Nonlinear Optical Activity of a Chiral Organic–Inorganic ([(NH3CH2CH2)3NH])2[MnBr5]Br5 Photoluminescent and Piezoelectric Crystal

The family of Mn-based organic–inorganic hybrids has greatly expanded due to their advantages in applications. They also show superior bright and size-tunable photoluminescence and can be considered a perfect alternative to toxic lead-based compounds. In this work, we present the detailed structural, optical, and electrical characterization of ([(NH3CH2CH2)3NH])2[MnBr5]Br5. The title compound exhibits a unique type of inorganic arrangement created by the trigonal bipyramids. It crystallizes in noncentrosymmetric space group R32, indicating its optical activity, piezoelectricity, and second-order optical nonlinearity proven by the second harmonic of light measurements. The studied crystals exhibit intense photoluminescence originating from the Mn(II) ion 4T1(G) → 6A1 transition. The measured lifetime of the photoluminescence emission is ≤1.5 ms, while the measured quantum yield for both powder and crystal samples reaches ∼70%.

We are grateful to all the Reviewers for their effort in carefully and critically reading the manuscript submitted to JPCL and associated ESI †.The comments and the remarks certainly helped us to make our contribution more clear and more scientifically sound.
The manuscript has been revised carefully according to the Reviewers' comments and improved by removal of some sentences and by shifting some parts of the text to the ESI †.We added 18 new references and changed the graphical abstract.The point-by-point replies to the comments are listed below.In addition, we have prepared the manuscript in two versions, one of which has the changes highlighted in red (Manuscript only for Reviewers).
According to the comments of the Referees, the following major changes were made in the paper:

Reviewer #1:
Comments: Rok et al. synthesized a bulk Mn-based perovskite single crystal (TMB) with a size about 2 cm.According to SC-XRD measurement, the Mn-based perovskite crystalizes in the high symmetry, chiral space group of R32 and the nonlinear optical activity, photoluminescent and piezoelectric property were studied.The piezoelectric properties of the TMB crystal were demonstrated by the characteristic resonance curves.The second harmonic generation and the second harmonic generation circular dichroism were observed on TMB.The temperature dependent photoluminescence of TMB single crystal was also measured and a high photoluminescence quantum yields of 70% was obtained.I would like to recommend the publication of their manuscript in JPCL after the following issues being properly addressed in a major revision.
(1) The TMB crystalized in a chiral space group of R32 using a nonchiral cation, according to the literature, enantiomers of single crystal should be found by repeating the single-crystal test.It is necessary to show the structure of all enantiomers.
file of the inverted structure (the structure of the second enantiomer), with the Flack parameter of 0.996(14), suggesting the wrong absolute structure.(2) The TMB exhibit a CD signal as shown in fig.6b, is it a real signal?It is better to take measurements on multiple samples to ensure the authenticity of the data and verify the existence of a symmetric CD spectrum.

Author note:
This is an important question.The signal observed in Fig. 6b, according to the discussion in the main manuscript does not origin from the chirality of any single component of the crystal, both organic cation as well as bipyramids are not chiral.CD signal is the result of structural chirality of the crystal as a whole as a result of chiral 3-fold axis around which the organic cations are distributed via symmetry.Similar situation occurs in chiral nematics (cf.J. Mater.Chem., 2012, 22, 7088-7097, DOI https://doi.org/10.1039/C2JM15962G), in which achiral molecules minimizing their interaction energy form helicoidal arrangements which are able to rotate linear polarization of light, i.e. show optical activity.CD signal which we presented in this work in Fig. 6 is known in literature as "apparent circular dichroism".We discuss its origin and cite relevant publications.At present we have no available good quality samples to perform new CD measurements, but we check that the signal is repeatable, changes its sign under crystal inversion with respect to incoming light and on tilting the crystal by +/ -30 deg, the signal drops significantly, what we show in Fig. S3 of ESI †.Please also read the information about this in chapter 1.4.1 Circular dichroism in TMB crystal of ESI †.
In the main manuscript on page 11 we put the sentence: "It must be noted that an "apparent CD" is dependent on a sample thickness and the crystal orientation with respect to the light incidence direction.Tilting off the crystal by 30 o in a CD spectrometer caused significant drop of the "apparent CD" (see Fig. S3 of ESI †).

