Halos show the path to perfection: Peripheral iodo-substituents improve the efficiencies of bis(diimine)copper(i) dyes in DSCs

Article abstract of DOI:10.1039/c4ra06823h

The homoleptic copper(i) complexes [CuL₂][PF₆] (L = 4,4′-bis(4-halophenyl)-6,6′-dimethyl-2,2′-bipyridine with halogen = F (2) and Cl (3)) have been prepared and characterized, and their absorption spectroscopic and electrochemical properties compared to that with L = 4,4′-bis(4-bromophenyl)-6,6′-dimethyl-2,2′-bipyridine (4). The synthesis of [CuL₂][PF₆] (L = 4,4′-bis(4-iodophenyl)-6,6′-dimethyl-2,2′-bipyridine, 5) resulted in a mixture of [Cu(5)₂][PF₆] and [Cu(5)(MeCN)₂][PF₆]; variable temperature ¹H NMR spectroscopy confirmed that the complexes are in equilibrium in CD₃CN solution. The structure of [Cu(5)(MeCN)₂][PF₆] was determined by single crystal X-ray crystallography, and confirms a distorted tetrahedral geometry for the copper(i) centre. The heteroleptic dyes [Cu(1)(2)]⁺, [Cu(1)(3)]⁺, [Cu(1)(4)]⁺ and [Cu(1)(5)]⁺ (1 = ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid)) have been assembled by ligand exchange between [CuL₂]⁺ and TiO₂ functionalized with the anchoring ligand 1, and the performances of the dyes in fully masked dye-sensitized solar cells (DSCs) have been measured and compared. On the day of DSC fabrication, the trend for the global efficiencies, η, depends on the halo-substituent in the order I > F ≈ Br > Cl. Ripening of the DSCs occurs and after 7 days, the dependence of η on the halo-atom is in the order I > Cl ≈ F ≈ Br; the highest η is 3.16% for [Cu(1)(5)]⁺ compared to 7.63% for N719. Compared to the other halo-functionalized dyes, [Cu(1)(5)]⁺ shows an extended spectral response to longer wavelength, with enhanced electron injection. The results of DFT calculations suggest that the better dye performance of [Cu(1)(5)]⁺ may be associated with improved electron transfer over the halogen of the aryl substituent from the reduced electrolyte. The assembly of anchored dye [Cu(1)(5)]⁺ by treating functionalized-TiO₂ with a 1:1 mixture of [Cu(MeCN)₄]⁺ and 5, yields a dye which gives a DSC performance that matches that made by ligand exchange using [Cu(5)₂][PF₆] and [Cu(5)(MeCN)₂][PF₆].

Full text of DOI:10.1039/c4ra06823h

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Halos show the path to perfection: peripheral iodo-
substituents improve the efficiencies of bis(diimine)
copper(I) dyes in DSCs†
Cite this: RSC Adv., 2014, 4, 48712
*
*
Frederik J. Malzner, Sven Y. Brauchli, Edwin C. Constable, Catherine E. Housecroft
and Markus Neuburger
The homoleptic copper(I) complexes [CuL2][PF6] (L ¼ 4,40-bis(4-halophenyl)-6,60-dimethyl-2,20-bipyridine
with halogen ¼ F (2) and Cl (3)) have been prepared and characterized, and their absorption spectroscopic
and electrochemical properties compared to that with L ¼ 4,40-bis(4-bromophenyl)-6,60-dimethyl-2,20-
bipyridine (4). The synthesis of [CuL2][PF6] (L ¼ 4,40-bis(4-iodophenyl)-6,60-dimethyl-2,20-bipyridine, 5)
resulted in
a
mixture of [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6]; variable temperature 1H NMR
spectroscopy confirmed that the complexes are in equilibrium in CD3CN solution. The structure of
[Cu(5)(MeCN)2][PF6] was determined by single crystal X-ray crystallography, and confirms a distorted
tetrahedral geometry for the copper(I) centre. The heteroleptic dyes [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+
and [Cu(1)(5)]+ (1 ¼ ((6,60-dimethyl-[2,20-bipyridine]-4,40-diyl)bis(4,1-phenylene))bis(phosphonic acid))
have been assembled by ligand exchange between [CuL2]+ and TiO2 functionalized with the anchoring
ligand 1, and the performances of the dyes in fully masked dye-sensitized solar cells (DSCs) have been
measured and compared. On the day of DSC fabrication, the trend for the global efficiencies, h, depends
on the halo-substituent in the order I > F z Br > Cl. Ripening of the DSCs occurs and after 7 days, the
dependence of h on the halo-atom is in the order I > Cl z F z Br; the highest h is 3.16% for [Cu(1)(5)]+
compared to 7.63% for N719. Compared to the other halo-functionalized dyes, [Cu(1)(5)]+ shows an
extended spectral response to longer wavelength, with enhanced electron injection. The results of DFT
calculations suggest that the better dye performance of [Cu(1)(5)]+ may be associated with improved
electron transfer over the halogen of the aryl substituent from the reduced electrolyte. The assembly of
anchored dye [Cu(1)(5)]+ by treating functionalized-TiO2 with a 1 : 1 mixture of [Cu(MeCN)4]+ and 5,
yields a dye which gives a DSC performance that matches that made by ligand exchange using
[Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6].
Received 8th July 2014
Accepted 18th September 2014
DOI: 10.1039/c4ra06823h
www.rsc.org/advances
greater global abundance and lower cost of copper compared to
ruthenium make copper-containing dyes relevant and topical. A
Introduction
ground-breaking photoconversion efficiency for a DSC of 4.66%
has recently been reported by Odobel, Boujtita and coworkers
using a heteroleptic copper(I) dye containing the sterically
demanding 6,60-dimesityl-2,20-bipyridine-4,40-dicarboxylic acid
anchoring ligand and a 2,20-bipyridine ancillary ligand with
hole-transporting triphenylamino units; the high efficiency of
the DSC was achieved in part by using the co-adsorbent che-
nodeoxycholic acid (cheno).17
In contrast to the HETPHEN approach used by the Odobel
group,17 we have developed a stepwise method of assembling
copper(I) dyes in n-type DSCs commencing with the absorption
of an anchoring ligand, Lanchor, onto the n-type semiconductor
surface. Subsequent reaction of the functionalized surface with
a labile homoleptic complex [Cu(Lancillary)2]+ in solution leads to
the formation of surface-bound dye [Cu(Lanchor) (Lancillary)]+.4
Isolation of the heteroleptic complex is, therefore, avoided;
indeed, isolation is not usually possible because of rapid
Pioneering studies by Sauvage and co-workers in the 1990s
established the potential use of copper(I) complexes in dye-
sensitized solar cells (DSCs).1 Nonetheless, ruthenium(II)-con-
taining photosensitizers remain the mainstay of conventional
2
¨
Gratzel-type DSCs. For sustainable future technologies, it is
necessary to develop a materials chemistry in which scarce
elements are replaced by more abundant ones. We and others
are currently addressing this by focusing attention on
the application of copper(I)-containing sensitizers in DSCs.3–14
While the photophysical properties of [Cu(N^N)2]+ (N^N ¼ diimine
ligand) are similar to those of ruthenium(II) complexes,15,16 the
Department of Chemistry, University of Basel, Spitalstrasse 51, CH4056 Basel,
Switzerland. E-mail: edwin.constable@unibas.ch; catherine.housecro@unibas.ch;
Tel: +41 61 267 1008
† CCDC [1000780]. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4ra06823h
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residual solvent peaks with respect to d(TMS) ¼ 0 ppm. Solution
absorption spectra were recorded with a Cary 5000 spectro-
photometer and FT-IR spectra of solid samples on a Perkin
Elmer UATR Two spectrometer. Electrospray ionization (ESI)
mass spectra were recorded on a Bruker esquire 3000plus
instrument.
