Conjugation-length dependence of spin-dependent exciton formation rates in pi-conjugated oligomers and polymers

We have measured the ratio, r = sigma(S)/sigma(T) of the formation cross section, sigma of singlet and triplet excitons from polarons in pi-conjugated oligomer and polymer films, using a spectroscopic technique we developed recently. We discovered a universal relation between r and the conjugation length (CL): r(-1) depends linearly on CL-1, irrespective of the chain structure. Since r is directly related to the maximum possible electroluminescence quantum efficiency in organic light emitting diodes (OLED), our results indicate that polymers have an advantage over small molecules in OLED applications.     

Triplet energies of pi-conjugated polymers

Using pulse radiolysis and triplet energy transfer has enabled us to measure the triplet energies in a broad range of different pi-conjugated polymers. In all cases we find that the 1 (3)B(u) is of order 0.6 to 1 eV below the 1 (1)B(u), indicative of localized triplet states with strong electron-electron correlation. We also observe that the 1 (1)A(g)-1 (3)B(u) gap decreases linearly as the 1 (1)A(g)-1 (1)B(u) gap decreases even though polymers with very different structure have been studied. This surprising result suggests that polymers with singlet gap <1.3 eV will have a triplet ground state.     

Photoinduced quantum interference antiresonances in pi-conjugated polymers

We observed photoinduced quantum interference antiresonances between several discrete infrared-active vibrations and the lower-polaron continuous absorption band in a series of pi-conjugated polymer films having superior planar orders, where the polaron transition energy is relatively small. The photoinduced Fano-type antiresonances are well explained by extending the amplitude mode model beyond the adiabatic limit. The agreement between the data and the model confirms the presence of a continuous electronic band above the polaron state. We show that high frequency modes are strongly coupled to electrons, with implications for superconductivity.     

Linear and nonlinear photoexcitation dynamics in pi-conjugated polymers

Linear and nonlinear recombination kinetics with various lifetime distributions were identified for long-lived photoexcitations in a series of pi-conjugated polymer films using modulation frequency and excitation intensity dependencies of the photoinduced absorption. This includes monomolecular, bimolecular, and defect-limited recombination processes that lead to saturation. Using generalized kinetics parameters, we found characteristic plots for all recombination processes. Specifically, the bimolecular recombination process shows superlinear intensity dependence away from the steady state; on the contrary, dispersive bimolecular recombination leads to sublinear dependence.     

Ultrafast spectroscopy of even-parity states in pi-conjugated polymers

Relaxation dynamics of even parity ( A(g)) states in poly( p-phenylene vinylene) derivatives are studied using a novel fsec transient spectroscopy, in which two different excitation pulses successively generate odd parity ( 1 (1)B(u)) excitons at 2.2 eV and then reexcite them to higher A(g) states. For reexcitation energies Planck's over 2piomega<1.1 eV ultrafast internal conversion back to 1 (1)B(u) takes place in accordance with Vavilov-Kasha's rule. However, for Planck's over 2piomega>1.1 eV the decay occurs in a nonemissive state identified as a polaron pair, showing that the A(g) states above 3.3 eV mediate charge transfer.     

Statistical model for intermolecular adhesion in pi-conjugated polymers

We propose an interchain binding mechanism in pi-conjugated polymers based on the intermolecular tunneling of the delocalized electrons occurring at points where the polymers cross. This mechanism predicts specific bound structures of chains that depend on whether they are semiconducting or metallic. Semiconducting chains should form polyacenelike states exhibiting binding at every other site, while (doped) metallic chains can bind at each site. We also show that solitons colocalize with the intermolecular binding sites thereby strengthening the binding effect and investigate the conformational statistics of the resulting bimolecular aggregates.     

Doping-dedoping-driven optic effect of pi-conjugated polymers prepared in cholesteric-liquid-crystal electrolytes

Chiral conjugated polymers were synthesized by electrochemical polymerization in a cholesteric-liquid-crystal (CLC) electrolyte. The polymers display a characteristic finger print texture similar to that of CLC. The natural optical activity of the polymers was successfully controlled by the electrochemical doping-dedoping procedure. The finding of this unprecedented mechanism, involving doping of pi-conjugated systems, can be an important advance in the development of plastic optoelectronics based on conductive polymers.     

Polaron spin-lattice relaxation time in pi-conjugated polymers from optically detected magnetic resonance

We describe a method for obtaining the polaron spin-lattice relaxation time T{SL} in pi-conjugated polymers by measuring the optically detected magnetic resonance (ODMR) dynamics as a function of microwave power and laser intensity. The peculiar ODMR dynamics is well described by a spin dependent recombination model where both recombination and spin relaxation rates determine together the response dynamics. We apply this method to the spin 1/2 ODMR in films of pristine 2-methoxy-5-(2{'}-ethylhexyloxy) phenylene vinylene [MEH-PPV] polymer, as well as MEH-PPV doped with various concentrations of radical impurities. We obtained T{SL} approximately 30 micros in pristine MEH-PPV, but substantially shorter when the magnetic impurities are added.     

