Meso-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Article abstract of DOI:10.1021/jo070593x

(Chemical Equation Presented) Understanding electronic communication among interacting chromophores provides the foundation for a variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin dyads has been investigated by a novel molecular design strategy that entails introduction of a ¹³C-atom (*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier (highest occupied) molecular orbital. The site of ¹³C substitution is at a meso-position, either the site of attachment of the linker (proximal, "P") or the site trans to the linker (distal, "D"). The substituents (R) at the non-linking meso-positions are mesityl, tridec-7-yl ("swallowtail"), or p-tolyl groups. Altogether five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy. The introduction of the meso-¹³C label provides a "clock" (via the hyperfine interactions) that allows investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural abundance ¹⁴N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer, which has been previously shown to be fast on the time scale of the ¹⁴N hyperfine clock (～220 ns), remains fast on the time scale of the ¹³C hyperfine clock (～50 ns).

Full text of DOI:10.1021/jo070593x

Meso-13C-Labeled Porphyrins for Studies of Ground-State Hole
Transfer in Multiporphyrin Arrays
Patchanita Thamyongkit,† Ana Z. Muresan,† James R. Diers,‡ Dewey Holten,*,§
David F. Bocian,*,‡ and Jonathan S. Lindsey*,†
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204,
Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and
Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4889
jlindsey@ncsu.edu; daVid.bocian@ucr.edu; holten@wuchem.wustl.edu
ReceiVed March 22, 2007
Understanding electronic communication among interacting chromophores provides the foundation for a
variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin
dyads has been investigated by a novel molecular design strategy that entails introduction of a 13C-atom
(*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier
(highest occupied) molecular orbital. The site of 13C substitution is at a meso-position, either the site of
attachment of the linker (proximal, “P”) or the site trans to the linker (distal, “D”). The substituents (R)
at the non-linking meso-positions are mesityl, tridec-7-yl (“swallowtail”), or p-tolyl groups. Altogether
five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of
one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy.
The introduction of the meso-13C label provides a “clock” (via the hyperfine interactions) that allows
investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural
abundance 14N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer,
which has been previously shown to be fast on the time scale of the 14N hyperfine clock (∼220 ns),
remains fast on the time scale of the 13C hyperfine clock (∼50 ns).
Introduction
and ground-state holes flow in opposite directions.2 Energy flow
is toward the anode where electron injection occurs and hole
flow is toward the cathode where electrons and holes recombine
to complete the circuit. Such a design should enhance solar-
energy-conversion efficiency because the energy-hole rectifica-
tion process mitigates deleterious charge-recombination reactions
at the anode.
One approach that we have employed to investigate the time
scale of ground-state hole-transfer in multiporphyrin arrays has
been to monitor the hyperfine splittings in the EPR spectra of
singly oxidized arrays such as Dyad-1 and Dyad-2 (Chart 1).
This approach does not explicitly yield the rate of hole transfer
but rather elucidates whether hole transfer is fast or slow on
the time scale defined by the hyperfine coupling. The basic
Understanding electronic communication between the indi-
vidual components of multicomponent molecular architectures
is essential for the rational design and tailoring of such materials
for use in molecular electronic and/or photonic devices. Toward
this goal, we have investigated a variety of porphyrin-based
arrays wherein the individual components are covalently linked
and electronic communication is primarily through-bond via the
linker.1 One objective of these studies has been to design arrays
that might function both as light-harvesting complexes and as
solar-energy transduction systems.2 In these arrays, the porphy-
rinic components are arranged such that excited-state energy
† North Carolina State University.
‡ University of California.
§ Washington University.
(1) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35,
57-69.
(2) Loewe, R. S.; Lammi, R. K.; Diers, J. R.; Kirmaier, C.; Bocian, D.
F.; Holten, D.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 1530-1552.
10.1021/jo070593x CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/19/2007
J. Org. Chem. 2007, 72, 5207-5217
5207

Thamyongkit et al.
CHART 1
the a1u versus the a2u orbital) because the spin density in the
2A1u ground state is localized primarily on the R- and â-pyrrole
carbon atoms.
The majority of 13C-labeled porphyrins that have been
prepared are labeled analogues of naturally occurring porphyrins
and have been prepared via biosynthetic procedures using
labeled precursors.5 By contrast, relatively few non-natural
porphyrins bearing 13C-labels in the macrocycle have been
prepared since the first examples nearly 30 years ago.4,6-16 meso-
Tetraphenylporphyrin,4,7,11 meso-tetra-p-tolylporphyrin,6 and
meso-tetramesitylporphyrin14 bearing four meso 13C-labels have
been prepared by condensation of pyrrole and the labeled
aldehyde. meso-Tetraphenylporphyrin bearing four pairs of
adjacent 13C-labeled pyrrole R- and â-carbons, or 13C labels at
all â-carbons, has been prepared by reaction of the labeled
pyrrole and benzaldehyde.7 Octaethylporphyrin bearing four
meso 13C-labels has been prepared by condensation of 13C-
labeled paraformaldehyde and 3,4-diethylpyrrole.12 An N-
confused porphyrin (2-aza-5,10,15,20-tetraphenyl-21-carbapor-
phyrin) bearing 13C-labels at each R- and â-pyrrole position (16
labels altogether) was prepared by condensation of per-13C-
labeled pyrrole and benzaldehyde.16 A mixture of etioporphyrin
isomers wherein each pyrrole contains one 13C label has been
prepared through use of the labeled pyrrole.13 Synthetic por-
phyrins bearing 13C labels at one or two specific meso-positions
have been prepared through use of labeled dipyrromethanes. A
trans-A2B2-porphyrin bearing two 13C labels (at trans meso-
positions) was prepared by self-condensation of a labeled
1-acyldipyrromethane.15 A protoporphyrin analogue bearing a
single meso-13C label has been prepared through use of a meso-
labeled dipyrromethane.8 Labeled analogues of other naturally
occurring porphyrins (bearing 8 â-substituents) have been
prepared from 1-methyl or 1-iminomethyl 13C-labeled bilenes9
or a,c-biladienes.10
concept is as follows. If hole transfer is slow on the EPR time
scale, the measured hyperfine coupling in a singly oxidized array
is similar to that of a singly oxidized monomeric porphyrin. As
the rate of hole transfer approaches the time scale defined by
the hyperfine coupling, the hyperfine splittings in the EPR
spectra are attenuated; as hole transfer becomes rapid, the
splittings collapses to 1/n of the value observed in a monomer,
where n is the number of centers over which the hole is
delocalized.
In typical porphyrin π-cation radicals, the only nuclei that
exhibit hyperfine interactions with the unpaired electron are 1H
and 14N.3 The relative magnitude of the interactions for the two
types of nuclei depends on the type of porphyrin π-cation
radical. In most meso-substituted porphyrins (which constitute
the bulk of the complexes we have investigated1), the unpaired
electron (and hole) resides in the a2u(π) HOMO; in other words,
2
the ground state of the monocation is A2u.3 This ground state
is characterized by substantial 14N hyperfine interactions and
In this Article, we report the preparation and characterization
of five different porphyrin dyads and companion benchmark
monomers in which a 13C label is incorporated at one of the
1
minimal H interactions due to the distribution of spin density
in the a2u(π) orbital, which is primarily at the meso-carbon and
pyrrole nitrogen atoms, with minimal density at the â-pyrrole
carbon atoms. A typical 14N hyperfine coupling for a porphyrin
(5) (a) Jordan, P. M. In Biosynthesis of Heme and Chlorophylls; Dailey,
H. A., Ed.; McGraw-Hill, Inc.: New York, 1990; pp 55-121. (b) Battersby,
A. R.; Leeper, F. J. Chem. ReV. 1990, 90, 1261-1274. (c) Scott, A. I. Acc.
Chem. Res. 1978, 11, 29-36. (d) Shrestha-Dawadi, P. B.; Lugtenburg, J.
Eur. J. Org. Chem. 2003, 4654-4663. (e) Rivera, M.; Caignan, G. A. Anal.
Bioanal. Chem. 2004, 378, 1464-1483.
2
π-cation with a A2u ground state is ∼1.6 G, the equivalent of
∼4.5 MHz. Thus, the 14N hyperfine interactions serve as a clock
that provides information as to whether hole transfer is fast or
slow on a time scale of ∼220 ns.
