A panchromatic, near infrared Ir(III) emitter bearing a tripodal C^N^C ligand as a dye for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.poly.2017.12.003

The synthesis of a new complex of the form [Ir(C^N^C)(N^N)Cl] [where C^N^C = 2-(bis(4-(tert-butyl)phenyl)methyl)pyridinato (dtBubnpy, L1) and N^N is diethyl [2,2′-bipyridine-4,4′-dicarboxylate (deeb)] is reported. The crystal structure reveals an unusual tripodal tridentate C^N^C ligand forming three six-membered rings around the iridium center. The photophysical and electrochemical properties suggest the use of this complex as a dye in dye-sensitized solar cells. Time-Dependent Density Functional Theory (TD-DFT) calculations have been used to reveal the nature of the excited-states.

Full text of DOI:10.1016/j.poly.2017.12.003

Accepted Manuscript
A Panchromatic, Near Infrared Ir(III) Emitter Bearing a Tripodal C^N^C ligand
as a Dye for Dye-Sensitized Solar Cells
Claus Hierlinger, Heather V. Flint, David B. Cordes, Alexandra M.Z. Slawin,
Elizabeth A. Gibson, Denis Jacquemin, Véronique Guerchais, Eli Zysman-
Colman
PII:
S0277-5387(17)30785-4
https://doi.org/10.1016/j.poly.2017.12.003
POLY 12959
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
14 November 2017
1 December 2017
Please cite this article as: C. Hierlinger, H.V. Flint, D.B. Cordes, A.M.Z. Slawin, E.A. Gibson, D. Jacquemin, V.
Guerchais, E. Zysman-Colman, A Panchromatic, Near Infrared Ir(III) Emitter Bearing a Tripodal C^N^C ligand as
a Dye for Dye-Sensitized Solar Cells, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.12.003
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A Panchromatic, Near Infrared Ir(III) Emitter Bearing a
Tripodal C^N^C ligand as a Dye for Dye-Sensitized Solar
Cells
a,b
Claus Hierlinger, Heather V. Flint, David B. Cordes, Alexandra M. Z. Slawin, Elizabeth A.
c
b
b
c
Gibson* Denis Jacquemin,* Véronique Guerchais,* Eli Zysman-Colman*
d
a
,b
a
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus
de Beaulieu, 35042 Rennes Cedex, France. E-mail: veronique.guerchais@univ-rennes1.fr
b
Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St
Andrews, Fife, KY16 9ST, UK. E-mail: eli.zysman-colman@st-andrews.ac.uk; Web:
http://www.zysman-colman.com
c
Chemistry, School of Natural and Environmental Science, Newcastle University, Newcastle
upon Tyne, NE1 7RU, UK. Elizabeth.gibson@ncl.ac.uk
d
UMR CNRS 6230, Université de Nantes, CEISAM, 2 rue de la Houssinière, 44322 Nantes
Cedex 3, France. E-mail: Denis.Jacquemin@univ-nantes.fr
Abstract
The synthesis of a new complex of the form [Ir(C^N^C)(N^N)Cl] [where C^N^C = 2-(bis(4-
(
tert-butyl)phenyl)methyl)pyridinato (dtBubnpy, L1) and N^N is diethyl [2,2'-bipyridine]-4,4'-
dicarboxylate (deeb)] is reported. The crystal structure reveals an unusual tripodal tridentate
C^N^C ligand forming three six-membered rings around the iridium center. The photophysical
and electrochemical properties suggest the use of this complex as a dye in dye-sensitized solar
1

cells. Time-Dependent Density Functional Theory (TD-DFT) calculations have been used to
reveal the nature of the excited-states.
Introduction
Dye-sensitized solar cells (DSSCs) represent a promising solar cell technology. The
majority of champion DSSCs, those showing power conversion efficiencies (PCE) greater than
1
0%, are based on ruthenium(II) complexes. Iridium(III) complexes, dominant as emitters in
electroluminescent devices,[1,2] have to date fared poorly as dyes in DSSCs.[3-13] This is
mainly because most iridium(III) complexes are not panchromatic, having absorption spectra that
tail off by 550 nm. This induces low short circuit currents in the DSSC and as a consequence
poor PCE; typically less than 4%. Indeed, there are very few examples of iridium(III) complexes
with significant absorption bands going up to the red or NIR parts of the visible spectrum.[14-
2
0,12]
We recently reported the development of tripodal C^N^C ligands, 2-benzhydrylpyridine
and its derivatives, which can coordinate to iridium, forming three six-membered chelate rings
through a double C-H bond activation.[21] When combined with a bidentate diimine ligand such
as 4,4'-ditertbutyl-2,2'-bipyridine (dtBubpy), a family of orange-to-red emitting neutral
[
Ir(C^N^C)(dtBubpy)Cl] complexes was formed with absorption bands tailing off at 600 nm.
Herein, we report an analogous complex showing panchromatic absorption, employing an
electron-poor ancillary ligand diethyl [2,2'-bipyridine]-4,4'-dicarboxylate (deeb), and study its
use as a DSSC dye.
2

