Charge Transfer Character Research Articles

Theoretical Insights into Ultrafast Separation of Photogenerated Charges in a Push-Pull Polarized Molecular Triad.

Herein, we propose a purely-organic donor-acceptor (D-A) molecular triad, with a light-absorbing polarized molecular wire (PMW) used as a central linkage, as a proof of concept for the possible future applications of the D-PMW-A arrangement in molecular photovoltaics. This work builds upon our earlier study on the PMW unit itself, which proved to be highly promising for the ultrafast photogeneration of free charge carriers. Quantum-chemical calculations performed for the D-PMW-A triad at a semi-empirical level of theory reveal a large electric dipole moment of the system, and show strong charge-transfer (CT) character of its lowest-energy excited electronic states, including the S1, which favours efficient dissociation of an exciton initially formed upon the absorption of light. The confirmation for this effect was found with nonadiabatic molecular dynamics simulations, revealing an ultrafast relaxation from higher, bright excited states to S1, completed on a subpicosecond timescale. The architecture of the proposed molecular triad enables its electronic coupling to the surrounding environment through chemical bonds, or noncovalent stacking interactions, which might open way for synthesis of a new class of D-PMW-A efficient molecular organic photovoltaic materials.

Theoretical exploration on the combined effect of isoelectron B-N bonds and B-N resonance skeleton on the thermally activated delayed fluorescence property

The multiple resonance thermally activated delayed fluorescence (MR-TADF) materials has become a hot spot in recent years depending on their potential in achieving an internal quantum efficiency of 100%. The combined effect of para B-N resonance skeleton and isoelectron B-N bonds on the TADF property was investigated in details by employing density functional theory/time-dependent density functional theory (DFT/TDDFT) in this work based on four polycyclic aromatic molecules. The results show that isoelectron B-N bonds are favourable to the enhancement of spin orbital coupling (SOC) constants between the first singlet state (S1) and the first triplet state (T1), while para B-N resonance skeleton is more conducive to reducing the energy gap between S1 and T1 (ΔEST) through realizing short-range charge transfer character and keeping electron and hole localized at different atoms. Accordingly the combination of isoelectron B-N bonds and para B-N resonance skeleton in m[B-N]N1 and m[B-N]N2 could realize faster intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes through larger SOC and lower ΔEST compared with BCz-BN and NBN-2 which only contain para B-N resonance skeleton and isoelectron B-N bonds, respectively. At the same time, the para B-N resonance skeleton help to stabilize the structure of polycyclic aromatic hydrocarbons and localize the vibration in low frequency region during emission from S1 to ground state. Thus larger nonradiative rates induced by the rotation of carbazole and tert-butyl carbazole in low frequency region in m[B-N]N1 and m[B-N]N2 could be easily reduced by replacing them by small functional group such as electron-donating (-CH3) and electron-withdrawing (-CN) groups. Therefore, we can obtain high TADF efficiency through combining para B-N resonance skeleton and isoelectron B-N bonds and suppressing low-frequency vibrations as in our designed molecules m[B-N]CH3 and m[B-N]CN.

Ultrafast Charge Separation Driven by Solvation-Coupled Intramolecular Torsion.

Photoinduced intramolecular charge separation in a pyrene- and triarylamine-based donor-acceptor dyad was studied by polarization-dependent femtosecond time-resolved transient absorption (TA) spectroscopy in polar solvents. Photoexcitation forms an excited state with charge transfer (CT) character due to the intrinsic electronic coupling between the triarylamine and pyrene groups, resulting in ultrafast charge separation (CS) in polar solvents. TA measurements reveal a correlation between the rate of CS and solvation dynamics, which implies that solvation is involved in the CS reaction. In addition, polarization-dependent TA spectroscopy was devoted to tracking the ultrafast anisotropy evolution of the cationic absorption band, which is attributed to intramolecular torsional motion and is proposed to be coupled to diffusive orientational solvent modes. The results therefore reveal that the evolution of the CT state in the condensed phase is driven by solvation-coupled excited-state structural relaxation. In other words, intramolecular torsional motion is directly confirmed to be involved in the reaction coordinate of the CS reaction in a strongly coupled donor-acceptor dyad.

