An approach has been developed to calculate the periods of rotational recurrences (RRs) in thermal ensembles of nonrigid molecules. Starting from the standard nonrigid rotor Hamiltonian with ...centrifugal distortion (CD) terms, the explicit analytical formulas were derived for CD-induced corrections to the periods of J, H, and K transients for symmetric top molecules. The corrections are shown to be proportional to the CD constants and the temperature. In addition, they depend considerably on the rotational constants of the molecule. Generally, the CDs are demonstrated to induce a shift in the periods of the RRs, which depends on the order (
n=1,2,…) of RRs. Only the first few RRs exhibit
n-independent shifts, and this is the situation in which our theory is exact. For higher order RRs, our approach starts to exaggerate slightly the values of the CD-induced corrections, giving their upper boundary.
We report the synthesis and electroluminescent properties of random poly(phenylenevinylene) copolymers, carrying fluorine substitutions. In particular, we used ...2,3,5,6-tetrafluoro-1,4-phenylenevinylene and 2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene moieties. The ratios of the two copolymer units were varied between 5:95 and 27:75. Electrochemical measurements showed an increase of the oxidation and a decrease of the reduction potentials of the copolymers upon increase of the fluorine content, as expected for the electron withdrawing effect of fluorine on the conjugated π-electron system. The copolymers showed emission in the red-orange region, with maximum photoluminescence efficiencies of 6.7%. We also observed lower turn-on voltages and higher electroluminescence efficiencies for lower fluorine contents.
In recent years, variously substituted derivatives of poly(1,4-phenylene vinylene)s have emerged as efficient candidates for the emissive layer in polymer light emitting diodes. The synthetic routes ...for these polymers divide between precursor routes and those leading to fully conjugated solvent-processible polymers. The Gilch dehydrohalogenation polycondensation has largely been used for the latter class. In this presentation, we describe a novel family of 2,3-disubstituted aromatic precursors, derived from catechol, and we report their efficient polymerisation as homo- and copolymers with, for example, silyl-substituted derivatives to give materials which are highly fluorescent and serve as interesting materials in polymer LEDs.
We report the electrochemical, optical, and electroluminescent properties of fluorine containing poly(phenylene vinylene) random copolymers. We used 2,3,5,6-tetrafluoro-1,4-phenylene vinylene and ...2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene moieties, with ratios of the two copolymer units varying between 5:95 and 27:75. The electron withdrawing effect of fluorine on the conjugated π-electron system was confirmed by electrochemical measurements, which showed an increase of the oxidation and a decrease of the reduction potentials, upon increase of the fluorine content. Light emitting diodes fabricated from the copolymers showed emission in the red-orange, with lower turn-on voltages and higher electroluminescence efficiencies for lower fluorine contents. We observed maximum photoluminescence efficiencies of 6.7%.
We report the fabrication of a two-dimensional periodic structure made from the conjugated, fluorescent polymer poly(p-phenylene vinylene), PPV, by direct-write near-field lithography of a ...photosensitive precursor of the conjugated polymer. The precursor film is illuminated with ultraviolet radiation using the apertured fibre of a home-built scanning near-field optical microscope (SNOM), and unexposed areas are subsequently dissolved in methanol. The fully conjugated polymer is obtained by thermal conversion under vacuum. The structure is approximately 10 mum X 10 mum, and consists of 30 nm tall pillars on a 330 nm periodicity lattice. We present a calculation of the photonic properties and propose that structures made with the method investigated here are promising candidates for photonic crystal lasers. We also report the results of preliminary investigations aimed at increasing the pillar height.