Results and Discussion
Synthesis
.
3
1
. IrCl H O / EtOCH CH OH / H O (3:1),
2
2
2
2
125 °C, 18 h
CO Et
2
N
2
. 125 °C, 6 h
N
N
Ir
N
EtO C
2
deeb
CO Et
2
N
N
CO Et
2
L1
Cl
1
(52%)
Scheme 1: Scheme for the one-pot synthesis of complex 1.
Compound L1[21] and deeb[22] were prepared by literature methods. Complex 1 was
obtained as a black solid in 52% yield using a two-step-one-pot protocol wherein a mixture of L1
and IrCl
3
.n6H
2
O in 2-ethoxyethanol/H
2
O (3:1) was heated at reflux for 19 h followed by the
addition of deeb and a further reaction time of 6 h (Scheme 1). Complex 1 was characterized by
1
13
H and C NMR spectroscopy, HR-ESI mass spectrometry, elemental analysis and melting point
determination [see Figures S1-6 in the Supporting Information (SI) for NMR and HR-ESI mass
spectra].
Crystal Structures
Single crystals of sufficient quality of 1 were grown from CH
structure of 1 was determined by single-crystal X-ray diffraction (Figure 1, Table S1). Complex
, [Ir(L1)(deeb)Cl], lies in a mirror plane; the pyridyl ring of L1, the iridium(III) and the
2
Cl
2
/Et
2
O at -18°C, and the
1
chloride all lying directly in the plane. The tridentate L1 shows a tripodal chelation motif,
3

analogous to that seen previously.[21] The remaining coordination sphere of 1 consists of the
deeb N^N ligand and a chloride anion. The arrangement of ligands is unusual, as the chloride
coordinates trans to the pyridine of L1 and not trans to a cyclometalated carbon ligand as
generally observed in tridentate complexes,[23-27] although this coordination arrangement was
found in our previous complexes based on L1.[21] The Ir1-Cl1 bond is 2.346(3) Å, which is
within the range of distances we have previously reported for [Ir(C^N^C)(dtBubpy)Cl]
complexes (where C^N^C is a substituted 2-benzhydrylpyridine),[21] and is similar to the Ir-Cl
2
+
distance seen in the related complex [Ir(tpy)(dmbpy)Cl] (where tpy = 2,2':6',2''-terpyridine and
dmbpy = 4,4'-dimethyl-2,2'-bipyridine).[28] The Ir-NN^N bonds [Ir1-N18: 2.134(6) Å] are in the
same range as both our related [Ir(C^N^C)(dtBubpy)Cl] complexes,[21] as well as the deeb
complex [Ir(topy)
2
(deeb)]PF [where topyH = 2-(p-tolyl)pyridine].[29] The Ir-N distance
6
involving the nitrogen from L1 [Ir1-N1: 2.038(8) Å] is markedly shorter than the Ir-NN^N
distance. The Ir-CC^N^C bonds [Ir1-C9: 2.027(7) Å] are shorter again than the Ir-N bonds, in
agreement with our previous observations.[21] The bite angle of the N^N ligand is 76.1(3)°, in
line with other iridium complexes possessing bidentate diimine ancillary ligands.[30-33,21] The
C-Ir-C angle [85.2(4)°] and N-Ir-C angle within L1 [88.0(2)°] are significantly larger due to the
formation of three 6 membered chelate rings in the C^N^C ligand.
4

Figure 1. Solid-state structure of complex 1, thermal ellipsoids are drawn at the 50 % probability
level. Hydrogen atoms and solvent molecules are omitted for clarity (color code: C = grey, N =
purple, O = red, Cl = green and Ir = blue).
Electrochemical properties
5

