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 a nd coumarin-2 chromophores at their periphery have been shown to be very ef ficient light-harvesting systems. Excitation of the peripheral coumarin-2 c hromophores results in energy transfer to the oligothiophene cores as a res ult of the large overlap between the donor emission spectrum and the accept or absorption spectrum, as well as the large transition dipole moments of t he oligothiophenes. Although these core dyes have low fluorescence quantum yields, their emission intensity is significantly enhanced by the ability o f the large light-harvesting dendron to funnel absorbed energy to the core. Because of the large Stokes shift of the oligothiophenes, the emission spe ctrum of the dendrimers was red-shifted by 200 nm from the excitation wavel ength. Additionally, it was found that oligothiophene orientation-end funct ionalization vs central functionalization-did not have a significant effect on energy-transfer efficiency.