# Controlling the Orientation of Polar Axis in Chiral Metal-Halides to Reveal Their Intrinsic Circularly Polarized Luminescence

Kostiantyn Tieriekhov, Maria Maniadi, Olivier Ségut, Sylvie Dabos-seignon, Arnaud Brosseau, Valérie Dupray, Nicolas Mercier, Alexandre Abhervé

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## ► To cite this version:

Kostiantyn Tieriekhov, Maria Maniadi, Olivier Ségut, Sylvie Dabos-seignon, Arnaud Brosseau, et al. Controlling the Orientation of Polar Axis in Chiral Metal-Halides to Reveal Their Intrinsic Circularly Polarized Luminescence. Advanced Optical Materials, 2025, 13 (13), pp.2403205. 10.1002/adom.202403205. hal-05076354

HAL Id: hal-05076354

https://hal.science/hal-05076354v1

Submitted on 21 May 2025

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# Controlling the Orientation of Polar Axis in Chiral Metal-Halides to Reveal Their Intrinsic Circularly Polarized Luminescence

Kostiantyn Tieriekhov, Maria Maniadi, Olivier Ségut, Sylvie Dabos-Seignon, Arnaud Brosseau, Valérie Dupray, Nicolas Mercier, and Alexandre Abhervé*

Finely modulating the structure of the organic cation in metal-halide semiconductors is a powerful strategy to influence the symmetry and polarity of the hybrid compound as well as its optoelectronic properties. In this study, chiral organic S/R-hydroxybutylammonium (S/R-HBA) cations are used to prepare a series of 1D metal-halide networks. The resulting chiral and polar (S/R-HBA)₂Pb₃X₈ (X = Br or I) compounds show both second harmonic generation (SHG), circular dichroism (CD), and circularly polarized luminescence (CPL). Contrary to the 1D networks previously prepared from S/R-2-hydroxypropyl-1-ammonium (S/R-HP1A) cations, the new compounds show no alteration of their CD signal upon sample flipping, highlighting the suitable orientation of the unique polar axis in both bulk crystal and thin film states. Since no macroscopic effect can interfere with the true chiroptical response when the optical axis of the compound is normal to the incident light beam, (S/R-HBA)₂Pb₃Br₈ presents a true CPL without sign modulation after flipping or rotating the sample, despite the inherent optical anisotropy in this family of crystalline materials.

## 1. Introduction

Chiral metal-halide semiconductors have been described in the last years as promising materials for the generation of circularly

polarized luminescence (CPL) and spin-polarized currents, due to the excellent optoelectronic properties of the metal-halide networks combined with the possibility to infer chirality through morphological (ligand and passivation or post-synthetic methods) or molecular approach (insertion of chiral organic cations).[1] The polarization of light and/or electric current by semiconductors is an important challenge for the next generation of spintronic devices. For example, chiral materials can replace ferromagnets in order to generate/transport spin-polarized currents, therefore allowing spintronic devices to work at room temperature. Still, the high efficiency of electron spin polarization in chiral metal-halides, resulting from chirality-induced spin selectivity (CISS effect), remains to be rationalized. On the other hand, the ability of these materials to generate CPL is still to be clearly determined, with only a few examples reported to date.[2] Most of the CPL-active metal-halide compounds reported up to date are based on 3D perovskite nanocrystals prepared by chiral ligand modification.[3] Still, this morphological approach limits the range of materials that can be afforded and has been mostly used to prepare CsPbX₃ (X=Cl, Br, I) chiral nanocrystals with monochromatic emission.[4,5] In order to modulate the structural and chiroptical properties and to investigate the structural origins of circular dichroism (CD) and CPL, a more powerful strategy consists of the use of chiral organic cations in low-dimensional metal-halide networks.[6] This molecular approach allows to finely tune the structural and physical properties of the resulting hybrid and could afford the record dissymmetry factors in metal-halides.[7] Indeed, tuning intermolecular interactions and distortion of the inorganic network can strongly affect the luminescence properties. For example, some of the authors decided to insert original chiral organic ammonium cations in metal-halide networks, modulating both dimensionality (2D vs 1D), nature (perovskite vs non-perovskite), and chiroptical properties of the hybrid compound.[8] Investigating the chiroptical properties of these solid-state materials remains challenging because they can simultaneously absorb (or emit) circularly polarized light of pure molecular origin and other contributions originating from macroscopic features and/or supramolecular assembly,[9] especially in the case of crystalline compounds such

K. Tieriekhov, M. Maniadi, O. Ségut, S. Dabos-Seignon, A. Brosseau,

N. Mercier, A. Abhervé

MOLTECH-Anjou

UMR 6200

CNRS

UNIV Angers

2 bd Lavoisier, ANGERS Cedex 49045, France

E-mail: alexandre.abhervé@univ-angers.fr

V. Dupray

SMS

UR 3233

Univ Rouen Normandie

Normandie Univ

Rouen F-76000, France

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The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202403205

© 2025 The Author(s). Advanced Optical Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adom.202403205
<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Structure of the cations described in this work.</figcaption></figure>

