Probing the rate of hole transfer in oxidized synthetic chlorin dyads via site-specific 13C-Labeling

Article abstract of DOI:10.1021/jo100527h

Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the φ-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein ¹H and ¹⁴N serve as the hyperfine clocks , and in arrays containing site-specific ¹³C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a ¹³C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with ¹³C-nitromethane. The resulting singly ¹³C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and ¹³C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a_1u-like for the chlorins versus a_2u-like for the porphyrins; the a_1u-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a_2u-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.

Full text of DOI:10.1021/jo100527h

pubs.acs.org/joc
Probing the Rate of Hole Transfer in Oxidized Synthetic
Chlorin Dyads via Site-Specific 13C-Labeling
Elıas J. Nieves-Bernier,† James R. Diers,‡ Masahiko Taniguchi,† 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
holten@wuchem.wustl.edu; david.bocian@ucr.edu; jlindsey@ncsu.edu
Received March 19, 2010
Understanding electronic communication among interacting constituents of multicomponent mole-
cular architectures is important for rational design in diverse fields including artificial photosynthesis
and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized
tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of
the π-cation radical. This strategy has been previously employed to probe the hole/electron-transfer
process in oxidized multiporphyrin arrays of normal isotopic composition, wherein 1H and 14N serve
as the hyperfine “clocks”, and in arrays containing site-specific 13C-labels, which serve as additional
hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins
(chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de
novo synthetic strategy has been employed to introduce a 13C label at the 19-position of the chlorin
macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with
13C-nitromethane. The resulting singly 13C-labeled chlorin was coupled with an unlabeled chlorin to
give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR
studies of the monocations of both the natural abundance and 13C-labeled zinc chlorin dyads and
benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns
range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron
transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO
is a1u-like for the chlorins versus a2u-like for the porphyrins; the a1u-like orbital exhibits little (or no)
electron/hole density at the site of linker attachment whereas the a2u-like orbital exhibits significant
electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide
insights into the structural features that influence the hole/electron-transfer process.
Introduction
properties and amenability to synthetic control.1-16 An effec-
tive light-harvesting array absorbs intensely and transfers
Understanding electronic communication among inter-
acting chromophores is essential for the rational design of
molecular architectures for artificial photosynthetic light-
harvesting and energy conversion. Porphyrinic macrocycles
have been widely employed in the construction of synthetic
light-harvesting arrays owing to their attractive physical
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DOI: 10.1021/jo100527h
r
Published on Web 04/29/2010
J. Org. Chem. 2010, 75, 3193–3202 3193
2010 American Chemical Society

Nieves-Bernier et al.
JOCFeatured Article
CHART 1. Porphyrin Dyads with One Meso-13C Label or Two R-13C Labels
1
the resulting electronic excited-state energy to a specific site
with high efficiency. Conversion of the harvested excited-
state energy to electrical energy then requires efficient electron
injection into the anode followed by efficient ground-state
hole migration away from the anode, thereby preventing
charge recombination. Accordingly, understanding hole mobi-
lity in prototypical light-harvesting and charge-separation
systems is of fundamental interest.
Over the past decade or more, our groups have investi-
gated ground-state hole/electron transfer in oxidized por-
phyrinic arrays using EPR spectroscopy.13 More recently,
we have developed transient-absorption spectroscopic ap-
proaches for examining the ground-state hole/electron-transfer
process.17 While the transient optical methods have the ad-
vantage of providing detailed insights into the hole/electron-
transfer process, these methods have the disadvantage of
complex experimental and data-interpretation protocols.
The simplicity of the EPR methods has prompted us to
explore other strategies for capitalizing on this method for
probing ground-state hole/electron transfer.
restricting the studies to measurements of 14N and/or H
interactions.13,18-20 The reliance on 14N and H hyperfine
1
interactions is a severe constraint because these couplings are
relatively small in porphyrin π-cation radicals.21 As a con-
sequence, the exact rates of hole/electron transfer could not
be extracted from the data; it could only be determined
whether the process is fast or slow on the EPR time scale.
More recently, we have attempted to mitigate the limitations
of the EPR method by embarking on molecular design
strategies that entail introduction of a 13C label at specific
sites in the porphyrin macrocycle where there is substantial
hole/electron density in the relevant frontier (highest
occupied) molecular orbital (HOMO).22,23 The incorpora-
tion of 13C labels has two advantages: (1) The 13C hyperfine
interactions in porphyrin π-cation radicals are typically
larger than those of 14N or 1H.21,24 (2) The introduction of
an additional hyperfine coupling affords more accurate
simulations of the EPR spectra.
Two previously prepared porphyrin dyads that contain
13C labels at specific sites in the macrocycle are shown in
Chart 1. In the dyad containing electron-rich trimesityl-
substituted porphyrins [ZnP(Mes)-Dyad-(13C)5], the 13C
label is introduced at the meso position; in the dyad con-
taining electron-deficient pentafluorophenyl-substituted
porphyrins [ZnP(F30)-Dyad-(13C)1,9] the 13C label is intro-
duced at selected R-pyrrolic positions. (The dyads were
previously termed Zn2D-Mes(5-13C) and Zn2D-F30(1,9-13C),
respectively.22,23) The rationale for these choices of 13C
label location is that the HOMO for the trimesityl-substituted
porphyrin is a2u whereas the HOMO for the pentafluoro-
phenyl-substituted porphyrin is a1u (D4h symmetry labels).
The former HOMO is characterized by large hole/electron
density at the meso positions, whereas the latter is char-
acterized by substantial electron/hole density at the R-
position(s) of the pyrrole rings (Chart 2). Accordingly, these
locations for the 13C labels yield the largest hyperfine
The use of EPR techniques to examine ground-state hole/
electron transfer relies on the measurement of the modula-
tion of hyperfine interactions via the hole/electron-transfer
process. In this regard, our early EPR studies utilized por-
phyrinic arrays of natural isotopic composition, thereby
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CHART 2. Metalloporphyrin a2u and a1u Orbitals
biosynthetic pathways26-29 and to prepare labeled porphy-
rins for spectroscopic studies.30-32 Studies devoted to the
preparation of isotopologues of chlorins are far fewer and
have typically employed three distinct approaches.
