Formation and spectroscopic characterisation of self-assembled phthalocyanine monolayers

A series of three Phthalocyanine (Pc) derivatives have been designed, synthesised and formulated as self-assembled monolayers (SAMs) on gold-coated optical waveguides. The alkyl chain tethering the Pc macrocycles to the gold surface via a thiol moiety varied in length viz. (CH2)(11), (CH2)(8) and (CH2)(3). The three Pc SAMs have been characterised using reflection absorption infrared spectroscopy and fluorescence spectroscopy excited via the evanescent wave. The infrared data have shown that the length of the alkyl tether has a profound effect on the orientation and packing density of the Pc macrocycle on the gold surface. Similarly, the intensity of fluorescence observed from the Pc monolayers was related to the length of the alkyl tether. A further investigation of the C-11 Pc derivative SAM has shown that the monolayer film was stable, exhibiting no oxidation and only minor orientation changes on the gold surface over a period of 24 months as determined by infrared and fluorescence spectroscopy. The longevity of the Pc SAM is thought to be due to the macrocyclic ring possibly preventing oxidation of the thiolate root. As simple alkanethiols are known to oxidise at gold surfaces, the stability results obtained for the Pc derivative would suggest that macrocyclic SAMs may have properties that are more suitable for long term applications.