Beyond templating: Electronic structure impacts of aromatic cations in organic–inorganic antimony chlorides

Organic cations influence the crystal packing and, less com-monly, the electronic structure of hybrid organic–inorganic materials. Two new hybrid compounds prepared from solution, (PEA)SbCl 6 and (PEA) 4 Sb 2 Cl 12 (PEA = phenylethylammonium), demonstrate how the aromatic PEA cation modifies the crystal and electronic structures relative to inorganic antimony chlorides. In (PEA)SbCl 6 the ethylammonium conformation results in a polar and chiral crystal structure, and the bandgap is characterized by organic-inorganic charge transfer. In the mixed-valence (PEA) 4 Sb III Sb V Cl 12 the structure is a uniaxial elongation of that of inorganic analogs and the optoelectronic properties combine features of intervalence charge transfer and organic–inorganic charge transfer.


Introduction
Hybrid organic-inorganic materials have gained a lot of interest in recent years due to their properties regarding applications in optics and electronics. A large focus has lately been on leadcontaining hybrid materials due to their intriguing optical and electronic properties. [1] Due to stability problems and the toxicity of lead salts lead-free alternatives are currently in the research focus. Lee et al. demonstrated the use of Cs 2 SnI 6 in solar cells, which renewed the interest in vacancy ordered perovskites as possible alternatives to lead halide perovskites. [2] Especially antimony halides gained more attention due to a large structural diversity. The ability of antimony to accommodate different polyhedral building units (in the case of oxidation state + 3: [SbX 4 ] À , [SbX 5 ] 2À , [SbX 6 ] 3À , [Sb 2 X 9 ] 3À , in the case of the oxidation state + 5: [SbX 6 ] À ) in combination with two possible oxidation states allows for a large number of compounds with interesting physical properties. [3] Antimony in oxidation state + 5 has recently been considered for optoelectronic applications because the relative energies of bands derived from halides, Sb s, and Sb p confer properties favorable for intermediate bandgap solar cells. [4] In (N-EtPy)SbBr 6 (N-EtPy = N-ethylpyridinium) isolated [SbBr 6 ] À octahedra are present. [5] Mixed-valence antimony halides have been of interest since the synthesis of Cs 2 SbCl 6 in the early 1900s. [6] Spectroscopic measurements pointed to the existence of Sb III Cl 6 and Sb V Cl 6 [7] , but only in 1983 was superlattice ordering of Sb(III) and Sb(V) revealed by neutron diffraction. [8] Mixed-valence antimony halides can be described by the formula A 2 SbX 6 where A is a monovalent cation, Sb is a nominal tetravalent antimony cation and X is a halide. [9] The compounds with small cations on the A position crystallise in a distorted K 2 PtCl 6 structure. [10] Several antimony based compounds with the formula A 4 Sb III Sb V X 12 (A = Cs, Rb, K, NH 4 + and X = Cl, Br) have been reported in the literature. [8,11,12] The structure of these compounds is built up by ordered [Sb III X 6 ] 3À and [Sb V X 6 ] À units. These mixed-valence compounds gained a lot of interest due to their deep purple color in A 4 Sb III Sb V Cl 12 and deep black in A 4 Sb III Sb V Br 12 . [9,13] Investigations into the optical absorption showed that this is due to electron transfer from Sb(III) to Sb(V). [14] The structure and the resulting dimensionality of the inorganic sublattice of organic-inorganic perovskite related materials is controlled by the organic cations. [15] A large family of compounds based on the phenylethylammonium (PEA) cation has been reported so far with varying metal cations and anions. [16,17,18] Most of the compounds feature a layered structure based on corner-sharing metal halide octahedra derived from the 3-D perovskite parent structure by taking h100i oriented layers. [15] In-between the layers PEA can exhibit three distinct conformations which can be divided in trans and gauche conformations depending on the rotation of the amine group. (PEA) 2 PbCl 4 [17] and (PEA) 2 CuCl 4 [18] exhibit two different trans conformations. In (PEA) 2 CuCl 4 the PEA cation possesses a mirror plane perpendicular to the phenyl ring and the ethylammonium group lies in the mirror. In contrast, in (PEA) 2 PbCl 4 the phenyl ring and the first ethylammonium carbon lie in a plane but the terminal ethylammonium carbon and the amine are located out of the plane.
