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A new diabatization scheme is proposed to calculate the electronic couplings for the singlet fission process in multichromophoric systems. In this approach, a robust descriptor that treats single and multiple excitations on an equal footing is adopted to quantify the localization degree of the particle and hole densities of the electronic states. By maximally localizing the particles and holes in terms of predefined molecular fragments, quasi-diabatic states with well-defined characters (locally excited, charge transfer, correlated triplet pair, etc.) can be automatically constructed as the linear combinations of the adiabatic ones, and the electronic couplings can be directly obtained. This approach is very general in that it applies to electronic states with various spin multiplicities and can be combined with various kinds of preliminary electronic structure calculations. Due to the high numerical efficiency, it is able to manipulate more than 100 electronic states in diabatization. The applications to the tetracene dimer and trimer reveal that high-lying multiply excited charge transfer states have significant influences on both the formation and separation of the correlated triplet pair and can even enlarge the coupling for the latter process by 1 order of magnitude.

pH buffer plays versatile roles in both biology and chemistry. In this study, we unravel the critical role of pH buffer in accelerating degradation of the lignin substrate in lignin peroxidase (LiP) using QM/MM MD simulations and the nonadiabatic electron transfer (ET) and proton-coupled electron transfer (PCET) theories. As a key enzyme involved in lignin degradation, LiP accomplishes the oxidation of lignin via two consecutive ET reactions and the subsequent C–C cleavage of the lignin cation radical. The first one involves ET from Trp171 to the active species of Compound I, while the second one involves ET from the lignin substrate to the Trp171 radical. Differing from the common view that pH = 3 may enhance the oxidizing power of Cpd I via protonation of the protein environment, our study shows that the intrinsic electric fields have minor effects on the first ET step. Instead, our study shows that the pH buffer of tartaric acid plays key roles during the second ET step. Our study shows that the pH buffer of tartaric acid can form a strong H-bond with Glu250, which can prevent the proton transfer from the Trp171-H•+cation radical to Glu250, thereby stabilizing the Trp171-H•+cation radical for the lignin oxidation. In addition, the pH buffer of tartaric acid can enhance the oxidizing power of the Trp171-H•+cation radical via both the protonation of the proximal Asp264 and the second-sphere H-bond with Glu250. Such synergistic effects of pH buffer facilitate the thermodynamics of the second ET step and reduce the overall barrier of lignin degradation by ∼4.3 kcal/mol, which corresponds to a rate acceleration of 103-fold that agrees with experiments. These findings not only expand our understanding on pH-dependent redox reactions in both biology and chemistry but also provide valuable insights into tryptophan-mediated biological ET reactions.

The variational quantum eigensolver (VQE) algorithm can simulate the chemical systems such as molecules in the noisy-intermediate-scale quantum devices and shows promising applications in quantum chemistry simulations. The accuracy and computational cost of the VQE simulations are determined by the underlying ansatz. Therefore, the most important issue is to generate a compact and accurate ansatz, which requires a shallower parametric quantum circuit and can achieve an acceptable accuracy. The newly developed adaptive algorithms (AAs) such as the adaptive derivative-assembled pseudo-Trotter VQE (ADAPT-VQE) can solve this issue via generating compact and accurate ansatzes. However, these AAs show very low computational efficiency because they require a large number of additional measurements. Here we propose an amplitude reordering (AR) strategy to accelerate the promising but expensive AAs by adding operators in a “batched” fashion in a way that their order is still quasi-optimal. We first introduce the AR method into ADAPT-VQE and build the AR-ADAPT-VQE algorithm. We then endow the energy-sorting VQE (ES-VQE) algorithm with the adaptive feature and introduce the AR into AES-VQE to form the AR-AES-VQE algorithm. To demonstrate the performance of these algorithms, we calculate the dissociation curves of three small molecules, LiH, linear BeH2, and linear H6, by using (AR-)ADAPT-VQE and (AR-)AES-VQE algorithms. It is found that all of the AR-equipped AAs (AR-AAs) can significantly reduce the number of iterations and subsequently accelerate the calculations with a speedup of up to more than ten times without the obvious loss of accuracy. The final ansatz generated by the AR-AAs not only avoids extra circuit depth but also maintains the computational accuracy; sometimes the AR-AAs even outperforms their original counterparts.

