Photochemical methods were developed to obtain a variation of the refractive index in aromatic polymer surfaces and a change in the photoluminescence characteristics of phenylenevinylene-based polymers. Films of aromatic polymers, among them polystyrene (PS), poly(2-vinylnaphthalene) (PVN) and derivatives of poly(p-phenylene-vinylene) (PPV) were UV irradiated in the presence of gaseous hydrazine (N2H4). The photoreaction led to a strong reduction of the refractive index of the polymers due to a hydrogenation of the aromatic units. In the case of PPV, we observed reductive photobleaching. This new technique was employed to produce photogenerated patterns in PPV. The results are compared to oxidative bleaching.

The enzyme horseradish peroxidase has been recently used for the polymerization of water-soluble conductive polyaniline. Hematin, an ironporphyrin molecule that catalyzes the oxidative polymerization of aniline and phenol, is a robust and inexpensive biomimetic alternative to horseradish peroxidase. Composite thin film assemblies were fabricated by electrostatic layer-by-layer (ELBL) alternate deposition of a polyelectrolyte, poly(dimethyl diallylammonium chloride), and hematin. These multilayer assemblies were then used for the polymerization of aniline in the presence of a polystyrene sulfonate as a template. It was found that the polymerization rate depends on both pH and the concentration of hematin deposited and the polyaniline synthesis was possible both onto the ELBL surfaces and in solution. It was also discovered that it is possible to reuse the ELBL assembled hematin substrates for multiple aniline polymerizations.

Charge transport in a composite solar cell (CSC) is analysed by considering independent and random migration of the photo-generated electrons/holes over interpenetrating conducting networks. It is shown that besides an interface area and the materials parameters the efficiency (average number of the carriers reaching the electrode per time unit) of the CSC depends on the effective dimensionality of the conducting channels. Our analysis shows that the 1-d network is ∼20% more effective than the 3-d one, and therefore the morphology of the 1-d type for the organic phase within a 3-d inorganic one is preferable for the CSC. It is shown that the CSC with bulk generation of excitons is potentially more efficient than a dye-sensitised solar cell. The highest efficiency of collection of photo-generated carriers for the CSC at highest possible current for the given material properties cannot exceed ∼40% and 34% for 1-d and 3-d networks respectively.

A number of material properties must be optimized before organic electro-optic materials can be used for practical device applications. These include electro-optic activity, optical transparency, and stability including both thermal and photochemical stability. Exploiting an improved understanding of the structure/function relationships, we have recently prepared materials exhibiting electro-optic coefficients of greater than 50 pm/V and optical loss values of less than 0.7 dB/cm at the telecommunication wavelengths of 1.3 and 1.55 microns. When oxygen is excluded to a reasonable extent, long-term photostability to optical power levels of 20 mW has been observed. Photostability is further improved by addition of scavengers and by lattice hardening. Long-term (greater than 1000 hours) thermal stability of poling-induced electro-optic activity is also observed at elevated temperatures (greater than 80°C) when appropriate lattice hardening is used. The successful improvement of organic electro-optic materials rests upon (1) attention to the design of chromophore structure including design to inhibit unwanted intermolecular electrostatic interactions and to improve chromophore instability and (2) attention to processing conditions including those involved in spin casting, electric field poling, and lattice hardening. A particularly attractive new direction has been the exploitation of dendrimer structures and particularly of multi-chromophore containing dendrimer structures. This approach has permitted the simultaneous improvement of all material properties. Development of new materials has facilitated the fabrication of a number of prototype devices and most recently has permitted investigation of the incorporation of electro-optic materials into photonic bandgap and microresonator structures. The latter are relevant to active wavelength division multiplexing (WDM). Significant quality factors (greater than 10,000) have been realized for such devices permitting wavelength discrimination at telecommunication wavelengths of 0.01 nm.

