The intramolecular aryl embrace: From light emission to light absorption

Article abstract of DOI:10.1039/c1dt11052g

6-(1-Methylpyrrol-2-yl)-2,2′-bipyridine, 3, and 6-(selenophene-2-yl)- 2,2′-bipyridine, 4, have been prepared and characterized in solution and by structural determinations. Copper(i) complexes [CuL₂][PF ₆] in which L is 2,2′-bipyridine substituted in the 6-position by furyl, thienyl, N-methylpyrrolyl, selenopheneyl, methyl or phenyl, (L = 1-6) have been synthesized. The complexes have been characterized by electrospray mass spectrometry, and solution NMR and UV-VIS spectroscopies. The single crystal structures of [Cu(1)₂][PF₆], [Cu(2) ₂][PF₆], [Cu(3)₂][PF₆], [Cu(5) ₂][PF₆] and [Cu(6)₂][PF₆] have been determined. In those compounds containing an aromatic substituent attached to the bpy unit, the substituent is twisted with respect to the latter. In [Cu(3)₂][PF₆] and [Cu(5)₂][PF₆], this results in intra-cation π-stacking between ligands which is very efficient in [Cu(3)₂]⁺ despite the steric requirements of the N-methyl substituents. Face-to-face stacking between the ligands in the [Cu(2)₂]⁺ ion is achieved by complementary substituent twisting and elongation of one Cu-N bond, but there is no analogous intra-cation π-stacking in [Cu(1)₂]⁺. Ligand exchange reactions between [CuL₂][PF₆] (L = 1-6) and TiO₂-anchored ligands 7-10 (L′ = 2,2′-bipyridine-based ligands with CO ₂H or PO(OH)₂ anchoring groups) have been applied to produce 24 surface-anchored heteroleptic copper(i) complexes, the formation of which has been evidenced by using MALDI-TOF mass spectrometry and thin layer solid state diffuse reflectance electronic absorption spectroscopy. The efficiencies of the complexes as dyes in DSCs have been measured, and the best efficiencies are observed for [CuLL′] with L′ = 10 which contains phosphonate anchoring groups. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11052g

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PAPER
The intramolecular aryl embrace: from light emission to light absorption†
Biljana Bozic-Weber, Edwin C. Constable,* Catherine E. Housecroft,* Peter Kopecky, Markus Neuburger and
Jennifer A. Zampese
Received 6th June 2011, Accepted 2nd September 2011
DOI: 10.1039/c1dt11052g
6-(1-Methylpyrrol-2-yl)-2,2¢-bipyridine, 3, and 6-(selenophene-2-yl)-2,2¢-bipyridine, 4, have been
prepared and characterized in solution and by structural determinations. Copper(I) complexes
[CuL2][PF6] in which L is 2,2¢-bipyridine substituted in the 6-position by furyl, thienyl,
N-methylpyrrolyl, selenopheneyl, methyl or phenyl, (L = 1–6) have been synthesized. The complexes
have been characterized by electrospray mass spectrometry, and solution NMR and UV-VIS
spectroscopies. The single crystal structures of [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6], [Cu(5)2][PF6]
and [Cu(6)2][PF6] have been determined. In those compounds containing an aromatic substituent
attached to the bpy unit, the substituent is twisted with respect to the latter. In [Cu(3)2][PF6] and
[Cu(5)2][PF6], this results in intra-cation p-stacking between ligands which is very efficient in [Cu(3)2]+
despite the steric requirements of the N-methyl substituents. Face-to-face stacking between the ligands
in the [Cu(2)2]+ ion is achieved by complementary substituent twisting and elongation of one Cu–N
bond, but there is no analogous intra-cation p-stacking in [Cu(1)2]+. Ligand exchange reactions
between [CuL2][PF6] (L = 1–6) and TiO2-anchored ligands 7–10 (L¢ = 2,2¢-bipyridine-based ligands with
CO2H or PO(OH)2 anchoring groups) have been applied to produce 24 surface-anchored heteroleptic
copper(I) complexes, the formation of which has been evidenced by using MALDI-TOF mass
spectrometry and thin layer solid state diffuse reflectance electronic absorption spectroscopy. The
efficiencies of the complexes as dyes in DSCs have been measured, and the best efficiencies are observed
for [CuLL¢] with L¢ = 10 which contains phosphonate anchoring groups.
by introduction of a [CuL2]+ complex which undergoes ligand
Introduction
exchange to give a surface-bound heteroleptic [CuLL¢]+ complex.
As part of our ongoing interest in Earth-abundant metals for ap-
This methodology should enable efficient screening of families
plications in dye-sensitized solar cells (DSCs), we are investigating
of complexes without the need for isolation of the heterolep-
a broad variety of copper(I) complexes. To date, there have been
tic species. Although the photophysical properties of copper(I)
only a few literature reports of copper(I)-based DSCs.1–9 Our past
oligopyridine complexes have been intensively studied and are
known to be critically dependent upon ligand substitution19 little
is known about the influence on their effectiveness as photosen-
sitisers. On the other hand, in the context of complexes in light
emitting electrochemical cells, we have shown that intramolecular
stabilization of cations by face-to-face p-stacking of aromatic
substituents at the 6-position of 2,2¢-bipyridines (an “aryl em-
brace”) leads to enhanced device performance.20 The enhancement
of the emission behaviour in copper(I) bis(1,10-phenanthroline)
complexes by the incorporation of appropiate substituents has
been demonstrated by McMillin and coworkers.21 In this paper,
we extend our studies to the use of 6-aryl-2,2¢-bipyridines in DSCs
and describe the synthesis and characterization of a series of
homoleptic [CuL2]+ complexes in which L is a 2,2¢-bipyridine
ligand, substituted in the 6-position by methyl, phenyl or an
aromatic heterocycle (L = 1–6, Scheme 1). We have then subjected
these complexes to exchange reactions on a TiO2 surface and
assessed the performance of the resulting heteroleptic complexes
in DSCs. Although we present solid-state structural data for the
efforts have focused on incorporating ligands functionalized with
substituents such as carboxylates to anchor the complex to a TiO2
surface.4,9,10 Copper(I) complexes are labile11,12 and this property
has been successfully applied for establishing dynamic libraries,13–15
and for preparing heteroleptic copper(I) complexes.16–18 In a
preliminary report,6 we showed that oligopyridine ligands undergo
rapid exchange at copper(I). This forms the basis of a new
strategy for the assembly of copper(I) complexes directly on
a semiconductor surface: initial binding of an oligopyridine
ligand, L¢, containing anchoring groups onto the surface, followed
Department of Chemistry, University of Basel, Spitalstrassse 51, CH-
4056, Basel, Switzerland. E-mail: catherine.housecroft@unibas.ch, edwin.
constable@unibas.ch; Fax: +41 61 267 1018; Tel: +41 61 267 1008
† Electronic supplementary information (ESI) available: Fig. S1–S5 show
thin layer solid state diffuse reflectance electronic absorption spectra
of TiO2-anchored ligands 7–10 after treatment with [CuL2][PF6] (L =
1, 2, 4–6). CCDC reference numbers 828945–828951. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11052g
12584 | Dalton Trans., 2011, 40, 12584–12594
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©

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which time bpy (1.56 g, 10.0 mmol) was added. The reaction
mixture turned red immediately. The suspension was allowed to
warm to room temperature and was then stirred for 12 h. After
quenching the reaction with H2O, the organic layer was separated
and the aqueous phase extracted 3 times with CH2Cl2. The
organic layers were combined and an excess of MnO2 was added.
