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Optical Mechanotransduction with Carbazole-Based Luminescent Liquid Single-Crystal Elastomers This article is dedicated to Prof. Heino Finkelmann on the occasion of his 70th birthday
Jaume Garcia-Amorós, Dolores Velasco* Carbazole-based liquid single-crystal elastomers (LSCEs) are valuable fluorescent flexible materials to perform optical mechanotransduction under ambient conditions. Indeed, the covalent incorporation of carbazole derivatives into nematic LSCEs allows to tune their luminescence on demand under mechanical control in a quick and reversible fashion. Specifically, the fluorescence intensity for these materials can be switched back and forth in less than a second. Moreover, such a process can be performed several times without detecting any sign of fatigue in the system. In addition, these materials show excellent resistance to aging; 2 years after their preparation they exhibit the very same mechanofluorescent behavior as when freshly prepared. In fact, the here reported fluorescent systems are highly sensitive; the application of a force of 70 mN decreases the fluorescence in the elastomeric material by 7%. Thus, mechanical forces are attractive external stimuli to modulate the fluorescence of nematic elastomers rapidly and reversibly enabling thereby mechanotransduction.
1. Introduction Mechanical forces are one of the most common external stimuli in nature. This is the reason why living organisms are endowed with a variety of force-sensitive cells, which enable them to be aware of what is nearby. More specifically, mechanosensory cells specialize in mechanotransduction, that is, the conversion of all incoming mechanical sensations into nerve signals that are sent across neurons to the brain for further processing and integration.[1] Resembling these biological systems, optical mechanotransducers emerge as valuable engineered materials capable of converting mechanical events, either tension or pressure, into readable and processable optical outputs. Dr. J. Garcia-Amorós, Prof. D. Velasco Grup de Materials Orgànics, Institut de Nanociència i Nanotecnologia (IN2UB), Departament de Química Orgànica, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain E-mail: [email protected] Macromol. Rapid Commun. 2015, 36, 755−761 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Indeed, smart materials that tune their optical properties upon deformation or compression are of great interest within actual technology since they allow easy access to unique applications, from damage sensing[2–5] to security inks[6,7] to rewritable paper.[8] Switching the material luminescence between two different emitting states, with either distinct color or intensity, by altering mechanically—shearing, grinding or rubbing—the microenvironment of fluorophores within solid materials can serve as a general protocol for optical mechanotransduction. The initial state of the system can be restored further either thermally (annealing) or chemically, e.g., by recrystallization of the material or by its exposure to vapors of a suitable solvent.[9–16] However, the working principle of such systems makes them too slow for efficient mechanotransduction since they require several hours or even days to switch back and forth. For a quick, efficient, and fully reversible mechanotransduction process to be accomplished, we propose the use of luminescent liquid single-crystal elastomers (LSCEs) as mechanosensitive materials. Indeed, LSCEs
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have been investigated intensively during recent years for a variety of force-related applications, for instance, artificial muscles,[17–19] nano-optomechanical systems[20] and refreshable Braille displays.[21] However, the potential role of LSCEs in optical mechanotransduction remains still unexplored. Briefly, nematic LSCEs are weakly crosslinked polymer networks exhibiting a macroscopic uniaxial orientation of the director, where the orientational long-range order in one dimension of nematic liquid crystals is coupled directly with the elasticity of conventional rubbers.[22] Interestingly, if fluorescent components are covalently attached to the main polymeric backbone of such materials, their luminescence can be switched on demand under mechanical control. In particular, fluorescence intensity switches off upon deformation in a few hundreds of milliseconds for our LSCEs. After the external force is removed, the system fully recovers its pre-stress emission intensity within the very same timescale. In fact, the response time of the LSCE-based optical mechanotransducers reported herein falls within the same range than those exhibited by some human mechanoreceptors like the Pacinian and Meissner's corpuscles (between 100 ms and 1 s), which perceive rapid vibratory pressure and light touch, respectively.[23] Thus, physical forces are valuable external stimuli to tune the fluorescence of nematic LSCEs in a quick and reversible fashion enabling thereby optical mechanotransduction.
2. Experimental Section 2.1. Preparation of the Monomers and Elastomers 4-methoxyphenyl-4-(3-butenyloxy)benzoate (M4OMe), 1,4-di(10-undecenyloxy)benzene (CL), and 9-(5-hexenyl)-9H-carbazole (CBZ6) were prepared as reported elsewhere.[24–26] M4OMe, CL (5% mol), CBZ6, and polyhydrogenomethylsiloxane (≈147 Si–H groups per chain) were dissolved in dry thiophene-free toluene (2 mL). Then, a solution of cyclooctadieneplatinum (II) chloride in dry dichloromethane (1%, 20 μL) was added. The reaction mixture was placed in a spinning Teflon mold and heated up to 70 °C for 2 h at 5000 rpm. Afterwards, the mold was cooled to room temperature and the elastomer (not totally cross-linked) was carefully removed from the wall. After that, a uniaxial stress along the polymer chains was applied to align the mesogenic molecules during the deswelling process. In order to fix this orientation, the cross-linking reaction was completed by leaving the sample under load in the oven at 70 ºC for 2 d. The nonreacted monomers were removed from the network by a swelling– deswelling process using toluene and hexane, respectively.
