The quantum interference in singlet fission (SF) among the multiple pathways from singlet excited states to correlated triplet pair states is comprehensively investigated. The analytical analysis reveals that this interference is strongly affected by the exciton–exciton coupling and is closely related to the property of J- and H-type of aggregates. Different from the interference in the spectra of aggregates, which depends only on the sign of exciton–exciton coupling, the interference in SF is additionally related to the signs of couplings between singlet excited states and triplet pair states. The interference dynamics is further demonstrated numerically by a time-dependent wavepacket diffusion method with electron–phonon interactions incorporated. Finally, we take a pentacene dimer as a concrete example to show how to adjust the constructive and destructive interferences in SF dynamics in terms of J-/H-aggregate behaviors. The results presented here may provide guiding principles for designing efficient SF materials through directly tuning quantum interference via morphology engineering.

This article is devoted to the theoretical study of the effects of a connection pattern and stereo hindrance of different π-bridges, nitrogen-containing donors, and boron-containing acceptors on the electrooptic properties of NB-type electronic asymmetric compounds in conventional D−π–A frameworks by the density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches. By introducing three different connection groups (−O–, −CH2–, and −CMe2−) and guided by structural rationality, we formed 30 NB-type molecules, which have been classified into four types: D−π–A, D–X1−π–A, D−π–X1–A, and D–X1−π–X2–A (Xn are connection groups). Then, the energy gaps (ΔEST) between the first singlet and triplet excited states were evaluated by TD-LC-ωPBE with the optimal values of ω*, as well as an approximate method, which only considers the interaction between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). It is found that for the compounds with strong vibronic coupling, the calculated ΔEST defined as the difference of vertical excitation energies largely deviates from the experimental result. The consistency between the estimated and experimental values indicates that ΔEST is predominantly determined by the frontier molecular orbitals, which can be tuned by adjusting the modular overlap between HOMO and LUMO or the orientation of the donor and acceptor groups. Accompanied with the other electronic and optical properties, our study suggests that the interaction mode, D−π–X1–A, the modified D−π–A system with a rigidly fixed acceptor and a relatively free donor, can serve as a valuable molecular design pattern for new blue-colored thermally activated delayed fluorescence (TADF) emitters. Specifically, our calculations predict that ARD-BZN-2CMe2-PYN and its relatives might have excellent potential as TADF emitters.Topics: Electric properties; Electronic structure; Energy level; Mechanical properties; Molecular structure-property relationship; Quantum mechanical methods; Quantum mechanical methods;

This work aims to explore the intrinsic properties of two-dimensional (2D)-layered perovskites, (PEA)2PbI4(N) and Cs2PbI4(N), and demonstrating how their structures and properties vary with N. The results reveal that both (PEA)2PbI4(N) and Cs2PbI4(N) are direct bandgap semiconductors, their band/optical gaps and exciton-binding energies vary linearly with 1/N at N ≥ 3, and the effective masses slowly vary with N. Compared to the bulk phases, the structures of ultrathin (PEA)2PbI4(N) are more flexible and deformable than Cs2PbI4(N). The giant spin-coupling effect greatly decreases the band gaps of both 2D materials; however, it only induces the spin splitting in the bands of (PEA)2PbI4(N). This work suggests that the ultrathin 2D materials can be a potential candidate for nano-optoelectronic devices, and that the nanoplates with N ≥ 3 could have similar performances with bulk materials in the carrier migration and exciton separation so that they can be effectively applied in photovoltaic cells.

The spectral densities of diagonal and nondiagonal exciton-phonon (e-p) coupling for tetracene crystal have been calculated by the harmonic oscillator (HO) model and ground-state MD-based approaches. We find that classical MD-based approaches overestimate the coupling of exciton with high-frequency vibrational modes and predict the strongest e-p coupling appeared above 1500 cm−1 whereas HO model and AIMD-based approach predict it appeared at ∼1400 cm−1. Additionally, the calculated spectral densities of nondiagonal e-p coupling for three different dimers show that they are continuously distributed in the range of 0–150 cm−1 and are 2–3 order of magnitude smaller than the maxima of diagonal e-p coupling.Download high-res image (83KB)Download full-size image

