  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.