2.2. Characterization of the Elastomers Differential scanning calorimetry (DSC) measurements were performed in a PerkinElmer DSC-7 apparatus under nitrogen atmosphere at heating rates of 9, 16, 25, and 36 K min−1. The transformation temperatures were determined by extrapolating
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to a heating rate of 0 K min−1. X-ray diffraction (XRD) measurements were carried out with the monochromatic Cu Kα radiation (λ = 1.5418 Å), using a 2D image plate system as a detector. The incident beam was normal to the surface of the elastomer. XRD experiments were realized at 298 K. Stress–strain measurements were performed using a Zwick/Roell Z100Allround instrument with a 10 N load cell.
2.3. Fluorescence Spectroscopy and Mechanofluorescent Experiments Both fluorescence spectroscopy and mechanofluorescent experiments were performed in a PTI 810 fluorimeter at 298 K. Polarized excitation spectra were collected by fixing the emission wavelength at λEm = 350 nm and placing a Glan Thompson polarizer between the light source and the sample. Emission spectra were registered by exciting the carbazole fluorophores at λEx = 290 nm. All mechanofluorescent experiments were carried out by fixing the elastomeric sample by both ends to a homemade measurement cell and further applying a uniaxial deformation stepwise. After deformation, the elastomeric sample was left to equilibrate for 10 min prior to the measurement. In all instances, carbazole fluorophores were excited at λEx = 290 nm, with light polarized parallel to the longest axis of the elastomeric material; the resulting luminescence was collected at λEm = 350 nm.
3. Results and Discussion For the present study, three novel elastomers have been prepared (ECBZ-5, ECBZ-10, and ECBZ-20, a in Figure 1) following the two-step cross-linking technique with a uniaxial stress applied after the first cross-linking stage developed by Küpfer and Finkelmann over the years.[27] All elastomeric materials are composed by a main polysiloxane backbone, which has the mesogen M4OMe attached end-on, the fluorogenic component CBZ6 attached sideon and CL as an isotropic cross-linker. Carbazole, a wellknown organic emitter, has been chosen as a fluorescent platform because, first, it can be easily modified chemically at the required position and, second, it exhibits not only a high-fluorescence quantum yield, even under aerated conditions, but also a great thermal and photochemical stability. The carbazole content in the elastomeric network has been varied from sample to sample (from 5% to 20% mol) in order to assess the influence of the fluorophore concentration on the mechanofluorescent response of the final transducing material. All elastomers were characterized by means of calorimetric and diffractometric methods as well as stress– strain experiments.[18a] DSC shows that both ECBZ-5 and ECBZ-10 exhibit a broad nematic phase between their glass transition temperature at Tg = 276 K and their nematic-to-isotropic phase transition temperature, TN-I, at 327 K and 313 K, respectively, covering thereby the
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Figure 1. a) Chemical composition of the fluorescent elastomers ECBZ-5, ECBZ-10, and ECBZ-20 and b,c) X-ray scattering patterns for ECBZ-10 (b) and ECBZ-20 (c) .
ambient temperature. On the other hand, ECBZ-20, with the highest carbazole content, presents uniquely a glass transition at Tg = 269 K, exhibiting, thus, an isotropic behavior at room temperature. XRD pattern for both ECBZ-5 and ECBZ-10 (b in Figure 1) at 298 K displays two equatorial reflexes in the wide angle X-ray scattering (WAXS) regime (spacing ≈ 4.4 Å), evidencing a macroscopic alignment of the nematic director, n, which lies parallel to the longest axis of the probe. From this reflection, the order parameter for the mesogenic units, S, was determined to be 0.74 for both ECBZ-5 and ECBZ-10. Contrarily, ECBZ-20 exhibits only a diffused halo (c in Figure 1), manifesting again its isotropic nature. Young's modulus (E) for both ECBZ-10 and ECBZ-20 was determined by stretching the samples parallel to their longest axis at 298 K. Specifically, E values of 0.53 and 0.30 MPa were determined for ECBZ-10 and ECBZ-20, respectively, evidencing, once again, the isotropic behavior of the latter.