Here we report the synthesis and characterisation of two novel hybrid antimony halides with PEA as organic cation, (PEA) SbCl 6 and (PEA) 4 Sb 2 Cl 12 . (PEA)Sb V Cl 6 crystallises as yellow platelike crystals with a polar and chiral crystal structure. The aromatic cation decreases the bandgap compared to CsSb V Cl 6 , and the bandgap is of charge transfer type with the unusual combination of an organic-derived valence band and an inorganic-derived conduction band.
Mixed-valence (PEA) 4 Sb III Sb V Cl 12 crystallises as dark red needle-like crystals in a layered structure of alternating planes of ordered [Sb III Cl 6 ] 3À and [Sb V Cl 6 ] À octahedra and molecular cation bilayers. This compound exhibits properties that combine features of intervalence charge transfer and organic-inorganic charge transfer.

Synthesis
(PEA)SbCl 6 and (PEA) 4 Sb 2 Cl 12 were prepared from a solution of Sb 2 O 4 and PEA in concentrated hydrochloric acid at 100°C. Upon cooling the solution yellow plate-like crystals of (PEA) SbCl 6 precipitated at room temperature (see Experimental section). Phase purity and composition were determined by powder X-ray diffraction (PXRD) and energy-dispersive X-ray spectroscopy (EDX, see Figure S1, Table S3, Figure S4, S5).
For (PEA) 4 Sb III Sb V Cl 12 the concentration of the starting materials was increased (see Experimental Section). After heating to 100°C and obtaining a clear solution the reaction mixture was cooled to room temperature. In the beginning only yellow plate-like crystals of (PEA)SbCl 6 precipitated. After leaving the mixture at room temperature for one week the dissolution of the yellow crystals and the formation of dark needle-like crystals of (PEA) 4 Sb III Sb V Cl 12 can be observed. To obtain a phase pure sample of (PEA) 4 Sb III Sb V Cl 12 the mixture was left to stand in the vial for one week until no yellow crystals were visible. Phase purity and composition were determined by PXRD and EDX (see Figure S2, Table S7, Figure S8, S9).

Crystal structure
The yellow plate-like crystals of (PEA)SbCl 6 crystallise in the monoclinic space group P2 1 (#4) with Z = 2 formula units in the unit cell (see Table 1 and Figure 1a, b). The Flack parameter was refined to 0.012 (12) and no additional symmetry elements were found with PLATON. [19] The asymmetric unit contains one PEA cation, one antimony cation and six chloride ions. The structure contains isolated [Sb V Cl 6 ] À octahedra (see Figure 1c). The SbÀ Cl bond lengths are in the range of 2.3389(9)-2.3911(10) Å, which indicate the oxidation state + 5 (see Table 2), [20,21] in line with the computed bond valence sum of 5.069.
The conformation of the PEA cation be described by the rotation of the terminal ethylammonium group as described by Ueda et al. In the most stable conformation, according to Ueda et al., the ethylammonium group lies in the mirror plane perpendicular to the phenyl ring. The rotation of the ammonium group around the ethyl bond is described by the angle ϕ a , as depicted in Figure 1d. Rotations around the bond connecting the phenyl and ethyl moieties, which rotate the plane of the phenyl ring with respect to the mirror plane given in Figure 1d, are described by the angle ϕ b . [22] ϕ a = 0°and ϕ b = 0°describe the most stable conformation for the free molecule (see Figure 1d). [22] Clockwise rotation from this conformation is denoted with positive angles and counterclockwise rotations with negative angles. For ϕ a > + 90°and < À 90°, the conformation is called gauche, while for ϕ a > À 90°and < 90°the conformation is called trans. [23] To accurately describe the overall conformation both angles need to be considered. The angles of the phenylethylammonium cation in (PEA)SbCl 6 are ϕ a = À 151.4(8)°and ϕ b = 61.5(7)°which corresponds to the gauche conformation of the cation (see Table 2). The cation creates NÀ H⋯Cl bonds to four chloride ions (see Table S11). Incorporation of the cation reduces the symmetry of (PEA) SbCl 6 compared to CsSbCl 6 (reported space group Cc; though we note in the ab initio calculations section below that the correct space group appears to be C2/c) to space group P2 1 and introduces chirality to the structure. [20] To address the origins of the chiral crystal structure, we consider whether the molecular building blocks are chiral or achiral, and how they assemble in the solid. [24,25] If any building block is chiral and enantiomerically pure, achiral crystal structures are forbidden. On the other hand, if the building blocks are achiral or chiral and racemic, both achiral and chiral crystal structures are permitted, as achiral building blocks can still assemble to a chiral crystal structure by a suitable rotation of bonds or helical arrangement. [26] For antimony halides Denhardt et al. reported the synthesis of several compounds based on the chiral cation (R)-1-(4-fluoro) phenylethylammonium and probed their nonlinear optical properties. [27] The crystal structures are chiral due to the chiral cation but the compounds crystallise in the acentric, achiral Sohncke group, P2 1 .
