Molecular Origin of UV-Induced Irreversible Phase Changes in a Chromonic Liquid Crystal

Abstract

Aqueous solutions of disodium cromoglycate (DSCG), a representative model system for chromonic liquid crystals, exhibit temperature- and concentration-dependent phase behaviors spanning isotropic, nematic, and columnar phases, as well as their coexistence regions. Nastishin et al. (2018) reported that UV irradiation can alter the phase diagram, transforming a nematic phase into a nematic-isotropic biphasic state due to weakened molecular attractions, accompanied by a slow post-irradiation relaxation. Here, we revisit this phenomenon and elucidate the molecular origin of this phase diagram shift: the UV-induced photodegradation of DSCG into specific photodegradation products, which we identify using liquid chromatography-mass spectrometry. Through an integrated approach combining in situ X-ray scattering and polarized optical microscopy, we demonstrate that these degradation products disrupt the self-assembly of DSCG aggregates, thereby expanding the isotropic and biphasic regions in the phase diagram. These findings demonstrate that chromonic assemblies and their phase behaviors are highly sensitive to minor chemical alterations, providing a potential route toward light-controlled self assembly of soft matter.

Figures (4)

• Figure 1: UV-induced changes in phase behavior. (a) Photographs of UV-irradiated DSCG solution contained in tubes according to the exposure time. The tubes are placed between two crossed polarizers. The color turns yellowish, and after 2 hours, an interface shown as a dashed line appears between the nematic phase (N) at the bottom and the isotropic phase at the top. The isotropic region expands with more UV exposure. (b) Phase-transition temperatures according to UV irradiation time. The biphase(B) regime (I+N) widens and shifts downward. Black circles represent the phase transition temperature $T_{\text{NB}}$ and red squares represent $T_{\text{BI}}$. (c) The phase diagram of the neat DSCG solution. As the concentration decreases, the biphase region shifts downward but narrows. The horizontal axis is reversed, so that concentration increases from right to left, for direct comparison with (b).
• Figure 2: (a) LC-MS chromatograms of DSCG solution. The chromatogram at the top is from the neat DSCG, and the bottom one from the DSCG after 3-hr UV irradiation. Peak no. 1 corresponds to the DSCG; peaks nos. 2 and 3 appear after UV exposure. (b) Molecular structure of DSCG. (c and d) Molecular structure of UV irradiation-induced photodegradation products corresponding to peaks no. 2 and 3 in (a). Two carboxyl groups of (a) are missing in (c), and one is missing in (d).
• Figure 3: Microstructural changes upon UV irradiation. (a) Changes in intermolecular distance $w$ and order parameter $<P_2>$ according to UV irradiation time. Red circles indicate $w$ when UV light is on, and blue triangles indicate $w$ after turning off the UV light. Hollow squares represent orientational order parameter $<P_2>$ calculated from the same WAXD data. Changes in (b) the intermolecular correlation length $\xi_{\text{L}}$, (c) interaggregate distance $d$, and (d) the interaggregate correlation length $\xi_\perp$ according to UV irradiation time. Black squares show reference data obtained from a UV-free specimen measured under X-ray exposure to assess beam-induced changes.
• Figure 4: Optical changes induced by global and local UV irradiation. The sample on the top row is exposed to UV light for 30 min, over the entire sample area (global irradiation), while the bottom one through an aperture (local irradiation). (a–c) and (h–k) show the evolution of birefringence and the growth of isotropic tactoids during UV exposure. The dark regions in (i) and (j) correspond to an aluminum mask with a small opening; oblique UV illumination can affect regions beneath the mask. (d–g) and (k–n) show the post-irradiation evolution. All images are acquired with crossed polarizers of which pass axes are shown with white double arrows in (a). The white scale bar indicates 50 µm.