as metal-halide semiconductors. In 2012, Harada et al. demonstrated that the proper CD or CPL characterization of single crystals was possible for compounds bearing their unique optical axis exactly parallel to the incident light beam.[10] For other non-polar compounds, such analysis would require access to the full Mueller matrix analytical method and/or electronic CD imaging from synchrotron beamlines.[11] As an alternative option, it is possible to discriminate between reciprocal circular absorption/emission of pure molecular origin and non-reciprocal effects by averaging two measurements performed before and after flipping the sample inside the standard CD spectrometer.[12] Only recently such a method was applied to accurately measure the optical polarization ability of metal-halide semiconductors.[13] For example, by using S/R-2-hydroxypropyl-1-ammonium (S/R-HP1A) cations, one of the present authors could confirm in thin films of 1D (S/R-HP1A)PbBr₃ and (S/R-HP1A)PbI₃ the presence of both CD<sub>iso</sub> (thickness-independent CD signal related to intrinsic chirality), aLDLB (the antisymmetric interference between linear dichroism LD and linear birefringence LB that change sign upon sample flipping), and sLDLB, another double-scattering event associated to the helical stacking of a given transition dipole.[14] This study allowed us to propose a complete experimental guide for measuring the chiroptical properties of metal-halide thin films.[15] Here we decided to slightly modify the structure of the organic cation by using S/R-hydroxybutylammonium (S/R-HBA) cations (Scheme 1) for the preparation of 1D edge-sharing metal-halide networks of formula (S/R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> and (S/R-HBA)<sub>2</sub>Pb<sub>3</sub>I<sub>8</sub>. Contrary to the 1D lead-bromide networks based on S/R-HP1A, (S/R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> present SHG and CPL properties. In addition, the very different behaviors in CD between both polar compounds (S/R-HP1A)PbBr<sub>3</sub> and (S/R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> allow us to disclose the effect of the position of the polar axis with respect to the light beam. We thus take benefit from the molecular approach over the morphological one, where the fine-tuning of the organic cation structure can lead to a series of hybrid compounds with modulation of individual chemical and structural parameters (nature of the halide, size of the 1D inorganic chains) and therefore to a relevant comparative study of chiroptical properties in this family of molecular materials.

# 2. Results and Discussion

## 2.1. Synthesis and Crystal Structures

The hybrid compounds were obtained using the liquid-vapor diffusion technique. A slow diffusion of ethyl acetate in a hydrobromic acid or hydroiodic acid solution containing PbBr<sub>2</sub>

or PbI<sub>2</sub> and S/R-HBA precursors afforded transparent needle-like crystals of the pure enantiomers. On the other hand, the racemic compound precipitated as a white powder and no single crystal suitable for X-ray diffraction measurement could be obtained. The isostructural (S/R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> and (S-HBA)<sub>2</sub>Pb<sub>3</sub>I<sub>8</sub> compounds crystallized in the monoclinic system in the polar and Sohncke space group I2 (Table S1, Supporting Information). The asymmetric unit is based on one organic HBA cation and one Pb<sub>1.5</sub>X<sub>4</sub> inorganic unit. Since one of the two metal centers is located on a special position, the inorganic 1D network is made of three edge-sharing PbX<sub>6</sub> octahedra extending along the *b* direction, therefore it can be described as a “triple-chain” metal-halide network (Figure 1; Figures S1, S2, Supporting Information). Such type of network is rather uncommon in the literature of 1D metal-halide semiconductors.[16,17] The two terminal octahedra appear to be strongly distorted with Pb-X bond distances ranging from 2.805(1) to 3.260(1) Å in bromide-based compounds and from 3.020(1) to 3.307(2) Å in iodide networks, while the central one presents bond distances in the range 2.952(1)-3.104(2) Å and 3.168(2)-3.256(2) Å in bromide and iodide-based compounds, respectively (Figure S3, Supporting Information). The shortest distances involve the X ions that are in close contact with the organic cation as highlighted by the numerous H-bonding interactions (Figure S4, Supporting Information), thus resulting in mirror-image distorted octahedra between compounds based on S and R cation enantiomers.

## 2.2. Optical Properties

Due to the presence of only one independent organic cation and one polar axis, all cations are oriented in the same direction along the *b*-axis (Figure S5, Supporting Information), suggesting an overall strong polarity in the bulk material. As a result, (S/R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> presents Second Harmonic Generation (SHG) properties. When illuminated at room temperature with a pulsed-femtosecond laser (λ<sub>exc</sub> = 900 nm), crystals of either (S-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> or (R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> were observed to emit light (Figure 2a). Spectral analysis of the emitted signal between 400 and 600 nm shows a sharp peak centered at λ<sub>exc</sub>/2 = 450 nm corresponding to a SHG signal (no two-photons fluorescence was observed for this excitation wavelength) thus confirming the non-centrosymmetric space group determined for these crystals (Figure S6, Supporting Information). In addition, a stronger intensity of the SHG response in (S-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub> was observed compared to (R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub>. Even if the phase-matching conditions in biaxial crystals are difficult to reach, the larger size of the crystals based on the S enantiomer and/or the influence of the polarization of the incident light could be a possible explanation for the enhanced response observed (S-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub>. Another explanation could be a slightly different enantiomeric excess in the amine precursors used for the crystallization of the hybrid compounds that couldn't be detected by circular dichroism (vide infra). In addition to SHG, these compounds show luminescence properties as observed for the crystals under UV irradiation (Figure 2b). Fluorescence measurements (λ<sub>exc</sub> = 360 nm) on polycrystalline samples of the two enantiomers-based compounds revealed a first emission band centered at 425 nm
<figure><img src="image_6.png" /><figcaption>**Figure 1.** Crystal structures of (S/R-HBA)₂Pb₃Br₈ show the propagation of the 1D chains along the crystallographic *b*-axis and the mirror-image structure between enantiomeric compounds.</figcaption></figure>

