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 not where n = 1 to 5 is the generation nu mber, 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 a cetonitrile/dichloromethane solution each dansyl unit of the dendrimers exh ibits the characteristic properties of a monodansyl model compound. Upon ad dition of trifluoroacetic or triflic acid, the absorption and emission band s 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 tit ration 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 proton ation first involves tertiary amine groups of the interior and then the dan syl units of the periphery. In the dendrimers containing both protonated an d unprotonated dansyl units, the fluorescence: of the protonated units is p artly quenched. Corrected excitation spectra have shown that the quenching is due to energy transfer fi om the excited protonated to the groundstate u nprotonated dansyl units, with a consequent amplification of the changes in the fluorescent signals that accompany the protonation/deprotonation of th e dansyl units.