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

Article abstract of DOI:10.1021/jo8012836

(Figure Presented) Insight into the electronic communication between the individual constituents of multicomponent molecular architectures is essential for the rational design of molecular electronic and/or photonic devices. To clock the ground-state hole/electron-transfer process in oxidized multiporphyrin architectures, a p-diphenylethyne-linked zinc porphyrin dyad was prepared wherein one porphyrin bears two ¹³C atoms and the other porphyrin is unlabeled. The ¹³C atoms are located at the 1- and 9-positions (α-carbons symmetrically disposed to the position of linker attachment), which are sites of electron/spin density in the a_1u HOMO of the porphyrin. The ¹³C labels were introduced by reaction of KS ¹³CN with allyl bromide to give the allyl isothiocyanate, which upon Trofimov pyrrole synthesis followed by methylation gave 2-(methyl-thio)pyrrole- 2-¹³C. Reaction of the latter with paraformaldehyde followed by hydrodesulfurization gave dipyrromethane-1,9-¹³C, which upon condensation with a dipyrromethane-1,9-dicarbinol bearing three pentafluorophenyl groups gave the tris(pentafluorophenyl)porphyrin bearing ₁₃C labels at the 1,9-positions and an unsubstituted meso (5-) position. Zinc insertion, bromination at the 5-position, and Suzuki coupling with an unlabeled porphyrin bearing a suitably functionalized diphenylethyne linker gave the regiospecifically labeled zinc porphyrin dyad. Examination of the monocation of the isotopically labeled dyad via electron paramagnetic resonance (EPR) spectroscopy (and comparison with the monocations of benchmark monomers, where hole transfer cannot occur) showed that the hole transfer between porphyrin constituents of the dyad is slow (<10⁶ s ^-1) on the EPR time scale at room temperature. The slow rate stems from the a_1u HOMO of the electron-deficient porphyrins, which has a node at the site of linker connection. In contrast, analogous dyads of electron-rich porphyrins (wherein the HOMO is a_2u and has a lobe at the site of linker connection) studied previously exhibit rates of hole transfer that are fast (>5 × 10⁷ s^-1) on the EPR time scale at room temperature.

Full text of DOI:10.1021/jo8012836

VOLUME 73, NUMBER 18
September 19, 2008
Copyright 2008 by the American Chemical Society
Regiospecifically r-13C-Labeled Porphyrins for Studies of
Ground-State Hole Transfer in Multiporphyrin Arrays
Ana Z. Muresan,† Patchanita Thamyongkit,† James R. Diers,‡ Dewey Holten,*,§
Jonathan S. Lindsey,*,† and David F. Bocian*,‡
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@wustl.edu
ReceiVed June 15, 2008
Insight into the electronic communication between the individual constituents of multicomponent molecular
architectures is essential for the rational design of molecular electronic and/or photonic devices. To clock
the ground-state hole/electron-transfer process in oxidized multiporphyrin architectures, a p-diphenylethyne-
linked zinc porphyrin dyad was prepared wherein one porphyrin bears two 13C atoms and the other
porphyrin is unlabeled. The 13C atoms are located at the 1- and 9-positions (R-carbons symmetrically
disposed to the position of linker attachment), which are sites of electron/spin density in the a1u HOMO
of the porphyrin. The 13C labels were introduced by reaction of KS13CN with allyl bromide to give the
allyl isothiocyanate, which upon Trofimov pyrrole synthesis followed by methylation gave 2-(methyl-
thio)pyrrole-2-13C. Reaction of the latter with paraformaldehyde followed by hydrodesulfurization gave
dipyrromethane-1,9-13C, which upon condensation with a dipyrromethane-1,9-dicarbinol bearing three
pentafluorophenyl groups gave the tris(pentafluorophenyl)porphyrin bearing 13C labels at the 1,9-positions
and an unsubstituted meso (5-) position. Zinc insertion, bromination at the 5-position, and Suzuki coupling
with an unlabeled porphyrin bearing a suitably functionalized diphenylethyne linker gave the regiospe-
cifically labeled zinc porphyrin dyad. Examination of the monocation of the isotopically labeled dyad
via electron paramagnetic resonance (EPR) spectroscopy (and comparison with the monocations of
benchmark monomers, where hole transfer cannot occur) showed that the hole transfer between porphyrin
constituents of the dyad is slow (<106 s-1) on the EPR time scale at room temperature. The slow rate
stems from the a1u HOMO of the electron-deficient porphyrins, which has a node at the site of linker
connection. In contrast, analogous dyads of electron-rich porphyrins (wherein the HOMO is a2u and has
a lobe at the site of linker connection) studied previously exhibit rates of hole transfer that are fast (>5
× 107 s-1) on the EPR time scale at room temperature.
Introduction
understanding of the electronic interactions between the con-
stituents.1 For systems that undergo electrochemical oxidation
as part of their functional properties, electronic communication
can be probed by assessing the mobility of the holes formed
The rational design of molecular photonic and electronic
devices composed of multiple constituents requires a deep
† 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.
10.1021/jo8012836 CCC: $40.75
Published on Web 08/22/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 6947–6959 6947

Muresan et al.
CHART 1
upon oxidation. Synthetic materials that undergo electrochemical
oxidation at low potential and support the migration of the
resulting holes can underpin a variety of novel molecular
devices. One example is information storage with molecular
materials, wherein information is stored in the discrete redox
states of the molecules.2 Another is efficient solar-energy
conversion, which requires that holes generated after excited-
state electron injection can move efficiently away from the
anode, thereby preventing charge recombination.3
In synthetic light-harvesting antennas, studies of hole transfer
have provided a basis for examining the extent of ground-state
electronic communication among constituent pigments.4 The studies
of synthetic materials also provide a backdrop for analogous studies
of natural systems. For example, chemical oxidation of natural
photosynthetic light-harvesting proteins has yielded a single hole
in the array of 32 bacteriochlorophyll molecules in the protein.5-7
This hole then migrates among the large number of pigments.
Magnetic resonance probing of the kinetics of hole migration yields
information about the interactions among the pigments and how
such interactions are influenced by changes in the matrix (liquid
versus solid). Accordingly, a detailed understanding of hole
migration may have important implications for the understanding
of natural photosynthetic systems as well as artificial solar cells.
Thus, understanding hole mobility in prototypical light-harvesting,
charge-separation, and charge-storage systems is of fundamental
interest.
To gain a deep understanding of hole mobility in molecular
architectures, the ground-state hole-transfer characteristics of
the monocations of several different types of tetrapyrrolic dyads
have been investigated using EPR spectroscopy. The structures
of the arrays are shown in Chart 1. All of the dyads are zinc
chelates linked by a diphenylethyne group via the meso-positions
of the macrocycle; the porphyrins differ only in the nature of
the nonlinking meso substituents. One dyad contains mesityl
FIGURE 1. Metalloporphyrin a2u and a1u orbitals.
substituents (Zn2D-Mes),8 one contains symmetrically branched
tridecyl (“swallowtail”) substituents (Zn2D-SWT),9 and the
other contains pentafluorophenyl substituents (Zn2D-F30)10 at
these positions.
In the case of the monocations of the dyads with mesityl and
swallowtail substituents, [Zn2D-Mes]+ and [Zn2D-SWT]+, the
hole resides in an a2u-type HOMO and the ground electronic
9,11
2
state is A2u.
In this ground state, the electron (and spin)
density resides primarily at the meso carbons and the pyrrole
nitrogens (Figure 1).12 Previous EPR studies of [Zn2D-Mes]+
and [Zn2D-SWT]+ have shown that the hole transfer between
the two porphyrins is fast (>107 s-1) on the time scale of the
EPR experiment. The EPR time scale is determined by the
nuclear hyperfine coupling between the unpaired electron and
the 14N nuclei of the pyrrole rings; this coupling is ∼4.2 MHz.11
The fast hole-transfer rates in the monocations of the dyads with
2A2u ground states are attributed to the fact that the linker that
joins the two porphyrins is at a site of large electron/spin density.
In the case of the monocation of the dyad with pentafluo-
rophenyl substituents, [Zn2D-F30]+, the hole resides in an a1u-
10
2
type HOMO and the ground electronic state is A1u. In this
(2) Roth, K. M.; Dontha, N.; Dabke, R. B.; Gryko, D. T.; Clausen, C.;
Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. J. Vac. Sci. Technol. B. 2000, 18,
2359–2364.
(3) 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.
(4) del Rosario Benites, M.; Johnson, T. E.; Weghorn, S.; Yu, L.; Rao, P. D.;
Diers, J. R.; Yang, S. I.; Kirmaier, C.; Bocian, D. F.; Holten, D.; Lindsey, J. S.
J. Mater. Chem. 2002, 12, 65–80.
ground state, the electron (and spin) density resides primarily
(8) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1996,
118, 11166–11180.
(9) 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.
(5) Srivatsan, N.; Weber, S.; Kolbasov, D.; Norris, J. R. J. Phys. Chem. B
2003, 107, 2127–2138.
(6) Kolbasov, D.; Srivatsan, N.; Ponomarenko, N.; Ja¨ger, M.; Norris, J. R.,
Jr. J. Phys. Chem. B 2003, 107, 2386–2393.
(7) Srivatsan, N.; Kolbasov, D.; Ponomarenko, N.; Weber, S.; Ostafin, A. E.;
Norris, J. R., Jr. J. Phys. Chem. B 2003, 107, 7867–7876.
(10) Strachan, J.-P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191–11201.
(11) Seth, J.; Palaniappan, V.; Wagner, R. W.; Johnson, T. E.; Lindsey, J. S.;
Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 11194–11207.
(12) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1979; Vol. 4, pp 197-256.
6948 J. Org. Chem. Vol. 73, No. 18, 2008

