# Transporting and concentrating vibrational energy to promote isomerization

## Abstract

Visible-light absorption and transport of the resultant electronic excitations to a reaction centre through Förster resonance energy transfer1,2,3 (FRET) are critical to the operation of biological light-harvesting systems4, and are used in various artificial systems made of synthetic dyes5, polymers6 or nanodots7,8. The fundamental equations describing FRET are similar to those describing vibration-to-vibration (V–V) energy transfer9, and suggest that transport and localization of vibrational energy should, in principle, also be possible. Although it is known that vibrational excitation can promote reactions10,11,12,13,14,15,16, transporting and concentrating vibrational energy has not yet been reported. We have recently demonstrated orientational isomerization enabled by vibrational energy pooling in a CO adsorbate layer on a NaCl(100) surface17. Here we build on that work to show that the isomerization reaction proceeds more efficiently with a thick 12C16O overlayer that absorbs more mid-infrared photons and transports the resultant vibrational excitations by V–V energy transfer to a 13C18O–NaCl interface. The vibrational energy density achieved at the interface is 30 times higher than that obtained with direct excitation of the interfacial CO. We anticipate that with careful system design, these concepts could be used to drive other chemical transformations, providing new approaches to condensed phase chemistry.

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## Data availability

All data generated and analysed during the current study are available from the corresponding author on request. Source data are provided with this paper.

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## Acknowledgements

We acknowledge the Max-Planck EPFL Center for Molecular Nanoscience and Technology for support.

## Author information

Authors

### Contributions

J.A.L. performed the presented infrared emission experiments and analysed the data; L.C. performed early-stage infrared emission experiments and built the instrument; A.C. performed infrared emission experiments; D.S. built the instrument and supervised the experimentation; V.B.V. developed the SNSPD used in this work; A.M.W. conceived the experiment; J.A.L. and A.M.W. wrote the initial draft of the manuscript; all authors participated in writing and revising the manuscript.

### Corresponding author

Correspondence to Alec M. Wodtke.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks the anonymous reviewers for their contribution to the peer review of this work.

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## Extended data figures and tables

### Extended Data Fig. 1 Wavelength-dependent relative detection efficiencies of the SNSPD-based emission spectrometer at different bias currents.

The curves for bias currents of 3.4 μA (black) and 5.0 μA (red) are applicable to overlayer and monolayer excitation, respectively.

### Extended Data Fig. 2 Fluorescence rate constants.

Overtone fluorescence rate constants, $${k}_{v}^{\Delta v=2}$$, of the 13C18O C-down (black squares) and O-down (red circles) isomers, the 13C18O overlayer (blue triangles) and the 12C16O overlayer (green triangles).

### Extended Data Fig. 3 Vibrational energy transfer efficiency.

a, The population distributions of the O-down and C-down isomers (blue) in the monolayer are compared to the overlayer population distribution (green) for m38o26 excitation. Where lines in the m38o26 emission spectrum are blended, no population is shown. b, Efficiency of transfer of vibrational excitation from the overlayer to the monolayer for different observation time windows after m38o26 excitation. The transfer efficiency is defined as the number of monolayer quanta, calculated from the blue bars, relative to the total number of quanta present in the system (in the overlayer and monolayer) shown in a. Error bars are estimated as described in Methods section 'Correction of emission spectra and conversion to relative population'.

### Extended Data Fig. 4 Overtone emission spectra of the m38o38 and m26o26 samples for excitation of the first layer and the overlayer.

a, Excitation of the C-down isomer in the first layer of the m38o38 sample (m38o38, 120 μJ per pulse) results in emission from the C-down (blue comb) and O-down (red comb) species in the monolayer and from the overlayer (green comb). b, Excitation of overlayer molecules in the m38o38 sample (m38o38, 100 μJ per pulse). The green comb shows the vibrational assignment for emission from 13C18O overlayer molecules and agrees with the frequencies observed in a. c, Excitation of the C-down isomer in the first layer of the m26o26 sample (m26o26, 120 μJ per pulse) results in emission from the C-down (blue comb) and O-down (red comb) species in the monolayer and from the overlayer (green comb). d, Excitation of overlayer molecules in the m26o26 sample (m26o26, 120 μJ per pulse). The green comb shows the assignment for emission from the 12C16O overlayer. All spectra are integrated over the first 500 μs after the laser pulse and were measured at 6.5 K.

### Extended Data Fig. 5 LIF excitation spectra.

LIF excitation spectra for excitation of the symmetric vibration of m38o26, m38o38, m26o26 and m26o38 transitions (integrated over the first 500 μs after excitation). The emission line (species and vibrational state) used to obtain each spectrum is indicated in parentheses. The combs underneath the spectra indicate the exciton splitting (left and right teeth) as observed in the FTIR absorption spectra (see Fig. 1), as well as a third tooth at the estimated absorption frequency should exciton splitting be neglected. The inset shows a comparison of the m38o26 spectrum of the main figure (black curve) with that seen after m38o26 excitation (red curve). The increased red shift is due to enhanced CO flipping.

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Lau, J.A., Chen, L., Choudhury, A. et al. Transporting and concentrating vibrational energy to promote isomerization. Nature (2021). https://doi.org/10.1038/s41586-020-03081-y

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