Molecular Thermometer Research Articles

Rethinking Assumptions: Assessing the Impact of Strong Magnetic Fields on Luminescence Thermometry.

Luminescence (nano)thermometry has exploded in popularity, offering a remote detection way to measure temperature across diverse fields like nanomedicine, microelectronics, catalysis, and plasmonics. A key advantage is its supposed immunity to strong electromagnetic fields, a crucial feature in many environments. However, this assumption lacks comprehensive experimental verification as most of the proposed luminescent thermometers rely on magnetic ions, such as lanthanides. Here, we address this gap by critically examining the thermometric response of the luminescent molecular thermometer [Tb0.93Eu0.07(bpy)2(NO3)3] (bpy = 2,2'-bipyridine) under high magnetic fields (up to 58 T). Our findings reveal that the conventional intensity-based method for Tb/Eu luminescent thermometers fails even under weak magnetic fields. However, careful data analysis identified specific transitions with minimal magnetic correlation, enabling the thermometer to operate across the entire temperature range up to 20 T, and with larger fields for temperatures exceeding 120 K. This study highlights the strong dependence of thermometric performance on material properties, urging caution, but also offers a path forward for developing robust luminescent thermometers in such environments.

Tetra dansylamides substituted cyclen and cyclam macrocycles as fluorescent sensing probes for metal ions and temperature-responsive materials in dopped polymers

Two novel tetra-dansyl derivatives incorporating cyclen (1,4,7,10-tetraazacyclododecane) and cyclam (1,4,8,11-tetraazacyclotetradecane) macrocycles have been synthesized, thoroughly characterized, and their photophysical properties examined, both in solution and in the solid state. These compounds exhibit fluorescence emission with quantum yields up to 40 %, varying significantly with different solvents. They also display positive solvatofluorochromic behavior, with emissions ranging from green to yellow colours. Kamlet-Taft studies were conducted to better understand solute-solvent interactions. Furthermore, aggregation-induced emission was observed in solutions with high water content, confirmed via dynamic light scattering. Given the intrinsic properties of these compounds, their potential for environmental remediation was explored through metal ion sensing studies. Compounds L1 and L2 demonstrated high sensitivity to Cu2+ and Hg2+ ions, significantly modulating their emission, with L2 capable of detecting and quantifying Hg2+ concentrations as low as 2–3 μM. Additionally, the solid-state emission of these compounds encouraged an investigation into their potential as temperature sensors. Several doped polymer thin films were fabricated, establishing a linear relationship with temperature beyond their melting point. These findings suggest that these tetra-chromophoric compounds hold promise as molecular thermometers.

Constructing [2.2]Paracyclophane‐Based Ultrasensitive Optical Fluorescent‐Phosphorescent Thermometer with Cucurbit[8]uril Supramolecular Assembly

AbstractThe pursuit of developing novel approaches to fully organic and efficient phosphorescent materials is in high demand. The optical activity of such functional, organic phosphorescent/fluorescent materials may exhibit great temperature dependence, allowing their application as advanced, highly sensitive molecular thermometers. In this study, a rational strategy involving host–guest complexation and polymerization of [2.2]paracyclophane (PCP) based molecules with cucurbit[8]uril (CB8) to suppress the molecular motion and promote temperature‐dependent phosphorescence is presented. The rigid cavity of CB8 provides an ideal microenvironment to host PCP molecules 1 and 2, significantly enhancing the photophysical performance after complexation. Co‐polymerizing phosphors 1 and 2 with acrylamide is an efficient method for improving phosphorescence. Incorporating CB8 into the resulting P‐1 and P‐2 polymers enhances phosphorescence performance. Importantly, the obtained materials exhibit a big structure‐dependent spectral shift and change of phosphorescence lifetimes with temperature, allowing novel, phosphorescence‐based, and purely organic optical thermometers to be developed. The practical applications of PCP‐based luminescent materials in temperature sensing via a multi‐parameter approach are showcased, i.e., using fluorescence spectral shift and changes in bandwidth, as well as phosphorescence lifetimes, exhibiting thermal sensitivity of ≈17.7 cm−1 °C−1, 47.8 cm−1 °C, and 5.2% °C−1, respectively.

