Factors controlling the photoresponse of copper(i) diimine dyes containing hole-transporting dendrons in dye-sensitized solar cells: Substituent and solvent effects

Article abstract of DOI:10.1039/c4ra03700f

Two series of 2,2′-bipyridine (bpy) ligands bearing different 6,6′-substituents (Me, ⁿBu, ^isoBu, hexyl, Ph and 2-naphthyl) and carrying first-generation (ligands 1-6) or second-generation (ligands 7-12) hole transporting dendrons in the 4,4′-positions are reported. They have been incorporated into homoleptic copper(i) complexes [CuL₂][PF₆]. FTO/TiO₂ electrodes functionalized with the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4, 4′-diyl)bis(4,1-phenylene))bis(phosphonic acid), 13, were dipped in either CH₂Cl₂ or acetone solutions of [CuL₂][PF ₆] to produce two series of surface-bound heteroleptic dyes. Their performances in dye-sensitized solar cells (DSCs) are assessed. Solid-state absorption spectra of dye-functionalized electrodes show that dye uptake is greater if acetone is used in the dye-dipping cycle rather than CH ₂Cl₂, and the DSCs made using acetone generally perform better than analogous DSCs made using CH₂Cl₂. Using acetone-dipping solutions, the best DSC efficiencies are obtained with the second-generation dyes [Cu(13)(L)]⁺ (L = 7-11 with Me, ⁿBu, ^isoBu, hexyl, Ph groups); [Cu(13)(12)]⁺ (12 contains 2-naphthyl groups in the 6,6′-positions) and its first-generation analogue [Cu(13)(6)]⁺ perform poorly. When CH ₂Cl₂ is used in the dipping cycle, DSCs with dyes [Cu(13)(1)]⁺ and [Cu(13)(7)]⁺ (6,6′-Me ₂-substituted) show the highest V_OC, J_SC and η values, and EQE spectra confirm electron injection over a wider energy range than for other dyes. For CH₂Cl₂ in the dipping cycle (but not for acetone), [Cu(13)(5)]⁺ (6,6′-Ph ₂-substituted) performs as well as [Cu(13)(1)]⁺. The overall results of the study indicate that a combination of small 6,6′-substituents and acetone in the dye-dipping cycle lead to the best performing dyes.

Full text of DOI:10.1039/c4ra03700f

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PAPER
Factors controlling the photoresponse of copper(I)
diimine dyes containing hole-transporting
dendrons in dye-sensitized solar cells: substituent
and solvent effects†
Cite this: RSC Adv., 2014, 4, 34801
*
Sven Y. Brauchli, Biljana Bozic-Weber, Edwin C. Constable, Nik Hostettler,
*
Catherine E. Housecroft and Jennifer A. Zampese
Two series of 2,20-bipyridine (bpy) ligands bearing different 6,60-substituents (Me, nBu, isoBu, hexyl, Ph and
2-naphthyl) and carrying first-generation (ligands 1–6) or second-generation (ligands 7–12) hole
transporting dendrons in the 4,40-positions are reported. They have been incorporated into homoleptic
copper(I) complexes [CuL2][PF6]. FTO/TiO2 electrodes functionalized with the anchoring ligand ((6,60-
dimethyl-[2,20-bipyridine]-4,40-diyl)bis(4,1-phenylene))bis(phosphonic acid), 13, were dipped in either
CH2Cl2 or acetone solutions of [CuL2][PF6] to produce two series of surface-bound heteroleptic dyes.
Their performances in dye-sensitized solar cells (DSCs) are assessed. Solid-state absorption spectra of
dye-functionalized electrodes show that dye uptake is greater if acetone is used in the dye-dipping cycle
rather than CH2Cl2, and the DSCs made using acetone generally perform better than analogous DSCs
made using CH2Cl2. Using acetone-dipping solutions, the best DSC efficiencies are obtained with the
second-generation dyes [Cu(13)(L)]+ (L ¼ 7–11 with Me, nBu, isoBu, hexyl, Ph groups); [Cu(13)(12)]+ (12
contains 2-naphthyl groups in the 6,60-positions) and its first-generation analogue [Cu(13)(6)]+ perform
poorly. When CH2Cl2 is used in the dipping cycle, DSCs with dyes [Cu(13)(1)]+ and [Cu(13)(7)]+ (6,60-Me2-
substituted) show the highest VOC, JSC and h values, and EQE spectra confirm electron injection over a
wider energy range than for other dyes. For CH2Cl2 in the dipping cycle (but not for acetone),
[Cu(13)(5)]+ (6,60-Ph2-substituted) performs as well as [Cu(13)(1)]+. The overall results of the study
indicate that a combination of small 6,60-substituents and acetone in the dye-dipping cycle lead to the
best performing dyes.
Received 23rd April 2014
Accepted 23rd July 2014
DOI: 10.1039/c4ra03700f
www.rsc.org/advances
a heteroleptic copper(I) dye containing 6,60-dimesityl-2,20-
bipyridine-4,40-dicarboxylic acid as the anchoring ligand and a
2,20-bipyridine ancillary ligand with triphenylamino domains
underlines the potential of copper(I) sensitizers.7 In the latter
case, the efficiency was enhanced by using the co-adsorbant
chenodeoxycholic acid.
We recently described the syntheses and characterization of
the two copper(I) diimine complexes incorporating ligands
containing rst- and second-generation hole-transport den-
drons (Scheme 1). For both ligands, semi-empirical MO calcu-
lations at the PM3 level showed that the HOMOs (degenerate
set) and LUMO are localized on the dendron and 2,20-bipyridine
(bpy) metal-binding unit, respectively.8 By applying a ligand
exchange strategy,9 we assembled dye-sensitized solar cells
(DSCs) containing dyes [Cu(Lancillary)(Lanchor)]+ anchored on
mesoporous TiO2. The combination of the hole-transport
substituents in the ancillary ligands with the anchoring ligand
6,60-dimethyl-2,20-bipyridine-4,40-bis(phosphonic acid) resulted
in power-to-current conversion efficiencies of 20.7% relative to
100% for N719 measured under the same conditions in fully
Introduction
¨
Conventional Gratzel dye-sensitized solar cells (DSCs) incorpo-
rate ruthenium(II) complexes as photosensitizers.1 Our ongoing
focus on DSCs containing copper(I) complexes as sensitizers is
predicated both upon their possessing similar photophysical
properties to ruthenium(II) complexes,2,3 and upon the greater
abundance of copper than ruthenium in the Earth. Sauvage and
co-workers pioneered the introduction of copper(I) complexes
in DSCs,4 and more recent uses of copper(I) in dyes in DSCs have
been surveyed by Robertson5 and by us.6 The recent report of a
remarkable photoconversion efficiency of 4.66% for a DSC with
Department of Chemistry, University of Basel, Spitalstrasse 51, CH4056 Basel,
Switzerland. E-mail: edwin.constable@unibas.ch; catherine.housecro@unibas.ch;
Tel: +41 61 267 1008
† Electronic supplementary information (ESI) available: Syntheses and
characterization
of
4,40-bis(4-bromophenyl)-6,60-di(naphthalen-2-yl)-2,20-bi-
pyridine and ligands 2–6, 8–12; Fig. S1. SEM image of commercial electrode.
Fig. S2–S6: additional J–V curves and EQE spectra; Tables S1–S4: DSC
parameters for all cells including duplicate data sets. See DOI: c4ra03700f
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wire and silver wire as the working, counter, and reference
electrodes, respectively. Substrates were dissolved in HPLC
grade CH2Cl2 (ca. 10ꢀ4 to 10ꢀ5 mol dmꢀ3) containing 0.1 mol
dmꢀ3 [nBu4N][PF6] as the supporting electrolyte; all solutions
were degassed with argon. Cp2Fe was used as internal reference.
The external quantum efficiency (EQE) measurements were
made using a Spe-Quest quantum efficiency instrument from
Rera Systems (Netherlands) equipped with a 100 W halogen
lamp (QTH) and a lambda 300 grating monochromator (Lot
Oriel). The monochromatic light was modulated to 3 Hz using a
chopper wheel (ThorLabs). The cell response was amplied with
a large dynamic range IV converter (CVI Melles Griot) and then
measured with a SR830 DSP Lock-In amplier (Stanford
Research).
