This PDF file contains the front matter associated with SPIE Proceedings Volume 6999, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

×The lasing threshold of a microcavity is mainly determined by the spontaneous emission factor β, which is inversely proportional to the mode volume V_c. We demonstrate an experimental way to decrease the mode volume via lateral structuring of the microcavity. This redistributes both number and density of the transversal cavity modes, which increases the amplitude of the internal electromagnetic field. Our samples are microcavities with an active layer of variable thickness (0.2 to 2 μm) made of tris-(8-hydroxyquinoline) aluminium (Alq₃) doped with 4-(dicyanomethylene)-2- methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). With thermal coevaporation through a shadow mask, this layer is structured into an array of photonic boxes square-shaped microcavities with an area of 55 μm₂. Using a microscope objective, we record the spatial distribution of the cavity transmission spectra with submicron resolution. The modes of the photonic boxes show a clear discretization, which is due to the multidimensional optical confinement. Under selective excitation of the DCM molecules via a focused pulsed laser (532 nm, 1.5 ns, 2 kHz , &diameter; ≈ 3μm), we record the spatially and spectrally resolved emission of single photonic boxes. The laser pulse energy is varied to obtain input-output curves of the cavity modes. At an excitation energy of ~30 pJ, we observe superlinear growth as well as a spectral narrowing of the emission from the lowest energy mode of a single photonic box. For this lasing transition, we determine a spontaneous emission factor β of ≈0.01.

We present the design of an optimized mixed-order photonic crystal laser structure. The lasing properties of this two-dimensional photonic crystal structure with an organic gain material are investigated theoretically and experimentally. A feedback structure fabricated in a thin film of Ta₂O₅ increases both the index contrast from the gain material as well as the optical confinement. Furthermore, by combining first order photonic crystal structures with second order ones losses occurring at the edge of the second order structure are dramatically reduced leading to a lower laser threshold and / or to a much smaller footprint of the laser.

We present single mode optically pumped lasing from a new resonator structure for polymer lasers employing nonperiodic circular Bragg gratings. Our devices, using a polyfluorene derivative (BN-PFO) as gain medium, are the first blue-emitting circular grating semiconductor lasers (either organic or inorganic). They exhibit feature sizes as small as 47 nm and emit azimuthally polarized beams with a spectral linewidth ≈ 0.2 nm. We find a minimum lasing threshold energy density of 1.2 μJ/cm² (10 Hz, 8 ns, 355 nm Nd:YAG laser excitation). The quality factor of the resonator modal fields is found to be at least 2200 for these devices.

Although optically pumped semiconductor organic lasers have been reported for a decade, no electrically pumped organic laser diode has been till now realized. Charge-induced and triplet excited state absorption have been identified as major bottlenecks: in this context exciton dynamic plays a key role. We report on the measurement of exciton diffusion lengths in the archetypal ambipolar material CBP, in the presence of an injected current in a working multilayer device. The technique is based on moving a thin red phosphorescent layer away from the recombination zone. The amount of emitted light depends on the layer position via the diffusion of triplet excitons. We demonstrate the crucial importance of designing the structure according to optical field calculations in order to measure diffusion lengths L_D. We measured a value of L_D = 16 nm +/- 4 nm in CBP, and also report on the variation of L_D with the injected current.

The primary result of UV-Visible photon absorption by complex organic molecules is the population of short-lived electronic excited states. Transportation of their excitation energy between single molecules, formally mediated by near-field interactions, may occur between the initial absorption and eventual fluorescence emission events, commonly on an ultrafast timescale. The routing of energy flow is typically effected by a sequence of pairwise transfer steps over numerous molecules, rather than a single step over the same overall distance. Directionality emerges when there is structure in the molecular organisation. For a chemically heterogeneous system with local order, and with suitable molecular dispositions, automatically unidirectional transfer can be exhibited as the result of a 'spectroscopic gradient'. However it is also possible to exert control over the directionality of excitation flow by the operation of external influences. Examples are the application of an electrical or optical stimulus to the system - achieved by the incorporation of an ancillary polar species, the application of a static electric field or electromagnetic radiation. Most significantly, based on the latter option, an all-optical method has recently been determined that enables excitation transportation to be completely switched on or off, such that the energy flow is subject to controllable photoactivated gating. It is already apparent that this photonic process, termed Optically Controlled Resonance Energy Transfer, has potentially numerous applications. For example, it represents a new basis for optical transistor action.

Light-induced mass transport in azobenzene functionalized polymers has been used for generation of surface relief gratings (SRG) for different optical applications. The effect of grating formation has been ascribed to the light-induced motion of the azobenzene chromophores involving the covalently bound polymer chains. We propose a concept of supramolecular materials for the effective all-optical generation of surface relief structures and optical anisotropy. The materials are based on the non-covalent interactions between charged photochromic azobenzene units and oppositely charged polymer matrix, for example polyelectrolytes including charged alkoxysilanes. This new supramolecular approach opens a new way for the simple, cost effective and environment friendly preparation from building blocks of a variety of materials for the effective formation of SRG. Up to 1.65 μm deep relief gratings were inscribed onto a few micrometers thick films of these materials. The high thermal stability of the induced structures has been explained in terms of the network of oppositely charged ions inherent to the materials. Also 2D-structures, for example square and hexagonal gratings, were inscribed by the successive recordings. The latter possibility was also used to generate gratings with non-sinusoidal profiles by Fourier transform technique. A new technique to control the grating profile has been developed based on the real-time process of grating formation in these materials. The gratings with sow-tooth like profiles were induced by this method. The diffraction efficiencies up to 60 % in one diffraction order were achieved.

Charge resonance is a characteristic feature of organic molecules where electron donor (D) and acceptor (A) groups are linked by π-conjugated bridges. Dipolar (D-π-A), quadrupolar (D-π-A-π-D or A-π-D-π-A) or, more generally, multipolar molecules are widely studied for applications in nonlinear optics. We propose a general model for multipolar chromophores based on an essential-state description of the electronic structure, and accounting for the coupling of electrons and molecular vibrations and polar solvation coordinates. Depending on the charge distribution on the molecule, multistable potential energy surfaces are found for the ground state and/or the lowest-energy excited state. The resulting symmetry breaking shows up with important solvatochromic effects in either absorption or fluorescence spectra. The essential-state model lends itself quite naturally to describe supramolecular interactions in aggregates, crystals, or ordered films. At variance with the standard excitonic pictures, we relax the dipole approximation for electrostatic intermolecular interactions and demonstrate important excitonic shifts of dark (two-photon allowed) states for quadrupolar dyes. Moreover, bound-biexciton states are found with huge two-photon absorption.