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Steady-state emission spectra of elastomers ECBZ-5, ECBZ-10, and ECBZ-20 (a–c in Figure 2) at 298 K shows a main emission band peaking at 350 and 367 nm upon excitation at λEx = 290 nm. In fact, identical bands are also present in the emission spectrum of a dichloromethane solution of CBZ6 (d in Figure 2). Such feature together with the absence of fluorescence in the 400–430 nm range, where the emission arising from possible carbazole aggregates or excimers might appear,[28,29] suggests that although the carbazole moieties are somehow organized within the elastomeric network due to the presence of the nematic mean field, they are essentially isolated. In addition, a weak emission band centered at 468 nm was also observed for all the elastomeric systems analyzed. Literature findings reveal that such band can be related to the phosphorescent emission of the carbazole units.[30,31] Mechanofluorescent experiments involve registering the evolution of the material fluorescence under
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nature. Therefore, and according to that abovementioned, nematic LSCE, ECBZ-10 is the most efficient optical mechanotransducer among those tested herein. Even though ECBZ-10 shows a similar sensitivity than ECBZ-5, the former exhibits a greater variation of its fluorescence intensity when it is stretched parallel to n. In order to unravel the source of the mechanofluorescent properties exhibFigure 2. Normalized emission spectrum of a) ECBZ-5, b) ECBZ-10, c) ECBZ-20 and a CH2Cl2 solution of d) CBZ6 at 298 K upon excitation at λEx = 290 nm and variation of the ited by both nematic LSCEs ECBZ-5 and excitation intensity at 295 nm against the angle between the polarization vector of the ECBZ-10, not only the orientation of the incoming light and the nematic director, n (e). In the latter experiment, the emission carbazole units within the elastomeric wavelength was fixed at 355 nm. network but also its possible modification upon deformation of the system was investigated. The steady-state excitation spectrum mechanical deformation. Such measurements were perof ECBZ-10 at 298 K shows a main band peaking at ca. formed by fixing the elastomeric sample by both ends 295 nm which, according to Johnson, can be assigned to the measurement cell and further applying a uniaxial to the S2 ← S0 transition of the carbazole moieties; the mechanical force stepwise. In all instances, carbazole fluorophores were excited at λEx = 290 nm, with light transition moment for such transition is placed along the longest axis of the carbazole molecule.[32] On the polarized parallel to the longest axis of the elastomeric material; the resulting luminescence was collected at other hand, the dependence of the excitation intensity against the angle formed by the polarization vector of λEm = 350 nm. Fluorescence at this particular wavelength the incoming light and the nematic director shows that diminished dramatically after deformation of the sample the maximum intensity for this band is achieved at 0° along the director direction for both nematic LSCEs, and 180° (e in Figure 2), respectively. Such feature sugECBZ-5 and ECBZ-10 (a,b in Figure 3). Indeed, the mechagests that the carbazole moieties are oriented within the nofluorescent behavior of these two uniaxially oriented elastomeric network with their longest axis parallel to n. elastomers can be fitted to a monoexponential decay Indeed, a local order parameter of 0.56 was determined function (Equation (1)): for the carbazole fluorophores from the corresponding (1) ΔI ε = ΔI ∞ + ΔI max × exp ( − s × ε ) polarized excitation spectra. The very same dependence was also studied during the deformation of the elastomeric sample in order to detect any change in the orienwhere ΔIε and ΔI∞ correspond to the relative fluorestation of the carbazole units. Maximum intensity for this cence intensity (ΔIε = ((Iε−I0)/I0)×100) at a determined band was attained when the incoming light is polarized deformation, ε, and at an infinite one, respectively. In at 0° and 180° with respect to n independently of the addition, fitting of Equation (1) to the experimental data sample deformation. This feature evidences that the caryields the maximum variation in the material luminesbazole molecules do not tilt during the deformation procence upon deformation (ΔImax in Figure 3) as well as the cess. As it is well known, nematic order in LSCEs increases mechanical sensitivity of the transducing material (s in upon stretching of the sample along the director direcFigure 3), which are both key parameters in the overall tion.[33] As a consequence, a more favorable overlap performance of optical mechanotransducers. Specifically, ΔImax values of −27% and −34% were obtained for elastobetween both carbazole units and surrounding mesogen molecules should occur. Such variation in the microenmers ECBZ-5 and ECBZ-10, respectively. Similar s values vironment where the carbazole fluorophores are located of ca. 11 were estimated for both systems. The mechanoresults in a quenching of their fluorescence originating, fluorescent response for ECBZ-10 was also investigated in turn, the macroscopic mechanofluorescent response upon deformation along the direction perpendicular to n. observed for these elastomeric systems. In this way, when In this case, only a residual mechanofluorescent response the isotropic elastomer ECBZ-20 is stretched either par(ΔImax ≈ −3%, d in Figure 3) was obtained due to the lack of allel or perpendicular to its longest axis as well as when order in the system along this spatial direction, in agreethe nematic LSCEs ECBZ-5 and ECBZ-10 are stretched in ment with its uniaxially oriented nematic structure. Such the direction perpendicular to n, where neither orienbehavior was also observed for the isotropic elastomer tational nor positional order exists, no mechanofluoresECBZ-20 independently of the stretching direction (ΔImax cent behavior is detected. Thus, it can be stated that the ≈3%, c,e in Figure 3), as a consequence of its disordered
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Figure 3. Mechanofluorescent response for elastomers a) ECBZ-5, b,d) ECBZ-10 and c,e) ECBZ-20 upon deformation along the direction a,c) parallel and d,e) perpendicular to the longest axis of the elastomeric sample and schematic representation (f) of the fluorescence quenching observed for the nematic LSCEs ECBZ-5 and ECBZ-10 when stretched along the direction parallel to n.
