Relaxation and trapping of excitons in J-aggregates of a thiacarbocynine dye Original Research Article

Exciton relaxation processes in J-aggregates of 3,3′,9-triethyl-5,5′-dichlorothiacarbocyanine iodide (TDC) dye have been studied by means of site-selective, steady-state and time-resolved spectroscopy. We found that TDC forms three different types of J-aggregates (J1,J2, and J3) in frozen solutions. Fluorescence polarization measurements showed that exciton diffusion between randomly oriented segments of aggregates plays a minor role in exciton relaxation. Optical properties of J3-aggregate fluorescent states are well described by the theoretical model of barrierless self-trapping of an excitation in a strictly one-dimensional discrete lattice. Upon optical excitation J3-aggregates also show thermally activated transformation to J2-aggregates. The height of the related potential barrier is approximately equal to 30 cm−1. Hence, two different exciton relaxation processes are proposed to take place in the system under study: barrierless self-trapping in J3-aggregates and thermally activated J3 → J2 photorearrangement.