Effect of solvation and intermolecular interactions on the structure and optical properties of PANI oligomers

Abstract

Polyaniline (PANI) has attracted lasting interest due to its unconventional electronic, optical, and electro-optical properties. A rigid backbone polymer, PANI exhibits very strong intra-and intermolecular interactions governing its overall behavior. The purpose of this study is to investigate theoretically the contribution of the various inter-and intramolecular interactions to the process of organization of PANI macromolecules in aqueous environment. The structure and spectral conduct of oligoanilines (tetramers) in different oxidation states have been calculated at the Monte Carlo/Molecular Mechanics (MC/MM) and quantum chemical (QC) semiempirical level. The effect of protonation on the optical properties of partially and fully oxidized oligomers is discussed. The role of intermolecular interactions and the influence of water as a solvent are estimated for a number of hydrated clusters. Computed results and experimental spectra are compared.

Theoretical study of the structure and electronic spectra of fully protonated emeraldine oligomers

“…Owing to the nonidentical atomic charges (Table I) of the two imino groups, protonation in the tetramer can be carried out in two stages leading to partially or fully protonated emeraldines. The theoretical study of partially protonated emeraldine systems has already been described in a previous work [31]. In the present article we extend the range of our investigations by taking into consideration the structural and spectral properties of fully (doubly) protonated emeraldine tetramers and the interchain interaction modeled as two stacked molecules.…”

“…Protonation of the imino nitrogen atoms in emeraldine base can occur upon exposure to strong acids [31]. Since amino groups have basic properties that are even weaker than those of ammonia, only imino nitrogen atoms are assumed protonated in the models.…”

“…For the stack assembly, the two tetramer chains are put at 3 Å from each other, as prescribed in the literature sources [31]. The mutual orientation of the chains is dictated by maximum overlap requirement between benzene and quinoid rings.…”

“…The chloride counterions are added to the optimized tetramers at 3.5 Å from the corresponding protons followed by a new force field geometry optimization (gradient 0.01 kcal/ mol. Å) with COC bonding parameters in AMBER96 modified to reproduce correctly the structure of the quinoid rings [31,43]. The AMBER96 optimized structures are then used for the construction of all clusters.…”

“…Thus, the search of better models, which take into account the end and solvation effects, is of great importance for the rationalization of theoretical results. In our previous studies [31], some of the mentioned drawbacks are corrected by design of model clusters of hydrated asymmetric non-and mono-protonated tetramers, where the water is accounted for explicitly (ϳ100 molecules per oligomer). In this article, we keep to the same calculation scheme and extend the range of our studies on doubly protonated emeraldine tetramers (DPE), which are models of the form responsible for PANI conductivity, i.e., the emeraldine salt.…”