(1) The biosynthesis approach entails labeling of naturally
occurring chlorins upon feeding labeled precursors to photo-
synthetic cells or plants. Thus, use of 13CH313COONa (or
13CO2) or NaH13CO3 in various cell types has afforded
per-13C-labeled chlorophyll a33,34 or per-13C-labeled bacter-
iochlorophyll c,35,36 respectively. Studies with more ad-
vanced precursors such as site-specifically labeled δ-amino-
levulinic acid37 or glutamic acid have provided chlorophyll
a labeled at multiple sites.38
(2) The semisynthesis approach entails chemical modifica-
tion of chlorophylls and has been used to prepare a wide
variety of chlorophyll analogues.39 Regioselective 18O-label-
ing at the 3-formyl, 3-hydroxymethyl, and 13-keto groups has
been achieved upon treatment with acidic H218O,40-42 but
the semisynthesis approach apparently has not been em-
ployed to introduce 13C atoms to the chlorophyll skeleton.
(3) The de novo synthesis approach to prepare chlorin
isotopologues has generally relied on routes to tetraphenyl-
chlorin or octaethylchlorin. Thus, zinc(II)tetraphenylchlorin
bearing 13C-labels at the four meso positions has been pre-
pared by synthesis of the meso-13C-labeledfreebasetetraphenyl-
porphyrin, diimide reduction43 to give the corresponding
free base chlorin and metalation to give the zinc chelate.44
Octaethylchlorin has been prepared with per-15N-labeling
(beginning with Na15NO2 in an established synthesis of
octaethylporphyrin45 followed by reduction46,47) or deuter-
ium labeling at some or all of the meso positions (by isotopic
CHART 3. HOMO for a Chlorin
interactions in porphyrin π-cation radicals that exhibit 2A2u
and 2A1u ground states, respectively.22,23
Hydroporphyrins (e.g., chlorins) are structurally more simi-
lar to natural photosynthetic pigments (e.g., chlorophylls)
than porphyrins; these structural features impart important
photophysical properties to the molecules.25 In particular,
chlorins contain one reduced pyrrole (i.e., pyrroline) ring.
Pyrrole-ring saturation results in a greatly enhanced oscillator
strength in the long-wavelength absorption relative to por-
phyrins. Accordingly, hydroporphyrins are potentially more
attractive for use as light-harvesting elements in photoconver-
sion systems. An additional characteristic of hydroporphyrins
is that the HOMO is the a1u-like orbital (a2 in C2v symmetry;
Chart 3), and therefore, the largest hole/electron density is
located on the R-carbons (versus the β- or meso-carbons).21 In
chlorins, the hole/electron density is large and of comparable
magnitude at the 4, 6, 9, 11, 16, and 19 positions; less density
resides at the remaining two R-carbons (1, 14). [Note that the
lower electron density at the 1 and 14 positions is not particu-
larly obvious from the chlorin HOMO electron-density diagram
in Chart 3; however, calculations show that the electron density
at the 1 or 14 position is less than half that at the 4, 6, 9, 11, 16, or
19 position.] This characteristic of chlorins prompted us to
pursue a chlorin dyad containing one unlabeled chlorin and
one chlorin containing a single 13C label at one of the afore-
mentioned six higher-electron-density R-positions.
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cycles has largely served as a means to elucidate tetrapyrrole
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SCHEME 1. Synthetic Considerations for Site-Specific
Labeling
JOCFeatured Article
CHART 4. Site-Specifically 13C-Labeled Chlorin Dyad and
Unlabeled Dyad
exchange under acidic conditions46,48-51) but 13C labeling
apparently has not yet been reported.
While each of the above methods has merit, to our know-
ledge no prior syntheses have placed a single 13C-label at the
R-position of the chlorin macrocycle. In general, even syn-
thetic porphyrins wherein the R- or β-carbon framework is
labeled with one or more 13C atoms have been little explored
given the requirement to begin the synthesis with isotopically
labeled precursors to pyrroles.23,52-54 Regardless, de novo
syntheses of site-specifically labeled chlorin isotopologues
are expected to be of considerable value. One example of
their utility is suggested by NMR studies of partially or
per-13C-labeled bacteriochlorophyll c pigments to eluci-
date their organization in self-assembled light-harvesting
antennas.35,36 We have developed a de novo synthetic route
to chlorins that in principle enables location of one or more
13C labels at any designated site.55 Herein, we employ the de
novo synthetic route for the preparation of a site-specifically
13C-labeled chlorin dyad [ZnC-Dyad-(13C)19, Chart 4] and
examine its hole/electron-transfer characteristics using EPR
spectroscopy. The dyad contains a diphenylethyne linker
that is identical to that in the isotopically labeled porphyrin
dyads shown in Chart 1. The 13C label was placed at the 19-
position owing to considerations of (1) HOMO hole/electron
density and (2) synthetic facility. The synthesis of the unlabeled
dyad (ZnC-Dyad) was achieved previously by Sonogashira
coupling of iodophenylchlorin and ethynylphenylchlorin
building blocks.56
Results and Discussion
Site-Specific Labeling Considerations. The de novo synthe-
sis of chlorins entails the reaction of a 1,3,3-trimethyl-
2,3,5,6-tetrahydrodipyrrin (Western half) and a 5-aryl-9-
bromodipyrromethane-1-carbinol (Eastern half).55 The
Western half contains a pyrrole ring and a gem-dimethyl-
substituted pyrroline ring; the latter becomes the pyrroline
ring of the chlorin. The presence of the geminal dimethyl
group stabilizes the chlorin against adventitious dehydro-
genation. The six possible locations of the 13C label are the 4,
6, 9, 11, 16, and 19 positions (Scheme 1). The introduction of
a
13C label at the 4, 6, 9, or 11 positions would entail labeling
of an R-carbon in a pyrrole derivative. Although such an
approach is feasible,23,57 the requirement to construct the
pyrroline ring regardless of labeling site, versus use of com-
mercially available pyrrole as starting material in the synthe-
ses, led to focus on positions 16 and 19 (which constitute the
R-positions of the reduced, pyrroline ring). Among these,
labeling the 16-position would require synthesis of mesityl
oxide bearing a 13C label at the carbonyl carbon, whereas the
19-position can be labeled upon use of 13C-labeled nitro-
methane, a commercially available compound. In this re-
gard, among all eight R-positions and eight β-positions of the
chlorin macrocycle, the 19-position is conveniently the most
accessible synthetically with regards to introduction of a
single 13C label. This synthetic convenience is felicitous given
that the 19-position has electron density comparable to or
greater than that at the other R-positions considered for
location of the label.
