Tin‐based Chiral Perovskites with Second‐Order Nonlinear Optical Properties

been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/adpr.202100056. This article is protected by copyright. All rights reserved Tin-based Chiral Perovskites with Second-order Nonlinear Optical Properties Liangliang Zhao, Xiao Han, Yongshen Zheng, Mei-Hui Yu*, Jialiang Xu* L. Zhao, Dr. X. Han, Dr. Y. Zheng, Dr. M.-H. Yu, Prof. J. Xu School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tongyan Road 38, Tianjin 300350, P.R. China E-mail: jialiang.xu@nankai.edu.cn; mh@nankai.edu.cn


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(Supporting Information, Figure S2b). Th shown in Figure S2c and 2d, as well as Figure S2e and 2f Accepted Article shown in Figure S2c and 2d, as well as Figure S2e and 2f individual inorganic SnBr6 2octahedral anions are isolated from each other, and periodically embedded in the organic MPEA + cations. There are no interactions between the optically active Sn 4+ halide species, and it should be mainly due to the distance between two S n 4+ centers being more than 1 nm. The ideal 0D core-shell structure is more clearly shown in the space-filled model

Reversible phase transition properties
The thermogravimetric analysis (TGA, powder samples) curves of the three compounds show their decomposition temperature at up to ~547 K ( Figure S3). Thermal analysis of three compounds via differential scanning calorimetry (DSC) reveals that (R-MPEA)2SnBr6 and (S-MPEA)2SnBr6 undergo two re versible phases transitions, while their racemic counterpart exhibits only one-step phase transition before decomposition. The DSC curve of (R-MPEA)2SnBr6 shows two distinct endothermic peaks at T1  404 K and T2  447 K in the heating run, and two conspicuous exothermic peaks at 359 K and 437 K in the cooling process (Figure 2a). The wide thermal hysteresis of 45 K/10 K reveals the feature of first-order phase transition. Similarly, the DSC curve of the enantiomer (S-MPEA)2SnBr6 also exhibits two pairs of reversible peaks, of which the endothermic peaks are at T1  398 K and T2  449 K in the heating process, and two exothermic peaks are at 348 K and 438 K in the cooling mode ( Figure 2c). But there is a negligible difference (about 6 K) between the pair of as-grown enantiomers at their first phase transition points. In addition, as shown in Figure S4a, the reversible phase transition points (T2  445 K, heating / 403 K, cooling) of racemic compound are very close to the second phase transition points of the pair of enantiomers. The sharp peaks and extremely wide thermal hysteresis at 42 K usually assign to the first-order feature. The molar heat capacity (CP) traces further confirm the first-order transition at T1 and T2 ( Figure S5). The enthalpy change (ΔH) and entropy change (ΔS) of thre e compounds are calculated and listed in  Accepted Article s merge into a single peak. Predictably, the variable-temperature PXRD of (S-MPEA)2SnBr6 shows almost the same behaviors as its enantiomer (Figure 2d). It is worth noting that the crystal lattice expands, and the interplanar crystal spacing becomes broader with the increase of temperature from 300 K to 448/452 K , which causes the blue shift of the XRD peaks of (R-MPEA)2SnBr6 and (S-MPEA)2SnBr6. However, as depicted in Figure S4b, the PXRD patterns of compound (rac-

Air stability
The phase stability of materials in the ambient atmosphere is of great concern for further device applications. Hence, to investigate our perovskites' phase stability, the milled sample powders were stored in plastic sample tubes and exposed to the air and sunlight for different periods.
Subsequently, PXRD data of (R-MPEA)2SnBr6 were collected after 7, 30, and 180 da ys, respectively. As shown in Figure S6a, the three experimental curves are nearly the same and match well with the calculated ones, implying the phase-stability of (R-MPEA)2SnBr6 in ambient air.

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Subsequently, PXRD data of Accepted Article data of respectively. As shown in Figure S6a, the three experimental curves are Accepted Article respectively. As shown in Figure S6a, the three experimental curves are Accepted Article Accepted Article ( Figure S6d, Supporting Information). All these results demonstrate the promising potential applications of these three compounds.

Linear optical properties
To investigate the linear optical and chiroptical properties of the chiral compounds, we firstly prepared thin films of (R-MPEA)2SnBr6 and (S-MPEA)2SnBr6, respectively (see details in Experimental Section). The PXRD patterns of these films agree well with the simulated ones, confirming the structural consistency of single crystals after being in the thin films ( Figure S9). The UV-vis absorption spectra of (R-MPEA)2SnBr6 and (S-MPEA)2SnBr6 thin films show similar features with an absorption peak at around 336 nm (Figure 3a). Figure 3c shows the corresponding circular dichroism (CD) spectra with strong CD response (at 268 and 352 nm) but opposite signs for the two chiral thin films. The CD spectra of chiral organic ligands R-/S-MPEA were measured at 226 and 257 nm in our previous work [40] . Comparatively, the CD signs of the prepared chiral materials originate from the Cotton effects near the exciton absorbance and bandgap region.
We also measured the diffusive reflectance spectrum (DRS) and PL emission spectra of the three compounds and simultaneously observed these compounds displaying almost identical behaviors. Figure S7a exhibits direct bandgap semiconductor characteristics as shown in the same type of 0D inorganic tin(IV) halide perovskites [48] . The optical bandgaps of these three compounds are around 2.68 eV ( Figure S7, inset and Figure S8, Supporting Information). However, they present not expectedly strong PL emission peaks at 470 nm with 260 nm short wave ultraviolet excitation ( Figure S7b).

