We report quantum chemical calculations of two kinds of N-alkyl imine-based light-driven rotary motors by using time-dependent density functional theory. A new transition structure was found in the thermal isomerization. Based on the latter, we present a '3-step' rotation process and reveal the full working mechanism in the ground state. Furthermore, we theoretically verified the robustness of the motors through pendant group substitution of the rotator in the fjord region. As a result, the free-energy barriers of the rate-limiting thermal steps are capable to hold their robustness, which makes these motors more promising in potential machine fabrication at molecular level.

Photo-oxidation of CH3OH on TiO2 has been extensively studied in order to understand the fundamental principles of heterogeneous photocatalysis.

Brookite is now recognized as an active phase of TiO2 which exhibits superior activities compared with anatase and rutile in some photocatalytic reactions. However, there is still little research and knowledge on its electron properties as well as the crucial role of defects in brookite. Using the ab initio many-body Green's function theory, we examined the quasiparticle structures of defects, including oxygen vacancies, Ti interstitials, and hydroxyl groups, in the bulk and the (210) surface of brookite. We discovered that small polarons may generate a deep defect band and a shallow one which are approximately 0.7 and 0.3 eV below the conduction band minimum (CBM). In brookite bulk, oxygen vacancy can only create a deep defect band which is 1.1 eV below CBM and induced by the sigma bonds formed between Ti 3d orbitals. These features are quite distinct from those in anatase and rutile. We also found that introducing hydroxyl groups into brookite bulk would make the band gap narrow by at least 0.4 eV, which may help to enhance its visible light absorption. The calculated band gap and defect levels of reduced brookite are in excellent agreement with the experiments.

Many kinds of modification techniques have been developed to improve the quantum efficiency of the graphitic carbon nitride (CN) for photocatalytic water splitting in recent years. In this work, we theoretically propose to incorporate polyyne (-C C-)(n) between the heptazine ring and the tertiary amino group of CN to form covalent organic frameworks (COFs) as a new strategy to further enhance the photocatalytic ability of CN. These COFs allow for a finetuning of their electronic and optical properties by altering the quantity of C C bonds. They could fulfill the three ultimate goals of an ideal photocatalyst for water splitting, i.e., broad optical absorption, overall water splitting, and low exciton binding energy. Most strikingly, in the visible region, these COFs possess a lot of strong dipole-allowed exciton transitions whose binding energies are just at the level of a few tens of meV and whose electron and hole are well separated in space by similar to 10 nm. This extremely low exciton binding energy is of the same magnitude as that of many three-dimensional inorganic photocatalysts, which is an important characteristic that distinguishes from the conventional two-dimensional semiconductors. Functionalization of the C C bonds in the linkers of COFs can further enhance the driving force for water oxidation.

Using the GW method within many-body perturbation theory, we investigate the quasiparticle structures of defects, including oxygen vacancy, Ti interstitial, and hydroxyl groups, in the anatase TiO2 (101) surface. We find that the deep defect state in this surface observed experimentally, which is 1 eV below the Fermi level, originates from the σ bond formed between 3d orbitals of the two under-coordinated Ti atoms at the surface oxygen vacancy. Different from the density functional theory modified with on-site Coulomb terms (DFT + U), the GW method predicts that the localized polaron in anatase (101) is a shallow defect state close to the conduction band bottom. Polaronic states play the role in pinning the Fermi level of anatase near the conduction band bottom. Our GW calculations can explain satisfactorily the coexistence of shallow and deep defect states in anatase as observed in experiments. We also find that the conduction band edge of anatase is drawn down greatly after the filling of original empty Ti 3d orbitals by excess electrons, making the calculated bandgap of the reduced anatase agree well with the experiments. This significant difference in the bandgap between the intact and the reduced anatase is missed in DFT + U.

Fission of the lowest-energy singlet exciton (S-1) to two lowest-energy triplet excitons (T-1) in pentacene has been expected to be a promising means for increasing the quantum efficiency of solar cells. Experiments find that S-1 and T-1 dissociate at quite different time scales at the donor/acceptor interface. Using the pentacene/TiO2 heterojunction as the model, we investigate the dissociation of pentacene excitons by a combination of many-body Green's function theory and the time-dependent Schrodinger equation. Singlet and higher-energy triplet (T-n) excitons of pentacene could dissociate at the same timescale of similar to 100 fs, benefiting from their capability to scatter into charge-transfer (CT) states and wealdy bound charge-separated (CS) states across the interface. Resonance of pentacene excitons with CS states could facilitate the creation of free charge carriers. However, dissociation of T-1 is hampered due to its poor density of states projected onto the interfacial states, preventing its scattering into CT and CS states. According to this phenomenon, we suspect that the electron transfer from T-1 to acceptor, as observed in experiments, might undergo two successive processes, promotion of T-1 to T-n by visible light and dissociation of T-n via scattering. Involvement of the additional light absorption process might result in the low dissociation rate of T-1.

