| In recent years organic conjugated polymers have emerged as a highly promising class of materials for electronic and optical applications. In a relatively short period of time, great achievements have been accomplished by a truly multidisciplinary community of chemists, physicists, and materials science investigators. Conjugated polymers have quasi-one-dimensional structure. In the doped state, their conductivity can be enhanced from typical semiconductor to metal levels. It has been known that in organic polymers the carriers are not electrons or holes but solitons, polarons or bipolarons depending upon the degeneracy of the ground state. For nondegenerate conjugated polymers, the carriers are polarons or bipolarons In relation to the length of the polymers the manner and the concentration of the redoxed charges. The competition among these excitations in stability has puzzled researchers both in theory and experimental for many years. This question becomes much prominent in the investigation of organic spintronics.Physical and chemical property of conjugated polymers could be understood to some extent by studying their corresponding oligomers. Oligomers are well-defined chemical systems: the conjugation chain length can be exactly controlled. Studies of the size dependence of optical and electronic properties of oligomers with well-defined lengths helps to check theoretical predictions and understand the neutral and charged states. It is significant for developing new organic polymer materials and technology to study oligomer.The band gap of poly(thienylene vivylene)(PTV) is smaller than that of PT and PPV ,as a result ,can be regarded as a low-band-gap polymer. Furthermore, an advantage of PTV is its high absorption in the visible range of the spectrum , making them excellent candidates for photovoltaic applications.In the doped state ,PTV has already proven to be an interesting conjugated polymer with a high conductivity and have a higher environment stability . Similarly to polyacetylene, PTV is nonluminescent polymer. moreover ,PTVs show relatively high-charge carrier mobility . basing on unique property of PTV ,it have potential application for organic solar cells, organic field effect transistor and all-polymer integrated circuits. In order to study the electronic and optical property for the neutral and charged oligothienylenevinylenes ,in the paper ,geometry optimizations are first carried out by mean of the semiempirical AM1 of gaussian 03 ,on the basis of the resulting geometries ,we present the ZINDO method to calculate the lowest excitated energy and study the absorption spectra.The semi-empirical Hartree-Fock Austin Model 1 (AM1) belongs to the Chemical-quantum calculation methods. This method provides good estimates of geometries and heats of formation for organic molecules in their ground state. The AM1 approach also yields a reasonable description of charged systems, the geometry optimizations are performed on isolated molecules, thus neglecting the possible effects of counterions whose impact on the geometry is expected to be weaker in solution than in solid state. So we employed the AM1 calculation method of Gaussian o3 to determine geometric and optical properties of neutral and charged states of oligomers and their copolymers. The results that we have obtained well agree with the related experimental results.The results of two semi-empirical calculation methods for oligomers and copolymers are as followed:1. Geometry optimization calculations are presented to determine the charged structure , the electronic structure or optical properties of oligothienylenevinylenes. Compared with the neutral states, the changes of the bond lengths in the charged states are evident. The singly oxidized state corresponds to a polaron and the doubly oxidized state to a bipolaron. even if the chain lengths increase to 16 units of PTV. The quadruple oxidized state corresponds to two separate bipolarons. The ZINDO calculation shows that transition energy will decrease with the increase of chains, which means that the optical absorptions emerge red shifts.2. Under different doping condition , Stability of polarons and bipolarons in oxidized oligothienylenevinylenes was discussed by means of Austin-Model 1 (AM1) method. It was found that their stability require definite condition . In actual application , transition between bipolarons and polarons would be possible by adjusting the multispin and the ways of doped positive charges , The investigation is helpful to understand spin-polarized transport in organic semiconductor materials.At one time , we have studied the evolution of electronic optical properties in -(PPP)m-(PT)n- .Copolymer could be classified two kinds :diblock and multiblock copolymers .A diblock copolymer consists of two segments of homopolymers ,having a configuration -(A)x-(B)y- .Heeger et al ,demonstrated that the electronic properties of diblock copolymers could be tuned by the lengths and ratios of segments of homopolymers .In the paper , the stable conformation of copolymer -(PPP)m-(PT)n-was analyzed by the semiempirical AM1 method in both the neutral and charged states,the torsional angle of the interface of the most stable conformation was 25° in the neutral state, it was 0° in the charged state.On the basis of the AM1 geometries, the effect of conformational changes of the copolymer in the interface on the optical properties was investigated by means of semiempirical ZINDO method in details. It is found that the absorption spectra of copolymer -(PPP)m-(PT)n- depends on changing in the torsional angles,excitation energies of the first three excited states is increased by varying the torsional angle from 0° to 90° in the neutral state. Moreover, this movement uptrend is more evident with decreasing chain length and increasing proportion of containing benzene rings; it is reduced with increasing torsional angles in the doubly charged states. |
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