Chiral polyalkylthiophenes are noncentrosymmetric organic materials which can be used both in second harmonic-generation devices and in polarized light emitting diodes. In this work we present the synthesis and the characterization of a polyalkylthiophene with a chiral center very close to the conjugated backbone: poly(3-[(S)-2-methylbutyl]thiophene) (PMBT). Circular dichroism (CD) measurements have been carried out to ascertain the chirality of these materials. The CD spectra show intense signals both in mixed solvents and in the solid state. The strong Cotton effect can be associated to a highly ordered aggregated phase whose nature is still under investigation. We also present the photo and electroluminescence characterization of single layer light emitting diode (LED) with the following configuration: ITO (Tin Indium Oxide)/PMBT/Al.

Artificial opal films have been prepared by sedimentation of monodisperse silica spheres in water suspension. Atomic force microscope images show a triangular packing of the spheres at the surface of the films. The presence and the energy position of an optical pseudo gap in incidence-angle-dependent transmittance and reflectance spectra is observed and accounted for by theoretical calculations of the photonic band structure. These calculations indicate that the pseudo gap is due to the splitting of the photonic bands in the L point of the Brillouin zone. The spectroscopic data show additional loss structures due to both other features of the Brillouin zone and the diffraction of the light from the regular surface of the sample. The effect of the infiltration of opals with polydiacetylene solutions is also reported.

Electrospinning has been used to prepare nanofibrous composite membranes of semi-conducting particles (TiO2) and photovoltaic dyes. Electrospinning is a relatively simple technique where electrical forces are used on polymeric solutions to produce nanoscale fibers. The resulting nanofibrous membranes have surface areas that are roughly one to two orders of magnitude higher than conventional thin films. It is believed that this higher surface will allow for more efficient light harvesting in photovoltaic devices. Our research has focused on the fabrication of organic/inorganic hybrid solar cells featuring dye sensitized nanocrystalline semiconductor particles using electrospinning. Phthalocyanine and N3 (cisdi(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate) ruthenium(II)) photoactive dyes were electrospun with semi-conductive TiO2 nanoparticles into a matrix polymer, polyacrylonitrile (PAN). Electron microscopy and elemental analysis of the electrospun membranes shows that each component is present and uniformly dispersed in the nanofibrous membranes. In general, the dye membranes electrospun with the TiO2 nanoparticles exhibited a greater photoelectric response than the membranes with dye only. The N3 dye membranes however showed the greatest photoresponse in comparison to the phthalocyanine dyes, with or without the TiO2 nanoparticles. Photoelectric responses on the order of 30 μA and 280 mV were achieved with dye-sensitized membranes and are believed to be the first demonstration of a photoelectric response from an electrospun nanofibrous membrane.

This work describes the synthesis and electrospinning of new fluorescent polymers and their use for the fabrication of optical chemical sensors. A new fluorescent monomer was first synthesized by coupling reactions between methacryloyl chloride and a pyrene derivative, 1-pyrene butanol. Fluorescent polymers containing pyrene molecules were then obtained by the copolymerization of this monomer with methylmethacrylate using 2,2'-azobisisobutyronitrile as the initiator. These polymers show distinct and well-defined fluorescence that is characteristic of the pyrene chromophores. Electrospinning was used to process these polymers into high surface area nanofibrous membranes for optical sensing. The resulting membranes show a high sensitivity to 2,4-dinitro toluene based on the fluorescence quenching of the pyrene chromophores. Fluorescence intensities decreased with increasing concentration of the 2,4-dinitro toluene. The quenching behavior follows Stern-Volmer bimolecular quenching kinetics. The synthesis, characterization, electrospinning fabrication, and sensing capability of these polymers will be discussed.