In the case of (PEA)SbCl 6 the free PEA cation in itself is achiral in the most stable conformation (ϕ a = 0°, ϕ b = 0°), but the conformationally flexible bonds of the ethylammonium group in the free cation all rotate in the same sense in the solid state, so this building block is chiral and enantiomerically pure, guaranteeing a chiral crystal structure. However, we note that in this case, the enantiomerically pure, chiral nature of the cation is intimately intertwined with the crystal packing: The free molecule is conformationally flexible, and could thus easily adopt both conformations equally (a racemate) or be disordered at finite temperature in such a way that an intramolecular mirror plane is restored. This is in contrast with molecules where chirality derives from the arrangement of the substituents and the barrier to exchanging handedness involves bond breaking and re-formation. In (PEA)SbCl 6 , the conformation of the cation, perhaps related to a favorable hydrogen bonding interaction arrangement, causes this chiral configuration to be favorable. There are no mirror planes and no inversion center in the crystal structure, which is therefore acentric and chiral, and the space group is the acentric, achiral Sohncke group, P2 1 . Strictly speaking, while the crystal structure is chiral, the space group itself is not as it does not belong to one of the 11 enantiomorphous pairs (e. g. P4 1 and P4 3 ). [24] The dark-red needle-shaped crystals of (PEA) 4 Sb III Sb V Cl 12 crystallise in the monoclinic spacegroup P2 1 /c (#14) with Z = 4 formula units in the unit cell (see Table 1). The asymmetric unit contains four PEA cations, two antimony cations, and twelve chloride ions. The structure contains isolated [Sb III Cl 6 ] 3À and [Sb V Cl 6 ] À octahedra (see Figure 2a, b). The antimony cations occupy two special crystallographic positions. The SbÀ Cl bond lengths for Sb02 is in the range of 2.3447(19)-2.3543(16) Å which are comparable to the bond lengths in (PEA)SbCl 6 and indicate the oxidation state + 5, while for Sb01 the SbÀ Cl bond lengths are in the range of 2.6321 (11).6739(10) Å which indicate the oxidation state + 3 (see Table 2). [8] Bond valence sum calculations led to the calculated valences of 2.637 for Sb01 and 5.231 for Sb02.
In comparison to (PEA)SbCl 6 the conformations are closer to the most stable conformation with the ethylammonium group in the mirror plane perpendicular to the phenyl ring. [22] The less stable conformation in (PEA)SbCl 6 could be the cause for the two-step synthesis of (PEA) 4 Sb III Sb V Cl 12 (see Figure S12c, d, e) in a manner analogous to Ostwald's step rule (strictly, the composition is not fixed).
