Zero-Dimensional Hybrid Organic-Inorganic Indium Bromide with Blue Emission

Article abstract of DOI:10.1021/acs.inorgchem.0c03164

Low-dimensional hybrid organic-inorganic metal halides have received increased attention because of their outstanding optical and electronic properties. However, the most studied hybrid compounds contain lead and have long-term stability issues, which mus

Full text of DOI:10.1021/acs.inorgchem.0c03164

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Article
Zero-Dimensional Hybrid Organic−Inorganic Indium Bromide with
Blue Emission
Hadiah Fattal,† Tielyr D. Creason,† Cordell J. Delzer, Aymen Yangui, Jason P. Hayward, Bradley J. Ross,
Mao-Hua Du, Daniel T. Glatzhofer, and Bayrammurad Saparov*
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ABSTRACT: Low-dimensional hybrid organic−inorganic metal halides have received
increased attention because of their outstanding optical and electronic properties.
However, the most studied hybrid compounds contain lead and have long-term stability
issues, which must be addressed for their use in practical applications. Here, we report a
new zero-dimensional hybrid organic−inorganic halide, RInBr4, featuring photoemissive
trimethyl(4-stilbenyl)methylammonium (R+) cations and nonemissive InBr4− tetrahedral
anions. The crystal structure of RInBr4 is composed of alternating layers of inorganic
anions and organic cations along the crystallographic a axis. The resultant hybrid
demonstrates bright-blue emission with Commission Internationale de l’Eclairage color
coordinates of (0.19, 0.20) and a high photoluminescence quantum yield (PLQY) of
16.36% at room temperature, a 2-fold increase compared to the PLQY of 8.15% measured for the precursor organic salt RBr. On the
basis of our optical spectroscopy and computational work, the organic component is responsible for the observed blue emission of
the hybrid material. In addition to the enhanced light emission efficiency, the novel hybrid indium bromide demonstrates
significantly improved environmental stability. These findings may pave the way for the consideration of hybrid organic In(III)
halides for light emission applications.
irradiation, ambient air, and other relevant conditions must be
further improved.22
INTRODUCTION
■
In recent years, low-dimensional metal halides have received
increased attention because of their outstanding light emission
properties attributed to strong charge localization and
enhanced excitonic properties.1−9 Among these, hybrid
organic−inorganic and all-inorganic lead halides have been
the focus of the most attention, with notable examples
including zero-dimensional (0D) Cs4PbBr6 with efficient
green emission and one-dimensional (TDMP)PbBr4 with
broadband white emission.10−13 However, despite remarkable
advancement in the development of lead halide phosphors, the
innate toxicity of lead and the issues regarding their long-term
air stability are obstacles to potential commercialization of
these materials.14 Unlike higher-dimensional metal halides,
there are virtually no structural restrictions for 0D metal
halides (e.g., size factors as outlined by the tolerance factor for
perovskites). Therefore, in principle, a far greater diversity of
chemical compositions and crystal structures can be explored
in the 0D metal halide family. In the search for lead-free light
emitters, brightly luminescent 0D metal halides have recently
been expanded to include halides of tin, copper, zinc,
manganese, etc.15−21 Among these, materials such as A2CuX3
(A = K, Rb; X = Cl, Br) demonstrate high photoluminescence
quantum yield (PLQY) values measuring up to unity, which
allows their consideration for light-emitting-diode and
radiation-detection applications.17,21−24 However, for potential
practical applications, the stability of metal halides under
In the literature, halides containing trivalent metal cations
have received less attention than halides based on divalent
metal cations.25 In particular, halides of Sb3+ and Bi3+ have
been studied as isoelectronic alternatives to unstable Sn2+ and
toxic Pb2+, respectively.26−30 In comparison, halides of the
group 13 elements (aluminum, gallium, and indium) have been
underexplored, with only a few known members reported in
the literature so far, including compounds Cs2InBr5·H2O,31
(C4H14N2)2In2Br10,32 (C4H7N2)4[InBr6][InBr4]·2H2O,33 and
(C4H10N)6[InBr6][InBr4]3·H2O34 and the mixed-valent in-
dium compounds CsInX3 (X = Br, Cl).35 Interestingly, indium
can form structures based on both the octahedral and
tetrahedral coordination geometry. Photoluminescence (PL)
studies on Cs2InBr5·H2O and (C4H14N2)2In2Br10 show that the
compounds emit in the red region (670−695 nm) with PLQY
values ranging from 3 to 33%, attributed to self-trapped
excitons (STEs) localized on the inorganic sublattice.31,32
Received: October 26, 2020
Published: January 5, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03164
© 2021 American Chemical Society
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Figure 1. Polyhedral view of the 0D crystal structure of RInBr4. Carbon, nitrogen, indium, and bromine atoms are shown in black, light blue, green,
and navy blue, respectively; hydrogen atoms are omitted for clarity.
