Stimuli-responsive Materials from Ferrocene-based Organic Small Molecule for Wearable Sensors

Stimuli-Responsive Materials from Ferrocene-Based Organic Small Molecule for Wearable Sensors. Small 2021, 2103125, which has in accordance with Wiley Terms and Conditions for self-archiving. Abstract : Stimuli-responsive crystals capable of energy conversion have emerged as promising materials for smart sensors, actuators, wearable devices, and robotics. Here we report a novel ferrocene-based organic molecule crystal ( Fc-Cz ) that possesses anisotropic piezoelectric, optical, and mechanical properties. We demonstrate that the new crystal Fc-Cz can be utilized as an ultrasensitive piezoelectric material in fabricating strain sensors and energy harvesters. The flexible sensor made of crystal Fc-Cz can detect small strains/deformations and motions with a fast response speed. Analysis based on density functional theory (DFT) indicates that an external pressure can affect the dipole moment by changing the molecular configuration of the asymmetric single crystal Fc-Cz at the crystalline state, leading to a change of polarity, thereby an enhanced dielectric constant. This work has demonstrated a new artificial organic small molecule for high-performance tactile sensors, indicating its great potential for developing low-cost flexible wearable sensors and energy harvesters. A novel ferrocene-based organic molecule crystal (Fc-Cz) that possesses anisotropic piezoelectric, optical, and mechanical properties. We demonstrate that the new crystal Fc-Cz can be utilized as an ultrasensitive piezoelectric material in fabricating strain sensors and energy harvesters.


Introduction
Stimuli-responsive materials have been widely used for actuators and sensors in advacned robotics, [1] structural health monitoring devices, [2] and human machine interfaces, [3] due to their high sensitivity and fast response to the variation of external environmental conditions (e.g., pressure, temperature, and pH). [4,5] As one common smart material, piezoelectric materials are utilized to develop pressure sensors of smart wearables, and much effort is focused on how to improve the sensitiveity, response speed, and operation stability through modifying material constitution and microstructures. Piezoelectric effect is atributed to the polarization charges of piezoelectric materials under mechanical deformations along the polarization direction in the non-centrosymmetric materials. [6] Many piezoelctric materials have been used for constructing E-skins, including carbon-based materials (carbon nanotubes, [7] graphene [8] ), polymer nanofibers, [9] perovskite, [10] 2D layered materials (such as α-In 2 Se 3 ), [11][12][13] inorganic complexes [14,15] as well as bio-organic and natural biomaterials. [16] Compared with traditional inorganic piezoelectric materials, synthesized small organic molecules may be better cadidates for novel piezoelectric materials because they i) possess tunable molecular frameworks, ii) adopt precise molecular structures with unique molecular conformations, and iii) exhibit controllable molecular packing in solid state. Moreover, theoretical calculations have revealed that the presence of hydrogen bonding plays a significant role in achieving a larger piezo-response. [17] Organic materials with numerous hydrogen bonds in their molecular packing can be efficient organic piezo-response materials for enhacing their piezoelectric properties.
Some bio-organic and natural biomaterials such as amino acids/proteins possess relatively weak piezoelectric effects and can be used for nanoelectronics. [18,19] For example, Thompson and coworkers utilized the piezoelectric properties of glycine polymorphs for biopiezoelectric sensing and energy harvesting. [20] Kholkin et al. observed that self-assembled diphenylalanine peptide nanotubes (PNTs) exhibited strong anisotropic piezoelectricity with the orientation of polarization along the axis of the tube. [21] However, developing unique molecular architectures with strong piezoelectricity based on man-made organic molecules remains challenging and rare solutions are reported.
Ferrocene, Fe(C 5 H 5 ) 2 , is an aromatic hydrocarbon compound with a sandwiched structure. The Fe atom between the two parallel cyclopentadienyl (Cp) rings exhibits an intriguing magnetic property and shows promise in electron transport and spin filtering. [22,23] Thus, various ferrocene-based complexes have been investigated for organic photoelectric-magnetic functional materials. On the other hand, due to the high-energy lone pair at the Fe atom in the ferrocene unit, ferrocene-containing compounds exhibit weak/non-emission through intramolecular photoinduced electron transfer (PET) reactions, which may be an excellent "turn-on" probe to detect the metal cation and anion by lowing the energy of the lone pair. [24] Aggregation-induced emission (AIE) is a new phenonma in which a series of twisted luminogens exhibited non-luminescent in solution but enhanced fluorescence intensity in solid state. [25][26][27] Cyanostilbene derivatives are widely used to develop new AIE luminogens, and these organic molecules exhibit high efficiency in mechanoluminescence (ML) and luminescence photoluminescence (PL) at solid state. Moreover, the conformation of the packing mode significantly affects the ML behavior of AIE. [28][29][30] However, it is not clear how AIE luminogens interact with organic photoelectric-magnetic materials.
Here, a new small organic molecule (Fc-Cz) is designed and readily synthesized by combining the cyanostilbene and the ferrocene motifs via Knoevenagel reaction. The crystal packing, thermal behaviors, photophysical properties, and electrical properties of the Fc-Cz are investigated in detail. Experimental results show that the crystalline Fc-Cz has strong anisotropic piezoelectricity under external stimuli, achiving a maximum peak current of 580 pA. Flexible tactile sensors are developed based on the advanced piezoelectric property of Fc-Cz, demonstrating its good sensitivity and fast response speed in detecting human motions. Figure 1A illustrates the synthetic route of the target compound (Fc-Cz) in 80% yield through a Knoevenagel reaction between ferrocene carboxaldehyde and 2-(4'-(8a,9a-dihydro-9H-carbazol-9-yl)-[1,10-biphenyl]-4-yl)acetonitrile (1) [31] in dry ethanol. The molecular structure Fc-Cz was fully characterized by 1 H/ 13 C NMR spectra, high resolution mass spectra (HRMS), and single crystal X-ray crystallography. Fc-Cz possesses excellent solubility in most common organic solvents, such as toluene, tetrahydrofuran (THF), dichloromethane, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF).

