Surface−Enhanced Polymerization via Schiff-Base Coupling at the Solid–Water Interface under pH Control

Abstract

On-surface polymerization realized at the solid-liquid interface represents a promising route to obtain stable and conductive organic layers with tunable properties. We present here spectroscopic evidence of π-conjugated polymer formation at the interface between an iodinemodified Au(111) and an aqueous solution. Schiff-base coupling has been used to drive the reaction by changing the pH. Scanning tunneling microscopy (STM) investigations show that the substrate acts as a template driving the formation of 1D ordered nanostructures. All the chemical states of the molecules on the surface have been identified and their evolution as a function of the pH has been monitored by synchrotron radiation X-ray photoelectron spectroscopy (XPS), demonstrating that two polymeric phases, undistinguishable by STM, exist on the surface: intermediate state and π-conjugated final product. The I/Au(111) substrate enhances the formation of π-conjugated polymers, as established comparing their production on the surface and in the bulk solution.

Probing the chemical structure of monolayer covalent-organic frameworks grown via Schiff-base condensation reactions

“…1b. 11 Hemiaminals are known intermediate states in the Schiff-base condensation reaction and have previously been identified in solution by Raman spectroscopy, mass spectroscopy and nuclear magnetic resonance (NMR). 12a-c Fig.…”

“…It should be noted that the BE of imine and Zn coordinated N are almost identical making it impossible to separate the contributions of these species. 10,11 Hemiaminal groups have been observed by XPS in solution grown 1D Schiff-base polymers. 11 However, the results presented here are the first evidence that hemiaminal covalent links are present in monolayer 2D-COFs grown via Schiff-base condensation at a solid-vapour interface.…”

Imine‐Based Architectures at Surfaces and Interfaces: From Self‐Assembly to Dynamic Covalent Chemistry in 2D

“…[83] Te rephthalaldehyde (6) and 4,4'-diaminostilbene (32)h ave been used as monomers for p-conjugated 1D polymers on an iodine-modified Au (111) surface. At low pH values most of the amino groups exist in their positively charged ammonium form (À NH 3 + )a nd they do not react with the aldehydes,w hereas by raising the pH, the ammonium ions get deprotonated to ÀNH 2 and can react with aldehydes to form Schiff bases through nucleophilic addition at the aldehyde.…”

“…At low pH values most of the amino groups exist in their positively charged ammonium form (À NH 3 + )a nd they do not react with the aldehydes,w hereas by raising the pH, the ammonium ions get deprotonated to ÀNH 2 and can react with aldehydes to form Schiff bases through nucleophilic addition at the aldehyde. [83] Te rephthalaldehyde (6) and 4,4'-diaminostilbene (32)h ave been used as monomers for p-conjugated 1D polymers on an iodine-modified Au (111) surface. [83] The pH of solutionst hat contained monomers was adjusted with HClo rN aOH.…”

“…[83] Te rephthalaldehyde (6) and 4,4'-diaminostilbene (32)h ave been used as monomers for p-conjugated 1D polymers on an iodine-modified Au (111) surface. [83] The pH of solutionst hat contained monomers was adjusted with HClo rN aOH. STM investigation of the samples prepared by drop-casting the aqueous mixture of 6 and 32 at pH 2.1 and 3.6 revealed the existenceo fo rdered 1D polymers.…”