Evaluating Electronic Couplings for Excited State Charge Transfer Based on Maximum Occupation Method ΔSCF Quasi-Adiabatic States

Electronic couplings of charge-transfer states with the ground state and localized excited states at the donor/acceptor interface are crucial parameters for controlling the dynamics of exciton dissociation and charge recombination processes in organic solar cells. Here we propose a quasi-adiabatic state approach to evaluate electronic couplings through combining maximum occupation method (mom)-ΔSCF and state diabatization schemes. Compared with time-dependent density functional theory (TDDFT) using global hybrid functional, mom-ΔSCF is superior to estimate the excitation energies of charge-transfer states; moreover it can also provide good excited electronic state for property calculation. Our approach is hence reliable to evaluate electronic couplings for excited state electron transfer processes, which is demonstrated by calculations on a typical organic photovoltaic system, oligo­thiophene/perylene­diimide complex

Impact of Phonon Dispersion on Nonlocal Electron–Phonon Couplings in Organic Semiconductors: The Naphthalene Crystal as a Case Study

Recent studies point to the impact that the dispersion of both optical and acoustic phonons can have on the nonlocal electron–phonon couplings in organic molecular semiconductors. Here, in order to further elucidate the influence of phonon dispersion, we have calculated the phonon modes in the entire Brillouin zone of the naphthalene crystal. The results demonstrate that the overall nonlocal couplings are underestimated by calculations in which only the phonon modes derived at the Brillouin zone center are considered. Moreover, the contributions of acoustic phonons to the overall strength of nonlocal electron–phonon couplings are calculated to be quantitatively very significant for parallel-stacked dimers, as high as 40% for holes and 47% for electrons

Hot Charge-Transfer States Determine Exciton Dissociation in the DTDCTB/C₆₀ Complex for Organic Solar Cells: A Theoretical Insight

To understand charge-transfer (CT) processes at the donor/acceptor interface of DTDCTB/fullerene solar cells, we have investigated the electronic couplings and the rates for exciton-dissociation and charge-recombination processes based on two representative intermolecular geometries of the DTDCTB/C₆₀ complex by means of quantum-chemical calculations. Consistent with the experimental measurements of the time scale of over subns or even ns for charge recombination (CR), the calculated CR rates are lower than 10¹⁰ s^–1 and in most cases, below 10⁹ s^–1. The calculated rates for exciton dissociation into the CT ground state are mostly lower than 10¹⁰ s^–1, which is, however, in sharp contrast with the ultrafast charge separation (∼100 fs) observed experimentally. Interestingly, our calculations point out that excitons are able to dissociate into a higher-energy excited CT state much faster, with the rates being as large as about 10¹² and 10¹⁴ s^–1 in all cases for excitons based on C₆₀ and DTDCTB, respectively. Thus, exciton dissociation in the DTDCTB/C₆₀ complex is determined by the hot CT states. As the excess energy of the excited CT state can facilitate the geminate electron and hole to further separate at the donor/acceptor interface, our theoretical results suggest that the high performance of the DTDCTB/fullerene-based solar cell can be mainly attributed to the fact that excitons dissociate via the hot CT states to effectively form mobile charge carriers

Role of the Dark 2A_g State in Donor–Acceptor Copolymers as a Pathway for Singlet Fission: A DMRG Study

The mechanism of intramolecular singlet fission in donor–acceptor-type copolymers, especially the role of the dark 2A_g state, is not so clear. In this Letter, the electronic structure of the benzodithiophene (B)-thiophene-1,1-dioxide (TDO) copolymer is calculated by density matrix renormalization group theory with the Pariser–Parr–Pople model. We find that the dark 2A_g state is the lowest singlet excited state and is nearly degenerate with the 1B_u state. So, a fast internal conversion from 1B_u to 2A_g state is highly possible. The 2A_g state has a strong triplet pair character, localized on two neighboring acceptor units, which indicates that it is an intermediate state for the intramolecular singlet fission process. With the increase of the donor–acceptor push–pull strength in our model, this triplet pair character of the 2A_g state becomes more prominent, and meanwhile the binding energy of this coupled triplet pair state decreases, which favors the separation into two uncoupled triplet states. We propose a model in which the competition between the singlet fission process and the nonradiative decay process from the 2A_g state would determine the final quantum yield

Solvent Effects on the Optical Spectra and Excited-State Decay of Triphenylamine-thiadiazole with Hybridized Local Excitation and Intramolecular Charge Transfer

The triphenylamine-thiadiazole molecule (TPA-NZP) is a newly popular, highly efficient OLED fluorescent emitter with exciton utilization efficiency exceeding the upper limit of spin statistics (25%). In this work, the optical spectra and the radiative and nonradiative decay rate constants have been investigated theoretically for TPA-NZP in hexane, ethyl ether, tetrahydrofuran, and dimethylformamide solvents, in comparison with the gas phase. We observed the evolutions of the excited states from the hybridized local and charge-transfer (HLCT) character to complete intramolecular charge transfer (CT) character with the increase of the solvent polarities. It is found that upon increasing the solvent polarity, the amount of red shift in the absorption peak is much less than that of emission, resulting in breakdown of the mirror symmetry. This is because that 0–0 transition energy is red-shifted but the vibrational relaxation increases with the solvent polarity, leading to subtraction in absorption while addition in emission. The radiative decay rate constant is calculated to be almost independent of polarity. The nonradiative decay rate increases by almost one order of magnitude from that in nonpolar hexane to the strongly polarized dimethylformamide, which is attributed to the dual effects of the red shift in the gap and enhancement of the vibrational relaxation by solvent polarity

