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# Energy funnelling within multichromophore architectures monitored with subnanometre resolution

## Abstract

The funnelling of energy within multichromophoric assemblies is at the heart of the efficient conversion of solar energy by plants. The detailed mechanisms of this process are still actively debated as they rely on complex interactions between a large number of chromophores and their environment. Here we used luminescence induced by scanning tunnelling microscopy to probe model multichromophoric structures assembled on a surface. Mimicking strategies developed by photosynthetic systems, individual molecules were used as ancillary, passive or blocking elements to promote and direct resonant energy transfer between distant donor and acceptor units. As it relies on organic chromophores as the elementary components, this approach constitutes a powerful model to address fundamental physical processes at play in natural light-harvesting complexes.

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## Data availability

The data supporting the findings of the present study can be found in Methods and the Extended Data. All the datasets are also available from the corresponding authors (A.R. and G.S.) upon request. Source data are provided with this paper.

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## Acknowledgements

We thank V. Speisser for technical support and A. Boeglin for discussions. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 771850) and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 894434. The Agence National de la Recherche (project organiso no. ANR-15-CE09-0017), the Labex NIE (contract no. ANR-11-LABX-0058_NIE) and the International Center for Frontier Research in Chemistry (FRC) are acknowledged for financial support.

## Author information

Authors

### Contributions

S.C., A.R., B.D. and G.S. conceived, designed and performed the experiments. S.C., A.R., M.R., F.S. and G.S. analysed the experimental data. F.S. and H.B. performed the oscillatory model. M.F. and F.C. synthesized the PdPc chromophores. All the authors discussed the results and contributed to the redaction of the paper.

### Corresponding authors

Correspondence to Anna Rosławska or Guillaume Schull.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 Spectra used to obtain the D-A distance dependence presented in Fig. 2i of the main manuscript.

a, STM image of the H2Pc–PdPc dimer, I = 5 pA, V = − 2.5 V. Scale bar 1 nm. b, RETeff values calculated from the spectra in (c). The horizontal error bars consider a 5 % error in the estimation of the molecule positions. The vertical error bars are smaller than the symbol size as the statistical error on the number of photon counts is less than 3 %. c, Normalized STML spectra acquired at positions marked in Fig. 2h, V = − 2.5 V, acquisition time t = 60 s, I = 50 pA (for R = 2.24 nm), I = 100 pA (for R = 1.68, 1.97, 3.2 nm), I = 250 pA (for R = 2.47, 2.72 nm). The spectra were normalized by the plasmonic response of the cavity41 to ensure a fair comparison between the intensities of the molecular emission lines, and scaled to unity. The splits observed in the PdPc spectra correspond to a partial lifting of the $$Q_{{{\rm{Pd}}}_{x}}$$ and $$Q_{{{\rm{Pd}}}_{y}}$$ degeneracy that seems to depend on details of the adsorption site. A similar effect has been reported previously 21 for H2Pc. RETeff values were estimated by integrating the light emission intensities in the spectral ranges indicated in blue and red. (d) Enlarged view of the H2Pc line displayed in (c).

### Extended Data Fig. 2 Normalized emission and absorption spectra.

a, PdPc–ZnPc (b) ZnPc–H2Pc and (c) PdPc–H2Pc dimers. d, Spectral overlaps (J), energy differences (ΔE) and RET efficiencies for the dimer configurations presented in Fig. 2.

### Extended Data Fig. 3 The effect of the relative dipole orientation on RET.

a, Geometrical configuration of the donor and acceptor transition dipoles in a dimer. R is the vector joining the centers of the dipoles. b, κ2 values calculated for the dimer configurations presented in (c). c, Top: STM image (V = − 2.5 V, I = 10 pA). Scale bar 1 nm. Bottom: ball-and-stick models of the PdPc–ZnPc donor–acceptor pair with the respective dipoles indicated. d,e, Plasmon-corrected STML spectra (V = − 2.5 V, I = 300 pA, acquisition time t = 300 s) recorded at the positions marked by a black dot (d) and grey star (e) in (c). The main emission lines of each molecule are highlighted in color in the spectra. These data are also presented in Fig. 2a,d of the main manuscript.

### Extended Data Fig. 4 Charge state of the ZnPc molecule.

a, STM image (V = − 2.5 V, I = 10 pA) of a PdPc-ZnPc dimer. Scale bar 1 nm. b, and (c) STML spectra recorded at positions marked in (a), V = − 2.7 V, I = 300 pA, acquisition time t = 180 s for (b) and t = 100 s (c). Note the different vertical scales.

### Extended Data Fig. 5 RET efficiency maps for H2Pc and ZnPc as acceptors in the PdPc-ZnPc-H2Pc trimer.

The circles mark the spatial extension of the acceptor, the arrows indicate the transition dipoles of the labelled chromophores. The high-intensity areas denote precise locations where the sub-molecular excitation of the donor results in an efficient energy transfer to the acceptor (H2Pc in (a) and ZnPc in (b)). The color scales range from 0 to 1.

### Extended Data Fig. 6 Modeling the role of an intermediate molecule with a classical oscillatory approach.

a, Graphical representation of the three-pendulum model. Oscillation amplitudes of the three pendulums as function of the normalized time t/TD for a large (b), a medium (c) and a small (d) eigenfrequency of the intermediate pendulum, where TD = 2π/ωD. e, Fraction of the excitation energy dissipated by the donor (D), the intermediate (I) and acceptor (A) pendulums as a function of the normalized intermediate pendulum eigenfrequency ωI/ωD.

### Extended Data Fig. 7 Comparison of the single-molecule and trimer dI/dV spectra.

a, Left panel: dI/dV spectra recorded on individual H2Pc, PdPc and ZnPc. Set-point: V = -3 V, I = 15 pA. Right panel: STM images of the corresponding individual molecules, the dots indicate the positions where the dI/dV spectra were acquired. V = -2.5 V, I = 5 pA. b, A series of 25 dI/dV spectra acquired along the H2Pc–PdPc–ZnPc trimer (following the black arrow in inset). Set-point: V = -3 V, I = 15 pA. Inset: STM image of the studied trimer, V = -2.5 V, I = 5 pA. All scale bars are 1 nm.

### Extended Data Fig. 8 RET excited ’at-distance’.

a, Sketch of the experiment. b, STML spectra acquired with the STM tip located at a distance of r = 2.2 nm (upper curve, I) and r = 4.0 nm (bottom curve, I0) from the center of the PdPc molecule. Inset: STM image V = -2.5 V, I = 5 pA of the investigated PdPc–H2Pc dimer. The dot and star mark the positions at which the spectra have been recorded. Scale bar 1 nm. c, Normalized spectrum I/I0.

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Cao, S., Rosławska, A., Doppagne, B. et al. Energy funnelling within multichromophore architectures monitored with subnanometre resolution. Nat. Chem. 13, 766–770 (2021). https://doi.org/10.1038/s41557-021-00697-z

• Accepted:

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• Issue Date:

• DOI: https://doi.org/10.1038/s41557-021-00697-z

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