Ground-State Hole Transfer in Multiporphyrin Arrays
CHART 2
at the R- and ꢀ-positions of the pyrrole rings; negligible spin
density resides at the pyrrole nitrogen atoms (Figure 1).12 The
absence of 14N nuclear hyperfine coupling in [Zn2D-F30]+
renders these nuclei unsuitable as a hyperfine clock for assessing
the hole-transfer characteristics. For this reason, our prior studies
of hole transfer in [Zn2D-F30]+ relied on the assessment of the
hyperfine coupling to the 1H nuclei at the ꢀ-pyrrole positions.10
These studies suggested that hole transfer in [Zn2D-F30]+ was
slow (<106 s-1) on the EPR time scale; however, this conclusion
was tentative because the 1H hyperfine couplings are not
resolved and appear to be relatively small.
One approach for utilizing the EPR spectral features for
assessing the hole-transfer characteristics in [Zn2D-F30]+ and
other types of dyads with 1A1u ground states is to incorporate a
suitable isotopic label at a site of large electron/spin density. In
this regard, we have previously used meso-13C labeling to refine
the time scale of hole transfer in monocations of porphyrin dyads
with 2A2u ground states.13 In these dyads, the hyperfine coupling
between the unpaired electron and the meso-13C nuclei is ∼16
MHz, which provides a hyperfine clock that is approximately
4 times faster than that of the 14N nuclei. The various meso-
13C-labeled dyads that have been previously prepared are shown
in Chart 2. In one set of dyads, one porphyrin incorporates a
13C atom (*) at the meso-position to which the diphenylethyne
linker is attached (Zn2D-Mes(5-13C), Zn2D-SWT(5-13C)). In
a second set of porphyrins, each porphyrin incorporates a 13C
atom (*) at the meso-position to which the diphenylethyne linker
is attached (Zn2D-Mes(5,5′-13C), Zn2D-SWT(5,5′-13C)). In a
third type of dyad (Zn2D-Mes/T(15,15′-13C)), each porphyrin
incorporates a 13C atom (*) at the meso-position distal to the
site of linker attachment. The EPR studies of the monocations
of the five isotopically labeled dyads shown in Chart 2 revealed
that hole-transfer is fast (>4 × 107 s-1) on the time scale of
the meso-13C hyperfine clock.13
In this paper, we implement the 13C-labeling approach in a
new synthetic design strategy that affords a definitive assessment
2
of the hole-transfer characteristics of the A1u monocation
[Zn2D-F30]+. In the isotopically labeled dyad, 13C atoms are
incorporated at specific R-pyrrole positions, which are the sites
of highest electron/spin density (Figure 1). The two dyads that
have been synthesized are shown in Chart 3. The two dyads
differ only in the presence of natural abundance H atoms at all
nonsubstituted sites in the porphyrins and linker of Zn2D-
F30(1,9-13C) versus per deuteriation of all such sites in Zn2D-
F30(1,9-13C)d24. The preparation of the dyads is accompanied
by a series of porphyrin monomers that incorporate R-pyrrole-
13C, ꢀ-pyrrole-2H, meso-carbon-2H, and/or a combination of
the various types of isotopic labels. The examination of the
deuteriated molecules allows a definitive assessment of the
magnitude of 1H hyperfine coupling and whether this coupling
is in fact reliable as a hyperfine clock for assessing the hole-
transfer characteristics in dyads such as [Zn2D-F30]+, which
2
exhibit A1u ground states.
Gaining access to regiospecifically R-13C-labeled porphyrins
and dyads required considerable synthetic development. While
(14) (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.
(13) Thamyongkit, P.; Muresan, A. Z.; Diers, J. R.; Holten, D.; Bocian, D. F.;
Lindsey, J. S. J. Org. Chem. 2007, 72, 5207–5217.
J. Org. Chem. Vol. 73, No. 18, 2008 6949

Muresan et al.
CHART 3
CHART 4
a number of 13C-containing isotopologues of naturally occur-
ring porphyrins have been prepared biosynthetically,14 most
synthetic porphyrins that bear 13C labels contain the label at
the meso-positions.13 The dearth of R- (or ꢀ-) 13C-labeled
synthetic porphyrins15 stems from the requirement to incorporate
the 13C label very early in a lengthy synthesis, namely in
precursors to the pyrrole, whereas meso-13C labels can be
incorporated via readily accessible 13C-labeled aldehyde or acyl
units that are reacted with pyrrole (or pyrrole-containing units
such as dipyrromethanes) in a late step of the porphyrin
synthesis. Indeed, to our knowledge, no prior syntheses have
placed 13C labels at two distinct R-positions of a tetrapyrrole
macrocycle. Collectively, the synthetic strategy described herein
and the EPR studies of [Zn2D-F30(1,9-13C)]+ and analogues
provide new insight into the hole-transfer characteristics of the
2A1u monocations.
differ only in the presence or absence of 13C labels in the
macrocycle. The preparation of such architectures requires a
directed rather than statistical approach for coupling the two
porphyrins. A synthetic strategy that has been widely used to
prepare analogous compounds (e.g., wherein the two porphyrins
differ only in the metalation state) entails the use of a
Sonogashira coupling of two porphyrin building blocks (Chart
4).8,16 In the Sonogashira approach, an iodophenylporphyrin and
an ethynylphenylporphyrin are joined to create the diphenyl-
ethyne linker. The synthesis of the two A3B-porphyrin building
blocks was envisaged via an established “2 + 2” dipyrrometh-
ane + dipyrromethanedicarbinol approach,17,18 wherein the
isotopic labels in one of the porphyrins would be introduced
via the dipyrromethane moiety. For the labeled dipyrromethane,
we planned to take advantage of a recent stoichiometric synthesis
of R,R′-bis(alkylthio)dipyrromethanes,19 which upon use of a
13C-labeled R-(alkylthio)pyrrole precursor would afford the
requisite regiocontrol over the isotopic label. The desired 13C-
labeled dipyrromethane bearing a meso-(4-iodophenyl) group
and R,R′-bis(methylthio) groups was prepared, but attempts to
Results
I. Synthesis. 1. Strategy. Each target architecture (Zn2D-
F30(1,9-13C), Zn2D-F30(1,9-13C)d24) shown in Chart 3 is unsym-
metrical owing to the presence of distinct porphyrins attached
to the diphenylethyne linker. The two porphyrins in each dyad
(16) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem.
1995, 60, 5266–5273.
(17) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org. Chem.
2000, 65, 7323–7344.
(18) Zaidi, S. H. H.; Fico, R. M., Jr.; Lindsey, J. S. Org. Process Res. DeV.
2006, 10, 118–134.
(19) Thamyongkit, P.; Bhise, A. D.; Taniguchi, M.; Lindsey, J. S. J. Org.
Chem. 2006, 71, 903–910.
(15) (a) Mispelter, J.; Momenteau, M.; Lhoste, J.-M. J. Chem. Soc., Dalton
Trans. 1981, 1729–1734. (b) Hu, S.; Mukherjee, A.; Piffat, C.; Mak, R. S. W.;
Li, X.-Y.; Spiro, T. G. Biospectroscopy 1995, 1, 395–412. (c) Calle, C.;
Schweiger, A.; Mitrikas, G. Inorg. Chem. 2007, 46, 1847–1855.
6950 J. Org. Chem. Vol. 73, No. 18, 2008