followed by a broadband emission in the range from 450 to 800 nm, with two maxima at 580 and 630 nm (Figure 2b). Such a large signal results in the overall orange emission (4605 and 3874 K for the *S* and *R* enantiomers, respectively) with CIE coordinates of (0.3528, 0.3329) and (0.3873, 0.3839). The origin of such broadband signal can be ascribed to the formation of self-trapped excitons resulting from electron-phonon coupling.[7] The blue emission centered at 425 nm was even absent in the racemic compound, and only the broadband could be observed. The different emission profiles might be ascribed to slightly different intermolecular interactions between cations and inorganic networks in the racemic compound compared to the pure enantiomers. Indeed, unit cell measurements performed on very thin single crystals of (*rac*-HBA)₂Pb₃Br₈ gave slightly different cell parameters than those of (S/R-HBA)₂Pb₃Br₈. In addition, the XRPD pattern of the racemic compound matched one of the enantiomer-based counterparts, but with a small shift in 2Theta values (Figure S7, Supporting Information). In addition, the excitation spectrum of (*rac*-HBA)₂Pb₃Br₈ presented a redshift of ≈15 nm compared to those of the pure enantiomers. These results confirmed that the racemic compound is based on the same hybrid network, probably with slightly different H-bonding interactions between cations and inorganic lattices. This can be the origin of the different optical properties, as previously observed in chiral metal-halide compounds.[19] Therefore, the racemic compound exhibits a more intense orange emission (2820 K) with CIE coordinates shifted at (0.4769, 0.4587). Both enantiopure and racemic compounds exhibit a solid-state quantum yield of less than 1% (Table S2, Supporting Information).

Still, these chiral and polar emissive materials represent good candidates for CPL. However, as mentioned in the introduction, strongly anisotropic materials such as metal-halide single crystals or thin films can present macroscopic interferences that strongly affect the chiroptical responses. It is thus important to verify the presence or absence of such effects by CD measurements on thin films of both (S/R-HBA)₂Pb₃Br₈ and (S/R-HBA)₂Pb₃I₈.

## 2.3. Morphological Characterization and Circular Dichroism of the Thin Films

Thin films were prepared by dissolving crystals of the compound in anhydrous DMF, dispersion of the resulting solution on FTO substrates by spin-coating, and subsequent annealing for 10 min at 95 °C. For both bromide and iodide-based compounds, several films were prepared starting from DMF solutions at different concentrations, in order to measure the CD on different film thicknesses and study the influence of the macroscopic effects. XRPD measurements were performed to control the nature and purity of the samples. While the results for the crystalline powders confirmed the phase purity of the starting materials (Figure S7, Supporting Information), XRPD patterns of (S/R-HBA)₂Pb₃Br₈ thin films showed quite unexpected results (Figure S8, Supporting Information). Indeed, while most of the diffraction peaks correspond to (h0l) planes from the described crystal structure of (S/R-HBA)₂Pb₃Br₈ (mainly the reticular distances of 11.30 and 9.26 Å at 2Theta = 7.82 and 9.55°, corresponding to the (101) and (00-2) planes), up to three extra peaks could be found at 8.82, 12.39 and 17.55°, particularly in the series of (R-HBA)₂Pb₃Br₈, with a more important relative intensity for the lowest concentrations of precursor solutions. Fortunately, these peaks disappear with time, as observed in the XRPD patterns obtained after a few days on the same-aged films (Figure S9, Supporting Information). In the case of the film prepared from a 0.05 M precursor solution, while the pattern was first dominated by the three extra peaks, these ones were strongly reduced but didn't vanish completely. On the other hand, for the films at 0.2 and 0.3 M, the relatively small extra peaks present in the fresh sample vanished after a few days. Finally, when increasing the annealing time and temperature to 1 h at 135 °C, an important increase in the intensity of the extra peaks was observed. These results suggest that they might be attributed to a high-temperature phase of (S/R-HBA)₂Pb₃Br₈. To confirm this hypothesis, thermal analysis of the crystalline powder has been undertaken. While thermogravimetric analysis confirmed the stability of the hybrid compounds with a first mass loss of 23% of the molecular weight
<figure><img src="image_7.png" /><figcaption>**Figure 2.** Optical characterization of (S/R-HBA)₂Pb₃Br₈. a) Microscopy images of (S-HBA)₂Pb₃Br₈ (left) and (R-HBA)₂Pb₃Br₈ (right) resulting from the superimposition of the transmitted diffuse scattering and emitted signal (λ<sub>SHG</sub> = 450 nm, in green) under a 0.5 W laser excitation (λ<sub>exc</sub> = 900 nm). b) 0.5 × 0.3 mm photography of a single crystal under normal and UV light. Emission (continuous lines) and excitation spectra (dashed lines) of (S/R/rac-HBA)₂Pb₃Br₈ crystalline powders, and associated CIE coordinates.</figcaption></figure>

(corresponding to two (HBA)Br salts) starting at 225 °C, differ-
ential scanning calorimetry showed the presence of an endother-
mic peak at 95 °C, as well as a small exothermic peak at 68 °C,
suggesting a reversible phase transition (Figure S10, Supporting
Information). On the other hand, temperature-dependent XRPD
showed the appearance at 110 °C of the extra peak at 2Theta =
8.43° (Figure S11, Supporting Information). These results dis-
card the probability of a DMF solvate in the thin films but sup-
port the existence of a high-temperature phase in the thin films.
This is also confirmed by the change in the aspect of the films
since optical microscopy suggested an increasing size of crys-
tal domains on the substrates. AFM measurements clearly con-
firmed that these samples were actually made of large needle-like
crystallites deposited in a non-homogeneous way (Figure 3a,b;
Figures S15-S17, Supporting Information). The size and con-
centration of these crystallites increase by increasing film thick-
ness, obviously resulting in an overall roughness in the order
of hundreds of nm (from 70 nm at the lowest concentration of
the precursor solution of 0.05 m to 190 nm at 0.3 m). Finally,
all films were kept in the dark under air condition and XRPD
patterns obtained after six months confirmed the long-term sta-
bility of the networks (Figure S12, Supporting Information). On
the other hand, thermal analysis of (S/R-HBA)₂Pb₃I₈ showed the
absence of any phase transition (Figure S13, Supporting Infor-
mation), while XRPD patterns of the thin films show the ex-