Lead-free halide double perovskites (LFDPs) arouse great interest as environmentally friendly substitutes to the state-of-the-art lead halide perovskites. However, most halide LFDPs feature large bandgaps that afford weak visible-light absorption. Herein, we synthesized and systemically studied the single-crystallographic, physical–chemical, band structural, and electronic properties of a series of multidimensional Au-based lead-free hybrid polymorphs, [(NH3CH3)2(AuI4)(AuI2)], [(NH2CHNH2)(AuI4)(AuI2)], [NH3(CH2)nNH3][(AuI4)2] (n= 2, 3), [NH3(CH2)nNH3][(AuI4)(I3)] (n= 4, 5, 6), and [NH3(CH2)nNH3]2[(AuI4)(AuI2)(I3)2] (n= 7, 8, 9). Among these two-dimensional (2D) perovskite-related and zero-dimensional nonperovskite polymorphs, we found an abnormal variation of structural dimensionality with increasing the size of A-site cations. These polymorphs displayed ultrabroad absorption ranges (200–1300 nm) and tunable low indirect bandgaps (0.93–1.18 eV). Combined single-crystallography and Hall effect measurements demonstrate that the I3–conduces to the formation of 2D perovskite structures related by connecting with the (AuI4)−sheets and also the charge transport properties. Consequently, [NH3(CH2)5NH3][(AuI4)I3] and [NH3(CH2)8NH3]2[(AuI4)(AuI2)(I3)2] showed high spectroscopic limited maximum efficiencies of 28.4 and 20.5%, respectively, with a film thickness of 500 nm. Our findings bring the Au-based polymorphs to the forefront of study on lead-free low-bandgap perovskites for optoelectronics and reveal the unique rule of dimensionality engineering in such materials.

Much effort has been dedicated to boost the development of lead-free perovskite solar cells. However, their performance and stability are still much less competitive to the lead-bearing counterparts. By exploiting a mixed Sn–Ge cation strategy for the development of lead-free perovskites, we perform ab initio electronic structure calculations and quantum dynamics simulations on MASn0.5Ge0.5I3and compare them to MASnI3. The calculations demonstrate that the hybrid cation strategy can improve simultaneously the perovskite stability and the lifetime of charge carriers. The stability increases due to a larger space of possible structures within the favorable range of the structural parameters, such as the Goldschmidt tolerance and octahedron factors. By exploring the larger structure space, mixed perovskites find stable configurations with lower free energies and better fitting components that exhibit reduced fluctuations around the equilibrium geometries. Charge carriers live longer in mixed perovskites because cation mixing results in an additional and moderate disorder that separates electrons and holes, reducing their interactions while still maintaining efficient band-like charge transport. These general and fundamental principles established by the analysis of the simulation results are useful for the design of advanced materials for solar energy and construction of optoelectronic devices.

Thermally activated delayed fluorescence (TADF) emitters have become a hot topic in organic light-emitting diode materials due to their advantages of high efficiency, long lifespan and low cost. Many investigations have shown that TADF efficiency is determined by the reverse intersystem crossing (ISC) rate from the triplet to singlet excited state where the direct spin-obit coupling (SOC) is a pivoted factor. This study demonstrates that nonadiabatic coupling (NAC) is critical in enhancing TADF efficiency, as elucidated through second-order perturbation theory. We start from the time-dependent correlation function approach for calculating ISC rate to incorporate SOC and NAC as well as the Herzberg-Teller-like vibronic coupling. From three typological molecular model simulations, we find that the NAC can even reach to 32.6% contribution to ISC rate, and these contributions are sensitive to the motions of vibrational modes. This finding may provide important reference and guidance for further optimizing material properties and designing new TADF emitters.