By introducing polyaniline as an additive, the perovskite solar cells exhibit a high PCE of 19.09% with improved humidity and heat stability.

Water clusters are an important species in the environment and atmosphere and take part in various chemical and biological reactions. How their optical properties vary with size is still an open question. Using the GW method and Bethe-Salpeter equation within the ab initio many-body Green's function theory, we study the electronic excitations in a series of water clusters (H2O)n with n = 1-48. We find that their absorption peaks blueshift with increasing cluster size due to the reducing electron-hole binding energy which arises from the enhanced electronic screening and gradually delocalized excitonic spatial distribution. The position of the first absorption peak has a close relation to the average number of hydrogen bonds per molecule. Off-diagonal matrix elements of the self-energy operator have pronounced effects on the unoccupied electronic levels and optical absorption for small clusters with n ≤ 10 when using density functional theory as the starting point for GW calculations. Although the optical absorption is predominated by delocalized excitons, highly localized excitons on a single water molecule are always present on the cluster surface in the vicinity of the absorption edge. These localized excitons may facilitate the photodissociation of water molecules. This can provide inspiration on the excited-state dynamics and photolysis in water clusters.

Defects play crucial roles in the photonic and chemical activities of ${\mathrm{TiO}}_{2}$. The origin of the deep band-gap defect state ${S}_{bg}$ in the rutile ${\mathrm{TiO}}_{2}$(110) surface has remained controversial for quite a long time. Using many-body Green's function theory, we believe that ${S}_{bg}$ can be attributed only to $\ensuremath{\sigma}$ bonds formed between $3d$ orbitals at the Ti interstitial, while the nonbonded Ti $3d$ defect states from the oxygen vacancy and polaron, which are held to be responsible for ${S}_{bg}$ by the present prevailing view, are shallow regardless of their spatial distribution. Especially, we discover the defect-induced appreciable downshift of unoccupied Ti $3d$ bands which should be the key for accurately describing the electronic structure of ${\mathrm{TiO}}_{2}$ but was missed in previous studies based on density functional theory. Our model could more consistently and more reasonably account for various experimental phenomena on rutile (110) than the current model based on the oxygen vacancy and polaron.

Defects play crucial roles in the photonic and chemical activities of TiO2. The origin of the deep band-gap defect state S-bg in the rutile TiO2(110) surface has remained controversial for quite a long time. Using many-body Green's function theory, we believe that S-bg can be attributed only to sigma bonds formed between 3d orbitals at the Ti interstitial, while the nonbonded Ti 3d defect states from the oxygen vacancy and polaron, which are held to be responsible for S-bg by the present prevailing view, are shallow regardless of their spatial distribution. Especially, we discover the defect-induced appreciable downshift of unoccupied Ti 3d bands which should be the key for accurately describing the electronic structure of TiO2 but was missed in previous studies based on density functional theory. Our model could more consistently and more reasonably account for various experimental phenomena on rutile (110) than the current model based on the oxygen vacancy and polaron.

Passivated codoping has never been reported to enhance the solar-to-hydrogen activity of g-C3N4, although it is an effective approach for other materials. In this letter, we use many-body Green's function theory to analyze the electronic structures, optical absorption spectra, and spatial distribution of electron-hole pair of the doped g-C3N4. Our results suggest that the passivated codoping, such as B+O, could not only extend absorption toward visible range but also promote the separation of photogenerated hole and electron to different parts of g-C3N4. Improvement of photocatalytic activity can be realized only when the n-type and p-type dopants reside in different tris-triazine units of g-C3N4. This manifests the important role of codoping microstructure on solar-to-hydrogen efficiency. These results could provide a guideline for the design of more efficient artificial photocatalysts.

