Ruthenium(II) complexes bearing a naphthalimide fragment: A modular dye platform for the dye-sensitized solar cell

Article abstract of DOI:10.1021/ic3024524

Cycloruthenated complexes of the type [Ru^II(N^N) ₂(C^N)]⁺ (N^N = substituted 2,2′-bipyridine; C^N = substituted 3-(2′-pyridyl)-1,8-naphthalimide ligand) are shown to generate high power conversion efficiencies (PCEs) in the dye-sensitized solar cell (DSSC). It is shown that substitution of the pyridine ring of the C^N ligand with conjugated groups can enhance molar absorption extinction coefficients, while the electron density imparted on the metal center is alleviated by the 1,8-naphthalimide fragment. This latter feature maintains a Ru(III)/Ru(II) redox couple more positive than 0.8 V versus NHE, thereby accommodating regeneration of the oxidized dye by an iodide-based redox mediator. This dye platform can consequently be modulated at various sites to enhance light absorption and suppress recombination between the redox mediator and the TiO₂ surface without compromising dye regeneration, thereby maintaining device PCEs as high as 7%. We also introduce a new phosphine-based coadsorbent, bis(2-ethylhexyl)phosphinic acid (BEPA), which is significantly easier to synthesize than the widely used bis(3,3-dimethylbutyl)phosphinic acid (DINHOP) while also facilitating high dye loading.

Full text of DOI:10.1021/ic3024524

Article
pubs.acs.org/IC
Ruthenium(II) Complexes Bearing a Naphthalimide Fragment: A
Modular Dye Platform for the Dye-Sensitized Solar Cell
Dmitry V. Pogozhev, Mate J. Bezdek, Phil A. Schauer, and Curtis P. Berlinguette*
́
́
Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive N.W., Calgary,
Canada T2N 1N4
S
* Supporting Information
ABSTRACT: Cycloruthenated complexes of the type
[RuII(N^N)2(C^N)]+ (N^N = substituted 2,2′-bipyridine; C^N =
substituted 3-(2′-pyridyl)-1,8-naphthalimide ligand) are shown to
generate high power conversion efficiencies (PCEs) in the dye-
sensitized solar cell (DSSC). It is shown that substitution of the
pyridine ring of the C^N ligand with conjugated groups can
enhance molar absorption extinction coefficients, while the electron
density imparted on the metal center is alleviated by the 1,8-
naphthalimide fragment. This latter feature maintains a Ru(III)/Ru(II) redox couple more positive than 0.8 V versus NHE,
thereby accommodating regeneration of the oxidized dye by an iodide-based redox mediator. This dye platform can consequently
be modulated at various sites to enhance light absorption and suppress recombination between the redox mediator and the TiO2
surface without compromising dye regeneration, thereby maintaining device PCEs as high as 7%. We also introduce a new
phosphine-based coadsorbent, bis(2-ethylhexyl)phosphinic acid (BEPA), which is significantly easier to synthesize than the
widely used bis(3,3-dimethylbutyl)phosphinic acid (DINHOP) while also facilitating high dye loading.
positioned more positively than +0.8 V vs NHE (all
electrochemical potentials in this manuscript are referenced
versus NHE) so that regeneration by the redox mediator is a
thermodynamically favorable process.6,26,27 [The relevant one-
electron redox couple here is likely I•/I− (E° ∼ +0.8 V); note
INTRODUCTION
■
There have been a number of significant recent advances in
chromophore design for the dye-sensitized solar cell (DSSC),1,2
including the discovery of high performance metal-free dyes3,4
and a porphyrin-based dye5 that produces a power conversion
efficiency (PCE) in excess of 12% in the DSSC.6 These findings
followed the discovery that isothiocyanate-free ruthenium(II)
dyes, such as [Ru(dcbpy)2(ppy)]1+ (1a; dcbpy = 4,4′-
dicarboxy-2,2′-bipyridine; ppy =2-phenylpyridine), exhibit
high PCEs in the DSSC.7 This finding is particularly important
from a commercial perspective because the lability of the NCS−
ligands that constitute conventional DSSC dyes, such as
[Ru(dcbpy)2(NCS)2] (N3), has been shown to be a source
of degradation in the device.8,9 Consequently, the ability to
reach high PCEs with dyes bearing chelating ligands10 (e.g., 1a)
in place of NCS− groups offers enormous opportunities for
enhancing the long-term stability of the DSSC.11−14 Moreover,
cycloruthenated dyes related to 1a offer the opportunity to
attenuate both the ground- and excited-state reduction
potentials through judicious substitution of the aryl ring
(Figure 1).15 This same procedure does not translate effectively
to N3 because the highest occupied molecular orbital
(HOMO) is delocalized over the isothiocyanato groups,
which cannot be as easily functionalized.
