Stability of J-aggregated species in an indocarbocyanine dye in Langmuir-Blodgett Films

Here, the stability behaviour of J-aggregate of an indocarbocyanine dye 1,1′-Dioctadecyl-3,3,3′,3′-Tetramethylindocarbocynine perchlorate (DiI) in fatty acid mixed (XDiI=0.4) Langmuir-Blodgett (LB) films under different condition was reported. Surface pressure-area per molecule (π-A) isotherm measurement suggest that the molecular interaction between the constituent molecules changes with subphase temperature. UV-vis absorption and fluorescence spectroscopy and Atomic Force Microscopy (AFM) were used to study the stability behaviour. It has been observed that prominent J-aggregate of DiI occurred in the mixed films at subphase temperature 15 °C. With the passage of time as well as post heat treatment DiI J-aggregate decayed to excimer in both mono- and multilayer LB films. Total transition from J-aggregate to excimer occurred in monolayer films, whereas, the transition was partial in multilayer films. The stability of DiI J-aggregate in monolayer films was increased by two ways - (i) depositing the DiI-SA mixed LB films at higher surface pressure, (ii) exposing the films at 50 °C for 5 min. It has been observed that after heat treatment the DiI J-aggregate remained almost stable even after 200 days. AFM investigations gave the compelling visual evidences of the same.