Research Article www.advopticalmat.de Tunable Thermofluorochromic Sensors Based on Conjugated Polymers Christian Bellacanzone, Jaume Ramon Otaegui, Jordi Hernando, Daniel Ruiz-Molina,* and Claudio Roscini* can be detected remotely with highly Even though thermofluorochromic materials are eternal candidates for their sensitive and fast responsive techniques use in multiple applications, they are still limited as they require complex and offers time and spatial resolution at the macro- (e.g., luminescent labels) and synthetic strategies to accomplish tunable optical properties and/or provide nanoscale (e.g., fluorescence microscopy). optical changes only over a very wide temperature range. By taking advan- Inherent thermofluorochromic mate- tage of the high sensitivity of the optical properties of conjugated polymers rials involve thermoresponsive molecular and oligomers to the external environment, herein phase change material dyes,[14–20] quantum dots, and lanthanide- (PCM)-based thermofluorochromic mixtures are created, where the solid-to- doped nanoparticles,[21] conjugated poly- liquid transition of the PCM host triggers a sharp fluorescence color change mers,[22] or polymorphic dyes.[23,24,25] Even though several reports of the materials of the dispersed polymers/oligomers. Fluorophore conjugation length, are available nowadays, their wide range concentration, and PCM nature can be used to vary the spectral properties of temperature responses and limited of the resulting materials along the visible region, covering a large part of the tunability of the color emission and/or CIE 1931 color space. For the preparation of functional devices, this behavior switching temperature still hamper their can be directly transferred to the solid state by soaking or printing cellulose complete implementation in commercial applications. papers with the obtained thermofluorochromic mixtures as well as by struc- This is the case of the solid films and turing them into solid lipid particles that can be dispersed within polymer particles of conjugated polymers,[26] matrices. The resulting materials show very promising features as thermal exhibiting relevant thermochromic sensors and anticounterfeiting labels. (e.g., polydiacetylene[27–31] and polythio- phenes[3,32–37]) and thermofluorochromicc (e.g., poly(phenylenevinylene))[38–41] prop- 1. Introduction erties with good emission efficiencies in the visible range. Even so, their implementation in thermal sensors has yet to explode Thermoresponsive optical materials are increasingly gaining due to their gradual optical changes over very broad tempera- interest in applications such as optical thermometers, diag- ture ranges, mainly due to continuous variation of their con- nostic tools, optoelectronic devices, and anticounterfeiting formation, aggregation degree, and/or crystalline phase with markers, among others.[1–13] Specially relevant are those whose temperature affecting their effective conjugation length. On emission can be controlled with temperature, as luminescence top of that, the fine-tuning of the emitted colors and switching temperature is intrinsically related to the chemical nature of the C. Bellacanzone, J. R. Otaegui, D. Ruiz-Molina, C. Roscini polymer so it requires time consuming and not cost-effective Catalan Institute of Nanoscience and Nanotechnology (ICN2) chemical modifications of the polymer backbone and/or the CSIC and The Barcelona Institute of Science and Technology (BIST) pending functional side groups. Campus UAB, Bellaterra, Barcelona 08193, Spain Herein, we report a novel material design to accomplish E-mail: dani.ruiz@icn2.cat; claudio.roscini@icn2.cat highly tunable dual-color thermofluorochromism in a broad J. R. Otaegui, J. Hernando region of the 1931 CIE color space. On the one hand, we aim Departament de Química Universitat Autònoma de Barcelona to employ conjugated polymers as emitters, which are emis- Cerdanyola del Vallès 08193, Spain sive both in homogeneous solutions and in the aggregated state The ORCID identification number(s) for the author(s) of this article and, more importantly, show fluorescence properties that are can be found under https://doi.org/10.1002/adom.202102423. highly dependent on the surrounding medium. On the other © 2022 The Authors. Advanced Optical Materials published by hand, we propose the use of phase change materials (PCM; Wiley-VCH GmbH. This is an open access article under the terms of e.g., polymers waxes) as thermally responsive surrounding the Creative Commons Attribution-NonCommercial-NoDerivs License, matrices. Phase change media (e.g., polymers and paraf- which permits use and distribution in any medium, provided the original fins) have already been used to develop thermofluorochromic work is properly cited, the use is non-commercial and no modifications or adaptations are made. materials, even for nonintrinsically thermoresponsive dyes or particles.