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.

Hierarchical assembly of a molecular material through the amorphous phase and the evolution of its fluorescence emission

“…We have also investigated the utility of mechanical control of the aggregation of an amphiphilic DADQ molecule at the air-water interface and its impact on the optical responses of the resulting monolayer Langmuir-Blodgett lms. 17 Aggregation induced enhancement of uorescence, 18,19 as opposed to its quenching, is of considerable interest from the perspectives of both molecular level interactions/assembly as well as applications such as displays and sensors. Enhancement of the uorescence of DADQ molecules from their solution to the crystalline solid state has been investigated in detail in our laboratory.…”

Efficient Bioimaging with Diaminodicyanoquinodimethanes: Selective Imaging of Epidermal and Stomatal Cells and Insight into the Molecular Level Interactions

“…The limited classes of molecules that exhibit strong fluorescence in the aggregated/solid states (often called aggregation-induced emission enhancement) include tetraphenylethenes 8 , diphenylbutadienes 9 , hexaphenylsilole 10 and diaminodicyanoquinodimethanes (DADQs). We have reported on the strong fluorescence of DADQs in crystals 11 , nanocrystals 12 , amorphous particles 13 and thin films 14 , 15 ; the critical role of specifically oriented aggregation in the fluorescence enhancement has been demonstrated recently 16 . DADQs are potential candidates for efficient bioimaging.…”

Establishing the Critical Role of Oriented Aggregation in Molecular Solid State Fluorescence Enhancement

“…Fluorescence enhancement of DADQs in the form of crystals, nanocrystals, colloids, and ultrathin films has been investigated; the fluorescence response served as a signature of amorphous‐to‐crystalline transformation and phase‐change materials . The impact of intermolecular excited state energy loss pathways in tuning the emission in the crystals, was demonstrated using DADQs having different molecular structures and hence assembly patterns .…”