Electrochemical measurements were made using a CH
Instruments 900B potentiostat with glassy carbon, platinum
wire and silver wire as the working, counter, and reference
electrodes, respectively. Substrates were dissolved in HPLC
grade CH2Cl2 (ca. 10ꢀ4 to 10ꢀ5 mol dmꢀ3) containing 0.1 mol
dmꢀ3 [nBu4N][PF6] as the supporting electrolyte; all solutions
were degassed with argon. Cp2Fe was used as internal reference.
The scan rate was 0.1 V sꢀ1
.
Ground state density functional theory (DFT) calculations
were performed using Spartan 14 (v. 1.1.3) at the B3LYP level
with a 6-31G* basis set in vacuum. Initial energy optimization
was carried out at a semi-empirical (PM3) level.
Scheme 1 Structures of ligands with atom labelling for NMR spec-
The external quantum efficiency (EQE) measurements were
made using a Spe-Quest quantum efficiency instrument from
Rera Systems (Netherlands) equipped with a 100 W halogen
lamp (QTH) and a lambda 300 grating monochromator (Lot
Oriel). The monochromatic light was modulated to 3 Hz using a
chopper wheel (ThorLabs). The cell response was amplied with
a large dynamic range IV converter (CVI Melles Griot) and then
measured with a SR830 DSP Lock-In amplier (Stanford
Research).
Ligands 1 6 and 4 5, [Cu(MeCN)4][PF6]19 and [Cu(4)2][PF6]
were prepared by literature methods. Compound 3 was
prepared by adapting the literature method,20 replacing 1-(2-
oxopropyl)pyridin-1-ium bromide by the corresponding chlo-
ride salt. 1-(2-Oxopropyl)pyridin-1-ium chloride was purchased
from Fluka.
troscopic assignments.
equilibration between homo- and heteroleptic cations in solu-
tion to give a statistical mixture of species. In an earlier inves-
tigation, surface-bound [Cu(Lanchor)(Lancillary)]+ species were
characterized by MALDI-TOF mass spectrometry and diffuse
reectance electronic absorption spectroscopy.18 The favoured
anchoring ligand is the bis(phosphonic acid) 1 (Scheme 1), with
the spacer between the 2,20-bipyridine and phosphonic acid
anchoring domains leading to enhanced performance of the
dye.6 Our dye assembly strategy is advantageous in that it
permits rapid screening of different families of ancillary
ligands, and has recently been implemented by the Robertson
group.9 However a disadvantage is the wastage of one equivalent
of ancillary ligand, and this is particularly unsatisfactory when
synthesis of the latter is a labour intensive multistep procedure.
Recently, we demonstrated that masked DSCs containing the
dye [Cu(1)(Lancillary)]+ in which Lancillary is 4,40-bis(4-bromo-
phenyl)-6,60-dimethyl-2,20-bipyridine reached power conversion
efficiencies of 2.31%, compared to 8.30% for standard dye N719.6
Since ancillary ligands in n-type dyes typically incorporate elec-
tron-donating domains, we were somewhat surprised that dyes
containing peripheral bromophenyl substituents (selected to
provide an active site for further derivatization) performed rela-
tively well.6 We were, therefore, prompted to study the effects of
altering the halo-substituent and now report the remarkably high
power conversion efficiencies of DSCs incorporating the series of
ancillary ligands 2–5 (Scheme 1). We also illustrate that complete
conversion of [Cu(MeCN)4]+ to [Cu(Lancillary)2]+ is not an essential
step prior to ligand exchange on the functionalized TiO2 surface,
and introduce a stepwise strategy for in situ assembly of the
surface-anchored copper(I) dye.
(1E,5E)-1,6-Bis(4-uorophenyl)hexa-1,5-diene-3,4-dione.
A
solution of butane-2,3-dione (1.98 mL, 22.7 mmol) in MeOH
(10 mL) was added dropwise over a 30 min. period to a vigor-
ously stirred solution of 4-uorobenzaldehyde (5.00 mL, 45.4
mmol) and piperidine (0.49 mL, 4.54 mmol) in MeOH (30 mL).
MeOH (10 mL) was added through the dropping funnel and the
reaction mixture was stirred at room temperature for 1 h and
then heated at reux overnight. The solution was slowly cooled
to room temperature over 1 h with stirring and was placed in the
fridge for a few minutes. The precipitate that formed was
collected by ltration, washed with Et2O and dried in air.
(1E,5E)-1,6-Bis(4-uorophenyl)hexa-1,5-diene-3,4-dione was iso-
lated as an orange solid (0.425 g, 1.42 mmol, 6.27%). Spectro-
scopic data were consistent with those reported.21
(1E,5E)-1,6-Bis(4-iodophenyl)hexa-1,5-diene-3,4-dione. The
method was as for (1E,5E)-1,6-bis(4-uorophenyl)hexa-1,5-
diene-3,4-dione, starting with 2,3-butanedione (0.57 mL, 6.46
mmol) in MeOH (8 mL) and a solution of 4-iodobenzaldehyde
(3.00 g, 12.9 mmol) and piperidine (0.13 mL, 1.29 mmol) in
MeOH (25 mL). The product was recrystallized in MeOH.
(1E,5E)-1,6-Bis(4-iodophenyl)hexa-1,5-diene-3,4-dione was iso-
lated as a brown solid (1.02 g, 1.98 mmol, 30.7%). m.p. 236 ꢁC.
Experimental
General
A Bruker Avance III-500 NMR spectrometer was used to record 1H NMR (500 MHz, CDCl3) d/ppm: 7.78 (d, J ¼ 16.1 Hz, 2H, Ha),
1H and 13C NMR spectra, and chemical shis were referenced to 7.78 (d, J ¼ 8.4 Hz, 4H, HA2), 7.50 (d, J ¼ 16.2 Hz, 2H, Hb), 7.37
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(d, J ¼ 8.4 Hz, 4H, HA3). 13C NMR (126 MHz, CDCl3) d/ppm: reduced pressure. The product was again precipitated by addi-
188.2 (CC]O), 146.6 (Ca), 138.3 (CA2), 133.7 (CA1), 130.3 (CA3), tion of Et2O, was washed with cold Et2O and dried in air.
119.8 (Cb), 98.1 (CA4). IR (n/cmꢀ1): 1670 (s), 1597 (s), 1578 (m), [Cu(3)2][PF6] was isolated as a dark red solid (276 mg, 0.289
1553 (m), 1480 (m), 1394 (m), 1312 (w), 1293 (w), 1178 (m), 1057 mmol, 71.7%). 1H NMR (500 MHz, CD3CN) d/ppm: 8.66 (d, J ¼
(w), 981 (s), 802 (s), 726 (s), 584 (s), 515 (w), 453 (m). Found: C 1.0 Hz, 4H, HA3), 8.00 (m, 8H, HB2), 7.81 (d, J ¼ 1.0 Hz, 4H, HA5),
40.57, H 2.48; C18H12I2O2$H2O requires C, 40.63, H, 2.65.
