Excitation Energy Transfer Coupling Research Articles

Effect of thienyl group on absorption spectra and excitation energy transfer of eco-friendly organic heterojunction from theoretical methods

Recently, more and more researchers have committed to designing higher-performance materials to improve the power conversion efficiencies (PCEs) of OSCs. However, the inherent laws of synthesising high-quality organic materials are still ambiguous. It is important to explore the structural effect on the photoelectric properties of the synthesised organic. In 2023, the Zou group designed two electron donors, 2DTBDT and 2DTBDT-2T, which are respectively without and with bithiophene in the molecular skeleton. The introduction of bithiophene into 2DTBDT-2T made the PCE of 2DTBDT-2T:Y6 heterojunction triple the 2DTBDT counterpart. Based on this, we selected eco-friendly electron acceptors, CBM, FBR, O-IDFBR and O-IDTBR, to compose the heterojunction with 2DTBDT and 2DTBDT-2T. The ionisation energies (IEs) and absorption spectra of all independent molecules and the excitation energy transfer (EET) couplings of heterojunction are accurately investigated from many-body Green’s function theory (MBGFT). We found that the electronic structure and excitation are sensitive to the resonance-antiresonance transitions and the electron–hole interactions. The thienyl group makes the molecule lower CT energy and the stronger EET coupling of heterojunctions, to promote the efficiency of relevant OSCs. The Dexter and overlap types couplings are very important to the study on the EET of organic heterojunctions.

Quantum mechanical analysis of excitation energy transfer couplings in photosystem II

We evaluated excitation energy transfer (EET) coupling (J) between all pairs of chlorophylls (Chls) and pheophytins (Pheos) in the protein environment of photosystem II based on the time-dependent density functional theory with a quantum mechanical/molecular mechanics approach. In the reaction center, the EET coupling between Chls PD1 and PD2 is weaker (|J(PD1/PD2)| = 79 cm−1), irrespective of a short edge-to-edge distance of 3.6 Å (Mg-to-Mg distance of 8.1 Å), than the couplings between PD1 and the accessory ChlD1 (|J(PD1/ChlD2)| = 104 cm−1) and between PD2 and ChlD2 (|J(PD2/ChlD1)| = 101 cm−1), suggesting that PD1 and PD2 are two monomeric Chls rather than a “special pair”. There exist strongly coupled Chl pairs (|J| > ∼100 cm−1) in the CP47 and CP43 core antennas, which may be candidates for the red-shifted Chls observed in spectroscopic studies. In CP47 and CP43, Chls ligated to CP47-His26 and CP43-His56, which are located in the middle layer of the thylakoid membrane, play a role in the “hub” that mediates the EET from the lumenal to stromal layers. In the stromal layer, Chls ligated to CP47-His466, CP43-His441, and CP43-His444 mediate the EET from CP47 to ChlD2/PheoD2 and from CP43 to ChlD1/PheoD1 in the reaction center. Thus, the excitation energy from both CP47 and CP43 can always be utilized for the charge-separation reaction in the reaction center.

Simulations of Frenkel to Wannier–Mott Exciton Transitions in a Nanohybrid System

Excitation energy transfer at a prototypical organic/inorganic interface is described theoretically. The nanohybrid system to be investigated is built up by a vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nanocrystal of 3903 atoms. To determine the time scale of excitation energy transfer all relevant electronic excitations of the organic and inorganic part are computed together with the related excitation energy transfer couplings. Values of the coupling lie in the millielectronvolt range or less. This motivates a Golden Rule description of the excitation energy transfer. Different Frenkel excitons are chosen as excitation energy donor levels. Due to the H-aggregate configuration of the organic part, the number of exciton wave function nodes increases with decreasing exciton energy. As a result, the couplings of the individual molecules to a certain ZnO electron–hole pair cancel each other more intensively and the overall transfer rate gets smaller. The highest exciton levels decay most rapidly and are characterized by lifetimes in the picosecond region. The lower part of the exciton band, however, has lifetimes in the nanosecond region. The Golden Rule description is finally compared to a direct solution of the time-dependent Schrödinger equation. The obtained transfer dynamics confirm those of the rate equation approach when the higher part of the Frenkel exciton band is considered. In the lower part, the reduced number of final electron–hole pair states in the inorganic part blocks the Frenkel exciton decay.