The structure is a uniaxial elongation of that of inorganic analog Cs 2 SbCl 6, which is derived from K 2 PtCl 6 with Sb(III) and Sb(V) ordering. The uniaxial elongation is along one h100i direction, which results in an a layered arrangement of ordered [Sb III Cl 6 ] 3À and [Sb V Cl 6 ] À octahedra in layers in (PEA) 4 Sb III Sb V Cl 12 . [8,9,11] The [Sb V Cl 6 ] À units and organic cations exhibit dynamic disorder. At room temperature two crystallographically-distinct chloride ions (the four equatorial chloride ions by symmetry) show large atomic displacement parameters and were refined with two positions each (see Figure S12b). This indicates dynamic disorder of the octahedra. The organic cations show positional disorder of the carbon atoms in the ethlyammonium group (see Figure S12a). The occupancy of the disordered carbon atoms in the ethylammonium group is 0.5. The libration of the [Sb V Cl 6 ] À unit and the ethylammonium group are frozen at 100 K (see Figure 2).
According to the Robin-Day classification system mixedvalence compounds can be categorized in three classes. [28] In Class I compounds the oxidation states are localized and there is no electronic coupling between the redox sites. Class II compounds can be characterized as compounds with localized oxidation states and electronic coupling between the redox sites. In Class III compounds the oxidation state is completely delocalized and there is strong electronic coupling. Due to the localized oxidation states from X-ray diffraction (and ab initio calculations, vide infra) this compound must belong to Class I or Class II. We tentatively assign Class II by analogy to the wellstudied inorganic analogs and because of the similarity of the coordination environments (implying facile interconversion), though assessing the degree of electronic coupling from optical and IR spectra is beyond the scope of this work. [8,29] Vibrational spectroscopy Figure 3 shows the infrared (IR) spectra of (PEA)SbCl 6 and (PEA) 4 Sb III Sb V Cl 12 . The major bands were assigned based on the literature. [30,31] In general the bands can be assigned to PEA cation in (PEA)SbCl 6 and (PEA) 4 Sb III Sb V Cl 12 . However, the bands of NH and CH 2 are broadened for (PEA) 4 Sb III Sb V Cl 12 due to the presence of two conformations of PEA cation in the structure. The bands in the fingerprint region are very similar in both compounds. In general, all vibrations with NH contribution are red shifted in (PEA)SbCl 6 compared to (PEA) 4 Sb III Sb V Cl 12 . The conformation of the cation in (PEA)SbCl 6 is stabilized by weak NH-π interactions which lower the energy of vibrations with NH contributions compared to (PEA) 4 Sb III Sb V Cl 12 . In (PEA) 4 Sb III Sb V Cl 12 the free NH stretching modes are at 3254 cm À 1 and 3185 cm À 1 , respectively while the free NH stretching mode in (PEA)SbCl 6 is at 3185 cm À 1 . The band at 3227 cm À 1 in (PEA)SbCl 6 corresponds to the NH stretching mode with NH-π interactions and is redshifted compared to the band at 3254 cm À 1 in (PEA) 4 Sb III Sb V Cl 12 , which is in accordance with the conformation of the cation and the resulting presence of NH-π interactions in (PEA)SbCl 6 . [30] Instead of distinct bands in (PEA) 4 Sb 2 Cl 12 broad signals in the range of 3000-3300 cm À 1 are present, which can be attributed to the NH stretch vibrations and aromatic CH stretch vibrations of the two conformations (see Figure 3b).
Furthermore, the bands in the range 850-900 cm À 1 and 1050-1080 cm À 1 are influenced by NH-π interactions (see Figure 3c). For (PEA)SbCl 6 a band at 870 cm À 1 is present which according to Chiavarino et al. can be attributed to bending vibration of NH with NH-π interactions. [30] The band at 1070 cm À 1 can be attributed to the NH bending in combination with CH 2 twisting and rocking. The bands described above are shifted to higher wavenumbers in (PEA) 4 Sb 2 Cl 12 , which is in agreement with the different conformations present in the two compounds.

Thermal stability
The thermal stability of (PEA)SbCl 6 and (PEA) 4 Sb 2 Cl 12 was characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in Argon atmosphere (see Figure 4).
