Improving the photoresponse of copper(i) dyes in dye-sensitized solar cells by tuning ancillary and anchoring ligand modules

Article abstract of DOI:10.1039/c3dt51416a

The syntheses of five homoleptic copper(i) complexes [CuL ₂][PF₆] are described in which L is a 4,4′-di(4- bromophenyl)-6,6′-dialkyl-2,2′-bipyridine ligand (compounds 1-4 with methyl, ⁿbutyl, ^isobutyl and hexyl substituents, respectively) or 4,4′-di(4-bromophenyl)-6,6′-diphenyl-2,2′- bipyridine (5). The new ligands 2-5 and copper(i) complexes [CuL ₂][PF₆] (L = 1-5) have been fully characterized. The single crystal structures of 2{[Cu(1)₂][PF₆]} ·3Me₂CO, [Cu(2)₂][PF₆], 2{[Cu(3) ₂][PF₆]}·Et₂O and [Cu(5) ₂][PF₆]·CH₂Cl₂ have been determined. The first three structures show similar distorted tetrahedral environments for the Cu⁺ ions with angles between the least squares planes of the bpy domains of 85.6, 86.4 and 82.9°, respectively; in contrast, the Cu⁺ ion in [Cu(5)₂][PF₆] ·CH₂Cl₂ is in a flattened coordinate environment due to intra-cation face-to-face π-interactions. The solution absorption spectra of the complexes with ligands 1-4 are virtually identical with an MLCT band with values of λ_max = 481-488 nm. In contrast, the absorption spectrum of [Cu(5)₂][PF₆] shows two broad bands in the visible region. Cyclic voltammetric data show that oxidation of the copper(i) centre occurs at a more positive potential in [Cu(2) ₂][PF₆], [Cu(3)₂][PF₆] and [Cu(4)₂][PF₆] than in [Cu(1)₂][PF₆] or [Cu(5)₂][PF₆] with the latter being oxidized at the lowest potential. The complexes have been used to prepare dye-sensitized solar cells (DSCs) incorporating heteroleptic dyes of type [Cu(L)(L _anchor)]⁺ where L is 1-5 and L_anchor is a 6,6′-dimethyl-2,2′-bipyridine functionalized in the 4- and 4′-positions with phosphonic acid groups with (L_anchor = 7) and without (L_anchor = 6) a spacer between the metal-binding and anchoring domains. The presence of the spacer results in enhanced performances of the dyes, and the highest energy conversion efficiencies are observed for the dyes [Cu(3)(7)]⁺ (η = 2.43% compared to 5.96% for standard dye N719) and [Cu(5)(7)]⁺ (η = 2.89% compared to 5.96% for N719). Measurements taken periodically over the course of a week indicate that the cells undergo a ripening process (most clearly seen for [Cu(5)(6)]⁺ and [Cu(5)(7)]⁺) before their optimum performances are achieved. IPCE (EQE) data are presented and confirm that, although the photo-to-current conversions are promising (37-49% for λ_max ≈ 480 nm), the copper(i) dyes do not realize the broad spectral response exhibited by N719.

Full text of DOI:10.1039/c3dt51416a

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Improving the photoresponse of copper(I) dyes in
dye-sensitized solar cells by tuning ancillary and
Cite this: Dalton Trans., 2013, 42, 12293
anchoring ligand modules†
Biljana Bozic-Weber, Sven Y. Brauchli, Edwin C. Constable,* Sebastian O. Fürer,
Catherine E. Housecroft,* Frederik J. Malzner, Iain A. Wright and
Jennifer A. Zampese
The syntheses of five homoleptic copper(I) complexes [CuL2][PF6] are described in which L is a 4,4’-di-
(4-bromophenyl)-6,6’-dialkyl-2,2’-bipyridine ligand (compounds 1–4 with methyl, nbutyl, isobutyl and
hexyl substituents, respectively) or 4,4’-di(4-bromophenyl)-6,6’-diphenyl-2,2’-bipyridine (5). The new
ligands 2–5 and copper(I) complexes [CuL2][PF6] (L = 1–5) have been fully characterized. The single crystal
structures of 2{[Cu(1)2][PF6]}·3Me2CO, [Cu(2)2][PF6], 2{[Cu(3)2][PF6]}·Et2O and [Cu(5)2][PF6]·CH2Cl2 have
been determined. The first three structures show similar distorted tetrahedral environments for the Cu+
ions with angles between the least squares planes of the bpy domains of 85.6, 86.4 and 82.9°, respect-
ively; in contrast, the Cu+ ion in [Cu(5)2][PF6]·CH2Cl2 is in a flattened coordinate environment due to
intra-cation face-to-face π-interactions. The solution absorption spectra of the complexes with ligands
1–4 are virtually identical with an MLCT band with values of λmax = 481–488 nm. In contrast, the absorp-
tion spectrum of [Cu(5)2][PF6] shows two broad bands in the visible region. Cyclic voltammetric data
show that oxidation of the copper(I) centre occurs at a more positive potential in [Cu(2)2][PF6], [Cu(3)2]-
[PF6] and [Cu(4)2][PF6] than in [Cu(1)2][PF6] or [Cu(5)2][PF6] with the latter being oxidized at the lowest
potential. The complexes have been used to prepare dye-sensitized solar cells (DSCs) incorporating hetero-
leptic dyes of type [Cu(L)(Lanchor)]+ where L is 1–5 and Lanchor is a 6,6’-dimethyl-2,2’-bipyridine functio-
nalized in the 4- and 4’-positions with phosphonic acid groups with (Lanchor = 7) and without (Lanchor = 6)
a spacer between the metal-binding and anchoring domains. The presence of the spacer results in
enhanced performances of the dyes, and the highest energy conversion efficiencies are observed for the
dyes [Cu(3)(7)]+ (η = 2.43% compared to 5.96% for standard dye N719) and [Cu(5)(7)]+ (η = 2.89% com-
pared to 5.96% for N719). Measurements taken periodically over the course of a week indicate that the
cells undergo a ripening process (most clearly seen for [Cu(5)(6)]+ and [Cu(5)(7)]+) before their optimum
performances are achieved. IPCE (EQE) data are presented and confirm that, although the photo-to-
current conversions are promising (37–49% for λmax ≈ 480 nm), the copper(I) dyes do not realize the
broad spectral response exhibited by N719.
Received 30th May 2013,
Accepted 28th June 2013
DOI: 10.1039/c3dt51416a
www.rsc.org/dalton
based on similarities between the photophysical properties of
copper(I) and ruthenium(II) complexes.4,5 The design of suit-
Introduction
There is currently significant interest in the use of earth-abun- able ligands for copper(I)-based dyes depends critically on
dant metals in dyes in dye-sensitized solar cells (DSCs).1,2 information gained from photophysical and theoretical
Sauvage and co-workers3 were the first to use copper(I) com- studies. The suitability of oligopyridine copper(I)-based com-
plexes as sensitizers. The rationale for their application is plexes as sensitizers has been confirmed by the results of
TD-DFT calculations.6,7 In addition to determining orbital
composition (in particular, confirming whether the LUMO of a
Department of Chemistry, University of Basel, Spitalstrasse 51, CH4056 Basel,
complex possesses contributions from the anchoring units as
required for efficient electron injection), DFT and TD-DFT cal-
Switzerland. E-mail: edwin.constable@unibas.ch, catherine.housecroft@unibas.ch;
Fax: +41 61 267 1018; Tel: +41 61 267 1008
culations provide insight into the consequences of changes in
†Electronic supplementary information (ESI) available: Fig. S1. Packing inter-
ligand conformations upon excitation of the complex,8 probe
action in 2{[Cu(1)2][PF6]}·3Me2CO. CCDC 942069–942073. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt51416a
energy matching of the HOMO and LUMO of the dye to the
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conduction band of the semiconductor and predict absorption
spectra of dyes.9–11
Our recent investigations of copper(I)-based dyes have capi-
talized on the lability of copper(I) bis(diimine) complexes in
solution which allows heteroleptic complexes containing
{CuI(bpy)2} cores (bpy = 2,2′-bipyridine) to be assembled in situ
by facile ligand exchange.9,12–14 One of the two ligands in the
complex is selected for its ability to anchor the complex to a
mesoporous TiO2 surface. Typical choices of anchoring units
are carboxylic acid, phosphonic acid or phenol groups, but we
have recently shown that DSCs incorporating dyes with phos-
phonic acid anchoring groups typically exhibit the best per-
formances,9,13,14 consistent with the observations of Grätzel.15
The ancillary ligand in the dye is designed to provide efficient
light-harvesting properties. It is well established that a 6,6′-
dimethyl substitution pattern is required in bpy ligands for
stabilization of copper(I).16 On the other hand, the photo-
physical and electrochemical properties of copper(I) bis-
(diimine) complexes are strongly influenced by the electronic
and steric effects of the substituents, viz. 6,6′-substituents in
bpy and 2,9-substituents in 1,10-phenanthroline (phen).8,17–22
Here we investigate the effects of varying the steric demands of
the 6,6′-substituents in the bpy derived ancillary ligands 1–4
(Scheme 1) and the effects of going from alkyl to phenyl substi-
tuents (5, Scheme 1), in conjunction with the effects of intro-
ducing an aromatic spacer in the phosphonic acid anchoring
ligands 6 and 7 (Scheme 1). The inclusion of the bromo-substi-
tuent in ancillary ligands 1–5 provides a built-in handle for
further derivatization and tuning of electronic properties of
the ligand as we have illustrated with the decoration of 1 with
first and second generation hole-transport dendrons.14 The
choice of 6,6′-substituents in 1–4 was made to explore the
effects of alkyl chain length and steric demands on the per-
formance of the surface-anchored dye. In anticipation that
longer chains might inhibit assembly of dye complexes on the
surface, we decided to trial a new anchoring ligand, 7, in
which a phenyl spacer extends the spatial separation of the
Scheme 2 Schematic representation of the ten dyes studied in this work.
metal centre from the anchoring units compared to dyes in-
corporating anchoring ligand 6 (Scheme 2).
Experimental
General
1H and 13C NMR spectra were recorded using a Bruker Avance
III-250, 400 or 500 NMR spectrometer with chemical shifts
referenced to residual solvent peaks with respect to δ(TMS) =
0 ppm. Absorption spectra were recorded on a Cary-5000
spectrophotometer, and FT-IR spectra on a Shimadzu 8400S
instrument (Golden Gate accessory for solid samples). Electro-
spray ionization (ESI) mass spectra were recorded on a Bruker
Esquire 3000plus instrument.
