Formation of benzene excimer following UV excitation of the neat liquid is monitored with femtosecond spectroscopy. A prompt rise component in excimer transient absorption, which contradicts the classical scenario of gradual reorientation and pairing of the excited monomers, is observed. Three-pulse experiments in which the population of evolving excimers is depleted by a secondary dump pulse demonstrate that the excimer absorption band is polarized along the interfragment axis. The experiments furthermore prove that the subsequent 4-fold increase in excimer absorption over ∼50 ps is primarily due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers forming excimers in a delayed fashion due to unfavorable initial geometry. Results are analyzed in light of recent studies of local structure in the liquid benzene combined with advanced electronic structure calculations. The prompt absorption rise is ascribed to excited states delocalized over nearby benzene molecules, which are sufficiently close and nearly parallel in the pure liquid. Such low-symmetry structures, which differ considerably from the optimized structures of isolated benzene dimer and solid benzene, are sufficiently abundant in liquid benzene. Electronic structure calculations confirm the orientation of transition dipoles of the excimers along the interparticle axis and demonstrate how slow refinement of the intermolecular geometry leads to a significant increase in the excimer absorption strength.

Photoinduced reactions play an important role in the photocycle of fluorescent proteins from the green fluorescent protein (GFP) family. Among such processes are photoisomerization, photooxidation/photoreduction, breaking and making of covalent bonds, and excited-state proton transfer (ESPT). Many of these transformations are initiated by electron transfer (ET). The quantum yields of these processes vary significantly, from nearly 1 for ESPT to 10–4–10–6 for ET. Importantly, even when quantum yields are relatively small, at the conditions of repeated illumination the overall effect is significant. Depending on the task at hand, fluorescent protein photochemistry is regarded either as an asset facilitating new applications or as a nuisance leading to the loss of optical output. The phenomena arising due to phototransformations include (i) large Stokes shifts, (ii) photoconversions, photoactivation, and photoswitching, (iii) phototoxicity, (iv) blinking, (v) permanent bleaching, and (vi) formation of long-lived intermediates. The focus of this review is on the most recent experimental and theoretical work on photoinduced transformations in fluorescent proteins. We also provide an overview of the photophysics of fluorescent proteins, highlighting the interplay between photochemistry and other channels (fluorescence, radiationless relaxation, and intersystem crossing). The similarities and differences with photochemical processes in other biological systems and in dyes are also discussed.

•Present the case for using multiple programming models to efficiently perform tensor contraction in quantum chemistry computations.•Show Libtensor multi-backend support using a unified interface.•Present Empirical measurements of data movement under the influence of programming models.•Analyze the performance of Libtensor framework on leading supercomputers.•Present a strategy to improve the energy-to-solution of tensor contraction computations.Coupled-cluster methods provide highly accurate models of molecular structure through explicit numerical calculation of tensors representing the correlation between electrons. These calculations are dominated by a sequence of tensor contractions, motivating the development of numerical libraries for such operations. While based on matrix–matrix multiplication, these libraries are specialized to exploit symmetries in the molecular structure and in electronic interactions, and thus reduce the size of the tensor representation and the complexity of contractions. The resulting algorithms are irregular and their parallelization has been previously achieved via the use of dynamic scheduling or specialized data decompositions. We introduce our efforts to extend the Libtensor framework to work in the distributed memory environment in a scalable and energy-efficient manner. We achieve up to 240× speedup compared with the optimized shared memory implementation of Libtensor. We attain scalability to hundreds of thousands of compute cores on three distributed-memory architectures (Cray XC30 and XC40, and IBM Blue Gene/Q), and on a heterogeneous GPU-CPU system (Cray XK7). As the bottlenecks shift from being compute-bound DGEMM’s to communication-bound collectives as the size of the molecular system scales, we adopt two radically different parallelization approaches for handling load-imbalance, tasking and bulk synchronous models. Nevertheless, we preserve a unified interface to both programming models to maintain the productivity of computational quantum chemists.

