Heteroleptic copper(I) sensitizers with one versus two hole-transporting units in functionalized 2,9-dimethyl-1,10-phenanthroline ancillary ligands

Article abstract of DOI:10.1039/c5ra12296a

A series of homoleptic [Cu(L)₂][PF₆] complexes in which L is a 2,9-dimethyl-1,10-phenanthroline fused at the 5,6-positions with a 2′-functionalized imidazole (ligands 1-4), or substituted at the 4,7-positions with electron-donating 4-(diphenylamino)phenyl groups (ligand 5) is described; the imidazole 2′-functionality in 1 is 4-bromophenyl, in 2 is 4-(diphenylamino)phenyl, in 3 is 4-(bis(4-n-butoxy)phenylamino)phenyl, and in 4 is 4-(carbazol-9-yl)phenyl. The copper complexes were characterized by mass spectrometry, NMR and absorption spectroscopies and cyclic voltammetry; the single crystal structure of ligand 4 has been determined. Compared to the solution absorption spectra of [Cu(1)₂][PF₆], [Cu(2)₂][PF₆], [Cu(3)₂][PF₆] and [Cu(4)₂][PF₆], that of [Cu(5)₂][PF₆] shows increased absorbance at wavelengths >375 nm. An on-surface strategy was used to assemble heteroleptic [Cu(6)(L)]⁺ dyes on TiO₂ electrodes where 6 is ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid); solid-state absorption spectra confirmed enhanced light-harvesting between 375 and 600 nm for [Cu(6)(5)]⁺ with respect to [Cu(6)(1)]⁺, [Cu(6)(2)]⁺, [Cu(6)(3)]⁺ and [Cu(6)(4)]⁺. Comparison of the performances of dye-sensitized solar cells (DSCs) containing [Cu(6)(2)]⁺, [Cu(6)(3)]⁺ and [Cu(6)(4)]⁺ with those with [Cu(6)(1)]⁺ indicate only a marginal influence of the diphenylamine or carbazole hole-transporting domains in 5,6-substituted phenanthroline dyes. The introduction of the 4-(diphenylamino)phenyl hole-transporting units in the 4- and 7-positions of the phen unit in 5 proves to be beneficial, with DSCs containing [Cu(6)(5)]⁺ performing better than those with the other four dyes; duplicate DSCs were tested for each dye to validate the results. While the values of the maximum external quantum efficiencies (EQE_max) for [Cu(6)(1)]⁺ and [Cu(6)(4)]⁺ are greater than for [Cu(6)(5)]⁺, the extension of the EQE spectrum for [Cu(6)(5)]⁺ to longer wavelengths results in higher short-circuit current densities (J_SC) compared to DSCs with [Cu(6)(1)]⁺, [Cu(6)(2)]⁺, [Cu(6)(3)]⁺ and [Cu(6)(4)]⁺.

Full text of DOI:10.1039/c5ra12296a

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Heteroleptic copper(I) sensitizers with one versus
two hole-transporting units in functionalized 2,9-
dimethyl-1,10-phenanthroline ancillary ligands†
Cite this: RSC Adv., 2015, 5, 69430
Sebastian O. Furer, Biljana Bozic-Weber, Markus Neuburger, Edwin C. Constable
¨
*
and Catherine E. Housecroft
A series of homoleptic [Cu(L)2][PF6] complexes in which L is a 2,9-dimethyl-1,10-phenanthroline fused at
the 5,6-positions with a 20-functionalized imidazole (ligands 1–4), or substituted at the 4,7-positions with
electron-donating 4-(diphenylamino)phenyl groups (ligand 5) is described; the imidazole 20-functionality
in 1 is 4-bromophenyl, in 2 is 4-(diphenylamino)phenyl, in 3 is 4-(bis(4-n-butoxy)phenylamino)phenyl,
and in 4 is 4-(carbazol-9-yl)phenyl. The copper complexes were characterized by mass spectrometry,
NMR and absorption spectroscopies and cyclic voltammetry; the single crystal structure of ligand 4 has
been determined. Compared to the solution absorption spectra of [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6]
and [Cu(4)2][PF6], that of [Cu(5)2][PF6] shows increased absorbance at wavelengths >375 nm. An
on-surface strategy was used to assemble heteroleptic [Cu(6)(L)]+ dyes on TiO2 electrodes where 6 is
((6,60-dimethyl-[2,20-bipyridine]-4,40-diyl)bis(4,1-phenylene))bis(phosphonic acid); solid-state absorption
spectra confirmed enhanced light-harvesting between 375 and 600 nm for [Cu(6)(5)]+ with respect to
[Cu(6)(1)]+, [Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+. Comparison of the performances of dye-sensitized
solar cells (DSCs) containing [Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+ with those with [Cu(6)(1)]+ indicate
only
a marginal influence of the diphenylamine or carbazole hole-transporting domains in
5,6-substituted phenanthroline dyes. The introduction of the 4-(diphenylamino)phenyl hole-transporting
units in the 4- and 7-positions of the phen unit in 5 proves to be beneficial, with DSCs containing
[Cu(6)(5)]+ performing better than those with the other four dyes; duplicate DSCs were tested for each
dye to validate the results. While the values of the maximum external quantum efficiencies (EQEmax) for
[Cu(6)(1)]+ and [Cu(6)(4)]+ are greater than for [Cu(6)(5)]+, the extension of the EQE spectrum for
[Cu(6)(5)]+ to longer wavelengths results in higher short-circuit current densities (JSC) compared to DSCs
with [Cu(6)(1)]+, [Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+.
Received 25th June 2015
Accepted 7th August 2015
DOI: 10.1039/c5ra12296a
www.rsc.org/advances
copper(I) dyes was rst recognized by Sauvage and coworkers,11
and in 2014, PCEs exceeding 3% (relative to z7.5% for refer-
ence dye N719) were achieved.12,13 Homoleptic copper(I)
Introduction
The development of dye-sensitized solar cells (DSCs) has pro-
complexes have also been used as redox mediators combined
with ruthenium(II) sensitizers in DSCs.14
¨
gressed from the prototype ruthenium dyes of Gratzel and
O'Regan,1,2 to the use of organic3 and porphyrin-containing4
dyes with solar-to-electrical power conversion efficiencies
(PCEs) reaching z12%.5 Recently, perovskite DSCs have excited
considerable attention, with PCEs of 18–20%.6–8 Our contribu-
tions to the advancement of DSCs focus on sustainable
components,9 in particular with copper-containing dyes10
replacing those containing precious metals. The potential of
The simplest copper(I) sensitizers are homoleptic complexes
of type [Cu(Lanchor)2]+ in which Lanchor is typically a diimine
ligand bearing a carboxylic or phosphonic acid substituent to
anchor the dye to the semiconductor surface.15 Dye perfor-
mance is most easily improved and tuned by employing heter-
oleptic [Cu(Lanchor)(Lancillary)]+ dyes, although these are oen
difficult to isolate because of the lability of bis(diimine)copper(I)
complexes.16 Two approaches to access heteroleptic dyes are
now successfully used. The rst is the HETPHEN strategy17
introduced by Odobel and coworkers13,18 which relies on bulky
ligands to hinder ligand exchange. Using this approach, a
remarkable efficiency of 4.66% (relative to 7.36% for N719) has
been recorded for the dye shown in Scheme 1a in the presence
of the co-adsorbant chenodeoxycholic acid.13 A second route to
Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel,
Switzerland. E-mail: catherine.housecro@unibas.ch; Tel: +41 61 267 1008
† Electronic supplementary information (ESI) available: Fig. S1: representative
HMQC and HMBC spectra. Fig. S2: cyclic voltammograms of copper(I)
complexes; Fig. S3: photographs of dye-functionalized electrodes; Fig. S4–S6: J–V
curves and EQE spectra. CCDC 1405837. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c5ra12296a
69430 | RSC Adv., 2015, 5, 69430–69440
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heteroleptic dyes is our ‘surface-as-ligand, surface-as-complex’
approach19–22 which involves a stepwise assembly of heteroleptic
metal complex dyes on electrode surfaces and has been used for
both copper(I)22 and zinc(II)23 sensitizers. The strategy provides a
straightforward means for rapid screening of different combi-
nations of anchoring and ancillary ligands. To assemble a
[Cu(Lanchor)(Lancillary)]+ dye, an electrode is initially soaked in a
solution of Lanchor, and then the functionalized electrode is
immersed in a dye-bath containing either [Cu(Lancillary)2]+ or a
mixture of [Cu(MeCN)4]+ and Lancillary
.