Electrochemical measurements were made on a CH Instru-
ments 900B potentiostat using glassy carbon, platinum wire
and silver wire as the working, counter, and reference electro-
des, respectively. Samples were dissolved in HPLC grade
CH2Cl2 (10−4 to 10−5 mol dm−3) containing 0.1 mol dm−3
[nBu4N][PF6] as the supporting electrolyte; all solutions were
degassed with argon. Cp2Fe was used as the internal reference.
(1E,5E)-1,6-Bis(4-bromophenyl)hexa-1,5-diene-3,4-dione and
compound 1 were prepared as previously reported.14 Com-
pound 623 was prepared by a modified literature method.24
1-(2-Oxohexyl)pyridinium iodide
Iodine (25.6 g, 101 mmol) was dissolved in pyridine under vig-
orous stirring (40.8 mL, 39.9 g, 505 mmol) and then hexan-2-
one (13 mL, 101 mmol) was added dropwise over a period of
5 min. The reaction mixture was heated at reflux overnight,
and then the solvent was removed under vacuum. The crude
material was used without purification for the preparation of
compound 2. 1H NMR (400 MHz, CDCl3) δ/ppm 9.17 (d, J =
5.5 Hz, 2H, Hpy2), 8.50 (m, 1H, Hpy4), 8.06 (m, 2H, Hpy3), 6.52
Scheme 1 Ancillary ligands 1–5 and anchoring ligands 6 and 7, with the label-
ling scheme for 1H NMR spectroscopic assignments. Alkyl chains are labelled
a, b... starting from the ring-attached CH2 unit.
CH2
(s, 2H, HNCH ), 2.85 (t, J = 7.4 Hz, 2H, H ), 1.68 (m, 2H,
2
CH2
HCH ), 1.38 (m, 2H, H ), 0.93 (t, J = 7.3 Hz, 2H, HMe).
2
12294 | Dalton Trans., 2013, 42, 12293–12308
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Compound 2
47.5 (Ca), 29.0 (Cb), 22.4 (Cc). IR (˜ν/cm−1) 3082 (w), 3053 (w),
2954 (m), 2928 (m), 2913 (w), 2864 (m), 1599 (m), 1543 (m),
1491 (m), 1462 (m), 1376 (m), 1326 (m), 1165 (m), 1010 (s), 896
(m), 828 (s), 814 (s), 765 (s), 744 (m), 658 (m), 590 (m), 526 (s).
ESI MS m/z 579.1 [M + H]+ (calc. 579.1). UV-VIS (CH2Cl2,
1.0 × 10−5 mol dm−3) λ/nm 256 (ε/dm3 mol−1 cm−1 58 350), 304
(14 360). Found: C 62.03, H 5.11, N 5.16; C30H30Br2N2 requires
C 62.30, H 5.23, N 4.84%.
1-(2-Oxohexyl)pyridinium iodide (3.00 g, 9.83 mmol) was dis-
solved in EtOH (60 mL) under vigorous stirring. (1E,5E)-1,6-Bis-
(4-bromophenyl)hexa-1,5-diene-3,4-dione (1.03 g, 2.46 mmol)
and NH4OAc (2.84 g, 36.9 mmol) were added followed by EtOH
(30 mL). The reaction mixture was heated at reflux for 2 days
and was then allowed to cool to room temperature while being
stirred. The precipitate was collected by filtration, washed with
cold EtOH, and then dried under a stream of air. The product
was recrystallized twice from EtOH, then from MeOH. Com-
pound 2 was isolated as an off-white solid (0.630 g, 1.09 mmol,
44.3%). m.p. 157 °C. 1H NMR (500 MHz, CDCl3) δ/ppm 8.48
(d, J = 1.6 Hz, 2H, HA3), 7.63 (m, 8H, HB2+B3), 7.34 (d, J =
1.7 Hz, 2H, HA5), 2.94 (t, J = 7.8 Hz, 4H, Ha), 1.83 (m, 4H, Hb),
1.47 (m, 4H, Hc), 0.99 (t, J = 7.4 Hz, 6H, Hd). 13C NMR
(126 MHz, CDCl3) δ/ppm 162.8 (CA6), 156.7 (CA2), 148.3 (CA4),
138.1 (CB1), 132.3 (CB2/3), 128.9 (CB2/3), 123.3 (CB4), 120.4 (CA5),
1-(2-Oxooctyl)pyridinium iodide
Iodine (16.2 g, 63.9 mmol) was dissolved in pyridine under vig-
orous stirring (25.8 mL, 25.2 g, 319 mmol), and then octan-
2-one (10.0 mL, 63.9 mmol) was added dropwise over a period
of 5 min. The reaction mixture was heated to reflux overnight.
The reaction mixture was cooled to room temperature over 1 h
and the solvent was removed under reduced pressure. The
crude material was used without purification for the synthesis
1
of compound 4. H NMR (400 MHz, CDCl3) δ/ppm 9.24 (d, J =
116.6 (CA3), 38.7 (Ca), 32.2 (Cb), 22.7 (Cc), 14.2 (Cd). IR (˜ν/cm−1
)
5.8 Hz, 2H, Hpy2), 8.54 (m, 1H, Hpy4), 8.07 (m, 2H, Hpy3), 6.47
2949 (w), 2918 (w), 2869 (w), 2853 (w), 1595 (m), 1544 (m),
1488 (m), 1404 (m), 1274 (m), 1298 (w), 1098 (w), 1008 (m),
821, (s), 758 (m), 480 (s). ESI MS m/z 579.1 [M + H]+ (calc.
579.1). UV-VIS (CH2Cl2, 1.0 × 10−5 mol dm−3) λ/nm 256 (ε/dm3
mol−1 cm−1 43 900), 302 (11 700). Found: C 61.04, H 5.22,
N 4.94; C30H30Br2N2·12H2O requires C 61.34, H 5.32, N 4.77%.
CH2
(s, 2H, HNCH ), 2.82 (t, J = 7.4 Hz, 2H, H ), 1.65 (m, 2H,
2
CH2+CH2+CH2
HCH ), 1.27 (overlapping m, 6H, H
6.5 Hz, 2H, HMe).
), 0.85 (t, J =
2
Compound 4
Compound 4 was prepared in the same manner as 2 starting
with 1-(2-oxooctyl)pyridinium iodide (5.47 g, 17.1 mmol) and
(1E,5E)-1,6-bis(4-bromophenyl)hexa-1,5-diene-3,4-dione (1.80 g,
4.28 mmol) and NH4OAc (4.95 g, 64.3 mmol). The reaction was
heated overnight at reflux. Compound 4 was isolated as an off-
white solid (0.173 g, 0.273 mmol, 6.36%). m.p. 135 °C. 1H
NMR (500 MHz, CDCl3) δ/ppm 8.48 (d, J = 1.7 Hz, 2H, HA3),
7.63 (m, 8H, HB2+B3), 7.33 (d, J = 1.7 Hz, 2H, HA5), 2.93 (t, J =
7.8 Hz, 4H, Ha), 1.85 (m, 4H, Hb), 1.44 (m, 4H, Hc), 1.35 (m,
8H, Hd+e), 0.99 (t, J = 7.1 Hz, 6H, Hf). 13C NMR (126 MHz,
CDCl3) δ/ppm 162.8 (CA6), 156.7 (CA2), 148.3 (CA4), 138.1 (CB1),
132.3 (CB2/B3), 128.9 (CB2/3), 123.3 (CB4), 120.4 (CA2), 116.6
(CA3), 38.65 (Ca), 31.92 (Cb), 29.99 (Cc), 29.29 (Cd), 22.80 (Ce),
14.29 (Cf). IR (˜ν/cm−1) 2949 (m), 2924 (m), 2866 (w), 2853 (m),
1597 (m), 1545 (m), 1490 (m), 1462 (m), 1407 (m), 1377 (m),
1069 (m), 1009 (m), 892 (m), 824 (s), 754 (m), 655 (m), 515 (m),
485 (m). ESI MS m/z 635.1 [M + H]+ (calc. 635.1). UV-VIS
(CH2Cl2, 1.0 × 10−5 mol dm−3) λ/nm 255 (ε/dm3 mol−1 cm−1
49 900), 305 (16 600). Found: C 62.91, H 5.86, N 4.62;
C34H38Br2N2·H2O requires C 62.58, H 6.18, N 4.29%.
