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PreprintCC BYChemRxiv

Single and Multiexciton Dynamics in Porphyrin Nanorings

  • Erico M. Braun1,2,3
  • Michael Jirasek4,5
  • Harry L. Anderson4,5
  • Stephen R. Meech2,3
  • Ismael A. Heisler1
  • Giovanni Bressan2,3
5 affiliations
  1. 1Departamento de Física, Universidade Federal do Rio Grande do Sul (UFRGS, Av. Bento Gonçalves, Porto Alegre, Brazil, 9500
  2. 2School of Chemistry, Pharmacy & Pharmacology
  3. 3University of East Anglia
  4. 4Department of Chemistry
  5. 5University of Oxford
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Abstract

Exciton transport in molecular assemblies is typically studied in linear chains and polydisperse aggregates, where chain ends perturb the electronic structure and structural inhomogeneity can obscure size-dependent trends. Molecular nanorings overcome both limitations: they are end-group-free macrocycles with well-defined sizes, while their dimensions can be synthetically tuned from a few to several tens of porphyrin units. Here, we use intensity-cycled femtosecond transient absorption spectroscopy to disentangle single- and multiexciton dynamics in a series of fully conjugated molecular nanorings containing 6–40 Zn-porphyrin units. By isolating the third-, fifth-, and seventh-order nonlinear responses, we independently track single-, two-, and three-exciton dynamics, enabling the ring-size dependence of exciton–exciton annihilation (EEA) to be determined without relying on a priori kinetic assumptions. The EEA rate decreases systematically with increasing ring size, accompanied by a crossover from annihilation-limited to diffusion-limited EEA between 20 and 30 porphyrin units. The smallest nanoring exhibits no resolvable multiexciton dynamics, consistent with exciton delocalisation across the entire ring or equivalent sub-instrument response EEA, whereas larger rings exhibit increasingly diffusive exciton motion as static disorder limits delocalisation. The retrieved exciton diffusion coefficients are two to three orders of magnitude higher than those we previously reported, indicating that the assumptions inherent to conventional intensity-dependent fitting can substantially underestimate exciton mobility.

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Cite

BibTeXRIS
BibTeX
@misc{braun2026single,
  title = {{Single and Multiexciton Dynamics in Porphyrin Nanorings}},
  author = {Erico M. Braun and Michael Jirasek and Harry L. Anderson and Stephen R. Meech and Ismael A. Heisler and Giovanni Bressan},
  howpublished = {ChemRxiv},
  year = {2026},
  month = oct,
  doi = {10.26434/chemrxiv.15007829/v2},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15007829/v2},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - Single and Multiexciton Dynamics in Porphyrin Nanorings
AU  - Erico M. Braun
AU  - Michael Jirasek
AU  - Harry L. Anderson
AU  - Stephen R. Meech
AU  - Ismael A. Heisler
AU  - Giovanni Bressan
PY  - 2026
DA  - 2026/10/08
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15007829/v2
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15007829/v2
AB  - Exciton transport in molecular assemblies is typically studied in linear chains and polydisperse aggregates, where chain ends perturb the electronic structure and structural inhomogeneity can obscure size-dependent trends. Molecular nanorings overcome both limitations: they are end-group-free macrocycles with well-defined sizes, while their dimensions can be synthetically tuned from a few to several tens of porphyrin units. Here, we use intensity-cycled femtosecond transient absorption spectroscopy to disentangle single- and multiexciton dynamics in a series of fully conjugated molecular nanorings containing 6–40 Zn-porphyrin units. By isolating the third-, fifth-, and seventh-order nonlinear responses, we independently track single-, two-, and three-exciton dynamics, enabling the ring-size dependence of exciton–exciton annihilation (EEA) to be determined without relying on a priori kinetic assumptions. The EEA rate decreases systematically with increasing ring size, accompanied by a crossover from annihilation-limited to diffusion-limited EEA between 20 and 30 porphyrin units. The smallest nanoring exhibits no resolvable multiexciton dynamics, consistent with exciton delocalisation across the entire ring or equivalent sub-instrument response EEA, whereas larger rings exhibit increasingly diffusive exciton motion as static disorder limits delocalisation. The retrieved exciton diffusion coefficients are two to three orders of magnitude higher than those we previously reported, indicating that the assumptions inherent to conventional intensity-dependent fitting can substantially underestimate exciton mobility.
ER  -