A study of dye anchoring points in half-squarylium dyes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c3ta15278b

This paper reports the synthesis of a series of new half-squaraine dyes (Hf-SQ) based around a common chromophoric unit consisting of linked indoline and squaric acid moieties. Carboxylate groups have been incorporated onto this core structure at four different points to study the influence of the anchoring group position on dye-sensitized solar cell (DSC) device performance. Dyes have been linked to TiO₂ directly through the squaric acid moiety, through a modified squaric acid unit where a vinyl dicyano group has replaced one carbonyl, via an alkyl carboxylate attached to the indole N or through a carboxylate attached to the 4 position of a benzyl indole. Contact angle measurements have been studied to investigate the hydrophobic/hydrophilic properties of the dyes and the results have been compared to N719 and Z907. Full characterization data of all the dyes and synthetic intermediates are reported including single-crystal X-ray structural analysis for dye precursors; the indole (2a) and the half-squarylium esters (3a) and (6b), as well as the dyes (4c), (8) and (12). Dye colours range from yellow to red/brown in solution (λ_max range from 430 to 476 nm) with ε ranging from 38000 to 133 p;100 M^-1 cm^-1. The performance of the dyes in DSCs shows the highest efficiency yet reported for a Hf-SQ dye (η = 5.0%) for 1 cm² devices with a spectral response ranging from 400 to 700 nm depending on the dye substituents. Co-sensitization of half-squarylium dye (7b) with squaraine dye (SQ2) resulted in a broader spectral response and an improved device efficiency (η = 6.1%). Density functional theory (DFT) calculations and cyclic voltammetry have been used to study the influence of linker position on dye HOMO-LUMO levels and the data has been correlated with I-V and EQE data.

Full text of DOI:10.1039/c3ta15278b

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PAPER
A study of dye anchoring points in half-squarylium
dyes for dye-sensitized solar cells†
Cite this: J. Mater. Chem. A, 2014, 2,
4055
a
a
Arthur Connell, Peter J. Holliman, Matthew L. Davies,a Christopher D. Gwenin,a
Sophie Weiss,a Mateusz B. Pitak,b Peter N. Horton,b Simon J. Colesb
and Graeme Cookec
*
This paper reports the synthesis of a series of new half-squaraine dyes (Hf-SQ) based around a common
chromophoric unit consisting of linked indoline and squaric acid moieties. Carboxylate groups have been
incorporated onto this core structure at four different points to study the influence of the anchoring
group position on dye-sensitized solar cell (DSC) device performance. Dyes have been linked to TiO2
directly through the squaric acid moiety, through a modified squaric acid unit where a vinyl dicyano
group has replaced one carbonyl, via an alkyl carboxylate attached to the indole N or through a
carboxylate attached to the 4 position of a benzyl indole. Contact angle measurements have been
studied to investigate the hydrophobic/hydrophilic properties of the dyes and the results have been
compared to N719 and Z907. Full characterization data of all the dyes and synthetic intermediates are
reported including single-crystal X-ray structural analysis for dye precursors; the indole (2a) and the half-
squarylium esters (3a) and (6b), as well as the dyes (4c), (8) and (12). Dye colours range from yellow to
red/brown in solution (lmax range from 430 to 476 nm) with 3 ranging from 38 000 to 133 100 Mꢀ1
cmꢀ1. The performance of the dyes in DSCs shows the highest efficiency yet reported for a Hf-SQ dye
(h ¼ 5.0%) for 1 cm2 devices with a spectral response ranging from 400 to 700 nm depending on the
dye substituents. Co-sensitization of half-squarylium dye (7b) with squaraine dye (SQ2) resulted in a
broader spectral response and an improved device efficiency (h ¼ 6.1%). Density functional theory (DFT)
calculations and cyclic voltammetry have been used to study the influence of linker position on dye
HOMO–LUMO levels and the data has been correlated with I–V and EQE data.
Received 18th December 2013
Accepted 28th January 2014
DOI: 10.1039/c3ta15278b
www.rsc.org/MaterialsA
¨
O'Regan and Gratzel's breakthrough in DSC devices used Ru complexes and have signicantly higher 3. This allows thinner
bipyridyl dye N3 to sensitize nanoparticulate TiO2.1 Since 1991, photo-electrodes to be used which can reduce recombination
Ru-bipy dyes have remained a widely used family of DSC dyes.2 losses and improve device voltages. High 3 is also advantageous
However, these dyes are expensive which is partly due to raw when co-sensitizing the TiO2 electrode to broaden spectral
material costs but also due to lengthy purication procedures response because if fewer dye sorption sites utilized for the dye
which hinder scaling and they have relatively low molar harvesting light at 400–650 nm, this leaves more space for near
extinction coefficients (3) and generally poor spectral response infrared (NIR) dyes which absorb at l > 650 nm.
above 600 nm leading to DSC efficiencies of 11.1% for N719
Extending DSC spectral response remains key to increasing
(ref. 3) and 11.4% for C101.4 This has led to the development of device efficiency. This has led to the development of panchro-
“Ru-free” organic DSC dyes (e.g. triphenylamines,5 coumarins6 matic dyes (e.g. “Black dye”,8 squaraine9–12). However, the
and indolines7) which absorb in the same region as Ru-bipy synthetic and purication procedures associated with such dyes
dyes (450–600 nm) where AM1.5 solar intensity is highest. make them expensive and ensuring optimized spectral
Generally, these organic dyes are simpler to purify than Ru-bipy response across 400–900 nm in a single dye has proved very
difficult. Co-sensitizing DSC photo-anodes with combinations
of two or more dyes is another approach to increasing light
aSchool of Chemistry, Bangor University, Gwynedd LL57 2UW, UK. E-mail: p.j.
harvesting13–16 which has led to the rst reports of ultra-fast co-
holliman@bangor.ac.uk; Fax: +44 (0)1248 370528; Tel: +44 (0)1248 382375
bUK National Crystallography Service, Chemistry, Faculty of Natural and
sensitization17,18 and h > 12% for a combination of porphyrin
and triphenylamine dyes.19
Environmental Sciences, University of Southampton, Higheld Campus,
Southampton, SO17 1BJ, UK
cWestCHEM, School of Chemistry, University of Glasgow, Glasgow, G12 8QQ, UK
Co-sensitization requires dyes which can harvest longer
wavelength photons. In this context, squaraines have been
studied as DSC dyes20–22 because of their high 3 and lmax
$
† Electronic supplementary information (ESI) available. CCDC 977417–977428.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ta15278b
650 nm where Ru-bipy dyes are less effective sensitizers.
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Scheme 1 Dye anchoring points on half-squaraine chromophore.
Half-squarylium (Hf-SQ) dyes are produced as an intermediate
during the synthesis of unsymmetrical squaraine dyes.23,24 Hf-
SQ dyes are known to be uorescent25 and have been tested in
ZnO giving h ¼ 0.27% (ref. 26) 0.53%,27 and, more recently, in
TiO2 DSC devices giving h ¼ 3.54%.28 The Hf-SQ unit is also
interesting because it offers a number of locations where a
carboxylate anchoring group can be positioned. It is widely
believed that the most efficient DSC dyes possess a donor–p
bridge–acceptor (D–p–A) conguration.29 In this paper, we have
used the synthetic exibility of the Hf-SQ moiety to investigate
the inuence of anchor position on DSC performance using a
common chromophoric unit. Scheme 1 shows the anchor
points tested (A/B ¼ indole aromatic moiety, C ¼ alkyl carbox-
ylate on the indole N, D ¼ carbonyl or vinyl dicyano moiety and
E ¼ H+ for an acid or an alkyl group for an ester). We believe this
the rst report of this type. We have also investigated the effect
of varying Hf-SQ side chain on dye/TiO2 surface hydrophobicity.