Figure 2. Cyclic voltammograms (in blue solid lines) and differential pulse voltammetry (in
-1
+
dotted black lines) carried out in degassed CH
2
Cl at a scan rate of 100 mV s , with Fc/Fc as the
2
internal reference, referenced to NHE (0.70 V vs. NHE).[34]
The electrochemical properties of 1 were evaluated by cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) in deaerated CH
2
Cl solution at 298 K at a scan rate of 100
2
-1
+
mV s using Fc/Fc as the internal reference and referenced with respect to NHE (0.70 V vs.
NHE).[34] The electrochemical data are summarized in Table 1 and the voltammograms are
shown in Figure 2. Complex 1 exhibits a quasi-reversible single electron oxidation wave at 1.21
V (∆̿ Ɣ 88 mV), which is assigned to the Ir(III)/Ir(IV) redox couple, with contributions from
+
the two phenyl rings of L1 and the chloro ligand. Compared to [Ir(L1)(dtBubpy)Cl], R1, (Figure
ox.
3
, E1/2 1.04 V vs. NHE)[21] the oxidation potential in 1 is significantly anodically shifted by
1
70 mV, reflecting the electron-withdrawing capacity of the ethyl ester groups of the N^N
ligand, which modifies the electron density on iridium. However, the oxidation potential of 1 is
ox.
less positive than that of [Ir(ppy)
2
(deeb)]PF , R2, (E1/2 = 1.57 V in deaerated MeCN vs NHE,
6
where ppy is 2-phenylpyridinato).[35] Upon scanning to negative potential, 1 shows a single
6

quasi-reversible reduction wave at -0.94 V (∆̿ = 99 mV), which is monoelectronic as inferred
+
from the DPV. The electron-withdrawing effect of the ethyl ester groups of the N^N ligand
red.
results in a large anodic shift of 610 mV in the reduction wave of 1 compared to R1 (E1/2 -1.58
V vs NHE).[21] Complex R2 showed two reversible reduction waves in MeCN. The first
reduction located at -0.76 V is assigned to the reduction of the deeb ligand while the second one
at -1.30 V is due to the reduction of the phenylpyridinato.[35] Thus, the reduction of the deeb
ligand in 1 is shifted to more negative potentials compared to the same reduction in R2. DFT
calculations of the previously reported R1 indicated that both the HOMO and HOMO-1 are close
in energy and involve the iridium and chlorine atoms and the two phenyl rings of L1.[21] As can
be seen in Figure 4 the same electron density distribution is found in 1. DFT calculations also
show that the three lowest unoccupied orbitals are exclusively localized on the deeb ligand in 1
(
Figure 4), while the LUMO+1 is primarily on the pyridyl of L1 in R1, illustrating the stronger
accepting character of deeb. The ∆̿ΠΓΒΝΦꢀfor 1 (2.18 eV) is markedly smaller than that of R2
(
∆̿ΠΓΒΝΦ = 2.33 V).[35]
PF6
N
N
Ir
N
2
CO Et
N
N
N
Ir
N
2
CO Et
Cl
R1
R2
Figure 3. Structures of reference complexes R1 and R2.
Table 1: Selected electrochemical properties of complex 1
a
Electrochemistry
7

ÅÎ
È»º
̊/̋
b
c
c
¡
/ V ∆¡ / mV ¡
/ V ∆¡ / mV ∆¡
/ V ¡¤«©« /eV
¡¨±©« / eV
̊
/̋
Æ
Æ
È»ºÅÎ
1
1.21
in degassed CH
88
Cl
-0.97
99
2.18
-5.31
-3.13
a
-1
+
2
2
at a scan rate of 100 mV s with Fc/Fc as internal reference, and
+
referenced with respect to NHE (Fc/Fc = 0.70 V in CH
2
Cl ); [36,34,37] [36,34,37] [36,34,37]
2
b
c
∆
E
is the difference (V) between first oxidation and first reduction potentials; E
vs Fc/Fc + 4.8] eV.[34]
= -
redox
ox/red
HOMO/LUMO
+
[
E
Figure 4. Frontier molecular orbitals of 1 computed through DFT (M06 functional, see the SI for
details) and represented using a contour threshold of 0.03 au.
Photophysical properties
The photophysical data for 1 recorded in CH
2
Cl at 298 K are shown in Figure 5 and the
2
data summarized in Table 2. The absorption profile of 1 differs significantly from that of R1.
4
-1
-1
Complex 1 shows intense high-energy absorption bands (ε on the order of 3.5 × 10 M cm )
1
1
below 250 nm, which are ascribed to π–π* ligand-centered ( LC) transitions localized on the
4
-1
-1
deeb ligand. A moderately intense band (ε on the order of 1.5 × 10 M cm ) at 319 nm is
assigned to a ligand-centered (LC) transition on the deeb with a small CT character (see below).
3
3
-1
-1
Weaker bands (ε on the order of 5-6 × 10 and 2 × 10 M cm ) in the region of 380 – 440 nm
8

