A Computational and Experimental Study of Thieno[3,4-b]thiophene as a Proaromatic π-Bridge in Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/chem.201503187

Four D-π-A dyes (D=donor, A=accpetor) based on a 3,4-thienothiophene π-bridge were synthesized for use in dye-sensitized solar cells (DSCs). The proaromatic building block 3,4-thienothiophene is incorporated to stabilize dye excited-state oxidation potentials. This lowering of the excited-state energy levels allows for deeper absorption into the NIR region with relatively low molecular weight dyes. The influence of proaromatic functionality is probed through a computational analysis of optimized bond lengths and nucleus independent chemical shifts (NICS) for both the ground- and excited- states. To avoid a necessary lowering of the TiO₂ semiconductor conduction band (CB) to promote efficient dye-TiO₂ electron injection, strong donor functionalities based on triaryl- and diarylamines are employed in the dye designs to raise both the ground- and excited-state oxidation potentials of the dyes. Solubility, aggregation, and TiO₂ surface protection are addressed by examining an ethylhexyl alkyl chain in comparison to a simple ethyl chain on the 3,4-thienothiophene bridge. Power conversion efficiencies of up to 7.8 % are observed.

Full text of DOI:10.1002/chem.201503187

DOI: 10.1002/chem.201503187
Full Paper
&
Dye-Sensitized Solar Cells
A Computational and Experimental Study of
Thieno[3,4-b]thiophene as a Proaromatic p-Bridge in Dye-
Sensitized Solar Cells
Phillip Brogdon,[a] Fabrizio Giordano,[b] George A. Puneky,[a] Amala Dass,[a]
Shaik M. Zakeeruddin,[b] Mohammad Khaja Nazeeruddin,[b] Michael Grätzel,[b]
Gregory S. Tschumper,[a] and Jared H. Delcamp*[a]
Abstract: Four D-p-A dyes (D=donor, A =accpetor) based
on a 3,4-thienothiophene p-bridge were synthesized for use
in dye-sensitized solar cells (DSCs). The proaromatic building
block 3,4-thienothiophene is incorporated to stabilize dye
excited-state oxidation potentials. This lowering of the excit-
ed-state energy levels allows for deeper absorption into the
NIR region with relatively low molecular weight dyes. The in-
fluence of proaromatic functionality is probed through
a computational analysis of optimized bond lengths and nu-
cleus independent chemical shifts (NICS) for both the
ground- and excited- states. To avoid a necessary lowering
of the TiO2 semiconductor conduction band (CB) to promote
efficient dye–TiO2 electron injection, strong donor function-
alities based on triaryl- and diarylamines are employed in
the dye designs to raise both the ground- and excited-state
oxidation potentials of the dyes. Solubility, aggregation, and
TiO2 surface protection are addressed by examining an ethyl-
hexyl alkyl chain in comparison to a simple ethyl chain on
the 3,4-thienothiophene bridge. Power conversion efficien-
cies of up to 7.8% are observed.
a porphyrin-based dye SM315[8] have demonstrated record
PCEs of 12.5–13.0% without the use of precious metals. The
success of these dyes is largely driven by the iterative use of
the ubiquitous, modular D-p-A dye design (D=donor, A=ac-
ceptor). PCE improvements over these impressive devices is
reasonable and would be aided through the careful develop-
ment of building blocks to allow tunable access to the NIR
region of the solar spectrum.[9] Proaromatic building blocks are
promising dye components that allow for tunability into the
NIR region from relatively low molecular weight dyes.
Introduction
Since the introduction of dye-sensitized solar cells (DSCs) in
1991,[1] tremendous efforts have been made to bring DSCs to
the forefront of alternative energy options.[2] The basic operat-
ing principle of a DSC device involves injection of electrons
from a photoexcited dye molecule into a metal oxide semicon-
ductor (typically TiO2). The electron then traverses an external
circuit to a counter electrode, where it is transferred back to
the dye molecule through the use of a redox shuttle, such as
À
the IÀ/I3 couple.[3] The dye plays many crucial roles, including
Several DSC photosensitizers have recently employed the
use of proaromatic structures.[8,11–13] These structures benefit
from added stability provided by a region of local aromaticity
in their excited-state (Figure 1). Due to this aromatically stabi-
lized excited state, photoexcitation occurs at significantly
lower energies, which leads to a narrowed optical bandgap.
The proaromatic conjugated building block 3,4-thienothio-
phene (3,4-TT) has been explored recently in several polymer
applications as a low-band-gap material for organic photovol-
taic devices,[14–16] and only once in DSC devices as a p-bridge
with an encouraging PCE (3.7%).[12]
controlling the light absorbed and charge separation, which di-
rectly affects power output.
Traditionally, ruthenium-based sensitizers have dominated
the DSC dye-design field by providing the highest device
power conversion efficiencies (PCEs of approximately 11 %).[4–6]
Recently, metal-free organic dyes ADEKA-1[7a] and C275[7b] and
[a] P. Brogdon,+ G. A. Puneky, Dr. A. Dass, Prof. G. S. Tschumper,
Prof. J. H. Delcamp
Department of Chemistry and Biochemistry
University of Mississippi, University, MS 38677 (USA)
E-mail: delcamp@olemiss.edu
Proaromatic materials often absorb longer wavelengths of
light than aromatic materials of similar conjugation length.
This attractive attribute is accomplished through the stabiliza-
tion of excited-state oxidation potentials (E(S+/S*), Table 1). How-
ever, a balance is needed between stabilizing E(S+/S*) values to
absorb shorter wavelengths and maintaining a necessary over-
potential (DGinj) for efficient electron injection from a dye excit-
ed state to the TiO2 conduction band (CB). A prior study on
[b] Dr. F. Giordano,+ Dr. S. M. Zakeeruddin, Prof. M. K. Nazeeruddin,
Prof. M. Grätzel
Laboratory for Photonics and Interfaces
Swiss Federal Institute of Technology, 1015 Lausanne (Switzerland)
[+] Authors contributed equally.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503187.
