The use of sublimable chlorotricarbonyl bis(phenylimino)acenaphthene rhenium(I) complexes as photosensitizers in bulk-heterojunction photovoltaic devices

Article abstract of DOI:10.1016/j.jorganchem.2009.04.037

A series of sublimable substituted chlorotricarbonyl bis(phenylimino)acenaphthene rhenium(I) complexes was synthesized and used in the fabrication of photovoltaic devices. The hole and electron carrier mobilities of these complexes are in the order of 10^-3 to 10^-4 cm² V^-1 s^-1. Heterojunction devices with CuPc/complex/C₆₀ (CuPc = copper phthalocyanine) as the active layer and bulk heterojunction devices with complex:C₆₀ as the active layer were fabricated. The rhenium complexes function as photosensitizer in the devices, and exhibit optical absorption in the region between 500 and 550 nm within which other components in the device do not absorb. Other devices with hole transport materials, exciton blocking materials, and different active layer thickness were also fabricated. Variation of substitution groups in the ligand did not show significant difference in device performance. The best power conversion efficiency of the devices was measured to be 1.29% under illumination of AM1.5 simulated solar light.

Full text of DOI:10.1016/j.jorganchem.2009.04.037

Journal of Organometallic Chemistry 694 (2009) 2770–2776
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The use of sublimable chlorotricarbonyl bis(phenylimino)acenaphthene
rhenium(I) complexes as photosensitizers in bulk-heterojunction
photovoltaic devices
Chris S.K. Mak a, Hei Ling Wong a, Qing Yun Leung a, Wing Yan Tam a, Wai Kin Chan a,
,
*
b
´
Aleksandra B. Djurišic
a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
b Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of sublimable substituted chlorotricarbonyl bis(phenylimino)acenaphthene rhenium(I) com-
plexes was synthesized and used in the fabrication of photovoltaic devices. The hole and electron carrier
mobilities of these complexes are in the order of 10ꢀ3 to 10ꢀ4 cm2 Vꢀ1 sꢀ1. Heterojunction devices with
CuPc/complex/C60 (CuPc = copper phthalocyanine) as the active layer and bulk heterojunction devices
with complex:C60 as the active layer were fabricated. The rhenium complexes function as photosensitizer
in the devices, and exhibit optical absorption in the region between 500 and 550 nm within which other
components in the device do not absorb. Other devices with hole transport materials, exciton blocking
materials, and different active layer thickness were also fabricated. Variation of substitution groups in
the ligand did not show significant difference in device performance. The best power conversion effi-
ciency of the devices was measured to be 1.29% under illumination of AM1.5 simulated solar light.
Ó 2009 Elsevier B.V. All rights reserved.
Received 26 February 2009
Received in revised form 28 April 2009
Accepted 28 April 2009
Available online 7 May 2009
Keywords:
Rhenium(I) carbonyl complexes
Photovoltaic devices
Sensitizers
1. Introduction
nium diimine complex that can function as photosensitizers in
photovoltaic cells [15,16]. In this paper, we report the synthesis
Research in organic solar cells has attracted considerable inter-
ests in the past decade because of their potential applications as
alternative light harvesting devices other than the conventional sil-
icon-based solar cells [1,2]. However, the performance and lifetime
of most organic solar cells there are not satisfactory. In general, or-
ganic materials exhibit low carrier mobilities, low absorbance in
near-IR region, short exciton diffusion length, and poor long-term
stability. In order to improve the performance of organic photovol-
taic devices, design of new photosensitizing/charge transport
materials and fabrication of devices with improved architecture
are essential.
of a series of chlorotricarbonyl rhenium complexes with different
substituted bis(phenylimino)acenaphthene (DIAN) ligands [17].
The neutral complexes can be sublimed in vacuum, allowing them
to be purified and processed into multilayer organic photovoltaic
cells. The effects of varying the structure of the ligands and device
structure to photovoltaic device performances were investigated.
2. Results and discussion
2.1. Synthesis and properties of the complexes
Other than organic molecules, transition metal complexes have
also been employed as photosensitizers for harvesting light en-
ergy, especially for dye-sensitized solar cells in which ruthenium
complexes adsorb on the surface of an inorganic semiconducting
materials [3,4]. The applications of metal complexes and metal
containing polymers for photovoltaic cells are also known [5–
11]. Our group previously reported the use of different rhenium
containing polymers for organic photovoltaic cell applications
[12,13]. However, there are only few examples of using rhenium
complexes in DSSC [14] or organic solar cells. We developed a rhe-
The substituted DIAN ligands 3a–3e were synthesized by the
reaction between acenaphthenequinone 1 and the corresponding
substituted aniline 2a–2e (Scheme 1) [18]. Rhenium complexes
4a–4e were synthesized by refluxing the ligand with rhenium(I)
pentacarbonyl chloride in toluene in good yield. All the Re(I) com-
plexes exhibit good thermal stability. The decomposition tempera-
tures of the complexes determined by thermal gravimetric analysis
are at least 250–300 °C (Table 1). Reasonable decomposition tem-
perature is the prerequisite for thermal evaporation of these mate-
rials under vacuum. All the complexes were characterized by 1H,
13C NMR, and FTIR spectroscopies and mass spectrometry. In their
FTIR spectra, three very strong absorption bands are observed in
* Corresponding author. Tel.: +852 28598943; fax: +852 28571586.
