Torsional effects on the optical properties of PPV and PPP
Highlights
► Optical properties of PPV and PPP are calculated using SIESTA program based on DFT approach. ► Conformational analysis and geometry optimization were done by minimizing the total energy. ► We report the influence of the chain length and the torsional angle on the band gap. ► Optical absorption spectrum for different conformation are compared to the experimental observations.
Introduction
Since the first detection of electroluminescence from poly(para-phenylene vinylene) (PPV) [1] and the first fabrication of a blue light-emitting device using poly(para-phenylene) (PPP) [2], the semiconducting conjugated polymers have emerged as a highly promising class of materials for electronic and optoelectronic applications. These two polymers are attractive materials for the theoretical study of the properties of conjugated polymers, since they take the form of relatively simple quasi-one-dimensional molecules arranged in a three-dimensional crystal structure. This arrangement makes it possible to study structural, electronic and optical properties in both the isolated chain and the crystalline phase. The interesting properties of these compounds come from the large conjugation and π-electron delocalization along the chain length. As a result, there is a considerable worldwide effort devoted to understanding the basic properties of these promising materials, and to developing efficient devices based on them. Ab initio or first principles approaches of the structural and electronic properties of polymers are very difficult, many aspects of the basic physics of these polymers are not well established and their properties are not fully understood. They are essentially complex materials where structural disorder is expected to play a major role in determining the physical properties.
However, attempts to enhance the performance of such devices have generally been frustrated by a limited knowledge of the microscopic mechanisms of charge transport, relaxation and recombination in these materials. Using a Hückel tight-binding Hamiltonian coupled to the negative factor counting (NFC) technique, Giro et al. [3] find that it is possible to obtain co-polymers of PPV and PPP with intermediate gap values of their parent structures. The band structure calculations for the PPV in the isolated and crystalline states were performed using the CASTEP plane wave code [4], and the results agree well with the other available theoretical data. De Carvalho et al. [5] use the large cell approach, in connection with the semi-empirical quantum method Extended Hückel (BICON-CEDiT code) and the Density Functional Theory (DFT) within the full-potential linearized augmented plane wave method (FPLAPW) as implemented in the computational code WIEN2K. Their results compare well with other calculations and optical measurements. Recently, films of PPV and nanocomposite materials based on PPV and carbon nanotubes have been studied experimentally and theoretically by Massuyeau et al. [6], [7], [8], [9], [10]. These experimental results give the motivation to investigate theoretically the electronic and optical properties of PPV for a good understanding of the microscopic origin of the very interesting electrical and optical properties of these compounds.
In this context, we perform ab initio calculations of infinite, isolated PPV and PPP chains focusing on electronic and optical properties. We use a formalism based on the Density Functional Theory (DFT) within the Local Density Approximation (LDA) for the exchange and correlation effects. We use pseudopotentials to describe the effects of core electrons which are not of interest in this work. We briefly describe in the next section the calculations details for the ab initio method used, and then in Section 3 we discuss the electronic and optical properties in relation with the geometry of the corresponding polymers. Finally, we summarize our work in conclusion.
Section snippets
Calculation method
We have investigated the optical properties of PPV and PPP polymers by using the SIESTAs (Spanish Initiative for Electronic Simulations with Thousands of Atoms) simulation package described in details elsewhere [11], [12]. It consists in DFT calculations using numerical atomic orbitals as basis sets to solve the single-particle Kohn–Sham equations.
In these calculations, we have used a double-ζ basis with single-ζ polarization functions. The core electrons were represented by nonlocal, norm
Results and discussion
There are two kinds of π-electrons in PPV chains: the first kind is distributed over all carbon atoms, forming an uninterrupted π-electron system over the interchain. In the second one π-states are delocalized only on the phenyl ring. The interesting properties of the conjugated polymers come essentially from π-electrons delocalization along the chain length. The mobility of these kind of electrons is very sensitive to all conformational changes in the chain: the conjugation and mobility of the
Conclusion
The theoretical study of the electronic and optical properties of infinite, isolated chain of poly(para-phenylene) and poly(para-phenylene vinylene) has been performed using the Density Functional Theory that employs linear combination of orbitals as basis set, norm-conserving pseudopotential and the Local Density Approximation (LDA) for exchange correlation. In particular, we have examined the effect of length chains and torsional angles on the optical properties of PPV and PPP polymers. An
Acknowledgement
The authors acknowledge the numerical support of the Centre de Calcul Intensif des Pays de la Loire (Nantes).
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