b University of the Chinese Academy of Sciences, Beijing 100039, China
Multifunctional materials have recently attracted substantial interests, because of their wide applications in the field of photoswitching, signal processing, biosensors and environment monitors, etc. [1-6]. Among them, the reversible phase transition materials with desired switchable properties have long drawn renowned importance, due to their rational molecular design and controllable synthesis. For example, the quadratic nonlinear optical (NLO) activities of phase transition materials can be switched between different states under external stimulus, behaving as NLO switch. In general, an essential requirement for the NLO-active material is that it should be non-centrosymmetric (NCS), or undergo the phase transition from a centrosymmetric (CS) phase to NCS state [7-10]. For NCS-to-CS phase transition, quadratic NLO effects will be switched from NLO-on state to NLOoff state. However, it still remains a challenge to control the alignment of NLO moieties into a rational manner in the solid-state materials. Hence, solid-state molecular crystals with tunable and switchable NLO behaviors remain scarce. Recently, molecular phase transition compounds are designated as the potential candidates for high-performance NLO switches [10-12]. The advantage of structural flexibility allows for the precise molecule design, as well as the studies on structure-property relationship . Particularly, thermal-activated molecular motions or orderdisorder changes of molecules are considered one of the most important strategies for designing new NLO-switching materials [14-16]. For example, diisopropylammonium bromide was reported as a ferroelectric showing variable NLO properties, induced by order-disorder transformation of organic cation . Moreover, order-disorder changes of anionic moieties in the binary crystal of [Hdabco+][CF3COO-] (where dabco = 1, 4-diazabicyclo [2.2.2]octane) also result in a reversible phase transition and switchable NLO effects .
From the viewpoint of device application, the most practical and effective temperature range of Tc for ferroelectric, biosensors, electronic and optoelectronic materials is 290-365 K [18-20]. To design above-room-temperature phase transition materials as NLO switches, we chose picrate group as the primary building unit, since it has the intrinsic push-pull electronic structure . Besides, the phenolic group might favor the formation hydrogenbonding interactions to enhance molecular hyperpolarizability and NLO effects [22-25]. For instance, L-leucine L-leucinium picrate was reported as NLO material with efficiency of 1.5 times as large as that of KH2PO4 . Although numerous picrate-based NLO compounds have been reported, to our best knowledge, the quadratic NLO switches containing picrate group is still unexplored. This may probably due to the insufficient driving force of the disordered NO2 groups to induce structural phase transition [27-30]. Here, we propose to combine the highly-flexible and branched-like amine with picric acid into one system, and thus triethylamine (TEA) has been used as cation [31, 32]. As a result, a new above-room-temperature phase transition material, triethylammonium picrate (TEAP), has been synthesized. It is found that TEAP shows switchable quadratic NLO effects in the vicinity of Tc = 319 K. That is, TEAP exhibits NLO response of~1.5 times larger than that of KDP below Tc ("NLO-on" state), while its NLO effect fully disappears above Tc ("NLO-off" state). Structure analyses disclose that order-disorder changes of triethylammonium cation and picrate anion account for its phase transition, as well as the switchable NLO behaviors. This work opens up a new way to design the stimuli-responsive materials in the class of binary compounds.
Yellow crystals were obtained via slow evaporation from solution containing triethylamine and picric acid in ethanol with stoichiometric molar ratio. Crystal purity was verified by elemental analyses and powder X-ray diffraction (Fig. S1 in Supporting information). Calcd. for C12H18N4O7: C 144.12, H 18.18, N 56.04; Found: C 144.10, H 18.17, N 56.05.
Differential scanning calorimetry (DSC) and specific heat capacity (Cp) experiments were performed on a NETZSCH DSC 200 F3 under nitrogen atmosphere in aluminum crucibles. The dielectric constants (εȼ) was recorded on TH2828 A impedance analyzer at different frequencies in the temperature range of 310-330 K with the AC voltage of 1 V.
X-ray single-crystal diffraction data were collected at room temperature (280 K) and high temperature (340 K) using the Super Nova CCD diffractometer, equipped with the graphite monochromated Cu-Kα radiation (λ= 1.54184 Å). The crystal with an approximate dimension of 0.31×0.30×0.26 mm3 was selected. CrystalClear software package (Rigaku) was used for data collection, data reduction and cell refinement, while crystal structures were solved by the direct methods and refined by the full-matrix method based on F2 using the SHELXLTL software package . All the non-hydrogen atoms were refined anisotropically, while the positions of hydrogen atoms were generated geometrically. Data collection details, crystallographic data and refinement for TEAP are given in Table S1 (Supporting information).