E-mail address: waichan@hku.hk (W.K. Chan).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.04.037

C.S.K. Mak et al. / Journal of Organometallic Chemistry 694 (2009) 2770–2776
2771
NH2
ZnCl2
AcOH
K2CO3/H2O
+
N
R
R
N
O
O
R
2a-e
3a-e
1
R =
4a OCH3
4b CH3
3a-e
+
Re(CO)5Cl
R
N
N
R
4c
4d
H
F
toluene
OC
Re Cl
4e CF3
OC CO
complexes 4a-e
Scheme 1.
Table 1
Properties of complexes 4a–4e.
Complex
R
Td (°C)
CH3CN
CHCl3
THF
Toluene
Thin film
HOMO
LUMO
4a
4b
4c
4d
4e
OCH3
CH3
H
F
CF3
336
345
333
346
253
464
474
477
480
482
493
499
503
508
517
493
502
505
508
513
514
517
521
523
549
475
500
503
508
516
ꢀ5.84
ꢀ5.83
ꢀ5.83
ꢀ5.85
ꢀ5.89
ꢀ3.73
ꢀ3.75
ꢀ3.77
ꢀ3.81
ꢀ3.92
the region between 1888 and 2028 cmꢀ1, which are assigned to the
CO stretching of the rhenium carbonyl moieties with facial config-
uration. Both electron donating and withdrawing groups were
introduced to the DIAN ligands. The UV–Vis absorption spectra of
some complexes are shown in Fig. 1, and Table 1 summarizes the
absorption maxima of different complexes in solid thin film and
in different solvents. In general, these complexes exhibit two major
absorption bands, of which the higher energy one at ca. 370 nm is
gand, while the lower energy one at ca. 500 nm is assigned to the
metal-to-ligand charge transfer (MLCT, 5d (Re)–
*(DIAN)) transi-
p
p
tion. Introduction of an electron donating methoxy group to the li-
gand results in a significant red shift in LC transition and blue shift
in the MLCT absorption peak compared to that of the unsubstituted
Re(I) complex. In contrast, the presence of electron withdrawing
trifluoromethyl groups results in slight red shift in the MLCT
absorption. On the other hand, from Table 1, it can also be seen that
the absorption peak maxima of these complexes exhibit solvato-
chromism. The observation suggests that the lower energy elec-
tronic transition has charge transfer character. Profound effect on
the photophysical and electrochemical properties of the complexes
with different substituents on the DIAN ligands was not observed.
This may be due to the fact that the two phenyl rings are almost
orthogonal to that of the acenaphthene ligand, resulting in little
overlap between the molecular orbitals which significantly reduces
the substitution effect of the ligands. Interestingly, the MLCT
absorption peak of the series of the complexes show hypsochromic
shift with an increase in the solvent polarity. The absorption max-
ima in CH3CN solutions are in general shifted 50 nm towards high-
er energy than those of toluene solutions. The result reveals that
the complexes exhibit negative solvatochromic effect and more po-
lar character in the ground state.
assigned to the ligand-centered (LC, p–p
*) transition of the DIAN li-
The effect of the geometry of the complexes can also be seen
from the electronic energy levels. The HOMO and LUMO levels of
the complexes were determined by cyclic voltammetry by compar-
ing the first oxidation and reduction potentials of the complexes
with those of ferrocene internal standard [19], and the data are
summarized in Table 1. In general, all complexes exhibit very sim-
ilar HOMO levels, which indicate that the HOMOs mainly localize
at the rhenium center. On the contrary, substitution of electron
withdrawing/donating groups varies the LUMO levels (from
ꢀ3.73 eV for 4a to ꢀ3.92 eV for 4e). The change of LUMO energies
with different substituents is due to the fact that the electron with-
drawing/donating groups have a great influence to the energy of the
Fig. 1. UV–Vis absorption spectra of rhenium complexes in CHCl3.

2772
C.S.K. Mak et al. / Journal of Organometallic Chemistry 694 (2009) 2770–2776
Table 2
Hole and electron carrier mobilities of different complexes measured under an
applied electric field strength of 300 kV/cm.
Complex Hole carrier mobility lh
Electron carrier mobility le
(10ꢀ4 cm2 Vꢀ1 sꢀ1
)
(10ꢀ4 cm2 Vꢀ1 sꢀ1
)
4a
4b
4c
4d
4e
2.9
4.2
19
3.5
2.5
3.4
3.6
17
3.9
4.8
Fig. 2. Calculated wave functions for the HOMO (left) and LUMO (right) of complex
4c.
