New theoretical insights on the nonradiative relaxation mechanism of the core structure of mycosporines: The amino-cyclohexenone central template

We present a comprehensive computational study describing the excited state dynamics and consequent photostability of amino-cyclohexenone (ACyO), the central template of mycosporine systems, widely recognized for their photoprotection of aquatic species. Photoexcitation to the first excited electronic state (S1, 1nπ*) of ACyO is considered an optically dark transition, while photoexcitation to the second excited electronic state (S21ππ*) is an optically bright 1ππ* transition and largely responsible for UV absorption properties of this molecule. We show that following initial photoexcitation to S2, ACyO relaxes via two competing deactivation mechanisms, each mediated by an S1/S0 conical intersection, which directs the excited state population to the electronic ground state (S0). Our ab initio computational results are supported with nonadiabatic dynamics simulation results, yielding an excited state lifetime of ∼280 fs for this system in vacuo. These results explain the inherent photostability of this core structure, commonplace in a wide range of microorganisms.

Ultrafast Hot Exciton Nonadiabatic Excited-State Dynamics in Green Fluorescent Protein Chromophore Analogue

The ultrafast high-energy nonadiabatic excited-state dynamics of the benzylidenedimethylimidazolinone chromophore dimer has been investigated using an electronic structure method coupled with on-the-fly quantitative wave function analysis to gain insight into the photophysics of hot excitons in biological systems. The dynamical simulation provides a rationalization of the behavior of the exciton in a dimer after the photoabsorption of light to higher-energy states. The results suggest that hot exciton localization within the manifold of excited states is caused by the hindrance of torsional rotation due to imidazolinone (I) or phenolate (P) bonds i.e., ΦI- or ΦP-dihedral rotation, in the monomeric units of a dimer. This hindrance arises due to weak π-π stacking interaction in the dimer, resulting in an energetically uphill excited-state barrier for ΦI- and ΦP-twisted rotation, impeding the isomerization process in the chromophore. Thus, this study highlights the potential impact of the weak π-π interaction in regulating the photodynamics of the green fluorescent protein chromophore derivatives.

Ab initio investigation of excited state charge transfer pathways in differently capped bithiophene cages

The electronic excitations of conformationally constrained bithiophene cage systems as previously investigated by Lewis et al. (J. Am. Chem. Soc. 143, 18548 (2021)) are revisited, employing the correlated ab initio Scaled Opposite-Spin Algebraic Diagrammatic Construction Second Order electronic structure method. Quantitative descriptors are determined to assess the extent of charge transfer between the bithiophene moieties and the capping domains, represented by either phenyl or triazine groups. The investigation substantiates intrinsic differences in the photophysical behavior of these two structural variants and reveals the presence of lower-energy excited states characterized by noteworthy charge transfer contributions in the triazine cage system. The manifestation of this charge transfer character is discernible even at the Franck-Condon geometry, persisting throughout the relaxation of the excited state. By examining isolated monomer building blocks, we confirm the existence of analogous charge transfer contributions in their excitations. Employing this methodological approach facilitates the prospective identification of potential wall/cap chromophore pairs, wherein charge transfer pathways can be accessed within the energetically favorable regime.

Exploring the ultrafast and isomer-dependent photodissociation of iodothiophenes via site-selective ionization

C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy. At greater C-I separations, beyond the critical distance, CT is no longer possible and the measured kinetic energies of the charged iodine atoms report on the neutral dissociation process. The time and momentum resolved measurements allow determination of the timescales and the respective product momentum and kinetic energy distributions for both isomers, which are interpreted in terms of rival 'direct' and 'indirect' dissociation pathways. The measurements are compared with a classical over the barrier model, which reveals that the onset of the indirect dissociation process is delayed by ∼1 ps relative to the direct process. The kinetics of the two processes show no discernible difference between the two parent isomers, but the branching between the direct and indirect dissociation channels and the respective product momentum distributions show isomer dependencies. The greater relative yield of indirect dissociation products from 262 nm photolysis of 3-iodothiophene (cf. 2-iodothiophene) is attributed to the different partial cross-sections for (ring-centred) π∗ ← π and (C-I bond localized) σ∗ ← (n/π) excitation in the respective parent isomers.

