New acceptor-π-porphyrin-π-acceptor systems for solution-processed small molecule organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2015.04.037

Two new conjugated acceptor-donor-acceptor (A-π-D-π-A) compounds having a Zn-porphyrin acting as donor and linked by ethynylenes to one or two units of thienylenevinylene and capped by dicyanovinylene groups as acceptor units have been synthesized and the

Full text of DOI:10.1016/j.dyepig.2015.04.037

Accepted Manuscript
New Acceptor-π-Porphyrin-π-Acceptor Systems for Solution-Processed Small
Molecule Organic Solar Cells
Susana Arrechea, Agustín Molina-Ontoria, Ana Aljarilla, Pilar de la Cruz, Fernando
Langa, Luis Echegoyen
PII:
S0143-7208(15)00167-9
10.1016/j.dyepig.2015.04.037
DYPI 4759
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 March 2015
Revised Date: 23 April 2015
Accepted Date: 27 April 2015
Please cite this article as: Arrechea S, Molina-Ontoria A, Aljarilla A, de la Cruz P, Langa F, Echegoyen
L, New Acceptor-π-Porphyrin-π-Acceptor Systems for Solution-Processed Small Molecule Organic Solar
Cells, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.04.037.
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ACCEPTED MANUSCRIPT
1a
η = 1.36 %
PC61BM
1b
η = 3.16 %
PC71BM

ACCEPTED MANUSCRIPT
New Acceptor-π-Porphyrin-π-Acceptor Systems for Solution-
Processed Small Molecule Organic Solar Cells
a
b
a
a
Susana Arrechea, Agustín Molina-Ontoria, Ana Aljarilla, Pilar de la Cruz, Fernando
a,*
b,*
Langa and Luis Echegoyen
a
Universidad de Castilla-La Mancha. Institute of Nanoscience, Nanotechnology and Molecular
Materials (INAMOL), Campus de la Fábrica de Armas, Toledo, Spain.
b
Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, USA.
Abstract: Two new conjugated acceptor-donor-acceptor (A-π-D-π-A) compounds having a Zn-
porphyrin acting as donor and linked by ethynylenes to one or two units of thienylenevinylene
and capped by dicyanovinylene groups as acceptor units have been synthesized and their
photophysical and electrochemical properties were investigated. These compounds were used as
donor materials and PC BM and PC BM were used as acceptors in solution-processed bulk-
6
1
71
heterojunction (BHJ) organic solar cells and the best photoconversion efficiency (PCE)
obtained was 3.21%.
Keywords: Photovoltaics, bulk heterojunction solar cells, small molecule, porphyrin.
Introduction
Efficient production of clean and sustainable energy is one of the most important scientific
challenges that the world faces today. Sunlight is the most abundant and one of the cleanest
sources of energy, thus the efficient utilization of solar energy to prepare solar cells has
1
-3
attracted much attention from the scientific community. The development of efficient organic
photovoltaic (OPV) solar cells has attracted considerable interest as potential alternative solar
energy sources to silicon-based solar cells, because of their distinct advantages of solution
processability, low-cost, flexibility and roll-to-toll production possibilities. The bulk
heterojunction (BHJ) approach is the most efficient to date, which consists of a nanometer scale
interpenetrating network of an electron-accepting, typically fullerene, phase and a light-
harvesting electron-donor phase, such as a polymer or a small molecule. Polymer solar cells
(
PSCs) underwent significant progress in the past decades, thanks to the judicious design of
narrow band-gap copolymers, optimization of the nanoscale morphologies of the photoactive
layers and enhanced carriers mobilities, which have led to power conversion efficiencies (PCEs)
4,5
6
of 10% for single-junction PSCs and up to 11% for tandem PSCs. Nonetheless, there are
issues with the synthesis of conjugated polymers for photovoltaic applications, such as difficult
purification, poor batch-to-batch reproducibility and so on.
1

ACCEPTED MANUSCRIPT
Solution processed small-molecule organic solar cells (SMBHJ), are an emerging alternative to
the polymer counterparts, and these have rapidly developed in recent years, which offer
potential advantages, such as better defined structures, easier purification and better
6
-9
reproducibility. Thus far, power conversion efficiencies exceeding 9% have been reported for
1
0-15
solution-processed small molecule single junction devices.
Inspired by natural photosynthetic organisms, where chlorophylls absorb light for energy
conversion, and because of intense Soret band at around 400 nm and the Q bands close to 600
nm, porphyrins and their derivatives are excellent building blocks for the construction of light
1
6
harvesting architectures. Porphyrins are among the best sensitizers in Dye Sensitized Solar
1
7-22
23
cells (DSSCs)
showing performances as high as 12%. However, the utilization of
2
4-33
porphyrins as active materials in solution-processed BHJ solar cells have been limited
3
4
despite their natural tendency to form aggregates by π-π stacking interactions.
In this article we report the synthesis of two new conjugated acceptor-donor-acceptor (A-π-D-π-
A) molecules 1a and 1b (Figure 1), along with their photophysical and electrochemical
properties, as well as their performance in solution-processed SMBHJ. Both compounds were
used as the p type layer with the Zn-porphyrin core acting as a donor linked by ethynylenes to
one or two units of thienylenevinylene and capped by dicyanovinylene groups as acceptor units.
Ethynylenes were chosen to make the systems planar and hexyl chains were attached on the
thiophene units to enhance their solubility. PC BM and PC BM were used as acceptor
6
1
71
components in the devices.
Figure 1.
Experimental Section
2
2
.1. Experimental details are given in the Supporting Information.
.2. Synthetic Procedures.
3
5
36
General synthetic procedure for 3a,b. To a solution of 5a,b (1 eq) in carbon tetrachloride
(
CCl , 1.25 mL/mmol) was added PhI(OCOCF ) (0.55 eq) and molecular iodine (I , 0.5 eq).
4
3 2
2
The mixture was stirred at room temperature. The reaction was quenched with a saturated
solution of sodium thiosulfate (Na SO sat.) while stirring for 15 min and extracted with
2
3
CH Cl . The combined organic phases were dried over anhydrous MgSO and filtered. The
2
2
4
solvent was removed by rotary evaporation.
2

