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Biogenic hydrocarbons are emitted into the Earth's atmosphere by terrestrial vegetation as by‐products of photosynthesis. Isoprene is one such hydrocarbon and is the second most abundant volatile organic compound emitted into the atmosphere (after methane). Reaction with ozone represents an important atmospheric sink for isoprene removal, forming carbonyl oxides (Criegee intermediates) with extended conjugation. In this manuscript, we argue that the extended conjugation of these Criegee intermediates enables electronic excitation of these compounds, on timescales that are competitive with their slow unimolecular decay and bimolecular chemistry. We show that the complexation of methacrolein oxide with water enhances the absorption cross section of the otherwise dark S1 state, potentially revealing a new avenue for forming lower volatility compounds via tropospherically relevant photochemistry. [ABSTRACT FROM AUTHOR]

Direct excitation of aromatic compounds grants access to high‐energy intermediates that can be utilised in organic synthesis. Understanding and predicting the substituent effects at the excited state for aromatic molecules remains challenging for the synthetic photochemist. In this work, we present an experimental and computational investigation of the excited state of the isomeric chloroanilines, which promptly react by losing the chloride when the amino group is in para position, but are non‐reactive and non‐emissive in the meta and ortho isomers. XMS‐CASPT2//CASSCF computations explain this apparent contradiction of the meta‐ortho selectivity rule of Zimmerman, which originates from the substituent effects lowering to a different extent the barrier to populate the prefulvenic conical intersection that deactivates non‐radiatively the singlet excited state of the chloroanilines. [ABSTRACT FROM AUTHOR]

SignificanceIn a theoretical study, we present an ultrafast technique for probing time-dependent molecular charge densities. An ultrafast optical pump first brings the molecule into an electronic nonstationary state. This is followed by coherent inelastic scattering of a broadband single-electron probe pulse with a variable delay T, which is detected spectrally. The technique is applied to reveal phase-sensitive background-free coherent electron beating in the conical intersection passage in uracil and reveals the otherwise elusive coherent beating of strongly coupled electrons and nuclei.

X-ray diffraction is routinely used for structure determination of stationary molecular samples. Modern X-ray photon sources, e.g., from free-electron lasers, enable us to add temporal resolution to these scattering events, thereby providing a movie of atomic motions. We simulate and decipher the various contributions to the X-ray diffraction pattern for the femtosecond isomerization of azobenzene, a textbook photochemical process. A wealth of information is encoded besides real-time monitoring of the molecular charge density for the cis to trans isomerization. In particular, vibronic coherences emerge at the conical intersection, contributing to the total diffraction signal by mixed elastic and inelastic photon scattering. They cause distinct phase modulations in momentum space, which directly reflect the real-space phase modulation of the electronic transition density during the nonadiabatic passage. To overcome the masking by the intense elastic scattering contributions from the electronic populations in the total diffraction signal, we discuss how this information can be retrieved, e.g., by employing very hard X-rays to record large scattering momentum transfers.

Conical intersections in two-state systems require a coordinate-dependent coupling. This paper identifies and investigates conical intersections in cyclic tight-binding system-bath Hamiltonians with an odd number of sites and a constant site-to-site coupling. In the absence of bath degrees of freedom, such tight-binding systems with a positive coupling parameter exhibit electronic frustration and a doubly-degenerate ground state. When these systems interact with a harmonic bath, the degeneracy becomes a conical intersection between the adiabatic ground and first excited states. Under weak system-bath coupling, overlapping wavefunctions associated with different sites give rise to distinct pathways with interfering geometric phases, which lead to considerably slower transfer dynamics. The effect is most pronounced in the presence of low-temperature dissipative baths characterized by a continuous spectral density. It is found that the transfer dynamics and equilibration time of a cyclic dissipative three-site system with a positive coupling exceeds that of a similar three-site system with a negative coupling, as well as that of cyclic four-site systems, by an order of magnitude. [ABSTRACT FROM AUTHOR]

The rates and outcomes of virtually all photophysical and photochemical processes are determined by conical intersections. These are regions of degeneracy between electronic states on the nuclear landscape of molecules where electrons and nuclei evolve on comparable timescales and thus become strongly coupled, enabling radiationless relaxation channels upon optical excitation. Due to their ultrafast nature and vast complexity, monitoring conical intersections experimentally is an open challenge. We present a simulation study on the ultrafast photorelaxation of uracil, based on a quantum description of the nuclei. We demonstrate an additional window into conical intersections obtained by recording the transient wavepacket coherence during this passage with an X-ray free-electron laser pulse. Two major findings are reported. First, we find that the vibronic coherence at the conical intersection lives for several hundred femtoseconds and can be measured during this entire time. Second, the time-dependent energy-splitting landscape of the participating vibrational and electronic states is directly extracted from Wigner spectrograms of the signal. These offer a physical picture of the quantum conical intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket in the vicinity of the conical intersection is directly mapped by the proposed experiment.

