Effects of the π-conjugation length of bipyridyl ligand on the photophysical properties of binuclear organotin(IV) complexes: Synthesis and characterization of dimethyltin(IV) complexes with bipyridyl

Article abstract of DOI:10.1016/j.ica.2014.02.032

The dimethyltin(IV) complexes of rigid bipyridyl-based ligands, 4,4′-bipyridine (4,4′-bipy) and 1,4-bis(4-pyridyl)-2,3-diaza-1,3- butadiene (4-bpdb), were prepared and characterized by ¹H, ¹³C and ¹¹⁹Sn NMR, IR, and UV spectroscopies, single-crystal X-ray diffraction and elemental analysis. The prepared complexes were used as green and yellow-orange electroluminescence dopant materials to fabricate organic light-emitting diodes (OLEDs) with a general configuration of ITO/PEDOT:PSS(90 nm)/PVK:PBD:tin-complexes (80 nm)/Al(200 nm). The effects of long and rigid bipyridyl-based ligands on the electroluminescence properties of the complexes were investigated. The maximum emission peaks showed that a good correlation exists between the π-conjugation length of bipyridyl ligands and emission wavelength. Replacing 4-bpdb with the long π-conjugation length, 4,4′-bipy, resulted in red shifts in both absorption and fluorescence spectra and moderately enhanced the photoluminescence intensity and the fluorescence quantum yield. Furthermore, complex of dimethyltin(IV) with 4-bpdb ligand was doped into a PVK:PBD blend as the host in two different concentrations. It was found that the high concentration of dopant caused a trap effect in the current voltage and led to a self-quenching of emission and reduced the efficiency of the device.

Full text of DOI:10.1016/j.ica.2014.02.032

Inorganica Chimica Acta 415 (2014) 52–60
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Effects of the
p-conjugation length of bipyridyl ligand on the
photophysical properties of binuclear organotin(IV) complexes:
Synthesis and characterization of dimethyltin(IV) complexes
with bipyridyl
b
b
c,d
Ezzatollah Najafi a, Mostafa M. Amini a, , Mohammad Janghouri , Ezeddine Mohajerani , Seik Weng Ng
⇑
a Department of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113, Iran
b Laser Research Institute, Shahid Beheshti University, G.C., Tehran 1983963113, Iran
c Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
d Department of Chemistry, Faculty of Science, King Abdulaziz University, PO Box 820203, Jaddah, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
The dimethyltin(IV) complexes of rigid bipyridyl-based ligands, 4,40-bipyridine (4,40-bipy) and 1,4-bis(4-
pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb), were prepared and characterized by 1H, 13C and 119Sn NMR,
IR, and UV spectroscopies, single-crystal X-ray diffraction and elemental analysis. The prepared com-
plexes were used as green and yellow-orange electroluminescence dopant materials to fabricate organic
light-emitting diodes (OLEDs) with a general configuration of ITO/PEDOT:PSS(90 nm)/PVK:PBD:tin-com-
plexes (80 nm)/Al(200 nm). The effects of long and rigid bipyridyl-based ligands on the electrolumines-
cence properties of the complexes were investigated. The maximum emission peaks showed that a good
Article history:
Received 7 November 2013
Received in revised form 5 February 2014
Accepted 22 February 2014
Available online 11 March 2014
Keywords:
Tin(IV) complex
Crystal structure
Photoluminescence
Electroluminescence
OLED
correlation exists between the
Replacing 4-bpdb with the long
p
p
-conjugation length of bipyridyl ligands and emission wavelength.
-conjugation length, 4,40-bipy, resulted in red shifts in both absorption
and fluorescence spectra and moderately enhanced the photoluminescence intensity and the fluores-
cence quantum yield. Furthermore, complex of dimethyltin(IV) with 4-bpdb ligand was doped into a
PVK:PBD blend as the host in two different concentrations. It was found that the high concentration of
dopant caused a trap effect in the current voltage and led to a self-quenching of emission and reduced
the efficiency of the device.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
for EL applications. However, little attention has been focused on
the EL properties of the group IVA metal complexes, in general,
Organic electroluminescent (EL) devices have recently received
a considerable amount of attention since they can be utilized to
produce emissions of all colors in accordance with a wide selection
of organic emitting materials [1–5]. One of their possible applica-
tions is in the fabrication of large-area light-emitting displays,
which are difficult to achieve using inorganic light-emitting cells.
Many attempts have been made to design and synthesize a wide
variety of coordination compounds as electronic materials for the
fabrication of low-cost OLED devices in convenient processes
[6–8]. Choosing a suitable ligand and metal is very important for
attaining a well-designed coordination compound with good appli-
cability in the fabrication of OLEDs [9]. A large number of metal
complexes of main and transition metal ions have been reported
and tin(IV) complexes, in particular [10–13]. The coordination
chemistry of tin is very interesting because of its variable valence
(divalent and tetravalent) states and structural diversity. Further-
more, tin has a higher work function than the group IIB and IIIA
elements due to the higher ionization potential of tin complexes
than those of other metal complexes [14,15]. Therefore, the syn-
thesis of tin(IV) complexes containing conjugated ligands (well-
known ligands in the field of photonic) should provide a basis for
luminescent studies and that could provide an opportunity to uti-
lize them in fabrication of OLED devices. It was found that the EL
properties of OLEDs are strongly coupled to the molecular struc-
tures of the dopants in the OLED devices [16–19]. To the best of
our knowledge, the relationship between the electroluminescent
properties of OLEDs and the structure of tin complexes has not
been reported. We are interested in learning how the structures
of the tin complexes influence the OLED’s brightness and in
⇑
Corresponding author. Tel.: +98 21 29903109; fax: +98 21 2243166.
