SELF-ASSEMBLY OF ALPHA-FUNCTIONALIZED TERTHIOPHENES ON GOLD

alpha-Functionalized terthiophenes containing disulfide (-S-T-3-H)(2) and alkanethiol (HS-(CH2)(11)-T-3-H) anchoring groups have been synthe sized for direct immobilization onto gold. Monolayer structures of the se compounds are prepared by spontaneous assembly from ethanol solutio ns on evaporated gold substrates and thoroughly characterized by ellip sometry, contact angle goniometry, infrared and X-ray photoelectron sp ectroscopy, and cyclic voltammetry. The two molecules coordinate to th e gold substrate exclusively via the anchoring groups upon formation o f gold-thiolate bonds. The kinetics of monolayer formation vary dramat ically for the two compounds. The alkanethiol analogue assembles rapid ly, within a few minutes, and forms a densely packed and highly organi zed monolayer, with the alkyl chains in an almost perfect all-trans co nformation and the C-alpha-C-alpha axis of the alpha-T-3 units tilted about 14 degrees away from the surface normal. The assembly process is much slower for the disulfide, but an organized monolayer with an ave rage alpha-T-3 chain tilt of about 33 degrees will eventually form whe n the assembly is allowed to equilibrate with a solution containing th e disulfide for at least 1 day. Moreover, the two monolayer assemblies also display a remarkably different electrochemical, behavior. The he terogeneous electron-transfer rate at the disulfide-covered gold subst rate is almost indistinguishable from that at bare gold, suggesting th at the assembly contains a large number of easily accessible defects. An alternative mechanism for explaining the large electron-transfer ra te involving electronic coupling via the conjugated pi-system of the a lpha-T-3 units is also proposed. The electrochemical response is signi ficantly reduced for the HS-(CH2)(11)-T-3-H assembly, but another type of defects, the so-called ''shallow defects'' originating from sparse ly populated areas on the electrode surface, can be identified.