Are alkynyl spacers in ancillary ligands in heteroleptic bis(diimine)copper(I) dyes beneficial for dye performance in dye-sensitized solar cells?

Article abstract of DOI:10.3390/molecules25071528

The syntheses of 4,4⁰-bis(4-dimethylaminophenyl)-6,6⁰-dimethyl-2,2⁰-bipyridine (1), 4,4⁰-bis(4-dimethylaminophenylethynyl)-6,6⁰-dimethyl-2,2⁰-bipyridine (2), 4,4⁰-bis(4-diphenylaminophenyl)-6,6⁰-dimethyl-2,2⁰-bipyridine (3), and 4,4⁰-bis(4-diphenylaminophenylethynyl)-6,6⁰-dimethyl-2,2⁰-bipyridine (4) are reported along with the preparations and characterisations of their homoleptic copper(I) complexes [CuL₂][PF₆] (L = 1-4). The solution absorption spectra of the complexes exhibit ligand-centred absorptions in addition to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu(5)(L_ancillary)]⁺ dyes in which 5 is the anchoring ligand ((6,6⁰-dimethyl-[2,2⁰-bipyridine]-4,4⁰-diyl)bis(4,1-phenylene))bis(phosphonic acid) and L_ancillary = 1-4 have been assembled on fluorine-doped tin oxide (FTO)-TiO₂ electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu(5)(1)]⁺ and [Cu(5)(3)]⁺. The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit current density (J_SC), confirmed by lower values of EQE_max. Addition of a co-absorbent (n-decylphosphonic acid) to [Cu(5)(1)]⁺ lead to no significant enhancement of performance for DSCs sensitized with [Cu(5)(1)]⁺. Electrochemical impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the analysis shows that more favourable electron injection into TiO₂ is observed for sensitizers without the alkynyl spacer and confirms higher J_SC values for [Cu(5)(1)]⁺,.

Full text of DOI:10.3390/molecules25071528

molecules
Article
Are Alkynyl Spacers in Ancillary Ligands in
Heteroleptic Bis(diimine)copper(I) Dyes Beneficial for
Dye Performance in Dye-Sensitized Solar Cells?
Guglielmo Risi , Mariia Becker , Catherine E. Housecroft and Edwin C. Constable *
Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, CH-4058 Basel, Switzerland;
guglielmo.risi@unibas.ch (G.R.); mariia.karpacheva@unibas.ch (M.B.); catherine.housecroft@unibas.ch (C.E.H.)
* Correspondence: edwin.constable@unibas.ch; Tel.: +41-61-207-1001
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Claudia Dragonetti and Alessia Colombo
Received: 11 March 2020; Accepted: 26 March 2020; Published: 27 March 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: The syntheses of 4,40-bis(4-dimethylaminophenyl)-6,60-dimethyl-2,20-bipyridine
(
1
),
diphenylaminophenyl)-6,60-dimethyl-2,20-bipyridine (
6,60-dimethyl-2,20-bipyridine (
) are reported along with the preparations and characterisations
of their homoleptic copper(I) complexes [CuL2][PF6] (L = ). The solution absorption spectra
4,40-bis(4-dimethylaminophenylethynyl)-6,60-dimethyl-2,20-bipyridine
(
2
),
4,40-bis(4-
3
), and 4,40-bis(4-diphenylaminophenylethynyl)-
4
1–4
of the complexes exhibit ligand-centred absorptions in addition to absorptions in the visible
region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic
[Cu(
4,40-diyl)bis(4,1-phenylene))bis(phosphonic acid) and Lancillary
5 5
)(Lancillary)]+ dyes in which is the anchoring ligand ((6,60-dimethyl-[2,20-bipyridine]-
= 1–4 have been assembled on
fluorine-doped tin oxide (FTO)-TiO2 electrodes in dye-sensitized solar cells (DSCs). Performance
parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells
for each dye) reveal that the best performing dyes are [Cu(
spacers are not beneficial, leading to a decrease in the short-circuit current density (JSC), confirmed by
lower values of EQEmax. Addition of a co-absorbent (n-decylphosphonic acid) to [Cu( )(
)]+ lead to
no significant enhancement of performance for DSCs sensitized with [Cu( )(
)]+. Electrochemical
5)(1 5)(3
)]+ and [Cu( )]+. The alkynyl
5
1
5
1
impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the analysis shows
that more favourable electron injection into TiO2 is observed for sensitizers without the alkynyl
spacer and confirms higher JSC values for [Cu(5)(1)]+.
Keywords: copper; dye-sensitized solar cell; 2,20-bipyridine; alkynyl group; phosphonic acid anchor
1. Introduction
Future energy strategies will rely increasingly upon sustainable methods of energy generation in
order to conform to the United Nations Sustainable Development Goals. Emerging energy economies
identify solar-to-electricity conversion as a key technology. Currently, the most widely deployed
technologies are based upon semiconductor photovoltaics, but a promising methodology is the so-called
dye-sensitized solar cell (DSC). DSCs [
semiconductor such as nanoparticulate TiO2 functionalized with a material that absorbs visible
light [ ]. The latter comprises the working electrode in an n-type DSC. Apart from the semiconductor
1] convert solar to electrical energy using a wide-bandgap
2–5
and the dye, the other crucial components of the device are the electrolyte (incorporating a redox
couple to facilitate electron and hole transport and to regenerate the ground-state of the dye after
excitation), and the counter electrode (typically conductive glass coated with a thin layer of platinum
to catalyse the regeneration of the reduced form of the redox couple). Some critical advances with
DSCs using aqueous electrolytes have been made [
6], but on the whole, electrolytes in the devices are
Molecules 2020, 25, 1528; doi:10.3390/molecules25071528
www.mdpi.com/journal/molecules

Molecules 2020, 25, 1528
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based on organic solvents or polymers and the most popular redox shuttle in the electrolyte remains
I3−/I− [
]. Using metal-free (i.e., organic), ruthenium(II) or zinc(II) porphyrin sensitizers, it has been
possible to achieve photoconversion efficiencies (PCE, ) of up to ca. 14% [ 14]. The parameters that
7
η
8–
characterize a DSC and contribute to the PCE are the open-circuit voltage (VOC) and the short-circuit
current density (JSC). However, it is difficult to predict how a change in the molecular structure of a dye
will influence JSC and VOC. Theoretical approaches providing insight into these parameters are critical,
for example, Quantum Theory of Atoms in Molecules (QTAIM) has been used to relate JSC and VOC to
the electronic properties of organic dyes and to provide strategies for better organic dye design [15].
Significant efforts have also been made in fabricating alternative working electrodes, for example,
by introducing graphene between the photoanode and dye, which leads to an increase in the rate of
electron injection into the semiconductor [16]. Dye design typically centres on fulfilling ‘push-pull’ or
donor-π-acceptor (D-π-A) characteristics (Scheme 1), but in addition, structural design should aim to
suppress recombination pathways. A relevant example is the design of T-shaped donor-acceptor dyes
incorporating phenoxazine/phenothiazine–triphenylamine donor domains [17].
Scheme 1. Schematic representation of a donor-acceptor (‘push-pull’) dye. In a heteroleptic
bis(diimine)copper(I) dye, the anchoring ligand is the anchor/acceptor domain, the copper(I) centre is
the linker, and the ancillary ligand is the donor.
While ruthenium dyes are still regarded as state-of-the-art, the abundance of ruthenium in
the Earth’s crust is only about 0.001 ppm [18], making explorations of sensitizers incorporating
Earth-abundant first-row metals, such as copper [19
Significant progress has been made in the last few years in improving the PCEs of DSCs sensitized with
copper(I) dyes. Although the best values of are below 5% [27 29], systematic efforts in modifying the
–22] and iron [23–26] extremely attractive.
η
–
sensitizer, redox couple and other electrolyte components, and the use of co-sensitizers have allowed
us [28–31] and others [27,32] to show that there is a viable future for copper(I) sensitizers.
A key factor in performance enhancement of copper(I)-based DSCs is systematic modification of
the structures of ‘push-pull’ dyes (Scheme 1). The ‘push-pull’ characteristics are readily tuned in a
heteroleptic copper(I) complex (in which the Cu atom is the linker) through structural modification of the
anchoring and ancillary ligands, such that in a charge-separated state, the hole is located on the ancillary
ligand and the electron on the anchoring ligand. Donor-
characteristics and introducing alkynyl units to optimize electronic communication and conjugation
has been found to be beneficial in some families of dyes [33]. The +I nature of alkynyl substituents [34
π-acceptor (D-π-A) dyes fulfil the ‘push-pull’
]
increases the electron density on the metal centre and enhance the population of the metal-to-ligand
charge transfer (MLCT) state [35]. Within the context of bis(diimine)copper(I) complexes, Castellano
and co-workers demonstrated that the incorporation of phenylethynyl groups into the 4,7-positions
of 2,3,7,8-tetraalkylsubstituted-1,10-phenanthrolines resulted in a significant red-shift in the MLCT
absorptions and shifts to more positive potentials for both the copper(I) oxidation and ligand-based
reduction processes compared to the parent bis(2,3,7,8-tetramethyl-1,10-phenanthroline)copper(I)
complex. Most importantly, these beneficial effects were gained without loss in the long excited-state
lifetimes, which result from the steric crowding imposed by the 2,3,7,8-tetraalkyl groups [36].
2,20-Bipyridine ligands bearing phenylethynyl or 4-substituted-phenylethynyl substituents in the 4,40-
or 5,5’-positions have been the focus of some attention, see for example [37
our knowledge, the use of related bis(diimine)copper(I) complexes in DSCs has not been explored.
We now report the preparation and characterization of ligands (Scheme 2), their homoleptic
–40], although, to the best of
1–4