Herein, we report the preparation and efficient sky-blue
emission properties of a novel hybrid organic−inorganic
indium halide, RInBr4, where R = trimethyl(4-stilbenyl)-
methylammonium cation, C14H22N+. In this study, an
alternative approach has been taken, wherein a previously
known efficient light emitter, stilbene, has been modified in a
novel fashion by adding a trimethylated ammonium group to
one of the two phenyl rings. Stilbene is known to exhibit high-
efficiency light emission properties but has poor photo-
stability.36,37 This stilbene derivative was then used as the
organic cation for the formation of novel RInBr4, which is, to
the best of our knowledge, one of the few known
pseudoternary hybrid organic−inorganic indium(III) halides
to date. The novel compound was structurally characterized
using X-ray diffraction methods. We find that RInBr4
demonstrates high-efficiency blue emission with a PLQY
value of 16.36%, which is a 2-fold increase compared to the
PLQY of 8.15% measured for the organic precursor salt RBr.
We discuss the origin of the sky-blue emission from RInBr4
based on our combined optical spectroscopy and density
functional theory (DFT) results.
Unlike trivalent bismuth and antimony halides, in which
metals predominantly adopt an octahedral coordination
environment, In3+ has a tetrahedral coordination in RInBr4.
Indium is known in the literature for its diversity of
coordination environments in bromides. For instance, In3+
takes a tetrahedral environment in InBr2, octahedral coordi-
nation in In7Br9, and both octahedral and tetrahedral
geometries in In4Br7.35,41−43 Only a few examples of ternary
indium halides are known to date. Among them is the mixed-
valent CsInBr3, in which In3+ has an octahedral environment.35
In Cs2InBr5·H2O, In3+ also adopts an octahedral coordination
environment.31 However, In3+ adopts both tetrahedral and
octahedral coordination environments in (C4H14N2)2In2Br10,32
(C4H7N2)4[InBr6][InBr4]·2H2O,33 and (C4H10N)6[InBr6]-
[InBr4]3·H2O.34 On the basis of Pauling’s Radius-Ratio Rule
(μ = RB/RX, where RB and RX are ionic radii of the B metal and
X anion), the acceptable octahedral factor μoct range is 0.414−
0.732 and the tetrahedral μtet range is 0.225−0.414. For
indium(III) bromides, μoct and μtet are 0.408 and 0.316,
respectively, which suggests a slight preference for the
tetrahedral coordination environment. However, the octahe-
dral factor of μoct = 0.408 is close to the threshold value of
0.414, which can explain the empirical observation of both the
octahedral and tetrahedral coordination geometries in indium
bromides.32,33,44 It must also be noted that, although the
Pauling rules are frequently used for crystal structure
rationalization, it has recently been shown that the Pauling
rules, including the Radius-Ratio Rule, have only a limited
predictive power when used for oxides.45 Important for future
work, the flexibility of the coordination environment of indium
provides a route for obtaining chemical compositions and
crystal structures that are not available through the use of
commonly used metal elements such as lead and tin in hybrid
organic−inorganic metal halides.
RESULTS AND DISCUSSION
■
RInBr4 crystallizes in the monoclinic space group P21/c, and its
structure is composed of molecular InBr4− anions separated by
R+ cations. Although there is no chemical bonding connectivity
between the tetrahedral InBr4− anions, i.e., the crystal structure
of RInBr4 is 0D (Figure 1). InBr4− anions and R+ cations group
into distinctly separate parts of the structure, forming
alternating inorganic anionic layers and organic cationic layers
along the a axis. Interestingly, similar pseudolayered crystal
structures have been reported for several other 0D halides of
group 12 metals.18,38−40 For RInBr4, the formation of such a
pseudo-two-dimensional-layered structure can be related to the
optimized electrostatic interactions between the ammonium
−
The InBr4 tetrahedra in RInBr4 are slightly distorted, with
InIII−Br bond distances ranging from 2.35 to 2.5 Å (Table S3).
