Transfer of rodlike aggregate phthalocyanines to hydrophobized gold and silicon surfaces: Effect of phenyl-terminated surface modifiers on thin film transfer efficiency and molecular orientation

We present here a study of the effects of surface modification on the efficiency of transfer of ultrathin Langmuir-Blodgett films of rodlike aggregates of the phthalocyanine (Pc) (2,3,9,10,16,17,23,24-octakis-((2-benzyloxy)ethoxy)phthalocyaninato)copper (1) to gold and silicon substrates. These surface modifications pact on (i) the molecular orientation of the individual Pcs within the rodlike aggregates, (ii) the coherence of the aggregates (providing for coherence lengths of some Pc rods of over 100 nm), and (iii) the optical anisotropy and the overall uniformity and lack of pinholes in Pc bilayer films, on the 100 μm distance scale. Methyl-terminated and phenyl-terminated surface modifiers were added to both Au and Si surfaces through thiol and silane chemistries, respectively. The phenyl-terminated modifier for Au surfaces, benzyloxyethanethiol, 2, mimics the side chain composition of Pc 1. Both methyl- and phenyl-terminated modifiers produced hydrophobic surfaces, as revealed by high water contact angles. FT-IR spectroscopy and AFM characterization of horizontally transferred bilayer LB thin films of I consistently showed higher degrees of coverage on the surfaces with phenyl-terminated modifiers. Reflection-absorption FT-IR of 1-5 bilayer films on modified Au substrates showed that the average tilt angle (ψ) and rotation angle (θ) of the individual Pc chromophores within these films varied with increasing Pc film coverage but that changes to ψ were significantly less on Au surfaces modified with 2. Phenyl-phenyl intermolecular interactions have been hypothesized to lead to the unique formation of rodlike aggregates of 1 and are shown here to be critical in establishing coherent thin films of this material during the transfer of the first bilayer.