The black suspension was stirred at room temperature overnight
and was then dried over MgSO4. The solids were separated by
filtration, the solvent removed in vacuo, and the product purified
by column chromatography (silica M60, CH2Cl2 changing to
CH2Cl2/MeOH 99 : 1–97 : 3). Compound 2 was obtained as a
brown oil as the 1st major fraction, and after recrystallization from
hexanes, 2 was isolated as white crystals (220 mg, 10%). The 2nd
major fraction was identified as 4-(thiophen-2-yl)-2,2¢-bipyridine35
(260 mg, 12%). Spectroscopic data for 2 were in agreement with
those published.23,35
Scheme 1 Ligand structures and labelling for 1H NMR spectroscopic
assignments.
homoleptic complexes, it is important to remember that the precise
geometries of the complexes in the films in the DSCs are unknown
and most unlikely to be microscopically identical to those in the
solid state. The choice of the heterocyclic substituents in this
study follows from the known ability of thiophene to tune the
electronic properties of 2,2¢-bipyridine.22 Although the chemistry
of 6-(2-thienyl)-2,2¢-bipyridine, 2, has been explored to some
extent, that of analogous ligand 1 has been little studied, while
derivatives 3 and 4 have not, to the best of our knowledge, been
reported. We note that2 exhibits a versatile coordination chemistry
in its reactions with ruthenium(II), ruthenium(III), palladium(II),
platinum(II) and gold(III).23–30
Compound 2: method 2
2-Acetylthiophene (1.26 g, 10 mmol) was added to a solution
of KOtBu (2.24 g, 20 mmol) in dry THF (30 ml). The mixture
was stirred for 2 h at room temperature during which time it
turned yellow. 1-(Pyridin-2-yl)-3-N,N-dimethylamino-prop-2-en-
1-one36–38 (1.76 g, 10 mmol) was added and the reaction mixture
was stirred for 14 h during which time it became deep red. Excess
solid NH4OAc and HOAc (25 ml) were added and the mixture was
stirred under reflux for 2 h to give a black solution. The solvents
were removed in vacuo and the black residue was suspended in H2O
and the pH adjusted to 8 by adding solid K2CO3. The aqueous
phase was extracted several times with CH2Cl2, the organic phases
were combined and dried over MgSO4. After removing the solvent
under reduced pressure, the resulting black oil was dissolved in
toluene and filtered through Celite. The solvent was evaporated to
give a brown paste which was passed through a column (SiO2 60 m,
18 cm, hexanes/EtOAc) to give two products, 2,6-bis(thiophen-
2-yl)-pyridine39 as the 1st major fraction (245 mg, 10% or 20%
relative to 2-acetylthiophene, respectively) and compound 2 as the
2nd major fraction (430 mg, 18%). Spectroscopic data for 2 were
in agreement with those published.23,35
Experimental
Infrared spectra were recorded on a Shimadzu FTIR 8400 S
Fourier-transform spectrophotometer with solid samples with a
Golden Gate ATR. 1H and 13C NMR spectra were recorded
on Bruker AM 400 or DRX 500 spectrometers; chemical shifts
are with respect to residual solvent peaks (TMS d 0 ppm).
77Se NMR spectra were recorded on a Bruker Avance III
NMR spectrometer operating 600.13 MHz proton frequency;
the instrument is equipped with a 5-mm broadband direct
observe probe (BBFO+); 77Se chemical shifts were referenced
externally, relative to selenophene in CD2Cl2 at a shift of 605 ppm.
Solution UV/VIS spectroscopic measurements were recorded
using a Varian Cary 5000 spectrophotometer. Solid state diffuse
reflectance electronic absorption spectra of Cu(I)-containing dyes
on TiO2 were measured using a Varian Cary 5000 with diffuse
reflectance accessory and a conducting glass with a TiO2 layer as
a blank. Emission spectra were recorded on Shimadzu 5301PC
spectrofluorophotometer. EI and electrospray (ESI) mass spectra
were recorded on Finnigan MAT 95, Finnigan MAT LCT or
LCQ mass spectrometers, and MALDI-TOF mass spectra with a
PerSeptive Biosystems Voyager instrument.
Compound 3
1-Methylpyrrole (2.03 g, 25.0 mmol) was added to dry THF
n
(20 ml) and the solution was cooled to 223 K. BuLi (18.8 ml,
1.6 M) was added and the reaction mixture was stirred for 1 h
and then warmed to 273 K. 2,2¢-Bipyridine (3.51 g, 22.5 mmol)
was added and the red suspension was allowed to warm to room
temperature after which it was stirred for 12 h. The reaction
mixture was quenched with water and the organic layer removed.
The aqueous layer was extracted three times with CH2Cl2. An
excess of MnO2 was added to the combined CH2Cl2 layers and
the resulting black suspension was stirred at room temperature
overnight. After drying over MgSO4, the mixture was filtered
and the filtrate collected. Solvent was removed in vacuo, and
the product purified by coloumn chromatography (silica M60,
pentane/ethyl acetate 95 : 5 changing to 9 : 1). 3 was isolated as
Ligand 1 was prepared by Kro¨hnke methodology31,32 as pre-
viously reported. [Cu(NCMe)4][PF6] was prepared by the litera-
ture method.33 Ligands 5 and 6 were made following literature
procedures.34
Compound 2: method 1
1
nBuLi (1.6 M in hexanes, 11 mmol, 6.3 ml) was added to a
solution of thiophene (0.80 ml, 10.0 mmol) in THF (20 ml)
at 223–233 K. The yellow solution was stirred for ª1 h, after
a white crystalline solid (0.907 g, 15.4%). H NMR (500 MHz,
CD2Cl2) d (ppm) 8.67 (dd, J = 4.7, 0.7 Hz, 1H, HA6), 8.44 (d, J =
7.9 Hz, 1H, HA3), 8.24 (d, J = 7.8 Hz, 1H, HB3), 7.84 (t, J = 7.7 Hz,
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1H, HA4), 7.79 (t, J = 7.8 Hz, 1H, HB4), 7.59 (d, J = 7.8, 1H, HB5),
HA6), 8.07 (br, 4H, HA4+B4), 7.86 (br, 2H, HB5), 7.51 (br, 2H, HA5),
7.07 (br, 2H, HC5), 6.85 (br, 4H, HC3), 6.05 (br, 2H, HC4); IR
(solid, n/cm-1) 3159w, 3132w, 2961w, 2932w, 2868w, 2361w, 2326w,
1981w, 1724w, 1599w, 1558w, 1497w, 1474w, 1452w, 1439w, 1412w,
1321w, 1298w, 1275w) 1248w, 1223w, 1165w, 1101w, 1074w, 1011w,
995w, 914w, 881w, 849 s, 825 s, 762 s, 744 s, 731w, 690w, 675w.
UV/VIS (CH2Cl2, 1.0 ¥ 10-5 mol dm-3) lmax/nm 240 (e/dm3 mol-1
cm-1 26 300), 275 (21 100), 300 (19 000), 328 (23 800), 420 (2600),
520 (1900). ESI MS: m/z 507.1 [M - PF6]+ (base peak, calc. 507.1).
Found: C 52.30, H 3.08, N 8.39; C28H20CuF6N4O2P requires C
51.50, H 3.09, N 8.58%.
7.32 (m, 1H, HA5), 6.79 (m, 1H, HC5), 6.65 (m, 1H, HC3), 6.18 (t,
1
J = 3.2 Hz, 1H, HC4), 4.13 (s, 3H, HMe); 13C{ H} NMR (126 MHz,
CD2Cl2) d (ppm) 156.9 (CA2), 155.4 (CB2), 152.6 (CB6), 149.6 (CA6),
137.8 (CB4), 137.4 (CA4), 132.2 (CC2), 127.0 (CC5), 124.2 (CA5), 121.8
(CB5), 121.3 (CA3), 117.9 (CB3), 111.4 (CC3), 108.1 (CC4), 37.9 (CMe);
IR (solid, n/cm-1) 2964 (w), 2937 (w), 2358 (w), 2331 (w), 1580
(m), 1566 (m), 1556 (m), 1456 (m), 1435 (m), 1157 (w), 1097 (w),
775 (w), 721 (w), 530 (s); UV/VIS (CH2Cl2, 1.0 ¥ 10-5 mol dm-3)
l
max/nm 237 (e/dm3 mol-1 cm-1 18 300), 281 (18 700), 303 (17 000);
emission (lex = 282 nm) lem = 457 nm; EI MS: m/z 235.1 [M]+ (calc.
235.1), 234.1 [M - H]+ (base peak, calc. 234.1). Found C 76.54, H
5.64, N 17.88; C15H13N3 requires C 76.57, H 5.57, N 17.86%.
[Cu(2)2][PF6]
[Cu(2)2][PF6] was isolated as red crystals (33 mg, 97%). 1H NMR
(500 MHz, CD2Cl2) d (ppm) 8.32 (br, 2H, HA3), 8.22 (br, 2H, HA6),
8.05 (br, 4H, HA4+B3), 7.94 (br, 2H, HB4), 7.61 (br, 2H, HB5), 7.51 (br,
2H, HA5), 7.27 (br, 2H, HC5), 6.99 (br, 2H, HC3), 6.61 (br, 2H, HC4).