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in order to evaluate if PCE diminishing aggregative effects
were present in final devices. Additionally, the alkyl groups on
the 3,4-TT ester p-bridge were varied in combination with each
donor to evaluate the structure–performance relationship of
longer versus shorter chain length on DSC device PCE.
Results and Discussion
The synthesis of the PB (ethylhexyl-substituted 3,4-TT) and DP
(ethyl-substituted 3,4-TT) series (depicted in Figure 2) proceeds
Figure 1. A comparison of a D-p-A dye designed with common aromatic
building blocks (such as L1[10]) with a dye containing a proaromatic building
block (PB1) leading to competing aromatic ground and excited states.
Table 1. Optical and electrochemical data for PB1, PB2, DP1 and DP2.
Absorbance data[a]
Electrochemical data
opt
lmax
e
lonset
[nm][b]
E(S+/S)
E(S+/S*)
[V][d]
Eg
[eV][e]
[nm]
[mÀ1 cmÀ1
]
[V][c]
PB1
PB2*
DP1
560
584
558
591
524
509
483
26000
30000
22000
20000
23000
35000
20000
665
670
650
670
600
590
610
+1.09
+1.08
+1.09
+1.08
+1.35[g]
+1.41[g]
+0.96
À0.79
À0.77
À0.79
À0.77
À0.70
À0.71
À1.21
1.88
1.85
1.88
1.85
2.05
2.12
2.17
DP2
MCT-1[f]
FTT-1[f]
Figure 2. Target dye structures with varying donor groups and chain lengths
on the proaromatic 3,4-TT p-bridge with cyanoacrylic acid.
LS-1[h]
*
[a] Measurements made in DCM. [b] The absorption onset (lonset) was
taken at the intercept of a tangent line on the low energy side of the
lmax absorption curve and the baseline. [c] E(S+/S) measurements were
made in DCM with a 0.1m Bu4NPF6 electrolyte and ferrocene as an inter-
nal standard. All values are reported versus NHE. [d] E(S+/S*) was calculated
in six synthetic steps according to Scheme 1. Lithium halogen
exchange with commercially available 3,4-dibromothiophene
and quenching with DMF leads to bromoaldehyde 1. A one-
pot Cu-catalyzed thiol cross-coupling/condensation reaction to
close the ring forms the 3,4-TT ester 2. The subsequent Vilsmei-
er–Haack reaction on 2 yields two regioisomers 3a and 3b (in
a 1:2 mixture), which vary by orientation of the proaromatic
ring sulfur on the opposite (3a) or same side (3b) as the alde-
opt
from the equation E(S+/S*) =E(S+/S)ÀEgopt. [e] Eg was calculated from the
equation Egopt =1240/lonset. [f] MCT-1 measurements are taken from refer-
ence [12]. Optical data and electrochemical data are reported in THF.
[g] Ground-state oxidation potentials calculated from the E0À0 and E0À0
*
values reported in reference 12. [h] LS1 measurements are reported in
reference [17] below. LS1 has an identical structure to DP2 and PB2 with
the 3,4-TT proaromatic bridge substituted with a simple thiophene. Note:
reference [17] uses a ferrocene reference set to +0.4 V vs. NHE. For con-
sistency in this manuscript, we have calibrated the LS1 numbers in our
table to a ferrocene reference of +0.7 V vs. NHE as reported by Gieger
and co-workers[18] with a conversion of SCE to NHE by addition of
+0.24 V. *Denotes that the dyes are identical with the exception of
changing the proaromatic 3,4-TT bridge to a thiophene.
1
hyde. The H NMR of these isomers yields three singlets in the
aromatic region with only slight shifts in ppm values for each
isomer. To unambiguously assign the structure to these iso-
mers NOE NMR and X-ray crystallography techniques were em-
ployed. A slight NOE response is observed between the alde-
hyde-H and proaromatic thiophene-H for isomer 3a with no
NOE response observed for any of the hydrogens in 3b
(Figure 3). X-ray quality crystals were obtained from isomer 3a
through the use of vapor infusion of hexanes into a solution of
3a in toluene to give long yellow needles. The NOE NMR re-
sults and X-ray crystallography results both confirm the major
isomer as desired product 3b (Scheme 1). Aldehydes 3a and
3b are separable by silica gel chromatography with 3b having
a slightly lower Rf. This regioselective route proved reliably
scalable to multiple gram batches of isomer 3b.
3,4-TT showed low energy E(S+/S*) values that require the addi-
tion of excessive Li+ to lower the TiO2 CB to boost DGinj
values. This results in low device open-circuit voltage values
and diminished PCEs. Coarse tuning of E(S+/S*) values may be
accomplished through the selection of electron-deficient func-
tionalities; however, E(S+/S*) values may be finely tuned through
donor building block selection. Increasing electron-donating
strength often leads to a weak destabilization of the dye E(S+
values. A series of four dyes based on 3,4-TT with strong
Compound 3b undergoes NBS bromination in good yield to
give the bromoaldehyde 4 in four steps and 18% overall yield.
