Properties and the nature of electric-field-induced variations in optical absorption spectra of polymethine dyes in polymer matrices

The effect of a permanent electric field on the absorption spectra of ionic (cationic and anionic) and intraionic symmetric and asymmetric polymethine dyes in poly-N-epoxypropylcarbazole, polyvinylbutyral, and polystyrene polymer films is studied. It is found that the electric field causes an increase in absorption of symmetric dyes in the short-wavelength region and a decrease in their absorption in the long-wavelength region. Asymmetric dyes exhibit opposite properties. These properties are common for these groups of dyes irrespective of their chemical structure and ionic nature. They are retained both in photoconducting polymers and in polymers that are incapable of charge photogeneration. These effects also take place in liquid solutions, although in this case they are much weaker. Various mechanisms of the influence of an electric field on the spectra, such as the electron transfer, Stark and Kerr effects, intermolecular interactions, and dye isomerization, are discussed. It is shown that the spectral effects are caused by the redistribution of the electron density in a chromophore of the dye in the ground state resulting in a change in the probability of vibronic transitions.     

Features of charge-carrier recombination in amorphous molecular semiconductors doped with polymethine dyes

The photogeneration and recombination of charge carriers in poly-N-epoxypropylcarbazole films with additions of a polymethine dye are investigated irradiation of films with blocking contacts by light both within and outside the absorption range of the dye. The kinetics of the accumulation and relaxation of electron-hole pairs, whose lifetimes exceed tens and hundreds of seconds, are studied. It is postulated that an increase in the recombination luminescence intensity occurs in an electric field as a result of an increase in the efficiency of the bimolecular radiative recombination stimulated by trapped electrons from photogenerated excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 629–635 (April 1998)     

Formation mechanisms of equilibrium component composition of molecular layers of polymethine dyes

We study the concentration dependences for the absorption spectra and component composition of molecular layers from three homologous series of symmetric polymethine dyes of different electron-donating ability of their terminal heterocyclic groups. We find that a change in the layer thickness leads to a change in the width and position of the spectrum due to a change in the number of absorption bands. The number of bands of monomers increases with increasing chain length and electron-donating ability. The concentration ratio of monomers and associated forms depends on the spatial orientation of molecules in the layer. The electron-donating ability of terminal groups affects the angle between the chromophores of molecules that form a dimer and the intensity ratio between the short- and long-wavelength absorption bands of dimers. We conclude that the effect of the thickness of the layer on its spectral parameters is determined by the degree of intramolecular electron asymmetry that arises as a result of the interaction of chemically symmetric molecules with charges of the substrate surface and upon intermolecular interaction. This asymmetry leads to changes in free energies of ground states of monomeric molecules and, as a consequence, to an increase in equilibrium concentrations of cis-stereoisomeric forms in the layer.     

Specific features of the radiative relaxation of excited states of polymethine dyes of different ionicity in films of photoconductive and nonphotoconductive polymers

The spectral and luminescent properties of film composites based on photoconductive poly-N-epoxipropylcarbazole and nonphotoconductive polyvinylbutyral with admixtures of cationic and anionic polymethine dyes, as well as the effect of an external magnetic field on these properties, are studied. It is found that the magnetic field affects the intensity and kinetics of the delayed fluorescence and recombination luminescence of the cationic dye in photoconductive films. This is explained by specific features of photogeneration of charge pairs, namely, by the participation of the singlet and triplet excited states of dye molecules in this process, as well as by the singlet-triplet conversion in dye molecules and photogenerated charge pairs.     

Special features of spectral and luminescent properties of squarilic dyes in polymer matrices

A study was made of spectral and luminescent properties of a squarilic dye on the basis of 1,3,3-trimethyl-3H-indoline in comparison with the cationic polymethine dye representing a derivative of the same heterocycle. The squarilic dye was introduced into polymer films of poly-N-epoxypropylcarbazole, which is capable of bearing photoproduced charges, and polystyrene, which lacks this ability. Luminescence spectra of films with different dye concentrations at temperatures of 4.2 and 300 K were studied. Spectral and luminescent properties of squarain in both types of polymers were found to be determined by the dye tendency to aggregation. For poly-N-epoxypropylcarbazole, this tendency is less pronounced than for polystyrene, which is associated with the enhancement of specific solvation of a bipolar squarain molecule by polar groups of poly-N-epoxypropylcarbazole. The structure with an angle between chromophore directions of 180° is shown to be the most likely spatial structure of associates. The advantage of this structure is that it is stabilized at both ends of bipolar molecules by electrostatic attractive forces between opposite charges.     

