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New dialkylterthiophene liquid crystalline compounds were synthesised and characterised using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarized optical microscopy (POM). Upon cooling from the isotropic state to the crystalline phase, these dialkylterthiophenes present different types of smectic mesophases. For the highest ordered smectic phase, XRD analyses have revealed a molecular organisation within the smectic layers and the key structural parameters are determined by molecular modelling simulations. The structural properties of these smectic phases are discussed in relation to the alkyl chain length.

Nearly 25 years after the discovery that an organic conjugated polymer can be “doped” (that is, in chemical terminology, oxidized or reduced) to metallic-like electrical conductivity, the field of π-conjugated oligomers and polymers has enjoyed a tremendous development, most remarkably underlined by the 2000 Nobel Prize in Chemistry awarded to Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa. Since, in this specific class of polymers, small variations in chemical structure play an essential role and the properties of interest directly depend on the electronic structure, quantum–chemical approaches, that take the chemical nature fully into account, have provided a most valuable input to the field. The results have helped to forge a fundamental understanding of the electronic and optical properties of π-conjugated materials and in guiding the experimental efforts toward novel compounds with enhanced characteristics. It is our main purpose in this contribution to illustrate the impact of quantum–chemical methods, more specifically in relation to transport processes. We do not dwell at all on the theoretical methodologies that have been designed, but rather on the concepts. We first recall the basic electronic-structure aspects of π-conjugated systems, that make their chemistry and physics so rich and fascinating. We then discuss the key role of interchain interactions and their impact on transport.

In this contribution, we review our recent quantum-chemical studies on intermolecular interactions and the way they impact the transport and optical properties of organic conjugated materials. When applied to the crystalline structures of two prototypical oligomeric materials, α-sexithienyl and a-a'bisdithieno[3,2-b:2',3'-d]thiophene, our approach illustrates how the variations in chain packing lead to dramatically different optical splittings and carrier mobilities. Various strategies to prevent solid-state luminescence quenching in light-emitting devices are proposed.

We investigate by means of quantum-chemical calculations the influence of intermolecular interactions on the properties of conjugated chains. We first focus on the optical absorption and emission of clusters formed by identical poly(paraphenylene vinylene), PPV, oligomers; we address the role played by the relaxation effects in the lowest excited state as well as the number and relative orientations of the interacting units. Several strategies are suggested that can be developed to avoid a substantial decrease in luminescence quantum yield in the solid state. In a second part, we illustrate by means of a simple theoretical model that charge or energy transfer can take place at the interface between two different PPV derivatives, depending on the nature of the substituents grafted on the PPV backbone

Phenylene-based π-conjugated materials are of current interest due to their wide range of possible applications, especially in light-emitting diodes, displays, and lasers. In this contribution, we first review the results of recent theoretical calculations on the geometric and electronic properties of phenylene-based chains, paying special attention to the influence of chemical doping on the geometry and optical absorption spectra. We then discuss an approach that allows us to characterize the electronic levels of finite size molecules in momentum space and to describe the chain-size evolution from molecular levels to band states in quasi-infinite one-dimensional chains. Finally, we characterize, at the molecular mechanics and molecular dynamics level, the structural properties of dendrimers synthesized with benzene rings as molecular building blocks; we analyze the shape persistence of dendrimers built on different cores by investigating the time evolution of the structures of their first and second generations.

In this contribution, we try to illustrate that quantum-chemistry calculations can prove useful to comprehend better the phenomena occurring in the active layer of polymer-based light-emitting diodes. We review the results of recent theoretical calculations conducted at the correlated level on conjugated oligomers that address several topics of prime interest in the search of new routes towards the achievement of devices with enhanced characteristics. These studies performed on oligomers representative of poly (para-phenylenevinylene) and polythiophene deal with: (i) the modeling of the optical properties of the conjugated polymers involved in the emitting layer; (ii) the investigation of the relative locations of, and relaxation phenomena taking place in, the lowest singlet and triplet excited states; and (iii) the analysis of the effects of derivatization of the conjugated backbone. The theoretical approach we follow allows us to rationalize a wide range of experimental measurements and to provide guidelines in the design of novel attractive materials.

By means of a high-level configuration interaction technique, we describe the nature of the lowest singlet and triplet excited states in oligo(phenylene vinylene)s and oligothiophenes. We first calculate the evolution with chain length of the singlet-singlet and triplet-triplet excitation energies, as well as that of the singlet-triplet energy difference. The theoretical results compare favorably with the available experimental data. We then investigate the geometry relaxation phenomena occurring in the lowest excited states of model oligomers. We found that the lowest triplet state is characterized by more pronounced and local lattice distortions than the singlet state, especially in the case of the phenylene vinylene oligomers.

