Azulene as an anomaly in triplet-state engineering via bromination and/or carbonylation: internal conversion overrides intersystem crossing

Abstract

Azulene, a non-benzenoid aromatic system, exhibits a unique electronic structure that gives rise to unconventional excited-state dynamics. Here, we investigate a series of halogenated and carbonylated azulenes using ultrafast spectroscopy and quantum chemical calculations. Despite incorporating substituents that are known to enhance intersystem crossing (ISC), halogenated/carbonylated azulenes exclusively display singlet excited-state dynamics. Femtosecond and nanosecond transient absorption measurements reveal rapid internal conversion without any detectable triplet population. Computational analysis using the Marcus formalism confirms that, although spin–orbit coupling increases upon bromination/carbonylation, ISC remains orders of magnitude too slow to compete with fast internal conversion. These findings highlight the intrinsic resistance of azulene derivatives to triplet-state formation, challenging established strategies for promoting ISC and offering new insights into substitution-dependent excited-state pathways in non-benzenoid aromatics.