Both compounds decompose at around 180°C. (PEA)SbCl 6 exhibits several peaks in the DSC, which are convoluted with the mass loss. This makes it impossible to unambiguously assign the thermal events in the heating process, but the mass loss can be safely attributed to the removal of volatile SbCl 5 and (PEA)Cl. (PEA) 4 Sb 2 Cl 12 exhibits two endothermic peaks in the DSC curve corresponding to the melting of the material at 163°C and subsequent decomposition. The peak at 250°C is in the range of the boiling point of SbCl 3 and phenylethylamine hydrochloride. The weight loss between the endothermic peaks can be attributed to the loss of SbCl 5 .

Optical properties
The Kubelka-Munk-transformed diffuse reflectance spectra for (PEA)SbCl 6 and (PEA) 4 Sb 2 Cl 12 are given in Figure 5, with the reported spectrum of inorganic Cs 2 SbCl 6 for comparison [14] 4 Sb III Sb V Cl 12 is very similar to that of (PEA)SbCl 6 when a Gaussian distribution capturing the intervalence charge transfer is subtracted. (*) The scaled, reported spectrum for mixed-valence Cs 2 SbCl 6 from Atkinson and Day is shown for comparison. [14] (b) Tauc plot assuming a direct, dipole-allowed gap.
(similar measurements for CsSbCl 6 are not available). Yellow (PEA)SbCl 6 absorbs somewhat in the blue, with the absorption rising significantly in the near-UV. The spectrum for dark red, mixed-valence (PEA) 4 Sb III Sb V Cl 12 looks rather similar in the UV. But there is also weaker, flat absorption in the blue, green, and yellow, which then diminishes gradually approaching the nearinfrared. The low energy absorption is likely due to intervalence Sb(III) to Sb(V) charge transfer, as corroborated by the similarity with the low energy feature for Cs 2 SbCl 6 , which has been studied in detail. [14] Three key features of the optical absorption are evident. First, compared to inorganic Cs 2 SbCl 6 , whose absorption drops significantly in the blue, violet, and near-UV before rising again above 3.5 eV, absorption of (PEA) 4 Sb 2 Cl 12 plateaus above the intervalence charge transfer, which peaks around 2.4 eV.
This explains the dark red color, compared to blue Cs 2 SbCl 6 . Second, after subtracting a Gaussian distribution which captures the broad, intervalence charge transfer feature (E 0 = 2.36 eV, FWHM = 0.91 eV), the spectrum of (PEA) 4 Sb III Sb V Cl 12 is remarkably similar to that of (PEA)SbCl 6 , suggesting strong similarity of some aspects of the electronic structures. In fact, these first two features are essentially one and the same: The "plateau" in (PEA) 4 Sb 2 Cl 12 absorption is fully explained by the additional absorption that is identical to that seen in (PEA)SbCl 6 . Third, excluding the intervalence charge transfer, the absorption onsets for the hybrid compounds (fit by Tauc analysis assuming a direct, dipole-allowed bandgap, as shown in Figure 5b) are bathochromically shifted by several hundred meV with respect to the UV absorption onset in Cs 2 SbCl 6 .
Clearly, the inclusion of the aromatic PEA cation has a significant impact on the optical properties. In the subsequent section, electronic structure calculations will be used to rationalize these optical features. In contrast to the behavior of some Sb(III) chlorides with alkali metal or crown ether countercations, [32] no photoluminescence was observed from either compound at room temperature.

Electronic structure
The electronic structure of both compounds was studied within the framework of density functional theory (DFT), under the generalized gradient approximation (GGA). The monoclinic symmetry and complex structures (196 atoms in the mixedvalence compound) rendered hybrid functionals prohibitively expensive. At the GGA level of theory, bandgaps are severely underestimated, and we focus here on qualitative features like the relative energetic ordering of various bands.
