Special lectures

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SL01

CHARGE-TRANSFER EXCITON TRAPPING AT VACANCIES IN MOLECULAR CRYSTALS: PHOTOCONDUCTIVITY QUENCHING, OPTICAL DAMAGE, AND DETONATION

R. W. MUNN and D. TSIAOUSIS

Department of Chemistry, UMIST, Manchester M60 1QD, United Kingdom

Charge-transfer (CT) excitons constructed from neighbouring molecular anion–cation or electron–hole pairs are obvious precursors to photoconduction. They also appear to be implicated in optical damage, where a mechanism involving multi-photon ionization has been invoked, and in detonation, where a recent mechanism proposes CT exciton formation as a more plausible alternative to the on-site ”local metallization” suggested previously. The Coulomb attraction between the electron and hole in a CT exciton is screened by polarization of the crystal, which therefore helps them to stay apart. However, vacancies remove polarizable material, so reducing the screening and increasing the attraction. Hence vacancies constitute traps for CT excitons. This trapping facilitates geminate recombination, which quenches photoconductivity. It also releases energy that is available to distort the lattice further, leading to more trapping, and eventually to optical damage. Alternatively, the energy released at such ”hot spots” may initiate chemical reaction leading to explosive decomposition, i.e. detonation.

Calculations are reported that confirm the existence of CT exciton trapping near vacancies and quantify the trap depths in three systems:


SL02

Materials for Polymer Electronics Applications – Semiconducting Polymer thin Films and Nanoparticles

Ullrich Scherf , Polymerchemie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, Haus 25, D-14476 Golm, Germany, e.mail: scherf@rz.uni-potsdam.de

The development of organic, semi-conducting polymer-based light emitting diodes (PLEDs) has demonstrated the enormous potential of this class of macromolecular materials. Besides PLEDs also organic materials-based field effect transistors (OFETs), solid state lasers, photodiodes, and solar cells came into the focus of scientific and industrial interest, especially for a future ”all-polymer electronics”.

The lecture reports novel promising results from our polymer chemistry group in Potsdam related to the synthesis, characterization and application of para-phenylene- and polythiophene-type polymers, as well as novel conjugated/conjugated block copolymers. Some of the materials are characterized by very high structural regularity, low defect concentrations, high charge carrier mobility, and exceptionally good photo- and electroluminescent properties.

Solid layers of semiconducting polymers have been successfully included as active layers into various electrical and electro-optical devices. While in most of these cases film preparation has been performed from solutions of the active components in organic solvents, the deposition from aqueous systems would be also desirable. We could now demonstrate film formation from semiconducting polymer nanoparticle (SPN) dispersions, which have been prepared in a miniemulsion process. The particle size of these nanospheres could be controlled in the range of 50-200 nm. Homogeneous layers could be prepared by spin coating the dispersions onto suitable substrates. Absorption and photoluminescence spectra of these novel layers showed no difference in relation to solution processed films indicating that the electronic structure of the polymer remains unchanged upon particle formation.

We propose that the concept of semiconducting polymer nanoparticles will allow to form multilayer structures by e.g. depositing a first layer from a solution of a polymer and overcoat it by semiconducting polymer nanospheres of a second polymer, followed by e.g. annealing. Most important, this will allow the formation of a multilayer structure from polymers, which are highly soluble in organic solvents, without introducing any additional chemical conversion steps.


SL03

OPTICAL GRATING RECORDING IN HIGHLY ORGANIZED THIN FILMS OF DISPERSE RED 1

A. MINIEWICZ a, M. SOLYGA a, H. TAUNAUMANG b, M.O. TIJA b

a Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, 50-370 Wroclaw, Poland, e-mail: miniewicz@kchf.ch.pwr.wroc.pl

b Department of Physics, Institut Teknologi Bandung, Jalan Ganesa no.10 Bandung 40132, Indonesia

Optical grating recording of micrometric dimensions which can handle gray-levels patterns is investigated in photochromic material made of Disperse Red 1 (DR1, 4-[N-ethyl-N-(2-hydroxyethyl)]amino-4’-nitro-azobenzene) molecules vacuum deposited on glass substrate. The properties of polymers containing DR1 have received enormous attention as they are suitable for applications such as holographic optical elements or data storage media. Up to our knowledge thin films of DR1 have never been investigated before in that respect, except our recent work [1]. Thin films of DR1 were prepared by vacuum deposition [2] and have thickness of 0.1 m. Holographic gratings of periods within the range of 0.6 m - 12 m were recorded by 514.5 nm light from cw Ar+ laser using degenerate two-wave mixing technique. Despite the very small recording layer thickness the diffraction efficiency measured within a Raman-Nath scattering regime reached 3.5 %. We established, using a grating translation technique [3], that the light scattering was mostly due to presence of periodic absorption changes (i.e. amplitude grating).

We studied kinetics of grating recording in function of incident light intensity and analyzed it using Fourier transforms. Grating profiles were analyzed in relation to exposure conditions and in correlation with molecular organization. Polarizing microscopic studies revealed the presence of light induced optical anisotropy. Following that we have checked that polarization sensitive recording is also possible in this medium. This discovery is a challenging one as it requires development of a model for grating build-up process in the DR1 films. We believe that this process is connected with local heat induced pre-melting then molecular rearrangement (reorientation in optical field) followed by layer re-crystallization.

[1] H. Taunaumang, M. Solyga, M.O. Tija, A. Miniewicz, On the efficient mixed amplitude and phase grating recording in vacuum deposited Disperse Red 1, in preparation

[2] H. Taunaumang, Herman, M.O. Tija, Optical Materials, 18 (2001) 343.

[3] K. Sutter, P. Gunter, J. Opt. Soc. Am. B. 7 (1990) 2274.