E-mail address: m-pouramini@sbu.ac.ir (M.M. Amini).
http://dx.doi.org/10.1016/j.ica.2014.02.032
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

E. Najafi et al. / Inorganica Chimica Acta 415 (2014) 52–60
53
evaluating the electroluminescence performance of the tin com-
plex-doped OLEDs devices. In continuation of our interest in the ef-
fect of the structures of tin complexes on the electroluminescence
(EL) properties of the organic light-emitting diode (OLED), we
chose two long-chain rigid bipyridyl-based ligands as primary
ligands, 4,40-bipyridine (4,40-bipy) and 1,4-bis(4-pyridyl)-2,3-dia-
za-1,3-butadiene (4-bpdb), and reacted them with dimethyltin(IV)
dichloride to prepare two new organotin complexes. Furthermore,
in order to enhance the optical properties of tin complexes,
cupferron was used as an auxiliary ligand in complexes. Then,
the prepared complexes were utilized as green and yellow-orange
EL dopant materials to fabricate OLED devices with a general
configuration of ITO/PEDOT:PSS(90 nm)/PVK:PBD:tin-complex
(80 nm)/Al(200 nm). Furthermore, we investigated the electronic
state energy levels (HOMO/LUMO) of the complexes via cyclic
voltammetry measurements, current–voltage curves (I–V), and
photoluminescence in solutions. From these data, the relationship
between the structures of tin compounds and the performance of
the devices was accomplished. A photophysical study of the pre-
pared complexes revealed that the intensity of the fluorescence
band of the complex containing the long conjugate rigid ligand
(2) was higher than the band of the complex with the short conju-
gate ligand (1). In addition, both absorption and emission peaks of
the complex 2 red shifted significantly in comparison with those of
the complex 1.
refinement of diffraction data from 4335 for 1 and 4882 for 2 un-
ique reflections. Data were collected to a maximum 2h value of
27.6° for 1 and 27.56° for 2. The numerical absorption coefficient,
l
, for Mo Ka
radiation is 1.243 mmꢀ1 for 1 and 1.158 mmꢀ1 for
2. A numerical absorption correction was applied using SADABS soft-
ware [23]. The data were corrected for Lorentz and Polarizing ef-
fects. The structures were solved by direct method and
subsequent difference Fourier map and then refined on F2 by a
full-matrix least-squares procedure using anisotropic displace-
ment parameters methods [24]. Subsequent refinement then con-
verged with R factors, and parameters errors significantly better
than for all attempts to model the solvent disorder. All refinements
were performed using the SHELXL97 crystallographic software pack-
age [25].
2.3. Synthesis of complexes
2.3.1. Preparation of [
l
-(4,40-bipy){Me2Sn(cupf)2}2]ꢁEtOH (1)
All manipulations were performed under an atmosphere of dry
nitrogen using standard Schlenk line technique. For preparation of
title complex, a solution of 4,40-bipyridine (0.16 g, 1.0 mmol) in
ethanol (20 mL) was added to a solution of dimethyltin(IV) dichlo-
ride (0.22 g, 1.0 mmol) in ethanol (10 mL). A white precipitation
was rapidly formed. The mixture was stirred for 10 min at room
temperature and then NH4[PhN(O)NO] salt (0.31 g, 2.0 mmol)
was added to the reactor. The NH4Cl promptly precipitated and
the resulting yellow solution was stirred for one hour at room tem-
perature. The precipitate was filtered off and the mother liquor was
left to evaporate slowly at room temperature. After one week,
yellow crystals of title complex were isolated (yield 75%; m.p.
