Synthesis of capped tetrachelate porphyrins: Towards "insulation" of the chromophore on semiconductor surfaces

Article abstract of DOI:10.1002/jccs.201000157

A novel "capped" metal-free tetraphenylporphyrin 1, was synthesized and characterized. The cap was designed to "insulate" the chromophoric unit and prevent aggregation as well as undesired contacts on the semiconductor surface. The cap was made of a benze

Full text of DOI:10.1002/jccs.201000157

Journal of the Chinese Chemical Society, 2010, 57, 1103-1110
1103
Invited Paper
Synthesis of Capped Tetrachelate Porphyrins: Towards “Insulation” of the
Chromophore on Semiconductor Surfaces
Chi-Hang Lee, Keyur Chitre and Elena Galoppini*
Department of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102, USA
Anovel “capped” metal-free tetraphenylporphyrin 1, was synthesized and characterized. The cap was
designed to “insulate” the chromophoric unit and prevent aggregation as well as undesired contacts on the
semiconductor surface. The cap was made of a benzene unit linked by four methylenic units (-(CH2)4-) to
the meso-phenyl ring of the porphyrin core through an ether group. The COOMe groups are in meta posi-
tion for an ideal bite angle to the semiconductor surface and the constrain imposed by the cap avoids
conformational freedom of the anchoring groups. The 1H NMR spectrum of 1 suggests that the metal free
macrocycle 1 is distorted from planarity. The solution UV-Vis and fluorescence emission spectra of 1 were
typical of a metal-free tetraphenylporphyrin.
Keywords: Porphyrin; Capped porphyrin; Sensitizer; Dye.
INTRODUCTION
Dyes bound to wide bandgap metal oxide semicon-
Specifically, our group,5,6 Diau, Lin and coworkers,4
Lindsey10,8b and others11 developed anchor-spacer-por-
phyrin model compounds, designed to study the influence
of binding geometry (distance and orientation) on charge
transfer events and solar cells efficiency.
ductor nanoparticles such as TiO2 or ZnO are of great inter-
est for the development of dye-sensitized (Grätzel) solar
cells.1 Metalloporphyrins of varying complexity, such as
Zn(II) tetraphenyl porphyrin (ZnTPP) and their deriva-
tives, bound to semiconductors though COOH groups have
attracted considerable attention as sensitizing chromo-
phores. Their favorable photophysical properties and good
stability makes them excellent compounds for efficient
photovoltaic applications.2 In addition, because of their
synthetic versatility, porphyrins are particularly well-
suited for the design of model systems to study fundamen-
tal charge transfer at the molecule/semiconductor interface.
The porphyrin macrocycle easily coordinates to different
transition metal ions (Zn2+, Mg2+, etc.…),3 and can be mod-
ified and functionalized to prepare model compounds of
varying complexity, from monomers to anchor-spacer-
porphyrin systems,4-6 to self-assembled7 or covalently
linked8 oligomeric arrays (antennae). Such arrays are stud-
ied for the sensitization of nanoparticle films as well as pla-
nar semiconductor surfaces that ultimately could lead to
thin film devices.2,8,9
In the case of ZnTPP derivatives the meso-phenyl
rings, which carry the anchor groups, are nearly electroni-
cally decoupled from the porphyrin macrocycle (the chro-
mophoric unit). In our previous work with porphyrin model
compounds that are perpendicular (rigid-rods)5 and planar6
(“spiders”) to the semiconductor surface, the fact that the
ZnTPP ring is decoupled from the anchoring units has al-
lowed us to explore the molecule/surface interaction and
binding orientation while maintaining the electronic prop-
erties virtually unchanged.
Previously, we reported that DSSCs prepared from
tetrachelated porphyrins such as the “spiders” in Fig. 1 ex-
hibited increased efficiency when compared to rod models
that bind perpendicularly.6a,b Recent charge injection and
computational studies12 of the “spider” compounds, how-
ever, suggest that this design needs improvement to (a)
limit the rotational mobility of the anchoring groups, (b) in-
hibit aggregation, especially at longer binding times, and
(c) decrease the possibility of direct contacts with the semi-
conductor in mesoporous films (i.e. head down binding or
“shorting” contacts with adjacent nanoparticles).