22,24
The incorporation of imidazo[40,50:5,6]-1,10-phenanthroline
ligands bearing electron-donating groups in the 20-position has
been shown to be advantageous in ruthenium-based sensi-
tizers,25 and these ligands are also attractive for copper(I)-based
DSCs.18,26 The imidazo[40,50:5,6]-1,10-phenanthroline unit is
readily extended with a 4-(diphenylamino)phenyl18 or other hole-
transporting unit, and Scheme 1b shows a copper(I) sensitizer
which is noteworthy for its broad absorption spectrum extending
beyond 700 nm; however, DSCs containing this dye gave effi-
ciencies of <0.3% (with respect to 6.55% for N719).18 Ligand 1 26
(Scheme 2) is a convenient precursor to 20-functionalized 2,9-
dimethyl-imidazo[40,50:5,6]-1,10-phenanthrolines for use as
ancillary ligands in [Cu(Lanchor)(Lancillary)]+ dyes. The 2,9-substit-
uents in the phen metal-binding domain stabilize copper(I) with
respect to oxidation by sterically hindering the transformation of
tetrahedral copper(I) to square planar copper(II). An additional
feature of 1 is the long N-alkyl substituent which helps to prevent
intermolecular aggregation of dye molecules on the semi-
conductor surface and also militates against charge recombi-
nation processes.27
We now report the development of heteroleptic copper(I)
dyes for DSCs with ancillary ligands derived through post-
functionalization of the peripheral bromo-substituent in 1.
We also demonstrate the effects of introducing hole-
Scheme
assignments.
2
Structures of ligands 1–5. Ring labelling is for NMR
transporting domains into the 4- and 7-positions of
2,9-dimethyl-1,10-phenanthroline.
Experimental
General
1H and 13C NMR spectra were recorded at 295 K on a Bruker
Avance III-500 NMR spectrometer with chemical shis refer-
enced to residual solvent peaks with respect to d(TMS) ¼ 0 ppm.
Solution and solid-state absorption spectra were recorded on
Perkin-Elmer Lambda 25 and Cary 5000 spectrophotometers,
respectively, and FT-IR spectra on a Perkin-Elmer Spectrum Two
spectrometer equipped with a UATR. Electrospray (ESI) mass
spectra (solution samples in MeOH with a drop of CH2Cl2
added) and high resolution ESI-MS were measured on Bruker
Esquire 3000plus and Bruker maXis 4G instruments,
respectively.
Electrochemical measurements were performed on a CHI
900B instrument by cyclic voltammetry (CV) using a glassy
carbon working electrode, platinum wire auxiliary electrode,
and a silver wire pseudo-reference electrode. HPLC grade, argon
degassed CH2Cl2 solutions (z10ꢀ4 mol dmꢀ3) of the copper
Scheme 1 Copper(I) sensitizers reported by Odobel and coworkers
(see text).
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complexes were used with 0.1 M [nBu4N][PF6] as supporting 2.82 (s, 6H). 13C NMR (126 MHz, DMSO-d6) 156.0 (CB2), 149.3
electrolyte; the scan rate was 0.1 V sꢀ1 and ferrocene was used as (Cimid-2), 143.1 (CB10a), 139.8 (CC9a), 137.6 (CA4), 129.9 (CB4),
an internal standard, added at the end of each experiment.
129.0 (CA1), 127.6 (CA2), 126.9 (CA3), 126.5 (CB5), 126.3 (CC2),
123.2 (CB3), 120.5 (CC4), 120.2 (CC3), 109.6 (CC1), 24.7 (CMe), (CB4a
not resolved). ESI MS m/z: 490.4 [M + H]+ (calc. 490.2).
Ligands and complexes
Compound 4 NaH (60% oil dispersion, 122 mg, 3.05 mmol)
Compound 1,26 2,26 2,9-dimethyl-1,10-phenanthroline-5,6-dione,28 was suspended and stirred vigorously for 1.5 h in DMF (5 ml)
4-(9H-carbazol-9-yl)benzaldehyde,29
4,7-dichloro-2,9-dimethyl- under N2. Compound 4a (200 mg, 0.409 mmol) was added and
1,10-phenanthroline,30 [Cu(MeCN)4][PF6]31 and [Cu(2)2][PF6]26 the suspension was stirred for 10 min. Then 1-bromo-n-octane
were synthesized as previously reported. 4,40-Di-n-butoxy- (0.711 ml, 789 mg, 4.09 mmol) was added and the mixture was
diphenylamine was prepared by the method reported for the heated at 70 ꢁC for 3 days. The black mixture was allowed to cool
analogous hexoxy derivative,32 and NMR spectra corresponded to room temperature and was diluted with water, giving a
to those published.33 Bis(dibenzylideneacetone)palladium(0), suspension which was extracted with CH2Cl2 (25 ml). The
[Pd(dba)2] was purchased from Strem Chemicals, and 4-(N,N- organic layer was washed with water (5 ꢂ 40 ml), dried over
diphenylamino)phenylboronic acid from Fluorochem.
