Highly Efficient Elastomeric Fluorescence Sensors for Force Detection

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Highly efficient elastomeric fluorescence sensors for force detection Daniel Heras, Marta Reig, Nuria LLorca-Isern, Jaume Garcia-Amorós, and Dolores Velasco ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00230 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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ACS Applied Polymer Materials

Highly Efficient Elastomeric Fluorescence Sensors for Force Detection Daniel Heras†, Marta Reig†, Núria Llorca-Isern‡, Jaume Garcia-Amorós† and Dolores Velasco†,* †

Grup de Materials Orgànics, Institut de Nanociència i Nanotecnologia (IN2UB),

Departament de Química Inorgànica i Orgànica (Secció de Química Orgànica), Universitat de Barcelona, Martí i Franquès 1, E-08028, Barcelona, Spain ‡

Departament de Ciència dels Materials i Química Física (Secció de Ciència dels Materials),

Universitat de Barcelona, Martí i Franquès 1, E-08028, Barcelona, Spain *[email protected] Abstract: Carbazole-containing nematic liquid single crystal elastomers (LSCEs) alter their luminescence upon the application of an external mechanical force. Therefore, they are valuable flexible materials to detect mechanical events with simple fluorescent measurements. In this work, we have focused our attention on the main principles underlying the operation of these materials and the development of novel design schemes to produce efficient elastomeric fluorescence sensors for force detection. In this context, comprehending and controlling the interactions established between the distinct components of the active material, i.e. mesogens and fluorophores, is essential to achieve force-sensitive materials with improved performances. With this purpose in mind, we have explored the role of two structural features on such phenomenon, namely, the type of connection (end-on or side-on) of the carbazole fluorophores to the elastomeric network and the length of the alkyl chain that binds them to the main polysiloxane backbone. As a whole, end-on carbazole fluorophores with short or medium flexible spacers enable a much better approximation to the mesogenic moieties upon deformation, promoting quenching, and resulting in more efficient force sensors. Keywords: carbazole, elastomers, fluorescence, liquid crystals, sensors, smart materials. 1 ACS Paragon Plus Environment

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Introduction Smart materials are high-tech materials possessing one or more characteristic features that can be tuned by a diversity of environmental changes such as mechanical forces. In particular, mechanical events can be designed to alter the interactions established between organic fluorophores and the host matrix where they are embedded. Thus, the material luminescence at the macroscopic level can be engineered to vary with mechanically-induced fluctuations in the molecular arrangement and intermolecular interactions at the nanoscale. 1−8 Under these conditions, mechanical inputs can be transduced into detectable and processable optical signals giving rise to promising force sensors. Currently, researchers are in enthusiastic pursuit of artificial materials that permit the finetuning of their optical properties under mechanical control. In fact, mechanoluminescent materials have sparked a great interest recently due to their widespread applications, ranging from clinical diagnostics to information processing to damage detection.9−35 Driven by the increasing demand for simple force sensors with high sensitivity and efficiency, we have been engaged in the development of novel mechanofluorescent liquid single crystal elastomers that might fullfill these requirements. However, even though the application of physical forces to elastomeric networks with covalently-attached fluorogenic components results in fast and fully-reversible variations in the material fluorescence with low mechanical demands, the switch in their fluorescence upon deformation does not exceed 30%,36,37 thus compromising their sensing abilities. In this article, we report on the elucidation of not only the driving force but also some structural features controlling the mechanoluminescent behavior of these functional materials, which is essential to rationalize this effect and possibly guide the future design of LSCE-based force sensors with improved performances. In particular, we have got insight into the intermolecular interactions established between the mesogenic units and the carbazole fluorophores as a main parameter governing this phenomenon. On the other hand, 2 ACS Paragon Plus Environment

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we have also explored how the anchoring mode (side-on or end-on) of the fluorogenic monomers to the elastomeric network and the length of the flexible alkyl chain that connects them to the main polymeric chain influences the sensing abilities of the final material.

