In situ scanning tunneling microscopy of 5-(Dodecyloxy)-2-(5-(4-(pentyloxy)phenyl)-1H-pyrazol-3-yl)phenol adsorbed on Au(1 1 1) electrode

The devise and advance of scanning tunneling microscopy (STM) have yielded insights of molecular adsorption at well-ordered electrified interface. This study employed in situ STM in 0.1 M HClO4 to examine the effect of potential on the spatial arrangements of 5-(Dodecyloxy)-2-(5-(4-(octyloxy)phenyl)-1H-pyrazol-3-yl)phenol (1) and 5-(Dodecyloxy)-2-(5-(4-(pentyloxy)phenyl)-1H-pyrazol-3-yl)phenol (2) on Au(1 1 1) electrode. High-quality molecular resolution STM imaging was used to study the anchoring configurations of these LC molecules on gold electrode. Only between 0 and 0.5 V (vs. reversible hydrogen electrode) did ordered molecular adlattices were found, including a chained phase formed between 0.5 and 0.25 V, and a lamellar phase formed between 0.05 and 0.25 V. The origin of this potential effect can be traced to the charge state of the gold electrode. Molecular resolution STM imaging of these ordered structures yielded intermolecular spacing and contour of admolecule, from which the adsorption configurations of 1 and 2 were deduced. They were adsorbed mostly parallel to the Au(1 1 1) substrate between 0.25 and 0.5 V, but their pyrazole moieties could turn upright as the potential was lowered to E < 0.25 V. The alkoxy side chains of adsorbed 1 and 2 were clearly imaged, which supports the notion that alkoxy interacted with Au(1 1 1) electrode more favorably at E < 0.25 V. This is the first report on the potential dependent adsorption of alkoxy moiety at metal electrode. Also, molecules comprising the lamellar phase were preferentially aligned with their long molecular axis normal to the lamellas, which resembles the molecular arrangement of the smectic A or C phase of liquid crystal.

► 5-(Dodecyloxy)-2-(5-(4-(octyloxy)phenyl)-1H-pyrazol-3-yl)phenol, a mesogenic molecule, is adsorbed spontaneously on a Au(1 1 1) electrode.
► In situ STM is used to probe the lateral structures of this molecule on Au(1 1 1) as a function of potential.
► A lamellar phase resembling closely the bulk smectic phase is identified at a negatively charged Au(1 1 1) electrode.
► The lamellar phase restructures to a chained phase as the potential is made more positive.