Donor-acceptor Electronic Coupling Research Articles

The Diverse Electrochemistry and Photophysics of Pyrrolopyrroles

Pyrrole is among the most popular molecular scaffolds for biology. Tetrapyrroles, such as porphyrins, corrins and their derivatives, are the quintessential redox centers and chromophores exhibiting a wide range of electrochemical and optical characteristics. Similarly, pyrrolopyrroles, built on condensed five-member rings with nitrogens as heteroatoms, offer scaffolds for organic chromophores with diversity of redox and photophysical properties. For example, pyrrolo[3,2-b]pyrroles are stable electron-rich UV absorbers with quadrupole symmetry (Acc. Chem. Res. 2017, 50, 2334-2345). They can act as immensely potent electron donors and attaching electron-accepting moieties to them leads to the emergence of a myriad of optical phenomena, such as excited-state symmetry breaking (Commun. Chem. 2020, 3, 190) and generating white fluorescence by tuning donor-acceptor electronic coupling (Chem. Sci. 2023). Attaching carbonyls to the pyrrolopyrrole rings drastically changes their electronic properties, making them pronouncedly electron deficient and shifting their absorption to the visible spectral region. For example, 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-diones, or diketopyrrolopyrroles (DPPs) for short, are excellent electron acceptors and photooxidants (Adv. Opt. Matter. 2015, 3, 2018-320). Their reduction potentials are susceptible to localized electric fields, which has allowed demonstrating dipole-induced rectification of photoinduced hole transfer (Angew. Chem. Int. Ed. 2018, 57, 12365-12369). This field susceptibility extends to the optical properties of DPPs, providing an extra dimension in their utility for photonics and optoelectronics. DPPs, and pyrrolopyrroles in general, are built on small bicyclic scaffolds, i.e., considerably smaller than those of other multi-pyrrole chromophores. The small size enhances the susceptibility of their redox properties to solvation and makes them attractive photoprobes. Therefore, this class of compounds have become indispensable agents for electrochemical and photonic science and engineering.

The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor-donor-acceptor fluorophores.

Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor-donor-acceptor or donor-acceptor-donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor-acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti-Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics.

Frustration Dynamics and Electron-Transfer Reorganization Energies in Wild-Type and Mutant Azurins.

Long-range electron tunneling through metalloproteins is facilitated by evolutionary tuning of donor–acceptor electronic couplings, formal electrochemical potentials, and active-site reorganization energies. Although the minimal frustration of the folding landscape enables this tuning, residual frustration in the vicinity of the metallocofactor can allow conformational fluctuations required for protein function. We show here that the constrained copper site in wild-type azurin is governed by an intricate pattern of minimally frustrated local and distant interactions that together enable rapid electron flow to and from the protein. In contrast, sluggish electron transfer reactions (unfavorable reorganization energies) of active-site azurin variants are attributable to increased frustration near to as well as distant from the copper site, along with an exaggerated oxidation-state dependence of both minimally and highly frustrated interaction patterns.

(Invited) Vibronic Effect of Donor-Acceptor Interaction Determines Fate of Mutiexciton Spins Generated By Singlet Fission