7.36 (m, 8H, HB3), 2.35 (s, 12H, HMe). 13C NMR (126 MHz,
Compound 2. 1-(2-Oxopropyl)pyridin-1-ium chloride (1.01 g, CD3CN) d/ppm: 164.8 (d, JCF ¼ 248.0 Hz, HB4), 158.7 (CA6), 153.4
5.92 mmol) was dissolved in EtOH (20.0 mL) and then (1E,5E)- (CA2), 150.0 (CA4), 134.4 (d, JCF ¼ 3.4 Hz, CB1), 130.5 (d, JCF ¼ 8.7
1,6-bis(4-uorophenyl)hexa-1,5-diene-3,4-dione (0.700 g, 2.35 Hz, CB2), 124.3 (CA5), 118.6 (CA3), 117.0 (d, JCF ¼ 22.0 Hz, CB3),
mmol) and NH4OAc (2.71 g, 35.2 mmol) were added under 25.2 (CMe). ESI MS m/z positive mode 807.6 [M ꢀ PF6]+ (calc.
vigorous stirring. Additional EtOH (30 mL) was then added and 807.2), 373.3 [2 + H]+ (calc. 373.1, base peak); negative mode
the reaction mixture was heated at reux overnight. Aer cool- 145.0 [PF6]ꢀ (calc. 145.0). UV-Vis (CH2Cl2, 1.0 ꢂ 10ꢀ5 mol dmꢀ3
)
ing to room temperature while being stirred, the mixture was l/nm 276 (3/dm3 molꢀ1 cmꢀ1 73 200), 321 (39 300), 347 (9500), 483
placed in a fridge for 1 h. The precipitate that formed was (12 600). Found: C 59.91, H 4.30, N 6.75; C48H36CuF10N4P$MeCN
separated by ltration, washed with cold EtOH and Et2O, and requires C 60.39, H 4.08, N 7.04.
dried in air. Compound 2 was isolated as a white solid (0.461 g,
[Cu(3)2][PF6]. [Cu(MeCN)4][PF6] (276 mg, 0.740 mmol) was
1
ꢁ
1.24 mmol, 52.8%). m.p. 232 C. H NMR (500 MHz, CDCl3) d/ added to a stirring solution of 3 (600 mg, 1.48 mmol) in a
ppm: 8.63 (broadened s, 2H, HA3), 7.83 (m, 4H, HB2), 7.45 (s, 2H, mixture of CH2Cl2 (16 mL) and MeCN (4 mL). The reaction
HA5), 7.22 (m, 4H, HB3), 2.81 (s, 6H, HMe). 13C NMR (126 MHz, mixture was stirred at room temperature overnight. The
CDCl3) d/ppm: 163.5 (d, J ¼ 249 Hz, CB4), 158.7 (CA6), 156.6 (CA2), CH2Cl2 was then removed under reduced pressure and the
148.6 (CA4), 135.0 (d, J ¼ 3.3 Hz, CB1), 129.1 (d, J ¼ 8.3 Hz, CB2), volume of MeCN was reduced to a minimum volume that
121.2 (CA5), 116.6 (CA3), 116.1 (d, J ¼ 21.6 Hz, CB3), 24.9 (CMe). IR retained the product in solution. The product was precipitated
(n/cmꢀ1): 1607 (m), 1595 (s), 1551 (s), 1511 (s), 1417 (w), 1382 by addition of Et2O and was separated by ltration, was
(m), 1303 (w), 1226 (s), 1163 (s), 1100 (w), 873 (m), 824 (s), 813 washed with cold Et2O and dried in air. [Cu(3)2][PF6] was iso-
(s), 720 (w), 566 (s), 551 (s), 512 (s), 471 (s). ESI MS m/z 373.3 [M + lated as a dark red solid (718 mg, 0.705 mmol, 95.2%). 1H NMR
H]+ (calc. 373.4). UV-Vis (CH2Cl2, 1.0 ꢂ 10ꢀ5 mol dmꢀ3) l/nm (500 MHz, CD3CN) d/ppm: 8.66 (s, 4H, HA3), 7.95 (d, J ¼ 8.2 Hz,
248 (3/dm3 molꢀ1 cmꢀ1 44 000), 302 (14 900). Found: C 74.12, H 8H, HB2), 7.82 (s, 4H, HA5), 7.63 (d, J ¼ 8.2 Hz, 8H, HB3), 2.35 (s,
4.90, N 7.53; C24H18F2N2$H2O requires C 73.83, H 5.16, N 7.18. 12H, HMe). 13C NMR (126 MHz, CD3CN) d/ppm: 158.8 (CA6),
Compound 5. The procedure was as for 2, starting with 1-(2- 153.4 (CA2), 149.7 (CA4), 136.7 (CB1), 136.5 (CB4), 130.3 (CB3),
oxopropyl)pyridin-1-ium chloride (0.668 g, 3.89 mmol), (1E,5E)- 130.0 (CB2), 124.4 (CA5), 118.6 (CA3), 25.2 (CMe). ESI MS m/z
1,6-bis(4-iodophenyl)hexa-1,5-diene-3,4-dione (0.800 g, 1.56 positive mode 873.5 [M ꢀ PF6]+ (calc. 873.1), 405.3 [3 + H]+
mmol) and NH4OAc (1.80 g, 23.3 mmol) were added. The (calc. 405.1, base peak); negative mode 144.9 [PF6]ꢀ (calc.
product was recrystallized from MeOH and again from CHCl3/ 145.0). UV-Vis (CH2Cl2, 1.0 ꢂ 10ꢀ5 mol dmꢀ3) l/nm 278 (3/dm3
Et2O. Further purication was needed. A suspension in CHCl3 molꢀ1 cmꢀ1 86 500), 322 (46 500), 353 sh (10 400), 485 (11 300).
was ltered and 5 was isolated by column chromatography Found: C 53.45, H 3.76, N 5.69; C48H36Cl4CuF6N4P$3H2O
(SiO2, 4 ꢂ 20 cm, CH2Cl2–EtOAc, 4 : 1) as an off-white solid (93.3 requires C 53.72, H 3.94, N 5.22.
