Light assisted synthesis of poly-para-phenylene on Ag(001)

Abstract

A detailed study of poly-para-phenylene (PPP) obtained by light-assisted on-surface-synthesis (OSS) on Ag(100) was carried out by scanning tunneling microscopy and spectroscopy together with density functional theory calculations. The use of light in combination with heat allows to lower by 50 K annealing temperature the each stage of the Ullmann coupling. Debromination of the 4,4″ dibromo-p-terphenyl precursors was thus realized at 300 K, the formation of the first oligomers from the organometallic intermediate by silver bridging atom release at 423 K and PPP by complete elimination of the silver at 473 K. This approach to lower the reaction temperature permits to enhance the Ag(100) surface reactivity to become comparable to that of Cu(111). The underlying mechanism of light effect was proposed to occur via surface mediated excitation, with the creation of photoexcited electrons known as hot electrons correlated with surface plasmon excitation. This original pathway combining both light and heat provides an additional parameter to control OSS by separating the precursor activation stage from the diffusion.

Unraveling the Mechanisms of On-Surface Photoinduced Reaction with Polarized Light Excitations

“…We speculate that the indirect excitation is not limited to the reaction we have explored currently but applies to other photoinduced reactions on metal surfaces. 25,26,50 This is supported by the following facts: Cross sections of optical adsorption are normally larger for the metal substrate than the adsorbates. The direct excitation often requires photons with shorter wavelength, while the hot electron excitation process offers a wider range of photon wavelengths for initiating reactions, which is mainly related to the nature of the metal surface.…”

“…Although it is fundamentally important to distinguish between the two main processes, direct experimental investigations are scarce. Previous reports of photoinduced debromination on surfaces have suggested that the underlying mechanism is a hot electron process, 25 but direct experimental evidence is difficult to provide. The reason is that unlike in the gas or liquid phase where the optical properties of molecules can be measured directly by optical spectroscopy, experimental evidence on surfaces often relies on ultrafast techniques such as two-photon emission spectroscopy.…”

“…A low molecular coverage favors the windmill self-assembly, and a higher molecular coverage tends to yield more herringbone structures, which is consistent with previous reports. 25,38 Eventually, the surface is fully covered by the herringbone nanostructure. The driving force to the windmill pattern is attributed to the synergic halogen (Br•••Br−C) and hydrogen (Br•••H−C) bonding, as demonstrated by the charge density difference map of the tetramer structure constituting the windmill pattern (see Figure S4), which is in line with similar brominated molecules, 39 while the herringbone pattern is stabilized by the hydrogen bonding, which allows for a denser packing of the molecules.…”

“…Different light sources have been introduced to induce chemical reactions on surfaces, including LED and laser lights with nominal wavelengths from the ultraviolet to visible regime and discharge lamps with high power and broad emission spectra. ,,, Here, we have used a laser light at a wavelength of 405 nm with the following considerations: (1) The violet visible spectrum ensures an easy and safe operation. (2) The optical absorption of the gold depends on the wavelength, and it is relatively high at the wavelengths around 400 nm within the visible light range .…”

“…Because of the advantages of photoinduced reactions, they are gaining increased attention in the field. Recent works, such as the cleavage of the C–Br bond, − initiation of Glaser coupling, tip-enhanced S–S bond cleavage, photodissociation of protecting groups, reversible [4 + 4] photocycloadditions, and photoinitiated radical polymerization on KCl, have already demonstrated the potential of on-surface photoreaction. Notably, given the importance of the C–Br bond dissociation, early efforts have demonstrated the photodissociation of C–Br bonds aiming for activation at room temperature .…”