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8-Oxoguanine is the most abundant oxidation product found in oxidatively damaged DNA. The study of the excited-state properties of the corresponding deoxyribonucleoside 8-oxo-deoxyguanosine is thus of important biological relevance. Herein, we present an ADC(2)-s ab initio study of the neutral and the anionic form of 8-oxo-deoxyguanosine, for each of which we have considered the intramolecularly 5'-O-H···N3 hydrogen-bonded syn conformer. We present energy profiles for a radiationless deactivation mechanism via intramolecular excited-state proton transfer. This mechanism is accessible in the neutral form, but it is unavailable in the anion. We present optimized structures for the proton-transfer conical intersection of the neutral form as well as the ring-puckered conical intersections inherent to the 8-oxoguanine moiety for the neutral and anionic forms. We highlight the possible relevance of the proton-transfer mechanism for the neutral form and discuss our results in light of several recently published computational and spectroscopic studies. Our results provide new insight into the photophysics of this biologically relevant nucleoside and pave the way for future nonadiabatic dynamics studies and for further spectroscopic investigations.

Transition metal tetrahalides are a class of highly symmetric molecules for which very few spectroscopic data exist. Exploratory ab initio calculations of electronic potential energy functions indicate that the equilibrium molecular geometries of the vanadium, niobium, and tantalum tetrafluorides (i.e., VF4, NbF4, and TaF4) exhibit strong distortions from the tetrahedral configuration in their electronic ground state (2E) and first excited state (2T2) along the nuclear displacement coordinates of e symmetry. The distortions result from the E × e and T2 × e Jahn–Teller (JT) effects, respectively. In addition, there are weaker distortions in the 2T2 state along the coordinates of t2 symmetry due to the T2 × t2 JT effect. The description of the large-amplitude dynamics induced by these JT effects requires the construction of JT Hamiltonians beyond the standard model of JT theory, which is based on Taylor expansions up to second order in normal-mode displacements. These higher-order JT Hamiltonians were constructed in this work by expansions of the electronic potentials of the title molecule in terms of symmetry invariant polynomials in symmetry-adapted nuclear displacement coordinates for the bending modes of VF4. A multi-configuration electronic structure method was employed to determine the coefficients of these high-order polynomial expansions from first principles. Using these large-amplitude Jahn–Teller Hamiltonians, the vibronic spectra of VF4 were computed. The spectra illustrate the effects of large-amplitude fluxional nonadiabatic dynamics due to exceptionally strong E × e and T2 × e JT couplings. In addition, the vibronic spectrum of the T2 × (e + t2) JT effect, including the bending mode of t2 symmetry, was computed. The spectrum displays strong inter-mode coupling effects exhibiting a vibronic structure, which is substantially different from that predicted by independent-mode approximation. These results represent the first ab initio study of dynamical Jahn–Teller effects in VF4.

The mechanisms of photoinduced reactions of adenine with water molecules in hydrogen-bonded adenine-water complexes were investigated with ab initio wave-function-based electronic-structure calculations. Two excited-state electron/proton transfer reaction mechanisms have been characterized: H-atom abstraction from water by photoexcited adenine as well as H-atom transfer from photoexcited adenine or the (adenine+H) radical to water. In the water-to-adenine H-atom transfer reaction, an electron from one of the p orbitals of the water molecule fills the hole in the n (π) orbital of the nπ* (ππ*) excited state of adenine, resulting in a charge-separated electronic state. The electronic charge separation is neutralized by the transfer of a proton from the water molecule to adenine, resulting in the (adenine+H)OH biradical in the electronic ground state. In the adenine-to-water H-atom transfer reaction, πσ* states localized at the acidic sites of adenine provide the mechanism for the photoejection of an electron from adenine, which is followed by proton transfer to the hydrogen-bonded water molecule, resulting in the (adenine-H)H3O biradical. The energy profiles of the photoreactions have been computed as relaxed scans with the ADC(2) electronic-structure method. These reactions, which involve the reactivity of adenine with hydrogen-bonded water molecules, compete with the well-established intrinsic excited-state deactivation mechanisms of adenine via ring-puckering or ring-opening conical intersections. By providing additional decay channels, the electron/proton exchange reactions with water can account for the observed significantly shortened excited-state lifetime of adenine in aqueous environments. These findings indicate that adenine possibly was not only a photostabilizer at the beginning of life, but also a primordial photocatalyst for water splitting.

Carbon nitride materials are of great interest for photocatalytic water splitting. Herein, we report results from first-principles simulations of the specific electron- and proton-transfer processes that are involved in the photochemical oxidation of liquid water with heptazine-based molecular photocatalysts. The heptazine chromophore and the solvent molecules have been described strictly at the same level of electronic structure theory. We demonstrate the critical role of solvent molecules for the absorption properties of the chromophore and the overall photocatalytic cycle. A simple model is developed to describe the photochemical water oxidation mechanism. Our results reveal that heptazine possesses energy levels that are suitable for the water oxidation reaction. We suggest design principles for molecular photocatalysts which can be used as descriptors in future experimental and computational screening studies.

