Synthesis of a novel unsymmetrical Zn(ii) phthalocyanine bearing a phenyl ethynyl moiety as sensitizer for dye-sensitized solar cells

Article abstract of DOI:10.1039/c0dt01292k

A new unsymmetrical zinc phthalocyanine sensitizer has been synthesised. The anchoring of the molecule to nanocrystalline TiO₂ films is realised by a carboxylic group connected to a phenyl ethynyl moiety. Density Functional Theory (DFT) calculations show significant and positive effects of such a functionalization. Electron injection into the semiconductor and photocurrent generation in DSSC are also presented.

Full text of DOI:10.1039/c0dt01292k

Supporting Information
Synthesis of a novel unsymmetrical Zn(II) Phthalocyanine bearing a phenyl ethynyl moiety as
efficient sensitizer for dye-sensitized solar cells
Gloria Zanotti,b Nicola Angelinia, Anna Maria Paolettia, Giovanna Pennesia, Gentilina Rossia, Aldo
Amore Bonapastaa, Giuseppe Mattiolia, Aldo Di Carlob, Thomas M. Brownb, Angelo Lembob, Andrea
Realeb .
Synthesis
t-Bu
t-Bu
CN
CN
N
N
N
N
CN
CN
CN
CN
ZnCl2
H2SO4, NaNO2
KI
Zn
a
+
t-Bu
DMAE, Δ
H2N
I
I
t-Bu
+
CO2Me
CO2Me
CO2Me
K2CO3
MeOH
SiMe3
Pd(II), Et3N
c
I
SiMe3
b
Pd(II), Et3N
t-Bu
t-Bu
t-Bu
t-Bu
THF/MeOH
N
N
N
N
N
N
Zn
Zn
NaOH/H2O, Δ
N
N
t-Bu
t-Bu
I
I
d
HO2C
MeO2C
Synthetic route for compound I
(a) 9(10),16(17),23(24)-Tri-tert-butyl-2-iodophthalocyaninatozinc(II) was synthesized as previously
1
described8 and characterized by NMR; H NMR (300 MHz, rt, Acetone d6): δ= 8.22-7.16 (m, 12H,
aromatic), 1.35 (s, 27H), C44H39N8ZnI·C4H8O2(dioxane)(959.67) calcd. C60.02, H 4.93, N 11.67;
found C 59.75, H 5.23, N 11.56.
(b) 4-(trimethylsilyl)ethynylmethylbenzoate: 700 mg (2.7 mmol) of 4-iodomethylbenzoate and 0.44
ml (3.2 mmol) of trimethylsilylacetylene were dissolved in 10 ml of Et3N in argon atmosphere. Then
37.7 mg (0.053 mmol) of bis(triphenylphosphine)palladium(II)dicloride and 5.1 mg (0.027 mmol) of
CuI were added and the mixture was stirred for 4 hours at room temperature. It was then concentrated