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit with power conversion efficiencies (PCE) ~33 % of single junction organic solar cells (OSC) because two separated triplet excitons (T+T) can be produced from one excited singlet state (S1S0) sharing its excitation energy with a neighboring ground-state chromophore. The intramolecular SF (iSF) dynamics have widely been studied using the transient absorption spectroscopic methods together with theoretical modeling taking into account the vibronic effects in the ultrafast regimes. The correlated intermolecular triplet pair 1(TT) generated with the singlet character is known to be converted to the quintet (Q) state as 5(TT). These TT pairs separate into individual triplets as the T+T state. In the previous study, we reported the iSF dynamics and subsequent separations to T+T in the pentacene dimers (PcD) bridged by a phenylene at ortho and meta positions by a combination of the ultrafast transient absorption spectroscopy and the time-resolved electron paramagnetic resonance (TREPR) method.1 Before the spin-conversions to the 5(TT) state, the initially populated 1(TT) is strongly coupled in the four unpaired spins of the two triplet excitons, i.e. the entangled spin-state by large spin-spin exchange coupling (J) with the large orbital overlap produced at femtoseconds regions. The modulations of the J-couplings are thought to be essential for the 1(TT) -> 5(TT) conversions. In an iSF system, we interpreted that the quintet ESPs were caused by the anisotropic spin conversions due to singlet-quintet spin-relaxations to the five sublevels at m S = +2, +1, 0, -1 and -2 in accordance with the anisotropy of the ZFS coupling which is highly dependent on the relative orientation of one triplet exciton (TB) with respect to the other triplet (TA) in the TATB multiexciton.2 Internal fluctuations in the J-coupling were concluded to be crucial on the quintet generations. Moreover, we reported the treatment of the EPR transitions in the weakly coupled T+T states in the framework of the spin-correlated triplet pair (SCTP) for the very weak exchange coupling including the superpositions of the nine basis functions of 5(TT) - 3(TT) - 1(TT), as treated in the spin-correlated radical pair model. 3,4 The above pioneering interpretations of the ESP mechanisms may pave a new avenue to clarify how the molecular conformations and their motions drive the spin-conversions and the ultimate T+T generations, both of which are essential but unknown for the photon-to-energy conversion process. In particular, because the J-coupling is anticipated to be sensitive to the TATB conformations, it is expected that the specific molecular motions or phonon modes may play roles for the ultimate T+T de-coupling, as the vibronic effects. So far, details of the TATB geometries, the vibration modes and frequencies, and the electronic couplings are unresolved in the iSF systems for generating the quintet multiexcitons and the subsequent T+T dissociations.We have characterized the molecular geometries, conformation dynamics and J-couplings of the intramolecular multiexcitons after the photoinduced iSF with the proposal of the density matrix analysis of ESP observed by the TREPR method. It is concluded that the J-coupling is determined by the donor-acceptor electronic coupling in the multiexciton and that terahertz TA-TB conformation dynamics plays a significant role for generating the quintet multiexcitons and for the individual T+T states caused by the J-fluctuation in the covalently linked systems.4 Sakai, H.; Inaya, R.; Nagashima, H.; Nakamura, S.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T., J. Phys. Chem. Lett. 2018, 9, 3354-3360.Matsui, Y.; Kawaoka, S.; Nagashima, H.; Nakagawa, T.; Okamura, N.; Ogaki, T.; Ohta, E.; Akimoto, S.; Sato-Tomita, A.; Yagi, S., et al., J. Phys. Chem. C 2019, 123, 18813-18823.Matsuda, S.; Oyama, S.; Kobori, Y., Chem. Sci. 2020, 11, 2934-2942.Kobori, Y.; Fuki, M.; Nakamura,; S. Hasobe, T. J. Phys. Chem. B 2020, 124, 9411–9419.

(Invited) Why Do Dipole Effects on Charge Transfer Matter?

Charge transfer (CT) and charge transport (CTr) are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible (Phys. Chem. Chem. Phys. 2020, 22, 21583-21629). In natural and manmade sensing systems, for example, well synchronized CT processes drive the conversion of mechanical, optical, chemical and thermal signals into electric outputs, which is essential for interfacing with processing modules. Concurrently, electric dipoles are everywhere and therefore, understanding how they affect CT and CTr is of principal importance for attaining and controlling the functionality of a wide range of materials and devices. Discussions of the idea about dipole effects on CT date back to the mid 20th century and reported experimental evidence from the 1990s and 2000s demonstrated their key importance. The dipole-generated localized electric fields induce Stark effects and modulate the electronic properties of the CT moieties in their vicinity. The notion for such effects focusses on dipole-induced changes in the reduction potentials of the donor and the acceptor, affecting the CT driving forces and thus, the Franck-Condon (FC) contributions to the CT kinetics. We recently demonstrated that to harness such effects, which are inherently enormous, (1) the dipoles should be placed as closed as possible to the electron donor and acceptor, and (2) the media polarity should be lowered (Angew. Chem. 2018, 57, 12365-12369). Polar media, indeed, stabilize charged states and in general enhances the rates of CT. Polar media, however, screen the field permeation and damps the dipole effect on the electron donor and acceptor. Using hydrocarbons as a medium, results in electron transfer rates along the dipole that are six times larger than the rates for the same system when in polar solvents, such as acetonitrile. The same localized field effects in non-polar medium completely shut down the electron transfer against the dipole. While in the vicinity of dipoles, the electric fields can reach GV/m, they fall of rapidly with distance. In our work, we incorporate the dipoles in the electron donors. This geometry provides the closest possible placement of the dipole to the CT moieties. Most importantly, it has also allowed us to discover for the first time dipole effects on donor-acceptor electronic coupling that dominate and oppose the established notion about effects on the FC contributions to the CT kinetics. These discoveries change the ways of thinking about dipole effects on CT and CTr, and set unprecedented paradigms for the development of electronic materials and devices.