[2,42–58] Paraffinic and nonparaffinic PCMs are of par- DOI: 10.1002/adom.202102423 ticular interest as their sharp and easily tunable solid–liquid Adv. Optical Mater. 2022, 10, 2102423 2102423 (1 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de solutions, films and particles exhibit largely redshifted and broader emission spectra with bands at λmax ≈600 nm (and a weaker shoulder at 640 nm, Figure 1b), which are ascribed to polymer aggregates or other interchain species (e.g., exciplexes, charge-separated polaron pairs, etc.).[77–80] And third, shorter oli- gomeric species of different conjugation length and aggregation- dependent luminescence[45,56,57] can be used for a control of the fluorescence properties along the whole visible range, upon son- ication of the parent MEH-PPV polymer.[81] Beyond the bulk studies, these mixtures endorsed the prepa- ration of thermofluorochromic papers or composite polymeric film (integrated as solid lipid particles, Figure 1c), to obtain thermal sensors over a large temperature range. 2. Results and Discussion 2.1. Switchable Fluorescence of MEH-PPV in PCM Mixtures The 20-carbon inert paraffin eicosane (EC, melting point TmEC = 36.5 °C[82]), already used to thermally modulate the aggregation and therefore fluorescence of small molecular dyes,[59,60,64] was Figure 1. a) Molecular structure of MEH-PPV. b) Emission spectra of a first chosen as a PCM in our work and mixed with 0.01 wt% CHCl3 solution and a film of MEH-PPV. c) Scheme of a polymeric film MEH-PPV (EC/MEH_0.01, see the Supporting Information for loaded with solid lipid particles containing MEH-PPV chains. Depending experimental details). In the solid state (20 °C), the mixture on the solid or liquid state of the surrounding PCM, the MEH-PPV chains showed a broad emission band at λmax = 603 nm and a shoulder lie in their aggregated or nonaggregated forms within the particles, which at 640 nm characteristic of the aggregated polymer (Figure S1, provide different emission colors. Supporting Information). Upon heating up to 60 °C, a little blueshift of the main luminescence band at λmax = 591 nm transition is a quite universal method to control the aggregation and a more defined and less intense shoulder at λ = 630 nm of the embedded fluorophores and their luminescence (e.g., were observed. This negligible change upon PCM melting was by switching between monomeric and excimer emission upon attributed to the low solubility of MEH-PPV in EC and the ten- dye aggregation).[59–68] Following this approach, up to now dency of the polymer to aggregate in both its liquid and solid organic nonpolymeric PCMs have been successfully combined phases, as confirmed by the formation of polymer particles in with small molecular fluorophores to yield single-color emis- suspension visible even by naked eye and the characteristic sion switching (off/on or on/off),[59,61,65] while multicolored spectral features reported for MEH-PPV films or poor-solvent thermal responses, with coarse color tunability, could only be solutions where the polymer is predominantly in its aggre- accomplished by using different fluorescent molecules.[62,66,68] gated state (Figure 1b).[70] Similar results were found in related Although conjugated polymers are known to manifest highly paraffins such as hexadecane (HD, TmHD = 18.2 °C)[83] and tet- medium-dependent emission properties, to our knowledge radecane (TD, TmTD = 5 °C) (0.01 wt%, Figure S1, Supporting the only so far reported examples of PCM-based thermofluoro- Information).[82] chromism using a semiconductive polymer (polydiphenylacety- To overcome this limitation we proposed the use of dodeca- lene) only provided a single-color on/off fluorescence modula- noic acid (DA, TmDA = 43 °C)[82] as a PCM, whose carboxylic acid tion, and/or it required additional chemical functionalization to functionality could enhance the MEH-PPV solubility, at least improve its miscibility with the surrounding PCM.[63,69] in the liquid state, through dipole–dipole interactions such as As a proof-of-concept to validate our approach, we have hydrogen bonds. Moreover, DA had already been used to modu- selected the commercially available and broadly used fluores- late the optical performance of proton-sensitive organic pho- cent conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)- tochromes[84] or yield reversible aggregation-induced quenching 1,4-phenylenevinylene (MEH-PPV, Figure 1a), as it shows three of fluorescent xanthene dyes.