Light-driven phenomena in organic molecular aggregates underpin several mechanisms relevant to optoelectronic applications. Modeling these processes is essential for aiding the design of new materials and optimizing optoelectronic devices. In this review, we cover the use of different atomistic models, excited-state dynamics, and transport approaches for understanding light-activated phenomena in molecular aggregates, including radiative and nonradiative decay pathways. We consider both intra- and intermolecular mechanisms and focus on the role of conical intersections as facilitators of internal conversion. We explore the use of the exciton models for Frenkel and charge transfer states and the electronic structure methods and algorithms commonly applied for excited-state dynamics. Throughout the review, we analyze the approximations employed for the simulation of internal conversion, intersystem crossing, and reverse intersystem crossing rates and analyze the molecular processes behind single fission, triplet-triplet annihilation, Dexter energy transfer, and Förster energy transfer. [ABSTRACT FROM AUTHOR]

Elementary events that determine photochemical outcomes and molecular functionalities happen on the femtosecond and subfemtosecond timescales. Among the most ubiquitous events are the nonadiabatic dynamics taking place at conical intersections. These facilitate ultrafast, nonradiative transitions between electronic states in molecules that can outcompete slower relaxation mechanisms such as fluorescence. The rise of ultrafast X-ray sources, which provide intense light pulses with ever-shorter durations and larger observation bandwidths, has fundamentally revolutionized our spectroscopic capabilities to detect conical intersections. Recent theoretical studies have demonstrated an entirely new signature emerging once a molecule traverses a conical intersection, giving detailed insights into the coupled nuclear and electronic motions that underlie, facilitate, and ultimately determine the ultrafast molecular dynamics. Following a summary of current sources and experiments, we survey these techniques and provide a unified overview of their capabilities. We discuss their potential to dramatically increase our understanding of ultrafast photochemistry. [ABSTRACT FROM AUTHOR]

We demonstrate how the wavelet transform, which is a powerful tool for compression, filtering, and scaling analysis of signals, may be used to separate large- and short-scale electron density features in X-ray diffraction patterns. Wavelets can isolate the electron density associated with delocalized bonds from the much stronger background of highly localized core electrons. The wavelet-processed signals clearly reveal the bond formation and breaking in the early steps of the photoinduced pericyclic ring opening reaction of 1,3-cyclohexadiene, which are not resolved in the bare signal.

In this work, we consider symmetric positive definite pencils depending on two parameters. That is, we are concerned with the generalized eigenvalue problem A (x) − λ B (x) v = 0 , where A and B are symmetric matrix valued functions in ℝ n × n , smoothly depending on parameters x ∈ Ω ⊂ ℝ 2 ; furthermore, B is also positive definite. In general, the eigenvalues of this multiparameter problem will not be smooth, the lack of smoothness resulting from eigenvalues being equal at some parameter values (conical intersections). Our main goal is precisely that of locating parameter values where eigenvalues are equal. We first give general theoretical results for the present generalized eigenvalue problem, and then introduce and implement numerical methods apt at detecting conical intersections. Finally, we perform a numerical study of the statistical properties of coalescing eigenvalues for pencils where A and B are either full or banded, for several bandwidths. [ABSTRACT FROM AUTHOR]

We report the topological properties, in terms of the Berry phase, of the 2D noninteracting system with electron–hole band inversion, described by the two-band generalized analogue of the low-energy Bernevig–Hughes–Zhang Hamiltonian, yielding the W-shaped energy bands in the form of two intersecting cones with the gap along the closed continuous loop. We identify the range of parameters where the Berry phase attains qualitatively different values: (a) the integer multiplier of 2 π , (b) the integer multiplier of π , and (c) the nontrivial value between the latter two, which depends on the system parameters. The system thus exhibits the anomalous quantum Hall effect associated with the nontrivial geometric phase, which is presumably tunable through the choice of parameters at hand. [ABSTRACT FROM AUTHOR]