Molecules 2020, 25, 1528
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[CuL2][PF6] complexes, and the application of these compounds for the in situ assembly of heteroleptic
copper(I) dyes on FTO/TiO2 photoanodes (FTO = fluorine-doped tin oxide) in DSCs. Ligands
1–4
contain peripheral electron-donating NR2 groups on the ancillary ligands, which are also known to be
beneficial in stabilizing the hole remote from the semiconductor surface [21].
Scheme 2. Structures of compounds
for 1–4.
1–5. Atom labelling for NMR spectroscopic assignments are given
2. Results and Discussion
2.1. Ligand Syntheses and Characterization
Compound was prepared using a Kröhnke approach [41] as shown in Scheme 3. For
the diphenylamino-derivative , the most convenient route was a palladium-catalysed coupling
of 4,40-bis(4-bromophenyl)-6,60-dimethyl-2,20-bipyridine and Ph2NH in the presence of base
(Scheme 4) [42 44]. Scheme 5 summarizes the syntheses of ligands and . The high-resolution
1
3
–
2
4
mass spectra of compounds 1, 2, 3 and 4 exhibited base peaks at m/z 423.2544, 471.2548, 671.3166 and
719.3162 corresponding to the [M + H]+ ions. The solid-state IR spectra of the ligands are shown
in Figures S1–S4 (see Supporting Information). The presence of the alkynyl spacer gives rise to an
absorption at 2198 cm−1 in
2 4, the stretching mode being IR active due to the
and 2207 cm−1 in
asymmetrical substitution.
1
The H and 13C{1H} NMR spectra of ligands
1–
4
were assigned using two-dimensional (2D)
1
methods. Figures S5–S8 (see Supporting Information) display the H NMR, NOESY, HMQC and
HMBC spectra of compound 1
, and atom labelling is given in Scheme 2. Protons HB2 and HB3 are
distinguished by the NOESY cross-peaks between signals for HB2 and HA3/HA5 and between HB3
and HNMe (Figure S6). The NOESY cross-peak between the signals for HA5 and HA6-Me (Figure S6)
distinguished HA5 from HA3. The resonances for HA5 from HA3 were assigned in a similar manner in
compounds
at 86.2 and 95.3 ppm in
2
–
4. In compounds
2
and
4
, the alkyne unit was characterized by 13C NMR resonances
; the signals were assigned from the HMBC
δ
2
and
δ
86. and 94.0 ppm in 4
2

Molecules 2020, 25, 1528
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cross-peaks to HA5 and HB2, respectively. NMR spectra for
Supporting Information.
2–
4
are shown in Figures S9–S16 in the
Scheme 3. Reaction scheme for the preparation of 1. Conditions: (i) piperidine, MeOH, reflux overnight;
(ii) 1-(2-oxopropyl)pyridin-1-ium chloride, NH4OAc in EtOH, reflux overnight. Atom labelling for
NMR spectroscopic assignments of the intermediate are given; see also Scheme 2.
Scheme 4. Reaction scheme for the preparation of 3
. Conditions: (i) Pd(tBu3P)2, NaOtBu, toluene,
100 ◦C overnight.
Scheme 5. Reaction scheme for the preparations of
CuI, PPh3, Me3SiC
CH in Et3N, 80 ◦C overnight; (ii) K2CO3 in MeOH, room temperature, 2 h;
(iii) Pd(dppf)Cl2, CuI, PPh3 in Et3N, 80 ◦C overnight. (dppf = 1,10-bis(diphenylphosphano)ferrocene).
2 (R = Me) and 4 (R = Ph). Conditions: (i) Pd(dppf)Cl2,
≡
The solution absorption spectra of compounds
absorptions assigned to
to intra-ligand charge-transfer (ILCT) arising from transfer of charge from the electron-donating Me2N
groups to the electron-poor bipyridine [21]. Introduction of NPh2 in place of NMe2 groups red-shifts
the absorption spectra with a concomitant increase in values of the extinction coefficient (Figure 1). A
1 and 2 (Figure 1) exhibit intense high-energy
←π transitions [45]. Absorptions on the edge of the visible region are assigned
π*
comparison of the spectra of
2 with 1, and of 4 with 3 shows the effect of extending the π-conjugation
as the alkynyl unit is introduced, extending the absorption spectra towards longer wavelengths.

Molecules 2020, 25, 1528
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Figure 1. Solution absorption spectra of 1–4 (CHCl3, concentrations 2.1–2.6 × 10−5 mol dm−3).
2.2. Syntheses and Characterization of Homoleptic Copper(I) Complexes
The [CuL2][PF6] compounds with L = and were prepared by reaction of [Cu(MeCN)4][PF6]
1,
2
,
3
4
with two equivalents of ligand in a mixture of CH2Cl2 and CH3CN. The dark red products were
precipitated from MeCN by addition of Et2O, and were isolated in yields of between 33.1% and
72.9%. The high-resolution electrospray mass spectra of the compounds exhibit peaks arising from
the [Cu(
Supporting Information). The IR spectra of the compounds exhibit a strong absorption at 838 cm−1 for
[Cu( )2][PF6],
)2][PF6], 842 cm−1 for [Cu( )2][PF6], 831 cm−1 for [Cu( )2][PF6], and 833 cm−1 for [Cu(
characteristic of the [PF6]− ion, and for [Cu(
)2][PF6] and [Cu( )2][PF6], absorptions at 2189 and
L
)2]+ ions with isotope distributions that agree with those predicted (Figures S17–S20 in
1
2
3
4
2
4
2200 cm−1 are consistent with the presence of the alkynyl unit (Figures S21–S24). The 1H and 13C{1H}
NMR spectra were assigned using NOESY, COSY, HMQC and HMBC methods (Figures S25–S36).
Coordination to copper(I) results in a broadening of the methyl signal (Figure 2), as well as shifting of
the signal arising from proton HA5
.
Figure 2. Comparison of the aromatic regions of the 1H NMR spectra (500 MHz, 298 K) of (
in CDCl3 and ( ) [Cu( )2][PF6] in CD3CN. The alkyl regions are shown in ( and ( ) [Cu(
See Scheme 2 for atom labelling. Scale: δ/ppm. * = residual CHCl3.
a
) ligand
4
b
4
c)
4
d
4)2][PF6].

Molecules 2020, 25, 1528
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The solution absorption spectra of the complexes were recorded in CHCl3 (Figure 3) and exhibit
intense, high-energy absorptions arising from ligand-centred, spin-allowed transitions in addition
to absorptions in the visible region. The lowest energy absorption around 500 nm is assigned to
metal-to-ligand charge-transfer (MLCT). However, the band exhibits a higher extinction coefficient
than is typical of bis(diimine)copper(I) complexes [19
observed in the free ligands (Figure 2). Inspection of Figure 3 reveals that the intensity of the absorption
at ca. 500 nm is greatest for [Cu( )2][PF6] and [Cu( )2][PF6], indicating that the Ph2N group plays a role.
We also note that on going from [Cu( )2][PF6] to [Cu( )2][PF6], and from [Cu( )2][PF6] to [Cu( )2][PF6],
-conjugation as
,46] due to overlap with the ligand-centred ILCT
3
4
1
2
3
4
the absorption spectra extends further towards the red, consistent with extension of
the alkynyl unit is introduced.
π
Figure 3. Solution absorption spectra of [CuL2][PF6] with L =
1, 1.0 × 10−5 mol dm−3 mol dm−3 for L = 2–4).
1–
4
(CHCl3, 3.4
×
10−5 mol dm−3 for L =
DFT calculations on [Cu(2
)2]+ as a representative complex confirmed that the orbital composition
in the HOMO-manifold contains both metal and Me2N character, while the LUMO and LUMO+1
are predominantly bpy character (Figure 4a). A restricted hybrid HF-DFT SCF calculation was also
performed on [Cu(
frontier molecular orbital ( MO) characteristics (Figure S37). Note that the DFT-optimized geometries
reveal that in ligand , the phenyl ring is twisted with respect to the pyridine ring to which it is bonded,
as expected for minimizing inter-ring H...H contacts. In contrast, in , the aromatic rings on either
2
)2]+ using a polarizable continuum solvation model (CHCl3) and revealed similar
1
2
side of the alkynyl unit are essentially coplanar. This is consistent with previously reported theoretical
results [47] and with the distribution of dihedral angles between alkynyl-connected phenyl rings for
compounds found in the Cambridge Structural Database (CSD v. 5.4.1 [48]) (Figure 5).

