Valdis Kokars

Triphenyl moieties as building blocks for obtaining molecular glasses with nonlinear optical activity

The incorporation of trityl and triphenylsilyl groups into low molecular weight molecules allows the formation of stable molecular glasses. A series of materials based on the N-phenyldiethanolamine core was synthesized bearing different azobenzenes and benzylydene-1,3-indandione as active chromophores. Molecular hyperpolarizability of the synthesized compounds was calculated by a restricted Hartree–Fock method with basis 6-31G(d,p) and measured in solutions by hyper-Rayleigh scattering. Non-linear optical (NLO) activity of the thin glassy films was confirmed after a corona poling procedure. Thermal sustainability of the NLO response of up to 85 °C was achieved. Quantum chemical calculations of the compounds revealed increased steric bulk and conformational freedom of the triphenylsilyl moiety. While the presence of the triphenylsilyl group results in more stable glasses and increased material nonlinearity, in the case of trityl groups, measured glass transition temperatures are higher.

New Figure of Merit for Tailoring Optimal Structure of the Second Order NLO Chromophore for Guest-Host Polymers

Reliability to forecast SHG efficiency using two sets of non linear optical (NLO) chromophore figures of merit (FOM) was tested. One of them predicts that SHG efficiency d 33 of the poled guest-host polymer is proportional to ground state dipole ∼ μg, another to ∼ 1/μg. Correlation of maximal achieved second order NLO efficiency of the PMMA based systems containing eight dimethylaminobenzylidene-1, 3-indandione (DMABI) related chromophores with proposed FOM have been analyzed. The best correlations were obtained with second set of FOM, especially if high dipole moment (μg > 7D) chromophores are included in analysis.

Nonlinear optical properties of low molecular organic glasses formed by triphenyl modified chromophores

The series of organic molecular glasses have been studied as possible candidates for nonlinear optical (NLO) applications. Amorphous phase formation of investigated materials is ensured by the presence of bulky triphenyl substituents in molecular structure of NLO chromophores. Linear optical properties as well as NLO coefficients and thermal stability of NLO activity for the 13 molecular materials in glassy thin solid films have been determined. For the benzylidene-1,3-indandione chromophore containing compound the highest d33 value equal to 280 pm/V was measured under the 1064 nm excitation. Among the investigated compounds uppermost achieved thermal sustainability of NLO response was 108 °C. The relationship between number of triphenyl substituents and increased thermal sustainability of nonlinear response was observed.

Synthesis and properties of 1,3-dioxo-1H-inden-2(3H)-ylidene fragment and (3-(dicyanomethylene)-5,5-dimethylcyclohex-1-enyl)vinyl fragment containing derivatives of azobenzene for holographic recording materials

New glassy 1,3-dioxo-1H-inden-2(3H)-ylidene fragment und (3-(dicyanomethylene)-5,5-dimethylcyclohex-1-enyl)vinyl fragment containing push-pull type derivatives of azobenzene able to create thin layers have been synthesized. Thin films of synthesized glasses for holographic recording were prepared using spin coating technique from saturated chloroform solution. Holographic grating recording in films of 6a-b, 7 and 12 has been experimentally studied at 633 and 532 nm in both transmission and reflection modes with p-p recording beam polarizations. The film 12 was found to be the most efficient at both wavelengths in transmission mode exhibiting the maximum self-diffraction efficiency of 9.9% at 633 nm, and 15.3% at 532 nm. The film of 6a was the most efficient in reflection mode with the maximum selfdiffraction efficiency of about 3%.

Stereoselective synthesis and properties of 1,3-bis(dicyanomethylidene)indane-5-carboxylic acid acceptor fragment containing nonlinear optical chromophores

A series of organic push–pull type chromophores using indane-1,3-dione 5-carboxylic acid (IDCA) and novel 1,3-bis(dicyanomethylidene)indane 5-carboxylic acid (CICA) electron acceptor fragments have been synthesized and characterized. NMR and X-ray analysis revealed that condensation reactions with the CICA fragment were stereoselective and yielded benzylidenes and azomethines with E double bond configurations. Due to the non-planar geometry these compounds are chiral and were acquired as a racemic mixture. The subsequent functionalization of the carboxylic acid group with 5,5,5-triphenylpentan-1-ol yielded solution-processable glass forming materials (6, 8, 10, 13) with glass transition temperature values of 76–134 °C. The nonlinear optical (NLO) properties of these compounds were characterized using quantum chemical calculations and second harmonic generation (SHG) measurements in corona-poled thin glassy films. The twisted geometry of the CICA based materials was shown to be beneficial to the macroscopic NLO performance due to the less pronounced solid phase stacking compared to the flat IDCA based compounds. The presence of site isolating groups at both the acceptor and donor ends of the molecule in compound 13 resulted in a considerable NLO efficiency increase. Non-centrosymmetric crystals of CICA based N,N-dimethylaminobenzylidene 7b were obtained and showed a SHG response comparable to urea.

