Maria Loi

Energy transfer in nanostructured oligothiophene inclusion compounds

Resonant energy transfers from terthiophene (T3) to quinquethiophene (T5) oligomers embedded in the nanochannels of perhydrotriphenylene (PHTP) crystals are studied by cw and femtosecond spectroscopy. The stringent geometry imposed by the host results in a very peculiar supramolecular organization of the guest molecules consisting of parallel linear arrays of chromophores. This provides a unique opportunity to make a quantitative analysis of the energy transfer phenomena and in particular to distinguish between heterotransfers and homotransfers. The experimental data fitted by a suitable theoretical analysis indicate that homotransfers become important in the long time dynamics and that the heterotransfer rate is satisfactorily described by the analytical solution obtained by assuming a continuous intermolecular spacing.

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 characterisation of poly-2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene

We have investigated the electronic properties and the photoluminescence (PL) decay of the poly-2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene (PDHPT) copolymer made up by the regular alternation of high and low band-gap units. Experimental data show that the large twisting of the neighbouring constituents of the polymer chain leads to a considerable shift of the optical transitions towards high energies. This blue shift is much more pronounced than the expected band-gap lowering caused by the low-gap thiophene units. Transient PL experiments show that inter-chain migration of excitons towards centers where excitons decay non-radiatively reduces the PL quantum yield (QY).

High pressure study of the intramolecular vibrational modes in sexithiophene single crystals

We report on the effects of a high hydrostatic pressure on the intra-molecular modes of a sexithiophene (T6) single crystal. Raman experiments were performed at low temperature up to a pressure of 80 kbar. We find that intermolecular interaction gives rise to a sizeable redistribution of the intensities of the strongest intra-molecular Raman lines. These changes are due to a slight deformation occurring in the terminal rings of the sexithiophene molecule. The frequencies of the bending modes involving sulfur atoms are the most affected by pressure. A Davydov doublet and a mixed inter/intra-molecular mode are observed. These results confirm the role played by the sulfur atoms in modulating the electronic overlap between chains.

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.

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.

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.