130–132 °C). Anal. Calc. for C40H46N10O9Sn2: C, 45.83; H, 4.42; N,
2. Experimental
2.1. Materials and physical measurements
Exception for the 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene
(4-bpdb) ligand which was prepared according to the literature
procedures [20], all other reagents were purchased from Merck
and used without further purification. All solvents were dried
and distilled under a nitrogen atmosphere prior to use, according
to a standard procedure [21]. Melting point was obtained with
an Electrothermal 9200 melting point apparatus and is not cor-
rected. Infrared spectra from 250 to 4000 cmꢀ1 were recorded on
a Shimadzu 470 FT-IR instrument, using KBr pellets. NMR spectra
were recorded at room temperature in DMSO on a Bruker AVANCE
300-MHz operating at 300.3 MHz. Elemental analyses was per-
formed with a Thermo Finnigan Flash-1112EA microanalyzer. The
thermal analysis (TGA-DTA) was carried out on a Bahr STA-503
instrument under air atmosphere. The thickness of the samples
was measured using a DekTak 8000 profilometer. The electrolumi-
nescence and photoluminescence spectra of fabricated OLEDs were
obtained on a HR4000 Oceanoptic and USB2000 spectrometers,
respectively. The current–voltage and luminance were checked
by Keithley 2400 and Minolta Luminance meter LS110, respec-
tively. Quantum yields were determined using quinine sulfate in
13.36. Found: C, 45.79; H, 4.39; N, 13.45%. IR (KBr, cmꢀ1):
O–H), 3480; (w, Cph–H), 3075; (m, C–H), 2920; (m, C@C),
1465; (m, N–N), 1334; (s, N@O), 1210; (s, ONNO), 1055; (m,
Sn–C), 520; (Sn–N), 461;
(Sn–O), 400. 1H NMR (DMSO, ppm):
m(m,
m
m
m
m
m
m
m
m
m
0.61 (6H, s, Sn-CH3,2J117/119Sn-H = 116 Hz), 7.38–8.74 (14H, m,
C6H5 and C5H5N). 13C NMR (DMSO-d6, ppm): 9.3 (CH3, 1J117/
119Sn–13C = 1097), 118.3 (C4, C8, C10, C14), 120.8 (C16, C18),
124.5 (C5, C7, C11, C13), 129.3 (C6, C12), 142.2 (C17), 148.4 (C15,
C19), 151.2 (C3, C9). 119Sn NMR (DMSO-d6, ppm): ꢀ382.5.
2.3.2. Preparation of [
l
-(4-bpdb){Me2Sn(cupf)2}2]ꢁMeOH (2)
Compound 2 was synthesized in the same way as compound 1
using 4-bpdb in instead of 4,40-bipyridine (yield 82%; m.p. 178–
180 °C). Anal. Calc. for C41H46N12O9Sn2: C, 45.25; H, 4.26; N,
15.44. Found: C, 45.48; H, 4.01; N, 15.91%. IR (KBr, cmꢀ1):
m
(m,
(m, C = C),
(s, ONNO), 1049;
(Sn–O), 402. 1H NMR (DMSO,
2 117/119Sn–H = 116 Hz), 7.79–8.75
(30H, m, C6H5 and C5H5N). 13C NMR (DMSO, ppm): 8.6 (CH3,
1 117/119Sn–13C = 1097 Hz), 117.2 (C4, C8, C10, C14), 120.7 (C16,
O–H), 3370;
14735; (m, N–N), 1301;
(m, Sn–C), 516; (Sn–N), 459;
ppm): 0.48 (12H, s, Sn–CH3,
m(w, Cph–H), 3065;
m(m, C–H), 2906; m
m
m
(s, N@O), 1228; m
m
m
m
J
1.0 N H2SO4 (/ = 0.54). UV–Vis spectra were recorded on
a
Shimadzu 2100 spectrometer. The ionization potential of tin com-
plexes was determined by a photoemission apparatus (Riken Keiki
AC).
J
C18), 126.1 (C5, C7, C11, C13), 126.9.3 (C6, C12), 143.1 (C17),
149.1 (C15, C19, C20), 150.1 (C3, C9). 119Sn NMR (DMSO, ppm):
ꢀ371.
2.2. X-ray crystallography
2.4. Fabrication of OLED by utilization of the prepared complexes
Single-crystal X-ray diffraction data were collected on a Bruker
SMART APEX with graphite monochromated Mo-K
100 K using APEX2 software [22]. For
-(4,40-bipy){Me2-
Sn(cupf)2}2]ꢁEtOH complex (1), a yellow prismatic crystal with a
-(4-bpdb){Me2-
Sn(cupf)2}2]ꢁMeOH complex (2), a yellow block crystal with a
dimension of 0.30 ꢂ 0.25 ꢂ 0.20 mm were mounted on a glass fiber
and used for data collection. Cell constants and an orientation
matrix for the data collection were obtained by least-squares
a
radiation at
Fig. S1 (Electronic Supplementary information; ESI) shows the
line drawing structures of the prepared complexes and axillary
materials that used for fabrication of OLED device. The structure
of the fabricated device is as follows: ITO/PEDOT:PSS(90 nm)/
PVK:PBD(80 nm)/Al(200 nm) and ITO/PEDOT:PSS(90 nm)/PVK:
PBD:tin complexes (80 nm)/Al(200 nm). The final sample structure
is shown in Fig. 1. The first step in the fabrication process was
cleaning ITO substrates by detergent, acetone, dichloromethane,
[
l
dimension of 0.25 ꢂ 0.25 ꢂ 0.25 mm and for [
l

54
E. Najafi et al. / Inorganica Chimica Acta 415 (2014) 52–60
Fig. 1. Layer arrangement and layer thickness of the OLED devices.
dichloroethane, ethanol, methanol and deionized water in ultra-
sonic bath. PEDOT:PSS as a hole-injection layer was then spin
coated on a clean ITO substrate at thickness of 90 nm. The coated
layer was then baked for one hour at 120 °C to reduce the surface
roughness. PVK as a hole-transporting material and PBD as an elec-
tron-transporting material were doped with tin complexes. The ra-
tio of tin complex for each type was 10 wt% in PVK:PBD (100:40).
