KEYWORDS: Liquid crystals, System on a chip, Americium, Modulation, Phase shift keying, Absorption, Temperature metrology, Molecules, Ultraviolet radiation, Molecular machines
Light- and temperature-driven multistable chiral materials composed of a binaphthyl moiety as a twisting part and an
azobenzene moiety as a photoresponsive part in a single component were investigated. It has been found that the
materials show reversible change in a dihedral angle of the twisting part at various temperatures. In contrast, no change
in the twisting angle of the materials during trans-cis photoisomerization was observed. Furthermore, the chiral
compounds were dissolved in nematic (N) liquid crystals (LCs) to produce a chiral N phase with a helical structure. The
mixture exhibited a dual molecular response to temperature and light. Helical pitch length of the chiral N LC decreased
with increasing temperature because of a molecular twisting motion of the binaphthyl moiety, resulting in a stabilization
of the LC helical structure. On the other hand, length of the helical pitch increased upon photoirradiation, and the
resulting LC mixture was found to show photoswitching between chiral N and N phases upon trans-cis isomerization of
the azobenzene moiety. The photoinduced deformation of the LC helical structure was derived from a bent shape of cis
isomer of the azobenzene moiety in the chiral dopant. This dynamic modulation of the self-organizing helical structure
was based on dual and selective molecular motions of the guest materials induced by external stimuli.
We designed and synthesized a novel type of a cyclic compound containing azobenzene and binaphthyl moieties. The most
significant characteristic of the compound is to possess a reversible change in a dihedral angle of a binaphthyl moiety by
means of photoisomerization of the azobenzene moiety. Furthermore, the compound shows change in a twisting motion in a
solution, a neat film, and a liquid-crystalline host by alternating irradiation of UV and visible light.
We investigated the carrier transport and photorefractive properties in multi-component materials containing 9-(2-ethylhexyl)carbazole (EHCz), so-called liquid carbazole. The electric-field dependence of the carrier mobility for EHCz and polyvinylcarbazole (PVK) was measured by the time-of-flight (TOF) technique. We obtained the higher carrier mobility of 4.2 x 10-6 cm2/Vs with an electric field of 2.5 x 105 V/cm for EHCz than that of 6.4 x 10-7 cm2/Vs for PVK. We also performed the two-beam coupling (TBC) technique to investigate photorefractive responses for the guest-host polymers containing PVK, EHCz, the electro-optic chromophore 4-piperidinobenzylidene malonitrile (PDCST), and the sensitizer C60. The measurement showed that the TBC gain depended on the mixture ratio of PVK and EHCz. The TBC gain was enhanced from 31 to 85 cm-1 at an electric field of 60 V/μm by increasing the concentration of EHCz from 10 to 15 wt%. These results show that EHCz plays important role for the optoelectronic and photorefractive materials as a hole transport materials as well as a plasticizer.
Vanadyl phthalocyanine derivatives having optically active side chains and the corresponding racemic isomers were synthesized and examined as nonlinear optical materials. These dyes were soluble in organic solvents and gave uniform thin films using spin coating. The thin films (neat or polymer doped) of each phthalocyanines showed the second- and third-order nonlinear optical responses under appropriate experimental conditions. The nonlinear optical susceptibilities of the optically active derivatives are larger than those of the corresponding racemic isomers. To clarify this enhancement phenomenon, we measured the electronic absorption- and circular dichloic spectra, and X-ray diffraction of the thin films. These results suggested that the optically active dyes forms one-dimensional columnar aggregates with one-handed helical sense and the columns further aligned into honeycomb-like chiral superstructures. It was surmised from the experimental results that the chiral superstructures enhance the nonlinear optical responses relative to the racemic analogues.