mechanofluorescent response observed for these elastomeric systems is coupled directly with the intrinsic liquid crystal order of the material. Time-resolved mechanofluorescent experiments, which involve registering the evolution of the material fluorescence over time upon applying or releasing an external physical force in a periodic fashion, allow to determine two other key parameters of mechanotransducers, namely, their response time and robustness. In fact, these two parameters will establish the potential applicability of the final material. Time-resolved mechanofluorescent experiments were carried out exclusively with the nematic LSCE ECBZ-10 since it showed the best performance for further technological applications. Fluorescence can be rapidly and reversibly modulated for ECBZ-10 by means of the presence or absence of a mechanical force. More specifically, when the system was stretched along the director up to ε = 0.05 (F = 75 mN), its
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relative fluorescence intensity decreased rapidly by 7% (a in Figure 4). Indeed, characteristic times of tON = 500 ms and tOFF = 500 ms were determined graphically for the decrease in intensity upon stretching of the sample and for the subsequent recovery of the prestress value upon releasing the force, respectively. More specifically, tON and tOFF are defined as the time required to undergo any of these two processes in an extension of 50%. Thus, a full cycle can be completed in approximately one second. Therefore, the elastomeric systems presented here are valuable systems to perform fast and efficient optical mechanotransduction under ambient conditions. In addition, the carbazole-based nematic LSCE ECBZ-10 can be also used as a storage device since the fluorescence intensity variation produced upon its deformation can be maintained for several hours and even more than a week (c in Figure 4). In this way, the information to be stored in this material can be rapidly written in the system within
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Figure 4. Time-resolved a) mechanofluorescent experiment, b) fatigue resistance test for the nematic LSCE ECBZ-10 and evaluation of the possible use of such elastomeric material for information storage purposes (c).
the millisecond timescale and kept there for the desired period of time. Once the external force is removed, the initial state of the system is fully restored thereby deleting the stored data in a quick fashion. The robustness of the ECBZ-10-based optical mechanotransducer was tested through the determination of its repeatability, fatigue, and aging resistance. Repeatability and fatigue resistance for such material were evaluated by applying and releasing a fixed force of 75 mN in a periodic fashion to the elastomeric material (b in Figure 4). Specifically, the optical mechanotransducer based on this nematic LSCE exhibited a great fatigue resistance and reversibility at 298 K since after several cycles neither its relative change in the fluorescence intensity nor its
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relaxation times were altered by the continuous work of the system (variation in ΔIε = 0.05 was less than 1% upon 15 consecutive cycles). In a further attempt to demonstrate the high robustness of the ECBZ-10-based optical mechanotransducer, aging tests were conducted. Indeed, mechanofluorescent experiments carried out 2 years after the fabrication of elastomer ECBZ-10 have shown no significant variation in the characteristic parameters of the mechanotransducer (ΔImax = 38% and s = 10).
4. Conclusion Carbazole-based LSCEs are valuable fluorescent flexible materials to perform optical mechanotransduction under
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ambient conditions. Such systems admit carbazole contents within their elastomeric networks up to 10% without losing their liquid-crystalline properties. The intrinsic nematic order of LSCEs allows the reorganization of the mesogenic molecules upon their deformation along the director direction forcing thereby a stronger interaction with the carbazole fluorophores. This feature results in a quenching of the luminescence of the latters. In this way, the application of external physical forces to carbazolebased LSCEs enables tuning the fluorescence of these materials and, thus, mechanotransduction. Remarkably, time-resolved mechanofluorescent experiments revealed that one full cycle can be completed for these systems in approximately one second approaching, therefore, to the response time of biological mechanoreceptors. Moreover, these materials are useful also to store information for weeks, which can be erased in a few hundreds of milliseconds when it is not needed anymore. In addition, the elastomeric materials reported here show an outstanding robustness exhibiting thereby a great resistance to fatigue and aging, enabling, thus, their further use in technological devices. Acknowledgements: Financial support for this research was obtained from the Ministerio de Economía y Competitividad (Spain, CTQ2012–36074). J.G.-A. is grateful for a Beatriu de Pinós post-doctoral grant from the Generalitat de Catalunya (Spain, 2011 BP-A-00270). Thanks are also due to Zwick/Roell for the determination of Young’s moduli. Received: December 23, 2014; Revised: January 23, 2015; Published online: February 19, 2015; DOI: 10.1002/marc.201400734
Keywords: elastomers; fluorescence; liquid-crystalline polymers (LCP); mechanochromism; transducers
[1] P. G. Guillespie, R. G. Walker, Nature 2001, 413, 194. [2] B. A. Samuel, M. C. Demirel, A. Haque, J. Micromech. Microeng. 2007, 17, 2324. [3] D. A. Davis, A. Hamilton, J. Yang, L. D. Cremar, D. Van Gough, S. L. Potisek, M. T. Ong, P. V. Braun, T. J. Martínez, S. R. White, J. S. Moore, N. R. Sottos, Nature 2009, 459, 68. [4] C. Weder, Nature 2009, 459, 45. [5] a) For a review see: D. R. T. Roberts, S. J. Holder, J. Mater. Chem. 2011, 21, 8256, and references therein; b) D. R. T. Roberts, M. Patel, J. J. Murphy, S. J. Holder, Sens. Actuators B 2012, 162, 43. [6] A. Kishimura, T. Yamashita, K. Yamaguchi, T. Aida, Nat. Mater. 2005, 4, 546. [7] S. Ye, Q. Fu, J. Ge, Adv. Funct. Mater. 2014, 24, 6430. [8] J. Luo, L.-Y. Li, Y. Song, J. Pei, Chem. Eur. J. 2011, 17, 10515.