We present the design, synthesis, and characterization of a class of heteroaromatic bichromophores in order to investigate intermolecular interactions and their effect on optical and nonlinear optical properties. As a design strategy we have linked two dipolar or quadrupolar units through a non-conjugated alkyl chain. The two units are connected either through their donor or their acceptor end-groups. This study represents a first step towards the design of bi- and multichromophoric systems with optimized NLO responses in order to exploit collective and cooperative effects from interchromophore interactions.

We haven been exploring polymethylmethacrylate, PMMA, doped with conjugated luminescent polymers for applications in plastic optical fibers (POFs) showing gain. In order to control loading and dispersion of the conjugated polymers in the PMMA matrix, new polyfluorene- PMMA copolymers were synthesised. In this communication we report on the optical properties of these copolymers, both in solution and in solid state. Furthermore, the properties of POFs fabricated with such copolymers are presented and compared with the properties of POFs based on PMMApolyfluorene blends.

A new ruthenium sensitizer based on a heteroaromatic-4,4'-π-conjugated 2,2'-bipyridine, bearing conjugated π-excessive heteroaromatic rings as donors is presented. Dye-sensitized solar cells have been fabricated based on the novel ruthenium complex [Ru(II)LL'(NCS)₂] (L = 4,4'-bis[(E)-2-(3,4-ethylenedioxythien-2-yl)vinyl]-2,2'-bipyridine, L' = 4,4'- (dicarboxylic acid)-2,2'-bipyridine) and their photoelectrochemical properties have been measured under various conditions. Using this sensitizer photovoltaic efficiencies up to 9.1 % under standard global AM 1.5 sunlight were obtained. DFT/TDDFT calculations have been performed for the sensitizer in solution. By calculating the excited states energy and character and comparing the results with the conduction band edge of a model TiO₂ nanoparticle, we were able to highlight the factors affecting the measured photovoltaic efficiencies.

The transparent electron transport material NTCDA (1,4,5,8-naphthalenetetracarboxylic dianhydride) was examined in order to find a suitable substitute for C60 which is today often used in small molecular organic solar cells as transport layer. Due to its wide band gap, NTCDA does not absorb in the visible range and is furthermore exciton blocking. By doping with AOB (acridine orange base), its conductivity was raised to about 1 • 10^-4S/cm. It can therefore simultaneously be used as electron transport material and optical spacer in p-i-n type solar cells, leading to power conversion efficiencies of up to 2.83%. Additionally, an investigation of the surface morphology using AFM was performed.

Organic photo-detectors (OPDs) have been discussed as high-speed photo-detectors fabricated by vacuum and solution processes. By vacuum process, OPD was fabricated on an ITO-coated glass substrate by organic molecular beam deposition (OMBD). Copper phthalocyanine (CuPc) and N,N'-bis(2,5-di-tert-butylphenyl) 3,4,9,10-perylene dicarboximide (BPPC) were used as a p-type and an n-type material. The photo response of OPD was evaluated using a laser (λ=650 nm) and clear response signals at 100 MHz were observed. The imaging signals were successfully received using the OPDs. For solution processed OPDs, poly(9,9-dioctylfluorene) and a phosphorescent iridium derivative were used as a host and a dopant material, respectively. A pulsed signal of 100MHz and amore than 40 MHz were observed by the OPDs fabricated by the vacuum processed and the solution processed devices, respectively.

Since their invention use of organic field-effect transistors (OFETs) has been restricted to applications that explore their unifunctional, i.e. current switching, characteristics. Recently, however, OFETs with additional functionalities have been designed and demonstrated with most notable examples the light-emitting (LE-OFET)^[1] and light-sensing (LS-OFET)^[2] transistors. These devices are of particular significance since design and fabrication of a new type of organic circuits can now be envisioned. Here we report on electro-optical circuits based on ambipolar LS-OFETs and unipolar OFETs. By carefully tuning the ambipolar transport of LS-OFETs their photosensitivity can be controlled and optimised. By going a step further and integrating LS-OFETs with unipolar OFETs we are able to demonstrate various optoelectronic circuits including electro-optical switches and logic gates. A unique characteristic of these gates is that their input signal(s) can be designed to be either all-optical or electro-optical. An additional advantage of the technology is that LS-OFETs can be integrated with the driving electronics using the same number of processing steps, hence eliminating the need of additional fabrication costs. This is one of the first demonstrations of organic circuits where signal processing involves the use of both optical and electrical input signals. Such optoelectronic devices/circuits could one day be explored in various applications including electro-optical transceivers and optical sensor arrays.

Organic single-crystal ambipolar light-emitting transistors show a great interest due to their unique features, spectral matching with their active material spectra and the quantum efficiency preservation during ambipolar operation at high current density operation in kA/cm² order. The development of ambipolar light emitting transistor based on high photoluminescent material, α,ω-bis(biphenylyl)terthiophene (BP3T) single crystal is reported. By using bottom-gated top-contact configuration, with Ca and Au opposed metal electrodes, high value of hole and electron mobility were obtained. Extremely bright light emission observed during ambipolar operation shows prospect for electrically driven amplified spontaneous emission from organic materials.

In this manuscript, we report on the performance of organic light-emitting diodes with remote metallic contact using N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C₁₃H₂₇) as well as perfluorohexyl-sustituted quaterthiophenes (DHF-4T) as the electron-transport material. Both materials exhibit high electron field-effect mobility. An electron mobility of 0.123 cm²/Vs is demonstrated for DHF-4T. The wide optical bandgap of DHF-4T allows reduction of the light absorption in the region of interest compared to PTCDI-C₁₃H₂₇. In this way an external quantum efficiency of 0.25% could be obtained. This is a factor of 10 larger than in PTCDI-C₁₃H₂₇ based devices.

