Organic chromophores based on a fused bis-thiazole core and their application in dye-sensitized solar cells

Article abstract of DOI:10.1002/ejoc.201201629

Four new D-π-A organic dyes incorporating either a thiazolo[5,4-d] thiazole bicyclic system (TTZ1-2) or a benzo[1,2-d:4,5-d′]bisthiazole tricyclic unit (BBZ1-2) have been synthesized and fully characterized. The key steps of the synthesis include an efficient MW-assisted preparation of the thiazolo[5,4-d]thiazole core and selective functionalization of two different dihalothienyl derivatives through Suzuki coupling. All the compounds showed photo- and electrochemical properties compatible with their employment in dye-sensitized solar cells. In addition, their electronic structure and transitions were investigated by means of TD-DFT calculations. Dye-sensitized solar cells fabricated with dye TTZ1 yielded an average power conversion efficiency corresponding to 43 and 60 % of the performances recorded under the same conditions for cells containing reference dyes N3 and D5, respectively. Efficiency measurements and electrochemical impedance spectroscopy (EIS) experiments indicated that use of chenodeoxycholic acid (CDCA) as a co-adsorbent was beneficial to disrupt dye-dye interactions and slow down recombination processes within the cells, thus allowing power conversion efficiencies up to 3.53 % to be obtained. Four D-π-A dyes incorporating fused-heterocyclic ring systems, such as thiazolothiazoles and benzobisthiazoles, have been synthesized and characterized. Their incorporation into dye-sensitized solar cells yielded devices with power conversion efficiencies up to 3.53 %. The average efficiencies of the cells were 43 and 60 % those of cells containing the reference dyes N3 and D5, respectively.

Full text of DOI:10.1002/ejoc.201201629

FULL PAPER
DOI: 10.1002/ejoc.201201629
Organic Chromophores Based on a Fused Bis-Thiazole Core and Their
Application in Dye-Sensitized Solar Cells
Alessio Dessì,[a] Gabriella Barozzino Consiglio,[a] Massimo Calamante,[a]
Gianna Reginato,*[a] Alessandro Mordini,[a] Maurizio Peruzzini,[a] Maurizio Taddei,[a,b]
Adalgisa Sinicropi,[b] Maria Laura Parisi,[b] Fabrizia Fabrizi de Biani,[b] Riccardo Basosi,[b]
Riccardo Mori,[c] Michele Spatola,[c] Mara Bruzzi,[c] and Lorenzo Zani*[d]
Keywords: Synthetic methods / Dyes/pigments / Fused-ring systems / Photochemistry / Sensitizers / Solar cells
Four new D-π-A organic dyes incorporating either a thi-
azolo[5,4-d]thiazole bicyclic system (TTZ1–2) or a benzo[1,2-
d:4,5-dЈ]bisthiazole tricyclic unit (BBZ1–2) have been synthe-
sized and fully characterized. The key steps of the synthesis
include an efficient MW-assisted preparation of the thiazolo-
[5,4-d]thiazole core and selective functionalization of two dif-
ferent dihalothienyl derivatives through Suzuki coupling. All
the compounds showed photo- and electrochemical proper-
ties compatible with their employment in dye-sensitized so-
lar cells. In addition, their electronic structure and transitions
were investigated by means of TD-DFT calculations. Dye-
sensitized solar cells fabricated with dye TTZ1 yielded an
average power conversion efficiency corresponding to 43
and 60% of the performances recorded under the same con-
ditions for cells containing reference dyes N3 and D5, respec-
tively. Efficiency measurements and electrochemical im-
pedance spectroscopy (EIS) experiments indicated that use
of chenodeoxycholic acid (CDCA) as a co-adsorbent was
beneficial to disrupt dye-dye interactions and slow down re-
combination processes within the cells, thus allowing power
conversion efficiencies up to 3.53% to be obtained.
production process and the requirement for highly pure
starting materials.[4]
Introduction
Since their appearance in 1991,[5] dye-sensitized solar
cells (DSSCs) have been considered a potential low-cost
and efficient alternative to traditional silicon-based photo-
voltaic devices.[6] In DSSCs, light harvesting is carried out
by a photosensitizer absorbed on a thin layer of a mesopo-
rous semiconductor (usually TiO2); the photoexcited sensi-
tizer transfers an electron to the semiconductor, thus
achieving charge separation. Therefore, effective light ab-
sorption by the sensitizer is essential to obtain highly-ef-
ficient DSSCs.
In the last decades, global energy demand has increased
at a fast pace.[1] Presently, the world’s growing energy needs
are mostly satisfied by employment of fossil fuels, however,
the resulting economic and environmental concerns are
stimulating intense research in the field of renewable energy
sources.[2] Among these, solar energy has often been de-
scribed as the most promising option.[3] Silicon-based pho-
tovoltaic cells were the first devices capable of converting
solar radiation into electricity; nowadays, they are a mature
and reliable technology, but they still present some draw-
backs, such as an expensive and energetically demanding
Until recently, the most efficient DSSCs described in the
literature incorporated polypyridyl complexes of RuII as the
sensitizers,[7] which allowed power conversion efficiencies
exceeding 11% to be reached.[8] Despite this, ruthenium-
based dyes still suffer from some important drawbacks,
such as difficult purification procedures, low molar extinc-
tion coefficients and high cost of the metal precursors; such
disadvantages prompted the search for alternative photo-
sensitizers. Organic dyes[9] offer the possibility of overcom-
ing many of the problems associated with the use of ruthe-
nium-containing sensitizers; they can often be prepared
from inexpensive starting materials using simple procedures
and have high molar extinction coefficients. Furthermore,
their stereoelectronic properties can be easily tuned by em-
ploying a flexible synthetic strategy. Not surprisingly, the
development of new organic compounds to be used as sen-
[a] CNR – Istituto di Chimica dei Composti Organometallici
(CNR-ICCOM),
Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
Fax: +39-055-5225203
E-mail: gianna.reginato@iccom.cnr.it
Homepage: www.iccom.cnr.it
[b] Dipartimento di Biotecnologie, Chimica e Farmacia, Università
degli Studi di Siena,
Via A. Moro 2, 53100 Siena, Italy
[c] Dipartimento di Ingegneria Industriale, Università degli Studi
di Firenze,
Via S. Marta 3, 50139 Florence, Italy
[d] CNR – Istituto per la Sintesi Organica e la Fotoreattività
(CNR – ISOF),
Via Piero Gobetti 101, 40129 Bologna, Italy
Fax: +39-051-6398349
E-mail: lorenzo.zani@isof.cnr.it
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201629.
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Fused Bis-Thiazole Chromophores
sitizers in DSSCs has become a very active field of re- tential as DSSC sensitizers, with the goal of determining
search.[10] Such compounds are usually characterized by a whether their interesting properties could also prove useful
D-π-A architecture, in which a donor group (D) is con- for such an application.
nected to an acceptor/anchoring group (A) through a con-
To this end, we designed compounds TTZ1–2 and
jugated bridge (π). Upon irradiation, such an arrangement BBZ1–2 (Figure 2). The well-characterized triarylamine
facilitates the generation of photoinduced intramolecular and cyanoacrylate groups were selected as the donor and
charge-transfer states, thus promoting electron injection acceptor/anchoring moieties, respectively. Thiophene rings
into the nanocrystalline semiconductor.
were placed next to the central heterocyclic core to extend
Thiazolo[5,4-d]thiazoles 1 are a class of heterocyclic the overall conjugation of the molecule. Finally, long alkyl
compounds endowed with a rigid and coplanar bicyclic chains were introduced on the thiophene rings to improve
scaffold giving rise to a highly extended π-electron system the solubility of the compound and to suppress dye aggre-
(Figure 1). The first compound of this class was prepared gation on the semiconductor surface, as previously re-
in 1891 by Ephraim,[11] but its correct structure was estab- ported.[28]
lished only in 1960 by Johnson et al.,[12] who were the first
to prepare a large number of analogues.[12,13] Thiazolo[5,4-
d]thiazoles were initially investigated for their potential bio-
logical activity,[14] but later they were especially used as
spacer units in semiconducting organic materials for poly-
mer light-emitting diodes (PLEDs)[15] and organic field-ef-
fect transistors (OFETs).[16] More recently, the thiazolo-
thiazole unit has been incorporated in donor/acceptor poly-
mers used as the photoactive components of organic bulk
heterojunction (BHJ) solar cells.[17] In particular, thiazolo-
thiazole-containing polymers showed remarkable charge
carrier diffusion lengths,[17e,17g,17h] and power conversion
efficiencies as high as 5.8% were achieved.[18]
Figure 2. Sensitizers TTZ1–2 and BBZ1–2 synthesized in this
work.
