Geometrically homogenous series of covalently linked zinc/free-base porphyrin dimers of varying length; design, synthesis and characterization

Article abstract of DOI:10.1002/ejoc.200600240

Singlet excitation energy transfer, SEET, can be mediated by a bridge, connecting an energy donor and acceptor, via a superexchange mechanism. The mediation efficiency depends on the energy difference between the first excited states of the donor and the bridge, ΔE_DB, as well as the donor-acceptor distance, R_DA. We have previously constructed a series of donor-bridge-acceptor, D-B-A, systems that allowed us to study how SEET depends on ΔE_DB. To expand this study into a second dimension, the distance dependence, a new series of D-B-A systems were constructed. This series was based on the same zinc/free-base porphyrin couple as the donor-acceptor pair in the previous series. Their relative orientation was also retained. In contrast to our first series, the bridges in the latter were of varying length. The bridges were oligo(phenyleneethynylene)phenylene (OPE) structures and the length was systematically changed by increasing the number of phenyleneethynylene units from 1 to 4. To obtain high quality samples, the D-B-A systems were assembled by a building block approach where the zinc and free-base porphyrins were introduced separately using Heck alkynylations. The performance of the OPE structure as a mediator and scaffold is discussed in terms of singlet excited state energies and flexibility. For the first time, when combining the topical D-B-A systems with our previous subset, a homogeneous series of D-B-A systems has been synthesized that allows for studies of both the distance dependence and the energy difference dependence of SEET. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200600240

FULL PAPER
DOI: 10.1002/ejoc.200600240
Geometrically Homogenous Series of Covalently Linked Zinc/Free-Base
Porphyrin Dimers of Varying Length; Design, Synthesis and Characterization
Thomas Ljungdahl,[a] Karin Pettersson,[b] Bo Albinsson,[b] and Jerker Mårtensson*[a]
Keywords: Bridge-mediated coupling / Cross-coupling / Donor–acceptor systems / Porphyrin
Singlet excitation energy transfer, SEET, can be mediated by
a bridge, connecting an energy donor and acceptor, via a
superexchange mechanism. The mediation efficiency de-
pends on the energy difference between the first excited
states of the donor and the bridge, ∆EDB, as well as the do-
nor–acceptor distance, RDA. We have previously constructed
a series of donor–bridge–acceptor, D–B–A, systems that al-
lowed us to study how SEET depends on ∆EDB. To expand
this study into a second dimension, the distance dependence,
a new series of D–B–A systems were constructed. This series
was based on the same zinc/free-base porphyrin couple as
the donor–acceptor pair in the previous series. Their relative
orientation was also retained. In contrast to our first series,
the bridges in the latter were of varying length. The bridges
were oligo(phenyleneethynylene)phenylene (OPE) struc-
tures and the length was systematically changed by increas-
ing the number of phenyleneethynylene units from 1 to 4. To
obtain high quality samples, the D–B–A systems were as-
sembled by a building block approach where the zinc and
free-base porphyrins were introduced separately using Heck
alkynylations. The performance of the OPE structure as a
mediator and scaffold is discussed in terms of singlet excited
state energies and flexibility. For the first time, when combin-
ing the topical D–B–A systems with our previous subset, a
homogeneous series of D–B–A systems has been synthesized
that allows for studies of both the distance dependence and
the energy difference dependence of SEET.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
bridge was therefore explored in the context of the orbital
overlap dependent exchange formalism derived by Dexter[2]
and the superexchange theory derived by McConnell.[3] The
EET was studied with respect to the dependence on bridge
length,[4–12] bridge conformation,[13] and the energy differ-
ences between the excited states of the donor and the
bridge,[14] affecting the overall magnitude of the electronic
donor–acceptor interaction.
Introduction
The covalently linked donor–bridge–acceptor, D–B–A,
systems that have been prepared to study excitation energy
transfer, EET, are numerous. In many of the earlier systems,
the bridge was assumed to be a mere spectator in the trans-
fer process. The systems were used to verify the dependence
of EET on the different parameters that appear in the ex-
pression for EET efficiency derived by Förster[1] based on
a through-space dipole–dipole interaction; e.g. the depen-
dence on distance and relative orientation between the do-
nor and the acceptor and the dependence on overlap be-
tween donor fluorescence and acceptor absorption. How-
ever, the value of the bridge as a mediator of electronic
interaction between the donor and the acceptor, as opposed
to a passive scaffold positioning the donor and acceptor at
a fixed distance and relative orientation, was soon recog-
nized. In latter systems, the mediation capacity of the
Excitation energy transfer according to the exchange
mechanism can be pictured as two simultaneous electron
transfer, ET, processes: (1) ET from the formally singly oc-
cupied LUMO of the photoexcited donor to the empty
LUMO of the acceptor and, (2) transfer of one electron
from the HOMO of the acceptor to the formally singly oc-
cupied HOMO of the donor, thus generating a ground state
donor and an acceptor in its first excited state. In fact, it
has been shown, both experimentally and theoretically, that
triplet EET is related to electron and hole transfer. In ad-
dition, ET via or through a bridge can be viewed as a quan-
tum mechanical tunneling process in which an electron es-
capes from one potential minimum to another separated by
an, in a classical sense, insurmountable potential barrier.
