In the present Article, a reversible transition behavior from Jaggregates to excimer of an indocarbocyanine dye 1,1′-dioctadecyl-3,3,3′,3′tetramethylindocarbocyanine perchlorate (DiI) in Langmuir−Blodgett (LB) films was reported. Surface pressure−area (π−A) isotherms, UV−vis, and fluorescence spectroscopies as well as atomic force microscopy (AFM) were used for characterizations of the films. π−A isotherms suggest a balance of interactions between DiI and fatty acids in the mixed monolayer at DiI mole fraction X DiI = 0.4, resulting in a stable and ideally mixed monolayer. It has been observed that pure DiI formed excimer in LB films, whereas both J aggregates and excimer were formed in LB films when DiI was mixed with long chain fatty acids, viz., stearic acid or arachidic acid. In fatty acid matrix at X DiI = 0.4, only J aggregates were formed in the LB films. This has been confirmed using deconvolution of spectroscopic results as well as using excitation spectroscopy. The coherent size of the J aggregate was found to be a maximum for the mixed film at the mole fraction 0.4 of DiI in fatty acid matrix. The J-aggregate domain in the LB film contains approximately (20 ± 5) coherent sizes. However, J aggregates were totally absent when DiI was mixed with cationic surfactant, polymer, or nanoclay.

In this communication, we report the design and synthesis as well as the supramolecular assembly behavior of a 2,4,5-triaryl imidazole derivative (compound 1) at the air-water interface and in thin films using Langmuir-Blodgett (LB) technique. The main idea for such a chemical structure is that the long alkyl chain and N-H of the imidazole core may help to form supramolecular architecture through the hydrophobic-hydrophobic interaction and hydrogen bonding, respectively. Accordingly, the interfacial behavior as well as morphology of 1 in thin films were studied through a series of characterization methods such as surface pressure-area (π-A) isotherm, hysteresis analysis, ultraviolet-visible (UV-vis) absorption and steady-state fluorescence spectroscopies, Fourier transform infrared, X-ray diffraction, Brewster angle microscopy (BAM), and atomic force microscopy (AFM) measurements, and so forth. Pressure-area isotherm is an indication toward the formation of supramolecular nanostructures instead of an ideal monolayer at the air-water interface. This has been confirmed by the hysteresis analysis and BAM measurement at the air-water interface. AFM images of 1 in the LB monolayer exhibits the formation of supramolecular nanowires as well as nanorods. By controlling different film-forming parameters, it becomes possible to manipulate these nanostructures. With the passage of time, the nanowires come close to each other and become straight. Similarly, nanorods come close to each other and form bundles of several rods in the LB films. H-bonding, J-aggregation, as well as compression during film formation might play a key role in the formation of such nanostructures. Electrical switching behavior of compound 1 was also observed because of the presence of an electron donor-acceptor system in 1. This type of organic switching behavior may be promising for next-generation organic electronics.

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In this communication we report the formation of fluorescent H -aggregates of a cyanine dye 3,3 -Dioctadecyloxacarbocyanine perchlorate (Oxa18) in ultrathin film and its effect on energy transfer between Oxa18 and Sulphorhodamine B monosodium salt (sRhb). Surface pressurearea per molecule isotherm revealed that Oxa18 forms stable Langmuir monolayer at air-water interface. Spectroscopic investigation revealed that Oxa18 forms H -aggregate in LangmuirBlodgett (LB) film which is enhanced in presence of nano clay laponite. Ideally H -aggregate do not fluoresce. However, due to imperfect stacking of Oxa18 molecules in the aggregates, fluorescence occurred from Oxa18 H -aggregates. This H -aggregated band has substantial effect on the enhancement of energy transfer from Oxa18 to sRhb both in solution and in ultrathin film.3

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