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

Luminescence 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.

limited to a spatial resolution of 10 µm, [26] the spatial resolution of luminescent nanothermometers can go down to ≈200 nm with common confocal microscopes. Luminescent nanothermometers can be used in combination with fluorescence microscopes for generating 2D and 3D images of temperature gradients. For example, fluorescent polymeric thermometers were used to visualize the temperature distribution within COS-7 cells. [27] Here, an increased temperature in the nucleus and in the mitochondria compared to the cytoplasm was observed. In addition to temperature imaging in cells, nanothermometers can also be used in other areas where conventional temperature sensors fail due to their size, such as imaging in microfluidic chips, [28][29][30][31] micro flow reactors [32] and microfluidic devices for continuous-flow polymerase chain reaction (PCR). [33] A large number of reported optical thermometers [34,35] can be divided into six readout groups depending on which luminescence property shows a temperature-dependent behavior: i) emission intensity, ii) spectral position of absorption and emission bands, iii) relative intensity between two emission bands, iv) emission bandwidth, v) anisotropy and vi) luminescence decay time. [36] Among these, measuring changes in a band shape and luminescence decay time are most popular due to robust read-out and self-referenced character. [26] Lifetimebased nanothermometers are particularly suitable for biological systems because this parameter is not affected by the dye concentration or by tissue scattering. [5] Intracellular lifetime-based temperature sensing has been realized with fluorescent dyes such as ER thermo yellow, [37] nanoparticle-embedded sulforhodamine derivative, [38] poly(N-isopropylamide) particles, [27,39,40] gold [41] and silver [42] nanoclusters and carbon dots. [43] Lifetimebased fluorescent and phosphorescent optical thermometers known outside of biological systems utilize ruthenium, [44] europium [45,46] and iridium [47] metal-organic complexes, inorganic phosphors, [48][49][50] silicon nanoparticles, [51] phase-change materials [52] and quantum dots. [53] A great variety of emitters that utilize thermally activated delayed fluorescence (TADF) have been reported for application in organic light emitting diodes (OLEDs) [54,55] but only recently have they started drawing attention as components of sensing materials. [56] Earlier reports demonstrated potential of TADF emitters (acridine yellow, [57] fullerene C70 [58,59] ) for optical temperature sensing. More recently, several optical thermometers have been reported using organic dyes derived from anthraquinone and dicyanobenzene, [60] platinum(II), palladium(II), and zinc benzoporphyrins, [61,62] polymers based on 1,8-naphthalimide, [63] carbon dot-based nanocomposite [43] and zinc complexes with Schiff bases. [64,65] Although the luminescence decay was shown to be strongly affected by temperature at ambient conditions (>2% K −1 ), practical usability of the state-ofthe-art systems is still limited. The challenges include moderate luminescence brightness, significant contribution of prompt fluorescence to the overall emission, bi-exponential decay in immobilized form, and often very long TADF decay times (>1 ms) which imply significant cross-talk to molecular oxygen even in case of polymer matrices with low gas permeability.
Recently, Milsmann and coworkers reported a new class of TADF emitters based on readily synthetically available complexes of zirconium(IV) with substituted pyridinedipyrrolide (PDP) ligands. [66][67][68] Remarkably, the Zr(PDP) 2 complexes emit pure TADF emission without any prompt fluorescence and have TADF lifetimes of hundreds of microseconds. We perceived that this class of compounds is of highest relevance for lifetime-based optical (nano)thermometry and may help to overcome the disadvantages of state-of-the-art systems.
In this work, we report a series of zirconium(IV) pyridinedipyrrolide complexes (Figure 1) that combine attractive spectral properties, high brightness, and excellent photostability with highly temperature-dependent TADF decay time. The lifetimes can be tuned by varying the substituents on the PDP ligand and via straightforward introduction of "heavy" halogen atoms. The oxygen crosstalk can be almost completely avoided by embedding the complexes with the shortest lifetimes into gas-blocking polymers. Application of water-dispersible nanothermometers for imaging of temperature in microfluidic devices and biological systems is demonstrated.