Na2SO4 and the solvent was removed. The remaining DMF was
Compound 3 a ask (50 ml) was charged with 1 (300 mg, removed by azeotrope distillation with toluene ve times. The
0.582 mmol), 4,40-di-n-butoxydiphenylamine (279 mg, brown residue was washed with petroleum ether and dried in
0.873 mmol), NaOtBu (140 mg, 1.45 mmol) and a catalytic vacuo. This gave a brown foam which was puried by column
amount of [Pd(dba)2] (18.4 mg, 0.0291 mmol) and was evacu- chromatography (three columns: alumina, CH2Cl2 changing to
ated for 15 min. Toluene (20 ml) and PtBu3 (0.03 ml of a 0.1 M CH2Cl2/2% MeOH; alumina, toluene/ethyl acetate 4 : 1
sol. in toluene, 23.9 mg, 0.0291 mmol) were added and the changing to 1 : 1; alumina, CH2Cl2/0.25% MeOH). Compound 4
1
reddish brown mixture was heated to 95 ꢁC for 8 days. The was isolated as a beige solid (45 mg, 0.075 mmol, 18.3%). H
mixture was allowed to cool to room temperature, ltered and NMR (500 MHz, DMSO-d6) d/ppm: 8.85 (d, J ¼ 8.3 Hz, 1H, HB7),
the solvent was removed. The remaining brown solid was 8.78 (d, J ¼ 8.6 Hz, 1H, HB4), 8.30 (d, J ¼ 7.9 Hz, 2H, HC4), 8.10
puried by column chromatography on alumina eluting with (m, 2H, HA2), 7.91 (m, 2H, HA3), 7.77 (d, J ¼ 8.3 Hz, 1H, HB3),
CH2Cl2 to yield the product as a yellow solid (223 mg, 298 mmol, 7.73 (d, J ¼ 8.0 Hz, 1H, HB8), 7.56 (d, J ¼ 8.2 Hz, 2H, HC1), 7.50
51.2%). 1H NMR (500 MHz, CDCl3) d/ppm 9.24 (br, 1H, HB4/B7), (t, J ¼ 7.6 Hz, 2H, HC2), 7.35 (t, J ¼ 7.3 Hz, 2H HC3), 4.82 (m, 2H,
8.48 (d, J ¼ 8.6 Hz, 1H, HB4/B7), 7.66 (d, J ¼ 8.2 Hz, 1H, HB3/B8), HNCH ), 2.84 (s, 3H, HMe-phen), 2.83 (s, 3H, HMe-phen), 1.85 (m, 2H,
2
CH2
7.62 (d, J ¼ 8.4 Hz, 1H, HB3/B8), 7.54 (m, 2H, HA2), 7.13 (m, 4H, HNCH CH ), 1.15–1.05 (m, 10H, H ), 0.68 (m, 3H, HMe-octyl). 13
C
2
2
HC2), 7.03 (m, 2H, HA3), 6.88 (m, 4H, HC3), 4.64 (m, 2H, HNCH ), NMR (126 MHz, DMSO-d6) d/ppm: 156.2 (CB2/B9), 155.7 (CB2/B9),
2
3.96 (t, J ¼ 6.5 Hz, 4H, HOCH ), 3.07 (s, 3H, HMe-phen), 3.01 (s, 3H, 152.4 (Cimid-2), 139.7 (CC9a), 137.8 (CA4), 131.6 (CA2), 130.0 (CB7),
2
HMe-phen), 1.95 (m, 2H, HNCH CH ), 1.78 (m, 4H, H
), 1.51 129.3 (CB4), 129.2 (CA1), 126.6 (CA3), 126.2 (CC2), 124.4 (CB5),
OCH2CH2
2
2
(m, 4H, HOCH CH CH ), 1.30–1.16 (m, 10H, HCH -octyl), 0.99 (t, J ¼ 123.5 (CB8), 123.0 (CB3), 122.8 (CC4a), 121.4 (CB6a), 120.5 (CC4),
2
2
2
2
7.4 Hz, 6H, HMe-butyl), 0.85 (t, J ¼ 6.9 Hz, 3H, HMe-octyl). 13C NMR 120.3 (CC3), 117.4 (CB4a), 109.5 (CC1), 45.7 (CNCH ), 30.9 (C ),
CH2
2
(126 MHz, CDCl3) d/ppm 157.5 (CB2+B9), 156.1 (CC4), 139.6 (CC1), 28.9 (CNCH CH ), 28.0 (C ), 25.0 (C ), 24.6 (overlapping
CH2
CH2
2
2
CH2
130.6 (CA2), 128.7 (CB4/B7), 127.6 (CC2), 124.5 (CB3/B8), 123.6 CMe-phen), 21.7 (CCH ), 18.4 (C ), 13.6 (CMe-octyl), (CB6, CB10a
,
)
2
(CB3/B8), 118.6 (CA3), 117.5 (CB4a/B6a), 115.6 (CC3), 67.9 (COCH ), CB10b not resolved). UV-Vis (CH2Cl2, 1.00 ꢂ 10ꢀ5 mol dmꢀ3
2
46.9 (CNCH ), 31.8 (CCH -octyl), 31.5 (COCH CH ), 30.2, (C
), l/nm (3/dm3 molꢀ1 cmꢀ1) 294 (55 000), 310 sh (35 500), 325 sh
NCH2CH2
2
2
2
2
29.2 (CCH -octyl), 29.0 (CCH -octyl), 26.4 (CCH -octyl), 25.3 (Cphen-Me), (29 900), 340 (23 000), 370 sh (4200). ESI MS m/z: 602.6 [M + H]+
2
2
2
25.1 (Cphen-Me), 22.7 (CCH -octyl), 19.4 (COCH CH CH ), 14.2 (calc. 602.3). Found: C 78.95, H 6.56, N 10.80; C41H39N5$H2O
2
2
2
2
(CMe-octyl), 14.0 (CMe-butyl), (other CQ not resolved). UV-Vis requires C 79.45, H 6.67, N 11.30%.
(CH2Cl2, 1.00 ꢂ 10ꢀ5 mol dmꢀ3) l/nm (3/dm3 molꢀ1 cmꢀ1
)
Compound 5 K3PO4 (343 mg, 1.62 mmol) was dissolved in
267 (35 700), 296 (40 700), 341 sh (23 800). HR ESI-MS m/z: H2O (2 ml) under N2, and N2 was bubbled through the solution
748.4582 [M + H]+ (calc. 748.4585). Found C 78.52, H 7.51, N for 10 min. In a separate ask, N2 was bubbled through
8.96; C49H57N5O2 requires C 78.68, H 7.68, N 9.36%.
Compound 4a 4-(9H-carbazol-9-yl)benzaldehyde (535 mg, dimethyl-1,10-phenanthroline (100 mg, 0.361 mmol), 4-(diphe-
1.97 mmol), 2,9-dimethyl-1,10-phenanthroline-5,6-dione nylamino)phenylboronic acid (230 mg, 0.794 mmol) and cata-
1,4-dioxane (5 ml) for 10 min and then 4,7-dichloro-2,9-
(481 mg, 2.02 mmol) and an excess of NH4OAc (5.04 g, lytic amounts of [Pd(dba)3] (24.7 mg, 0.0238 mmol, 6.6 mol%)
65.4 mmol) were added to EtOH (100 ml). The yellow mixture and tricyclohexylphosphine (15.2 mg, 0.0542 mmol) were
was heated at reux overnight aer which time the solvent was added. The aqueous solution of K3PO4 was then added to the
removed. The remaining orange solid was washed with water reaction mixture and this was heated at reux at 95 ꢁC for 15 h.
and Et2O. The product was puried using column chromatog- The orange mixture was allowed to cool down to room
raphy (alumina, CH2Cl2 with 3% MeOH) and was isolated as a temperature and water (10 ml) was added. The mixture was
1
yellow solid (282 mg, 0.575 mmol, 29.2%). H NMR (500 MHz, extracted with CH2Cl2 (5 ꢂ 15 ml) and the organic layer was
DMSO-d6) d/ppm 13.8 (br, NH), 8.84 (d, J ¼ 8.2 Hz, 2H, HB4), 8.56 dried over Na2SO4 and the solvent then removed. The product
(m, 2H, HA2), 8.29 (m, 2H, HC4), 7.91 (m, 2H, HA3), 7.72 (m, 2H, was recrystallized from CH2Cl2/MeOH (1 : 1) and 5 was isolated
HB3), 7.55 (m, 2H, HC1), 7.49 (m, 2H, HC2), 7.34 (m, 2H, HC3), as a yellow solid (97.0 mg, 0.140 mmol, 38.7%). 1H NMR
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(500 MHz, CDCl3) d/ppm 8.00 (s, 2H, HB5), 7.55 (s, 2H, HB3), 7.41 [M ꢀ PF6]+ (calc. 1557.8315). Satisfactory elemental analysis
(m, 4H, HC2), 7.32 (m, 8H, HA3), 7.19 (m, 12H, HA2+C3), 7.10 (m, could not be obtained.
