Synthesis and photovoltaic properties of new metalloporphyrin-containing polyplatinyne polymers

Article abstract of DOI:10.1021/ma2006206

Three new solution-processable platinum(II) polyyne polymers containing zinc(II) porphyrinate chromophores P1, P2, and P3 and their corresponding dinuclear model complexes were synthesized via the CuI-catalyzed dehydrohalogenation reaction of the platinum(II) chloride precursor and each of the respective bis(ethynyl)-zinc(porphyrin) metalloligands. The thermal, photophysical (absorption, excitation and emission spectra), electrochemical, and photovoltaic properties of P1-P3 were investigated. These results are also correlated by time-dependent density functional theory (TDDFT) calculations. The computations corroborate the presence of moderate conjugation in the π-systems, somewhat more accentuated for P3 where more favorable dihedral angles between the porphyrin and thiophene rings are noted. Moreover, the computed excited states are predicted to be π-π* in nature with some charge transfer components from the trans-[-C≡CPt(L)₂C≡ C-]_n unit to the porphyrin rings. The optical bandgaps range from 1.93 to 2.02 eV for P1-P3. Intense π-π*-localized fluorescence emissions typical of the Q-bands of the polymers were observed. The effect of thiophene ring along the polymer chain on the extent of π-conjugation, luminescent and photovoltaic properties of these metalated materials was also examined. Bulk heterojunction solar cells using these metallopolymers as an electron donor blended with a methanofullerene electron acceptor were studied. In one case, the metallopolymer P3 showed a power conversion efficiency of 1.04% with the open-circuit voltage of 0.77 V, short-circuit current density of 3.42 mA cm^-2 and fill factor of 0.39 under illumination of an AM 1.5 solar cell simulator.

Full text of DOI:10.1021/ma2006206

ARTICLE
pubs.acs.org/Macromolecules
Synthesis and Photovoltaic Properties of New
Metalloporphyrin-Containing Polyplatinyne Polymers
Hongmei Zhan,† Simon Lamare,‡ Annie Ng,§ Tommy Kenny,‡ Hannah Guernon,‡ Wai-Kin Chan,f
Aleksandra B. Djuriꢀsiꢁc,*,§ Pierre D. Harvey,*,‡ and Wai-Yeung Wong*,†
†Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and
Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Kowloon Tong,
Kowloon, Hong Kong, P.R. China
‡Dꢁepartement de Chimie, Universitꢁe de Sherbrooke, 2550 Boul. Universitꢁe, Sherbrooke, PQ, J1K 2R1 Canada
§Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China
fDepartment of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China
S Supporting Information
b
ABSTRACT: Three new solution-processable platinum(II)
polyyne polymers containing zinc(II) porphyrinate chromo-
phores P1, P2, and P3 and their corresponding dinuclear model
complexes were synthesized via the CuI-catalyzed dehydroha-
logenation reaction of the platinum(II) chloride precursor and
each of the respective bis(ethynyl)-zinc(porphyrin) metalloli-
gands. The thermal, photophysical (absorption, excitation and
emission spectra), electrochemical, and photovoltaic properties
of P1ꢀP3 were investigated. These results are also correlated
by time-dependent density functional theory (TDDFT) calcu-
lations. The computations corroborate the presence of moderate conjugation in the π-systems, somewhat more accentuated for P3
where more favorable dihedral angles between the porphyrin and thiophene rings are noted. Moreover, the computed excited states
are predicted to be πꢀπ* in nature with some charge transfer components from the trans-[ꢀCtCPt(L)2CtCꢀ]n unit to the
porphyrin rings. The optical bandgaps range from 1.93 to 2.02 eV for P1ꢀP3. Intense πꢀπ*-localized fluorescence emissions
typical of the Q-bands of the polymers were observed. The effect of thiophene ring along the polymer chain on the extent of
π-conjugation, luminescent and photovoltaic properties of these metalated materials was also examined. Bulk heterojunction solar
cells using these metallopolymers as an electron donor blended with a methanofullerene electron acceptor were studied. In one case,
the metallopolymer P3 showed a power conversion efficiency of 1.04% with the open-circuit voltage of 0.77 V, short-circuit current
density of 3.42 mA cmꢀ2 and fill factor of 0.39 under illumination of an AM 1.5 solar cell simulator.
’ INTRODUCTION
electrons results in minimal structural change.2 Porphyrin has
rich and extensive optical absorption in the visible spectrum and
high mobility.3ꢀ5 The typical absorption spectra of porphyrin units
exhibit sharp and strong Soret bands (410ꢀ430 nm) and weak
Q-bands (530ꢀ540 nm) without absorption features between
them. In addition, efficient photoinduced electron separation and
transfer between porphyrins and fullerene derivatives have also
been reported.6ꢀ8 Therefore, organic solar cells using porphyrin-
containing molecules,9ꢀ11 oligomers,12 and polymers13ꢀ15as photo-
active layers have been extensively investigated in recent years.
Unfortunately, only low power conversion efficiencies (PCEs)
were obtained so far. For example, the PCEs of the devices based
onporphyrin triad,9 liquid-crystalline porphyrin,4 porphyrin den-
drimer,10 porphyrin-containing oligomers,12 and main-chain
Organic solar cells have attracted much attention in recent
years owing to their advantages of low-cost fabrication by
solution processing and easy chemical tailoring, as well as
potential applications in flexible, lightweight and large-area
energy-harvesting devices. Bulk heterojunction devices fabri-
cated by simply blending organic donor materials and methano-
fullerene acceptor materials, such as [6,6]-phenyl C61-butyric
acid methyl ester (PCBM), in the same organic solvents has
become one of the most successful device architectures devel-
oped in this field, which result in the efficient photoinduced
electron transfer from donor materials to fullerene derivatives.1
Most of the efforts have been focused on developing novel donor
materials with sufficient solubility, low optical band gap and high
hole mobility, which would lead to efficient solar cells.
Porphyrins contain an extensively conjugated two-dimen-
sional π-system which renders them suitable for light-harvesting
and efficient electron transfer because the uptake or release of
Received: March 17, 2011
Revised:
May 22, 2011
Published: June 13, 2011
r
2011 American Chemical Society
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porphyrin polymers14 are only 0.035%, 0.775%, 0.32%, 0.000081%
and 0.3%, respectively. More recently, new advances have been
realized using supramolecular and nanocomposite systems. Novel
organic photovoltaic systems using supramolecular complexes of
porphyrin-peptide oligomers with fullerene clusters assembled as
three-dimensional arrays onto SnO2 films have been constructed
with the PCE reaching 1.6%16 whereas photovoltaic cells using
composite nanoclusters of porphyrins and fullerenes with gold
nanoparticles were also developed to afford a PCE of 1.5%.3
On the other hand, platinum metallopolyyne polymers of the
form trans-[ꢀPt(L)2CtCRCtCꢀ]n (L is an auxiliary phos-
phine ligand, and R is an aromatic spacer unit) have attracted a
great deal of research attention. The interest derives from the fact
that incorporation of platinum into the conjugated polymers can
result in good overlap between the d-orbital of Pt with the
p-orbital of the alkyne unit, and so the two alkyne units can
mutually interact through the Pt dxy and dyz orbitals, which lead
to efficient electronic π-conjugation and delocalization along the
polymer chain.17 Moreover, due to strong spinꢀorbit coupling
resulting from the presence of platinum, intersystem crossing is
enhanced which enables the spin-forbidden triplet emission to
become partially allowed,18ꢀ24 and can extend the exciton
diffusion length. These features enable platinum-containing
metallopolyyne polymers more feasible to be used as the donor
materials in photovoltaic cells, and encouraging progress has
been made in recent years.25ꢀ28
In view of the considerations above, the use of metallopor-
phyrins as the building block in combination with linear con-
jugated systems of transition metal-alkyne polymers for the
design of new p-type photovoltaic active materials serves as a
good illustration of the recent trend toward solution-processable
functional polymers. We present here the synthesis, character-
ization and theoretical modeling studies of several platinum
polyyne polymers coupled with zinc(II) porphyrinate chromo-
phores, and their photovoltaic properties have been investigated.