The photocatalytic splitting of water into molecular hydrogen and molecular oxygen with sunlight is the dream reaction for solar energy conversion. Since decades, transition-metal-oxide semiconductors and supramolecular organometallic structures have been extensively explored as photocatalysts for solar water splitting. More recently, polymeric carbon nitride materials consisting of triazine or heptazine building blocks have attracted considerable attention as hydrogen-evolution photocatalysts. The mechanism of hydrogen evolution with polymeric carbon nitrides is discussed throughout the current literature in terms of the familiar concepts developed for photoelectrochemical water splitting with semiconductors since the 1970s. We discuss in this perspective an alternative mechanistic paradigm for photoinduced water splitting with carbon nitrides, which focusses on the specific features of the photochemistry of aromatic N-heterocycles in aqueous environments. It is shown that a water molecule which is hydrogen-bonded to an N-heterocycle can be decomposed into hydrogen and hydroxyl radicals by two simple sequential photochemical reactions. This concept is illustrated by first-principles calculations of excited-state reaction paths and their energy profiles for hydrogen-bonded complexes of pyridine, triazine and heptazine with a water molecule., Comment: 21 pages, 2 tables, 8 figures

Recent phase-locked femtosecond double-pump experiments on individual light-harvesting complexes LH2 of purple bacteria at ambient temperature revealed undamped oscillatory responses on a time scale of at least 400 fs [ Hildner et al. Science 2013 , 340 , 1448 ]. Using an excitonic Hamiltonian for LH2 available in the literature, we simulate these signals numerically by a method that treats excitonic couplings and exciton-phonon couplings in a nonperturbative manner. The simulations provide novel insights into the origin of coherent dynamics in individual LH2 complexes. MOE (Min. of Education, S’pore) Accepted version

An analytic expression for the single-passage nonadiabatic transition probability in the vicinity of a G3/2 × (t2 + e) conical intersection in tetrahedral or octahedral systems containing heavy elements is derived in the quasi-classical approximation. The four-channel dynamical equations are solved for a straight-line nuclear trajectory in the five-dimensional space of the t2 and e vibrational modes. The analytic results obtained in the present work may be useful for trajectory surface-hopping simulations of the dynamics of the G3/2 × (t2 + e) Jahn–Teller effect.

We consider a classical particle bilinearly coupled to a harmonic bath. Assuming that the evolution of the particle is monitored on a timescale which is longer than the characteristic bath correlation time, we derive a Markovian master equation for the probability density of the particle. The master equation is fully specified by the time correlation function of the momenta of the particle. We find the functional form of the momentum correlation function which yields the Keilson–Storer master equation (Keilson and Storer, 1952). We show that the parameters of this master equation can directly be related to the characteristic memory time of the bath.

The Jahn–Teller effect in the electronic ground state of the P 4 + radical cation, which is one of the strongest E × e Jahn–Teller effects known in nature, has been revisited in this work with computational methods. The relevance of the Jahn–Teller coupling terms beyond second order in normal-mode displacements has been investigated. An elegant and efficient scheme based on polynomial invariant theory has been employed to expand the E × e potential energy matrix up to arbitrarily high orders in normal mode displacements. Using the state-averaged complete-active-space self-consistent-field method and a correlation consistent double- ζ basis set, an accurate ab initio adiabatic E × e Jahn–Teller potential-energy surface was obtained. It is shown that a polynomial expansion of least up to sixth order is necessary to account for the pronounced anharmonicity of the ab initio potential-energy surface for large amplitude displacements of the Jahn–Teller active vibrational mode. The vibronic structure of the X 2 E band of the photoelectron spectrum of P 4 has been computed using a time-dependent wave-packet propagation method. The results reveal the significance of the higher-order Jahn–Teller coupling terms for the high-resolution vibronic spectrum as well as for the low-resolution band shape.

Considering an electronic two-level system coupled to vibrational degrees of freedom and driven by short and intense non-overlapping laser pulses, we introduce the concept of nonperturbative response functions. These response functions are expressed in terms of effective electronic transition dipole moments which depend on the strength of the field-matter coupling and on the pulse durations. It is shown that the nonlinear polarization representing four-wave-mixing signals can elegantly be expressed in terms of these nonperturbative response functions to all orders in the field-matter coupling. The nonperturbative response functions generalize the interpretative power of the familiar third-order response functions to four-wave-mixing experiments with intense laser pulses.