(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)的荧光以及长余辉发光性能

采用高温固相法制备了一系列(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)(0≤x≤0.25)绿色荧光粉,并研究了Mg离子对(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)的结构、荧光以及长余辉发光性能的影响。Mg离子取代Zn进入Zn_2GeO_4晶格,形成(Zn_(1-x),Mg_x)_2GeO_4固溶体,并产生了晶格畸变。光谱分析结果表明,样品中位于533 nm的绿色荧光源于Mn~(2+)的~4T_1(~4G)→~6A_1(~6S)跃迁。随着Mg离子浓度的增加,(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)样品的激发光谱出现了蓝移现象,说明Mg离子进入到Zn_2GeO_4晶格中对其晶格结构产生了影响,导致(Zn_(1-x),Mg_x)_2GeO_4的带宽发生改变。发射光谱则表明Mg离子进入Zn_2GeO_4晶格引起Mn~(2+)的~4T_1(~4G)→~6A_1(~6S)跃迁绿色荧光发光强度的增强。Zn_2GeO_4基质中的氧空位缺陷陷阱深度由于基质带宽的变化而变深,样品具有良好的长余辉发光效果。通过热释光谱分析研究了材料中缺陷陷阱的特征,进一步证实了(Zn_(1-x),Mg_x)_2GeO_4中缺陷陷阱深度发生改变。根据光谱分析结果给出了(Zn_(1-x),Mg_x)_2GeO_4∶Mn~(2+)中荧光与余辉发光的产生机理。     

Phosphorescence of cycloalkanones

The ³n, π¹ phosphorescence of a series of cycloalkanones has been studied in EPA at 77 K with the aim of deducing the importance of ring strain and substitution on radiationless decay processes. Results indicate that the contour of the phosphorescence spectrum (unstructured) remains the same (≈450 nm maximum) for the series cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, and cyclo-octanone, and that whereas α-methyl substitution decreases the phosphorescence intensity relative to the parent cycloalkanone, β-substitution results in increasing phosphorescence. The phosphorescence yield of cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, cyclo-octanone, increase in the ratio 1 : 63 : 22 : 201 : 290. The low phosphorescence yield of cyclobutanone (ø_p=0.003) is considered to be an indication of a small triplet yield relative to the remaining molecules. The results suggest that the α-CH stretching mode is a factor in radiationless deactivation from the lowest triplet and that radiotionless deactivation processes may be different in the singlet and triplet states.     

Phosphorescence of thin benzophenone films

Institute of Physics, National Academy of Sciences of Ukraine, 46, Prospekt Nauki, Kiev 252650 Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 3, pp. 117–121, May–June, 1995.     

Intrinsic room-temperature electrophosphorescence from a pi-conjugated polymer

Electrically induced phosphorescence from a poly(para-phenylene) ladder-type polymer is observed for the first time and characterized using time resolved spectroscopy. Short-lived phosphorescence is also observed in gated fluorescence spectra and is found to be quenched reversibly by oxygen. Thermally activated triplet diffusion to covalently bound palladium sites, which are formed at a concentration of about 80 ppm in a side reaction during polymer synthesis, is believed to be the cause of this novel effect, which suggests a new approach to the design of efficient electroluminescent materials.     

Solid-Surface Room Temperature Phosphorescence

Abstract

Luminescence can be defined as radiation emitted from atoms or molecules as they undergo a radiative transition from an excited state to a lower energy state. After the absorption of incident radiation, the emission of a photon during the radiative transition is called photoluminescence. The process of photoluminescence encompasses both fluorescence and phosphorescence.     

Phosphorescence Peculiarities and Structure of Glassy Benzophenone

Journal of Applied Spectroscopy - Results of a comparative analysis of spectral-luminescent properties of crystalline and glassy benzophenone are presented. The main spectral characteristics...     

吲哚-3-丁酸无保护流体室温燐光和胶束增稳室温燐光性质的比较

仔细研究了吲哚-3-丁酸(IBA)的无保护流体室温燐光(NP-RTP)及以高分子分散剂聚乙二醇-200,聚乙二醇-400和非离子表面活性剂Tween-20,Tween-40, Tween-80,Tween-85,Brij35和乳化剂OP为介质的流体室温燐光性质。表面活性剂和高分子分散剂能抑制IBA燐光猝灭,使其具有更低的检出限,同时也使燐光强度-激发光照射时间曲线发生改变,但不影响IBA燐光光谱特性。无论是否存在表面活性剂或高分子分散剂,TlNO₃都不能诱导IBA产生燐光,KI却能诱导其产生强烈燐光。用于强化水样和土壤样品中IBA的测定,回收率95.2%～104%,相对标准偏差2.4%～4.0%。     

Phosphorescence of Enaminodicarbonyl-8-Azasteroids and Related Compounds

We present the results of the investigation of the spectral-luminescent properties of some 8-azasteroids and model compounds in ethanol solution. The absorption and luminescence spectra of these molecules have been investigated. It is shown that all the compounds under investigation have common absorption and phosphorescence spectra that are close to the corresponding spectra of the related enaminodycarbonyl compounds. The results obtained allow to conclude that the main channel of singlet state deactivation is the intersystem crossing. It has been found that the 8-azasteroids are unstable under laser irradiation.     

压力使固体产生磷光

Recent development of theory ofluminescence demands that latticedefects are directly responsible forthe phenomena of phosphoresccence.There are many evidences supportingthis view.It seems to us there is anew method of testing this theory.This method is to investigate whetherit is possible to produce the phosphores-cence by purely mechanical means.Following is a report of some experi-mental results concerning this question.     

Phosphorescence of X-modification and amorphous films of benzophenone

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 5, pp. 811–815, November, 1991.