mg, 0.160 mmol, 10.3%). Dec. > 296 ꢁC. 1H NMR (500 MHz,
[Cu(5)2][PF6] and [Cu(5) (MeCN)2][PF6]. The method and
CDCl3) d/ppm: 8.81 (broadened s, 2H, HA3), 7.87 (m, 4H, HB3), initial purication was as for [Cu(2)2][PF6], starting with
7.66 (m, 4H, HB2), 7.52 (s, 2H, HA5), 2.90 (s, 6H, HMe). 13C NMR [Cu(MeCN)4][PF6] (15.8 mg, 0.043 mmol) and 5 (50 mg, 0.085
(126 MHz, CDCl3 d/ppm: 158.0 (CA6), 155.3 (CA4), 153.5 (CA2), mmol). Aer the second precipitation with Et2O (as in puri-
138.4 (CB3), 136.2 (CB1), 129.2 (CB3), 122.6 (CA5), 121.9 (CB4), cation of [Cu(2)2][PF6]), the product was redissolved again in
119.4 (CA3), 23.2 (CMe). IR (n/cmꢀ1): 1594 (m), 1569 (w), 1542 (w), MeCN and the solution split into 4 vials. Addition of Et2O
1486 (m), 1376 (m), 1004 (s), 817 (s), 745 (s), 511 (w), 472 (s). ESI resulted in the formation of a white precipitate; the red
MS m/z 589.2 [M + H]+ (calc. 589.0). UV-Vis (CH2Cl2, 1.0 ꢂ 10ꢀ5 solution was decanted from each vial and le to stand at room
mol dmꢀ3) l/nm 260 (3/dm3 molꢀ1 cmꢀ1 46 700), sh 309 temperature. Orange crystals grew in each vial and some of X-
(23 200). Found: C 48.87, H 3.34, N 4.27; C24H18I2N2 requires C ray quality were selected. The red mother liquors were
49.00, H 3.08, N 4.76.
combined and solvent removed to yield a red solid which
[Cu(2)2][PF6]. [Cu(MeCN)4][PF6] (150 mg, 0.430 mmol) was analysed as a mixture of [Cu(5)2][PF6] and [Cu(5) (MeCN)2][PF6]
added to a stirring solution of 2(300 mg, 0.806 mmol) in a (see text). 1H NMR (500 MHz, CD3CN, 295 K cations in
mixture of CH2Cl2 (16 mL) and MeCN (4 mL). The reaction exchange) d/ppm 8.61 (broad, HA3), 7.97 (d, J ¼ 7.6 Hz, 8H, HB3),
mixture was stirred at room temperature overnight. The CH2Cl2 7.85 (broad, HA5), 7.71 (d, J ¼ 7.3 Hz, 8H, HB2), 2.52 (v br, FWHM
was then removed under reduced pressure and the volume of ꢃ145 Hz, HMe). 13C NMR (126 MHz, CD3CN, 295 K cations in
MeCN was reduced to a minimum volume that retained the exchange) d/ppm: 150.0 (CA4), 137.6 (CB1), 130.2 (CB3), 130.2
product in solution. The product was precipitated by addition of (CB2), 124.5 (CA5), 118.6 (CA3), 96.7 (HB4), signals for CA6, CA2 and
Et2O and was separated by ltration, was washed with cold Et2O CMe not resolved. ESI MS m/z positive mode 1239.3 [Cu(5)2]+
and dried in air. The crude product was redissolved in MeCN, (calc. 1238.8), 692.2 [Cu(5)(MeCN)]+ (calc. 691.9), 589.2 [5 + H]+
the solution was ltered and the ltrate concentrated under (calc. 589.0, base peak); negative mode 144.9 [PF6]ꢀ (calc. 145.0).
48714 | RSC Adv., 2014, 4, 48712–48723
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For crystals of [Cu(5) (MeCN)2][PF6]: found C 38.09, H 3.02, N
5.84; C28H24CuF6I2N4P requires C, 38.27, H, 2.75, N, 6.38.
Results and discussion
Ligand synthesis and characterization
28
¨
We have previously used the strategy of Krohnke to prepare
compounds 3 20 and 4 5 and now extend the series of halo-
derivatives to compounds 2 and 5 (Scheme 2). The yield of 2 was
only moderate (52.8%), and 5 proved very difficult to purify and
pure material was obtained in only 10.3% yield. The electro-
spray mass spectra of 2 and 5 exhibited base peaks at m/z 373.3
and 589.2, respectively, corresponding to [M + H]+. The 1H and
13C NMR spectra were assigned by COSY, HMQC and HMBC
methods and are consistent with the disubstitution pattern
shown in Scheme 2. Broadening of the signals for HA3 and HA5
(see Scheme 1 for atom labelling) is most likely associated with
rotation of the 4-halophenyl groups on the NMR timescale. At
500 MHz, values of FWHM for the signals for HA3 and HA5 are 36
and 12 Hz, respectively in 2, and 21 and 17 Hz in 5. The solution
absorption spectra of compounds 2–5 are compared in Fig. 1,
the intense high energy bands arising from p* ) p and p* ) n
transitions. The highest energy absorption shis from 248 nm
in 2 to 260 nm in 5.
Crystallography
Single crystal data were collected on a Bruker APEX-II
diffractometer with data reduction, solution and renement
using the programs APEX22 and CRYSTALS.23 The ORTEP-
type diagram and structure analysis used Mercury v. 3.0.24,25
[Cu(5)(MeCN)2][PF6]. C28H24CuF6I2N2P, M ¼ 878.84, orange
block, monoclinic, space group C2/c, a ¼ 9.9980(4), b ¼
3
ꢁ
˚
˚
29.1275(18),c ¼ 13.5247(8) A, b ¼ 130.048(2) , U ¼ 6080.0(7) A , Z
¼ 8, Dc ¼ 1.920 mg mꢀ3, m(Cu-Ka) ¼ 18.021 mmꢀ1, T ¼ 123 K.
Total 28 350 reections, 5520 unique, Rint ¼ 0.027. Renement of
5460 reections (380 parameters) with I > 3s(I) converged at nal
R1 ¼ 0.0250 (R1 all data ¼ 0.0251), wR2 ¼ 0.0261 (wR2 all data ¼
0.0263), gof ¼ 1.0853.
DSC fabrication and measurements
26,27
¨
DSCs were prepared modifying the method of Gratzel.
Synthesis and characterization of copper(I) complexes
Commercial Solaronix Test Cell Titania Electrodes were used.
The electrodes were rinsed with ꢁEtOH and sintered at 450 ꢁC
for 30 min, then cooled to z80 C and immersed in a 1 mM
DMSO solution of the anchoring ligand 1 for 24 h. The col-
ourless electrode was removed from the solution, washed with
DMSO and EtOH and dried at z60 ꢁC (heat gun). The elec-
trode with adsorbed 1 was immersed in a 0.1 mM CH2Cl2
solution of each copper(I) complex for z68 h. For the nal set
of DSCs (see text), the electrode with adsorbed anchoring
ligand was immersed in a CH2Cl2 solution containing equi-
molar (0.1 mM) amounts of [Cu(MeCN)4][PF6] and ancillary
ligand 5 for 68 h; during this time, the electrodes turned pale
red-orange.
We have described the complex [Cu(4)2][PF6] previously,6 and
the same simple strategy was adopted to prepare analogous
homoleptic complexes containing ligands 2, 3 or 5. Treatment
of [Cu(MeCN)4][PF6] with two equivalents of 2 or 3 gave
[Cu(2)2][PF6] or [Cu(3)2][PF6] as orange-red solids in 71.7 and
95.2% yields. The highest mass peak envelope in the electro-
spray mass spectrum of each complex corresponded to the
[M ꢀ PF6]+ ion, and isotope patterns matched those calculated.
The base peak in each spectrum arose from [L + H]+ (L ¼ 2 or 3).
