Michel Belletête

Steady-state and time-resolved studies of 2,7-carbazole-based conjugated polymers in solution and as thin films: determination of their solid state fluorescence quantum efficiencies

The UV–Vis absorption spectra and the luminescence properties of poly(N-octyl-2,7-carbazole) (POC) and poly(N-octyl-2,7-carbazole-alt-9,9-dioctyl-2,7-fluorene) PCF have been investigated in solution and in the solid state (thin films). No aggregate and/or excimer formation has been detected in these polymeric systems. From time-resolved fluorescence measurements in solution and in the solid state, the fluorescence efficiencies of the thin films have been estimated. It is found that the fluorescence efficiencies of these polycarbazoles in the solid state are quenched, as compared to those measured in fluid solutions, but remain relatively high (φF≅0.40), making them promising materials for electroluminescent devices.

Novel polyacetylenes containing pendant 1-pyrenyl groups: synthesis, characterization, and thermal and optical properties

Abstract We report the synthesis as well as the thermal, optical and photophysical properties of four different polyacetylenes with pendant 1-pyrenyl groups: poly(1-ethynylpyrene) (PEP), poly(1-(trimethylsilanylethynyl)pyrene) (PTMSEP), poly(1-(4-(trimethylsilanyl-buta-1,3-diynyl)pyrene) (PTMSBDP) and poly(1-buta-1,3-diynylpyrene) (PBDP). Polymerizations were carried out with W and Ta catalysts, respectively, for mono- and disubstituted monomers. Soluble poly(1-ethynylpyrene) with high molecular weights (up to 4×10 5 ) and extended conjugation of the main chain was obtained with good yields. Lower molecular weights (up to 6×10 3 ) were obtained with the other polymers. Oligomers and polymers displayed high thermal stability. From the absorption spectra of the various polymers, it is found that PEP possesses a higher degree of conjugation than the other polyacetylenes. Molecular interactions occur between pyrene units present in each polymer giving rise to an emission due to associated pyrenes. These interactions are affected by the steric hindrance present in the polymer backbone. Excitation spectra combined with fluorescence decay profiles show that these interactions occur in the ground state (excited complex).

TOWARDS A THEORETICAL DESIGN OF THERMOCHROMIC POLYTHIOPHENES

Abstract Ab initio calculations have been performed on several bithiophene derivatives which are the building blocks of substituted polythiophenes in order to correlate their torsional potential energy curves with the thermochromic properties of the parent polymers. It is found that the occurrence of thermochromism in polythiophenes can be related to the values of the energy barrier calculated for the dimer models. Moreover, trends in the thermally induced blue-shift observed with some polythiophene derivatives can be related to the difference in the torsional angles between a co-planar conformation and the more stable conformation of the corresponding bithiophene moieties.

Conformational analysis and electronic properties of bithiophene and terthiophene in their ground state as well as in their first excited singlet and triplet states

Abstract For the first time a joint theoretical and experimental method is proposed that allows one to elucidate the conformations of oligothiophenes in their respective ground and first excited singlet and triplet states. Semiempirical calculations (AM1 and ZINDO/S) are used to obtain the ground state conformation (S0) of bithiophene and terthiophene. Similar potential energy curves are obtained for both molecules. A higher rotational barrier between adjacent rings is calculated for terthiophene suggesting that a higher conjugation length is effective for this molecule. This is confirmed by comparison with experiments. ZINDO/S is used to calculate the energy of the S1 ← S0 electronic transitions as well as the S1 and Tn excited state energies. These calculations suggest that, after excitation, both molecules relax to a planar S1 conformation. The geometry of the molecules in their T1 states is also predicted to be planar. From the S1−Tn energy gaps and the reported values of the intersystem crossing rate constants (kisc) of bithiophene and terthiophene, it is suggested that the Franck-Condon factors are not playing an important role in the intersystem crossing probability of these thiophene oligomers.

Study of the interactions between substituted 2,2′-bithiophenes and cyclodextrins

Abstract The interactions of 4,4′-dimethoxy-2,2′-bithiophene (DMO44BT), 4,4′-dibutoxy-2,2′-bithiophene (DBO44BT), and 2,2′-bithiophene (BT) with α- and/or β-cyclodextrins (CD) were investigated by absorption and steady-state fluorescence spectroscopy. The absorption spectra in all cases are only slightly shifted going from water to the CD cavity showing that the ground state conformations of the bithiophenes in the CD cavity remain similar to that found in water. From fluorescence measurements, it was found that DMO44BT and DBO44BT form 1:2 (guest:host) complexes with α-CD, while DMO44BT and BT form 1:1 complexes with β-CD. The nature of the interactions involved in the inclusion complexes are discussed.

