Methylene excitation energies,transition probabilities,and frequency dependent polarizabilities

The electronic structure of the low-lying valence states of the polyatomic free radical methylene (CH₂) is investigated at two different geometries using multiconfigurational self-consistent field (MC SCF) and multiconfigurational random phase approximation (MC TDHF MC RPA) techniques. Oscillator strengths and polarizabilities of CH₂ are also examined with the MC TDHF MC RPA.     

Photoionization cross sections involving an explicitly correlated initial state: Combination of multiconfigurational linear response and the stieltjes—tchebycheff moment theory

We have performed outer valence photoionization cross section calculations for N₂ and O₂. To do this we have combined several linear response techniques, in particular time-dependent Hartree—Fock (TDHF), multiconfigurational time-dependent Hartree—Fock (MC TDHF), and a modification of MC TDHF (MMC TDHF) with Stieltjes—Tchebycheff moment theory (STMT). To our knowledge, these MC TDHF and MMC TDHF calculations are the first which combine explicitly correlated Green function approaches with STMT. Since, in addition, these calculations are fully coupled, we expect the MC TDHF and in particular the MMC TDHF—STMT results to be highly reliable. For both N₂ and O₂ our MC TDHF—STMT and MMC TDHF—STMT results are in overall agreement with previous static exchange STMT results; however, there are a few significant differences and differences in detail in the partial and total photoionization cross sections. In particular, for example, for N₂ we note that the MMC TDHF—STMT does not give a “hump” resonance in the cross section for the (1π_u⁻¹)A²Π_u ionic state. In O₂ we note that the (3σ_g⁻¹) cross section obtained using MMC TDHF—STMT is substantially lower than the static exchange results.     

Self consistent field perturbation theory for Frenkel and charge transfer states of hydrogen bonded crystals

Comparison is presented of energies, oscillator strengths, and polarization directions calculated using the dipole length and dipole velocity formulas for the lowest 4–7ππ ^* transitions in eight 6π-electron molecules of C _s symmetry using various amounts of CI, from singly excited configurations only (SCI) to complete CI (CCI), and also using the time-dependent Hartree-Fock (TDHF) method. The standard simple PPP approximation was used. For the strongest transitions, SCI and especially TDHF give results in fairly good agreement with CCI. For weaker transitions both SCI and TDHF give similar results, quite different from the CCI solutions. Rational methods for selecting a small number of configurations needed to reproduce correct order of excited states (SECI-1) or a somewhat larger number reproducing correct order, energies, oscillator strengths and polarization directions of transitions (SECI-2) are presented. Both, and particularly the latter, give similar results from both dipole length or dipole velocity formulas when Linderberg's relation is used for matrix elements of the linear momentum operator and in this respect resemble the TDHF and CCI methods for which both formulas necessarily give identical results.     

Theoretical analysis of excited states and energy transfer mechanism in conjugated dendrimers

The excited states of the phenylene ethynylene dendrimer are investigated comprehensively by various electronic‐structure methods. Several computational methods, including SCS‐ADC(2), TDHF, TDDFT with different functionals (B3LYP, BH&HLYP, CAM‐B3LYP), and DFT/MRCI, are applied in systematic calculations. The theoretical approach based on the one‐electron transition density matrix is used to understand the electronic characters of excited states, particularly the contributions of local excitations and charge‐transfer excitations within all interacting conjugated branches. Furthermore, the potential energy curves of low‐lying electronic states as the functions of ethynylene bonds are constructed at different theoretical levels. This work provides us theoretical insights on the intramolecular excited‐state energy transfer mechanism of the dendrimers at the state‐of‐the‐art electronic‐structure theories. © 2014 Wiley Periodicals, Inc.     

Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices

Transition metal complexes are vital components in a wide range of photooptical applications; these range from targeted drug delivery to devices for the conversion of solar energy to electrical and/or stored chemical energy. Metal centered (MC) ligand field excited states play important roles in the photophysics of those complexes having partially filled d-orbitals. This review offers a broad perspective on key investigations that have characterized the chemistry and physics of MC excited states in d³ and d⁶ transition metal complexes. It will also illustrate the impact of these excited states on various photooptical applications and highlight efforts to understand, control, and tune these MC excited states in the context of such applications.     

