Direct evidence of interfacial hydrogen bonding in proton-electron concerted 2D organic bilayer on Au substrate

Abstract

Recent advances in the molecular design of organic materials have uncovered various novel functional properties. One of them is the coupling of proton dynamics and electrical conductivity, which can only be achieved in 3D organic crystals. However, reduction of dimensionality to two dimensions is essential in organic electronics application. In this study, we prepared and characterized a 2D organic bilayer with “proton-electron” concerted functionality on a solid surface. It consisted of catechol-fused bis(methylthio)tetrathiafulvalene (H2Cat-BMT-TTF) deposited onto an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on a Au surface. Direct evidence of inter-facial hydrogen bonding (H-bonding) was obtained by scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. STM images showed the deposited H2Cat-BMT-TTF molecules as grains with the thickness of a single molecular layer. The OH stretching vibrational modes of H2Cat-BMT-TTF in the IRAS spectra showed a large red shift and substantial broadening upon adsorption on Im-SAM, indicating that the OH groups of H2Cat-BMT-TTF act as the H+ donor sites. The counterpart H+ acceptor sites were pinpointed by N K-edge NEXAFS. The π* peak of the imino N atoms of the imidazole rings in Im-SAM shifted to higher energy upon the adsorption of H2Cat-BMT-TTF. Therefore, H-bonds form between the imino N atoms (H+ acceptor sites) of Im-SAM and the OH groups (H+ donor sites) of H2Cat-BMT-TTF. The present work is a steady step toward the realization of 2D organic functional materials, and the experimental methods adopted herein will serve as powerful tools for the detection of their functions.