Heat Resistant Organic Dyes for High Temperature Luminescent Temperature Sensing

Compliant and large‐area high‐temperature gradient sensing is essential for scientific and industrial applications but remains a big challenge. Although organic luminophores have intrinsic advantages of flexibility and solution processability, they generally suffer from significant emission quenching at high temperatures due to thermally facilitated nonradiative decay. Herein, a heat‐resistant blue emitter of C3 based on triarylphosphine oxide has been developed, due to the thermal population of the higher emissive state from its lowest excited state. Based on this, hybridization of C3 with a faster thermally‐deactivated yellow dye of T4AC which exhibits a large Stokes shift enables blocking of energy transfer and independent thermal response of the two respective emitters. Thus, sensitive ratiometric film thermometers for high‐temperature sensing can be constructed. The relative sensitivity (Sr) reaches 1.27%°C−1 at 128 °C and the temperature resolution is < 0.77 °C in a wide sensing range of 20–240°C. Moreover, naked‐eye thermal mapping and multiple anti‐counterfeiting of these ratiometric films have also been demonstrated.


Introduction
[20][21][22][23][24][25][26] The fluorescence thermometers based on the luminescence decay require expensive instruments and longer processing time.29] However, the luminescence intensity is inevitably affected by the quantity of the luminophores, excitation power, and the sample morphology.[37][38][39] Another facile technology is to implement infrared thermometers based on the principle of blackbody radiation but has the drawbacks of low spatial resolution, high noise, low image contrast, blurred visual effect, and a narrow grayscale. [40][43][44] Nevertheless, organic luminophores generally exhibit significant emission quenching upon heating as a result of thermo-facilitated nonradiative decay, which limits their applications in high-temperature measurements.[49] However, the energy transfer generally much reduces the original significant difference in their distinct temperature sensitivity when they are in the isolated state, thus leading to lower sensitivity.

Compound Synthesis and Preliminary Photophysical Properties
The fluorophore compound C3 has been synthesized by the substitution reaction of pyrrole-substituted pyreneyl lithium with triethyl phosphite, followed by subsequent oxidation by hydrogen peroxide (Scheme 1).The molecular structure and purity were verified by 1 H NMR spectra, MALDI-TOF MS, and HRMS (see the supporting information).
The preliminary photophysical properties of the C3 were first studied in solutions.Both the absorption and emission spectra show obvious bathochromism as the polarity of the solvent increases (Figure 1).In the absorption spectra, the longestwavelength absorption band exhibits a significant bathochromic shift from nonpolar n-hexane to polar solvents but shows rela-tively small shifts when moving from one polar solvent to another.This lowest-energy absorption is peaked at 405 nm in the nonpolar solvent of n-hexane, then shifted to 422 -438 nm in polar solvents (422, 428, 436, 437, and 438 nm in tetrahydrofuran, ethanol, isopropyl alcohol, dichloromethane, and acetonitrile, respectively) (Figure 1a).The photoluminescence (PL) spectra show a similar change tendency.The emission peak is located at 455 nm in nonpolar n-hexane, and dramatically shifts to 483 nm in the polar solvent of tetrahydrofuran (Figure 1b).According to the time-resolved emission spectra (TRES) spectra of the solution, the emission spectra actually consist of two emissions -one shorter-wavelength emission of high-lying S 2 state and the other longer-wavelength emission of low-lying S 1 state (Figure S1a, Supporting Information).In the nonpolar solvent of hexane, the emission in the short-wavelength region peaked ≈ 455 nm is dominated by the high-lying S 2 state.With the increase of solvent polarity, the S 2 state exhibits a larger bathochromism than the longer-wavelength S 1 state, due to its much larger ratio of CT character as seen from the Natural transition orbitals (Figure 2a).Thus, in polar solvents, the significantly red-shifted S 2 emission with decreasing intensity merges into the S 1 emission, and the whole emission spectra are dominated by the longer wavelength emission.Further, this merged emission undergoes a small bathochromism from 483 to 490 nm in polar solvents as the polarity of the solvent increases.(483, 484, 485, 490, and 490 nm in tetrahydrofuran, dichloromethane, isopropyl alcohol, ethanol, and acetonitrile, respectively) (Figure 1b).This is characteristic of a typical hybrid local and charge transfer (HLCT) feature, which is consistent with the theoretical calculations (Figure 2a; Table S1 and Figure S2, Supporting Information).In this work all the quantum-chemical calculations were performed using the Gaussian 09 suite of programs. [56]Based on the natural transition orbital analysis, it was clear that, for the transition S 0 → S 1 , the holes and particles were both overlapping and spread over the two pyrene groups and show small variations of the charge distribution.This indicates that S 1 is an HLCT state that exhibits the main component of the local excited state (LE) mixed with a small amount of charge transfer (CT) state (Figure 2a).This low-lying HLCT S1 state corresponds to the long wavelength emission.In contrast, for the transition S 0 → S 2 , the holes are mainly located at one pyrene group, with only a few distributed on the other, while the particles are almost all located on the other one (Figure 2a).This suggests that the S 2 state, which corresponds to the short wavelength emission, is also HLCT state but contains more CT character.In solution, the high-lying HLCT state (the short wavelength emission peak) and low-lying HLCT state (the long wavelength emission peak) exhibit a longer and shorter fluorescence lifetime, respectively, according to the TRES spectra (Figure S1a, Supporting Information).Consistently, the fluorescence decay data are also characterized by a two-component decay, consisting of shorter (2.5 ns) and longer lifetimes (12 ns), respectively (Figure S3, Supporting Information).

