Regulation of Photophysical Properties for Naphthyl Decorated PAMAM Dendrimers by Pseudorotaxane Formation at Periphery

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Abstract Dendrimers are regularly and hierarchically branched synthetic macromolecules with monodispersity, globular shape, interior pockets, and numerous peripherial functionality, which endow them with great potential in miscellaneous applications such as host-guest chemistry, chemosensor, catalysis, drug delivery, microreactor and light-harvesting systems. In the present work, a series of naphthyl terminal-decorated polyamidoamine (PAMAM) dendrimers Gn-NA (n=1~3) and model compound were synthesized. The water-soluble PAMAM backbone was constructed by the classic divergent method and the terminal modification of dendrimers by naphthyl groups was accomplished through a reaction of peripheral amino units with naphthyl succinimidyl ester efficiently. The average functionalization extents of the peripheral groups were 100%, 97% and 93% for G1~G3, respectively. The structures of Gn-NA (n=1~3) were characterized by H NMR, C NMR, MALDI-TOF mass spectra and IR. Steady state and time-resolved photophysical studies reveal that strong interactions among peripheral naphthyls occur within Gn-NA, resulting in the excimer formation and the naphthyl fluorescence quenching. The quenching efficiency of naphthyl fluorescence increases with increasing generations. The assembly of the peripheral naphthyl group with cucurbit[7]uril (CB[7]) forms a pseudorotaxane with ratio of 1∶1 and association constants (an application of B-H equation) of 768, 887 and 823 mol•L from generation 1 to generation 3, respectively. The lifetime of the pseudorotaxane of NA-Model with CB[7] is slightly longer than that of NA-Model, indicating that the formation of pseudorotaxane can reduce the nonradiative decay at a certain degree. The lifetimes of naphthyl group and its pseudorotaxane with CB[7] of Gn-NA were resolved to be 2.5~2.7 and 11.4~11.7 ns, respectively, and the lifetimes of the intramolecular naphthyl excimer of Gn-NA were analyzed to be ca. 17 ns. Combining the results of the remarkably increased lifetime of the naphthyl group in its pseudorotaxane form and those of the NA-Model system, we conclude that the peripheral pseudorotaxane formation depresses both the formation of intramolecular naphthyl excimer and the excited state quenching by solvent, thus enhancing the fluorescence emission of naphthyl dramatically. This study provides a potential strategy for developing controllable light-emitting dendritic system with efficient synthesis and improvable photophysical properties.