The development of novel coherent and brilliant sources, such as soft X-ray free electron laser (FEL) and high harmonic generation (HHG), enables new ultrafast analysis of the electronic and structural dynamics of a wide variety of materials. Soft X-ray FEL delivers high-brilliance beams with a short pulse duration, high spatial coherence and photon energy tunability. In comparison with FELs, HHG X-ray sources are characterized by a wide spectral bandwidth and few- to sub-femtosecond pulses. The approach will lead to the time-resolved reconstruction of molecular dynamics, shedding light on different photochemical pathways. The high peak brilliance of soft X-ray FELs facilitates investigations in a nonlinear regime, while the broader spectral bandwidth of the HHG sources may provide the simultaneous probing of multiple components. Significant technical breakthroughs in these novel sources are under way to improve brilliance, pulse duration, and to control spectral bandwidth, spot size, and energy resolution. Therefore, in the next few years, the new generation of soft X-ray sources combined with novel experimental techniques, new detectors, and computing capabilities will allow for the study of several extremely fast dynamics, such as vibronic dynamics. In the present review, we discuss recent developments in experiments, performed with soft X-ray FELs and HHG sources, operating near the carbon K-absorption edge, being a key atomic component in biosystems and soft materials. Different spectroscopy methods such as time-resolved pump-probe techniques, nonlinear spectroscopies and photoelectron spectroscopy studies have been addressed in an attempt to better understand fundamental physico-chemical processes. [ABSTRACT FROM AUTHOR]

The photochemistry of pyruvic acid has received a large attention due to its relevance to atmospheric chemistry. Pyruvic acid is produced in the troposphere from both biogenic and anthropogenic sources and is a prototypical model for the family of α ${\alpha }$ ‐dicarbonyls. What makes the photochemistry of pyruvic acid particularly interesting from a gas‐phase perceptive is its expected decarboxylation upon sunlight absorption. The exact photodynamics leading to this release of CO2 remains elusive. In this work, we used a combination of excited‐ and ground‐state ab initio molecular dynamics to unravel the possible mechanisms leading to the decarboxylation of pyruvic acid. Our calculations highlight the importance of a proton‐coupled electron transfer mechanism taking place in the first excited electronic state and triggering a nonadiabatic transfer of the molecule to the ground electronic state. The decarboxylation takes place in the ground‐electronic state with the concomitant formation of methylhydroxycarbene. We also calculate the photoabsorption cross‐section and wavelength‐dependent quantum yields for pyruvic acid, highlighting the limits of our theoretical formalism. [ABSTRACT FROM AUTHOR]

Triplet‐triplet annihilation (TTA) is a spin‐allowed conversion of two triplet states into one singlet excited state, which provides an efficient route to generate a photon of higher frequency than the incident light. Multiple energy transfer steps between absorbing (sensitizer) and emitting (annihilator) molecular species are involved in the TTA based photon upconversion process. TTA compounds have recently been studied for solar energy applications, even though the maximum upconversion efficiency of 50 % is yet to be achieved. With the aid of quantum calculations and based on a few key requirements, several design principles have been established to develop the well‐functioning annihilators. However, a complete molecular level understanding of triplet fusion dynamics is still missing. In this work, we have employed multi‐reference electronic structure methods along with quantum dynamics to obtain a detailed and fundamental understanding of TTA mechanism in naphthalene. Our results suggest that the TTA process in naphthalene is mediated by conical intersections. In addition, we have explored the triplet fusion dynamics under the influence of strong light‐matter coupling and found an increase of the TTA based upconversion efficiency. [ABSTRACT FROM AUTHOR]

