Dynamics of formation of 1,1′-diethyl-2,2′-cyanine iodide J-aggregates in solution

The dynamics of the formation of 1,1'-diethyl-2,2'-cyanine iodide (PIC) J-aggregates in water solutions have been studied using UV/vis spectroscopy. The appearance and growth of the J-aggregates' band at 573 nm took place simultaneously with the decrease of the absorption of the sample at 525 and 485 nm. The experimental dependence of the intensity of the absorption at 573 nm versus time has the shape of a reverse exponential curve, J(573) = J(0)(1 - exp(-kt)). Two possible explanations of the experimental data are given. The first one is that if J-aggregates are simple chains of molecules, then only three PIC molecules in aggregate may be enough to produce a J-band in the absorption spectrum of the solution. The second one is that J-aggregates are not simple chains of molecules but consist of unit cells of nonlinear structure. At a high concentration of PIC the full width at half-maximum of the J-band increased with time, but the position of its maximum did not change. Possible explanations of this phenomenon are discussed.