1-(4-Methyl-2-oxopentyl)pyridinium iodide
Iodine (30.4 g, 120 mmol) was dissolved in pyridine under vig-
orous stirring (48.5 mL, 47.4 g, 599 mmol) and then 4-methyl-
pentan-2-one (15.0 mL, 120 mmol) was added dropwise over a
period of 5 min. The reaction mixture was heated at reflux
overnight, after which time it was cooled slowly (over 1 h) to
room temperature. The solvent was removed under reduced
pressure. Diethyl ether was added and the mixture was poured
into a flask; the liquid phase was decanted and the solid
residue was dissolved in CHCl3. After filtration, the solvent was
removed under reduced pressure. The crude material was used
without purification for the synthesis of compound 3. 1H NMR
(400 MHz, CDCl3) δ/ppm 9.22 (d, J = 5.7 Hz, 2H, Hpy2), 8.58 (t,
J = 7.8 Hz, 1H, Hpy4), 8.09 (m, 2H, Hpy3), 6.38 (s, 2H, HNCH ),
2
2.70 (d, J = 6.8 Hz, 2H, HCH ), 2.20 (m, 1H, HCH), 0.93 (d, J =
2
6.7 Hz, 6H, HMe).
Compound 3
Compound 3 was prepared in the same manner as 2 starting
with 1-(4-methyl-2-oxopentyl)pyridinium iodide (4.16 g,
1-(2-Phenyl-2-oxoethyl)pyridinium iodide
14.3 mmol) and (1E,5E)-1,6-bis(4-bromophenyl)hexa-1,5-diene- Iodine (38.2 g, 150 mmol) was dissolved in pyridine (75.0 mL,
3,4-dione (1.50 g, 3.57 mmol) and NH4OAc (4.13 g, 73.7 g, 932 mmol). The solution was stirred until all I2 had dis-
53.6 mmol). The reaction time at reflux was 1 day. Compound solved. Then acetophenone (17.5 mL, 18.0 g, 150 mmol, 1.0
3 was isolated as an off-white solid (0.401 g, 0.693 mmol, eq.) was added to the solution which was then heated to reflux
19.4%). m.p. 181 °C. 1H NMR (500 MHz, CDCl3) δ/ppm 8.48 for 1 h. The heating was stopped and the solution was cooled
(d, J = 1.7 Hz, 2H, HA3), 7.63 (m, 8H, HB2+B3), 7.30 (d, J = to rt. The remaining pyridine was removed under vacuum. The
1.7 Hz, 2H, HA5), 2.80 (d, J = 7.2 Hz, 4H, Ha), 2.27 (m, 2H, Hb), solid was washed with ice water and acetone. The product was
1.01 (d, J = 6.6 Hz, 12H, Hc). 13C NMR (126 MHz, CDCl3) dried under vacuum and 1-(2-phenyl-2-oxoethyl)pyridinium
δ/ppm 161.6 (CA6), 156.5 (CA2), 148.1 (CA4), 137.8 (CB1), 132.1 iodide was isolated as an off-white solid (43.6 g, 134 mmol,
(CB2/B3), 128.8 (CB2/B3), 123.1 (CB4), 121.1 (CA5), 116.5 (CA3), 90%). 1H NMR (250 MHz, CD3CN) δ/ppm 8.77 (dd, J = 6.6,
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1.3 Hz, 2H, Hpy2), 8.65 (m, 1H, Hpy4), 8.17–8.08 (m, 4H, 1539 (m), 1447 (w), 1382 (m), 1364 (w), 1254 (m), 1242 (s),
Hpy3+Ph2/3), 7.78 (m, 1H, HPh4), 7.64 (m, 2H, HPh2/3), 6.4 (s, 2H, 1214 (w), 1136 (w), 1121 (m), 1113 (w), 1053 (m), 1023 (s), 1017 (s),
HNCH ).
955 (s), 898 (w), 890 (w), 830 (s), 798 (m), 781 (s), 747 (m),
681 (m), 603 (m), 571 (s). ESI MS m/z 631.2 [M + Na]+ (calc.
631.2), 609.3 [M + H]+ (base peak, calc. 609.2). Found C 63.81,
2
Compound 5
Compound 5 was prepared in the same manner as 2 starting H 6.34, N 4.84; C32H38N2O6P2 requires C 63.15, H 6.29,
with 1-(2-phenyl-2-oxoethyl)pyridinium iodide (1.61 g, N 4.60%.
4.95 mmol) and (1E,5E)-1,6-bis(4-bromophenyl)hexa-1,5-diene-
3,4-dione (1.04 g, 2.48 mmol) and NH4OAc (2.68 g,
Compound 7
37.1 mmol). The reaction time at reflux was 1 day. Compound Compound 7a (200 mg, 0.33 mmol) was dissolved in aqueous
5 was isolated as an off-white solid (0.443 g, 0.718 mmol, HCl (6 M, 20 mL) and the solution was heated at reflux for
1
29.0%). m.p. 318 °C. H NMR (500 MHz, CD2Cl2) δ/ppm 8.91 48 h. The solvent was removed under reduced pressure to leave
(d, J = 1.5 Hz, 2H, HA3), 8.23 (m, 4H, HC2), 8.04 (d, J = 1.6 Hz, a pale yellow residue. The residue was treated with AcOH
2H, HA5), 7.81 (m, 4H, HB2), 7.72 (m, 4H, HB3), 7.57 (m, 4H, (15 mL) at reflux in the presence of 5 drops of concentrated
HC3), 7.52 (m, 2H, HC4). 13C NMR (126 MHz, CD2Cl2) δ/ppm aqueous HCl prior to addition of water. This resulted in the
157.6 (CA6), 155.6 (CA2), 150.1 (CA4), 138.8 (CC1), 137.9 (CB1), precipitation of 7 as a white powder (163 mg, >99%). m.p.
132.7 (CB3), 129.8 (CC4), 129.5 (CB2), 129.2 (CC3), 127.6 (CC2), >300 °C. 1H NMR (500 MHz, DMSO-d6) δ/ppm 8.53 (s, 2H,
124.2 (CB4), 119.5 (CA5), 118.9 (CA3). IR (˜ν/cm−1) 3061 (w), 3035 HA3), 7.95 (dd, JPH = 3.1 Hz, JHH = 8.4 Hz, 2H, HB2), 8.15 (dd,
(w), 1594 (s), 1571 (m), 1542 (s), 1488 (s), 1405 (m), 1375 (m), JPH = 12.5 Hz, JHH = 8.2 Hz, 2H, HB3), 7.73 (s, 2H, HA5), 2.68 (s,
1238 (w), 1181 (w), 1074 (m), 1025 (m), 1008 (m), 884 (m), 818 6H, HMe). 13C NMR (126 MHz, DMSO-d6) δ/ppm 158.7 (CA6),
(s), 775 (s), 739 (s), 690 (s), 637 (m), 557 (m), 538 (m), 476 (m). 140.0 (CB1), 135.9 (CA4/B4), 134.4 (CA4/B4), 131.3 (CB3), 126.5
ESI MS m/z 619.1 [M + H]+ (calc. 619.0). UV-VIS (CH2Cl2, 1.0 × (CB2), 121.2 (CA5), 115.2 (CA3), 24.1 (CMe), CA2 not resolved. IR
10−5 mol dm–3) λ/nm 265 (ε/dm3 mol−1 cm−1 79 400), 321 (solid, ν/cm−1) 3244 (w), 3063 (w), 2717 (br, w), 1645 (m), 1627
(16 400). Found: C 64.86, H 3.67, N 4.54; C34H22Br2N2·12H2O (m), 1597 (s), 1544 (w), 1454 (w), 1385 (m), 1360 (w), 1312 (w),
requires C 65.09, H 3.70, N 4.47%.
1276 (w), 1238 (m), 1164 (m), 1146 (s), 1137 (s), 1089 (w), 1041 (w),
931 (s), 913 (s), 880 (s), 826 (vs), 761 (m), 728 (w), 680 (vs),
Compound 7a
564 (s). Found C 55.06, H 4.96, N 5.46; C24H22N2O6P2·1.5H2O
1
Compound
1
(1.00 g, 2.02 mmol), Pd(PPh3)4 (213 mg, requires C 55.07, H 4.81, N 5.35%. Protonated 7, see text: H
0.18 mmol) and Cs2CO3 (1.44 g, 4.41 mmol) were combined in NMR (500 MHz, CF3CO2D) δ/ppm 8.67 (s, 2H, HA3), 8.33 (s, 2H,
anhydrous THF (15 mL) in a 10 or 20 mL microwave vial HA5), 8.15 (dd, JPH = 14.0 Hz, JHH = 8.0 Hz, 2H, HB3), 8.00 (dd,
equipped with a stirrer bar under argon. Diethylphosphite JPH = 2.7 Hz, JHH = 7.6 Hz, 2H, HB2), 3.04 (s, 6H, HMe).
(1.12 g, 8.09 mmol) was added by syringe before the vial was 13C NMR (126 MHz, CF3CO2D) δ/ppm 161.6 (CA4), 160.8 (CA6),
sealed and the reaction mixture heated to 110 °C for 90 min. 144.2 (CA2), 140.2 (d, JPC = 3.2 Hz, CB1), 135.2 (d, JPC = 11.4 Hz,
The reaction mixture was filtered to give a yellow solution and CB3), 134.3 (d, JPC = 187 Hz, CB4), 130.6 (d, JPC = 16.2 Hz, CB2),
then the solvent was evaporated under reduced pressure. The 130.2 (CA5), 126.7 (CA3), 21.2 (CMe).
resulting brown residue was dissolved in CH2Cl2 (20 mL) and
stirred with decolourising charcoal for 30 minutes then filtered
[CuL2][PF6]: general procedure
over celite prior to removal of the solvent under reduced In a typical reaction, [Cu(NCMe)4][PF6] was dissolved in a
pressure to produce an oily bright yellow residue. The residue mixture of MeCN and CH2Cl2 and ligand was added. The solu-
was dissolved in acetone (5 mL) and filtered through a short tion turned deep red immediately. It was stirred overnight,
silica plug eluting with acetone (25 mL) to give a pale yellow after which time the volume of solvent was reduced in vacuo
solution. The solvent volume was reduced to ≈5 mL and a and the product was precipitated by addition of Et2O. The
mixture of pentane and hexane (1 : 1) was added until the solu- solid was redissolved in MeCN and precipitated again by
tion became turbid. It was heated until clear and then allowed addition of Et2O. The dark red product was collected by fil-
to cool, resulting in the precipitation of 7a as a white powder tration and was washed with cold Et2O and dried in a stream
(802 mg, 65%). Single crystals were grown by diffusion of of air.
pentane–hexane (1 : 1) into a concentrated acetone solution of
[Cu(1)2][PF6]
7a. m.p. 174 °C. 1H NMR (500 MHz, CD3CN) δ/ppm 8.57 (d, J =
1.2 Hz, 2H, HA3), 7.92 (overlapping m, 8H, HB2+B3), 7.59 (d, J = [Cu(NCMe)4][PF6] (85.0 mg, 0.228 mmol) was reacted with 1
1.4 Hz, 2H, HA5), 4.10 (m, 8H, HEt(CH2)), 2.68 (s, 6H, HMe), 1.31 (225 mg, 0.456 mmol) in MeCN (4 mL) and CH2Cl2 (16 mL).
(t, J = 7.0 Hz, 12H, HEt(CH3)). 13C NMR (126 MHz, CD3CN) [Cu(1)2][PF6] was isolated as a dark red solid (217 mg,
δ/ppm 159.9 (CA6), 157.0 (CA2), 149.0 (CA4), 143.2 (CB1), 133.2 0.181 mmol, 79.6%). Decomp. > 320 °C. 1H NMR (500 MHz,
(d, JPC = 10.1 Hz, CB2/B3), 130.5 (d, JPC = 187.7 Hz, CB4), 128.2 CD3CN) δ/ppm 8.66 (br, 4H, HA3), 7.88 (d, J = 7.9 Hz, 8H, HB2/B3),
(d, JPC = 15.0 Hz, CB2/B3), 122.3 (CA5), 116.8 (CA3), 63.0 (d, JPC
=
7.83 (br, 4H, HA5), 7.78 (d, J = 7.7 Hz, 8H, HB2/B3), 2.37 (s,
5.3 Hz, CEt(CH2)), 24.7 (CMe), 16.6 (d, JPC = 6.3 Hz, CEt(CH3)). IR 12H, Ha). 13C NMR (126 MHz, CD3CN) δ/ppm 165.1 (CA6),
(˜ν/cm−1) 2980 (w), 2936 (w), 2905 (w), 1700 (w), 1591 (m), 1570 (w), 153.7 (CA2), 149.8 (CA4), 137.1 (CB1), 133.3 (CB2/B3), 130.2
12296 | Dalton Trans., 2013, 42, 12293–12308
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(CB2/B3), 124.5 (CB4), 124.8 (CA5), 118.7 (CA3), 25.1 (Ca). ESI MS m/z N 4.18; C68H76Br4CuF6N4P·1MeCN requires C 55.32, H 5.21,
2
1050.8 [M − PF6]+ (calc. 1050.9), 495.0 [1 + H]+ (calc. 495.0). N 4.21%.
