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Here, we used collinear and noncollinear density functional theory (DFT) methods to explore the interfacial properties of two heterojunctions between a fullerene (C60 and C70) and the MAPbI3(110) surface. Methodologically, consideration of the spin–orbit interaction has been proven to be required to obtain accurate energy-level alignment and interfacial carrier dynamics between fullerenes and perovskites in hybrid perovskite solar cells including heavy atoms (such as Pb atoms). Both heterojunctions are predicted to be the same type-II heterojunction, but the interfacial electron transfer process from MAPbI3 to C60 is completely distinct from that to C70. In the former, the interfacial electron transfer is slow because of the associated large energy gap, and the excited electrons are thus trapped in MAPbI3 for a while. In contrast, in the latter, the smaller energy gap induces ultrafast electron transfer from MAPbI3 to C70. These points are further supported by DFT-based nonadiabatic dynamics simulations including the spin–orbit coupling (SOC) effects. These gained insights could help rationally design superior fullerene–perovskite interfaces to achieve high power conversion efficiencies of fullerene–perovskite solar cells. [ABSTRACT FROM AUTHOR]

A wide bandgap and short-lived charge carrier constitutes two major issues for restricting anatase titanium dioxide (TiO2) photocatalytic activity in the ultraviolet light region of the solar spectrum. Interestingly, experiments reported that anatase TiO2 doping with substitutional N can achieve a high visible-light photocatalytic activity but the mechanism remains controversial and unclear yet. N substituting oxygen creates a mid-gap state, which typically acts as a nonradiative charge recombination center. Using the nonadiabatic (NA) molecular dynamics, we demonstrate that charge carrier lifetimes of N-doped anatase TiO2 are notably prolonged regardless of the oxidation states of the N dopant, but it operates by different mechanisms. The neutral and negatively charged N dopant reduces the bandgap by creating either an electron or a hole trap state prior to recombination with free holes and free electrons, respectively. While the direct recombination of free electrons and free holes, bypassing the electron trap state, dominates the nonradiative relaxation in the neutral N-doped TiO2 and extends the charge carrier lifetimes over 4-fold compared to the pristine anatase TiO2; the hole-trap-assisted electron–hole recombination beats the direction pathway in the negatively charged N-doped TiO2 system, delaying the nonradiative charge recombination over (22 times slower) the pristine system. Our simulations point out to changes in the relative values of NA coupling, decoherence times and energy gaps as determinants of the aforementioned changes in the carrier lifetimes. In this way, our study rationalizes the long-term debate on the enhanced visible-light photocatalytic activity of the TiO2 doping with N and, therefore, it contributes to rational defect engineering for design of high performance of photocatalytic and optoelectronic devices based on TiO2 and other metal oxides. [ABSTRACT FROM AUTHOR]

Hybrid carbon materials are found to exhibit novel optoelectronic properties at their interfaces, but the related interfacial carrier dynamics is rarely explored theoretically. In this contribution, we have employed density functional theory (DFT) and DFT-based nonadiabatic dynamics methods to explore photoinduced interfacial electron transfer processes at interfaces between a single-walled carbon nanotube with chiral index (6,5) and C60 or C70 (C60@CNT65 and C70@CNT65). We have found that with low E11 excitation, electron transfer takes place from CNT65 to C60 and C70 in both heterojunctions. This process is ultrafast and completed within about 200 fs, which is consistent with recent experiments. Differently, high E22 excitation does not induce electron injection to C60 in C60@CNT65; instead, "hot" electrons produced within CNT65 will be trapped in its higher conduction band for a while because of slow inter-band relaxation. By contrast, in C70@CNT65, high E22 excitation still can lead to ultrafast electron transfer to C70, but only a comparable amount of electrons are transferred (ca. 30%). Interestingly, electrons either remaining on CNT65 or transferred to C70 are trapped in the higher conduction band for a while, similarly, due to slow inter-band relaxation. The present results could be useful to guide the design of excellent interfaces of mixed-dimensional hybrid carbon materials for various optoelectronic applications. [ABSTRACT FROM AUTHOR]

The perovskite CH3NH3PbI3 excited‐state lifetimes exhibit conflicting experimental results under humid environments. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that the interplay between lead vacancy and water can rationalize the puzzle. The lead vacancy reduces NA coupling by localizing holes, slowing electron–hole recombination. By creating a deep electron trap state, the coexistence of a neutral lead vacancy and water molecules enhances NA coupling, accelerating charge recombination by a factor of over 3. By eliminating the mid‐gap state by accepting two photoexcited electrons, the negatively charged lead vacancy interacting with water molecules increases the carrier lifetime over 2 times longer than in the pristine system. The simulations rationalize the positive and negative effects of water on the solar cell performance exposure to humidity. [ABSTRACT FROM AUTHOR]