Photoinduced electron transfer in fluorescent proteins from the GFP family can be regarded either as an asset facilitating new applications or as a nuisance leading to the loss of optical output. Photooxidation commonly results in green-to-red photoconversion called oxidative redding. We discovered that yellow FPs do not undergo redding; however, the redding is restored upon halide binding. Calculations of the energetics of one-electron oxidation and possible electron transfer (ET) pathways suggested that excited-state ET proceeds through a hopping mechanism via Tyr145. In YFPs, the π-stacking of the chromophore with Tyr203 reduces its electron-donating ability, which can be restored by halide binding. Point mutations confirmed that Tyr145 is a key residue controlling ET. Substitution of Tyr145 by less-efficient electron acceptors resulted in highly photostable mutants. This strategy (i.e., calculation and disruption of ET pathways by mutations) may represent a new approach toward enhancing photostability of FPs.

Electronic factors controlling singlet fission (SF) rates are investigated in covalently linked dimers of tetracene. Using covalent linkers, relative orientation of the individual chromophores can be controlled, maximizing the rates of SF. Structures with coplanar and staggered arrangements of tetracene moieties are considered. The electronic structure calculations and three-state kinetic model for SF rates provide explanations for experimentally observed low SF yields in coplanar dimers and efficient SF in staggered dimers. The calculations illuminate the role of the excimer formation in SF process. The structural relaxation in the S1 state leads to the increased rate of the multi-exciton (ME) state formation, but impedes the second step, separation of the ME state into independent triplets. The slower second step reduces SF yield by allowing other processes, such as radiationless relaxation, to compete with triplet generation. The calculations of electronic couplings also suggest an increased rate of radiationless relaxation at the excimer geometries. Thus, the excimer serves as a trap of the ME state. The effect of covalent linkers on the electronic factors and SF rates is investigated. In all considered structures, the presence of the linker leads to larger couplings, however, the effect on the overall rate is less straightforward, since the linkers generally result in less favorable energetics. This complex behavior once again illustrates the importance of integrative approaches that evaluate the overall rate, rather than focusing on specific electronic factors such as energies or couplings.

Electronic factors controlling singlet fission (SF) rates are investigated in four-chromophore model systems comprising two covalently linked dimers of tetracene. By using adiabatic framework and wave function analysis tools, we show that the lowest singlet multiexciton states are localized on the individual molecules. The intermolecular ME states, in which the triplet excitons reside on separate moieties, are higher in energy and are not accessible from the lowest singlet excitonic state. Although the electronic states from the excitonic/charge-resonance band are delocalized over the four chromophores, the calculations suggest that the essential electronic properties controlling the rates of SF depend largely on the electronic properties of the individual covalently linked chromophores, owing to the strong through-bond couplings.

We apply high-level ab initio methods to describe the electronic structure of small clusters of ammonia and dimethyl ether (DME) doped with sodium, which provide a model for solvated electrons. We investigate the effect of the solvent and cluster size on the electronic states. We consider both energies and properties, with a focus on the shape of the electronic wave function and the related experimental observables such as photoelectron angular distributions. The central quantity in modeling photoionization experiments is the Dyson orbital, which describes the difference between the initial N-electron and final (N–1)-electron states of a system. Dyson orbitals enter the expression of the photoelectron matrix element, which determines total and partial photoionization cross-sections. We compute Dyson orbitals for the Na(NH3)n and Na(DME)m clusters using correlated wave functions (obtained with equation-of-motion coupled-cluster model for electron attachment with single and double substitutions) and compare them with more approximate Hartree-Fock and Kohn-Sham orbitals. We also analyze the effect of correlation and basis sets on the shapes of Dyson orbitals and the experimental observables.

The effective fragment potential (EFP) approach, which can be described as a nonempirical polarizable force field, affords an accurate first-principles treatment of noncovalent interactions in extended systems. EFP can also describe the effect of the environment on the electronic properties (e.g., electronic excitation energies and ionization and electron-attachment energies) of a subsystem via the QM/EFP (quantum mechanics/EFP) polarizable embedding scheme. The original formulation of the method assumes that the system can be separated, without breaking covalent bonds, into closed-shell fragments, such as solvent and solute molecules. Here, we present an extension of the EFP method to macromolecules (mEFP). Several schemes for breaking a large molecule into small fragments described by EFP are presented and benchmarked. We focus on the electronic properties of molecules embedded into a protein environment and consider ionization, electron-attachment, and excitation energies (single-point calculations only). The model systems include chromophores of green and red fluorescent proteins surrounded by several nearby amino acid residues and phenolate bound to the T4 lysozyme. All mEFP schemes show robust performance and accurately reproduce the reference full QM calculations. For further applications of mEFP, we recommend either the scheme in which the peptide is cut along the Cα–C bond, giving rise to one fragment per amino acid, or the scheme with two cuts per amino acid, along the Cα–C and Cα–N bonds. While using these fragmentation schemes, the errors in solvatochromic shifts in electronic energy differences (excitation, ionization, electron detachment, or electron-attachment) do not exceed 0.1 eV. The largest error of QM/mEFP against QM/EFP (no fragmentation of the EFP part) is 0.06 eV (in most cases, the errors are 0.01–0.02 eV). The errors in the QM/molecular mechanics calculations with standard point charges can be as large as 0.3 eV.