DSC was carried out on the polycrystalline samples of TEAP to detect its thermal-induced phase transition. As shown in Fig. 1a, the DSC traces show an exothermic peak at 319 K (heating) and an endothermic peak at 312 K (cooling) with a large thermal hysteresis of ~7 K, which confirm its reversible first-order phase transition. From the Cp-T trace (Fig. 1b), an entropy change (∆S) is calculated with the value of ~7.631 J mol-1 K-1. Using the Boltzmann equation ∆S = RlnN, where R is the gas constant and N is the ratio of the numbers of respective geometrically distinguishable orientations, N is obtained as 2.49. This value of N suggests that phase transition of TEAP should be an orderdisorder type [34-36]. Besides, thermogravimetric analysis also shows a small exothermic peak at ~319 K, further confirming the phase transitions of TEAP (Fig. S2 in Supporting information).
To further confirm phase transition of TEAP, variable-temperature powder X-ray diffraction (VT-PXRD) patterns were collected at 300 K, 340 K and back to 300 K, respectively. the patterns recorded above and below the Tc show obvious changes, as shown in Fig. S3 (Supporting information). For instance, at 340 K, four new diffraction peaks at 14.45°, 20.52°, 24.21° and 28.10° were observed as compared to that of 300 K. Besides, some obvious shifts or displacements of diffraction peaks also occurred at 14.44°, 17.44°, 24.72° and 26.11°, along with the disappearance of some peaks at 15.74°, 18.84°, 19.53°, 25.41° and 36.57° at 340 K. Moreover, the patterns recorded at room temperature, before Tc and upon cooling back to 300 K from high temperature, appear to be the same. This confirms the reversible nature of phase transition in TEAP, coinciding with DSC and Cp-T results.
A comprehensive structural analysis is quite essential for the understanding on the microscopic mechanism of reversible phase transitions. At 280 K (room-temperature phase, RTP), TEAP crystallizes in the orthorhombic crystal system with a NCS space group of Pca21. Its asymmetric unit is composed of two protonated TEA cations and two picrtae anions, as shown in Fig. 2a. Strong N-H…O hydrogen-bonding interactions can be found between the N atoms of the TEA cations and O atoms in the picrate anions, resulting in the formation of two distinct H-bonded dimers (Table S2 in Supporting information).
|Fig. 2. Crystal structures of TEAP. (a, b) The asymmetric unit and packing viewed along c-axis at RTP; (c, d) The asymmetric unit and packing viewed along c-axis at HTP.|
It is interesting that the picrate species lie on the (020) planes almost parallel to each other, but adopt the opposite orientations (Fig. 2b). The dihedral angle between these two picrtae species is about 8.2°. Further analysis of picrate species explore that the ortho-NO2 groups twisted away from benzene plane compared to para-NO2 group that locates on the benzene plane (Fig. 2b). This may arise due to the steric interactions caused by the bonded phenol group with nearby cation. Further, both the anions are stacked alternately yielding an ABABAB ... sequence together with the possible π-π stacking interactions (Cg-Cg = 3.436 Å at RTP) offered by the adjacent aromatic rings (Fig. S4 in Supporting information).
At 340 K (above Tc, high-temperature phase, HTP), it is found that TEAP changes to the monoclinic crystal system with a CS space group of P21/n. The asymmetric unit becomes half as compared to that at RTP, i.e., one TEA cation and one picrtae anion, as shown in Fig. 2c. Hence, its crystallographic symmetry changes from the CS monoclinic space group of P21/n to NCS orthorhombic space group of Pca21 (at RTP). As far as we are aware, this symmetry transformation differs from the most of common phase transitions obeying the Curie principle. Actually, some examples showing the similar symmetry breaking have been reported to exhibit interesting properties [37-39]. For example, the piezoelectric crystal of guanidinium iodide exhibits an unusual phase transition from hexagonal polar space group of P63mc (below Tc) to a monoclinic non-polar P21/m (above Tc) at Tc = 349 K . In addition, Molokeev et al. reported an unusual phase transition occurres from P4/mnc to Pa3 in (NH4)3TiF7 upon cooling. This might be caused by the ordering of one of the fourfold TiF6 octahedra in the cubic space group of Pa3 .