Shallow distance-dependent triplet energy migration mediated by endothermic charge-transfer

Conventional wisdom posits that spin-triplet energy transfer (TET) is only operative over short distances because Dexter-type electronic coupling for TET rapidly decreases with increasing donor acceptor separation. While coherent mechanisms such as super-exchange can enhance the magnitude of electronic coupling, they are equally attenuated with distance. Here, we report endothermic charge-transfer-mediated TET as an alternative mechanism featuring shallow distance-dependence and experimentally demonstrated it using a linked nanocrystal-polyacene donor acceptor pair. Donor-acceptor electronic coupling is quantitatively controlled through wavefunction leakage out of the core/shell semiconductor nanocrystals, while the charge/energy transfer driving force is conserved. Attenuation of the TET rate as a function of shell thickness clearly follows the trend of hole probability density on nanocrystal surfaces rather than the product of electron and hole densities, consistent with endothermic hole-transfer-mediated TET. The shallow distance-dependence afforded by this mechanism enables efficient TET across distances well beyond the nominal range of Dexter or super-exchange paradigms.

Binding Orientation of a Ruthenium-Based Water Oxidation Catalyst on a CdS QD Surface Revealed by NMR Spectroscopy.

We report on the binding of a Ru-based water oxidation catalyst (WOC) to CdS quantum dots (QDs) revealed by 1H NMR spectroscopy. Spin centers within the WOC exhibit correlated trends in chemical shift and T2 lifetime shortening upon QD binding. These effects are a highly directional function of proton position within the WOC, thus uncovering orientation information relative to the QD surface. The data suggest that the WOC interacts with the QD surface via the Ru terpyridine ligand, an unexpected orientation that has important implications for interfacial charge transfer and subsequent catalysis. This binding motif enables strong enough donor-acceptor electronic coupling for ultrafast photoinduced hole transfer while maintaining electronically distinct functional subunits.

Theoretical investigation of non-Förster exciton transfer mechanisms in perylene diimide donor, phenylene bridge, and terrylene diimide acceptor systems.

The rates of exciton transfer within dyads of perylene diimide and terrylene diimide connected by oligophenylene bridge units have been shown to deviate significantly from those of Förster's resonance energy transfer theory, according to single molecule spectroscopy experiments. The present work provides a detailed computational and theoretical study investigating the source of such a discrepancy. Electronic spectroscopy data are calculated by time-dependent density functional theory and then compared with experimental results. Electronic couplings between the exciton donor and the acceptor are estimated based on both the transition density cube method and transition dipole approximation. These results confirm that the delocalization of the exciton to the bridge parts contributes to significant enhancement of donor-acceptor electronic coupling. Mechanistic details of exciton transfer are examined by estimating the contributions of the bridge electronic states, vibrational modes of the dyads commonly coupled to both donor and acceptor, inelastic resonance energy transfer mechanism, and dark exciton states. These analyses suggest that the contribution of common vibrational modes serves as the main source of deviation from Förster's spectral overlap expression.

(Invited) Why Do Dipole Effects on Charge Transfer Matter?