Ground-State Hole Transfer in Multiporphyrin Arrays
SCHEME 1
remove the methylthio groups resulted in concomitant deiodi-
nation (vide infra). Hence, the Sonogashira route was abandoned.
A second strategy was pursued wherein a Suzuki coupling
reaction is employed to join the labeled porphyrin and the
unlabeled porphyrin bearing a diphenylethyne substituent. While
more laborious, this route circumvented the limitations associ-
ated with removal of the methylthio groups that were encoun-
tered in the Sonogashira route. Suzuki couplings have been used
to prepare a variety of porphyrin dyads, although heretofore
most such dyads contained phenylene linkers rather than
diphenylethyne linkers.20 Other strategies for preparing por-
phyrin dyads have been investigated.21
Initially, we envisaged using a building block approach to
prepare an analogue of Zn2D-F30(1,9-13C) wherein all ꢀ-pyrrole
protons were replaced with deuterium. The preparation of
deuteriated building blocks with retention of the deuterium
during the various synthetic procedures proved difficult. Ulti-
mately, we found that dyad Zn2D-F30(1,9-13C) could be
converted to dyad Zn2D-F30(1,9-13C)d24 by global deuteriation
wherein not only the ꢀ-pyrrole protons but also the protons of
the diphenylethyne linker were replaced with deuterium. The
presence of deuterium on the diphenylethyne linker was not
deleterious to the processes under investigation.
2. Labeled Porphyrin Monomers. The synthesis of a
dipyrromethane bearing 13C labels at the 1- and 9-positions
cannot rely on the traditional one-flask condensation22 of an
aldehyde with excess pyrrole. Such a reaction with an R-13C-
labeled pyrrole would lead to a mixture of products wherein
the labels would be located at the 1,9-, 1,6-, and 4,6-positions.
Moreover, the one-flask condensation requires a large excess
of pyrrole (20-100 molar equiv) to suppress the self-oligo-
merization and polymerization of the pyrrole-carbinol inter-
mediate, which for a 13C-containing pyrrole would be prohibi-
tively expensive. The recent synthesis19 of dipyrromethanes by
condensation of stoichiometric quantities of R-(methylthio)-
pyrrole and an aldehyde followed by removal of the R-meth-
ylthio groups appeared as a reasonable solution. The precursor
to the R-(methylthio)pyrrole is an isotopologue of allyl isothio-
cyanate (mustard oil),23 which can be prepared from allyl
bromide and potassium thiocyanate24 (Scheme 1).
lar cyclization. Subsequent treatment with water protonated
selectively the nitrogen, and alkylation with methyl iodide
afforded the isotopically labeled 2-(methylthio)pyrrole 2 in 56%
yield.
With the isotopically labeled 2-(methylthio)pyrrole 2 in hand,
the synthesis of 1,9-bis(methylthio)dipyrromethanes bearing 13
C
labels at the R-positions is thus possible. The condensation of
2 and 4-iodobenzaldehyde was carried out with a catalytic
amount of InCl3 at room temperature in the absence of any
solvent. The reaction was monitored by TLC, and after 52 h,
dipyrromethane 3a was obtained in 30% yield. A similar
reaction with paraformaldehyde afforded dipyrromethane 3b,
which was used in crude form in the subsequent reaction. The
presence of the 13C atom at the R-position of the methyl-
thiopyrrole 2 and the 1,9-bis(methylthio)dipyrromethanes 3a,b
Treatment of allyl bromide with KS13CN at 55 °C gave the
allyl thiocyanate, which isomerized to allyl isothiocyanate 1.
The isomerization of alkyl thiocyanates to alkyl isothiocyanates
has been known since 1875. The reaction is lengthy (3 days);
nevertheless, the product formed in high yield (86%) in a one-
flask process, and no side products were observed except a trace
1
of allyl thiocyanate (1% according to H NMR and GC-MS
1
was clearly displayed by H NMR spectroscopy, where the
analyses). Given the volatile nature of 1, the crude product (99%
protons of the methyl group resonated as a doublet (J ) 4.8
1
purity as determined by H NMR and GC-MS analysis) was
Hz) owing to coupling with the 13C label.
carried forward to the next step. Following Trofimov pyrrole
synthesis,26 isothiocyanate 1 underwent deprotonation with
KN(i-Pr)2/t-BuOLi (generated in situ) followed by intramolecu-
The methylthio protecting groups of dipyrromethanes 3a,b
were cleaved by hydrodesulfurization using excess Raney Ni
in a slurry at ambient conditions.19 The desulfurization reaction
requires use of at least a stoichiometric quantity of Raney Ni.19,27
With iodophenyldipyrromethane 3a, the reaction gave dipyr-
romethane 4a wherein the two methylthio groups and the iodo
(20) Yu, L.; Lindsey, J. S. Tetrahedron 2001, 57, 9285–9298.
(21) Thamyongkit, P.; Yu, L.; Padmaja, K.; Jiao, J.; Bocian, D. F.; Lindsey,
J. S. J. Org. Chem. 2006, 71, 1156–1171.
(22) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.; Lindsey,
J. S. Org. Process Res. DeV. 2003, 7, 799–812.
(23) Remaud, G. S.; Martin, Y.-L.; Martin, G. G.; Naulet, N.; Martin, G. J.
J. Agric. Food Chem. 1997, 45, 1844–1848.
(24) Guy, R. G. In The Chemistry of Cyanates and Their Thio DeriVatiVes;
Patai, S., Ed.; John Wiley & Sons, Inc.: New York, 1977; Part 2, pp 819-
886.
(25) (a) Gerlich, G. Justus Liebigs Ann. Chem. 1875, 178, 80–91. (b) Smith,
P. A. S.; Emerson, D. W. J. Am. Chem. Soc. 1960, 82, 3076–3082. (c) Gurudutt,
K. N.; Rao, S.; Srinivas, P. Indian J. Chem. 1991, 30B, 343–344.
(26) (a) Nedolya, N. A.; Brandsma, L.; Verkruijssse, H. D.; Trofimov, B. A.
Tetrahedron Lett. 1997, 38, 7247–7248. (b) Nedolya, N. A.; Brandsma, L.;
Trofimov, B. A. Russ. J. Org. Chem. 1998, 34, 900–901. (c) Trofimov, B. A.
J. Heterocycl. Chem. 1999, 36, 1469–1490. (d) Klyba, L. V.; Bochkarev, V. N.;
Brandsma, L.; Nedolya, N. A.; Trofimov, B. A. Russ. J. Gen. Chem. 1999, 69,
1805–1809.
(27) Bru¨ckner, C.; Posakony, J. J.; Johnson, C. K.; Boyle, R. W.; James,
B. R.; Dolphin, D. J. Porphyrins Phthalocyanines 1998, 2, 455–465.
J. Org. Chem. Vol. 73, No. 18, 2008 6951

Muresan et al.
SCHEME 2
SCHEME 3
Yb(OTf)3 in CH2Cl2 gave the corresponding porphyrin P1-
F15(1,9-13C) (20%) or P2-F15(1,9-13C) (crude form). Subsequent
metalation with zinc acetate gave the corresponding zinc
porphyrin ZnP1-F15(1,9-13C) (87% yield) or ZnP2-F15(1,9-13C)
(18% overall yield). Bromination31 of porphyrin ZnP2-F15(1,9-
13C) with NBS at 0 °C proceeded regioselectively (as evidenced
by the disappearance of the singlet for the meso-H, δ ) 10.43
ppm) to give the meso-bromoporphyrin ZnP3-F15(1,9-13C) in
85% yield. Each porphyrin shown in Scheme 2 contains 13C
atoms at the 1- and 9-positions of the macrocycle.
group had been cleaved as shown by 1H NMR and mass
spectrometric analyses. The loss of the iodo group precluded
use of the Sonogashira route outlined in Chart 4; however, the
phenyldipyrromethane (4a) so obtained provided the precursor
for an additional benchmark porphyrin for dyad studies. On the
other hand, hydrodesulfurization of the more robust 3b gave
dipyrromethane 4b in 29% overall yield (from pyrrole 2). For
3. Unlabeled Porphyrin Monomer. For the Suzuki coupling
strategy, the unlabeled porphyrin building block must contain
a boronate-derivatized diphenylethyne linker. The corresponding
dipyrromethane was prepared as shown in Scheme 3. The known
ethynylphenyldipyrromethane 617 was coupled with 1-bromo-
4-iodobenzene under Sonogashira conditions [THF/triethylamine
(TEA) (1:1) in the presence of Pd2(dba)3, AsPh3 and CuI] to
afford dipyrromethane 7 in 86% yield (Scheme 3). A comple-
mentary route to dipyrromethane 7 employed Pd-mediated
coupling of 4-ethynylbenzaldehyde (8)32 with 1-bromo-4-
iodobenzene and PdCl2(PPh3)2, TEA, and CuI in benzene at
room temperature to obtain aldehyde 9 in 82% yield. Aldehyde
9 is known33 but with limited characterization data. Condensa-
1
both labeled dipyrromethanes (4a, 4b), the H resonance from
the R-protons was characteristic: in 4a a doublet of multiplets
was observed at 6.66 ppm (1H-13C J-coupling of 184.8 Hz),
and likewise, for 4b the signal observed was 6.69 ppm Hz
(1H-13C J-coupling of 184.4 Hz). The appearance of this
resonance as a doublet of multiplets is due to the 13C incorpora-
tion at the R-loci. Additionally, gHETCOR and gHSQC
experiments indicated the correlation of the two signals corre-
sponding to the R-protons with the enhanced signal of the 13C
atom. The values of the chemical shift and coupling constants
for the dipyrromethanes agree with expectations on the basis
of comparison with the analogous 2-methylpyrrole.28
22
tion of aldehyde 9 with excess pyrrole in the presence of InCl3
Porphyrins bearing 13C labels were prepared by condensation
of a 13C-labeled dipyrromethane (4a, 4b) and a dipyrromethane-
dicarbinol (Scheme 2). The dibutyltin complex of the tris(pen-
tafluorophenyl)-substituted 1,9-diacyldipyrromethane (5)29 was
reduced with NaBH4 to give the dipyrromethane-dicarbinol,
which upon condensation18,30 with 4a or 4b in the presence of
afforded dipyrromethane 7 in 58% yield.
The required A3B-porphyrin building block was prepared as
shown in Scheme 4. Thus, reduction of 5 gave the corresponding
dipyrromethane-dicarbinol, which upon condensation18,30 with
dipyrromethane 7 gave the free base porphyrin bearing the
1-bromo-p-diphenylethyne moiety. Subsequent zinc insertion
(28) Chadwick, D. J. In Pyrroles. Part One. The Synthesis and the Physical
and Chemical Aspects of the Pyrrole Ring; Jones, R. A., Ed.; John Wiley &
Sons, Inc.: New York, 1990; pp 1-103.
(31) Tomizaki, K.-Y.; Lysenko, A. B.; Taniguchi, M.; Lindsey, J. S.
Tetrahedron 2004, 60, 2011–2023.
(32) Austin, W. B.; Bilow, N.; Kelleghan, W. J.; Lau, K. S. Y. J. Org. Chem.
1981, 46, 2280–2286.
(33) Terazono, Y.; Liddell, P. A.; Garg, V.; Kodis, G.; Brune, A.; Ham-
bourger, M.; Moore, A. L.; Moore, T. A.; Gust, D. J. Porphyrins Phthalocyanines
2005, 9, 706–723.
(29) Tamaru, S.-I.; Yu, L.; Youngblood, W. J.; Muthukumaran, K.; Taniguchi,
M.; Lindsey, J. S. J. Org. Chem. 2004, 69, 765–777.
(30) Geier, G. R., III; Callinan, J. B.; Rao, P. D.; Lindsey, J. S. J. Porphyrins
Phthalocyanines 2001, 5, 810–823.
6952 J. Org. Chem. Vol. 73, No. 18, 2008