Molecules 2020, 25, 1528
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Figure 4. Orbital compositions of the highest occupied and lowest unoccupied molecular orbitals
in ( ) [Cu( ) [Cu( )(
)2]+ and ( )]+ calculated using DFT (B3LYP level, 6-31G* basis set, in vacuum).
Geometry optimizations were at the same DFT level of calculation.
a
2
b
5 2
Figure 5. Distribution of torsion angles (defined in the inserted structure and considered for a range
0–90o) for alkynyl-connected phenyl rings in the Cambridge Structural Database (CSD v. 5.4.1), with
analysis using Conquest, version 2.0.4 [49].
2.3. Solid-State Absorption Spectra of the Surface-Bound Dyes and DSC Performances
The homoleptic compounds [CuL2][PF6] where L =
ancillary ligand to assemble surface-bound heteroleptic dyes [Cu(
ligands, surfaces-as-complex (SALSAC) ligand exchange methodology [50
anchoring ligand (Scheme 2) was selected because of its superior binding to TiO2 with respect to
1
–4
, were used as a source of copper(I) and
)(L)]+ using the surfaces-as-
51]. The phosphonic acid
1
–
4
5
,
5
carboxylic acids [51], and because copper(I) dyes with phosphonic acids or phosphonates [52] with a
phenyl spacer between the PO(OH)2 and bpy units are superior to carboxylic and cyanoacrylic acids in
terms of DSC performance and/or ease of synthesis [53–55].
The solid-state absorption spectra of the heteroleptic dye-functionalized FTO/TiO2 electrodes
were recorded. In order to better differentiate the absorption maxima in the visible region from the
higher-energy tail arising from TiO2, the first-derivative spectra [56] were determined (Figure 6). The
maxima at around 510 nm in [Cu(
confirm that a red-shift in the absorptions accompanies the introduction of the alkyne spacer in the
5)(1 5)(3 5)(2 5)(4
)]+ and [Cu( )]+, and 520 nm in [Cu( )]+ and [Cu( )]+

Molecules 2020, 25, 1528
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heteroleptic compounds. Absorption maxima for the surface-bound dyes compare with solution values
of 500 nm and 493 nm in [Cu( )2][PF6] and [Cu( )2][PF6], respectively, and 515 nm and 507 nm in
1
3
[Cu(2)2][PF6] and [Cu(4)2][PF6], respectively (Figure 3).
Figure 6. First-derivative solid-state absorption spectra of transparent FTO/TiO2 electrodes
functionalized with the [Cu(5)(L)]+ dyes (L = 1, 2, 3 and 4).
DFT calculations on [Cu(
contain both metal and ancillary-ligand character, while the LUMO is localized on the anchoring ligand
(Figure 4b); the LUMO + 1 exhibits mainly ancillary ligand bpy character. Similar orbital characters
are revealed for [Cu( )(
)]+ (Figure S38 in the Supporting Information), indicating that the frontier
5)(2
)]+ show that the orbital compositions of the HOMO and HOMO–1
5
5
1
orbital characteristics of the heteroleptic dyes are not significantly perturbed by the presence of the
alkynyl spacers. Thus, the localization of LUMO character on the anchoring domain is consistent with
what is desired for efficient electron injection from the anchoring ligand to the semiconductor in a DSC.
Table 1 summarizes the performances of sets of four DSCs sensitized with the dyes [Cu(
and [Cu( )(
)]+ compared with the performance of a DSC containing the reference ruthenium(II)
dye N719. In the final column of Table 1, we present relative efficiency values with N719 set at an
arbitrary 100%. We [57] and others [58 59] find it useful to include relative values so as to permit
5)(1
)]+
5
2
,
valid comparisons of data recorded on different solar simulators or in different laboratories [57]. The
first point to note in Table 1 is the reproducibility of the DSC performance parameters for a given
sensitizer. This is also observed in the J–V (J = current density, V = voltage) plots in Figure 7 and
Figure S39. The most noticeable feature is that the DSCs containing [Cu(
and VOC values than those with [Cu( )(
)]+, indicating that the introduction of the alkynyl spacer in
is not beneficial. The PCEs for DSCs sensitized with [Cu( )(
)]+ are in the range 1.66–1.79%, which
5)(2
)]+ have both lower JSC
5
1
2
5
1
corresponds to 31.0–33.4% of the PCE of the reference DSC containing the ruthenium(II) dye N719.
Figure 8 and Figure S40 show the external quantum efficiency (EQE) spectra for the cells in Table 1,
with values of λmax
≈
490 nm and EQEmax
≈
38% for [Cu(
5
)(
1
)]+, and λmax
≈
470 nm and EQEmax
≈
34% for [Cu(5)(2)]+. These data are consistent with the differences in the J–V curves for the two dyes.

Molecules 2020, 25, 1528
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Table 1. Performance parameters (under 1 sun illumination) of duplicate, masked DSCs on the day of
sealing the cells with the dyes [Cu( )( )(
)]+ and [Cu( )]+. The data are compared to a DSC containing
N719 and relative η values are with respect to N719 set at 100%.
5
1
5 2
SC/mA cm−2
Dye
Cell Number
V
OC/mV
ff/%1
η/%
Relative η/%
J
[Cu(5)(1)]+
[Cu(5)(1)]+
[Cu(5)(1)]+
[Cu(5)(1)]+
[Cu(5)(2)]+
[Cu(5)(2)]+
[Cu(5)(2)]+
[Cu(5)(2)]+
N719
1
2
3
4
1
2
3
4
4.54
4.69
4.74
4.55
3.59
3.64
3.54
3.57
14.13
541
539
539
524
514
508
513
514
602
67.6
68.1
70.1
69.7
70.7
69.6
71.2
68.4
63.0
1.66
1.72
1.79
1.66
1.30
1.29
1.29
1.25
5.36
31.0
32.1
33.4
31.0
24.3
24.0
24.1
23.4
100
1ff = fill-factor.
Figure 7. J–V (J = current density, V = voltage) curves for sets of four dye-sensitized solar cells (DSCs)
sensitized with [Cu(
the cells.
5)(1 5)(2
)]+ (blue curves) and [Cu( )]+ (red curves) measured on the day of sealing
Figure 8. External quantum efficiency (EQE) spectra for sets of four DSCs sensitized with [Cu(
(blue curves) and [Cu(5)(2)]+ (red curves) measured on the day of sealing the cells.
5)(1
)]+
A critical question is why the introduction of the alkynyl spacer into the dye leads to a blue-shifted
EQE maximum (Figure 8) rather than the red-shift observed in the absorption spectra (Figures 3 and 6).
This reveals that the injection of electrons into the semiconductor is not benefitting from an extension
of light absorption towards longer wavelengths. It is informative to compare the performance data and
EQE spectra for DSCs with [Cu(5)(1 5)(2 5
)]+ and [Cu( )]+ with those sensitized with [Cu( )(dmbpy)]+

Molecules 2020, 25, 1528
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where dmbpy is 6,60-dimethyl-2,20-bipyridine (Scheme 6). Using a common electrolyte, redox shuttle
and sun simulator as employed in the present study (see Materials and Methods section), duplicate
DSCs with [Cu(
and = 1.46% and 1.35% versus 5.91% for N719 (i.e., relative
values were 42% and 38% with a value of λmax = 470 nm [57]. These data are reminiscent of those
for DSCs containing [Cu( )(
)]+. We therefore see an enhancement of DSC performance on going
from the dye [Cu( )( )(
)(dmbpy)]+ to [Cu( )]+, but not from [Cu( )(dmbpy)]+ to [Cu( )]+. The
comparison of EQE spectra shown in Figure 9 illustrates a gain in EQE arising from photon harvesting
at longer wavelengths on going from [Cu( )(
)(dmbpy)]+ to [Cu( )]+, but this enhancement is lost
once the alkynyl spacer is introduced with the EQE spectrum of the DSC with [Cu( )(
)]+ looking
very similar to that of the cell with [Cu(
)(dmbpy)]+. An additional comparison can be made between
the EQE spectra of DSCs sensitized with the dyes [Cu( )( )(
)]+ and [Cu( )]+ (see Scheme 6 for the
structures of and ), [Cu( )( )( corresponding to EQEmax is 470 nm
)]+ and [Cu( )]+. The value of
for [Cu( )(
)]+ [60], the same as for [Cu( )(dmbpy)]+ and [Cu( )]+. Introducing electron-donating
4-methoxy groups on going from ancillary ligand to shifts λmax in the EQE spectrum of DSCs with
[Cu( )( )( )(
)]+ to 480 nm [60], consistent with the red-shift seen on going from [Cu( )]+ to [Cu( )]+.
The photoconversion efficiencies of DSCs with [Cu( )(
)]+ (peripheral NMe2 groups, Table 1) are
similar to those sensitized with [Cu( )(
)]+ (peripheral OMe groups [60]); the electrolyte and redox
5
)(dmbpy)]+ exhibited values of JSC = 3.46 and 3.79 mA cm−2, VOC = 522 and 527 mV,
η
η = 24.7% and 22.8%). The EQEmax
5
2
5
5
1
5
5 2
5
5 1
5
2
5
5
6
5 7
6
7
5
1
5
2
λ
5
6
5
5)(2
6
7
5
7
5
6
5 1
5
1
5
7
shuttle are constant throughout. The comparison of this series of dyes reveals the beneficial effects of
introducing the electron-donating, peripheral substituents, but the ‘blocking’ effect that the alkynyl
group imparts on electron transfer and ultimate injection.
Scheme 6. Structures of ancillary ligands dmbpy, 6, and 7, and the co-adsorbent n-decylphosphonic
acid (DPA).
Figure 9. A comparison of the EQE spectra for DSCs sensitized with [Cu(
5 1 5)(2
)]+ (blue curve), [Cu( )]+
)(
(red curve) and [Cu(
the cells.
5
)(dmbpy)]+ (black curve, data from ref [57]), measured on the day of sealing

