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.