Herein, we report the synthesis of the new compounds,
as well as their experimental and computational characteri-
zation. Furthermore, we present the results of efficiency
measurements carried out on photovoltaic devices built
using TTZ1–2 and BBZ1–2 as the light-harvesting materi-
als.
Figure 1. The thiazolo[5,4-d]thiazole (1) and benzo[1,2-d:4,5-dЈ]-
bisthiazole (2) ring systems.
Benzo[1,2-d:4,5-dЈ]bisthiazoles 2 feature another interest-
ing fused heterocyclic system, in which π-conjugation is fur-
ther extended by the presence of an additional aromatic
ring (Figure 1). Although the synthesis of these compounds
was claimed in 1903,[19] subsequent studies proved the cor-
responding structural assignment to be incorrect and de-
scribed alternative procedures with which to obtain the
products with the correct regiochemistry.[20,21] In analogy
with thiazolothiazoles, benzo[1,2-d:4,5-dЈ]bisthiazoles have
Results and Discussion
Synthesis of the New Organic Sensitizers
Compounds TTZ1–2 were prepared according to the
also been extensively investigated for their applications in synthetic sequence shown in Scheme 1. Treatment of 3-hex-
nonlinear optics[22] and organic (opto)electronics.[23] Fur- ylthiophene with lithium 2,2,6,6-tetramethylpiperidide, gen-
ther reports focused on their employment as fluorescent erated in situ from tert-butyllithium and 2,2,6,6-tetrameth-
probes,[24] near-IR chromophores,[25] and liquid crystal pre- ylpiperidine, and quenching of the resulting thienyllithium
cursors.[26] Moreover, they have recently been incorporated species with N,N-dimethylformamide (DMF) afforded 4-
in photoactive polymers used in BHJ solar cells, reaching a hexylthiophene-2-carbaldehyde (3) as a single regioisomer.
maximum power conversion efficiency of 3.0%.[27]
Interestingly, the same transformation using nBuLi as the
Despite the large body of work outlined above, to the base gave, instead, the product as a mixture of 2,4- and 2,3-
best of our knowledge the employment of thiazolo[5,4-d]- regioisomers. Compound 3 was then reacted in the presence
thiazoles and benzo[1,2-d:4,5-dЈ]bisthiazoles as photosensi- of dithiooxamide (4) to provide thiazolothiazole 5. The re-
tizers for DSSCs has not yet been described. Therefore, we action was performed under solvent-free conditions at
decided to prepare new organic dyes containing these two 180 °C with microwave heating, giving the desired product
heterocyclic systems as spacer units and to assess their po- in good yield.
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Scheme 1. Synthetic sequence employed for the preparation of sensitizers TTZ1–2.
It should be pointed out that this is the first microwave- coupled products. As a consequence, the reactions could
assisted synthesis of a thiazolo[5,4-d]thiazole heterocycle re- not generally be driven to completion and provided prod-
ported to date. Indeed, all previous preparations employed ucts 9a–b in only moderate yields (24–28%).
conventional heating and most of them either used a large
At this stage, a second Pd-catalyzed Suzuki coupling
excess of aldehyde or were carried out in high-boiling sol- with 5-formyl-2-thiopheneboronic acid (10) was used to in-
vents.[12–18,29] Using the conditions described above, we troduce a formyl group, leading to compounds 11a–b. The
were able to both reduce reaction time and improve product latter transformation was found to proceed well when KF
yield compared with the previous methodology.[15] Unfortu- was employed as the base and heating was performed by
nately, use of an excess amount of aldehyde was still re- microwave irradiation. Under these conditions the desired
quired, but the latter could be recovered during product products could be obtained in moderate to good yields (40–
purification by means of flash column chromatography and 74%) within short reaction times (20–40 min); use of dif-
reused for the synthesis of compound 5.
ferent bases, such as sodium carbonate, and classical heat-
Iodination of heterocycle 5 by treatment with N-iodosuc- ing conditions, resulted in lower yields or longer reaction
cinimide (NIS) at 40 °C overnight led to the formation of times. Finally, compounds 11a–b were converted in good
bis-iodo compound 6 in 80% yield. Compound 6 then yields into sensitizers TTZ1–2 by Knoevenagel condensa-
underwent a Pd-catalyzed Suzuki coupling to introduce the tion with excess cyanoacetic acid in the presence of ammo-
triarylamine donor moiety. Two different organoboron nium acetate and glacial acetic acid as the solvent.[31]
compounds were employed for this transformation: com-
The synthesis of compounds BBZ1–2 was carried out in
mercially available 4-(diphenylamino)benzeneboronic acid a similar manner, although some steps required slightly dif-
(7) was used to prepare triphenylamino derivative 9a (R = ferent conditions (Scheme 2). The starting material for the
H), while boronic ester 8, in turn obtained from the corre- preparation of benzobisthiazole 13 was 2,5-diaminobenz-
sponding arylbromide,[30] was used for the synthesis of bis- ene-1,4-dithiol bis-hydrochloride (12), which was treated
methoxy-substituted analogue 9b (R = MeO). In both with aldehyde 3 in DMF under thermal conditions. Unfor-
cases, it was necessary to carefully monitor the reactions by tunately, microwave heating did not prove beneficial in this
TLC to ensure good conversion of the starting materials case. Electrophilic iodination of 13 with NIS proved to be
and, at the same time, minimize formation of the double extremely slow, and the starting material was not fully con-
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Fused Bis-Thiazole Chromophores
Scheme 2. Synthetic sequence employed for the preparation of sensitizers BBZ1–2.
verted even after a prolonged reaction time. For this reason, gion can be attributed to a delocalized π–π* transition from
bis-bromo derivative 14 was prepared instead by reaction the triarylamine and the central heterocyclic moiety to the
with N-bromosuccinimide (NBS), which required only four cyanoacrylic function. These assignments are mainly based
hours at room temperature. Bis-bromide 14 was then used on the results of computational studies carried out on the
as the key intermediate for the preparation of benzobisthi- sensitizers (see below). Thiazolothiazole species TTZ1–2
azoles BBZ1–2, which were obtained in moderate yields.
showed a redshifted absorption compared with benzobisthi-
All the compounds were isolated in quantities large azole derivatives BBZ1–2 (472–476 vs. 446–455 nm); the
enough to allow full chemical characterization and fabrica- difference can be attributed to the presence of an additional
tion of the corresponding photovoltaic devices. Compounds aromatic ring in the latter structures, which stabilizes the
TTZ1–2 and BBZ1–2 were highly soluble in tetra- HOMO–1 and HOMO orbitals relative to the LUMO, thus
hydrofuran (THF) but only moderately soluble in chlorin- causing the observed hypsochromic shift. In addition, it can
ated solvents such as CH2Cl2 and CHCl3, and almost com- be observed that compounds TTZ2 and BBZ2, featuring
pletely insoluble in all other common organic solvents; methoxy-substituted terminal benzene rings, had absorp-
therefore, TiO2 sensitization proved possible only from tion maxima localized at slightly higher wavelengths com-
THF solutions of the dyes (see the Exp. Section).
pared with TTZ1 and BBZ1. In this case, the discrepancy
probably arises from the presence of electron-rich substitu-
ents on the triarylamine moiety, which increase its donor
ability and cause the observed redshift. For all new sensitiz-
Optical Properties
The UV/Vis absorption spectra of compounds TTZ1–2 ers, the visible bands were characterized by good to high
and BBZ1–2 in THF solution are displayed in Figure 3; the values of molar absorptivity (ε = 2.76ϫ104
m
–1 cm–1 for
relevant optical data are summarized in Table 1. All com- TTZ1; 4.36ϫ104 for TTZ2; 3.64ϫ104 for BBZ1; 6.32ϫ104
pounds showed two main absorption bands: the absorption for BBZ2). Such values are comparable to those of most
band at around 300 nm is probably due to a localized aro- organic sensitizers described to date, such as the well-
matic π–π* transition typical of the triarylamine frag- known organic dye D5, which is reported to have ε =
ment,[32] whereas the intense band found in the visible re- 3.8ϫ104
m
–1 cm–1 in CH3CN solution.[33] Interestingly,
Eur. J. Org. Chem. 2013, 1916–1928
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G. Reginato, L. Zani et al.
