Poly(propylene amine) dendrimers with peripheral dansyl units: Protonation, absorption spectra, photophysical properties, intradendrimer quenching, and sensitization processes

We report the preparation, absorption spectra, and photophysical properties (fluorescence spectrum, quantum yield, and lifetime) of five dendrimers of the poly(propylene amine) family (POPAMs) functionalized with dansyl units at the periphery. Each dendrimer nD, where n = 1 to 5 is the generation number, comprises 2(n+1) (i.e., 64 for 5D) dansyl functions in the periphery and 2(n+1) - 2 (i.e., 62 for 5D) tertiary amine units in the branches. In acetonitrile/dichloromethane solution each dansyl unit of the dendrimers exhibits the characteristic properties of a monodansyl model compound. Upon addition of trifluoroacetic or triflic acid, the absorption and emission bands of the dansyl units are gradually replaced by the absorption and emission bands of the protonated dansyl unit. In the case of the dendrimers the titration curves obtained from the changes in emission intensity do not match those obtained from the changes in absorbance. Furthermore, each dendrimer exhibits a different titration curve. The results obtained show that protonation first involves tertiary amine groups of the interior and then the dansyl units of the periphery. In the dendrimers containing both protonated and unprotonated dansyl units, the fluorescence of the protonated units is partly quenched. Corrected excitation spectra have shown that the quenching is due to energy transfer from the excited protonated to the ground-state unprotonated dansyl units, with a consequent amplification of the changes in the fluorescent signals that accompany the protonation/deprotonation of the dansyl units.