Variable metric optimization of molecular geometry in electronically excited states

The variable metric (VM) method is used to optimize molecular geometry in electronically excited states. A general expression for the first derivative of energy in the particular excited state is derived, considering configuration interaction of all singly excited configurations. A special expression for the excited states energy derivative is given for calculations with semiempirical methods of CNDO type. The geometry optimizations of a set of molecules in various excited states have been carried out by the CNDO/2 method. The results of computations have been discussed and compared with the available experimental data. A good agreement of the calculated geometries with the experimental ones has been shown in the first excited states and a relatively good agreement in the higher states, with some exceptions. Some special features of the proposed method are discussed.     

Constraint Variational Calculations on Excited States

The difficulties of the SCF and MCSCF calculations on excited states have been reviewed. Three constraint variational methods are then developed which can perform the SCF and MCSCF calculations on excited states in the scheme of the generalized Brillouin theorem method. The proper constraints and the ways to incorporate the constraints into the variational calculations on excited states have been studied and the comparison with other approaches have been accessed.     

Electronic states and radiative transitions in LiAr

Ab initio MRD-CI calculations have been carried out on the ground and the eight lowest excited electronic states of LiAr, correlating with excited Li atom states up to 3d ²D. The ground (X²∑⁺ (2s)) and 2 ²Σ⁺ (2p) electronic states are repulsive while the higher excited states show shallow Rydberg minima. Rates of radiative bound-bound and bound-free transitions have been also calculated.     

Double proton transfer and charge-transfer transitions in biological hydrogen-bonded systems guaninecytosine (GC) and GCMg2+ systems

Potential energy curves for intermolecular proton transfer have been calculated within a modified INDO scheme. Analysis of the nature of the excited states, with emphasis on charge-transfer transitions, has been performed. The proton-transfer probability was found to be markedly greater in some excited states than for the ground state. With the metal ions the double proton transfer should be most effective in charge-transfer states.     

Second virial coefficients of oxygen—oxygen and carbon—oxygen interactions in different electronic states

Second virial coefficients of oxygen—oxygen and carbon—oxygen interactions in different electronic states have been calculated up to 20000 K. The results show that the virial coefficients of excited states are greater than the corresponding ground state values. The influence of the excited states on the total virial coefficients, calculated by assuming a Botlzmann distribution along the different states, has been found strong (up to 40%) in the carbon—oxygen system, while a minor effect (up to 3%) has been observed for the oxygen—oxygen system.     

Expanded electronic states of the fluorine molecule

Potential curves of electronically excited states of F₂ with an expanded outer orbital have been calculated using a modified frozen core technique: The ionic core has been described with a two-determinant wave function and for the excited states a mixing of configurations with different cores has been employed. An investigation of the valence shell states of F₂ is presented and potential curves for a singly excited as well as a doubly excited V-state of ¹Σ_u⁺ symmetry have been calculated. Further a low lying two-configuration state resulting from simultaneous excitation to a valence and a Rydberg orbital is predicted.     

Excited states in electron-transfer reaction products: ultrafast relaxation dynamics of an isolated acceptor radical anion

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.     

Double proton transfer and charge transfer transitions in hydrogen-bonded systems: formic acid dimer

The barriers for double proton transfer in the ground and lowest Π-Π^* and Π-Π^* excited states of the formic acid dimer have been calculated within a modified INDO scheme. Analysis of the nature of the excited electronic states, with emphasis on charge-transfer transitions, has been performed. The results indicate a lower barrier in the excited Π-Π^* states than in the ground state.     

Theoretical calculation about the valence and Rydberg excited states of hydrogen cyanide

The singlet and triplet excited states of hydrogen cyanide have been computed by using the complete active space self-consistent field and completed active space second order perturbation methods with the atomic natural orbital (ANO-L) basis set. Through calculations of vertical excitation energies, we have probed the transitions from ground state to valence excited states, and further extensions to the Rydberg states are achieved by adding 1s1p1d Rydberg orbitals into the ANO-L basis set. Four singlet and nine triplet excited states have been optimized. The computed adiabatic energies and the vertical transition energies agree well with the available experimental data and the inconsistencies with the available theoretical reports are discussed in detail.     

