Synthesis of Zinc Tetraphenylporphyrin Rigid Rods with a Built-In Dipole

Article abstract of DOI:10.1021/jp5112982

Three Zn(II) tetraphenylporphyrins (ZnTPP) were synthesized to study the influence of a molecular dipole on the energy level alignment of a chromophore bound to a metal oxide semiconductor: ZnTPP-PE(DA)-IpaOMe (1), ZnTPP-PE-IpaOMe (2), and ZnTPP-PE(AD)-Ip

Full text of DOI:10.1021/jp5112982

Article
pubs.acs.org/JPCB
Synthesis of Zinc Tetraphenylporphyrin Rigid Rods with a Built-In
Dipole
†
†
†
†
†
†
Keyur Chitre, Alberto Batarseh, Andrew Kopecky, Hao Fan, Hao Tang, Roger Lalancette,
‡
,†
Robert A. Bartynski, and Elena Galoppini*
†
Chemistry Department, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States
‡
Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers University, 136 Frelinghuysen Road,
Piscataway, New Jersey 08854, United States
*
S Supporting Information
ABSTRACT: Three Zn(II) tetraphenylporphyrins (ZnTPP) were synthesized to study
the influence of a molecular dipole on the energy level alignment of a chromophore
bound to a metal oxide semiconductor: ZnTPP-PE(DA)-IpaOMe (1), ZnTPP-PE-
IpaOMe (2), and ZnTPP-PE(AD)-IpaOMe (3). Each contained a rigid-rod linker made
of a p-phenylene ethynylene (PE) moiety terminated with the methyl ester of an
isophthalic acid unit (Ipa). Porphyrins 1 and 3 contained an intramolecular dipole in the
central phenyl ring, which was built by introducing electron donor (D, NMe ) and
2
acceptor (A, NO ) substituents in para position to each other. In 1 and 3, the relative
2
position of the D and A substituents, and therefore the dipole direction, was reversed.
Porphyrin 2, without substituents in the linker, was synthesized for a comparison. The structures of precursors to 1 and 3 and the
structure of 1 were determined by single crystal X-ray analysis. Solution UV−vis and steady-state fluorescence spectra of 1−3
were identical to each other and exhibited the spectral features typical of the ZnTPP chromophore and their electrochemical
properties were also very similar. Methyl esters 1−3 were hydrolyzed to the corresponding carboxylic acids for binding to metal
oxide semiconductors.
1
. INTRODUCTION
by UV photoemission spectroscopy (UPS), is illustrated in
Figure 1b. A monolayer of molecules with a built-in dipole in
the linker and bound to ZnO forms an electrostatic potential
that shifts the HOMO and LUMO of the porphyrin
chromophore by (±) ∼100 meV with respect to the band
edges of the semiconductor, when compared to the homologue
with no dipole. When the direction of the dipole in the linker
was reversed, the shift direction was reversed by the same
Interfaces between organic molecules and transition-metal
oxide semiconductors are important for applications ranging
from solar cells, to photocatalysts, sensors, and molecular
electronics. The alignment of the energy levels between
molecules and metal-oxides influences charge transfer and
other electronic processes that, in turn, affect the device’s
performance. For this reason there is a widespread effort
aimed at controlling and understanding the factors influencing
energy level alignment at molecules/semiconductor, as well as
other interfaces, such as metals.
dipoles play an important role and are extensively inves-
tigated.
electron transfer across molecule/metal boundaries in experi-
ments that often involved clever molecular design to introduce
strong dipoles and to probe the effects of dipole reversals.
1
,2
27
amount.
Here we describe the synthesis and properties of the dipole-
containing porphyrins ZnTPP-PE(DA)-IpaOMe (1) and
ZnTPP-PE(AD)-IpaOMe (3) used in that study and shown
in Figure 1a. Each contains a phenylene ethynylene (PE) rigid-
rod bridge terminated with the methyl ester of an isophthalic
acid unit (Ipa), which is an anchoring group used for binding to
the semiconductor surface. The more soluble methyl esters 1−
3 were used in solution studies and were converted into the
corresponding carboxylic acids just prior to binding. Porphyrins
1 and 3 contained an intramolecular dipole in the central PE
unit, which was obtained by introducing electron donating (D,
NMe ) and accepting (A, NO ) substituents in the para
3
−8
In this context, interface
9
−13
Molecular dipoles have been used to influence
14−19
On transition metal oxide semiconductors, the adsorption of
structurally simple molecules to form polar layers was found to
shift the position of the semiconductor conduction band and
influence the open circuit photovoltage and other parameters in
2
0−26,11,13
dye-sensitized solar cells.
2
2
As part of our interest in molecules/semiconductor
interfaces, we recently demonstrated that two Zn(II)
tetraphenylporphyrins having the structure porphyrin-linker-
anchor with a reversible built-in dipole in the linker shift a
molecule’s energy levels alignment on transition-metal oxide in
position to each other. In 1 and 3, the relative position of the D
Special Issue: John R. Miller and Marshall D. Newton Festschrift
Received: November 11, 2014
Revised: January 27, 2015
27
a predictable manner. The proof-of-concept experiment,
which was carried out on a ZnO(11−20) single crystal surface
©
XXXX American Chemical Society
A
DOI: 10.1021/jp5112982
J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B
Article
2
. EXPERIMENTAL SECTION
.1. General. Reagents and Methods. Zn(II) 5-(4-
2
28
iodophenyl)-10,15,20-triphenylporphyrin (9), dimethyl 5-
ethynylisophthalate (5), 2,5-dibromo-4-nitroaniline,
28
43,44
and
28
ZnTPP-PE-IpaOMe (2) were synthesized according to
reported procedures. All commercially available chemicals and
solvents were used as received unless otherwise noted.
