Picosecond spectroscopic studies of electronic energy relaxation in H-aggregates of 1,1′-diethyl-2,2′-dicarbocyanine on colloidal silica

Polarized transient bleaching measurements of H-aggregates of 1,1′-diethyl-2,2′-dicarbocyanine on colloidal silica were performed using picosecond pump-probe spectroscopy. The aggregates were nonfluorescent but exhibited a broad absorption band, between 550 and 600 nm, which was blue-shifted from the monomer band at 711 nm. These spectral properties are consistent with the simple theory for one-dimensional excitons. A fit of the transient bleaching signals to a stretched exponential, exp[-(t/τ_p)α], was obtained for α = 0.46 ± 0.02 and τ_p = 17 ± 5 ps. The nonexponential signals are attributed to a distribution of aggregates with different lifetimes, with the most probable lifetime being equal to 120-130 ps. The absorption anisotropy was constant and equal to 0.30 ± 0.04. The anisotropy is discussed in terms of the coherence properties of excitons.