The optical microcavities and plasmonic arrays have emerged as new ways to manipulate and control the molecular properties by exploiting radiation–matter coupling. Here, the time-dependent wave-packet diffusion method is extended to investigate the influence of the optical microcavity on the singlet fission (SF) dynamics in organic aggregates. The results from a model system show that the photon–exciton coupling can dramatically attenuate the dependence of SF rates on molecular packing mode and there is an optimal photon–exciton coupling at which the SF rate reaches its maximum, and the cavity effect becomes less obvious as the modulatory role of charge-transfer states on SF dynamics is incorporated. The possible mechanism is assigned to the joint effect of photon–exciton and exciton–exciton interactions on initial singlet exciton distribution. Furthermore, we simulate the SF dynamics of a realistic pentacene dimer with a medium-strength (VPE= 100 meV) and strong (VPE= 300 meV) photon–exciton couplings. The numerical results verify that SF rates are susceptible to VPE.

Surface charge transfer doping (SCTD) is a promising technique to construct high-performance nanodevices because of its high reproducibility and high spatial selectivity and because it does little harm to the host semiconductor. Here, we performed a first-principles theoretical investigation to assess the effects of SCTD on the properties of two-dimensional (2D) arsenene, antimonene, and arsenene/antimonene van der Waals heterostructure as well. It was found that doping O or S on the surfaces of arsenene and antimonene could achieve efficient p-type doping, while doping Cs2CO3on them could achieve n-type doping. Furthermore, when O and Cs2CO3were co-doped on the two sides of the arsenene/antimonene heterostructure, a typical type-ii energy band alignment can be formed in O-arsenene/Cs2CO3-antimonene heterostructure, which effectively extends the range of the light absorption into the near-infrared region and facilitates the spatial separation of photogenerated electron–hole pairs. O- or S-doped arsenene and antimonene have tunable band gaps varying from 1.20 to 0.54 eV because of the doping-induced change of the conduction band minima (CBM), and Cs2CO3-doped arsenene and antimonene have band gaps of 2.02 and 1.36 eV, respectively, because of the changes of both valence band maxima and CBMs. This work offers a way to design p–n junctions with a tunable character, and the 2D p–n/p–n O-arsenene/Cs2CO3-antimonene heterostructure might be applied to electronic and optoelectronic nanodevices.

The absorption spectra of a series of tetrahedral-shaped aluminum nanoparticles (ANPs) with side lengths ranging from L= 1.4 to 141.4 nm have been calculated. The size-dependent evolution of structures and spectra has been demonstrated. The plasmon resonance of ANPs is highly sensitive to the particle size and spans a wide spectral region from ultraviolet to visible. As Lincreases from 70 to 141 nm, an apparent spectral red shift is still observed, although their spectral line shapes do not change much. For the small clusters Alnwith n≤ 560 (or with L≤ 3.68 nm), their absorption spectra have been calculated by both the real-time time-dependent density functional theory (RT-TDDFT) scheme and the finite difference time domain (FDTD) classical electrodynamics method, whereas for the larger nanoparticles, their absorption spectra have only been calculated by the FDTD method. Alnwith n≤ 560 has two main absorption bands attributed to two kinds of localized surface plasmon resonance modes, vertexes and edges, revealed by the corresponding induced electron densities. RT-TDDFT and FDTD approaches produce obvious spectral differences, and RT-TDDFT predicts more intensive low-energy absorption bands and broader high-energy bands than FDTD. The effect of geometrical relaxation on the absorption spectra of small clusters is visible, which tends to blue shift and broaden the spectra. The smaller the cluster, the larger is the geometrical change.