According to the theory for such tunnel effect phenomena,
the probability for ET will depend on the width and the
height of the barrier, where the barrier width is the distance
between donor and acceptor (RDA) and the barrier height
is the energy splitting between the relevant states of the do-
nor and bridge (∆EDB).
[a] Department of Chemical and Biological Engineering/Organic
Chemistry, Chalmers University of Technology,
41296 Göteborg, Sweden
Fax: +46-31-7723657
E-Mail: jerker@chembio.chalmers.se
[b] Department of Chemical and Biological Engineering/Physical
Chemistry, Chalmers University of Technology,
41296 Göteborg, Sweden
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Figure 1. An overview of the two D–B–A series designed for EET studies with the systems featuring the repeating phenylethynyl-unit
being the topical series. The two series share a common member in ZnP-3B-H2P.
Because exchange EET via or through a bridge can be systems is discussed. Then the synthesis of these systems is
pictured as two simultaneous ET processes and ET can be presented. Finally, some important structural features of
treated as an electron tunneling process, it should also be the bridge relevant to EET are discussed.
possible to treat exchange EET as a tunneling phenomenon.
The probability for electron exchange shows exponential
distance dependence as well as inverse dependence on the
barrier height that parallels the dependence expressed in the
Results and Discussion
Dexter and the superexchange formalism for EET in a D–
Design: In our continued efforts to understand the de-
B–A system. The distance dependence of singlet EET, pendence of the exchange type of excitation energy transfer,
SEET, in covalently linked D–B–A systems has been exten- EET, on the medium between the donor and the acceptor,
sively studied.[4–12] Also the inverse dependence on the en- a new subset of porphyrin dimers has been designed to al-
ergy splitting has been studied, although far less compre- low for exploration of the impact of the energy barrier
hensively. However, to the best of our knowledge, no re- width on such transfer. The new subset of donor–bridge–
ports have appeared in the literature where the dependence acceptor, D–B–A, systems are composed of a zinc 15-aryl-
of SEET on both these parameters have been studied in a 2,3,7,8,12,13,17,18-octaalkylporphyrin as the donor (ZnP)
single homogeneous series of D–B–A systems in which both and the corresponding free-base porphyrin as the acceptor
have been varied systematically.
(H2P). The bridges (nB) linking them together via the 5-
We have previously reported the synthesis and SEET positions are oligo(phenyleneethynylene)phenylenes (OPEs)
studies of D–B–A systems (ZnP-RB-H2P, see Figure 1) in of different lengths. The bridges vary from a total of two to
which the donor–acceptor distances were kept the same, but five spacing phenylene units, 2B, 3B, 4B and 5B, respec-
the energy splitting between the donor and the bridge, i.e. tively (Figure 1). As a general design principle for the new
the barrier height, was varied systematically. Now, we are D–B–A systems, to allow for conclusive interpretations of
interested in studying the distance, i.e. the barrier length, the data from the photophysical measurements, the varia-
dependence in systems based on the same donor–acceptor tions in all other parameters known to affect the EET pro-
couple and with the same relative donor–acceptor orienta- cess than these under scrutiny are kept to a minimum.
tion to enable easy and productive comparison with our Therefore, the D–B–A systems are constructed in such a
previous results. Therefore, we would like to extend our pre- way that (1) the relative donor–acceptor orientation is inde-
vious series by varying the bridge length with the prospect pendent of the bridge, and (2) simple π-conjugation be-
to explore the interplay between the effects of distance and tween the three components; donor, bridge, and acceptor,
energy splitting in a single series of homologous D–B–A is minimized such as to preserve the spectroscopic identity
systems (ZnP-nB-H2P, see Figure 1).
of the three components in the system. However, it has not
This paper deals with the synthesis and characterization been possible to keep the energy splitting constant through-
of this new subset of D–B–A systems and is organized as out the series of new systems. Therefore, in the systems re-
follows. First, the design of the porphyrin based D–B–A ported in this paper, all parameters relevant to EET other
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Covalently Linked Zinc/Free-Base Porphyrin Dimers
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than the donor–acceptor length and the energy splitting are
kept constant.
To meet these requirements the construction of the sys-
tems is based on the following: Firstly, because of the use of
triple bonds in the bridge structure, the relative orientation
between the donor and acceptor is the same in the different
systems. Quantum mechanical calculations show that the
angular distribution between the porphyrin planes and the
central unit are almost uniform. The low rotational barrier
calculated for rotation around a triple bond (Ͻ1 kcal/mol)
in the bridge structures indicates that at room temperature
all relative orientations of the porphyrin planes are energeti-
cally attainable and that fast interconversion between all
possible conformations is to be expected.[14] Secondly, the
π-conjugation between the components is minimized be-
cause of the steric effects imposed by the β-methyl groups
closest to the bridge. This is shown by the conservation of
the absorption spectra of the components in the assembly
compared to the ones of the “free” components. Finally,
the use of the phenyleneethynylene motif allows for a sub-
stantial variation in barrier width without affecting the rela-
tive donor–acceptor orientation. The donor–acceptor cen-
ter-to-centre distances have been calculated to be 19.7, 26.5,
Scheme 1. (a) 2 mol-% [Pd2(dba)3·CHCl3], 4 equiv. PPh3 per Pd,
4 mol-% CuI, NEt3, room temp., 1 h. (b) i. 1 equiv. of TIPS-acetyl-
ene, 2 mol-% [Pd2(dba)3·CHCl3], 4 equiv. PPh3 per Pd, 4 mol-%
CuI, NEt3, room temp., overnight; ii. 1 equiv. aqueous NaOH (1 )
in THF/EtOH, 1:1, room temp., 1 h.