Temperature Sensing Properties in Solution
Temperature-dependent emission spectra were first measured in 2-methoxyethyl ether (MOE) solution.Two main emissions peaked ≈ 435 and 482 nm, respectively, are clearly observed in the emission spectra (Figure 3a).With the increase in temperature, the emission spectra abnormally show a gradual enhancement, which is more prominent in the shorter-wavelength emission (435 nm) than that in the longer-wavelength (482 nm) one (Figure S4a, Supporting Information).This indicates a specific high heat tolerance of C3, considering most organic emitters suffer from significant emission quenching upon heating due to thermally facilitated nonradiative decay.Meanwhile, the longer-wavelength emission ≈ 482 nm shows a slight blue shift to 477 nm when the temperature increases from 30 to 100 °C (Figure 3a; Figure S4a, Supporting Information).Here, we select two fixed wavelengths for simplification to obtain the ratiometric fluorescence.The relationship between the fluorescence intensity ratio of the emission peaked at 435 nm (I 435 ) to that at 478 nm (I 478 ) and temperature (T) was fitted with high accuracy using the function I 435 /I 478 = 0.04 + 0.01 T-7.49 × 10 −5 T 2 + 6.70 × 10 −7 T 3 -1.31× 10 −8 T 4 + 6.72 × 10 −11 T 5 , with a correlation coefficient of 0.9993 (Figure 3b).
The maximum relative sensitivity (S r ) and absolute sensitivity (S a ) are determined to be 3.09% K −1 (at 303 K/30 °C) and 8.8 × 10 −1 K −1 (at 303 K/30 °C), according to equation of S5a, Supporting Information). [57]Thus, by measuring the ratio of fluorescence intensity related to temperature, the working temperature (T) in this temperature sensing region can be obtained.The relative errors are calculated to be < 2% at temperatures > 40°C (Figure S5b, Supporting Information).Moreover, the temperature resolution (T, calculated from equation of °C in the range of 30 -70°C, with the minimum T of 0.52°C (at 50°C) (Figure S5c, Supporting Information). [57]In addition, this solution thermometer exhibits good reversibility for recyclable applications.In multiple cycles between 30 and 70°C, the ratiometric emission intensity at each temperature remains relatively stable (Figure S5d, Supporting Information).This temperaturedependent behavior of the ratiometric fluorescence is promising for liquid temperature sensing.
Further, time-dependent density functional theory calculation was performed to explain this temperature dependence of the fluorescence and its high heat tolerance (Figure 2).The energy levels of the lowest (S 1 ) and higher excited state (S 2 ) are determined to be 3.34 and 3.48 eV, respectively.When the temperature increases, the S 1 excited states (the low-lying HLCT state) can probably be populated to the S 2 states (the high-lying HLCT state).This is further verified by the TRES and temperature-dependent emission decay spectra (Figures S1, S3, and Table S2, Supporting Information).The TRES spectra indicate that the longerwavelength emission exhibits a faster fluorescence decay while the shorter-wavelength emission presents a slower emission decay (Figure S1, Supporting Information).With the increase of temperature, the total emission decay lifetime is lengthened with the ratio of longer lifetime components increased (Figure S3 and Table S2, Supporting Information).This suggests that the highlying S 2 emissive state is thermally populated.As a result, the ratio of shorter-wavelength emission gradually increased at heating (Figure 3a; Figure S4).Therefore, it can be concluded that with the increase of temperature, the absolute emission enhancement of the solution is mainly caused by the prevailing effect of thermal populating from S 1 to S 2 over that of thermally facilitated nonradiative decay (Figure 3a,b). [16]This atypical high heat resistance of solution as well its ratiometric temperature sensing ability prompt us to further investigate the potential of C3 for high-temperature film thermosensing.Here, it should be noted that the nanosecond-scale fluorescence decay precludes the effect of thermally activated delayed fluorescence (TADF) on the heat-induced emission enhancement (Figure S3, Supporting Information).

