Structures of (4-Y-C6H4CH2NH3)2PbI4 {Y = H, F, Cl, Br, I}: Tuning of Hybrid Organic Inorganic Perovskite Structures from Ruddlesden-Popper to Dion-Jacobson Limits

Article abstract of DOI:10.1021/acs.chemmater.9b01564

In analogy to their oxide counterparts, two-dimensional (2D) hybrid organic-inorganic perovskites have been classified, in many cases, as either Dion-Jacobson (DJ) or Ruddlesden-Popper (RP) structures. We quantified the offset of the inorganic layers to allow the structures of hybrid organic inorganic perovskite to be consistently related to these two structure types. We report the structures of a family of 2D hybrid structures, (4-Y-C₆H₄CH₂NH₃)₂PbI₄ (where Y = F, Cl, Br, I), which consist of single ?100?-terminated perovskite sheets separated by p-halobenzylammonium cations. In contrast to the previous RP structure of (C₆H₅CH₂NH₃)₂PbI₄, where the inorganic layers are offset from each other, the Y = F, Cl, and Br examples tend toward the DJ structure, in which successive layers eclipse each other, despite the use of an organic monocation. Close Y···I approaches suggest that halogen bonding plays a role in these structures. Use of Y = I, for which stronger halogen bonding is expected and is also suggested by a more linear C-Y···I angle, results in an RP-like structure. The stability of the (4-Y-C₆H₄CH₂NH₃)₂PbI₄ derivatives under ambient conditions is substantially higher for Y = Br and I than for Y = H, F, and Cl.

Full text of DOI:10.1021/acs.chemmater.9b01564

Article
pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Structures of (4-Y‑C6H4CH2NH3)2PbI4 {Y = H, F, Cl, Br, I}: Tuning of
Hybrid Organic Inorganic Perovskite Structures from Ruddlesden−
Popper to Dion−Jacobson Limits
†
‡
Marie-Helene Tremblay, John Bacsa, Boqin Zhao,†,§ Federico Pulvirenti,† Stephen Barlow,
́
̀
and Seth R. Marder*,†
†Center for Organic Photonics and Electronics (COPE), School of Chemistry and Biochemistry, Georgia Institute of Technology,
Atlanta, Georgia 30332-0400, United States
‡Crystallography Lab, Emory University, 201 Dowman Drive, Atlanta, Georgia 30322, United States
S
* Supporting Information
ABSTRACT: In analogy to their oxide counterparts, two-dimensional
(2D) hybrid organic−inorganic perovskites have been classified, in
many cases, as either Dion−Jacobson (DJ) or Ruddlesden−Popper
(RP) structures. We quantified the offset of the inorganic layers to allow
the structures of hybrid organic inorganic perovskite to be consistently
related to these two structure types. We report the structures of a family
of 2D hybrid structures, (4-Y-C6H4CH2NH3)2PbI4 (where Y = F, Cl,
Br, I), which consist of single ⟨100⟩-terminated perovskite sheets
separated by p-halobenzylammonium cations. In contrast to the
previous RP structure of (C6H5CH2NH3)2PbI4, where the inorganic
layers are offset from each other, the Y = F, Cl, and Br examples tend
toward the DJ structure, in which successive layers eclipse each other, despite the use of an organic monocation. Close Y···I
approaches suggest that halogen bonding plays a role in these structures. Use of Y = I, for which stronger halogen bonding is
expected and is also suggested by a more linear C−Y···I angle, results in an RP-like structure. The stability of the (4-Y-
C6H4CH2NH3)2PbI4 derivatives under ambient conditions is substantially higher for Y = Br and I than for Y = H, F, and Cl.
formula A2′′MO4 (M = hexacoordinated ion, A′′ = interlayer
metal ion). Repulsion between the two layers of A′′ cations
present between each layer of MO6 octahedra leads to an offset
of one layer of MO6 octahedra from the next by one-half of a
M-to-M repeat distance in both of the M-O-M directions; this
is often referenced to as a (1/2, 1/2) offset. On the other hand,
DJ oxides exhibit perfect stacking with no offset between
successive layers (0, 0), or a displacement of (1/2, 0) (i.e., a
shift of half a M-to-M repeat distance in one of the two M-O-
M directions). This can be attributed to the single layer of ions
between each layer of octahedra found in these structures; the
general formula is A′′′MO4.
Despite the very different interlayer ion sizes and shapes
used in HOIPs, their structures have often been classified in a
similar way, i.e., as either as RP or DJ structures. A large
number of HOIPs incorporating organic monocationsthe
formulas of which parallel those of RP oxideshave staggered,
RP-like structures (although others have been described as
“RP”, apparently without consideration of the stacking motif).
