Synthesis and Investigation of Solar-Cell Photosensitizers Having a Fluorazone Backbone

Article abstract of DOI:10.1002/ejoc.201601622

A synthetic sequence for the preparation of fully conjugated 2,7-disubstituted fluorazone (9H-pyrrolo[1,2--a]indol-9-one) derivatives has been developed comprising an Elming–Clauson–Kaas-type pyrrole formation, POCl₃-mediated ring closure, selective halogenation and elongation of the conjugated backbone through cross-coupling reactions. As a proof of principle, this methodology was used to prepare for the first time two organic D–π–A dyes containing the fluorazone moiety. The new compounds display broad absorption bands in the visible-light region when adsorbed on nanocrystalline TiO₂ and electrochemical properties compatible with their employment as photosensitizers in dye-sensitized solar cells. Small-scale photovoltaic devices fabricated with the fluorazone dyes yielded power conversion efficiencies in the range of 2.1–2.4 %, which correspond to approximately 70 % of the efficiency obtained with the reference organic dye DF15 under the same conditions.

Full text of DOI:10.1002/ejoc.201601622

A Journal of
Accepted Article
Title: Synthesis and Investigation of New Solar Cell Photosensitizers
Having a Fluorazone Backbone
Authors: Béla Mátravölgyi, Tamás Hergert, Angelika Thurner, Bálint
Varga, Nicola Sangiorgi, Riccardo Bendoni, Lorenzo Zani,
Gianna Reginato, Massimo Calamante, Adalgisa Sinicropi,
Alessandra Sanson, Ferenc Faigl, and Alessandro Mordini
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601622
Link to VoR: http://dx.doi.org/10.1002/ejoc.201601622
Supported by

10.1002/ejoc.201601622
European Journal of Organic Chemistry
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Synthesis and Investigation of New Solar Cell Photosensitizers
Having a Fluorazone Backbone
Béla Mátravölgyi,[a] Tamás Hergert,[b] Angelika Thurner,[a] Bálint Varga,[b] Nicola Sangiorgi,[c] Riccardo
Bendoni,[c] Lorenzo Zani,[d] Gianna Reginato,[d] Massimo Calamante,[d] Adalgisa Sinicropi,[d,e]
Alessandra Sanson,[c] Ferenc Faigl,[a,b],* Alessandro Mordini[d],*
Abstract: A synthetic sequence for the preparation of fully conjugated, redox reactions.4 Due to its multifaceted biological activity,5 the
2,7-disubstituted
fluorazone
(9H-pyrrolo[1,2-a]indol-9-one)
preparation and functionalization of the fluorazone core have
been the subject of significant synthetic efforts; however, studies
concerning the extension of its conjugate system6 and the
enhancement of its photophysical properties have so far been
missing. In particular, no report describing the preparation of
nonsymmetrical, donor-acceptor-type structures incorporating the
fluorazone core was reported in the literature. Despite that, we
reasoned that fluorazone, thanks to its planar and fused structure,
could be used as a linker in conjugated (hetero)aromatic systems
with potential application in organic electronics. Therefore, we
designed a novel synthetic route able to provide conjugated 2,7-
disubstituted fluorazone derivatives through the intermediacy of
the corresponding 2,7-halogenated species. As a proof of concept,
we applied such sequence to the preparation of D-π-A type
compounds HT157 and TA1314 (Figure 1), having the fluorazone
core in the central part of the molecule, and featuring a π-bridge
of different length (one thiophene ring for HT157 and two for
TA1314), aiming to apply them as organic photosensitizers for
testing in dye-sensitized solar cells (DSSC).7,8
derivatives was developed, comprising Elming-Clauson-Kaas type
pyrrole formation, POCl3-mediated ring closure, selective
halogenation and elongation of the conjugate backbone via cross-
coupling reactions. As a proof of principle, such methodology was
used to prepare for the first time two organic D-π-A dyes containing
the fluorazone moiety. The new compounds displayed broad
absorption of visible light when adsorbed on nanocrystalline TiO2 and
electrochemical properties compatible with their employment as
photosensitizers in dye-sensitized solar cells. Small-scale
photovoltaic devices fabricated with the fluorazone dyes yielded
power conversion efficiencies in the 2.1-2.4% range, corresponding
to approx. 70% of the efficiency obtained with reference organic dye
DF15 under the same conditions.
Introduction
1-Aryl-1H-pyrrole-containing compounds serve as an important
framework for the synthesis of various complex molecules, such
as chiral ligands for enantioselective catalytic reactions,1,2 and
exhibit a wide array of biological activities.3 From an electronic
standpoint, the conjugation between their aromatic and
heteroaromatic rings can be significantly increased if they are
fixed in the same plane in a fused ring system, as it can be
observed in the case of fluorazone (9H-pyrrolo[1,2-a]indol-9-one)
and its analogues. As a consequence, fluorazone derivatives can
show interesting photochemical properties, which prompted
testing of this type of compounds as photosensitizing agents in
Figure 1. Structure of new DSSC sensitizers HT157 and TA1314.
Both compounds have an alkoxy-substituted triphenylamine
group as the donor part of the molecule and a typical cyanoacrylic
acid anchoring group, which should make them suitable for the
proposed application. Structures featuring the pyrrole ring
connected to the donor group and the benzene ring attached to
thiophene were preferred over their regioisomers (with an
“inverted” central fluorazone core) based on the results of a TD-
DFT computational analysis (see below). The synthesis, full
spectroscopic and electrochemical characterization, and
employment of the compounds in DSSC devices are described
below.
[a]
[b]
MTA-BME Organic Chemical Technology Research Group,
Hungarian Academy of Sciences, 1111 Budapest, Budafoki út. 8.,
Hungary.
E-mail: ffaigl@mail.bme.hu
Department of Organic Chemistry and Technology, Budapest
University of Technology and Economics, 1111 Budapest, Budafoki
út. 8., Hungary.
Institute of Science and Technology of Ceramic Materials (CNR-
ISTEC), Via Granarolo 64, 48018 Faenza, Italy.
Institute of Chemistry of Organometallic Compounds (CNR-ICCOM),
Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy.
E-mail: alessandro.mordini@iccom.cnr.it
[c]
[d]
[e]
Department of Biotechnology, Chemistry and Pharmacy, University
of Siena, Via A. Moro 2, 53100 Siena, Italy.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201601622
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Scheme 1. Synthesis of fluorazones 5a and 5b.
Results and Discussion
Synthesis of Compounds HT157 and TA1314
A number of synthetic strategies have been developed for the
preparation of (un)substituted-fluorazones and analogues,9-11
among which the most appealing appear those featuring ortho-
(1H-pyrrol-1-yl)aryl and heteroaryl carboxylic acids, in turn
obtained by pyrrolation of ortho-aminoaryl (anthranilic) and ortho-
aminoheteroaryl carboxylic acids, respectively.12–17 Our synthetic
pathway was based on the preparation of two previously
unreported key intermediates: 2,7-dihalo-9-oxo-9H-pyrrolo[1,2-
a]indoles 5a-b, from which the target compounds could be
obtained by consecutive cross-coupling reactions in the presence
of palladium catalysts, followed by the introduction of the
cyanoacrylic acid moiety into the acceptor part of the molecules.
Accordingly, an efficient synthesis of 5a and 5b was developed
(Scheme 1).
The pyrrolidine amide derivatives of 5-halogeno-2-pyrrolobenzoic
acid (3a or 3b) were prepared starting from the methyl ester of 5-
bromo- or 5-iodoanthranilic acid (1a or 1b), which was condensed
with 2,5-dimethoxytetrahydrofuran (formation of 2a,b) according
to the Elming-Clauson-Kaas-type pyrrole formation protocol,18
followed by reaction with pyrrolidine, which, however, proceeded
only with moderate yield. Amides 3a-b were then treated with
phosphorous oxychloride to yield fluorazones 4a and 4b in 81-
78% yield. Regioselective monohalogenation of the β-position of
the pyrrole ring was finally accomplished applying the mild
conditions developed by our laboratory for the halogenation of 1-
phenyl-1H-pyrrole derivatives.19 It should be pointed out that 2,7-
dihalofluorazone derivatives such as 5a-b were never reported
previously in the literature, and that in this case they could be
accessed thanks to the selectivity of such monohalogenation
protocol.
