New cyclometalated iridium(III) dye chromophore complexes for p-type dye-sensitised solar cells

Article abstract of DOI:10.1016/j.dyepig.2017.01.011

The synthesis of seven iridium complexes [Ir(C?N)₂(N?N)][PF₆] (AS9-15) designed as dyes for p-type DSSC is described. These complexes comprise a 4-(pyrid-2-yl)benzoic acid as the cyclometalating/anchoring ligand with different diimin

Full text of DOI:10.1016/j.dyepig.2017.01.011

Dyes and Pigments 140 (2017) 269e277
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New cyclometalated iridium(III) dye chromophore complexes for
p-type dye-sensitised solar cells
,
, **
Alessandro Sinopoli a, Christopher J. Wood b, Elizabeth A. Gibson b *, Paul I.P. Elliott a
a Department of Chemistry, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK
b School of Chemistry, Newcastle University, Newcastle, NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of seven iridium complexes [Ir(C^N)2(N^N)][PF6] (AS9-15) designed as dyes for p-type DSSC
is described. These complexes comprise a 4-(pyrid-2-yl)benzoic acid as the cyclometalating/anchoring
ligand with different diimine ligands acting as electron accepting ancillary ligands. DFT analysis together
Received 15 December 2016
Received in revised form
6 January 2017
Accepted 7 January 2017
Available online 17 January 2017
with photophysical investigations reveal how using different
p-systems on the ancillary ligand it is
possible to tune the absorption spectra of these complexes and to enhance the spatial separation be-
tween the HOMO and LUMO. Computational studies demonstrate an ideal HOMO to LUMO charge
transfer directionality for the presented [Ir(C^N)2(N^N)]þ frameworks, promoting a favourable hole
transfer into the NiO cathode valence band upon photoexcitation in p-type DSSC devices. Preliminary
tests on NiO-based p-type DSSCs have been carried out confirming the potential use of complexes AS9-15
as a basis for continued development as DSSC chromophores.
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
molecular orbital (HOMO) of the dye significantly extends over the
anchoring groups in order to overlap with the valence band (VB) of
There is currently a growing interest in the development of dye-
sensitised solar cells (DSSCs) based on p-type inorganic semi-
conductors (p-SCs) such as nickel oxide [1]. This type of solar cell is
based on hole injection into the valence band of the p-SC from the
photoexcited sensitiser (Fig. 1). Accordingly, the specific electronic
properties of the ideal sensitiser for p-SCs significantly differ from
the semiconductor. On the other hand, to minimize the charge
recombination, the region of the molecule where the lowest un-
occupied molecular orbital (LUMO) resides, should be kept away
from the NiO surface.
The attention toward the development of NiO p-type DSSCs has
increased thanks to their potential use in tandem DSSC, involving
the combination of a sensitised photocathode with a sensitised
photoanode for which, ideally, their absorption should be com-
plementary. Although improvements have been made [7e9], tan-
dem and p-type cells are plagued with low efficiency values, partly
due to rapid (ps timescale [10]) recombination of the photocathode
semiconductor hole with the reduced dye [7]. In order to overcome
this fundamental obstacle in the tandem and p-type system, work
has been focused on increasing the lifetime of both the reduced dye
and charge in the semiconductor, designing chromophores that
localize the electron farther away from the semiconductor surface
[9,11e13].
Significantly longer charge separation can be achieved in dyes
with charge transfer (CT) character, the longest being with weakly
coupled, separate donor and acceptor units. The latter have been
long-lived enough to allow the use of redox couples other than Iꢀ3 /Iꢀ
[14,15], most notably cobalt polypyridine complexes that have
recently given breakthrough results in TiO2 DSSCs [16,17] and could
€
those designed for classical Gratzel cells based on the sensitization
of a n-type semiconductor (n-SC) such as titanium dioxide. In the
latter case, the dye must be a strong reducing agent in its excited-
state in order to inject an electron in the conduction band of the n-
SC [2,3]. The electronic requirements of the sensitisers for p-SCs are
just the opposite of those for n-SC. In addition, the sensitisers must
be functionalized with anchoring groups, first to firmly graft them
on the semiconductor surface and second to provide significant
electronic coupling with the semiconductor wavefunction in order
to ensure an efficient charge tunnelling from the sensitiser excited-
state into the semiconductor [4,5]. For hole injection into a p-SC,
the electron coupling is optimized when the highest occupied
* Corresponding author.
** Corresponding author.
E-mail addresses: elizabeth.gibson@ncl.ac.uk (E.A. Gibson), p.i.elliott@hud.ac.uk
(P.I.P. Elliott).
http://dx.doi.org/10.1016/j.dyepig.2017.01.011
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

270
A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
then allow extension to p-type or tandem cells with Co(III/II)
electrolytes [18].
Iridium bis cyclometalated heteroleptic complexes are inter-
esting in this regard as they have intrinsic CT excited state char-
acter. The main advantages include the fact that isomers are not a
problem as synthesis is regioselective which makes CT direction-
ality highly predictable. They also offer opportunities for photo-
physical and electronic tunability through ligand functionalisation
for further optimization. The broad colour tunability and typically
high phosphorescence quantum yields for this class of compounds
[19,20], make them very attractive for luminescent applications in
organic light-emitting diodes (OLEDs) [21e24], phosphorent ma-
terials [25,26], and, more recently, photodynamic anticancer agents
[27e29], generating an explosion of new cyclometalated iridiu-
m(III) complexes reported in the literature in the last decade [30].