The realization of stable, high efficiency blue organic light emitting devices (OLEDs) remains challenging. The efficiency of blue fluorescent OLEDs is fundamentally limited, and blue phosphors are typically less stable. We discuss potential solutions to these problems. We show that device engineering can be used to optimize the color of a relatively stable and efficient bluegreen phosphor. In addition, we demonstrate the ability to manipulate the fraction of excitons which form as singlets in fluorescent materials by inserting a mixing layer that affects only the precursors to excitons.

Magnetic field effects in organic light emitting diodes have emerged as subject of intense research activities. We investigated the recently discovered organic magnetoresistance effect, i. e. the phenomenon that the presence of an external magnetic field can influence both the current flow through an organic light emitting diode and the light emission from the device. Magnetoresistance measurements were performed in different device structures as a function of magnetic field and driving voltage. We demonstrate that electrical conditioning can be used as an efficient method to enhance the organic magnetoresistance effect in devices based on polymers and small molecules. Depending on duration and intensity of the conditioning process the magnetoresistance effect can be increased from ~1% to values exceeding 15% at 40mT in devices with poly(paraphenylene-vinylene) as light emitting polymer. Qualitatively the increase in magnetoresistance can be correlated with a decrease in luminance during the conditioning process. From this we conclude that degradation of the bulk emitter material is responsible for the enhancement of organic magnetoresistance. In addition, we show a dependence of the magnetoresistance effect on the charge carrier balance within the device. In bipolar devices the magnetoresistance effect is significantly larger than in hole-dominated devices which suggests that electron-hole pairs play an important role in the fundamental mechanism causing the organic magnetoresistance effect.

A way to reach highly efficient and stable red bottom emission organic light emitting diodes (OLEDs) is the use of doped transport layers, charge and exciton blockers, and phosphorescent emitter materials to combine low operating voltage and high quantum yield. We will show how efficiency and lifetime of such devices can be further increased. In our contribution, we report on highly efficient red p-i-n type organic light emitting diodes using an iridium-based electrophosphorescent dye, Ir(MDQ)₂(acac), doped in α-NPD as host material. By proper adjustment of the hole blocking layer, the device performance may be enhanced to 20 % external quantum efficiency at an operation voltage of 2.4 V and a brightness of 100 cd/m². At the same time, a power efficiency of 37.5 lm/W is reached. The quantum efficiency is well above previously reported values for this emitter. We attribute this high efficiency to a combination of a well-adjusted charge carrier balance in the emission layer and a low current density needed to reach a certain luminance due to the use of doped transport layers. High chemical stability of the blocker material assures a long device lifetime of 32.000 hours at 1.000 cd/m² initial luminance.

Charge mobilities in disordered organic conductors have a high impact on the functionality of the number of devices, but simulation methods for a detailed analysis of the transport mechanisms and material-dependent effects are still in their infancy. Here we present a model of charge transport in organic solids which explicitly considers the packing and electronic structure of individual molecules. We first develop molecular models for disordered films of tris(8-hydroxyquinoline) aluminium (Alq₃ ) on the basis of all-atom simulations. We use density functional theory to determine the electronic couplings between molecules and simulate the time-of-flight mobility measurement. We find electron mobilities in the crystalline and disordered phases of ~ 1 cm² V^-1 s^-1 and ~ 10^-4 cm² V^-1 s^-1 respectively. A detailed analysis of the conduction pathways suggests the existence of kinetic traps for electrons and holes on localized molecular clusters. Our results suggest that charge transport in disordered Alq₃ is dominated by a few highly conducting pathways.

Self-assembled monolayers (SAMs) of covalently bound organic molecules are rapidly becoming an integral part of organic electronic devices. There, SAMs are employed to tune the work function of the inorganic electrodes in order to adjust the barriers for charge-carrier injection into the active organic layer and thus minimize undesired onset voltages. Moreover, in the context of molecular electronics, the SAM itself can carry device functionality down to a few or even a single molecule. In the present contribution, we review recent theoretical work on SAMs of prototype π-conjugated molecules on noble metals and present new data on additional systems. Based on first-principles calculations, we establish a comprehensive microscopic picture of the interface energetics in these systems, which crucially impact the performance of the specific device configuration the SAM is used in. Particular emphasis is put on the modification of the substrate work function upon SAM formation, the alignment of the molecular levels with the electrode Fermi energy, and the connection between these two quantities. The impact of strong acceptor substitutions is studied with the goal of lowering the energy barrier for the injection of holes from a metallic electrode into the subsequently deposited active layer of an organic electronic device.

The currently starting technical exploitation of organic electronic devices requires a deep understanding of ageing and degradation mechanisms. In addition to extrinsically caused ageing processes, such as the penetration of oxygen and water in organic layers and subsequent (electro)chemical reactions, further degradation channels exist in such devices, which are based on intrinsic chemical reactions of the materials used in the devices. At this time, we know the degradation mechanisms of only few organic materials applied in organic light emitting devices (OLEDs). To detect specific reaction products, we introduced laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS), a method which allows to distinguish between desired and undesired compounds in thin film organic devices. We use LDITOF- MS to detect the degradation products of different Iridium based emitter materials like Ir(MDQ)₂acac (red emitter) and FIrpic (light blue) in dc driven OLEDs and adapted test sample structures. Due to the dissociation behaviour of some Ir complexes and the ability of their fragments to form complexes with several hole blocking materials, the degradation mechanisms of the devices can be understood in terms of such chemical complex formation between the emitter molecules and neighbouring materials. On the other hand, the knowledge about these mechanisms can be used to select the right combination of materials for the benefit of long-living devices as we will show at the end of this work.

In this study a high efficient p-i-n type orange organic light emitting diode (OLED) is presented. It is based on doped charge transport layers to realize low operating voltage and emitting layer which consists of alpha-NPD(4,4-bis [N-(1- naphtyl)-N-phenylamino]biphenyl) and Iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) as a host and a phosphorescence dye dopant respectively. Organic layers are vacuum-sublimed on ITO-coated glass substrates in vertical inline deposition tool, and aluminium is deposited directly on organic layer by DC magnetron sputtering to form a cathode. Since sputter deposition of top electrode is known to damage organic layers and degrade OLED performance, various sputter process parameters are selected and applied for cathode formations, and the OLEDs are characterized by means of I-V-L measurements. The OLED characteristics are evaluated with the plasma factors based on sputter process parameters in order to explain the damage sources from sputtering process. The characteristics of OLEDs that cathodes are deposited by sputtering and evaporation are compared. The fabricated OLED which has the lowest damage level exhibits almost comparable result to the OLED that the cathode is deposited by evaporation. The OLED shows good performances of driving voltage of 4.25 V and luminous efficacy of 7.77 lm/W and current efficiency of 10.68 cd/A at 1000cd/m².

