Optical Thermometry with Organoeuropium(II) Complexes

Abstract

The measurement of temperature is of critical importance to a diverse array of fields and industries, such as heating, ventilation, and cooling, nutrition, molecular biology, medicine, engineering of all denominations, chemistry, climate science, and computing technologies, to name a few. As technologies in all these areas progress, measuring temperature more accurately and precisely under increasingly challenging conditions is an ever-evolving problem. One strategy to address these challenges is optical thermometry, which correlates changes in the optical characteristics (e.g. luminescence intensity, emission lifetime, colour, etc.) of a thermosensitive system to changes in temperature. In contrast to common thermometric methods employing thermistors, thermocouples, or even infrared thermography, optical thermometry is a remote-detection, minimally invasive technique with very high attainable spatial resolution, making it well-suited to probing temperature in difficult environments and sensitive systems. Due to their favourable (magneto-)optical properties, lanthanide-based materials are well-studied as optical thermometers. However, most of these materials operate via a ratiometric approach monitoring thermosensitive emission intensities on account of comprising trivalent lanthanides, which exhibit characteristic narrow, Laporte-forbidden 4f-4f absorbance and emission bands whose positions are almost invariant with temperature. This thesis explores a less-studied approach to lanthanide-based optical thermometry, exploring divalent europium and its Laporte-allowed, environment-sensitive, intense and broad 5d-4f transitions for thermal sensing, with a focus on emission band-shift. Moreover, organometallic complexes were targeted for these studies, as these are also not well-known for optical thermometry and the unique structure and bonding of such systems was envisioned to lead to unique properties. Starting from the brightly emissive, synthetically convenient starting material [EuII(BH4)2(THF)2] (which was revealed to itself be a well-performing luminescence thermometer), the hemimetallocene dimer [Cp*Eu(μ-BH4)(THF)2]2 (Cp* = pentamethylcyclopentadienyl) was prepared and characterized. This complex exhibits strongly thermochromic luminescence, with the maximum of the 5d4f emission band red-shifting by nearly 1900 cm-1 as the temperature is increased from 60-320 K. This defines a thermometric system that demonstrates the highest absolute sensitivity of any lanthanide-based band shift thermometer to date, reaching 8.2 cm-1 K-1 at 320 K. An opto-structural correlation study via variable-temperature single-crystal X-ray diffraction revealed that small changes in metrical parameters with temperature were correlated to these rather large changes in emission wavelength/energy, unveiling the potential of organoeuropium(II) systems as sensitive luminescence thermometers. Building upon this discovery, a series of isostructural hemimetallocene dimers [CpREu(μ-BH4)(solv)n]m (solv = tetrahydrofuran, 1,2-dimethoxyethane; n = 1 or 2; m = 2 or ∞) bearing systematically varying ligand sets was synthesized and examined for their thermometric performance, with an eye toward elucidating some of the factors governing the thermochromic luminescence that is clearly intrinsic to this type of system. Members of this series show widely varying emission properties despite their compositional similarities, demonstrating the tunability of the europium(II) hemimetallocene architecture. Optostructural correlation studies via variable-temperature single-crystal X-ray diffraction revealed a convoluted dependence of the emission properties on various factors, which are difficult to disambiguate. However, a few factors of particular importance are identified, such as the distance between the europium(II) ion and the basal plane of the square-pyramidal coordination polyhedron, the presence of pendant electron density that might further interact with the excited-state 5d orbitals, and metal-ligand flexibility. These findings represent a step toward enabling the rational design of organoeuropium(II) luminescence thermometers for bespoke characteristics. Finally, one of the substituted cyclopentadienides from the previous study, 1-(bis(dimethylamino)phosphino)-2,3,4,5-tetramethylcyclopentadienyl, was selected for further investigation due to the good thermometric performance exhibited by its hemieuropocene(II), and the possibility of further functionalizing the free phosphine moiety. Oxidizing this moiety with elemental sulfur, selenium, or tellurium yielded a corresponding series of novel phosphinochalcogenoylcyclopentadienides, which were coordinated to europium(II) to give a family of homoleptic europocenes(II) bound by both the cyclopentadienyl fragment and the pendant phosphinochalcogenoyl arms of the ligands. These complexes were characterized structurally and optically and, though they are not suited to band-shift luminescence thermometry, turn out to be extremely sensitive magneto-optical thermometers via magnetic circular dichroism, with relative sensitivity values reaching 170 % K-1 at ultralow temperatures inaccessible to conventional luminescence thermometry. This result corroborates our previous work introducing the concept of magnetic circular dichroism thermometry, and further strengthens the position of europium(II) as a valuable foundation for thermosensitive optical materials.