The computed band structures and partial charge densities associated with key bands are shown in Figure 6. In agreement with the experimental colors and optical absorption, the bandgap of (PEA)SbCl 6 is significantly wider than that of mixedvalence (PEA) 4 Sb III Sb V Cl 12 . As expected for the nominally isolated [Sb V Cl 6 ] À and [Sb III Cl 6 ] 3À octahedra in both compounds, band widths are relatively narrow (~200 meV in the conduction band). Both bandgaps are computed to be direct, but the crystal momenta at the band edges could conceivably change with a costlier functional. Atkinson and Day have previously observed that the breadth of the intervalence charge transfer optical absorption in similar mixed-valence antimony chlorides is both wide and T-dependent (e. g. FWHM = 710 meV at 4 K and 970 meV at 300 K in [CH 3 NH 3 ] 2 Sb x Sn 1À x Cl 6 ), and have fit the T-dependent widths to a model involving substantial distortion of the excited state. [14] They further note that no vibronic structure was resolved at low temperature. While the band widths computed here are narrow with respect to conventional semiconductors, they are far from the zero width that would occur for a completely localized system. Indeed, these band widths (~300 meV in the valence band,~200 meV in the conduction band) could account already for a substantial fraction of the breadth of the optical absorption feature, simultaneously explaining the residual breadth and the lack of vibronic structure at low temperature. This is not to say that there are not additionally substantial distortions of the excited state, but rather that both factors play a role, with the band widths setting a lower bound on the breadth of the absorption feature. These calculated band widths underscore the fact that though these systems have isolated polyanions at first glance, halogen-halogen interactions between neighboring octahedral anions cannot be ignored, much as they give rise to substantial band widths in structurally-related Sn(IV) and Te(IV) iodides. [9] Interestingly given the very small aromatic π system of the molecular cation, the valence band of (PEA)SbCl 6 ("VB1" in Figure 6) is derived from the phenyl ring, rather than from the filled chloride p bands (vide infra). The conduction band ("CB1") derives from the antibonding interaction of Sb s and Cl p. Thus excitations across the bandgap result in charge transfer from organic to inorganic sublattices. This is contrary to most of the studied hybrid organic-inorganic main-group halides, where the organic molecular cations do not contribute states near the band edges. The π* orbitals of the phenyl ring ("CB3") and the Sb p states ("CB7") lie significantly higher in energy.
On the other hand, the bandgap of (PEA) 4 Sb III Sb V Cl 12 is as expected for such a mixed-valence system: The valence band derives from the antibonding Sb(III) s-Cl p interaction, and the conduction band from the corresponding Sb(V) s-Cl p antibonding interaction. This results in the similar low energy optical absorption to that of Cs 2 SbCl 6 . The next lowest excited states ("CB3" in Figure 6) derive from the antibonding Sb(III) p-Cl p interaction and lie several eV higher. As in (PEA)SbCl 6 , the π orbitals of the phenyl ring fall slightly above the manifold of Cl p bands in energy, but they no longer make up the valence band due to the higher filled states from Sb(III). However, transitions from the phenyl π orbitals to the empty Sb(V) s orbitals would explain the blue/near-UV absorption in both compounds (and thus, the "plateau" in (PEA) 4 Sb III Sb V Cl 12 absorption, which is really the sum of the intervalence charge transfer and this blue/near-UV organic-to-inorganic charge transfer).
To further examine the electronic structures, we compute the densities of states (DOS) for the title compounds and for the inorganic analogs CsSbCl 6 and Cs 2 SbCl 6 , which have similar structures with isolated, SbÀ Cl octahedra. The results are given in Figure 7. Information on CsSbCl 6 is quite limited, but it is reported to be colorless and to crystallize in space group Cc (#9). [20] Relaxing the reported structure resulted in a higher symmetry, centro-symmetric space group (C2/c, #15), which matches the entry for CsSbCl 6 in the Materials Project database. [33] Given that the reported structure appears to be separated from this centrosymmetric, relaxed structure only by a small distortion with no activation barrier, we have studied the centrosymmetric structure as it is likely the correct one (no experimental confirmation of non-centrosymmetry was provided, e. g. by anomalous scattering or second harmonic generation measurements). Cs 2 SbCl 6 and related mixed-valence compounds have been studied somewhat extensively. The crystal structure is derived from the K 2 PtCl 6 structure, with Sb(III) and Sb(V) ordering resulting in a structure of I4 1 /amd (#141) space group symmetry. The Sb(III)/Sb(V) ordering persists at least up to 423 K. [11] In agreement with the differences in color (yellow (PEA) SbCl 6 , colorless CsSbCl 6 ), the bandgap of (PEA)SbCl 6 is somewhat narrower than that of CsSbCl 6 due to the valence band states from the phenyl ring. On the other hand, the electronic structures of (PEA) 4 Sb III Sb V Cl 12 and Cs 2 SbCl 6 are very similar, with narrow bandgaps due to Sb(III)!Sb(V) charge transfer, and the frontier states from the phenyl ring occurring away from the band edges of the solid. However, as noted above, these additional states from the phenyl π orbitals are shallow in energy, and explain the absorption in the blue and near-UV seen in both compounds. (PEA)SbCl 6 is seen to absorb visible light because of the staggered alignment of the bands derived from the inorganic and organic sublattices: Both the inorganic (as in colorless CsSbCl 6 ) and organic (as in colorless benzene) parts would be colorless on their own, but together they make up a yellow charge transfer semiconductor. On the other hand, in (PEA) 4 Sb III Sb V Cl 12 the filled Sb(III) s bands fall above the π bands from the phenyl group, and the compound absorbs light like the simple sum of the intervalence Sb(III)-Sb(V) charge transfer and the organic-inorganic charge transfer seen for (PEA)SbCl 6 (recall the difference curves in Figure 5).