Electron attachment to closed-shell molecules is a gateway to various important processes in the gas and condensed phases. The properties of an electron-attached state, such as its energy and lifetime as well as the character of the molecular orbital to which the electron is attached, determine the fate of the anion. In this experimental and theoretical study of copper and silver fluoride anions, we introduce a  new type of metastable anionic state. Abrupt changes in photoelectron angular distributions point to the existence of autodetaching states. Equation-of-motion coupled-cluster singles and doubles calculations augmented by a complex absorbing potential identify some of these states as Σ and Π dipole-stabilized resonances, a new type of shape resonance. In addition, these molecules support valence and dipole-bound states and a Σ resonance of charge-transfer character. By featuring five different types of anionic states, they provide a vehicle for studying fundamental properties of anions and for validating new theoretical approaches for metastable states.

Red fluorescent proteins (RFPs) are indispensable tools for deep-tissue imaging, fluorescence resonance energy transfer applications, and super-resolution microscopy. Using time-resolved optical spectroscopy this study investigated photoinduced dynamics of three RFPs, KillerRed, mRFP, and DsRed. In all three RFPs, a new transient absorption intermediate was observed, which decays on a microsecond–millisecond time scale. This intermediate is characterized by red-shifted absorption at 1.68–1.72 eV (λmax = 720–740 nm). On the basis of electronic structure calculations, experimental evidence, and published literature, the chemical nature of the intermediate is assigned to an unusual open-shell dianionic chromophore (dianion-radical) formed via photoreduction. A doubly charged state that is not stable in the isolated (gas phase) chromophore is stabilized by the electrostatic field of the protein. Mechanistic implications for photobleaching, blinking, and phototoxicity are discussed.

The effect of morphology on singlet fission (SF) efficiency was investigated by using a combination of high-level electronic structure methods and a simple three-state kinetic model. The calculations reproduce the observed differences in SF efficiency in different polymorphs of 1,3-diphenylisobenzofuran (DPBF) and 1,6-diphenyl-1,3,5-hexatriene (DPH), as well as make predictions about 5,12-diphenyltetracene (DPT). The analysis of different factors contributing to the rates reveals that (i) there is more than one pair of adjacent chromophores that contribute to SF; (ii) not only slip-stacked configurations show efficient fission; and (iii) both electronic couplings and energy differences are responsible for different rates. The model predicts that the difference in SF efficiency in DPBF and DPH polymorphs increases at low temperature. In contrast, temperature dependence of the relative rates in the two DPT forms is predicted to be small. Our model predicts similar rates for the two polymorphs of DPT, although one form features much more favorable electronic couplings. This prediction depends strongly on the magnitude of Davydov’s splitting; small changes in its value may change the ratio in favor of faster SF in xylene-grown crystals of DPT.

A simple three-state model for the dynamics of the singlet fission (SF) process is developed. The model facilitates the analysis of the relative significance of different factors, such as electronic energies, couplings, and the entropic contributions. The entropic contributions to the rates are important; they drive the SF process in endoergic cases (such as tetracene). The anticipated magnitude of entropic contributions is illustrated by simple calculations. By considering a series of three acenes (tetracene, pentacene, and hexacene), we explained the experimentally observed 3 orders of magnitude difference in the rate of SF in tetracene and pentacene and predicted that the rate in hexacene will be slightly faster than in pentacene. This trend is driven by the increased thermodynamic drive for SF (Gibbs free energy difference of the initial excitonic state and two separated triplets). The model also explains experimentally observed fast SF in 5,12-diphenyltetracene. Consistently with the experimental observations, the model predicts weak temperature dependence of the multiexciton formation rate in tetracene as well as a reduced rate of this step in solutions and in isolated dimers.

