Black hybrid iodobismuthate containing linear anionic chains

Article abstract of DOI:10.1039/c7nj04948j

Three hybrid 1,1′-(1,n-alkanediyl)bis(4-methylpyridinium) iodobismuthates 1-3 were prepared by a facile solution route and showed thermal stability in air up to 230 °C. The structures of solids 1 and 3 contain zero-dimensional anions, and the structure of 2 contains one-dimensional linear anionic chains [BiI₅]_n^2n-. Photoluminescence (PL) in the spectral range between 600 and 750 nm was observed for 1 and 2. DFT calculations and optical studies confirmed that compounds 1-3 are semiconductors with band gaps of 1.73-2.10 eV, which correspond with their intense black (for 2) or red (for 1 and 3) colors. The optical absorption of 2 in the red spectral range is primarily due to charge transfer from the I5p orbitals at the top of the valence band to the Bi6p orbitals at the bottom of the conduction band.

Full text of DOI:10.1039/c7nj04948j

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DOI: 10.1039/C7NJ04948J
Black Hybride Iodobismuthate Containing Linear Anionic Chains
Vitalii Yu. Kotov, a,b* Andrey B. Ilyukhin,a Alexander A. Korlyukov,c Alexander F.
Smol’yakovc,d and Sergey A. Kozyukhina,e
aN.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
119991, Moscow, Russian Federation
eNational Research University Higher School of Economics, 101000, Moscow, Russian
Federation
cA.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
119991, Moscow, Russian Federation
dFaculty of Science, RUDN University, 117198, Moscow, Russian Federation
eDepartment of Chemistry, Tomsk State University, 634050, Tomsk, Russian Federation
ABSTRACT: Three hybrid 1,1'ꢀ(1,nꢀalkanediyl)bis(4ꢀmethylpyridinium) iodobismuthates 1 ꢀ 3
were prepared by a facile solution route and showed thermal stability in air up to 230°C. The
structures of solid 1 and 3 contain zeroꢀdimensional anions, and the structure of 2 contains oneꢀ
dimensional linear anionic chains [BiI5]n2nꢀ. Photoluminescence (PL) in the spectral range
between 600 and 750 nm was observed for 1 and 2. DFT calculations and optical studies
confirmed that compounds 1−3 are semiconductors with band gaps of 1.73−2.10 eV, which
corresponds with their intense black (for 2) or red (for 1 and 3) colors. The optical absorption of
2 in the red spectral range is primarily due to charge transfer from the I5p orbitals at the top of
the valence band to the Bi6p orbitals at the bottom of the conduction band.
Introduction
Hybrid haloplumbates with perovskite structure 1, 2 stand among the most promising photoactive
materials. Solar cells based on hybrid haloplumbates show high efficiency of solar energy
conversion (up to 20%) due to the unique set of fundamental properties of these materials,
namely, high optical absorption in the visible, outstanding charge transport properties
comparable to those of "classic" inorganic semiconductors, and high photosensitivity. Low
production costs of hybrid haloplumbate solar cells are also very important. Weber 3 was the first
who synthesized methylammonium haloplumbates with perovskite structure in 1978. Subsequent
research was aimed at studying the influence of the nature of halogen on the properties of these
4
perovskites and at studying the physicochemical properties of hybrid haloplumbates, in