Electrical doping of organic layers is now a well established method for building highly efficient and long living OLEDs. A unique class of OLED devices called PIN-OLEDs based on redox doping technology is emerging as one key technology for OLED applications. These devices exhibit high power efficiency and long life time, which are critical parameters for commercial success. Moreover, PIN OLEDs offer high degree of freedom in choosing layer structures for optimizing the device performance for specific lighting and display applications. For example, optimizing color and power efficiency of OLEDs can be easily achieved without compromising the device operating voltage. It is worth to mention that PIN OLEDS, especially the red emitting PIN OLEDs, exhibit record breaking half life time of more than one million hours with the starting device brightness of 1000 cd/m². The doping technology also offers benefits to other organic electronic devices such as OTFTs and photovoltaic devices. This paper briefly discusses the improvements made on the OLED device performance such as power efficiency and lifetime using doped transport layers. Few examples of device optimization using doped layers are presented in detail. In addition, a brief discussion on performance of doped transport layers in photovoltaics is also presented.

Light emitting devices (LEDs) based on colloidal semiconductor nanocrystals represent a matter of technological interest for the development of flat panel display and lighting systems. The appealing features of these materials are the high fluorescence efficiency, narrow ban edge emission, potential chemical stability, and tunable light emission across the visible spectrum. However the integration of these materials in the very promising PIN technology is still challenging due to the lack of an appropriate QD deposition technique. So far only wet deposition methods such as spin-coating and drop-casting have been exploited to realize QD thin film. Moreover QD thermal evaporation is not possible because of their high molecular weight. In this scenario we developed a dry, simple, and inexpensive deposition technique to transfer semiconductor QDs on organic semiconductor materials. We exploited this technique to fabricated an organic/inorganic hybrid red emitting device whit a doped hole transport layer.

The details of the arrangement of mixtures of semiconducting materials in thin-films have a major influence on the performance of organic heterojunction solar cells. Here, we exploit the phenomenon of spinodal dewetting during spin coating of blends of PCBM and a cyanine dye for the design of phase separated morphologies with increased interfacial area. AFM snapshots of as-prepared films and after selective dissolution suggest that the solution separates into transient bilayers, which destabilize due to long-range intermolecular interactions. We propose that film destabilization is effectively driven by electrostatic forces that build up due to mobile ions that cross the junction and dissolve partially in PCBM. The resulting morphology type is mainly dependent on the ratio between the layer thicknesses, whereas the dominant wavelengths formed are determined by the absolute film thickness. Solar cells were fabricated from films with known structure and a power conversion efficiency of η = 0.29 % was measured for a vertically segregated film consisting of a cyanine layer covering the anode and an upper phase composed of dewetted PCBM domains. We explain the merits of this structure in contrast to a lateral separated blend morphology where the efficiency was 3 times smaller.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid (PEDOT:PSS) has been widely used as hole collector in organic solar cells. We report a study of the PEDOT:PSS quality grade on the current voltage characteristics of a planar heterojunction type photovoltaic cells made of discotic molecules. Electroabsorption (EA) studies were performed on these components in order to understand the correlation between electrode materials and Voc. From the experimental results it can be shown a high open circuit voltage of 1.5 V_oc.

In this work, we present the evolution of optical constants varying with distinct annealing temperature for poly (3- hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM). With calculation of the transmission and reflection spectra in P3HT:PCBM photovoltaic device , the optical properties correlation to annealing temperature is studied. The solar cell power conversion efficiency and optical absorption is compared simultaneously. Finally, the electric field amplitude in the device is discussed for detailed explanation of thermal annealing effects on the organic photovoltaic device performance.

To reach higher performances in organic solar cells, each layer has to be optimised with respect to its purpose. In the case of a p-i-n structured solar cell, the layers are the absorber system, the doped electron and hole transport layers, and the bottom and top contacts. This work focuses on the investigation and characterisation of the transparent hole transport materials PV-TPD, PV-TPDoM, Di-NPB, and MeO-Spiro-TPD, as used in organic p-i-n solar cells. The motivation is to replace the hole transport material MeO-TPD, which has been used so far despite its morphological instability at elevated temperatures, with an energetically and morphologically more suitable material. The hole transport materials were investigated for dopability, hole mobility, absorption, reflection, cyclic voltammetry, and glass transition temperature. Further specific material properties were determined with simplified structures, e.g. m-i-p diodes, and the standard solar cells, consisting of the fullerene C60 as acceptor and ZnPc as the donor material. The Di-NPB has turned out to be the best choice with respect to its intrinsic properties and device parameters. The deep lying HOMO, the high hole mobility of μ = 1.9 • 10^-4 cm²/V s, the morphological stability of T_g = 158°C, and the excellent results of the C₆₀:ZnPc bulk heterojunction solar cell makes the Di-NPB highly suitable for replacement of the MeO-TPD in organic

It is shown that the introduction of functionalized single-walled carbon nanotubes SWNTs in bulk heterojunction polymer-fullerene photovoltaic devices results in an improvement of both the short circuit current density and the fill factor. An optimum performance with a power conversion efficiency of 1.4% is obtained with 0.5 wt% of SWNTs in a (1:1) P3HT/PCBM mixture. The results indicate that the addition of nanotubes enhances the performance of polymerfullerene photovoltaic cells by means of both the electron accepting feature of fullerenes and the high electron transport capability of SWNTs.

Conjugated polymer materials have great potential to be suitable candidates for use in all-optical network communications. They possess ultra-fast response times and a large third order non-linearity compared to minerals (third order susceptibility χ⁽³⁾ of conjugated polymers can be 4 orders of magnitude larger than that of fused silica). Such large non-linearities would allow the fabrication of compact all-optical devices at low power levels. Here we present the first investigations into the creation of a conjugated polymer single mode optical waveguide based on Poly(3- AlkylThiophene) (P3AT). We first synthesized the P3AT and tried to control the chemical reaction conditions in order to improve polymer solubility in common organic solvents. In parallel, we studied the engineering of P3AT single mode waveguide structures made by photolithography techniques which requires the adjustment of P3AT thermo-mechanical properties. Recently, we have been able to fabricate and measure the parameters of Strip-Loaded and buried waveguides with usual polymers. We intend to adapt these processes to obtain the first P3AT single mode optical waveguide.

