5-(Methylidene)barbituric acid as a new anchor unit for dye-sensitized solar cells (DSSC)

Article abstract of DOI:10.3998/ark.5550190.p008.686

Novel dyes bearing a 5-(methylidene)barbituric acid moiety as a new acceptor/anchor fragment were obtained and exhibited remarkable photophysical properties, according to a preliminary assessment of their sensitization activity as elements for dye-sensitized solar cells. ARKAT-USA, Inc.

Full text of DOI:10.3998/ark.5550190.p008.686

General Papers
ARKIVOC 2014 (v) 123-131
5-(Methylidene)barbituric acid as a new anchor unit
for dye-sensitized solar cells (DSSC)
Roman A. Irgashev,a,b,* Grigory A. Kim,a Gennady L. Rusinov,a,b
and Valery N. Charushina,b
a I. Postovsky Institute of Organic Synthesis, Ural Division, Russian Academy of Sciences, S.
Kovalevskoy Str., 22, Ekaterinburg, 620041, Russia.
bUral Federal University named after the First President of Russia B.Eltsin, Mira St. 19,
Ekaterinburg, 620002, Russia
E-mail: irgashev@ios.uran.ru
This paper is dedicated to Professor Oleg N. Chupakhin on the occasion of his 80th birthday
Abstract
Novel dyes bearing a 5-(methylidene)barbituric acid moiety as a new acceptor/anchor fragment
were obtained and exhibited remarkable photophysical properties, according to a preliminary
assessment of their sensitization activity as elements for dye-sensitized solar cells.
Keywords: Barbituric acid, carbazole, pyrimidine, anchor group, push-pull structure, dye-
sensitized solar cells
Introduction
Dye-Sensitized Solar Cells (DSSCs)1-4 based on organic dyes adsorbed on nanocrystalline
semiconductor (e.g. TiO2, SnO2, ZnO) electrodes are considered to be promising electronic
devices having a number of advantages, such as a high efficiency of solar light-to-electricity
conversion, light weight, low cost and nontoxic manufacturing. The photochemical properties of
a variety of organic sensitizers have been extensively investigated.2,4 However, the design of new
dye-sensitizers with a visible light absorption coupled to long-lived excited states is still very
important for improving of DSSCs. Typically, the metal-free organic dyes consist of donor (D)
and acceptor/anchor (A) moieties connected with a π-bridge system, forming a so-called push–
pull structure. Due to this structure of dye molecules, intramolecular electron-charge transfer
takes place on absorption of light (Figure 1).
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Figure 1. The diagramatic structure of a push-pull organic dye-sensitizer.
It should be noted that the acceptor moiety of a dye-sensitizer has a significant influence for
electron-transfer processes and optical absorption of the dye. Besides that, it serves for anchoring
of dye molecules onto the semiconductor surface. In contrast to a diversity of electron-donating
groups and π-bridges for effective sensitizers, a limited number of acceptors have been reported
in the literature. One of the commonly used acceptor/anchor moieties is 2-cyanoacrylic acid.5-7 In
this connection the development of novel acceptors/anchors for dye-sensitizers is a very crucial
task to enhance performance of DSSCs.
Results and Discussion
In this paper we report the synthesis of two novel dyes bearing a 5-(methylidene)barbituric acid
fragment, as a new acceptor/anchor group, and a carbazole unit, as a donor, which has recently
been used successfully for the design of efficient dye-sensitizers.8-10 Moreover, two novel related
sensitizers with 2-cyanoacrylic acid, as a classical acceptor/anchor group, were prepared for
comparison of their properties. Thus, the carbazol-3-ylboronic acid 4 was obtained from 3-
bromocarbazole 3 according to a slightly modified literature procedure.11 The brominated
compound 3 was prepared by using successive bromination with N-bromosuccinimide (NBS)
and standard N-alkylation procedures from carbazole 1 (Scheme 1).12
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B(OH)2
N
H
N
Et
1
4
1. n-BuLi / DME
2. B(OEt)3
NBS / DMF
3. HCl / H2O
Br
Br
EtI + KOH
Me2CO
N
N
H
Et
2
3
Scheme 1. Preparation of carbazol-3-ylboronic acid 4.
The aromatic aldehydes 5,6 bearing the carbazole unit were obtained in high yields using the
Suzuki reaction between boronic acid 4 and 5-bromothiophene-2-carbaldehyde or 4-bromo-
benzaldehyde, respectively. These cross-coupling reactions were carried out at ambient
temperature in air for 24 hours, with 2 mol% trans-bis(dicyclohexylamine)palladium(II) acetate
(DAPCy)13 as the catalyst (Scheme 2).