Temperature Dependence of the C3 Pristine Films
The emitter of C3 was then examined in solid film (with a total concentration of 1 wt.% in the PMMAPMMA matrix) to explore their thermal responsiveness, which is crucial for the determination of their practical applicability. [58]Notably, different from the solution, the film emission spectra are dominated by the longerwavelength emission ≈ 480 nm, that is, originated mainly from the low-lying HLCT excited state (Figure 3c).When the film is heated, only a slight blue shift within 8 nm (from 482 to 474 nm) is detected (Figure S4b, Supporting Information).However, in film the short-wavelength high-lying HLCT emission can still be detected by the TRES spectra (Figure S1b, Supporting Information).Similar to the solution, the longer wavelength emission in film ≈ 465 -475 nm appeared in a shorter time range (<7 ns), while the shorter-wavelength emission ≈ 405 nm emerged in the longer time range (8 -20 ns).Thus, it can be inferred that the single emission band ≈ 474 -484 nm in film steady PL spectrum is actually resulted from the merging of the low-and high-lying HLCT states.
Next, we characterized the temperature dependence of the C3 film.When the temperature was increased from room temperature to 60 °C, the emission intensity showed an abnormal increase, which is 8% higher than the initial emission intensity at 20 °C (assumed as 100%) (Figure 3c,d).This is because upon heating some of the lowest excited states (i.e., the low HLCT state) transit to the high-level HLCT emissive state rather than undergo a thermally facilitated nonradiative deactivation to the ground state.This feature allows the film luminescence efficiency to be kept at a higher value in the high-temperature region.As the temperature was raised from 60 to 80°C, the emission intensity remained relatively constant.In this stage, the thermal population of the high-lying excited state is comparable to the thermally promoted non-radiative decay.After that, the emission intensity began to decline, which suggests that the effect of the thermally facilitated nonradiative decay prevails that of the thermal population of the high-lying HLCT states.As the temperature further increased from 80 to 140 °C, the fluorescence intensity showed a relatively slow decrease, with an average decay rate of 0.47%°C −1 .The relative fluorescence intensity at 140 °C still remained 81% of the initial value (at 20 °C).With further heating (140 -200 °C), the fluorescence decayed faster due to intensely facilitated nonradiative deactivation, with an average decay rate of 1.09%°C −1 .When the temperature was further heated to 240 °C, the decrease of the fluorescence intensity of the C3 film began to slow down again.The offset between two opposite thermal effects, that is, the thermal population of the high-lying excited state and the thermally induced nonradiative decay, allows C3 to exhibit sustainable fluorescence at high temperatures which is still detectable by the naked eye (Figure 3d).This high heat resistance indicates good potential for high-temperature sensing.Consistent to the thermally increased population of the highlying excited state, the whole emission spectrum profile exhibits a slight blue shift with an increase of temperature (Figure S4b, Supporting Information).A function of I T /I 0 = 0.92 + 0.46 × 10 2 T -1.26 × 10 −5 T 2 -1.10 × 10 −7 T 3 -1.51× 10 −9 T 4 + 6.69 × 10 −12 T 5 accurately describes the temperature dependence of the ratio (I T /I 0 ) of the emission intensity at the operating temperature T (I T ) to the initial value at 20°C (I 0 ), yielding a correlation coefficient of 0.9891 (Figure 3d).The pristine C3 film thermometer exhibits a maximum S r of 6.34% K −1 (513 K/ 240 °C), S a of 0.65 × 10 −2 K −1 (513 K/ 240 °C), and T of 0.47 °C (453 K/180°C) in the high-temperature region, with high thermal stability and remarkable reversibility (Figure S6, Supporting Information).This shows its potential applications in film thermosensing.