The charge carrier mobilities and singlet fission (SF) dynamics in newly synthesized three thienoquinoidal compounds (ThBF, TThBF, and BThBF) are theoretically characterized by combining a time-dependent wavepacket diffusion method and electronic structure calculations. It is found that all three compounds have quite large electron and hole mobilities and that ThBF possesses the property of p-type semiconductor, whereas TThBF and BThBF show the behavior of n-type semiconductor. For the SF efficiency, TThBF should be the best candidate among the three molecular crystals due to a strong electronic coupling. The obvious correlation between charge carrier mobilities and the SF dynamics is not found; however, the expansion of exciton wave function in the aggregates of these compounds can accelerate the SF process, consistent with other experimental and theoretical observations.

Tin-based perovskites are one of the most promising candidates for the development of lead-free perovskite solar cells (PSCs) and have attracted lots of attention. Despite the extensive research efforts, the performance of tin-based PSCs (Sn-PSCs) still lags far behind the lead containing counterparts due to the poor stability and self-p-doping. Here, we perform first-principles density functional theory calculations combined with non-adiabatic molecular dynamics simulations to unveil the origins of the instability of tin iodine perovskite when exposed to O2. It is found that O2is more thermally favorable to be adsorbed on iodine-vacancy (VI) defect sites in the defective surface rather than the pristine surface, and the generation of peroxide species on the VIsites dramatically accelerates the structural decomposition. The presence of VIdefects on the surface of CsSnI3decreases the band gap by inducing a local shallow state around conduction band minimum, significantly accelerating the electron–hole recombination. The adsorption of O2on VIsite slightly increases the band gap compared to that of the pristine one and decreases the influence of VIon the surface’s optoelectronic properties and the electron–hole recombination rate but significantly accelerates structural decomposition by weakening the defect tolerance ability of the [SnI6]4–octahedra as well as the carriers’ relaxation. The increased structural instability in combined VIand O2points to the surface VIdefect passivation as the main optimization scheme for efficient and stable Sn-PSCs.

A time-dependent wavepacket diffusion method is used to investigate the effects of charge transfer (CT) states, singlet exciton, and multiexciton migrations on singlet fission (SF) dynamics in organic aggregates. The results reveal that the incorporation of CT states can result in SF dynamics different from the direct interaction between singlet exciton and multiexciton, and an obvious SF interference is also observed between the direct channel and the indirect channel mediated by CT states. In the case of direct interaction, although the fast population transfer of singlet exciton in monomers, caused by the increase of exciton–exciton interaction, can accelerate the SF process, the spatial coherence alternatively has a counter-productive effect, and their competition leads to an optimal exciton–exciton interaction at which SF has a maximal rate. This trade-off relationship in SF dynamics is further analyzed from different perspectives, specifically in spatial and energy representations, and is also confirmed through the indication that static energy disorders can speed up SF process by destructing the coherence. Meanwhile, it is found that the couplings among multiexciton states decrease SF rates by the multiexciton coherence and backward conversion from multiexciton to singlet exciton states.

A systematically theoretical investigation has been performed to study the dynamic chemical enhancement of surface-enhanced Raman spectroscopy (SERS) of pyridine, pyrimidine, 2-mercaptopyridine, and 4-mercaptopyridine absorbed on a silver cluster of Ag20. The influences of different structural configurations (V and S), different intermolecular charge-transfer (CT) excited states, and the different approximations [Franck–Condon (FC) or FC+ Herzberg–Teller (FCHT) approximations] to spectral cross sections have been examined. It is found that the photoexcitation can easily produce the intermolecular CT excited states, leading to the absorption maxima red-shift, and their intensities decrease compared with that of Ag20. Furthermore, we observe that the absolute Raman intensities are sensitive to the systems’ structural configurations, exchange-correlation functionals, CT excited states, and the FC/FCHT approximations as well. However, the relative Raman intensities and dominant vibrational structures of CT resonance RS (RRS) are mainly determined by the adsorbates. The modes which can have a larger enhancement in all CT RRS are those related to ring stretch and ring breathing. The ring-stretching mode at around 1600 cm–1in four molecule–Ag20systems is evidently enhanced compared to that of bare molecules which can be considered as a hint of the presence of the CT resonance enhancement. Additionally, we observe that HT effects dominate the resonance enhancement and it could explain the coupling between the plasmonic and chemical enhancement mechanisms, but the FCHT approximation does not significantly change the relative RRS intensities obtained through the FC approximation.