Violet-to-red photoluminescence of spiro-TAD organic films doped with different organic dyes

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of undoped spiro-TAD films and spiro-TAD films doped either by the organic dyes coumarin 7 or DCM as well as by both of these dyes simultaneously were investigated at different dye concentrations. A widened PL spectrum caused by doping was encountered and violet-to-red emission was obtained. It was established that excitation of the dyes is realized most efficiently through spiro-TAD. The overall integral PL intensity of the coumarin 7 doped films increased with dye concentration due to the suppression of nonradiative recombination in the film caused by a transfer of spiro-TAD excitation energy to the dye molecules. Mainly radiative energy transfer from semiconductor to dye molecules occurs in the case of DCM doping. No mutual influence on the luminescence of both dyes in the spiro-TAD film was observed and as a consequence, the PL band intensity of each dye can be adjusted separately.     

Unusual temperature dependences of the photoconductivity and recombination luminescence of amorphous molecular semiconductors doped with ionic dyes

A nonexponential increase in photoconductivity with increasing temperature is discovered for poly(N-epoxypropylcarbazole) (PEPK) films doped with polymethine dyes. It is postulated that traps for nonequilibrium charge carriers form in these films during irradiation and are destroyed as the temperature is raised. Such traps are manifested by broadening of the high-temperature shoulder on the thermally stimulated luminescence (TSL) curves following the preliminary irradiation of PEPK films doped with polymethine and xanthene ionic dyes in the visible or UV range at 250–320 K and by the appearance of a new narrow TSL maximum near the preliminary irradiation temperature. These TSL features disappear after prolonged storage of the films in the dark or heating to higher temperatures. Fiz. Tverd. Tela (St. Petersburg) 41, 203–209 (February 1999)     

Photoelectric Characteristics of Composite Carbazole Polymerfilms Containing 1,8-Naphthoylene-1′,2′-Benzimidazole

The electric conduction and photoconduction of sandwich-type poly-N-epoxypropylcarbazole (PEPC) and 3,6-di-Br-poly-N-epoxypropylcarbazole (3,6-di-Br-PEPC) films containing 33 wt.% of the dye 1,8-naphthoylene-1′, 2′-benzimidazole (NBI) have been investigated. It has been established that the PEPC films, as compared to the 3,6-di-Br-PEPC films, have a higher electric conduction and a lower photosensitivity and an electric field exerts a stronger quenching effect on the photoluminescence of the first-mentioned films. Analysis of the voltampere and lux-ampere characteristics and the kinetics of electric conduction of these films has shown that the space charge and recombination via dye molecules in them influence their electric conduction and photoconduction. The volume recombination of charges is radiative in character. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 627–631, September–October, 2005.     

Thermochromism and thermofluorochromism of merocyanines with a positive solvatochromism

The thermochromism and thermofluorochromism of ethanol solutions of di-, tetra-, and hexamethine merocyanines based on 1,3-dihydro-1,3,3-trimethyl-(2H)-indol-2-yliden and malonodinitrile, having a positive solvatochromism as compared to the corresponding cationic and anionic symmetric dyes, are studied at T = 348, 293, and 4.2 K. It is found that, upon a decrease in the temperature, the absorption spectra exhibit bathochromic shifts of bands, an increase in the vinylene shifts, and a decrease in deviations, whereas the fluorescence spectra exhibit opposite changes. In this case, the differences in the shapes of the absorption and luminescence bands decrease so considerably that the bands become virtually mirror-symmetric. It is shown that the spectral and luminescent effects observed arise because, at high temperatures, the electronic structure of the ground state of merocyanines approaches that of neutral polyenes, whereas, at low temperatures, this structure is similar to the structure of ideal polymethine. Such a transformation of the structure is caused by an increase in specific electrostatic interactions (nucleophilic and electrophilic solvations) with a solvent, as a result of which the alternation of the charges of a chromophore increases, while its bond orders become equalized.     