Ground-State Hole Transfer in Multiporphyrin Arrays
SCHEME 4
ꢀ-deuteriated porphyrin building blocks (e.g., analogues of
ZnP3-F15(1,9-13C) and ZnP5-F15) resulted in loss of significant
amounts of the deuterium labels. The failure of the building-
block approach eventually prompted examination of direct
deuteriation of the dyad Zn2D-F30(1,9-13C). Treatment of Zn2D-
F30(1,9-13C) with TFA-d, TFA-d/D2O, or acetic acid-d4 resulted
only in demetalation of the zinc porphyrins, whereas D2SO4 at
150 °C gave higher molecular weight side products. To our
delight, treatment of Zn2D-F30(1,9-13C) with D2SO4 at room
temperature for 7 h afforded the exhaustively deuteriated dyad,
wherein the 16 protons at the pyrrole positions and the 8 protons
of the diphenylethyne linker underwent exchange. Exchange of
porphyrin meso-aryl protons under such conditions has been
reported.36 Remetalation with zinc acetate afforded Zn2D-
F30(1,9-13C)d24 in 80% overall yield and 95% isotopic purity.
The extensive level of deuterium incorporation and the absence
of unexchanged protons precluded characterization by 1H NMR
spectroscopy. Characterization was achieved by absorption
spectroscopy, fluorescence spectroscopy, laser-desorption mass
spectrometry in the absence of a matrix (LD-MS),37 and 13C
NMR spectroscopy. Substitution of H atoms with D atoms is
known to affect the 13C NMR spectra.38 The 13C signal of Zn2D-
F30(1,9-13C)d24 appears as a multiplet at 150.1-150.2 ppm. This
route to per deuteriated dyads may be useful because the
resulting array contains only two nuclei with spin 1/2, which is
anticipated to simplify the EPR spectra, although the amounts
of material obtained in the present case were quite limited.
5. Porphyrin Benchmarks. EPR studies required the syn-
thesis of porphyrin monomers for assessing the magnitude of
the contribution of 1H hyperfine interactions and for comparison
with the properties of the two dyads. One benchmark was
perdeuterio-meso-tetrakis(pentafluorophenyl)porphyrin, which
could be prepared by condensation of pentafluorobenzaldehyde
SCHEME 5
39
and either pyrrole-d5 or pyrrole itself (with in situ deuteria-
tion).36 The time-honored method for meso-deuteriation of
octaethylporphyrin entails treatment with D2SO4 at room
temperature.40 Treatment of meso-tetrakis(pentafluorophe-
nyl)porphyrin (P6-F20) with D2SO4 at 150 °C for 22 h resulted
in deuterium exchange of the pyrrole hydrogens; aqueous
workup afforded the desired P6-F20d8 (Scheme 6). A strong
signal at -2.93 ppm and a weak signal at 8.92 ppm were
gave porphyrin ZnP4-F15 in 22% yield. Finally, borylation34
of ZnP4-F15 by treatment with bis(pinacolato)diboron,
PdCl2(PPh3)2 and TEA at 75 °C for 40 h afforded ZnP5-F15 in
60% yield.
1
observed in the H NMR spectrum, and the LD-MS spectrum
4. Porphyrin Dyads. The isotopically labeled porphyrin
ZnP3-F15(1,9-13C) and unlabeled porphyrin ZnP5-F15 were
reacted under Suzuki conditions,20 which consist of Pd(PPh3)4
and K2CO3 at 90 °C for 27 h (Scheme 5). Analytical-scale size-
exclusion chromatography (SEC)16 was employed to monitor
the reaction and to assess the purity of the dyad. Purification
by a series of three columns (adsorption chromatography,
preparative SEC, and adsorption chromatography) afforded the
isotopically labeled porphyrin dyad Zn2D-F30(1,9-13C) in 50%
yield.
exhibited a peak at m/z ) 982.4. Integration of the NMR peaks
indicated the porphyrin P6-F20d8 was 97% isotopically pure.
Subsequent zinc metalation gave ZnP6-F20d8 quantitatively.
A second benchmark was a ꢀ-deuteriated A3B-porphyrin
monomer bearing 13C atoms at the 1,9-positions. Treatment of
ZnP1-F15(1,9-13C) to mild deuteriation conditions such as acetic
acid-d4 or TFA-d at room temperature for 1 week, TFA-d at 60
°C for 48 h, or D2SO4 at room temperature proved to be
unsuccessful for the ꢀ-deuteriation. Treatment with D2SO4 at
110 °C for 48 h followed by remetalation with zinc acetate
A ꢀ-deuteriated analogue of dyad Zn2D-F30(1,9-13C) was
sought for EPR studies. The use of perdeuteriated porphyrin
monomers in the synthesis of a perdeuteriated triply fused
porphyrin dimer has been reported.35 In our case, however,
various Pd-mediated coupling reactions (Suzuki, borylation) with
(36) Gross, Z.; Kaustov, L. Tetrahedron Lett. 1995, 36, 3735–3736.
(37) (a) Fenyo, D.; Chait, B. T.; Johnson, T. E.; Lindsey, J. S. J. Porphyrins
Phthalocyanines 1997, 1, 93–99. (b) Srinivasan, N.; Haney, C. A.; Lindsey, J. S.;
Zhang, W.; Chait, B. T. J. Porphyrins Phthalocyanines 1999, 3, 283–291.
(38) (a) Hansen, P. E. Prog. NMR Spectrosc. 1981, 14, 175–296. (b)
So¨derman, O. J. Magn. Reson. 1986, 68, 296–302.
(39) (a) Shirazi, A.; Goff, H. M. J. Am. Chem. Soc. 1982, 104, 6318–6322.
(b) Asano, M. S.; Abe, H.; Kaizu, Y. J. Label. Compd. Radiopharm. 2006, 49,
595–601.
(40) Fuhrhop, J.-H.; Smith, K. M. In Porphyrins and Metalloporphyrins;
Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; pp 757-869.
(41) (a) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C.
Inorg. Chem. 1980, 19, 386–391. (b) Collman, J. P.; Hampton, P. D.; Brauman,
J. I. J. Am. Chem. Soc. 1990, 112, 2986–2998.
(34) (a) Yu, L.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin,
E.; Diers, J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. Inorg.
Chem. 2003, 42, 6629–6647. (b) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org.
Chem. 1995, 60, 7508–7510. (c) Chng, L. L.; Chang, C. J.; Nocera, D. G. J.
Org. Chem. 2003, 68, 4075–4078.
(35) Frampton, M. J.; Accorsi, G.; Armaroli, N.; Rogers, J. E.; Fleitz, P. A.;
McEwan, K. J.; Anderson, H. L. Org. Biomol. Chem. 2007, 5, 1056–1061.
J. Org. Chem. Vol. 73, No. 18, 2008 6953

Muresan et al.
TABLE 1. 13C Resonances of Labeled Compounds
SCHEME 6
entry
compd type
compd
13C resonance (δ, ppm)
1
2
isothiocyanate
pyrrole
1
2
132.4
121.5
3
4
5
6
dipyrromethane 3a
dipyrromethane 3b
dipyrromethane 4a
dipyrromethane 4b
121.6
121.0
117.4
117.4
7
8
9
10
11
12
porphyrin
porphyrin
porphyrin
dyad
dyad
porphyrin
ZnP1-F15(1,9-13C)
ZnP2-F15(1,9-13C)
ZnP3-F15(1,9-13C)
Zn2D-F30(1,9-13C)
Zn2D-F30(1,9-13C)d24
ZnP2-F15(1,9-13C)d9
150.1
150.2
150.5
150.2
150.16-150.22
150.11-150.18
CHART 5
SCHEME 7
afforded the corresponding ꢀ-deuteriated porphyrin ZnP2-
F15(1,9-13C)d9 in 67% yield (Scheme 7). Analysis by negative
ion FAB-MS gave an observable signal but the intensity was
insufficient for high-resolution data, whereas MALDI-MS in
the negative ion mode gave a good quality signal albeit at low
resolution. The 13C resonances for all isotopically labeled
compounds prepared herein are presented in Table 1.
niques.42 Room-temperature EPR spectra of the monomer
[ZnP7-F15]+ and the dyad [Zn2D-F30]+ are shown in Figure 2.
As we have previously reported, the spectra of neither [ZnP7-
F15]+ nor [Zn2D-F30]+ exhibit any resolved hyperfine structure,
and the peak-to-peak line widths (∆Hpp) for the two monocations
are quite similar.10 The finding that the peak-to-peak line widths
(and underlying hyperfine splittings) are similar for [Zn2D-F30]+
and [ZnP7-F15]+ suggests that ground-state hole transfer in
[Zn2D-F30]+ is slow on the EPR time scale. If hole transfer in
[Zn2D-F30]+ were rapid on the EPR time scale, the line width
All of the porphyrin monomers suitable herein as benchmarks
for EPR spectroscopy are shown in Chart 5. ZnP1-F15(1,9-13C)
and ZnP2-F15(1,9-13C) were prepared as shown in Scheme 2.
ZnP2-F15(1,9-13C)d9 was prepared from ZnP2-F15(1,9-13C) as
shown in Scheme 7. ZnP6-F20 was obtained by metalation of
the free base porphyrin P6-F20.41 ZnP6-F20d8 was prepared as
shown in Scheme 6. ZnP7-F15 was prepared as described in
the literature.10
(42) 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–
147.
II. EPR Studies. Monocations of the porphyrin monomers
and dyads were prepared by standard electrochemical tech-
6954 J. Org. Chem. Vol. 73, No. 18, 2008