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SCHEME 3. Synthesis of the Eastern Halves
SCHEME 2. Synthesis of the 13C-Labeled Western Half
Synthesis. The unlabeled dyad was prepared over 7 years
ago.56 Since then, refined procedures have been developed
for multiple steps in the synthesis of Eastern halves58-60 and
the Western half.61 The refined procedures were used here
to prepare the labeled and unlabeled Western halves and
both of the Eastern halves. The synthesis of the labeled
Western half is shown in Scheme 2. Nitro-aldol condensation
of pyrrole-2-carboxaldehyde (1) and 13C-nitromethane
followed by reduction with NaBH4 afforded 2-(2-nitroethyl)-
pyrrole-(2-13C) [2(13C)] in 32% yield. Michael addition of
2(13C) and mesityl oxide afforded the corresponding labeled
nitrohexanone 3(13C) in 87% yield. Reductive cyclization of 3
using Zn and HCOONH4 afforded the labeled tetrahydro-
dipyrrin [Western half, 4(13C)] in 60% yield.
and iodophenyldipyrromethane (5b),63 prepared via a
streamlined synthesis,60 were treated with EtMgBr followed
by the known Mukaiyama reagent (6)55 to give the corres-
ponding 1-acyldipyrromethanes. 1-Acyldipyrromethanes
afford amorphous foams and are prone to streak upon
attempted chromatography but typically give crystalline
solids upon dialkylboron complexation.59 Accordingly,
treatment with Bu2BOTf gave the dibutylboron complexes
7a and 7b in 39% and 47% overall yield, respectively.
Bromination58 of 7a,b using NBS gave Eastern half pre-
cursors 8a,b in 65% and 59% yield, respectively. Inter-
mediates 7a,b and 8a,b are new compounds. Reduction of
the carbonyl group afforded the Eastern halves 8a-OH and
8b-OH, which were not isolated owing to the typical
instability of dipyrromethanecarbinols but were used imme-
diately in chlorin formation.
The Eastern half contains the functional group (iodophenyl,
ethynylphenyl) that serves to form the linker joining the
chlorins. Both Eastern halves have been prepared previously56
and were prepared here via refined procedures (Scheme 3).
The known (TMS-ethynylphenyl)dipyrromethane (5a)62
(58) Zaidi, S. H. H.; Muthukumaran, K.; Tamaru, S.-I.; Lindsey, J. S.
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Lindsey, J. S. J. Org. Chem. 1999, 64, 7890–7901.
(63) 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.
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SCHEME 4. Formation of Chlorin Building Blocks and
TABLE 1. 13C NMR Chemical Shifts of Labeled Compounds
13C-Labeled Dyad
a13C NMR spectrum in toluene-d8 at room temperature.
ethynylphenyl ZnC-Ar5PE10(13C)19 and iodophenylchlorin
ZnC-Ar5PI10. Some difficulties were encountered upon puri-
fication of the resulting dyad ZnC-Dyad-(13C)19. The puri-
fication process consisted of a three-column sequence:65
silica gel chromatography to remove the monomers, size-
exclusion chromatography (SEC) to remove high molecular
weight material, and silica gel chromatography to remove
particles derived from the SEC column. The resulting crude
dyad showed some impurities in the aromatic region upon 1H
NMR spectroscopic examination. Suspension of the mixture
in methanol and sonication followed by decanting the super-
1
natant afforded the target dyad with purity >95% (by H
NMR spectroscopy) and expected dominant peak due to the
molecular ion in the mass spectrum.
Chemical Characterization. All synthetic compounds were
1
characterized by mass spectrometry and by H NMR and
1
13C NMR spectroscopy. H NMR spectra of the labeled
compounds 2(13C)-4(13C) exhibited expected additional
coupling with the proton(s) bonded to the 13C atom. The
13C NMR spectra exhibited strongly enhanced intensity of
the peak due to the 13C atom versus that of the natural
abundance analogues (2-4).61 The chemical shifts of the 13
C
atom of the labeled compounds are shown in Table 1. The
13C NMR spectrum of the dyad [ZnC-Dyad-(13C)19] was not
characterized in detail, but the signal from the 13C atom was
observed as expected upon comparison with that of the
labeled Zn-chlorin monomer [ZnC-Ar5PE10(13C)19].
EPR Studies. The chlorin monocations were prepared in a
glovebox via quantitative coulometric bulk electrolysis at
a Pt electrode (see the Experimental section). The EPR
spectra of the monocations of the unlabeled chlorin mono-
mer66 (ZnC-T5M10, Chart 5) and unlabeled chlorin dyad
(ZnC-Dyad, Chart 4) are shown in Figure 1 (left traces). The
spectrum of the monocation of the monomer exhibits a
three-line pattern due to hyperfine interactions with the
two H atoms [I(1H)=1/2] on the pyrroline ring. In contrast,
the spectrum of the monocation of the dyad exhibits a (poorly
resolved) five-line pattern indicative of rapid hole/electron
The synthesis of chlorins55 entails a two-step procedure of
acid-catalyzed condensation of the Eastern half and the
Western half followed by metal-mediated oxidative cycliza-
tion (Scheme 4). The reaction of TMS-ethynylphenyl East-
ern half 8a-OH and labeled Western half 4(13C) gave the
ethynylphenylchlorin bearing a single 13C label [ZnC-Ar5-
PE10(13C)19] in 40% yield. The TMS group was cleaved dur-
ing the reaction of 8a-OH and 4(13C). The analogous reaction
of iodophenyl Eastern half 8b-OH and unlabeled Western
half 461 gave the iodophenylchlorin ZnC-Ar5PI10 in 37%
yield.
The dyad was prepared via Pd-mediated coupling under
copper-free conditions64 of the two chlorin building blocks,
(65) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc.
1996, 118, 11166–11180.
(64) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J. S. Chem. Mater.
1999, 11, 2974–2983.
(66) Strachan, J.-P.; O’Shea, D. F.; Balasubramanian, T.; Lindsey, J. S.
J. Org. Chem. 2000, 65, 3160–3172.
3198 J. Org. Chem. Vol. 75, No. 10, 2010

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FIGURE 2. Room temperature EPR spectra of the monocations of
the 13C-labeled chlorin monomer and 13C-labeled chlorin dyad.
