Dielectric and electro-optical properties of Mn12-acetate and ferroelectric liquid crystal composite

ABSTRACT The impact of single-molecule magnet (SMM), Mn12-acetate, on the dielectric properties of ferroelectric liquid crystal (FLC) has been investigated by dielectric spectroscopy and electro-optical techniques in a dual electrode sample cell (DESC). The temperature-dependent dielectric studies on Mn12-acetate/FLC composite have revealed the enhancement in the ferroelectric (smectic C*) to paraelectric (smectic A*) phase transition temperature by 3.5°C. The relaxation process corresponding to Goldstone mode in the ferroelectric phase of the composite is found to be slower compared to pure FLC sample. The electrical response for an input triangular wave shows the existence of one extra polarisation peak in Mn12-acetate/FLC composite which is ascribed to the induced dipole moment in Mn12-acetate molecule. The electro-optical texture of Mn12-acetate/FLC composite revealed that the incorporation of SMM in FLC significantly improve the memory effect. GRAPHICAL ABSTRACT


Introduction
Single-molecule magnets (SMMs) with diameter in the range of~0.5-5.0 nm, displaying magnetic behaviour of molecular origin below their blocking temperature (T B ), have drawn a considerable interest of researchers in the past two decades due to their potential applications in high-density data storage, quantum computing and magnetic refrigeration [1,2]. The first compound found to have SMM properties is a polynuclear complex, Mn 12 -acetate, [Mn 12 O 12 (CH 3 CO 2 ) 16 (H 2 O) 4 ]·2CH 3 COOH ·4H 2 O (Figure 1) [3], synthesised by Lis in 1980 [4]. Mn 12 -acetate, has a total ground-state spin, S = 10 which comes from antiferromagnetic coupling of four inner core Mn 4+ ions (S = 3/ 2) with eight outer Mn 3+ ions (S = 2), with zero-field splitting parameter (D) of −0.5 cm −1 and a blocking temperature of 2.7 K [5]. It is most widely studied SMM till date [2].
Recently, Mn 12 -acetate has been encapsulated into carbon nanotubes to form new hybrid structures and is expected to revolutionise the field of nano-electronics with exciting opportunities in spintronics and data storage devices [6]. Candini et al. have fabricated the multiple field effect nano-transistor by functionalising terbium-based SMM on graphene whose gate characteristics can be controlled magnetically [7]. In addition, new SMMs with liquid crystalline properties have also been designed. A chiral derivative of terbium complex material has shown the properties of both single molecule magnet at low temperature and liquid crystal phase at room temperature whose magnetic properties are affected by varying the structural environment [8]. Terrazi et al. have shown the synthesis of liquid crystalline Mn 12 cluster by introducing mesogens and aliphatic spacers to its core structure [9]. Further, the studies have shown the improved characteristics of ferroelectric liquid crystal (FLC) materials on incorporation of nanomaterials [10][11][12]. Gold nanomaterials have enhanced the relative dielectric behaviour in the low-frequency range and contribute to a long-lasting memory effect along with the internal field-induced charge transfer that has been observed in the SmC* [13].
Dielectric spectroscopy is widely used to study the molecular dynamics of FLC materials and their composites, particularly Goldstone and soft modes in the SmC* and SmA* phases, respectively [10,[14][15][16]. Mn 12acetate exhibits the dielectric relaxations in the low frequency range i.e. 100 Hz to 1 kHz at room temperature due to the presence of peripheral ─CH 3 groups and solvated H 2 O molecules [17]. We envision that dielectric properties of Mn 12 -acetate on interfacing with FLC molecule may impart new functionalities in FLC in the form of altered dielectric and electro-optical properties. Further, owing to its molecular nature Mn 12 -acetate is not expected to affect the alignment of FLC material as observed in micro/nanomaterials interfaced with FLC that restricts the practical applications of liquid crystal composite [18,19].
In this article, we present the effect of SMM, Mn 12acetate, specifically on the dielectric properties of FLC material (ZLI 3654) in the SmC*, that is, ferroelectric phase and its transition from ferroelectric to paraelectric phase. It is observed that ferroelectric to paraelectric phase transition gets shifted towards higher temperature, that is, from 62°C to 65.5°C as shown by relaxation frequency and dielectric strength analysis. The change in relaxation behaviour of Mn 12 -acetate/ FLC composite is attributed to the coupling of flexible alkyl chain of FLC with ─CH 3 group of Mn 12 -acetate via van der Waals interaction (as shown in Scheme 1) [20]. We have observed enhanced memory effect in the designed composite.