Figure 2. Structures of intermediates 6-8.
sequential cross-coupling reactions. We chose the Suzuki-
Miyaura cross-coupling protocol due to its robustness and ease
of application, and the fact that it had been previously applied by
us to the preparation of organic DSSC sensitizers with good
results.20,21 Thus, the mono- (6) and dithiophene (7) moieties of
the acceptor parts were prepared according to slightly modified
literature methods (Figure 2). Subsequently, boronic ester 8,
containing the donor portion, was assembled following a multistep
literature procedure from 4-hexyloxybromobenzene, 4-
hexyloxyaniline and 4-iodobromobenzene via consecutive
palladium-catalyzed C-N coupling reactions and Miyaura
borylation (Figure 2, see SI for details on the preparation of
compounds 6-8).
For the preparation of compound HT157, we established that the
best synthetic sequence was the one starting with the introduction
of the acceptor unit: for this reason, in compound 6 the aldehyde
function was protected as a cyclic acetal. Coupling of compound
5b with boronic acid 6 under typical Suzuki-Miyaura conditions
using Pd(PPh3)4 as catalyst proceeded selectively on the C-I bond
to afford intermediate 9 in moderate yield. The latter was then
subjected to reaction with boronic ester 8: in this case, the catalyst
species was generated by mixing palladium(II) acetate with 2.0
eq. of dppf, and the reaction proceeded in the presence of a sub-
At this stage, the preparation of the donor and acceptor parts of
dyes HT157 and TA1314 was carried out, with the intention of
joining them to the central part of the molecule via efficient,
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10.1002/ejoc.201601622
European Journal of Organic Chemistry
FULL PAPER
Scheme 2. Synthesis of fluorazone dyes HT157 and 13 (piperidine salt of TA1314).
stoichiometric amount of CuCl22 to provide, after acidic workup,
2,7-disubstituted fluorazone 10 in 40% yield. Direct treatment of
aldehyde 10 with cyanoacetic acid and ammonium acetate in
acetic acid/chloroform resulted in the desired product HT157 as
the free acid (Scheme 2).
use by simple treatment of 13 with diluted aq. HCl followed by
extraction in organic solvents.
Optical and electrochemical characterization
With compounds HT157 and TA1314 in hand, we carried out their
optical and electrochemical characterization. The relevant results
are reported in Table 1. Initially, we recorded their UV-Vis
absorption spectra in dichloromethane solution (Figure 3a). Both
dyes showed a relatively wide absorption band in the visible
region covering the range between 300 and 550 nm: in each case
the spectrum displayed two main absorption peaks, likely arising
from different electronic transitions. While the first peak had
almost the same position for both dyes (352 nm for HT157 and
349 nm for TA1314), the second one was significantly red-shifted
in the case of the latter compound (400 nm for HT157 vs. 444 nm
for TA1314), which could be obviously attributed to its longer
conjugated structure (for a detailed computational analysis, see
below). Both compounds absorbed light quite intensely, and their
In the case of compound TA1314 the best yield was
obtainedwhen the order of cross-coupling transformations was
reversed. Thus, 5a was reacted with boronic ester 8 in the
presence of Pd(PPh3)4/potassium carbonate suspension in
tetrahydrofuran, and product 11 was isolated from the organic
solution with satisfactory yield (Scheme 2). Then, the bithiophene
moiety (7) was connected to fluorazone 11 with a second Suzuki-
Miyaura coupling, this time carried out in toluene, to obtain
aldehyde 12. It was found that condensation of the latter
compound with cyanoacetic acid was best accomplished in the
presence of piperidine in acetonitrile solution, leading to isolation
of the desired product TA1314 as its piperidine salt (13, Scheme
2). The free acid form of TA1314 could then be released prior to
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10.1002/ejoc.201601622
European Journal of Organic Chemistry
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Figure 3. (a) UV-Vis absorption spectra of compounds HT157 (red squares) and TA1314 (blue circles) in CH2Cl2 solution; concentrations: HT157 - 2.38 × 10–5 M,
TA1314 - 2.02 × 10–5 M. (b) Tauc plots for compounds HT157 and TA1314; intersection of the fitting straight line with the horizontal axis provides an estimation of
the compounds optical band-gaps. (c) Normalized UV-Vis absorption spectra of compounds HT157 and TA1314 adsorbed on nanocrystalline TiO2, obtained by
difference from the spectrum of the bare semiconductor. (d) Cyclic voltammetries of compounds HT157 and TA1314 obtained in in CH2Cl2 solution.
semiconductor layer, which was much more pronounced in the
Table 1. Optical and electrochemical characterization data for compounds
case of compound TA1314 (58 nm vs. only 11 nm for HT157).
Such hypsochromic shift could be attributed to the formation of H-
aggregates on TiO2,24 which appears to be much more severe for
compound TA1314, probably due to its longer and flatter structure
around the acceptor region, resulting from the introduction of an
extra thiophene ring. The absorption spectra of both compounds
were remarkably wide, extending well above 600 nm, which
proved positive in view of their potential use in DSSCs. TA1314
displayed a slightly broader band compared to HT157, in
agreement with the shape of the spectra recorded in solution.
Finally, characterization of the dyes was completed by measuring
their oxidation potential at the ground state (Eox) by means of
cyclic voltammetry (Figure 3d). Such measurement was carried
out to determine if the new compounds were able to participate to
the electron injection and regeneration processes occurring in a
DSSC (see below for a brief discussion).7 Eox value was
determined by averaging the anodic and cathodic peak potentials.
As could be expected from their very similar structure and
identical donor groups, the compounds presented very close Eox
values, in the range of 1.01-1.02 V vs. NHE. As a consequence,
both sensitizers appeared to provide ample driving force (more
than 0.6 V) for the ground state regeneration reaction, given the
redox potential of the I–/I3– couple typically used in the electrolyte
(0.35 V vs. NHE).25 The excited state oxidation potential (E*ox) of
HT157 and TA1314.
Comp.
λmax
(sol)
[nm]a
ε × 104
E0-0
λmax
(TiO2)
[nm]
Eox
[V]
E*ox
[V]c
[M–1 cm–1
]
[eV]b
HT157
TA1314
400
444
3.56
3.96
2.75
2.47
389
386
1.02d
1.01d
–1.73
–1.46
[a] Measured in CH2Cl2 solution. [b] Estimated from the corresponding Tauc
plot. [c] Determined from the equation: E*ox = Eox – E0-0. [d] Potentials vs.
NHE. They were obtained by adding 0.2 V to potentials vs. the Ag/AgCl (sat.
KCl) reference electrode used in the study.
molar extinction coefficients reached the respectable values of
3.5-4.0 × 104 M–1 cm–1. The optical band gap of the dyes could be
estimated from their absorption spectra by calculating the
corresponding Tauc plot Figure 3b),23 and resulted of 2.47 eV for
TA1314 and 2.75 eV for HT157.
The optical properties of the new dyes were also measured after
adsorption on a nanocrystalline TiO2 film, similar to that employed
in a typical DSSC device (see Experimental Section for details on
film deposition). As can be seen from their spectra (Figure 3c),
both dyes exhibited a significant blue-shift upon adsorption on the
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10.1002/ejoc.201601622
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the dyes was estimated from the difference between their optical
band-gap and their ground state oxidation potential (E0-0 – Eox),
and was found to be –1.73 V for HT157 and –1.46 V for TA1314,
respectively. Such values were largely sufficient to ensure
effective electron injection from the excited dyes into the
conduction band of TiO2, thus making the new compounds
suitable for employment in DSSC devices.