UV-VIS (CH2Cl2, 1.0 × 10−5 mol dm−3) λ/nm 280 (ε/dm3 mol−1
[Cu(5)2][PF6]
cm−1 82 600), 324 sh (42 200), 488 (11 300). Found: C 47.55,
H 3.16, N 4.93; C48H36Br4CuF6N4P·H2O requires C 47.45, H 3.15, [Cu(NCMe)4][PF6] (90.4 mg, 0.243 mmol) was reacted with 5
N 4.61%.
(300 mg, 0.485 mmol) in MeCN (5 mL) and CH2Cl2 (40 mL).
[Cu(5)2][PF6] was isolated as a dark red solid (206 mg,
0.143 mmol, 58.9%). m.p. 325 °C. H NMR (500 MHz, CDCl3)
[Cu(2)2][PF6]
1
[Cu(NCMe)4][PF6] (36.1 mg, 0.097 mmol) was reacted with 2 δ/ppm 8.23 (d, J = 1.2 Hz, 2H, HA3), 7.77 (overlapping m, 8H,
(112 mg, 0.194 mmol) in MeCN (10 mL) and CH2Cl2 (10 mL). HB2+B3), 7.64 (d, J = 1.2 Hz, 2H, HA5), 7.62 (d, J = 7.5 Hz, 4H,
[Cu(2)2][PF6] was isolated as a dark red solid (50.9 mg, HC2), 7.05 (t, J = 7.4 Hz, 2H, HC4), 6.93 (t, J = 7.6 Hz, HC3).
0.037 mmol, 38.5%). Decomp. > 308 °C. 1H NMR (500 MHz, 13C NMR (126 MHz, CDCl3) δ/ppm 156.9 (CA6), 153.6 (CA2),
CD3CN) δ/ppm 8.67 (broadened d, 4H, HA3), 7.89 (d, J = 8.6 Hz, 149.5 (CB1), 137.9 (CC1), 135.6 (CA4), 132.9 (CB2/3), 129.5 (CC4),
8H, HB2), 7.84 (d, J = 1.5 Hz, 4H, HA5), 7.78 (d, J = 8.6 Hz, HB3), 128.9 (CB2/3), 127.7 (CC3), 127.6 (CC2), 124.8 (CB4), 122.5 (CA5),
2.70 (br, 8H, Ha), 1.40 (br, 8H, Hb), 0.88 (br, 8H, Hc), 0.44 (br, 119.4 (CA3). ESI MS m/z 1299.2 [M − PF6]+ (calc. 1399.0), 618.8
12H, Hd). 13C NMR (126 MHz, CD3CN) δ/ppm 163.0 (CA6), [5 + H]+ (calc. 619.0). UV-VIS (CH2Cl2, 1.0 × 10−5 mol dm−3
)
153.6 (CA2), 150.1 (CA4), 137.1 (CB1), 133.4 (CB3), 130.3 (CB2), λ/nm 245 sh (ε/dm3 mol−1 cm−1 57 000), 268 (72 400), 289
125.0 (CB4), 124.1 (CA5), 119.1 (CA3), 40.3 (Ca), 32.6 (Cb), 23.3 (76 000), 335 (29 300), 423 (5840), 573 (4350). Found: C 55.02,
(Cc), 13.7 (Cd). ESI MS m/z 1219.0 [M − PF6]+ (calc. 1219.1), H 3.20, N 4.14; C68H44Br4CuF6N4P·2H2O requires C 55.14,
579.1 [2 + H]+ (calc. 579.1). UV-VIS (CH2Cl2, 1.0 × 10−5 mol H 3.27, N 3.78%.
dm−3) λ/nm: 281 (ε/dm3 mol−1 cm−1 101 500), 325 sh (48 000),
486 (13 500). Found: C 52.08, H 4.53, N 4.29; C60H60Br4CuF6N4-P·
Crystallography
H2O requires C 52.10, H 4.52, N 4.05%.
Data were collected on a Bruker APEX-II diffractometer with
data reduction, solution and refinement using the programs
APEX25 and SHELXL97 or SHELX-13.26 ORTEP-type diagrams
[Cu(3)2][PF6]
[Cu(NCMe)4][PF6] (58.7 mg, 0.158 mmol) was reacted with 3 and structure analysis used Mercury v. 3.0.27,28
(182 mg, 0.315 mmol) in MeCN (10 mL) and CH2Cl2 (10 mL).
7a
[Cu(3)2][PF6] was isolated as a dark red solid (186 mg,
1
0.136 mmol, 86.4%). m.p. 338 °C. H NMR (500 MHz, DMSO- C32H38N2O6P2, M = 608.58, colourless plate, triclinic, space
d6) δ/ppm 9.08 (d, J = 1.4 Hz, 4H, HA3), 8.08 (m, 8H, HB2), 8.03 group P1, a = 7.7810(7), b = 7.9273(7), c = 14.5543(13) Å, α =
ˉ
(d, J = 1.4 Hz, 4H, HA5), 7.85 (m, 8H, HB3), 2.52 (s, 8H, Ha), 75.197(3), β = 88.156(3), γ = 64.398(3)°, U = 779.57(12) Å3, Z = 1,
1.72 (septet, d, J = 6.8 Hz, 4H, Hb), 0.47 (d, J = 6.6 Hz, 24H, Hc). Dc = 1.296 Mg m−3, μ(Cu-Kα) = 1.646 mm−1, T = 123 K. Total
13C NMR (126 MHz, DMSO-d6) δ/ppm 160.6 (CA6), 153.3 (CA2), 10 179 reflections, 2739 unique, Rint = 0.0401. Refinement of
148.7 (CA4), 135.9 (CB1), 132.7 (CB3), 130.2 (CB2), 124.3 (CB4), 2681 reflections (233 parameters) with I > 2σ(I) converged at
123.5 (CA5), 118.1 (CA3), 47.6 (Ca), 27.5 (Cb), 21.5 (Cc). ESI MS final R1 = 0.0391 (R1 all data = 0.0395), wR2 = 0.1074 (wR2 all
m/z 1219.1 [M − PF6]+ (calc. 1219.1), 579.1 [3 + H]+ (calc. data = 0.1078), gof = 1.058. CCDC 942072.
579.1). UV-VIS (CH2Cl2, 1.0 × 10−5 mol dm−3) λ/nm 282 (ε/dm3
2{[Cu(1)2][PF6]}·3Me2CO
mol−1 cm−1 98 200), 323 sh (44 950), 481 (12 100). Found:
C 52.51, H 4.52, N 4.38; C60H60Br4CuF6N4P requires C 52.78, C105H90Br8Cu2F12N8O3P2, M = 2568.09, yellow plate, triclinic,
ˉ
H 4.43, N 4.10%.
space group P1, a = 11.7229(6), b = 14.2639(8), c = 15.9549(8) Å,
α = 93.364(3), β = 100.301(3), γ = 91.589(3)°, U = 2618.4(2) Å3,
Z = 1, Dc = 1.629 Mg m−3, μ(Cu-Kα) = 4.998 mm−1, T = 123 K.
[Cu(4)2][PF6]
[Cu(NCMe)4][PF6] (27.4 mg, 0.074 mmol) was reacted with 4 Total 36 736 reflections, 9192 unique, Rint = 0.0380. Refine-
(93.4 mg, 0.147 mmol) in MeCN (10 mL) and CH2Cl2 (10 mL). ment of 7490 reflections (657 parameters) with I > 2σ(I) con-
[Cu(4)2][PF6] was isolated as a dark red solid (40.2 mg, verged at final R1 = 0.0575 (R1 all data = 0.0703), wR2 = 0.1655
1
0.027 mmol, 45.2%). m.p. 270 °C. H NMR (500 MHz, CD3CN) (wR2 all data = 0.1785), gof = 1.038. CCDC 942070.
δ/ppm 8.70 (s, 4H, HA3), 7.90 (d, J = 8.5 Hz, 8H, HB2), 7.85 (s,
[Cu(2)2][PF6]
4H, HA5), 7.79 (d, J = 8.5 Hz, 8H, HB3), 2.68 (s, 8H, Ha), 1.41 (s,
8H, Hb), 0.89 (s, 8H, He), 0.84 (s, 8H, Hc), 0.72 (s, 8H, Hd), 0.55 C60H60Br4CuF6N4P, M = 1365.24, orange block, monoclinic,
(t, J = 6.6 Hz, 12H, Hf). 13C NMR (126 MHz, CD3CN) δ/ppm space group P21/n, a = 17.7430(13), b = 16.0855(12), c =
163.2 (CA6), 153.6 (CA2), 150.1 (CA4), 137.0 (CB1), 133.3 (CB3), 20.9134(16) Å, β = 105.193(3)°, U = 5760.2(8) Å3, Z = 4, Dc =
130.2 (CB2), 124.9 (CB4), 124.1 (CA5), 118.9 (CA3), 40.7 (Ca), 32.0 1.574 Mg m−3, μ(Cu-Kα) = 4.560 mm−1, T = 123 K. Total 63 279
(Cd), 30.6 (Cb), 30.0 (Cc), 22.9 (Ce), 14.0 (Cf). ESI MS m/z 1331.0 reflections, 10 359 unique, Rint = 0.0513. Refinement of 9481
[M − PF6]+ (calc. 1331.2), 635.2 [5 + H]+ (calc. 635.1). UV-VIS reflections (747 parameters) with I > 2σ(I) converged at final R1
(CH2Cl2, 1.0 × 10−5 mol dm−3) λ/nm 282 (ε/dm3 mol−1 cm−1 = 0.0372 (R1 all data = 0.0406), wR2 = 0.0958 (wR2 all data =
85 000), 325 sh (41 100), 486 (10 800). Found: C 55.44, H 5.21, 0.0985), gof = 1.037. CCDC 942069.