Metal-Ligand Covalency in the Valence Excited States of Metal Dithiolenes Revealed by S 1s3p Resonant Inelastic X-ray Scattering.

Metallo dithiolene complexes with biological and catalytic relevance are well-known for having strong metal-ligand covalency, which dictates their valence electronic structures. We present the resonant sulfur Kβ (1s3p) X-ray emission spectroscopy (XES) for a series of Ni and Cu bis(dithiolene) complexes to reveal the ligand sulfur contributions to both the occupied and unoccupied valence orbitals. While S K-edge X-ray absorption spectroscopy played a critical role in identifying the covalency of the unoccupied orbitals of metal dithiolenes, the present focus on XES explores the occupied density of states. For a series of [Cu(mnt)2]n- and [Ni(mnt)2]n- anions and dianions, a comparison of the nonresonant and resonant S Kβ XES spectra highlights the dramatic improvement in spectral resolution and corresponding ability to differentiate subtle changes in occupied electronic structure across the series. Furthermore, the use of resonant inelastic X-ray scattering (RIXS) probes the valence excited states and the core-valence couplings of the complexes. By employing a theoretical approach based on time-dependent density functional theory to interpret the RIXS spectra, we reveal how metal-ligand covalency influences the excited state energies and covalencies. We identify the low energy excited states as having the same symmetry as the nominal "ligand field" or "d-d" states that typically dominate the photophysics of 3d metal complexes but with significant metal-ligand charge transfer character dictated by their covalency. These results suggest that strong metal-ligand covalency can be used to influence the charge-transfer photochemistry of first row transition metal complexes.

Estimation of Electric Dipole Moment by Solvatochromism, Computational Method, and Study of the Effect of Solvents by Preferential Solvation of 6 - Methoxy-4-(4-Nitro-Phenoxy Methyl)-Chromen-2-One (6mnpm).

At room temperature, the absorption and fluorescence properties of coumarin 6-Methoxy-4-(4-nitro-phenoxy methyl)-chromen-2-one (6MNPM) are investigated in pure organic solvents and a combination of acetonitrile (ACN) and tetrahydrofuran (THF). The pure solvents' influence on spectral characteristics is examined by applying theories such as Kamlet and Catalan's multiple linear regression techniques, Reichardt's microscopic solvent polarity parameter, and the Lippert-Mataga polarity function. The significant role of solute-solvent interactions in pure solvents, particularly dielectric interaction and hydrogen bonding. However, hydrogen bonding interactions dominate the contribution of dielectric interactions. The electric dipole moments of both the ground as well as excited states had been calculated using the Solvatochromic method. The value of the excited state electric dipole moment and the redshifts of the emission spectra show that the emitting singlet state has an intramolecular charge transfer (ICT) character. From Catalan's linear regression, we found that di-polarity has a much smaller influence than polarizability. By solvation study, we conclude that Tetrahydrofuran solvent is preferred over Acetonitrile.

Electron Spin Polarization in the Triplet State of Methoxy-substituted Phosphorus(V) Tetraphenyl Porphyrins

Photoexcited triplet states of porphyrins are of great relevance in various applications due to their high yield, long lifetime, and strong electron spin polarization. This study delves into properties and spin dynamics of the triplet state of a series of hypervalent phosphorus(V) porphyrins. Transient Electron Paramagnetic Resonance (TrEPR) measurements, supported by quantum chemical calculations as well as by optical absorption/luminescence experiments, reveal that, unlike singlet states, the lowest triplet state does not exhibit charge-transfer (CT) character upon photoexcitation. However, the presence of excited CT singlet states alters the intersystem crossing in phosphorus(V) porphyrins, leading to a sign change in the initial multiplet polarization of the photoexcited triplet state. TrEPR results further demonstrate that significant net polarization develops in the triplet states of the phosphorus(V) porphyrins due to the dynamic Jahn-Teller effect. Yet, this effect remains largely unaffected by differences in their molecular structures.