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prepared thin films of ( Accepted Article prepared thin films of (

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Accepted Article Experimental Section). The PXRD patterns of these films agree well with the simulated ones,

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). The PXRD patterns of these films agree well with the simulated ones,

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We also measured the diffusive reflectance spectrum (DRS) and

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We also measured the diffusive reflectance spectrum (DRS) and Accepted Article excitation ( Figure S7b).
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Second-order NLO properties
The introduced chiral amine group successfully breaks the intrinsic centrosymmetry of typical halide perovskite materials. Due to our bulk perovskites crystalize in the chiral space group (P21), the bulk effect SHG response is typically analyzed, compared to surface effect [49] . We performed SHG measurement to prove second-order NLO effects of our chiral perovskites at room temperature and used the Y-cut quartz plate (5 × 5 × 0.5 mm 3 ) as the reference. Firstly, taking (R-MPEA)2SnBr6 crystal as an example, the DRS reveals that its absorption band edge is ~450 nm ( Figure S7a). This result indicates that (R-MPEA)2SnBr6 should be suitable for SHG measurements with a femtosecond pulsed laser (wavelength tunable from 690 nm to 1040 nm) in a reflection acquisition mode.
We utilized a home-built set-up equipped with a confocal laser scanning microscope to probe the NLO properties of (R-MPEA)2SnBr6 (see details in the Experimental Section). The scanned mapping image of the SHG signal clearly shows the outline of this compound and suggests the second-order NLO activity of this chiral material (Figure 4a). The wavelength-dependent SHG spectra demonstrate that thi s chiral compound represents a sharp SHG signal band when the excitation wavelength tunes between 800 nm and 1020 nm at 20-nm steps with the pumping power . This value is about 18 times α-SiO2 [50] and slightly lower than the chiral 2D perovskite analog. [40,51] The power dependence (Figure 4c) suggests a quadratic dependence for SHG intensity on laser power and reveals the two-photo nature of this NLO process.
It is worth noting that the SHG intensity does not decrease until the pump power is higher than ~420 mW (840 nm). Therefore, we estimate the LDT of as-tested chiral perovskite to be ~1.34 × 10 5 W cm -2 , supposing the excited laser spot of ~20 µm in diameter. This result reveals that the optical stability is much higher than that of perovskite-type NLO materials. [52] The polarization dependence tests were conducted by rotating the λ/2 plate to generate linearly polarized light with diff erent directions, in which p-polarized (parallel to the plane of incidence) input polarization at 0º angle and fixed linear-polarized output polarization. The polarization plots ( Figure 4d) accorded well with cos 4  function, [40] demonstrate the anisotropy of materials with P21 space group [50,53] (see details in Supporting Information). The polarization ratio defined as ρ = (Imax -Imax)/(Imax + Imax) is ~0.83 for a linearly polarized analyzer and suggests a comparably high sensitivity of SHG to the crystal symmetry. Accepted Article accorded well [50,53] Accepted Article [50,53] (see details in Supporting Information).

Conclusion
In summary, we successfully designed and manufactured three chiral lead-free perovskites

Experimental Section
Materials: The following chemicals were commercially available unless otherwise indicated.

Compounds Preparation:
The precursors R-/S-/rac-MPEABr were synthesized by reference to our previous work. [40]  Accepted Article potential commercial application value.

Experimental Section
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Experimental Section
The following chemicals were

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The following chemicals were

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. [40] Accepted Article [40] The 2:1 molar ratio of obtained precursors and SnCl Accepted Article  Single Crystal and Powder X-ray Diffraction: Single-crystal XRD data of (R-MPEA)2SnBr6, (S-MPEA)2SnBr6, and (rac-MPEA)2SnBr6 were collected with Cu Kα radiation (λ  1.5418 Å) at 100 K on a SuperNova diffractometer. The final crystal structures were resolved by direct methods, the SHELXL programs, and full-matrix least-squares on F 2 with Olex2 [54] software. PXRD measurements were performed on a MiniFlex600 powder X-ray diffractometer with Cu Kα radiation (λ  1.5418 Å) at a scanning rate of 5º min -1 from 3º to 40º.
Linear Optical Measurements: For powder absorption, the prepared products were ground by a mortar and pestle. UV-vis DRS spectra were performed in a Hitachi U-4100 spectrophotometer using BaSO4 as the 100% reflectance standard. The measurements were conducted in the wavelength range of 230-800 nm at room temperature. For thin film samples, UV-vis absorption spectra were collected in a SHIMADZU UV-3600 spectrophotometer, operating in the 190-900 nm region. Quartz substrates were used as 100% transmittance standard. PL emission spectra were obtained in a Hitachi F-2500 spectrophotometer with an ultraviolet excitation wavelength of 260 nm. CD measurements were recorded with a MOS-450 spectrophotometer in the transmission mode.
The CD spectra were operated in the 500-190 nm region and with 1 nm resolution.
Thermal Measurements: TGA measurements were conducted using Thermo Plus EVO 2 with a heating rate of 10 K min -1 from 300 K to 800 K under Ar atmosphere.
DSC measurements were conducted before decomposition using METTLER TOLEDO DSC1 with a temperature range from 300 K to 470 K under N2 atmosphere with a scanning rate of 10 K min -1 .

NLO Measurements:
The NLO properties of chiral perovskites were investigated with a home-built set-up [8] with a femtosecond laser pump (Mai Tai HP, 100 fs, 80 MHz, 690-1040 nm). The measurements were conducted under a reflection geometry at a 45º angle of both incidence and detection.

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K on a SuperNova diffractometer.

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K on a SuperNova diffractometer.
SHELXL programs, and full