The rational design of charge transfer semiconductors by selecting conjugated molecules and inorganic components of appropriate energetic alignment is a desirable goal. In this context, the new compounds reported here suggest that in the localized limit (and barring peculiarities of crystal packing that could lead to vastly differing electrostatic environments for the various ions), the π orbitals of PEA lie above Cl p orbitals and only slightly below Sb(III) s orbitals in energy. Less electronegative anions may lead the phenyl HOMO to be lower than the highest anion p orbitals, while larger bandwidths from either aromatic π-π stacking or condensation of the inorganic sublattice may have more complex impacts. It appears that (PEA)SbCl 6 , a charge transfer semiconductor with an organicderived valence band and inorganic-derived conduction band, is somewhat unusual. Of the relatively few charge transfer organic-inorganic halides that have been prepared, most appear instead to have an inorganic-derived valence band and an organic-derived conduction band, including tropylium halides (Sn(IV) and Pb(II) iodides, [34] Bi(III) and Sb(III) chlorides, bromides, and iodides [35] ) and Pb(II) iodides of tetrathiafulvalene. [36]

Conclusions
In summary, two novel hybrid organic-inorganic antimony chlorides based on phenylethylamine have been prepared and the influence of the cation on the crystal packing and the electronic structure was elucidated. In (PEA)SbCl 6 , the cation has a chiral conformation, and the crystal packing results in a chiral crystal structure. In the case of mixed-valence (PEA) 4 Sb III Sb V Cl 12 , the structure can be seen as an elongation of the K 2 PtCl 6 structure along one h100i direction, with two conformations of the organic cation preserving centrosymmetry. Inclusion of the aromatic cation causes (PEA)SbCl 6 to be a charge transfer semiconductor, with an unusual energetic order of the organic and inorganic frontier bands. (PEA) 4 Sb III Sb V Cl 12 exhibits optical properties that are a combination of typical intervalence Sb(III)-Sb(V) charge transfer and the organicinorganic charge transfer seen in (PEA)SbCl 6 . These two new compounds show how the same relatively simple organic cation can have complex and varying impacts on crystal packing and electronic structure. The computed energetic alignment of bands from organic and inorganic components, corroborated by the observed optical properties, provides guidance for the rational design of new charge transfer semiconductors.

Experimental Section
Starting materials were procured from standard commercial sources and used as received. Sb 2 O 4 was prepared from Sb 2 O 3 by heating in air to 550°C for 12 hours.
