Excitons in molecular aggregates of 3,3'-bis-[3-sulfopropyl]-5,5'-dichloro-9 ethylthiacarbocyanine (THIATS): Temperature dependent properties

J-aggregates of the dye THIATS (triethylammonium salt of 3,3 ' -bis-[3-sulf opropyl]-5,5 ' -dichloro-9-ethylthia-carbocyanine) with a two-component Dav ydov splitting of the exciton band were investigated in the temperature ran ge from 5 to 130 K and at room temperature. A wide set of excitonic and opt ical characteristics (absorption line broadening, fluorescence line broaden ing, Stokes shift, coherence length, exciton migration rate, and wavelength dependence of the fluorescence decay time) of the same J-aggregates is pre sented. The exciton migration rate was found to be the most temperature sen sitive property. The temperature dependence of a whole set of exciton prope rties reveals two critical temperatures: 30 and 70 K. The observed phenomen a are described qualitatively as an interplay of static and dynamic disorde r effects. At low temperature (T < 20 K) static disorder is the main factor which limits the coherence length and exciton-exciton annihilation rate an d determines the absorption width. An intraband, subnanosecond exciton rela xation toward the lower energy states is observed. Below 20 K only a limite d number of exciton states of the molecular ensemble are reached by the exc iton during downhill relaxation. While the temperature increases from 30 to 70 K, a wider set of states becomes accessible for the exciton during its relaxation. The "internal" structure of the exciton band becomes blurred by homogeneous broadening and the coherence length decreases. Very fast excit on wave packet motion occurs over 10(6)-10(7) molecules. At temperatures hi gher than 80 K, we suggest dynamical processes to play the most important r ole. The Stokes shift becomes temperature independent. Exciton migration st arts to be strongly blocked by scattering on optical phonons. The effective , long distance exciton migration in THIATS J-aggregates as well as peculia rities of the Stokes shift and line broadening temperature dependence allow us to conclude that no exciton self-trapping process occurs at temperature s higher than 20 K.