Role of the low-energy excited states in the radiolysis of aromatic liquids

The contribution of the low-energy excited states to the overall product formation in the radiolysis of simple aromatic liquids--benzene, pyridine, toluene, and aniline--has been examined by comparison of product yields obtained in UV-photolysis and in γ-radiolysis. In photolysis, these electronic excited states were selectively populated using UV-light excitation sources with various energies. Yields of molecular hydrogen and of "dimers" (biphenyl, bibenzyl, dipyridyl for benzene, toluene, pyridine, respectively, and of ammonia and diphenylamine for aniline) have been determined, since they are the most abundant radiolytic products. Negligibly small production of molecular hydrogen in the UV-photolysis of aromatic liquids with excitation to energies of 4.88, 5.41, 5.79, and 6.70 eV and the lack of a scavenger effect suggest that this product originates from short-lived high-energy singlet states. A significant reduction in "dimer" radiation-chemical yields in the presence of scavengers such as anthracene or naphthalene indicates that the triplet excited states are important precursors to these products. The results for toluene and aniline suggest that efficient dissociation from the lowest-energy excited triplet state leads to noticeable "dimer" production. For benzene and pyridine, the lowest-energy triplet excited states are not likely to fragment into radicals because of the relatively large energy gap between the excited state level and corresponding bond dissociation energy. The "dimer" formation in the radiolysis of benzene and pyridine is likely to involve short-lived high-energy triplet states.     

Electronic structure of benzaldehyde: A comparative study of the lowest excited singlet π* ← π and π* ← n states

VE-PPP, CNDO/2, and CNDO/s-CI methods have been used to investigate the electronic spectrum and structure of benzaldehyde. Electronic charge distributions and bond orders in the ground and lowest excited singlet π* ← π and π* ← n states of the molecule have been studied. The molecule has been shown to be nonplanar in the lowest π* ← n excited singlet state, in agreement with the conclusions drawn from the study of vibrational spectra. Dipole moments in both excited states have been shown to be larger than the ground-state value. Thus, the ambiguity in the experimental result for the π* ← π n excited singlet state dipole moment has been resolved. It has been shown that the n orbital is mainly localized on the CHO group. Furthermore, charge distributions, dipole moments, and molecular geometries are shown to be very different in the excited singlet π* ← π and π* ← n states.     

Photophysics and spectroscopy of the higher electronic states of zinc metalloporphyrins: a theoretical and experimental study

The photophysics of the S2 and S1 excited states of zinc porphyrin (ZnP) and five of its derivatives (ZnOEP, ZnTBP, ZnTPP, ZnTFPP, ZnTCl8PP) have been investigated by measuring their steady-state absorption and fluorescence spectra, quantum yields and excited state lifetimes at room temperature in several solvents. The radiative and radiationless decay constants of the fluorescent excited states accessible in the visible and near UV regions of the spectrum have been obtained. Despite the similarities in the Soret spectra of these compounds, their S2 excited state radiationless decay rates differ markedly. Although the S2-S1 electronic energies of a given zinc porphyrin vary linearly with the Lippert (refractive index) function of the solvent, the S2 radiationless decay rates of the set of compounds do not follow the energy gap law of radiationless transition theory. Calculations, using time-dependent density functional theory (TDDFT), of the energies and symmetries of the complete set of excited states accessible by 1- or 2-photon absorption in the near UV-visible have also been carried out. Substitution on the porphyrin macrocycle framework affects the ground state geometry and alters the electron density distributions, the orbital energies and the relative order of the excited electronic states accessible in the near UV-blue regions of the spectrum. The results are used to help interpret both the nature of the electronic transitions in the Soret region, and the relative magnitudes of the radiationless transition rates of the excited states involved.     

