The roles played by spatially anisotropic intermolecular electrostatic interactions, chromophore shape, host dielectric constant, and poling field strength in defining maximum achievable electro-optic activity for electrically poled chromophore/polymer materials are investigated by equilibrium and Monte-Carlo quantum statistical mechanical calculations. Even simple Hamiltonians reproduce critical qualitative features such as the existence of a maximum in plots of electro-optic activity versus chromophore number density in a polymer matrix. Comparison of theoretical results for various methods provides a useful check on the validity of approximations employed with individual methods. The most significant conclusion to derive from a comparison of experimental and theoretical results is the dependence of maximum achievable electro-optic activity upon chromophore shape. Theoretical calculations suggest a new paradigm for the design of optimum electro-optic chromophores; realization of the desired shapes may be facilitated by dendritic synthetic approaches. In the presence of intermolecular electrostatic interactions, the dependence of electro-optic activity upon material dielectric permittivity and electric poling field strength is more complex than in the absence of such interactions. Of particularly, interest are conditions that lead to second order phase transitions to lattices containing centrically (antiferroelectricallly) ordered chromophore domains. Such phase transitions can lead to further complications in the attempted preparation of device quality materials but can be effectively avoided by utilization of theoretically derived phase diagrams.

This paper reports on a series of compounds based on Mn(II) complexes with nitronyl nitroxide free radicals which afford layered structures and exhibiting spontaneous magnetization up to 50K.

A soluble and processable polydiacetylene (PDA) with functionalizable N,N-dialkylaniline and glutamate ester substituted alkylurethane side groups was synthesized. This PDA was further reacted with tetracyanoethylene to effect postpolymerization functionalization. This new strategy further functionalizes conjugated polymers and introduces a tricyanovinyl (TCV)-aniline chromophore. The resulting polymer was soluble in common organic solvents. Asymmetric multilayers of the tricyanovinylated PDA were fabricated using the Langmuir-Blodgett (LB) technique. Thickness, visible absorption, and second harmonic (SH) intensity were found to increase linearly as a function of the number of layers. Polarized UV-visible and IR studies revealed that the main chain preferentially orients vertical to the compression direction with out-of-plane orientation of the TCV-aniline group with respect to the glass substrate. The resonant enhanced second-order nonlinear optical (NLO) coefficient (d33) of the 20 LB layers was found to be 21 pm/V at 1.064 µm, while no SHG signal was observed for cast or spincoated films.

A novel approach for the deposition of high quality films of oligothiophenes has been developed. It is based on an original seeded supersonic molecular beam combined with a UHV deposition chamber. This method gives an unprecedented control on film properties. Several T4 films under different beams conditions have been prepared and characterized by optical absorption, photoluminescence and TMAFM. Morphology, structure and optical response are controlled by the beam's parameters. Highly ordered films, up to several hundreds of nm thick, show well resolved vibronic structures in low temperatures PL as only the best published data on films a few monolayers thick do.

The synthesis of poly(1,4-phenylene vinylene)s (PPVs) containing a 2,3-dialkoxy substitution pattern has been developed. Poly[2,3-bis(2-ethylhexyloxy)-1,4-phenylene vinylene)] (BEH-PPV, 4) was prepared and its optical properties compared with the recently discussed poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV, 1), which showed in contrast to classical 2,5-substituted PPV derivatives a notable blue-shift of the longest-wavelength absorption (λmax) and a considerable increase of the solid-state photoluminescence (PL) efficiency. The two statistical PPV copolymers 5 and 6 comprising 2,3-dibutoxy and dimethyloctylsilyl- or 2,5-bis(dimethyloctylsilyl) side-chains, respectively, were synthesized via the Gilch dehydrohalogenation route. Double-layer light-emitting devices (LEDs) were demonstrated to combine high electroluminescence efficiencies with low turn-on voltages.

A series of new regioregular polyaryleneethynylene obtained by condensation of 3-hexyl multisubstituted oligothiophene derivatives with benzene and anthracene compounds were prepared. A complete NMR characterization of the starting monomers and model compounds lead to a deep investigation of the corresponding macromolecules. Both small and large molecules were optically characterized, the former, particularly when containing anthracene, moieties display noticeable photoluminescence quantum yield. In the polymers the photoluminescence quantum yield is lower than that of the starting molecules when anthracene is present, while the polymers containing benzene are more luminescent.