Synthesis: (PEA)SbCl 6 and (PEA) 4 Sb 2 Cl 12 were prepared by solutionbased synthesis. For the synthesis of (PEA)SbCl 6 phenylethylamine (1 mmol, 0.126 mL) and Sb 2 O 4 (0.25 mmol, 76.9 mg) were heated in conc. HCl (10 mL) to 100°C until a clear solution was obtained. Upon cooling at room temperature yellow plate like crystals precipitated from the solution. The crystals were filtered off and dried at 60°C in air. For the preparation of (PEA) 4 Sb III Sb V Cl 12 the concentration of the reagents was increased to 4 mmol (0.503 mL) for phenylethylamine and 0.5 mmol (154 mg) for Sb 2 O 4 . The solution was heated to 100°C until everything was dissolved. Upon cooling at room temperature yellow crystals of (PEA)SbCl 6 precipitated. The reaction mixture was left in the vial for one week. After one day the formation of dark needles-like crystals of (PEA) 4 Sb III Sb V Cl 12 can be observed. To obtain a phase pure sample the reaction mixture was left in the vial for a week until no more yellow crystals were visible.
X-ray crystallography: Single-crystal X-ray diffraction data were collected at room temperature and at 100 K on a Bruker D8 Venture diffractometer equipped with a rotating anode generator with Mo K α radiation (λ = 0.71073 Å). The diffraction intensities were integrated using the SAINT software package and a multiscan absorption correction was applied with SADABS-2016/2 (Bruker,2016/2). The crystal structure was solved using direct methods (SHELXS) [37] for (PEA)SbCl 6 and using intrinsic phasing (SHELXT) [37] for (PEA) 4 Sb 2 Cl 12 and refined against F 2 by applying the full-matrix least-squares method (SHELXL) [37] using the software OLEX2 [38] . Hydrogen atoms were inserted at idealised positions and refined using a riding model. All non-hydrogen atoms were refined anisotropically using full-matrix least-squares. Crystallographic data and refinement details are summarised in the supporting information in Table S13 ((PEA) 4 Sb 2 Cl 12 100 K), Table S14 ((PEA) 4 Sb 2 Cl 12 ) and UV-Vis: Diffuse reflectance spectra were obtained on an integrated sphere attached to a double monochromator spectrofluorometer (FLS980, Edinburgh Instruments) working in synchronous mode. The double monochromator configuration, both in the excitation and in the collection, allows us to discard any photoluminescence signal coming from the sample. The diffuse reflectance spectra were converted using the Kubelka-Munk function: F(R) = (1-R) 2 /(2R).
STA: STA measurements were obtained with a STA 449 F5 Jupiter instrument by Netzsch. Corundum crucibles were filled with the sample. They were heated dynamically with heating rates of 10 K · min À 1 under Ar flow. The data was analysed with the Netzsch Proteus 61 software package.
DFT: Electronic structure calculations were performed with the Vienna Ab initio Simulation Package (VASP) [39] , which implements the Kohn-Sham formulation of density functional theory (DFT) using a plane wave basis set and the projector augmented wave formalism [40] . The generalized gradient approximation (GGA) exchange and correlation functional of Perdew, Burke, and Ernzerhof (PBE) was employed. [41] The plane wave basis set cutoff energy was 700 eV to accurately model the short bonds in the molecular cations. Structures were relaxed to a force tolerance on the ions of 10 meV Å À 1 and the resulting symmetries checked against those of the input structures using FINDSYM. [42] Spin-orbit coupling was not included. Reciprocal-space integration was performed on Γ-centered Monkhorst-Pack grids [43] with densities of 50 to 200 k-points per reciprocal atom for convergence of the self-consistent charge density, and 5000 to 8000 k-points per reciprocal atom for calculation of the electronic density of states (DOS). The LOBSTER program [44] was used to compute DOS orbital projections. [45] Manipulation of crystal structures and LOBSTER outputs was performed with the pymatgen package. [33] Crystal structures and charge densities were visualised with VESTA. [46] PXRD: The X-ray powder patterns were obtained with a Stoe Stadi-P diffractometer in the Debye-Scherrer geometry, employing Cu Kα 1 radiation, a Ge(111) monochromator, and a Mythen 1k detector,).
EDX: Scanning electron microscopy of polycrystalline samples was performed on a Carl Zeiss EVO-MA 10 instrument with a SE detector, which was controlled by the SmartSEM software. [47] The microscope was equipped with a Bruker Nano EDX detector (X-Flash detector 410-M) for EDX investigations using the QUANTAX 200 software to collect and evaluate the spectra. [48] Elements contained in the sample holder and adhesive carbon pads were disregarded.