This work investigates the performance of equation-of-motion coupled-cluster (EOM-CC) methods for describing the changes in the potential energy surfaces of the penta-2,4-dieniminium cation, a reduced model of the retinal chromophore of visual pigments, due to dynamical electron correlation effects. The ground-state wave function of this model includes charge-transfer and diradical configurations whose weights vary along different displacements and are rapidly changing at the conical intersection between the ground and the first excited states, making the shape of the potential energy surface sensitive to a balanced description of nondynamical and dynamical correlation. Recently, variational (MRCISD) and perturbative (MRPT2) approaches for including dynamical correlation in CASSCF-based calculations were tested along three representative ground state paths. Here, we use the same three paths to compare the performance of single-reference EOM-CC methods against MRCISD and MRCISD+Q. We find that the spin-flip variant of EOM-CCSD with perturbative inclusion of triple excitations (dT or fT) produces potential energy profiles of the two lowest electronic states in quantitative agreement with MRCISD+Q (our highest-quality reference method). The nonparallelity errors and differences in vertical energy differences of the two surfaces along these scans are less than 1.4 kcal/mol (EOM-SF-CCSD(dT) versus MRCISD+Q). For comparison, the largest error of MRCISD versus MRCISD+Q is 1.7 kcal/mol. Our results show that the EOM-CC methods provide an alternative to multireference approaches and may be used to study photochemical systems like the one used in this work.

Motivated by the discrepancies in recent experimental and theoretical studies of photodetachment from isolated model chromophores of the green fluorescent protein (GFP), this study reports calculations of the electron detachment energies and photoelectron spectra of the phenolate and deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) anions. The spectra were computed using double-harmonic parallel normal mode approximation. High-level coupled-cluster methods as well as density functional theory were used to compute vertical and adiabatic detachment energies of the phenolate anion serving as a model system representing anionic GFP-like chromophores (HBDI). The benchmark calculations reveal that the basis set has significant effect on the computed detachment energies, whereas the results are less sensitive to the level of electron correlation treatment. At least aug-cc-pVTZ basis set is required. The best ωB97X-D and CCSD(T) estimates of phenolate’s adiabatic detachment energy are 2.12 and 2.19 eV; these values are very close to the experimental value, 2.253 eV [Gunion et al. Int. J. Mass Spectrom. Ion Proc. 1992, 117, 601]. The best estimate of the vertical detachment energy of deprotonated HBDI is 2.76 eV, which supports bound character of the bright excited state in the Franck–Condon region. The most intense transition in the computed photoelectron spectra of both phenolate and deprotonated HBDI is the 0–0 S0–D0 transition, which is 0.11 eV below vertical detachment energy. Therefore, the position of the maximum of the photoelectron spectrum does not represent vertical detachment energy, and the direct comparison between theory and experiment must involve spectrum modeling.

Understanding the chromophore maturation process in fluorescent proteins is important for the design of proteins with improved properties. Here, we present the results of electronic structure calculations identifying the nature of a blue intermediate, a key species in the process of the red chromophore formation in DsRed, TagRFP, fluorescent timers, and PAmCherry. The chromophore of the blue intermediate has a structure in which the π-system of the imidazole ring is extended by the acylimine bond, which can be represented by the model N-[(5-hydroxy-1H-imidazole-2yl)methylidene]acetamide (HIMA) compound. Ab initio and QM/MM calculations of the isolated model and protein-bound (mTagBFP) chromophores identify the anionic form of HIMA as the only structure that has absorption that is consistent with the experiment and is stable in the protein binding pocket. The anion and zwitterion are the only protonation forms of HIMA whose absorption (421 and 414 nm, or 2.95 and 3.00 eV) matches the experimental spectrum of the blue form in DsRed (the absorption maximum is 408 nm or 3.04 eV) and mTagBFP (400 nm or 3.10 eV). The QM/MM optimization of the protein-bound anionic form results in a structure that is close to the X-ray one, whereas the zwitterionic chromophore is unstable in the protein binding pocket and undergoes prompt proton transfer. The computed excitation energy of the protein-bound anionic form of the mTagBFP-like chromophore (3.04 eV) agrees with the experimental absorption spectrum of the protein. The DsRed-like chromophore formation in red fluorescent proteins is revisited on the basis of ab initio results and verified by directed mutagenesis revealing a key role of the amino acid residue 70, which is the second after the chromophore tripeptide, in the formation process.

