Polarized Absorption Research Articles

Relation between Crystal Packing and Optical Anisotropy for Schiff Base-Nickel Complexes that Form Various Ladder-like Hydrogen-Bonding Networks

A series of nickel(II) complexes of a 2,6-dihydroxynaphtho-1-aldehyde Schiff base was synthesized. By means of in situ formation of metal-containing tectons, some of those complexes afforded sub-millimeter-scale single crystals directly from the reaction mixture. The complexes consisted in part of various ladder-like hydrogen-bonded networks, which forced the chromophore to arrange in anisotropic fashion, with the effect dependent on the length of the alkyl chains. The crystals exhibited a marked linear dichroism, and their solid-state polarization absorption spectra were recorded with an optical microscope. Using data from quantum chemical calculations and the crystal structure, we were able to elucidate the relationship between crystal packing and optical anisotropy. Analyses based on theoretical equations allowed us to explain the observed absorbance and to interpret the mechanism of the dichroism in detail.

Highly Oriented J-Aggregates of Bisazomethine Dye on Aligned Poly(tetrafluoroethylene) Surfaces

Highly oriented J-aggregates have been obtained in solid, thin films of a bisazomethine dye on aligned poly(tetrafluoroethylene) (PTFE) surfaces. The aggregates show remarkable optical anisotropy resulting from their high orientational order: the dichroic ratios at their J peaks in polarized absorption spectra reach 25. Moreover, this orientation also brings about a remarkable increase in the third-order nonlinear optical susceptibility [χ(3)] of the films of over 10 times that of their isotropic films. Major molecules do not form the aggregates, but rather, form crystalline domains where the aggregates should be dispersed. Polarized absorption spectra (visible and IR) and X-ray diffraction measurements reveal that the molecules are oriented with their long axis parallel to the PTFE chains both in the aggregates and in the crystalline domains ([2 0 −1] axis ∥ PTFE chains, (010) plane ∥ substrate surface). The growth mechanism of the aggregates is discussed on the basis of both the morphology observed by atomic force microscopy and the structural affinity between the crystal and the aggregate.

Passive mode-locked laser based on quantum dot superlattice

Passive mode-locking is achieved in two sectional lasers with an active layer based on superlattice formed by ten layers of quantum dots. Tunnel coupling of ten layers changes the structural polarization properties: the ratio between the TE and TM polarization absorption coefficients is less by a factor of 1.8 in the entire electroluminescence spectrum range for the superlattice.

Photoinduced Symmetry‐Breaking Charge Separation: The Direction of the Charge Transfer

Even flow: Photoinduced symmetry-breaking charge separation takes place in a few picoseconds in a 1,3-bis(perylene)propane dyad in polar solvents. Polarized transient absorption measurements show that the direction of the charge flow is random and entirely governed by the fluctuations of the solvent orientation around the dyad.

Controlling Chain Conformation in Conjugated Polymers Using Defect Inclusion Strategies

The Horner method was used to synthesize random copolymers of poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) that incorporated different backbone-directing monomers. Single-molecule polarization absorption studies of these new polymers demonstrate that defects that preserve the linear backbone of PPV-type polymers assume the highly anisotropic configurations found in defect-free MEH-PPV. Rigid defects that are bent lower the anisotropy of the single chain, and saturated defects that provide rotational freedom for the chain backbone allow for a wide variety of possible configurations. Molecular dynamics simulations of model defect PPV oligomers in solution demonstrate that defect-free and linearly defected oligomers remain extended while the bent and saturated defects tend toward more folded, compact structures.

Enhanced Optical and Nonlinear Optical Responses in a Polyelectrolyte Templated Langmuir−Blodgett Film

Optical and nonlinear optical properties like fluorescence and second harmonic generation (SHG) of molecular materials can be strongly influenced by the mode of assembly of the molecules. The Langmuir-Blodgett (LB) technique is an elegant route to the controlled assembly of molecules in ultrathin films, and complexation of ionic amphiphiles in the Langmuir film by polyions introduced in the aqueous subphase provides a simple and efficient access to further control, stabilization, and optimization. The monolayer LB film of the hemicyanine-based amphiphile, N-n-octadecyl-4-[2-(4-(N,N-ethyloctadecylamino)phenyl)ethenyl]pyridinium possessing a "tail-head-tail" structure, shows fluorescence as well as SHG response. The concomitant enhancement of both of these linear and nonlinear optical attributes is achieved through templating with the polyanion of carboxymethylcellulose. Brewster angle and atomic force microscopy reveal the influence of polyelectrolyte templating on the morphology of the Langmuir and LB films. Polarized absorption and fluorescence spectroscopy provide insight into the impact of complexation with the polyelectrolyte on the orientation and deaggregation of the hemicyanine headgroup leading to fluorescence and SHG enhancement in the LB film.