Compounds 1 and 7,8 anchoring ligand 13,10 [Cu(MeCN)4][PF6],11
[Cu(1)2][PF6]8 and [Cu(7)2][PF6]8 were prepared as previously
reported. The syntheses and characterization data of the ligands
and the numbering scheme for the NMR assignments are given
in the ESI.†
Scheme
1 Homoleptic copper(I) diimine complexes with ligands
containing first- and second-generation hole-transport dendrons.8
[Cu(2)2][PF6]
[Cu(MeCN)4][PF6] (35.2 mg, 0.094 mmol) was reacted with 2
(165 mg, 0.189 mmol) in a mixture of MeCN (15 mL) and CH2Cl2
(15 mL). The solution turned red immediately and was stirred
overnight at room temperature. Then, the volume of the solvent
was reduced in vacuo and the product was precipitated by
addition of Et2O. The solid was separated by ltration, was
washed with Et2O and was dried in a stream of air. [Cu(2)2][PF6]
was isolated as a red solid (145 mg, 0.074 mmol, 78%). 1H NMR
(500 MHz, CDCl3) v/ppm: 8.27 (d, J ¼ 1.2 Hz, 4H, HA3), 7.59 (d,
J ¼ 8.8 Hz, 8H, HB2), 7.57 (d, J ¼ 1.2 Hz, 4H, HA5), 7.14 (d, J ¼ 9.0
Hz, 16H, HC2), 7.02 (d, J ¼ 8.8 Hz, 8H, HB3), 6.89 (d, J ¼ 9.0 Hz,
16H, HC3), 3.83 (s, 24H, OMe), 2.62 (t, J ¼ 8.2 Hz, 8H, Ha), 1.44–
1.32 (m, 8H, Hb), 0.90 (tq, J ¼ 7.4 Hz, 8H, Hc), 0.48 (t, J ¼ 7.4 Hz,
12H, Hd).13C NMR (126 MHz, CDCl3) v/ppm: 161.5 (CA6), 156.8
(CC4), 152.6 (CA2), 150.8 (CB4), 150.2 (CB1), 139.8 (CC1), 127.8
(CB2), 127.5 (CC2), 127.3 (CA4), 121.8 (CA5), 119.4 (CB3), 116.3
(CA3), 115.1 (CC3), 55.7 (OMe), 39.8 (Ca), 31.9 (Cb), 22.7 (Cc), 13.6
(Cd). IR (~n/cmꢀ1): 3187 (w), 3036 (w), 2997 (w), 2952 (w), 2924 (w),
2866 (w), 2853 (w), 2832 (w), 1595 (s), 1501 (s), 1462 (m), 1440
(m), 1321 (m), 1237 (s), 1195 (m), 1179 (m), 1102 (m), 1031 (m),
838 (s), 824 (s), 728 (m), 574 (m), 557 (m), 527 (m). ESI MS (m/z):
1813.8 [M ꢀ PF6]+, (calc. 1813.8). UV-VIS (CH2Cl2, 1.0 ꢁ 10ꢀ5
mol dmꢀ3): lmax/nm 306 (3/dm3 molꢀ1 cmꢀ1 67 500), 386
(60 400), 480 sh (27 500). Found: C, 71.69; H, 6.32; N, 5.52;
masked DSCs.8 We have also established that the photo-
response of a [Cu(Lancillary)(Lanchor)]+ dye can be signicantly
improved by introducing an aromatic linker between the bpy
and phosphonate domains of the anchoring ligand, and by
replacing the 6- and 60-methyl substituents in the ancillary
ligand by isobutyl or phenyl groups.10 We now describe a
systematic extension of these studies in which the ancillary bpy
ligands (1–12 in Scheme 2) contain (i) rst- or second-
generation hole-transport dendrons and (ii) alkyl or aromatic
substituents of varying steric bulk in the 6,60-positions. The
anchoring ligand in the [Cu(Lancillary)(Lanchor)]+ dyes is 13
(Scheme 3) which has shown the greatest potential in recent
studies. The heteroleptic complex is assembled using ligand
exchange between surface-anchored ligand Lanchor and a
homoleptic complex [Cu(Lancillary)2]+. This approach circum-
vents the need to isolate the heteroleptic species (which is oen
not possible because of the rapid establishment of statistical
solution equilibria between homo- and heteroleptic species).
Characterisation of related surface species has previously been
carried out.9
Experimental
General
C
116H116N8O8CuPF6 requires C, 71.13; H, 5.97; N, 5.72 (%).
A Bruker Avance III-500 NMR spectrometer was used to record
[Cu(3)2][PF6]
1H and 13C NMR spectra, and chemical shis were referenced to
residual solvent peaks with respect to v(TMS) ¼ 0 ppm. Solution The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6]
absorption spectra were recorded with a Cary 5000 spectro- (47.9 mg, 0.128 mmol) and 3 (225 mg, 0.257 mmol) was isolated
photometer and FT-IR spectra of solid samples on a Perkin as red solid (105 mg, 0.054 mmol, 42%).1H NMR (500 MHz,
Elmer UATR Two spectrometer. MALDI-TOF and electrospray CDCl3) v/ppm: 8.27 (s, 4H, HA3), 7.60 (d, J ¼ 8.5 Hz, 8H, HB2),
ionization (ESI) mass spectra were recorded on Bruker Dal- 7.52 (s, 4H, HA5), 7.14 (d, J ¼ 8.7 Hz, 16H, HC2), 7.02 (d, J ¼ 8.6
tonics microex and Bruker esquire 3000plus instruments, Hz, 8H, HB3), 6.89 (d, J ¼ 8.4 Hz, 16H, HC3), 3.83 (s, 24H, HOMe),
respectively. Electrochemical measurements were made using a 2.47 (d, J ¼ 7.4 Hz, 8H, Ha), 1.69 (m, 4H, Hb), 0.54 (d, J ¼ 6.7 Hz,
CH Instruments 900B potentiostat with glassy carbon, platinum 24H, Hc).13C NMR (126 MHz, CDCl3) v/ppm: 160.5 (CA6),
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Scheme 2 Synthetic route to compounds 1–12. Conditions: (i) NaOtBu; (ii) Pd(bda)2, PtBu3, toluene, 100 ꢂC, 16 h. Pd(bda)2 ¼ bis(dibenzylide-
neacetone)palladium(0).
[Cu(4)2][PF6]
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6]
(31.7 mg, 0.085 mmol) and 4 (158 mg, 0.17 mmol). [Cu(4)2][PF6]
was isolated as a red solid (153 mg, 0.074 mmol, 86%). 1H NMR
(500 MHz, CDCl3) v/ppm: 8.26 (d, J ¼ 1.4 Hz, 4H, HA3), 7.58 (d,
J ¼ 8.9 Hz, 8H, HB2), 7.56 (d, J ¼ 1.4 Hz, 4H, HA5), 7.13 (d, J ¼ 9.0
Hz, 16H, HC2), 7.02 (d, J ¼ 8.9 Hz, 8H, HB3), 6.89 (d, J ¼ 9.0 Hz,
Scheme 3 Structure of anchoring ligand 13.
16H, HC3), 3.83 (s, 24H, HOMe), 2.61 (t, J ¼ 8.2 Hz, 8H, Ha), 1.38
(m, 8H, Hb), 0.95 (m, 8H, Hd), 0.86 (m, 8H, Hc), 0.77 (m, 8H, He),
0.62 (t, J ¼ 7.3 Hz, 12H, Hf). 13C NMR (126 MHz, CDCl3) v/ppm:
156.9 (CC4), 153.1 (CA2), 149.9 (CB1, CB4), 139.9 (CC1), 127.9 (CB2),
161.6 (CA6), 156.8 (CC4), 152.6 (CA2), 150.8 (CB4), 150.2 (CB1),
139.8 (CC1), 127.8 (CB2), 127.5 (CC2), 127.2 (CA4), 121.8 (CA5),
119.4 (CB3), 116.2 (CA3), 115.1 (CC3), 55.7 (COMe), 40.1 (Ca), 31.5
(Ce), 30.0 (Cb), 29.5 (Cc), 22.6 (Cd), 14.1 (Cf). IR (~n/cmꢀ1): 3187
(w), 3036 (w), 2997 (w), 2947 (w), 2924 (w), 2853 (w), 2835 (w),
1594 (s), 1504 (s), 1461 (m), 1327 (m), 1294 (m), 1239 (s), 1200
(m), 1179 (m), 1102 (m), 1031 (m), 838 (s), 825 (s), 729 (m), 663
(m), 573 (m), 557 (m), 535 (m). ESI MS (m/z): 1926.0 [M ꢀ PF6]+
(base peak, calc. 1924.5), 932.2 [4 + H]+ (calc. 931.5). UV-VIS
(CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3): lmax/nm 306 (3/dm3 molꢀ1
cmꢀ1 65 700), 388 (60 000), 480 sh (28 000). Found: C, 68.93; H,
127.6 (CA4, CC2), 122.8 (CA5), 119.5 (CB3), 116.5 (CA3), 115.1 (CC3),
55.7 (COMe), 48.8 (Ca), 28.5 (Cb), 22.2 (Cc). IR (~n/cmꢀ1): 3190 (w),
3033 (w), 2997 (w), 2953 (w), 2929 (w), 2866 (w), 2834 (w), 1594
(s), 1504 (s), 1463 (m), 1440 (m), 1322 (m), 1289 (m), 1239 (s),
1196 (m), 1179 (m), 1102 (m), 1032 (m), 825 (s), 597 (m), 574 (m),
557 (m), 531 (m). ESI MS (m/z): 1814.8 [M ꢀ PF6]+ (calc. 1813.8),
875.6 [3 + H]+ (base peak, calc. 875.5). UV-VIS (CH2Cl2, 1.0 ꢁ
10ꢀ5 mol dmꢀ3): lmax/nm 305 (3/dm3 molꢀ1 cmꢀ1 63 000), 393
(58 700), 480 sh (28 200). Found: C, 70.14; H, 6.30; N, 5.48;
C
116H116N8O8CuPF6$H2O requires C, 70.48; H, 6.02; N, 5.67 (%).
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6.12; N, 5.46; C124H132N8O8CuPF6$4H2O requires C, 69.50; H, v/ppm: 8.34 (d, J ¼ 1.0 Hz, 4H, HA3), 7.65 (d, J ¼ 8.8 Hz, 8H, HB2),
6.58; N, 5.23 (%).
7.60 (d, J ¼ 1.0 Hz, 4H, HA5), 7.11 (d, J ¼ 8.8 Hz, 8H, HB3), 7.07
(d, J ¼ 9.0 Hz, 32H, HD2), 7.02 (d, J ¼ 8.9 Hz, 16H, HC2), 6.90
(d, J ¼ 8.9 Hz, 16H, HC3), 6.84 (d, J ¼ 9.0 Hz, 32H, HD3), 3.77
(s, 48H, HOMe), 2.64 (t, J ¼ 8.1 Hz, 8H, Ha), 1.39 (m, 8H, Hb), 0.90
(m, 8H, Hc),0.48 (t, J ¼ 7.3 Hz, 12H, Hd). 13C NMR (126 MHz,
CD2Cl2) v/ppm: 162.0 (CA6), 156.5 (CD4), 153.1 (CA2), 151.0 (CB4),
150.5 (CA4), 146.2 (CC4), 141.4 (CD1), 139.9 (CC1), 128.2 (CB2),
128.0 (CB1), 127.2 (CC2), 126.9 (CD2), 122.1 (CA5+C3), 120.1 (CB3),
116.8 (CA3), 115.2 (CD3), 56.0 (COMe), 40.2 (Ca), 32.4 (Cb), 23.1
(Cc), 13.8 (Cd). IR (~n/cmꢀ1): 3036 (w), 2997 (w), 2954 (w), 2929 (w),
2905 (w), 2834 (w), 1595 (m), 1497 (s), 1313 (m), 1237 (s), 1033
(m), 824 (s), 576 (m), 539 (m). MALDI-TOF MS (m/z): 1728.2
[Cu(8)]+ (calc. 1726.7), 1664.8 [8 + H]+ (calc. 1664.8). UV-VIS
(CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3): lmax/nm 225 (3/dm3 molꢀ1
cmꢀ1 170 200), 309 (159 600), 340 sh (134 200), 480 sh (39 600).
