Anomalous energy transfer dynamics due to torsional relaxation in a conjugated polymer

In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale-exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.     

共轭高分子中激子演化过程的非绝热动力学研究

有机共轭高分子受光激发或被电荷掺杂后可能会产生各种激发状态的激子,激子的演化过程对有机发光光谱有着至关重要的影响.通过非绝热动力学演化的方法模拟了受光激发后有机高分子中激子驰豫的动力学过程,结果表明高激发态激子不稳定,由于电声耦合作用,高激发态激子会持续向低激发态激子演化,同时,低激发态激子的复合发光会发生红移.稳定的激子复合发光光谱中,基态激子发光强度最大,可高达70-80%;第一激发态激子及其它激发态激子发光强度的总和不超过20%.     

External energy transfer in amplified spontaneous emissions from MEH-PPV conjugated polymer

We observed amplified spontaneous emissions (ASEs) in solutions of conjugated polymer under high power neodymium-doped yttrium aluminum garnet (Nd:YAG) laser excitation. A solution of coumarin-485 dye (C₄₈₅) and conjugated polymer (MEH-PPV) was prepared in tetrahydrofuran (THF). Here, C₄₈₅ acted as a donor (with different ratios) while MEH-PPV served as an acceptor (with a constant ratio). Without the donor, MEH-PPV exhibited ASE only in the excimeric state; with the donor, however, it produced ASE in both the monomeric and excimeric states simultaneously. These results clearly show that it is possible to monitor and manipulate the excited state behavior of molecules by using an energy transfer process using laser-excited conjugated polymer.     

Heterocharge accumulation and relaxation and nonlinear transfer in polymer films

St. Petersburg State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 15–19, April, 1994.     

Positron annihilation studies of a conjugated polymer

Lineshapes of the Doppler broadened annihilation radiation (DBAR) of a conjugated polymer polydiacetylene, 4-butoxycarbonylmethylurethane (P-4BCMU), in its amorphous state, were measured over the temperature range of 30 to 190°C. A differential-scanning-calorimeter (DSC) was used to correlate the transition temperatures. The DBAR lineshapes show a distinct change during the structural transitions of the polymer. A two states trapping model is used to qualitatively describe the temperature dependence of the lineshape. The nature of formation of the trapped states of positrons above T_c, through the interplay of electronic structure/conformation change, is discussed.     

共轭聚合物单分子构象和能量转移特性研究

利用基于宽场显微光学系统的单分子散焦成像技术测量了不同构象poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole](PFO-DBT)共轭聚合物单分子的光物理与动力学特性.通过分析共轭聚合物单分子的荧光轨迹和对应的发射偶极取向变化识别共轭聚合物单分子发光单元,发现延伸构象下的单分子呈现多发色团发光特性,而折叠构象下的单分子保持高效链间能量转移,呈现单个发色团发光特性.共轭聚合物单分子构象对能量转移效率的影响可用于研究基于共轭聚合物的光电器件和分子器件.     

Photoluminescence quenching through resonant energy transfer in blends of conjugated polymer with low-molecular acceptor

A model is proposed for photoluminescence quenching due to resonant energy transfer in a blend of a conjugated polymer and a low-molecular energy acceptor. An analytical dependence of the normalized photoluminescence intensity on the acceptor concentration is derived for the case of a homogeneous blend. This dependence can be described by two fitting parameters related to the Förster radii for energy transfer between conjugated segments of the polymer and between the conjugated polymer segment and the energy acceptor. Asymptotic approximations are obtained for the model dependence that make it possible to estimate the contribution from the spatial migration of excitons to the photoluminescence quenching. The proposed model is used to analyze experimental data on the photoluminescence quenching in a blend of the soluble derivative of poly(p-phenylene vinylene) and trinitrofluorenone [13]. The Förster radius for resonant energy transfer between the characteristic conjugated segment of poly(p-phenylene vinylene) and the energy acceptor is determined to be r _F = 2.6 ± 0.3 nm.     