4H, HA4), 3.14 (s, 6H, HMe). 13C (126 MHz, CDCl3) d/ppm 158.8
[Cu(4)2][PF6]. The method and purication were as for
(CB2), 150.2 (CB4), 148.9 (CC4), 147.3 (CA1), 142.9 (CB10a), 130.8 [Cu(1)2][PF6]; reagents and solvents were [Cu(NCMe)4][PF6]
(CC2), 129.7 (CA3), 125.2 (CA2), 125.1 (CB4a), 124.8 (CB3), 123.9 (CA4), (12.4 mg, 0.0332 mmol) in MeCN (5 mL) and 4 (40.0 mg,
123.5 (CB5), 122.5 (CC3), 24.7 (CMe), (CC1 not resolved). UV-Vis 0.0665 mmol) in CH2Cl2 (5 ml). [Cu(4)2][PF6] was isolated as a
(CH2Cl2, 1.00 ꢂ 10ꢀ5 mol dmꢀ3) l/nm (3/dm3 molꢀ1 cmꢀ1) 297 dark red solid (46.9 mg, 0.0330 mmol, 100%). 1H NMR
(49 000), 360 sh (18 100), 500 sh (4950). HR ESI-MS m/z: 695.3174 (500 MHz, CD3CN) d/ppm: 9.18 (d, J ¼ 8.2 Hz, 2H, HB7), 8.99 (d,
[M + H]+ (calc. 695.3169). Satisfactory elemental analysis was not J ¼ 8.6 Hz, 2H, HB4), 8.27 (dt, J ¼ 7.8, 1.0 Hz, 4H, HC4), 8.11 (m,
obtained.
4H, HA2/A3), 7.97 (d, J ¼ 8.5 Hz, 2H, HB3), 7.94 (overlapping m,
[Cu(1)2][PF6]. A solution of [Cu(NCMe)4][PF6] (16.2 mg, 6H, HA2/A3+B8), 7.64 (m, 4H, HC1), 7.51 (ddd, J ¼ 8.3, 7.0, 1.2 Hz,
0.0435 mmol) in MeCN (2 ml) was added dropwise to a solution 4H, HC2), 7.36 (ddd, J ¼ 8.0, 7.0, 0.9 Hz, 4H, HC3), 4.90 (t, J ¼
of 1 (44.8 mg, 0.0870 mmol) in CHCl3 (1 ml). The dark red 7.2 Hz, 4H, HNCH ), 2.505 (s, 6H, HMe-Phen), 2.50 (s, 6H, HMe-Phen),
2
solution was stirred for 3 h and then the solvent was removed. 2.04 (m, 4H, HNCH CH ), 1.30 (m, 4H, H
), 1.25–1.16 (m,
NCH2CH2CH2
2
2
The product was puried by column chromatography (alumina, 16H, HCH -octyl), 0.76 (t, J ¼ 6.9 Hz, 6H, HMe-octyl). 13C NMR
2
CH2Cl2 with 1% MeOH) and [Cu(1)2][PF6] was isolated as a dark (126 MHz, CD3CN) d/ppm 156.5 (CB2/B9), 155.5 (CB2/B9), 154.2
red solid (38.0 mg, 0.0310 mmol, 70.5%). H NMR (500 MHz, (Cimid-2), 141.5 (CB10b), 141.1 (CB10a), 140.6 (CC9a), 139.0 (CA4),
1
CD3CN) d/ppm 9.10 (d, J ¼ 8.2 Hz, 2H, HB7), 8.91 (d, J ¼ 8.7 Hz, 136.2 (CB6), 131.7 (CA2), 131.2 (CB7), 130.6 (CB4), 129.5 (CA1),
2H, HB4), 7.91 (d, J ¼ 8.6 Hz, 2H, HB3), 7.89 (d, J ¼ 8.3 Hz, 2H, 127.3 (CA3), 126.3 (CC2), 126.0 (CB8), 125.6 (CB3), 125.3 (CB5),
HB8), 7.82 (m, 4H, HA3), 7.73 (m 4H, HA2), 4.73 (t, J ¼ 7.3 Hz, 4H, 123.5 (CC4a), 123.1 (CB6a), 120.5 (CC4), 120.3 (CC3), 119.2 (CB4a),
HNCH ), 2.44 (s, 6H, HMe-phen), 2.43 (s, 6H, HMe-phen), 1.91 (m, 4H, 109.8 (CC1), 46.6 (CNCH ), 31.3 (CCH -octyl), 29.6 (CNCH CH ), 28.5
2
2
2
2
2
HCH ), 1.24–1.18 (m, 8H, H ), 1.16–1.11 (m, 12H, H ), 0.83 (2CCH -octyl), 25.6 (CNCH CH CH ), 25.0 (overlapping CMe-Phen), 22.3
CH2
CH2
2
2
2
2
2
(t, J ¼ 7.2 Hz, 6H, HMe-octyl). 13C NMR (126 MHz, CD3CN) d/ppm (CCH -octyl), 13.3 (CMe-octyl). UV-Vis (CH2Cl2, 1.00
ꢂ
10ꢀ5
2
157.1 (CB2/B9), 156.6 (CB2/B9), 154.6 (Cimid-2), 142.5 (CB10a/B10b), mol dmꢀ3) l/nm (3/dm3 molꢀ1 cmꢀ1) 262 (106 400), 292
142.0 (CB10a/B10b), 137.1 (CB4a/B6a), 133.1 (CA3), 132.8 (CA2), 132.1 (104 300), 338 (52 900), 475 (14 500). ESI MS m/z: 1266.0
(CB7), 131.4 (CB4), 130.6 (CA1), 126.9 (CB8), 126.4 (CB3), 125.0 [M ꢀ PF6]+ (calc. 1265.6). HR ESI-MS: m/z 1265.5687 [M ꢀ PF6]+
(CA4), 124.8 (CB4a/B6a), 123.9 (CB6), 120.0 (CB5), 47.5 (CNCH ), 32.2 (calc. 1265.5701). Found C 69.00, H 5.78, N 9.74; C82H78CuF6-
2
(CCH ), 29.4 (2C ), 26.6 (C ), 25.8 (2CMe-phen), 23.3 (CCH ), N10P$0.5H2O requires C 69.30, H 5.60, N 9.86%.
CH2
CH2
2
2
14.3 (CMe-octyl). UV-Vis (CH2Cl2, 1.00 ꢂ 10ꢀ5 mol dmꢀ3) l/nm
[Cu(5)2][PF6]. A solution of [Cu(NCMe)4][PF6] (22.8 mg,
(3/dm3 molꢀ1 cmꢀ1) 282 (87 200), 304 (95 300), 474 (13 400). ESI 0.0612 mmol) in MeCN (2 mL) was added dropwise to a solution
MS m/z: 1093.7 [M ꢀ PF6]+ (calc. 1093.3). Found C 57.02, H 5.38, of 28 (85.0 mg, 0.122 mmol) in CHCl3 (2 mL). The dark red
N 9.01; C58H62Br2CuF6N8P requires C 56.20, H 5.04, N 9.04%.
solution was stirred for 0.5 h and the solvent was then removed.