that the I−/I3 couple at 0.4 V that defines the voltage of the
−
cell is a two-electron couple catalyzed by the counter-
electrode.28] Consequently, electron-withdrawing groups
(EWGs) on the aryl ring of these types of dyes are usually
needed in order to achieve high PCEs.7,11,20 Unfortunately, this
prerequisite imposes significant constraints on how the balance
of the dye scaffold may be designed (e.g., electron-rich light-
harvesting substituents on a polypyridyl ligand can shift the
HOMO energy too negative to be regenerated by I−),29 thereby
prompting us to seek out more weakly σ-donating anionic
ligands.
Following this line of inquiry, we report herein a new class of
cycloruthenated compounds that consist of a naphthalimide
fragment31 (Figure 1 and Chart 1). We view this class of
sensitizer to be a platform for sensitizing TiO2 because the
metal-based oxidation potentials are sufficiently positive in
energy (i.e., >+0.8 V) to enable the structure (e.g., -R, -R1, and
-R2 in Chart 1) to be manipulated without compromising
DSSC performance. Moreover, the electron-rich portion of the
molecule can be modulated to potentially affect chemical
interactions with the electrolyte. The new opportunities for
optimizing both the light-harvesting properties and dye
A major focus of our program (and others16−24) has been
centered on developing ruthenium complexes bearing anionic
chelating ligands, such as deprotonated forms of ppy and 6-
phenyl-2,2′-bipyridine.14,25 One of the fundamental issues with
these types of dyes in conventional DSSCs, however, is the
need to maintain a metal-centered reduction potential
Received: November 9, 2012
© XXXX American Chemical Society
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Figure 1. Three classes of ruthenium-based DSSC sensitizers illustrating the similarities of the frontier molecular orbitals (orbital isosurfaces plotted
at 0.05 au; DHTB = 4,4′-(5″-n-hexyl-2″-thienyl)-2,2′-bipyridine, methyl analogue of DHTB shown for simplicity; HL1 = N-benzyl-3-(2′-pyridyl)-
1,8-naphthalimide).7,11,30
regeneration steps offered by these novel organometallic
sensitizers, which can reach PCEs that exceed that of champion
dyes measured under the same conditions, are outlined below.
The UV−vis absorption spectra of the ester derivatives are
shown in Figure 2a. The spectrum of 2 is characterized by an
intense band centered at 544 nm (denoted λmax1; ε = 1.4 × 104
M−1 cm−1), which, on the basis of TDDFT calculations for 2a,
we attribute predominantly to transitions from the naphthali-
mide fragment and substituent to the bipyridine ligands (see
Supporting Information). The band centered at ∼415 nm
(λmax2; ε = 1.2 × 104 M−1 cm−1) bears greater MLCT character,
wherein transitions to the bipyridyl fragments arise from both
ruthenium- and naphthalimide-centered orbitals. Intense π−π*
transitions dominate the spectrum at λ < 350 nm. Heteroleptic
RESULTS AND DISCUSSION
■
The cyclometalating proligands, C^N, for each of the title
complexes were prepared by a Stille coupling of the relevant N-
substituted-3-bromo-1,8-naphthalimide and 2-(trimethylstann-
yl) pyridine fragments in accordance with a previously reported
procedure.31 In cases where alkylthiophene substituents were
used, a Suzuki cross-coupling scheme involving N-(3′,5′-
dimethylphenyl)-3-(4′′,4′′,5′′,5′′-tetramethyl-1′′,3′′-dioxaboro-
lan-2′′-yl)-1,8-naphthalimide and 2-chloro-4-(5'-hexylthiophen-
2'-yl)-pyridine was needed to access the relevant C^N
proligand. The reaction of these proligands with [(η6-p-
cymene)RuCl2]2 or [(η6-benzene)RuCl2]2 furnished the cyclo-
metalated precursor [Ru(C^N)(MeCN)4]+ as the major
product; [Ru(C^N)(η6-p-cymene)(MeCN)2]+ was not formed
in significant quantities when the former metal reagent was
employed. The esterified derivative, 2, of dye 2a was isolated by
reacting [Ru(C^N)(MeCN)4]+ with 2 equiv of 4,4′-dicarbox-
yethyl-2,2′-bipyridine (DCEB) in ethanol. The family of
heteroleptic ruthenium complexes, 3−6, were obtained by the
synchronous addition of DCEB and 4,4′-bis(5′′-hexylthiophen-
2′′-yl)-2,2′-bipyridine (DHTB) to the cyclometalated precursor
in ethanol (Figure S5). These esterified derivatives, 2−6, were
converted to the corresponding dyes, 2a−6a, in high yields,
after being left to reflux in a DMF/H2O/Et3N (3:1:1) mixture.