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5ꢀ7
particular their optical, electric, and dielectric properties.
There was much debate about the
optoelectronic mechanisms that enable these lowꢀtemperature materials to deliver such high
efficiencies. The debate concerned the roles that crystal sizes, stoichiometry, defects, and
8,9
ferroelectric domains could play.
Hybrid haloplumbates were also shown to have some
inherent disadvantages. For example, they contain toxic lead; they are not stable in air, especially
in a humid environment; and they cannot be applied to coat large surface areas. Thus, researchers
10
were motivated to search for alternative, leadꢀfree materials. Lately, hybrid iodobismuthates
have been considered to be potential substitutes for methylammonium and formamidinium
11ꢀ16
iodoplumbates as lightꢀharvesting materials for solidꢀstate solar cells.
The efficiency of
bismuthꢀcontaining cells does not exceed 1ꢀ1.5%. Meanwhile, bismuthꢀcontaining hybrid
iodobismuthates have several advantages: first of all, they are less toxic than leadꢀcontaining
compounds; in addition, they are more resistant to moisture and can be prepared from aqueous
solutions. Recently, we turned to study the properties of 1,1'ꢀ(1,nꢀalkanediyl)bis(4ꢀ
17
.
2+
cyanopyridinium) halobismuthates. We found
that PyC3BiBr5 H2O (PyCn = 1,1'ꢀ(1,nꢀ
alkanediyl)bis(4ꢀcyanopyridinium cation) prepared from an aqueous solution has 1D BiBr5
anions in its structure, and its optical band gap is Eg = 2.24 eV. Substituting iodine for bromine
17
in this compound, we obtained a product with Eg = 1.70 eV. An even lower value of Eg (1.59
eV) was found for 1,1'ꢀ(1,5ꢀpentanediyl)bis(4ꢀcyanopyridinium) iodobismuthate. 18 However, we
have not succeeded in crystallizing the prepared 1,1'ꢀ(1,nꢀalkanediyl)bis(4ꢀcyanopyridinium)
iodobismuthates. The main reason for our failure was likely the hydrolysis of nitrile groups of
the cation, which prevented the recrystallization of these compounds from acidic or hot aqueous
solutions. Therefore, we turned to study 1,1'ꢀ(1,nꢀalkanediyl)bis(4ꢀmethylpyridinium)
iodobismuthates, apparently not expecting to meet any obstacles in crystallizing reaction
products from acidic solutions19. Of additional interest was the question, whether the decrease of
electron affinity of 1,1'ꢀ(1,5ꢀpentanediyl)bis(4ꢀmethylpyridinium) as compared to 1,1'ꢀ(1,5ꢀ
pentanediyl)bis(4ꢀcyanopyridinium) (Fig.S1 in ESIǂ) would affect a value of Eg for
corresponding iodobismuthates.
Results and discussion
The structure of the cations used in this study is presented in Scheme 1. 1,1'ꢀ(1,nꢀ
Alkanediyl)bis(4ꢀmethylpyridinium) bromides (PiСnBr2), n = 2–6, were synthesized from 4ꢀ
methylpyridine and α,ωꢀdibromoalkanes in acetonitrile (ESIǂ).

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+
+
H3C
N
(CH2)n
N
CH3
Scheme 1. 1,1'ꢀ(1,nꢀAlkanediyl)bis(4ꢀmethylpyridinium) cation.
Synthesis and thermal stability
Reactions of 1,1'ꢀ(1,nꢀalkanediyl)bis(4ꢀmethylpyridinium) (n = 2ꢀ6) cations with iodobismuthate
anions in concentrated HI occur with the formation of red solutions. However, the only solids
isolated from these solutions were red crystals of 1 (n = 2) and black crystals of 2 (n = 5) (ESIǂ):
2+
2ꢀ
2PiC2 + 2BiI5
=
(PiC2)2Bi2I10↓
PiC5BiI5↓
(Compound 1)
(Compound 2)
2+
2ꢀ
PiC5 + BiI5
=
From a solution containing dilute HI, black crystals of 2 as the major product and red crystals of
3 as a byproduct (both for n = 5) were isolated (ESIǂ):
2ꢀ
2PiC52+ + 4BiI5
=
(PiC5)2Bi4I16↓ + 4Iꢀ
(Compound 3)
In aqueous KI solution, reactions between 1,1'ꢀ(1,nꢀalkanediyl)bis(4ꢀmethylpyridinium) cations
and iodobismuthate anions (n = 2ꢀ6) yielded nonꢀcrystalline solids. When kept in contact with
the solution for several hours, compound 3 (n = 5) transformed to black product 2a (ESIǂ).
According to DSCꢀTG results, the heating of a sample of compound 1 and a sample of
compound 2 results in melting at t=265±1oC for 1 and t=232±1oC for 2; ꢁH= 5.04 cal/g (2). Both
compounds decompose at temperatures above 260±5oC (Fig.S2 in ESIǂ).
Xꢀray diffraction study
4ꢀ
The structure of 1 is formed by 1,1'ꢀethaneꢀ1,2ꢀdiylbis(4ꢀmethylpyridinium) cations and Bi2I10
centrosymmetric anions (Fig.1). The Bi atom is in an octahedral environment. The lengths of Biꢀ
I bonds are unequal: the longest bonds are found for the I(3) bridging atoms, while the shortest
bonds are formed by the I atoms in the trans position to the bridge. The aliphatic moiety has a
gauche conformation with the torsion angle N(1)C(6)C(7)N(2) of 58.5o (here and below
parameters referring to 100 K). The short contacts in cation chains [N(1)…N(1') 3.435, 3.501 Å;