PVK, PBD and tin complexes with the ratio of 100:40:10 were
blended in DMF and then spin coated and baked at 80 °C for 1 h.
Finally, a metallic cathode of Al was deposited on the emissive
layer at 8 ꢂ 10ꢀ5 mbar by thermal evaporation. The active area of
the prepared samples was 6 ꢂ 6 mm2.
1H NMR spectra of the complexes showed the expected aliphatic
and aromatic peaks with the correct integration, and the 2J117/
119Sn–H coupling constant of 116 Hz for both complexes is consis-
tent with pentagonal bipyramidal geometry about the tin atom.
Based on the magnitude of J
2 117/119Sn-H, the C-Sn-C angle is esti-
mated to be 171° for both compounds. In the 13C NMR spectra of
the compounds, the signals of ligands are shifted only slightly from
their position in the spectra of the free donor ligands which can be
attributed to the coordination of cupferron and bipyridyl ligands to
tin and formation of Sn–N and Sn–O bonds.
3.3. Single-crystal X-ray analyses
3. Results and discussion
The molecular structures of 1 and 2 were established with sin-
gle-crystal X-ray diffraction analysis and results are shown in
Figs. 2 and 3 along with the atom numbering schemes. Crystal data
and refinement details and selected bond distances and angles for
1 and 2 are listed in Tables 1–3.
3.1. Synthesis
Both complexes were isolated in moderate yields from the reac-
tion of dimethyltin(IV) dichloride, corresponding bipyridyl ligand
and cupferron, and were fully characterized by various techniques.
These reactions were fast and facile, and their progress was moni-
tored through the appearance of fluorescence colors. The elemental
analyses and the IR and NMR spectral data of the 1 and 2 are in
Compound 1 crystallized in the monoclinic system in the space
group of P21/n. The structure of complex consists of centrosymmet-
ric binuclear [(CH3)2Sn(O2N2(C6H5)2]2.4,40-bipy units, which the
asymmetric unit contains one independent tin atom. The coordina-
tion geometry around the tin atom is distorted pentagonal bipyra-
midal and the tin hepta-coordinated axially by two tin-bound
methyl carbon atoms and equatorially by the O4N donor set of
the cupferronato and the bipy ligands. The sum of the bond angles
of O(1)–Sn(1)–O(2), 67.95(5)°; O(1)–Sn(1)–O(4), 74.97(5)°; O(4)–
Sn(1)–O(3), 66.92(5)°, O(3)–Sn(1)–N(5) 74.47(6)° and O(2)–
Sn(1)–N(5), 75.88(6)° is 360.1°. This shows that the Sn(1), O(1),
O(2), O(3), O(4) and N(5) atoms are coplanar. Therefore, the coor-
dination at Sn(1) atom can be best described as a slightly distorted
pentagonal bipyramidal geometry. The nitrosohydroxylaminato
groups of the cupferronato anions are essentially planar, while
the phenyl ring is nearly coplanar with the plane of the N(O)NO
group, in spite of some variation in torsion angles of O(2)–N(2)–
C(3)–C(8) and O(4)–N(4)–C(9)–C(10). In the structure of 1, each
half of molecule contains two SnO2N2 units as five-membered che-
late rings (A and B (Fig. 2)) with O–Sn(1)–O bite angles of 67.95(5)°
(A) and 66.92(5)° (B). The four Sn–O bond lengths around the Sn(1)
are not equal. The lengths of the two Sn–O bond in five-membered
accordance with the formulas [
and [
l
-(4,40-bipy){Me2Sn(cupf)2}2]ꢁEtOH
l
-(4-bpdb){Me2Sn(cupf)2}2]ꢁMeOH, respectively. The pre-
pared complexes are stable in air and soluble in chloroform and
can be accumulated at room temperature for an indistinct period
of time. The good stability of the complexes is probably associated
with the overall molecule, which is linked together by covalent Sn-
O and Sn-N bonds that provide adequate thermodynamic and ki-
netic stability for existence in the solid state.
3.2. General characterization
In the IR spectra of the prepared complexes, the absence of a
stretching vibration at the 3160 cmꢀ1 (associated with the ammo-
nium salt of the cupferron ligand) clearly indicates the formation of
the Sn–O bonds. The stretching vibration band of Sn–O is impor-
tant data and can be related to the bonding mode of the ligand.
When the cupferronato anion coordinates to the tin center in the
common bidentate chelating mode, the
the 399–406 cmꢀ1 range, while, for a bridging coordination pat-
tern, the
(Sn–O) band appears around 389 cmꢀ1 [26]. In 1 and 2,
m
(Sn–O) band appears in
chelate ring A are nearly equal (
membered chelate ring B are slightly different (
D
= 0.003 Å), while those in five-
= 0.048 Å). The
D
m
Sn–O bond lengths are similar to such bond lengths found in other
diorganotin(IV) cupferronato complexes [27]. The Sn–N bond
length of 2.415(18) Å is slightly longer than the 2.402(2) Å ob-
served in the polymeric [Me2SnCl2(4,40-bipy)]n complex [28]. All
Sn–C bond lengths in the title compound almost are identical,
and they are in the typical range of such bonds lengths found in
diorganotin(IV) complexes.
