Synthesis of strapped porphyrins: Toward isolation of the chromophore on semiconductor surfaces

Article abstract of DOI:10.1021/jo100434t

Figure presented Strapped porphyrins, designed to bind planar to semiconductor surfaces, were synthesized as model dyes to study the sensitization processes on metal oxide semiconductor nanoparticles surfaces. The strap was added to limit porphyrin-porphy

Full text of DOI:10.1021/jo100434t

pubs.acs.org/joc
Synthesis of Strapped Porphyrins: Toward Isolation of the Chromophore
on Semiconductor Surfaces
Chi-Hang Lee and Elena Galoppini*
Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102
galoppin@rutgers.edu
Received March 12, 2010
Strapped porphyrins, designed to bind planar to semiconductor surfaces, were synthesized as model
dyes to study the sensitization processes on metal oxide semiconductor nanoparticles surfaces. The
strap was added to limit porphyrin-porphyrin stacking and contacts with the semiconductor, while
constraining the anchoring groups’ orientation. Diester ZnPorOMe2 (2b) was made of a Zn(II)
5,10,15,20-tetraphenylporphyrin (ZnTPP) with the 5 and 15 meso-phenyl groups tethered by a 1,4-
phenylenebis(butyl-4-oxy) unit (the strap) and carrying a methylbenzoic ester (the anchoring group)
in the meta position. The synthesis involved a useful intermediate, dibromide ZnPorOBr2 (2a). The
X-ray structure of the corresponding metal-free H2PorOBr2 shows that, in the crystal, the strap is
located on one side above the macrocycle and tilted with respect to the porphyrin plane. The
1
1
characterization included H-1H COSY, H-13C HMQC, and DEPT. The spectroscopic and
electrochemical properties of 2b varied little when compared to ZnTPP, suggesting that the properties
are not greatly influenced by the tether or the anchor groups. The HOMO-LUMO gap for 2b, at
2.14 V, was typical for a ZnTPP derivative. Compound 2b was bound to TiO2 and ZrO2 nanoparticle
films. On TiO2, the fluorescence emission was fully quenched, while the emission on insulating ZrO2
was similar to the solution spectra.
Introduction
the sensitization process of semiconductors used in dye-sensitized
solar cells (DSSCs). For instance, we reported that DSSCs pre-
pared from tetrachelated Zn(II) tetraphenylporphyrin (ZnTPP)
derivatives 1a-c, Figure 1, designed to bind planar to the semi-
conductor surface, were more efficient than those prepared
from rigid-rod ZnTPP models designed to bind perpendicu-
larly.10,11a,13a,13b This was observed on cells prepared from
TiO2 nanoparticles13b as well as from TiO2 nanotube13a elec-
trodes. For example, IPCE values at the Q bands of 1a
(m-ZnTCPP) and 1b were 20-fold higher with respect to the
IPCE values observed in the same region for p-ZnTCPP, which
is binding normal to the surface, Figure 1.13a Additionally,
p-ZnTCPP and porphyrin rigid-rods aggregated considerably,
even at low surface coverage.11a,13a,13b The orientation effect on
DSSCs efficiencies for m- and p-ZnTCPP was first probed by
Officer and co-workers.1b
The study of porphyrins as sensitizing dyes for wide band gap
metal oxide semiconductor nanoparticles (typically TiO2, SnO2,
or ZnO) has attracted much interest in view of the potential
technological applications for renewable energy.1 Porphyrins
are excellent sensitizers from a practical and fundamental
standpoint because they possess favorable photophysical pro-
perties, including excellent light harvesting in the visible, and are
photostable.2 In addition, the porphyrin macrocycle can host a
variety of metal ions,3 and its synthetic versatility is ideal for the
design of model compounds of varying complexity. For in-
stance, porphyrin oligomeric arrays are studied for numerous
applications, ranging from antennae for artificial photosynth-
esis to light-harvesting arrays in renewable energy projects.1,4-8
More recently, anchor-spacer-porphyrin systems1b,d,e,i,5,9-13
have been developed as model dyes to understand the kinetics of
3692 J. Org. Chem. 2010, 75, 3692–3704
Published on Web 05/07/2010
DOI: 10.1021/jo100434t
r
2010 American Chemical Society

Lee and Galoppini
JOCArticle
porphyrins 2 and 3,15 designed to bind planar to the surface
of the semiconductor and having the top plane of the ring
protected by a single tether (strap) or a double tether (cap),
respectively, Figure 2. The main goal is to use such model
compounds to study charge-transfer kinetics on TiO2 or
ZnO, as 2 and 3 may provide better control over the binding
geometry, and, ultimately, to a better understanding of
injection and recombination dynamics. The strap was added
to limit or prevent porphyrin-porphyrin stacking and con-
tacts with the semiconductor while providing a constraint on
the conformation of the meso-phenyl rings.
One attractive aspect of this design is that the phenyl ring
in the strap (or cap) could eventually be replaced by a photo-
or redox-active unit (i.e., an electron donor, for instance). We
have followed Lindsey’s classification16 and named 2 as
“strapped” and 3 as “capped” porphyrins. We recently
reported the synthesis of milligram amounts of metal-free
3.15 Typically, however, the syntheses of capped porphyrins
similar to 3 require long and low-yielding pathways.17
FIGURE 1. Tetrachelate porphyrins 1a-c bind planar to the semi-
conductor surface. The calculated height increases from 3.6 A (1a)
˚
13b
˚
to 7.1 A (1b) to 9.1 A (1c).
˚
Recent charge-transfer and computational studies14 of the
tetrachelated ZnTPP compounds, however, suggest that it is
necessary to design an improved generation of models to (a)
inhibit porphyrin stacking, (b) limit the conformational
mobility of the phenyl rings carrying the anchor group,
and (c) decrease the possibility of direct contacts with the
semiconductor in mesoporous films (i.e., head-down bind-
ing or “shorting” contacts10 with adjacent nanoparticles).
To improve the design, we have now synthesized ZnTPP
(9) (a) Schmidt-Mende, L.; Campbell, W. M.; Wang, Q.; Jolley, K. W.;
€
Officer, D. L.; Nazeeruddin, Md. K.; Gratzel, M. ChemPhysChem 2005, 6,
1253–1258. (b) Chou, J.-H., Nalwa, H. S.; Kosal, M. E.; Rakow, N. A.; Suslick, K.
S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: New York, 2000; Vol. 6, pp 43-131. (c) Balanay, M. P.;
Dipaling, C. V. P.; Lee, S. H.; Kim, D. H.; Lee, K. H. Sol. Energy Mater. Sol.
Cells 2007, 91, 1775–1781. (d) Montanari, I.; Nelson, J.; Durrant, J. R. J. Phys.
Chem. B 2002, 106, 12203–12210.
(1) (a) Kamat, P. V. J. Phys. Chem. C. 2007, 111, 2834–2860. (b) Campbell,
W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. Rev.
2004, 248, 1363–1379. (c) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem.
Res. 2001, 34, 40–48. (d) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Durrant,
J. R.; Klug, D. R. J. Phys. Chem. B. 2000, 104, 1198–1205. (e) Campbell,
W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K. C.;
(10) (a) Lin, C.-Y.; Lo, C.-F.; Luo, L.; Lu, H.-P.; Hung, C.-S.; Diau, E.
W.-G. J. Phys. Chem. C 2009, 113, 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, 11524–11531. (c) Lo, C.-F.; Luo, L.; Diau, E.
W.-G.; Chang, I.-J.; Lin, C.-Y. Chem. Commun. 2006, 13, 1430–1432.
(11) (a) Loewe, R. S.; Ambroise, A.; Muthukumaran, K.; Padmaja, K.;
Lysenko, A. B.; Mathur, G.; Li, Q.; Bocian, D. F.; Misra, V.; Lindsey, J. S. J.
Org. Chem. 2004, 69, 1453.
€
Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Gratzel, M.; Officer,
D. L. J. Phys. Chem. C. 2007, 111, 11760–11762. (f) Cherian, S.; Wamser,
ꢀ
C. C. J. Phys. Chem. B. 2000, 104, 3624–3629. (g) Odobel, F.; Blart, E.; Lagree,
(12) (a) Rochford, J.; Galoppini, E. Langmuir 2008, 24, 5366–5374. (b)
Rangan, S.; Katalinic, S.; Thorpe, R.; Bartynski, R. A.; Rochford, J.;
Galoppini, E. J. Phys. Chem. C 2010, 114, 1139–1147.
M.; Villieras, M.; Boujtita, H.; El Murr, N.; Caramori, S.; Bignozzi, C. A.
J. Mater. Chem. 2003, 13, 502–510. (h) Hasobe, T.; Fukuzumi, S.; Kamat,
P. V. J. Phys. Chem. B 2006, 110, 25477–25484. (i) Nazeeruddin, Md.
K.; Humphry-Baker, R.; Officer, D. L.; Campbell, W. M.; Burrell, A. K.;
(13) (a) de Tacconi, N. R.; Chanmanee, W.; Rajeshwar, K.; Rochford, J.;
Galoppini, E. J. Phys. Chem. C 2009, 113, 2996–3006. (b) Rochford, J.; Chu,
D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129, 4655–4665. (c)
Galoppini, E.; Rochford, J.; Chen, H.; Saraf, G.; Lu, Y.; Hagfeldt, A.;
Boschloo, G. J. Phys. Chem. B 2006, 110, 16159–16161.
€
Gratzel, M. Langmuir 2004, 20, 6514–6517. (j) Clifford, J. N.; Palomares, E.;
Nazeeruddin, Md. K.; Gratzel, M.; Nelson, J.; Li, X.; Long, N. J.; Durrant,
€
J. R. J. Am. Chem. Soc. 2004, 126, 5225. (k) Durrant, J. R.; Haque, S. A.;
Palomares, E. Coord. Chem. Rev. 2004, 248, 1247. (l) Hasobe, T.; Hattori, S.;
Kamat, P. V.; Urano, Y; Umezawa, N.; Nagano, T.; Fukuzumi, S. Chem.
Phys. 2005, 319, 243. (m) Hasobe, T.; Hattori, S.; Kamat, P. V.; Fukuzumi,
S. Tetrahedron 2006, 62, 1937. (n) Eu, S.; Hayashi, S.; Umeyama, T.; Oguro,
A.; Kawasaki, M.; Kadota, N.; Matano, Y.; Imahori, H. J. Phys. Chem. C
2007, 111, 3528. (o) Kira, A.; Tanaka, M.; Umeyama, T.; Matano, Y.;
Yoshimoto, N.; Zhang, Y.; Ye, S.; Lehtivuori, H.; Tkachenko, N. V.;
Lemmetyinen, H.; Imahori, H. J. Phys. Chem. C 2007, 111, 13618. (p) Tanaka,
M.; Hayashi, S.; Eu, S.; Umeyama, T.; Matano, Y.; Imahori, H. Chem.
Commun. 2007, 2069.
€
(14) Sundstrom, V.; Persson, P. Unpublished results. Some of the results
were presented at the 2009 International Symposium on Dye-Sensitized
Solar Cells , 22-23 Oct 2009, Jhongli City, Taoyuan, Taiwan.
(15) A viable approach to porphyrin 2, designed to have all anchoring
groups toward the same direction, is in progress, although we have synthe-
sized a few milligrams of the metal-free homologue of 2. Lee, C.-H.; Chitre,
K.; Galoppini, E. Submitted for publication.
(16) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. Tetrahedron 1997, 53,
6755–6790.
(17) (a) Johnson, M. R.; Seok, W. K.; Ibers, J. A. J. Am. Chem. Soc. 1991,
113, 3998–4000. (b) Slebodnick, C.; Duval, M. L.; Ibers, J. A. Inorg. Chem.
1996, 35, 3607–3613. (c) Maruyama, K.; Kobayashi, F.; Osuka, A. Bull.
Chem. Soc. Jpn. 1990, 63, 2672–2681. (d) Battersby, A. R.; Howson, W.;
Hamilton, A. D. Chem. Commun. 1982, 21, 1266–1268. (e) Zaitseva, S. V.;
Zdanovich, S. A.; Semeikin, A. S.; Golubchikov, O. A. Russ. J. Gen. Chem.
2003, 73, 467–472. (f) Baldwin, J. E.; Cameron, J. H.; Crossley, M. J.; Dagley,
I. J.; Hall, S. R.; Klose, T. Dalton Trans. 1984, 8, 1739–1746. (g) Crossley, M.
J.; Thordarson, P. Angew. Chem., Int. Ed. 2002, 41, 1709–1712. (h) Shimizu,
M.; Basolo, F. Inorg. Chim. Acta 1984, 91, 247–250. (i) Almog, J.; Baldwin, J.
E.; Dyer, R. L.; Peters, M. J. Am. Chem. Soc. 1975, 97, 226–227. (j) Nagasaki,
T.; Fujishima, H.; Shinkai, S. Chem. Lett. 1994, 23, 989–992. (k) Panda, P.
K.; Lee, C.-H. Org. Lett. 2004, 6, 671–674. (l) Leighton, P.; Sanders, J. K. M.
Chem. Commun. 1984, 13, 854–856. (m) Collman, J. P.; Zhang, X.; Lee, V. J.;
Brauman, J. I. Chem. Commun. 1992, 13, 1647–1649. (n) Lee, C. H.; Garcia,
B.; Bruice, T. C. J. Am. Chem. Soc. 1990, 112, 6434–6435. (o) Blasko, A.;
Garcia, B.; Bruice, T. C. J. Org. Chem. 1993, 58, 5738–5747. (p) Garcia, B.;
Lee, C. H.; Blasko, A.; Bruice, T. C. J. Am. Chem. Soc. 1991, 113, 8118–8126.
(q) Wytko, J. A.; Graf, E.; Weiss, J. J. Org. Chem. 1992, 57, 1015–1018. (r)
Giraudeau, A.; Gisselbrecht, J. P.; Gross, M.; Weiss, J. Chem. Commun.
1993, 13, 1103–1105. (s) Zhang, H.-Y.; Blasko, A.; Yu, J.-Q.; Bruice, T. C. J.
Am. Chem. Soc. 1992, 114, 6621–6630. (t) Zhang, H.-Y.; Bruice, T. C. Inorg.
Chim. Acta 1996, 247, 195–202.
(2) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.,
Ed.; Academic Press: New York, 2000; Vols. 8 and 9.
€
(3) (a) Dean, M. L.; Schmink, J. R.; Leadbeater, N. E.; Bruckner, C.
Dalton Trans. 2008, 1341–1345. (b) McCarthy, J. R.; Melfi, P. J.; Capetta, S.
€
H.; Bruckner, C. Tetrahedron 2003, 59, 9137–9146.
(4) Mozer, A. J.; Griffith, M. J.; Tsekouras, G.; Wagner, P.; Wallace, G.
G.; Mori, S.; Sunahara, K.; Miyashita, M.; Earles, J. C.; Gordon, K. C.; Du,
L.; Katoh, R.; Furube, A.; Officer, D. L. J. Am. Chem. Soc. 2009, 131, 15621–
15623.
(5) (a) Stromberg, J. R.; Marton, A.; Kee, H. L.; Kirmaier, C.; Diers, J.
R.; Muthiah, C.; Taniguchi, M.; Lindsey, J. S.; Bocian, D. F.; Meyer, G. J.;
Holten, D. J. Phys. Chem. C. 2007, 111, 15464–15478. (b) Muthiah, C.;
Taniguchi, M.; Kim, H.-J.; Schmidt, I.; Kee, H. L.; Holten, D.; Bocian, D. F.;
Lindsey, J. S. Photochem. Photobiol. 2007, 83, 1513.
(6) Drain, C. M.; Varotto, A.; Radivojevic, I. Chem. Rev. 2009, 109, 1630–
1658.
(7) Brune, A.; Jeong, G.; Liddell, P. A.; Sotomura, T.; Moore, T. A.;
Moore, A. L.; Gust, D. Langmuir 2004, 20, 8366.
(8) (a) Koehorst, R. B. M.; Boschloo, G. K.; Savenije, T. J.; Goossens, A.;
Schaafsma, T. J. J. Phys. Chem. B 2000, 104, 2371–2377. (b) Hasselman, G.
M.; Watson, D. F.; Stromberg, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J.
S.; Meyer, G. J. J. Phys. Chem. B 2006, 110, 25430–25440.
J. Org. Chem. Vol. 75, No. 11, 2010 3693

