Photoswitchable Carbohydrate‐Based Macrocyclic Azobenzene: Synthesis, Chiroptical Switching, and Multistimuli‐Responsive Self‐Assembly

Abstract

A one-pot O-alkylation mediated macrocyclization approach has been used for the synthesis of carbohydrate-based macrocyclic azobenzene. The synthesized macrocycle can be reversibly isomerized between E and Z isomers upon UV or visible irradiation with excellent photostability and thermal stability (t =51 days at 20 °C for the Z isomer). A chirality transfer from the chiral sugar unit to azobenzene was observed by circular dichroism (CD). DFT and TD-DFT calculations were performed to calculate the optimal geometry and the theoretical absorption and CD spectra. Comparison of the experimental CD spectra with the theoretical ones suggests that both E- and Z-macrocycles adopt preferentially P-helicity for the azobenzene moiety. Furthermore, the macrocycle showed gelation ability in cyclohexane and ethanol with multistimuli-responsive behavior upon exposure to environmental stimuli including thermal-, photo-, and mechanical responses. Moreover, these organogels display temperature-dependent helical inversion, which can be tuned by a repeated heating-cooling procedure.

Sweet switching: Development of photoswitchable glycoconjugates ‐ Mini‐Review

“…All macrocycles showed excellent fatigue resistance since photoswitching cycles have been repeated more than 15 times without any noticeable degradation. [13,14] We have also observed chiroptical properties for macrocyclic glycoazobenzenes. [13,14] As demonstrated by circular dichroism (CD) spectra, the 4,6-O-bridged manno derivative 1, [13] as well as the gluco macrolactones 2 and 4 [14] showed a negative band at 454 nm and a positive band (~350 nm) corresponding respectively to n* and * transitions of the E-azobenzene (figure 1b, green line, table 1), revealing a chirality transfer from the sugar ring to the azobenzene moiety.…”

“…[12] Four azobenzeneembedded glycomacrolactones with the di-ohydroxy azobenzene linked to 4,6 or 2,3-position of manno (1,3) and gluco (2,4) derivatives have been prepared through the one-pot O-alkylation mediated macrocyclization reaction between di-Obromoacetyl thio--or -D-glycopyranosides and 2,2'-dihydroxy azobenzene with 22-39% yield (Scheme 1). [13,14] Scheme 1: One-pot synthesis of photoswitchable glycomacrolactones 1-4…”

“…E-isomer (green line), PSS365 (red line), PSS435 (yellow line). (d) DFT-optimized geometries of E-and Z-(P)-4 [13,14] Irradiation at 435 nm led to the back ZE photoisomerization (figure 1a, yellow line), with E:Z ratio varied from 53:47 to 70:30 at the PSS435 (table 1). This lower efficiency is due to the comparable level of molar absorption coefficient of both E and Z isomers at 435 nm.…”

“…The EZ photoisomerization led to a large conformational change of the macrocycles ( figure 1d), while the sugar 4 C1 conformation is conserved in both E-and Z-isomers, as confirmed by 1 H NMR studies. [13,14] Furthermore, we have accidently observed the gelation ability of the 4,6-O-bridged manno macrolactone 1 in cyclohexane or in EtOH, with a reversible solutiongel transition through repetitive heating and cooling procedure (figure 2). Light irradiation at 365 nm induced an irreversible gelsolution transition in cyclohexane, leading to an opaque solution due to the reduced solubility of the more polar Z-macrocycle; while the gelsolution transition can be partially switched back to the gel state in EtOH after illumination at 435 nm.…”

“…The principle of fluorescence photoswitching is based on absorption and emission spectral features of the chromophoric units to induce efficient We have successfully synthesized azobenzeneembedded glycomacrolactones which displayed remarkable photochromic properties and bistability. Although only small numbers of photoswithchable glycomacrocycles have been reported to date, [13,14,25,26] these 'smart' glycomacrocycles have already shown very interesting properties like chirality transfer and supramolecular chirality, photocontrolled molecular shape or solubility. The azobenzene functionalized glycolipids also displayed good photoswitchable functionality.…”