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N(1)…C(1') 3.499, 3.547 Å; and C(10)…C(10') 3.393, 3.488 Å] can be interpreted as stacking
interactions. The short contacts between anions [I(1)…I(2') 3.831 Å and I(4)…I(5') 3.714 Å]
form a pseudoꢀ3D structure of the anionic sublattice (Fig.S3 in ESIǂ). Each Bi2I104ꢀ has eight such
contacts, and the thusꢀformed sublattice can be reasonably described as derived from the bodyꢀ
centered cubic lattice.
Figure 1. Fragment of the structure of compound 1. Thermal ellipsoids are shown at 50%
probability. Selected bond lengths and bond angles are given in Table S1 in ESIǂ
2nꢀ
In the structure of 2 (BiI5)n chains are parallel to the b axis (Fig.2b,с and Fig.S4b in
ESIǂ). Since bismuth atoms and bridging iodine atoms are located on the 2ꢀfold axis, the average
length of bridging bonds is b/4=3.362 Å. Atoms Bi(1) and I(3,4) are disordered over two
positions in the ratio 1:1. If we assume that in a crystal there are coexisting chains formed by Bi
atoms of the same type, then the distances Bi(1)…Bi(1') are 6.093 and 7.357 Å, and distances
Bi(1B)…Bi(1B') are 6.192 and 7.258 Å, accordingly (here B denotes one of the positions). Such
large values suggest that one of the BiꢀI distances in the chain is significantly greater than 4 Å.
We have not found any such example in the literature. If positions Bi(1) and Bi (1B) alternate in
the chain, the BiꢀBi distances are 6.676 and 6.774 Å. These values are closer to the analogous
20
values of 6.433 and 6.477 Å in the structures (MV)BiBr3.2I1.8 (MVꢀmethylviologen)
and
(MV)BiI5 21. Taking into account the disorder in the structure, there are four sets of Bi...I
distances for each Bi: Bi(1)…I(3) 2.90 and 3.19, Bi(1)…I(4) 3.51 and 3.84; Bi(1B)…I(3) 3.49
and 3.77, Bi(1B)…I(4) 2.93 and 3.26 Å. Since the nonuniformity of bond lengths is caused by
the different degree of stereochemical activity of the unshared electron pair of Bi, the BiꢀI bond
lengths in the chain should alternate. In our case one can propose two possible chain structures
with the following sets of bond lengths: 2.93, 3.77, 2.90, and 3.84 Å (Fig.2b) and 3.19, 3.51,

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3.26, and 3.49 Å (Fig.2c). There are three compounds with BiX5 chains (X=Cl, Br, I), containing
bridging fragments BiꢀIꢀBi with I atoms in transꢀpositions in the CCDC (version 5.38, May
2017) 22 ꢀ (MV)BiBr3.2I1.8 20, (MV)BiI5 21 and (MV)BiCl2I3 23. The lengths of bridging BiꢀI bonds
in these structures are 2.87 and 3.60 Å in (MV)BiBr3.2I1.8; 2.87 and 3.66 Å in (MV)BiI5 and 2.97,
3.43, 2.93, and 3.32 Å in (MV)BiCl2I3, respectively. The short bonds in these compounds are
similar to the corresponding short bonds in our compound (Fig.2b). There are compounds with
identical BiꢀX bond lengths20, but the iodobismuthates with short bonds about 3.2 Å and
analogues halobismuthates are unknown. Taking into account these data, we consider the chain
structure shown in Fig.2b and in Fig.S4b in ESIǂ as the most probable one.
The terminal I(1,2) atoms in the structure 2, which take part in nonꢀcovalent interactions
with other structural units, occupy ordered positions. Since the population of the Biꢀpositions is
the same and no superstructure reflections were found, it can be asserted that there is a statistical
distribution of chain orientations in the crystal The chain direction was defined as the direction
.
of the shortest BiꢀI vector. All attempts to eliminate the disorder in the structure 2 by lowering
the symmetry and allowing for twinning lead to the appearance of a large number of correlations
between the refined parameters associated with elements of the pseudoꢀsymmetry of atoms and
the unrealistic geometric parameters of 1,1'ꢀpentaneꢀ1,5ꢀdiylbis(4ꢀmethylpyridinium) cation.
The aliphatic moiety N(1)...N(2) has a gꢀtꢀtꢀg conformation with torsion angles of ꢀ60.9, ꢀ
176.9, ꢀ176.9, and ꢀ60.9o. Corrugated cationic layers (Fig.S4 in ESIǂ) that frame the chain anions
can conventionally be recognized in the structure.
a