Each reference electrode was prepared by dipping a Solar-
onix Test Cell Titania Electrode in a 0.3 mM EtOH solution of
standard dye N719 (Solaronix) for z68 h. The electrodes were
washed with the same solvent used in the dipping period and
dried at z60 ꢁC (heat gun). A Solaronix Test Cell Platinum
Electrode was used for the counter electrode, and it was heated
ꢁ
for 30 min at 450 C (heating plate) to remove impurities.
The two electrodes were combined using thermoplast hot-
melt sealing foil (Solaronix Test Cell Gaskets) by heating while
pressing them together. The electrolyte was a mixture of LiI
(0.1 mol dmꢀ3), I2 (0.05 mol dmꢀ3), 1-methylbenzimidazole
(0.5 mol dmꢀ3) and 1-butyl-3-methylimidazolinium iodide (0.6
mol dmꢀ3) in 3-methoxypropionitrile; it was introduced into
the DSC by vacuum backlling. The hole on the counter elec-
trode was sealed using hot-melt sealing foil (Solaronix Test
Cell Sealings) and a cover glass (Solaronix Test Cell Caps).
Measurements were made by irradiating from behind using a
light source SolarSim 150 (100 mW cmꢀ2 ¼ 1 sun). The
power of the simulated light was calibrated by using a refer-
ence Si cell.
Scheme 2 Synthesis of ligands 2 and 5 using the Krohnke approach.
Atom numbering for NMR assignments of the precursors.
¨
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pure [Cu(5)2][PF6]. In contrast to the sharp singlet for the methyl
protons in [Cu(2)2][PF6] and [Cu(3)2][PF6], the room temperature
1H NMR spectrum of the iodo-containing product showed a very
broad signal centred at d 2.52 ppm with FWHM ꢃ145 Hz. The
13C NMR resonance for CMe could not be resolved at 295 K and
no cross peak for the HMe signal was observed in the HMQC
spectrum. The signals for HA3 and HA5 were also broad (Fig. 2c).
Cooling the sample to 240 K led to collapse of the broad peaks
and appearance of two sets of signals (Fig. 3), consistent with
two environments for coordinated ligand 5 (the free ligand is
poorly soluble in acetonitrile and therefore signals arising from
ligand can be discounted). The separations of pairs of signals
for HMe indicate that the two methyl groups are signicantly
different in the two species (d 2.26 and 2.83 ppm), and similarly
for the two HA3 protons (d 8.42 and 8.66 ppm). The local envi-
ronment of HA5 appears similar in the two species, and similarly
for HB2 and HB3. We propose that the bulk material contains a
mixture of [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6], and that at
295 K, ligand exchange occurs between [Cu(5)(MeCN)2]+ and
[Cu(5)2]+. This is supported by the electrospray mass spectrum
of the bulk material which exhibited peak envelopes with a
characteristic copper isotope pattern at m/z 1239.3 and 692.2
arising from [Cu(5)2]+ and [Cu(5)(MeCN)]+; the base peak (m/z
589.2) was assigned to [5 + H]+.
The identity of [Cu(5)(MeCN)2][PF6] as one component of the
mixture was conrmed by a single crystal structure determina-
tion of orange blocks that grew during recrystallization of the
bulk material (see Experimental section). Fig. 4 shows the
structure of the [Cu(5)(MeCN)2]+ cation in [Cu(5)(MeCN)2][PF6].
The complex crystallizes in the monoclinic space group C2/c.
Atom Cu1 is in a distorted tetrahedral environment; the bite
angle of the bpy unit is 81.06(7)ꢁ, and other N–Cu–N bond
angles range from 106.24(9) to 124.31(8)ꢁ (caption to Fig. 4). The
bpy domain deviates from planarity with the angle between the
planes of the pyridine rings being 13.8ꢁ. The phenyl ring con-
taining C12 is twisted through 15.5ꢁ with respect to the plane of
the pyridine ring with N1, while the corresponding angle for the
Fig. 1 Absorption spectra of CH2Cl2 solutions of ligands 2–5 (1.0 ꢂ
10ꢀ5 mol dmꢀ3).
The 1H and 13C NMR spectra of a CD3CN solution of
[Cu(2)2][PF6] were consistent with a single ligand environment,
1
and the aromatic region of the H NMR spectrum is shown in
Fig. 2a. Coupling to 19F gives characteristic signals for protons
HB2 and HB3 and doublets for all ring B resonances in the 13C NMR
spectrum (see Scheme 1 for numbering), and the assignments
were conrmed through the HMBC and HMQC spectra. The
methyl groups give rise to sharp singlets at d 2.35 and 25.2 ppm in
the 1H and 13C{1H} NMR spectra, respectively, of [Cu(2)2][PF6], and
these shis are unchanged on going to [Cu(3)2][PF6]. The aromatic
region of the 1H NMR spectrum of a CD3CN solution of
[Cu(3)2][PF6] is shown in Fig. 2b. The change from uoro to chloro
substituent has the greatest effect on HB3 in keeping with
expectations.29
Preparation of the iodo-containing complex [Cu(5)2][PF6]
proved more problematical. The method used was as for the
uoro-, chloro- and bromo-containing complexes, but repeated
purication of the bulk sample failed to produce analytically
Fig. 2 500 MHz 1H NMR spectra (aromatic region only) of CD3CN Fig. 3 Variable temperature 500 MHz 1H NMR spectra of the bulk
solutions of (a) [Cu(2)2][PF6], (b) [Cu(3)2][PF6] and (c) [Cu(5)2][PF6] (see material containing [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6]. * ¼ residual
text).
H2O in CD3CN.
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addition this anion exhibits a short F/I contact (F3/I2i ¼
˚
3.406(1) A, symmetry code i ¼ 1/2 ꢀ x, ꢀ1/2 + y, 1/2 ꢀ z). In
contrast, the anion containing P2 (red in Fig. 5b) sits between
the p-stacked cations and interacts with methyl groups of both
coordinated ligands 5 and MeCN.
Solution absorption spectra and electrochemistry
Absorption spectroscopic and electrochemical data were recor-
ded only for [Cu(2)2][PF6] and [Cu(3)2][PF6] since the homoleptic
complex with ligand
5 could not be obtained free of
[Cu(5)(MeCN)2][PF6]. The absorption spectra are shown in Fig. 6
and are compared with that of [Cu(4)2][PF6].6 Approximate
doubling of the extinction coefficients on going from the free
ligands (Fig. 1) to the complexes is consistent with the forma-
tion of the homoleptic species. The intense, high-energy bands
are similar for all the complexes, and a small red-shi is
observed for the MLCT band from 483 nm for the uoro-con-
taining complex to 488 nm 6 for the bromo-derivative. The
mixture of [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6] has an MLCT
maximum at 488 nm.
The electrochemical behaviour of [Cu(2)2][PF6], [Cu(3)2][PF6]
and [Cu(4)2][PF6] are compared in Table 1, and a representative
cyclic voltammogram (with internal Fc/Fc+ reference) is shown
in Fig. 7. Each complex exhibits a copper-centred oxidation
process and the introduction of the F, Cl or Br substituents has
only a small effect on its potential. In each of [Cu(2)2][PF6] and
[Cu(3)2][PF6], the irreversible ligand-centred reduction process
ca. ꢀ2.0 V is more pronounced in the rst cycle than in
subsequent scans.