[65] crucial characteristics required to achieve PCM-based thermo- The emission spectrum of the solid mixture of MEH-PPV fluorochromism. First, it strongly fluoresces in both homoge- (0.1 wt%) in DA (MEH/DA_0.1), measured at 20 °C, showed neous solutions (e.g., ΦFtoluene = 0.34)[70] and in the aggregated the characteristic pattern of aggregated polymer species with state (e.g., film, ΦFfilm = 0.10–0.15).[71] Second, its luminescence a broad and intense band at λmax = 614 nm and a shoulder is highly environment-dependent: i) diluted MEH-PPV liquid at 650 nm (Figure 2a and Table 1). Above its melting point, a solutions (e.g., in CHCl3) present a high energy emission band homogeneous liquid solution was obtained, without forming at λ1,max ≈555 nm and weaker shoulders at around λ2 ≈600 and apparent macroscopic aggregates despite the 10-fold concentra- λ3 ≈655 nm,[72–76] mainly arising from the 0–0, 0–1, and 0–2 tion increase used relative to the previous paraffin mixtures. vibronic progression of the S1 → S0 transition of the single intra- The fluorescence spectrum of the molten solution showed chain exciton emission (Figure 1b);[72] ii) MEH-PPV concentrated a maximum at λmax = 595 nm, a new high-energy band at Adv. Optical Mater. 2022, 10, 2102423 2102423 (2 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Figure 2. Images and normalized emission spectra of the a) MEH/DA_0.1, b) MEH/SA_0.1, and c) MEH/NA_0.1 mixtures below and above their respective TmPCM under UV light (λexc = 365 nm). Normalized emission spectra of the d) MEH/DA_0.01, e) MEH/SA_0.01, and f) MEH/NA_0.01 mix- tures below and above their respective TmPCM under UV light (λexc = 365 nm). λmax = 546 nm and the low-intensity shoulder at λmax = 634 nm. strongly supports the formation of ground-state polymer (static) These features differed significantly from the emission spec- aggregates in the solid PCMs, though the further presence of trum of solid MEH/DA_0.1 mixture and are reminiscent of dynamic aggregates cannot be excluded. The fact that the band the vibronic fluorescent bands of MEH-PPV in homogeneous of the liquid MEH/DA_0.1 mixtures is broader than that of the organic solutions,[70] though with a different I0-1/I0-0 intensity CHCl3 solution confirms that MEH-PPV aggregates are still band ratio that we ascribed to the remaining aggregated species present in the liquid DA, as previously showed by the emission in melted MEH/DA_0.1 (Figure 2a and Table 1). Macroscopi- spectra manifesting I0-1/I0-0 ratio different from those described cally, these aggregation changes resulted in a sharp (within for CHCl3 homogeneous solutions. 10 °C) emission color transition from orange to yellow observed Worth to mention is that the aggregation/disaggregation pro- by naked-eye (Figure 2a) –, i.e., thermofluorochromism. cesses did not affect the emission efficiency, as demonstrated To further corroborate the temperature-induced fluorescence by the absolute fluorescence quantum yield values, which did modulation is caused by the polymer aggregation/redissolu- not vary significantly when passing from the solid (ΦF = 0.15) tion in the solid/liquid PCM, we performed absorption and to the liquid (ΦF = 0.19) state (Table 1). The slightly higher diffused reflectance spectra of the PCM mixture in its liquid value registered for the liquid solution agrees with quantum and solid state, respectively (Figure S2, Supporting Informa- yield data previously reported for MEH-PPV organic solutions tion). Reflectance spectra were converted in F(R) functions and films.[70,71] This result, combined with the evident spectral through the Kubelka–Munk equation. Broad F(R) spectra with changes observed upon melting the PCM, makes this system a λmax = 520–550 nm were obtained for the PCM solid mixture. good dual-color thermofluorochromic material. On top of that, Above the TmPCM the spectra blueshifted to λmax = 490–500 nm these mixtures manifested quite good stability, as corroborated and became narrower, approaching the λmax position of the by the i) fluorescence quantum yield values in the solid (ΦF = absorption spectrum of a homogeneous solution of MEH- 0.15) and liquid (ΦF = 0.18) state for the same mixture, meas- PPV in CHCl3 (Figure S2, Supporting Information). This ured 1 week later, which remained practically constant; and ii) Adv. Optical Mater. 