[Cu(3)2][PF6]. The method and purication were as for The residue was suspended in water and the mixture then
[Cu(1)2][PF6]; reagents and solvents were [Cu(MeCN)4][PF6] ltered. The lter cake was collected by dissolving it in a mixture
(12.5 mg, 0.0334 mmol) in MeCN (2 ml) and 3 (50 mg, 0.0668 of MeCN and CHCl3 and then ltering the solution through a P3
mmol) in CHCl3 (2 ml). [Cu(3)2][PF6] was isolated as a red solid glass lter (16–40 mm). Aer removing the solvent from the
(41.4 mg, 24.3 mmol, 72.7%). 1H NMR (500 MHz, CDCl3) d/ppm ltrate in vacuo, [Cu(5)2][PF6] was isolated as a red solid (91.0 mg,
9.24 (br, 2H, HB7), 8.85 (d, J ¼ 8.6 Hz, 2H, HB4), 7.94 (d, J ¼ 0.057 mmol, 93.0%). 1H NMR (500 MHz, CD2Cl2) d/ppm 8.21 (br,
8.5 Hz, 2H, HB3), 7.78 (d, J ¼ 8.3 Hz, 2H, HB8), 7.54 (m, 4H, HA2), 4H, HB5), 7.72 (br, 4H, HB3), 7.51 (m, 8H, HC2), 7.35 (m, 16H,
7.15 (m, 8H, HC2), 7.07 (m, 4H, HA3), 6.89 (m, 8H, HC3), 4.78 (t, HA3), 7.23 (m, 24H, HA2+C3), 7.14 (m, 8H, HA4), 2.53 (s, 12H,
OCH2
J ¼ 6.8 Hz, 4H, HNCH ), 3.97 (t, J ¼ 6.4 Hz, 8H, H
), 2.45 (s, HMe-phen). 13C NMR (126 MHz, CD2Cl2) d/ppm 157.2 (CB2), 149.6
2
6H, HMe-phen), 2.43 (s, 6H, HMe-phen), 2.00 (m, 4H, HNCH CH ), (CB4), 149.3 (CC4), 147.5 (CA1), 144.5 (CB10a), 131.0 (CC2), 130.0
2
2
1.79 (m, 8H, HOCH CH ), 1.52 (m, 8H, H
), 1.31 (m, 4H, (CA3), 126.0 (CB4a), 125.8 (CC3/A2), 125.7 (CB3), 124.4 (CA4), 122.5
OCH2CH2CH2
2
2
HNCH CH CH ), 1.27–1.14 (m, 16H, HCH -octyl), 0.99 (t, J ¼ 7.4 Hz, (CC3/A2), 124.1 (CB5), 26.2 (CMe-phen), (CC1 not resolved). UV-Vis
12H, HMe-butyl), 0.84 (t, J ¼ 6.8 Hz, 6H, HMe-octyl). 13C NMR (CH2Cl2, 1.00 ꢂ 10ꢀ5 mol dmꢀ3) l/nm (3/dm3 molꢀ1 cmꢀ1) 295
(126 MHz, CDCl3) d/ppm 156.1 (CC4), 155.3 (CB2/B9), 155.2 (119 200), 384 (54 000), 486 (23 600). ESI MS m/z: 1452.1
(Cimid-2), 154.8 (CB2/B9), 150.4 (CA4), 141.1 (CB10b), 139.6 (CC1), [M ꢀ PF6]+ (calc. 1452.6). Found C 72.47, H 4.79, N 7.01;
2
2
2
2
131.7 (CB7), 130.5 (CA2), 129.9 (CB4), 127.4 (CC2), 125.6 (CB3),
125.3 (CB8), 124.8 (CB5), 122.8 (CB6a), 120.0 (CA1), 119.2 (CB4a),
C100H76CuF6N8P$3H2O requires C 72.69, H 5.00, N 6.78%.
NCH2
118.8 (CA3), 115.6 (CC3), 67.8 (COCH ), 46.7 (C
), 31.6
2
(CCH -octyl), 31.2 (COCH CH ), 30.0 (C
), 29.0 (CCH -octyl), 28.9
NCH2CH2
Crystallography
2
2
2
2
(CCH -octyl), 26.0 (CCH -octyl), 25.9 (CMe-phen), 25.7 (CMe-phen), 22.4 Single crystal data were collected on a Bruker APEX-II diffrac-
2
2
(CCH -octyl), 18.9 (COCH CH CH ), 13.9 (CMe-octyl), 13.7 (CMe-butyl), tometer with data reduction, solution and renement using the
(CB6 CB10a not resolved). UV-Vis (CH2Cl2, 1.00 10ꢀ5 programs APEX34 and CRYSTALS.35 Structure analysis used
mol dmꢀ3) l/nm (3/dm3 molꢀ1 cmꢀ1) 256 (73 200), 292 (88 800), Mercury v. 3.5.1.36,37
2
2
2
2
,
ꢂ
342 sh (58 700), 469 (16 100). HR ESI-MS m/z: 1557.8334
Compound 4 C41H39N5, M ¼ 601.79, colourless needle,
ꢀ
triclinic, space group P1, a ¼ 11.7635(14), b ¼ 11.8126(14), c ¼
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ꢁ
˚
12.2155(14) A, a ¼ 72.343(4), b ¼ 86.285(4), g ¼ 83.168(4) , U ¼ butoxydiphenylamine was prepared by a copper(I) iodide/L-proline
1605.25(18) A , Z ¼ 2, Dc ¼ 1.245 Mg mꢀ3, m(Cu-Ka) ¼ catalysed32 Ullmann coupling (Scheme 3a) and was then used in
3
˚
0.570 mmꢀ1, T ¼ 123 K. Total 18 881 reections, 5769 unique, the Hartwig–Buchwald amination shown in Scheme 3b.
Rint ¼ 0.027. Renement of 5522 reections (415 parameters) Compound 3 was isolated in 51.2% yield. Attempts to prepare 4 by
with I > 2s(I) converged at nal R1 ¼ 0.0375 (R1 all data ¼ reaction of 1 with 9H-carbazole using Hartwig–Buchwald condi-
0.0391), wR2 ¼ 0.0416 (wR2 all data ¼ 0.0477), gof ¼ 1.1112. tions led to very low yields of 4; changing the catalyst from
CCDC 1405837.†
[Pd(dba)2]/PtBu3 to [Pd(OAc)2]/PtBu3 or [Pd(PPh3)4] gave mixtures
from which pure 4 could not be separated. We therefore opted for
the alternative route to 4 shown in Scheme 4. Compound 4a was
formed by treatment of 4-(9H-carbazol-9-yl)benzaldehyde with 2,9-
dimethyl-1,10-phenanthroline-5,6-dione and NH4OAc; subse-
quent alkylation of the imidazole gave 4 in 18.3% yield. The
electrospray mass spectra of 3 and 4 exhibited ions arising from
DSC fabrication and measurements
Solaronix Test Cell Titania Electrodes were used for the photo-
anodes. They were washed with milliQ H2O and heated at
450ꢁ C for 30 min, then cooled to z80 ꢁC and soaked in a
1.0 mM DMSO solution of 6 for 24 h at room temperature. Aer
removal of the electrodes from the solution, they were washed
with DMSO and EtOH and dried in a stream of N2. Each func-
tionalized electrode was then soaked for 3 days in a 0.1 mM
MeCN solution of [Cu(Lancillary)2][PF6] (Lancillary ¼ 1–5) at room
temperature. The electrodes were removed from the dye-bath,
washed with MeCN and dried in a stream of N2.
N719 reference electrodes were made by dipping Solaronix
Test Cell Titania Electrodes in a 0.3 mM EtOH solution of N719
(Solaronix) for 3 days. The electrodes were removed from the dye-
bath, and were washed with EtOH and dried in a stream of N2.