to remove the solvent. After purification with column chromatography on silica gel using hexane-
toluene 3:2 as eluent, 600 mg of desired product were obtained. (yield 97%).
1H NMR (300 MHz, rt, CDCl3):δ= 7.97 (d, J = 8.7 Hz, 2H aromatic) 7.49 (d, J = 8.7 Hz, 2H aromatic),
3.91 (s, 3H, CH3), 0.26 (s, 9H, Si(CH3)3); IR (nujol, cm-1 2160 C≡C, 1732 C=O)
(c) 4-ethynylmethylbenzoate: 600 mg of 4-(trimethylsilyl)ethynylmethylbenzoate (2.58 mmol) were
dissolved in 12 ml of methanol and then 156 mg of anhydrous K2CO3 were added. The mixture was
stirred under argon atmosphere at room temperature for 3 hours and then diluted with
dichloromethane and washed 3 times with water. The aqueous phase was extracted with
dichloromethane and the organic phase dried over Na2SO4 and concentrated to afford 346 mg of
product, which was used without any purification, (yield 84%).1H NMR (300 MHz, rt, CDCl3): δ=
7.99 (d, J = 9.1 Hz, 2H aromatic) 7.54 (d, J = 9.1 Hz, 2H aromatic), 3.92 (s, 3H, CH3), 3.22 (s, 1H,
≡CH); IR (nujol cm-1, 2102 C≡C, 1700 C=O)
(d) 9(10),16(17),23(24)-Tri-tert-butyl-2-[ethynyl-(4-carboxymethyl)phenyl]phthalocyaninatozinc(II)
In argon atmosphere, 73 mg of a (0.084 mmol) were dissolved in 2 ml of Et3N and then 16 mg of c
(0.1 mmol), 1.2 mg of (PPh3)2PdCl2 (0.00168 mmol) and 0.16 mg of CuI (0.00084 mmol) were added.
The mixture was stirred for 5 hours in argon atmosphere and then it was concentrated to afford 137
mg of crude product. After purification with column chromatography using hexane-dioxane 9:1 and
1
4:1 as eluents, the desired product was obtained as a green solid. (75 mg, 99% yield). H NMR (300
MHz, rt, THF d8): δ= 9.42-8.86 (m, 8H, ring), 8.30-8.11 (m, 6H, phthalocyanine macrocycle and
benzoic ring), 7.93-7.87 (m, 2H, benzoic ring), 3.94 (s, 3H, CH3), 1.87 (s, 27H, C(CH3)3)
IR (KBr cm-1, 2206 C≡C,),UV-Vis (THF, nm): 671, 686; ISI/MS (m/z) [M+H+] 903.2-909.2 ;
elemental analysis: C54H46N8O2Zn·2C4H8O2(dioxane) (1060.39), calcd. C 70.16, H 6.08, N 10.56;
found C 70.20, H 6.15, N 10.01.
(I) 9(10),16(17),23(24)-Tri-tert-butyl-2-[ethynyl-(4-carboxy)phenyl]phthalocyaninatozinc(II)
20 mg of ester (0.022 mmol) were dissolved in 5 ml of a 1:1 THF-methanol solution and after the
addiction of 190 mg of NaOH dissolved in 5 ml of H2O the resultant mixture was refluxed under
stirring until the TLC showed the complete disappearance of the reagent. Then it was concentrated to
eliminate THF and methanol and the resulting aqueous phase was acidified to allow the precipitation
of the product which was collected by filtration and washed several times with water. The resulting 19
mg of crude were purified by column chromatography using silica gel as stationary phase and 4:1
hexane-dioxane and 1:1 chloroform-methanol as mobile phases and the blue-greenish solid obtained
1
was washed several times with water and methanol. (13 mg, yield 66.5%); H NMR (300 MHz, rt,
THF d8): δ= 9.86-7.67 (m, 16H, aromatics), 1.67 (s, 27H, C(CH3)3)
IR (nujol cm-1, 2203 C≡C)
UV-Vis (THF, nm): 671, 685

Optical characterization
UV-vis spectra of solution (∼10-4M) were recorded on a Perkin-Elmer Lambda 950 UV-vis-NIR
spetrophotometer. Fluorescence spectra (∼10-6M) were recorded on a Jobin-Yvon Spectrofluorometer
Fluorolog 3-22.
120000
200000
1.0
150000
100000
100000
80000
50000
0
500
60000
600
700
Wavelenghts (nm)
0.5
0.0
40000
20000
0
600
750
900
Wavelength (nm)
UV-vis absorption (red) and steady-state fluorescence emission (blue) (λexc = 670 nm) spectra of I in
THF solution, in the inset fluorescence excitation spectrum (λem= 710 nm).
Electrochemical measurements
Cyclic voltammograms were recorded at 25 °C with a computer aided system (AMEL
INSTRUMENTS 2059 potentiostat/galvanostat, 568 function generator) in a conventional three
electrodes cell, a platinum disk was used as working electrode together with a platinum wire as
auxiliary electrode. The reference electrode was Ag/AgNO3(0.01M) in distilled acetonitrile (E0 vs
SCE = 0.298 V). The samples solutions were about 10-4 M in distilled anhydrous THF and tetra(n-
butyl)ammonium tetrafluoroborate (TBATFB, 0.1 M) was used as supporting electrolyte. The
solutions were previously purged for 10 minutes with nitrogen and all measurements were performed
in inert atmosphere.

