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.118513007930086Wang, H., Xu, Y., Tsuboi, T., Xu, H., Wu, Y., Zhang, Z., Miao, Y., Xu, B., Energy Transfer in Polyfluorene Copolymer Used for White-Light Organic Light Emitting Device (2013) Org. Electron., 14, pp. 827-838Stanley, J.M., Holliday, B.J., Luminescent Lanthanide-Containing Metallopolymers (2012) Coord. Chem. Rev., 256, pp. 1520-1530Graffion, 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 β-Diureido-2,2β-bipyridine Isomers: Application for Luminescent Solar Concentrators (2011) Chem. Mater., 23, pp. 4773-4782Ho, C.-L., Wong, W.-Y., Charge and Energy Transfers in Functional Metallophosphors and Metallopolyynes (2013) Coord. Chem. Rev., 257, pp. 1614-1649Jaque, D., Vetrone, F., Luminescence Nanothermometry (2012) Nanoscale, 4, pp. 4301-4326Kim, T.H., Shin, Y.W., Jung, J.H., Kim, J.S., Kim, J., Crystal-to-Crystal Transformation between Three CuI Coordination Polymers and Structural Evidence for Luminescence Thermochromism (2008) Angew. Chem., Int. Ed., 47, pp. 685-688Brites, 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 (2011) New J. Chem., 35, pp. 1177-1183Kristensen, P.K., Pedersen, T.G., Zhu, K., Yu, D., Energy Transfer from Polyfluorene Based Polymer to Europium (2007) Eur. Phys. J.: Appl. Phys., 59, pp. 57-59Liu, Y., Li, J., Li, C., Song, J., Zhang, Y., Peng, J., Wang, X., Zhu, W., Highly Efficient Sharp Red Electroluminescence from Europium (2007) Chem. Phys. Lett., 433, pp. 331-334McGehee, 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 (1999) Adv. Mater., 11, pp. 1349-1354Peng, J., Takada, N., Minami, N., Red Electroluminescence of a Europium Complex Dispersed in Poly(N -vinylcarbazole) (2002) Thin Solid Films, 405, pp. 224-227Chan, C.-M., Chan, M.-Y., Zhang, M., Lo, W., Wong, K.-Y., The Performance of Oxygen Sensing Films with Ruthenium-Adsorbed Fumed Silica Dispersed in Silicone Rubber (1999) Analyst, 124, pp. 691-694Chen, X.-Y., Yang, X., Holliday, B.J., Photoluminescent Europium-Containing Inner Sphere Conducting Metallopolymer (2008) J. Am. Chem. Soc., 130, pp. 1546-1547Bettencourt-Dias, A., Lanthanide-Based Emitting Materials in Light-Emitting Diodes (2007) Dalton. Trans, pp. 2229-2241Ho, C., Wong, W., Metal-Containing Polymers: Facile Tuning of Photophysical Traits and Emerging Applications in Organic Electronics and Photonics (2011) Coord. Chem. Rev., 255, pp. 2496-2502Turchetti, D.A., Rodrigues, P.C., Berlim, L.S., Zanlorenzi, C., Faria, G.C., Atvars, T.D.Z., Schreiner, W.H., Akcelrud, L.C., Photophysical Properties of a Fluorene-Bipyridine Copolymer and Its Complexes with Europium (2012) Synth. Met., 162, pp. 35-43Babudri, F., Farinola, G.M., Naso, F., Synthesis of Conjugated Oligomers and Polymers: The Organometallic Way (2004) J. Mater. Chem., 14, pp. 11-34Tian, L., Zhang, W., Yang, B., Lu, P., Zhang, M., Lu, D., Ma, Y., Shen, J., Zinc(II)-Induced Color-Tunable Fluorescence Emission in the -Conjugated Polymers Composed of the Bipyridine Unit: A Way to Get White-Light Emission (2005) J. Phys. Chem. B, 109, pp. 6944-6947Ling, Q.D., Kang, E.T., Neoh, K.G., Huang, W., Synthesis and Nearly Monochromatic Photoluminescence Properties of Conjugated Copolymers Containing Fluorene and Rare Earth Complexes (2003) Macromolecules, 36, pp. 6995-7003Luo, J., Yang, C., Zheng, J., Ma, J., Liang, L., Lu, M., Synthesis and Photophysics Properties of Novel Bipolar Copolymers Containing Quinoline Aluminum Moieties and Carbazole Segments (2011) Eur. Polym. J., 47, pp. 385-393Wang, L.-H., Wang, W., Zhang, W.-G., Kang, E.