Because trivalent indium has no lone pair of s electrons (i.e.,
In3+ has an electronic configuration of [Kr] 5s0), the observed
functional groups of R+ and InBr4 and the nonpolar
−
interactions between the nonpolar tails of the organic cations,
which is a common observation for low-dimensional metal
halide perovskites.25
−
distortions of InBr4 are attributed to the aforementioned
optimized electrostatic interactions and packing of the organic
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Figure 2. (a) PL and PLE spectra of RInBr4 and RBr. (b) Normalized PLQY values of RInBr4 and RBr under continuous irradiation at their
respective PLEmax
.
molecular cations and inorganic molecular anions. This
conjecture is supported by the fact that, in binary indium(III)
bromides such as In7Br9 and InBr2, the InIIIBr4 tetrahedra are
regular, with InIII−Br bond distances ranging from 2.4 to 2.5 Å,
in agreement with the estimates based on the sum of the
Shannon ionic radii.46 In comparison, indium(III)-containing
pseudoternary hybrid bromides such as (C4H7N2)4[InBr6]-
[InBr4]·2H2O, (C4H10N)6[InBr6][InBr4]3·H2O, and
(C4H14N2)2In2Br10 feature more distorted coordination
polyhedra, pointing to the important role of constituent
organic cations in the observed distortions of the inorganic
building blocks.32−34 The InIII−Br bond lengths in InBr4
tetrahedra in these compounds range from 2.47 to 2.62 Å,
which are comparable to that observed for RInBr4. The slightly
longer distances reported in the literature can be attributed to
the higher data collection temperatures of 150 K. The trends in
the tetrahedral bond angles also largely follow the same trends
a fwhm value of 95.3 nm, giving it a lighter blue tone with the
Commission Internationale de l’Eclairage (CIE) color
coordinates of (0.19, 0.20), which can be seen in the emission
color for RInBr4 (Figure S2a,b).
Immediately noticeable is that the PL peak shape for RInBr4,
including two peaks at 437 and 451 nm and a broader shoulder
at 506 nm, is nearly identical with that measured for RBr.
Therefore, on the basis of the measured PL results, the origin
of the emission in RInBr4 seems to be the organic component.
This is sharply different from the reported PL properties for
most of the hybrid metal halides, which typically exhibit PL
emission stemming from the inorganic structural segments.47,48
As mentioned above, In3+ is capable of adapting both the
octahedral or tetrahedral coordination geometries; however,
only the octahedrally coordinated InIIIBr6 unit was shown to be
emissive in (C4H14N2)2In2Br10.32 This is attributed to the more
pronounced structural distortions of the InBr6 octahedra
compared to the InBr4 tetrahedra, which helps to stabilize the
STEs in the former. In the case of RInBr4, the presence of
emissive organic counteraction, together with nonemissive
−
as those described for the bond distances, showing that InBr4
tetrahedra undergo distortions when placed in a hybrid
matrix.33,34
−
The optical band gap of RInBr4 was estimated using diffuse-
reflectance measurements at room temperature. The band gap
extrapolated from the linear region of the Kubelka−Munk plot
is 3.1 eV (Figure S1a). The onset of optical absorption is fairly
sharp, suggesting the direct nature of the band gap in RInBr4.
The estimated band gaps from direct and indirect fittings of
the Tauc plots are 3.2 and 3.3 eV, respectively (Figure S1b,c).
Room temperature PL emission (PL) and excitation (PLE)
spectra for RInBr4 and the precursor organic salt RBr are
presented in Figure 2a. Both excitation and emission spectra
for RInBr4 and RBr have similar wavelength ranges and peak
shapes, suggesting that the organic cations may be contributing
to the observed blue emission in RInBr4. However, there are
subtle differences between the two materials as they apply to
their optical properties. The maximum excitation wavelength
(PLEmax) of 391 nm for RInBr4 matches the onset of optical
absorption measured via diffuse reflectance (Figure S1). In
comparison, PLEmax of 398 nm was measured for the precursor
RBr. The PL spectrum for the precursor salt RBr exhibits a
broad emission peak with PLmax of 440 nm and a full width at
half-maximum (fwhm) value of ∼71.3 nm. Notably, the PL
spectrum of RBr contains two characteristic peaks at 432 and
457 nm and a broad shoulder located at 488 nm. In contrast,
RInBr4 has a broader emission peak with PLmax of 437 nm and
InBr4 tetrahedra, may explain the observed PL properties.