Molecules 2020, 25, 1528
11 of 24
A similar trend in DSC performances is observed for dyes [Cu(
)( )(
)]+ versus [Cu( )]+. Introduction of the alkynyl unit on going from
SC values although values of VOC are little affected (Figure 10 and Table 2). The change from Me2N
(ligands and ) to Ph2N (ligands and ) substituents results in a small improvement in DSC
performance, and this is more noticeable on going from [Cu( )( )( )(
)]+ to [Cu( )]+ than from [Cu( )]+
to [Cu( )(
)]+ (Tables 1 and 2). This has its origins in a small increase in JSC, which, for four cells, lies
in the range of 3.34–3.64 mA cm−2 for [Cu( )]+, and 3.96–4.24 mA cm−2 for [Cu( )]+. This trend
)( )(
is confirmed in the EQE spectra (Figure 10), which extend further to longer wavelengths for DSCs
containing [Cu( )( )( )(
)]+. As was observed for cells with dyes [Cu( )]+ and [Cu( )]+, values of λmax
in the EQE spectra for [Cu( )( )(
)]+ and [Cu( )]+ are around 470 nm and 490 nm, respectively. A
comparison of Figures 8 and 11 reveals an increase in EQEmax for [Cu( )(
)]+ (up to 42%) compared
to [Cu( )( 34%) as well as a significant extension of the spectrum for [Cu( )(
)]+ ( )]+ towards the
5
)(3
)]+ versus [Cu(
to
5
)(
4
)]+ as for
[Cu(
5
1
5
2
3
4 leads to lower
J
1
2
3
4
5
2
5
4
5 1
5
3
5
2
5 4
5
4
5
1
5 2
5
3
5 4
5
4
5
2
≈
5 4
red. These data are consistent with the improvement in JSC as the peripheral electron-donating NMe2
substituents are replaced by NPh2 groups.
Figure 10. J–V curves for sets of four DSCs sensitized with [Cu(5)(3 5)(4
)]+ (blue curves) and [Cu( )]+
(red curves) measured on the day of sealing the cells.
Table 2. Performance parameters (under 1 sun illumination) of duplicate, masked DSCs on the day of
sealing the cells with the dyes [Cu( )( )(
)]+ and [Cu( )]+. The data are compared to a DSC containing
N719 and relative η values are with respect to N719 set at 100%.
5
3
5 4
SC/mA cm−2
Dye
Cell Number
V
OC/mV
ff/%
η/%
Relative η/%
J
[Cu(5)(3)]+
[Cu(5)(3)]+
[Cu(5)(3)]+
[Cu(5)(3)]+
[Cu(5)(4)]+
[Cu(5)(4)]+
[Cu(5)(4)]+
[Cu(5)(4)]+
N719
1
2
3
4
1
2
3
4
4.82
5.25
4.64
4.89
4.24
3.96
4.08
4.05
14.25
532
523
536
533
535
530
534
531
603
67.9
70.3
68.6
66.8
69.3
68.6
66.8
70.3
60.3
1.74
1.93
1.71
1.74
1.57
1.44
1.46
1.51
5.18
32.5
36.0
31.8
32.4
29.3
26.8
27.2
28.2
100

Molecules 2020, 25, 1528
12 of 24
Figure 11. EQE spectra for sets of four DSCs sensitized with [Cu(5)(3 5)(4
)]+ (blue curves) and [Cu( )]+
(red curves) measured on the day of sealing the cells.
2.4. Effects of Adding a Co-Adsorbent
It is well established that introducing co-adsorbents with dyes on the semiconductor surface
increases values of VOC for ruthenium(II) dyes [61] and zinc(II) porphyrin dyes [62 63]. Similarly,
,
the role of the co-adsorbent chenodeoxycholic acid was critical to achieving the high PCE of the
[Cu(Lanchor)(Lancillary)]+ dye in which Lanchor was 6,60-dimesityl-2,20-bipyridine-4,40-dicarboxylic acid
and Lancillary was a 2,20-bipyridine containing electron-donating triphenylamino groups [21]. We were
therefore interested to see whether use of a co-adsorbent could enhance the performance of the dyes in
the present investigation, and we chose to investigate the dye [Cu(
(cheno) is commonly employed as a co-adsorbent and contains a carboxylic acid anchoring unit. In
contrast, anchoring ligand bears a phosphonic acid group. Since we have shown that phosphonic
5)(1
)]+. Chenodeoxycholic acid
5
anchoring ligands displace carboxylic anchors in TiO2 [51], we opted to use n-decylphosphonic acid
(DPA, Scheme 6) instead of cheno as the co-adsorbent.
Table 3 gives the DSC performance parameters for four DSCs sensitized with [Cu(5)(1
)]+ in
the presence of the co-adsorbent DPA, and the J–V curves for the cells are shown in Figure 12. The
ranges of JSC (4.42 to 4.75 mA cm−2) and JSC values (534 to 548 mV) are similar to those for cells with
no co-adsorbent (Table 1) leading to similar overall photoconversion efficiencies. The EQE spectra
(Figure 13) confirm reproducible behaviour for the four DSCs with values of EQEmax around 43%
at a value of λmax = 490 nm. Again, these results reflect those of the devices in the absence of DPA.
Evidence that the co-adsorbent is present on the semiconductor surface comes from the electrochemical
impedance spectroscopic data presented below.
Table 3. Performance parameters (under 1 sun illumination) of duplicate, masked DSCs on the day of
sealing the cells the dye [Cu(
N719 and relative η values are with respect to N719 set at 100%.
5)(1
)]+ and co-adsorbent DPA. The data are compared to a DSC containing
SC/mA cm−2
Dye
Cell Number
V
OC/mV
ff/%
η/%
Relative η/%
J
[Cu(5)(1)]+
[Cu(5)(1)]+
[Cu(5)(1)]+
[Cu(5)(1)]+
N719
1
2
3
4
4.75
4.75
4.42
4.73
14.13
534
548
545
543
602
65.1
68.9
65.9
68.8
63.0
1.65
1.79
1.59
1.77
5.36
30.8
33.4
29.6
32.9
100

Molecules 2020, 25, 1528
13 of 24
Figure 12. J–V curves for a set of four DSCs sensitized with [Cu(
measured on the day of sealing the cells.
5)(1
)]+ with DPA as a co-adsorbent,
Figure 13. EQE spectra for a set of four DSCs sensitized with [Cu(5)(1
)]+ with DPA as a co-adsorbent,
measured on the day of sealing the cells.
2.5. Electrochemical Impedance Spectroscopy (EIS)
As discussed above, the introduction of the alkynyl spacer into the dyes results in a red-shifted
absorption (Figures 3 and 6), but a blue-shifted EQE maximum (Figure 8) as well as a decrease of
EQEmax. The lowering of the charge transfer energy results in more difficult electron injection into
the semiconductor, and the values of JSC decrease. By using electrochemical impedance spectroscopy
(EIS), it is possible to investigate the multiple interfacial electronic processes in a DSC [64,65] and
we therefore decided to apply EIS in an attempt to gain insight into the origin of the poorer DSC
performances for dyes containing the alkynyl spacers. EIS is a technique that can describe electronic
processes in terms of electron and hole diffusion in the counter electrode/electrolyte/dye-semiconductor
interfaces of DSCs. Parameters including the recombination resistance (Rrec), chemical capacitance
(Cµ), electron/hole transport resistance (Rtr), electron lifetime (τ) and electron diffusion length (Ld) can
be extracted from the fits of the experimental data typically presented as Nyquist and Bode plots. All
measurements were performed at VOC conditions and a light intensity of 22 mW cm−2. The equivalent
circuit model used for fitting the measurements is shown in Figure 14.