In most of the porphyrin arrays we have studied to date, hole
transfer is fast on the 220 ns time scale of the 14N hyperfine
coupling;1 thus, it would be desirable to have a faster hyperfine
clock. Practical considerations severely restrict the types of
porphyrin isotopomers that (1) can be prepared and (2) have
the requisite hyperfine coupling characteristics. All things
considered, 13C incorporation is the best choice for isotopic
substitution. In 2A2u porphyrin π-cation radicals, the spin density
at the meso-carbon atoms is much larger than at the pyrrole
nitrogen atoms.3 As a result, the hyperfine coupling for meso-
13C is ∼5.7 G (or ∼16 MHz), affording the opportunity to
(6) Mispelter, J.; Momenteau, M.; Lhoste, J.-M. J. Chem. Soc., Chem.
Commun. 1979, 808-810.
(7) Mispelter, J.; Momenteau, M.; Lhoste, J.-M. J. Chem. Soc., Dalton
Trans. 1981, 1729-1734.
(8) Smith, K. M.; Kehres, L. A. J. Chem. Soc., Perkin Trans. 1 1983,
2329-2335.
(9) Clezy, P. S.; van Thuc, L. Aust. J. Chem. 1984, 37, 2085-2092.
(10) Smith, K. M.; Minnetian, O. M. J. Org. Chem. 1985, 50, 2073-
2080.
(11) (a) Atamian, M.; Donohoe, R. J.; Lindsey, J. S.; Bocian, D. F. J.
Phys. Chem. 1989, 93, 2236-2243. (b) Seth, J.; Bocian, D. F. J. Am. Chem.
Soc. 1994, 116, 143-153. (c) Bell, S. E. J.; Al-Obaidi, A. H. R.; Hegarty,
M. J. N.; McGarvey, J. J.; Hester, R. E. J. Phys. Chem. 1995, 99, 3959-
3964. (d) Rivera, M.; Caignan, G. A.; Astashkin, A. V.; Raitsimring, A.
M.; Shokhireva, T. K.; Walker, F. A. J. Am. Chem. Soc. 2002, 124, 6077-
6089.
2
monitor events on the ∼60 ns time scale in A2u radicals.4
Furthermore, 13C incorporation affords essentially the only
means of monitoring hole transfer in porphyrin π-cation radicals
with 2A1u ground states. The latter situation occurs for â-pyrrole-
versus meso-substituted porphyrins (for which the HOMO is
(12) Rohrer, A.; Ocampo, R.; Callot, H. J. Synthesis 1994, 923-
925.
(13) Hu, S.; Mukherjee, A.; Piffat, C.; Mak, R. S. W.; Li, X.-Y.; Spiro,
T. G. Biospectroscopy 1995, 1, 395-412.
(14) Nakamura, M.; Ikeue, T.; Ikezaki, A.; Ohgo, Y.; Fujii, H. Inorg.
Chem. 1999, 38, 3857-3862.
(3) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.;
Academic: New York, 1979; Vol. 4, pp 197-256.
(4) Atamian, M.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. Inorg.
Chem. 1988, 27, 1510-1512.
(15) Sharada, D. S.; Muresan, A. Z.; Muthukumaran, K.; Lindsey, J. S.
J. Org. Chem. 2005, 70, 3500-3510.
(16) Calle, C.; Schweiger, A.; Mitrikas, G. Inorg. Chem. 2007, 46, 1847-
1855.
5208 J. Org. Chem., Vol. 72, No. 14, 2007

Meso-13C-Labeled Porphyrins
CHART 2
four meso-positions of the porphyrin macrocycle. The new dyads
are shown in Chart 2. Two such isotopic substitutions were
investigated: the meso-position where a linker is appended
(proximal “P”) and the meso-position opposite to that where
the linker is appended (distal “D”). The rationale for preparing
singly 13C-labeled species is that the EPR spectra are less
complicated than those that would be observed for a porphyrin
in which all meso-carbon atoms are 13C-labeled.4 The types of
dyads investigated include unsymmetrical isotopomers, wherein
the 13C is incorporated in only one of the two constituent
porphyrins of the dyad, and symmetrical isotopomers, wherein
13C is incorporated in both porphyrins. Collectively, the studies
illustrate a novel molecular design strategy wherein modified
hyperfine clocks are employed for studies of hole transfer in
multiporphyrin arrays.
Results and Discussion
I. Synthesis. 1. Strategy. Each target porphyrin dyad bears
a diphenylethynyl interporphyrin linkage and one or two 13C
labels (Chart 2). Hence, Sonogashira coupling of an iodo-
porphyrin monomer and an ethynyl-porphyrin monomer is the
reaction of choice for joining the porphyrin constituents.17,18
A
13C atom is present at one or both of the linking meso-positions
J. Org. Chem, Vol. 72, No. 14, 2007 5209

Thamyongkit et al.
SCHEME 1
SCHEME 2
in four dyads (Dyad-1-P, Dyad-1-PP, Dyad-2-P, and Dyad-
2-PP), and at the non-linking, distal meso-positions of the Dyad-
3-DD. The substituents at the non-linking meso-positions include
mesityl, swallowtail, and p-tolyl groups. Mesityl17,18 and swal-
lowtail19,20 groups provide facial encumbrance and thereby
increase the solubility of porphyrins and accompanying multi-
porphyrin arrays. The porphyrins in Dyad-1-P and Dyad-1-PP
or Dyad-2-P and Dyad-2-PP are of the A3B-type and include
three mesityl or three swallowtail groups. The porphyrins in
Dyad-3-DD are of the trans-AB2C type and include two mesityl
groups at the flanking (“winged”) positions and a p-tolyl group
at the site distal to the interporphyrin linker.
Rational synthetic methods are available for the synthesis of
porphyrins bearing up to four different meso-substituents.21,22
The methods accommodate a variety of substituents, but are
not yet compatible with three mesityl or three swallowtail
groups. Accordingly, the A3B-porphyrins bearing mesityl or
swallowtail groups (required for the synthesis of Dyad-1P,
Dyad-1PP, Dyad-2P, and Dyad-2PP) were prepared via
statistical methods. In each case, the singly 13C-labeled A3B-
porphyrin was prepared through use of a non-linking “A”
aldehyde and the labeled “B” aldehyde. The trans-AB2C-
porphyrin required for the synthesis of Dyad-3-DD was prepared
via a rational route where the labeled unit was introduced via
9-acylation of a 1-acyldipyrromethane.
2. Labeled Porphyrin Monomers. The preparation of an
aldehyde precursor is shown in Scheme 1. The 13C atom was
introduced by cyanation of 1,4-diiodobenzene with 1.1 equiv
of KCN-13C in the presence of CuI23 in 1-methyl-2-pyrrolidi-
none24 to give the labeled cyano derivative 1 in 47% yield.
Reduction of the latter using DIBALH followed by Pd-coupling
of the resulting aldehyde 2 with 2-methylbut-3-yn-2-ol gave
aldehyde 3. These two 13C-labeled aldehydes were used in the
syntheses of the precursors of the target dyads.
Following the published procedure for preparing the non-
labeled analogues,25,26 a mixed-condensation of mesitaldehyde,
pyrrole, and aldehyde 2 in the presence of BF3-ethanol
cocatalysis27 (BF3‚O(Et)2 in CHCl3 containing ethanol) followed
by DDQ oxidation afforded the mixture of free base porphyrins.
A mixture composed of porphyrins bearing substituents that give
a graded degree of steric hindrance is more easily separable
upon conversion to the zinc chelates versus the free base species
given the affinity of the apical zinc site for adsorption
chromatographic media.17 Thus, metalation with zinc acetate
gave the corresponding zinc porphyrins, which upon chroma-
tography on silica gave the 13C-containing iodo-porphyrin Zn-
4-P in 8% yield (Scheme 2). Analogous reaction with aldehyde
(17) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
(18) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem.
1995, 60, 5266-5273.
(23) (a) Supniewski, J. V.; Salzberg, P. L. Org. Synth. 1941, 1, 46-47.
(b) Kurumaya, K.; Okazaki, T.; Seido, N.; Akasaka, Y.; Kawajiri, Y.;
Kajiwara, M.; Kondo, M. J. Labelled Compd. Radiopharm. 1989, 27, 217-
235.
(19) Thamyongkit, P.; Speckbacher, M.; Diers, J. R.; Kee, H. L.;
Kirmaier, C.; Holten, D.; Bocian, D. F.; Lindsey, J. S. J. Org. Chem. 2004,
69, 3700-3710.
(24) Carr, R. M.; Cable, K. M.; Wells, G. N.; Sutherland, D. R. J.
Labelled Compd. Radiopharm. 1994, 34, 887-897.
(20) Thamyongkit, P.; Lindsey, J. S. J. Org. Chem. 2004, 69, 5796-
5799.
(25) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J. S. Chem.