33.4, and 40.3 Å for the 2B-, 3B-, 4B-, and 5B-bridged di- ges. The bridge was elongated by one phenyleneethynylene
mers, respectively.[15,16]
unit in a two step sequence; the sequence starts with a stan-
Synthesis: The D–B–A systems were assembled from dard Sonogashira cross-coupling to the product from the
three intermediates or building blocks: (1) the donor build- very first step (TMS-ethynyl-1B-I), followed by removal of
ing block in the form of an iodo functionalized zinc por- the trimethylsilyl group by aqueous sodium hydroxide. This
phyrin, (2) the corresponding free-base porphyrin as the ac- two-step reaction sequence is repeated until a bridge build-
ceptor building block, and (3) an oligo(phenyleneethyny- ing block of the desired length is obtained. To obtain the
lene)ethynyl compound as the bridge building block. To en- terminated bridge building blocks (ethynyl-nB), the bridge
sure the precise metallation state required to keep the pho- building blocks of appropriate lengths were coupled to
tophysical measurements uncompromised, one porphyrin at either iodobenzene or the monofunctionalized 1-iodo-4-
a time was coupled to the bridge building block. To ac- phenylethynyl-benzene (I-2B)[18] following TMS removal,
complish this sequential assembly without concomitant for- see Scheme 2. The terminated bridge building blocks except
mation of troublesome symmetrical porphyrin dimers, the the longest one are either commercially available (ethynyl-
bridge building block had to be protected at one end in the 1B = phenylacetylene) or obtained via literature procedures
first step of the D–B–A assembly.
(ethynyl-2B and ethynyl-3B).[19] No reports on the isolation
Bridge building blocks: The bridge building blocks of ethynyl-4B could be found in the literature and its syn-
(TIPS-ethynyl-nB-ethynyl, see Scheme 1) used in the por- thesis is therefore included in the experimental section.
phyrin dimer synthesis were prepared according to a litera- However, the deprotected ethynyl-4B proved to be very in-
ture procedure and the yields and NMR-data were in ac- soluble and unstable. Therefore, all characterization was
cordance with that reported.[17]
performed on its TIPS-protected precursor (TIPS-ethynyl-
The synthesis commenced with a cross-coupling between 4B) and all subsequent reactions including this compound
1,4-diiodobenzene and trimethylsilylethyne under normal were carried out immediately after deprotection.
Sonogashira reaction conditions yielding a statistical mix-
The bridge reference compounds (nB, see Figure 2) are
ture of non-, mono- and disubstituted products. The mono- either commercially available (2B = tolan) or available via
substituted 1-iodo-4-(trimethylsilylethynyl)benzene (TMS- literature procedures (3B[18] and 4B[20]). Although described
ethynyl-1B-I) was then subjected to a second cross-coupling in the literature,[21] the product 5B has been prepared using
under the same conditions, albeit this time with the triiso- a different methodology and its synthesis is therefore in-
propylsilyl (TIPS) protected alkyne. Subsequent deprotec- cluded in the experimental section.
tion with aqueous sodium hydroxide selectively removed the
Porphyrin compounds: The larger structures comprising
trimethylsilyl protective group affording the monoprotected one or more porphyrin moieties were assembled using a
diethynylbenzene (TIPS-ethynyl-1B-ethynyl). This is used as modified Sonogashira coupling protocol to avoid metalla-
the bridge building block in the synthesis of the donor– tion/transmetallation-problems associated with the copper
bridge–acceptor, D–B–A, system with the second shortest presence.[22] Apart from this, the synthetic route was similar
bridge (ZnP-3B-H2P). It is also the starting point for the to that of the bridge building blocks with successive depro-
synthesis of the bridge building blocks for the longer brid- tections and couplings. The assembly of the series of D–B–
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Figure 2. The bridge reference chromophores nB.
A systems, ZnP-nB-H2P, began with a palladium-catalyzed
cross-coupling between the arylic iodide moiety of the por-
phyrin building block and the terminal alkyne of one of the
mono protected bridge building blocks (see Scheme 3).
The conditions used for this coupling were the ones de-
veloped by Wagner et al.[23] featuring a high loading of tri-
phenylarsane and [Pd2(dba)3·CHCl3] together with triethyl-
amine as base and in the absence of cuprous iodide. The
triisopropylsilyl protected alkyne was then deprotected
using fluoride ions followed by a final cross-coupling per-
formed under the same conditions as the previous one. The
first porphyrin coupled to a bridge building block was in
all cases but one pre-metallated with zinc. Consequently the
last porphyrin units coupled to complete the diporphyrins
were in all cases but one a free-base porphyrin. The reversed
order of coupling for one of the systems was based on solu-
Scheme 2. (a) 2 mol-% [Pd2(dba)3·CHCl3], 4 equiv. PPh3 per Pd,
4 mol-% CuI, NEt3, room temp., overnight. (b) 2 equiv. of TBAF
in THF, room temp., 2 h. [ء] Prepared using the same procedure
but with iodobenzene instead of I-2B.
