Quantitative self-sorting of bis(N-arylpyridinium) chromophore guests under double cucurbit[8]uril (CB[8]) clamping was shown in 2020 to produce discrete, pseudostatic heterodimers in water in which photoexcitation of a 1,4-naphthyl donor is followed by non-radiative energy transfer to a stacked 9,10-anthracenyl acceptor with an efficiency above 85%. The microscopic origin of this transfer was left open: whether it proceeds by short-range Dexter-type electron exchange, by longer-range Förster-type dipole–dipole resonance coupling, or by a combination of both, with the authors stating that the nature (short- and/or long-ranged) of the transfer was under investigation with time-resolved techniques. Later work has applied and structurally diversified CB[8]-mediated (hetero)dimerization, but no time-resolved or theoretical determination of the operative mechanism, the transfer rate constants, or their dependence on the enforced stacking geometry has been published to date. The problem is to determine this mechanism and the associated transfer rate constants, thereby converting an efficient but mechanistically unexplained supramolecular energy-transfer scaffold into a predictable design platform.

Background

Cucurbit[n]urils (CB[n]) are pumpkin-shaped macrocyclic hosts made of glycoluril units; CB[8] is large enough to encapsulate two aromatic guests simultaneously, forming so-called homo- or heteroternary complexes. Ordinarily such two-guest complexes are highly dynamic, so any interchromophore electronic coupling is averaged out before excited-state processes, which occur on femtosecond-to-nanosecond timescales, can exploit it. A strategy introduced by the Cambridge group around 2017–2020 overcomes this: guests of the bis(N-arylpyridinium) (BAP) family — rigid rods with an aromatic chromophore core and pyridinium ends — are stacked face-to-face and 'clamped' by two CB[8] macrocycles in a 2:2 complex, giving chromophore dimers that persist as single entities for more than 30 ms ('pseudostatic' dimers) with a fixed interchromophore spacing of about 4 Å set by the CB[8] cavity. Femtosecond and nanosecond transient absorption (monitoring the time evolution of absorption changes after a pump pulse) plus time-correlated single-photon counting studies of the corresponding homodimers showed mono-exponential, excimer-like excited states — excimers being dimeric complexes that are bound only in the excited state — with elongated lifetimes and high quantum yields, establishing the platform's photophysics but not addressing directional donor-to-acceptor transfer.

In the 2020 source work, quantitative heterodimer formation (rather than a statistical AA/AB/BB mixture) was achieved by shape-complementary self-sorting: one guest carries bulky end groups and its partner small ones, so that self-recognition is sterically disfavored. Mixing the 1,4-naphthyl-core guest Np14CMe2 with the 9,10-anthracenyl-core guest Ant910H and CB[8] quantitatively yields the heterochromophore dimer. Steady-state spectroscopy — comparing the naphthyl emission of the real ('physical') mixture with the spectral superposition ('mathematical') mixture under excitation at an isosbestic point (a wavelength at which both chromophores absorb equally, so each receives the same photon flux) — showed that excitation of the naphthyl absorption band produces anthracenyl emission with an estimated transfer efficiency above 85%, i.e. highly efficient non-radiative energy transfer over a fixed, host-enforced geometry.

Two limiting microscopic mechanisms are standard in molecular photophysics. Long-ranged Förster (dipole–dipole resonance) energy transfer proceeds through Coulombic coupling of the donor and acceptor transition dipoles, falls off with the inverse sixth power of the separation, and requires spectral overlap of donor emission and acceptor absorption. Short-ranged Dexter energy transfer proceeds through direct electron exchange requiring orbital (wavefunction) overlap, and therefore decays roughly exponentially with separation, becoming significant only at contact distances of a few ångströms. At the ≈4 Å, partially slipped stacking enforced by CB[8] — where the end groups are known from earlier work to control the slippage along the molecular long axis — both channels are in principle active, and their relative contributions and the associated transfer rate constants were explicitly left undetermined: the paper states only that 'the nature of this observed energy transfer (short- and/or long-ranged) is of great importance for the design of future systems, which is currently being investigated with time-resolved techniques.'

Problem Statement

Determine the microscopic nature of the highly efficient (estimated >85%) non-radiative energy transfer from the photoexcited 1,4-naphthyl donor to the 9,10-anthracenyl acceptor within the pseudostatic heterodimer formed by quantitative self-sorting of Np14CMe2 and Ant910H under double cucurbit[8]uril clamping in aqueous solution — specifically, whether the transfer is short-ranged (Dexter-type electron exchange), long-ranged (Förster-type dipole–dipole resonance energy transfer), or a combination of both, with the relative contributions of the two channels and the energy-transfer rate constants on the relevant femtosecond-to-nanosecond timescales quantified.