A terpyridyl-imidazole based europium tris-(β-diketonate) complex as an efficient molecular luminescent thermometer and single component white light emitter via synergy in energy transfer between ligands and Eu3.

The thermosensing and thermochromic behavior of one of our recently reported terpyridyl-imidazole based ternary europium tris-(β-diketonate) complexes of the composition [Eu(tta)3(tpy-HImzphen)] (tta = 2-thenoyltrifluoroacetone and tpy-HImzphen = 2-(4-[2,2':6',2''] terpyridin-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole) has been thoroughly investigated in this work. The said Eu(III) complex exhibits magnificent thermosensing as well as thermochromic properties and can be recommended as an excellent temperature sensor in a wide temperature domain of 273-343 K in terms of both emission intensity ratio (Sm = 5.78% K-1 at Tm = 343 K, δT = 0.012 K) and lifetime values (Sm = 3.36% K-1 at Tm = 333 K, δT = 0.009 K) or even in terms of its emitting color (red at 268 K, violet at 303 K, and blue at 343 K). Additionally, it displays remarkable solvent-induced luminescence behavior by displaying various emitting colors instead of its sole characteristic red emission upon varying the nature of the solvent. Finally, amalgamating these two features, we are able to attain white light emission (Commission Internationale de l'Eclairage coordinates: x = 0.34, y = 0.38) at 283 K from a single component. A plausible energy transfer mechanism has also been proposed in light of the existence of the ligand-to-metal charge transfer (LMCT) state as the quencher.

A new optical nickel(II) cyclen-naphthalene probe for molecular thermometry using fluorescence and NMR techniques

A new luminescent molecular thermometer (LMT) was prepared, characterized, and investigated. The sensor works based on the temperature effect on the spin-crossover equilibrium between two spin states of a nickel-cyclen derivative (cyclen = 1,4,7,10-tetraazacyclododecane) containing a naphthalene fragment as the emitting unit. DFT and TD– DFT calculations revealed that photoelectron transfer (PET) can partially quenches the fluorescence of the naphthalene fragment to varying degrees in both high- and low-spin states of the sensor. UV–visible spectra in coordinating solvents such as acetonitrile revealed that 80% of the predominant species adopt five-coordination in the triplet state ([Ni(cycna)(CH3CN)]2+) at 20 °C, while 20 % of the molecules are square planar in the singlet spin state. The fluorescence spectra showed excellent correlation of luminescence intensity with temperature (R2 = 1.00) from 10 to 70 °C, proving that the coordination compound can function as LMT.

Phosphinine vs Pyridine in Luminescent CuI Complexes and Application in Lifetime‐Based Molecular Thermometer

AbstractLuminescent CuI complexes have attracted significant interest due to their adjustable emission properties, diverse structures, and reasonable cost. In this work, two class of complexes, namely phosphinine and pyridine ligated CuI complexes that differ by only one atom, were synthesized and characterized. Photophysical analysis and theoretical studies reveal an emissive phosphinine‐localized triplet states for CuI phosphinine complexes, and a temperature‐dependent interplay between metal‐to‐ligand charge‐transfer (MLCT) singlet and triplet states for CuI pyridine complexes. In general, the CuI phosphinine complexes exhibit a longer lifetime and greater temperature‐dependent lifetime changes than the pyridine complexes. A molecular thermometer incorporating CuI phosphinine complexes 2 c as indicator was fabricated. This thermometer exhibits a rare linear correlation between temperature and lifetime ranging from 77–297 K with a high sensitivity of −13.5 μs K−1.

Development of fluorochromic polymer doped materials as platforms for temperature sensing using three dansyl derivatives bearing a sulfur bridge