Iridium based photosensitisers have recently been reported for p-
type DSSCs, but with modest results [31,32]. Moreover, the majority
of dyes tested with n- and p-type semiconductors are anchored
through carboxylic acid groups attached on an ancillary ligand
[1,8,9,33] which is not necessarily optimal for hole injection since
being electron withdrawing they generally favour localization of
the lowest unoccupied molecular orbital (LUMO) on these ligands
[4,5].
Fig. 1. (Left) Main operating principles of a p-type dye-sensitised solar cell. D, Dꢀ, D*
represent the photosensitiser in the ground, and the reduced and excited states,
respectively.(1) photon absorption, (2) reduced dye regeneration, (3) regeneration of
the electrolyte, (4) hole injection into NiO valence band, (5) electron transfer to the
dye. (Right) Potentials for oxidation/reduction of a sensitiser (dye AS9 from this work,
vide infra) compared to NiO valence band and iodide/triiodide electrolyte potentials
[6].
The design of a p-type dye reported here (Fig. 2) comes mainly
from the need to prevent the recombination of the injected hole
with the excited electron together with a reasonable absorption
of the visible light. The carboxylate anchoring groups are placed
on the electron donating anionic aryl groups. This should main-
tain efficient anchoring onto the electrode but ensure localization
of the HOMO on these moieties. Use of a neutral electron
accepting ancillary ligand trans to the anchoring arylcarboxylate
rings thus ensures localization of the LUMO away from the
anchoring groups thereby maximising charge separation and
minimising recombination. Examples of this Ir(pyba)2(N^N) type
architecture (pybaH ¼ 4-(pyrid-2-yl)benzoic acid) have been
previously reported with applications in MOF oxygen sensors,
luminogenic probes and ion probes but have not been applied as
DSSC chromophores [34e36]. This electron donor moiety is then
combined with neutral electron accepting ligands (Fig. 3). These
range from the simple and ubiquitous bipyridine and phenan-
throline (AS9 and AS10) to more electron withdrawing ligands
such as dipyrido[3,2-a:20-30-c]-phenazine (dppz) (AS11) [37] as
well as nitro-substituted bpy ligands (AS12 and AS13) to red shift
the absorption spectra [38]. We have also included in this series
BIAN-type [bis(arylimino)acenaphthene] ligands known to yield
panchromatic absorption in their Ir(III) cyclometalated complexes
(AS14 and AS15) [39].
Fig. 2. Main design of complexes AS9-15, donor fragment (blue), acceptor fragment
(red). (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
Fig. 3. Iridium biscyclometalated complexes prepared (AS9-15). Ancillary ligands used for p-type complexes synthesis (N^N): (AS9) bipyridine; (AS10) phenanthroline; (AS11)
dipyrido[3,2-a:20-30-c]-phenazine; (AS12) 4-nitrobipyridine; (AS13) 4,4’-dinitrobipyridine; (AS14) N,N'-bis(4-methoxyphenylimino)acenaphthene; (AS15) N,N'-bis(phenylimino)
acenaphthene.

A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
271
Scheme 1. Synthetic route of complexes AS9-15.
Table 1
2. Results & discussion
Summarised photophysical absorption and emission data for complexes AS9-15 in
aerated acetonitrile.
The cyclometalating ligand precursor designed for the reported
p-type dyes, has been prepared by oxidizing the tolylpyridine to
pyridylbenzoic acid (pybaH) with KMnO4. The cyclometalated
complexes AS9-15 were prepared using a two-step procedure that
is depicted in Scheme 1 [40]. Iridium (III) chloride and the 4-(2-
pyridyl)benzoic acid ligand were combined in ethoxyethanol/wa-
ter (3:1) and heated to reflux.
/nm (ε/dm3 molꢀ1 cmꢀ1
)
l
/nm
t/ns
abs
em
Dye
l
max
max
AS9
265 (57200), 289 (43400), 356 (7933), 417 (4250)
508, 535 146
505, 536 144
AS10 267 (53300), 306 (24300), 354 (7390), 405 (4600)
AS11 274 (86202), 362 (17538), 381 (16936), 433 (5380)
AS12 267 (46014), 302 (28981), 391 (6871)
AS13 253 (47529), 291 (34300), 390 (10000)
AS14 265 (56517), 295 (37402), 402 (16210), 431 (14514)
AS15 256 (29463), 295 (19024), 347 (11653), 401 (8139)
e
e
e
e
e
e
e
e
e
e
The reaction mixtures are observed to change from dark red to
bright orange within 15e30 min once the reaction reaches the
point of reflux. To ensure complete consumption of the metal salt,
the reactions were heated for four hours and then allowed to cool to
room temperature. The crude dimer solution [Ir(pyba)2Cl]2 was
reduced under vacuum to a minimal volume and recrystallised
from dichlormethane/hexane, and the solid collected by filtration
and was used without further purification or characterisation. The
dimer was then reacted with the corresponding ancillary ligand in
refluxing dichloromethane/methanol. The product was purified by
column chromatography on silica and the resulting solids were
dissolved in a minimum volume of methanol and NH4PF6 was
added. The products were recrystallised by addition of diethyl ether
to yield from yellow to dark red solids in modest yields. The com-
plexes [Ir(pyba)2(N^N)]PF6, AS9-15, were then fully characterised
using 1H and 13C NMR spectroscopy and mass spectrometry.