Table 3
Performance of heterojunction devices ITO/HTM(10 nm)/4c/C60(10 nm)/Al(60 nm)
with different hole transport material (HTM) and active layer thickness.
p
* orbital of the ligand than that of the d
p orbital of the Re(I) metal
Complex thickness (nm)
HTM
Jsc (mA cmꢀ2
)
Voc (V)
FF
gp (%)
center. It is reported that the electron-withdrawing substituents on
the diimine stabilizes the ligand p* level [20]. The decrease in
absorption and LUMO energies of the complexes with electron
withdrawing groups in our study (Table 1) is consistent with that
of the literature reported.
The effect of different substituents to the HOMO and LUMO was
further investigated by theoretical calculations. DFT calculation of
complex 4c shows that the HOMO is largely localized at the rhe-
nium center with some contribution from the carbonyl and chlo-
ride ligands (Fig. 2), while LUMO delocalizes between rhenium
center and the DIAN ligand. This further suggests that the lowest
excited state is not a pure MLCT state, which is mixed with ligand
center character. It can also be seen that for both MOs, the contri-
bution from the phenyl moieties is very small, and that the substit-
uents have little effect to the electronic properties of the
complexes.
15
25
35
40
50
25
25
CuPc
CuPc
CuPc
CuPc
CuPc
TPD
0.69
0.59
0.59
0.15
0.17
0.14
0.11
0.44
0.48
0.44
0.42
0.45
0.44
0.60
0.32
0.39
0.29
0.16
0.28
0.36
0.33
0.10
0.11
0.07
0.01
0.02
0.02
0.02
NPB
rier mobilities of complex 4c are higher than those of other com-
plexes. However, the difference in the carrier mobilities between
4c and other complexes cannot be correlated to the structure or
HOMO/LUMO energy levels of the complexes. One possible expla-
nation is that the mobilities are affected by the solid-state mor-
phology of the materials. Further experimental works to support
this argument will be needed. From these results, it is envisaged
that after exciton generation, electrons will be captured by fuller-
ene molecules, while holes may be transported via the complexes
before reaching the hole transport layer.
2.2. Charge carrier mobility
Understanding the charge carrier mobilities of the materials in
photovoltaic cells is important to the design of high performance
devices by optimizing the charge transport and minimizing the
recombination losses. Free carriers may be extracted by the hole
and electron transport layers more efficiently, which can minimize
the charge recombination process. It has been demonstrated by us
that some rhenium diimine complexes for light emitting devices
exhibit bipolar charge transport character [21]. The hole and elec-
tron carrier mobilities of the complexes were determined by time-
of-flight experiment, and the results are summarized in Table 2.
The complexes exhibit comparable hole and electron carrier mobil-
ities of the order of 10ꢀ4 cm2 Vꢀ1 sꢀ1. It was shown that these com-
plexes are able to act as charge transport species by the metal
center oxidation (hole hopping) or by the ligand center reduction
(electron hopping) processes [22]. It can be observed that the car-
2.3. Fabrication of photovoltaic cells
In preliminary studies, we first fabricated different heterojunc-
tion photovoltaic devices using complex 4c as the photosensitizing
material. Different hole transport materials including copper
phthalocyanine (CuPc), N,N0-diphenyl-N,N0-di-m-tolyl-p-benzidine
(TPD), and N,N0-diphenyl-N,N0-bis(1-naphthyl)-1,10-biphenyl-4,400-
diamine (NPB) were used. The results are summarized in Table 3.
Heterojunction devices with 4c as the sensitizer show improved
performance compared to those devices with CuPc/C60 active layer
[23]. In addition, devices with CuPc hole transport layer show bet-
ter performance than other hole transport materials, which may be
due to the higher HOMO levels of CuPc (ꢀ5.3 eV) [24] than those of
TPD and NPB (ꢀ5.5 and ꢀ5.4 eV, respectively) [24,25]. In addition,
Table 4
Effect of complex 4c:C60 ratio and active layer thickness to the performance of the devices ITO/CuPc/4c:C60/C60/Al.
