The Aromatic-to-Quinonoid Transformation in π-Conjugated Polymer Chains Is Not Necessarily Related to a Topological Phase Transition

A topological aromatic-to-quinonoid (AQ) phase transition has recently been reported in several one-dimensional π-conjugated polymers that display low bandgaps. An intriguing question then arises: Does the AQ transformation universally indicate a topological phase transition in such systems, specifically in those with wide bandgaps? To address this issue, poly(para-phenylene) (PPP) is considered here as a representative model system. Using first-principles calculations and tight-binding models, its intra- and inter-ring electronic interactions, topological invariant characteristics, bandgap evolution during the AQ transformation, and emergence of topological boundary states are systematically evaluated. Our results demonstrate that PPP exhibits intraring interactions that are stronger than inter-ring interactions in both its aromatic and quinonoid configurations. Also, the topological invariant remains unchanged between the two configurations, and no bandgap closure and reopening are observed during the AQ transformation. Additionally, the conditions governing the emergence of topological end modes in finite chains are identical for both configurations, indicating their topological equivalence. Overall, these findings firmly establish that in PPP, the aromatic and quinonoid configurations belong to the same topological class. What this means is that the AQ transformation in π-conjugated polymers does not universally correspond to a topological phase transition, which points out the importance of a comprehensive approach in evaluating the topological properties of low-dimensional organic materials.