Light-induced nonadiabatic phenomena arise when molecules or molecular ensembles are exposed to resonant external electromagnetic fields. The latter can either be classical laser or quantized cavity radiation fields, which can couple to either the electronic, nuclear or rotational degrees of freedom of the molecule. In the case of quantized radiation fields, the lightâ€"matter coupling results in the formation of two new hybrid lightâ€"matter states, namely the upper and lower ‘polaritons’. Light-induced avoided crossings and light-induced conical intersections (CIs) between polaritons exist as a function of the vibrational and rotational coordinates of single molecules. For ensembles of N molecules, the N âˆ' 1 dark states between the two optically active polaritons feature, additionally, so-called collective CIs, involving the coordinates of more than one molecule to form. Here, we study the competition between intramolecular and collective light-induced nonadiabatic phenomena by comparing the escape rate from the Franckâ€"Condon region of a single molecule and of a molecular ensemble coupled to a cavity mode. In situations where the polaritonic gap would be large and the dark-state decay channels could not be reached effectively, the presence of a seam of light-induced CI between the polaritons facilitates again the participation of the dark manifold, resulting in a cooperative effect that determines the overall non-radiative decay rate from the upper into the lower polaritonic states. [ABSTRACT FROM AUTHOR]

We explore the excited-state intramolecular proton transfer process of quinophthalone theoretically. This molecule possesses three low-lying singlet excited states ( S 1 , S 2 and S 3 ) in a narrow energy gap of less than the N–H stretching frequency. Dynamics simulations show nonadiabatic wavepacket transfer to S 2 and S 3 upon initiating the wavepacket on S 1 . Multiple accessible conical intersections that lie in the Franck–Condon region facilitate the nonadiabatic wavepacket transfer. Nuclear densities associated with the proton transfer promoting vibrations would start accumulating on S 2 and S 3 within a few tens of femtoseconds, validating the involvement of these vibrations in the nonadiabatic events that occur before the proton transfer process. Our findings emphasize the necessity of refined kinetic models for assigning the time constants of ultrafast transient spectroscopy measurements due to the simultaneous evolution of nonadiabatic events and proton transfer kinetics in quinophthalone. [ABSTRACT FROM AUTHOR]

Carbonyl oxides, or Criegee intermediates, play an important role in many physicochemical processes occurring in the Earth's atmosphere. Criegee intermediates are chemically active compounds that easily react with other atmospheric components, promoting the formation of OH and CH3 radicals, toxic compounds of nitrogen, and various acids. Traditionally, the literature considers reactions involving only those carbonyl oxides that are the most stable in the troposphere under the standard atmospheric conditions. In this study, it is shown that in the mesosphere and ionosphere, where the total concentration of molecules is low and the intensity of UV radiation and the number of free electrons are high, reactions involving electronically excited states of the Criegee intermediates CH3CHOO play a significant role. In this case, we should take into account the features of the decomposition of all isomers of the CH3CHOO molecule. [ABSTRACT FROM AUTHOR]

The choice of the electronic representation in on-the-fly quantum dynamics is crucial. The adiabatic representation is appealing since adiabatic states are readily available from quantum chemistry packages. The nuclear wavepackets are then expanded in a basis of Gaussian functions, which follow trajectories to explore the potential energy surfaces and approximate the potential using a local expansion of the adiabatic quantities. Nevertheless, the adiabatic representation is plagued with severe limitations when conical intersections are involved: the diagonal Born–Oppenheimer corrections (DBOCs) are non-integrable, and the geometric phase effect on the nuclear wavepackets cannot be accounted for unless a model is available. To circumvent these difficulties, the moving crude adiabatic (MCA) representation was proposed and successfully tested in low energy dynamics where the wavepacket skirts the conical intersection. We assess the MCA representation in the case of non-adiabatic transitions through conical intersections. First, we show that using a Gaussian basis in the adiabatic representation indeed exhibits the aforementioned difficulties with a special emphasis on the possibility to regularize the DBOC terms. Then, we show that MCA is indeed able to properly model non-adiabatic transitions. Tests are done on linear vibronic coupling models for the bis(methylene) adamantyl cation and the butatriene cation. This article is part of the theme issue 'Chemistry without the Born–Oppenheimer approximation'. [ABSTRACT FROM AUTHOR]

Non-adiabatic couplings between Born–Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating 'reactive' outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase H2O molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products. This article is part of the theme issue 'Chemistry without the Born–Oppenheimer approximation'. [ABSTRACT FROM AUTHOR]