Scheme 3. (a) 15% [Pd2(dba)3·CHCl3], 4 equiv. AsPh3 per Pd, toluene/NEt3, 3:1, 40 °C, 5–18 h (b) i. TBAF, THF, room temp., 30 min;
ii. 15% [Pd2(dba)3·CHCl3], AsPh3 4 equiv. per Pd, ZnP-1B-I or H2P-1B-I, toluene/NEt3, 3:1, 40 °C, 5–18 h, [ء] H2P-1B-ethynyl-TMS
was produced by direct condensation of a TMS-protected p-ethynyl-substituted benzaldehyde in the porphyrin ring synthesis. Therefore
only step (b) in the Scheme is applicable to the synthesis of ZnP-2B-H2P.
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bility and purification considerations. The zinc/free-base di- where.[24] The high purity is most clearly indicated by that
mers were by this approach produced directly in a precise the decay of the singlet excited state of the ZnP donor could
state of metallation. The shortest dimer, ZnP-2B-H2P (x = be fitted to a single exponential decay for all of the newly
0, Scheme 3), was prepared from the cross-coupling of the prepared systems except one. The short lifetime of ZnP-2B-
functionalized porphyrin building blocks ZnP-1B-I and H2P makes the studies of this much more sensitive to trace
H2P-1B-ethynyl using the same modified Sonogashira pro- amounts of impurities and a second exponential decay with
tocol. All of the porphyrin building blocks together with a minute preexponential factor was used to fit the data from
the dimer ZnP-3B-H2P were available from our previous this. A building block approach was used to ensure the in-
work on donor–bridge–acceptor systems.[18] The reference tegrity of the metallation states. The presence of two dis-
1
compound ZnP-3B with x = 1 shown in Scheme 4 was also tinct signals corresponding to the meso protons in the H
available from this work. The other reference compounds, NMR spectrum in all cases and the overall nice fit to a
ZnP-nB, were assembled using the same modified Sonoga- single exponential decay confirmed this. Furthermore, the
shira protocol as the dimers, the only difference being the ground state absorption spectra of the different porphyrinic
use of truncated bridge building blocks (ethynyl-nB) instead compounds were identical to the sum of the three compo-
of difunctionalized ones (see Scheme 4). The coupling was nents, the donor ZnP, the bridge nB, and the acceptor H2P.
performed using a pre-metallated iodo-functionalized por- This also indicates that the design was successful in pre-
phyrin in all cases but one. For the shortest bridge, x = 0, venting π-conjugation between the bridge and the donor or
metallation of the porphyrin was performed in the last step acceptor.
due to purification advantages. Regardless of in which order
The difference between the previous system ZnP-RB-
the synthesis was performed, the zinc insertion produced H2P and the topical system ZnP-nB-H2P lies in the nature
excellent yields.
of the bridges. The energy splitting between the lowest ex-
cited states of the donor and the bridge, ∆EDB, is of crucial
importance in determining the operating mechanisms for
EET (hopping vs. direct). Furthermore, we have previously
shown the bridge mediated EET rate to correlate to the
inverse quadratic energy splitting (∆EDB–2) in the ZnP-RB-
H2P series.[14,25] All of the new bridges have higher lying
first excited states than the donor. If the energies for the S0
Ǟ S1 transitions in the topical bridges are taken as the mid-
point between the corresponding absorption and emission
bands, as indicated in Figure 3, then ∆EDB for 2B, 3B, 4B,
and 5B is 15 800, 11 600, 9 700, and 8 800 cm–1, respec-
tively. The differences in energies are all large enough to
prevent a hopping mechanism for the energy transfer, and
superexchange is the only bridge dependent mechanism to
be expected. Thus, these systems are well suited for studying
bridge mediation effects, but the variation in ∆EDB ac-
companying the variation in length has to be considered
when evaluating the data.
To reveal the bridge mediating effects, the shares of EET
accounted for by bridge dependent through-bond and
bridge independent through-space mechanisms have to be
established. Singlet–singlet EET can be efficiently transfer-
Scheme 4. (a) 15% [Pd2(dba)3·CHCl3], 4 equiv. AsPh3 per Pd, tolu-
ene/NEt3, 3:1, 40 °C, 5–18 h. (b) Zn(OAc)2·2H2O/MeOH, CHCl3,
room temp., 2 h.
Characterization: The H and 13C NMR spectroscopic red through-space over long distances without the aid of
data were in accordance with what would be expected. any bridge via a Coulombic dipole–dipole interaction, i.e.
Furthermore, HRMS confirmed that the desired products the via the Förster mechanism. A higher EET rate than
were obtained. Spectra from 13C NMR and FAB+ HRMS what would be expected from the calculated Förster rate
are provided in the Supporting Information. The purity of could in these systems be attributed to a “mediation effect”
the products obtained via the present protocol were very of the bridges, i.e. to a superexchange/through-bond mecha-
good as judged from NMR spectroscopy. This was also nism. To calculate the rate constant for the bridge mediated
confirmed by more sensitive methods, i.e. steady-state and EET, kmed, the rate constant, kFör, for EET according to
time-resolved spectroscopy. Both methods were employed Förster [Equation (1)] is subtracted from the experimental
to characterize the systems further and to reveal their EET rate constant k for EET, calculated from the measured life-
properties. The photophysical studies in combination with times of the excited donor.