Importantly, our results seem to confirm the validity of our
original material design concept, which was based on the
utilization of emissive organic molecules for the preparation of
a rare example of a highly luminescent hybrid metal halide in
which the emission originates from the organic component.
In order to further ascertain the mechanism of emission for
RInBr4, measurements of PL on single crystals and powdered
samples, PL measurements at low temperature, and excitation-
dependent PL were carried out (Figures 3 and S3 and S4). PL
spectra measured on single crystals and powdered samples are
nearly identical with a minor drop in intensity, ruling out the
possible contribution from surface defects to the PL emission
of RInBr4 (Figure S5). We notice that changing the excitation
wavelength does not cause a shift in the PL peak position
(Figures 3 and S4). In fact, notwithstanding the emergence of
a higher-energy shoulder peak when a higher excitation energy
is used, the PL spectra are largely independent of the excitation
wavelength. The peak that emerges at a higher excitation
energy is present in the literature concerning various stilbene-
based compounds and is attributed to a π−π* transition.49
Note that when the optimal excitation energy is used (i.e., at
PLEmax), this shoulder is cut off by the allowable emission
monochromator scan range. More specifically, our excitation
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Figure 4 shows the time-resolved photoluminescence
(TRPL) measurements for RInBr4. The decay profiles were
fitted using the equation
n
t
A e−(t−t )/τ dt′
i
′
I(t) =
IRF(t′)
∑
∫
i
−∞
i=1
where IRF is the instrument response function, Ai is the
coefficient of exponential decay, and τi is the PL lifetime.
According to the refinement results, τ1 is the dominant lifetime
at 178 ns. The radiative and nonradiative rates were calculated
to be 9.19 × 105 and 4.70 × 106 s−1, respectively. They were
calculated using the equation
1
τ
1
τ0
=
+ Anr
where τ is the lifetime, τ0 is the radiative lifetime, and Anr is the
nonradiative rate. Full fitting parameters for TRPL can be
found in Table S4. Previous studies report that organic
molecules typically have faster emission lifetimes.52,53 There-
fore, the observed longer emission lifetime is unusual and,
possibly, indicative of greater lattice coupling of the photo-
emissive state in RInBr4.
Figure 3. Excitation-wavelength-dependent PL spectra of RInBr4. The
optimal excitation wavelength is determined to be PLEmax = 390 nm.
wavelength dependence of the PL spectra shows a peak
appearing at 395 nm when our excitation source is below 300
nm. This observation is in agreement with the PL behavior of
previously studied stilbene-based compounds.50 Therefore, our
excitation-dependent PL results suggest that there is only one
emission mechanism - emission from the organic component.
This is further supported by our low-temperature PL results
taken at 4 K (Figure S3). The comparison of room
temperature and low-temperature PL spectra suggests that
the peak shape is largely preserved upon cooling; i.e., there are
no new low-temperature PL peaks attributable to a possible
inorganic emission center. The lack of significant thermal
broadening supports the assignment of the emission to the
organic molecules in RInBr4. This is because organic molecules
have stronger bonds and are less deformable by an exciton,
and, therefore, phonon participation in the exciton emission is
not as strong as that in the inorganic anion.51 Note that the
reported fluorescence spectra for various parent trans-stilbene
compounds are nearly identical in shape with the one reported
in this paper.49,50
Important for practical applications, the incorporation of an
emissive organic molecule into a hybrid metal halide can yield
substantial improvements in both the optical properties and
material’s stability. Thus, although PL and PLE are largely
similar for RBr and RInBr4, the light emission efficiency is
doubled in the hybrid RInBr4 with a PLQY value of 16.36%
compared to a PLQY value of 8.15% for RBr. The high room
temperature PLQY value of 16.36% demonstrated by RInBr4 is
comparable to that shown by several other recently reported
hybrid organic−inorganic blue emitters (Table 1). Figure 2b
shows periodic PLQY measurements under a continuous
exposure to light over the course of 1 h for both RBr and
RInBr4. The PLQY of RBr drops 40% within 1 h, whereas the
PL emission intensity of RInBr4 remains almost unchanged
with the PLQY dropping only 4%. The photoinstability of
stilbene (and stilbene-derived organics) has been attributed to
its photochemical changes (e.g., photoisomerization and
photodimerization) that quench PL; for example, Saltiel et
al. reported that the photoisomerization of stilbene resulted in