The cyclometalation of pyba the ligand is evidenced by 1H NMR
spectroscopy whereby the corresponding complexes show the loss
of one proton in the aryl fragment together with a shift to higher
field by ~0.5 ppm for the signals corresponding to the protons
adjacent to the coordinated carbon and to the coordinated nitrogen.
In the 1H NMR spectra, AS9-12 and AS14e15 complexes show a
single set of resonances for the two cyclometalated ligands as
consequence of the C2 symmetry of the complexes. The peaks
distribution for the coordinated diimine ligands shows that half of
the ligand is in an unique NMR environment, further reflecting this
symmetry. Complex AS13 is the only asymmetric complex due to
the unsymmetrical NO2-bpy ligand, for this reason all its protons
exhibit unique resonances.
UVeVisible absorption spectra were recorded for all complexes
in aerated acetonitrile solutions at room temperature and are
presented in Fig. 4 with data summarised in Table 1. The absorption
spectra of these compounds show intense bands in the ultraviolet
region between 250 and 350 nm (Fig. 4). These bands are attributed
to spin-allowed
p-p
* ligand-centred (1LC) transitions. These as-
signments were made on assessment of closely related metal
complexes in the literature [41,42]. The less intense, lower energy
Fig. 4. Absorption spectra of AS9-15 dyes in acetonitrile.

272
A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
centred) plus a structured 1MLCT band at 433 nm. Complexes AS12
and AS13 have similar profile and, as expected, the presence of two
nitro groups for AS13 helps to enhance the absorption,
(ε390nme10000 Mꢀ1 cmꢀ1 for AS13 vs 6800 Mꢀ1 cmꢀ1 for AS12).
Complex AS14 has clearly the strongest absorption in the visible
range within the complexes presented in this work with a strong
band across 430 nm followed by a tail beyond 550 nm. BIAN based
ligands have been used previously by Zysman-Colman et al. [39] to
yield panchromatic absorbing complexes, able to shift the absorp-
tion of the complex into the red and near-IR region. The absorption
profile of complex AS15 is clearly affected by the absence of the
MeO groups on the phenyl rings, in agreement with the results
showed by Zysman-Colman et al. [39], hence the complex ab-
sorption profile is not as extensive and is not as intense
(ε401nme8100 Mꢀ1 cmꢀ1) as that for AS14.
Normalised emission spectra are presented in Fig. 5. Complexes
AS9 and AS10 exhibit intense structured emission bands, with
vibronic progression, with maxima at 508 nm and 535 nm which is
characteristic of phosphorescent emission from 3MLCT/3ILCT states,
dominated by the [Ir(pyba)2] fragment [34,41]. Luminescence life-
times were also determined in aerated acetonitrile with a value of
ca 145 ns for both AS9 and AS10, consistent with phosphorescent
emission from a triplet state. The carboxylic acid EWGs, placed on
the pyba ligands, bring down in energy the pyba unoccupied or-
bitals compared to those of 2-phenylpyridine (ppy), such that the
excited electron in the emissive excited state must be localised on
the pyridyl rings of the pyba instead of the ancillary bpy as is the
case for [Ir(ppy)2(bpy)]þ which exhibits 3MLCT/3LLCT-based emis-
sion at 591 nm [44]. All the other complexes are non-emissive and
Fig. 5. Normalised emission spectra for complexes AS9-10 in aerated acetonitrile at RT
(
lex ¼ 365 nm).
absorption features from 350 to 500 nm are due to 1MLCT/1LLCT
charge-transfer (CT) transitions. Two types of CT transitions can be
distinguished in the spectra: bands of moderate intensity between
350 and ~450 nm, assigned to 1MLCT/1LLCT transitions, and tran-
sitions with weaker intensity (~450e550 nm), tentatively assigned
as being spin forbidden 3MLCT in character [43].
AS9 and AS10 have very similar absorption profiles and their
absorptions in the visible region is limited to a weak band
(ε410nme4200 Mꢀ1 cmꢀ1) about 410 nm. The absorption profile of
AS11 is dominated by a strong transition at 274 nm (1LC dppz
Fig. 6. Plots of HOMO (bottom) and LUMO (top) orbitals for complexes AS9-15.
Fig. 7. Molecular orbital energy level diagram for complexes AS9-15.

A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
273
Fig. 8. TDDFT calculated transitions for complexes AS9-15 with experimental spectra overlaid.

274
A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
Table 2
here we expect that the greater electron-withdrawing character of
the ancillary ligands results in 3MLCT/3LLCT states which must
decay non-radiatively to the ground state.
HOMO, LUMO and HOMO-LUMO gap energy values for complexes AS9-15.
Dye
HOMO/eV
LUMO/eV
D/eV
Electrochemical analysis was complicated by limited solubility
of the dyes and therefore did not yield satisfying informative re-
sults. Some reversible or quasi reversible oxidation waves for AS9-
14, corresponding to the one-electron IrIII/IrIV couple could be
observed, however, occurring in the range between þ1.02
and þ 1.19 V (vs Fc/Fcþ) consistent with the expectation of delo-
calization of the HOMO on the cyclometalating fragment.