As such, this paper reports the synthesis of new Hf-SQ dyes
along with detailed characterization, calculations of HOMO–
LUMO energy levels and electron density distributions which
have been correlated with DSC device testing. The co-sensiti-
zation of Hf-SQ and squaraine dyes has also been studied which
offers the potential to reduce dye cost whilst also improving
spectral response and device efficiency due to both sensitizers
being prepared from the same synthetic pathway.
Results and discussion
Dye synthesis
The synthetic routes used to produce the Hf-SQ used in this
work are shown in Schemes 1 and 2. The rst step is the
esterication of squaric acid in ethanol to give (1) which was
prepared according to the method described by Terpetschnig
et al. in good yield with spectroscopic data in line with the
literature.30 Compounds (2a–c) and (5a–b) were prepared using
a general N-alkyl substitution reaction at the indole nitrogen
with various iodo-alkanes.31 All of the materials were prepared
Scheme 2 Synthetic pathways to half-squaraine dyes.
in high yield (>90%) with spectroscopic data in line with the the Hf-SQ dyes (4a–c) and (7a–b), respectively. The target
1
literature.31 The molecular structures of (2a), (2b) and (5a) have compounds (4a–c) and (7a–b) were analyzed using H and 13C
been conrmed by single crystal X-ray analysis (Fig. 1). NMR, high resolution mass spectrometry, UV-visible spectros-
Compounds (2a–c) and (5) were reacted with (1) to prepare (3a– copy, I.R. spectroscopy and melting points. In addition, single
c) and (6), respectively. In addition, single crystal analysis of (3a) crystal analysis of (4b) and (4c) sodium salts show the expected
shows the expected conguration (Fig. 1). Finally, hydrolysis of molecular conguration as shown in (Fig. 3 and 4, ESI†). The
(3a–c) and (6a–c) using NaOH as described elsewhere20,30 gives synthetic routes to (8), (10) and (12) are shown in Scheme 2.
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Fig. 1 Crystal structures of dye precursors and dyes (a) (2a), (b) (2b), (c) (3a), (d) (5a), (e) (6b), (f) (9) and (g) (12). Displacement ellipsoids – 50%
probability.
In situ hydrolysis of (6c) together with nucleophilic substitution indole, respectively have been conrmed by, single crystal X-ray
of a squaric acid carbonyl with malononitrile was used to analysis (Fig. 1) and (Fig. 6 ESI†). Whilst some structurally-
prepare compound (8). These reaction conditions have been related Hf-SQ dyes and their precursors have been reported
reported elsewhere in the literature for the preparation of previously,20,30 (5), (6b), (7b) and (8) are variants (see ESI†) with a
related vinyl dicyano substituted Hf-SQ molecules.23,32 Dye (10) dodecyl chain attached to the nitrogen of the indole. The
and (12) were prepared by reacting (9)33 and (11)34 with (1), spectroscopic data for these new compounds are in line with the
respectively. All of the dyes were isolated in high yield (>75%) previously reported data (see ESI for references†). Dye (10) is
and characterised using 1H, 13C NMR, high resolution mass signicantly different to previously reported Hf-SQ dyes due to
spectrometry, melting points, infrared and UV-visible spec- the linker being attached to the nitrogen of the indole. However,
1
troscopy. The expected molecular conguration (8) (sodium the H and 13C NMR of (10) show resonances in the expected
salt) and (12) with the carboxylate linker groups sited on a vinyl regions for the various functional groups in the molecule and
dicyano-derivatized squaraine unit and on the benzyl of the high resolution mass spectrometry has identied the expected
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mass for this compound. In addition, the expected functional
groups, including the carboxylate moiety, have been identied
using infrared spectroscopy and (10) has a lmax of 440 nm and
an extinction coefficient of 116 000 Mꢀ1 cmꢀ1 in ethanol
measured using UV-visible spectroscopy.
The new dyes in this paper have been designed with bulky
side groups to try to minimize inter-molecular interactions
which can give rise to dye aggregation and increased recombi-
nation processes between adsorbed dye molecules on the TiO2
surface. This is an established approach to DSC dye design,
which has been described previously.2,3 However, this can make
it more difficult to grow single crystals for X-ray analysis which
hinders structural studies of DSC dyes. Using careful purica-
tion and slow evaporation, it was possible to grow suitable
crystals of several HF-SQ dyes and precursors reported
including; (4c), the esteried precursor of (7b) i.e. (6b), (8) and
(12). Analysis of the crystal structure obtained from (4c) i.e. the
molecule containing the longest and potentially most hydro-
phobic alkyl unit (Fig. 4, ESI†) shows a complex structure where
the negatively charged squaric acid moiety is balanced by
sodium ions but with several molecules H-bonded together
through the carbonyl and C–OH of this acyloin moiety (ESI,
Fig. 5 and Table 4†). The hydrophobic hexadecyl alkyl chain is
then arranged pointing away from the hydrophilic, electrostat-
ically charged squaric acid linker group in the same way that
might be observed in a surfactant. Interestingly, this localized
arrangement is then extended to the wider crystal packing
arrangement so that alternating layers of hydrophilic squaric
acid and hydrophobic alkyl chains can be observed (Fig. 5,
ESI†). This structural arrangement is important in the context
of DSC devices because these dyes were designed with the hope
that the charged, hydro-philic squaric acid moiety of the Hf-SQ
dyes (4a–c), (7a–b) would chemisorb to the TiO2 surface through
Fig. 2 Contact angle measurements and pictures for TiO2 surfaces
dyed with (a) N719, (b) Z907, (c) (7b), (d) (8), (e) (10) and (f) (12).
Table 1 Contact angles for dyes sorbed on TiO2 photo-electrodes
Surface
Contact angle/ꢁ
N719
Z907
(7b)
(8)
(10)
(12)
45.8 ꢂ 3.0
110.1 ꢂ 5.0
42.3 ꢂ 3.0
25.5 ꢂ 3.0
38.7 ꢂ 3.0
106.2 ꢂ 4.0
a bi-dentate ester-like linkage whilst the long chain alkyl moiety believed to include the orientation of individual dye molecules
might point away from the surface (Fig. 3). It was also hoped on the surface which will, in turn, inuence the orientation of
that this localized structure might extend to the inter-molecular the alkyl chain to the surface and, hence, any inter-molecular
packing arrangement of dyes on the TiO2 surface creating a dye interactions and wider surface arrangement of dyes. All of
hydrophobic surface and that this might affect electrolyte–dye these factors are important because contact angles only
interactions and device stability in a similar manner to that measure average surface hydrophobicity which results from the
which occurs for Z907 versus N719.35 To test this hypothesis, collective action of many molecules across the surface. For DSC
contact angles have been measured for TiO2 electrodes dyed dyes, such as Z907, where a dye monolayer is believed to form, it
either with N719, Z907, (7b), (8), (10) and (12) are shown in Fig. 2 is reported that the alkyl chains are orientated towards the
and Table 1. The data show that lms dyed with N719 have a periphery of the adsorbed dye such that these groups will be the
lower contact angle and are relatively hydrophilic by compar- rst part of the molecule in contact with any species
ison to lms dyed with Z907 in line with literature reports35,36 for approaching the TiO2 surface. For the Hf-SQ dyes, the poor
these and other dyes incorporating various hydrophobic hydrophobicity of (7b) and (8) by comparison to (10) could be
molecular entities such as the alkyl chains on Z907.35 Interest- due to the alkyl chains being orientated in a way that does not
ingly, although (7b) and (8) both contain dodecyl alkyl chains point them towards the periphery of the molecule, or because a
attached to the nitrogen of the indole moiety, neither dye complete monolayer has not formed and/or because the alkyl
displays the expected hydrophobic properties that Z907 does chains do not form a collective surface. This is clearly not the
with lower contact angles observed than for N719. Neither do case for (12) and therefore the position of the linker group and
(7b) or (8) show enhanced hydrophobicity relative to (10), which alkyl chains result in an orientation of the alkyl chain to repel
does not contain a long chain alkyl unit. By comparison, (12) hydrophilic species. Equilibrium dye loading measurements
does show a contact angle similar to Z907 despite possessing a show that 81.7 mg cmꢀ2 of (7b) sorbs to titania whilst (10) and
shorter (octyl) alkyl chain compared to the longer, dodecyl unit (12) show comparable and much lower dye loadings at 33.9 and
in (7b) and (8). This suggests that hydrophobicity is also inu- 34.0 mg cmꢀ2. It was not possible to measure the dye loading of
enced by other factors as well as alkyl chain length. These are (8) because this dye bleached rapidly and completely on
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ensure the redox couple (I3ꢀ/Iꢀ) is able to interact with the dye-
oxide surface avoiding any dye regeneration limitations on
device performance.
