Mechanical control of molecular aggregation and fluorescence switching/enhancement in an ultrathin film

Abstract

Optical responses of molecular aggregates and assemblies are often different from that of the individual molecules. Self-assembly approaches provide little physical control on the extent of aggregation. Mechanical compression of amphiphilic molecules (with chromophore/fluorophore head groups) at the air–water interface, followed by transfer as Langmuir–Blodgett (LB) films, should prove to be an elegant route to molecular assemblies with systematically tunable aggregation and optical responses. This concept is demonstrated using monolayer LB films of a diaminodicyanoquinodimethane (DADQ)-based amphiphile fabricated at different surface pressures. Films deposited above a threshold pressure exhibit a strong blue-shift in the absorption and fluorescence relative to those deposited below; computational investigations suggest that this is due to the formation of 2-dimensional close-packed assemblies. Significantly, the blue emission of the films deposited above the threshold pressure increases with compaction, demonstrating aggregation-induced fluorescence enhancement in ultrathin films, a phenomenon well-established in crystals and nanocrystals of selected classes of molecules including the DADQs. The sharp contrast with aggregation-induced fluorescence quenching observed with most dye molecules is illustrated by a parallel investigation of LB films of a hemicyanine-based amphiphile. The present study illustrates the efficacy of simple mechanical compression and the LB technique in fabricating ultrathin films with tailored supramolecular assembly and optical responses.