complexes 3−6 exhibit similar absorption profiles (λmax1
=
413−419 nm and λmax2 = 555−561 nm), but differ in
absorption intensities (Figure 2, Table 1). Indeed, the superior
light harvesting properties of 3−6 relative to 2 reveal the
benefit of the two alkylthiophene units about the auxiliary bpy
ligand. DFT calculations suggest the HOMO to reside
predominantly on the ruthenium (ca. 30%) and naphthalimide
(ca. 50%) fragments for complexes 3a, 5a, and 6a, whereas the
HOMO is localized on the electron-rich dimethoxyphenyl
substituent in complex 4a. Though not altering the overall
absorption profile in comparison to the other title complexes,
the decreased molar absorptivity of complex 4 is plausibly
attributable to the shift in electron density away from the
naphthalimide core and ruthenium center orbitals that
dominate the UV−vis transitions. The presence or absence of
a methylene spacer between the terminal substituent and core
does not appear to significantly affect molecular absorption
properties (3 cf. 5, Figure 2a), as is to be expected by the
B
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different trends in intensities due largely to solubility issues.
Complexes 2a−6a are weakly emissive upon excitation at
wavelengths corresponding to λmax1 and generate emission
maxima over the λ = 737−757 nm range (Figure 2b, Table 1).
The two compounds bearing a benzyl moiety appended to the
naphthalimide fragment demonstrate two similar lifetimes (τ1 =
17.59 and 9.36 ns and τ2 = 65.96 and 67.73 ns for 2a and 3a,
respectively), while 4a−6a each demonstrates a single ∼65 ns
lifetime.
Chart 1. Numbering Scheme for the Cycloruthenated
Complexes under Investigation
The electrochemical behavior of 2−6 and the corresponding
dyes were measured by cyclic voltammetry; square wave
voltammograms of the dyes attached to TiO2 were also
evaluated (see Figure 4). A reversible one-electron oxidation
process was observed at +1.17 V in the cyclic voltammogram
(CV) for 2. Of relevance to the DSSC is the fact that this value
is shifted more positively by ∼200 mV relative to that of 1.
Consequently, the heteroleptic compounds 3−6, which bear
additional electron-rich light-harvesting units, exhibit reversible
oxidation waves at ca. 1.05 V, a value that is still sufficiently
positive to be regenerated by the relevant iodide-based redox
mediator (i.e., >+0.8 V vs NHE). This potential is decreased by
50−100 mV upon saponification of the esters, but sufficiently
positive values are still maintained. Successive one-electron
reduction processes involving the DCEB and DHTB ligands
were also observed for each complex (Table S1). The E0−0
(determined from intersections of the emission and absorption
curves; Figure 2b) and E(S+/S) values for each of the dyes
indicate E(S+/S*) levels over the −0.88 to −1.00 V range for
2a−6a, all of which are suitably positioned for efficient
electron-injection into TiO2.32,33
Current−voltage traces were collected on DSSCs con-
structed by immersing TiO2 photoelectrodes in a solution
containing dyes 1a−6a (0.2 mM) in absolute ethanol (Table
2), and utilizing iodide-based electrolyte compositions [i.e.,
DMII (0.6M), NaI (0.6M), I2 (0.06M), t-BP (0.5M), GuSCN
(0.1M) in MeCN]. Chenodeoxycholic acid (CDCA) and the
novel bis(2-ethylhexyl)phosphinic acid (BEPA; Figure 5) were
each examined as coadsorbents in this study. The latter
compound can be viewed as an analogue of the proven
coadsorber, bis(3,3-dimethyl-butyl)phosphinic acid (DINHOP;
Figure 5),35,36 but BEPA has the distinct advantage of being
easier to synthesize than DINHOP while facilitating higher dye
loading than CDCA (vide infra).
absence of π-conjugation through the naphthalimide nitrogen
atom. The inclusion of the 5-hexyl-2-thienyl moiety at the para
position of the C^N pyridyl ring for complex 6 clearly results in
enhanced light absorption, in agreement with (TD)DFT
calculations for 6a that show orbital character extended from
the metal over the thienyl moiety (Figure 3). The MLCT band
of 1 is bathochromically shifted relative to each of the title
complexes, which corroborates the more weakly σ-donating
character of the naphthalimide fragment relative to 2-phenyl-
pyridine.