Ligand Design and Synthesis of Zr(PDP) 2 Complexes
As became evident from the previous work, [60,65] TADF emitters of different dye classes are promising self-referenced molecular thermometers since they feature very high-temperature dependency on the emission decay time. For the envisaged applications such emitters should combine chemical stability with attractive spectral properties and high brightness of TADF. Moreover, it is desirable that the probes possess a relatively short TADF decay time (<1 ms and ideally <100 µs) in order to minimize crosssensitivity to oxygen for polymer-immobilized dyes. Thus, we  (Figure 1) is known to have a significant effect on the photophysical properties and the stability of the complexes. [67] In fact, mesityl-groups (Mes) in the 3-positions were demonstrated to strongly enhance the hydrolytic stability making the Zr( Mes PDP Ph ) 2 -complex stable under ambient conditions. On the other hand, substitution at the 5-position of the pyrrole rings can be used for finetuning the TADF lifetimes and quantum yields. For example, the compounds with an electron-withdrawing C 6 F 5 -group and an electron-donating methyl-group in the 5-positions were shown to have shorter (313 µs) and longer (576 µs) lifetimes, respectively, compared to the unsubstituted complex (τ TADF = 474 µs). [67] The lifetimes of several hundred microseconds appear too long for applications as molecular thermometers since even embedded in gas-blocking polymers like polyacrylonitrile (PAN) and poly(vinylidene chloride-co-acrylonitrile) (PVDC-co-AN) these dyes would show some oxygen crosstalk.
Therefore, in order to tune the TADF decay times we combined two different strategies: i) introducing different electronwithdrawing and electron-donating groups at the 5-position of the PDP pyrrole rings and ii) additionally substituting the 4-position of the pyrroles with heavy atoms (bromine or iodine). Considering that the metal complex contains four pyrrole moieties, the latter modification was expected to be a viable strategy to significantly reduce the emission decay times. All the complexes were substituted with a mesityl-group at the 3-positions to ensure air and moisture stability. Overall, nine new Zr(PDP) 2 complexes were synthesized.
An overview of the dye synthesis is given in Scheme 1. In the first step, R 1 -substituted aldehyde and 2′,4′,6′-trimethylacetophenone were condensed to the corresponding chalcone in aldol condensation. Then, the ligand H 2 Mes PDP R 1 was formed in a two-step one-pot synthesis (Stetter and Paal-Knorr reactions) according to the reported protocol. [69] The reaction time of the second step (Paal-Knorr reaction) could be improved from >72 h to <8 h by refluxing the mixture in a synthesis reactor at 170 °C and a pressure of ≈20 bar. Next, H 2 Mes PDP R 1 was complexed with tetrabenzylzirconium (ZrBn 4 ) to give Zr( Mes PDP R 1 ) 2 similar to literature. [68] Finally, in case of Zr( Mes PDP t-BuPh ) 2 and Zr( Mes PDP C6F5 ) 2 the protons in the pyrrole rings were substituted with bromine/iodine using a corresponding succinimide derivative. Spectroscopic characterization of the complexes and intermediates by NMR-spectroscopy and mass spectroscopy (APCI-MS or MALDI-TOF) can be found in supporting information (Figures S19-S66 and Figures S67-S90 in Supporting Information, respectively).