Computational analysis
To better understand the results of the dyes optical and
electrochemical characterization, we subjected them to
computational analysis, and compared the resulting data with
those obtained for our previously reported organic dye DF15
(Figure S1, Supporting Information).26 First, the structures of
HT157 and TA1314 were optimized in vacuo by means of DFT
calculations at the B3LYP/6-31G* level, using the Gaussian 09
program package.27 From this analysis, it resulted that the
compounds conjugated backbones were not perfectly planar, with
dihedral angles of approx. 24-26° existing between the central
fluorazone moiety and the flanking phenyl and thienyl groups (see
Figure S2). The values of the frontier orbital energy gaps for the
computed structures were then calculated and are reported in
Figure 4, while the wave function plots of the corresponding
orbitals are presented in Figure S2. While the HOMO orbitals
were mostly localized on the donor moiety for both dyes, and had
therefore a comparable energy (in agreement with the results of
cyclic voltammetry experiments, see above), the HOMO–1
orbitals extended mostly on the central tricyclic portion of the
molecules, with a smaller contribution from the donor (a bit more
pronounced in the case of HT157), and had an energy difference
of approx. 0.2 eV. The LUMO orbitals, instead, were clearly
located on the acceptor parts of both compounds, with that of
TA1314 being slightly stabilized relative to that of HT157
(probably due to the longer conjugation). As a consequence, the
energy gap between frontier orbitals was smaller for TA1314
compared to HT157.
Figure 4. Frontier orbital energies and energy gaps for dyes DF15,26 HT157
and TA1314. The wave function plots of the orbitals are presented in Figure S2.
Table 2. Experimental (λexpmax) and computed (MPW1K/6-311+G(2d,p))
(λamax) absorption maxima, oscillator strengths (f) and vertical excitation
energies (Eexc) for dyes HT157, TA1314 and DF15 (taken as a reference) in
CH2Cl2 solution.
Compd.
λexp
λa
Eexc
f
main transition
max
max
[nm]
[nm]
[eV]
HT157
400
444
392
456
3.16
2.72
0.8547
0.9758
H–1 → L 63%
TA1314
H → L+1 41%
H–1 → L 33%
Subsequently, absorption maxima (λamax), vertical excitation
energies (Eexc) and oscillator strengths (f) in CH2Cl2 solution were
calculated on the optimized structures via time-dependent DFT
(TD-DFT) at the MPW1K/6-311+G(2d,p) level (Table 2). Solvent
effects have been included by using the polarizable continuum
model (PCM).28 Interestingly, calculations showed that the main
visible band in the spectrum of both dyes was generated by a
mixed transition, whose foremost component was usually not the
HOMO → LUMO, but rather the HOMO–1 → LUMO and HOMO
→ LUMO+1 excitations. This was different from the situation for
dye DF15, whose visible absorption was red-shifted compared to
the new compounds, and was mostly attributed to a HOMO →
LUMO transition. In good agreement with the experimental
spectra, the lowest-energy transition for HT157 was computed to
take place at a shorter wavelength compared to TA1314, and
calculated vertical excitation energies for the two dyes could
reproduce the experimental values with relatively small
differences (0.01-0.07 eV).
DF15
530a
533
2.33
1.5344
H → L
89%
[a] Value taken from ref. 26.
the regioisomeric compound showed a visible absorption band
mostly due to a HOMO–1 → LUMO transition. However, the band
gap was larger than that of HT157 (3.14 eV vs. 3.03 eV), and the
HOMO–1 orbital was only located on the π-bridge and acceptor
group, thus reducing the degree of charge transfer upon
photoexcitation (Figure S3). Since no clear advantages against
the original dyes could be appreciated from the computational
analysis, the synthesis of the regioisomeric forms of compounds
HT157 and TA1314 was not pursued further.
Photoelectrochemical Measurements
As mentioned above, we intended to test compounds HT157 and
TA1314 as organic photosensitizers for dye-sensitized solar cells
(DSSC).7 In such devices, a dye (organic or organometallic in
nature) is used to sensitize
semiconductor (most commonly TiO2) towards the absorption of
Finally, as briefly mentioned in the introduction, it should be noted
that a TD-DFT computational analysis was carried out also on the
regioisomeric form of dye HT157, featuring an “inverted”
fluorazone core (reg-HT157, Figure S3). Similar to HT157, also
a wide band-gap inorganic
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Figure 5. Current-voltage curves for compounds HT157 (red squares), TA1314 (blue circles) and reference compound DF15 (black triangles), recorded in the
absence (a) and presence (b) of CDCA as a disaggregating agent.
Table 3. Photovoltaic and electrochemical parameters for DSSCs built with dyes HT157, TA1314 and DF15.
Compound
CDCA
Jsc [mA cm–2
4.6±0.2
4.5±0.3
5.1±0.2
5.2±0.6
6.6±0.1
7.1±0.2
]
Voc [mV]
635±5
639±7
610±2
629±7
672±5
672±4
ff [%]
70±1
74±2
72±1
73±1
72±1
70±1
η [%]
R3 [Ω]b
22.4
23.4
16.5
16.1
18.1
18.9
–
2.09±0.10
2.13±0.08
2.26±0.12
2.39±0.26
3.17±0.09
3.40±0.10
HT157
a
+
–
TA1314
DF15
a
+
–
a
+
[a] Obtained by staining the photoanode with a dye solution containing 2 mM CDCA. [b] Determined by electrochemical
impedance spectroscopy (EIS) measurements (see text for discussion).
visible light: transfer of an electron from the photoexcited dye to
depositing a Pt-layer on the conductive side via sputtering,
followed by thermal treatment. Cell assembly was then completed
by sandwiching the photo-anode and the counter-electrode with a
Surlyn® sealing gasket and vacuum cell filling (through the pre-
drilled hole) with a commercial electrolyte containing the iodide /
triiodide redox couple (Solaronix Z-100).
Photoanode sensitization was carried out either in the presence
or in the absence of chenodeoxycholic acid (CDCA, 2 mM), which
was used as a disaggregating agent. In addition, measurements
were performed with our previously reported organic dye DF15 as
well, which was used as a reference. The corresponding J-V
curves are reported in Figure 5, while the relevant photovoltaic
parameters are provided in Table 3.
Considering first the results obtained without CDCA (Figure 5a),
it can be observed that cells built with the two new dyes gave
similar efficiencies, with TA1314 yielding a slightly higher value,
and were characterized by excellent fill factors. While HT157
produced a higher photovoltage, TA1314 gave a higher current
density, which could be due either to its superior light-harvesting
efficiency (Figure 3a,c) or to a more efficient electron injection into
the semiconductor (also called “electron injection”) represents the
key charge-separation step taking place within the cell.
Regeneration of the dye from a suitable redox shuttle must take
place in order to close the circuit, allowing production of an electric
current. Both these steps require a precise alignment of energy
levels among all cell components29 which, in the present case,
was ensured by the results of the electrochemical measurements
described above.
The complete cell assembly procedure is reported in the
Experimental Section. Briefly, photoanodes were prepared by
deposing a TiO2 blocking layer on a conductive glass (FTO) sheet
by spin-coating of a TiCl4 alcoholic solution, and then screen-
printing a 6 μm thick nanocrystalline TiO2 film using a commercial
paste (Solaronix T/SP), which was then subjected to thermal
sintering; the preparation procedure was completed by spin-
coating a second time a TiCl4 alcoholic solution and then
sensitizing the electrode by immersion in a 0.2 mM solution of the
relevant dye. Counter-electrodes were prepared by drilling a hole
in the glass sheet for the subsequent electrolyte filling and
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Figure 6. IPCE curves for compounds HT157 (red squares), TA1314 (blue circles) and reference compound DF15 (black triangles), recorded in the absence (a)
and presence (b) of CDCA as a disaggregating agent.