pected diffraction peaks at 7.43° ((101) plane corresponding to
a reticular distance of 12.1 Å) and 9.15° ((00-2) plane and retic-
ular distance of 9.0 Å), together with distinguishable peaks at
10.45, 12.90, 15.71, 20.83, 25.33, 25.89 and 27.08°, all attributed
to (h0l) planes, confirming the phase purity as well as the direc-
tionality of the chains, always parallel to the substrate (Figure
S14, Supporting Information). Not that the film prepared from
a concentration c = 0.025 m of the precursor solution displays
no peak in XRPD and is out of detection range for absorbance
and CD (vide infra). XRPD patterns of (S/R-HBA)₂Pb₃Br₈ display
sharper diffraction peaks than those of (S/R-HBA)₂Pb₃I₈. FWHM
values for the diffraction peaks belonging to the (101) and (00-
2) planes are ≈0.10° in the bromide-based compound and ≈0.15
and 0.14° in the iodide-based ones. This confirms the presence
of larger crystallites in the former compound and of a homoge-
neous film in the latter. Indeed, (S/R-HBA)₂Pb₃I₈ thin films ex-
hibit a more regular aspect than those of (S/R-HBA)₂Pb₃Br₈, as
observed by optical microscopy (Figure 3c,d). Finally, AFM mea-
surements revealed a smoother surface with an overall roughness
of ≈30 nm for the thinnest film, and up to 70 nm for the thick-
est one (Figures S18, S19, Supporting Information). Finally, pro-
filometry measurements on all films confirmed the regular in-
crease of the particle/film size across the series of concentrations
(Figure S20, Supporting Information). For (S/R-HBA)₂Pb₃Br₈,
an average particle size of 200, 350, and 500 nm was found
<figure><img src="image_8.png" /><figcaption>**Figure 3.** Morphology of the (S/R-HBA)₂Pb₃X₈ thin films. Optical microscopy image and AFM topography images in a) (S/R-HBA)₂Pb₃Br₈ thin film prepared from a 0.1 M concentration and b) from a 0.3 M concentration, c) (S/R-HBA)₂Pb₃I₈ thin film prepared from a 0.1 M concentration and d) from a 0.3 M concentration.</figcaption></figure>

starting from the concentration of 0.1, 0.2 and 0.3 M of the precursor solution, respectively, while a film thickness of ≈50, 90, and 150 nm was obtained from the concentration of 0.1, 0.2 and 0.3 M of (S/R-HBA)₂Pb₃I₈ solutions, respectively.

UV-vis absorption spectra of the films also confirmed the nature of the 1D metal-halide compounds, showing the typical ex-

citonic band with maximum absorption at ≈330 and 415 nm for lead-bromide and lead-iodide networks, respectively (Figure S21, Supporting Information). On the other hand, fluorescence measurements on (S/R-HBA)₂Pb₃Br₈ revealed that only the broadband emission centered at ≈600 nm can be observed in the thin films. The CD was analyzed in transmission mode using a 180°
<figure><img src="image_9.png" /><figcaption>**Figure 4.** CD characterization of (S/R-HBA)₂Pb₃Br₈ and (S/R-HBA)₂Pb₃I₈. a) CD measurements of (R-HBA)₂Pb₃Br₈ thin film in the “front” configuration with a variation of the azimuth angle θ. b) Same for the “back” configuration. c) CD<sub>iso</sub> and aLDLB in (R-HBA)₂Pb₃Br₈. d) CD<sub>iso</sub> in (S/R-HBA)₂Pb₃Br₈. e) CD<sub>iso</sub> in a 30 nm-thick (S/R-HBA)₂Pb₃I₈ film and f) in a 120 nm-thick (S/R-HBA)₂Pb₃I₈ film. Normalized absorbance is represented by a gray area.</figcaption></figure>

geometry, which is the ideal configuration to prevent artifacts on the chiroptical measurements. However, as previously reported by one of the authors, analyzing the CD response over different azimuth angles, different positions along the light axis, and on thin films of different thicknesses is the only way to reveal or discard the presence of artifacts and aLDLB/sLDLB effects.[15] First, rotating the substrate by an azimuth angle of 90° (following the direction normal to the light beam) didn't affect the CD responses in both (S/R-HBA)₂Pb₃Br₈ and (S/R-HBA)₂Pb₃I₈ thin films, thus discarding any possible artifacts related to LD (Figure 4a; Figure S22, Supporting Information). Only a small modulation on the relative intensity of the CD signal can be observed in (S/R-HBA)₂Pb₃Br₈, which can be rather attributed to the non-homogeneous deposition on the substrate. On the other hand, the absence of aLDLB was confirmed by flipping the film and measuring the CD spectra in a “back” configuration, with the film pointing toward the detector (Figure 4b; Figure S22, Supporting Information). Indeed, aLDLB change sign upon sample flipping, therefore the semi-sum of both CD<sub>front</sub> and CD<sub>back</sub> spectra gives calculated CD<sub>iso</sub> spectra, while the semi-difference allows isolating the CD signal originating from aLDLB only.[12] Since the “back” measurements presented the same response as for the “front” configuration (film pointing toward the light source), the calculated semi-difference spectra always show an almost flat response allowing to discard of any possible macroscopic interference in the CD signals (Figure 4c; Figure S22, Supporting Information). The CD<sub>iso</sub> spectra of (S/R-HBA)₂Pb₃Br₈ finally displayed a clear signal in the absorption region of the inorganic network (Figure 4d). On the other hand, absorbance spectra were

not well defined which confirmed the nature of the deposition on the substrate. Indeed, as sometimes observed in powder samples, absorbance spectra can be strongly affected by scattering and diffuse reflectance, resulting here in a residual absorbance signal at wavelengths higher than 340 nm. For this reason, the precise calculation of g<sub>abs</sub> values for these compounds is rather elusive and might be underrated. Still, analysis of the results for the different thicknesses clearly indicates that the CD responses are not affected by LDLB effects (Figure S23, Supporting Information), with a similar range of CD values for the three samples (0.1, 0.2 and 0.3 м, the sample from the smallest concentration of 0.05 м being out of detection range). Since the nature of the deposition prevents the formation of stacks along the direction of light propagation, we can conclude that no sLDLB could occur in such non-homogeneous samples.