This can be rationalized if one assumes that the organic
monocation consists of a charge-bearing headgroup (usually an
INTRODUCTION
■
Three-dimensional (3D) hybrid organic−inorganic perovskites
(HOIPs), BPbX3 (B = small organic cation; X = halide) have
attracted much interest, since they have already shown
promising properties in a broad variety of optoelectronic
devices.1−8 Two-dimensional (2D) HOIPs in which each
planar (“(100)-oriented”) layer of corner-sharing PbX6
octahedra is separated by a layer of larger organic monocations
or dications (A or A′, respectively), resulting in compositions
of formula A2PbX4 or A′PbX4, (along with so-called “quasi-2D”
structures of formula A2Bn−1PbnX3n+1 or A′Bn−1PbnX3n+1) have
also been examined as electroactive layers in solar cells9−18 and
light-emitting diodes,19,20 and they show higher stability then
their 3D analogues, coupled with modest performance. Many
other aspects of these materials have also been inves-
tigated.21−27 The choice of the A cation or the A′ dication
strongly influences the final structure of the HOIP. Multiple
examples of cation−cation interactions (steric,28−30 aromatic−
aromatic,31 van der Waals,32 hydrogen bonding29,33,34) or
cation-halide interactions (hydrogen bonding35,36 halogen
bonding,37 nitro-halide interaction31) have been reported in
materials of this type and presumably help direct the structure.
Layered oxide perovskites have been classified into different
families, notably the Ruddlesden−Popper (RP) and Dion−
Jacobson (DJ) structures.38,39 n = 1 RP perovskites have the
Received: April 24, 2019
Revised: August 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.chemmater.9b01564
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(C7H9N)2PbI4: C, 18.06%; H, 2.17%; N, 3.01%. Found: C, 18.01%; H
1.98%; N, 3.00%.
ammonium group) that interacts with the inorganic layer
(electrostatically and/or through hydrogen bonding) and a
relatively rigid tail perpendicular to the layer; staggering the
layers allows the most efficient packing of the tail groups
(Figure 1). On the other hand, a dication in which two head
Bis(4-fluorophenylmethylammonium) Tetraiodoplumbate,
(F-PMA)2PbI4. The combination of PbI2 (131 mg, 0.28 mmol) in HI
(1 mL), F-PMA (65 mg, 0.52 mmol) in HI (1 mL), and MeOH (0.8
mL) gave dark orange crystals (82 mg, 30%). Anal. Calcd for
(C7H9NF)2PbI4: C, 17.39%; H, 1.88%; N, 2.90%. Found: C, 17.34%;
H 1.75%; N, 2.96%.
Bis(4-chlorophenylmethylammonium) Tetraiodoplumbate,
(Cl-PMA)2PbI4. The combination of PbI2 (124 mg, 0.28 mmol) in HI
(2 mL), Cl-PMA (78 mg, 0.55 mmol) in HI (2 mL), and MeOH (1.2
mL) gave orange crystals (96 mg, 29%). Anal. Calcd for
(C7H9NCl)2PbI4: C, 16.81%; H, 1.81%; N, 2.80%. Found: C,
16.80%; H 1.67%; N, 2.82%.
Bis(4-bromophenylmethylammonium) Tetraiodoplumbate,
(Br-PMA)2PbI4. The combination of PbI2 (127 mg, 0.28 mmol) in HI
(2 mL), PMA (103 mg, 0.55 mmol) in HI (2 mL), and MeOH (1.2
mL) gave orange crystals (135 mg, 55%). Anal. Calcd for
(C7H9NBr)2PbI4: C, 15.44%; H, 1.67%; N, 2.57%. Found: C,
15.42%; H 1.57%; N, 2.59%.
Bis(4-iodophenylmethylammonium) Tetraiodoplumbate,
(I-PMA)2PbI4. The combination of PbI2 (202 mg, 0.45 mmol) in
HI (4 mL), I-PMA (197 mg, 0.73 mmol) in HI (0.3 mL), and MeOH
(4 mL) gave orange crystals (302 mg, 44%). Anal. Calcd for
(C7H9NI)2PbI4: C, 14.21%; H, 1.53%; N, 2.37%. Found: C, 14.46%;
H 1.54%; N, 2.38%.
Figure 1. (Left) Schematic representation of a Ruddlesden−Popper
(RP) and a Dion−Jacobson (DJ) n = 1 layered HOIP. (Right)
Schematic representation of the different possibilities when mixing
different offsets and different cations. [Legend: MDJ = monocation-
DJ, DRP = dication-RP, nDJ = near-DJ (x < 1/4, y < 1/4), and nRP =
1
1
near-RP (x > /4, y > /4).]
groups are separated by a rigid bridge would be expected to
hold adjacent inorganic layers in registry with each other
(Figure 1). Indeed, several HOIPs with A′PbX4 formulas that
parallel those of n = 1 DJ oxides have been found to exhibit
offsets typical of that class. However, the organic A and A′
monocations and dications used in HOIPs do not generally
have rigid tails or bridges held perpendicular to the layers; in
view of their irregular shapes and/or conformational flexibility,
and the wealth of possible cation−cation and cation−inorganic
interactions possible, it is perhaps surprising that so many
examples of HOIPs appear to exhibit offsets similar to the ideal
expectations for oxides with analogous formulas.
