Folding of Alternating Dialkylsilylene-Spaced Donor-Acceptor Copolymers: The Oligomer Approach
Date Issued
2012
Date
2012
Author(s)
Chen, Wen-Hao
Abstract
A series of oligomers with donor and acceptor moieties divided by different dialkylsilylene spacers (Me2Si- vs iPr2Si-) have been designed and synthesized to inspect the folding behavior of the corresponding copolymers by investigating their photophysical properties. 4-Aminostyrene moiety was chosen as the donor and stilbene moiety was selected as the acceptor. Sequential Rh-catalyzed hydrosilylation of terminal alkyne with silylhydride afforded these oligomers. Terminal alkynes were prepared by Sonogashira reaction. Silylhydrides were introduced by lithiation of corresponding aryl bromides with nBuLi followed by silylation with dialkylchlorosilanes. Unsymmetrical stilbenes and bisalkyne were delicately designed and synthesized for constructing these oligomers.
Both local excited (LE) emission of the acceptor chromophore and emission from the charge-separated state (CT emission) have been observed in Me2Si-spaced oligomers, just like that of Me2Si-spaced copolymer. As degree of oligomerization increased, the relative intensity of the CT emission compared to LE emission of Me2Si-spaced oligomers displayed prominent enhancement. When compared with Me2Si-spaced copolymers with different degrees of polymerization (DP = 14 and DP = 30), this tendency reached a plateau rather than increased proportionally.
On the other hand, albeit the bulky isopropyl group introduced on the silicon atom, iPr2Si-spaced monomer and dimer showed similar photophysical behaviors to those of Me2Si-spaced oligomers and copolymers. However, iPr2Si-spaced trimer and tetramer exhibited charge-transfer absorption, which is different from the intrinsic absorption of the donor and acceptor chromophores. Furthermore, tetramer showed essentially same emission from charge-transfer complex as that of iPr2Si-spaced copolymers. The charge-transfer complex emission is attributed to the strong electronic interaction between the donor and acceptor. The change of local conformation triggered by substitution of bulky isopropyl group on silicon atom and the extension of molecular structure resulted in this variation.
These results suggest that the electronic interactions between adjacent donor and acceptor should be affected by the folding nature, which could be tuned by not only the steric effect of the substituents on the silicon atoms but also the extending molecular structures. In addition, the differences in the small energetic barriers for each of the conformational states may be amplified by extending the distance of the folding structure, thus resulting in a more stable conformation.
Subjects
oligomer
photoinduced electron transfer
polymer folding
silylene-spaced copolymer
Thorpe-Ingold Effect
Type
thesis
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