Ground-State Hole Transfer in Multiporphyrin Arrays
FIGURE 2. Room-temperature EPR spectra of [ZnP7-F15]+ and
[Zn2D-F30]+.
FIGURE 4. Room-temperature EPR spectra of [ZnP6-F20]+ and
[ZnP6-F20d8]+.
positions is somewhat uncertain. Therefore, the additional
studies described below were performed.
The issues regarding the magnitude of the 1H hyperfine
coupling are resolved by studies of the various deuteriated
porphyrins. This point is illustrated in Figure 4, which compares
the EPR spectra of one pair of representative monomers, [ZnP6-
F20]+ and [ZnP6-F20d8]+. These spectral data show that
deuteriation of the ꢀ-pyrrole positions results in very little
change in the overall EPR line width. Similar results were
obtained for all of the other deuteriated versus protonated
complexes (data not shown). Self-consistent spectral simulations
for [ZnP6-F20]+ and [ZnP6-F20d8]+ (aD/aH ) 0.15 and a fixed
12
1
line width ) indicate that the largest value possible for the H
hyperfine is significantly smaller than the inhomogeneous line
width. Thus, the ꢀ-pyrrole 1H hyperfine coupling cannot be used
reliably as a hyperfine clock to assess the hole-transfer
characteristics of [Zn2D-F30]+.
The assessment of the ground-state hole-transfer character-
istics of [Zn2D-F30]+ is resolved by incorporation of the 13C
labels at the two equivalent R-pyrrole positions. This is
illustrated in Figure 5, which compares the room-temperature
EPR spectra of the monomer [ZnP2-F15(1,9-13C)d9]+ and dyad
[Zn2D-F30(1,9-13C)]+. The comparison of the spectra of the
deuteriated monomer versus protonated dyad is arbitrary;
deuteriation has no significant effect on the line shape of any
of the monomers or dyads. [Note that the lower S/N observed
for the isotopically labeled complexes (Figure 5) versus the
unlabeled complexes (Figure 2) is due to the smaller quantities
available for the former complexes.] Inspection of Figure 5
reveals that the line shape of the monomeric cation, while
exhibiting no resolved hyperfine structure, is broader than that
of [ZnP7-F15]+ (and the other benchmark monomers), which
lack the R-13C labels (Figure 2). Self-consistent spectral
simulations for [ZnP7-F15]+ and [ZnP2-F15(1,9-13C)d9]+ in-
FIGURE 3. Temperature dependence of the EPR spectra of [Zn2D-
F30]+.
for the dyadic monocation would be expected to be narrower
than that of the monomeric monocation.10 Elevating the tem-
perature to 320 K (or lowering the temperature to 220 K) has
no effect on the peak-to-peak line width of [Zn2D-F30]+, as is
illustrated in Figure 3, which shows the EPR spectra of the
dyadic monocation over the range 220 to 320 K. [Note that
temperatures higher than 320 K cannot be accessed owing to
boiling of the solvent (CH2Cl2).] Thus, hole transfer in [Zn2D-
F30]+ appears to be slow under all of the conditions used to
acquire the EPR spectra. However, as noted in the Introduction,
deriving a definitive lower limit on the rate is difficult in this
case because (1) the hyperfine coupling to the 14N nuclei of the
13
C
13C
dicate that the a is ∼2.35 G (∼6.2 MHz). The value of a
∼6.2 MHz observed for R-13C-labeled [ZnP2-F15(1,9-13C)d9]+
13C
is smaller than the value of a ∼16 MHz observed for the
monocations of the meso-13C-labeled porphyrins shown in Chart
2,13 as expected because the spin density at the R-pyrrole C
2
2
pyrrole rings is negligible for the A1u monocation and (2) the
atoms of the A1u cations is significantly less than that at the
1
2
magnitude of the coupling to the H nuclei at the ꢀ-pyrrole
meso-C atoms of the A2u cations.12
J. Org. Chem. Vol. 73, No. 18, 2008 6955

Muresan et al.
porphyrin formation followed by elaboration of the regiospe-
cifically labeled porphyrin into dyads. Examination of the
resulting meso-diphenylethyne-linked porphyrin dyads by EPR
spectroscopy established that the hole-transfer rate is <106 s-1
at room temperature. This hole-transfer rate is at least 50-fold
slower than that for meso-diphenylethyne-linked, electron-rich
porphyrin dyads, wherein the HOMO is a2u, which has
substantial electron/spin density at the meso-carbons. The new
synthetic methodology for preparing 1,9-13C-labeled porphyrins
provides an entre´e for examining hole transfer in other types of
electron-deficient porphyrin arrays in which different aspects
of the molecular structure, such as the site of linker attachment,
are varied.
Experimental Section
Electrochemistry. The electrochemical measurements (cyclic
voltammetry and bulk electrolysis) were performed using techniques
and instrumentation previously described.42 The solvent was CH2Cl2
containing 0.1 M Bu4NPF6 as the supporting electrolyte. The bulk
oxidized complexes were prepared in a glovebox using techniques
previously described. Upon oxidation, the samples were transferred
to an EPR tube and sealed in the glovebox.
EPR Spectroscopy. The EPR spectra were recorded on an
X-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.
FIGURE 5. Room-temperature EPR spectra of [ZnP2-F15(1,9-13C)d9]+
and [Zn2D-F30(1,9-13C)]+.
Further inspection of Figure 5 reveals that the line shape of
[Zn2D-F30(1,9-13C)]+ is generally similar to that of [ZnP2-
F15(1,9-13C)d9]+. Simulations of the spectrum of the dyad
indicate that the line shape can be accounted for by a
superposition of the EPR spectra of two monomeric monocations
s one of normal isotopic composition, the other containing two
R-13C labels with a
∼2.35 G. In other words, the EPR
13C
spectrum of the dyadic monocation is the same as that of a 50/
50 mixture of two monomers each exhibiting hyperfine cou-
plings characteristic of one or the other half of the isotopically
unsymmetrical dyad. These characteristics of the EPR spectrum,
particularly the fact that the 13C hyperfine coupling in the dyadic
monocation is the same as that of a monomeric monocation,
definitively show that hole transfer is slow (<106 s-1) on the
Allyl Isothio(cyanate-13C) (1). A sample of KS13CN (99%
isotopic purity) was dried overnight in a high vacuum oven at 140
°C. A solution of allyl bromide (0.760 mL, 9.00 mmol) in dry THF
(9 mL, 1 M) was treated with KS13CN (1 g, 0.01 mol) and heated
to 55 °C. The reaction was monitored by TLC, GC-MS, 1H NMR,
and 13C NMR analyses. After 3 days, the reaction mixture was
cooled to room temperature and filtered. The filtrate was concen-
trated under reduced pressure at room temperature (to avoid loss
of product; expected bp ) 150 °C) to a yellow oil (0.776 g, 86%
2
EPR time scale for the A1u dyadic monocation. If the hole
1
transfer were rapid on the EPR time scale, the line shape would
instead be characteristic of a single species (rather than two
species) wherein the effective hyperfine couplings of all nuclei
are one-half the value observed for a monomer.10 The slower
yield, 99% purity according to GC-MS and H NMR spectros-
copy): 1H NMR δ 4.13-4.17 (m, 2H), 5.27-5.30 (m, 1H),
5.38-5.42 (m, 1H), 5.81-5.88 (m, 1H); 13C NMR δ 47.1, 117.7,
130.5, 132.4 (enh); GC-MS tR ) 4.74 min, m/z ) 100; calcd 100.0
(C313CH5NS).
2
hole-transfer rate for the A1u monocation [Zn2D-F30]+ com-
2
CAUTION. Allyl isothiocyanate is reported to be toxic and
flammable,23 is a strong lachrymator, and should be handled in a
well-vented hood with care.
pared with those of the A2u monocations [Zn2D-Mes]+ and
[Zn2D-SWT]+ is attributed to the fact that in the former dyad,
the linker site (meso position) is at a position of negligible
electron/spin density (nodal plane) in the HOMO, whereas in
the latter dyads, the linker is at a position of large electron/spin
density in the HOMO (Figure 1).
2-(Methylthio)pyrrole-2-13C (2). Following a reported proce-
dure26 with modification, diisopropylamine (855 µL, 6.00 mmol)
was added at room temperature under argon to a mixture of t-BuOK
(673 mg, 6.00 mmol) in dry THF (3.75 mL, 0.800 M), and the
mixture was cooled to -100 °C (using a cooling bath of ethanol
and liquid nitrogen). A sample of n-BuLi (3.75 mL, 6.0 mmol, 1.6
M in hexanes) was added dropwise, whereupon the mixture turned
yellow. The reaction vessel was placed in a cooling bath consisting
of dry ice and acetonitrile. When the reaction mixture attained -40
°C, a solution of allyl isothiocyanate 1 (300 mg, 3.00 mmol, in
2.40 mL of dry THF, 1.25 M) was added dropwise via a 25 µL
syringe over a period of 50 min, maintaining the temperature at -35
to -25 °C. The cooling bath was removed, and after reaching room
temperature, the reaction mixture was heated to 40 °C for 40 min.
The heat source was removed and H2O (480 µL) was added. Stirring
was continued at room temperature for 35 min. Then CH3I (155
µL, 2.70 mmol) was added to the reaction vessel. The reaction
mixture was heated at 45 °C for 40 min. The reaction mixture was
allowed to cool to room temperature, treated with cold H2O (5 mL),
and extracted with diethyl ether. The organic phase was dried
(K2CO3) and concentrated to a brown oil. Column chromatography
Conclusions
A strategy has been demonstrated for assessing the ground-
state hole-transfer rate in cationic π-radical systems wherein
the redox-active constituents lack substantial nuclear-hyperfine
couplings. The strategy entails introduction of 13C labels at sites
of substantial electron/spin density in the HOMO. For electron-
deficient porphyrins, the HOMO is a1u, wherein the largest
electron/spin density resides at the R-pyrrolic carbons; negligible
electron/spin density resides at the meso-carbons or pyrrole
nitrogens, both of which are in nodal planes of the HOMO. A
new synthetic approach, beginning with 13C-labeled KSCN, was
devised to prepare a dipyrromethane bearing 13C labels specif-
ically at the two R- (1- and 9-) positions. The dipyrromethane-
1,9-13C could then be employed in standard methods for
6956 J. Org. Chem. Vol. 73, No. 18, 2008