Columnar Self-Assembly and Alignment of Planar Carbenium Ions in Langmuir−Blodgett Films

Structural and optical properties of multilayer Langmuir-Blodgett (LB) films of two amphiphilic carbenium salts 2-didecylamino-6,10-bis(dimethylamino)-4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-1) and 2,6-bis(decylmethylamino)-10-dimethylamino-4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-2) are described. The LB films were prepared on lipophilic glass by standard vertical dipping. Grazing incidence X-ray diffraction (GIXD) measurements show that the planar organic cores, in spite of their positive charge, form closely packed columns with a repeating distance of ∼3.45 Å. Specular X-ray reflectivity (SXR) reveals the LB multilayers to consist of Y-type bilayers with thickness 31 Å for ATOTA-1 and 41 Å for ATOTA-2. This significant difference is ascribed to the different packing motifs of the alkyl chains in the two LB films. GIXD and polarized UV-vis absorption and emission spectroscopy show that the columnar aggregates in the LB films are oriented along the dipping direction. This alignment is attributed to shear effects during LB transfer. The main absorption band of the LB films is blue-shifted compared to that in solution, while the fluorescence is red-shifted by more than 100 nm. These findings suggest the presence of H-aggregates in agreement with the cofacial packing derived from the X-ray measurements. Polarized absorption spectroscopy with variable angle of incidence was used to resolve two perpendicular optical transitions in the visible range, one at 460 nm polarized perpendicular to the columnar direction, in the plane of the film, and one at 420 nm polarized along the film normal.

Single wall carbon nanotube absorber with polarization absorption grown by vertical evaporation method

We have demonstrated single wall carbon nanotube absorber fabricated by vertical evaporation method. The saturation intensity of the absorber is 600 μJ/cm2 at 940 nm and 200 μJ/cm2 at 1060 nm. The modulation depth of the absorber is as high as 4% at 940 and 1060 nm. Furthermore, we found the Modulated probe signal is related to the polarization of the pump laser in pump-probe experiment.

Spectroscopic studies of KGd(WO 4) 2:Ho 3+ single crystals

Single crystals of KGd(WO 4) 2 doped with Ho 3+ ions were grown by the top seeded solution growth method. Polarized room temperature absorption spectra were analyzed by means of the conventional Judd–Ofelt theory taking into account strong dependence of the host refractive index on the wavelength. In addition to the intensity parameters Ω 2, Ω 4, Ω 6, the branching ratios and radiative lifetimes were estimated for all possible transitions in the studied spectral region. The transitions predicted by the phenomenological model as potential transitions for laser applications are discussed. Emission spectra in the green, red, and near-infrared spectral regions were recorded for different excitation wavelengths. Comparison with spectroscopic properties of Ho 3+ ion in other crystals is discussed.

Synthesis, growth, and characterization of Nd-doped SrGdGa3O7 crystal

A disordered Nd:SrGdGa3O7 (Nd:SGGM) laser crystal was grown by the Czochralski method. The space group and effective segregation coefficient of Nd3+ were determined to be P4¯21m and 1.36, respectively. Thermal properties, including the average linear thermal expansion coefficient, thermal diffusivity, specific heat, and thermal conductivity were measured. It was found that the thermal conductivity increases with increasing temperature, indicating glasslike behavior. Sellmeier’s equations were fitted by measuring the refractive indices in the range of 253–2325 nm. The polarization absorption and emission spectra were measured at room temperature, and Judd–Ofelt analysis was carried out to calculate the fluorescence branching ratios from the upper F43/2 state and the fluorescence lifetime. The stimulated emission cross section for the F43/2→I411/2 transition was calculated to be 2.00×10−20 cm2. Finally, a diode-pumped laser experiment at 1.06 μm is described. Thermal, optical, and laser properties have shown that Nd:SGGM crystals are promising for use as disordered laser materials.

Optical properties of [formula omitted] ions in [formula omitted] for completely-solid-state cw UV laser emission

Eu-doped BaY 2 F 8 single crystals have been grown with two different Eu ion concentrations: 0.5%, 1% Eu doping levels. It was found that part of the Eu 3 + ions added in the melt were reduced during the growth process and the ratio between the Eu 2 + and Eu 3 + content in the crystal depends on the duration of the growth process. A complete room-temperature polarized spectroscopic characterization of the divalent Eu ions in this host crystal is presented with particular insights in the laser potentialities of the compound in the UV region. Polarized absorption, emission and excitation spectra are presented together with the lifetime measurement of the emitting level. It was found that due to the weak nephelauxetic effect the 4 f 7 → 4 f 6 5 d 1 band is located at higher energy than the 4 f → 4 f ( 8 S 7 / 2 → 6 P 7 / 2 ) transition. As a result, when pumped in the 200– 300 nm range, this crystal shows an interesting emission that extends from 351 to 366 nm and has a decay-time as long as τ = ( 1.5 ± 0.1 ) ms . This opens the possibility to obtain completely-solid-state laser emission in the UV region with potentialities for cw or mode-locked emission.

