Senior researcher at Institute of Solid State Physics, Univ. of Latvia
SPIE Involvement:
Author
Area of Expertise:
Thin organic films ,
Electroluminescence in organic compounds ,
Photoluminescence properties of organic meterials ,
Organic photovoltaic ,
Charge carrier trap states in organic films ,
Charge carrier mobility
Publications (28)
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KEYWORDS: Temperature metrology, Quantum efficiency, Metals, Thin films, Luminescence, Copper, Chemical species, System on a chip, Scattering, Phosphorescence
Materials for organic light-emitting diodes could be divided into several generations were last 3rd generation organic compounds exhibiting TADF which exhibit up to 100% singlet and triplet state conversation to light without the presence of heavy metal. Organometallic complexes consisting of Cu metal atom is a conceptually different class of low-cost TADF emitters. While this element does not exhibit fast room-temperature phosphorescence, it still can promote a rapid TADF, despite the considerable ΔEST gaps met in such compounds (< 0.05 eV). In this work, we present a novel approach towards TADF emitters where charge transfer through-space and metal-assisted take place. Cu complexes bearing carbazolide and carbene ligands were synthesized. Imidazole-based N-heterocyclic carbenes (NHCs) were used, with electron-accepting sulphonyl groups introduced at 4-position of the N-bound 2,6-diisopropylphenyl (Dipp) substituents. Photoluminescence bands, decays and quantum yields of complexes in various solutions, as well as PMMA-doped films with emitter concentration 5 wt%, were taken. Emission properties of thin films in the temperature diapason from 10 to 300 K were measured to estimate the charge transfer mechanism. For PL bands at room temperature excitation wavelength was either 300 or 375 nm. For temperature-induced CT emission band shifts in vacuum 345 nm excitation wavelength was used, delay 50 ns and width 10 μs. To get PL decays at room temperature a laser with a wavelength of 375 nm was used and decay was measured at the peak wavelength. For temperature-induced decays in vacuum used excitation was 345 nm, time range 50 µs. To get quantum yields at room temperature, emission and scattering ranges of the sample were measured separately with 375 nm excitation. Photoluminescence quantum yield from 0.2 to 0.3 was obtained in solutions but in thin films, it increased up to 0.9 due to the decreased torsional degree of freedom. Emission decays could be described by three exponential functions in all temperature intervals. One featuring a prompt and two delayed emission components. For both delayed emissions, the fastest and slowest emission rate decrease is observed throughout the cooling range, designating a thermally activated nature of the emissive process. The corresponding ΔEST values were estimated from Arrhenius plots and are in the range of 0.0062−0.0075 eV. By combining through-space CT architecture, giving compounds the characteristic low ΔEST values, with the presence of a heavy metal atom, which provides small, but still present SOC.
Organic solids can be used as emitters in organic solid-state lasers. As one of the most popular red light-emitting organic laser dyes is DCM that has been widely used in a dye laser. It has been used in solid-state lasers. The active material was host-guest systems where DCM was doped at low concentration due to large intermolecular interactions. We previously developed original glass-forming DCM derivatives were attached bulky triphenyl groups reduced intermolecular interaction thus it was possible to observe amplified spontaneous emission (ASE) in the neat films.
In this work, we are presenting new derivatives with modified electron acceptor and donor groups. The investigated compounds are pyranyliden derivatives with malononitrile and 1H-indene-1,3(2H)-dione as acceptor and bis(5,5,5-triphenylpentyl)amino- and 5,5,5-triphenylpentyl-piperazin-1-yl- as donor. Photoluminescence and ASE were investigated in thin films. All compounds emit light in red spectral regain with the photoluminescence quantum yield of up to 6%. Amplified spontaneous emission excitation threshold energy was about 100 μJ/cm2.
Silver nanoparticles were used to increase the performance of the compounds. Triangular nanoparticles with the surface plasmon resonance (SPR) overlapping the absorption and emission spectra of compounds were used. Metal nanoparticles were dissolved in the organic solution with investigated compounds. The solution was ultrasonicated prior to deposition on a glass substrate by the spin-coating method.