Stark spectroscopy of excited-state transitions in a conjugated polymer

Stark spectroscopy, which is well established for probing transitions between the ground and excited states of many material classes, is extended to transitions between transient excited states. To this end, it is combined with femtosecond pump-probe spectroscopy on a conjugated polymer with appropriately introduced traps which harvest excitation energy and build up a sufficient excited state population. The results indicate a significant difference in the effective dipole moments between two short lived excited states.     

Band bending in conjugated polymer layers

We use the Kelvin probe method to study the energy-level alignment of four conjugated polymers deposited on various electrodes. Band bending is observed in all polymers when the substrate work function exceeds critical values. Through modeling, we show that the band bending is explained by charge transfer from the electrodes into a small density of states that extends several hundred meV into the band gap. The energetic spread of these states is correlated with charge-carrier mobilities, suggesting that the same states also govern charge transport in the bulk of these polymers.     

Polaron dynamics in a system of coupled conjugated polymer chains

The motion of excitations such as polarons is believed to be of fundamental importance for the transport properties of conjugated polymers for the use in, e.g., polymer based LED's. We have investigated polaron dynamics in a system of coupled polymer chains in the presence of an external electric field. In particular, we focus on how a polaron migrates through the polymer lattice, i.e., the situation in which a polaron reaches a chain end and is scattered to the surrounding chains. We show that the outcome of this event strongly depends on the strength of the electric field, and we identify three different cases for the polaron migration.     

共轭高聚物链中极化子生成的掺杂效应

在紧束缚近似下,用绝热动力学演化的方法研究了共轭高聚物链中低浓度掺杂对极化子生成位置的影响.研究发现,掺入杂质与注入电子的先后次序不同,极化子生成的位置会有很大差别,先注入电子后掺入杂质的情况下,在链端自由区与过渡区,极化子生成位置受杂质影响较小;而先杂质后电子的次序下,极化子的生成位置明显受到杂质的控制.该研究表明共轭高聚物链中极化子的生成位置可通过调节掺杂与电荷注入的次序加以控制,从而可达到间接改变载流子迁移率的目的.     

共轭高聚物链中极化子生成的掺杂效应

在紧束缚近似下,用绝热动力学演化的方法研究了共轭高聚物链中低浓度掺杂对极化子生成位置的影响。研究发现,掺入杂质与注入电子的先后次序不同,极化子生成的位置会有很大差别,先注入电子后掺入杂质的情况下,在链端自由区与过渡区,极化子生成位置受杂质影响较小;而先杂质后电子的次序下,极化子的生成位置明显受到杂质的控制。该研究表明共轭高聚物链中极化子的生成位置可通过调节掺杂与电荷注入的次序加以控制,从而可达到间接改变载流子迁移率的目的。     

Electronic structure of polyisonaphtothiophene: Design of a new conjugated organic polymer

ab initio band structure results of polyisonaphtothiophene (PINTP) are presented. Comparison of its electronic properties such as band gap, ionisation potential and electron affinity with those of polyisothianaphthene (PITN) and polythiophene (PTP) indicates PINTP to be the best intrinsic electrical conductor of these three polymers. PINTP is also predicted to be the most dopant-philic of them. The effect of the hetero substitution and the substitution at the naphthalene ring on the conduction properties of PINTP is also discussed.     

Coherent control of optical emission from a conjugated polymer

We have observed that resonant Rayleigh scattering dominates the emission from poly(p-phenylene vinylene) excited with photons at energies below the threshold at which excitonic migration is reduced. The intensity of the resonant emission decays exponentially with a lifetime of up to 450 fs after pulsed excitation. The coherent nature of the emission was confirmed by angular variations in the far-field emission intensity-bright and dark speckles. Persistence of a coherent polarization was demonstrated by coherent control using phase-locked pulses.     