Anhydrous grade, stabilizer-free tetrahydrofuran (THF) was
passed through a MBraun solvent purification system
immediately prior to use. CH Cl and hexane used for column
2
2
chromatography were distilled. Ethyl acetate for column
chromatography was HPLC grade and used as received. Silica
gel plates for thin-layer chromatography (TLC) and silica gel
(
230−400 mesh) for column chromatography were purchased
from Sorbent Technologies. All reactions involving air and
moisture sensitive reagents were performed under nitrogen
atmosphere in oven-dried or flame-dried glassware and using
anhydrous solvents. 1
Instrumentation. H and 13C NMR spectra were recorded
on a Varian INOVA NMR spectrometer operating at 499.896
1
13
MHz for H and 125.711 MHz for C using the indicated
solvent as an internal reference. Chemical shifts were reported
relative to the central line of the solvent: CDCl (δ 7.26 ppm
3
1
13
for H and δ 77.0 for C spectra) and THF-d8 (δ 3.58 ppm for
H and δ 67.60 for C spectra). The coupling constants (J) for
H NMR are reported in Hertz. High-resolution mass spectra
1
1
13
(
ESI and MALDI, using 2,5-dihydroxybenzoic acid, 2,5-DBH,
as the matrix) were recorded on a Bruker Daltonics Apex-Qe
series Fourier Transform Mass Spectrometer. UV−vis
absorption spectra were collected on a Varian Cary 500 UV−
vis−NIR spectrophotometer at room temperature. Steady-state
fluorescence emission spectra were collected on a Horiba
Fluorolog-3 instrument equipped with a Xenon short-arc lamp
source at room temperature. The excitation wavelength was set
to 430 nm with a monochromator bandwidth of 3 nm for both
excitation and emission monochromators. UV−vis and
fluorescence spectra were corrected by subtracting a baseline
spectrum, which was determined for a solution containing all
chemicals but porphyrins 1−3. Single attenuated total
reflectance infrared (FTIR-ATR) spectra were collected on a
Thermo Electron Corporation Nicolet 6700 FT-IR (ZnSe
Figure 1. (a) Molecular structure of the porphyrins used in the
experiment and described here. (b) Schematic diagram of how a dipole
in a linker shifts the energy level of a porphyrin chromophore with
respect to the conduction band edge of a wide band gap metal oxide
27
semiconductor. The shift direction is indicated by a red arrow.
and A substituents, and therefore the dipole direction, was
reversed. Comparisons were made with the previously reported
−1
crystal, 128 scans averaged, resolution at 4 cm ).
28
Electrochemistry. Cyclic voltammetry of methyl esters 1−3
was performed on a BAS CV27 potentiostat. The experiments
were conducted under nitrogen at room temperature with a
conventional three-electrode configuration at a scan rate of 50
ZnTPP-PE-IpaOMe (2), which is structurally identical to 1
and 3, but that has no substituents, hence no intramolecular
dipole in the linker.
The primary requirement for the molecular design of 1, 2,
and 3, and the rationale for choosing ZnTPP derivatives, was
the ability to introduce an internal dipole in the linker without
making significant changes in the size of the HOMO−LUMO
gap of the chromophoric unit. This was possible in 1, 2, and 3
because the dipole-containing linker was attached on a meso-
phenyl group which is nearly orthogonal to the porphyrin plane
−1
mV s . The data were collected in dichloromethane solutions
with a 0.1 M Bu NPF supporting electrolyte. A glassy-carbon
4
6
working electrode (2 mm diameter) was used along with a
platinum wire auxiliary electrode and a Ag wire reference
electrode. Ferrocene was used as the internal reference. The
half-wave redox potentials (E1/2) were determined as (Epa
+
Epc)/2, where Epa and Epc are the anodic and cathodic peak
potentials, respectively. The reduction processes are not
reported because of the poor quality of the cyclic voltammetry
scans at negative potentials.
Single-Crystal X-ray Studies. For each compound, a single
crystal was mounted in a Cryoloop using Paratone-N oil. Data
28
and therefore electronically decoupled from the macrocycle.
Second, the study of 1, 2, and 3 allowed a direct comparison
with our previous work on ZnTPP derivatives bound to TiO
2
2
8−31
and ZnO.
Finally, porphyrin dyes with chromophore-
bridge-anchor structures similar to the ones described here have
exhibited record efficiencies in photovoltaic applications, are
were collected at 100 K with the crystal in a stream of N gas on
2
32−37
useful models in fundamental charge transfer studies,
and
a Bruker APEX2 diffractometer with CCD detector using Cu
Kα radiation (λ = 1.54178 Å); cell refinement was done by
continue to be of great interest as photocatalysts and in artificial
38−42
45
photosynthesis projects.
APEX2. Numerical absorption corrections from face indices
B
DOI: 10.1021/jp5112982
J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B
Article
4
6
were applied using SADABS, and data reduction was done by
mmol), Pd(PPh ) Cl (12 mg, 0.017 mmol), CuI (6.5 mg
3
2
2
4
7
48
SAINT. SHELXTL was used to solve and refine the
structures, to manipulate the molecular graphics, and to prepare
the material for publication.
.2. Synthesis. 2,5-Dibromo-N,N-dimethyl-4-nitroaniline
4). To a stirring solution of 2,5-dibromo-4-nitroaniline (see the
Supporting Information) (2.0 g, 6.74 mmol) in THF (7 mL),
NaH (0.485 g, 20.2 mmol) was added slowly in small portions
∼150 mg each) under positive pressure of nitrogen. MeI
4.78g, 2.10 mL, 33.68 mmol) was added via syringe. The
mixture was stirred at room temperature for 2 h under a
nitrogen atmosphere and monitored by TLC. Water was added
to the solution, a yellow precipitate formed, and THF was
removed in vacuo to ease the extraction. The reaction mixture
was extracted with ethyl acetate, the organic layers were dried
over sodium sulfate, and the solvents were removed in vacuo.