FULL PAPER
Table 1. Optical and electrochemical properties of compounds TTZ1–2 and BBZ1–2.
Cmpd
λabs (εϫ 104 m–1 cm–1) [nm][a]
λemi [nm]
SS [cm–1][b]
E0–0 [eV][c]
Eox [V][d]
E*ox [V][d,e]
TTZ1
TTZ2
BBZ1
BBZ2
472 (2.76)
476 (4.36)
446 (3.64)
455 (6.32)
581
605
537
534
3975
4479
3780
3251
2.33
2.31
2.51
2.46
1.14 (0.93)
0.93 (0.72)
1.15 (0.94)
0.92 (0.71)
–1.19
–1.38
–1.36
–1.54
[a] UV/Vis spectra were recorded on a 2.5ϫ10–5 m solution of the dye in THF. [b] Stokes Shift. [c] Estimated from the intersection of
the normalized absorption and emission spectra. [d] Potentials vs. NHE (potentials vs. Ag/AgCl NaCl 3 m are given in parentheses)
measured in CH2Cl2 solution. [e] Calculated from Eox – E0–0
.
benzobisthiazole derivatives BBZ1–2 showed higher molar
absorptivities compared with their thiazolothiazole ana-
logues TTZ1–2. In addition, compounds containing meth-
oxy-substituents absorbed light more intensely than com-
pounds featuring simple phenyl rings. The UV/Vis spectrum
of compound TTZ1 on TiO2 film features a broadened ab-
sorption band compared with that recorded in solution,
with a slightly redshifted maximum (477 vs. 472 nm) and a
more clearly redshifted onset (see the Supporting Infor-
mation, Figure S1), suggesting a tendency to form J-aggre-
gates on the semiconductor surface.[34]
Figure 4. Normalized fluorescence emission spectra of compounds
TTZ1–2 and BBZ1–2 in THF solution. TTZ1: black squares;
TTZ2: red circles; BBZ1: green rhombs; BBZ2: blue triangles.
Electrochemical Measurements
The electrochemical characteristics of the new sensitizers
were investigated by means of cyclic voltammetry (CV) in
CH2Cl2 solution. The cyclic voltammogram of compound
TTZ1 is presented in Figure 5, while the relevant electro-
chemical data for all compounds are reported in Table 1
(for the cyclic voltammograms of compounds TTZ2 and
BBZ1–2 see Figure S2). All dyes displayed a first oxidation
Figure 3. UV/Vis absorption spectra of compounds TTZ1–2 and
BBZ1–2 in THF solution (c = 2.5ϫ10–5 m). TTZ1: black squares;
TTZ2: red circles; BBZ1: green rhombs; BBZ2: blue triangles.
Fluorescence emission spectra (Figure 4) were recorded
by irradiating the samples at their maximum absorption
wavelengths. Compounds TTZ1–2 emitted at higher wave-
lengths compared with BBZ1–2, and were also charac-
terized by larger Stokes shifts (Table 1). Thiazolothiazole
TTZ2 had both the most redshifted emission at 605 nm and
the largest Stokes shift at 4479 cm–1, suggesting the occur-
rence of an intramolecular charge transfer excitation pro-
cess.[35]
From the intersection between the normalized absorp-
tion and emission spectra, the optical band gap of the novel
sensitizers (corresponding to the zero-zero transition en-
ergy, E0–0) was calculated. Thiazolothiazoles had much
smaller optical band gaps compared with benzobisthiazoles,
whereas the optical band gaps of triphenylamine species
were slightly larger than those of methoxy-substituted com-
pounds. The corresponding values are reported in Table 1.
Figure 5. Cyclic voltammogram of compound TTZ1 in CH2Cl2
solution.
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Eur. J. Org. Chem. 2013, 1916–1928

Fused Bis-Thiazole Chromophores
process (Eox) in the 0.92–1.15 V range (vs. NHE), which can suggesting that TTZ1+ radical cation is reasonably stable
be attributed to removal of one electron from the triarylam- under these conditions. The appearance of this broad ab-
ine moiety. Indeed, methoxy-substituted dyes TTZ2 and sorption has been previously observed[36,37] and has been
BBZ2 showed less positive ground-state oxidation poten- assigned to the HOMO/(HOMO–1)ǞSOMO transitions in
tials than triphenylamine-bearing dyes TTZ1 and BBZ1 triarylamine radical cations, in agreement with our in-
(0.92–0.93 V compared with 1.14–1.15 V, vs. NHE). Clearly, terpretation of TTZ1 redox behavior. A similar behavior
the presence of electron-rich methoxy-substituents on the has also been observed for compounds TTZ2 and BBZ1–2,
donor group makes the corresponding compounds easier to with methoxy-substituted compounds having the new band
oxidize than those lacking such functionalities. On the appearing at lower energy compared with their tri-
other hand, the presence of such substituents also affects phenylamine counterparts.
the chemical reversibility of the redox process, as indicated
by the ipc/ipa ratio measured at 0.2 Vs–1, which decreases
from 1 (for TTZ1 and BBZ1) to 0.8 (for TTZ2 and BBZ2).
Computational Analysis
The ground-state oxidation potentials of all the new
compounds are more positive than that of the I–/I3 redox
The electronic structure of the new sensitizers was
–
couple (0.4 V vs. NHE), thus assuring dye regeneration dur- studied by means of density functional theory (DFT) calcu-
ing operation of a DSSC. Based on the experimental optical lations using the Gaussian 09 software package.[38] First,
band gap values (2.31–2.51 eV), we calculated the excited the ground-state geometries of compounds TTZ1–2 and
state oxidation potential (E*ox) of the dyes to be between BBZ1–2 were optimized in vacuo by using the B3LYP func-
–1.54 and –1.19 V (vs. NHE; Table 1). Such values are suffi- tional and the 6-31G* basis set. From such optimization,
ciently more negative than that of the conduction band of it became apparent that the dyes assume a largely planar
TiO2 (–0.5 V) to guarantee electron injection from the dye structure along most of the conjugated system, with small
to the semiconductor.
dihedral angles between adjacent rings (Ͻ20°); in all com-
To further characterize the spectroscopic features and the pounds, the only substantial deviation from coplanarity was
redox behavior of the new compounds, spectroelectrochem- found at the thiophene-phenyl junction, where the two rings
ical measurements were conducted by observing the UV/ form a 47–48° dihedral angle for steric reasons (see Figure
Vis spectral changes of all the compounds upon oxidation S3 in the Supporting Information).
in CH2Cl2 solution; as an example, the spectra obtained for
The complete set of wave function plots for all the
compound TTZ1 are shown in Figure 6. As previously studied compounds is reported in Figure S4. From an in-
noted, the UV/Vis spectrum of this species shows two dis- spection of the computed frontier orbital electron density
tinct bands, which, under these experimental conditions, are distributions, we can conclude that, for all compounds, the
found at 300 and 486 nm and could be attributed to local- HOMO–1 orbital is mainly located on the conjugated
ized and delocalized aromatic πǞπ* transitions, respec- bridge, with a sizeable contribution coming from the ter-
tively.[35] As shown in Figure 6, removal of the first electron minal thiophene ring (see insets in Figure 7 and Figure S4);
from TTZ1 causes a change in its absorption properties, in the HOMO, the electron density is mainly found on the
resulting in a decrease of both these bands. At the same donor triarylamine group, whereas for the LUMO it is con-
time, a broad band appears at lower energy (centered at centrated on the acceptor cyanoacrylic group. As far as or-
680 nm) and one isosbestic point is maintained at 550 nm, bital energies are concerned (Figure 7), our calculations
indicate that the HOMO–1 (localized on the central hetero-
cycle) is strongly stabilized moving from the thiazolothiaz-
ole- to benzobisthiazole-containing compounds, as a conse-
quence of the increase in aromaticity due to the presence of
the central benzene ring. Both the HOMO and the
HOMO–1 are instead destabilized when methoxy-substitu-
ents are placed on the terminal phenyl ring. LUMO ener-
gies, on the other hand, are relatively unaffected by the
changes in molecular structure. As a consequence, meth-
oxy-substituted compounds TTZ2 and BBZ2 display re-
duced energy gaps, leading to slightly redshifted absorption
maxima, whereas benzobisthiazoles BBZ1–2 exhibit larger
energy gaps than their thiazolothiazole counterparts
TTZ1–2, leading to a blueshifted absorption.