Arylamine donor building blocks were installed through Suzuki
coupling reactions in excellent yields to give intermediate 5,
/S*)
donor motifs dihexlyoxytriphenylamine (TPA) and indoline
were selected as target materials. The concise indoline donor
was selected to compare with the larger triphenylamine donor
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Scheme 1. Synthetic scheme for dyes PB1, PB2, DP1, and DP2. a) i. nBuLi (1.5 equiv), 0.25m Et2O, À788C, 30 min; ii. DMF (excess), À788C to RT, 2 h; 74%.
b) Ethylhexyl mercaptoacetate (or ethyl mercaptoacetate, 1.1 equiv), CuO nanopowder (3%), K2CO3 (1.5 equiv), 0.5m DMSO, 608C, 16 h; 56% (EtHexyl); 80%
(Et). c) i. POCl3 (1.05 equiv), DMF (1.05 equiv), 0.4m DCE, 08C, 2.5 h; ii. 0.4m KOH (1.0m, aq), RT, 15 min; 48% (EtHexyl); 71% (Et). d) NBS (1.2 equiv), 0.2m DMF,
08C to RT; 62% (EtHexyl); 82% (Et). e) TPA-Bpin (or Ind-Bpin, 1.1 equiv), K3PO4 (3.0 equiv), [Pd2(dba)3] (4%), X-Phos (16%), 0.05m toluene, 1.15m H2O, 808C,
6 h; 75% (TPA-EtHexyl); 93% (TPA-Et); 84% (Ind-EtHexyl); 26% (Ind-Et). f) Cyanoacetic acid (3.0 equiv), piperidine (7.0 equiv), 0.06m CHCl3, 908C; 77% (TPA-
EtHexyl); 51% (TPA-Et); 63% (Ind-EtHexyl); 35% (Ind-Et).
Figure 4. UV/Vis absorption spectra of PB1, PB2, DP1 and DP2 in dichloro-
methane.
Figure 3. NOE NMR spectrum of intermediate 3a (top) with the aldehyde
proton irradiated (bottom).
This trend of increasing molar absorptivity is expected based
on previous studies regarding an increase in alkyl chain
and the final dyes (PB1, PB2, DP1, and DP2) were completed
through Knoevenagel condensation with cyanoacetic acid
(CAA) in high yield in six linear steps from commercial starting
materials in up to about 13% overall yield.
length.[19] Installation of a stronger indoline donor in place of
the diheyxloxyTPA donor of DP1 and PB1 resulted in red-shift-
ed lmax and lonset values (by approximately25 nm) for both DP2
and PB2. Introduction of extended alkyl chains on the 3,4-TT
bridge again resulted in higher molar absorptivity values for
PB2 than DP2 with shorter alkyl chains (30000 versus
Optical and electrochemical data
20000mÀ1 cmÀ1). Importantly, the dihexyloxyTPA and indoline
opt
UV/Vis absorption spectra were collected for PB1, PB2, DP1,
and DP2 in dichloromethane (Figure 4, Table 1). The use of di-
hexyloxyTPA as a strong donor on dye DP1 gave a lmax and
lonset of about 560 and 650 nm respectively. A longer alkyl
chain with a stereogenic center on the 3,4-TT bridge (PB1)
yielded identical lmax and lonset values to DP1; however, the
molar absorptivity increased from 22000 to 26000mÀ1 cmÀ1.
donors show substantially diminished optical band gaps (Eg )
when compared with a simple TPA donor on the 3,4-TT-CAA p-
bridge-acceptor (about 1.85 versus 2.12 eV, Table 1). Dye LS-
1 (identical to DP2 and PB2 when the 3,4-TT p-bridge is re-
place with a simple thiophene) was used for comparison of
the proaromatic 3,4-TT with a common p-bridge (Table 1). A
lmax red-shift of >100 nm is observed when the 3,4-TT building
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block is used, while the molar absorptivities are similar. Electro-
chemical analysis is conducted to better understand the effects
of the 3,4-TT building block on the ground- and excited-state
oxidation potentials when compared to thiophene.
Solvatochromic effects of the proaromatic 3,4-TT bridge
were examined and compared to that of thiophene through
the comparison of PB2 and LS1, which differ only at the p-
bridge (Figures 5 and 6 in the text and Tables S2 and S3 in the
Figure 6. Emission spectra of PB2 (A) and LS1 (B) in various solvents. EA is
ethyl acetate and Tol is toluene. The LS1 absorption spectrum in DMF is
omitted due to solubility issues, see Supporting Information for the spec-
trum.
the Et2O data point due to apparently low solubility. The differ-
ence in the solvatochromic effects of these two dyes were fur-
ther highlighted though analysis of the Stokes shifts observed.
Dye LS1 gave Stokes shifts ranging broadly from 0.38 to
0.98 eV, while those of PB2 were observed over a significantly
narrower range of 0.27 to 0.52 eV. The dramatic difference in
solvatochromic effects from the emission curve sets and
Stokes shifts for these two dyes may in part be due to
a weaker stabilization of the charge-separated excited-state
from the solvent with proaromatic bridges as the dye excited-
state is already stabilized by aromaticity.
Figure 5. UV/Vis absorption spectra of PB2 (A) and LS1 (B) in various sol-
vents. EA=ethyl acetate and Tol =toluene. The LS1 absorption spectrum in
DMF is omitted due to solubility issues, see Supporting Information for the
spectrum.
Supporting Information). For both PB2 and LS1 the absorption
maxima varied by approximately 100 nm with dichlorome-
thane giving the longest wavelength absorption for both dyes
and DMSO/DMF giving the shortest wavelength absorptions
(Figure 5). This indicates that the solvent stabilization extent is
similar between the two dyes in the ground-state geometry. It
should be noted that a clear trend based on solvent polarity
was not apparent. A substantial difference was noted when
comparing the range of the emission maxima shifts in various
solvents (Figure 6). The non-proaromatic LS1 gave emissions
over roughly a 125 nm range (approximately 0.24 eV), while
PB2 shows a relatively narrow emission range based on sol-
vent of about 50 nm (approximately 0.11 eV) after excluding
The ground-state oxidation potentials (E(S+/S)) were mea-
sured to determine if suitable electron regeneration could take
À
place between the dye and the IÀ/I3 redox shuttle. E(S+/S)
values are critical in determining the thermodynamic driving
force for dye regeneration (DGreg) in a functioning DSC device
and calculating E(S+/S*) values. Cyclic voltammetry was used to
evaluate the ground-state oxidation potential of PB1, PB2,
DP1, and DP2 in DCM with a 0.1m Bu4NPF6 electrolyte
(Table 1). Ground-state potentials were found to range from
+1.08 to +1.09 V versus NHE. These potentials are substantial-
ly higher in energy than previously measured for unsubstituted
TPA-3,4-TT dyes (+1.41 to +1.35 V vs. NHE).[12] Thus, a lower
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overpotential for DGreg of 0.73–0.74 eV is observed for the PB
and DP dyes. The optical band gap for these dyes ranged from
1.88-1.85 eV with PB2 and DP2 having the narrowest bandgap.