Three novel bis-dansyl derivatives bearing a sulfur bridge have been synthesized, fully characterized, and their photophysical characterization studied in solution, as well as, in the solid state. All compounds exhibit fluorescence emission with quantum yields up to 60%, which vary significantly depending on the solvent used, and the inherent molecular structure. Moreover, these compounds demonstrate positive solvatofluorochromic behaviour emitting from bluish-green to yellow. Kamlet-Taft studies were performed to better understand the solute–solvent interactions. Due to the intrinsic characteristics of the compounds, efforts were made to understand their potential usefulness for environmental remediation and thus metal ion sensing studies were investigated. Compounds L1 and L2 showed high sensitivity to Cu2+ and Hg2+ ions and were found to modulate their emission extensively, with L2 capable of detecting and quantifying up to 4 µM of Hg2+. Considering the solid-state emission of these compounds, the application towards temperature sensing was put forth. L3 was found to quench its emission in a linear relation with temperature up to 170 °C. Several doped polymer thin films were fabricated, which served as a platform to establish a linear relation with temperature beyond their melting point. Polymethylmetacrylate (PMMA) films emitted up to temperatures of 218 °C, which could be fully restored at room temperature. These results suggest the potential application of these bis-chromophoric compounds as molecular thermometers.

Luminescent Single-Molecule Magnets as Dual Magneto-Optical Molecular Thermometers.

Luminescent thermometry allows the remote detection of the temperature and holds great potential in future technological applications in which conventional systems could not operate. Complementary approaches to measuring the temperature aiming to enhance the thermal sensitivity would however represent a decisive step forward. For the first time, we demonstrate the proof-of-concept that luminescence thermometry could be associated with a complementary temperature readout related to a different property. Namely, we propose to take advantage of the temperature dependence of both magnetic (canonical susceptibility and relaxation time) and luminescence features (emission intensity) found in Single-Molecule Magnets (SMM) to develop original dual magneto-optical molecular thermometers to conciliate high-performance SMM and Boltzmann-type luminescence thermometry. We highlight this integrative approach to concurrent luminescent and magnetic thermometry using an air-stable benchmark SMM [Dy(bbpen)Cl] (H2 bbpen=N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethyl-enediamine)) exhibiting Dy3+ luminescence. The synergy between multiparametric magneto-optical readouts and multiple linear regression makes possible a 10-fold improvement in the relative thermal sensitivity of the thermometer over the whole temperature range, compared with the values obtained with the single optical or magnetic devices.

Luminescent Single‐Molecule Magnets as Dual Magneto‐Optical Molecular Thermometers

AbstractLuminescent thermometry allows the remote detection of the temperature and holds great potential in future technological applications in which conventional systems could not operate. Complementary approaches to measuring the temperature aiming to enhance the thermal sensitivity would however represent a decisive step forward. For the first time, we demonstrate the proof‐of‐concept that luminescence thermometry could be associated with a complementary temperature readout related to a different property. Namely, we propose to take advantage of the temperature dependence of both magnetic (canonical susceptibility and relaxation time) and luminescence features (emission intensity) found in Single‐Molecule Magnets (SMM) to develop original dual magneto‐optical molecular thermometers to conciliate high‐performance SMM and Boltzmann‐type luminescence thermometry. We highlight this integrative approach to concurrent luminescent and magnetic thermometry using an air‐stable benchmark SMM [Dy(bbpen)Cl] (H2bbpen=N,N′‐bis(2‐hydroxybenzyl)‐N,N′‐bis(2‐methylpyridyl)ethyl‐enediamine)) exhibiting Dy3+ luminescence. The synergy between multiparametric magneto‐optical readouts and multiple linear regression makes possible a 10‐fold improvement in the relative thermal sensitivity of the thermometer over the whole temperature range, compared with the values obtained with the single optical or magnetic devices.

Ribosomal proteins can hold a more accurate record of bacterial thermal adaptation compared to rRNA.

Ribosomal genes are widely used as 'molecular clocks' to infer evolutionary relationships between species. However, their utility as 'molecular thermometers' for estimating optimal growth temperature of microorganisms remains uncertain. Previously, some estimations were made using the nucleotide composition of ribosomal RNA (rRNA), but the universal application of this approach was hindered by numerous outliers. In this study, we aimed to address this problem by identifying additional indicators of thermal adaptation within the sequences of ribosomal proteins. By comparing sequences from 2021 bacteria with known optimal growth temperature, we identified novel indicators among the metal-binding residues of ribosomal proteins. We found that these residues serve as conserved adaptive features for bacteria thriving above 40°C, but not at lower temperatures. Furthermore, the presence of these metal-binding residues exhibited a stronger correlation with the optimal growth temperature of bacteria compared to the commonly used correlation with the 16S rRNA GC content. And an even more accurate correlation was observed between the optimal growth temperature and the YVIWREL amino acid content within ribosomal proteins. Overall, our work suggests that ribosomal proteins contain a more accurate record of bacterial thermal adaptation compared to rRNA. This finding may simplify the analysis of unculturable and extinct species.