The TD-CAM-B3LYP/6-31G* absorption maxima
(λamax), excitation energies (Eexc) and oscillator strengths (f)
of dyes TTZ1–2 and BBZ1–2, both in vacuo and in THF,
are shown in Table 2.
For all dyes, the excitation process in the visible region is
mainly due to the HOMO–1ǞLUMO transition, account-
Figure 6. UV/Vis spectra recorded upon stepwise oxidation of
TTZ1 in a OTTLE cell in CH2Cl2 solution; supporting electrolyte:
[NBu4][PF6] (0.2 m).
Eur. J. Org. Chem. 2013, 1916–1928
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G. Reginato, L. Zani et al.
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Figure 7. HOMO–1, HOMO and LUMO energies of compounds TTZ1–2 and BBZ1–2; the values for D5 are reported for comparison.
Computed B3LYP and TD-CAM-B3LYP, both in vacuo (in parentheses) and in THF {in brace}, HOMO–LUMO (bottom left) and
HOMO–1-LUMO (top right) energy differences (in eV) are shown next to vertical arrows. The conduction band edge (CBE) level of the
TiO2 (–4.0 eV) and the redox potential of the I–/I3 couple (ca. –4.8 eV) are indicated by the dashed green and purple lines.[39] The wave
–
function plots for compounds TTZ1 and BBZ1 are also shown.
Table 2. Experimental and TD-CAM-B3LYP computed absorption Photoelectrochemical Measurements
maxima (λamax), excitation energies (Eexc) and oscillator strengths
(f) for dyes TTZ1–2 and BBZ1–2 in vacuo and in THF.
Once their characterization was complete, the new dyes
were used for the fabrication of a series of test solar cells to
verify their photovoltaic performances. It should be pointed
out that the cells produced during this work were not fully
optimized, for example by introducing a scattering titania
layer or by adjusting the electrolyte composition. Indeed,
our aim at this stage was to evaluate the potential of com-
pounds TTZ1–2 and BBZ1–2 as dyes for DSSCs by com-
paring their performance with those of reference sensitizers,
such as Ru complex N3 and organic compound D5. The
corresponding results are summarized in Table 3.
λabs [nm][a]
λa
[nm]
Eexc [eV]
f
max
TTZ1
TTZ2
BBZ1
BBZ1
472
THF
in vacuo
THF
in vacuo
THF
in vacuo
THF
in vacuo
466
2.66
2.57
2.64
2.50
2.83
2.75
2.82
2.74
2.35
2.45
2.39
2.55
2.42
2.47
2.50
2.55
482
470
486
437
451
440
453
476
446
455
[a] UV/Vis spectra were recorded on a 2.5ϫ10–5 m solution of the
dye in THF.
Initially, we prepared two series of cells by using dye
TTZ1 and a iodide/triiodide-based electrolyte, either in the
ing for 51–68% of the total, whereas the HOMOǞLUMO absence or in the presence of the well-known co-adsorbent
transition contributes only 9–29%, depending on the mole- chenodeoxycholic acid (CDCA). The average efficiency of
cule studied. Both these transitions are productive in terms the cells built with TTZ1 alone reached 43% of the per-
of electron injection because they transfer electron density formance yielded by a N3-containing cell[7] measured under
from either the spacer or the donor group to the acceptor the same conditions; it also corresponded to 60% of the
cyanoacrylic group, which is in closest proximity to the efficiency observed for the standard organic dye D5.[33] In
semiconductor surface. TD-DFT values are in good agree- the case of organic dyes, co-adsorption with cholic acid de-
ment with the experimental results with an error of at most rivatives has often been reported to improve the efficiency
0.1 eV.
of the corresponding DSSCs, thanks to their tendency to
1922
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Eur. J. Org. Chem. 2013, 1916–1928

Fused Bis-Thiazole Chromophores
Table 3. Photovoltaic parameters of DSSCs built using the new or- sity of adsorbed methoxy-containing dyes TTZ2 and
ganic sensitizers as well as reference dyes D5 and N3.[a]
BBZ2 on TiO2, and found values of 4.0ϫ10–7 and
2.9ϫ10–7 molcm–2, respectively. Thus, the lower density of
adsorbed benzobisthiazole dyes compared with thiazolothi-
azoles could at least in part explain the difference in the
observed efficiencies. In addition, despite their superior
light-harvesting properties, methoxy-substituted species
TTZ2 and BBZ2 gave rise to less efficient photovoltaic cells
in comparison with triphenylamine-containing dyes TTZ1
and BBZ1. Possibly, the destabilization induced in the
ground-state HOMO level by the methoxy substituents (see
above) hindered dye regeneration by the electrolyte, thus
leading to the observed decrease in photovoltaic perform-
ance.
Sensitizer
Additive
Jsc
Voc
(mV)
ff
η
[%]
[mA cm–2]
TTZ1
TTZ1
TTZ2
BBZ1
BBZ2
D5
–
5.48
6.92
5.24
4.76
3.76
8.92
11.80
0.583
0.621
0.591
0.594
0.564
0.614
0.639
0.72
0.74
0.73
0.74
0.71
0.71
0.71
2.56
3.53
2.53
2.32
1.67
4.29
5.93
CDCA
CDCA
CDCA
CDCA
–
N3
–
[a] J-V curves measured under AM 1.5G simulated solar irradia-
tion (90 mWcm–2) at room temperature; film thickness approx.
10 μm; active area 0.25 cm2.
The encouraging results described above suggest that the
central thiazolothiazole core could find use as a spacer unit
in efficient organic DSSC sensitizers, provided that further
structural optimization is carried out on the dyes.
disrupt unfavorable dye-dye interactions, thus facilitating
electron injection.[40] We found that cells fabricated by using
TTZ1 in combination with CDCA showed improved pho-
tovoltaic parameters compared with those not containing
the co-adsorbent (Table 3 and Figure 8), which resulted in
a considerable increase of power conversion efficiency (η)
from 2.56 to 3.53%. Based on the increase both in short-
circuit current (Jsc) and open-circuit voltage (Voc), we con-
cluded that addition of CDCA was necessary to minimize
detrimental dye-dye interactions and to hinder reverse elec-
tron transfer from the semiconductor to triiodide, thereby
reducing the dark current. An analogous behavior has been
previously observed by adding CDCA to DSSCs sensitized
with compound D5.[41] Based on these results, CDCA was
used in the fabrication of all the following photovoltaic de-
vices.
Electrochemical Impedance Spectroscopy
Understanding charge transfer processes taking place
within a solar cell is crucial to explain the variation in pho-
tovoltaic performance of devices built under different con-
ditions. Electrochemical impedance spectroscopy (EIS) is a
powerful technique that allows the interfacial charge trans-
fer and internal electron transport processes that occur dur-
ing operation of a DSSC to be characterized.[42] We carried
out measurements on cells built using the most effective
sensitizers, namely thiazolothiazoles TTZ1–2, over the
100 kHz to 0.1 Hz frequency range in the dark at –0.6 V
forward bias voltage; the resulting Nyquist plots are pre-
sented in Figure 9.
Figure 8. Representative current/voltage curves of DSSCs built
using sensitizers TTZ1–2 and BBZ1–2. TTZ1: hollow squares;
TTZ1+CDCA: black squares; TTZ2+CDCA: red circles;
BBZ1+CDCA: green rhombs; BBZ2+CDCA: blue triangles.
Figure 9. EIS plots of cells built using sensitizers TTZ1–2. TTZ1:
black triangles; TTZ1+CDCA: black squares; TTZ2+CDCA: hol-
low circles.