quenching of the quantum yield.64 In order for the photo-
chemical reactions to occur, the stilbene molecules must be at
an optimal distance from one another (between 3.5 and 4.2 Å),
whereas any distance above 4.7 Å will not result in harmful
changes.65 In the hybrid compound RInBr4, the longer
intermolecular distances between stilbene molecules (>4.7
Å), therefore, explain the improved stability. Therefore, these
results suggest a substantial improvement of the photostability
of the hybrid material compared to the precursor organic salt
RBr. The observed optical property improvements in RInBr4
compared to RBr are also in line with the literature reports of a
synergistic combination of the photoluminescent properties of
organic and inorganic units and improved stability of the
resultant low-dimensional hybrid organic−inorganic metal
halides.40 Briefly, low-dimensional hybrid metal halides exhibit
increased charge localization compared to the organic and
inorganic precursors (irrespective of the emission origin),
which leads to enhanced excitonic properties and improved
light emission properties. In the case of RInBr4, the greater
separation of organic R+ units not only improves its light
emission efficiency but also can lead to reduced harmful PL
Figure 4. Room temperature TRPL data for RInBr4 obtained using
3.5 eV excitation energy.
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Table 1. Comparison of the Photophysical Properties of Blue-Emitting Hybrid Organic−Inorganic Halides3,18,40,54−59
compound
PLQY (%)
PLmax (nm)
PLEmax (nm)
Stokes shift (nm)
fwhm (nm)
95.3
ref
RInBr4
16.36
19.18
0.32
3.07
32
21.3
58.3
54.1
3.430
83
437
438
501
491
441
390
475
422
373
470
450
391
320
399
386
288
300
-
376
325
348
375
46
118
102
105
153
100
-
46
48
122
75
this work
(C5H7N2)2ZnBr4
18
40
40
54
55
56
57
58
3
[C6(CH3)5CH2N(CH3)3]2CdBr4·DMSO
(C6(CH3)5CH2N(CH3)3)ZnBr3(DMSO)
[(NH4)2]CuPbBr5
[BAPrEDA]PbCl6
PEA2(Rb0.6Cs0.4)2Pb3Br10
Cl-MA3Bi2Br9
(R-mBZA)2PbBr4
(C9NH20)7(PbCl4)Pb3Cl11
(C6N2H16Cl)2SnCl6
164
188
107
73
20
41
48
84
125
8.1
59
quenching photochemical reactions, which are observed for
stilbene-derived organics, resulting in improved photostability.
In order to test for the thermal stability of the compound,
simultaneous thermogravimetric analysis (TGA)/differential
scanning calorimetry (DSC) studies were performed on
RInBr4 and RBr (Figures S6 and S7). For RInBr4, the onset
temperature at 177.65 °C represents the melting transition of
the compound, also confirmed by measurements on our Mel-
Temp melting point apparatus, which yields a value of 178.5
°C. The hybrid RInBr4 demonstrates improved thermal
stability with no significant loss in weight up to 250 °C
compared to the precursor salt RBr, which shows a significant
weight drop above 200 °C. The onset point of 209.67 °C in
Figure S7 likely represents the trimethylamine group being
thermally removed from the material. This is corroborated by
the weight loss of 18%, an exact match to the mass percent of
trimethylamine. Therefore, the measured thermal properties of
RInBr4 suggest a significant advantage of this material, and
potentially its other hybrid indium(III) bromide derivatives,
RInBr4 may be amenable for melt processing, which has a set
of distinct advantages over solution processing.60,61 Further-
more, as another test for the thermal and air stability, we
carried out experiments aimed at studying RInBr4 under
constant heating in ambient air. For this, single crystals and
powdered samples of RInBr4 were placed on a hot plate at 100
°C for 1 week. The powder X-ray diffraction (PXRD)
measurements taken before and after heating indicate that
although the sample crystallinity decreased upon continuous
heating in air, as judged by lowered diffraction peak intensities,
RInBr4 remained crystalline with no signs of impurity
evolution (Figures S8 and S9). In addition to the structural
stability, only a modest drop in the measured PLQY of RInBr4
from 16.36% to 10.76% is observed after heating at 100 °C for
1 week. Longer-term air stability studies with no heating
suggest that RInBr4 is stable up to 2 weeks, after which an
impurity peak appears (Figure S10). For the samples kept in
ambient air for 1 week, a PLQY decrease of only 0.13% was
recorded. A clear degradation of the sample purity and
crystallinity can be observed at 2 months (Figure S10). These
air stability and photostability metrics for RInBr4 compare
favorably to those of many of the hybrid lead halide phosphors
reported in the literature.62,63
Figure 5. (a) Electronic band structure and (b) DOS of RInBr4
calculated using the PBE functional as well as (c) the DOS calculated
using the more advanced hybrid PBE0 functional. Note that the PBE
band gap in parts a and b is underestimated, and the band gap is
corrected by the PBE0 calculation in part c.