Nonlinear optical dendritic macromolecules, called “azobenzene dendrons”, having a branching structure modified with a second-order nonlinear optical chromophore has been synthesized. An electron donor and acceptor azobenzene chromophore having a large molecular hyperpolarizability of β0=150E-30 esu, was chosen. The electronic structure of synthesized dendrons, which were expected to become dipolar due to their intermolecular attractive interaction, was proven by second-order nonlienar optical properties. The molecular hyperpolarizabilities of azobenzene dendrons were measured by the hypr-Rayleigh scattering technique. The highest molecular hyperpoalrizability was found to be 3,010E-30 esu for an azobenzene dendron having 15 chromophoric unit, where each chromophore coherently contribute to the second harmonic generation. This level of the molecular hyperpolarizability was much larger than that for a monomeric azobenzene chromophore.

Using the ultrafast photoinduced electron transfer with long-living charge separation in the conjugated polymer/fullerene thin films, photovoltaic devices have been fabricated. The photoinduced charge separation happens with internal quantum efficiency near unity. The performance of such “bulk heterojunction” photovoltaic devices is critically dependent on the transport properties of the interpenetrating network. The influence of the variation of different donor / acceptor materials on the sample morphology is monitored by atomic force microscopy (AFM), while I/V characteristics have been studied to evaluate the conversion efficiency.

Despite the fast-paced new developments and improvements of applications incorporating conjugated organic materials, rather little effort is made to obtain well-defined structures and thin films of these in order to exploit the highly anisotropic properties (e.g. conductivity, polarized light emission) of most conjugated organic materials. It has been shown that oligomers, which can easily be evaporated under ultrahigh vacuum conditions, exhibit heteroepitaxial growth on various substrates. Depending on the nature of the interaction of the organic molecules with the substrate different orientations can be obtained, as well as by simply changing the conditions of evaporation. Therefore, by going from amorphous or polycrystalline structures to highly textured or single crystalline ones new insights into the physical properties of the materials can be expected, and also new applications. We studied the formation of well-defined films of the electroluminescent oligomer p -sexiphenyl on various metal and semiconductor surfaces and determined the orientation of the molecules by means of low energy electron diffraction (LEED), infrared reflection absorption spectroscopy (IRRAS) and atomic force microscopy (AFM). Depending on the substrate material we found that the molecules are oriented either parallel or perpendicular to the substrate surface. In addition to that, highly oriented 6P films were doped with potassium, and the change in the molecular vibrational signature was investigated with IR-RAS.

The conductive polymer, polyaniline, has been proposed as a material for applications such as flexible batteries, anti-static coatings and anti-corrosion paints. Polyaniline (PAN), however, is an intractable solid. Its rigid backbone and high cohesive energy lead to insoluble polymers. Dispersing polyaniline in solution has been a problem for industrial scale applications.

Observation of electroluminescence (EL) in single layer of molecularly doped polymer (MDP) films is reported. The EL diodes constructed by spin coating technique contain Bisphenol-A-polycarbonate (PC), N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) and 2-(4-biphenyl)-5-(4-terbutylphenyl)1,3,4-oxadiazole (PBD) with the configuration: ITO/PC(20%):TPD(40%):PBD(40%)/Ca, Ag. The EL emission ranges in the green-yellow part of the spectrum and is red shifted respect to the photolu minescence (PL) both of PC:TPD and of PC:PBD. It can be assigned to emission from an exciplex state resulting from the interaction between TPD and PBD, as confirmed by stationary and pulsed fluorescence spectroscopy, and also from other states (triplets, traps or ‘electoplexes’) emitting at lower energies.

Magnetic -field induced orientation of crystalline polymers recently reported by our group is briefly summarized and a possible application of this new phenomenon is presented. We have recently reported a new finding that crystalline polymers including poly(ethylene-2,6-naphthalate), isotactic polystyrene, isotactic polypropylene, poly(ethylene terephthalate), and paraffin align under a strong magnetic field during the phase transition from melt to crystal. We have assumed that some ordered structure, most likely liquid-crystalline-like structure, appearing transiently during the transition is responsible for this phenomenon. Aligned films were transparent and exhibited retardation of several hundred nanometers, suggesting a possible application to fabrication of materials demanded for optical use.

Improvement of electroluminescence (EL) efficiency by utilizing quantum confined structure, so-called type I superlattice, has been discussed. Superlattice structures, which consist of 8-hydroxyquinoline aluminum (Alq3) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), and of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) and Alq3, and of (1,10-phenanthroline)-tris -(4,4,4-trifluoro -1-(2-thienyl)-butane-1,3-dionate) europium (III) (Eu(TTA )3phen) and (N,N'-disalicylidene-1,6-hexanediaminate) zinc (II) (1AZM -Hex) are studied. As a result, strong EL emission is observed in the type I superlattice structures. The mechanism of enhancement is discussed using energy band structure.