1
1
and tailing to 500 - 600 nm are attributed to a mixture of ( MLCT/ LLCT) and spin-forbidden
3
3
(
MLCT/ LLCT) transitions involving the deeb ligand. Iridium(III) complexes often do not show
absorption onsets lower in energy than 550 nm;[38-40] though, there are known examples of
neutral Ir(III) complexes showing absorption bands beyond 550 nm.[41,25,42,43]
The assignments for complex 1 were confirmed by TD-DFT calculations (see the ESI for
technical details). The two lowest singlet states, computed at 623 and 611 nm, present relatively
small intensities (oscillator strengths, f, of 0.010 and 0.056, respectively) and mainly correspond
to HOMO-1 to LUMO and HOMO to LUMO transitions. As can be seen in Figure 4, this clearly
corresponds to a mixed CT process from the metal and the phenyl rings of the C^N^C ligand
towards the deeb. The following significant vertical absorption are predicted by TD-DFT at 496
nm (f=0.071), 456 nm (f=0.027) and 443 nm (f=0.084) and these bands can be ascribed to
HOMO-2 to LUMO, HOMO to LUMO+1 and HOMO-1 to LUMO+1 transitions, respectively,
and therefore all involve strong CT character towards the deeb moiety. The more intense and
resolved band at 319 nm experimentally (see Table 2) is computed at 315 nm by TD-DFT
(
(
f=0.162) and corresponds to a more LC excitation from a low-lying orbital centered on the deeb
and partly on chlorine atom) towards the LUMO centered on the deeb as well.
Upon photoexcitation at 420 nm, 1 exhibits a broad featureless profile, indicative of an
emission with mixed CT character, with a maximum at λem = 731 nm, an emission that is
-1
significantly redshifted (99 nm, 2194 cm ) compared to R1 (λem = 630 nm).[21] The red-shifted
luminescence is due to the presence of the presence of the π-accepting deeb. The emission of 1 is
-
1
likewise red-shifted (51 nm, 2194 cm ) compared to that of R2 (λem = 680 nm).[35] The DFT
9

calculations returns an emission of the T state at 762 nm, close to the experimental value,
1
confirming emission from the lowest triplet excited state. The topology of this state, in terms of
localization of the excess α electrons, is displayed in Figure 6. As can be seen, the spin density is
mostly localized on the Ir and Cl atoms and on the ancillary ligand, the tridentate ligand playing
only a minor role in this state. This localization is consistent with the observed red-shift in
emission compared to R1 and R2. The measured photoluminescence quantum yield (ΦPL) of 1 is
0
.5%, lower than those of R1 (6%) and R2 (5%). This finding is a logical consequence of the
energy gap law, which states that the nonradiative decay rate increases with decreasing emission
energy.[44,45] Among near-infrared emissive cationic Ir(III) emitters with λem beyond 700 nm
bearing diimines as ancillary ligand, most examples exhibit ΦPL values less than 4%.[40,46-49]
However, NIR-emitting neutral Ir(III) complexes of the form [Ir(C^N) (O^O)] (where O^O a
2
substituted β-diketonate ancillary ligand) employing highly conjugated C^N ligands have
reached ΦPL of up to 16%.[50,41,51] Complex 1 exhibits a multiexponential emission decay, a
reflection of the large non-radiative decay rate constant.
10

Figure 5. The absorptivity (solid line) and photoluminescence spectra (dotted line) of 1 in
-5
CH
2
Cl at 298 K (c = 10 M).
2
Figure 6. DFT computed spin density difference plots for the lowest triplet state of 1. Both side
-3
and top views are shown and they have been drawn with a contour threshold of 3 × 10 au.
11