A comparative calculation on excited state energies of some conjugated hydrocarbons

The energies of the single-configuration lowest π – π* singlet and triplet states of some conjugated hydrocarbons have been calculated by the MC-SCF method using the conjugate-gradient technique of minimisation. The results are compared with those calculated by other methods currently in use, like (a) single-configuration calculation with V_N?1 potential for virtual orbitals; (b) CI calculation involving singly excited states; and (c) TDHF method. It has been concluded that the results for the MC-SCF method are very good, considering that only a single open-shell configuration is involved.     

Dimensionality dependence of optical properties and quantum confinement effects of hydrogenated silicon nanostructures

The excited state properties of linear, planar, and spherical hydrogenated silicon nanostructures are studied systematically with use of a time-dependent Hartree-Fock (TDHF) approach with a semiempirical Hamiltonian. The calculated optical gaps decrease significantly from linear, planar, to spherical silicon structures, showing that the optical gap is dimensionality dependent and hence it can be varied by solely managing the shape of the nanostructures. Remarkably, the calculated exciton sizes of the lowest dipole-allowed excited states for both silicon chains and planes are approximately 26 A, revealing that the quantum confinement effect should be significantly enhanced when the sizes of silicon nanostructures are smaller than this value but not dependent on the dimensionality. A similar trend is also observed for hydrogenated silicon spherical clusters.     

Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited-State Decay

Square-planar Ni^II complexes are interesting as cheaper and more sustainable alternatives to Pt^II luminophores widely used in lighting and photocatalysis. We investigated the excited-state behavior of two Ni^II complexes, which are isostructural with two luminescent Pt^II complexes. The initially excited singlet metal-to-ligand charge transfer (¹MLCT) excited states in the Ni^II complexes decay to metal-centered (³MC) excited states within less than 1 picosecond, followed by non-radiative relaxation of the ³MC states to the electronic ground state within 9–21 ps. This contrasts with the population of an emissive triplet ligand-centered (³LC) excited state upon excitation of the Pt^II analogues. Structural distortions of the Ni^II complexes are responsible for this discrepant behavior and lead to dark ³MC states far lower in energy than the luminescent ³LC states of Pt^II compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited-state relaxation in four-coordinate Ni^II complexes could be decelerated such that luminescent ³LC or ³MLCT excited states become accessible. These insights are relevant to make Ni^II fit for photophysical and photochemical applications that relied on Pt^II until now.     

Ab initio study of the NMR shielding constants and spin-spin coupling constants in cyclopropene

Summary Ab initio calculations of parameters which characterize the NMR spectrum are presented for the cyclopropene molecule. The London orbitals CHF (or GIAO-CHF, Gauge-Independent Atomic Orbital Coupled Hartree-Fock) results for the shielding constants are in good agreement with the experimental data, accurately determined, and with otherab initio values. The calculations of the NMR spin-spin coupling constants have been performed using the Multiconfiguration Time-Dependent Hartree-Fock (MC TDHF) approach. Different basis sets and MC SCF wavefunctions were used to estimate the accuracy of the results. Good agreement is obtained with the coupling constants estimated using the available experimental data.Dedicated to Professor Werner Kutzelnigg on the occasion of his 60th birthday     

Semiempirical NDDO/MC calculations of the excited states of the chromate ion

A new semiempirical method of calculating the excited states of transition metal complexes is developed. This technique uses the configuration interaction and semiempirical NDDO/MC methods to obtain the ground state of a set of Slater type valence spd-orbitals chosen from the optical spectra of transition metals together with the corresponding core integrals. The method is tested in calculations of the electronically excited states of the chromate ion. Good agreement with the experimental energies of vertical transitions and the results of ab initio calculations is achieved.     