Charge transfer (CT) and charge transport (CTr) are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible. In natural and manmade sensing systems, for example, well synchronized CT processes drive the conversion of mechanical, optical, chemical and thermal signals into electric outputs, which is essential for interfacing with processing modules. Conversely, electric dipoles are everywhere and therefore, understanding how they affect CT and CTr is of principal importance for attaining and controlling the functionality of a wide range of devices. Discussions of the idea about dipole effects on CT date back to the mid 20th century and reported experimental evidence from the 1990s and 2000s demonstrated their key importance. The dipole-generated localized electric fields induce Stark effects and modulate the electronic properties of the CT moieties in their vicinity. The notion for such effects focusses on dipole-induced changes in the reduction potentials of the donor and the acceptor, affecting the CT driving forces and thus, the Franck-Condon (FC) contributions to the CT kinetics. We recently demonstrated that to harness such effects, which are inherently enormous, (1) the dipoles should be placed as closed as possible to the electron donor and acceptor, and (2) the media polarity should be lowered. Polar media, indeed, stabilize charged states and in general enhances the rates of CT. Polar media, however, screen the field permeation and damps the dipole effect on the electron donor and acceptor. Using hydrocarbons as a medium, results in electron transfer rates along the dipole that are six times larger than the rates for the same system when in polar solvents, such as acetonitrile. The same localized field effects in non-polar medium completely shut down the electron transfer against the dipole. While in the vicinity of dipoles, the electric fields can reach GV/m, they fall off rapidly with distance. In our work, we incorporate the dipoles in the electron donors. This geometry provides the closest possible placement of the dipole to the CT moieties. Most importantly, it has also allowed us to discover for the first time dipole effects on donor-acceptor electronic coupling that dominate and oppose the established notion about effects on the FC contributions to the CT kinetics. These discoveries change the ways of thinking about dipole effects on CT and CTr, and set unprecedented paradigms for the development of electronic materials and devices.

(Invited) Organic Molecular Electrets: Multifaceted Complexity of Dipole Effects on Charge Transfer

Electric dipoles are everywhere. While they provide potentially important paradigms for controlling charge transfer (CT), dipole effects on CT still remain largely unexplored. CT is among the most fundamental processes responsible for sustaining life on earth and for ensuring the functionality of a range of energy and electronic devices that are an intricate part of everyday life. Therefore, we pioneer the concept of bioinspired molecular electrets that, in addition to possessing large macrodipoles, also can mediate long-range CT. (Electrets are systems with ordered electric dipoles, i.e., they are the electrostatic analogies of magnets.) Our electret designs are based on polypeptides composed of non-native aromatic beta-amino acids. We observe that even a single electret residue induces rectification of CT that surpasses what is reported for similar systems utilizing polypeptide helices composed of native amino acids. While dipoles generate enormous fields around them, the strength of these localized fields significantly decays with distance. Therefore, we incorporate the dipoles within the participants in the CT steps. It allows us to observe unprecedentedly large dipole effects on the CT kinetics.5 A decrease in solvent polarity increases the rates of CT. It appears counterintuitive, and it should be the other way around: polar media stabilizes the formed charged CT states and thus increases the rates of photoinduced CT in the Marcus normal region. Concurrently, polar media screens the dipole-generated fields and suppresses their effects on CT. For non-polar solvents, changing the dipole orientation can alter the reduction potentials of the photosensitizers with hundreds of millivolts. Therefore, a careful balance between the two opposing polarity effects allows us to demonstrate CT in lipophilic media, such as toluene. For non-polar solvent, we observe enhancement of electron-transfer (ET) rates along the dipole, while ET against the dipole is completely suppressed. By modifying the semi-classical CT theory, we quantify such experimentally observed dipole effects for the first time. In addition to affecting the reduction potentials and thus the Franck-Condon aspects of the CT kinetics, the molecular electrets reveal that dipoles can also drastically change the CT rates by altering the donor-acceptor electronic coupling. This discovery contradicts the accepted notion about dipole effects on CT, where the focus is on the Franck-Condon nuclear aspects of the kinetics and the electronic contributions are considered insignificant. Our findings not only lead to rethinking the fundamentals of dipole effects on CT, but also open new avenues for development of unprecedented paradigms for organic electronics and energy materials.

