Abstract
The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol−1) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.
Highlights
The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films
Thiol-based thin-film-on-metal biosensors employing surface plasmon resonance (SPR) effects are routinely used for the detection of nanogram quantities of biomolecules in real time without the need for labelling[5,6]; challenges with film stability lead to low shelf life, poor bilayer film quality and increased nonspecific adsorption
Seeking to develop alternative methods to the use of strong base for the generation of free carbenes through deprotonation (1a to 2a, Fig. 1a), we explored literature procedures for the preparation of imidazolium hydrogen carbonates via salt metathesis between imidazolium iodide salts and KHCO3
Summary
NHC hydrogen carbonate synthesis and film generation. Seeking to develop alternative methods to the use of strong base for the generation of free carbenes through deprotonation (1a to 2a, Fig. 1a), we explored literature procedures for the preparation of imidazolium hydrogen carbonates via salt metathesis between imidazolium iodide salts and KHCO3 (ref. 16). Compound 3a, prepared, is an odourless, bench-stable, free-flowing solid that forms an NHC film on Au(111) by immersing the Au surface in a methanol solution of 3a in air at room temperature without any special precautions. These films have identical chemical and electrochemical stability to films deposited using the free carbene method[11], resisting refluxing solvents, water, acid, base and oxidant (Supplementary Fig. 1)[11]. Benzimidazolium hydrogen carbonate 3a was used to generate films in vacuo from the solid state by connecting a solid doser containing 3a to a ultra high vacuum chamber and warming to 325 K (Supplementary Fig. 2). Both the coverage and thermal stability of NHC were analysed using temperature programmed desorption (TPD) from the Au surface
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