A theoretical investigation of benzothiadiazole derivatives for high efficiency OLEDs.

It is of great importance and worthy of efforts to give a clear structure-property relationship and microscopic mechanism of fluorescence emitters with high quantum yield. In this work, we perform a detailed computational investigation to give an explanation to the high efficiency of a fluorescence emitter XBTD-NPh based TADF sensitized fluorescence (TSF) OLEDs, and construct a symmetry structure DSBNA-BTD. Theoretical calculations show that XBTD-NPh is a long-time phosphorescent material at 77 K and TADF is attributed to the RISC of T1 to S1 state. For DSBNA-BTD, excitons arrived at T1 state comes to a large rate of nonradiatively path to the ground state, meaning it is may not be an efficient TADF molecule. For both molecules, the fast IC between T2 and T1 state results in that the hot exciton channel T1-Tn-S1 makes no contribution to the TADF.

Identification of the interchromophore interaction in the electronic absorption and circular dichroism spectra of bis-phenanthrenes.

We characterize the low-lying excited electronic states of a series of bis-phenanthrenes using our newly developed diabatic scheme called the fragment particle-hole density (FPHD) method and calculate both the electronic absorption and circular dichroism (ECD) spectra using the time-dependent density functional theory (TDDFT) and the FPHD-based exciton model which couples intrachromophore local excitations (LEs) and the interchromophore charge-transfer excitations (CTEs). TDDFT treats each bis-phenanthrene as a single molecule while the mixed LE-CTE exciton model partitions the molecule into two phenanthrene-based aromatic moieties, and then applies the electronic coupling between the various quasi-diabatic states to cover the interactions. It is found that TDDFT and the mixed LE-CTE model reproduce all experimentally observed trends in the spectral profiles, and the hybridization between LE and CTE states is displayed differently in absorption and ECD spectral intensities, as it usually decreases the absorption maxima and affects the positive/negative extrema of the ECD irregularly. By comparing the results yielded by the LE-CTE model with and without the LE-CTE coupling, we identify the contribution of CTE on the main dipole-allowed transitions.

Time-dependent Kohn-Sham electron dynamics coupled with nonequilibrium plasmonic response via atomistic electromagnetic model.

Computational modeling of plasmon-mediated molecular photophysical and photochemical behaviors can help us better understand and tune the bound molecular properties and reactivity and make better decisions to design and control nanostructures. However, computational investigations of coupled plasmon-molecule systems are challenging due to the lack of accurate and efficient protocols to simulate these systems. Here, we present a hybrid scheme by combining the real-time time-dependent density functional theory (RT-TDDFT) approach with the time-domain frequency dependent fluctuating charge (TD-ωFQ) model. At first, we transform ωFQ in the frequency-domain, an atomistic electromagnetic model for the plasmonic response of plasmonic metal nanoparticles (PMNPs), into the time-domain and derive its equation-of-motion formulation. The TD-ωFQ introduces the nonequilibrium plasmonic response of PMNPs and atomistic interactions to the electronic excitation of the quantum mechanical (QM) region. Then, we combine TD-ωFQ with RT-TDDFT. The derived RT-TDDFT/TD-ωFQ scheme allows us to effectively simulate the plasmon-mediated "real-time" electronic dynamics and even the coupled electron-nuclear dynamics by combining them with the nuclear dynamics approaches. As a first application of the RT-TDDFT/TD-ωFQ method, we study the nonradiative decay rate and plasmon-enhanced absorption spectra of two small molecules in the proximity of sodium MNPs. Thanks to the atomistic nature of the ωFQ model, the edge effect of MNP on absorption enhancement has also been investigated and unveiled.

Comment on "A posteriori localization of many-body excited states through simultaneous diagonalization".

We comment on an excited-state localization method recently proposed by Blanc et al. (J. Comput. Chem. 2023, 44, 105). Elaborate comparisons are made to demonstrate that their method is a less-comprehensive version of the diabatization method proposed by us 2 years earlier (J. Phys. Chem. Lett. 2021, 12, 1032).

Analytical derivative couplings within the framework of time-dependent density functional theory coupled with conductor-like polarizable continuum model: Formalism, implementation, and applications.

