A Photophysical Interpretation of the Thermochromism of a

Article pubs.acs.org/JPCC

A Photophysical Interpretation of the Thermochromism of a Polyfluorene Derivative−Europium Complex Denis A. Turchetti,† Raquel A. Domingues,‡ Cristiano Zanlorenzi,† Bruno Nowacki,† Teresa D. Z. Atvars,‡ and Leni C. Akcelrud*,† †

Paulo Scarpa Polymer Laboratory (LaPPS), Federal University of Parana, CP 19081, Curitiba/PR, 81531-980, Brazil Institute of Chemistry, State University of Campinas (Unicamp), CP 6154, Campinas/SP, 13084-971, Brazil

‡

S Supporting Information *

ABSTRACT: The thermochromism of a polyfluorene derivative complexed with a europium ion was interpreted using the photophysical properties of the noncomplexed polymer and a low molecular mass model compound having the same structure of the complexed site in the polymer. To the naked eye, the thermochromism was characterized by a strong red at low temperatures (170− 260 K, due to Eu3+ ion) and a blue color at higher ones (280−330 K, due to fluorene). Absorption and emission spectra, time-resolved measurements, and theoretical simulations showed that the polymer chain does not affect the europium photophysical properties, but the opposite occurred: the ion insertion precluded interchain aggregation, and the backbone emission did not vary with temperature variation, in the range of 170−330 K. To the best of our knowledge, this effect is reported for the first time and opened a new avenue for the design of nanothermometers, since the polymer can act as a “built-in standard”, thus allowing the construction of much simpler devices.

1. INTRODUCTION The search for new functional materials with specific applications with low cost and simplicity of manufacturing is the prime goal of the materials science. In the field of optoelectronics, large area displays for illumination,1,2 photovoltaic cells,3,4 and temperature sensors5−7 are particularly relevant. In this context, polymer systems containing metallic ions have been the focus of attention because they can combine the metallic characteristics with those of plastics, such as the capability of film formation, and can be processed by the usual techniques, such as casting, jet printing, and others. The literature reports a large number of examples where the metallic ion is dispersed in the polymer matrix.8−11 This approach has the advantage of facile experimental procedures, but at the same time presents serious problems related to homogeneity and solubility of the final polymer−ion dispersions.12−14 The natural route to circumvent this situation leads to the search of materials where the metallic ion is chemically bound to a polymer backbone. Specifically, in the case of emissive properties, the metal−polymer association provides mechanical stability and processability which are not presented by the low molecular mass ligand counterparts. This class of materials has been named metallopolymers,13,15 where the metal is linked directly to the polymer backbone16−18 or as a lateral side chain.19−22 When the organic part is a π-conjugated polymer, the polymer may emit upon excitation, or the metallic ion, through specific mechanisms of energy transfer,23,24 and we can get a predominant emission of the polymer chain25,26 or from the metal ion.27,28 © 2014 American Chemical Society

Rare earth metals, specifically lanthanides, display high luminescence with narrow emission bands. Europium(III) and terbium(III) are attractive luminescent materials because of their red and green emission, respectively. Because f → f transitions of lanthanide are Laporte-forbidden, the direct excitation of ions is weak. One way to overcome this shortcoming and greatly enhance the luminescence intensity is to employ a sensitizing chromophore or antenna as a ligand in a lanthanide complex.29−31 Regarding europium complexes, this kind of process has been addressed in terms of ligand energy absorption that, through intersystem crossing, goes to its triplet state that, in turn, goes to the central ion responsible for the red emission.32−34 In addition to the photoemission process, there are several reported cases of thermochromism of dispersed rare earths in polymer matrixes,35−37 but situations where these metallic components are chemically bound to a conjugated polymer chain are still in the initial stage.13,38 Recent data have shown that the energy levels of the excitated state of the Ln3+ ions are highly sensitive to temperature, and that this property can be explored in temperature sensors, using Tb 3+ and Eu 3+ complexes.39,40 However, the disassociation of the metal− ligand bond of small molecular ligand−Ln3+ complexes at high temperatures restricted their thermal reversibility.41 To overcome this difficulty, blends of luminescent lanthanide Received: August 11, 2014 Revised: November 3, 2014 Published: November 24, 2014 30079

dx.doi.org/10.1021/jp508136q | J. Phys. Chem. C 2014, 118, 30079−30086

The Journal of Physical Chemistry C

Article

Figure 1. (a) Structure of poly(9,9′-dihexylfluorene-diyl-alt-3,5-bipyridinevinylene): (LaPPS34). (b) Structure of LaPPS34 complexed with europium: (LaPPS34Eu). (c) Structure of model compound Eu(DBM)3bipy: (LaPPS34M).