Vibronic coupling and ultrafast relaxation dynamics in the first five excited singlet electronic states of bithiophene

The vibronic structure and nuclear dynamics in the first five excited singlet electronic states of bithiophene (2T) are investigated here. Specifically, considerations are given to comprehend the first two structureless and broad electronic absorption bands and the role of nonadiabatic coupling in the excited state relaxation mechanism of 2T in the gas phase. Associated potential energy surfaces (PESs) are established by constructing a model vibronic coupling Hamiltonian using 18 vibrational degrees of freedom and extensive ab initio electronic structure calculations. The topographies of these PESs are critically examined, and multiple conical intersections are established. The nuclear dynamics calculations are performed by propagating wave packets on the coupled electronic manifold. The present theoretical results are in good agreement with the experimental observations. It is found that strong nonadiabatic coupling between the S1-S4 and S1-S5 states along totally symmetric modes is predominantly responsible for the structureless and broad first absorption band, and overlapping S2, S3, S4, and S5 states form the second absorption band. Photorelaxation from the highly excited S5 to the lowest S1 state takes place through a cascade of diabatic population transfers among the S1-S4-S5 electronic manifold within the first ∼100 fs. Totally symmetric C=C stretching, C-S stretching, C-H wagging, ring puckering, and inter-ring bending modes collectively drive such relaxation dynamics.

A Local Diabatisation Method for Two-State Adiabatic Conical Intersections

A methodology to locally characterize conical intersections (CIs) between two adiabatic electronic states for which no nonadiabatic coupling (NAC) vectors are available is presented. Based on the Hessian and gradient at the CI, the branching space coordinates are identified. The potential energy surface around the CI in the branching space is expressed in the diabatic representation, from which the NAC vectors can be calculated in a wave-function-free, energy-based approach. To demonstrate the universality of the developed methodology, the minimum-energy CI (MECI) between the first (S1) and second (S2) singlet excited states of formamide is investigated at the state-averaged complete active space self-consistent field (SA-CASSCF) and extended multistate complete active space second-order perturbation theory (XMS-CASPT2) levels of theory. In addition, the asymmetrical MECI between the ground state (S0) and S1 of cyclopropanone is evaluated using SA-CASSCF, as well as (ME)CIs between the S1 and S2 states of benzene using SA-CASSCF and time-dependent density functional theory (TDDFT). Finally, a CI between the S1 and S2 excited states of thiophene was analyzed using TDDFT.

On the description of conical intersections between excited electronic states with LR-TDDFT and ADC(2)

Conical intersections constitute the conceptual bedrock of our working understanding of ultrafast, nonadiabatic processes within photochemistry (and photophysics). Accurate calculation of potential energy surfaces within the vicinity of conical intersections, however, still poses a serious challenge to many popular electronic structure methods. Multiple works have reported on the deficiency of methods like linear-response time-dependent density functional theory within the adiabatic approximation (AA LR-TDDFT) or algebraic diagrammatic construction to second-order [ADC(2)]-approaches often used in excited-state molecular dynamics simulations-to describe conical intersections between the ground and excited electronic states. In the present study, we focus our attention on conical intersections between excited electronic states and probe the ability of AA LR-TDDFT and ADC(2) to describe their topology and topography, using protonated formaldimine and pyrazine as two exemplar molecules. We also take the opportunity to revisit the performance of these methods in describing conical intersections involving the ground electronic state in protonated formaldimine-highlighting in particular how the intersection ring exhibited by AA LR-TDDFT can be perceived either as a (near-to-linear) seam of intersection or two interpenetrating cones, depending on the magnitude of molecular distortions within the branching space.

