E-mail addresses:(Z.Sun),(J.Luo).

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 example,the quadratic nonlinear optical(NLO)activities of phase transition materials can be switched between different states under external stimulus,behaving as NLO 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 NLO-off ,it still remains a challenge to control the alignment of NLO moieties into a rational manner in the solid-state ,solid-state molecular crystals with tunable and switchable NLO behaviors remain ,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[13].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[8].Moreover,order-disorderchanges 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[17].

From the viewpoint of device application,the most practical and effective temperature range of Tcfor 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[21].Besides,the phenolic group might favor the formation hydrogenbonding interactions to enhance molecularhyperpolarizability 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[26].Although numerous picrate-based NLO compounds have been reported,to our best knowledge,the quadratic NLO switches containing picrate group is still may probably due to the insufficient driving force of the disordered NO2groups 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 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 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 was verified by elemental analyses and powder X-ray diffraction(Fig.S1 in Supporting information). 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 dielectric constants(ε￠)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.?).The crystal with an approximate dimension of 0.31×0.30×0.26 mm3was 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 F2using the SHELXLTL software package[33].All the non-hydrogen atoms were refined anisotropically,while the positions of hydrogen atoms were generated 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 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 the Cp-T trace(Fig.1b),an entropy change(ΔS)is calculated with the value of～7.631 the Boltzmann equationΔS=R ln N,where R is the gas constant and N is the ratio of the numbers of respective geometrically distinguishable orientations,N is obtained as value of N suggests that phase transition of TEAP should be an order disorder 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).