Two strategies are compared for the formation of self-assembled monolayers (SAMs) of silicon phthalocyanines on gold. Silicon phthalocyanines were synthesized with thiol anchoring groups in either eight peripheral side chains (the "octopus") or with one short thiol in an axial position (the "umbrella"). Both approaches gave phthalocyanines capable of forming SAMs on gold surfaces. The orientation and coverage of the phthalocyanines were compared using ellipsometry, X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman scattering (SERS). The octopus silicon phthalocyanines form poorly organized SAMs in which the phthalocyanine (Pc) rings are strongly tilted with respect to the gold surface. On average, between 3 and 4 of the thiol "arms" fail to bind to the gold surface, even when limiting coverage is achieved after 7 days of soaking. The film thickness is 22 ( 5 Å. In contrast, the umbrella silicon phthalocyanine produces close-packed SAMs within 1 h in which the Pc rings lie parallel to the gold surface. The average thickness of the later SAMs is 11 ( 3 Å, and each phthalocyanine ring occupies an average area of 284 Å 2 .

The axial reactivity of peripherally octasubstituted soluble silicon(IV) phthalocyanines (SiPcZ 8 , Z ) OC 5 H 11 or Z ) C 5 H 11 ) was studied in order to explore new synthetic routes for preparing axially functionalized SiPc monomers and dimers. Self-exchange of the axial methoxide in methylmethoxy SiPcZ 8 was about 10 times faster for Z ) OC 5 H 11 than for Z ) C 5 H 11 . The Pc rings of these two complexes do not differ in electron-richness; their first oxidation potentials ( -0.880 and -0.875 V, respectively) are the same within experimental error. The axial alkoxide ligand in methylisopropoxy SiPcZ 8 monomer (1c′′, 2c′′) could be quantitatively replaced by mercaptoethanol to yield O-bound methylmercaptoethoxy SiPcZ 8 (1h, 2h). Condensation of methylhydroxy SiPcZ 8 monomers yielded µ-oxo-bridged bismethyl SiPcZ 8 dimers (1k, 2k) without formation of oligomeric byproducts. Photolysis of such methyl-capped dimers in the presence of mercaptoethanol yielded µ-oxo-bridged bismercaptoethoxy SiPcZ 8 dimers (1n, 2n) which are of interest as precursors for oriented self-assembled monolayers (SAMs) of oligomeric phthalocyanines.

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