Azo-Bridged Dextran: A Photoresponsive Sustainable Material with Photo-Tunable Mechanical Properties

Abstract

We report on the synthesis, characterization, and properties of dextran polymers, which are covalently bridged/cross-linked by azobenzene moieties. The reversible photoactivity of the azo moiety is retained in the polymers, and the kinetics of the E/Z isomerization depend on the dextran/azobenzene ratio. Together with the simple preparation, our approach provides convenient access to photoresponsive sustainable materials. Moreover, based on the water-soluble polymers, we have prepared photoresponsive hydrogels, which soften upon UV irradiation. Our findings are based on spectroscopy (UV/vis, IR, and NMR/DOSY), size exclusion chromatography, and rheology.

Scheme 1. E/Z Photoisomerization of the Bifunctional 4,4-Azobenzene Dicarboxylic Acid (B)

Scheme 2. Coupling Reaction of B to Dextran

Figure 1

FTIR spectra of B, D, DBD(7.4), and DBD(16.6).

Figure 2

¹H NMR (top) and ¹H DOSY (bottom) spectrum of DBD(0.3) in DMSO-d₆.

Figure 3

(a) SEC of native dextran D, a lightly cross-linked derivative DBD(0.3), a highly cross-linked derivative DBD(7.4), and of the non-cross-linked sample DM. Apart from the chromatogram for DBD(7.4), which is shifted toward smaller elution times, the obtained curves are mostly identical. (b) Plot of diffusion constants determined by DOSY NMR against the M_w obtained via SEC, showing the linear relationship between the two properties. Photographs indicate the corresponding samples.

Figure 4

Amplitude scan (a), loss tangent (b), frequency scan (c), and photographs (d) of DBD(9.9) and DBD(16.6). Error bars omitted for clarity (see Supporting Information).

Figure 5

(a) UV–vis spectra of B and M (aqueous pH = 9 borate buffer), as well as DBD(7.4) and DM (recorded in water). (b) In black: spectrum of a freshly prepared solution of B (aqueous pH = 9 borate buffer). In blue: spectrum after irradiation at 355 nm. In red: spectrum after subsequent irradiation at 450 nm.

Figure 6

Time traces at 355 nm for the E to Z conversion of DBD samples compared to the references DM, B, and M.

Scheme 3. Reactions That Occur Upon Illumination of an Azobenzene Solution: Photochemical E to Z and Z to E Conversion As Well As Thermal Z to E Conversion

Figure 7

(a) Logarithmic plot of the curves shown in Figure 6 (the regions used for the fit are marked with the corresponding lines). (b) Relative rate constants for reference compounds M, B, DM, and DBD(0.3–7.4); error bars are taken from the linear fits.

Figure 8

Cyclic irradiation of DBD(0.7) at 355 nm followed by 450 nm at pH = 7.

Figure 9

(a) UV/vis spectrum before (orange) and after (violet) illumination with a UV lamp for 30 min. (b) Photo-dependent frequency scans of the DBD(9.9) gel before (orange) and after (violet) illumination.