Entry
4c:C60 ratio
Thickness of (4c:C60) layer
Jsc (mA cmꢀ2
)
Voc (V)
FF
gp (%)
1
2
3
4
5
6
7
8
1:9
1:9
1:9
1:9
3:7
3:7
3:7
3:7
1:1
1:1
1:1
1:1
9:1
25
50
75
100
25
50
75
100
25
50
75
4.13
2.67
1.48
2.30
4.03
2.72
2.93
0.03
3.0
5.07
4.34
3.23
1.3
0.37
0.37
0.30
0.3
0.42
0.44
0.46
0.30
0.47
0.51
0.42
0.46
0.59
0.49
0.40
0.39
0.46
0.45
0.52
0.43
0.12
0.33
0.51
0.42
0.32
0.20
0.74
0.40
0.17
0.31
0.76
0.62
0.58
0.01
0.46
1.29
0.76
0.47
0.15
9
10
11
12
13
100
50

C.S.K. Mak et al. / Journal of Organometallic Chemistry 694 (2009) 2770–2776
2773
Bulk-heterojunction photovoltaic devices were fabricated by
blending the donor- and acceptor in one single layer. There is much
larger contact surface area between donor and acceptor. However,
the amount of donor and acceptor has to be carefully controlled be-
cause it directly affects the formation of charge percolation path-
ways in the mixed layer. Bulk-heterojunction photovoltaic cells
with CuPc:C60 as the active layers have been reported, and the effi-
ciencies are in the order of 1–2% [26–28]. We fabricated different
series of bulk-heterojunction photovoltaic cells with the device
configuration ITO/CuPc/4c:C60/C60/Al by varying the ratio between
complex 4a and C60 in the active layer. The thickness of the active
layer in these devices was varied between 25 and 100 nm, and the
data are summarized in Table 4. For all devices, when the active
layer thickness is higher than 75 nm, the efficiencies of the devices
decrease rapidly. For the device with 4c:C60 ratio of 9:1, due to the
small amount of electron transport material in the active layer, low
fill factor and efficiency were measured. The optimum thickness of
the active 4c:C60 layer appears to be dependent on the 4c:C60 ratio.
For the devices with 4c:C60 ratio of 1:9 and 3:7, highest efficiencies
were obtained when the active layer thickness was 25 nm, while
for the device with 1:1 4c:C60 ratio the optimum thickness for
the active layer was 50 nm. Fig. 3a shows the current–voltage char-
acteristics of different device with different 4c:C60 ratio. The en-
ergy levels of different materials in the device are shown in
Fig. 3b, which shows that after exciton dissociation, transport of
electrons from the complex to C60 and holes from complex to CuPc
are favorable processes.
For comparison purpose, complexes 4a, 4b, 4d, and 4e were fab-
ricated into bulk-heterojunction photovoltaic cells ITO/CuPc/com-
plex:C60 (1:1)/C60/Al with active layer thickness of 50 nm. The
device performance data are summarized in Table 5. It can be seen
that substitution with electron donating (4a and 4b) or withdraw-
ing (4d and 4e) groups did not result in significant effect to device
efficiency, and the performance of these device measured are in the
range 0.62–0.79%.
We then attempted to further optimize the device based on
complex 4c by varying the hole transport material (HTM) or by
insertion of an exciton diffusion blocking layer (EBL) between C60
and metal electrode. For the hole transport layer, TPD, NPB, zinc
phthalocyanine (ZnPc), and 4,40-N,N0-dicarbazolyl-biphenyl (CBP)
were used (Table 5, entries 6–9). For devices with TPD, NPB, and
CBP as the HTM, the device efficiencies are lower than 0.1% (entries
7–9), which is due to the low absorbance of these materials in the
visible region. The device based on ZnPc showed the best perfor-
Fig. 3. (a) Current–voltage characteristics of the device ITO/CuPc/4c:C60/C60/Al with
different complex:fullerene ratios under illumination of simulated AM1.5 solar
light. (b) Schematic diagram showing the energy levels (in eV) of different materials
in the device.
CuPc exhibits high absorbance in the visible region, which may
contribute to photocurrent generation (see discussion below).
The optimum thickness for CuPc layer is between 15 and 25 nm,
and the highest efficiency measured was 0.11%.
mance among these four (FF = 0.51, gp = 0.59%). However, it is still
slightly lower than that the device with CuPc as HTM. It has been
demonstrated that insertion of EBL between electron acceptor and
metal cathode in photovoltaic cells can improve device efficiency
Table 5
Performance of photovoltaic cells with different rhenium complexes, hole transport material (HTM), and exciton blocking layer (EBL) ITO/HTM (10 nm)/complex:C60 (1:1)
(50 nm)/C60 (10 nm)/EBL (10 nm)/Al (60 nm).
Entry
Complex
HTM
EBL
Jsc (mA cmꢀ2
)
Voc (V)
FF
gp (%)
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4c
4c
4c
4c
4c
4c
4c
4c
CuPc
CuPc
CuPc
CuPc
CuPc
ZnPc
TPD
NPB
CBP
CuPc
CuPc
CuPc
CuPc
–
–
–
–
–
–
–
–
3.25
2.84
5.07
2.76
2.70
2.41
0.20
0.38
0.014
0.18
0.21
2.59
0.038
0.46
0.52
0.51
0.47
0.44
0.48
0.48
0.72
0.34
0.30
0.40
0.44
0.66
0.53
0.50
0.51
0.48
0.54
0.51
0.36
0.29
0.26
0.51
0.54
0.46
0.18
0.79
0.73
1.29
0.62
0.64
0.59
0.035
0.08
0.01
0.27
0.45
0.53
0.004
9
–
10
11
12
13
BCP
BCP/Ag*
BCP/Mg:Ag (6:1)*
PTCDI
*
Aluminium electrode was not deposited in these devices.