Accurate Calculation of Excited-State Absorption for Small-to-Medium-Sized Conjugated Oligomers: Multiconfigurational Treatment vs Quadratic Response TD-DFT

Excited-state absorption (ESA) spectra of π-conjugated compounds are frequently calculated by (quadratic response) time-dependent density functional theory, (QR) TD-DFT, often giving a reasonable representation of the experimental results despite the (known) incomplete electronic description. To investigate whether this is inherent to the method, we calculate here the ESA spectra of small-to-medium-sized oligophenylenevinylenes (nPV) and oligothiophenes (nT) using QR TD-DFT as well as CASPT2 based on CASSCF geometries. CASPT2 gives indeed a reliable, theoretically correct description of the ESA features for all compounds; the computational effort can be reduced without significant loss of accuracy using TD-DFT geometries. QR TD-DFT, based on BHandHLYP and CAM-/B3LYP functionals, fails on short nTs but provides a reasonable description for spectral positions of nPVs and long nTs. The failure on short nTs is, however, only partly due to the incomplete configuration description but, in particular, related to an improper MO description, resulting in an asymmetric energy spacing of the occupied vs unoccupied MOs in the DFT scheme. Longer nTs, on the other side, adapt approximately the MO scheme for alternant hydrocarbons just like in nPVs, while contributions by two triplet excitations combined to a singlet (which inhibits an accurate treatment of polyenes with standard TD-DFT) do not play a relevant role in the current case. For such "well-behaved" systems, a reasonable representation of ESA spectra is found at the QR TD-DFT level due to the rather small energy shifts when including higher-order excitations.

On the propensity of formation of cyclobutane dimers in face-to-face and face-to-back uracil stacks in solution

UV irradiation of RNA leads to the formation of intra- and inter-strand crosslinks of cyclobutane type. Despite the importance of this reaction, relatively little is known about how the mutual orientation of the two bases affects the outcome of the reaction. Here we report a comparative nonadiabatic molecular dynamics study of face-to-back (F2B) and face-to-face (F2F) stacked uracil-water clusters. The computations were performed using the second-order algebraic-diagrammatic-construction (ADC(2)) method. We found that F2B stacked uracil-water clusters either relax non-reactively to the ground state by an ethylenic twist around the CC bond or remain in the lowest nπ* state in which the two bases gradually move away from each other. This finding is consistent with the low propensity for the formation of intra-strand cyclobutane dimers between adjacent RNA bases. On the contrary, in F2F stacked uracil-water clusters, in addition to non-reactive deactivation, we found a pro-reactive deactivation pathway, which may lead to the formation of cyclobutane uracil dimers in the electronic ground state. On a qualitative level, the observed photodynamics of F2F stacked uracil-water clusters explains the greater propensity of RNA to form inter-strand cyclobutane-type crosslinks.

Nonradiative relaxation mechanisms of the elusive silole molecule

Silole derivatives have been extensively employed for developing organic optoelectronics, but few studies focused on the photophysical properties of the silole molecule. In this work, we investigate these properties by computing the absorption spectra and performing nonadiabatic molecular dynamics of silole employing the algebraic diagrammatic construction [ADC(2)] and extended multi-state XMS-CASPT2 ab initio electronic structure methods. For vertical excitations and excited state optimizations, the equation of motion coupled-cluster singles and doubles (EOM-CCSD) was also used. The nuclear ensemble and the fewest-switches surface hopping molecular dynamics methods coupled with the first two high-level electronic structure methods were applied to probe the relaxation mechanisms of silole. We could reproduce the experimental first absorption maximum value and found an ultrafast relaxation process occurring exclusively through ring-puckering distortions without breaking ring bonds or hydrogen elimination. Minimum energy conical intersection optimizations were carried out and potential energy curves, including triplet states, were calculated to further elucidate the relaxation process of silole.

Caveat when using ADC(2) for studying the photochemistry of carbonyl-containing molecules

Several electronic-structure methods are available to study the photochemistry and photophysics of organic molecules. Among them, ADC(2) stands as a sweet spot between computational efficiency and accuracy. As a result, ADC(2) has recently seen its number of applications booming, in particular to unravel the deactivation pathways and photodynamics of organic molecules. Despite this growing success, we demonstrate here that care has to be taken when studying the nonradiative pathways of carbonyl-containing molecules, as ADC(2) appears to suffer from a systematic flaw.