Fig. 4 Structure of the [Cu(5)(MeCN)2]+ cation in [Cu(5)(MeCN)2][PF6]
with ellipsoids plotted at 50% probability level. Selected bond
parameters: Cu1–N1 ¼ 2.0515(19), Cu1–N2 ¼ 2.0527(18), Cu1–N3.
1.949(2), Cu1–N4 ¼ 1.968(2), I1–C15 ¼ 2.092(2), I2–C21 ¼ 2.098(2) A˚;
N1–Cu1–N2 ¼ 81.06(7), N1–Cu1–N3 ¼ 124.31(8), N2–Cu1–N3 ¼
114.59(8), N1–Cu1–N4
¼
110.66(8), N2–Cu1–N4
¼
119.27(8),
N3–Cu1–N4 ¼ 106.24(9)ꢁ.
rings containing C18 and N2 is 38.7ꢁ. The difference in twist
angles is associated with the packing of the cations. Face-to-face
p-stacking occurs between phenylbpy domains involving the
phenyl ring containing atom C12 and this leads to innite
columns of cations running parallel to the c-axis (Fig. 5a). The
stacks are built up by alternating operations of a 2-fold axis
followed by an inversion centre. The asymmetric unit contains
two half-anions; each [PF6]ꢀ is ordered, and atoms P1 and P2
are each located on a 2-fold axis. Cation/anion interactions
involve extensive CH/F contacts. For the anion containing P1
(green in Fig. 5b), these contacts involve arene CH units; in
Comparison of the performances of the copper(I) dyes in DSCs
Surface-immobilized copper(I) dyes were assembled on
commercial titania electrodes by initial adsorption of the
phosphonic acid anchoring ligand 1 followed by treatment with
CH2Cl2 solutions of the labile30–32 homoleptic complexes
[CuL2][PF6] (L ¼ 2, 3 or 4) or a mixture of [Cu(5)2][PF6] and
[Cu(5) (MeCN)2][PF6] (Scheme 3). All cells were fully masked,33
and were prepared in duplicate. The electrolyte contained the
I3ꢀ/Iꢀ redox shuttle. The DSC parameters measured for each cell
are given in Table 2 and are compared to data for a DSC with
standard dye N719. The right-hand column in Table 2 gives the
Fig. 5 (a) Face-to-face p-stacking of phenylbpy domains (space-
filling representation) involving the phenyl ring with C12 leading to
infinite assemblies following the c-axis. View down the c-axis showing Fig. 6 Solution (CH2Cl2) absorption spectra of the fluoro, chloro and
the two different [PF6]ꢀ environments. Anion with P1 is shown in green, bromo-containing complexes [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6].
and with P2, in red; CH/F contacts are shown by red hashed lines.
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Table 1 Cyclic voltammetric data for [CuL2][PF6] with L ¼ 2–4 with Table 2 Performance data for two independent sets of sealed and
respect to Fc/Fc+; CH2Cl2 solutions with [nBu4N][PF6] as supporting masked DSCs with copper(I) anchored dyes; data are with respect to
electrolyte and scan rate of 0.1 V sꢀ1. Processes are reversible unless standard dye N719 measured under the same conditions. JSC ¼ short-
otherwise stated (ir ¼ irreversible)
circuit current density; VOC ¼ open-circuit voltage; ff ¼ fill factor
Complex
E1o/x2/V (Epc ꢀ Epa/mV)
Er1e/2d/V
Er1e/2d/V
Ref.
Relative
h/%
Dye
JSC/mA cmꢀ2
VOC/mV
ff
h/%
[Cu(2)2][PF6]
[Cu(3)2][PF6]
[Cu(4)2][PF6]
+0.39 (74)
+0.43 (67)
+0.42 (94)
ꢀ2.08ir
ꢀ2.06ir
ꢀ2.16ir
ꢀ2.42ir
ꢀ2.42ir
this work
this work On the day of sealing the cell
6
[Cu(1)(2)]+
[Cu(1)(2)]+
[Cu(1)(3)]+
[Cu(1)(3)]+
N719a
6.70
6.37
5.59
6.00
16.66
6.64
6.49
6.68
6.92
16.08
527
544
519
522
637
514
525
582
576
641
68.3
70.0
71.5
69.5
67.2
69.3
71.7
74.0
72.6
72.0
2.41
2.42
2.08
2.18
7.13
2.37
2.45
2.88
2.89
7.43
33.8
33.9
29.2
30.6
100
[Cu(1)(4)]+
[Cu(1)(4)]+
[Cu(1)(5)]+
[Cu(1)(5)]+
N719b
31.9
33.0
38.8
38.9
100
3 days aer sealing the cell
[Cu(1)(2)]+
[Cu(1)(2)]+
[Cu(1)(3)]+
[Cu(1)(3)]+
N719a
6.49
6.19
6.28
6.69
16.82
5.85
6.18
6.81
7.07
15.87
544
570
566
562
667
554
562
604
586
664
69.8
69.5
69.7
68.0
67.7
68.1
70.3
74.0
72.6
72.3
2.46
2.45
2.48
2.56
7.59
2.21
2.44
3.04
3.01
7.61
32.4
32.2
32.7
33.7
100
[Cu(1)(4)]+
[Cu(1)(4)]+
[Cu(1)(5)]+
[Cu(1)(5)]+
N719b
29.0
32.1
39.9
39.6
Fig. 7 Cyclic voltammogram for a CH2Cl2 solution of [Cu(2)2][PF6];
scan rate 0.1 V sꢀ1 and referenced internally to Fc/Fc+.
100
7 days aer sealing the cell
[Cu(1)(2)]+
[Cu(1)(2)]+
[Cu(1)(3)]+
[Cu(1)(3)]+
N719a
6.62
6.20
6.27
6.95
16.42
6.55
6.46
7.10
7.42
16.21
556
578
559
565
692
559
561
604
579
662
69.1
68.9
68.1
66.2
66.3
61.6
67.5
73.6
71.5
71.1
2.54
2.47
2.39
2.60
7.53
2.26
2.45
3.16
3.07
7.63
33.7
32.8
31.7
34.5
100
[Cu(1)(4)]+
[Cu(1)(4)]+
[Cu(1)(5)]+
[Cu(1)(5)]+
N719b
29.6
32.1
41.4
40.2
100
a
N719 reference cell used in conjunction with DSCs containing dyes
[Cu(1)(2)]+ and [Cu(1)(3)]+. N719 reference cell used with dyes
b
[Cu(1)(4)]+ and [Cu(1)(5)]+.
different solar simulators.34 For each DSC, the efficiencies were
measured on the day of sealing the cell and then three and
seven days later.
The rst point to note is that the DSC parameters for the dye
[Cu(1)(4)]+ (bromo-substituents) are comparable with those we
have previously reported,6 despite differences in electrode
origins. In the present work, the photoanodes are commercially
available titania electrodes which include a scattering layer; in
our previous study, screen-printed electrodes with scattering
layer were prepared in our laboratory. A second important point
is that the data in Table 2 for corresponding pairs of DSCs
conrm reproducibility of measurements.
Scheme 3 Assembly of heteroleptic copper(I) dyes using anchoring
ligand 1.