The dynamics of the dimethyl methylphosphonate (DMMP) radical cation after production by strong field adiabatic ionization have been investigated. Pump-probe experiments using strong field 1300 nm pulses to adiabatically ionize DMMP and a 800 nm non-ionizing probe induce coherent oscillations of the parent ion yield with a period of about 45 fs. The yields of two fragments, PO2C2H7+ and PO2CH4+, oscillate approximately out of phase with the parent ion, but with a slight phase shift relative to each other. We use electronic structure theory and nonadiabatic surface hopping dynamics to understand the underlying dynamics. The results show that while the cation oscillates on the ground state along the P=O bond stretch coordinate, the probe excites population to higher electronic states that can lead to fragments PO2C2H7+ and PO2CH4+. The computational results combined with the experimental observations indicate that the two conformers of DMMP that are populated under experimental conditions exhibit different dynamics after being excited to the higher electronic states of the cation leading to different dissociation products. These results highlight the potential usefulness of these pump-probe measurements as a tool to study conformer-specific dynamics in molecules of biological interest. [ABSTRACT FROM AUTHOR]

Many chemical reactions exhibit nonadiabatic effects as a consequence of coupling between electronic states and/or interaction with light. While a fully quantum description of nonadiabatic reactions is unfeasible for most realistic molecules, a more computationally tractable approach is to combine a classical description of the nuclei with a quantum description of the electronic states. Combining the formalisms of quantum and classical dynamics is however a difficult problem for which standard methods (such as Ehrenfest dynamics and surface hopping) may be insufficient. In this article, we review a new trajectory-based approach developed in our group that is able to describe nonadiabatic dynamics with a higher accuracy than previous approaches but for a similar level of computational effort. This method treats the electronic states with a phase-space representation for discrete-level systems, which in the two-level case is analogous to a spin-½. We point out the key features of the method and demonstrate its use in a variety of applications, including ultrafast transfer through conical intersections, damped coherent excitation under coupling to a strong light field, and nonlinear spectroscopy of light-harvesting complexes.

The crystal structure of 5-azacytosine was proposed by the X-ray diffraction pattern of the compound and the performed Rietveld analysis. The crystal structure solutions were achieved with the simulated annealing procedure. The model molecule for the procedure was taken from the BLYP/aug-cc-pVDZ optimization of the N1(H) oxo-amino tautomer of the compound. The refined unit cell parameters are: a = 12.82916 Å, b = 9.47282 Å, c = 3.66363 Å, and space group P212121. The studied excited-state reaction paths of the H-detachment processes of two oxo-amino tautomers show a possibility for mutual phototransformation of the two oxo-amino tautomers: N1(H) and N5(H). The driven states of this photoreaction are the 1nσ* and 1πσ* excited states. [ABSTRACT FROM AUTHOR]

The compound 6,6'-dibromoindigo, which is the main component of the pigment Tyrian purple, was studied theoretically at the TDDFT level of theory. We found that the absorption maximum of the compound about 600 nm underwent a red-shift when moving from the gas phase in solution. With the located two conical intersections S0/S0 we investigated (TD-DFT) the mechanisms connecting these structures with the ground state equilibrium geometry. It was established that the conical intersections are not accessible along any of the excited-state reaction paths which implies optical deactivation of the first excited state of the compound. With respect to these mechanisms the pigment should exhibit high photostability when exposed to visible light. [ABSTRACT FROM AUTHOR]

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s–3p excitations of Al+ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1. On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH+(H2O)n‐1 and are transparent in the investigated energy range. For n=4–8, a mixture of Al+(H2O)n and HAlOH+(H2O)n‐1 is present in the experiment. [ABSTRACT FROM AUTHOR]

The development of novel coherent and brilliant sources, such as soft X-ray free electron laser (FEL) and high harmonic generation (HHG), enables new ultrafast analysis of the electronic and structural dynamics of a wide variety of materials. Soft X-ray FEL delivers high-brilliance beams with a short pulse duration, high spatial coherence and photon energy tunability. In comparison with FELs, HHG X-ray sources are characterized by a wide spectral bandwidth and few- to sub-femtosecond pulses. The approach will lead to the time-resolved reconstruction of molecular dynamics, shedding light on different photochemical pathways. The high peak brilliance of soft X-ray FELs facilitates investigations in a nonlinear regime, while the broader spectral bandwidth of the HHG sources may provide the simultaneous probing of multiple components. Significant technical breakthroughs in these novel sources are under way to improve brilliance, pulse duration, and to control spectral bandwidth, spot size, and energy resolution. Therefore, in the next few years, the new generation of soft X-ray sources combined with novel experimental techniques, new detectors, and computing capabilities will allow for the study of several extremely fast dynamics, such as vibronic dynamics. In the present review, we discuss recent developments in experiments, performed with soft X-ray FELs and HHG sources, operating near the carbon K-absorption edge, being a key atomic component in biosystems and soft materials. Different spectroscopy methods such as time-resolved pump-probe techniques, nonlinear spectroscopies and photoelectron spectroscopy studies have been addressed in an attempt to better understand fundamental physico-chemical processes.