ps) was used with emission signals collected at λem = 450 nm. The sample decay signal was deconvoluted from the laser pulse signal using a Ludox scatterer. The experimental curves were treated using the F900 software provided by Edinburgh Instruments, by fitting the decays with multiple exponential functions (eq 1) using nonlinear least-squares routines minimizing χ2. Good fits were obtained when χ2 is close to 1.

complexes with polymer matrixes were tested, but in this case, the polymer chains do not participate in the transfer mechanisms and served only as a “mechanical support”.42,43 This is mainly due to the difficulty of finding π-conjugated polymers with adequate energy levels for the excitation of the lanthanides, and at the same time with the proper geometrical configuration of the complexing sites. 2,2′-Bipyridine,44 1,10phenanthroline,45 diketones,46 and carboxylic groups21,45 represent chemical sites where complexation can occur. In this contribution, the photophysical properties, with special emphasis on thermochromism of the system poly[9,9′dihexylfluorene-diyl-alt-3,5-bipyridinevinylene] and its complexes with the europium ion, were studied.

N

F (t ) =

⎛ −t ⎞ ⎟ ⎝ τi ⎠

∑ Bi exp⎜ i=1

(1)

Bi is a pre-exponential factor representing the fractional contribution to decay of the component with a lifetime τi and t is the time. The emission decay in the ms time scale was measured using a multichannel system (FluoroHub-B) with a delay of 5.333 μs. The emission decay curves were obtained with a pulsed 150 W xenon lamp using the multichannel system and 1024 channels. A wavelength excitation of λexc = 350 and 390 nm was used, and the emission signals were collected at λem = 615 nm for both samples LaPPS34M and LaPPS34Eu.

2. EXPERIMENTAL SECTION 2.1. Materials. The preparation of all materials used was published elsewhere.16 Briefly, the copolymers were obtained through the Wittig polycondensation route, using 2,7-bis[(ptriphenylphosphonium)methyl]-9,9′-di-n-hexylfluorene and 2,2′-bipyridine-4,4′-dicarboxaldehyde as monomers. 2.2. Equipment and Measurements. The UV−vis spectra were taken in a Shimadzu model NIR 3101 spectrophotometer. Steady-state fluorescence spectroscopy was performed on a Shimadzu model RF5301-PC spectrophotometer, using a square cuvette of 1 cm. The spectral range was 300−550 nm for the excitation spectra and from 400 to 700 nm for emission spectra. Slits were selected for a spectral resolution of ±1 nm in excitation and emission modes. The DSC equipment used was a Netzsch DSC 204 F1. All samples were heated from 293 to 523 K at a rate of 10 K/min in a nitrogen atmosphere, and then cooled down to 293 at 10 K/min. This procedure was repeated, and the second run was recorded. Fluorescence measurements of all samples were carried out at 170−330 K at increments of 10 K within a closed He-cycle cryosystem from APD Cryogenics, using a SPEX 0.5 ms spectrograph. Fluorescence decays were recorded using time-correlated single photon counting in an Edinburg Analytical Instruments FL 900 spectrofluorimeter with MCP−PMT in Peltier housing, featuring a Hamamatsu R3809U-50. Measurements were performed with a wavelength excitation of λexc = 340 nm (pulsed diode model EPLED-340 with width = 12.5 nm, 840 ps) and λexc = 405 nm (pulsed diode laser model EPL-375 with width = 2.5 nm, 80.0 ps), and the emission signals were collected at λem = 450 nm, for LaPPS34Eu samples. For LaPPS34 samples, a wavelength excitation of λexc = 405 nm (pulsed diode laser model EPL-375 with width = 2.5 nm, 80.0

