Conformation and Packing of Porphyrin Polymer Chains Deposited Using Electrospray on a Gold Surface

Abstract

Conjugated porphyrin polymers have stimulated great interest due to their potential applications in nonlinear optics, light harvesting and nanoscale charge transport. As with many other organic materials, interfacial properties are likely to play an important role in their applications in molecular electronics. However, it has not so far been possible to study these effects due to the difficulty in preparing suitable monolayers, since the relevant polymers and oligomers cannot be sublimed. A question of particular interest relates to the influence of the flexibility of such a large molecule on the ordering within interfacial regions. We have investigated the adsorption of two oligomers, a porphyrin tetramer (P4, N= 4; see Figure 1 for structural diagrams), a hexamer (P6, N= 6), and a polymer Pn (N = 30–50) on the Au(111) surface using scanning tunneling microscopy (STM). The porphyrin units have long octyloxy side chains to promote solubility in organic solvents. Our experiments are performed at room temperature under ultrahigh vacuum (UHV) conditions (base pressure 5 10 11 Torr) and we use UHV electrospray deposition (UHV-ESD) to transfer the oligomers and polymers directly from solution onto a surface. In our approach to UHV-ESD, a volatilized mixture of solvent and solute molecules is produced in atmosphere by electrospray. This mixture enters the UHV system through a small aperture and is passed through a series of differentially pumped chambers, to the Au(111) substrate (for further details see Supporting Information). UHV-ESD and related approaches have been used to introduce nanotubes, fullerenes, dye molecules, and polymers into a UHV environment. Images acquired after deposition of a sub-monolayer coverage of P6 (Figure 2) show that, despite their large size, the porphyrin oligomers diffuse on the surface and form Figure 1. a) Structure of porphyrin oligomers and polymers. b) P6 molecule with the trihexylsilyl end groups truncated to trimethylsilyl groups for clarity.