Figure 7. (a) Equivalent circuit of DSSC used for EIS curve fitting. (b) Nyquist plot for EIS measurements of cells built in the absence of CDCA, together with fitting
curves. (c) Nyquist plot for EIS measurements of cells built in the presence of CDCA, together with fitting curves. In both (b) and (c) plots: HT157, red squares;
TA1314, blue circles; DF15, black triangles..
the semiconductor (see below the description of EIS
measurements for a discussion). Addition of CDCA to the
sensitizing bath (Figure 5b) had a very small effect on the
efficiency of the HT157-containing cell, while a somewhat more
pronounced influence could be observed for TA1314, resulting in
the best η value of 2.39%: such performance improvement could
be due to a better suppression of aggregation, which, in the case
of TA1314, seemed more relevant than for HT157, as indicated
by the large blue-shift of its absorption spectrum on TiO2 relative
to that in solution (see above). Finally, it should be observed that,
under the conditions used in this study, dye DF15 gave power
conversion efficiencies of 3.17-3.40%, superior to those obtained
with the new fluorazone sensitizers, with the best efficiency
measured for TA1314 reaching approx. 70% of that recorded with
DF15.
Although the efficiencies obtained in this study were not very high
when compared to the best results reported in the literature, it
must be observed that some of the conditions employed here (i.e.:
absence of a TiO2 scattering layer, use of a relatively thin TiO2
–
film and a commercial I–/I3 electrolyte) were far from those
required to achieve record efficiencies, and were mainly selected
to ensure a simple and reproducible cell fabrication procedure.
Thus, the results obtained with sensitizers HT157 and TA1314
should be considered as a basis for potential future improvements,
especially in view of a possible refinement of the dyes structure
resulting from the data collected in this study.
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The IPCE curves recorded for the new dyes follow the same
trends observed for the photocurrent produced by the cells, with
TA1314 giving rise to slightly higher values compared to HT157
(Figure 6). It can be observed that all curves had a similar onset
around 650 nm, but those relative to TA1314 were broader and
higher in the 500-600 nm range, consistent with the UV-Vis
absorption spectra on TiO2. Furthermore, the curves obtained in
the presence of CDCA were very close to those recorded in the
absence of the additive, in agreement with the similar current
densities observed for the dyes in both cases.
and TA1314. The compounds incorporated the tricyclic moiety in
their middle conjugate section, where it was connected to one or
two thiophene rings. The structure of the dyes was then
completed by typical donor (4-alkoxy-substituted triphenylamine)
and acceptor (cyanoacrylic acid) units.
Dyes HT157 and TA1314 both displayed intense absorption of
visible light in dichloromethane solution, with the latter showing a
significantly red-shifted maximum absorption peak. Moreover,
their spectra on TiO2 were much broader than those in solution,
which was promising in view of the utilization of the dyes as DSSC
sensitizers. In agreement with DFT computational analysis, cyclic
voltammetry measurements, combined with the optical band-gap
obtained from the UV-Vis spectroscopy experiments, suggested
that the compounds had energy levels correctly aligned to be
used in photovoltaic cells.
DSSCs built with the new dyes displayed moderate efficiencies in
the 2.09-2.39% range, with TA1314 giving the best result. The
highest efficiency reached approx. 70% of that recorded with
reference dye DF15. The successful development of an efficient
synthetic route, as well as the spectroscopic and electrochemical
data reported in the present study, will hopefully demonstrate
useful for the design and synthesis of more optimized organic
functional materials containing the fluorazone moiety.
Finally, DSSCs produced with the new dyes were subjected also
to Electrochemical Impedance Spectroscopy (EIS) analysis30
under illumination at Voc, using Z-View software to fit the
experimental curves with the equivalent circuit represented in
Figure 7a. In such circuit, R1 represents the sum of all resistances
due to the electrode layer materials and the external set-up of the
circuit, while R2/CPE2 (constant-phase element) describe the
properties of the counter-electrode and R3/CPE3 the properties
of the TiO2/dye/electrolyte interface. The corresponding Nyquist
plots are also reported in Figure 7, both in the absence and in the
presence of CDCA. Fitting the data, it is possible to obtain the
value for the charge-transfer resistance at the dye-TiO2 interface
(R3 in the equivalent circuit), which is provided by the diameter of
the mid-frequency semicircle in the plot. Under illumination, R3
should correlate with the photocurrent values (the lower the
resistance, the higher the Jsc), provided that all other factors
influencing the current are similar among different dyes. In the
present case, R3 was consistently found to be lower for TA1314
than for HT157 (16.5 Ω vs. 22.4 Ω in the absence of CDCA; 16.1
Ω vs. 23.4 Ω in the presence of CDCA, Table 3): considering that
dye regeneration should be almost equal for the two sensitizers
since their Eox values were very close, we can thus suggest that
the higher photocurrent produced by TA1314 is due to an
enhanced electron injection rate caused by the smaller resistance
at the TiO2/dye interface, accompanied by a slightly superior light
harvesting efficiency (as shown by its broader spectrum on TiO2,
see Figure 3c). On the other hand, the R3 values found for DF15
were intermediate between those for TA1314 and HT157 (18.1-
18.9 Ω): the higher Jsc value of this dye should therefore mainly
be attributed to a different factor, such as, for example, its
stronger light absorption ability compared to the two new dyes (as
highlighted by its red-shifted absorption both in solution and on
TiO2),26 or a more efficient dye regeneration by the electrolyte.
Experimental Section
General Synthetic Remarks
All commercial starting materials were purchased from Sigma-Aldrich Kft.
Hungary and Merck Kft. Hungary and were used without further purification.
The organometallic reactions were carried out in Schlenk-flasks under a
dry nitrogen atmosphere. Solvents were typically freshly distilled or dried
over molecular sieves. Tetrahydrofuran was obtained anhydrous by
distillation from sodium wire after the characteristic blue colour of the in
situ generated sodium diphenylketyl radical was found to persist. All
reactions were monitored by thin-layer chromatography. TLC was carried
out on Kieselgel 60 F254 (Merck) aluminium sheets (visualization of the
product was made by exposing the plate to UV radiation or by staining it
with the aqueous solution of (NH4)6Mo7O24, Ce(SO4)2 and sulfuric acid).
Flash column chromatography was performed using a CombiFlash
(Teledyne ISCO). Routine 1H- and 13C-NMR spectra were obtained on a
Bruker AV 300 or DRX 500 spectrometer. The chemical shifts () are
reported in parts per million (ppm) and the coupling constants (J) in Hz.
Deuterated chloroform (CDCl3) with tetramethylsilane (TMS), 1,4-dioxane-
d8 and dimethylsulfoxide-d6 (DMSO-d6) were used as solvents, and signal
positions were measured relative to the signals of TMS or solvents (TMS
0 ppm, chloroform = 7.26 ppm, dioxane = 3.53 ppm, DMSO = 2.50 ppm for 1H-
NMR and TMS = 0 ppm, chloroform = 77.0 ppm, Dioxane = 66.7 ppm, DMSO
39.4 ppm for 13C-NMR). Melting points were determined in capillary tubes,
using a Gallenkamp melting point apparatus. High-resolution mass spectra
(HRMS) were recorded on Waters LCT Premier XE spectrometer in
electrospray ionization (ESI, 2.5 kV) mode, using water (0.035%
trifluoroacetic acid)/acetonitrile (0.035% trifluoroacetic acid) as eluent in
gradient elution (5%–95% acetonitrile); samples were prepared in
acetonitrile.
