenothiazine and methylviologen was found to be only weakly dependent on the ligand substitutions. Intramolecular long‐range electron transfer from phenothiazine to photoexcited iridium(III) in the dyads is slow due to the occurrence of a Coulomb barrier. Consequently, an electron‐transfer photoproduct is only observable in the transient absorption spectrum of a donor–bridge–acceptor molecule with a fluorinated photosensitizer that exhibits a very long excited‐state lifetime. A flash‐quench technique is necessary for detection of an electron‐transfer product in the dyad with a non‐fluorinated photosensitizer. The occurrence of a Coulomb barrier associated with intramolecular (excited‐state) long‐range electron transfer in the dyads with cyclometalated iridium(III) photosensitizers represents an important difference to previously investigated similar donor–bridge–acceptor molecules with photosensitizers based on d6 metal diimine complexes. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2009)

Variation of charge transfer kinetics in structurally closely related dyads with rhenium photosensitizers

The kinetics for photoinduced charge separation and thermal charge-recombination processes in three donor–bridge–acceptor molecules with rhenium(I) tricarbonyl diimine photosensitizers were investigated. Time resolved luminescence and transient absorption spectroscopies reveal that in addition to driving force effects, differences in bridge-mediated electronic donor–acceptor coupling among the three dyads play important roles. Notably, it is seen that charge separation depends strongly on whether initial photoexcitation involves promotion of an electron towards or away from a phenothiazine electron donor. Thermal charge recombination rates are found to differ by a factor of 2 between two isomeric dyads due to an electronic coupling effect.

Solvent Dynamical Effects on Electron Transfer in U-Shaped Donor-Bridge-Acceptor Molecules

This study explores how the electron transfer in a class of donor-bridge-acceptor (DBA) supermolecules is affected by the dynamical response of the solvent. These DBA molecules have a pendant group juxtaposed between the donor and acceptor groups (Figure 1). The pendant provides intermediate electronic coupling strengths of a few hundred wavenumbers by way of its nonbonded contacts with the donor and acceptor and it can be tuned by substituents added to the pendant. This design allows the measurement of electron transfer rates from a regime in which the mechanism is nonadiabatic to a regime in which the solvent friction modifies the rate substantially. The rate constants and mechanistic parameters are compared with the expectations of models for solvent dynamical effects on the reaction rate.

On the effect of nuclear bridge modes on donor–acceptor electronic coupling in donor–bridge–acceptor molecules

We report a theoretical study of intra-molecular electronic coupling in a symmetric DBA (donor–bridge–acceptor) complex, in which a donor electronic site is coupled to an acceptor site by way of intervening orbitals of a molecular bridge unit. In the off-resonant (deep tunneling) regime of electronic transport, the lowest unoccupied molecular orbitals (MO’s) of the DBA system are split into distinguishable donor/acceptor and bridge orbitals. The effect of geometrical changes at the bridge on the donor/acceptor electronic energy manifold is studied for local stretching and bending modes. It is demonstrated that the energy splitting in the manifold of donor/acceptor unoccupied MOs changes in response to such changes, as assumed in simple McConnell-type models. Limitations of the simple models are revealed where the electronic charging of the bridge orbitals correlates with increasing donor/acceptor orbital energy splitting only for stretching but not for bending bridge modes.

DENSITY MATRIX ANALYSIS AND SIMULATION OF ELECTRON DYNAMICS IN MULTIPLE-BRIDGED DONOR/ACCEPTOR MOLECULES UNDER DISSIPATIVE ENVIRONMENTS

Feynman and Vernon path integral approach is adopted to investigate electron transfer dynamics in donor–bridge–acceptor molecules under dissipative environments. Especially, we focus on the solvent effect on the superexchange process of electron transfer. The results reveal that at high enough bridge energies or low enough temperature, electron can transfer with a superexchange mechanism no matter whether solvent is incorporated or not. However, the superexchange changes from coherent to incoherent limits when the dissipative strength increases, and electron transfer rates are much dependent on the dissipative strength.