FIGURE 1. Room-temperature EPR spectra of the monocations
of the unlabeled chlorin monomer and unlabeled chlorin dyad.
dyads establish that the time scale for hole/electron transfer
is in the 4-7 ns range.
The time scale for the hole/electron-transfer process in the
monocations of the prototypical chlorin dyad (4-7 ns) is
significantly longer than that measured (via optical techni-
ques) for the analogous process in the monocation of a
prototypical porphyrin dyad (∼0.8 ns) that contains the
same linker (diphenylethyne) and linker attachment site
(meso carbon position).67-69 As noted in the Introduction,
a key difference between porphyrins (with the types of meso
substituents present on the monomers and dyads investi-
gated herein) and chlorins is that the HOMO in the former
molecule is a2u, whereas the HOMO in the latter is the
a1u-like a2 orbital. Thus, the hole/electron density at the site
of linker attachment is much larger for the porphyrin dyad
than the chlorin dyad (Chart 3). The resulting larger through-
bond electronic coupling in the porphyrin versus chlorin
dyads is consistent with the faster rates of hole/electron
transfer in the monocations of the former dyads. In this
connection, previous studies of excited-state energy trans-
fer in neutral Zn-free base analogues of the bis-Zn porphy-
rin and chlorin dyads have shown that the time constant for
the energy-transfer process is ∼5-fold shorter in the por-
phyrin (24 ps) versus chlorin (110 ps) dyads.56 Accordingly,
the relative time scales for excited-state energy transfer in
the neutral porphyrin versus chlorin dyads are in a similar
range as those for ground-state hole/electron transfer in the
monocations of the same dyads.
CHART 5. Unlabeled Chlorin Monomer
transfer on the time scale of the H-atom hyperfine clock.
Simulations of the EPR spectra of the monocations of
the monomer and dyad are also shown in Figure 1 (right
traces). The spectrum of the monocation of the monomer is
1
fit with an H hyperfine coupling of ∼6.2 G. This value is
comparable to that measured previously for the 1H hyper-
fine coupling in monocations of other types of chlorins.21
Simulations of the EPR spectrum of the monocation of the
dyad are reasonably well accounted for (see below for addi-
tional discussion) with a rate constant for hole/electron
transfer, kHT ∼ 2.6 ꢀ108 s-1 (time constant, τHT ∼ 3.8 ns).
The EPR spectra of the monocations of the R-13C-labeled
chlorin monomer ZnC-Ar5PE10(13C)19 and labeled dyad
ZnC-Dyad-(13C)19 are shown in Figure 2 (left traces). The
spectrum of the monocation of the monomer exhibits addi-
tional structure due to hyperfine interactions with the single
13C [I(13C) = 1/2] nucleus. The monocation of the labeled
dyad exhibits a line shape that is less well resolved than that
of the unlabeled dyad. Simulations of the EPR spectra of the
monocations of the 13C-labeled monomer and dyad are also
shown in Figure 2 (right traces). The spectrum of the mono-
cation of the monomer is fit with a 13C hyperfine coupling of
∼4.8 G in addition to the 1H hyperfine coupling of ∼6.2 G.
Simulations of the EPR spectrum of the monocation of the
dyad are reasonably well accounted for with a rate constant
for hole/electron transfer, kHT ∼ 1.4ꢀ108 s-1 (time constant,
τHT ∼ 7.1 ns). Attempts to obtain a more self-consistent fit of
the EPR spectra of the monocations of the labeled versus
unlabeled dyads were unsuccessful. The simulations shown
represent the best compromise for self-consistent fits of the
spectra of the two dyads. Regardless, the collective simula-
tions of the EPR spectra of both the labeled and unlabeled
Summary and Outlook
The studies reported herein establish the methodology for
preparation of chlorins that contain a 13C label selectively
placed at the 19-position of the macrocycle. EPR studies of
the monocations of both the labeled and unlabeled dyads
establish that the time constant for hole/electron transfer is
in the 4-7 ns range. This general approach can be extended
to chlorin analogues, particularly 17-oxochlorins.70 The
(67) Song, H.-E.; Taniguchi, M.; Kirmaier, C.; Bocian, D. F.; Lindsey,
J. S.; Holten, D. Photochem. Photobiol. 2009, 85, 693–704.
(68) Song, H.-E.; Kirmaier, C.; Taniguchi, M.; Diers, J. R.; Bocian, D. F.;
Lindsey, J. S.; Holten, D. J. Am. Chem. Soc. 2008, 130, 15636–15648.
(69) Song, H.-E.; Kirmaier, C.; Diers, J. R.; Lindsey, J. S.; Bocian, D. F.;
Holten, D. J. Phys. Chem. B 2009, 113, 54–63.
(70) Taniguchi, M.; Kim, H.-J.; Ra, D.; Schwartz, J. K.; Kirmaier, C.;
Hindin, E.; Diers, J. R.; Prathapan, S.; Bocian, D. F.; Holten, D.; Lindsey,
J. S. J. Org. Chem. 2002, 67, 7329–7342.
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Nieves-Bernier et al.
JOCFeatured Article
incorporation of the 13C label is essential for studies of hole/
electron transfer in the 17-oxochlorins because H atoms are
not present on the pyrroline ring of these macrocycles. Thus,
the 1H hyperfine clock that is present in the chlorins (owing
to the methylene at the 17-position) is absent in the oxo-
chlorins. The collective studies of the porphyrins, chlorins,
oxochlorins, and ultimately bacteriochlorins (two saturated
pyrrole rings) will elucidate how the structural features of the
different macrocycles modulate the hole/electron-transfer
rates in the monocations of dyads (and larger arrays) of
these molecular architectures.
3J = 6.6 Hz, 2H), 6.00-6.02 (m, 1H), 6.13-6.16 (m, 1H),
6.71-6.72 (m, 1H), 8.07-8.22 (br, 1H); 13C NMR δ 25.3, 25.7,
75.4 (13C), 107.0, 108.9, 117.9; ESI-MS obsd 141.0615, calcd
141.0619 [(M þ H)þ, M = C5C13H8N2O2].
3,3-Dimethyl-2-nitro-1-(2-pyrrolyl)-5-hexanone-(2-13C) (3(13C)).