Scheme 2. Preparation of aldehydes 5,6.
The aldehydes 5 and 6 were used for the synthesis of push-pull dyes, as building-blocks
containing the carbazole unit. Novel dyes 7,8 were obtained easily in excellent yields (compound
7, 96%; compound 8, 91%) by using the Knoevenagel condensation of aldehydes 5,6 with
barbituric acid in glacial acetic acid under catalysis with piperidine at reflux for 5 hours. In a
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similar way, dyes 9,10 were prepared in good yields under the same reaction conditions, starting
from appropriate aldehydes and 2-cyanoacetic acid (compound 9, 81%; compound 10, 74%)
(Scheme 3). It should be noted that incorporation of the fragments of 5-(methylidene)barbituric
and 2-cyanoacrylic acids into the structure of dyes, as acceptor/anchor groups proceeded very
smoothly.
O
O
HN
NH
HN
O
O
NH
O
O
N
(Het)Ar
Et
7,8
piperidine
AcOH,
O
∆
N
(Het)Ar
Et
5,6
NC
(Het)Ar
S
NCCH2CO2H
5,7,9: (Het)Ar =
6,8,10: (Het)Ar =
CO2H
N
Et
9,10
Scheme 3. Synthesis of dyes 7-10.
The UV-visible absorption spectra of dyes 7-10 in ethanol solution (2×10-5 mol/L) and
adsorbed on TiO2 nanoparticles were recorded at ambient temperature (Figure 2 and 3).
Adsorption of these dyes on a TiO2 surface was carried out from their THF solutions (2×10-5
mol/L) at ambient temperature for 24 hours. Samples of TiO2 coated with the dyes were washed
with ethanol and dried at 120 °C under vacuum.
Figure 2. UV-vis spectra of dyes 7,9 and 8,10 in EtOH solution (2×10-5 mol/L).
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The UV-spectra of dyes demonstrate maximum absorption wavelengths (λmax) at 493 nm for
7, 489 nm for 8, 411 nm for 9, and 362 nm for 10 (Figure 2). These peaks are ascribed to the
intramolecular charge transfer from the electron-donating parts in these molecules to their
acceptor fragments. The corresponding maximum molar extinction coefficients (ε) for dyes 7-10
are 2.92×104, 1.89×104, 1.25×104, and 0.84×104 Lmol-1cm-1, respectively. The red-shifts of the
absorption band for 5-(methylidene)barbituric dyes 7,8 are approximately 90–120 nm in
comparison to 2-cyanoacrylic dyes 9,10, respectively. This trend continued after the dyes were
anchoring on a TiO2 surface, and λmax for dyes 7-10 were 531, 495, 426 and 418 nm, respectively
(Figure 3). Compared to the spectrum in ethanol solution, a slight red-shift and broadening of the
absorption peak was observed for all dyes on a TiO2 surface, which can be attributed to the
formation of J-type aggregates.14 These results show that 5-(methylidene)barbituric acid unit
exhibits a stronger acceptor ability in the dye, thus increasing intramolecular charge transfer, and
leading to the red-shift of absorption maximum with enhancement of molar extinction. It should
be noted that the maximal visible light absorption is one of the most important characteristics of
a dye-sensitizer for DSSC.
Figure 3. UV-vis spectra of dyes 7,9 and 8,10 anchoring on TiO2.
Conclusions
We have obtained two novel dyes bearing a 5-(methylidene)barbituric acid fragment, as
acceptor/anchor group, which have better characteristics for absorbtion visible light in
comparison with those of the related dyes containing a 2-cyanoacrylic acid fragment (according
to the maximum absorption wavelengths and corresponding molar extinction coefficients (ε) in
their UV-spectra). In this respect, the 5-(methylidene)barbituric acid fragment is a new,
promising acceptor/anchor unit for the design and further studies of novel dye-sensitizers for
solar cell applications.
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Experimental Section
1
General. H and 13C NMR spectra were obtained on Bruker DRX-400 and AVANCE-500
spectrometers with TMS as the internal standard. Elemental analysis was carried on a Eurovector
EA 3000 automated analyzer. Melting points were determined on Boetius combined heating
stages and are not corrected. IR spectra of samples (solid powders) were recorded on a Spectrum
One Fourier transform IR spectrometer (Perkin Elmer) equipped with a diffuse reflectance
attachment (DRA). Spectrum processing and band intensity determination were carried out using
the special software supplied with the spectrometer. UV-visible spectra were recorded for a
2×10-5 M EtOH solution with Shimadzu UV-2401PC spectrophotometer (Diffuse Reflection
with Shimadzu integrating sphere for solid samples).
