The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Excited-state properties and environmental effects for protonated schiff bases: a theoretical study.

Complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), density functional theory (DFT), time dependent DFT (TDDFT) and the singles and doubles coupled-cluster (CC2) methodologies have been used to study the ground state and excited states of protonated and neutral Schiff bases (PSB and SB) as models for the retinal chromophore. Systems with two to four conjugated double bonds are investigated. Geometry relaxation effects are studied in the excited pipi* state using the aforementioned methods. Taking the MRCI results as reference we find that CASSCF results are quite reliable even though overshooting of geometry changes is observed. TDDFT does not reproduce bond alternation well in the pipi* state. CC2 takes an intermediate position. Environmental effects due to solvent or protein surroundings have been studied in the excited states of the PSBs and SBs using a water molecule and solvated formate as model cases. Particular emphasis is given to the proton transfer process from the PSB to its solvent partner in the excited state. It is found that its feasibility is significantly enhanced in the excited state as compared to the ground state, which means that a proton transfer could be initiated already at an early step in the photodynamics of PSBs.     

Distinguishing yy‐G tautomers by their spectroscopic signatures: A theoretical investigation

A comprehensive theoretical study of electronic transitions of naphtho‐homologated yyG and its five possible tautomers (yyG‐AO7, yyG‐AEc, yyG‐AEt, yyG‐IcO17, and yyG‐ItO17) was performed. The nature of the low‐lying excited states is discussed, and the results are compared to that of y‐bases. Geometry optimizations were performed on the lowest excited singlet ππ* states. Finally, the effects of methanol solution and hydrogen bonding with cytosine on the absorption and emission spectra were examined. The ground state structures were optimized using both the DFT and ab initio HF methods, whereas the excited‐state structures were optimized using the CIS method. The methanol solution was found to red‐shifts both the absorption and emission maxima of the studied bases except for yyG, for which the absorption and emission maxima were blue‐shifted after solvation. In addition, hydrogen bonding with cytosine was found to blue‐shifts both the absorption and emission maxima of yyG, yyG‐AO7, yyG‐IcO17, and yyG‐ItO17. © 2013 Wiley Periodicals, Inc.     

Alkalized borazine: A simple recipe to design closed‐shell superalkalis

Superalkalis are hypervalent species, possessing smaller ionization energy (IE) than alkali metal. These species are typically designed by electronegative atoms with excess electropositive ligands. Typical examples include FLi₂, OLi₃, NLi₄, etc. Herein, we study successive alkali metal substitution at H atoms in borazine (B₃N₃H₆). Our B3LYP and MP2 calculations demonstrate that the vertical ionization energy (VIE) of B₃N₃H_6‐xLi_x decreases with the increase in x for x = 1–6. For x ≥ 4, the VIE of B₃N₃H_6‐xLi_x becomes lower than that of Li atom, thereby indicating their superalkali nature. More interestingly, all these species are planar with closed‐shell structure such that the NICS_zz value at the ring's center is reduced. We have also studied B₃N₃M₆ (M = Li, Na, and K) species and found that the VIE is further reduced in case of Na and K substitutions. These findings should suggest a simple yet effective route to the design of species with lower ionization energies than alkali metal.     

Ab initio study on the electronic structures of styrene in the Franck-Condon region

The electronic structures of styrene in the Franck‐Condon region have been theoretically examined by means of ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations. The optimized structure of styrene in S₀ is planar but the torsional motion of the phenyl group is very floppy. The S₁ state is assigned to the local π–π* excitation within the benzene ring. On the other hand, S₂, above S₁ by 0.561 eV, is assigned to a state that resembles the so‐called V‐state of ethylene. The transition intensity of S₀–S₁ is weak, while that of S₀–S₂ is strong. This is in good agreement with the experimental absorption spectrum where the S₀–S₁ and S₀–S₂ transitions are in the energy range of 290–220 nm. The optimized geometry of S₁, characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S₁ state. © 2002 Wiley Periodicals, Inc. J Comput Chem 9: 928–937, 2002     

Electronic Structure of the Ground and Excited States of β‐Carboline

Coupled‐cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of β‐carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.     