The introduction of thiophene unit into the porphyrin-based
polymer main chain is expected to extend the π-conjugation and
cover the missing absorption region (430ꢀ530 nm) or enhance
the absorption of the weaker Q-bands. Phenyl rings at positions
10 and 20 of the zinc porphyrin ring are used for enhancing the
molecular ordering, hence improving the interpenetrating
network with the fullerene derivative. This work represents
the first example of porphyrin-containing polymetallaynes
used for harvesting solar energy in solution-processed photo-
voltaic devices.
General Synthetic Procedures of Porphyrin Com-
pounds.14 5,15-Bis(1,4-trimethylsilylethynylbenzene)-10,20-bis(phenyl)-
porphyrin (L1H-TMS). A solution of 4-(2-(trimethylsilyl)ethynyl)-
benzaldehyde (119 mg, 0.59 mmol) and meso-phenyldipyrromethane
(139 mg, 0.59 mmol) in CH2Cl2 (60 mL) was purged with nitrogen for
30 min, and then trifluoroacetic acid (TFA) (47 mg, 0.41 mmol) was
added. The mixture was stirred for 3 h at room temperature, and then
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (267 mg, 1.2 mmol)
was added. After the mixture was stirred at room temperature for an
additional 30 min, the reaction was quenched by adding triethylamine
(0.6 mL). The solvent was removed, and the residue was purified by flash
column chromatography on silica gel using CH2Cl2/hexane (1:1, v/v) as
the eluent. Recrystallization from CH2Cl2/methanol gave L1H-TMS as
a purple solid (45 mg, 11%). ν(CtC) 2153 cmꢀ1. 1H NMR (CDCl3,
400 MHz, δ): 8.86 (d, J = 5.2 Hz, 4H, Ar), 8.81 (d, J = 5.0 Hz, 4H, Ar),
8.21ꢀ8.19 (m, 4H, Ar), 8.18ꢀ8.15 (m, 4H, Ar), 7.86 (d, J = 8.1 Hz, 4H,
Ar), 7.78ꢀ7.74 (m, 6H, Ar), 0.38 (s, 18H, TMS), ꢀ2.82 (s, 2H, NH)
ppm. FAB-MS: m/z 807.5 (Mþ).
The same procedures were applied to prepare L2H-TMS and L3H-
TMS from their corresponding aromatic aldehyde derivatives and meso-
phenyldipyrromethane.
5,15-Bis(1,4-(2,5-trimethylsilylethynylthienyl)benzene)-10,20-bis-
(phenyl)porphyrin (L2H-TMS). Purple solid, yield: 10%. ν(CtC)
2141 cmꢀ1. 1HNMR(CDCl3, 400MHz, δ): 8.91ꢀ8.89(m, 4H, Ar), 8.87ꢀ
8.85 (m, 4H, Ar), 8.21 (d, J = 7.8 Hz, 8H, Ar), 7.94 (d, J = 7.9 Hz, 4H,
Ar), 7.77ꢀ7.74 (m, 6H, Ar), 7.45 (d, J = 3.7 Hz, 2H, Ar), 7.35 (d, J =
3.7 Hz, 2H, Ar), 0.31 (s, 18H, TMS), ꢀ2.76 (s, 2H, NH) ppm. FAB-MS:
m/z 971.5 (Mþ).
5,15-Bis(2,5-trimethylsilylethynylthiophene)-10,20-bis(phenyl)por-
phyrin (L3H-TMS). Purple solid, yield: 14%. ν(CtC) 2144 cmꢀ1. 1H
NMR (CDCl3, 400 MHz, δ): 9.06 (d, J = 4.7 Hz, 4H, Ar), 8.84 (d, J =
4.7 Hz, 4H, Ar), 8.20ꢀ8.19 (m, 4H, Ar), 7.81ꢀ7.77 (m, 6H, Ar), 7.75 (d, J=
3.6 Hz, 2H, Ar), 7.64 (d, J = 3.6 Hz, 2H, Ar), 0.35 (s, 18H, TMS), ꢀ2.76
(s, 2H, NH) ppm. FAB-MS: m/z 819.4 (Mþ).
General Synthetic Procedures of ZnꢀPorphyrin Com-
plexes. Zinc(II) 5,15-Bis(1,4-trimethylsilylethynylbenzene)-10,20-bis-
(phenyl)porphyrin (L1-TMS). To a solution of L1H-TMS (41 mg, 0.05
mmol) in CH2Cl2 (16 mL) was added a solution of Zn(OAc)2 H2O
3
(25 mg, 0.11 mmol) in methanol (1 mL). The reaction mixture was stirred
at room temperature for 5 h. Evaporation of the solvent and purification
by column chromatography using CH2Cl2/hexane (1:1, v/v) as the
eluent afforded the product L1-TMS (44 mg, 98%) as a purple solid.
ν(CtC) 2156 cmꢀ1. 1H NMR (CDCl3, 400 MHz, δ): 8.97 (d, J = 4.8 Hz,
4H, Ar), 8.91 (d, J = 4.8 Hz, 4H, Ar), 8.22ꢀ8.19 (m, 4H, Ar), 8.17ꢀ8.15
(m, 4H, Ar), 7.87 (d, J = 8.0 Hz, 4H, Ar), 7.77ꢀ7.75 (m, 6H, Ar), 0.38 (s,
18H, TMS) ppm. 13CNMR(CDCl3, 125 MHz, δ): 150.21, 149.81, 143.03,
142.56, 134.29, 134.19, 132.18, 131.67, 130.15, 128.82, 127.51, 126.51,
121.31, 120.27 (Ar), 105.02, 95.32 (CtC), 0.09 (TMS) ppm. FAB-MS:
m/z 869.4 (Mþ).
’ EXPERIMENTAL SECTION
The same procedures were used to prepare L2-TMS and L3-TMS
from their corresponding starting materials L2H-TMS and L3H-TMS.
Zinc(II) 5,15-Bis(1,4-(2,5-trimethylsilylethynylthienyl)benzene)-10,20-bis-
(phenyl)porphyrin (L2-TMS). Purple solid, yield: 93%. ν(CtC)
2142 cmꢀ1. 1H NMR (CDCl3, 400 MHz, δ): 9.02 (m, 4H, Ar), 8.97ꢀ
8.95 (m, 4H, Ar), 8.24ꢀ8.21 (m, 8H, Ar), 7.95 (d, J = 7.9 Hz, 4H, Ar),
7.79ꢀ7.73 (m, 6H, Ar), 7.45 (d, J = 3.8 Hz, 2H, Ar), 7.35 (d, J = 3.8 Hz,
2H, Ar), 0.32 (s, 18H, TMS) ppm. 13C NMR (CDCl3, 125 MHz, δ):
150.36, 150.12, 145.70, 142.76, 135.11, 134.49, 134.10, 132.96, 132.28,
131.87, 128.96, 127.61, 126.66, 124.10, 123.38, 122.77, 121.43, 120.47 (Ar),
99.99, 97.82 (CtC), 0.09 (TMS) ppm. FAB-MS: m/z 1033.4 (Mþ).
Zinc(II) 5,15-Bis(2,5-trimethylsilylethynylthiophene)-10,20-bis(phenyl)-
porphyrin (L3-TMS). Purple solid, yield: 96%. ν(CtC) 2145 cmꢀ1. 1H
NMR (CDCl3, 400 MHz, δ): 9.16 (d, J = 4.7 Hz, 4H, Ar), 8.94 (d, J =
4.7 Hz, 4H, Ar), 8.21ꢀ8.19 (m, 4H, Ar), 7.80ꢀ7.76 (m, 6H, Ar), 7.74
Materials. All reactions were carried out under a nitrogen atmo-
sphere by using standard Schlenk techniques. Solvents were dried and
distilled from appropriate drying agents under an inert atmosphere prior
to use. Glassware was oven-dried at about 120 °C. All reagents and
chemicals, unless otherwise stated, were purchased from commercial
sources and used without further purification. 5-Bromothiophene-2-
carbaldehyde29 and meso-phenyldipyrromethane30 trans-[PtCl(Ph)-
(PEt3)2]31 and trans-[Pt(PBu3)2Cl2]32 were prepared according to the
literature methods. All reactions were monitored by thin-layer chroma-
tography (TLC) with Merck precoated glass plates. Flash column
chromatography and preparative TLC were carried out using silica gel
from Merck (230ꢀ400 mesh).
