Relative stabilities and the spectral signatures of stacked and hydrogen-bonded dimers of serotonin

The O–H???N hydrogen-bonded dimer of serotonin is shown to be more stable than the stacked dimer in its ground electronic state, by using the Møller–Plesset second-order perturbation theory (MP2) and the 6–31g^** basis set. The vertical excitation energy for the lowest π?→?π* transition for the monomer as well as the dimer is predicted by time-dependent density functional theory. The experimentally observed red shift of excitation wavelength on oligomerisation is explained in terms of the change in the HOMO–LUMO energy gap due to complex formation. The impact of dimer formation on the proton magnetic resonance spectrum of serotonin monomer is also examined.     

In search of the dark state of 5-methyl-2-hydroxypyrimidine using a numerical DFT/MRCI gradient

In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S₁ population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses ³(n?→?π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying ³(π?→?π*) state whose PEH crosses both, the S₁ ¹(n?→?π*) and T₁ ³(n?→?π*) PEHs.     

Structure and electronic absorption spectra of isotruxene dyes for dye-sensitized solar cells: Investigation by the DFT, TDDFT, and QTAIM methods

The electronic structure of two photosensitizing isotruxene dyes used in Grätzel photoelectro-chemical cells has been investigated by the quantum-chemical methods of the density functional theory (DFT) and Bader’s topological analysis of the electron density. It has been shown that intramolecular interactions in the dye molecules stabilize the planar relative arrangement of isotruxene and cyano-acrylic structural fragments, which provides π-conjugation over the whole molecular backbone. Based on the time-dependent density functional theory (TDDFT) calculations and analysis of the Kohn-Sham orbitals, it has been found that the first vertical electronic transition in the spectra of the isotruxene dyes is a combination of a poorly resolved lπ*-type excitation and a well-resolved π-π* excitation. The π-π* excitation admixture is associated with the asymmetric structure of the isotruxene fragment and contributes significantly to the intensity of the first electronic transition.     

CNDO/S Method Interpretation of Ultraviolet Spectra of Bithiazoles

Abstract

The uv spectra of five bithiazoles are reported. These experimental results are interpreted using CNDO/S method (Complete Neglect of Differential Overlap for Spectroscopy). Calculations show that the observed transitions are π→π? type transitions.     

CNDO/S Method Interpretation of Ultraviolet Spectra of New 5-Nitroimidazoles

Abstract

The uv spectra of three new highly active against anaerobic bacteria 5-nitroimidazoles are reported. These experimental results are interpreted using CNDO/S method (Complete Neglect of Differential Overlap for Spectroscopy). Calculations show that the observed transitions, except one, are π→π? type transitions.     

Ab initio SCF CI study of the electronic spectra of nitroethylene

Configuration interaction studies of ground, n→π* and π→π* electronically excited states are reported for nitroethylene in its ground-state equilibrium geometry. In case of the n→π* transitions, the two symmetry combinations of oxygen lone pair orbitals are found to give almost the same transition energies of 4·42 eV and 4·48 eV. These are in good agreement with the experimental transition energy of 4·20 eV. The calculated transition energy to the lowest π→π* excited ¹ A′ state of 5·48 eV is also in good agreement with the reported experimental value of 5·12 eV. Numerous other singlet states as well as the triplet states have also been calculated.     

Bridge Effect in Charge Transfer Absorption Bands. Para-substituted Benzylideneacetones

Abstract

The electronic absorption charge transfer bands in a series of para - substituted benzalketones are analyzed in order to stablish the role of the electron-donor substkuent as well as the electronic properties of the molecular structure of the π-conduction channel.

Absorption bands assignment of the π-π^? electronic transitions in the near ultraviolet spectral region is carry out from an experimental and theoretical point of view. The photo-induced charge transfer spectral bands in these aromatic compounds follow the same spectral pattern than the para-substituted benzaldehydes and acetophenones and the electronic transition takes place in the π,π^?(¹L_a) excited state. However, our semiempirical M.O. calculations show that this charge transfer process involve the electron-acceptor carbonyl group and the olefinic bond bridge as a second electron-acceptor group.     

New small organic molecules based on thieno[2,3-b]indole for efficient bulk heterojunction organic solar cells: a computational study

ABSTRACT

This study aims to evaluate the characteristics of novel organic D-π-A-π-D class small-molecules by using carefully the density functional theory, and time-dependent density functional theory calculations. Thedesigned sequence of (D-A) BHJ-1a to BHJ-4a in organic Bulk Heterojunction (BHJ) solar cells has been comprehensively analysed. Thiéno[2,3-b]indole (TI) has been used as donor, and Diketopyrrolopyrrole (DPP) as acceptor for all compounds. In order to improve the electronic, photovoltaic, and opticalproperties, we have substituted thiophene unit with furan, thieno[2,3-b]thiophene, thiazole and thiazolothiazole as π-bridge moieties. Thus, the result shows that the wise choice of the π-bridge units plays a significant role in improving E_gap, producing a high bathochromic shift, and increasing V_OC as well as a theoretical power conversion efficiency (PCE) over 7%. Interestingly, BHJ-4a with suitable π-bridge presents the optimal electronic properties with low band gap (1.870?eV) and high V_OC (1.534?eV). Furthermore, we have modelled a Bulk heterojunction organic photovoltaic cells based on donor-PCBM complex in order to achieve the optimum E_gap and V_OC. Consequently, the obtained results provide a new way to design BHJ small molecule donors with higher power conversion efficiency.     

