Evanescent wave excited fluorescence from self-assembled phthalocyanine monolayers

Novel phthalocyanine (Pc) molecules have been designed and synthesized to incorporate a thiol moiety for the formation of monolayer films onto gold-coated optical waveguides via the technique of self-assembly. The thiol moiety is connected to the Pc macrocycle by either a (CH2)(11) or (CH2)(3) hydrocarbon chain. Both Pc SAMs have been characterized on the metal surface using reflection absorption infrared (PAIR) spectroscopy. The RAIR spectra suggest that the two Pc molecules lie in a different orientation with respect to the gold surface. This behavior can be rationalized with consideration of the different structures (the different alkyl connecting chain lengths and different macrocyclic peripheral side chains) of the two Pc molecules. The fluorescence emission spectrum of each of the Pc SAMs has been obtained by exciting each monolayer along the longitudinal axis of the optical waveguide via laser-induced evanescent wave stimulation. The use of the longer mercaptoalkyl connecting chain appears to inhibit quenching of the electronically excited state through energy transfer to the metal layer. It is thought that the presence of peripheral alkyl side chains (six hexyl or seven decyl chains, respectively) minimizes self-quenching in the single-component SAM film. It has also been shown that the fluorescence signal of the (CH2)(11) mercaptoalkyl Pc SAM can be reversibly quenched by exposure to gaseous nitrogen dioxide (NO2). The decrease in fluorescence intensity has been related to increasing NO2 concentration by a Stern-Volmer relation ship. The Pc SAM can detect concentrations of NO2 down to 10 ppmv with no interferent effect seen from other environmentally relevant gases: cai bon monoxide and carbon dioxide. These results suggest that SAMs of specifically designed materials, such as Pc's, may be used as sensitive receptor molecules to form the basis of a novel technology for the development of optical chemical sensors for gaseous species.