AS9
ꢀ6.04
ꢀ6.05
ꢀ6.07
ꢀ6.16
ꢀ6.26
ꢀ5.94
ꢀ6.02
ꢀ2.63
ꢀ2.65
ꢀ3.09
ꢀ3.66
ꢀ3.90
ꢀ3.30
ꢀ3.39
3.41
3.40
2.98
2.50
2.36
2.64
2.63
AS10
AS11
AS12
AS13
AS14
AS15
To aid in our understanding of the photophysical properties of
the complexes prepared we carried out gas-phase density func-
tional theory (DFT) calculations. The orbital analysis reveals the
spatial distribution of the HOMO and LUMO are perfectly set to
promote charge transfer directionality away from the NiO surface
and minimize the recombination (Fig. 6). The HOMO localised on
the anchoring ligand, as expected, makes the electronic distribution
on these complexes ideal for their hole injection into the nickel
oxide electrode when incorporated in a DSSC device.
Looking at the orbital distribution on complex AS11, it is inter-
esting to note that for the dppz ligand the LUMO is localised on the
pyrazine rather than on the bpy-like part of the ligand, as previ-
ously reported in literature for similar dppz complexes, which
should lead to increased charge separation [37]. In complexes
AS12e13, the LUMO is delocalised on the dinitro-bpy for AS13 and
on the nitro-pyridine fragment for AS12, as consequence of its
asymmetry. The computational data obtained for complexes AS14
and AS15 are in line with those reported by Zysman-Colman et al.
[39,45] for [Ir(ppy)2(BIAN)][PF6] and [Ir(ppy)2(MeOBIAN)][PF6]. In
particular the LUMO is localised on the BIAN fragment and the
HOMO on the pyba ligands.
beyond 450 nm of 1MLCT/1ILCT character, with a substantial
contribution of the MeO-BIAN fragment.
The photovoltaic properties of the complexes as dyes in p-type
NiO based DSSC were evaluated in preliminary test devices. Com-
plex AS15 wasn't tested because of its low solubility. The dye coated
film was prepared using a water/ethanol solution of NiCl2 and a
triblock co-polymer, F108. Platinised fluorine-doped tin oxide (FTO)
glass was used as the counter electrode and the electrolyte was a
solution of 0.1 M I2 and 1.0 M LiI in acetonitrile. The cells were
thermally sealed before measurements. Photovoltaic measure-
ments were performed on an AM 1.5 solar simulator
(100 mW cmꢀ2) and the incident light was calibrated by using a Si
photodiode reference. JV curves were obtained by applying an
external bias to the cells and measuring the generated photocur-
rent with a Keithley digital source meter.
Fig. 9 reports the IPCE data for complexes AS9-14, which all
exceed the IPCE for un-dyed NiO; this suggests they each photo-
sensitise NiO. The IPCE values are between 10 and 25% and their
profiles reflect the corresponding absorption spectra of the dyes.
Unfortunately the presented dyes have their main absorption
bands at or below ~500 nm and so the IPCE traces similarly tail off
in this region. Only AS13e14, which present the highest extinction
coefficients in the visible region, generate reasonable IPCE values.
Dye aggregation can be an important factor in determining device
performance [46,47]. Absorption spectra of the adsorbed dyes
exhibit similar features to IPCE and solution absorption spectra
(Supporting Information) but do not provide any evidence of ag-
gregation effects peculiar to the dyes described.
Despite the emissive state for AS9-10 being 3MLCT/3ILCT in
character, the ground state LUMO for these complexes is also on the
diimine ligand. However, the order of excited states doesn't
necessarily follow the order of the ground state orbitals associated
with them, when their respective unoccupied orbitals are brought
into close energetic proximity.
The energy levels of the frontier orbitals are shown in Fig. 7. The
corresponding HOMO-LUMO energies are reported in Table 2. The
calculated HOMO energies remain relatively unchanged as would
be expected for the consistency of the pyba ligand across the series
with a value of about ꢀ6.2 eV. However, the introduction of elec-
tron withdrawing nitro substituents on the bipyridyl skeleton in
AS12e13, leads to a significant stabilization of the LUMO relative to
those of complexes AS9 and AS10 by more than 1 eV. For complexes
AS14 and AS15, the BIAN fragment brings down in energy the
LUMO, compared to the bpy analogue, by approximately 0.7 eV.
Time-dependent DFT (TDDFT) calculations were carried out on
the optimized ground state geometries of each complex in order to
determine vertical excitation energies. From TDDFT analysis ( Fig. 8
and ESI), the predicted transitions correlate well with the experi-
mental spectra.