Lee and Galoppini
JOCArticle
FIGURE 2. Meso-strapped and meso-capped porphyrins.
In this paper, we describe a practical synthesis of strapped
porphyrins 2a and 2b, their characterization, and proper-
ties in solution and bound. Porphyrin dibromide 2a does
not have anchor units to bind to a semiconductor surface
but is a useful precursor, as the Br group can be used in
cross-coupling reactions to introduce a variety of anchor
units.
Strapped porphyrins have been reported before, although
only a few carry functional groups on the tethered meso rings.
They are dividedinto two main categories: β-strapped,21 with
the tethers attached to the pyrrole units, and meso-strapped
(as in 2), with the tethers attached to the meso-phenyl rings.
We selected the latter design to build models that are directly
comparable to the ZnTPP models that we studied pre-
viously11 and to determine the influence of the conforma-
tional restriction of the strap on the meso-phenyl rings
carrying the anchor groups (Figure 2).14 In addition, since
the meso-phenyl rings are electronically decoupled from the
chromophore,11 the presence of the strap would not result in
large spectroscopic changes.
While the synthetic complexity of 2 and 3 prevents their
use for purely practical applications, our interest is to devel-
op novel model compounds for fundamental charge transfer
studies and that are designed to decrease binding hetero-
geneity and aggregation and to achieve better control over
the porphyrin/semiconductor interface. We do not anti-
cipate an improvement in DSSCs efficiencies, rather in
improved binding geometry control and, possibly, less com-
plex injection and recombination dynamics. Recent years
have seen an increased effort toward the synthesis of model
anchor-spacer-dyes to tune the HOMO-LUMO gap, the
spacer’s length and conjugation, and the orientation of the
dye unit on the semiconductor surface because such struc-
tural variations influence key parameters of DSSCs devices
and the dyes’ properties.18 The design of porphyrin dyes
1-3 is part of our continuing effort to achieve a greater
control and understanding over this important interface.
In addition, it will be interesting to determine to what
extent the cap and tether unit can protect the chromophoric
macrocycle on the semiconductor surface, in what can be
called an “isolation effect”. This approach, which is little
explored, involves the study of dyes19 or redox active20
molecules encapsulated in host macrocycles and bound to
semiconductors.
Meso-strapped porphyrins were used to model the photo-
synthetic center,22 to mimic the catalytic activity of naturally
occurring iron porphyrins,17d,f,23 and to develop new types of
polymers,24 self-assembled monolayers,25-27 and light har-
vesting arrays.28,29 They also have found use in molecular
(21) (a) Tang, H.; Dolphin, D. Inorg. Chem. 1996, 35, 6539–6545. (b)
Wijesekera, T. P.; Paine, J. B., III; Dolphin, D. J. Am. Chem. Soc. 1983, 105,
6747–6749. (c) Traylor, T. G.; Koga, N.; Deardurff, L. A.; Swepston, P. N.;
Ibers, J. A. J. Am. Chem. Soc. 1984, 106, 5132–5143. (d) Morgan, B.;
Dolphin, D.; Jones, R. H.; Jones, T.; Einstein, F. W. B. J. Org. Chem.
1987, 52, 4628–4631. (e) Wijesekera, T. P.; Paine, J. B., III; Dolphin, D. J.
Org. Chem. 1988, 53, 1345–1352. (f) David, S.; Dolphin, D.; James, B. R.;
Paine, J. B., III; Wijesekera, T. P. Can. J. Chem. 1986, 64, 208–212. (g)
Einstein, F. W. B.; Jones, T. Acta Crystallogr. 1984, C40, 696–700. (h) Cruse,
W. B.; Kennard, O.; Sheldrick, G. M.; Hamilton, A. D.; Hartley, S. G.;
Battersby, A. R. Chem. Commun. 1980, 15, 700–701. (i) Chang, C. K.; Koo,
M. S.; Ward, B. Chem. Commun. 1982, 13, 716–719. (j) Chiang, L.-C.;
Konishi, K.; Aida, T.; Inoue, S. Chem. Commun. 1992, 3, 254–256.
(22) Staab, H. A.; Feurer, A.; Krieger, C.; Kumar, A. S. Liebigs. Ann.
1997, 11, 2321–2336.
ꢀ
(23) Prevot, L.; Jaquinod, L.; Fischer, J.; Weiss, R. Inorg. Chim. Acta
1998, 283, 98–104.
(24) Sugiyasu, K.; Takeuchi, M. Chem.;Eur. J. 2009, 15, 6350–6362.
(25) Ikeda, T.; Asakawa, M.; Miyake, K.; Goto, M.; Shimizu, T. Langmuir
2008, 24, 12877–12882.
(26) Ikeda, T.; Asakawa, M.; Miyake, K.; Shimizu, T. Chem. Lett. 2004,
33, 1418–1419.
(27) Redman, J. E.; Sanders, J. K. M. Org. Lett. 2000, 2, 4141–4144.
(28) Shao, X.-B.; Jiang, X.-K.; Zhu, S.-Z.; Li, Z.-T. Tetrahedron 2004, 60,
9155–9162.
(18) Zhang, Y.; Galoppini, E. ChemSusChem 2010, 3, 410-428.
(19) (a) Pagba, C.; Zordan, G.; Galoppini, E.; Piatnitski, E. L.; Hore, S.;
Deshayes, K.; Piotrowiak, P. J. Am. Chem. Soc. 2004, 126, 9888–9889. (b)
Choi, H.; Kang, S. O.; Ko, J.; Gao, G.; Kang, H. S.; Kang, M. S.;
€
Nazeeruddin, Md. K.; Gratzel, M. Angew. Chem., Int. Ed. 2009, 48, 5938–
5941.
(20) Freitag, M.; Galoppini, E. Langmuir 2010, ASAP. DOI: 10.1021/
la904671w
(29) Springs, S. L.; Andrievsky, A.; Kral, V.; Sessler, J. L. J. Porphyrins
Phthalocyanines 1998, 2, 315–325.
3694 J. Org. Chem. Vol. 75, No. 11, 2010