In recent years there has been significant interest in the ability to switch the second-order nonlinear optical (NLO) response at the molecular level. A compound can be considered as an NLO switch when the response can be turned to an 'on 'and 'off' state. Several switching schemes at the molecular level have been envisioned. Earlier schemes used isomerisation and tautomerisation, causing changes in the nature and/or degree of conjugation between electron donor and acceptor. An alternative approach is based on lowering the electron donating capacity of the electron donor or the withdrawing capacity of the acceptor group. Here we will present results based on protonation/deprotonation and oxidation/reduction of the donor group.

In holographic memories, photopolymer is a hopeful material as a recording medium. To use a photopolymer for holographic memories as practical recording media, it is necessary to clarify the design condition of recording/reproduction characteristics. The coupled-wave analysis (CWA) and the rigorous coupled-wave analysis (RCWA) are widespread methods to analyze diffraction characteristics of volume holographic gratings. However, holographic grating is more complex than simple grating that is presumed in CWA and RCWA. In this study, we analyzed the index change of photopolymer based on a diffusion model and clarified the diffraction characteristics by using the finite-difference time-domain (FDTD) method.

Organic film deposition in vacuum is fast developing scientific and industrial domain. We developed installation for deposition of organic films equipped with optical spectrometer for measurements in situ. We are developing new dyes aimed for application in waveguide sensor, nonlinear optics and studying film organisation during deposition. Fluorinated azo-dyes and azomethine dyes were synthesized at University of Applied Sciences Wildau and at the Institute of Organic Chemistry, Kyiv. Compounds were evaporated at a pressure of 10-³ Pa using resistive heated crucible. Glass and glass covered with polytetrafluoroethylene (PTFE) film are used as substrates. The films were studied with Polytec and StellarNet spectrometers and an atomic force microscope. Optical spectra of the dye films revealed, that some compounds were decomposed during evaporation. Several kinds of dyes were evaporated and deposited without decomposition. Some deposited films formed H-aggregates and other types of aggregates. AFM images of dye films showed that their morphology depends on the chemical structure of the compounds and on the nature of the substrate on which the film was grown.

Second harmonic generation (SHG) in the oriented film of symmetric squaraine (Sq) was studied. Oriented Sq film on aligned polytetrafluoroethylyne (PTFE) sublayer prepared by vacuum deposition with subsequent rubbing using a cloth has been obtained. However, the mechanisms of orientation and SHG are still not clear. Methyl and ethyl substituted hydroxyl-Sq (OHSq) compounds formed oriented films with dichroic ratio of 8 on PTFE layer but with dichroic ratio of 1,5 on Teflon AF. Second layer deposition of Me-OHSq on Et-OHSq or of Et-OHSq on Me-OHSq led to an increase of the film dichroic ratio. Only the film, where Me-OHSq was first layer, exhibits an increase of SHG signal after deposition of second layer. Small differences in bi-layered OHSq films structure was detected by X-Ray diffraction (XRD) spectra.

New outstanding possibilities are emerging by the synthesis of organo-inorganic polymeric materials from a buildingblock approach. Our approach has been to assemble extended solids from bent flexible arenedicarboxylate linkers that offer totally new topologies with helical channels. A new family of Rare-earth Polymeric Framework (RPF-4) has been obtained and its structure solved. The framework is formed by a Lanthanide (Ln) matrix, with the Ln atoms well separated in two directions. The crystals are highly stable in air and the decomposition temperature is above 350°C. Under UV excitation, the linker presents a bluish-white emission peaking around 450 nm. The emission corresponding to the different crystals either present an emission similar to the linker, slightly modified depending on R, or an intense emission due to Ln localized crystal field transitions. In the second case, an energy transfer from the linker to the Ln ions, which relax radiatively with a very efficient emission, seems to occur. The observed emission properties and crystal stability are of interest for applications as light emitting materials. The peculiarities of the structure may avoid the concentration quenching of the luminescence since Ln ions form well separated chains along a-axis with interchain distances around 12.5 Å.

Much progress has been made in improving the molecular hyperpolarizability by constructing larger structures with lots of pi-delocalized electrons. This bigger-is-better approach does not answer a more fundamental question of what intrinsic molecular properties yield the largest response. We introduce a novel analysis (the quantum limits analysis) which is simple to apply and combines first principles with experimental results. The quantum limits analysis allows us to determine the intrinsic nonlinear efficiency of a structure and highlights the underlying physical principles behind the nonlinear response of molecules.

A variety of pyridinium and benzothiazolium-substituted conjugated donor-π-acceptor dipoles are studied by hyper- Rayleigh scattering (HRS) in solution. We have investigated their potential as strong electron acceptor moiety and a complete series has been synthesized and characterized. An elongation of the conjugated π-system for each molecule is made and characterized to see the influence of the elongation of the conjugated π-system. A leveling off of the hyperpolarizability β is already observed after three subsequent double bonds.

SHG efficiency of the poled guest - host polymer system is proportional to the concentration and orientation degree of NLO active molecules (chromophores). Corona poling realized at elevated temperatures could cause concentration decrease of NLO- active molecules due to centrosymmetric crystallization. Our studies showed that number density of crystallites is depending on orientation procedure. To obtain the best orientation procedure for guest - host systems containing four different chromophores based on dimethylaminobenzylidene 1, 3 - indandione we have compared optical images and SHG efficiency of corona poled films. According to our observations external poling electric field applied from the very beginning of the sample heating process can reduce crystallite grow. The optical quality is improved and SHG efficiency in some cases is up to 1.6 (depending on molecule structure) times larger after our suggested orientation sequence compared to classic corona poling procedures.

