Density-Functional Theory and Car-Parinello Study of Electronic, Structural, and Dynamical Properties of the Hexapyrrole Molecule

Abstract

Electronic and structural properties of the ground state of the neutral and singly ionized hexapyrrole molecule are studied using density functional theory implemented with plane waves and pseudopotentials to include the effects of the core electrons. The calculated properties are compared with those of an infinite pyrrole chain in a regular 3D lattice of polypyrroles. The dynamical evolution of a charged defect along the chain of an isolated hexapyrrole created with a structural defect on a terminal ring is simulated using Car -Parrinello molecular dynamics. Information is obtained on coherence, time scale, and general features of the motion of the charge along the chain. The aim of this paper is, first, to compare the electronic and structural properties of an isolated, finite chain of pyrrole rings with those of the corresponding infinite chain in a regular 3D lattice of polypyrroles, thus identifying the differences that can be attributed mainly to environment effects. Second, using CarParrinello (CP) molecular dynamics, 1 we shall study the time evolution of a charged defect along the chain of an isolated hexapyrrole created with a structural defect on a terminal ring. The aim is to obtain information on coherence, time scale, and general features of the motion of a charge suddenly produced inside the distorted chain of a conjugated heterocycle polymer. In a previous paper, 2 hereafter referred to as I, we have performed a density functional theory (DFT) study of neutral and doped polypyrroles, each of which is represented as a 3D lattice of infinite chains of pyrrole rings defined in terms of a fixed unit cell with internal degrees of freedom. The polypyrrole has been chosen as a prototype of the organic conjugated polymers that become conducting when properly doped and, thus, have attracted much interest both from a theoretical and an experimental point of view. 3,4,5 It is well-known from several sets of data that real polypyrroles are materials with low degrees of crystallinity, being mixtures of chains with large weight dispersion and different morphology. The presence of several chemical and structural defects makes it difficult to understand whether a property is intrinsically associated with the polymer or caused by defects. For these reasons, a crystalline lattice of infinite regular chains represents a model that contains only some of the peculiar features of the real systems and should be complemented by studying the properties of isolated pyrrole oligomers. Moreover, regular ( R-R ′)-trans-dipyrrole oligomers, with up to seven pyrrole rings, have recently been synthesized and unambiguously characterized. 6 These compounds represent ideal systems for a theoretical study based on ab initio methods because their properties are intrinsically associated with those of an isolated chain and are affected only slightly by the environment. In this paper, we investigate electronic, structural, and dynamical properties of an isolated hexapyrrole both neutral and singly ionized, and perform CP molecular dynamics in order to simulate transport phenomena inside a small chain and obtain information useful for polymers. We have chosen this oligomer because its structure is large enough to exhibit a conformational behavior resembling that of a typical polypyrrole chain, and the electronic properties are sufficiently similar to those of the polymer, as shown by our test calculations. 7,8