ACCEPTED MANUSCRIPT
5
-Iodo-3,4-dihexyl-2-thiophencarboxaldehyde (3a). Using the general procedure previously
described, 5a (1.8 mmol, 500 mg) was reacted with PhI(OCOCF ) (1.0 mmol, 416 mg) and I
2
3
2
(
(
0.9 mmol, 226 mg) in 2.2 mL of CCl . The product was purified by column chromatography
4
silica gel, hexane-CHCl , 7:3). 3a was obtained as a yellow oil (610 mg, 1.51 mmol, 84%
3
1
yield). H NMR (400 MHz, CDCl ) δ/ppm: 9.90 (s, 1H), 2.94-2.90 (m, 2H), 2.57-2.53 (m, 2H),
3
1
3
1
1
1
.64‐1.57 (m, 4H), 1.39‐1.31 (m, 12H), 0.93‐0.88 (m, 6H). C NMR (100 MHz, CDCl ) δ/ppm:
3
81.2, 150.5, 148.4, 143.0, 89.7, 32.3, 31.5, 31.4, 30.6, 29.7, 29.3, 29.3, 27.8, 22.6, 22.5, 14.1,
+
4.0. MALDI-TOF MS (m/z): [M] calculated for C H IOS: 406.08; found: 406.08. FT-IR
17
27
-1
(
ATR) υ/cm : 2923, 2854, 1654, 1523, 1461, 1427, 1365, 1222, 1126, 1079, 721, 674.
(
E)‐1‐(5‐Formyl‐3,4‐dihexyl‐2‐thienyl)‐2‐(5-iodo-3’,4’‐dihexyl‐2’‐thienyl)ethylene
(3b).
Using the general procedure previously described, 5b (0.6 mmol, 338 mg) was reacted with
PhI(OCOCF ) (0.3 mmol, 142 mg), I (0.3 mmol, 77 mg) in 0.75 mL of CCl . The product was
3
2
2
4
purified by column chromatography (silica gel, hexane-CHCl , 3:2). 3b was obtained as a
3
1
yellow oil (255 mg, 0.37 mmol, 62% yield). H NMR (400 MHz, CDCl ) δ/ppm: 9.98 (s, 1H),
3
7
.19 (d, 1H, J = 15.5 Hz), 6.97 (d, 1H, J = 15.5 Hz), 2.84 (t, 2H, J = 7.7 Hz), 2.64 (t, 2H, J = 7.7
Hz), 2.58 (t, 2H, J = 7.7 Hz), 2.49 (t, 2H, J = 7.7 Hz), 1.50-1.27 (m, 32H), 0.94-0.88 (m, 12H).
1
3
C NMR (100 MHz, CDCl ) δ/ppm: 181.9, 153.0, 147.7, 146.6, 141.8, 141.6, 141.0, 134.8,
3
1
1
23.1, 118.8, 75.5, 32.3, 31.6, 31.5, 31.4, 31.1, 31.0, 29.8, 29.3, 27.9, 27.1, 26.4, 22.6, 22.5,
+
4.1, 14.05. MALDI-TOF MS (m/z): [M] calculated for C H IOS : 682.27; found: 683.49.
3
5
55
2
-1
FT-IR (ATR) υ/cm : 2923, 2850, 1654, 1600, 1523, 1461, 1403, 1375, 1249, 1211, 933, 725,
78, 663.
6
3
7
38
Synthesis of 5,15-dimesitylporphyrin. A solution of corresponding dipyrromethane (14.4
mmol, 2.10 g) and 2,4,6-trimethylbenzaldehyde (14.4 mmol, 2.1 mL) in 1.40 L of CHCl was
3
treated with BF O(C H ) , (4.7 mmol, 0.6 mL). The mixture was stirred for 3 h. DDQ (21.5
3
2
5 2
mmol, 4.9 g) was added and the reaction mixture was stirred for 1 h. Et N (2.0 mL) was added
3
and stirred during 30 min. The solvent was removed by rotary evaporation and the solid was
purified by column chromatography (silica gel, hexane-CHCl , 1:1). 5,15-Dimesitylporphyrin
3
1
was obtained as a purple solid (1400 mg, 2.56 mmol, 35% yield). H NMR (400 MHz, CDCl )
3
δ/ppm: 10.25 (s, 2H), 9.35 (d, 4H, J = 4.5 Hz), 8.91 (d, 4H, J = 4.5 Hz), 7.35 (s, 4H), 2.69 (s,
1
3
6
H), 1.87 (s, 12H), -3.04 (s, 2H). C NMR (100 MHz, CDCl ) δ/ppm: 146.8, 145.4, 139.5,
3
+
1
37.8, 137.6, 131.8, 130.0, 127.8, 117.3, 104.6, 21.7, 21.5. MALDI-TOF MS (m/z): [M]
-1
calculated for C H N : 546.09; found: 546.28. FT-IR (ATR) υ/cm : 3305, 1828, 1604, 1411,
3
8
34
4
1
373, 1319, 1234, 1052, 950, 852, 782, 736, 694, 617.
3
9
Synthesis of [5,15-dimesitylporphyrinato] zinc (II).
To
a
solution of 5,15-
dimesitylporphyrin (2.5 mmol, 1.35 g) in 209 mL of CHCl , was added a solution of
3
3