Lee and Galoppini
JOCArticle
recognition,30 ion sensing,31 host-guest complex32 studies,
and other applications.31b,33-35 Meso-strapped porphyrins
were made with a variety of connections of the tethers:
trans-23,28,30,31a,32a,33c,33d or cis-36 connected single straps, cis-
RR- or Rβ-straps,35a,b,37 trans-RR-38 or Rβ-straps,24,31a,39,40 and
so-called gyroscope straps.41 The tethers were made from a
variety of chains, including alkanes,24,31a,36,42 ethylene
oxides,26,28,29 diimides,43 disulfides,27 and peptides.44 In
some cases, in the tethers were embedded aromatic rings
(pyridine,45 phenyl,31a,33a,33d,46 biphenyl23), chromo-
phores (binaphthyl,17n,35a,35b,35d fullerene,47 phenanthro-
line,17r,s,32b,48,57 anthracene,17e porphyrin49), or coordi-
nating hosts (crown32c/aza-crown ethers,40,50 TREN,51
and calix/cavitands17k,14l,32a). Fewer had functional
groups on the meso-phenyl rings, as the compounds des-
cribed in Figure 2.24,25,28,30,34a,41,46 Strapped porphyrins
with substituent groups on the tethered meso-phenyl rings
are more rare.
The main general strategies for the syntheses of
strapped porphyrins are illustrated in Figure 3. The for-
mation of the porphyrin ring is achieved by condensation
of tethered tetra-aldehydes and pyrrole, as in Figure 3,
routes A-C and E. Alternatively, the tether is added to a
porphyrin ring using substitution or condensation reac-
tions of ester,17d,m azide,52 or amide23,38 groups, Figure 3,
route D.53
Here, we report the synthesis and preliminary spectro-
scopic and electrochemical studies of novel strapped por-
phyrins 2a-c in solution and semiconductor-bound and
the crystal structure of 2a. The synthetic route described
here is practical, as it proceeds through a useful inter-
mediate, bromo derivative 2a, that is amenable to syn-
thetic modifications to attach the anchor units, and it
proceeds in satisfactory yields, leading to half-gram
amounts of products.
(30) Shao, X.-B.; Jiang, X.-K.; Wang, X.-Z.; Li, Z.-T.; Zhu, S.-Z. Tetra-
hedron 2003, 59, 4881–4889.
(31) (a) Takeuchi, M.; Shioya, T.; Swager, T. M. Angew. Chem., Int. Ed.
€
2001, 40, 3372–3376. (b) Gunter, M. J.; Farquhar, S. M.; Mullen, K. M. New
J. Chem. 2004, 28, 1443–1449.
(32) (a) Middel, O.; Verboom, W.; Reinhoudt, D. N. J. Org. Chem. 2001,
66, 3998–4005. (b) Froidevaux, J.; Ochsenbein, P.; Bonin, M.; Schenk, K.;
Maltese, P.; Gisselbrecht, J.-P.; Weiss, J. J. Am. Chem. Soc. 1997, 119, 12362–
12363. (c) Gunter, M. J.; Jeynes, T. P.; Johnston, M. R.; Turner, P.; Chen, Z.
Perkin Trans. 1 1998, 1945–1958.
(33) (a) Gunter, M. J.; Johnston, M. R. Chem. Commun. 1992, 17, 1163–
1165. (b) Flamigni, L.; Talarico, A. M.; Serroni, S.; Puntoriero, F.; Gunter,
M. J.; Johnston, M. R.; Jeynes, T. P. Chem.;Eur. J. 2003, 9, 2649–2659. (c)
Gunter, M. J.; Jeynes, T. P.; Turner, P. Eur. J. Org. Chem. 2004, 1, 193–208.
(d) Gunter, M. J.; Hockless, D.C. R.; Johnston, M. R.; Skelton, B. W.;
White, A. H. J. Am. Chem. Soc. 1994, 116, 4810–4823. (e) Gunter, M. J.;
Farquhar, S. M.; Jeynes, T. P. Org. Biomol. Chem. 2003, 1, 4097–4112.
(34) (a) Shao, X.-B.; Jiang, X.-K.; Zhao, X.; Zhao, C.-X.; Chen, Y.; Li,
Results and Discussion
a. Synthesis. For the synthesis of 2b, our strategy followed
route B in Figure 3, and involved the condensation of the
brominated, tethered bis(dipyrrolylmethylphenyl) compound 8
(Scheme 1) with benzaldehyde. The reaction conditions were
based on recent work by Crooks and co-workers.54 The strap is
made of a benzene ring tethered in para through two C4 satu-
rated alkyl chains, and attached to the tetraphenyl porphyrin
macrocycle (TPP) through an ether group (Ph[-(CH2)4O-]2-
TPP). The use of an ether to attach the tether was a compro-
mise. Groups such as esters, ketones, or amides, although
in some cases synthetically more convenient, were avoided
because they could physisorb or bind to metal oxides, leading
to unwanted binding interactions with the surface. Conversely,
the synthesis of a nonreactive, and chemically more robust,
tether made of methylene units (Ph[-(CH2)n]2TPP) would
have been considerably complex.42 The tether’s length was
carefully selected. Shorter chains inhibit the cyclization step
necessary to form the porphyrin ring,55 while longer chains
would have promoted intermolecular reactions and would not
provide the conformational constrain needed in the final
product. The anchoring PhCOOMe groups in 2b were placed
in the meta-position on the meso ring to ensure a better bite
angle to the metal oxide surface.56
€
Z.-T. J. Org. Chem. 2004, 69, 899–907. (b) Mullen, K. M.; Gunter, M. J. J.
Org. Chem. 2008, 73, 3336–3350.
(35) (a) Rose, E.; Ren, Q.-Z.; Andrioletti, B. Chem.;Eur. J. 2004, 10,
224–230. (b) Ren, Q.-Z.; Wang, A.-Q.; Liu, S.-Y.; Ding, X.-J. Chin. J. Chem.
2009, 27, 895–899. (c) Konishi, K.; Oda, K.-I.; Nishida, K.; Aida, T.; Inoue,
S. J. Am. Chem. Soc. 1992, 114, 1313–1317. (d) Inoue, S.; Aida, T.; Konishi,
K. J. Mol. Catal. 1992, 74, 121–129.
(36) Lee, C.-H.; Lee, C.-K. Bull. Korean Chem. Soc. 1992, 13, 352–354.
(37) (a) Halime, Z.; Lachkar, M.; Roisnel, T.; Richard, P.; Boitrel, B.
Inorg. Chem. 2007, 46, 6338–6346. (b) Simonis, U.; Walker, F. A.; Lee, P. L.;
Hanquet, B. J.; Meyerhoff, D. J.; Scheidt, W. R. J. Am. Chem. Soc. 1987, 109,
2659–2668.
(38) Uemori, Y.; Kyuno, E. Inorg. Chim. Acta 1989, 165, 115–122.
(39) Boitrel, B.; Lecas, A; Rose, E. Chem. Commun. 1989, 6, 349–350.
(40) Balieu, S.; Hijazi, I.; Motreff, N.; Lachaud, F.; Even-Hernandez, P.;
Boitrel, B. Org. Lett. 2010, 12, 8–11.
(41) (a) Osuka, A.; Nagata, T.; Maruyama, K. Chem. Lett. 1991, 10,
1687–1690. (b) Nagata, T. Bull. Chem. Soc. Jpn. 1992, 65, 385–391.
(42) Ma, W.; Slebodnick, C.; Ibers, J. A. J. Org. Chem. 1993, 58, 6349–
6353.
(43) Staab, H. A.; Nikolic, S.; Krieger, C. Eur. J. Org. Chem. 1999, 6,
1459–1470.
(44) (a) Karpishin, T. B.; Vannelli, T. A.; Glover, K. J. J. Am. Chem. Soc.
1997, 119, 9063–9064. (b) Geier, G. R., III; Sasaki, T. Tetrahedron Lett.
1997, 38, 3821–3824.
(45) Gunter, M. J.; Mander, L. N. J. Org. Chem. 1981, 46, 4792–4795.
(46) Gonςalves, D. P. N.; Sanders, J. K. M. Synlett 2007, 4, 591–594.
(47) (a) Fazio, M. A.; Durandin, A.; Tkachenko, N. V.; Niemi, M.;
Lemmetyinen, H.; Schuster, D. I. Chem.;Eur. J. 2009, 15, 7689–7705. (b)
Cheng, P.; Wilson, S. R.; Schuster, D. I. Chem. Commun. 1999, 1, 89–90.
(48) (a) Ochsenbein, P.; Bonin, M.; Schenk, K.; Froidevaux, J.; Wytko, J.;
Graf, E.; Weiss, J. Eur. J. Inorg. Chem. 1999, 7, 1175–1179. (b) Brandel, J.;
Trabolsi, A.; Melin, F.; Elhabiri, M.; Weiss, J.; Albrecht-Gary, A.-M. Inorg.
Chem. 2007, 46, 9534–9536. (c) Koepf, M.; Melin, F.; Jaillard, J.; Weiss, J.
Tetrahedron Lett. 2005, 46, 139–142. (d) Melin, F.; Boudon, C.; Lo, M.;
Schenk, K. J.; Bonin, M.; Ochsenbein, P.; Gross, M.; Weiss, J. J. Porphyrins
Phthalocyanines 2007, 11, 212–221.
(49) Collman, J. P.; Chong, A. O.; Jameson, G. B.; Oakley, R. T.; Rose,
E.; Schmittou, E. R.; Ibers, J. A. J. Am. Chem. Soc. 1981, 103, 516–533.
(50) (a) Andrioletti, B.; Ricard, D.; Boitrel, B. New J. Chem. 1999, 23,
1143–1150. (b) Collman, J. P.; Herrmann, P. C.; Fu, L.; Eberspacher, T. A.;
Eubanks, M.; Boitrel, B.; Hayoz, P.; Zhang, X.; Brauman, J. I.; Day, V. W. J.
Am. Chem. Soc. 1997, 119, 3481–3489.
The synthesis of 8 is shown in Scheme 1. Commer-
cially available 1,4-diiodobenzene was reacted with excess
(52) Webb, J. E. A.; Maharaj, F.; Blake, I. M.; Crossley, M. J. Synlett
2008, 14, 2147–2149.
(53) (a) Lindsey, J. S.; Mauzerall, D. C. J. Am. Chem. Soc. 1982, 104,
4498–4500. (b) Almog, J.; Baldwin, J. E.; Crossley, M. J.; Debernardis, J. F.;
Dyer, R. L.; Huff, J. R.; Peters, M. K. Tetrahedron 1981, 37, 3589–3601.
(54) Zhang, Z.; Zheng, G.; Pivavarchyk, M.; Deaciuc, A. G.; Dwoskin, L.
P.; Crooks, P. A. Biorg. Med. Chem. Lett. 2008, 18, 5753–5757.
€
(55) Schnatter, W. F. K.; Almarsson, O.; Bruice, T. C. Tetrahedron 1991,
47, 8687–8700.
(56) Thyagarajan, S.; Galoppini, E.; Persson, P.; Giaimuccio, J. M.;
Meyer, G. J. Langmuir 2009, 25, 9219–9226.
(51) (a) Andrioletti, B.; Boitrel, B.; Guilard, R. J. Org. Chem. 1998, 63,
1312–1314. (b) Ricard, D.; Andrioletti, B.; L’Her, M.; Boitrel, B. Chem.
€
Commun. 1999, 16, 1523–1524. (c) Collman, J. P.; Broring, M.; Fu, L.; Rapta,
M.; Schwenninger, R.; Straumanis, A. J. Org. Chem. 1998, 63, 8082–8083.
(57) (a) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827–836. (b) Lindsey, J. S.;
MacCrum, K. A.; Tyhonas, J. S.; Chuang, Y.-Y. J. Org. Chem. 1994, 59, 579–
587.
J. Org. Chem. Vol. 75, No. 11, 2010 3695

