The optical characterization of metal-mediated aggregation behaviour of amphiphilic Zn(ii) phthalocyanines

Abstract

Ag⁺ mediated aggregation behaviour of two different amphiphilic zinc phthalocyanines (Zn–Pcs) (symmetric and asymmetric) has been investigated in solution, at the air–water interface and in Langmuir–Blodgett (LB) films transferred onto glass substrates. A 4 : 1 ([Ag⁺]/[Pc]) complexation was observed for both symmetric and asymmetric Zn–Pcs at high [Ag⁺] concentrations which led to the formation of H-aggregates in solution. At the air–water interface, both symmetric and asymmetric Zn–Pcs showed a tilted, edge-on orientation in columnar stacks. Addition of Ag⁺ to the subphase enhanced the order in the monolayers. LB films on glass substrates showed a split Q-band indicating the presence of “herring-bone” type aggregation consisting of both H- and J-aggregates. For LB films deposited from a Ag⁺ containing subphase, a shoulder appeared in the absorption spectra at longer wavelengths which indicates that the fraction of J-aggregates was enhanced by Ag⁺ in LB films. The molecular orientation in LB films was investigated by polarized absorption spectroscopy and a tilt angle was calculated to be 49° with the substrate normal for symmetric and 74.6° for the asymmetric Zn–Pc. The presence of Ag⁺ cations in the subphase decreased the tilt angles slightly by 4–5°. These results indicate that Ag⁺ induced the aggregation of Zn–Pcs and acted as a linker between Pc molecules in the ultrathin films of both symmetric and asymmetric Zn–Pcs. These results are important in inducing J-aggregates for the fabrication of molecular devices based on phthalocyanine thin films.