In this work, we study the influence of organic solvents on the absorption and emission spectra of solutions and films of a soluble poly-(p-phenylenevinylene)- PPV derivative, the poly(3-methoxy-5-dodecoxy-p-phenylene-vinylene)- MD-PPV. We carried out absorption in the UV-Vis range, photoluminescence (PL) and photoluminescence excitation (PLE) measurements of MD-PPV solutions and cast films from five different solvents at room temperature. UV-Vis measurements show a solvatochromic effect, presenting a blue shift of the lowest-energy edge of the φ-φ* transition band for different solvents. PL spectra, on the other hand, show a multi-peak behavior, which is fitted considering an excitonic transition model followed by vibronic structure replica. It is observed that the first vibronic peak intensity tends to increase in comparison to the excitonic peak as δH increases, indicating that the solubility parameter of the solvent has a direct influence on the electron-phonon coupling. In addition, deviations from the intrachain model were observed for aromatic solvents, suggesting that aggregate formation can take place in the emission spectrum. Excitation spectra show that a new excitation peak comes out around 550 nm for poorer solvent solutions at higher PLE detection wavelengths, which is consistent with aggregate states emission.

We review several applications of microstructured photonic crystal optical fibers that incorporate active materials infused into the air-holes. The tunable optical characteristics of the materials combined with the unique structure of the fiber enable a number of functionalities including reconfigurability and tunability for various fiber device applications. We describe a brief characterization of the modes and discuss the experimental results of the fibers.

Ultrafast photoinduced charge transfer from conjugated polymers to fullerenes has led to extensive studies of these systems as photovoltaic devices. The charge transfer process prevents radiative electron-hole recombination, resulting in free, mobile charges. A limiting factor is the exciton diffusion distance, which is of the order of 10 nm. If the fullerene is not within this distance of the optical excitation site, no charge separation will take place. The simplest system for such devices is a bilayer system in which a film of C60 is sublimed onto a spin-cast film of MEH-PPV, for example. We describe studies in which the polymer is heated above the glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created. The photoluminescence and the short circuit currents of the devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface.

A novel rigid and planar chromophore, p-phenylene benzobisthiazole unit with donor and acceptor groups, has been synthesized for second-order nonlinear optical applications. The first hyperpolarizability, β, of the nonlinear optical (NLO) chromophore has been measured by a Hyper-Rayleigh Scattering technique at 1064 nm in 1,1,1,3,3,3-hexafluoro-2-propanol solution. The β value of the NLO chromophore is 405E-30 esu, which is about seventeen times that of the p-NA (nitroaniline) value. The chromophore showed good thermal stability up to 330 °C in nitrogen atmosphere.

We report optical switching by a silica microsphere optical resonator coated by a conjugated polymer. Microspheres were fabricated by melting the tip of an optical fiber and coated by dipping in a 1 mg/ml toluene solution of poly(2,5-dioctyloxy-1,4-phenylenevinylene) (DOOPPV). The resonator properties were characterized by evanescently coupling 1.55 μm light propagating along a stripline-pedestal anti-resonant reflecting optical waveguide into optical whispering gallery modes (WGMs). WGM linewidths less than 2 MHz were measured, corresponding to cavity Q > 108. WGM resonant frequency shifts as large as 3.2 GHz were observed when 405 nm pump light with a power density of ∼100 mW/cm2 was incident on the microsphere. The time constant of the observed frequency shifts is approximately 0.165 seconds, leading us to attribute the frequency shift to thermo-optic effects. Such a system should be capable of thermo-optically switching at speeds on the order of 10 kHz.

The effects of processing conditions on the properties of organic light emitting devices (LEDs) based on rubrene-doped poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] and a new electron transporting material, 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]pyridine, are reported. These dual-layer LEDs exhibited a higher quantum efficiency than observed for structures incorporating the more widely used electron transport compound 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7). However, the as-prepared devices degraded relatively rapidly on storage (10-1 mbar, no applied bias). Thermal annealing of the degraded devices at 160 °C for 30 minutes restored the currents and light outputs close to those measured for fresh devices. The annealed LEDs exhibited a significant increase in their operating lifetime. Lifetime improvements could also be achieved by increasing the deposition rate and thickness of the thermally evaporated aluminium top electrode. These effects are attributed to better adhesion between the aluminium top electrode and the underlying electron transport layer.

