Synthesis, structures, and optical properties of cadmium iodide/phenethylamine hybrid materials with controlled structures and emissions

Article abstract of DOI:10.1021/ic7007304

A new type of organic-inorganic hybrid materials based on cadmium iodide (CdI₂) and phenethylamine (PEA) has been synthesized and characterized. The reaction of CdI₂ with PEA in a 1:2 molar ratio yields a four-coordinate hybrid material CdI₂(PEA)₂ (1) with extended 1D (CdI₂)_n chains, while the reaction of CdI₂ with PEA in a 1:4 molar ratio produces a six-coordinate hybrid material CdI₂(PEA)₄ (2) with a discrete linear structure of CdI₂ moiety. By introducing a trace amount of Na₂S to the reaction for CdI₂(PEA)₂, we obtained a new compound [CdI₂(PEA)₂]-(CdS)_0.038 (3) with uniformly doped CdS nanoparticles. Steady and transient photoluminescence studies reveal that compounds 1 and 2 exhibit bright blue (465 nm) and green (512 nm) fluorescent emissions in solid state at room temperature, respectively, while compound 3 gives a broad and complex emission ranging from 450 to 700 nm. Theoretical studies of electronic structures were carried out using density functional theory in order to gain a good understanding of the luminescent behaviors of these hybrid materials.

Full text of DOI:10.1021/ic7007304

Inorg. Chem. 2007, 46, 10252−10260
Synthesis, Structures, and Optical Properties of Cadmium Iodide/
Phenethylamine Hybrid Materials with Controlled Structures and
Emissions
Wei Wang,† Juan Qiao,*,†,‡ Liduo Wang,† Lian Duan,† Deqiang Zhang,† Weitao Yang,‡ and Yong Qiu*,†
Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education,
Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and
Department of Chemistry, Duke UniVersity, Durham, North Carolina 27708-0354
Received April 17, 2007
A new type of organic−inorganic hybrid materials based on cadmium iodide (CdI2) and phenethylamine (PEA) has
been synthesized and characterized. The reaction of CdI2 with PEA in a 1:2 molar ratio yields a four-coordinate
hybrid material CdI2(PEA)2 (1) with extended 1D (CdI2)n chains, while the reaction of CdI2 with PEA in a 1:4 molar
ratio produces a six-coordinate hybrid material CdI2(PEA)4 (2) with a discrete linear structure of CdI2 moiety. By
introducing a trace amount of Na2S to the reaction for CdI2(PEA)2, we obtained a new compound [CdI2(PEA)2]-
(CdS)0.038 (3) with uniformly doped CdS nanoparticles. Steady and transient photoluminescence studies reveal that
compounds 1 and 2 exhibit bright blue (465 nm) and green (512 nm) fluorescent emissions in solid state at room
temperature, respectively, while compound 3 gives a broad and complex emission ranging from 450 to 700 nm.
Theoretical studies of electronic structures were carried out using density functional theory in order to gain a good
understanding of the luminescent behaviors of these hybrid materials.
Introduction
diversity is particularly evident in cadmium(II) halide
complexes from 0D,4 1D,5-6 to 2D7 by choosing proper
organic and inorganic portions. Complexes of CdX2L2 (X
) Cl, Br, and I; L ) pyridine or similar small, monodentate
ligands) typically form extended chain structures with the
metal center in six-coordinate pseudo-octahedral coordination
or four-coordinate tetrahedral coordination.8 Recently, a
family of cadmium halide/pseudohalide bpy (bpy ) 2,2′-
bipyridine or 4,4′-bipyridine) compounds have been synthe-
sized and characterized.9 These compounds show interesting
structural modes and potential applications in organic op-
toelectronics. Optical properties of CdI2(bpy) and ZnI2(bpy)
have also been investigated as these materials are fluorescent
and might be used for electroluminescent (EL) devices.
Organic-inorganic hybrid materials of metal halide/amine
systems such as MX2(L)n (M ) Zn, Cd, Cu, Co, Fe, etc.; X
) Cl, Br, I; L ) organic ligands containing one or more N
atoms as the coordination atoms; n ) 1, 2) have been widely
studied due to their interesting crystal structures and potential
applications as photoactive materials.1-3 Group 12 (IIB)
metal halides are regarded to be particularly promising, due
to the variety of coordination numbers and geometries
provided by the d10 configuration of the metal centers. This
structural diversity is closely dependent on factors such as
the dimension of the metal ions and halides, nature of ligands,
and the availability of other structure controlling interactions,
such as hydrogen bonding and charge transfer. The structure
In our work, we report novel CdI2 based hybrid materials
with phenethylamine (PEA) as the organic ligand. We choose
* To whom correspondence should be addressed. Fax: (+86)-10-
62795137. E-mail: qiuy@mail.tsinghua.edu.cn (Y.Q.), qjuan@mail.tsinghua.
edu.cn (J.Q.).
(4) Ni, J.; Xie, Y.; Liu, X.; Liu, Q. Appl. Organomet. Chem. 2003, 17,
315.
(5) Matsuda, K.; Shinkai, Y.; Irie, M. Inorg. Chem. 2004, 43, 3774.
(6) Seitz, M.; Kaiser, A.; Stempfhuber, S.; Zabel, M.; Reiser, O. J. Am.
Chem. Soc. 2004, 126, 11426.
† Tsinghua University.
‡ Duke University.
(1) Bailey, R. D.; Hook, L. L.; Powers, A. K.; Hanks, T. W.; Pennington,
W. T. Cryst. Eng. 1998, 1, 51.
(2) Handley, D. A.; Hitchcock, P. B.; Lee, T. H.; Leigh, G. J. Inorg. Chim.