ACCEPTED MANUSCRIPT
Zn(OAc) ·2H O (12.3 mmol, 2.26 g) in 6.2 mL of MeOH. The mixture was stirred 18 hours.
2
2
The reaction was quenched with water and extracted with CHCl (3 x 100 mL). The combined
3
organic extract was dried over anhydrous MgSO and filtered. The solvent was removed by
4
rotary evaporation. The product was purified by column chromatography (silica gel, hexane-
CHCl , 1:1). The product [5,15-dimesitylporphyrinato] zinc (II), was obtained as a purple solid
3
1
(
1510 mg, 2.47 mmol, 99% yield). H NMR (400 MHz, CDCl ) δ/ppm: 10.27 (s, 2H), 9.41 (d,
3
1
3
4
H, J = 4.4 Hz), 9.00 (d, 4H, J = 4.4 Hz), 7.35 (s, 4H), 2.69 (s, 6H), 1.85 (s, 12H). C NMR
(
100 MHz, CDCl ) δ/ppm: 149.8, 149.4, 139.3, 138.8, 135.7, 132.1, 131.3, 127.7, 118.2, 105.5,
3
+
2
1.7, 21.5. MALDI-TOF MS (m/z): [M] calculate for C H N Zn: 608.57; found: 608.19. FT-
3
8
32
4
-1
IR (ATR) υ/cm : 1816, 1608, 1438, 1388, 1318, 1211, 1052, 856, 786, 728, 701, 617.
Synthesis of [5,15-dibromo-10,20-dimesitylporphyrinato] zinc (II). To a solution of [5,15-
dimesitylporphyrinato] zinc (II) (1.5 mmol, 920 mg) in 80 mL of CHCl , N-bromosuccinimide
3
(
NBS) (3.0 mmol, 536 mg) and 1 mL of pyridine were added. The mixture was stirred 30
minutes and quenched with acetone. The solvent was removed by rotary evaporation and the
solid was purified by column chromatography (silica gel, hexane-CHCl , 7:3). The product
3
[
5,15-dibromo-10,20-dimesitylporphinato] zinc (II) was obtained as a purple solid (1117 mg,
1
1
4
1
.45 mmol, 98% yield). H NMR (400 MHz, CDCl ) δ/ppm: 9.59 (d, 4H, J = 4.6 Hz), 8.71 (d,
3
1
3
H, J = 4.6 Hz), 7.80 (s, 4H), 2.65 (s, 6H), 1.80 (s, 12H). C NMR (100 MHz, CDCl ) δ/ppm:
3
50.5, 149.9, 139.1, 138.8, 137.6, 133.3, 132.0, 127.7, 122.3, 120.1, 104.1, 55.4, 21.6, 21.4.
+
MALDI-TOF MS (m/z): [M] calculate for C H Br N Zn: 766.73; found: 766.01. FT-IR
3
8
30
2
4
-1
(
ATR) υ/cm : 1808, 1604, 1423, 1373, 1315, 1203, 1072, 995, 860, 786, 740, 694, 628, 609.
Synthesis of [5,15-bis-(trimethylsilyl)ethynyl)-10,20-dimesitylporphyrinato] zinc (II) (2). In
a schlenk tube under an argon atmosphere, a solution of [5,15-dibromo-10,20-
dimesitylporphyrinato] zinc (II) (1.4 mmol, 1.10 g) in 62 mL of THF and 3 mL of Et N was
3
added over a mixture of Pd(PPh ) Cl (0.1 mmol, 54 mg) and CuI (0.07 mmol, 13 mg).
3
2
2
Afterwards, trimehylsilylacethylene (5.4 mmol, 0.8 mL) was added. The mixture was stirred 18
h. The solvent was removed by rotary evaporation and the solid was purified by column
chromatography (silica gel, hexane-CHCl , 7:3) and recrystallized from CH Cl /MeOH. 2 was
3
2
2
1
obtained as a purple solid (1.07 g, 1.35 mmol, 97% yield). H NMR (400 MHz, CDCl ) δ/ppm:
3
9
.68 (d, 4H, J = 4.6 Hz), 8.79 (d, 4H, J = 4.6 Hz), 7.33 (s, 4H), 2.68 (s, 6H), 1.86 (s, 12H), 0.62
1
3
(
s, 18H). C NMR (100 MHz, CDCl ) δ/ppm: 152.1, 149.9, 139.0, 138.4, 137.6, 131.9, 131.7,
3
+
1
31.4, 127.7, 120.9, 107.4, 101.5, 100.6, 21.5, 21.5, 0.3. MALDI-TOF MS (m/z): [M]
-1
calculated for C H N Si Zn: 800.58; found: 800.27. FT-IR (ATR) υ/cm : 2136, 1808, 1604,
4
8
48
4
2
1
438, 1380, 1334, 1203, 1072, 995, 860, 786, 736, 694, 617.
4

