Emission light from Ag/ Rhodamine-B (RB) Langmuir-Blodgett (LB) films due to surface plasmon (SP) excitations has been investigated using the Kretschmann configuration of the attenuated total reflection (ATR) method and the reverse irradiation of laser beam from air to the samples. The conventional photoluminescence (PL) of the RB LB films showed a broad spectrum and the peak wavelength was about 600 nm. In the reverse irradiation, emission light was observed through the prism, and the intensities and the spectra strongly depended upon the emission angle where the light was observed. The wavelengths of the emission light became shorter as the emission angles increased. The relation between the wavelength and the emission angle agreed with the resonant condition of excitations of SPs in the Kretschmann configuration of ATR method. It was concluded that the emission light was caused by excitations of multiple SPs at the Ag/RB LB films.

Langmuir-Blodgett (LB) films of poly(p-phenylene vinylene) (PPV) were prepared using an amphiphilic precursor, where the counterion chloride was partially replaced by a long chain dodecylbenzenesulfonate ion. These LB-PPV films are highly ordered along the dipping direction as demonstrated by linear dichroism experiments using linearly polarized optical absorption and emission. The intensity ratio between emitted light parallel and perpendicular to the dipping direction was ca. 17, which is much higher than the dichroic ratio of 3.6 found in the polarized absorption experiment. These values indicate an efficient energy and/or charge transfer between low conjugated segments (disordered) and highly conjugated (ordered) ones after excitation. In addition, the LB films displayed large circular polarization with an asymmetry factor ge which varies from 0.6 to –0.2 when the sample temperature is increased from 30 to 300 K.

Two types of core-shell structured dendrimer, 2,4-bis-{3,5-bis-[3,5-bis-(3,5-di-tert- butylbenzylthio)benzylthio]benzyloxy}-6,8-dinitroquinazoline (7a) and 2,4-bis-{3,5-bis-[3,5-bis- (3,5-di-tertbutylbenzyloxy)benzyloxy]benzyloxy}-6,8-dinitroquinazoline (7b) as a referential compound were synthesized. These compounds consist of a 6,8-dinitroquinazoline ring as the functional unit and a 3,5-dimercaptobenzyl or a 3,5-dihydroxybenzyl group as the dendritic unit. In order to protect the sulfur atom in dendrimer (7a) from the oxidation reaction, a 3,5-di-tert- butylbenzyl group was designed as the outer-most unit. The synthesized dendrimer containing 3,5-dimercaptobenzyl blocks (7a) is completely novel, and is interesting as a photoelectronic material to examine what kind of influence the d-orbital electron properties give to the whole molecule.

The external quantum efficiency in conventional double-layer type organic light-emitting devices with different layer thickness was carefully evaluated. From the measurements of luminance and emission spectra, external quantum efficiency (uncorrected) was obtained.

The correction due to the modification of spatial emission distribution was added from the measurements of spatial emission patterns. The external quantum efficiency, both uncorrected and corrected, showed strong dependency on the distance between a metal electrode and an emissive region. This variation was ascribed to the change of coupling-out efficiency. Independent experimental evaluations of apparent couple-out efficiency using an integrating sphere supported this interpretation.

Measurement and theory are presented for accumulation mode P3AT TFTs with Schottky and ohmic contacts. The polymer has been allowed to turn p type through exposure to air. Sample characteristics for gold contact TFTs are presented. Mobility values are 0.2 cm2/Vs and are comparable to the best reported.

Using Schottky contacts as the source and drain electrodes can reduce off currents due to the existence of potential barriers at the contact. The presence, however, of the Schottky depletion region changes the mode of operation. A model is presented to describe the sub- threshold and gradual channel regions. Devices constructed using titanium show good agreement with the model. Transconductance for these devices is reduced when compared to ohmic contact devices with calculated mobilities of 0.01 cm2/Vs. Much smaller channel lengths are, however, possible in this technology. Off currents are found to be completely independent of gate voltage indicating good control of the off current by the Schottky junctions. Little or no gate modulation has been observed, to date, for aluminium and chromium contact devices.

We report on the influence of Bias-Temperature Stress (BTS) on the pentacene Thin Film Transistors (TFTs) electrical characteristics and on their 1/f noise level. The gate BTS primarily affects the TFT threshold voltage, leaving both mobility and sub-threshold slope values almost unchanged. The degree of the threshold voltage shift induced by the positive or negative BTS depends on the TFT design and the BTS parameters. The current-voltage characteristics time dependence of the organic TFTs, subjected to the BTS, resembles that for amorphous-Si TFTs. The results of the 1/f noise measurements in the organic TFTs allowed us to conclude that the gate BTS primarily affects the TFT contact regions, resulting in the increase of both the contact noise and the contact resistance.