The fabrication of surface phase junctions has proven to be an efficient strategy for the enhancement of photocatalytic activity; however, some questions about these systems are not yet well understood. In this study, the photocatalytic overall water splitting reaction was achieved on photocatalysts with pure and mixed phase compositions of cubic γ-Ga2O3and monoclinic β-Ga2O3. All the Ga2O3photocatalysts can split water stoichiometrically into H2and O2; however, the phase-mixed γ/β-Ga2O3photocatalyst with a small amount of β phase shows the lowest activity. This is opposite from that of the reported α/β-Ga2O3system in which the phase-mixed α/β-Ga2O3photocatalyst with a small amount of β phase shows much higher photocatalytic activity than the individual phases. Much more disordered structure is found between the γ and β phases in the γ/β-Ga2O3photocatalyst with low content of β phase due to the defective spinel structure of γ phase. Spectroscopic characterization and theoretical calculations indicate that the disordered structure serves as defect and charge recombination centers resulting in the decrease of photocatalytic activity. Based on the analysis of α/β-Ga2O3and γ/β-Ga2O3systems, it is proposed that a prerequisite for the formation of phase junction boosting photocatalytic reactions is that the interfacial structure between two phases should not be disordered or defective.

Molecular dynamics simulations and combined quantum mechanics and molecular mechanics calculations are employed to investigate dimethoxy-tetraphenylethylene (DMO-TPE) molecules in water solution for their detailed aggregation process and the mechanism of aggregation-induced emission. The molecular dynamics simulations show that the aggregates start to appear in the nanosecond time scale, and small molecular aggregates appear at low concentration; whereas the large aggregates with a chain-type structure appear at high concentration, and the intramolecular rotation is largely restricted by a molecular aggregated environment. The average radical distribution demonstrates that the waters join the aggregation process and that two types of hydrogen bonds between DMO-TPE and water molecules are built with the peaks at about 0.5 and 0.7 nm, respectively. The spectral features further reveal that the aggregates dominantly present J-type aggregation although they fluctuate between J-type and H-type at a given temperature. The statistical absorption, emission spectra, and the aggregation-induced emission enhancement with respect to the solution concentration agree well with the experimental measurements, indicating the significant effect of molecular environments on the molecular properties.

It has been suggested that the exotic aggregation-induced emission (AIE) phenomenon was caused by the restriction on the nonradiative decay through intramolecular vibrational/rotational relaxation. There have been other proposed mechanisms such as J-aggregation or excimer formation, etc. Through computational studies, we propose a direct approach to verify the AIE process, namely, using resonance Raman spectroscopy (RRS) to explore the microscopic mechanism of AIE. Taking examples of AIE-active 1,2-diphenyl-3,4-bis(diphenylmethylene)-1-cyclobutene (HPDMCb) and AIE-inactive 2,3-dicyanopyrazino phenanthrene (DCPP) for comparison, we found that for the AIEgen, after aggregation into cluster, the intensities of low-frequency peaks in RRS are evidently reduced relative to the high-frequency peaks, along with a remarkable blueshift. However, the RRS of non-AIEgen remains almost unaffected upon aggregation. Such distinctive spectroscopic characteristics can be ascribed to the intramolecular vibrational relaxation which is hindered for AIEgen, especially for the low-frequency ring-twisting motions, because the RRS amplitude is proportional to the mode vibrational relaxation energy times frequency λjωj. Thus, RRS is a direct way to clarify the recent dispute on the AIE mechanism. If such predictions are true, it will clearly validate the earlier proposed restriction on the nonradiative decay through an intramolecular vibration/rotation relaxation mechanism.