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Communication
Optical Mechanotransduction with Carbazole-Based Luminescent Liquid Single-Crystal Elastomers This article is dedicated to Prof. Heino Finkelmann on the occasion of his 70th birthday
Jaume Garcia-Amorós, Dolores Velasco* Carbazole-based liquid single-crystal elastomers (LSCEs) are valuable fluorescent flexible materials to perform optical mechanotransduction under ambient conditions. Indeed, the covalent incorporation of carbazole derivatives into nematic LSCEs allows to tune their luminescence on demand under mechanical control in a quick and reversible fashion. Specifically, the fluorescence intensity for these materials can be switched back and forth in less than a second. Moreover, such a process can be performed several times without detecting any sign of fatigue in the system. In addition, these materials show excellent resistance to aging; 2 years after their preparation they exhibit the very same mechanofluorescent behavior as when freshly prepared. In fact, the here reported fluorescent systems are highly sensitive; the application of a force of 70 mN decreases the fluorescence in the elastomeric material by 7%. Thus, mechanical forces are attractive external stimuli to modulate the fluorescence of nematic elastomers rapidly and reversibly enabling thereby mechanotransduction.
1. Introduction Mechanical forces are one of the most common external stimuli in nature. This is the reason why living organisms are endowed with a variety of force-sensitive cells, which enable them to be aware of what is nearby. More specifically, mechanosensory cells specialize in mechanotransduction, that is, the conversion of all incoming mechanical sensations into nerve signals that are sent across neurons to the brain for further processing and integration.[1] Resembling these biological systems, optical mechanotransducers emerge as valuable engineered materials capable of converting mechanical events, either tension or pressure, into readable and processable optical outputs. Dr. J. Garcia-Amorós, Prof. D. Velasco Grup de Materials Orgànics, Institut de Nanociència i Nanotecnologia (IN2UB), Departament de Química Orgànica, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain E-mail: [email protected] Macromol. Rapid Commun. 2015, 36, 755−761 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Indeed, smart materials that tune their optical properties upon deformation or compression are of great interest within actual technology since they allow easy access to unique applications, from damage sensing[2–5] to security inks[6,7] to rewritable paper.[8] Switching the material luminescence between two different emitting states, with either distinct color or intensity, by altering mechanically—shearing, grinding or rubbing—the microenvironment of fluorophores within solid materials can serve as a general protocol for optical mechanotransduction. The initial state of the system can be restored further either thermally (annealing) or chemically, e.g., by recrystallization of the material or by its exposure to vapors of a suitable solvent.[9–16] However, the working principle of such systems makes them too slow for efficient mechanotransduction since they require several hours or even days to switch back and forth. For a quick, efficient, and fully reversible mechanotransduction process to be accomplished, we propose the use of luminescent liquid single-crystal elastomers (LSCEs) as mechanosensitive materials. Indeed, LSCEs
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have been investigated intensively during recent years for a variety of force-related applications, for instance, artificial muscles,[17–19] nano-optomechanical systems[20] and refreshable Braille displays.[21] However, the potential role of LSCEs in optical mechanotransduction remains still unexplored. Briefly, nematic LSCEs are weakly crosslinked polymer networks exhibiting a macroscopic uniaxial orientation of the director, where the orientational long-range order in one dimension of nematic liquid crystals is coupled directly with the elasticity of conventional rubbers.[22] Interestingly, if fluorescent components are covalently attached to the main polymeric backbone of such materials, their luminescence can be switched on demand under mechanical control. In particular, fluorescence intensity switches off upon deformation in a few hundreds of milliseconds for our LSCEs. After the external force is removed, the system fully recovers its pre-stress emission intensity within the very same timescale. In fact, the response time of the LSCE-based optical mechanotransducers reported herein falls within the same range than those exhibited by some human mechanoreceptors like the Pacinian and Meissner's corpuscles (between 100 ms and 1 s), which perceive rapid vibratory pressure and light touch, respectively.[23] Thus, physical forces are valuable external stimuli to tune the fluorescence of nematic LSCEs in a quick and reversible fashion enabling thereby optical mechanotransduction.
