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

Abstract

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–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 XDiI = 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 XDiI = 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.