THE INTERCALATION OF PbI2 WITH 2,2-BIPYRIDINE EVIDENCED
BY PHOTOLUMINESCENCE, FT-IR AND RAMAN SPECTROSCOPY
N. PREDA, L. MIHUT, M. BAIBARAC, I. BALTOG, M. HUSANU, C. BUCUR, T. VELULA
National Institute for Physics of Materials, Lab. Optics and Spectroscopy,
Bucharest-Magurele, P.O. Box MG-7, R-77125, Romania,
E-mail: [email protected]
Received October 10, 2008
Hybrid material based on PbI2 intercalated with 2,2-bypiridine (BIPY) was
investigated by correlated studies of photoluminescence, infrared absorption and
Raman spectroscopy. The PbI2(BIPY) intercalated compound has been synthesized
by the chemical reaction of KI and Pb(NO3)2 in aqueous BIPY solution. The optical
studies reveal different properties for the hybrid material in comparison with those of
pure PbI2 and BIPY. In the photoluminescence spectrum of the intercalated
compound, recorded at liquid nitrogen temperature a new intense band emission with
maximum at 2.07 eV is observed. The excitation spectrum reveals a broad band
featured by several maxima at 2.77, 3.34 and 3.70 eV. New absorption bands at about
1589, 1489, 1435, 1312, 1009 cm–1 are observed in the IR spectrum of PbI2(BIPY).
The Raman spectrum of intercalated compound discloses new lines at 67, 83, 127 cm–1
and the shift of two Raman lines from 994 cm–1 to 1010 cm–1 and from 1045 cm–1 to
1060 cm–1. A charge transfer process, leading to the formation of lead-BIPY
coordination complexes, is considered as responsible for the strong host-guest
interaction revealed by almost all experimental data.
Key words: lead iodide, 2,2-bipyridine, intercalation.
1. INTRODUCTION
The intercalation of layered inorganic materials with different organic
molecules represents a useful method to synthesize inorganic/organic compounds
with novel properties. Recent studies suggest that such hybrid compounds have a
great potential for the synthesis of functional materials [1] using the wide variety
of properties associated with each component.
In the last years, much attention has been paid to the intercalation of
inorganic layered semiconductors as host with different organic molecules
(polymers, amines, etc.) as guest. Among the layered semiconductors, the PbI2 is
a challenging material, its optical properties being substantial changed upon the

Paper presented at the National Conference of Physics, 10–13 September, 2008,
Bucharest–Mãgurele, Romania.
Rom. Journ. Phys., Vol. 54, Nos. 7– 8 , P. 667–675, Bucharest, 2009
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intercalation [2–4]. In the bulk form, the structure of lead iodide consists in
sandwiched layers. Each layer contains a plane of metal ions sandwiched
between two planes of hexagonally arranged iodide ions. The bonds within
I-Pb-I layers are strong while those between adjacent layers are weak. The van
der Waals-type interlayer bonding allows an easy insertion of different guest
molecules, resulting in the expansion of interlayer space along the crystal c axis.
Recently, the intercalation of lead iodide with polycyclic heteroaromatic
molecules, such as 2,2 bipyridine (BIPY) and 1,10-phenantroline, was reported
[5, 6]. Structural informations regarding the new compounds have been achieved
using X-ray diffraction, thermo-gravimetric analysis and differential thermal
analysis [5, 6]. However, the studies on the vibrational and optical properties of
lead iodide intercalated with the above mentioned molecules are still rather
scarce.
The goal of this paper is to synthesize the PbI2(BIPY) hybrid compound by
a chemical way and to investigate its photoluminescence, infrared absorption and
Raman spectroscopic properties.
2. EXPERIMENTAL
The PbI2(BIPY) intercalated compound has been synthesized by the
chemical reaction between an aqueous solution of Pb(NO3)2 (5 cm3; 0.01 M) and
an aqueous solution of KI (5 cm3; 0.05 M) carried out under vigorously
ultrasonic homogenizing in aqueous BIPY solution (100 cm3; 0.5%) as buffer.