2. Experimental Section 2.1. Preparation of the Monomers and Elastomers 4-methoxyphenyl-4-(3-butenyloxy)benzoate (M4OMe), 1,4-di(10-undecenyloxy)benzene (CL), and 9-(5-hexenyl)-9H-carbazole (CBZ6) were prepared as reported elsewhere.[24–26] M4OMe, CL (5% mol), CBZ6, and polyhydrogenomethylsiloxane (≈147 Si–H groups per chain) were dissolved in dry thiophene-free toluene (2 mL). Then, a solution of cyclooctadieneplatinum (II) chloride in dry dichloromethane (1%, 20 μL) was added. The reaction mixture was placed in a spinning Teflon mold and heated up to 70 °C for 2 h at 5000 rpm. Afterwards, the mold was cooled to room temperature and the elastomer (not totally cross-linked) was carefully removed from the wall. After that, a uniaxial stress along the polymer chains was applied to align the mesogenic molecules during the deswelling process. In order to fix this orientation, the cross-linking reaction was completed by leaving the sample under load in the oven at 70 ºC for 2 d. The nonreacted monomers were removed from the network by a swelling– deswelling process using toluene and hexane, respectively.
2.2. Characterization of the Elastomers Differential scanning calorimetry (DSC) measurements were performed in a PerkinElmer DSC-7 apparatus under nitrogen atmosphere at heating rates of 9, 16, 25, and 36 K min−1. The transformation temperatures were determined by extrapolating
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to a heating rate of 0 K min−1. X-ray diffraction (XRD) measurements were carried out with the monochromatic Cu Kα radiation (λ = 1.5418 Å), using a 2D image plate system as a detector. The incident beam was normal to the surface of the elastomer. XRD experiments were realized at 298 K. Stress–strain measurements were performed using a Zwick/Roell Z100Allround instrument with a 10 N load cell.
2.3. Fluorescence Spectroscopy and Mechanofluorescent Experiments Both fluorescence spectroscopy and mechanofluorescent experiments were performed in a PTI 810 fluorimeter at 298 K. Polarized excitation spectra were collected by fixing the emission wavelength at λEm = 350 nm and placing a Glan Thompson polarizer between the light source and the sample. Emission spectra were registered by exciting the carbazole fluorophores at λEx = 290 nm. All mechanofluorescent experiments were carried out by fixing the elastomeric sample by both ends to a homemade measurement cell and further applying a uniaxial deformation stepwise. After deformation, the elastomeric sample was left to equilibrate for 10 min prior to the measurement. In all instances, carbazole fluorophores were excited at λEx = 290 nm, with light polarized parallel to the longest axis of the elastomeric material; the resulting luminescence was collected at λEm = 350 nm.
3. Results and Discussion For the present study, three novel elastomers have been prepared (ECBZ-5, ECBZ-10, and ECBZ-20, a in Figure 1) following the two-step cross-linking technique with a uniaxial stress applied after the first cross-linking stage developed by Küpfer and Finkelmann over the years.[27] All elastomeric materials are composed by a main polysiloxane backbone, which has the mesogen M4OMe attached end-on, the fluorogenic component CBZ6 attached sideon and CL as an isotropic cross-linker. Carbazole, a wellknown organic emitter, has been chosen as a fluorescent platform because, first, it can be easily modified chemically at the required position and, second, it exhibits not only a high-fluorescence quantum yield, even under aerated conditions, but also a great thermal and photochemical stability. The carbazole content in the elastomeric network has been varied from sample to sample (from 5% to 20% mol) in order to assess the influence of the fluorophore concentration on the mechanofluorescent response of the final transducing material. All elastomers were characterized by means of calorimetric and diffractometric methods as well as stress– strain experiments.[18a] DSC shows that both ECBZ-5 and ECBZ-10 exhibit a broad nematic phase between their glass transition temperature at Tg = 276 K and their nematic-to-isotropic phase transition temperature, TN-I, at 327 K and 313 K, respectively, covering thereby the
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Figure 1. a) Chemical composition of the fluorescent elastomers ECBZ-5, ECBZ-10, and ECBZ-20 and b,c) X-ray scattering patterns for ECBZ-10 (b) and ECBZ-20 (c) .
ambient temperature. On the other hand, ECBZ-20, with the highest carbazole content, presents uniquely a glass transition at Tg = 269 K, exhibiting, thus, an isotropic behavior at room temperature. XRD pattern for both ECBZ-5 and ECBZ-10 (b in Figure 1) at 298 K displays two equatorial reflexes in the wide angle X-ray scattering (WAXS) regime (spacing ≈ 4.4 Å), evidencing a macroscopic alignment of the nematic director, n, which lies parallel to the longest axis of the probe. From this reflection, the order parameter for the mesogenic units, S, was determined to be 0.74 for both ECBZ-5 and ECBZ-10. Contrarily, ECBZ-20 exhibits only a diffused halo (c in Figure 1), manifesting again its isotropic nature. Young's modulus (E) for both ECBZ-10 and ECBZ-20 was determined by stretching the samples parallel to their longest axis at 298 K. Specifically, E values of 0.53 and 0.30 MPa were determined for ECBZ-10 and ECBZ-20, respectively, evidencing, once again, the isotropic behavior of the latter.