Here, we report the structures of a family of 2D HOIPs, (Y-
PMA)2PbI4 (Y-PMA = 4-Y-C6H4CH2NH3; Y = H, F, Cl, Br, I).
Despite the presence of A-site monocations, and, therefore,
RP-like formulas, adjacent layers of the F, Cl, and Br examples
more closely approach DJ-type stacking; we describe this class
of compounds, in which the chemical formula of a RP
perovskite is combined with DJ-like stacking, as monocationic-
near-DJ (MnDJ) structures. In contrast, the Y = I analogue has
a near-RP structure. We also classify several structures from the
literature in a similar manner. In addition, we describe the
cation−inorganic and cation−cation interactions in the
crystals, the distortions found in the inorganic layers, and the
optical and stability characteristics of the materials.
General Method for Growth of (Y-PMA)2PbI4Single Crystals.
PbI2 in HI (1 mL of a 25 mg mL−1 solution) was added to a vial.
MeOH (2 mL) was added on top. Y-PMA then was dissolved in a
minimum of MeOH and added slowly into the vial. Orange crystals
typically started to appear at the interface between the HI and MeOH
after 24 h.
General Method for Obtaining (Y-PMA)2PbI4 Thin Films. A
solution made by dissolving (Y-PMA)2PbI4 (10 mg) in DMF (100
μL) was spin-coated at 4000 rpm for 45 s onto a clean glass or
sapphire substrate, precleaned using ultaviolet-treated ozone (UV-
ozone), in a N2-filled glovebox. Films were annealed for 30 min at 100
°C.
Single-Crystal X-ray Diffraction. Suitable crystals (see Figure S1
in the Supporting Information) were selected from the reaction
mixtures obtained using the general procedure above and were
mounted on a loop with paratone oil on a XtaLAB Synergy-S,
Dualflex, HyPix diffractometer. Most crystals were heated to 100 K
during data collection. However, crystals of (Cl-PMA)2PbI4 were
found to undergo a phase transition upon cooling, with a loss of
crystallinity being observed at <215 K; therefore, the crystal was
cooled to 225 K during the data collection. Using OLEX2,40 the
structure was solved with the ShelXT41 structure solution program
using intrinsic phasing and refined with the ShelXL42 refinement
package using least-squares minimization. Details of the crystals, data
collection, and data refinements are summarized in Table S1 in the
Supporting Information. Simulated powder patterns were calculated
by Mercury software, using the single-crystal X-ray CIF data.43
Powder X-ray Diffraction (PXRD). PXRD patterns (see Figures
S2 and S3 in the Supporting Information) were acquired on a
Panalytical XPert PRO Alpha-1 XRD diffractometer, using Cu K X-
ray tube radiation at a voltage of 45 kV and 40 mA, with an incident
beam Johannsson monochromator and an X’Celerator solid-state
detector. The diffraction pattern was scanned over the angular range
of 3°−40° with a step size of 0.016°, at room temperature.
Thermal Studies. The thermal behavior of the HOIPs was
examined by differential scanning calorimetry using a Mettler Toledo
Instrument (DSC 3+ Star system), with heating and cooling rates of
10 °C min−1 and a nitrogen gas flow of 80 mL min−1 (see Figure S4 in
the Supporting Information), and by thermogravimetric analyses on a
Pyris 1 TGA (Perkin−Elmer) at a heating rate of 5 °C min−1 under a
nitrogen atmosphere (see Figure S5 in the Supporting Information for
data).
EXPERIMENTAL SECTION
■
General Method of Synthesis of Bulk Crystalline (Y-
PMA)2PbI4. PbI2 (1 equiv) was dissolved in HI (57 wt % aqueous
solution) and stirred at 100 °C for 2 min. A solution of Y-PMA (2
equiv) in HI and MeOH was added to the hot PbI2 solution, and the
resultant mixture was stirred at 130 °C for 2 min. The solution was
then allowed to crystallize at room temperature for 12 h. The solids
were filtered and dried under high vacuum. Small peaks were often
obtained at low angle, close to that at which the (00l) peaks are
observed (see Figure S2 in the Supporting Information). This minor
impurity did not affect the elemental analysis of the products.