The porphyrin macrocycle is particularly suitable for
the preparation of model dyes designed to control the ori-
entation of the porphyrin unit with respect to the surface.
Special Issue for the 2009 International Symposium of Dye-Sensitized Solar Cells
* Corresponding author. galoppin@rutgers.edu

1104 J. Chin. Chem. Soc., Vol. 57, No. 5B, 2010
Lee et al.
Here we present the synthesis of the first example of a
“capped” porphyrin designed to have all anchoring groups
towards the same direction, with ideal bite angle of the
COOH groups for the semiconductor (in meta position) and
with a “cap”, intended to prevent aggregation and contacts
with the semiconductor, Fig. 2. While the synthetic com-
plexity of the target molecule clearly prevents its use for
purely practical DSSCs applications, the scope of this work
is to develop an improved model compound for fundamen-
tal charge transfer studies that is designed to decrease het-
erogeneity, aggregation, and conformational effects on the
surface of semiconductors, thereby “insulating” the chro-
mophoric unit on the semiconductor surface. Additionally,
the aromatic ring in the cap could be eventually replaced by
a photo- or redox-active unit (electron donor, or acceptor).
Capped porphyrins have been reported, especially to
mimic naturally-occurring catalytic centers,13-24 A few
crystal structures of capped porphyrins have been pub-
lished.13 However, examples of this kind of system in the
literature remain relatively few, because the addition of the
cap requires longer, and low yielding (< 10%), synthetic
pathways, and imposes limitations on the functional groups
that can be introduced on the molecule. Porphyrins have
been capped with calix[4]arenes,14 4,4¢-bipyridyl,15 binap,16
biphenyl,17 phenanthroline,18 bicyclo[2.2.2.]octane,19 and
salen,20 There are examples of bidentate21 as well as tetra-
dentate22 caps. The most challenging step in such syntheses
is the formation of the porphyrin ring: this has been done by
condensation of tetra-aldehydes and pyrrole, or by adding
the cap later with ester, azide23 or amide linkages.24
We describe here the design and synthesis of a novel
metal-free capped porphyrin 1. While the results are still
preliminary, as yields are very low, we introduce here a
novel compound that can be used to test an approach to sen-
sitization studies where the dye unit is “insulated” on the
semiconductor surface. The cap is made of a benzene ring
linked to the macrocycle through four alkyl chains linked
to the porphyrin core through an ether group. Group as es-
ters, ketones, amino, amides, etc, were synthetically more
accessible, but were avoided because they may physisorb,
or bind, to metal-oxide surfaces, leading to head-down
binding or unwanted interactions to the surface. Instead,
the synthesis of a cap made of only methylene linkages,
with no functional groups, would have required a consider-
ably more complex synthesis.25 In compound 1, the chain
linking the porphyrin macrocyle to the cap is made of four
methylenic units (-(CH2)4-). It has been reported that
shorter chains may inhibit the cyclization step, and longer
chains would not provide the conformational constrain that
was needed.26 The anchoring COOMe groups are in meta-
position for a better bite angle to the metal oxide surface.27
Fig. 1. Tetrachelate porphyrins designed to bind pla-
nar to the semiconductor surface.6
RESULTS AND DISCUSSION
Synthesis
For the synthesis of 1 we adapted a route reported by
Crook and coworkers, Scheme I.28 Benzene was iodinated
with periodic acid and potassium iodide in concentrated
Fig. 2. (a) Chemical structure (left) and schematic dia-
gram (right) of 1. (b) Minimized molecular
model of 1 (Spartan ’08, MM2).

