Synthesis and steady-state photophysical properties of dye-labeled dendrimers having novel oligothiophene cores:: A comparative study

Novel chromophore-labeled dendrimers with penta- and heptathiophene cores and coumarin-2 chromophores at their periphery have been shown to be very efficient light-harvesting systems. Excitation of the peripheral coumarin-2 chromophores results in energy transfer to the oligothiophene cores as a result of the large overlap between the donor emission spectrum and the acceptor absorption spectrum, as well as the large transition dipole moments of the oligothiophenes. Although these core dyes have low fluorescence quantum yields, their emission intensity is significantly enhanced by the ability of the large light-harvesting dendron to funnel absorbed energy to the core. Because of the large Stokes shift of the oligothiophenes, the emission spectrum of the dendrimers was red-shifted by 200 nm from the excitation wavelength. Additionally, it was found that oligothiophene orientation-end functionalization vs central functionalization-did not have a significant effect on energy-transfer efficiency.