Syntheses. The syntheses of all the ligand precursors are given in
the Supporting Information.
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ARTICLE
(d, J = 3.6 Hz, 2H, Ar), 7.64 (d, J = 3.6 Hz, 2H, Ar), 0.35 (s, 18H, TMS)
ppm. 13C NMR (CDCl3, 125 MHz, δ): 150.81, 150.61, 145.51, 142.43,
134.43, 133.27, 132.56, 131.90, 131.80, 127.74, 126.69, 125.18, 122.11,
111.47 (Ar), 99.92, 97.61 (CtC) ppm. FAB-MS: m/z 881.2 (Mþ).
Synthesis of Zn(II) Porphyrinate Ligands. The ligands were synthe-
sized by deprotection reaction of L1-TMS to L3-TMS using tetrabuty-
lammonium fluoride (TBAF) as a base in THF. A typical example was
given for L1.
Zinc(II) 5,15-Bis(1,4-ethynylbenzene)-10,20-bis(phenyl)porphyrin (L1).
TBAF (0.13 mL, 1 M in THF) was added to a stirred solution of L1-
TMS (54 mg, 0.06 mmol) in THF (8 mL). After stirring for 5 min, water
(30 mL) was added to quench the reaction. The solution was extracted
with chloroform, washed with water and dried over anhydrous MgSO4.
After evaporation of the solvent, the residue was purified by column
chromatography using CH2Cl2/hexane (1.2:1, v/v) as the eluent to give
L1 (41 mg, 90%) as a purple solid. IR (KBr): ν(CtC) 2106 cmꢀ1. 1H
NMR (CDCl3, 400 MHz, δ): 8.97ꢀ8.95 (m, 4H, Ar), 8.93 (m, 4H, Ar),
8.22ꢀ8.17 (m, 8H, Ar), 7.88 (d, J = 7.8 Hz, 4H, Ar), 7.77ꢀ7.74 (m, 6H,
Ar), 3.31 (s, 2H, CtCH) ppm. 13C NMR (CDCl3, 125 MHz, δ): 150.29,
149.83, 143.44, 142.62, 134.40, 134.32, 132.29, 132.06, 131.63, 130.08,
127.59, 126.59, 121.41, 120.18 (Ar), 83.73, 78.16 (CtC) ppm. FAB-
MS: m/z 725.0 (Mþ). Anal. Calcd for C48H28N4Zn: C, 79.39; H, 3.89;
N, 7.72. Found: C, 79.21; H, 4.05; N, 7.57.
Zinc(II) 5,15-Bis(1,4-(2,5-ethynylthienyl)benzene)-10,20-bis(phenyl)por-
phyrin (L2). Purple solid, yield: 85%. IR (KBr): ν(CtC) 2098 cmꢀ1. 1H
NMR (CDCl3, 400 MHz, δ): 9.02ꢀ9.00 (m, 4H, Ar), 8.98ꢀ8.95 (m,
4H, Ar), 8.25ꢀ8.22 (m, 8H, Ar), 7.97 (d, J = 8.1 Hz, 4H, Ar), 7.77 (m,
6H, Ar), 7.48 (d, J = 3.8 Hz, 2H, Ar), 7.41 (d, J = 3.7 Hz, 2H, Ar), 3.49 (s,
2H, CtCH) ppm. 13C NMR (CDCl3, 125 MHz, δ): 145.56, 142.18,
142.12, 141.98, 135.27, 134.63, 134.11, 133.16, 127.86, 127.82, 126.80,
126.78, 124.22, 123.47, 122.85, 120.44, 120.32, 119.47 (Ar), 100.07,
97.78 (CtC) ppm. FAB-MS: m/z 889.4 (Mþ). Anal. Calcd for C56H32-
N4S2Zn: C, 75.54; H, 3.62; N, 6.29. Found: C, 75.34; H, 3.79; N, 6.42.
Zinc(II) 5,15-Bis(2,5-ethynylthiophene)-10,20-bis(phenyl)porphyrin
(L3). Purple solid, yield: 87%. ν(CtC) 2101 cmꢀ1. 1H NMR (CDCl3,
400 MHz, δ): 9.17 (d, J = 4.7 Hz, 4H, Ar), 8.96 (d, J = 4.7 Hz, 4H, Ar),
8.21ꢀ8.19 (m, 4H, Ar), 7.79ꢀ7.77 (m, 8H, Ar), 7.68 (d, J = 3.6 Hz, 2H,
Ar), 3.57 (s, 2H, CtCH) ppm. 13C NMR (CDCl3, 125 MHz, δ):
150.75, 150.61, 145.75, 142.36, 134.38, 133.11, 132.58, 132.22, 131.74,
127.72, 126.66, 123.99, 122.13, 111.26 (Ar), 82.13, 77.10 (CtC) ppm.
FAB-MS: m/z 737.2 (Mþ). Anal. Calcd for C44H24N4S2Zn: C, 71.59; H,
3.28; N, 7.59. Found: C, 71.68; H, 3.50; N, 7.45.
7.44 (m, 2H, Ar), 7.00 (m, 2H, Ar), 2.24ꢀ2.22 (m, 12H, PBu3),
1.71ꢀ1.67 (m, 12H, PBu3), 1.61ꢀ1.54 (m, 12H, PBu3), 1.01 (t, J =
7.2 Hz, 18H, PBu3) ppm. 31P NMR (CDCl3, 162 Hz, δ): 3.42 (1JPꢀPt
=
2328 Hz) ppm. Anal. Calcd for (C80H84N4P2S2PtZn)n: C, 64.57; H,
5.69; N, 3.76. Found: C, 64.76; H, 5.85; N, 3.68. GPC (THF): Mw =
50820, Mn = 13620, PDI = 3.73, DP = 9.
1
P3. Green solid, yield: 35%. IR (KBr): ν(CtC) 2084 cmꢀ1. H
NMR (CDCl3, 400 MHz, δ): 9.30ꢀ9.25 (m, 4H, Ar), 8.93ꢀ8.88 (m,
4H, Ar), 8.21ꢀ8.19 (m, 4H, Ar), 7.82ꢀ7.66 (m, 8H, Ar), 7.26 (m, 2H,
Ar), 2.29ꢀ2.18 (m, 12H, PBu3), 1.73ꢀ1.70 (m, 12H, PBu3), 1.57ꢀ1.53
(m, 12H, PBu3), 1.03ꢀ0.88 (m, 18H, PBu3) ppm. 31P NMR (CDCl3,
162 Hz, δ): 3.30 (1JPꢀPt = 2325 Hz) ppm. Anal. Calcd for
(C68H76N4P2S2PtZn)n: C, 61.14; H, 5.73; N, 4.19. Found: C, 61.34;
H, 5.56; N, 4.24. GPC (THF): Mw = 112250, Mn = 32470, PDI = 3.46,
DP = 24.
Synthesis of Platinum Model Complexes (M1ꢀM3). All of them
were synthesized following the dehydrohalogenating coupling between
trans-[PtCl(Ph)(PEt3)2] and their corresponding diterminal alkynes. A
typical procedure was given for M1 starting from L1.