1
quantum chemical studies, and a thorough discussions on
the interplay between the effects of length and the energy
difference between the donor and the bridge, ∆EDB, on the
superexchange mechanism for energy transfer is given else-
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the β values obtained by fitting a linear function to the plot
of the natural logarithm of the transfer rates vs. either Rcc
or Ree is small for these systems. Therefore, in the subse-
quent discussion only the centre-to-centre distances will be
used to avoid the discussion to become unnecessarily com-
plex.
The donor–acceptor distance is a crucial factor in de-
termining the rate of EET transfer by either the through-
space or the through-bond mechanisms. In this new subset
of molecules studied, viz. ZnP-2B-H2P, ZnP-3B-H2P, ZnP-
4B-H2P, and ZnP-5B-H2P (Figure 1), the DFT-calculated
centre-to-centre distances vary from 19 to 40 Å.[24] These
values give good estimations of the donor-acceptor dis-
tances, but their validity in calculations using the Förster
theory depend on the rigidity of the OPE structural ele-
ments. Oligo(phenyleneethynylene) is often considered as a
rigid structure and we have also chosen to treat it as such.
However, NMR-studies reported in the literature suggest a
relatively high flexibility in these kinds of structures.[26] Be-
cause the donor–acceptor centre-to-centre distance Rcc is
one of the dominating parameters governing EET via the
Förster mechanism, it thus seems appropriate to discuss the
potential variation in Rcc due to conformational changes
and its effect on the EET. There are several ways to estimate
the dynamic average donor–acceptor distances. NMR-stud-
ies[26] have been used and extensive computational model-
ling could be an alternative. However, it is also possible to
use the data that we have from our previous studies on the
ZnP-RB-H2P system (see Figure 1) if a few simplifications
are made. Furthermore, because one of the members of the
ZnP-nB-H2P series, ZnP-2B-H2P, is very similar to the di-
porphyrin used in the NMR-study performed by Bothner-
By et al.,[26] a comparison to check how well the estimates
from the different approaches match is possible. Our esti-
mates are derived from a small discrepancy between the ex-
perimentally determined SEET rate constant for the ZnP-
OB-H2P system (see Figure 1) and the one calculated using
the Förster approximation (vide supra).[14] For a full de-
scription of the model used to obtain dynamic average do-
nor–acceptor distances for the members of the ZnP-nB-
H2P series, see Supporting Information.
Figure 3. Normalized absorption spectra for the four bridge mole-
cules and the ZnP donor in toluene. The energy difference, ∆EDB
,
between the S0 Ǟ S1 transition for the donor and the bridges are
indicated by vertical bars.
The quantum yield, ΦD, and the lifetime, τD, of the donor
are both defined in absence of the acceptor. NA is Avo-
gadros number, n the refractive index of the solvent and Rcc
the centre-to-centre distance between donor and acceptor.
The orientation factor, κ2, describes the relative orientation
of the donor and acceptor transition moments, i.e. the two
interacting dipoles. It has been shown that κ2 varies be-
tween 2/3 and 1 for the different configurations of the por-
phyrin planes of this particular donor–acceptor system.
Therefore, the dynamic average of 5/6 is used for κ2 in the
calculations. Further, J is the Förster spectral overlap inte-
gral defined as [Equation (2)].
The through-bond electronic coupling for the ZnP-OB-
H2P system is so small that it does not result in any detect-
able triplet EET.[27] Because triplet EET is governed by or-
bital overlap dependent mechanisms, this result indicates
that the only available SEET mechanism for this system is
the Förster mechanism. Consequently, any rate enhance-
ment compared to the rate constant calculated according
to the Förster theory could be assumed to be an effect of
conformational dynamics. From these data, we can calcu-
late the dynamic average donor–acceptor distance for the
ZnP-OB-H2P system if the following three assumptions are
made: (1) The discrepancy between the experimental and
(3) the calculated rate constants is not caused by the use of
any erroneous numerical values of the parameters in the
where FD(λ) is the fully corrected fluorescence intensity of
the donor, and εA(λ) is the acceptor molar absorptivity.
In distance dependence studies where the bridge is ex-
pected to mediate the exchange interaction, i.e. via superex-
change,[3] it is customary to evaluate the bridge efficiency
as a mediator or conduit by calculating its resistance or
attenuation factor. The electronic coupling for superex-
change is assumed to approximately decay exponentially
with distance, thus the rate constant is generally described
by the following expression [Equation (3)].