Ribonucleic acids, often called a biological jack of all trades, contribute intimately to every aspect of gene expression, including the synthesis of other polypeptide biocatalysts. The fundamental importance of recurring structural motifs and the kinetics and energetics of the complex secondary and tertiary structure of RNA have been shown to be intimately linked with its functions in vivo. We have developed a novel enzymatic synthetic approach for covalent attachment of photoresponsive units into the RNA backbone. The synthetic conditions of this approach are extremely mild, involving the reverse micellar solubilization of nucleic acid along with lipase in apolar hydrocarbon solvents. Lipase catalyzed acylation of the 2' hydroxyl group in the ribose sugars of the RNA molecule has been used to incorporate photo-isomerizable azobenzene groups into the RNA strands. This micellar approach was envisaged for RNA functionalization while maintaining the conformational integrity of the macromolecular backbone in neutral buffer solution. The modification of RNA using covalently attached chromophores or fluorophores can be extended to other biomacromolecular matrices leading to the development of more versatile photoactive biopolymers. The photo-isomerizable groups incorporated in the RNA molecule can serve as optical ‘handles’ for the manipulation of the conformation of RNA and open new opportunities for biophotonic device applications.

Ionically self-assembled monolayer (ISAM) films have been recently shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Here, we study the effects of replacing the NLO-active polymer layers with layers of dianionic molecules. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. The incorporation of dianionic molecules within the ISAM films affords greater χ(2) due to increased net polar orientation of the chromophores.

We have used a long chain dodecylbenzenesulfonate ion (DBS) to produce self-assembled multilayer structures of poly(p-phenylene vinylene) (PPV) with special features. Rather than alternating layers provided of a PPV-precursor (the poly(xylylidene tetrahydrothiophenium chloride or PTHT) and a polyanion, the alternated adsorption process was carried out with the PPV-precursor and the DBS ions in aqueous solution. The internal layer structure of these PTHT- DBS films is therefore different from conventional layer-by-layer films, since DBS is expected to be incorporated into the PTHT chain via counterion exchange. As a consequence, the conjugation length could also be controlled by controlling the DBS content in the film. Further advantage of introducing DBS lies in the possibility of converting films with a high conjugation length at 115 oC within only 3 min. The luminescence and absorption spectra at low temperatures show a very intense well-resolved vibronic structure. We also demonstrated the band-gap control of PPV films with a possible heterostructure formation.

The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene), PPV, have been determined by photoconductivity excitation profile spectroscopy as a function of light polarization, applied electric field, and temperature. The spectral signature of the exciton is a narrow peak (100 meV full width at half maximum) that emerges just below the band edge upon increasing the external field, the temperature or the defect density. The exciton peak is observed only for light polarized parallel to the chain axis. The exciton binding energy is obtained from the energy of the exciton peak with respect to the band edge and, independently from analysis of the field dependence of the exciton dissociation. It is Eb ≈ 60 meV.

A study was made of third-order nonlinear optical susceptibilities of new tetrathiafulvalene (TTF) derivatives, using the degenerate four wave mixing (DFWM) method, as well as complex quantum chemical calculations. To understand the physical nature of the optical nonlinearities, we separated their electronic and nuclear contributions. We found that the electronic contribution to these nonlinearities predominated. Our investigations suggest that TTF may be a highly promising material for nonlinear optics (NLO).

We investigate the electrical properties and the OLED application of controlledly doped amorphous hole transporters. Thin films of starburst amine, 4,4',4“-tris(N,N-diphenyl- amino) triphenylamine (TDATA), doped by a fully fluorinated form of tetracyano- quinodimethane (F4-TCNQ), are characterized in situ by temperature dependent conductivity and Seebeck measurements. The conductivity and hole concentration increase with dopant concentration and are many orders of magnitude higher than those of undoped material. OLED devices with the layer sequence ITO/TDATA(200 nm)/Alq3(65 nm)/LiF(1 nm)/Al were fabricated. The use of p-doped TDATA thin films with high bulk conductivity and hole concentration reduces the resistance of the devices and leads to a thinner space charge layer which facilitates injection of holes from the ITO anode.

Electronic devices based on organic materials are presently being developed. In these devices, the cathode interface and the nature of the charge carrying species are often of crucial importance. In that context, the behavior of lithium atoms deposited on the surfaces of ultra-thin spin-coated films of poly(dioctyl-fluorene), PFO, and of condensed molecular solid films of tris(8-hydroxyquinoline) aluminum, Alq3, have been studied. The Li-atoms donate charges to the organic systems, leading to doping-induced electronic states in the otherwise forbidden energy gap. The changes in the electronic structure induced by charge transfer from the Li-atoms are different in the two materials studied, and depend upon the localization of the electronic states to which the electrons are transferred. In the case of the delocalized wave functions of the ∼-system of PFO, at low doping levels, the added charges lead to the formation of polaron states, while at higher doping concentrations, bipolaron states are formed. In the case of Alq3, however, up to a level of three added electrons per molecule, the added electrons reside in states localized on each of the three ligands. The role of thin (∼5 Å) interfacial layers of LiF or CsF between poly(9,9- dioctylfluorene) polymer film and an aluminum electrode was studied as well. LiF-deposition on poly(9,9-dioctyl-fluorene) did not cause doping of the polymer films, nor did the LiF dissociate at the interface. No significant shifts in the binding energy of the core levels nor any changes in the work function occurred. Al-deposition on LiF/PFO films did not cause dissociation of LiF, unlike the case for Alq3. We found that CsF does not dissociate when deposited on a polymer film. However, deposition of aluminum on CsF will cause dissociation with cesium n-doping the PFO film at the interface and fluorine likely reacting with aluminum to form AlF3. When deposited onto either of sputter-cleaned Al surface or Al with native oxide layer (AlxOy), CsF also was found to dissociate at the interface. CsF does not dissociate for Au/CsF/PFO and CsF/Au interfaces, which emphasizes the crucial role aluminum plays in the process. The observed decomposition of CsF occurring upon Al deposition and following Cs-doping of the surface region of PFO likely enhances injection of electrons into organic layer improving device performance. Since CsF dissociation is independent of the underlying material, the Al/CsF/emissive-material structure could be effective for almost all types of polymer/organic based electronic devices.