We synthesized novel ligand-based mixed valence (LBMV) CrIII-dioxolene complexes, [Cr(X4SQ)(X4Cat)(4,4'-di-tert-butyl-2,2'-bpy)] (SQ = semiquinone, Cat = catecohol, 2,2'-bpy = 2,2'-bipyridine; X = Cl (2a) and Br (2b)) and [Cr(X4SQ)(X4Cat)(4,4'-dinonyl-2,2'-bpy)] (X = Cl (3a) and Br (3b)), and prepared thin films for investigating their third-order nonlinear optical (NLO) properties in terms of the mixed valence states. Electronic absorption spectra of these complexes in solution and solid states showed an intervalence charge-transfer (IVCT) band from Cat2- to SQ•- at the IR region, indicating of a coexistence of SQ and Cat ligands, namely, LBMV state of the complexes. These complexes were well soluble in nonpolar organic solvent, which allowed us to prepare thin films by spin coating. The obtained films showed the electronic absorption spectra similar to those in solution and were amorphous because of steric hindrance of halogen and alkyl substituents in o-dioxolene and 2,2'-bpy moieties, respectively. The x(3) values of the films of 3a and 3b with a thickness of 30 ~ 40 nm were determined for 1.0 × 10-12 esu at 1.907 μm.
We discussed the electric and optical properties of three types of conjugated carbazole polymers: substituted polyacetylene with carbzole moiety, main-chain polymers having conjugated carbazole trimer units, and triindole units. Substituted polyacetylenes can be expected to show the enhanced optoelectronic responses due to the extended conjugated systems in the main chain and the additional chromophores in the side chain. The hole mobility of a substituted polyacetylene containing carbazole moiety was determined to be about 10-4 cm2/Vs with the electric field of 4 x 105 V/cm by time-of-flight technique, while polyvinylcarbazole showed hole mobility of about 10-6 cm2/Vs. The carrier mobility is presumably enhanced by the extended conjugation system. Hyperbranched polymers with carbazole trimer units exhibited the enhanced two beam coupling gain of 60 cm-1 in comparison with main-chain polymer with carbazole units. Since triindole is an electron-donative molecule containing three carbazole substructures with smaller ionization energy than that of carbazole, we can expect the application to a variety of optical and electrical materials. Main-chain polymers with triindole units exhibited the two-beam coupling gain of over 100 cm-1. Therefore, theses conjugated carbazole polymers are one of the good candidates for the new class of electronic and photonic materials.
We studied linear and nonlinear optical properties of four different phthalocyanines: vanadyl and copper phthalocyanines substituted with chiral branched side chains, (S)(OMeBu)8VOPc, (S)(OMeBu)8CuPc; a racemic analogue (R,S)(OMeBu)8VOPc; vanadyl phthalocyanine substituted with linear side chains, (OBu)8VOPc. We investigate the molecule packing and their third-order nonlinear optical response in terms of chirality, planarity, and side chain structures. Molecular arrangement of (S)(OMeBu)8VOPc in the thin films was determined to be a columnar phase with rectangular 2D crystals by X-ray diffraction studies. The thin films of (S)(OMeBu)8VOPc diplayed CD activity. While, a chloroform solution of this compound did not show any CD. Therefore, we conclude that the CD in the films must result from the chiral aggregation of the molecules. The Χ(3) value of the flims of (S)(OMeBu)8VOPc was determined for 6.7×10-11 esu by third harmonic generation at 1.907 μm and this value was larger than those of (R,S)(OMeBu)8VOPc, (R,S)(OMeBu)8CuPc, and (OBu)8VOPc.
The polarization and depolarization behavior of electric field-induced polar alignment in a stable photorefractive polymer containing carbazole was measured by thermally stimulated current (TSC) and electro-optic (EO) modulation experiments. A relaxation peak was observed around the glass transition temperature (Tg) for the poled photorefractive polymer in a TSC experiment. The apparent TSC was not observed at room temperature. The polarization calculated from the relaxation peak linearly increased with the strength of the poling electric field. This result was consistent with the EO behavior which showed the linear dependence of the EO coefficients on the poling electric field. The polar alignment induced by the electric field at an elevated temperature was stable at room temperature, which could lead to the long-term stability of the photorefractive responses without an external applied electric field.