Found: C, 73.53; H, 6.33; N, 6.03; C220H204CuF6N16O16P$3H2O
requires C, 73.59; H, 5.89; N, 6.24 (%).
[Cu(5)2][PF6]
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6]
(45.9 mg, 0.123 mmol) and 5 (225 mg, 0.246 mmol) in CH2Cl2
(40 mL), and the reaction mixture turned green-black. [Cu(5)2]
[PF6] was isolated as a green-black solid (225 mg, 0.11 mmol,
90%). 1H NMR (500 MHz, CDCl3) v/ppm: 7.95 (d, J ¼ 1.2 Hz, 4H,
HA3), 7.61 (d, J ¼ 1.2 Hz, 4H, HA5), 7.58 (m, 8H, HD2), 7.56 (d, J ¼
8.8 Hz, 8H, HB2), 7.17 (d, J ¼ 9.0 Hz, 16H, HC2), 7.05 (d, J ¼ 8.8
Hz, 8H, HB3), 7.03 (m, 4H, HD4), 6.92 (d, J ¼ 9.0 Hz, 16H, HC3),
7.03 (m, 8H, HD3), 3.84 (s, 24H, HOMe). 13C NMR (126 MHz,
CDCl3) v/ppm: 157.0 (CA6), 156.9 (CC4), 153.6 (CA2), 150.9 (CB4),
149.9 (CB1), 139.8 (CC1), 138.7 (CD1), 129.2 (CD4), 127.8 (CB2),
127.7 (CD2, CD3), 127.6 (CC2), 127.2 (CA4), 121.5 (CA5), 119.4 (CB3),
117.8 (CA3), 115.1 (CC3), 55.7 (COMe). IR (~n/cmꢀ1): 3036 (w), 2999
(w), 2950 (w), 2931 (w), 2903 (w), 2832 (w), 1593 (s), 1504 (s), 1321
(m), 1238 (s), 1196 (m), 1179 (m), 1104 (m), 1030 (m), 824 (s), 773
(m), 741 (m), 729 (m), 696 (m), 576 (m), 557 (m), 527 (m). ESI MS
(m/z): 1894.4 [M ꢀ PF6]+ (calc. 1893.7), 915.9 [5 + H]+ (base peak,
calc. 915.4). UV-VIS (CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3): lmax/nm
286 (3/dm3 molꢀ1 cmꢀ1 79 300), 331 (51 600), 401 (55 600), 560
sh (7000). Found: C, 73.04; H, 5.39; N, 5.30; C124H100N8O8CuPF6
requires C, 73.05; H, 4.94; N, 5.50 (%).
[Cu(9)2][PF6]
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6]
(31.9 mg, 85.7 mmol) and 9 (285 mg, 171 mmol) in a mixture of
MeCN (10 mL) and CH2Cl2 (20 mL). [Cu(9)2][PF6] was isolated as
red solid (251 mg, 70.9 mmol, 83%). 1H NMR (500 MHz, CD2Cl2)
v/ppm: 8.35 (d, J ¼ 1.3 Hz, 4H, HA3), 7.66 (d, J ¼ 8.9 Hz, 8H, HB2),
7.56 (d, J ¼ 1.3 Hz, 4H, HA5), 7.11 (d, J ¼ 8.8 Hz, 8H, HB3), 7.07 (d,
J ¼ 9.0 Hz, 32H, HD2), 7.02 (d, J ¼ 9.0 Hz, 16H, HC2), 6.89 (d, J ¼
9.0 Hz, 16H, HC3), 6.84 (d, J ¼ 9.0 Hz, 32H, HD3), 3.77 (s, 48H,
HOMe), 2.49 (d, J ¼ 7.2 Hz, 8H, Ha), 1.72 (m, 4H, Hb), 0.54 (d, J ¼
6.6 Hz, 24H, Hc). 13C NMR (126 MHz, CD2Cl2) v/ppm: 160.9
(CA6), 156.5 (CD4), 153.6 (CA2), 151.1 (CB4), 150.2 (CA4), 146.2
(CC4), 141.5 (CD1), 139.9 (CC1), 128.2 (CB2), 127.8 (CB1), 127.2
(CC2), 126.9 (CD2), 123.2 (CA5), 122.1 (CC3), 120.2 (CB3), 117.0
(CA3), 115.2 (CD3), 56.0 (COMe), 49.2 (Ca), 28.8 (Cb), 22.4 (Cc). IR
(~n/cmꢀ1): 3030 (w), 2997 (w), 2970 (w), 2952 (w), 2926 (w), 2903
(w), 2866 (w), 2834 (w), 1595 (m), 1497 (s), 1312 (m), 1236 (s),
1034 (m), 824 (s), 576 (m), 528 (m). MALDI-TOF MS (m/z): 3397.1
[M ꢀ PF6]+ (calc. 3391.5), 1729.1 [Cu(9)]+ (calc. 1726.7), 1666.0
[9 + H]+ (calc. 1664.8). UV-VIS (CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3):
lmax/nm 225 (3/dm3 molꢀ1 cmꢀ1 164 000), 309 (159 900), 340 sh
(134 000), 480 sh (41 400). Found: C, 74.30; H, 6.03; N, 6.49;
[Cu(6)2][PF6]
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6]
(27.6 mg, 0.074 mmol) and 6 (150 mg, 0.148 mmol) in CH2Cl2
(40 mL). The reaction mixture was green-black, and [Cu(6)2][PF6]
was isolated as a green-black solid (153 mg, 0.068 mmol, 92%).
1H NMR (500 MHz, CDCl3) v/ppm: 8.22 (s, 4H, HD1), 7.62 (m, 4H,
HD3), 7.57 (m, 8H, HA5+D5/D8), 7.43 (d, J ¼ 8.6 Hz, 4H, HD4), 7.32 (m,
16H, HA3+B2+D6/D7), 7.20 (d, J ¼ 8.9 Hz, 16H, HC2), 7.14 (m, 8H,
HD5/D8+D6/D7), 7.03 (d, J ¼ 8.7 Hz, 8H, HB3), 6.95 (d, J ¼ 9.0 Hz, 16H,
HC3), 3.86 (s, 24H, HOMe). 13C NMR (126 MHz, CDCl3) v/ppm:
156.9 (CC4), 156.5 (CA6), 153.5 (CA2), 150.8 (CB4), 149.8 (CB1), 139.9
(CC1), 136.0 (CD2), 133.3 (CD4a/D8a), 132.2 (CD4a/D8a), 128.1
(CD5/D6/D7/D8), 127.9 (CB2), 127.6 (CC2), 127.5 (CD1+D4+D5/D8),
127.2 (CA4), 127.1 (CD6/D7), 126.4 (CD5/D6/D7/D8), 125.1 (CD3),
121.2 (CA5), 119.2 (CB3), 117.4 (CA3), 115.2 (CC3), 55.7 (COMe). IR
(~n/cmꢀ1): 3038 (w), 2999 (w), 2950 (w), 2929 (w), 2905 (w), 2832
(w), 1592 (s), 1504 (s), 1320 (m), 1239 (m), 1196 (m), 1179 (m),
C
220H204N16O16CuPF6 requires C, 74.71; H, 5.81; N, 6.34 (%).
1102 (m), 1032 (m), 824 (s), 781 (m), 755 (m), 741 (m), 576 (m), [Cu(10)2][PF6]
557 (m), 533 (m), 477 (m). ESI MS (m/z): 2094.8 [M ꢀ PF6]+ (base
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6]
peak, calc. 2093.8.), 1016.1 [6 + H]+ (calc. 1015.4). UV-VIS
(CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3): lmax/nm 290 (3/dm3 molꢀ1
cmꢀ1 103 100), 340 sh (55 400), 408 (58 000), 576 (5900). Found:
C, 73.11; H, 5.18; N, 4.98; C140H108N8O8CuPF6$3H2O requires C,
73.33; H, 5.01; N, 4.89 (%).
(29.5 mg, 79.1 mmol) and 10 (272 mg, 158 mmol) in a mixture of
MeCN (20 mL) and CH2Cl2 (20 mL). [Cu(10)2][PF6] was isolated
as red solid (242 mg, 66.2 mmol, 84%). 1H NMR (500 MHz,
CD2Cl2) v/ppm: 8.34 (d, J ¼ 1.2 Hz, 4H, HA3), 7.65 (d, J ¼ 8.9 Hz,
8H, HB2), 7.61 (d, J ¼ 1.2 Hz, 4H, HA5), 7.11 (d, J ¼ 8.8 Hz, 8H,
HB3), 7.07 (d, J ¼ 9.0 Hz, 32H, HD2), 7.02 (d, J ¼ 9.0 Hz, 16H,
HC2), 6.90 (d, J ¼ 9.0 Hz, 16H, HC3), 6.84 (d, J ¼ 9.0 Hz, 32H,
[Cu(8)2][PF6]
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6] HD3), 3.77 (s, 48H, HOMe), 2.63 (t, J ¼ 8.2 Hz, 8H, Ha), 1.40 (m,
(27.6 mg, 73.9 mmol) and 8 (246 mg, 0.148 mmol) in MeCN 4H, Hb), 0.95 (m, 8H, He), 0.87 (m, 8H, Hc), 0.79 (m, 8H, Hd),
(18 mL) and CH2Cl2 (18 mL). [Cu(8)2][PF6] was isolated as a 0.61 (t, J ¼ 7.3 Hz, 12H, Hf). 13C NMR (126 MHz, CD2Cl2) v/ppm:
red solid (202 mg, 57.2 mmol, 77%). 1H NMR (500 MHz, CD2Cl2) 162.1 (CA6), 156.5 (CD4), 153.1 (CA2), 151.0 (CB4), 150.5 (CA4),
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146.2 (CC4), 141.4 (CD1), 139.9 (CC1), 128.2 (CB2), 127.9 (CB1), 525 (m), 477 (m). MALDI-TOF MS (m/z): 3676.2 [M ꢀ PF6]+ (calc.