Acta 2001, 314, 14.
(3) Zhai, Q.; Wu, X.; Chen, S.; Lu, C.; Yang, W. Cryst. Growth Des.
2006, 6, 2126.
(7) Bailey, R. D.; Pennington, W. T. Polyhedron 1997, 16, 417.
(8) Hu, C.; Li, Q.; Englert, U. CrystEngComm 2003, 5, 519.
(9) Chen, W.; Wang, M.; Liu, X.; Guo, G.; Huang, J. Cryst. Growth Des.
2006, 6, 2289.
10252 Inorganic Chemistry, Vol. 46, No. 24, 2007
10.1021/ic7007304 CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/01/2007

CdI2/PEA Hybrid Materials with Controlled Structures
dropped into the CdI2/methanol solution in a three-necked flask
under magnetic stirring. The mixed solution was heated to reflux
for 1 h and filtrated immediately. The hot filtrate was cooled to
room temperature to obtain 2.33 g of needle crystals which were
filtered again and dried in vacuo. Yield: 48.6% (based on cadmium
iodide). Chemical analysis of the product CdI2(PEA)2 yielded the
following. Calcd: C, 31.58; H, 3.64; N, 4.60. Found: C, 31.64; H,
3.62; N, 4.59. IR peaks (KBr, cm-1): 3431 (m), 3307 (w), 3227
(s), 3139 (w), 2937 (w), 2880 (w), 1581 (s), 1498 (s), 1455 (s),
985 (s), 750 (s), 700 (s), 500 (m). Slow evaporation of the second
time filtrate for 3 days yielded colorless, rectangle crystals which
were suitable for X-ray measurement.
PEA as the ligand because the hybrids containing PEA are
luminescent and easily dissolved in organic solvents such
as methanol and ethanol. It was found that the inorganic
frameworks in the hybrids can be tuned from (CdI2)n chains
(1D) to CdI2 dot (0D) just by changing the molar ratio of
PEA and CdI2 in the reaction mixture. Herein, we describe
the synthesis and characteristics of CdI2(PEA)2 (1) and CdI2-
(PEA)4 (2). Compound 1 has a linear crystal structure with
tetrahedral coordination of the metal center such as previ-
ously reported hybrid materials,1,8,10 while compound 2 has
a pseudo-octahedral coordination with discrete CdI2 moieties
which are surrounded by four PEA molecules. We have also
measured the absorption and photoluminescence of the
compounds and found that compound 1 exhibits blue
fluorescence while compound 2 gives green fluorescence at
room temperature. By doping a very small amount of CdS
into compound 1, a new compound of CdS-doped CdI2-
(PEA)2 (3) was obtained which exhibits orange fluorescence
at room temperature. Molecular orbital calculations of ground
and excited electronic states were carried out in combination
of transient photoluminescence to understand the emission
mechanism of these hybrids.
(b) CdI2(PEA)4 (2). A 1.24 g amount of compound 2 was
prepared with CdI2 (1.44 g, 3.95 mmol) and PEA (2 mL, 15.78
mmol) by a method similar to that described above for 1. Note,
the CdI2/methanol solution was inversely dropped into the PEA/
methanol solution and the cooling process of the hot filtrate was
very slow; otherwise only compound 1 was obtained. Yield: 35.8%
(based on cadmium iodide). Chemical analysis of the product CdI2-
(PEA)4 yielded the following. Calcd: C, 45.17; H, 5.21; N, 6.58.
Found: C, 45.27; H, 5.11; N, 6.56. IR peaks (KBr, cm-1): 3433
(m), 3323 (s), 3240 (w), 3150 (w), 2933 (w), 2875 (w), 1595 (s),
1487 (s), 1451 (s), 970 (s), 748 (s), 702 (s), 544 (m). Slow
evaporation of the second time filtrate for 3 days yielded yellow,
transparent, hexagon crystals which were suitable for X-ray
measurement.
Experimental Section
(c) CdS-Doped CdI2(PEA)2 (3). Compound 3 was prepared by
introducing trace Na2S in the PEA solution. Na2S‚9H2O (0.038 g,
0.16 mmol) and PEA (1 mL, 7.89 mmol) were dissolved together
in 10 mL of anhydrous methanol and CdI2 (1.44 g, 3.95 mmol) in
another 10 mL of anhydrous methanol. The former solution was
added to the latter one, and the mixed solution was heated to reflux
for 1 h and filtered immediately. The filtrate was kept in the
refrigerator (4 °C) for 1 day and yielded light yellow, transparent,
needle crystals which were suitable for X-ray measurement.
Yield: 50.9% (based on cadmium iodide). Chemical analysis of
compound 3 yielded C, 31.52; H, 3.66; N, 4.55; S, 0.2. ICP-AES
analysis revealed the content of Na is lower than the limit of
detection (1 ppm). As a result, the formula of compound 3 can be
expressed as [CdI2(PEA)2](CdS)0.038. IR peaks (KBr, cm-1): 3430
(m), 3240 (s), 3289 (w), 3238 (w), 2939 (w), 2877 (w), 1580 (s),
1497 (s), 1458 (s), 992 (s), 752 (s), 695 (s), 504 (s).
Measurements. All of the chemicals are readily available from
commercial sources and were used without further purification.