Bis(phenylmethylammonium) Tetraiodoplumbate,
(PMA)2PbI4. The combination of PbI2 (129 mg, 0.28 mmol) in HI
(1 mL), PMA (238 mg, 2.23 mmol) in HI (1 mL), and MeOH (0.2
mL) gave dark orange crystals (90 mg, 35%). Anal. Calcd for
B
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Figure 2. (A) Crystal structures of (Y-PMA)2PbI4 (data for Y = H taken from ref 44): (top) side view parallel to the inorganic layers (along the
short axis) and (bottom) view along an axis perpendicular to the inorganic planes. (B) Schematic representation of the halogen bonding between
the iodine of the inorganic sheet and the cation. (C and D) Representations of the inorganic sheets for (PMA)2PbI4 (panel (C)) and (Br-
PMA)2PbI4 (panel (D)) (cations have been removed for the sake of clarity).
Figure 3. Left: Quantification of the offset of the two inorganic layers on the x- and y-axes for some literature A2PbX4 and A′PbX4 HOIPs, and for
the (Y-PMA)2PbI4 HOIPs for which structures are reported in this work. The diagonal line indicates cases where x = y. The cross symbols (×)
represent the new structures from this report, the open blue circles represent structures with DJ or nDJ stacking, and the solid red squares represent
structures with RP or nRP stacking. The image on the right is a schematic showing that offsets of (1/2, 1/2) are found for ideal RP structures and (0,
0) or (1/2, 0) ideal DJ structures (purple = halide ions, gray = Pb ions).
h. Films of the HOIPs were obtained through spin-coating
onto glass substrates from dimethylformamide. All reflections
seen in the Bragg-geometry PXRD patterns of the films can be
indexed as (00l) reflections, corresponding to the layer
stacking direction and indicating a strong preferential growth
in the (001) plane to form platelike crystallites, that then lie
parallel to the substrate. This is consistent with numerous
previous studies of n = 1 RP and DJ systems, indicating that
the orientation of the layers parallel to the substrate is strongly
favored over the perpendicular one.17 The films were used to
study the stability of the materials (see Figure S3 in the
Supporting Information). Ambient stability of the films in
darkness was tested and analyzed using PXRD. After 1 week
under ambient conditions (ca. 50% humidity, ca. 20 °C) in
darkness, PbI2 peaks were prominent and the HOIP Bragg
peaks almost disappeared for films of the Y = H, F, and Cl
derivatives. On the other hand, the Br- and I-analogues were
unchanged, showing an increased stability for these two
compounds, perhaps because of halogen-bonding interactions
RESULTS AND DISCUSSION
■
Synthesis. The HOIPs were synthesized using a 1:2 ratio
of PbI2 and Y-PMA iodide (where Y = F, Cl, Br, I) in a mixture
of methanol and hydroiodic acid. Crystallization afforded the
2D HOIPs as bright to dark orange plates. The crystals were
analyzed directly to obtain their single-crystal structures (see
Figures 2, 3, and 4 where they are compared to the previously
reported structure of their Y = H analogue;44 see Figure S1 and
Tables S1 and S2 in the Supporting Information) or filtered,
ground, and analyzed by powder X-ray diffraction (PXRD)
(see Figure S2 in the Supporting Information). The single-
crystal structural data was acquired at 100 K, except in the case
of (Cl-PMA)2PbI4, where this was precluded by a phase
transition at ca. 175 K and where a temperature of 225 K was
used instead.
All the crystals were stable in the presence of the solution
from which they were grown over several days, except those
based on Cl-PMA, which decomposed on a time scale of ca. 48
C
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(vide infra) between the heavier halogens and the iodide
ions.45 This may be related to a slight increase in hydro-
phobicity, as measured by the water contact angle, in the order
of Y = F < Cl < Br < I (see Figure S6 in the Supporting
Information).
Organic−Inorganic Interactions. For the four (Y-
PMA)2PbI4 compounds presented in this work, along with
44
the previously reported structure of (PMA)2PbI4 and in
common with the structure of many other 2D HOIPs, there is
hydrogen bonding between the inorganic I− ions and the
ammonium groups. While the “head” of each cation, the
ammonium moiety, is directed toward the inorganic sheet, the
orientation of the tail is governed by the balance of “tail−tail”
and, in some cases, “tail−inorganic” interactions that are
possible. “Tail−inorganic” interactions for A2PbX4 HOIPs are
relatively scarce. Indeed, in many such structures, including
those of (Y-PEA)2PbI4 (Y-PEA = 2-(4-halophenyl)-
ethylammonium, where Y = F, Cl, Br),56 which contain A-
site cations differing from the Y-PMA cations used in this work
by an extra CH2 group, the organic cations are arranged in
bilayers between successive inorganic layers, and, thus, do not
form head-to-tail bridges between the PbI4 sheets. In the
present series, however, there are hydrogen bonds from the
Interlayer Stacking. In the previously reported structure
of (PMA)2PbI4, in which the Y = H unsusbtituted phenyl-
methylammonium (benzylammonium) cation is employed, a
1
RP structure, with perfectly offset (1/2, /2) layers is obtained.