The optical properties of the dyes (7b), (8), (10) and (12) have
been studied both in solution and adsorbed to translucent TiO2
lms (Fig. 3). (12) has a lmax at 428 nm (133 100 Mꢀ1 cmꢀ1) in
solution which red shis to 444 nm when adsorbed to TiO2.
Bathochromic shis are observed for (7b) (lmax 442 nm, 38 000
Mꢀ1 cmꢀ1) and (10) (lmax 440 nm, 116 000 Mꢀ1 cmꢀ1) in solu-
tion which are attributed to the increased conjugation from the
naphthyl indole by comparison to the benzyl indole in (12). (10)
also exhibits narrower absorption peaks than (7b) and the
solution appears bright orange by comparison to the more
diffuse yellow hue of (7b) and (12). Furthermore, a decrease in
the extinction coefficient is observed for (7b) compared to (6b)
which is similar to observations reported in the literature for
other de-esteried Hf-SQ molecules.26,28 A relatively large bath-
ochromic shi and signicantly broadened absorption peaks
are observed for (7b) and (10) when adsorbed to TiO2 (Fig. 3). To
help understand these effects, the HOMO and LUMO maps for
molecules (7a), (8), (10) and (12) have been calculated (Fig. 4).
All of the dyes show that both the HOMO and LUMO are delo-
calized throughout the p-framework although there is signi-
cantly greater electron density for the LUMO around the squaric
acid unit and at the nitrogen of the indole group. This should
result in greater electronic interaction between the anchoring
unit and TiO2 surface which contributes to the broader
absorption spectra for (7a), (8) and (10) which link via these
groups by comparison to (12) which links via a benzyl carbox-
ylate. Interestingly, the Hf-SQ–TiO2 interface has also been
investigated using DFT calculations by Cicero et al.28 which also
shows that the HOMO is delocalized through the p-framework
of the whole structure while the LUMO contains a notable
electron density contribution arising from the squaric acid
group.28 These workers reported that the HOMO–LUMO tran-
sition should therefore move electron density to the anchoring
group resulting in directional electron transfer to the TiO2
surface.28 A similar phenomenon should also occur in the dyes
reported here. Finally, incorporating the vinyl dicyano entity
into the Hf-SQ central unit in (8) results in the largest peak
broadening and bathochromic shi both in solution (lmax
476 nm, 57 700 Mꢀ1 cmꢀ1) and when adsorbed to TiO2 lms.
Whilst this inuence of the electron donating vinyl dicyano
group is similar to that reported for squaraines,23,32 to the best
of our knowledge, this effect has not yet been reported for
Fig. 3 (a) Colours and molecular configurations of selected dyes in
solution and sorbed to TiO2 and normalized UV-visible spectra of (b)
dyes in solution and (c) dyes adsorbed onto TiO2 films. Data show (7b)
dotted line, (8) long dashes, (10) short dashes and (12) solid line.
exposure to alkali. Dye loading correlates with Jsc for (7b), (10) half-squaraines.
and (12) with the highest dye loading and Jsc for (7b) whilst
lower dye loadings and Jsc are observed for (10) and (12). In
addition, the broader spectral response of (10) relative to (12)
DSC device testing
explains the increased Jsc of (10) compared to (12). The dye Table 2 shows I–V data for selected Hf-SQ devices. Initially, DSC
loading data further suggest that dye orientation is important devices were prepared with and without 5 mM the additive che-
and appears to be more inuential than dye loading when nodeoxycholic acid (CDCA). However, the power conversion effi-
inuencing contact angles (assuming dye uptake is complete). ciencies of devices without CDCA were always at least 25% lower
Interestingly, very low contact angles were observed for all dyes than those with CDCA; see e.g. (10) or (12) due to slight improve-
when measurements were made using the electrolyte solvent ments in Jsc, Voc and FF. This is in line with published reports that
methoxypropionitrile instead of water suggesting complete CDCA suppresses aggregation28,32,37 and affects the position of the
wetting of the surface should occur in DSC devices. This should conduction band resulting in improved electron injection.32,37
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Table 2 I–V data for DSC devices prepared using various half-
squaraine dyes. Errors on device efficiencies are ꢂ0.2%
Jsc/
Dye
h/%
FF
Voc/V
mA cmꢀ2
Single dye devices
(4a) + CDCA
(4b) + CDCA
(4c) + CDCA
(7a) + CDCA
(7b)
(7b) + CDCA
(8)
(10)
(10) + CDCA
(12)
(12) + CDCA
N719 only
3.3
3.8
3.5
4.0
4.5
5.0
2.8
3.5
3.9
2.4
2.7
6.4
0.56
0.62
0.67
0.61
0.66
0.68
0.47
0.65
0.68
0.74
0.72
0.60
0.64
0.66
0.63
0.60
0.65
0.71
0.53
0.67
0.68
0.61
0.68
0.73
9.20
9.22
7.92
10.90
10.40
10.25
11.11
8.12
8.38
5.31
5.44
14.69
Co-sensitized devices
SQ2 + CDCA
(7) + SQ2 + CDCA
3.4
6.1
0.67
0.66
0.62
0.63
8.25
14.61
electrolyte but this is only has a relatively small effect on
h. However, aer synthesis, this dye was also the most difficult
to purify from the iodohexadecane precursor by comparison to
the shorter chain iodoalkane dyes and precursors which may
also have had a slight negative effect on Jsc for (4c).
Studying the effects of alkyl chain length on the naphthyl
indoles of dyes (7a) and (7b) shows two main differences.
Firstly, a signicant upli in h is observed for the napthyl
indoles (4.0–5.0%) compared to their benzyl indoles which
mainly results from increased Jsc (10.2–10.9 mA cmꢀ2). This Jsc
increase is attributed to a slight broadening of the absorption
spectra due to the extended conjugation of the napthyl indole.
This is in line with literature data for squaraines which report
improved h for the napthyl indole squaraine, SQ2 versus the
benzyl indole, SQ1.20,21 Interestingly, for the napthyl indoles,
increasing the length of the alkyl chain attached to the indole N
from an ethyl group in (7a) to a dodecyl chain in (7b) increases
Voc from 0.61 to 0.71 V respectively. This is the highest Voc
observed for the Hf-SQ dyes reported here which, along with an
improved FF, gives the highest single dye efficiency (h ¼ 5.0%).
In addition, the Voc of 0.61 V observed for (7a) is close to the
literature value of 0.64 V reported elsewhere for this dye.28 We
believe this to be the highest efficiency Hf-SQ reported to date.
The improved Voc and FF may be due to decreased dye aggre-
gation on the TiO2 photo-electrode which could be caused by
the longer alkyl chain in (7b) sterically reducing inter-dye
Fig. 4 Optimized geometries and HOMO and LUMO maps of the
methyl indoline derivatives of selected dyes calculated using DFT.