The excited-state potentials (À0.77 to À0.79 V vs. NHE) are cal-
opt
culated from the equation E(S+/S*) =E(S+/S)ÀEg and show a de-
sirable electron injection driving force into the TiO2 conduction
band (DGinj =0.27–0.29 eV, Figure 7). It is worth noting that the
Figure 8. HOMO (A) and LUMO (B) of PB1/DP1 analogues as well as HOMO
(C) and LUMO (D) of PB2/DP2 analogues. All extended alkyl chains were
truncated to methyl chains. Iso values set to 0.04. See Supporting Informa-
tion for orbitals of L1, FTT-1 and fully alkylated PB1.
HOMO and LUMO, which suggests efficient intramolecular
charge transfer is feasible. In order to examine the validity of
the valence bond theory argument in Figure 1 for proaromatic
materials exhibiting excited-state aromaticity, DFT and TD-DFT
computations with the B3LYP/6-311+G(d,p) functional/basis
set were performed to probe the aromaticity in the singlet
ground state (S0) as well as the first excited singlet (S1) and
triplet (T1) states of the 3,4-TT CT dyes and in comparison with
L1. For example, valence bond theory suggests bonds 4 and 5
(Figure 9) will elongate upon excitation to the S1 state as
Figure 7. Comparison of ground-state (positive values) and excited-state
(negative values) oxidation potentials of PB and DP series dyes with previ-
ously reported MCT-1 and FTT-1. Values for MCT-1 and FTT-1 are reported
as shown in Table 1 of Reference [12] with the ground-state oxidation poten-
tials calculated from the E0À0 and E0À0* reported therein.
E(S+/S) values are similar for the 3,4-TT dyes PB2/DP2 and the
non-pro-aromatic dye LS-1 (within ꢀ100 mV); however, the
E(S+/S*) values are dramatically shifted by >400 mV to more
stable values energies when the proaromatic 3,4-TT bridge is
incorporated into the dye structure.
Computational studies
In addition to well-positioned oxidation potentials, dyes should
also have an orbital distribution that allows for electron injec-
tion into TiO2 and restrict back electron transfer. Ideally, the
LUMO should be positioned near the acceptor/anchor at the
TiO2 surface to allow for efficient injection and the HOMO
should be centered predominately on the donor and spatially
separated from the TiO2 surface to diminish back electron
transfer from the TiO2 semiconductor.[2] DFT calculations em-
ploying the B3LYP functional and 6-311G+(d,p) basis set show
an orbital arrangement that is ideal for electron injection into
the TiO2 semiconductor. As shown in Figure 8, the HOMO is
centered on the donor region and the LUMO is centered on
the acceptor side of the dye for both PB1/DP1 and PB2/DP2
analogues as well as for the fully alkylated PB1, FTT-1 and L1
(see Supporting Information). The HOMO and LUMO orbitals
appear to have good overlap across the 3,4-TT bridge as a sig-
nificant accumulation of orbital density is present for both the
Figure 9. Comparison of 3,4-TT bond lengths in PB1/DP1 and L1 dyes in
ground state versus the singlet excited stated. TPA is bis(methoxyphenyl)-
phenylamine and CAA is cyanoacrylic acid. Sh=shortens; L=lengthens.
double-bond character is lost due to intramolecular charge
transfer (ICT). Conversely, bond 3 is expected to shorten as the
donor/acceptor-substituted thiophene loses local aromaticity.
These predicted bond-length changes are indeed observed
when the DFT optimized S0 structure of PB1 is compared to
the corresponding TD-DFT optimized S1 and T1 structures. Simi-
lar changes for L1 are predicted and observed with an overall
decrease in analogous bond-length change (Table 2). The full
set of computed bridge-related bond-length changes for both
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The B3LYP/6-311 +G(d,p) ground-state optimized geometry
of the PB1/DP1 and PB2/DP2 with truncated alkyl chain ana-
logues were used for all geometry and NICS computations.
Evaluation of PB1 with full alkyl chain lengths gave near identi-
cal NICS values when compared with the truncated alkyl
chains of the PB1/DP1 analogue indicating a minimal effect on
proaromaticity in this system (see the Supporting Information
for DFT geometries, TD-DFT evaluation, and NICS graphs). For
comparison, FTT-1 and L1 dyes were analyzed with the same
method/functional/basis set combination. The magnetic shield-
ing tensor was computed at the geometric center of each ring
as well as 20 evenly spaced points (0.25 Š apart) “above” and
“below” the ring plane (41 points altogether) for both the S0
and T1 states with the same functional and basis set. These
NICS(R) and NICS(R)zz values are plotted in Figure 10 and in the
Table 2. Comparison of CÀC bond lengths in PB1/DP1 and L1 ground
state (S0), singlet excited-state (S1), triplet excited-state (T1) and the
changes in lengths.
Bond
S0 [Š]
S1 [Š]
DRS1 [Š]
T1 [Š]
DRT1 [Š]
dye: PB1/DP1 analogue
1
2
3
4
1.361
1.430
1.433
1.399
1.397
1.373
1.425
1.426
1.416
1.411
0.012
À0.006
À0.007
0.018
1.379
1.416
1.408
1.442
1.437
0.018
À0.014
À0.024
0.043
5
0.014
0.040
dye: L1
3
4
5
1.401
1.390
1.391
1.394
1.391
1.408
À0.006
0.001
0.017
1.369
1.419
1.443
À0.032
0.029
0.052
dyes are shown in Table 2. The CÀC bond lengths of the proar-
omatic thiophene are shifted toward more homogeneous
values, with the length of bond 1 increasing by 0.012 Š, while
the lengths of bonds 2 and 3 decrease by 0.006 and 0.007 Š,
respectively, for the S1 state. In further support of this analysis,
CÀC bonds 4 and 5 found in the donor–acceptor thiophene
lengthen by 0.018 and 0.014 Š, respectively, upon losing aro-
maticity. The T1 state shows bond-length changes consistent
with those for the S1 state, but slightly larger in magnitude.