多孔硅后处理对其镶嵌染料光学特性的影响

对多孔硅进行真空退火处理和暴露大气快速退火处理,将有机染料香豆素102(C102)镶嵌其中,研究镶嵌复合膜发光特性的变化。通过比较多孔硅退火处理前、后傅立叶红外(FT-IR)吸收光谱的变化,从多孔硅与镶嵌染料分子间的能量传递方式角度出发,解释了PS表面态氧化能改善镶嵌复合膜发光特性的原因。实验通过改变多孔硅表面态,提高了复合膜的发光效率和多孔硅基体的透明程度,证明了多孔硅是一种良好的载体,在发展固体激光器方面有一定的应用,同时为实现硅基蓝绿发光开辟一条新的途径。     

Mutual modification of silver-nanoparticle plasmon resonances and the absorptive properties of polymethine-dye molecular layers on a sapphire surface

Extinction spectra of a hybrid material comprised of molecular layers of polymethine dyes and granulated silver films preliminarily deposited onto sapphire substrates are studied experimentally. Specific features of the dye layer composition and broadening of plasmon absorption bands are discussed. In hybrid samples, both an increase and a decrease in the silver-nanoparticle plasmon resonance frequency upon spreading of dyes have been observed, as well as a considerable increase in the dye molecule absorption. Possible mechanisms of the phenomena are discussed.     

Optical, electrical, and photophysical properties of films of polycomplexes of azobenzene derivatives with cobalt

The absorption spectra, photoconductivity, and electrical conductivity of films of new polycomplexes of 4-methacyloyloxy-(4′-carboxy-3′-oxy)azobenzene and 4-methacryloyloxy-2-nitro-(4′-carboxy-3′-oxy)azobenzene with cobalt were investigated with switched-off and switched-on external electric fields. The effect of an external electric field on the transmission of linearly polarized light through films near the long-wavelength absorption edge is revealed. It is shown that the change in the light transmission under the action of an external electric field is due to the spatial reorientation of the dipole moments of azobenzene groups photoinduced by polarized light in an external electric field. The effect of an electric field on the light transmission is explained by the formation of forces acting on the dipole moments of azobenzene groups and on metal ions. The increase in the dipole moments of azobenzene groups as a result of introduction of nitro groups into their composition decreases the effect of cobalt ions on the electro-optical properties of polycomplex films.     

Effect of electric field on the optical absorption of amorphous molecular semiconductors doped with polymethine dyes

The effect of a dc electric field on the evolution of the electronic absorption spectra of poly(N-epoxypropylcarbazole) (PEPC), poly(styrene) (PS), and poly(vinyl butyral) (PVB) films doped with methine dyes is investigated. It is revealed that these spectra in the absorption range of the dye undergo transformations depending on the dipole moment and the symmetry of the π-electron structure of dye molecules. The inference is made that the electron density in the dye molecule is redistributed in an external electric field. This redistribution gives rise to the electric field-induced anisotropy in the spatial distribution function of photogenerated electron-hole pairs in amorphous molecular semiconductors.     

Luminescence properties of anodic aluminum oxide films with organic luminophores embedded into pores

Luminescence properties of porous anodic aluminum oxide films formed in a 0.6 M solution of citric acid and luminescence of paraterphenyl, perylene, coumarin 7, and rhodamine 6G dyes adsorbed by the films are investigated. The nature of emitting centers in anodic aluminum oxide is revealed. Intense photoluminescence of all tested dyes embedded into pores of anodic aluminum oxide has been found. A redshift of fluorescence spectra of dyes adsorbed by the matrix and emergence of an additional longwave band have been detected. Data obtained can be used in developing new thin-film luminescent coatings for future applications in optoelectronics and molecular electronics. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No 4, pp. 483–488, July–August, 1997.     

Proton-irradiation effect on the luminescence of the MEH-PPV conjugated polymer

We report proton-irradiation effect on luminescence in the MEH-PPV conjugated polymer. While the luminescence was severely weakened by the low-energy irradiation with a high dosage, it was affected only weakly by the high-energy irradiation with a low dosage. The electroluminescence was affected by the irradiation more severely than the photoluminescence was, preserving the external quantum efficiency. While the luminescence spectrum of the severely damaged polymer was broadened with a blue-shift, a weak damage resulted only in a decrease in the luminescence intensity apparently preserving the spectral shape. The change in the luminescence spectra may be explained by chain conformational disorders as well as chain scission induced by the irradiation.     