ACCEPTED MANUSCRIPT
Synthesis of [5,15-bis-(ethynyl)-10,20-dimesitylporphyrinato] zinc (II). To a solution of 2
(
0.7 mmol, 550 mg) in 150 mL of CH Cl , was added TBAF 1 M in THF (1.65 mmol, 1.65
2 2
mL). The solution was stirred at room temperature for 2 h and treated with CaCl (8.2 mmol,
2
9
12 mg). The reaction was hydrolyzed with water and extracted with CHCl (3 x 150 mL). The
3
combined organic extract was dried over anhydrous MgSO and filtered; finally, the solvent was
4
removed by rotary evaporation. The deprotected product was quantitatively obtained and it was
used in the next synthetic step without further purification.
General Procedure for the Sonogashira coupling reactions. Under an argon atmosphere, a
solution of [5,15-bis-(ethynyl)-10,20-dimesitylporphyrinato] zinc (II) (1 eq), the corresponding
aldehyde (3a-b) (3 eq) and freshly distilled Et N (45 mL/mmol) in THF (230 mL/mmol) was
3
added over a mixture of Pd (dba) (0.6 eq) and AsPh (3.8 eq). The reaction mixture was
2
3
3
refluxed for 18 h. The solvent was removed by rotary evaporation and the solid was purified by
column chromatography (silica gel, hexane-CH Cl , 7:3) followed by recrystallization from
2
2
CH Cl /MeOH.
2
2
4
a: Using the general procedure previously described, [5,15-bis-(ethynyl)-10,20-
dimesitylporphyrinato] zinc (II) (0.34 mmol, 658 mg), 3a (1.02 mmol, 417 mg), 15 mL of Et N
3
in 79 mL of THF were allowed to react. 4a was obtained as a green solid (337 mg, 0.22 mmol,
1
8
4
1
6
1
1
1
1
1
1% yield). H NMR (400 MHz, CDCl ) δ/ppm: 9.49 (d, 4H, J = 4.4 Hz), 9.16 (s, 2H), 8.75 (d,
3
H, J = 4.4 Hz), 7.33 (s, 4H), 2.99 (t, 4H, J = 7.6 Hz), 2.80 (t, 4H, J = 7.6 Hz), 2.69 (s, 6H),
.89 (s, 12H), 1.60 (t, 4H, J = 7.2 Hz), 1.54 (t, 4H, J = 7.2 Hz), 1.43‐ 1.22 (m, 24H), 0.93 (t,
1
3
H, J = 6.4 Hz), 0.77 (t, 6H, J = 7.0 Hz). C NMR (100 MHz, CDCl ) δ/ppm: 207.2, 181.2,
3
51.7, 151.6, 150.1, 148.2, 139.00, 138.1, 137.9, 137.3, 131.9, 131.2, 129.4, 127.8, 122.0,
03.1, 100.1, 89.5, 32.3, 31.8, 31.6, 30.9, 30.9, 29.7, 29.4, 28.7, 27.7, 22.7, 22.6, 21.6, 21.5,
+
4.1, 14.0. MALDI-TOF MS (m/z): [M] calculated for C H N O S Zn: 1212.53; found:
7
6
84
4
2 2
-1
212.53. FT-IR (ATR) υ/cm : 2919, 2854, 2175, 2053, 1650, 1612, 1489, 1438, 1403, 1330,
284, 1207, 1150, 995, 937, 848, 790, 709.
4
b: Using the general procedure previously described, [5,15-bis-(ethynyl)-10,20-
dimesitylporphyrinato] zinc (II) (0.34 mmol, 658 mg), 3b (1.02 mmol, 702 mg), 15 mL of Et N
3
1
in 79 mL of THF. 4b was obtained as a green solid (246 mg, 0.14, 71%). H NMR (400 MHz,
CDCl3) δ/ppm: 9.56 (s, 1H), 9.55 (d, 4H, J = 4.6 Hz), 8.73 (d, 4H, J = 4.6 Hz), 7.32 (s, 4H),
7
.32 (d, 2H, J = 15.5 Hz), 7.08 (d, 2H, J = 15.5 Hz), 3.05 (t, 4H, J = 7.9 Hz), 2.79 (t, 4H, J = 7.9
Hz), 2.75 (t, 4H, J = 8.3 Hz), 2.68 (s, 6H), 2.66 (t, 4H, J = 7.2 Hz), 1.91 (t, 4H, J = 7.2 Hz), 1.89
1
3
(
s, 12H), 1.67-1.31 (m, 60H), 1.00-0.91 (m, 18H), 0.80 (t, 6H, J = 7.2 Hz). C NMR (100 MHz,
CDCl3) δ/ppm: 181.7, 153.1, 151.6, 149.7, 148.5, 146.9, 142.7, 141.9, 139.0, 138.3, 137.7,
1
37.3, 134.6, 131.5, 131.1, 127.7, 132.5, 121.5, 119.0, 118.9, 101.2, 100.8, 90.7, 32.2, 31.8,
5

