Self-assembly of conjugated molecular rods: A high-resolution STM study

The formation, properties, and potential applications of self-assembled monolayers (SAMs) on metal and metal oxide surfaces have recently been demonstrated and reviewed, and it is clear that the investigation of these structures represents a target of opportunity for the discovery of new physical phenomena and technologies. In this regard,the determination of the nature and degree of order in a SAM can be a prerequisite for its eventual utilization. Along these lines, we have undertaken investigations of the structures and properties of monolayers derived from conjugated arylthiol derivatives, and specifically, those related to the series of oligo(phenylethynyl) benzenethiols represented by 1 (n ≥ 1), derived from 1a-c which reveals that the degree of order in these systems increases with chain length and that, for 1c, an ordered SAM is formed. Due to the conjugated, rigid rod framework of 1c, this SAM represents an important new platform upon which a variety of optical/electronic studies related to nanostructured materials can be based. In conclusion, we find that, while a monolayer is formed in all cases, the STM indicates an increase in order from 1a to 1b to 1c, culminating in the formation of a highly ordered SAM for 1c. This represents the first direct observation of order in a SAM that is not based on an n-alkanethiol derivative. The design of new SAMs that are patterned after the general structural motif of 1c represents an opportunity to investigate new electrical/ optical phenomena, at both the molecular and monolayer level, that might arise from the highly ordered, conjugated nature of this particular array.