Any admissible experimental or theoretical route may be used to obtain the assignment and rate constants. The assignment and the rate constants are to be related to the host-enforced geometry — the ≈4 Å interchromophore spacing and the end-group-induced slippage along the molecular long axis. The goal is a mechanistic assignment — short-range, long-range, or a quantified combination — supported by quantitative energy-transfer rate constants; the question concerns this heterodimer at its native CB[8]-enforced geometry rather than a variable-geometry dimer series or a design-rule-grade geometry–coupling relationship.

References

  1. Guanglu Wu, Zehuan Huang, Oren A. Scherman, 'Quantitative Supramolecular Heterodimerization for Efficient Energy Transfer', Angewandte Chemie International Edition 2020, 59, 15963–15967, DOI 10.1002/anie.202006530, https://doi.org/10.1002/anie.202006530 (open-access full text at PubMed Central PMC7540262, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7540262/)
  2. Guanglu Wu, Youn Jue Bae, Magdalena Olesińska, Daniel Antón-García, István Szabó, Edina Rosta, Michael R. Wasielewski, Oren A. Scherman, 'Controlling the Structure and Photophysics of Fluorophore Dimers Using Multiple Cucurbit[8]uril Clampings', Chemical Science 2020, 11, 812–825, DOI 10.1039/C9SC04587B, https://doi.org/10.1039/C9SC04587B (open-access full text at PubMed Central PMC8146025, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8146025/)
  3. Guanglu Wu, Fei Li, Bohan Tang, Xi Zhang, 'Molecular Engineering of Noncovalent Dimerization', Journal of the American Chemical Society 2022, 144, 14962–14975, DOI 10.1021/jacs.2c02434, https://doi.org/10.1021/jacs.2c02434
  4. Kehuan Li, Hao Yu, Ningxu Han, Jiaqi Li, Tong Lu, Junjuan Shi, Houyu Zhang, Guanglu Wu, Ming Wang, 'Metal–Organic Dimerization of Dissymmetrical Ligands toward Customized Through-Space Chromophore Interactions', Chemistry of Materials 2023, 35, 1788–1795, DOI 10.1021/acs.chemmater.2c03668, https://doi.org/10.1021/acs.chemmater.2c03668
  5. Chenjia Yin, Zhiqin Wu, Zi-Ang Yan, Ping Jiang, Yuxi Ding, Tao Li, Yanjie Chen, He Tian, Xiang Ma, 'Cucurbit[8]uril-Induced Self-Sorting Heterodimer for Aqueous Red/NIR Delayed Emission', Journal of the American Chemical Society 2025, 147, 43286–43294, DOI 10.1021/jacs.5c07836, https://doi.org/10.1021/jacs.5c07836
  6. Tangxin Xiao, Zehuan Huang, Liangliang Zhang, Shuang Jin, Xiaoyi Chen, Hongwei Qian, Guanglu Wu, Jade A. McCune, Oren A. Scherman, 'How many fluorophores are required to achieve AIE?', Chemical Science 2025, 16, 22058–22064, DOI 10.1039/d5sc04053a, https://doi.org/10.1039/d5sc04053a (open-access full text at PubMed Central PMC12539275, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12539275/)
  7. Tianyi Yang, Jacob F. Jones, Song Zhang, Bao Li, Sijia Li, Yizhuo Yu, Yibin Sun, Haichao Liu, Bing Yang, Thomas A. A. Oliver, Guanglu Wu, 'Supramolecular Control of Dual Emission in Macrocycle-Confined Dimers', JACS Au 2026, 6, 3059–3066, DOI 10.1021/jacsau.6c00474, https://doi.org/10.1021/jacsau.6c00474 (open-access full text at PubMed Central PMC13213490, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13213490/)
  8. Haigen Nie, Die Hu, Zhiyao Zeng, Yan Fan, Yaxin Zhai, Xin-Long Ni, 'Cucurbit[8]uril-regulated face-to-face dimerization assembly enhanced photosensitization and photocatalysis', Science China Chemistry 2024, 67, 1605–1612, DOI 10.1007/s11426-023-1926-9, https://doi.org/10.1007/s11426-023-1926-9