Molecular dynamics calculations of pressure effects on mStrawberry and mCherry fluorescent proteins are reported. The simulations reveal that mStrawberry has much floppier structure at atmospheric pressure, as evidenced by larger backbone fluctuations and the coexistence of two conformers that differ by Ser146 orientation. Consequently, pressure increase has a larger effect on mStrawberry, making its structure more rigid and reducing the population of one of the conformers. The most significant effect of pressure increase is in the hydrogen-bonding network between the chromophore and the nearby residues. The quantum-mechanics/molecular mechanics calculations of excitation energies in mStrawberry explain the observed blue shift and identify Lys70 as the residue that has the most pronounced effect on the spectra. The results suggest that pressure increase causes an initial increase of fluorescence yield only for relatively floppy fluorescent proteins, whereas the fluorescent proteins that have more rigid structures have quantum yields close to their maximum. The results suggest that a low quantum yield in fluorescent proteins is dynamic in nature and depends on the range of thermal motions of the chromophore and fluctuations in the H-bonding network rather than on their average structure.

The electronic structure of the three representative isomers of the ionized uracil dimers is characterized by high-level electronic structure calculations. Noncovalent interactions between the fragments lower the vertical ionization energies by 0.13−0.35 eV, the largest drop being observed for the stacked and the T-shaped isomers. The initial hole is delocalized in the stacked and the H-bonded isomers and is localized in the T-shaped one. The ionization induces significant structural relaxation and increases the binding energies. The stacked dimer cation relaxes to the symmetric structure bound by 22.7 kcal/mol. The T-shaped dimer cation has a binding energy of 25.1 kcal/mol. Thus, the relative order of the stacked and T-shaped isomers is reversed upon ionization. Finally, the H-bonded isomer, which relaxes to the proton-transferred structure, is bound by 37.0 kcal/mol. The electronic spectra of all three isomers characterized at the vertical and the relaxed geometries show different patterns, which may be exploited in spectroscopic probing of ionization-induced dynamics in these species.

A combined theoretical and experimental study of the ionized dimers of thymine and adenine, TT, AA, and AT, is presented. Experimentally observed and computed adiabatic and vertical ionization energies (IEs) for monomers and dimers as well as thresholds for the appearance of the protonated species are reported and analyzed. Non-covalent interactions strongly affect the observed IEs. The magnitude and the nature of the effect is different for different isomers of the dimers. The computations reveal that for TT, the largest changes in vertical IEs (0.4 eV) relative to the monomer occur in asymmetric H-bonded and symmetric π-stacked isomers, whereas in the lowest-energy symmetric H-bonded dimer the shift in IEs is much smaller (0.2 eV). The origin of the shift and the character of the ionized states is different in asymmetric H-bonded and symmetric stacked isomers. In the former, the initial hole is localized on one of the fragments, and the shift is due to the electrostatic stabilization of the positive charge of the ionized fragment by the dipole moment of the neutral fragment. In the latter, the hole is delocalized, and the change in IE is proportional to the overlap of the fragments’ MOs. Relative to TT, the shifts in AA and AT are much smaller due to a less efficient overlap, smaller dipole of A and the large energy gap between ionized states of A and T monomers in the case of AT dimer. The ionization of the H-bonded dimers results in barrierless (or nearly barrierless) proton transfer, whereas the π-stacked dimers relax to structures with the hole stabilized by the delocalization or electrostatic interactions.