Fluorinated Epoxy waveguides doped with Nd complexes have been studied for optical amplification applications. The fluorescent complex was Nd(TTA)3phen (TTA = thenoyltrifluoroacetone, phen = 1, 10-phenanthroline), which was mixed with the host material 6-FDA (6-fluorinated-dianhydride ). The solution was spin coated in order to obtain Nd(TTA)3phen-doped 6-FDA/epoxy slab and channel waveguides. The emission spectra of the Nd-complex doped waveguides were measured at different pump powers by pumping at 800nm, and emission was observed at 890nm, 1060nm and 1330nm. The luminescence lifetime of the Nd complex within the waveguides was experimentally determined. The results demonstrate that the neodymium ions within the polymer host have good transition properties. Based on experimentally obtained parameters the optical gain of the Nd-complex doped waveguides was estimated with the aid of rate equations. The results show that Nd complex doped polymer waveguides are promising gain media for optical amplification.

Volume holography is a research topic that was generated considerably a big interest of the storage information during the last decades. The different systems of storage holographic are principally determined by the physic and chemical characteristics of the storage material. We use an organic conductive material based on polyvinyl alcohol (PVA) for the volume holographic storage. We presented some electro-optics results and physic - chemicals properties of the material used for holographic storage. The influences in the diffraction efficiency parameter as change different variables into material and the qualitative results of other electro-optical parameters examined.

Electroabsorption (EA) spectroscopy is a useful tool to investigate electronic properties of solvated dyes. Following Liptay, EA spectra can be fitted from the linear absorption spectrum and its first and second derivatives, to extract the variation of the dipole moment and of the molecular polarizability upon photoexcitation. In the lack of specific models, the Liptay approach, developed for dipolar dyes, is currently applied to multipolar dyes. We discuss EA spectra of quadrupolar and octupolar dyes, based on essential state models. A perturbative treatment of the applied field allows to express the numerically exact EA spectra of multipolar dyes: the second-derivative contribution vanishes for quadrupolar dyes, and for both quadrupolar and octupolar dyes a contribution appears due to the electric field-induced absorption to a dark (two-photon allowed) state. A new fitting procedure is then proposed for EA spectra of quadrupolar and octupolar chromophores, leading to reliable estimates of the molecular parameters. The method is applied to a few dyes of interest for two-photon absorption applications.

Artificial opals are a simple and cheap playground to manipulate the propagation of light. The interest in these kind of photonic crystals is further increased by the possibility to be infiltrated with highly polarisable media like organic semiconductors, i.e. conjugated polymers, push-pull molecules and multipolar chromophores. In this work, we report on the optical properties of polystyrene opals infiltrated with a heteroaromatic quadrupolar derivative endowed with strong nonlinear optical properties (two-photon absorption) in solution. The insertion of tris(ethylene glycol)monomethyl ether chains on the conjugated skeleton allows the molecule to be soluble in water, a non-solvent for polystyrene. This condition is fundamental in order to attempt opal infiltration. Variable angle transmittance and photoluminescence spectroscopy are used to characterize the system. The bathochromic shift of the opal stop band upon immersion in the chromophore solution confirms that the infiltration process easily takes place preserving a dielectric contrast suitable for further investigations. Photoluminescence spectra recorded at different emission angle with respect to the normal of the sample for both the chromophore solution and opals infiltrated with such solution show interesting characteristics. The presence of opal modifies the chromophore emission spectrum by filtering the light for wavelengths corresponding to those of the stop band and according to its dispersion.

The integration of top-emitting OLEDs on CMOS substrates is of interest for a variety of applications. Whereas OLEDbased microdisplays have already been commercialized, OLEDs could also be used to realize sensor applications, optocouplers, etc. Red top-emitting OLED structures were deposited on CMOS substrates. The OLED technology includes phosphorescent emitters and doped transport layers. This approach results in high efficiencies and low operating voltage. The CMOS top metal is crucial for this type of devices since this layer is the interface between CMOS and OLED technology. In a first step, OLED process development was carried out on passive substrates without transistor circuit but CMOS compatible interface. Luminance values of 100cd/m2 and 1000cd/m2 are reached at 2.45V and 3.1V, respectively. Current efficiency at these luminance values is 14.2 cd/A and 13.4 cd/A, respectively, with a peak wavelength of 627nm. This OLED stack was then successfully prepared on full-CMOS-substrates. luminance values is 14.2 cd/A and 13.4 cd/A, respectively, with a peak wavelength of 627nm. This OLED stack was then successfully prepared on full-CMOS-substrates.

We report a solution processed blue stilbenoid dendrimer based on a 1, 3, 5 - benzene core and endowed with a periphery of electron donating and solubilizing alkoxy chains. Raman analysis it is revealed as a helpful tool to investigate changes from the pristine material to the material in the OLED structure, explaining the differences between the dendrimer single layer thin film photoluminescence (PL) and the electroluminescence (EL) dendrimer active layer emission in the device. We report a blue EL emission (439 nm) and a very promising effective mobility value of 2.55 × 10^-5 cm²/(V•s) suggesting good transport properties for non doped blue OLEDs that use air stable Al as the cathode.

ITO (tin doped indium oxide) coatings with a sheet resistance of 2 to 3 kΩ(square) were produced by gravure printing process on PET and PEN foil. The printing paste consisted of ITO nanoparticles which were dispersed in a solvent by using a surfactant. The dispersion was mixed with a binder and a photo initiator before printing. The printed films were hardened under UV-irradiation at low temperatures (< 130°C). The sheet resistance could be decreased by heat treatment at 120°C under forming gas atmosphere (N₂/H₂) to 1.5 kΩ(square). The transmission of the ITO coated PET and PEN foils is more than 80 % in the visible range. The ITO films were directly used as the bottom electrode in an organic photodiode (OPD). The setup of the OPD originates from the well known Tang photodiode, consisting of a stacked layer of copper phthalocyanine (p-type material) and perylene tetracarboxylic bisbenzimidazole (n-type material). The photodiodes are characterised via current-voltage (I-V) characteristics. The performance of the photodiodes with printed ITO on plastic substrates could be improved by the deposition of a PEDOT/PSS layer (Baytron^(R) P) on the ITO coated foils and was then comparable to the performance of photodiodes with semi-transparent gold as anode on PET substrates. These results demonstrate the suitability of the printed ITO layers as anode for organic photodiodes.