Molecules 2020, 25, 1528
14 of 24
Rs
Rpt
DX1
Ws1
CPE1
Figure 14. Equivalent circuit used to fit the electrochemical impedance spectroscopy (EIS) data, where
Rs is the series resistance, RPt and CPE1 are the resistance and constant phase element of the platinum
counter electrode, DX1 is an extended distributed element that represents the TiO2/electrolyte interface
and WS is the Warburg element associated with diffusion of the electrolyte.
We first focused our attention on a comparison of DSCs sensitized with [Cu(5)(1 5)(2
)]+ and [Cu( )]+,
and parameters extracted from fitting of the experimental Nyquist plots (Figure 15) are given in Table S1
(all data) and Table 4 (the most representative DSC of a set of four). Going from the DSCs with
[Cu(
[Cu(
5
5
)(
)(
1
1
)]+ to those [Cu(
)]+ than [Cu(
)(
5
2
)(
)]+. This shows that more electron density is located in the conductive band
)(
)]+, which contributes to greater JSC values. The
2
)]+, the values of Rrec increase. At the same time, values of Cµ are higher for
5
of the semiconductor in the case of the dye [Cu(
5 1
short-circuit density depends not only on the charge injection, but also on the collection of photoinjected
electrons at the anode [66], and the electron diffusion length has to be greater than thickness of TiO2
for the efficient collection of electrons. The Ld value depends on Rrec and Rtr and can be calculated
from the Equation (1) where d (≈ 12 µm) is the thickness of the TiO2 layer [67].
p
Ld = d Rrec/Rtr
(1)
Figure 15. Nyquist plots of the DSCs containing dyes [Cu(5)(1 5)(2 5)(1
)]+ and [Cu( )]+, and [Cu( )]+
with the co-adsorbent DPA. Solid lines represent fitted curves and the solid-circles represent
experimental data.
Table 4. EIS parameters1 obtained from fitting the experimental data for DSCs with dyes [Cu(
5)(1
)]+
and [Cu( )( )(
5
2
)]+, and with [Cu( )]+ with the co-adsorbent n-decylphosphonic acid (DPA). Data
5 1
are for the most representative DSC of a set of four; data for all the DSCs are given in Table S1 (see
Supporting Information).
Dye
Rrec/Ω Cµ/µF Rtr/Ω τ/ms τ /ms Ld/µm Rs/Ω
R
Pt/Ω
CPt/µF
−V2 OC/mV
η/%
J
SC/mA cm
t
[Cu(
[Cu(
5
5
)(
)(
1
2
)]+
)]+
125
200
386
328
28
30
48
66
11
10
25
31
20
10
24
16
5
6
4.69
3.54
539
513
1.72
1.29
[Cu(5)(1)]+
+ DPA
384
252
39
97
10
38
10
21
6
4.75
548
1.79
1
Rrec = recombination resistance; Cµ = chemical capacitance; Rtr = transport resistance;
τ
= lifetime’ Ld = electron
diffusion length; Rs = series resistance; RPt and CPt = Pt counter electrode resistance and capacitance.

Molecules 2020, 25, 1528
15 of 24
The diffusion length does not vary significantly between the dyes [Cu(
5)(1
)]+ and [Cu(
5)(
2
)]+ and
is about two times greater than d, despite the difference in EQEmax values and red-shifted enhancement
in the range of the spectrum for [Cu( )(
)]+. The addition of the co-adsorbent DPA in the DSCs with
dyes [Cu( )(
)]+ results in a significant difference in EQEmax as well as in Ld. The diffusion length
and EQEmax values for [Cu( )( )(
)]+ with co-adsorbent increase with respect to [Cu( )]+ with no
5
1
5
1
5
1
5 1
co-adsorbent. However, this in accompanied by a decrease in Cµ and a large increase in Rrec and this
results in similar overall performances for DSCs with and without co-adsorbent.
For a well-performing DSC, the electron lifetime (
τt). This results in the efficient transport of the photoinjected electrons through the semiconductor,
and this is the case for all the DSCs (Table 4 and Table S1). The extremely high for [Cu( )(
)]+ with a
co-adsorbent (range of values = 79–99 ms for four cells) is observed due to the high Rrec values for this
dye (range = 293–384 ). The trend in lifetime values can be confirmed from the Bode plots (Figure 16)
since is related to the maximum frequency. A high in combination with increased Ld values result
τ) needs to be longer than electron transport time
(
τ
5 1
Ω
τ
τ
in a small charge loss in the semiconductor. The parameters for the counter electrode stay constant for
all DSCs.
Figure 16. Bode plots of the DSCs containing dyes [Cu(5)(1 5)(2 5)(1
)]+ and [Cu( )]+, and [Cu( )]+ with the
co-adsorbent DPA. Solid lines represent fitted curves and the solid-circles represent experimental data.
The most significant differences in EIS parameters are observed for values of the recombination
resistance and chemical capacitance, and we conclude that [Cu(5)(1)]+ has the most favourable electron
injection into the conduction band of the semiconductor.
3. Materials and Methods
1H and 13C and NMR spectra were recorded on a Bruker Avance iii-500 spectrometer (Bruker
BioSpin AG, Fällanden, Switzerland) at 298 K. The 1H and 13C NMR chemical shifts were referenced
with respect to residual solvent peaks (δ TMS = 0). A Shimadzu LCMS-2020 and a Bruker maXis
4G QTOF instrument (Bruker BioSpin AG, Fällanden, Switzerland) were used to record electrospray
ionization (ESI) and HR-ESI mass spectra, respectively. FT-infrared (IR) and absorption spectra were
measured using PerkinElmer UATR Two (Perkin Elmer, 8603 Schwerzenbach, Switzerland), Cary-5000
(Agilent Technologies Inc., Santa Clara, CA, United States) and UV-2600 (Shimadzu Schweiz GmbH,
4153 Reinach, Switzerland) spectrophotometers.
All reactions were carried out with chemicals used as received from Sigma Aldrich (Sigma Aldrich
Chemie GmbH, 89555 Steinheim, Germany) or Fluorochem (Chemie Brunschwig AG, 4052 Basel,
Switzerland) without further purification. Biotage Sfär silica HC D was purchased from Biotage
(Biotage EU, 75103 Uppsala, Sweden).