Mater. 1999, 11, 2974-2983.
(21) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org.
Chem. 2000, 65, 7323-7344.
(22) Zaidi, S. H. H.; Fico, R., Jr.; Lindsey, J. S. Org. Process Res. DeV.
2006, 10, 118-134.
(26) Youngblood, W. J.; Gryko, D. T.; Lammi, R. K.; Bocian, D. F.;
Holten, D.; Lindsey, J. S. J. Org. Chem. 2002, 67, 2111-2117.
(27) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.
5210 J. Org. Chem., Vol. 72, No. 14, 2007

Meso-13C-Labeled Porphyrins
SCHEME 3
SCHEME 4
as an acylating reagent for the preparation of a porphyrin
monomer to be used in the synthesis of Dyad-3-DD.
The synthesis of the porphyrin building blocks bearing two
mesityl groups, one p-tolyl group attached to a 13C-labeled meso
site, and an iodophenyl or ethynylphenyl unit, is shown in
Scheme 5. 5-Mesityldipyrromethane (12)32 was acylated previ-
ously under standard Grignard-mediated conditions with iodo-
benzothioate 13a21 and TMS-ethynylbenzothioate 13b21 to give
the known compounds 14 and 15.21 1-Acyldipyrromethanes
streak extensively upon chromatography and afford amorphous
solids, rendering purification difficult. We recently developed
methodology wherein a dialkylboron complex is prepared of
the 1-acyldipyrromethanes, affording nonpolar crystalline sol-
ids.33 Accordingly, the resulting 1-acyldipyrromethane was
readily isolated in the form of a dialkylboron complex (Bu2B-
1433 or Bu2B-15) in 89% or 63% yield, respectively.
The Grignard-mediated 9-acylation34 of dialkylboron complex
Bu2B-14 or Bu2B-15 with benzothioate 11 led to the labeled
1,9-diacyldipyrromethane-dialkylboron complex Bu2B-16 or
Bu2B-17 in 41% or 45% yield, respectively. Reduction of
complex Bu2B-16 or Bu2B-17 with NaBH4 gave the corre-
sponding dipyrromethane-1,9-dicarbinol, which upon condensa-
tion with dipyrromethane 12 in the presence of Yb(OTf)3,21,35
oxidation with DDQ, and metalation of the resulting porphyrin
with Zn(OAc)2‚2H2O gave zinc iodo-porphyrin Zn-18-D or zinc
TMS-ethynyl-porphyrin Zn-19-D in 13% or 18% yield, respec-
tively. TMS-cleavage from Zn-19-D with TBAF gave ethynyl-
porphyrin Zn-20-D in 98% yield.
3 gave TMS-ethynyl-porphyrin Zn-5-P in 14% yield. Depro-
tection of Zn-5-P with NaOH in refluxing toluene gave ethynyl-
porphyrin Zn-6-P in 96% yield.
In a similar manner,19 condensation of pyrrole, aldehyde 2,
and 7-formyltridecane28 followed by DDQ oxidation gave the
mixture of free base porphyrins. Zinc metalation and chromato-
graphic separation gave iodo-porphyrin Zn-7-P in 15% yield
(Scheme 3). Pd-coupling of the latter with (trimethylsilyl)-
acetylene led to TMS-ethynyl-porphyrin Zn-8-P, which upon
TMS-deprotection with TBAF gave ethynyl-porphyrin
Zn-9-P.
3. Unlabeled Porphyrin Monomers. Several porphyrin
monomers required for the synthesis of the singly labeled dyads
were synthesized previously. The monomers include iodo-
porphyrin Zn-21,25 TMS-ethynyl-porphyrin Zn-22,19 and ethy-
nyl-porphyrin Zn-23,19 which are shown in Chart 3.
Following a procedure for the synthesis of unlabeled p-toluic
acid,29 p-bromotoluene underwent cyanation with KCN-13C and
CuI23 in pyridine30 followed by 75% aqueous H2SO4 to obtain
labeled p-toluic acid 10 in 50% yield (Scheme 4). Reaction31
of the latter with 2,2′-dipyridyl disulfide in the presence of PPh3
afforded benzothioate 11 in 96% yield. The latter was employed
(32) Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391-1396.
(33) Muthukumaran, K.; Ptaszek, M.; Noll, B.; Scheidt, W. R.; Lindsey,
J. S. J. Org. Chem. 2004, 69, 5354-5364.
(28) Kato, M.; Komoda, K.; Namera, A.; Sakai, Y.; Okada, S.; Yamada,
A.; Yokoyama, K.; Migita, E.; Minobe, Y.; Tani, T. Chem. Pharm. Bull.
1997, 45, 1767-1776.
(29) Codington, J. F.; Mosettig, E. J. Org. Chem. 1952, 17, 1035-1042.
(30) Reid, J. C.; Weaver, J. C. Cancer Res. 1951, 11, 188-194.
(31) Rao, P. D.; Littler, B. J.; Geier, G. R., III; Lindsey, J. S. J. Org.
Chem. 2000, 65, 1084-1092.
(34) Zaidi, S. H. H.; Muthukumaran, K.; Tamaru, S.-I.; Lindsey, J. S. J.
Org. Chem. 2004, 69, 8356-8365.
(35) Geier, G. R., III; Callinan, J. B.; Rao, P. D.; Lindsey, J. S. J.
Porphyrins Phthalocyanines 2001, 5, 810-823.
J. Org. Chem, Vol. 72, No. 14, 2007 5211

Thamyongkit et al.
SCHEME 5
CHART 3
5. Chemical Characterization of the Labeled Porphyrins
and Dyads. Each labeled porphyrin was characterized by
1
absorption spectroscopy, fluorescence spectroscopy, H NMR
spectroscopy, 13C NMR spectroscopy, and mass spectrometry.
The latter included laser desorption mass spectrometry (LD-
MS) without a matrix36 and high-resolution FAB-MS. The dyads
were similarly characterized including use of analytical SEC
to establish purity but without FAB-MS given their higher mass.
The availability of 13C-labeled porphyrins, porphyrin precursors,
and porphyrin dyads provides a valuable set of data for
unambiguous assignment of 13C NMR resonances for these
compounds. A table of such data is provided in the Supporting
Information.
II. EPR Spectra and Hole-Transfer Characteristics of the
Dyads. The EPR spectra of the monocation radicals of all of
the 13C-labeled monomers and dyads were examined. Ambient-
temperature spectra of selected monomers and dyads are shown
in Figures 1 and 2, respectively. In both figures, the top trace
is the spectrum of the monocation radical of an unlabeled
analogue of one of the labeled complexes. The complexes for
which spectra are shown in the figures were chosen because
the monocation radicals of these species exhibit the full
complement of spectral features observed for all of the singly
oxidized complexes.
The EPR spectrum of the unlabeled complex, [Zn-22]+,
exhibits the characteristic nine-line hyperfine pattern due to
interaction of the four 14N nuclei with the unpaired electron
(Figure 1, top trace).2-4 The spectra of the two 13C-labeled
complexes, [Zn-8-P]+ and [Zn-19-D]+, exhibit this same pattern
superimposed on each member of the doublet that results from
interaction of the single 13C nucleus with the unpaired electron.
The 13C hyperfine splittings for both of the labeled complexes
4. Labeled Porphyrin Dyads. The Sonogashira coupling of
an iodo-porphyrin and an ethynyl-porphyrin was carried out to
give the target dyads. The coupling was performed under
copper-free conditions in the presence of Pd2(dba)3 and P(o-
tol)3 in toluene/TEA at 35 °C.25 The reaction was monitored
by analytical size exclusion chromatography (SEC), which
showed the complete consumption of the monomer after 22 h.
The reaction mixtures were purified by a three-column sequence
consisting of adsorption chromatography and preparative SEC.
The dyads were obtained in reasonable yield, with the exception
of Dyad-1-PP where, due to the poor solubility of the
compound, the reaction mixture could not be completely
redissolved after the complete removal of the solvent. The results
are summarized in Table 1.
(36) (a) Srinivasan, N.; Haney, C. A.; Lindsey, J. S.; Zhang, W.; Chait,
B. T. J. Porphyrins Phthalocyanines 1999, 3, 283-291. (b) Fenyo, D.; Chait,
B. T.; Johnson, T. E.; Lindsey, J. S. J. Porphyrins Phthalocyanines 1997,
1, 93-99.
5212 J. Org. Chem., Vol. 72, No. 14, 2007

Meso-13C-Labeled Porphyrins
TABLE 1. Synthesis of Labeled Porphyrin Dyadsa
iodo-porphyrin
ethynyl-porphyrin
R10, R20
entry
no.