Synthesis and characterization of Fc-Cz crystal
Therefore, a single crystal of Fc-Cz can be cultivated by slow evaporation of a mixture of hexane and chloroform. The molecular structure and crystal pattern ( Figure 1B) were confirmed by X-ray crystallography. The basic crystallographic data and structure refinement parameters are summarized in Table S1. .031 Å. The crystals form an ordered tight crystal-state packing along the a-axis and the c-axis directed by weak C-H‧‧‧π interactions ( Figure 1C-D), while adopt a loose packing pattern with unconspicuous overlaps and weak intermolecular interactions along the b-axis. [32] Moreover, Fc-Cz adopts a herringbone pattern along the b-axis ( Figure S3-2), and this packing contributes to the improvment of the mechanical properties of the crystal. [33]

UV-Vis measurements of Fc-Cz
The UV-visible absorption spectra of compound Fc-Cz were respectively measured in a THF solution (10 -5 M) and in the solid state. As shown in Figure S4 Figure S4). This may be attrributed to the synergic effects of the strong twisted intramolecular charge transfer (TICT) effect and the strong PET from the ferrocene to the phenyl-phenyl carbazole fragment.

Polarization-dependent photoluminescence (PL) spectrum of Fc-Cz crystal
The molecule Fc-Cz crystallizes in the monoclinic P2 1 /c space group, and its low symmetry in a unit cell indicates that the anisotropic excitons will dominate the excitonic state and enhance the excitonic effects. [34,35] Thus, polarization-dependent photoluminescence (PL) is performed to understand the anisotropic optical properties via angle-resolved polarized photoluminescence spectroscopy (ARPPS) (Figure 2A). [36] A high-quality, single rod-like crystal Fc-Cz (15 × 50 μm) is transfered to a SiO 2 /Si substrate, the orientation-dependent photoluminescence property is measured under a 532 nm excitation laser ( Figure 2B). During the measurements, the rotation angle (θ) from 0°to 360°(every 10°) of the sample is operated in a clockwise direction. The PL diagram is presented in Figure 2C.
In particular, a strong and intensive PL peak at 581 nm is observed under un-polarization. When the excitation laser polarization direction (i.e., the angle φ between the incident light and PL detector) is fixed at 0°, as the rotation angle (

