Femtosecond transient absorption studies of energy transfer within chromophore-labeled dendrimers

Dendrimers possessing coumarin-2 dyes at the periphery, and either a coumarin-343 or a heptathiophene dye at the core, have been studied by femtosecond transient absorption spectroscopy in order to elucidate energy-transfer rate constants. These measurements indicated that energy transfer occurs with average correlation times of less than 6 ps in the first three generations, and slows to a correlation time of approximately 18 ps in the fourth. The transient spectral features arising from excitation of the peripheral donor chromophores at 330 nm are described. Additionally, these measurements provide a quantitative comparison between observed and theoretical energy-transfer rate constants (k(ET)), indicating a close correlation of the experimental results with Forster theory.