Table 2 shows device data for TiO2 devices sensitized with interactions. Due to the improved device performance of (7b) by
Hf-SQs containing either benzyl or napthyl indoles with alkyl comparison to (4a–c) or (7a), the dye (7b) was taken forward for
chains of varying in length (ethyl to octadecyl). Comparing the further study using co-sensitization studies and for comparison
benzyl indole dyes (4a–c) shows very similar device performance with dyes (8), (10) and (12).
data for all three dyes with h ranging from 3.3–3.8%. The vari-
Cyclic voltammetry (CV) measurements of (7b), (8), (10) and
ance was mainly due to differences in Jsc which ranges from 7.9– (12) in solution (10 mM in degassed THF) have been carried out
9.2 mA cmꢀ2 with very similar Voc values observed for all three to compare dye oxidation and reduction processes with spectral
dyes (ca. 0.65 V). The slightly lower Jsc value was observed for the and DSC device data along with theoretical DFT calculations to
indole with the longest alkyl chain (hexadecyl) may reect try to further examine any structure–activity relationships
slightly poorer electron injection or dye regeneration from the arising from changes to the dye linker position. The CV data for
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device efficiency was consistently improved due to increased Voc
as has been widely reported in the literature for various dye
systems.38,39 Further desorption of (8) was also observed when
an electrolyte was introduced into the device which meant that
the I–V data for (8) was measured immediately aer electrolyte
addition. The desorption problems of (8) meant that, despite
having the highest photocurrent of all dyes reported in this
study (Jsc > 11 mA cmꢀ2), the overall efficiency was limited to h ¼
2.8% as a result of much lower Voc and FF which must reect the
signicant changes to the photo-electrode surface during
desorption. These problems are ascribed to the addition of the
vinyl dicyano group to one carbonyl of the squaric acid unit
which in (8) must act as the linker group to the TiO2 surface.
This new group, rich in p-related electron density, presumably
increases steric hindrance at the dye sorption site. Whilst this
does not prevent the formation of an ester link between the dye
and TiO2 surface, it does appear to weaken this interaction to
the extent that dye desorption occurs more easily. Indeed,
desorption in the presence of the electrolyte suggests a dye
binding energy of the same order as iodide which means the dye
and electrolyte anions are competing for surface sorption sites.
The instability of (8) is further evidenced by the fact that it
bleaches on exposure to alkali (see earlier dye loading
discussion).
Fig. 5 Cyclic voltammetry data for 5 mM of (8) in degassed THF with
[Bu4N] [PF6] as supporting electrolyte (solid line). Dashed line shows
CV of supporting electrolyte only.
(7b) show three anodic waves (Fig. 5) similar to that reported in
the literature.27
Shis in the anodic and cathodic waves were observed for (8)
and (10) which were expected from the changes in the electronic
spectra of these dyes (Fig. 3). The CV data have been used to
calculate the HOMO–LUMO potentials and band gaps versus the
NHE and the band gap energies obtained correlate well with the
onset of the absorption measured from the solution UV-Visible
spectroscopy (Table 3). For example, the bandgap calculated
from the absorption onset obtained for (7b), (8) and (10) is 2.20,
2.35 and 2.60 (eV) respectively which correlates with the
bandgap energies of 2.23, 2.4 and 2.6 (eV) calculated from the
CV data. By comparison, the anodic and cathodic waves
measured for (12) are not as clear as for the other dyes (see
ESI†). However, a sensible estimation of the HOMO–LUMO
energy gap i.e. 2.62 (eV) could still be made by using the
bandgap of 2.6 (eV) calculated from absorption onset obtained
from solution UV-visible spectroscopy. Furthermore, both the
DFT and CV data (Fig. 4 and 5) suggest that the LUMO levels of
(7b), (8), (10) and (12) should be at the correct energy for
successful electron injection from the dye into the TiO2
conduction band and that the energy of the HOMO levelꢀs
should enable successful regeneration of the dye using an Iꢀ/I3
redox couple in the electrolyte.
Fig. 6 shows that the spectral responses of (7b), (8), (10) and
(12) are all slightly red shied by comparison to the corre-
sponding absorption spectra of TiO2 dyed lms (Fig. 3). The
To study the inuence of linker position, the performance of
(7b), (8), (10) and (12) were evaluated in DSC devices with and
without CDCA (Table 2). Interestingly, it was not possible to
include CDCA with (8) as this caused desorption of the sensi-
tizer from the TiO2 surface. However, where CDCA was present,
Table 3 Band-gap (EB) calculated from the absorption onset, cyclic
voltammetry and DFT calculations for dyes (7b), (8), (10) and (12)
l
EB calc.
EB calc.
Dye
onset/nm
from l onset/eV
from CV/eV
(7b)
(8)
(10)
(12)
556
526
476
472
2.20
2.35
2.60
2.62
2.23
2.40
2.60
2.5
Fig. 6 EQE of (a) (7b) dotted line, (8) long dashes, (10) short dashes and
(12) solid line and (b) (7b) dotted line and (7b) + SQ2.
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Journal of Materials Chemistry A
Paper
narrowest spectral response and absorption data are observed
for (12) which correlate with the lowest Jsc and h of these four
dyes. Interestingly, the carboxylate linker in (12) is placed on the
benzene ring and so is expected to lie furthest from greatest
electron density of the LUMO on the squaraine moiety. In
addition, (12) has the least conjugation of the 4 dyes tested here
which limits the extent of spectral response. This is despite this
dye having the highest 3 of the four dyes tested here (133 100
Mꢀ1 cmꢀ1) which reects the importance of electron injection
over simply light harvesting. By comparison, the broadest
spectral response (out to ca. 650 nm) and absorption data are
observed for (8) which reect the much greater conjugation in
this molecule arising from the addition of the napthyl indole
and vinyl dicyano moieties. The broader light harvesting gives
rise to the highest Jsc despite this dye having a moderate 3 of
(57 700 Mꢀ1 cmꢀ1). Dye (10) shows a slightly broader spectral
response compared to (12) at ca. 400 nm along with a shoulder
at ca. 575 nm which is reected in the higher Jsc; 8.12 versus
5.44 mA cmꢀ2. These two dyes have almost identically high 3
relative to (7b) and (8) and although Jsc is improved for (10), it is
lower than for (7b) which presumably is related the fact that the
linker group is attached to the N of the indole. This means the
linker is closer to the LUMO electron density on the squaraine
ester but that electron injection must occur through the N of the
indole. In squaraine dyes, it has been reported that charge
generation occurs around the squaraine moiety during photo-
excitation40,41 but that electron transfer through the indole
nitrogen such as that which occurs in the dye B1 is less efficient
than through a pathway composed entirely of carbon and
hydrogen atoms as occurs in SQ1.20,33 The highest spectral
response is observed for (7b) which approaches 75% between
ca. 410 and 550 nm. This is despite (7b) having the lowest 3 of
these 4 dyes (38 000 Mꢀ1 cmꢀ1). This is ascribed to the linker
group being the squaric acid moiety where DFT calculations
(Fig. 4) show that the greatest LUMO electron density is found.
Hence, electron injection should be enhanced by the proximity
of the LUMO to the TiO2 surface resulting in increased Jsc (over
10 mA cmꢀ2) despite signicantly narrower spectral response
Experimental
Instrumentation and chemicals
All chemicals were purchased from Aldrich and used as
supplied unless otherwise stated. Anhydrous solvents were
used as purchased except THF which was dried with Na wire.
NMR spectra were recorded on a Bruker AC500 instrument
operating at 500 MHz for H and 125 MHz for 13C. Chemical
1
shis (d) are given in ppm relative to tetramethylsilane (for 1H
and 13C). J values are given in Hz and refer to JH,H unless
otherwise indicated. Mass spectra were recorded using elec-
tron impact, chemical ionisation (NH3), fast atom bombard-
ment, electrospray, matrix-assisted laser desorption ionisation
and laser desorption ionisation at the EPSRC National Mass
Spectrometry Service at the University of Swansea. Infrared
spectra were recorded on a Perkin-Elmer 1600 series FTIR
spectrometer. Elemental analysis was performed on a Carlo
Erba EA1108 elemental analyser. Dye loadings were measured
by desorption from 1 cm2 TiO2 electrodes using 40 mM
Bu4NOH in 1 : 1 v/v ethanol–water according the method
described previously.42 The concentration of dye in solution
was measured using UV/Visible spectroscopy and dye loadings
were calculated (mg cmꢀ2) using the appropriate 3 (see ESI†)
and the Beer–Lambert Law.