The crude product was purified using column chromatography
0.034 mmol), and THF (8 mL) under nitrogen atmosphere.
Then diisopropylamine (0.8 mL) was added in one portion via
syringe, and the solution turned dark brown. At each addition,
the flask was degassed and maintained under a nitrogen
atmosphere. This mixture was stirred overnight at 50 °C under
nitrogen atmosphere, then cooled and filtered through Celite
and a thin layer of silica gel. The organic solvent was
evaporated in vacuo, and the crude solid was purified with
silica gel column chromatography using a 1:2 mixture of ethyl
2
(
(
(
1
acetate and hexane, v/v, to obtain 7a (172 mg, 41%). H NMR
(
1
500 MHz, chloroform-d): δ 8.61 (s, 1H), 8.29 (s, 2H), 8.26 (s,
H), 6.90 (s, 1H), 3.96 (s, 6H), 3.24 (s, 6H), 0.29 (s, 9H). C
1
3
NMR (CDCl ): δ 165.68, 156.01, 140.13, 136.16, 133.01,
3
1
9
31.37, 130.65, 124.09, 121.17, 120.30, 110.19, 104.40, 100.88,
4.57, 89.38, 52.84, 42.95, −0.11. HRMS (ESI): calculated for
+
C H N O Si, 479.1633 [M + H] ; found, 479.1635 [M +
H] .
2
5
26
2
6
(
(
7
1
dichloromethane:hexane 2:1 v/v) to obtain 4 as a yellow solid.
+
1
1.42 g, 65.3%). H NMR (500 MHz, CDCl ): δ 8.24 (s, 1H),
3
TMS-PE(AD)-IpaOMe (7b). To a stirring solution of 6b (330
1
3
.19 (s, 1H), 2.98 (s, 6H). C NMR (126 MHz, CDCl ): δ
3
mg, 0.72 mmol) in THF (15 mL), Pd(PPh ) Cl (10 mg, 0.014
3
2
2
55.90, 141.92, 132.52, 124.78, 115.50, 113.54, 43.45. HRMS
mmol), CuI (5.5 mg 0.029 mmol), and diisopropylamine (7.5
mL) were added in one portion, sequentially. Then the flask
was degassed, flushed with nitrogen, and trimethylsilyl
acetylene (628 mg, 0.91 mL, 6.40 mmol) was added with a
syringe. The solution turned dark brown. The reaction mixture
was stirred overnight at 50 °C under nitrogen atmosphere. The
reaction mixture was then cooled, filtered through Celite and a
thin layer of silica gel, the organic layer was dried in vacuo, and
the crude solid was purified with silica gel column
chromatography using a 2:7 mixture of ethyl acetate and
+
+
(
ESI ): calculated for C H Br N O , 324.9005 [M + H] ;
8 8 2 2 2
+
found, 324.8986 [M + H] .
2
-Bromo-N,N-dimethyl-4-nitro-5-((trimethylsilyl)ethynyl)-
aniline (6a). To a stirring solution of 4 (300 mg, 0.926 mmol)
in THF (10 mL) at room temperature, Pd(PPh ) Cl (28 mg,
3
2
2
0
(
.04 mmol), CuI (17.6 mg 0.09 mmol), and diisopropylamine
1.0 mL) were added sequentially. The solution turned orange-
red. To this reaction mixture trimethylsilyl acetylene (99.9 mg,
.143 mL, 1.01 mmol) was added in one portion. The solution
turned dark brown. At each addition,the flask was degassed and
flushed with nitrogen. The resulting reaction mixture was
stirred at room temperature under nitrogen atmosphere for 1 h
then filtered through Celite. The organic layer was dried in
vacuo, and the resulting crude solid was purified with silica gel
column chromatography using a 1:4 mixture of ethyl acetate
0
1
hexane, v/v to obtain 7b (257 mg, 75%). H NMR (500 MHz,
chloroform-d): δ 8.68 (s, 1H), 8.43 (s, 2H), 8.29 (s, 1H), 6.94
13
(
s, 1H), 3.98 (s, 6H), 3.23 (s, 6H), 0.28 (s, 9H). C NMR (126
MHz, CDCl ): δ 165.47, 156.13, 138.96, 136.84, 133.46,
3
1
31.07, 130.87, 123.71, 120.22, 119.34, 110.82, 102.87, 102.55,
1
94.53, 87.88, 52.61, 42.62, −0.33. HRMS (ESI): calculated for
and hexane v/v to obtain 6a (173 mg, 55%). H NMR (500
+
C H N O Si, 479.1633 [M + H] ; found, 479.1602 [M +
H] .
2
5
26
2
6
MHz, CDCl ): δ 8.33 (s, 1H), 7.10 (s, 1H), 2.97 (s, 6H), 0.30
3
+
s, 9H). 13C NMR (CDCl ): δ 155.50, 142.47, 131.35, 124.23,
(
3
General TMS Deprotection Procedure. To a stirring
solution of 7a or 7b (300 mg, 0.627 mmol) in a 1:1 mixture
of dichloromethane and MeOH (26 mL), K CO (433 mg,
1
18.77, 114.95, 104.01, 99.81, 43.31, −0.37. HRMS (ESI):
+
calculated for C H BrN O Si, 343.0295 [M + H] ; found,
13
17
2
2
+
2
3
343.0278 [M + H] .
3
.13 mmol) was added in one portion at room temperature.