IR (solid, n/cm-1) 3109w, 2348w, 1669w, 1593 m, 1558 m, 1526w,
1480w, 1447 s, 1423 m, 1394 m, 1353w, 1258 m, 1228 m, 1177 m,
1162 m, 1120 w, 1092 w, 1057 w, 1002 w, 825 s, 814 s, 770 s, 740 s,
702 s, 690 s, 655 m, 633 m; UV/VIS (CH2Cl2, 2.1 ¥ 10-5 mol dm-3)
Compound 4
nBuLi (1.6 M in hexanes, 1.6 ml, 2.6 mmol) was added to a solution
of selenophene (190 ml, 2.2 mmol) in THF (12 ml) at 190 K.
The pale yellow solution was stirred for 1 h, after which bpy
(310 mg, 2.0 mmol) was added. The reaction mixture slowly turned
red. The suspension was allowed to warm to room temperature
and then was stirred for 12 h during which time it became deep
red. After quenching the reaction with H2O, the organic layer
was separated and the aqueous phase was extracted 3 times with
CH2Cl2. The organic layers were combined and an excess of MnO2
was added. The black suspension was stirred at room temperature
overnight, and dried over MgSO4. The precipitate was removed by
filtration and the filtrate collected. After solvent removal in vacuo,
the product was purified by column chromatography (silica M60,
pentane/ethylacetate 95 : 5 to 9: 1). Compound 4 was isolated as
l
max/nm 242 (e/dm3 mol-1 cm-1 32 400), 270 (22 600), 320 (22 400),
410 (2500), 530 (1600). ESI MS: m/z 539.0 [M - PF6]+ (base peak,
calc. 539.0). Found: C 49.12, H 2.99, N 7.90; C28H20CuF6N4PS2
requires C 49.09, H 2.94, N 8.18%.
[Cu(3)2][PF6]
The synthesis was carried out on a 213 mmol scale. [Cu(3)2][PF6]
was isolated as red crystals (107 mg, 74%). 1H NMR (500 MHz,
CD2Cl2) d (ppm) 8.61 (br, 2H, HA6), 8.15 (br, 2H, HB4), 8.08 (br,
2H, HA4), 7.91 (br, 4H, HA3+B3), 7.61 (br, 2H, HA5), 7.41 (br, 2H,
HB5), 6.10 (br, 2H, HC5), 5.76 (br, 2H, HC3), 5.55 (br, 2H, HC4), 3.28
1
an yellow solid (160 mg, 28%). H NMR (500 MHz, CD2Cl2) d
(ppm) 8.67 (d, J = 4.1 Hz, 1H, HA6), 8.51 (d, J = 7.9 Hz, 1H, HA3),
8.31 (d, J = 7.8 Hz, 1H, HB3), 8.10 (d, J = 5.6 Hz, 1H, HC5), 7.87 (t, J
7.7 Hz, 1H, HA4), 7.83 (m, 2H, HB4+C3), 7.72 (d, J = 7.8 Hz, 1H, HB5),
1
1
7.41 (m, 1H, HC4), 7.34 (m, 1H, HA5); 13C{ H} NMR (101 MHz,
(s, 6H, HMe); 13C{ H} NMR (126 MHz, CD2Cl2) d (ppm) 153.3
CD2Cl2) d 156.2 (CA2/B2), 156.1 (CA2/B2), 153.7 (CC2), 152.7 (CB6),
(CA2), 152.7 (CB2), 150.8 (CB6), 149.0 (CA6), 138.2 (CA4), 132.3 (CC2),
126.6 (CA5+B5), 125.3 (CC5), 122.7 (CC4), 120.3 (CA3+B3), 111.4 (CC3),
108.3 (CC4), 35.2 (CMe). IR (solid, n/cm-1) 3124 w, 3090 w, 2939 w,
2919 w, 2360 w, 2331 w, 1988 w, 1663 m, 1599 m, 1569 m, 1558 m,
1487 m, 1456 s, 1444 s, 1423 m, 1390 m, 1321 m, 1299 m, 1248 m,
1222w, 1177 m, 1157 m, 1090 m, 1066 m, 1053 m, 1004 m, 990 w,
914 m, 893 m, 835 s, 833 s, 820 s, 800 s, 770 s, 745 s, 719 s, 678 m,
624 m; UV/VIS (CH2Cl2, 5.0 ¥ 10-5 mol dm-3) lmax/nm 242 (e/dm3
mol-1 cm-1 27 700), 268 (29 700), 297 (27000), sh 330 (17 000), 430
(3200), 530 (1800). ESI MS m/z 533.0 [M - PF6]+ (base peak, calc.
533.2). Found C 52.13, H 4.01, N 12.44; C30H26CuF6N6P requires
C 53.06, H 3.86, N 12.38%.
149.6 (CA6), 138.2 (CB4), 137.5 (CA4), 133.4 (CC5), 131.4 (CC4), 126.7
(CC3), 124.5 (CA5), 121.5 (CA3), 119.7 (CB3), 118.1 (CB5); 77Se{ H}
1
NMR (114 MHz, CD2Cl2) d (ppm) 603.1; IR (solid, n/cm-1) 3049
w, 2922 w, 2851 w, 1580 w, 1562 s, 1541 w, 1454 w, 1423 s, 1265
(w), 1225 w, 1151 w, 1092 w, 1078 w, 1045 w, 989 w, 968 w, 912 w,
847 w, 824 w, 771 s, 735 s, 704 s, 677 s, 648 w. UV/VIS (CH2Cl2,
1.0 ¥ 10-5 mol dm-3) lmax/nm 241 (e/dm3 mol-1 cm-1 11 500), 275
(13 500), 312 (11 000). ESI MS: m/z 309.0 [M + Na]+ (base peak,
calc. 309.0), 287.3 [M + H]+ (calc. 287.0). Found C 58.89, H 3.57,
N 9.68; C14H10N2Se requires C 58.96, H 3.53, N 9.82%.
Copper complexes: general method
[Cu(4)2][PF6]
Each synthesis was carried out on an 50 mmol scale unless
otherwise stated. [Cu(NCMe)4][PF6] (18.6 mg, 50 mmol) was
dissolved in MeCN and the ligand (100 mmol) was added. The
solution turned deep red immediately. It was stirred for 2 h with
TLC monitoring until all free ligand has been consumed. Solvent
was removed in vacuo, leaving a red or black residue. The product
was recrystallized from EtOH when necessary.
[Cu(4)2][PF6] was isolated as red crystals (37 mg, 95%). 1H NMR
(500 MHz, CD2Cl2) d (ppm) 8.30 (br, 2H, HA6), 8.23 (br, 2H,
HB4), 8.06 (br, 2H, HA3+B3), 7.99 (br, 2H, HA4), 7.76 (br, 2H, HC5),
7.67 (br, 2H, HB5), 7.48 (br, 4H, HC3+A5), 6.92 (br, 2H, HC4). IR
(solid, n/cm-1) 3115 w, 3101 w, 2926 w, 2854 w, 2361 w, 2338 w,
1724 w, 1653 w, 1593 w, 1558 w, 1539 w, 1481 w, 1448 s, 1433 w,
1394 w, 1340 w, 1294 w, 1258 w, 1225 w, 1178 w, 1161 w, 1121 w,
1078 w, 1053 w, 1016 w, 999 w, 972 w, 827 s, 768 w, 741 w, 692
w, 677 w, 654 w, 608 w. UV/VIS (CH2Cl2, 1.0 ¥ 10-5 mol dm-3)
[Cu(1)2][PF6]
[Cu(1)2][PF6] was isolated as red crystals (28 mg, 95%). 1H NMR
(500 MHz, CD2Cl2) d (ppm) 8.32 (br, 4H, HA3+B3), 8.15 (br, 2H,
l
max/nm 247 (e/dm3 mol-1 cm-1 29 000), 273 (25 300), 322 (22 200),
12586 | Dalton Trans., 2011, 40, 12584–12594
This journal is The Royal Society of Chemistry 2011
©

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413 (3100), 531 (2300). ESI MS: m/z 635.0 [M - PF6]+ (base peak,
calc. 634.9). Found: C 43.37, H 2.73, N 6.88, C28H20CuF6N4PSe2
requires C 43.18, H 2.59, N 7.19%.
parameters) with I > 2s(I) converged at final R1 = 0.0832 (R1 all
data = 0.0923), wR2 = 0.0787 (wR2 all data = 0.1079), gof = 0.9314.