We investigate femtosecond double-pump single-molecule signals in the strong-field regime, which is characterized by nonlinear scaling of the signal with the intensity of the pump pulses. The signals can be decomposed into population and coherence contributions. In contrast to the weak-field regime (in which only the coherence contribution is important) both contributions are relevant in the strong-field regime and reveal the vibrational dynamics of the chromophore. Other than in the weak-field regime, the detection of vibrational beatings is not limited by the electronic dephasing time of the chromophore. Moreover, the signals in the strong-field regime are more robust with respect to the environment-induced modulation of the chromophore parameters. It is shown that excited-state absorption in chromophores with three electronic states is reflected in the phase dependence of single-molecule signals. The simulations reveal that the information content of femtosecond double-pump single-molecule signals is enhanced in the strong-coupling regime.

Trajectory surface hopping (TSH) methods have been widely used for the study of nonadiabatic molecular dynamics. In the present work, the accuracy of two TSH algorithms, Tully’s fewest switching algorithm and an algorithm based on the Landau-Zener formula, has been critically evaluated in comparison with exact nonadiabatic quantum dynamics calculations for a model of the photoinduced hydrogen-atom dissociation reaction in phenol. The model consists of three electronic states (S0, 1ππ*, 1πσ*) and two nuclear degrees of freedom (the OH stretching coordinate and CCOH dihedral angle) and displays two successive conical intersections (1ππ*/1πσ* and 1πσ*/S0). Considering instantaneous photoexcitation from different vibrational levels of the S0 state to the 1ππ* state, we examined the time-dependent electronic population dynamics as well as the branching ratio of the two dissociation channels. The results of fully converged trajectory calculations are compared with the results of exact quantum wave-packet calcul...

Impressive progress has recently been achieved in photocatalytic hydrogen evolution with polymeric carbon nitride materials consisting of heptazine building blocks. However, the fundamental mechanistic principles of the catalytic cycle are as yet poorly understood. Here, we provide first-principles computational evidence that water splitting with heptazine-based materials can be understood as a molecular excited-state reaction taking place in hydrogen-bonded heptazine–water complexes. The oxidation of water occurs homolytically via an electron/proton transfer from water to heptazine, resulting in ground-state heptazinyl and OH radicals. It is shown that the excess hydrogen atom of the heptazinyl radical can be photodetached by a second photon, which regenerates the heptazine molecule. Alternatively to the photodetachment reaction, two heptazinyl radicals can recombine in a dark reaction to form H2, thereby regenerating two heptazine molecules. The proposed molecular photochemical reaction scheme within hy...

We theoretically investigate the feasibility of characterizing conical intersections with time-resolved resonant femtosecond stimulated Raman spectroscopy (FSRS) using an intense actinic pump pulse. We perform nonperturbative numerical simulations of FSRS signals for a three-electronic-state two-vibrational-mode model, which is inspired by the S 2 ( π π * )- S 1 ( n π * ) conical intersection in pyrazine. Our results show that moderately strong actinic pulses increase the intensity of vibrational fingerprint lines in FSRS transients. They facilitate the extraction of useful spectroscopic information by enhancing peaks revealing the coupling and tuning modes of the conical intersection.

It has recently been shown that low-lying dark charge-separated singlet excited states of nπ* and ππ* character exist in the hydrogen-bonded pyridine-water complex in addition to the familiar nπ* and ππ* excited states of the pyridine chromophore. The former have been shown to promote the transfer of a proton from water to pyridine, resulting in the pyridinyl-hydroxyl radical pair. In the present work, the potential-energy surfaces of the triplet excited states of the pyridine-water complex have been explored with the same ab initio electronic-structure methods (ADC(2), CASPT2). Minimum-energy reaction paths for excited-state H atom transfer, energy surfaces in the vicinity of the barrier for H atom transfer, as well as multistate surface crossings have been characterized. The photochemical reaction mechanisms on the singlet and triplet potential-energy surfaces are compared, and their relevance for photoinduced water oxidation with the pyridine chromophore are discussed.

Kynurenines are UV filters found in the human ocular lens which protect the retina from radiation damage. We report on ab initio investigations of the photochemistry of the cis and trans conformers of kynurenine and of an intramolecularly hydrogen-bonded conformer of 3-hydroxykynurenine O-β-d-glucoside. We have explored the excited-state reaction paths for several radiationless excited-state deactivation processes in kynurenines. We show that electron-driven proton-transfer processes mediated by an excited state of charge-transfer character exhibit negligible barriers and that the relevant potential-energy profiles are lower in energy than the lowest absorbing ππ* state. In these proton-transfer processes, a proton moves from one of the amino groups of kynurenine to the keto group. We also report on nonadiabatic trajectory-surface-hopping molecular-dynamics simulations for photoexcited kynurenine. These simulations show that the cis and trans conformers of kynurenine deactivate on a femtosecond-to-picosecond time scale preferably via electron-driven proton transfer from one of the amino groups to the keto group. Cis kynurenine deactivates via a ring-N—H···O═C proton-transfer process. Trans kynurenine tends to undergo trans → cis isomerization before deactivating via the same process. These results suggest that the deactivation process involving the ring-amino group in the cis conformer of kynurenine is the most efficient excited-state deactivation process in kynurenines. The joint electronic-structure calculations and dynamics simulations provide a new level of mechanistic insight into the efficient UV-filtering capacity of kynurenines.