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2{[Cu(3)2][PF6]}·Et2O
removed from the solution and were washed with CH2Cl2 and
dried under a stream of N2.
C124H130Br8Cu2F12N8OP2, M = 2804.61, yellow plate, monoclinic,
space group P21/c, a = 15.5265(7), b = 23.9138(9), c =
17.7749(7) Å, β = 114.893(2)°, U = 5986.6(4) Å3, Z = 2, Dc = 1.556
Mg m−3, μ(Cu-Kα) = 4.410 mm−1, T = 123 K. Total 46 225 reflec-
tions, 10 612 unique, Rint = 0.0480. Refinement of 8760 reflec-
tions (761 parameters) with I > 2σ(I) converged at final R1
0.0467 (R1 all data = 0.0581), wR2 = 0.1168 (wR2 all data =
0.1244), gof = 1.037. CCDC 942071.
Each counter electrode was prepared from an FTO glass
plate (Solaronix TCO22-7, 2.2 mm thickness, sheet resistance
≈7 Ω square−1) with a previously drilled hole. Residual organic
impurities were removed by heating for 15 min at 500 °C on a
heating plate and the perforated plate was washed with water,
then 0.1 M HCl solution in EtOH and finally ultrasonicated in
an acetone bath for 15 min. It was dried on a heating plate at
500 °C for 15 min. The Pt catalyst was deposited on the FTO
glass plate by coating with Platisol T (2 × 25.0 μL), Solaronix,
and dried on a heating plate at 500 °C for 15 min.
=
[Cu(5)2][PF6]·CH2Cl2
The dye-covered TiO2 electrode and Pt counter-electrode
were assembled using a thermoplast hot-melt sealing foil
(Solaronix, Meltonix 1170-25 Series, 25 microns thick) by
heating while pressing them together. The electrolyte com-
prised LiI (0.1 mol dm−3), I2 (0.05 mol dm−3), 1-methylbenzi-
midazole (0.5 mol dm−3) and 1-butyl-3-methylimidazolinium
iodide (0.6 mol dm−3) in methoxypropionitrile, and was intro-
duced into the cell by vacuum backfilling. The hole on the
counter electrode was finally sealed using the hot-melt sealing
foil and a cover glass.
C69H46Br4Cl2CuF6N4P, M = 1530.12, yellow plate, orthorhom-
bic, space group Pbcn, a = 7.5560(15), b = 28.392(7), c = 27.854
(6) Å, U = 5976(2) Å3, Z = 4, Dc = 1.701 Mg m−3, μ(Cu-Kα) =
5.284 mm−1, T = 123 K. Total 37 392 reflections, 5233 unique,
Rint = 0.318. Refinement of 1637 reflections (395 parameters)
with I > 2σ(I) converged at final R1 = 0.0825 (R1 all data =
0.2739), wR2 = 0.1280 (wR2 all data = 0.1881), gof = 0.933.
CCDC 942073.
Solar cell fabrication
The solar cell measurements and testing protocol were per-
The fabrication of DSCs was based on the method of Grätzel formed using fully masked cells. A black coloured copper
and coworkers.29 The TiO2 paste was prepared adapting the sheet was used for masking with a single aperture of an
published method using a three-roll mill (50 EC, EXAKT, average area of 0.06012 cm2 (with a standard deviation of 1%)
Germany), a sonicator bath and terpineol (CAS: 8000-41-7). placed over the screen printed dye-sensitized TiO2 circle. The
Each working electrode was made from an FTO glass plate area of the aperture in the mask was smaller than the active
(Solaronix TCO22-7, 2.2 mm thickness, sheet resistance ≈7 Ω area of the dye-sensitized TiO2 dot (0.288 cm2). For complete
square−1) which was cleaned by sonicating in Hellmanex® sur- masking, tape was also applied over the edges and rear of the
factant (2% in milliQ water), and rinsed with milliQ water and cell. Current density–voltage (I–V) measurements were made
EtOH. After surface activation in a UV-O3 system (Model 256- by irradiating from behind using a light source SolarSim 150
220, Jelight Company Inc.) for 20 min, the FTO plate was (100 mW cm−2 = 1 sun). The power of the simulated light was
immersed in aqueous TiCl4 (40 mmol dm−3) at 70 °C for calibrated by using a reference Si photodiode. The standard
30 min, and then washed with milliQ water and EtOH. The dye N719 was purchased from Solaronix.
FTO plate was dried and a layer of TiO2 paste was screen
The quantum efficiency measurements were performed on
printed (90 T, Serilith AG, Switzerland). The printed plate was an Spe-Quest quantum efficiency setup from Rera Systems
kept in an EtOH chamber for 3 min to reduce surface irregula- (Netherlands) equipped with a 100 W halogen lamp (QTH)
rities of the printed layer and dried for 6 min at 125 °C on a and a lambda 300 grating monochromator from Lot Oriel. The
heating plate. The screen printing process was repeated 8 monochromatic light was modulated to 3 Hz using a chopper
times and then the electrodes were gradually heated at 75 °C wheel from ThorLabs. The cell response was amplified with a
for 30 min, at 135 °C for 15 min, at 325 °C for 5 min, at 375 °C large dynamic range IV converter from CVI Melles Griot and
for 5 min, at 450 °C for 15 min and at 500 °C for 15 min. then measured with an SR830 DSP Lock-In amplifier from
After the final sintering, the thickness of the TiO2 layer was Stanford Research.
14–16 μm (measured with a Tencor Alpha-Step 500 profil-
ometer). The annealed TiO2 film was post-treated with
40 mmol dm−3 aqueous TiCl4 solution (see above), rinsed with
milliQ water and EtOH and sintered at 500 °C for 30 min.
The electrodes were cooled to ca. 80 °C and immersed in a
1 mM DMSO solution of the anchoring ligand 6 or 7 for 20 h.
Results and discussion
Synthesis and characterization of ancillary ligands
The colourless electrode was removed from the solution, We have previously described the synthesis of compound 1
washed with DMSO and EtOH and dried under a stream of N2. using Krönhke methodology30 by the treatment of (1E,5E)-
The electrode with the adsorbed anchoring ligand was 1,6-bis(4-bromophenyl)hexa-1,5-diene-3,4-dione with 1-(2-oxo-
immersed in a 1 mM MeCN solution of [Cu(L)2][PF6] (L = 1–4) propyl)pyridinium chloride and ammonium acetate.14 Com-
and in a 1 mM CH2Cl2 solution of [Cu(5)2][PF6] for 4 days to pounds 2, 3, 4, and 5 were prepared according to Scheme 3
produce red-orange coloured electrodes. The electrodes were using the appropriate “pyridacylpyridinium salts” to introduce
12298 | Dalton Trans., 2013, 42, 12293–12308
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the higher energy band is consistent with the latter arising
from π*←π transitions. The lower energy absorption shows no
change in intensity along the series of five compounds, and is
assigned to π*←n(py) transitions.
In terms of ligand design for dyes for DSCs, compound 5
appears superior to 1–4 in terms of red-shifting of the spectral
response, and we later confirm that this trend is preserved in
the homoleptic copper(I) complexes of these ligands.
Synthesis and characterization of anchoring ligand 7
Compatibility between anchoring and ancillary ligands is of
prime importance in our copper-containing heteroleptic dyes.
We have previously made extensive use of anchoring ligand
6
9,13,14 and have shown that, even though 6 binds the copper(I)
ion close to the TiO2 surface, it remains suited to ancillary
ligands with sterically demanding substituents.9 On the other
hand, we argued that the in situ assembly of copper(I) dyes in
which the bpy-derived ancillary ligand contains long chain or
aromatic 6,6′-substituents could be enhanced by introducing a
spacer between the metal-binding domain of the anchoring
ligand and the phosphonic acid anchors. With this in mind,
we prepared ligand 7 (Scheme 4) as the first in a series of new
phosphonic acid anchoring ligands.
Scheme 3 Synthetic route to compounds 2–5.
the desired
bipyridine.
R groups at the 6,6′-positions of the 2,2′-
The electrospray mass spectrum of each of compounds 2–5
exhibited a peak envelope as the base peak which corre-
sponded to [M + H]+ and with an isotope pattern characteristic
of the presence of two bromine atoms. The H and 13C NMR
1
Scheme 4 summarizes the synthesis of 7 from the corres-
ponding ester, 7a (in turn obtained from ligand 1); preparation
of the latter was based upon a literature method.31 After
spectra were in accord with a 4,4′,6,6′-substituted 2,2′-bipyri-
dine, and the spectra were assigned using COSY, NOESY,
DEPT, HMQC and HMBC methods. The 1H NMR resonances
for protons HA5 and HA3 were distinguished by the appearance
of a cross peak in the HMBC spectrum of each of compounds
2–4 between signals for Ca and HA5. In the 500 MHz H NMR
1
spectra of 2, 3 and 4, the signals for HB2 and HB3 overlap, and
unambiguous assignments of the CB2 and CB3 resonances were
not made from the HMQC or HMBC spectra.
Compounds 1–4 each contain alkyl substituents in the 6,6′-
positions and their electronic absorption spectra are similar,
being dominated by an intense band at 255 nm with a weaker
band close to 305 nm (Fig. 1). Introduction of the 6- and
6′-phenyl substituents in compound 5 red-shifts both absorp-
tions (to 265 and 321 nm). The enhancement of intensity of
Scheme 4 Synthesis of 7: (i) [Pd(PPh3)4], Cs2CO3, HPO3Et2, THF, microwave,
110 °C, 90 min; (ii) aqueous HCl, reflux, 48 h.
Fig. 1 Absorption spectra of ligands 1–5 (CH2Cl2, 1.0 × 10−5 mol dm−3).
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dimethyl-2,2′-bipyridine),32 and we have recently observed that
addition of aliquots of trifluoroacetic acid to a CDCl3 solution
of a ligand closely related to 7 leads to an initial downfield
shift of the signals for HA3 from δ 8.39 to 8.53 ppm, followed
by an upfield shift to δ 8.34 ppm corresponding to formation
of first the monoprotonated and then the diprotonated
ligand.33 Fully assigned NMR spectroscopic data for proto-
nated 7 are given in the Experimental section.