A new, but archetype compound [Ir(ppy-F₂)₂Me₄phen]PF₆, where ppy-F₂ is 2-(2',4'- fluorophenyl)pyridine and Me₄phen is 3,4,7,8-tetramethyl-1,10-phenanthroline, was synthesized and used to prepare a solid-state light-emitting electrochemical cell (LEC). This complex emits blue light with a maximum at 476 nm when photoexcited in a thin film, with a photoluminescence quantum yield of 52 %. It yields an efficient single-component solid-state electroluminescence device with a current efficiency reaching 5.5 cd/A and a maximum power efficiency of 5.8 Lm/Watt. However, the electroluminiscence spectrum is shifted with respect to the photoluminiscence spectrum by 80 nm resulting in the emission of green light. We demonstrate that this unexpected shift in emission spectrum is not originating from the way of excitation, nor from the presence of large concentrations of ions, but is related to the concentration of the ionic transition metal complex in the thin film. The origin of the concentration dependent emission is extensively commented and argued to be related to the population of either ³LC π-π* or ³MLCT triplet states, in diluted and concentrated films, respectively. Using quantum chemical calculations we demonstrate that three low-energy triplet states are present with only 0.1 eV difference in energy and that their associated emission wavelengths differ by as much as 60 nm from each other.

Organic light-emitting diodes (OLEDs) based on small-molecule materials are currently developed for applications in flat panel displays and general lighting sources. However, a large number of efficient deep blue emitters still suffer from rather fast degradation and thus, requires further improvement. The aim of the present work is to gain a fundamental understanding of the intrinsic degradation processes causing the low stability of blue OLED emitters. For this purpose we study the photoluminescence (PL) degradation instead of the most often investigated electroluminescence (EL) degradation to separate electrically and optically generated effects. We show a newly developed PL lifetime measurement system which allows the study of degradation processes under the influence of either electron or hole currents. Using this set-up we demonstrate the very high PL stability of the highly efficient blue singlet emitter 2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene (Spiro- DPVBi) under electron and hole currents and compare this to the lifetime of OLEDs using the same emitter material.

Bright and efficient stacked color-tunable organic light emitting devices (OLEDs) using intermediate Al/Au electrode have been reported. The effects of the thickness of Al and Au layer on the luminance characteristics have been comprehensively studied. After the optimization, After the optimization, the bottom-emission single-unit OLED of 4,4',4"-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine /N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)- 4,4'-diamine /tris(8-hydroxyquinoline) aluminum has a maximum luminance efficiency (ηL) of 3.37 by using Al/Au as the cathode and 2.92 cd/A by using Al/Au as the anode. By introducing the optimized intermediate Al/Au electrode into the stacked color-tunable OLEDs, red unit with maximum ηL of 4.73 cd/A and blue unit with maximum ηL of 3.96 cd/A have been obtained. The color can be tuned efficiently along a linear route from pure red with the Commission Internationale de l'Eclairage (CIE) coordinates of (0.662, 0.330) to sky blue with the CIE coordinates of (0.155, 0.340). This scheme can be a potential candidate for achieving high brightness and efficient stacked color-tunable OLEDs.

High-efficiency fluorescent white organic light-emitting diodes (OLED) were fabricated by using double holetransporting- layers (HTLs), poly(3,4-ethylene- dioxythiophene)-poly-(styrenesulfonate) (PEDOT) and N,N'-bis-(1- naphthyl)-N,N'-diphenyl-1,10-biphenyl-4-4'-diamine (NPB). The diodes were composed of a single emissive-layer (EML), with 0.5 wt% red 4-(dicyanomethylene)-2-tbutyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran doped in a mixed-host of 25% trans-1,2-bis(6-(N,N-di-p-tolylamino)-Naphthalene-2-yl)ethene and 75% 1-butyl-9,10-naphthaleneanthracene. The device structure comprised a 125 nm anode layer of indium tin oxide, a 25 nm first HTL of PEDOT, a 0 to 10 nm second HTL of NPB, a 30 nm EML, a 40 nm electron-transporting-layer of 2,2',2"-(1,3,5-benzenetriyl)-tris(1- phenyl-1-H-benzimidazole), a 1 nm electron-injection-layer of lithium fluoride and a 150 nm cathode layer of aluminum. With the addition of a 7.5 nm second HTL (NPB), the resultant power-efficiency at 100 cd/m², for example, was increased from 11.9 to 18.9 lm/W, an improvement of 59%. The improvement was even more marked at 1,000 cd/m², i.e. that the power-efficiency was increased from 9.1 to 16.5 lm/W, an improvement of 81%. The marked efficiency improvement may be attributed to a better balance of carrier-injection in the desired emissive zone since the addition of the NPB layer in between the first HTL and the EML may have effectively reduced the injection of excessive holes into the EML due to the relatively high energy-barrier to hole, which was 0.5 eV, at the interface of the two HTLs. The resultant hole-blocking function was plausibly more effective at higher voltage so that comparatively much less holes would be injected into the EML, leading to a much better balanced carrier-injection and consequently a higher efficiency-improvement at the higher brightness.

In a conventional bottom emitting organic light emitting diode only about half of the generated photons are emitted into the glass substrate (out of which 25% are extracted into air), the other half being wave-guided and dissipated in the OLED stack. This is due to the refractive index mismatch between the organic layers (n=1.7-1.9) and the glass substrate (n=1.5). By matching the refractive index of the substrate (n=1.8) and organic layers and augmenting the distance of the emission zone to the cathode to suppress plasmonic losses light extraction into the substrate can be increased to 80- 90%. This is shown by simulation and experiment. Furthermore the effect of pyramidal structures on the light extraction from the substrate into air is studied by experiment and simulation. Ultimately it is limited by the reflectance of the OLED stack. The experimental results for monochromatic light are well corroborated by simulations. The main conclusion is that most photons can be out-coupled from the organic stack into an index matched substrate. The OLED light extraction problem is thus reduced to an effective extraction from the substrate into air.