Plasmon assisted control of photo-induced excitation energy transfer in a molecular chain

The strong and ultrafast laser pulse excitation of a molecular chain in close vicinity to a spherical metal nano-particle (MNP) is studied theoretically. Due to local-field enhancement around the MNP, pronounced excited-state formation has to be expected for the part of the chain which is in proximity to the MNP. Here, the description of this phenomenon will be based on a uniform quantum theory of the MNP–molecule system. It accounts for local-field effects due to direct consideration of the strong excitation energy transfer coupling between the MNP and the various molecules. The molecule–MNP distances are chosen in such a way as to achieve a correct description of the MNP via dipole–plasmon excitations. Short plasmon life-times are incorporated in the framework of a density matrix approach. By extending earlier work the present description allows for multi-exciton formation and multiple dipole–plasmon excitation. The region of less intense and not-too-short optical excitation is identified as being best suited for excitation energy localization in the chain.

Distributed Multipolar Expansion Approach to Calculation of Excitation Energy Transfer Couplings

We propose a new approach for estimating the electrostatic part of the excitation energy transfer (EET) coupling between electronically excited chromophores based on the transition density-derived cumulative atomic multipole moments (TrCAMM). In this approach, the transition potential of a chromophore is expressed in terms of truncated distributed multipolar expansion and analytical formulas for the TrCAMMs are derived. The accuracy and computational feasibility of the proposed approach is tested against the exact Coulombic couplings, and various multipole expansion truncation schemes are analyzed. The results of preliminary calculations show that the TrCAMM approach is capable of reproducing the exact Coulombic EET couplings accurately and efficiently and is superior to other widely used schemes: the transition charges from electrostatic potential (TrESP) and the transition density cube (TDC) method.

Nanotubular J-aggregates and quantum dots coupled for efficient resonance excitation energy transfer.

Resonant coupling between distinct excitons in organic supramolecular assemblies and inorganic semiconductors is supposed to offer an approach to optoelectronic devices. Here, we report on colloidal nanohybrids consisting of self-assembled tubular J-aggregates decorated with semiconductor quantum dots (QDs) via electrostatic self-assembly. The role of QDs in the energy transfer process can be switched from a donor to an acceptor by tuning its size and thereby the excitonic transition energy while keeping the chemistry unaltered. QDs are located within a close distance (<4 nm) to the J-aggregate surface, without harming the tubular structures and optical properties of J-aggregates. The close proximity of J-aggregates and QDs allows the strong excitation energy transfer coupling, which is around 92% in the case of energy transfer from the QD donor to the J-aggregate acceptor and approximately 20% in the reverse case. This system provides a model of an organic-inorganic light-harvesting complex using methods of self-assembly in aqueous solution, and it highlights a route toward hierarchical synthesis of structurally well-defined supramolecular objects with advanced functionality.

Estimation of Electronic Coupling for Singlet Excitation Energy Transfer

Electronic coupling is a key parameter that controls the efficiency of excitation energy transfer (EET) and exciton delocalization. A new approach to estimate electronic coupling is introduced. Within a two-state model, the EET coupling V of two chromophores is expressed via the vertical excitation energies (Ei and Ej), transition dipole moments (Mi and Mj) of the system and transition moments (μA and μB) of the individual chromophores: V = (Ei – Ej){[(MiMj)(μA2 – μB2) – (μAμB)(Mi2 – Mj2)]/[(Mi2 – Mj2)2 + 4(MiMj)2]}. These quantities are directly available from quantum mechanical calculations. As the estimated coupling accounts for both short-range and long-range interactions, this approach allows for the treatment of systems with short intermolecular distances, in particular, π-stacked chromophores. For a system of two identical chromophores, the coupling is given by V = (Ei – Ej)[(EiFj – EjFi)/(EiFj + EjFi)][1/(2 cos θ)] where Fi and Fj are the corresponding oscillator strengths and cos θ is determined by the relative position of the chromophores in the dimer. Thus, the coupling can be derived from purely experimental data. The developed approach is used to calculate the EET coupling and exciton delocalization in two π-stacks of pyrimidine nucleobases 5′-TT-3′ and 5′-CT-3′ showing quite different EET properties.