Dimethyl 5-((4-bromo-5-(N,N-dimethylamino)-2-
The reaction mixture was stirred at room temperature, under
nitrogen atmosphere for about 2.5 h and monitored by TLC for
the disappearance of the starting material. Deionized water
nitrophenyl)ethynyl)isophthalate (6b). To a stirring solution
of 4 (300 mg, 0.926 mmol) and 5 (202 mg, 0.926 mmol) in
THF (5 mL), Pd(PPh ) Cl (20 mg, 0.02 mmol), CuI (10.0
3
2
2
(
250 mL) was added, and the product was extracted with
mg 0.03 mmol), and triethylamine (5.0 mL) were added in one
portion sequentially. The solution turned dark brown. After
each addition the flask was brought degassed and flushed with
nitrogen. This mixture was stirred overnight at room
temperature under a nitrogen atmosphere. Then the reaction
mixture was filtered through Celite, the organic layer was dried
in vacuo, and the solid was purified with silica gel column
chromatography using a 4:1 mixture of dichloromethane and
hexane, v/v, to obtain 6b (220 mg, 51%). H NMR (CDCl ): δ
.69 (s, 1 H), 8.44 (s, 2 H), 8.43 (s, 1 H), 7.18 (s, 1 H), 3.99 (s,
H), 3.03 (s, 6 H). C NMR (126 MHz, CDCl ): δ 165.43,
dichloromethane (3 × 50 mL). The collected organic layers
were washed with water (2 × 50 mL), dried over sodium
sulfate, and the solvent was removed in vacuo. The solid crude
was purified with silica gel column chromatography using a 1:2
mixture of ethyl acetate and hexane, v/v, to obtain 8a or 8b,
respectively, as yellow solids.
1
H-PE(AD)-IpaOMe 8a. Yellow solid (204 mg, 80%). H
1
NMR (500 MHz, chloroform-d): δ 8.65 (s, 1H), 8.33 (d, J =
3
8
6
1
1
1.4 Hz, 2H), 8.32 (s, 1H), 6.97 (s, 1H), 3.98 (s, 6H), 3.56 (s,
1
3
13
1H), 3.26 (s, 6H). C NMR (126 MHz, CDCl
): δ 165.45,
3
3
55.71, 141.85, 136.82, 131.77, 131.15, 130.99, 123.72, 123.52,
18.41, 115.09, 94.73, 86.92, 52.62, 43.36. HRMS (ESI):
155.73, 139.69, 135.95, 132.97, 131.16, 130.49, 123.79, 121.35,
119.20, 110.12, 94.45, 88.99, 85.28, 79.82, 52.66, 42.71. HRMS
+
+
calculated for C H BrN O , 461.0343 [M + H] ; found,
(ESI): calculated for C H N O , 407.1238 [M + H] ; found,
2
0
17
2
6
22 18
2
6
+
+
4
61.0327 [M + H] .
TMS-PE(DA)-IpaOMe (7a). A round-bottom flask was
charged with 6a (300 mg, 0.879 mmol), 5 (374 mg, 1.72
407.1223 [M + H] .
1
H-PE(AD)-IpaOMe 8b. Yellow solid (201 mg, 79%). H
NMR (500 MHz, chloroform-d): δ 8.68 (s, 1H), 8.43 (s, 2H),
C
DOI: 10.1021/jp5112982
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The Journal of Physical Chemistry B
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8
6
.32 (s, 1H), 6.97 (s, 1H), 3.98 (s, 6H), 3.51 (s, 1H), 3.22 (s,
H).
solution (5 mL, 10 mmol) was then added, and the reaction
mixture was stirred for 3 h at reflux (87 °C). After the reaction
was completed (TLC) organic solvents were removed in vacuo.
The unreacted ester was extracted using chloroform, and the
carboxylate salt remained in the water layer as a fine
suspension. The aqueous layer was then acidified by cautious,
dropwise addition of 2 N HCl aqueous to pH ∼ 3. The fine
solid was collected by vacuum filtration on a 0.45 μm PTFE
filter and washed with copious amounts of water to yield a
purplish-brown solid:
13
C NMR (126 MHz, CDCl ): δ 165.48, 156.40, 139.07,
3
1
9
36.88, 133.81, 131.08, 130.94, 123.65, 120.38, 119.74, 109.83,
4.68, 87.70, 84.95, 81.41, 52.63, 42.75. HRMS (ESI):
+
calculated for C H N O , 407.1238 [M + H] ; found,
2
2
18
2
6
+
407.1222 [M + H] .
ZTPP-PE(DA)-IpaOMe (1). Zn(II) 5-(4-iodophenyl)-
10,15,20-triphenylporphyrin (9) (300 mg, 0.373 mmol, 1
equiv), H-PE(DA)-IpaOMe (8a) (303 mg, 0.746 mmol, 2.0
equiv), PdCl (PPh ) (5.24 mg, 0.0075 mmol 0.02 equiv), and
1
ZnTPP-DA-Ipa (1-acid). 33 mg, 79%. H NMR (THF-d8): δ
2
3 2
CuI (2.9 mg, 0.015 mmol, 0.04 equiv) were added to a 50 mL
-necked round-bottom flask that was flushed with nitrogen.
THF (10 mL) and diisopropyl amine (2 mL) were added via
syringe under nitrogen atmosphere. The reaction was heated to
10.82 (s, 1H, carboxylic acid), 8.88 (m, 4H), 8.84 (s, 4H), 8.66
(s, 1H), 8.45 (s, 1H), 8.39 (s, 2H), 8.27 (d, J = 7.8,2H), 8.20
(m, 6H), 8.01 (d, J = 7.4, 2H), 7.75 (d, J = 5.6, 9H), 7.28 (s,
1H), 3.37 (s, 6H). FT-IR-ATR: v(O−H) weak 3500−3000
2
−1
−1
−1
60 °C with constant stirring overnight under nitrogen. The
cm ; v(C−H) 2924 cm ; v(CC) 2208 cm ; v(CO)
−
1
−1
−1
reaction mixture was cooled to room temperature and then
filtered through Celite, and the solvents were evaporated in
vacuo to obtain a crude solid. It was purified with column
chromatography using a 2:3 mixture of ethyl acetate and
1714 cm ; v(C−C, aromatic) 1593 cm ; v(N−O) 1541 cm ;
−1
v(C−O) 1259 cm .