A theoretical study on three conformational structures of 2,6‐distyrylpyridine

Ab initio methods at the levels HF/cc‐pVDZ, HF/6‐31G(d,p), MP2/cc‐pVDZ, and MP2/6‐31G(d,p), as well as methods based on density functional theory (DFT) employing the hybrid functional B3LYP with the basis sets cc‐pVDZ and 6‐31G(d,p), have been applied to study the conformers of 2,6‐distyrylpyridine. Bond distances, bond angles, and dihedral angles have been calculated at the B3LYP level. The calculated values were in good agreement with those measured by X‐ray diffraction analysis of 2,6‐distyrylpyridine. The values calculated using the Hartree‐Fock method and second‐order perturbation theory (MP2) were inconsistent. The optimized lowest‐energy geometries were calculated from the reported X‐ray structural data by the B3LYP/cc‐pVDZ method. Three conformations, A, B, and C, were proposed for 2,6‐distyrylpyridine. Calculations at the three levels of theory indicated that conformation A was the most stable structure, with conformations C and B being higher in energy by 1.10 and 2.57 kcal/mol, respectively, using the same method and basis function. The same trend in the relative energies of the three possible conformations was observed at the two levels of theory and with the different basis sets employed. The reported X‐ray data were utilized to optimize total molecular energy of conformation A at the different calculation levels. The bond lengths, bond angles, and dihedral angles were then obtained from the optimized geometries by ab initio methods and by applying DFT using the two basis functions cc‐pVDZ and 6‐31G(d,p). The values were analyzed and compared. The calculated total energies, the relative energies of the molecular orbitals, the gap between them, and the dipole moment for each conformational structure proposed for 2,6‐distyrylpyridine are also reported. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Photochemical and electronic properties of conjugated bis(azo) compounds: an experimental and computational study

We have investigated the photophysical, photochemical and electrochemical properties of two bis(azo) derivatives, (E,E)-m-1 and (E,E)-p-1. The two compounds, which can be viewed as being composed of a pair of azobenzene units sharing one of their phenyl rings, differ only for the relative position of the two azo groups on the central phenyl ring-meta and para for m-1 and p-1, respectively. The UV-visible absorption spectra and photoisomerisation properties are noticeably different for the two structural isomers; (E,E)-m-1 behaves similarly to (E)-azobenzene, while (E,E)-p-1 exhibits a substantial red shift in the absorption bands and a decreased photoreactivity. The three geometric isomers of m-1, namely the E,E, E,Z and Z,Z isomers, cannot be resolved in a mixture by absorption spectroscopy, while the presence of three distinct species can be revealed by analysis of the absorption changes observed upon photoisomerisation of (E,E)-p-1. Quantum chemical ZINDO/1 calculations of vertical excitation energies nicely reproduce the observed absorption changes and support the idea that, while the absorption spectra of the geometrical isomers of m-1 are approximately given by the sum of the spectra of the constituting azobenzene units in their relevant isomeric form, this is not the case for p-1. From a detailed study on the E-->Z photoisomerisation reaction it was observed that the photoreactivity of an azo unit in m-1 is influenced by the isomeric state of the other one. Such observations indicate a different degree of electronic coupling and communication between the two azo units in m-1 and p-1, as confirmed by electrochemical experiments and quantum chemical calculations. The decreased photoisomerisation efficiency of (E,E)-p-1 compared to (E,E)-m-1 is rationalised by modelling the geometry relaxation of the lowest pi-pi* state. These results are expected to be important for the design of novel oligomers and polymers, based on the azobenzene unit, with predetermined photoreactivity.     

Theoretical study of the low‐lying electronic states of the molecular ion KRb+

The potential energy curves of the molecular ion KRb⁺ have been investigated for the 60 lowest molecular states of symmetry ²Σ⁺, ²Π, ²Δ, and Ω = 1/2, 3/2, and 5/2. Using an ab initio method, the calculation has been done in a one active electron approach based on nonempirical pseudopotentials with core valence effects taken into account through parameterized l‐dependent polarization potentials. Using the canonicals functions approach a rovibrational study is done by calculating the eigenvalues E_v, the rotational constants B_v, the centrifugal distortion constants D_v (up to 135 vibrational levels), and the spectroscopic constants ω_e and B_e for the five electronic states (1)²Σ⁺, (3)²Σ⁺, (1)²Π, (1)Ω = 1/2, and (1)Ω = 3/2. No comparison of these values with other results is yet possible because they are given here for the first time. Extensive tables of energy values of E_v, B_v, and D_v are displayed at http://hplasim2.univ‐lyon1.fr/allouche . © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