Fig. 2 ORTEP representation of compound 7a (ellipsoids plotted at 40% prob-
ability level; only the major occupancy sites are shown). Symmetry code i = 1 −
x, 1 − y, 1 − z. Selected bond distances and angles: N1–C2 = 1.3389(14), N1–C6
= 1.3442(13), C10–P1 = 1.7888(9), P1–O1 = 1.4629(9), P1–O3 = 1.5457(16), P1–
O2 = 1.5885(8), O2–C13 = 1.4515(14), O3–C15 = 1.461(3) Å; C2–N1–C6 =
117.68(8), O1–P1–O3, 107.04(8), O1–P1–O2 = 115.27(4), O3–P1–O2 = 103.00
(7), O1–P1–C10 = 113.61(5), O3–P1–C10 = 110.31(6), O2–P1–C10 = 107.07(5)°.
Synthesis and characterization of homoleptic copper(I)
complexes
Reaction of each of ligands 1–5 with [Cu(NCMe)4][PF6] in
CH2Cl2 at room temperature resulted in the formation of a red
complex characterized as [CuL2][PF6]. In the electrospray mass
spectrum of each complex, the base peak corresponded to
[M − PF6]+. The combination of copper and bromine isotopes
leads to the two most intense peaks in the isotope pattern for
the [M − PF6]+ peak being of almost equal intensity. This is
exemplified for [Cu(3)2][PF6] in Fig. 3. The lower m/z value of
each pair is listed in the Experimental section.
deprotection by treatment with aqueous HCl and workup, 7a
was isolated in 65% yield. The base peak in the ESI mass spec-
trum (m/z 609.3) arose from the [M + H]+ ion and a much less
intense peak at m/z 631.2 was assigned to [M + Na]+.
The 1H and 13C NMR spectra of the complexes were consist-
ent with a single ligand environment in each complex. The
spectra have been fully assigned using routine 2D methods.
X-ray quality crystals of 2{[Cu(1)2][PF6]}·3Me2CO were grown
by Et2O diffusion into an acetone solution of the complex
at room temperature, and Fig. 4 shows the structure of the
[Cu(1)2]+ cation. The distorted tetrahedral coordination geo-
metry of atom Cu1 is less flattened than in [Cu(dmbpy)2]+
(dmbpy = 6,6′-dimethyl-2,2′-bipyridine); in [Cu(1)2]+, the angle
between the least squares planes containing Cu1 and each bpy
unit is 85.6°, compared to angles of 74.3° in [Cu(dmbpy)2]-
[PF6],9 80.9° in [Cu(dmbpy)2][BF4]34 and 80.9° in [Cu(dmbpy)2]-
[ClO4].35 The flexibility of the Cu(bpy)2-unit is consistent with
the suggestion35 that packing forces are responsible for the
detailed geometry at the copper(I) centre. The two bpy units
are close to planar (angles between the planes of the rings con-
taining N1/N2 and N3/N4 = 5.5 and 7.9°, respectively), and the
angles between the planes of the pairs of bonded phenyl and
pyridine rings range from 10.1 to 34.9°. The variation in the
latter is a consequence of packing interactions. Pairs of phenyl-
pyridine units containing N1/C13 and N1i/C13i (symmetry
X-ray quality crystals of 7a were grown by diffusion of a
mixture of pentane and hexane (1 : 1 by volume) into a concen-
trated acetone solution of the compound. Structural determi-
nation confirmed the structure shown in Fig. 2. The bpy unit
adopts the expected trans conformation and the two halves of
the molecule are related by an inversion centre. Compound 7a
is disordered and the OEt group containing O3 has been mod-
elled over two sites with 49 and 51% occupancies; the methyl
group of the second OEt unit has been modelled over two sites
of occupancies 74 and 26%. The phenyl ring is twisted
through 29.3° with respect to the pyridine ring. Although the
molecules pack with the aromatic domains over one another,
the presence of the methyl substituents prevents efficient face-
to-face π-stacking interactions. The dominant packing forces
involve CH⋯O and CH⋯N contacts.
Quantitative conversion of the ester 7a into phosphonic
acid 7 was achieved using aqueous HCl (6 M) at reflux followed
by treatment of the residue obtained with aqueous acetic acid.
This latter step appears to be essential for the isolation of pure
7. Compound 7 is poorly soluble in most common solvents
and it was not possible to obtain an electrospray mass spec-
trum. However, satisfactory elemental analysis was obtained
for the hydrate 7·H2O. The solubility of 7 in DMSO-d6 was
sufficient to permit a 1H NMR spectrum to be recorded, but
13C NMR resonances could only be assigned using HMQC and
HMBC methods; no cross peaks associated with CA2 (see
Scheme 1 for atom labelling) in the HMBC spectrum were
1
observed. Compound 7 dissolved readily in CF3CO2D. The H
NMR spectrum was consistent with protonation of 7, although
it is difficult to assess from the change in the chemical shift of
the signal for HA3 (δ 8.53 ppm for 7 in DMSO-d6 to δ 8.67 ppm
for the compound in CF3CO2D) whether the species present is
[H27]2+ (trans-conformation of the bpy unit) or [H7]+ (cis-bpy).
These conformational changes have been confirmed structu-
rally for salts of [Hdmbpy]+ and [H2dmbpy]2+ (dmbpy = 6,6′-
Fig. 3 The base peak envelope in the ESI mass spectrum of [Cu(3)2][PF6].
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Fig. 4 ORTEP diagram of the [Cu(1)2]+ cation in 2{[Cu(1)2][PF6]}·3Me2CO
(H atoms omitted and ellipsoids plotted at 40% probability level; only the major
occupancy sites are shown). Bond parameters in the coordination sphere: Cu1–
N4 = 2.010(4), Cu1–N1 = 2.027(4), Cu1–N3 = 2.034(4), Cu1–N2 = 2.038(4) Å;
N4–Cu1–N1 = 130.67(18), N4–Cu1–N3 = 80.68(16), N1–Cu1–N3 = 124.04(16),
N4–Cu1–N2 = 129.03(16), N1–Cu1–N2 = 81.49(17), N3–Cu1–N2 = 116.14(17)°.
code i = 1 − x, −y, 1 − z) sandwich one acetone solvent mole-
cule which is disordered over two sites positioned about an
inversion centre. The phenyl ring of the phenylpyridine unit
containing N2 is π-stacked over its symmetry related counter-
part (symmetry code = 1 − x, 1 − y, 1 − z) at a separation of
3.4 Å (Fig. S1†), and a similar interaction occurs between the
phenylpyridine units containing N3 and N3ii (symmetry code
ii = 3 − x, 1 − y, 2 − z). The phenylpyridine unit with N4 shows
the greatest deviation from planarity and accommodates a
hydrogen-bonded pair of acetone molecules over the pyridine
ring (C102H10E...O100iii = 1.65 Å; symmetry code iii = 2 − x,
−y, 2 − z). The space-filling diagram in Fig. 5a illustrates the
shielding of the copper(I) centre by the methyl substituents.
Single crystals of [Cu(2)2][PF6] were grown by Et2O diffusion
into an acetone solution of the complex; the structure of the
[Cu(2)2]+ cation is shown in Fig. 6. The distorted tetrahedral
coordination geometry of Cu1 is essentially the same as that in
[Cu(1)2]+ with an angle between least squares planes contain-
ing Cu1 and each bpy unit of 86.4°. Each bpy unit is slightly
twisted (angles between the planes of the rings containing N1/
N2 and N3/N4 = 7.1 and 12.3°, respectively), and the angles
between the planes of the pairs of bonded phenyl and pyridine
rings lie in the range 11.1–29.8°, again a consequence of
packing interactions. The butyl substituents of the ligand con-
taining N3 and N4 are disordered; each of the three terminal C
and attached H atoms have been modelled over two sites (C31,
C32 and C33 with fractional occupancies 0.54 and 0.46, and
C46, C47 and C48 with fractional occupancies 0.51 and 0.49).
The presence of the disorder makes it meaningless to
comment on the relative orientations of the butyl groups.
Nonetheless, using the major occupancy sites, Fig. 5b illus-
trates how the butyl chains wrap around the copper(I) centre.
Single crystals of 2{[Cu(3)2][PF6]}·Et2O were obtained by
Et2O diffusion into an acetone–chloroform solution of the
complex. Fig. 7 shows the structure of the [Cu(3)2]+ cation. The
Fig. 5 Space-filling representation of the (a) [Cu(1)2]+, (b) [Cu(2)2]+, (c)
[Cu(3)2]+ and (d) [Cu(5)2]+ cations. In [Cu(2)2]+, only the major occupancy sites
of the two disordered butyl chains are shown.
Fig. 6 ORTEP representation of the [Cu(2)2]+ cation in [Cu(2)2][PF6] (H atoms
omitted and ellipsoids plotted at 40% probability level; only the major occu-
pancy sites are shown). Bond parameters in the coordination sphere: Cu1–N1 =
2.0368(14), Cu1–N2 = 2.0391(14), Cu1–N3 = 2.0283(15), Cu1–N4 = 2.0485(13)
Å; N3–Cu1–N1 = 130.38(6), N3–Cu1–N2 = 126.86(6), N1–Cu1–N2 = 81.35(6),
N3–Cu1–N4 = 81.16(6), N1–Cu1–N4 = 117.95(6), N2–Cu1–N4 125.08(5)°.
distorted tetrahedral coordination environment is similar to
those in 2{[Cu(1)2][PF6]}·3Me2CO and [Cu(2)2][PF6]; the angle
between the least squares planes containing Cu1 and each bpy
unit is 82.9°. However, one bpy unit in [Cu(3)2]+ is significantly
more twisted than in the complexes containing 1 and 2: angles
between the planes of the rings with N1/N2 and N3/N4 of 12.3
and 29.0°, and this is probably a consequence of steric
crowding originating from the four isobutyl groups which
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Fig. 7 ORTEP representation of the [Cu(3)2]+ cation in [Cu(3)2][PF6] (H atoms
omitted and ellipsoids plotted at 40% probability level). Bond parameters in the
coordination sphere: Cu1–N2 = 2.029(3), Cu1–N4 = 2.039(3), Cu1–N1 = 2.049(3),
Cu1–N3 = 2.063(3) Å; N2–Cu1–N4 = 124.31(11), N2–Cu1–N1 = 81.30(11),
N4–Cu1–N1 = 125.99(11), N2–Cu1–N3 = 122.66(11), N4–Cu1–N3 = 81.21(11),
N1–Cu1–N3 = 127.62(11)°.
