Folding of Alternating Dialkylsilylene-Spaced Donor-Acceptor Copolymers: The Oligomer Approach

A series of oligmers with donor–acceptor pairs separated by diisopropylsilylene (iPr2Si) spacers, composed of monomer 4 b, dimer 5, trimer 6, and tetramer 7, were synthesized to scrutinize the folding behavior. Monomer 4 a with a dimethylsilylene (Me2Si) spacer was also prepared for comparison. The 4‐aminostyrene moiety was used as the donor and the stilbene moiety as the acceptor. Both steady‐state and time‐resolved fluorescence spectroscopic measurement were made. Regardless of the substituents on the silicon atom, the emission spectra of 4 a and 4 b exhibit both local excited (LE) emission of the acceptor chromophore and emission from the charge‐separated state (CT emission), which are similar to that of the corresponding Me2Si‐spaced copolymer 2 a with the same donor and acceptor chromophores, but different from that of the copolymer with the iPr2Si spacer 2 b. Dimer 5 behaves like 4 and 2 a. As the chain length of the oligomers increases, the emission properties of the higher homologues become prone to that of 2 b. Thus, tetramer 7 exhibits emission from the charge‐transfer complex, which is essentially same as that of 2 b. Moreover, charge‐transfer absorptions emerge in 6 and 7. These results suggest that the folding nature of oligomers approaches that of the corresponding polymer, as the degree of oligomerization increases, and the electronic interactions between adjacent donor–acceptor pairs are controlled by the steric effect of the substituents on the silicon atoms and concomitant amplification of the stabilizing energy by extending the distance of the folding structure. Size control: A series of oligomers containing alternating donor and acceptor (2:1) chromophores separated by diisopropylsilylene moieties were synthesized to scrutinize the folding behavior of the corresponding polymers. The photophysical properties of the tetramer approach those of corresponding diisopropylsilylene‐containing polymers, whereas those of monomer and dimer behave similarly to those of dimethylsilylene‐incorporated polymers. Both the Thorpe–Ingold effect and folding of polymers may be responsible for this behavior (see picture).