127.1 (CC2), 126.9 (CD2), 122.2 (CA5), 122.1 (CC3), 120.2 (CB3), 3671.4), 1869.9 [Cu(12)]+ (calc. 1867.7), 1805.9 [12 + H]+ (calc.
116.6 (CA3), 115.2 (CD3), 56.0 (COMe), 40.6 (Ca), 32.0 (Cd), 30.5 1804.8). UV-VIS (CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3): lmax/nm: 226
(Cb), 29.9 (Cc), 23.0 (Ce), 14.3 (Cf). IR (~n/cmꢀ1): 3037 (w), 2997 (w), (3/dm3 molꢀ1 cmꢀ1 276 400), 240 sh (237 300), 306 (194 300), 340
2950 (w), 2928 (w), 2853 (w), 2834 (w), 1594 (m), 1495 (s), 1312 sh (170 200), 448 (56 000), 580 sh (10 700). Found: C, 75.83; H,
(m), 1235 (s), 1032 (m), 822 (s), 575 (m), 536 (m). MALDI-TOF MS 5.42; N, 5.81; C244H196N16O16CuPF6$2H2O requires C, 76.06; H,
(m/z): 1784.7 [Cu(10)]+ (calc. 1782.8), 1720.9 [10 + H]+ (calc. 5.23; N, 5.82 (%).
1720.9). UV-VIS (CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3): lmax/nm 225
(3/dm3 molꢀ1 cmꢀ1 164 100), 309 (155 400), 340 sh (131 200), Crystallography
480 sh (42 200). Found: C, 73.54; H, 6.25; N, 6.14; C228H220
N16O16CuPF6$4H2O requires C, 73.60; H, 6.18; N, 6.02 (%).
-
Single crystal data were collected on a Bruker APEX-II diffrac-
tometer with data reduction, solution and renement using the
programs APEX12 and SHELX-13.13 The ORTEP-type diagram
and structure analysis used Mercury v. 3.0.14,15
[Cu(11)2][PF6]
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6]
(27.8 mg, 74.5 mmol) and 11 (254 mg, 149 mmol) in CH2Cl2
(40 mL); the reaction mixture turned green-black. [Cu(11)2][PF6]
was isolated as a green-black solid (233 mg, 64.3 mmol, 86%).1H
NMR (500 MHz, CD2Cl2) v/ppm: 8.02 (d, J ¼ 1.7 Hz, 4H, HA3),
7.66 (d, J ¼ 1.7 Hz, 4H, HA5), 7.63 (m, 16H, HB2+E2), 7.14 (d, J ¼
8.5 Hz, 8H, HB3), 7.09 (d, J ¼ 9.0 Hz, 32H, HD2), 7.05 (m, 20H,
HC2+E4), 6.92 (m, 24H, HC3+E3), 6.86 (d, J ¼ 9.0 Hz, 32H, HD3),
3.78 (s, 48H, HOMe). 13C NMR (126 MHz, CD2Cl2) v/ppm: 157.4
(CA6), 156.5 (CD4), 154.2 (CA2), 151.1 (CB4), 150.2 (CA4), 146.6
(CC4), 141.5 (CD1), 139.8 (CC1), 139.2 (CE1), 129.5 (CE4), 128.2
(CE2), 128.0 (CE3), 127.9 (CB1), 127.2 (CC1), 126.9 (CD2), 122.1
(CC3), 121.9 (CA5), 120.1 (CB3), 118.4 (CA3), 115.2 (CD3), 55.9
(COMe). IR (~n/cmꢀ1): 3033 (w), 3004 (w), 2989 (w), 2952 (w), 2929
(w), 2903 (w), 2834 (w), 1593 (m), 1497 (s), 1314 (m), 1238 (s),
1032 (m), 824 (s), 765 (m), 750 (m), 576 (m), 538 (m). MALDI-
TOF MS (m/z): 3478.9 [M ꢀ PF6]+ (calc. 3471.4), 1769.1
[Cu(11)]+ (calc. 1766.7), 1705.7 [11 + H]+ (calc. 1704.7). UV-VIS
(CH2Cl2, 1.0 ꢁ 10ꢀ5 mol dmꢀ3): lmax/nm 226 (3/dm3 molꢀ1
cmꢀ1 225 900), 308 (190 100), 340 sh (166 000), 435 sh (57 000),
Compound 3
ꢀ
C58H58N4O4, M ¼ 875.08, yellow block, triclinic, space group P1,
˚
a ¼ 9.9980(4), b ¼ 11.2625(4), c ¼ 11.6848(5) A, a ¼ 64.9850(10),
3
ꢂ
˚
b ¼ 80.227(2), g ¼ 89.948(2) , U ¼ 1171.31(8) A , Z ¼ 1, Dc ¼
1.241 Mg mꢀ3, m(Cu-Ka) ¼ 0.611 mmꢀ1, T ¼ 123 K. Total 23 653
reections, 4153 unique, Rint ¼ 0.0258. Renement of 3800
reections (302 parameters) with I > 2s(I) converged at nal
R1 ¼ 0.0343 (R1 all data ¼ 0.0376), wR2 ¼ 0.0910 (wR2 all data ¼
0.0942), gof ¼ 1.040. CCDC 987649.
DSC fabrication and measurements
¨
DSCs were prepared adapting the method of Gratzel and
coworkers.16,17 Solaronix Test Cell Titania Electrodes made from
TCO22-7 FTO coated glass, prepared by screen-printing for a
homogenous surface using Ti-Nanoxide pastes, active layer
from Ti-Nanoxide T/SP covered by
a reective layer of
Ti-Nanoxide R/SP, active area: 6 ꢁ 6 mm, thickness: titania layer
9 mm plus scattering layer 3 mm (Fig. S1†) were used. The elec-
trodes were rinsed with EtOH and sintered at 450 ꢂC for 30 min,
then cooled to ca. 80 ꢂC and immersed in a 1 mM DMSO
solution of the anchoring ligand 13 for 24 h. The colourless
electrode was removed from the solution, washed with DMSO
and EtOH and dried at 60 ꢂC. The functionalized electrode was
580 sh (11 500). Found: C, 73.04; H, 5.16; N, 6.16; C228H188
-
N16O16CuPF6$2CH2Cl2 requires C, 72.96; H, 5.11; N, 5.92 (%).
[Cu(12)2][PF6]
The method was as for [Cu(2)2][PF6] starting with [Cu(MeCN)4][PF6] immersed in either a 0.1 mM CH2Cl2 or a 0.1 mM acetone
(25.3 mg, 67.8 mmol) and 12 (245 mg, 136 mmol) in CH2Cl2 solution of each homoleptic copper(I) complex for z68 h.
(100 mL); the reaction mixture was green-black. [Cu(12)2][PF6] Reference cells were prepared by dipping a commercial elec-
was isolated as green-black solid (231 mg, 60.6 mmol, 89%). 1H trode into an EtOH solution (0.3 mM) of N719 (Solaronix) for
NMR (500 MHz, CD2Cl2) v/ppm: 8.29 (d, J ¼ 1.4 Hz, 4H, HE1), z68 h. The electrodes were nally washed with the same
7.69 (dd, J ¼ 8.5, 1.6 Hz, 4H, HE3), 7.60 (d, J ¼ 1.3 Hz, 4H, HA5), solvent as used for dye-assembly and dried at 60 C. Solaronix
ꢂ
7.58 (d, J ¼ 8.2 Hz, 4H, HE5/E8), 7.45 (d, J ¼ 8.5 Hz, 4H, HE4), 7.43 Test Cell Platinum Electrodes were used for the counter-
(d, J ¼ 1.3 Hz, 4H, HA3), 7.36 (d, J ¼ 8.7 Hz, 8H, HB2), 7.33 (m, electrodes, and residual organic impurities were removed by
4H, HE6/E7), 7.21 (m, 8H, HE5/E8+E6/E7), 7.11 (m, 56H, HB3+C2+D2), heating at 450 C for 30 min.
ꢂ
6.95 (d, J ¼ 8.9 Hz, 16H, HC3), 6.87 (d, J ¼ 9.0 Hz, 32H, HD3), 3.79
The dye-covered TiO2 electrode and Pt counter-electrode
(s, 48H, HOMe). 13C NMR (126 MHz, CD2Cl2) v/ppm: 156.8 (CA6), were assembled using thermoplast hot-melt sealing foil (Solar-
156.6 (CD4), 154.1 (CA2), 151.0 (CB4), 150.1 (CA4), 146.2 (CC4), 141.5 onix Test Cell Gaskets) by heating while pressing them together.
