Photophysical Properties Of Functional Dendrimers With Noncovalent Decoration

Dendrimers are well-defined and successively branched macromolecules with the possibility of modifying functional units in predetermined sites of their treelike structure. The unique structure of the dendrimer makes it a mimic of natural photosynthetic systems, which are surrounded by plentiful antenna chromophores. Improving light-harvesting dendrimers is an important research field of the artificial photosynthetic research. In addition, dendrimers are widely applied in the studies of ion senors and metallic nanocomposites. Understanding the relationship between photophysical properties and dendrimer structures and revealing the metal complexation processes with dendrimers are essential for those research fields.In this dissertation, light-harvesting poly(amidoamine) dendrimers with naphthyl decorated at periphery, generation 0?3, were synthesized. These light-harvesting dendrimers have been noncovalently modified by cucurbit[7]uril (CB[7]) through peripheral pesudorotaxane formation resulting in enhancement of energy utilization. The interactions among components of dendrimers and the metal complexation were investigated by steady-state spectroscopy.1. Synthesis of the target compounds. Naphthyl terminal-decorated poly(amidoamine) dendrimers, generation 0?3 (GnNap, n = 0?3), were synthesized divergently using diaminododecane as the core. Corresponding water-soluble light-harvesting dendrimers (GnNapH, n = 0?3) were obtained after the protonation of GnNap. Host molecule for noncovalent modification, cucurbit[7]uril (CB[7]), was also synthesized. Structures of all compounds were characterized. 2. Enhancement of energy utilization in light-harvesting dendrimers by noncovalent modification. Fluorescence studies reveal that strong interactions among peripheral chromophores occur in these dendrimers according to the intensive excimer emission and the low fluorescence quantum yields (Φf = 0.12, 0.097, 0.065 and 0.062 for G0–3NapH, respectively). Through assembly of dendrimers with cucurbit[7]uril (CB[7]), the well-defined pseudorotaxane assemblies GnNapH?xCB[7] (x = 4, 8, 16, 32 for n = 0–3, respectively) form and the energy dissipation is entirely suppressed resulting in a dramatic increase of the fluorescence quantum yield of dendrimers (Φf = 0.18, 0.19, 0.19 and 0.20 for 0–3 generations, respectively). The noncovalent modification is a reversible process and CB[7] can be unthreaded from the dendrimer periphery by adding 1-amimoadamantane (AD) which can form a more stable complex with CB[7]. Furthermore, 9-anthracenecarboxylic acid (AN), an energy acceptor, was introduced into the dendritic system to investigate the harvested energy utilization. Steady-state fluorescence investigations demonstrate that the energy transfer efficiencies from naphthyl to AN in G3NapH?32CB[7]–AN and G2NapH?16CB[7]–AN are enhanced 100% and 70% compared with those without CB[7] complexation.3. Protonation investigation of GnNap. Steady-state photophysical studies indicate that the fluorescence of naphthyl is quenched by the amine units of dendrimers GnNap via the intramolecular electron transfer, and a naphthyl-amine exciplex is formed with a structureless emission around 450 nm. The protonation of GnNap by addition of excess trifluoroacetic acid makes the fluorescence intensity of dendrimers increase dramatically due to suppression of the photoinduced electron transfer process and the exciplex formation, a weak naphthyl excimer emission with maximum at ca. 400 nm can be observed indicative of close-packed periphery and stretched conformation of protonated dendrimers.4. Investigation on the complexation of GnNap (n = 0, 3) with metal ions. GnNap show a special bingding behavior toward Hg2+. First, Hg2+ and peripheral secondary amine groups form a complex with an 1:2 binding mode resulting in an increase of the fluorescence of GnNap. After full complexation of peripheral secondary amine is reached, Hg2+ coordinates with tertiary amine units within dendrimer backbone in an 1:1 binding model and consequently quenched the emission of GnNap. Fe2+, Co2+, Ni2+, Cu2+ and Zn2+ can complex with GnNap and evidently affect the photophysical properties of dendrimers. Cd2+, Mn2+, alkali metal and alkaline-earth metal ions have no obvious effect on the photophysical properties of GnNap in methanol indicative of no obvious interactions between GnNap and those ions.