On the structure-to-property relationship of polyanilines. A modern quantum chemistry perspective

We employ state-of-the-art quantum chemistry methods to study the structure-to-property relationship in polyanilines (PANIs) of different lengths and oxidation states. Specifically, we focus on leucoemeraldine, emeraldine, and pernigraniline in their tetramer and octamer forms. We scrutinize their structural properties, HOMO and LUMO energies, HOMO-LUMO gaps, and vibrational and electronic spectroscopy using various Density Functional Approximations (DFAs). Furthermore, the accuracy of DFAs is assessed by comparing them to experimental and wavefunction-based reference data. For that purpose, we performed large-scale orbital-optimized pair-Coupled Cluster Doubles (oo-pCCD) calculations for ground and electronically excited states and conventional Configuration Interaction Singles (CIS) calculations for electronically excited states in all investigated systems. Furthermore, we augment our study with a quantum informational analysis of orbital correlations in various forms of PANIs. Our study highlights the growing multi-reference nature of PANIs with the length of the polymer. While structural and vibrational features of the investigated PANIs, regardless of their oxidation states, are adequately modeled in the tetramer forms, the length of the PANI chain profoundly affects electronic spectra. Specifically, polymer elongation changes the character of the leading transitions in the lowest-lying excited states in all investigated PANIs.