Tailoring CIPSI Expansions for QMC Calculations of Electronic Excitations: The Case Study of Thiophene

The perturbatively selected configuration interaction scheme (CIPSI) is particularly effective in constructing determinantal expansions for quantum Monte Carlo (QMC) simulations with Jastrow–Slater wave functions: fast and smooth convergence of ground-state properties and balanced descriptions of ground and excited states of different symmetries have been reported. In particular, accurate excitation energies have been obtained by the pivotal requirement of using CIPSI expansions with similar second-order perturbation corrections for each state, that is, a similar estimated distance to the full configuration interaction limit. Here, we elaborate on the CIPSI selection criterion for excited states of the same symmetry as the ground state, generating expansions from a common orbital set. Using these expansions in QMC as determinantal components of Jastrow–Slater wave functions, we compute the lowest, bright excited state of thiophene, which is challenging due to its significant multireference character. The resulting vertical excitation energies are within 0.05 eV of the best theoretical estimates, already with expansions of only a few thousand determinants. Furthermore, we relax the ground- and excited-state structures following the corresponding root in variational Monte Carlo and obtain bond lengths that are accurate to better than 0.01 Å. Therefore, while the full treatment at the CIPSI level of this system is quite demanding, in QMC, we can compute high-quality excitation energies and excited-state structural parameters building on affordable CIPSI expansions with relatively few, well-chosen determinants.

Exploring the effects of quantum decoherence on the excited-state dynamics of molecular systems

Fewest-switches surface hopping (FSSH) is employed in order to investigate the nonadiabatic excited-state dynamics of thiophene and related compounds and hence to establish a connection between the electronic system, the critical points in configuration space and the deactivation dynamics. The potential-energy surfaces of the studied molecules were calculated with complete active space self-consistent field and time-dependent density-functional theory. They are analyzed thoroughly to locate and optimize minimum-energy conical intersections, which are essential to the dynamics of the system. The influence of decoherence on the dynamics is examined by employing different decoherence schemes. We find that irrespective of the employed decoherence algorithm, the population dynamics of thiophene give results which are sound with the expectations grounded on the analysis of the potential-energy surface. A more detailed look at single trajectories as well as on the excited-state lifetimes, however, reveals a substantial dependence on how decoherence is accounted for. In order to connect these findings, we describe how ensemble averaging cures some of the overcoherence problems of uncorrected FSSH. Eventually, we identify carbon–sulfur bond cleavage as a common feature accompanying electronic transitions between different states in the simulations of all thiophene-related compounds studied in this work, which is of interest due to their relevance in organic photovoltaics.

Trajectory Surface Hopping Approach to Condensed-Phase Nonradiative Relaxation Dynamics Using Divide-and-Conquer Spin-Flip Time-Dependent Density-Functional Tight Binding

Nonradiative relaxation of excited molecules is central to many crucial issues in photochemistry. Condensed phases are typical contexts in which such problems are considered, and the nonradiative relaxation dynamics are expected to be significantly affected by interactions with the environment, for example, a solvent. We developed a nonadiabatic molecular dynamics simulation technique that can treat the nonradiative relaxation and explicitly include the environment in the calculations without a heavy computational burden. Specifically, we combined trajectory surface hopping with Tully's fewest-switches algorithm, a tight-binding approximated version of spin-flip time-dependent density-functional theory, and divide-and-conquer (DC) spatial fragmentation scheme. Numerical results showed that this method can treat systems with thousands of atoms within reasonable computational resources, and the error arising from DC fragmentation is negligibly small. Using this method, we obtained molecular insights into the solvent dependence of the photoexcited-state dynamics of trans-azobenzene, which demonstrate the importance of the environment for condensed-phase nonradiative relaxation.