The theory for the calculation of vibronic absorption spectra within a Jahn–Teller (JT) active electronic state from first principles has been developed. The infrared absorption spectra of the 5E′ ...

Urocanic acid is a UV filter found in human skin that protects the skin from UV damage but has also been linked to the onset of skin cancer and to photoimmunosuppression. We report on ab initio investigations of two rotameric forms of each of the two tautomers of neutral (E)- and (Z)-urocanic acid. We have computed the vertical singlet excitation energies of eight isomers and have explored the singlet excited-state reaction paths of several photochemical processes for radiationless excited-state deactivation: the E/Z photoisomerization, an electron-driven proton transfer for an intramolecularly hydrogen-bonded Z isomer, as well as the hydrogen-atom detachment process and the ring-puckering process involving the NH group inherent to the imidazole moiety. We have optimized the S1/S0 conical intersections for each of these processes and located additional ππ*/nπ* conical intersections. Because of the reversed energetic order of the nπ* and ππ* states in the N3H and N1H tautomers, an energy window exists where the N3H tautomers can be excited to the nπ* state, from which only the photoisomerization process is accessible, while the N1H tautomers can be excited to the ππ* state, from which several deexcitation processes compete from the onset of the absorption. These results explain the unusual dependence of the quantum yield for E→Z photoisomerization on the excitation wavelength. The present work provides novel insight into the complex photochemistry of this biomolecule and paves the way for future computational studies of the photoinduced excited-state dynamics of urocanic acid.

Recently, resonant two-photon ionization experiments on isolated adenine and adenosine suggested that adenosine exhibits a significantly shorter excited-state lifetime than adenine, which indicates the existence of an efficient excited-state deactivation mechanism in adenosine that is not existent in adenine. We report on ab initio investigations on a syn and an anti conformer of adenosine exhibiting an intramolecular O-H···N3 hydrogen bond. For both conformers, we have identified the existence of a barrierless excited-state deactivation mechanism that involves the forward-backward transfer of a proton along the intramolecular hydrogen bond and ultrafast radiationless deactivation through conical intersections. The S1/S0 conical intersection associated with the proton-transfer process is lower in energy than the known S1/S0 conical intersections associated with the excited-state deactivation processes inherent to the adenine moiety. These results support the conjecture that the photochemistry of hydrogen bonds plays a decisive role for the photostability of the molecular building blocks of RNA and DNA, which have been selected at the earliest stages of the chemical evolution of life.

In the present work we explored the mechanisms of the formations of the uracil cyclobutane and oxetane cyclodimers using ab initio methods through excited states and the S 0 – S 1 conical intersections. The excited-state reaction paths showed that the 1 ππ * excited states of the stacked dimers can relax, in a barrierless manner, to the ground states via S 0 – S 1 conical intersections. We found that the destruction of the cyclodimers occurs through 1 ππ * excited states and the same S 0 – S 1 conical intersections. In other words, the formation and the destruction of the uracil cyclodimers are competitive reactions initiated by UV light. Excited-state reaction paths show that the production of the cyclobutane dimer is much more favorable than the oxetane adduct.

The photodynamics of ethylene has been studied by means of ab initio surface-hopping dynamics using extended multireference configuration interaction wavefunctions. At the highest level, the explicit possibility of excited-state CH dissociation and consideration of the Rydberg π−3s state was included into the electronic wavefunction. The initial dynamics is characterised by the torsional motion and the crossing between the bright π−π * state with S 1, the latter having primarily Rydberg character with only a minor contribution of the repulsive valence π−σ * state. Due to back-rotation to planar structures after 17 fs, part of the population flows into the Rydberg states. The lifetime for this fraction of trajectories is significantly longer than that for the valence population. An analysis of the latter population shows that the decay to the ground state proceeds mainly at the pyramidalised conical intersection. Thus, no major qualitative mechanistic changes as compared to previous dynamics simulations ar...