Phenethylamine (99.5%) was purchased from Acros Organics, CdI2
(99.5%) was from Beijing Chemical Reagents Co., and Na2S‚9H2O
(98%) was from Beijing Modern Eastern Fine Chemical. C, H, and
N analyses were carried out with an Exeter Analytical CE-440
elemental analyzer. S analysis was carried out with an X-ray
fluorescence spectrometer (XRF-1700). Na analysis was carried out
with a Vista-MPX inductive coupled plasma atomic emission
spectrometer (ICP-AES). The infrared spectra (KBr pellet) were
recorded on a Perkin-Elmer spectrum GX FT-IR spectrophotometer
over the frequency range of 400-4000 cm-1. The SEM (scanning
electron microscope) image was recorded on a Hitachi S-4500
scanning electron microscope using an accelerating voltage of 15
kV. High-resolution transmission electron microscopy investigations
were carried out with a JEOL-2011 instrument. Thermal gravimetric
analyses were performed on a TGA2050 of TA Instrument Corp.
The diffuse reflection absorption spectra were recorded on a Hitachi
U-3010 spectrophotometer equipped with an integrating sphere at
room temperature (298 K). The absorption spectra were calculated
by the Kubelka-Munk function:11 R/S ) (1 - R)2/2R, where R is
the absorption coefficient, S is the scattering coefficient, which is
practically wavelength independent when the particle size is larger
than 5 µm, and R is the reflectance. Photoluminescence (PL) spectra
were recorded at room temperature (298 K) on a Jobin Yvon
FluoroMax-3 spectrofluorometer using a xenon arc lamp as the light
source. The time-resolved fluorescence decays were measured with
a time correlated single photon counting (TCSPC) system on
Edinburgh Instruments (FLS920) and analyzed by the software
provided by the supplier.
X-ray Crystallography. The crystals of compounds 1-3 with
the approximate dimensions of 0.02 × 0.2 × 0.6, 0.15 × 0.2 ×
0.3, and 0.2 × 0.2 × 0.7 mm3, respectively, were selected under a
microscope and attached to the end of a quartz fiber. The room
temperature (294 ( 1 K) single-crystal X-ray experiments were
performed on a Bruker P4 diffractometer equipped with graphite-
monochromatized Mo KR radiation (λ ) 0.710 73 Å) using the ω
scan technique. Direct phase determination yielded the positions
of Cd, I, N, and most of the C atoms. The remaining atoms were
located in successive-difference Fourier syntheses. Hydrogen atoms
were generated theoretically and rode on their parent atoms in the
final refinement. All non-hydrogen atoms were subjected to
anisotropic refinement. The structural solutions and refinements
were performed using the SHELXTL NT v 5.10 program package
(Bruker, 1997).12 A summary of the refinement details and the
resulting factors are given in Table 1.
Syntheses. (a) CdI2(PEA)2 (1). CdI2 (2.89 g, 7.89 mmol) and
PEA (2 mL, 15.78 mmol) were dissolved in 20 mL of anhydrous
methanol, respectively. Then the PEA/methanol solution was
Calculation Details. Calculations on the ground and excited
electronic states of compounds 1-3 (omit the CdS dopant) were
(10) Altun, A.; Golcuk, K.; Kumru, M. Vib. Spectrosc. 2003, 33, 63.
(11) Kortu¨m, G. Reflectance Spectroscopy; Springer-Verlag: Berlin,
Heidelberg, Germany, 1969.
(12) Sheldrick, G. M. SHELXTL, version 5.1; Bruker Analytical X-ray
System: Madison, WI, 1997.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10253

Wang et al.
Table 1. Crystallographic Data for Compounds 1 and 2 and the
CdI2(PEA)2 Matrix of Compound 3
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds
1 and 2a and the CdI2(PEA)2 Matrix of Compound 3
CdI2(PEA)2
matrix of compd 3
Compound 1
param
1
2
C(1)-C(2)
1.401(1)
2.246(3)
120.9(1)
129.1(2)
107.6(1)
C(8)-N(1)
1.480(5)
2.771(1)
112.9(4)
102.1(1)
107.0(1)
empirical formula
mol mass
color
cryst syst
space group
a, Å
C
16H22N2CdI2
608.58
C
32H44N4CdI2
C16H22N2CdI2
608.58
Cd(1)-N(1)
Cd(1)-I(1)
850.94
yellow
monoclinic
C2/c
31.424(2)
12.483(1)
8.991(1)
96.043(1)
3507.4(4)
4
C(1)-C(2)-C(3)
N(1)-Cd(1)-N(1)1
N(1)-Cd(1)-I(1)
N(1)-C(8)-C(7)
N(1)-Cd(1)-I(1)1
I(1)1-Cd(1)-I(1)
colorless
monoclinic
C2/c
33.349(5)
4.846(1)
13.075(2)
105.65(1)
2034.7(5)
4
light yellow
monoclinic
C2/c
29.832(3)
5.137(1)
27.674(3)
111.04(1)
3958.2(6)
8
Compound 2
C(3)-C(4)
1.394 (4)
C(1)-N(2)
1.472 (3)
b, Å
c, Å
C(11)-C(12)
1.392 (5)
2.393(2)
3.030 (1)
180.0 (0)
180.0 (0)
87.4 (2)
C(9)-N(1)
1.461 (3)
2.372 (2)
3.030 (1)
180.0 (0)
94.9 (1)
91.7 (2)
Cd(1)-N(1)
Cd(1)-N(2)
â, deg
Cd(1)-I(1)2
Cd(1)-I(1)
V, Å3
I(1)-Cd(1)-I(1)2
N(2)-Cd(1)-N(2)2
I(1)-Cd(1)-N(2)
N(1)-Cd(1)-N(1)2
I(1)-Cd(1)-N(1)
N(1)-Cd(1)-N(2)
Z
d
calcd, g cm-3
1.987
4.104
1.611
2.249
2.042
4.219
0.0295
abs coeff (µ), mm-1
Rint
0.0418
0.0253
CdI2(PEA)2 Matrix of Compound 3
R1, wR2 [I g 2σ(I)]a 0.0424, 0.1069 0.0408, 0.0994 0.0499, 0.1390
C(1)-C(2)
1.374 (9)
1.373 (1)
2.244 (5)
2.762 (1)
C(8)-N(1)
1.451 (8)
1.477 (6)
2.247 (4)
2.739 (1)
110.1 (6)
114.3 (4)
110.4 (1)
104.3 (1)
116.1 (3)
R1, wR2 (all data)a
0.0466, 0.1104 0.0499, 0.1065 0.0755, 0.1534
C(9)-C(10)
C(16)-N(2)
Cd(1)-N(1)
Cd(1)-N(2)
Cd(1)-I(2)
N(1)-C(8)-C(7)
N(2)-C(16)-C(15)
N(1)-Cd(1)-I(2)
N(1)-Cd(1)-I(1)
C(16)-N(2)-Cd(1)
2
2
2
a R1 ) Σ|Fo| EnDash |Fc|/Σ|Fo|; wR2 ) [Σw(Fo - Fc )2/Σ(Fo )2]1/2
.