Photophysical Properties
All complexes show spectral properties that are highly attractive for utilization in sensing ( UV and blue-green parts of the electromagnetic spectrum and show orange emission (Figure 2). Substitution in the 5-position of the PDP pyrrole rings with electron-withdrawing or electron-donating groups only minimally affects the shape and position of absorption and emission bands ( Figure 2; Figure S1, Supporting Information). Here, the stabilizing effects of the electron-withdrawing and slightly destabilizing effects of the electron-donating groups on the HOMO and HOMO+1 energies are almost completely compensated by an equivalent change in the LUMO and LUMO+1 energies, resulting in similar HOMO/LUMO energy gaps. [67] Substitution of hydrogen atoms in the 4-position of the pyrroles with bromine and iodine does not affect the spectral properties either. All the complexes are efficiently excitable with a variety of common LEDs (470, 505, or 525 nm) and Ar laser (488 and 514 nm lines). The dyes feature one broad emission peak (full width at half maximum: ≈2300 cm −1 ) in the orange to red part of the electromagnetic spectrum which on the basis of luminescence decays (Figure 3) is attributed purely to TADF emission. This property is remarkable since virtually all reported TADF emitters show prompt fluorescence in addition to TADF. [54,55] It should be noted that existence of prompt fluorescence on the cost of TADF would reduce the analytically useful signal in case of time-domain measurement and strongly reduce the phase resolution in frequency domain measurements. [70] The quantum yields (Φ TADF ) of the complexes approach 50% and are very close to that of the parent compound Zr( Mes PDP Ph ) 2 (Φ = 0.46 in toluene, Table 1, and 0.45 in tetrahydrofuran [68] ). The brightness (ε · Φ) is between 7800 and 10 200 M −1 cm −1 and is comparable to that of typical phosphorescent emitters such as platinum(II) meso-pentafluorophenyl porphyrin and ruthenium(II) tris-4,7-diphenyl-1,10-phenanthroline [71] and TADF emitters based on dicyanobenzene derivatives. [60] Figure 3 and Figure S2 (Supporting Information) show that the luminescence of the dissolved dyes decays monoexponentially. In contrast to the extreme similarity in spectral properties, the TADF decay times (τ TADF ) are strongly affected by substitution in the 4-and 5-position of the pyrroles ( Table 1). Substitution of the phenyl rings in the 5-positions with 4-tert-butylphenyl, 3,4,5-trifluorophenyl, and 3,4-didodecyloxyphenyl substituents result in very minor decrease of the decay time (<5%). Compared to Zr( Mes PDP Ph ) 2 the decay time decreases by ≈10% in case of 3,5-bis-trifluoromethyl substituents. The strongest effect is observed for pentafluorophenyl and 9,9-dimethylfluorene substituents: 31% and 24% decrease of the decay time for Zr( Mes PDP C6F5 ) 2 and Zr( Mes PDP FluorenePh ) 2 , respectively.
The introduction of heavy halogen atoms in the 4-position of the pyrroles results in a strong reduction of the TADF decay time. This is not unexpected, since heavy atoms promote more efficient intersystem crossing due to higher spin-orbit coupling [72] and the number of introduced heavy atoms is significant (four per molecule). In fact, the decay time decreases by 46% and 54% upon bromination and iodination, respectively, of Zr( Mes PDP t-BuPh ) 2 . Similar decrease (51%) is observed upon bromination of Zr( Mes PDP C6F5 ) 2 indicating that substitution effects in the 4-and 5-positions can be used independently for tuning the TADF decay time. Zr( Mes BrPDP C6F5 ) 2 shows ≈3-fold lower lifetime compared to the reported Zr( Mes PDP Ph ) 2 (96 µs and 282 µs, respectively, at 25 °C) which is substantial considering that other photophysical properties are very similar.