Table 2. Photophysical properties of complex 1.
-
1
-1
a
b
b,c
d
e
Complex λ / nm (ε / M cm )
λem / nm ΦPL / %
τ / ns
abs
1
237 (34819), 319 (14647), 384 (5105),
731 0.5
36 (73 %)
78 (19 %)
4
34 (5607), 504 (2176), 597 (1925)
3
92 (8 %)
a
b
Recorded in aerated CH Cl at 298 K; Recorded at 298 K in deaerated CH
2
Cl
2
solution (λexc = 420
2
2
c
d
nm); [Ru(bpy)
3
](PF
6
)
2
in MeCN as the reference (ΦPL = 1.8% in aerated MeCN at 298 K)[52]; λexc
=
3
78 nm.
Dye-sensitized solar cells (DSSCs)
Sandwich-type solar cells were assembled using 1-sensitised nanocrystalline TiO as the
2
working electrodes, platinized conducting glass as the counter electrode and iodide/triiodide in
acetonitrile as electrolyte. The photovoltaic performances of solar cells based 1 and N719, as
benchmark sensitizer, are summarized in Table 3. Figure 7 shows the current–voltage
-2
characteristics of the dyes under AM 1.5 simulated sunlight (100 mW cm ) and in the dark.
Table 3. Photovoltaic performance of 1.
a
J
SC / mA
a
a
a
DYE
-2
V
OC / V
FF
η / %
cm
1
0.995
8.84
0.67
0.81
0.74
0.61
0.49
4.4
N719
a
Jsc is the short-circuit current density at the V = 0 intercept, V
oc
is the open-circuit voltage at the J = 0 intercept, FF is the
device fill factor, η is the power conversion efficiency.
12

1
0
8
6
4
2
0
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Potential / V
-
Figure 7. Current-voltage curves for DSSCs constructed using 1 (orange) and N719 (red) in the
-2
dark (dashed line) and under simulated sunlight (solid line, AM1.5, 100 mW cm ).
The photovoltaic efficiency (η = 0.49%) obtained with 1 is low, but comparable with
results for iridium sensitizers reported elsewhere.[10,9,8,7,5] Both charge injection from the
excited dye into TiO and regeneration by the electrolyte are thermodynamically favourable. The
2
fill factor for the N719 device (0.61) was slightly lower than that typically obtained in optimized
devices (0.70-0.75) and the shape of the current-voltage curve is consistent with high series
resistance. Procedures used in optimized N719 devices, such as mixed solvents or additives such
as chenodeoxycholic acid in the electrolyte or dye bath are likely to improve the performance of
the N719 devices, however the conditions chosen were optimal for compound 1. We therefore
attribute the reason for the low efficiency for 1 compared to the benchmark Ru dye to be the
weak absorption in the visible region, compared to ruthenium-based photosensitizers such as
N719. The absorption spectrum of the TiO
2
electrode after immersion in the dye solution is
provided in Figure S13 and the spectral response of the DSSC is given in Figure S14. The low
13

incident photon-to current conversion efficiency (IPCE < 2%) is consistent with the poor light-
harvesting at λ > 500 nm. While these dyes absorb broadly across the visible spectrum, the low ε
-1
-1
-1
-1
(
ε ~ 2 000 M cm ) compared to ruthenium dyes (ε > 10 000 M cm ) is a limitation to their
solar cell performance.
Conclusions
In conclusion, a new panchromatically absorbing, NIR luminescent iridium(III)
complexes bearing a tripodal tris(six-membered) chelate ligand has been obtained and
comprehensively characterized, including by single crystal X-ray diffraction. The absorption
spectrum tails off at 700 nm, much further than most neutral iridium complexes while the
emission is significantly shifted into the NIR, with a maximum of 731 nm. DSSCs using 1 as the
dye achieved only modest efficiency of 0.49%, comparable to other Ir(III) dyes. This was
attributed to the modest absorption coefficient, which leads to weak light harvesting in the
visible region and low short-circuit current.
Appendix A. Supplementary data
CCDC 1583853 contains the supplementary crystallographic data for 1. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. NMR and MS spectra for 1, Supplementary
crystallographic data, supplementary electrochemical and photophysical data. Description of the
DFT/TD-DFT protocol. Experimental details for the DSSC assembly and testing. Plots of the
absorption spectra of 1-sensitized TiO and IPCE spectrum of the DSSC.
2
14

Acknowledgements
C.H. acknowledges the Région Bretagne, France for funding. EZ-C acknowledges the University
of St Andrews and EPSRC (EP/M02105X/1) for financial support. We thank Umicore AG for
the gift of materials. We thank the EPSRC UK National Mass Spectrometry Facility at Swansea
University for analytical services. This research used computational resources of 1) the GENCI-
CINES/IDRIS, 2) the CCIPL (Centre de Calcul Intensif des Pays de Loire), 3) a local Troy
cluster. EAG and HVF thank the ERC for a Starting Grant (p-TYPE, 715354).
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TOC
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A panchromatic absorbing, NIR emitting neutral iridium complexes bearing a tripodal 2-(bis(4-
tert-butyl)phenyl)methyl)pyridinato (dtBubnpy) ligand, a bidentate diethyl [2,2'-bipyridine]-
,4'-dicarboxylate (deeb)] ligand and a chloro ligand is reported. This complex was used as a dye
in a dye-sensitized solar cell.
(
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