Author note:
Thank you for mentioning the two important papers.We include them into our reference list.However, the described in these publications' cases are much different from ours.In Angew.Chem.Int.Ed. 2023, 62 the space group is P21 and chiral cations are used to promote chirality, while in Science China-Chemistry, 2024 the space group is P1 and also the chirality is transferred from chiral cations.Moreover, none of the compounds contains Mn 2+ ion.Our discussion is different because the chirality is not induced by chiral molecule and the space group is also different R32.This makes that the CD spectra are much different and discussion cannot be based on chiral cations.In our work we have proposed the modelling of SHG-CD in different way due to different crystal symmetry, but generally the results are quite similar.In our work we point out that chiroptical effects are dependent on geometry of experiment and universal measures of chirality are sometime hard to asses and hard to define by simple anisotropy factor gSHG-CD.Because we did not perform quantum chemical calculations of the bands structure of the studied compound and any calculations of second order susceptibilities we are unable to deepened the discussion.But we hope that this can be done in the future.
(4) The author claims that the TMB have piezoelectric properties, thus the piezoelectric coefficient of TMB should be given.

Author note:
We devoted the chapter 1.3.2"Piezoelectric effect" in ESI †, to comment about the piezoelectricity in the studied crystal.The measurement of the piezoelectric effect is not a simple task as it requires large crystal and a few proper precision cuts, so it is a subject for a separate publication.The only relatively easy applied method in our case of platelet shape of TMB crystal would be the method based on AFM, when a conducting tip being in contact with the crystal surface is making stress generating surface charge.The drawback of this method (available in our laboratory) is that it is limited to accurately measure only the d333 piezoelectric coefficient, which due to symmetry reasons is absent in the studied crystal.The remark about piezoelectricity has been addressed also by Reviewer #3, remark 2, so please refer to our response to this Reviewer.We are citing here also one sentence: "From the work of de Jong, et al. and Fig. 6  In the main manuscript on page 10 we add the following sentence: "We simply observe that at certain crystal temperatures the used frequency of AC electric field matches perfectly with the mechanical crystal resonance (local ε′(T)max) followed by the associated anti-resonance (local ε′(T)min) as is clearly seen in Fig. 5a.The additional information about the piezoelectric effect in chiral space group R32 is described in ESI † chapter 1.3.2." (5) According to the previous work, the SHG-CD signal will be influenced by the anisotropic of crystal, it is better to measure the SHG-CD spectra with different single crystal orientation or rotating the single crystal.

Author note:
Thank you for this comment.Obviously, it is correct that the SHG-CD signal will be influenced by the anisotropy of the crystal.The natural large face of the TMB crystal is perpendicular to the chiral c-axis, therefore for the simplicity of calculations we chose this preferential geometry.Any departure of the k-vector of incident light with respect to the c-axis produced asymmetric polarimetric response in SHG-CD signal.Therefore, we put a lot of effort to properly align the crystal with respect to incoming 1064 nm laser light beam and we resign from focusing incoming light to avoid spurious SHG signal connected to crystal birefringence.Please note that the crystal is optically uniaxial with optical axis (extraordinary refractive index) along the c-axis, by symmetry reasons.Similar problems we faced when linear effect of CD dichroism was measured.In this case, we performed several experiments in turning the crystal in the holder with respect to the incoming light and the CD signal was sensitive to the tilt angle as expected.SHG-CD will be absent at any direction of laser IR light beam being perpendicular to the crystal c-axis.

Author note:
Thank you for this important remark.Yes, in fact we missed in our discussion these very important publications, which enlarged our knowledge about possible applications of chiral crystals.
In the revised version of our manuscript we put a following text on page 5: "The chiroptical and chiral-related electric and magnetic properties have recently attracted very wide attention due to both scientific interest as well as potential applications in various fields (see the recently published review publications and references therein) [32][33][34][35][36] .Long et al. 32 in the excellent review concerning research and applications of chiral-perovskites in optoelectronics gives the broad outlook on different aspects of these very promising materials.Among the unique achievements one can mention reports on spin control in reduced-dimensional chiral perovskites 37 , their topological quantum properties 38 , spintronics, report on chiral-perovskite photodetectors with responsivity 100 times higher than that of chiral metasurface photodetectors 39,40 and third order nonlinear optical effect of two-photon absorption mostly due to delocalized cloud of π electrons (e.g."push-pull" or D-π−A architecture systems).In the studied compound the organic H4tren cation does not contain any extended π electronic system.It rather represents the nondipolar molecule known as octupolar one.All dipolar-like quantities, i.e. the dipole moment µ0, and the vector part of βiii, vanish for a purely octupolar molecule and only the symmetry-allowed octupolar components can contribute to second-order NLO effects. 67Therefore, the second order NLO effects are not large.The contribution from the inorganic part could arise from the field induced asymmetry of trigonal bipyramidal [MnBr5] 3-groups.However, without detailed quantum-chemical calculations it is difficult to judge how large contribution to SHG susceptibilities χ (2) they can add. 68The role of the N-H⋯Br hydrogen bonds linking [MnBr5] 3-anions with cations could not be prevailing due to the weakness of these bonds (see Table S3 of ESI †).
structure and all unit cells, as the structure of a crystal is composed of a repeated by translation vectors (a, b, c) unit cells in three dimensions.
We put the following sentence in the main manuscript on page 4 and 5: "Note that chirality of this compound is neither imparted by incorporation of any chiral organic molecules nor induced by tuning the environmental conditions, e.g. by using chiral solvents or external stimuli (such as strain)." 2. What is the structure-property relationship for piezoelectricity?I assume the audience is probably not familiar with it, so it would be better to add the discussion.