The nonadiabatic phenomena, which are characterized by a strong coupling between electronic and nuclear motions, are ubiquitous. The nonadiabatic effect of the studied system can be significantly affected by the surrounding environment, such as solvents, in which such nonadiabatic process takes place. It is essential to develop the theoretical models to simulate these processes while accurately modeling the solvent environment. The time-dependent density functional theory (TDDFT) is currently the most efficient approach to describe the electronic structures and dynamics of complex systems, while the polarizable continuum model (PCM) represents one of the most successful examples among continuum solvation models. Here, we formulate the first-order derivative couplings (DCs) between the ground and excited states as well as between two excited states by utilizing time-independent equation of motion formalism within the framework of both linear response and spin flip formulations of TDDFT/CPCM (the conductor-like PCM), and implement the analytical DCs into the Q-CHEM electronic structure software package. The analytic implementation is validated by the comparison of the analytical and finite-difference results, and reproducing geometric phase effect in the protonated formaldimine test case. Taking 4-(N,N-dimethylamino)benzonitrile and uracil in the gas phase and solution as an example, we demonstrate that the solvent effect is essential not only for the excitation energies of the low-lying excited-states but also for the DCs between these states. Finally, we calculate the internal conversion rate of benzophenone in a solvent with DC being used. The current implementation of analytical DCs together with the existing analytical gradient and Hessian of TDDFT/PCM excited states allows one to study the nonadiabatic effects of relatively large systems in solutions with low computational cost.

Unequal Perylene Diimide Twins in a Quadruple Assembly.

Natural light-harvesting (LH) systems can divide identical dyes into unequal aggregate states, thereby achieving intelligent "allocation of labor". From a synthetic point of view, the construction of such kinds of unequal and integrated systems without the help of proteinaceous scaffolding is challenging. Here, we show that four octatetrayne-bridged ortho-perylene diimide (PDI) dyads (POPs) self-assemble into a quadruple assembly (POP)4 both in solution and in the solid state. The two identical PDI units in each POP are compartmentalized into weakly coupled PDIs (P520) and closely stacked PDIs (P550) in (POP)4 . The two extreme pools of PDI chromophores were unambiguously confirmed by single-crystal X-ray crystallography and NMR spectroscopy. To interpret the formation of the discrete quadruple assembly, we also developed a two-step cooperative model. Quantum-chemical calculations indicate the existence of multiple couplings within and across P520 and P550, which can satisfactorily describe the photophysical properties of the unequal quadruple assembly. This finding is expected to help advance the rational design of dye stacks to emulate functions of natural LH systems.

Exploring the photocatalytic properties and carrier dynamics of 2D Janus XMMX' (X = S, Se; M = Ga, In; and X' = Te) materials.

Recently, two-dimensional (2D) Janus structures have been extensively explored because of their robust electron mobility and unique photocatalytic properties. In spite of the increasing interest, the origin of high photocatalytic activities and the behaviors of photoinduced carriers in this kind of materials have not been well understood. Herein, we present a step-by-step protocol based on the first-principles calculations combined with the ab initio non-adiabatic molecular dynamics (NAMD) simulations to unveil the origin of high photocatalytic activity of highly stable typical 2D Janus XMMX' structures (X = S, Se; M = Ga, In; and X' = Te). Their band structures, optical properties, exciton binding energies, carrier effective masses, solar-to-hydrogen efficiency, hot carrier relaxation and recombination times, etc. have been calculated. We find that the difference between X and X' atoms on the two surfaces of the XMMX' monolayer not only builds an out-of-plane electric field, which significantly affects the charge distributions on the valence band maxima (VBM) and the conduction band minima (CBM) and subsequently decreases the exciton binding energy, but also transforms the indirect band structures of XM into the direct ones with well suitable energy gaps for visible-light absorption as well as endows the XMMX' structures with unequal electron and hole mobility, rapid hot carrier relaxation and slow electron-hole recombination processes on a timescale of tens of nanoseconds. The current work suggests that Janus XMMX' monolayers are good photocatalytic materials for overall water splitting and provides a guide to regulate the materials' properties for efficient energy harvesting and optoelectronic applications.

Vibrationally resolved absorption spectra and ultrafast exciton dynamics in α-phase and ß-phase zinc phthalocyanine aggregates.

