Kaspars Traskovskis

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.

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.

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.

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.

Triphenylmethyl and triphenylsilyl based molecular glasses for photonic applications

Triphenylmethyl and triphenylsilyl structural fragments can be used to obtain glass forming, solution processable materials from polar chromophore molecules. Large number of compounds has been synthesized taking advantage of this approach, making it possible to identify some structure-property relations. Regarding the non-linear optical (NLO) properties of the given materials it is evident that triphenylmethyl groups help shielding unwanted NLO efficiency limiting dipolar interactions between polar chromophores in solid films. Chromophore stacking is still observed for compounds with large dipole momentum values. The glass transition temperatures of the compounds increase with the molecular weight for the studied material class, reaching values up to 130°C.

Solution-processable green phosphorescent iridium(III) complexes bearing 3,3,3-triphenylpropionic acid fragment for use in OLEDs

New solution-processable materials based on well-known green iridium(III) heteroleptic complexes (ppy)2Ir(acac) and (ppy)2Ir(pic) were acquired by chemical modification of ppy ligand with functionable hydroxyl groups and subsequent esterification with 3,3,3-triphenylpropionic acid fragment. Photoluminescence quantum efficiencies up to 0.90 were measured for the compounds in solution. Emission characteristics in pure solid films and different guest-host systems with hole transporting materials were investigated. Green light emitting OLEDs (organic light emitting devices) was prepared and characterized.

3,3'-Bicarbazole structural derivatives as charge transporting materials for use in OLED devices

In this study we report novel 3,3′-bicarbazole based charge transporting materials mainly designed for a use in systems containing phosphorescent iridium (III) complex emitters. A low-cost oxidative coupling reaction using FeCl3 was employed in the synthesis of 3,3′-bicarbazole compounds. Different derivatives of 3,3′-bicarbazole with 4-ethoxyphenyland ethyl- substituents at 9,9′- positions and (2,2-diphenylhydrazono)methyl- and 4-(dimethylamino)styryl- substituents at 6,6′- positions were synthesized. Obtained (2,2-diphenylhydrazono)methyl- derivatives exhibit glass transition temperatures that are sufficient for applications in electronic devices. Thin amorphous films of good optical quality can be produced from synthesized materials using spin-coating method. The effect of (2,2-diphenylhydrazono)methylsubstituents at 6,6′- and 4-ethoxyphenyl- substituents at 9,9′- positions on the charge transport properties of the 3,3′-bicarbazole derivatives was investigated. With the introduction of both electron acceptor and donor moieties to 3,3′-bicarbazole structure material electron and hole drift mobilities reach approximately 1·10-5 cm2/V·s. Molecule ionization (If) levels and electron affinity (EAf) levels in thin films were determined using photoelectric effect experiment. Depending on the nature of substituents at 6,6′- and 9,9′- positions If levels range from -5.19 to -5.13 eV and EAf levels are from -2.44 to -2.38 eV.

Dark Relaxation of Holographic Gratings in Azobenzene and Chalcogenide Films

Studies of transmission holographic grating (HG) relaxation (recorded with s-s, p-p, s-p, L- L, L-R laser beam polarizations) have been carried out in organic molecular azobenzene glassy films W- 50 and W-75, and in chalcogenide glassy a-As2S3 film. Results previously obtained with azobenzene molecular glassy K-RJ-9 films also are used for comparison. HG with the periods of 0.50, 2.0 and 8.6 μm were studied in these films. It is found that recording efficiency in the same sa mple depends on both recording polarizations and HG period. The HG stability in the course of relaxation also depends on these factors. Anticorrelation has been observed of HG recording efficiency and stability. Recording was efficient with p-p or L-R polarizations at 2.0 μm HG period whereas it was stable with s-s L-L polarizations at 8.6μm HG period. As a rule, HG stability was higher in the samples with higher glass transition temperature. The HG dark (relaxational) self-enhancement of HG was observed in the case of W-75 (L-R polarizations, 0.50μm; s-s polarizations, 2.0μm) and in previously studied K-RJ-9 (p-p polarizations, 8.6μm) samples. Relaxational changes in diffraction efficiency are explained in terms of post-recording mass transfer under the influence of photoinduced spatially periodic mechanical stress.

Recording and self-enhancement of dynamic polarization gratings in degenerate four-wave-mixing geometry

Scalar and vectorial degenerate four-wave-mixing (DFWM) in azobenzene molecular K-RJ-4-3 [N,N-bis(5,5,5- tryphenylpentyl)-4-((4-tritylphenyl)diazenyl)aniline] and in chalcogenide a-As2S3 glassy films is experimentally investigated. A coherent self-enhancement (CSE) of holographic gratings (HG) in this geometry was experimentally confirmed, for the first time to our knowledge, thus establishing a new method of HG recording, the DFWM CSE recording. Scalar linear s-s, p-p and circular R-R(right) polarizations and orthogonal linear s-p and circular L(left)-R polarizations were used for HG recording with 2 μm period at 532 nm. In the case of K-RJ-4-3 film L-R polarizations were the most efficient enabling the maximum DFWM efficiency ρmax=14.5% whereas in the case of a-As2S3 film s-s polarizations were the best with ρmax=6.0%. DFWM CSE recording has exhibited a different polarization dependence compared to normal DWFM recording. CSE factor ρ/ρ0 (ρ0 being the initial ρ) was the highest for K-RJ-4-3 film (6.8) with s-s polarizations compared to 3 for a-As2S3 film with p-p polarizations. Two-wave HG recording was also studied for comparison.

An improved molecular design of obtaining NLO active molecular glasses using triphenyl moieties as amorphous phase formation enhancers

New molecular design of obtaining molecular glasses has been developed by linking triphenylmethyl moieties to chromophore core by flexible C-C bridge. Compounds capable of forming stable amorphous phase with good optical quality have been acquired with increased chemical and thermal sustainability compared to the previously reported design. NLO activity of compounds has been measured after corona discharge polling. Compared to previously synthesized trityloxy fragment containing compounds increase of d33 coefficient by up to 17 times was achieved for the same chromophore core containing compounds.