Fluorescence emission and excitation spectra of para-phenylene vinylenes nPV with nϭ1 -4 styryl units are investigated experimentally and theoretically as a function of the temperature and the polarizability of the solvent. At low temperatures, the vibronic structures of the S 0 ↔S 1 emission and excitation bands are mirror symmetrical with negligible 0-0 energy gaps. The frequencies of the prominent vibrational modes are assigned to the second longitudinal acoustic phonon modes of the entire molecules and to localized carbon-carbon stretching vibrations. The complete vibronic structures of the spectra are calculated at the ab initio Hartree-Fock (HF/6-311G*) and restricted configuration interaction singles (RCIS/6-311G*) levels of theory assuming planar C 2h molecular symmetry. The theoretically predicted spectra are in good agreement with the experiments. At room temperature, a 0-0 energy gap between the first band maxima opens, and the mirror symmetry between absorption and emission is lost. The vibronic band shapes and 0-0 band gaps are successfully simulated with a combination of Gaussian and exponential broadening of the low temperature spectra. The exponential term reflects the differences in thermal population of the phenyl-vinyl torsional modes in the S 0 and S 1 electronic states. Spectral shifts upon changes in temperature and solvents are quantitatively explained by changes in the refractive index of the environment. From extrapolation of the experimental data the vertical and adiabatic transition energies of the oligomers in vacuo are obtained and compared to RCIS and semiempirical quantum chemical calculations, respectively.

No abstract

No abstract

Using electronic absorption and fluorescence spectroscopic techniques, as well as quantum chemical calculations, we have studied the electronic spectra of thia-bridged stilbenophane (TSP) with close cofacial contact of two trans-stilbene (t-SB) units. Compared to the t-SB monomer, the experimental consequences of the cofacial arrangement are (i) a splitting of the main absorption band with a weakly allowed emitting state, and (ii) a strongly red-shifted, unstructured emission spectrum with long fluorescence decay times. According to the theoretical investigations, the two t-SB units are strongly bent in the electronic ground state (S 0 ), because of repulsive π-π overlap. In the first excited state (S 1 ), the t-SB units become almost planar, because of attractive π*-π* overlap. As a consequence, the symmetry-forbidden S 0 T S 1 transition couples strongly to interchromophore breathing modes of low frequency (ν 1 ) 67 cm -1 , ν 2 ) 117 cm -1 ), yielding structureless spectra with large Stokes shifts. The features of the calculated spectra are in good agreement with the experimental data. The results indicate that strong intermolecular vibronic coupling is also responsible for "excimer-like" emission in organic molecular crystals of cofacially arranged molecules. Furthermore, the different geometries in the S 0 and S 1 states of TSP give evidence for the mechanism of [2+2]photodimerization of t-SB in solutions.

As a test for the applicability of the density functional theory to the system containing intramolecular hydrogen bonds, calculations were performed on propen-1,2,3-triol, the feasible intermediate in the epimerization of dihydroxyacetone and glyceraldehyde enantiomers. A comparison is made between results obtained by Becke's three parameter hybrid functional (for exchange) with gradient corrections provided by the LYP correlation functional (B3LYP) and those predicted at the ab initio Møller−Plesset second-order (MP2) level. The calculated minimum energy structures are in excellent agreement with respect to both energy and geometries of hydrogen-bonded structures. Earlier and recent studies suggest that, generally, the nonlocal B3LYP approximation leads to a very accurate overall description of intramolecular hydrogen-bonded systems. We propose a new, more efficient computational protocol, which may be useful in the study of the biologically important molecules at a level of accuracy usually only provided by traditional post-Hartree−Fock ab initio methods.

No abstract

The synthesis of FC(O)OONO2 is accomplished by the photolysis of a mixture of (FCO)2, NO2, and O2. The pure product is isolated after trap-to-trap condensation and the removal of byproducts by treatment of the crude product with O3 and AsF5. The colorless liquid freezes at −105 °C; its boiling point is 32 °C. At 20 °C and a few millibars, FC(O)OONO2 decomposes in the gas phase with a half-life of 20 h. FC(O)OONO2 is characterized by vapor pressure measurements, vibrational, 19F NMR, 13C NMR and UV spectroscopies as well as by mass spectrometry. According to the vibrational and NMR spectra, the compound exists at room temperature only as a syn conformer. The molecular structure of FC(O)OONO2 is determined by gas electron diffraction. The molecule possesses a skew structure with a dihedral angle of φ(COON) = 86.2(14)°. The short O−O bond (1.420(6) Å) and the long N−O bond (1.514(6) Å) are consistent with the chemical properties of this compound. The experimental geometry is reproduced reasonably well by quantum chemical.

No abstract