Synthesis of combined single-molecule magnets and multi-metal phthalocyanine systems for future applications in dual purpose gas sensing and data storage

Abstract

Gas sensing devices are widely integrated into modern society due to their roles in monitoring gas composition in aerospace, detecting toxic gases in industrial settings, ensuring environmental and human safety, and diagnosing diseases in biomedical applications. Among the most critical gases for monitoring are ammonia (NH_₃) and nitrogen oxides (NO_x), as they pose significant risks to human health and can be detected in human breath in measurable amounts Inhalation of NH_₃ can lead to conditions such as asthma and heart disease, while elevated NH_₃ levels in breath may indicate kidney failure or renal disease, emphasizing the need for sensitive and selective detection methods. Similarly, increased NO levels in breath signal airway inflammation, offering a means to track the progression of respiratory diseases like lung cancer and pneumonia. NO is readily oxidized to NO_₂ a toxic gas known to accelerate cognitive decline, cardiovascular disease, and diabetes. Environmental sensors capable of detecting NO_₂ are therefore essential for minimizing exposure and associated health risks.<br /> <br /> Although the field of gas sensing has ancient origins, recent research has concentrated on improving sensor sensitivity, selectivity, cost efficiency, and response time. Metal oxide sensors, some of the earliest developed, exhibit good sensitivity and selectivity but demand substantial energy inputs for precise measurements. Metal phthalocyanines (MPcs) emerge as promising alternatives due to their tunability, ease of synthesis, and low production costs. MPcs, which feature a metal atom within a phthalocyanine cavity, form square planar structures when using transition metals like Fe or Co. These structures possess open axial octahedral positions at the metal center for gas coordination. By changing metal centers and substituents on the aryl rings, MPcs semiconducting properties can be tuned towards specific gases. When incorporated into extended networks like covalent organic frameworks (COFs) or metal organic frameworks (MOFs), MPcs can achieve enhanced sensitivity and selectivity, particularly with heterometallic materials. Further advancements can be achieved by integrating molecular storage devices into MPc networks. This approach improves gas sensing response times using sensitive electromagnetic systems and sheds light on the behavior of magnetic components like single molecule magnets (SMMs) in functional materials. Studies have shown that combining SMMs with ferromagnetic materials such as Co films boosts the magnetic properties of SMMs. Incorporating SMMs into semiconducting MPc composite materials allows researchers to explore their effects on gas sensing capabilities and magnetism. Additionally, varying substituents, metal centers, and crystal phases of these composite building blocks provides valuable insights into the interplay between structural modifications and the gas sensing and magnetic properties of MPc systems.<br /> <br /> This research aimed to deepen the understanding of the interplay between metal phthalocyanine (MPc) gas sensors and single-molecule magnets (SMMs) to develop a dual purpose material for breath sensing and molecular data storage. To achieve this, three specific objectives were pursued: first, MPc and SMM building blocks were synthesized and characterized to ensure suitability for composite materials; second, composite materials were developed using covalent organic framework (COF) and metal-organic framework (MOF) approaches, incorporating magnetic molecules like SMMs; and third, the gas sensing and magnetic properties of both building blocks and composites were investigated to understand the effects of combining them.<br /> <br /> To accomplish these goals four families of building blocks were synthesized: MPcF₁₆, MPc(COOH)_x, LnPc₂, and Mn_₁₂. MPcF₁₆ is a known n-type semiconductor with selective detection of NH_₃ at concentrations as low as 40 ppb, while MPc(COOH)_x is a p-type semiconductor with preferential coordination to NO_₂. These substituents were chosen for their facile synthesis and ability to coordinate into extended networks. Mn_₁₂ was selected as it is the first and most well studied SMM, facilitating magnetic characterization, while LnPc₂ was selected due to its structural and electrical similarities to MPcs using transition metals. Notably, LnPc₂ is a known semiconductor, while Mn_₁₂ is an insulator, preventing loss of conductivity.