Insulating behaviour in room temperature rhombohedral LiNiO2 cathodes is driven by dynamic correlation

Abstract Inspired by the experimental finding of a paramagnetic insulating state in rhombohedral LiNiO2, a lithium-ion battery cathode of great interest, we calculate the electronic, magnetic, and optical properties of LiNiO2 employing a range of single-particle and many-body methods. Within density-functional theory (DFT) using the generalized-gradient approximation (GGA), meta-GGA, and hybrid functionals, we obtain a ferromagnetic half-metallic ground state for rhombohedral LiNiO2, as has been seen previously. Self-consistent GW calculations including self-interaction corrections beyond DFT for various flavours show an electronic band gap albeit with a small quasiparticle peak at the Fermi energy. Moving beyond this, room temperature state-of-the-art dynamical mean-field theory (DMFT) calculations on rhombohedral LiNiO2 show for the first time a gap of combined Mott and charge-transfer character. The paramagnetic insulating state has a band gap of ~0.6 eV, in excellent agreement with experiments and is in sharp contrast to DFT calculations that require the presence of an extra structural symmetry breaking in the form of Jahn–Teller distortions to open a gap. We observe Ni to be in a +2 state in a d 8L configuration, with a charge-transfer ligand hole in O p, and identify the ligand hole state from the DMFT DOS. We further show that whereas DFT shows the presence of an unphysical metallic Drude peak in the optical absorption spectra, DMFT calculations capture the correct form of the optical absorption spectra, and have an excellent match with the calculated band gap as well. Our results clarify that at room temperature, it is the charge transfer gap with a Mott character that causes rhombohedral LiNiO2’s insulating nature; a structural distortion is not required.

Monosilicon Derivatives of Phenanthrene and Pyrene as Potential Singlet Fission Materials for High-Performance Solar Cells.

Singlet fission (SF) is a process in which the energy of a singlet-excited molecule is divided into two triplet excitations. This is a special case of an internal conversion that is spin-allowed and extremely fast. Ideally, this process utilizes one photon to produce two electron-hole pairs. In tandem with a layer of singlet fission material, conventional solar cells can achieve improved efficiency by utilizing higher-energy photons. This density functional theory study provides information about additional efficient SF chromophores that were theoretically modeled by functionalizing phenanthrene and pyrene via site-specific monosilicon substitutions. The SF capabilities of the derivatives were evaluated by calculating the SF thermodynamic driving force (ΔESF) and the excited state's molecular planarity. The most promising monosilicon derivatives with SF capabilities are 3-silaphenanthrene and 1-silapyrene for each family, respectively. All phenanthrene and pyrene monosilicon derivatives are strong closed-shell species, because their multiple diradical characteristics are close to zero. Based on these results, 3-silaphenanthrene and 1-silapyrene were selected for electron excitation analysis, which further demonstrated that the monosilicon functionalization of phenanthrene and pyrene led to a transfer from local excitation characters to hybridized local and charge-transfer characters of the excited states, resulting in a significant change from endoergic to exoergic in the SF chromophores.

Intramolecular Charge Transfer and Spin-Orbit Coupled Intersystem Crossing in Hypervalent Phosphorus(V) and Antimony(V) Porphyrin Black Dyes.

Porphyrin dyes with strong push-pull type intramolecular charge transfer (ICT) character and broad absorption across the visible spectrum are reported. This combination of properties has been achieved by functionalizing the periphery of hypervalent and highly electron-deficient phosphorus(V) and antimony(V) centered porphyrins with electron-rich triphenylamine (TPA) groups. As a result of the large difference in electronegativity between the porphyrin ring and the peripheral groups, their absorption profiles show several strong charge transfer transitions, which in addition to the porphyrin-centered π → π* transitions, make them panchromatic black dyes with high absorption coefficients between 200 and 800 nm. Time-resolved optical and electron paramagnetic resonance (EPR) studies show that the lowest triplet state also has ICT character and is populated by spin-orbit coupled intersystem crossing.