On the day of cell fabrication, all four dyes (Table 2) perform
relatively well with the global efficiencies, h, dependent upon
the halo-substituent in the order I > F z Br > Cl. Upon aging of
the DSCs, there is a general trend for improvement of
energy conversion efficiencies, h, relative to N719 arbitrarily set
to 100%. We have recently adopted this presentation of results
to provide a valid means of comparing data recorded using
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performance (Table 2). Over a three day period, the DSCs con- signicant features in Fig. 8 are the enhancements in both
taining the iodo-substituted dye [Cu(1)(5)]+ show a ripening short-circuit current density and open-circuit voltage over time
effect with h increasing from 2.88 to 3.01%, and 2.89 to 3.04% for the chloro- and iodo-containing dyes (Fig. 8b and c). For
for the two independent DSCs. Aer a further four days, the [Cu(1)(5)]+ (iodo), a maximum value of VOC ¼ 604 mV is achieved
efficiencies increase to 3.16 and 3.07%, respectively. This aer 3 days with no further improvement over the next 4 days
enhancement is likely associated with initial dye aggregation on (Fig. 8c); 604 mV compares with VOC(max) ¼ 664 mV for N719
the surface followed by molecular reorganization over time.35–37 measured under the same conditions are the copper(I) dyes
In terms of h, the best performing DSCs aer 7 days are those (Table 2). Enhancement in the open-circuit voltage with aging of
with iodo-dye [Cu(1)(5)]+ which show power conversion effi- the DSC has also been noted for other copper(I) dyes.38
ciencies of 40.2 or 41.4% with respect to N719 set at 100%. For
the aged DSCs, the dependence of h on the halo-substituent a week following cell fabrication. All show EQE maxima corre-
follows the trend I > Cl z F z Br. sponding to lmax in the range 460–480 nm (Table 3). The values
The EQE spectra of the DSCs were measured over a period of
Fig. 8 shows J–V curves for DSCs containing anchored dyes of EQEmax for the copper(I) dyes compare to EQEmax z 75% for
[Cu(1)(2)]+, [Cu(1)(3)]+ and [Cu(1)(5)]+; data for the bromo-con- N719 (Fig. 9). Within experimental error, the EQE values for the
taining dye [Cu(1)(4)]+ essentially replicate those already pub- cells containing [Cu(1)(2)]+ and [Cu(1)(4)]+ do not change with
lished.6 All J–V curves show good ll factors. The most time, remaining around 46–47%. For DSCs with [Cu(1)(3)]+
(chloro) or [Cu(1)(5)]+ (iodo), a small enhancement in the EQE is
observed up to 50 or 51%, respectively aer 3 days with no
further improvement (Fig. 9). Fig. 10 compares the EQE spectra
for DSCs containing [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ or
[Cu(1)(5)]+. The most signicant feature is the extended spectral
response to longer wavelength for [Cu(1)(5)]+ compared to the
other halo-functionalized dyes. This indicates improved elec-
tron injection and is consistent with the higher JSC values
observed for DSCs containing [Cu(1)(5)]+ (Table 2 and Fig. 8).
The differences in the EQE spectra in Fig. 10 also correspond to
the dependence of h on the halo-substituent (I > Cl z F z Br)
for the aged cells described earlier in this section.
In situ dye assembly
Because the assembly of the best-performing dye [Cu(1)(5)]+
was carried out using
a
mixture of [Cu(5)2]+ and
[Cu(5)(MeCN)2]+ in the dipping process, we decided to assess a
new strategy which eliminates the need to prepare the
homoleptic copper(I) complex. The electrode functionalized
with anchoring ligand 1 was immersed in a CH2Cl2 solution
containing a 1 : 1 mixture of [Cu(MeCN)4][PF6] and 5 (Scheme 4);
the concentration of each was 0.1 mM. Over the dipping period
of 68 hours, the electrode became pale red-orange and the
colour persisted aer drying. Duplicate cells were prepared and
Table 3 EQE maxima for two independent sets of DSCs containing
dyes [Cu(1)(L)]+ L ¼ 2, 3, 4 or 5 measured on the day of cell fabrication
and 3 days later
Day 0
Day 3
Anchored dye
lmax/nm
EQEmax/%
lmax/nm
EQEmax/%
[Cu(1)(2)]+
[Cu(1)(2)]+
[Cu(1)(3)]+
[Cu(1)(3)]+
[Cu(1)(4)]+
[Cu(1)(4)]+
[Cu(1)(5)]+
[Cu(1)(5)]+
480
480
480
470
470
470
470
470
46.1
46.4
44.1
47.8
48.3
48.1
48.0
48.2
470
470
470
460
460
470
470
470
46.6
46.0
46.9
50.2
46.2
47.9
49.4
51.4
Fig. 8 J–V curves for two independent DSCs containing dye (a)
[Cu(1)(2)]+ (fluoro substituents), (b) [Cu(1)(3)]+ (chloro substituents) and
(c) [Cu(1)(5)]+ (iodo substituents) measured over a 7 day period after
sealing the cells.
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Fig. 9 EQE spectra for duplicate DSCs functionalized with [Cu(1)(5)]+
measured on the day of sealing the cell (day 0) and 3 and 7 days later,
compared to the EQE spectrum of a DSC with N719.
Scheme 4 Stepwise assembly of titania-anchored [Cu(1)(5)]+.
Table 4 Performance data for two independent sets of sealed and
masked DSCs with dye [Cu(1)(5)]+ assembled in a stepwise manner;
data are with respect to standard dye N719 measured under the same
conditions
Fig. 10 Comparison of the EQE spectra for DSCs containing dyes
[Cu(1)(2)]+ (fluoro), [Cu(1)(3)]+ (chloro), [Cu(1)(4)]+ (bromo) and
[Cu(1)(5)]+ (iodo) measured 3 days after sealing the cells.
Relative
h/%
Dye
JSC/mA cmꢀ2
VOC/mV
ff
h/%
the DSC characteristics compared to an N719 standard are given
in Table 4.
On the day of sealing the cell
The performances of the duplicate DSCs are similar and
exhibit efficiencies of 2.80 or 2.71% aer 7 days. One cell
shows an enhanced performance over the 7 days aer sealing
the DSCs (Table 4) and both cells exhibit improved VOC but
little change in JSC (Fig. 11). The EQE spectra for the two DSCs
show maxima at 46.0 and 46.1% (lmax ¼ 470 nm) on the day of
cell fabrication and these values vary little over a 7 day period
(Fig. 12). Overall, the performances of the DSCs containing
dye [Cu(1)(5)]+ assembled in situ are comparable with those of
the DSCs made by ligands exchange. The differences in
performance (compare the entries for [Cu(1)(5)]+ in Tables 2
and 4) are not signicant and imply that isolation of the
homoleptic copper(I) complex is not an essential part of the
cell-assembly process.
