Studies On Intramolecular Electron Transfer And Energy Transfer In Poly(Aryl Ether) Dendrimers

The specific structure of dendrimers makes it a mimic light-harvesting system, where the antenna chromophores surround the central reaction center. The particular attention has been paid to the photoactive dendrimers, and light-harvesting ability is one of the most specific characters of dendrimers. It's very important to study electron transfer and energy transfer in dendrimer system.In the present work, we synthesized several series of poly(aryl ether) dendrimers. The intramolecular electron transfer and energy transfer in dendrimers were examined by photophysical and photochemical methods. We also investigated the intermolecular interactions of some poly(aryl ether) dendrimers with various metal ions and alkyl viologen.1. Design and synthesis of poly(aryl ether) dendrimers. Several series of poly(aryl ether) dendrimers with carbazole as the peripheral group(CZ-Gn-OH, n = 1-3), carbazole and norbornadiene(CZ-Gn-NBD, n = 1-3),carbazole and quadricyclane (CZ-Gn-QC, n = 1-3),carbazole and viologen(CZ-Gn-V²⁺, n = 1-3),carbazole and crown ether(CZ-Gn-CR, n = 1-3) attaching to the periphery and the core, respectively, and tetradecyl viologen, total 16 new compounds, were synthesized by convergent method. All new compounds were characterized by IR, 1H NMR, MS (EI or MALTI-TOF) and elemental analysis.2. Studies on the intramolecular electron transfer and triplet energy transfer in poly(aryl ether) dendrimers (CZ-Gn-NBD n = 1-3). Selective excitation of the carbazole units in CZ-Gn-NBD resulted in a singlet electron transfer from CZ to NBD at room temperature, and an intersystem crossing followed a triplet-triplet energy transfer from CZ to NBD in glassy 2-methyltetrahydrofuran at 77 K. Both singlet electron transfer and triplet energy transfer processes lead to the isomerization of the norbornadiene group to the quadricyclane (CZ-Gn-QC). The efficiencies and the rate constants for singlet electron transfer are ca. 88%, 80%, 74%, and 1.8×109, 6.1×108, 4.0×108 s-1 for generation 1-3, respectively. The quantum yields of the intramolecular photosensitized isomerization are measured to be ca. 0.013, 0.012 and 0.011, and the efficiencies of the triplet norbornadiene formation via the singlet electron transfer are ca. 0.070, 0.065, and 0.059 for generation 1-3, respectively. The light-harvesting ability of CZ-Gn-NBD increases with the generation due to an increase of the number of peripheral chromophores. In glassy 2-methyltetrahydrofuran at 77 K, the triplet-triplet energy transfer proceeds with the efficiencies of ca. 0.86, 0.64, 0.36, and the rate constants of 0.96, 0.25, 0.08 s-1 for generation 1-3, respectively. The intramolecular singlet electron transfer and the triplet energy transfer in CZ-Gn-NBD proceed mainly via a through-space mechanism involving the proximate donor (folding back conformation) and acceptor groups.3. Studies on the electron transfer in poly(aryl ether) dendrimers (CZ-Gn-V²⁺, n = 1-3). Selective excitation of the CZ units in CZ-Gn-V²⁺ resulted in a singlet electron transfer from CZ to V²⁺ with the efficiencies of ca. 98%, 95% and 95%, and the rate constants of 8.2×109 s-1, 3.1×109 s-1, 3.1×109 s-1 for generation 1-3, respectively. The efficiencies of the singlet electron transfer do not decrease obviously with the generation increase, which means that CZ-Gn-V²⁺ is a high-efficient light-harvesting system. The intramolecular singlet electron transfer is proposed to proceed mainly via a through space mechanism, involving the closest donor and acceptor.4. Studies on the interactions of CZ-Gn-CR and CZ-Gn-OH with metal ions and alkyl viologen. The photophysical studies reveal that there are strong interactions between dendrimers and Eu³⁺, Tb³⁺ and Cu²⁺ in the ground state, and the core crown ether structure enhances interactions. The fluorescence of dendrimers is quenched by Eu³⁺, Tb³⁺ and Cu²⁺ through static quenching process. Li+, Na+, K+, Ca²⁺and Zn²⁺ have no measurable influence to the absorption and fluorescence spectra of dendrimers. Dendrimers can also form 1:1 complexes with alkyl viologen in the ground state with the binding constants about 103. The fluorescence of dendrimers is quenched by alkyl viologen through the electron transfer process.