Parts a and b of Figure 5 show the electronic band structure
and density of states (DOS) of RInBr4 calculated using the
Perdew−Burke−Ernzerhof (PBE) functional. Because the PBE
calculation typically underestimates the band gap, the DOS
was further corrected by using the hybrid PBE0 functional
(Figure 5c), which was shown to improve the band-gap
description significantly.66−68 The band structure (Figure 5a)
shows extremely flat bands, indicating very weak intermolec-
ular coupling, as expected for a 0D hybrid metal halide
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featuring large, bulky organic cations. The PBE0-corrected
band gap is about 4.1 eV. The top valence band is derived from
π orbitals of the organic cation R+, while the filled states of
InBr4 are much lower in energy between −4 and −1 eV, as
shown in the DOS (Figure 5c). The conduction band is made
characterized. RInBr4 shows a visibly bright blue emission at
room temperature with a measured PLQY of 16.36%, a 2-fold
improvement over the precursor organic salt RBr. On the basis
of our combined computational and experimental work, the
blue emission from RInBr4 is attributed to the organic
component, which is the main contributor to both the valence
and conduction band-edge states. In addition to the 2-fold
improvement in the PLQY, the incorporation of an organic
emitter into a hybrid metal halide material results in a
substantial improvement of the thermal stability and photo-
stability. Of special note is that RInBr4 melts at 177.65 °C and
shows no signs of mass loss up to 250 °C. Together with the
promising light emission properties, the improved air and
thermal stability of hybrid indium(III) halides make this class
of compounds promising for future fundamental research and
practical applications. More broadly, our work also highlights
the potential benefits of using luminescent organics based on
stilbene, which can be incorporated into hybrid metal halide
structures, and affords large single crystals through low-
temperature solution methods.
up of both R+ and InBr4 states. Because the energy gap
−
between the filled and empty InBr4− states is much larger than
that of R+, the emission and lowest-energy excitation should be
due to the molecular cation R+, consistent with our
experimental result. Our computational work reveals that the
observed blue emission is due to the transition between π
orbitals on stilbene (Figure 6). In contrast, the reported band
METHODS
■
Synthesis of RBr. A round-bottom flask was charged with (E)-4-
methylstilbene (3.9 g), N-bromosuccinimide (3.7 g), 2,2′-azobis(2-
methylpropionitrile) (0.060 g), and 50 mL of benzene. A condenser
was attached, and the solution was heated to gently reflux solvent
overnight. The solution was cooled, transferred to a separatory funnel,
and washed with a solution of NaOH (3.0 g) in 50 mL of distilled
water. The benzene layer was dried over sodium sulfate and filtered,
and the solvent was removed under reduced pressure. Most of the
solid residue (5.1 g) was dissolved in 90 mL of acetonitrile in a round-
bottom flask fitted with an inlet adaptor and a magnetic stirrer. The
flask and contents were cooled in an ice bath, and with stirring,
trimethylamine gas, generated from trimethylamine hydrochloride
(4.0 g) and NaOH (1.8 g) in a small amount of water, was allowed to
condense into the flask. After 24 h, the procedure was repeated,
followed by another 24 h of stirring. Diethyl ether (70 mL) was added
with stirring, and the solid white precipitate was collected using
suction filtration, washed twice with diethyl ether, dried by pulling air
through it, and stored in a desiccator to give 4.80 g (72% overall
yield) of the desired trimethylammonium bromide salt RBr. Mp:
227−228 °C (lit.69 226 °C). Scheme 1 shows the synthesis of the
organic salt RBr. 1H NMR (300 MHz, DCCl3): δ 7.65 (d, 2H, J = 8.3
Hz), 7.59 (d, 2H, J = 8.0 Hz), 7.55−7.51 (m, 2H), 7.42−7.29 (m,
3H), 7.19 (d, 1H, J = 16.5 Hz), 7.10 (d, 1H, J = 16.3 Hz), 5.05 (s,
2H), 3.43 (s, 9H).