Cyclic voltammetry was measured on dyes (5 mM) at room
temperature in N2-saturated anhydrous tetrahydrofuran (THF)
using 0.1
M
tetrabutylammonium hexauorophosphate
(Bu4NPF6) as the supporting electrolyte. Measurements were
performed using an Autolab PGSTAT 30 computer-controlled
electrochemical measurement system (Eco Chemie, Holland).
with a sweep rate of 0.05 V sꢀ1 in a three-electrode cell with a
glassy carbon (1 cm2) working electrode, a silver/silver chloride
(Ag/AgCl) reference electrode and a Pt sheet (1 cm2) counter
electrode. All voltammetric potentials were re-calculated and
are reported versus NHE.
X-ray crystallography
than (8). To the best of our knowledge (7b) gives rise to the Single-crystal X-ray diffraction analyses of 3a, 4c_salt and 5a
highest efficiency reported for a Hf-SQ DSC dye to date. were performed at 120 K using a Bruker APEXII CCD diffrac-
In an attempt to improve device and broaden the spectral tometer mounted at the window of a Bruker FR591 rotating
˚
response further towards the NIR, (7b) was co-sensitized with anode (Mo Ka, l ¼ 0.71073 A) and equipped with an Oxford
squaraine dye SQ2 using a fast pump dyeing method reported Cryosystems Cryostream device. Data were processed using the
elsewhere.17 The spectral response for a co-dyed (7b)/SQ2 device COLLECT package.43
shows signicantly greater light harvesting out to ca. 700 nm
The X-ray data for compounds 2a, 2b, 4b_salt, 6b, 6c, 9, 12(I)
leading to a signicant increase in Jsc to 14.61 mA cmꢀ2 and h ¼ and 12(II) were collected at 100 K on Rigaku AFC12 goniometer
6.1% (Table 2). By comparison, an efficiency of 6.4% was equipped with an enhanced sensitivity (HG) Saturn 724+
obtained for a device sensitized using N719. In addition, the detector mounted at the window of an FR-E+ Superbright MoKa
performance of this co-dyed device is very similar to many rotating anode generator with HF Varimax optics.44
panchromatic SQ dyes reported9–11 but has the advantage that a
The X-ray data for compound 8_salt were collected with the
complicated synthesis is not required to incorporate the use of synchrotron radiation at Diamond Light Source UK,
˚
molecular entities required for higher energy photon harvesting beamline I19 (l ¼ 0.6889 A) on a Crystal Logic diffractometer
across the solar spectrum. This could encourage research and Rigaku Saturn 724+ detector.
efforts to be directed towards incorporating molecular entities
Unit cell parameters were rened against all data. For 3a,
into dye systems that broaden the spectral response further into 4c_salt, 5a an empirical absorption correction was carried out
the near infrared as another approach alongside the develop- using SADABS,45 whereas for 2a, 2b, 4b_salt, 6b, 6c, 9, 12(I) and
ment of panchromatic responses from individual dye systems. 12(II) CrystalClear46 soware was used.
4062 | J. Mater. Chem. A, 2014, 2, 4055–4066
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Paper
Journal of Materials Chemistry A
All structures were solved by direct methods and rened on ¼ 0.0958; Rw(F2, all data) ¼ 0.2473; Drmin/max ¼ 0.926/ꢀ0.404 e
Fo2 by full-matrix least-squares renements using programs of
the SHELX-97/SHELX-2013 soware.47 All non-hydrogen atoms
A
ꢀ3; CCDC: 977421.
˚
˚
˚
˚
5a: a ¼ 15.3349(3) A, b ¼ 7.5158(2) A, c ¼ 13.7564(2) A, a ¼
3
ꢁ
ꢁ
ꢁ
˚
were rened with anisotropic displacement parameters. All 90 , b ¼ 92.5450(10) , g ¼ 90 ; V ¼ 1583.92(6) A , monoclinic,
hydrogen atoms were added at calculated positions and rened P21/c, Z ¼ 4, rcalc ¼ 1.532 Mg mꢀ3; m ¼ 2.010 mmꢀ1; T ¼ 120 K;
using a riding model with isotropic displacement parameters qmax ¼ 27.48ꢁ, 14 035 measured reections, 2792 unique
based on the equivalent isotropic displacement parameter (Ueq
)
reections [Rint ¼ 0.0390], 2592 with F2 > 2s, R(F, F2 > 2s) ¼
of the parent atom.
The crystal structure of 4c_salt is non-merohedraly twinned
(180ꢁ rotation around 001 reciprocal vector). EvalCCD48 was
0.0217; Rw(F2, all data) ¼ 0.0528; Drmin/max ¼ 0.366/ꢀ0.367 e
ꢀ3; CCDC: 977422.
˚
A
˚
˚
˚
6b: a ¼ 19.2363(14) A, b ¼ 10.2830(7) A, c ¼ 29.048(2) A, a ¼
3
ꢁ
ꢁ
ꢁ
˚
used to integrate the domains and the structure was rened 90 , b ¼ 92.704(2) , g ¼ 90 ; V ¼ 5739.5(7) A , monoclinic, C2/c,
using the HKLF 5 instruction in Shelxl.47 The rened percentage Z ¼ 8, rcalc ¼ 1.161 Mg mꢀ3; m ¼ 0.073 mmꢀ1; T ¼ 100 K; qmax
¼
ratio of the twin domains was 25 : 75 (rened BASF ¼ 0.25714). 25.02ꢁ, 15 920 measured reections, 5046 unique reections
The crystal data for 8_salt is not of the highest quality, but is [Rint ¼ 0.0461], 4129 with F2 > 2s, R(F, F2 > 2s) ¼ 0.0401; Rw(F2,
all data) ¼ 0.1077; Drmin/max ¼ 0.194/ꢀ0.182 e Aꢀ3; CCDC:
˚
sufficient to prove structural connectivity.
The long aliphatic chains present in the crystal structures of 977423.
8_salt and 12(II) are disordered and modelled over two sites.
˚
˚
˚
6c: a ¼ 9.802(13) A, b ¼ 9.8259(10) A, c ¼ 15.69(2) A, a ¼
3
ꢁ
ꢁ
ꢁ
˚
For disordered components vibrational restraints (SIMU/ 83.509(10) , b ¼ 89.40(9) , g ¼ 66.40(6) ; V ¼ 1375(3) A ,
DELU), similar displacement restraints (EADP) and distance/ triclinic, P1, Z ¼ 2, rcalc ¼ 1.178 Mg mꢀ3; m ¼ 0.074 mmꢀ1; T ¼
ꢀ
angle restraints DFIX/DANG used to maintain sensible geome- 100 K; qmax ¼ 25.03ꢁ, 15 659 measured reections, 4847 unique
tries and atomic displacement ellipsoids. Some atoms required reections [Rint ¼ 0.0554], 3510 with F2 > 2s, R(F, F2 > 2s) ¼
ISOR restraint to approximate isotropic behaviour. Crystal 0.0848; Rw(F2, all data) ¼ 0.2128; Drmin/max ¼ 0.593/ꢀ0.330 e
ꢀ3; CCDC: 977424.