Cd(1)-I(1)
C(1)-C(2)-C(3)
C(9)-C(10)-C(11)
N(1)-Cd(1)-N(2)
N(2)-Cd(1)-I(2)
I(2)-Cd(1)-I(1)
121.1 (7)
121.6 (6)
116.2 (2)
108.7 (1)
112.2 (1)
carried out using density functional theory (DFT) and time-
dependent DFT (TD-DFT) at the B3LYP level.13-14 “Double-ê”
quality basis sets were employed for the C, H, and N (6-31G*)
and the Cd, I (LANL2DZ). The initial ground-state geometries were
directly obtained from the X-ray crystal structures. The geometries
were fully optimized with C1 symmetry constraints. All calculations
were performed with Gaussian 03 software package using a spin-
restricted formalism.15 The electron density diagrams of molecular
orbitals were obtained with the ChemOffice 2002 graphics program.
a Symmetry transformations used to generate equivalent atoms: (1)
3
3
3
compound 1, -x + 2, y, -z + /2; (2) compound 2, -x + /2, -y + /2,
-z.
octahedrally coordinated CdI2(pyridine)21 and CdI2(pyrazine).7
The lower coordination number allows a closer approach of
the ligands to the metal center. The bond length of Cd1-
N1 and Cd1-I1 is 2.246 and 2.771 Å, respectively. The
complex also consists of infinite CdI2-based chains, not being
doubly bridged by the two iodine atoms. The extended CdI2
chains are kept by intermolecular hydrogen bonding of N-H‚
‚‚I and π-π stacking of the organic ligands (as shown in
Figure 2a). The hydrogen bonding in single CdI2 chain is
moderate with the bond length of H1ba‚‚‚I1 (x, y - 1, z) of
2.946 Å, comparing with the van der Waals radii of H (1.06
Å) and I (1.98 Å).17 The organic ligands (PEA) are well-
arranged via π-π stacking interaction (3.537 Å). Moreover,
there is a weak Cd-I‚‚‚Cd interaction (3.897 Å) between
the adjacent CdI2 due to the d10 configuration of Cd atom.
The distance of adjacent Cd atom is 4.846 Å, and more
details of the packing of CdI2 in compound 1 are shown in
Figure S1 of the Supporting Information. The chains are
linked together by intermolecular hydrogen bonding of N1a-
Results and Discussion
Crystal Structures. The molecular structures of com-
pounds 1-3 were elucidated by X-ray crystallography. The
crystal data and selected bond lengths and angles are listed
in Tables 1 and 2, respectively. The perspective drawings
of the structures are shown in Figure 1 and the molecular
packings of the crystal structures are shown in Figures 2, 3,
and 5. The complexes with group 12 metal elements (Zn,
Cd, and Hg) as the centered atom have different coordinate
numbers from 4 to 8 due to the d10 configurations, and the
structures of the complexes are diversified.16 The coordinate
number of this kind of organic-inorganic hybrid materials
can be 4,8 5,3 or 6,7-9 depending on the organic ligand.
CdI2(PEA)2 (1). Compound 1 crystallizes in monoclinic
space group with centric C2/c symmetry. The molecule has
a monomeric structure with pseudo-tetrahedral coordination
about the cadmium atom, which is quite different from the
1
H1aa‚‚‚I1 (x, -y, z + /2) with the bond length of 3.050 Å
(shown in Figure 2b). More information of the hydrogen
bonding can be found in Table S1 of the Supporting
Information.
CdI2(PEA)4 (2). Compound 2 has a nomomeric structure
with octahedral coordination about the cadmium atom, which
is quite different from compound 1. To our best knowledge,
Cd(γ-picoline)4I2 and [CdI2(pyridine)4](pyridine)2 were
hitherto reported as monomeric six-coordinate compounds
of group 12 metal halide/organic ligand systems. Moreover,
(13) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(14) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennuci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui,
Q.;Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Oritz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. GAUSSIAN03, revision C.02; Gaussian: Pittsburgh,
PA, 2003.
18
19
(17) Zhang, M. B.; Zhao, D. X.; Yang, Z. Z. J. Theor. Comput. Chem.
2005, 4, 281.
(18) Liptay, G.; Borbe´ly-Kuszmann, A.; Wadsten, T.; Losonczi, J. Ther-
mochim. Acta 1988, 133, 353.
(16) Shriver, D. F.; Atkins, P. W. Inorganic Chemistry; Oxford University
Press: Oxford, U.K., 2006.
(19) Ito, M.; Shibata, T.; Saito, Y. Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 1984, 40, 2041.