3. RESULTS AND DISCUSSION 3.1. Characterization. In a previous publication, the synthesis and structural characterization of the alternated fluorene and bipyridine copolymer and of its complex with the europium ion were described, along with the optoelectronic properties.16 In Figure 1a−c, the structure of the noncomplexed backbone (LaPPS34), of the complexed one (LaPPS34Eu), and of the model compound, representing the complexed site of the polymer (LaPPS34M), are displayed. DSC curves of the LaPPS34 and LaPPS34Eu showed the onset of glass transition at about 398 and 448 K, respectively. The end temperature of this same transition was about at 413 K for LaPPS34 and 466 K for the LaPPS34Eu (Figure S1, Supporting Information). The increase of the glass transition temperature indicates that the copolymerization with the europium complex produces an increase of the backbone rigidity. 3.2. Photophysical Characterization. The UV−vis absorption spectra of LaPPS34M (model compound) in THF solution (10−5 mol/L) exhibited a broad band centered at 342 nm assigned to the ligands of the europium complex.47,48 Some absorption from the direct excitation of the ion could be anticipated, but that was not the case. The literature reports that, under some conditions, two sharp peaks around 464 and 531 nm49,50 appear, corresponding to the f → f absorption transitions (7F0 → 5D2 and 7F0,1 → 5D1, respectively). The 30080



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612 nm, which was attributed to the 5D0 → 7F2 transition of the europium emission,55,56 as illustrated in Figure 3. LaPPS34Eu (polyfluorene with the europium complex) showed characteristic emission peaks of both from the polymer (LaPPS34) and from the europium complex (LaPPS34M). The emission arising of the fluorene backbone is red-shifted in comparison with the conjugated polymer (LaPPS34), suggesting that there is a planarization of the backbone and an increase of the average chain conjugation distance. The emissions of all compounds are independent of the excitation wavelengths (Figure 3). Fluorescence lifetimes were also determined for the compounds dissolved in THF solution (10−3 mol/L) at room temperature and at 77 K. Monoexponential fluorescence decays were observed for all samples, with a lifetime at room temperature shorter than those measured at 77 K. The emission lifetimes for europium in both materials (LaPPS34M and LaPPS34Eu) were 0.14 ms at room temperature and 0.36 ms at 77 K, showing that they are not influenced by the presence of a polymeric chain. The decay curves are presented in the Supporting Information, labeled as S2−S15. As a general trend, the emission decay of the polymer chain is in the nanosecond time scale. Comparing the lifetimes obtained for the polymer (τ = 3.89 ns) and for the polymer with the complex (τ = 3.16 ns), one can see that they are similar (Table 1). This means that the disturbance of the polymer photophysics by the complex is very small, suggesting that they behave almost independently.

absence of those peaks was ascribed to the fact that, under direct excitation, the absorption of the ion was much weaker in comparison with that of the ligands, and could not be detected. The LaPPS34 (polymer backbone without complexed sites) spectrum showed two bands centered at 386 and 406 nm, with a typical profile of polyfluorenes.51−53 The absorption of the complexed copolymer (LaPPS34Eu) is the sum of the corresponding spectra of individual components (LaPPS34 and LaPPS34M), as clearly seen in Figure 2.

Figure 2. Normalized absorption spectra of LaPPS34 (blue line) related to fluorene, of LaPPS34M (black line) related to the ligands of the model compound, and LaPPS34Eu (red line) in THF solutions (10−5 mol/L).

Table 1. Some Photoemission Properties of the Polymer (LaPPS34), the Model Compound (LaPPS34M), and the Polymer with the Complex (LaPPS34Eu) in THF Solutions (10−3 mol/L)

Steady-state fluorescence spectra in a solution of THF at room temperature showed that the LaPPS34 (noncomplexed polymer) emits in the range of 400−550 nm (Figure 3), with two well-resolved vibronic peaks at 422 and 446 nm, characteristic of polyfluorene derivatives.51,54 Regarding the LaPPS34M (europium complex - model compound), the lines were distributed mainly in the 570−640 nm range, which were associated with the 4f → 4f transitions of the 5D0 excited state to 7F0−2 levels of the europium ion, with the strongest peak at

sample

T (K)

λexc (nm)

λem (nm)

LaPPS34M

R.T. 77 R.T. 77 R.T. 77 R.T. 77 R.T. 77 R.T. 77 R.T. 77

350 350 390 390 350 350 390 390 340 340 405 405 405 405

615 615 615 615 615 615 615 615 450 450 450 450 450 450

LaPPS34Eu

LaPPS34

τ 0.14 0.36 0.14 0.36 0.14 0.36 0.14 0.36 2.30 3.03 2.24 3.16 2.36 3.89

χ2 ms ms ms ms ms ms ms ms ns ns ns ns ns ns

0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.989 0.979 0.989 0.989 0.986 0.997

R.T.: room temperature. χ2 shows the quality of the fitting of the decay curve.

