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.
Interactions between 3H-indole derivatives, their
cations and anions, and microheterogeneous environments
like micelles and vesicles have been studied extensively in our
laboratory recently. We report herein the
interactions of
2-(p-aminophenyl)-3,3-dimethyl-5-cyano-3H-indole
(1) and
2-[p-(dimethylamino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (2) with aqueous
solutions of β-cyclodextrin (β-CD), studied by absorption
and
fluorescence steady-state and time-resolved measurements. The
stoichiometries of the cyclodextrin:guest
inclusion complexes have been determined by steady-state fluorescence
measurements. The data reveal that
two types of complexes, i.e., 1:1 and 2:1 types are formed.
Thermodynamic parameters are calculated at six
different temperatures. Spectral characteristics, bandwidths, and
photophysical parameters indicate that
molecule 2 is better protected against hydrophilic
interactions. Protonation reactions carried out at
different
concentrations of β-CD show that the protonation is inhibited at the
indolic nitrogen, contrary to what was
observed in other microheterogeneous media. Time-resolved
measurements and global analysis of the results
are best described by a discrete triple exponential decay law clearly
indicating that the guest molecules
experience three different environments in aqueous solutions: bulk
water and a stepwise 1:1 and 2:1 (β-cyclodextrin:guest) inclusion complexation. The effective polarity
of the cyclodextrin cavity is equivalent
with the polarity of an 80:20 methanol−water mixture at the β-CD
rim where the indolic (tertiary) nitrogen
is likely to be located near the “alcoholic” secondary rim of the
macrocycle.
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).
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