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

Laser photodissociation spectroscopy of the I^‐·guanine complex has been conducted for the first time across the regions above the electron detachment threshold to explore the excited states and whether vertical ionisation occurs from the iodide or the nucleobase. The photofragment spectra reveal a prominent dipole-bound excited state (I) close to the calculated vertical electron detachment energy (～4.0?eV) and a second excited (II) centred around 4.8?eV, which we assign to π-π* nucleobase-localised transitions. The ionic photofragments are identified as I^‐ and I^‐·[G-H], with the later fragment being produced significantly more strongly than the former. Both photofragments are observed across the two excited states, with production of the iodide being attributed to internal conversion to the ground state followed by evaporation. We trace the formation of the I^‐·[G-H] photofragment to initial vertical ionisation of guanine, followed by ejection of a proton. This two-step process is important as it follows known steps in radiation-induced damage to DNA, namely initial formation of a guanine radical cation which then forms a free radical [G-H] moiety through deprotonation. Production of the I^‐·[G-H] photofragment is pronounced through II indicating that its formation is enhanced by coupling of the π-π* transitions to the electron detachment continuum.     

Computational study of chiral molecules with high intrinsic hyperpolarizabilities

We have investigated the electronic transition, chiroptical properties, and the second-order nonlinear optical (NLO) properties of eight novel chiral diborate compounds and elucidated structure–property relationships from the micromechanism. These compounds show calculated first hyperpolarizabilities (β) ranging from 2738.52 to 83976.45?×?10^?33?esu, which means that subtle structural modifications can substantially enhance the first hyperpolarizability. The cooperativity of intramolecular charge transfer and an effective way to enhance the NLO response were also systemically investigated. The linear correlation between the first hyperpolarizability and the inverse of the electronic transition energy suggests that the electronic transition energy plays a key role in determining the NLO response. These compounds have the potential to be excellent second-order NLO materials from the standpoint of the large β values, high transparency and the intrinsic non-centrosymmetry. The electronic transition and chiroptical properties have been assigned and analysed. The main UV–visible absorption features are best described as π?→?π* transitions. Moreover, the effects of different functionals and basis sets on the first hyperpolarizability were investigated.     

Vibrational and Electronic Spectral Study on 3,5-Dihydro-4H-Imidazol-4-One Derivatives Containing Organo-Selenium

The Fourier transform infrared, Raman, and UV-visible absorption spectral analyses were carried out on five 3,5-dihydro-4H-imidazol-4-one derivatives containing organo-selenium. The ground-state geometries, vibrational wavenumbers, and infrared and Raman intensities were calculated by density functional theory using the B3PW91 functional and 6-311G(2df,p) basis set. Spectral analysis shows that the large conjugation effect makes the C=N stretching vibration of the skeleton shift by 50–100 cm^?1 toward the lower wavenumber. The UV-visible absorption spectra were studied by the time-dependent density functional theory method with the polarizable continuum model using B3LYP and PBE0 functionals. Theoretical calculations reveal that the strong absorption bands at 368–411 nm arise from the n → π* transition, and the sharp bands at 208–211 nm arise from the π → π* transition.     

Study on Stiffness of the Symmetry Energy at High Densities by π－/π+ Ratio

using the isospin–and momentum–dependent transport model, for three different symmetry energies, we have calculated the π^–/π⁺ ratio as a function of time and tranverse–momentum in central 132sn+124 sn collision at e_beam/A=400mev. the results show that the π^–/π⁺ ratios is very sensitive to the stiffness of the symmetry energy. thus we can probe the stiffness of the symmetry energy at high densities by using the π–/π+ ratio at different time or transverse momentum more accurately than before. comparing experimental data with the calculations will reveal crucial information about the stiffness of the symmetry energy more accurately.hadronic transport model, symmetry energy, π–/π+ ratio     

ESR Detection of Triplet States in Rare Earth Chelates

Abstract

Energy transfer from ligand molecule to ion in rare earth chelates has been assumed to involve a molecular triplet state. ^1,2 These triplet states manifest themselves optically as long-lived T_{π, π? →S}π,π emission if the lowest resonance level of the ion is of higher energy than the molecular triplet. ³ If the resonance level lies below the triplet, longlived emission is absent or severely reduced in intensity. ^4,5     

Rotationally averaged linear absorption spectra beyond the electric-dipole approximation

ABSTRACT

Based on the recently developed implementation of the full semi-classical field–matter interaction operator, we present a numerically accurate yet efficient scheme to perform rotational averaging of linear absorption spectra beyond the electric-dipole approximation. This allows for a gauge-origin independent determination of UV/vis and X-ray absorption spectra for randomly oriented systems such as multilayers, liquids, and gas phase samples. The approach is illustrated by the determination of spectral intensities of electric-dipole allowed π → π* transitions and electric-dipole forbidden n → π* transitions in the UV–vis region of the spectrum as well as electric-dipole forbidden 1s → 3d transitions in the X-ray region of the spectrum. The employed Lebedev quadrature scheme shows very fast convergence with respect to the number of symmetry-independent quadrature points – in all considered cases, the oscillator strengths for the randomly oriented systems are fully converged with use of only seven quadrature points.     

