Exciton Dynamics and Self-Trapping of Carbocyanine J-Aggregates in Polymer Films

Spectral properties and morphologies of 1,1'-disulfobutyl-3,3'-diethyl-5,5',6,6'-tetrachlorobenzimidazolyl-carbo-cyanine (TDBC) J-aggregates formed in two different types of polymer films (neutral spin-coated and charged layered) were studied using steady-state, picosecond, and femtosecond spectroscopies as well as optical and atomic force microscopies. It was found that the J-aggregates adopt different morphologies in the films: quasi one-dimensional rod-like in the spin-coated films and two-dimensional island-like in the layered films. The TDBC J-aggregates exhibit very similar absorption and fluorescence spectra in different films, but their fluorescence lifetimes and quantum yields differ strongly (similar to 4% for spin-coated films and similar to 0.5% for layered films). By pump-probe spectroscopy, three different contributions to the population dynamics and relaxation to lower lying levels are identified with exciton-exciton annihilation dominating the observed kinetics. Low-temperature experiments reveal strong exciton self-trapping in the J-aggregates due to large exciton-phonon coupling. However, the different morphologies of the TDBC J-aggregates in the films result in different behaviors of exciton self-trapping. While in spin-coated films barrierless self-trapping is dominant, in layered films it is associated with overcoming an energy barrier.