Wavelength-Dependent Photodissociation of Benzoic Acid Monomer in α C−O Fission

In concert with the latest laser-induced fluorescence (LIF) experiment [Wei et al. J. Phys. Chem. A 2008, 112, 4727], we investigated the photodissociation mechanics of the benzoic acid monomer (BAM) with α C−O fission by means of state-of-the-art ab initio calculations. Complete active space self-consistent-field (CASSCF) and multireference CASSCF second-order perturbation theory (MSCASPT2) calculations were performed on the ground and a number of low-lying excited states of BAM. Our calculations indicated that α C−O fission from the S1 state is in competition with the fission from the T2 state upon the 266−284 nm wavelength photon. This differs from the conclusion of the previous theoretical investigation and clarified the vague experimental conclusion made earlier. According to our calculations, α C−O fission mainly occurs at the T2 state upon photoexcitation at 284−294 nm, and the photon with a wavelength longer than 294 nm is unable to present the α C−O fission. This conclusion agrees with the LIF experimental observation

Tuning the Nonradiative Electron–Hole Recombination with Defects in Monolayer Black Phosphorus

We use nonadiabatic (NA) molecular dynamics to demonstrate that the nonradiative electron–hole recombination is delayed and accelerated by the Stone-Wales (SWs) and phosphorus divacancy (DV-(5|7)) defects in monolayer black phosphorus (BP). Both types of defects increase the bandgap by 0.1 eV without creating midgap states. Driven by P–P stretching vibrations, the recombination proceeds within 1 ns in the SW and within 100 ps in the DV-(5|7), respectively, which occurs within 332 ps in BP. The SW defect slows down recombination because the notably reduced NA coupling combined with a large bandgap competes to the long-lived coherence. In contrast, the DV defect accelerates recombination since long-lived coherence is superior to the slightly decreased NA coupling correlated with a tiny increased bandgap. The diverse time scales rationalize the broad range of charge carrier lifetimes reported experimentally. The study provides a strategy to engineer excited-state dynamics for improving the BP-based optoelectronics

Great Influence of Pressure and Isotope Effects on Nonradiative Charge Loss in Hybrid Organic–Inorganic Perovskites

The intrinsic softness of hybrid organic–inorganic perovskites (HOIPs) allows their lattice and optoelectronic performance to be tunable to external pressure. Using nonadiabatic (NA) molecular dynamics, we demonstrate that a mild pressure accelerates hot electron relaxation and suppresses nonradiative electron–hole recombination in CH3NH3PbI3. Both processes are governed by NA coupling, which is enhanced between the electronic states of the quasi-continuous bands while is decreased between the band-edge states by reducing the electron–hole wave function overlap. Hydrogen/deuterium isotope exchange alleviates the pressure-induced NA coupling by increasing lattice rigidity and decreasing wave function overlap, slowing down both the hot electron relaxation and electron–hole recombination processes. The simulated time scales of sub-3 ps for hot electron relaxation and half nanoseconds for recombination agree well with the experiments. The study suggests that the isotope exchange can mitigate the pressure-caused fast losses of hot electrons and further prolong the charge carrier lifetime in HOIPs

Nonadiabatic Molecular Dynamics Simulation of Charge Separation and Recombination at a WS₂/QD Heterojunction

Two-dimensional transition metal dichalcogenides (TMDs), such as WS₂, are appealing candidates for optoelectronics and photovoltaics. The strong Coulomb interaction in TMDs is however known to prevent electron–hole pairs from dissociating into free electron and hole. The experiment demonstrates that combination of WS₂ and quantum dots (QDs) can achieve efficient charge separation and enhance photon-to-electron conversion efficiency. Using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics, we model electron and hole transfer dynamics at a WS₂/QD heterojunction. We demonstrate that both electron and hole transfer are ultrafast due to strong donor–acceptor coupling. The photoexcitation of the WS₂ leads to a 75 fs electron transfer, followed by a 0.45 eV loss within 90 fs. The photoexcitation of QD results in 240 fs hole transfer, but loses only 0.15 eV of energy within 1 ps. The strong charge–phonon coupling and a broad range of phonon modes involved in electron dynamics are responsible for the faster electron transfer than the hole transfer. The electron–hole recombination across the WS₂/QD interface occurs in several 100 ps, ensuing a long-lived charge-separated state. Particularly, the hole transfer is threefold magnitude faster than the electron–hole recombination inside QD, ensuing that QD can be an excellent light-harvester. The detailed atomistic insights into the photoinduced charge and energy dynamics at the WS₂/QD interface provide valuable guidelines for the optimization of solar light-harvesting and photovoltaic efficiency in modern nanoscale materials

The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radicals from NO₂ and H₂O

In the present work, we report a quantitative understanding on how to generate hydroxyl radicals from NO2 and H2O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory and density functional theory. The conical intersections dominate the nonadiabatic relaxation processes after NO2 irradiated at ∼410 nm in the troposphere and further control the generation of OH radical by means of hydrogen abstraction. In agreement with two-component fluorescence observed by laser techniques, there are two different photophysical relaxation channels along decreasing and increasing O−N−O angle of NO2. In the former case, the conical intersection between B̃2B1 and Ã2B2 (CI (2B2/2B1) first funnels NO2 out of the Franck−Condon region of B̃2B1 and relaxes to the Ã2B2 surface. Following the primary relaxation, the conical intersection between Ã2B2 and X̃2A1 (CI(2B2/2A1)) drives NO2 to decay into highly vibrationally excited X̃2A1 state that is more than 20 000 cm−1 above zeroth-order |n1,n2,n3 = 0⟩ vibrational level. In the latter case, increasing the O−N−O angle leads NO2 to relax to a minimum of B̃2B1 with a linear O−N−O arrangement. This minimum point is also funnel region between B̃2B1 and X̃2A1 (CI(2B1/2A1)) and leads NO2 to relax into a highly vibrationally excited X̃2A1 state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40−50 kcal/mol) from water vapor, producing OH (2Π3/2) radicals. The collision between NO2 and H2O molecules not only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersections. The faster internal conversion favors more energy transfer from electronically excited states into highly vibrationally excited X̃2A1 states. The collision (i.e., the heat motion of molecules) functions as the trigger and accelerator in the generation of OH radicals from NO2 and H2O in the troposphere