2022, 10, 2102423 2102423 (3 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Table 1. Thermofluorochromic properties of the MEH/DA, MEH/NA, and MEH/SA mixtures: emission spectral maxima, I0-1/I0-0 intensity ratio for the liquid state, chromaticity coordinates, and absolute fluorescence quantum yield values. Mixture MEH-PPV conc. [wt%] T [°C] λ1a) [nm] λ2a) [nm] λ3a) [nm] I0-1/I0-0 1931 CIE coordinates (x,y) ΦF MEH/DA 0.1 20 — 614 650 — (0.605, 0.394) 0.15 60 546 595 634 2.56 (0.550, 0.448) 0.19 0.01 20 — 610 — — (0.539, 0.455) 0.16 60 548 581 643 1.04 (0.469, 0.524) 0.19 MEH/NA 0.1 0 — 610 648 — (0.599, 0.391) 0.17 20 549 592 641 1.66 (0.526, 0.471) 0.19 0.01 0 — 589 640 — (0.513, 0.476) 0.27 20 548 588 638 1.01 (0.475, 0.519) 0.35 MEH/SA 0.1 20 550 617 650 — (0.616, 0.383) 0.17 80 542 604 629 1.42 (0.485, 0.494) 0.19 0.01 20 567 580 617 — (0.529, 0.419) 0.18 80 544 580 640 0.94 (0.453, 0.540) 0.17 a) For the monomeric emission in the molten mixtures, bands are assigned to vibronic transitions 0-0, 0-1, and 0-2. reproducible thermal luminescence responses of the MEH- The reversible thermofluorochromic properties of MEH-PPV PPV/PCM mixtures upon repetitive heating/cooling cycles in aliphatic carboxylic acids were preserved upon 10-fold dilu- (Figure S3, Supporting Information). tion of the polymer content (0.01 wt%, Figure 2d–f and Table 1; The color transition temperature was successfully modu- and Figure S5, Supporting Information), though with some lated using two analogous PCMs of strikingly different Tm, differences. In the solid state, all the mixtures (MEH/NA_0.01, stearic acid (MEH/SA_0.1, TmSA = 69.3 °C)[82] and nonanoic acid MEH/SA_0.01, and MEH/DA_0.01) showed a blueshift of the (MEH/NA_0.1, TmNA = 12.4 °C),[82] and the same 0.1 wt% MEH- broad emission band (λmax ≈580–610 nm) compared to the PPV concentration. The emission spectra of both mixtures more concentrated samples, probably due to lower aggrega- in the solid state show broad unstructured emission bands tion effects. For the same reason, the liquid diluted mixtures at λmax = 617 nm (MEH/SA_0.1 at 20 °C) and λmax = 610 nm presented larger relative intensities of the high-energy band (MEH/NA_0.1 at 0 °C), with the former displaying a larger band at λmax ≈550 nm related to intrachain emission (see I0-1/I0-0, broadening and redshift probably due to the lower solubility of Table 1). Absolute quantum yield values of the solid and liquid MEH-PPV in the long-alkyl chain SA (Figure 2b,c and Table 1). mixtures were also comparable and slightly higher than those An increase of the temperature over the melting point, up to of the more concentrated mixtures, possibly due to a decrease 80 and 30 °C for MEH/SA_0.1 and MEH/NA_0.1, respectively, of ground-state aggregates that are more prone to undergo induced a 18 and 13 nm hypsochromic shift of the main lumi- excited state relaxation through nonradiative decays. This con- nescence bands and the appearance of new high-energy bands clusion was supported by the F(R) spectra measured for the corresponding to monomer emission at λmax = 542 and 549 nm. diluted mixtures, which displayed narrower and blueshifted Actually, the larger contribution of such high-energy bands rela- (λmax = 500–520 nm) bands than the corresponding concen- tive to what observed for the MEH/DA_0.1 mixture, was attrib- trated solutions, thus further suggesting a lower degree of uted to a larger degree of polymer disaggregation because of aggregation or the formation of smaller aggregates (Figure S6, its enhanced solubility in short-alkyl chain NA and at the high Supporting Information). Also in this case, the melted solutions temperature required to melt SA. Again the spectrally different displayed a blueshift of the respective absorption bands (λmax = emission observed in the solid and liquid state, showed similar 480–490 nm), which was associated to the redissolution of the efficiency in quantum yield experiments (Table 1). polymer molecules, as it happens in homogeneous CHCl3 solu- Once more, the narrowing and blueshift of absorption tion. Overall, this allowed obtaining concentration-dependent spectra of the MEH/SA_0.1 and MEH/NA_0.1 upon melting thermally-induced optical changes, where the switching colors confirmed the fluorescence spectral variations observed upon of the solid and liquid mixtures were displaced toward the PCM transition are related to the aggregation/disaggregation yellow region of the CIE 1931 color space upon dilution of the processes (Figure S2, Supporting Information). MEH-PPV/PCM mixture, i.e., fine tuning of the thermofluoro- Finally, temperature-dependent fluorescence measurements chromic behavior without the need of varying the fluorophore obtained every 10 °C over a large thermal range for the low- of choice (Table 1; and Figure S7, Supporting Information). (MEH/NA_0.1) and high-Tm (MEH/SA_0.1) mixtures revealed negligible spectral changes once below or above the PCM melting point, corroborating that the thermally-induced optical 2.2. Switchable Fluorescence of Oligomers in PCM Mixtures changes are triggered by the PCM solid-to-liquid transition (Figure S4, Supporting Information),[85] occurring, in all cases, To further generalize this approach, we also prepared PCM within 10 °C. mixtures with shorter oligomeric species straightforwardly Adv. Optical Mater. 