Ultrafast Excited-State Dynamics in Halogen Terminated Polyynes: Influence of the Halogen Group

Carbyne, the ideal infinite one-dimensional chain of sp-hybridized carbon atoms, is the missing carbon allotrope besides graphite and diamond. It is predicted to exhibit exceptional mechanical, thermal, and electronic properties due to its extended π-electron conjugation and strong electron-phonon coupling [1]. Two main electronic configurations exist: the cumulenic form =(C=C)<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</inf>=, an ideal metallic structure with zero bandgap, and the polyynic form -(C=C)<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</inf>-, an alternated structure with a finite bandgap. Their stabilization is challenging, and it can be modulated by terminal groups. Among these, halogen end-capped polyynes (halopolyynes) are particularly interesting due to their selective reactivity and suitability as precursors for functionalization. Understanding their structural, electronic, and vibrational properties well as their excited-state dynamics, is thus relevant for practical applications. Vibrational properties of halogenated carbynes have been recently addressed [2], while the excited-state relaxation has been elucidated in prototypical linear carbon chains [3] and very few different systems (e.g. ref. [4]), with the role of halogen terminations still unexplored.