2774
C.S.K. Mak et al. / Journal of Organometallic Chemistry 694 (2009) 2770–2776
photovoltaic devices was demonstrated. Insertion of a rhenium-
complex:C60 blend as the active layer in the device resulted in sig-
nificant improvement in device performance compared to those
devices based on CuPc:C60 active layer. We attribute the improve-
ment in performance to the fact that these complexes may play the
role of photosensitizers and charge transport materials in the de-
vices. The best power conversion efficiency was measured to be
1.29%. The performance of the devices may be improved by the
use of complexes with higher absorption coefficient and carrier
mobilities, and further investigation in the photophysical proper-
ties of these complexes will be required.
4. Experimental
4.1. Materials
Acenaphthenequinone tech. (95%) and zinc chloride (98%) were
purchased from ABCR. p-Anisidine was purchased from Merck
KGaA. p-Toluidine (99%) was purchased from Aldrich Chemical
Co. 4-Aminobenzotrifluoride (99%) was purchased from Acros
Organics. 4-Fluoroaniline (99%) and aniline (99+%) were purchased
from Lancaster Synthesis Ltd. Rhenium pentacarbonyl chloride
(98%) was purchased from Strem Chemicals. For thermal evapora-
tion, copper phthalocyanine (sublimation grade, dye content 99%)
was purchased from Aldrich Chemical Co. Fullerene (>99.5%) was
purchased from Bucky USA. Aluminium wire (99.999%) was pur-
chased from Strem Chemicals. Unless otherwise specified, all the
chemicals were used as received.
Fig. 4. Plot of IPCE as the function of wavelength for the device ITO/CuPc/4c:C60
(1:1)/C60/Al under illumination of simulated solar light. The absorption spectra of
thin films of 4c and CuPc are also shown.
[29]. In our studies, devices with bathocuproine (BCP) and pery-
lene-3,4,9,10-tetracarboxylic diimide (PTCDI) as the EBL were fab-
ricated (entries 10–13). From Table 5, it can be seen that insertion
of EBL did not result in any significant improvement in device effi-
ciency. Poor performance was also observed in the device fabri-
cated with PTCDI EBL. It is possible that for different HBL, there
exists an optimum thickness that has to be carefully controlled,
and further investigation may be necessary.
In order to understand which functionality is responsible for the
generation of photocurrent in the device, we measured the inci-
dent photon-to-electron conversion efficiency (IPCE) as the func-
tion of incident light wavelength. IPCE is defined as the number
of electron generated per incident photon, and is represented by
4.2. Synthesis of ligands (3a–3e)
Synthesis of bis(4-methoxyphenylimino)acenaphthene 3a: [1] A
mixture of acenaphthenequinone 1 (2.0 g, 11 mmol), anhydrous
ZnCl2 (3.0 g, 22 mmol), and p-anisidine 2a (3.4 g, 28 mmol) was
heated under reflux in glacial acetic acid (20 ml) for 1 h. The sus-
pension was cooled to room temperature and the solid, bis(4-
methoxyphenylimino)acenaphthene zinc chloride, was filtered
off, washed with acetic acid (2 ꢁ 10 ml) and diethyl ether
(4 ꢁ 20 ml), and then air-dried. It was then added to an aqueous
potassium carbonate solution (25 g in 25 ml water) and the mix-
ture was refluxed with vigorous stirring for 2 h. The mixture was
cooled to room temperature and the solid was filtered off and
washed with water (5 ꢁ 30 ml). The product was dried under vac-
uum and recrystallized with ethanol. The product was collected as
red needle (3.4 g, 79%). 1H NMR (400 MHz, CDCl3): d = 7.89 (d,
J = 8.2 Hz, 2H), 7.40 (t, J = 7.8 Hz, 2H), 7.10 (m, 4H), 7.02 (m, 6H),
3.90 (s, 6H). 13C NMR (75 MHz, CDCl3): d = 161.6, 156.9, 144.9,
128.8, 127.6, 123.6, 119.8, 114.6, 55.5. MS (EI): m/z: 393 [M]+.
Ligands 3b–3e were synthesized by the same procedure as for
3a using different substituted anilines 2d–2e.