2022, 10, 2102423 2102423 (4 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de obtained from MEH-PPV through a sonochemical strategy the solid and liquid phases (λmax ≈475 nm). This result was already reported by us (see the Supporting Information for ascribed to the increased solubility of the shorter MEH-1.5 oli- details).[81] Applying this methodology, we synthesized three dif- gomer and higher solvation capability of NA compared to DA, ferent types of MEH-PPV oligomers, which we named MEH-4, thus leading to well dispersed MEH-1.5 molecules showing MEH-2, and MEH-1.5 in decreasing order of polymer length. intrachain emission even in the solid state of the sample upon Interestingly, such oligomers exhibit different emission and dilution (Table 2; and Figure S12b, Supporting Information). absorption properties due to the modification of the effective conjugation length (Figure S8, Supporting Information), which particularly decreased from that of the untreated polymer 2.3. Modification of the PCM Nature (nECL = 10–17) to nECL = 2 for the shorter oligomers.[81] The solid mixtures of MEH-4 in DA at 0.1 wt% (MEH-4/ The larger solubility of MEH-4 in organic media with respect DA_0.1) and 0.01 wt% (MEH-4/DA_0.01) showed concentra- to the parent MEH-PPV polymer also allowed us to use paraf- tion-dependent fluorescence with broad and nonstructured fins as host PCM matrices to accomplish thermofluorochromic emission bands centered at λmax = 581 (orange emission color) materials. MEH-4 was thus dissolved in EC at different con- and 571 nm (green emission color), respectively (Figure 3a,b centrations, obtaining MEH-4/EC_0.1 (0.1 wt%) and MEH-4/ and Table 2). Heating above the DA melting point induced a EC_0.01 (0.01 wt%) mixtures. The first remarkable feature 43 and 48 nm blueshift of the emission band of the MEH-4/ found for these samples was the ≈20 nm emission blueshift DA_0.1 (λmax = 538 nm) and MEH-4/DA_0.01 (λmax = 523 nm) registered for the solid MEH-4/EC_0.01 mixture with respect to liquid mixtures, respectively. The fluorescence quantum yield its DA counterpart, revealing an intrinsic matrix-induced solva- values of the oligomer mixtures did not vary significantly tochromic effect prior to any thermal treatment. In addition, passing from the solid (0.14 and 0.21 for MEH-4/DA_0.1 and they showed the characteristic thermal switching behavior pre- MEH-4/DA_0.01 samples, respectively) to the liquid state (0.19, viously observed in long alkyl chain carboxylic acids, with 46 nm 0.30), confirming that these oligomer mixtures could also be (MEH-4/EC_0.1) and 33 nm (MEH-4/EC_0.01) blueshifts upon used as dual-color thermofluorochromic materials. In addition, heating above TmEC (Figure 3c and Table 2; and Figure S13a, the reduction of oligomer concentration also yielded higher Supporting Information). emission quantum yield values in both solid and liquid mix- When the same MEH-4 oligomer was dissolved at 0.1 wt% tures, probably due to a decrease in the degree of aggregation. concentration in HD (TmHD = 18.2 °C),[82] the resulting MEH-4/ As exemplified by MEH-4/DA_0.1, the emission switch took HD_0.1 mixture also displayed two main characteristics: i) a place within a narrow temperature range of 10 °C around TmDA 33 nm fluorescence switch upon melting from λmax = 560 to (Figure S9a, Supporting Information) and was fully revers- λmax = 527 nm (Figure 3d), and ii) a slightly blueshift of the ible upon several heating/cooling cycles (Figure S9b, Sup- band maxima of the solid and liquid mixtures compared to the porting Information). Further tuning of the emission color corresponding EC solution. On the other hand, the less con- was obtained with the use of the MEH-2/DA_0.1 (0.1 wt%) centrated MEH-4/HD_0.01 solution displayed negligible ther- and MEH-2/DA_0.01 (0.01 wt%) mixtures, which presented mofluorochromism, probably due to the higher solubility of fluorescence maxima at λmax = 550 and 539 nm in the solid the oligomer in solid HD that yielded the blueshifted monomer state (ΦF = 0.02 and 0.04), respectively, that are blueshifted by emission in both phases of the PCM (Table 2; and Figure S13b, almost 30 nm in comparison with MEH-4/DA samples. More- Supporting Information). over, these values were further blueshifted by 45 and 34 nm, From these results, we can infer that the nature of the PCM respectively, upon melting at 50 °C (ΦF = 0.04 for both concen- critically determines the performance of the thermofluoro- trations), thus demonstrating that the thermofluorochromic chromic mixtures with conjugated polymers and oligomers behavior was preserved even when decreasing nECL (Table 2; as it allows both fine tuning of the switching temperature