R10, R20
R15
no.
R15
product
yield
1
2
3
4
5
Zn-4-P
Zn-21
Zn-7-P
Zn-7-P
Zn-18-D
Mes
Mes
SWT
SWT
Mes
Mes
Mes
SWT
SWT
p-Tol
Zn-6-P
Zn-6-P
Zn-9-P
Zn-23
Mes
Mes
SWT
SWT
Mes
Mes
Mes
SWT
SWT
p-Tol
Dyad-1-PP
Dyad-1-P
Dyad-2-PP
Dyad-2-P
23%
80%
71%
73%
51%
Zn-20-D
Dyad-3-DD
a Mes ) mesityl; SWT ) swallowtail (7-tridecyl); p-Tol ) p-tolyl.
FIGURE 2. Ambient temperature EPR spectra of [Dyad-2]+, [Dyad-
2-P]+, [Dyad-1-PP]+, and [Dyad-3-DD]+.
FIGURE 1. Ambient temperature EPR spectra of [Zn-22]+, [Zn-8-
P]+, and [Zn-19-D]+.
transfer.2 As expected, the EPR spectra of the three different
13C-labeled dyads, [Dyad-2-P]+, [Dyad-1-PP]+, [Dyad-3-DD]+,
also fail to exhibit resolved 14N hyperfine splittings. More
importantly, the spectra of the three dyads also fail to exhibit
resolved 13C hyperfine splittings. Spectral simulations indicate
that the 13C splittings have been reduced from ∼6.2 to ∼3.1 G.
This result is consistent with hole transfer remaining fast on
the time scale defined by the 13C hyperfine interactions
(∼50 ns).
are ∼6.2 G, which is much larger (as expected4) than the 14N
hyperfine splittings, which are ∼1.5 G. [Note that the meso-
13C hyperfine couplings observed for [Zn-8-P]+ and [Zn-19-
D]+ are slightly larger than those observed for the monocation
radical of zinc tetraphenylporphyrin (∼5.7 G).2] The 14N
hyperfine pattern for [Zn-8-P]+ is much better resolved than
that for [Zn-19-D]+. This difference is not due to the slightly
larger 14N hyperfine couplings for complexes that contain
swallowtail
The temperature dependence of the EPR spectra of the
monocations of the various dyads was also examined. The
spectra of a selected (and representative) dyad, [Dyad-2-P]+,
obtained in the 120-295 K range are shown in Figure 3. As
the temperature is lowered from 295 to 190 K, near the freezing
point of the solvent, the hyperfine splittings become slightly
better resolved; however, the overall spectral line width remains
approximately constant. This observation indicates that hole
transfer remains fast on the time scale of the 13C (and 14N) clock
at reduced temperature. The line width is appreciably broadened
at 120 K, where the solvent is frozen; however, no hyperfine
splittings are resolved. This result is consistent with previous
([Zn-8-P]+ ∼1.7 G) versus aryl ([Zn-19-D]+ ∼1.5 G) non-
linker substituents, but rather to a difference in line width that
appears to be characteristic of placement of the 13C label
proximal versus distal to the appended linker. This assessment
is based on the fact that [Zn-5-P]+ and [Zn-6-P]+, each of which
contains a proximal 13C label and aryl non-linker substituents,
exhibit well-resolved spectra similar to that observed for [Zn-
8-P]+. We have no explanation for this difference in line widths.
The EPR spectrum of the unlabeled dyad, [Dyad-2]+, does
not exhibit any resolved 14N hyperfine splittings (Figure 2, top
trace) because of collapse of these splittings due to rapid hole
J. Org. Chem, Vol. 72, No. 14, 2007 5213

Thamyongkit et al.
(250 mL). After drying over Na2SO4, the solvent was evaporated.
Flash column chromatography [silica, hexanes f hexanes/CH2Cl2
1
(2:1)] afforded a white solid (1.65 g, 47%): mp 124-125 °C; H
NMR (400 MHz) δ 7.38 (dd, J ) 8.4 Hz, J ) 5.2 Hz, 2H), 7.86
(d, J ) 8.4 Hz, 2H); 13C NMR δ 118.2 (enh), 133.1, 133.2, 138.45,
138.51. Anal. Calcd for C613CH4IN: C, 36.98; H, 1.75; N, 6.09.
Found: C, 37.04; H, 1.73; N, 5.94.
4-Iodobenzaldehyde-formyl-13C (2). A solution of 1 (3.00 g,
13.0 mmol) in CH2Cl2 (43 mL) was cooled in an ice bath and slowly
treated with DIBAL-H (15.65 mL, 1.0 M in hexanes). The ice bath
was removed. The reaction mixture was stirred at room temperature.
After 3 h, the mixture was poured into a mixture of crushed ice
(100 g) and 6 N HCl (250 mL) and stirred for 1 h. The organic
phase was separated, washed with 5% NaHCO3 and brine, dried
over Na2SO4, and concentrated to dryness. The crude product was
purified by column chromatography [silica, CH2Cl2/hexanes (1:
1
2)], resulting in a white solid (2.64 g, 87%): mp 77-78 °C; H
NMR (400 MHz) δ 7.60 (dd, J ) 8.0 Hz, J ) 4.8 Hz, 2H), 7.93
(d, J ) 8.4 Hz, 2H), 9.96 (d, J ) 175.2 Hz, 1H); 13C NMR δ
130.78, 130.82, 138.37, 138.42, 191.4 (enh). Anal. Calcd for
C613CH5O: C, 36.51; H, 2.16. Found: C, 36.21; H, 2.08.
4-(3-Methyl-3-hydroxybut-1-yn-1-yl)benzaldehyde-formyl-
13C (3). A mixture of 2 (500 mg, 2.15 mmol), Pd(PPh3)2Cl2 (151
mg, 0.215 mmol), and CuI (21.0 mg, 0.110 mmol) was degassed
in a Schlenk flask. Dry TEA (4.3 mL) was added, and the resulting
mixture was treated with 2-methylbut-3-yn-2-ol (250 µL, 2.58
mmol). The reaction mixture was heated to 40 °C and stirred for 2
h. After being cooled to room temperature, the mixture was filtered.
The filtered material was washed with diethyl ether. The filtrate
was concentrated to dryness and purified by column chromato-
graphy (silica, CH2Cl2) to obtain a yellow oil, which solidified upon
cooling (342 mg, 84%): 1H NMR (400 MHz) δ 1.64 (s, 6H), 2.35
(s, 1H), 7.55 (d, J ) 8.0 Hz, 2H), 7.80 (dd, J ) 8.0 Hz, J ) 4.8
Hz, 2H), 9.99 (d, J ) 174.8 Hz, 1H); 13C NMR δ 31.3, 65.6, 81.3,
97.8, 129.1, 129.4, 129.5, 132.09, 132.14, 135.1, 135.6, 191.5 (enh);
FAB-MS obsd 190.0941, calcd 190.0949 [(M + H)+, M )
C1113CH12O2].
FIGURE 3. Temperature dependence of the EPR spectrum of [Dyad-
2-P]+.
studies of unlabeled dyads, which indicate that hole transfer
becomes slow on the EPR time scale upon freezing of the
solvent.37 These previous studies have also revealed that the
slowing of hole transfer occurs abruptly upon solvent freezing,
similar to the behavior observed herein. The abrupt slowing of
hole transfer is attributed to the significantly higher reorganiza-
tion energy of the frozen versus liquid solvent.
Experimental Section
5-(4-Iodophenyl)-10,15,20-trimesitylporphinatozinc(II)-(5-
13C) (Zn-4-P). Following a standard procedure,25 a mixture of
pyrrole (346 mg, 5.16 mmol), mesitaldehyde (572 mg, 3.86 mmol),
and 2 (300 mg, 1.29 mmol) in CHCl3 (515 mL) was degassed for
5 min and treated with BF3‚O(Et)2 (215 µL, 1.70 mmol) at room
temperature. After 1 h, DDQ (875 mg, 3.85 mmol) was added.