Unfortunately, the other photosensitizers investigated in
this study provided inferior photovoltaic performance com-
pared with dye TTZ1. Benzobisthiazole sensitizers BBZ1–2
gave worse results than their thiazolothiazole counterparts
In such plots, the large semicircle in the mid-frequency
(Table 3), yielding both lower photocurrent and photovolt- domain reflects the charge transfer resistance relative to
age. The decrease in Jsc could be attributed to a less efficient both electron diffusion across the nanocrystalline titania
adsorption of benzobisthiazole sensitizers on the semicon- layer and the electron recombination reaction between the
ductor surface. To prove this point, we measured the den- semiconductor and the electrolyte at the sensitized-TiO2/
Eur. J. Org. Chem. 2013, 1916–1928
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1923

G. Reginato, L. Zani et al.
FULL PAPER
metallic sodium, methanol was distilled from metallic magnesium
in the presence of a catalytic amount of iodine, and CH2Cl2 was
distilled from CaCl2. 2,2,6,6-Tetramethylpiperidine (TMP) was
purified by distillation over CaH2 and kept over KOH under a ni-
trogen atmosphere. Acetonitrile, valeronitrile and tert-butylpyridine
were distilled before use. DMF was stored under nitrogen over 4 Å
molecular sieves. 4-Bromo-N,NЈ-bis(4-methoxyphenyl)aniline[30]
was prepared according to a published procedure. All other chemi-
cals employed were commercially available and used as received.
Petroleum ether (PE) was the 40–60 °C boiling fraction. Thin-layer
solution interface. As shown in Figure 9, the size of the
semicircles decreases in the order TTZ1+CDCA
Ͼ
TTZ2+CDCA Ͼ TTZ1, which is in agreement with the or-
der of the corresponding Voc values reported in Table 3. In
particular, the semicircle relative to the cell in which TTZ1
was used in combination with CDCA is significantly larger
than that corresponding to TTZ1 alone. Such increase in
diameter indicates a higher resistance to interfacial electron
transfer from the conduction band of TiO2 to triiodide in
the electrolyte solution. This finding is in agreement with chromatography was carried out on aluminum-supported Merck
60 F254 plates; detection was achieved using UV light and by stain-
ing with either permanganate or molybdophosphoric acid solutions
followed by heating. Flash column chromatography[44] was per-
formed using Merck Kieselgel 60 (300–400 mesh) as the stationary
phase. 1H NMR spectra were recorded at 300 or 400 MHz, and
13C NMR spectra were recorded at 75.5 or 100.6 MHz, respectively,
with Bruker Avance or Varian Mercury series instruments. Chemi-
cal shifts were referenced to the residual solvent peak (CHCl3: δ =
7.26 ppm for 1H NMR and δ = 77.16 ppm for 13C NMR; [D8]THF:
δ = 1.72 and 3.58 ppm for 1H NMR, δ = 67.21 and 25.31 ppm
for 13C NMR). FTIR spectra were recorded with a Perkin–Elmer
Spectrum BX instrument in the range 4000–400 cm–1 with a 2 cm–1
resolution. ESI-MS spectra were obtained by direct injection of the
sample solution using a Thermo Scientific LCQ-FLEET instru-
ment and are reported in the form m/z. HRMS spectra were mea-
sured with a Thermo Scientific LTQ Orbitrap (FT-MS) instrument.
UV/Vis spectra were recorded with a Varian Cary 400 spectrometer,
and fluorescence spectra were recorded with a Varian Eclipse in-
strument, irradiating the sample at the wavelength corresponding
to maximum absorption in the UV spectrum.
the different Voc values obtained from the J/V curves (Fig-
ure 8), and confirms that adsorption of CDCA is useful to
reduce the amount of dark current when sensitizer TTZ1 is
employed.
Conclusions
Four new photosensitizers, TTZ1–2 and BBZ1–2, con-
taining either a thiazolo[5,4-d]thiazole- or a benzo[1,2-
d:4,5-dЈ]bisthiazole spacer, have been prepared and charac-
terized. Their synthesis featured a novel and efficient MW-
assisted preparation of the thiazolo[5,4-d]thiazole core as
well as the double functionalization of two different dihalo-
thienyl derivatives through Suzuki coupling. The new com-
pounds displayed interesting photo- and electrochemical
properties, which made them applicable for the construction
of dye-sensitized solar cells. TD-DFT calculations were car-
ried out to probe the electronic structure and transitions of
the dyes, and suggested that visible absorption was mostly
due to a HOMO–1ǞLUMO transition. Dye-sensitized so-
lar cells built with compounds TTZ1–2 and BBZ1–2
yielded power conversion efficiencies up to 3.53%; such val-
ues were promising but did not exceed the photovoltaic per-
formance of reference organic dye D5. An increase in effi-
ciency was observed for the cells when TiO2 staining was
conducted in the presence of chenodeoxycholic acid
(CDCA), probably due to minimization of unfavorable
dye···dye interactions and reduction of reverse electron
transfer from the semiconductor to triiodide, as seen by
J/V and EIS measurements. Despite their drawbacks, the
straightforward synthesis presented for these molecules and
their encouraging photophysical properties suggest that the
thiazolothiazole nucleus can be employed as a new core
structure in organic π-dyes for DSSCs. The preparation of
new sensitizers, based on the introduction of more efficient
and suitable donor and acceptor groups and on the re-
duction of molecular aggregation, is underway in our
laboratories and will be reported in due course.
Synthetic Procedures
2,5-Bis(4-hexylthiophen-2-yl)thiazolo[5,4-d]thiazole (5): In a micro-
wave vial equipped with a magnetic stirrer were introduced di-
thiooxamide (4; 0.218 g, 1.82 mmol, 1.0 equiv.) and 4-hexylthio-
phene-2-carbaldehyde (3; 1.43 g, 7.26 mmol, 4.0 equiv.). The re-
sulting mixture was heated under microwave irradiation at 180 °C
for 15 min, after which it was allowed to cool to room temperature.
TLC analysis (SiO2; PEt/Et2O, 15:1) showed the presence of the
desired compound; the reaction mixture was diluted with H2O
(15 mL) and CH2Cl2 (15 mL), and the layers were separated. The
aqueous phase was extracted with CH2Cl2 (2ϫ 15 mL) and the
combined organic layers were washed with water (30 mL) and brine
(30 mL), and dried with Na2SO4. Removal of the solvent in vacuo
afforded a dark thick oil, which was purified by flash column
chromatography (SiO2; PE/Et2O, 22:1) to give 5 as a brown solid
1
(0.461 g, 0.97 mmol, 54%). H NMR (300 MHz, CDCl3): δ = 7.40
(d, J = 1.2 Hz, 2 H), 7.04 (d, J = 1.2 Hz, 2 H), 2.62 (t, J = 7.5 Hz,
4 H), 1.56–1.72 (m, 4 H), 1.25–1.43 (m, 12 H), 0.87–0.92 (m, 6
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 162.8, 149.8, 144.6,
137.2, 128.2, 123.6, 31.8, 30.5, 30.4, 29.1, 22.7, 14.2 ppm. The data
are in agreement with those reported in the literature.[15]
2,5-Bis(4-hexyl-5-iodothiophen-2-yl)thiazolo[5,4-d]thiazole (6): In a
Schlenk flask equipped with a magnetic stirrer were introduced
2,5-bis(4-hexylthiophen-2-yl)thiazolo[5,4-d]thiazole (5; 0.094 g,
0.2 mmol, 1.0 equiv.) and N-iodosuccinimide (0.111 g, 0.99 mmol,
5.0 equiv.), followed by anhydrous CHCl3 (0.5 mL) and glacial ace-
tic acid (2.0 equiv.). The reaction mixture was heated at 40 °C and
stirred overnight. After cooling to room temperature, the solvents
were removed under reduced pressure and the residue was purified
by flash column chromatography (SiO2; PE/toluene, 3:1 to 1:1) to
afford 6 as a yellow solid (0.115 g, 0.16 mmol, 80%), m.p. 131–
Experimental Section
General Synthetic Remarks: All reactions were performed under an
inert nitrogen atmosphere in flame- or oven-dried apparatus using
Schlenk techniques.[43] Microwave-assisted transformations were
carried out with a CEM Discover Bench-Mate reactor at fixed tem-
perature and variable power. THF was distilled from metallic so-
dium in the presence of benzophenone; toluene was distilled from
1924
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 1916–1928

Fused Bis-Thiazole Chromophores
1
133 °C. H NMR (300 MHz, CDCl3): δ = 7.17 (s, 2 H), 2.54 (t, J
139.6, 137.0, 136.9, 134.2, 132.6, 129.9, 129.8, 129.6, 127.1, 126.9,
= 7.5 Hz, 4 H), 1.52–1.71 (m, 4 H), 1.28–1.44 (m, 12 H), 0.84–0.97 125.1, 123.7, 122.6, 31.8 (ϫ2), 31.0, 30.3, 30.1, 29.4, 29.3, 29.1, 22.8
(m, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 161.9, 150.0, (ϫ2), 14.3 (ϫ2) ppm. IR (neat): ν = 3050, 2951, 2922, 2852, 1667,
˜
148.7, 141.9, 126.9, 79.4, 32.5, 31.7, 30.0, 29.0, 22.7, 14.2 ppm. IR 1591, 1547, 1511, 1487, 1467, 1436, 1333, 1286, 1266, 1224,
(KBr): ν = 2950, 2918, 2847, 1653, 1550, 1473, 1459, 1395, 1369, 1062 cm–1. ESI-MS: m/z = 827.55 [M]+.