Ligand 4
[Cu(5)2][PF6]
C14H10N2Se, M = 285.21, colourless needle, orthorhombic, space
˚
group Pna21, a = 10.715(5), b = 19.191(8), c = 5.731(3) A, U =
[Cu(5)2][PF6] was isolated as red crystals (30 mg, 89%). 1H NMR
(500 MHz, CD2Cl2) d (ppm) 8.53 (d, J = 4.7 Hz, 2H, HA6), 8.16
(d, J = 8.1 Hz, 2H, HA3), 8.08 (t, J = 7.7 Hz, 2H, HA4), 7.92 (m,
J = 4.5 Hz, 4H, HB3+B5), 7.57 (m, 2H, HA5), 7.52 (m, 2H, HB4), 7.29
(d, J = 7.5 Hz, 4H, HC2), 7.02 (t, J = 7.4 Hz, 2H, HC4), 6.80 (t,
3
-3
-1
˚
1178.4(9) A , Z = 4, Dc = 1.608 Mg m , m(Mo-Ka) = 3.162 mm ,
T = 123 K. Total 19 837 reflections, 2811 unique, Rint = 0.092.
Refinement of 1755 reflections (155 parameters) with I > 2s(I)
converged at final R1 = 0.0458 (R1 all data = 0.0774), wR2 = 0.0421
(wR2 all data = 0.0805), gof = 1.102.
J = 7.6 Hz, 4H, HC3); 13C{ H} NMR (126 MHz, CD2Cl2) d (ppm)
1
158.2 (CB6), 152.8 (CA2/B2), 152.6 (CA2/B2), 148.9 (CA6), 139.5 (CC1),
138.7 (CB5), 138.4 (CA4), 129.6 (CC4), 128.1 (CC3), 127.8 (CC2), 126.8
(CA5), 125.4 (CB4), 122.8 (CA3), 120.7 (CB3); IR (solid, n/cm-1) 3055
w, 2959 w, 2926 w, 2868 w, 2366 w, 2326 w, 1724 w, 1595 w, 1570
w, 1558 w, 1477 w, 1445 s, 1391 w, 1288 w, 1277 w, 1240 w, 1184 w,
1161 w, 1122 w, 1076 w, 1043 w, 1014 w) 1003 w, 928 w, 903 w, 878
w, 831 s, 814 s, 779 s, 754 s, 741 s, 698 s, 636 w. UV/VIS (CH2Cl2,
1.0 ¥ 10-5 mol dm-3) lmax/nm 232 (e/dm3 mol-1 cm-1 38 100), 263
(28 500), 308 (26 200), 419 (4400), 526 (3000). ESI MS: m/z 527.2
[M - PF6]+ (base peak, calc. 527.1). Found C 57.22, H 3.95, N
7.41; C32H24CuF6N4P·EtOH requires C 56.79, H 4.20, N 7.79%.
[Cu(1)2][PF6]
C28H20CuF6N4O2P, M = 653.00, red plate, monoclinic, space group
˚
P21/c, a = 11.6298(5), b = 8.6881(3), c = 13.7113(6) A, b =
◦
3
-3
˚
107.915(2) , U = 1318.23(9) A , Z = 2, Dc = 1.645 Mg m , m(Mo-
Ka) = 0.967 mm-1, T = 123 K. Total 61 626 reflections, 9825 unique,
Rint = 0.036. Refinement of 8405 reflections (191 parameters) with
I > 2s(I) converged at final R1 = 0.0394 (R1 all data = 0.0470),
wR2 = 0.0354 (wR2 all data = 0.0613), gof = 0.9996.
[Cu(2)2][PF6]
C28H20CuF6N4PS2, M = 685.13, red block, triclinic, space group
[Cu(6)2][PF6]
¯
˚
P1, a = 7.1898(2), b = 14.0902(3), c = 14.6598(4) A, a = 103.270(2),
◦
3
˚
[Cu(6)2][PF6] was isolated as red crystals (25 mg, 91%). 1H NMR
(500 MHz, CD2Cl2) d (ppm) 8.49 (ddd, J = 5.0, 1.5, 0.8 Hz, 2H,
HA6), 8.34 (d, J = 8.2 Hz, 2H, HA3), 8.19 (d, J = 8.0 Hz, 2H, HB3),
8.10 (td, J = 7.8, 1.6 Hz, 2H, HA4), 8.01 (t, J = 7.8 Hz, 2H, HB4),
7.55 (ddd, J = 7.5, 5.1, 1.1 Hz, 2H, HA5), 7.48 (d, J = 7.7 Hz,
b = 99.206(2), g = 98.373(2) , U = 1400.92(7) A , Z = 2, Dc =
1.624 Mg m-3, m(Mo-Ka) = 1.053 mm-1, T = 123 K. Total
22 905 reflections, 7740 unique, Rint = 0.046. Refinement of 4821
reflections (114 parameters) with I > 2s(I) converged at final R1 =
0.0404 (R1 all data = 0.0770), wR2 = 0.0389 (wR2 all data = 0.0696),
gof = 1.0610.
2H, HB5), 2.26 (s, 6H, HMe); 13C{ H} NMR (126 MHz, CD2Cl2)
1
d (ppm) 158.1 (CB6), 152.9 (CA2), 151.9 (CB2), 149.3 (CA6), 138.8
(CB4), 138.6 (CA4), 126.8 (CA5), 126.6 (CB5), 122.6 (CA3), 119.6 (CB3),
25.6 (CMe); IR (solid, n/cm-1) 3080 w, 2920 w, 2853 w, 2361 w, 2336
w, 1653 w, 1595 w, 1558 w, 1558 w, 1452 s, 1379 w, 1300 w, 1252 w,
1238 w, 1180 w, 1161 w, 1101 w, 1053 w, 1009 w, 910 w, 876 w, 827
s, 812 s, 766 s, 723 w, 652 w, 631 w. UV/VIS (CH2Cl2, 1.0 ¥ 10-4
mol dm-3) lmax/nm 249 (e/dm3 mol-1 cm-1 20 000), 265 (25 600),
295 (32 200), sh 309 (20 000), 458 (6400), 530 (1400). ESI MS: m/z
403.2 [M - PF6]+ (base peak, calc. 403.1). Found C 48.27, H 3.81,
N 10.05; C22H20CuF6N4P requires C 48.14, H 3.67, N 10.21%.
[Cu(3)2][PF6]
C30H26CuF6N6P, M = 679.08, red block, monoclinic, space group
˚
C2/c, a =◦ 15.5702(6), b = 12.5528(5), c = 15.1905(6) A, b =
3
-3
˚
102.575(2) , U = 2897.8(2) A , Z = 4, Dc = 1.556 Mg m , m(Mo-
Ka) = 0.880 mm-1, T = 123 K. Total 79 592 reflections, 7731 unique,
Rint = 0.040. Refinement of 6961 reflections (200 parameters) with
I > 2s(I) converged at final R1 = 0.0319 (R1 all data = 0.0345),
wR2 = 0.0339 (wR2 all data = 0.0466), gof = 1.0580.
[Cu(5)2][PF6]
Crystal structure determinations
C32H24CuF6N4P, M = 673.07, red plate, monoclinic, space group
Data were collected on a Bruker-Nonius KappaAPEX diffrac-
tometer, with data reduction, solution and refinement using the
programs APEX2,40 SIR9241 and CRYSTALS,42 or on a Stoe
IPDS diffractometer with data reduction, solution and refinement
using Stoe IPDS software43 and SHELXL97.44 ORTEP figures
were drawn using Ortep-3 for Windows,45 and structures were
analysed with the program Mercury v. 2.3.46,47
˚
C2/c, a = 8.0516(13), b = 27.170(6), c = 13.159(2) A, b =
◦
3
-3
˚
96.239(13) , U = 2861.7(9) A , Z = 4, Dc = 1.562 Mg m , m(Mo-
Ka) = 0.889 mm-1, T = 173 K. Total 22 088 reflections, 3452 unique,
Rint = 0.0572. Refinement of 3218 reflections (201 parameters) with
I > 2s(I) converged at final R1 = 0.0358 (R1 all data = 0.0390),
wR2 = 0.0944 (wR2 all data = 0.0967), gof = 1.073.