Bright and Photostable TADF‐Emitting Zirconium(IV) Pyridinedipyrrolide Complexes: Efficient Dyes for Decay Time‐Based Temperature Sensing and Imaging

AbstractLuminescence thermometry represents a technique of choice for measurements in small objects and imaging of temperature distribution. However, most state‐of‐the‐art luminescent probes are limited in spectral characteristics, brightness, photostability, and sensitivity. Molecular thermometers of the new generation utilizing air and moisture‐stable zirconium(IV) pyridinedipyrrolide complexes can address all these limitations. The dyes emit pure thermally activated delayed fluorescence without any prompt fluorescence and show a unique combination of attractive features: a) visible light excitation and emission in the orange/red region, b) high luminescence brightness (quantum yields ≈0.5 in toluene and 0.8–1.0 in polystyrene matrix), c) excellent photostability, d) suitability for two‐photon excitation and e) mono‐exponential decay on the order of tens to hundreds of microseconds with strongly temperature‐dependent lifetimes (between −2.5 and −2.9% K−1 in polystyrene at 25 °C). Immobilization in gas‐blocking polymers yields sensing materials for self‐referenced decay time read‐out that are manufactured in two common formats: planar optodes and water‐dispersible nanoparticles. Positively charged nanoparticles are demonstrated to be suitable for nanothermometry in live cells and multicellular spheroids. Negatively charged nanoparticles represent advanced analytical tools for imaging temperature gradients in samples of small volumes such as microfluidic devices.

Asymmetry-enhanced 59Co NMR thermometry in Co(iii) complexes.

Design strategies for molecular thermometers by magnetic resonance are essential for enabling new noninvasive means of temperature mapping for disease diagnoses and treatments. Herein we demonstrate a new design strategy for thermometry based on chemical control of the vibrational partition function. To do so, we performed variable-temperature 59Co NMR investigations of four air-stable Co(iii) complexes: Co(accp)3 (1), Co(bzac)3 (2), Co(tBu2-acac)3 (3), and Co(acac)3 (4) (accp = 2-acetylcyclopentanonate; bzac = benzoylacetonate; tBu2-acac = 2,2,6,6-tetramethyl-3,5-heptanedionate and acac = acetylacetonate). We discovered 59Co chemical shift temperature sensitivity (Δδ/ΔT) values of 3.50(2), 3.39(3), 1.63(3), and 2.83(1) ppm °C-1 for 1-4, respectively, at 100 mM concentration. The values observed for 1 and 2 are new records for sensitivity for low-spin Co(iii) complexes. We propose that the observed heightened sensitivities for 1 and 2 are intimately tied to the asymmetry of the accp and bzac ligands versus the acac and tBu2-acac ligands, which enables a larger number of low-energy Raman-active vibrational modes to contribute to the observed Δδ/ΔT values.

A palette of site-specific organelle fluorescent thermometers.

Intracellular micro-temperature is closely related to cellular processes. Such local temperature inside cells can be measured by fluorescent thermometers, which are a series of fluorescent materials that convert the temperature information to detectable fluorescence signals. To investigate the intracellular temperature fluctuation in various organelles, it is essential to develop site-specific organelle thermometers. In this study, we develop a new series of fluorescent thermometers, Thermo Greens (TGs), to visualize the temperature change in almost all typical organelles. Through fluorescence lifetime-based cell imaging, it was proven that TGs allow the organelle-specific monitoring of temperature gradients created by external heating. The fluorescence lifetime-based thermometry shows that each organelle experiences a distinct temperature increment which depends on the distance away from the heat source. TGs are further demonstrated in the quantitative imaging of heat production at different organelles such as mitochondria and endoplasmic reticulum in brown adipocytes. To date, TGs are the first palette batch of small molecular fluorescent thermometers that can cover almost all typical organelles. These findings can inspire the development of new fluorescent thermometers and enhance the understanding of thermal biology in the future.