•The absorption of DTE in Pt complex is red shifted by ring closing.•Ring-closing process in the S0 state of DTE gets easier in Pt complex.•The first ring-closing is a little harder than the second ring-closing process.•The ring closing processes in the ionic and triplet states are easier.Stepwise ring closing processes of platinum(II) alkynyl arkynyl bridged DTEs (OO→OC→CC) were studied theoretically with density functional theory method in this work for better understanding the Pt(II) alkynyl coordination effect on the photophysical and photochemical properties of dithienylethene (DTE). It is observed that the absorption spectrum of OO complex is red-shifted compared with that of ring-opened DTE monomer because of the decreased HOMO-LUMO gap energy. The first S0→S1 absorption of OC is red shifted than that of OO and is blue shifted than that of CC complex because of the increased conjugation of the ring-closed DTE moiety. In addition, potential energy surface (PSE) studies show that the energy barrier for the ring-closing process in the ground state of Pt(II) complex is smaller than that of the isolated DTE monomer and the isomerization process is changed from endothermic to exothermic reaction. Moreover, the energy barrier for the first ring-closing process is a little larger than the second ring-closing process involved in the DTE moieties. More important, similar with that in the DTE monomers, the ring closing processes in the ionic and triplet states of Pt(II) complexes is easier than that in the neutral case.Download high-res image (127KB)Download full-size image

With efficiencies exceeding 20% and low production costs, lead halide perovskite solar cells (PSCs) have become potential candidates for future commercial applications. However, there are serious concerns about their long-term stability and environmental friendliness, heavily related to their commercial viability. Herein, we present a theoretical investigation based on the ab initio molecular dynamics (AIMD) simulations and the first-principles density functional theory (DFT) calculations to investigate the effects of sunlight and moisture on the structures and properties of MAPbI3 perovskites. AIMD simulations have been performed to simulate the impact of a few water molecules on the structures of MAPbI3 surfaces terminated in three different ways. The evolution of geometric and electronic structures as well as the absorption spectra has been shown. It is found that the PbI2-terminated surface is the most stable while both the MAI-terminated and PbI2-defective surfaces undergo structural reconstruction, leading to the formation of hydrated compounds in a humid environment. The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species, and the hydrated crystals MAPbI3·H2O and MA4PbI6·2H2O scarcely absorb the visible light. The electronic excitation in the bare and water-absorbed MAPbI3 nanoparticles tends to weaken Pb–I bonds, especially those around water molecules, and the maximal decrease of photoexcitation-induced bond order can reach up to 20% in the excited state in which the water molecules are involved in the electronic excitation, indicating the accelerated decomposition of perovskites in the presence of sunlight and moisture. This work is valuable for understanding the mechanism of chemical or photochemical instability of MAPbI3 perovskites in the presence of moisture.

A step-by-step theoretical protocol based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) has been performed to study a Ruthenium polypyridyl complex named cis-dithiocyanatobis-(4,4′-dicarboxy-2,2′-bipyridine) ruthenium(II) (N3) sensitized TiO2 solar cell including dye excitations and electronic injection. Three binding structures of N3 anchored to a TiO2 nanoparticle (TiO2)38 have been adopted. The electronic structures and optical properties of N3 dye in gas phase, in solutions (with the explicit and implicit solvent models), and interfaced with TiO2 have been calculated. The hybrid DFT exchange-correlation (XC) functional B3LYP and PBE0, the newly-modified PBE0 functional PBE0-1/3, and the long-range corrected DFT (LRC-DFT) XC functional Cam-B3LYP have been applied. It is found that both the DFT XC functionals and molecular environments play a crucial role on molecular properties, which affect both the spectral lineshapes and peak positions. Coupled with PCM model, TD-PBE0-1/3 produces the best result compared with the experimental spectrum of N3 in CH2Cl2, TD-B3LYP slightly underestimates the excitation energies, and TD-Cam-B3LYP overestimates the excitation energies of 0.4–0.5 eV. As the solvent polarity increases, the electronic absorption spectra of N3 dye blue shift. The absorption manners of N3 anchored to (TiO2)38 affect the electronic excitations, leading to different electronic injection pathways. Two of three anchoring manners favor the interfacial electronic transfer. In the end, three possible electronic injection pathways from the different parts of the excited dye to TiO2 nanoparticle have been suggested, indicating the different timescales of electron injections.