The reaction mixture was stirred for 1 h, and then TEA (0.50 mL)
was added. The mixture was passed through a silica pad (CH2Cl2)
to give the mixture of porphyrins. The solvent was removed. The
resulting crude mixture was dissolved in CHCl3 (40 mL) and treated
overnight with a solution of Zn(OAc)2‚2H2O (1.37 g, 6.24 mmol)
in methanol (10 mL) at room temperature. The reaction mixture
was washed with water, dried (Na2SO4), and concentrated to
dryness. Column chromatography [silica, CH2Cl2/hexanes (1:4)]
afforded the title compound as the second purple band (100 mg,
8%): 1H NMR (400 MHz, THF-d8) δ 1.84 (s, 12H), 1.85 (s, 6H),
2.63-2.64 (m, 9H), 7.28-7.29 (m, 6H), 7.96 (dd, J ) 8.0 Hz, J )
1.6 Hz, 2H), 8.08 (d, J ) 8.4 Hz, 2H), 8.71 (s, 4H), 8.77 (d, J )
4.8 Hz, 2H), 8.84 (d, J ) 4.4 Hz, 2H); 13C NMR (THF-d8) δ 21.5,
21.66, 21.75, 119.0 (enh), 127.6, 130.75, 130.81, 131.15, 131.22,
131.71, 131.79, 131.81, 135.58, 135.63, 136.0, 136.1, 137.4, 137.7,
138.87, 138.95, 139.2, 149.00, 149.04, 149.7, 149.8, 149.9, 150.0;
LD-MS obsd 929.2 [M+], 1734.2 [(2M - I)+]; FAB-MS obsd
919.1989, calcd 919.2014 (C5213CH45IN4Zn); λabs 423, 549, 593 nm;
λem (λex ) 550 nm) 594, 645 nm.
Electrochemistry. The electrochemical measurements were
performed using techniques and instrumentation previously de-
scribed.38 The solvent was CH2Cl2, CH2Cl2/THF (9:1), or CH2Cl2/
o-dichlorobenzene (9:1), depending on the solubility of the complex,
containing 0.1 M Bu4NPF6 as the supporting electrolyte. The bulk
oxidized complexes were prepared in a glove box. Upon oxidation,
the samples were transferred to an EPR tube and sealed in the glove
box.
EPR Spectroscopy. The EPR spectra were recorded on an
E-band spectrometer (Bruker EMX) equipped with an NMR
gaussmeter and microwave frequency counter. The EPR spectra
were obtained on samples that were typically 0.2 mM at both
ambient and cryogenic temperatures. The microwave power and
magnetic field modulation amplitude were typically 5.7 mW and
0.32 G, respectively.
4-Iodobenzonitrile-cyano-13C (1). Following a standard proce-
dure24 with slight modification, a mixture of 1,4-diiodobenzene
(5.00 g, 15.2 mmol), KCN-13C (1.10 g, 16.7 mmol), and CuI (1.44
g, 7.56 mmol) in NMP (38 mL) was refluxed in a Schlenk flask
for 5.5 h. After the mixture was cooled to room temperature, ethyl
acetate (200 mL) was added. The resulting reaction mixture was
washed with several solutions in the following sequence: aqueous
FeCl3 (2 × 250 mL, 10 w/v %), water (250 mL), aqueous Na2S2O3
(2 × 250 mL, 10 w/v %), water (250 mL), and saturated NaCl
5-[4-(3-Methyl-3-hydroxybut-1-yn-1-yl)phenyl]-10,15,20-tri-
mesitylporphinatozinc(II)-(5-13C) (Zn-5-P). Following a standard
procedure,26 a mixture of pyrrole (355 mg, 5.29 mmol), mesital-
dehyde (587 mg, 3.96 mmol), and 3 (250 mg, 1.32 mmol) in CHCl3
(72 mL) was degassed for 5 min and treated with BF3‚O(Et)2 (164
µL, 1.29 mmol) at room temperature for 1 h. DDQ (901 mg, 3.97
mmol) was added, and the reaction mixture was stirred for 1 h.
(37) (a) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey,
J. S.; Bocian, D. F. J. Am. Chem. Soc. 1994, 116, 10578-10592. (b) Seth,
J.; Palaniappan, V.; Wagner, R. W.; Johnson, T. E.; Lindsey, J. S.; Bocian,
D. F. J. Am. Chem. Soc. 1996, 118, 11194-11207.
(38) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann, S.; Kim, D.; Holten,
D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines 1999, 3,
117-47.
5214 J. Org. Chem., Vol. 72, No. 14, 2007

Meso-13C-Labeled Porphyrins
TEA (0.50 mL) was added. The mixture was passed through a short
silica column (CH2Cl2), affording a first band (faint, containing
meso-tetramesitylporphyrin) and a second band (intense, containing
a mixture of A3B, A2B2, AB3, and B4 free base porphyrins). The
solvent was removed. The resulting crude purple material was
dissolved in CHCl3 (40 mL) and treated overnight with a solution
of Zn(OAc)2‚2H2O (1.45 g, 6.61 mmol) in methanol (10 mL) at
room temperature. The reaction mixture was washed with water
and dried (Na2SO4). Column chromatography [silica, CH2Cl2/
hexanes (1:1)] afforded the title compound as the first purple band.
The resulting solid was suspended in methanol; the suspension was
sonicated and then decanted to afford a purple solid (165 mg,
14%): 1H NMR (400 MHz, THF-d8) δ 1.77 (s, 6H), 1.84 (s, 12H),
1.86 (s, 6H), 2.37 (s, 1H), 2.64 (s, 9H), 7.28 (s, 6H), 7.81 (d, J )
8.0 Hz, 2H), 8.18 (dd, J ) 8.0 Hz, J ) 3.6 Hz, 2H), 8.71 (s, 4H),
8.76 (d, J ) 4.4 Hz, 2H), 8.83 (d, J ) 4.4 Hz, 2H); 13C NMR
(THF-d8) δ 21.5, 21.7, 21.8, 31.6, 119.1 (enh), 127.6, 129.74,
129.78, 129.82, 130.68, 130.73, 130.8, 131.1, 131.2, 131.75, 131.79,
131.81, 131.83, 134.28, 134.3, 137.40, 137.45, 137.47, 139.0, 139.2,
139.3, 149.1, 149.7, 149.9, 150.0; LD-MS obsd 885.3 [M+]; FAB-
MS obsd 885.3508, calcd 885.3466 (C5713CH52N4OZn); λabs 423,
550, 592 nm; λem (λex ) 550 nm) 595, 645 nm.
5-(4-Ethynylphenyl)-10,15,20-trimesitylporphinatozinc(II)-(5-
13C) (Zn-6-P). A solution of Zn-5-P (100 mg, 113 µmol) in toluene
(7.5 mL) was treated with powdered NaOH (76.6 mg, 1.92 mmol)
under reflux. After 3.5 h, TLC analysis showed the complete
consumption of Zn-5-P. The reaction mixture was passed through
a silica pad (toluene) whereupon the title compound eluted as the
first band. Removal of the solvent led to a purple solid (90 mg,
96%): 1H NMR (400 MHz, THF-d8) δ 1.85 (s, 12H), 1.86 (s, 6H),
2.64 (s, 9H), 3.32 (s, 1H), 7.29 (s, 6H), 7.88 (d, J ) 8.0 Hz, 2H),
8.20 (dd, J ) 8.4 Hz, J ) 4.0 Hz, 2H), 8.72 (s, 4H), 8.77 (d, J )
4.4 Hz, 2H), 8.84 (d, J ) 4.4 Hz, 2H); 13C NMR (THF-d8) δ 21.5,
21.67, 21.75, 118.9 (enh), 121.1, 127.6, 130.27, 130.32, 130.7,
130.8, 131.1, 131.2, 131.7, 131.8, 134.29, 134.30, 137.4, 138.9,
139.0, 139.2, 149.0, 149.7, 149.8, 149.90, 149.93, 149.95, 150.0;
LD-MS obsd 827.3 [M+]; FAB-MS obsd 827.3045, calcd 827.3047
(C5413CH46N4Zn); λabs 423, 550, 592 nm; λem (λex ) 550 nm) 595,
645 nm.