and Figure S10, Supporting Information). The band maxima of and emission response, and modulation of the position of the the F(R) spectra of the solid DA mixtures were broader and/or emission bands owing to solvatochromism. Such tunability more redshifted than the absorption bands of the liquid solu- can be further increased, by varying the emitter conjugation tions, which again evidences the formation of the aggregates of length and concentration, as illustrated for the different MEH- the MEH-PPV oligomers in the solid PCM, especially at higher PPV polymer and oligomer samples analyzed. Altogether, this concentrations (Figure S11, Supporting Information). Finally, enables a controlled and broad variation of the thermofluoro- the fair agreement between the absorption spectrum of the chromic behavior of the materials developed herein along the liquid DA mixtures with those of MEH-4 and MEH-2 in CHCl3 CIE 1931 color space, as shown in Figure 3e. suggests the good redissolution of these oligomers in the liquid PCM (Figure S11, Supporting Information). As for the MEH-1.5 oligomer, experiments were conducted 2.4. Cellulose-Based Temperature Sensors in this case in NA to tune the solid–liquid transition down to 12.4 °C. At 0.1 wt% concentration, the MEH-1.5/NA mixture Once proven the efficiency of our strategy toward thermofluoro- (MEH-1.5/NA_0.1) provided a further blueshifted thermore- chromism based on conjugated polymers and PCMs, we aimed sponsive switching upon melting (from λmax = 495 to 478 nm, to integrate these mixtures into solid substrates to develop easy- Figure S12a, Supporting Information). However, this was not to-handle sensor devices. The substrate of choice for this was observed for MEH-1.5/NA_0.01 with a 10-fold decrease in cellulose paper, a cheap, biodegradable, and fully-scalable mate- emitter concentration, which showed similar spectra in both rial that has been rediscovered for the fabrication of added-value Adv. Optical Mater. 2022, 10, 2102423 2102423 (5 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Figure 3. a,b) Images and normalized emission spectra of a) MEH-4/DA_0.1 and b) MEH-4/DA_0.01 before and after PCM melting (λexc = 365 nm). The pictures of the cuvettes at 45 °C show the still incomplete solid-to-liquid transition of the PCM mixtures with clearly fractioned emitted colors: the bottom part, still solid, presents the corresponding initial emissions, while the top one, already melted, clearly displays the luminescence of the final liquid mixtures. c,d) Images and normalized emission spectra of c) MEH-4/EC_0.1 and d) MEH-4/HD_0.1 below and above the respective TmPCM under UV light (λexc = 365 nm). e) 1931 CIE chromaticity coordinates of the emission color of the following mixtures below and above their TmPCM. 1: MEH/DA_0.1; 2: MEH/NA_0.1; 3: MEH/SA_0.1; 4: MEH/DA_0.01; 5: MEH/NA_0.01; 6: MEH/SA_0.01; 7: MEH-4/DA_0.1; 8: MEH-2/DA_0.1; 9: MEH-4/ DA_0.01; 10: MEH-2/DA_0.01; 11: MEH-4/EC_0.1; 12: MEH-4/HD_0.1; 13: MEH-4/EC_0.01; 14: MEH-4/HD_0.01; 15: MEH-1.5/NA_0.1; 16: MEH-4/ NA_0.01. The chromaticity coordinates for the solid and melted samples are shown in black and red, respectively. sensors, i.e., the so called smart papers.[86] As a proof of con- allow for PCM solidification. Interestingly, the resulting smart cept, we prepared four different cellulose papers by soaking papers retained the colors and switching behavior around them with melted MEH/SA_0.1, MEH/DA_0.01, MEH/NA_0.1, the respective TmPCM of the corresponding bulk mixtures and MEH-4/EC_0.1 mixtures, which were selected according to (Figure 4a), though slight discrepancies in emission color their rich and differential emission and interconversion tem- and/or color contrast were observed that can be attributed to a perature. Afterward, the systems were cooled down to 0 °C to variety of factors, e.g., the distinct thicknesses of the papers and Adv. Optical Mater. 2022, 10, 2102423 2102423 (6 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Table 2. Thermofluorochromic properties of the mixtures of MEH-PPV oligomers with PCMs: emission spectral maxima below and above their TmPCM, emission spectral shifts upon heating and emission chromaticity coordinates. Mixture Oligomer conc. [wt%] Temperature [°C] λmax [nm] Spectral shift [nm] 1931 CIE coordinates (x,y) MEH-4/DA 0.1 20 581 43 (0.535, 0.463) 60 538 (0.389, 0.573) 0.01 20 571 48 (0.484, 0.509) 60 523 (0.380, 0.574) MEH-2/DA 0.1 20 550 45 (0.419, 0.533) 60 505 (0.260, 0.439) 0.01 20 539 34 (0.351, 0.512) 60 505 (0.250, 0.425) MEH-1.5/NA 0.1 0 495 17 (0.260, 0.384) 20 478 (0.201, 0.292) 0.01 0 476 1 (0.197, 0.275) 20 475 (0.201, 0.304) MEH-4/EC 0.1 20 578 46 (0.494, 0.500) 60 532 (0.336, 0.578) 0.01 20 554 33 (0.412, 0.558) 60 521 (0.301, 