Alkynyl Linkers as a Design Tool to Gain Control over the Self-Assembly of Meso-Substituted Porphyrins on HOPG.

Self-assembled monolayers (SAMs) fall generally into two broad categories: those that are covalently bound either to the surface or to each other and those that rely on weaker forces such as hydrogen bonding or van der Waals forces. The engineering of the structure of SAMs formed from weaker forces is an exciting and complex field that often utilizes long alkane substituents bound to core moieties. The core provides the unique optical, electronic, or catalytic property desired, while the interdigitation of the alkane chains provides the means for creating well-regulated patterns of cores on the substrate. This design technique sometimes fails because some of the alkane substituents remain extended into solution rather than become interdigitated on the substrate. One contributor to this is steric hindrance between elements of the core and of the alkane chain. It is shown that the use of an alkyne linker between the core and the alkane chain can, in the case of meso-substituted porphyrins, significantly reduce this steric barrier and allow more stable and predictable surface structures to form. In particular, 5,15-bis(1-octynyl)porphyrin and 5,15-bis(1-tetradecynyl)porphyrin are shown to form significantly more stable SAMs than their alkane-linked counterparts. Scanning tunneling microscopy is used to provide detailed surface structures. Temperature and solution concentration dependence of the surface coverage is also reported. Density functional theory (DFT) is used to determine the energetic effects associated with alkane substitution at both the meso and beta positions and the beneficial energetic effects of the alkyne linker.