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Steady-state emission spectra of elastomers ECBZ-5, ECBZ-10, and ECBZ-20 (a–c in Figure 2) at 298 K shows a main emission band peaking at 350 and 367 nm upon excitation at λEx = 290 nm. In fact, identical bands are also present in the emission spectrum of a dichloromethane solution of CBZ6 (d in Figure 2). Such feature together with the absence of fluorescence in the 400–430 nm range, where the emission arising from possible carbazole aggregates or excimers might appear,[28,29] suggests that although the carbazole moieties are somehow organized within the elastomeric network due to the presence of the nematic mean field, they are essentially isolated. In addition, a weak emission band centered at 468 nm was also observed for all the elastomeric systems analyzed. Literature findings reveal that such band can be related to the phosphorescent emission of the carbazole units.[30,31] Mechanofluorescent experiments involve registering the evolution of the material fluorescence under
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nature. Therefore, and according to that abovementioned, nematic LSCE, ECBZ-10 is the most efficient optical mechanotransducer among those tested herein. Even though ECBZ-10 shows a similar sensitivity than ECBZ-5, the former exhibits a greater variation of its fluorescence intensity when it is stretched parallel to n. In order to unravel the source of the mechanofluorescent properties exhibFigure 2. Normalized emission spectrum of a) ECBZ-5, b) ECBZ-10, c) ECBZ-20 and a CH2Cl2 solution of d) CBZ6 at 298 K upon excitation at λEx = 290 nm and variation of the ited by both nematic LSCEs ECBZ-5 and excitation intensity at 295 nm against the angle between the polarization vector of the ECBZ-10, not only the orientation of the incoming light and the nematic director, n (e). In the latter experiment, the emission carbazole units within the elastomeric wavelength was fixed at 355 nm. network but also its possible modification upon deformation of the system was investigated. The steady-state excitation spectrum mechanical deformation. Such measurements were perof ECBZ-10 at 298 K shows a main band peaking at ca. formed by fixing the elastomeric sample by both ends 295 nm which, according to Johnson, can be assigned to the measurement cell and further applying a uniaxial to the S2 ← S0 transition of the carbazole moieties; the mechanical force stepwise. In all instances, carbazole fluorophores were excited at λEx = 290 nm, with light transition moment for such transition is placed along the longest axis of the carbazole molecule.[32] On the polarized parallel to the longest axis of the elastomeric material; the resulting luminescence was collected at other hand, the dependence of the excitation intensity against the angle formed by the polarization vector of λEm = 350 nm. Fluorescence at this particular wavelength the incoming light and the nematic director shows that diminished dramatically after deformation of the sample the maximum intensity for this band is achieved at 0° along the director direction for both nematic LSCEs, and 180° (e in Figure 2), respectively. Such feature sugECBZ-5 and ECBZ-10 (a,b in Figure 3). Indeed, the mechagests that the carbazole moieties are oriented within the nofluorescent behavior of these two uniaxially oriented elastomeric network with their longest axis parallel to n. elastomers can be fitted to a monoexponential decay Indeed, a local order parameter of 0.56 was determined function (Equation (1)): for the carbazole fluorophores from the corresponding (1) ΔI ε = ΔI ∞ + ΔI max × exp ( − s × ε ) polarized excitation spectra. The very same dependence was also studied during the deformation of the elastomeric sample in order to detect any change in the orienwhere ΔIε and ΔI∞ correspond to the relative fluorestation of the carbazole units. Maximum intensity for this cence intensity (ΔIε = ((Iε−I0)/I0)×100) at a determined band was attained when the incoming light is polarized deformation, ε, and at an infinite one, respectively. In at 0° and 180° with respect to n independently of the addition, fitting of Equation (1) to the experimental data sample deformation. This feature evidences that the caryields the maximum variation in the material luminesbazole molecules do not tilt during the deformation procence upon deformation (ΔImax in Figure 3) as well as the cess. As it is well known, nematic order in LSCEs increases mechanical sensitivity of the transducing material (s in upon stretching of the sample along the director direcFigure 3), which are both key parameters in the overall tion.[33] As a consequence, a more favorable overlap performance of optical mechanotransducers. Specifically, ΔImax values of −27% and −34% were obtained for elastobetween both carbazole units and surrounding mesogen molecules should occur. Such variation in the microenmers ECBZ-5 and ECBZ-10, respectively. Similar s values vironment where the carbazole fluorophores are located of ca. 11 were estimated for both systems. The mechanoresults in a quenching of their fluorescence originating, fluorescent response for ECBZ-10 was also investigated in turn, the macroscopic mechanofluorescent response upon deformation along the direction perpendicular to n. observed for these elastomeric systems. In this way, when In this case, only a residual mechanofluorescent response the isotropic elastomer ECBZ-20 is stretched either par(ΔImax ≈ −3%, d in Figure 3) was obtained due to the lack of allel or perpendicular to its longest axis as well as when order in the system along this spatial direction, in agreethe nematic LSCEs ECBZ-5 and ECBZ-10 are stretched in ment with its uniaxially oriented nematic structure. Such the direction perpendicular to n, where neither orienbehavior was also observed for the isotropic elastomer tational nor positional order exists, no mechanofluoresECBZ-20 independently of the stretching direction (ΔImax cent behavior is detected. Thus, it can be stated that the ≈3%, c,e in Figure 3), as a consequence of its disordered
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Figure 3. Mechanofluorescent response for elastomers a) ECBZ-5, b,d) ECBZ-10 and c,e) ECBZ-20 upon deformation along the direction a,c) parallel and d,e) perpendicular to the longest axis of the elastomeric sample and schematic representation (f) of the fluorescence quenching observed for the nematic LSCEs ECBZ-5 and ECBZ-10 when stretched along the direction parallel to n.