Photostability of dyes is another important parameter, particularly in applications requiring relatively high light intensity such as microscopy. Evaluation of photostability was conducted for solutions of dyes in toluene that were irradiated by a metal-halide lamp. The decrease in the dye concentration was determined by UV-vis spectroscopy ( Figure S3, Supporting Information). Tetramethylrhodamine ethyl ester perchlorate (TMR) dissolved in water was selected as a reference dye because it is generally considered as fairly photostable [73] and is frequently used as a fluorescence probe in microscopy. [74] The photobleaching quantum efficiencies (Φ bl ) were accessed for several selected dyes from the slope of the curves (amount of bleached dye vs amount of absorbed photons, Figure 4) and are summarized in Table 1. Remarkably, all dyes show better photostability than TMR (Φ bl = 2.0 × 10 −6 µmol µmol −1 ) ( Figure 4). Although photostability of Zr( Mes PDP Ph ) 2 and Zr( Mes PDP t-BuPh ) 2 is 4-5-fold better than that of TMR, further  2 . In fact, it becomes challenging to reliably access such small photobleaching rates even using a high-power light source. Improvement of photostability upon substitution of phenyl groups by pentafluorophenyl in the 5-positions and introduction of halogens in the 4-positions may be due to the combined effect of i) electron-withdrawing substituents that prevent oxidation of the chromophore by photosensitized singlet oxygen and ii) reduction in the lifetime of the triplet state that makes reactions involving significantly more reactive excited states less likely.

Evaluation of Temperature-Sensing Properties for Polystyrene-Immobilized Dyes
Temperature-sensing materials were prepared by immobilizing the dyes into polymer hosts. Polystyrene (PS) was used as a model substrate for evaluation of temperature-sensing properties of immobilized dyes. The dyes were dissolved in chloroform along with PS and the so-called "cocktails" were knife-coated on a transparent polyethylene terephthalate support. The materials were characterized under anoxic conditions, due to significant oxygen permeability of PS. The photophysical properties of these materials are summarized in Table 2.
Immobilization into a rigid matrix does not affect the emission spectra but leads to improvement of the TADF quantum yields that exceed 0.8. The three dyes with halogens in the 4-position of the pyrroles (Zr( Mes BrPDP t-BuPh ) 2 , Zr( Mes IPDP t-BuPh ) 2, and Zr( Mes BrPDP C6F5 ) 2 ) show the highest quantum yields of around unity.
All immobilized dyes show mono-exponential TADF decays ( Figure 5A for Zr( Mes BrPDP C6F5 ) 2 as an example) that are beneficial for sensing applications. This is in contrast to previously reported TADF emitters based on anthraquinones, [60] dicyanobenzenes, [60] and Zn(II) Schiff base complexes [64,65] which all showed bi-or tri-exponential decays in the immobilized form. Such behavior of the donor-acceptor dyes is attributed to existence of at least two different rotamers in the immobilized dyes since the rigid matrix prevents the dyes from rotation. [75] It is therefore likely that the zirconium complexes, which lack donor-acceptor character exist only in one form when immobilized into polymeric matrices.
Similar to other TADF emitters, the Zr(PDP) 2 complexes show high-temperature dependencies of their decay times ( Figure 5B; Figure S5, Supporting Information). The sensitivity (dτ dT −1 ) at 25 °C is between −2.5 and −2.9% K −1 (Table 1) which exceeds the values shown by most state-of-the-art lifetime-based temperature sensing materials with the exception of TADF emitters. [65] In contrast to the high-temperature dependency on the TADF lifetimes, the emission intensities of the embedded dyes remain almost constant (<5% variation) in the temperature range between 10 °C and 50 °C (Figure 6; Figure S6, Supporting Information). This implies very stable signals and signal-to-noise ratios over a wide temperature range. Generally, TADF emitters show strong enhancement of delayed fluorescence with temperature [76,77] which is also the case for the zirconium pyridinedipyrrolide complexes, albeit in sub-zero temperature range. [68] At ambient conditions, the emission is thus fully "activated". In comparison, most of the reported optical thermometers including phosphorescent and fluorescent dyes and inorganic phosphors show a strong decrease in emission intensity with increasing temperature due to enhancement in the radiationless deactivation ("thermal quenching"). This typical behavior is exemplified in Figure 6 for an Eu(III) complex (Eu(tta) 3 DEADIT) [78] immobilized in PS. In fact, the luminescence intensity decreases ≈3-fold on going from 10 °C to 50 °C.