SL04

PHOTONIC ENGINEERING OF MOLECULAR MATERIALS

FRANÇOIS KAJZAR

Commissariat à l’Energie Atomique

DRT – LIST, DECS/SEMM/LCO

CEA/Saclay, 91191 Gif sur Yvette, France

The observed recently fast development of photonics, comprised as science and technology connected with handling of photons, can be compared to what electronics has known in the last centaury. This is not only due to the immense development and large needs for information processing, transmission and storage, but also to the increasing use of photons in the everyday life. Indeed, photons are successfully used for atom cooling, material processing, in medicine, dentistry, biology, industry, on battlefield, etc. They become also to be largely used in the material engineering. In this talk we will describe and discuss some aspects and very recent developments in photonic engineering such as control of molecular order in polymeric media, important for practical applications of these materials in e.g. optical signal processing as well as the way how photons can be used to move molecules, or their parts, thus to create well defined structures or mechanical functions.


SL05

Nano Photo Science: Thin Organic Films for Data Storage

L. Brehmer, P. Karageorgiev, B. Stiller

Physics of Condensed Matter, Institute of Physics, University of Potsdam, Am Neuen Palais 10, Potsdam 14469, Germany, e-mail: brehmer@rz.uni-potsdam.de

Local induced photo reactions in thin organic films are studied and the efficiency for data storage arrangements is discussed.

The modification of surfaces in nanometer scale using Scanning Probe Microscopes (SPM) is a new powerful way for developing storing information devices. Special tip-sample effects can be used. The effect of local intensity enhancement of electromagnetic field near a conductive object much smaller than wavelength of the field allowed a concentration of optical energy into the tip-substrate gap within an area not more than several tenth of a nanometer in diameter. For the first time a light-induced local modification of the surface potential of an azobenzene-containing film in the near field of a scanning force microscope tip has been observed [1].

The E/Z isomerisation of azobenzenes is one of the best investigated molecular switching process.

Typical organic materials displaying reversible photoisomerisation are azobenzene derivatives. We used such materials to prepare alternating LB films. By illuminating with light of specific wavelength and power an optical induced isomerisation of azo groups will take place, witch is correlated with a change of molecular dipole moment. In our case (in non centrosymmetrical structure) this will generate a change of surface potential, which can be measured by Kelvin probe microscope.

The local isomerisation (writing) was done by using the local intensity enhancement effect [2, 3]. Therefore the whole surface will be illuminated with low power light (1...10 mW/cm2 ) while the tip is close but not in contact with the surface and is scanning a small area. To read out the written information, the light has to be switched off and the microscope has to scan a larger area. The written information is ”visible” as an area with lower surface potential. The information can be erased by illuminating the surface with light of higher power and the film is ready for a second writing process.

Thus, it is shown the possibility of optical writing and erasing of nanostructures in thin organic films by using a scanning Kelvin microscope with nm-resolution.

[1] Karageorgiev, P.; Stiller, B.; Prescher, D.; Dietzel, B.; Schulz, B.; Brehmer, L.; Langmuir 16, Nr. 13, p 5515-5518, (2000)

[2] Stiller, B.; Karageorgiev, P.; Perez, E.; Valez, M.; Reiche, J.; Prescher, D.; Dietzel, B.; Brehmer, L.; Surface and Interface Analysis 30, Nr. 1, p 549-551, (2000)

[3] Stiller, B.; Knochenhauer, G.; Markava, E.; Gustina, D.; Muzikante, I.; Karageorgiev, P.; Brehmer, L Materials Science and Engineering C 8-9, p 385-389, (1999)


SL06

MOLECULAR NANOWIRES OF CHARGE TRANSFER COMPLEX

T. AKUTAGAWA,a T. HASEGAWA,a T. NAKAMURA,a J. BECHERb

aMolecular Electronics Laboratory, Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan

bDepartment of Chemistry, University of Southern Denmark, Odense, Denmark

In the recent years, molecular based nano-electronics devices have been attracting mach attention as a possible candidate of primary parts for the next-generation computing systems. We have been interested in the application of molecular conductors and magnets to the namo-electronic devices, and have already reported highly conducting Langmuir-Blodgett (LB) films of metallic transport properties.1 Since the overlap of the frontier orbitals between the molecules is essential for exhibiting the functions of molecular conductors and magnets, a novel approach different from those for single-molecular electronics is necessary to apply the molecular assemblies in the practical devices.

We have been proposing "Integrated Molecular System (IMS)" approach, in which molecular systems are constructed through the self-assembly process of programmed (designed) molecules. In this paper, we present "nanowires" composed of molecular charge transfer complex as an example of IMS. The nanowires are constructed through self-assembly process, which orients specific direction on single crystalline surface by recognizing periodic ion array.

We designed the TTF-bisannulated molecule 1, which has both the electrically active TTF units and macrocyclic moiety of ion-recognition ability. We have found that the F4TCNQ complex of molecule 1 forms nanowire structures, which grows on mica recognizing K+ array of six-fold symmetry.

  1. T. Nakamura, Handbook of Organic Conductive Molecules and Polymers pp.727-780, H. S. Nalwa (ed.), vol. 1, John Wiley & Sons, Chichester (1997).
  2. T. Akutagawa, T. Ohta, T. Hasegawa, T. Nakamura, C. A. Christensen and J. Becher, Proc. Natl. Acad. Sci. USA, 99, 5028-5033 (2002).

SL07

THEORY OF AN ALL-CARBON MOLECULAR SWITCH: FULLERENES BETWEEN NANOTUBE ELECTRODES

R. GUTIERREZ, 1 G. FAGAS, 2 R. SCHMIDT, 1 and K. RICHTER 2

1 Institute for Theoretical Physics, Technical University of Dresden, D-01062 Dresden, Germany

2 Institute for Theoretical Physics, University of Regensburg, D-93040 Regensburg,Germany

We have recently shown in model calculations that geometry-induced interfacial effects on electron transport across molecular junctions are greatly enhanced in the presence of mesoscopic electrodes [1]. In particular, for molecules between carbon nanotube electrodes, which support low-dimensional transport and share a rich topology, the conductance depends crucially on the exact atomic configuration at the interface. This owes to the importance of quantum intereference at the molecular scale [2].