General procedure for the preparation of aldehydes 5,6. K3PO4 (890 mg, 4.2 mmol) was
added to a solution of 5-bromothiophene-2-carbaldehyde (270 mg, 1.4 mmol) or 4-
bromobenzaldehyde (260 mg, 1.4 mmol), (9-ethylcarbazol-3-yl)boronic acid 4 (360 mg, 1.5
mmol) and trans-bis(dicyclohexylamine)palladium(II) acetate (16 mg, 0.028 mmol, 2 mol%) in
MeOH (15 mL). The resulting suspension was stirred at ambient temperature for 24 h. MeOH
was evaporated under vacuum and the residue was suspended in CH2Cl2 (20 mL), filtered from
inorganic salts and concentrated under vacuum. The resulting residue was purified by flash
chromatography of silica gel with CHCl3/n-hexane (1:1) to remove by-products and then with
EtOAc to give the appropriate aldehyde, 5 or 6.
5-(9-Ethyl-9H-carbazol-3-yl)thiophene-2-carbaldehyde (5). Orange needles, yield 365 mg
1
(85%), mp 145-6 °C (EtOAc); H NMR (500 MHz, CDCl3) δ 9.88 (s, 1H, CHO), 8.40 (d, J 1.7
Hz, 1H), 8.14 (d, J 7.7 Hz, 1H), 7.78 (dd, J 8.5, 1.9 Hz, 1H), 7.75 (d, J 3.9 Hz, 1H), 7.54 – 7.47
(m, 1H), 7.46 – 7.41 (m, 3H), 7.30 – 7.26 (m, 1H), 4.38 (q, J 7.3 Hz, 2H, CH2), 1.46 (t, J 7.3 Hz,
3H, CH3); 1H NMR (400 MHz, DMSO-d6) δ 9.90 (s, 1H, CHO), 8.68 (d, J 1.6 Hz, 1H), 8.29 (d,
J 7.7 Hz, 1H), 8.06 (d, J 4.0 Hz, 1H), 7.91 (dd, J 8.6, 1.7 Hz, 1H), 7.79 (d, J 4.0 Hz, 1H), 7.72 (d,
J 8.6 Hz, 1H), 7.66 (d, J 8.2 Hz, 1H), 7.51 (t, J 7.7 Hz, 1H), 7.26 (t, J 7.4 Hz, 1H), 4.48 (q, J 7.1
13
Hz, 2H, CH2), 1.34 (t, J 7.1 Hz, 3H, CH3); C NMR (126 MHz, CDCl3) δ 182.6, 156.3, 141.2,
140.5, 137.8, 126.4, 124.4, 124.1, 122.9, 122.7, 120.7, 119.6, 118.6, 109.0, 108.9, 37.8, 13.8;
IR(DRA): 416, 504, 551, 583, 625, 663, 725, 742, 789, 867, 917, 1066, 1054, 1155, 1126, 1231,
1271, 1345, 1382, 1431, 1474, 1523, 1597, 1628, 1657, 1733, 1869, 2797, 2969, 3059 cm-1;
Anal. Calcd for C19H15NOS: C, 74.73; H, 4.95; N, 4.59. Found: C, 74.34; H, 4.92; N, 4.94.
4-(9-Ethyl-9H-carbazol-3-yl)benzaldehyde (6). Colourless crystals, yield 375 (89%), mp 152-3
°C (EtOAc); 1H NMR (400 MHz, CDCl3) δ 10.06 (s, 1H, CHO), 8.38 (d, J 1.4 Hz, 1H), 8.16 (d,
J 7.8 Hz, 1H), 7.97 (d, J 8.4 Hz, 2H), 7.88 (d, J 8.3 Hz, 2H), 7.76 (dd, J 8.5, 1.8 Hz, 1H), 7.59 –
7.38 (m, 3H), 7.30 – 7.25 (m, 1H), 4.40 (q, J 7.2 Hz, 2H, CH2), 1.47 (t, J 7.2 Hz, 3H, CH3); 13C
NMR (126 MHz, CDCl3) δ 192.0, 148.27, 140.5, 140.1, 134.5, 130.6, 130.4, 127.6, 126.2, 125.2,
123.7, 123.0, 120.6, 119.4, 119.3, 109.0, 108.8, 37.7, 13.9; IR(DRA): 521, 689, 737, 757, 802,
836, 905, 942, 1024, 1084, 1126, 1168, 1213, 1235, 1282, 1306, 1333, 1379, 1449, 1475, 1594,
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1688, 2743, 2838, 2978, 3054 cm-1; Anal. Calcd for C21H17NO×0.05CH3CO2C2H5: C, 83.82; H,
5.77; N, 4.61. Found: C, 83.74; H, 5.62; N, 4.76.