Synthesis of Capped Tetrachelate Porphyrins
J. Chin. Chem. Soc., Vol. 57, No. 5B, 2010 1105
Scheme I Synthesis of porphyrin 1
sulfuric acid to yield 1,2,4,5-Tetraiodobenzene (2) in 40%
yields.29 Tetraiodide 2 was coupled with excess 3-butyn-
1-ol in Sonogashira Pd-catalyzed cross coupling condi-
tions to yield 1,2,4,5-tetrakis(4-hydroxylbuty-3-nyl)ben-
zene (3) in moderate (46%) yields. Attempts to synthesize
3 by cross-coupling 1,2,4,5-tetrabromobenzene with 3-bu-
tyn-1-ol resulted in lower yields. Tetra-ethyne (3) was hy-
drogenated in the presence of 10% Pd/C at room tempera-
ture and atmospheric pressure to yield 1,2,4,5-tetrakis(4-
hydroxylbutyl)benzene (4) in 52% yield. Tetrol 4 was re-
acted with carbon tetrabromide and triphenylphosphine to
yield the corresponding tetrabromide, 1,2,4,5-tetrakis(4-
bromobutyl)benzene (5), in 59% yield. Nucleophilic sub-
stitution of 5 with excess methyl 3-formyl-4-hydroxyben-
zoate in the presence of potassium carbonate in DMF
yielded tetraaldehyde 6 in almost quantitative yields. Com-
pound 6 underwent cyclo-tetramerization with pyrrole in
propinoic acid with the addition of DDQ30 to form the
metal-free capped porphyrin (1). Unfortunately, the cycliza-
tion step proceeded in very low yields (less than 1 percent).
We are currently working on improving the yield of the
cyclization step to obtain 1 in sufficient amount for DSSCs
studies, and an alternate route to capped porphyrins is in
progress.
1H NMR
Metal-free porphyrins are not planar, and the distor-
1
tion from planarity was detected in the H NMR and the
spectroscopic spectra (absorption and emission) of 1. For
instance, we observed that in the metallated tetrachelated

1106 J. Chin. Chem. Soc., Vol. 57, No. 5B, 2010
Lee et al.
porphyrins, such as m-ZnTPPA, the splitting of the of pro-
tons of the pyrrolic ring is less than 0.05 ppm while in
metal-free porpyrins splitting of the pyrrolic protons of the
macrocycle is larger (about 0.2 ppm).31 In capped porphy-
rin 1, protons were split into two sets (K and M, at 8.55 and
8.78 ppm), Fig. 3. Also, the CH2 groups next to the ether
linkage (G and H, at 4.0 and 4.2 ppm) were split into two
sets, due to the ring current, Fig. 4.
Fig. 3. Selected region of the 1H NMR Spectrum of 1.
Fig. 4. Selected region of the 1H NMR Spectrum of 1.

Synthesis of Capped Tetrachelate Porphyrins
J. Chin. Chem. Soc., Vol. 57, No. 5B, 2010 1107
Absorption and Emission Properties of 1
EXPERIMENTAL SECTION
General Methods
Reactions were carried out in flame-dried glassware
The UV-Vis spectrum of 1 (Fig. 5) is typical of a
metal-free porphyrin (Q bands region) and it was recorded
in CHCl3 at varying concentrations (from 0.2 mM to 10
mM). A plot of the absorption against concentration shows
a linear relation in the range of concentrations studied (Fig.
5, inset). The lack of spectral shifts in the absorption spec-
trum and the linearity of the Lambert-Beer’s plot suggests
that, as anticipated, the porphyrin macrocycle did not ag-
gregate in diluted chloroform solutions.
under nitrogen atmosphere. THF was distilled over so-
dium/benzophenone. Dichloromethane was distilled over
calcium hydride. Melting points are uncorrected. 1H NMR
(500 MHz) and 13C NMR (125 MHz) spectra were col-
lected by a Varian INOVA 500 spectrometer operating at
499.896 MHz for 1H, 125.711 MHz for 13C at room temper-
ature in CDCl3, THF-d8 or acetone-d6 as noted. Chemical
shifts were reported relative to internal tetramethylsilane (d
0.00 ppm) or CDCl3 (d 7.26 ppm), THF-d8 (d 1.73 ppm) or
acetone-d6 (d 2.05 ppm) for 1H and CDCl3 (d 77.0 ppm),
THF-d8 (d 25.37 ppm) or acetone-d6 (d 29.92 ppm) for 13C.