Inorganic hole transport material (HTM) NiO is superior to traditional organic HTMs due to its high stability and conductivity. Efficient charge separation and slow charge recombination are two key important steps in determining the efficiency of solar cells but they often show strong temperature dependence. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we have demonstrated that the time scales for both hole transfer and electron–hole recombination show weak temperature dependence in MAPbI3/NiO perovskite solar cells. The hole transfer occurs on sub-100 fs at both high and low temperatures. Hole transfer proceeds slightly faster at high temperature than low temperature due to enhanced NA coupling. Notably, the hole remains hot over 200 fs energy relaxation before cooling to interact with the remaining electrons, form excitons and accelerate electron–hole recombination, providing an excellent advantage for solar energy applications. Following charge separation, the electron–hole recombination takes place in several nanoseconds at both low and high temperatures. The acceleration only by a factor of below 2 arises because increased atomic motions cause rapid loss of coherence, which competes successfully with the enhanced NA coupling. The detailed atomistic understanding of the reported results allows us to generalize the conclusions to other hole transport materials and suggests weak temperature-dependent charge dynamics properties enabling the high performance of perovskite solar cells. [ABSTRACT FROM AUTHOR]

The stationary and intersection structures on the S0 and S1 potential energy surfaces of CH3COCH2Cl have been determined by the CAS(10,8)/cc-pVDZ optimizations and their relative energies are refined by the CASPT2//CAS(10,8)/cc-pVDZ single-point calculations. Non-adiabatic molecular dynamics simulations were performed on the basis of the state-averaged CAS(10,8)/cc-pVDZ calculated energies, energy gradients, and Hessian matrix for the S0 and S1 states. It is found that the features of the S1 potential energy surface and non-adiabatic effect control the selectivity of the two α-C-C bond fissions, which provides a reasonable explanation why one α-C-C bond was observed as a primary channel and the other is ruled out even if CH3COCH2Cl is excited at 193 nm. The β-C-Cl fission is determined to be a dominant channel once the CH3COCH2Cl molecule is excited to the S1 state and the β-C-Cl:α-C-C branching ratio is estimated by the RRKM rate theory to be 15:1 at 193 nm, which is overestimated in comparison with the value of ∼11:1 inferred experimentally. The present calculation reveals that the α-C-C fission might take place in the ground electronic state as a result of the S1 → S0 internal conversion upon photolysis at 308 nm. However, the measured kinetic energy distributions of the α-C-C fission products suggest that the fission does not involve internal conversion to the ground state. To solve this issue, we need to perform non-adiabatic quantum dynamics simulation on accurate S0, S1, and S2 potential energy surfaces, which is still a challenging task currently. [ABSTRACT FROM AUTHOR]

In this study, the conformational preferences and photochemistry of acrylic acid (AA, CH2=CHCOOH) monomer isolated in cryogenic argon and krypton matrices were interpreted, based on results of quantum chemical calculations. Natural bond orbital analysis allowed to shed light on the main electronic effects determining the relative stability of the conformers of the molecule in the ground electronic state. The conformational isomerization taking place upon UV-irradiation of the matrix-isolated compound (λ ∼ 243 nm) was explained, based on theoretical complete active space self-consistent field/complete active space with second order perturbation theory (CASSCF/CASPT2) and time-dependent density functional theory (TD-DFT) results, allowing to rationalize the nearly equal populations of the two lowest energy conformers of the molecule observed in the photostationary state. Besides, details of the infrared spectra of the compound were reinterpreted based on the calculated spectra for the two most stable conformers of the molecule. In particular, the assignments for the out-of-plane A″ symmetry vibrations were revised. [ABSTRACT FROM AUTHOR]

In the present paper, different electronic structure methods have been used to determine stationary and intersection structures on the ground (S0) and 1ππ* (S2) states of 4-methylpyridine, which is followed by adiabatic and nonadiabatic dynamics simulations to explore the mechanistic photoisomerization of 4-methylpyridine. Photoisomerization starts from the S2(1ππ*) state and overcomes a small barrier, leading to formation of the prefulvene isomer in the S0 state via a S2/S0 conical intersection. The ultrafast S2 → S0 nonradiative decay and low quantum yield for the photoisomerization reaction were well reproduced by the combined electronic structure calculation and dynamics simulation. The prefulvene isomer was assigned as a long-lived intermediate and suggested to isomerize to 4-methylpyridine directly in the previous study, which is not supported by the present calculation. The nonadiabatic dynamics simulation and electronic structure calculation reveal that the prefulvene isomer is a short-lived intermediate and isomerizes to benzvalene form very easily. The benzvalene form was predicted as the stable isomer in the present study and is probably the long-lived intermediate observed experimentally. A consecutive light and thermal isomerization cycle via Dewar isomer was determined and this cycle mechanism is different from that reported in the previous study. It should be pointed out that formation of Dewar isomer from the S2(1ππ*) state is not in competition with the isomerization to the prefulvene form. The Dewar structure observed experimentally may originate from other excited states. [ABSTRACT FROM AUTHOR]