Following a general procedure,61 a mixture of 2(13C) (890 mg,
6.4 mmol) and mesityl oxide (0.81 mL, 7.0 mmol) was treated
with DBU (2.92 g, 19.2 mmol). The temperature rose immedi-
ately, and the reaction mixture darkened. The reaction mixture
was stirred at room temperature for 24 h, diluted with ethyl
acetate (100 mL), and washed with water. The organic layer
was dried (Na2SO4) and concentrated. Excess mesityl oxide
was removed under high vacuum. The resulting oil was dis-
solved in a minimal amount of CH2Cl2 and filtered through a
silica pad [ethyl acetate/hexanes (1:3)] to afford a light brown
oil (1.32 g, 87%): 1H NMR (400 MHz) δ 1.12 (s, 3H), 1.25 (s,
3H), 2.15 (s, 3H), 2.41 (AB, J=17.4 Hz, 1H), 2.59 (AB, J=17.4
Hz, 1H), 3.04 (ABX, 3J=2.4 Hz, 2J=15.4 Hz, 1H), 3.35 (ABX,
3J=11.6 Hz, 2J=15.4 Hz, 1H), 5.13 (ddd, 1J(13C-1H)=152
Hz, 3J = 11.6 Hz, 3J = 2.4 Hz, 1H), 5.95-5.99 (m, 1H),
6.08-6.10 (m, 1H), 6.65-6.67 (m, 1H), 8.10-8.13 (br, 1H);
13C NMR δ 24.3, 24.6, 26.8, 32.1, 36.9, 51.6, 94.9 (13C), 107.5,
108.9, 118.0, 126.2, 207.2; ESI-MS obsd 239.1349, calcd
239.1351 [(M þ H)þ, M = C11C13H18N2O3].
Experimental Section
Electrochemistry. The electrochemical measurements were
performed using techniques and instrumentation previously
described.71 The samples were contained in a three-compartment
cell equipped with a Pt wire working electrode, a Pt mesh
counter electrode, and a Ag/Agþ (butyronitrile) reference
electrode. The solvent was CH2Cl2 containing 0.1 M Bu4NPF6
as the supporting electrolyte. The bulk-oxidized complexes
were prepared in a glovebox via quantitative coulometric bulk
electrolysis at a Pt mesh electrode. The integrity of the samples
was checked by cyclic voltammetry after each successive oxida-
tion. Upon oxidation, the samples were transferred to an EPR
tube and sealed in the glovebox.
EPR Spectroscopy and Simulations. 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. The microwave power and magnetic field modulation
amplitude were typically 5.7 mW and 0.32 G, respectively.
The EPR spectra of the monocations of the chlorin mono-
mers were simulated with the WinSim program.72 The hyperfine
coupling constants obtained from these simulations were then
used as parameters in the simulations of the EPR spectra of the
monocations of the dyads. The hole/electron-transfer rates for
the monocations of the dyads were obtained from these simula-
tions using the ESR-EXN program.73
2,3,4,5-Tetrahydro-1,3,3-trimethyldipyrrin-(4-13C) (4(13C)).
Following a general procedure,61 a stirred suspension of HCO-
ONH4 (4.73 g, 75.0 mmol) and nitrohexanone 3(13C) (0.715 g,
3.00 mmol) in THF (12 mL) was treated in one portion with
zinc dust (9.76 g, 75.0 mmol). The resulting mixture was stirred
vigorously at room temperature. After 2 h, ethyl acetate (40 mL)
was added, and the mixture was filtered. The filter cake was
washed with ethyl acetate. The filtrate was washed (water,
brine) and dried (Na2SO4). The solvent was concentrated to
afford a light brown oil, which slowly solidified upon standing.
The crude product (1.65 g) was chromatographed (silica, ethyl
acetate) to afford a light brown oil which solidified to a pale
yellow-orange solid (0.345 g, 60%): mp 55-56 °C (lit.61 mp
1
53-54 °C for 4); H NMR (300 MHz) δ 0.94 (s, 3H), 1.11 (s,
3H), 2.03 (s, 3H), 2.27 (AB, J=16.8 Hz, 1H), 2.37 (AB, J=16.8
3
2
Hz, 1H), 2.57 (ABX, J = 11.6 Hz, J = 14.8 Hz, 1H), 2.77
(ABX, 3J=3.2 Hz, 2J=14.8 Hz, 1H), 3.62 (dm, 1J(13C-1H)=
138 Hz, 1H), 5.92-5.94 (m, 1H), 6.08-6.11 (m, 1H), 6.69-6.71
(m, 1H), 9.75-9.83 (br, 1H); 13C NMR δ 20.5, 22.8, 27.3, 28.2,
41.7, 54.4, 80.3 (13C), 105.2, 107.3, 116.4, 131.7, 174.2; ESI-MS
obsd 191.1505, calcd 191.1503 [(M þ H)þ, M=C11C13H18N2].
S-2-Pyridyl 3,5-Di-tert-butylbenzothioate (6).55 Following a
general procedure,58 a solution of 2-mercaptopyridine (2.78 g,
25.0 mmol) in THF (10 mL) was treated with 3,5-di-tert-butyl-
benzoyl chloride (6.31 g, 25.0 mmol) in THF (15 mL) over 30 min.
The organic phase was isolated, washed with water, and dried
(Na2SO4). The solvent was removed to afford a yellow solid,
which upon recrystallization (hexanes) afforded a yellow solid
2-(2-Nitroethyl)pyrrole-(2-13C) (2(13C)). Following a general
procedure,61 a stirred solution of acetic acid (0.69 mL, 12 mmol)
in methanol (1.3 mL) under argon at 0 °C was treated dropwise
with n-propylamine (0.90 mL, 11 mmol). The resulting n-propyl-
ammonium acetate mixture was stirred at 0 °C for 5 min and
then added dropwise to a stirred solution of 1 (1.90 g, 20.0 mmol)
in 13C-nitromethane (99% 13C; 1.10 mL, 20.0 mmol) at 0 °C. The
resulting mixture was stirred at 0 °C. After 15 min, the cooling
bath was removed, and stirring was continued at room tem-
perature. The color changed from yellow to dark orange during
the course of the reaction. After 2 h, CHCl3 (100 mL) was
added, and the organic phase was washed with water and brine.