Fig. 8 ORTEP representation of the [Cu(5)2]+ cation in [Cu(5)2][PF6]·CH2Cl2
(H atoms omitted and ellipsoids plotted at 30% probability level). Bond parameters
in the coordination sphere: Cu1–N1 = 2.013(6), Cu1–N2 = 2.017(6) Å; N1–Cu1–
N1i = 83.6(4), N1–Cu1–N2i = 138.5(2), N1–Cu1–N2 = 112.1(2), N2i–Cu1–N2 =
82.0(4)°. Symmetry code i = −x, y, 3/2 − z.
wrap around the metal centre (Fig. 6c). The isobutyl group con-
taining C1 and C2 is disordered and has been modelled over
two positions with site occupancies of 0.59 and 0.41; the
carbon atom of the CH2 group is common to both positions.
The distortion of the ligand containing N3 and N4 extends to
the bromophenyl unit with Br4; the phenylpyridine unit is sig-
nificantly bowed. The origin of the distortion is not clear but
may be associated with interactions with the ordered [PF6]−
ion which is sandwiched between the phenyl rings containing
C52 and C58 with three short CH⋯F contacts in the range 2.59
and 2.71 Å. In addition, atom Br4 exhibits a short contact with
Br2i (Br4⋯Br2i = 3.6922(7) Å, symmetry code i = 1 − x, + y,
1
2
− z, van der Waals radius of Br = 1.95 Å).36 As in the two
1
2
previous structures, the extent that each phenylpyridine unit
deviates from planarity is controlled by packing forces with
twist angles lying in the range 20.1–40.3°. The Et2O molecule
is disordered and has been modelled over two sites on a
special position.
Fig. 9 Absorption spectra of CH2Cl2 solutions (1.0 × 10−5 mol dm−3) of [CuL2]-
[PF6] for L = 1–5.
Crystals of [Cu(5)2][PF6]·CH2Cl2 suitable for X-ray diffraction
were grown by diffusion of Et2O into a CH2Cl2 solution of the
complex. Fig. 8 shows the structure of the [Cu(5)2]+ cation and {Cu(2,9-Ph2phen)2} and related complexes,20,21,37–50 although
within these examples, the efficiency of the intra-cation π-inter-
bond lengths and angles involving atom Cu1 are given in the
caption. The complex crystallizes in the orthorhombic space actions is variable. In [Cu(5)2][PF6]·CH2Cl2, face-to-face π-stack-
group Pbcn with atom Cu1 lying on a two-fold axis. In contrast ing occurs between pendant phenyl rings of neighbouring
cations leading to columns of [Cu(5)2]+ cations running paral-
to [Cu(1)2]+, [Cu(2)2]+ and [Cu(3)2]+, the [Cu(5)2]+ cation
possesses a flattened structure (Fig. 5) with an angle between lel to the a-axis. Additional packing interactions involve short
the least squares planes containing Cu1 and each bpy unit of Br⋯Br contacts, and close CH⋯F and CH⋯Cl contacts; both
the [PF6]− ion and the CH2Cl2 molecule are ordered.
only 44.1°. This is a consequence of a π-stacking interaction
between each 6- (or 6′-) phenyl-substituent and a pyridine ring
of the adjacent ligand. To achieve four such interactions, the with L = 1–5 are shown in Fig. 9. Spectra for the complexes
containing the aliphatic side chains are virtually superimposa-
The solution electronic absorption spectra of [CuL2][PF6]
bpy domains deviate from planarity (19.6 and 30.3°, respecti-
vely) and each phenyl ring twists (31.2 and 62.4°) with respect ble with high energy bands assigned to ligand-based π*←π
to the pyridine ring to which it is bonded. Despite these con- transitions and a broad MLCT band in the visible region (λmax
= 481–488 nm). The dual bands observed in the visible region
formational changes, optimal π-stacking interactions with par-
allel rings are not achieved. The structural features of the of the spectrum [Cu(5)2][PF6] and the red-shift to 573 nm
copper(I) coordination sphere in [Cu(5)2]+ are consistent
are consistent with features in the absorption spectrum of
[Cu(dpp)2]+ (dpp = 2,9-diphenyl-1,10-phenanthroline) which
with the flattening observed in a range of {Cu(6,6′-Ph2bpy)2}+,
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are explained in terms of the flattened, low-symmetry structure substituents being the easiest to oxidize: [Cu(1)2]+ < [Cu(2)2]+ ≈
of the complex.9,20 The spectrum of [Cu(5)2][PF6] retains the [Cu(3)2]+ ≈ [Cu(4)2]+. The oxidation potential for [Cu(1)2][PF6]
enhanced spectral response in the higher energy bands (+0.42 V) compares to +0.50 V (vs. Fc/Fc+ in CH2Cl2) for
observed for ligand 5 compared to 1–4 (Fig. 1).
[Cu(dmp)2][PF6] (dmp = 2,9-dimethyl-1,10-phenanthroline).
All the complexes are electrochemically active and cyclic vol-
tammetric data are presented in Table 1. It is well established
that in copper(I) diimine complexes, the presence of substitu-
ents adjacent to the N,N-donor sites (e.g. 6,6′-positions in 2,2′-
bpy or 2,9-sites in 1,10-phenanthroline) stabilizes copper(I)
with respect to oxidation by sterically hindering the flattening
of the coordination sphere that accompanies the d10 (Cu+) to
d9 (Cu2+) electronic configurational change.19,22 Cyclic voltam-
mograms were recorded in CH2Cl2 to avoid problems associ-
Table 1 Cyclic voltammetric data for [CuL2][PF6] with L = 1–5 with respect to
Fc/Fc+; CH2Cl2 solutions with [nBu4N][PF6] as the supporting electrolyte and a
scan rate of 0.1 V s−1. Processes are reversible unless otherwise stated (ir =
irreversible)
Complex
E1o/x2/V (Epc − Epa/mV)
Eo1/x2/V
Er1e/2d/V
[Cu(1)2][PF6]
[Cu(2)2][PF6]
+0.42 (94)
+0.52 (87)
+0.54 (96)
+0.54 (84)
+0.36 (86)
+1.49ir
+1.47ir
+1.47ir
−2.16ir
−2.16ir
−2.20ir
ated with coordinating solvents such as MeCN.19 The [Cu(3)2][PF6]
[Cu(4)2][PF6]
[Cu(5)2][PF6]
+1.49ir
−2.14ir
potentials for the metal-centred oxidation process in [CuL2]-
[PF6] with L = 1–4 follow the expected trend, with the com-
plex containing the least sterically demanding (methyl)
—
a
a No reduction process observed within the solvent accessible window.
Table 2 Sealed and masked DSC cell performances using anchoring ligands 6 and 7 with respect to standard dye N719 measured under the same conditions
On the day of sealing the cell
[CuL2]+
Anchor
JSC/mA cm−2
VOC/mV
ff
η/%
Relative η/%
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
N719
6
6
6
6
6
7
7
7
7
7
3.21
3.64
4.32
3.57
0.35
5.09
5.50
6.73
5.07
2.74
15.06
477
478
515
475
473
497
497
541
507
605
634
0.72
0.71
0.69
0.71
0.56
0.72
0.71
0.70
0.71
0.66
0.69
1.10
1.24
1.54
1.21
0.09
1.82
1.95
2.55
1.83
1.10
6.62
16.6
18.8
23.2
18.3
1.38
27.5
29.5
38.5
27.7
16.6
100.0
3 days after sealing the cell
[CuL2]+
Anchor
JSC/mA cm−2
VOC/mV
ff
η/%
Relative η/%
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
N719
6
6
6
6
6
7
7
7
7
7
4.44
3.51
4.35
3.97
2.22
5.85
5.17
7.02
5.54
6.41
14.80
500
506
543
505
534
519
524
566
496
600
673
0.72
0.67
0.65
0.70
0.68
0.72
0.67
0.63
0.68
0.73
0.61
1.60
1.19
1.54
1.40
0.81
2.19
1.82
2.49
1.87
2.80
6.09
26.3
19.5
25.3
23.0
13.3
36.0
29.9
40.8
30.6
45.9
100.0
4 days after sealing the cell
[CuL2]+
Anchor
JSC/mA cm−2
VOC/mV
ff
η/%
Relative η/%
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
N719
6
6
6
6
6
7
7
7
7
7
4.53
3.56
4.44
4.21
3.53
6.01
5.30
7.06
6.07
6.70
14.66
507
502
549
514
536
527
525
571
520
592
679
0.72
0.66
0.66
0.69
0.68
0.73
0.65
0.60
0.60
0.73
0.60
1.65
1.18
1.61
1.50
1.29
2.30
1.80
2.43
1.90
2.89
5.96
27.7
19.8
27.0
25.2
21.6
38.6
30.2
40.8
31.9
48.5
100.0
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Table 3 DSC performance data for an independent set of sealed and masked cells using anchoring ligand 7; data are with respect to standard dye N719 measured
under the same conditions
On the day of sealing the cell
[CuL2]+
Anchor
JSC/mA cm−2
VOC/mV
ff
η/%
Relative η/%
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
N719
7
7
7
7
7
4.77
4.22
6.42
4.80
3.97
15.37
503
482
555
502
605
648
0.73
0.74
0.73
0.72
0.69
0.71
1.76
1.49
2.60
1.74
1.67
7.08
24.9
21.1
36.7
24.6
23.6
100.0
1 day after sealing the cell
[CuL2]+
Anchor
JSC/mA cm−2
VOC/mV
ff
η/%
Relative η/%
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
N719
7
7
7
7
7
6.09
5.61
6.74
5.92
6.81
17.10
520
499
571
510
611
677
0.74
0.74
0.73
0.73
0.72
0.71
2.33
2.06
2.81
2.19
3.01
8.22
28.4
25.1
34.2
26.6
36.6
100.0
2 days after sealing the cell
[CuL2]+
Anchor
JSC/mA cm−2
VOC/mV
ff
η/%
Relative η/%
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
N719
7
7
7
7
7
5.93
5.42
6.91
5.51
6.96
17.2
525
501
576
521
606
684
0.74
0.73
0.72
0.73
0.74
0.71
2.31
1.99
2.87
2.09
3.10
8.30
27.8
24.0
34.6
25.2
37.4
100.0
7 days after sealing the cell
[CuL2]+
Anchor
JSC/mA cm−2
VOC/mV
ff
η/%
Relative η/%
[Cu(1)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(5)2]+
N719
7
7
7
7
7
5.73
5.51
6.69
5.44
7.03
17.23
530
513
584
531
590
696
0.74
0.74
0.73
0.71
0.73
0.71
2.26
2.09
2.85
2.05
3.01
8.45
26.8
24.7
33.7
24.3
35.6
100.0
Oxidation of the copper(I) ion in [Cu(5)2]+ occurs at lower with the labile53,54 homoleptic complexes [CuL2][PF6] (L = 1–5)
potential than in [Cu(1)2]+, and the value of +0.36 V is similar resulting in ligand exchange and surface immobilisation of
to that reported for [CuL2]+ where L = 4,4′,6,6′-tetraphenyl-2,2′- the heteroleptic dyes [Cu(L)(6)]+ and [Cu(L)(7)]+ shown in
bipyridine (+0.39 V vs. Fc/Fc+ in CH2Cl2, corrected51 from the Scheme 2. The proximity to the surface of the copper(I) centre
published value40 of +0.77 V vs. SCE). The structural reorganiz- and of the 6,6′-substituents in the ancillary ligand is related to
ation that accompanies metal oxidation is less pronounced in the presence or not of the aryl spacer in the anchoring ligand.
