Chirality Transfer and Electron Transfer in Dendritic Complexes with Stable Secondary Structure
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The control of secondary structure in macromolecules is always desired when constructing functional macromolecules and when trying to mimic what nature has to offer in biological environments. Dendrimers, because of their highly branched structure, versatile morphology and almost endless functionalization possibilities, have always been an interesting target for researchers.
Most dendrimers constructed with aliphatic linkers and focal units are too flexible to control their conformation. However, our group has developed two types of focal units, 2-methoxyisophthalamide and pyridine-2,6-dicarboxamide units that have strong energetic preferences (4.6 and 10.1 kcal/mol, respectively) to stay in the syn-syn arrangement. This provides a good building block to build dendrimers with stable secondary structures. Furthermore, our group has also discovered that by inserting anthranilamide turn units into the system, the packing can be further improved.
The first chapter of this thesis describes creating a dendrimer that the global chirality of the dendrimer is essentially reflected by the core of the dendrimer, i.e. only the core is chiral and by using building blocks with stable secondary structure the chirality from the core can be propagated to the periphery. There had been a published example (19) from our group that featured a (S)-BINOL core and dendrons with bulky terminal groups. However that dendrimer had several places that could be improved: (1) the linker between dendron and BINOL is based on a labile phenolic ether, (2) the bulky t-butyl ester end group is acid labile, and (3) the higher generation is unreachable due to extreme steric effects. Based on that design, several modified targets were attempted which finally leads to 62, which has a (R)-BINOL core with extra phenol turn units that was necessary to put the dendrons into close proximity to induce packing. End groups were also switched to stable, bulky diisopropyl amides.
The major advantage of 29 over 19 is synthetic versatility. Because the dendrons are now connected to the chiral core with a C-C bond, it allowed wide varieties of 29 to be made. First, higher generations are now accessible, with both type I (without additional anthranilamide turn unit) and type II (with anthranilamide turn unit). Second, different end group variations for 29 were also carried out. However, the degree of chirality transfer for 29 was only comparable to that of 19.
The second chapter focuses on studying the effect of structure and conformation on electron and energy transfer behavior of triads that features two zinc porphyrin electron donors and naphthalene diimide acceptors. The previously synthesized triads by Yingqi Wu feature the 2-methoxy-isophthalamide focal unit (72 and 129) that have lower energy preferences for the syn-syn conformation. Therefore new triads (89 and 92) were prepared with the pyridine-2,6-dicarboxamide focal unit.
NOESY experiments were performed for all of the triads. It was found that the type I triads (72 and 89) had the more defined "Y" type conformation. Even though the 2-methoxyisophthalamide focal unit has lower energy preference for the syn-syn arrangement, it was still capable of holding the porphyrin units apart to maintain the "Y" shape even though slight back folding of the porphyrin was observed with 72. On the other hand, in the type II triads (73 and 92), due to the inserted anthranilamide turn units that allow the porphyrins to get closer, a backfolded conformation with porphyrin donor stacking on top of naphthalene diimide (NDI) acceptor were observed for 73, but a more extended conformation was found for 92 due to the higher energy preference of the pyridine-2,6-dicarboxamide to stay in the syn-syn arrangement.
Different control compounds that do not have the NDI acceptor and with only one porphyrin unit for these triads were also prepared because when porphyrin units in the traids were excited, different electron transfer mechanisms competed that both electron transfer from porphyrin to NDI and energy transfer from porphyrin to porphyrin then electron transfer from porphyrin to NDI could happen. Laser spectroscopic analyses were carried out by Dr. David A. Modarelli's lab for all of the triads and their control compounds, and were found that in general, triads with overall shorter porphyrin to NDI distance showed better electron transfer capability, suggesting a through-bond electron transfer mechanism. On the other hand, triads with pyridine-2,6-dicarboxamide focal unit showed better energy transfer because the conformations were more defined.
Document number: osu1222186119
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=osu1222186119
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