Molecules 2020, 25, 1528
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Compound
5
[
53], N,N-dimethyl-4-((trimethylsilyl)ethynyl)aniline [68], 4-ethynyl-N,N-
dimethylaniline [69], 4,40-dibromo-6,60-dimethyl-2,20-bipyridine [70] and [Cu(MeCN)4][PF6] [71
]
were prepared as previously reported in literature.
3.1. (1E,5E)-1,6-Bis(4-(dimethylamino)phenyl)hexa-1,5-diene-3,4-dione
4-(Dimethylamino)benzaldehyde (2.24 g, 15 mmol, 2.0 eq) and piperidine (148
µL, 1.5 mmol,
0.2 eq) were dissolved in MeOH (15 mL). Butan-2,3-dione (656 mL, 7.5 mmol, 1.0 eq) was dissolved in
in MeOH (12 mL) and added to the reaction over 10 min. The reaction mixture was stirred for 1.5 h and
then heated under reflux overnight. The precipitate that formed was separated by filtration, washed
with Et2O and dried under vacuum. The product was isolated as a red solid (411 mg, 1.18 mmol,
15.7%). Decomposes > 248 ◦C. 1H NMR (500 MHz, CDCl3):
(d, J = 8.8 Hz, 4H, HB2), 7.23 (d, J = 15.9 Hz, 2H, Ha), 6.71 (d, J = 8.5 Hz, 4H, HB3), 3.06 (s, 12H, HNMe).
13C{1H} NMR (126 MHz, CDCl3): /ppm 190.3 (CCO), 152.1 (CB4), 148.2 (Cb), 131.1 (CB2), 123.0 (CB1),
115.5 (Ca), 112.2 (CB3), 40.4 (CNMe). UV-VIS (CHCl3, 3.3 10−5 mol dm−3 /dm3 mol−1 cm−1
/nm 262 (
12,500), 327 (
/dm3 mol−1 cm−1 14,600), 348 ( /dm3 mol−1 cm−1 14,800), 364 ( /dm3 mol−1 cm−1 14,059),
δ
/ppm 7.79 (d, J = 15.9 Hz, 2H, Hb), 7.55
δ
×
)
λ
ε
ε
ε
ε
454 (ε/dm3 mol−1 cm−1 43,700). HRMS m/z 349.1905 [M + H]+ (calc. 349.1911).
3.2. 4,40-Bis(4-dimethylaminophenyl)-6,60-dimethyl-2,20-bipyridine (1)
1-(2-Oxopropyl)pyridin-1-ium chloride (1437 mg, 8.38 mmol, 2.5 eq) was dissolved in EtOH (150 mL)
under vigorous stirring. The (1E,5E)-1,6-Bis(4-(dimethylamino)phenyl)hexa-1,5-diene-3,4-dione
(1167 mg, 3.35 mmol, 1.0 eq.) and ammonium acetate (3873 mg, 50.3 mmol, 15 eq.) were then
added followed by additional EtOH (30 mL). The reaction mixture was heated under reflux overnight,
after which it was cooled to room temperature while stirring. The precipitate that had formed was
separated by filtration, washed with cold MeOH and then cold Et2O. The product was recrystallized
from N,N-dimethylformamide (DMF) and after filtration the product was washed with cold EtOH and
dried. The compound was isolated as a fine yellow powder (131 mg, 0.31 mmol, 9.25%). Decomposes >
248 ◦C. 1H NMR (500 MHz, CDCl3):
HA5), 6.81 (d, J = 8.7 Hz, 4H, HB3), 3.05 (s, 12H, HNMe), 2.76 (s, 6H, HA6-Me). 13C{1H} NMR (126 MHz,
CDCl3):
/ppm 157.5 (CA6), 151.6 (CB4), 128.7 (CB2), 124.5 (CB1), 120.6 (CA5), 117.2 (CA3), 112.7 (CB3),
40.7 (CNMe), 23.8 (CA6-Me); CA2, CA4 not resolved in HMBC. UV-VIS (CHCl3, 2.5 10−5 mol dm−3
/nm 291 sh (
/dm3 mol−1 cm−1 28,800), 336 (42,700), 429 (2,300). HRMS m/z 423.2544 [M + H]+ (calc.
δ
/ppm 8.51 (s, 2H, HA3), 7.76 (d, J = 8.7 Hz, 4H, HB2), 7.47 (s, 2H,
δ
×
)
λ
ε
423.2543). IR spectrum: see Figure S1 in the Supporting Information.
3.3. 4,40-Bis(4-diphenylaminophenyl)-6,60-dimethyl-2,20-bipyridine (3)
4,40-Bis(4-bromophenyl)-6,60-dimethyl-2,20-bipyridine (100 mg, 203
(103 mg, 609 µ µ µmol,
µ
mol, 1.0 eq), diphenylamine
mol, 3.0 eq), Pd(tBu3P)2 (4.15 mg, 8.12 mol, 4 mol%), and NaOtBu (64.4 mg, 670
3.3 eq) were loaded into a heat-dried microwave vial, previously degassed and purged with N2 three
times. After a further 15 min of N2 purging, toluene (6.9 mL) was added to the mixture, and the
◦
mixture was stirred at 100 C overnight. After cooling to room temperature, the reaction mixture
was separated from the solid residues with a Pasteur pipette, then dried under reduced pressure.
The residue was then boiled in EtOH (25 mL ca.) and the organic layer was then separated from the
suspension by decantation and removal with Pasteur pipette. The remaining solid was then boiled
in CHCl3 (25 mL ca.), filtered through a Celite plug (to remove Pd dust), and finally removed under
reduced pressure. The product was isolated as a sticky yellow-brown solid (97.3 mg, 145
1H NMR (500 MHz, CDCl3): /ppm 8.52 (br, 2H, HA3), 7.68 (br d, 4H, HB2), 7.40 (br, 2H, HA5), 7.30
(t, J = 7.7 Hz, 8H, HC3), 7.15 (overlapping m, 12H, HC2+B3), 7.08 (t, J = 7.2 Hz, 4H, HC4), 2.74 (s, 6H,
HMe). 13C{1H} NMR (126 MHz, CDCl3): /ppm 147.3 (CC1), 132.0 (CB1), 129.5 (CC3), 128.1 (CB2), 124.9
(CC2), 123.4 (CC4), 123.0 (CB3), 120.4 (CA5), 116.5 (CA3), 24.6 (CMe), signals for CA2, CA4, CA6, CB4
not resolved. UV-VIS (CHCl3, 2.5 /nm 308 sh (
10−5 mol dm−3 /dm3 mol−1 cm−1 30,000), 362
µmol, 71.4%).
δ
δ
×
)
λ
ε

Molecules 2020, 25, 1528
17 of 24
(51,000), 457 (4700). HRMS m/z 671.3166 [M + H]+ (calc. 671.3169). IR spectrum: see Figure S3 in the
Supporting Information.
3.4. N,N-Diphenyl-4-((trimethylsilyl)ethynyl)aniline
N,N-Diphenyl-4-((trimethylsilyl)ethynyl)aniline was prepared in the same manner as
N,N-dimethyl-4-((trimethylsilyl)ethynyl)aniline [68] rather than by the previously described
method [72]. 4-Bromophenylamine (1.002 g, 3.09 mmol, 5.0 eq), Pd(dppf)Cl2 (136 mg, 185
6 mol%), CuI (58.8 mg, 309 mol, 0.5 eq) and PPh3 (81.0 mg, 309 mol, 0.5 eq) were loaded in a 10 mL
microwave vial, degassed and purged with N2 three times. After 15 min of N2 purging, 2 mL of Et3N
µmol,
µ
µ
were added to the mixture and set under stirring. Finally, trimethylsilylethyne (910 mg, 9.27
15 eq) was added to the reaction mixture and was set at 80 C overnight. After cooling down to
µ
mol,
◦
room temperature, the mixture was poured in water (10 mL) and extracted with AcOEt (3
×
10 mL),
the organic layers were washed further with brine (30 mL) and back-extracted. The organic layers
were collected together, dried over MgSO4, and dried under reduced pressure. The crude mixture
was purified by column chromatography (Biotage Sfär silica HC D, petroleum ether/CH2Cl2, CH2Cl2
increasing gradient up to 10%). The product was isolated as a brown solid (1.027 g, 3.007 mmol,
1
97.30%). H NMR (500 MHz, CDCl3):
δ/ppm 7.33 (d, J = 8.7 Hz, 2H), 7.27 (m, 4H), 7.08 (m, 4H), 7.06
(m, 2H), 6.95 (d, J = 8.7 Hz, 2H), 0.26 (s, 9H) as reported in the literature [73].
3.5. 4-Ethynyl-N,N-diphenylaniline
Deprotection of N,N-diphenyl-4-((trimethylsilyl)ethynyl)aniline to give 4-ethynyl-N,N-
1
diphenylaniline was carried out as previously described [72]. H NMR (500 MHz, CDCl3):
δ/ppm 7.32
(m, 2H), 7.26 (m, 4H), 7.09 (d, J = 7.7 Hz, 2H), 7.06 (t, J = 7.6 Hz, 2H), 6.96 (m, 2H), 3.01 (s, 1H).
3.6. 4,40-Bis(4-dimethylaminophenylethynyl)-6,60-dimethyl-2,20-bipyridine (2)
4,40-Dibromo-6,60-dimethyl-2,20-bipyridine (150 mg, 439
dimethylaniline (159 mg, 1.1 mmol, 2.5 eq), Pd(dppf)Cl2 (19.3 mg, 26.3
43.9 mol, 10 mol%), and PPh3 (11.5 mg, 43.9 mol, 0.1 eq) were loaded into a 5 mL microwave vial,
previously degassed and purged with N2 three times. After 15 min of N2 purging, Et3N (875 L)
was added to the reaction mixture, which was then stirred at 80 ◦C overnight. After cooling to room
temperature, the reaction mixture was poured into water (10 mL) and extracted with CH2Cl2 (3
µmol, 1.0 eq), 4-ethynyl-N,N-
µmol, 6 mol%), CuI (8.36 mg,
µ
µ
µ
×
10 mL). The organic layers were combined, washed with brine (10 mL), dried over MgSO4, and then
dried in vacuo. The crude product was purified by column chromatography (SiO2, CH2Cl2 with a
gradient of AcOEt (0 changing to 39:1 to 19:1 to 9:1 to 2:1). The product was isolated as a red solid
1
(78.1 mg, 166
µ
mol, 37.8%). H NMR (500 MHz, CDCl3):
δ
/ppm 8.28 (s, 2H, HA3), 7.45 (d, J = 8.7 Hz,
4H, HB2), 7.25 (s, 2H, HA5), 6.68 (d, J = 8.5 Hz, 4H, HB3), 3.02 (s, 12H, HNMe), 2.65 (s, 6H, HA6-Me).
13C{1H} NMR (126 MHz, CDCl3): /ppm 158.0 (CA6), 150.7 (CB4), 133.4 (CB2), 124.6 (CA5), 120.2 (CA3),
111.9 (CB3), 109.0 (CB1), 95.3 (Cb), 86.2 (Ca), 40.3 (CNMe), 24.6 (CA6-Me); CA2, CA4 not resolved in
HMBC. UV-VIS (CHCl3, 2.2
439 (3,100). HR ESI-MS m/z 471.2548 [M + H]+ (calc. 471.2543). IR spectrum: see Figure S2 in the
δ
×
10−5 mol dm−3 /dm3 mol−1 cm−1 34,550), 358 (40,500),
) λ/nm 303 (ε
Supporting Information.
3.7. 4,40-Bis(4-diphenylaminophenylethynyl)-6,60-dimethyl-2,20-bipyridine (4)
4,40-Dibromo-6,60-dimethyl-2,20-bipyridine (100 mg, 292
mol, 6 mol%), CuI (5.56 mg, 29.2 mol, 10 mol%), 4-ethynyl-N,N-diphenylaniline (181 mg,
mol, 2.3 eq) and PPh3 (7.66 mg, 29.2 mol, 10 mol%) were loaded into a heat-dried microwave
L)
was added to the reaction mixture, which was then stirred at 80 ◦C overnight. After cooling to room
temperature, the reaction mixture was poured into water (10 mL) and extracted with CH2Cl2 (3
10 mL). The organic layers were combined and washed with brine (10 mL). After back-extraction of the
µmol, 1.0 eq), Pd(dppf)Cl2 (12.8 mg,
17.5
672
µ
µ
µ
µ
vial, previously degassed and purged with N2 three times. After 15 min of N2 purging, Et3N (583
µ
×