Structural defects in carbazolyl-containing polymers with polymethine dye impurities

Photoluminescence spectra and thermoluminescence curves of pure and doped carbazolyl-containing polymers polyvinyl carbazole (PVC) and poly-N-epoxypropyl carbazole (PEPC) are investigated in the temperature range 5–300 K. The impurities are cationic indocarbocyanines with various lengths (n) of the polymethine chain: HIC (n=1), HID (n=2), HIT (n=3) and squaryl dye HISq with the same polymethine chain length and the same structure of heterocyclic end groups as for the HID dye. It is found that solvation of dye molecules by polar groups of a polymer is accompanied by conformation changes in the polymer, which are considerably enhanced with increasing n, as well as upon a transition from a rigid-chain PVC macromolecule to a more flexible PEPC molecule. As a result, the concentration of structural defects, viz., excimer-forming sites playing the role of traps for singlet excitons and charge carriers, increases considerably in doped PEPC films. This leads to the emergence of a band with a peak at 460 nm in the luminescence spectra of PEPC films with HID and HIT and at 480 nm for films with the HISq impurity, while the thermoluminescence curve for PEPC with HISq acquires an additional band with a peak at 275 K.     

Effect of the external electric field on the kinetics of recombination of photoexcited carriers in a ZnSe/BeTe type II heterostructure

The kinetics of the radiative recombination of photoexcited electrons and holes for a spatially direct transition in a ZnSe/BeTe type II heterostructure in an external electric field has been analyzed. A strong decrease (more than two orders of magnitude) in the photoluminescence intensity, as well as a decrease in the duration of the relaxation of the direct transition, is observed when the electric field is applied. The energy levels and wavefunctions of electrons and holes in the ZnSe/BeTe heterostructure subjected to the electric field have been numerically calculated. It has been shown that the observed decrease in the photoluminescence intensity and duration of the relaxation of the direct transition is due to both an increase in the radiative recombination time and an increase in the rate of escape of photoexcited holes from the above-barrier level in the ZnSe layer to the BeTe layer.     

Characteristic features of the photoconductivity and spin conversion of electron-hole pairs in doped amorphous molecular semiconductors in the dye absorption region

The photoconductivity of doped poly-N-epoxypropylcarbazole films in the absorption region of a cationic polymethine dye is observed to increase when the BF ₄ ⁻ ion is replaced by Cl⁻ and I⁻. The effect of a magnetic field on the photoluminescence is investigated at room temperature. The photoconductivity is studied as a function of the electric field intensity and wavelength of light. The dependence of the photoconductivity on the nature of the anion can be explained by a change in the probability of intercombination conversion in ionic and electron-hole pairs. Fiz. Tverd. Tela (St. Petersburg) 41, 44–48 (January 1999)     

电场调谐InAs单量子点的发光光谱

采用稳态和时间分辨发光光谱,研究了生长在p-i-n二极管结构内的InAs单量子点发光光谱的电场调谐特性.随着电场强度的增加,观察到量子点中激子发光的Stark 效应.通过选择不同波长的激光线激发量子点样品,发现随着电场强度的增加导致量子点发光强度的减弱,这是由于量子点俘获载流子概率的减小所致,而激子寿命的增加源于电场导致激子的Stark效应. 关键词： InAs单量子点 Stark效应 电子-空穴分离     

Luminescence and Electrical Properties of CdS Films Doped with Potassium and Sodium

CdS films, both pure and doped with the alkali metals Na and K, with Cl and without it, obtained by the aerosol pyrolysis method from thiourea coordination compounds at a temperature of 250 to 500°C are investigated. The photoluminescence spectra and the dependence of the luminescence intensity on the duration of exposure of samples to a high-power UV radiation have been obtained. The doping of samples results in a tenfold increase in the intensity of the luminescence band in the visible spectrum range (650–850 nm) as compared to pure samples of CdS. Upon irradiation of samples by high-power UV radiation for 1 h, no noticeable change in the intensity of luminescence of CdS films was observed. This is indicative of the stability of the films under study against the action of UV radiation. The luminescent properties of the films, obtained pyrolytically using thiourea coordination complexes, are determined by the synthesis conditions as well as by the chemical nature of the original substances. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 509–513, July–August, 2005.