SPR enhance the absorption and emission of the investigated compounds. Accordingly, it was observed as at least two times increase in the PLQY of mixed thin films and more than two times were decreased excitation threshold energy of ASE. More detailed results and metal nanoparticle influence on emission properties of investigated compounds will be discussed.
Metal nanoparticles have been very active research object due to the local surface plasmon resonance effect that can be used in various applications, such as sensors, photovoltaic and Raman spectroscopy. Metallic nanoparticles enhanced fluorescence is an important effect to improve the emission properties of the semiconductor. It offers higher emission efficiency and decreased lifetime via the local electric field enhancement effect of metal nanoparticles induced by its surface plasmon resonance. Initially, inorganic semiconductors were used to show the emission enhancement, but recently more systems consist of organic semiconductors. It is driven by the growing potential of organic materials in the light-emitting diodes, photovoltaics, and solid-state lasers. Synthesis of specific size and shape of nanoparticles is important because it determines the spectrum of surface plasmon resonance which in turn should match the emission spectrum of an organic dye to get the highest enhancement. Silver nanoparticles typically are synthesized in aqueous solution and their transfer to organic solvents is required for application purposes in organic luminophores emission intensity enhancement. We studied prismatic and spherical silver nanoparticles transfer from aqueous to organic media by ultrasonic treatment and shell changing processes. Photoluminescence properties – emission maps, photoluminescence quantum yield and lifetime of obtained nanoparticles solutions mixtures with organic luminophore DWK-1-TB (2-(2-(4-(bis(2-(trityloxy)ethyl)amino)styryl)-6-methyl-4H-pyran-4-ylidene)malononitrile) were studied. Improvements of photoluminescence properties was observed in nanoparticles based organic luminophore solutions. Presence of nanoparticles increased emission intensity, improved photoluminescence quantum yield and shifted excitation wavelength.
The luminescence of organic semiconductors can be enhanced by surface plasmon resonance on metal nanoparticles, so it can pave the way for the development of light modulators, laboratory chips or sensors based on organic semiconductors. Silver nanoprisms and nanospheres are two-dimensional plasmonic nanostructures that have attracted massive attention due to their strong shape-dependent related applications and optical properties. When the lateral dimension of silver nanoplates is much larger than the thickness, they possess a very great degree of anisotropy, which favors a high tunability of their localized surface plasmon resonance and maximum electromagnetic-field enhancement therefore is generated. Within this work, silver nanoparticles (nanoprisms and nanospheres) of different sizes were obtained in aqueous solution, which were stabilized with sodium citrate and PVP (Polyvinipyrrolidone). Most luminophores are insoluble in water, so nanoparticles had to be transferred to the organic environment. This was achieved using the ultrasonic method. The obtained nanoparticle solutions were added to organic luminophore DWK-1-TB solution. Before nanoparticles were applied to thin films together with organic luminophore (DWK-1-TB), their properties (absorption, quantum yield, excitation, emission) were studied. Thin films were coated from these solutions. Thin films were applied by spin coating and blade casting method. Different concentrations of silver nanoparticles solutions were coated, as well as films of different thickness. PMMA (polymethyl methacrylate) was added because the nanoparticles efficiently transfer into the PMMA solution and its presence at low concentrations improves the homogeneity of the thin film. Quantum yield, change in luminescence depending on the type of nanoparticles, as well as film thickness were studied in the obtained samples.
The synthesis of a novel imidazolidine type N-heterocyclic carbene (NHC) Cu (I) complex with asymmetrically attached phenylsulfonyl- acceptor group is presented. The asymmetrical ligand was used for the preparation two Cu(I) carbene-metal-amide (CMA) complexes with carbazolide (complex 1) or 1,8-dimethylcarbazolide (complex 2) as the amide donors. Obtained complexes exhibit efficient thermally activated delayed fluorescence (TADF) with luminescence quantum yields (QY) reaching 0.80 in PMMA matrix. Metal promoted through-space charge transfer approach in the emitter design lowers the ΔΕST gap and the small spin-orbit coupling (SOC) provided by metal atom enables high radiative rates (kr = 2.21×105 s-1 for complex 2). The photophysical properties of the asymmetrical (1-2) and previously reported symmetrical (3-6) molecular designs are compared. Obtained results suggest a closely similar photophysical behavior for both the asymmetrical and symmetrical CMA complexes.