Electronic spectroscopy of DNA bases in the gas phase provides detailed information about the electronic excitation, which places the molecule in the Franck-Condon region in the excited state and thus prepares the starting conditions for excited-state dynamics. Double resonance or hole-burning spectroscopy in the gas phase can provide such information with isomer specificity, probing the starting potential energy landscape as a function of tautomeric form, isomeric structure, or hydrogen bonded or stacked cluster structure. Action spectroscopy, such REMPI, can be affected by excited-state lifetimes.

The article titled "Front Cover: Meta-Ortho Effect on the Excited State Pathways of Chloroanilines" explores the rates at which different isomeric chloroanilines can reach their respective conical intersections for non-radiative decay. The front cover of the journal showcases the isomers as light-fueled hovering vehicles navigating a potential energy surface landscape. The research article, authored by S. Crespi et al., provides more detailed information on the topic. [Extracted from the article]

Electronic spectroscopy of DNA bases in the gas phase provides detailed information about the electronic excitation, which places the molecule in the Franck-Condon region in the excited state and thus prepares the starting conditions for excited-state dynamics. Double resonance or hole-burning spectroscopy in the gas phase can provide such information with isomer specificity, probing the starting potential energy landscape as a function of tautomeric form, isomeric structure, or hydrogen bonded or stacked cluster structure. Action spectroscopy, such REMPI, can be affected by excited-state lifetimes.

The imino tautomers of isocytosine were objects of investigation at the TD-DFT level of theory - TD BLYP/6-311++G(d,p). We studied the mechanisms of the H1-N detachment in these tautomers through excited-state reaction paths. It was proposed that these transformations occur through the 1ps* excited-state reaction paths of the imino tautomers. The mechanisms involve dissociations of the N1-H bonds in the tautomers and lead to crossings between the reaction paths of 1ps* and S0 electronic states. One can suppose that such processes would facilitate the tautomerizations of the imino tautomers if further mechanisms have been found. [ABSTRACT FROM AUTHOR]

The dynamics of the dimethyl methylphosphonate (DMMP) radical cation after production by strong field adiabatic ionization have been investigated. Pump-probe experiments using strong field 1300 nm pulses to adiabatically ionize DMMP and a 800 nm non-ionizing probe induce coherent oscillations of the parent ion yield with a period of about 45 fs. The yields of two fragments, PO2C2H7+ and PO2CH4+, oscillate approximately out of phase with the parent ion, but with a slight phase shift relative to each other. We use electronic structure theory and nonadiabatic surface hopping dynamics to understand the underlying dynamics. The results show that while the cation oscillates on the ground state along the P=O bond stretch coordinate, the probe excites population to higher electronic states that can lead to fragments PO2C2H7+ and PO2CH4+. The computational results combined with the experimental observations indicate that the two conformers of DMMP that are populated under experimental conditions exhibit different dynamics after being excited to the higher electronic states of the cation leading to different dissociation products. These results highlight the potential usefulness of these pump-probe measurements as a tool to study conformer-specific dynamics in molecules of biological interest.

Light-induced nonadiabatic phenomena arise when molecules or molecular ensembles are exposed to resonant external electromagnetic fields. The latter can either be classical laser or quantized cavity radiation fields, which can couple to either the electronic, nuclear or rotational degrees of freedom of the molecule. In the case of quantized radiation fields, the light–matter coupling results in the formation of two new hybrid light–matter states, namely the upper and lower ‘polaritons’. Light-induced avoided crossings and light-induced conical intersections (CIs) between polaritons exist as a function of the vibrational and rotational coordinates of single molecules. For ensembles of N molecules, the N − 1 dark states between the two optically active polaritons feature, additionally, so-called collective CIs, involving the coordinates of more than one molecule to form. Here, we study the competition between intramolecular and collective light-induced nonadiabatic phenomena by comparing the escape rate from the Franck–Condon region of a single molecule and of a molecular ensemble coupled to a cavity mode. In situations where the polaritonic gap would be large and the dark-state decay channels could not be reached effectively, the presence of a seam of light-induced CI between the polaritons facilitates again the participation of the dark manifold, resulting in a cooperative effect that determines the overall non-radiative decay rate from the upper into the lower polaritonic states.