Electronic relaxation of troponoids: Tropolone fluorescence

Absorption spectra, excitation and fluorescence spectra of tropolone, in water at pH 3 and pH 12, and in cyclohexane, are reported. An assumption for the superposition of two (π, π¹) absorption transitions is made, and the oscillator strengths are determined for these transitions. Radiative and non-radiative processes following the excitation are investigated. Results are discussed in terms of the existence of an (n, π¹) fluorescent state lying below the first (π, π¹) state.     

A Partial Wave Analysis of the Reaction π+p→ (π+π−π0) Δ++ at 16 GeV/c

A partial wave analysis of the 3π-system has been performed for the reaction π⁺p→ (π⁺π^?π⁰) Δ⁺⁺ (1232) at 16 GeV/c. Beside the well-established A₂⁰ (1300), the resonant state ω* with isospin I = 0 and spin-parity J^P = 3^? decaying mainly into (?π) has been found. Its mass and width have been determined to be M = (1.71 ± 0.03) GeV and Γ = (0.22 ± 0.10) GeV. The cross section for the reaction π⁺p→ ω* (1700) Δ⁺⁺ (1232) is σ = (12 ± 6) μb.     

A Theoretical Study of the Electronic Spectrum of Photochromic 1,2-Dihydroquinoline

Abstract

Recently, Kolc and Becker¹ reported photochromism of 1,2-dihydro quinoline. The primary act of this reaction appears to be a dissociation of the C – N sigma bond as a result of π→π? excitation which leads to a fully conjugated molecule absorbing in the visible region. This coloured form can be eradicated thermally and the whole process can be repeated. A detailed explanation of this photochromic process obviously requires a good knowledge of the electronic states and structure of the considered molecule. A theoretical study thus appeared quite useful.     

Structural dynamics of 4‐pyrimidone in lower‐lying excited States

The structural dynamics of 4‐pyrimidone (4PMO) in the A‐ and B‐band absorptions was studied by using the resonance Raman spectroscopy combined with quantum chemical calculations to better understand whether the excited state intramolecular proton‐transfer (ESIPT) reaction occurs in Franck–Condon regions or not. The transition barrier for the ground state proton‐transfer tautomerization reaction between 3(H) (I) and hydroxy (II) was determined to be 165 kJ·mol⁻¹ in vacuum on the basis of the B3LYP/6‐311++G(d,2p) level of theory calculations. Two ultraviolet absorption bands of 4PMO were, respectively, assigned as π_H→π*_L and π_H→π*_L+1 transitions. The vibrational assignments were done on the basis of the Fourier transform (FT)‐Raman and FT‐infrared (IR) measurements, the density‐functional theory computations and the normal mode analysis. The A‐ and B‐band resonance Raman spectra of 4PMO were measured in water, methanol and acetonitrile. The structural dynamics of 4PMO was obtained through the analysis of the resonance Raman intensity pattern. We discuss the similarities in the structural dynamics of 4PMO and 2‐thiopyrimidone (2TPM), and the results were used to correlate to the intramolecular hydrogen‐atom‐transfer process as observed by matrix‐isolation IR experiments for 4PMO. A variety of NH/CH bend modes + C = O stretch mode mark the hydrogen‐detachment‐attachment or ESIPT reaction initiated in Franck–Condon region for 4PMO and 2TPM. Copyright © 2013 John Wiley & Sons, Ltd.     

Ab initio SCF CI study of the electronic spectrum of s-tetrazine

Configuration interaction (CI) studies of ground, n→ π* and π→ π* electronically excited states are reported for s-tetrazine. The first n→ π* singlet excited state (¹ B _3u ), which is responsible for the purple-red colour of the molecule, is calculated at 2·80 eV, compared to the experimental transition energy of 2·22–2·70 eV. The singlet-triplet split of the first n→ π* states (¹ B _3u and ³ B _3u states) is calculated to be 0·76 eV.

The interaction of nitrogen lone pair orbitals (n-orbitals) is studied in terms of the ordering of the n π* excited states and found that the SCF orbital ordering is qualitatively in accord with the ordering of the n π* excited states in the CI level.

The first π→ π* excited state (¹ B _2u ) is calculated at 5·99 eV, slightly above the observed range of absorption. Numerous other high-lying singlet states as well as the triplet states have been calculated and they are used to verify several proposals relating to the excited state dynamics in the photo-physical studies of s-tetrazine.