Author note:
Due to the fact that we did not study the piezoelectric effect and merely we noted its presence by observation of piezoelectric resonances during measurements of complex electric permittivity, we did not discuss structure-relationship property in this case.Instead we prepared a kind of tutorial text about piezoelectric effect in R32 chiral space group and put it to ESI † in chapter 1.3.2.: "Piezoelectricity is the ability of electric charge accumulation in non-centrosymmetric solid materials in response to the applied mechanical stress or strain.Stress and strain are related with electric field via third order tensor: where D, E, , σ and T represent the electric displacement field, the electric field, the strain tensor, the stress tensor and the temperature, respectively.In the (experimental) literature the piezoelectric strain constant is usually denoted as dijk.These can be readily related to the eijk constants if the elastic compliances   , of the material are known, then: crystal.In our experiment we observed the reverse piezoelectric effect, the internal generation of a mechanical strain was caused by externally applied electric field (E3 || c-axis) used for measurement of dielectric response." Considering the structure-property relationship it is reasonable to assume that the observed strongest piezoelectric resonance at 133 kHz for TMB single crystal is related to the lowest acoustic resonance with acoustic wave propagating within the direction perpendicular to the caxis.Taking into account that the resonance is broad one may suppose that it can be a mixture of several acoustic waves being in resonance with TMB crystal edges seen in Fig. 1 of the main manuscript.For the inducement of a piezoelectricity can be responsible the shearing strain that can deform the triangular base of bipyramids formed by Mn 2+ and three Br1 atoms, however other charged units (organic parts) may be involved as well.It is well known that the procedure of measurements as well as calculations of piezoelectric crystal response is a difficult task and the results may differ due to material imperfections, defects, domains, etc.
In the main manuscript we add the following text and reference to ESI †: "We simply observe that at certain crystal temperatures the used frequency of AC electric field matches perfectly with the mechanical crystal resonance (local ε′(T)max) followed by the associated anti-resonance (local ε′(T)min) as is clearly seen in Figure 5 (a).The additional information about the piezoelectric effect in this chiral space group R32 is described in ESI †." 3. Why the SHG intensity in Figure 9b is twice of that in Figure 9d?I assume the intensity should not change obviously by reversing the crystal to the light beam.

Author note:
This assumption is not correct in this case, reversing the crystal to the IR light beam causing SHG of light will have no effect for achiral crystal and chiral crystal illuminated only by linearly polarized light.Here we have illumination by LCP and RCP light that make a fundamental difference.This is a direct result which is expected when the same TMB purely chiral crystal is reversed by 180 degrees along the axis perpendicular to the c-axis and measured by SHG observation under constant rotation of a quarter wave retardation plate.This large difference is only observed for circularly polarized light and not for linearly polarized light.In the latter case the SHG response is the same irrespective of linear polarization azimuth (please see the Fig. 8  c and d).For the observed difference in SHG are responsible nonlinear optical processes due to coupling of electric and magnetic fields with a chiral structure, i.e. magneto-electric second order susceptibilities.The theory presented in ESI † and fitting of SHG-CD results is devoted entirely to explain these differences noticed by the Reviewer.Please also read the paper by Valev et al.Adv Mater 2013, 25, 2517-2534 where this problem is addressed.

Fig. A .
Fig. A. Orientation of crystallographic directions of the single crystal.
are only two independent piezoelectric coefficient that have to be measured.From the work of de Jong, et al. and Fig.6therein it follows that in the chiral point group 32 the piezoelectric tensor �   � largest values do not exceed 2.5 C/m 2 for any until now measured