this band was observed at about 400 cmꢀ1. The new band at about
461 cmꢀ1 in comparison with the IR spectra of the free bipyridyl li-
gands was assigned to the Sn–N vibration, and this unambiguously
proved the coordination of nitrogen to tin. The presence of a single
Sn–C stretching vibration band at about 520 cmꢀ1 in the infrared
spectra of the complexes indicates that the C–Sn–C bond angle is
close to 180°. This is in accordance with the X-ray diffraction re-
sults. For the determination of the structural features of 1 and 2
in solution the 1H, 13C and 119Sn NMR spectra were recorded. The
Complex 2 crystallized in the triclinic system in the space group
ꢀ
of P1. As can be seen in Fig. 3, crystal structure of 2 is similar to that
of 1. Although there are many main and transition metal

E. Najafi et al. / Inorganica Chimica Acta 415 (2014) 52–60
55
Fig. 2. Molecular structure of 1. Thermal ellipsoids are drawn at 40% probability.
Fig. 3. Molecular structure of 2. Thermal ellipsoids are drawn at 40% probability.
complexes of bipyridil ligand [29–34], to the best of our knowl-
edge, 2 is the first organotin complex with 1,4-bis(4-pyridyl)-2,3-
diaza-1,3-butadiene ligand. The structural similarity between 1
and 2 is reflected by the practically unaltered O(1L)–Sn(1)–O(2L)
bite angles [68.40(2)° for A and 65.46(5)° for B] and differential
175 °C, respectively. The first mass loss between 80 and 90 °C in
the TGA curve of both complexes, which is accompanied with an
endothermic peak in their DTA curve, is associated with the loss
of lattice-solvent molecules. The second endothermic peak in the
DTA curves of 1 and 2 at 135 and 180 °C, respectively, without
mass loss in their TGA curves is associated with the melting of
complexes. Finally, the distinct mass loss from 270 to 360 °C for
1 and 360 to 500 °C for 2, with a broad exothermic peak in the
same temperature range in their DTA curves, is attributed to the
elimination of ligands. The solid residue formed at 500 °C is prob-
ably SnO2. This investigation shows that the prepared complexes
have good thermal stability. Furthermore, it is interesting to note
that these complexes are soluble in many solvents, including CHCl3
and MeOH. Therefore, the thin films of these complexes can be pre-
pared readily by spin coating in addition to vacuum deposition.
These properties make them potentially good green and yellow-or-
ange emitters in OLEDs.
Sn–O distances (
D = 0.026 Å for A, D = 0.038 Å for B).
The main factor that controls the packing in structures is the
presence of weak intra- and intermolecular interactions. The
hydrogen bonding between the C–H and oxygen and nitrogen
atoms (Tables S1 and S2, ESI), weak face-to-face
p–p stacking
interactions (Fig. S2, ESI) between pyridine rings of dipyridil li-
gands and phenyl rings of cupferronato anion of two adjacent mol-
ecules of 1 and 2 with centroid–centroid distance of 3.97 and
3.70 Å and C– interactions (C–Hꢁ ꢁ ꢁCg distance of 2.82 and 2.75 Å)
caused more growth of the structures to a 2D supramolecular
network.
3.4. Thermal behavior
3.5. Optical properties
Thermal analysis (TGA-DTA) was employed to study the behav-
ior of 1 and 2 during heat treatment. As thermograms of the com-
plexes illustrate (Fig. S3, ESI), 1 and 2 are stable up to ca. 120 and
(Fig. 4) shows the absorption spectra of the prepared complexes
and bare ligands in dichloromethane solution. The UV–Vis spectra

56
E. Najafi et al. / Inorganica Chimica Acta 415 (2014) 52–60
Table 1
Crystal data and refinement details for 1 and 2.
Complex
1
2
Molecular formula
Formula weight
Crystal dimensions (mm)
Crystal color
Crystal system
Space group
a (Å)
C
40H46N10O9Sn2
C42H50N12O10Sn2
1120.32
1048.29
0.25 ꢂ 0.25 ꢂ 0.25
0.30 ꢂ 0.25 ꢂ 0.20
yellow
yellow
monoclinic
P21/n
11.2083(3)
11.5885(3)
16.3461(4)
triclinic
ꢀ
P1
9.4894(2)
10.9004(4)
12.8940(5)
b (Å)
c (Å)
a
(°)
67.664(4)
b (°)
90.113(2)
75.771(3)
c
(°)
69.785(3)
1147.56(8)
V (Å3)
2123.15(9)
Z
2
1
F (000)
1056
566
Limiting indices
Unique reflections
Calculated density (g cmꢀ3
ꢀ13 6 h 6 14, ꢀ11 6 k 6 15, ꢀ20 6 l 6 21
12 6 h 6 12, ꢀ14 6 k 6 14, ꢀ16 6 l 6 16
4335
4882
)
1.64
1.243
2.2–27.6
1.132
1.621
1.158
2.31–27.56
1.042
Absorption coefficient (mmꢀ1
h Range (°)
)
Goodness of fit (GOF) on F2
Data/restraints/parameters
1/18/292
1/0/305
Final R indices [I > 2
r
(I)]
R1 = 0.0253, wR2 = 0.0709
R1 = 0.0237, wR2 = 0.0501
Table 2
Selected bond distances and angles for 1.