2-Aminooxazole has recently been proposed as a hypothetical precursor of RNA nucleotides on early earth. UV irradiation was considered as a crucial environmental factor in the proposed reaction sequence. We report on state-of-the-art multireference quantum-chemical calculations elucidating the possible nonradiative deactivation channels of this compound. According to our findings, the gas-phase photochemistry of 2-aminooxazole should be dominated by the photodetachment of the hydrogen atom of the NH2 group via a 1πσNH* state leading either to ultrafast nonradiative deactivation, phototautomerization, or photodissociation of a hydrogen atom. We also identified a possible ring-opening reaction and a ring-puckering process that could occur after electronic excitation. These reactions seem to be less probable because they are driven by a higher-lying excited singlet state and are inherently slower than the hydrogen-atom dynamics.

We present the wavefunction (WF) version of the equation-of-motion phase-matching approach (EOM-PMA) for the calculation of four-wave-mixing (4WM) optical signals. For the material system, we consider a general electronic-vibrational Hamiltonian, comprising the electronic ground state, a manifold of singly-excited electronic states, and a manifold of doubly-excited electronic states. We show that the calculation of the third-order polarization for particular values of the pulse delay times and in a specific phase-matching direction requires 6 independent WF propagations within the rotating wave approximation. For material systems without optical transitions to doubly-excited electronic states, the number of WF propagations is reduced to 5. The WF EOM-PMA automatically accounts for pulse-overlap effects and allows the efficient numerical calculation of 4WM signals for vibronically coupled multimode material systems. The application of the method is illustrated for model systems with strong electron-vibrational and electronic inter-state couplings.

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The effect of a dissipative environment on the ultrafast nonadiabatic dynamics at conical intersections is analyzed for a two-state two-mode model chosen to represent the S2(ππ*)–S1(nπ*) conical intersection in pyrazine (the system) which is bilinearly coupled to infinitely many harmonic oscillators in thermal equilibrium (the bath). The system–bath coupling is modeled by the Drude spectral function. The equation of motion for the reduced density matrix of the system is solved numerically exactly with the hierarchy equation of motion method using graphics-processor-unit (GPU) technology. The simulations are valid for arbitrary strength of the system–bath coupling and arbitrary bath memory relaxation time. The present computational studies overcome the limitations of weak system–bath coupling and short memory relaxation time inherent in previous simulations based on multi-level Redfield theory [A. Kühl and W. Domcke, J. Chem. Phys. 2002, 116, 263]. Time evolutions of electronic state populations and time-dependent reduced probability densities of the coupling and tuning modes of the conical intersection have been obtained. It is found that even weak coupling to the bath effectively suppresses the irregular fluctuations of the electronic populations of the isolated two-mode conical intersection. While the population of the upper adiabatic electronic state (S2) is very efficiently quenched by the system–bath coupling, the population of the diabatic ππ* electronic state exhibits long-lived oscillations driven by coherent motion of the tuning mode. Counterintuitively, the coupling to the bath can lead to an enhanced lifetime of the coherence of the tuning mode as a result of effective damping of the highly excited coupling mode, which reduces the strong mode–mode coupling inherent to the conical intersection. The present results extend previous studies of the dissipative dynamics at conical intersections to the nonperturbative regime of system–bath coupling. They pave the way for future first-principles simulations of femtosecond time-resolved four-wave-mixing spectra of chromophores in condensed phases which are nonperturbative in the system dynamics, the system–bath coupling as well as the field-matter coupling.

Carbohydrates are essential building blocks of life that assume a multitude of biological functions in all living organisms found on Earth. It was recently reported that ribose was identified in UV-irradiated interstellar ice analogs, which suggests that it can be found on comets and that it may have been transported to Earth via the impact of comets. Herein, we present computational results obtained with multiconfigurational ab initio quantum-chemical methods showing that various photochemical processes for radiationless deactivation are available for photoexcited ribose. These processes are driven by nσ* states and involve either O-H- or endocyclic or exocyclic C-O-bond elongation whereby a conical intersection with the electronic ground state becomes accessible. The local topography of the potential-energy surfaces around these conical intersections suggests that these intersections mediate efficient radiationless deactivation and favor regeneration of the initially photoexcited ground-state reactant. These findings indicate that ribose found in interstellar space can be expected to be highly photostable upon irradiation with UV starlight, which could be of relevance in the field of astrobiology.

The availability of non-radiative decay mechanisms by which photoexcited molecules can revert to their ground electronic state, without experiencing potentially deleterious chemical transformation, is fundamental to molecular photostability. This Perspective Article combines results of new ab initio electronic structure calculations and prior experimental data in an effort to systematise trends in the non-radiative decay following UV excitation of selected families of heterocyclic molecules. We start with the prototypical uni- and bicyclic molecules phenol and indole, and explore the structural and photophysical consequences of incorporating progressively more nitrogen atoms within the respective ring structures en route to the DNA bases thymine, cytosine, adenine and guanine. For each of the latter, we identify low energy non-radiative decay pathways via conical intersections with the ground state potential energy surface accessed by out-of-plane ring deformations. This is followed by summary descriptions and illustrations of selected rival (electron driven H atom transfer) non-radiative excited state decay processes that demand consideration once the nucleobases are merely components in larger biomolecular systems like nucleosides, and both individual and stacked base-pairs.

