A new mechanism of photosensitized formation of thymine (Thy) dimers is proposed, which involves generation of a delocalized triplet excited state as the key step. This is supported by chemical evidence obtained by combining one benzophenone and two Thy units with different degrees of freedom, whereby the photoreactivity is switched from a clean Paternò–Büchi reaction to a fully chemo‐, regio‐, and stereoselective [2+2] cycloaddition. 相似文献
A series of oligothiophenes that incorporate cyclopenta[c]thiophene‐based units bearing spiro‐substituted dialkylfluorene was synthesized. Photophysical measurements indicated that there was no interruption in the conjugation along the oligothiophene backbones, irrespective of the number or position of this unit. Electrochemical measurements showed that the thiophene 7‐mers and 11‐mer exhibit reversible multi‐oxidation waves. The formation of cationic species was clearly observed from UV/Vis/NIR measurements. Furthermore, the UV/Vis/NIR spectra at 223 K under one‐electron oxidation conditions revealed that the unsubstituted thiophene or bithiophene units remained in the absence of intermolecular π–π interactions, whereas the formation of π‐dimeric species was observed for the thiophene 7‐mer containing an unsubstituted terthiophene ( U3 ) unit. Theoretical calculations indicated that the combination of the U3 unit and the all‐trans conformation decreased the intermolecular steric repulsion between the fused cyclopentene ring and its facing thiophene, which may contribute to the formation of the dimeric structure. 相似文献
A novel route for the synthesis of poly(ethylene glycol)‐b‐polystyrene copolymer, starting from commercially available poly(ethylene glycol) methyl ether and azido terminated polystyrene prepared by atom transfer radical polymerization and subsequent nucleophilic substitution, is applied with simplicity and high efficiency. The combination of photoinduced copper (I)‐catalyzed alkyne‐azide cycloaddition (CuAAC) and ketene chemistry reactions proceeds either simultaneously or sequentially in a one‐pot procedure under near‐visible light irradiation. In both cases, excellent block copolymer formations are achieved, with an average molecular weight of around 7000 g mo1−1 and a polydispersity index of 1.20.
An ultraviolet (UV)‐cleavable bottlebrush polymer is synthesized using the “grafting‐onto” strategy by combining living radical polymerization and copper‐catalyzed azide‐alkyne cycloaddition (CuAAC). In this approach, reversible addition‐fragmentation chain transfer polymerization is used to prepare a poly(methylacrylate) backbone with azide side groups, while atom transfer radical polymerization is employed to prepare polystyrene (PS) side chains end‐functionalized with o‐nitrobenzyl (UV‐cleavable) propargyl groups. CuAAC is then used to graft PS side chains onto the polymer backbone, producing the corresponding bottlebrush polymers with UV‐cleavable PS side chains. The formation of the bottlebrush polymer is characterized by 1H nuclear magnetic resonance spectroscopy, gel permeation chromatography (GPC), and Fourier transform infrared spectroscopy. The cleavage behavior of the bottlebrush polymer is monitored in tetrahydrofuran solution under UV irradiation by GPC and viscosity measurements.
We have developed an efficient method for preparation of triazolamers using a dicobalt hexacarbonyl complex of propargyl azides (CPA) as a synthetic component. Several types of CPAs possessing the side chains found in the natural amino acids were prepared by Nicholas reaction as well as by a reaction of propargyl azides with dicobalt octacarbonyl. Triazolamers with both amino and carboxyl termini were readily synthesized by repetitive reaction sequence involving a copper-catalyzed 1,3-dipolar cycloaddition followed by an oxidative deprotection. 相似文献
The tunable self-assembly of a fullerene (C(60))-N,N-dimethylaminoazobenzene (DPNME) molecular system as a function of solvent polarity in THF/water binary solvent is reported. Gradual increase of the volume fraction of the nonsolvent water in a 1×10(-5) M THF solution of DPNME at a mixed dielectric constant ε(mix)≈42 resulted in initial redshifting of the (1)(π→π(*)) absorption band, which signified the 1D head-to-tail or J-type arrangement of the DPNME molecular system. Further increase in the solvent polarity to ε(mix)≈66 evidenced formation of an antiparallel head-to-tail or H-type molecular arrangement in conjunction with the J-aggregates, thereby establishing a solvent-polarity-dependent dynamic equilibrium between the monomer ? J-aggregate ? H-aggregate. The controlled aggregation was governed by the synergetic effect of intermolecular donor-acceptor interaction between the electron-deficient fullerene ring and the electron-rich N,N-dimethylamino-substituted aromatic ring; typically, van der Waals and π-π interactions between the molecules constituting a pair of dimers were envisaged. An agreement between the semiempirically calculated drastically reduced oscillator strength of the DPNME H-dimer in the antiparallel configuration (0.69 vs. 1.29 in the monomeric DPNME) and the experimental electronic absorption spectra beyond ε(mix)=66 further strengthened this assignment to the hitherto forbidden antiparallel H-dimer. Complementing the above, the periodicity of molecular self-assembly dictated a monoclinic unit cell in the single-crystal XRD packing pattern with a C2/c space group; the molecules packed laterally with mutual interdigitation with the donor (E)-N,N-dimethyl-4-(p-tolyldiazenyl)aniline (AZNME) parts in an antiparallel fashion (contrary to the usual expectation for H-aggregates) with strong inter- and intrapair van der Waals and π-π interactions between the constituent fullerene moieties. Unlike those of porphyrin/phthalocyanine bowl-like donor-initiated architectures, a rare class of DPNME dyadic supramolecular self-assemblies was realized with π-extended 2D fullerene networks, in which the linear geometry of the AZNME donor and the conformational rigidity of the fullerene acceptor played crucial roles. 相似文献
Density functional calculations have been performed to comparatively investigate two possible pathways of Au(I)‐catalyzed Conia‐ene reaction of β‐ketoesters with alkynes. Our studies find that, under the assistance of trifluoromethanesulfonate (TfO), the β‐ketoester is the most likely to undergo Model II to isomerize into its enol form, in which TfO plays a proton transfer role through a 6‐membered ring transition state. The coordination of the Au(I) catalyst to the alkynes triple bond can enhance the eletrophilic capability and reaction activity of the alkynes moiety, which triggers the nucleophilic addition of the enol moiety on the alkynes moiety to give a vinyl‐Au intermediate. This cycloisomerizaion step is exothermal by 21.3 kJ/mol with an energy barrier of 56.0 kJ/mol. In the whole catalytic process, the protonation of vinyl‐Au is almost spontaneous, and the formation of enol is a rate‐limiting step. The generation of enol and the activation of Au(I) catalyst on the alkynes are the key reasons why the Conia‐ene reaction can occur in mild condition. These calculations support that Au(I)‐catalyzed Conia‐ene reactions of β‐ketoesters with alkynes go through the pathway 2 proposed by Toste. 相似文献