The properties of 4′-N,N-dimethylaminoflavonol in the ground and excited states

The mechanism of protonation of 4-N,N-dimethylaminoflavonol and the structure of its protolytic forms in the ground and excited states were studied by electron absorption and fluorescence (steady-state and time-resolved) spectroscopy and with the use of the RM1 quantum-chemical method. A comparison of equilibrium constants and the theoretical enthalpies of formation showed that excitation should be accompanied by the inversion of the basicity of the electron acceptor groups of this compound and, as a consequence, changes in the structure of its monocationic form. An analysis of the spectral parameters of the protolytic 4-N,N-dimethylaminoflavonol forms, however, showed that their structure and the sequence of protonation in the excited state were the same as in the ground state. Changes in the structure of the monocation in the excited state were not observed because of the fast radiationless deactivation of this form and the occurrence of excited state intramolecular proton transfer in aprotic solvents.     

The acid—base equilibria of dimethylphenols in the ground and the first excited singlet states

Measurements of the acidity constants in the ground (pK_a(S₀)) and the lowest excited singlet (pK_a(S₁)) states for 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dimethylphenols in aqueous solution have been carried out spectrophotometrically at 25°C. The pK_a values in S₀ have been derived from the absorption spectra and the pK_a values in S₁ were estimated by means of the Forster cycle. It is found that the hydroxy group is more acidic in the first excited singlet than in the ground state.     

Investigation of β-cyclodextrin complex formation with 2,2′-dipyridine in ground and excited triplet states

An exfoliation/restacking synthesis route has been developed for the fabrication of Ni–Al layered double hydroxide (LDH) intercalated Cu(II) tetrasulfophthalocyanine (CuPcs) hybrid by using exfoliated LDH nanosheets and guest molecules as building blocks. The structural and morphological features of the resulting hybrid have been investigated by varieties of analytical techniques such as XRD, SEM, UV–Vis and thermal analysis. Interlayer spacings determined from XRD patterns reveal a perpendicular orientation of the CuPcs macrocycles to the Ni–Al LDH layer. Then the obtained nanohybrid was utilized as photocatalyst for the decolorization of Rhodamine 6G (Rh6G) aqueous solutions. The effects of H₂O₂, time, substrate concentration, catalyst dose, were studied as a function of percentage of decolorization under irradiation and the corresponding dark controls were also carried out for comparison. The decolorization percentage of Rh6G increases with irradiation time and can reach to 70% at 6 h as against to 18% in dark control.     

The optical activity of β,γ-enones in ground and excited states using circular dichroism and circularly polarized luminescence

The circularly polarized luminescence (CPL) and electronic circular dichroism (CD) spectroscopic parameters corresponding to the n←π* and n→π* transitions, respectively, have been calculated for selected β,γ-enones using density functional theory. For the smallest β,γ-enone, (1R,4R)-bicyclo[2.2.1]hept-5-en-2-one (norbornenone), coupled-cluster calculations have also been carried out. The excited-state potential energy surface for three of the five enones studied reveals two minima with different C[double bond, length as m-dash]OC[double bond, length as m-dash]C dihedral angles, and with rotatory strengths of opposite sign. The relative energies of the minima determine the sign of the CPL intensity, which may be the same or opposite as in the CD spectrum, in agreement with experimental data. The results obtained in this first computational study of CPL demonstrate its usefulness as an indicator of excited-state structures of chiral species.     

Molecular parameters of excited molecules and ions—III. Geometries of dihaloethylenes in some ionic states

The vibrational fine structures of the photoelectron bands of gem-, cis- and trans-dihaloethylenes were used in Franck—Condon factor calculations leading to the deduction of ionic geometries of these molecules. Separability of electronic and nuclear motion, constancy of electronic transition moment and photoionization cross section within vibrational progressions, as well as the harmonic oscillator treatment, were assumed to be valid in the calculations. In the determination of the signs of the changes in bond lengths and bond angles of the molecules upon ionization, different criteria were utilized with the inclusion of the nodal repulsive force model for bond angle changes. Moreover, the two mechanisms, namely, the change in bond order and the change in electrostatic interaction as proposed by Coulson and Luz were investigated and were found to be important in geometric changes upon ionization.     