Lee and Galoppini
JOCArticle
FIGURE 3. General strategies for the synthesis of strapped porphyrins.
3-butyn-1-ol in Sonogashira Pd-catalyzed cross coupling
conditions to yield 4,40-(1,4-phenylene)dibut-3-yn-1-ol (4)
in excellent (92%) yields. White, needle-like crystals of 4
were obtained by crystallization from THF/MeOH. After
crystallization, 4 became stable in the presence of air and
light.
ZnPorBr2 (2a) was obtained in 27% yields, together with
a small amount of the corresponding metal-free porphyrin
H2PorBr2 (typically less than 10%), Scheme 2. Metalation
of the H2PorBr2/ZnPorBr2 mixture with zinc acetate di-
hydrate in refluxing chloroform/methanol yielded quanti-
tatively ZnPorBr2 (2a). Suzuki coupling of dibromide 2a
with 4-methylcarboxyphenylboronic acid in the presence
of a catalytic amount of Pd(PPh3)4 and Na2CO3 led to
diester ZnPorOMe2 (2b), a purple solid which was soluble
in common organic solvents.
Alternatively, metal-free porphyrin dibromide H2PorBr2
was synthesized directly from 8 and then metalated to yield
2a (Scheme 2) or, using the same Suzuki coupling condi-
tions, reacted with 4-methylcarboxyphenylboronic acid to
afford metal-free H2PorOMe2. The latter was then meta-
lated in the presence of zinc acetate to yield 2b in excellent
yields. In conclusion, Zn(II) and metal-free strapped por-
phyrins were accessible through various approaches, as
shown in Scheme 2.
Hydrogenation of 4 in the presence of 10% Pd/C at
room temperature and atmospheric pressure yielded
4,40-(1,4-phenylene)dibutan-1-ol (5) in 76% yield. Diol 5
was reacted with carbon tetrabromide and triphenylpho-
sphine to prepare the corresponding dibromide, 1,4-bis-
(4-bromobutyl)benzene (6), in 81% yield. Nucleophilic
substitution of 6 with excess 5-bromo-2-hydroxybenzal-
dehyde in the presence of potassium carbonate in DMF led
to 7 in excellent yields (96%). Tethered dialdehyde 7 was
reacted with excess pyrrole in the presence of a catalytic
amount of trifluoroacetic acid in CH2Cl2 to yield the
tethered bis(dipyrrolylmethylphenyl) units24 8 in 64%
yields, Scheme 1. The bromine groups in 8 are necessary
to introduce the anchor groups on the phenyl rings after
formation of the porphyrin.
Purple, needle-shaped crystals of H2PorBr2 were grown by
layering MeOH over a CHCl3 solution of H2PorBr2. A
picture of the structure of H2PorBr2 obtained by small-angle
X-ray diffraction is shown in Figure 4.58
The one-pot cyclization and metalation of the highly
insoluble 8 was done in propinoic acid/methylene chloride
solvent mixture, with excess benzaldehyde, and in the
presence of zinc acetate dihydrate and oxidizing agent
DDQ,57 Scheme 2. The brominated Zn(II) porphyrin
(58) Lee, C.-H.; Lalancette, R.; Galoppini, E. Manuscript in prepara-
tion.
3696 J. Org. Chem. Vol. 75, No. 11, 2010

Lee and Galoppini
JOCArticle
SCHEME 1. Synthesis of the Brominated, Tethered
Bis(Dipyrrolylmethylphenyl) Derivative 8
FIGURE 4. Single-crystal X-ray structure of metal-free porphyrin
dibromide H2PorBr2.
SCHEME 2. Synthesis of Strapped Porphyrin ZnPorOMe2
Although there are examples of crystal structures of strapped
porphyrins,21a,23,32b,32c,33c,33d,37b,43,48a,48d,49,50b,59,60 to the best of
our knowledge, this is the first example a crystal structure of a
strapped porphyrin with substituent group on the tethered
meso-phenyl rings. The X-ray structure indicates that, at least
in the crystal packing, the phenyl group of the strap is not
centered above the porphyrin and is tilted with respect to the
porphyrin plane at a 66° angle. The data (collected at 100 K)
indicate that the alkyl chain is disordered as the result of
conformational mobility. In summary, the structure in Figure 4
(59) (a) Johnson, M. R.; Seok, W. K.; Ma, W.; Slebodnick, C.; Wilcoxen,
K. M.; Ibers, J. A. J. Org. Chem. 1996, 61, 3298–3303. (b) Slebodnick, C.;
Fettinger, J. C.; Peterson, H. B.; Ibers, J. A. J. Am. Chem. Soc. 1996, 118,
€
3216–3224. (c) Staab, H. A.; Krieger, C.; Anders, C.; Ruckemann, A. Chem.
Ber. 1994, 127, 231–236. (d) Staab, H. A.; Weiser, J.; Baumann, E. Chem. Ber.
1992, 125, 2275–2283. (e) Krieger, C.; Dernbach, M.; Voit, G.; Carell, T.;
Staab, H. A. Chem. Ber. 1993, 126, 811–821. (f) Schappacher, M.; Fischer, J.;
ꢀ
Weiss, R. Inorg. Chem. 1989, 28, 389–390. (g) Jaquinod, L.; Prevot, L;
is not representative of the solution conformation (see NMR).
The structure shows that, in the crystal, the metal-free macro-
cyclic ring is not significantly distorted from planarity.61 Two
chloroform molecules (not shown) were trapped in each unit
cell, and the crystal lattice was characterized by channels.58 The
two porphyrinato hydrogens were shared almost equally bet-
ween the four pyrrole units, and the nitrogens of the pyrrole
units were identical.
Fischer, J.; Weiss, R. Inorg. Chem. 1998, 37, 1142–1149. (h) Woo, L. K.;
Maurya, M. R.; Jacobson, R. A.; Ringrose, S. L. Inorg. Chim. Acta 1993, 212,
337–340. (i) Ricard, L.; Weiss, R.; Momenteau, M. Chem. Commun. 1986, 11,
818–820. (j) Evans, D. R.; Drovetskaya, T.; Bau, R.; Reed, C. A.; Boyd, P. D.
W. J. Am. Chem. Soc. 1997, 119, 3633–3634. (k) Momenteau, M.; Scheidt, W.
R.; Eigenbrot, C. W.; Reed, C. A. J. Am. Chem. Soc. 1988, 110, 1207–1215. (l)
Comte, C.; Gros, C. P.; Koeller, S.; Guilard, R.; Nurco, D. J.; Smith, K. M.
New J. Chem. 1998, 22, 621–626. (m) Benson, D. R.; Valentekovich, R.;
Knobler, C. B.; Diederich, F. Tetrahedron 1991, 47, 2401–2422. (n) Jaquinod,
L; Kyritsakas, N.; Fischer, J.; Weiss, R. New J. Chem. 1995, 19, 453–460. (o)
Schmitt, P.; Mandon, D.; Fischer, J.; Weiss, R. New J. Chem. 1992, 16, 763–
765.
(60) Lo, M.; Mahajan, D.; Wytko, J. A.; Boudon, C.; Weiss, J. Org. Lett.
2009, 11, 2487–2490.
(61) Slebodnick, C.; Kim, K.; Ibers, J. A. Inorg. Chem. 1993, 32, 5338–
5342.
J. Org. Chem. Vol. 75, No. 11, 2010 3697