Planar and Fiber-Optic Temperature Sensors
Sensing materials based on PS cannot be used for temperature measurements under aerated conditions due to the significant oxygen permeability of PS (1.9 × 10 −13 cm 3 (STP) cm cm −2 s −1 Pa −1 [79] ). To overcome this problem, Zr( Mes BrPDP C6F5 ) 2 , the dye with the shortest lifetime among all investigated complexes, was immobilized into PVDC-co-AN. The combination of the very low oxygen permeability of PVDC-co-AN (0.0031 × 10 −13 cm 3 (STP) cm cm −2 s −1 Pa −1 [80] ) and the short lifetimes of Zr( Mes BrPDP C6F5 ) 2 drastically reduces the cross-talk to oxygen. In fact, the luminescence lifetimes in temperature range between 10 °C and 50 °C decrease by < 2% on going from deoxygenated to air-saturated water (Figure 7). Without knowing the exact oxygen concentration, this corresponds to the maximum a) Lifetimes at 37 °C were obtained by interpolating the temperature calibration curves ( Figure 5 and Figure  conditions. However, in many applications, the oxygen concentration remains nearly constant so that the error can be neglected. The same material was used to prepare a fiber-optic sensor. A layer of Zr( Mes BrPDP C6F5 ) 2 -PVDC-co-AN on polyethylene terephthalate support was coated with an additional scattering layer (titanium dioxide particles in silicone rubber) to achieve higher signals when read-out with a custom version of a compact phase fluorometer (Firesting, PyroScience) that was equipped with a blue LED (465 nm) for excitation. A sensor spot prepared from a planar foil and fixed with a metal cap on an optical fiber operated fully reversibly ( Figure S7, Supporting Information). Possible applications of such a sensor include measurements of temperature in strong magnetic fields or local measurements in small objects like microfluidic chips. [28][29][30]

Water-Dispersible Nanoparticles
Immobilization of the zirconium complexes in nanoparticles based on oxygen-blocking polymers would enable "nanothermometry", i.e., temperature measurement in small objects such as live cells, tissues, and microfluidic chips. Since nanoparticles prepared from PVDC-co-AN are rather large and lack stability, particularly in the presence of salts, the polymer was modified with charged groups to ensure stability in aqueous dispersion and to enable cell interactivity. For this purpose, the gas-blocking PVDC-co-AN polymer was modified to additionally incorporate either ≈10 wt.% methacrylic acid (MAA) or ≈10 wt.% [2-(methacryloyloxy)-ethyl]-trimethylammonium chloride (TMA). This leads to negatively-and positively-charged copolymers, abbreviated as PVA-MAA and PVA-TMA, respectively. Nanoprecipitation technique [81] offers a simple way to obtain dye-loaded polymeric nanoparticles. Fast addition of water to the solution of a copolymer and a dye in acetone or its mixtures with other solvents and subsequent removing of the organic solvents via evaporation gives particles of 34 nm (PVA-MAA, zeta potential: -16 mV) and 42 nm (PVA-TMA, zeta potential: +35 mV) ( Figures S8 and S9, Supporting Information). We note that in the second case, the size distribution is not homogenous, and significantly larger particles are also  present, which can require the need for further optimization of the polymer composition and nanoprecipitation procedure.
In order to show the potential of the new tools for imaging of temperature distribution, we incorporated Zr( Mes IPDP t-BuPh ) 2 into negatively charged cell-impermeable PVA-MAA nanoparticles and applied them for in-chip imaging. Zr( Mes IPDP t-BuPh ) 2 and Zr( Mes BrPDP C6F5 ) 2 were embedded into positively charged PVA-TMA nanoparticles for intracellular imaging.