(CD1), 139.9 (CC1), 136.4 (CE4a/E8a), 133.7 (CE2), 132.6 (CE4a/E8a), The electrolyte (LiI (0.1 mol dmꢀ3), I2 (0.05 mol dmꢀ3), 1-
128.5 (CE5/E8/E6/E7), 128.2 (CB2), 128.0 (CE1), 127.8 (CE5/E8), 127.7 methylbenzimidazole (0.5 mol dmꢀ3
)
and 1-butyl-3-
(CE4+E6/E7), 127.5 (CB1), 127.2 (CC2), 127.0 (CD2+E6/E7/E5/E8), 125.6 methylimidazolinium iodide (0.6 mol dmꢀ3) in 3-methoxy-
(CE3), 122.1 (CC3), 121.5 (CA5) 120.0 (CB3), 118.0 (CA3), 115.2 (CD3), propionitrile) was introduced into the DSC by vacuum back-
56.0 (COMe). IR (~n/cmꢀ1): 3036 (w), 3002 (w), 2991 (w), 2950 (w), lling. The hole in the counter electrode was sealed using hot-
2926 (w), 2903 (w), 2833 (w), 1592 (m), 1496 (s), 1314 (m), 1275 melt sealing foil (Solaronix Test Cell Sealings) and a cover
(m), 1261 (m), 1237 (s), 1033 (m), 823 (s), 765 (s), 751 (s), 575 (m), glass (Solaronix Test Cell Caps). Measurements were made by
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irradiating from behind using a light source SolarSim 150 adopts a trans-conguration which switches to a cis-conforma-
(100 mW cmꢀ2 ¼ 1 sun). The power of the simulated light was tion in the monoprotonated ligand, and the resonance for
calibrated by using a reference Si cell.
proton HA5 (see Scheme 2 and ESI† for atom numbering) is
particularly sensitive to the protonation state of the bpy unit.19
A
NOESY cross peak from HC1 to HA5 distinguishes the signals for
HA5 (v 8.66 ppm) and HA3 (v 8.68 ppm). The close chemical
shis of these signals contrasts with their signicant separation
in 4,40-bis(4-bromophenyl)-6,60-dialkyl-2,20-bipyridines (HA3 and
HA5 appear at v 8.48 and 7.34 ꢄ 0.03 ppm, respectively for alkyl
Scanning electron microscopy
Secondary electron SEM micrograph images were recorded
under vacuum (ꢃ1 ꢁ 10ꢀ6 mbar) using an FEI Nova Nano SEM
230 at an accelerating voltage of 5 keV and magnication of
ꢃ9000ꢁ. The sample was prepared by scoring and fracturing
the glass electrode in order to image the cross section of the
different layers.
n
¼ Me, nBu, isoBu, hexyl) and in 4,40-bis(4-bromophenyl)-6,60-
diphenyl-2,20-bipyridine (v 8.91 and 8.04 ppm).8,10
The reactions shown in Scheme 2 proceeded smoothly to
give, aer purication, the rst-generation ligands 2–5 in >85%
and 6 in 60%. Yields for the second-generation compounds
8–12 ranged from 65 to 81%. All the ligands are readily soluble
in CHCl3 and CH2Cl2. The electrospray mass spectrum of each
of 2–6 exhibited a base peak assigned to [M + H]+. For the
second-generation ligands, peaks were observed for [M + 2H]2+
and, for 9 and 12, an additional peak arising from [M + H]+.†
The 1H and 13C NMR spectra of CDCl3 solutions of the
compounds were assigned by 2D techniques. The spectra†
indicate a symmetrical bpy domain and signal integrals are
consistent with the bis-functionalized structures shown in
Scheme 2. Representative spectra for a pair of rst- and second-
generation ligands are shown in Fig. 1.
Results and discussion
Ligand synthesis and characterization
We have previously described the syntheses of the rst- and
second-generation ligands 1 and 7 by treatment of 4,40-bis(4-
bromophenyl)-6,60-dimethyl-2,20-bipyridine with either bis(4-
methoxyphenyl)amine or 4,40-bis(N,N-bis(4-methoxyphenyl)-
amino)diphenylamine.8 An analogous strategy was adopted for
the preparation of rst-generation ligands 2–6 and second-
generation ligands 8–12. The appropriate 4,40-bis(4-bromo-
phenyl)-6,60-dialkyl-2,20-bipyridine or 4,40-bis(4-bromophenyl)-
6,60-diaryl-2,20-bipyridine precursors have previously been
reported10 with the exception of 4,40-bis(4-bromophenyl)-6,60-
X-Ray quality single crystals of 3 were grown by Et2O diffu-
sion into an acetone/chloroform solution of the compound. The
structure of 3 is shown in Fig. 2. Selected bond parameters are
given in the gure caption. The compound crystallizes in the
di(naphthalen-2-yl)-2,20-bipyridine (6a, Scheme 4). This was
18
¨
prepared by Kronhke methodology starting from (1E,5E)-1,6-
bis(4-bromophenyl)hexa-1,5-diene-3,4-dione as shown in
Scheme 4. The product is poorly soluble in most common
organic solvents and NMR spectra† were recorded in TFA-d1 and
ꢀ
space group P1 with half of the molecule in the asymmetric unit;
the second half is generated through an inversion centre and
the bpy unit necessarily adopts a trans-conformation and is
planar. The phenylene unit is twisted 36.7ꢂ with respect to the
pyridine ring to which it is bonded, thereby minimizing inter-
ring H/H interactions. Atom N2 is in a planar environment,
consistent with delocalization of the lone pair into the arene
p-systems. The twisted arrangement of the three arene rings
bonded to N2 is expected on steric grounds. The presence of the
thus represent data for the protonated ligand [H(6a)]+. The H
1
and 13C NMR spectra were assigned using COSY, NOESY,
HMQC and HMBC methods. The resonance for HA3 is a sensi-
tive probe for the conformation of the bpy domain. Neutral bpy
Fig. 1 Aromatic region of the 500 MHz 1H NMR spectra (CDCl3, 295 K)
Scheme 4 Synthesis of the precursor 4,40-bis(4-bromophenyl)-6,60- of (a) first-generation 4 and (b) second-generation 10. See Scheme 2
di(naphthalen-2-yl)-2,20-bipyridine, 6a, by Kronhke methodology.
and ESI† for ring labelling. * ¼ residual CHCl3.
¨
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Fig. 4 Solution absorption spectra of the first-generation ligands 1–6
in CH2Cl2 (1 ꢁ 10ꢀ5 mol dmꢀ3).
Fig. 2 Structure of compound 3 with ellipsoids plotted at 50% prob-
ability; H atoms omitted for clarity. Symmetry code i ¼ 1 ꢀ x, 2 ꢀ y,
2 ꢀ z. Selected bond parameters: N1–C5 ¼ 1.3451(14), N1–C9 ¼
1.3453(14), N2–C13 ¼ 1.4006(14), N2–C22 ¼ 1.4262(14), N2–C16 ¼
1.4302(14), C19–O1 ¼ 1.3649(14), C25–O2 ¼ 1.3697(14), O1–C28 ¼
1.4225(16), O2–C29 ¼ 1.4274(15) A˚; C13–N2–C22 ¼ 121.22(9), C13–
N2–C16 ¼ 120.38(9), C22–N2–C16 ¼ 118.16(9), C19–O1–C28 ¼
117.40(10), C25–O2–C29 ¼ 116.05(9)ꢂ.
isobutyl groups prevents face-to-face interactions between bpy
domains of neighbouring molecules. Dominant packing inter-
actions involve methoxy CH/ppyridine contacts (CH/centroid
˚
¼ 2.39 A) which lead to a centrosymmetric embrace between
adjacent molecules (Fig. 3a). These interactions lead to the
assembly of hydrogen-bonded chains which slice obliquely
through the unit cell (Fig. 3b).
Fig. 5 Solution absorption spectra of the second-generation ligands
7–12 in CH2Cl2 (1 ꢁ 10ꢀ5 mol dmꢀ3).
The solution absorption spectra of the rst- and second-
generation ligands are displayed in Fig. 4 and 5, respectively.
The spectra are all broad with intense bands which tail into the
visible region. The observed absorptions originate from p* ) p
and p* ) n transitions, and the spectra of 1–4 (which contain
6- and 60-alkyl substituents) are similar, as are those of the
second-generation ligands 7–10. Extension of the aromatic
domains on going from 1 to 7 (both with methyl substituents)
leads to the anticipated increase in extinction coefficient (red
curves in Fig. 4 and 5). Similar trends are observed upon
comparing the spectra of 2 and 8, 3 and 9, or 4 and 10. As
expected, the intensities of the highest energy bands in the
spectra of 5 and 11 (phenyl substituents, purple curves in Fig. 4
and 5) and 6 and 12 (naphthyl substituents, black curves in
Fig. 4 and 5) are substantially larger than those of the alkyl-
substituted ligands.
Synthesis and characterization of homoleptic copper(I)
complexes
We have previously reported the syntheses and properties of
[Cu(1)2][PF6] and [Cu(7)2][PF6].8 Homoleptic copper(I)
complexes containing ligands 2–6 and 8–12 were prepared by
treatment of [Cu(NCMe)4][PF6] with two equivalents of the
appropriate ligand. Yields of [CuL2][PF6] were typically high.
The electrospray mass spectra of the complexes containing the
rst-generation ligands 2–6 each exhibited a peak envelope
corresponding to [M ꢀ PF6]+ with the correct isotopic pattern. In
addition, peaks assigned to [3 + H]+, [4 + H]+, [5 + H]+ or [6 + H]+
were observed in the spectra of the respective complexes.
Ligand loss was observed in the mass spectra of the complexes
with the second-generation ligands and the spectra exhibited
Fig. 3 (a) Centrosymmetric pair of molecules of 3 embrace through
CHOMe/ppyridine contacts. (b) The interactions extend between
molecules to generate hydrogen bonded chains.
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Table 1 Comparison of chemical shifts of bpy protons HA3 and HA5 in
free ligand, L, in CDCl3 and [CuL2][PF6] complexes in CDCl3 for L ¼ 1–6
and CD2Cl2 for L ¼ 7–12
peaks for [CuL]+ and [L + H]+ for L ¼ 8, 9, 10, 11 or 12. For L ¼ 9,
11 and 12, a peak envelope arising from [M ꢀ PF6]+ was also
observed.
The 1H and 13C NMR spectra of [CuL2][PF6] with L ¼ 2–6 were
recorded in CDCl3, making the spectra directly comparable with
those of the free ligands. Although the complexes containing
the second-generation ligands are soluble in CDCl3, signals in
the 1H NMR spectra were broadened, necessitating a solvent
change to CD2Cl2 to obtain well-resolved spectra. Fig. 6
compares the aromatic regions of the 1H NMR spectra of 2 and
[Cu(2)2][PF6]. Shis in the signals for HA3 and HA5 are consistent
with the change in conformation of the bpy unit upon coordi-
nation. Resonances for the peripheral dendron protons are
essentially unaffected; the signal for the OMe protons appears
at d 3.82 ppm in 2 and at d 3.83 ppm in [Cu(2)2][PF6]. The trends
are typical of the spectra of all ligand–complex pairs in which
the ligand bears 6- and 60-alkyl substituents (Table 1). The shi
to lower frequency for HA3 on comparing ligand with complex
(Table 1) is more pronounced for ligands 5 and 11 (phenyl
substituents) than for the pairs of alkyl-substituted ligands, and
is even more so for 6 and 12 (2-naphthyl substituents). The
structure of [Cu(6)2]+ modelled using Spartan 14 (v. 1.1.3, MMFF
level) is shown in Fig. 7. The HA3 protons on one bpy ligand are
located within a cle between the two naphthyl domains of the
second ligand and thus lie in the shielding region of their ring
currents. The same effect is observed for both rst- and second-
generation ligands and their complexes (Table 1).
