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

Dendrimers possessing coumarin-2 dyes at the periphery, and either a coumar in-343 or a heptathiophene dye at the core, have been studied by femtosecon d transient absorption spectroscopy in order to elucidate energy-transfer r ate constants. These measurements indicated that energy transfer occurs wit h average correlation times of less than 6 ps in the first three generation s, and slows to a correlation time of approximately 18 ps in the fourth. Th e transient spectral features arising from excitation of the peripheral don or chromophores at 330 nm are described. Additionally, these measurements p rovide a quantitative comparison between observed and theoretical energy-tr ansfer rate constants (k(ET)), indicating a close correlation of the experi mental results with Forster theory.