The availability of non-radiative decay mechanisms by which photoexcited molecules can revert to their ground electronic state, without experiencing potentially deleterious chemical transformation, is fundamental to molecular photostability. This Perspective Article combines results of new ab initio electronic structure calculations and prior experimental data in an effort to systematise trends in the non-radiative decay following UV excitation of selected families of heterocyclic molecules. We start with the prototypical uni- and bicyclic molecules phenol and indole, and explore the structural and photophysical consequences of incorporating progressively more nitrogen atoms within the respective ring structures en route to the DNA bases thymine, cytosine, adenine and guanine. For each of the latter, we identify low energy non-radiative decay pathways via conical intersections with the ground state potential energy surface accessed by out-of-plane ring deformations. This is followed by summary descriptions and illustrations of selected rival (electron driven H atom transfer) non-radiative excited state decay processes that demand consideration once the nucleobases are merely components in larger biomolecular systems like nucleosides, and both individual and stacked base-pairs.

Femtosecond two-dimensional (2D) spectroscopy has become a widely employed method for the investigation of the dynamics of complex chemical and biological systems. In 2D spectroscopy, the sample is excited with three phase-locked femtosecond pulses, and the signal is heterodyned with the local oscillator field. The 2D spectrum is obtained by double Fourier transform with respect to the time delay between the first two pulses and the time delay between the third pulse and the local oscillator field. We show that 2D optical signals can alternatively be measured and computationally simulated as four-wave-mixing signals generated by two femtosecond pulses and two one-sided continuous-wave (CW) pulses. The first femtosecond pulse and one-sided CW pulse create the doorway state, while the second femtosecond pulse and one-sided CW pulse create the window state. This picture relates 2D spectroscopy to other mixed time-frequency-domain techniques, which is useful for the interpretation of the corresponding signals. Moreover, it allows a computationally efficient evaluation of 2D spectra.

We give a detailed theoretical analysis of the simplest variant of femtosecond stimulated Raman spectroscopy, where a picosecond Raman pump pulse and a femtosecond Raman probe pulse are applied resonantly to a chromophore in thermal equilibrium in the ground electronic state. We demonstrate that this technique is capable of the detection of dephasing-free Raman-like lines revealing vibrational modes not only in the electronic ground state but also in the excited electronic state of the chromophore. The analytical results obtained with simplifying assumptions for the shape of the laser pulses are substantiated by numerical simulations with realistic laser pulses, employing the equation-of-motion phase-matching approach.

We present a description of resonant femtosecond stimulated Raman spectra, which is based on the solution of the nonperturbative equation of motion of the chromophore in the laser fields. The theory is applicable for arbitrary shapes and durations of the Raman pulses, accounts for excited-state absorption, and describes nonstationary preparation of the system by an actinic pulse. The method is illustrated by the calculation of femtosecond stimulated Raman spectra of a model system with a conical intersection.

The photocatalytic splitting of water into H and OH radicals in hydrogen-bonded chromophore-water complexes has been explored with computational methods for the chromophores acridine orange (AO) and benzacridine (BA). These dyes are strong absorbers within the range of the solar spectrum. It is shown that low-lying charge-transfer excited states exist in the hydrogen-bonded AO H2O and BA H2O complexes which drive the transfer of a proton from water to the chromophore, which results in AOH OH or BAH OH biradicals. The AOH and BAH radicals possess bright ππ∗ excited states with vertical excitation energies near 3.0 eV which are predissociated by a low-lying repulsive πσ∗ state. The conical intersections of the πσ∗ state with the ππ∗ excited states and the ground state provide a mechanism for the photodetachment of the H-atom by a second photon. Our results indicate that AO and BA are promising chromophores for water splitting with visible light.

The Hamiltonian for linear and quadratic Jahn–Teller (JT) and pseudo-JT (PJT) coupling as well as zeroth-order and linear spin–orbit (SO) coupling in the 2 E + 2 B 2 electronic multiplet in D 2 d systems is derived. The SO coupling is described by the microscopic Breit–Pauli operator. It is shown that the 2 E state exhibits a 2 E × e JT effect which is of relativistic origin, that is, it arises from the SO operator. The relativistic PJT coupling of the 2 E and 2 B 2 states involves the normal modes of b 1 , b 2 and e symmetry. The 2 E × ( b 1 + b 2 + e ) JT Hamiltonian is analytically transformed to a SO-adapted electronic basis in which the zeroth-order SO operator is diagonal. In the special case of 2 E - 2 B degeneracy, the ( 2 E + 2 B 2 ) × ( b 1 + b 2 + e ) JT + PJT Hamiltonian also is transformed to a SO-adapted basis.