CD spectra of (S/R-HBA)₂Pb₃I₈ show much more intense signals compared to their bromide-based counterparts (Figure 4e,f). The fact that 1D metal-iodide networks exhibit higher CD intensity was previously reported[18] and recently attributed to the higher film quality and subsequent presence of sLDLB.[15] When metal-bromide networks usually present higher roughness and the presence of large crystal domains. Very surprisingly, the main CD peak at 425 nm vanishes completely by decreasing thickness (Figure S24, Supporting Information). It results in very different spectra over the full range of wavelength, as highlighted by the comparison between 30 nm-thick films (Figure 4e) and 120 nm-thick films (Figure 4f). This strong modulation is reflected by the change in the order of magnitude of g<sub>abs</sub> at 425 nm (from 10⁻³ to 10⁻²) and suggests that this signal may originate
<table><caption>**Table 1.** Chiroptical features of metal-halide thin films.</caption><thead><tr><th>Compounds</th><th>Tilt Angle Between Polar Axis and Excitation Light</th><th>|g<sub>abs</sub>| [CD<sub>iso</sub>]</th><th>Maximum |g<sub>abs</sub>| [CD<sub>iso</sub> + sLDLB]</th><th>aLDLB</th></tr></thead><tbody><tr><td>(S/R-HBA)<sub>2</sub>Pb<sub>3</sub>Br<sub>8</sub></td><td>90°</td><td>5.10<sup>-4</sup> – 1.10<sup>-3</sup></td><td>10<sup>-3</sup> (no sLDLB)</td><td>No</td></tr><tr><td>(S/R-HP1A)PbBr<sub>3</sub></td><td>Between 0 and 45°</td><td>10<sup>-3</sup></td><td>10<sup>-3</sup> (no sLDLB)</td><td>Strong</td></tr><tr><td>(S/R-HBA)<sub>2</sub>Pb<sub>3</sub>I<sub>8</sub></td><td>90°</td><td>5.10<sup>-3</sup> (450 nm) 10<sup>-3</sup> (425 nm)</td><td>5.10<sup>-3</sup> (450 nm) 10<sup>-2</sup> (425 nm, strong sLDLB)</td><td>No</td></tr><tr><td>(S/R-HP1A)PbI<sub>3</sub></td><td>Between 0 and 45°</td><td>10<sup>-3</sup></td><td>4.10<sup>-2</sup> (strong sLDLB)</td><td>No</td></tr></tbody></table>

from interactions between 1D chains, leading to the so-called sLDLB. Another possible origin for this strongly modulated signal might be the third-order term in the exponential form of the Mueller matrix that has been described recently by Robinson et al.[19] This “pairwise contribution” can be significant in strongly linearly and circularly dichroic materials as well as in thick films. The latter hypothesis could explain the absence of similar CD variation in the bromide-based compounds. To confirm this possible origin for such high CD modulation, a complete analysis should be undertaken using Mueller matrix polarimeters, which is beyond the scope of the present study. Still, we can conclude that strong macroscopic interferences may occur in (S/R-HBA)₂Pb₃I₈, resulting in a CD signal at 425 nm that can be opposite in sign compared to the chirality-induced CD<sub>iso</sub>. On the other hand, at the edge of the maximum absorption (≈450 nm), we can observe the conservation of the negative and positive band for S and R enantiomers, respectively, and the continuous value of g<sub>abs</sub> (±5.10<sup>-3</sup>) at all thicknesses, suggesting that a true CD signal can be attributed in the higher-wavelength region of absorbance (Figure S25, Supporting Information). These results are in line with those obtained in the series of lead-bromide and lead-iodide 1D networks based on HP1A cations[15] and confirm the more homogeneous deposition of 1D lead-iodide compounds on FTO substrates. However, in HP1A-based compounds, the macroscopic anisotropy had a much stronger effect on the chiroptical response, especially with a strong aLDLB in (S/R-HP1A)PbBr₃ (Table 1). With the presence of large crystal domains in the new compounds (S/R-HBA)₂Pb₃Br₈, one should expect strong interferences in their CD spectra. Nevertheless, the absence of these effects and conservation of the CD response can be rationalized by a careful analysis of the XRPD patterns, which show that the orientation of the 1D inorganic network on the substrate is very different between (S/R-HBA)₂Pb₃Br₈ and (S/R-HP1A)PbBr₃. The peaks observed in (S/R-HBA)₂Pb₃Br₈ thin films are all attributed to (h0l) planes (Figure S9, Supporting Information), therefore the unique polar axis (along the crystallographic b direction) always remains parallel to the substrate. In other words, the polar axis is normal to the incident light beam (Figure 5a). As stated in the introduction, a proper characterization of the chiroptical properties (CD and CPL) of single crystal compounds is possible especially when the unique optical axis is exactly parallel to the light beam.[10] However, in light of the present results, it is clear that having the optical axis perpendicular to the light beam is also an ideal configuration to avoid birefringence and subsequent double refraction.[20] This is the reason why no aLDLB could be observed in the series of (S/R-HBA)₂Pb₃Br₈ thin films, independently of the thickness and inhomogeneous deposition on the substrate. Such a situation is opposite to the one described in thin films of (S/R-HP1A)PbBr₃,

for which the main diffraction peaks were attributed to the (0-1-1) plane followed by a series of (0kl) planes. Since these compounds crystallized in the polar P₂₁ space group, the tilt angle between the unique polar axis and the plane of the substrate was between 45 and 90°, leading to an angle between the polar axis and incident light ≤ 45° (Figure 5b). As a consequence, the antisymmetric LDLB was affecting the chiroptical response, showing an increasing effect with sample thickness. Finally, due to the large thickness-dependent signal in (S/R-HBA)₂Pb₃I₈, we can conclude that the anisotropy-induced symmetric effects (such as sLDLB or pairwise interferences) can still occur no matter the orientation of the polar axis with respect to the light beam. For this reason, it remains critical to characterize the chiroptical response on a series of thin films of various thicknesses in order to disclose between thickness-modulable and thickness-independent CD signals.