Voltage-modulated millimeter-wave spectroscopy on a polymer diode: mesoscopic charge transport in conjugated polymers

We present a new technique to determine the carrier mobility mu in semiconducting, undoped, conjugated polymers in the millimeter-wave frequency range, 10-500 GHz. This method probes charge conduction on mesoscopic length scales, a regime inaccessible to other transport experiments. The experiment is based on the detection of millimeter-wave absorption of field-induced charge in polymer diodes, and is applied to poly( p-phenylene vinylene). We demonstrate that locally mu can be as high as 10(-4) m(2)/V s, and deduce the typical hopping parameters in the mesoscopic high- mu regions in the material.     

共轭高分子链中大极化子与小极化子的弹性碰撞

基于Su-Schrieffer-Heeger(SSH)模型,考虑电子-电子相互作用,以及在哈密顿量中引入Brazovskii-Kivova对称破缺项,采用动力学演化的方法研究了在外电场作用下共轭高聚物分子链中大极化子与小极化子之间的弹性散射过程.研究发现,载流子的载荷性质不但决定大小极化子之间的相互作用性质,也深刻地影响了其发生散射后的载流子输运性质.在较低电场的作用下,当两载流子电性相同时,发生碰撞后,小极化子将推动着大极化子一起运动;而当两者电性相反时,碰撞后,大极化子将拖曳着小极化子一起运动.当电场强度超过临界值时,将产生量子隧穿效应,即,大小极化子相互迅速穿透,且不受载流子的载荷影响.     

共轭高分子中激子形成的非绝热动力学研究

有机共轭高分子被光激发后,其电子结构会发生明显的变化,通过非绝热数值计算的方法模拟了受光激发后有机高分子中激子生成的动力学过程.结果表明,激子的生成一般需要数十飞秒时间,激子的生成周期随分子链长度的增大而增大.高激发态激子可以根据生成周期分为两类,第一类激子的生成周期在数十到上百飞秒之间,第二类激子的生成周期可达上千飞秒.对于超过200个碳原子的高分子,第二类激子的生成周期可达5000飞秒以上.这个计算结果说明,相比于有机高分子,有机小分子中更容易观测到激子.     

Dielectric relaxation in PEO-based polymer electrolytes

Dielectric properties of cross-linked poly(ethylene oxide) (PEO) with different mesh sizes, doped with lithium salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), have been studied in frequency region between 0.1 and 10⁷ Hz and in broad temperature range. Results were compared with linear PEO of 1000 g/mol. Dielectric responses of the systems are dependent on frequency and thermally activated. Systems exhibit different responses in semi-crystalline and molten state. Increase of temperature promotes polarization; whereas, increase of frequency lessens it. In other words, polarization is thermally activated and local conductivity reduced. Generally, one observes enhanced dc conductivity in linear PEO as compared to cross-linked PEO at high temperature and the opposite at low temperature. Resonance responses are observed in low-molecular cross-linked PEO and in linear PEO at low temperature. These responses lead to splitting of polarization relaxation at frequencies beyond low-frequency range. Salt-comprising systems display only relaxation-type dielectric response. Imaginary parts of response spectra show distribution of relaxation times. It turns out that this distribution is independent of temperature in the low-frequency range, but depends on concentration of salt in the cross-linked polymer. In both systems, neat cross-linked and linear polymer of low-molecular mass, one observes coexistence of non-local and local motions of charged entities even at very low temperature.     

共轭高分子链中电荷迁移的热效应

从理论上研究了共轭高聚物链中在电场作用下极化子运动的热效应.基于SSH模型以及通过绝热动力学演化的方法,模拟了共轭高聚物链中极化子在电场作用下从链左端向右端运动的过程.晶格受到的热扰动作用假设为通过局域的晶格范围内原子位移的随机涨落来实现.结果发现,晶格中的局域热涨落对于运动中的极化子而言等效于一个势垒.势垒高度由高分子中受到热扰动的区域的范围大小以及该区域与其周围环境的温差来决定.当分子中存在热吸收不均匀的现象时,链内极化子迁移率在低电场范围内随电场的变化遵循对数曲线变化规律.