Computational Assessment of MLCT versus MC Stabilities in First-to-Third-Row d6 Pseudo-Octahedral Transition Metal Complexes

An accurate modeling of metal-to-ligand-charge-transfer (MLCT) and metal-centered (MC) excited state energies is key to predict the photoinduced response in transition metal complexes (TMCs). Herein, the importance of the ground state and excited state reference geometries is addressed for three-prototype d⁶ pseudo-octahedral TMCs, each displaying a different potential energy landscape of MLCT versus MC relative stabilities. Several functionals are used within the time-dependent density functional theory (TDDFT), as well as multireference wave-function theory (MS-CASPT2), applied to [Mn(im)(CO)₃(phen)]⁺, [Ru(im)₂(bpy)₂]²⁺, and [Re(im)(CO)₃(phen)]⁺, (im: imidazole, bpy: bypiridine, phen: phenantroline). The results revel that TDDFT is robust except when using B3LYP functional for first-row d⁶ TMCs. In contrast, MS-CASPT2 calculations are strongly biased in those cases with competitive MLCT/MC states. The results reinforce the reliability of B3LYP to describe the excited states in d⁶ TMCs, but question the validity of assessing the density functional theory (DFT)/TDDFT performance via direct comparison with MS-CASPT2 performed at the same DFT reference geometry as a standard strategy. © 2019 Wiley Periodicals, Inc.     

Excited state properties of 7-hydroxy-4-methylcoumarin in the gas phase and in solution. A theoretical study

TDDFT/B3LYP and RI-CC2 calculations with different basis sets have been performed for vertical and adiabatic excitations and emission properties of the lowest singlet states for the neutral (enol and keto), protonated and deprotonated forms of 7-hydroxy-4-methylcoumarin (7H4MC) in the gas phase and in solution. The effect of 7H4MC-solvent (water) interactions on the lowest excited and fluorescence states were computed using the Polarizable Continuum Method (PCM), 7H4MC-water clusters and a combination of both approaches. The calculations revealed that in aqueous solution the pi pi* energy is the lowest one for excitation and fluorescence transitions of all forms of 7H4MC studied. The calculated excitation and fluorescence energies in aqueous solution are in good agreement with experiment. It was found that, depending on the polarity of the medium, the solvent shifts vary, leading to a change in the character of the lowest excitation and fluorescence transition. The dipole-moment and electron-density changes of the excited states relative to the ground state correlate with the solvation effect on the singlet excited states and on transition energies, respectively. The calculations show that, in contrast to the ground state, the keto form has a lower energy in the pi pi* state as compared to enol, demonstrating from this point of view the energetic possibility of proton transfer from the enol to the keto form in the excited state.     

A multi-configuration LCAO–MO study for complex unsaturated molecules. II. Application to the benzene cation

The method of the MC –LCAO –MO approach, described in the preceding paper, is further applied to the benzene cation. Through the iteration process the π-electron energies and the molecular shapes are computed for the ground and two lowest excited states of the cation in both D_6h and D_2h geometries. A remarkable fact obtained is that a comparatively small variation of the geometrical structure (c. 0.010 – 0.013 Å bond length difference) brings about a considerable change of the energy value (c. 0.85 – 1.25 eV). The π-electronic excitation energies obtained from the iteration process are compared with the transition energies calculated from the usual method in which the structures of the excited states are assumed to be the same as the corresponding ground state structures. The difference in the excitation energy between the cation and the anion, and the CI effect on the excited states, are discussed. It is found that the doubly excited configurations play an important role in CI , which is somewhat different from that of the singly excited configurations. The stabilization energy due to the Jahn–Teller distortion is estimated for the ground state of the cation.     

Some remarks on multiconfiguration time-dependent Hartree–Fock theory

Recent progress in multiconfiguration time-dependent Hartree–Fock (MC –TDHF ) theory is reviewed. The generality of a formalism first put forward by Moccia [Int. J. Quantum Chemistry 8 , 293 (1974)] is emphasized, and illustrative applications are discussed. A calculation of the frequency-dependent electric-dipole polarizability of the helium atom shows that even with a modest amount of CI, it is possible to achieve an accuracy comparable to that obtained by use of Hylleraas-type wave functions.     