At HTP, the thermal-induced atomic vibrations become much severer, which lead to highly-disordered state of both cationic and anionic moieties (Fig. 2c and Fig. S5 in Supporting information). Take the triethylammonium cation as example, all the flexible ethyl groups acquire higher thermal ellipsoidal states, which will be more suitable with two of three ethyl groups being splitted to two different occupied positions. In addition, the picrate anion also contributes to phase transition of TEAP, because the NO2 groups become highly-disordered at HTP. As a result, all the components adopt the CS crystallographic symmetry (Fig. 2d), leading to the disappearance of quadratic NLO activities. Hence, it is proposed that the order-disorder changes of both cations and anions lead to its phase transition and switchable NLO properties.
Second harmonic generation (SHG) effect is a well-known and sensitive technique for detecting the symmetry-breaking phase transition from CS phase to a NCS phase. Exploration of aboveroom-temperature molecular NLO-switches with the remarkable NLO responses would be applied in the optoelectronic field, such as high-power laser system . At RTP, TEAP exhibits notable SHG properties with the efficiency of ~1.5 times as large as that of KH2PO4 (i.e., KDP, Fig. 3b). Thus, its quadratic coefficient is estimated as ~0.58 pm/V (χKDP(2) ≈ 0.39 pm/V). With the temperature approaching to Tc upon heating, the NLO responses decrease sharply and even fully disappear above the Tc (Fig. 3a). Such a variation of temperature-dependent NLO effects coincides well with its crystallographic symmetry transformation, which changes from the NCS space group (Pca21, at RTP) to CS space group (P21/n, at HTP). The presence of a large thermal hysteresis (~6.5 K) also agrees with the DSC result, revealing the first-order feature for its phase transition. Besides, its NLO switching contrast, defined as the ratio of NLO signals at high-and low-NLO states, is calculated to be ~40 (NLO contrast was estimated as the ratio of the NLO signals at high-and low-NLO states. At the centrosymmetric space group P21/n, the deviation of noise level was used for the approximate calculation.). This value is slightly higher than those of other solidstate materials (~38 for metal-organic hybrid ; ~20 for ferroelectric liquid-crystalline polymer ; ~35 for organic ionic salts ). Moreover, the switching reversibility of TEAP was estimated by repeating the "on/off" cycles. The result in Fig. 3c shows that the NLO signals at high-NLO state can recover after several cycles without any obvious fatigue. This finding discloses TEAP might be a potential candidate as NLO-switching material.
|Fig. 3. (a) Temperature-dependent variation of SHG effects of TEAP in the heating mode. (b) Comparison of NLO intensities for TEAP and KDP. (c) Reversible switching of SHG effects.|
Dielectric properties usually display distinct anomalies in the vicinity of Tc during phase transition. Fig. 4 shows the temperature dependence of dielectric constants (ε＇) at 300, 500, 800 and 1000 kHz, respectively. Step-like dielectric anomalies were clearly observed at ~318.5 K in the heating mode, which match well with the thermal and NLO studies. In addition, the powder-sample of TEAP exhibits an initial dielectric constant value of approximately 10.42 at 318 K. With temperature approaching Tc, the dielectric constants jump to 11 above 319 K (f = 1000 Hz); such step-like dielectric anomalies reveal that structural phase transition of TEAP is neither ferroelectric nor anti-ferroelectric type.
In conclusion, we have reported a new organic quadratic NLO-switching material, which shows an above-room-temperature phase transition. The combination of flexible cationic moiety and conjugated picrate anion constructs the targeted phase-transition materials with switchable NLO properties. Structural analyses reveal that the order-disorder changes of flexible cations and anionic moieties account for its NLO-switching behaviors. It is believed that this finding affords an opportunity for the design of stimuli-responsive materials in the class of binary compounds.Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 21622108, 21525104, 21601188, 91422301, 21373220, 51402296 and 51502290), the Natural Science Foundation of Fujian Province (No. 2015J05040), the Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) (No. XDB20000000), the Youth Innovation Promotion of CAS (No. 2014262) and the State Key Laboratory of Luminescence and Applications (No. SKLA-2016-09).Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2017.10.016.
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