=
=
Conclusions
In this paper, we reported a new methodology for the synthesis of
fully conjugated molecules containing the fluorazone (9H-
pyrrolo[1,2-a]indol-9-one) moiety. The synthetic sequence
featured a Elming-Clauson-Kaas type pyrrole formation, a POCl3-
mediated ring closure and a selective halogenation, which
allowed to access two previously unreported 2,7-dihalofluorazone
derivatives. Extension of the conjugated system was then carried
out by means of Suzuki-Miyaura cross-coupling reactions. The
new procedure was applied to the synthesis of the first fluorazone-
containing D-π-A sensitizers for dye-sensitized solar cells, HT157
Methyl
5-bromo-2-(1H-pyrrole-1-yl)benzoate
(2a).31
2,5-
Dimethoxytetrahydrofuran (2.98 g, 22.6 mmol, 2.92 mL) was added into a
solution of methyl 2-amino-5-bromobenzoate (1a, 4.33 g, 18.8 mmol) in
glacial acetic acid (22 mL). The solution was stirred at boiling temperature
for 2 hours (TLC: hexane/ethyl acetate 3:1), then evaporated in vacuo. The
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201601622
European Journal of Organic Chemistry
FULL PAPER
residue was dissolved in dichloromethane (40 mL) and washed with a 5%
aqueous solution of sodium carbonate (3 × 20 mL). The organic phase was
dried over sodium sulphate and concentrated in vacuo. The crude product
(6.7 g) was purified by flash column chromatography (silica gel,
hexane/ethyl acetate 3:1) to afford 2a as a pale yellow oil. Yield: 81% (4.29
g); 1H-NMR (500 MHz, CDCl3): H = 7.92 ( d, J = 2.4 Hz, 1H), 7.67 (dd, J
= 8.5, 2.4 Hz, 1H), 7.25 (d, J = 8.5 Hz, 1H), 6.77 (t, J = 2.2 Hz, 2H), 6.31
(t, J = 2.2 Hz, 2H), 3.72 (s, 3H). Spectroscopic data are in agreement with
those reported in the literature.31
127.7, 119.9, 118.5, 116.5, 114.7, 111.7. HRMS (ESI): m/z calcd for
C11H7BrNO [M+H]+ 247.9706, found 247.9704.
7-Iodo-9H-pyrrolo[1,2-a]indol-9-one (4b).12 It was prepared from 3b
(2.65 g, 7.24 mmol), using the method described for compound 4a. The
crude product was purified by silica gel column chromatography
(hexane/ethyl acetate 4:1) to give 4b as a light brown solid. Yield: 78%
(1.67 g). Further purification can be accomplished by recrystallization from
toluene (15 mL) to afford a yellow solid; mp: 182–183 °C; 1H-NMR (500
MHz, DMSO-d6): H = 7.90 (dd, J = 8.1, 1.8 Hz, 1H), 7.73 (d, J = 1.8 Hz,
1H), 7.66 (dd, J = 2.7, 0.6 Hz, 1H), 7.37 (d, J = 8.1 Hz, 1H), 6.85 (dd, J =
3.7, 0.6 Hz, 1H), 6.38 (dd, J = 3.7, 2.7 Hz, 1H). 13C-NMR (126 MHz,
DMSO-d6): C = 177.0, 142.74, 142.71, 131.9, 131.3, 130.4, 122.4, 116.5,
114.5, 113.9, 89.4. HRMS (ESI): m/z calcd for C11H7INO [M+H]+ 295.9527,
found 295.9545.
Methyl 5-iodo-2-(1H-pyrrole-1-yl)benzoate (2b). It was prepared using
the above described method starting from 1b (5.21 g, 18.8 mmol). The
product 2b is a pale yellow oil. Yield 80% (4.92 g); 1H-NMR (500 MHz,
CDCl3) H = 8.10 (d, J = 1.8 Hz, 1H), 7.85 (dd, J = 8.4, 1.8 Hz, 1H), 7.11
(d, J = 8.4 Hz, 1H), 6.76 (t, J = 1.9 Hz, 2H), 6.31 (t, J = 1.9 Hz, 2H), 3.71
(s, 3H). 13C-NMR (126 MHz, CDCl3): C = 165.9, 141.1, 139.9, 139.1, 129.4,
128.2, 121.8, 110.1, 91.1, 52.6. HRMS (ESI): m/z calcd for C12H11INO2
[M+H]+ 327.9835, found 327.9838.
7-Bromo-2-iodo-9H-pyrrolo[1,2-a]indol-9-one (5a). N-iodosuccinimide
(0.48 g, 2.12 mmol) was added into a solution of 4a (0.50 g, 2.02 mmol) in
anhydrous N,N-dimethylformamide (15 mL) and the mixture was stirred for
48 hours at 70 °C (TLC: hexane/ethyl acetate 3:1). The mixture was
poured into distilled water (80 mL), the resulting precipitate was filtered off
and washed with ice-cold distilled water. The residue was dissolved in
ethyl acetate (20 mL) and washed with distilled water (2 × 15 mL). The
organic phase was dried over sodium sulphate and concentrated in vacuo
to give 0.68 g of crude product, which was recrystallized from isopropanol
to give 5a as orange coloured crystals. Yield 73% (0.55 g); mp: 176°C
(decomp); 1H-NMR (500 MHz, CDCl3): H = 7.70 (d, J = 1.5 Hz, 1H), 7.57
(dd, J = 8.5, 1.5 Hz, 1H), 7.15 (s, 1H), 6.99 (d, J = 8.5 Hz, 1H), 6.89 (s,
1H). 13C-NMR (75 MHz, CDCl3): C = 176.8, 142.0, 136.7, 132.5, 130.5,
128.1, 123.7, 122.4, 120.6, 119.1, 111.9. HRMS (ESI): m/z calcd for
C11H4BrINO [M–H]– 371.8526, found 371.8549.
(5-Bromo-2-(1H-pyrrole-1-yl)phenyl)(pyrrolidine-1-yl)methanone (3a).
A solution of methylester 2a (3.0 g, 10.7 mmol) in pyrrolidine (3.0 g, 42.0
mmol, 3.5 mL) was stirred for 7 hours at 90 °C (TLC: hexane/ethyl acetate
4:1). After cooling down to room temperature, water (60 mL) and ethyl
acetate (60 mL) were added. The phases were separated and the organic
solution was washed with 0.1 M aqueous hydrochloric acid solution (45
mL), water (45 mL) and
a 1% aqueous solution of sodium
hydrogencarbonate (30 mL). The organic phase was dried over sodium
sulphate and concentrated in vacuo to give 3a as a light yellow solid, which
was used in the next step without further purification. Yield: 47% (1.62 g);
m.p. 92–93 °C; 1H-NMR (300 MHz, CDCl3): H = 7.65–7.50 (m, 2H), 7.24
(d, J = 8.4 Hz, 1H), 6.93 (d, J = 2.1 Hz, 2H), 6.29 (d, J = 2.1 Hz, 2H), 3.46
(t, J = 6.8 Hz, 2H), 2.72 (br. s, 2H), 1.75 (quint, J = 6.6 Hz, 2H), 1.61 (quint,
J = 6.6 Hz, 2H). 13C-NMR (75 MHz, CDCl3): C = 165.8, 136.3, 133.8, 133.3,
131.5, 125.7, 121.0 (2C), 120.1, 110.7 (2C), 47.1, 45.5, 25.6, 24.2. HRMS
(ESI): m/z calcd for C15H16BrN2O [M+H]+ 319.0446, found 319.0453.
2-Bromo-7-iodo-9H-pyrrolo[1,2-a]indol-9-one
(5b).