Additionally, nucleus independent chemical shifts (NICS)
have been computed as another means to assess changes in
the aromaticity of these systems. Since first proposed in
1996,[20] a variety of NICS indices have been introduced as
a means to quantify aromaticity based on magnetic criteria.[21]
For (quasi-) planar organic ring systems, the zz component of
the magnetic shielding tensor and the total isotropic absolute
shielding evaluated at various points R Š above/below the
geometric center of the ring (typically denoted NICS(R)zz and
NICS(R), respectively) have emerged as effective metrics of aro-
maticity that can readily be computed with DFT methods in
a variety of popular software packages.[21c,22] Unfortunately, TD-
DFT magnetic shielding tensors are not as widely available,
which prevents us from directly assessing the aromaticity in
the S1 excited state of these 3,4-TT-based molecules. It is possi-
ble, however, to compute these NICS(R) and NICS(R)zz values
for the T1 excited state, because, unlike S1, the different spin
symmetry prevents variational collapse to the S0 ground state.
Baird’s Rule predicts that Hückel’s p-electron-counting rules
are reversed in both the S1 and T1 states when compared with
the S0 state (i.e., [4n] is aromatic and [4n+2] is antiaromatic in
excited-states).[23] This expectation has, in fact, been verified
computationally for benzene.[24] For vertical excitations (i.e.,
when the geometry of the excited states is constrained to that
of the ground state), both the S1 and T1 states demonstrate
strong antiaromatic character. Due to these similarities, analysis
of the T1 state in this manner provides a correlated indicator of
aromatic behavior in the S1 state, which is typically the most
populated state in these types of dyes. Because of the difficul-
ties in directly computing the NICS indices in the S1 excited
state, the general strategy was adopted here to compare the
S0 and T1 aromaticty of both 3,4-TT rings in PB1/DP1.
Figure 10. NICS(R) calculations demonstrating changes in aromaticity for 3,4-
TT TPA/CAA-substituted thiophene ring (A) and the proaromatic thiophene
ring (B) upon excitation to the T1 state. * Denotes the ring undergoing NICS
computational analysis at multiple points extending 5 Š above and below
the plane of the ring.
Supporting Information, respectively, over the range spanning
5 Š “below” to 5 Š “above” the rings. The large negative iso-
tropic NICS indices (ca. À9 ppm) at the center of the ring (i.e.,
NICS(0)), as well as slightly above and below the ring (e.g.,
NICS(Æ1)) indicate both the TPA/CAA-functionalized thiophene
ring and the proaromatic thiophene ring of 3,4-TT have appre-
*
ciable aromatic character in the S0 state ( data in Figure 10).
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This is expected as the 3,4-TT building block has a 10-p elec-
tron periphery (Figure 1). Upon vertical excitation to the T1 ex-
cited state, the TPA/CAA-functionalized thiophene ring experi-
ences a significant decrease in aromaticity as the NICS(R)
values near the ring plane become small in magnitude and
even change sign (” data in Figure 10a). Again, this is consis-
tent with predictions from valence bond theory as charge is
transferred from the donor to the acceptor, the local cyclic
conjugation is lost. Interestingly, vertical excitation to the T1
state does not significantly change the aromatic character of
the proaromatic ring of 3,4-TT as indicated by the similarity of
*
the T1 (”) and S0 ( ) NICS(R) data in the bottom panel of
Figure 10, signifying a preservation of aromaticity in the excit-
ed-state dye. This preservation of aromaticity leads to a stabi-
lized excited-state energy and promotes longer wavelength
absorption in these dyes when compared with dyes of similar
conjugation lengths. Additional NICS computational analysis is
available for PB2/DP2, L1, and FTT-1 in the Supporting Infor-
mation. Changes in NICS values based on the donor strengths
of the aryl amine donors were subtle by NICS analysis.
TD-DFT was used to evaluate the influence of increasing
donor strength in combination with a proaromatic p-bridge on
the lowest energy vertical transition position and the oscillator
strength. While additional effects such as alkyl chain length
affect experimental values for molar absorptivities, computa-
tional evaluation shows primarily the electronic effects of the
conjugated system irrespective of alkyl chains. Thus computa-
tional analysis was used to explore the effects of various
donors and proaromatic versus aromatic bridges. Electron-
donor strength proceeds in the following order: FTT-1<PB1/
DP1<PB2/DP2. The changes in vertical transition are predict-
ed to be subtle (<0.1 eV difference); however, the changes in
oscillator strength are apparent and follow the predicted
donor strength trend ranging from 0.8537 to 0.9330. When the
oscillator strengths of L1 and PB1 (analogous with the change
of 3,4-TT to thiophene) are compared, that of the proaromatic
system are significantly higher at 0.8775 versus 0.7506. The in-
crease in oscillator strength may in part be attributed to the in-
fluence of the proaromatic bridge promoting a strong charge
transfer.
Figure 11. Top: J–V curve of dyes PB1, PB2, DP1 and DP2. A 450 W Xe lamp
at a simulated AM 1.5 G sun illumination was used as the light source. Elec-
trolyte containing 1.0m 1,3-dimethylimidazolium iodide (DMII), 100 mm LiI,
30 mm I2, 0.5m tert-butylpyridine,0.1m guanidinium thiocyanate (GNCS) in
acetonitrile was used to measure PB1, DP1, PB2 and DP2. Bottom: IPCE
spectrum of PB1-, PB2-, DP1-, and DP2-based DSCs.