ACCEPTED MANUSCRIPT
3
1
1.7, 31.6, 31.5, 31.2, 30.9, 29.7, 29.4, 29.3, 29.2, 29.1, 27.5, 27.1, 26.4, 22.7, 22.6, 21.5, 21.4,
+
4.2, 14.1, 14.0, 13.9. MALDI-TOF MS (m/z): [M] calculated for C H N O S Zn: 1764.92;
1
12 140
4
2 4
found: 1765.07. FT-IR (ATR) υ/cm-1: 2919, 2854, 2175, 1646, 1592, 1500, 1454, 1388, 1338,
292, 1207, 1095, 998, 929, 852, 790, 709.
1
General procedure for the Knoevenagel condensations. To a solution of 4a-b (1 eq) in 25
mL/mmol of CH Cl , malonitrile (3 eq) and 3 drops of Et N were added. The reaction mixture
2
2
3
was stirred for 18 h and quenched by the addition of water and extracted with CHCl (3 x 150
3
mL). The combined organic extract was dried over anhydrous MgSO and filtered. The solvent
4
was removed by rotary evaporation. The product was purified by column chromatography
(
silica gel, hexane-CH Cl , 1:1) and recrystallized with CH Cl : MeOH.
2 2 2 2
1
a: Using the general procedure previously described, 4a (0.08 mmol, 100 mg) in 2 mL of
CH Cl were allowed to react with malonitrile (0.25 mmol, 16 mg). 1a was obtained as a green
2
2
1
solid (87 mg, 0.06 mmol, 81% yield). M. p. >300°C. H NMR (400 MHz, CDCl ) δ/ppm: 9.16
3
(
s, 4H), 8.75 (d, 4H, J = 4.2 Hz), 7.89 (s, 2H), 7.37 (s, 4H), 3.84 (m, 4H), 2.76 (m, 4H), 2.71 (s,
6
6
1
3
H), 1.96 (s, 12H), 1.76 (m, 4H), 1.64-1.74 (m, 12H), 1.27-1.22 (m, 16H), 0.97 (m, 6H), 0.76 (t,
13
H, J = 6.7 Hz). C NMR (100 MHz, CDCl ) δ/ppm: 154.3, 151.6, 150.3, 147.7, 147.4, 138.9,
3
38.1, 137.8, 132.2, 131.0, 130.8, 127.9, 122.8, 115.1, 113.9, 106.3, 99.5, 89.4, 75.2, 32.3, 31.7,
1.6, 31.5, 30.9, 30.7, 30.2, 29.7, 29.6, 29.1, 28.8, 22.6, 21.6, 21.5, 14.1, 14.0. MALDI-TOF
+
-1
MS (m/z): [M] calculated for C H N S Zn: 1308.53; found: 1308.56. FT-IR (ATR) υ/cm :
8
2
84
8 2
2
9
923, 2854, 2225, 2154, 2053, 1612, 1489, 1465, 1403, 1330, 1288, 1249, 1203, 1083, 998,
37, 850, 790, 711.
1
b: Using the general procedure previously described, 4b (0.2 mmol, 295 mg) in 3 mL of
CH Cl , malonitrile (0.5 mmol, 33mg. 1b was obtained as a green solid (246 mg, 0.06 mmol,
2
2
1
7
9% yield). M. p. > 300°C. H NMR (400 MHz, CDCl ) δ/ppm: 9.46 (d, 4H, J = 4.4 Hz), 8.68
3
(
d, 4H, J = 4.4 Hz), 7.69 (s, 2H), 7.28 (s, 4H), 7.21 (d, 2H, J = 15.6 Hz), 6.96 (d, 2H, J = 15.6
Hz), 2.96 (t, 4H, J = 7.7 Hz), 2.70 (t, 4H, J = 7.7 Hz), 2.62 (s, 6H), 2.57 (m, 8H), 1.85 (s, 12H),
1
3
1
.60-1.54 (m, 8H), 1.49-1.26 (m, 56H), 0.94-0.56 (m, 18H), 0.73 (t, 6H, J = 7.0 Hz). C NMR
(
100 MHz, CDCl ) δ/ppm: 155.9, 151.6, 149.8, 149.1, 148.8, 147.2, 144.1, 141.6, 139.0, 138.2,
3
1
7
2
1
1
37.8, 137.2, 131.6, 131.1, 128.3, 127.8, 125.7, 120.1, 118.2, 115.6, 114.2, 101.4, 101.2, 90.8,
3.1, 32.2, 31.8, 31.6, 31.6, 31.5, 31.0, 31.9, 29.7, 29.5, 29.3, 29.2, 27.8, 27.5, 26.8, 22.8, 22.7,
+
2.6, 22.6. MALDI-TOF MS (m/z): [M] calculated for C H N S Zn: 1860.94; found:
1
18 140
8 4
-1
861.94. FT-IR (ATR) υ/cm : 2928, 2856, 2217, 2171, 1552, 1500, 1457, 1399, 1336, 1288,
209, 1091, 998, 929, 854, 794, 711.
Results and discussion
6

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3
.1. Synthesis and Characterizations.
Scheme 1 illustrates the synthetic route used to obtain organic semiconductor compounds 1a
and 1b, starting from bis-trimethylsilyl porphyrin 2. The trimethylsilyl group was quantitatively
removed by hydrolysis with TBAF and, without further purification, reacted with the
corresponding iodoaldehyde 3a-b under Pd-catalyzed Sonogashira coupling conditions to afford
1
the bisaldehydes 4a-b in 81% and 71% yields, respectively. The H NMR spectra of 4a-b show
the expected signals for both the porphyrin and the thienylenevinylene moieties and the
aldehyde protons are observed at 9.16 ppm and 9.56 ppm, respectively. Compound 4b, only
showed the trans configuration of the double bond, which was confirmed by a coupling
constant of 15.6 Hz. Finally, the target compounds 1a-b were obtained by Knoevenagel
condensations of 4a-b with malononitrile in the presence of triethylamine in 81% and 79%
1
yields, respectively. In the H NMR spectra of 1a-b, the aldehyde hydrogen signals are not
observed and new vinylic hydrogens signals were observed at 7.89 ppm and 7.69 ppm,
respectively, indicating the successful condensation. The mass spectrum of compounds 1a
showed the molecular ion peak at m/z 1308.56 amu, while that of compound 1b exhibited the
molecular ion peak at m/z 1861.94 amu.
Scheme 1.
Previously, iodoaldehydes 3a-b were prepared by the reaction of the aldehydes 5a-b, with
molecular iodine (I ), and bistrifluoroacetoxiiodobenzene (PhI(OCOCF ) ) in 84% and 62%
2
3 2
3
5
yield according to Scheme 2.
Scheme 2.
1
13
All compounds were satisfactorily characterized by H and C NMR, FT-IR and MALDI-MS
spectrometry (see the Experimental section and the Supporting Information for synthetic details
and full analytical and spectroscopic characterizations).
The thermal stabilities of compounds 1a and 1b were evaluated by thermogravimetric analysis
(
TGA). Compounds 1a–b display excellent thermal stabilities up to 300 ºC with Td of 369 ºC
and 374 ºC for 1a and 1b, respectively (Figure S29), which are appropriate for photovoltaic
applications.
Optical Properties
7