Ring Rearrangement Reactions of 4-Alkenylisocoumarins and Photophysical Evaluation of Multi-Substituted Anthracene Products.

Herein we report that readily available 4-alkenylisocoumarins can be regarded as potent dienolate equivalents. For example, lactol silyl ethers derived from 4-alkenylisocoumarins were selectively converted to the corresponding benzo-homophthalates through a fluoride-induced ring opening step that was followed by a ring closure through a vinylogous intramolecular aldol condensation. Likewise, nucleophilic activation of 4-alkenylisocoumarins directly yields diversely poly-substituted naphthalenes and anthracenes without formation of any regioisomer. Photophysical evaluation of a set of thus obtained 1,3-di- and 1,3,4-trisubstituted anthracenes reveals their distinct intramolecular charge transfer (ICT) character during light absorption in polar solutions and excimer emission from the solid state when a face-to-face π-stacked molecular assembly is present in the crystal packing.

Effect of Pyrrolidinedithiocarbamate Ligand on the Luminescence Properties of Heteroligand Samarium and Europium Complexes: Experimental and Theoretical Study.

The photophysical properties of two isostructural heteroligand lanthanide complexes of general formula Ln(pdtc)3(phen) (pdtc = pyrrolidinedithiocarbamate anion, phen = 1,10-phenanthroline), Ln = Sm3+ (1), Eu3+ (2)) were studied in solid state and dichloromethane (DCM) solution. The two lanthanide complexes were investigated by experimental techniques for structural (single-crystal X-ray diffraction analysis of 1, powder XRD, TG-DTA) and spectroscopic [electron paramagnetic resonance (EPR), infrared (IR), ultraviolet-visible (UV-vis), photoluminescence (PL)] characterization. DFT/TDDFT/ωB97xD and multireference SA-CASSCF/NEVPT2 calculations with perturbative spin-orbit coupling corrections were applied to construct the Jablonski energy diagrams and to discuss the excited state energy transfer mechanism with competing excited state processes and possible sensitized mechanism of metal-centered emission. The first excited state (S1) involved in the excited state energy transfer L(antenna)-to-Ln was predicted to have interligand (pdtc-to-phen) charge transfer character in contrast to the previously predicted ligand-to-metal charge transfer character. The theoretical consideration showed similar relaxation paths and luminescence quenching channels and appropriate Donor*(phen)-Acceptor*(Ln3+) energy gap for 1 and 2. The experimental measurements in the solid state, however, showed efficient luminescence and good ability to convert UV to visible light only for the Sm(pdtc)3(phen) complex. The minor emission of 2 was explained by partial reduction of Eu3+, confirmed by EPR and calculated electron density distribution data.

Picosecond Metal-to-Ligand Charge-Transfer Deactivation in Co(ppy)3 via Jahn-Teller Distortion.

The excited-state dynamics of fac-Co(ppy)3, where ppy = 2-[2-(pyridyl)phenyl], are measured with femtosecond UV-vis transient absorption spectroscopy. The initial state is confirmed with spectroelectrochemistry to have significant metal-to-ligand charge transfer (MLCT) character, unlike other Co complexes that generally have ligand-to-metal charge transfer or ligand-field transitions in this energy range. Ground-state recovery occurs in 8.65 ps in dichloromethane. Density functional theory calculations show that the MLCT state undergoes Jahn-Teller distortion and converts to a five-coordinate triplet metal-centered state in which one Co-N bond is broken. The results highlight a potential pitfall of heteroleptic bidentate ligands when designing strong-field ligands for transition-metal chromophores.