Table 3. Photovoltaic device[a] measurements under AM 1.5 G incident
light.
VOC [mV]
JSC [mAcm2]
FF
PCE [%][b]
PB1
PB2
DP1
DP2
704
648
680
697
12.1
12.7
10.9
13.7
0.75
0.75
0.75
0.76
6.50
6.24
5.61
7.41
Device data
Having demonstrated favorable thermodynamic properties and
orbital positions for the use of the 3,4-TT dye series in DSC de-
vices, we then evaluated their performance in devices with
[a] Devices fabricated using electrolyte containing 1.0m 1,3-dimethylimi-
dazolium iodide (DMII), 100 mm LiI, 30 mm I2, 0.5m tert-butylpyridine,
0.1m guanidinium thiocyanate (GNCS) in acetonitrile. [b] PCEs were calcu-
Voc Jsc
I0
lated using the formula:
FF. All devices were measured at 1 sun.
À
TiO2 and IÀ/I3 through I–V curve, incident photon-to-current
conversion efficiency (IPCE), and electron lifetime measure-
ments. As shown in Figure 11, the highest photocurrent densi-
ty of the 3,4-TT series of dyes was observed with DP2 followed
values (Table 3, Figure 11). Dye DP1 gave the lowest Jsc value
due to a narrowed range of absorbance and the lowest IPCE
peak value of 72%. The integrated IPCE spectrum areas are in
good agreement with the values measured by the current–
voltage curve. A dip in absorbance is seen for all dyes present-
ed from 400–500 nm with PB1 showing significantly better ab-
sorbance in this range allowing for a boost in Jsc relative to
DP1 with a similar IPCE onset (Table 3, Figure 11). The trend in
by PB2, PB1, and DP1 with a range of 13.7 to 10.9 mAcmÀ2
.
The red-shifted absorption of indoline-donor-based DP2 and
PB2 in part leads to the enhanced photocurrent generation
(Tables 1 and 3 and Figures 4 and 11). Dye DP2 reached the
highest peak IPCE value of 81%. Dyes PB1 and DP1 demon-
strate noticeably lower photocurrent breadths when the IPCE
was analyzed, which led to lower short-circuit current (Jsc)
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À
open-circuit voltages (Voc) observed is PB1>DP2>DP1>PB2
and ranged from 704 to 648 mV. To better understand this
trend in open circuit voltages, we studied electron lifetime
versus capacitance trends to evaluate the dye structure effect
on electron recombination rates with the redox shuttle
(Figure 12). The longer alkyl chains present on 3,4-TT with PB1
overpotential from the IÀ/I3 redox shuttle. Due to the in-
creased energy of E(S+/S*) for the PB/DP series, Li+ doping is
not required to artificially lower the conduction band of TiO2
(100 mm LiI in PB1/DP1 and PB2/DP2, 500 mm LiI in non-sub-
stituted triarylamine dyes). These improved electronic parame-
ters lead to an overall increased performance ranging from
5.61% (DP1) to 7.41% (DP2) with a DP2 champion cell reach-
ing 7.8% PCE.
Conclusion
We have synthesized four new dyes for use in DSC devices
based on the 3,4-TT proaromatic building block. NOE NMR and
X-ray crystallography studies were required to accurately char-
acterize isomers resulting from a Vilsmeier–Haack reaction on
the 3,4-TT building block. The valence bond theory prediction
that an aromatic structure is present in the excited state of
these proaromatic dyes was supported through computational
analysis of bond lengths and NICS values in both the ground
and excited states. The computational procedures set forward
here will aid the development of future organic materials by al-
lowing the evaluation of excited-state energy influences.
Through judicious donor selection, we observed a decrease in
opt
the Eg
relative to analogous dyes with weaker donor
strengths. We observe a noticeable red shift in the lonset of the
more strongly donated indoline-based dyes (PB2 and DP2) rel-
ative to their TPA-based counterparts (PB1 and DP1). Higher
Voc values were obtained from DSC devices due to destabiliza-
tion of the excited-state oxidation potentials allowing for effi-
cient electron injection into the conduction band of TiO2 with-
out the use of high Li+ concentrations. Through DFT calcula-
tions, it was determined that these dyes have ideal orbital ar-
rangements for allowing electron injection into the TiO2 CB.
These simple structural changes aided the 3,4-TT p-bridged
dyes in reaching PCEs between 5.61–7.80%, which is a dramatic
improvement for the 3,4-TT-based building block. In order to
further improve PCEs of dyes containing 3,4-thienothiophene,
work has begun on extending the conjugation length of these
dyes through the implementation of simple p-bridges to
extend absorption breadth. Furthermore, the use of cobalt
redox shuttles allowing for higher maximum Voc will be exam-
ined with the use of surface protecting donor groups.
Figure 12. Electron lifetime versus capacitance of PB1, PB2, DP1 and DP2
based cells.
is shown to slow recombination between the redox shuttle
and electrons present on the surface of TiO2 relative to DP1.
The compact dye DP2 with minimal alkyl chain lengths (one
ethyl and one methyl) gave rise to a comparable electron life-
time to that of DP1. The shortest electron lifetimes were ob-
served for PB2. Prior studies have shown introduction of
a longer-alkyl-substituted group leads to longer electron life-
times, which is attributed to slowing recombination of the
redox shuttle with TiO2.[25,26] Surprisingly, this is not observed
with the ethylhexyl-substituted PB2 as DP2 demonstrates
a longer electron lifetime with a simple ethyl group. This sug-
gests PCE diminishing aggregative effects are not inhibitive for
the 3,4-TT-bridge-based dyes as the more compact DP2 gives
improved electron lifetimes presumably through a closer pack-
ing of the dye on the TiO2 surface to allow for good surface
protection from the redox shuttle. Dye PB1 behaves as is typi-
cally expected, having the highest electron lifetime among all
of the dyes measured due to the combined sterics of its ethyl-
hexyl-substituted p-bridge and its hexyloxy-substituted TPA
donor. This slow recombination rate allows PB1 to generate
a higher voltage relative to the other dyes in this series.