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b
c
Figure 2. Molecular structure of 1,1'ꢀ(pentaneꢀ1,5ꢀdiyl)bis(4ꢀmethylpyridinium) cation in 2 (a).
1D network structure of [BiI5]n in 2 (b,c). Thermal ellipsoids are shown at 50% probability.
Selected bond lengths and bond angles are given in Table S1 in ESIǂ.
A Bi4I164ꢀ tetranuclear anion in 3 has usual "defective dicubane" geometry (Fig. 3). The values of
BiꢀI bond lengths can be divided into three nonꢀoverlapping intervals: µ3ꢀI(3) (3.33ꢀ3.36 Å), µ2ꢀ
I(4,5) (3.04ꢀ3.27 Å), and I(6ꢀ10) (2.88ꢀ2.93 Å). The tꢀtꢀgꢀt conformation of the nꢀpentane moiety
in 1,1'ꢀpentaneꢀ1,4ꢀdiylbis(4ꢀmethylpyridinium) cation, with torsion angles of ꢀ173.7o, ꢀ167.5o,
77.4o, and ꢀ178.8o, differs from the gꢀt‒tꢀg conformation found in the structure of 2. The Bi4I16

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anion in the structure of 3 is surrounded by an array of eight cations (Fig.S5 in ESIǂ). The
shortest C...C(N) contacts, equal to 3.59 Å, involve methyl carbon atoms.
Figure 3. Fragment of the structure of compound 3. Thermal ellipsoids are shown at 50%
probability. Selected bond lengths and bond angles are given in Table S1 in ESIǂ.

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Table 1. Crystal data and details of structure refinement for 1ꢀ3.
Sample notation
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
a, Å
1_100K
1_150K
1_298K
2
3
C28H36Bi2I10N4
2115.57
C28H36Bi2I10N4
2115.57
C28H36Bi2I10N4
2115.57
C17H24BiI5N2
1099.86
C34H48Bi4I16N4
3379.08
100(2)
150(2)
298(2)
150(2)
150(2)
0.71073
0.71073
0.71073
0.71073
0.71073
Monoclinic
P21/n
Monoclinic
P21/n
Monoclinic
P21/n
Orthorhombic
I222
Monoclinic
P21/n
12.5628(6)
12.2070(6)
15.2026(7)
93.0000(10)
2328.19(19)
2
12.5792(4)
12.2266(4)
15.2453(5)
92.9530(10)
2341.63(13)
2
12.6544(16)
12.2914(15)
15.3829(19)
92.656(4)
2390.1(5)
2
12.2018(4)
13.4499(4)
16.2572(5)
90
16.8086(9)
12.0003(6)
17.7036(9)
107.294(2)
3409.5(3)
2
b, Å
c, Å
β, °
Volume, Å3
2668.02(14)
4
Z
D (calc), Mg/m3
µ, mmꢀ1
3.018
3.000
2.940
2.738
3.291
14.202
14.120
13.834
12.399
17.561
F(000)
1856
1856
1856
1952
2920
Crystal size, mm
θ range, °
0.24 x 0.18 x 0.01
2.328, 34.378
ꢀ19<=h<=19
ꢀ19<=k<=19
ꢀ24<=l<=23
64771
0.1 x 0.08 x 0.04
2.324, 31.518
ꢀ18<=h<=18
ꢀ17<=k<=17
ꢀ22<=l<=22
46334
0.24 x 0.18 x 0.01
2.038, 30.524
ꢀ18<=h<=18
ꢀ17<=k<=17
ꢀ21<=l<=21
45437
0.3 x 0.12 x 0.1
2.254, 30.980
ꢀ16<=h<=16
ꢀ18<=k<=18
ꢀ22<=l<=22
18203
0.2 x 0.16 x 0.08
2.081, 30.554
ꢀ24<=h<=23
ꢀ17<=k<=17
ꢀ25<=l<=25
47874
Index ranges
Reflections collected
Independent reflections, Rint
9555, 0.0493
7699, 0.0492
7283, 0.0486
3743, 0.0325
10422, 0.0655