We present a step-by-step theoretical protocol based on the first-principles methods to reveal the insight into the origin of the high photocatalytic activity achieved by the mixed-phase TiO2, consisting of anatase and rutile. The interfacial geometries, density of states, charge densities, optical absorption spectrum, electrostatic potential, and band offsets have been calculated. The most stable mixed-phase structures have been identified, the interfacial tensile strain-dependent electronic structures have been observed, and the energy level diagram of band alignment has been given. We find that the geometrical reconstruction around the interfacial area has a negligible influence on the light absorption of the heterojunction and the interfacial sites seem not to dominantly contribute to the band-edge states. For the most stable heterojunction, the calculated valence-band maximum and conduction-band minimum of rutile, respectively, lie 0.52 and 0.22 eV above those of anatase, which agrees well with the experimental measurements and other theoretical predications. The good match of band energies to reaction requirements, large driving force for the charge immigration across the interface, and the difference of electrostatic potentials around the interface successfully explain the high photocatalytic activity achieved by the mixed-phase TiO2.

We review our recent work on the methodology development of the excited-state properties for the molecules in vacuum and liquid solution. The general algorithms of analytical energy derivatives for the specific properties such as the first and second geometrical derivatives and IR/Raman intensities are demonstrated in the framework of the time-dependent density functional theory (TDDFT). The performance of the analytical approaches on the calculation of excited-state energy Hessian has also been shown. It is found that the analytical approaches are superior to the finite-difference method on the computational accuracy and efficiency. The computational cost for a TDDFT excited-state Hessian calculation is only 2–3 times as that for the DFT ground-state Hessian calculation. With the low computational complexity of the developed analytical approaches, it becomes feasible to realize the large-scale numerical calculations on the excited-state vibrational frequencies, vibrational spectroscopies and the electronic-structure parameters which enter the spectrum calculations of electronic absorption and emission, and resonance Raman spectroscopies for medium-to large-sized systems.

This project aims to attack the frontiers of electronic structure calculations on the excited states of large molecules and molecular aggregates by developing novel theoretical and computational methods. The methodology development is especially based on the time-dependent density functional theory (TDDFT) and valence bond (VB) theory, and is expected to be computationally effective and accurate as well. Research works on the following related subjects will be performed: (1) The analytical energy-derivative approaches for electronically excited state within TDDFT will be developed to reduce bypass the computational costs in the calculation of molecular excited-state properties. (2) The ab initiomethods for electronically excited state based on VB theory and hybrid TDDFT-VB method will be developed to overcome the limitations of current TDDFT in simulating photophysics and photochemistry. (3) For larger aggregates, neither ab initiomethods nor TDDFT is applicable. We intend to build the effective model Hamiltonian by developing novel theoretical and computational methods to calculate the involved microscopic physical parameters from the first-principles methods. The constructed effective Hamiltonian is then used to describe the excitonic states and excitonic dynamics of the natural or artificial photosynthesized systems, organic or inorganic photovoltaic cell. (4) The condensed phase environment is taken into account by combining the developed theories and algorithms based on TDDFT and VB with the polarizable continuum solvent models (PCM), molecular mechanism (MM), classical electrodynamics (ED) or molecular dynamics (MD) theory. (5) Highly efficient software packages will be designed and developed.

We present here a brief summary of a National Natural Science Foundation Major Project entitled “Theoretical study of the low-lying electronic excited state for molecular aggregates”. The project focuses on theoretical investigation of the electronic structures and dynamic processes upon photo- and electric-excitation for molecules and aggregates. We aim to develop reliable methodology to predict the optoelectronic properties of molecular materials related to the electronic excitations and to apply in the experiments. We identify two essential scientific challenges: (i) nature of intramolecular and intermolecular electronic excited states; (ii) theoretical description of the dynamic processes of the coupled motion of electronic excitations and nucleus. We propose the following four subjects of research: (i) linear scaling time-dependent density-functional theory and its application to open shell system; (ii) computational method development of electronic excited state for molecular aggregates; (iii) theoretical investigation of the time evolution of the excited state dynamics; (iv) methods to predict the optoelectronic properties starting from electronic excited state investigation for organic materials and experimental verifications.