As a new class of multi-functional compounds, three kinds of calix[4]arene molecules were newly synthesized: calix[4]arenes containing two 3-nitro-N-allyl carbazoles, two N- allyl carbazoles and one of each at the lower rim. These compounds take cone conformation in solution because of the 1,3-di-O-substitution at the lower rim. Those compounds take amorphous state as proved by differential scanning calorimetry and powder X-ray diffraction and they can be made into films without supporting the polymer matrix. Second- harmonic generation signals were observed in the films of the two calix[4]arenes containing nitro groups after electric poling at a temperature above glass transition point. Enhancement of d33 in the calix[4]arene with nitrocarbazole derivatives compared with the calix[4]arene with both carbazole and nitrocarbazole derivatives was observed, which might be attributed partially to some extent to the conformational regulation between the two carbazole chromophores. Using two beam couple experiment, photorefractive effect was observed in the film of the calix[4]arene with both carbazole and nitrocarbazole derivatives doped with 2,4,7-trinitro-9-fluorenone. This is the first report on the photorefractive property of calix[4]arene compound.
Anisotropy of third-order nonlinear optical properties in di-n-butyl fluorescein derivatives was investigated by electroabsorption spectroscopy. In a series of fluoresceins, the derivatives with Cl substitution at phenyl ring presented a new intense absorption band at the wavelength range from 550 to 620 nm, which is at longer wavelength side of the main absorption band in other derivatives. This new absorption band was attributed to intramolecular charge transfer transition due to Cl substitution at the phenyl ring, according to the spectral profile of electroabsorption which was proportional to the second derivative of linear absorption spectrum. Polarization dependence of normalized absorption coefficient was about 2:1 at the main absorption band in all derivatives. However, in the charge transfer transition band, the polarization dependence was about 1:1,2, suggesting that the charge transfer direction is nearly perpendicular to the otpical transition. This result is consistent with structural aspect that the charge transfer direction is located along to the bond between the Cl-substituted phenyl ring and condensed ring, while the optical transition is along to the longitudinal axis of the condensed ring.
Anisotropy of transient absorption change and electroabsorption response in metallophthalocyanine (MPc) aggregates were investigated in terms of the influence of intermolecular interaction to the 2D character of the molecule. In an MPc aggregate intermolecular interaction with less symmetry might result in appearance of 1D character, depending on the nature of interaction and arrangement of molecules. In experiment, polarization- dependent electroabsorption response in various MPcs doped in PMMA presented a 1D character at the red-shifted absorption band due to coplanar intermolecular interaction and a 2D one at the blue-shifted band due to cofacial interaction. Polarization-dependent subpicosecond transient absorption change in various aggregate vanadylphthalocyanine, evaporated and doped-polymer films in phase-I and phase-II aggregation, presented a variety of anisotropy: in phase-I aggregate doped in polystyrene, a wavelength-dependent anisotropy similar to that of electroabsorption response was obtained; in phase-I aggregates doped in PMMA or evaporated, a 2D character was observed in almost whole wavelength region; in phase-II aggregates both 1D and 2D characters were in the red-shifted absorption band. These result are partly elucidated by the exciton coupling model in 2D molecules. It is also suggested that pump wavelength dependence of anisotropy and existence of two or more aggregate species might play important rolls.
We report a series of polymers with linear, macrocyclic and hyperbranched structures for electroluminescent (EL) applications. The polymers are polycarbazoles (PC) containing different substituents. The polymers are amorphous and are soluble in common organic solvents such as chloroform, and tetrahydrofuran (THF). High optical quality films were obtained by spin-coating from the polymer solutions of chloroform or THF. All these polymers show strong photoluminescence under a UV-lamp illumination. Single and bilayer EL devices consisting of anode/hole transfer layer/electron transfer layer/cathode have been fabricated and characterized. The effects of polymer structures on the energy levels and EL properties are discussed. The results indicated that macrocyclic oligomers and hyperbranched polymers are new candidates for EL devices.
