Self-assembled two-dimensional nanoporous molecular arrays and photoinduced polymerization of 4-bromo-4′-hydroxybiphenyl on Ag(111)

Abstract

Self-assembled two-dimensional molecular arrays and photoinduced polymerization of 4-bromo-4'-hydroxybiphenyl on Ag(111) were studied using low-temperature scanning tunneling microscopy combined with density functional theory calculations. Square-like self-assembled structures of 4-bromo-4'-hydroxybiphenyl stabilized by intermolecular hydrogen and halogen bonds were transformed into hexagonal nanopores of biphenyl biradicals by 266 nm UV laser irradiation at 80 K. The biradicals further coupled to each other and formed covalently linked polyphenylene polymer chains at room temperature.

Micrometre-long covalent organic fibres by photoinitiated chain-growth radical polymerization on an alkali-halide surface

“…In fact, most of the reactions described so far are initiated by thermal annealing which however often creates defects in the formed structures due to the increased diffusion of the precursors on the surface. In order to circumvent this problem, some work was reported where the polymerization was initiated by UVillumination 12,25,26,27,28 or current injection by means of an scanning tunnelling microscopy tip. 7,29,30,31 The main limitations of the so far created structures in view of future applications in nanoscale electronic and optical devices are (i) the use of metal substrates (for instance, leading to non-radiative quenching 32 ), (ii) the high number of defects in the formed covalent structures, and (iii) the side-products of some reactions which might remain on the substrate surface.…”

Thermal- vs Light-Induced On-Surface Polymerization

“… 6 , 8 , 9 Heat, i.e., thermal energy, is the preferred stimulus for activation, but alternative activation modes have also been explored, for instance, a scanning tunneling microscope (STM) tip (via voltage pulses), 10 an electron beam, 11 or light. 12 …”

“… 29 Simultaneous C–Br and C–OH bond cleavage of hydroxyphenyl molecules on Ag(111) was triggered by UV light. 12 This led to the formation of biphenyl biradical species that assemble into porous networks, stabilized by interactions with substrate Ag atoms at 80 K. At RT, the same radical species couple to each other and form covalently linked polyphenylene chains.…”

“…Chemical bonds within adsorbed molecules have been dissociated using either the STM tip, 10 , 30 heat, 2 or light 12 or by a combination of voltage pulses from a STM tip and light. 31 , 32 The subsequent synthesis of covalently linked nanostructures is achieved by thermal activation 4 or without heating (i.e., at RT) in the case of photoactivated species.…”

Photochemistry Highlights on On‐Surface Synthesis

“…In order to demonstrate that photo‐activation of chemical bonds can be a powerful approach to obtain long π‐conjugated polymers at room temperature, Shen et al. deposited the 4‐bromo‐4′‐hydroxy‐biphenyl molecule (Figure a), as starting precursor on a Ag(111) surface . α,ω‐disubstitued polyphenylenes are well‐known to lead to very long π‐conjugated polymers by thermally induced Ullmann cross‐coupling …”