For the counter electrodes, Solaronix Test Cell Platinum
Electrodes were used, and volatile organic impurities were
1
[M + H]+, and the compounds were characterized by 13C and H
NMR spectroscopies using COSY, NOESY, HMQC and HMBC
techniques. The alkyl chain desymmetrizes the phen unit, giving
rise to pairs of signals in both the 1H and 13C NMR spectra for H/
CB3/B8, H/CB4/B7, and H/CMe-phen. For example, the methyl groups
in the 1H NMR spectrum appear at d 3.01 and 3.07 ppm in 3, and d
2.83 and 2.84 ppm in 4. In contrast to 4, the 1H NMR spectrum of
precursor 4a (see Experimental section) reects the C2 symmetry
that results from the tautomerism of the imidazole ring.
The synthetic approach to compound 5 (Scheme 2) was based
on that described in the patent literature for 4,7-bis(4-(diphe-
nylamino)phenyl)-1,10-phenanthroline.40 Suzuki coupling of
4,7-dichloro-2,9-dimethyl-1,10-phenanthroline with two equiva-
lents of 4-(N,N-diphenylamino)phenylboronic acid (Scheme 5)
gave 5 in moderate yield. The high-resolution electrospray mass
spectrum conrmed the presence of the [M + H]+ ion at m/z ¼
695.3174. Fig. 1 shows the 1H NMR spectrum of 5 which is
consistent with a C2-symmetric molecule. Protons HB3 and HB5
were distinguished using the NOESY cross peak between HMe
ꢁ
removed by heating on a heating plate at 450 C for 30 min.
The dye-covered TiO2 electrode and Pt counter-electrode
were combined using thermoplast hot-melt sealing foil (Solar-
onix Test Cell Gaskets, 60 mm) by heating while pressing them
together. The electrolyte (LiI (0.1 M), I2 (0.05 M), 1-methyl-
benzimidazole (0.5 M), 1-butyl-3-methylimidazolinium iodide
(0.6 M) in 3-methoxypropionitrile) was introduced into the DSC
by vacuum backlling. The hole in the counter electrode was
sealed with hot-melt sealing foil (Solaronix Test Cell Sealings)
and a cover glass (Solaronix Test Cell Caps).
For each dye, duplicate DSCs were made and the cells were
completely masked.38,39 Measurements were made by irradi-
ating the DSC from behind using a SolarSim 150 (Solaronix)
light source previously calibrated with a silicon reference cell to
100 mW cmꢀ2 (1 sun).
External quantum efficiency (EQE) measurements were
made using a Spe-Quest quantum efficiency instrument from
Rera Systems (Netherlands) 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 measured
with a SR830 DSP Lock-In amplier (Stanford Research).
Results and discussion
Synthesis and characterization of ancillary ligands 1–5
We have previously reported the preparation and characterization
of the bromo-derivative 1 and subsequent Hartwig–Buchwald
amination with diphenylamine to give 2 (Scheme 2).26 The same
Scheme
3
(a) Synthetic route to 4,40-di-n-butoxydiphenylamine;
conditions (i) K2CO3, CuI, L-proline in DMSO, 90 ꢁC, 24 h. (b) Conditions
for Hartwig–Buchwald coupling to 3: (ii) 4,40-di-n-butoxydiphenyl-
strategy was used for the synthesis of 3. 4,40-Di-n- amine, NaOtBu, catalytic [Pd(dba)2]/PtBu3, toluene, reflux 15 h.
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Fig. 1 500 MHz 1H NMR spectrum of 5 (CDCl3, 295 K). See Scheme 2
for atom labelling; * is residual CHCl3. Chemical shifts are in d/ppm.
photoresponse of the ligands in the region between 325 and
400 nm, but the most signicant improvement in absorption
towards the red-region is observed in compound 5 which
absorbs down to z550 nm.
Single crystal structure of 4
The single crystal structure of 4 was determined from crystals
grown from a DMSO-d6 solution in an NMR tube. The
ꢀ
compound crystallizes in the triclinic space group P1 and the
structure is shown in Fig. 3. Bond lengths (caption to Fig. 3) and
bond angles are unremarkable, and the phenyl ring containing
C19 is twisted through 53.4ꢁ with respect to the plane through
the imidazole ring consistent with minimizing inter-ring H.H
contacts. The n-octyl chain is folded over the N-phenylcarbazole
unit (Fig. 3), with an orientation which mimics that in 1 26 and
this leads to close CHalkyl.p contacts, both to the phenyl
Scheme 4 Synthetic route to compound 4; conditions (i) excess
NH4OAc, EtOH; (ii) NaH, DMF under N2; (iii) nC8H17Br, DMF, 75 ꢁC, 15 h.
and HB3; HC2/HB3 and HC2/HB5 NOESY cross peaks were used to
discriminate between HC2 and HC3. Assignment of the 13C NMR
spectrum was made using HMBC and HMQC methods.
The solution absorption spectra of the ve ligands are
compared in Fig. 2. Introduction of the diphenylamino or
carbazole units on going from 1 to 2, 3 or 4 enhances the
˚
spacer (CH.centroid ¼ 3.47 A) and to the heterocyclic ring of
˚
the carbazole (CH.centroid ¼ 3.32 A). Two packing motifs are
of importance. Firstly, carbazole and phen units in adjacent
molecules engage in face-to-face p-stacking interactions,
leading to the assembly of chains parallel to the c-axis (Fig. 4a).
As Fig. 3a illustrates, the carbazole and phen units are slipped
with respect to one another giving an optimum conguration
for p-interactions; the carbazolecentroid.phenplane distance is
˚
3.40 A. Adjacent chains interact through p-stacking and this is
best described in terms of the quadruple-decker stack shown in
Fig. 4b. The central interaction is between a centrosymmetric
pair of 1H-phenanthro[9,10-d]imidazole domains (interplane
Scheme 5 Synthetic route to compound 5; conditions (i) K3PO4,
catalytic [Pd(dba)2]/P(C6H11)3, 1,4-dioxane/H2O (ratio 5 : 2), reflux, Fig. 2 Solution absorption spectra of ligands 1–5 (CH2Cl2, 1 ꢂ 10ꢀ5
95 ꢁC, 15 h.
mol dmꢀ3).
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Fig. 3 Crystal structure of 4 with H atoms omitted and ellipsoids
plotted at 50% probability level. Selected bond lengths: N1–C1 ¼
1.3250(14), N1–C5 ¼ 1.3529(13), N2–C6 ¼ 1.3551(14), N2–C10 ¼
1.3271(14), N3–C13 ¼ 1.3892(13), N3–C15 ¼ 1.3783(13), N3–C34 ¼
1.4644(13), N4–C14 ¼ 1.3758(13), N4–C15 ¼ 1.3152(14), N5–C19 ¼
1.4216(13), N5–C22 ¼ 1.3905(13), N5–C33 ¼ 1.3954(13) A˚.
1
Fig. 5 Comparison of the aromatic regions of the 500 MHz H NMR
spectra of 4 (in DMSO-d6) and [Cu(4)2][PF6] (in CD3CN). See Scheme 2
for atom labelling.
˚
separation ¼ 3.33 A). Extension beyond the quadruple-decker
unit is prevented by the CH.p contacts from the terminal
methyl group of the n-octyl chain (Fig. 4b).
solubility properties of free ligands and complexes precluded
the use of common solvents for recording NMR spectra of
ligands and copper(I) complexes. Nonetheless, the shis to
Synthesis and characterization of homoleptic copper
complexes
higher frequencies for the signals of the phen unit (HB3, HB8
,
HB4 and HB7) upon complexation are characteristic, as illus-
trated for 4 to [Cu(4)2][PF6] in Fig. 5.
