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Peer-reviewedCC BY-NC 3.0Chemical Science

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

  • Kavya Vinod1
  • Claire Tonnelé2,3
  • Anup Rana4
  • T. Amalnadh1
  • P. E. Swathi Krishna1
  • Juan Carlos Roldao2
  • Henrik Ottosson4
  • David Casanova2,3
  • Mahesh Hariharan1
4 affiliations
  1. 1School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER TVM) Maruthamala P.O., Vithura Thiruvananthapuram 695551 Kerala India
  2. 2Donostia International Physics Center (DIPC) 20018 Donostia Euskadi Spain
  3. 3IKERBASQUE− Basque Foundation for Science 48009 Bilbao Euskadi Spain
  4. 4Department of Chemistry - Ångström Laboratory, Uppsala University Box 523 Uppsala 751 20 Sweden
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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.

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Cite

BibTeXRIS
BibTeX
@article{vinod2026azulene,
  title = {{Azulene as an anomaly in triplet-state engineering via bromination and/or carbonylation: internal conversion overrides intersystem crossing}},
  author = {Kavya Vinod and Claire Tonnelé and Anup Rana and T. Amalnadh and P. E. Swathi Krishna and Juan Carlos Roldao and Henrik Ottosson and David Casanova and Mahesh Hariharan},
  journal = {Chemical Science},
  year = {2026},
  month = sep,
  doi = {10.1039/d6sc00694a},
  url = {https://doi.org/10.1039/d6sc00694a},
  note = {CC BY-NC 3.0},
}
RIS
TY  - JOUR
TI  - Azulene as an anomaly in triplet-state engineering via bromination and/or carbonylation: internal conversion overrides intersystem crossing
AU  - Kavya Vinod
AU  - Claire Tonnelé
AU  - Anup Rana
AU  - T. Amalnadh
AU  - P. E. Swathi Krishna
AU  - Juan Carlos Roldao
AU  - Henrik Ottosson
AU  - David Casanova
AU  - Mahesh Hariharan
PY  - 2026
DA  - 2026/09/07
JO  - Chemical Science
DO  - 10.1039/d6sc00694a
UR  - https://doi.org/10.1039/d6sc00694a
AB  - 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.
SN  - 2041-6520
SN  - 2041-6539
ER  -