The corresponding acid derivatives, 2a−6a, each exhibit
similar spectroscopic characteristics although they do display
Benchmark devices (using CDCA as the coadsorbent)
reached PCE = 3.1% and PCE = 5.7% for 1a and
Figure 2. (a) UV−vis absorption spectra of 1−6 recorded in MeOH. (b) UV−vis absorption and ambient temperature emission spectra of 2a and 3a
measured in MeOH.
C
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a
Table 1. Spectroscopic and Electrochemical Properties of Title Complexes
UV−vis data
emission data
compd
λmax1 (nm)
λmax2 (nm)
τ1 (ns)
τ2 (ns)
χ2
λem (nm)
E (S+/S) (V vs NHE)
E (S+/S*) (V vs NHE)
1
1a
2
582 (1.7)
577 (1.2)
544 (1.4)
532 (1.2)
561 (2.5)
540 (2.1)
555 (2.0)
536 (1.8)
556 (2.6)
547 (1.7)
557 (2.8)
543 (2.4)
505 (1.1)
504 (1.0)
415 (1.2)
408 (1.0)
419 (2.2)
408 (2.0)
413 (1.8)
416 (1.6)
416 (2.3)
420 (1.6)
415 (3.1)
426 (2.5)
+0.96
+0.84
+1.17
+1.09
+1.05
+1.01
+1.03
+0.90
+1.03
+0.89
+1.02
+0.89
82715
757
751
739
743
737
2a
3
17.59
9.36
65.96
67.73
1.150
1.006
1.028
1.031
1.070
−0.88
−0.89
−0.96
−1.00
−0.97
3a
4
4a
5
65.00
66.53
62.80
5a
6
6a
a
UV−vis absorption data recorded in MeOH at 298 K. ε values indicated in parentheses with units of ×104 M−1 cm−1. Electrochemical data collected
n
on glassy carbon electrode using 0.1 M Bu4NBF4 DMF solutions at a scan rate of 200 mV/s and referenced to 1,1′,2,2′,3,3′,4,4′-octamethyl-
−
ferrocene [oFc]+/[oFc] internal standard followed by conversion to NHE ([oFc]+/[oFc] vs NHE = +0.29 V). Counterion is PF6 for all cationic
complexes.
Figure 4. (a) Cyclic and (b) square wave voltammogram of 6 in
solution and on TiO2, respectively. Data was recorded in DMF in both
cases, with the internal [oFc]+/[oFc] reference shown at +0.29 V vs
NHE.
surface coverage. Notably, superior efficiencies were obtained
with BEPA than with CDCA, a feature we ascribe to the higher
surface coverage using the phosphine derivative.
While comparable PCEs were measured for 3a and 5a using
CDCA, cell efficiencies were markedly lower for dye 4a.
Although the role of these −OMe groups remains under
investigation, the finding casts light on the sensitivity of PCE to
substituents about the anionic ligand. Dye 4a notwithstanding,
Figure 3. Calculated and experimental UV−vis absorption spectrum of
6a with pertinent frontier molecular orbitals. Details of calculated
transitions (theoretical wavelength in nm, oscillator strength, %
contribution to transition): λ3 HOMO → LUMO + 1 (552, 0.0303,
74%); λ4 HOMO − 2 → LUMO (527, 0.1654, 77%); λ5 HOMO − 1
→ LUMO + 1 (502, 0.1951, 41%); λ10 HOMO − 1 → LUMO + 3
(443, 0.1040, 42%); λ18 HOMO − 1 → LUMO + 5 (402, 0.1139,
68%); λ41 HOMO − 4 → LUMO + 2 (336, 0.1900, 39%).
each of the heteroleptic dyes displayed higher PCEs relative to
2a, a feature that is manifest in the additional light-harvesting
units driving up the Jsc values. As expected, the alkyl
substituents of the heteroleptic dyes enhance the Voc values
by impeding undesirable recombination processes. The
incident-photon-to-current-efficiency (IPCE) traces for devices
based on 3a and 6a are shown in Figure 6 and show an onset at
ca. 780 nm with the highest response over the ∼450−600 nm
range (maxima of 68.5% and 71.1% are observed at 540 and
530 nm, respectively). The congruent IPCE traces for these
two complexes, taken together with the substantial Voc increase
in the case of 6a, are consistent with the increase in PCE being
a manifestation of the alkyl groups suppressing the recombi-
nation processes.