Absorption spectra were obtained at room temperature by a
VARIAN Cary-500 spectrophotometer. Emission spectra
were recorded at room temperature on a VARIAN Cary-
Eclipse fluorescence spectrophotometer. Infrared (IR) spec-
tra were obtained at room temperature on a Thermo Elec-
tron Corporation’s Nicolet 6700 FT-IR spectrometer using
ZnSe crystal or KBr in pellet. Mass spectra were obtained
by GC-MS or ESI-MS. Data are reported for the molecular
ion or protonated molecular ion. GC-MS were collected
from a HP 6890 gas chromatograph with a HP 5973 MS de-
tector. High resolution mass spectra were recorded on a
high-resolution mass spectrometer in electron impact mode
(Hunter College MS Facility, NY). Elemental analyses
were determined by CHN Analyzer with Autobalance from
a commercial company (Quantitative Technologies, Inc.).
Flash column chromatography was performed with silica
gel (230-400 mesh) and TLC on aluminum backed silica
gel (200 mm thick).
The emission spectrum of 1 in Fig. 6 is also typical of
metal-free non-aggregated tetraphenyl porphyrins.32
Fig. 5. UV-Vis spectra of 1 at different concentration
in CHCl3. Inset: plot of absorption vs. concen-
tration.
1,2,4,5-Tetraiodobenzene (2)31
To a stirring solution of concentrated H2SO4 (300
mL) at 0 °C, periodic acid (12.8 g, 56 mmol) was added in 1
portion. Crushed potassium iodide (27.9 g, 168 mmol) was
added in small portions followed by dropwise addition of
HPLC-grade benzene (5 mL) to the resulting dark brown
solution. The solution was stirred for 3 days at 40 °C. Then,
the mixture was poured onto ice/water. Excess sodium
metabisulfite was added cautiously and in small portions
into the mixture to remove the excess iodine. Then, the
crude pink precipitate was filtered, triturated with metha-
nol and filtered. The crude product was triturated in THF/
MeOH to yield a pale yellow solid (17.2 g, 40%). The pu-
Fig. 6. UV-Vis (solid line) and fluorescence emission
1
(dotted line) spectra of 1 in CHCl3 (5 mM; lexc
=
rity of the compound was determined by GC-MS and H
410 nm).
NMR (the spectra matched those reported in literature, ref

1108 J. Chin. Chem. Soc., Vol. 57, No. 5B, 2010
Lee et al.
31).
tion was filtered and the filtrate was evaporated to yield 5
as a viscous yellow liquid. The liquid was purified by sil-
ica-gel column chromatography (hexane and then gradient
to CHCl3). The product was eluted as the second compo-
nent (0.25 g, 59%). 1H NMR (CDCl3, d = 7.26) 6.91 (s, 2
H), 3.44-3.47 (t, J = 7 Hz, 8 H), 2.57-2.61 (t, J = 8 Hz, 8 H),
1.92-1.98 (m, 8 H), 1.70-1.76 (m, 8 H); 13C NMR (CDCl3, d
= 77.00) 137.2, 130.1, 33.7, 32.6, 31.3, 29.7; HRMS (ESI+
1,2,4,5-Tetrakis(4-hydroxylbuty-3-nyl)benzene (3)
To a stirring solution of 1,2,4,5-tetraiodobenzene
(10.0 g, 17 mmol) in THF (250 mL), were added 3-butyn-
1-ol (12.04 g, 172 mmol), PdCl2(PPh3)2 (1.21 g, 1.7 mmol),
copper(I) iodide (1.65 g, 3.4 mmol), triphenylphosphine
(0.90 g, 3.4 mmol) and triethylamine (150 mL). The reac-
tion mixture was stirred for 2 days at 50 °C, concentrated in
vacuo, and partitioned between CHCl3 and water. The or-
ganic solution was then evaporated and purification by sil-
ica-gel column chromatography (gradient from hexane to
THF) yielded 3 as a pale yellow powder. The crude product
was triturated with THF to yield 3 as white solid (2.79 g,
46%). Compound 3 was not stable at ambient temperature
when exposed to light. 1H NMR (acetone-d6, d = 2.05) 7.38
(s, 2 H), 3.96-3.99 (t, J = 6 Hz, 4 H), 3.73-3.77 (q, J = 6.5
Hz, 8 H), 2.64-2.67 (t, J = 6.5 Hz, 8 H); 13C NMR (ace-
tone-d6, d = 29.92) 135.6, 126.4, 94.5, 80.2, 61.5, 24.7;
HRMS (ESI+ - TOF) m/z: Calcd for C22H22O4+ = 350.1518;
Found 350.1516 [M]+; IR (cm-1) n = 2881, 2231, 1629,
1486, 1403, 1328, 1182, 1039, 894, 847; Anal. Calcd for
C22H22O4: C (75.41%) H (6.33%). Found: C (75.03%) H
(6.44%). M.p. 176-178 °C.