Molecules 2020, 25, 1528
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aqueous phase, the organic layers were dried over MgSO4 and then dried under reduced pressure. The
product was recrystallized from a mixture of EtOH and CHCl3 and isolated as a dark orange powder
1
(104 mg, 145
µ
mol, 49.5%). H NMR (500 MHz, CDCl3):
δ
/ppm 8.28 (s, 2H, HA3), 7.40 (d, J = 8.7 Hz,
4H, HB2), 7.29 (m, 4H, HC3), 7.25 (s, 2H, HA5), 7.13 (d, J = 7.5 Hz, 8H, HC2), 7.09 (t, J = 7.4 Hz, 4H, HC4),
7.01 (d, J = 8.9 Hz, 4H, HB3), 2.64 (s, 6H, HMe). 13C{1H} NMR (126 MHz, CDCl3): /ppm 158.0 (CA6),
148.9 (CB4), 147.0 (CC1), 132.9 (CB2), 129.5 (CC3), 125.2 (CC2), 124.8 (CA5), 123.9 (CC4), 121.8 (CB3), 114.8
(CB1), 94.0 (Cb), 86.8 (Ca), 24.7 (CMe); CA2, CA4 not resolved in HMBC. UV-VIS (CHCl3, 2.2 10−5 mol
dm−3 /dm3 mol−1 cm−1 41,600), 380 (63,750), 468 (3300). HR EIS-MS m/z 719.3162 [M + H]+
/nm 302 (
δ
×
)
λ
ε
(calc. 719.3169). IR spectrum: see Figure S4 in the Supporting Information.
3.8. [Cu(1)2][PF6]
Compound
1
(110 mg, 260
µmol, 2.0 eq) was dissolved in 100 mL of CH2Cl2/CH3CN (2:1, total
volume 100 mL) in a round bottomed flask. After the addition of [Cu(MeCN)4][PF6] (48.5 mg, 130
µmol,
1.0 eq), the solution was stirred for 3 h at room temperature. Then, CH2Cl2 was removed in vacuo and
the volume of CH3CN was reduced in vacuo. Then, Et2O was added to the reaction mixture to afford
precipitation. The precipitate was collected by filtration, washed with Et2O and dried. [Cu(
1
)2][PF6]
1
was isolated as a dark red powder (71.8 mg, 6.80
µmol, 52.4%). H NMR (500 MHz, CD3CN):
δ/ppm
8.56 (br, 4H, HA3), 7.89 (d, J = 7.8 Hz, 8H, HB2), 7.75 (br, 4H, HA5), 6.89 (d, J = 8.1 Hz, 8H, HB3), 3.01
(s, 24H, HNMe), 2.33 (br, 12H, HA6-Me). 13C{1H} NMR (126 MHz, CD3CN): /ppm 153.1 (CB4), 150.6
(CA4), 129.0 (CB2), 124.4 (CB1), 122.2 (CA5), 116.4 (CA3), 113.3 (CB3), 40.4 (CNMe); CA2, CA6, CA6-Me not
resolved in HMBC. UV-VIS (CHCl3, 3.4 /nm 313 (
10−6 mol dm−3 /dm3 mol−1 cm−1 51,600), 370
(44,800), 500 sh (13,700). HRMS m/z 907.4228 [M
PF6]+ (calc. 907.4231), 423.2546, [ + H]+ (calc.
δ
×
)
λ
ε
−
1
423.2543, base peak). IR spectrum: see Figure S22 in the Supporting Information.
3.9. [Cu(2)2][PF6]
The preparation of [Cu(
2
)2][PF6] was as for [Cu(
1
)2][PF6] starting with
mol, 0.5 eq). The product was isolated as a dark red solid
/ppm 8.61 (s, 4H, HA3), 7.68 (s, 4H, HA5),
2 (62.0 mg, 132 µmol,
1.0 eq) and [Cu(MeCN)4][PF6] (24.6 mg, 65.9
µ
1
(47.2 mg, 41
µ
mol, 36%). H NMR (500 MHz, acetone-d6):
δ
7.51 (d, J = 8.8 Hz, 8H, HB2), 6.88 (broadened m, 8H, HB3), 3.09 (s, 24H, HNMe), 2.35 (br, 4H, HA6-Me).
13C{1H} NMR (126 MHz, acetone-d6): /ppm 158.4 (CA6), 152.7 (CA2), 152.2 (CB4), 135.4 (CA4), 134.3
(CB2), 127.9 (CA5), 122.3 (CA3), 113.2 (CB3), 109.6 (CB1), 99.5 (Cb), 86.7 (Ca), 40.5 (CNMe), 25.2 (CA6-Me).
δ
UV-VIS (CHCl3, 1.0
515 (15,200). ESI-MS m/z 1003.4226 [M
×
10−5 mol dm−3
)
λ
/nm 263 (
ε
/dm3 mol−1 cm−1 39,400), 326 (40,700), 409 (41,800),
+ H]+ (calc. 471.2543, base
−
PF6]+ (calc. 1003.4231), 471.2545 [
2
peak). IR spectrum: see Figure S23 in the Supporting Information.
3.10. [Cu(3)2][PF6]
[Cu(
3
)2][PF6] was prepared in the same manner as [Cu(
1
)2][PF6] starting with
mol, 1.0 eq). [Cu( )2][PF6] was isolated as a dark red
/ppm 8.57 (s, 4H, HA3), 7.84 (d,
3 (88.2 mg, 131 µmol,
2.0 eq) and [Cu(MeCN)4][PF6] (24.5 mg, 65.7
powder (34.0 mg, 22.0
µ
3
1
µ
mol, 33.1%). H NMR (500 MHz, CD3CN):
δ
J = 8.5 Hz, 8H, HB2), 7.75 (s, 4H, HA5), 7.36 (t, J = 7.9 Hz, 16H, HC3), 7.17–7.12 (m, 24H, HC2+C4), 7.11
(d, J = 8.7 Hz, 8H, HB3), 2.32 (s, 12H, HMe). 13C{1H} NMR (126 MHz, CD3CN): /ppm 149.8 (CB4),
147.8 (CC1), 130.7 (CC3), 129.5 (CB1), 129.2 (CB2), 126.2 (CC2/C4), 125.3 (C C2/C4), 123.3 (CA5), 122.9 (CB3),
117.6 (CA3), 25.4 (CA6-Me); CA2, CA4, CA6, CB4 not resolved in HMBC. UV-VIS (CHCl3, 1.0 10−5 mol
dm−3 /dm3 mol−1 cm−1 67,050), 319 sh (63,400), 394 (56,000), 493 (19,100). HRMS m/z
/nm 309 (
1405.5492 [M
PF6]+ (calc. 1404.5551, base peak, see Figure S19). IR spectrum: see Figure S24 in the
δ
×
)
λ
ε
−
Supporting Information.
3.11. [Cu(4)2][PF6]
[Cu(4)2][PF6] was prepared following the methods described for [Cu(1)2][PF6] starting with
4
(83.2 mg, 116
µ
mol, 2.0 eq) and [Cu(MeCN)4][PF6] (21.6 mg, 57.9 mol, 1.0 eq). [Cu(4
µ
)2][PF6] was