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.

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.

A step-by-step theoretical protocol based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) at both the molecular and periodic levels have been performed to study a zinc porphyrin complex (named YDoc) sensitized TiO2solar cell including dye excitations, electron injection, the regeneration of photooxidized dyes and the effect of electrolyte additives. Our study reveals the possibility of a favorable electron transfer from the excited dye to the semiconductor conduction band (CB) and suggests three possible pathways of the electron injection from the dye to the nanoparticle (TiO2)38. One is the direct one-step injection by photoexcitation, and the other two are from the different parts of the excited dye to the nanoparticle. The influence of the electrolyte composition on the geometric and electronic features of the dye/TiO2system has also been studied. It is found that, with the additive of the lithium ion, the energy gap between the LUMO of dye and the TiO2CB edge increases, which subsequently increases the driving force for the ultrafast excited-state electron injection, contrary to the effect of 4-tert-butylpyridine additive. The computational results of the oxidized dye interacting with I–and I2–reveal that there are a few possible mechanisms for the regeneration of oxidized dye. The effective mechanisms of the regeneration are suggested.

Efficient quantum dynamical and electronic structure approaches are presented to calculate resonance Raman spectroscopy (RRS) with inclusion of Herzberg–Teller (HT) contribution and mode-mixing (Duschinsky) effect. In the dynamical method, an analytical expression for RRS in the time domain is proposed to avoid summation over the large number of intermediate vibrational states. In the electronic structure calculations, the analytic energy-derivative approaches for the excited states within the time-dependent density functional theory (TDDFT), developed by us, are adopted to overcome the computational bottleneck of excited-state gradient and Hessian calculations. In addition, an analytic calculation to the geometrical derivatives of the transition dipole moment, entering the HT term, is also adopted. The proposed approaches are implemented to calculate RR spectra of a few of conjugated systems, phenoxyl radical, 2-thiopyridone in water solution, and free-base porphyrin. The calculated RR spectra show the evident HT effect in those π-conjugated systems, and their relative intensities are consistent with experimental measurements.

Triplet–triplet energy transfer in benzophenone–fluorene and benzophenone–fluorene–naphthalene molecules is theoretically investigated by using the rate theories and electronic structure calculations established for electron transfer. From the calculated electronic couplings for the single-step tunneling and multistep hopping pathways of the energy transfer from the donor benzophenone to the acceptor naphthalene, it is found that the tunneling comes from the direct electronic couplings between the donor and acceptor states, other than the coupling via the virtual bridge state in the conventional superexchange mechanism. The mode-specific reorganization energy calculations reveal that only the several high-frequency modes dominate the energy transfer, leading to an important nuclear tunneling effect. Succeedingly, with use of the obtained parameters, Fermi’s golden rule predicts the consistent energy transfer rates with experimental ones.

Because the feature sizes of noble metal nanoparticles (NPs) are smaller than a few of nanometers, simulations including quantum effects and atomistic details are inevitable. In this work, we report a detailed electronic structure study on the plasmon resonance of isolated gold hollow nanoparticles (NPs) and NP pairs. The long-range-corrected (LRC) density functional theory (DFT) has been employed. We find that the plasmon resonance of small-size gold NPs is very sensitive to NP sizes and interparticle distances. When the NP’s size changes from Au32 to Au17–, the high-energy absorption maximum blue shifts 50 nm and when the interparticle distance of Au17–NP pairs changes from 1.15 to 0.83 nm, the corresponding blue shift is ∼40 nm. The spectral line width becomes narrower as the NP size increases and the interparticle distance reduces. The insight of how the plasmon-resonance peaks of a NP pair are formed and how they are sensitive to the interparticle separation is revealed by the plots of transition densities and frontier molecular orbitals (MOs) as a function of the interparticle distances. As the two NPs approach near touching contact, they are strongly coupled and a bond-forming step takes place, which is verified by the significant overlap between the unoccupied MOs. The strong coupling between the wave functions results in the electrons to redistribute. As a result, we observe that a large number of electrons are localized in the gap and the nearest neighboring atoms of a closely spaced NP pair. The localized electrons enhance the electromagnetic field in the gap of the NP pair, leading to a pronounced red shift and increasing polarizability for the plasmon-resonance peaks, and many new absorption peaks appeared in low-energy range.

