On the basis of theoretical models and calculations, several alternating polymeric structures have been investigated to develop optimized poly(2,7-carbazole) derivatives for solar cell applications. Selected low band gap alternating copolymers have been obtained via a Suzuki coupling reaction. A good correlation between DFT theoretical calculations performed on model compounds and the experimental HOMO, LUMO, and band gap energies of the corresponding polymers has been obtained. This study reveals that the alternating copolymer HOMO energy level is mainly fixed by the carbazole moiety, whereas the LUMO energy level is mainly related to the nature of the electron-withdrawing comonomer. However, solar cell performances are not solely driven by the energy levels of the materials. Clearly, the molecular weight and the overall organization of the polymers are other important key parameters to consider when developing new polymers for solar cells. Preliminary measurements have revealed hole mobilities of about 1 x 10(-3) cm2 x V(-1) x s(-1) and a power conversion efficiency (PCE) up to 3.6%. Further improvements are anticipated through a rational design of new symmetric low band gap poly(2,7-carbazole) derivatives.

The ground and excited states of covalently linked fluorene-based dimers were investigated by theoretical methods and by UV-vis and fluorescence spectroscopies. The optimized structures and the characterization of frontier molecular orbitals were obtained by HF/6-31G* ab initio calculations. All derivatives are nonplanar in their ground electronic states. The extent of nonplanarity depends on the nature of the aromatic ring bonded to the fluorene unit. All frontier orbitals involved both subunits of the dyads. The HOMO of each compound possesses an antibonding character between subunits, while the LUMO shows bonding character. The nature and the energy of the first 10 singlet-singlet electronic transitions have been obtained by ZINDO/S semiempirical calculations performed on the HF/6-31G* optimized geometries. All electronic transitions are of the ππ* type and involve both subunits of the molecules. For each derivative, excitation to the S 1 state corresponds mainly to the promotion of an electron from the HOMO to the LUMO, and the S 1 r S 0 electronic transition is strongly favored and polarized along the long axis of the molecular frame. The energy of the first electronic transition of all derivatives follows the HOMO-LUMO energy gap computed from HF/6-31G* ab initio calculations. The absorption and fluorescence spectra of the fluorene derivatives have been recorded in cyclohexane. The first absorption band of each derivative can be assigned to the S 1 r S 0 electronic transition computed from ZINDO/S calculations. The overall shape of the absorption and fluorescence spectra suggests a smaller distribution of conformers in the S 1 state than in the ground state. The fluorescence quantum yield and lifetime in cyclohexane have been obtained. In these systems, the photophysical properties are mainly governed by nonradiative processes.

The thermochromic and solvatochromic properties of a series of fluorene-based conjugated polymers have been investigated. Both poly(2,7-(9,9-dioctylfluorene)) and poly(2,5-(thiophene)-alt-2,7-(9,9-dioctylfluorene)) have revealed interesting chromic properties which, on the basis of theoretical calculations, seem to be related to a relatively flexible backbone. On the other hand, the sterically hindered, nonplanar poly(2,5-(3,4-dimethylthiophene)-alt-2,7-(9,9-dioctylfluorene)) does not exhibit any significant solvatochromic and thermochromic properties which could be explained by an important energy barrier against planarity. Finally, investigations on poly(2,5-(3,4-ethylenedioxythiophene)-alt-2,7-(9,9-dioctylfluorene)) have revealed a highly conjugated conformational structure which is not significantly modified upon temperature or solvent changes.

A detailed analysis of the optical and photophysical properties of 2,2‘:5‘:2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene (QT), 3,3‘ ‘‘-dimethoxy-2,2‘:5‘:2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene (DMOQT), 3,3‘ ‘‘-dimethyl-2,2‘:5‘:2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene (DMQT) and 3‘,4‘ ‘-didecyl-2,2‘:5‘:2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene (DDQT) in various environments is reported. In solution at room temperature, the optical properties of the free molecules are obtained and discussed in terms of the effect of the substitution on the conformation adopted by each derivative in the electronic ground and first excited states. In a tetradecane matrix at 77 K, the optical properties are obtained for the isolated molecules in this rigid medium where the oligothiophenes adopt conformations similar to those found in the solid state. The optical properties of the quaterthiophene derivatives in their aggregated forms and in the solid state are also reported and discussed in terms of the substitutional effect on the intermolecular interactions, which affect the spectral and photophysical properties of the isolated molecules. For the first time, a β,β‘-disubstituted oligothiophene (DMQT) showing an excitonic splitting similar to that obtained for QT is reported. All other substituted oligothiophenes presented show a conformational change, following the aggregation process. This difference is explained by more disordered crystalline forms for DMOQT and DDQT. Theoretical calculations using the ZINDO/S semiempirical method are also performed on the crystalline structure of each derivative in an attempt to correlate the optical properties of these molecules in their aggregated forms and in the solid state with the molecular arrangement found in the crystal.