kex = A exp(–βRcc)
where β is the attenuation factor. The donor–acceptor dis- calculation of the Förster rate constant. (2) There are no
tance used in this approximation is usually the edge-to-edge changes in numerical values of these parameters, other than
distance, Ree. However the difference in magnitude between in the donor–acceptor distance, due to the structural dy-
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namics. (3) The through-bond coupling is negligible. In observations, because of the smaller energy differences,
other words, the EET rate enhancement is due exclusively ∆EDB, between the new bridges and the donor, compared
to a reduction of the donor–acceptor distance. The third to that of the previously mentioned OB-bridge
assumption is supported by several structural features of (17600 cm–1). This observation is also supported by the
the OB-bridge disfavoring its efficacy as conduit for elec- work of Lindsey et al. who have reported that EET is the
tronic coupling; the s-cis conformation into which the bicy- major deactivation pathway for systems similar to ZnP-2B-
cloctane moiety is looked,[13] the clear separation of the H2P, with an even larger mediation contribution.[29]
coupling pathway into two π-systems separated by a σ-skel-
When the somewhat shorter dynamic average donor–ac-
eton,[28] and its high singlet excitation energy compared to ceptor distances are used in recalculations of the through-
that of the donor.[14] The absence of triplet EET in the ZnP- space transfer rates and mediation contributions, a constant
OB-H2P system clearly reflects this insulating property.[27]
mediation contribution of ca. 53% is observed for the two
The dynamic average donor–acceptor distance can then shortest systems, whereas a drop in the mediation contri-
be obtained as the product of the donor–acceptor distance bution to 41% and 26% is seen for ZnP-4B-H2P and ZnP-
in the extended ZnP-OB-H2P system and the sixth root of 5B-H2P, respectively (Supporting Information, Table S2).
the quotient between the calculated Förster rate constant This is an effect of the larger Förster rate constant caused
for the extended conformation and the experimentally de- by the reduction in through-space donor–acceptor distance.
termined one. The donor-acceptor distance for the fully ex- However, it is important to note that the through-bond
tended and energy-optimized conformation of the ZnP- length is not affected, or only marginally so, by conforma-
OB-H2P system is found to be 26.4 Å (DFT B3LYP/6- tional bending, only the through-space distance is affected.
31G*), the EET rate constant in CH2Cl2 was calculated to
When the DFT-calculated centre-to-centre distances are
2.6·108 s–1, and experimentally found to be 3.7·108 s–1.[14] used an attenuation factor β = 0.20 Å–1 is obtained.[24] If
The dynamic average donor-acceptor distance is then calcu- the estimated dynamic average donor–acceptor distances
lated to 24.8 Å.
are used instead the attenuation factor becomes β =
This reasoning can be extrapolated to cover the nB- 0.28 Å–1 with an excellent fit. Although the attenuation fac-
bridged systems if we assume the overall bending force con- tor β is system specific and depends on system parameters
stants for the OB and the 3B bridges to be identical. Fur- such as ∆EDB, it has often been regarded as a measure of
ther, if the bridges are pictured as rods consisting of a series how efficient a specific class of bridge act as conduit. A
of identical structural elements all characterized by the comparison could therefore be informative and give at least
same bending force constant, then the overall bending force some qualitative measure of how efficient mediator the
constant is equal to the bending force constants for the con- bridge type is compared to others. Attenuation factors for
stituent structural elements divided by the number of ele- EET along different conjugated bridges have been reported
ments. The number of structural elements is arbitrarily cho- by other groups; β = 0.32 Å–1 and β = 0.33 Å–1 for oligo-
sen as the number of atoms of the bridges, including the (phenylene) bridges,[9,10] β = 0.17 for alkyne bridges,[11] and
meso carbons that lie on the line connecting the porphyrin β = 0.11 Å–1 for dialkoxy-substituted oligo(p-phenylene-
centres, i.e. the contribution from the aromatic rings and ethynylene) bridges.[12] The obtained β value for SEET via
bicyclooctane is neglected. And finally, the available energy oligo(phenyleneethynylene) thus indicates that this bridge
for bending motion is, at a given temperature, the same for type is a better conduit in this context than oligo(pheny-
all systems. According to this model, the reduction in do- lene) and less efficient than oligo(ethynylene) or its dialk-
nor–acceptor distances increases with increasing bridge oxy-substituted counterpart. The β-values of the OPE motif
length. The reduction in donor–acceptor distances are be- fits in at the same position in this general pattern either if
tween 4–9% of the centre-to-centre distances estimated the DFT-calculated distances or the dynamic average esti-
from the geometry optimized structures. There are large un- mates are used.
certainties in the calculated dynamic average donor–ac-
Despite a proven significant mediation contribution, a
ceptor distances, because of the minute difference between slope of –5.9 was obtained by linear fitting to the plot of
the experimental and the calculated transfer rate upon lnk vs. lnRcc when the DFT-calculated donor–acceptor dis-
which the model is based. However, it is worth noting that tances were used.[24] When the recalculated values using the
through this simplistic treatment of the structural dynamics dynamic average donor–acceptor distances are used, a slope
we obtain a dynamic average bending of 28° for the ZnP- of –6.5 is obtained. Regardless of the distance used, the
2B-H2P system, in very good agreement with the 26° found value is close to –6.0, the theoretical value for dipole–dipole
by the elegant NMR studies reported by Bothner-By et coupling as described by the Förster equation, often used
al.[26]
as an indicator for through-space transfer. It is also worth
For all members of the ZnP-nB-H2P series, the rate con- noting that if the DFT-calculated centre-to-centre distances
stant for mediation, kmed – the difference between the k and are used in the evaluations, the share of through-space
kFör – explains more than 60% of the donor fluorescence transfer is smaller, but the slope is closer to the theoretical
quenching when the DFT-calculated centre-to-centre dis- value for dipole–dipole coupling as described by the Förster
tances are used (Supporting Information, Table S2).[24] equation. This shows that fitting lnk vs. lnR and taking the
Such a rate enhancement caused by a superexchange-medi- slope of –6 as a proof for the Förster mechanism can be
ated through-bond contribution is in line with our previous misleading.