Molecular MC SCF calculations by direct minimization

Calculations on the H₂CO ground state and excited states at the equilibrium planar geometry, using the single excitation MC SCF method, are described for two basis sets. The results obtained by a full calculation including all the electrons are compared with those obtained by a pseudopotential version of the method including only the valence electrons. The results agree quite well both with each other and with the observed values.     

Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering

Light-driven chemical transformations provide a compelling approach to understanding chemical reactivity with the potential to use this understanding to advance solar energy and catalysis applications. Capturing the non-equilibrium trajectories of electronic excited states with precision, particularly for transition metal complexes, would provide a foundation for advancing both of these objectives. Of particular importance for 3d metal compounds is characterizing the population dynamics of charge-transfer (CT) and metal-centered (MC) electronic excited states and understanding how the inner coordination sphere structural dynamics mediate the interaction between these states. Recent advances in ultrafast X-ray laser science has enabled the electronic excited state dynamics in 3d metal complexes to be followed with unprecedented detail. This review will focus on simultaneous X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) studies of iron coordination and organometallic complexes. These simultaneous XES-XSS studies have provided detailed insight into the mechanism of light-induced spin crossover in iron coordination compounds, the interaction of CT and MC excited states in iron carbene photosensitizers, and the mechanism of Fe–S bond dissociation in cytochrome c.

Ultrafast X-ray scattering and spectroscopy captures photophysical and photochemical transformations of 3d transition metal complexes with atomistic detail.     

Application of the MC SCF method to the π → π* excitation energies of ethylene

The MC SCF method is employed to calculate the N → T and N → V π → π^* vertical excitation energies of ethylene. To obtain accurate excitation energies it is found to be necessary to utilize an expanded valence space containing two π and two π^* orbitals. Relatively small MC SCF calculations, allowing at most one-electron excitations from the sigma space, are found to yield excitation energies and spatial extents of the excited states in excellent agreement with the predictions of large multi-reference or iterative-natural-orbital CI calculations. These results show that within an MC SCF framework σ-σ correlation is unimportant for describing the π → π^* processes. We also conclude that the neglect of the effects of unlinked cluster terms in some of the CI calculations may have introduced small, but important, errors in the excitation energies and predictions of the spatial extent of the V state.     

Excited-state proton transfer behavior of 7-hydroxy-4-methylcoumarin in AOT reverse micelles

The excited-state proton transfer and phototautomerization of 7-hydroxy-4-methylcoumarin (7H4MC) dye has been studied in the confined water pools of AOT reverse micelles using steady-state and time-resolved fluorescence measurements. In the "dry" reverse micelles ([water]/[AOT], w(0) = 0), only the neutral form of the dye is present both in the ground and the excited states. At higher w(0) values, three prototropic forms, namely, neutral, anionic, and tautomeric, can be identified in the excited state, although only the neutral form of the dye is present in the ground state. From steady-state fluorescence results and time-resolved area-normalized emission spectra (TRANES), it is indicated that the anionic and tautomeric forms of the dye are the excited-state reaction products and that they arise apparently independently from the excited neutral form of the dye. In bulk water, however, there is no evidence of the tautomeric species and only the anionic form is observed in the excited state. The fluorescence quenching results of the three forms of 7H4MC by the different quenchers, potassium iodide, aniline, and N, N-dimethylaniline, suggest that the distribution of 7H4MC molecules in the reverse micelles is not diverse but that the different prototropic forms arise from the same population of the excited dye in the interfacial region.     

Ab initio calculations on the three lowest states of HO+2

Ab initio calculations were performed for the three lowest lying states of HO⁺₂. The ground state was found to be a bend ³A″ state. The first excited ¹A′ state cannot appropriately be described by a single determinant, therefore a MC SCF calculation was employed.