Theoretical methods.
The structure and the electronic properties of the ZnPc molecule have been investigated by using ab
initio Density Functional Theory methods in the generalized gradient approximation (GGA) and a
supercell approach.[1] Total energies have been calculated by using ultrasoft pseudopotentials,[2]
plane-wave basis sets, and the PBE gradient corrected exchange-correlation functional.[3]
Satisfactorily converged results have been achieved by using cutoffs of 30 Ry on the plane waves and
of 240 Ry on the electronic density as well as the Γ point for the k-point sampling of the Brillouin
zone. Geometry optimization procedures have been performed by fully relaxing the positions of all of
the atoms in a large (60 a.u. edge) cubic supercell as well as by applying a Makov-Payne correction to
the total energy, in order to minimize the occurrence of spurious interactions between periodically
repeated images. The electronic properties have been investigated by analyzing the electronic
eigenvalues and eigenfunctions calculated at the Γ point. The electronic eigenvalues could be affected,
in principle, by two limits of the DFT-GGA methods: i) a wrong ordering of the frontier molecular
orbitals, due to a self-interaction error inducing a bad description of the strongly localized d orbitals of
the central metal,[4] and ii) the well known GGA limit leading to too small values of the molecule
HOMO-LUMO gap. However, the former limit does not affect the results achieved for the
investigated metallo-organic molecules carrying a Zn atom, whose fully occupied d shell is quite
deeply embedded in the region of the occupied ligand orbitals[5]. The frontier HOMO-LUMO belong
indeed to the Pc macrocycle and are not affected by the d orbitals of the central atom. Regarding the
latter limit, the GGA error on the energy gaps, our HOMO-LUMO gap (1.42 eV) reasonably
compares with the experimental values (1.7-1.9 eV). The HOMO and LUMO eigenfunctions have
been used to calculate the ⏐ψ⏐2 isosurfaces reported in Fig. 2 of the paper. Similar isosurfaces are
plotted in the following figure for the HOMO and LUMO ⏐ψ⏐2 estimated in the case of the
unsubstituted ZnPc.
Electron density isosurfaces of the π HOMO (left) and of the doubly degenerate π* LUMO (right)
of the unsubstituted ZnPc molecule (isosurfaces are sampled at 0.0005 e/a.u.3).
[1] Giannozzi, P.; et al. J. Phys.: Condens. Matter, 2009, 21, 395502. http://www.quantum-
espresso.org.
[2] D. Vanderbilt, Phys. Rev. B, 1990, 41, 7892.
[3]J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.
[4] N. Marom, O. Hod, G. E. Scuseria, and L. Kronik, J. Chem. Phys., 2008, 128, 164107.
[5] G. Mattioli et al. Chem. Mater., 2009, 21, 4555–4567.

DSSC fabrication.
Sensitization was carried out in 0.1mM EtOH solution of I at rt and different time depending on the
Chenodeoxycholic Acid (CDCA) equivalents (16h for 0 and 1eqs, 48-72h for 5 and 10 eqs). Two
different TiO2 pastes were used: 18NR-T (transparent TiO2 paste; 7 µm thickness, 0.25 cm2 active
area) and 18NR-AO (opaque TiO2 paste; 14µm thickness, 0.88cm2 active area) purchased by Dyesol.
Electrolyte used was the I3-/I- based EL-HSE, while the platinum for the counter electrode was PT-1
catalyst, both purchased by Dyesol.
IV curve of I/DSSCs as function of CDCA equivalents
(TiO2 paste 18NR-T 0.25 cm2)
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
I-V curve of I/ DSSCs with 18NR-T
(transparent) photoanode (active
area: 0.25 cm2, thickness 7µm)
Pc + 0 eq. CDCA
Pc + 1 eq. CDCA
Pc + 5 eq. CDCA
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55
Voc (Volt)
IV curve of I/DSSCs as function of CDCA equivalents
2.75
(TiO2 paste 18NR-AO 0.88 cm2)
2.50
2.25
2.00
1.75
1.50
1.25
1.00
I-V curve of I/DSSCs with 18NR-
AO (opaque) photoanode (active
area: 0.88 cm2, thickness 14µm).
0.75
Pc + 0 eq. CDCA
Pc + 1 eq. CDCA
Pc + 5 eq. CDCA
0.50
0.25
0.00
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
Voc (Volt)