[Cu(5)2]+ than in [Cu(1)2]+. This assumption is based upon the All cells were fully masked to prevent overestimation of conver-
flattened geometry in the former brought about by π-stacking sion efficiency.55 Table 2 summarizes the DSC characteristics
interactions that occur between the phenyl groups of one measured for each cell over a period of 4 days and with respect
ligand and the bpy domain of the other. This has been to the standard dye N719 (in the table, VOC = open circuit
observed in a number of related structures.37–41 Each of the voltage, JSC = short circuit current density, ff = fill factor). The
complexes containing aliphatic substituents exhibits an irre- final column of the table shows the energy conversion efficien-
versible oxidation process at higher potential (Table 1) most cies relative to N719 which is arbitrarily set to 100%.
likely associated with the oxidation of the 4-bromophenyl sub-
stituent to the corresponding radical cation.52
The surface-bound heteroleptic dyes [Cu(1)(6)]+, [Cu(2)(6)]+,
[Cu(3)(6)]+ and [Cu(4)(6)]+ show a gain in values of JSC and VOC
over the 4 day period, but overall there is little change to the
fill factor for each cell. Of the four alkyl-substituted ligands, 3
(with isobutyl substituents) produces the most efficient dye
with a relative energy efficiency, η, reaching 25.2% with respect
Performance of the dyes in DSCs
Heteroleptic dyes were assembled by stepwise adsorption of
the anchoring ligand 6 or 7 onto TiO2 followed by treatment
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and coworkers also conclude that electronic charge transferred
from copper to dpp ligands does not reside on the phenyl
rings since conjugation between phen and Ph domains is lost
when the ligand changes its conformation. In our case, there
is an additional degree of freedom associated with the rotation
about the interannular C–C bond of the bpy ligand.
In order to check the reproducibility of our results, a new
set of masked solar cells was prepared using anchoring ligand
7, and cell performances were measured over a 7 day period
(Table 3). A comparison of the data in Tables 2 and 3 reveals
analogous trends, although absolute efficiencies are lower for
the second batch of cells than for the first. Fig. 10a shows
plots of current density against open-circuit voltage for
[Cu(3)(7)]+, illustrating the initial maturing of the cell, followed
by consistent behaviour for a week. The ripening process of
the cell containing [Cu(5)(7)]+ is illustrated in the I–V curve in
Fig. 10b and replicates that observed for the first set of
cells. Good fill factors were obtained for all cells in both sets
(Tables 2 and 3).
The incident photon-to-current conversion efficiency (IPCE)
or external quantum efficiency (EQE) spectra were measured
for each of the solar cells detailed in Table 3. The spectra
shown in Fig. 11a for [Cu(3)(7)]+ are representative of those of
cells containing the dyes [Cu(1)(7)]+, [Cu(2)(7)]+, [Cu(3)(7)]+ and
[Cu(4)(7)]+. The peak maxima in the IPCE spectra at ≈470 nm
correlate with the observed maxima (λmax = 481–488 nm) in
the solution absorption spectra of the homoleptic complexes
[Cu(1)2]+, [Cu(2)2]+, [Cu(3)2]+ and [Cu(4)2]+ (Fig. 9). IPCE peak
maxima of between 39 and 42% are achieved for [Cu(1)(7)]+,
38–40% for [Cu(2)(7)]+, 47–50% for [Cu(3)(7)]+ and 38–39% for
[Cu(4)(7)]+, corresponding to the trends in efficiencies given in
Table 3. Fig. 11b shows the IPCE spectra for [Cu(5)(7)]+ from
the day of sealing to a week after sealing the cell. Initially an
IPCE maximum of 28% is observed but, consistent with the
ripening of the cell described above, this increases to 46%
after a week. It is noticeable that the shape of the IPCE
curve for [Cu(5)(7)]+ is close to those of the other complexes,
i.e. the differences observed in the solution absorption spectra
between the homoleptic complexes containing alkyl and
phenyl substituents are not replicated in the IPCE curves of
the heteroleptic complexes. To gain more insight into this, the
absorption spectra of the surface-anchored heteroleptic dyes
[Cu(3)(7)]+ and [Cu(5)(7)]+ were recorded. The anodes were
prepared as described in the Experimental section (solar cell
fabrication), and their absorption spectra are shown in Fig. 12.
The absorption spectrum of the photoanode with adsorbed
[Cu(5)(7)]+ also exhibits an enhanced spectral response at ca.
600 nm as also seen in the homoleptic complex [Cu(5)2]+ in
solution (Fig. 9).
Fig. 10 Current density (JSC) against open-circuit voltage (VOC) curves for the
dyes (a) [Cu(3)(7)]+ and (b) [Cu(5)(7)]+ over a period of 7 days.
to N719 measured under the same conditions. The efficiency
of the dye is significantly improved by incorporating the aryl
spacer into the anchoring ligand (i.e. going from [Cu(3)(6)]+ to
[Cu(3)(7)]+), and after 4 days, the cell performs with a relative η
of 40.8% (Table 2). A comparison of the values of JSC, VOC and
η in Table 2 for a given dye containing 1, 2, 3 or 4 over the 7
day period reveals a small enhancement as the cell initially
matures. Unexpectedly, the DSCs containing ligand 5 per-
formed poorly on the first day, but as Table 2 shows, energy
conversion efficiency improved considerably as the cell aged.
This is mainly due to an increase in the short-circuit current
density, for both [Cu(5)(6)]+ and [Cu(5)(7)]+. Once again, the
presence of the aryl spacer in the phosphonic acid anchoring
ligand is beneficial and a relative efficiency of 48.4% was
achieved, compared to 21.6% without the spacer. The highly
promising performances of dyes with the diphenyl-substituted
ancillary ligand 5 are of interest in the context of recent find-
ings of Chen and coworkers.8 Their key conclusions are that
the photoinduced structural changes that occur to [Cu(dpp)2]+
(dpp = 2,9-diphenyl-1,10-phenanthroline) are small, and that
changes in the orientations of the phenyl rings upon excitation
result in a copper centre protected from solvent attack, thereby
prolonging the MLCT lifetime in coordinating solvents. Chen
Fig. 13 compares the IPCE spectra of cells containing the
five copper(I)-containing dyes with that of N719. The IPCE
curves indicate improved response in the red region of the
spectrum consistent with the red-shifting of the absorption
spectra of the dyes containing ancillary ligand 5. While good
photon-to-current quantum efficiency is achieved with the
copper(I) dyes over the range 450–550 nm, the dyes do not
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Fig. 13 IPCE spectra of cells containing the copper(I)-containing dyes com-
pared to a cell with standard dye N719.
shifting of the absorption and also an increase in the
absorptivity.
Conclusions
We have described the syntheses and characterizations of
the homoleptic complexes [CuL2][PF6] with L = 1–5. The solid
state structures of 2{[Cu(1)2][PF6]}·3Me2CO, [Cu(2)2][PF6], and
2{[Cu(3)2][PF6]}·Et2O confirm distorted tetrahedral environ-
ments for the Cu+ ions. In [Cu(5)2][PF6]·CH2Cl2, the cation
possesses a significantly flattened geometry caused by π-inter-
actions between the pendant phenyl rings and bpy domains of
adjacent ligands. The solution absorption spectrum of each
of [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6]
shows an MLCT band at around 485 nm. Greater absorptivity
in the red region of the spectrum is achieved by replacing the
6- and 6′-alkyl substituents with phenyl groups in the complex
[Cu(5)2][PF6] and results in the appearance of two broad bands
in the visible region, the longer wavelength band at 573 nm.
The efficiencies, I–V curves and IPCE spectra of DSCs contain-
ing photoanodes with [Cu(L)(Lanchor)]+ dyes in which L is 1–5
and Lanchor is 6 or 7 have been measured. Cell performance
is enhanced by (i) an aromatic spacer in the anchoring ligand
(i.e. anchoring ligand 7 vs. 6), and (ii) either isobutyl or phenyl
substituents in the 6- and 6′-positions of the ancillary ligand,
L. The highest energy-conversion efficiencies are observed for
dyes [Cu(3)(7)]+ (η = 2.43% compared to 5.96% for standard
dye N719) and [Cu(5)(7)]+ (η = 2.89% compared to 5.96% for
N719). It is important to note that cells incorporating
[Cu(5)(7)]+ undergo a ripening process leading to an optimum
performance only after a number of days.
Fig. 11 IPCE spectra for DSCs containing the anchored dyes (a) [Cu(3)(7)]+ and
(b) [Cu(5)(7)]+ over a period of a week after sealing the solar cells.
Fig. 12 Normalized absorption spectra of photoanodes with surface-anchored
heteroleptic dyes [Cu(3)(7)][PF6] and [Cu(5)(7)][PF6].
Acknowledgements
achieve the broad spectra response exhibited by the state-of-
the-art ruthenium(II) dye N719. Specifically, aspects of mole-
cular design that need to be addressed include the further red-
We thank the European Research Council (Advanced Grant
267816 LiLo), the Swiss National Science Foundation, the
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Swiss Nanoscience Institute and the University of Basel for 19 M. K. Eggleston, D. R. McMillin, K. S. Koenig and
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