Here, we first give an overview of our studies on electron transport across a carbon molecular junction consisting of a C60 molecule sandwiched between two semi-in nite metallic carbon nanotubes [3]. Within a tight-binding approach parametrised by density functional theory, it is shown that the Landauer conductance of this carbon hybrid system can be tuned within orders of magnitude not only by varying the tube-C60 distance, but more importantly at fixed distances by either i) changing the orientation of the Buckminsterfullerene or ii) rotating one of the tubes around its cylinder axis. Furthermore, it is explicitely shown that structural relaxation determines qualitatively the transmission spectrum of such devices and a detailed account of the parameters which control the conductance is given. Finally, we focus on the contribution of additional states appearing within the HOMO-LUMO gap of an isolated C60 molecule, which are attributed to the specifics of the molecular junction and lie in the vicinity of the equilibrium Fermi energy [4]. This contrasts the general view that electron transport takes place in a resonant tunneling fashion via the molecular orbitals, and reveals an analogy to the metal-induced gap states (MIGS) at a metal-semiconductor-metal junction. We also show that for carbon nanotube electrodes terminated by polyhedral caps the conductance is dominated by the presence of such states and may be efficiently controlled by applying a gate potential to the fullerene.

[1] G. Fagas, G. Cuniberti, and K. Richter, Phys. Rev. B 63, 045216 (2001).
[2] G. Cuniberti, G. Fagas, and K. Richter, to be published in Chem. Phys. (2002).
[3] R. Gutierrez, G. Fagas, G. Cuniberti, F. Grossmann, R. Schmidt, and K. Richter, Phys. Rev. B 65, 113410 (2002)
[4] R. Gutierrez, G. Fagas, R. Schmidt, and K. Richter, in preparation


SL08

TRAPS FOR CHARGE CARRIERS IN MOLECULAR MATERIALS FORMED BY DIPOLAR SPECIES: TOWARDS LIGHT-DRIVEN MOLECULAR SWITCH

J. SWORAKOWSKIa, S. NEŠPŮrEKb

aInstitute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, PL-50-370 Wroclaw 51, Poland. E-mail: sworakowski@pwr.wroc.pl

bInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského nám. 2, CZ-162 06 Praha 6, Czech Republic. E-mail: nespurek@imc.cas.cz

Earlier work of the present authors [1-5] demonstrated that the presence of polar species in molecular materials should result in creation of traps for charge carriers localised on neighbouring molecules, their depths and cross sections depending on the dipole moment of the polar guest molecule. Based on the concept of ‘dipolar traps’, a scheme of a light-driven current switch is put forward. The switch would consist of a molecular wire (e.g., a conjugated polymer chain) with suitably chosen photochromic moieties placed in its vicinity or chemically attached to the main chain. A reversible photochemical reaction occurring in the photochromic species should reversibly change their dipole moment resulting in creation of trapping sites on the chain.

The architecture of the light-driven molecular switch and basic requirements for its segments will be discussed. In particular, the competition between dipolar and chemical trapping will be considered.

[1] J. Sworakowski and S. Nešpůrek, Polish J. Chem. 72 (1998) 163.

[2] J. Sworakowski, IEEE Trans. Diel. Electr. Insul. 7 (2000) 531.

[3] S. Nešpůrek, J. Sworakowski, A. Kadashchuk, IEEE Trans. Diel. Electr. Insul. 8 (2001) 432.

[4] S. Nešpůrek, J. Sworakowski, Thin Solid Films 393 (2001) 168.

[5] J. Sworakowski, S. Nešpůrek, in: Molecular Low Dimensional and Nanostructured Materials for Advanced Applications. (Eds. A. Graja, V. M. Agranovich, F. Kajzar), NATO ASI Series, Kluwer Acad. Publ., Dordrecht 2002, in press.


SL09

Charge transfer processes and environmental degreEs of freedom: cooperativity and non-linearity

ANNA PAINELLI, FRANCESCA TERENZIANI

Dipartimento di Chimica GIAF, Università di Parma,

viale delle Scienze 17/A, 43100 Parma, Italy; INSTM UdR Parma

The charge- (or electron-) transfer is at the heart of the behavior of molecular materials for advanced applications. Understanding the subtle interplay between electrons and environmental degrees of freedom (including vibrations) is fundamental to understand and fully exploit the interesting properties of these materials. Here we concentrate on a paradigmatic class of materials: donor-acceptor (DA) conjugated chromophores.

In DA chromophores the intermolecular charge-transfer state is responsible for strong absorption bands in the visible region, whose large dependence on the polarity of the environment is known since date. This same state is also responsible for the large NLO responses of these molecules, and dominates their linear and non-linear spectral properties. Within a two-state picture for the electronic problem, we demonstrate a large amplification of the non-linearity of the electronic system as due to the interaction with slow degrees of freedom. The spectroscopic signatures of this amplified non-linearity are shortly discussed with reference to experimental data on DA chromophores in solution.

In dense samples, including e.g. crystals, (poled) films, functionalized polymers, or Langmuir Blodgett films, inter-chromophore interactions are an additional source of non-linearity. Each chromophore in fact feels the electric field generated by the surrounding chromophores and then readjusts its polarity, leading to a variation of the environmental field. Adding intermolecular electrostatic interactions to the two-state model for the isolated chromophore, leads to an interesting model for interacting polar and polarizable chromophores. By varying the relative chromophore orientation and/or the strength of electrostatic interactions, the system is easily driven from a neutral (N) ground state towards an ionic (I) ground state, dominated by the zwitterionic form. The charge-transfer process is localized within each chromophore, but the N-I transition we observe in our model shares the same physics as the N-I transition observed in mixed-stack charge-transfer salts, where the charge-transfer interaction is delocalized along the chain. Inter-chromophore interactions not only affect ground state properties, but also show up in spectral properties with exotic excitonic effects.


SL10

CORRELATIONS BETWEEN LARGE POLARONS IN MOLECULAR CHAINS

L. BRIZHIK, A. EREMKO

Bogolyubov Institute for Theoretical Physics , Ukrainian National Academy of Sciences, Metrologichna Str., 14-b, 03150 Kyiv, Ukraine

The self-trapped (spontaneously localised) electron states in one-dimensional molecular chains are studied with account of electron correlation. It is shown that the autolocalized states of a few extra electrons in a deformable chain are described within the zero adiabatic approximation by the many-component nonlinear Schroedinger equations.