The impact of metal atom on the photophysical properties of luminescent organometallic carbene-metal-amide complexes exhibiting through-space charge transfer is investigated. The substitution of copper atom with gold alters the excited state energy level configuration of the emitter. While in the copper-based emitters the lowest triplet excited state (T1) is related to a through-space charge transfer between the carbazolide donor and carbene-bound phenylsulphonyl acceptor, in the gold-bearing structural analogue T1 level is accompanied by an additional closely situated triplet state T2, which features a charge transfer between the carbazolide donor and imidazolidine carbene acceptor. Because of a significant spin-orbit coupling provided by Au atom T2 state exhibits relatively fast phosphorescence rate of 8×104 s-1. Consequently, the emissive process for the gold-functionalized compound can be characterized with a co-occurring thermally activated delayed fluorescence (TADF) and phosphorescence, in contrast to the copper-based structural analogues, where only TADF is observed.
A series of 2-(1-benzyl-2-(styryl)-6-methylpyridin-4(1H)-ylidene) fragment containing glassy organic compounds have been synthesized from relevant luminescent 4H-pyran-4-ylidene derivatives and investigated as potential solution processable emitters. Glass transition temperatures of synthesized 1H-pyridine compounds are above 100°C with thermal stabilities higher than 260°C. In the solutions of dichloromethane their absorption bands are in the range from 350 nm to 500 nm with photoluminescence from 500 nm to 650 nm. In a contrary to the 4H-pyran-4-ylidene derivatives, the incorporation of various electron acceptor fragments within the 1H-pyridine fragment containing molecules only slightly influenced absorption and photoluminescence band shifts. Based on these compounds, neat spin-cast films were obtained and investigated as light-emitting mediums which show amplified spontaneous emission (ASE) with λmax in the range from 603 nm to 615 nm with ASE threshold values as low as 46 μJ/cm2. Synthesized 1H-pyridine derivatives show perspective to be applied as solution-processable components for light-amplification studies.
A series of 2,6-bis-styryl-4H-pyran-4-ylidene fragment containing glass-forming organic compounds with bonded amorphous phase promoting bulky triphenyl moieties through piperazine structural fragment [2-(2-(4-(bis(2-(trityloxy)ethyl)amino)styryl)-6-methyl-4H-pyran-4ylidene)malononitrile derivatives (DWK)-T dyes] in a form of (5,5,5-triphenylpentyl)piperazin-1-yl)styryl)-substituent attached to the 4H-pyran-4-yliden fragment in two-position have been synthesized and investigated as the potential light-amplification medium for organic solid-state lasers. DWK-T dye physical properties also depend on the structure of the other styryl-substituent attached to the 4H-pyran-4-ylidene backbone fragment in six-position. Thermal stability of synthesized dyes is above 312°C with the glass transitions from 97°C and up to 109°C. Obtained neat pure spin-cast films based on these compounds show photoluminescence with λmax in range from 672 to 695 nm, ASE λmax from 690 to 704 nm with ASE threshold values in range from 327 to 1091 μJ / cm2, which are mostly influenced by the nature of the electron transition characteristics of various four substituents in a 6-styryl-fragment. The proposed synthetic approach could be useful for obtaining chemically stable and covalently bonded bulky triphenyl group containing glassy dyes, while the synthetic design allows to acquire different nonsymmetric 2,6-bis-styryl-4H-pyran-4-ylidene fragment-containing compounds for red and infrared light-emitting and light amplification applications.