The vibrationally resolved absorption spectra and ultrafast exciton dynamics in α-phase and ß-phase zinc phthalocyanine (ZnPc) aggregates are theoretically investigated using a non-Markovian stochastic Schrödinger equation combined with first-principles calculations. It is found that although similar double-peak structures arise in the Q-band region of the absorption spectra in both phases, these peaks are different in nature and exhibit distinct types of behavior with respect to the aggregation length. The analysis on the basis of an effective two-state model indicates that the two absorption peaks in the α phase are from mixing between the charge-transfer (CT) state and the bright Frenkel exciton (FE) state. By contrast, in the ß-phase, the low-energy peak is solely contributed by a low-lying bright FE state, whereas the high-energy peak originates from the interplay between the CT state and another high-lying bright FE state. For the relaxation processes right after photoexcitation from the Q-band region, it is found that within the first dozens of femtoseconds the ZnPc aggregates of both phases tend to temporarily fall into some intermediate states where the population distribution and average electronic energy do not obviously evolve. In addition, it is found that the optical transition of the low-lying bright FE state in the ß phase is not favorable for the formation of bound CT states due to the absence of enough driving forces.

Substituent Effect on Vibrationally Resolved Absorption Spectra and Exciton Dynamics of Dipyrrolonaphthyridinedione Aggregates.

Dipyrrolonaphthyridinedione (DPND) thin films exhibit interesting photophysical properties and singlet fission (SF) processes. A recent experimental work found that the alkyl substitution in the DPND skeleton has the remarkable influence on the characteristics of electronic absorption spectra and SF rates. Here, we theoretically elucidate the microscopic mechanism of the substituent effect on the optical properties and exciton dynamics of materials by combining the electronic structure calculations and the quantum dynamics simulations. The results show that the alkyl substituent has a minor effect on the single molecular properties but dramatically changes those of DPND aggregates via varying the intermolecular interactions. The aggregates of DPND with and without alkyl side chains exhibit the more likely characters of H-type aggregations. In the former (DPND6), the weak degree of mixing of intramolecular localized excited (LE) states and intermolecular charge transfer (CT) states makes the low-energy absorption band possess the predominant optical absorption, while in the latter (DPND), the CT and LE states are close in energy, together with their strong interaction, resulting in the substantial state-mixing, so that its two low-energy absorption bands have nearly equal oscillator strengths and a wide energy spacing of more than 0.5 eV. The simulation of exciton dynamics elucidates that the photoinitiated states in both aggregates cannot generate the free charge carrier because of the lack of enough driving forces. However, the population exchanges between LE and CT states in DPND aggregates are much faster than in DPND6 aggregates, indicating the different SF behaviors, consistent with the experimental observation.

Vibronic Coupling Effect on the Vibrationally Resolved Electronic Spectra and Intersystem Crossing Rates of a TADF Emitter: 7-PhQAD.

Assessing and improving the performance of organic light-emitting diode (OLED) materials require quantitative prediction of rate coefficients for the intersystem crossing (ISC) and reverse ISC (RISC) processes, which are determined not only by the energy gap and the direct spin-orbit coupling (SOC) between the first singlet and triplet excited-states at a thermal equilibrium position of the initial electronic state but also by the non-Condon effects such as the Herzberg-Teller-like vibronic coupling (HTVC) and the spin-vibronic coupling (SVC). Here we apply the time-dependent correlation function approaches to quantitatively calculate the vibrationally resolved absorption and fluorescence spectra and ISC/RISC rates of a newly synthesized multiple-resonance-type (MR-type) thermally activated delayed fluorescence (TADF) emitter, 7-phenylquinolino[3,2,1-de]acridine-5,9-dione (7-PhQAD), with the inclusion of the Franck-Condon (FC), HTVC, and Duschinsky rotation (DR) effects. The SVC effect on the rates has also been approximately evaluated. We find that the experimentally measured ISC rates of 7-PhQAD originate predominantly from the vibronic coupling, consistent with the previous reports on other MR-type TADF emitters. The SVC effect on ISC rates is about 10 times larger than the HTVC effect, and the latter increases the ISC rates by more than 1 order of magnitude while it slightly affects the vibrationally resolved absorption and fluorescence spectra. The discrepancy between the theoretical and experimental results is attributed to inaccurately describing excited-states calculated by the time-dependent density functional theory as well as to not fully accounting for the complex experimental conditions. This work provides a demonstration of what proportion of ISC and RISC rate coefficients of a MR-type TADF emitter can be covered by the HTVC effect, and it opens design routes that go beyond the FC approximation for the future development of high-performance OLED devices.