General procedure for the preparation of dyes 7-10. The appropriate aldehyde 5 (153 mg, 0.5
mmol) or 6 (150 mg, 0.5 mmol) and barbituric acid (128 mg, 1 mmol) or 2-cyanoacetic acid (85
mg, 1 mmol) were added to glacial acetic acid (10 mL) and the suspension was warmed to obtain
a clear solution. Piperidine (170 mg, 0.2 mL, 2 mmol) was added and the reaction mixture was
stirred at 120 °C for 5 h. The precipitate of product was filtered off and washed with MeOH (5×5
mL) and then dried at 100 °C to give a pure appropriate dye, 7-10.
5-{[5-(9-Ethyl-9H-carbazol-3-yl)thiophen-2-yl]methylene}pyrimidine-2,4,6(1H,3H,5H)-
1
trione (7). Dark-red powder, yield 200 mg (97%), mp 242-1 °C (AcOH); H NMR (500 MHz,
DMSO-d6) δ 11.26 (s, 1H, NH), 11.24 (s, 1H, NH), 8.70 (d, J 1.6 Hz, 1H), 8.51 (s, 1H), 8.33 (d,
J 7.7 Hz, 1H), 8.22 (d, J 4.2 Hz, 1H), 7.96 (dd, J 8.6, 1.7 Hz, 1H), 7.86 (d, J 4.1 Hz, 1H), 7.74 (d,
J 8.6 Hz, 1H), 7.66 (d, J 8.3 Hz, 1H), 7.55 – 7.48 (m, 1H), 7.27 (t, J 7.4 Hz, 1H), 4.49 (q, J 7.0
Hz, 2H, CH2), 1.34 (t, J 7.1 Hz, 3H, CH3); 13C NMR (126 MHz, DMSO-d6) δ 163.6, 163.2,
160.6, 150.3, 147.9, 145.7, 140.4, 140.1, 134.4, 126.5, 124.4, 124.1, 123.8, 122.9, 122.1, 120.9,
119.5, 118.6, 110.0, 109.5, 109.4, 37.2, 13.7; IR(DRA): 463, 525, 509, 552, 622, 750, 729, 797,
855, 967, 1086, 1124, 1155, 1191, 1235, 1297, 1258, 1347, 1393, 1448, 1471, 1506, 1544, 1595,
1657, 1692, 1734, 1942, 2171, 2830, 2974, 3058, 3188, 3410 cm-1; Anal. Calcd for
C23H17N3O3S×0.5H2O: C, 65.08; H, 4.27; N, 9.90. Found: C, 65.15; H, 4.02; N, 9.87.
5-[4-(9-Ethyl-9H-carbazol-3-yl)benzylidene]pyrimidine-2,4,6(1H,3H,5H)-trione (8). Red
powder, yield 185 mg (91%), mp 211-2 °C (AcOH); 1H NMR (500 MHz, DMSO-d6) δ 11.39 (s,
1H, NH), 11.26 (s, 1H, NH), 8.69 (d, J 1.5 Hz, 1H), 8.37 – 8.32 (m, 3H), 8.29 (d, J 7.7 Hz, 1H),
7.97 (d, J 8.6 Hz, 2H), 7.94 (dd, J 8.6, 1.7 Hz, 1H), 7.73 (d, J 8.6 Hz, 1H), 7.65 (d, J 8.2 Hz, 1H),
7.52 – 7.46 (m, 1H), 7.25 (t, J 7.4 Hz, 1H), 4.49 (q, J 7.1 Hz, 2H, CH2), 1.35 (t, J 7.1 Hz, 3H,
13
CH3); C NMR (126 MHz, DMSO) δ 163.6, 161.9, 154.5, 150.17, 145.1, 140.1, 139.8, 134.9,
130.5, 129.4, 126.1, 125.9, 124.8, 122.94, 122.4, 120.7, 119.1, 119.0, 117.8, 109.7, 109.3, 37.1,
13.7; IR(DRA): 461, 510, 531, 551, 569, 632, 664, 707, 723, 744, 796, 840, 941, 967, 1055,
1079, 1121, 1134, 1158, 1190, 1234, 1261, 1305, 1346, 1396, 1442, 1477, 1529, 1593, 1669,
1730, 2868, 2973, 3063, 3185 cm-1; Anal. Calcd for C25H19N3O3×0.2H2O: C, 72.70; H, 4.73; N,