5-(4-Iodophenyl)-10,15,20-tris(tridec-7-yl)porphinatozinc(II)-
(5-13C) (Zn-7-P). Following a standard procedure,19 a mixture of
pyrrole (411 mg, 6.13 mmol), 7-formyltridecane (2.60 g, 12.3
mmol), and 2 (357 mg, 1.53 mmol) in CHCl3 (613 mL) was
degassed for 5 min and treated with BF3‚O(Et)2 (257 µL, 2.03
mmol) at room temperature. After 1 h, DDQ (1.04 g, 4.60 mmol)
was added, and the mixture was stirred for 1 h. TEA (0.50 mL)
was added. The reaction mixture was filtered through a silica pad
(CH2Cl2). The resulting mixture of free base porphyrins was
dissolved in CHCl3 (48 mL) and treated overnight with a solution
of Zn(OAc)2‚2H2O (1.68 g, 7.66 mmol) in methanol (12 mL) at
room temperature. The reaction mixture was washed with water,
dried (Na2SO4), and concentrated to dryness. Column chromatog-
raphy [silica, CH2Cl2/hexanes (1:8)] afforded the title compound
as the second purple band (263 mg, 15%): 1H NMR (400 MHz) δ
0.70-0.75 (m, 18H), 1.02-1.64 (m, 48H), 2.71-2.84 (m, 6H),
2.88-3.02 (m, 6H), 5.14-5.27 (m, 3H), 7.93 (dd, J ) 8.0 Hz, J )
3.6 Hz, 2H), 8.08 (d, J ) 8.0 Hz, 2H), 8.84-8.90 (m, 2H), 9.60-
9.84 (m, 6H); 13C NMR δ 13.9, 22.5, 29.6, 29.7, 29.8, 29.9, 31.7,
42.5, 42.6, 47.1, 47.2, 47.3, 47.5, 116.9 (enh, t, J ) 70.5 Hz), 124.3,
124.5, 129.1, 129.3, 129.6, 130.0, 130.3, 130.6, 131.0, 131.3, 135.5,
136.0, 142.7, 143.2, 146.6, 146.8, 147.1, 147.5, 148.6, 148.8, 149.0,
149.4, 149.6, 151.0, 151.1, 151.6; LD-MS obsd 1121.6; FAB-MS
obsd 1121.6035, calcd 1121.5770 (C6413CH93IN4Zn); λabs 425, 555,
593 nm; λem (λex ) 550 nm) 602, 652 nm.
flask followed by (trimethylsilyl)acetylene (30.4 µL, 307 µmol).
After reaction at 35 °C for 16 h, the reaction mixture was
concentrated to dryness and purified by column chromatography
[silica, CH2Cl2/hexanes (1:9)], affording a purple solid (90 mg,
53%): 1H NMR δ 0.43 (s, 9H), 0.70-0.76 (m, 18H), 1.02-1.66
(m, 48H), 2.75-2.86 (m, 6H), 2.88-3.00 (m, 6H), 5.14-5.30 (m,
3H), 7.88 (dd, J ) 7.8 Hz, 2H), 8.15 (dd, J ) 7.8 Hz, J ) 3.6 Hz,
2H), 8.82-8.98 (m, 2H), 9.58-9.88 (m, 6H); 13C NMR δ 0.1, 13.9,
22.5, 29.6, 29.7, 29.8, 29.9, 31.7, 42.5, 42.6, 47.1, 47.2, 47.4, 47.5,
95.0, 105.4, 117.7 (enh, t, J ) 69.0 Hz), 122.0, 124.1, 124.3, 124.6,
129.1, 129.3, 129.4, 130.07, 130.11, 130.4, 130.6, 130.9, 131.0,
131.2, 131.3, 134.2, 143.5, 144.2, 146.9, 147.5, 148.8, 149.5, 149.6,
151.2, 151.7; LD-MS obsd 1092.2; FAB-MS obsd 1091.6882, calcd
1091.7199 (C6913CH102N4SiZn); λabs 425, 555, 593 nm; λem (λex
)
550 nm) 604, 654 nm.
5-(4-Ethynylphenyl)-10,15,20-tris(tridec-7-yl)porphinatozinc-
(II)-(5-13C) (Zn-9-P). A solution of Zn-8-P (70.0 mg, 64.0 µmol)
in CHCl3/THF (7.0 mL, 3:1) was treated with a solution of TBAF
(96.0 µL, 96.0 µmol, 1.0 M in THF) at room temperature over 1 h.
The reaction mixture was washed with 10% NaHCO3 and water,
dried (Na2SO4), and concentrated to dryness. The resulting crude
product was purified by column chromatography [silica, CH2Cl2/
hexanes (1:8)] to obtain a purple solid (40 mg, 60%): 1H NMR
(400 MHz) δ 0.69-0.74 (m, 18H), 1.02-1.64 (m, 48H), 2.72-
2.84 (m, 6H), 2.87-3.01 (m, 6H), 3.33 (s, 1H), 5.16-5.26 (m,
3H), 7.89 (d, J ) 7.6 Hz, 2H), 8.16 (dd, J ) 7.6 Hz, J ) 4.0 Hz,
2H), 8.81-8.90 (m, 2H), 9.48-9.84 (m, 6H); 13C NMR δ 13.9,
22.5, 29.6, 29.8, 29.9, 31.7, 42.5, 42.6, 47.1, 47.2, 47.3, 47.5, 77.8,
84.0, 117.5 (enh, t, J ) 68.1 Hz), 124.0, 125.1, 129.1, 129.3, 129.5,
130.21, 130.25, 130.6, 131.0, 131.3, 134.2, 143.8, 144.5, 146.8,
147.2, 147.5, 148.7, 149.0, 149.5, 149.7, 150.5, 151.0, 151.10,
151.18, 151.24; LD-MS obsd 1021.3; FAB-MS obsd 1019.7037,
calcd 1019.6803 (C6613CH94N4Zn); λabs 425, 555, 593 nm; λem
(λex ) 550 nm) 605, 653 nm.
p-Toluic-carboxy-13C Acid (10). Following a known procedure,29
a solution of CuCN-13C (2.41 g, 26.6 mmol) in pyridine (1.82 mL,
22.5 mmol) at 150-155 °C was treated with 4-bromotoluene (3.50
g, 20.5 mmol). The reaction mixture was refluxed at 185-190 °C
for 6 h. After the mixture cooled to room temperature, 75% aqueous
H2SO4 (12 mL) was added, and the mixture was stirred at 150 °C
for 2 h. The reaction mixture was poured into ice-water (20 mL)
and subsequently treated with 5% aqueous NaOH (30 mL), resulting
in a yellow precipitate. After being stirred at room temperature for
15 min, the mixture was filtered. The filtrate was acidified with
50% aqueous HCl until pH < 4 was obtained. A colorless
precipitate was collected and dissolved in ethyl acetate (50 mL).
The ethyl acetate solution was dried (Na2SO4) and concentrated to
1
dryness, affording a white solid (1.41 g, 50%): mp 180 °C; H
NMR δ 2.44 (s, 3H), 7.29 (d, J ) 8.1 Hz, 2H), 8.02 (dd, J ) 8.1
Hz, J ) 4.2 Hz, 2H); 13C NMR δ 21.8, 126.1, 127.0, 129.17,
129.23, 130.2, 144.6, 172.2 (enh). Anal. Calcd for C713CH8O2: C,
70.06; H, 5.88. Found: C, 70.05; H, 5.94.
S-2-Pyridyl 4-Methylbenzo-carbonyl-13C Thioate (11). Fol-
lowing a standard procedure,31 a solution of 10 (0.750 mg, 5.47
mmol), 2,2′-dipyridyl disulfide (2.41 g, 10.9 mmol), and PPh3 (2.84
g, 10.9 mmol) in dry THF (8 mL) was stirred at room temperature
for 16 h. After removal of the solvent, the resulting yellow oil was
chromatographed [silica, ethyl acetate/hexanes (1:1)] to afford a
colorless solid (1.21 g, 96%): mp 61 °C; 1
H NMR δ 2.44 (s, 3H),
7.28-7.36 (m, 3H), 7.72-7.82 (m, 2H), 7.93 (dd, J ) 8.4 Hz, J )
4.5 Hz, 2H), 8.68-8.69 (m, 1H); 13C NMR δ 21.7, 123.5, 127.6,
127.7, 129.4, 129.5, 130.9, 133.5, 134.4, 137.1, 144.9, 150.5, 151.5,
188.9 (enh). Anal. Calcd for C1213CH11NO2S: C, 67.80; H, 4.81;
N, 6.08. Found: C, 67.57; H, 4.77; N, 6.03.