To a solution of L1 (6 mg, 0.008 mmol) and trans-[PtCl(Ph)(PEt3)2]
(10 mg, 0.018 mmol) in Et3N (2 mL) and CH2Cl2 (2 mL) was added
CuI (1.0 mg) under nitrogen. After stirring overnight at room tempera-
ture, all volatile components were removed under reduced pressure. The
residue was dissolved in CH2Cl2 and purified by preparative silica TLC
plates using CH2Cl2/hexane as the eluent. The product M1 was
obtained as a purple solid (6 mg, 46%). ν(CtC) 2091 cmꢀ1 1H
;
NMR (CDCl3, 400 MHz, δ): 9.04 (d, J = 4.6 Hz, 4H, Ar), 8.93 (d, J =
4.7 Hz, 4H, Ar), 8.24ꢀ8.22 (m, 4H, Ar), 8.05 (d, J = 8.1 Hz, 4H, Ar),
7.79ꢀ7.73 (m, 6H, Ar), 7.69ꢀ7.67 (m, 4H, Ar), 7.40 (d, J = 7.1 Hz, 4H,
Ar), 7.02 (t, J = 7.4 Hz, 4H, Ar), 6.85 (t, J = 7.2 Hz, 2H, Ar), 1.92ꢀ1.87
(m, 24H, PEt3), 1.25ꢀ1.21 (m, 36H, PEt3) ppm; 31P NMR (CDCl3,
162 Hz, δ): 10.08 (1JPꢀPt = 2639 Hz) ppm; FAB-MS: m/z 1739.7 (Mþ).
Anal. Calcd for C84H96N4P4Pt2Zn: C, 57.95; H, 5.56; N, 3.22. Found: C,
58.12; H, 5.43; N, 3.45.
M2. Purple solid, yield: 38%. ν(CtC) 2079 cmꢀ1. 1H NMR (CDCl3,
400 MHz, δ): 9.05ꢀ9.03 (m, 4H, Ar), 8.96ꢀ8.94 (m, 4H, Ar), 8.22 (d,
J = 6.5 Hz, 4H, Ar), 8.18 (d, J = 7.7 Hz, 4H, Ar), 7.93 (d, J = 8.1 Hz, 4H,
Ar), 7.79ꢀ7.75 (m, 6H, Ar), 7.41 (d, J = 3.7 Hz, 2H, Ar), 7.35 (d, J =
7.0 Hz, 4H, Ar), 7.01ꢀ6.97 (m, 6H, Ar), 6.83 (t, J = 7.2 Hz, 2H, Ar),
1.84ꢀ1.74 (m, 24H, PEt3), 1.25ꢀ1.19 (m, 36H, PEt3) ppm. 31P NMR
(CDCl3, 162 Hz, δ): 10.01 (1JPꢀPt = 2627 Hz) ppm. MALDIꢀTOF:
m/z 1905.4798 [M þ H]þ, calculated: 1904.4922. Anal. Calcd for
C92H100N4P4S2Pt2Zn: C, 57.99; H, 5.29; N, 2.94. Found: C, 57.76; H,
5.34; N, 3.20.
Synthesis of Platinum Polyyne Polymers (P1ꢀP3). The polymers
were prepared by the dehydrohalogenative polycondensation between
trans-[Pt(PBu3)2Cl2] and each of the ligands (L1ꢀL3). A typical
procedure was given for P1 starting from L1.
M3. Purple solid, yield: 48%. ν(CtC) 2081 cmꢀ1. 1H NMR (CDCl3,
400 MHz, δ): 9.30 (d, J = 4.7 Hz, 4H, Ar), 8.92 (d, J = 4.7 Hz, 4H, Ar),
8.22ꢀ8.19 (m, 4H, Ar), 7.79ꢀ7.73 (m, 6H, Ar), 7.64 (d, J = 3.5 Hz, 2H,
Ar), 7.36 (d, J = 7.0 Hz, 4H, Ar), 7.27 (d, J = 3.5 Hz, 2H, Ar), 6.99 (t, J =
7.4 Hz, 4H, Ar), 6.83 (t, J = 7.2 Hz, 2H, Ar), 1.87ꢀ1.80 (m, 24H, PEt3),
1.18ꢀ1.12 (m, 36H, PEt3) ppm. 31P NMR (CDCl3, 162 Hz, δ): 10.03
(1JPꢀPt = 2630 Hz) ppm. FAB-MS: m/z 1751.7 (Mþ). Anal. Calcd for
C80H92N4P4S2Pt2Zn: C, 54.81; H, 5.29; N, 3.20. Found: C, 54.98; H,
5.10; N, 3.12.
Physical Measurements. Fast atom bombardment (FAB) mass
spectra were recorded on a Finnigan MAT SSQ710 system. NMR
spectra were measured in CDCl3 on a Bruker AVANCE 400 MHz FT-
NMR spectrometer using tetramethylsilane as an internal standard for
1H and 13C nuclei or 85% H3PO4 as an external standard for 31P nucleus.
UVꢀvisible spectra were obtained on an HP-8453 diode array spectro-
photometer. The solution emission spectra of the compounds were
measured on a Photon Technology International (PTI) Fluorescence
QuantaMaster Series QM1 spectrophotometer. Thermogravimetric
analysis (TGA) measurements were performed on thermal gravimetric
analyzer (model Perkin-Elmer TGA-6) under a nitrogen flow at a
Polymerization was carried out by mixing L1 (30 mg, 0.04 mmol),
trans-[Pt(PBu3)2Cl2] (28 mg, 0.04 mmol) and CuI (3.00 mg) in Et3N/
CH2Cl2 (12 mL, 1:1, v/v). After stirring at room temperature for 24 h
under nitrogen, the solution mixture was evaporated to dryness. The
residue was redissolved in CH2Cl2 and filtered through a short alumina
column using the same eluent to remove ionic impurties and catalyst
residues. After removal of the solvent, the crude product was purified by
precipitation in CH2Cl2 from MeOH twice to give the polymer P1
(15 mg, 27%) as a purple solid. IR (KBr): ν(CtC) 2099 cmꢀ1. 1H NMR
(CDCl3, 400 MHz, δ): 9.05ꢀ8.95 (m, 8H, Ar), 8.24ꢀ8.09 (m, 8H, Ar),
7.78ꢀ7.71 (m, 10H, Ar), 2.42ꢀ2.38 (m, 12H, PBu3), 1.70ꢀ1.58 (m,
12H, PBu3), 1.51ꢀ1.42 (m, 12H, PBu3), 0.94 (t, 18H, PBu3) ppm. 31
P
NMR (CDCl3, 162 Hz, δ): 3.16 (1JPꢀPt = 2352 Hz) ppm. Anal. Calcd for
(C72H80N4P2PtZn)n: C, 65.32; H, 6.09; N, 4.23. Found: C, 65.45; H,
5.87; N, 4.10. GPC (THF): Mw = 22325, Mn = 9500, PDI = 2.35, DP = 7.
1
P2. Purple solid, yield: 32%. IR (KBr): ν(CtC) 2086 cmꢀ1. H
NMR (CDCl3, 400 MHz, δ): 9.05ꢀ9.03 (m, 4H, Ar), 8.98ꢀ8.95 (m,
4H, Ar), 8.22ꢀ8.20 (m, 8H, Ar), 7.95 (m, 4H, Ar), 7.78ꢀ7.76 (m, 6H, Ar),
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Scheme 1. Synthetic Routes to Ligand Precursors
heating rate of 15 °C minꢀ1. The gel permeation chromatography
(GPC) measurements were performed on the Agilent 1050 HPLC
system with VWD, using THF as eluent and polystyrene standards as
calibrants. Electrochemical measurements were carried out on a deoxy-
genated solution of [nBu4N]PF6 (0.1 M) in acetonitrile using a
computer-controlled electrochemical workstation, a eDAQ EA161
potentiostat electrochemical interface equipped with a thin film coated
indium tin oxide (ITO) covered glass working electrode, a Pt wire as the
counter electrode, and a Ag/AgCl (in 3 M KCl) as the reference
electrode (at the scan rate of 100 mV sꢀ1). Polymer film was prepared
by dipping a glass plate in the polymer solution of chlorobenzene and
then dried under vacuum, and one side of the glass plate had been casted
ITO film beforehand. The onset oxidation and reduction potentials were
used to determine the HOMO and LUMO energy levels using the equa-
tions EHOMO =[ꢀ(Eonset, ox (vs Ag/AgCl) ꢀ Eonset (NHE vs Ag/AgCl))] ꢀ 4.50 eV
and ELUMO = [ꢀ(Eonset, red (vs Ag/AgCl) ꢀ Eonset (NHE vs Ag/AgCl))] ꢀ 4.50 eV,
where the potentials for NHE versus vacuum and NHE versus Ag/AgCl
are 4.50 and ꢀ0.22 V, respectively.33
active area of the devices (2 mm diameter circle). All the fabrication
procedures (except drying, PEDOT:PSS annealing, and Al deposition)
and cell characterization were performed in air. PCE was determined
from JꢀV curve measurement (using a Keithley 2400 sourcemeter)
under white light illumination (at 100 mW cmꢀ1). For white light
efficiency measurements, an Oriel 66002 solar light simulator with
an AM1.5 filter was used. The light intensity was measured by a
Molectron Power Max 500D laser power meter.