0.591) MEH-4/HD 0.1 0 560 33 (0.392, 0.542) 20 527 (0.340, 0.580) 0.01 0 523 2 (0.333, 0.545) 20 521 (0.323, 0.551) bulk mixtures, where excitation light can also penetrate differ- small percentage of MEH-PPV polymers/oligomers toward the ently and lead to differential contributions of the reabsorption cellulose fibers of the papers, which will be no longer affected processes to the final colors emitted; the non-negligible autoflu- by PCM melting. In spite of this, the clear and large thermo- orescence of the cellulose substrates used; or the migration of a fluorochromic responses registered for the free-standing papers Figure 4. a) Cellulose papers soaked with the MEH/NA_0.1, MEH/DA_0.01, MEH/SA_0.1, MEH-4/EC_0.1 mixtures, whose emission below and above their TmPCM is shown (λexc = 365 nm). b) Emission of a cellulose paper wax-printed with the MEH-2/DA_0.1 mixture, below and above the TmDA. The background blue emission is due to the cellulose paper autofluorescence. Adv. Optical Mater. 2022, 10, 2102423 2102423 (7 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de prepared make them usable as temperature sensing platforms drop casting a colloidal dispersion of MEH@DA_SLMs in an that are accessible through a simple and scalable approach. In aqueous PVA solution and allowing the water to evaporate (see addition, though the price of commercially available conjugated the Supporting Information). polymers such as MEH-PPV might be often high, the little The presence of phase-separated MEH@DA_SLMs amount required for the fabrication of these smart papers (typi- embedded within the PVA layer was confirmed by differen- cally, 16 mg cm−2) makes them also of low cost. tial scanning calorimetry (DSC), where we clearly observed On a further step, even more complex thermoresponsive the signal arising from DA solid–liquid transition at T ≈44 °C fluorescent patterns were created on cellulose papers by wax- (Figure S16, Supporting Information). This guarantees that the printing the desired MEH-PPV/PCM using a plotter with an microparticles remain confined within the resulting MEH@ attached pen, which was coated with aluminum foil and con- DA@PVA free-standing film, thereby retaining the thermally- nected to a power supply to induce Joule heating. With this, we induced fluorescence modulation of the bulk MEH-PPV/DA were able to print well-resolved thermoresponsive fluorescent mixtures (Figure 5e,f). writings and patterns by using the MEH-2/DA_0.1 mixture, To illustrate the applicability of the composite films prepared, which modified its emission color upon reaching the TmDA one of them was used to successfully sense the temperature of (Figure 4b). Noticeably, the resolution of the drawings was pre- an aqueous solution heated at 60 °C. As shown in Figure 5g, served even after several heating-cooling cycles, thus indicating the fraction of the film that was not submerged in the solution the good encapsulation of the PCM mixtures within the cellu- remained at room temperature and, consequently, exhibited lose fibers. the red-orange emission arising from MEH@DA_SLMs in the solid state; by contrast, the part of the polymer layer immersed into the heated sample experienced the phase transition of the 2.5. Microstructuration and Integration into Free-Standing Films embedded MEH@DA_SLMs and, therefore, the corresponding for Temperature Sensing switch to yellow colored fluorescence. Beyond smart papers, we aimed to explore the incorporation of MEH-PPV/PCM mixtures into free-standing polymeric films 3. Conclusions that are highly suitable for the preparation of optical tempera- ture sensing devices. However, this is not as simple as in the In summary, herein we have reported a straightforward and case of cellulose papers, since polymeric films are not porous general strategy to obtain sharply changing dual-color fluores- and therefore do not adsorb the thermofluorochromic mix- cent thermoresponsive systems based on the semiconductive tures. On the other hand, the direct mixing of MEH/PCM polymer MEH-PPV dissolved in phase change materials. The solutions with matrix polymers would alter the final composi- thermally-induced emission switch of these mixtures is fully tion and, as a result, their emission switching properties. For reversible and relates to the different environment generated all this, we then structured the MEH/PCM mixtures into solid around the fluorophores during the solid–liquid transition lipid microparticles (SLMs) that can be easily dispersed within of the PCM: large aggregates emitting at lower energies are inert polymer films, following a similar approach already used formed in the solid state while non-/less aggregated systems in our group with core–shell microcapsules,[10,87–89] liquid nan- with