The Langmuir-Blodgett (LB) films of tolan derivatives were studied by a new second-harmonic generation (SHG) phase measurement in terms of the directional sense of the net dipole moment and molecular structures. 4-Docosylamino-tolan derivatives with 4'-nitro group (ANT-22) or 4'-dicyano-vinyl (DCNANT-22) group were transferred by horizontal lifting. In rotational Maker-fringe, the envelope of ANT-22 LB film shows an in-plane polar anisotropy, and that of DCNANT-22 LB film shows a polar anisotropy perpendicular to the substrate. Nonlinear optical chromophore in ANT-22 is bound to lie almost flat upon the plane of the substrate, while DCNANT-22 molecules in thick multilayers tend to align perpendicular to the substrate. It was found by SHG phase measurement that the directional sense of the net dipole moment of ANT-22 was perpendicular to the compression direction.
Two-beam coupling gain obtained from an orientational index grating was observed without applying an external field in a photorefractive polymeric material with ow glass transition temperature. A novel carbazole derivative with a photorefractive function was used as the material doped with a sensitizer for photocarrier generation. The mechanism to form the grating and to give the gain was investigated by two-beam coupling measurements.
Photocarrier generation properties and an electro-optic response were studied in acceptor-substituted carbazole materials. Two kinds of carbazole systems were investigated: molecularly doped polymer and main-chain polymer. The photoconductive sensitivity of 1.2x1012(S cm1)I(W cm2) was obtained with a field strength of 5x1O5V/cm at a
wavelength of 534nm. The electro-optic coefficient 33 of l.lpmJV was obtained with a field strength of 5x1O5V/cm at a wavelength of 532nm. These results show the monolithic photorefractive polymer could be developed by using acceptor-substituted carbazoles.
We have developed novel multifunctional materials for photorefractive applications. Two kinds of molecular systems were studied: acceptor-substituted carbazole main-chain polymers and oligomers. The photocarrier generation properties and second-order nonlinear optical responses in acceptor-substituted carbazoles were examined by xerographic discharge, second-harmonic generation and electro-optic measurements. The multifunctionalities of the main-chain polymers and oligomers with carbazole moieties were confirmed, which fulfill all the requirements for photorefraction. Photorefractive effects were confirmed by two-beam coupling and four-wave mixing measurements. These multifunctional carbazole main-chain polymers and oligomers enable us the development of monolithic photorefractive materials.
The transient absorption spectrum of a nonaggregated vanadylphthalocyanine solution has been measured by a subpicosecond pump-probe spectroscopy technique. A biexponential response with time constants of 4 ps and 800 ps has been observed. The dependence of the polarizations of pump and probe light has been investigated. The parallel-to- perpendicular ratio, i.e. the ratio of transient absorption change between configurations where pump and probe polarizations are parallel and perpendicular to each other, respectively, is 4:3. This is different from the ratio in ordinary 1D molecules, reflecting the 2D character of the vanadylphthalocyanine molecule.
Novel two-dimensional conjugated systems have been developed for second- and third-order nonlinear optics. Four types of asymmetrically substituted macrocycles and non-aggregated phthalocyanines were synthesized and their second- and third-order nonlinear optical properties were characterized: meso-monosubstituted octaethyl metalloporphyrins, (beta) - monosubstituted tetraphenyl metalloporphyrins, mono (tert-butyl) vanadyl phthalocyanine, tris (tert-butyl) mononitro metallophthalocyanines, and hexadeca (trifluoroethoxy) metallophthalocyanines. Control of molecular packing in asymmetric vanadyl phthalocyanine was described in terms of the enhancement of the third-order nonlinear optical susceptibility. No evidence of molecular aggregation was observed in optical absorption and electroabsorption spectra for hexadeca (trifluoroethoxy) metal-free and vanadyl phthalocyanines. The effects of the substituents and metallation on the dipole moment for asymmetric porphyrins is discussed. A preliminary investigation of the second-order nonlinear optical responses for Z-type Langmuir-Blodgett films and poled molecularly doped polymer is presented.