HA3 in
HA5 in
L
HA3 in L
HA5 in L
[CuL2]+
[CuL2]+
First-generation
1
2
3
4
5
6
8.39
8.44
8.46
8.44
8.82
8.89
7.33
7.31
7.28
7.31
7.95
8.11
8.25
8.27
8.27
8.26
7.95
7.32
7.59
7.57
7.52
7.56
7.61
7.57
Second-generation
7
8
9
10
11
12
8.40
8.46
8.46
8.46
8.83
8.93
7.33
7.32
7.28
7.32
7.96
8.12
8.26
8.34
8.35
8.34
8.02
7.43
7.57
7.60
7.56
7.61
7.66
7.60
going from ligand to complex. The shoulder at around 480 nm
in the spectra of [CuL2]+ for L ¼ 1–4 is absent in the spectra
of the ligands and is assigned to the MLCT band. The
energy is consistent with the band at 483 nm observed in
[Cu(6,6-Me2bpy)2]+, the MLCT character of which has been
conrmed from TD-DFT calculations.20 For [Cu(5)2][PF6] and
[Cu(6)2][PF6], a low intensity band at 560 and 576 nm, respec-
tively, (Fig. 8) is assumed to arise from the MLCT. The absorp-
tion spectra of [CuL2]+ for second-generation ligands 7–12 are
dominated by high energy bands originating from ligand-based
p* ) p and p* ) n transitions (Fig. 9). Broad shoulders in the
spectra of CH2Cl2 solutions of [Cu(7)2][PF6], [Cu(8)2][PF6],
[Cu(9)2][PF6] and [Cu(10)2][PF6] centred around 480 nm
are assigned to MLCT transitions. (Note that the previously
reported spectrum for [Cu(7)2][PF6] with lmax ¼ 505 nm was
recorded in MeCN.)8 The MLCT bands for [Cu(11)2][PF6] and
[Cu(12)2][PF6] are assigned to the low intensity absorption
maxima at 580 nm (Fig. 9), consistent with the rst-generation
analogues (Fig. 8). The broad and intense spectral responses of
The absorption spectra of CH2Cl2 solutions of the copper(I)
complexes are shown in Fig. 8 (rst-generation ligands) and
Fig. 9 (second-generation). The broad absorption bands extend
signicantly further to longer wavelength than for the free
ligands. The approximate doubling of the extinction coefficients
on comparing the spectra of ligands with complexes is consis-
tent with the homoleptic complexes [CuL2]+. At high energies
(l < 450 nm), the absorption spectra of [CuL2]+ for L ¼ 1–6
possess similar band-shapes to those of the rst-generation
ligands 1–6 (compare Fig. 8 with Fig. 4). The ligand-centred
band at z360 nm undergoes a red-shi of 10–30 nm on
Fig. 6 Aromatic region of the 500 MHz 1H NMR spectra (CDCl3, 295 K) Fig. 7 Modelled structure of [Cu(6)2]+. The HA3 protons (in green) and
of (a) [Cu(2)2][PF6] and (b) free ligand 2. See Scheme 2 and ESI† for ring bpy domain of one ligand and the two 2-naphthyl groups of the
labelling. * ¼ residual CHCl3.
second ligand are shown in space-filling representation.
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containing the methyl substituents (ligands 1 and 7). The metal-
centred oxidation in [Cu(2)2][PF6], [Cu(3)2][PF6], [Cu(4)2][PF6],
[Cu(8)2][PF6], [Cu(9)2][PF6] and [Cu(10)2][PF6] (n-butyl, isobutyl
or n-hexyl substituents) occurs at similar potentials (+0.56 ꢄ
0.02 V), noticeably higher than in the methyl, phenyl or 2-
naphthyl-containing complexes. The trends are consistent with
data for [CuL2][PF6] with L ¼ 4,40-bis(4-bromophenyl)-6,60-dia-
lkyl-2,20-bipyridines (alkyl ¼ Me, nBu, isoBu, nhexyl) or bis(4-
bromophenyl)-6,60-diphenyl-2,20-bipyridine.10 For the alkyl
substituents, the trend follows the steric bulk of the group, with
the complexes containing the least sterically demanding
substituents being the easiest to oxidize. We have previously
commented10 that the metal centre in [CuL2]+ with L ¼ 4,40-
Fig. 8 Solution absorption spectra of in CH2Cl2 solutions of (1 ꢁ 10ꢀ5
mol dmꢀ3) [CuL2][PF6] where L ¼ 1–6 (first-generation ligands).
bis(4-bromophenyl)-6,60-diphenyl-2,20-bipyridine possesses
a
attened geometry in the solid state as a consequence of
p-stacking of 6-phenyl groups of one ligand and the bpy domain
of the other. This leads to entatic22 lower energy metal oxidation
as the coordination geometry is already enroute to that favoured
by Cu2+
.
For complexes containing the rst-generation ligands, a
ligand-based oxidation process is observed close to +0.30 V
similar to that observed in the free ligand.8 For [CuL2][PF6]
complexes with second-generation ligands 7–12, four ligand-
centred oxidation processes are observed in each complex
(Table 2), again assigned by analogy with the free ligand.8 The
potential for each ligand oxidation process shows little variation
across the series [Cu(7)2][PF6] to [Cu(12)2][PF6].
Ligand reduction processes in the complexes lie close to the
edge of the solvent accessible window and are typically poorly
dened. Each complex exhibits an irreversible reduction
process close to ꢀ2.05 V which appears only in the rst cycle
(Fig. 10).
Fig. 9 Solution absorption spectra of in CH2Cl2 solutions of (1 ꢁ 10ꢀ5
mol dmꢀ3) [CuL2][PF6] where L ¼ 7–12 (second-generation ligands).
all the complexes, especially those containing ligands 7–12,
suggest that they should be good candidates as precursors to
dyes in DSCs.
DSC fabrication and solid-state absorption spectra
Heteroleptic dyes for DSC measurements were assembled by
stepwise adsorption of anchoring ligand 13 onto commercial
titania electrodes followed by treatment with a solution of the
Electrochemistry
Cyclic voltammetric data for the homoleptic copper(I) labile23,24 homoleptic complexes [CuL2][PF6] (L ¼ 1 to 12).
complexes are presented in Table 2. Cyclic voltammograms Ligand exchange occurs with the formation of surface-bound
were recorded CH2Cl2 to preclude association of the metal ion heteroleptic dyes (Scheme 5). Our strategy for surface-
with coordinating solvents,21 and are always referenced to immobilized dye assembly8–10,20 has recently been employed by
internal ferrocene. For the rst- or second-generation Robertson and coworkers.25 In the present study, we observed
complexes, three or ve oxidation processes are observed, that dye uptake is dependent on the solvent used during the
respectively. A representative pair of CVs for rst-generation dipping process. Fig. 11 shows the appearance of DSCs which
[Cu(3)]2][PF6] and second-generation [Cu(9)2][PF6] is shown in have been assembled using CH2Cl2 or acetone solutions of
Fig. 10. By comparison with related systems, the oxidation [CuL2][PF6] (L ¼ 1 to 12). In every pair of CH2Cl2/acetone dipped
process in the range 0.40–0.58 V is assigned to the Cu+/Cu2+ cells, the intensity of colour was reproducibly greater for the
couple. In copper(I) diimine complexes, incorporation of acetone-dipping.
substituents ortho to the N-donor sites (e.g. 6,60-positions in
FTO/TiO2 electrodes (without a scattering layer) with adsor-
bpy) stabilizes tetrahedral Cu+ with respect to square-planar bed dye were prepared and their solid-state absorption spectra
copper(II) by sterically encumbering the attening of the coor- recorded, each spectrum being corrected for the background
dination environment. The potentials for the Cu+/Cu2+ couple spectrum of a blank electrode. Absorption spectra for electrodes
in [CuL2][PF6] with L ¼ 1–6 follow the same trend as in the series with rst-generation dyes made using CH2Cl2 or acetone solu-
[CuL2][PF6] with L ¼ 7–12. The complexes containing the phenyl tions of the homoleptic complexes [CuL2][PF6] with L ¼ 1–6 are
or 2-naphthyl substituents (ligands 5, 6, 11 and 12) exhibit the shown in Fig. 12a and b, respectively. For a given ancillary
lowest metal oxidation potentials, followed by the complexes ligand, the general trend is for enhanced absorptivity when
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Table 2 Cyclic voltammetric data for [CuL2][PF6] with L ¼ 1–12 with respect to Fc/Fc+; CH2Cl2 solutions with [nBu4N][PF6] as supporting
electrolyte and scan rate of 0.1 V sꢀ1. Processes are reversible unless otherwise stated (qr ¼ quasi-reversible; ir ¼ irreversible)
E1/2ox/V
(Epc ꢀ Epa/mV)
E1/2ox/V
(Epc ꢀ Epa/mV)
E1/2ox/V
(Epc ꢀ Epa/mV)
E1/2ox/V
(Epc ꢀ Epa/mV)
E1/2red/V
(Epc ꢀ Epa/mV)
Complex
E1/2ox/V (Epc ꢀ Epa/mV)
[Cu(1)2][PF6]
[Cu(2)2][PF6]
[Cu(3)2][PF6]
[Cu(4)2][PF6]
[Cu(5)2][PF6]
[Cu(6)2][PF6]
[Cu(7)2][PF6]
[Cu(8)2][PF6]
[Cu(9)2][PF6]
[Cu(10)2][PF6]
[Cu(11)2][PF6]
[Cu(12)2][PF6]
+0.29 (96)
+0.29 (79)
+0.29 (86)
+0.29 (89)
+0.32a
+0.44 (87)
+0.57 (65)
+0.58 (66)
+0.57 (74)
+0.40a
+1.00ir
+0.98 (94)
ꢀ2.04ir
ꢀ2.09ir
ꢀ2.07ir
ꢀ2.08ir
ꢀ2.25ir
ꢀ2.07ir
ꢀ2.03ir
ꢀ2.07ir
ꢀ2.06ir
ꢀ2.06ir
ꢀ2.19ir
ꢀ2.08ir
qr
+0.99 (102)qr
+0.96 (106)qr
+0.99 (83)qr
+0.97 (96)qr
+1.03ir
+0.31a
+0.40a
ꢀ0.07 (78)
ꢀ0.07 (83)
ꢀ0.08 (65)
ꢀ0.07 (79)
ꢀ0.07 (81)
ꢀ0.08 (63)
+0.20 (75)
+0.19 (83)
+0.18 (88)
+0.18 (89)
+0.19 (103)
+0.19 (85)
+0.47 (129)
+0.54 (99)
+0.55 (90)
+0.55 (84)
+0.39(81)
+0.39 (67)
+0.69 (168)
+0.69 (155)
+0.68 (157)
+0.69 (150)
+0.69 (170)
+0.68 (168)
+1.01 (111)qr
+1.02 (85)qr
+1.03 (86)qr
+1.03 (69)qr
+1.02ir
a
1st and 2nd oxidation processes overlap.