Conformational, optical and photophysical properties of a substituted terfluorene isolated and incorporated in a polyester

Abstract The optical and photophysical properties of a terfluorene, isolated and incorporated in a polyester, are reported. A similar oligofluorene having carboxylic acid groups at each end of the molecule has also been studied. This molecule provides a better correlation with its corresponding polyester. The fully relaxed single-bond torsional potentials of both oligofluorenes were evaluated with the aid of semiempirical calculations (AM1). The incorporation of the terfluorene in the polyester does not change its conformation and spectral properties. On the other hand, the luminescence intensity of the polyester ( φ f =0.76) is slightly weaker than that of the terfluorenes ( φ f =0.88 and 0.93) but remains intense.

Interfacial micropolarity in the micellar region of the sodium (2-ethylhexyl) sulfosuccinate (AOT) inverted micelles

Abstract The fluorescence lifetime (τF) of 2-(p-dimethylaminophenyl)-3,3-dimethyl-3H-indole (1) in various AOT aqueous micellar solutions is reported in methylcyclohexane and n-heptane. It is shown that the probe resides at the interfacial region of the inverted AOT micelle. The variation of τF−1 with the molar fraction of water follows a single experimental exponential law which is similar to the one observed in p-dioxane/water mixtures. This shows that no dielectric enrichment of the probe is taking place in the reversed micellar solution and that the “solvation” of the probe in AOT is similar to that in p-dioxane. This allows one to obtain dielectric constants at the micellar interface using the fluorescence lifetime in p-dioxane/water mixtures as a polarity ruler. Moreover it was shown that the dielectric constant increases linearly with the mole fraction of water.

Solubilization of two cyano 3H-indole molecular probes in SDS and CTAB micelles: spectral and photophysical characterization

An absorption and fluorescence spectral study on the solubilization behaviour of 2-[(p-amino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (1) and 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (2) in sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) aqueous micelles has been carried out. Using spectral correlations with polarity parameters, bandwidths and photophysical parameters, the nature of the binding site of the molecules in micelles has been discussed. Both molecules occupy sites in SDS and CTAB that are less polar than water but accessible to it. The dimethylamino substituent helps in pushing the molecule toward the micellar core in both micelles. pH effects on the ring-nitrogen protonation (neutral–monocation equilibrium) and terminal nitrogen deprotonation (neutral–monoanion equilibrium) in SDS and CTAB, respectively, have also been discussed and found to be consistent with both the pseudophase ion-exchange (PIE) and electrostatic models.

SEMIEMPIRICAL DETERMINATION OF TORSIONAL POTENTIALS AND ELECTRONIC PROPERTIES OF BITHIOPHENE, TERTHIOPHENE AND 3',4'-DIHEXYL-2,2' :5',2 -TERTHIOPHENE IN THEIR GROUND AND FIRST EXCITED SINGLET AND TRIPLET ELECTRONIC STATES

Abstract We report a detailed study of the conformational behavior of bithiophene (BT), terthiophene (TT) and 3′,4′-dihexyl-2,2′:5′,2″-terthiophene (DHTT) in their ground (S 0 ) and first excited singlet (S 1 ) and triplet (T 1 ) states. Ground state fully optimized torsional potentials are calculated using the AM1, PM3 and ZINDO/S semiempirical calculation methods. ZINDO/S is also used to obtain the energy of the S 1 ← S 0 electronic transitions as well as the S 1 and T 1 excited state energies. The calculated singlet-singlet transition energies are compared with UV spectra. It is found that the PM3 method does not reproduce accurately the torsional potential of oligothiophenes. In contrast, AM1 calculations reproduce minima of the BT potential curve at similar dihedral angles to those measured in the vapor phase. AM1 calculations indicate that BT and TT, in their ground state, have slightly twisted trans conformations (torsional angle between adjacent rings ≈ 150 °) whereas DHTT is predicted to be much more twisted due to steric hindrance caused by the presence of the hexyl chains. ZINDO/S performed on AM1 optimized conformers suggests that, after excitation, all the molecules relax to planar S 1 and T 1 conformations such that the excited state energy barriers are predicted to be higher than those observed for the ground electronic states of these oligomers.

A conformational study of ethyl-substituted bithiophenes. Semi-empirical versus ab initio methods

Abstract Semi-empirical (AM1, PM3) and ab initio calculations (STO-3G, 3-21G ∗ ) are employed to obtain the equilibrium optimized geometries and the torsional potential surfaces of 2,2′-bithiophene as well as its 3,4′- and 3,3′-ethyl-substituted derivatives. For the unsubstituted molecule, ab initio calculations have also been performed at the HF/6-31G ∗ level. The geometries were completely optimized along the torsional potential curves to account for the molecular relaxation, yielding a physically meaningful picture of the nonrigid rotation. The results given by each theoretical method are compared and discussed. It is found that ethyl substitution causes rather small changes in the thiophene ring structure. Contrary to these results, ethylation dramatically influences the overall shape of the torsional potentials, leading to a large tilt from planarity. The barrier against planarity is found much higher for the 3,3′-ethyl derivative. It is also observed that the steric hindrance created by ethyl groups is much higher than that induced by methyl substituents.