Results and Discussion Scheme 1 summarizes the structures of the different elastomeric materials. For the present study, two series of nematic liquid single crystal elastomers have been considered. The first one covers LSCEs ECBZ-O6 and ECBZ-OMe-N6, and will permit to explore how an end-on or a side-on connection of the carbazole monomers to the polysiloxane network impacts the sensing abilities of the material. The second one includes elastomers ECBZ-O3, ECBZ-O6 and ECBZ-O11 (denoted as ECBZ-OX in Scheme 1), where the carbazole-based fluorogenic component is attached end-on through flexible spacers of systematically-increased length. All LSCEs have been prepared following the synthetic methodology reported previously by Küpfer and Finkelmann. 38 The different elastomeric materials contain the nematogen M4OMe attached end-on (80% mol), the isotropic cross-linker CL (10% mol) and the corresponding fluorogenic comonomer attached either end-on (CBZ-OX, 10% mol) or sideon (CBZ-OMe-N6, 10% mol). In all instances, the main polymeric backbone is polyhydrogenomethylsiloxane. The chemical structure of the different monomers is shown in Scheme S1. The liquid-crystalline phase exhibited by the LSCEs at room temperature has been determined by X-ray diffraction. As a representative example, the X-ray diffraction pattern for the LSCE ECBZ-O3 is shown in Figure S1. The presence of a broad reflex in the wide-angle region [2θ = 19.7−20.0º, spacing 4.4−4.5 Å (d in Table 1)] of the X-ray scattering pattern points out the existence of a nematic mesophase at 298 K. In addition, the highly anisotropic azimuthal intensity distribution of this reflex indicates that the mesogens are homogeneously organized 3 ACS Paragon Plus Environment


all over the elastomer, i.e. that monodomain elastomeric materials have been obtained. Specifically, the order parameter (S in Table 1) of the mesogenic units has been estimated to be between 0.56 and 0.69. The thermal range of stability of the nematic mesophase has been established by differential scanning calorimetry (DSC). DSC thermograms reveal that the system adopts an enantiotropic nematic mesophase between a glass transition temperature (Tg in Table 1) at 279−283 K, with an associated isobaric specific heat (ΔCP in Table 1) of 0.3−0.6 J g−1 K−1, and a nematic-to-isotropic phase transformation temperature at 318−330 K (TN−I in Table 1), with an associated enthalpy (ΔHN−I in Table 1) of 0.9−1.6 J g−1. Thus, DSC analyses confirm that all LSCEs exhibit a nematic mesophase at 298 K. Mechanofluorescent experiments involve monitoring the emission intensity (IF) of the elastomeric material when a uniaxial force is applied stepwise along its longest axis, that is, parallel to the anisotropic direction of the sample (Figure 1a). Carbazole fluorophores have been excited with UV light (λEx = 315 nm), polarized perpendicular to the director n. As a whole, IF (or in relative terms ΔIε = (Iε−I0/I0)·100, where Iε and I0 correspond to the fluorescence intensity of the stretched and unstretched sample, respectively) drops dramatically upon deformation (Figure 1b and c) until a plateau is eventually reached. The constant value of ΔIε achieved (ΔIMax in Figure 1c) corresponds to the maximum variation in the relative emission intensity of the LSCE that can be produced upon mechanical stimulation. Specifically, ECBZ-O6 diminishes its fluorescence at a λEm of 360 nm by 93% (Figure 1c). Such a huge variation in the emission intensity before and after elongation generates two interconvertible states for the system with excellent contrast and enables to switch its fluorescence from on to off under mechanical control. Actually, this particular LSCE is, to the best of our knowledge, not only the most efficient elastomer-based force sensor reported in the literature heretofore, but is also able to operate under ambient conditions.