Ab initio potential energy surface and excited vibrational states for the electronic ground state of Li_2H

A 285-pomt multi-reference configuration-interaction involving single and double excitations ( MRS DCI) potential energy surface for the electronic ground state of L12H is determined by using 6-311G (2df,2pd)basis set.A Simons-Parr-Finlan polynomial expansion is used to fit the discrete surface with a x2 of 4.64×106 The equn librium geometry occurs at Rc=0.172 nm and,LiHL1=94.10°.The dissociation energy for reaction I2H(2A)→L12(1∑g)+H(2S) is 243.910 kJ/mol,and that for reaction L12H(2A')→HL1(1∑) + L1(2S) is 106.445 kl/mol The inversion barrier height is 50.388 kj/mol.The vibrational energy levels are calculated using the discrete variable representation (DVR) method.     

Flickering dipoles in the gas phase: structures, internal dynamics, and dipole moments of β-naphthol-H2O in its ground and excited electronic states

Described here are the rotationally resolved S(1)-S(0) electronic spectra of the acid-base complex cis-β-naphthol-H(2)O in the gas phase, both in the presence and absence of an applied electric field. The data show that the complex has a trans-linear O-H???O hydrogen bond configuration involving the -OH group of cis-β-naphthol and the oxygen lone pairs of the attached water molecule in both electronic states. The measured permanent electric dipole moments of the complex are 4.00 and 4.66 D in the S(0) and S(1) states, respectively. These reveal a small amount of photoinduced charge transfer between solute and solvent, as supported by density functional theory calculations and an energy decomposition analysis. The water molecule also was found to tunnel through a barrier to internal motion nearly equal in energy to kT at room temperature. The resulting large angular jumps in solvent orientation produce "flickering dipoles" that are recognized as being important to the dynamics of bulk water.     

Bond functions in molecular excited states: MRD CI calculations for the A3Σ+u,B3Πg and W3Δu states of N2

Using double-zeta plus polarization (DZP) basis sets systematically augmented with a variety of bond functions, the term dissociation energies are calculated for the A³Σ⁺_u, B³Π_g and W³Δ_u states of N₂. It is found that the best agreement with literature values is generally with a basis set composition of DZP augmented by a set of s, p and d orbitals at the bond midpoint. The excited state potential energy curves and spectroscopic constants for the B³Π_g state are calculated from this basis and compared with experimental values. Good agreement was obtained, considering the small basis set size, with the spectroscopic constants ω_e, ω_eχ_e, ω_ey_e, B_e and α_e and the dissociation energy D_e (e.g., D_e = 3.38 (3.681, exp.), 4.75 (4.897) and 4.77(4.873) eV for the A, B and W stages, respectively). Poorer agreement was obtained for the term energy T′₀ (7.92 versus 7.35 eV, exp., for the B state). The error in term energy arises largely from an error in the calculated ⁴S → ²D splitting (2.705 versus 2.383 eV, exp.), and shifting the potential curve for the B state by a constant amount leads to much improved agreement relative to the ground state. The counterpoise correction applied to the potential curve of the B state causes a drastic deterioration of the results and shows qualitatively incorrect behaviour, and is therefore not recommended for calculations of this type.     

Estimation of electrooptical parameters of EХ<Subscript>3</Subscript> molecules in ground and excited states from empirical correlation equations

Basing on the experimental data, it has been shown that for EХ₃ (E = N, P, As, or Sb; X = F, Cl, Br, I, CH₃, С₆Н₅, or SiH₃) compounds the value of the difference between first ionization potentials of atoms and molecules is a function of the bond angle and dipole moment of the molecule. The calculated values of dipole moments of charged atoms and groups are regularly changed within the considered compounds series. The change of the dipole moment of the molecules during the vibrational excitation has been determined, and intensities of symmetric stretching and deformation bands in IR spectra of EF₃ and ECl₃ compounds have been estimated.     

