Exciton dynamics and trapping in J-aggregates of carbocyanine dyes

Exciton relaxation processes in J-aggregates of two thiacarbocyanine dyes (TDC and THIATS) in water/ethylene glycol low temperature glass have been studied by means of steady-state, site-selective and time-resolved spectroscopy. The results of different experiments can be rationalized within the framework of exciton self-trapping (ST) in a 1D molecular chain. The measured Stokes shift is found to be proportional to the exciton - phonon coupling strength squared that is in accordance with the model of large radius ST excitons. A negative fluorescence anisotropy was observed for J-aggregates of both investigated dyes upon excitation outside the J-band. The observed monotonic dependence of the fluorescence anisotropy on the excitation wavelength is interpreted quantitatively within the framework of intraband absorption and Davydov splitting.