A series of 2,6-bis-styryl-4H-pyran-4-ylidene fragment containing glassy organic compounds with chemically stable bonding of amorphous phase promoting bulky triphenyl moieties through piperazine structural fragment (DWK-T dyes) in a form of 2-(5,5,5-triphenylpentyl)piperazin-1-yl)styryl)-substituent have been synthesized and investigated as the potential gain medium component for organic solid state laser applications. Physical properties of the dyes vary and are mostly depending from the other styryl-substituent attached to the 4H-pyran-4-ylidene backbone fragment in 6-position. Thermal stability of synthesized dyes is above 312°C with the glass transitions from 97°C to 109°C. Obtained neat spin-cast films based on these compounds exhibit photoluminescence with λmax in range from 672 nm to 695 nm, ASE λmax from 690 nm to 704 nm with ASE threshold values in range from 327 μJ/cm2 to 1091 μJ/cm2. Parameters are mostly influenced by the electron affinities of various 4-substituents in 6-styryl-fragment. The proposed synthetic approach could be useful for obtaining stable covalently bonded bulky triphenyl group containing glassy dyes while the synthetic design allows to acquire different non-symmetric 2,6-bis-styryl-4H-pyran-4-ylidene fragment containing compounds for infra-red light-emitting and light amplification systems.
Organic light emitting diodes (OLED) have found their applications in the mobile and TV screens. Till now the commercially available diodes are made by expensive thermal evaporation in a vacuum. The costs of OLED fabrication could be decreased by applying low-cost wet casting methods, for example, spin-coating. In this work, we have studied a group of blue light emitting purine derivatives which could potentially be used in OLEDs. The advantage of these compounds is their ability to form amorphous thin films from solutions. All the thin films were prepared by the spincoating method from chloroform solution on ITO glass. The position of hole and electron transport energy levels is important for efficient OLED fabrication. Ionization energy was determined using photoelectron yield spectroscopy. The gap between ionization energy and electron affinity was determined using photoconductivity measurements. Electron affinity (Ea) then was calculated as a difference between ionization energy (I) and photoconductivity threshold value (Eth). Changes in the energy level values depending on the molecule structure were investigated. The position of electron acceptor group strongly affects the gap between ionization energy and electron affinity, while with the help of the attached substitute groups it is possible to alter the ionization energy. Fine “tuning” of the ionization energy values can be achieved by altering the length of the “tail” where the inactive bulky group is attached.
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.
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.
During the last two decades, small organic molecules have been widely studied for potential applications in organic solid-state lasers due to low-cost production, simple processing possibility and physical property tuning ability through chemical structure synthetic modifications. One of the most investigated and applied compound in dye lasers is 4- (dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). It has shown remarkable properties as a dye in solid-state lasers. One of the drawbacks of this compound is high intermolecular interactions which reduce emission efficiency. Therefore it can be applied only in doped systems in low concentration (around 2 wt%). Recently we have demonstrated that incorporation of bulky triphenyl groups in the low molecular mass organic compounds enables the ability to form good optical quality transparent glassy films by solution processing. Additional such bulky groups reduce intermolecular interaction thus increase photoluminescence quantum yield in the thin film. In the presentation, we will show optical properties of new 2-cyanoacetic derivatives where two different bulky groups (9H-carbazole fragment and triphenyl group) are attached to molecule electron donating and accepting parts. Synthesized compounds have light absorption from 400nm to 600nm and photoluminescence from 600 nm up to 800 nm. Dyes with only one incorporated electron donating fragment showed 16% PLQY and ASE excitation threshold values (below 52 μJ/cm2) in neat thin films. Two electron donating fragment containing molecules have PLQY of 7% and ASE excitation threshold 223 μJ/cm2.
In this work photoluminescence and amplified spontaneous emission properties of new original 2-cyanoacetic acid derivative in different concentration mixed in polyvinyl carbazole (PVK) matrix were investigated. Ethyl 2-(2-(4-(bis(2- (trityloxy)ethyl)amino)styryl)-6-tert-butyl-4H-pyran-4-ylidene)-2-cyanoacetate (KTB) is recently synthesised nonsymmetric red light emitting laser dye, that in previous experiments with neat thin films showed low amplified spontaneous emission (ASE) threshold value. Based on PVK high refractive index it has been used as a polymer to ensure the preparation of good planar waveguide. Luminescence quenching is expected in neat amorphous thin films according to previous experiments which reduces photoluminescence quantum yield and increases ASE excitation threshold energy. It could be overcome by a decrease of the intermolecular interactions between laser active molecules by doping them in polymer matrix thereby decreasing photoluminescence quenching effect in the system by increasing distance between organic molecules which in turn results in lowering ASE excitation threshold energy. The lowest threshold value of ASE was achieved at 20wt% of KTB molecule in PVK matrix. Ability to significantly decrease intermolecular interactions and excitation threshold energy of investigated compound in host-guest systems makes it promising to be used as a laser dye in preparation of organic solid state lasers.
