SPECTROSCOPIC CHARACTERIZATION OF AGGREGATION BEHAVIOR IN HEMICYANINE DYE MONOLAYER AND MULTILAYER SYSTEMS

The aggregation behavior of the hemicyanine dye N-(3-sulfopropyl)-4-(p-(dioctylamino) styryl)pyridinium, fabricated in Langmuir-Blodgett (LB) monolayer and multilayer systems and directly physisorbed onto quartz substrates has been studied by absorption and fluorescence spectrocopy. The LB monolayer films prepared on quartz substrates show entirely H-aggregate structures, characterized by a large blue-shifted (Delta E similar to 9000 cm(-1)) electronic transition at 340 nm, not only for the pure-dye system but also for the dye diluted in stearic acid at molar ratios down to 17%. The LB multilayer systems with pure dye show an identical absorption band (340 nm) to the LB monolayers. However, excitation of the aggregate bands yields fluorescence emission at 530 nm for monolayers, and two emission bands, at 400 and 530 nm, for multilayers. The presence of the blue emission band indicates that a twisted intramolecular charge-transfer (TICT) state is accessed in multilayer assemblies. In addition, submonolayer coverages of the dye on fused-quartz substrates are obtained by physisorption from CHCl3 solution. Similar to the LB film, physisorbed dye self-aggregates at the air-quartz interface with a rate of aggregation, which is critically temperature dependent.