In the present structures where the para position of the phenyl
ring is substituted with F, Cl, or Br, the layers more closely
approach DJ-type stacking (MnDJ) (see Figure 2), while the I-
PMA derivative again has RP-like layer offsets (nRP). In Figure
3, we have quantified the offsets of adjacent inorganic layers as
(x, y), where these coordinates correspond to the mean
translation of the offsets of the Pb atoms in one plane, relative
to that in the next along the two Pb−I−Pb directions (which
do not necessarily correspond to the crystallographic axes),
expressed as fractions of the Pb−I−Pb repeat distances.
Offset data for several previously reported structures are
plotted, along with those for the present (Y-PMA)2PbI4
structures. In addition to (PMA)2PbI4, the monocations (p-
tolylmethylammonium)2PbI4 (1),46 (thien-2-ylmethylammo-
nium)2PbI4 (2),46 (cyclopropylammonium)2PbI4 (3),47 have
+
NH3 group of the cation to one PbI4 sheet and a close
approach (see Table 1) between the organic halo-substituent
Table 1. Selected Distances and Angles in the Crystal
Structures of (Y-PMA)2PbI4
I···Y
distance
(Å)
1
an “ideal” RP offset of (1/2, /2). (PEA)2PbI4 (4, PEA =
sum of Y and I VdW
I···Y−C4
interlayer I···I
distance (Å)
phenylethylammonium)48 deviates significantly from the ideal
with an offset of (0.37, 037) and therefore can be regarded as
nRP. Two previously reported dication structures(naph-
thalene-1,5-diyl ammonium)PbI4 (5)49 and (4-
(ammoniomethyl)piperidinium)PbI4 (6)16are “ideal” DJ
structures with translations of precisely (0, 0). Another
reported dication structure, (4-(2-ammonioethyl)-1H-
imidazolium)PbBr4 (7),50 is close to the (1/2, 0) DJ limit,
with a translation of (0.42, 0), and therefore is best described
as having “near-DJ” (nDJ) stacking. Another dication structure,
(3-(ammoniomethyl)piperidinium)PbI4 (8),16 deviates more
significantly from the “ideal” DJ values, but its translation of
(0.18, 0.18) is still closer to one of the ideal DJ values than the
RP limit and can therefore also be regarded as nDJ. Figure 3
shows that the offset for (F-PMA)2PbI4 is not particularly close
to any of the ideal values; however, it is closest to the (1/2, 0)
DJ limit and therefore is best considered to be a nDJ structure.
(Y-PMA)2PbI4 (Y = Cl, Br) are also nDJ structures, but more
closely approach the (0, 0) than the (1/2, 0) extreme and
exhibit offsets similar to that seen for (3-(ammoniomethyl)-
piperidinium)PbI4. Given that (Y-PMA)2PbI4 (Y = F, Cl, Br)
incorporate organic monocations, resulting in a RP-like
formula, but DJ-like layer offsets, we classify these materials
as monocationic near-DJ (MnDJ) structures. A search of the
literature revealed several 2D HOIPs that can also be classified
as MnDJ structures, although the DJ-like aspects of their
structures were not commented upon in the original
publications. The offsets calculated for the structures of (2-
hydroxyethylammonium)2PbI4 (9),51 (octylammonium)2PbI4
(10),52 and (3-bromopyridinium)2PbI4 (11),53 are shown in
Figure 3. We also found one example of a perfect MDJ: (5-
iodopentylammonium)2PbI4 (12).51 Finally, the translation
seen for (I-PMA)2PbI4 is close to, but falls slightly short of, the
ideal RP value of (1/2, 1/2) and this structure can, therefore, be
classified as near-RP (nRP). Again, our search of the literature
revealed several 2D HOIPs that can be classified as nRP
structures, such as (2-cyanoethylammonium)2PbI4 (13)54 and
(pentylammonium)2PbI4 (14).55
a
Y
radii (Å)
angle (deg)
b
H
F
3.488
3.478,
3.954
4.039,
3.933
3.894,
3.805
3.07
3.45
140.78
118.7, 157.2
9.197
7.777
Cl
Br
I
3.73
3.83
3.96
146.41,
164.49
150.67,
164.49
9.546
9.553
3.931
168.2
10.771
a
van der Waals radii for halogens and hydrogen were taken from refs
57 and 58, respectively. Data taken from ref 44.