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On the wake of these inspiring results, we have explored how the distinct linkage (end-on or side-on) of the carbazole monomers to the elastomeric network affects the mechanofluorescent behavior, and in turn the sensing abilities, of the resulting material. A comparison between nematic LSCEs ECBZ-O6 and ECBZ-OMe-N6 yields that the former system, with the carbazole fluorophores attached end-on, is significantly more efficient than the latter. Indeed, while emission intensity at 360 nm diminishes by 93% for ECBZ-O6 (a in Figure 2) upon deformation, a ca. 3-fold smaller ΔIMax value of 28% was registered for its side-on counterpart ECBZ-OMe-N6 (b in Figure 2). In this way, the mode by which the carbazole fluorophores are connected to the main polymeric backbone is crucial to achieve elastomeric materials with improved sensing abilities. The mechanofluorescence exhibited by these effective materials depends also on the length of the flexible alkyl chain that binds the carbazole fluorogenic moieties to the polysiloxane network. In order to explore the effect of this particular structural factor, LSCEs ECBZ-O3 and ECBZ-O11 have been considered and compared with parent ECBZ-O6 (see above). As a whole, all three LSCEs show essentially the same qualitative behavior. Data displayed in Figure 3, however, reveals that the length of the flexible spacer impacts noticeably ΔIMax and, in turn, the response of the final force-sensitive material. Specifically, ΔIMax values of 87% and 55% have been registered for ECBZ-O3 and ECBZ-O11 (a and b in Figure 3), respectively. In this way, a remarkably large mechanofluorescence is detected when short (m = 3) and medium (m = 6) flexible spacers connect the carbazole fluorophores to the elastomeric network while it decreases significantly when long flexible spacers (m = 11) are used instead. Two different causes might be responsible for not only the efficient mechanoluminescence exhibited by LSCEs ECBZ-OX but also the differential behavior with their side-on analogue ECBZ-OMe-N6. Specifically, luminescence suppression after deformation can occur either 5 ACS Paragon Plus Environment


because of mechanically-induced spatial reorientations of the carbazole units when the external force is applied on the elastomeric material or due to fluorescence quenching produced by a more effective mesogen-fluorophore interaction upon stretching. In order to investigate the first hypothesis, the distinct electronic transitions involved in the fluorescent emission of these elastomeric materials have been analyzed. For this purpose, both absorption and emission spectra of model compounds 9-methyl-9H-carbazole and 2-methoxy9-methyl-9H-carbazole (CBZ-NMe and CBZ-OMe respectively, Figure 4) have been registered. The absorption spectrum (a in Figure 4) of CBZ-NMe in CH2Cl2 at 25 ºC shows two main absorption bands peaking at 293 and 345 nm, with molar absorption coefficients of 12.0 and 3.1 mM−1 cm−1. Literature findings reveal that these absorptions can be ascribed to the S0 → S2 and S0 → S1 transitions of carbazole, respectively. 39 On the other hand, these absorption bands overlap significantly for CBZ-OMe due to the presence of the electrondonating methoxy group. Specifically, the corresponding absorption spectrum (b in Figure 4) reveals signals centered at 303 nm (S0 → S2) and 334 nm (S0 → S1), with molar absorption coefficients of 13.2 and 2.7 mM−1 cm−1. In both instances, excitation at a λEx positioned within these absorptions results in significant fluorescence along the UV region of the electromagnetic spectrum, arising from the S1 → S0 transition of the carbazole fluorophore. In particular, the emission spectra of CBZ-NMe and CBZ-OMe (c and d in Figure 4) display bands peaking at 349/365 and 342/358 nm, respectively, under these experimental conditions. The orientation of the transition dipole moments for the S0 → S1 and S0 → S2 transitions within the carbazole molecular framework has been determined by doping CBZ-NMe and CBZ-OMe into rigid polypropylene (PP) polymeric matrixes. The resulting probes were subsequently elongated to induce a proper alignment of the carbazole moieties along the stretching direction.40,41 Specifically, the rod-like carbazole fluorophores orient in such a way that their longest molecular axis lies parallel to the stress axis. 42,43 The orientation of the 6 ACS Paragon Plus Environment