5-[4-[2-(Trimethylsilyl)ethynyl]phenyl]-10,15,20-tris(tridec-7-
yl)porphinatozinc(II)-(5-13C) (Zn-8-P). Following a standard
procedure19 with modification,18 a mixture of Zn-7-P (200 mg, 178
µmol), Pd2(dba)3 (24.4 mg, 26.6 µmol), and AsPh3 (66.2 mg, 216
µmol) in dry toluene/TEA (72 mL, 5:1) was placed in a Schlenk
10-(Dibutylboryl)-5-mesityl-1-[4-[2-(trimethylsilyl)ethynyl]-
benzoyl]dipyrromethane (Bu2B-15). Following a standard pro-
cedure,33 a solution of 12 (550 mg, 2.08 mmol) in dry THF (2.10
mL) was treated with EtMgBr (4.2 mL, 4.2 mmol, 1.0 M in THF)
at room temperature for 10 min. The reaction mixture was cooled
J. Org. Chem, Vol. 72, No. 14, 2007 5215

Thamyongkit et al.
to -78 °C and treated with a solution of 13b (650 mg, 2.09 mmol)
in THF (2.10 mL). The cooling bath was removed, and the reaction
mixture was stirred at room temperature for 30 min. Standard
workup including treatment with TEA (695 µL) and dibutylboron
triflate (4.16 mL, 4.16 mmol, 1.0 M in CH2Cl2) followed by flash
column chromatography [silica, hexanes/CH2Cl2 (3:1) f CH2Cl2]
afforded a yellow solid (771 mg, 63%): mp 55 °C (dec); 1H NMR
δ 0.29 (s, 9H), 0.56-1.26 (m, 18H), 2.16 (s, 6H), 2.27 (s, 3H),
5.89 (s, 1H), 5.90-5.94 (m, 1H), 6.19 (dd, J ) 4.5 Hz, J ) 1.8
Hz, 1H), 6.49 (d, J ) 3.3 Hz, 1H), 6.68-6.72 (m, 1H), 6.83 (s,
2H), 7.19 (d, J ) 3.3 Hz, 1H), 7.61 (dd, J ) 4.8 Hz, J ) 1.5 Hz,
2H), 7.83 (brs, 1H), 8.12 (d, J ) 6.3 Hz, 2H); 13C NMR δ -0.2,
14.0, 14.3, 20.7, 21.5, 25.9, 26.0, 27.1, 27.3, 39.9, 99.2, 103.9,
107.8, 108.7, 116.5, 116.8, 122.4, 128.6, 129.3, 129.9, 130.1, 130.4,
132.4, 134.6, 135.3, 136.7, 137.0, 152.5, 174.4; LD-MS obsd 588.1
[M+]. Anal. Calcd for C38H49BN2OSi: C, 77.53; H, 8.39; N, 4.76.
Found: C, 77.27; H, 8.39; N, 4.73.
Cl2 (228 mL) was treated with Yb(OTf)3 (453 mg, 0.730 mmol) at
room temperature for 15 min, whereupon DDQ (388 mg, 1.71
mmol) was added. After 1 h, TEA (0.50 mL) was added. The
mixture was filtered through a silica pad (CH2Cl2), and the resulting
purple fraction was collected. The resulting crude product was
dissolved in CHCl3 (100 mL) and treated overnight with a solution
of Zn(OAc)2‚2H2O (625 mg, 2.85 mmol) in methanol (25 mL) at
room temperature. The reaction mixture was washed with water,
dried (Na2SO4), concentrated, and chromatographed [silica, CH2-
Cl2/hexanes (1:2)] to afford a purple solid (68 mg, 13%): 1H NMR
(THF-d8) δ 1.84 (s, 12H), 2.53 (s, 9H), 2.61 (s, 3H), 2.68 (s, 3H),
7.29 (s, 4H), 7.54 (d, J ) 7.8 Hz, 2H), 7.71-7.73 (m, 2H), 7.97
(d, J ) 7.8 Hz, 2H), 8.04-8.11 (m, 4H), 8.17-8.19 (m, 2H), 8.64-
8.67 (m, 4H), 8.76 (d, J ) 4.5 Hz, 2H), 8.79 (d, J ) 4.5 Hz, 2H);
13C NMR (THF-d8) δ 21.7, 22.1, 119.3, 119.5, 121.0, 121.1, 121.4
(enh), 127.2, 127.9, 128.0, 128.2, 128.6, 128.7, 130.71, 130.76,
130.81, 131.1, 132.5, 132.8, 132.9, 133.0, 133.1, 135.4, 135.5,
136.5, 137.3, 137.76, 137.84, 138.17, 138.23, 140.0, 140.8, 140.9,
144.3, 144.7, 150.6, 150.7, 150.9, 151.0, 151.60, 151.64; LD-MS
obsd 775.6 [(M - I)+], 901.5 [M+]; FAB-MS obsd 901.1707, calcd
901.1701 (C5013CH41IN4Zn); λabs 423, 549, 593 nm; λem (λex ) 550
nm) 596, 644 nm.
10-(Dibutylboryl)-9-(4-iodophenyl)-5-mesityl-1-(4-methylben-
zoyl-carbonyl-13C)dipyrromethane (Bu2B-16). Following a stan-
dard procedure,34 a solution of Bu2B-14 (853 mg, 1.38 µmol) and
HMDS (288 µL, 1.38 µmol) in dry THF (2.76 mL) was treated
with EtMgBr (2.8 mL, 2.8 mmol, 1.0 M in THF) at room
temperature for 10 min. The reaction mixture was treated with a
solution of 11 (635 mg, 2.76 mmol) in THF (2.76 mL) and stirred
at room temperature for 2 h. Standard workup followed by flash
column chromatography [silica, CH2Cl2/hexanes (1:1) f CH2Cl2]
10,20-Dimesityl-15-p-tolyl-5-[4-[2-(trimethylsilyl)ethynyl]phe-
nyl]porphinatozinc(II)-(15-13C) (Zn-19-D). Following a standard
procedure,21,35 a solution of Bu2B-17 (420 mg, 0.593 mmol) in THF/
MeOH (24 mL, 10:1) was treated with NaBH4 (449 mL, 11.9 mmol)
at room temperature for 40 min. The reaction mixture was poured
into a mixture of saturated aqueous NH4Cl (30 mL) and CH2Cl2
(30 mL) and stirred for 10 min. The organic phase was separated,
washed with water, dried (Na2SO4), and concentrated. The resulting
crude product was dissolved in CH2Cl2 (238 mL), and 12 (157 mg,
0.594 mmol) was added. After a homogeneous solution was
obtained, Yb(OTf)3 (472 mg, 0.761 mmol) was added, and the
reaction mixture was stirred at room temperature for 15 min. DDQ
(405 mg, 1.78 mmol) was added, and stirring was continued for 1
h. TEA (0.50 mL) was added. The mixture was filtered through a
silica pad (CH2Cl2). The resulting purple crude material was
dissolved in CHCl3 (100 mL) and treated overnight with a solution
of Zn(OAc)2‚2H2O (651 mg, 2.97 mmol) in methanol (25 mL) at
room temperature. The reaction mixture was washed with water,
dried (Na2SO4), and chromatographed [silica, CH2Cl2/hexanes
(1:1)]. The resulting solid was suspended in methanol; the suspen-
sion was sonicated and then decanted to afford a purple solid (93
mg, 18%): 1H NMR (THF-d8) δ 0.35 (s, 9H), 1.84 (s, 12H), 2.53
(s, 3H), 2.60 (s, 3H), 2.67 (s, 3H), 7.29 (s, 4H), 7.54 (d, J ) 7.8
Hz, 2H), 7.81 (d, J ) 7.8 Hz, 2H), 8.04-8.08 (m, 2H), 8.17 (d, J
) 7.8 Hz, 2H), 8.64-8.66 (m, 4H), 8.76 (d, J ) 4.8 Hz, 2H), 8.79
(d, J ) 4.5 Hz, 2H); 13C NMR (THF-d8) δ 0.2, 21.6, 22.0, 95.1,
106.4, 110.3, 119.4, 120.8, 121.3 (enh), 121.7, 123.2, 127.8, 127.9,
128.5, 130.7, 130.9, 132.4, 132.9, 133.0, 135.27, 135.35, 137.7,
138.1, 139.9, 140.7, 141.9, 145.1, 150.47, 150.57, 150.64, 150.8,
151.5; LD-MS obsd 871.6 [M+]; FAB-MS obsd 871.3128, calcd
1
afforded a yellow solid (420 mg, 41%): mp 89-90 °C (dec); H
NMR δ 0.30-1.26 (m, 18H), 2.16-2.17 (m, 6H), 2.28 (s, 3H),
2.44 (s, 3H), 5.95-6.01 (m, 2H), 6.46 (dd, J ) 4.2 Hz, J ) 3.0
Hz, 1H), 6.82-6.84 (m, 1H), 6.86 (s, 1H), 7.20 (d, J ) 3.9 Hz,
1H), 7.24 (d, J ) 4.2 Hz, 1H), 7.29 (s, 1H), 7.56-7.61 (m, 1H),
7.65-7.70 (m, 1H), 7.79 (dd, J ) 8.1 Hz, J ) 3.9 Hz, 2H), 7.89-
7.96 (m, 2H), 8.20-8.23 (m, 1H), 9.07 (brs, 1H); 13C NMR δ 14.1,
14.2, 20.7, 21.5, 26.0, 27.0, 27.4, 40.0, 102.0, 111.0, 117.2, 119.8,
121.5, 121.9, 124.7, 128.9, 129.0, 129.1, 129.7, 130.6, 130.8, 133.4,
134.0, 135.4, 136.1, 137.0, 137.2, 137.3, 138.5, 139.0, 139.3, 142.2,
149.2, 149.9, 183.9 (enh); LD-MS obsd 613.5 [(M - I)+]; FAB-
MS obsd 738.2826, calcd 738.2809 [(M + H)+, M ) C4013CH46-
BIN2O2].