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Completeness to θ = 25.242°
99.9 %
99.9 %
100.0 %
99.9 %
100.0 %
Absorption correction
Semiꢀempirical
from equivalents
0.1626, 0.0757
Fullꢀmatrix
Semiꢀempirical
from equivalents
0.4942, 0.3129
Fullꢀmatrix
Semiꢀempirical
from equivalents
0.1561, 0.0744
Fullꢀmatrix
Semiꢀempirical
from equivalents
0.0526, 0.0127
Fullꢀmatrix
Semiꢀempirical
from equivalents
0.7461, 0.4793
Fullꢀmatrix
Max,. min. transmission
Refinement method
leastꢀsquares on F2 leastꢀsquares on F2 leastꢀsquares on F2
leastꢀsquares on F2
3743 / 0 / 127
1.387
leastꢀsquares on F2
10422 / 0 / 264
1.008
Data / restraints / parameters
Goodnessꢀofꢀfit
9555 / 0 / 201
1.014
7699 / 0 / 199
1.016
7283 / 0 / 199
1.077
R1, wR2 [I>2sigma(I)]
R1, wR2 (all data)
0.0237, 0.0478
0.0334, 0.0507
0.0248, 0.0471
0.0375, 0.0509
0.0297, 0.0652
0.0455, 0.0714
0.0425, 0.1137
0.0454, 0.1149
0.287(16)
0.0349, 0.0623
0.0563, 0.0678
Absolute structure parameter
Largest diff. peak and hole, e.Åꢀ3 1.522, ꢀ1.125
1.074, ꢀ1.087
1.809, ꢀ1.347
2.245, ꢀ2.263
2.293, ꢀ1.752

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Optical properties
The diffuse reflectance spectra (DRS) of 1,1'ꢀ(1,5ꢀpentanediyl)bis(4ꢀmethylpyridinium)
iodobismuthates are shown in Fig.4 as KubelkaꢀMunk function versus photon energy. The
KubelkaꢀMunk equation is expressed as follows: 24
,
where Rd is the absolute reflectance of the sample layer. The DRS of 1 is shown in Fig.S14 in
ESIǂ. The optical band gap energies (Eg) were estimated by extrapolating the linear portions of
the corresponding curves to F(Rd) = 0. Their values are 2.08 eV (1), 1.73 eV (2), and 2.10 eV (3)
(Fig.4 and Fig.S6 in ESIǂ).
Figure 4. Reflectance spectra of compound 2, which was isolated from concentrated HI (a);
compound 2a, which was isolated from aqueous KI solution (b); and compound 3 (c). The
corresponding spectrum for compound 1 is found in Fig.S6 in ESIǂ.
The equal values of Eg for 2 and 2a (Fig.4) allowed us to assume that 2 can be obtained from
aqueous solutions. The refinements were carried out using TOPAS program.25 Details of the
refinement for 2 (Fig.S8a in ESIǂ), are as follows: I222, a=12.1971(3), b=13.6078(3),
c=16.4240(4); Rexp = 6.77%, Rwp = 7.99%, Rp = 6.23%, GOF = 1.18. Details of the refinement
for 2a (Fig.S8b in ESIǂ): I222, a=12.2051(2), b=13.6162(4), c=16.4189(3); Rexp = 6.65%, Rwp
=
9.59%, Rp = 7.49%, GOF = 1.44. The results of refinements are similar. Thus, compounds 2 and
2a are identical.

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The Eg values found for 1 and 3 are typical of compounds having 0D iodobismuthate anions. The
lower Eg value of 2 can arise from the exciton band appearing due to the 1D structure of the
anion. We found 1 to be a thermochromic material. Compound 1 had a red color at room
temperature, while at the boiling point of liquid nitrogen, it reversibly turned yellow signifying
an increase in its Eg at low temperatures. A comparison of the structures of 1 at 100 K, 150 K
and 298 K (Table 1 and Fig.S7 in ESIǂ) shows no phase transitions in the range of temperatures
studied. The origin of the thermochromic properties of hybrid iodobismuthates in the absence of
phase transitions was discussed earlier 26.
Photoluminescence properties
The photoluminescence spectra of samples 1 and 2 were recorded at room temperature. We
observed PL emission bands between 600 and 800 nm (Fig.5 and Fig.6).
b
2500
2000
1500
1000
500
a
0
450
500
550
600
650
700
750
wavelength (nm)
Figure 5. Excitation spectrum (curve a, λem= 680 nm) and PL spectrum (curve b, λexc=525 nm)
of 1 at room temperature.