10254 Inorganic Chemistry, Vol. 46, No. 24, 2007

CdI2/PEA Hybrid Materials with Controlled Structures
Figure 1. ORTEP drawings of compounds 1 (a) and 2 (b) and the CdI2(PEA)2 matrix of compound 3 (c) with 30% probability ellipsoids, showing the
atomic numbering scheme. The doped CdS cannot be observed in the crystal structure due to the small amount.
Figure 2. Hydrogen bondings and π-π stacking in the crystal of compound 1 for (a) the single chain and (b) the adjacent chains. The thin dashed lines
represent the hydrogen bondings, and the thick dashed lines represent the π-π stacking interaction.
only [CdI2(pyridine)4](pyridine)2 was reported with crystal
structure, which contains enclathrated pyridine guest mol-
ecules. In contrast, compound 2 crystallizes without any
enclathrated solvent molecules. PEA ligand coordinates to
the cadmium atom through the amidogen, and the flexible
ethylic group is conducive to reducing the steric hindrance
and electron density at the metal center caused by the high
coordination.20 The bond lengths of Cd1-N1, Cd1-N2, and
Cd1-I1 are 2.393, 2.372, and 3.030 Å, respectively, which
are considerably longer than those in four-coordinated
compound 1. The Cd atom and four nitrogen atoms form a
square-planar configuration, and the two iodine atoms are
in the trans postion with I-Cd-I of 180.0°, indicating the
existence of CdI2N4 octahedron.
stacking interactions. As shown in Figure 3a, the hydrogen
atoms linked with N1a and N2 involve in the hydrogen
bondings with the same iodide atom of the neighboring
molecule. However, the two hydrogen bondings are different
1
in the bond length and angle. The N2-H8‚‚‚I1 (x, y + /2,
1
z - /2) hydrogen bonding with bond length of 2.939 Å is
1
somewhat stronger than the N1a-H4b‚‚‚I1 (x, y + /2, z -
1/2) with bond length of 3.009 Å. There is no intramolecular
interaction between the aromatic rings, but strong π-π
stacking intermolecular interactions exist (3.105 Å, Figure
3b).
CdS-Doped CdI2(PEA)2 (3). Doping the metal sulfide
nanoparticles such as CdS, PbS, ZnS, and SnS2 to organic
semiconductors have drawn much attention as the metal
sulfides have excellent optical and electrical properties.21-22
These discrete CdI2N4 octahedra pack together by inter-
molecular hydrogen bondings of N-H‚‚‚I and strong π-π
(21) Caseri, W. R. Mater. Sci. Technol. 2006, 22, 807.
(22) Aharon, E.; Kalina, M.; Frey, G. L. J. Am. Chem. Soc. 2006, 128,
15968.
(20) Strasdeit, H.; Saak, W.; Pohl, S.; Driessen, W. L.; Reedijk, J. Inorg.
Chem. 1988, 27, 1557.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10255

Wang et al.
Figure 3. π-π stacking (a) and hydrogen bondings (b) in the crystal of compound 2. The thin dashed lines represent the hydrogen bonding, and the thick
dashed lines represent the π-π stacking interaction.
Figure 5. Hydrogen bondings and π-π stacking in the CdI2(PEA)2 matrix
of compound 3 for (a) the single chain and (b) the adjacent chains. The
thin dashed lines represent the hydrogen bonding, and the thick dashed
lines represent the π-π stacking interaction.
croscope (HR-TEM). As shown in Figure 4a, CdS nanopar-
ticles uniformly disperse in the CdI2(PEA)2 matrix. Their
diameter varies from 5 to 40 nm (Figure 4b). The lattice
spacing of the CdS nanoparticles is 1.57 Å (Figure 4c,d),
corresponding to the (2, 0, 2) plain spacing of wurtzite CdS.23
The origin of the morphology of CdS nanoparticles may be
related to the geometric structure of the CdI2(PEA)2 matrix,
and the reaction of CdI2 with Na2S was confined into a
nanoscale volume due to the linear structure of CdI2 chains.24
Such a small amount of S (in the form of CdS) cannot
exhibit in the powder diffraction analysis and single crystal
analysis, which only gave the diffraction information of the
CdI2(PEA)2 matrix. The main composition of compound 3
is still CdI2(PEA)2 which is similar to compound 1. However,
the crystal structure of the CdI2(PEA)2 matrix has been
obviously changed by the incorporation of CdS nanoparticles.
CdI2(PEA)2 in compound 3 belongs to the C1 point group
(while compound 1 belongs to C2V), and one of the PEA
Figure 4. SEM (a) and HR-TEM (b-d) images of the CdS nanocrystals
doped to the CdI2(PEA)2 matrix of compound 3. Measured lattice spacings
of images c and d correspond to (202) spacing in wurtzite CdS.
Herein, we have successfully prepared organic-inorganic
hybrid CdI2(PEA)2 doped with nanocrystalline CdS. As
described in the Experiment Section, the reaction was carried
out at a relatively low temperature (the reflux temperature
of methanol) and the reactants are safe and easily obtained.
The trace amount of S is difficult to determine by elemental
analysis, so we carried out X-ray fluorescence measurement,
which gave the S amount of 0.2%. ICP-AES analysis
revealed the absence of Na. The formula of compound 3
can be then expressed as [CdI2(PEA)2](CdS)0.038. The doped
CdS can be directly observed by the scanning electron
microscopy and high-resolution transmission electron mi-
(23) Grun, M.; Hetterich, M.; Becker, U.; Giessen, H.; Klingshirn, C. J.
Cryst. Growth 1994, 141, 68.
(24) Nie, Q.; Yuan, Q.; Chen, W.; Xu, Z. J. Cryst. Growth 2004, 265,
420.