Scheme 5 Anchored heteroleptic dyes. For the first-generation, Ar ¼
4-MeOC6H4 in ligands 1–6 (see Scheme 2); for the second-genera-
tion, Ar ¼ 4-MeOC6H4 in ligands 7–12 (see Scheme 2).
DSC performances: rst- versus second-generation ancillary
ligands
Fig. 10 Cyclic voltammograms of CH2Cl2 (degassed) solutions of (a)
[Cu(3)2][PF6] and (b) [Cu(9)2][PF6] for first- and second cycles. Scan rate
The enhanced absorption observed using acetone in the dye
dipping cycle leads us to initially focus on the performances of
these DSCs. All DSCs were masked to avoid overestimation of
efficiencies.26 In order to ensure that DSC parameters were
reproducible, measurements were made using duplicate cells
for each dye-solvent combination. A complete set of DSC
parameters is presented in Table S1,† and Table 3 summarizes
representative DSC characteristics using anchoring ligand 13,
rst-generation ancillary ligands and acetone in the dye dipping
cycle, measured over a period of three weeks and with respect to
the standard dye N719. The nal column in the table shows
¼ 0.1 V sꢀ1
.
acetone is used in the dipping solution during cell fabrication. A
similar trend is observed for complexes containing the second-
generation ligands 7–12. The enhancement of absorption
observed on changing from CH2Cl2 to acetone coupled with that
on going from rst- to second-generation ligand in the surface-
immobilized heteroleptic complex, is exemplied in Fig. 13
with [Cu(13)(4)]+ and [Cu(13)(10)]+.
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Table 3 DSC performance data for sealed and masked cells using
anchoring ligand 13 and first-generation ancillary ligands, and acetone
in the [CuL2][PF6] dipping cycle. Relative efficiencies (last column) are
with respect to 100% for standard dye N719 measured under the same
conditions. See also ESI Table S1†
Anchored dye JSC/mA cmꢀ2 VOC/mV
ff
h/% Relative h/%
On the day of sealing the cell
[Cu(13)(1)]+
[Cu(13)(2)]+
[Cu(13)(3)]+
[Cu(13)(4)]+
[Cu(13)(5)]+
[Cu(13)(6)]+
N719
6.00
4.41
4.22
5.60
3.22
4.29
16.72
510
482
475
542
468
508
641
70.3 2.15
69.4 1.48
69.6 1.39
71.8 2.18
68.6 1.03
67.0 1.46
29.4
20.2
19.0
29.8
14.1
19.9
68.4 7.32 100
22a days aer sealing the cell
Anchored dye JSC/mA cmꢀ2 VOC/mV
ff
h/% Relative h/%
[Cu(13)(1)]+
[Cu(13)(2)]+
[Cu(13)(3)]+
[Cu(13)(4)]+
[Cu(13)(5)]+
[Cu(13)(6)]+
N719
5.27
4.42
4.18
5.05
4.00
4.10
16.65
520
516
487
532
485
504
671
71.1 1.94
70.7 1.61
69.6 1.42
70.8 1.90
69.9 1.36
68.0 1.41
24.4
20.3
17.9
23.9
17.1
17.7
Fig. 11 Cells after dipping in acetone or CH2Cl2 solutions of the
homoleptic complexes: (a) first-generation ligands 1–6 (left to right),
(b) second-generation ligands 7–12 (left to right).
71.2 7.95 100
a
For [Cu(13)(1)]+, the second measurements were made aer 18 days.
A comparison of Tables 3 and 4 reveals that, with the
exception of [Cu(13)(6)]+ and [Cu(13)(12)]+ (6 and 12 ¼ 6,60-bis(2-
naphthyl) substituted ligands), the dyes containing the second-
generation ancillary ligands give higher global efficiencies. This
corresponds to an increase in the solid-state absorbance of the
Fig. 12 Solid-state absorption spectra of electrodes with anchored
dyes [Cu(13)(L)]+ where L ¼ 1–6. The electrodes were made using (a)
CH2Cl2, or (b) acetone solutions of [CuL2][PF6].
Table 4 DSC performance data for masked cells using anchoring
ligand 13 and second-generation ancillary ligands, and acetone in the
[CuL2][PF6] dipping cycle. Relative efficiencies (last column) are with
respect to 100% for standard dye N719 measured under the same
conditions. See also ESI Table S2†
Anchored dye JSC/mA cmꢀ2 VOC/mV
ff
h/% Relative h/%
On the day of sealing the cell
[Cu(13)(7)]+
[Cu(13)(8)]+
[Cu(13)(9)]+
[Cu(13)(10)]+
[Cu(13)(11)]+
[Cu(13)(12)]+
N719
6.46
6.08
5.48
5.25
4.01
3.04
16.52
515
506
475
488
459
444
608
67.9 2.26
70.8 2.18
69.9 1.82
70.9 1.81
70.2 1.29
69.2 0.93
32.8
31.6
26.4
26.2
18.7
13.5
68.8 6.90 100
22a days aer sealing the cell
Anchored dye JSC/mA cmꢀ2 VOC/mV
ff
h/% Relative h/%
[Cu(13)(7)]+
[Cu(13)(8)]+
[Cu(13)(9)]+
[Cu(13)(10)]+
[Cu(13)(11)]+
[Cu(13)(12)]+
N719
5.94
5.97
6.31
5.47
4.99
2.77
16.98
536
532
541
522
487
466
674
70.3 2.23
71.6 2.27
71.0 2.42
70.6 2.02
71.3 1.73
70.2 0.91
27.5
28.0
29.8
24.9
21.3
11.2
Fig. 13 Solid-state absorption spectra of electrodes with the
anchored first- and second-generation dyes [Cu(13)(4)]+ and
[Cu(13)(10)]+. The electrodes with anchored dye were prepared using
(a) CH2Cl2, or (b) acetone solutions of [CuL2][PF6] (L ¼ 4 or 10).
relative efficiencies with respect to N719 set to 100%. Perfor-
mance data for the dyes containing the second-generation
ligands 7–12 are given in Tables 4 and S2.†
70.9 8.11 100
a
For N719, the second measurement was made aer 15 days.
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DSCs as illustrated in Fig. 12. Aer three weeks, the DSCs with
the ancillary ligands 7–9 and 11 exhibit higher JSC values than
those with 1–3 and 5 (Fig. 14, 15, S1 and S2†), while JSC for dyes
containing the n-hexyl subtituents (4 and 10) are similar
(Fig. S3†). For [Cu(13)(9)]+ (isobutyl substituents, second-
generation), there is also a signicant gain in VOC over the
three week period (Fig. 14). The dyes with the methyl (Fig. 15) or
n-butyl (Fig. S1†) substituents perform the best, with enhance-
ment on going from rst- to second-generation ancillary
ligands. [Cu(13)(1)]+ and [Cu(13)(7)]+ (methyl substituted ancil-
lary ligands) suffer from reduced JSC as the DSCs age, but benet
from increased VOC. This latter enhancement is also observed in
[Cu(13)(2)]+ and [Cu(13)(8)]+ (n-butyl groups).
The dyes which incorporate the sterically hindering 2-
naphthyl groups exhibit poorer device performances than the
other dyes, and this is associated with lower JSC values (Fig. 16).
As the DSCs age, the efficiencies decrease, with this trend
observed for both generations of ligands; these observations are
conrmed for the duplicate DSCs (Tables S1 and S2†). The data
also suggest that extending the dendron on going from 6 to 12 is
detrimental to overall dye performance (Fig. 16).
Fig. 15 J–V curves for dyes [Cu(13)(1)]+ and [Cu(13)(7)]+ (methyl
substituents, acetone used in the [CuL2][PF6] dipping cycle) on day of
sealing the DSC (day 0) and after 18 or 22 days.
domain. Consistent with the earlier discussion, EQE data
conrm poorer electron injection on going from 6 to 12
(naphthyl-groups).
Fig. 17 shows the EQE spectra of freshly sealed DSCs for all
the dyes; the spectra were also recorded aer 22 days (Fig. S4
and S5†). Only small changes in EQE parameters (Tables 5 and
6) are observed for a given DSC over a 22 day period. Except for
2-naphthyl-containing dyes, on going from the rst- to second-
generation ligand in each pair (e.g. [Cu(13)(1)]+ to [Cu(13)(7)]+),
values of EQEmax typically increase. Irrespective of the ancillary
ligand, the dyes exhibit values of lmax in the range 470–490 nm.
