The aggregation behavior of the amphiphilic poly (amidoamine) (PAMAM) dendrons (ABn-Nap

Dan-ABn and Dan-ABn-Nap

n denotes the generation of the dendrons

Dan = Dansyl

Nap = Naphthyl) in water was explored and their energy transfer property was studied by using UV-Vis

fluorescence spectroscopy and transmission electron microscopy (TEM) .It was found that the modified dendritic molecules aggregated to irregular sphere-like assemblies with the size range of 150 ～ 650 nm.The critical aggregation concentration (CAC) values of ABn-Nap and Dan-ABn-Nap decreased with the generation of dendron increasing

which indicated the enhanced aggregation ability of them.Specially

Dan-AB4 displayed the lowest CAC value

indicating the dendron possessed an appropriate hydrophilic-hydrophobic balance that was a key factor for the aggregation.Notably

the efficient energy transfer occurred from the peripheral naphthyl groups to the focal dansyl unit in the aggregates.While Dan-ABn-Nap aggregates were excited at 282 nm

the fluorescence of naphthyl groups was quenched greatly

instead

strong fluorescence (400 ～ 650 nm) of dansyl groups appeared.The energy transfer efficiencies of different generation dendrons at different concentrations in water were calculated by using Frster theory.Energy transfer occurred with low efficiency when the concentration of dendrons was lower than CAC

for example

the energy transfer efficiency of Dan-AB8-Nap was only 5.7% at the concentration of 1.0 × 10-7 mol/L.However

when the concentration was above CAC (1.0 × 10-4 mol/ L)

the energy transfer efficiency improved greatly (77.1%) .Interestingly

the energy transfer efficiency of the aggregates in water was higher than that in organic solution at the same concentration.These results can be attributed to the more ordered stacking of the chromophores

closer distance between chromophores and both intermolecular and intramolecular energy transfer in the aggregates.