10256 Inorganic Chemistry, Vol. 46, No. 24, 2007

CdI2/PEA Hybrid Materials with Controlled Structures
ligands has a largely distorted configuration. Its amidogen
folds down to the phenyl ring, leading to a smaller dihedral
angle of 50.2° between the two phenyl rings of two PEA
ligands than that in compound 1 (86.3°). The bond angle of
N1-Cd1-N2 (116.2°) is smaller than that in compound 1.
The CdI2N2 tetrahedrons are also linearly linked together by
intermolecular hydrogen bonding and π-π stacking interac-
tion. There are three different hydrogen bondings, as shown
in Figure 5a. The hydrogen bondings of N1-H1b‚‚‚I2 (x, y
- 1, z) and N1a-H1ba‚‚‚I1 (x, y - 1, z) turn out to be
different and weaker, while in compound 1 they are identical.
The bond lengths of H1a‚‚‚I2 (x, y + 1, z) and H2a‚‚‚I1 (x,
y + 1, z) are 3.158 and 3.209 Å, respectively. Note the
hydrogen bonding of N1-H1b‚‚‚I1 (x, y + 1, z) is a new
one with a much smaller bond length of H1b‚‚‚I1 (2.852
Å). The π-π stacking interaction (3.345 Å) occurs between
the phenyl/phenyl rings of the PEA ligands, and the
interaction is somewhat stronger than that in compound 1.
However, the distance of adjacent Cd atoms (5.137 Å) in a
single CdI2 chain is fairly longer than that in compound 1
(4.846 Å). There is no interchain hydrogen bonding due to
the increased distance between the nearest two CdI2 chains.
Figure 6. Solid-state diffuse reflectance spectra of compounds 1-3 at
room temperature.
0D structure of CdI2 in compound 2, while compound 1 has
a 1D structure of CdI2 chains.26 In the diffuse reflectance
spectrum of compound 3, there are two absorption edges
from the onsets of the two linear increases with the values
of 2.92 and 4.38 eV, which correspond to the Eg values of
CdS27 and the matrix of CdI2(PEA)2, respectively. Note that
although the content of CdS is low, the absorption coefficient
is as high as 1.6 × 105 cm-1.28 As a result, the signal of
CdS nanoparticles in the solid-state diffuse reflectance
spectrum is detectable.
Photoluminescence Spectroscopy. The photolumines-
cence spectra of compounds 1-3 were investigated in solid
state at room temperature. As shown in Figure 7, compounds
1-3 exhibit blue, green, and orange emissions in the solid
states at room temperature, respectively. Compound 1 has a
strong emission band centered at 465 nm with an excitation
band at 375 nm. Compound 2 exhibits a strong emission
maximum at 512 nm (λex ) 432 nm) which is red-shifted
by 47 nm compared with that of compound 1. The difference
in emission energies is most likely due to the iodide ligands
acting as different roles (a bridging or terminal ligand) for 1
and 2, respectively. In this context, the lower p-orbital energy
for bridging iodide compared with the terminal one may be
responsible for the blue shift in emission energies from 2
(512 nm) to 1 (465 nm). The emission spectrum of compound
3 features an overlapping band formed by two bands centered
at 480 and 577 nm upon photoexcitation at 406 and 365 nm
(Figure 7c). The emission band centered at 480 nm might
be originated from the CdI2(PEA)2 matrix and the other
emission band centered at 577 nm corresponds to the
emission of CdS nanoparticles with the diameter varying
from 5 to 40 nm in the solid state.29
It is amazing that such large variations in the overall crystal
structure are caused by the small amount of dopant. Recently,
a similar phenomenon was found in rhodamine 6G adsorbed
on silver nanoparticles.25 The angles between the xanthene
plane and the phenyl substituent of rhodamine 6G are
different when the molecules are adsorbed on two different
adsorption sites. Enlightened by this work, we suggested a
possible mechanism for the distorted configuration of CdI2-
(PEA)2 matrix in compound 3. As CdS has a smaller
solubility than CdI2(PEA)2, it precipitated from the solution
first and formed nanoparticles. Some CdI2(PEA)2 molecules
may be adsorbed on the surface of the CdS nanoparticles
and begin to crystallize. Because of the spatial hindrance of
the CdS nanoparticles, the dihedral angle of two PEA groups
turns out to be smaller than that in compound 1.
Thermal Analysis. Thermal gravimetric analyses (TGA)
were performed to determine the thermal stability of these
compounds (as shown in Figure S2 of the Supporting
Information). Compounds 1-3 thermally decompose by
simple loss of the organic ligands, followed by sublimation
of the metal halides. In compounds 1 and 3, the PEA ligand
is removed approximately at 120 °C, while in compound 2
the PEA ligand appears to be lost at a lower temperature of
91 °C due to the relatively longer bond length of Cd-N
(2.372-2.393 Å). The cadmium iodide portion sublimes at
approximately 400 °C for all the three compounds.
We have also measured the time-resolved radiative decays
of compounds 1-3 in solid state (as shown in Figure S3 of
the Supporting Information). The radiative decay parameters
including excitation and emission wavelengths, lifetimes,
fractions, and average lifetimes are shown in Table 3. The
decay curves of compounds 1 and 2, and the CdI2(PEA)2
matrix of compound 3 are biexponetial, with lifetimes of
UV-Vis Spectroscopy. The solid-state diffuse reflectance
spectra of compounds 1-3 were investigated at room
temperature as shown in Figure 6. The band gap energy (Eg)
value was determined by extrapolation from the linear portion
of the absorption edge. The Eg values of compounds 1 and
2 are 4.36 and 4.45 eV, respectively. The larger Eg value of
compound 2 compared with compound 1 may be due to the
(26) Papavassiliou, G. C. Prog. Solid State Chem. 1997, 25, 125.