Electroabsorption spectra have been measured on poly(methyl methacrylate) thin films doped with three kinds of metallophthalocyanines: tetrakis (tert-butyl) metal-free and lead phthalocyanines, and hexadeca (trifluoroethoxy) metal-free phthalocyanine. The third-order nonlinear optical susceptibility, (Chi) (3) (-(omega) ; (omega) , O, O), was determined from the electric field-induced absorption change. The considerable differences in absorption, electroabsorption, and complex (Chi) (3) (-(omega) ; (omega) , O, O) spectral profiles of these metallophthalocyanines are attributable to the difference in molecular aggregation states.
The nonlinear optical channel waveguide consisting of the lead phthalocyanine derivative doped polymer layer over the ion-exchanged glass substrate was fabricated. The single-mode propagation was observed in this polymeric channel waveguides. Propagation loss was estimated to be 2 dB/cm at 830 nm. Nonlinear optical properties are investigated by measuring the intensity-dependent refractive index change using a femtosecond interferometric method.
Second- and third-order nonlinear optical responses of photoconductive materials were studied by optical second and third harmonic generation and femtosecond time-resolved spectroscopy. Four types of photoconductive molecular systems were investigated: vanadylphthalocyanine thin films, polymeric charge-transfer complex, single crystals of 9-hydroxyethylcarbazole derivatives, and carbazole head-to-tail polymers.
Optical Kerr effect were applied to an all-optical switching device in the form of nonlinear waveguide directional couplers. The nonlinear directional coupler had a nonsymmetrical waveguide whose structure consists of a quartz thin gap between two Corning 7059 guided layers on the pyrex substrate with ion-milled grating and organic thin film as a top layer. The vacuum-deposited polydioacetylene film was used as an organic nonlinear material. Switching phenomena in this nonlinear directional coupler were confirmed for 10 ns pulse Nd:YAG laser and 150 fs pulse duration of mode-locked Ti:Sapphire laser.
The molecular design and assembly of metallophthalocyanine systems that have the enhanced macroscopic third-order nonlinear susceptibility (chi) (3)ijkl(-(omega) 4; (omega) 1, (omega) 2, (omega) 3) and show ultrafast responses are described. Enhancement of the third harmonic susceptibility (chi) (3)1111(-3(omega) ; (omega) , (omega) , (omega) ) was observed in vanadyl phthalocyanine vacuum-deposited film with the staggered stacking arrangement induced by thermal treatment. Processable polymeric system based on tert-butyl mono-substituted rich vanadyl phthalocyanine was developed and the favorable staggered stacking arrangement was induced in a polymer matrix to enhance (chi) (3)1111(- 3(omega) ;(omega) ,(omega) ,(omega) ). Femtosecond-time-resolved spectroscopy was performed on vanadyl phthalocyanine thin films with different morphological forms in order to elucidate the exciton dynamics. The exciton decay kinetics consists of three processes: exciton-exciton annihilation, exciton-phonon coupling, and much slower triplet state formation. The decay rate of the exciton population via exciton-exciton annihilation was found to have a time dependence of t-1/2. The vacuum-deposited film and spin-coated film of vanadyl phthalocyanine derivatives showed similar decay behavior, whereas molecular beam epitaxy film showed a faster decay with a time constant of several hundreds of femtoseconds.
Nonlinear optically active compounds newly designed and synthesized in Frontier
Research Program were reviewed: single crystal of dicyanovinyl anisole (DIVA) for
blue light generation, one- or two-dimensional r -conjugated systems such as polythiophenes,
polydiynes, soluble phthalocyanines and annulenes, for third order
optical nonhinearities and waveguide application. As a promising technique to control
the molecular orientation and assembly, the organic molecular beam epitaxy (OMBE) is
proposed.
The third order optical susceptibilities XIfli(-3w ; co, a, )
for PMMA films doped with a soluble metallophthalocyanine derivative
are determined by optical third harmonic generation measurements at
wavelengths of 1543nm and 1907nm. The x3 values are 6-7 times
enhanced at resonant and nonresonant wavelengths, concurrent with an
absorption change induced by solvent vapor treatment. The enhancement
of third order optical nonlinear susceptibilities in phthalocyanine
thin films is discussed in terms of molecular stacking.
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