Thiazolo[5, 4-d]thiazole based dye modified microspheres as metal nanoparticle reactor template and hybrid catalyst

Highlights

• PVBC microparticles for benzylation of Py₂TTz molecule to prepare MNP reactor template.

• PyTT usage as reducing agent for Ag⁺, Au³⁺, Pt²⁺ metal cations.

• AgNP decorated Py₂TTz-PVBC microparticles as hybrid catalyst for p-nitrophenol reduction.

Abstract

2,5-di(pyridin-4-yl)thiazolo[5,4-d]thiazole (Py₂TTz) molecule was synthesized and used to modify poly(4-vinyl benzylchloride) microparticles (PVBC-MPs) thanks to its superior interaction with 4-vinyl benzyl chloride monomer containing active –Cl atom. PVBC spherical microparticles were successfully synthesized by suspension polymerization method and they were modified via two different approaches as in-situ and post-modification. Monodispersity of particles via in-situ modification is a challenging approach due to the low solubility of Py₂TTz. Therefore, post-modification of PVBC microparticles with Py₂TTz dye was carried out. The particles act as metal nanoreactor in the absence of any reducing agent both at room temperature and 50 °C. The particle morphology was determined with electron microscopies before and after reduction process. Metal nanoparticles (MNP) on MPs were characterized via XRD analysis. Gold, silver and platinum nanoparticles (AuNP, AgNP and PtNP) decorated microparticles were successfully synthesized. According to the ICP-MS results, 25.6% Ag, 24.7% Pt and 0.33% Au were found in the microparticles treated with metal cations. These MNP decorated MPs were used as hybrid heterogeneous catalyst for 4-nitrophenol reduction into 4-aminophenol. The highest catalytic efficiency was obtained with AgNPs decorated MPs as also consistent with the ICP-MS data. The catalytic activities of the MNP decorated MPs were determined with time dependent absorption measurents via UV–Vis spectrophotometer.

Introduction

Thiazolo[5,4-d]thiazole (TTz) based fluorophores, which consist of two adjacent thiazole rings, have been among the subjects of interest in the field of organic electronics due to their planar and rigid structure, optic and electronic properties. Interest in TTz compounds has increased dramatically in the last decade [1], [2], with the reporting of effective studies on their use in optoelectronic devices such as organic light emitting diodes (OLEDs) [3] and organic field effect transistors (OFETs). TTz is an electron-deficient molecule with strong intermolecular π-π stacking and cofacial geometry and TTz derivative molecule has a much higher ground state oxidation potential than the nitrogen-free thieno-thiophene counterpart [4], [5]. So, TTz derivatives have been extensively studied in conjugated polymers for photocatalytic applications, two-photon absorption, nonlinear optics and photovoltaics [6], [7]. TTz based fluorophores consisting of polymeric and small organic molecules have also lots of superior properties such as high electron mobility and better stability, optical properties and high quantum efficiency provided by high oxidation potentials. In this way, it is preferred as a photo-active material in optoelectronic applications [5], [8], [9], [10], [11], [12], [13], [14], sensor studies [15], organic dye removal [16] and electrochromic applications [17]. Solvatochromic effect with large Stokes shift [18], and various optical properties in tautomeric form by excited state intramolecular proton transfer (ESIPT) has been also reported with thiazolo[5,4-d]thiazole based fluorophores [8]. In the most recent years, structures based on TTz and coordination polymers have attracted great attention with their different applications. By preparing a photocatalyst / biocatalyst system based on microporous polymer consisting of two molecules with a TTz rigid skeleton, selective and sustainable photocatalytic reduction of CO₂ to methanol and NADH regeneration efficiency with 82% high efficiency within five minutes was achieved [19]. Similarly, effective catalytic activity for CO₂ selective adsorption and Knoevenagel condensation reactions in solvent-free environment was provided by TTz-based molecules [20]. The 2D covalent organic skeleton materials consisting TTz units have been used as electrocatalyst [21] photo-electrochemical and photo-chemical catalyst for hydrogen production [22], [23], [24]. In addition to all these applications, there are very few reported studies on the dye removal, biological applications, optical and sensor applications of these molecules [17], [18], [25], [26], [27].

Vinyl benzyl chloride (VBC) is a chloromethylated type of styrene. Thanks to the benzene ring and benzyl chlorine in its structure, it enables numerous electrophilic and nucleophilic substitution reactions for poly(vinylbenzyl chloride) (PVBC). Since it has a vinyl group and chlorine atom, it is open to many modifications and can be easily polymerized [28]. Chloromethyl groups (-CH₂Cl) are highly active in VBC and can react with –NH₂ groups so that quaternary ammonium groups can enter the copolymer matrix [29]. Both homopolymers and copolymers of PVBC can be synthesized with the radical polymerization method [30]. PVBC has many usage areas, such as using it as an anion exchange membranes [31], [32], [33], composite membrane for fuel cells [34], redox flow batteries [35], boron-selective adsorbent materials [36], preparation of reactive adhesives [37] and water electrolysers [38].

Many quaternized structure containing polymers can be used in different applications [39], [40], [41], [42], [43]. PVBC can be used for quaternization reactions and preparation of ionic polymers [31], [33], [38], [44]. Poly(vinyl benzyl trimethylammonium chloride) as a water soluble polymer having highly positive charge can be prepared by quarternization of PVBC.

Benzyl chloride has been used for bonding to pyridine rings of Py₂TTz to increase both solubility and photoluminescent quantum yield. This molecule exhibits electrochromism when it gets electron in a electrochemically route [45]. Eventhough this electron accepting benzylated Py₂TTz is a highly promising molecule for different applications, preparation of polymer consisting a lot of benzylated Py₂TTz structures in the chains may result with superior properties. Electron accepting nature of benzylated Py₂TTz resulting with radical formation [45] can be used for metal nanoparticle (MNP) formation.

Hybrid heterogeneous catalysts are promising structures due to their reusability and easily separation from the reaction medium. Polymeric microparticles can be decorated with metallic nanoparticles [46], [47], [48] and can be used as a catalyst system [49], [50], [51], [52]. Au and Ag@Au nanocomposite hybrid structures were obtained using poly(N-isopropylacrylamide)/polyethyleneimine microgels, and this system exhibited controllable catalytic activity in the reduction reaction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) [53].

Herein, Py₂TTz molecule usage in the polymeric MPs via benzylation of Py₂TTz is reported for the first time and used to prepare MNPs formation on the surface of MPs without reducing agent. PVBC is highly suitable polymer for modification and prepared successfully by suspension polymerization of 4-vinylbenzyl chloride monomer. These microparticles were functionalized by both in-situ and post-modification methods with Py₂TTz molecule. By using these Py₂TTz-modified PVBC microparticles synthesized via benzylation of Py₂TTz, MNP decorated MPs were obtained from metal salt solutions without any reducing agent. The benzylated MPs are assumed as forming radical electrons which is reducing the metal cations into nanoparticles. Py₂TTz molecule containing -N and -S atoms can act as anchor point for metal nanoparticle formation on the surface of MPs. The usage of these hybrid structures as heterogeneous catalyst which is effectively reduce p-NP to p-AP is reported. AgNPs decorated MPs are determined as quite effective hybrid catalyst system.