Ground-State Hole Transfer in Multiporphyrin Arrays
[silica hexanes/CH2Cl2 (1:2)] gave a yellow oil (172 mg, 56%):
1H NMR δ 2.33 (d, J ) 4.8 Hz, 3H), 6.19-6.22 (m, 1H), 6.34-6.37
(m, 1H), 6.78-6.81 (m, 1H), 8.23 (br, 1H); 13C NMR δ 21.9, 114.8,
115.5, 120.18, 121.5 (enh); FAB-MS obsd 115.0442, calcd
115.0405 [(M + H)+; M ) C413CH7NS]; GC-MS tR ) 9.18 min;
m/z ) 114. Note: Dimerization of allyl isothiocyanate is a significant
possible side reaction. Departure from the specified conditions
(temperature, addition, concentrations, stoichiometry, etc.) can lead
to lower yields.
(m, 2H), 6.15-6.18 (m, 2H), 6.70-6.72 (m, 2H), 7.19 (d, J ) 8.3
Hz, 2H), 7.36-7.39 (m, 2H), 7.45-7.48 (m, 4H), 7.93 (br s, 2H);
13C NMR δ 44.1, 88.6, 90.5, 107.6, 108.8, 117.7, 121.8, 122.4,
122.7, 128.7, 131.8, 132.1, 133.2, 142.8; FAB-MS obsd 400.0564,
calcd 400.0575 (C23H17BrN2).
Route 2 to Dipyrromethane 7. Following a reported proce-
dure,22 a sample of aldehyde 9 (200 mg, 0.701 mmol) and pyrrole
(4.8 mL, 70 mmol) was degassed for 10 min. InCl3 (15 mg, 0.070
mmol) was added, and the mixture was stirred at room temperature
under argon. After 1.5 h, the reaction mixture was treated with
NaOH (84 mg, 2.1 mmol), and the stirring was continued for 1 h.
The reaction mixture was filtered, and the filtrate was concentrated.
The resulting solid was triturated with hexanes and the volatile
components were evaporated. Crystallization from ethanol-water
afforded the product as a yellow solid (162 mg, 58%). Character-
ization data were identical with those described above.
4-[2-(4-Bromophenyl)ethynyl]benzaldehyde (9). Following a
reported procedure,33 a sample of aldehyde 8 (140 mg, 1.06 mmol),
1-bromo-4-iodobenzene (300 mg, 1.06 mmol), PdCl2(PPh3)2 (40
mg, 0.025 mmol), and CuI (24 mg, 0.12 mmol) were weighed into
a 50 mL Schlenk flask which was then pump-purged three times
with argon. Dry, degassed benzene/TEA (7 mL, 3:1) was added to
the Schlenk flask, and the reaction mixture was stirred at room
temperature under argon for 18 h. The reaction mixture was treated
with CH2Cl2 followed by 0.1 M HCl. The organic layer was washed
(water, brine), separated, and dried (Na2SO4). The solvent was
evaporated. Purification by column chromatography (silica, CH2Cl2)
afforded a pale-yellow solid (247 mg, 82%): mp 167-169 °C; 1H
NMR δ 7.40-7.44 (m, 2H), 7.50-7.53 (m, 2H), 7.66 (d, J ) 8.4
Hz 2H), 7.87 (d, J ) 8.1 Hz, 2H), 10.03 (s, 1H); 13C NMR δ 89.8,
92.5, 121.6, 123.6, 129.4, 129.8, 132.0, 132.3, 133.4, 135.8, 191.6;
GS-MS tR ) 22.9 min, m/z ) 284; ESI-MS obsd 284.9914, calcd
284.9909 [(M + H)+, M ) C15H9BrO].
1,9-Bis(methylthio)-5-(4-iodophenyl)dipyrromethane-1,9-13C
(3a). Following a reported procedure,19 a mixture of pyrrole 2 (285
mg, 2.50 mmol) and 4-iodobenzaldehyde (290 mg, 1.25 mmol) was
treated with InCl3 (58 mg, 0.25 mmol) and stirred at room
temperature under an argon atmosphere. After 52 h, TLC analysis
showed the complete consumption of the starting material. The
reaction mixture was treated with hexanes/THF (6 mL, 2:1) and
filtered. The filtrate was concentrated to dryness. Column chroma-
tography [silica, hexanes/CH2Cl2 (1:2)] afforded a yellow-orange
1
solid (165 mg, 30%): mp 109-112 °C; GC tR ) 24.9 min; H
NMR δ 2.29 (d, J ) 4.8 Hz, 6H), 5.31 (s, 1H), 5.81-5.85 (m,
2H), 6.24-6.27 (m, 2H), 6.93 (d, J ) 8.3 Hz, 2H), 7.64 (d, J )
8.3 Hz, 2H), 7.90 (br s, 2H); 13C NMR δ 21.9, 44.1, 92.9, 109.5,
115.2, 115.9, 120.6, 121.6 (enh), 130.6, 133.8, 138.1, 141.0; FAB-
MS obsd 441.9933, calcd 441.9945 (C1513C2H17IN2S2).
5-Phenyldipyrromethane-1,9-13C (4a). Following a reported
procedure,19 Raney nickel (∼1.5 g, slurry in EtOH) was added to
a solution of 3a (150 mg, 0.339 mmol) in EtOH (15 mL) at room
temperature. After 30 min, TLC indicated the complete consumption
of 3a and the presence of the product. The reaction mixture was
treated with hexanes/THF (2:1) and filtered through a Celite pad.
The filter cake was washed with diethyl ether. The filtrate was
concentrated. Column chromatography (silica, CH2Cl2) gave a gray
1
solid (30 mg, 40%): mp 99-102 °C; H NMR δ 5.51 (s, 1H),
5-Phenyl-10,15,20-tris(pentafluorophenyl)porphyrin-1,9-13C
(P1-F15(1,9-13C)). Following general procedures,18,29,30 reduction
of tin complex 5 (100 mg, 0.109 mmol) in THF/MeOH (4.4 mL,
3:1) with NaBH4 (0.161 g, 4.36 mmol) for 1.5 h afforded a yellow
solid. The latter was treated with 4a (24.0 mg, 0.108 mmol) in
CH2Cl2 (44 mL, 2.5 mM) containing Yb(OTf)3 (90.0 mg, 0.145
mmol) at room temperature for 2 h, followed by oxidation with
DDQ (74 mg, 0.32 mmol). The reaction mixture was stirred for 30
min, and TEA (100 µL) was added. Column chromatography (silica,
CH2Cl2) afforded a purple solid (20 mg, 20%): 1H NMR δ -2.79
(s, 2H), 7.77-7.84 (m, 3H), 8.20-8.22 (m, 2H), 8.79-8.86 (m,
2H), 8.88-8.89 (m, 4H), 8.96-8.99 (m, 2H); 13C NMR δ 150.1
(enh); LD-MS obsd 886.6; FAB-MS obsd 886.1147, calcd 886.1124
5.90-5.93 (m, 2H), 6.13-6.18 (m, 2H), 6.69 (dm, J ) 184.4 Hz,
2H), 7.18-7.24 (m, 2H), 7.28-7.35(m, 3H), 7.93 (br s, 2H); 13C
NMR δ 44.1, 107.4, 108.3, 108.9, 116.7, 117.4 (enh), 118.2, 127.2,
128.6, 128.9; ESI-MS obsd 225.1297, calcd 225.1296 [(M + H)+;
M ) C1313C2H14N2].
Dipyrromethane-1,9-13C (4b). Following a reported procedure,19
a sample of pyrrole 2 (0.179 g, 1.57 mmol) and paraformaldehyde
(22 mg, 0.75 mmol) was heated at 50 °C until a homogeneous
mixture was obtained. The mixture was treated with InCl3 (33 mg,
0.15 mmol) and stirred at room temperature for 18 h. The reaction
mixture was treated with hexanes/THF (2:1) and filtered. The filter
cake was washed with diethyl ether. The filtrate was concentrated
to give a crude product with satisfactory characterization data [1H
NMR δ 2.31 (d, J ) 4.8 Hz, 6H), 3.91 (s, 2H), 5.98-6.01 (m,
2H), 6.25-6.29 (m, 2H), 7.95 (br, 2H); 13C NMR δ 106.6, 108.3,
108.9, 121.0 (enh), 127.6]. The resulting crude mixture was
dissolved in THF (4 mL) and treated with Raney nickel (∼200
mg) slurry in THF at room temperature for 45 min. Raney nickel
was removed by filtration through a Celite pad. The filter cake was
washed with THF. The filtrate was concentrated to dryness. Column
chromatography [silica, hexanes/CH2Cl2 (1:2)] gave a gray solid
(32 mg, 29%): mp 67-70 °C; 1H NMR δ 3.98 (s, 2H), 6.02-6.05
(m, 2H), 6.12-6.17 (m, 2H), 6.66 (dm, J ) 184.8 Hz, 2H), 7.83
(br s, 2H); 13C NMR δ 26.6, 106.6, 108.2, 108.9, 116.7, 117.4 (enh),
118.2; GC-MS tR ) 14.17 min, m/z ) 148; ESI-MS obsd
149.0985, calcd 149.0983 [(M + H)+, M ) C713C2H10N2].
5-[4-[2-(4-Bromophenyl)ethynyl]phenyl]dipyrromethane (7).
Dipyrromethane 6 (123 mg, 0.500 mmol), 1-bromo-4-iodobenzene
(141 mg, 0.500 mmol), Pd2(dba)3 (14 mg, 0.015 mmol), AsPh3 (36
mg, 0.12 mmol), and CuI (28 mg, 0.15 mmol) were weighed into
a 10 mL Schlenk flask which was then pump-purged three times
with argon. Dry, degassed THF/TEA (5 mL, 1:1) was added to the
Schlenk flask, and the reaction mixture was stirred at 35 °C under
argon for 24 h. The solvent was evaporated. Purification by column
chromatography (silica, CH2Cl2) afforded a gold-yellow solid (172
(C4213C2H15F15N4); λabs 418, 511, 541, 587, 642 nm; λem (λex
)
418 nm) 650, 720 nm.
5-Phenyl-10,15,20-tris(pentafluorophenyl)porphinatozinc(II)-
1,9-13C (ZnP1-F15(1,9-13C)). Following a general procedure,29
a
sample of P1-F15(1,9-13C) (15 mg, 0.017 mmol) in CHCl3 (3.4 mL)
was treated overnight with methanolic Zn(OAc)2 ·2H2O (18 mg,
0.085 mmol) at room temperature. Column chromatography (silica,
CH2Cl2) afforded a purple solid (14 mg, 87%): 1H NMR δ
7.78-7.84 (m, 3H), 8.21-8.24 (m, 2H), 8.90-8.93 (m, 2H),
8.97-9.10 (m, 4H), 9.05-9.10 (m, 2H); 13C NMR δ 126.9, 128.3,
130.5, 131.0, 131.8, 132.07, 132.11, 134.61, 134.68, 150.1 (enh),
151.1; LD-MS obsd 948.8; FAB-MS obsd 948.0208, calcd 948.0259
(C4213C2H13F15N4Zn); λabs 421, 553 nm; λem (λex ) 421 nm) 580,
650 nm.
5,10,15-Tris(pentafluorophenyl)porphinatozinc(II)-1,9-13C(ZnP2-
F15(1,9-13C)). As described above for P1-F15(1,9-13C), reduction
of tin complex 5 (186 mg, 0.200 mmol) in dry THF/MeOH (8 mL,
3:1) with NaBH4 (295 mg, 8.00 mmol) for 1.5 h gave a yellow
material after workup. The yellow material was dissolved in dry
CH2Cl2 (80 mL, 2.5 mM) and treated with 4b (30 mg, 0.20 mmol)
and Yb(OTf)3 (164 mg, 0.264 mmol) at room temperature. After
1.5 h, DDQ (136 mg, 0.600 mmol) was added. After 30 min, TEA
(180 µL) was added to the reaction mixture. The crude reaction
1
mg, 86%): mp 132-134 °C; H NMR δ 5.48 (s, 1H), 5.90-5.91
J. Org. Chem. Vol. 73, No. 18, 2008 6957

