Ground and excited-state electronic interactions in poly(propylene amine) dendrimers functionalized with naphthyl units: effect of protonation and metal complexation.

We report the absorption spectra and the photophysical properties (fluorescence spectrum, quantum yield, and lifetime) of four dendrimers of the poly(propylene amine) family (POPAMs) functionalized at the periphery with naphthylsulfonamide (hereafter called naphthyl) units. Each dendrimer Gn, where n = 1 to 4 is the generation number, comprises 2n + 1 (i.e., 32 for G4) naphthyl functions in the periphery and 2n + 1--2 (i.e., 30 for G4) tertiary amine units in the branches. All the experiments have been carried out in acetonitrile solutions. Comparison with two reference compounds (N-methyl-naphthalene-2-sulfonamide, A, and N-(3-dimethylamino-propyl)-2-naphthalene-1-sulfonamide, B) has shown that the absorption spectra of the dendrimers are significantly different from those expected from the component units. Furthermore, the intense fluorescence band of the naphthyl unit (lambda max = 343 nm; phi = 0.15, tau = 8.5 ns) is strongly quenched in the dendrimers. The quenching effect increases with increasing generation and is accompanied by the appearance of a weak and broad emission tail at lower energy. Protonation of the amine units of the dendrimers by addition of CF3SO3H (triflic) acid causes a strong increase in the intensity of the naphthyl luminescence and a change in the form of the emission tail. The shapes of the titration curves depend on dendrimer generation, but in any case, the effect of the acid can be fully reversed by successive addition of a base (tributylamine). The results obtained show that in the dendrimers there are interactions (both in the ground and excited states) between naphthyl units as well as between naphthyl units and amine units of the branches; this gives rise to dimer/excimer and charge-transfer/exciplex excited states. Titration with Zn(CF3SO3)2 has the same effect as acid titration, as far as the final emission spectrum is concerned, but a much higher concentration of Zn(CF3SO3)2 has to be used and the shapes of the titration plots are very different. Titration with Co(NO3)2.6H2O causes a much smaller increase in the intensity of the naphthyl fluorescence compared with Zn(CF3SO3)2. The results obtained have shown that protonation and metal coordination can reveal the presence of ground and excited state electronic interactions in functionalized poly(propylene amine) dendrimers, and that the presence of photo-active units in the dendrimers can be useful to reveal some peculiar aspects of the protonation and metal coordination processes.