A. Piaggi

Conformational analysis and optical characterization of oligothiophene inclusion compounds

Abstract The conformations and the optical properties of quinquethiophene molecules included in a peryhydrotriphenylene matrix are discussed from a critical comparison of experimental and theoretical data and compared to the situation encountered in solutions and crystals. The complex absorption and emission patterns and the lack of a mirror symmetry between the two spectra are consistent with the presence of two different absorbing and emitting conformations characterized by a different degree of inter-ring twisting. These new results provide insight into the properties of single oligothiophenes upon photoexcitation.

Optical properties of para-hexaphenyl polycrystalline films

We investigate the optical properties and intermolecular coupling of para-hexaphenyl polycrystalline films by site-selective spectroscopy, time-resolved photoluminescence and photoinduced absorption measurements. Our results point to (i) a short-range orientational and energetic disorder and (ii) the molecular character of the light emission, with no evidence of intermolecular excited states.

Radiative defects and optical response in oriented para-hexaphenyl films

The optical properties of an ordered para-hexaphenyl film, grown with molecules lying parallel to the substrate, are investigated. The absorption spectrum exhibits two peaks at 3.1 (L) and 4.35 (H) eV, featuring a different degree of polarization. The photoluminescence emission and the absorption at 3.1 eV are highly polarized. The presence of the L peak is strongly related to the growth conditions, and is favored at high deposition rates and low substrate temperatures. Upon 4.66 eV ultraviolet irradiation in air, the photoluminescence intensity and the absorption at 3.1 eV are reduced. Conversely, transitions at higher energies are almost unaffected by irradiation. Experimental data show that the L peak and emission arise from a crystal defect.

Photoinduced generation of radical ions in α-sexithyenil

Abstract The cw photoinduced absorption technique has been applied to α-sexithyenil (T 6 ) molecules both in vacuum evaporated films and KBr pellets. We observed new IR active vibrations as well as new electronic transitions at low energy. We assigned these structures to charged species separated on different molecular layers and trapped to defects. The narrow line shape of the photoinduced features is indicative of the molecular solid character of T 6 thin films and it provides an upper limit for the charge symmetry breaking of 0.04 eV. A comparison with the longer chain polymer (polythyophene) is discussed.

RELAXATION PROCESSES IN THIOPHENE-BASED RANDOM COPOLYMERS

The relaxation dynamics of soluble polyalkylthiophenes obtained by the random copolymerisation of 3,4-dibutylthiophene and 3-butylthiophene monomers is investigated. In these systems, the effective conjugation length, the optical gap and the non-radiative decay rate are controlled by varying the content of disubstituted monomers, the steric hindrance of which induces a twisting angle between thiophene rings. Several indications are reported in favour of spectral diffusion of the photoexcitations. Migration processes mainly occur within a few tens of picoseconds.

Photoexcitations in conjugated polymer superlattices

Abstract We present evidence of quantum confinement of the photo-generated states in a conjugated block copolymer consisting of regularly alternating thiophenic and phenylenic moieties of various lengths, thus giving rise to a sort of one-dimensional superperiodicity. CW photoinduced spectra (both in the infrared and in the visible) have revealed localization effects when the width of the potential well exceeds five thiophenic rings. A discussion of the new features in the copolymer photoexcitation spectrum with respect to the parent polymers is also presented.

Optical properties and structure of thiophene substituted copolymers

The optical properties and the structure of random copolymers consisting of mono- and disubstituted alkylthiophenes have been studied by optical and NMR spectroscopy. While the optical data are consistent with the presence of randomly distributed mono and disubstituted units, NMR spectra reveal that preferential enchainment between monosubstituted units takes place.

Temporal dynamics of photoexcitations in π-conjugated regular copolymers

We investigate relaxation processes in thiophene-based copolymers through site-selective and time-resolved photoluminescence spectroscopies. We observe a large Stokes-shift between the absorption and luminescence peaks that can be explained in terms of spectral relaxation of excitons within an inhomogeneously broadened density of states. The incoherent motion of the photoexcitations toward non radiative centres leads to a non exponential decay of the photoluminescence intensity. The decay rate of the photoluminescence signal strongly depends on temperature except for long times. The interplay among thermally activated jumps, slow transfer to non nearest neighbour sites and molecular recombination explains satisfactorily these findings.

Control of the luminescence properties in polythiophenes solutions by interring rotational disorder

The photoluminescence properties of thiophene-based polymers obtained by the random copolymerization of 3,4-dibutylthiophene and 3-butylthiophene are reported. Optical absorption spectra, cw photoluminescence (PL) spectra, PL quantum efficiencies and PL decays have been measured in dilute solutions. By varying the copolymer chemical composition it is possible to tune the intensity and the color of the luminescence spectra in the whole visible range. The authors evidence the role of disorder, controlled by interring rotations, as the tuning factor for the disexcitation pathway of the photoexcitations in these materials.

Photomodulation Spectroscopy in α-Sexithyenil

Abstract The CW photoinduced absorption technique has been applied to α-sexithyenil (T 6) vacuum evaporated films. We observed new electronic transitions which appear deep in the gap. We assigned these structures to charged species separated on different molecular layers and trapped to defects. The narrow line shape of the photoinduced features is indicative of the molecular solid character of T 6 thin films and it provides an upper limit for the charge symmetry breaking of 0.04 eV.