Nicolas DiCésare

Structural, spectroscopic and photophysical analyses of substituted terthiophenes and quinquethiophenes as well as their corresponding polyesters

Abstract In this study, we present spectroscopic and photophysical results on oligothiophenes (trimers and pentamers) substituted with alkyl side chains as well as on the respective molecules incorporated in polyesters. The same oligothiophenes having two different electron acceptor groups, namely the carbonyl chloride and the acid substituent, at each end of the molecules have also been studied. These molecules provide a better correlation with their corresponding polyesters. Absorption and fluorescence spectra in solution are used to discuss the effect of the substitution as well as the length of the thiophene chain on the molecular conformation. Semiempirical calculations (AM1 and PM3) have also been performed to obtain the torsional potentials of the oligomers. Consequences of the conformational changes on the spectral and photophysical properties (fluorescence quantum yields, lifetimes and decay constants) of the various oligothiophenes and polyesters are examined. It is found that insertion of two alkyl chains on the central thiophene ring creates an important molecular twisting in the molecules. The length of the thiophene chain as well as the presence of end-substituents do not significantly influence the ground state molecular conformation. In the excited state, all molecules relax to more planar conformations. The fluorescence quantum yields and lifetimes are smaller for the alkyl-substituted molecules (without end-substituents) giving rise to higher values of the nonradiative decay constants compared to those of the unsubstituted molecules. This behavior might involve a decrease of the singlet–triplet energy gap for the twisted molecules which could enhance the intersystem crossing process. The incorporation of terthiophene and quinquethiophene derivatives in polyesters does not significantly change their conformation in the ground and excited states as well as their optical and photophysical properties.

HF/3-21G* ab initio calculations on methoxy-substituted bithiophenes

Abstract We report, for the first time, the lowest energy structures as well as a detailed conformational analysis of several bithiophenes substituted with methoxy and methyl groups in various positions. For each molecule, geometrical parameters and potential energy surfaces were obtained from HF/3-21G* ab initio calculations. The effect of the position of substituents on structural parameters and on torsional potentials was examined. These calculations give results, which are in good agreement with available experimental data. For oligomers having methoxy groups in positions 3,3′ and 3,4′, planar anti conformations are well stabilized with large rotational barriers at 90°. This phenomenon was interpreted in terms of charge transfer in the thiophene units induced by the methoxy groups in the 3- and/or 3′-positions and delocalization throughout the molecular frame. For 4,4′-methoxy-2,2′-bithiophene, the alkoxy groups do not show any influence on the electronic structure of the molecule and do not create steric hindrance such that the most stable conformer is very similar to that of the unsubstituted bithiophene molecule. In contrast, it is found that the most stable conformer of 3,4′-dimethoxy-4,3′-dimethyl-2,2′-bithiophene is much twisted owing to the presence of the methyl group in the 3′-position. This work also reports on theoretical results performed at lower levels (AM1 and PM3) as these levels of calculation are often the only one practically available for large substituted oligothiophenes. It is shown that these calculations fail to predict accurate conformations of the molecules studied here mainly because of the methoxy group substituent. A ZINDO/S analysis performed on the geometries obtained by the various theoretical methods employed is also reported. It is observed that, for the same torsional angle, the ZINDO/S method is very sensitive to the starting geometry.

THERMOCHROMIC PROPERTIES OF QUATERTHIOPHENE, AN ALKYL-SUBSTITUTED QUATERTHIOPHENE DERIVATIVE AND ITS CORRESPONDING POLYMER

Abstract We report for the first time the thermochromic properties of a substituted oligothiophene. The temperature-dependent absorption spectra of 3′,4″-didecyl-2,2′:5′,2″:5″,2‴-quaterthiophene (DDQT) show the appearance of a red-shifted band that is caused by conformational changes. The changes in the optical properties of DDQT are compared with those observed in the unsubstituted quaterthiophene (QT). In the latter, a parallel type excitonic model is used to explain the changes in the optical spectra. The thermochromism of DDQT is also compared with that of the corresponding polymer (PDDQT). For all compounds, the fluorescence spectra also show a new red-shifted band. The origin of this new emission is discussed.

Temperature-dependent spectroscopy and photophysics of substituted terthiophenes in fluid solution

Abstract The temperature-dependent optical and photophysical properties of terthiophene (TT) as well as three substituted terthiophene derivatives, namely 3′,3″-dimethoxy-2,2′:5′,2″-terthiophene (DMOTT), 3′,3″-dimethyl-2,2′:5′,2″-terthiophene (DMTT) and 3′,4′-dihexyl-2,2′:5′,2″-terthiophene (DHTT) in fluid solution are reported. For TT, it is shown that a change of conformation is induced at low temperature giving rise to a red-shift of the absorption band. This conformational change is explained by an increase of the interaction between TT and solvent molecules at low temperatures. Since the relaxed excited singlet state is expected to be nearly planar for this molecule, no shift is observed in the temperature-dependent fluorescence spectra. DMOTT, which is nearly planar in the ground state, also shows a red-shift of its absorption spectrum at low temperatures. This behavior is interpreted in terms of small conformational changes involving the thiophene rings and methoxy groups. For the alkyl-substituted terthiophene derivatives, which are much twisted in their ground states, changes in the absorption spectra are less important because twisted conformers are still present at low temperatures. The potential energy barrier against planarity for these substituted oligothiophenes does seem to be correlated qualitatively to specific solute–solvent molecular interactions at various temperatures. The fluorescence quantum yield ( φ F ), at room temperature, is higher for DMOTT but smaller for alkyl-substituted terthiophenes compared to that of TT. As the temperature is lowered, TT and alkyl derivatives show an important increase of φ F while DMOTT does not show any significant changes in its φ F . Fluorescence quantum yields are also reported in an isopentane glass at 77 K. Results show that φ F of TT and DMOTT are higher in the isopentane glass compared to those measured in fluid solution. On the other hand, the alkyl derivatives show an important decrease of their fluorescence quantum yields in the glassy medium. For all the compounds investigated, a regular increase of the fluorescence lifetime is observed as the temperature is lowered.