In the first application, a Zr( Mes IPDP t-BuPh ) 2 -PVA-MAA particle suspension (≈22 °C) was pumped at different flow rates (between 0 and 10 µL s −1 ) through a microfluidic chip which was heated to ≈37 °C. The temperature distribution was imaged with a frequency domain fluorescence lifetime imaging (FLIM) camera from PCO Computer Optics (Figure 8). Figure 8 demonstrates that at low flow rates (1-2 µL s −1 ), the colder nanoparticles solution quickly achieved the set temperature within the chip. At higher flow rates, constant temperature in the chip was no longer uniformly sustained and drastic gradients of >10 °C were observed. Considering the growing importance of temperature for cell metabolism studies and as a parameter affecting analytical response of every optical sensor, these new materials can be applied to many tissue-on-a-chip experimental platforms.
In the second application, we evaluated applicability of the positively charged nanoparticles for the live cell nanothermometry. First, we looked at the effect of the nanoparticle surface charge (PVA-MAA and PVA-TMA particles) on the interaction with the cells (Figure S11, Supporting Information). As expected, negatively charged nanoparticles displayed much lower staining of the human colon cancer HCT116, endothelial HUVEC, and dental pulp stem cells. In contrast, quaternary ammonium groups harboring Zr( Mes IPDP t-BuPh ) 2 -PVA-TMA nanoparticles promoted efficient endo-and lysosome-like staining (Figure 9A-C; Figure S12, Supporting Information), similar to other previously described nanosensors. [9,12,38,82] As with the sulforhodamine-based temperature probe, [38] we saw no effect of the positively charged nanoparticles on the cell viability ( Figure S13, Supporting Information). We also observed relatively long retention of the nanoparticles inside the cells, lasting longer than 48 h ( Figure S14, Supporting Information). These features prompted us to see if we could also measure temperature in more complex cell-based models, such as multicellular 3D spheroids produced from HCT116 cells (Figure 9D-F; Figure S15, Supporting Information).
Despite somehow patchy distribution in the spheroids, we managed to measure the emission lifetimes (≈49 µs) at the periphery of the live spheroids using a 485 nm excitation and confocal microscope ( Figure 9E,F).
To improve the imaging quality and expand the scope of potential biological applications with spheroids, related organoid models, or intravital measurements, two-photon (2P) excitation can be used alternatively. [1,83] We thus tested the compatibility of Zr( Mes IPDP t-BuPh ) 2 embedded into PVDC-co-AN with the 2P excitation. The material was compared to Rhodamine B immobilized in the same polymer. The latter was  selected as a standard since its emission spectrum is similar to that of the Zr( Mes IPDP t-BuPh ) 2 complex. Rhodamine B is reported to show two-photon absorption (2PA) maximum at ≈820-840 nm with the estimated 2PA cross-sections between 200 GM [84,85] and 500 GM. [86] Figure S16 (Supporting Information) shows that both dyes are excitable in approximately the same part of the spectrum. Zr( Mes IPDP t-BuPh ) 2 shows rather a broad excitation which is most efficient from 710 to 810 nm. Emission spectra ( Figure S17, Supporting Information) show much higher luminescence intensity in case of the reference dye obtained for the same concentrations, excitation wavelength (800 nm), and laser power. In fact, except for the lowest concentration of 0.5 µmol g −1 polymer the signal from Rhodamine B was too high at the excitation power of 5% ( Figure S17B, Supporting Information). Considering that the emission quantum yields of both dyes are quite similar, the 2PA cross-section for Zr( Mes IPDP t-BuPh ) 2 is estimated to be ≈25-fold lower than for Rhodamine B, i.e., in the range of 10-20 GM. Nevertheless, these values are generally sufficient for 2P microscopy and are higher than for many dyes bearing no additional 2P antennas. [87] Indeed, we found that nanoparticle-stained multicellular spheroids were sufficiently bright for measurements by 2P phosphorescence lifetime imaging microscopy ( Figure S15, Supporting Information).