Developing Quinoidal Fluorophores with Unusually Strong Red/Near-Infrared Emission

Despite the dominant position of aromatic fluorophores, we report herein the design and synthesis of quinoidal fluorophores based on rarely emissive quinoidal bithiophene. Quinoidal bitheno­[3,4-<i>b</i>]­thiophene, <b>QBTT-C6</b>, consisting of cruciform-fused (<i>E</i>)-1,2-bis­(5-hexylthiophen-2-yl)­ethene and quinoidal bithiophene, shows a fluorescence quantum yield of 8.5%, 25-fold higher than that of the parent quinoidal <b>QBT</b> chromophore, but its maximum emission is at similar wavelengths. <b>QBTT-Ar</b>’s featuring intramolecular charge transfer can further shift the maximum emission into the near-infrared region. The intramolecular charge transfer is programmably enhanced by tuning the substituents on the aryl groups from the electron-withdrawing trifluoromethyl to the electron-donating methoxy groups. Unexpectedly, a positive relationship between intramolecular charge transfer and fluorescence quantum yield is observed; as a result, <b>QBTT-FL</b> gives an unprecedentedly high fluorescence quantum yield of up to 53.1% for quinoidal oligothiophenes. With detailed photophysical and theoretical investigations, we demonstrate that the nonradiative intersystem crossing (S₁ → T₂) is significantly restrained in <b>QBTT-Ar</b>’s, which can be attributed to the faster reverse intersystem crossing (T₂ → S₁) characteristic of a small activation energy. This work reveals the possibility for developing red/near-infrared fluorophores from the less explored quinoidal molecules because of their intrinsically narrow bandgaps

Ultrafast Excited-State Energy Transfer in DTDCTB Dimers Embedded in a Crystal Environment: Quantum Dynamics with the Multilayer Multiconfigurational Time-Dependent Hartree Method

Photoinduced excited-state energy transfer (EET) processes play a key role in the solar energy conversion of small molecule organic solar cells. We investigated intermolecular EET dynamics in the 2-[[7-(5-<i>N</i>,<i>N</i>-ditolylaminothiophen-2-yl)-2,1,3-benzothiadiazol-4-yl]­methylene]­malononitrile (DTDCTB) dimer embedded in a crystal environment using full quantum dynamics, i.e., the multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) method. Two different stacking statuses of the DTDCTB dimers, which occur along the OA axis in the DTDCTB crystal, were considered. We built a vibronic diabatic Hamiltonian using the projection method based on quantum mechanics/molecular mechanics results. Different model Hamiltonians were considered in the full quantum dynamics studies. First, reduced-dimensional models were constructed by simply including more of the important vibrational modes. Second, we tried to construct a continuous spectral density based on the vibronic coupling strengths of different modes and then created a set of “pseudomodes” to represent electron–phonon couplings. The dynamics results based on these reduced models were compared with the results obtained with the full dimensional model. Our theoretical descriptions demonstrated that ultrafast intermolecular EET dynamics takes place in the well-stacked DTDCTB dimers. This work deepens our understanding of the photoinduced ultrafast EET dynamics of realistic organic photovoltaic systems at the full quantum mechanical level

Prediction of Remarkable Ambipolar Charge-Transport Characteristics in Organic Mixed-Stack Charge-Transfer Crystals

We have used density functional theory calculations and mixed quantum/classical dynamics simulations to study the electronic structure and charge-transport properties of three representative mixed-stack charge-transfer crystals, DBTTF–TCNQ, DMQtT–F₄TCNQ, and STB–F₄TCNQ. The compounds are characterized by very small effective masses and modest electron–phonon couplings for both holes and electrons. The hole and electron transport characteristics are found to be very similar along the stacking directions; for example, in the DMQtT–F₄TCNQ crystal, the hole and electron effective masses are as small as 0.20 and 0.26 <i>m</i>₀, respectively. This similarity arises from the fact that the electronic couplings of both hole and electron are controlled by the same superexchange mechanism. Remarkable ambipolar charge-transport properties are predicted for all three crystals. Our calculations thus provide strong indications that mixed-stack donor–acceptor materials represent a class of systems with high potential in organic electronics

Electronic Properties of Mixed-Stack Organic Charge-Transfer Crystals

The electronic structures of a series of donor–acceptor mixed-stack crystals have been investigated by means of density functional theory calculations. The results highlight that a number of the donor–acceptor crystals under consideration are characterized by wide valence and conduction bands, large hole and electron electronic couplings, and as a result very low hole and electron effective masses. The fact that the effective masses and electronic couplings for holes and electrons are nearly equal along the stacking directions implies that the hole and electron mobilities in these systems are also similar. In addition, in several of these crystals, charge transport has a two-dimensional character. The impact on the charge transport properties of the electronic couplings between donor and acceptor frontier orbitals and of the related energy gaps is also discussed

Electronic and Charge-Transport Properties of the Au₃(CH₃NCOCH₃)₃ Crystal: A Density Functional Theory Study

Density functional theory was used to investigate the electronic and charge-transport properties of the trinuclear gold Au₃(CH₃NCOCH₃)₃ crystal. Hole transport is found to be anisotropic and characterized by a very small effective mass of about 0.21 <i>m</i>₀ along the stacking direction of the Au₃ molecules. Interestingly, the calculations suggest an isotropic character of electron transport, for which the effective mass is about 1 <i>m</i>₀. We show that while the interstack interactions facilitate electron transport in the directions perpendicular to the stacks, they act to diminish this transport along the stacking directions. Overall, the present results indicate that this compound is a promising ambipolar material for application in electronic devices