The copper(I) complexes [CuL2][PF6] with L ¼ 1, 3–5 were
prepared by dropwise addition of an MeCN solution of
[Cu(MeCN)4][PF6] to a solution of the ligand in CHCl3 or
CH2Cl2. The preparation of [Cu(2)2][PF6] has previously been
reported.26 The homoleptic complexes were isolated in
70.5–100% yield, and in the electrospray mass spectrum of
each, the highest mass peak envelope corresponded to the
The solution absorption spectra of [CuL2][PF6] with L ¼ 1–5
are shown in Fig. 6. The approximate doubling in the values of
the extinction coefficients for the high-energy, ligand-centred
absorptions (assigned to p* ) p transitions) on going from L
(Fig. 4) to [CuL2]+ (Fig. 6) is consistent with the formation of the
homoleptic complexes. The absorption around 470–475 nm for
[Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6] arises
from metal-to-ligand charge transfer (MLCT) and this undergoes a
bathochromic shi to 486 nm on going to [Cu(5)2][PF6]. The
enhanced spectral response of [Cu(5)2][PF6] at wavelengths
above 375 nm is noteworthy in terms of the incorporation of the
{Cu(5)} unit in TiO2-bound sensitizers (see later).
1
[M ꢀ PF6]+ ion. H and 13C NMR spectra were assigned using
COSY, NOESY, HMQC and HMBC methods (Fig. S1†). Differing
The copper(I) complexes are redox active and cyclic voltam-
mograms were recorded in CH2Cl2 to avoid possible involve-
ment by coordinating solvents such as MeCN. [Cu(1)2][PF6]
exhibits a reversible oxidation process (Fig. 7) at +0.48 V
Fig. 4 (a) In 4, chains run along the c-axis, assembled through
p-stacking interactions between phen and carbazole units. (b) One Fig. 6 Solution absorption spectra of complexes [CuL2][PF6] for L ¼
quadruple-decker p-stack in 4.
1–5 (CH2Cl2, 1 ꢂ 10ꢀ5 mol dmꢀ3).
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Table 1 Performance parameters of masked DSCs with the copper(I)
dyes; two DSCs were fabricated for each dye. Data are compared to a
DSC containing N719. See Schemes 2 and 6 for ligand structures
Dye
JSC/mA cmꢀ2
VOC/mV
ff/%
h/%
Relative h/%
On the day of sealing the DSCs
[Cu(6)(1)]+
[Cu(6)(1)]+
[Cu(6)(2)]+
[Cu(6)(2)]+
[Cu(6)(3)]+
[Cu(6)(3)]+
[Cu(6)(4)]+
[Cu(6)(4)]+
[Cu(6)(5)]+
[Cu(6)(5)]+
N719
5.38
5.82
5.41
4.71
4.75
5.65
5.82
5.89
6.81
6.40
17.94
542
547
562
558
523
540
556
562
557
558
642
71
72
75
73
73
68
73
72
72
73
70
2.08
2.30
2.29
1.92
1.81
2.07
2.35
2.38
2.73
2.62
8.06
25.8
28.6
28.4
23.9
22.5
25.7
29.1
29.5
33.9
32.5
100.0
Fig. 7 Cyclic voltammogram of [Cu(1)2][PF6] in CH2Cl2 solution (Epc
ꢀ
Epa ¼ 74 mV) with respect to Fc/Fc+; scan rate ¼ 0.1 V sꢀ1
.
assigned to the Cu+/Cu2+ redox couple. The value is close to the
3 days aer sealing the DSCs
reported value of +0.50 V for 2,9-dimethyl-1,10-phenanthro- [Cu(6)(1)]+
5.32
5.57
4.84
4.24
4.47
5.11
5.54
5.26
6.47
6.17
17.77
548
555
570
567
539
546
558
570
567
564
700
71
72
75
72
72
68
72
71
71
73
68
2.06
2.23
2.06
1.74
1.73
1.88
2.22
2.14
2.59
2.54
8.49
24.3
26.3
24.3
20.4
20.4
22.2
26.1
25.2
30.5
30.0
100.0
[Cu(6)(1)]+
[Cu(6)(2)]+
[Cu(6)(2)]+
[Cu(6)(3)]+
[Cu(6)(3)]+
line,41 indicating that the 2-(4-bromophenyl)-1-octyl-1H-imidazo
unit in 1 has little effect on the oxidation potential of the cop-
per(I) centre. For [Cu(2)2][PF6], [Cu(3)2][PF6], [Cu(4)2][PF6] and
[Cu(5)2][PF6], a number of quasi-reversible or irreversible
oxidation processes were observed (Fig. S2†) consistent with the [Cu(6)(4)]+
introduction of the diphenylamino or carbazole functionaliza-
tions; these were not investigated in detail. Ligand-based
reduction processes are poorly dened (Fig. 7) for all the
complexes.
[Cu(6)(4)]+
[Cu(6)(5)]+
[Cu(6)(5)]+
N719
Preparation of DSCs and solid-state absorption spectra of dye-
functionalized electrodes
The solid-state absorption spectra of the sensitized elec-
trodes are shown in Fig. 8; Fig. S3† shows photographs of the
electrodes, all of which had the same soaking conditions in the
dye baths. The values of lmax for the MLCT bands of [Cu(1)2]+,
[Cu(2)2]+, [Cu(3)2]+ and [Cu(4)2]+ in solution (470–475 nm) are
consistent with those of the on-surface dyes [Cu(6)(1)]+,
[Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+ (466–469 nm). Pleasingly,
the enhanced absorption between 375–600 nm shown by
[Cu(5)2]+ compared to the other homoleptic dyes in solution
(Fig. 6) is also observed for the surface-anchored [Cu(6)(5)]+
(Fig. 8). However, none of the dyes absorbs as far into the red as
N719.
The [Cu(Lanchor)(Lancillary)]+ dyes, in which Lanchor is the phos-
phonic acid anchoring ligand 6 42 (Scheme 6) and Lancillary is 1–
5, were assembled on TiO2 electrodes by rst soaking them in a
solution of Lanchor followed by immersion in solutions of
[Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6], [Cu(4)2][PF6] or
[Cu(5)2][PF6]. Dye-bath concentrations and dipping times were
the same for all electrodes. Commercial TiO2 electrodes with or
without a scattering layer were used for DSC measurements or
solid-state absorption spectroscopy, respectively. Electrodes
with the reference dye N719 were prepared by soaking the TiO2
electrodes in solutions of the sensitizer. Although we have
previously shown that DSCs (with screen-printed TiO2) incor-
–
porating [Cu(6)(2)]+ perform similarly using either I–/I3 or
[Co(bpy)3]2+/3+ electrolytes,26 we chose in the present work to use
a standard I–/I3 electrolyte (see Experimental section).
–
Fig.
8 Solid-state absorption spectra of dye-functionalized TiO2
Scheme 6 Structure of the anchoring ligand 6.
electrodes.