Doping is an effective way to extend the optical absorption of TiO2 to the visible range. Doping of TiO2 by carbon has been found to enhance the water splitting efficiency significantly in experiment. However, the mechanism behind this is elusive. Using the ab initio many-body Green's function theory, we find that the C2 dimer formed on the TiO2 surface produces a shallow delocalized occupied Ti 3d state just below the bottom of the conduction bands. Therefore, band-gap narrowing in carbon-doped TiO2 is caused by the opposite shifts of both valence and conduction bands simultaneously, which is in contrast to the generally accepted idea that anionic dopants can only affect the valence band of TiO2. Optical absorption in the infrared region is also increased compared to reduced TiO2. The spatially well-separated photogenerated electrons and holes might help to reduce the recombination rate of carriers, in favor of improvement in photocatalysis efficiency. This novel behavior of anionic dopants is distinct from previous understandings and may guide the engineering of TiO2.

As a typical photocatalyst, TiO2 has been intensively studied for water splitting. However, the mechanism, especially how photogenerated holes participate in, behind the reaction remains highly debated. It is fundamental to identify the capability of groups at the TiO2/water interface to attract photogenerated holes. In this work, we applied the first-principles many-body Green's function theory to investigate the behaviors of free photo generated holes and coupled electron-hole pairs at three kinds of TiO2/water interfaces. Our calculations show that interaction between TiO2 and water elevates their respective valence band edges significantly. The groups derived from water, including Ti-OH, OH-, and H2O, cannot be oxidized by holes from photons at the optical absorption edge of TiO2. With higher-energy UV light, holes may be trapped by Ti-OH and OH-. However, these holes might eventually decay onto three-coordinated lattice O atoms both with and without the presence of photoelectrons if they cannot participate in the oxygen evolution reaction in time. Species related to water may only act as reactants in the photooxidation reaction. There must be some other configurations on TiO2(110), which are unknown yet and may emerge with the excited-state dynamics process, that provide active sites for holes to promote the photooxidation reaction.

Quasiparticle band structures of the defective anatase TiO2 bulk with O vacancy, Ti interstitial and H interstitial are investigated by the GW method within many-body Green's function theory. The computed direct band gap of the perfect anatase bulk is 4.3 eV, far larger than the experimental optical absorption edge (3.2 eV). We found that this can be ascribed to the inherent defects in anatase which drag the conduction band (CB) edge down. The occupied band-gap states induced by these defects locate close to the CB edge, excluding the possible contribution of these bulk defects to the deep band-gap state below CB as observed in experiments.

Using the GW method within many-body perturbation theory, we investigate the electronic properties of the rutile (011) surfaces with different reconstruction patterns. We find that keeping the Ti:O ratio on the reconstructedsurface to 1:2 enlarges the bandgap of the rutile (011) surface to ca. 4.0 eV. Increasing the content of O atoms in the surface can turn rutile into a semi-metal. For some surfaces, it is important to apply self-consistent GW calculation to get the correct charge distributions for the frontier orbitals, which are relevant to the photocatalytic behavior of TiO2.

It is unclear whether there is an intermediate dark state between the S 2 and S 1 states of carotenoids. Previous two-dimensional electronic spectroscopy measurements support its existence and its involvement in the energy transfer from carotenoids to chlorophylls, but there is still considerable debate on the origin of this dark state and how it regulates the energy transfer process. Here we use ab initio calculations on excited-state dynamics and simulated two-dimensional electronic spectrum of carotenoids from purple bacteria to provide evidence supporting that the dark state may be assigned to a new A g + state. Our calculations also indicate that groups on the conjugation backbone of carotenoids may substantially affect the excited-state levels and the energy transfer process. These results contribute to a better understanding of carotenoid excited states.

Acene is a type of important organic semiconductor which has promising applications in various optoelectronic devices. The fission of a singlet to triplet in it has been expected to elevate the quantum efficiency of organic solar cells. However, the quantum efficiency is still very low and the fission process is still under debate. Controversies also exist on the energies of the singlet and triplet states in acene. Using the many-body Green's function theory, which includes the GW method and Bethe-Salpeter equation (BSE), we compared the electronic excited states of several kinds of acene molecules (naphthalene to pentacene) at geometries optimized by different approaches. The energies of both the singlet and triplet depend strongly on the geometries of the molecules and their stacking. The non-negligible contribution from the resonant and anti-resonant transition coupling can cause large errors of the Tamm-Dancoff approximation, and the full BSE is required to get accurate results which are consistent with experiments. We found that accurate ionization energies and exciton energies can only be obtained when the geometries optimized by the Hartree-Fock approach are used. Singlet fission may be realized in isolated molecules, clusters, and surfaces, but it is hard in perfect pentacene crystals energetically. We provide a methodology for future research on acene-based solar cells and other optoelectronic devices.