Abstract: The non-Condon effect plays an important role in the process of electron transfer (ET). Several theoretical models have been proposed to investigate its effect on ET rates. In this paper, we overview a theoretical method for the calculations of the non-Condon ET rate constants proposed by us, and its applications to organic semiconductors. First, full quantum expressions of the non-Condon ET rates are presented with the electronic couplings having exponential, Gaussian and linear dependences in terms of the nuclear coordinates, respectively. The proposed formulas have closed forms in time domain and they thus can be easily applied in multi-mode systems. Then, the driving force dependences of the ET rates involving the non-Condon effect are calculated with the use of full quantum mechanical formulas. It is found that these dependences show very different properties from the Marcus one. As an example of applications, the approaches are used to investigate the non-Condon effect on the mobility of the organic semiconductor dithiophene-tetrathiafulvalene (DT-TTF). The results manifest that the non-Condon effect enhances ET rates compared with the Condon approximation, and static fluctuations of electronic coupling dominate the ET rate in the DT-TTF, which has been confirmed by the molecular dynamics simulation.

Abstract: Absorption and photoluminescence spectroscopies are useful tools to study the photo-physical properties of materials. The theoretical methods for calculation of the spectra of molecules/supermolecules and aggregates, whose structures can differ significantly, are reviewed from the viewpoint of computational efficiency. Several model compounds/multimers are taken as examples for the spectral calculations. The numerical results achieve a satisfactory agreement between the theory and experiment.

The ultrafast electron-transfer (ET) processes in three dye-sensitized TiO 2systems (pycooh−, catechol−, and alizarin−) are studied by using the real-time time-dependent density functional theory (RT-TDDFT). TiO 2cluster models are used to substitute TiO 2nanocrystals in order to check the quantum size effect on ET. The initial-state geometrical optimization for the individual constituents and coupled systems and the subsequent calculations for IR spectra and the density of states (DOS) are performed at the B3LYP/Lanl2dz theory level. The calculated IR spectra, the DOS, and the low-lying excited states reveal that the couplings between three dyes and TiO 2clusters are very strong so that an ultrafast electron injection from the excited dyes to TiO 2clusters is favored. By following the electronic motion of coupled systems after the photoexcitation of adsorbates in real time without allowing the nuclei to move, we predict an electronic injection time of a few femtoseconds for the present finite systems, which is slightly longer than the experimental measurements and other theoretical predications for the ET time on the same dye-sensitized bulk TiO 2systems due to the small clusters used in our simulation. We find that the ET time is appreciably dependent on the cluster size when the cluster is quite small. However, the size effects on ET time reduce dramatically as the cluster size reaches to a moderate middle size, for example, (TiO 2) 14. The electron−nuclear coupled movement does not play a significant role in the initial ET process in these three systems. The effects of different initial excited states on electronic dynamics are also discussed.

Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.

The localized-density-matrix (LDM) method has been developed to calculate the excited state properties of very large systems containing thousands of atoms. It is particularly suitable for simulating the dynamic electronic processes in nanoscale materials, and has been applied to poly(p-phenylenevynelene) (PPV) aggregates and carbon nanotubes. Absorption spectra of PPVs and carbon nanotubes have been calculated and compared to the experiments.

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.

Review: 55 refs.