N-
bromosuccinimide (0.19 g, 1.08 mmol) was added to a solution of 4b (0.30
g, 1.03 mmol) in anhydrous N,N-dimethylformamide (5 mL) at 0 °C and the
mixture was stirred for 20 minutes at the same temperature (TLC:
hexane/ethyl acetate 4:1). The mixture was poured into distilled water (60
mL), the resulting precipitate was filtered off and washed with ice-cold
distilled water. The residue was dissolved in ethyl acetate (20 mL) and
washed with distilled water (2 × 15 mL). The organic phase was dried over
sodium sulphate and concentrated in vacuo to give the crude product,
which was recrystallized from isopropanol to give 5b as yellow coloured
crystals. Yield: 84% (0.32 g); mp: 162–163 °C; 1H-NMR (300 MHz, CDCl3):
(5-Iodo-2-(1H-pyrrole-1-yl)phenyl)(pyrrolidin-1-yl)methanone (3b). It
was prepared starting from 2b (8.37 g, 26.5 mmol), using the same method
described for compound 3a. Product 3b was obtained as a light yellow
solid, which was used in the next step without further purification. Yield:
56% (5.43 g); mp: 132–133 °C; 1H-NMR (300 MHz, CDCl3): H = 7.89–
7.62 (m, 2H), 7.11 (d, J = 8.7 Hz, 1H), 6.93 (t, J = 2.1 Hz, 2H), 6.29 (t, J =
2.1 Hz, 2H), 3.46 (t, J = 6.7 Hz, 2H), 2.71 (br. s, 2H), 1.84–1.67 (m, 2H),
1.67–1.48 (m, 2H). 13C-NMR (75 MHz, CDCl3): C = 165.7, 139.2, 137.3,
136.9, 133.9, 125.8, 120.9 (2C), 110.7 (2C), 90.9, 47.1, 45.5, 25.5, 24.2.
HRMS (ESI): m/z calcd for C15H16IN2O [M+H+] 367.0307, found 367.0305.

H = 7.86 (d, J = 1.3 Hz, 1H), 7.77 (dd, J = 8.1, 1.3 Hz, 1H), 7.09 (s, 1H),
6.88 (d, J = 8.1 Hz, 1H), 6.78 (s, 1H). 13C-NMR (75 MHz, CDCl3): C
=
177.3, 142.9, 142.8, 133.7, 130.6, 130.4, 119.1, 115.7, 112.3, 103.7, 89.1.
HRMS (ESI): m/z calcd for C11H4BrINO [M–H]– 371.8526, found 371.8523.
7-Bromo-9H-pyrrolo[1,2-a]indol-9-one (4a).17 A solution of 3a (2.31 g,
7.24 mmol) was stirred in phosphorous oxychloride (27.7 g, 181 mmol,
16.9 mL) for 8 h at 70 °C (TLC: hexane/ethyl acetate 3:1). The mixture was
poured into ice before being extracted with toluene (100 mL). The
undissolved tarry material was filtered off. The aqueous phase (500 mL)
was treated at 0 °C with a concentrated solution of sodium hydroxide (50
g) in water (pH = 10), the sodium chloride precipitate was filtered of and
washed with toluene. The aqueous phase was extracted with toluene (2 ×
100 mL). The collected organic solutions were washed with distilled water
(2 × 50 mL), dried over sodium sulphate and concentrated in vacuo to give
4a as a light brown solid. Yield: 81% (1.46 g). Further purification can be
accomplished by recrystallization from toluene (15 mL) to afford a yellow
solid; mp: 145–146 °C; 1H-NMR (300 MHz, CDCl3): H = 7.66 (d, J = 1.7
Hz, 1H), 7.53 (dd, J = 8.2, 1.9 Hz, 1H), 7.06 (dd, J = 2.6, 0.6 Hz, 1H), 7.00
(d, J = 8.2 Hz, 1H), 6.80 (dd, J = 3.7, 0.6 Hz, 1H), 6.33 (dd, J = 3.7, 2.6 Hz,
1H). 13C-NMR (75 MHz, CDCl3): C = 177.9, 142.4, 136.4, 132.0, 131.9,
2-Bromo-7-(5-(dioxolan-2-yl)thiophene-2-yl)-9H-pyrrolo[1,2-a]indol-
9-one (9). Under a nitrogen atmosphere, Pd(PPh3)4 (23 mg, 0.02 mmol)
was added into a solution of fluorazone 5b (0.37 g, 0,98 mmol) in
degassed dry tetrahydrofuran (5 mL), followed by the addition of distilled
and degassed water (2.5 mL) and dry sodium carbonate (0.21 g, 1.96
mmol). The suspension was stirred for 30 min at 40 °C before being treated
with a solution of boronic acid 6 (0.29 g, 1.47 mmol) in degassed dry
tetrahydrofuran (3 mL). After being stirred for 12 h at 70 °C (TLC:
hexane/ethyl acetate 4:1) the mixture was quenched with distilled water
(15 mL) and extracted with ethyl acetate (3 × 15 mL). The organic phase
was dried over sodium sulphate and concentrated in vacuo. The crude
product was purified by flash column chromatography (silica gel, hexane
→ hexane/ethyl acetate 5:1) to give product 9 as an orange solid. Yield:
43%, (0.17 g); mp: 169-170 °C; 1H-NMR (500 MHz, 1,4-dioxane-d8): H
=
7.88 (d, J = 1.8 Hz, 1H), 7.69 (dd, J = 8.1, 1.8 Hz, 1H), 7.40 (d, J = 0.7 Hz,
1H), 7.33 (d, J = 3.7 Hz, 1H), 7.21 (d, J = 8.1 Hz, 1H), 7.11 (d, J = 3.7 Hz,
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201601622
European Journal of Organic Chemistry
FULL PAPER
1H), 6.89 (d, J = 0.7 Hz, 1H), 6.03 (s, 1H), 4.09–4.01 (m, 2H), 3.98–3.90
(m, 2H). 13C-NMR (75 MHz, 1,4-dioxane-d8): C = 178.6, 144.0, 143.5,
143.3, 133.3, 132.4, 131.7, 130.6, 127.9, 124.1, 122.3, 120.1, 115.2, 111.7,
103.2, 100.8, 65.8 (2C). HRMS (ESI): m/z calcd for C18H12BrNO3S [M+H]+
401.9799, found 401.9807.
(15 mL) and extracted with ethyl acetate (2 × 15 mL). The organic phase
was dried over sodium sulphate and concentrated in vacuo. The crude
product was purified by flash column chromatography (silica gel, hexane
→ hexane/ethyl acetate 5:1) to give bromide 11 as a red oil. Yield 56%
(159 mg); 1H-NMR (300 MHz, CDCl3): H = 7.85 (d, J = 0.9 Hz, 1H), 7.75–
7.62 (m, 2H), 7.55 (d, J = 8.3 Hz, 1H), 7.39 (d, J = 8.8 Hz, 2H), 7.16 (s,
1H), 7.08 (d, J = 8.8 Hz, 4H), 6.98 (d, J = 8.6 Hz, 2H), 6.84 (d, J = 8.8 Hz,
4H), 3.94 (t, J = 6.5 Hz, 4H), 1.78 (dt, J = 14.3, 6.5 Hz, 4H), 1.46 (br. s,
4H), 1.40 – 1.28 (m, 8H), 0.91 (t, J = 6.1 Hz, 6H). 13C-NMR (75 MHz,
CDCl3): C = 181.9, 155.7 (2C), 140.5 (2C), 137.2, 136.2, 133.2, 132.6,
130.9, 128.1, 127.0 (2C), 126.8 (4C), 125.1, 122.9, 120.4 (2C), 115.5,
115.3 (4C), 111.5, 111.3, 108.0, 68.3 (2C), 31.6 (2C), 29.7 (2C), 25.8 (2C),
22.6 (2C), 14.1 (2C). HRMS (ESI): m/z calcd for C41H44BrN2O3 [M+H]+
691.2535, found 691.2543.