[Cu(6)2][PF6]
Ligand 3
¯
C22H20CuF6N4P, M = 548.94, red block, triclinic, space group P1,
¯
˚
C15H13N3, M = 235.29, colourless block, triclinic, space group P1,
a = 8.9873(7), b = 10.3479(7), c = 12.9086(10) A, a = 98.400(4), b =
a = 7.7042(8), b = 10.2596(11), c◦= 15.3283(14) A, a = 107.978(8),
3
˚
˚
b = 97.393(8), g = 100.067(8) , U = 1112.90(19) A , Z = 2,
◦
3
Dc = 1.638 Mg m-3, m(Mo-Ka) = 1.122 mm-1, T = 173 K. Total
23 465 reflections, 4574 unique, Rint = 0.0547. Refinement of 4378
reflections (309 parameters) with I > 2s(I) converged at final
˚
99.933(4), g = 90.639(4) , U = 1169.00(15) A , Z = 4, Dc = 1.337 Mg
m-3, m(Mo-Ka) = 0.082 mm-1, T = 123 K. Total 32 162 reflections,
7705 unique, Rint = 0.033. Refinement of 6116 reflections (325
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R1 = 0.0414 (R1 all data = 0.0428), wR2 = 0.1117 (wR2 all data =
0.1130), gof = 1.049.
mass spectrum of 3 appeared at m/z 234.1 and was assigned to
[M - H]+. The intensity of the peak at m/z 235.1 was too great
to arise only from 13C in the [M - H]+ ion and so was assigned to
a combination of the latter and the parent ion [M]+. The 1H and
13C NMR spectra were assigned by 2D methods. In the NOESY
spectrum, cross peaks between the signals for HC3 and HB5, and
for HC5 and HMe (see Scheme 1) permitted resonances for HC3 and
HC5 to be unambiguously assigned.
Preparation of solar cells
Nanocrystalline TiO2 electrodes were prepared by doctor blading
a colloidal TiO2 paste (Solaronix Nanooxide-T, colloidal anatase)
onto a conducting glass slide (F-doped SnO2, FTO, Hartford
glass company, Tec 8, 8 X cm-2) to produce a film 6 mm thick.
After annealing at 450 ◦C for 30 min, each slide was cooled
to ª80 ◦C and dipped into a DMSO solution of ligand 7, 8, 9
or 10 (3 mmol dm-3) overnight. The colourless slide was then
removed from the solution, washed with DMSO and then EtOH,
and dried. The functionalized electrode was then dipped into
an EtOH solution of [CuL2]+ (L = 1–6) (1 mmol dm-3) and
left to stand for 24 h during which time the slide became red
in colour. The slide was removed from the solution, and was
washed with EtOH. The solar cells were made using LiI (0.5
mol dm-3), I2 (0.05 mol dm-3), 1-methylbenzimidazole (0.5 mol
dm-3) and 1-butyl-3-methylimidazolinium iodide (0.6 mol dm-3)
in 3-methoxypropionitrile as electrolyte, the latter being chosen to
produce the best comparison with the state-of-the-art optimized
systems based on N719 (standard dye purchased from Solaronix).
Cathodes (size-matched to the anodes) were constructed from
FTO glass pieces platinized using H2PtCl6 (5 mmol dm-3) in
propan-2-ol, followed by heating to 280 ◦C for 15 min. The anode
and cathode were assembled using Surlyn (Dupont) plastic by
heating while pressing them together. Measurements were made by
irradiating from behind using a light source SolarSim 150 (100 mW
cm-2 = 1 sun).
Single crystals of 3 were grown by slow evaporation of a hexane
solution of the ligand. X-Ray diffraction confirmed the structure
¯
depicted in Fig. 1. The compound crystallizes in the triclinic P1
space group with the asymmetric unit containing two independent,
but structurally similar, molecules (A and B). The bpy domain
adopts the expected trans-configuration with the two rings in each
unit deviating slightly from planarity (angles between the least
squares planes in independent molecules A and B are 14.14(8) and
5.79(8)◦, respectively). The pyrrole ring lies close to the bpy plane
(angles between the least squares planes of rings containing atoms
N2 and N3, and corresponding rings in molecule B, are 3.19(9)
and 11.62(9)◦). The orientation of the pyrrole ring is such that the
methyl substituent faces the lone pair of atom N2 (Fig. 1), and sim-
ilarly in the second independent molecule. The methyl H atoms are
too distant from N2 for there to be weak C–H ◊ ◊ ◊ N interactions.
Thus, the preference for the observed orientation is presumably
associated with minimizing repulsive H ◊ ◊ ◊ H interactions while
maintaining an approximately planar molecular geometry to
optimize intermolecular interactions. The crystal packing is such
that pairs of molecules A associate through face-to-face p-stacking
across an inversion centre, and similarly for pairs of molecules B.
˚
The centroid ◊ ◊ ◊ plane distances are 3.37 and 3.30 A, for A and
B, respectively. Molecules further assemble into ribbons through
weak CHpyrrole ◊ ◊ ◊ Nbpy hydrogen bonds (C14H141 ◊ ◊ ◊ N1i = 2.62,
Results and discussion
◦
i
i
˚
C14 ◊ ◊ ◊ N1 = 3.579(2) A, C14–H141 ◊ ◊ ◊ N1 = 176 , symmetry code
i = x, 1 + y, z between molecules A; corresponding parameters for
Ligand synthesis and characterization
ii
ii
˚
Three of the general strategies for the preparation of 6-aryl-
2,2¢-bipyridines are Kro¨hnke methodology,31 direct coupling of
a lithiated aryl compound and bpy,34 and Jameson’s strategy.36,48
The choice between these routes depends upon the substituent.
Furyl derivative 1 is most efficiently synthesized by the Kro¨hnke
route as detailed by Lee and coworkers.32 Ligand 2 has previously
been prepared using a Kro¨hnke synthesis,23,35 but we have found
it more convenient to use a one-pot method using direct cou-
pling of 2-lithiothiophene (prepared in situ)49 and 2,2¢-bipyridine,
following the general strategy used by Sauvage and coworkers
for the preparation of 2,2¢-bipyridines and 2,9-disubstituted 1,10-
phenanthrolines.34 In contrast to 1 and 2, pyrrolyl derivative 3 and
selenopheneyl derivative 4 have not previously been reported.
6-(3,5-Diphenyl-2-pyrrolyl)-2,2¢-bipyridine has been prepared
in 67% yield by Nagata et al. using a Kro¨hnke synthesis.50 Overall
yields of 10–40% were obtained for 3 using a multi-step Kro¨hnke
synthesis over a period of 4–7 days. Using a two-step Jameson
strategy, yields of 3 of up to 50% were achieved, but again, reaction
and purification required a week. We therefore turned to the one-
molecules B are C29H291 ◊ ◊ ◊ N4 = 2.69, C29 ◊ ◊ ◊ N4 = 3.655(2) A,
C29–H291 ◊ ◊ ◊ N4ii = 175◦, symmetry code ii = x, -1 + y, z). The
two sets of ribbons are approximately orthogonal to one another
(Fig. 2).
n
pot reaction of 1-methylpyrrole and BuLi at 223 K, followed
Fig. 1 Structure of one (molecule A) of two independent molecules
of 3; ellipsoids plotted at 40% probability level. Selected bond lengths
and angles: N1–C1 = 1.338(2), N1–C5 = 1.347(2), N2–C6 = 1.3460(19),
N2–C10 = 1.341(2), N3–C11 = 1.3866(19), N3–C14 = 1.372(2), N3–C15 =
by treatment with bpy at 273–295 K which led, after workup
and purification, to 3 which was isolated as a white crystalline
solid. The yield of 15.4% was disappointing, but the method
was significantly more efficient in terms of time (<3 days) than
either of the Kro¨hnke or Jameson routes. The base peak in the EI
˚
1.460(2) A; C11–N3–C14 = 109.00(13), C11–N3–C15 = 128.76(13),
C14–N3–C15 = 122.20(14)◦.