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700
600
500
400
300
200
100
0
a
c
d
b
500
550
600
650
700
750
800
wavelength (nm)
Figure 6. Excitation and emission spectra of 2 at room temperature: curves a and b show
excitation spectra for λem=736 nm and λem=672 nm, respectively; curves c and d show PL spectra
for λexc=525 nm and λexc=600 nm, respectively. The brown curve shows the relative absorbance
spectrum of 2.
Intense PL emission in the spectral range between 600 and 700 nm was observed for 1 (Fig.5).
Compound 1 emits with the emission intensity far higher than for 2. According to Fig.6, the
spectrum of 2 has two lowꢀintensity red luminescence bands with peaks at 736 nm and 672 nm.
The relative intensities of these peaks depend on the excitation energy: under 525ꢀnm excitation,
the peak intensity at 672 nm is higher than at 736 nm; under 600ꢀnm excitation on the contrary,
the peak intensity at 736 nm is higher. We assign the 736ꢀnm emission peak to exciton transition
in the iodobismuthate anion since the excitation band peak (600 nm) has almost the same
position as the exciton band peak in the absorbance spectrum (as indicated by the red arrow in
fig.6) and since Eem = 1.68 eV is roughly equal to Eg = 1.73 eV.
DFT calculations and electronic structure
Analysis of Xꢀray diffraction and optical data for 2 and 3 allowed us to infer that the different
natures and strengths of intermolecular contacts can be responsible for the different band gap
values. To test this inference, we performed DFT calculations using the plane wave basis set.
Atomic coordinates and total energies of optimized structures are presented in supporting
materials. After optimizing of atomic coordinates, we calculated the densities of states (DOS)

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and band structures for 2 and 3. To overcome the problem of disorder of BiI5 the symmetry of
crystal structure of 2 was lowered to P1. The geometry of organic cations in calculated structures
of 2 and 3 are in close agreement with experimental ones. Concerning the latter cations the most
prominent difference between the experimental and calculated structures is related to CꢀH bond
4ꢀ
value, which is ∼0.1 Å longer than the value derived from Xꢀray studies. The structure of Bi4I16
anions in 3 are very close to those obtained in Xꢀray diffraction study. Besides, calculated BiꢀI
bonds are 0.02–0.04 Å longer compared to their experimental values. The situation with
structure 2 is somewhat different as compared to its Xꢀray. According to analysis of the positions
of the bismuth atoms in disordered experimental structure we assumed that there is an alternation
of the BiꢀI bonds lengths and Bi…I interatomic distances between pentaiodobismuthate anions in
2nꢀ
the structure of (BiI5)n (Fig. 2b). In calculated structure of 2 coordination polyhedron of
bismuth atoms corresponds to the distorted square pyramide as it was assumed. However, the
magnitude of above mentioned alternation of BiꢀI bonds and Bi…I interatomic distances is
negligible. Indeed, the mean values of apical BiꢀI bonds are 2.972 and 2.975 Å. The distances
between bismuth and iodine atoms of neighboring BiI5 fragments are equal to 3.752 and 3.766 Å.
We believe that disagreement of the structure of infinite chains in calculated and experimental
structures is related to use of P1 symmetry in our computations. Thus, the infinite chains in
structure 2 were formed by weak Bi…I interactions between apical iodine atoms and
neighboring bismuth atoms.
The comparison of DOS plots (Fig.7 and Fig.8) has shown that the band gap of 3 is
somewhat wider than that in 2. According to our calculations, the Fermi energies are 1.09 and
1.52 eV, while the band gaps are 1.39 and 1.85 eV for 2 and 3, respectively. The difference
between the band gap values is close to the value measured by the DRS method (0.37 eV).
According to the results of electron localization, the bands in the range from ꢀ2.0 to ꢀ0.5 eV are
populated by iodine 5p electrons. At the same time, the contributions of bismuth 6p electrons are
noticeable in bands located in the range between 1.5 and 3.5 eV, so it can be assumed that
electron transition between I and Bi orbitals plays a significant role in the observed optical
spectra. In view of the symmetry of BiI5 moiety, it can be assumed that the py electrons of the
bismuth and axial iodine atom are responsible for the formation of infinite anionic chains in a
crystal. So, it is reasonable to compare the contributions of the py electrons of Bi and I atoms in 2
with the respective values for 3. The difference can be illustrated by the localized DOS plots for
higher lying orbitals (Fig.7 and Fig.8).