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The optical properties of 1a and 1b in CH Cl solutions as well as of their thin-films are shown
2
2
in Figure 1 and the characteristics of semiconductors 1a and 1b are collected in Table 1. The
UV-Vis absorption spectra of the precursor aldehydes (4a-b) and of the final compounds 1a-b
in solution exhibit a panchromatic absorption in the visible region. These spectra show the Soret
band (λmax = 465 nm and 499 nm respectively) bathochromically shifted, with respect to the
Soret band in the precursor porphyrin 2 (λmax = 435 nm, Figure S30). New intense broad bands
are observed (at λmax = 668 nm for 4a and 698 nm for 4b), assigned to intramolecular charge
transfer (ICT) (Figure S30). In solution, 1a and 1b show absorption ranges from 400 nm to 750
nm with a valley centered at 600 nm. The introduction of the malononitrile fragments (1a-b)
lead to a bathochromic shift of both bands (λmax = 499 and 698 for 1a and 497 and 705 nm for
1
b) with respect to the corresponding aldehydes (4a-b) as a consequence of the extension of the
conjugation and due to the strong electron-withdrawing properties of the dicyanomethylene.
Increasing the -conjugation upon increasing the length of the thienylenevinylene in 1b results
π
in a wider absorbance.
a
a
b
Table 1. UV-Vis, Fluorescence Emission and OSWV data for compounds 1a-b.
1
b, c
ox
d
e
λmax soln
nm)
497
05
542
31
log
λmax film λem
E
E
HOMO
E
LUMO
E
0-0
(
(ε)
(nm)
516
(nm) (V)
(eV)
(eV)
(eV)
1.75
1
1
a
5.26
5.04
5.17
5.24
717
0.40
-5.50
-3.76
7
b
734
747
0.26
-5.36
-3.67
1.69
7
a
-5
b
-3
+
1
0
M, in dichloromethane; 10 M in ODCB-acetonitrile (4:1) versus Fc/Fc (E = 0.04 V) glassy carbon, Pt
ox
o
-1
c
d
counter electrode, 20 C, 0.1 M Bu NClO , scan rate = 100 mV s ; Nonreversible processes; calculated with
4
4
40 e
respect to ferrocene, EHOMO: –5.1 eV;
estimated from the intersection between the normalized absorption and
normalized emission spectra at λmax
.
In the solid state, the absorption maximum for 1a is bathocromically shifted relative to those in
solution, by 13 nm. 1b shows a pronounced absorbance between of 400-850 nm, with a red-
shifted maximum of 33 nm (764 nm). These results suggest that the extended backbone in 1b
8

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results in stronger intermolecular π-π stacking interactions than in 1a (Figure 2). The estimated
optical band gaps were calculated from the thin film absorbance onsets, and were 1.57 eV for 1a
and 1.50 eV for 1b.
Figure 2.
The fluorescence spectra display a red-shift of the emission band for compound 1b in
comparison to that for 1a, around 30.2 nm, due to the increased conjugation by one more
thienylenevinylene unit. If the emission spectra compounds 1a-b are compared with those of the
precursor aldehydes 4a-b (Figure S33), a significant quenching of the emission is observed,
attributed to more efficient electron transfer processes.
Electrochemical Properties
The electrochemical properties of 1a and 1b were investigated using Cyclic Voltammetry (CV)
and Osteryoung Square Wave Voltammetry (OSWV) in o-DCB-acetonitrile (4:1) (Table 1,
Figures 3 and S34). In the anodic scan, both compounds show a first reversible one-electron
+
oxidation wave at 0.40 V for 1a (Figure 4) and 0.26 V for 1b (vs Fc/Fc in all cases) which
correspond to the first oxidation of the porphyrin. For 1b, the extended conjugation gives rise to
a decrease of the E value by 14 mV with respect to 1a. A second reversible oxidation wave is
ox
observed at 0.78 V for 1a and at 0.62 V for 1b. Compound 1b shows two more non reversible
oxidation waves at 0.93 and 1.02 V attributed to the oxidation of the thienylenevinylene
moieties. The estimated EHOMO values were calculated with respect to ferrocene as reference
40
(
EHOMO: -5.1 eV) and were determined to be -5.50 eV for 1a and -5.36 eV for 1b, in good
agreement with the onset oxidation potentials.
Figure 3.
Low-lying HOMO levels should result in high open-circuit voltages (V ) and are, therefore,
oc
41-43
desired.
The HOMO-LUMO gaps, optically determined, are as narrow as 1.75 and 1.69 eV
for 1a and 1b, respectively. The ELUMO of the dyes are higher than that ELUMO of PCBM (-3.9
eV), with values of -3.75 and -3.67 eV for 1a and 1b, respectively. Hence, the LUMO energy
9