[Cu(1)(5)]+
[Cu(1)(5)]+
N719
6.82
7.01
16.88
551
559
641
69.1
70.2
69.6
2.59
2.75
7.54
34.4
36.5
100
3 days aer sealing the cell
[Cu(1)(5)]+
[Cu(1)(5)]+
N719
6.97
6.93
16.54
584
588
671
69.6
67.6
68.2
2.83
2.75
7.58
37.3
36.3
100
7 days aer sealing the cell
[Cu(1)(5)]+
[Cu(1)(5)]+
N719
6.85
6.96
16.55
587
587
684
69.7
66.3
68.7
2.80
2.71
7.77
36.0
34.9
100
previously shown that the choice of atomic orbital basis set (6-
311++G** basis set on all atoms, 6-311++G** on copper and 6-
31G* basis set on C, H and N, or 6-31G* basis set on all atoms)
has a strong inuence on the calculated absorption spectra of
representative bis(diimine) copper(I) dyes, while the orbital
characteristics of HOMOs and LUMOs are essentially
HOMO and LUMO characteristics
We have used ground state DFT calculations to gain some
insight into the origin of the surprisingly good performance
of dyes containing iodo-substituted ligand 5. We have
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Fig. 11 J–V curves measured over a 7 day period for one of the DSCs
containing [Cu(1)(5)]+ formed in situ by stepwise assembly.
Fig. 13 (a) LUMO, (b) LUMO+1, and (c) HOMOꢀ3 of [Cu(1)(5)]+ (ligand
5 is at the top of each diagram).
Fig. 12 EQE spectra measured over a 7 day period for one of the DSCs
containing [Cu(1)(5)]+ formed in situ by stepwise assembly.
improved hole transfer over the halogen of the aryl substituent
to the reduced electrolyte.
unaffected.39 Thus, for qualitative assessment of the MOs of
[Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ and [Cu(1)(5)]+, we opted to
use a 6-31G* basis set on all atoms for reasons of computa-
tional efficiency.
Conclusions
DFT calculations on the ground state, energy minimized The synthesis and characterization of [Cu(2)2][PF6] and
structures of [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ and [Cu(1)(5)]+ [Cu(3)2][PF6] have been reported and the spectroscopic and
showed that the energies and characteristics of the LUMO and electrochemical properties compared with those of the
LUMO+1 of the four complexes are similar. The LUMO is cen- bromo-analogue
[Cu(4)2][PF6].
The
preparation
of
tred on the anchoring ligand while the LUMO+1 is principally [Cu(5)2][PF6] from [Cu(MeCN)4][PF6] and 5 leads to a mixture
localized on the bpy domain of the ancillary ligand. These MOS of [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6] which, in MeCN
are shown for the iodo-complex in Fig. 13a and b. The close solution, are in equilibrium at room temperature. The
similarity in the orbital characteristics for the four dyes suggests performance of
DSCs containing surface-anchored
that the enhanced performance of [Cu(1)(5)]+ is not associated heteroleptic dyes [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ and
with a tuning of the properties of the lowest lying vacant MOs [Cu(1)(5)]+ depend upon the halo-substituent. Initially, the
leading to improved electron injection.
global efficiencies, h, of the dyes follow the order I > F z Br > Cl.
The DFT calculations indicate that the HOMO of each Ripening of the DSCs occurs and aer 7 days, the dependence of
ground-state dye is mainly based on copper, as are the next two h on the halo-substituent is I > Cl z F z Br; the highest h is
highest occupied MOs. Ancillary ligand character is present in 3.16% for [Cu(1)(5)]+ compared to 7.63% for N719 (a relative
HOMOꢀ3 and HOMOꢀ4 in [Cu(1)(5)]+, with a dominant efficiency of 41.4% versus N719 set at 100%). Compared to the
contribution from the iodophenyl substituent (Fig. 13c). other halo-substituted dyes, [Cu(1)(5)]+ exhibits an extended
Signicantly, the corresponding contributions by ligands 2, 3 spectral response to longer wavelength, with enhanced electron
or 4 to these MOs is smaller. This leads us to suggest that the injection. The performance of [Cu(1)(5)]+ is surprising consid-
better dye performance of [Cu(1)(5)]+ may be associated with ering the structural simplicity of ancillary ligand 5 and the lack of
a peripheral electron donating group. DFT calculations have been
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used to establish the HOMO and LUMO characteristics of the 10 K. A. Wills, H. J. Mandujano-Ramirez, G. Merino, D. Mattia,
ground-states of [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ and
[Cu(1)(5)]+. The results show that ancillary ligand character is
T. Hewat, N. Robertson, G. Oskam, M. D. Jones, S. E. Lewis
and P. J. Cameron, RSC Adv., 2013, 3, 23361.
present within the HOMO manifold of [Cu(1)(5)]+, with a domi- 11 A. Colombo, C. Dragonetti, D. Roberto, A. Valore, P. Biagini
nant contribution from the iodophenyl substituent; corre-
and F. Melchiorre, Inorg. Chim. Acta, 2013, 407, 204.
sponding contributions from ligands 2, 3 or 4 to these MOs in 12 M. Sandroni, M. Kayanuma, A. Planchat, N. Szuwarski,
[Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ is smaller. This suggests that
the improved dye performance of [Cu(1)(5)]+ may result from
E. Blart, Y. Pellegrin, C. Daniel, M. Boujtita and F. Odobel,
Dalton Trans., 2013, 42, 10818.
better electron transfer over the halogen of the aryl substituent 13 J. Baldenebro-Lopez, N. Flores-Holguin, J. Castorena-
from the reduced electrolyte.
Gonzalez and D. Glossman-Mitnik, J. Photochem.
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The observation that DSCs containing [Cu(1)(5)]+ give
global efficiencies >3% is unexpected and has signicant 14 L. N. Ashbrook and C. M. Elliott, J. Phys. Chem. C, 2013, 117,
potential in terms of the use of a synthetically very accessible 3853, erratum: 2013, 117, 11883.
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with [Cu(1)(5)]+ assembled using
1 : 1 mixture of 16 N. Armaroli, Top. Curr. Chem., 2007, 280, 69.
[Cu(MeCN)4]+ and 5 in place of [Cu(5)2]+. This not only avoids 17 M. Sandroni, L. Favereau, A. Planchat, H. Akdas-Kilig,
a
the need to prepare the homoleptic complex, but also prevents
the wastage of one equivalent of ancillary ligand. Our next
N. Szuwarski, Y. Pellegrin, E. Blart, H. Le Bozec,
M. Boujtita and F. Odobel, J. Mater. Chem. A, 2014, 2, 9944.
challenge is the optimization of the DSCs with [Cu(1)(5)]+ and 18 B. Bozic-Weber, E. C. Constable, C. E. Housecro,
related dyes, starting with an investigation of the role of co-
adsorbents such as cheno.
P. Kopecky, M. Neuburger and J. A. Zampese, Dalton
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19 G. J. Kubas, Inorg. Synth., 1990, 28, 68.
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Acknowledgements
The European Research Council (Advanced Grant 267816 LiLo),
Swiss National Science Foundation and University of Basel are 22 Bruker Analytical X-ray Systems, Inc., APEX2, version2 User
´
acknowledged for nancial support. Roche Walliser and Nik
Manual, M86-E01078, Madison, WI, 2006.
Hostettler are thanked for recording 500 MHz NMR spectra. Dr 23 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout and
Biljana Bozic-Weber is acknowledged for helpful discussions
and technical support.
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RSC Adv., 2014, 4, 48712–48723 | 48723