Homogenous blends of a processable methanofullerene, [6,6]-Phenyl C61 - butyric acid methyl ester (PCBM), with poly (p-phenylene vinylene) (PPV) made from a novel nonionic precursor route were investigated by optical spectroscopy and by photocurrent measurements. The conversion process of the precursor polymer to the PPV in the blends with PCBM was monitored by IR absorption. In composites of PPV/PCBM strong quenching of the PPV luminescence was observed. LESR (light induced electron spin resonance) and PIA (photoinduced absorption) studies confirmed the occurrence of photoinduced electron transfer from the PPV to PCBM. Photovoltaic devices made from PPV/PCBM blends showed monochromatic power conversion efficiencies of app. 0.3%. The spectral photocurrent was observed to follow the absorption profile of the PPV.

We have used electron paramagnetic resonance (EPR) spectroscopy for investigating the properties of spins, such as those carried by polarons which carry both spin and charge in an electrosynthesized poly (meta/para phenylene) PMPP.Indeed, the study of their mobility provides an insight into the charge transport properties of the conjugated polymer in a microscopic scale.Moreover, we report a correlation between thin film morphology, chainlength and optical gap.Particularly, we show a blue shift upon decreasing chainlength. Furthermore, we have observed a blue shift of the energy band gap with the decrease of the surface grain size deduced from atomic force microscopy (AFM) analyses. This result has been explained in terms of quantum confinement.

Substituent effects are quite important in fine tuning the photonic properties of conjugated molecules. In designing new chromophores, electron-donating or withdrawing substituents affect the electron distribution in the conjugation sequence, and previous studies to establish structure- property relationships have noted that when second row elements replace first row elements in the structure (e.g. S for O in donor groups), large enhancements of both the second and third order optical nonlinearity are observed. However, along with the observed enhancement of the nonlinearity, a red-shift in the absorption spectra occurs, often with peak broadening and tailing. This absorptivity-nonlinearity trade-off has been a constant concern in proposing organic materials for electro-optic device applications. In this presentation we will review our recent activity in designing new chromophores wherein P replaces N in donor groups and dendrimer building blocks, and the consequences and opportunities resulting from the observed large blue shifts in the spectra.

Hydrogen-terminated silicon surfaces exposing a molecular film of covalently grafted pyrrole units were formed by a solution-phase reaction using the organolithium reagent 5-(N-pyrrolyl)pentyllithium. Electrochemical polymerization of pyrrole onto native hydrogen-terminated silicon surfaces and these chemically modified supports produced silicon/polypyrrole junctions that showed diode-like characteristics, with those formed on the latter substrate exhibiting higher current densities and better ideality factors. The presence of the grafted pyrrole films on silicon provided a better electrochemical control over the polymerization process and yielded smoother polypyrrole films than on the native hydrogen-terminated silicon supports. Impedance measurements revealed that the electrical improvements to the silicon/polypyrrole junctions were a consequence of incorporating sites on the silicon surface for direct contact between the conducting polymer and the semiconductor, as the barrier height of the junction appeared to be unaffected by the chemical modification.

We have studied the effect of a magnetic field on the photocurrent of MDMO-PPV (poly(2-methoxy,5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene) and MDMOPPV/fullerene diodes (plastic solar cells) and performed photocurrent detected magnetic resonance (PCDMR) experiments on these devices consisting of an active layer sandwiched between a low and a high workfunction metal electrode. Results clearly show that the magnetic field effect (MFE) on the photocuttent as well as PCDMR signals are quenched in conjugated polymer/fullerene composite devices in contrast to large signals observed in the pristine conjugated polymer devices. Photoinduced electron transfer from conjugated polymers onto fullerene is proposed to be responsible for this quenching.

The conventional molecular ion doped polyaniline undergoes dedoping in sea water because the sea water, with pH = 8, induces deprotontation of polyaniline. In this article we report polyaniline complexes that remain conductive in sea water. These complexes are useful for corrosion protection of metals in marine environment and for chromate replacement coatings. The reason for the pH stability of these polyaniline complexes was discussed. The polymeric complexes of polyaniline were used as an additive in an epoxy primer coating on AL7075-T6 without surface pre-treatment of the metal. Corrosion tests show that these polymeric complexes of polyaniline are effective for improving the corrosion inhibition in seawater for aluminum alloys.

Novel conjugated light-emitting polymers were synthesized via a Wittig-Horner condensation reaction between a binaphthyl dicarbaldehyde and a series of electron-rich or electron-deficient aryl diphosphonates. After comparing these materials with the model compound, 4c, it was revealed that the introduction of a twisted, non-coplanar binaphthyl structure provided an effective approach for tailoring the spectral characteristics of the polymers and improving their solubility and thermal stability (glass transition temperature, Tg, 200°C). Furthermore, the band structures of the polymers could be fine-tuned by changing the electronic properties of the bridging aromatic units. The electroluninescence (EL) and device characteristics were also reported in this paper.