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levels of these small-molecules match quite well the LUMO energy of PC BM (-3.9 eV) and
6
1
PC BM (-4.0 eV), which suggest an energetically favorable electron transfer from 1a and 1b to
71
the acceptor moiety that should in turn favor the exciton dissociation.
Theoretical Calculations
Theoretical calculations were carried out by density functional (DFT) at the B3LYP 6-31G*
level in vacuo using Gaussian 03W to determine the more stable geometries for both dyes 1a
and 1b (Figure 4), the HOMO and LUMO energy levels and their molecular orbital contours.
Figure 4.
The optimized structure of both 1a and 1b, show that the core is almost perfectly flat, with the
aryl groups bounded to the porphyrin core being perpendicular to the macrocycle (dihedral
o
angle of 90 ), while the thienylenevinylene fragments are almost in the same plane with respect
o
to the porphyrin rings with a dihedral angle around 0.5 . This planarity allows an extension of
the conjugation between the porphyrin and the dicyanovinylene fragments. The bond lengths of
the π-conjugated bridge between the porphyrin and the acceptor units are around 1.40 Å, both
for single and double bonds, revealing a quinoid character. This fact suggests some zwitterionic
contribution to the ground state.
The distribution of the orbital coefficients of the HOMO and LUMO states (Figure 5) show that
the charge density of the HOMO of 1a and 1b is delocalized over the whole conjugated system,
the porphyrin and thienylenevinylene moieties. Similar to the HOMO, the LUMO spreads over
the π-conjugated system. Since both orbitals, HOMO and LUMO (Figure 5) are somewhat
overlapped, this favors the HOMO to LUMO electronic transitions.
The theoretical HOMO-LUMO gaps are similar for both dyes, being slightly lower for
compound 1b (ꢀE = 1.72 eV) than for 1a (ꢀE = 1.87 eV). This fact is mainly due to the more
extended conjugation, increasing the HOMO level in 1b and is related to the bathochromic shift
of the maximum absorption wavelength of compound 1b with respect to that for dye 1a
(
according with the experimental data), which improves the light harvesting behaviour. Finally,
the offset between the LUMO of the donor (1a or 1b) and the LUMO of the acceptor (PC BM
6
1
4
1
or PC BM), from 0.54 eV to 1.01 eV, ensures efficient exciton dissociation at the D/A
71
interface.
10

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Figure 5.
Photovoltaic properties
To explore the potential photovoltaic (PV) properties of 1a and 1b, solar cells were fabricated
using the conventional sandwich structure of ITO/poly(3,4-
ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/small molecule: acceptor/Ca/Al.
The active layer was spin-coated from chlorobenzene solutions. The ratio of 1a and 1b to
PC BM was adjusted, ranging from 1:1 to 1:4 (w/w), and the optimized value was found to be
61
1
:2 for both of them. The optimized ratio was employed in a blend of 1a and 1b with PC BM.
71
The photovoltaic devices were measured under an ambient atmosphere employing AM 1.5G
2
simulated illumination at an intensity of 100 mW/cm . The current density-voltage (J-V)
characteristics and the external quantum efficiency (EQE) for the conventional device are
shown in Figures 6a–d and the performance parameters are summarized in Table 2 as a function
of the weight ratios of Donor:PC BM.
6
1
Figure 6.
Figure 6b and Figure 6d illustrate the best photovoltaic performances at varying D/A ratios.
Photovoltaic devices containing 1a:PC BM exhibited high V (0.86 V) at the optimized blend
6
1
oc
-2
ratio of 1:2 w/w, with a short circuit current (J ) of 5.72 mA cm , and a fill factor (FF) of
sc
2
8.4% for an average PCE of 1.45%. Increasing as well as decreasing the amount of PC BM
61
resulted in a lower PCE. In contrast, photovoltaic devices incorporating a blend of 1b and
PC BM exhibited an average PCE of 2.70%, with a V of 0.82 V, FF of 35.2% and a
61
oc
-2
noteworthy Jsc of 9.79 mA cm , because of its more efficient light absorbing properties.
Average values were taken from 12 devices. The high values observed for the V using 1a are
oc
in agreement with the deeper HOMO level of 1a (-5.50 eV) vs 1b (-5.36 eV). The electron
acceptor (PC BM) was replaced by PC BM due to the broader absorbance and the higher
6
1
71
extinction coefficient of the latter in the visible range. SMBHJ devices were fabricated with an
architecture of ITO/PEDOT:PSS/1b or 1a:PC BM (1:2 w/w)/Ca/Al. Photovoltaic devices
7
1
based on 1b and PC BM yielded an increased PCE of 3.21%, a remarkable 14% improvement,
7
1
-2
with an average J of 10.83 mA cm , V of 0.82 V and a slightly improved FF (Figure 7).
sc
oc
Surprisingly, no improvement was observed employing 1a:PC BM (1:2 w/w) as the
7
1
photoactive layer (see Figure S36). In order to support the performances of the above devices,
incident photo-to-current efficiency (IPCE) measurements were also conducted. As shown in
Figure 6b, the IPCE response of 1a with PC BM covers the visible spectrum ranging, from 300
6
1
nm to 800 nm, which matches the absorption spectra. The devices incorporating 1b exhibited
11