Contribution of the Two-Photon Absorption to the Third Order Nonlinearity of Au Nanoparticles Embedded in TiO2 Films and in Ethanol Suspension

We measured the absorptive and refractive nonlinearities in Au nanoparticles when they are embedded in a TiO2 film and when they are suspended in ethanol. The morphology of the nanoparticles was estimated by using HRTEM microscopy. Different contributions related to electronic polarization and two-photon absorption were observed in the samples using a self-diffraction technique with pulses of 26 ps at 532 nm. Transmittance experiments were performed in order to confirm the mechanisms of optical absorption. The Au nanoparticles were grown by the photoreduction of TiO2 sol−gel solutions which contain Au3+ ions. The thin films were prepared by using the dip coating technique with fresh UV exposed sol−gel solutions. We observed that when the sample does not present any important nonlinear absorption it is possible to enhance the participation of the nanoparticles in the optical Kerr response of the media.

In Plane Orientation of SWCNT Ultrathin Film Fabricated Using a Liquid–Liquid Interface

Non-chemically treated single-walled carbon nanotubes (SWCNTs) were assembled at a liquid–liquid interface in the shape of an ultrathin film. SWCNTs were dispersed into water phase by aid of sodium dodecyl sulfate (SDS) and were assembled at a hexane-water interface with addition of ethanol. The assembled film was transferred onto a solid substrate at various dipping speeds. Polarized absorption spectra of the transferred film indicated that the SWCNTs aligned with their long axis parallel to the dipping direction when the assembly was transferred at the dipping speed of 50 mm/min.

A Versatile Molecular Layer-by-Layer Thin Film Fabrication Technique Utilizing Copper(I)-Catalyzed Azide−Alkyne Cycloaddition

We have developed a rapid and versatile layer-by-layer (LbL) thin film fabrication method using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) or "click" chemistry in the construction of molecular multilayer assemblies. Multilayers containing synthetic porphyrins, perylene diimides, and mixtures of the two have been constructed in order to highlight the versatility of this method. Characterization of thin films using UV-vis absorption, water contact angle, and electrochemical techniques indicate that multilayer growth is consistent over tens of layers. Preliminary X-ray reflectivity measurements yield an average bilayer thickness of 2.47 nm for multilayers of 1 and 3 grown on glass. Polarized absorption measurements suggest that the dense thin films exhibit moderate ordering in their molecular structure with partial alignment with respect to the surface normal.

Novel liquid crystalline hyperbranched polyesters with interior 4,7-diphenyl-2,1,3-benzothiadiazole units

Novel hyperbranched (HB) polyesters with interior 4,7-diphenyl-2,1,3-benzothiadiazole (DBT) units were prepared by melt polycondensation of a dimethyl ester derivative of DBT (BMBT) (A2 monomer) with 2-hydroxymethyl-2-methyl-1,3-propanediol (HMP) (B3 monomer) at various mole ratios (A2/B3) via an A2 + B3 approach. The liquid crystalline (LC) and optical properties were investigated. FT-IR and 1H NMR spectroscopies revealed that the HB polymers have good solubility in chloroform, and can be synthesized without gelation during the polymerization with a degree of branching (DB) of 36–94%. The branched structures of HB polymers are dependent on the feed mole ratio. The HB polymer prepared from a mole ratio of A2/B3 = 1/3 had the highest number average molecular weight (Mn) and a more linear structure than the other HB polymers (A2/B3 = 3/1, 2/1, 3/1 and 1/2), which had the lowest DB values. DSC measurements, polarizing microscopy observations and X-ray analyses suggested that the HB polymer (A2/B3 = 1/3) forms the smectic A phase, whereas the others formed the smectic C phase. UV-vis and photoluminescent (PL) spectra in the solutions and in films indicated that the HB polymers show maximum absorbances and yellow-light emission attributable to the DBT unit. The PL spectra of HB polymers in film showed another broad peak, which was likely to be due to interchain interactions or aggregations, at longer wavelengths. Relative quantum yields (Φ) of the HB polymers in the chloroform solutions were lower than those of HB polymers comprising the 2-phenylbenzothiazole unit. Polarized absorption and fluorescent spectra revealed that the HB polymers have poor orientation properties and the luminescent properties are independent of the orientation structures of HB polymers.