10-(Dibutylboryl)-5-mesityl-1-(4-methylbenzoyl-carbonyl-13C)-
9-[4-[2-(trimethylsilyl)ethynyl]benzoyl]dipyrromethane (Bu2B-
17). Following a standard procedure,34 a solution of Bu2B-15 (771
mg, 1.31 mmol) and HMDS (273 µL, 1.31 mmol) in dry THF (2.60
mL) was treated with EtMgBr (2.6 mL, 2.6 mmol, 1.0 M in THF)
at room temperature for 10 min. The reaction mixture was treated
with a solution of 11 (603 mg, 2.62 mmol) in THF (2.76 mL) and
stirred at room temperature for 2 h. Standard workup followed by
column chromatography [silica, CH2Cl2/hexanes (1:1) f CH2Cl2]
1
gave a yellow solid (420 mg, 45%): mp 100-101 °C (dec); H
NMR (400 MHz) δ 0.29 (s, 9H), 0.30-1.26 (m, 18H), 2.17 (s,
6H), 2.28 (s, 3H), 2.44 (s, 3H), 5.95-6.01 (m, 2H), 6.46-6.47
(m, 1H), 6.83-6.84 (m, 1H), 6.86 (s, 2H), 7.20 (d, J ) 3.3 Hz,
1H), 7.29 (s, 2H), 7.63 (d, J ) 8.0 Hz, 2H), 7.78-7.81 (m, 2H),
8.14 (d, J ) 8.0 Hz, 2H), 9.07 (brs, 1H); 13C NMR δ -0.2, 14.1,
14.2, 20.7, 21.1, 21.55, 21.58, 26.0, 27.0, 27.4, 40.0, 99.4, 103.9,
111.0, 117.2, 120.0, 121.7, 129.0, 129.4, 129.9, 132.4, 133.3, 135.5,
137.0, 137.2, 139.2, 142.2, 149.2, 149.6, 175.2, 183.9 (enh); LD-
MS obsd 597.5 [(M - 2(n-butyl))+], 653.5 [(M - n-butyl)+], 708.1
[M+]; FAB-MS obsd 708.4280, calcd 708.4238 [(M + H)+, M )
C4513CH55BN2O2Si].
871.3130 (C5513CH50N4SiZn); λabs 425, 552, 593 nm; λem (λex
550 nm) 602, 648 nm.
)
5-(4-Ethynylphenyl)-10,20-dimesityl-15-p-tolylporphinatozinc-
(II)-(15-13C) (Zn-20-D). A solution of Zn-19-D (50.0 mg, 57.2
µmol) in CHCl3/THF (6.4 mL, 3:1) was treated with TBAF (86.0
µL, 86.0 µmol) at room temperature for 1 h. The reaction mixture
was washed with 10% aqueous NaHCO3 and water, dried (Na2-
SO4), and concentrated to dryness. Purification by column chro-
matography [silica, CH2Cl2/hexanes (1:1)] afforded a purple solid
(45 mg, 98%): 1H NMR (THF-d8) δ 1.84 (s, 12H), 2.53 (s, 3H),
2.60 (s, 3H), 2.64 (s, 3H), 3.78 (s, 1H), 7.29 (s, 4H), 7.54 (d, J )
7.8 Hz, 2H), 7.84 (d, J ) 8.1 Hz, 2H), 8.06 (dd, J ) 8.1 Hz, J )
3.6 Hz, 2H), 8.18 (d, J ) 7.8 Hz, 2H), 8.65 (t, J ) 4.8 Hz, 4H),
8.78 (dd, J ) 8.1 Hz, J ) 4.5 Hz, 4H); 13C NMR (THF-d8) δ 21.6,
22.0, 30.7, 79.7, 84.5, 119.4, 119.6, 120.8, 121.3 (enh), 121.7, 122.6,
127.8, 127.9, 128.5, 130.66, 130.70, 130.8, 130.9, 132.4, 132.9,
133.0, 135.3, 135.4, 137.7, 138.1, 139.9, 140.7, 141.2, 145.1,
15-(4-Iodophenyl)-10,20-dimesityl-5-p-tolylporphinatozinc(II)-
(5-13C) (Zn-18-D). Following a standard procedure,21,35 a solution
of Bu2B-16 (420 mg, 0.570 mmol) in THF/MeOH (23 mL, 10:1)
was treated with NaBH4 (431 mg, 11.4 mmol) at room temperature
for 40 min. The reaction mixture was poured into a mixture of
saturated aqueous NH4Cl (50 mL) and CH2Cl2 (50 mL), and then
stirred for 10 min. The organic phase was separated, washed with
water, dried (Na2SO4), and concentrated to dryness. A solution of
the resulting foam-like solid and 12 (150 mg, 0.568 mmol) in CH2-
5216 J. Org. Chem., Vol. 72, No. 14, 2007

Meso-13C-Labeled Porphyrins
150.49, 150.58, 150.66, 150.74, 151.5; LD-MS obsd 799.6 [M+];
FAB-MS obsd 799.2750, calcd 799.2734 (C5213CH42N4Zn); λabs 424,
550, 593 nm; λem (λex ) 550 nm) 601, 648 nm.
119.2, 119.3, 119.4, 122.4, 127.6, 129.6, 130.5, 130.8, 130.9, 131.9,
134.1, 136.9, 138.5, 138.6, 142.3, 142.7, 143.1, 148.9, 149.3, 149.4,
149.8; LD-MS obsd 1629.0 [M+], calcd avg mass 1631.7 (C107
-
Dyad-1-P. Following a standard procedure,25 a mixture of Zn-
6-P (26.7 mg, 32.2 µmol), Zn-21 (30.0 mg, 32.2 µmol), Pd2(dba)3
(4.5 mg, 4.9 µmol), and P(o-tol)3 (11.8 mg, 38.8 µmol) in dry
toluene/TEA (13 mL, 5:1) was reacted in a Schlenk flask at 35 °C.
After 12 h, identical portions of catalyst and ligand were added,
and the reaction mixture was stirred for 10 h. The product was
purified by a three-column chromatography sequence, without
complete evaporation of solvent from intermediate fractions. Thus,
after removal of about one-half of the solvent, the resulting reaction
mixture was passed through a silica column [CH2Cl2/toluene (1:
1)]. The first purple band was collected and concentrated to obtain
a clear solution of minimal volume (1-2 mL). Preparative SEC
(THF) gave the title compound as the second band. After the
removal of one-half of the solvent, the resulting purple solution
was purified by column chromatography [silica, CH2Cl2/hexanes
(4:1)]. The resulting product was suspended in methanol; the
suspension was sonicated and then decanted to afford a purple solid
(34 mg, 80%): 1H NMR (CS2) δ 1.92 (s, 36H), 2.69 (m, 18H),
7.28 (m, 12H), 8.06 (d, J ) 7.8 Hz, 4H), 8.31 (d, J ) 8.1 Hz, 4H),
8.73 (s, 8H), 8.80 (d, J ) 4.5 Hz, 4H), 8.96 (d, J ) 4.8 Hz, 4H);
13C NMR (CS2) δ 21.5, 22.7, 91.1, 118.5, 118.6, 119.0 (enh), 119.1,
13CH90N8Zn2); λabs 426, 550, 589 nm; λem (λex ) 550 nm) 596,
646 nm.
Acknowledgment. This research was supported by grants
from the Division of Chemical Sciences, Office of Basic Energy
Sciences, Office of Energy Research, U.S. Department of Energy
to D.H. (DE-FG02-05ER15661), D.F.B. (DE-FG02-05ER15660),
and J.S.L. (DE-FG02-96ER14632). Mass spectra were obtained
at the Mass Spectrometry Laboratory for Biotechnology at North
Carolina State University. Partial funding for the NCSU Facility
was obtained from the North Carolina Biotechnology Center
and the NSF.
Supporting Information Available: Experimental procedures
for the synthesis of four dyads; 1H NMR spectra for selected new
compounds; and LD-MS and SEC data for all new porphyrin dyads.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO070593X
J. Org. Chem, Vol. 72, No. 14, 2007 5217