Effects of Donor–Acceptor Quantum Coherence and Non-Markovian Bath on the Distance Dependence of Resonance Energy Transfer

Accurate information on the distance dependence of resonance energy transfer (RET) is crucial for its utilization as a spectroscopic ruler of nanometer scale distances. In this regard, understanding the effects of donor–acceptor quantum coherence and non-Markovian bath, which become significant at short distances, has significant implications. The present work investigates this issue theoretically by comparing results from a theory of coherent RET (CRET) with a nonequilibrium version of Forster’s RET (FRET) theory, both accounting for non-Markovian bath effects. Even for a model where the donor–acceptor electronic coupling is of transition dipole interaction form, it is shown that the RET rate in general deviates from the inverse sixth power distance dependence as opposed to the prediction of the original FRET. On the one hand, it is shown that the donor–acceptor quantum coherence makes the distance dependence steeper than the sixth power, although detailed manner of enhancement is sensitive to specific v...

Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency

Semiconductor quantum dots (QDs) functionalized by metal–organic dyes show great promise in photocatalytic and photovoltaic applications. However, the charge transfer direction and rates—key processes governing the efficiency of energy conversion—are strongly affected by the QD–dye interactions, insights on which are challenging to obtain experimentally. We use density functional theory (DFT) and constrained DFT calculations to investigate a degree of sensitivity of the electronic level alignment and related QD–dye electronic couplings to binding conformations of N719 dye at the surface of the 1.5 nm CdSe QD. Our calculations reveal a lack of direct correlations between the strength of the QD–dye interaction in terms of their binding conformations and the donor–acceptor electronic couplings. While the QD–dye binding conformations are the most stable when the N719 dye is attached to the QD via two carboxylate groups, the strongest electronic coupling between the QD as an electron donor and the dye as an el...

Fluctuating hydrogen-bond networks govern anomalous electron transfer kinetics in a blue copper protein

We combine experimental and computational methods to address the anomalous kinetics of long-range electron transfer (ET) in mutants of Pseudomonas aeruginosa azurin. ET rates and driving forces for wild type (WT) and three N47X mutants (X = L, S, and D) of Ru(2,2'-bipyridine)2 (imidazole)(His83) azurin are reported. An enhanced ET rate for the N47L mutant suggests either an increase of the donor-acceptor (DA) electronic coupling or a decrease in the reorganization energy for the reaction. The underlying atomistic features are investigated using a recently developed nonadiabatic molecular dynamics method to simulate ET in each of the azurin mutants, revealing unexpected aspects of DA electronic coupling. In particular, WT azurin and all studied mutants exhibit more DA compression during ET (>2 Å) than previously recognized. Moreover, it is found that DA compression involves an extended network of hydrogen bonds, the fluctuations of which gate the ET reaction, such that DA compression is facilitated by transiently rupturing hydrogen bonds. It is found that the N47L mutant intrinsically disrupts this hydrogen-bond network, enabling particularly facile DA compression. This work, which reveals the surprisingly fluctional nature of ET in azurin, suggests that hydrogen-bond networks can modulate the efficiency of long-range biological ET.

Remarkable Dependence of the Final Charge Separation Efficiency on the Donor-Acceptor Interaction in Photoinduced Electron Transfer.

The unprecedented dependence of final charge separation efficiency as a function of donor-acceptor interaction in covalently-linked molecules with a rectilinear rigid oligo-p-xylene bridge has been observed. Optimization of the donor-acceptor electronic coupling remarkably inhibits the undesirable rapid decay of the singlet charge-separated state to the ground state, yielding the final long-lived, triplet charge-separated state with circa 100% efficiency. This finding is extremely useful for the rational design of artificial photosynthesis and organic photovoltaic cells toward efficient solar energy conversion.

Electron-Transfer Dynamics for a Donor-Bridge-Acceptor Complex in Ionic Liquids.

Intramolecular photoinduced electron transfer from an N,N-dimethyl-p-phenylenediamine donor bridged by a diproline spacer to a coumarin 343 acceptor was studied using time-resolved fluorescence measurements in three ionic liquids and in acetonitrile. The three ionic liquids have the bis[(trifluoromethyl)sulfonyl]amide anion paired with the tributylmethylammonium, 1-butyl-1-methylpyrrolidinium, and 1-decyl-1-methylpyrrolidinium cations. The dynamics in the two-proline donor-bridge-acceptor complex are compared to those observed for the same donor and acceptor connected by a single proline bridge, studied previously by Lee et al. (J. Phys. Chem. C 2012, 116, 5197). The increased conformational freedom afforded by the second bridging proline resulted in multiple energetically accessible conformations. The multiple conformations have significant variations in donor-acceptor electronic coupling, leading to dynamics that include both adiabatic and nonadiabatic contributions. In common with the single-proline bridged complex, the intramolecular electron transfer in the two-proline system was found to be in the Marcus inverted regime.