A Time‐Dependent Picture of the Ultrafast Deactivation of keto‐Cytosine Including Three‐State Conical Intersections

Using mixed quantum–classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto‐cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S₁ and S₂ states, with minor contributions from the S₃. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto‐cytosine to the ground state. Non‐adiabatic transfer between the ππ* and nπ* excited states and deactivation via three‐state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S₂ state. The latter two events can be connected to the fs and ps transients observed experimentally.     

How important is the dispersion interaction for cyclobis(paraquat‐p‐phenylene)‐based molecular “shuttles”? A theoretical study

Inclusion complexes of cyclobis(paraquat‐p‐phenylene) and various aromatic molecules in their neutral and oxidized form were studied at the LMP2/6‐311+G**//BHandHLYP/6‐31G* level of theory, which represents the highest level theoretical study to date for these complexes. The results show that it is dispersion interaction that contributes most to the binding energy. One electron oxidation of a guest molecule leads to complete dissociation of inclusion complex generating strong repulsion potential between guest and host molecules. Electrostatic interactions also can play an important role, provided the guest molecule has a dipole moment; however, dispersion interactions always dominate in binding energy. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Theoretical study of the electronic excited states of tetracyanoethylene and its radical anion.

The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.     

Photophysics of eumelanin: ab initio studies on the electronic spectroscopy and photochemistry of 5,6-dihydroxyindole.

Excited-state reaction paths and energy profiles of 5,6-dihydroxyindole (DHI), one of the elementary building blocks of eumelanin, have been determined with the approximated singles-and-doubles coupled-cluster (CC2) method. 6-Hydroxy-4-dihydro-indol-5-one (HHI) is identified as a photochromic species, which is formed via nonadiabatic hydrogen migration from the dangling OH group of DHI to the neighboring carbon atom of the six-membered ring. It is shown that HHI is a typical excited-state hydrogen-transfer (ESIHT) system. HHI absorbs strongly in the visible range of the spectrum. A barrierless hydrogen transfer in the (1)pipi* excited state, followed by barrierless torsion of the hydroxyl group, lead to a low-lying S(1)-S(0) conical intersection and thus to ultrafast internal conversion. This very efficient mechanism of excited-state deactivation provides HHI with a high degree of intrinsic photostability. It is suggested that the metastable photochemical product HHI plays an essential role for the photoprotective biological function of eumelanin.     

(P-Bis(pentafluorophenyl) substituted) PCP-pincer Ru(II) complexes: A theoretical study of the molecular structure and electronic properties

The differences between the molecular structures of the PCP-pincer complex [RuCl{C₆H₃(CH₂P(C₆H₅)₂)₂-2,6}(PPh₃)] ([RuCl(PCP^H)(PPh₃)], 1) and its tetrakis-pentafluorophenyl substituted analogue [RuCl{C₆H₃(CH₂P(C₆F₅)₂)₂-2,6}(PPh₃)] ([RuCl(PCP^F20)(PPh₃)], 2) have been rationalised by performing calculations on the cations [Ru(PCP^H)(PPh₃)]⁺ (1cat) and [Ru(PCP^F20)(PPh₃)]⁺ (2cat). The molecular interactions between the chloride ligand and the axial rings, as found in 1 and 2, respectively, have been studied computationally in the model systems [(C₆X₅PH₂)₂Cl⁻] (X = H, F). The calculations on 2cat show that in 2 it is most likely the attractive electrostatic interaction between the chloride ligand and the fluorinated phenyl rings that forces the C_ipso atom to occupy an axial position rather than an equatorial one in the observed (X-ray of 2) square pyramidal arrangement. In 1, however, repulsive steric hindrance forces the PPh₃ ligand to take the apical position. The applicability of the TD-DFT method for the calculation of the electronic spectra of the PCP-pincer compounds 1 and 2 has been tested. The results indicate that the excitation energies calculated for both complexes are in a reasonable agreement with the experimental absorption maxima. However, for 1, all the calculated transition energies are underestimated.     