It is proved that in the case of two extra electrons the minimum of total energy of the system corresponds to the singlet bipolaron (bisoliton) state, which describes bound state of electrons localised in a single potential well, created by the self consistent chain deformation. In the same zero adiabatic approximation, two electrons in the triplet state form two almost free solitons (large polarons), localised in distant potential wells. Between such solitons the repulsion exists, that decreases exponentially with distance between the solitons centre of mass coordinates. It is shown that the account of nonadiabatic terms in the Hamiltonian results in the attraction between polarons via virtual phonon exchange. In the result, the two-soliton state is created, which is described by the double-hump localised function. The equilibrium distance between function maxima is determined both by the adiabatic and nonadiabatic terms of the Hamiltonian. This stabilisation occurs due to the competition between Pauli repulsion and the mutual attraction of polarons arising from electron-phonon interaction.

Similarly, four electrons bind into two bound bisolitons with the finite distance between them. As a result of electron correlation, arising from the Pauli principle and electron-phonon interaction, at zero temperature Ne extra electrons in a chain create the periodic lattice of bisolitons in the form of the Froehlich charge density wave.


SL11

THERMOLUMINESCENCE OF ORGANIC MATERIALS FOR OPTO-ELECTRONIC APPLICATIONS

J. ULANSKI, I. GLOWACKI, E. DOBRUCHOWSKA, B. LUSZCZYNSKA

Department of Molecular Physics, Technical University of Lodz,

Zeromskiego Str. 116, 90-924 Lodz, Poland

Thermoluminescence (TL) was developed as a method of investigation of charge carrier trapping, and up to now it is regarded as one of the most direct methods for characterisation of trapping sites. In this work we demonstrate, that this technique yields also an insight into other phenomena, important for performance of opto-electronic devices.

We analyse our TL investigations for a broad range of organic materials, which can be used as components in various opto-electronic applications: photovoltaic cells, LED or FET. Both non-conjugated polymers, like poly(N-vinylcarbazole) and its derivatives, or N-carbazolyl substituted silane homo- and copolymers, and also -conjugated polymers, like various poly(arylenevinylene)s, polyfluorenes and poly(para-phenylene)s, as well as p- and n-type discotic liquid crystals were investigated.

It is shown, that detailed analysis of TL (by means of partial-heating technique and spectral analysis of emitted light), performed in a broad temperature range (from LH2 temperatures range) and combined with other techniques, like thermally stimulated currents, photo-discharge, photoluminescence, electroluminescence and dielectric spectroscopy, may allow to determine the mechanisms of: charge carrier recombination; nature of undesired, long-wavelength emitting sites; relationship between charge carrier trapping and polymer morphology and molecular relaxations (‘wet-dog’ effect); or photo- and thermal degradations.


SL12

COMPARATIVE THERMOLUMINESCENT STUDY OF p - AND s -CONJUGATED POLYMERS

A. KADASHCHUKa, H. BÄSSLERb, S. NEŠPŮREKc

aInstitute of Physics, National Academy of Sciences of Ukraine, Prospect Nauki 46, 03028 Kiev, Ukraine, e-mail: kadash@iop.kiev.ua

bPhilipps-Universität Marburg, Hans-Meerwein-Strasse, D35032 Marburg, Germany

cInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq.2, 16206 Prague 6, Czech Republic

The charge-carrier transport in both p - and s - conjugated polymers has been believed to be controlled by charge hopping through intrinsic states derived from domainlike segments of the main chain. There are certain indications that extrinsic traps could also play an important role in these polymers, however the nature of trapping states is still unclear. It was recently showed that thermally stimulated luminescence (TSL) could be an efficient tool for studying localized states and their energy distribution in disordered semiconducting polymers. In the present work we report on comparative TSL study of selected p -conjugated polymers, as methyl-substituted ladder-type poly(para-phenylene) (MeLPPP) and different PPV derivatives, and some s - conjugated polysilanes.

The obtained TSL data for p -conjugated polymers are interpreted in terms of recently developed model of thermally assisted hopping in disordered material, which allows quantitative evaluation of the distribution of localized states for charge carriers. In particular, we found that the distribution of localized states for charge carriers in a series of substituted PPVs can be described by a double-peak Gaussian DOS function. The shallower peak is due to the inhomogeneously broadened distribution of intrinsic hopping sites while the deeper one can be ascribed to extrinsic traps which are most probably of ”aggregate” origin and inherent for PPV-type polymers implying that charge transport is also affected by such traps.

TSL results for studied s -conjugated polysilanes suggest importance of polaronic effects in these polymers. A model of low-temperature energy relaxation of polarons in a disordered hopping system was formulated. The model proves that the relaxation of polarons is much slower than that of charge carriers in similarly disordered but rigid hopping system and explains observed characteristic features of TSL in polysilanes.


SL13

SPECTROSCOPIC STUDIES OF CHARGE-ORDERING SYSTEM IN ORGANIC CONDUCTORS

K. YAKUSHIa, K. YAMAMOTOa, R. SWIETLIKa, R. WOJCIECHOWSKIa, K. SUZUKIa, T. KAWAMOTOb, and T. MORIb , Y. MISAKIc, K. TANAKAc

aInstitute for Molecular Science, Nishigo-naka, Myodaiji, Okazaki, 444-8585, Japan

b Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552 Japan

cDeparment of Molecular Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501 Japan

In molecular conductors, the molecular orbital barely overlap with those of neighbor molecules. Owing to this very small overlap integral, the molecular conductors are located at the boundary between a metal (delocalized state) and insulator (localized state). When the charge is localized, the charge often induces a charge disproportionation and eventually produces a inhomogeneous charge distribution. This localized state is called as a charge-ordered (CO) state, since the localized charges form a new periodic structure.

Recently, metal-insulator phase transitions accompanied by such CO are found in several organic conductors. We are investigating the CO phase transition by using C=C stretching modes as probes of the charge distribution. The authors have reported the phase transitions in -(BDT-TTP)2Cu(NCS)21) and -(ET)2RbZn(SCN)42) by means of polarized reflection and Raman spectroscopy. Both compounds exhibit drastic changes in the Raman spectra below the CO phase transitions. The single Raman band splits into several bands reflecing the lowering of symmetry accompanying the localization of charge. In this manner, we have demonstrated that the vibrational spectroscopy is a power tool to study the CO phase transition. High pressure is an effective method to change the electronic state of organic conductors, because they are located at the boundary between a metal and insulator. Using a high-pressure Raman spectroscopic method, we are investigating organic CO systems such as -(ET)2TlZn(SCN)4, -(ET)2IBr2, -(ET)2I3, and (TTM-TTP)I3. We will present several recent results.