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Table 1 Selected bond parameters for the homoleptic copper(I) com-
plexes. In each of [Cu(1)2]+ and [Cu(3)2]+, the two ligands are related by a
2-fold axis
[Cu(1)2][PF6] [Cu(2)2][PF6]a
[Cu(3)2][PF6]
atom X = O1 atom X = S1, S2 Atom X = N3
˚
Bond distance/A
Cu1–N1
2.0046(6)
2.0932(5)
2.0046(6)
2.0932(5)
1.3759(8)
1.3690(10)
1.980(2)
2.200(2)
2.006(2)
2.095(2)
1.720(3)
1.712(3)
1.7111(6)
1.7001(4)
1.9918(6)
2.0758(6)
1.9918(6)
2.0758(6)
1.3800(10)
1.3713(11)
Cu1–N2
Cu1–N3/N1i
Cu1–N4/N2i
C11–X
C14–X
C31–X
C34–X
Bond angle/deg
N1–Cu1–N2
N1–Cu1–N3/N1i
N2–Cu1–N3/N1i
N1–Cu1–N4/N2i
N2–Cu1–N4/N2i
81.05(2)
80.41(8)
128.44(9)
133.10(8)
143.07(9)
94.74(8)
80.97(9)
92.14(16)
92.690(9)
81.43(2)
124.53(4)
137.70(2)
137.70(2)
102.26(3)
134.50(4)
129.55(2)
129.55(2)
101.79(3)
81.43(2)
Fig. 2 Part of the packing diagram for 3. Ribbons of molecules A and B
are shown in green and orange, respectively.
N3/N1i –Cu1–N4/N2i 81.05(2)
Since the lithiation route was successful for the preparation of 3,
we also chose this strategy to synthesize compound 4. Treatment of
selenophene with nBuLi followed by bpy resulted in the isolation of
4 in 28% after workup. The electrospray mass spectrum exhibited
peaks at m/z 309.0 and 287.3 corresponding to [M + Na]+ and
[M + H]+, and the isotope distributions were consistent with those
C11–X–C14
C31–X–C34
106.59(6)
108.77(7)
a Major occupancy atoms only included.
analogous conformation to 2 in which the S ◊ ◊ ◊ N separation in
1
calculated. The H and 13C NMR spectra were in accord with
the solid state is 2.895(3) A. Both 4 and its sulfur analogue30
30
˚
the structure shown in Scheme 1 and were fully assigned by 2D
methods. The 1H NMR resonance for HC5 (i.e. the proton closest
to the Se atom) exhibited satellites with coupling to 77Se of 45 Hz.
crystallize in the space group Pna21 with similar cell dimensions,
and the molecular packing in the two compounds is essentially the
same involving C–H ◊ ◊ ◊ p and C–H ◊ ◊ ◊ N interactions. Despite the
planarity of ligands 2 and 4, there is no face-to-face p-stacking.
1
In the 77Se{ H} NMR spectrum, a single resonance at d 603.1 ppm
was observed.
Crystals of 4 suitable for X-ray diffraction grew overnight by
evaporation of an Et2O solution of the compound, and Fig. 3
depicts the structure. The molecule is planar (angles between
the least squares planes of the rings containing N1/N2 and
N2/Se1 are 3.4(3) and 5.2(3)◦) and fully ordered. The bpy
domain is in the expected trans-configuration. The orientation
of the selenium-containing heterocycle results in an Se1 ◊ ◊ ◊ N2
Homoleptic copper(I) complexes with heteroaromatic substituents
The reactions of ligands 1, 2, 3 or 4 with [Cu(NCMe)4][PF6] in
MeCN at room temperature were monitored by thin layer chro-
matography and each was complete within two hours. Complexes
[Cu(1)2][PF6], [Cu(2)2][PF6] and [Cu(4)2][PF6] were isolated in near
quantitative yields, and [Cu(3)2][PF6] in 74% yield. The electro-
spray mass spectrum of each complex exhibited a peak envelope
corresponding to [M - PF6]+ with an isotope distribution matching
that simulated. Single crystals of [Cu(1)2][PF6], [Cu(2)2][PF6],
[Cu(3)2][PF6] were grown by recrystallization from EtOH, by
layering a CHCl3 solution of the compound with toluene, and
from a CH2Cl2/Et2O solution kept at 2 ◦C, respectively. All three
complexes crystallize without solvent, and the structures and
packing can therefore be directly compared. Fig. 4–6 illustrate
the structures of the [Cu(1)2]+, [Cu(2)2]+ and [Cu(3)2]+ cations,
and important bond lengths and angles are given in Table 1.
[Cu(1)2][PF6] and [Cu(3)2][PF6] crystallize in monoclinic space
groups P21/c and C2/c, respectively, and the two ligands in each of
[Cu(1)2]+ and [Cu(3)2]+ are related by a 2-fold axis passing through
atom Cu1. In [Cu(2)2][PF6], one of the two ligands suffers from
a 2-fold rotational disorder of the thienyl substituent. This has
been modelled over two positions with fractional occupancies of
0.869(2) and 0.131(2). Only the major occupancy site is considered
in Fig. 5 and Table 1. The hexafluoridophosphate anion in each
structure is ordered.
˚
separation of 2.982(5) A (compare the sum of the van der
˚
Waals radii of 3.54 A). The observed separation is shorter than
intramolecular Se ◊ ◊ ◊ N contacts observed in related molecules:
51
˚
˚
3.490(2) A in 2-((mesitylselanyl)methyl)quinoline, 3.406(2) A
in 1,3-bis(2-pyridylmethyl)triselane,51 and 3.448(4) A in 2,6-
bis((phenylselanyl)methyl)pyridine.52 Significantly, 4 adopts an
˚
Fig. 3 Structure of 4; ellipsoids plotted at 30% probability level. Selected
bond lengths and angles: C11–Se1 = 1.864(7), C14–Se1 = 1.866(7), C1–N1 =
˚
Inspection of the bond distances in Table 1 shows that the Cu1–
1.333(9), C5–N1 = 1.340(8), C6–N2 = 1.359(6), C10–N2 = 1.328(8) A;
C5–N1–C1 = 116.8(6), C6–N2–C10 = 118.3(5), C14–Se1–C11 = 86.3(3)◦.
N2 bond in [Cu(2)2][PF6] is noticeably longer (2.200(2) A) than
˚
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Fig. 4 Structure of the [Cu(1)2]+ cation in [Cu(1)2][PF6] with H atoms
omitted (ellipsoids plotted at 40% probability level). Symmetry code i = 1
- x, y, 3/2 - z.
Fig. 6 Structure of the [Cu(3)2]+ cation in [Cu(3)2][PF6] with H atoms
omitted (ellipsoids plotted at 40% probability level). Symmetry code i = 1
- x, y, 3/2 - z.
sites, the orientation of one of the heterocyclic rings permits a
˚
short Cu ◊ ◊ ◊ Se contact of ª3.2 A.
The extent of inter-ligand p-stacking within each cation is
illustrated in Fig. 7. In [Cu(1)2][PF6], each furyl group is twisted
16.36(4)◦ with respect to the plane of the pyridine ring to which
it is attached, but as Fig. 7a shows, the mutual twisting of the
heterocyclic rings within the ligands does not result in inter-ligand
stacking within the [Cu(1)2]+ cation. We have already noted that
in [Cu(2)2]+, both thienyl substituents are involved in intra-cation
face-to-face interactions (Fig. 7b). However, the most effective
intramolecular aryl embrace is achieved in [Cu(3)2]+, despite the
presence of the N-methyl substituents. Each pyrrolyl unit twists
54.87(4)◦ out of the plane of the central pyridine ring in coordi-
nated 3. The ligand attains a conformation that permits a face-to-
face p-interaction between the pyrrolyl domain of one ligand and
the central pyridine ring of the other: the interplane angle is 2.8◦
Fig. 5 Structure of the [Cu(2)2]+ cation in [Cu(1)2][PF6] with H atoms
omitted (ellipsoids plotted at 40% probability level). The ring containing
atom S2 is disordered and only the major occupancy site is illustrated.
˚
and the pyrrolyl centroid ◊ ◊ ◊ pyridine plane separation is 3.46 A.
˚
the remaining Cu–N bonds, all of which are close to 2.0 A. The
elongation is associated with a deformation of the corresponding
bpy ligand away from planarity: the angle between the least squares
planes of the two rings in this copper-bound bpy is 27.16(14)◦◦
compared to 16.36(13)◦ for the second bpy in [Cu(2)2]+, 14.62(4)
in [Cu(1)2]+ and 8.54(4)◦ in [Cu(3)2]+. As a result of these structural
changes, each thienyl substituent in the [Cu(2)2]+ cation engages
in face-to-face p-stacking with the central pyridine ring of the
second ligand, the thienyl centroid ◊ ◊ ◊ pyridine plane separations
˚
being 3.55 and 3.58 A.