Figure 6. (a) Conduction band minimum and (b) valence band
maximum at the Γ point for RInBr4.
Synthesis of RInBr4. The reactants were dissolved in methanol in
separate vials. Indium(III) bromide (>99%, Aldrich) dissolved
completely in methanol, forming a dark-yellow solution. The organic
precursor salt RBr had to be sonicated briefly (<5 min) for complete
dissolution in methanol. The resulting solution was light yellow. Both
solutions were filtered, and the indium(III) bromide solution was
added dropwise to the organic solution. The reaction mixture was
sonicated for 30 min and left to evaporate at room temperature. Light-
yellow block-shaped crystals were formed overnight (Figure S11).
Single-Crystal X-ray Diffraction (SXRD) Measurements. The
SXRD data were collected at 100(2) K on a Bruker D8 Quest
structures for hybrid indium bromides such as that for
(C4H14N2)2In2Br10 show valence and conduction bands
dominated by the inorganic components, leading to more
dispersive bands.
32
CONCLUSIONS
■
In conclusion, a novel hybrid organic−inorganic 0D metal
halide, RInBr4, featuring a photoemissive organic R+ cation and
−
a nonemissive tetrahedral InBr4 anion was prepared and
Scheme 1. Synthesis of the Organic Salt RBr
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diffractometer with a Kappa geometry goniometer, an Incoatec Imus
X-ray source [graphite-monochromated Mo Kα (λ = 0.71073 Å)
radiation], and a Photon II detector. The data were corrected for
absorption using a semiempirical method based on equivalent
reflections, and the structures were solved by intrinsic phasing
methods (SHELXT), as embedded in the APEX3, version 2015.5-2,
program. All atoms were refined with anisotropic displacement
parameters, and the site-occupancy factors were checked by freeing
the occupancies of each unique crystallographic site. Details of the
data collection, crystallographic parameters, and atomic coordinates
are summarized in Tables S1−S3. Additional information on the
crystal structure investigations can be obtained in the form of a CIF
file, which was deposited in the Cambridge Crystallographic Data
Centre as CCDC 1991390.
PXRD Measurements. In order to establish the bulk purity,
PXRD measurements were performed on a Rigaku Miniflex600
benchtop diffractometer with a D/tex detector and a Ni-filtered Cu
Kα radiation source. The scans were done with room temperature
measurements in the 3−90° (2θ) range using a step size of 0.02°. The
PXRD data were fitted using the Pawley method (Figure S12). Room
temperature PXRD measurements were also taken on a single crystal
using the above-described conditions. In order to test for the air and
moisture stability of the compounds, periodic PXRD measurements
were taken on samples stored in open air (thermostat set to 20 °C and
30% relative humidity).
Thermal Property Measurements. Simultaneous TGA and
DSC measurements were performed on polycrystalline powdered
samples of RInBr4 and RBr on a TA Instruments SDT 650 thermal
analyzer system. The samples were heated from 25 to 250 °C under
an inert flow of dry nitrogen gas at a rate of 100 mL/min, with a
heating rate of 5 °C/min.
Melting point measurements for RInBr4 and RBr were run on a
Mel-Temp apparatus (110/120VAC; 50/60 Hz and 200 W). The
heating element was set to 50 V, and the measurements took 15 and
20 min for RInBr4 and RBr, respectively. The samples were loaded
into the capillary tubes (0.8−1.1 × 90 mm).
was used to describe the interaction between ions and electrons.71
The lattice parameters were fixed at the experimentally measured
values, while the atomic positions were optimized until the force on
each atom was less than 0.02 eV/Å. The electronic band structure and
DOS of RInBr4 were calculated using the PBE exchange-correlation
functional.72 The band gap was further corrected, and the DOS was
recalculated by using the hybrid PBE0 functional, which has 25%
nonlocal Fock exchange. The inclusion of a fraction of Fock exchange
significantly improves the calculation of the band-gap energy.66−68
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03164.