One of the well-known red light emitting laser dyes is 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4Hpyran (DCM). Amplified spontaneous emission (ASE) has been widely investigated of DCM molecules or its derivatives in polymer or low molecular weight matrix. The main issue for these molecules is aggregation which limits doping concentration in matrix. Lowest ASE threshold values within concentration range of 2 and 4 wt% were obtained. In this work ASE properties of two original DCM derivatives in poly(N-vinylcarbazole) (PVK) at various concentrations will be discussed. One of the derivatives is the same DCM dye with replaced butyl groups at electron donor part with bulky trytiloxyethyl groups (DWK-1). These groups do not influence electron transitions in the dye but prevent aggregation of the molecules. Second derivative (DWK-2) consists of two equal donor groups with the attached trytiloxyethyl groups. All results were compared with DCM:PVK system. Photoluminescence quantum yield (PLQY) is almost three times larger for DWK-1 concentration up to 20wt% with respect to DCM systems. PLQY was saturated on 0.06 at higher DWK-1 concentrations. Bulky trytiloxyethyl groups prevent aggregation of the molecules thus decreasing interaction between dyes and numbers of non-radiative decays. Red shift of photoluminescence and amplified spontaneous emission at higher concentrations were observed due to the solid state solvation effect. Increases of dye density in matrix with smaller lose in PLQY resulted in low ASE threshold energy. The lowest threshold value was obtained around 29 μJ/cm2 in DWK-1:PVK films.
Photogeneration efficiency and charge carrier extraction from active layer are the parameters that determine the efficiency of organic photovoltaics (OPVs). Devices made of organic materials often consist of thin (up to 100nm) layers. At this thickness different interface effects become more pronounced. The electron affinity and ionization energy shift can affect the charge carrier transport across metal-organic interface which can affect the performance of the entire device. In the case of multilayer OPVs, energy level compatibility at the organic-organic interface is as important. Photoemission yield spectroscopy was used for organic-organic interface study by ionization energy measurements. In this work we studied “sandwich” type samples of two well-known organic photovoltaic materials- poly(3- hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Ionization energy changes at the P3HT/PCBM interface depending on PCBM layer thickness were studied. P3HT layer was obtained by spin-coating while PCBM was deposited on the P3HT by thermal evaporation in vacuum. No ionization energy shift of P3HT was observed. On the contrary, PCBM at the interface with P3HT created additional 0.40eV barrier for hole transport from PCBM to P3HT.
Small D-π-A type organic molecules with incorporated 4H-pyan-4-ylidene (pyranylidene) fragment in their structures show potential in organic photonics - such as materials for organic light emitting diode application studies and organic solid state lasers. Additional incorporation of bulky triphenyl- groups in their structures gives them the ability to form thin amorphous solid films from volatile non-polar organic solvents. Unfortunately, there is still no clear relation between compound organic structures and their thermal and optical properties. In order to investigate the above mentioned regularities we have synthesized a series of tripheyl- group containing derivates of 2,6-bis-styryl-4H-pyran-4- ylidene with different stryryl- substituents and investigated their physical properties. The thermal decomposition temperatures of the obtained glassy 4H-pyran-4-ylidene compounds are in range from 198o to 312oC and depend from electron acceptor and styryl-groups while their glass transition temperatures are in range from 104o to 132oC and are mostly influenced by the electron acceptor fragment. The light absorption of synthesized 4Hpyran- 4-ylidene compounds in solutions of dichloromethane as well as in their solid state are in range from 400 nm to 550 nm. Their photoluminescence spectra in the solid state (from 600 nm to 800 nm) are red-shifted by approximately 50 nm comparing to their photoluminescence spectra in solutions of dichloromethane (from 550 nm to 750 nm) and mostly depends on the N,N-ditrityloxyethyl-aminostyryl-electron donor substituent, 4H-pyran-4-ylidene π-system and on the electron-acceptor group. Synthesized compounds could be used as potential materials for light amplification.
