Optical dynamics of excitons in J aggregates of a carbocyanine dye

Abstract

Results of temperature and wavelength dependent fluorescence lifetime and accumulated photon-echo experiments on the J aggregate of the dye 5,5′,6,6′-tetrachloro-1,1′-diethyl-3,3′-di(4–sulfobutyl) -benzimidazolocarbocyanine (TDBC) in an ethylene glycol/water glass are reported. Additionally, the temperature dependent relative fluorescence quantum yield was determined. Using two-color pump–probe spectroscopy, the intersite correlation of the frequency disorder and the size of the coherence domains were estimated. Both the phase and the population relaxation times are frequency dependent in the echo and the single-photon-counting experiments. The dependence of the fluorescence lifetime on detection wavelength is restricted to temperatures below 80 K, indicating a temperature activated process. From our experiments and by comparison with previously published results we conclude that the dispersive nature of both the dephasing and the depopulation is caused by intraband relaxation processes. At higher temperatures this wavelength dependence vanishes due to thermalization. The temperature dependence of the relative fluorescence quantum yield of TDBC is equal to the one of pseudoisocyanine (PIC). Using a motional narrowing model for disordered molecular aggregates with consideration of intersite correlation, at 1.5 K, the two-color pump–probe experiments indicate a very high correlation in the frequency disorder in TDBC J aggregates with a correlation length of several hundred molecules. From pump–probe experiments the delocalization length of the exciton is determined to be between 30 and 45 molecules at 1.5 K.