Photoinduced Electron Transfer as a Probe for the Folding Behavior of Dimethylsilylene-Spaced Alternating Donor–Acceptor Oligomers and Polymers

A series of oligomers and polymers having dimethylsilylene-spaced alternating 4-aminostyrene donor and stilbene acceptor chromophores (two to one) are regioselectively synthesized, and the two donor chromophores are separated by different bridges between two donors. Photophysical tools have been used to examine the folding behavior of these copolymers. Both steady-state and time-resolved fluorescence spectroscopic measurements were examined. The relative intensities (<i>I</i>_CT/<i>I</i>_LE) between emission from charge-separated state (CT emission) and local excited emission of acceptor chromophore (LE emission) increase with increasing number of repeating units, and reach a plateau, when the linkers between the two aminostyrene chromophores are trimethylene bridges. Replacements of these by dimethylene or tetramethylene linker reduce the relative intensities of CT emission of the polymers, owing to the different folding behavior of these polymers. The CT emission intensity of the polymer with rigid piperazine linkers is much lower than that with trimethylene-bridged copolymer of the same degree of polymerization. Slight conformational change of these polymers would lead to slight variation of the distance between donor and acceptor chromophores so that the nonadiabatic interactions in the excited state between donor–acceptor pairs in these oligomers and polymers would be perturbed by such change of conformations

Substituent-Dependent Photophysical Properties Due to the Thorpe–Ingold Effect on Foldings of Alternating Substituted Methylene–Diethynylbenzene Copolymers: A Comparison of Carbon versus Silicon Tethers

Alternating <i>tert</i>-butyl- and methyl-substituted alkoxy­methylene–diethynyl­benzene copolymers with different degrees of polymerization and the corresponding dimers are synthesized. The <i>tert</i>-butyl-substituted polymers show prominent emissions around 350–400 owing to ground state interactions between adjacent chromophores separated by a substituted methylene group. The Thorpe–Ingold effect exerted by the bulky <i>tert</i>-butyl group would compress the bond angle at the methylene tether and may alter the overall folding scaffold of the polymer. The interactions between adjacent chromophores would be significantly enhanced in these <i>tert</i>-butyl-substituted copolymers. On the other hand, the corresponding less bulky, methyl-substituted alkoxy­methylene tethered copolymers exhibit emission around 400–450 nm attributed to the through-space interactions between nonadjacent diethynyl­benzene chromophores. The variations of folding nature of these two kinds of copolymers are determined by the size of the substituents, methyl versus <i>tert</i>-butyl, resulting in different photophysical behaviors. The emission properties of the methyl-substituted copolymers behave similarly to those of related silylene-tethered copolymers in the literatures, albeit the relative intensity in the blue light emission is somewhat smaller in methylene-bridged copolymers than in silylene-linked copolymers

Stereoselective Iterative Convergent Synthesis of <i>Z</i>‑Oligodiacetylenes from Propargylic Dithioacetals

A series of ^<i>t</i>Bu-substituted <i>Z</i>-oligodiacetylenes (<i>Z</i>-ODAs) are synthesized from the reactions of allenyl/propargylic zinc reagents, obtained from the corresponding propargylic dithiolanes and BuLi, with dithiolane-substituted propargylic aldehydes followed by stereospecific elimination of β-thioalkoxy alcohols under Mitsunobu conditions. The stereochemical assignments are based on NOE experiments. The X-ray structure of the hexamer further supports the <i>Z</i> configuration for each of the double bonds in these ODAs. The photophysical properties of these <i>Z</i>-ODAs have been examined and are compared with known related <i>E</i>- and <i>Z</i>-ODAs with different substituents