| It has demonstrated that non-covalent interactions, such as hydrogen bonds and salt bridges, exist extensively in biological supramolecular systems. Among these, salt bridges have been attracting more interests due to their remarkable role in stabilizing biologic structures. The electron transfer and energy transfer processes play remarkable role in supramolecular systems, and determine the characters of the system. The salt bridge formed between carboxylate and amidinium is stable enough to investigate the energy/electron transfer processes even in polar or protonic solvents. Compared with the research of electron transfer and singlet energy transfer in salt bridge system, the role of the amidinium-carboxylate salt bridge in triplet-triplet energy transfer process has rarely been investigated although it is very important.In this dissertation, several supramolecular systems in which the energy donor and acceptor are connected by an amidinium-carboxylate salt bridge were built, and their photophysical and photochemical properties were examined by steady-state and time-resolved spectroscopy measurements, laser flash photolysis and photosensitized isomerization.1. Triplet-triplet energy transfer in BP-(amidinium-carboxylate)-NBD salt bridge system. A supramolecular dyad, BP-(amidinium-carboxylate)-NBD was constructed, in which benzophenone (BP) and norbornadiene (NBD) are connected via an amidinium-carboxylate salt bridge. The phosphorescence of the BP chromophore was efficiently quenched by the NBD group in BP-(amidinium-carboxylate)-NBD via salt bridge. Time-resolved spectroscopy measurements indicated that the lifetime of the BP triplet state in BP-(amidinium-carboxylate)-NBD was shortened due to the quenching by the NBD group. Selective excitation of the BP chromophore resulted in an isomerization of the NBD group to quadricyclane (QC). All of these observations suggest that the triplet-triplet energy transfer occurs efficiently in BP-(amidinium- carboxylate)-NBD salt bridge system. The triplet-triplet energy transfer process proceeds with efficiencies of approximately 0.87, 0.93 and the rate constants 1.8×103 s-1, 4.7×105 s-1 at 77 K and room temperature, respectively. The mechanism for the triplet-triplet energy transfer is proposed to proceed via a'through-bond'electron exchange process, and amidinium-carboxylate salt bridge can mediate the triplet-triplet energy transfer process effectively.2. Triplet-triplet energy transfer in BP-(carboxylate-amidinium)-NA and BP-(amidinium-carboxylate)-NA. Supramolecular dyads with different salt bridge directions, BP-(carboxylate-amidinium)-NA and BP-(amidinium-carboxylate)-NA were constructed, in which benzophenone (BP) and naphthalene (NA) are connected via amidinium-carboxylate salt bridge. The phosphorescence of the BP chromophore was efficiently quenched by the NA group via salt-bridge. Time-resolved spectroscopy measurements indicated the triplet-triplet energy transfer proceeds with efficiencies of approximately 0.91, 0.81 and the rate constants of 1.7×103 s-1, 1.1×103 s-1 at 77 K for BP-(carboxylate-amidinium)-NA and BP-(amidinium-carboxylate)-NA, respectively. At room temperature, the efficiencies (ΦET) are 0.94 and 0.84 with the rate constants of 6.7×105 s-1 and 1.6×105 s-1 for BP-(carboxylate-amidinium)-NA and BP-(amidinium-carboxylate) -NA, respectively. The mechanism for the triplet-triplet energy transfer is proposed to proceed via a'through-bond'electron exchange process. We cannot make a conclusion on the directionality and the influence of the amidinium-carboxylate salt bridge in triplet energy transfer process.3. Singlet-singlet energy transfer in salt bridge systems. Steady state and time-resolved fluorescence studies demonstrate that the singlet-singlet energy transfer from NA to BP can occur efficiently with efficiencies of 0.998, 0.970 and rate constants of 6.8×1010 s-1 and 1.5×1010 s-1 for NA-(amidinium-carboxylate)-BP and NA-(carboxlate-amidinium)-BP supramolecular systems, respectively. We infer that the singlet-single energy transfer proceeds via a'through-bond'electron exchange mechanism, and the amidinium-carboxylate salt bridge mediates the energy transfer process with directionality. Similar results were obtain in NA-(amidinium-carboxylate)-An. The detection of the anthracene fluorescence gives a direct observation of the singlet energy transfer within the salt bridge supramolecular system. |
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