A double-layer photonic crystal (PhC) possessing incomplete photonic band gap (PBG) has been prepared by successive formation of one opaline film on top of another and subsequent impregnation with CdTe colloidal nanocrystals as the light source. Transmission, reflection and photoluminescence (PL) spectra of this nanocomposite have been measured from 1.8 to 2.5 eV at different angles of the light incidence and detection. The suppression of emission intensity has been found at both stop bands of the PhC heterostructure. Depending on the excitation power, either suppression or enhancement of the emission rate at the stop band has been observed.

In this paper, we present the syntheses of the conjugated polymers of different “effective molecular weights.” The goal is to be able to optimize the thin film coating process without compromising the electronic, optical and electroactive properties.

Our strategy for controlling the effective molecular weight is to synthesize a double-strand conducting polymer that is an inter-polymer complex of a polymeric dopant and a conjugated polymer. The first strand of the two-strand complex is either a polyaniline or a polypyrrole chain, while the second strand is a poly(acrylic acid). The molecular weight of poly(acrylic acid) is used as a parameter for adjusting the coating and film-forming properties.

We found that the inter-polymer complexes are dispersible in water. The dispersion stability is strongly dependent on the molecular weight of the poly(acrylic acid) strand. The dispersion is more stable for polymers synthesized with higher molecular weight of poly(acrylic acid). We also found that the particle size and particle morphology are dependent on the effective molecular weight.

By using the Trznadel method, it has been possible to increase molecular weight, the regio-regularity, and probably to decrease the residual catalyst of poly-3-hexylthiophene thin films. The drift mobility of holes, normal to the surface of cast films, in air, has been found using Schottky diodes, and field-effect mobility has been measured with Thin-Film Transistors. Three types of film have been studied using the two methods of assessing mobility. The as-synthesised films are compared with those that have been fractionated. The third set of films involves doping with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The doped films show a field effect mobility of 0.2 cm2V-1s-1: all others being lower. Field effect mobility is approximately two orders of magnitude greater than that in the bulk normal to the plane of the film. Doping levels in the films are found to be similar, probably because of the process conditions.

We here studied photosensitized radical reaction of poly(L-lactic acid) (PLA) using spectrophotometry, electron spin resonance (ESR), gel permeation chromatography (GPC) measurements, and semiempirical molecular orbital calculation. PLA doped with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was irradiated at 77 K using UV light. After the irradiation, we observed a new broad absorption band that is assigned to TMPD radical cation produced by two-photon ionization. ESR spectrum of the irradiated sample also indicated presence of main chain scission radical of PLA as well as the TMPD radical cation. By increasing the irradiation time, the ESR spectrum changed by another radical species which is produced by dehydrogenation of PLA main chain. Spectral simulation for the obtained ESR spectra revealed changes of relative amounts. The GPC measurements clearly indicated decrease in molecular weight of PLA after the irradiation. The semiempirical molecular orbital calculation was used to confirm the reaction scheme of radical anion decomposition.

For the first time simulations of chirally-induced excitons have been carried out, using molecular dynamic simulations and self-consistent quantum chemical calculations within the RHF [Restricted Hartree-Fock ] and DFT [Density Functional Theory] approaches. We found that the DFT approach gave substantially better agreement than the RHF. Comparison of theoretical and experimental results for biscyanine dye with a Shiff base showed a sufficiently good level of agreement.

Spectroscopic properties of conjugated molecules/polymers have varying degrees of sensitivity to backbone conformation. Optical studies are presented as a function of temperature and hydrostatic pressure, using photoluminescence and Raman scattering from two polymers with distinct differences in their backbone conformation, namely, polyfluorene (PF) and ladder type poly(para-phenylene)(m-LPPP). In contrast to the photoluminescence (PL) vibronics in mLPPP, the 0-0 PL vibronic peak in PF shows a red-shift with increasing temperatures. Pressure studies reveal that the PL spectrum of PF red-shifts and broadens with increasing pressures. The phonon lines in PF show an antiresonance effect at higher pressures indicating a strong electron-phonon interaction.