1240 ꢁ ISC
IPCE ð%Þ ¼
ꢁ 100%
k ꢁ
U
where Isc is the short circuit current density (in mA/cm2), k is the
wavelength of incident light (in nm) and is the light intensity
U
(in mW/cm2). Fig. 4 shows the plot of IPCE vs. incident light wave-
length for the bulk heterojunction device fabricated from 4c. The
absorption spectra of thin films of 4c and CuPc are also shown for
comparison. Maximum IPCE of 25% is observed at 620 nm, and
the curve is very similar to the absorption spectrum of CuPc. This
strongly indicates that optical absorption by the CuPc layer also
plays an important role in the photocurrent generation process. This
may also explain the observations that devices fabricated from
other HTMs (except ZnPc) showed poor performances. However, it
should be emphasized that the rhenium complexes are also impor-
tant to photocurrent generation. Complex 4c strongly absorbs in the
region between 450 and 500 nm, in which CuPc shows almost no
absorption at all. The IPCEs measured are higher than 5% in the en-
tire visible region. This shows that the rhenium complexes and CuPc
exhibit complementary absorption in the visible region, and more
photons can be harvested as a result.
Bis(4-methylphenylimino)acenaphthene 3b: (70%). 1H NMR
(300 MHz, CDCl3): d = 7.91 (d, J = 8.3 Hz, 2H), 7.40 (t, J = 7.3 Hz,
2H), 7.30 (m, 4H), 7.05 (d, J = 8.2 Hz, 4H), 6.95 (d, J = 7.2 Hz, 2H),
2.47 (s, 6H). 13C NMR (75 MHz, CDCl3): d = 161.7, 149.6, 134.3,
131.6, 130.4, 129.1, 128.0, 124.3, 118.6, 21.5. MS (EI): m/z: 361 [M]+.
Bis(phenylimino)acenaphthene 3c: (71%). 1H NMR (300 MHz,
CDCl3): d = 7.90 (d, J = 8.3 Hz, 2H), 7.48 (t, J = 7.8 Hz, 4H), 7.37 (t,
J = 8.1 Hz, 2H), 7.14 (d, J = 7.8 Hz, 4H), 6.84 (d, J = 7.2 Hz, 2H). 13C
NMR (100 MHz, CDCl3): d = 161.3, 151.8, 141.8, 131.2, 129.4,
129.0, 128.6, 127.7, 124.4, 124.0, 118.2; MS (EI): m/z: 333 [M]+.
Bis(4-fluorophenylimino)acenaphthene 3d: [30] (65%). 1H NMR
(400 MHz, CDCl3): d = 7.93 (d, J = 8.2 Hz, 2H), 7.42 (t, J = 7.8 Hz,
2H), 7.19 (m, 4H), 7.10 (m, 4H), 6.94 (d, J = 7.3 Hz, 2H). 13C NMR
(75 MHz, CDCl3): d = 162.2, 159.0, 148.0, 131.7, 129.7, 128.7,
128.1, 124.3, 120.2, 120.1, 116.9, 116.6; MS (EI): m/z: 369 [M]+.
3. Conclusions
The use of sublimable rhenium diimine complexes based on
bis(phenylimino)acenaphthene ligands as the active materials in

C.S.K. Mak et al. / Journal of Organometallic Chemistry 694 (2009) 2770–2776
2775
Bis((4-trifluoromethyl)phenylimino)acenaphthene 3e: (58%). 1H
NMR (300 MHz, CDCl3): d = 7.97 (d, J = 8.2 Hz, 2H), 7.76 (d,
J = 8.3 Hz, 4H), 7.44 (t, J = 7.8 Hz, 2H), 7.23 (d, J = 8.2 Hz, 4H), 6.85
(d, J = 7.2 Hz, 2H). 13C NMR (100 MHz, CDCl3): d = 161.3, 154.4,
142.0, 131.3, 129.7, 127.9, 126.8, 124.2, 118.3; MS (EI): m/z: 469
[M]+.
(e
) = 517 nm (4100 dm3 molꢀ1 cmꢀ1). MS (FAB-MS): m/z: 774
[M]+, 739 [MꢀCl]+. Anal. Calc. for C29H14N2ClF6O3Re: C, 45.00; H,
1.82; N, 3.62. Found: C, 44.73; H, 1.77; N, 3.27%.