Ab initio study of phosphorescent emitters based on rare-earth complexes with organic ligands for organic electroluminescent devices

An ab initio approach is developed for calculation of low-lying excited states in Ln(3+) complexes with organic ligands. The energies of the ground and excited states are calculated using the XMCQDPT2/CASSCF approximation; the 4f electrons of the Ln(3+) ion are included in the core, and the effects of the core electrons are described by scalar quasirelativistic 4f-in-core pseudopotentials. The geometries of the complexes in the ground and triplet excited states are fully optimized at the CASSCF level, and the resulting excited states have been found to be localized on one of the ligands. The efficiency of ligand-to-lanthanide energy transfer is assessed based on the relative energies of the triplet excited states localized on the organic ligands with respect to the receiving and emitting levels of the Ln(3+) ion. It is shown that ligand relaxation in the excited state should be properly taken into account in order to adequately describe energy transfer in the complexes. It is demonstrated that the efficiency of antenna ligands for lanthanide complexes used as phosphorescent emitters in organic light-emitting devices can be reasonably predicted using the procedure suggested in this work. Hence, the best antenna ligands can be selected in silico based on theoretical calculations of ligand-localized excited energy levels.     

Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin-orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.     

High-order non-born-oppenheimer electric polarizabilities for excited states of H+2 via the perturbational-variational rayleigh-ritz formalism

Large-order perturbation theory in the form of the perturbational-variational Rayleigh-Ritz (PV RR) formalism has been applied to excited energy levels in the Stark effect for H⁺₂,yielding non-Born-Oppenheimer electric polarizabilities through 16th, 8th, 4th, and 2nd order, respectively, for the first four excited non-rotational states; these non-Born-Oppenheimer excited states correspond to what are normally known as vibrationally excited levels of the electronic ground state. The excited-state polarizability series are found to be more divergent in each corresponding order than the ground-state one, becoming increasingly so for the higher excited states. Methods for obtaining the excited-state polarizabilities to still higher order within the framework of PV RR are discussed.     

A Theoretical Study on Electronically Excited States of the Hydrogen-Bonded Clusters for Fluorenone and Fluorenone Derivatives in Methanol Solvent

The time-dependent density functional theory (TDDFT) method has been carried out to study the hydrogen-bonding of fluorenone (FN) and FN derivatives (FODs) in hydrogen-donating methanol solvent. The ground-state geometry structure optimizations, electronic excitation energies and corresponding oscillation strengths of the low-lying electronically excited states for the isolated FN, FODs and methanol monomers and their corresponding complexes have been calculated using DFT and TDDFT methods respectively. Comparing FODs with FN, we have obtained the strength change of the hydrogen bonds and the electronic spectral shift in different excited states. At the same time, the nature of the FODs in the electronic excited states and the influence of the different substituent group have been summed up.     

Chemically Produced Excited States

Electronically excited states of organic molecules are formed in many chemical reactions. Such chemically produced excited states are (with one exception) identical to light produced excited states, and they undergo the molecular transformations expected of such states (“photochemistry without light”). The excited states can also be used in energy transfer experiments. This review covers the generation of chemically produced excited states, the chemical reactions they undergo, and the possible role of chemically produced excited states in biology.     

Electron transfer dynamics from organic adsorbate to a semiconductor surface: zinc phthalocyanine on TiO2(110)

The femtosecond time evolutions of excited states in zinc phthalocyanine (ZnPC) films and at the interface with TiO2(110) have been studied by using time-resolved two-photon photoelectron spectroscopy (TR-2PPE). The excited states are prepared in the first singlet excited state (S1) with excess vibrational energy. Two different films are examined: ultrathin (monolayer) and thick films of approximately 30 A in thickness. The decay behavior depends on the thickness of the film. In the case of the thick film, TR-2PPE spectra are dominated by the signals from ZnPC in the film. The excited states decay with tau = 118 fs mainly by intramolecular vibrational relaxation. After the excited states cascaded down to near the bottom of the S1 manifold, they decay slowly (tau = 56 ps) although the states are located at above the conduction band minimum of the bulk TiO2. The exciton migration in the thick film is the rate-determining step for the electron transfer from the film to the bulk TiO2. In the case of the ultrathin film, the contribution of electron transfer is more evident. The excited states decay faster than those in the thick film, because the electron transfer competes with the intramolecular relaxation processes. The electronic coupling with empty bands in the conduction band of TiO2 plays an important role in the electron transfer. The lower limit of the electron-transfer rate was estimated to be 1/296 fs(-1). After the excited states relax to the states whose energy is below the conduction band minimum of TiO2, they decay much more slowly because the electron-transfer channel is not available for these states.