In conclusion, the similar g<sub>abs</sub> calculated for both (S/R-HBA)₂Pb₃X₈ and (S/R-HP1A)PbX₃ compounds suggest that the intrinsic CD<sub>iso</sub> is mainly affected by the dimensionality of the metal-halide network. In this regard, it was recently theoretically proposed that g<sub>abs</sub> magnitude is essentially determined by the effective (chirality-induced) spin-orbit coupling (SOC) as well as by exciton broadening, the latter being directly related to quantum confinement.[21] More precisely, the effective SOC strength increases by increasing exciton dipole moment and/or decreasing the distance between adjacent excitons, leading to higher g<sub>abs</sub>. Therefore, for the same metal-halide composition (i.e., same exciton dipole moment), reducing the distance between adjacent excitons can increase SOC strength and g<sub>abs</sub>. This might be the case in (S/R-HBA)₂Pb₃X₈ due to the smaller distance between neighboring 1D chains than in (S/R-HP1A)PbX₃ (Figure S26, Supporting Information). On the other hand, exciton broadening is expected to slightly increase in the larger triple-chain network of (S/R-HBA)₂Pb₃X₈ compared to the double-chain of (S/R-HP1A)PbX₃, leading to a decrease of quantum confinement and of g<sub>abs</sub> in the former. These two competing effects might explain the similar dissymmetry factors experimentally obtained. The same trend can be observed by comparing these results with the examples of (S/R-HP2A)₃PbBr₅ previously reported by some of us (S/R-HP2A = S/R-1-hydroxypropyl-2-ammonium), where the single-chain perovskite networks presented the same g<sub>abs</sub> of ±1.10<sup>-3</sup> (Table S3, Supporting Information).[22] A comparative (and more quantitative) study, together with an in-depth theoretical understanding of all the parameters influencing g<sub>abs</sub> magnitude, must be undertaken based on a larger series of metal-halide networks. Finally, the LDLB effects can be induced by the presence of large crystal domains on the surface of the thin film, only if the unique optical axis is placed in an oblique position with respect to the incident light beam. Even though a strong anisotropy
<figure><img src="image_10.png" /><figcaption>**Figure 5.** Orientation of the 1D inorganic networks and of the polar axis in the thin films of (S/R-HBA)₂Pb₃Br₈ and (S/R-HP1A)PbBr₃ during CD and CPL measurements. Substrates are represented as orange rectangular shapes. a) For (S/R-HBA)₂Pb₃Br₈, the two possible orientations for the chains on the 2D plane of the substrate are presented. In both cases, the plane of the substrate contains the polar axis. b) For (S/R-HP1A)PbBr₃, the main orientation for the chains is presented, with the lowest tilt of the polar axis (45° from the 2D plane of the substrate). For all other possible orientations highlighted by XRPD, the polar axis is always tilted from the 2D plane of the substrate by at least 45°.</figcaption></figure>

can be observed in the morphology of the film, if the unique polar axis is oriented normally to the light propagation, the linear polarization effects are minimized and thus strongly reduce the probability of observing LDLB effects in the chiroptical response. Since our setup for CPL characterization is the same as for CD (using a spectrophotometer with the 180° geometry), we can expect to minimize the effects from linearly polarized light emission. Precisely, if we assume that the optical axis of fluorescence is exactly identical to the electronic absorption axis,[10] we can anticipate that no artifacts would arise during CPL measurements on these compounds.

## 2.4. Circularly Polarized Luminescence on Single Crystals and Thin Films

As stated in the introduction, measuring the CPL of anisotropic solid-state samples remains challenging for the interpretation of the observed polarization.[23] The fact that CPL usually presents a smaller magnitude than the total emission of the compound, the signal can be easily affected by artifacts (especially in the case of photoselection) or macroscopic effects, therefore large changes in CPL intensity or even a change in sign can be expected.[24] For example, L. Mao et al. recently observed on non-polar hy-

brid manganese-bromides that $g_{lum}$ could increase by one order of magnitude depending on the size of the selected single crystal, showing the strong effect of linearly polarized light (LPL) due to large anisotropy.[25] Still, having control over the possible artifacts in polar crystals is possible by adopting the same analytical method as previously described for CD. For example, a combined theoretical and experimental analysis of benzil single crystals could demonstrate a method to avoid artifacts and macroscopic interferences from the polarized emission.[10] While a single crystal with its optical axis placed parallel to the light beam shows no measurable LD or LPL, a small tilt from this configuration can result in a strongly modulated CD or CPL signal, therefore requiring an analytical method based on Stokes-Mueller matrix approach to reveal the true CD or CPL. In the needle-like single crystals of (S/R-HBA)₂Pb₃Br₈, indexation of the hkl faces shows the (010) direction at the top plane of the needle, an indication of the crystal growth along the direction of the polar axis (i.e., the crystallographic *b* axis) and the propagation of the inorganic chains (Figure 6a). Therefore, with our CPL setup, placing the optical axis of the single crystal along the direction of the incident light appeared to be very challenging. However, as demonstrated in the CD characterization, another well-adapted configuration to avoid artifacts consists in placing the unique polar axis perpendicular to the excitation light, which was the situation for
<figure><img src="image_11.png" /><figcaption>**Figure 6.** CPL measurements on (S/R-HBA)₂Pb₃Br₈ single crystals. a) Top and lateral view of the crystal structure of (S-HBA)₂Pb₃Br₈ along with the hkl planes of the selected needle-like crystals. b) Measurements on (S-HBA)₂Pb₃Br₈ in the “front” configuration at various azimuth angles θ. c) Accumulation of 20 spectra for both “front” and “back” positions for (S-HBA)₂Pb₃Br₈ (red line, orange, and magenta dashed lines) and (R-HBA)₂Pb₃Br₈ (blue line, dark blue and cyan dashed lines). d) *g*<sub>lum</sub> values of (S-HBA)₂Pb₃Br₈ obtained for “front”, and “back” configurations and after averaging both measurements. e) Same for (R-HBA)₂Pb₃Br₈.</figcaption></figure>

