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.
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.
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.
The optically induced switching of material properties is important for investigations of opto-electronic effects and optomechanical
properties. Investigated organic materials contain chromophore dipole consisting of acceptor and donor
groups bridged by a delocalized &pgr;-electron system. Both calculations and experimental data show a reversible highly
dipolar photoinduced intra molecular charge transfer in indandione type molecules (DMABI) accompanied by change of
the sign and the value of the dipole moment. Investigations of optical properties of thin host-guest polymer films show
that the photoinduced process of DMABI is related to the photoinduced switching between two equally stable states of
the molecule. In this work first results of formation of the surface relief in polymer films incorporated with DMABI
derivatives will be presented. The refractive index gratings of DMABI host-guest films show that red light is less
diffracted than blue one. The reversible surface potential changes on irradiation in photoinduced intramolecular electron
transfer band in polymer host-guest films is observed. The DMABI molecules in solid state have nonlinear optical
properties, which can be used and investigated in host-guest polymer matrix. The influence of concentration of DMABI
molecules on photoinduced processes is discussed.
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.
During the last years, there has been an increasing interest in the photoinduced switching effects. Among the objects of
concern azobenzene derivatives, isomerizing under UV and visible light, occupy a prominent position. To understand the
photoresponse of these materials in the condensed phase, their spectroscopic and electrical properties are studied. It is shown
that the photoresponse depends on the orientation of the molecules and their packing. A number of novel azobenzene
derivatives containing a N,N-dicyclohexyl sulfonamide moiety is synthesized. The derivatives differ in the length of alkyl
chains between the azobenzene moiety and SH or COOH groups. The morphology and the photoinduced switching of the
surface potential of the self-assembled monolayers and Langmuir-Blodgett multilayers are investigated.
The optically induced switching of organic materials with large dipole moments consisting of acceptor and donor groups
bridged by a delocalized π-electron system is important for investigations of optoelectronic effects. To the contrary of
azobenzene molecule, photoinduced change of dipole moment of indandione molecule does not need free volume in thin
films. To modify the dipole moment and also to optimise properties of film formation, a number of novel indandione
derivatives is investigated. Reversible switching of the surface potential of polymer films with incorporated or
chemically attached indandione molecules is observed at irradiation in photoinduced intramolecular electron transfer
wavelength.
The excited state dynamics of N-(4-azaindan-1,3-dion-2-yl)pyridinium betaine (4N-IPB) in various solvents were studied using steady state absorption, fluorescence, and transient absorption measurements. The 4N-IPB molecules in solutions show weak fluorescence and possess a very large Stokes-shift of the fluorescence band even in non-polar solvents with low dielectric constants. The relaxation rate of 4N-IPB in alcohols faintly depends on the solvent viscosity. The excited state relaxation mechanism involving two excited states is similar to that of the IPB molecule, however the excited state relaxation of 4N-IPB is slightly faster.
We have synthesized and investigated a novel electrondonor -- electronacceptor type photoactive amphiphilic azobenzene derivative containing the hydrophobic dicyclohexylamine moiety along with the hydrophylic carboxyl group at the opposite end of the molecule. Reversible trans/cis photoisomerization of this compound in solutions and in Langmuir-Blodgett multilayers has been observed on irradiation with alternating UV (360 nm) and visible (450 nm) light. The reversible changes of absorbance of main absorption band and surface potential of Langmuir-Blodgett multilayers on alternate irradiation is demonstrated.
Organic materials have received considerable attention because of their large dipole moments and optical nonlinearities. Organic materials for photonic applications contain chromophore dipoles consisting of acceptor and donor groups bridged by a delocalized π-electron system. Both calculations and experimental data show a reversible highly dipolar photoinduced intramolecular charge transfer in betaine type molecules and trans/cis photoisomerization in azobenzene derivatives, accompanied by change of the sign and the value of the dipole moment. The switching is important for optoelectronic effects including second harmonic generation. Arrangement of polar molecules in films is studied by surface potential measurements.