The potential energy surfaces for Cl2CO dissociation into CO+Cl+Cl in the lowest two electronic singlet states (S0 and S1) have been determined by the complete active space self-consistent field, coupled-cluster method with single and double excitations (CCSD), and equation-of-motion CCSD calculations, which are followed by direct ab initio molecular dynamics simulations to explore its photodissociation dynamics at 230 nm. It is found that the C–O stretching mode is initially excited upon irradiation and the excess internal energies are transferred to the C–Cl symmetric stretching mode within 200 fs. On average, the first and the second C–Cl bonds break completely within subsequent 60 and 100 fs, respectively. Electronic structure and dynamics calculations have thus provided a strong evidence that the photoinitiated dissociation of Cl2CO at 230 nm or shorter wavelengths is an ultrafast, adiabatic, and concerted three-body process. Since the two C–Cl bonds begin to break at the same time and the time interval between the two C–Cl bond broken fully is very short (∼40 fs), the photoinitiated dissociation of Cl2CO to CO+2Cl can be considered as the synchronous concerted process. [ABSTRACT FROM AUTHOR]

Ab initio calculations reveal that radiative lifetime of the lowest excited singlet state of 2-aminopyridine molecule should be around 20 ns, consistent with the molecules of the same type but is about one order of magnitude larger than the claimed experimental fluorescent lifetime in recent years. An S1/S0 conical intersection close to the S1 state has been located, which could be the possible nonradiative channel that is responsible for the fast decay observed in the experiment. [ABSTRACT FROM AUTHOR]

The potential energy surfaces for the BrCH[SUB2]COC1 dissociations into Br+CH[SUB2]COC1, BrCH[SUB2]CO +C1, and BrCH[SUB2]+COC1 in the S[SUB0], S[SUB1], and T[SUB1] states have been investigated at the complete-active-space self-consistent field, density functional theory, and multireference configuration interaction levels with the 6-31G* and cc-pVDZ basis sets, which provide some new insights into the mechanism of the BrCH[SUB2]COC1 photodissociation at 248 nm. It is found that the most probable pathway is the S[SUB1] C-C1 α and C-Br β bond fissions, which are a pair of competitive dissociation channels with some preference of the α C-C1 bond cleavage. The C-C α bond fission can take place along the S[SUB1] pathway upon photoexcitation ar 248 nm, but it is not in competition with the C-C1 α bond cleavage. These results are consistent with the experimental findings. The relative strength of the C-C and C-C1 α bonds is one of the factors that influences the selectivity of the α bond fissions. However, the selectivity is mainly determined by the mechanism of dissociation upon n→&pie;* excitation. The preference of the C-C1 α bond fission over the C-Br β bond cleavage was attributed to the nonadiabatic recrossing in previous studies. The present calculations predict that the distance dependence of intramolecular energy relaxation is another important factor that influences the selectivity of the α and β bond cleavages. [ABSTRACT FROM AUTHOR]

Potential energy surfaces of the HClCO dissociation to H+ClCO and Cl+HCO in the lowest three electronic states (S[sub 0], S[sub 1], and T[sub 1]) have been investigated with ab initio molecular orbital method at the levels of the complete active space self-consistent-field, single- and multi-reference MP2. The results show that the C–Cl bond cleavage on the S[sub 1] surface is the most probable mechanism for the HClCO photodissociation at 230–320 nm. In comparison with formyl fluoride (HFCO), a significant difference was found in photodissociation dynamics between HClCO and HFCO, and this has been discussed in detail. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report transient resonance Raman spectra and density functional theory calculations for the photoproducts produced after ultraviolet excitation of CHBr[sub 2]Cl and CCl[sub 3]Br in cyclohexane solution. Comparison of the computed vibrational frequencies to the experimental Raman frequencies revealed that the iso-CHBrCl-Br and iso-CHClBr-Br species are mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CHBr[sub 2]Cl. Similar comparisons for CCl[sub 3]Br showed the iso-CCl[sub 2]Cl-Br species is mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CCl[sub 3]Br. Additional density functional theory computations were done to examine the chemical reactions of iso-CH[sub 2]Br-Cl and iso-CH[sub 2]Cl-Br with ethylene to give cyclopropane and Br-Cl product. We briefly discuss the possibility for release of reactive halogens into the atmosphere via the photochemical and chemical reactions of iso-polyhalomethane molecules formed after ultraviolet excitation of polyhalomethanes in condensed phase environments. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]