Design, synthesis and characterization of luminescent tetrazole ligands and their coordination compounds

Abstract

Coordination compounds exhibiting the spin crossover (SCO) phenomenon are studied not only for their intriguing theoretical background but especially for their potential practical applications in data storage devices, sensors and magnetic technologies. To enhance their applicability, recent research in the SCO field has focused on synergistically combining additional functionalities – such as electrical conductivity, host-guest chemistry or photoluminescence (PL) – with SCO behavior. The latter is of particular interest, as a synergistic interplay between SCO and PL enables monitoring the electronic state of a system, in the form of a spin-state-dependent PL modulation. Compared to conventional methods for tracking spin states, SCO-PL systems offer several advantages, including high contrast,sensitivity, spatial and temporal resolution, and even the ability to detect the spin state of single molecules. These features are especially valuable when investigating size-dependent effects in nanomaterials. Although several SCO-PL systems exhibiting such synergy have been reported, their formation remains largely serendipitous, and the underlying energy transfer mechanisms are often poorly understood. To address these challenges, novel single-molecule SCO-PLsystems with tunable SCO and PL properties must be developed and systematically investigated. This thesis focuses on the design, synthesis and characterization of PL-active ligands – based on the well-established coordinating unit 1-substituted-1H-tetrazoles (tz) – suitable for tunable Fe(II) single-molecule SCO-PL systems and their corresponding coordination compounds. Based on the favorable photophysical properties of the luminophore class 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) – which had not yet been explored in the context of SCOPL research – this class was selected as the photoluminescent core. By varying the spacer length and type between the PL and the coordinating unit, four different BODIPY-tz ligands were synthesized. Their structures and photophysical properties were analyzed in detail, and the coordination behavior of the ligand with the shortest spacer was further investigated with Ag(I) and Fe(II). A total of eleven distinct Ag(I) coordination compounds, differing in the counterion, demonstrated versatile coordination modes of the BODIPY-tz ligand while retaining its emission properties upon coordination. Due to the high steric demand of this ligand, Fe(II)coordination preferentially yielded octahedral heteroleptic complexes, and the homoleptic complex could only be synthesized under special conditions. In addition to chemical modification of the BODIPY-tz ligand itself, further modifications – such as co-ligand exchange and counterion variations – within the heteroleptic Fe(II) systems are feasible and will be explored in future studies to assess their impact on SCO and PL tunability. As a second luminophore, anthracene – commonly used in SCO-PL systems – was employed. The connection to the tz coordinating unit was achieved via a methylene (-CH2-) bridge, and the resulting Fe(II) complex exhibited a heteroleptic coordination environment. Interestingly, the anthracene-tz ligand and its Fe(II) complex undergo photodimerization under UV irradiation. The two resulting coordination polymers (1D and 2D) exhibited synergistic SCO-PL properties. As with the BODIPY-tz system, the SCO behavior of these anthracene-tz Fe(II) compounds is expected to be tunable through co-ligand exchange and counterion variation.