The photophysical formation of surface relief grating (SRG) was compared between isocyanate-based polymeric and non-polymeric thin films of low molecular weight organics containing azobenzene group. The non-polymeric film forms faster and more efficient surface grating formation than the polymeric film when exposed to an interference pattern of polarized Ar+ laser beams at 488 nm. However, the polymeric materials exhibited higher stability of SRG and orientation of azobenzenes than non-polymeric ones. The relation between the rate of photoinduced surface modulation and orientation of azobenzenes is discussed.

Both photoluminescence (PL) and electroluminescence (EL) properties of 6,6””-dimethyl-4', 4”'-diphenyl-2,2':6',2”:6”,2”':6”',2””-quinquepyridine (DMDPQPY), a 2,6-linked oligopy- ridine derivative, have been investigated. Two maxima are observed from its PL spectra in solid state, but only one peak appears in its PL in solutions. Emission from the single layer device with indium-tin-oxide (ITO) and Al as the respective anode and cathode (ITO/DMDPQPY/Al) is very weak, but introduction of a hole transporting layer polyvinylcarbazole (PVK), in the double layer device (ITO/PVK/DMDPQPY/Al), increases the luminance significantly indicating a better charge balance. Two peaks appear in the EL spectra, one at 419 nm and another at 536 nm, ascribed to PVK and DMDPQPY respectively.

Current-voltage characteristics of organic semiconductor pentacene were studied as a function of applied tip force using conducting probe atomic force microscope. I-V measurements were performed on 150Å and 300Å pentacene films. No electrical contact was observed until 20 nN of tip force for the 150Å film coverage, whereas electrical contact was established easily with a few nN of tip force values for the 300Å film coverage. On both films, once an electrical contact was obtained, the conductance was observed to increase with increasing load. At about 50 nN and higher loads we lost electrical contact in both cases in a reproducible manner. While I-V on 150Å pentacene film showed a rectifying behavior, I-V on 300Å pentacene looked like typical I-V curves of tunneling phenomenon. From these measurements, we have estimated a hole injection barrier of about 0.76 eV and a band gap of about 2.00 eV for pentacene. These results are in agreement with those in the literature.

The synthesis of poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) has allowed the study of this material as the active layer in a multiple layer device using a composite combination of polymer and Alq3 with an oxadiazole as electron injection/transport layers. The luminous efficiency of the resulting device was significantly improved as a result of this combination of transport layers. An entirely new approach to the synthesis of monomers for PPV precursors is described in which the 2- and 5- substituents are introduced by directed metallation groups (DMGs) which ultimately become the bis(halomethyl) groups.

Rigid rod-like poly(2,5-pyridinediyl), semi-rigid polyaniline and flexible poly(4- vinylpyridine) are nitrogen-containing polymers that with selected amphiphilic oligomers form self-organized comb-shaped supramolecules due to protonation, hydrogen bonding and polar- nonpolar effects combined. Luminescent or conductive ordered structures are demonstrated in thin films. The structures are characterized using small-angle x-ray scattering (SAXS) and grazing-incidence small-angle x-ray scattering (GISAXS). The uniformity is studied using atomic force microscopy and scanning near-field optical microscopy (SNOM).

Ionically self-assembled monolayers (ISAMs), fabricated by alternate adsorption of cationic and anionic components, yield exceptionally homogeneous thin films with sub- nanometer control of the thickness and relative spatial location of the component materials. Using organic electrochromic materials such as polyaniline, we report studies of electrochromic responses in ISAM films. Reversible changes in the absorption spectrum are observed with the application of voltages on the order of 1.0 V. Measurements are made using both liquid electrolytes and in all-solid state devices incorporating solid polyelectrolytes such as poly(2- acylamido 2-methyl propane sulfonic acid) (PAMPS).

Photoinduced changes in electrical rectification were measured for a 5-layer Langmuir-Blodgett multilayer of hexadecylquinolinium tricyanoquinodimethanide, sandwiched between opaque and semitransparent Al electrodes, as a function of bias and wavelength. The current increases with the light intensity, but this may be simply due to heating of the sample, rather than to a photon-induced enhancement of the rectification.