The fluorescence spectra of LaPPS34, LaPPS34M, and LaPPS34Eu in THF solutions (10−3 mol/L) were recorded for every 10 K from 170−330 K during sample heating (Figure 4). All data were obtained in dilute THF solutions (10−3 mol/L) in order to avoid additional complexities that arise from solid-state conditions. Among those, the most significant is the macromolecular interactions that arise from chain proximity. In the liquid state, the interchain interactions are usually very weak so that the energy difference between LUMO and HOMO is mainly determined by intrachain interactions (effective conjugation length). In the solid state, the interchain interactions become, however, so strong that the energy

Figure 3. Emission spectra of the copolymers and model compound in THF solution (10−3 mol/L), excitation wavelengths at maximum absorption of the noncomplexed backbone (λexc = 390 nm), and of the europium complex (λexc = 350 nm). 30081



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from 430 to 560 nm, for both the complexed (LaPPS34) and noncomplexed (LaPPS34Eu) forms, was measured, as shown in Figure 5a. Additionally, to describe the temperature depend-

Figure 5. Normalized and integrated fluorescence intensity versus temperature of LaPPS34Eu (red square), LaPPS34 (blue triangle), and LaPPS34M (black circle) in THF solutions (10−3 mol/L). (a) 430− 560 nm and (b) 570−650 nm spectral ranges.

ence of the europium emission band, the intensity was defined as the integrated area of the emission bands, from 570 to 650 nm, for both the complexed form (LaPPS34Eu) and the model low molecular weight (LaPPS34M), as shown in Figure 5b. The first observation from Figure 5a related to the temperature dependence of the emission of the noncomplexed and complexed forms is that they have different profiles. In the former, the polymer deactivation is thermally activated (the intensity decreases with the increase of the temperature), and in the latter, the emission intensity is practically temperatureindependent. In general, the deactivation process in conjugated polymers is induced by torsional motions of the polymer backbone.63 The hindering of these motions brings about a decrease in the efficiency of the electron−phonon coupling, inhibiting the temperature dependence of the nonradiative pathways responsible by the thermal deactivation of the electronic excited state. In general, it has been proposed that the electron−phonon couplings in conjugated polymers are responsible by their deactivation and that this process is greatly improved when torsional optical modes are thermally

−3

Figure 4. Emission spectra, in THF solution (10 mol/L), as a function of temperature (170−330 K): (a) noncomplexed backbone (LaPPS34), λexc = 400 nm, (b) model compound (LaPPS34M) representing the complexed sites, λexc = 350 nm, (c) complexed polymer (LaPPS34Eu), λexc = 350 nm.

difference between the LUMO and HOMO depends not only on intrachain interactions but also on interchain interactions.57−59 As a general trend, there is an intensity decrease as the temperature increases and this behavior is controlled by two sets of properties: some were intrinsically dependent on the photophysical properties of the lumophore, including the relaxation process of the polymer chain, and some were dependent on the extrinsic properties, that is, the properties of the surrounding medium.60−62 In order to describe the temperature dependence of the fluorescence intensity, the intensity of the integrated area of the entire emission band corresponding to the polymer emission, 30082



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Figure 6. (a) Representation of the frontier orbitals of an oligomer of LaPPS34Eu with an arbitrary torsion, simulating the conformation necessary for complexation of a unity of bipyridine, forming (DBM)3bipy. (b) Representation of the frontier orbitals of an oligomer of the noncomplexed polymer, LaPPS34.