Muresan et al.
mixture was passed through a silica pad. The resulting purple solid
was dissolved in CHCl3 (5 mL) and treated overnight with
methanolic Zn(OAc)2 ·2H2O (44 mg, 0.20 mmol, 2.0 mL of MeOH)
at room temperature. Column chromatography [silica, hexanes/
(K2CO3) and concentrated to give the free base porphyrin. The
resulting product was dissolved in CHCl3 (1 mL) and treated
overnight with methanolic Zn(OAc)2 ·2H2O (5 mg, 0.02 mmol,
0.200 mL of MeOH) at room temperature. Column chromatography
[silica, hexanes/CH2Cl2 (1:2)] afforded a bright-pink solid (10 mg,
1
CH2Cl2 (1:1)] afforded a purple solid (32 mg, 18%): H NMR δ
9.01-9.07 (m, 6H), 9.52-9.56 (m, 2H) 10.43 (s, 1H); 13C NMR δ
131.3, 131.8, 132.0, 134.4, 150.2 (enh); ESI-MS obsd 873.0013,
calcd 873.0019 [(M + H)+, M ) C3613C2H9F15N4Zn]; LD-MS obsd
872.2, 854.2 (M - F)+; FAB-MS obsd 871.9958, calcd 871.9946;
λabs 421, 553 nm; λem (λex ) 421 nm) 585, 641 nm.
67% yield, 95% isotopic purity): H NMR δ 9.01-9.07 (weak
1
signals), 9.52-9.56 (weak signals); 13C NMR δ 150.11-150.18
(m, enh); LD-MS obsd 880.9, calcd 881.0511 (C3613C2D9F15N4Zn);
λ
abs 416, 542, 576 nm; λem (λex ) 416 nm) 580, 636 nm. The ESI-
MS data for the title compound were not satisfactory; however,
data were satisfactory for the free base analogue (P2-F15(1,9-13C)d9)
obtained as an intermediate and used in the metalation described
here: ESI-MS obsd 820.1446, calcd 820.1448 [(M + H)+, M )
C3613C2H2D9F15N4].
5-Bromo-10,15,20-tris(pentafluorophenyl)porphinatozinc(II)-
1,9-13C (ZnP3-F15(1,9-13C)). Following a general procedure,31
a
solution of ZnP2 (21 mg, 0.024 mmol) in dry CHCl3 (4.8 mL, 5.0
mM) was treated with NBS (0.0240 mmol, 117 µL stock solution
in dry CHCl3, 0.205 M) and anhydrous pyridine (7 µL, 0.08 mmol)
at 0 °C. The reaction was monitored by TLC and LD-MS. After
30 min the reaction was quenched by the addition of acetone. The
solvent was evaporated. Column chromatography [silica, hexanes/
meso-Tetrakis(pentafluorophenyl)porphyrin-2,3,7,8,12,13,
17,18-d8 (P6-F20d8). A sample of P6-F20 (11 mg, 0.011 mmol)
was treated with D2SO4 (1 mL) and stirred at 150 °C for 22 h. The
LD-MS spectrum of the crude reaction mixture exhibited one peak
(m/z ) 983.9) corresponding to the fully deuteriated porphyrin
containing 10 deuterons (ꢀ-pyrrole sites and the two inner pyrrolic
nitrogens); calcd 984.1214 (C44D10F20N4). Neutralization of the
reaction mixture with saturated aqueous NaHCO3 and washing with
H2O resulted in proton/deuterium exchange at the inner nitrogen
atoms. The product was isolated as a purple solid (10 mg, 92%
1
CH2Cl2 (1:2)] gave a red-purple solid (19 mg, 85%): H NMR δ
8.91-8.97 (m, 6H), 9.87-9.91 (m, 2H); 13C NMR δ 131.6, 132.1,
132.39, 132.42, 150.5 (enh); LD-MS obsd 951.2; FAB-MS obsd
949.9084, calcd 949.9051 (C3613C2H8BrF15N4Zn); λabs 425, 551 nm;
λ
em (λex ) 425 nm) 597, 648 nm; SEC tR ) 26.7 min (three-column
series).
5-[4-[2-(4-Bromophenyl)ethynyl]phenyl]-10,15,20-tris(pen-
1
yield, 97% isotopic purity): H NMR δ -2.93 (s, 2H), 8.92 (s,
tafluorophenyl)porphinatozinc(II) (ZnP4-F15). Following a re-
ported procedure,29 reduction of tin complex 5 (204 mg, 0.219
mmol) in THF/MeOH (9 mL, 3:1) with NaBH4 (324 mg, 8.76
mmol) afforded a yellow solid after the standard workup. The latter
was then treated with 7 (87.0 mg, 0.219 mmol) in CH2Cl2 (88 mL,
2.5 mM) containing Yb(OTf)3 (170 mg, 0.287 mmol) at room
temperature. After 1.5 h, DDQ (149 mg, 0.657 mmol) was added
to the reaction mixture. After 30 min, the reaction mixture was
neutralized with TEA (200 µL). The crude reaction mixture was
passed through a silica pad. The resulting purple solid was dissolved
in CHCl3 (9 mL) and treated overnight with Zn(OAc)2 ·2H2O (50.0
mg, 0.219 mmol) and MeOH (2 mL) at room temperature. Column
chromatography (silica, CH2Cl2) gave a purple solid (55 mg, 22%):
1H NMR δ 7.53-7.59 (m, 4H), 7.96 (d, J ) 8.3 Hz, 2H), 8.22 (d,
J ) 8.3 Hz, 2H), 8.92-8.94 (m, 2H), 8.98-9.01 (m, 4H),
9.07-9.09 (m, 2H); 13C NMR δ 90.0, 90.4, 104.1, 116.7, 122.3,
123.07, 123.11, 123.23, 130.3, 130.9, 131.8, 132.0, 132.2, 133.4,
134.3, 134.7, 136.4, 138.9, 142.2, 143.4, 145.5, 148.0, 149.9, 150.2,
150.5, 150.9; LD-MS obsd 1126.6; FAB-MS obsd 1123.9594, calcd
1123.9610 (C52H16BrF15N4Zn); λabs 422, 554 nm; λem (λex ) 422
nm) 588, 644 nm.
0.2H); LD-MS obsd 982.4; ESI-MS obsd 983.1160, calcd 983.1161
[(M + H)+; M ) C44H2D8F20N4]; λabs 417, 508, 585, 641 nm; λem
(λex ) 416 nm) 643, 713 nm.
meso-Tetrakis(pentafluorophenyl)porphinatozinc(II)-2,3,7,8,12,
13,17,18-d8 (ZnP6-F20d8). Following a general procedure,29
a
solution of P6-F20d8 (8 mg, 0.008 mmol) in CHCl3 (1.6 mL) was
treated with a solution of Zn(OAc)2 ·2H2O (9 mg, 0.04 mmol) in
methanol (0.3 mL) at room temperature for 40 h. The reaction
mixture was washed with water, dried (Na2SO4) and concentrated
to dryness. Purification by column chromatography [silica, CH2Cl2/
hexanes (2:1)] afforded a bright-pink solid (8 mg, quantitative yield,
97% isotopic purity): 1H NMR δ 9.0 (s, weak signal); LD-MS obsd
1043.8; FAB-MS obsd 1044.0205, calcd 1044.0215
(C44D8F20N4Zn); λabs 421, 546 nm; λem (λex ) 421 nm) 586, 640
nm.
Zn2D-F30(1,9-13C). Following a reported procedure,20 samples
of ZnP3-F15(1,9-13C) (10 mg, 0.010 mmol), ZnP5-F15 (12 mg,
0.010 mmol), Pd(PPh3)4 (4 mg, 0.003 mmol), and anhydrous K2CO3
(6 mg, 0.04 mmol; dried overnight at 150 °C in the high vacuum
oven) were weighed into a 5 mL Schlenk flask which was then
pump-purged three times with argon. Toluene/DMF [1 mL (2:1)]
was added (the solvents were purged with argon for 30 min) and
the reaction mixture was stirred at 90 °C under argon. After 27 h,
analytical SEC indicated the presence of the product as the major
component, traces of starting material, and some trimer material.
Column chromatography (silica, CH2Cl2) removed the unreacted
porphyrin monomers. The fraction containing the dyad was
concentrated and then further purified by preparative SEC (toluene).
A final adsorption chromatography column (silica, CH2Cl2) afforded
5-[4-[2-[4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phe-
nyl]ethynyl]phenyl]-10,15,20-tris(pentafluorophenyl)porphi-
natozinc(II) (ZnP5-F15). Following a reported procedure34 with
modification, samples of ZnP4-F15 (40 mg, 0.035 mmol), bis(pina-
colato)diboron (71 mg, 0.28 mmol), and PdCl2(PPh3)2 (3 mg, 0.002
mmol) were weighed into a 10 mL Schlenk flask which was then
pump-purged three times with argon. Anhydrous, degassed 1,2-
dichloroethane (3.8 mL, 10 mM) and TEA (12 equiv, 60 µL, 0.42
mmol) were added to the Schlenk flask. The resulting mixture was
stirred at 75 °C for 40 h under argon. Column chromatography
[silica, hexanes/CH2Cl2 (1:3)] afforded a pink-purple solid (25 mg,
60%): 1H NMR δ 1.23 (s, 12H), 7.68 (d, J ) 8.3 Hz, 2H), 7.88 (d,
J ) 8.3 Hz, 2H), 7.96 (d, J ) 8.3 Hz, 2H), 7.21 (d, J ) 8.3 Hz,
2H), 8.91-8.93 (m, 2H), 8.95-8.99 (m, 4H), 9.06-9.08 (m, 2H);
LD-MS obsd 1172.9; FAB-MS obsd 1172.1356, calcd 1172.1357
(C58H28BF15N4O2Zn); λabs 424, 548 nm; λem (λex ) 424 nm) 592,
646 nm; SEC tR ) 25.1 min (three-column series).
1
a purple solid (9 mg, 50%): H NMR δ 8.13 (d, J ) 8.0 Hz, 4H),
8.32 (d, J ) 7.6 Hz, 4H), 8.95-9.02 (m, 12H), 9.15-9.17 (m,
4H); 13C NMR δ 150.2 (enh); LD-MS obsd 1916.5, calcd 1916.03
(C8813C2H24F30N8Zn2); λabs 427, 553 nm; λem (λex ) 427 nm) 598,
650 nm; SEC tr ) 23.7 min (three-column series).
Zn2D-F30(1,9-13C)d24. A sample of Zn2D-F30(1,9-13C) (5 mg,
0.002 mmol) was treated with D2SO4 (0.340 mL, 7.6 mM) and
stirred at room temperature. The reaction was monitored by LD-
MS. After 7 h, the reaction mixture was treated with saturated
aqueous NaHCO3 and CH2Cl2. The organic layer was separated,
dried (K2CO3) and the solvent was evaporated to give a purple solid.
The resulting free base porphyrin dyad was examined by 1H NMR
spectroscopy; comparison of the signals from the inner NH protons
versus residual protons of the porphyrin ꢀ-pyrrole positions gave
an estimate of 95% deuteriation. The free base porphyrin dyad was
5,10,15-Tris(pentafluorophenyl)porphinatozinc(II)-1,9-13C-
2,3,7,8,12,13,17,18,20-d9 (ZnP2-F15(1,9-13C)d9). A sample of ZnP2-
F15(1,9-13C) (15 mg, 0.017 mmol) was treated with D2SO4 (0.40
mL, 0.02 M) and stirred at 110 °C for 48 h. The reaction mixture
was cooled to room temperature, and treated with saturated aqueous
NaHCO3 and CH2Cl2. The organic layer was separated, dried
6958 J. Org. Chem. Vol. 73, No. 18, 2008