<br /> <br /> The synthesized building blocks were characterized to confirm structure and purity using a variety of techniques, including UV/Vis spectroscopy, FTIR, Raman spectroscopy, and PXRD before their integration into extended networks. Composite materials were synthesized using COF and MOF based approaches, reacting MPcF_₁₆ and MPc(COOH)_ₓ together or with metal salts to form extended networks. Solid-state techniques such as FTIR, SEM, XRD, HRTEM, and BET analysis confirmed the structure, purity, and porosity of these materials. However, results revealed 1D chains without pores, instead of the anticipated 2D or 3D frameworks A new building block, Gd(Pc(COOH))₄)₂, was synthesized as an analog for LnPc₂ SMMs to reduce sensitivity and simplify reaction conditions. The Gd(Pc(COOH)₄)₂ product was characterized and incorporated into similar COF and MOF networks. Mn₁₂ composites were synthesized using ligand exchange, adding CoPc(COOH)_₈ to Mn_₁₂, with FTIR, SEM, and XRD confirming their structure and purity.<br /> <br /> Gas sensing experiments assessed the selectivity and sensitivity of these materials to NH_₃ and NO_₂ by depositing them on gold interdigitated electrodes (IDEs) via power vapor deposition or drop casting, and monitoring current changes when exposed to the gases. Additional experiments examined how annealing or ultrasonic vibrations of FePcF₁₆ nanowires influenced gas sensing response. Annealing FePcF_₁₆ nanowires to 300°C using a stepwise approach significantly improved response and recovery times by enhancing crystallinity, alignment, and interactions between nanowires and electrodes (published in <em>MDPI Chemosensors</em>).Vibrations slightly enhanced sensing performance by creating dislocations in the crystalline lattice, increasing the number of active sites, despite reduced crystallinity. Results from these studies provide valuable insights into optimizing gas sensing and magnetic properties of these innovative materials.<br /> <br /> Preliminary gas sensing experiments on the drop-cast MPc composite material, synthesized by combining CoPc(COOH)₈ and FePcF₁₆ revealed a reduction of the gas sensing selectivity for NH_₃ and NO_₂. While the building block materials showed good sensitivity and recovery towards their preferential gases, coordinating the p-type CoPc(COOH)₈ and n-type FePcF₁₆significantly reduced both response and recovery. This decrease in recovery was attributed to free, unreacted carboxylic acid groups interacting strongly with the target gases. Although these findings did not align with the anticipated improvements, reducing the number of carboxylic acid groups in the composite material may lead to enhanced response and recovery, as observed in other MPc COF materials.<br /> <br /> In collaboration with Dr. Rodolphe Clérac at the Cetre de Recherche Paul Pascal in Bordeaux, France, funded by the Chateaubriand Fellowship, magnetic experiments were conducted using a SQUID magnetometer to investigate the effect of substituent and crystallinity variations on the magnetism of MPcs. The experiments demonstrated that changes to substituents influenced magnetism depending on the metal center. For example, Fe, as a magnetically sensitive metal center, exhibited noticeable variations where substituent modifications or conversions from powder to nanowires reduced the spin state from S = 1 for FePc to S = ½ for FePcF_₁₆. In contrast, Co, being less magnetically sensitive, showed negligible changes in magnetism under similar conditions. Additionally, LnPc₂ (Ln = Gd, Dy, Tb) and Mn_₁₂ were magnetically characterized to confirm their purity. While the synthesized Mn_₁₂ displayed expected SMM behavior, such as slow relaxation at low temperatures and hysteresis, DyPc_₂ and TbPc_₂ did not exhibit similar traits. This deviation is attributed to bulk impurities or the presence of a non-CH_₂Cl_₂ counter-ion inhibiting SMM behavior.<br /> <br /> The research herein reports the successful synthesis of MPcF₁₆(M = Co, Fe), MPc(COOH)_x (M = Co, Fe, Gd), LnPc₂ (Ln = Gd, Dy, Tb), Mn_₁₂, and new composite materials from the coordination of MPcF₁₆ and MPc(COOH)_x. Structural and purity characterizations were performed, alongside studies of their gas sensing and magnetic properties. The findings pave the way for further exploration, including the continued synthesis of MPc-based MOF and COF materials for gas sensing and magnetism, the refinement of substituted LnPc₂ syntheses for SMM applications or gas sensing, and the enhancement of gas sensing sensitivity by coordinating SMMs and MPcs into extended networks.