induced.61,64,65 Therefore, although the backbone and the complex are apparently not interacting, the chain conformation is disturbed by the complex, inducing changes in the electron− phonon coupling and, consequently, influencing also the temperature dependence of the emission band. The emission intensity from the europium site (570−650 nm), either in the “free” state (LaPPS34M) or as inserted via complexation onto the polymer backbone (LaPPS34Eu), decreases with temperature increases, as usual.35,38 These two curves (integrated intensity band versus temperature) presented in Figure 5b fit each other due to the noninfluence of polymeric chain in europium emission, as shown previously by the lifetime results. At lower temperatures, the europium emission turns as intense as that of the backbone (see Figure 4b). It is noteworthy that the insertion of the ion precludes the interchain aggregation, probably to induced chain planarization, as noted, and demonstrated previously.16 This brings about a well-defined emission of the fluorene peak at around 450 nm. The discussion of this effect cannot be done by comparing to literature data, since no similar phenomenon was found, nor described or mentioned. Several approaches will then be addressed as attempts to interpret the effects observed. The degree of insertion was 80% (molar basis).16 Therefore, the presence of the complex at each side of the fluorene moiety is very like hindering local relaxations or vibrations that, in turn, will preclude electron−phonon interactions. As a matter of fact, the rise in the glass transition temperature from the noncomplexed to the complexed material was from 398 to 448 K, indicating that substantial chain stiffening had occurred. In the same direction, the spectral red-shift also confirms the planarization of the polymer backbone, and consequently favors the chain rigidity.

As noted, the polymeric chain does not have any influence on europium properties, and consequently, the maximum wavelength emission is the same with or without the presence of the polyfluorene derivative, according to Figure S16b (Supporting Information). The presence of the ion, however, changes the emission λmax of the polymer: the material containing europium showed a spectral blue-shift as the temperature increases (LaPPS34Eu, Figure S16a), and a spectral blue-shift in the absence of the complex (LaPPS34, Figure S16a). There are several parameters that can explain the spectral shifts with the temperature. One parameter is conjugation length. When the conjugation length is longer, the emission band is more redshifted. The other parameter is related to the electron−phonon coupling.66−68 In the polymer, there is a remarkable red-shift of the emission band, which is a usual behavior for the polyfluorenes, and it is associated with the relaxation processes of the polymer chain and due to the electron−phonon coupling between the electronic excited state and the vibrational modes of the polymer chains. In other words, the decay of the excited state is originated from more relaxed Franck−Condon states. On the other hand, in the polymer chain with the complex, there is a blue-shift of the spectrum as the temperature increases. These more planar and hindered chains have their conjugated length decreasing at higher temperatures because of the thermally induced torsional movements. This shortening of the conjugated lengths increases the band gap, and the spectrum is blue-shifted. At the same time, apart from the rigidity imposed by the bulkiness of the complexed site and the high degree of ion insertion (80% molar), one may consider that the excited state of the complexed copolymer differs from the noncomplexed, even if the chromophores are not electronically coupled, as shown by the time decays, due to the rotation required 30083



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(179°)16 to the second pyridine ring, to afford the proper geometry for the europium insertion. In order to determine the electronic structure of the polymers, a series of quantum mechanical calculations were performed using DFT methods, based on the B3LYP functional and 6-311g(d,p) basis set. Initially, a geometry optimization of the LaPPS34 tetramer was performed to determine its ground state and electronic levels. Based on this geometry, an arbitrary torsion to 0° was made in one of the NCCN dihedral angles to simulate the coordination to Eu3+ in the LaPPS34Eu, and based on this geometry, single point calculations were performed. The results of theoretical simulations showed that the electronic delocalization at the LUMO level of the distorted configuration of the backbone, that is, the one in which the second pyridine ring is rotated 179° to allow for the complexation, is more delocalized than the original one, in which the nitrogen atoms are in opposite directions, as shown in Figure 6. This result strengthens the assumption of the increased rigidity of the complexed copolymer, thus decreasing the electron−phonon interactions bringing about the elimination of the nonradiative channels, even at higher temperature conditions (330 K).69 Apart from the assignment of the optical mechanism operating in the so far described phenomena, it is noteworthy the forecast application of LaPPS34Eu in nanothermometry. The reported methods to sense temperature variations in the scale of metabolic activity comprise nanotechnology based systems using rare earth as quantum dots,70,71 or fluorescent probes in the form of nanoparticles,72 among others. In every case, a calibration has to be made, using an invariant emission as standard, leading to a rather complex set of photophysical pathways. The steady intensity of the fluorene emission in the present case is a “built-in” standard for this purpose, and thus will not require any other component to be added for the construction of a nanothermometer. Along with further studies to probe the photophysics of the series LaPPS34, its application in nanothermometry is under current investigation.

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ASSOCIATED CONTENT

S Supporting Information *

Decay curves and DSC analyses of materials. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +554130270650 (L.C.A.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge INEO (National Institute for Organic Electronics), CNPQ (National Research Council) for financial support and CCAD (High Performance Computing Center) for computational facilities.