Piezoelectric properties of Fc-Cz crystal
The transport properties of organic meterials such as charge carrier mobility have significnt effect on the performance of organic semiconducting devices. [12,13,[36][37][38] In our study, we fabricat a flexible piezoelectric sensor using a metal-organic semiconductor-metal (MOM) structure on a polyimide (PI) substrate. Based on the crystal structure, the compound Fc-Cz interacts with one another by a series of C-H···πinteraction and the van der Waals force, and exhibits good mechanical performance, with an average Young's modulus of 3.23 Gpa. In the fabrication of the designed pressure sensor, silver paste is used for the electrodes, and the rod-like crystal Fc-Cz is aligned along the z-axis (001-plane) to connect the two metal electrodes ( Figure 3A). The channel length of the device was about ca. 25 µm (Figure S7-1). Figure 3B shows the induced piezoelectric current as a fucntion of the applied strain under a bending load on the device. As shown in Figure 3C-D and Figure S6-4, when the pressure sensor is subjected to a cyclic strain (bending-releasing) for one hundred cycles, the crystal Fc-Cz shows a stable piezoelectric signal. Furthermore, we found that the piezoelectric signal output (I ds ) has slightly changed when the strain changing is less than 0.25%-0.3%, and the bent ferrocene will not only generate a piezoelectric signal, but also the piezoelectric signal The voltage signal (current signal) along the x-axis is a=1, b=2 (a=49, b=3) ( Figure S6-8).
When the applied strain is along the z-axis of the crystal Fc-Cz, a negative current output is rapidly generated, and a positive output was observed as the strain is removed. The response time is <40 ms. As shown in Table S3, the piezoelectric sensitivity of crystal Fc-Cz can be as high as 350 pA/ε (up to 94 mV/ε) when the strain is 1.7%, which is superior to that of most 2D materials and has a larger working strain range (0.2%~1.7% strain). Figure 3 and  Figure   S6-3). However, under the same strain, the maximum peak current (or voltage) is ca. 2-fold of that along the z-axis (001) compared to x-axis (100). This is attributed to the anisotropy of the crystal Fc-Cz, in agreement with the previous reports for 2D layered materials. [38] When the applied strain is up to 1.7% along the z-axis, the maximum peak current (or voltage) reaches > 580 pA (or 140 mV), which is about 80 times (or 23 times) higher than the induced current at a strain of 0.7%. This indicates that the piezoelectric perfromance of the crystal Fc-Cz is better than other reported materials such as two-dimensional (2D) ultrathin semiconductors (GaS, GaSe, In 2 Se 3 , MoS 2 , MoSe 2 , and graphene-based composites [12][13][14][36][37][38][39] ), leading to possible ultrasensitive piezoelectric devices (Table S3). We also fabricate a wearable tactile sensor by transferring a thin Fc-Cz film onto a PDMS substrate to monitor human motions. As shown in Figure 3E, the tactile sensor worn on the forefinger is encapsulated by a polyimide film. When the forefinger performs a cyclic bending-unbending motion, the induced piezoelectric current changes with the variation of the bending angle. It is demonstrated that a maximum current of~3600 pA can be generated when the finger bending angle reaches~70°. The flexible tactile sensor also exhibits good stability, high sensitivity, and excellent repetitivity in detecting the continuous motions of a human hand (see Movie 1 in SI). Therefore, this small molecular organic semiconductor Fc-Cz has the great potential for developing future wearable devices.