Experimental section
2.1. Material synthesis 2.1.1. Mn 12 -acetate Mn 12 -acetate was synthesised using the method reported by Lis [4] with slight modifications and characterised using standard techniques. Long black crystals of size 0.3 × 0.3 × 5.0 mm 3 were isolated. The preparation and characterizations of Mn 12 -acetate are given in detail in supplementary information.

Ferroelectric liquid crystal
The commercially available FLC material ZLI 3654 has been used in the present study. Following is the temperature phase sequence of the ZLI 3654 FLC material: Crystal -30°C SmC * 62°CSmA * 76°C Nematic 86°CIsotropic Helical pitch value of the material is about 3 μm at 25°C . Spontaneous polarisation of the material at room temperature is about 29 nC/m 2 .

Mn 12 -acetate/ferroelectric liquid crystal composite
A 40 µL solution of Mn 12 -acetate in acetonitrile (1 mg/ mL) is mixed manually with 4 mg FLC (i.e. 1% wt/wt Mn 12 -acetate/FLC), at room temperature and then by ultrasonication. Further, the mixture was heated above isotropic phase transition of FLC, that is, at 92°C for proper mixing of FLC and SMM as well as removal of acetonitrile (boiling point 82°C). The composite is cooled naturally to room temperature and is used as obtained. No agglomeration of Mn 12 -acetate in FLC mixture was observed by optical microscopy under transmission mode.

Sample cell preparation and experimental techniques
Dual electrode sample cells (DESCs) for the present study were fabricated using low-resistant (10 Ω/) indium tin oxide (ITO)-coated λ/2 glass substrates as illustrated in Scheme 2. The two ITO electrodes pattern in the form of square shape (4.5 mm × 4.5 mm) was designed using photolithography technique. The designed DESC consists of two working electrodes on the same substrate for better comparative studies and to avoid variations in two separate cells. The pattern of counter electrode was taken large as compared to patterns on dual electrode glass substrate so that the whole working area of dual electrode substrate could be covered by reference counter electrode. Both the plates were treated with silane solution for better adhesion of nylon 6/6 polymer film used for homogeneous alignment of FLC material. Both the substrates treated with nylon 6/6 polymer were rubbed unidirectional using velvet cloth to achieve homogeneous alignment of the FLC molecules. The DESC was assembled by placing the ITO electrodes in capacitive arrangement where uniform capacitive cell gap is maintained by using Mylar spacers of~7 μm thickness. A long spacer was placed in the spacing between two electrodes of square shape on the same substrate to avoid the intermixing of pure FLC and Mn 12 -acetate/FLC composite.
The calibration of cells was carried out using air as standard reference. The Mn 12 -acetate/FLC composite and FLC material were sandwiched in between two substrates on both sides of the sample cell (as shown in Scheme 2) at 90°C, that is, above nematic to isotropic phase transition of FLC through capillary action. The homogeneous alignment was obtained by keeping the sample cell at 90°C up to 30 min and then cooling it to room temperature very slowly at the rate of 1 o C/ min. The DESC was sealed using UV sealant to avoid any leakage of material.
The dielectric measurements of the sample cells were recorded using impedance precession analyser Wayne Kerr 6540A (20 Hz to 120 MHz) in the frequency range 20 Hz to 1 MHz. Before the systematic measurement, the analyser was calibrated using open and short circuit to avoid the effect of cables and connections. Electrical response was observed using multifunctional electrical wave generator in conjugation with oscilloscope (HEMAG, Germany) equipped with computer-controlled automation setup. Temperature-dependent dielectric properties were performed using inbuilt computer-controlled precision impedance analyser (Wayne Kerr 6450A, UK). The temperature of the sample cell was controlled using temperature controller (Julabo 25, Germany) with temperature accuracy of ±0.01°C. The electro-optical response was recorded by crossed polarised optical microscope (Axioscope-40, Carl Zeiss, Germany) fitted with computer-controlled cannon digital camera.