Eur. J. Org. Chem. 2006, 3087–3096
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T. Ljungdahl, K. Pettersson, B. Albinsson, J. Mårtensson
FULL PAPER
ArH), 7.54 (m, 2 H, ArH) ppm; elemental analysis calcd. (%) for
C41H38Si (558.8); C 88.12 H 6.85; found: C 87.94, H 6.81.
Conclusions
A series of zinc/free-base porphyrin dimers have been
synthesized for the purpose of excitation energy transfer,
EET, studies. A building block approach using palladium-
catalyzed alkynylations gave the diporphyrins of high purity
Ethynyl-4B: A sample of TIPS-ethynyl-4B (73 mg, 130 µmol) was
dissolved in THF (5 mL). Bu4NF (260 µmol, 260 µL of a 1.0 
solution in THF) was added and the mixture was stirred at room
temp. for 2 h. The product ethynyl-4B is highly insoluble in almost
in a precise state of metallation and in good yields. The all organic media and precipitates rapidly. The white solid was sep-
arated by means of centrifugation and was used directly in the syn-
thesis of ZnP-5B without further purification and characterization
due to its instability and exceedingly poor solubility.
donor–bridge–acceptor, D–B–A, systems were designed to
enable delineation of the impact of critical geometric and
electronic parameters on the EET transfer rate. The OPE
structural element of the bridge allowed a stepwise increase
in length, with a limited decrease in energy of the first ex-
cited singlet state, thus allowing studies of a mediating ef-
fect in the absence of a hopping mechanism. This structural
element was shown to be somewhat flexible, but stiff
enough to give comparable results when the evaluations
were made using dynamic average or DFT-calculated do-
nor–acceptor distances. Estimates of the dynamic average
donor–acceptor distances were obtained from a simple me-
chanical model of the bridge, where the bending force con-
stant was derived from a difference in a calculated and ob-
served EET transfer rate.
5B: [Pd2(dba)3·CHCl3] (5 mg, 5 µmol) was added to a deaerated
solution of ethynyl-3B (79 mg, 0.261 mmol), I-2B (79 mg,
0.261 mmol), PPh3 (10 mg, 40 µmol) and CuI (1.9 mg, 10 µmol) in
toluene/NEt3, 1:1 (10 mL). The reaction mixture was stirred over-
night at room temp. The solvent was evaporated and the residue
triturated with methanol to remove unwanted material. The re-
maining residue was triturated with PhCl using a soxhlet apparatus.
The pure product 5B crystallized directly from the solution upon
cooling to room temperature affording a white crystalline solid
(50 mg, 0.104 mmol, 40%); elemental analysis calcd. (%) for C38H22
(478.6); C 95.37 H 4.63; found: C 95.23, H 4.58
ZnP-2B-H2P: [Pd2(dba)3·CHCl3] (6 mg, 6 µmol) was added to a
deaerated solution of H2P-1B-ethynyl (35 mg, 0.046 mmol), ZnP-
1B-I (42 mg, 0.045 mmol) and AsPh3 (15 mg, 48 µmol) in dry tolu-
ene/NEt3 (3:1, 12 mL). The reaction mixture was stirred overnight
at 40 °C and the solvent was removed in vacuo. Chromatography
(silica gel, CH2Cl2 followed by 2% MeOH in CH2Cl2) and SEC
gave ZnP-2B-H2P as a red solid (39 mg, 0.025 mmol, 56%); 1H
NMR (CDCl3): δ = –2.39 ppm (br. s, 2 H, NH), 1.52 (s, 18 H, tBu),
1.53 (s, 18 H, tBu), 1.81 (m, 24 H, -CH2CH3), 2.46 (s, 6 H, pyrrole-
CH3), 2.48 (s, 6 H, pyrrole-CH3), 2.63 (s, 6 H, pyrrole-CH3), 2.65
(s, 6 H, pyrrole-CH3), 4.06 (m, 16 H, -CH2CH3), 7.83 (m, 2 H,
ArH), 7.93 (d, 4J = 2 Hz, 2 H, ArH), 7.95 (d, 4J = 2 Hz, 2 H,
ArH), 8.13 (m, 4 H, ArH), 8.21 (m, 4 H, ArH), 10.25 (s, 2 H,
meso), 10.28 (s, 2 H, meso) ppm. HRMS (FAB) calcd. for
[C106H120N8Zn]+: 1568.8927, found: 1568.8944.
Experimental Section
Materials: Diethyl ether, toluene and tetrahydrofuran (THF) were
dried by distillation from sodium/benzophenone under nitrogen.
Dichloromethane and triethylamine were dried by distillation from
calcium hydride under nitrogen. Dried solvents were used immedi-
ately following distillation. Other commercially available reagents
were purchased from Aldrich or Lancaster and used without fur-
ther purification.