Immediately a deposit is formed on the bottom of the preparation vessel. The
mixture was filtered off and the solid was washed with water several times and
then dried in air. The final compound, PbI2(BIPY), was a bright yellow solid. By
the same chemical reaction between Pb(NO3)2 and KI in water PbI2 microcrystals
were prepared. All starting chemicals, Pb(NO3)2, KI and BIPY powders, were
reagent grade, purchased from Alfa-Aesar, and used without further purification.
The photoluminescence (PL) and photoluminescence excitation (PLE)
spectra at liquid nitrogen temperature (LNT) and room temperature (RT) were
recorded using a Horiba Jobin Yvon Fluorolog 3-22 spectrofluorimeter.
The Fourier transformed-infrared (FT-IR) spectra were recorded with a
FT-IR Bruker Vertex 70 spectrometer in the 4000–400 cm–1 range with 4 cm–1
resolution.
The Raman studies were performed at room temperature using a FT Raman
Bruker RFS 100/S spectrometer equipped with a YAG:Nd laser (1064 nm
excitation wavelength) and a liquid N2 cooled Ge detector. The laser power at
the samples was kept at 50 mW. The spectra were recorded in the 3500–50 cm–1
range with 4 cm–1 resolution.
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Intercalation of PbI 2 with 2,2-bipyridine
669
3. RESULTS AND DISSCUSSION
The PL and PLE spectra of PbI2(BIPY) intercalated compound recorded at
LNT (solid line) and RT (dot lines) are presented in Fig. 1a and Fig. 1b,
respectively. One can clearly see that at LNT, the typical emission of PbI2 [7]
disappears and a new intense emission band with maximum at 2.07 eV is
observed. At RT, the emission is found slightly shifted towards higher energy. It
has to be mentioned that the shape and the position of the band are preserved
when the excitation wavelength is varied from 435 nm to 335 nm. We are
tempted to associate this broad and intense emission band with a complex
structure resulted from a strong interaction between the PbI2 lattice and the BIPY
molecules. Thus, the intercalation may involve a charge transfer from the
non-bonding electron pair of nitrogen atom of the organic molecule to the
incompletely filled 6s orbital of lead ion. As consequence of the charge transfer,
a lowering of the Pb2+ initial coordination geometry is expected to occur,
favoring the appearance of a new extended one-dimensional chain structure.
Such one-dimensional structures, involving linked coordination complexes, were
reported for a compound resulted from the interaction between lead iodide and
pyridine in Ref. [8].
The PLE spectra of the hybrid compound were measured at 620 nm
(2.00 eV) emission wavelength. Regardless the working temperature (LNT or RT)
Fig. 1 – PL (a) and PLE (b) spectra of PbI2(BIPY) compound recorded at LNT (solid
lines) and RT (dot lines). PL spectra were obtained at 435 nm excitation wavelength.
PLE spectra were recorded with the emission wavelength fixed at 620 nm (2.00 eV).
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the spectrum consists in a broad band with the maximum at 3.34 eV. A similar
PLE spectrum featured by a peak at 3.37 eV was associated with the
coordination complex mentioned in Ref. [8]. So, we are right in considering the
broad band observed in the PLE spectrum as an evidence for the formation of
lead-BIPY coordination complex.
If indeed the intercalation process of PbI2 with BIPY leads to a
coordination complex, new vibrational bands should be observed in the IR and
Raman spectra of intercalated compound.
The PbI2 crystalline matrix is transparent in the IR region. Consequently,
the infrared absorption must be related either to BIPY or to the new compound
Fig. 2 – FTIR spectra of BIPY powder (a) and PbI 2(BIPY) compound (b).
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Intercalation of PbI 2 with 2,2-bipyridine
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resulted from the interaction between BIPY and PbI2. The FTIR spectra of BIPY
and PbI2(BIPY) hybrid compound are presented in Fig. 2a and Fig. 2b,
respectively.