4-(dicyanomethylene)-2-methyl- 6-(p-dimethylaminostyryl)-4H-pyran (DCM) is well known red laser dye which can be used also in solid state organic lasers. The lowest threshold value of amplified spontaneous emission was achieved by doping 2wt% of DCM molecule in tris-(8-hydroxy quinoline) aluminium (Alq3) matrix. Further increase of dye concentration also increases threshold value. It is due to large intermolecular interaction which reduce photoluminescence quantum yield. Compounds with small intermolecular interaction and which exhibit similar amplified spontaneous properties as DCM could be useful for solid state organic lasers. In the work photoluminescence and amplified spontaneous emission properties of two DCM derivatives in poly (methyl methacrylate) (PMMA) matrix were investigated. Bulky trityloxyethyl groups are attached to the donor part of investigated molecules. These groups reduce intermolecular distance wherewith reduce photoluminescence quenching. More than one order of magnitude lower excitation threshold energy of the amplified spontaneous emission was achieved in doped polymer films with investigated compound in comparison to doped polymer with DCM. It means that the investigated compound is more perspective as a laser material compared to previously study.
Organic materials are becoming more popular due to their potential application in electronics. Low molecular weight materials possible produce from solution are in special consideration. It gives the possibility to avoid both thermal evaporation in vacuum, and use of polymers in thin film preparation process. Indandione fragment containing azobenzene compounds are one of such materials. These compounds are good candidates for use in design of novel molecular electronic devices due to their possibility to form amorphous structure from solution thus allowing developing flexible, small size systems with low production costs. In this work three indandione fragment containing azobenzene compounds were investigated. Difference between these compounds is bulky groups which assist formation of amorphous thin film. Absorption spectra of the investigated compounds are similar to P3HT but with higher absorption coefficient. Molecule ionization and electron affinity levels of these compounds are around -5.45eV and -3.80eV, respectively. Combining PCBM with investigated compounds could lead to difference between electron affinity levels maximum of 0.15eV. It is several times less compared to ~1eV for P3HT:PCBM system. Higher difference between the donor ionization level and the acceptor affinity level could also be obtained which should lead to the higher open circuit voltage.
In this work we present simple preparation of original trityl group containing glassy luminescent 6-styryl substituted
derivatives of 2-(2-tert-butyl-4H-pyran-4-ylidene)malononitrile
(DWK-1TB), 2-(2-tert-butyl-4H-pyran-4-ylidene)-2-
ethyl-2-cyanoacetate (KWK-1TB),
2-(2-tert-butyl-4H-pyran-4-ylidene)-1H-indene-1,3(2H)-dione
(ZWK-1TB) and
5-(2-tert-butyl-4H-pyran-4-ylidene)pyrimidine-2,4,6(1H,3H,5H)-trione (JWK-1TB). Their optical properties have been
investigated. The absorption maxima of synthesized glasses is in region from 425 nm to 515 nm and emission maxima is
from 470 nm to 625 nm in solution of dichloromethane. But absorption maxima of their solid films is from 425 nm to
500 nm and emission maxima is in range from 570 nm to 710 nm.
Incorporation of bulky trityloxy ethyl groups combining with existing tert-butyl groups results in thin solid films
formation of synthesized glasses from volatile organic solvents (chloroform, dichloromethane) without them being doped
in any polymer. This makes them perspective for potential applications in organic light-emitting diodes and organic
lasers by simple luminescent layer composition with cheap
wet-casting approaches.
All glasses show good thermal stability with thermal decomposition temperatures from 264° to 318°C and glass
transition values up to 158°C for DWK-1TB. These thermal properties of synthesized glasses could make them also
useful for potential applications in other optical materials such as materials for nonlinear optics.