Deciphering the Chemical Basis of Fluorescence of a Selenium-Labeled Uracil Probe when Bound at the Bacterial Ribosomal A-Site

We unveil in this work the main factors that govern the turn-on/off fluorescence of a Se-modified uracil probe at the ribosomal RNA A-site. Whereas the constraint into an "in-plane" conformation of the two rings of the fluorophore is the main driver for the observed turn-on fluorescence emission in the presence of the antibiotic paromomycin, the electrostatics of the environment plays a minor role during the emission process. Our computational strategy clearly indicates that, in the absence of paromomycin, the probe prefers conformations that show a dark S₁ electronic state with participation of nπ* electronic transition contributions between the selenium atom and the π-system of the uracil moiety.

The Photoisomerization Pathway(s) of Push-Pull Phenylazoheteroarenes*

Azoheteroarenes are the most recent derivatives targeted to further improve the properties of azo-based photoswitches. Their light-induced mechanism for trans-cis isomerization is assumed to be very similar to that of the parent azobenzene. As such, they inherited the controversy about the dominant isomerization pathway (rotation vs. inversion) depending on the excited state (nπ* vs. ππ*). Although the controversy seems settled in azobenzene, the extent to which the same conclusions apply to the more structurally diverse family of azoheteroarenes is unclear. Here, by means of non-adiabatic molecular dynamics, the photoisomerization mechanism of three prototypical phenyl-azoheteroarenes with increasing push-pull character is unraveled. The evolution of the rotational and inversion conical intersection energies, the preferred pathway, and the associated kinetics upon both nπ* and ππ* excitations can be linked directly with the push-pull substitution effects. Overall, the working conditions of this family of azo-dyes is clarified and a possibility to exploit push-pull substituents to tune their photoisomerization mechanism is identified, with potential impact on their quantum yield.

Fast Nonadiabatic Molecular Dynamics via Spin-Flip Time-Dependent Density-Functional Tight-Binding Approach: Application to Nonradiative Relaxation of Tetraphenylethylene with Locked Aromatic Rings

Nonadiabatic dynamics around conical intersections between ground and excited states are crucial to understand excited-state phenomena in complex chemical systems. With this background in mind, we present an approach combining fewest-switches trajectory surface hopping and spin-flip (SF) time-dependent (TD) density-functional tight binding (DFTB), which is a simplified version of SF-TD density functional theory (DFT) with semiempirical parametrizations, for computationally efficient nonadiabatic molecular dynamics simulations. The estimated computational time of the SF-TD-DFTB approach is several orders of magnitude lower than that of SF-TD-DFT. In addition, the proposed method reproduces the time scales and quantum yields in photoisomerization reactions of azobenzene at a level comparable with conventional ab initio approaches, demonstrating reasonable accuracy. Finally, we report a practical application of the developed technique to explore the nonradiative relaxation processes of tetraphenylethylene and its derivative with torsionally locked aromatic rings and discuss the effect of locking the rings on the excited-state lifetime.

Simulation of time-resolved x-ray absorption spectroscopy of ultrafast dynamics in particle-hole-excited 4-(2-thienyl)-2,1,3-benzothiadiazole

To date, alternating co-polymers based on electron-rich and electron-poor units are the most attractive materials to control functionality of organic semiconductor layers in which ultrafast excited-state processes play a key role. We present a computational study of the photoinduced excited-state dynamics of the 4-(2-thienyl)-2,1,3-benzothiadiazole (BT-1T) molecule, which is a common building block in the backbone of π-conjugated polymers used for organic electronics. In contrast to homo-polymer materials, such as oligothiophene, BT-1T has two non-identical units, namely, thiophene and benzothiadiazole, making it attractive for intramolecular charge transfer studies. To gain a thorough understanding of the coupling of excited-state dynamics with nuclear motion, we consider a scenario based on femtosecond time-resolved x-ray absorption spectroscopy using an x-ray free-electron laser in combination with a synchronized ultraviolet femtosecond laser. Using Tully's fewest switches surface hopping approach in combination with excited-state calculations at the level of configuration interaction singles, we calculate the gas-phase x-ray absorption spectrum at the carbon and nitrogen K edges as a function of time after excitation to the lowest electronically excited state. The results of our time-resolved calculations exhibit the charge transfer driven by non-Born-Oppenheimer physics from the benzothiadiazole to thiophene units during relaxation to the ground state. Furthermore, our ab initio molecular dynamics simulations indicate that the excited-state relaxation processes involve bond elongation in the benzothiadiazole unit as well as thiophene ring puckering at a time scale of 100 fs. We show that these dynamical trends can be identified from the time-dependent x-ray absorption spectrum.