The location and the attribution of the IR absorption bands of BIPY and
intercalated compound experimentally observed are summarized in Table 1. For
comparison the main characteristic absorptions for [Mn(BIPY)3]2+, another
coordination complex of BIPY, was also included in the Table 1. Looking at the
IR data one can easy observe that the positions of the absorption bands of our
intercalated compound are very close with those mentioned for [Mn(BIPY)3]2+
[9, 10]. Based on this result we can consider that the bands at about 1589, 1489,
1435, 1312, 1009 and 770 cm–1 are in their turn the signature of a coordination
complex, lead-BIPY, resulted from the host-guest interaction.
Table 1
Comparison of the IR absorption data for PbI 2(BIPY), BIPY and [Mn(bipy)3]2+
Assignment [9]
ring stretching
(C=C, C=N)
BIPY
PbI2(BIPY)
[Mn(BIPY)3]2+ [9]
1578
1557
1589
1570
1597
1574
1562
1491
1473
1489
1472
ring stretching + H bend
1452
1414
1435
1439
1425
1312
1244
1315
1249
N-H+ out of plane bend
res. deformation ring stretching
1240
H in plane bend
1210
1171
1157
1101
1176
1154
1098
ring stretching + H bend
1063
1069
1060
ring breathing mode
991
1009
1011
ring stretching + H bend
H out of plane bend
ring bend
911
893
756
741
652
770
733
895
774
737
644
627
644
621
The Raman spectra of PbI2 microcrystals, BIPY powder and PbI2(BIPY)
hybrid compound are shown in Fig. 3a, Fig. 3b and Fig. 3c1-c2, respectively. The
Raman spectrum of PbI2 microcrystals (Fig. 3a) discloses four lines situated at
75, 96, 112 and 164 cm–1 attributed to the E21, A11, A12 and 2E21 vibration modes,
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Fig. 3 – Raman spectra at 1064 nm excitation wavelength: PbI 2 microcrystals (a);
BIPY powder (b); PbI2(BIPY) compound (c1 and c2).
respectively [11]. In the same domain, the Raman spectrum of the intercalated
compound exhibits three new lines situated at 67, 83 and 127 cm –1.
The insertion of the organic molecules between iodine layers may activate
new Raman vibrations linked with the interface modes [12]. Thus, for a PbI2
crystal submitted to a laser irradiation, a Raman band situated at 83 cm–1 was
associated to the interface phonons, which propagate along the planar defects
produced by the stacking faults. Starting from this we are tempted to link the
Raman band situated at 83 cm–1 to the stacking faults resulting from the insertion
of BIPY between PbI2 layers.
Regarding the Raman line situated at 127 cm–1, this could be a vibrational
mode belonging to the lead-BIPY coordination complex. An argue sustaining
such assignment is the appearance of a Raman line at about 135 cm–1 in the case
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Intercalation of PbI 2 with 2,2-bipyridine
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of coordination complexes formed by PbI2 with other nitrogen-containing
molecules, like ammonia or pyridine [13].
The most convincing proof sustaining the generation of the lead-BIPY
coordination complex during intercalation process is given by the Raman spectra
of BIPY (Fig. 3b) and PbI2(BIPY) compound (Fig. 3c2). In the Table 2, the
positions and assignments of the main bands are shown. For comparison, Table 2
also includes the Raman bands observed in the case of another intercalated
compound, Zn(BIPY)Cl2 [14].