˚
A
structure of 4b_salt exhibits positional disorder which has been
modelled over two sites. Additionally, in 4b_salt highly diffused
˚
˚
˚
8_salt: a ¼ 11.9454(11) A, b ¼ 12.4652(11) A, c ¼ 24.378(2) A,
3
ꢁ
ꢁ
ꢁ
˚
solvent electron density has been removed with PLATON/ a ¼ 90.54(2) , b ¼ 101.50(2) , g ¼ 118.10(3) ; V ¼ 3115.3(5) A ,
SQUEEZE routine.49
triclinic, P1, Z ¼ 2, rcalc ¼ 1.210 Mg mꢀ3; m ¼ 0.084 mmꢀ1; T ¼
ꢀ
2a: a ¼ 6.944(2) A, b ¼ 11.434(4) A, c ¼ 15.743(6) A, a ¼ b ¼ g 100 K; qmax ¼ 24.21ꢁ, 22 317 measured reections, 10 395
˚
˚
˚
3
¼ 90 ; V ¼ 1249.9(8) A , orthorhombic, P212121, Z ¼ 4, rcalc
¼
unique reections [Rint ¼ 0.2525], 2921 with F2 > 2s, R(F, F2 > 2s)
ꢁ
˚
1.600 Mg mꢀ3; m ¼ 2.527 mmꢀ1; T ¼ 100 K; qmax ¼ 27.48ꢁ, 8462 ¼ 0.1277; Rw(F2, all data) ¼ 0.4276; Drmin/max ¼ 0.459/ꢀ0.422 e
ꢀ3; CCDC: 977425.
˚
A
measured reections, 2810 unique reections [Rint ¼ 0.0230],
2696 with F2 > 2s, R(F, F2 > 2s) ¼ 0.0151; Rw(F2, all data) ¼
9: a ¼ 7.530(2) A, b ¼ 13.635(3) A, c ¼ 34.164(8) A, a ¼ b ¼ g ¼
˚
˚
˚
0.0385; Drmin/max ¼ 0.370/ꢀ0.230 e Aꢀ3; CCDC: 977417.
90 ; V ¼ 3507.9(15) A , orthorhombic, Pbca, Z ¼ 8, rcalc ¼ 1.550
3
ꢁ
˚
˚
2b: a ¼ 7.4137(5) A, b ¼ 13.6430(12) A, c ¼ 29.756(3) A, a ¼ b Mg mꢀ3; m ¼ 1.832 mmꢀ1; T ¼ 100 K; qmax ¼ 27.48ꢁ, 21 886
˚
˚
˚
3
ꢁ
˚
¼ g ¼ 90 ; V ¼ 3009.7(5) A , orthorhombic, Pbca, Z ¼ 8, rcalc
¼
measured reections, 3992 unique reections [Rint ¼ 0.0666],
1.585 Mg mꢀ3; m ¼ 2.123 mmꢀ1; T ¼ 100 K; qmax ¼ 25.02ꢁ, 12 057 3503 with F2 > 2s, R(F, F2 > 2s) ¼ 0.0595; Rw(F2, all data) ¼
measured reections, 2644 unique reections [Rint ¼ 0.1625], 0.0896; Drmin/max ¼ 0.908/ꢀ0.631 e Aꢀ3; CCDC: 977426.
˚
1333 with F2 > 2s, R(F, F2 > 2s) ¼ 0.0486; Rw(F2, all data) ¼
12 (polymorph I): a ¼ 12.7035(9) A, b ¼ 18.2291(13) A, c ¼
˚
ꢁ
˚
0.0690; Drmin/max ¼ 0.974/ꢀ0.595 e Aꢀ3; CCDC: 977418.
11.1693(8) A, a ¼ 90.0 , b ¼ 111.529(2) , g ¼ 90.0 ; V ¼ 2406.1(3)
ꢁ
ꢁ
˚
˚
3
3a: a ¼ 7.2314(2) A, b ¼ 9.4760(3) A, c ¼ 11.6165(4) A, a ¼ A , monoclinic, P21/c, Z ¼ 4, rcalc ¼ 1.213 Mg mꢀ3; m ¼ 0.083
˚
˚
˚
˚
3
82.094(2) , b ¼ 82.082(2) , g ¼ 78.631(2) ; V ¼ 767.95(4) A , mmꢀ1; T ¼ 100 K; qmax ¼ 27.48ꢁ, 30 172 measured reections,
ꢁ
ꢁ
ꢁ
˚
triclinic, P1, Z ¼ 2, rcalc ¼ 1.286 Mg mꢀ3; m ¼ 0.087 mmꢀ1; T ¼ 5504 unique reections [Rint ¼ 0.0562], 4195 with F2 > 2s, R(F, F2
ꢀ
120 K; qmax ¼ 27.48ꢁ, 13 277 measured reections, 3509 unique > 2s) ¼ 0.0406; Rw(F2, all data) ¼ 0.1137; Drmin/max ¼ 0.305/
reections [Rint ¼ 0.0435], 2951 with F2 > 2s, R(F, F2 > 2s) ¼ ꢀ0.199 e Aꢀ3; CCDC: 977427.
˚
0.0542; Rw(F2, all data) ¼ 0.1243; Drmin/max ¼ 0.261/ꢀ0.212 e
12 (polymorph II): a ¼ 10.8885(8) A, b ¼ 17.3070(11) A, c ¼
˚
˚
ꢁ
ꢀ3; CCDC: 977419.
27.892(2) A, a ¼ 78.372(3) , b ¼ 80.234(3) , g ¼ 77.874(3) ; V ¼
ꢁ
ꢁ
˚
A
˚
3
4b_salt: a ¼ 8.810(4) A, b ¼ 12.003(5) A, c ¼ 17.977(8) A, a ¼ 4988.8(6) A , triclinic, P1, Z ¼ 2, rcalc ¼ 1.178 Mg mꢀ3; m ¼ 0.081
˚
˚
˚
˚
ꢀ
3
ꢁ
ꢁ
ꢁ
82.678(14) , b ¼ 86.660(17) , g ¼ 74.635(14) ; V ¼ 1817.5(14) A , mm 1; T ¼ 100 K; qmax ¼ 25.028ꢁ, 47 382 measured reections,
˚
triclinic, P1, Z ¼ 2, rcalc ¼ 1.211 Mg mꢀ3; m ¼ 0.108 mmꢀ1; T ¼ 17 294 unique reections [Rint ¼ 0.0964], 9296 with F2 > 2s, R(F,
ꢀ
100 K; qmax ¼ 25.06ꢁ, 26 587 measured reections, 6434 unique F2 > 2s) ¼ 0.1066; Rw (F2, all data) ¼ 0.3071; min/max ¼ 1.222/
reections [Rint ¼ 0.0518], 4237 with F2 > 2s, R(F, F2 > 2s) ¼ ꢀ0.468 e Aꢀ3; CCDC: 977428.
˚
0.0751; Rw(F2, all data) ¼ 0.2457; Drmin/max ¼ 0.747/ꢀ0.525 e
ꢀ3; CCDC: 977420.
˚
A
Theoretical calculations
˚
˚
ꢁ
˚
4c_salt: a ¼ 8.6755(7) A, b ¼ 10.3795(8) A, c ¼ 35.240(3) A, a ¼
3
92.172(2) , b ¼ 91.370(2) , g ¼ 99.956(3) ; V ¼ 3121.7(4) A , All calculations were performed using Spartan '08.50 Geometries
ꢁ
ꢁ
˚
triclinic, P1, Z ¼ 2, rcalc ¼ 1.155 Mg mꢀ3; m ¼ 0.087 mmꢀ1; T ¼ were rst optimized semi-empirically (AM1) and then re-opti-
ꢀ
120 K; qmax ¼ 24.71ꢁ, 10 081 measured reections, 10 081 mized using DFT (B3LYP/6-31G*) level of theory. The optimized
unique reections [Rint ¼ 0.000], 8831 with F2 > 2s, R(F, F2 > 2s) structures were shown to be local minima on their respective
This journal is © The Royal Society of Chemistry 2014
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Journal of Materials Chemistry A
Paper
potential energy surfaces, as none of the vibrational frequencies [M + H]+ calculated ¼ 474, [M + H]+ observed ¼ 474, m/z accurate
in the optimized geometries generated imaginary frequencies. mass (FTMS+), reference compound: NH4OAc, [M + H]+ calcu-
lated ¼ 474.3003, [M + H]+ observed ¼ 474.2998. FT-IR (KBr) n/
Preparation of 1,1,2-trimethyl-1-dodecyl-1H-benzo[e]indole
(5b)
cmꢀ1 3473–3010 (m), 2960 (m), 1740 (s), 1600 (s). UV-Visible
lmax 442 nm (38 000 Mꢀ1 cmꢀ1) in ethanol.