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The three aromatic alde-
hyde derivatives were prepared from 4-bromobenzaldehyde and
thiophene-2-carbaldehyde according to the pathways depicted in
Scheme 1. Following appropriate chemical modifications of the
published synthetic procedures, the key trans-substituted por-
phyrins L1H-TMS, L2H-TMS and L3H-TMS were synthesized
by the acid-catalyzed condensation of meso-phenyldipyrro-
methane with the aromatic aldehyde derivatives, followed by
oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) at room temperature.14 The free base porphyrins were
then coordinated with zinc(II) ion from zinc acetate in the
solvent mixture of CH2Cl2 and MeOH to form the trimethylsi-
lylethynyl-containing zinc(II) porphyrins L1-TMS to L3-TMS
in high yields. The diethynyl ligands L1, L2 and L3 were then
prepared by removing the protecting groups with tetrabutylam-
monium fluoride (TBAF) as shown in Scheme 2.
Scheme 3 shows the synthetic routes toward Pt(II)-containing
Zn(II)-porphyrinate polymers and their corresponding model
complexes. By CuI-catalyzed dehydrohalogenation method,
model compounds M1ꢀM3 were prepared from ligands
L1ꢀL3 and trans-[PtCl(Ph)(PEt3)2] (in a 1:2.2 stoichiometry)
in CH2Cl2 and Et3N (1:1, v/v) in moderate yields. Using the
same approach, the reaction of metalloligands with trans-[Pt(n-
Bu3P)2Cl2] in a 1:1 ratio gave the respective polymers P1, P2,
and P3 in 27ꢀ35% yields. All of the metal alkynyl complexes and
polymers are stable and generally exhibit good solubility in
chlorocarbons CH2Cl2 and CHCl3. For P3, it is more soluble
in THF and chlorobenzene than in chloroalkanes.
Computational Details. Calculations were performed with the
Gaussian 0934 program at the Universitꢁe de Sherbrooke with Mam-
mouth super computer supported by le Rꢁeseau Quꢁebꢁecois de Calculs de
Haute Performances. The DFT35ꢀ38 and TDDFT39ꢀ41 were calculated
with the B3LYP42ꢀ44 method. 3-21G*45ꢀ50 basis sets were used for
C, H, S, N and Zn. Polarized VDZ (valence double ζ)51with SBKJC
effective core potentials52ꢀ55 were applied for Pt as well. The predicted
phosphorescence wavelengths were obtained by energy differences
between the total energy of the triplet and singlet optimized states.56
The calculated absorption spectra and related molecular orbital (MO)
contributions were obtained from the TDDFT/singlets output file and
Gausssum 2.1.57
Fabrication and Characterization of Bulk Heterojunction
Solar Cells. The device structure was ITO/poly(3,4-ethylene-dioxy-
thiophene):poly(styrenesulfonate) (PEDOT:PSS)/polymer:PCBM
blend/Al. Indium tin oxide (ITO) coated glass substrates (10 Ω per
square) were cleaned by sonication in toluene, acetone, ethanol, and
deionized water, dried in an oven, and then cleaned with UV ozone for
300 s. As-received PEDOT:PSS solution was passed through the
0.45 μm filter and spin-coated on patterned ITO substrates at
5000 rpm for 2 min, followed by baking in N2 at 120 °C for 20 min.
P1ꢀP3:PCBM (1:4 by weight) active layer was prepared by spin-
coating the chlorobenzene solution (4 mg mLꢀ1 of polymer, 16
mg mLꢀ1 of PCBM) at 1000 rpm for 2 min. The substrates were dried
at room temperature under high vacuum (10ꢀ5 to 10ꢀ6 Torr) for 2 h
and then stored in a glovebox under Ar atmosphere overnight. An Al
electrode (100 nm) was evaporated through a shadow mask to define the
FTIR spectra show that the ν(CtC) stretching frequencies
for the platinum polymers at about 2086 cmꢀ1 and model
compounds at about 2081 cmꢀ1 are lower than those for the
free diethynyl ligands near 2101 cmꢀ1, which reveal a higher
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Scheme 2. Synthetic Pathways to Diethynyl Ligands L1ꢀL3
Scheme 3. Synthetic Pathways to Pt(II) Polyynes P1ꢀP3 and Diynes M1ꢀM3
degree of conjugation in the former. Moreover, the absence of
the stretching vibrations for the terminal acetylenic CꢀH bonds
at around 3286 cmꢀ1 further confirms the successful formation
of platinumꢀcarbon bond. 31P NMR spectra of the platinum-
containing complexes and polymers exhibit a strong 31P singlet
signal flanked with two satellites, consistent with a trans-geometry
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Figure 1. Absorption (black), excitation (red) and fluorescence (blue) spectra of P1 (up), P3 (middle), and P2 (bottom) in 2-MeTHF at 298 (left) and
77 K (right).
of the square-planar Pt unit, and their 1JPꢀPt values between 2627
Table 1. Spectral (Absorption, Top; Fluorescence, Bottom)
and Photophysical Parameters
to 2639 Hz for the PEt3 moieties and 2325 and 2352 Hz for the
PBu3 moieties are typical of those for related trans-PtP2
systems.58 The Pt model complexes were also successfully
characterized by mass spectrometry in which the respective
molecular ion was observed in each case.
polymer in 2-MeTHF
polymer temp (K)
Soret, λ (nm)
Q-region, λ (nm)
P1
P3
P2
298
77
398 sh, 430
508 sh, 550, 593
522 sh, 558, 598
555, 607
Molecular weights of these polymers were determined by
GPC in THF solution using polystyrene standards for the
method calibration. P2 has a relatively higher degree of polym-
erization than that of P1. This is probably ascribed to the more
reactive terminal acetylenic CꢀH bond at the R-position of
thiophene ring than that of benzene ring in the dehydrohalo-
genative polymerization reaction. This is also corroborated with
the data of P3, which has a Mn of 32470 with a higher degree of
polymerization of 24.
342
406 sh, 432, 442
298
77
297, 348 434, 456 sh
364
377
382
438, 460
522, 562, 612
552, 610
298
77
401, 426
410, 432, 448
556, 600
298 K
77 K
polymer
λF (nm)
ΦF
τF (ns)
λF (nm)
τF (ns)
The thermal properties of the polymers were determined by
thermogravimetric analysis (TGA). The polymers P1, P2, and
P3 have relatively good thermal stability with onset decomposi-
tion temperatures of 305, 348, and 366 °C, respectively.
Absorption and Photoluminescence Spectra. The absorp-
tion and emission data for the three polymers are presented in
Figure 1 and Table 1. All the polymers show a sharp and strong
Soret band at about 430 nm (π f π* transition, S0 f S2) and a
set of weak Q-bands between 540 and 635 nm (π f π*
transition, S0 f S1), which is a typical absorption profile for
P1
P3
P2
608, 655
639
0.081
0.029
0.085
0.28
<0.10
0.35
612, 642 663, 699
644, 705
0.87
0.40
1.02
609, 655
615, 665
porphyrin-type compounds.60 Compared to the absorption
features of the corresponding diethynyl ligands, the Soret band
and Q-bands of the polymers occur with a slight red-shift of
1ꢀ5 nm. This observation illustrates that these polymers do not
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exhibit significant π-conjugation because the large arylꢀporphyrin
dihedral angles, which result from steric interactions with the
β-hydrogens, lead to the nonplanarity.59a The similar phenomenon
takes place in the absorption features of the model complexes
(see Supporting Information).