(S/R-HBA)₂Pb₃Br₈. Several single crystals of both enantiomers have been tested and their crystal faces indexed by single-crystal X-ray diffraction in order to confirm the orientation of the 1D chains with respect to the light beam. In this way, the “front” configuration could be defined as the (10-2) plane of the crystal facing the light source and the (-102) plane toward the detector (Figure 6a; Figure S27, Supporting Information). To confirm the absence of artifacts, CPL was recorded at eight different θ angles (turning every 45°), and no measurable difference could be observed, either on the CPL or DC current responses (Figure 6b; Figure S28, Supporting Information). The DC current revealed a more separated dual emission with respect to crystalline powder measurements (vide supra) with new maxima at 540 and 680 nm. Finally, the same measurements were performed with the single crystal positioned in the “back” configuration (Figure 6c; Figure S28, Supporting Information). All measurements led to the conclusion that these compounds present a slightly higher intensity of the CPL signal in the “front” configuration, with a *g*<sub>lum</sub> factor of +5.10<sup>-3</sup> and -5.10<sup>-3</sup> for the S and R enantiomers, respectively (Figure 6d,e). Since DC currents are the same for both “front” and “back”, the *g*<sub>lum</sub> values decrease to ±1.10<sup>-3</sup> for the “back” configuration, resulting in the average *g*<sub>lum</sub> of +3.10<sup>-3</sup> for (S-HBA)₂Pb₃Br₈ and -3.10<sup>-3</sup> for (R-HBA)₂Pb₃Br₈ (Figure 6c). It thus appears that LPL slightly affected the intensity of the recorded CPL signal. The reproducibility of these measurements indicates that such modulation is not related to any tilt from the ideal positioning of the crystal. Instead, it might be a clue for a possible change in the molecular geometry between ground and excited states, which de facto would result in a different LPL axis with respect to the LD axis.[10] Finally, the same procedure has been applied to measure the CPL of thin films (Figure S29, Supporting Information). The consistency with the results observed for the single crystals is highlighted by the same tendency be-

tween “front” and “back” configuration, with *g*<sub>lum</sub> of ±3.10<sup>-3</sup> and ±1.10<sup>-3</sup>, respectively, resulting in the average *g*<sub>lum</sub> of +2.10<sup>-3</sup> for (S-HBA)₂Pb₃Br₈ and -2.10<sup>-3</sup> for (R-HBA)₂Pb₃Br₈. It should be noted that for the same film, a different emission profile could be observed between “front” and “back” configurations, the former presenting always a broadband emission centered at 640 nm, while the latter shows the dual emission previously observed for single crystals (Figure S30, Supporting Information). This better resolution in the “back” configuration may be related to a different focus of the excitation light when it first passes through the glass substrate.

### 3. Conclusion

The structural and optical properties of a series of chiral and polar metal-halide compounds showing a triple-chain 1D network based on the new chiral cations S/R-HBA could be described. The crystal structure of (S/R-HBA)₂Pb₃X₈ showed strong polarity and distortion of the inorganic network. As a result, optical characterization revealed properties of SHG and a broadband emission leading to an orange luminescence in (S/R-HBA)₂Pb₃Br₈. At the difference of the previously reported (S/R-HP1A)PbX₃ compounds, the unique polar axis in the triple-chain network of (S/R-HBA)₂Pb₃X₈ is contained in the lateral face of the crystal or in the 2D plane of the film. As a result, CD measurements could be performed with the polar axis being normal to the light beam, allowing us to record CD without LDLB effects. Despite the weak luminescence in the 1D lead-bromide compounds, the absence of linear polarization effects on the chiroptical properties allows us to reveal their true CPL. By averaging “front” and “back” measurements, the true CPL of these compounds finally revealed a dissymmetry factor *g*<sub>lum</sub> = ±2.10<sup>-3</sup>. These values are within the range of those previously obtained using the 180°
geometry in 1D and 2D metal-halides (between 10⁻⁴ and 8.10⁻³), the record values of up to ±2.10⁻² being reported for manganese-based compounds showing coordinated structures[25] and one example of lead-bromide 1D network based on a chiral dication.[7] However, it remains elusive to lead any comparative study since the very few examples of CPL studies on chiral metal-halide networks often deal with non-polar compounds and report the maximum $g_{lum}$ without necessarily performing the full set of measurements required to discard any possible artifacts. Nevertheless, the present study confirmed the potential of using the molecular approach for the preparation of a larger family of chiral metal-halide semiconductors, as well as the methodology to reveal the true CPL performances in this class of hybrid materials.

## 4. Experimental Section

**Materials and Methods:** The precursors PbBr₂, PbI₂, S-(+)-2-amino-1-butanol (97%), R-(−)-2-amino-1-butanol (98%), hydrobromic acid (HBr, 48% in H₂O, Fluorochem), hydroiodic acid (HI, 57 wt. % in H₂O, distilled, stabilized, 99.95%, TCI), ethyl acetate (EtOAc) and anhydrous dimethylformamide (DMF extra dry, 99.8%, water< 50 ppm) were commercially available and used without further purification. Thermogravimetric analysis (TGA) measurements were performed on a TG 209 F3 Tarsus (from Netzsch) from 25 to 600 °C with a ramp of 10 °C min⁻¹ under nitrogen flow (20 mL min⁻¹). Differential scanning calorimetry (DSC) measurements were performed on a DSC 3500 Sirius (from Netzsch) from 0 to 155 °C with a ramp of 10 °C min⁻¹.