Vibronic spectra,ab initio calculations,and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part V: Oxalyl chloridefluoride (COCl–COF)

The structure and conformational dynamics of the COCl–COF molecule in the ground and lowest excited electronic states were investigated theoretically by the CASPT2/cc-pVTZ method. The equilibrium geometric parameters, harmonic vibrational frequencies, potential functions of internal rotation, and adiabatic transition energies were obtained. According to the results of calculations, the molecule in the ground electronic state exist as the trans and gauche (dOCCO ~30–40°) conformers with a low potential barrier to gauche–gauche transition therefore it is impossible to exclude existence of the cis conformer (instead of gauche) with a very broad and flat potential minimum. For all the investigated excited electronic states of oxalyl chloridefluoride molecule calculations predicted the trans and cis conformers. The strong coupling of internal rotation around the C–C bond and non-planar vibrations of carbonyl fragments was found for the excited electronic states. The results of calculation were utilized for reanalysis of experimental \( \tilde{A}^{1} A^{\prime \prime} \leftarrow \tilde{X}^{1} A^{\prime} \) and \( \tilde{a}^{3} A^{\prime \prime} \leftarrow \tilde{X}^{1} A^{\prime} \) vibronic spectra reported in Kidd and King (J Mol Spectrosc 50:209–219 (1974), and ibid. 48:592–599 (1973)). The vibrational assignment that does not contradict the vibrational spectroscopy data and results of calculations was obtained.     

Bond shortening (1.4 Å) in the singlet and triplet excited states of [Ir2(dimen)4]2+ in solution determined by time-resolved X-ray scattering

Ground- and excited-state structures of the bimetallic, ligand-bridged compound Ir2(dimen)4(2+) are investigated in acetonitrile by means of time-resolved X-ray scattering. Following excitation by 2 ps laser pulses at 390 nm, analysis of difference scattering patterns obtained at eight different time delays from 250 ps to 300 ns yields a triplet excited-state distance between the two Ir atoms of 2.90(2) ? and a triplet excited-state lifetime of 410(70) ns. A model incorporating the presence of two ground-state structures differing in Ir–Ir separation is demonstrated to fit the obtained data very well, in agreement with previous spectroscopic investigations. Two ground-state isomers with Ir–Ir separations of 3.60(9) and 4.3(1) ? are found to contribute equally to the difference scattering signal at short time delays. Further studies demonstrate the feasibility of increasing the effective time resolution from the 100 ps probe width down to the 10 ps regime by positioning the laser pump pulse at selected points in the X-ray probe pulse. This approach is used to investigate the structures of both the singlet and the triplet excited states of Ir2(dimen)4(2+).     

Hamiltonian for the problem on the ground and excited electron–nuclear states of nano-objects of finite size with periodic structure: Oligomers

A Hamiltonian has been proposed to study ground and excited states of nano-objects with a periodic structure, which makes it possible to find a general solution as a linear combination of basis functions in the form of normal waves. Terms that allow obtaining easily the solution with varying degrees of accuracy are explicitly defined. The method is suitable for the analysis of both finite and infinitely extended objects, including segments with a periodic structure.     

Hydrogen bonding and reactivity of water to azines in their S1 (n,π*) electronic excited states in the gas phase and in solution

A unified picture is presented of water interacting with pyridine, pyridazine, pyrimidine, and pyrazine on the S(1) manifold in both gas-phase dimers and in aqueous solution. As (n,π*) excitation to the S(1) state removes electrons from the ground-state hydrogen bond, this analysis provides fundamental understanding of excited-state hydrogen bonding. Traditional interpretations view the excitation as simply breaking hydrogen bonds to form dissociated molecular products, but reactive processes such as photohydrolysis and excited-state proton coupled electron transfer (PCET) are also possible. Here we review studies performed using equations-of-motion coupled-cluster theory (EOM-CCSD), multireference perturbation theory (CASPT2), time-dependent density-functional theory (TD-DFT), and excited-state Monte Carlo liquid simulations, adding new results from symmetry-adapted-cluster configuration interaction (SAC-CI) and TD-DFT calculations. Invariably, gas-phase molecular dimers are identified as stable local minima on the S(1) surface with energies less than those for dissociated molecular products. Lower-energy biradical PCET minima are also identified that could lead to ground-state recombination and hence molecular dissociation, dissociation into radicals or ions, or hydration reactions leading to ring cleavage. For pyridine.water, the calculated barriers to PCET are low, suggesting that this mechanism is responsible for fluorescence quenching of pyridine.water at low energies rather than accepted higher-energy Dewar-benzene based "channel three" process. Owing to (n,π*) excitation localization, much higher reaction barriers are predicted for the diazines, facilitating fluorescence in aqueous solution and predicting that the as yet unobserved fluorescence from pyridazine.water and pyrimidine.water should be observable. Liquid simulations based on the assumption that the solvent equilibrates on the fluorescence timescale quantitatively reproduce the observed spectral properties, with the degree of (n,π*) delocalization providing a critical controlling factor.     