Evaluation of molecular photophysical and photochemical properties using linear response time-dependent density functional theory with classical embedding: Successes and challenges.

Time-dependent density functional theory (TDDFT) based approaches have been developed in recent years to model the excited-state properties and transition processes of the molecules in the gas-phase and in a condensed medium, such as in a solution and protein microenvironment or near semiconductor and metal surfaces. In the latter case, usually, classical embedding models have been adopted to account for the molecular environmental effects, leading to the multi-scale approaches of TDDFT/polarizable continuum model (PCM) and TDDFT/molecular mechanics (MM), where a molecular system of interest is designated as the quantum mechanical region and treated with TDDFT, while the environment is usually described using either a PCM or (non-polarizable or polarizable) MM force fields. In this Perspective, we briefly review these TDDFT-related multi-scale models with a specific emphasis on the implementation of analytical energy derivatives, such as the energy gradient and Hessian, the nonadiabatic coupling, the spin-orbit coupling, and the transition dipole moment as well as their nuclear derivatives for various radiative and radiativeless transition processes among electronic states. Three variations of the TDDFT method, the Tamm-Dancoff approximation to TDDFT, spin-flip DFT, and spin-adiabatic TDDFT, are discussed. Moreover, using a model system (pyridine-Ag20 complex), we emphasize that caution is needed to properly account for system-environment interactions within the TDDFT/MM models. Specifically, one should appropriately damp the electrostatic embedding potential from MM atoms and carefully tune the van der Waals interaction potential between the system and the environment. We also highlight the lack of proper treatment of charge transfer between the quantum mechanics and MM regions as well as the need for accelerated TDDFT modelings and interpretability, which calls for new method developments.

Analytic high-order energy derivatives for metal nanoparticle-mediated infrared and Raman scattering spectra within the framework of quantum mechanics/molecular mechanics model with induced charges and dipoles.

This work is devoted to deriving and implementing analytic second- and third-order energy derivatives with respect to the nuclear coordinates and external electric field within the framework of the hybrid quantum mechanics/molecular mechanics method with induced charges and dipoles (QM/DIM). Using these analytic energy derivatives, one can efficiently compute the harmonic vibrational frequencies, infrared (IR) and Raman scattering (RS) spectra of the molecule in the proximity of noble metal clusters/nanoparticles. The validity and accuracy of these analytic implementations are demonstrated by the comparison of results obtained by the finite-difference method and the analytic approaches and by the full QM and QM/DIM calculations. The complexes formed by pyridine and two sizes of gold clusters (Au18 and Au32) at varying intersystem distances of 3, 4, and 5 Å are used as the test systems, and Raman spectra of 4,4'-bipyridine in the proximity of Au2057 and Ag2057 metal nanoparticles (MNP) are calculated by the QM/DIM method and compared with experimental results as well. We find that the QM/DIM model can well reproduce the IR spectra obtained from full QM calculations for all the configurations, while although it properly enhances some of the vibrational modes, it artificially overestimates RS spectral intensities of several modes for the systems with very short intersystem distance. We show that this could be improved, however, by incorporating the hyperpolarizability of the gold metal cluster in the evaluation of RS intensities. Additionally, we address the potential impact of charge migration between the adsorbate and MNPs.

Understanding the mechanism of plasmon-driven water splitting: hot electron injection and a near field enhancement effect.