By applying molecular dynamics (MD) simulations and quantum chemical calculations, we have characterized the states and processes involved in the excited-state proton transfer (ESPT) of LSSmKate1. MD simulations identify two stable structures in the electronic ground state of LSSmKate1, one with a protonated chromophore and the other with a deprotonated chromophore, thus leading to two separate low-energy absorption maxima with a large energy spacing, as observed in the calculated and experimentally measured absorption spectra. Proton transfer is induced by electronic excitation. When LSSmKate1 is excited, the electrons in the chromophore are transferred from the phenol ring to the N-acylimine moiety; the acidity of a phenolic hydroxyl group is thus enhanced. The calculated potential energy curves (PECs) exhibit energetic feasibility in the generation of the fluorescent species in LSSmKate1, and the exact agreement between the calculated and experimentally measured values of the large Stokes shift further provides solid theoretical evidence for the ESPT process taking place in photoexcited LSSmKate1. The molecular environments play a significant role in the geometries and absorption/emission energies of the chromophores. Overall, TD-ωB97X-D/molecular mechanics (MM) provides a better description of the optical properties of LSSmKate1 than TD-B3LYP/MM, although it always overestimates the excitation energies.

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.

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.

Osmapentalyne cations synthesized recently show remarkable optical properties, such as near-infrared emission, unusual large Stokes shift and aggregation-enhanced emission. Here, the mechanisms behind those novel optical behaviors are revealed from the combined molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations. The results demonstrate that the large Stokes shift in the gas phase comes from a photoexcitation-induced deformation of the osmium plane, whereas in solution it corresponds to the variation of osmium ring symmetry. Although the central chromophore ring dominates the absorption and emission processes, the protecting groups PPh3 join the emission. As osmapentalyne cations are aggregated together in solution, the radical distribution functions of their mass-central distances display several peaks immersed in a broad envelope due to different aggregation pathways. However, the chromophore centers are protected by the PPh3 groups, the aggregation structures do not affect the Stokes shift too much, and the calculated aggregate-enhanced emission is consistent with experimental measurements.

Recently, solar cells with hybrid organic–inorganic lead halide perovskites have achieved a great success and their power conversion efficiency reaches about 17.9%. For practical applications, one has to avoid the toxicology issue of lead, to develop lead-free perovskite solar cells by using metal substitution. It has been shown that tin is one of possible candidates as a replacement for lead. Herein, a step-by-step protocol based on the first-principles calculations is performed to investigate the geometrical and electronic properties of mixed Sn and Pb perovskite MAPbxSn1−xI3 with different crystal symmetries. At first, a GGA functional with the inclusion of the van der Waals interaction, vdW-DF3, is used to optimize the geometries and it reproduces closely the unit cell volume. Then, a more accurate hybrid functional PBE0 combined with the spin–orbit coupling effect is used to perform electronic-structure calculations. The calculated results reveal that the band gaps of MAPbxSn1−xI3 are sensitive to the ratio of Sn/Pb, and are proportional to the x component, consistent with the previous reports. Further investigations show that the crystal symmetry can also modify the band gap in an order of Pnma > I4cm > P4mm at x = 0.5. The random rotation of organic cations, which makes the band alignments in the compounds, facilitates the separation and transfer of holes and electrons. Interestingly, the computed binding energies of the unrelaxed exciton have the same trend as band gaps, which decreases with decreasing x, the binding energies of MAPb0.5Sn0.5I3 also decrease as the crystal symmetry decreases, implying a faster exciton dissociation with lower x and lower symmetry at an ambient temperature.