The relationship between piezoelectricity, dielectric and polarizability
Theortically, the external forces play a key role in the flexible organic molecular pattern in the crystal/aggregation state [40] , leading to some macroscopic photophysical or chemical behaviors like mechanoluminescence. [41][42][43] It is measured that the capacitance of the flexible piezoelectric nanogenerator can be enhanced from 6 to 30 μF as the applied strain increases along the x-axis, following the trends of dynamic piezoelectric signals. The relationship between capacitance and dielectric constant can be expressed as where ε r is the relative dielectric constant, C is the capacitance, ε 0 is the vacuum dielectric constant (8.854 ×10 -12 F/m), and S is the area of a sample. The polarizability X is calculated by It can be rewritten as Thus, the polarizability X is positively associated with the relative dielectric constant of the material. It is assumed that the relative polarizability and the dipole moment of crystal Fc-Cz change with the applied strain. [44] To reveal the relationship between the piezoelectricity, dielectriccity, and polarizability of crystal Fc-Cz, we perform density functional theory (DFT) analysis at the PBE0/def2-SVP level. Analytical frequency calculations are also performed at the same level of theory to verify that the optimized structure is at the minimum point. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are isolated and mainly located on the carbazole unit and the diphenyl fragment, respectively. The calculated HOMO and LUMO energies are respectively-5.93 eV and -2.20 eV, and the energy gap between them is 3.73 eV ( Figure 4A). Furthermore, the HOMO level of the compound Fc-Cz is confirmed to be -5.72 eV by XPS. The optical energy gap derived from the lowest energy absorption onset in the absorption spectra is 3.14 eV, therefore, the corresponding LUMO level is -2.58 eV. These results varify a good agreement between the experimental and theoretical studies. Starting from the optimal structure, different structures are scanned along the ferrocene group (dihedral angle ∠1-2-3-4, as shown in Figure 4B) at the same level. Their dipole moments are further evaluated at the PBE0/def2-SVPD level. The polarizability and the surface of the polarizability tensor are analyzed using the Multiwfn package and VMD software. [43,44] The ferrocene unit is almost perpendicular to the phenyl ring at an

Conclusion
In this paper, we have synthesized a a small organic molecular crystal (Fc-Cz) and demonstrated its high anisotropy-piezoelectric properties (~580 pA) and fast response speed

Experimental Section
Materials Preparation. Unless otherwise stated, all reagents were purchased from commercial sources and were used without further purification. The starting material 2-(4'-(8a,9a-dihydro-9H-carbazol-9-yl)-[1,10-biphenyl]-4-yl)acetonitrile was prepared following the procedures in a previous report. [31] Materials Characterization. 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE III 600M spectrometer using chloroform-d solvent and tetramethylsilane as internal references. J-values are given in Hz. High-resolution mass spectra (HRMS) were taken on a LC/MS/MS, which consisted of a HPLC system (Ultimate 3000 RSLC, Thermo Scientific, USA) and a TSQ Endura QQQ mass spectrometer. UV-vis absorption spectra and Photoluminescence (PL) spectra were recorded on a Shimadzu UV-2600 and the Hitachi F-4700 fluorescence spectrometer. The quantum chemistry calculations were performed using the Gaussian 16W (PBE0/def2-SVPD basis set) software package. [47,48] X-ray Crystallography. Crystallographic data for the compound was collected on a Bruker APEX 2 CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) in the ω scan mode. [49,50] The structure was solved by charge flipping or direct methods algorithms and refined by full-matrix least-squares methods on F 2 . [49] All esds (except the esd in the dihedral angle between two l.s. planes) were estimated using the full covariance matrix.
The cell esds were considered individually in the estimation of esds in distances, angles and torsion angles. Correlations between esds in cell parameters were only used when they were defined by crystal symmetry. An approximate (isotropic) treatment of cell esds was used for estimating esds involving l.s. planes. The final cell constants were determined through global refinement of the xyz centroids of the reflections harvested from the entire data set. Structure solution and refinement were carried out using the SHELXTL-PLUS software package. [50] Data (excluding structure factors) on the structures reported here had been deposited with the   Supporting Information ((delete if not applicable)) Supporting Information is available from the Wiley Online Library or from the author.

TOC
A novel ferrocene-based organic molecule crystal (Fc-Cz) that possesses anisotropic piezoelectric, optical, and mechanical properties. We demonstrate that the new crystal Fc-Cz can be utilized as an ultrasensitive piezoelectric material in fabricating strain sensors and energy harvesters.