Table 2
Comparison of the Raman data for PbI 2(BIPY), BIPY and Zn(BIPY)Cl2
BIPY
PbI2(BIPY)
Zn(BIPY)Cl2 [14]
ring stretching (C-C, C-N) + N-H + in-plane
deformation
Assignment [14]
1591
1589
1599
ring stretching (C-C, C-N)
1573
1570
1569
ring stretching (C-C, C-N) + C-H in plane
deformation
1483
1490
1493
ring stretching (C-C, C-N) + C-H in plane
deformation
1448
1431
1447
C-C inter-ring stretching + ring stretching
(C-C, C-N) + C-H in plane deformation
1301
1307
1299
1261
1266
ring stretching (C-C, C-N) + C-C inter-ring
stretching + C-H in plane deformation
1238
ring stretching (C-C, C-N) + C-C inter-ring
stretching + C-H in plane deformation
1219
C-H in plane deformation + ring stretching
1147
C-H in plane deformation + ring stretching
1095
C-H in plane deformation + ring stretching and
deformation
1045
1060
1063
ring breathing
995
1010
1030
C-H out of plane deformation
816
814
814
in plane ring deformation
765
763
765
in plane ring deformation
615
647
551
639
547
out of plane deformation
441
449
461
ring-ring stretching
334
347
331
1158
1101
It is known that in crystalline state and in aqueous solution, the BIPY
molecule is found in its trans-conformation [15]. The Raman fingerprint of this
conformation is represented by three lines situated at 615, 1238 and 1448 cm–1.
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N. Preda et al.
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Or, in the Raman spectrum of the PbI2(BIPY) these lines are not observed. The
lack of the mentioned lines indicates that in the intercalated compound, the BIPY
molecule is found in another form, namely its cis-conformation. A passage from
trans-to-cis conformation was signaled in the complexation process of BIPY
with metal ions [16]. Based on this fact, we can state that a similar process
leading to a lead-BIPY coordination complex takes place at the intercalation of
PbI2 with BIPY molecules. The formation of such coordination complex is
illustrated by the following reaction:
Comparison between the Raman spectra of the PbI2(BIPY) complex and
BIPY helps in the characterization of the two conformations. The appearance of
the complex is accompanied by the shift to higher frequencies of several Raman
peaks. Thus, the ring breathing mode shifts from 994 cm–1 (Fig. 3b) to 1010 cm–1
(Fig. 3c2) indicating a stronger interaction between the nitrogens of the rings and
the lead ions. Also, the shift of the medium intensity band, associated with the
ring stretching and deformation mode, from 1045 cm–1 (Fig. 3b) to 1060 cm–1
(Fig. 3c2) sustains the formation of a coordination complex involving electrostatic
interactions between the cations and the nitrogen atoms of BIPY molecules.
Other observations concerning the Raman spectrum of the PbI2(BIPY)
compound are: i) the two strong peaks at 1573 and 1591 cm–1 are replaced by a
strong band at 1589 cm–1 and another band, of medium intensity, at 1570 cm–1,
ii) the two strong bands at 1448 and 1483 cm–1 are replaced by a single band at
1490 cm–1 and iii) the Raman band situated at 816 cm–1 does not shift
significantly due to the complexation.
Returning to Table 2, a noticeable fact is the similarity of the Raman band
positions for PbI2(BIPY) and Zn(BIPY)Cl2. Based on the fact that in Ref. [14],
Zn(BIPY)Cl2 is treated as a coordination complex we found an additional
argument sustaining the formation of lead-BIPY complex.
4. CONCLUSION
PbI2(BIPY) hybrid material was synthesized by a chemical way and
characterized by photoluminescence under continuous excitation, infrared
absorption and Raman spectroscopy. The main PL signature of the PbI2(BIPY)
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Intercalation of PbI 2 with 2,2-bipyridine
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compound consists in an intense broad emission band peaking at LNT at about
2.07 eV. In the IR spectrum of PbI2(BIPY), new bands at 1589, 1489, 1435,
1312, 1009 and 770 cm–1 are observed. The Raman spectrum of PbI2(BIPY)
discloses new lines at 67, 83, 127 cm–1 and the shift of two Raman lines from
994 cm–1 to 1010 cm–1 and from 1045 cm–1 to 1060 cm–1. Almost all experimental data sustain a strong interaction between PbI2 host lattice and BIPY guest
molecules, leading to the appearance of a coordination complex. The crucial role
in the formation of such complexes is played by the non-bonding electron pair of
nitrogen atom belonging to BIPY molecule.
Acknowledgments. This project is funded by the Romanian Ministry of Education and
Research, CEEX Program, Project no. 2-CEx06-11-19/25.07.2006.
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