1
ZnTPP-AD-Ipa (3-acid). 27 mg, 53%. H NMR (500 MHz,
THF-d8): δ 8.95−8.86 (m, 4H), 8.85 (s, 4H), 8.69 (s, 1H),
8.47 (s, 1H), 8.44 (s, 2H), 8.27 (d, J = 7.8 Hz, 2H), 8.21 (d, J =
7.0 Hz, 6H), 7.97 (d, J = 7.8 Hz, 2H), 7.75 (d, J = 6.2 Hz, 9H),
7.25 (s, 9H), 3.42 (s, 6H). FT-IR-ATR: v(O−H) weak 3500−
1
hexane, v/v to obtain 1 as a purple solid (109 mg, 27%). H
NMR (500 MHz, chloroform-d): δ 9.07−8.89 (m, 8H), 8.62 (s,
1H), 8.44 (s, 1H), 8.31 (s, 2H), 8.26 (m 8H), 8.04 (d, J = 7.9
−
1
−1
−1
Hz, 2H), 7.78 (m, 9H), 7.15 (s, 1H), 3.97 (s, 6H), 3.34 (s, 6H).
3000 cm ; v(C−H) 2926 cm ; v (CC) 2198 cm ; v(C
1
3
−1
−1
C NMR (126 MHz, CDCl3): δ 165.24, 155.88, 150.29,
50.23, 150.19, 149.77, 144.06, 142.79, 142.77, 139.29, 135.73,
34.69, 134.47, 133.12, 132.27, 132.09, 132.06, 131.68, 130.77,
30.33, 130.09, 127.53, 126.58, 123.73, 121.71, 121.35, 121.26,
20.59, 120.26, 119.95, 109.66, 97.66, 94.18, 89.32, 87.38,
O) 1699 cm ; v(C−C, aromatic) 1597 cm ; v(N−O) 1543
−1 −1
1
1
1
1
5
cm ; v(C−O) 1261 cm .
2.3. X-ray Crystal Structure Determination. ORTEP
diagrams with the numbering schemes of the crystal structures
of (8a), (8b), and (1) are shown in Figures 2 and 3.
−1
2.54, 42.78. FT-IR-ATR: v(C−H) 2922 cm ; v(CC) 2206
For (8a). A yellow plate of C22H N O was obtained by
18 2 6
−1
−1
−1
cm ; v(CO) 1728 cm ; v(C−C, aromatic) 1591 cm ;
slow evaporation from THF. Unit cell parameters were
determined by least-squares refinement of 6246 reflections
between θ of 3.18 and 72.07°. Intensity data were collected,
integrated, and corrected for absorption as well as Lorentz and
polarization. Standard procedures and programs were used to
−1
−1
v(N−O) 1541 cm ; v(C−O) 1242 cm . HRMS (MALDI):
calculated for C H N O Zn, 1080.2608 [M]; found,
6
6
44
6
6
1080.2619 [M].
ZnTPP-PE(AD)-IpaOMe (3). Zn(II) 5-(4-iodophenyl)-
0,15,20-triphenylporphyrin 9 (195 mg, 0.242 mmol, 1.0
45−48
1
solve and refine the structure.
There were a total of 18872
equiv), H-PE(AD)-IpaOMe (8b) (197 mg, 0.485 mmol, 2.0
reflections and 3492 independent observations. The solution
was found by direct methods. The final refinement on F
2
equiv), PdCl (PPh ) (3.4 mg, 0.004 mmol, 0.02 equiv), and
2
3 2
CuI (2 mg, 0.0095 mmol, 0.04 equiv) were dissolved in
anhydrous THF (10 mL), and the solution was flushed with
nitrogen. Diisopropylamine (2 mL) was added in one portion,
and the reaction mixture was heated at 60 °C overnight under
nitrogen atmosphere. The resulting solution was allowed to
cool to room temperature and then filtered through Celite, and
the solvents were removed in vacuo. The dark brown crude was
purified with silica gel column chromatography using a 2:3
mixture of ethyl acetate and hexane, v/v, to obtain 3 as a purple
involved an anisotropic model for all nonhydrogen atoms. All
the H atoms were found in electron-density difference maps
and were then placed as riding models on their respective C
atoms. With 275 parameters, the refinement converged to a
conventional R value of 5.3%. Cell dimensions and all of the
metrical data are shown in Table S1 of the Supporting
Information. Planes and angles for the selected atoms are given
in Table S2 of the Supporting Information.
For (8b). A yellow plate of C H N O was obtained from
2
2
18
2
6
1
solid (121 mg, 46%). H NMR (500 MHz, chloroform-d): δ
THF. Cell dimensions, data, etc., were handled exactly as above.
Unit cell parameters were determined by least-squares
refinement of 9837 reflections between θ of 3.55 and 71.68°.
There were a total of 15006 reflections and 3454 independent
observations. With 275 parameters, the refinement converged
to a conventional R value of 4.4%. Cell dimensions and the
metrical data are also shown in Table S1 of the Supporting
Information. Planes and angles for the selected atoms are given
in Table S2 of the Supporting Information.