Photoelimination of Nitrogen from Diazoalkanes: Involvement of Higher Excited Singlet States in the Carbene Formation

Although diazoalkanes are important carbene precursors in organic synthesis, a comprehensive mechanism of photochemical formation of carbenes from diazoalkanes has not been proposed. Synergies of experiments and computations demonstrate the involvement of higher excited singlet states in the photochemistry of diazoalkanes. In all investigated diazoalkanes, excitation to S₁ results in nonreactive internal conversion to S₀. On the contrary, excitation to higher-lying singlet states (S_n, n > 1) drives the reaction toward a different segment of the S₁/S₀ conical intersection seam and results in nitrogen elimination and formation of carbenes.

Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials

Optically active molecular materials, such as organic conjugated polymers and biological systems, are characterized by strong coupling between electronic and vibrational degrees of freedom. Typically, simulations must go beyond the Born-Oppenheimer approximation to account for non-adiabatic coupling between excited states. Indeed, non-adiabatic dynamics is commonly associated with exciton dynamics and photophysics involving charge and energy transfer, as well as exciton dissociation and charge recombination. Understanding the photoinduced dynamics in such materials is vital to providing an accurate description of exciton formation, evolution, and decay. This interdisciplinary field has matured significantly over the past decades. Formulation of new theoretical frameworks, development of more efficient and accurate computational algorithms, and evolution of high-performance computer hardware has extended these simulations to very large molecular systems with hundreds of atoms, including numerous studies of organic semiconductors and biomolecules. In this Review, we will describe recent theoretical advances including treatment of electronic decoherence in surface-hopping methods, the role of solvent effects, trivial unavoided crossings, analysis of data based on transition densities, and efficient computational implementations of these numerical methods. We also emphasize newly developed semiclassical approaches, based on the Gaussian approximation, which retain phase and width information to account for significant decoherence and interference effects while maintaining the high efficiency of surface-hopping approaches. The above developments have been employed to successfully describe photophysics in a variety of molecular materials.

Electronic structure and VUV photoabsorption measurements of thiophene

The absolute photoabsorption cross sections for thiophene in the 5.0-10.7 eV range were measured using synchrotron radiation. New theoretical calculations performed at the time-dependent density functional theory level were used to qualitatively interpret the recorded photoabsorption spectrum. The calculations facilitated a re-analysis of the observed vibronic and Rydberg structures in the photoabsorption spectrum. Here a number of features have been re-assigned, while a number of other features have been assigned for the first time. This represents the most comprehensive and self-consistent assignment of the thiophene high-resolution photoabsorption spectrum to date.

Effect of extending conjugation via thiophene-based oligomers on the excited state electron transfer rates to ZnO nanocrystals

Despite spectral differences of a series of thiophene dyes, their excited state electron transfer rates to ZnO nanocrystals are similar.

Conformationally controlled ultrafast intersystem crossing in bithiophene systems

Bithiophenes serve as model systems for larger polythiophenes used in solar cell applications and molecular electronics. We report a study of ultrafast dynamics of two bithiophene systems measured with femtosecond time-resolved photoelectron spectroscopy, and show that their intersystem crossing takes place within the first few picoseconds after excitation, in line with previous studies. We show that the intersystem crossing rate can be explained in terms of arguments based on symmetry of the S1 minimum energy geometry, which depends on the specific conformation of bithiophene. Furthermore, this work shows that the minor cis-conformer contributes to an even higher intersystem crossing rate than the major trans conformer. The work presented here can provide guiding principles towards the design of solar cell components with even faster formation of long-lived excited states for solar energy harvesting.