Then 2-propanol (33 mL) was added to the crude mixture of
2-(2-nitrovinyl)pyrrole, to which silica (18 g) was added. The
mixture was stirred vigorously, and NaBH4 (1.51 g, 40.0 mmol)
was added in one batch. After 1 h, the mixture was filtered. The
filter cake was washed with CH2Cl2. The filtrate was concen-
trated, and the resulting dark oil was filtered through a silica
pad (CH2Cl2) to afford an orange oil (890 mg, 32%): 1H NMR
(300 MHz) δ 3.30-3.34 (m, 2H), 4.61 (dt, 1J(13C-1H) =147 Hz,
1
(5.10 g, 62%): mp 73-77 °C (lit.55 mp 73-74 °C); H NMR
(400 MHz) δ 1.37 (s, 18H), 7.32-7.35 (m, 1H), 7.69-7.70 (m,
1H), 7.74-7.82 (m, 2H), 7.86-7.88 (m, 2H), 8.67-8.69 (m,
1H); 13C NMR δ 32.0, 35.7, 122.6, 124.2, 128.9, 131.4, 136.9,
137.7, 151.1, 152.3, 152.5, 190.8. Anal. Calcd for C20H25NOS:
C, 73.35; H, 7.69; N, 4.28. Found: C, 73.46; H, 7.69; N, 4.28.
10-(Dibutylboryl)-1-(3,5-di-tert-butylbenzoyl)-5-[4-[2-(trimethyl-
silyl)ethynyl]phenyl]dipyrromethane (7a). Following a general
procedure,59 EtMgBr (10.0 mL, 1.0 M in THF) was added to a
solution of 5a (1.59 g, 5.00 mmol) in dry THF (10 mL) at room
temperature under argon. The mixture was stirred at room
temperature for 10 min and then cooled to -78 °C. A solution
of 6 (1.64 g, 5.00 mmol) in dry THF (10 mL) was added. The
reaction mixture was maintained at -78 °C for 10 min, and
then the cooling bath was removed. After 1 h, the reaction
was quenched with 50 mL of saturated aqueous NH4Cl. The
(71) 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.
(72) (a) Duling, D. R. J. Magn. Reson. B 1994, 104, 105–110. (b) http://
www.niehs.nih.gov/research/resources/software/tools/index.cfm (accession date
03/18/2010).
(73) (a) Heinzer, J. Mol. Phys. 1971, 22, 167–177. (b) Heinzer, J. Quantum
Chemistry Program Exchange, No. 209, 1972.
3200 J. Org. Chem. Vol. 75, No. 10, 2010

Nieves-Bernier et al.
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reaction mixture was extracted with ethyl acetate. The organic
extract was washed with water, dried (Na2SO4), and concen-
trated under reduced pressure to give a dark foam. The crude
product was dissolved in CH2Cl2 (10 mL) and then treated with
TEA (1.67 mL, 12.0 mmol) followed by Bu2BOTf (10.0 mL, 1.0 M
in CH2Cl2), with stirring at room temperature. After 1 h, the
mixture was poured onto a pad of silica eluting with hexanes/
CH2Cl2 (1:1). Column chromatography [silica packed with
hexanes/CH2Cl2 (4:1)] afforded a yellow solid (1.55 g, 47%):
mp 80 °C (dec.); 1H NMR (400 MHz) δ 0.25 (s, 9H), 0.46-0.62
(m, 4H), 0.66 (t, J = 7.3 Hz, 3H), 0.74 (t, J = 7.3 Hz, 3H),
0.78-0.99 (m, 4H), 1.06-1.19 (m, 4H), 1.40 (s, 18H), 5.60
(s, 1H), 5.84-5.85 (m, 1H), 6.14-6.16 (m, 1H), 6.41 (d, J =
4.0 Hz, 1H), 6.70-6.72 (m, 1H), 7.20-7.22 (m, 3H), 7.40-7.43
(m, 2H), 7.74 (t, J = 1.8 Hz, 1H), 7.77-7.83 (br, 1H), 8.03 (d,
J=1.8 Hz, 2H); 13C NMR δ 14.12, 14.23, 22.4, 22.6, 26.0, 26.9,
27.2, one of the butyl groups was not observed perhaps owing
to overlap, 31.3, 35.1, 43.9, 94.2, 104.9, 107.9, 108.6, 117.4,
117.7, 118.9, 121.9, 124.0, 128.5, 128.7, 130.0, 131.5, 132.2,
134.5, 141.8, 149.0, 151.9, 177.9. Anal. Calcd for C43H59BN2O-
Si: C, 78.39; H, 9.03; N, 4.25. Found: C, 78.26; H, 9.06; N, 4.24.
10-(Dibutylboryl)-1-(3,5-di-tert-butylbenzoyl)-5-(4-iodophenyl)-
dipyrromethane (7b). Following a general procedure,59 EtMgBr
(20.0 mL, 1.0 M in THF) was added to a solution of 5b (3.48 g,
10.0 mmol) in dry THF (10 mL) at room temperature under
argon. The mixture was stirred at room temperature for 10 min
and then cooled to -78 °C. A solution of 6 (3.27 g, 10.0 mmol)
in dry THF (10 mL) was added. The reaction mixture was
maintained at -78 °C for 10 min, and then the cooling bath was
removed. After 1 h, the reaction was quenched by the addition
of 50 mL of saturated aqueous NH4Cl. The reaction mixture
was extracted with ethyl acetate, washed with water, dried
(Na2SO4), and concentrated under reduced pressure to give a
dark foam. The crude product was dissolved in CH2Cl2 (20 mL)
and then treated with TEA (3.34 mL, 24.0 mmol) followed by
Bu2BOTf (20.0 mL, 20.0 mmol, 1.0 M in CH2Cl2) with stirring
at room temperature. After 1 h, the mixture was poured onto a
pad of silica eluting with hexanes/CH2Cl2 (1:1). Column chro-
matography [silica packed with hexanes/CH2Cl2 (4:1)] afforded
a 13C NMR spectrum could not be acquired due to decompo-
sition in CDCl3, acetone-d6, THF-d8, or CD2Cl2. Anal. Calcd
for C43H58BBrN2OSi: C, 70.01; H, 7.92; N, 3.80. Found: C,
69.87; H, 7.89; N, 3.72.
