The relationship between structure and excited-state properties in polyanilines from geminal-based methods

TL;DR

This study uses advanced quantum chemistry methods to analyze how the structure of polyanilines influences their excited-state properties, revealing charge transfer behaviors and the impact of polymer length and oxidation state.

Contribution

It introduces orbital-optimized pair-Coupled Cluster Doubles and quantum informational analysis to better understand excited states in polyanilines, surpassing traditional DFA limitations.

Findings

Charge transfer dominates low-lying spectra of emeraldine and pernigraniline.

Polymer elongation alters the nature of electronic transitions.

Orbital-optimized methods clarify transition character beyond DFA capabilities.

Abstract

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. We perform 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. The EOM-pCCD+S approach with pCCD-optimized orbitals allows us to unambiguously identify charge transfer and local transitions across the investigated PANI systems -- an analysis not possible within a delocalized canonical molecular orbital basis obtained, for instance, by DFAs. We show that the low-lying part of the emeraldine and pernigraniline spectrum is dominated by charge transfer excitations and that polymer elongation changes the character of the leading transitions. Furthermore, we augment our study with a quantum informational analysis of orbital correlations in various forms of PANIs.