Molecules 2020, 25, 1528
19 of 24
1
isolated as a dark red powder (40.0 mg, 24.0
µ
mol, 40.0%). H NMR (500 MHz, CD3CN):
δ/ppm 8.33
(s, 4H, HA3), 7.55 (s, 4H, HA5), 7.45 (d, J = 8.7 Hz, 8H, HB2), 7.36 (t, J = 7.9 Hz, 16H, HC3), 7.19–7.12
(overlapping m, 24H, HC2+C4), 6.97 (d, J = 8.7 Hz, 8H, HB3), 2.28 (s, 12H, HMe). 13C{1H} NMR (126 MHz,
CD3CN):
122.2 (CA3), 121.8 (CB3), 113.7 (CB1), 97.3 (Cb), 86.4 (Ca), 25.5 (CMe); CA2, CA4, CA6 not resolved in
HMBC. UV-VIS (CHCl3, 1.0 /nm 298 (
10−5 mol dm−3 /dm3 mol−1 cm−1 88,440), 331 (72,250), 422
PF6]+ (calc. 1500.5517, base peak). IR spectrum: see
δ
/ppm 150.1 (CB4), 147.2 (CC1), 134.0 (CB2), 130.7 (CC3), 127.9 (CA5), 126.7 (CC2), 125.6 (CC4),
×
)
λ
ε
(83,500), 507 (33,000). HRMS m/z 1500.5499 [M
−
Figure S25 in the Supporting Information.
3.12. Calculations
Ground state density functional theory DFT calculations were carried out using Spartan ‘18 (v.
1.3) [74] at the B3LYP level with a 6-31G* basis set in vacuum. We have previously demonstrated
that for bis(diimine)copper(I) complexes, the choice of atomic orbital basis set (6-311++G** basis
set on all atoms, 6-311++G** on Cu and 6-31G* basis set on C, H and N, or 6-31G* basis set on all
atoms) has a negligible effect on the calculated MO compositions, while significantly influencing the
calculated absorption spectra [75]. Hence, a 6-31G* basis set on all atoms was chosen to optimize
computer time. Geometry optimization was also carried out at the DFT level after an initial geometry
energy optimization had been completed at a semi-empirical (PM3) level. For the complex [Cu(2
)2]+, a
restricted hybrid HF-DFT SCF calculation was also performed using Pulay DIIS plus Geometric Direct
Minimization Polarizable Continuum solvation model in CHCl3 using Spartan ‘18 (v. 1.3) [74].
3.13. DSC Fabrication
FTO/TiO2 electrodes (Solaronix Test Cell Titania Electrodes, Solaronix SA, Aubonne, Switzerland)
were washed with milliQ water and EtOH, heated at 450 ◦C for 30 min, and then cooled to ca. 80 ◦C.
The electrodes were immediately immersed in a DMSO solution of 5 (1.0 mM) for 24 h, after which they
were removed, washed with DMSO and EtOH, and finally dried in a stream of N2. To assemble the
adsorbed heteroleptic dyes, each functionalized electrode was placed in a CH2Cl2 solution (0.1 mM)
of [CuL2][PF6] (L = 1, 2, 3 or 4) for 3 days at room temperature, and was then removed, washed
with CH2Cl2 and dried under an N2 stream. For the reference dye N719 (Solaronix SA, Aubonne,
Switzerland), FTO/TiO2 electrodes (Solaronix Test Cell Titania Electrodes, Solaronix SA, Aubonne,
Switzerland) were immersed in a solution of N719 (EtOH, 0.3 mM) for 1 day. Then the electrodes
were removed from the solution, washed with EtOH and dried in a stream of N2. Counter electrodes
(Solaronix Test Cell Platinum Electrodes, Solaronix SA, Aubonne, Switzerland) were washed with
EtOH and then heated at 450 ◦C for 30 min to remove volatile organic impurities.
The working and counter-electrode for each DSC were joined together using thermoplast hot-melt
sealing foil (Solaronix Test Cell Gaskets, 60 µm, Solaronix SA, Aubonne, Switzerland) and the gap
between them was filled with electrolyte (LiI (0.1 M), I2 (0.05 M), 1-methylbenzimidazole (0.5 M),
1-butyl-3-methylimidazolinium iodide (0.6 M) in 3-methoxypropionitrile) by vacuum backfilling
through a hole in the counter-electrode. Finally, the hole was sealed (Solaronix Test Cell Sealings and
Solaronix Test Cell Caps, Solaronix SA, Aubonne, Switzerland).
3.14. Electrodes for Solid-State Absorption Spectroscopy
The method in Section 3.13 was used to fabricate dye-functionalized electrodes, but starting with
Solaronix Test Cell Titania Electrodes Transparent (Solaronix SA, Aubonne, Switzerland).
3.15. DSCs with Co-Adsorbent n-Decylphosphonic Acid
The method in Section 3.13 was used to fabricate dye-functionalized electrodes, but with one
change to the dye bath procedure. In place of a DMSO solution of
5
(1.0 mM), a DMSO solution of
containing both
24 h.
5 (1.0 mM) and n-decylphosphonic acid (1.0 mM) was used. The dipping time was

Molecules 2020, 25, 1528
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3.16. DSC, EQE and EIS Measurements
The DSCs were all masked before measurements. The mask was made from a black-coloured
copper sheet with an accurately calibrated aperture smaller than the surface area of TiO2. Cells were
also masked on the top and on the sides using black tape. Performance measurements were made by
irradiating the DSC from behind with a LOT Quantum Design LS0811 instrument (LOT-QuantumDesign
GmbH, Darmstadt, Germany, 100 mW cm−2 = 1 sun, AM1.5 G conditions) and the simulated light
power was calibrated with a silicon reference cell.
EQE measurements used a Spe Quest quantum efficiency setup (ReRa Systems, Nijmegen,
The Netherlands) with a 100 W halogen lamp (QTH) and a lambda 300 grating monochromator
(LOT-Oriel GmbH & Co. KG, Darmstadt, Germany). The monochromatic light was modulated to 3 Hz
using a chopper wheel (ThorLabs Inc., Newton, NJ, USA), and the cell response was amplified with a
large dynamic range IV converter (Melles Griot B.V., Didam, the Netherlands) and measured with a
SR830 DSP Lock-In amplifier (Stanford Research Systems Inc., Sunnyvale, CA, USA).
EIS measurements were performed using a ModuLab® XM PhotoEchem photoelectrochemical
measurement system (Solartron Metrology Ltd., Leicester, UK). The impedance was measured in
galvanostatic mode at open-circuit potential of the cell at a light intensity of 22 mW cm−2 (590 nm)
in the frequency range 0.05 Hz to 400 kHz using an amplitude of 10 mV. The impedance data were
analysed using ZView® software (Scribner Associates Inc., Southern Pines, NC, USA).
4. Conclusions
We have described the syntheses and characterisations of ligands
copper(I) complexes [Cu( )2][PF6], [Cu( )2][PF6], [Cu( )2][PF6] and [Cu( )2][PF6]. The ligands feature
electron-donating Me2N or Ph2N groups, and compounds and contain alkynyl spacers. A
comparison of the solution absorption spectra of with , and of with confirms that increasing the
-conjugation as the alkynyl units are introduced extends the absorption towards longer wavelengths.
The solution absorption spectra of the complexes exhibit absorptions in the visible region assigned to a
combination of ILCT and MLCT. DSCs containing heteroleptic [Cu(
)(Lancillary)]+ dyes with Lancillary
, and a phosphonic acid anchoring ligand were fabricated. The best performing DSCs were
sensitized with [Cu( )( )( )(
)]+ and [Cu( )]+. The presence of the alkynyl spacers in the dyes [Cu( )]+
and [Cu( )( )( )(
)]+ results in lower values of JSC and EQEmax than for [Cu( )]+ and [Cu( )]+. Addition
of a co-absorbent (n-decylphosphonic acid) to [Cu( )(
)]+ in the DSCs did not significantly improve
performance. Using EIS, we have demonstrated a more favourable electron injection into TiO2 when
the dye is [Cu( )(
)]+, and have confirmed that the introduction of the alkynyl spacers is not beneficial.
1–4 and the homoleptic
1
2
3
4
2
4
2
1
4
3
π
5
=
1–
4
5
5
1
5
3
5 2
5
4
5
1
5 3
5
1
5
1
Supplementary Materials: The following are available online: Figures S1–S4: IR spectra of compounds
Figures S5–S16: NMR spectra of compounds ; Figures S17–S20: high-resolution electrospray mass spectra
of [CuL2][PF6] with L = ; Figures S21–S24: IR spectra of [CuL2][PF6] with L = ; Figures S25–S36: NMR
spectra of [CuL2][PF6] with L = ; Figure S37: orbital compositions of HOMOs and LUMOs in [Cu(
)2]+ using a
polarizable continuum solvation model; Figure S38: orbital compositions (DFT) of the highest occupied and lowest
unoccupied MOs in [Cu( )( )(
)]+; Figures S39 and S40: J–V curves and EQE spectra for DSCs with [Cu( )]+ and
[Cu(5)(2)]+ compared to N719; Table S1. EIS parameters for all DSCs.
1–4;
1–4
1–
4
1–4
1
–4
2
5
1
5 1
Author Contributions: Project conceptualization, administration, supervision and funding acquisition, E.C.C.
and C.E.H.; investigation, G.R.; data analysis, G.R. and M.B.; DFT calculations, C.E.H.; writing, C.E.H.; G.R. and
M.B.; manuscript editing, E.C.C, G.R. and M.B. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was partially funded by the Swiss National Science Foundation, grant number
200020_182000.
Acknowledgments: We gratefully acknowledge the support of the University of Basel. We thank Frederik J.
Malzner for preliminary studies of ligand 1 and its copper(I) complexes.
Conflicts of Interest: The authors declare no conflict of interest.

Molecules 2020, 25, 1528
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