4.4. Measurements
1H and 13C NMR spectra (1H NMR 298 K, 13C NMR 298 K) were
collected on a Bruker AM-500 (500-MHz), a Bruker AV-400 (400-
MHz) or a Bruker DPX-300 (300-MHz) multinuclear Fourier trans-
form nuclear magnetic resonance (NMR) spectrometers. Chemical
shifts (d) are reported in ppm relative to tetramethylsilane (TMS)
as the internal standard and coupling constants (J) in Hz. Multiplic-
ities are reported as singlet (s), doublet (d), triplet (t), quartet (q),
and multiplet (m). Fourier transform infrared (FTIR) spectra were
recorded as KBr pellet on a Bio-Rad FTS-7 FTIR spectrometer
(4000–400 cmꢀ1) and are reported in cmꢀ1. UV–Visible absorption
spectra were recorded on a Hewlett–Packard 8452A diode array
UV–Visible spectrophotometer or a Varian Cary 50 UV–Visible
spectrophotometer. Positive ion FAB mass spectra were recorded
on a Finnigan MAT95 mass spectrometer. Thermal analyses were
performed on Perkin Elmer TGA7 thermal analyzers. The thermo-
gravimetric analysis (TGA) was performed under a nitrogen atmo-
sphere with the heating rate of 15 °C minꢀ1. Cyclic voltammetry
(CV) was performed on a Princeton Applied Research 270 potentio-
stat. A conventional three-electrode system was adopted, which
consisted of a glassy carbon working electrode and two platinum
electrodes as the counter and reference electrodes. The salt bridges
of reference electrode and counter electrode compartments were
separated from the working electrode compartment by a vycor
glass. Cyclic voltammograms were recorded at a scan rate of
100 mV/s. HPLC grade acetonitrile was used as the solvent and tet-
rabutylammonium tetrafluoroborate (0.1 M) was used as the sup-
porting electrolyte. Ferrocene was added as the internal standard.
For the DFT calculations, GAUSSIAN98 program was used. The density
functional theory calculations for the ground state of the com-
plexes were performed by using B3LYP method with a LanL2DZ ba-
sis set.
4.3. Synthesis of complexes (4a–e)
Synthesis of chlorotricarbonyl bis(4-methoxyphenylimino)ace-
naphthene rhenium(I) 4a: Under a nitrogen atmosphere, a mixture
of 3a (0.8 g, 2 mmol) and rhenium pentacarbonyl chloride (0.8 g,
2 mmol) in toluene (15 ml) was heated under reflux for 24 h. After
cooling, the solid was filtered off and washed with toluene, ether,
and then dried under vacuum. The product was collected as
brownish solid. (1.3 g, 87%). Td = 336 °C; 1H NMR (400 MHz,
CDCl3): d = 8.05 (d, J = 8.2 Hz, 2H), 7.65 (m, 2H), 7.51 (t, J = 7.8 Hz,
2H), 7.32 (m, 2H), 7.17 (d, J = 7.3 Hz, 2H), 7.11 (dd, J = 1.5, 7.6 Hz,
2H), 3.96 (s, 6H); 13C NMR (125 MHz, CDCl3): d = 196.0, 186.3,
159.6, 144.8, 142.2, 131.4, 130.9, 129.0, 128.5, 128.2, 126.4,
125.3, 124.6, 123.1, 121.7, 115.0, 55.6; FTIR (KBr pellet):
m
= 2020, 1921, 1888 cmꢀ1 (metal carbonyl CO stretching); UV–
Vis (CHCl3): kmax (e
) = 493 nm (7200 dm3/mol cm); MS (FAB): m/z
(%): 698 [M]+, 663 [MꢀCl]+. Anal. Calc. for C29H20N2ClO5Re: C,
49.89; H, 2.89; N, 4.01. Found: C, 49.66; H, 2.54; N, 4.17%.
Chlorotricarbonyl bis(4-methylphenylimino)acenaphthene rhe-
nium(I) 4b: (83%). 1H NMR (300 MHz, CDCl3): d = 8.07 (d,
J = 8.3 Hz, 2H), 7.62 (d, J = 6.2 Hz, 2H), 7.52 (t, J = 7.8 Hz, 2H), 7.43
(d, J = 8.6 Hz, 4H), 7.11 (d, J = 7.3 Hz, 2H), 7.09 (d, J = 8 Hz, 2H),
2.53 (s, 6H). 13C NMR (125 MHz, CDCl3): d = 196.0, 186.2, 172.8,
146.6, 144.9, 138.6, 131.4, 131.0, 130.8, 130.3, 128.5, 126.3,
124.8, 121.3, 119.9, 21.4. FTIR (KBr pellet):
(metal carbonyl CO stretching). UV–Vis (CHCl3): kmax
m
= 2026, 1920 cmꢀ1
(e
) = 499 nm (11 000 dm3 molꢀ1 cmꢀ1). MS (FAB-MS): m/z: 667
[M]+, 632 [MꢀCl]+. Anal. Calc. for C29H20N2ClO3Re: C, 52.29; H,
3.03; N, 4.21. Found: C, 51.94; H, 2.88; N, 4.06%.