Fluorescence lifetimes (rF) of 2-[p-(dimethylamino)phenyl]-3,3-dimethyl-3//-indole (1) have been extensively studied in a two-component mixture of p-dioxane/water and also in a three-component mixture of n-heptane/AOT (sodium bis(2-ethylhexyl) sulfosuccinate)/water. We have applied the concept of "dielectric enrichment" to solute/mixed solvents systems in order to evaluate the effective dielectric constants at the AOT interfaces in «-heptane. We have shown that the effective dielectric constant (D) at the interface increases from 2.3 (water molar ratio W = 0) to approximately 9.0 before reaching a plateau at W = 12. This was explained by the solvation of the polar head groups of the surfactant. Equations have been given for the variation of D vs W and for the variation of D vs [AOT] at various AOT and water concentrations, respectively. The validity of these results has been discussed by taking into account the role played by the probe molecule in the stabilization of the solvation energy (dielectric enrichment effect).

We report a conformational analysis of several substituted terthiophenes using ab initio calculations performed at the HF/3-21G* level. Geometries of terthiophenes having methoxy substituents in 3,3‘‘ positions (DMOTT), methyl groups in the same positions (DMTT), and ethyl substituents in 3‘,4‘ positions (DETT) are compared with that of the unsubstituted molecule (TT). For all these symmetrical molecules, it is observed that the two dihedral angles are independent of each other. The most stable conformation of TT is found for dihedral angles θ = φ = 147.2°, whereas three maxima are located at 0°, 90°, and 180°. The insertion of methoxy groups in 3,3‘‘ positions favors a more planar conformation with a higher rotational barrier at 90°. This behavior is explained by the electron donor properties of the methoxy groups. By contrast, the addition of two methyl groups at the same positions induces a twisting in the molecule which is caused by the steric hindrance between the methyl substituents and the sulfur atom. The presence of two ethyl groups in 3‘,4‘ positions creates an even stronger steric effect, giving rise to a more twisted conformation for DETT compared to that of DMTT. Absorption and fluorescence spectra of each terthiophene derivative are also reported and are correlated with their respective potential energy surfaces. The more planar molecule (DMOTT) shows a red-shifted absorption band with a higher vibrational resolution and a smaller bandwidth. For more twisted molecules, the blue shift and the bandwidth of the absorption bands increase with twisting while the absorption coefficient decreases. The fluorescence bands, in all molecules, show a better vibrational resolution with a smaller bandwidth compared to their absorption counterparts, while their maximum wavelengths are practically the same, showing that in the first excited singlet state, all molecules relax to a more planar conformation.

The absorption and fluorescence spectra 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), isolated in an alkane matrix and in their aggregated forms are reported. The thermochromic properties of substituted oligothiophenes are reported for the first time. The optical changes observed for the isolated molecules in the alkane matrix are interpreted in terms of conformational changes caused by the packing effect. After aggregation, TT exhibits a splitting in the excitation spectrum, which is provoked by an excitonic effect. On the other hand, it is observed that the changes in the optical properties of substituted terthiophenes are close to those found in the alkane matrix. This strongly suggests that the aggregation process does not produce any significant optical change, other than those provoked by conformational changes, in the excitation spectra of these molecules. It is suggested that the presence of the side chains weaken the intermolecular interactions, which are responsible for the excitonic effect. Finally, the fluorescence spectra of all the molecules investigated show a new red-shifted band, when aggregated. The new emission cannot be explained by conformational changes and has its origin in the aggregation process itself.

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