Utilizing plasmon-generated hot carriers to drive chemical reactions has currently become an active area of research in solar photocatalysis at the nanoscale. However, the mechanism underlying exact transfer and the generation dynamics of hot carriers, and the strategies used to further improve the quantum efficiency of the photocatalytic reaction still deserve further investigation. In this work, we perform a nonadiabatic excited-state dynamics study to depict the correlation between the reaction rate of plasmon-driven water splitting (PDWS) and the sizes of gold particles, the incident light frequency and intensity, and the near-field spatial distribution. Four model systems, H2O and Au20@H2O separately interacting with the laser field and the near field generated by the Au nanoparticle (NP) with a few nanometers in size, have been investigated. Our simulated results clearly unveil the mechanism of PDWS and hot-electron injection in a Schottky-free junction: the electrons populated on the antibonding orbitals of H2O are mandatory to drive the OH bond breaking and the strong orbital hybridization between Au20 and H2O creates the conditions for direct electron injection. We further find that the linear dependence of the reaction rate and the field amplitude only holds at a relatively weak field and it breaks down when the second OH bond begins to dissociate and field-induced water fragmentation occurs at a very intensive field, and that with the guarantee of electron injection, the water splitting rate increases with an increase in the NP size. This study will be helpful for further improving the efficiency of photochemical reactions involving plasmon-generated hot carriers and expanding the applications of hot carriers in a variety of chemical reactions.

Nonlinear features of Fano resonance: a QM/EM study.

The nonlinear Fano effects on the absorption of hybrid systems composed of a silver nanosphere and an indoline dye molecule have been systematically investigated by the hybrid approach, which combines the quantum mechanics method (QM) with the computational electromagnetic method (EM). The absorption spectra of the dye molecule in the proximity of an Ag nanoparticle have been calculated by changing the incident field intensity, the phenomenological dephasing of molecular excitation, and the enhancement ratio of the near field. The contribution of molecular nonlinear response properties and the quantum interferences of the incident and scattered fields and of resonant plasmon-molecular excitations to the spectra has been identified. It is in no doubt that Fano resonance due to the plasmon-molecular interaction can appear in both the weak and strong field regimes; however, the Fano effect is more pronounced in the strong field regime where quantum interference leads to a nonlinear Fano effect controlled by a complex field-dependent Fano factor. When the incident field is strong enough, the resonance antisymmetry structure is spectrally resolved, and it changes with the change of the field intensity. As the field intensity varies from weak to strong, the Fano lineshape's asymmetry increases with increasing intensity in the beginning, and then decreases with a further increase of the field intensity attributed to the increase of the detuning energy induced by the integrated energy shift upon field dressing during the excitation. Decreasing the enhancement ratio of the near field or the dephasing of molecular excitation can also control the spectral lineshape transformation from an asymmetric profile to a symmetric Lorentzian lineshape. These findings are consistent with previous experimental and theoretical observations arisen by quantum interferences and are expected to stimulate further work toward exploring the plasmon-molecular interplay and the applications of Fano resonance in optical switching and sensing.

Correction: Analysis and visualization of energy densities. I. Insights from real-time time-dependent density functional theory simulations.

Correction for 'Analysis and visualization of energy densities. I. Insights from real-time time-dependent density functional theory simulations' by Junjie Yang et al., Phys. Chem. Chem. Phys., 2020, 22, 26838-26851, DOI: .

Vibrationally Resolved Absorption Spectra and Exciton Dynamics in Zinc Phthalocyanine Aggregates: Effects of Aggregation Lengths and Remote Exciton Transfer.

The vibrationally resolved absorption spectra and exciton dynamics in the α-zinc phthalocyanine aggregates are theoretically investigated by using a non-Markovian stochastic Schrödinger equation. The model Hamiltonian adopted for spectral and dynamic simulations explicitly includes the couplings for both nearest-neighbor and remote exciton transfer, and it is parametrized from first-principles calculations. The results indicate that aggregation lengths and remote exciton transfer significantly influence the relative energy alignment between delocalized Frenkel exciton (FE) and charge transfer (CT) states, which in turn strongly affects the relative intensities of the two absorption peaks in the Q-band region. Analytical formulas are derived to establish quantitative structure-spectra relationships in aggregates, and they offer simple patterns to extract electronic-state properties directly from absorption spectra. The dynamics simulations reveal that the light absorption can directly generate mixed states with both FE and CT features, but it is hard for the photoexcitation from the Q-band region to generate free carriers due to the high energies of charge-separated states.

Potassium doping-induced variations in the structures and photoelectric properties of a MAPbI3 perovskite and a MAPbI3/TiO2 junction.

