Reversible Transition between Excimer and J‑Aggregate of Indocarbocyanine Dye in Langmuir–Blodgett (LB) Films

In the present Article, a reversible transition behavior from J-aggregates 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–<i>A</i>) isotherms, UV–vis, and fluorescence spectroscopies as well as atomic force microscopy (AFM) were used for characterizations of the films. π–<i>A</i> isotherms suggest a balance of interactions between DiI and fatty acids in the mixed monolayer at DiI mole fraction <i>X</i>_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 <i>X</i>_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