8
8
1
.98 (m, 8H), 8.69 (s, 1H), 8.52 (s, 1H), 8.47 (s, 2H), 8.40−
.07 (m, 8H), 7.93 (d, J = 5.2 Hz, 2H), 7.77 (m, 9H), 7.08 (s,
1
3
H), 3.99 (s, 6H), 3.43 (s, 6H). C NMR (126 MHz, CDCl ):
3
δ 165.49, 155.98, 150.33, 150.30, 150.20, 149.77, 143.63,
1
1
1
9
42.71, 139.40, 136.87, 134.64, 134.43, 132.91, 132.26, 132.15,
32.10, 131.61, 131.06, 130.87, 129.40, 127.57, 126.59, 123.76,
21.97, 121.45, 121.32, 120.42, 119.89, 119.23, 111.35, 97.10,
4.64, 88.39, 88.00, 52.62, 42.85. FT-IR-ATR: v(C−H) 2924
−1
−1
−1
cm ; v(CC) 2218 cm ; v(CO) 1730 cm ; v(C−C,
For (1). A red plate of C H N O Zn was obtained from
7
4
60
6
8
−1
−1
aromatic) 1595 cm ; v(N−O) 1541 cm ; v(C−O) 1236
THF. Cell dimensions, data, etc., were accumulated exactly as
above. Unit cell parameters were determined by least-squares
refinement of 9993 reflections between θ of 2.77 and 69.31°.
There were a total of 29037 reflections and 17401 independent
observations. The structure was solved in the space group P1,
with 2 molecules in the asymmetric unit; there are two THF
−
1
cm . HRMS (MALDI): calculated for C H N O Zn,
6
6
44
6
6
1
080.2608 [M]; found, 1080.2577 [M].
General Hydrolysis Procedure. Ester 1 (42 mg, 0.04 mmol)
or ester 3 (52.5 mg, 0.05 mmol) was dissolved in a mixture of
THF and methanol (1:1 ratio, 4 mL). A 2 N NaOH aqueous
D
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Article
also twinned, with a twinning ratio 0.66:0.34(6). Cell
dimensions and the metrical data are also shown in Table S1
of the Supporting Information. Planes and angles for the
selected atoms are given in Table S2 of the Supporting
Information.
3
. RESULTS AND DISCUSSION
.1. Synthesis. The synthesis of the rigid rod porphyrins 1
and 3, illustrated in Schemes 1 and 2, followed the general
3
28
strategy previously reported for 2, which involves a series of
Pd-catalyzed cross-coupling reactions. The modular approach
involved a 3-step procedure in which previously synthesized
building blocks were assembled to yield 1 and 3 in 5% and 14%
total yield from 4, respectively. A requirement for the synthesis
of porphyrins 1 and 3 was the ability to introduce the internal
dipole in the linker unit by functionalizing the central phenyl
moiety with an electron donor and an acceptor group in para
position with respect to each other and to be able to switch the
position of these groups with respect to the Ipa anchor unit.
This was accomplished by using N,N-dimethyl-2,5-dibromo-
4
-nitroaniline (4) as the precursor to the central unit of the
linker. 2,5-Dibromo-4-nitroaniline was reported to exhibit
enhanced reactivity of the bromine in ortho to the nitro
group in Pd catalyzed cross-coupling reactions with alkynes, a
property that we anticipated would be retained by the N,N-
dimethyl derivative, 4. The amine was methylated to enhance
the electron donation ability of the amino group. The enhanced
reactivity of the bromine in ortho to the nitro group [Br(5),
indicated with an arrow in Scheme 1] allowed for the
regioselective coupling either with trimethylsilyl (TMS)
acetylene to form 6a, leaving Br(2) available for introduction
of the anchor Ipa unit, or to couple it directly with the anchor
Ipa unit to form 6b, leaving Br(2) available for reaction with
TMS acetylene, for subsequent coupling with the ZnTPP
chromophore. In summary, the reversal of the donor and
acceptor positions on the linker was obtained by reversing the
position of the anchor and the chromophore on the linker unit
on precursor 4. The two sequences are shown in Scheme 1.
Methyl esters 1−3 were used for solution characterization and
spectral studies (UV−vis, fluorescence) because they exhibited
good solubility in organic solvents and were hydrolyzed to the
corresponding carboxylic acids for binding to metal oxide
semiconductors. Whereas samples of 1 and 3 exhibited some
degradation over time, depending on the storage conditions
Figure 2. X-ray crystal structure of (a) H-PE(DA)-IpaOMe (8a) and
(
b) H-PE(AD)-IpaOMe (8b). Left: front-view showing the distortion
of the backbone. Right: the respective edge-on views showing the
nonlinearity of the whole molecule; planes and angles for the named
atoms are given in Table S2 of the Supporting Information.
(
vide infra), no changes were observed for samples of 2 stored
for years, suggesting that 2 is chemically more stable than 1 and
3
.
3
.2. Single Crystal X-ray Structure. Crystals of 8a, 8b,
and 1 (see Figure 2, Figure 3, and Supporting Information)
were obtained from THF solutions either cold or at room
temperature. The crystals of 1 were obtained by storing the
THF solution cold (about 4 °C). Crystals of 8a and 8b were
also obtained from ethyl acetate/hexane or dichloromethane
but were not of satisfactory quality for single crystal X-ray
structure. The crystal structures of 8a and 8b confirmed the
dipole reversal and show significant distortions both along the
molecular backbone [C1−C2−C3−C4−C5−C6 vs C13−
C16−C19−C20] (“twist” angle = 17.04 deg for 8a and 7.01
deg for 8b) and of the plane of the molecule (14.4 deg for 8a
and 8.7 deg for 8b) ascribed to crystal packing and/or the
presence of the nitro and N,N-dimethylamino substituents on
the phenyl ring. Both types of distortions are evident in Figure
2 through the front-view and edge-on-view of the two
Figure 3. X-ray crystal structure of ZnTPP-PE(DA)-IpaOMe (1).
molecules bonded to each Zn ion. There are also 5 THF
molecules of crystallization (mostly disordered, even at 100 K)
associated with these molecules; these were handled with the
49
program SQUEEZE. This removed the disordered THF
solvent molecules from the structure. These solvent molecules
cause the symmetry between the two molecules to be
̅
pseudosymmetric and the space group to be almost P1;
however, there was enough disorder in the THF molecules to
destroy this symmetry. With 1432 parameters, the refinement
converged to a conventional R value of 12.1%. The crystal is
E
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Article
a
Scheme 1. Synthesis of Dipole-Containing Linkers H-PE(DA)-IpaOMe (8a) and H-PE(AD)-IpaOMe (8b)
a
Reagents and conditions: (i) Pd(PPh ) Cl , CuI, (iPr) NH, THF and (ii) (a) K CO , CH Cl , MeOH 2.5 h, rt. (b) HCl.