Depleted Oxygen Defect State Enhancing Tungsten Trioxide Photocatalysis: A Quantum Dynamics Perspective

Oxygen vacancies generally create midgap states in transition metal oxides, which are expected to decrease the photoelectrochemical water-splitting efficiency. Recent experiments defy this expectation but leave the mechanism unclear. Focusing on the photoanode WO3 as a prototypical system, we demonstrate using nonadiabatic molecular dynamics that an oxygen vacancy suppresses nonradiative electron–hole recombination, because the defect acts as an electron reservoir instead of a recombination center. The occupied midgap electrons prefer to be populated a priori compared to the band edge transition because of a larger transition dipole moment, converting to depleted/unoccupied trap states that rapidly accept conduction band electrons and then cause trap-assisted recombination by impeding the bandgap recombination regardless of oxygen vacancy configurations. The reported results provide a fundamental understanding of the “realistic” role of the oxygen vacancies and their influence on charge-phonon dynamics and carrier lifetime. The study generates valuable insights into the design of high-performance transition metal oxide photocatalysts

Stereoselective Excited-State Isomerization and Decay Paths in <i>cis</i>-Cyclobiazobenzene

Herein, we have employed OM2/MRCI-based full-dimensional nonadiabatic dynamics simulations to explore the photoisomerization and subsequent excited-state decay of a macrocyclic cyclobiazobenzene molecule. Two S1/S0 conical intersection structures are found to be responsible for the excited-state decay. Related to these two conical intersections, we found two stereoselective photoisomerization and excited-state decay pathways, which correspond to the clockwise and counterclockwise rotation motions with respect to the NN bond of the azo group. In both pathways, the excited-state isomerization is ultrafast and finishes within ca. 69 fs, but the clockwise isomerization channel is much more favorable than the counterclockwise one with a ratio of 74% versus 26%. Importantly, the present work demonstrates that stereoselective pathways exist not only in the photoisomerization of isolated azobenzene (AB)-like systems but also in macrocyclic systems with multiple ABs. This finding could provide useful insights for understanding and controlling the photodynamics of macrocyclic nanostructures with AB units as the main building units

Excited-State Ring-Opening Mechanism of Cyclic Ketones: A MS-CASPT2//CASSCF Study

We have employed complete active space self-consistent field (CASSCF) and its second-order perturbation (MS-CASPT2) methods to study the S₁ and T₁ excited-state ring-opening mechanisms and S₁ excited-state deactivation channels of cyclopropanone, cyclobutanone, cyclopentanone, and cyclohexanone. On the basis of optimized minima, transition states, conical intersections, refined energies, and relaxed two-dimensional S₁ and T₁ potential energy surfaces, we find that, with the ring-strain decrease from cyclopropanone to cyclohexanone, (1) the ring-opening S₁ and T₁ barrier increases from 0.0 and 0.0 to 19.7 and 10.4 kcal/mol, respectively; (2) the electronic state responsible for the dominant ring-opening reaction varies from the S₁ state of cyclopropanone to the T₁ state of cyclopentanone and cyclohexanone; and (3) the S₁ ring opening gradually becomes inefficient even blocked in cyclopentanone and cyclohexanone. This work shows that these dissimilar excited-state dynamics could originate from different ring strain of small cyclic ketones

Mechanistic Photochemistry of Methyl-4-hydroxycinnamate Chromophore and Its One-Water Complexes: Insights from MS-CASPT2 Study

Herein we computationally studied the excited-state properties and decay dynamics of methyl-4-hydroxycinnamate (OMpCA) in the lowest three electronic states, that is, ¹<i>ππ</i>*, ¹<i>nπ</i>*, and S₀ using combined MS-CASPT2 and CASSCF electronic structure methods. We found that one-water hydration can significantly stabilize and destabilize the vertical excitation energies of the spectroscopically bright ¹<i>ππ</i>* and dark ¹<i>nπ</i>* excited singlet states, respectively; in contrast, it has a much smaller effect on the ¹<i>ππ</i>* and ¹<i>nπ</i>* adiabatic excitation energies. Mechanistically, we located two ¹<i>ππ</i>* excited-state relaxation channels. One is the internal conversion to the dark ¹<i>nπ</i>* state, and the other is the ¹<i>ππ</i>* photoisomerization that eventually leads the system to a ¹<i>ππ</i>*/S₀ conical intersection region, near which the radiationless internal conversion to the S₀ state occurs. These two ¹<i>ππ</i>* relaxation pathways play distinct roles in OMpCA and its two one-water complexes (OMpCA-W1 and OMpCA-W2). In OMpCA, the predominant ¹<i>ππ</i>* decay route is the state-switching to the dark ¹<i>nπ</i>* state, while in one-water complexes, the importance of the ¹<i>ππ</i>* photoisomerization is significantly enhanced because the internal conversion to the ¹<i>nπ</i>* state is heavily suppressed due to the one-water hydration