[Ru(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)(NCS)3](Bu4N)3
(black dye), respectively (Table 2). The title dyes 2a−6a
demonstrate comparably higher PCEs. The highest PCE (7.0%
at 1 Sun; 7.3% at 0.6 Sun) was observed for the dye 6a,
characterized by the best optical properties and the lowest
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Table 2. Photovoltaic Data Obtained for DSSCs Sensitized by 1a−6a Using Different Coadsorbents and at Different Light
a
Intensities
entry
dye
Sun
coadsorbent1
Voc (V)
Jsc (mA cm−2
)
FF
PCE (%)
1
2
3
4
5
6
7
8
9
1a
2a
3a
4a
5a
5a
6a
6a
1
CDCA
BEPA
CDCA
BEPA
CDCA
CDCA
BEPA
BEPA
CDCA
0.53
0.59
0.65
0.60
0.70
0.68
0.68
0.66
0.68
8.49
13.6
16.3
9.9
0.53
0.59
0.56
0.64
0.65
0.67
0.65
0.68
0.59
3.1
4.8
6.3
3.9
6.6
7.0
7.0
7.3
5.7
1
1
1
1
14.5
10.1
15.4
10.3
13.77
0.6
1
0.6
1
black dye34
a
Electrolyte compositions used for device fabrication: DMII (1,3-dimethylimidazolium iodide) (0.6 M), NaI (0.6 M), I2 (0.06 M), t-BP (4-tert-
butylpyridine) (0.5 M), GuSCN (guanidinium thiocyanate) (0.1 M) in MeCN; 0.1 equiv of BEPA or 10 equiv of CDCA was used as coadsorbent as
indicated. DSSC substrate consists of 12-μm active and 3-μm scattering TiO2 layers; 0.28 cm2 active area.
the chemical reactivity of the R group (see Chart 1) can affect
the regeneration step with the redox mediator, a point that will
be elaborated in future studies. In addition to this new dye
platform containing an uncommon cyclometalating ligand,31 we
also introduce here an effective new coadsorbent, BEPA, for use
in the DSSC. This coadsorbent was found to be superior to the
widely used CDCA in certain cases, and is more synthetically
accessible than the DINHOP precursor that is also pervasive in
the field. These collective findings are important advances for
the sensitization of TiO2 in energy conversion schemes.
Figure 5. Chemical structures of bis(3,3-dimethyl-butyl)phosphinic
acid (DINHOP) and bis(2-ethylhexyl)phosphinic acid (BEPA). It is
shown that the latter acts as an effective coadsorbent in this study.
CONCLUSIONS
■
ASSOCIATED CONTENT
* Supporting Information
Synthetic and computational details, and full characterization of
complexes and devices. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
This study describes a series of cycloruthenated DSSC
sensitizers ligated by substituted 3-pyridyl-1,8-naphthalimide.
The physicochemical properties of this novel dye scaffold can
be manipulated by modifying various sites of the complex while
not sacrificing performance. The best performing dye of this
particular series in the DSSC was found to be the tris-
heteroleptic complex 6a. This dye was decorated with light-
harvesting alkylthienyl moieties on both the auxiliary bipyridine
ligand and pyridyl-naphthalimide ligand to produce substantial
extinction coefficients and to also suppress charge recombina-
tion processes. While the presence of three electron-rich
substituents in most cycloruthenated sensitizers would push the
HOMO level to unsatisfactorily high energies for regeneration
of the photo-oxidized dye by iodide, the naphthalimide
fragment is sufficiently electron-withdrawing to maintain a
thermodynamically favorable regeneration process thus reach-
ing a performance of 7.0% under AM1.5 sunlight. This
fragment also presents the opportunity to examine whether
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: cberling@ucalgary.ca.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Canadian Natural
Science and Engineering Research Council (NSERC), Canada
Research Chairs, Canadian Foundation for Innovation (CFI),
Alberta Ingenuity, Alfred P. Sloan Foundation, and the Canada
School of Energy and Environment (CSEE).
Figure 6. IPCE and photovoltaic (inset) data for 3a (left) and 6a (right).
E
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