Ground-State Hole Transfer in Multiporphyrin Arrays
dissolved in CHCl3 (0.750 mL) and treated overnight with metha-
nolic Zn(OAc)2 ·2H2O (5 mg, 0.02 mmol) and stirred at room
temperature for 48 h. Column chromatography [silica, hexanes/
U.S. Department of Energy to D.H. (DE-FG02-05ER15661),
J.S.L. (DE-FG02-96ER14632), and D.F.B. (DE-FG02-
05ER15660). Mass spectra were obtained at the Mass Spec-
trometry 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.
1
CH2Cl2 (1:2) afforded a bright-pink solid (4 mg, 80% yield): H
NMR δ 8.12-8.14 (m, weak signals), 8.31-8.33 (m, weak signals),
8.98-8.99 (m, weak signals), 9.14-9.16 (m, weak signals); 13C
NMR δ 150.16-150.22 (m, enh), LD-MS obsd 1939.2, calcd
1940.18 (C8813C2D24F30N8Zn2); λabs 424, 547 nm; λem (λex ) 424
nm) 587, 642 nm; SEC tr ) 41.7 min (five-column series).
Supporting Information Available: General procedures and
spectral data for new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by grants
from the Chemical Sciences, Geosciences and Biosciences
Division, Office of Basic Energy Sciences, Office of Science,
JO8012836
J. Org. Chem. Vol. 73, No. 18, 2008 6959