˜
1347, 1308, 1248, 1233, 1199, 1172, 1115, 1084 cm–1. ESI-MS: m/z
2-Cyano-3-{5-[4-hexyl-2-(5-{4-hexyl-5-[4-(diphenylamino)phen-1-yl]-
thiophen-2-yl}thiazolo[5,4-d]thiazol-2-yl)thiophen-5-yl]thiophen-2-
= 727.17 [M + 1]+.
2-{4-Hexyl-5-[4-(diphenylamino)phen-1-yl]thiophen-2-yl}-5-(4-hexyl-
5-iodothiophen-2-yl)thiazolo[5,4-d]thiazole (9a): In a Schlenk tube
equipped with a magnetic stirrer were placed [Pd(PPh3)4] (18 mg,
0.015 mmol, 0.1 equiv.) and 2,5-bis(4-hexyl-5-iodothiophen-2-yl)
thiazolo[5,4-d]thiazole (6; 112 mg, 0.15 mmol, 1.0 equiv.), which
were dissolved in toluene (3.0 mL). 4-(Diphenylamino)benzenebo-
ronic acid (7; 45 mg, 0.15 mmol, 1.0 equiv.) was then added, fol-
lowed by Na2CO3 (2.0 m aqueous solution, 0.39 mL, 0.77 mmol,
5.0 equiv.). The resulting mixture was heated at 90 °C and stirred
for 5 h, during which the progress of the reaction was monitored
by TLC (SiO2; PE/toluene, 2:1). The reaction mixture was allowed
to cool to room temperature, then diluted with H2O (5 mL), and
CH2Cl2 (5 mL) and stirred for 5 min. The phases were separated,
and the aqueous layer was washed with CH2Cl2 (3ϫ 5 mL). The
combined organic layers were washed with water (10 mL) and brine
(10 mL), and dried with Na2SO4. After filtration and evaporation
of the solvent, a dark-red oil was obtained, which was purified
by flash column chromatography (SiO2; PE/toluene, 2:1) to give
triarylamine 9a (37 mg, 0.044 mmol, 28% yield) as an orange oil.
1H NMR (300 MHz, CDCl3): δ = 7.43 (s, 1 H), 7.26–7.34 (m, 6
H), 7.13–7.21 (m, 5 H), 7.05–7.12 (m, 4 H), 2.68 (t, J = 7.5 Hz, 2
H), 2.55 (t, J = 7.5 Hz, 2 H), 1.57–1.62 (m, 4 H), 1.22–1.45 (m, 12
H), 0.82–0.98 (m, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
163.2, 161.3, 150.0, 148.7, 147.9, 147.5, 142.0, 139.6, 138.5, 134.4,
129.9, 129.7, 129.6, 127.6, 127.2, 126.7, 125.1, 123.6, 122.7, 79.1,
32.5, 31.7 (ϫ2), 31.0, 30.0, 29.3, 29.0 (ϫ2), 22.7 (ϫ2), 14.2
yl}prop-2-enoic Acid (TTZ1): In a Schlenk flask equipped with a
magnetic stirrer were placed aldehyde 11a (16 mg, 0.019 mmol,
1.0 equiv.), cyanoacetic acid (7 mg, 0.077 mmol, 4.0 equiv.), ammo-
nium acetate (2 mg, 0.025 mmol, 1.3 equiv.) and glacial acetic acid
(1.5 mL). The reaction mixture was heated to 110 °C and stirred
for 24 h; at this point, TLC analysis (SiO2; PE/EtOAc, 2:1) showed
the disappearance of the starting material. The reaction mixture
was cooled to room temperature, water (6 mL) was added and stir-
ring was continued for 20 min. A precipitate formed, which was
collected by vacuum filtration and washed with small portions of
water and methanol. The crude product was purified by crystalli-
zation from toluene followed by washing with pentane, to afford
acid TTZ1 (11 mg, 0.012 mmol, 65%) as a dark amorphous solid.
1H NMR (300 MHz, [D8]THF): δ = 8.37 (s, 1 H), 7.87 (d, J =
4.1 Hz, 1 H), 7.62 (s, 1 H), 7.60 (s, 1 H), 7.43 (d, J = 4.1 Hz, 1 H),
7.37 (s, 1 H), 7.35 (s, 1 H), 7.21–7.30 (m, 4 H), 7.00–7.15 (m, 8 H),
2.91 (t, J = 7.8 Hz, 2 H), 2.70 (t, J = 7.8 Hz, 2 H), 1.63–1.81 (m,
2 H), 1.45–1.54 (m, 2 H), 1.24–1.43 (m, 12 H), 0.83–0.97 (m, 6
H) ppm. 13C NMR (75 MHz, [D8]THF): δ = 163.7, 163.5, 161.4,
152.5, 150.6, 148.8, 148.2, 146.0, 144.9, 143.8, 142.7, 140.2, 139.0,
138.0, 137.6, 137.0, 133.5, 130.8, 130.5, 130.4, 130.1, 127.9, 127.6,
125.7, 124.2, 123.2, 116.4, 100.3, 32.4, 31.5, 31.0, 30.7, 30.1, 30.0,
29.6, 29.5, 23.3 (ϫ2), 14.3 (ϫ2) ppm. IR (KBr): ν = 3429, 3059,
˜
3031, 2952, 2919, 2852, 2207, 1726, 1586, 1471, 1429, 1325, 1312,
1259, 1178, 1091, 1021 cm–1. HRMS: m/z calcd. for C50H46N4O2S5
[M+] 894.2224; found 894.2219.
(ϫ2) ppm. IR (neat): ν = 3059, 3035, 2954, 2925, 2854, 1733, 1645,
˜
Note: In the 1H NMR spectra of compound TTZ1 recorded in
[D8]THF, the multiplet at δ = 1.63–1.81 ppm was partially or com-
pletely covered by the signal belonging to THF.
1593, 1552, 1479, 1328, 1315, 1280, 1192, 1178, 1116, 1093,
1075 cm–1. ESI-MS: m/z = 844.32 [M + 1]+.