(27) (a) Xie, R.; Kolb, U.; Li, J.; Basche¨, T.; Mews, A. J. Am. Chem. Soc.
2005, 127, 7480. (b) Tiwari, S.; Tiwari, S. Cryst. Res. Technol. 2006,
41, 78.
(28) Matsumoto, H.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem.
1996, 100, 13781.
(25) Zhao, J.; Jensen, L.; Sung, J.; Zou, S.; Schatz, G. C.; Van, Duyne, R.
(29) Wang, W.; Germanenko, I.; El-Shall, M. S. Chem. Mater. 2002, 14,
3028.
P. J. Am. Chem. Soc. 2007, 129, 7647.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10257

Wang et al.
Figure 7. Photoluminescence spectra of compounds 1 (a), 2 (b), and 3 (c) in the solid state at room temperature Inset: excitation bands.
Table 3. Radiative Decay Parameters (Lifetimes, Fractions, and Average Lifetimes and Excitation and Emission Wavelengths) for Compounds 1-3
compd
excitation wavelength/nm
emission wavelength/nm
τ1/ns
τ2/ns
R1/%
R2/%
τav a/nsa
ø b
1
2
3
375
432
406
365
465
512
450
660
1.445
1.704
1.508
6.096
4.353
5.759
43.68
75.94
66.53
100
56.32
24.06
33.47
4.06
2.34
2.93
1.073
1.008
1.111
1.119
174.6
b
a τav ) (R1τ1 + R2τ2)/(R1+ R2). Goodness of fit parameter; ø2 ) 1 for a good fit.
1.445 (43.68%) and 6.096 ns (56.32%) for compound 1,
Typically, the emission mechanisms of cadmium halides
with N-donor ligands are difficult to be attributed due to
many factors.33 However, the theoretical studies can give us
a clear understanding of the photophysical properties of these
compounds.34 We performed theoretical studies of ground
and excited electronic states of compounds 1-3. For the
calculated ground-state geometries, the electronic structure
is examined in terms of the highest occupied molecular
orbitals (HOMOs) and lowest unoccupied molecular orbitals
(LUMOs). The nature of the low-lying excited states is then
explored using TD-DFT approach to derive both absorption
and emission spectra, which are compared with the above
spectroscopic data.
Figure 8 illustrates the characteristics of the LUMOs and
HOMOs of compounds 1 and 2 and the CdI2(PEA)2 matrix
of 3. Apparently, there is a large disparity in the distribution
of the frontier orbitals between compounds 1 and 2. For
compound 1, the electron densities of the HOMO are located
on the iodine atoms, whereas those of the LUMO are
distributed mainly on the cadmium atom, indicating that the
lowest electronic transition might be attributed to iodide-to-
1.704 (75.94%) and 4.353 ns (24.06%) for compound 2, and
1.508 (66.53%) and 5.759 ns (33.47%) for the CdI2(PEA)2
matrix of 3. The average lifetimes τav 30 are obtained out of
the two decay components for compounds 1, 2, and the CdI2-
(PEA)2 matrix of 3 and are determined to be 4.06, 2.34, and
2.93 ns, respectively, as shown in Table 3. The nanosecond
scale of the decay times point out that all the above emission
bands are from singlet excited states. Notice that the average
lifetime of the CdI2(PEA)2 matrix of compound 3 is shorter
than that of compound 1 due to the doped CdS nanoparticles.
In addition, the emission at 660 nm from the doped CdS
decays according to a single-exponential function and the
lifetime is 174.6 ns, which is longer than the former reported
lifetimes of undoped CdS (93-117 ns).31 The changes of
the lifetimes of the CdI2(PEA)2 matrix and CdS dopant in
compound 3 strongly indicate the existence of energy transfer
from the CdI2(PEA)2 matrix to the doped CdS nanopar-
ticles.32
(30) Kishore, V. V. N. R.; Narasimhan, K. L.; Periasamy, N. Phys. Chem.
Chem. Phys. 2003, 5, 1386.
(31) Lakowicz, J. R.; Gryczynski, I.; Gryczynski, Z.; Murphy, C. J. J. Phys.
Chem. B 1999, 103, 7613.
(32) Jiu, T.; Li, Y.; Gan, H.; Li, Y.; Liu, H.; Wang, S.; Zhou, W.; Wang,
C.; Li, X.; Liu, X.; Zhu, D. Tetrahedron 2007, 63, 232.
(33) Yu, J.; Ye, L.; Bi, M.; Hou, Q.; Zhang, X.; Xu, J. Inorg. Chim. Acta
2007, 360, 1987.
(34) Xiao, B.; Hou, H.; Fan, Y.; Tang, M. Inorg. Chem. Commun. 2007,
10, 376.
10258 Inorganic Chemistry, Vol. 46, No. 24, 2007

CdI2/PEA Hybrid Materials with Controlled Structures
figurations, wherein a linear combination of several occupied-
to-virtualMOexcitationscomprisesagivenopticalabsorption.36
Assignment of the characteristics of each excited state was
based on the compositions of the occupied and virtual MOs
of the dominant configurations for that excited state. For
compound 1 and the CdI2(PEA)2 matrix of compound 3, all
of the selected excited states are designated to be XMCT,
since all the involved occupied orbitals are iodine-based and
all the involved virtual orbitals are metal-based.37 For
compound 2, although the first singlet excited state corre-
sponding to the transition from HOMO to LUMO is
forbidden (au f au) due to the spatial symmetry selection
rules,38 the other selected excited states with greatest oscil-
lator strengths are designated to be XLCT, because all of
the involved occupied orbitals are iodine-based but all the
involved virtual orbitals are PEA-based π*.39 That is
consistent with the deduction from the electronic structures
of ground states. The calculated lowest energy absorption
peaks are 293 nm (4.24 eV) for compound 1, 274 nm (4.53
eV) for compound 2, and 292 nm (4.25 eV) for the CdI2-
(PEA)2 matrix of compound 3, which are in good agreement
with the experimental energy gaps for these three materials
from absorption spectra.
