Electronic dynamics in long linear and cyclic polyynes towards the carbyne limit

TL;DR

This study investigates the electronic dynamics of long linear and cyclic polyynes approaching the carbyne limit, revealing topology-dependent behaviors and minimal excited-state structural changes at 48 carbons.

Contribution

It provides new insights into the properties of long polyynes, highlighting the influence of topology on their electronic and vibrational dynamics near the carbyne limit.

Findings

Highly delocalized ground states in both topologies

Weaker Peierls distortions in long chains

Rapid self-localisation of excited states depending on topology

Abstract

Carbyne-the one-dimensional sp-hybridised allotrope of carbon-has long been predicted to exhibit unique properties, yet its synthesis remains elusive. To probe its behaviour, finite sp-carbon chains such as cumulenes and polyynes have been studied, but work to date has focused almost exclusively on short, linear systems far from the infinite carbyne limit and without considering topology. Here, we investigate long (48-carbon) linear and cyclic polyynes using steady-state and ultrafast, temperature- and polarization-resolved optical and vibrational spectroscopy. We find highly delocalized ground states in both topologies, with Peierls distortions markedly weaker than in short chains. In contrast, excited states undergo rapid self-localisation, with the localisation pathway and subsequent intersystem crossing strongly dependent on chain length and topology. Unlike shorter polyynes, excited-state structural rearrangements are minimal, and comparison with theoretical predictions shows that properties, such as Huang-Rhys factors, have plateaued by 48 carbons. Our results reveal how topology influences the electronic dynamics of long polyynes and refines our understanding of sp-carbon systems approaching the carbyne limit