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

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.