Current-voltage characteristics of polyspiroblue SB -based light-emitting diodes with the structure: ITO/PEDOT:PSS/SB/cathode have been analyzed. Several cathodes were used (Al, LiF with different thicknesses, and Ba) in order to change the barrier for electron injection. As expected, the inclusion of a thin (0.5-1 nm) LiF layer between SB and Al, or the use of Ba, modifies the electron barrier as derived from the increment in the turn-on voltage (related to the built-in potential) with respect to that observed for Al cathode. For hole-only devices (Au cathode) J-V characteristics are interpreted in terms of bulk-limited SCLC transport with hole mobility of the order of 10^-6 cm²/V s. When J-V characteristics obtained using different cathodes are compared the current level observed are consistent with the mobility observed for the hole only device. This implies that the device operation is mainly determined by the hole conduction. However, the electroluminescence observed for these devices employing different cathodes differs over four orders of magnitude. Our results suggest that the electron mobility is much smaller than the hole mobility and that the recombination process is confined to a thin layer near the cathode. Additionally, the results obtained from simple device modeling are also presented.

The internal quantum efficiency of organic light-emitting diodes (OLEDs) can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used, however the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. In this work we use numerical simulations to identify and quantify different loss mechanisms. Changing various simulation parameters, for example layer thicknesses, enables us to study their influence on the fraction of light leaving the OLED. With these simulations we therefore can enhance the light output of the OLED stack. We present simulations of bottom-emitting OLEDs based on the green emitter tris-(8-hydroxyquinoline) aluminum (Alq₃) with transparent indium tin oxide anode and a metallic cathode, as well as microcavity devices with two metallic electrodes. The results of the simulations are compared with experimental data on the angular dependent emission spectra and published effi;ciency data.

A novel class of bottom emission electroluminescent device is described in which a metal oxide is used as the electron injecting contact. The preparation of such a device is simple, and consists of the deposition of a thin layer of a metal oxide on top of an indium tin oxide covered glass substrate, followed by the solution processing of the light emitting layer and subsequently the deposition of a high workfunction (air stable) metal anode. This architecture allows for a low cost electroluminescent device as no rigorous encapsulation is required. Electroluminescence with a high brightness level reaching 6500 cd/m² is observed at voltages as low as 8 V, demonstrating the potential of this new approach to OLED devices. Unfortunately the device efficiency is rather low caused by the high current density flowing through the device. We show that the device only operates after the insertion of an additional hole injection layer in between the light emitting polymer and the metal anode. A simple model that explains the observed experimental results and provides avenues for further optimization of these devices is described. It is based on the idea that the barrier for electron injection is lowered by the formation of the space charge field over the metal oxide-light emitting polymer interface due to the build up of holes in the light emitting polymer layer.

Low out-coupling efficiency is one of the most critical problems in organic light-emitting device (OLED) application. Only 20~30% of the emitting light from OLED can propagate into air [1]. Therefore, several methods have been utilized to extract more light from device. Here, we use the microlens array attached on device to couple out wave-guiding mode in the glass substrate. We found that, the luminous enhancement behavior has great dependence on OLED structure. When light emitted in the layered structure of OLED, the wide angle interference and multi-beam interference occurred, and far-field emission profile change simultaneously. For different emission profile, microlens array film shows a different enhancement behavior. For a conventional OLED device, the most critical interference will occur at the electron transport layer (ETL). We fabricated a series of OLEDs with different ETL thicknesses to investigate the influence to the optical properties, such as spectrum, CIE coordinate change, and emission profile at different view angles. By controlling the emission dipole position, we investigate the relation between the emission profile and the efficiency enhancement by microlens array attachment. When increasing the ETL thicknesses from 30nm to 150nm, the weaker micro cavity effect results in broader spectrum and more light extracted. In these devices, the luminous enhancement varies from 25.1% to 51.3%.

The application of organic materials as solid state lasers critically relies on a low lasing threshold. We investigate the characteristics of emission from an organic vertical cavity surface emitting laser. The microcavity studied here consists of two highly reflective distributed Bragg reflectors enclosing a wedge-shaped active layer of Alq₃:DCM. Lasing of the DCM molecules is induced via two different pump regimes, either exciting Alq3 at a wavelength of 400 nm or pumping directly into the absorption band of DCM at 532 nm. By a variation of the pump beam position with respect to the microcavity surface, we demonstrate a continuous wavelength tuning in the organic microcavities in a range of 55 nm. The continuously variable cavity thickness allows us to study the thickness dependence of the input-output characteristics in a single sample. These data are obtained at a certain emission wavelength, λ, close to the maximum of the gain spectrum, for a number of cavity thicknesses, which correspond to different multiples of λ/2. For a decreasing thickness of the active layer, one-dimensional optical confinement is expected to result in an increased spontaneous emission factor. On the other hand, the loss rate through the mirrors increases with decreasing thickness resulting in a minimum threshold value for an active layer thickness of approximately 3/2 λ. This lower threshold limit is set by nonradiative losses as well as residual absorption.

A study of a group of compounds based on the 1,4-bis(phenylethynyl)benzene (1) architecture was undertaken to improve our understanding of their photophysics and the factors which control their geometry and hence the π- conjugation pathway in the ground and excited state of these compounds. 1 exists as a range of molecular rotamers in the ground state, resulting from the low barrier to rotation around their C(sp)-C(sp2) bonds. These compounds are highly conjugated systems with good electron conducting properties, due to delocalisation of the HOMO and LUMO over the molecule. In the electronic excited state they are capable changing their molecular conformation and will adopt a planar, or near planar, low energy conformation prior to fluorescence emission in solution. In a glassy matrix at 77 K with sterically hindering substituents on the benzene rings of 1, emission form high and low energy conformations are observed. 1 is highly emissive owing to the high oscillator strength of the S₁→S₀ transition. All the compounds studied maintained their C≡C character in the excited singlet and triplet states. The substitution of the central benzene ring in 1 with a thiophene moiety increases the singlet oxygen generation quantum yield, which is consistent with greater intersystem crossing to the triplet excited state.

Using femtosecond pump probe spectroscopy with sub-20 fs resolution, we probe fundamental properties of the E11 exciton in (6,5) single walled carbon nanotubes, prepared by density gradient ultracentrifugation. From the initial photobleaching signal, measured faster than any relaxation process, we obtain the one-dimensional size of the excitonic wavefunction along the nanotube. Exciton decay is found pump-intensity dependent only at elevated pump intensities. Numerical modelling of decay kinetics yields an exciton diffusion coefficient of about 0.1 cm^2/s. Anisotropy measurements in highly purified samples show that there is virtually no depolarisation of the E11 bleach over 40 ps. A photoinduced absorption (PA) band, blueshifted against the E11 bleach, shows only weak anisotropy.