b
(Y) of the same cation and an I− ion of the next PbI4 sheets; in
some cases, this approach may reflect a halogen bonding
interaction in which Y is the halogen-bond donor (Lewis acid)
and I− the acceptor (Lewis base). In the case of (PMA)2PbI4,
the H···I distance exceeds the sum of the van der Waals radii
(from refs 57 and 58) and therefore likely does not represent a
significant interaction. In the case of (F-PMA)2PbI4, one of
two inequivalent Y···I distances is close to the sum of van der
Waals radii, but the corresponding C−F···I angle is far from
the ideal 180° expected for a halogen bond. In this case, a
hydrogen bonding interaction between the F atom and the
NH3+ group of a neighboring cation may be responsible for the
close F···I approach. In the case of (Cl-PMA)2PbI4, the Cl···I
distances are both somewhat larger than the sum of the van der
Waals radii, but both C−Cl···I interactions are more linear
than the short contact seen for Y = F, with the shortest of the
two being the more linear. For (Br-PMA)2PbI4 and (I-
PMA)2PbI4, the Y···I contacts are similar to or shorter than van
der Waals contacts. The C−Br···I angles are similar to the C−
Cl···I angles, while the C−I···I interaction is the most linear of
any of the C−Y···I interactions seen here. The tendency
toward shorter (relative to van der Waals radii) and more
linear interactions seen for the heavier halogens is consistent
with the strength of C−Y bonds as halogen bond donors,
which increases in the order of F < Cl < Br < I (see Figure
4).59
D
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the increased molecular volume by stronger halogen bonding
in the Br-PMA derivative, which results in Br···I contacts that
are shorter than the Cl···I distances.
However, despite the evidence for halogen bonding in some
of these structures, (Br-PMA)2PbI4 and (I-PMA)2PbI4, in
which the strongest halogen bonding is both suggested by the
crystallographic data and expected based on theoretical
considerations, exhibit very different structures, notably
exhibiting nDJ and nRP offsets, respectively. In contrast, (Cl-
PMA)2PbI4 and (Br-PMA)2PbI4 adopt similar packing and
nDJ translations, despite their different halogen-bonding
strengths. Thus, although halogen bonding allows the Y-
PMA monocations to act as interlayer bridges, potentially
affecting the stacking of the inorganic layers, the conforma-
tional flexibility of the cations, combined with the other
cation−cation interactions present, steric hindrance, and the
various distortions of the PbI4 layers that can occur, means that
halogen bonding is not the only factor. Interestingly, while the
present series shows an evolution from RP to MnDJ to nRP
with increasing halogen size and potential halogen-bonding
strength, a series of (CH3(CH2)m−1NH3)2PbI4 struc-
tures,52,55,60,61 with m = 4−10, 12, 14, 16, 18 (the m = 2
and 3 cations lead to compounds containing one-dimensional
(1D) chains of face-sharing octahedra of formula APbI3, rather
than 2D HOIPs62,63) shows a similar evolution from ideal RP
(m = 4) to near-ideal MnDJ (m = 6) and back to ideal RP (m
= 10+) with increasing alkyl chain length and increasing alkyl−
alkyl van der Waals interaction (see Figure S7 in the
Supporting Information). Even in the (Y-PEA)2PbI4 series
(Y = H, F, Cl, Br),48,56 where there is a bilayer of organic
cations, but in which the cation−cation interaction strength
likely increases with the halogen polarizability, the deviation
from ideal RP stacking increases from Y = H to Y = F, with
ideal RP stacking for Y = Cl, and Br (see Figure S7).
Cation−cation interactions and contacts in the (Y-
PMA)2PbI4 structures are shown in Figure S8 in the
Supporting Information. As noted above, C−Y···H hydrogen
bonding is present in some structures. Another interesting
feature is the presence of π−π interactions between F-PMA
cations; this is a rare interaction for HOIPs, with only a few
examples given in the literature.31 The distortions in the
inorganic portion of the structures are discussed in the
following section.
Distortions within the PbI4 Layers. Various distortions
have been found in the PbX4 layers of HOIPs and, in various
series of compounds, have been correlated with optical
properties. One important structural parameter that can be
Figure 4. Representation of the possible close Y···I approaches
between the halo substituent of the organic cation and the inorganic
sheet of (Y-PMA)2PbI4 (from left to right, Y = H, F, Cl, Br, and I).
Note that for Y = F, Cl, and Br, there are two inequivalent cations in
the asymmetric unit, whereas there is only one for Y = H and I.
Figure 5 shows a related observation. For 2D HOIPs
incorporating increasingly large cations of similar structure, a
Figure 5. (A) Relationship between the interlayer stacking distance
and the molar volume for alkylammonium HOIPs (black squares) and
for (Y-PMA)2PbI4 derivatives (red circles). The interlayer stacking
distance is extracted from the reported crystal structure of
butylammonium (BA),60 pentylammonium (PA),55 hexylammonium
(HA),55 and nonylammomium (NA).52
linear trend is often observed between the interlayer stacking
distance and molecular volume of the cation (see black squares
for the alkylammonium series in Figure 5). In the present
series, the trend is not observed (see red circles in Figure 5); in
particular, the Cl- and Br-PMA derivatives show very similar
interlayer spacings, despite an increase in cation volume of 7.7
cm3 mol−1. This can be attributed to the counterbalancing of
Figure 6. Comparisons of (A) the Pb−I−Pb angles and (B) the ammonium penetration past the peripheral iodine for (Y-PMA)2PbI4 (where Y =
H, F, Cl, Br, I). The ammonium penetration is the distance between the nitrogen of the ammonium and the plane made by the four iodine of the
octahedron. Because of the crystallographic symmetry of the crystal structures, there are two values for the Cl and Br compounds. (C) Relationship
between the bond angle variance and the quadratic elongation.