Effect of thiophene substitution on the intersystem crossing of arene photosensitizers

Thiophene substitution gives energy level-matched S₁/T₂ states and the ISC is enhanced, which was not observed with phenyl substitution.

Exploring the Dynamics of the Photoinduced Ring-Opening of Heterocyclic Molecules

Excited states formed by electron promotion to an antibonding σ* orbital are now recognized as key to understanding the photofragmentation dynamics of a broad range of heteroatom containing small molecules: alcohols, thiols, amines, and many of their aromatic analogues. Such excited states may be populated by direct photoexcitation, or indirectly by nonadiabatic transfer of population from some other optically excited state (e.g., a ππ* state). This Perspective explores the extent to which the fast-growing literature pertaining to such (n/π)σ*-state mediated bond fissions can inform and enhance our mechanistic understanding of photoinduced ring-opening in heterocyclic molecules.

Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

Exciting a molecule with an ultraviolet photon often leads to bond fission, but the final outcome of the bond cleavage is typically both molecule and phase dependent. The photodissociation of an isolated gas-phase molecule can be viewed as a closed system: Energy and momentum are conserved, and the fragmentation is irreversible. The same is not true in a solution-phase photodissociation process. Solvent interactions may dissipate some of the photoexcitation energy prior to bond fission and will dissipate any excess energy partitioned into the dissociation products. Products that have no analog in the corresponding gas-phase study may arise by, for example, geminate recombination. Here, we illustrate the extent to which dynamical insights from gas-phase studies can inform our understanding of the corresponding solution-phase photochemistry and how, in the specific case of photoinduced ring-opening reactions, solution-phase studies can in some cases reveal dynamical insights more clearly than the corresponding gas-phase study.

Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad

Understanding electronic energy transfer (EET) is an important ingredient in the development of artificial photosynthetic systems and photovoltaic technologies. Although EET is at the heart of these applications and crucially influences their light-harvesting efficiency, the nature of EET over short distances for covalently bound donor and acceptor units is often not well understood. Here we investigate EET in an orthogonal molecular dyad (BODT4), in which simple models fail to explain the very origin of EET. On the basis of nonadiabatic ab initio molecular dynamics calculations and ultrafast fluorescence experiments, we gain detailed microscopic insights into the ultrafast electrovibrational dynamics following photoexcitation. Our analysis offers molecular-level insights into these processes and reveals that it takes place on time scales ≲100 fs and occurs through an intermediate charge-transfer state.

Mechanism of ultrafast non-reactive deactivation of the retinal chromophore in non-polar solvents

Counterion sensitive photodynamics of the retinal chromophore in solution.

Ab initio molecular dynamics of thiophene: the interplay of internal conversion and intersystem crossing

Ab initio on-the-fly molecular dynamics reveals that excited thiophene decays via low lying conical intersections and via intersystem crossing. Open-ring structures are responsible for the observed long life times.

Visualizing Excited-State Dynamics of a Diaryl Thiophene: Femtosecond Stimulated Raman Scattering as a Probe of Conjugated Molecules

Conjugated organic polymers based on substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization. Subsequent slow opening of the thiophene ring through a cleavage of the carbon-sulfur bond triggers an intersystem crossing to the triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained, combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information provided by time-dependent density functional theory calculations.

Low-Lying ππ* States of Heteroaromatic Molecules: A Challenge for Excited State Methods

The description of low-lying ππ* states of linear acenes by standard electronic structure methods is known to be challenging. Here, we broaden the framework of this problem by considering a set of fused heteroaromatic rings and demonstrate that standard electronic structure methods do not provide a balanced description of the two (typically) lowest singlet state (L_a and L_b) excitations. While the L_b state is highly sensitive to correlation effects, L_a suffers from the same drawbacks as charge transfer excitations. We show that the comparison between CIS/CIS(D) can serve as a diagnostic for detecting the two problematic excited states. Standard TD-DFT and even its spin-flip variant lead to inaccurate excitation energies and interstate gaps, with only a double hybrid functional performing somewhat better. The complication inherent to a balanced description of these states is so important that even CC2 and ADC(2) do not necessarily match the ADC(3) reference.