Photoinduced charge and energy transfer in molecular wires.

Exploring charge and energy transport in donor-bridge-acceptor systems is an important research field which is essential for the fundamental knowledge necessary to develop future applications. These studies help creating valuable knowledge to respond to today's challenges to develop functionalized molecular systems for artificial photosynthesis, photovoltaics or molecular scale electronics. This tutorial review focuses on photo-induced charge/energy transfer in covalently linked donor-bridge-acceptor (D-B-A) systems. Of utmost importance in such systems is to understand how to control signal transmission, i.e. how fast electrons or excitation energy could be transferred between the donor and acceptor and the role played by the bridge (the "molecular wire"). After a brief description of the electron and energy transfer theory, we aim to give a simple yet accurate picture of the complex role played by the bridge to sustain donor-acceptor electronic communication. Special emphasis is put on understanding bridge energetics and conformational dynamics effects on the distance dependence of the donor-acceptor electronic coupling and transfer rates. Several examples of donor-bridge-acceptor systems from the literature are described as a support to the discussion. Finally, porphyrin-based molecular wires are introduced, and the relationship between their electronic structure and photophysical properties is outlined. In strongly conjugated porphyrin systems, limitations of the existing electron transfer theory to interpret the distance dependence of the transfer rates are also discussed.

Photoinduced electron transfer within a zinc porphyrin-cyclobis(paraquat-p-phenylene) donor-acceptor dyad.

Understanding the mechanism of efficient photoinduced electron-transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor-acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT(4+) ) acceptor. The X-ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad. Photoinduced electron transfer within the dyad in MeCN was investigated by femtosecond and nanosecond transient absorption spectroscopy, as well as by transient EPR spectroscopy. Photoexcitation of the dyad produced a weakly coupled ZnP(+.) -CBPQT(3+.) spin-correlated radical-ion pair having a τ=146 ns lifetime and a spin-spin exchange interaction of only 0.23 mT. The long radical-ion-pair lifetime results from weak donor-acceptor electronic coupling as a consequence of having nine bonds between the donor and the acceptor, and the reduction in reorganization energy for electron transfer caused by charge dispersal over both paraquat units within CBPQT(3+.) .

Molecular Structure – Optical Property Relationships for a Series of Non-Centrosymmetric Two-photon Absorbing Push-Pull Triarylamine Molecules

This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.

Estimation of Electronic Coupling for Charge Transfer in Bioinorganic Molecules

Fundamental biochemical processes such as respiration and photosynthesis rely on electron transfer (ET) between cofactors containing transition metal atoms. Donor-acceptor electronic coupling VDA is a key parameter that controls the ET rate. Accurate estimation of this parameter is challenging. In the paper, we consider different methods used to calculate the coupling matrix element for ET between transition-metal centers in proteins and discuss their advantages and limitations. Keywords: Electron coupling, electronic transfer, structural dynamics, transition metal cofactors, quantum-mechanical calculations.

Dynamical Simulation of Electron Transfer Processes in Alkanethiolate Self-Assembled Monolayers at the Au(111) Surface

Electron transfer is investigated in a series of self-assembled monolayers (SAMs) consisting of nitrile-substituted short chain alkanethiolate molecules adsorbed at the Au(111) surface. Using first-principles methods and a model electron transfer Hamiltonian, we analyze the main factors controlling, at the molecular level, the electron injection times from donor states localized at the tail group of the SAM into the Au(111) substrate. We show that the donor–acceptor electronic couplings depend significantly on the orbital symmetry of the donor state and the length of the aliphatic spacer chain of the SAM. The dependence on the donor state symmetry and on the molecular structure of the linker can be used to control the electron injection times even in situations where the energy separation between the donor states is smaller than their width.