We also investigated the non-linear properties of Zr( Mes PDP FluorenePh ) 2 to see if the fluorene moiety can serve as a 2P antenna. Indeed, the 2P excitation spectrum of this dye is different from Zr( Mes IPDP t-BuPh ) 2 ( Figure S18, Supporting Information). Much broader excitation band and maximum at ≈700 nm indicate that under 2P both the chromophore and the fluorene antenna can be excited. However, the 2PA cross-sections are estimated to be lower than for Zr( Mes IPDP t-BuPh ) 2 , so such modification appears not to lead to improvement of the non-linear properties.

Conclusion
In summary, we have synthesized and characterized a series of nine substituted zirconium(IV) pyridinedipyrrolide complexes bearing various electron-donating and electron-withdrawing groups in the 4-and 5-position of the PDP pyrrole rings. The complexes can be excited in the blue and green regions of the electromagnetic spectrum, are highly photostable, and show excellent brightness under one-photon excitation and acceptable characteristics under two-photon excitation mode. Moreover, the dyes show a unique combination of photophysical properties that are particularly attractive for optical temperature sensing applications: i) pure TADF emission without prompt fluorescence; ii) mono-exponential luminescence decay; iii) high-temperature dependency of the TADF decay time between −2.5 and −2.9% change per K at 25 °C; and iv) virtually constant TADF intensity in the relevant temperature range.
The introduction of a strongly electron-withdrawing C 6 F 5group in the 5-positions and heavy atoms (Br and I) in the 4-positions, led to a drastic reduction in the TADF lifetime (up to ≈3-fold) compared to the previously reported Zr( Mes PDP Ph ) 2 complex. Such substitution is beneficial for temperature sensing applications since it significantly reduces potential cross-talk with oxygen but has almost no effect on other photophysical properties. Additionally, it was found that heavy atoms could further enhance photostability which can be beneficial for other photonics applications such as OLEDs and photocatalysis.
Immobilization of the new probes into gas-blocking copolymer of vinylidene chloride and acrylonitrile gives materials for temperature sensing and imaging, that can be realized in a variety of formats such as planar foils, fiber-optic sensors, and water-dispersible nanoparticles. The latter can harbor anionic or cationic functional groups to ensure stability in water and addressable character in biological applications. We demonstrated application of nanoparticles in imaging of temperature gradients within a microfluidic chip and in cellular imaging. We found the positively charged nanoparticles compatible with the lifetime-based nanothermometry of 3D cultures of the live cells. Collectively, presented materials can be useful for a broad range of biological applications, complementing studies of cell metabolism, senescence, and development, under static and fluidic flow conditions.
PS Planar Sensor Foils: A "cocktail" was prepared by dissolving the dye (0.5 wt.% with respect to the polymer) and polystyrene (10 wt.% with respect to the solvent) in chloroform. The "cocktail" was knife-coated on dust-free PET support with a wet film thickness of 25 µm and the foils were dried for one day at room temperature.
PVDC-co-AN Planar Sensor Foils: Dye (1 wt.% in respect to the polymer) and PVDC-co-AN (10 wt.% in respect to the solvent) were dissolved in dry THF and the "cocktail" was knife-coated in a glovebox on dust-free PET support (wet film thickness of 25 µm). After drying the foil for 12 h at room temperature, the foil was transferred from the glovebox to a drying chamber and was dried for another 12 h at 70 °C. Then a scattering layer (TiO 2 powder:silicone E4:heptane, 1:1:1 w/w/w) was knife-coated over the sensing layer with a wet film thickness of 75 µm and the foil was dried in a drying chamber for 6 h at 70 °C.
PVA-TMA Nanoparticles: 20 mg PVA-TMA was pre-dissolved in 200 mg hexafluoro-2-propanol, diluted with 10 mL acetone and 0.2 mg Zr( Mes IPDP t-BuPh ) 2 of Zr( Mes BrPDP C6F5 ) 2 were added. Then, water (40 mL) was quickly added to the solution under vigorous stirring and the organic solvents were removed under reduced pressure.