Preliminary crystallographic data were obtained for
[Cu(4)2][PF6], but problems with twinning resulted in
a
poorly refined dataset. We are, however, able to confirm the
gross structural features of the cation. The copper(I) ion is in a
flattened tetrahedral environment with Cu–N bond distances in
˚
Fig. 7 Representations of the (a) [Cu(1)2]+, (b) [Cu(2)2]+, and (c)
[Cu(3)2]+ cations, illustrating the enhancement of inter-ligand face-to-face
p-interactions on going from furyl to thienyl to pyrrolyl substituent.
the range ª1.9 to 2.0 A. Both selenophene-2-yl substituents are
rotationally disordered over two positions, but in each ring, one
position is dominant. Considering only the major occupancy
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The crystal packing in all three complexes involves extensive
cation ◊ ◊ ◊ anion CH ◊ ◊ ◊ F contacts and some degree of inter-cation
p-stacking, although in [Cu(2)2][PF6] this is rather inefficient.
1
At room temperature, signals in the the CD2Cl2 solution H
NMR spectra of [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6] and
[Cu(4)2][PF6] are broadened (Fig. 8a–8d), in particular those of the
thienyl derivative (Fig. 8b). The spectra are better resolved at low
temperatures. These observations may arise from slow rotation
of the heteroaromatic substituent about the Cbpy–Chetero bond.
However, if this is the case, it initially appears surprising that the
most bulky substituent, i.e. the N-methylpyrrole in [Cu(3)2][PF6],
presents the least broadened spectrum, and that a sharp, well-
resolved spectrum is observed for [Cu(5)2][PF6] (Fig. 8e, discussed
below). An alternative explanation is that the heteroatom in lig-
ands 1, 2 and 4 interacts with the copper(I) centre as observed in the
solid state for [Cu(4)2][PF6], and that this interaction hinders the
rotation of the O-, S- and Se-containing substituents. A detailed
study of the dynamics of these and model systems is in progress.
Fig. 9 Structure of the [Cu(5)2]+ cation in [Cu(5)2][PF6] with H atoms
omitted (ellipsoids plotted at 30% probability level). Symmetry code
3
i = 1 - x, y,
/
2
- z. Selected bond parameters: Cu1–N1 = 2.0278(14),
i
˚
Cu1–N2 = 2.0530(14) A; N1–Cu1–N2 = 80.82(6), N1-Cu1–N1 = 119.11(8),
N1–Cu1–N2i = 135.29(6), N1i–Cu1–N2i = 80.82(6), N2–Cu1–N2i
=
114.09(8)◦.
Fig. 8 Room temperature 500 MHz NMR spectra (d 8.8–5.4 ppm) of
CD2Cl2 solutions of (a) [Cu(1)2][PF6], (b) [Cu(2)2][PF6], (c) [Cu(3)2][PF6],
(d) [Cu(4)2][PF6], (e) [Cu(5)2][PF6] and (f) [Cu(6)2][PF6]. The signal at d
5.49 ppm in some of the spectra is one of the 13C satellites from CDHCl2.
Homoleptic copper(I) complexes containing 6-Phbpy or 6-Mebpy
Treatment of [Cu(NCMe)4][PF6] with two equivalents of ligand
5 or 6 (Scheme 1) resulted in the isolation of red crystalline
[Cu(5)2][PF6] or [Cu(6)2][PF6] in ª90% yield. The base peak in
the ESI mass spectra of the complexes arose from the [M - PF6]+
ion (m/z 527.2 and 403.2, respectively) and the isotope patterns
matched those simulated.
Single crystals of [Cu(5)2][PF6] and [Cu(6)2][PF6] were grown
from CH2Cl2 solutions of the complexes layered with Et2O. The
structures of the [Cu(5)2]+ and [Cu(6)2]+ cations are depicted in
Fig. 9 and 10, and bond parameters for the metal coordination
spheres are listed in the figure captions. The copper(I) coordination
sphere in [Cu(6)2]+ is close to tetrahedral, with the angle between
the two bpy domains being 87.31(3)◦ (Fig. 11a). In contrast, Fig.
11b illustrates significant flattening of the ligand arrangement, the
corresponding angle being 65.47(2)◦. The difference originates in
intra-cation arene stacking in [Cu(5)2]+. The phenyl substituent
of one ligand 5 (twisted 45.02(10)◦ with respect to the bpy unit)
lies over the pyridine ring containing N2 of the second ligand.
Fig. 10 Structure of the [Cu(6)2]+ cation in [Cu(6)2][PF6] with H atoms
omitted (ellipsoids plotted at 30% probability level). Selected bond
parameters: Cu1–N3 = 1.996(2), Cu1–N2 = 1.998(2), Cu1–N4 = 2.044(2),
˚
Cu1–N1 = 2.0443(18) A; N3–Cu1–N2 = 137.88(9), N3–Cu1–N4 = 81.64(9),
N2–Cu1–N4 = 116.07(9), N3–Cu1–N1 = 126.69(8), N2–Cu1–N1 =
81.78(8), N4–Cu1–N1 = 115.82(8)◦.
Although there is a face-to-face p-interaction (centroid to ring
˚
distance = 3.66 A), it is not optimal since the planes of the two rings
subtend an angle of 10.8◦ with one another. The crystal packing
in [Cu(5)2][PF6] is dominated by C–H ◊ ◊ ◊ F contacts rather than
p-stacking interactions. In contrast, the packing in [Cu(6)2][PF6]
involves efficient face-to-face p-stacking between centrosymmetric
pairs of cations, involving bpy domains containing atoms N3/N4
and N3i/N4i (symmetry code i = 1 - x, 1 - y, 1 - z; inter-
˚
plane separation = 3.36 A). Extension of the p-stacking between
bpy units of adjacent cations involving N3/N4 and N3ii/N4ii
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The efficient preparation of heteroleptic complexes is therefore
of importance. We have demonstrated the lability in solution of
diimine and phosphane ligands in copper(I) complexes, and the
formation of heteroleptic complexes from equilibrium mixtures of
two homoleptic species.6,17 Following these observations, we have
adopted a simple strategy for rapid screening of the performance
of heteroleptic copper(I) complexes in DSCs, using a common
anchoring ligand, L¢, and a variable second ligand, L, which is
introduced as [CuL2]+. In this study, for the assembly of surface-
bound [CuLL¢]+, L¢ is one of the ligands 7–10 shown in Scheme
2, and L = 1–6. Ligands 7–9 contain carboxylic acid (carboxylate)
anchoring groups while in10, anchoring is through the phosphonic
acid (phosphonate) units. Gra¨tzel has established that absorption
of dyes onto TiO2 is enhanced by the presence of phosphonate
groups.53
Fig. 11 Comparison of the coordination spheres in (a) [Cu(6)2]+ and (b)
[Cu(5)2]+ in the hexafluoridophosphate salts.
˚
(symmetry code ii = 2 - x, 2 - y, 2 - z; inter-plane distance = 3.29 A)
results in the propagation of chains slicing obliquely through the
unit cell.
Electronic absorption spectroscopy
The electronic absorption spectra of CH2Cl2 solutions of
[CuL2][PF6] with L = 1–6 (Fig. 12) exhibit high energy absorption
bands arising from ligand-based p*←p transitions. For each
complex, two broad absorptions with values of emax ranging from
1700 to 3800 dm3 mol-1 cm-1 are observed around 400–435 nm and
513–550 nm and are assigned to MLCT transitions.
Scheme 2 Structures of the anchoring ligands, L¢, 7–10. The anchoring
groups are the carboxylic or phosphonic acids or their conjugate bases.
The anodes were prepared by dipping annealed conducting
glass slides (see experimental section) into a DMSO solution of
the anchoring ligand L¢ (7–10). After washing and drying the
slides, the latter were left to stand for 24 h in EtOH solutions
of the homoleptic complexes [CuL2][PF6] (L = 1–6), allowing
the equilibrium in Scheme 3 to be established. The appearance
of the slide changed from colourless to red, consistent with the
presence of a surface adsorbed copper(I) complex and the red
colour persisted after the slide was washed with EtOH.