3
2
2
2
2
3
2
2
molecules. We believe that the nitro group in the ortho position
in (8a) exhibits electronic repulsion of the C11−C12 acetylenic
bond (the distance of O5 to C12 is only 2.69 Å), leading to the
nonlinearity of the molecule as a whole; the “tilt” angle [defined
by the angle C11−C2−C16] is ∼3.7 deg for 8a, as shown in
Figure 2a. In 8b, the nitro group is far removed from the same
acetylenic moiety, and therefore, there is less distortion of the
molecule; the same “tilt” angle is ∼2.4 deg, as shown in Figure
reduces the symmetry to being that of the noncentrosymmetric
space group.
3.3. Spectroscopic and Electrochemical Properties.
The UV−vis absorption and emission spectra of acetonitrile
27
solutions of 1, 2, and 3 were recently reported, and these data
are overlaid in Figure 4 (panels a and c, respectively) and
compared with the spectra of the linker units in Figure 4b.
The UV−vis spectra of the linkers containing the D and A
substituents exhibited an additional absorption band centered
at 407 nm for 8a and 396 nm for 8b, compared to the linker
without substituents. The shift is not surprising as the dipole
reversal with respect to the Ipa unit occurs in a highly
conjugated system. The UV−vis spectra of 1, 2, and 3 exhibited
characteristic features of the ZnTPP chromophore, with bands
centered at 422 nm (Soret), 556 and 597 nm (Q bands), in
addition to higher energy bands assigned to the π−π*
transitions of the PE conjugated bridge. The steady-state
fluorescence emission spectra exhibited two bands at 603 and
2b. The crystal packing of 8a and 8b, shown in Figure S31 of
the Supporting Information and Figure S32 of the Supporting
Information, respectively, differ considerably: in molecule 8a,
all of the nitro groups in 1/2 of the cell point toward the a cell
direction and in the other 1/2 of the cell (by symmetry), these
nitro groups point in the −a cell direction. In 8b, the nitro
groups alternate throughout the cell going toward the b cell
direction and then toward the −b cell direction, and then
toward the b direction again. It seems that there is more of an
antiparallel orientation of the dipole for 8a than for 8b. Given
the distances between the molecular planes in the lattice (∼3.8
Å), however, it is unlikely that dipole−dipole interactions are
responsible for this arrangement.
The crystal structure of 1 in Figure 3 shows the presence of
large channels in the lattice that were filled with about five
molecules of the crystallization solvent (THF). The axial
coordination of the central zinc metal ion with two THF
molecules and an antiparallel arrangement of the internal
molecular dipoles are also evident. There is ∼4.5 Å between
molecules, leaving a large channel to accommodate the THF
molecules. The space group is P1 but is very close to being the
6
56 nm, which are consistent with the Q(0,0) and Q(1,0)
vibrational states transitions observed for the ZnTPP macro-
cycle.
Cyclic voltammetry was performed on the methyl esters 1−3
because of their solubility in organic solvents and to prevent
physisorption and decarboxylation processes usually associated
28
with the carboxylic acids. The electrochemical properties
were studied in dichloromethane with 0.1 M Bu NBF
4
4
supporting electrolyte and were referenced to saturated calomel
electrode (SCE). Porphyrins 1−3 exhibited very similar redox
28
behavior, consistent with that of ZnTPP porphyrins, and with
the first and second oxidation reversible and in the same ranges
(see Table 1).
̅
centrosymmetric P1; the presence of the THF molecules
F
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The Journal of Physical Chemistry B
Article
Scheme 2. Synthesis of 1 and 3 and Hydrolysis to the
a
Corresponding Carboxylic Acids
Figure 4. (a) Expanded Q-band region of the UV−vis absorption
spectra for 1, 2, and 3 measured in acetonitrile normalized to 0.1 at
423 nm. Inset: UV−vis absorption spectra for 1, 2, and 3. Adapted
from data reported in ref 27. (b) Absorption spectra of three linker
molecules in acetonitrile. (c) Fluorescence emission spectra
normalized at 603 nm for 1, 2, and 3 in acetonitrile (λexc = 556
nm). Adapted from data reported in ref 27.
a
Reagents and conditions: (i) Pd(PPh ) Cl , CuI, (iPr) NH, THF and
3
2
2
2
(
ii) 6 N NaOH in MeOH, CH Cl , 3h.
2 2
In summary, the electrochemical and spectroscopic proper-
ties for 1, 2, and 3 are virtually indistinguishable, indicating that
the ZnTPP chromophore properties are not affected by the
functionalization of the linker with the donor and acceptor
groups or by the dipole reversal. As described in Introduction,
this was expected since the ZnTPP chromophore is electroni-
cally decoupled from substituents on the meso phenyl rings. In
particular, the data indicate that the energy gap between excited
states and ground states are the same for all three compounds.