Thymine photochemistry is important for understanding DNA photodamage. In the gas phase, thymine undergoes a fast non-radiative decay from S2 to S1. In the S1 state, it gets trapped for several picoseconds until returning to the ground-state S0. Here, we explore the electrostatic effects of nanomeric droplets of methanol and water on the excited states of thymine. For this purpose, we develop and implement an electrostatic embedding TD-DFT/MM method based on a QM/MM coupling defined through electrostatic potential fitting charges. We show that both in methanol and water, the mechanism is similar to the gas phase. The solvent molecules participate in defining the branching plane of S0/S1 intersection and have a negligible effect on the S1/S2 intersection. Despite the wrong topology of the ground/excited state intersections, electrostatic embedding TD-DFT/MM allows for a fast exploration of the potential energy surfaces and a qualitative picture of the photophysics of thymine in solvent droplets.

The mechanism of aggregation‐induced emission, which overcomes the common aggregation‐caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward–Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications. [ABSTRACT FROM AUTHOR]

It remains highly challenging to identify small molecule‐based photothermal agents with a high photothermal conversion efficiency (PTCE). Herein, we adopt a double bond‐based molecular motor concept to develop a new class of small photothermal agents to break the current design bottleneck. As the double‐bond is twisted by strong twisted intramolecular charge transfer (TICT) upon irradiation, the excited agents can deactivate non‐radiatively through the conical intersection (CI) of internal conversion, which is called photoinduced nonadiabatic decay. Such agents possess a high PTCE of 90.0 %, facilitating low‐temperature photothermal therapy in the presence of a heat shock protein 70 inhibitor. In addition, the behavior and mechanism of NIR laser‐triggered molecular motions for generating heat through the CI pathway have been further understood through theoretical and experimental evidence, providing a design principle for highly efficient photothermal and photoacoustic agents. [ABSTRACT FROM AUTHOR]

Within three functionals (TD‐B3LYP, TD‐BHandHLYP, and TD‐CAM‐B3LYP) in combination with four basis sets (3‐21g, 6‐31g, 6‐31g(d), and cc‐pvdz), global switching (GS) trajectory surface hopping molecular dynamics has been performed for cis‐to‐trans azobenzene photoisomerization up to the S1(nπ*) excitation. Although all the combinations show artificial double‐cone structure of conical intersection between ground and first excited states, simulated quantum yields and lifetimes are in good agreement with one another; 0.6 (±5%) and 40.5 fs (±10%) by TD‐B3LYP, 0.5 (±10%) and 35.5 fs (±4%) by TD‐BHandHLYP, and 0.44 (±9%) and 35.2 fs (±10%) by TD‐CAM‐B3LYP. By analyzing distributions of excited‐state population decays, hopping spots, and typical trajectories with performance of 12 functional/basis set combinations, it has been concluded that functional dependence for given basis set is slightly more sensitive than basis set dependence for given functional. The present GS on‐the‐fly time‐dependent density functional theory (TDDFT) trajectory surface hopping simulation can provide practical benchmark guidelines for conical intersection driven excited‐state molecular dynamics simulation involving in large complex system within ordinary TDDFT framework. © 2019 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Unlike carbonyl compounds, it has long been common understanding that excited imines show virtually no photoreactivity, and hence their properties and potential utility in chemical science remain largely unexplored. Now, a strategy is presented for eliciting latent photoreactivity of imines based on the introduction of a donor–acceptor (D‐A) structure to extend the lifetime of their photoexcited states. A series of spectroscopic analyses and density functional theory calculations reveal unique photophysical properties of the D‐A‐type imines. Furthermore, the reactivity of the D‐A‐type imines is demonstrated by using them as a photoredox catalyst for atom‐transfer radical addition. These findings illuminate a previously neglected chemical space in the field of photochemistry, which will be exploited by taking advantage of the inherent structural modularity of imines. [ABSTRACT FROM AUTHOR]