In the photoluminescence spectra, the three polymers exhibit
strong luminescence bands between 575 and 790 nm. Since
internal conversion is rapid, the photoluminescence seems to
occur at the onset positions of their absorption Q-bands.14,59b
Similarly, the luminescence bands of the polymers undergo a
relatively modest red-shift of about 12 nm as compared to that of
their corresponding ligands, indicating that the conjugation is
moderate.
TDDFT calculations (vide infra). No phosphorescence band was
observed in these polymers, even at 77 K. Despite the absence of
phosphorescence in P1, P2, and P3, the triplet states are none-
theless populated. In a recent work, a series of polymer closely
related to P1 of this work were investigated.59c Evidence for T1
population was made from the measurements of the weak
phosphorescence band at 77 K, but also from T1ꢀTn transient
absorption spectroscopy. In addition, Schanze and co-workers
demonstrated the involvement of the triplet state for energy
conversion in solar cells for polymer of the type (ꢀCtCꢀ
PtL2ꢀCtCꢀthiopheneꢀ)n (L = PBu3).25 They also demon-
strated the very rapid intersystem crossing rate constant (kISC
∼
1011
s
ꢀ1) in materials containing the fragment ArꢀCtCꢀ
The excitation spectra superimpose the absorption ones,
which indicate that the observed emissions arise from the species
generating the absorption spectra (i.e., there are no obvious
impurities). The fact that the emission lifetimes are in the short
ns time scale and that the energy gap between the lowest energy
absorption peak and the highest energy emission signal is relatively
small (i.e., notably 10ꢀ15 nm for P1 and P2) indicate that the
luminescence is indeed fluorescence. Moreover, the very short
fluorescence lifetimes, τF, are consistent with what is generally
encountered for low-bandgap Pt-containing polymers.59c,60
The dinuclear model complexes behave almost the same way
as described above for the polymers and no further description is
needed for these materials (see Supporting Information).
The comparison between P1 and P3 indicates more red-
shifted Soret band and Q-bands by 10ꢀ15 nm for the latter, and
by 30 nm or so for the emission bands for the latter as well,
witnessing the substituent effect of the thiophene residue onto
the ππ*-type transitions of the zinc(II)-porphyrin unit. This
effect may also be ascribed to the better dihedral angle between
the average planes for the thiophene versus the metalloporphyrin
one in P3, in comparison with the planes between the phenyl
versus metalloporphyrin ring in P1. In the absence of X-ray
crystal structures, density functional theory (DFT) geometry
optimizations were performed instead (vide infra). Indeed, the
calculated dihedral angle in P1 (average of 71.5°) is less favorable
for conjugation whereas in P3 this angle varies from 64 to 71°
(averaging 68.3°). The emission band for the polymer P3
appears broader than that for P1 exhibiting less vibronic struc-
tures even at 77 K. This is indicative of low-frequency vibrational
modes being FranckꢀCondon active in the emission band of P3.
One of the surprising features is that the absorption and
emission spectra for P2 are not more red-shifted with respect to
P1 and P3. In fact, the positions of the peaks are closer to that of
P1 for the Soret band and to P3 for the Q-bands (Table 1). The
shapes of the emission spectra of P2 are reminiscent of those for
P1 at 298 K and P3 at 77 K, indicating that both substituents
induce some effects on the luminescent state. The comparison of
the photophysical parameters is also presented in Table 1. The
striking feature is that both the fluorescence quantum yields, ΦF,
and lifetimes, τF, at 298 and 77 K for polymers P1 and P2 are very
similar, whereas those for P3 are very different. Overall, the
photophysical features of P2 are more similar to those for the P1
one. The reason for this notable resemblance may come from the
polymer structure itself where the immediate substituent around
the central zinc(porphyrin) chromophore is the same between
P1 and P2 (i.e., a phenyl group attached at the meso-position of
the zinc(porphyrin) in both cases). Evidence for weak conjuga-
tion associated with the large dihedral angle of the meso-phenyl
and -thiophene substituent is clearly addressed by means of
PtL2ꢀCtCꢀAr where L = PBu3 and Ar = one or many aromatic
groups.19b So, the triplet state is readily populated, but this is not
a guarantee that the materials will be phosphorescent.
Spectroscopic and Computational Analyses. The frontier
MOs going from HOMO-5 to LUMOþ3 are presented in
Figures 2ꢀ4 for the three polymers. The HOMO, HOMO-1,
HOMO-2 and HOMO-3 exhibit atomic contributions of the π-
systems spreading from a metalloporphyrin chromophore to one
or two Pt-containing spacer ꢀArꢀCtC-PtL2ꢀCtCꢀArꢀ (Ar =
C6H4, C4H2S, C6H4ꢀC4H2S; L = PBu3). This computed feature
suggests the presence of conjugation, which is consistent with
the substituent effect as discussed above, but also indicates that
the conjugation does not spread over a very long segment of the
chain, which is consistent with the lack of band shifting between
P1 and P2.
The MOs placed at lower energies such as HOMO-4 and
HOMO-5 for P1 and P3 exhibit rather localized atomic con-
tributions associated with π-systems centered on a metallopor-
phyrin chromophore. Similarly, the LUMO, LUMOþ1,
LUMOþ2, and LUMOþ3 exhibit atomic contributions pretty
much localized onto the zinc(porphyrin) unit. For P3, some
minor atomic contributions located on adjacent metalloporphyr-
ins are also computed. All in all, these computations suggest that
the excited states are localized on the zinc(porphyrin) unit. This
observation is consistent with the strong resemblance between
the absorption and fluorescence spectra of the polymers and a
single zinc(porphyrin) unit. The exception for P3 may be
explained, in part, by the presence of π-system atomic contribu-
tions over more than one zinc(porphyrin) unit in the LUMO,
LUMOþ1, LUMOþ2 and LUMOþ3 (Figure 3).
TDDFT was employed to address the nature of the lowest S1
excited states for the three polymers to see whether these
correspond to zinc(porphyrin)-localized ππ*-type electronic
transitions or spacer-to-porphyrin charge transfers as it would
simply be suggested by the HOMOs and LUMOs presented in
Figures 2ꢀ4. Tables 2ꢀ4 show the computed electronic transi-
tions along with their positions and oscillator strength (f). The
key feature is that the first 4 and 3 electronic transitions for P1
and P2, respectively, are all from lower energy HOMOs and
LUMO, LUMOþ1, LUMOþ2, or LUMOþ3. These are all
zinc(porphyrin)-localized ππ*-type electronic transitions. These
common features are consistent with the resemblance in the
spectral and photophysical data as discussed above. Conversely,
among the 10 lowest energy transitions computed for P3, two
are zinc(porphyrin)-localized ππ*-type electronic transitions
(numbers 1 and 3), and the others are spacer-to-porphyrin charge-
transfer in nature. The lowest energy electronic transition is com-
puted at 566 nm and is of zinc(porphyrin)-localized ππ*-type.
The next transition is placed at 561 nm and is spacer-to-porphyrin
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Figure 2. Frontier MO representations for an oligomeric model compound of P1 containing three units.
Figure 3. Frontier MO representations for an oligomeric model compound of P3 containing three units.
charge transfer in origin. The close proximity of these two excited
states promotes mixing and therefore it would not be surpris-
ing to see charge transfer behavior such as that has recently
been observed for several polymers and oligomers containing
the ꢀArꢀCtCꢀPtL2ꢀCtCꢀArꢀ spacer (Ar = carbazole,
quinone diimine).60ꢀ63
The computed and observed lowest energy peaks are not equal
(Table 5). There is in fact a difference of an approximate
1500 cmꢀ1 (i.e., about 45 nm) between the two values. The
computed positions of the lowest energy transitions show a blue-
shift of about 45 nm; a difference that cannot be accounted for
only by the use of gas phase conditions in the calculations.