5-(2-(4-(Bis(4-(hexyloxy)phenyl)amino)phenyl)-9-oxo-9H-pyrrolo[1,2-
a]indol-7-yl)thiophene-2-carbaldehyde (10). Under
a
nitrogen
atmosphere, to a solution of bromide 9 (0.17 g, 0.42 mmol) in degassed
dry N,N-dimethylformamide (5 mL) were added boronic ester 8 (0.48 g,
0.85 mmol), Pd(OAc)2 (5 mg, 0.02 mmol), dppf ligand (23 mg, 0.04 mmol),
copper(I) chloride (21 mg, 0.21 mmol) and dry cesium carbonate (0.55 g,
1.68 mmol). After being stirred for 16 h at 100 °C (TLC: hexane/ethyl
acetate 4:1) the mixture was quenched with distilled water (10 mL) and
extracted with ethyl acetate (3 × 10 mL). The organic phase was
concentrated (to approximately 5 mL) and water 1 (mL) and acetic acid
(100 μL) were added to deprotect the aldehyde. After stirring for 4 h the
phases were separated, the organic layer was washed with solution of 1%
NaHCO3/H2O (5 mL), dried over sodium sulphate and concentrated in
vacuo. The crude product was purified by flash column chromatography
(silica gel, hexane → hexane/ethyl acetate 5:1) to give aldehyde 10 as a
red amorphous solid. Yield: 40% (0.12 g); 1H-NMR (300 MHz, CDCl3): H
= 9.87 (s, 1H), 7.90–7.61 (m, 4H), 7.44–7.21 (m, 3H), 7.14 (d, J = 8.0 Hz,
1H), 7.10–6.97 (m, 5H), 6.93 (d, J = 8.0 Hz, 2H), 6.82 (d, J = 8.1 Hz, 4H),
3.93 (t, 6.0 Hz, 4H), 1.78 (dt, J = 12.9 Hz, 6.2 Hz, 4H), 1.59–1.40 (m, 4H),
1.40–1.11 (m, 8H), 0.91 (t, J = 5.5 Hz, 6H). 13C-NMR (75 MHz, CDCl3): C
= 182.6, 178.6, 155.6 (2C), 152.3, 148.2, 143.9, 142.6, 140.5 (2C), 137.4,
133.3, 132.6, 132.1, 131.0, 130.6, 126.6 (4C), 125.8 (2C), 124.9, 124.2,
122.0, 120.6 (2C), 115.4, 115.3 (4C), 111.6, 110.7, 68.3 (2C), 31.6 (2C),
29.3 (2C), 25.8 (2C), 22.6 (2C), 14.1 (2C). HRMS (ESI): m/z calcd for
C46H47N2O4S [M+H]+ 723.3257, found 723.3253.
5’-(2-(4-(Bis(4-(hexyloxy)phenyl)amino)phenyl)-9-oxo-9H-
pyrrolo[1,2-a]indol-7-yl)-2,2’-bithiophene-5 carbaldehyde (12). To a
solution of bromide 11 (150 mg, 0.25 mmol) in degassed toluene (5 mL)
was added boronic ester 9 (96 mg, 0.30 mmol) and the catalyst (Pd(PPh3)4,
17 mg, 0.015 mmol). The red coloured solution was stirred at room
temperature for 20 min, then a solution of sodium carbonate (0.185 g, 1.75
mmol) in degassed distilled water was added. The mixture was stirred at
80 °C for 48 h (TLC: hexane/ethyl acetate 3:1), quenched with distilled
water (15 mL) and extracted with ethyl acetate (2 × 15 mL). The organic
phase was dried over sodium sulphate and concentrated in vacuo. The
crude product was purified by flash column chromatography (silica gel,
hexane → hexane/ethyl acetate 5:1) to give aldehyde 12 as a crude
product. According to the 1H-NMR spectra the product contained just a
small amount of starting material 11 and thus it was used without further
purification in the next step. Yield 36% (0.071 g); 1H-NMR (300 MHz,
CDCl3): H = 9.87 (s, 1H), 7.82 (dd, J = 11.1, 1.7 Hz, 1H), 7.69 (d, J = 4.0
Hz, 2H), 7.67 (dd, J = 8.4, 1.8 Hz, 1H), 7.34 (d, J = 3.8 Hz, 1H), 7.31 (d, J
= 8.7 Hz, 2H), 7.28 (d, J = 7.4 Hz, 2H), 7.14 (d, J = 8.0 Hz, 1H), 7.06 (d, J
= 8.9 Hz, 4H), 7.02 (d, J = 5.2 Hz, 1H), 6.95 (d, J = 8.7 Hz, 2H), 6.83 (d, J
= 8.9 Hz, 4H) 3.94 (t, J = 6.5 Hz, 4H), 1.78 (quint, J = 6.7 Hz, 4H), 1.47
(quint, J = 7.2 Hz, 4H), 1.40–1.28 (m, 8H), 0.91 (t, J = 6.9 Hz, 6H). 13C-
NMR (126 MHz, CDCl3) C = 182.4, 178.9, 162.6, 155.5, 155.4, 148.0,
146.6, 144.3, 142.9, 141.7, 140.6, 140.5, 137.3, 132.9, 131.0, 126.6, 126.5,
125.7, 124.3, 124.2, 120.6, 120.5, 115.3, 115.3, 111.3, 111.1, 110.6, 110.5,
68.3, 31.6, 29.3, 25.8, 22.6, 14.1. HRMS (ESI): m/z calcd for C50H49N2O4S2
[M+H]+ 805.3133, found 805.3126.
3-(2-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)-9-oxo-9H-pyrrolo[1,2-
a]-indol-7-yl)thiophen-5-yl)-2-cyanoacrylate (HT157). Under a nitrogen
atmosphere, to a suspension of the protected aldehyde precursor 10 (0.13
g, 0.17 mmol) in acetic acid (1.50 mL) were added cyanoacetic acid (19
mg, 0.22 mmol), ammonium acetate (3 mg, 0.04 mmol) and chloroform (3
mL). The solution was refluxed for 10 h at 100 °C (TLC: hexane/ethyl
acetate 2:1). After cooling, the reaction mixture was extracted with
chloroform. The organic layer was dried over anhydrous Na2SO4. The
filtrate was concentrated under reduced pressure. Purification was
performed by recrystallization in ethanol to yield HT157 as a dark red
crystalline solid. Yield: 96% (0.125 g); mp: 202-203 °C; 1H-NMR (300 MHz,
DMSO-d6): H = 9.87 (1H, s), 8.42 (s, 1H), 8.04 (s, 1H), 7.94 (d, J = 4.5 Hz,
2H) 7.85–7.70 (m, 2H), 7.53 (d, J = 8.0 Hz, 1H), 7.41 (d, J = 8.1 Hz, 2H),
7.20 (s, 1H), 6.96 (d, J = 8.3 Hz, 4H), 6.86 (d, J = 8.3 Hz, 4H), 6.75 (d, J =
8.1 Hz, 2H), 3.90 (t, J = 6.0 Hz, 4H), 1.68 (dt, J = 13.2, 6.2 Hz, 4H), 1.45–
1.35 (m, 4H), 1.35–1.22(m, 8H), 0.87 (t, J = 5.4 Hz, 6H). 13C-NMR (75 MHz,
DMSO-d6): C = 177.8, 163.5, 157.0, 155.1 (2C), 147.2, 143.4, 140.8,
139.9 (2C), 132.5, 132.0, 131.6, 130.3, 129.8, 127.3, 126.5 (4C), 125.7,
125.7 (2C), 125.3, 125.0, 120.9, 119.7 (2C), 117.9, 116.7, 115.5, 115.4
(4C), 112.1, 111.3, 67.6 (2C), 31.0 (2C), 28.7 (2C), 25.2 (2C), 22.1 (2C),
13.9 (2C). HRMS (ESI): m/z calcd for C49H46N3O5S [M–H]– 788.3164,
found 788.3139.