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ABBREVIATIONS LaPPS34, poly[9,9′-dihexylfluorene-diyl-alt-3,5-bipyridinevinylene]; LaPPS34Eu, poly[9,9′-dihexylfluorene-diyl-alt-3,5-bipyridinevinylene] complexed with Eu(DBM)3 in the bipyridine sites; LaPPS34M, Eu(DBM)3bipy; THF, tetrahydrofuran

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REFERENCES

(1) Wang, H.; Xu, Y.; Tsuboi, T.; Xu, H.; Wu, Y.; Zhang, Z.; Miao, Y.; Hao, Y.; Liu, X.; Xu, B.; et al. Energy Transfer in Polyfluorene Copolymer Used for White-Light Organic Light Emitting Device. Org. Electron. 2013, 14, 827−838. (2) Stanley, J. M.; Holliday, B. J. Luminescent Lanthanide-Containing Metallopolymers. Coord. Chem. Rev. 2012, 256, 1520−1530. (3) Graffion, J.; Cattoën, X.; Wong Chi Man, M.; Fernandes, V. R.; André, P. S.; Ferreira, R. A. S.; Carlos, L. D. Modulating the Photoluminescence of Bridged Silsesquioxanes Incorporating Eu3+Complexed N,N′-Diureido-2,2′-bipyridine Isomers: Application for Luminescent Solar Concentrators. Chem. Mater. 2011, 23, 4773−4782. (4) Ho, C.-L.; Wong, W.-Y. Charge and Energy Transfers in Functional Metallophosphors and Metallopolyynes. Coord. Chem. Rev. 2013, 257, 1614−1649. (5) Jaque, D.; Vetrone, F. Luminescence Nanothermometry. Nanoscale 2012, 4, 4301−4326. (6) Kim, T. H.; Shin, Y. W.; Jung, J. H.; Kim, J. S.; Kim, J. Crystal-toCrystal Transformation between Three CuI Coordination Polymers and Structural Evidence for Luminescence Thermochromism. Angew. Chem., Int. Ed. 2008, 47, 685−688. (7) Brites, C. D. S.; Lima, P. P.; Silva, N. J. O.; Millan, A.; Amaral, V. S.; Palacio, F.; Carlos, L. D. Lanthanide-Based Luminescent Molecular Thermometers. New J. Chem. 2011, 35, 1177−1183. (8) Kristensen, P. K.; Pedersen, T. G.; Zhu, K.; Yu, D. Energy Transfer from Polyfluorene Based Polymer to Europium. Eur. Phys. J.: Appl. Phys. 2007, 59, 57−59. (9) Liu, Y.; Li, J.; Li, C.; Song, J.; Zhang, Y.; Peng, J.; Wang, X.; Zhu, M.; Cao, Y.; Zhu, W. Highly Efficient Sharp Red Electroluminescence from Europium. Chem. Phys. Lett. 2007, 433, 331−334. (10) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. Narrow Bandwidth Luminescence from Blends with Energy Transfer from Semiconducting Conjugated Polymers to Europium Complexes. Adv. Mater. 1999, 11, 1349−1354. (11) Peng, J.; Takada, N.; Minami, N. Red Electroluminescence of a Europium Complex Dispersed in Poly(N-vinylcarbazole). Thin Solid Films 2002, 405, 224−227.

4. CONCLUSIONS The europium insertion in the pyridine sites of poly(9,9′dihexylfluorene-diyl-alt-3,5-bipyridinevinylene) in almost all the repeating units (80% molar) brought about a thermochromism in the range of 170−330 K that can be described as a strong red color emission at low temperatures (170−260 K) and a blue one at higher ones (280−330 K). The most striking feature of this system is that the europium ion behaves as the usual chromophores do. That is, the emission intensity decreases with temperature increases, due to the activation of nonradiative channels, but the polymer backbone becomes insensitive to temperature variations. This effect was interpreted as a strong stiffening of the polymer chain that hindered electron−phonon interactions. The stiffening was due to the rotation of 179° of one pyridine ring necessary to afford the adequate configuration for the Eu3+ ion insertion and also to the bulkiness of the ion and its ligands hindering chain movements. The most important conclusion that can be drawn is that “anomalous” invariance of emission intensity with temperature can be utilized for the construction of nanothermometers, without the need of a standard, since the polymer itself would be used as a “built-in” standard. 30084



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A Photophysical Interpretation of the Thermochromism of a

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