Re-excitation In Conjugated Organic Polymers, And The Lattice Vibration Research

Organic functional material, conducting polymers have attracted much comprehensive attention recently because of their applied foreground. Recently, numerous high-performance electro-photonic devices are fabricated from organic polymers and molecular crystals have been made, including light-emitting diodes and electrochemical cells, display panels, photovoltaic cells, photodetectors, transistors, lightemitting field-effect transistors, biosensors, imaging devices, solid-state lasers and spin valves. Duo to their obvious market potential, organic photovoltaic materials have drawn interest in the applied and theoretical investigation.During the past few years, the perception of the nature of the physical basis of the unique electronic properties of these conjugated polymers, both as isolated "molecules" and as molecular solids, has developed to a somewhat more sophisticated level of understanding. The essential ideas about the nature of the unusual charge species, and of the excited states of conjugated systems, have been discussed intensely over the past twenty years.Prior to the discovery of conducting polymers, polymer science and technology had focused on "saturated" polymers. In "saturated" polymers the valence of the carbon atoms in the main chain of the repeat unit is fully saturated, i.e. every carbon is bonded to four other atoms in the sp~3 hybridized configuration. Each carbon is σ -bonded to two neighboring carbons and two hydrogen atoms. Saturated polymers are insulators, they tend not to have interesting electronic or optical properties.Conducting polymers differ from saturated polymers in that each carbon of the main chain is bonded to only three other atoms. In conducting polymers, three of the electrons on each carbon reside in σ-bonding orbitals while the fourth electron resides in a delocalized p_z-orbital. The p_z-orbital of neighboring carbons overlap to form conjugated π bond, so conducting polymers can be called as conjugated polymers. Conjugated polymers have various optical, electronic and magnetic properties, so it has been the theoretical focus on the static and dynamic properties ofthe excitations in conjugated polymers.Since 1970's, SSH Hamiltonian, the tight-binding semi-empirical calculation method that was found by Su, Schrieffer and Heeger has been demonstrated successfully for determining the electronic structures and optical properties in conjugated polymers. In the later years, Bishop, Conwell, Sun and Xie et al have extended the SSH Hamiltonian to research the static and dynamic process of excitations. The further research in this field not only can broaden our understanding of the microcosmic physical world but also can have a substantial impact on the applications on organic polymer devices.In this paper, Some unrevaled properties of organic polymers are found, such as the re-excitation of solitons or polarons, dispersion relation of a copolymer chain and the effect of perturbation on the vibration modes in one dimensional systems. The detailed research and main results are given below: 1. Re-excited states in conjugated polymersFemtosecond technology has provided a powerful tool to study an ultrafast process, especially to explore new properties of excited states. The transient and dynamical process in the excited states can be revealed in detail. By using femtosecond spectroscopy, it is found that in polymers the primary photoexcitations are intrachain excitons. And very recently, the biexciton, which is a sequential reexcitation of the intrachain exciton, is observed in PPV. A dynamical simulation in polymers shows photoinduced carrier fission where a positive carrier (polaron) is split into two carriers by photoexcitation, which is called "Charge flipping of spin carrier". Duo to polymer's one-dimensional nature, the lattice configuration is sensitively dependent on the electronic state, it is thus essential to study the re-excited of solitons n polarons and bipolarons.? Reexcitation of a soliton There are two kinds of solitons, a charged soliton5* and a neutral one 5°. An important fact is that a soliton has a reverse spin-charge relation different from that of an electron or hole, i.e., a charged soliton S* has one unit charge but no spin, while a neutral soliton has aspin 1/2. We have studied the re-exciting process of solitons, the following processes will take place: S° +hv => S± + P? , S° +hv=> S° +S° +S°,S°+hv=>K+)/4+K'-2+K+}'4 , Si+hv=>S°+P±S- +hv=>K°+K->2+K-u\ where s°, S* and P± express a neutralsoliton, charged solitons and charged polarons separately. K denotes a kink in the chain.? Reexcitation of a polaron There are two splitting energy levels when a polaron forms, which are symmetrical with respect to the center of the gap.the following processes will take place: P± + hv => P* + BP±2 ,pt+hv^pt+E0, pt+hv^BE0 +Pi, where BP±2, E° and BE0express a charged bipolaron, exciton and bieceiton separately.? Reexcitation of a bipolaron There are two deeper splitting energy levels comparing with the polaron case. For the re-exciting process of bipolarons,the following processes will take place: BPt2 +hv=> P* + P* ,BPt2 +hv^> BP±2 + E° , BP±2 +hv=>P±+Pr , where P* express aexcited state. A bipolaron is split into two polarons.. 2.Phonon states and vibration modes in one-dimensional copolymer Two or more different homopolymer segments can be combined together to form a hetero-structure through physical and chemical methods. Such kind of hetero-structure is usually called a copolymer. It is expected by controlling of hetero-structures to improve the physical properties. It is very significative to study the lattice vibrate for understanding the properties of compolymer. My investigations include:? For diblock copolymer -(PPP)OT-(PA)n-, by studying all the vibration modes, we can divide them into three categories: (la) modes confined only in segment PA, (lb) modes only in segment PPP, and (2) modes spreading along the whole chain (hybridized modes). If the copolymer consists of veryshort homopolymer segments, it was found that all the vibration modes would spread throughout the whole copolymer chain and there are no confined modes.? From the symmetry point of view, segment PPP will meet symmetry broken of interface from left to right, and segment PA will meet symmetry broken from right to left at interface. Such kind of symmetry broken will make lattice distortion at interface. The lattice distortion may create some new vibration modes in interfacial region. Interfacial modes appear only whenfi > 1. In the case of p = 1.2, there is only one interfacial mode and itsfrequency lies in the lower band gap. Two interfacial modes appear for fi = 1.3, one with frequency at the lower gap and another at the higher gap.? For triblock copolymer -(PPP)m-(PA)n-(PPP)m-, it has a barrier-well-barrier structure and thus presents a characteristic of quantum well. Three kinds of modes were obtained. One case hybridized modes spreading over the whole system; others are modes confined in the central well or modes confined in the barrier range. If we shorten the lengths of segments, all the confined modes will become hybridized modes, as the coupling among segments enhances in this case.? For copolymer -[(PPP)m-(PA)m],-, the vibration modes spread throughout the whole chain when m is small, and in this case we say that the system presents a characteristic of organic superlattice. Oppositely, when m is large, some localized vibration modes will be confined in segment PA or PPP, and the system presents a characteristic of organic quantum well.3. The intense infrared-activity absorption induced by solitons or polarons is due to the localized vibrational phonon modes around the structural defect. In actual polymer samples, there exist chain breaks and various conjugation defects. Whether is the localized vibrational modesof the solitons or polarons affected by temperature, defects or adulteration? We studied the effects of disturbance by. a square distribution.? The numerical calculation showed that the well-known Goldstone mode g,will be pinned. The weak mode g4 of a soliton is enhanced. Two new localized modes gx and g\ were found in the acoustic-frequency branch, they havethe same node and symmetry with g, and g2.It was found that the Goldstone mode of polaron is pinned and its frequency is shifted up to a finite value. The staggered modes keep their localization even there is a large lattice fluctuation.