Lee and Galoppini
JOCArticle
FIGURE 5. 1H NMR (CDCl3) spectrum of ZnPorOBr2 and assign-
ments (see the Supporting Information).
b. NMR Study of 2a and 2b. Figure 5 shows the 1H NMR
spectrum of 2a with the corresponding assignments.
The assignments for dibromide 2a were determined by 2D-
1
NMR, specifically H-1H COSY (Figure S19, Supporting
Information) and 1H-13C HMQC (Figure S20, Supporting
1
Information) in CDCl3. The H-1H COSY spectrum indi-
cates that signal O (8 H) belongs to the pyrrole units protons
and that M (4 H) and I (6 H), which are correlated, belong to
the aromatic protons of the unsubstituted meso-phenyl rings.
Signals H, K, and N (each 2 H) were assigned to the bromo-
substituted meso-phenyl rings. Signal K is a double of doub-
let (J1 = 2.4, J2 = 8.8 Hz) split by H (d, J = 8.9 Hz) and N (d,
J = 2.4 Hz). The coupling constant of ortho aromatic
protons is typically 6-10 Hz, and long-range coupling in
aromatics is 1-3 Hz (meta) and 0-1 Hz (para).62 This
suggests that signal N can be assigned to the isolated proton
of the bromo-substituted phenyl ring, with a long-range
coupling and weak correlation to K, and that K and H,
which are strongly coupled, are ortho to each other. In the
1H-13C HMQC spectrum, the F/G signal is correlated to a
carbon at 125.7 ppm in the Ph proton region and was
assigned to the protons of the phenyl ring of the strap.
Interestingly, the F/G protons are considerably shifted up-
field by the porphyrin ring current, an observation consistent
with literature available on capped porphyrins.17f,j,35c,46 Protons
A, B, C, and E were assigned to the CH2 groups of the tether
units (Ph[-CH2CH2CH2CH2O]2-). Methylenic protons E
were observed at 3.9 ppm, with the corresponding carbon
(1H-13C HMQC spectrum) at 68.8 ppm. E was therefore assig-
ned to the CH2 bound to the oxygen. Signal C, which is shifted
downfield with respect to A and B, was assigned to the PhCH2-
on the tether. Signal E correlates only with B, while A is corre-
lated with B and C. In summary, the assignment of the
FIGURE 6. NMR spectra of 2b (CDCl3). The 9-7 ppm region of
the 1H (top) and 1H-1H COSY (bottom) spectra.
1
(Figure 6, bottom and Figure 7, bottom), H-13C HMQC
(Figure 8), and DEPT (Figure 9), and the spectral assign-
ments for 2b were done following the spectral assignments of
2a. For instance, M (4 H) and I (6 H) were assigned to the
protons from the unsubstituted meso-phenyl ring of the
porphyrin.
Signals K and L are overlapping (4 þ 2 H), with K-H and
L-J correlations. L and J were assigned to the protons from
the additional phenyl rings (PhCOOMe) added in the Suzuki
coupling step. H, K, and N were assigned as aforementioned.
The protons F/G (4 H) in the 1H-1H COSY in Figure 7 were
assigned to the protons on the phenyl ring of the cap as F/G
1
correlates to the Ph carbon at 125.8 ppm in the H-13C
HMQC in Figure 8. Signal D (6 H), the only primary carbon
in the 13C/DEPT (Figure 9), was readily assigned to the
methyl group of the ester (COOCH3). The assignment of the
methylenic unit of the tether was PhCH2CCH2ACH2B-
CH2EO, following the same assignment for 2a. The pyrrolic
protons of ZnPorBr2 (2a) and ZnPorOMe2 (2b) were split
into double doublets (J = 4.6, 11.5 Hz for 2a and J = 4.6,
19.8 Hz for 2b). However, for H2PorOMe2, the pyrrolic
proton was split into two sets of double doublets at 8.94-
9.00 (dd, J = 4.6, 23.0 Hz, 4H) and at 8.83-8.90 (dd, J = 4.2,
27.5 Hz, 4H). This splitting may be due to some distortion
of the metal-free porphyrin ring. Also, most notably, the
methylenic units of the tether is as follows: Ph[-CH2CCH2A
-
CH2BCH2EO]2.
Figure 6 (top) and Figure 7 (top) show two regions of the
1H NMR spectrum of 2b with the corresponding assign-
ments. Diester 2b was characterized by 1H-1H COSY
(62) Silverstein, R. M.; Webster, F. X.; Jiemle, D. J. Spectrometric
Identification of Organic Compounds, 7th ed.; John Wiley & Sons, Inc.:
New York, 2005; p 198.
(63) Kadish, K. M.; Royal, G.; Caemelbecke, E. V.; Gueletti, L. In The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: New York, 2000; Vol. 9, pp 85-86.
3698 J. Org. Chem. Vol. 75, No. 11, 2010

Lee and Galoppini
JOCArticle
FIGURE 9. 13C/DEPT NMR spectrum of 2b.
pulse voltammetry (DPV) and were compared to data ob-
tained for the parent compound ZnTPP as the reference,
Figure 10 and Table 2.
The formation of the radical cation was observed at 0.8 V
vs SCE, and the dication at about 1.1 V vs SCE. In the
negative potential region, three irreversible/quasireversible
reduction processes were observed in the range -1.31 to
-1.78 V vs SCE.64 The reduction processes were more clearly
discernible by DPV. The electrochemical data indicate that
ZnTPP and 2b have very similar electrochemical properties,
and the HOMO-LUMO gap was calculated to be 2.14 V.
d. Absorption and Emission Properties in Solution and
Bound to TiO2. The solution (CHCl3) UV-vis and steady-
state fluorescence emission spectra of 2a and 2b are shown in
an overlay in Figure 11, and their selected photophysical
properties are listed in Table 3.
The absorption spectra were typical of a Zn(II) tetraphe-
nylporphyrin with the strong Soret absorption band at ∼421
nm corresponding to the S0 f S2 transition and the Q bands
corresponding to the two vibrational modes of the S0 f S1
transition at ∼550 nm (Q(1,0)) and ∼590 nm (Q(0,0)), Figure 12.
The absorption and emission spectra of dibromide 2a exhibited
a modest (∼ 4 nm) red shift compared to the spectra of diester
2b. Significant spectral changes upon substitution on the meso
phenyl rings are not observed in tetraphenylporphyrins,12a,13b
but a very modest (few nm) red shift upon substitution with
electron-withdrawing groups, such as a Br group, has been
reported before.24,65
FIGURE 7. NMR spectra of 2b (CDCl3). The aliphatic region of
the 1H (top) and 1H-1H COSY (bottom).
The emission bands at 605 nm (0-0 emission) and 650 nm
(0-1 emission) in the fluorescence spectra of 2 are typical of
ZnTPP derivatives. The stacking of Zn porphyrins produces
typical shifts in the absorption and emission spectra,66 and
the lack of spectral shifts in solution spectra collected in
CHCl3 at varying concentrations (Figure S15, Supporting
Information) indicated that, as anticipated, 2a and 2b do not
aggregate in solutions of a polar solvent.
The TiO2 and ZrO2 films were prepared according to
previously reported procedures.11a The films were immersed
in 0.4 mM CH2Cl2 solutions of 2a and 2b for variable periods
of time (30 min to 24 h) and rinsed with neat solvent until
FIGURE 8. 2D 1H-13C HMQC NMR spectrum of 2b.
coupling constant in H2PorOMe2 and ZnPorOMe2 (∼20
Hz) was considerably larger compared to that of ZnPorBr2.
Comparisons between selected signals in the 1H NMR
spectra of 2a and 2b are listed in Table 1.
c. Electrochemistry. The electrochemical properties of 2b
were studied by cyclic voltammetry (CV) and differential
(64) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982–2991.
(65) Liu, F.; Duan, L.; Wang, Y.-L.; Zhang, Q.; Wang, J.-Y. Synth.
Commun. 2009, 39, 3990–3998.
(66) (a) Lo, C.-F.; Luo, L.; Diau, E. W.-G.; Chang, I.-J.; Lin, C. Y. Chem.
Commun. 2006, 1430. (b) Aratani, N.; Osuka, N.; Cho, H. S.; Kim, D. J.
Photochem. Photobiol. C: Photochem. Rev. 2002, 3, 25.
J. Org. Chem. Vol. 75, No. 11, 2010 3699

Lee and Galoppini
JOCArticle
TABLE 1. Comparison of Selected Signals (in ppm) in the 1H NMR Spectra and Coupling Constants J (in Hz) of 2a and 2b
porphyrin
D
F/G
H
I
J
K, L
M
N
O
ZnPorBr2 2a
4.14
s
7.17
d
J = 8.9
7.71-7.78
7.85-7.87 8.19-8.21 8.28
8.92-8.95
dd
J1 = 4.6
J2 = 11.5
8 H
m
dd
m
d
J = 2.4
J1 = 2.4
J2 = 8.8
2 H
4 H
2 H
6 H
4 H
2 H
H2PorOMe2
3.91-3.92 4.03-4.23 7.35-7.38 7.70-7.77 7.87-7.88 8.03-8.06 8.17-8.24 8.50-8.51 8.83-8.90 8.94-9.00
d
J = 3.7
m
d
J = 7.8
m
d
J = 8.5
dt
m
dd
dd
J1 = 4.2
dd
J1 = 4.6
J1 = 2.5
J2 = 8.6
6 H
8.03-8.07 8.19-8.25 8.50
m
J1 = 2.5
J2 = 8.9
2 H
J2 = 27.5 J2 = 23.0
4H
6 H
ZnPorOMe2 2b 3.87
4 H
4.25
s
2 H
7.37
d
6H
4 H
4 H
4H
8.95-9.00
dd
7.71-7.76 7.86
m
s
d
J = 8.6
dd
d
J = 2.3
J = 8.7
J1 = 7.5
J2 = 26.7
4 H
J1 = 4.6
J2 = 19.8
8 H
6 H
4 H
2 H
6H
4 H
6 H
2 H
FIGURE 10. CV (black solid line) and DPV (blue solid line) of
ZnPorOMe2 (2b) vs SCE.
FIGURE 11. Normalized UV-vis (black) and steady-state fluor-
escence (red) spectra of CHCl3 solutions of 2a (ZnPorOBr2, 2 ꢀ
10-6 M, solid lines) and 2b (ZnPorOMe2,1 ꢀ 10-6 M, dotted lines),
λ
ex = 410 nm.
TABLE 2. Solution Redox Potentials of Porphyrin Methyl Esters in
Dichloromethane Reported vs SCE (V)
before.67 The most notable features in the absorption spectra
on TiO2 films were the red shift (∼10 nm) of the Soret band
observed upon binding and the general broadening of the
spectrum, Figure 12. Broadening of the absorption spectra
are a typically observed in dyes bound to metal oxide
nanoparticle surfaces.
oxidation (V)
reduction(V)
2nd
1st
2nd
1st
3rd
ZnTPP63
ZnTPP (exp)
2b
0.82
0.79
0.79
1.14
1.10
1.08
-1.3
-1.32
-1.31
-1.70
-1.68
-1.59
-1.78
-1.78
-1.71
The origin of the observed shift is not clear. The simplest
explanation of the red shift would be a side-by-side interaction
of the porphyrin rings (J-aggregates), since the formation of
H-aggregates (face to face stacking) between porphyrin mole-
cules leads to blue shifts.11a,68,66a Longer binding times (12 h,
not shown) produced very intensely colored films, but no
changes in the observed shift and broadening. The spectral
shift may indicate that the single strap is not sufficient to
completely avoid dye-dye interaction. We suppose that, since
2b is a dichelate (binds through two anchoring groups), the
porphyrin rings are tilted on the surface, leading to some degree
of J-overlap. A second possible explanation is that the strap
leads to conformationally constrained and geometrically dis-
torted geometry upon binding. The synthesis of capped tetra-
chelate 3, still in progress, should clarify this point. In addition,
UV-vis spectra of the solvent showed no leaching of the dye.
The absorption spectra in Figure 12, with binding times up to
1 h, show that dibromide 2a, which does not have an anchor
group, did not bind or physisorb significantly to the surface
of the semiconductor. Diester 2b did bind, again indicating
that the methyl ester of a carboxylic acid can be an effective
anchor group for TiO2 nanoparticles. The binding of methyl
esters on metal oxides nanoparticle films has been observed
(67) (a) Wang, D.; Mendelsohn, R.; Galoppini, E.; Hoertz, P. G.; Carlisle,
R. A.; Meyer, G. J. J. Phys. Chem. B 2004, 108, 16642–16653. (b) Galoppini,
E.; Guo, W.; Zhang, W.; Hoertz, P. G.; Qu, P.; Meyer, G. J. J. Am. Chem.
Soc. 2002, 124, 7801–7811. (c) Piotrowiak, P.; Galoppini, E.; Qei, Q.; Meyer,
ꢀ
G. J.; Wiewior, P. J. Am. Chem. Soc. 2003, 125, 5278–5279. (d) Clark, C. C.;
Meyer, G.; Wei, Q.; Galoppini, E. J. Phys. Chem. B 2006, 10, 11044–11046.
(e) Hoertz, P. G.; Carlisle, R. A.; Meyer, G. J.; Wang, D.; Piotrowiak, P.;
Galoppini, E. Nano Lett. 2003, 3, 325–330. (f) Galoppini, E.; Guo, W. J. Am.
Chem. Soc. 2001, 123, 4342–4343.
(68) Aratani, N.; Osuka, A.; Cho, H. S.; Kim, D. J. Photochem. Photobiol.
C: Photochem. Rev. 2002, 3, 25–52.
3700 J. Org. Chem. Vol. 75, No. 11, 2010