One Luminescent Cadmium Iodide with Free Bifunctional Azole Sites as a Triple Sensor for Cu2+, Fe3+, and Cr2O72- Ions.

The exploration of an excellent triple sensor for monitoring Cu2+, Fe3+, and Cr2O72- ions is of exceeding significance because of their serious effects on the human body. Herein, optically active 1H-3,5-bis(pyrazinyl)-1,2,4-triazole (Hbpt) with triazolyl and pyrazinyl groups was applied for the construction of a new type of organic hybrid cadmium iodide [Cd6I8(bpt)4(H2O)4]·2H2O (1) incorporating a hitherto-unknown [Cd3I4(H2O)2]2+ trimeric-cationic unit, which shows an orange light emission at 589 nm with a large Stokes shift of 246 nm. In virtue of the existence of free bifunctional azole sites as the receptors, 1 exhibits a highly selective and sensitive sensing property toward Cu2+, Fe3+, and Cr2O72- ions in aqueous solution with lower detection limits of 0.70∼4.46 ppm, which offers the sole example of cadmium iodide as an excellent triple sensor for detecting Cu2+, Fe3+, and Cr2O72- ions. Moreover, temperature-dependent luminescent determinations also reveal that 1 can be used as the potential luminescent molecular thermometer.

Record Chemical-Shift Temperature Sensitivity in a Series of Trinuclear Cobalt Complexes

Designing spins that exhibit long-lived coherence and strong temperature sensitivity is central to designing effective molecular thermometers and a fundamental challenge in the chemistry/quantum-information space. Herein, we provide a new pathway to both properties in the same molecule by designing a nuclear spin, which possesses a robust spin coherence, to mimic the strong temperature sensitivity of an electronic spin. This design strategy is demonstrated in the group of trinuclear Co(III) spin-crossover compounds [(CpCo(OP(OR)2)3)2Co](SbCl6) where Cp = cyclopentadienyl and R = Me (1), Et (2), i-Pr (3), and t-Bu (4). Nuclear magnetic resonance analyses of the 59Co nuclear spins reveal 59Co chemical-shift temperature sensitivity (Δδ/ΔT) values that span from 101(1) ppm/°C in 1 to 149(1) ppm/°C in 2 and 150(2) ppm/°C in 4, where the latter two are record temperature sensitivities for any nuclear spin. Additionally, complexes 2 and 4 have T2* values of 74 and 78 μs in solution at ambient temperatures surpassing those from electron-spin-based complexes, which typically display long coherence times only at extremely low temperatures. Our results suggest that spin-crossover phenomena can enable electron-spin-like temperature sensitivities in nuclear spins while retaining robust coherence times at room temperature.

Fluorescent Polymers Conspectus.

The development of luminescent materials is critical to humankind. The Nobel Prizes awarded in 2008 and 2010 for research on the development of green fluorescent proteins and super-resolved fluorescence imaging are proof of this (2014). Fluorescent probes, smart polymer machines, fluorescent chemosensors, fluorescence molecular thermometers, fluorescent imaging, drug delivery carriers, and other applications make fluorescent polymers (FPs) exciting materials. Two major branches can be distinguished in the field: (1) macromolecules with fluorophores in their structure and (2) aggregation-induced emission (AIE) FPs. In the first, the polymer (which may be conjugated) contains a fluorophore, conferring photoluminescent properties to the final material, offering tunable structures, robust mechanical properties, and low detection limits in sensing applications when compared to small-molecule or inorganic luminescent materials. In the latter, AIE FPs use a novel mode of fluorescence dependent on the aggregation state. AIE FP intra- and intermolecular interactions confer synergistic effects, improving their properties and performance over small molecules aggregation-induced, emission-based fluorescent materials (AIEgens). Despite their outstanding advantages (over classic polymers) of high emission efficiency, signal amplification, good processability, and multiple functionalization, AIE polymers have received less attention. This review examines some of the most significant advances in the broad field of FPs over the last six years, concluding with a general outlook and discussion of future challenges to promote advancements in these promising materials that can serve as a springboard for future innovation in the field.