For the dye containing 1 (methyl groups), Fig. 17a shows a
pronounced low energy shoulder at 590 nm, conrming
improved photoresponse and electron injection consistent with
the relatively high conversion efficiency observed for anchored
dye [Cu(13)(1)]+ (Table 4). The dyes with the second-generation
alkyl-functionalized ancillary ligands all exhibit enhanced EQE
to lower energies than the rst-generation analogues (Fig. 17b
versus 17a). This observation appears not to carry through to
dyes 5 and 11 with phenyl substituents where there is little gain
in EQE at higher wavelength upon extending the hole transport
DSC performances: inuence of solvent in the dye dipping
cycles
We now return to the inuence of the solvent during the dye
dipping cycle when making the solar cells. Tables S3 and S4†
give DSC parameters for cells (including duplicates) which were
fabricated using CH2Cl2 solutions of the homoleptic dyes.
Measurements were made over a three week period and Fig. S6
and S7† summarize the J–V characteristics. Data for the best
performing cells on the day of sealing are presented in Table 7,
and Fig. 18 and 19 show J–V curves for the rst- and second-
generation dyes, respectively. A comparison of the data in
Table 7 with those in Tables 3 and 4 indicates that, in general,
the conversion efficiencies are higher when acetone is used
during cell fabrication. It is difficult to see unambiguous trends
between the DSC performances and the 6,60-substituents in the
ancillary ligand. However, in all cases, the dyes containing the
methyl substituents perform well. The worst performing dyes
contain the bulky 2-naphthyl groups, irrespective of solvent,
Fig. 14 J–V curves for dyes [Cu(13)(3)]+ and [Cu(13)(9)]+ (isobutyl
substituents, acetone used in the dye-dipping cycle) on day of sealing Fig. 16 J–V curves for dyes [Cu(13)(6)]+ and [Cu(13)(12)]+ (naphthyl
the DSC (day 0) and after 18 or 22 days showing enhancement of JSC substituents, acetone used in the [(CuL)2][PF6] dipping cycle) on day of
with second-generation ligand.
sealing the DSC (day 0) and after 22 days.
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Table 7 DSC performance data for masked cells using anchoring
ligand 13 and first or second-generation ancillary ligands, and CH2Cl2
in the [CuL2][PF6] dipping cycle. Relative efficiencies (last column) are
with respect to 100% for standard dye N719 measured under the same
conditions. See also ESI Tables S3 and S4†
Anchored dye JSC/mA cmꢀ2 VOC/mV
ff
h/% Relative h/%
First-generation ligands; on the day of sealing the cell
[Cu(13)(1)]+
[Cu(13)(2)]+
[Cu(13)(3)]+
[Cu(13)(4)]+
[Cu(13)(5)]+
[Cu(13)(6)]+
N719
5.47
4.65
3.94
4.07
5.35
2.94
16.31
490
475
479
491
492
485
637
69.4 1.86 25.1
69.8 1.54 20.8
69.6 1.31 17.7
71.8 1.43 19.3
70.4 1.85 25.0
68.3 0.97 13.1
71.3 7.41 100.0
Second-generation ligands; on the day of sealing the cell
Anchored dye JSC/mA cmꢀ2 VOC/mV
h/% Relative h/%
ff
[Cu(13)(7)]+
[Cu(13)(8)]+
[Cu(13)(9)]+
[Cu(13)(10)]+
[Cu(13)(11)]+
[Cu(13)(12)]+
N719
4.32
4.08
2.60
2.73
1.78
1.63
16.44
509
469
428
459
418
411
647
68.1 1.50 20.3
67.9 1.30 17.6
65.3 0.73 9.9
68.5 0.86 11.6
67.2 0.50 6.8
61.1 0.41 5.5
69.5 7.40 100.0
Fig. 17 EQE spectra (on the day of sealing) of DSCs with dyes con-
taining ancillary ligands (a) 1–6 and (b) 7–12. Acetone was used in the
[CuL2][PF6] dipping cycle.
Table 5 EQE maxima for DSCs containing first-generation dyes
Day 0
Day 22
Anchored dye
lmax/nm
EQEmax/%
lmax/nm
EQEmax/%
[Cu(13)(1)]+
[Cu(13)(2)]+
[Cu(13)(3)]+
[Cu(13)(4)]+
[Cu(13)(5)]+
[Cu(13)(6)]+
470
470
470
470
470
470
35.2
34.4
34.2
40.6
29.0
35.0
470
470
470
470
470
470
31.3
32.8
32.0
37.2
32.7
31.2
Fig. 18 J–V curves for dyes [Cu(13)(L)]+ with L ¼ 1–6 on day of sealing
the DSCs (day 0). CH2Cl2 was used in the [CuL2][PF6] dipping cycle.
Table 6 EQE maxima for DSCs containing second-generation dyes
Day 0
Day 22
Anchored dye
lmax/nm
EQEmax/%
lmax/nm
EQEmax/%
[Cu(13)(7)]+
[Cu(13)(8)]+
[Cu(13)(9)]+
[Cu(13)(10)]+
[Cu(13)(11)]+
[Cu(13)(12)]+
470-490
470
470
470
470
42.2
41.3
40.3
38.6
36.1
27.6
480
470
470
480
480
470
41.8
41.7
46.5
41.3
41.4
24.3
460-470
and the EQE spectra in Fig. 17 and 20 conrm the poorest
electron injection as a function of wavelength for dyes with
ligands 6 and 12.
Fig. 19 J–V curves for dyes [Cu(13)(L)]+ with L ¼ 7–12 on day of
sealing the DSCs (day 0). CH2Cl2 was used in the [CuL2][PF6] dipping
cycle.
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For the second-generation dyes (Fig. 19), the DSC perfor- interactions involving the phenyl substituents and dendron in
mance corresponds quite well to the steric demands of the 6,60- the second-generation 11.
substituents, the best VOC, JSC and h values being observed when
methyl groups are used. The EQE spectra for these dyes
(Fig. 20b) are also consistent with this performance ordering,
Conclusions
and we again (compare red curves in Fig. 17b and 20b) observe We have prepared and characterized two series of homoleptic
extension of the EQE spectrum of the dye containing 7 to lower [CuL2]+ complexes containing bpy-derived ligands with
energy compared to the remaining dyes.
different 6,60-substituents, and either rst-generation (ligands
One of the most dramatic differences between performances 1–6) or second-generation (ligands 7–12) hole transporting
of DSCs assembled using acetone or CH2Cl2 copper(I) complex dendrons. FTO/TiO2 electrodes were functionalized with the
solutions is seen with the 6,60-diphenyl substituted ancillary phosphonic acid 13. Ligand exchange using either CH2Cl2 or
ligands. The performance of DSCs containing [Cu(13)(5)]+ (rst- acetone solutions of [CuL2]+ gave two series of surface-bound
generation, phenyl) improves when CH2Cl2 is used instead of heteroleptic dyes. Solid-state absorption spectra of dye-
acetone. For the CH2Cl2-derived dyes, a ripening effect is covered electrodes show that uptake of the dye is improved if
observed (Table S3†) with h increasing from 1.85 to 2.06% (25.0 acetone rather than CH2Cl2 is used in the dye-dipping cycle.
to 26.0% relative to N719), mainly due to an increase in VOC
When acetone is used in the dye-soaking process, the best
from 492 to 537 mV (Fig. S6†). For the CH2Cl2 dye solutions, DSC efficiencies are obtained using the second generation dyes
[Cu(13)(5)]+ is the optimum dye. However, extending the hole except for those with 2-naphthyl groups where low JSC values
transporting domain appears to be unfavourable with losses in contribute to a poor performance. The enhancement on going
both VOC and JSC (Fig. 18 versus 19, purple curves). The values of from rst- to second-generation dendron is consistent with
EQEmax are 42.2% (at lmax ¼ 470 nm) for [Cu(13)(5)]+ and 18.0% increased absorbance in the solid-state absorption spectra. The
(at lmax ¼ 480 nm) for [Cu(13)(11)]+. DSCs with [Cu(13)(5)]+ dyes that perform most efficiently, both for rst and generation-
(rst-generation, phenyl) show similar values of VOC , JSC and h ancillary ligands, are those with the methyl or n-butyl substit-
to those with [Cu(13)(1)]+ (rst-generation, methyl). We attri- uents. The EQE spectra of the DSCs containing the methyl-
bute the loss in performance on going from [Cu(13)(5)]+ to containing ancillary ligands show extension of the band to
[Cu(13)(11)]+ to dye aggregation arising from intermolecular lower energy, consistent with electron injection over a wider
range of wavelengths compared to the other dyes.
Overall, the DSC performances for cells made using acetone in
the dye-dipping cycle are better than those made using CH2Cl2
solutions. For dyes assembled in CH2Cl2, those with ancillary
ligands 1 and 7 (methyl groups) perform well, and exhibit the
highest VOC, JSC and h values as well as EQE spectra that extend
further towards higher wavelengths than the other dyes. The EQE
spectra indicate that dyes containing the 2-naphthyl groups in
Lancillary show the poorest electron injection, consistent with low
JSC values. We conclude that incorporation of these large
aromatic substituents is detrimental to DSC performance. In
contrast, the rst-generation 6,60-diphenyl-substituted ancillary
ligand, 5, gives a dye that performs as well as that containing 1
(methyl groups), and for CH2Cl2-derived dyes, [Cu(13)(5)]+ is the
optimum dye. However, the performance dramatically falls on
going to the second-generation analogue 11.
Our overall conclusions are as follows. Among the
[Cu(13)(Lancillary)]+ dyes studied, the most promising Lancillary
ligands are 1 and 5 (methyl or phenyl substituents) if CH2Cl2 is
used in the dye-dipping cycle. Generally, acetone is favoured
over CH2Cl2 in the dye-assembly process, and gives adsorbed
dyes that exhibit higher absorbances in the solid-state absorp-
tion spectra and enhanced conversion efficiencies. Use of the
bulky 2-naphthyl substituents militates against good DSC
performance. We will report on the effects of adding the co-
adsorbant cheno to these DSC systems in the near future.
Acknowledgements
Fig. 20 EQE spectra (on the day of sealing) of DSCs with dyes con-
taining ligands (a) 1–6 and (b) 7–12. CH2Cl2 was used in the [CuL2][PF6]
dipping cycle.
The European Research Council (Advanced Grant 267816 LiLo),
Swiss National Science Foundation and University of Basel are
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