E
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Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
used to describe HOIPs is the Pb−I−Pb angle (see Figure 6A
and Table 2), where larger Pb−I−Pb angles represent less
(PL) spectroscopy67 (see Figure 7 and Table 3 for film data;
data for powders are shown in Figure S10 in the Supporting
Information).
Table 2. Distortion Parameters for the PbI4 Layers in the
a
Crystal Structures of (Y-PMA)2PbI4
σoct (deg2)
2
Y
Pb−I−Pb (deg)
r(Pb−I),avg (Å)
λoct
b
H
F
158.43
3.2082
3.2090
3.1907
3.1868
1.0035
1.0041
1.0021
1.0024
1.0031
12.0819
12.7554
6.8871
8.2175
10.8138
158.00, 158.69
153.91, 155.07
151.34, 154.75
156.11
Cl
Br
I
3.1840
a
b
Calculated using VESTA.65 Data taken from ref 44.
distortion; smaller distortions have been correlated with lower
exciton energy for both RP and DJ perovskites.64 In the series
presented here, these distortions are fairly large, and fall into a
relative narrow range of 150°−160°. The Br-PMA compound
has the smallest Pb−I−Pb angle (151°), while its F- and H-
analogues have the largest (ca. 158°). The ammonium
penetration past the peripheral iodine is also related to the
Pb−I−Pb angle.48 As mentioned previously, the (Cl-
PMA)2PbI4 and (Br-PMA)2PbI4 structures are less symmetric
than the Y = H and Y = I analogues, perhaps due to the
asymmetrical ammonium penetration (see Figure 6B). (F-
PMA)2PbI4 is also slightly asymmetrical. Because of the large
penetration on one side, the octahedra tilt away from each
other on that side, which results in a tilt toward each other on
the opposite face of the PbI4 layer, allowing only a small
penetration of the other ammonium. In this series, as the
ammonium penetration is increased (taking, where relevant,
the largest of the two values), the Pb−I−Pb angle is reduced,
showing more distortion.
The distortion of the geometry of the PbI6 octahedra from
regularity has also been discussed in the literature and
correlated with optical properties. This distortion can be
gauged through the quadratic elongation (λoct) or the bond
angle variance (σ2oct):66
2
6
i
j
y
z
d
d0
1
6
i
j
j
z
z
λoct
=
∑
j
j
z
z
(1)
k
{
i=1
12
1
11
2
σ
=
(α − 90)2
∑
oct
i
(2)
i=1
where di is the Pb−I bond length, d0 the center-to-vertex
distance of a regular polyhedron of the same volume, and αi
the I−Pb−I angle. In the present series, there is good
correlation between the two measures of distortion (Figure
6C), consistent with that seen for most other haloplumbate
HOIPs.66 However, the distortions are all relatively small and
there is no obvious trend with the identity of Y (see Table 2
and Figure 6C). We recently demonstrated that the 207Pb
NMR chemical shift (δ(207Pb)) for planar and corrugated
A2PbI4 and A′PbI4 compounds correlates linearly with λoct. As
shown in Figure S9 in the Supporting Information, data for
(Br-PMA)2PbI4 also fit this relation.
Optical Properties. The photophysical properties of the
HOIP films and powders (the PXRD patterns of which were,
in each case, consistent with those calculated using the relevant
single-crystal parameters, as shown in Figure S2) were
investigated by UV-vis absorption (films) or diffuse reflectance
(powders) spectroscopy and steady-state photoluminescence
Figure 7. (A) Absorption and (B) PL spectra of the HOIP films. (C)
Relationship between the band gap and the average Pb−I−Pb angle.
Table 3. Optical Properties of (Y-PMA)2PbI4 Films
Y
Eg (eV) Eex (eV) photoluminescence, PL (eV)
color
H
F
Cl
Br
I
2.66
2.69
2.73
2.74
2.66
2.29
2.33
2.33
2.28
2.29
2.32
2.36
2.36
2.36
2.32
dark orange
dark orange
pale orange
pale orange
dark orange
F
DOI: 10.1021/acs.chemmater.9b01564
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
The exciton energy (Eex) is estimated by extrapolating the
low-energy edge of the low-energy discrete peak in the UV-vis
absorption spectra to the energy axis, while the bandgap (Eg) is
estimated by extrapolating the edge of the higher-energy
feature to the energy axis. Overall, there is relatively little
variation in the energies in the present series. As shown in
Figure 7C, values of Eg (but not of Eex or the PL) correlate
fairly well with the Pb−I−Pb angle over the series, which is a
relationship that has previously been suggested;64 the
deviations from this relationship may be due to the competing
effects of other structural features, but it should be emphasized
that the ranges of Pb−I−Pb angles and Eg values are both
relatively small, compared to those seen in ref 64 Regardless,
the Cl-PMA and Br-PMA derivatives, which exhibit the
smallest (most distorted) Pb−I−Pb angles of this series do
exhibit the highest values of Eg. The values of Eg also show
interactions in order to obtain whichever stacking pattern is
desired. Further systematic structural studies of A2PbX4 HOIPs
that contain families of closely related cations will be required
to validate or refute this idea.