Fabrication of Non-Energy Transferred Dual Emitting Films
The heat-resistant blue fluorophore of C3 was hybridized with a yellow excited state intramolecular proton transfer (ESIPT) emitter of T4AC (see its molecular structure in Figure S7, Supporting Information) which display an enormous Stokes shift and a more remarkable thermal quenching of emission, [4] to construct a ratiometric film thermometer for sensitive high-temperature detection.The yellow emitter T4AC, which is obtained from our previous study, [59] exhibits a large Stokes shift of 195 nm (Figure 4a), attributed to the ESIPT process involving a fast fourlevel photophysical cycle between the enol and keto tautomers. [53]herefore, the spectrum overlap (J) between the blue and yellow emitters is almost negligible, which is been estimated to be J = 8.90 × 10 10 cm −1 nm 4 , according to the equation of J = This hinders the energy transfer from C3 to T4AC (Figure 4a).The block of energy transfer can be proved by the following experimental investigations.1) Despite enhancing T4AC mass ratio to 10 times higher than that of C3, the blue emission of the hybrid film is still prominent, with its intensity about one-third of the yellow emission (Figure 4b).2) Moreover, the excitation spectrum profiles of the hybrid detected at the yellow emission region closely resemble the isolated T4AC film, rather than the isolated C3 film, indicating the direct excitation from the yellow fluorophores in the hybrid films (Figure 4c). 3) Finally, the luminescence lifetimes of the hybrid films detected at the blue emissions remained almost constant with increasing concentrations of the yellow emitter T4AC (Figure 4d).This is because either Foster or Dexter energy transfer will result in the decrease of the fluorescence decay lifetime of the donor. [62]All these results strongly suggest that the energy transfer between the two emitters are completely impeded.This may greatly reduce their mutual interference and preserve their individual and distinct thermal-deactivation response in the hybrid films, which is highly promising for sensitive ratiometric thermosensing in films.
In comparison to the C3 films, the T4AC film exhibits a faster emission decay at heating.Across the temperature range of 20 -160 °C, the emission first decays very quickly with a deactivation speed of 0.61%°C −1 which is 2.1 times higher than that of the C3 film (0.29%°C −1 ).Then, it decays relatively slowly with a deactivation speed of 0.13%°C -1 when the temperature is increased further from 160 -240 °C (in this temperature range, the C3 films show a deactivation speed of 0.59%°C −1 ).Heating the T4AC film to a high temperature of 240 °C renders it almost invisible, maintaining only 3.99% of its initial fluorescence at 20 °C (Figure 5).In contrast, the C3 film still remains at 11.49% (at 240 °C) of its initial fluorescence at 20 °C (Figure 5).Based on the above investigations, with integration of the maintaining the different thermal sensitivities of the two colored emitters and hindrance of energy transfer, these hybrid films may be able to be applied in sensitive high-temperature ratiometric thermal detection.