Characterization of electronic structure and physicochemical properties of antiparasitic nifurtimox analogues: A theoretical study

American trypanosomiasis, also known as Chagas' disease, is caused by Trypanosoma cruzi (T. cruzi). It is well known that trypanosomes, and particularly T. cruzi, are highly sensitive towards oxidative stress, i.e., to compounds than are able to produce free radicals. Generally, nifurtimox (NFX) and benznidazol are most effective in the acute phase of the disease; therefore, nitroheterocycles constitute good models to design other nitrocompounds with specific biological characteristics. Thus, we have performed an ab initio study at the Hartree‐Fock and Density Functional Theory levels of theory of several NFX analogues recently synthesized, to characterize them by obtaining their electronic, structural, and physicochemical properties, which might be linked to the observed antichagasic activity. The antitrypanosomal activity scale previously reported for the NFX analogues studied in this work is in good agreement with our theoretical results, from which we can conclude that the activity seems to be related to the reactivity along with the acidity observed for the most active molecules. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

An Ab initio theoretical investigation on the geometrical and electronic structures of BAun−/0 (n = 1–4) clusters

An ab initio theoretical investigation on the geometrical and electronic structures and photoelectron spectroscopies (PES) of BAu_n^?/0 (n = 1–4) auroboranes has been performed in this work. Density functional theory and coupled cluster method (CCSD(T)) calculations indicate that BAu (n = 1–4) clusters with n‐Au terminals possess similar geometrical structures and bonding patterns with the corresponding boron hydrides BH. The PES spectra of BAu (n = 1–4) anions have been simulated computationally to facilitate their future experimental characterizations. In this series, the T_d BAu anion appears to be unique and particularly interesting: it possesses a perfect tetrahedral geometry and has the highest vertical electron detachment energy (VDE = 3.69 eV), largest HOMO‐LUMO gap (ΔE_gap = 3.0 eV), and the highest first excitation energy (E_ex = 2.18 eV). The possibility to use the tetrahedral BAu unit as the building block of Li⁺[BAu₄]^? ion‐pair and other [BAu₄]^?‐containing inorganic solids is discussed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ground and Electronically Excited Singlet‐State Structures of 5‐Fluoroindole Deduced from Rotationally Resolved Electronic Spectroscopy and ab Initio Theory

The structure and electronic properties of the electronic ground state and the lowest excited singlet state (S₁) of 5‐fluoroindole (5FI) were determined by using rotationally resolved spectroscopy of the vibration‐less electronic origin of 5FI. From the parameters of the axis reorientation Hamiltonian, the absolute orientation of the transition dipole moment in the molecular frame was determined and the character of the excited state was identified as L_b.     

Evaluation of the restricted virtual space approximation in the algebraic‐diagrammatic construction scheme for the polarization propagator to speed‐up excited‐state calculations

The applicability and limitations of the restricted virtual space (RVS) approximation within the algebraic‐diagrammatic construction (ADC) scheme for the polarization propagator up to third order is evaluated. In RVS‐ADC, not only the core but also a substantial amount of energetically high‐lying virtual orbitals is restricted in excitation energy calculations of low‐lying excited electronic states. Using octatetraene, indole, and pyridine as representative examples and different standard basis sets of triple‐zeta quality, RVS‐ADC(2) turns out to be highly useful and to have negligible effects on ππ* excited states. However, for nπ* or πσ* states, the RVS approximation is generally less reliable but better at third‐order than second‐order ADC level. In addition, a unified, basis‐set independent, thus normalized virtual orbital threshold (value) is introduced, making the RVS approximation more controllable and a priori applicable. © 2017 Wiley Periodicals, Inc.     

A combined gas-phase photoelectron spectroscopic and theoretical study of Zeise's anion and its bromine and iodine analogues

Shining light on Zeise: In a study of Zeise's anion, [PtCl(3)(C(2)H(4))](-), and its bromine and iodine analogues, electronic structure information for each species, derived from spectral features, is assigned through calculations at the coupled cluster level of theory. The calculations indicate that the electron binding energies decrease with halogen size and that there is a synergistic η(2) interaction between C(2)H(4) and the PtX(3)(-) anions.