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Figure 7. DOS plot calculated for the structure of 2. The contributions of the bismuth and iodine
py electrons are marked by the red line and green line, respectively.
Figure 8. DOS plot calculated for the structure of 3. The contributions of the bismuth and iodine
py electrons are marked by the red line and green line, respectively.

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A more detailed explanation of possible electron transition came from a critical examination of
the band structure of 2. The band structure of 2 calculated for several points that correspond to
bonding between BiI5...BiI5 moieties is presented in Fig.9. It can be seen that the I5py electrons
partially occupy the valence bands while the Bi6py electrons partially occupy the conduction
bands. The energy difference is the lowest in the region near the iodine atom. Unfortunately, it is
impossible to accurately determine the minimum corresponding to the highest occupied bands
since they are almost horizontal to the x axis. However, it will be a reasonable conclusion that
BiI5...BiI5 interactions leading to the formation of anionic chains, favor a decrease in the Fermi
energy and the corresponding energy of electron transition.
Figure 9. Band structure of 2. The coordinates of points are chosen to show bands in the
directions of Bi…I interactions and BiꢀI bonds (A = 0 0.5 0.5, B = 0 1 0.5, C = 0.25 0.75 1).
To evaluate the energy of weak Bi...I interactions between BiI5 moieties in crystal, we carried
out the analysis of electron density in terms of Bader’s “atoms in molecules” theory. 27 The bond
critical points that indicate the formation of anionic chains lie on the line between bismuth and
iodine atoms. The distances from these atoms to the bond critical points are 1.78 and 1.99 Å. The
values of electron density and its Laplacian (∼0.011 and 0.018 a.u., respectively) are rather low
compared to those for axial and equatorial BiꢀI bonds (0.019 and 0.030 a.u.). Thus, both BiꢀI
bonds and Bi...I interactions have ionic nature. We evaluated the energies of BiꢀI bonds in BiI5
fragments and Bi...I weak interactions using the method proposed by Espinosa et al., 28 based on
the values of electron density and its Laplacian in bond critical points. As a result, we obtained a

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mean value of 9.1 kcal/mol for the BiꢀI bond energy, while for Bi...I interaction, this value is 1.4
kcal/mol. In addition, we found few weak I…I interactions between infinite BiI5…BiI5 chains in
2. The I…I distances are ca. 4.90 Å and corresponding energies are ca. 0.2 kcal/mol. Thus, we
can consider that their role in energy transfer is much lower than that of coordination Bi…I
bond.
Conclusions
We have prepared and characterized three new hybrid iodobismuthates. One of them, namely
2ꢀ
1,1'ꢀpentaneꢀ1,5ꢀdiylbis(4ꢀmethylpyridinium) iodobismuthate, has (BiI5 )n linear polymer chains
in its structure generated by Bi...I bonding and Bi...I interactions, which were observed for the
first time. The 1,1'ꢀpentaneꢀ1,5ꢀdiylbis(4ꢀmethylpyridinium) iodobismuthate crystals have black
color and a low value of Eg, which will make them a prospective candidate for use as a lightꢀ
harvesting material in solar cells. The electronic structure calculations have enabled us to
determine the energy band structure of the prepared compounds, and showed that electron
transition between I5py and Bi6py orbitals plays a considerable role in the observed optical
spectra. The cation orbitals are hardly involved in the observed electron transition. Thus, the
electron affinity of the cation do not affect the value of Eg in the compounds under study. The
possibility of linear anionic chain formation is determined by the cation geometry.
Our preliminary results demonstrated that no thinꢀfilm structures with low Eg can be
obtained from solutions of hybrid iodobismuthates in dimethylsulfoxide or dimethylformamide.
However, the water vapor treatment of coatings with high Eg leads to the decrease of Eg values
(Fig. S9 in ESIǂ). Further experimental efforts will be aimed at developing methods for preparing
thinꢀfilm structures on the basis of the prepared hybrid iodobismuthates and at studying their
optoelectronic and photovoltaic properties. In view of the fact that the black iodobismuthate
described in this work is the fourth prepared 1,1'ꢀ(1,nꢀalkanediyl)bis(4ꢀXꢀpyridinium)
halobismuthate with Eg in the range from 1.59 to 1.74 eV, we hope that a further search for lightꢀ
harvesting materials among the halobismuthates of divalent organic cations would provide
compounds to substitute for hybrid haloplumbates in solar cells.
Experimental
The reflectance spectra were measured on a Cary 5000 spectrophotometer in the frequency range
10 000ꢀ50 000 cmꢀ1 at room temperature.