Can a single-reference approach provide a balanced description of ground and excited states? A comparison of the completely renormalized equation-of-motion coupled-cluster method with multireference quasidegenerate perturbation theory near a conical intersection and along a photodissociation coordinate in ammonia

We calculated the two lowest electronically adiabatic potential energy surfaces of ammonia in the region of the conical intersection and at a sequence of geometries along which one of the N-H bonds is broken. We employed both a multireference (MR) method and a single-reference (SR) method. The MR calculations are based on multiconfiguration quasidegenerate perturbation theory (MC-QDPT) with a 6-311+G(3df,3pd) basis set. The SR calculations, carried out with the same basis, employ the completely renormalized equation-of-motion coupled-cluster method with singles and doubles, and a noniterative treatment of triples, denoted CR-EOMCCSD(T). At 91 geometries used for comparison, including geometries near a conical intersection, the surfaces agree to 7% on average.     

A New Dynamic Covalent Bond of Se?N: Towards Controlled Self‐Assembly and Disassembly

A new kind of Se? N dynamic covalent bond has been found that can form between the Se atom of a phenylselenyl halogen species and the N atom of a pyridine derivative, such as polystyrene‐b‐poly(4‐vinylpyridine). This Se? N dynamic covalent bond can be reversibly and rapidly formed or cleaved under acidic or basic conditions, respectively. Furthermore, the bond can be dynamically cleaved by heating or treatment with stronger electron‐donating pyridine derivatives. The multiple responses of Se? N bond to external stimuli has enriched the existing family of dynamic covalent bonds. It can be used for controlled and reversible self‐assembly and disassembly, which may find potential applications in a number of areas, including self‐healing materials and responsive assemblies.     

Ultrafast excited state dynamics of spirilloxanthin in solution and bound to core antenna complexes: Identification of the S∗ and T(1) states

Ultrafast excited state dynamics of spirilloxanthin in solution and bound to the light-harvesting core antenna complexes from Rhodospirillum rubrum S1 were investigated by means of femtosecond pump-probe spectroscopic measurements. The previously proposed S? state of spirilloxanthin was clearly observed both in solution and bound to the light-harvesting core antenna complexes, while the lowest triplet excited state appeared only with spirilloxanthin bound to the protein complexes. Ultrafast formation of triplet spirilloxanthin bound to the protein complexes was observed upon excitation of either spirilloxanthin or bacteriochlorophyll-a. The anomalous reaction of the ultrafast triplet formation is discussed in terms of ultrafast energy transfer between spirilloxanthin and bacteriochlorophyll-a.     

Global ab initio potential energy surfaces for both the ground (X̃1A') and excited (Ã1A') electronic states of HNO and vibrational states of the Renner-Teller Ã1A'-X̃1A' system

The global potential energy surfaces for both the ground (X?(1)A(')) and excited (A?(1)A(')) electronic states of the HNO molecule have been constructed by three-dimensional cubic spline interpolation of more than 17,000 ab initio points, which have been calculated at the internal contracted multi-reference configuration interaction level with the Davidson correction using an augmented correlation-consistent polarized valence quadruple zeta basis set. The low-lying vibrational energy levels for the two electronic states of HNO have also been calculated on our potential energy surfaces including the diagonal Renner-Teller terms. The calculated results have shown a good agreement with the experimental vibrational frequencies of HNO and its isotopomers.