SXRD data and refinement parameters, PXRD, diffuse
reflectance, low-temperature- and excitation-wavelength-
dependent PL, TGA/DSC, and air and moisture stability
test results (PDF)
Accession Codes
CCDC 1991390 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Bayrammurad Saparov − Department of Chemistry and
Biochemistry, University of Oklahoma, Norman, Oklahoma
73019, United States; orcid.org/0000-0003-0190-9585;
Email: saparov@ou.edu
Optical Property Measurements. PL emission (PL) and
excitation (PLE) measurements were performed at room temperature
using a Jobin Yvon Fluorolog-3 spectrofluorometer (Horiba Co.)
equipped with a xenon lamp and a Quanta-φ integrating sphere. PL
and PLE experiments were conducted on a single crystal and
polycrystalline powdered sample of RInBr4 and a powdered sample of
the precursor organic salt RBr. For lifetime measurements, a time-
correlated single photon counting (TCSPC) system, including a
DeltaHub DH-HT high-throughput TCSPC controller and a
NanoLED NL-C2 pulsed diode controller, was used. For a light
source, a 350 nm NanoLED diode was selected, which has a <1.2 ns
pulse duration. Low-temperature PL was collected under excitation by
the 325 nm line of a HeCd laser (Kimmon Electric HeCd dual-
wavelength laser, model IK552R-F). The sample was placed on the
coldfinger of a helium closed-cycle cryostat, and the measurements
were performed at 4 K.
For the photostability measurements, a single crystal of RInBr4 and
a powdered sample of RBr were placed inside the Quanta-φ
integrating sphere on the Jobin Yvon Fluorolog-3 spectrofluorometer.
The samples were then exposed to the full power of the xenon lamp of
the spectrofluorometer at PL excitation maxima of 391 and 398 nm
for RInBr4 and RBr, respectively. Periodic PLQY measurements were
taken every 5 min under these conditions over 60 min.
Authors
Hadiah Fattal − Department of Chemistry and Biochemistry,
University of Oklahoma, Norman, Oklahoma 73019, United
States
Tielyr D. Creason − Department of Chemistry and
Biochemistry, University of Oklahoma, Norman, Oklahoma
73019, United States
Cordell J. Delzer − Department of Nuclear Engineering,
University of Tennessee, Knoxville, Tennessee 37996, United
States
Aymen Yangui − Chemical Physics and NanoLund, Lund
University, Lund 22100, Sweden
Jason P. Hayward − Department of Nuclear Engineering,
University of Tennessee, Knoxville, Tennessee 37996, United
States
Bradley J. Ross − Department of Chemistry and Biochemistry,
University of Oklahoma, Norman, Oklahoma 73019, United
States
Mao-Hua Du − Materials Science and Technology Division,
Oak Ridge National Laboratory, Oak Ridge, Tennessee
37831, United States; orcid.org/0000-0001-8796-167X
Daniel T. Glatzhofer − Department of Chemistry and
Biochemistry, University of Oklahoma, Norman, Oklahoma
73019, United States
Room temperature diffuse-reflectance spectra of a polycrystalline
powder of RInBr4 were measured using a high-resolution PerkinElmer
LAMBDA 750 UV−vis−NIR spectrometer equipped with a 100 mm
InGaAs integrating-sphere attachment. The diffuse-reflectance data
were converted to pseudoabsorption spectra according to the
Kubelka−Munk equation FI = α/S = (1 − R)2/2R, where R is the
reflectance, α is the absorption coefficient, and S is the scattering
coefficient.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03164
Computational Work. Our calculations are based on DFT, as
implemented in the VASP code.70 The kinetic energy cutoff of the
plane-wave basis is 400 eV. The projector-augmented-wave method
Author Contributions
†These authors contributed equally.
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Article
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the University of Oklahoma (OU)
startup funds. C.J.D., J.P.H., and M.-H.D. were supported by
the Department of Energy National Nuclear Security
Administration through the Nuclear Science and Security
Consortium under Award DE-NA0003180. We thank Dr.
Vincent R. Whiteside and Dr. Ian R. Sellers for technical
assistance with low-temperature PL measurements. We
acknowledge Dr. Douglas R. Powell for assistance with the
SXRD measurements (instrument support from NSF Grant
CHE-1726630).
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