All the applied complexes, under irradiation, were able to
generate a photocurrent. The photovoltaic performances of solar
cells based on these iridium complexes AS9-14 are summarised in
Table 3. Fig. 10 shows the currentevoltage characteristics of the
dyes under AM 1.5 illumination. Both IPCE and efficiency values for
AS11 and AS12 may have been affected by their low solubility as a
The excitations to the S1 state all complexes are primarily
HOMO/LUMO in character, however, they are of low oscillator
strength and will therefore contribute little to the observed ab-
sorption spectra. The S1 transition for complexes AS9-11 takes place
below 500 nm, whereas the S1 for all the other complexes, occurs
beyond 630 nm (see ESI). The major transitions observed for all
complexes between 350 and 500 nm are primarily of 1MLCT char-
acter (see ESI), with generally charge transfer from Ir/aryl to
ancillary ligand. Below 300 nm most of the transitions are 1LC in
character, for example AS11 shows a transition at 283 nm with a
high oscillator strength which is ILCT in character with charge
transfer from the distal phenazine to the proximal phenantroline
moieties of the dppz ligand. Only AS14 shows a strong transition
Fig. 9. IPCE spectra for p-type cells sensitised using iridium complexes AS9-14.

A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
275
C6 complex [Ir(C6)2(dcb)]þ [32]. The methodology reported here
will allow for facile tuning of photophysical and photovoltaic
properties and produce dyes with greater ease than existing
ruthenium based systems. Results are encouraging for the devel-
opment of a new generation of p-type dyes based on this general
design. Future research will focus on improving on these initial
studies and increasing the light absorbing properties of these dyes
together with the substitution of the triiodide/iodide electrolyte for
a one-electron, outer sphere redox couple such as cobalt poly-
pyridyl complexes, in order to relieve the constraint that the
complexities of the iodine-based system impose on the energetic
requirements of the dye [6].
Table 3
Photovoltaic parameters of tested dyes.
Dye
J
sc/mA cmꢀ2
Voc/V
FF/%
h/%
AS9
0.68
0.66
0.45
0.36
0.82
1.12
0.09
0.09
0.07
0.09
0.10
0.10
36.6
37.6
38.1
40.1
38.7
36.8
0.022
0.022
0.013
0.013
0.032
0.043
AS10
AS11
AS12
AS13
AS14
Acknowledgements
The authors thank the University of Huddersfield for funding
this research. Through membership of the UK Materials Chemistry
Consortium (EPSRC grant no EP/L000202) also acknowledges the
ARCHER UK National Supercomputing Service (http://www.archer.
ac.uk) as well as the University of Huddersfield High Performance
Computing Research Group for computational resource utilised in
this work.
Materials and general synthetic procedures
Chemicals were purchased from Aldrich and Acros, iridium was
purchased from Precious Metals Online (Australia) and used as
received. All complexation reactions were carried out under ni-
trogen. Dppz dipyrido[3,2-a:20-30-c]-phenazine [48], 4-nitro-
bipyridine and 4,4’-dinitro-bipyridine [49,50], BIAN [bis(arylimino)
acenaphthene] [39,51] were prepared according to literature
methods.
All the compounds prepared in this work were characterised by
1H NMR and 13C NMR spectrometry; spectra were obtained on a
Bruker Advance 400 MHz instrument. Mass spectrometry data
were collected on a Bruker MicroTOF-Q instrument and Agilent
6210 TOF MS. UVeVisible absorption spectra were recorded on an
Agilent Cary 60 UVeVis spectrophotometer and corrected emission
spectra were recorded on a Horiba Fluoromax-4 Spectrofluorom-
eter. Luminescent lifetime measurements were carried out using an
Edinburgh Instruments Mini-Tau spectrometer.
Fig. 10. Current-Voltage curves DSSC constructed using iridium complexes AS9-14.
dilute dye solution presumably limits the dye loading on the
immersed electrodes. As expected from their very similar struc-
tures, AS9 and AS10 showed nearly identical results in terms of IPCE
and efficiency. Complex AS13 showed better performances in terms
of both IPCE value and efficiency compared to the mono-nitro bpy
based AS12, this can be attributed to the presence of two electron-
withdrawing nitro groups on AS13 able to more effectively stabilise
the LUMO, narrowing the HOMO-LUMO gap and extending the
absorption, and hence IPCE, further towards the red in the visible
region. Complexes AS13 and AS14 showed the best efficiencies of
0.032% and 0.043% respectively, attributable to their enhanced
absorption in the visible spectrum compared to the rest of the se-
ries. The reported efficiencies seem to follow the trend of IPCE in-
tensities shown in Fig. 9.
Dye sensitised solar cell fabrication
NiO electrodes were made by applying the precursor solution
described in previous literature [52] onto conducting glass sub-
3. Conclusions
strates (Pilkington TEC15, sheet resistance 15 U/sq) by doctor blade
We have prepared and characterised, by 1H and 13C NMR
spectroscopy, mass spectrometry and investigated the photo-
physics by UVeVisible absorption and emission spectroscopy,
seven new iridium(III) dyes with various ancillary ligands.