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more efficient photoconversion efficiency than those based on 1a, due to broader and higher
IPCE response in the 300-600 nm range. Therefore, the addition of an extra unit of
thienylenevinylene leads to a stronger light-harvesting small-molecule which results in a
significant improvement in the power conversion efficiency. In addition, the IPCE values were
further improved for a 1b:PC BM blend with a remarkable IPCE of 60% at 427 nm (Figure 7).
7
1
The theoretical Jsc values integrated from the EQEs are in good agreement with those
experimentally observed in all cases (± 5% mismatch).
Table 2. Summary of the photovoltaic performance of 1a and 1b blended with PC BM and
6
1
2
PC BM under the illumination of AM1.5G, 100 mW/cm .
71
-
2
Active layer
Voc (V)
J (mA cm )
FF (%)
PCE [Highest] (%)
1.16±0.1[1.26]
1.36±0.1[1.48]
0.84±0.1[0.91]
2.21±0.1[2.34]
2.70±0.1 [2.82]
1.35±0.1[1.48]
1.24±0.1[1.34]
3.16±0.1 [3.21]
sc
1
1
1
a:PC BM (1:4)
0.78±0.03
0.86±0.01
0.63±0.02
0.79±0.03
0.82±0.02
0.84±0.02
0.84±0.01
0.82±0.01
5.38±0.33
5.67±0.12
4.62±0.13
8.41±0.21
9.44±0.26
5.84±0.30
5.56±0.12
10.83±0.24
27.8±0.27
28.1±0.22
29.1±0.21
33.2±0.29
35.0±0.22
27.3±0.21
26.4±0.20
35.7±0.24
6
1
a:PC BM (1:2)
6
1
a:PC BM (1:1)
6
1
1
1
1
1
b:PC BM (1:4)
61
b:PC BM (1:2)
6
1
b:PC BM (1:1)
6
1
a:PC BM (1:2)
7
1
1
b:PC BM (1:2)
71
Figure 7.
Inspecting the morphology of the photoactive layers by atomic force microscopy using the
tapping mode (AFM) provided some explanation for lower power conversion efficiencies
4
4
obtained for the photovoltaic devices incorporating 1a and 1b when blended with PC BM and
6
1
PC BM (Figure 8). The topography images for PC BM and PC BM reveal the formation of
71
61
71
coarse morphologies with faceted islands features showing a root-mean-square roughness (rms)
of 1.9 and 1.7 nm, respectively. Large aggregates of several hundred nanometers were observed
which limit the exciton dissociation efficiencies and the charge transport, explaining the lower
FF and J values measured for these devices. On the other hand, the surface morphology of
sc
devices containing 1b:PC BM and 1b:PC BM exhibit a smoother surface topography (rms of
61
71
1
.0 and 0.9 nm, respectively) with smaller aggregate sizes indicating better morphological
features for exciton dissociation at the donor-acceptor interfaces, which correlate with the
higher FF and J and higher power conversion efficiencies observed.
sc
12

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Figure 8.
Conclusions
In conclusion, we have synthesized two new conjugated acceptor-donor-acceptor (A-π-D-π-A)
compounds having a Zn-porphyrin acting as donor and linked by ethynylenes to one or two
units of thienylenevinylene and capped by dicyanovinylene groups as acceptor units.
Incorporation of electron-accepting dicyanovinylene moieties through the thiophene based
bridges shift the absorption profiles bathocromically to the NIR due to an intense intramolecular
charge transfer band. 1a and 1b exhibit not only excellent light harvesting properties but also
thermal stability and low-lying HOMO levels at -5.50 and -5.36 eV. Photovoltaic devices
incorporating 1b blended with PC BM displayed a moderate PCE of 3.21% compared to that of
71
1
a (PCE of 1.4%). Extension of the conjugation in 1b results in a significantly much higher Jsc
and FF compared to that of 1a. In order to chase higher efficiencies, further variations in the
molecular structure are currently underway in our laboratories.
Acknowledgements
Financial support from the Ministry of Science and Innovation of Spain, (CTQ2013-48252-P)
and Junta de Comunidades de Castilla-La Mancha (PEII-2014-014-P) is gratefully
acknowledged. SA thanks to the Fundación Carolina for a grant. LE wishes to thank the Robert
A. Welch Foundation for an endowed chair (AH-0033) and the National Science Foundation,
grant DMR-1205302 (PREM Program) for generous financial support.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
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Figure 1. Structures of 1a and 1b.
Scheme 1. Synthetic route to chromophores 1a and 1b.
Scheme 2. Synthesis of precursor aldehydes 3a,b.
Figure 2. Normalized UV-Vis spectra of 1a (a) and 1b (b) in DCM solution (black line) and in
thin films (red line).
Figure 3. CV plot of compound 1a.
Figure 4. Optimized geometries of dyes 1a and 1b.
Figure 5. Electronic density contours and energy levels for HOMO-1, HOMO, LUMO, and
LUMO+1 calculated at the B3LYP/6-31G** level for dyes 1a and 1b compared with the energy
levels of the PC BM and PC BM frontier orbitals.
6
1
71
Figure 6. EQE and J-V characteristics of the SMBHJ for 1a and 1b blending with PC BM at
61
different ratios.
Figure 7. EQE (a) and J-V (b) characteristics of the SMBHJ for 1b blending with PC BM and
6
1
PC BM in a 1:2 ratio.
71
Figure 8. Tapping mode AFM height images (2 µm x 2 µm) of 1a films spin-coated from
chlorobenzene blended with (a) PC BM and (b) PC BM; Tapping mode AFM height images
6
1
71
(
2 µm x 2 µm) of 1b films spin-coated from chlorobenzene blended with (c) PC BM and (d)
61
PC BM.
71
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•
•
•
The molecules 1a and 1b exhibit excellent light harvesting properties, thermal stability
and low-lying HOMO levels.
Photovoltaic devices incorporating 1b blended with PC BM displayed a moderate PCE
7
1
of 3.21% compared to that of 1a (PCE of 1.4%).
Extension of the conjugation in 1b results in a significantly much higher J and FF
sc
compared to that of 1a.