It has been experimentally demonstrated that mixed metallic cation modification could be an effective strategy to enhance the performance and stability of perovskite-based solar cells (PSCs). However, there is limited microscopic understanding at the atomic/molecular level of the behavior of small radius alkali metal cation doping in both perovskite materials and perovskite/TiO2 junctions. Here, we perform a first-principles density functional theory study on the doping-induced variation of the geometric and electronic structures of MAPbI3 (MA = methylammonium) and the MAPbI3/TiO2 junction. The impacts of different doping methods, and different charge states and locations of the given dopants have been investigated. At first, we theoretically confirm that the structures doped by K+ are the most thermally stable compared to the structures doped by the other charge states of K, and that K+ dopants prefer to modify the perovskite lattice interstitially and stay near the MAPbI3/TiO2 interface. Meanwhile, we find that a severe geometric deformation occurs if two doped lattices come into contact directly, indicating that the lattice may rapidly collapse from the interior if the doping concentration is too high. Additionally, we observe that K+ doped interstitially near the MAPbI3/TiO2 interface causes the intensive distortion of the surface Ti-O bonds and severe bond-length fluctuations. Consequently, this results in distorted TiO2 bands of the interfacial layer and a slight decrease of the band offset of conduction bands between two phases. This work complements experiments and provides a better microscopic understanding of the doping modification of the properties of perovskite materials and PSCs.

Unveiling the effect of electron tunneling on the plasmonic resonance of closely spaced gold particles.

Recent experimental techniques enable the nanoparticles' (NPs) ensembles to be made with an angstrom-level of interparticle gap widths. The theoretical description of their optical properties becomes more challenging because of the nonnegligible quantum mechanism (QM) effects such as electron tunneling and nonlocal screening. To demonstrate the microscopic mechanism of QM effects and quantitatively elucidate their impact on a variety of surface plasmon resonance (SPR) models of gold particle oligomers, in this work we performed a theoretical study on closely spaced Au cluster dimers and NP oligomers by the first-principles approach, and the classical or quantum-corrected electromagnetic models (CEM or QCM), respectively. Through the first-principles calculation on a series of AuN dimers with different interparticle distances d and N, we depicted the variation of the possibility of direct electron tunneling (DET) across the junction constructed by two nearest NPs with d and N, and found that it exactly follows an exponential decay and the decay rate linearly varies with 1/N. The impact of the QM effect on the SPR models excited along the dimer axis is much more profound than those perpendicular to the dimer axis. CEM/QCM calculations on strongly coupled NP dimers and symmetric and asymmetric trimers demonstrated the evolution of their optical properties with variable NP sizes, gap separations and light polarization, as well as the QM effect on the major SPR modes. The side-by-side comparison between the results from time-dependent density functional theory and CEM/QCM models sheds light on understanding the origin of a variety of SPR models of gold NP oligomers and the QM effect on those modes, and makes a connection between the calculations of small cluster and large NP oligomers.

Analysis and visualization of energy densities. II. Insights from linear-response time-dependent density functional theory calculations.

Inspired by the analysis of Kohn-Sham energy densities by Nakai and coworkers, we extended the energy density analysis to linear-response time-dependent density functional theory (LR-TDDFT) calculations. Using ethylene-tetrafluoroethylene and oxyluciferin-water complexes as examples, distinctive distribution patterns were demonstrated for the excitation energy densities of local excitations (within a molecular fragment) and charge-transfer excitations (between molecular fragments). It also provided a simple way to compute the effective energy of both hot carriers (particle and hole) from charge-transfer excitations via an integration of the excitation energy density over the donor and acceptor grid points.

Analysis and visualization of energy densities. I. Insights from real-time time-dependent density functional theory simulations.

In this article, we report a scheme to analyze and visualize the energy density fluctuations during the real-time time-dependent density functional theory (RT-TDDFT) simulations. Using Ag4-N2 complexes as examples, it is shown that the grid-based Kohn-Sham energy density can be computed at each time step using a procedure from Nakai and coworkers. Then the instantaneous energy of each molecular fragment (such as Ag4 and N2) can be obtained by partitioning the Kohn-Sham energy densities using Becke or fragment-based Hirshfeld (FBH) scheme. A strong orientation-dependence is observed for the energy flow between the Ag4 cluster and a nearby N2 molecule in the RT-TDDFT simulations. Future applications of such an energy density analysis in electron dynamics simulations are discussed.