**Synthesis:** Crystals of (S/R-HBA)₂Pb₃Br₈ and (S/R-HBA)₂Pb₃I₈ were prepared by a liquid-vapor slow diffusion method. At room temperature in a vial, PbBr₂ (110 mg, 0.30 mmol) or PbI₂ (138 mg, 0.30 mmol) was dissolved in HBr or HI solution (0.5 mL). S-(+)-2-amino-1-butanol or R-(−)-2-amino-1-butanol (142 µL, 1.50 mmol) was added and the mixture was stirred for 5 min. The vial was placed into a saturated EtOAc vapor atmosphere. After two or three days, needle-like crystals were obtained, filtered, washed three times with ethyl acetate, and dried under vacuum. The thin needles of (rac-HBA)₂Pb₃Br₈ and of (R-HBA)₂Pb₃I₈ were too small to be analyzed by single-crystal X-ray diffraction. Yields: (S-HBA)₂Pb₃Br₈ (88 mg, 61%), (R-HBA)₂Pb₃Br₈ (97 mg, 68%), (rac-HBA)₂Pb₃Br₈ (89 mg, 62%), (S-HBA)₂Pb₃I₈ (130 mg, 72%), (R-HBA)₂Pb₃I₈ (120 mg, 66%).

**X-Ray Structure Determinations:** Single crystals of (S-HBA)₂Pb₃Br₈, (R-HBA)₂Pb₃Br₈, and (S-HBA)₂Pb₃I₈ were mounted on glass fiber loops using a viscous hydrocarbon oil to coat the crystal and then transferred directly to the cold nitrogen stream for data collection. Data collections were performed at 150 or 230 K on an Agilent Supernova with CuKα (λ = 1.54184 Å). Intensities were corrected for Lorentz-polarization effects, as well as for absorption effect (multi-scan method using CrysAlisPro program -CrysAlisPro, Agilent Technologies, V1.171.38.46, 2015). The structures were solved by direct methods with the SIR97 program and refined against all F² values with the SHELXT programs.[26] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and refined isotropically with a riding model. The structures were deposited in the Cambridge Crystallographic database. CCDC 2377873 ((S-HBA)₂Pb₃Br₈), CCDC 2377874 ((R-HBA)₂Pb₃Br₈) and CCDC 2377875 ((S-HBA)₂Pb₃I₈) contain the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystallographic data of all compounds are summarized in Table S1 (Supporting Information). X-ray powder diffraction (XRPD) analysis on powders and films was performed on a Bruker D8 ADVANCE (Cu Kα, λ = 1.5406 Å) at 20 °C. It is mounted in the Bragg-Brentano geometry in θ-2θ measurement.

**SHG Measurements:** Second harmonic generation (SHG) microscopy was performed by coupling an Insight X3 single laser with automated dispersion compensation (Spectra-physics, Santa Clara, USA) and a TCS SP8

confocal microscope (Leica Systems, Wetzlar, Germany). The laser emited pulsed light with a repetition rate of 80 MHz and a temporal width ≈120 fs. For the measurements, an excitation wavelength of 900 nm, an average power of 0.33–0.50 W depending on the samples, and a long working distance dry Leica objective (HC PL Fluotar 5x NA 0.15) were used. Two detectors were used to record images. One was a Leica hybrid descanned detector (HyD) which collects light between 380 and 780 nm by dispersing photons with a prism and by selecting the spectral band with a motorized split mirror. The second detector was a photomultiplier detector operating in transmission and giving an image of the sample in shades of gray (bright field image).

**Luminescence Measurements:** Fluorescence measurements were performed on a Shimadzu RF-6000 spectrofluorometer. Quantum yield measurements were performed on a JASCO FP-8500 spectrofluorometer with an integration sphere ILF-835. The powder was inserted between two quartz slides. For each measurement, the absorptance was adjusted in order to be between 0.10 and 0.25.

**Thin Films Preparation:** 15 × 15 mm FTO substrates were subsequently washed by ultrasonication in soap water, and ethanol and further dried under N₂ flow. The cleaning was followed by an ozone treatment for 15 min prior to the deposition of the material. The precursor solutions were prepared by dissolving crystals of the compounds in anhydrous DMF. 50 µL of the solution was deposited on the substrate and spin coating was performed at 2000 rpm (10 s) and 6000 rpm (20 s). The films were placed on a 95 °C hot plate for 10 min for annealing.

**AFM:** Atomic force microscopy (AFM) experiments were performed using the Nano-Observer device (CSInstrument, Les Ulis, France). The topographic images were obtained at room temperature in contact mode using the FORTA SPM Probe (AppNano, USA). Images were processed with the Gwyddion free SPM data analysis software (v2.64, Czech Metrology Institute, Brno, Czech Republic).

**CD and CPL Measurements:** UV–vis absorption and CD spectra were recorded in transmission mode on a JASCO J-1500 spectrometer at 20 °C. For each sample, at least eight scans were performed, at four different angles for both “front” and “back” configurations. CPL was recorded on a JASCO CPL-300 spectrophotometer at 20 °C. Measurements were performed under excitation wavelength λ = 340 nm, with excitation and emission slit widths of 3000 and 2000 µm, respectively, data integration time of 2 s, and scanning speed of 50 nm min⁻¹. For both “front” and “back” configurations, the final spectra were the accumulation of 20 scans.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work was supported by the CNRS, the University of Angers and the project "Etoiles Montantes" from Région Pays la Loire (CHAOS project, postdoctoral grant to K.T.).

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

chirality, circular dichroism, circularly polarized luminescence, metal halides, second harmonic generation
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