mechanofluorescent response observed for these elastomeric systems is coupled directly with the intrinsic liquid crystal order of the material. Time-resolved mechanofluorescent experiments, which involve registering the evolution of the material fluorescence over time upon applying or releasing an external physical force in a periodic fashion, allow to determine two other key parameters of mechanotransducers, namely, their response time and robustness. In fact, these two parameters will establish the potential applicability of the final material. Time-resolved mechanofluorescent experiments were carried out exclusively with the nematic LSCE ECBZ-10 since it showed the best performance for further technological applications. Fluorescence can be rapidly and reversibly modulated for ECBZ-10 by means of the presence or absence of a mechanical force. More specifically, when the system was stretched along the director up to ε = 0.05 (F = 75 mN), its
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relative fluorescence intensity decreased rapidly by 7% (a in Figure 4). Indeed, characteristic times of tON = 500 ms and tOFF = 500 ms were determined graphically for the decrease in intensity upon stretching of the sample and for the subsequent recovery of the prestress value upon releasing the force, respectively. More specifically, tON and tOFF are defined as the time required to undergo any of these two processes in an extension of 50%. Thus, a full cycle can be completed in approximately one second. Therefore, the elastomeric systems presented here are valuable systems to perform fast and efficient optical mechanotransduction under ambient conditions. In addition, the carbazole-based nematic LSCE ECBZ-10 can be also used as a storage device since the fluorescence intensity variation produced upon its deformation can be maintained for several hours and even more than a week (c in Figure 4). In this way, the information to be stored in this material can be rapidly written in the system within
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Figure 4. Time-resolved a) mechanofluorescent experiment, b) fatigue resistance test for the nematic LSCE ECBZ-10 and evaluation of the possible use of such elastomeric material for information storage purposes (c).
the millisecond timescale and kept there for the desired period of time. Once the external force is removed, the initial state of the system is fully restored thereby deleting the stored data in a quick fashion. The robustness of the ECBZ-10-based optical mechanotransducer was tested through the determination of its repeatability, fatigue, and aging resistance. Repeatability and fatigue resistance for such material were evaluated by applying and releasing a fixed force of 75 mN in a periodic fashion to the elastomeric material (b in Figure 4). Specifically, the optical mechanotransducer based on this nematic LSCE exhibited a great fatigue resistance and reversibility at 298 K since after several cycles neither its relative change in the fluorescence intensity nor its
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relaxation times were altered by the continuous work of the system (variation in ΔIε = 0.05 was less than 1% upon 15 consecutive cycles). In a further attempt to demonstrate the high robustness of the ECBZ-10-based optical mechanotransducer, aging tests were conducted. Indeed, mechanofluorescent experiments carried out 2 years after the fabrication of elastomer ECBZ-10 have shown no significant variation in the characteristic parameters of the mechanotransducer (ΔImax = 38% and s = 10).
4. Conclusion Carbazole-based LSCEs are valuable fluorescent flexible materials to perform optical mechanotransduction under
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ambient conditions. Such systems admit carbazole contents within their elastomeric networks up to 10% without losing their liquid-crystalline properties. The intrinsic nematic order of LSCEs allows the reorganization of the mesogenic molecules upon their deformation along the director direction forcing thereby a stronger interaction with the carbazole fluorophores. This feature results in a quenching of the luminescence of the latters. In this way, the application of external physical forces to carbazolebased LSCEs enables tuning the fluorescence of these materials and, thus, mechanotransduction. Remarkably, time-resolved mechanofluorescent experiments revealed that one full cycle can be completed for these systems in approximately one second approaching, therefore, to the response time of biological mechanoreceptors. Moreover, these materials are useful also to store information for weeks, which can be erased in a few hundreds of milliseconds when it is not needed anymore. In addition, the elastomeric materials reported here show an outstanding robustness exhibiting thereby a great resistance to fatigue and aging, enabling, thus, their further use in technological devices. Acknowledgements: Financial support for this research was obtained from the Ministerio de Economía y Competitividad (Spain, CTQ2012–36074). J.G.-A. is grateful for a Beatriu de Pinós post-doctoral grant from the Generalitat de Catalunya (Spain, 2011 BP-A-00270). Thanks are also due to Zwick/Roell for the determination of Young’s moduli. Received: December 23, 2014; Revised: January 23, 2015; Published online: February 19, 2015; DOI: 10.1002/marc.201400734
Keywords: elastomers; fluorescence; liquid-crystalline polymers (LCP); mechanochromism; transducers
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