1) J. Ouyang, K. Yakushi, Y. Misaki, and K. Tanaka, Phys. Rev. B, 63, 054301 (2001).

2) K. Yamamoto, K. Yakushi, K. Miyagawa, K. Kanoda, and A. Kawamoto, Phys. Rev. B, 65, 085110 (2002).


SL14

ELECTRICAL AND SPECTRAL PROPERTIES OF ORGANIC SALTS FORMED FROM BEDT-TTF AND MAGNETIC ANIONS

A. GRAJAa, A. ŁAPIŃSKIa AND V.A. STARODUBb

aInstitute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland, graja@ifmpan.poznan.pl

bDepartment of Chemistry, Kharkov National Karazin University, 61077 Kharkov, Ukraine, vladimir.a.starodub@univer.kharkov.ua

Structural dimensionality, and thus the electronic dimensionality, is of great importance for the physical properties of low-dimensional molecular solids. Recently, much attention being devoted to the exploration of the novel lattice architectures and physical properties resulting from the association of organic cation radicals (e.g. bis(ethylenedithio)tetrathiafulvalene, BEDT-TTF) with bulky anions (e.g. polyoxometalates). One of the reasons for the interest in polyoxometalate-based materials, containing both localized and delocalized electrons is the possibility of creating compounds where the coexistence of co-operative magnetic and electrical properties such as ferromagnetism and superconductivity can be observed.

A structure of these hybrid material is usually built of alternating organic and inorganic layers. The organic layers are created by peculiar arrangement of BEDT-TTF species with short intermolecular contacts. The inorganic ones are formed of polyoxometalate anions and solvent molecules. Both, organic and inorganic building blocks show a structural disorder which is so important for the electron distribution in these blocks.

Structural, electrical and spectral properties of two organic/inorganic hybrid crystals are presented and discussed. The new organic CT salt (BEDT-TTF)6(Mo8O26)(DMF)3, where DMF is a dimethyl formamide, is characterised. It exhibits metallic properties with anomalies at about 180 and 60 K. The anomaly at 180 K, confirmed by IR spectral investigation, is caused by changes in a charge carriers scattering mechanism (domination of electron-phonon scattering for T > 180 K, and electron-electron scattering for T < 180 K). The distinct anomaly at 60 K, not observed by spectral methods, could be explained by spin transition and/or rearrangements of organic and inorganic sublattices. IR and Raman spectra of single crystals are analysed and an assignment of the vibrational features is proposed. The T-dependencies of the transport parameters are obtained using the Drude - Lorenz model. For comparison, the spectral properties of the hybrid semiconducting salt (BEDT-TTF)2(W6O19) are also presented and discussed.


SL15

CALCULATIONS OF NONLINEAR OPTICAL PROPERTIES

OF ORGANIC STRUCTURES

V.M. YARTSEV

Centro de Física, Instituto Venezolano de Investigaciones Científicas (IVIC),

Apartado 21827, Caracas 1020-A, Venezuela; e-mail: syartsev@ivic.ve

Organic structures present fascinating prospects of customer-adapted practical applications due to their ability to be modified on the molecular level in a very precise and subtle way. Our recent work [1-6] on modeling organic structures will be evaluated with respect to understanding relative significance of the following factors: size, chemical composition, configuration, dimensionality, symmetry, electronic correlation, electron coupling to internal vibrations. The molecular dimer is a very convenient system for investigation of the electronic correlation: the model is simple enough to allow (in several important limiting cases) analytical relations for the first and second hyperpolarizabilities as a function of the transfer integral, Hubbard on-site repulsion energy, and small polaron binding energies. However, this model cannot account for changes in the molecular configuration which may be made either permanently by an appropriate chemical synthesis or induced by an external light irradiation. In this case, the model of four sites is working well: two sites describe the central part (bridge) and the other two the attached electroactive groups which may have different angles with respect to the bridge. Other parameters include donor and/or acceptor abilities of the side groups, intersite transfer integrals, bridge length, intrasite flexibility. These parameters have a clear physical meaning, can be estimated independently, and may be modified in a controlled way by an appropriate molecular engineering. Hyperpolarizability tensors are calculated and the results suggest what kind of changes in geometry are needed for optimization of specific components. The role of interaction between organic molecules in adjacent layers in the Langmuir-Blodgett films is also discussed. Taken together, modeling of organic structures by a set of interacting sites offers a relatively simple way to obtain a theoretical design of molecular structures with high nonlinear optical properties.

[1] V.M. Yartsev, Chem. Phys. Lett. 313, 241 (1999).

[2] V.M. Yartsev, A. Marcano, Synth. Metals 115, 197 (2000).

[3] V.M. Yartsev, Nonlin. Optics 26, 107 (2000).

[4] V.M. Yartsev, A. Marcano O., Rev. Mex. Fis. 47 (S.1), 48 (2001).

[5] V.M. Yartsev, M.R. Singh, Synth. Metals 127, 115 (2002).

[6] V.M. Yartsev, M.R. Singh, Chem. Phys. 276, 293 (2002).