Bond lengths (Å)
Bond angles (°)
Sn(1)–C(1)
Sn(1)–C(2)
Sn(1)–O(1)
Sn(1)–O(2)
Sn(1)–N(5)
Sn(1)–O(3)
2.117(3)
2.118(3)
2.2698(15)
2.2668(15)
2.4152(18)
2.3284(17)
C(1)–Sn(1)–C(2)
C(1)–Sn(1)–O(2)
C(2)–Sn(1)–O(2)
C(1)–Sn(1)–O(1)
C(2)–Sn(1)–O(1)
O(2)–Sn(1)–O(2)
179.27(9)
89.67(8)
89.88(8)
92.17(7)
88.20(8)
67.95(5)
Table 3
Selected bond distances and angles for 2.
Bond lengths (Å)
Bond angles (°)
Sn(1)–C(1)
Sn(1)–C(2)
Sn(1)–O(1)
Sn(1)–O(2)
Sn(1)–N(5)
Sn(1)–O(3)
2.125(2)
2.112(2)
2.2628(13)
2.2362(14)
2.4517(16)
2.3075(13)
C(1)–Sn(1)–C(2)
C(1)–Sn(1)–O(2)
C(2)–Sn(1)–O(2)
C(1)–Sn(1)–O(1)
C(2)–Sn(1)–O(1)
O(2)–Sn(1)–O(1)
173.93(8)
91.82(7)
94.07(7)
89.44(7)
94.11(7)
68.40(5)
of 4,40-bipy, 4-bpdb, and cupferron ligands exhibit bands at 273,
282, and 292 nm, respectively. The absorbency maximums of 1
and 2 are at 302 and 334 nm. The observed bands in the UV–Vis
spectra of ligands and complexes can be assigned to
p ?
p⁄ transi-
tions of the aromatic rings [35–38]. Interestingly, the absorption
bands of ligands after coordination to tin red shifted, owing to
the formation of the rigid conjugated systems [39]. This confirms
that the UV spectra of the complexes are essentially reflections of
the absorption of the ligand. The reason for these changes can be
explained by the overlap of the absorption spectra region of the
ligands. This resulted in a red shift of the major
p ?
p⁄ electronic
transition, indicating that the energy of the p⁄ orbitals increased
after the coordination of the nitrogen atom of bipyridyl ligands
to the tin atom.
The photoluminescence properties of both complexes and bare
ligands and the effect of the
p-conjugation length of bipyridyl
ligands on photophysical properties of complexes have been
investigated. Fig. 5 shows the photoluminescence spectra of the
prepared complexes and bare ligands in dichloromethane solution.
The emission spectra of 4,40-bipy, 4-bpdb, and cupferron ligands
Fig. 4. Absorption spectra of 1,
2
and bare ligands in CH2Cl2 solution
(10ꢀ5 mol litꢀ1).

E. Najafi et al. / Inorganica Chimica Acta 415 (2014) 52–60
57
exhibit bands at 492, 528, and 537 nm, respectively. The emission
spectra of the complexes were collected from 430 to 800 nm with
excitation by 100 mW laser at 405 nm. Quantum yields of 0.27 and
0.32 were determined for 1 and 2, respectively, using quinine sul-
fate as the standard in all measurements. The quantum yield of
red shifted with respect to an increase of the
of the fluorescent cores as observed in the absorption spectra.
p
-conjugation length
3.6. Energy levels of prepared complexes
complex 2 with a longer
of complex 1 with a shorter
p
-conjugation length is higher than that
A photoemission apparatus was used for determination of the
ionization potential of tin complexes. A thin film of complexes
takes over a range of energies using a monochromatic light source.
The photocurrent produced from the samples at each energy level
was measured. The ionization potential is found by extrapolating
the photocurrent until it intercepts the energy axis (Table 4). The
electron affinity of tin complexes was appraised by subtracting
the energy gap from the ionization potential, where the energy
gap was determined by the absorption edge of the UV absorption
spectrum of the tin compounds [40,41]. The energy level data for
the tin complexes are shown in Table 4.