Experimental Section
General information: All commercially obtained reagents were
used as received. 3,4-dibromothiophene was purchased from
Matrix Scientific. Ethylhexyl mercaptoacetate was purchased from
TCI. Thin-layer chromatography (TLC) was conducted with Sigma T-
6145 pre-coated silica gel 60 F254 polyester sheets and visualized
with UV. Flash column chromatography was performed by Sorbent
A Voc increase of up to 144 mV and a Jsc increase of up to
3.30 mAcmÀ2 is observed for the PB/DP series when compared
with previously reported 3,4-TT dyes. The success of the PB/DP
series is owed to a destabilization of the excited-state potential
and a narrowed optical band gap achieved through an in-
creased donor strength (Figure 7). These potentials allow for
more efficient injection of electrons into the TiO2 semiconduc-
tor CB as well as decreasing the dye regeneration excessive
1
Tech P60, 40–63 mm (230–400 mesh). H NMR spectra were record-
ed on a Bruker Avance-300 (300 MHz) spectrometer and a Bruker
Avance-500 (500 MHz) spectrometer and are reported in ppm
using solvent as an internal standard (CDCl3 at 7.26 ppm). Data re-
ported as: s=singlet, d=doublet, t=triplet, q=quartet, p=
pentet, m=multiplet, br=broad, ap=apparent, dd=doublet of
doublets; coupling constant(s) in Hz; integration. UV/Vis spectra
Chem. Eur. J. 2016, 22, 694 – 703
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were measured with a Cary 5000 UV/Vis spectrometer. Cyclic vol-
tammetry was measured with a C-H Instruments electrochemical
analyzer.
30 min. Cyanoacetic acid (15 mg, 0.18 mmol) and piperidine
(0.04 mL, 0.42 mmol) were then added to the reaction mixture. The
flask was then heated to 908C and stirred for 1 h. The reaction mix-
ture was then diluted with dichloromethane and acetic acid was
added to the mixture. The mixture was extracted with dichlorome-
thane and water, and dried over MgSO4. The product mixture was
then purified through a silica gel plug with 100% dichlorome-
thane!3%methanol/dichloromethane!10% methanol/2% acetic
acid/dichloromethane. The solvent was evaporated under reduced
pressure. The dye was then extracted in hexanes and water and
dried over MgSO4 to give the final dye (DP1) in the form of
Computational details: All density functional theory (DFT) and
time-dependent DFT (TD-DFT) data reported here were obtained
with the B3LYP[27,28] exchange-correlation functional and the 6–
311+G(d,p) basis set as implemented in Gaussian 09.[29] The
B3LYP/6–311+G(d,p) magnetic shielding tensors were computed
with the gauge-independent atomic orbital (GIAO) method.[30,31] In-
itial geometries for the PB1, PB2, DP1 and DP2 structures were ob-
tained from MM2 optimizations in ChemBio3D Ultra (version:
13.0.2.3021). Dihedral angles for all relevant groups were set to
values between the global minimum and the next local maximum
on the conformational energy diagram as calculated by Chem-
Bio3D. These structures were then sequentially refined with B3LYP
optimizations using first the 3-21G basis set, followed by the 6-
31G(d,p) basis set and finally the 6-311+G(d,p) basis set. The de-
fault convergence criteria were used for these preliminary ground
state optimizations and for the TD-DFT geometry optimizations of
the S1 and T1 excited states of PB1.
1
a purple solid (0.02 g, 51%). H NMR (500 MHz, CDCl3): d=8.36 (s,
1H), 7.98 (s, 1H), 7.60 (m, 2H), 7.11 (d, J=8.85 Hz, 4H), 6.90 (m,
6H), 4.40 (q, J=7.2 Hz, 2H), 3.95 (t, J=6.3 Hz, 4H), 1.78 (t,
J=6.9 Hz, 4H), 1.43 (m, 12H), 0.90 (m 6H), 0.85 ppm (t, J=8.94,
3H); IR (neat): n˜ =3040.5 (br), 2929.3, 2862.1, 2212.9, 1712.1,
1565.8, 1502.6 cmÀ1
;
HRMS m/z calcd for C43H46N2O6S2 [M]+:
750.2797; found: 750.2878; UV/Vis (CH2Cl2): lmax =558 nm
(e=22000mÀ1 cmÀ1), lonset =650 nm; cyclic voltammetry (0.1m
Bu4NPF6 in CH2Cl2, sweep width 1.1 to À2.0 V, 0.1 VsÀ1 scan rate):
E
(S+/S) =1.09 V (vs. NHE). E(S+/S*) =À0.79 V [vs. NHE, calcd from
Ethyl
thieno[3,4-b]thiophene-2-carboxylate (5, DP1-CHO): Compound
(103 mg, 0.26 mmol), 4-(hexyloxy)-N-[4-(hexyloxy)phenyl]-N-[4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]aniline (TPA-
4-(4-[bis{4-(hexyloxy)phenyl}amino]phenyl)-6-formyl
E(S+/S*) =(E(S+/S)ÀEgopt)].
(E)-2-Cyano-3-{2-(ethoxycarbonyl)-4-{4-(p-tolyl)-1,2,3,3a,4,8b-hex-
ahydrocyclopenta[b]indol-7-yl}thieno[3,4-b]thiophen-6-yl}acrylic
acid (DP2): Synthesis follows same procedure as DP1. Final prod-
uct was a purple solid (8 mg, 35%). 1H NMR (300 MHz, CDCl3):
d=8.33 (s, 1H), 8.01 (s, 1H), 7.51 (br, 2H), 7.20 (brs, 4H), 6.83 (d,
J=9.24 Hz, 1H), 4.92 (m, 1H), 4.40 (q, J=6.33 Hz, 2H), 3.87 (m,
1H), 2.42 (s, 3H), 1.88 (m, 2H), 1.62 (m, 4H), 1.41 ppm (t, J=7.1 Hz,
4
Bpin) (160 mg, 0.28 mmol), K3PO4 (165 mg, 0.78 mmol), toluene
(6 mL), and H2O (0.23 mL), were added to a round-bottomed flask,
and the mixture was degassed with N2 for 20 min. [Pd2(dba)3]
(10 mg, 0.011 mmol) and X-phos (20 mg, 0.042 mmol) were then
added simultaneously to the reaction mixture. The temperature
was then increased to 808C and the mixture was allowed to stir for
6 h. The reaction was then diluted with ethyl acetate (200 mL) and
extracted using water. The organic layer was separated and dried
using MgSO4. The solvent was removed under reduced pressure.
