Soft discotic liquid crystal interface for photocatalytic dye degradation

Abstract

In line with conventional photocatalysts such as excitonic, heterojunction, polymeric, and soft photocatalysts, Tricycloquinazoline (TCQ) based room-temperature discotic liquid crystals (DLCs) can also exhibit promising photocatalytic properties. TCQ DLCs material exhibits a hexagonal columnar (Colh) mesophase with C3 symmetry, an electron-deficient nature, and high thermal and chemical stability. In this study, TCQ DLC has beeninvestigated as a soft photocatalytic interface for the degradation of methylene blue (MB) and rhodamine B (RhB) dyes under UV irradiation and sunlight. Spectroscopic analysis reveals that TCQ DLC exhibits absorption peaks at 396 nm (far-UV) and 420 nm (near-visible), with an optical bandgap of 3.04 eV, as confirmed by UV-Visible spectroscopy. Photoluminescence (PL) spectra exhibit dual emission peaks at 565 nm (green-yellow) and 610 nm (red) when excited with 396 nm and 420 nm, indicating efficient charge transfer among multiple emissive species. The photocatalytic performance of TCQ DLC has been evaluated against MB and RhB at dye concentrations of 1 mg/L and 10 mg/L under both UV light and direct sunlight. For lower dye concentrations (1 mg/L), degradation efficiency exceeded 99 % within 60 min for MB and 40 min for RhB under UV light. The degradation kinetics followed a pseudo-first-order model (-0.12570 s 1 for MB and 0.15206 s 1 for RhB) at 1 mg/L and a pseudo-zero-order model (-0.00393 s 1 for MB and 0.02983 s 1 for RhB) at 10 mg/L. Radical trapping experiments confirmed the equal involvement of superoxide radicals (O•2 ), holes (h⁺), and hydroxylradicals (OH•) with a degradation efficiency of 56.61 %, 60.45 %, and 63.96 % using ethanol, IPA, and p-BQ, respectively, in the photocatalytic degradation process. The TCQ DLC was found to be structurally stable after photocatalytic degradation, as confirmed by POM, FTIR, and PL spectra, showing recyclability up to 5 cycles with an efficiency of ~79 %. This study highlights the potential of soft discotic interfaces in photocatalytic applications and purification technologies, opening new avenues for sustainable environmental solutions, complementary to the usual photocatalysts.