2-{4-Hexyl-5-[4-(diphenylamino)phen-1-yl]thiophen-2-yl}-5-[4-hexyl-
5-(5-formylthiophen-2-yl)thiophen-2-yl]thiazolo[5,4-d]thiazole (11a):
In a microwave vial equipped with a magnetic stirrer were intro-
duced compound 9a (22 mg, 0.026 mmol, 1.0 equiv.) and
PdCl2(dppf)·CH2Cl2 (ca. 2.0 mg, 2.6 μmol, 0.1 equiv.), which were
dissolved in anhydrous toluene (1.0 mL). Meanwhile, into a
Schlenk flask under inert atmosphere were placed 5-formyl-2-thi-
ophenboronic acid (10; 12 mg, 0.078 mmol, 3.0 equiv.) and KF
(9 mg, 0.16 mmol, 6.0 equiv.), which were dissolved in MeOH
(1.0 equiv.). The resulting pink solution was transferred into the
microwave vial, and the reaction mixture was heated under micro-
wave irradiation at 70 °C for 20 min. The progress of the reaction
was monitored by TLC (SiO2; PE/EtOAc, 10:1), which showed that
the starting material had been completely consumed. After cooling
to room temperature, the reaction mixture was diluted with water
(5 mL) and CH2Cl2 (5 mL), the layers were separated and the aque-
ous layer was extracted with CH2Cl2 (2ϫ5 mL). The combined
organic layers were washed with brine (15 mL) and dried with
Na2SO4. Removal of the solvent in vacuo yielded a dark-red oil,
which was purified by flash column chromatography (SiO2; PE/
2-Cyano-3-(5-{4-hexyl-2-[5-(4-hexyl-5-{4-[bis(4-methoxyphenyl)-
amino]phen-1-yl}thiophen-2-yl)thiazolo[5,4-d]thiazol-2-yl]thiophen-
5-yl}thiophen-2-yl)prop-2-enoic Acid (TTZ2): In a Schlenk flask
equipped with a magnetic stirrer were placed aldehyde 11b (18 mg,
0.02 mmol, 1.0 equiv.), cyanoacetic acid (8 mg, 0.08 mmol,
4.0 equiv.), ammonium acetate (2 mg, 0.025 mmol, 1.3 equiv.) and
glacial acetic acid (1.5 mL). The reaction mixture was heated to
110 °C and stirred for 24 h; at this point, TLC analysis (SiO2; PE/
EtOAc, 2:1) showed the disappearance of the starting material. The
reaction mixture was cooled to room temperature, water (6 mL)
was added and stirring was continued for 20 min. A precipitate
formed, which was collected by vacuum filtration and washed with
small portions of water and methanol. The crude product was puri-
fied by crystallization from toluene followed by washing with meth-
anol and pentane, to afford acid TTZ2 (12 mg, 0.013 mmol, 65%)
as a dark amorphous solid. 1H NMR (400 MHz, [D8]THF): δ =
8.37 (s, 1 H), 7.87 (d, J = 3.9 Hz, 1 H), 7.62 (s, 1 H), 7.58 (s, 1 H),
7.43 (d, J = 3.9 Hz, 1 H), 7.27 (d, J = 8.5 Hz, 2 H), 7.09 (d, J =
8.5 Hz, 4 H), 6.83–6.94 (m, 6 H), 3.77 (s, 6 H), 2.91 (t, J = 7.8 Hz,
EtOAc, 10:1 to 5:1) to give the desired product 11a (16 mg, 2 H), 2.69 (t, J = 7.8 Hz, 2 H), 1.63–1.81 (m, 2 H), 1.43–1.55 (m,
1
0.019 mmol, 74%) as a red oil. H NMR (300 MHz, CDCl3): δ =
9.89 (s, 1 H), 7.71 (d, J = 4.0 Hz, 1 H), 7.43 (s, 1 H), 7.38 (s, 1 H),
7.20–7.35 (m, 7 H), 7.02–7.19 (m, 8 H), 2.81 (t, J = 7.8 Hz, 2 H),
2.67 (t, J = 7.8 Hz, 2 H), 1.55–1.78 (m, 4 H), 1.19–1.50 (m, 12 H),
2 H), 1.22–1.41 (m, 12 H), 0.82–0.96 (m, 6 H) ppm. 13C NMR
(100 MHz, [D8]THF): δ = 163.69, 163.65, 161.3, 157.6, 151.2,
150.6, 149.8, 146.0, 144.9, 143.8, 143.4, 141.0, 139.8, 138.9, 137.7,
137.0, 134.8, 133.4, 130.8, 130.5, 130.1, 127.9, 127.6, 125.5, 119.7,
0.81–0.97 (m, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 182.8, 116.4, 115.4, 100.3, 55.4, 32.4, 31.5, 31.0, 30.6, 30.5, 30.0, 29.9,
163.5, 161.0, 150.5, 150.0, 147.9, 147.4, 145.3, 143.1, 143.0, 142.2,
29.5, 23.3 (ϫ2), 14.2 (ϫ2) ppm. IR (KBr): ν = 3444, 2957, 2852,
˜
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1925

G. Reginato, L. Zani et al.
FULL PAPER
2210, 1651, 1505, 1318, 1260, 1242, 1093, 1035 cm–1. HRMS: m/z
31.1, 30.6, 30.01, 29.95, 29.5, 23.3 (ϫ2), 14.2 (ϫ2) ppm. IR (KBr):
calcd. for C52H50N4O4S5 [M]+ 954.2436; found 954.2428.
ν = 3418, 2951, 2925, 2854, 2208, 1584, 1505, 1495, 1435, 1312,
˜
1241, 1192, 1059, 1036 cm–1. HRMS: m/z calcd. for C56H52N4O4S5
Note: In the 1H-NMR spectra of compound TTZ2 recorded in
[D8]THF, the multiplet at δ = 1.63–1.81 ppm was partially or com-
pletely covered by the signal belonging to THF.
[M]+ 1004.2587; found 1004.2596.
Note: In the 1H-NMR spectra of compound BBZ2 recorded in
[D8]THF, the multiplet at δ = 1.72–1.78 ppm was partially or com-
pletely covered by the signal belonging to THF.
2-Cyano-3-{5-[4-hexyl-2-(5-{4-hexyl-6-[4-(diphenylamino)phen-1-yl]-
thiophen-2-yl}benzobis(1,2-d;4,5-dЈ)thiazol-2-yl)thiophen-5-yl]thio-
phen-2-yl}prop-2-enoic Acid (BBZ1): Into a Schlenk flask equipped
with a magnetic stirrer was placed aldehyde 16a (38 mg,
Cyclic Voltammetry and Spectroelectrochemical Measurements: An-
hydrous 99.9%, HPLC grade dichloromethane for electrochemistry
0.043 mmol, 1.0 equiv.). Cyanoacetic acid (73 mg, 0.86 mmol) and was purchased from Aldrich. The supporting electrolyte used was
ammonium acetate (22 mg, 0.28 mmol) were dissolved in glacial
acetic acid (7.5 mL) at room temperature and an aliquot (1.5 mL)
of the resulting solution was then added to the substrate, corre-
sponding to the following quantities: cyanoacetic acid (14.6 mg,
0.17 mmol, 4.0 equiv.), ammonium acetate (4.3 mg, 0.056 mmol,
1.3 equiv.). The reaction mixture was heated to reflux and stirred
for 48 h, after which it was allowed to cool to room temperature.
The reaction mixture was poured into water (ca. 10 mL) and the
resulting precipitate was filtered off, washed with water and dried.
Recrystallization from toluene, followed by washing with methanol
and pentane afforded carboxylic acid BBZ1 (15 mg, 0.016 mmol,
37 %) as a dark-red amorphous solid. 1H NMR (400 MHz,
[D8]THF): δ = 8.47 (s, 1 H), 8.46 (s, 1 H), 8.38 (s, 1 H), 7.88 (d, J
electrochemical grade [N(Bu)4]PF6 obtained from Fluka. Cyclic
voltammetry was performed in a three-electrode C-3 BAS Cell hav-
ing a platinum working electrode, a platinum counter electrode and
an aqueous Ag/AgCl NaCl (3 m) reference electrode. A BAS 100A
electrochemical analyser was used as a polarizing unit. Under these
experimental conditions, the one-electron oxidation of ferrocene
occurs at E°’ = +0.42 V. UV/Vis spectroelectrochemical measure-
ments were carried out with a Perkin–Elmer Lambda 2 UV/Vis
spectrophotometer and a Optically Transparent Thin Layer spec-
troelectrochemical (OTTLE) cell in quartz glass with an optical
path-length of 1 mm, equipped with a Pt-minigrid working elec-
trode, a Pt auxiliary electrode and an Ag/AgCl NaCl (3 m) reference
electrode. A nitrogen-saturated CH2Cl2 solution of the compound
= 4.0 Hz, 1 H), 7.67 (s, 1 H), 7.65 (s, 1 H), 7.47 (d, J = 4.0 Hz, 1 under study was used with [N(Bu)4][PF6] (0.2 m) as the supporting
H), 7.39 (d, J = 8.6 Hz, 2 H), 7.25–7.31 (m, 4 H), 7.02–7.16 (m, 8 electrolyte. The in situ spectroelectrochemistry was recorded by col-
H), 2.92 (t, J = 7.9 Hz, 2 H), 2.73 (t, J = 7.8 Hz, 2 H), 1.70–1.78 lecting spectra in the spectral window 250–1000 nm during the
(m, 4 H), 1.30–1.42 (m, 12 H), 0.86–0.94 (m, 6 H) ppm. 13C NMR
(100 MHz, [D8]THF): δ = 163.8, 162.6, 161.2, 153.1, 152.6, 148.8,
stepwise oxidation of the compound (Ei = +0.55 V, ΔE = 50 mV,
Δt = 1 min). A BAS 100A electrochemical analyzer was used as the
148.2, 145.9, 144.8, 143.75, 143.68, 140.2, 138.9, 137.4, 137.2, polarizing unit during all the experiments.