Engineering Single Component Luminogens to Multicomponent Charge‐transfer Co‐crystal Substrate as New Frontiers for Sensitive SERS Detection

AbstractOrganic charge‐transfer (CT) co‐crystals have demonstrated remarkable physical properties and have found applications in numerous fields. Yet their utility as a Surface‐Enhanced Raman Spectroscopy (SERS) substrate, a powerful and versatile analytical tool, has never been explored. Herein, three twisted molecular donors are synthesized, that exhibit well‐controlled switchable optical properties including aggregation‐induced emission (AIE), mechanochromic luminescence (MCL), and color‐specific polymorphism. Rapid production of charge‐transfer co‐crystals is also established with a π‐acceptor TCNQ and utilized conceptually as a SERS substrate for methylene blue (MB) detection, exhibiting a very high enhancement factor of 109 and limit of detection of 10−13 m, respectively, due to the presence of low‐lying excited state, exhibit an 80% CT character, originating from the HOMO of the co‐crystal and interacting with the LUMO of the MB molecule. This approach using CT co‐crystals as a SERS substrate presents newer frontiers that require minuscule levels of rapid detection and impact allied areas, helping us understand and optimize the fascinating properties of such multicomponent materials for newer technologies.

Synthesis, optical properties, DNA, β-cyclodextrin interactions, and antioxidant evaluation of novel isoxazolidine derivative (ISoXD2): A multispectral and computational analysis

ISoXD2 are well explored among versatile and outstanding class of pharmacophores for the preparation and discovery of drugs. Herein, the electronic absorption and emission spectra of ISoXD2 were investigated in three different solvents. The observed transition was attributed to π-π* with charge transfer character. Changes in the excited state and shift of the absorption and emission peaks to longer wavelengths are observed as a result of increasing solvent polarity, due to the interactions between the ISoXD2 molecule and the solvent molecules surrounding it. Changing the solvent confirms its solvatochromic effect. UV–vis and fluorescence analysis revealed that ISoXD2 binds to deoxyribonucleic acid (DNA) by intercalation mode, with a stoichiometric ratio of 1:1.5. Moreover, the fluorescence intensity of DNA bound to ethidium bromide (EB) in the presence of ISoXD2 was investigated. From this analysis, the Stern-Volmer quenching constant (Ksv), quenching rate constant (kq), binding constant (Kb) and binding sites number (n) were found to be 5.654 × 103 M−1, 2.827 × 1011 M−1 s−1, 3.81 × 104 M−1 and 1.225, respectively. The interaction between ISoXD2 and β-CD was investigated through absorption spectra analysis. Kb for this interaction was determined to be 4.9 × 104 M−1. The free radical-scavenging ability of the prepared ISoXD2, examined by DPPH and ABTS assays have shown a good antioxidant activity. Furthermore, modeling study was conducted to explore their plausible binding mechanism with ISoXD2 and to support the experimental results.

Using Iron L-Edge and Nitrogen K-Edge X-ray Absorption Spectroscopy to Improve the Understanding of the Electronic Structure of Iron Carbene Complexes.

Iron-centered N-heterocyclic carbene compounds have attracted much attention in recent years due to their long-lived excited states with charge transfer (CT) character. Understanding the orbital interactions between the metal and ligand orbitals is of great importance for the rational tuning of the transition metal compound properties, e.g., for future photovoltaic and photocatalytic applications. Here, we investigate a series of iron-centered N-heterocyclic carbene complexes with +2, + 3, and +4 oxidation states of the central iron ion using iron L-edge and nitrogen K-edge X-ray absorption spectroscopy (XAS). The experimental Fe L-edge XAS data were simulated and interpreted through restricted-active space (RAS) and multiplet calculations. The experimental N K-edge XAS is simulated and compared with time-dependent density functional theory (TDDFT) calculations. Through the combination of the complementary Fe L-edge and N K-edge XAS, direct probing of the complex interplay of the metal and ligand character orbitals was possible. The σ-donating and π-accepting capabilities of different ligands are compared, evaluated, and discussed. The results show how X-ray spectroscopy, together with advanced modeling, can be a powerful tool for understanding the complex interplay of metal and ligand.

Theoretical Study of the Exciplex Formation in an m-MTDATA–BPHEN OLED

The formation of charge-transfer (CT) exciplexes in m-MTDATA–BPhen (4,4',4''-tris[(3-methylphenyl)phenylamino]-triphenylamine)–bathophenanthroline) donor-acceptor pair is studied using density functional theory (DFT) and multireference quantum chemistry. It is shown that the lowest excited singlet S1 and one of the low-lying triplets have the CT character, while the lowest triplet is localized on the BPhen molecule, which agrees with the experimental fluorescence and phosphorescence data. A multireference treatment agrees with the DFT calculations and shows that the CT character of the exciplex states in such donor-acceptor systems is not an artifact of the functional.

Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes.

To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

Stimuli-Responsive Oligourea Molecular Films.

We have designed and synthesized a helical cysteamine-terminated oligourea foldamer composed of ten urea residues featuring side carboxyl and amine groups. The carboxyl group is located in proximity to the C-terminus of the oligourea and hence at the negative pole of the helix dipole. The amine group is located close to the N-terminus and hence at the positive pole of the helix dipole. Beyond the already remarkable dipole moment inherent in oligourea 2.5 helices, the incorporation of additional charges originating from the carboxylic and amine groups is supposed to impact the overall charge distribution along the molecule. These molecules were self-assembled into monolayers on a gold substrate, allowing us to investigate the influence of an electric field on these polar helices. By applying surface-enhanced infrared reflection-absorption spectroscopy, we proved that molecules within the monolayers tend to reorient themselves more vertically when a negative bias is applied to the surface. It was also found that surface-confined oligourea molecules affected by the external electric field tend to rearrange the electron density at urea groups, leading to the stabilization of the resonance structure with charge transfer character. The presence of the external electric field also affected the nanomechanical properties of the oligourea films, suggesting that molecules also tend to reorient in the ambient environment without an electrolyte solution. Under the same conditions, the helical oligourea displayed a robust piezoresponse, particularly noteworthy given the slender thickness of the monolayer, which measured approximately 1.2 nm. This observation demonstrates that thin molecular films composed of oligoureas may exhibit stimulus-responsive properties. This, in turn, may be used in nanotechnology systems as actuators or functional films, enabling precise control of their thickness in the range of even fractions of nanometers.

Characterization of Excited-State Electronic Structure in Diblock π-Conjugated Oligomers with Adjustable Linker Electronic Coupling.

Diblock conjugated oligomers are π-conjugated molecules that contain two segments having distinct frontier orbital energies and HOMO-LUMO gap offsets. These oligomers are of fundamental interest to understand how the distinct π-conjugated segments interact and modify their excited state properties. The current paper reports a study of two series of diblock oligomers that contain oligothiophene (Tn) and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) segments that are coupled by either ethynyl (-C≡C-) or trans-(-C≡C-)2Pt(II)(PBu3)2 acetylide linkers. In these structures, the Tn segment is electron rich (donor), and the TBT is electron poor (acceptor). The diblock oligomers are characterized by steady-state and time-resolved spectroscopy, including UV-visible absorption, fluorescence, fluorescence lifetimes, and ultrafast transient absorption spectroscopy. Studies are compared in several solvents of different polarity and with different excitation wavelengths. The results reveal that the (-C≡C-) linked oligomers feature a delocalized excited state that takes on a charge transfer (CT) character in more polar media. In the (-C≡C-)2Pt(II)(PBu3)2-linked oligomers, there is weak coupling between the Tn and TBT segments. Consequently, short wavelength excitation selectively excites the Tn segment, which then undergoes ultrafast energy transfer (~1 ps) to afford a TBT-localized excited state.

Excited-State Charge Transfer Coupling from Quasiparticle Energy Density Functional Theory.

The recently developed Quasiparticle Energy (QE) scheme, based on a DFT calculation with one more (or less) electron, offers a good description of excitation energies, even with charge transfer characters. In this work, QE is further extended to calculate electron transfer (ET) couplings involving two excited states. We tested it with a donor-acceptor complex, consisting of a furan and a 1,1-dicyanoethylene (DCNE), in which two low lying charge transfer and local excitation states are involved. With generalized Mülliken-Hush and fragment charge-difference schemes, couplings from the QE approach generally agree well with those obtained from TDDFT, except that QE couplings exhibit better exponential distance dependence. Couplings from half-energy gaps with an external field are also calculated and reported. Our results show that the QE scheme is robust in calculating ET couplings with greatly reduced computational time.