+
- TOF) m/z: Calcd for C22H34Br4 = 613.9393; Found
613.9396 [M]+; IR (cm-1) n = 3052, 2940, 2866, 1503,
1460, 1439, 1266, 896, 728, 705; Anal. Calcd for C22H34Br4:
C (42.75%) H (5.54%). Found: C (43.19%) H (5.60%).
Tetramethyl 4,4¢,4²,4¢²-(4,4¢,4²,4¢²-(benzene-1,2,4,5-
tetrayl)tetrakis(butane-4,1-diyl))tetrakis(oxy)tetra-
kis(3-formylbenzoate) (6)
To a stirred solution of 5 (1.0 g, 1.6 mmol) in dry
DMF, were added methyl 3-formyl-4-hydroxybenzoate
(2.62 g, 14.6 mmol) and K2CO3 (4.02 g, 29.1 mmol). The
solution was stirred at 60 °C for 4 days. It was poured into
water and then filtered. The white solid residue was used
without further purification. The product was insoluble in
most solvents and was used directly in the next step. HRMS
(ESI+ - TOF) m/z: Calcd for C58H62O16 = 1014.4037;
+
1,2,4,5-tetrakis(4-hydroxylbutyl)benzene (4)
Found 1014.4029 [M]+; IR (cm-1) n = 2939, 2873, 1725,
1685, 1609, 1499, 1440, 1423, 1314, 1271, 1193, 1128,
1028, 958, 928, 932, 766; Anal. Calcd for C58H62O16: C
(68.63%) H (6.16%). Found: C (68.27%) H (5.88%). M.p.
190-191 °C.
To a stirred solution of 3 (0.80 g, 2.3 mmol) in ethanol
(30 mL), was added palladium on charcoal (10%, 0.49 g) in
one portion. The flask was evacuated in vacuo and loaded
with hydrogen, and the reaction mixture was stirred under
H2 atmosphere overnight. The solution was filtered through
Celite, concentrated and purified by silica gel column chro-
matography (gradient: hexane, ethyl acetate:hexane (20:
80) and then THF) to yield 4 as a white solid as the final
portion (0.44 g, 52%). 1H NMR (acetone-d6, d = 2.05) 6.94
(s, 2 H), 3.56-3.60 (p, J = 5.5 Hz, 8 H), 3.43-3.45 (t, J = 6.5
Hz, 4 H), 2.58-2.61 (t, J = 7 Hz, 8 H), 1.58-1.65 (m, 16); 13C
1: To a stirred, refluxing solution of 6 (0.5 g, 0.5
mmol) in refluxing propanoic acid (217 mL), was added
pyrrole (0.27 mL, 3.9 mmol) dropwise. The solution changed
from yellow, to brown, to purple within 1.5 h. DDQ (1
mmol) was added and the solution was refluxed for an addi-
tional 30 min. After that, the solution was cooled with con-
tinuous bubbling of air to oxygenate the solution and en-
hance the formation of porphyrins. The solution was con-
centrated in vacuo to remove most of the solvent (pro-
panoic acid). The solution was filtered through Celite and
the filtrate was concentrated and column chromatographed
with CH2Cl2 and then THF with alumina. 1H NMR (CDCl3,
d = 7.26) 8.78 (s, 4 H), 8.61 (s, 4 H), 8.55 (s, 4 H), 8.47-8.49
(d, J = 8.5 Hz, 4 H), 8.36-8.38 (d, J = 8.5 Hz, 4 H), 4.15-
4.25 (m, 4 H), 3.95-4.05 (m, 4 H), 3.88 (s, 12 H), 3.81 (s,
capped proton, 2 H), 1.20-1.30 (m, 4 H), 1.05-1.15 (m, 8