We report a combined theoretical and experimental study of ionization of cytosine monomers and dimers. Gas-phase molecules are generated by thermal vaporization of cytosine followed by expansion of the vapor in a continuous supersonic jet seeded in Ar. The resulting species are investigated by single photon ionization with tunable vacuum-ultraviolet (VUV) synchrotron radiation and mass analyzed using reflectron mass spectrometry. Energy onsets for the measured photoionization efficiency (PIE) spectra are 8.60 ± 0.05 eV and 7.6 ± 0.1 eV for the monomer and the dimer, respectively, and provide an estimate for the adiabatic ionization energies (AIE). The first AIE and the ten lowest vertical ionization energies (VIEs) for selected isomers of cytosine dimer computed using equation-of-motion coupled-cluster (EOM-IP-CCSD) method are reported. The comparison of the computed VIEs with the derivative of the PIE spectra suggests that multiple isomers of the cytosine dimer are present in the molecular beam. The calculations reveal that the large red shift (0.7 eV) of the first IE of the lowest-energy cytosine dimer is due to strong inter-fragment electrostatic interactions, i.e., the hole localized on one of the fragments is stabilized by the dipole moment of the other. A sharp rise in the protonated cytosine ion (CH+) signal at 9.20 ± 0.05 eV is ascribed to the formation of protonated cytosine by dissociation of the ionized dimers. The dominant role of this channel is supported by the computed energy thresholds for the CH+ appearance and the barrierless or nearly barrierless ionization-induced proton transfer observed for five isomers of the dimer.

The implementation of the effective fragment potential (EFP) method within the Q-CHEM electronic structure package is presented. The EFP method is used to study noncovalent π−π and hydrogen-bonding interactions in DNA strands. Since EFP is a computationally inexpensive alternative to high-level ab initio calculations, it is possible to go beyond the dimers of nucleic acid bases and to investigate the asymptotic behavior of different components of the total interaction energy. The calculations demonstrated that the dispersion energy is a leading component in π-stacked oligomers of all sizes. Exchange-repulsion energy also plays an important role. The contribution of polarization is small in these systems, whereas the magnitude of electrostatics varies. Pairwise fragment interactions (i.e., the sum of dimer binding energies) were found to be a good approximation for the oligomer energy.

We present the results of quantum chemical calculations of the electronic properties of the anionic form of the green fluorescent protein chromophore in the gas phase. The vertical detachment energy of the chromophore is found to be 2.4−2.5 eV, which is below the strongly absorbing ππ* state at 2.6 eV. The vertical excitation of the lowest triplet state is around 1.9 eV, which is below the photodetachment continuum. Thus, the lowest bright singlet state is a resonance state embedded in the photodetachment continuum, whereas the lowest triplet state is a regular bound state. Based on our estimation of the vertical detachment energy, we attribute a minor feature in the action spectrum as due to the photodetachment transition. The benchmark results for the bright ππ* state demonstrated that the scaled opposite-spin method yields vertical excitation within 0.1 eV (20 nm) from the experimental maximum at 2.59 eV (479 nm). We also report estimations of the vertical excitation energy obtained with the equation-of-motion coupled cluster with the singles and doubles method, a multireference perturbation theory corrected approach MRMP2 as well as the time-dependent density functional theory with range-separated functionals. Expanding the basis set with diffuse functions lowers the ππ* vertical excitation energy by 0.1 eV at the same time revealing a continuum of “ionized” states, which embeds the bright ππ* transition.

We present quantum chemical calculations of the properties of the anionic form of the green fluorescent protein (GFP) chromophore that can be directly compared to the results of experimental measurements: the cis−trans isomerization energy profile in water. Calculations of the cis−trans chromophore isomerization pathway in the gas phase and in water reveal a problematic behavior of density functional theory and scaled opposite-spin-MP2 due to the multiconfigurational character of the wave function at twisted geometries. The solvent effects treated with the continuum solvation models, as well as with the water cluster model, are found to be important and can reduce the activation energy by more than 10 kcal/mol. Strong solvent effects are explained by the change in charge localization patterns along the isomerization coordinate. At the equilibrium, the negative charge is almost equally delocalized between the phenyl and imidazolin rings due to the interaction of two resonance structures, whereas at the transition state the charge is localized on the imidazolin moiety. Our best estimate of the barrier obtained in cluster calculations employing the effective fragment potential-based quantum mechanics/molecular mechanics method with the complete active space self-consistent field description of the chromophore augmented by perturbation theory correction and the TIP3P water model is 14.8 kcal/mol, which is in excellent agreement with the experimental value of 15.4 kcal/mol. This result helps to resolve previously reported disagreements between experimental measurements and theoretical estimates.

