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A direct pathway to molecular photodissociation on metal surfaces using visible light Emiko Kazuma, Jaehoon Jung, Hiromu Ueba, Michael Trenary, and Yousoo Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12680 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 7, 2017
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Journal of the American Chemical Society
A direct pathway to molecular photodissociation on metal surfaces using visible light Emiko Kazuma†, Jaehoon Jung*,‡, Hiromu Ueba§, Michael Trenary⊥, and Yousoo Kim*,† †
Surface and Interface Science Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
‡
Department of Chemistry, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 680-749, Republic of Korea
§
Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan
⊥
Department of Chemistry, University of Illinois at Chicago, 845 W Taylor Street, Chicago, Illinois 60607, United States
ABSTRACT: We demonstrate molecular photodissociation on single-crystalline metal substrates, driven by visible-light irradiation. The visible-light induced photodissociation on metal substrates has long been thought to never occur, either because visible-light energy is much smaller than the optical energy gap between the frontier electronic states of the molecule or because the molecular excited states have short lifetimes due to the strong hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S-S bond in dimethyl disulfide adsorbed on both Cu(111) and Ag(111) surfaces was dissociated through direct electronic excitation from the HOMO-derived MO (the nonbonding lone-pair type orbitals on the S atoms (nS)) to the LUMOderived MO (the antibonding orbital localized on the S-S bond (σ*SS)) by irradiation with visible light. A combination of scanning tunneling microscopy and density functional theory calculations revealed that visible-light-induced photodissociation becomes possible due to the interfacial electronic structures constructed by the hybridization between molecular orbitals and the metal substrate states. The molecule-metal hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which results in longer excited-state lifetimes.
INTRODUCTION Molecular photodissociation is a crucial reaction for the effective use of solar energy, a clean and renewable energy resource. However, the photodissociation of small molecules (H2O, O2, NOx, SOx, etc.) in the gas and liquid phases with visible light is not feasible because of the wide energy gap between the frontier molecular orbitals, such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Thus, although photodissociation has been studied extensively, most researchers have focused on irradiation by ultraviolet (UV) light. Molecular adsorption onto solid surfaces can provide a new opportunity for photochemical reactions with visible light, based on reconstructing the interfacial electronic structures due to the molecule-surface interaction. Visiblelight-induced photodissociation has recently been achieved using both plasmonic noble metal1–3 and non-plasmonic transition metal4,5 nanoparticles, and these have attracted significant attention as a novel route to practical solar energy uses. Although a fundamental understanding of the relationship between the reaction profiles and the molecular electronic structure is crucial to elucidate the underlying mechanism of photodissociation on metal nanoparticles, the determination of the mechanism has been hampered by the non-uniform surface morphologies. In contrast, the atomically well-defined surfaces of metal single crystals enable both scanning tunneling microscopy (STM) and density functional theory (DFT) calculations to investigate the photoinduced
surface dynamics and the accompanying changes in electronic structure necessary to obtain molecular-scale mechanistic insights.6–8 Molecular photochemistry of dissociation, desorption and rearrangement, induced by UV light, on single crystalline metal surfaces has been extensively studied.9–17 The UV-light photodissociation of small molecules such as O2,9,10 Cl2CO11 and OCS12 has been observed even at low temperatures (30– 100 K), although the excited states of the adsorbates on metal surfaces tend to relax rapidly (