1-Bromo-10-(dibutylboryl)-9-(3,5-di-tert-butylbenzoyl)-5-(4-
iodophenyl)dipyrromethane (8b). Following a general proce-
dure,58 a solution of 7b (2.07 g, 3.00 mmol) in 30 mL of dry
THF was cooled to -78 °C under argon. NBS (534 mg, 3.00
mmol) was added, and the reaction mixture was stirred for 1 h
at -78 °C. Hexanes (20 mL) and water (20 mL) were added,
and the mixture was allowed to warm to room temperature.
The organic phase was extracted with Et2O and dried (Na2SO4).
The solvent was removed under reduced pressure without heat-
ing. Column chromatography [silica; hexanes/CH2Cl2 (4:1)]
afforded a light brown powder (1.35 g, 59%): mp 55 °C (dec.); 1H
NMR (300 MHz) δ 0.43-0.60 (m, 4H), 0.66 (t, J=7.3 Hz, 3H),
0.75 (t, J=7.3 Hz, 3H), 0.79-0.97 (m, 4H), 1.09-1.15 (m, 4H),
1.40 (s, 18H), 5.48 (s, 1H), 5.75-5.77 (m, 1H), 6.06-6.08 (m,
1H), 6.40 (d, J=4.4 Hz, 1H), 7.00 (d, J=8.1 Hz, 2H), 7.20 (d,
J = 4.0 Hz, 2H), 7.65 (d, J = 8.1 Hz, 2H), 7.75 (t, J = 1.8 Hz,
2H), 8.03 (d, J=1.8 Hz, 1H); a 13C NMR spectrum could not
be acquired due to decomposition in CDCl3, acetone-d6, THF-
d8, or CD2Cl2. Anal. Calcd for C38H49BBrIN2O: C, 59.47; H,
6.44; N, 3.65. Found: C, 59.43; H, 6.47; N, 3.56.
Zn(II)-17,18-Dihydro-10-(4-ethynylphenyl)-18,18-dimethyl-
5-(3,5-di-tert-butylphenyl)porphyrin-(19-13C) (ZnC-Ar5PE10(13C)19).
Following a standard procedure,55 a sample of 8a (737 mg, 1.00
mmol) was reduced with NaBH4 (757 mg, 20.0 mmol) in 10 mL
of anhydrous THF/methanol (3:1). The resulting 8a-OH was
dissolved in 10 mL of anhydrous CH3CN, and then 4(13C) (190 mg,
1.00 mmol) and TFA (78 μL, 1.0 mmol) were added. The reaction
mixture was stirred at room temperature for 30 min, whereupon
551 mg of the crude tetrahydrobilene intermediate was obtained.
Then the crude tetrahydrobilene (215 mg) was treated with AgOTf
(385 mg, 1.50 mmol), Zn(OAc)2 (826 mg, 4.50 mmol), and 2,2,6,6-
tetramethylpiperidine (0.76 mL, 4.5 mmol) in CH3CN (30 mL).
The resulting mixture was refluxed for 18 h. The reaction mixture
was concentrated under reduced pressure. The residue was chro-
matographed [silica, hexanes/CH2Cl2 (2:1)] to afford a blue solid
(84 mg, 40%): 1H NMR (300 MHz) δ 1.49 (s, 18H), 2.02 (s, 6H),
3.26 (s, 1H), 4.50 (s, 2H), 7.72 (t, J=1.5 Hz, 1H), 7.81 (d, J=8.1
Hz, 2H), 7.92 (t, J=1.5 Hz, 2H), 8.03 (d, J=8.1 Hz, 2H), 8.35 (d,
J=4.5 Hz, 1H), 8.45 (d, J=4.5 Hz, 1H), 8.58 (s, 1H), 8.59-8.62
(m, 2H), 8.64 (s, 1H), 8.66 (d, J=4.5 Hz, 1H), 8.72 (d, J=4.5 Hz,
1H); 13C NMR δ30.9, 31.7, 34.7, (d, 44.9 and 45.3), 50.2, 77.8, 83.9,
(d, 94.4 and 95.1), 96.7, 120.8, 122.9, 126.1, 126.86, 126.91, 128.2,
128.9, 129.2, 130.4, 131.4, 131.8, 133.5, 133.8, 141.3, 143.3, 145.7,
146.3, 146.9, 147.7, 148.6, 153.5, 154.1, 159.7, 170.9 (13C); ESI-MS
obsd 691.2713, calcd 691.2734 [(M þ H)þ, M=C43C13H42N4Zn].
Zn(II)-17,18-Dihydro-10-(4-iodophenyl)-18,18-dimethyl-5-(3,5-
di-tert-butylphenyl)porphyrin (ZnC-Ar5PI10). Following a stan-
dard procedure,55 a sample of 8b (767 mg, 1.00 mmol) was reduced
with NaBH4 (757 mg, 20.0 mmol) in 10 mL of anhydrous THF/
methanol (3:1). The resulting 8b-OH was dissolved in 10 mL of
anhydrous CH3CN, and then 4 (190 mg, 1.00 mmol) and TFA
(78 μL, 1.0 mmol) were added. The reaction mixture was stirred at
room temperature for 30 min, and then the reaction mixture was
diluted with 100 mL of CH3CN. AgOTf (771 mg, 3.00 mmol),
Zn(OAc)2 (2.75 g, 15.0 mmol), and 2,2,6,6-tetramethylpiperidine
(2.55 mL, 15.0 mmol) were added. The resulting mixture was re-
fluxed for 18 h. The reaction mixture was concentrated under re-
duced pressure. The residue was chromatographed [silica, hexanes/
CH2Cl2 (2:1)] to afford a blue solid (292 mg, 37%): 1H NMR (300
MHz) δ 1.49 (s,18 H), 2.03 (s, 6H), 4.51 (s, 2H), 7.72 (t, J=1.5 Hz,
1H), 7.80 (d, J=8.1 Hz, 2H), 7.91 (t, J=1.5 Hz, 2H), 8.01 (d, J=
8.1 Hz, 2H), 8.35 (d, J=4.5 Hz, 1H), 8.44 (d, J=4.5 Hz, 1H), 8.58
(s, 1H), 8.61-8.63 (m, 2H), 8.65-8.69 (m, 2H), 8.72 (d, J=4.5 Hz,
1H); 13C NMR δ 30.9, 31.7, 35.0, 45.1, 50.3, 93.5, 94.8, 96.8, 120.8,
1
a yellow solid (2.68 g, 39%): mp 145-146 °C; H NMR (400
MHz) δ 0.34-0.61 (m, 4H), 0.65 (t, J=7.3 Hz, 3H), 0.74 (t, J=
7.3 Hz, 3H), 0.81-0.98 (m, 4H), 1.01-1.20 (m, 4H), 1.40 (s,
18H), 5.54 (s, 1H), 5.83-5.85 (m, 1H), 6.13-6.18 (m, 1H), 6.41
(d, J=4.4 Hz, 1H), 6.70-6.72 (m, 1H), 7.00-7.04 (m, 2H), 7.20
(d, J=4.1 Hz, 1H) 7.62-7.66 (m, 2H), 7.66-7.68 (br, 1H), 7.74
(t, J=1.8 Hz, 1H), 8.02 (d, J=1.9 Hz, 2H); 13C NMR δ 14.10,
14.19, 22.3, 22.6, 25.95, 26.00, 26.9, 27.2, 31.3, 35.1, 43.6, 92.5,
107.9, 108.6, 117.5, 117.6, 118.7, 124.0, 128.8, 129.9, 130.6,
131.3, 134.5, 137.6, 141.2, 148.9, 151.9, 178.0. Anal. Calcd for
C38H50 BIN2O: C, 66.29; H, 7.32; N, 4.07. Found: C, 66.49; H,
7.57; N, 4.04.
