Optical absorption in boron clusters B6_{6} and B6+_{6}^{+} : A first principles configuration interaction approach

The linear optical absorption spectra in neutral boron cluster B$_{6}$ and cationic B$_{6}^{+}$ are calculated using a first principles correlated electron approach. The geometries of several low-lying isomers of these clusters were optimized at the coupled-cluster singles doubles (CCSD) level of theory. With these optimized ground-state geometries, excited states of different isomers were computed using the singles configuration-interaction (SCI) approach. The many body wavefunctions of various excited states have been analysed and the nature of optical excitation involved are found to be of collective, plasmonic type.Comment: 22 pages, 38 figures. An invited article submitted to European Physical Journal D. This work was presented in the International Symposium on Small Particles and Inorganic Clusters - XVI, held in Leuven, Belgiu

Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di -n,n-octyl)fluorene)

X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the beta phase in poly(9,9-(di n,n-octyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperature-dependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called beta phase. Rapid thermal quenching is employed for PF8 studies of pure alpha phase samples while extended low-temperature annealing is used for improved beta phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the beta phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative beta phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.Comment: Submitted to PRB, 12 files; 1 tex, 1 bbl, 10 eps figure

Opto-Electronic Properties of Fluorene-Based Derivatives as Precursors for Light-Emitting Diodes

This paper reports optical absorption spectra of oxidized fluorene copolymers obtained by chemical oxidation with Ce(IV) and by pulse radiolysis experiments in chlorinated solvents. Comparison of the results observed by the two techniques is used to provide spectral data on the copolymer radical ions and information on stability of the oxidized species. In addition, a detailed quantum chemical characterization is presented, concerning the electronic and optical properties of three series of charged oligomers containing alternating fluorene and phenylene or thienylene or benzothiadiazole units, respectively. The introduction of the comonomer strongly influences the optical properties, leading to a red shift in the absorption spectra of the charged oligomers. This shift is more pronounced in the case of fluorene benzothiadiazole anions due to the strong electron-accepting character of the benzothiadiazole moieties. The charge distribution of the fluorene benzothiadiazole anion is different from that corresponding to fluorene phenylene and fluorene thienylene anions. The negative charge of the latter oligomers is evenly distributed over the fluorene units, while the former oligomer localizes the negative charge on the benzothiadiazole units. The charge distribution correlates with the optical absorption spectra. When the positive charge is localized on a different unit than the negative charge, the cation and anion spectra are different. Similar spectra are obtained if both the positive and negative charges are localized on the same unit