SL16

The lipid bilayer principle: From Soap bubbles to black lipid membranes (BLMs) to biosensor Applications

H.Ti. Tien, Angelica Ottova

Department of Physiology, Biomedical and Physical Sciences Building, Michigan State University, East Lansing, MI Michigan 48824

In the past decade there have been a number of reports on self assembled molecules or structures as 'advanced materials' or 'smart sensors' [1]. Without question, the inspirations for these exciting findings come from the biological world, where, the lipid bilayer of cell membranes plays a pivotal role. Self assembled bilayer lipid membranes (BLMs or planar lipid bilayers) on solid or gel supports may be formed by a number of methods [2]. A,s an example, it has been demonstrated that by doping an s-BLM or sb-BLM with fullerenes such as C60, it is possible to construct sensor probes, and to investigate redox reactions and light induced electron transfer across the lipid bilayer. The C60-containing s-BLM or sb-BLM, considered basically as a 'molecular device', is a light sensitive diode that is capable of photoinduced charge separation that undergoes redox reactions across the substrate/hydrophobic lipid bilayer/aqueous solution junctions. Using cyclic voltammetry, our results show that C60 embedded in the BLM can act as an excellent electron carrier or mediator and should be useful for electrochemical biosensor and molecular electronics device development. Supported BLMs, possessing the structural and dynamic properties of conventional planar lipid bilayers, are excellent models for investigating basic membrane-mediated ligand-receptor contact interactions [2]. They are ideal systems for incorporating a host of compounds including fullerenes, semiconducting rianoparticles, receptor proteins, and polymeric materials for practical applications [3].

References

  1. H. T. Tien and A. L. Ottova, Membrane Biophysics:as viewed from experimental bilayer lipid membranes (planar lipid bilayers and spherical liposomes), Elsevier Science, Amsterdam, the Netherlands, 2000 .
  2. H. T. Tien and A. Ottova "The Lipid Bilayer Concept and Its Experimenta.l Realization: From Soap Bubbles, the Kitchen Sink, to Bilayer Lipid Membranes", J. Membrane Science, 189 (2001) 83
  3. Ottova and H. T. Tien, The 40th anniversary of bilayer lipid membrane research, Bioelectrochemistry. 56, (2002)

SL17

NOVEL ELECTRICAL AND OPTICAL PROPERTIES OF CONJUGATED MOLECULES AND POLYMERS DEPENDENT ON MOLECULAR STRUCTURES AND NANOSCALE STRUCTURES.

EFFECTS OF NANOSCALE PERIODIC STRUCTURE-

K. YOSHINO, H. TAKEDA, M. KASANO, N. TAKAMOTO, T. UMEDA, Y. YOSHIDA, T. MATSUI, Y. NISHIHARA, A. FUJII, M. OZAKI

Department of Electronic Engineering, Graduate School of Engineering, Osaka University

2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

Novel electrical and optical properties of conjugated molecules and polymers with various molecular structures in nano-scale periodic structures are studied in details.

Nano-scale periodic structures of the conjugated polymers and carbons as 3-dimensional photonic crystals were prepared by infiltrating the conjugated polymers and carbons in the interconnected nano-scale voids in synthetic opals and inverse opals of the conjugated polymers and carbons made by removing silica from infiltrated opals by HF.

These infiltrated opals and inverse opals made of the conjugated polymers and molecules exhibited the novel electrical and optical properties such as unique tunable optical reflection and transmission spectra, unique gel like behavior and lasing upon optical excitation.

Nano-scale periodic structures of conjugated polymers and molecules were also prepared by irradiation interfering optical beam on the surface of the polymer film having azo-chromophore in the structures and also doped with the conjugated polymers or molecules. Their surface relief structures can be interpreted to be a two-dimensional photonic crystals and various novel optical properties have been observed.

Cholesteric liquid crystals and ferroelectric smectic liquid crystals doped with conjugated molecules exhibits the characteristics as one-dimensional photonic crystals and even tunable lasing was observed in these systems.

To understand their characteristics photonic band schemes have been theoretically calculated and various new effects were demonstrated.


SL18

ORGANIC MEDIA FOR 3-D OPTICAL MEMORY: MODERN TRENDS

V.BARACHEVSKY

Photochemistry Center, Russian Academy of Sciences, 7a, Novatorov Street, Moscow, 117421, Russia

barva@photonics.ru

Using literature and own data the state of the art and future for making light - sensitive recording organic media which are suitable for 3-dimensional archive and working optical memory are analyzed. At present these media have aroused considerable interest in connection with the intensive development of telecommunication networks and Data Bases with super high information capacity.

The analysis shows that dry photopolymerizable materials containing photobleaching dyes as photoinitiators for polymerization provide registration of deep hologram with a high diffraction efficiency and thus a high angle selectivity during registration and read-out holograms. Application of two-photon polymerization and irreversible photochemical transformations of organic compounds is acceptable for bitwise 3-D optical memory too. In this case read-out of optical information is achieved by use of photoinduced refraction or luminescence.

On the development of recording media for working 3-D optical memory photochromic materials are best suited to application. These media provide bitwise registration of optical information as a result of reversible transformation of organic compounds exhibiting photoinduced refraction or luminescence too.

At last, the possibility for realization bitwise frequency - selective 3-D optical memory based on processes of photoinduced aggregation - deaggregation as well photochemical burning organic compounds is discussed

Achieved results in the above mentioned science lines are illustrated by certain examples.


SL19

ROUGH ELECTRODE SURFACE: EFFECT ON CHARGE CARRIER INJECTION AND TRANSPORT IN ORGANIC DEVICES

S.V. NOVIKOV

A.N. Frumkin Institute of Electrochemistry, Leninsky prosp. 31, 119071 Moscow,

Russia, email: vanlab@online.ru

Effect of electrode roughness on charge carrier injection and transport is considered. Explicit formula connecting roughness profile of an electrode with the distribution of the electric field at its surface (and electrostatic potential in the bulk of transport layer) is derived for the case of smooth roughness, when typical height of roughness element is small in comparison to its size across a surface (this is a very typical situation). This formula gives us a possibility to measure the electrode surface profile (e.g., by AFM) and then calculate various injection properties of this particular electrode for any kind of injection rate (e.g., Fowler-Nordheim or Schottky-Richardson): total injection current, surface distribution of injection peaks etc. Smooth roughness leads to relatively small spatial fluctuation of electric field at the surface, yet because of highly nonlinear relation between electric field and injection current even these small fluctuations may result in giant fluctuation of the current. In some cases effect of smooth roughness is so strong that the functional dependence of the total injection current on electric field is completely changed. For this reason, calculation of injection parameters from the injection current data, carried out without respect of the electrode roughness, may produce wrong numbers. General properties of electrode - organic layer interface in the case of significant (not smooth) roughness are considered and a suitable numeric procedure for the calculation of the surface electric field distribution is suggested. It is shown also that rough surface of electrodes generates an additional energy disorder in the bulk of transport layer. This principal result indicates that electrode roughness affects not only carrier injection but carrier transport as well. Magnitude of the disorder is proportional to voltage applied to the device and decays as inverse distance to the electrode for distances much greater than surface correlation length. Roughness-induced energetic disorder produces a channel-like structure in the vicinity of the electrode (typical scale being the surface correlation length, 50-100 nm in many cases), thus providing the separation of electrons and holes. Such separation should decrease charge recombination rate and, hence, emitting efficiency of light emitting devices. At the same time, the separation is favorable for solar cells. Implications of electrode roughness on optimal design of devices are considered.