Temperature Dependence of the Hybrid Films
In order to fabricate ratiometric luminescent thermometers, the C3/T4AC hybrid films are prepared with a total doping ratio of 1 wt.% in a PMMA film.The hybrid film, with a mixing ratio of 1:6 (blue to yellow dye), was analyzed for its temperature dependence of fluorescence.The emission spectra of the C3/T4AC film reveal dual emissions from respective emitters, with the short-wavelength blue and the longer-wavelength yellow emissions peaking ≈ 470 and 540 nm, respectively (Figure 6a).As the temperature increases, the yellow emission decreases rapidly due to the thermo-induced intramolecular rotation and vibration in T4AC.In contrast, the blue emission initially shows a reverse change tendency of emission enhancement with increasing the temperature from 20 to 60 °C, which is attributed to the thermal population of the high-lying HLCT states.Accordingly, the emission intensity ratio (I 470 /I 540 ) of the blue (I 470 ) to yellow emission peak (I 540 ) of C3/T4AC increases, although not significant (Figure 6a,b).By further heating, both the blue and the yellow emission intensities decrease (Figure 6a).In the temperature range of 20 -80 °C, the ratiometric intensity (I 470 /I 540 ) shows a slight fluctuation (Figure 6b), with a small enhancement in the temperature range of 20 -60 °C and thereafter a slight decrease at 60 -80 °C.Correspondingly, the luminescence color is mainly located in the yellow emission region, with the Commission International de l'Eclairage (CIE) coordinates in the range of (0.330, 0.4204) and (0.328, 0.414) (Figure 6c,d).Subsequently, with the temperature increasing from 80 to 180 °C, the blue emitter exhibits a much lower thermo-deactivation rate than the yellow emitter, due to its high thermal resistance.Consistently, the emission intensity ratio (I 470 /I 540 ) shows a rapid increase from 0.631 to 1.618.As a result, the luminescence color changes from yellow, yellow-green, green-white to blue-white emission, with the CIE coordinates gradually shifting from (0.328, 0.414) at 80 °C, (0.293, 0.345) at 140 °C, (0.293, 0.345) at 160 °C to (0.275, 0.308) at 180 °C.By further heating from 200 to 240 °C, the increase of I 470 /I 540 begins to slow down (Figure 6a,b).At the same time, the emission color of the hybrid film becomes cyan blue, with the CIE coordinates in the range of (0.266, 0.2842) and (0.263, 0.253) (Figure 6c,d).
To precisely evaluate the performance of this ratiometric temperature sensor, a function of I b /I y = 0.1453 + 0.0472 T -9.5710 × 10 −4 T 2 + 8.9739 × 10 −6 T 3 -3.4366× 10 −8 T 4 + 4.6242 × 10 −11 T 5 , with a correlation coefficient of 0.9975, can well describe the relationship between the fluorescence intensity ratio I b /I y of blue (I 470 ) to yellow emission (I 540 ) and temperature (T) (Figure 6b).The T is < 0.77 °C with a calculated error of < 1.8% in the wide temperature range of 20 -240°C (Figure 7a,c).Moreover, the maximum S r and S a of the temperature-sensitive film are determined to be 1.27%K −1 (401 K/128 °C) and 1.38 × 10 −2 K −1 (423 K/150 °C), respectively (Figure 7b).The T reaches as low as 0.29 at 180 °C (Figure 7c).The film sample exhibited substantially unchanged fluorescence spectra upon 3 h continuous heating at a high temperature of 200 °C (Figure S8, Supporting Information), and displayed good stability during multiple cycles between 20 and 200°C (Figure 7d).This indicates that the hybrid film not only offers high sensitivity and high-temperature resolution but also holds the potential for reliable and repeatable high-temperature thermosensing in an ambient atmosphere.
Similar temperature dependence was observed in the film with a different mixing ratio (C3: T4AC (wt.%: wt.%) = 1: 10) (Figure S9    (Figure S9c, Supporting Information).Additionally, the relative error remains < 2% within the whole operating range of 20 -300°C (Figure S9d, Supporting Information).The doping film exhibits a high-temperature resolution with T < 1.51 °C, which is determined as 0.58 at 240 °C (Figure S9e, Supporting Information).Meanwhile, its fluorescence spectrum presents good stability during the 4 h continuous heating at 200 °C (Figure S9f, Supporting Information).These results imply sensitivity-tunable and less demanding preparation conditions for these hybrid film temperature probes.

Gradient Temperature Sensing
To create a prominent temperature gradient across a large surface area, a probing film, cast by the hybrid solution of C3/T4AC (wt.%:wt% = 1:6) onto a 4.5 × 4.5 cm 2 quartz plate, was laid on the edge of a steel block to form a 20°angle to the hot stage (Figure 8a).After stably setting the temperature of the hot stage to 240 °C, the gradient distribution of luminescence color was clearly observed (Figure 8b).The side in contact with the heating stage exhibits a deep blue color, corresponding to a higher temperature of 240 °C, while the other side in contact with the steel block displays a yellow color, corresponding to a lower temperature of 80 °C.Thus, with the assistance of the temperature-dependent CIE coordinate plot (Figure 6d), the fluorescence thermal mapping enables coarse estimation of temperature gradient distribution in large areas by the naked eye.Note that the molecular fluorescence-based signal change of the intensity ratio provides high spatial resolution since it is limited only by diffraction.Furthermore, we tried applications in thermal patterning.By placing a quartz slide coated with doped film onto columnar steel letters of "NJUPT" and then heating them at 150 °C on a hot plate, differential luminescence coloration was achieved between the cold and hot regions (Figure S10, Supporting Information).Under 365 nm UV light illumination, the thermally-patterned region appears white emitting, while the non-heated area remains yellow-green emissive.This indicates potential for serving as a heat-sensitive pattern material and also for heat-induced anti-counterfeiting and information encryption.