Excitation energy transfer in GFP‐X‐CFP model peptides (X = amino acids): Direct Versus through‐bridge energy transfers

AbstractThe excitation energy transfer (EET) between a fluorescent resonance energy transfer (FRET) pair, cyan fluorescent protein chromophore (CFPc)/ green fluorescent protein chromophore (GFPc), is used for measuring molecular proximity in experimental studies. In this article, the role of the bridging protein media has been investigated using the quantum chemical calculations for the EET electronic coupling. We adopted a model peptide connecting GFPc and CFPc, a GFPc‐X‐CFPc model, and examined the X (amino acids) dependence. The major part of the EET coupling element arises from the direct interaction between GFPc and CFPc via the Förster mechanism. The magnitude of the through‐X interactions vary for the type of residues. The contribution of the X was, however, at most 10% of the total value. Together with the contributions from through‐peptide and charge‐transfer types, X gives minor modifications to the total coupling via the super‐exchange mechanism. © 2012 Wiley Periodicals, Inc.

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

The theoretical description of the initial steps in photosynthesis has gained increasing importance over the past few years. This is caused by more and more structural data becoming available for light-harvesting complexes and reaction centers which form the basis for atomistic calculations and by the progress made in the development of first-principles methods for excited electronic states of large molecules. In this Review, we discuss the advantages and pitfalls of theoretical methods applicable to photosynthetic pigments. Besides methodological aspects of excited-state electronic-structure methods, studies on chlorophyll-type and carotenoid-like molecules are discussed. We also address the concepts of exciton coupling and excitation-energy transfer (EET) and compare the different theoretical methods for the calculation of EET coupling constants. Applications to photosynthetic light-harvesting complexes and reaction centers based on such models are also analyzed.

Conformational dependence of the electronic coupling for singlet excitation energy transfer in DNA. An INDO/S study

Using the INDO/S method, we study the effects of structural fluctuations on the interaction of singlet excited states in homogeneous poly(dA)-poly(dT) and alternating poly(dAdT)(2) stacks. The coupling for excitation energy transfer (EET) between intra- and inter-strand nucleobases is derived with the fragment excitation difference scheme (Hsu, et al. J. Phys. Chem. C 2008, 112, 1204). In this approach, both Coulomb and short-range contributions to the EET coupling are properly accounted for. 15 000 conformations for each nucleobase dimer were considered. Conformational fluctuations of DNA are shown to result in a large variation of the transfer integral. The root mean square coupling values are used to characterize the interaction of pi-pi* states in DNA. The intra-strand and inter-strand couplings between adenines are found to be significantly smaller than those for thymines. Our findings suggest that (1) EET couplings in DNA are significantly more sensitive to conformational changes of the pi stack than is estimated within the dipole-dipole scheme; (2) singlet excitation energy transfer in poly(dA)-poly(dT) should dominantly occur through thymine bases; (3) pi-pi* excited states in homogeneous stacks are more delocalized than in alternating sequences; (4) structural fluctuations can strongly affect the exciton distribution.

The Electronic Couplings in Electron Transfer and Excitation Energy Transfer

The transport of charge via electrons and the transport of excitation energy via excitons are two processes of fundamental importance in diverse areas of research. Characterization of electron transfer (ET) and excitation energy transfer (EET) rates are essential for a full understanding of, for instance, biological systems (such as respiration and photosynthesis) and opto-electronic devices (which interconvert electric and light energy). In this Account, we examine one of the parameters, the electronic coupling factor, for which reliable values are critical in determining transfer rates. Although ET and EET are different processes, many strategies for calculating the couplings share common themes. We emphasize the similarities in basic assumptions between the computational methods for the ET and EET couplings, examine the differences, and summarize the properties, advantages, and limits of the different computational methods. The electronic coupling factor is an off-diagonal Hamiltonian matrix element between the initial and final diabatic states in the transport processes. ET coupling is essentially the interaction of the two molecular orbitals (MOs) where the electron occupancy is changed. Singlet excitation energy transfer (SEET), however, contains a Frster dipole-dipole coupling as its most important constituent. Triplet excitation energy transfer (TEET) involves an exchange of two electrons of different spin and energy; thus, it is like an overlap interaction of two pairs of MOs. Strategies for calculating ET and EET couplings can be classified as (1) energy-gap-based approaches, (2) direct calculation of the off-diagonal matrix elements, or (3) use of an additional operator to describe the extent of charge or excitation localization and to calculate the coupling value. Some of the difficulties in calculating the couplings were recently resolved. Methods were developed to remove the nondynamical correlation problem from the highly precise coupled cluster models for ET coupling. It is now possible to obtain reliable ET couplings from entry-level excited-state Hamiltonians. A scheme to calculate the EET coupling in a general class of systems, regardless of the contributing terms, was also developed. In the past, empirically derived parameters were heavily invoked in model description of charge and excitation energy drifts in a solid-state device. Recent advances, including the methods described in this Account, permit the first-principle quantum mechanical characterization of one class of the parameters in such descriptions, enhancing the predictive power and allowing a deeper understanding of the systems involved.