SL20

ELECTRICAL VS STRUCTURAL AND MECHANICAL PROPERTIES OF POLYANILINE-BASED CONDUCTING SYSTEMS

P. Fedorko, J. Fraysse, J. Planes, D. Djurado, B. Dufour, P. Rannou, A. Pron, J.P. Travers

Laboratoire de Physique des Métaux Synthétiques, UMR 5819 (CEA-CNRS-UJF), DRFMC, CEA-Grenoble, France

In recent years, several routes have been investigated with the goal of obtaining processible conducting polymers showing both high electrical (quasi-metallic) conductivity and good mechanical properties. In the case of polyaniline, one can cite: i) the use of tailor-made protonating agents which combine doping and plasticizing properties (internal plasticizers), ii) the addition of an external plasticizer during the processing of doped polyaniline, i.e. molecules which do not dope polyaniline but only plasticizes it, and iii) the preparation of polyaniline blends with classical insulating polymer matrix. We will present and discuss the results of transport, structural and mechanical studies in a series of materials belonging to the three categories.

Using molecular engeneering concepts we synthesized series of plasticizing dopants for polyaniline. Films cast from solutions of doped polyaniline exhibit both high quasi-metallic conductivity, and an excellent flexibility. As an example, films with an elongation at break l/l0 » 200% were obtained. These films are semicristalline. Nanocristallites present a lamellar structure with an alternance of polyaniline chain and dopant layers.

The addition of external plasticizers induces dramatic changes in the transport properties. First, the room temperature conductivity increases significantly with increasing amount of plasticizer whereas the relative proportion of conducting chains in the material decreases. Second, we show for the first time that a veritable insulator-metal transition (IMT) is induced by the addition of plasticizers, leading to polymers which present a finite conductivity at T= 0. However, mechanical properties are only slightly improved.

Finally, blends of polyaniline with classical insulating polymers are well known for presenting an extremely low percolation threshold (<0.1 wt % of polyaniline). In this case, one tries to maximize the conductivity while avoiding a loss in the mechanical properties of the matrix. As a matter of fact, in polyaniline / PMMA blends, some kind of ”mechanical percolation” is observed for a polyaniline content higher than that corresponding to the electrical percolation threshold. Above the percolation, the polyaniline network appears as a mechanical reinforcement of the matrix. The scaling law of percolation has been checked as a function of temperature. While the percolation threshold remains constant, a continous increase of the critical exponent is observed. We propose a unique model, based on the continuous percolation theory which accounts for the whole set of data.


SL21

Deformation-Induced Molecular Alignment in Pentacene Thin Films Characterized by Electron Microscopy and Diffraction

Lawrence F. Drummy, Paul K. Miska and David C. Martin

Department of Materials Science&Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, Michigan MI 48109-2136

Pentacene is a material of current interest as the semiconducting layer in all-plastic thin film electronic devices. We have investigated the plastic deformation and resulting molecular alignment of scratched and rubbed polycrystalline pentacene thin films using optical and electron microscopy and electron diffraction.

Polycrystalline films of pentacene were produced by vacuum sublimation onto amorphous carbon coated substrates in a glass tube, and the films had grain sizes of 500-5000 nm. Before deformation the films are textured with the (001) planes parallel to the substrate and the long axis of the molecules nearly perpendicular to the substrate.

Inside the plastically deformed zone near the scratch, the molecules lie down with their long axis parallel to the substrate, and they are oriented in the scratch direction. We used electron diffraction and high resolution electron microscopy to quantify the extent of alignment in the deformed region and to directly image defects visible after alignment. The grain size of the film is reduced after deformation.

Outside the scratched zone the films are uniform on a sub-micron length scale with density fluctuations in the form of grain boundaries and holes on a length scale greater than one micron. Inside the scratched zone the films are uniformly dense on a length scale greater than one micron with density fluctuations in the form of grain boundaries on a nanocrystalline length scale.


SL22

DESIGN OF MOLECULAR MAGNETS

J.V. YAKHMI

Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai (Bombay) – 400 085, India. (E-mail: Yakhmi@magnum.barc.ernet.in)

The conventional magnetic materials used in present-day technology, such as, Fe, Fe2O3, Cr2O3, SmCo5, NdFe14B etc. are all atom-based, whose synthesis requires high temperature routes. Employing ambient temperature synthetic organic chemistry, it has recently become possible to engineer a bulk molecular material with designer characteristics, including magnets. One essentially makes use of the weak nature of intermolecular interactions in a molecular solid, which lets the molecules retain their individuality, making it possible to derive the properties of a molecular solid from those of the molecular sub-units. Typical synthetic approach to design molecule-based magnets consists of choosing molecular precursors, each bearing an unpaired spin, and assembling them in such a way that there is no compensation of spins at the scale of the crystal lattice. Magnetism being a co-operative effect, the spin-spin interaction must extend to all the three dimensions, either through space or through bond. Specific occurrence of ‘spin delocalisation’ and ‘spin polarisation’ in molecular lattices is helpful in bringing about ferromagnetic interaction by facilitating necessary intermolecular exchange interactions.

Since the first successful synthesis of molecular magnets in 1986, a variety of them have been synthesized, which can be classified on the basis of the chemical nature of the magnetic units involved: organic-, metal-based systems, or mixed organic-inorganic systems. A number of molecular magnets have been assembled, through polymerisation process using ferrimagnetic chains employing heterobimetallic species, in which two different spin carriers are bridged by extended bisbidentate ligands such as oxamato, oxamido, or oxalato, etc.