H), 0.6-0.7 (m, 4 H), 0.2-0.4 (m, 8 H); 13C NMR (CDCl3, d
= 77.00) 167.1, 162.6, 136.4, 134.9, 132.0, 131.6, 129.0,
121.5, 114.8, 111.3, 69.7, 53.4, 52.0, 30.3, 29.7, 28.2, 27.9;
NMR (acetone-d6, d = 29.92) 138.4, 131.0, 62.4, 33.8,
+
32.9, 28.8; HRMS (ESI+ - TOF) m/z: Calcd for C22H38O4
=
366.2770; Found 366.2763 [M]+; IR (cm-1) n = 3340, 3029,
2930, 2862, 1501, 1479, 1433, 1347, 1074, 1064, 1055,
1033, 934, 920, 889, 880; Anal. Calcd for C22H38O4: C
(72.09%) H (10.45%). Found: C (72.35%) H (10.02%).
M.p. 176-178 °C.
1,2,4,5-tetrakis(4-bromobutyl)benzene (5)
To a solution of 4 (0.25 g, 0.7 mmol) in CH2Cl2, were
added CBr4 (1.80 g, 5.4 mmol) and triphenylphosphine
(1.40 g, 5.4 mmol) at 0 °C in one portion. The solution was
stirred overnight and then concentrated in vacuo. The solu-

Synthesis of Capped Tetrachelate Porphyrins
J. Chin. Chem. Soc., Vol. 57, No. 5B, 2010 1109
HRMS (ESI+-TOF) m/z: Calcd for C74H68N4O12
=
=
+
Diau, E. W. G. J. Phys. Chem. C 2009, 113(1), 755-764. (b)
Chang, C.-W.; Luo, L.; Chou, C.-K.; Lo, C.-F.; Lin, C.-Y.;
Hung, C. S.; Lee, Y.-P.; Diau, E. W.-G. J. Phys. Chem. C
2009, 113(27), 11524-11531. (c) Lo, C.-F.; Luo, L.; Diau, E.
W.-G.; Chang, I.-J.; Lin, C.-Y. Chem. Commun. 2006, 13,
1430-1432.
1204.4834; Found 1204.4831 [M]+); UV-Vis: log e420
4.35.
CONCLUSIONS
We propose the application of capped porphyrins in
DSSCs studies, and describe here the first approach to such
class of model sensitizers. Capped porphyrin 1 was synthe-
sized and characterized, but the yield of the cyclization
step, the last step of the synthesis, was extremely low. Mod-
ifications of reaction conditions and other reaction path-
ways are in progress to prepare 1 or homologues for DSSCs
studies.
5. (a) Rochford, J.; Galoppini, E. Langmuir 2008, 24(10),
5366-5374. (b) Rangan, S.; Katalinic, S.; Thorpe, R.;
Bartynski, R. A.; Rochford, J.; Galoppini, E. J. Phys. Chem.
C 2010, 114, 1139-1147.
6. (a) de Tacconi, N. R.; Chanmanee, W.; Rajeshwar, K.; Roch-
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ACKNOWLEDGEMENT
This work was funded 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.
7. Drain, C. M.; Varotto, A.; Radivojevic, I. Chem. Rev. 2009,
109(5), 1630-1658.
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Supporting Information Available
NMR spectra of 1, 3, 4, and 5.
Received December 3, 2009.
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31. This behavior was observed in a second capped porphyrin
compound the synthesis of which is in progress in our lab,
Lee and Galoppini, unpublished results.
32. For Zn(II) porphyrins, two Q bands are observable ~ 600 and
640 nm.
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