Photoinduced processes in thiouracil derivatives have lately attracted considerable attention due to their suitability for innovative biological and pharmacological applications. Here, sub‐20 fs broadband transient absorption spectroscopy in the near‐UV are combined with CASPT2/MM decay path calculations to unravel the excited‐state decay channels of water solvated 2‐thio and 2,4‐dithiouracil. These molecules feature linear absorption spectra with overlapping ππ* bands, leading to parallel decay routes which we systematically track for the first time. The results reveal that different processes lead to the triplet states population, both directly from the ππ* absorbing state and via the intermediate nπ* dark state. Moreover, the 2,4‐dithiouracil decay pathways is shown to be strongly correlated either to those of 2‐ or 4‐thiouracil, depending on the sulfur atom on which the electronic transition localizes. [ABSTRACT FROM AUTHOR]

Basic knowledge of mankind so far relates to the description of electrons and atoms in the material in a state of equilibrium, where the behavior changes slowly over time. The electron diffraction with a high temporal and space resolution has opened the possibility of direct observation of the processes occurring in the transient state of the substance (molecular movie). Here it is necessary to provide a temporary resolution of the order of 100 fs, which corresponds to the transition of the system through the energy barrier of the potential surface, which describes the chemical reaction - the process of the breaking and the formation of new bonds between the interacting agents. Thus, the possibility of the investigation of the coherent nuclear dynamics of molecular systems and the condensed matter can be opened. In the past two decades, it has been possible to observe the nuclear motion in the temporal interval corresponding to the period of the nuclear oscillation. The observed coherent changes in the nuclear system at such temporal intervals determine the fundamental shift from the standard kinetics of chemical reactions to the dynamics of the phase trajectory of a single molecule, the molecular quantum state tomography.

Donor‐acceptor Stenhouse adducts (DASAs) have emerged in the last years as novel reversible photoswitches characterized by the light‐induced interconversion between a linear open‐chain isomer and a compact closed‐ring isomer. Despite the considerable interest for potential applications (due to their changes in size, color and polarity), several steps of the photoswitching mechanism are still not completely understood, hence limiting the rational design of these compounds. Herein we propose a complete computational study of the switching mechanism by means of TD‐DFT and CASPT2//CASSCF calculations. Special attention is paid to the excited state pathway, leading to a previously unknown general scenario with common features for different DASAs: After light absorption, a rotational energy barrier needs to be overcome in order to reach a conical intersection region with the ground state, whose topology and relative energy is investigated based on different solvents and acceptor moieties. Then, the evolution of the compounds in the ground state is considered by calculating different minima until the formation of the final closed form. Finally, the eventual reverse thermal path was also considered. We offer herein a complete mechanistic description which allows for an overall comparison with available experimental results. [ABSTRACT FROM AUTHOR]

Unrestricted DFT (UDFT), time‐dependent DFT (TDDFT) and spin‐flip TDDFT (SF‐TDDFT) were used to investigate the potential energy surfaces of the ground and first two electronic excited states of the naphthalene, pyrene and perylene radical cations. In particular, conical intersections (which play a central role in the photophysics of these cations) were located with these DFT‐based methods. The results are consistent with accurate multiconfigurational wavefunction‐based ab initio methods. These show that naphthalene and pyrene cations can quickly relax nonradiatively from their excited states back down to the original ground state species through easily accessible conical intersections, but the perylene cation cannot do so, due to the absence of any accessible funnels between the lowest excited state and the ground state, leaving radiative decay as the most probable photophysical pathway. This study paves the way for using computationally efficient density functional theory (DFT)‐based methods in future investigations of the photophysics of much larger polycyclic aromatic hydrocarbons, for which multiconfigurational wavefunction‐based methods become prohibitively expensive. [ABSTRACT FROM AUTHOR]

Optical absorption and emission properties as well as fluorescence quenching via introduction of an additional unsaturated five‐membered ring in some azaacenes are investigated computationally. It is shown that non‐radiative decay via conical intersections or intersystem crossing is rather unlikely. Instead, the additional double bond alters the electronic structure of the first excited singlet state, turning it into a dark state with long lifetime and hence low fluorescence rate, that cannot be seen anymore with the naked eye. [ABSTRACT FROM AUTHOR]