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transition dipole moments was deciphered by means of polarized fluorescence spectroscopy. The excitation (e and f in Figure 4) and emission (g and h in Figure 4) spectra of PP films doped with CBZ-NMe and CBZ-OMe show essentially the same spectroscopic signature observed in CH2Cl2 (a-d in Figure 4). The dependence of the fluorescence intensity at two different excitation wavelengths (292 and 342 nm for CBZ-NMe and 304 and 331 nm for CBZ-OMe, respectively) with β (Figure 4), i.e. the angle between the polarization vector of the excitation beam and the stress axis of the sample, has been monitored. These two sets of excitation wavelengths are positioned within the absorption bands arising from the S0 → S2 and S0 → S1 transitions of the carbazole moieties, respectively. For CBZ-NMe, fluorescence intensity at 292 and 342 nm (i and j in Figure 4) becomes maximum when β is equal to 0° and 90°, respectively (Figure 4). Thus, the transition moment for the S0 → S2 and S0 → S1 transitions is placed along the longest and shortest axes of the carbazole heterocycle respectively, and perpendicular to each other, in perfect agreement with previous literature reports.39 On the other hand, maximum fluorescence intensity at 304 and 331 nm (k and l in Figure 4) for CBZ-OMe is achieved at 10° and 70°, respectively. In this case, the transition dipole moments of the S0 → S2 and S0 → S1 transitions for CBZ-OMe are not orthogonal but form an angle of 60° instead (Figure 4). In order to detect any mechanically-induced spatial reorientation of the carbazole units when the external force is applied on the elastomeric material, we registered the polarized excitation spectra for the end-on carbazole-containing LSCEs ECBZ-O3−ECBZ-O11 before and after deformation. Figure 5 shows the evolution of the emission intensity at 345 nm with β for LSCE ECBZ-O3. This particular wavelength falls within the absorption band ascribed to the S0 → S1 transition of the carbazole fluorophores. In all instances, the emission wavelength was set at λEm = 370 nm. Maximum fluorescence intensity at 345 nm is achieved at β = 87°, independently of the deformation applied to the sample (a-c in Figure 5). This fact indicates 7 ACS Paragon Plus Environment


that the carbazole molecules do not tilt further during the deformation of the elastomeric material and, in turn, that the huge weakening of the fluorescence detected experimentally is not a result of any physical reorientation of the carbazole units with respect to the polarization direction of the excitation beam. LSCEs ECBZ-6 and ECBZ-11 exhibited the very same behavior as ECBZ-O3. In the second hypothesis proposed, fluorescence quenching might occur due to the establishment of more effective mesogen-fluorophore intermolecular interactions upon stretching of the LSCE. In this case, we have assumed that the mesogenic units, which are in the highest proportion within the elastomeric network, should be the quencher. To corroborate this idea, we recorded the emission spectrum of a 15 μM solution of CBZ-OMe in CH2Cl2 before and after the addition of specific volumes of a 55 mM solution of M4OMe (Figure 6) in the very same solvent. Indeed, as the concentration of the quencher increases fluorescence is rapidly suppressed. Importantly, this experiment reveals that very small concentrations of the mesogen M4OMe reduces significantly the fluorescence of CBZ-OMe and, therefore, that the mesogenic units present in the elastomeric networks are effective quenchers of the carbazole luminescence. As it is widely reported, the uniaxial deformation, along the director direction, of an LSCE increases its nematic order with a concomitant stronger interaction between the mesogenic units.44 Similarly, the deformation of the elastomeric material should also result in a more favorable carbazole-mesogen interaction. In this way, and considering the previous experiments in solution, such mechanically-enhanced intermolecular interactions of the carbazole units with the mesogenic platforms at the nanoscale are responsible of their fluorescence quenching, producing the mechanoluminescent response observed at the macroscopic level.