135.4, 135.3, 135.1, 134.5, 132.9, 132.4, 130.4, 130.1, 128.5, 128.0,
Computational Details: DFT calculations were performed with the
127.8, 125.7, 124.2, 123.2, 116.4, 115.9, 115.6, 32.4, 31.5, 31.1, 30.6,
Gaussian09 program package.[38] Geometry optimizations were
carried out in vacuo using the B3LYP functional[45] and the stan-
dard 6-31G* basis set for all atoms. The absorption maxima
(λamax), vertical excitation energies (Eexc) and oscillator strengths
(f) were calculated, both in vacuo and in THF, on the optimized
structure by time-dependent DFT (TD-DFT) at the CAM-B3LYP/
6-31G* level.[46] Solvent effects were included by using the polariz-
able continuum model (PCM).[47]
30.5, 30.0, 29.9, 29.5, 23.3 (ϫ2), 14.3 (ϫ2) ppm. IR (KBr): ν =
˜
3434, 2961, 2924, 2852, 2206, 1729, 1580, 1489, 1432, 1310, 1261,
1087, 1020 cm–1. HRMS: m/z calcd. for C54H48N4O2S5 [M + 1]+
945.2454; found 945.2451.
Note: In the 1H-NMR spectra of compound BBZ1 recorded in
[D8]THF, the multiplet at δ = 1.70–1.78 ppm was partially or com-
pletely covered by the signal belonging to THF.
2-Cyano-3-(5-{4-hexyl-2-[5-(4-hexyl-6-{4-[bis(4-methoxyphenyl)- DSSC Fabrication and Photoelectrochemical Measurements: FTO-
amino]phen-1-yl}thiophen-2-yl)benzobis(1,2-d;4,5-dЈ)thiazol-2-yl]-
thiophen-5-yl}thiophen-2-yl)prop-2-enoic Acid (BBZ2): Into a
Schlenk flask equipped with a magnetic stirrer was placed aldehyde
16b (25 mg, 0.027 mmol, 1.0 equiv.), which was dissolved in glacial
acetic acid (2.0 mL) at room temperature. Cyanoacetic acid (30 mg,
0.35 mmol, 13.0 equiv.) was added, followed by ammonium acetate
(50 mg, 0.4 mmol, 14.4 equiv.). The reaction mixture was heated to
reflux and stirred for 48 h, after which it was allowed to cool to
room temperature. The reaction mixture was poured into water (ca.
10 mL) and the resulting precipitate was filtered off, washed with
water and dried. Recrystallization from toluene, followed by wash-
ing with a large volume of methanol and pentane afforded carbox-
ylic acid BBZ2 (10 mg, 0.01 mmol, 37%) as a dark-red amorphous
coated conducting glass (TCO 30–8, Solaronix) was cut into
20ϫ20 mm slides by using a diamond point. The slides were
cleaned by immersing them in ethanol for 15 min and, sub-
sequently, in acetone for 15 min using an ultrasound bath, followed
by rinsing with water and ethanol. The slides were then immersed
in a 0.04 m TiCl4 aqueous solution at 70 °C for 30 min, followed
by rinsing with water and ethanol. A commercially available nano-
crystalline TiO2 paste (Ti-Nanoxide D/SP, Solaronix) was printed
on the cleaned FTO glass by means of squeegee printing. The
active area of the resulting nanocrystalline TiO2 films was 0.25 cm2.
The films were kept in a clean box for 5 min with ethanol so that
the film could relax, and were then heated on a hot plate at 125 °C
for 15 min to remove any trace of water. Subsequently, they were
sintered for 30 min at 280 °C, followed by 30 min at 450 °C. The
coated glass slides were again immersed in a 0.04 m TiCl4 aqueous
solution at 70 °C for 30 min, followed by rinsing with water and
ethanol. Finally, they were sintered at 450 °C for 30 min and al-
lowed to cool under air until they reached a temperature of 70 °C.
At this point, the electrodes were immersed in a 5ϫ10–4 m solution
of the appropriate dye in THF for at least 10 h; in the case of
measurements performed in the presence of a co-adsorbent, the
solution contained also chenodeoxycholic acid (CDCA) in a
2.5ϫ10–3 m concentration. Once the staining procedure was com-
1
solid. H NMR (300 MHz, [D8]THF): δ = 8.48 (s, 1 H), 8.46 (s, 1
H), 8.39 (s, 1 H), 7.89 (d, J = 4.0 Hz, 1 H), 7.68 (s, 1 H), 7.64 (s,
1 H), 7.48 (d, J = 4.0 Hz, 1 H), 7.30 (d, J = 8.7 Hz, 2 H), 7.05–
7.14 (m, 4 H), 6.83–6.95 (m, 6 H), 3.77 (s, 6 H), 2.93 (t, J = 7.8 Hz,
2 H), 2.71 (t, J = 7.8 Hz, 2 H), 1.72–1.78 (m, 2 H), 1.48–1.54 (m,
2 H), 1.23–1.45 (m, 12 H), 0.83–0.97 (m, 6 H) ppm. 13C NMR
(100 MHz, [D8]THF): δ = 163.7, 162.6, 161.1, 157.6, 153.2, 152.5,
149.9, 146.0, 144.9, 144.3, 143.8, 141.0, 139.8, 138.9, 137.4, 137.2,
135.4, 135.2, 134.6, 134.5, 132.9, 132.4, 130.2, 127.9, 125.6, 119.7,
116.4, 115.8, 115.5, 115.4, 111.4, 100.5, 55.4, 32.44, 32.42, 31.6,
1926
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 1916–1928

Fused Bis-Thiazole Chromophores
plete, the electrodes were rinsed with ethanol and dried under a
flow of air. For the preparation of the counter electrodes, the FTO
conducting glass was cut and the resulting slides were cleaned as
described above; a small hole (approx. 1 mm diameter) was then
drilled on every slide by using a diamond-point drill. A commercial
platinum-based paint (Platisol T, Solaronix) was deposed on the
conductive side of the FTO-coated glass by using a brush, followed
by drying at 125 °C for 15 min and sintering at 400 °C for 30 min.
The two electrodes were assembled into a sandwich type cell by
heating at 250 °C with a hot-melt ionomer film (Meltonix 1170-25,
Solaronix) as a spacer between them. A drop of the appropriate
electrolyte solution was placed on the drilled hole on the back of
the counter electrode and was driven into the cell by vacuum back-
filling. The hole was finally sealed by using additional sealing film
and a small glass cover (0.1 cm2).
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Supporting Information (see footnote on the first page of this arti-
cle): Complete experimental procedures for the synthesis of com-
pounds 3, 8, 9b, 11b, 13, 14, 15a–b and 16a–b, copies of 1H and
13C NMR spectra of compounds TTZ1–2 and BBZ1–2, additional
computational, spectroscopic and electrochemical data.
Acknowledgments
M. L. P. and M. C. are grateful to Regione Toscana within the
POR-FSE 2007-2013 program for
a postdoctoral fellowship
(FOTOSENSORG project). Dr. Jonathan Filippi (ICCOM-CNR)
is gratefully acknowledged for his help in performing EIS measure-
ments. A. S., M. L. P. and R. B. acknowledge CINECA awards
N. HP10C9HMGL and N. HP10CJ65S2, 2011 for the availability
of high performance computing resources and support.
Eur. J. Org. Chem. 2013, 1916–1928
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1927

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Received: December 3, 2012
Published Online: February 18, 2013
1928
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