-T., Huang, W., Synthesis and Luminescence Properties of Novel Eu-Containing Copolymers Consisting of Eu(III)-Acrylate-Îβ-Diketonate Complex Monomers and Methyl Methacrylate (2000) Chem. Mater., 12, pp. 2212-2218Ling, Q., Yang, M., Wu, Z., Zhang, X., Wang, L., Zhang, W., A Novel High Photoluminescence Effciency Polymer Incorporated with Pendant Europium Complexes (2001) Polymer, 42, pp. 4605-4610Complex, E.L., Nonat, A., Regueiro-Figueroa, M., Esteban-Gómez, D., De Blas, A., Definition of an Intramolecular Eu-to-Eu Energy Transfer within a Discrete [Eu2L] Complex in Solution (2012) Chem. - Eur. J., 18, pp. 8163-8173Souza, E.R., Sigoli, F.A., Princípios Fundamentais e Modelos de Transferência de Energia Inter e Intramolecular (2012) Quim. Nova, 35, pp. 1841-1847Qin, L., Zhu, Y., Yang, H., Ding, L., Sun, F., Shi, M., Yang, S., White-Light Phosphorescence from Binary Coordination Polymer Nanoparticles (2013) Mater. Chem. Phys., 139, pp. 345-349Ley, K.D., Schanze, K.S., Photophysics of Metal-Organic -Conjugated Polymers (1997) Coord. Chem. Rev., 171, pp. 287-307Cai, Q.J., Ling, Q.D., Li, S., Zhu, F.R., Huang, W., Kang, E.T., Neoh, K.G., Chemical States and Electronic Properties of the Interface between Aluminium and a Photoluminescent Conjugated Copolymer Containing Europium Complex (2004) Appl. Surf. Sci., 222, pp. 399-408Ma, D., Lu, K., Guo, H., Pan, Y., Liu, J., Controlled Syntheses, Structures and Photoluminescence of Two Europium Coordination Polymers Based on 2,4-Dcp (2,4-Dichlorophenoxyacetate) and 4,4β-Bpy (4,4β-Bipyridine) Ligands (2012) J. Mol. Struct., 1021, pp. 179-186Shao, G., Zhang, N., Lin, D., Feng, K., Cao, R., Gong, M., A New Europium(III)-Îβ-Diketonate Complex Based on Diphenylethyne as Red Phosphors Applied in LED (2013) J. Lumin., 138, pp. 195-200Yip, Y.-W., Wen, H., Wong, W.-T., Tanner, P.A., Wong, K.-L., Increased Antenna Effect of the Lanthanide Complexes by Control of a Number of Terdentate N-Donor Pyridine Ligands (2012) Inorg. Chem., 51, pp. 7013-7015Shunmugam, R., Tew, G.N., Dialing in Color with Rare Earth Metals: Facile Photoluminescent Production of True White Light (2007) Polym. Adv. Technol., 18, pp. 940-945Yang, C., Xu, J., Ma, J., Zhu, D., Zhang, Y., Liang, L., Lu, M., The Effect of Two Additional Eu3+ Lumophors in Two Novel Trinuclear Europium Complexes on Their Photoluminescent Properties (2013) Photochem. Photobiol. Sci., 12, pp. 330-338Stanimirov, S.S., Hadjichristov, G.B., Petkov, I.K., Emission Efficiency of Diamine Derivatives of Tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono]europium (2007) Spectrochim. Acta, Part A, 67, pp. 1326-1332Kumar, R., Makrandi, J.K., Singh, I., Khatkar, S.P., Preparation and Photoluminescent Properties of Europium Complexes with Methoxy Derivatives of 2β-Hydroxy-2-phenylacetophenones (2008) J. Lumin., 128, pp. 1297-1302Lopez, I.S., Luisa Mendonća, A., Fernandes, M., Bermudez, V., De, Z., Morgado, J., Del Pozo, G., Cabanillas-Gonzalez, J., Europium Complex-Based Thermochromic Sensor for Integration in Plastic Optical Fibres (2012) Opt. Mater. (Amsterdam, Neth.), 34, pp. 1447-1450Boilot, J.P., Gacoin, T., Perruchas, S., Synthesis and Sol-Gel Assembly of Nanophosphors (2010) C. R. Chim., 13, pp. 186-198Basu, B.B.J., Vasantharajan, N., Temperature Dependence of the Luminescence Lifetime of a Europium Complex Immobilized in Different Polymer Matrices (2008) J. Lumin., 128, pp. 1701-1708Balamurugan, A., Reddy, M.L.P., Jayakannan, M., -Conjugated Polymer-Eu3+ Complexes: Versatile Luminescent Molecular Probes for Temperature Sensing (2013) J. Mater. Chem. A, 1, pp. 2256-2266Cui, Y., Xu, H., Yue, Y., Guo, Z., Yu, J., Chen, Z., Gao, J., Chen, B., A Luminescent Mixed-Lanthanide Metal-Organic Framework Thermometer (2012) J. Am. Chem. Soc., 134, pp. 3979-3982Brites, C.D.S., Lima, P.P., Silva, N.J.O., Millán, A., Amaral, V.S., Palacio, F., Carlos, L.D., A Luminescent Molecular Thermometer for Long-Term Absolute Temperature Measurements at the Nanoscale (2010) Adv. Mater., 22, pp. 4499-4504Borisov, S.M., Wolfbeis, O.S., Temperature-Sensitive Europium(III) Probes and Their Use for Simultaneous Luminescent Sensing of Temperature and Oxygen (2006) Anal. Chem., 78, pp. 5094-5101Chem, J.M., Xie, Z., Xu, H., Taubert, A., A Transparent, Flexible, Ion Conductive, and Luminescent PMMA Ionogel Based on a Pt/Eu Bimetallic Complex and the Ionic Liquid [Bmim][N(Tf)2] (2012) J. Mater. Chem., 22, pp. 8110-8116Sato, T., Higuchi, M., A Vapoluminescent Eu-Based Metallo-Supramolecular Polymer (2012) Chem. Commun., 48, pp. 4947-4949Pei, J., Liu, X., Yu, W., Lai, Y., Niu, Y., Efficient Energy Transfer to Achieve Narrow Bandwidth Red Emission from Eu3+-Grafting Conjugated Polymers (2002) Macromolecules, 35, pp. 7274-7280Ritchie, J., Ruseckas, A., André, P., Münther, C., Van Ryssen, M., Vize, D.E., Crayston, J., Samuel, I.D.W., Synthesis and Lanthanide-Sensing Behaviour of Polyfluorene/1,10-Phenanthroline Copolymers (2009) Synth. Met., 159, pp. 583-588Xu, S., Evans, R.E., Liu, T., Zhang, G., Demas, J.N., Trindle, C.O., Fraser, C.L., Aromatic Difluoroboron Îβ-Diketonate Complexes: Effects of -Conjugation and Media on Optical Properties (2013) Inorg. Chem., 52, pp. 3597-3610Lima, P.P., Junior, S.A., Carlos, L.D., Ferreira, R.A.S., Pavithran, R., Reddy, M.L.P., Synthesis, Characterization, and Luminescence Properties of Eu3+ 3-Phenyl-4-(4-toluoyl)-5-isoxazolonate Based Organic-Inorganic Hybrids (2006) Eur. J. Inorg. Chem., pp. 3923-3929Carlos, L.D., Ferreira, R.A.S., Bermudez, V.D.Z., Ribeiro, S.J.L., Lanthanide-Containing Light-Emitting Organic-Inorganic Hybrids: A Bet on the Future (2009) Adv. Mater., 21, pp. 509-534Lima, P.P., Nolasco, M.M., Paz, F.A.A., Ferreira, R.A.S., Longo, R.L., Malta, O.L., Carlos, L.D., Photo-Click Chemistry to Design Highly Efficient Lanthanide Îβ-Diketonate Complexes Stable under UV Irradiation (2013) Chem. Mater., 25, pp. 586-598Pacold, J.I., Tatum, D.S., Seidler, G.T., Raymond, K.N., Zhang, X., Stickrath, A.B., Mortensen, D.R., Direct Observation of 4f Intrashell Excitation in Luminescent Eu Complexes by Time-Resolved X-ray Absorption Near Edge Spectroscopy (2014) J. Am. Chem. Soc., 136, pp. 4186-4191Nowacki, B., Iamazaki, E., Cirpan, A., Karasz, F., Atvars, T.D.Z., Akcelrud, L., Highly Efficient Polymer Blends from a Polyfluorene Derivative and PVK for LEDs (2009) Polymer, 50, pp. 6057-6064Chen, X., Wan, H., Li, H., Cheng, F., Ding, J., Yao, B., Xie, Z., Zhang, J., Influence of Thermal Annealing Temperature on Electro-optical Properties of Polyoctylfluorene Thin Film: Enhancement of Luminescence by Self-Doping Effect of Low-Content α Phase Crystallites (2012) Polymer, 53, pp. 3827-3832Quites, F.J., Domingues, R.A., Ferbonink, G.F., Nome, R.A., Atvars, T.D.Z., Facile Control of System-Bath Interactions and the Formation of Crystalline Phases of Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(9,9-di-{5β-pentanyl}-fluorenyl-2,7-diyl)] in Silicone-Based Polymer Hosts (2013) Eur. Polym. J., 49, pp. 693-705Song, H.J., Kim, D.H., Lee, T.H., Moon, D.K., Emission Color Tuning of Copolymers Containing Polyfluorene, Benzothiadiazole, Porphyrin