Fig. 12 Electronic absorption spectra of CH2Cl2 solutions of [CuL2][PF6]
for L = 1–6. Key: Solid red line, L = 1; solid black line, L = 2, hashed black
line, L = 3; dotted red line, L = 4, hashed red line, L = 5; dotted black line
L = 6. See experimental section for solution concentrations.
DSCs with heteroleptic copper(I) complexes
Complexes for use as dyes in DSCs incorporate substituents
that anchor the complex to the TiO2 semiconductor surface, and
carboxylate and phosphonate functionalities are popular choices.
Our recent studies of potential dyes for DSCs have focused on
copper(I) complexes incorporating bpy-based and related ligands
with carboxylic acid or carboxylate functionalities.1,4,9 While the
synthesis of homoleptic complexes of type [CuL2]+ is straightfor-
ward, the requirement to incorporate anchoring groups limits the
range of ligands and, therefore, of complexes that can be screened.
The sample prepared with L = 4 and L¢ = 10 was subjected
to MALDI-TOF mass spectrometric analysis. A sample of red-
coloured TiO2 was scratched from the surface of the slide and the
powder was suspended in CH2Cl2 and mixed with the MALDI
matrix (2,5-dihydroxybenzoic acid). The mass spectrum showed
peaks at m/z 348.8, 634.7 and 762.7 which were assigned to
[Cu(4)]+ (calc. 348.9), [Cu(4)2]+ (calc. 634.9) and [Cu(4)(10 - 4H
+ Et + 2Na)]+ (calc. 763.9). Isotope patterns consistent with these
ions were observed.
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Table 2 DSC efficiency data in comparison to N719 measured under the
same conditions (see experimental section). [CuL2]+ are introduced for
surface ligand exchange as hexafluoridophosphate salts
[CuL2]+
L¢
I
SC/A cm-2
V
OC/V
ff
h/%
[Cu(1)2]+
[Cu(1)2]+
[Cu(1)2]+
[Cu(1)2]+
[Cu(2)2]+
[Cu(2)2]+
[Cu(2)2]+
[Cu(2)2]+
[Cu(3)2]+
[Cu(3)2]+
[Cu(3)2]+
[Cu(3)2]+
[Cu(4)2]+
[Cu(4)2]+
[Cu(4)2]+
[Cu(4)2]+
[Cu(5)2]+
[Cu(5)2]+
[Cu(5)2]+
[Cu(5)2]+
[Cu(6)2]+
[Cu(6)2]+
[Cu(6)2]+
[Cu(6)2]+
N719
7
0.005
0.001
0.001
0.004
0.005
0.001
0.001
0.004
0.005
0.001
0.002
0.004
0.005
0.001
0.002
0.004
0.004
0.001
0.002
0.004
0.005
0.001
0.002
0.004
0.015
0.506
0.506
0.451
0.561
0.488
0.414
0.433
0.524
0.524
0.469
0.488
0.579
0.488
0.433
0.433
0.543
0.525
0.506
0.451
0.561
0.506
0.487
0.469
0.561
0.715
0.54
0.58
0.62
0.60
0.53
0.62
0.59
0.63
0.45
0.57
0.57
0.57
0.57
0.58
0.65
0.63
0.49
0.56
0.60
0.59
0.55
0.56
0.59
0.56
0.43
1.25
0.42
0.40
1.51
1.17
0.22
0.35
1.45
1.21
0.25
0.64
1.34
1.31
0.32
0.49
1.31
1.07
0.39
0.48
1.42
1.28
0.31
0.55
1.20
4.50
8
9
10
7
8
9
10
7
8
9
10
7
8
9
10
7
8
9
10
7
8
9
10
Scheme 3 Ligand exchange leading to surface anchored heteroleptic
copper(I) dyes. L = 1–6, L¢ = 7–10; X = anchoring group (see Scheme
2).
Diffuse reflectance electronic absorption spectra of the surfaces
of the slides were also recorded. Fig. 13 is representative of the
results and illustrates the spectra of the TiO2-anchored ligands
7–10 after treatment with [Cu(3)2][PF6]; corresponding spectra for
the remaining 20 devices are given in Fig. S1–S5, ESI.† The spectra
were compared to that of a TiO2 electrode which had been dipped
into a DMSO solution of the homoleptic complex [Cu(10)2]Cl54
for four hours. The differences between the spectra shown as solid
lines in Fig. 13 provide evidence for the presence of different
surface species, and the tail into the visible region is consistent
with surface-binding of complexes of copper(I) as opposed to
free ligand, the latter being colourless. Although in Fig. 13, the
spectra for combinations of 7 and [Cu(3)2][PF6], and of 10 and
[Cu(3)2][PF6] look similar, we can rule out the possibility that the
spectra arise from different surface coverages of [Cu(3)2][PF6] on
the basis that ligand 3 has no anchoring groups. Significantly, each
of the spectra for the combinations of 7, 8, 9 or 10 after treatment
with [Cu(3)2][PF6] shows a low energy shoulder which coincides
with the dominant band in the diffuse reflectance spectrum of
TiO2-supported [Cu(10)2]+. This further supports the presence of
the heteroleptic species.
variations in the efficienices of the solar cells. However, for a
common ligand L, the choice of the anchoring ligand has a
significant effect on the solar cell efficiency, with ligands 7 and
10 performing the best. Anchoring ligands 9 and 10 differ only in
the anchoring groups themselves, i.e. CO2H versus PO(OH)2 (or
conjugate bases thereof). A comparison of the data confirms that
high efficiencies are achieved with the phosphonate binding groups
which should be considered the anchors of choice for future DSC
design.
Conclusions
We have prepared and fully characterized 6-(1-methyl-1H-pyrrol-
2-yl)-2,2¢-bipyridine, 3, and 6-(selenophene-2-yl)-2,2¢-bipyridine,
4. Copper(I) complexes [CuL2][PF6] in which L is 2,2¢-bipyridine
substituted in the 6-position by methyl, phenyl or an aromatic
heterocyclic substituent (L = 1–6) have been synthesized. These
have been characterized by electrospray mass spectrometry,
solution NMR and electronic absorption spectroscopies, and,
for [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6], [Cu(5)2][PF6] and
[Cu(6)2][PF6], single crystal X-ray diffraction structure determi-
nations. In each of [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6] and
[Cu(5)2][PF6], the heterocyclic or phenyl substituent is twisted out
of the plane of the bpy unit. In [Cu(3)2][PF6] and [Cu(5)2][PF6],
this results in inter-ligand p-stacking which is particularly efficient
in the former, despite the presence of the N-methyl substituents.
In [Cu(2)2][PF6], inter-ligand face-to-face stacking is achieved by a
combination of substituent twisting and elongation of one Cu–N
bond. In contrast, no intra-cation p-stacking between ligands is
observed in [Cu(1)2][PF6]. The copper(I) ion in [Cu(6)2][PF6] is in
an approximately tetrahedral coordination environment.
Fig. 13 Thin layer solid state diffuse reflectance electronic absorption
spectra of TiO2-anchored ligands 7 (black), 8 (green), 9 (red) and 10 (blue)
after treatment with [Cu(3)2][PF6]. The hashed line is for [Cu(10)2]Cl.
Table 2 presents the DSC efficiency data for the solar cells
and also the data for standard dye N719 measured under the
same conditions as the copper(I) complexes. The measurements
were made in open cells and the configuration is different to
optimized cells reporting >10% efficiency for N719. For a common
anchoring ligand L¢, changing ligand L leads to only small
Complexes [CuL2][PF6] (L = 1–6) undergo ligand exchange
reactions with TiO2-anchored ligands 7–10 (L¢) to generate a
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series of 24 surface-anchored heteroleptic copper(I) complexes.
MALDI-TOF mass spectrometry and diffuse reflectance UV-VIS
spectroscopy have been used to provide evidence of the formation
of the latter, and their efficiencies as dyes in DSCs have been
screened. The greatest efficiencies were observed for [CuLL¢] in
which L¢ = 10 which contains phosphonate anchoring groups.
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Acknowledgements
We thank the Swiss National Science Foundation, the European
Research Council (Advanced Grant 267816 LiLo) and the Univer-
sity of Basel for financial support. 77Se NMR spectra were recorded
by PD Dr Daniel Ha¨ussinger. Michael Liebetanz is thanked for
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©