Fluorescence and reflectance spectra of dipolar N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecular crystals of α and β crystallographic modifications have been studied over a wide temperature range. The luminescence spectral properties have been discussed by means of the self-trapped exciton model. The crystal phase transition in both α and β modifications resulting in the deeply-trapped excitonic state formation has been observed at low temperatures, below 60 K.
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.
The excitation dynamics in indandione-1,3 Pyridinium Betaine (IPB) intramolecular charge transfer molecules in various environments was studied. By comparing the excitation properties of the IPB molecules in solution and those in the crystal form, the influence of the intermolecular interactions on the excited state dynamics is considered. Two types of excited states are revealed in the IPB crystal: the Frenkel exciton states, which cause ultrafast nonradiative excitation decay, and the intermolecular charge transfer exciton states positioned below the Frenkel states, that have a longer lifetime and are responsible for pronounced photocurrent efficiency of IPB solids.
The present publication is devoted to the studies of the azo- hydrazone tautomerism of some 3,5-dialkyl substituted azobenzenes. Four new 3,5-dialkyl-4-hydroxy azobenzenes containing electron acceptor SO2 groups were synthesized and investigated in solutions, vacuum evaporated (VE) films and doped polymer films. We have observed that azo-hydrazone tautomeric equilibrium depends on the structure of the molecule and the polarity of the solvent. VE films of compounds containing a primary sulfonamide group SOP2NH2 exhibit predominating azo form absorption band at about 365 nm, which decreases or disappears on UV irradiation giving rise to characteristic hydrazone absorption band at 430 nm. For the thin VE films of related derivatives containing a secondary sulfonamide group SO2NHC4H9 we have observed predominating hydrazone form absorption band at about 430 nm or a broad band at 380 - 430 nm suggesting a tautomeric mixture. Reversible trans-cis isomerization of 3,5-dimethyl-4- hydroxy azobenzenes containing secondary sulfonamide group in the doped PMMA film was investigated.
The present publication is devoted to the studies of the structure of the Langmuir-Blodget films of (methylacylamino)azobenzene derivatives and their electrical properties. Structural studies of monolayer have been carried out by atom force microscopy. The molecules are closely packed in a hexagonal crystalline array. UV irradiation causes a rearrangement of the molecules and a significant structural modification in the monolayer is observed in the frontier region between two crystalline domains of different orientations. The anisotropy of dc conductivity sigma is observed in multilayer assemblies. A low dc conductivity in the bulk sample of the order of sigma(perpendicular) approximately equal to 10-1010-13 S/cm is obtained. In the case of in-plane conductivity the value of conductivity is of the order of sigma(parallel) approximately equals 10-8- 10-10 S/cm. The several sets of hole trapping states are situated at energy depth of Et1 equals 0.55 plus or minus 0.03 eV and Et2 equals 0.48 plus or minus 0.03 eV with the value of the total density of given set of traps of the order of 1012 cm-3. The frequency dependence of capacitance and conductance is nonlinear, with capacitance values decreasing exponentially with higher frequencies while the conductance increases gradually. The behavior is one typical for low mobility materials and the conduction mechanism can be explained in terms of hopping conduction between localized states. The linear dependence of dc in-plane conductivity on relative humidity was observed.
Five amphiphilic alpha- or beta-amino acid derivatives containing photoactive azobenzene moiety and bearing amide or sulfonamide functional groups were synthesized and mono- and multilayers, prepared by Langmuir-Blodgett technique, were investigated. The reversible photoisomerization processes in solutions of these derivatives were observed. Reversible trans/cis photoisomerization of 4-[4-(hexyl- palmitoylamino)- phenylazo]benzenesulfonyl-(beta) -alanine in multilayer assemblies has been observed on alternate irradiation with UV light (centered at 360 nm) and visible light (452 nm).
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