Ultrafast Vibrational Energy Transfer in Catalytic Monolayers at Solid–Liquid Interfaces

Abstract

We investigate the ultrafast vibrational dynamics of monolayers from adsorbed rhenium-carbonyl CO-reduction catalysts on a semiconductor surface (indium-tin-oxide (ITO)) with ultrafast two-dimensional attenuated total reflection infrared (2D ATR IR) spectroscopy. The complexes are partially equipped with isotope-labeled (C) carbonyl ligands to generate two spectroscopically distinguishable forms of the molecules. Ultrafast vibrational energy transfer between the molecules is observed via the temporal evolution of cross-peaks between their symmetric carbonyl stretching vibrations. These contributions appear with time constant of 70 and 90 ps for downhill and uphill energy transfer, respectively. The energy transfer is thus markedly slower than any of the other intramolecular dynamics. From the transfer rate, an intermolecular distance of ∼4-5 Å can be estimated, close to the van der Waals distance of the molecular head groups. The present paper presents an important cornerstone for a better understanding of intermolecular coupling mechanisms of molecules on surfaces and explains the absence of similar features in earlier studies.

Plasmonic Substrates Do Not Promote Vibrational Energy Transfer at Solid–Liquid Interfaces

“…These cross peaks originate from vibrational energy transfer between the different molecules at the interface. 8 Interestingly, the relative intensity between the cross peaks and the diagonal peaks is very similar for the TiO2/ITO samples and slightly smaller for the Au layer. To elucidate the detailed kinetics of vibrational energy transfer, we measured 2D ATR IR spectra at a full series of population delays that covers multiples of the vibrational lifetime of the complexes.…”

“…[1][2][3]5,6 For vibrational spectroscopy in the mid-infrared (IR) range, intermolecular vibrational energy transfer through space is, however, much less frequently observed, but still holds great promise to facilitate structural investigations of samples, such as relative molecular distances and orientations. 4,[7][8][9][10][11][12][13][14][15] It is therefore strongly desirable to develop experimental strategies, through which vibrational energy transfer can be engineered.…”

“…16,17 Thus, molecular monolayers are prime candidates to observe vibrational energy transfer, and quite some experimental effort went into this direction recently. 8,[18][19][20][21][22][23][24][25] We have shown in Ref. 8 that in addition to a short distance, a large transition dipole µ is required to compete with the generally short lifetime of vibrational transitions.…”

“…8,[18][19][20][21][22][23][24][25] We have shown in Ref. 8 that in addition to a short distance, a large transition dipole µ is required to compete with the generally short lifetime of vibrational transitions. As of today, intermolecular vibrational energy transfer for a molecular monolayer has been observed only for a metal-carbonyl complex, owing to its exceptionally large transition dipole in combination with a comparably long lifetime.…”

“…Comparing the dynamics of vibrational energy transfer on these surfaces with otherwise as identical as possible sample conditions allows us to study the impact of substrate polarizability and plasmonic surface enhancement on the vibrational energy transfer mechanism. 8 , but have been re-measured for consistency reasons. For all samples, the 2D ATR IR spectra at early population delays (0.5 ps) exhibit ground state bleach (blue) and excited state absorption signals (red) on the diagonal line.…”