higher energy emission are generated in the liquid PCM odroplets,[90] and solid lipid nanoparticles[91] to preserve the phase. Therefore, the PCM matrix assures the control and tun- optical properties of macroscopic solutions in solid composites. ability of the transition temperature, as well as guarantees a First of all, SLMs were prepared from the representative sharp (within 10 °C) thermal switching behavior for the conju- MEH/DA_0.1 and MEH/SA_0.1 mixtures through the emul- gated polymer emission, which to date had only been showed sion-cooling method (see the Supporting Information for more to display progressive thermofluorochromic changes over wide details). SEM and optical microscopy images of the obtained temperature ranges. MEH/DA_SLMs (Figure 5a,b) and MEH/SA_SLMs (Figure 5c; Noticeably, by taking advantage of the very high sensitivity and Figure S14, Supporting Information) confirmed the suc- of conjugated polymer molecules to the external environment, cessful formation of the particles (diameter ≈10–50 µm). More- our strategy enables fine modulation of the color and magni- over, the red spots observed in the core of the microparticles by tude of the thermofluorochromic response by simply control- optical microscopy suggest the successful encapsulation of the ling the fluorophore concentration and/or the PCM type. In MEH-PPV polymer. Accordingly, both SLMs undergo the fluo- addition, the color palette of these thermoresponsive fluores- rescence switch observed in bulk once they are heated above the cent switching materials can be further expanded with MEH- TmPCM, with the blueshift of the main band and the formation PPV oligomers to other regions of the CIE 1931 chromaticity of the high energy band at 547 nm (Figure 5d; and Figure S15, diagram without the need for the tedious synthesis or func- Supporting Information). However, some differences of the tionalization of alternative fluorophores. Noticeably, the MEH- emission spectra are observed compared to the measurements PPV/PCM mixtures can be easily transferred to cellulose of the bulk mixtures. This was ascribed to the partial loss of the papers for the development of paper-based color-changing polymer during the SLMs preparation, which caused the reduc- temperature sensors. Furthermore, they are also suitable for tion of the polymer final concentration in the structured PCM. wax-printing, which opens the door for the preparation of Next, the SLMs were incorporated into a hydrophilic poly(vinyl patterns of interest for sensing and anticounterfeiting labels. alcohol) (PVA) polymer matrix, which is not miscible with the Finally, our PCM-based materials can be structured as solid liquid and solid phases of the SLMs. This was achieved upon lipid microparticles, which upon incorporation into polymeric Adv. Optical Mater. 2022, 10, 2102423 2102423 (8 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Figure 5. a) Optical microscopy and b) SEM images of MEH@DA_SLMs. c) Optical microscopy image and d) temperature-dependent emission spectra of MEH@SA_SLMs. e) Image (λexc = 365 nm) and f) temperature-dependent emission spectra of a MEH@DA@PVA film. g) Demonstration of the use of the MEH@DA@PVA film as a fluorescent temperature sensor (λexc = 365 nm). matrices enable obtaining free-standing films with thermo- Supporting Information fluorochromic behavior that could be directly used as temper- Supporting Information is available from the Wiley Online Library or ature sensors. from the author. Adv. Optical Mater. 2022, 10, 2102423 2102423 (9 of 11) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Acknowledgements [17] D. Li, W. Hu, J. Wang, Q. Zhang, X.-M. Cao, X. Ma, H. Tian, Chem. Sci. 2018, 9, 5709. This work was supported by Grant Nos. RTI2018-098027-B-C21 and [18] J. Wang, N. Wang, G. Wu, S. Wang, X. Li, Angew. Chem., Int. Ed. PID2019-106171RB-100 funded by No. MCIN/AEI/10.13039/501100011033 2019, 58, 3082. and by ERDF A way of making Europe. The ICN2 is funded by the [19] Y. Cao, J. K. Nagle, M. O. Wolf, B. O. Patrick, J. Am. Chem. Soc. CERCA programme/Generalitat de Catalunya. The ICN2 is supported 2015, 137, 4888. by the Severo Ochoa Centres of Excellence programme, Grant No. SEV- [20] L. Liu, X. Wang, N. Wang, T. Peng, S. Wang, Angew. Chem., Int. Ed. 2017-0706 funded by No. MCIN/AEI/10.13039/501100011033. This work 2017, 56, 9160. was also supported by Generalitat de Catalunya (No. 2017 SGR00465 [21] B. del Rosal, E. Ximendes, U. Rocha, D. Jaque, Adv. Opt. Mater. project). J.R.O. aknowledge the Generalitat de Catalunya (AGAUR) for 2017, 5, 1600508. his predoctoral FI fellowship. [22] J.-Y. Lin, B. Liu, M.-N. Yu, X.-H. Wang, L.-B. Bai, Y.-M. Han, C.-J. Ou, L.-H. Xie, F. 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