The mediated excitation energy transfer: Effects of bridge polarizability

The observation of bridge-mediated excitation energy transfer (EET) has raised questions on the physical origin of such an effect. In this work, we studied the effect of bridge fragments in the Coulomb coupling, the major contribution to the electronic coupling in an EET process. For a series of ortho-phenyleneethynylene oligomers spaced donor-acceptors, we found that a large influence of the bridge fragment in EET coupling is through changes in the Coulomb couplings. Both enhancement and screening effects of the bridge were observed as the EET rates were modified by a factor of 0.3-23 with an intervening bridge in our calculations. The dependency of EET couplings on the orientation of transition dipoles of the donor and acceptor from quantum mechanical computations is very similar to that of a simple classical dielectric model. Our work shows that the bridge fragments can modify the Coulomb coupling with their polarizability by providing an optical dielectric medium between the donor and acceptor. In particular, when the transition dipoles of the donor and acceptor were longitudinal to a polarizable bridge, the EET rates were enhanced by one order of magnitude, as compared to the values of through-space models. Our results offer important insights into the design of efficient energy transfer systems.

Photophysical Properties of Natural Light-Harvesting Complexes Studied by Subsystem Density Functional Theory

In this study, we investigate the excited states and absorption spectra of a natural light-harvesting system by means of subsystem density functional theory. In systems of this type, both specific interactions of the pigments with surrounding protein side chains as well as excitation energy transfer (EET) couplings resulting from the aggregation behavior of the chromophores modify the photophysical properties of the individual pigment molecules. It is shown that the recently proposed approximate scheme (J. Chem. Phys. 2007, 126, 134116) for coupled excitations within a subsystem approach to time-dependent DFT is capable of describing both effects in a consistent manner, and is efficient enough to study even the large assemblies of chromophores occurring in the light-harvesting complex 2 (LH2) of the purple bacterium Rhodopseudomonas acidophila. A way to extract phenomenological coupling constants as used in model calculations on EET rates is outlined. The resulting EET coupling constants and spectral properties are in reasonable agreement with the available reference data. Possible problems related to the effective exchange-correlation kernel are discussed.

Characterization of the Short-Range Couplings in Excitation Energy Transfer

The electronic coupling in excitation energy transfer (EET) is composed of a Coulomb coupling (which can be approximated as the Forster's dipole coupling), a Dexter's exchange coupling, and a term arising from the orbital overlap of the donor and acceptor fragments. We have developed a new scheme to account for the EET coupling by generalizing the fragment charge difference scheme [Voityuk, A. A.; Rosch, N. J. Chem. Phys. 2002, 117, 5607] for electron-transfer coupling. As a result, the EET coupling can be calculated in a general class of systems irrespective of the molecular symmetry. The short-range coupling, defined as the contribution from Dexter's exchange coupling and the overlap effect, was obtained as the difference of the EET coupling from this new scheme and a precise account of the Coulomb coupling. For a pair of stacked naphthalenes, the short-range coupling is very similar to the triplet−triplet energy-transfer coupling in both magnitudes and the distance dependences. We have observed cases i...