Long-Term Operations of Polymeric Thin-Film Transistors: Electric-Field-Induced Intrachain Order and Charge Transport Enhancements of Conjugated Poly(3-hexylthiophene)

This study focuses on the microstructural modifications of regioregular poly(3-hexylthiophene) (rr-P3HT) in the small active channel of thin-film transistors (TFTs) during operations. Polarized absorption and micro-Raman spectroscopy analyses allow us to probe directly the conformation transitions of rr-P3HT chains parallel or perpendicular to the channel by means of exciton bandwidth, interchain electronic coupling, and effective conjugation length. The results of absorption spectra and a joint experimental−theoretical study of Raman spectra show that an external source-to-drain electric field can align rr-P3HT chains parallel to the channel, improving electrical performance after long-term operations, especially charge transport properties. In comparison, the applied external gate field induced an increase in amorphous fraction of the rr-P3HT films. After the analysis, we propose a chain rearrangement model driven by an external electric field to interpret the changes of the effective conjugation length of rr-P3HT, rather than thermal annealing. Our observations provide a thorough explanation for the previously unknown relationships of structure−electronic properties under the extended operations of polymer TFT devices.

Theory-inspired development of organic electro-optic materials

Real-time, time-dependent density functional theory (RTTDDFT) and pseudo-atomistic Monte Carlo–molecular dynamics (PAMCMD) calculations have been used in a correlated manner to achieve quantitative definition of structure/function relationships necessary for the optimization of electro-optic activity in organic materials. Utilizing theoretical guidance, electro-optic coefficients (at telecommunication wavelengths) have been increased to 500 pm/V while keeping optical loss to less than 2 dB/cm. RTTDDFT affords the advantage of permitting explicit treatment of time-dependent electric fields, both applied fields and internal fields. This modification has permitted the quantitative simulation of the variation of linear and nonlinear optical properties of chromophores and the electro-optic activity of materials with optical frequency and dielectric permittivity. PAMCMD statistical mechanical calculations have proven an effective means of treating the full range of spatially-anisotropic intermolecular electrostatic interactions that play critical roles in defining the degree of noncentrosymmetric order that is achieved by electric field poling of organic electro-optic materials near their glass transition temperatures. New techniques have been developed for the experimental characterization of poling-induced acentric order including a modification of variable angle polarization absorption spectroscopy (VAPAS) permitting a meaningful correlation of theoretical and experimental data related to poling-induced order for a variety of complex organic electro-optic materials.

Extended X-ray Absorption Fine Structure of Copper(II) Complexes at the Air-Water Interface by a Polarized Total-Reflection X-ray Absorption Technique

Copper(II) complexes spread on an aqueous solution surface were studied by a polarized total-reflection X-ray absorption fine structure (TR-XAFS) technique. The polarized TR-XAFS spectra at the Cu-K edge for copper(II) porphyrins and copper(II) chlorophyllin in a monolayer were measured in situ at the air-water interface. The polarization dependences of X-ray absorption near-edge structure (XANES) involving a 1s-->4p(z) transition allowed us to estimate the molecular orientation and the local coordination structure around the copper(II) atom in the polarization plane selectively. The extended X-ray absorption fine structure (EXAFS) region of the polarized TR-XAFS spectra for the metal complexes present at the air-water interface was successfully analyzed for the first time. The relative coordination number for the copper center evaluated from the EXAFS analysis indicated larger values in the vertical polarization than in the horizontal one, in agreement with the standing-up molecular orientation at the air-water interface estimated from the XANES region.

A Study of Simultaneous Patterning and Alignment of Semiconductor Nanorods via Polymerization-Induced Phase Separation

We report simultaneous patterning and alignment of semiconductor nanorods (NRs) in nanorod-polymer films by using photolithographic polymerization-induced phase separation (PIPS). Exposure of the nanoparticle-monomer mixture to UV irradiation through a mask resulted in the site-specific photoinitiated polymerization of the monomer, which was followed with flow of the NRs from the areas rich in polymer to the areas rich in monomer. The orientation of NRs in the direction of flow was trapped in the polymerized films and characterized in polarization absorption experiments.

Green polarized electroluminescence from poly （9, 9-dioctylfluorene-alt- benzothiadiazole）

Polarized green electroluminescence has been achieved at the peak wavelength of 550 nm from alternate copolymer poly （9, 9-dioctylfluorene-alt- benzothiadiazole） （PFBT）, which was synthesized by polycondensation. The liquid crystalline state of this copolymer under certain thermal treatment results in the uniaxial orientation of the conjugated backbones on an alignment layer. A rubbed-hole injection layer poly（3,4-ethylenedioxythiophene）:polystyrenesulfonate （PEDOT:PSS） was utilized as the alignment layer. Polarized absorption and photoluminescence spectra indicate the good alignment of the copolymer films with an ordering parameter of 075. Polarized polymer light-emitting diodes were fabricated with the structure of ITO/PEDOT:PSS/PFBT/LiF/Al. An electroluminescence polarization ratio of about 4 was achieved at a voltage of 15 V, with the luminance of 450 cd/m2 and luminous efficiency of 021 cd/A.