A real-time time-dependent density functional theory coupled with the classical electrodynamics finite difference time domain technique is employed to systematically investigate the optical properties of hybrid systems composed of silver nanoparticles (NPs) and organic adsorbates. The results demonstrate that the molecular absorption spectra throughout the whole energy range can be enhanced by the surface plasmon resonance of Ag NPs; however, the absorption enhancement ratio (AER) for each absorption band differs significantly from the others, leading to the quite different spectral profiles of the hybrid complexes in contrast to those of isolated molecules or sole NPs. Detailed investigations reveal that the AER is sensitive to the energy gap between the molecular excitation and plasmon modes. As anticipated, two separate absorption bands, corresponding to the isolated molecules and sole NPs, have been observed at a large energy gap. When the energy gap approaches zero, the molecular excitation strongly couples with the plasmon mode to form the hybrid exciton band, which possesses the significantly enhanced absorption intensity, a red-shifted peak position, a surprising strongly asymmetric shape of the absorption band, and the nonlinear Fano effect. Furthermore, the dependence of surface localized fields and the scattering response functions (SRFs) on the geometrical parameters of NPs, the NP–molecule separation distance, and the external-field polarizations has also been depicted.

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–1 in four molecule–Ag20 systems 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 rational design and fabrication of mixed-phase oxide junctions is an attractive strategy for photocatalytic applications. A new tuneable α–β mixed-phase Ga2O3 has recently been discovered to have high activity for photocatalytic water splitting. Here we perform a first-principles study to reveal the nature of the efficient separation of photogenerated carriers achieved by the mixed-phase Ga2O3. It is found that the strain and lattice misfit at the interface junctions significantly tune their energy bands. As the interior angles between two components change, the characteristics of the valence band-edge states can be significantly different. Through analysis of the bonding strength of the bonds near the interfaces, and the comparison of calculated and experimentally-observed carrier migration directions, we suggest a favorable junction for the efficient separation of photogenerated carriers. This junction has a type-II band alignment with a valance band of α-Ga2O3 that is 0.35 eV higher than that of β-Ga2O3, and a conduction band offset of only 0.07 eV. It seems that electron migration across the phase boundary from α- to β-Ga2O3 mainly follows an adiabatic electron-transfer mechanism, due to strong orbital coupling between the conduction bands of the two phase materials.

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.Keywords: band alignment; band offsets; electronic properties; interfacial geometries; mixed-phase TiO2; separation of photogenerated carrier

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.

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 TiO2 solar 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/TiO2 system 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 TiO2 CB 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.

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 initio methods 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 initio methods 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.

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.

With efficiencies exceeding 20% and low production costs, lead halide perovskite solar cells (PSCs) have become potential candidates for future commercial applications. However, there are serious concerns about their long-term stability and environmental friendliness, heavily related to their commercial viability. Herein, we present a theoretical investigation based on the ab initio molecular dynamics (AIMD) simulations and the first-principles density functional theory (DFT) calculations to investigate the effects of sunlight and moisture on the structures and properties of MAPbI3 perovskites. AIMD simulations have been performed to simulate the impact of a few water molecules on the structures of MAPbI3 surfaces terminated in three different ways. The evolution of geometric and electronic structures as well as the absorption spectra has been shown. It is found that the PbI2-terminated surface is the most stable while both the MAI-terminated and PbI2-defective surfaces undergo structural reconstruction, leading to the formation of hydrated compounds in a humid environment. The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species, and the hydrated crystals MAPbI3·H2O and MA4PbI6·2H2O scarcely absorb the visible light. The electronic excitation in the bare and water-absorbed MAPbI3 nanoparticles tends to weaken Pb–I bonds, especially those around water molecules, and the maximal decrease of photoexcitation-induced bond order can reach up to 20% in the excited state in which the water molecules are involved in the electronic excitation, indicating the accelerated decomposition of perovskites in the presence of sunlight and moisture. This work is valuable for understanding the mechanism of chemical or photochemical instability of MAPbI3 perovskites in the presence of moisture.