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According to this model, an end-on connection of the carbazole fluorophores to the polymer network, the very same as the mesogens, enables a much stronger interaction with the surrounding mesogenic molecules upon deformation than that established when the formers are linked side-on. Such feature explains the much more efficient mechanofluorescent response observed for the end-on LSCE ECBZ-O6 in comparison to that of its side-on analogue ECBZ-OMe-N6. On the other hand, the different mechanoluminescent behavior observed for LSCEs ECBZ-O3−ECBZ-O11 can be interpreted in terms of the conformational freedom of the corresponding flexible spacer. When short (m = 3) or medium (m = 6) spacers are used, the force applied to the elastomeric material is effectively transmitted from the polymer chains to the pendant carbazole units producing, thus, a major decrease in the material luminescence. As the length of the flexible spacer increases (m = 11), the high conformational freedom of the linker can dissipate the mechanical input leading, therefore, to lower ΔIMax values. In this way, not only the connection mode (side-on or endon) of the carbazole monomers to the elastomeric network but also the length of the alkyl chain that binds them to the polymeric backbone are two key structural features to consider when efficient fluorescence-based force sensors are sought-after. The response time of the force sensors has been determined by means of time-resolved mechanofluorescent measurements. In these experiments, the evolution of the material fluorescence at λEm = 360 nm is monitored over time before and after the application of an external physical force along the director direction. Figure 7a shows a representative timeresolved mechanofluorescent experiment for the end-on nematic LSCE ECBZ-O3. Experimental data evidences that fluorescence changes rapidly either when the external mechanical force is applied (ε = 0.015) or when it is released. Indeed, characteristic times36 (τON and τOFF in Figure 7a) of 500 ms have been estimated for both processes. In this way,



these mechanoresponsive materials can complete a full operating cycle in approximately one second. All LSCEs showed identical τON and τOFF values. The robustness of the sensors has been tested by stretching and relaxing the LSCE in a periodic fashion (Figure 7b) and monitoring the effect on the characteristic features of the elastomeric material. The continuous and prolonged work of the system did not affect neither ΔIMax nor τON / τOFF. Therefore, the design schemes here presented have evolved into a new family of efficient and robust fluorescence-based force sensors capable of converting mechanical forces into optical signals under ambient conditions.

Conclusion Carbazole-containing LSCEs are invaluable materials to convert mechanical inputs into optical signals. The uniaxial deformation of the material along its anisotropic direction forces a closer mesogen-fluorophore interaction, thereby leading to the quenching of the carbazole luminescence. Remarkably, the connection mode, i.e. side-on or end-on, of the carbazole fluorogenic monomers to the elastomeric network and the length of the flexible alkyl chain that connects them to the main polymeric backbone are two structural features that govern such interaction and, thus, they are two essential parameters to consider in the engineering of novel mechanofluorescent systems with improved sensing abilities under ambient conditions. Specifically, LSCEs containing end-on carbazole fluorophores with short or medium flexible spacers (m ≤ 6) are the best performing force sensors. Indeed, these elastomeric materials are able to turn their fluorescence from on to off at room temperature under mechanical control.

Acknowledgments Financial support from the Ministerio de Economía y Competitividad (Spain, grant CTQ201565770-P MINECO/FEDER) is gratefully acknowledged. Authors thank Dr. Xavier Alcobé for performing the XRD experiments. 10 ACS Paragon Plus Environment

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Supporting Information Supporting

Information

Available:

synthesis

of

the

monomers,

preparation

and

characterization of the LSCEs, absorption and emission spectroscopies, preparation of the fluorophore-doped polypropylene films for the determination of the transition dipole moment orientation, and mechanofluorescent experiments.

