Gold (I) pyrazolate complex with optical and liquid crystalline properties and fabrication on thin nanoparticle film composites as catalysts for reduction of 4-nitrophenol

Abstract

This research aims to synthesize a trinuclear gold(I) pyrazolate complex having both room temperature liquid crystalline and phosphorescent properties as well as fabricate thin mesoporous silica/gold nanoparticles (AuNPs) film nanocomposites for the catalytic reduction of 4-nitrophenol (4-NP) with high reusability. In the first part, the gold complex 1 with its anisotropic arrangement as 2 was found to exhibit a phosphorescent peak centered at 699 nm (lifetime of 7.9 ± 0.8 μs) and mesogenic properties covering a wide room temperature range upon characterization using differential scanning calorimetry (DSC). Polarized optical microscope indicated that 2 displayed a fan-shaped texture for typical arrangements of discotic hexagonal columnar of liquid crystals, in good agreement with small-angle X-ray scattering (SAXS) analysis. Likewise, the DSC and SAXS analyses demonstrated that hexagonal columnar mesophase of 2 could be recovered even after the heating and cooling for two cycles. In the second part, 1 as self-assembly of 2 was used as a template to form thin mesoporous silica/gold film nanocomposites (3) via sol-gel synthesis. X-ray diffraction (XRD) analysis showed that formation of 3 resulted in hexagonal characteristics with parallel direction upon coated on glass (3/glass) and perpendicular orientation on combination of anodized aluminium oxide (AAO)-glass substrate (3/AAO-glass). In the third part, AuNPs was formed using simple thermal treatments. In contrast to calcination methods from 190 to 450 ºC (4cal), thermal hydrogen reduction at 210 ºC produced the best quality of thin mesoporous silica/AuNPs film composites (4red), as observed from the intense d100 diffraction peak at 2θ = 2.41º of XRD diffractogram and hexagonal image from transmission electron microscopy (TEM). In addition, the presence of d111 diffraction peak at 2θ = 38.20º, surface plasmon resonance peak from 500 to 580 nm, and spherical shape of TEM image as well as pink to purple colored materials indicated the formation of AuNPs in all the films. In the last part, composite films of 4 were simply dipped in catalytic systems for reduction of 4-NP to 4-aminophenol. Interestingly, the kinetic study revealed that the resulting high quality film on AAO-glass substrate after thermal hydrogen reduction at 210 ºC (4red/AAO-glass), exhibited almost 2.5 times higher catalytic activity with rate constant of 4.62 x 10-2 min-1 and high reusability up to 20 times compared to other films. Therefore, high quality composites containing small size AuNPs with protection of AAO membrane shed light on utilization of thin film catalysts as versatile and efficient catalytic platforms.