The effects of linear and higher-order Jahn–Teller couplings as well as spin–orbit coupling in orbitally degenerate electronic states of the transition-metal trifluorides MnF3 and CoF3 have been systematically explored with multi-configuration ab initio methods. The adiabatic potential-energy surfaces of low-lying 5E′ and 5E″ states have been calculated with the CASSCF method. The spin–orbit coupling is described by matrix elements of the Breit–Pauli operator with nonrelativistic CASSCF wave functions. The Jahn–Teller coupling parameters of the Jahn–Teller active in-plane bending and stretching modes have been determined up to sixth order and fourth order, respectively, in the normal-mode expansion. Vibronic spectra have been computed, employing a Jahn–Teller Hamiltonian up to sixth order in the degenerate bending mode and fourth order in the degenerate stretching mode. These results represent the first ab initio study of the dynamical Jahn–Teller effect in transition-metal trifluorides with inclusion of spin–orbit coupling.

We have extended the doorway-window representation of optical pump-probe spectroscopy with weak pulses toward N-wave-mixing spectroscopy with temporally well-separated pulses of arbitrary strength. The expressions for the signals in the strong-pulse doorway-window representation are derived in the framework of the nonperturbative theory of N-wave-mixing spectroscopy. The strong-pulse doorway-window representation is complementary to the equation-of-motion phase-matching approach. The latter fully accounts for pulse-overlap effects in signals induced by weak pulses but is computationally more expensive. The performance of the doorway-window approximation for temporally well-separated strong pulses is illustrated for an electronic two-level system with an underdamped Condon-active vibrational mode.

It is shown that orbitally triply degenerate spin-doublet states (2T2) in tetrahedral systems exhibit, in addition to the well-known electrostatic Jahn–Teller effects, Jahn–Teller couplings which are of relativistic origin, that is, they arise from the spin-orbit operator. The linear relativistic Jahn–Teller Hamiltonian of a 2T2 state involving vibrational modes of E and T2 symmetry is derived with group-theoretical methods. The 2T2 × (E + T2) Jahn–Teller Hamiltonian is transformed to a SO-adapted basis in which the zeroth-order spin-orbit-coupling matrix is diagonal. For the 4 × 4 sub-matrices corresponding to the G3/2 irreducible representation of the spin double group T d ′ , the adiabatic potential-energy surfaces and the geometric phases of the adiabatic electronic wave functions are given in analytic form.

A high-order expansion of the three potential-energy surfaces of the T 2 × e Jahn–Teller effect in tetrahedral systems is presented. It is shown that the expansion of the vibronic matrix can be obtained from two diagonal matrices determined from symmetry-invariant polynomials. The method is applied to the methane cation in its triply degenerate electronic ground state, which is known to exhibit an exceptionally strong Jahn–Teller effect. The potential-energy surfaces exhibit a highly anharmonic structure and multiple seams of intersections, rendering a high-order expansion of the potential matrix indispensable. An analytic expansion of the potential-energy surfaces up to 10th order has been fitted to accurate ab initio data.

CASSCF//CASPT2 pathways for a two-glycine minimal model system show that photoinduced electron-driven forward and backward proton transfer could play an important role for the stability of proteins against damage by UV radiation, when a hydrogen bond is located between the two amino acids. The overall photoinduced process involves two electron and proton transfer processes (forward and backward) and results in the reformation of the initial closed-shell electronic structure of the system.

The excited-state potential-energy surfaces of the lowest-energy conformer of the tryptophan-glycine dipeptide have been investigated with ab initio electronic-structure methods. The calculations reveal a potentially very efficient excited-state deactivation mechanism via conical intersections of the excited states of the indole chromophore with locally-excited and charge-transfer states of the peptide backbone. These findings suggest that the excited-state lifetime of the lowest-energy conformer of the Trp-Gly dipeptide may be too short to allow the detection of a resonant two-photon ionization signal. It is proposed that the efficient excited-state deactivation enhances the photostability of the dipeptide.

The photochemical reaction mechanisms of a supramolecular model system consisting of a truncated chlorophyll, imidazole, and benzoquinone have been explored with ab initio electronic-structure methods. The calculations indicate that a photoinduced electron-driven proton-transfer process leads to the oxidation of imidazole and reduction of the quinone. The repeated photoinduced reduction of benzoquinone is a mechanism which can lead to the decomposition of water by sunlight.

