Highly branched stilbene dendrimers were synthesized and their photochemical behavior was studied. Even the stilbene dendrimer with molecular weight over 6500 underwent trans-cis isomerization in the excited singlet state within the lifetime of 10 ns. The photoisomerization of C=C double bond of stilbene dendrimers in the excited state may proceed by a volume-conserving novel mechanism such as hula-twist rather than conventional 180 degrees rotation around the C=C double bond based on fluorescence and isomerization experiments.
The energy-releasing process and the conformational dynamics of stilbene dendrimers (tetramethoxystilbene
(generation (G) 0, G1, G2, and G4) upon photoexcitation are studied by the time-resolved transient grating
method. For all dendrimers, the excited triplet (T1) state formation was observed. From the time-resolved
studies on energy and the molecular volumes changes, it is found that the conformational change completes
with the decay of the T1 state for G0−G2. However, the dynamics is slightly slower for G4, which is attributed
to the conformational change of the dendron part. The molecular volume change due to the conformational
rearrangement of the dendron upon the trans to cis isomerization is slightly positive (molecular expansion).
The expansion is explained in terms of the creation of the void volume surrounded by the dendron side
chains. Although the diffusion coefficient (D) of the trans and cis forms of G0 is almost the same, the difference
becomes larger with increasing generation. This is clear evidence for the large conformational change upon
isomerization. The larger D of the trans form compared with that of the cis form is explained in terms of the
surface roughness of the dendrimers.
The third generation of novel photo-responsive water-soluble stilbene dendrimer (trans- and cis-G3 WSD) undergoes unusual one-way trans-to-cis isomerization to give 100% of cis isomer at the photostationary state on UV irradiation in water.
A straightforward synthesis of 1,2-dicyanoalkanes by reacting nitroalkenes with trimethylsilyl cyanide in the presence of tetrabutylammonium fluoride is described. The reaction proceeds through a tandem double Michael addition under mild conditions. Employing the hypervalent silicate generated from trimethylsilyl cyanide and tetrabutylammonium fluoride is essential for achieving this transformation. Mechanistic studies suggest that a small amount of water included in the reaction media plays a key role. This protocol is applicable to various types of substrates including electron-rich and electron-deficient aromatic nitroalkenes, and aliphatic nitroalkenes. Moreover, vinyl sulfones were found to be good alternatives, particularly for electron-deficient nitroalkenes. The broad substrate scope and functional group tolerance of the reaction makes this approach a practical method for the synthesis of valuable 1,2-dicyanoalkanes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.