10.17. Found: C, 72.68; H, 4.49; N, 10.24.
2-Cyano-3-[5-(9-ethyl-9H-carbazol-3-yl)thiophen-2-yl]acrylic acid (9). Red powder, yield
150 mg (81%), mp 195-6 °C (AcOH); 1H NMR (500 MHz, DMSO-d6) δ 13.66 (br.s, 1H, CO2H),
8.65 (d, J 1.7 Hz, 1H), 8.50 (s, 1H), 8.31 (d, J 7.7 Hz, 1H), 8.05 (d, J 4.1 Hz, 1H), 7.88 (dd, J
8.6, 1.8 Hz, 1H), 7.81 (d, J 4.0 Hz, 1H), 7.72 (d, J 8.6 Hz, 1H), 7.66 (d, J 8.2 Hz, 1H), 7.55 –
7.48 (m, 1H), 7.26 (t, J 7.4 Hz, 1H), 4.48 (q, J 7.1 Hz, 2H, CH2), 1.34 (t, J 7.1 Hz, 3H, CH3); 13C
NMR (126 MHz, DMSO-d6) δ 163.84, 155.28, 146.69, 141.86, 140.23, 140.15, 133.23, 126.47,
124.34, 123.84, 123.29, 122.86, 122.05, 120.93, 119.43, 118.48, 116.67, 110.00, 109.53, 96.78,
37.16, 13.69; IR(DRA): 460, 513, 533, 550, 573, 589, 629, 654, 722, 729, 752, 797, 818, 894,
930, 972, 1065, 1023, 1125, 1089, 1157, 1272, 1293, 1350, 1390, 1433, 1471, 1492, 1506, 1577,
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1627, 1663, 1687, 2219, 2508, 2543, 2597, 2815, 2965, 3062, 3088 cm-1; Anal. Calcd for
C22H16N2O2S: C, 70.95; H, 4.33; N, 7.52. Found: C, 70.82; H, 4.35; N, 7.65.
2-Cyano-3-[4-(9-ethyl-9H-carbazol-3-yl)phenyl]acrylic acid (10). Orange powder, yield 135
1
mg (74%), mp 190-1 °C (AcOH); H NMR (400 MHz, DMSO-d6) δ 13.93 (s, 1H, CO2H), 8.68
(s, 1H), 8.39 (s, 1H), 8.28 (d, J 7.7 Hz, 1H), 8.18 (d, J 8.4 Hz, 2H), 8.06 (d, J 8.4 Hz, 2H), 7.93
(d, J 8.6 Hz, 1H), 7.74 (d, J 8.6 Hz, 1H), 7.65 (d, J 8.2 Hz, 1H), 7.50 (t, J 7.5 Hz, 1H), 7.25 (t, J
7.5 Hz, 1H), 4.49 (q, J 7.1 Hz, 2H, CH2), 1.34 (t, J 7.1 Hz, 3H, CH3); 13C NMR (126 MHz,
DMSO-d6) δ 163.48, 153.88, 145.57, 140.05, 139.77, 131.49, 129.39, 129.15, 126.94, 126.10,
124.78, 122.93, 122.35, 120.67, 119.09, 119.06, 116.41, 109.67, 109.34, 102.16, 37.07, 13.67;
IR(DRA): 459, 517, 552, 580, 615, 626, 671, 726, 748, 766, 786, 798, 846, 894, 946, 1022,
1064, 1085, 1123, 1132, 1155, 1190, 1234, 1294, 1348, 1384, 1428, 1476, 1492, 1515, 1546,
1569, 1629, 1697, 2223, 2548, 2618, 2676, 2816, 2971, 3054 cm-1; Anal. Calcd for C24H18N2O2:
C, 78.67; H, 4.95; N, 7.65. Found: C, 78.32; H, 4.92; N, 7.49.
Acknowledgments
This work was supported by the Ural Division of the Russian Academy of Sciences (Grants №
12-P-3-1014, 12-P-3-1030, 12-T-3-1025 and 12-T-3-1031), the Russian Foundation for Basic
Research (research projects No. 13-03-12434-ofi_m2, 13-03-96049-r_ural_a, 14-03-01017_A,
14-03-00479_A), and the Scientific Council of the President of the Russian Federation (grant
МК-3043.2014.3).
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