Influence of chromophore dipole moments in parameters of organic light emitting devices based on phenyl and methyl modified pyrazoloquinoline.

Absorption, photo- and electroluminescence spectra of some trityl substituted 1H-pyrazolo[3,4-b]quinolines derivatives (methyl- and phenyl substituted) and fabrication of the single layered organic light emitting diodes are reported. The bulky trityl substituent was introduced to prevent aggregation and crystallization of the dopant in polymer matrix. Role of ground state dipole moments in the observed red Stokes shift, electroluminescent features and photocarrier transport is explored. The maximally achieved brightness about 50Cd/m(2) is observed in the spectral range extending from 443nm up to 462nm. The voltage threshold was varied from 7.8V up to 10V. The brightness-current dependences show an existence of at least two types of carrier injections.

Optical absorption of bisphenol A based pyrazoloquinoline dimers.

The paper presents the measured absorption spectra for several newly synthesized derivatives of bisphenol A based pyrazoloquinoline dimers (BAPQD). The experimental results are compared with quantum chemical calculations performed by semiempirical method AM1 and PM3. Molecular mechanic analysis shows that BAPQD dyes possess by a large number of equilibrium molecular conformations with similar total energies and small energy barriers between these states; thus at room temperature (T=300 K) the molecular dynamics exhibits slow temporal evolution passing a large number of intermediate slightly non-equilibrium conformational states. For this reason the semiempirical quantum chemical methods have been combined with the molecular dynamics simulations. The semiempirical PM3 method exhibits the best agreement with the experimental data, especially in the prediction of the spectral position of the first absorption band (absorption threshold). Taking into account that such good agreement is found for a set of more complex organic systems like BAPQD this may be of substantial interest for chemical engineering which deals with the designing of new efficient organic luminophores for electroluminescent applications.

Spectral emission properties of 4-aryloxy-3-methyl-1-phenyl-1H-pyrazolo[3,4-b]quinolines.

Spectral emission properties of novel 4-aryloxy-1H-pyrazolo[3,4-b]quinolines were investigated. All of the compounds exhibit strong blue luminescence in the solution and in the solid state as well. Pyrazoloquinolines were used as dopants in PVK matrices in electroluminescent devices with ITO/PVK:PQ/Ca/Al light emitting diode configuration.

Photovoltaic response and values of state dipole moments in single-layered pyrazoloquinoline/polymer composites.

We report the photovoltaic response of composite films formed by polymer transport matrices poly(3-octylthiophene) (P3OT) and poly(3-decylthiophene) (PDT) with incorporated 1H-pyrazolo[3,4-b]quinoline (PAQ) chromophore (see the first figure). The photovoltage (PV) data were obtained for different substituted PAQ possessing different state dipole moments. The photovoltaic cells were formed between ITO and aluminum electrodes. We found that the PV signal of polymer/PAQ substantially depends on the state dipole moments of the pyrazoloquinoline chromophore. This fact indicates on a possibility of significant enhancement of PV efficiency by appropriate variations of the state dipole moments of chromophore. This results in photoinduced electron transfer from polymer serving as donors to PAQ being the electron acceptor. Despite an efficiency of the PV devices is below 1%, however, it may be substantially enhanced in future varying the chromophore state dipole moments appropriately.

Photoinduced transparency in polypyrrole films.

For the first time we have discovered drastic photoinduced effects in the transparency for the pyrrole films. Photoinduced investigations of transparency were performed for the polypyrroles with different number of the pyrrole rings. We have established a considerable increase of the transparency from 70-75% up to 82-84% for the wavelengths of the probing lasers operating within the 530-1040 nm spectral range. The photoexcitation was performed by nanosecond pulsed polarized Nd-YAG laser generating at 1.34 microm. The phototransparent changes are completely reversible and disappear after switching off the laser treatment. The typical relaxation time for the photoinduced transparency is equal to about 10-20 micros and the changes of the transparency are strictly related with the values of state dipole moments of the polypyrroles. As a possible mechanism for explanation of the observed dependences one can consider existence of the charged trapping levels intra the forbidden energy gaps which effectively interact with the electric strength of the external polarized optical field and state dipole moments of the particular polymers.

Influence of bond lengths between substituents and mother molecule on spectral properties of pyrazoloquinolines.