Zinc Donor–Acceptor Schiff Base Complexes as Thermally Activated Delayed Fluorescence Emitters

Four new zinc(II) Schiff base complexes with carbazole electron donor units and either a 2,3-pyrazinedicarbonitrile or a phthalonitrile acceptor unit were synthesized. The donor units are equipped with two bulky 2-ethylhexyl alkyl chains to increase the solubility of the complexes in organic solvents. Furthermore, the effect of an additional phenyl linker between donor and acceptor unit on the photophysical properties was investigated. Apart from prompt fluorescence, the Schiff base complexes show thermally activated delayed fluorescence (TADF) with quantum yields up to 47%. The dyes bearing a phthalonitrile acceptor emit in the green–yellow part of the electromagnetic spectrum and those with the stronger 2,3-pyrazinedicarbonitrile acceptor—in the orange–red part of the spectrum. The emission quantum yields decrease upon substitution of phthalonitrile with 2,3-pyrazinedicarbonitrile and upon introduction of the phenyl spacer. The TADF decay times vary between 130 µs and 3.5 ms at ambient temperature. The weaker phthalonitrile acceptor and the additional phenyl linker favor longer TADF decay times. All the complexes show highly temperature-dependent TADF decay time (temperature coefficients above −3%/K at ambient conditions) which makes them potentially suitable for application as molecular thermometers. Immobilized into cell-penetrating RL-100 nanoparticles, the best representative shows temperature coefficients of −5.4%/K at 25 °C that makes the material interesting for further application in intracellular imaging.

Toward Opto-Structural Correlation to Investigate Luminescence Thermometry in an Organometallic Eu(II) Complex

Lanthanide-based luminescent materials have unique properties and are well-studied for many potential applications. In particular, the characteristic 5d → 4f emission of divalent lanthanide ions such as EuII allows for tunability of the emissive properties via modulation of the coordination environment. We report the synthesis and photoluminescence investigation of pentamethylcyclopentadienyleuropium(II) tetrahydroborate bis(tetrahydrofuran) dimer (1), the first example of an organometallic, discrete molecular EuII band-shift luminescence thermometer. Complex 1 exhibits an absolute sensitivity of 8.2 cm-1 K-1 at 320 K, the highest thus far observed for a lanthanide-based band-shift thermometer. Opto-structural correlation via variable-temperature single-crystal X-ray diffraction and fluorescence spectroscopy allows rationalization of the remarkable thermometric luminescence of complex 1 and reveals the significant potential of molecular EuII compounds in luminescence thermometry.

Employing three-blade propeller lanthanide complexes as molecular luminescent thermometers: study of temperature sensing through a concerted experimental/theory approach

The synthesis, structures, magnetic and luminescence (including thermometry) properties of a series of three-blade propeller homoleptic lanthanide complexes are reported.

Isotopomeric Elucidation of the Mechanism of Temperature Sensitivity in 59Co NMR Molecular Thermometers.

Understanding the mechanisms governing temperature-dependent magnetic resonance properties is essential for enabling thermometry via magnetic resonance imaging. Herein we harness a new molecular design strategy for thermometry─that of effective mass engineering via deuteration in the first coordination shell─to reveal the mechanistic origin of 59Co chemical shift thermometry. Exposure of [Co(en)3]3+ (1; en = ethylenediamine) and [Co(diNOsar)]3+ (2; diNOsar = dinitrosarcophagine) to mixtures of H2O and D2O produces distributions of [Co(en)3]3+-dn (n = 0-12) and [Co(diNOsar)]3+-dn (n = 0-6) isotopomers all resolvable by 59Co NMR. Variable-temperature 59Co NMR analyses reveal a temperature dependence of the 59Co chemical shift, Δδ/ΔT, on deuteration of the N-donor atoms. For 1, deuteration amplifies Δδ/ΔT by 0.07 ppm/°C. Increasing degrees of deuteration yield an opposing influence on 2, diminishing Δδ/ΔT by -0.07 ppm/°C. Solution-phase Raman spectroscopy in the low-frequency 200-600 cm-1 regime reveals a red shift of Raman-active Co-N6 vibrational modes by deuteration. Analysis of the normal vibrational modes shows that Raman modes produce the largest variation in 59Co δ. Finally, partition function analysis of the Raman-active modes shows that increased populations of Raman modes predict greater Δδ/ΔT, representing new experimental insight into the thermometry mechanism.