Photoelectron spectra of the cis and trans isomers of HCOH were computed using vibrational wave functions calculated by diagonalizing the Watson Hamiltonian, including up to four mode couplings. The full-dimensional CCSD(T)/cc-pVTZ potential energy surfaces were employed in the calculation. Photoionization induces significant changes in equilibrium structures, which results in long progressions in the ν5, ν4, and ν3 modes. The two isomers show progressions in different modes, which leads to qualitatively distinguishable spectra. The spectra were also calculated in the double harmonic parallel-mode (i.e, neglecting Duschinsky rotation) approximation. Calculating displacements along the normal coordinates of the cation state was found to give a better approximation to the vibrational configuration interaction spectrum; this is due to the effects of Duschinsky rotations on the vibrational wave functions.

Multiphoton ionization and dissociation processes in diazirine have been studied experimentally via 304−325 nm two-photon absorption and theoretically by using the EOM-CCSD and B3LYP methods. The electronic structure calculations identified two excited valence states and four Rydberg states in the region 4.0−8.5 eV. In one-photon excitation, the strongest absorption is to the 21A1(3px ← n) Rydberg state, whereas in two-photon absorption at comparable energies the first photon excites the low-lying 11B2 (π* ← n) valence state, from which the strongest absorption is to the dissociative valence 11A2 (π* ← σNN) state. The diazirine ion is calculated to be rather unstable, with a binding energy of only 0.73 eV and a geometry that resembles a weakly bound CH2+···N2 complex. In the experimental studies, resonance-enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH2+ ions from dissociative photoionization. It is proposed that weak one-photon absorption to the 11B2 state is immediately followed by more efficient absorption of another photon to reach the 11A2 state from which competition between ionization and fast dissociation takes place. Strong signals of CH+ ions are also detected and assigned to 2 + 1 REMPI via the D2Π (v′ = 2) ← ← X2Π (v′′ = 0) two-photon transition of CH fragments. Velocity map CH+ images show that CH(X, v′′ = 0, N′′) fragments are born with substantial translational energy, indicating that they arise from absorption of two photons in diazirine. It is argued that two-photon processes via the 11B2 intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the 11A2 state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN2. In agreement with other theoretical papers, we recommend revisions of the heats of formation of diazirine and diazomethane.

Equilibrium structure, vibrational frequencies, and ionization energies of the para-benzyne radical anion are characterized by coupled-cluster and equation-of-motion methods. Vibronic interactions with the low-lying excited state result in a flat potential energy surface along the coupling mode and even in a lower-symmetry C2v structures. Additional complications arise due to Hartree–Fock instabilities and near-instabilities. The magnitude of vibronic interactions was characterized by geometrical parameters, charge localization patterns and energy differences between the D2h and C2v structures. The observed trends suggest that the C2v minimum predicted by several theoretical methods is an artifact of incomplete correlation treatment. The comparison between the calculated and experimental spectrum confirmed D2h structure of the anion, as well as accuracy of the coupled-cluster and spin-flip structures, frequencies and normal modes of the anion and the diradical. Density functional calculations (B3LYP) yielded only a D2h minimum, however, the quality of the structure and vibrational frequencies is poor, as follows from the comparison to high-level wave function calculations and the calculated spectrum. The analysis of charge localization patterns and the performance of different functionals revealed that B3LYP underestimates the magnitude of vibronic interactions due to self-interaction error.

Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multireference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation. Our best estimates for the vertical excitation energies for the lowest singlet n → π* and π → π* are 5.0 ± 0.1 eV and 5.3 ± 0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and ±0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A′′ and A′ states leading to intensity borrowing by the forbidden transition.

The photodissociation dynamics of H2CO is known to involve electronic states S1, T1 and S0. Recent quasiclassical trajectory (QCT) calculations, in conjunction with experiment, have identified a “roaming” H-atom pathway to the molecular products, H2+CO [Townsend; et al. Science 2004, 306, 1158.]. These calculations were initiated at the global minimum (GM) of S0, which is where the initial wave function is located. The “roaming” mechanism is not seen if trajectories are initiated from the molecular transition state saddle point (SP). In this Letter we identify the minimum energy-crossing configurations and energy of the T1/S0 potentials as a step toward studying the multisurface nature of the photodissociation. QCT calculations are initiated at these configurations on a revised potential energy surface and the results are compared to those initiated, as previously, from the S0 GM as well as the S0 SP. The product state distributions of H2 + CO from trajectories initiated at the T1/S0 crossing are in excellent agreement with those initiated at the S0 GM.