Absorption and luminescent spectra of several new synthesized pyrazolo-quinoline possessing different substituents are studied. Absorption spectra of all the considered compounds possess five relatively strong absorption bands at about 430, 320, 270, 253 and about 230 nm. A correlation between the bond lengths between the substituent molecule and mother molecule with the observed spectral shifts was found. Theoretical spectra obtained within semi-empirical quantum chemical AM1 calculation methods seem to be more widened compared to the experimental ones due to electron-vibration interactions. However generally a good coincidence between spectral positions between experimental and calculated spectral peak positions was achieved. The corresponding experimental spectra have an absorption edges situated at about 430 nm which appears in fairly good agreement with quantum chemical simulations, namely for absorption spectra calculated by semi-empirical AM1-method. The red shifts in the experimental luminescence spectra are a consequence of electron-vibration interactions which increase with the effective radius and polarizabilities of the particular substituents.

Optical spectra of some 6-styrylo-1H-pyrazolo[3,4-b]quinolines.

It was proposed a method of synthesis several 6-styrylo1-1H-pyrazolo[3,4-b]quinolines as promising material for optoelectronics. Particularly, 6-styryl-1,3-diphenyl-1H-pyrazolo[3,4-b]quinolines were prepared by Wadsworth-Emmons reaction. One of advantageous of these materials is stability of their optical spectra versus the possible cis- and trans-transformation. Theoretical and experimental studies of optical absorption and photoluminescence excitation spectra for styryloquinolines were done. They show that the AM1 semi-empirical method gives better results compared to other approaches. We have found that the solvents do not play a role in the behavior of the spectra. The backside-substituted groups do not influence substantially the observed optical spectra.

Optical poling effect and optical absorption of cyan, ethylcarboxyl and tert-buthyl derivatives of 1H-pyrazolo[3,4-b]quinoline: experiment and quantum-chemical simulations.

We present here results of experimental studies and quantum-chemical simulations of optical absorption and optical poling effects performed on a new synthesized cyan, ethylcarboxyl and tert-buthyl derivatives of 1H-pyrazolo[3,4-b]quinoline incorporated into polymer matrix or dissolved in organic solutions. The efficiency of second-order optical susceptibility d vs photoinduced power density I(p) clearly saturates to certain magnitude d(eff) at sufficient power densities (I(p) > or = 1.3 GW cm(-2)). Comparing experimental data and results of semiempirical quantum-chemical simulations one can conclude that there exists generally a good correlation between the magnitude of saturated susceptibilities d(eff) and macroscopic hyperpolarizabilities for all compounds except the chromophore 1,3-dimethyl-6-cyano-[PQ] only. The discrepancy for this compound may reflect a specific contribution of surrounding polymer matrix. According to the quantum chemical analysis the methyl-containing cyan and ethylocarcoxyl derivatives reveal four/five strong absorption bands in the spectral range 200-500 nm. A substitution of the methyl groups by the phenyl group causes the substantial changes of the absorption spectra mainly in the spectral range 240-370 nm. Measured and calculated absorption spectra manifest rather good agreement mainly in the part regarding the spectral positions of the first oscillator (absorption threshold). The quantum-chemical PM3 method shows the best agreement with experiment. At the same time a considerable broadening almost of all absorption bands appears as a characteristic feature of all measured spectra. The discrepancies between the calculated and the measured spectra are attributed to electron-vibronic coupling as well as to a specific rotational dynamics of phenyl rings.

Optical absorption of 1H-pyrazolo[3,4-b]quinoline and its derivatives.

The results of experimental studies and quantum chemical simulations of the absorption spectra of 1H-pyrazolo[3,4-b]quinoline and its derivatives are presented. The quantum chemical calculations (semi-empirical AM1 and PM3 methods) show similarity in the absorption spectra of 1H-pyrazolo[3,4-b]quinoline and 1,3-dimethyl-1H-pyrazolo[3,4-b]quinoline which are characterized by five strong absorption bands in the spectral range 200-500 nm. A substitution of the methyl groups by at least one phenyl group causes the drastic changes of the absorption spectra mainly within the spectral range 240-370 nm. We attribute these differences to additional molecular double bonding segments C=C of the substituted phenyl groups, i.e. to pi --> pi* transitions. A comparison of measured and the calculated absorption spectra manifests quite satisfactory agreement for all compounds in the part regarding the spectral position of the first oscillator (absorption threshold). At the same time, the measured spectra demonstrate the considerable broadening practically of all absorption bands and even complete damping some of them in the case of phenyl derivatives. The experiments performed with highly and weakly polar organic solvents shows that the solvent effect on the absorption spectra is small. For this reason the discrepancies between the calculated and the measured spectra are attributed to electron-vibronic coupling as well as to rotational dynamics of phenyl rings.