Apart from the fact that a molecule is the ultimate unit for data-storage, the design of molecular magnets has also opened the doors for the unique possibility of designing poly-functional molecular materials, such as magnets exhibiting second-order optical nonlinearity, liquid crystalline magnets, or chiral magnets. Solubility of molecular magnets, their low density and biocompatibility are attractive features. Being weakly colored, unlike the opaque classical magnets, possibilities of photomagnetic switching can be envisaged. Persistent efforts continue to design the ever-elusive polymer magnets which when made would have a huge impact on applications in industry.

This talk will highlight some recent developments in the field of molecular magnetism, with some examples from the author’s own lab.

Acknowledgements. This talk is dedicated to the memory of the late Prof. Olivier Kahn, a pioneer in the area of molecular magnetism, and a great source of inspiration.


SL23

DESIGN, SYNTHESIS, AND CHARACTERIZATION OF CATIONIC WATER-SOLUBLE CONJUGATED POLYMERS

B. LIU1, Q.-L. FAN1, S. LU1, W.-L. YU1, S.-J. CHUA1, W. HUANG1,2

1 Institute of Materials Research and Engineering, National University of Singapore, 3 Research Link, Singapore 117602, Republic of Singapore

2 Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore. chehw@nus.edu.sg

In this talk, we report our recent studies on the design, synthesis, and characterization of novel cationic water-soluble conjugated polymers. Through the functionallization of polyfluorene (PF) and poly(p-phenylene vinylene) (PPV) derivatives with amino-terminal groups, cationic water-soluble polymers with different color emissions have been achieved, with the water-solubility realized through the post-polymerization method, which permits the full structural characterization and the control of the cationic degree. Herein the PPV derivatives present the first examples of the cationic water-soluble polymers, which emit red and green light. It was demonstrated that the ionization of the side chains reduced the effective conjugation of the backbones of the polymers. The complexation of PPV-NEt3Br with the oppositely charged polyanions, such as PAANa, in aqueous solution induces the PPV chain to vary from isolated state to aggregated state and then to be surrounded by PAANa chains with increasing amount of PAANa. The complexation dependent spectral change provides a strategy for studying complexation behavior between oppositely charged polymers and has potential applications in sensory technologies.

Breakthroughs have also been achieved in the synthesis of blue emission conjugated polymers with tunable water-solubility, which provide enriched materials for the self-assembly study. It was also found that the neutral polymers had good solubility in water upon the addition of some weak organic acids. Instead of forming the quaternized salt, only weak interaction exists between the acid and the amino-terminal groups. Detailed characterization results have shown good thermal stability and reversible electrochemical properties for both the neutral and the water-soluble polymers. The special solubility and intense fluorescence both in solutions and as films endow this series of materials most attractive candidates as the transporting/emitting layer in the multi-layer device fabrication.


SL24

Giant Optical Rotatory Power and Light Modulation Properties in Biodegradation of Poly(Lactic Acid) Films

Y. TAJITSU

Department of Polymer Science & Engineering, Yamagata University

Yonezawa, Yamagata 992-8510, JAPAN

e-mail: tajitsu@yz.yamagata-u.ac.jp, FAX: 81-238-26-3410

Every year, the amount of industrial waste is increasing, and as a result, environmental pollution has become more serious than ever. As a material that is expected to solve environmental pollution, biodegradable polymers have attracted much attention. Polylactic acid (PLA) is one polymer material on which biodegradability research has been most advanced. PLA is a chiral polymer in which molecules containing asymmetric carbon atoms have a helical orientation. Two optical isomers exist in PLA. One is PLLA, and the other is PDLA. For a long time, chiral polymers have been expected to exhibit optical rotary power, , in their solid state, compared with inorganic low-molecular-weight crystals. A typical substance with a large is -quartz (-SiO2).  The of -quartz is due to the helical structure formed by molecules. Therefore, it is believed that of a chiral polymer is large. However, no PLA film with a large had been prepared to date, because it was very difficult to control its higher order structure. In this study, we prepared PLLA with a molecular weight Mw of 100,000 ~ 600,000 and an isomer (D-type) content (ID) of 0.001 ~ 1.000%. Then, using physical processes, we fabricated various samples such as oriented PLLA film, PLLA fiber, rolled PLLA film and a cylindrical plate. Finally, we observed a large and high-speed light modulation (LM) in the cylindrical plate fabricated using a new physical process. The findings for the cylindrical plates fabricated using a new physical process are summarized as follows:

1) Existence of significantly large in the fiber axis direction of PLA films

PLLA and PDLA films possess levorotatory power and dextrorotatory power, respectively. The of PLLA film is 7200˚/mm and approximately 300 times larger than that of -quartz.

2) Demonstration of LM by PLLA films

We confirmed LM up to 10 MHz. We believe that the LM speed in the GHz range of a highly modified PLLA sample fabricated using the new physical process is possible because the PLLA film provides a rapid response.


SL25

FEATURES OF CHARGE CARRIER CONCENTRATION

AND MOBILITY IN p -CONJUGATED POLYMERS.

G.JUŠKAa, K.GENEVIČIUSa, K.ARLAUSKASa, R.ÖSTERBACKAb, H.STUBBb

a Department of Solid State Electronics, Vilnius University, Saulėtekio 9, LT-2040 Vilnius, Lithuania gytis.juska@ff.vu.lt

b Åbo Academy University, Department of Physics, Porthansgatan 3, FIN-20500

Turku, Finland rosterba@abo.fi

We will represent the experimental results of investigation of photoexcited and of equilibrium charge carrier mobility and concentration by charge carrier extraction by linearly increasing voltage method (CELIV) in pi-conjugated polymers (PPV, RRPOT, RRPHT, RRaPHT, etc.). From the numerical modelling and experimental investigation by CELIV the nature of mobility dependence on electric field can be estimated [1].

The following obtained experimental results will be discussed:

[1] G.Juška et al. Phys.Rev. B62, R16235 (2000)

Fig.1. Schematic view of current transients of CELIV and photo-CELIV.