Holographic recording of surface relief gratings in stilbene azobenzene derivatives at 633 nm

Holographic recording in stilbene azobenzene derivatives by He-Ne 633 nm laser light has been experimentally studied. It was found that surface relief gratings (SRG) can be recorded by red light. Usually shorter wavelengths are used to induce the trans-cis photo-isomerization in organic materials. SRG with 2 μm period and an amplitude of 130 nm have been recorded with 0.88 W/cm2 light in about 20 minutes in amorphous films of 3-(4-(bis(2-(trityloxy)ethyl)amino)phenyl)-2-(4-(2-bromo-4-nitrophenyl)diazenyl)phenyl)acrylonitrile spin-coated on glass substrates. Self-diffraction efficiency up to 17.4% and specific recording energy down to 114 J/(cm2%) were measured. The recorded SRG were stable as proved by subsequent AFM measurements. The photo-induced changes in absorption spectra did not reveal noticeable signs of trans-cis transformations. Rather, spectrally uniform bleaching of the films took place. We conclude that a photothermally stimulated photo-destruction of chromophores is responsible for the SRG recording. The recording of stable SRG in the stilbene azobenzene derivatives we studied is accompanied by the recording of relaxing volume-phase gratings due to the photo-orientation of chromophores by the linearly polarized recording light. It should also be noted that holographic recording efficiency in stilbene azobenzene derivatives exhibit an unusual non-monotonic sample storage-time dependence presumably caused by the peculiarities of structural relaxation of the films.

Hologram recording in azobenzene oligomers

Elementary hologram (holographic grating) recording and their coherent optical erasure have been experimentally studied in azobenzene oligomer (ABO) layers differing by their chemical composition, matrices and by the connection type of azobenzene chromophores to the matrix (dispersed or covalently bound). The best holographic parameters (7.9% diffraction efficiency and 86 J/cm2 specific recording energy) were achieved in the samples with covalent bonding to the matrix. Vector recording is also possible. Recording is unstable and reversible. The coherent optical erasure studies have shown its efficiency dependencies on the initial diffraction efficiency, erasing beam intensity and grating period which are different for three groups of ABO samples. The conclusion is made that recording is due to the photoinduced alignment of the azobenzene chromophores followed by refractive index changes. These are the first results and further studies are in progress.

Pyranylidene indene-1,3-dione derivatives as an amorphous red electroluminescence material

The organic light-emitting diode (OLED) has promising applications in flat-panel displays and novel light sources. Thus far, OLED structures have mostly been made by thermal evaporation in vacuum. An alternative approach is to use small molecules that form amorphous (glassy) structures from solutions. Such compounds can be used in ink-jet printing technologies and result in reduced OLED prices. We present an original red fluorescent organic compo- und 2-(2-(4-(bis(2-(trityloxy)ethyl)amino)styryl)-6-methyl-4H-pyran-4-ylidene)-1H-indene-1, 3(2H)-dione (ZWK1), and its derivative 2-(2,6-bis(4-(bis(2-(trityloxy)ethyl) amino)styryl)-4H- pyran-4-ylidene)-1H-indene-1,3(2H)-dione (ZWK2), where the methyl group is replaced with a 4-substituted-styryl group. This change could improve the formation of glassy structures. The thickness of the electroluminescent layer in the device is optimized to the higher power efficiency and obtains: ITO/PEDOT:PSS (40 nm)/ZWK1 (95 nm)/LiF (1 nm)/Al (100 nm), and ITO/PEDOT:PSS (40 nm)/ZWK2 (85 nm)/LiF (1 nm)/Al (100 nm). The maximum of electroluminescence (EL) spectra for the device with the ZWK1 compound is 667 nm, which corresponds to the CIE coordinates x = 0.65 and y = 0.34. The power and luminance efficiency at a luminance of 100 cd/m 2 is 0.63 lm/W and 1.78 cd/A, respectively. Adding an additional 4-substituted-styryl group to the ZWK1 molecule shifts the maximum of EL spectra to the red region (705 nm) and decreases the efficiencies by one order. C 2011 Society of Photo-Optical

Effect of Light Polarization on Holographic Recording in Glassy Azocompounds and Chalcogenides

Light polarization effects on a holographic grating recording in a glassy chalcogenide a-As40S15Se45 film has been experimentally studied and compared with previously studied glassy molecular azobenzene film 8a at 633, using s − s,p − p, CE-1 and CE-2 circular-elliptic recording-beam polarizations (differing by light electric field rotation directions). The azocompound exhibited much higher self-diffraction efficiency (SDE) and diffraction efficiency whereas chalcogenide was more sensitive. Their recording efficiency polarization dependences also were different. SDE up to 45% was achieved in 8a with p − p and up to 2.6% in a-As40S15Se45 with CE-2 polarized recording beams. The polarization changes in the diffraction process were studied as well in these and other materials (11, 16, 19 and a-As2S3 film, LiTaO3:Fe crystal). It was found that light polarization changes in the process of diffraction from gratings recorded vectorially by s−p polarizations depended on chemical composition, wavelength, and exposure time. Vector gratings with SDE up to 25% were recorded in 8a, rotating a linear polarization by 90°. No light polarization changes were found in azobenzene 19 and chalcogenide films and in LiTaO3:Fe crystal, thus showing a vector recording of scalar holograms. The recording mechanisms in azocompounds and chalcogenides are discussed and compared.

Synthesis of Red Luminescent Non Symmetric Styryl-4H-Pyran-4-Ylidene Fragment Containing Derivatives for Organic Light-Emitting Diodes

New organic glassy non symmetric styryl- derivatives of 2(2,6-substituted-4H-pyran-4-ylidene)-malononitrile, 2(2,6-substituted-4H-pyran-4-ylidene)-1H-indene-1,3(2H)-dione and 2(2,6-substituted-4H-pyran-4-ylidene)-pyrimidine-2,4,6(1H,3H,5H)-trione were synthesized. They form thin solid amorphous films from volatile organic solvents (DCM and chloroform). Their spectral properties have been studied.