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Tables Table 1. Glass transition (Tg) and nematic-to-isotropic phase transformation (TN–I) temperature, change of the isobaric specific heat during the glass transition (ΔCP), nematic-toisotropic phase transformation enthalpy (ΔHN–I), distance between the mesogenic units (d) and nematic order parameter (S) for the nematic LSCEs ECBZ-O3, ECBZ-O6, ECBZ-O11 and ECBZ-OMe-N6.

LSCE

Tg [K]

ΔCP [J g−1 K−1]

TN−I [K]

ΔHN−I [J g−1]

d [Å]

S

ECBZ-O3

283

0.4

329

1.3

4.5

0.64

ECBZ-O6

282

0.6

330

1.6

4.5

0.67

ECBZ-O11

279

0.3

330

1.0

4.4

0.69

ECBZ-OMe-N6

282

0.5

318

0.9

4.5

0.56



Schemes and Figures Scheme 1. Chemical structure of the end-on LSCEs ECBZ-O3, ECBZ-O6 and ECBZ-O11, and their side-on analogue ECBZ-OMe-N6.


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Figure 1. Mechanofluorescent behavior of the LSCEs: (a) elongation of the nematic LSCE ECBZ-O6 by applying a uniaxial force along n. Evolution of the polarized emission spectrum (b) and the relative emission intensity (c) at λEm = 360 nm upon deformation of the nematic LSCE ECBZ-O6 (λEx = 315 nm).



Figure 2. Effect of the fluorophore anchoring mode (side-on or end-on) on the mechanofluorescent response: evolution of the relative emission intensity, ΔIε, at a fixed λEm of 360 nm, for the end-on ECBZ-O6 (a) and side-on ECBZ-OMe-N6 (b) nematic LSCEs upon deformation (ε) along their longest axis (λEx = 315 nm). The maximum variation in the relative emission intensity upon deformation, ΔIMax, is also indicated for each elastomeric sample.


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Figure 3. Effect of the fluorophore flexible spacer on the mechanofluorescent response: evolution of the relative emission intensity, ΔIε, at a fixed λEm of 360 nm, for the end-on LSCEs ECBZ-O3 (a) and ECBZ-O11 (b) upon deformation (ε) along the director direction (λEx = 315 nm). The maximum mechanofluorescent response, ΔIMax, displayed by these two elastomers is also indicated; ΔIMax for parent end-on LSCE ECBZ-O6 is 93% (see Figure 2).



Figure 4. Orientation of the transition dipole moments for the S0 → S1 and S0 → S2 transitions within the carbazole molecular framework: absorption (a and b) and emission (c and d, λEx = 290 nm) spectra of dichloromethane solutions (30 μM, 25 ºC) of CBZ-NMe (a and c) and CBZ-OMe (b and d). Excitation (e and f, λEm = 370 nm) and emission (g and h, λEx = 290 nm) spectra of polypropylene films doped with either CBZ-NMe (e and g) or CBZ-OMe (f and h) and evolution of their fluorescence intensity (i-l) with the polarization angle (β) at 292 (i) and 342 nm (j) [CBZ-NMe], and at 304 (k) and 331 nm (l) [CBZ-OMe].


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Figure 5. Fluorescence intensity at 345 nm (λEm = 370 nm) at different angles between the polarization vector of the incoming light and the nematic director (β) for the liquid single crystal elastomer ECBZ-O3 before deformation (a), at a deformation ε = 0.013 (b) and after the corresponding ΔIMax value is reached (c).



Figure 6. Evolution of the emission spectrum (λEx = 290 nm) of a 15 μM solution of CBZOMe in dichloromethane with the successive addition of a 55 mM solution of M4OMe in the very same solvent at 25 ºC.


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Figure 7. Time-resolved mechanofluorescent experiment (a, λEx = 315 nm, λEm = 360 nm) and fatigue resistance test (b, ε = 0.015) for the end-on nematic LSCE ECBZ-O3 under ambient conditions.



Graphical Abstract


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Highly Efficient Elastomeric Fluorescence Sensors for Force Detection

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