A mixture of 1,1,2-trimethyl-1H-benzo[e]indole (8 g, 38 mmol)
and dodecyl iodide (25 g, 87 mmol) was heated under reux
overnight under nitrogen in anhydrous acetonitrile (80 ml).
Aer cooling, the solvent was removed in vacuo and the product
Preparation of triethylammonium-2-[(1-dodecyl-3,3-dimethyl-
1,3-dihydro-2H-indol-2-ylidene)-3-(dicyanomethylidene)-4-
oxocyclobut-1-en-1-olate (8)
was re-crystallized from ethyl acetate to give a green solid. (Yield Triethylamine (1 ml, 7.14 mmol) was added dropwise to a
17.3 g, 90%.) mixture of (6b) (2 g, 6.15 mmol) and malononitrile (440 mg, 6.66
1H NMR (400 MHz, CDCl3) d: 0.86 (q, J ¼ 6.7 Hz, 2H), 1.03– mmol) in ethanol (35 ml) and stirred for 2 h at room tempera-
1.41 (m, 14H), 1.41–1.60 (m, 1H), 1.73–1.91 (m, 4H), 1.97 (dd, J ¼ ture. The solvent was removed in vacuo and the product was
15.2 Hz, 7.8, 1H), 3.13–3.22 (m, 2H), 4.77 (t, J ¼ 7.7 Hz, 1H), 7.72 obtained as an orange solid aer purication by column chro-
(ddd, J ¼ 29.6 Hz, 17.8 Hz, 8.1 Hz, 2H), 7.99–8.15 (m, 2H). 13C matography (SiO2) with chloroform and methanol as eluent.
NMR (101 MHz, CDCl3) d 7.41, 14.10, 15.27, 16.90, 22.64, 22.66, (Yield 4.15 g, 75%).
22.73, 26.80, 28.18, 28.52, 29.13, 29.27, 29.31, 29.39, 29.44,
1H NMR (400 MHz, CDCl3) d 0.89 (t, J ¼ 6.8 Hz, 4H), 1.06–1.61
29.53, 29.59, 30.48, 31.85, 31.88, 33.55, 50.37, 55.30, 55.95, (m, 33H), 1.61–2.01 (m, 11H), 3.33 (q, J ¼ 7.3 Hz, 7H), 3.94
76.83, 77.15, 77.36, 77.46, 112.53, 119.75, 122.49, 122.89, (s, 2H), 6.21 (s, 1H), 7.22 (d, J ¼ 8.7 Hz, 1H), 7.28 (s, 3H), 7.35
124.41, 126.33, 127.65, 127.84, 128.69, 130.06, 131.47, 133.69, (d, J ¼ 7.6 Hz, 1H), 7.51 (t, J ¼ 7.6 Hz, 1H), 7.85 (dd, J ¼ 19.3 Hz,
137.17, 138.21, 195.17. MS (FTMS+) M+ calculated ¼ 378, M+ 8.4, 2H), 8.12 (d, J ¼ 8.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) d
observed ¼ 378. m/z accurate mass (FTMS+), M+ calculated ¼ 8.91, 14.12, 15.83, 22.68, 26.80, 29.33, 29.51, 29.60, 31.90, 39.20,
378.1355, M+ observed ¼ 378.1359.
46.83, 69.71, 76.72, 77.04, 77.24, 77.35, 116.92, 118.39, 172.74,
179.07, 181.96, 192.83. MS (FTMS+) M+ calculated ¼ 623, M+
observed ¼ 623. m/z accurate mass (FTMS+), M+ calculated ¼
623.4320, M+ observed ¼ 623.4317. FT-IR (KBr) n/cmꢀ1 3473–
3010 (s), 2960 (s), 2230 (s), 2210 (s), 1740 (s), 1600 (s). UV-Visible
lmax 476 nm (57 700 Mꢀ1 cmꢀ1) in ethanol.
Preparation of 3-ethoxy-4-[(1-dodecyl-1,3-dihydro-3,3-
dimethyl-2H-benzo[e]indol-2-ylidene)methyl]-3-cyclobutene-
1,2-dione (6b)
(Yield 3 g, 65%). 1H NMR (400 MHz, CDCl3) d 0.89 (t, J ¼ 6.8 Hz,
3H), 1.27 (s, 16H), 1.38–1.54 (m, 4H), 1.58 (t, J ¼ 7.1 Hz, 3H),
1.96–1.60 (m, 9H), 3.95 (t, J ¼ 7.5 Hz, 2H), 4.95 (q, J ¼ 7.1 Hz,
2H), 5.48 (s, 1H), 7.20–7.31 (m, 1H), 7.39 (t, J ¼ 7.4 Hz, 1H), 7.55
(t, J ¼ 7.3 Hz, 1H), 7.87 (dd, J ¼ 13.4 Hz, 8.5, 2H), 8.12 (d, J ¼ 8.5
Preparation of 3-ethoxy-(2-carboxyethyl)-[1,1,2-trimethyl-1H-
benzo[e]indolium iodide-2-ylidene)methyl]-3-cyclobutene-1,2-
dione (10)
Hz, 1H), 13C NMR (101 MHz, CDCl3) d 14.13, 16.01, 22.68, 26.71, (3 g, 70%). H NMR (400 MHz, THF) d 1.42 (t, J ¼ 7.3 Hz, 8H),
26.99, 28.05, 29.32, 29.47, 29.54, 29.61, 31.91, 43.12, 49.84, 1.47–1.89 (m, 38H), 1.94 (s, 13H), 2.08 (s, 1H), 2.76 (t, J ¼ 7.3 Hz,
69.86, 76.78, 77.10, 77.30, 77.42, 80.90, 109.80, 122.22, 123.77, 5H), 3.16 (q, J ¼ 7.3 Hz, 6H), 3.30 (s, 1H), 3.62 (s, 23H), 4.32 (t, J
127.19, 128.59, 129.58, 129.74, 130.82, 132.51, 139.82, 170.33, ¼ 7.3 Hz, 5H), 4.90 (q, J ¼ 7.1 Hz, 5H), 5.53 (s, 2H), 7.35 (t, J ¼ 7.5
173.30, 187.09, 187.14, 192.71. MS (FTMS+) [M + H]+ calculated Hz, 3H), 7.46–7.58 (m, 5H), 7.89 (dd, J ¼ 8.3 Hz, 5.7, 5H), 8.22 (d,
¼ 488, [M + H]+ observed ¼ 488, m/z accurate mass (FTMS+), J ¼ 8.5 Hz, 2H). 13C NMR (101 MHz, THF) d 5.83, 13.37, 24.18,
reference compound: NH4OAc, [M + H]+ calculated ¼ 488.3159, 28.53, 36.63, 43.61, 47.56, 67.60, 79.00, 108.23, 120.23, 121.28,
[M + H]+ observed ¼ 488.3153. FT-IR (KBr) n/cmꢀ1 2988 (w), 1736 124.79, 126.79, 127.34, 127.59, 129.09, 129.82, 138.01, 166.88,
1
(m), 1707 (s).
169.68, 171.21, 185.32, 186.53, 189.90. MS (FTMS+) [M + H]+
calculated ¼ 406, [M + H]+ observed ¼ 406, m/z accurate mass
(FTMS+), reference compound: NH4OAc, [M + H]+ calculated ¼
406.1649, [M + H]+ observed ¼ 406.1647. FT-IR (KBr) n/cmꢀ1
3543–3060 (s), 2978 (w), 1738 (s), 1707 (s). UV-Visible lmax 440
nm (116 000 Mꢀ1 cmꢀ1) in ethanol.