Anti-Counterfeiting Application
The digital "8888" was painted onto a quartz plate by casting pure T4AC and C3/T4AC mixture (both within the PMMA matrix) onto different regions (Figure 8c).At room temperature (20 °C), the whole digital of "8888" is yellow emitting under a 365 nm UV lamp illumination (Figure 8d).At a higher temperature of 220 °C, the areas painted with the C3/T4AC blend turn sky blue, while the regions painted with pure T4AC are still yellow emissive (Figure 8e).The fluorescence image shows a two-colored digital of "8888" which uncovers a hidden cyan emitting digital of "1807".By further heating to 280 °C, the entire T4AC section (originally yellow emitting) became dark due to significant thermal deactivation, and the C3/T4AC regions are still visibly luminescent giving deep blue emission.This eventually results in a blue luminescent digital of "1807"(Figure 8f), which differs entirely from the original yellow "8888" at room temperature (Figure 8d).As we can see, with increasing temperature, both the digital region and color undergo changes, with the fluorescence image displaying multiple changes.Given the intrinsic advantages of lightweight, flexibility, as well as easy and inexpensive preparation of organic materials, these film thermometers indicate significant potential for advanced opto-/electronic applications in multiple anti-counterfeiting, data encryption, and information security.

Conclusion
In general, a novel blue-emitting fluorophore of C3 based on pyrrole-substituted aryl phosphine oxide has been developed to possess high heat resistance, which is attributed to the thermal coupling between its low-and high-lying HLCT states.The C3 film maintains sustainable luminescence at high temperatures even up to 240°C.On this basis, by hybridizing with a yellowemitting ESIPT compound, high-temperature organic ratiometric film thermometers based on the mechanism of blocking energy transfer have been fabricated.They exhibit favorable sensitivity with S r > 0.5% K −1 in the higher temperature region (90 -189°C) and high-temperature resolution with T < 0.77 °C in the whole operating temperature range (20 -240 °C).The maximum *-reaches 1.27% K −1 at 128 °C and the T is only 0.29 at 180 °C.Moreover, in an ambient atmosphere, these blockedenergy-transferred hybrid films can be feasibly applied in noninvasive naked-eye gradient thermal imaging, thermal patterning, and multiple anti-counterfeiting.This work indicates good potential in practical high-temperature thermal mapping and information encryption.

Figure 2 .
Figure 2. a) Natural transition orbitals (NTOs) of S 0 →S 1 and S 0 →S 2 of the C3, b) Energy level diagram showing the changes in the emission energy with increasing temperature caused by the thermal population of the higher-energy S 2 state.

Figure 3 .
Figure 3. Temperature-dependent emission spectra a) and ratiometric fluorescence intensity I 435 /I 478 b) of C3 in MOE solution (1.0 × 10 −5 m), temperature-dependent emission spectra c) and relative fluorescence intensity I T /I 0 d) of C3 in PMMA film.The excitation wavelength is 370 nm.

Figure 5 .
Figure 5. a) Temperature-dependent emission spectra of T4AC, b) The fluorescence intensity decay curves of C3 and T4AC films.

Figure 8 .
Figure 8. a,b) Fluorescence thermal mapping (under 365 nm UV illumination) of cast with the C3/T4AC film, which is cast on the quartz plate (4.5 × 4.5 cm 2 ) by titling its bottom side on the hot stage and placing its top side on a steel block.Inset: side view.c-f) Temperature-controlled digital information in fluorescence photographs by painting C3/T4AC (wt.%:wt.%= 1:6) and T4AC at different regions onto quartz plates.