Results and discussion
Dielectric spectroscopy has been widely known for the analysis of dielectric relaxation processes in liquid crystal, that is, collective (Goldstone mode and soft mode) and individual (short and long molecular orientation) [21]. The temperature-dependent dielectric studies of Mn 12 -acetate/FLC composite have been explored for the analysis of dielectric relaxation and phase transition of Mn 12 -acetate/FLC composite system. In general, such complex dielectric phenomena can be described by the following relation: where ε 1 and Δε i , are the permittivity at high frequency, and dielectric strength for a particular mode, respectively. G and S stand for Goldstone and soft mode processes. α stands for the distribution parameter. ω is the angular frequency of measuring electric field. τ i is the relaxation distribution time for a particular mode. The complex dielectric permittivity (ε Ã ðωÞ) could further be separated in real and imaginary part for the analysis of dielectric parameters. Figure 2(a,b) shows the real part of the relative dielectric permittivity (ε′) for pure FLC and Mn 12 -acetate/FLC composite as a function of frequency (20-10 kHz) at different temperatures ranging from 20°C to 67°C. Above 10 kHz frequency, the permittivity for both the samples becomes independent of frequency and temperature. Equation (1) has been used for the fitting of experimental permittivity data (real part) for the analysis of dielectric parameters. The fitted data have been tabulated in Table 1a and b in the supplementary information. Further, Figure 2(a) shows that the ε′ for pure FLC at low frequency drops by two order of magnitude after 62°C which confirms a transition from ferroelectric (SmC*) to paraelectric (SmA*) phase, whereas Figure 2(b) shows that the phase transition temperature for Mn 12 -acetate/FLC composite increased to 65.5°C which is higher than the pure FLC at 62°C. This change in phase transition temperature is further confirmed by comparing the permittivity difference δε 0 ¼ ε 0 FLC À ε 0