Chlorotricarbonyl bis(phenylimino)acenaphthene rhenium(I) 4c:
(96%). 1H NMR (400 MHz, CDCl3): d = 8.06 (d, J = 8.2 Hz, 2H), 7.73
(d, J = 8.3 Hz, 2H), 7.63 (t, J = 8.0 Hz, 4H), 7.51 (t, J = 7.3 Hz, 2H),
7.48 (d, J = 8.1 Hz, 2H), 7.38 (d, J = 8.1 Hz, 2H), 7.00 (d, J = 7.3 Hz,
2H). 13C NMR 125 MHz, CDCl3): d = 195.8, 186.0, 173.0, 148.9,
145.0, 131.4, 131.1, 130.4, 129.9, 129.0, 128.6, 128.2, 126.2,
4.5. Carrier mobility measurements
The thin film for charge carrier mobility measurement was fab-
ricated by evaporating the material on an ITO coated glass sub-
strate under vacuum. Typical thickness of the thin film was
approximately 500 nm. A layer of 100 nm metal electrode was
coated on the organic film surface by thermal evaporation. Charge
carrier mobility was determined by the time-of-flight experiment
[31]. A Laser Science VSL-337 nitrogen laser was used to generate
124.8, 121.3, 120.0. FTIR (KBr pellet):
m
= 2019, 1921 cmꢀ1 (metal
) = 503 nm
carbonyl CO stretching). UV–Vis (CHCl3): kmax
(e
(7500 dm3 molꢀ1 cmꢀ1); MS (FAB-MS): m/z: 638 [M]+, 603
[MꢀCl]+. Anal. Calc. for C27H16N2ClO3Re: C, 50.82; H, 2.53; N,
4.39. Found: C, 50.43; H, 2.21; N, 4.11%.
Chlorotricarbonyl bis(4-fluorophenylimino)acenaphthene rhe-
nium(I) tricarbonyl chloride 4d: (90%). 1H NMR (300 MHz, CDCl3):
d = 8.10 (d, J = 8.2 Hz, 2H), 7.57 (m, 2H), 7.54 (td, J = 1.1, 7.8 Hz,
2H), 7.30 (m, 6H), 7.07 (d, J = 7.3 Hz, 2H); 13C NMR (125 MHz,
CDCl3): d = 195.6, 185.7, 173.4, 163.2, 161.2, 145.1, 144.9, 131.5,
131.4, 129.6, 128.7, 126.0, 124.8, 123.5, 123.4, 121.9, 121.8,
a pulsed laser source (wavelength = 337 nm, pulse energy = 120 lJ,
and pulse width full width at half-maximum (fwhm) = 3 ns). The
transient photocurrent profiles were recorded with an oscilloscope
(Tektronix TDS 3052B).
4.6. Photovoltaic device characterizations
117.6, 117.4, 117.1, 116.9; FTIR (KBr pellet):
(metal carbonyl CO stretching). UV–Vis (CHCl3): kmax
m
= 2022, 1906 cmꢀ1
The ITO coated glass substrates with sheet resistance of 15
X/
(
e
) = 508 nm (9000 dm3 molꢀ1 cmꢀ1). MS (FAB-MS): m/z: 675
square (Tinwell Technology Ltd.) were cleaned prior to use. The
substrates were rinsed with toluene, ethanol, and acetone in se-
quence. It was then cleaned in an ultrasonic bath for 8 min in a
3% Decon 90 glass–detergent solution, 8 min in ultrapure water
[M]+, 640 [MꢀCl]+. Anal. Calc. for C27H14N2ClF2O3Re: C, 48.11; H,
2.09; N, 4.16. Found: C, 47.83; H, 1.96; N, 4.23%.
Chlorotricarbonyl
bis((4-trifluoromethyl)phenylimino)acenaph-
thene rhenium(I) 4e: (92%). 1H NMR (400 MHz, CDCl3): d = 8.14 (d,
J = 8.3 Hz, 2H), 7.93 (d, J = 8.5 Hz, 4H), 7.87 (d, J = 8.4 Hz, 2H),
7.57 (t, J = 7.8 Hz, 2H), 7.50 (d, J = 7.3 Hz, 2H), 6.99 (d, J = 7.3 Hz,
2H). 13C NMR (125 MHz, DMSO-d6): d = 196.8, 185.9, 173.6,
151.3, 144.9, 132.3, 131.1, 129.0, 128.4, 125.2, 124.8, 122.8,
(18.2 MX), 5 min in ethanol, and 5 min in acetone in sequence.
The substrates were rinsed thoroughly with ultrapure water after
each cleaning step. All the solvents used for cleaning were analyt-
ical grade. The substrates were dried under a nitrogen atmosphere.
Before the ITO substrates were loaded into an evaporator, it was
treated with UV ozone for 15 min. The multilayer photovoltaic de-
vices were fabricated by evaporating different organic materials on
121.8, 121.3, 112.7. FTIR (KBr pellet):
(metal carbonyl CO stretching). UV–Vis (CHCl3): kmax
m
= 2028, 1930, 1897 cmꢀ1

2776
C.S.K. Mak et al. / Journal of Organometallic Chemistry 694 (2009) 2770–2776
the ITO coated glass substrates inside
a vacuum chamber
References
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Acknowledgements
The work reported in this paper has been substantially sup-
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Theme, University Development Fund for Clean Energy Research
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.04.037.