1-Bromo-10-(dibutylboryl)-9-(3,5-di-tert-butylbenzoyl)-5-[4-[2-
(trimethylsilyl)ethynyl]phenyl]dipyrromethane (8a). Following
a general procedure,58 a solution of 7a (1.32 g, 2.00 mmol) in
30 mL of dry THF was cooled to -78 °C under argon. NBS
(356 mg, 2.00 mmol) was added, and the reaction mixture was
stirred for 1 h at -78 °C. Hexanes (20 mL) and water (20 mL)
were added, and the mixture was allowed to warm to room
temperature. The organic phase was extracted with Et2O and
dried (Na2SO4). The solvent was removed under reduced pres-
sure without heating. Column chromatography [silica; hexanes/
CH2Cl2 (2:1)] afforded a light brown powder (950 mg, 65%):
mp 55 °C (dec.); 1H NMR (300 MHz) δ 0.24 (s, 9H), 0.46-0.61
(m, 4H), 0.66 (t, J = 7.3 Hz, 3H), 0.75 (t, J = 7.3 Hz, 3H),
0.79-0.98 (m, 4H), 1.01-1.17 (m, 4H), 1.40 (s, 18H), 5.54 (s,
1H), 5.77 (t, J= 2.9 Hz, 1H), 6.07 (t, J =2.9 Hz, 1H), 6.41 (d,
J=4.0, 1H), 7.21 (s, 1H), 7.22 (d, J=8.4 Hz, 2H), 7.46 (d, J=
8.4 Hz, 2H), 7.69 (s, 1H), 7.75 (s, 2H), 8.03 (d, J=1.8 Hz, 1H);
J. Org. Chem. Vol. 75, No. 10, 2010 3201

Nieves-Bernier et al.
JOCFeatured Article
122.4, 126.4, 126.90, 127.03, 128.1, 128.7, 129.2, 132.9, 133.8, 135.2,
135.8, 141.3, 142.1, 145.7, 146.3, 146.9, 147.7, 148.6, 153.6, 154.1,
159.8, 170.9; ESI-MS obsd 792.1639, calcd 792.1662 [(M þ H)þ,
M=C44C13H41IN4Zn].
NMR (300 MHz, toluene-d8) δ 1.53 (s, 36H), 1.88 (s, 12H), 4.17
(s, 4H), 7.95 (t, J=1.7 Hz, 2H), 7.99 (d, J=8.0 Hz, 4H), 8.13 (d,
J = 8.0 Hz, 4H), 8.30 (d, J =1.7 Hz, 4H), 8.43 (s, 2H), 8.45 (s,
2H), 8.57-8.61 (m, 6H), 8.75 (d, J=4.4 Hz, 2H), 8.85 (d, J=4.4
Hz, 2H), 8.98 (d, J=4.4 Hz, 2H); MALDI-MS (matrix of 1,4-
bis(5-phenyloxazol-2-yl)benzene)74 obsd 1355.0, 1354.52 calcd
(C86H82N8Zn2).
10-[4-[2-[4-[17,18-Dihydro-18,18-dimethyl-5-(3,5-di-tert-butyl-
phenyl)porphinatozinc(II)-10-yl]phenyl]ethynyl]phenyl]-17,18-di-
hydro-18,18-dimethyl-5-(3,5-di-tert-butylphenyl)porphinatozinc(II)-
(19-13C) (ZnC-Dyad-(13C)19). Following a reported procedure,56,64
samples of ZnC-Ar5PE10(13C)19 (23.0 mg, 33.0 μmol) and ZnC-
Ar5PI10 (26.3 mg, 33.0 μmol) were coupled using Pd2(dba)3
(4.5 mg, 4.95 μmol) and P(o-tol)3 (11.0 mg, 36.3 μmol) in toluene/
triethylamine (5:1, 12 mL) at 35 °C under argon in a Schlenk
flask. Analytical SEC showed that the reaction had leveled off
after 4 h. The product was purified by a three-column chroma-
tography sequence, without complete evaporation of solvent
from intermediate fractions. Thus, the concentrated crude reac-
tion mixture was passed through a silica column [hexanes/CH2Cl2
(1:1)]. The second purple band was collected and concentrated
to obtain a solution of small volume. Preparative SEC (THF)
gave the compound as the second band. The product solution
was concentrated (but not to dryness), and used directly for
silica gel column chromatography (CH2Cl2). The resulting
product was suspended in methanol, the suspension was soni-
cated and then filtered to afford a purple solid (10 mg, 22%): 1H
Acknowledgment. This work was supported by grants from
the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, of the U.S. Department of
Energy to D.F.B. (DE-FG02-05ER15660), D.H. (DE-FG02-
05ER15661), 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 facility was obtained from the North Carolina
Biotechnology Center and the National Science Foundation.
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.
(74) Srinivasan, N.; Haney, C. A.; Lindsey, J. S.; Zhang, W.; Chait, B. T.
J. Porphyrins Phthalocyanines 1999, 3, 283–291.
3202 J. Org. Chem. Vol. 75, No. 10, 2010