Low molecular mass organic compounds which make thin films from volatile organic solutions would be great benefit in future
organic light emitting systems. Two most important advantages could be mentioned. First - the repetition of synthesis of small
molecules is better than for polymers. Second - wet casting methods could be used.
In this work we are presenting optical, electroluminescence and amplified spontaneous emission properties of four original glassy
forming compounds containing 2-tert-butyl-6-methyl-4H-pyran-4-ylidene fragment as backbone of the molecule. They has the same
N,N-dialkylamino electron donating group with incorporated bulky trityloxy ethyl groups. The difference of these compounds is in
electron acceptor group. One has 1H-indene-1,3(2H)-dione group, second has pyrimidine-2,4,6(1H,3H,5H)-trione group, third has
malononitrile group and fourth has 2-ethyl-2-cyanoacetate.
Absorption maximum of the compounds is between 420 and 500 nm and is red shifted from weaker acceptor group to stronger one.
The electroluminescence efficiency for simple device ITO/PEDOT:PSS/Organic compound/BaF/Al is low. For the best one with
malononitrile group it was 0.13 cd/A and 0.036 lm/W. It could be increased by optimising the sample geometry or adding addition
layers for charge carrier transport and exciton blocking. But nevertheless the use of these compounds in organic light emitting devices
in neat films is unlikely.
Attached bulky trityloxy ethyl groups and tert-butyl group decrease interaction between the molecules thus allowing to obtain
amplified spontaneous emission in neat thin films for all investigated compounds.
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/m2 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.
SHG efficiency of the poled guest - host polymer system is proportional to the concentration and orientation degree of
NLO active molecules (chromophores). Corona poling realized at elevated temperatures could cause concentration
decrease of NLO- active molecules due to centrosymmetric crystallization. Our studies showed that number density of
crystallites is depending on orientation procedure. To obtain the best orientation procedure for guest - host systems
containing four different chromophores based on dimethylaminobenzylidene 1, 3 - indandione we have compared
optical images and SHG efficiency of corona poled films. According to our observations external poling electric field
applied from the very beginning of the sample heating process can reduce crystallite grow. The optical quality is
improved and SHG efficiency in some cases is up to 1.6 (depending on molecule structure) times larger after our
suggested orientation sequence compared to classic corona poling procedures.
Many organic compounds in solid state have nonlinear optical properties due to the orientation of the molecules in a
polymer matrix. In this work, all-optical poling and second harmonic generation in a composition consisting of 1 mass%
of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) compound in poly(methyl methacrylate) (PMMA) matrix
were studied. Thin films were prepared by solvent casting. The 1.064-μm fundamental and 532-nm second harmonic
wavelengths of a Nd:YAG laser were used. It is shown that DMABI molecules can be oriented by the method of all-
optical poling, and that the process is related to the photoinduced switching between two equally stable states of the
molecule.
Second order non linear optical (NLO) properties of sPMMA based host-guest systems containing eight dimethylaminobenzylidene -1, 3 - indandione (DMABI) related chromophores have been investigated by means of quantum chemical calculations and SHG experimental characterization. Ab initio calculations with basis set 6-31G were used for molecular geometry determination as well as in the calculations of molecular hyperpolarizability by FF approach. Influence of the chromophore concentration on the host-guest film NLO performance was obtained by SHG Maker fringe experiments. The highest value of d53233 =80 pm/V (frequency corrected value d033 =12 pm/V) at chromophore concentration 15 %wt have been recorded for tret-butyl substituted DMABI chromophore.
The studies of two betaine molecules for optically induced intramolecular electron transfer important in photosynthesis and photoelectricity are presented. The investigated betaine molecules possess a large permanent dipole moment changing the sign and value at excitation within the intramolecular charge transfer band (380 - 410 nm). The molecules are mixed with poly(methylmetacrylate) polymer and solvent to cast thin films. The optical density of the intramolecular charge transition band and the change of surface potential of the prepared films are found to decrease at irradiation of the same wavelength. The decrease of optical density is avoided by protecting the polymer film from ambient oxygen. Photo-oxidation of betaine molecules is discussed.
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