Computational calculations were fundamental tools in the theo-
retical characterisation of the new complexes and in the prediction
and explanation of their proprieties. Diimine complexes show good
tuning of absorption wavelengths and HOMO-LUMO gap, and
thanks to their design, these complexes show spatial charge
transfer directionality together with energy levels in accordance
with their intended application in p-DSSC. The use of these com-
plexes as sensitisers in DSSC devices has been investigated. All
tested complexes achieved modest efficiencies with AS13 and AS14
showing the best efficiencies of 0.032% and 0.043% respectively.
and are encouraging results when related to the 0.068% efficiency
reported for [Ir(ppy)2(N^N)]þ -based complexes, bearing methyl
phosphonic acid as anchoring groups, by Gennari et al. [31] and to
the 0.020% previously reported by our group, using the coumarin
using adhesive tape (3M) as spacer (0.2 cm2 active area), followed
by sintering in a NaberthermB150 oven at 450 ꢁC for 30 min. NiO
films were immersed in dye solutions (0.3 mM) for 12 h. The dyed
NiO electrode was assembled face-to-face with a platinised counter
electrode (Pilkington TEC8, sheet resistance 8
U/sq) using a 30 mm
thick thermoplastic frame (Surlyn 1702). The electrolyte, containing
LiI (1.0 M) and I2 (0.1 M) in acetonitrile, was introduced through the
predrilled hole in the counter electrode, which was sealed after-
ward. The active area was 0.25 cm2.
Computational methods
DFT calculations were carried out using the NWChem 6.3 soft-
ware package [53]. Calculations were carried out using the B3LYP
hybrid functional (20% HartreeeFock) [54], Stuttgart relativistic
small core ECP [55] for iridium and 6-311G* basis sets [56] for all
other atoms. Molecular structures and molecular orbitals were
visualized using the ECCE graphical user interface. The ground state

276
A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
geometries of all complexes were first optimized. TDDFT calcula-
tions on optimized structures in CH3CN by using the COSMO sol-
vation model [57], built in to the NWChem software, were used to
obtain the electronic spectra and molecular orbital energy levels.
Pyba-H). 13C {1H} NMR (400 MHz, CD3CN)
d:166.62, 152.12, 150.38,
149.55, 149.29, 147.27, 139.50, 139.31, 132.83, 132.23, 128.86, 127.98,
127.41, 126.65, 125.21, 125.15, 124.92, 121.6. MS (ESI) m/z calcd for
[C36H24IrN4O4] + 769.142, found 769.2015 (M+).
Synthesis of 4-(2-pyridyl)benzoic acid (pyba)
Synthesis of [Ir(pyba)2(2,5-bis(2-pyridyl)pyrazine)]PF6 (AS11)
The synthetic procedure was the same as that for AS9, except
that 2,5-bis(2-pyridyl)pyrazine (0.043 g, 0.154 mmol), which
replaced 2,2'-bipyridine, was used. The resulting product was pu-
rified and precipitated as for AS9, yielding a dark yellow powder.
This synthetic procedure was adapted from Ref. [58]. Tolyl-
pyridine was oxidized with KMnO4 as follows: a mixture of tolyl-
pyridine (0.514 g) and KMnO4 (1.54 g) in water 50 mL was heated at
70 ꢁC for 24 h. After filtration of the reaction mixture and washing
the filtrate with aqueous NaOH (1M, 5 mL), the combined water
fraction were collected and washed three times with CHCl3 to
remove unconverted starting material. The aqueous solution was
neutralized with HCl (1M) and concentrated (30 mL). After acidi-
fying to pH 4, the precipitate was collected, washed with ethanol
and dried in vacuum yielding a white powder. (Yield 85%) 1H NMR
(28 mg, yield 17.8%) 1H NMR (400 MHz, CD3CN)
d
:9.80 (d, 3J ¼ 8.22,
2H, Dppz-H); 8.51e8.47 (m, 2H, Dppz-H); 8.41 (d, 3J ¼ 4.40, 2H,
Dppz-H); 8.23 (d, 3J ¼ 8.03, 2H, Pyba-H); 8.18e8.14 (m, 2H, Dppz-
H); 8.04e7.90 (m, 6H, Dppz-H&Pyba-H); 7.75 (d, 3J ¼ 8.22, 2H,
Pyba-H); 7.71 (d, 3J ¼ 4.97, 2H, Pyba-H); 7.05 (t, 3J ¼ 6.50, 2H, Pyba-
H); 7.00 (s, 2H, Pyba-H). [Low solubility] 13C {1H} NMR (400 MHz,
CD3CN): 166.05, 152.80, 150.07, 149.61, 148.92, 148.44, 142.80,
139.93, 139.11, 135.38, 132.57, 132.25, 131.34, 129.65, 128.39, 124.77,
124.67, 124.50, 121.16. MS (ESI) calcd for [C42H26IrN6O4]+ 871.1644,
found 871.1631 (M+).
(400 MHz, CD3OD)
d
:8.57 (d, 3J ¼ 4.46, 1H, Py-H); 8.05 (d, 3J ¼ 8.23,
2H, Ar-H); 7.97 (d, 3J ¼ 8.23, 2H, Ar-H); 7.87 (d, 3J ¼ 3.43, 1H, Py-H);
7.37e7.33 (m, 2H, Py-H). 13C {1H} NMR (400 MHz, DMSO-d6):
168.04, 154.59, 148.75, 141.78, 139.90, 131.51, 130.44, 129.98, 127.55,
124.50, 122.69. HRMS (ESI) m/z calcd 199.0633 for C12H9NO2, found
200.0706 (M+H)+
Synthesis of [Ir(pyba)2(4-nitro-2,2'-bipyridine)]PF6 (AS12)
The synthetic procedure was the same as that for AS9, except
that 4-nitro-2,2'-bipyridine (0.031 g, 0.16 mmol), which replaced
2,2'-bipyridine, was used. The resulting product was purified and
precipitated as for AS9, yielding an orange powder. (28.6 mg, yield
Synthesis of [Ir(4-(2-pyridyl)benzoic acid)2Cl]2
353 mg of IrCl3$3H2O (1 mmol) were dissolved in 35 mL 2-
ethoxyethanol:water 3:1 in 2 necked round bottom flask and
degassed with N2 at 80 ꢁC for 20 min 2 equivalents (400 mg) of 4-
(2-pyridyl)benzoic acid were added and heated to reflux temper-
ature for 4 h. The solution was reduced to a minimal volume and
recrystallised from DCM/Hexane. The dimer was used without any
further characterisation or purification.