Derivatives (2012) Eur. Polym. J., 48, pp. 1485-1494Naveen, K., Sivaiah, K., Buddhudu, S., Structural, Thermal and Optical Properties of Tb3+, Eu3+ and Co-Doped (Tb3++Eu3+): PEO+PVP Polymer Films (2014) J. Lumin., 147, pp. 316-323Li, J., Song, F., Wang, L., Jiao, J., Cheng, Y., Zhu, C., Excitation Induced Emission Color Change Based on Eu(III)-Zn(II)-Containing Polymer Complex (2012) Macromol. Rapid Commun., 33, pp. 1268-1272Assaka, A.M., Rodrigues, P.C., Oliveira, A.R.M.D., Ding, L., Hu, B., Karasz, F.E., Akcelrud, L., Novel Fluorine Containing Polyfluorenes with Efficient Blue Electroluminescence (2004) Polymer, 45, pp. 7071-7081Hu, B., Zhang, X., Zhou, Y., Jin, C., Zhang, J., Pressure Dependence of the Photoluminescence of Polyparaphenylene (1991) Phys. Rev. B, 43, pp. 14001-14008Akcelrud, L., Electroluminescent Polymer Systems (2007) Physical Properties of Polymers Handbook, pp. 757-782. , Mark, J. E. Springer: Cincinnati, OHAtvars, T.D.Z., Abraham, S., Hill, A.J., Pas, S.J., Chesta, C., Weiss, R.G., Modulation of the Photophysical Properties of Pyrene by the Microstructures of Five Poly(alkyl methacrylate)s over a Broad Temperature Range (2011) Photochem. Photobiol., 89, pp. 1346-1353Domingues, R.A., Yoshida, I.V.P., Atvars, T.D.Z., Synthesis, Photophysical Properties and Thermal Relaxation Processes of Carbazolyl-Labeled Polysiloxanes (2011) J. Photochem. Photobiol., A, 217, pp. 347-355Faria, G.C., Plivelic, T.S., Cossiello, R.F., Souza, A.A., Atvars, T.D.Z., Torriani, I.L., De Azevedo, E.R., A Multitechnique Study of Structure and Dynamics of Polyfluorene Cast Films and the Influence on Their Photoluminescence (2009) J. Phys. Chem. B, 113, pp. 11403-11413Hagler, T., Pakbaz, K., Voss, K., Heeger, A., Enhanced Order and Electronic Delocalization in Conjugated Polymers Oriented by Gel Processing in Polyethylene (1991) Phys. Rev. B, 44, pp. 8652-8666Balzer, F., Pogantsch, A., Rubahn, H.-G., Temperature Dependent Analysis of Three Classes of Fluorescence Spectra from P-6P Nanofiber Films (2009) J. Lumin., 129, pp. 784-789Peterman, E.J.G., Pullerits, T., Van Grondelle, R., Van Amerongen, H., Electron-Phonon Coupling and Vibronic Fine Structure of Light-Harvesting Complex II of Green Plants: Temperature Dependent Absorption and High-Resolution Fluorescence Spectroscopy (1997) J. Phys. Chem. B, 101, pp. 4448-4457Renge, I., Thermal Effects on Zero-Phonon Holes in the Optical Spectra of Molecular Probes in Polymer Glasses (2003) Phys. Rev. B, 68, p. 064205Guha, S., Rice, J., Yau, Y., Martin, C., Chandrasekhar, M., Chandrasekhar, H., Guentner, R., Scherf, U., Temperature-Dependent Photoluminescence of Organic Semiconductors with Varying Backbone Conformation (2003) Phys. Rev. B, 67, p. 125204Huang, K., Rhys, A., Theory of Light Absorption and Non-Radiative Transitions in F-Centres (1950) Proc. R. Soc. A, 204, pp. 406-423Gettinger, C.L., Heeger, A.J., Drake, J.M., Pine, D.J., The Effect of Intrinsic Rigidity on the Optical Properties of PPV Derivatives (1994) Mol. Cryst. Liq. Cryst., 256, pp. 507-512Yang, J.-M., Yang, H., Lin, L., Quantum Dot Nano Thermometers Reveal Heterogeneous Local Thermogenesis in Living Cells (2011) ACS Nano, 5, pp. 5067-5071Brites, C.D.S., Lima, P.P., Silva, N.J.O., Millan, A., Amaral, V.S., Palacio, F., Carlos, L.D., Thermometry at the Nanoscale (2012) Nanoscale, 4, pp. 4799-4829Peng, H., Stich, M.I.J., Yu, J., Sun, L., Fischer, L.H., Wolfbeis, O.S., Luminescent Europium(III) Nanoparticles for Sensing and Imaging of Temperature in the Physiological Range (2010) Adv. Mater., 22, pp. 716-719