Intermolecular Coulomb Couplings from Ab Initio Electrostatic Potentials: Application to Optical Transitions of Strongly Coupled Pigments in Photosynthetic Antennae and Reaction Centers

An accurate and numerically efficient method for the calculation of intermolecular Coulomb couplings between charge densities of electronic states and between transition densities of electronic excitations is presented. The coupling of transition densities yields the Förster type excitation energy transfer coupling, and from the charge density coupling, a shift in molecular excitation energies results. Starting from an ab initio calculation of the charge and transition densities, atomic partial charges are determined such as to fit the resulting electrostatic potentials of the different states and the transition. The different intermolecular couplings are then obtained from the Coulomb couplings between the respective atomic partial charges. The excitation energy transfer couplings obtained in the present TrEsp (transition charge from electrostatic potential) method are compared with couplings obtained from the simple point-dipole and extended dipole approximations and with those from the ab initio transition density cube method of Krüger, Scholes, and Fleming. The present method is of the same accuracy as the latter but computationally more efficient. The method is applied to study strongly coupled pigments in the light-harvesting complexes of green sulfur bacteria (FMO), purple bacteria (LH2), and higher plants (LHC-II) and the "special pairs" of bacterial reaction centers and reaction centers of photosystems I and II. For the pigment dimers in the antennae, it is found that the mutual orientation of the pigments is optimized for maximum excitonic coupling. A driving force for this orientation is the Coulomb coupling between ground-state charge densities. In the case of excitonic couplings in the "special pairs", a breakdown of the point-dipole approximation is found for all three reaction centers, but the extended dipole approximation works surprisingly well, if the extent of the transition dipole is chosen larger than assumed previously. For the "special pairs", a large shift in local transition energies is found due to charge density coupling.

Toward a Molecular Scale Interpretation of Excitation Energy Transfer in Solvated Bichromophoric Systems

This paper presents a quantum-mechanical study of the intramolecular excitation energy transfer (EET) coupling in naphthalene-bridge-naphthalene systems in gas phase and in solution. ZINDO and TDDFT response schemes are compared using both an exact and an approximate solution. The approximate solution based on a perturbative approach uses the single chromophore properties to reconstruct the real system coupling thus neglecting possible through-bond effects which conversely are accounted for in the exact solution. The comparison of the results of the two approaches with the experiments allows a detailed analysis of the relative importance of through-bond and through-space effects as well as a more complete understanding of the modifications in the EET coupling with the size of the system, the chromophore-chromophore distance, and solvation.

Theoretical Study of Photosynthetic Light‐Harvesting Processes: Application of Time‐Dependent Density Functional Theory

AbstractWe review recent progress in the area of theoretical investigations of light‐harvesting processes. The application of time‐dependent density functional theory (TDDFT) in the calculation of excitation energy transfer coupling factor will be emphasized. The effect of a dielectric environment can be obtained through a response theory based on TDDFT formalism. As an example of applicability of TDDFT, the role of a transition forbidden S1 state of carotenoid in a light‐harvesting complex LH2 is discussed in detail. The applicability of TDDFT to states with charge‐transfer character is also discussed with a few examples in the literature. Finally we briefly review works on the applications of TDDFT for excitations in chlorophylls and bacteriochlorophylls.

The Role of the S1 State of Carotenoids in Photosynthetic Energy Transfer: The Light-Harvesting Complex II of Purple Bacteria

Using time-dependent density functional theory (TDDFT), we obtained the excitation energy transfer coupling (Coulombic coupling) between the S1 state of rhodopin glucoside (RG) and the Qy state of bacteriochlorophylls (BChl) in the light-harvesting complex II (LH2) of purple photosynthetic bacterium Rhodopseudomonas (Rps.) acidophila. Our results suggest that the small mixing of S2 character arising from symmetry-breaking of the carotenoid plays an important role in the Coulombic coupling. As a result the carotenoid (car) S1 couplings to chlorophylls are similar to a set of scaled down Car(S2)−BChl(Qy) couplings. We also report results for 6,10,15,19-tetramethyl-2-cis-4,6,8,10,12,14,16,18,20-all trans-22-cis-tetracosaundecaene, the polyene backbone of RG with six methyl groups attached, in two different structures: an optimized planar structure and the crystal structure of RG with hydrogen atoms replacing the two end groups, which is distorted from its planar structure. The mixing of S2 configuration is ...