20%) 1H NMR (400 MHz, CD3CN)
d
:9.81 (d, 3J ¼ 1.88, 1H, Bpy-H);
8.77 (d, 3J ¼ 8.15, 1H, Bpy-H); 8.28 (d, 3J ¼ 5.95, 1H, Bpy-H);
8.26e8.20 (m, 3H, Bpy-H); 8.10 (dd, 3J ¼ 5.95, 4J ¼ 1.88, 1H, Bpy-H);
8.04 (d, 3J ¼ 5.33, 1H, Bpy-H); 8.01e7.92 (m, 4H, Pyba-H); 7.73e7.66
(m, 4H, Pyba-H); 7.60 (t, 3J ¼ 6.42, 1H, Bpy-H); 7.18 (t, 3J ¼ 6.42, 1H,
Pyba-H); 7.16 (t, 3J ¼ 6.58, 1H, Pyba-H); 6.89 (d, 3J ¼ 1.10, 1H, Pyba-
H); 6.85 (d, 3J ¼ 1.10, 1H, Pyba-H). [Low solubility] 13C {1H} NMR
Synthesis of [Ir(pyba)2(2,2'-bipyridine)]PF6 (AS9)
2,2'-bipyridine (0.025 g, 0.16 mmol) was dissolved in
DCM:MeOH 2:1 (6 mL) and [Ir(pyba)2(Cl)]2 (0.100 g, 0.08 mmol)
was added. The mixture was heated at reflux under nitrogen at-
mosphere, in the dark. After 6 h, the yellow solution was cooled to
room temperature. The crude solid was purified by column on silica
eluted with MeCN:H2O:KNO3 7:1:0.5. The yellow fraction was dried
under vacuum, redissolved in acetonitrile and filtered. The solution
was dried again and the solid was dissolved in a minimal amount of
methanol and NH4PF6 (130 mg, 0.9 mmol) was added allowed to
stir overnight. The solution was left in the fridge for two hours,
recrystallised with ether and then filtered on sintered glass. A
yellow powder was collected by filtration. (42 mg, yield 30%) 1H
(400 MHz, CD3CN) d:166.58, 165.90, 165.79, 159.63, 154.78, 154.24,
153.78, 151.35, 149.95, 149.79, 148.16, 148.47, 139.91, 139.33, 139.31,
132.14, 131.90, 130.94, 129.69, 126.13, 124.90, 124.84, 124.79, 124.77,
124.66, 124.58, 121.62, 121.31, 118.27. MS (ESI) calcd for
[C34H23IrN5O6]+ 790.17, found 790.1271 (M+).
Synthesis of [Ir(pyba)2(40,4-dinitro-2,2'-bipyridine)]PF6 (AS13)
The synthetic procedure was the same as that for AS9, except
that 40,4-dinitro-2,2'-bipyridine (0.039 g, 0.16 mmol), which
replaced 2,2'-bipyridine, was used. The resulting product was pu-
rified and precipitated as for AS9, yielding a dark red powder
(40.3 mg, yield 25%) 1H NMR (400 MHz, CD3CN)
d:9.43 (s 2H, Bpy-
NMR (400 MHz, CD3CN)
d
:8.55 (d, 3J ¼ 8.36, 2H, Bpy-H); 8.20 (d, 3J ¼
H); 8.34 (d, 3J ¼ 6.03, 2H, Bpy-H); 8.23 (d, 3J ¼ 8.03, 2H, Pyba-H);
8.18 (dd, 3J ¼ 6.03, 4J ¼ 1.67, 2H, Bpy-H); 8.00 (t, 3J ¼ 8.03, 2H, Pyba-
H); 7.98e7.95 (m, 2H, Pyba-H); 7.73 (d, 3J ¼ 8.03, 2H, Pyba-H); 7.67
(d, 3J ¼ 5.36, 2H, Pyba-H); 7.18 (t, 3J ¼ 6.19, 2H, Pyba-H); 6.85 (s, 2H,
8.36, 2H, Pyba-H); 8.16 (t, 3J ¼ 6.18, 2H, Bpy-H); 8.00e7.94 (m, 4H,
Bpy-H&Pyba-H); 7.93 (d, 3J ¼ 8.36, 2H, Pyba-H); 7.71e7.67 (m, 4H,
Pyba-H); 7.51 (t, 3J ¼ 6.84, 2H, Bpy-H); 7.17 (t, 3J ¼ 6.59, 2H, Pyba-H);
6.89 (s, 2H, Pyba-H).13C {1H} NMR (400 MHz, CD3CN): 167.15,
166.58, 156.29, 151.48, 150.21, 149.79, 149.29, 140.14, 139.61, 132.63,
131.29, 129.10, 125.32, 125.30, 125.26, 124.85, 121.70. MS (ESI) m/z
calcd for [C34H24IrN4O4] + 745.1426, found 745.1427 (M+).