Preparation of 4-[(1-dodecyl-1,3-dihydro-3,3-dimethyl-2H-
benzo[e]indol-2-ylidene)methyl]-3-cyclobutene-1,2-dione (7b)
1
(Yield 2.5 g, 70%). H NMR (400 MHz, MeOD) d 0.89 (t, J ¼ 6.8
Hz, 4H), 0.99–1.52 (m, 57H), 1.42 (dq, J ¼ 14.6 Hz, 7.3, 19H),
1.56–1.91 (m, 18H), 1.95 (d, J ¼ 5.9 Hz, 10H), 3.24 (dd, J ¼ 10.0
Hz, 7.1 Hz, 14H), 3.33 (dt, J ¼ 3.2 Hz, 1.6, 2H), 3.94 (t, J ¼ 7.3 Hz,
Device manufacture and testing
2H), 4.90 (s, 2H), 5.73 (s, 1H), 7.28 (dd, J ¼ 7.9 Hz, 5.0 Hz, 2H), Photo-electrodes (0.5 ꢃ 2 cm ¼ 1.0 cmꢀ2) were prepared by
7.47 (s, 1H), 7.76–7.89 (m, 2H), 8.12 (d, J ¼ 8.5 Hz, 1H). 13C NMR doctor blading two layers of TiO2 colloid (DSL18NRT, Dyesol)
(101 MHz, MeOD) d 12.58, 13.08, 19.31, 19.32, 19.34, 21.36, followed by one layer of scattering TiO2 colloid (WER4-O, Dye-
22.32, 23.39, 26.34, 26.61, 27.93, 29.02, 29.17, 29.19, 29.26, sol) onto uorine tin oxide-coated glass (TEC15, 15 U ,ꢀ1).
29.32, 31.66, 42.11, 46.98, 47.20, 47.41, 47.62, 47.84, 48.05, Each TiO2 layer was calcined at 723 K for 30 min, then
48.26, 48.72, 48.75, 49.25, 58.08, 58.11, 58.14, 82.60, 109.52, immersed in TiCl4 : THF2 in deionised water (50 mM) at 343 K
121.66, 122.32, 126.27, 128.93, 128.96, 129.39, 130.29, 130.73, for 30 min, rinsed with de-ionised water, air-dried for 10 min
140.97, 164.76, 176.55, 179.71, 196.59, 203.63. MS (FTMS+) and re-sintered at 723 K for 30 min. Counter electrodes were
4064 | J. Mater. Chem. A, 2014, 2, 4055–4066
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prepared by drilling two holes ca. 2 cm apart through TEC8™
glass (NSG), washing with iPrOH and drying in air before
depositing Pt colloid (Pt-1, Dyesol) and sintering at 400 ꢁC for 30
min. The two electrodes were sealed together using a Surlyn™
(DuPont) gasket at 120 ꢁC. Contacts were applied onto both
electrodes using conductive silver paint (Agar).
Current–voltage characteristics were measured using an
ABET Solar Simmulator with Xe arc lamp and a Keithley 2400 at
100 mW cmꢀ2 or 1 Sun between 0 and 1 V. Spectral response
measurements were made on a QEX10 Quantum Efficiency
Measurement System in DC mode at resolution of 10 nm.
Lamps were calibrated to 1 Sun (100 mW cmꢀ2) using a certied
(Oriel 91150V) monocrystalline silicon reference cell traceable
to the National Renewable Energy Laboratory (NREL).
4 Y. Cao, Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao and
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7 S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska,
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¨
M. Takata, H. Miura, S. Uchida and M. Gratzel, Adv. Mater.,
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´
¨
8 M. K. Nazeeruddin, P. Pechy and M. Gratzel, Chem.
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2874.
Conclusions
10 J. H. Delcamp, Y. Shi, J.-H. Yum, T. Sajoto, E. Dell'Orto,
¨
This study shows that the position of dye anchoring points in
S. Barlow, M. K. Nazeeruddin, S. R. Marder and M. Gratzel,
half-squarylium dyes inuences hydrophobicity and contact
Chem. – Eur. J., 2013, 19, 1819.
angle of dyes sorbed to TiO2 surfaces. These phenomena are 11 Y. Shi, R. B. M. Hill, J.-H. Yum, A. Dualeh, S. Barlow,
¨
ascribed to the orientation of both individual dyes on the
surface and the way in which sorbed dyes interact with each
M. Gratzel, S. R. Marder and M. K. Nazeeruddin, Angew.
Chem., 2011, 123, 6749.
other such that hydrophobic surfaces consist of dye monolayers 12 T. Maeda, S. Arikawa, H. Nakao, S. Yagi and H. Nakazumi,
with hydrophobic chains perpendicularly arranged to the New J. Chem., 2013, 37(71), 701–708.
surface. Cyclic voltammetry measurements suggest the LUMO 13 M. Guo, P. Diao, Y.-J. Ren, F. Meng, H. Tian and S.-M. Cai,
levels of all the dyes tested should be correctly positioned to
Sol. Energy Mater. Sol. Cells, 2005, 88, 23.
enable electron injection from the dye excited state. However, 14 Y. Chen, Z. Zeng, C. Li, W. Wang, X. Wang and B. Zhang, New
the position of the linker group on the chromophore does affect J. Chem., 2005, 29, 773.
the dye performance in DSC devices where improved Jsc and Voc 15 D. Kuang, P. Walter, F. Nuesch, S. Kim, J. Ko, P. Comte,
¨
¨
are observed for dyes where DFT calculations suggest the LUMO
is located near to the squaraine linker moiety. Finally, we
S. M. Zakeeruddin, M. K. Nazeeruddin and M. Gratzel,
Langmuir, 2007, 23, 10906.
believe this is the rst report of the ultra-fast co-sensitization of 16 J.-J. Cid, J.-H. Yum, S.-R. Jang, M. K. Nazeeruddin, E. M. Nez-
¨
a dye precursor (Hf-SQ) with the associated full dye (squaraine)
which is important in the context of extending spectral response
Ferrero, E. Palomares, J. Ko, M. Gratzel and T. Torres, Angew.
Chem., Int. Ed., 2007, 46, 8358.
whilst minimizing dye costs. Thus, co-sensitizing with two dyes 17 P. J. Holliman, M. L. Davies, A. Connell, B. Vaca Velasco and
from the same synthetic pathway reduces synthetic costs and T. M. Watson, Chem. Commun., 2010, 7256.
results in similar device performance to panchromatic squar- 18 P. J. Holliman, M. Mohsen, A. Connell, M. L. Davies, K. Al-
aines reported in the literature, which require much more
complex and expensive synthetic procedures.
Salihi, M. B. Pitak, G. J. Tizzard, S. J. Coles,
R. W. Harrington, W. Clegg, C. Serpa, O. H. Fontes,
C. Charbonneau and M. J. Carnie, J. Mater. Chem., 2012,
22(26), 13318.
Acknowledgements
19 A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran,
M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh,
We gratefully acknowledge ERDF-WAG LCRI funding for SPARC
(AC), EPSRC/TSB funding for SPECIFIC (EP/I019278/1) for MLD
¨
S. M. Zakeeruddin and M. Gratzel, Science, 2011, 334, 629.
and AC, and the EPSRC UK National Mass Spectrometry Facility 20 J.-H. Yum, P. Walter, S. Huber, D. Rentsch, T. Geiger,
¨
¨
at Swansea University. We thank EPSRC for funding the
National Crystallography Service (Southampton and Newcastle),
F.
Nuesch, F.
De
Angelis,
M.
Gratzel
and
M. K. Nazeeruddin, J. Am. Chem. Soc., 2007, 129, 10320.
and Diamond Light Source for access to synchrotron facilities. 21 T. Geiger, S. Kuster, J.-H. Yum, S.-J. Moon,
¨
¨
M. K. Nazeeruddin, M. Gratzel and F. Nuesch, Adv. Funct.
Mater., 2009, 19, 2720.
22 H. Choi, J.-J. Kim, K. Song, J. Ko, M. K. Nazeeruddin and
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