The distortions of the inorganic layers in the (Y-PMA)2PbI4
series have also been examined; in particular, when the
ammonium penetration is increased, the Pb−I−Pb angle is
reduced, showing more distortion in the plane of the sheet.
Further understanding of the role of cation−cation and cation-
inorganic sheet interactions may help engineer new HOIPs;
the findings presented here underscore the importance of
increasing the library of HOIP structures, in particular of
MnDJ structures, available in order to attempt to understand
the factors determining their structural characteristics. The
optical properties of the series show relative minor variations,
consistent with the large interlayer spacings and highly
distorted Pb−I−Pb angles found in each case.
2
some correlation with λoct and σoct (see Figure S11 in the
Supporting Information). The PL spectrum for (F-PMA)2PbI4
is the broadest one among those collected (Figure 7B),
following the expected trend where the structure with the most
distorted octahedra, as measured by the quadratic elongation,
has a larger full-width at half-maximum (fwhm).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.9b01564.
■
S
Note that some DJ A′PbI4 structures have been previously
found to exhibit lower band gaps than typical RP HOIPs.
However, this is attributed to closer I···I interlayer distances
(arising from relatively small bridging dications) and to less
distorted inorganic frameworks, rather than being a direct
consequence of the different stacking motifs.16 In the present
series, the largest band gaps are found for the MnDJ structures
(Y = F, Cl, Br). However, the interlayer distances in the
present MnDJ structures are much greater than those in
previously studied DJ examples (and, as shown in Figure 5,
there is no clear pattern in this distance between (n)RP and
MnDJ structure types), and they are likely sufficiently large to
preclude the layer−layer interactions thought to be responsible
for the red-shift of previous DJ examples. In addition, two of
the MnDJ structures (Y = Cl, Br) exhibit the most distorted
Pb−I−Pb angles and least distorted octahedra, while the other
nDJ structure (Y = F) has the least-distorted Pb−I−Pb angles
and most distorted octahedra.
Crystallographic data for (4-F-PMA)2PbI4 (CIF)
Crystallographic data for (4-Cl-PMA)2PbI4 (CIF)
Crystallographic data for (4-Br-PMA)2PbI4 (CIF)
Tables of crystallographic data, additional figures
showing structural features, 207Pb SS-NMR, and stability
data (PDF)
Crystallographic data for (4-I-PMA)2PbI4 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: seth.marder@chemistry.gatech.edu.
ORCID
John Bacsa: 0000-0001-5681-4458
Stephen Barlow: 0000-0001-9059-9974
Seth R. Marder: 0000-0001-6921-2536
■
Present Address
§Beijing National Laboratory for Molecular Sciences, State Key
Laboratory of Rare Earth Materials and Applications, College
of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, PRC.
CONCLUSION
■
In summary, we have reported the structures, optical
properties, film formation, and stability data of a series of
A2PbI4 HOIPs employing Y-PMA cations, where Y = F, Cl, Br,
and I, and compared them to those of their Y = H analogue.
Short contacts are seen between the halo substituents of the
cations and the iodides of the inorganic sheets; in particular,
the Y···I distances and C−Y···I geometries seen for Y = Br and
I are suggestive of halogen bonding. Moreover, we have
quantified the DJ- vs RP-like nature of 2D HOIPs, according to
the offsets of subsequent inorganic layers; this may prove
useful in classifying other HOIPs. We classify the Y = F, Cl,
and Br derivatives as “MnDJ” on the basis that they include
organic monocations, but exhibit stacking of the inorganic
layers that is closer to that found in DJ perovskites than in RP
perovskites. For the X = I example, a more RP-like stacking is
observed. Examination of the literature crystal structures
reveals some similar behavior in other series, suggesting that
the trends in the stacking motif can perhaps be correlated with
those in the strength of organic−inorganic or organic−organic
interactions. Based on the stacking obtained for a given cation,
it may even be possible to engineer stronger or weaker
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by NSERC (ES. D. Scholarship for
MHT) and the AFOSR (No. FA9550-18-1-0499).
■
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DOI: 10.1021/acs.chemmater.9b01564
Chem. Mater. XXXX, XXX, XXX−XXX