Deactivation pathways of thiophene and oligothiophenes: internal conversion versus intersystem crossing

Quantum chemical calculations reveal that excited thiophene decays via a low lying conical intersection seam. In oligothiophenes barriers inhibit this passage while deactivation pathways via intersystem crossing channels open.

How does tetraphenylethylene relax from its excited states?

Photocyclization play a key role in the deactivation mechanism of tetraphenylethylene.

Mechanistic studies of the photo-electrochemical hydrogen evolution reaction on poly(2,2′-bithiophene)

The realisation of poly(2,2′-bithiophene) (PBTh) as an effective photo-electrocatalyst for the hydrogen evolution reaction is a novel discovery [Ng et al., Int. J. Hydrogen Energy, 2014, 39, 18230]; however, the underlying mechanism for this catalysis remains unknown.

Assessment of Approximate Coupled-Cluster and Algebraic-Diagrammatic-Construction Methods for Ground- and Excited-State Reaction Paths and the Conical-Intersection Seam of a Retinal-Chromophore Model

As a minimal model of the chromophore of rhodopsin proteins, the penta-2,4-dieniminium cation (PSB3) poses a challenging test system for the assessment of electronic-structure methods for the exploration of ground- and excited-state potential-energy surfaces, the topography of conical intersections, and the dimensionality (topology) of the branching space. Herein, we report on the performance of the approximate linear-response coupled-cluster method of second order (CC2) and the algebraic-diagrammatic-construction scheme of the polarization propagator of second and third orders (ADC(2) and ADC(3)). For the ADC(2) method, we considered both the strict and extended variants (ADC(2)-s and ADC(2)-x). For both CC2 and ADC methods, we also tested the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) variants. We have explored several ground- and excited-state reaction paths, a circular path centered around the S1/S0 surface crossing, and a 2D scan of the potential-energy surfaces along the branching space. We find that the CC2 and ADC methods yield a different dimensionality of the intersection space. While the ADC methods yield a linear intersection topology, we find a conical intersection topology for the CC2 method. We present computational evidence showing that the linear-response CC2 method yields a surface crossing between the reference state and the first response state featuring characteristics that are expected for a true conical intersection. Finally, we test the performance of these methods for the approximate geometry optimization of the S1/S0 minimum-energy conical intersection and compare the geometries with available data from multireference methods. The present study provides new insight into the performance of linear-response CC2 and polarization-propagator ADC methods for molecular electronic spectroscopy and applications in computational photochemistry.

0-0 Energies Using Hybrid Schemes: Benchmarks of TD-DFT, CIS(D), ADC(2), CC2, and BSE/GW formalisms for 80 Real-Life Compounds

The 0-0 energies of 80 medium and large molecules have been computed with a large panel of theoretical formalisms. We have used an approach computationally tractable for large molecules, that is, the structural and vibrational parameters are obtained with TD-DFT, the solvent effects are accounted for with the PCM model, whereas the total and transition energies have been determined with TD-DFT and with five wave function approaches accounting for contributions from double excitations, namely, CIS(D), ADC(2), CC2, SCS-CC2, and SOS-CC2, as well as Green's function based BSE/GW approach. Atomic basis sets including diffuse functions have been systematically applied, and several variations of the PCM have been evaluated. Using solvent corrections obtained with corrected linear-response approach, we found that three schemes, namely, ADC(2), CC2, and BSE/GW allow one to reach a mean absolute deviation smaller than 0.15 eV compared to the measurements, the two former yielding slightly better correlation with experiments than the latter. CIS(D), SCS-CC2, and SOS-CC2 provide significantly larger deviations, though the latter approach delivers highly consistent transition energies. In addition, we show that (i) ADC(2) and CC2 values are extremely close to each other but for systems absorbing at low energies; (ii) the linear-response PCM scheme tends to overestimate solvation effects; and that (iii) the average impact of nonequilibrium correction on 0-0 energies is negligible.