Lee and Galoppini
JOCArticle
TABLE 3. Selected Photophysical Properties of 2a, 2b, and Reference Compound 1a
Soret λmax, nm
Q(1,0) λmax, nm
Q(0,0) λmax, nm
Q(0,0)*, Q(0,1)*
max, nm
porphyrin
(ε ꢀ 105, M-1 cm-1
)
(ε ꢀ 104, M-1 cm-1
)
(ε ꢀ 104, M-1 cm-1
)
λ
E0-0, eV
ZnPorOBr2 (2a)
ZnPorOMe2 (2b)
423 (4.16)
421(4.19)
423 (4.44)
550 (1.95)
548 (1.87)
558 (2.09)
593 (0.43)
584 (0.33)
597 (0.66)
600, 645
595, 643
604, 657
2.22
2.24
2.07
m-ZnTCPPa (1a),1111a
aAcquired from the triethylammonium salt in methanol.
The decrease in intensity of one of the two emission bands,
the 605 nm S1(0-0) emission, has been observed before in
our previous study of tetrachelated porphyrins bound to
ZrO2.13b The bound spectrum (2b/ZrO2) in Figure 13 also
exhibits a moderate blue shift (few nm) upon binding.
Similar changes in intensity in the 0-0 and 0-1 emission
bands and blue shifts have been observed in Zn(II) tetra-
phenylporphyrin “compartmentalized” in highly constrain-
ing polymer microenvironments,70 where the polymer chains
imposed conformational distortions on the molecule result-
ing in a high energy excited state. We speculate that the
contraints imposed by the tether and the binding may result
in a similar effect. Forthcoming photophysical studies of 2b
will address this question.
The fluorescence emission of 2b bound to TiO2 was fully
quenched, Figure 13. Prior results with 1a-c and other
porphyrins suggest that quenching is indicative of efficient
electron injection into the semiconductor.10,11,13 This result
suggests that it will be possible to carry out charge transfer
and DSSCs studies with the more soluble esters, rather than
the carboxylic acids.
FIGURE 12. Selected UV-vis spectra of zinc-strapped porphyrins
in solution and bound.
Conclusions
Novel strapped ZnTPP porphyrins model dyes, with
functional or anchoring groups at the meta position on the
meso-phenyl rings connected by the tether, were synthesized
and characterized by 2D NMR, electrochemistry, MS, and
X-ray crystallography. Their UV-vis and fluorescence emis-
sion spectra were recorded in solution and bound to TiO2
and ZrO2 films. Diester 2b did bind efficiently to TiO2 or
ZrO2 surfaces, and the fluorescence emission was fully
quenched on TiO2 nanoparticles films. The spectral data
suggested that there is no significant aggregation on the
surface of semiconductor nanoparticles films, but some
degree of dye-dye interaction may be present. The electro-
chemical and spectral properties of strapped porphyrin 2b
were very similar to the properties of ZnTPP, indicating that
the modifications of the two meso- phenyl rings connected by
the strap do not influence significantly the properties of the
chromophoric unit. DSSCs and charge transfer studies of 2b
are in progress.
FIGURE 13. Normalized emission spectra of ZnPorOMe2 bound
to TiO2 (black) and ZrO2 surfaces (red, dotted line) and in CH2Cl2
(blue); λexc = 420 nm.
binding studies on TiO2 (rutile) planar crystal surfaces with 2b
and the corresponding acid will help characterize the binding.69
The fluorescence emission spectra of diester 2b bound to
TiO2 and ZrO2 films are shown in Figure 13, in an overlay
with the solution emission spectrum. Because of the much
wider bandgap (Ebg ∼5 eV for ZrO2, compared to ∼3 eV for
TiO2), ZrO2 behaves as an insulator preventing electron
injection and quenching. Therefore, ZrO2 films are an ex-
cellent substrate to study the emission of dyes bound to a
semiconductor surface with morphology very similar to the
TiO2 films. The fluorescence emission on ZrO2 resembled the
solution spectra, and the dramatic spectral changes (merging
of the two emission bands) that are associated with por-
phyrin aggregation11a,13c were not observed.
Experimental Section
General Methods. Air- and moisture-sensitive reactions were
carried out in flame-dried glassware under nitrogen atmosphere.
THF was freshly distilled over sodium/benzophenone. Dichloro-
methane and Et3N were distilled over calcium hydride. Propinoic
acid and other solvents were used as received. Commercially avail-
able chemicals, including 5-bromo-2-hydroxybenzaldehyde and
1,4-diiodobenzene, were used as such or purified. Melting points
were uncorrected. Flash column chromatography was performed
(70) Morishima, Y.; Saegusa, K.; Kamachi, M. Macromolecules 1995, 28,
1203–1207. In this case, however, the emission spectra exhibited a blue-shift.
(69) Bartynski, R. Private communication.
J. Org. Chem. Vol. 75, No. 11, 2010 3701