The product was purified through silica gel chromatography with
10% ethyl acetate/hexanes (0.10 g, 93%). 1H NMR (500 MHz,
CDCl3): d=9.83 (s, 1H), 7.98 (s, 1H), 7.53 (d, J=8.6 Hz, 2H), 7.10 (d,
J=8.8 Hz, 4H), 6.93 (d, J=8.7 Hz, 2H), 6.87 (d, J=8.8 Hz, 4H), 4.38
(q, J=7.2 Hz, 2H), 3.94 (t, J=6.6 Hz, 4H), 1.78 (t, J=7.1 Hz, 4H),
1.46 (m, 4H), 1.39 (t, J=7.1 Hz, 3H), 1.34 (m, 8H), 0.90 ppm (m,
6H); 13C NMR (125 MHz, CDCl3): d=178.8, 162.6, 156.5, 150.7,
148.5, 140.8, 139.2, 128.4, 127.6, 127.5, 123.4, 123.3, 118.9, 118.7,
115.6, 115.5, 68.5, 61.9, 31.6, 29.3, 25.8, 22.6, 14.1 ppm (br); IR
(neat): n˜ =3039.7, 2930.1, 2858.2, 1711.8, 1505.6, 1240.6 cmÀ1; MS
m/z calcd for C40H45NO5S2 [M+H]+: 683.3; found: 683.4.
3H); IR (neat): n˜ =3478.5, 2920.6, 2850.9, 2063.3, 1638.2 cmÀ1
.
HRMS m/z calcd for C31H26N2O4S2 [M]+: 555.14; found: 555.14; UV/
Vis (CHCl3): lmax =591 nm (e=20000mÀ1 cmÀ1), lonset =670 nm;
cyclic voltammetry (0.1m Bu4NPF6 in CH2Cl2, sweep width 1.1 to
À2.0 V, 0.1 VsÀ1 scan rate):
E(S+/S) =1.08 V (vs. NHE). E(S+/S*) =
À0.77 V [vs. NHE, calcd from E(S+/S*) =(E(S+/S)ÀEgopt)].
Acknowledgements
P.B. and F.G. contributed equally to this work. The authors
J.H.D., P.B., G.A.P., and G.S.T. thank the Mississippi NSF-EPSCOR
program (EPS-0903787) and the University of Mississippi for
funding. J.H.D., P.B., and G.A.P. thank the NSF-CAREER program
(NSF-1455167) for support. G.A.P. thanks the UM Sally McDon-
nell Barksdale Honors College for funding. G.S.T. thanks the
NSF MRI award (CHE-1338056) for funding and the Mississippi
Center for Supercomputing Research. We also thank Brandon
Stamper and Chesney Petkovsek for preliminary X-ray crystallo-
graphic analysis. M.K.N., M.G., and F.G. thank the European
Community’s Seventh Framework Programme (FP7/2007–2013)
ENERGY.2012.10.2.1, NANOMATCELL, Grant agreement no:
308997.
Ethyl 6-formyl-4-[4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[-
b]indol-7-yl]thieno[3,4-b]thiophene-2-carboxylate
(DP2-CHO):
Synthesis follows the same conditions as for 5, using 7-(4,4,5,5-tet-
ramethyl-1,3,2-dioxaborolan-2-yl)-4-(p-tolyl)-1,2,3,3a,4,8b-hexahy-
drocyclopenta[b]indole (Ind-Bpin) instead of TPA-Bpin (0.04 g,
1
50%). H NMR (300 MHz, CDCl3): d=9.83 (s, 1H), 8.04 (s, 1H), 7.49
(s, 1H), 7.46 (s, 1H), 7.21 (s, 4H), 6.87 (d, J=8.4 Hz, 1H), 4.91 (t,
J=6.4 Hz, 1H), 4.41 (q, J=7.1 Hz, 2H), 3.90 (t, J=8.6 Hz, 1H), 2.37
(s, 3H), 1.93 (m, 2H), 1.74 (m, 2H), 1.59 (m, 2H), 1.41 ppm (t,
J=7.11 Hz, 3H); 13CNMR (125 MHz, CDCl3): d=178.8, 162.9, 150.7,
150.0, 140.4, 139.1, 136.5, 133.4, 130.2, 128.4, 124.3, 123.9, 122.6,
121.5, 107.6, 107.5, 69.9, 62.0, 45.2, 35.6, 33.5, 29.9, 24.5, 14.5 ppm;
IR (neat): n˜ =3025.8, 2952.6, 2927.2, 2859.3, 1706.3, 1635.6,
1600.3 cmÀ1; HRMS m/z calcd for C28H25NO3S2 [M+Na]+: 510.1174;
found: 510.1280.
Keywords: dye-sensitized solar cells · NICS · proaromaticity ·
solvatochromism · thienothiophene
(E)-3-{4-(4-[Bis{4-(hexyloxy)phenyl}amino]phenyl)-2-(ethoxycar-
bonyl)thieno[3,4-b]thiophen-6-yl}-2-cyanoacrylic acid (6, DP1):
Compound 5 (45 mg, 0.06 mmol) and chloroform (1 mL) were
added to a round-bottom flask. The flask was degassed with N2 for
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