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Figure 4. Frontier MO representations for an oligomeric model compound of P2 containing three units.
Table 2. Computed Positions and Oscillator Strength (f) of the 10 Lowest-Energy Electronic Transitions of the Model Compound
Used for P1 as Shown in Figure 2
no.
ν (cmꢀ1
)
λ (nm)
f
major contributions (%)
1
2
18 237
18 253
18 369
18 436
19 503
19 511
19 703
19 709
21 568
21 720
548
548
544
542
517
513
508
507
464
460
0
H-6fLþ1 (87), H-5fLþ1 (11)
0.0001
0
H-6fLUMO (88)
3
H-4fLþ2 (97)
H-4fLþ3 (96)
4
0
5
0.0132
0.0605
0.0069
0.0181
0.1044
0.0011
H-5fLUMO (31), H-1fLþ1 (13), HOMOfLþ1 (30)
H-5fLþ1 (29), H-1fLUMO (13), HOMOfLUMO (32)
H-3fLþ3 (41), H-1fLþ2 (35), HOMOfLþ2 (16)
H-3fLþ2 (41), H-1fLþ3 (35), HOMOfLþ3 (17)
H-2fLþ3 (22), HOMOfLþ3 (60)
H-2fLþ2 (21), HOMOfLþ2 (68)
6
7
8
9
10
Table 3. Computed Positions and Oscillator Strength (f) of the 10 Lowest-Energy Electronic Transitions of the Model Compound
Used for P2 as Shown in Figure 4
no.
ν (cmꢀ1
)
λ (nm)
f
major contributions (%)
H-7fLUMO (31), H-5fLUMO (68)
1
2
18 077
18 122
18 211
18 213
19 177
19 245
19 328
19 328
20 393
20 455
553
552
549
549
521
520
517
517
490
489
0
0.0001
0
H-4fLþ1 (96)
3
H-7fLþ2 (30), H-5fLþ2 (70)
4
0
H-4fLþ3 (97)
5
0.4474
0.0002
0.0072
0.0092
0.1477
0.0118
H-6fLþ3 (12), H-1fLUMO (13), H-1fLþ1 (11), HOMOfLUMO (19), HOMOfLþ1 (20)
H-7fLþ2 (10), H-6fLþ3 (13), H-1fLUMO (15), HOMOfLUMO (13), HOMOfLþ1 (21)
H-7fLUMO (25), H-5fLUMO (11), H-1fLþ2 (25), HOMOfLþ2 (22)
H-6fLþ1 (32), H-2fLþ3 (14), H-1fLþ3 (18), HOMOfLþ3 (31)
H-3fLUMO (12), H-2fLUMO (10), HOMOfLUMO (43)
H-6fLþ3 (10), H-2fLþ1 (19), HOMOfLUMO (13), HOMOfLþ1 (40)
6
7
8
9
10
Previous works on this type of spacer demonstrated that the
difference between gas phase and solvated species is about several
nanometers.64,65
The computed spectra for the polymers using the model
compounds shown in Figures 2ꢀ4 are presented in Figure 5.
These are generated by computing the first 100 electronic
transitions and then a bar graph was generated by plotting the
intensity versus the position in nm. Many of these transitions
have no intensity. By assigning a thickness to these transitions,
one can generate the computed spectra as depicted in Figure 5.
The calculated absorption spectra (Figure 5) exhibit features
as discussed above. First, the positions of the lowest-energy electronic
transitions for P1 and P2 are almost the same, whereas that for P3 is
more red-shifted. The second striking feature lies in the presence of
two well separated peaks in the Soret region of P2 (red curve in
Figure 5). This split is clearly seen in the 77 K spectra of both the P3
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Table 4. Computed Positions and Oscillator Strength (f) of the 10 Lowest-Energy Electronic Transitions of the Model Compound
Used for P3 as Shown in Figure 3
no.
ν (cmꢀ1
)
λ (nm)
f
major contributions (%)
1
2
17 672
17 829
18 370
18 450
18 464
18 612
18 898
18 998
20 049
20 649
566
561
544
542
542
537
529
526
499
484
0.0001
0.0018
0.0015
0.4214
0.0023
0.0154
0.0095
0.0059
0.0056
0.0906
H-6fLUMO (61), H-5fLUMO (26), H-4fLUMO (12)
H-3fLþ1 (95)
3
H-6fLþ2 (62), H-5fLþ2 (24), H-4fLþ2 (14)
4
H-2fLþ3 (10), H-1fLUMO (16), HOMOfLUMO (19), HOMOfLþ1 (41)
H-3fLþ3 (94)
H-1fLUMO (28), HOMOfLUMO (20), HOMOfLþ1 (28)
H-2fLþ1 (30), H-1fLþ3 (11), HOMOfLþ3 (54)
H-5fLUMO (19), H-4fLUMO (13), H-1fLþ2 (37), HOMOfLþ2 (23)
H-4fLUMO (11), H-1fLUMO (13), HOMOfLUMO (47)
H-7fLþ1 (24), H-2fLþ3 (14), H-1fLUMO (11), H-1fLþ1 (15), HOMOfLþ1 (14)
5
6
7
8
9
10
Table 5. Comparison between the Experimental and Calcu-
lated Positions of the Lowest Energy Q-Bands.a
λabs (nm)
λabs (cmꢀ1
)
λcalcd
(cmꢀ1
b
77 K
77 K
λcalcd (nm)
)
Δ (cmꢀ1
)
P1
P3
P2
598
612
600
16 722
16 340
16 667
548
566
552
18 253
17 829
18 122
1531
1489
1455
a The 77 K data are selected for better accuracy. Only calculated data for
the gas phase are investigated. Previous experience demonstrated that
the incorporation of a solvent has a minor effect (several nm).64,65 b Δ is
the difference of the calculated and experimental positions in cmꢀ1
.
Figure 5. Computed spectra of P1 (green), P3 (blue), and P2 (red). Note
that no transition was calculated at wavelengths smaller than 290 nm or so.
This is due to the limitation in the computed electronic transitions (100).
and P2 spectra. These splits are due to exciton couplings.66 The
presence of such couplings indicates partial electronic communica-
tion that allows the transition moment of the electronic transition to
couple with the neighboring ones.
bandgaps (1.93ꢀ2.02 eV) within experimental error, as well as
the HOMOꢀLUMO energy gaps as obtained from the DFT
computations above (see Figures. 2ꢀ4). Compared to P1, poly-
mer P3 has a smaller energy gap because of the stronger electron-
donating nature of thiophene than that of benzene, which can
increase the energy of the HOMO level and hence decrease the
bandgap.68 From cyclic voltammetry, P2 has the lowest HOMO
(close to that for P1) and the lowest LUMO energy levels (close
to that for P3), which may be partially attributed to the difficulty
in estimating the true potentials for irreversible waves. The
relative electrochemical oxidation and reduction peak potentials
allow us to state which of the polymers is easier to oxidize and
which is easier to reduce. The oxidation peak potentials are P3
(þ0.81) < P1 (þ0.86) < P2 (þ0.90 V vs Ag/AgCl) indicating
that P3 is easier to oxidize, which corroborates the fact that this
system exhibits better π-conjugation as discussed above. The
DFT computations indicate that the corresponding model for P3
exhibits the highest HOMO level and hence it is also the easiest
to oxidize. The reduction peak potentials are P2 (ꢀ0.99) ≈ P3
(ꢀ1.01) < P1 (ꢀ1.08 V vs Ag/AgCl). Meanwhile, the DFT
computations indicate that the corresponding model for P3
exhibits the lowest LUMO level of the three computed models,
suggesting that P3 is predicted to be the easiest to reduce.
Experimentally, both P2 and P3 are the easiest ones to reduce
with close LUMO levels as determined by the electrochemical
method. Although the relative order for the computed LUMO
energies for P2 and P3 and the reduction peak positions appears
to be reversed, this can readily be explained by the fact that the
All in all, the TDDFT computations support the ππ* assign-
ments localized within the zinc(porphyrin) ring for P1 and P2. In
the case of P3, the presence of conjugation is demonstrated and
despite the fact that localized ππ* transition within the zinc(por-
phyrin) unit was also observed, some mixing with spacer-to-por-
phyrin charge transfer character is suspected due to the presence
of neighboring transitions placed close in energy, which is consistent
with the difference in band-shape of the fluorescence.