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Photoluminescence measurements were performed on an LS55 (Perkin–Elmer) spectrometer in
the range 200–800 nm at room temperature with 0.5ꢀnm resolution; the slit width was varied
from 10 to 12 nm. A front surface accessory was used for solidꢀstate luminescence
measurements.
The thermal behavior of the samples was studied by thermogravimetric analysis (TG) and
differential thermal analysis (DTA) using an SDT Q600 V8.3 Build 101 Module DSCꢀTGA
thermal analyzer. Test samples were heated from 25 to 300oC at 10oC/min under air in open
alumina pans.
29
Singleꢀcrystal Xꢀray diffraction data were collected on a Bruker SMART APEX2 instrument
30
(Table 1). Absorption correction was applied from semiempirical equivalents using SADABS.
The structures were determined using a combination of the direct method and Fourier syntheses.
The positions of CꢀH hydrogen atoms were calculated from geometrical considerations. The
structures were refined by the fullꢀmatrix anisotropic leastꢀsquares method. All calculations were
carried out using SHELXSꢀ2014, SHELXSꢀ2016, SHELXLꢀ2014, and SHELXLꢀ2016 software.
31 For compound 1, three experiments were carried out with two crystals, with one crystal at 100
and 298 K and the other at 150 K. Xꢀray powder diffraction experiment was carried out on a
Bruker D8 ADVANCE XꢀRay diffractometer (CuKα, Niꢀfilter, LYNXEYE detector, reflection
geometry) with a horizontal goniometer axis in the 2θ range 3 – 70° at a 2θ step of 0.01 ꢀ 0.02°
and a counting time of 0.3 – 0.5 s per step. Fullꢀprofile analysis of the Xꢀray diffraction patterns
of crystalline substances were performed using TOPAS 4.2 software. All DFT calculations were
performed within the PBE exchangeꢀcorrelation functional using VASP 5.4.1. 32ꢀ36 Atomic cores
were described using PAW potentials. Valence electrons (2s and 2p for O and N atoms; 5d, 6p,
and 6s for Bi; 5s and 5p for I; and 1s for H) were described in terms of a planeꢀwave basis set
(the kinetic energy cutoff was at 900 eV). The structure of 2 has disordered BiI5 anions, so we
could not directly optimize the atomic positions. To avoid this limitation for the structure of 2,
the symmetry was lowered and the disorder of BiI5 was removed by hand. Then, atomic
positions were optimized. The bond lengths, bond angles, and other interatomic distances in the
optimized structure were in good agreement with the respective values in the experimental
structure. There were no such limitations in the case of 3, so this structure was optimized in
terms of space group P21/n. The electron density function suitable for analysis in terms of
QTAIM theory was obtained in separate singleꢀpoint calculations of the optimized structure of 2
using the fast Fourier transform grid that was twice as dense as the default values (the distances
between points in direct space were ∼0.03 Å). For complete experimental methods of Xꢀray
crystallography, DRS, TGAꢀDTA, optimized atomic coordinates of structures and synthesis
procedure see the Electronic Supplementary Information.

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CCDC 1567568ꢀ1567572 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk.
Acknowledgements
Authors are grateful to Shared Facilities Center of the N.S. Kurnakov Institute of General and
Inorganic Chemistry (all experimental studies) and Samara Centre for Theorethical Materials
Science (computational resources). This publication was supported by the Ministry of Education
and Science of the Russian Federation (the Agreement number 02.a03.21.0008) and RFBR
(project number 16ꢀ03ꢀ00691).
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Crystal structure of new hybrid iodobismuthate contains linear anionic chains. This black
compound is semiconductor (Eg = 1.73 eV) and can be propose as promising material for
solar cells.