We report a combined theoretical and experimental study of ionization of cytosine monomers and dimers. Gas-phase molecules are generated by thermal vaporization of cytosine followed by expansion of the vapor in a continuous supersonic jet seeded in Ar. The resulting species are investigated by single photon ionization with tunable vacuum-ultraviolet (VUV) synchrotron radiation and mass analyzed using reflectron mass spectrometry. Energy onsets for the measured photoionization efficiency (PIE) spectra are 8.60 ± 0.05 eV and 7.6 ± 0.1 eV for the monomer and the dimer, respectively, and provide an estimate for the adiabatic ionization energies (AIE). The first AIE and the ten lowest vertical ionization energies (VIEs) for selected isomers of cytosine dimer computed using equation-of-motion coupled-cluster (EOM-IP-CCSD) method are reported. The comparison of the computed VIEs with the derivative of the PIE spectra suggests that multiple isomers of the cytosine dimer are present in the molecular beam. The calculations reveal that the large red shift (0.7 eV) of the first IE of the lowest-energy cytosine dimer is due to strong inter-fragment electrostatic interactions, i.e., the hole localized on one of the fragments is stabilized by the dipole moment of the other. A sharp rise in the protonated cytosine ion (CH+) signal at 9.20 ± 0.05 eV is ascribed to the formation of protonated cytosine by dissociation of the ionized dimers. The dominant role of this channel is supported by the computed energy thresholds for the CH+ appearance and the barrierless or nearly barrierless ionization-induced proton transfer observed for five isomers of the dimer.

A combined theoretical and experimental study of the ionized dimers of thymine and adenine, TT, AA, and AT, is presented. Experimentally observed and computed adiabatic and vertical ionization energies (IEs) for monomers and dimers as well as thresholds for the appearance of the protonated species are reported and analyzed. Non-covalent interactions strongly affect the observed IEs. The magnitude and the nature of the effect is different for different isomers of the dimers. The computations reveal that for TT, the largest changes in vertical IEs (0.4 eV) relative to the monomer occur in asymmetric H-bonded and symmetric π-stacked isomers, whereas in the lowest-energy symmetric H-bonded dimer the shift in IEs is much smaller (0.2 eV). The origin of the shift and the character of the ionized states is different in asymmetric H-bonded and symmetric stacked isomers. In the former, the initial hole is localized on one of the fragments, and the shift is due to the electrostatic stabilization of the positive charge of the ionized fragment by the dipole moment of the neutral fragment. In the latter, the hole is delocalized, and the change in IE is proportional to the overlap of the fragments’ MOs. Relative to TT, the shifts in AA and AT are much smaller due to a less efficient overlap, smaller dipole of A and the large energy gap between ionized states of A and T monomers in the case of AT dimer. The ionization of the H-bonded dimers results in barrierless (or nearly barrierless) proton transfer, whereas the π-stacked dimers relax to structures with the hole stabilized by the delocalization or electrostatic interactions.

Electronic factors controlling singlet fission (SF) rates are investigated in covalently linked dimers of tetracene. Using covalent linkers, relative orientation of the individual chromophores can be controlled, maximizing the rates of SF. Structures with coplanar and staggered arrangements of tetracene moieties are considered. The electronic structure calculations and three-state kinetic model for SF rates provide explanations for experimentally observed low SF yields in coplanar dimers and efficient SF in staggered dimers. The calculations illuminate the role of the excimer formation in SF process. The structural relaxation in the S1 state leads to the increased rate of the multi-exciton (ME) state formation, but impedes the second step, separation of the ME state into independent triplets. The slower second step reduces SF yield by allowing other processes, such as radiationless relaxation, to compete with triplet generation. The calculations of electronic couplings also suggest an increased rate of radiationless relaxation at the excimer geometries. Thus, the excimer serves as a trap of the ME state. The effect of covalent linkers on the electronic factors and SF rates is investigated. In all considered structures, the presence of the linker leads to larger couplings, however, the effect on the overall rate is less straightforward, since the linkers generally result in less favorable energetics. This complex behavior once again illustrates the importance of integrative approaches that evaluate the overall rate, rather than focusing on specific electronic factors such as energies or couplings.