Abstract
The Langmuir–Blodgett (LB) films with J aggregates of the amphiphilic MC2 merocyanine have been deeply investigated for more than two decades, mainly because of their interest for applications in photonic devices. To extend the excitation/emission wavelength range, we have studied an homologous dye, MC1, based on a shorter merocyanine chromophore, aiming at checking the ability of this compound to arrange into J aggregates within stable LB films. In this comparative MC1/MC2 investigation, we have addressed the structural and spectroscopic properties of the monomers in solution, the thermodynamic and morphological properties of the Langmuir monolayers at the air–water interface and the spectroscopic and photophysical properties, and the structural features thence obtainable, of the aggregates in the LB films. 1H NMR experiments have shown that, in chloroform solutions, both dyes adopt planar conformations with the hydrophilic and hydrophobic groups pointing to opposite directions. Strongly attractive interactions are exhibited by both dyes towards arachidic acid in monolayers at the air–subphase interface, with a maximum stability for the 1 : 2 MC(1/2) : AA mixture. Relatively homogeneous LB multilayers have been obtained from 1 : 4 and 1 : 2 MC : AA mixtures with good transfer ratios. Absorption and emission of the LB films of MC1 are dominated by J aggregates, while those of MC2 show some H aggregate contributions too. The J aggregates of both compounds exhibit a tendency to align with the transition dipoles along the film dipping direction, only slightly tilted relative to the substrate surface; however, this tendency is more pronounced for the MC1 J aggregates. Overall, MC1 yields slightly more stable monolayers and more solid multilayers compared to MC2.
Highlights
Merocyanines are push–pull compounds able to form aggregates, both H and J,1 in cast[2] and Langmuir–Blodgett (LB) films
After an 1H NMR characterization of the structures, and a comparative study of absorption and fluorescence properties of the two dyes as monomers in solution, we describe the thermodynamics of merocyanine–arachidic acid mixing in Langmuir monolayers formed on a subphase containing ammonia, and some spectroscopically monitored structural features of the LB films, including the extent of dye aggregation within them
The amphiphilic MC2 dye, having pentamethine-merocyanine as the basic chromophore, has been long known to form stable J aggregates in LB films which can be of prime interest for applications in photonic devices
Summary
Merocyanines are push–pull compounds able to form aggregates, both H and J,1 in cast[2] and Langmuir–Blodgett (LB) films (see the literature quoted in ref. 3 and ref. 4). Because of the crucial role of the optical properties of merocyanines and their aggregates in the applications of their LB films, it is important to extend the wavelength range for these applications beyond that of MC2, without compromising LB film stability To this aim, we have obtained a shorter and more rigid quasi-homologue of merocyanine, MC1, which absorbs and emits, as a monomer, around 100 nm to the blue of MC2 in a chloroform solution. We have obtained a shorter and more rigid quasi-homologue of merocyanine, MC1, which absorbs and emits, as a monomer, around 100 nm to the blue of MC2 in a chloroform solution Both dyes exhibited the formation of H-type aggregates in films cast from such solutions.[2] their solubilization in water by complexation with p-sulfonated calixarenes has been investigated in view of the preparation of electrostatically self-assembled multilayers incorporating these dyes.[19]. The effects of the differences between the two merocyanines on these properties are highlighted and discussed
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