p
-conjugation length. According to
the quantum yields and fluorescence spectra of the prepared com-
plexes, both complexes have relatively good fluorescence proper-
ties. As expected, based on the PL characterization, fluorescence
emissions are observed at the longer wavelengths with red chro-
maticity rather than at the absorption spectra. The maximum
emission wavelengths in CH2Cl2 solution at room temperature
for 1 and 2 are 500 and 548 nm, respectively, and they show that
a good correlation exists between the
p-conjugation length of
bipyridyl ligands and emission wavelengths. As can be seen in
Fig. 5, the utilization of 4-bpdb instead of 4,40-bipy in the structure
of 1 enhances the luminescence. In addition, both absorption and
emission peaks of 2 are found to be significantly red shifted in
comparison with those of 1. These results can be explained in
terms of the decreasing HOMO–LUMO energy gap and the exten-
3.7. Electroluminescence characteristic
In order to study the effect of the prepared complexes on the
electroluminescence (EL) properties of the organic light-emitting
diodes (OLEDs) and to demonstrate their potential application in
OLEDs, the prepared complexes were used to fabricate OLED de-
vices with a general configuration of ITO/PEDOT:PSS(90 nm)/
PVK:PBD:tin-complex (80 nm)/Al(200 nm). Fig. 6 shows the nor-
malized electroluminescence spectra of the tin complexes. An
emissive layer without a tin complex was fabricated to record
the EL spectrum of PVK:PBD and to find a relation between the
EL spectra of the tin complex and PVK:PBD EL in order to separate
it from the emission of the tin complex. The EL spectrum of the
PVK:PBD-based device showed a broad peak in the blue region,
and the EL of the 1- and 2-based devices showed a peak in the
green and yellow-orange regions, respectively. The emission char-
acteristics of the devices depended strongly on the structure of the
tin complexes’ dopants. The EL spectral peak shifted from the high-
wavelength region to the low-wavelength region when the bridged
ligand changed from 4-bpdb to 4-bipy. This tendency is similar to
that of the PL spectra of the corresponding tin complexes. In the EL
spectra, the tin complexes showed a long red shift rather than
PVK:PBD EL spectra, demonstrating that effective energy transfer
was taking place in the emissive layer. As driving voltage was
biased to electrodes, the electrons and holes began to inject into
the bulk. Some of the electrons and holes under the electric forces
formed excitons, which emitted light with their annulation be-
tween the HOMO and LUMO energy levels of the layers. The forma-
tion of excimers is possible in PVK molecules, since there is an
overlap between its absorption and emission spectra. This overlap
caused the reduction of EL, which affected the hosting property of
the PVK. The effect of excimer formation is decreased when PBD
was added to PVK. To achieve a good balance of holes and elec-
trons, both holes and electron-transporting functions should be
incorporated into a single bipolar host material [42]. It should be
noted that the high ratio of PBD led to lower stability of the device
due to the decrease in the ratio of PVK. The ratio of
PVK:PBD = 100:40 is suggested as an efficient ratio [43,44], which
reduces the risk of excimer formation and increases the stability
of the device. The use of a PEDOT:PSS interlayer also helped to
establish a better hole injection into the emissive layers, and this
resulted in an increase of current and in the probability of exciton
sion of delocalization of the p-electron system along the backbone.
In comparison to the absorption maximum of 1, the absorption
maximum of 2 is about 32 nm red shifted. This indicates that the
p-electrons are delocalized over the entire conjugated backbone.
The photoluminescence spectrum of 2 exhibited a strong emission
in the green region with the emission maximum being gradually
Table 4
Energy level for the tin complexes.
Complex
Ea (eV)
IP (eV)
Eg (eV)
1
2
3.14
3.13
6.3
6.0
3.16
2.87
Fig. 5. The photoluminescence spectra of 1 and 2 and bare ligands in CH2Cl2
solution (10ꢀ5 mol litꢀ1).

58
E. Najafi et al. / Inorganica Chimica Acta 415 (2014) 52–60
Fig. 8. Current density versus applied voltage for 1 and 2 based devices.
Fig. 6. The electroluminescence spectra of 1 and 2.
level of the tin complexes. Finally, electron–hole recombination oc-
curs in the molecules of the tin complexes. As a result, the green
and yellow–orange electroluminescence bands are observable.
Apparently, an appropriate energy level of PVK:PBD ensures better
charge injection for both hole and electron injection into the emis-
sion layer. Therefore, more holes and electrons can recombine in
the emission layer, and more excitons can form in this layer. Due
to the efficient energy transfer from host to dopant, more emis-
sions from the tin complexes can be obtained.
The electrical characteristics of the tin complexes into PVK:PBD
are shown in Fig. 8. By applying voltage, the electron–hole was in-
jected into the layers, and current density increased. At an applied
voltage of 11 V, the current density of 2 was higher than 1. Appar-
ently, 1 required a larger driving voltage than 2, indicating that the
electron injection barrier increased. This induced the space charge
and prevented further injections of carriers from electrodes to bulk
[45]. The high operating voltage of 1 will shorten the device life-
time due to thermal aging and impurity diffusion under a high
electric field [46].
formation. Complexes 1- and 2-doped PVK:PBD showed emissions
at 525 and 578 nm with FWHM (full width at half maximum) of
12.1 and 10.2 eV, respectively. The EL spectra resembles the PL
spectra of complexes, indicating that emission comes from the tet-
ravalent tin complex singlet excited states, which formed by the
recombination of injected electrons and holes. This can be ex-
plained from the energy level diagram of the multi-layer devices
(Fig. 7). The energy level of HOMO of PVK and tin complexes is
about 5.5 and 6.3 eV and the energy level of LUMO of PVK and
tin complexes is about 2.0 and 3.14 eV, respectively. As Fig. 7
shows, in the energy level diagram of the devices, the HOMO and
LUMO levels of the tin complexes are located in the PVK:PBD
range. This indicates that the injected holes and electrons can di-
rectly recombine on the dopant molecules. The injected holes
and electrons can directly recombine on the tin complexes’ mole-
cules and produce green and yellow-orange lights due to the suit-
able energy level of the PVK:PBD host and the tin complexes’
dopant molecules.