The rational design and fabrication of mixed-phase oxide junctions is an attractive strategy for photocatalytic applications. A new tuneable α–β mixed-phase Ga2O3 has recently been discovered to have high activity for photocatalytic water splitting. Here we perform a first-principles study to reveal the nature of the efficient separation of photogenerated carriers achieved by the mixed-phase Ga2O3. It is found that the strain and lattice misfit at the interface junctions significantly tune their energy bands. As the interior angles between two components change, the characteristics of the valence band-edge states can be significantly different. Through analysis of the bonding strength of the bonds near the interfaces, and the comparison of calculated and experimentally-observed carrier migration directions, we suggest a favorable junction for the efficient separation of photogenerated carriers. This junction has a type-II band alignment with a valance band of α-Ga2O3 that is 0.35 eV higher than that of β-Ga2O3, and a conduction band offset of only 0.07 eV. It seems that electron migration across the phase boundary from α- to β-Ga2O3 mainly follows an adiabatic electron-transfer mechanism, due to strong orbital coupling between the conduction bands of the two phase materials.

Osmapentalyne cations synthesized recently show remarkable optical properties, such as near-infrared emission, unusual large Stokes shift and aggregation-enhanced emission. Here, the mechanisms behind those novel optical behaviors are revealed from the combined molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations. The results demonstrate that the large Stokes shift in the gas phase comes from a photoexcitation-induced deformation of the osmium plane, whereas in solution it corresponds to the variation of osmium ring symmetry. Although the central chromophore ring dominates the absorption and emission processes, the protecting groups PPh3 join the emission. As osmapentalyne cations are aggregated together in solution, the radical distribution functions of their mass-central distances display several peaks immersed in a broad envelope due to different aggregation pathways. However, the chromophore centers are protected by the PPh3 groups, the aggregation structures do not affect the Stokes shift too much, and the calculated aggregate-enhanced emission is consistent with experimental measurements.

Recently, solar cells with hybrid organic–inorganic lead halide perovskites have achieved a great success and their power conversion efficiency reaches about 17.9%. For practical applications, one has to avoid the toxicology issue of lead, to develop lead-free perovskite solar cells by using metal substitution. It has been shown that tin is one of possible candidates as a replacement for lead. Herein, a step-by-step protocol based on the first-principles calculations is performed to investigate the geometrical and electronic properties of mixed Sn and Pb perovskite MAPbxSn1−xI3 with different crystal symmetries. At first, a GGA functional with the inclusion of the van der Waals interaction, vdW-DF3, is used to optimize the geometries and it reproduces closely the unit cell volume. Then, a more accurate hybrid functional PBE0 combined with the spin–orbit coupling effect is used to perform electronic-structure calculations. The calculated results reveal that the band gaps of MAPbxSn1−xI3 are sensitive to the ratio of Sn/Pb, and are proportional to the x component, consistent with the previous reports. Further investigations show that the crystal symmetry can also modify the band gap in an order of Pnma > I4cm > P4mm at x = 0.5. The random rotation of organic cations, which makes the band alignments in the compounds, facilitates the separation and transfer of holes and electrons. Interestingly, the computed binding energies of the unrelaxed exciton have the same trend as band gaps, which decreases with decreasing x, the binding energies of MAPb0.5Sn0.5I3 also decrease as the crystal symmetry decreases, implying a faster exciton dissociation with lower x and lower symmetry at an ambient temperature.

A real-time time-dependent density functional theory coupled with the classical electrodynamics finite difference time domain technique is employed to systematically investigate the optical properties of hybrid systems composed of silver nanoparticles (NPs) and organic adsorbates. The results demonstrate that the molecular absorption spectra throughout the whole energy range can be enhanced by the surface plasmon resonance of Ag NPs; however, the absorption enhancement ratio (AER) for each absorption band differs significantly from the others, leading to the quite different spectral profiles of the hybrid complexes in contrast to those of isolated molecules or sole NPs. Detailed investigations reveal that the AER is sensitive to the energy gap between the molecular excitation and plasmon modes. As anticipated, two separate absorption bands, corresponding to the isolated molecules and sole NPs, have been observed at a large energy gap. When the energy gap approaches zero, the molecular excitation strongly couples with the plasmon mode to form the hybrid exciton band, which possesses the significantly enhanced absorption intensity, a red-shifted peak position, a surprising strongly asymmetric shape of the absorption band, and the nonlinear Fano effect. Furthermore, the dependence of surface localized fields and the scattering response functions (SRFs) on the geometrical parameters of NPs, the NP–molecule separation distance, and the external-field polarizations has also been depicted.