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Table 2 EQE maxima for two independent sets of DSCs containing
dyes [Cu(6)(Lancillary)]+ with Lancillary ¼ 1–5 measured 3 days after cell
fabrication
Anchored dye
EQEmax/%
lmax/nm
[Cu(6)(1)]+
[Cu(6)(1)]+
[Cu(6)(2)]+
[Cu(6)(2)]+
[Cu(6)(3)]+
[Cu(6)(3)]+
[Cu(6)(4)]+
[Cu(6)(4)]+
[Cu(6)(5)]+
[Cu(6)(5)]+
N719
41.0
39.4
37.9
35.3
37.3
34.7
39.0
39.1
480
480
480
480
480
480
480
480
Fig. 9 J–V curves for DSCs containing the sensitizers [Cu(6)(1)]+,
[Cu(6)(2)]+, [Cu(6)(3)]+, [Cu(6)(4)]+ and [Cu(6)(5)]+; see also Fig. S4†.
Data were recorded on the day of DSC fabrication.
36.6 (sh. 19.5)
37.1 (sh. 19.5)
74.4
490 (sh. 570)
490 (sh. 570)
530
DSC performances
Table 1 gives the performance parameters for duplicate DSCs
containing [Cu(6)(1)]+, [Cu(6)(2)]+, [Cu(6)(3)]+, [Cu(6)(4)]+ and
[Cu(6)(5)]+ on the day of assembly (day 0) and aer 3 days. All
DSCs show similar ll factor (ff) values. Values of the open-
circuit voltage (VOC) for all the copper(I) dyes lie in the range
523–562 mV on day 0 and show a small gain over a 3 day ageing
period (Table 1). Improved performance over time is a known
phenomenon for copper(I)-containing dyes combined with an
screen printed TiO2. The values of JSC, VOC, ff and h of the
DSCs with [Cu(6)(2)]+ (Table 1) using commercial electrodes
–
with scattering layer and an I–/I3 electrolyte, compare favour-
ably with parameters (JSC ¼ 5.11 mA cmꢀ2, VOC ¼ 574 mV, ff ¼
–
71%, h ¼ 2.08%) for DSCs containing an I–/I3 electrolyte with
the dye [Cu(6)(2)]+ anchored on 4-layer screen-printed elec-
trodes post-treated with 40 mmol dmꢀ3 H2O–TiCl4.26
The current density/potential (J–V) curves recorded on the
day of device fabrication are shown in Fig. S4;† J–V curves for the
best performing device from each pair of duplicate DSCs are
displayed in Fig. 9. The DSCs sensitized with [Cu(6)(5)]+
outperform the other solar cells, the main contributing factor
being enhanced JSC values. This is consistent with extended
light absorption towards the red for complexes containing 5
(Fig. 8) and is conrmed by the higher external quantum effi-
ciencies (EQE) of the DSCs. EQE spectra for all devices are
shown Fig. S5,† and Fig. 10 depicts the spectra for the best
performing DSC of each pair; values of EQEmax and lmax are
given in Table 2. Although the values of EQEmax for [Cu(6)(1)]+
and [Cu(6)(4)]+ are higher than for [Cu(6)(5)]+ (Table 2), the
extension of the EQE spectrum of the DSCs with [Cu(6)(5)]+ to
longer wavelengths (Fig. 10 and S5†) leads to higher JSC values
with respect to DSCs with the other dyes. Fig. S6† shows a
comparison of the EQE spectra of duplicate DSCs containing
[Cu(6)(5)]+ with the EQE spectrum of an N719 sensitized DSC,
and demonstrates the origins of the lower values of JSC for the
copper(I) dye versus the ruthenium(II) reference dye.
–
I–/I3 electrolyte;24 it has also been reported for ruthenium(II)
dyes and appears to arise from disaggregation and reorganiza-
tion of the surface-bound dye molecules.43 For DSCs with
[Cu(6)(1)]+, [Cu(6)(2)]+, [Cu(6)(3)]+, [Cu(6)(4)]+ and [Cu(6)(5)]+,
the improved VOC values are not complemented by an increase
in the short-circuit current density (JSC) over time, and the
overall efficiencies (h) remain similar or decrease from day 0 to
day 3 (Table 1).
An important point is that these studies can be used to
validate comparisons between DSC data from our laboratory
where we use both commercial electrodes and those made in-
house. Images obtained using scanning electron microscopy
conrm that the z9 mm thickness of the transparent layer of a
commercial electrode44 corresponds to 4-layers of in-house
Conclusions
We have reported the synthesis and characterization of a series
of homoleptic [Cu(L)2][PF6] complexes in which
L is a
2,9-dimethyl-1,10-phenanthroline substituted in either the
5,6-positions with a peripherally-functionalized imidazole unit
or in the 4,7-positions with electron-donating 4-(diphenyla-
mino)phenyl groups. The solution absorption spectrum of
[Cu(5)2][PF6] exhibits a greater spectral response above 375 nm
than those of [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6] and
[Cu(4)2][PF6]. The heteroleptic dyes [Cu(6)(L)]+ were assembled
in a stepwise manner on TiO2 electrodes, and solid-state
Fig. 10 EQE spectra of DSCs containing the sensitizers [Cu(6)(1)]+,
[Cu(6)(2)]+, [Cu(6)(3)]+, [Cu(6)(4)]+ and [Cu(6)(5)]+; see also Fig. S5.† The
spectra were recorded 3 days after cell assembly.
69438 | RSC Adv., 2015, 5, 69430–69440
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absorption spectra conrmed enhanced absorption between
375–600 nm for [Cu(6)(5)]+ compared to [Cu(6)(1)]+, [Cu(6)(2)]+,
C. E. Housecro, P. Kopecky, M. Neuburger and
J. A. Zampese, Dalton Trans., 2011, 12584.
[Cu(6)(3)]+ and [Cu(6)(4)]+. Comparison of the performances of 11 N. Alonso-Vante, J.-F. Nierengarten and J.-P. Sauvage, J.
DSCs containing [Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+ with
those with [Cu(6)(1)]+ suggests only a marginal inuence of the 12 F. J. Malzner, S. Y. Brauchli, E. C. Constable, C. E. Housecro
diphenylamine or carbazole hole-transporting domains in and M. Neuburger, RSC Adv., 2014, 4, 48712.
5,6-substituted phenanthroline dyes. In ancillary ligand 5, the 13 M. Sandroni, L. Favereau, A. Planchat, H. Akdas-Kilig,
Chem. Soc., Dalton Trans., 1994, 1649.
4-(diphenylamino)phenyl hole-transporting units are intro-
duced directly into the 4- and 7-positions of the phen unit, and
N. Szuwarski, Y. Pellegrin, E. Blart, H. Le Bozec,
M. Boujtita and F. Odobel, J. Mater. Chem. A, 2014, 2, 9944.
this combined with a phosphonic anchoring domain in 14 A. Colombo, C. Dragonetti, M. Magni, D. Roberto,
[Cu(6)(5)]+ leads to the best performing DSCs of those investi-
gated. Although the values of EQEmax for [Cu(6)(1)]+ and
F. Demartin, S. Caramori and C. A. Bignozzi, ACS Appl.
Mater. Interfaces, 2014, 6, 13945.
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spectrum of the DSCs with [Cu(6)(5)]+ towards the red-end of the
spectrum results in higher JSC values with respect to DSCs with
the other dyes. We are currently exploring the effects on DSC
performance of introducing other substituents in the 4,7-posi-
tions of 2,9-dimethyl-1,10-phenanthrolines used as ancillary
ligands in heteroleptic copper(I) sensitizers, and are also
focusing on electrolyte optimization.
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Acknowledgements
We acknowledge nancial support from the European Research
Council (Advanced Grant 267816 LiLo), the Swiss National
Science Foundation (Grant 200020_144500) and the University
of Basel.
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69440 | RSC Adv., 2015, 5, 69430–69440
This journal is © The Royal Society of Chemistry 2015