The multimode vibronic coupling problem is discussed and compared with the standard single-mode problem. It is demonstrated for two typical examples that the simultaneous consideration of two vibrational modes is essential to explain the experimental findings. The coupling of the involved electronic states via these modes gives rise to a conical intersection of the potential energy surfaces. The presence of the conical intersection strengthens considerably the nonadiabatic effects in the nuclear motion. Conical intersection of potential energy surfaces constitute a widespread phenomenon which should be considered when investigating vibronic coupling effects in realistic polyatomic molecules.

The vibronic Hamiltonians describing linear and quadratic Jahn–Teller coupling as well as spin–orbit coupling in ME states of trigonal systems are derived for the spin multiplicities M = 3 , 4 , 5 , starting from the microscopic Breit–Pauli spin–orbit operator. The 2 M × 2 M Hamiltonian matrices in a diabatic spin-electronic basis are obtained by the expansion of the Hamiltonian in powers of the Jahn–Teller active normal mode. It is shown that the ME × E Jahn–Teller Hamiltonians can be made block-diagonal by the transformation to an alternative diabatic basis which mixes electronic orbital and spin projections. The adiabatic potential-energy surfaces and the adiabatic electronic wave functions are obtained in analytical form. In systems with an odd number of electrons (and thus even spin multiplicity) the Jahn–Teller effect is quenched by strong spin–orbit coupling and the adiabatic potential-energy surfaces are strictly two-fold degenerate (Kramer’s degeneracy). In systems with an even number of electrons (and thus odd spin multiplicity), two of the 2 M adiabatic potentials exhibit an E × E Jahn–Teller effect which is unaffected by the spin–orbit interaction. The geometric phases of the adiabatic electronic wave functions are those of the 2E × E Jahn–Teller effect with and without spin–orbit coupling, respectively.

The glycine–(H 2 O) 2 cluster has been considered as a model for the study of the photochemistry of neutral and zwitterionic glycine in an aqueous environment. It has been found that the detachment of neutral NH 3 in the S 1 state of zwitterionic glycine leads in an essentially barrierless manner to a low-lying conical intersection of the S 1 and S 0 energy surfaces. This conical intersection can provide the mechanism for efficient radiationless deactivation of the excited state, leading to internal conversion or deamination (loss of ammonia). These results provide a mechanistic explanation of the high quantum yield of ammonia in the UV photolysis of non-aromatic amino acids in aqueous solution.

We propose a simple model which allows a comprehensive interpretation of the available experimental photon-echo data on Nile Blue. Homodyne signals such as two- and three-pulse photon echo, transient grating and three-pulse peak shift are addressed. An explanation of their time evolution is provided with a single parameter set. A new interpretation of two-dimensional (2D) heterodyne experimental spectra is proposed which emphasizes the strong electron–vibrational coupling of the 590 cm−1 mode of Nile Blue. The model is used for predictions of 2D correlation and relaxation signals monitored with improved resolution (low temperature, short pulse durations). It is argued that such measurements could provide the first experimental detection of the signatures of electron–vibrational coupling in 2D photon-echo spectra.

The role of electron- and proton-transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. Adopting indole, pyridine, and ammonia as molecular building blocks, we discuss generic mechanisms of the photophysics of isolated aromatic chromophores (indole), complexes of pi systems with solvent molecules (indole-ammonia, pyridine-ammonia), hydrogen-bonded aromatic pairs (indole-pyridine), and intramolecularly hydrogen-bonded pi systems (7-(2'-pyridyl)indole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections, and the properties of frontier orbitals. A common feature of the photochemistry of the various systems is the electron-driven proton-transfer (EDPT) mechanism: highly polar charge-transfer states of 1pipi*, 1npi*, or 1pisigma* character drive the proton transfer, which leads, in most cases, to a conical intersection of the S1 and S0 surfaces and thus ultrafast internal conversion. In intramolecularly hydrogen-bonded aromatic systems, out-of-plane torsion is additionally needed for barrierless access to the S1-S0 conical intersection. The EDPT process plays an essential role in diverse photophysical phenomena, such as fluorescence quenching in protic solvents, the function of organic photostabilizers, and the photostability of biological molecules.

The vibronic structure of the A ∼ 3 Π excited electronic state of CCX (X = O, S, Se) and CNY (Y = N, P, As) has been calculated by considering Renner–Teller coupling together with spin–orbit coupling. The vibronic and spin–orbit coupling parameters have been determined by accurate ab initio electronic-structure calculations. The effect of the linear (in the bending mode) spin–orbit coupling mechanism [S. Mishra et al., J. Chem. Phys. 126 (2007) 134312] on the vibronic structure of the 3 Π electronic state has been discussed. The vibronic structures of the A ∼ 3 Π electronic state of CCSe and CNAs are found to exhibit pronounced perturbations due to the linear spin–orbit coupling term.