G. Bongiovanni

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.

The origin of radiative emission of quaterthiophene ultra-thin films

The origin of the excited species giving rise to radiative emission in ultra-thin films of quaterthiophene grown on potassium acid phthalate single crystal by organic molecular beam deposition is investigated. In the steady-state photoluminescence (PL) spectrum collected at 80 K two emission peaks at 2.52 eV and 2.43 eV with their vibronic progression, are detected. The PL emission at 2.52 eV decays on the time scale of 500 ps, while the lifetime of the second one is only 4 ps. These results allow to relate the two emissions with localised and delocalised states, respectively.

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.

A Comparative Study of the Photophysics in Polyfluorenes and Polyfluorenes with Polyphenylene Dendron Sidechains

The cw absorption, steady state photoluminescence (PL), photoinduced absorption (PA), PL-detected magnetic resonance (PLDMR), and the time resolved PL of a novel polyfluorene (PF) prepared with bulky polyphenylene dendrimer substituents are compared with those of (PF) with ethyl-hexyl substituents. We show that the dendronic sidechains suppress the contribution from unwanted low energetic emission, yielding a polymer with pure blue emission. The sidechains also strongly alter the dynamics of the excited entities. In particular, the time-resolved PL and temperature-dependence of the cw PL from 20-320 K reveal distinct singlet exciton (SE) dynamics in the polymer films, while the behavior in solution is essentially the same. However, the PA results show that the dynamics of polarons and triplet excitons (TEs) are similar, and the PLDMR shows that the interaction between the SEs and polarons are also similar.

Optical properties and photoluminescence of tetrahexyl-sexithiophene allotropes

The optical absorption, Raman scattering and photoluminescence of two phases of tetrahexyl-sexithiophene (4HT6) display properties coherently related to the different molecular conformations imposed by the chain packing. We analyse the temperature dependence of the optical properties of a sample in which the two phases coexist. In the photoluminescence the lower energy emission from the phase possessing planar molecules is thermally activated at room temperature, while the emission from the phase possessing less planar molecules becomes dominant at low temperature due to effective excitation confinement. The time resolved photoluminescence study indicates that exciton migration processes within the inhomogeneous density of states of the two phases sample introduce relevant non-radiative quenching effects.

High pressure effects on the photoluminescence intensity of sexithiophene single crystals

We report on the influence of interchain interactions on the light emission properties of a sexithiophene single crystal. The strength of the intermolecular interactions is controlled by applying hydrostatic pressure. The combined use of both steady-state and time-resolved photoluminescence techniques permits to show that the pressure-induced quenching of the photoluminescence is caused by a reduction of the radiative recombination rate.

Energy transfer in oligothiophene inclusion compounds

Energy transfer between terthiophene and quinquethiophene oligomers embedded in the nanochannels of perhydrotriphenylene crystals is investigated by fs-time-resolved photoluminescence spectroscopy. Excitonic effects in these compounds are suppressed due to the large intermolecular distances imposed by the host crystal. Very efficient transfers of the electronic excitations from donor (terthiophene) to acceptor (quinquethiophene) molecules are observed. A quantitative analysis of the experimental data show that energy migration mainly occurs via direct donor-acceptor single-step transfer.

Colloidal synthesis and characterization of Bi2S3 nanoparticles for photovoltaic applications

Bismuth sulfide is a promising n-type semiconductor for solar energy conversion. We have explored the colloidal synthesis of Bi2S3 nanocrystals, with the aim of employing them in the fabrication of solution-processable solar cells and to replace toxic heavy metals chalcogenides like PbS or CdS, that are currently employed in such devices. We compare different methods to obtain Bi2S3 colloidal quantum dots, including the use of environmentally benign reactants, through organometallic synthesis. Different sizes and shapes were obtained according to the synthesis parameters and the growth process has been rationalized by comparing the predicted morphology with systematic physical-chemistry characterization of nanocrystals by X-ray diffraction, FT-IR spectroscopy, Transmission Electron Microscopy (TEM).

Pressure-induced quenching of the photoluminescence in sexithiophene single crystals observed by femtosecond spectroscopy

The influence of interchain interactions on the photoluminescence processes in a sexithiophene single crystal is studied by applying hydrostatic pressure up to 60 kbar, Steady state and femtosecond-time resolved optical spectroscopies indicate that an ultrafast (100 fs) formation of intermolecular species induced by pressure accounts for the observed strong decrease of the photoluminescence quantum yield.

White fluorescent nano-fibers prepared by periodic organic hetero-epitaxy

To preserve the morphological properties of para-hexaphenyl (p-6P) based nano- bers and simultaneously tune their emission wavelength, periodic organic-organic hetero-epitaxy was utilized. Multilayer structures of p-6P and -sexithiophene (6T) have been prepared by hot wall epitaxy and analyzed by scanning force microscopy, uorescence microscopy, X-ray di raction and time resolved spectroscopy. We demonstrate that organic heteroepitaxy can be applied to produce multilayered nano- bers with high crystallinity, well de ned epitaxial relationships along di erent material phases, molecular azimuthal order, and long-range morphological homogeneity. It is shown, that it is possible to precisely control and tune the highly polarized photoluminescence emission of the nano- bers from the blue to the green and orange spectral regime by a variation of the 6T concentration. Remarkably, it is possible to prepare nano- bers emitting white polarized light.