Lee and Galoppini
JOCArticle
with silica gel (230-400 mesh) and TLC on aluminum-backed
was purified by column chromatography (silica gel, gradient
from hexane to THF) to yield 1 as a pale yellow powder. The
solid was crystallized in THF/MeOH to yield white needle-like
crystals (9.0 g, 92%). Before crystallization, 4 degraded when
stored at ambient temperature and exposed to light. The crystals
of 4 were stable: mp 207-208 °C; 1H NMR (acetone-d6, δ 2.05)
7.34 (s, 4H), 3.99-4.01(t, J = 5.5 Hz, 2H), 3.70-3.74 (dd, J =
6.5, 13.5 Hz, 4H), 2.59-2.62 (t, J = 6.5 Hz, 4H); 13C NMR
(acetone-d6, δ 29.92) 132.3, 124.3, 90.3, 81.7, 61.4, 24.5; HRMS
(ESIþ-TOF) m/z calcd for C14H14O2þ 214.0994, found 214.0990
[M]þ; IR (cm-1) ν 3400 (broad), 2946, 2882, 2231, 1484, 1403,
1330, 1264, 1183, 1030, 891, 842, 777, 639. Anal. Calcd for
C14H14O2: C, 78.48; H, 6.59. Found: C, 78.85; H, 6.17.
1
silica gel (200 μm thick). H NMR (500 MHz) and 13C NMR
(125 MHz) spectra (1D or 2D) were collected on an NMR spectro-
meter operating at 499.896 MHz for 1H, and 125.711 MHz for 13
C
at room temperature in CDCl3, THF-d8, or acetone-d6, as noted.
Chemical shifts were reported relative to the central line of the
solvent: CDCl3 (δ 7.26 ppm), THF-d8 (δ 1.73 ppm) or acetone-d6
(δ 2.05 ppm) for 1H spectra, and CDCl3 (δ 77.0 ppm), THF-d8 (δ
25.37 ppm) or acetone-d6 (δ 29.92 ppm) for 13C spectra. Infrared
(IR) spectra were obtained at room temperature on a FT-IR
spectrometer using KBr in pellets. MS data are reported for the
molecular ion or the protonated molecular ion. GC-MS data were
collected from a gas chromatograph with a MS detector. High-
resolution mass spectra (ESI) were recorded on the departmental
FTMS facility. Elemental analyses were determined by a commer-
cial facility.
Spectroscopic Measurements. Absorption spectra were col-
lected at room temperature on a UV-vis spectrophotometer.
Emission spectra were recorded at room temperature on a
fluorescence spectrophotometer. The excited-state oxidation
potentials E1/2 (Pþ/P*) of all porphyrins in MeOH were calcu-
lated from eq 1
4,40-(1,4-Phenylene)dibutan-1-ol (5). To a stirred solution
of 4 (3.0 g, 14 mmol) in ethanol (50 mL) was added Pd/C 10%
(0.92 g) in 1 portion. The flask was charged with H2 by three
vacuum-hydrogen cycles. The mixture was stirred under H2
atmosphere overnight. The reaction mixture was filtered
through Celite and concentrated in vacuo, and the crude pro-
duct was purified by column chromatography (silica gel, gradient
from hexane, then ethyl acetate/hexane (20/80) and then THF) to
yield a white solid as the final fractions (2.36 g, 76%): mp 203-
204 °C; 1H NMR (CDCl3, δ 7.26) 7.09 (s, 4H), 3.64 (s, 4H), 2.59-
þ
E1=2ðP =P Þ ¼ E1=2ðPþ=PÞ - E0 - 0
ð1Þ
ꢁ
2.62 (t, J = 7 Hz, 4H), 1.65-1.69 (m, 4H), 1.58-1.62 (m, 4H); 13
C
where E1/2 (Pþ/P) is equivalent to the first oxidation potential of
the ground-state porphyrin chromophore and E0-0 is the zero-
zero excitation energy.71 E0-0 was calculated from the intersec-
tion of the porphyrin absorption spectrum with the fluorescence
emission spectrum in MeOH at normalized absorption/emission
intensity.
NMR (CDCl3, δ 77.0) 139.6, 128.3, 6þ2.8, 35.2, 32.3, 27.5; HRMS
(ESIþ-TOF) m/z calcd for C14H22O2 222.1614, found 222.1620
[M]þ; IR (cm-1) ν 3272 (broad), 2930, 2857, 1500, 1478, 1450,
1429, 1348, 1282, 1253, 1191, 1159, 1055, 1029, 1000, 953, 936, 921,
887, 878, 825, 761, 736. Anal. Calcd for C14H22O2: C, 75.63; H,
9.97. Found: C, 75.81; H, 9.87.
1,4-Bis(4-bromobutyl)benzene (6). To a stirring solution of 5
(3.0 g, 14 mmol) in CH2Cl2 (600 mL) was added CBr4 (17.9 g, 54
mmol) at 0 °C. Triphenylphosphine (14.1 g, 36 mmol) was added
in small portions. The solution was stirred overnight under
nitrogen and then concentrated in vacuo. THF was added,
and the precipitate was filtered and discarded. The filtrate was
concentrated in vacuo to yield a viscous, yellow oil. The oil was
purified by column chromatography (silica gel, hexane, and
then gradient to CHCl3). The product was the second band
eluting from the column (3.8 g, 81%): 1H NMR (CDCl3, δ 7.26)
7.10 (s, 4H), 3.41-3.44 (t, J = 6.5 Hz, 4H), 2.60-2.63 (t, J = 7.5
Hz, 4H), 1.79-1.88 (m, 4H), 1.74-1.76 (m, 4H); 13C NMR
(CDCl3, δ 77.00) 139.3, 128.4, 34.5, 33.7, 32.2, 29.9; HRMS
(ESIþ) m/z for [C14H20Br2 þ Na]þ 368.9824, found 368.9145
[M þ Na]þ); IR (cm-1) ν 3011, 2938, 2849, 1508, 1460, 1431,
1333, 1249, 1211, 1187, 1119, 1056, 1019, 970, 902, 815, 750, 736,
698. Anal. Calcd for C14H20Br2: C, 48.30; H, 5.79. Found: C,
48.51; H, 5.81.
Electrochemistry. The electrochemical properties of reference
compound ZnTPP and of diester 2b were studied by cyclic
voltammetry on a potentiostat. All cyclic voltammograms were
measured at room temperature at 0.1 or 0.05 V/s in dichlor-
omethane with 0.1 M Bu4NClO4 electrolyte. The measurements
were carried out with potential calibration using the ferrice-
nium/ferrocene redox couple, i.e., E0 (Fe3þ/Fe2þ) in CH2Cl2 as
þ0.45 V (ref to SCE) in 0.1 M Bu4NClO4 solution at room
temperature under Ar using a three-electrode system: Ag/AgCl
electrode (auxiliary), glassy carbon (working), and Pt (reference).
The half-wave redox potentials (E1/2) were determined according
to eq 2
E1=2 ¼ ðEpa þ EpcÞ=2
ð2Þ
where Epa and Epc are the anodic and cathodic peak potentials,
respectively. All potentials were referenced to SCE. When E1/2
could not be calculated, due to irreversible oxidation or reduction
processes, Epa and Epc are reported, respectively. All redox pro-
cesses were also studied by differential pulse voltammetry (DPV),
using the relationship in eq 3
6,60-(4,40-(1,4-Phenylene)bis(butane-4,1-diyl))bis(oxy)bis(3-bro-
mobenzaldehyde) (7). To a stirred solution of 6 (1.11 g, 3.2 mmol)
in dry DMF (20 mL) were added 5-bromo-2-hydroxybenzalde-
hyde (2.55 g, 12.7 mmol) and K2CO3 (3.95 g, 28.6 mmol) under
nitrogen. The mixture was stirred at 50 °C for 4 days under
nitrogen. The reaction mixture was then poured into water and
then filtered. The white solid obtained was used in the next step
E1=2 ¼ Emax þ ΔE=2
ð3Þ
where Emax is the peak maxima in the DPV scan and ΔE is the pulse
amplitude.72
Synthesis. 4,40-(1,4-Phenylene)dibut-3-yn-1-ol (4). To a stir-
ring solution of 1,4-diiodobenzene (15.0 g, 45 mmol) in THF
(250 mL) were added 3-butyn-1-ol (15.9 g, 227 mmol), PdCl2-
(PPh3)2 (3.19 g, 4.5 mmol), copper(I) iodide (1.73 g, 9.1 mmol),
triphenylphosphine (2.38 g, 9.1 mmol), and triethylamine (150
mL) under nitrogen. The reaction mixture was stirred for 2 days
at 50 °C under nitrogen. The mixture was concentrated in vacuo
and partitioned between CHCl3 and water. The organic layer
was then separated and dried over Na2SO4. The crude product
1
without further purification (1.8 g, 96%): mp 123-125 °C; H
NMR(THF-d8, δ 1.73) 10.37 (s, 2H), 7.81-7.82 (d, J = 2.5 Hz,
2H), 7.64-7.66 (dd, J = 2.5, 9 Hz, 2H), 7.10 (s, 4H), 7.07-7.09 (d,
J = 9 Hz, 2H), 4.12-4.14 (t, J = 6.5 Hz, 4H), 2.65-2.68 (t, J =
7.5 Hz, 4H), 1.80-1.87 (m, 8H); 13C NMR (THF-d8, δ 25.4)
187.6, 161.4, 140.4, 138.8, 131.0, 129.2, 127.5, 116.2, 113.8, 69.7,
35.9, 29.6, 28.8; HRMS(ESIþ) m/z calcd for [C28H28Br2O4 þ H]þ
587.0427, found 587.0402 [M þ H]þ); IR (cm-1) ν 2938, 1679,
1590, 1519, 1488, 1471, 1405, 1390, 1271, 1250, 1181, 1119, 1031,
1000, 906, 837, 810, 791, 767, 740, 656. Anal. Calcd for C28H28-
Br2O4: C, 57.16; H, 4.80. Found: C, 57.32; H, 4.98.
(71) Kuciauskas, D.; Monat, J. E.; Villahermosa, R.; Gray, H. B.; Lewis,
N. S.; McCusker, J. K. J. Phys. Chem. B. 2002, 106, 9347–9358.
(72) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications, 2nd ed.; Wiley: New York, 2001.
1,4-Bis(4-(4-bromo-2-(di(1H-pyrrol-2-yl)methyl)phenoxy)butyl)-
benzene (8). To a stirring solution of 7 (1.8 g, 3.1 mmol) in pyrrole
3702 J. Org. Chem. Vol. 75, No. 11, 2010