It is worth noticing that a small, broad shoulder centered at
about 635 nm in the absorption spectra was observed for solid
samples of 1 and 3 stored for several months at room
temperature and exposed to light. The shoulder band was
completely suppressed by adding a few drops of a ZnOAc2
MeOH solution, suggesting that it was related to the presence
of free porphyrins that could be metallized again.
Table 1. Solution Redox Potentials of 1-3 in
a
Dichloromethane Reported vs SCE
porphyrin
oxidation (V)
1st
2nd
1
2
3
0.87
0.86
0.85
1.15
1.14
1.15
a
All cyclic voltammograms were measured at a scan rate of 50 mV s−1
using a glassy carbon working electrode
withdrawing (nitro) groups in the central phenyl ring of the
linker, and the dipole reversal was achieved by switching the
position of the two groups. The UV−vis and steady-state
emission properties of the dipole-containing ZnTPP com-
pounds 1, 2, and 3 were identical. The X-ray crystal structure of
the linker units shows a distortion due to the presence of the
nitro and N,N-dimethylamino substituents on the central
phenyl ring, but the orientation of the molecules in the packing
does not seem to be influenced by the presence of internal
dipoles. Compounds 1−3 were studied to demonstrate the
4
. CONCLUSIONS
Rigid-rod Zn(II) tetraphenylporphyrins (ZnTPP) 1 and 3,
containing an intramolecular dipole in the linker, were
synthesized in a modular approach and were compared to the
previously reported 2, which is identical in structure but does
not contain substituents in the linker. The dipole was obtained
by introducing electron donor (N,N-dimethylamino) and
G
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Article
influence of a molecular dipole on the energy level alignment of
(11) Holm, A. H.; Ceccato, M.; Donkers, R. L.; Fabris, L.; Pace, G.;
Maran, F. Effect of peptide ligand dipole moments on the redox
potentials of Au38 and Au140 Nanoparticles. Langmuir 2006, 22,
27
a chromophore bound to a metal oxide semiconductor, and
time-resolved electron transfer studies of 1−3 bound to TiO2
nanoparticle films to determine the influence of such dipoles on
charge transfer kinetics are currently in progress.
1
(
0584−10589.
12) De Angelis, F.; Fantacci, S.; Selloni, A.; Gratzel, M.;
̈
Nazeeruddin, M. K. Influence of the sensitizer adsorption mode on
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ASSOCIATED CONTENT
Supporting Information
■
*
S
(13) Arnaud, G. F.; De Renzi, V.; del Pennino, U.; Biagi, R.;
Corradini, V.; Calzolari, A.; Ruini, A.; Catellani, A. Nitrocatechol/ZnO
interface: The role of dipole in a dye/metal-oxide model system. J.
Phys. Chem. C 2014, 118, 3910−3917.
Synthetic procedures for the synthesis of 2,5-dibromo-4-
1
13
nitroaniline; H and C NMR and HRMS (ESI or MALDI)
for 4, 6a, 6b, 7a, 7b, 8a, 8b, 1, 3, 1-acid, 3-acid; and FTIR-ATR
spectra of compounds 1, 3, 1-acid, and 3-acid; tables with cell
dimensions, angles, and all of the metrical data for crystal
structures of 8a, 8b, and 1; and packing diagrams of 8a and 8b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) Becucci, L.; Guryanov, I.; Maran, F.; Guidelli, R. Effect of a
strong interfacial electric field on the orientation of the dipole moment
of thiolated Aib-oligopeptides tethered to mercury on either the N- or
C-Terminus. J. Am. Chem. Soc. 2010, 132, 6194−6204.
(
́
15) Bartucci, A.; Florian, J.; Ciszek, J. W. Spectroscopic evidence of
work function alterations due to photoswitchable monolayers on gold
surfaces. J. Phys. Chem. C 2013, 117, 19471−19476.
AUTHOR INFORMATION
Corresponding Author
E-mail: galoppin@rutgers.edu.
■
(16) Evans, S. D.; Ulman, A. Surface Potential studies of alkyl-thiol
monolayers adsorbed on gold. Chem. Phys. Lett. 1990, 170, 462−466.
(17) Imanishi, Y.; Miura, Y.; Iwamoto, M.; Kimura, S.; Umemura, J.
Surface potential generation by helical peptide monolayers and
multilayers on gold surface. Proc. Jpn. Acad., Ser. B 1999, 75, 287−290.
*
Notes
The authors declare no competing financial interest.
(18) Risko, C.; Zangmeister, C. D.; Yao, Y.; Marks, T. J.; Tour, J. M.;
Ratner, M. A.; van Zee, R. D. Experimental and theoretical
identification of valence energy levels and interface dipole trends for
a family of (oligo)phenylene-ethynylenethiols adsorbed on gold. J.
Phys. Chem. C 2008, 112, 13215−1322.
ACKNOWLEDGMENTS
■
The authors are grateful to Prof. Jonathan Rochford for
assistance with the CV measurements and to Dr. Sylvie Rangan
for discussions about the dipole effect. Funding by the Division
of Chemical Sciences, Geosciences, and Biosciences, Office of
Basic Energy Sciences of the U.S. Department of Energy
through Grant DE-FG02-01ER15256 is gratefully acknowl-
edged. A.B. thanks Rutgers University for the Summer 2013
FASN Dean’s Undergraduate Research Fellowship and the
Chemistry Department for the 2014 Ian Fryer Memorial
Award.
(
19) Yasutomi, S.; Morita, T.; Imanishi, Y.; Kimura, S. A Molecular
photodiode system that can switch photocurrent direction. Science
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Molecular modification on dye-sensitized solar cells by phosphonate
self-assembled monolayers. J. Mater. Chem. 2012, 22, 2915−2921.
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DOI: 10.1021/jp5112982
J. Phys. Chem. B XXXX, XXX, XXX−XXX