Pyba-H). [Low solubility] 13C {1H} NMR (400 MHz, CD3CN)
d:165.63,
157.76, 157.51, 155.05, 154.17, 154.13, 150.07, 150.04, 139.62, 139.22,
131.92, 131.12, 124.94, 124.92, 122.78, 121.48, 119.56. MS (ESI) m/z
calcd for [C34H22IrN6O8]+ 835.1128, found 835.1135 (M+).
Synthesis of [Ir(pyba)2(1,10-phenanthroline)]PF6 (AS10)
Synthesis of [Ir(pyba)2(N,N'-bis(4-methoxyphenylimino)
acenaphthene)]PF6 (AS14)
The synthetic procedure was the same as that for AS9, except
that 1,10-phenanthroline (0.027 g, 0.16 mmol), which replaced 2,2'-
bipyridine, was used. The resulting product was purified and
precipitated as for AS9, yielding a bright yellow powder. (20 mg,
The synthetic procedure was the same as that for AS9, except
that 4-MeOPh-BIAN (0.063 g, 0.16 mmol), which replaced 2,2'-
bipyridine, was used. The resulting product was purified and
precipitated as for AS9, yielding a brown powder. (35 mg, yield 21%)
yield 13%) 1H NMR (400 MHz, CD3CN)
d
:8.72 (dd, 3J ¼ 8.23, 4J ¼ 1.27,
2H, Phen-H); 8.31 (dd, 3J ¼ 8.5, 4J ¼ 1.27, 2H, Phen-H); 8.26 (s, 2H,
Phen-H); 8.19 (d, 3J ¼ 8.00, 2H, Pyba-H); 7.96 (d, 3J ¼ 8.23, 2H, Pyba-
H); 7.90 (td, 3J ¼ 7.30, 4J ¼ 1.51, 2H, Phen-H); 7.86 (dd, 3J ¼ 8.24, 4J ¼
5.03, 2H, Phen-H); 7.72 (dd, 3J ¼ 8.23, 4J ¼ 1.62, 2H, Pyba-H); 7.51
(dd, 3J ¼ 5.45, 4J ¼ 0.70, 2H, Pyba-H); 7.00 (td, 3J ¼ 5.33, 4J ¼ 1.27, 4H,
1H NMR (400 MHz, CD3CN)
d
: 8.84 (d, 3J ¼ 5.12 Hz, 2H, Pyba -H);
8.31 (d, 3J ¼ 8.28 Hz, 2H, BIAN-H); 8.07e8.01 (m, 4H, Pyba -H); 7.60
(t, 3J ¼ 7.84 Hz, 2H, BIAN-H); 7.52 (d, 3J ¼ 7.68 Hz, 2H, Pyba-H);
7.47e7.41 (m, 4H, Pyba-H); 7.35 (d, 3J ¼ 7.56 Hz, 2H, BIAN-H); 6.99
(bs, 2H, BIAN-H); 6.73 (s, 2H, Pyba-H); 6.63 (bs, 4H, BIAN-H); 6.11

A. Sinopoli et al. / Dyes and Pigments 140 (2017) 269e277
2011;334:629e34.
277
(bs, 2H, BIAN-H); 3.74 (s, 6H, BIAN-CH3). 13C {1H} NMR (400 MHz,
CD3CN)
€
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d:173.23, 165.73, 159.33, 151.64, 148.12, 147.63, 139.34,
137.77, 131.92, 131.54, 131.28, 130.27, 128.92, 127.27, 125.12, 124.06,
123.84, 123.80, 120.66, 114.69, 55.34. MS (ESI) m/z calcd for
[C50H34IrN4O6]+ 981.2264, found 981.2380 (M+).
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Synthesis of [Ir(pyba)2(N,N'-bis(phenylimino)acenaphthene)]PF6
(AS15)
The synthetic procedure was the same as that for AS9, except
that Ph-BIAN (0.054 g, 0.16 mmol), which replaced 2,2'-bipyridine,
was used. The resulting product was purified and precipitated as for
AS9, yielding a dark green powder. (35 mg, yield 21%) 1H NMR
(400 MHz, DMSO-d6)
d
:9.06 (d, 3J ¼ 5.49, 2H, Pyba-H); 8.40 (d, 3J ¼
8.30, 2H, BIAN-H); 8.24 (d, 3J ¼ 7.60, 2H, Pyba-H); 8.18 (d, 3J ¼ 7.04,
2H, Pyba-H); 7.71e7.64 (m, 2H, BIAN-H); 7.63e7.53 (m, 4H, Pyba-
H); 7.30e7.18 (m, 8H, Pyba-H&BIAN-H); 7.15e7.08 (m, 2H, BIAN-H);
7.03e6.95 (m, 2H, BIAN-H); 6.53 (d, 3J ¼ 6.25, 2H, Pyba-H);
6.05e5.93 (m, 2H, BIAN-H). [Low solubility] 13C {1H} NMR not
recorded. HRMS (ESI) m/z calcd for [C48H32IrN4O4]+ 921.2052, found
921.2204 (M+).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.01.011.
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