Lee and Galoppini
JOCArticle
2a from H2PorBr2. To a stirred solution of H2PorBr2 (0.13 g,
(20 mL) and CH2Cl2 (20 mL) under nitrogen and at 0 °C was
added trifluoroacetic acid (0.14 mL, diluted with CH2Cl2, 2 mL)
dropwise. The solution was allowed to stir overnight. The organic
solution was extracted with ∼1 M NaOH(aq) until the water layer
was basic and then with water. The organic layer was separated and
then dried over Na2SO4 and filtered. The solution was evaporated
in vacuo at 50 °C. Addition of hexane led to the formation of a
precipitate which was collected by filtration. The crude solid
product was purified by column chromatography (silica gel,
hexane, then hexane/ethyl acetate (1/1) to yield a brown solid
(1.6 g, 64%): mp 154-156 °C; 1H NMR (THF-d8, δ 1.73) 9.69 (s,
4H), 7.24-7.26 (dd, J = 2.5, 9 Hz, 2H), 7.09 (d, J = 2.5 Hz, 2H),
7.05 (s, 4H), 6.81-6.93 (d, J = 8.5 Hz, 2H), 6.56-6.57 (d, J = 1.5
Hz, 4H), 5.91-5.92 (dd, J = 2.5, 5.5 Hz, 4H), 5.77 (s, 2H), 5.63 (s,
4H), 3.89-3.92 (t, J = 6 Hz, 4H), 2.55-2.58 (t, J = 7 Hz, 4H),
1.65-1.73 (m, 8H); 13CNMR(THF-d8, δ25.4) 156.6, 140.4, 136.0,
133.0, 132.7, 130.8, 129.1, 117.6, 114.3, 113.3, 108.0, 107.7, 69.2,
38.1, 36.0, 29.8, 28.6; HRMS (ESIþ) m/z calcd for C44H44Br2-
N4O2þ 818.1826, found 818.1820 [M]þ); IR (cm-1) ν 3410, 2930,
2857, 1488, 1468, 1399, 1315, 1280, 1248, 1196, 1171, 1117, 1084,
1030, 955, 809, 794, 717, 650. Anal. Calcd for C44H44Br2N4O2: C,
78.48; H, 6.59. Found: C, 78.85; H, 6.17.
0.13 mmol) in CH2Cl2 (20 mL) and MeOH (2 mL) was added
Zn(OAc)2 H2O (0.14 g, 0.66 mmol). The solution was refluxed
3
under nitrogen overnight and then dried in vacuo. The solid was
partitioned between water and chloroform. The organic layer
was separated, dried over Na2SO4, filtered, and concentrated in
vacuo. The crude product, a purple powder, was purified by
column chromatography (silica gel, hexane/CH2Cl2, 1/1) to
1
yield 2a as a purple solid (0.15 g, 98%): H NMR(CDCl3, δ
7.26) 8.92-8.95 (dd, J = 4.6, 11.5 Hz, 8H), 8.285-8.290 (d, J =
2.4 Hz, 2H), 8.19-8.21 (m, 4H), 7.85-7.87 (dd, J = 2.4, 8.8 Hz,
2H), 7.71-7.78 (m, 6H), 7.16-7.18 (d, J = 8.9 Hz, 2H), 4.14 (s,
4H, Ar-capped), 3.88-3.90 (t, J = 5.1 Hz, 4H), 1.26-1.30 (m,
4H), 0.83-0.88 (m, 4H), 0.23-0.29 (m, 4H); 13C NMR(CDCl3, δ
77.0) 158.4, 150.3, 150.2, 142.8, 137.4, 137.3, 134.5, 134.3, 132.3,
132.1, 131.5, 127.4, 126.5, 125.7, 121.0, 115.7, 113.5, 111.8,
68.8,33.3, 27.7, 26.2; HRMS (ESIþ) m/z calcd for [C58H44Br2-
N4O2Zn þ H]þ 1051.1195, found [M þ H]þ 1051.0136; IR (cm-1
)
ν 2913, 2905, 2857, 1249, 1013, 964, 794, 736, 704.
H2PorOMe2. To a stirring solution of H2PorBr2 (0.25 g, 0.25
mmol) in THF (10 mL) were added tetrakis(triphenylphos-
phine)palladium(0) (29.2 mg, 0.025 mmol), sodium carbonate
(0.12 g, 1.1 mmol), 4-methylcarboxyphenylboronic acid (0.14 g,
0.76 mmol), and water (4 mL) under nitrogen. The mixture was
refluxed overnight. The reaction mixture was dried in vacuo and
then partitioned between water and chloroform. The organic
layer was separated and then dried over Na2SO4 and filtered.
The crude product was purified by column chromatography
(silica gel, hexane/CH2Cl2 (1:1), then CH2Cl2) to yield 2b as a
purple product (0.2 g, 72%): 1H NMR (CDCl3, δ 7.26)
8.94-9.00 (dd, J = 4.6, 23.0 Hz, 4H), 8.83-8.90 (dd, J = 4.2,
27.5 Hz, 4H), 8.50-8.51 (dd, J = 2.5, 3.9 Hz, 2H), 8.17-8.24
(m, 4H) (proton M), 8.09-8.12 (m, 4H), 8.03-8.06 (dt, J = 2.5,
8.6 Hz, 42H), 7.87-7.88 (d, J = 8.5 Hz, 4H) (proton J), 7.70-
7.77 (m, 6H), 7.35-7.38 (d, J = 7.8 Hz, 2H), 4.03-4.23 (4H, Ar-
capped), 3.94-3.98 (m, J = 5.1 Hz, 4H), 3.91-3.92 (d, J = 3.7
Hz, 6H), 1.32-1.36 (m, 4H), 0.85-0.90 (m, 4H), 0.31-0.41 (m,
4H), -2.52 (s, 2H); 13C NMR(CDCl3, δ 77.00): 167.0, 159.5,
150.4, 150.3, 145.2, 145.1, 143.0, 142.3, 137.3, 137.0, 134.6,
134.5, 134.3, 133.9, 132.8, 132.1, 132.0, 131.6, 130.9, 130.8,
130.2, 128.5, 128.4, 128.3, 127.6, 127.3, 126.7, 126.6, 126.5,
125.8, 120.7, 119.6, 116.8, 115.8, 112.2, 68.5, 67.8, 52.0, 33.4,
29.7, 27.9, 27.8, 26.2, 25.8, 25.5; HRMS (ESIþ) m/z calcd for
[C74H60N4O6 þ H]þ 1101.4586, found [M þ H]þ 1101.5184;
IR (cm-1) ν 1719, 1602, 1490, 1464, 1434, 1273, 1183, 1148,
1109, 1053, 1020, 1000, 969, 831, 798, 770, 750, 729, 716, 701,
663.
2b from ZnPorBr2. To a stirring solution of 2a (0.10 g, 0.09
mmol) in THF (2 mL) were added tetrakis(triphenylphosphine)-
palladium(0) (11 mg, 0.01 mmol), sodium carbonate (45 mg,
0.4 mmol), 4-methyl carboxyphenylboronic acid (51 mg, 0.28
mmol), and water (4 mL). The mixture was refluxed overnight
and evaporated in vacuo. The solid residue was partitioned
between water and chloroform. The organic layer was sepa-
rated, dried over Na2SO4, and filtered. The crude product was
purified by column chromatography (silica gel, hexane/CH2Cl2
(1:1), then CH2Cl2) to yield 2b as a purple solid (0.08 g, 77%).
2b from H2PorOMe2. To a stirred solution of H2PorOMe2
(0.19 g, 0.17 mmol) in CH2Cl2 (20 mL) and MeOH (2 mL) was
H2PorBr2 was synthesized directly from 8, as in Scheme 2: To
a stirring solution of 8 (0.50 g, 0.61 mmol) in propinoic acid
(50 mL) at 0 °C under nitrogen was added trifluoroacetic acid
(0.2 mL, 2.7 mmol). A solution of benzaldehyde (0.19 mL, 1.8
mmol) in CH2Cl2 (20 mL) was added dropwise over 1-2 h.
under nitrogen. The mixture was stirred at 0 °C for an additional
2 h and then overnight at room temperature, during which time
the mixture changed from yellow to purple. The mixture was
then refluxed for 2 h, and the purple color intensified. DDQ
(0.41 g, 1.82 mmol) was added, and the mixture was stirred for
an additional 6 h. The color of the mixture changed from purple
to black, and some precipitate formed. Finally, water was added
into the solution, and excess NaHCO3 was added in small
portions until the aqueous layer was basic. The organic layer
was separated and then concentrated in vacuo. The crude
product was purified by column chromatography (silica gel,
1
CHCl3) to yield H2PorBr2 as a purple solid (0.13 g, 22%): H
NMR (CDCl3, δ 7.26) 8.78-8.92 (s, 8H), 8.299-8.303 (d, J =
2.0 Hz, 2H), 8.17-8.19 (t, J = 5.8 Hz, 4H), 7.85-7.87 (dd, J =
2.1, 8.8 Hz, 2H), 7.72-7.77 (m, 6H), 7.13-7.15 (d, J = 8.9 Hz,
2H), 3.97-4.01 (d, J = 17.5 Hz, 4H, Ar-capped), 3.84-3.86 (t,
J = 4.8 Hz, 4H), 1.28-1.31 (t, J = 7.0 Hz, 4H), 0.74-0.84 (m,
4H), 0.28-0.36 (m, 4H), -2.61(s, 2H); 13C NMR (CDCl3, δ
77.0) 158.5, 142.1, 137.4, 137.0, 134.6, 134.3, 133.6, 132.6, 127.6,
126.7, 136.6, 125.8, 125.7, 120.0, 114.7, 113.5, 111.8, 68.7, 33.3,
27.7, 25.7, HRMS (ESIþ) m/z calcd for [C58H46Br2N4O2 þ H]þ
989.2060, found [M þ H]þ 989.2044).
2a and H2PorBr2 Mixture. To a stirred solution of 8 (0.50 g,
0.61 mmol), trifluoroacetic acid (0.54 mL, 7.3 mmol), and
Zn(OAc)2 2H2O (0.53 g, 2.4 mmol) in 50 mL of propinoic
3
acid at 0 °C was added benzaldehyde (0.19 mL, 1.8 mmol) in
20 mL of CH2Cl2 dropwise over 1-2 h under nitrogen. The
solution was stirred at 0 °C for 2 h and then overnight at room
temperature, during which time it changed from yellow to
purple and then refluxed for 2 h. The purple color was deeper
at the end of the reaction. DDQ (0.41 g, 1.82 mmol) was
added, and the mixture was stirred for additional 6 h, during
which time the color changed from purple to black and some
precipitate formed. Finally, water was added to the mixture,
and excess NaHCO3 was added in small portions until the
aqueous layer was basic. The organic layer was separated
and then concentrated in vacuo. The crude product was
purified by column chromatography (silica gel, CHCl3) to
yield a purple solid which was a mixture of metal-free
H2PorBr2 (from traces to less than 10%) and ZnPorBr2 (2a,
0.17 g, 27%).
added Zn(OAc)2 H2O (0.18 g, 0.84 mmol). The solution was
3
refluxed under nitrogen overnight and then dried in vacuo. The
solid was partitioned between water and chloroform. The
organic layer was separated, dried over Na2SO4, filtered, and
concentrated in vacuo. The crude product, a purple powder, was
purified by column chromatography (silica gel, hexane/CH2Cl2,
1/1) to yield 2b as a purple solid (0.19 g, 97%): 1H NMR (CDCl3,
δ 7.26) 8.95-9.00 (dd, J = 4.6, 19.8 Hz, 8H), 8.50 (d, J = 2.3,
2H), 8.19-8.25 (dd, J = 7.5, 26.7 Hz, 4H), 8.03-8.07 (m, 6H),
J. Org. Chem. Vol. 75, No. 11, 2010 3703

Lee and Galoppini
JOCArticle
7.85-7.87 (d, J = 8.6 Hz, 4H), 7.71-7.76 (m, 6H), 7.37-7.38
(d, J = 8.7 Hz, 2H), 4.25 (s, 4H, Ar-capped), 3.97-3.99 (t, J =
5.0 Hz, 4H), 3.87 (s, 6H), 1.33-1.36 (t, J = 7.3 Hz, 4H), 0.89-
0.94 (m, 4H), 0.31-0.35 (m, 4H); 13C NMR (CDCl3, δ 77.00)
167.0, 159.5, 150.4, 150.2, 154.1, 143.0, 137.4, 134.5, 134.3,
132.8, 132.0, 131.7, 130.8, 130.2, 128.3, 127.4, 126.7, 126.5,
125.8, 120.8, 116.8, 112.2, 68.6, 52.0, 33.4, 27.9, 26.3; HRMS
(ESIþ) m/z calcd for C74H58N4O6Znþ 1162.3642, found [M]þ
1162.4308); IR (cm-1) ν 2946, 2857, 1711, 1600, 1491, 1460,
1429, 1340, 1273, 1190, 1143, 1109, 1068, 1055, 1021, 998, 904,
852, 834, 808, 800, 769, 754, 728, 717, 702, 670.
of Basic Energy Sciences of the U.S. Department of
Energy through Grant No. DE-FG02-01ER15256. We
thank Dr. Lalancette for solving the crystal structure of 2a and
Dr. Cliff Soll for the MS for compounds 4 and 5.
Supporting Information Available: 1H NMR and 13C NMR
spectra of 4-8, H2PorBr2, H2PorOMe2, 2a, and 2b; 1H-1H
COSY NMR spectra of H2PorBr2, H2PorOMe2, 2a, and 2b;
1H-13C HMQC NMR spectra of H2PorBr2, H2PorOMe2, 2a,
and 2b; UV-vis spectra of H2PorBr2, H2PorOMe2, 2a, and 2b;
emission spectra of H2PorBr2, H2PorOMe2, 2a, and 2b. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. This work was funded by the Division
of Chemical Sciences, Geosciences, and Biosciences, Office
3704 J. Org. Chem. Vol. 75, No. 11, 2010