R. Tubino

Optical Properties and Photoexcitations in Regularly Alternating Conjugated Copolymers

We report in this short paper a preliminary account on the design, preparation, optical characterization and photoexcitation of a class of novel conjugated polymers consisting of regularly alternating thiophene and benzene sequences, which Might represent Models for one-dimensional superlattices. Optical absorption and emission spectra suggest that the barriers produced by the phenylene Moieties do not interrupt the conjugation but can produce localization effects. Resonant Raman spectra indicate that a selective response of the thiophene Moiety occurs upon excitation with visible light. Moreover these Measurements confirm the presence of a confinement on the thiophene blocks, while the biphenil Moiety does not appear to feel the energy barrier and behaves as the corresponding polymer poly (paraphenylene) (PPP). CW photomodulation spectra show that the formation of long-lived bipolarons takes place only if the thienylenic Moiety is long enough to accomodate the lattice distortion.

Optical and vibrational properties of conjugated polymeric heterostructures

We present an optical and vibrational analysis of two copolymers consisting of alternating thienylene and phenylene blocks linked by azomethines moieties which might represent models of a one‐dimensional superlattice. Both the pristine polymers and their GaCl3 complexes are studied. Their electronic structure is investigated with absorption‐emission spectra, preresonant and resonant Raman scattering. Though the backbone conjugation is not interrupted by the phenylene moieties, resonant Raman modes of the thienylene and phenylene blocks are selectively enhanced by exciting in the visible and in the UV regions, respectively. Radiative recombination occurs within the thienylenic blocks which act as efficient trapping centers for the photoexcitations. While the thienylene ring vibrations feel the presence of the potential barriers and are confined within the thienylenic blocks of the copolymers, phenylene vibrations are characteristic of a phenylenic chain segment longer than the biphenylene block. By GaCl3 c...

Resonant Raman spectra of soluble trans polyacetylenea)

Resonant Raman spectra of a soluble form of trans polyacetylene are here reported. Accurate excitation profiles have been obtained by using the 1196 and the 1370 cm−1 bands of toluene as an internal standard. Both the integrated and sliced excitation profiles exhibit a resolved vibronic structure, whose shape is interpreted in terms of the magnitude of the electron–phonon coupling and of the inhomogenous distribution of the electronic energies due to the interaction with the solvent. The present data are compared with those obtained from solid samples.

Novel erbium complexes with low band-gap for infrared laser and optical amplifiers

Two new NIR luminescent erbium(III) crown-ethers have been synthesized and studied by optical spectroscopies. Both the complexes show absorption bands in the visible spectral region (absorption edge at about 500 nm and 600 nm) and exhibit the NIR lanthanide luminescence following excitation of the organic ligand. The absorption bands of these complexes, well in the visible region of the spectrum, make them ideal candidates as active molecules for efficient optically pumped laser and optical amplifier by excitation with commercially available lasers.

Optical properties of oriented thin films of oligothiophenes

Abstract Optical absorption and photoluminescence of both unsubstituted and alkyl-substituted oligohiophenes thin films, grown on various inorganic and organic substrates with the Molecular Beam Deposition technique, have been studied. Dramatic orientation effects, associated with large dichroism in the linear optical properties, are observed upon molecular deposition onto properly cleaved potassium acid phtalate substrates. X-ray diffraction data and optical anisotropy are consistent with a card pack supramolecular organization, in which the separation between the molecules and their orientation with respect to the substrate are strongly affected by the lateral substitutions.The polarization behavior of the optical excitations in films of both unsubstituted and laterally substituted oligothiophenes provides novel information on the electronic structure of these compounds

Growth of quaterthiophene thin films on potassium acid phthalate by organic molecular beam deposition

Absorption measurements and atomic force microscopy images of quaterthiophene (4T) thin films grown by molecular beam deposition on an organic single crystal substrate provide interesting information on the growth mode of these films, which is demonstrated to be strongly influenced by the interaction between the 4T molecules and the substrate. Evidence of a macroscopic film orientation and local crystal order is achieved.

Orientation and order of β-tetrahexyl-sexithiophene molecules deposited by organic molecular beam deposition

Highly ordered and oriented deposition of substituted sexithiophene molecules is obtained on single crystal organic substrates. The study of X-ray diffraction and of the optical absorption of the films measured under different angles of incidence and with different directions of light polarization permits a complete structural model to be proposed. In addition, some interesting conclusions about the average molecular orientation with respect to the substrate are drawn.

Ultrafast spectroscopy of photoexcitations in α-sexithienyl films: evidence for excitons and polaron-pairs

We have used transient and steady state photomodulation techniques for studing the photoexcitations in thin polycrystalline films of α-sexithienyl. We show that singlet Frenkel excitons and charge transfer excitons are formed within 1 ps. The initial Frenkel exciton population decays via a bimolecular annihilation process followed by a slower, trap-mediated recombination. The charge transfer state is long lived and gives rise to separated charges in the nanosecond time domain.

Steady state and fast photoconductive response of highly oriented polyacetylene

Abstract The steady state and transient response of highly oriented polyacetylene was measured for different orientations of the external dc field with respect to the stretching direction of the sample and different temperatures. When the current flow is parallel to the chains the cw photoresponse is not ohmic and the picosecond response is independent of the temperature. These experimental results are described by existing models for the photogeneration of the charges and their motion in conjugated polymers.

Depolarization ratios of the resonance Raman bands of soluble trans‐polyacetylene

Depolarization ratios for the totally symmetric Raman lines in soluble polyacetylene have been measured for various exciting wavelengths in the resonance region. The origin of the unusually high values of the depolarization ratios and their variations with the exciting wavelength are discussed in terms of two overlapping electronic transitions in the visible region.