No phosphorescence band was observed in this work, even when
the time-resolved spectra were measured at 77 K. The calculated
positions of the phosphorescence peaks for the model complexes
of P1, P2, and P3 are 674.9, 752.8, and 981.1 nm, respectively.
Electrochemical Properties. Cyclic voltammetry was em-
ployed to determine the oxidation and reduction potentials and
qualitatively estimate the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
energy levels of the metalloporphyrin-containing polymetal-
laynes. Each of the three polymers shows the irreversible oxidation
and reductive waves, probably attributable to the presence of the
electron-rich porphyrin units.14 From Table 6, the Pt/Zn mixed-
metal polymers have HOMO energy level at ꢀ5.53 to ꢀ5.62 eV
and LUMO energy level at ꢀ3.64 to ꢀ3.73 eV, the latter of which
is more than 0.3 eV higher than that of the PCBM acceptor, indicat-
ing that the energy level positions of the donor and acceptor
are suitable for charge transfer and separation at the interface
between the donor and acceptor.67 The electrochemical band-
gaps between 1.82 and 1.94 eV matched well with their optical
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Table 6. Electrochemical Onset Potentials and Electronic Energy Levels of the Polymersa
polymer
λonset (nm)
Egopt (eV)
Eox/EHOMO (V)/(eV)
Ered/ELUMO (V)/(eV)
Egec (eV)
P1
P2
P3
616
619
643
2.02
2.00
1.93
0.86/ꢀ5.58
0.90/ꢀ5.62
0.81/ꢀ5.53
ꢀ1.08/ꢀ3.64
ꢀ0.99/ꢀ3.73
ꢀ1.01/ꢀ3.71
1.94
1.89
1.82
a Onset oxidation potential vs Ag/AgCl (Eox), onset reduction potential vs Ag/AgCl (Ered), bandgaps derived from the difference between onset
potentials of oxidation and reduction (Egec), EHOMO = ꢀ(Eox þ 4.72) eV, ELUMO = ꢀ(Ered þ 4.72) eV, and the optical bandgap was obtained from the
equation Egopt = 1240/λonset, where λonset is the onset value of the absorption spectrum in CH2Cl2.
Table 7. Photovoltaic Performance of Metalloporphyrin-
Containing Polyplatinynes
polymer/PCBM
Voc (V)
Jsc (mA cmꢀ2
)
FF
PCE (%)
P1 (1:4)
P2 (1:4)
P3 (1:4)
0.72
0.78
0.77
2.74
3.02
3.42
0.34
0.30
0.39
0.68
0.71
1.04
Figure 6. JꢀV curves of devices based on P1ꢀP3/PCBM blends.
reduction peaks are large and are located at very close values (P2
(ꢀ0.99) < P3 (ꢀ1.01 V vs Ag/AgCl); i.e. a shift of 0.02 V only)
coupled to the fact that gas phase computations and electroche-
mical measurements in acetonitrile are not ideal for comparison.
Polymer Photovoltaic Behavior. Since the light-induced
intramolecular electron transfer could easily occur from electron
donor to electron acceptor through the π-bridge which favors the
photocurrent generation and photoelectronic energy conversion
in photovoltaic devices, polymer solar cells were fabricated by
using P1, P2, and P3 as the electron donor and PCBM as the
electron acceptor with a blend ratio of 1:4. The hole-collection
electrode consisted of indium tin oxide (ITO) with a spin-coated
poly(3,4-ethylene-dioxythiophene)/poly(styrenesulfonate)
(PEDOT/PSS) layer, whereas Al served as the electron-collect-
ing electrode. The current density (J) versus voltage (V) curves
of the solar cells with the blend layer of P1/PCBM, P2/PCBM,
and P3/PCBM are displayed in Figure 6. The open-circuit
voltage (Voc), short-circuit current density (Jsc), fill factor
(FF), and PCE of the devices are summarized in Table 7.
Polymer P2 shows a relatively larger Voc than P1 and P3,
consistent with its lowest HOMO energy level, because Voc is
linearly correlated with the difference of the HOMO of the donor
and the LUMO of the acceptor.69 In the polymer series, P3
exhibited the highest efficiency of 1.04% resulting from the Voc of
0.77 V, Jsc of 3.42 mA cmꢀ2 and FF of 0.39. This is probably due
to its stronger absorption features with a broader Soret band and
stronger Q-bands. Such PCE of over 1% is among the highest for
bulk heterojuntion solar cells based on porphyrin-containing
conjugated polymers to date in the literature. In all diodes, FFs
are not very impressive, partly because all processing (except
PEDOT:PSS annealing and electrode deposition) and measure-
ments have been done under ambient atmosphere, which likely
results in the presence of traps. We expect FF to improve for
fabrication and characterization to be performed in an inert gas
environment. Additional possible reason for the relatively low fill
Figure 7. External quantum efficiency (EQE) versus wavelength curve
for the solar cell based on a P2/PCBM blend. The inset shows the
absorption spectrum of the polymer blend.
factor include unbalanced charge transport for electrons and
holes,70,71 which cannot be entirely excluded. Comprehensive
study of charge transport and the influence of traps is necessary
to further improve FF and overall device performance.
The absorption curve of the blend films and external quantum
efficiency (EQE) curves of the solar cells based on these Pt/Zn-
based polymers and PCBM were also studied for P1 and P2
(Figure 7). For instance, the P1 photodiode shows three main EQE
peaks, which correspond to the peaks in the absorption spectrum
of the polymer. The absorption is enhanced in the high-energy
absorption range 350ꢀ415 nm after blending with 80% PCBM.
Moreover, the shape of the EQE curve is similar to that of the
absorption curve, illustrating that all the light energy absorbed by
the polymer/PCBM blend film is to some extent converted into
electricity. The excitons produced by absorption in the polymer are
dissociated into charge carriers at the contact between polymer
and PCBM in the active layer, and are subsequently collected at
the electrodes. The highest EQE values of P1 and P2 are 42.4% at
443 nm. The absorption bands of these polymers at 448 nm are
narrow, and the Q-bands of porphyrin are still weak in the film
absorption spectra. Therefore, the photovoltaic efficiency should
be increased by improving the absorption property of porphyrin-
containing polymers in the visible region, such as broadening and
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enhancing the Q-bands or covering the missing region (430ꢀ
530 nm) by using perylenetetracarboxylic diimide residues, which
should give rise to a strong absorption band between 450 and
600 nm. This is the subject of future attention.72
also supported by the Areas of Excellence Scheme, University
Grants Committee, Hong Kong (AoE/P-03/08). We also thank
the Natural Sciences and Engineering Research Council of
Canada (NSERC), le Fonds Quꢁebꢁecois de la Recherche sur la
ꢁ
Nature et les Technologies (FQRNT), and the Centre d’Etudes
des Matꢁeriaux Optiques et Photoniques de l’Universitꢁe de
Sherbrooke. This work was also supported by Small Project Fund-
ing, Strategic Research Theme, and University Development
Fund of the University of Hong Kong.
’ CONCLUDING REMARKS
A series of soluble platinum metallopolyynes containing Zn-
(porphyrin) chromophores and electron-rich aromatic rings
(benzene and/or thiophene) were synthesized and systemati-
cally characterized. These polymers exhibit a sharp and strong
Soret band near 430 nm and two weak Q-bands between 540 and
635 nm, which is a typical absorption profile for porphyrinate
compounds. The optical bandgaps for these polymers vary from
1.93 to 2.02 eV, consistent with the electrochemical data. The
best photovoltaic performance of devices based on the P3/
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*Corresponding authors. E-mail: (W.-Y.W.) rwywong@hkbu.edu.hk;
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W.-Y.W. acknowledges the support from the Hong Kong
Research Grants Council for the General Research Grant
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