Mn12Àacetate=FLC
of pure FLC and Mn 12 -acetate/FLC composite as a function of temperature at three selective frequencies, that is, 100 Hz, 1 kHz and 10 kHz as shown in Figure 3. At frequencies 100 Hz and 1 kHz, the Goldstone mode process is the dominating dielectric phenomenon in both the samples. As can be seen from Figure 3, the permittivity of pure FLC is higher at lower frequencies compared to the permittivity of Mn 12 -acetate/FLC composite. There is a slight increment in permittivity of FLC close to the phase transition temperature (62°C) due to the contribution of soft mode process in SmC* phase [18]. The permittivity difference, δε′, at 100 Hz becomes negative after 62°C which is attributed to increase in ferroelectric to paraelectric phase transition in Mn 12acetate/FLC composite. The δε′ remains negative up tõ 65.5°C that further confirms the shift in ferroelectric to paraelectric phase transition. On the other hand, the magnitude of δε′ at higher frequencies is very small indicating the characteristic of SmA* phase. Figure 4 reveals the imaginary part of complex dielectric permittivity (εʺ) of pure FLC and Mn 12 -acetate/FLC composite as a function of frequency and temperature. The peaks in the εʺ versus frequency plots in both the cases are due to the existence of Goldstone mode relaxation process [13,16]. There are two visible peaks at low temperature in both the FLC and Mn 12 -acetate/FLC composite plots. The lower frequencies (57 and 89 Hz at 45°C and 50°C, respectively, for pure FLC materials whereas 30, 35, 45 and 50 Hz at   temperatures 32, 35, 45 and 50°C for Mn 12 -acetate/FLC composite, respectively) peak is attributed to some part of FLC mixture which might not be properly conjugated with the main component of FLC [22]. This peak is observed even after several heating and cooling cycles of the FLC. The magnitude of lower frequency peak is highly bias-dependent and is suppressed by applying 1 V DC bias during dielectric measurements. In the present study, the temperature dependence of the lower frequency peak shows an increase in frequency with the increase in temperature and merges with the Goldstone mode at higher temperatures. The possibility of ionic contribution is ruled out because of the reason that if low-frequency relaxation peak is due to the ionic contribution then it should have been observed in SmA phase at about same frequency. But this relaxation is merged with Goldstone mode process in SmC* phase and disappears with the Goldstone mode after phase transition to SmA phase omitting out the possibility of ionic contribution. The second peak at higher frequency in both the materials as shown in Figure 5 is attributed to the Goldstone mode process and is in agreement with the reported literature [15].
The temperature-dependent Goldstone mode relaxation behaviour of FLC and its composite is revealed in Figure 5. Goldstone mode relaxation of Mn 12 -acetate/FLC composite is found to be slowed down compared to pure FLC. The temperature dependent behaviour of inverse of dielectric strength of Mn 12 -acetate/FLC composite is shown in Figure 6. It is observed that at low temperature, that is, in deep SmC* phase, the dielectric strength of Mn 12 -acetate/FLC composite is slightly lower compared to the dielectric strength of pure FLC at the same temperature which is attributed to the fact that the M 12acetate has affected the local molecular interactions. The increment of 3.5°C in phase transition temperature from ferroelectric to paraelectric in Mn 12 -acetate/FLC composite suggests that the modified molecular interactions are forcing the FLC molecules to retain SmC* phase. A shift of about 3.5°C in SmC* to SmA phase transition is further corroborated by differential scanning calorimetry (DSC), the data are given in supplementary information ( Figure S6). A shift of~3.5°C has also been observed in the SmA-nematic phase transition revealing the intermolecular interaction between FLC molecules mediated by Mn 12 -acetate.
The electrical response of the pure FLC and Mn 12acetate/FLC composite has been analysed using the input triangular wave pulse to the designed DESC. Figure 7(a) exhibits the comparative electrical response of the sample cell at 2 V peak to peak and 1 Hz frequency of the input signal. In pure FLC, two polarisation peaks (Peak A and Peak B) are observed. The appearance of Peak A in electrical response of pure FLC could be attributed to the ionic contribution, whereas Peak B is due to dipole moment of FLC molecule. On the other hand, Mn 12 -acetate/FLC sample exhibits only one polarisation peak (Peak B′) at the same field strength with a minor phase lagging corresponding to Peak B in the pure FLC. The Peak B′ in Mn 12 -acetate/FLC composite reveals the reduction in switching voltage compared to pure FLC.  On increasing the applied voltage to 5 V peak to peak, one prominent extra polarisation peak (Peak C) in the output signal of Mn 12 -acetate/FLC composite is observed along with two other peaks (Peak A′ and Peak B′) corresponding to Peak A and Peak B of pure FLC. It is noticeable here that Peak A′ corresponding to Peak A of FLC is also visible in Mn 12 -acetate/FLC composite at higher voltage (Figure 7(b)) indicating a defined interaction between FLC and Mn 12 -acetate molecules, which we refer to as van der Waals interactions. Further Peak A′ in Mn 12 -acetate/FLC is lagging behind in phase with Peak A in pure FLC reinforcing the fact that Mn 12 -acetate is somehow softening the restoring forces as observed in dielectric relaxation frequency response. The appearance of Peak C (Figure 7(b)) at higher amplitude of applied pulse is attributed to the probable dipole-induced dipole interactions between the FLC and Mn 12 -acetate molecules, that is, van der Waals interaction as shown in Figure 8. Since the attachment of Mn 12acetate with FLC molecules is through weak interactions, therefore it is expected that the composite dipole moment will not form a well-defined resultant dipole. A stronger electric field,Ẽ ¼ τ ! =p j jsinθ, (where τ ! ,p, and θ are the torque, dipole vectors and angle between direction of electric field dipolar axis), is required to rotate the induced dipole (weak dipolar strength) in comparison to permanent dipoles (strong dipolar strength) of FLC molecule which is reflected from the appearance of Peak C (Figure 7 (b)). Furthermore, the phase difference of about~39°b etween Peak B′ and Peak C is calculated from Figure 7(b) and schematically represented in Figure 8, which shows that the induced dipole moment of Mn 12 -acetate is not coupled with permanent dipole moment of FLC molecule.
The significance of molecular interactions between FLC and Mn 12 -acetate is further realised in the electrooptical measurements, as seen in Figure 9(a-f). Figure 9 (a-c) shows the electro-optical switching of FLC sample, whereas Figure 9(d-f) shows the electro-optical response by Mn 12 -acetate/FLC composite sample cell. Figure 9(c) mimics the texture of FLC as seen in Figure 9(a), 1 min after the removal of the applied electric field suggesting no retention of memory state of Figure 9(b). However, in Figure 9(f), there is retention of switched state observed in Figure 9(e) at 5 V even after 1 min. This is attributed to the weakening of intra-layer restoring forces due to the presence of Mn 12 -acetate.

Conclusion
In summary, we have observed the significant variations in dielectric and electrical responses in Mn 12acetate/FLC composite compared to pure FLC. The electrical response of the Mn 12 -acetate/FLC composite to a triangular pulse of amplitude 5 V peak to peak exhibits additional new polarisation peak at higher amplitude. The interfacial dynamics of the FLC with Mn 12 -acetate molecules causes the generation of this new peak due to induced dipole moments in Mn 12acetate which also allows Mn 12 -acetate/FLC composite to remain in the switched state (memory effect) for longer time. Further, the optical texture indicates that Mn 12 -acetate does not affect the alignment of FLC molecule as the size of Mn 12 -acetate is of the order of FLC molecule. In conclusion, we have shown that Mn 12 -acetate interacts with FLC molecule through van der Waals interactions and results in modification of visco-electric properties of FLC that may find potential applications in display and non-display devices.