3-(5’-(2-(4-(Bis(4-(hexyloxy)phenyl)amino)phenyl)-9-oxo-9H-
pyrrolo[1,2-a]-indol-7-yl)-2,2’-bithiophen-5-yl)-2-cyanoacrylate
(13,
piperidine salt of TA1314). To a suspension of aldehyde 12 (70 mg,
0.087 mmol) in dry acetonitrile (2.5 mL) were added cyanoacetic acid (40
mg, 0.47 mmol), piperidine (39 mg, 0.46 mmol, 45 μL) and chloroform (2
mL). The resulting solution was refluxed for 3 h and stirred overnight at
50 °C (TLC: hexane/ethyl acetate 2:1). The solvent was evaporated in
vacuo. The precipitated crystals were filtered off and washed with ice-cold
acetonitrile. Product 13 was isolated as a dark red solid. Yield: 74% (0.061
g); mp: 160-161 oC; 1H-NMR (500 MHz, CDCl3): H = 9.68 (br. s, 1H), 8.15
(s, 1H), 7.76 (dd, J = 9.0, 0.9 Hz, 1H), 7.62 (dd, J = 7.9, 1.3 Hz, 1H), 7.53
(d, J = 4.0 Hz, 1H), 7.28 (d, J = 8.8 Hz, 2H), 7.25 (s, 1H), 7.23 (d, J = 3.9
Hz, 1H), 7.17 (d, J = 3.9 Hz, 1H), 7.10 (d, J = 8.2 Hz, 1H), 7.05 (d, J = 8.9
Hz, 4H), 6.98 (s, 1H), 6.93 (d, J = 8.2 Hz, 2H), 6.82 (d, J = 8.9 Hz, 4H),
3.93 (t, J = 6.5 Hz, 4H), 3.30 (s, 1H), 3.21–3.14 (m, 4H), 2.01 (s, 1H), 1.93–
1.83 (m, 4H), 1.78 (dt, J = 14.5, 6.6Hz, 4H), 1.68 (br. s, 2H), 1.47 (quint, J
= 7.1 Hz, 4H), 1.39–1.32 (m, 8H), 0.91 (t, J = 6.8 Hz, 6H). 13C-NMR (75
MHz, CDCl3): C = 179.0, 168.1, 158.1, 155.5, 147.3 (2C), 143.5, 141.0,
140.6 (2C), 135.8, 132.9, 132.4, 130.9, 130.1, 129.6, 127.4, 126.6 (4C),
126.5, 125.8, 125.8 (2C), 125.4, 125.2, 124.1, 121.3, 120.7 (2C), 119.7,
118.5, 116.9, 116.3, 115.3 (4C), 112.9, 111.4, 68.3 (2C), 44.4 (2C), 31.6
(2C), 29.4 (2C), 27.4 (2C), 25.8 (2C), 22.7, 22.6 (2C), 14.1 (2C). HRMS
2-(4-(Bis(4-(hexyloxy)phenyl)amino)phenyl)-7-bromo-9H-pyrrolo[1,2-
a]indol-9-one (11). Under a nitrogen atmosphere, to a solution of
fluorazone 5a (175 mg, 0.47 mmol) in degassed dry tetrahydrofuran (3 mL)
were added Pd(PPh3)4 (35 mg, 0.03 mmol), distilled and degassed water
(1.5 mL) and dry potassium carbonate (195 mg, 1.41 mmol). The
suspension was stirred for 30 min at 40 °C before being treated with a
solution of boronic ester 8 (0.32 g, 0.56 mmol) in degassed dry
tetrahydrofuran (3 mL). After being stirred for 24 h at 80 °C (TLC:
hexane/ethyl acetate 3:1) the mixture was quenched with distilled water
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10.1002/ejoc.201601622
European Journal of Organic Chemistry
FULL PAPER
(ESI): m/z calcd for C53H50N3O5S2 [M+H]+ 872.3186 (carboxylic acid),
found 872.3173.
apparatus (Bentham, UK). Electrochemical Impedance Spectroscopy
(EIS) measurements were carried out under illumination at open circuit
potential with an Autolab PGSTAT302N+FRA32 electrochemical
workstation (Metrohm); spectra were recorded over a frequency range of
10−1 Hz to 105 Hz with an amplitude of 10 mV. Data fitting was carried out
using the Z-Lab software.
Spectroscopic and Electrochemical measurements
UV-Vis spectra were recorded with a Shimadzu UV-2600 spectrometer.
UV-Vis spectra of the compounds on TiO2 were recorded in transmission
mode after sensitization of thin, transparent semiconductor films prepared
with a procedure similar to that employed for the fabrication of the
photovoltaic devices (see below). Cyclic voltammetry experiments were
Acknowledgements
conducted in dichloromethane solution with
a PARSTAT 2273
electrochemical workstation, employing a three-electrode cell having a
glassy carbon working electrode, a platinum counter electrode, and the
aqueous Ag/AgCl (sat. KCl) reference electrode. The supporting
electrolyte used was electrochemical grade [N(Bu)4]PF6. Under these
experimental conditions, the one-electron oxidation of ferrocene occurs at
E0’ = 0.62 V.
We gratefully acknowledge the Hungarian Scientific Research
Fund (OTKA K 104528 and OTKA PD 121217) for their financial
support of the synthetic research work. This project is also
connected to the scientific program of the New Széchenyi Plan
(Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002). Moreover,
we also thank Regione Toscana (“SELFIE” Project, FAR-FAS
2014 grant) and Ente Cassa di Risparmio di Firenze
(“ENERGYLAB” project) for financial support, as well as CINECA
and CREA (Colle Val d’Elsa, Italy) for the availability of high
performance computing resources and financial support. Finally,
we thank Dr. J. Filippi (CNR-ICCOM) for his collaboration in
performing CV experiments.
Solar Cells Fabrication
Transparent conducting glass, whose conductive layer was constituted by
a nanometric layer of FTO (Fluorine-doped Tin Oxide, SnO2:F), was cut in
square sheets of 2.5 cm × 2.5 cm dimensions. The substrates were
cleaned with both basic and acidic treatments to eliminate organic
impurities. A “blocking layer” of TiO2 was then deposed by spin-coating of
a TiCl4 alcoholic solution (0.05 M in EtOH) and subsequent thermal
treatment at 450°C for 30’.32 At this stage, a nanocrystalline TiO2 layer was
screen-printed on the substrates employing a commercial paste (Solaronix
T/SP) and was then sintered at 450°C in an oven. The final dimensions of
the TiO2 films were: thickness 6 μm measured by profilometer (Optical 3D
Microscope, Bruker), area 0.25 cm2. The screen-printed films were then
treated once again by dip-coating process with a TiCl4 alcoholic solution
(0.04 M in EtOH) and subjected to thermal treatment to produce a
protective layer on the nanocrystalline TiO2 electrode.
Keywords: Organic dyes • Fluorazone derivatives • Elming-
Clauson-Kaas pyrrole formation • Cross-coupling • Dye-
sensitized solar cells.
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Entry for the Table of Contents
FULL PAPER
A
synthetic sequence for the
preparation of two fully conjugated,
2,7-disubstituted fluorazone (9H-
Heterocyclic synthesis, Dye-
sensitized solar cells
Béla Mátravölgyi, Tamás Hergert,
Angelika Thurner, Bálint Varga, Nicola
Sangiorgi, Riccardo Bendoni, Lorenzo
Zani, Gianna Reginato, Massimo
Calamante, Adalgisa Sinicropi,
Alessandra Sanson, Ferenc Faigl,*
Alessandro Mordini,*
pyrrolo[1,2-a]indol-9-one) derivatives
was developed. Thanks to their broad
absorption of visible light and their
favorable electrochemical properties,
the new compounds could be
successfully used as sensitizers for
dye-sensitized solar cells.
Page No. – Page No.
Synthesis and Investigation of New
Solar Cell Photosensitizers Having a
Fluorazone Backbone
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