Electrons from the Al are injected into the LUMO level of the
PBD as an electron transport layer and then into the LUMO level
of the tin complexes, while holes from the ITO are injected into
the HOMO level of the PEDOT:PSS and the PVK as the hole-injec-
tion and hole transport layer, respectively, and then into the HOMO
Fig. 9 shows the luminescence–voltage characteristics of the de-
vices. The maximum brightness of emission levels are 900 and
1098 cd/m2 for 1- and 2-based devices, respectively. The device
performance is dependent on the
ligands in tin complexes. The turn-on voltage, defined as the bias
voltage at L = 1 cd/m2, changed with the change of
-conjugation
p-conjugation length of bipyridyl
p
length of bipyridyl ligands. It was lower for complex 2 with the
long conjugate ligand and higher for complex 1 with the short con-
jugate ligand. Importantly, the maximum brightness of the devices
with different tin complex dopants showed a similar structure ef-
fect to the photoluminescence of tin complexes; brighter for the
device with tin complex possessing the long conjugate ligand,
and less bright for the device with tin complex possessing the short
conjugate ligand. These results may provide grounds to consider
new tin complexes as promising materials for the design of effi-
cient OLEDs. Finally, the prepared complexes showed good thermal
stability according to their melting point measurements and ther-
mal analysis, and they also are stable upon exposure to moisture
and air. Therefore, a good level of luminance can be sustained with
these complexes.
3.8. The effect of doping concentration on the EL properties of the
devices
In order to understand the effect of the doping concentration on
Fig. 7. Schematically energy diagram shows the various barriers for charge
injection in devices.
the EL properties of the OLEDs, 2 with concentrations of 10% and

E. Najafi et al. / Inorganica Chimica Acta 415 (2014) 52–60
59
Fig. S6 shows atomic force microscope (AFM) images of the pre-
mixed of PVK:PBD:complex 2 with a scanning area of 5 ꢂ 5 m.
l
The average root-mean-square (RMS) surface morphologies of
the films with 10% and 20% (w/w) of 2 as the dopant were about
8.2 and 11.2 nm, respectively. This showed that the surface rough-
ness of the layer with a concentration of 20% was certainly larger
than the layer with a concentration of 10%. As a result, more effec-
tive electron injection should be achieved at a layer with 10% con-
centration because of the increased contact area between the
electron transport layer (ETL) and the cathode.
4. Conclusions
Two new organotin(IV) complexes have been synthesized and
characterized by spectroscopic methods in addition to their sin-
gle-crystal structures. The intensity of the fluorescence band of 2
containing the long conjugated rigid ligand is higher than the band
of 1. In addition, both absorption and emission peaks of 2 are found
to be significantly red shifted in comparison with those of 1. The
Fig. 9. Luminescence–voltage characteristic of 1 and 2 based devices.
results showed that the p-conjugation length of ligand is an effec-
tive parameter in tuning of the emission wavelength. The EL spec-
tra of tin complexes illustrated a broad red shift rather than a
PVK:PBD blend. It has been demonstrated that the concentration
of the prepared complex changes the driving voltage and also the
shift of the I-V diagram to high voltage by increasing the complex
concentration. The prepared complexes showed fluorescence emis-
sions at room temperature in solution and have good stability in air
and in a high-temperature environment, which make them suit-
able for the fabrication of OLEDs. Finally, all properties and features
suggest that this novel tin(IV) complex is a good candidate as an
emitting layer material in OLEDs and other devices.
20% (w/w) was used as a dopant to fabricate two OLED devices, and
their electroluminescence properties were investigated. The EL
spectra of the devices with 10% and 20% (w/w) of 2 as the dopant
showed a yellow-orange emission at 578 and 589 nm with FWHM
(full width at half maximum) of 10.2 and 10.7 eV, respectively. As
can be seen in Fig. 10, the height concentration of the dopant gen-
erally caused a trap effect in the current voltage and led to the self-
quenching of emission and a reduction in the efficiency of the de-
vice. Fig. S4 shows the current density versus the voltage of the
two OLEDs. Apparently, there is an inverse relationship between
the complex concentration and the current density change at a
specific voltage. Notably, the increase of complex concentration re-
sulted in an increase of in surface roughness and a decrease in car-
rier transport and device efficiency. The device with 10% of 2 had a
much lower operation voltage than that of the device with 20% of
2. This suggests improved electron injection, and the result is in
accordance with an increase in the number of exciton recombina-
tion mechanisms. As Fig. S5 illustrates, the maximum brightness of
the devices with 10% and 20% (w/w) of 2 are 1098 and 940 cd/m2,
respectively. It can be seen that the luminance of the OLEDs de-
creases with the increase in the doping concentration of 2 at the
same voltage. This can be understood by considering the effects
of concentration quenching and trapping of dopant molecules.
Acknowledgment
The authors thank the Vice-President’s Office for Research Af-
fairs of Shahid Beheshti University for supporting this work.
Appendix A. Supplementary material
CCDC 904685 and 928776 contains the supplementary crystal-
lographic data for 1 and 2. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2014.02.032.
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