Figure 8. Surfaces of HOMOs and LUMOs in compounds 1 [CdI2(PEA)2],
2 [CdI2(PEA)4], and the matrix of 3 [CdI2(PEA)2] obtained at the B3LYP
level. All the MO surfaces correspond to an isocontour value of |ψ| )
0.03 au.
cadmium charge transfer (XMCT).35 In contrast, for com-
pound 2, although the electron densities of the HOMO are
also distributed on the iodine atoms, those of the LUMO
are located at the four phenyl rings of PEA ligands,
suggesting that the lowest electronic transition might be
dominated by halide-to-ligand charge transfer (XLCT).9 The
PEA ligand of compound 2 can be considered as a good
electron acceptor for XLCT. We have also tried to use
alkylamines (such as methylamine, ethylamine, and n-
butylamine) instead of PEA as the ligands, but the products
are optically inert. To further clarify the excited-state nature
of these hybrid materials, we have also performed theoretical
studies of CdI2(MA)4 (MA ) methylamine). For CdI2(MA)4,
the electron densities of the HOMO are located on the iodine
atoms and those of the LUMO mainly on the cadmium atom
(as shown in Figure S4), indicating the XMCT mechanism.
Compared with PEA, methylamine is a weak acceptor and
not suitable for the XLCT mechanism which exists in CdI2-
(PEA)4. Although the CdI2(PEA)2 matrix of compound 3 has
some distortion in one PEA ligand compared with compound
1, the electron densities of the HOMO and LUMO exhibit
almost the same distribution as that in compound 1, explain-
ing that the high-energy emission band of compound 3 might
also result from the XMCT of the CdI2(PEA)2 matrix.
Conclusion
This paper reports the synthesis and properties of three
kinds of organic-inorganic hybrid materials based on CdI2
and PEA (phenethylamine). By controlling the stoichiometric
ratio used, CdI2(PEA)2 (1) and CdI2(PEA)4 (2) with quite
different structures were successfully synthesized. By intro-
ducing a small amount of Na2S to the reaction mixture, CdS
nanoparticles can be easily doped to the CdI2(PEA)2 matrix
to obtain a new compound 3 with the formula of [CdI2-
(PEA)2](CdS)0.038. The existence of CdS nanoparticles give
an amazing effect on the crystal structure of the CdI2(PEA)2
matrix of compound 3. Compounds 1-3 display bright blue,
green, and orange emissions in the solid state at room
temperature, respectively. The molecular orbital calculations
make it clear that the emission of compound 1 and the CdI2-
(PEA)2 matrix of compound 3 are originated from a singlet
“CdI2-centered” halide-to-metal charge transfer (XMCT)
excited state, while the emission of compound 2 is designated
to a halide-to-ligand charge transfer (XLCT). It is worthwhile
to note that the complex emission of compound 3 might be
ascribed to a mixture of the CdI2(PEA)2 matrix and the doped
CdS nanopaticles. Our work proves that the structures and
photoelectronic properties of the organic-inorganic hybrid
materials can be finely tuned by controlling the stoichiometric
ratio used and doping inorganic semiconductor nanoparticles.
What is more, the synthesis of compound 3 provides a new
and simple method to prepare CdS nanoparticles.
TDDFT calculations are employed to examine the low-
lying singlet excited states of compounds 1 and 2, and the
CdI2(PEA)2 matrix of compound 3. Selected three singlet
excited states with the greatest oscillator strengths are
displayed in Table S2 of the Supporting Information. The
energy of each excited state is vertical excitation energy in
electronvolts from the ground state. The excited-state elec-
tronic structures are best described in terms of multicon-
(36) Nelson, S. F.; Weaver, M. N.; Yamazaki, D.; Komatsu, K.; Rathore,
R.; Bally, T. J. Phys. Chem. A 2007, 111, 1667.
(37) Bardaji, M.; Calhorda, M. J.; Costa, P. J.; Jones, P. G.; Laguna, A.;
Pe´rez, M. R.; Villacampa, M. D. Inorg. Chem. 2006, 45, 1059.
(38) Steiner, E.; Fowler, P. W. J. Phys. Chem. A 2001, 105, 9553.
(39) Zhao, Q.; Liu, S.; Shi, M.; Wang, C.; Yu, M.; Li, L.; Li, F.; Yi, T.;
Huang, C. Inorg. Chem. 2006, 45, 6152.
(35) Wu, T.; Li, D.; Ng, S. W. CrystEngComm 2005, 7, 514.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10259

Wang et al.
inorganic frameworks of compounds 1 and 3, TGA curves of
compounds 1-3, time-resolved radiative decays of compounds 1-3,
theoretical calculation results of CdI2(MA)4 (MA ) methylamine),
hydrogen bonding of compounds 1-3, and TDDFT calculation
results of compounds 1-3. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (Grant Nos.
50433020 and 50403001), the National Basic Research
Program of China (Grant No. 2006CB806200), and the
National Science Foundation of the United States (Grant
CHE-06-16849).
Supporting Information Available: Listings of X-ray crystal-
lographic data in CIF format for compounds 1-3, packing of the
IC7007304
10260 Inorganic Chemistry, Vol. 46, No. 24, 2007