We investigate the electric-field-induced change in the absorbance (i.e. electroabsorption response) of samples of β-carotene and α,ω-bis(1,1-dimethylheptyl) -1,3,5,7,9,11,13,15-hexadecaoctaene dispersed in a polystyrene matrix. From the change in absorbance, the refractive index and the thickness of the polymer film, the third-order nonlinear d.c. Kerr susceptibility, χ (3) (-ω; 0,0,ω), can be calculated via the Kramers-Kronig relationship. Results are also presented for the simulated electroabsorption of β-carotene, derived from calculation of the absorption both in the presence and absence of an applied electric field.

The effects of doping poly(cyanoterephthalylidene) with sodium in ultrahighvacuum been studied by optical absorption spectroscopy. Upon doping, new optical transitions are observed within the bandgap; the characteristics of these transitions are consistent with the formation of bipolarons. The optical absorption results are confirmed by direct measurements of the doping-induced gap states using ultraviolet photoelectron spectroscopy.

We present opticalabsorption and electron spin resonance studies of iodine and AsF5 doped β-carotene films. We show that stable soliton-like defects are formed upon p-doping. The spin to charge ratio found experimentally is ∼-0.02, indicating that nearly all defects are spinless. The absorption spectrum is characterized by a single intragap band and set of infrared active vibration lines; evidence that there is significant structural relaxation upon charge transfer. A possible spinless species is the dication in which two soliton defects are formed. The small number of spins observed are probably due to β-carotene radical cations.

We present a new efficient protocol for the synthesis of two- and three-ring oligo(phenylenevinylene) derivatives incorporating heterocycles. The versatility of this synthetic route allows for the design of a large number of chemical structures differing by the position and number of the heterocycles as well as by the para versus meta connections between the phenylene rings; π-donor and n-acceptor substituents can also be attached on the conjugated backbones. The synthesized oligomers have been characterized by means of optical absorption and emission spectroscopy and corresponding quantum-chemical calculations.

When an exciton diffuses to the interface between two conjugated polymers, either energy- or charge-transfermay take place. We present an experimental investigation of these processes in various binary polymer systems. These results are consistent with calculations of the relative energies of the intrachain and interchain excited states on pairs of different PPV-related chains. In particular, we show experimentally and theoretically that charge transfer occurs at the interface between MEH-PPV and a cyano-substituted PPV derivative, whereas energy transfer occurs at the interface of PPV with the same cyano-substituted polymer.

In this contribution, we propose a unified description of the nature of the intrinsic species photogenerated in the lowest-lying one-photon allowed excited state of isolated conjugated chains, by closely confronting the results of correlated quantum-chemistry calculations to related experimental measurements. We specifically focus on the characterization of the 1 B u excited state of oligo(phenylenevinylene)s and trans-polyenes and extrapolate the calculated oligomer properties to the scale of the corresponding polymers.

The singlet and triplet excited states in cyano-substituted phenylene vinylene oligomers are characterized by means of configuration interaction calculations. We first evaluate the vertical singlet-singlet. S 0 –S 1 , singlet-triplet. S 0 –T 1 , and triplet-triplet T 1 -T n , excitation energies in oligomers ranging in size from two to five phenylene rings. We then describe the lattice distortions taking place in the lowest excited states. Finally, we estimate the spatial extent of the S 1 , T 1 . and T 1 excitons on the basis of a simple analysis of their wavefunctions. In each case, the results are compared to those obtained for the corresponding unsubstituted oligo(phenylene vinylene)s.

We report photoinduced electron transfer in films of poly(3-decyloxy-1,4-phenylene) (DO-PPP) blended with C 60 , observed by infrared photoexcitation spectroscopy. The photoinduced IR-active vibrational (IRAV) structure and electronic subgap absorption features of the DO-PPP are greatly enhanced upon adding just two percent C 60 - The spectral signature of the C 60 anion at 1.15 eV is clearly observed as a peak superimposed on the weak background between the midgap states of the polymer. The system is pumped at 2.41 eV, below the bandgap of the polymer, but within the visible absorption tails of both the polymer and of the C 60 .

Summary form only given. We have studied films of neutral 6T in order to compare the electronic structure of its photoexcited state with that of the radical cations and dications, on one hand, and with the photogenerated defects observed for the neutral polythiophene. Using the photoinduced absorption spectroscopy, we have found two strong absorption bands very similar in energy (0.80 and 1.54 eV) and shape (both have distinct vibronic shoulders) to those found for the radical cation 6T/sup middot/+/. In addition, there is a very weak separate band (at 1.1 eV) close in energy to the position of the dication related band (1.2 eV). Photoinduced infrared active vibrations were also observed, confirming that these defects are charged. Good agreement with the calculated transitions of the oligothiophene radical cations is obtained. We conclude from our measurements that in the time domain of our steady state measurements (I msec), most of the photogenerated defects are polarons (radical cations). We further conclude that, at least in the 6T oligomer, the characteristic absorption of the polaron contains two strong transitions, whereas that of bipolaron (dication) contains only a single band.