Methods: Deaeration of reaction mixtures was achieved by bub-
bling argon through the solution for 30 minutes. Palladium-cata-
lyzed reactions were performed under argon and protected from
light by aluminium foil. Column chromatography and flash
chromatography was generally performed using silica gel (Matrex,
LC 60 Å/35–70 micron). Chromatography of porphyrins was per-
formed using silica gel (Merck, grade 60, 230–400 mesh). Size ex-
clusion chromatography (SEC) was performed on BioRad Bio-Be-
ads SX–3 in toluene. 1H (400 MHz) and 13C (100.6 MHz) NMR
spectra were recorded at room temp. in CDCl3, [D5]chlorobenzene
or [D2]tetrachloroethane, using a Jeol Eclipse 400 NMR spectrome-
ter and a Varian UNITY 400 NMR spectrometer. Chemical shifts
are reported relative to tetramethylsilane (δH = 0 ppm) and all
coupling constants given are for (H,H) couplings. Positive FAB
high resolution mass spectra were obtained on a JEOL SX102 mass
spectrometer at Instrumentstationen, Lund University, Sweden.
Samples were desorbed from a 3-NBA matrix using 6 kV xenon
atoms. Elemental analyses were performed by H. Kolbe Mikroana-
lytisches Laboratorium, Mülheim an der Ruhr, Germany.
ZnP-3B-ethynyl-TIPS: Pd2(dba)3·CHCl3 (8 mg, 8 µmol) was added
to a deaerated solution of ZnP-1B-I (50 mg, 0.054 mmol), TIPS-
ethynyl-2B-ethynyl (31 mg, 0.081 mmol), AsPh3 (20 mg, 65 µmol)
and toluene/NEt3 (16 mL, 3:1). The reaction mixture was stirred
for 5 h at 40 °C and the solvent removed in vacuo. Chromatography
on silica (pentane/diethyl ether, 10:1) followed by SEC gave ZnP-
3B-ethynyl-TIPS as a red solid (40 mg, 0.034 mmol, 63%); 1H
NMR (CDCl3): δ = 1.15 [m, 21 H, Si(iPr)], 1.52 (s, 18 H, tBu),
1.77 (m, 12 H, -CH2CH3), 2.45 (s, 6 H, pyrrole-CH3), 2.51 (s, 6 H,
pyrrole-CH3), 4.01 (m, 8 H, -CH2CH3), 7.50 (m, 4 H, ArH), 7.60
3
3
(d, J = 8 Hz, 2 H, ArH), 7.69 (d, J = 8 Hz, 2 H, ArH), 7.82 (t,
4J = 2 Hz, 1 H, ArH), 7.94 (m, 4 H, ArH), 8.10 (d, 3J = 8 Hz,
2 H, ArH), 10.18 (s, 2 H, meso) ppm. HRMS (FAB) calcd. for
[C79H88N4SiZn]+: 1184.6070, found: 1184.6067.
ZnP-3B-ethynyl:
A sample of ZnP-3B-ethynyl-TIPS (23 mg,
0.021 mmol) was dissolved in THF (3 mL). Bu4NF (25 µmol, 25 µL
of a 1.0  solution in THF) was added and the mixture was stirred
at room temp. for 30 minutes during which the colour turned
darker red. Methanol was added to precipitate the porphyrin and
TIPS-ethynyl-4B: [Pd2(dba)3·CHCl3] (4 mg, 4 µmol) was added to
a
deaerated solution of ethynyl-2B-ethynyl-TIPS (77 mg,
0.200 mmol), I-2B (61 mg, 0.200 mmol), PPh3 (8 mg, 32 µmol) and subsequent centrifugation gave ZnP-3B-ethynyl as a red solid
CuI (1.5 mg, 8 µmol) in NEt3 (15 mL). The reaction mixture was
stirred overnight, filtered and concentrated. Flash chromatography
(SiO2, pentane followed by CH2Cl2) and recrystallization from
EtOAc yielded the desired product, TIPS-ethynyl-4B, as a white
solid (77 mg, 0.138 mmol, 69%); H NMR (CDCl3): δ = 1.13 (m,
21 H, iPr), 7.36 (m, 3 H, ArH), 7.46 (m, 4 H, ArH), 7.52 (m, 8 H,
(17 mg, 0.018 mmol, 86%); The product was used directly in the
synthesis of ZnP-4B-H2P without other characterization than
1
FAB-MS and 1H NMR due to its instability; H NMR (CDCl3): δ
= 1.52 (s, 18 H, tBu), 1.80 (m, 12 H, -CH2CH3), 2.47 (s, 6 H,
1
pyrrole-CH3), 2.54 (s, 6 H, pyrrole-CH3), 3.21 (s, 1 H, alkyne-H),
3
4.03 (m, 8 H, -CH2CH3), 7.53 (m, 4 H, ArH), 7.62 (d, J = 8 Hz,
3094
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Covalently Linked Zinc/Free-Base Porphyrin Dimers
FULL PAPER
3
4
2 H, ArH), 7.71 (d, J = 8 Hz, 2 H, ArH), 7.84 (t, J = 2 Hz, 1 H, tion of the solvent gave ZnP-2B as a red solid (14 mg, 0.015 mmol,
ArH), 7.96 (m, 4 H, ArH), 8.13 (d, 3J = 8 Hz, 2 H, ArH), 10.21 (s, 2
100%); H NMR (CDCl3): δ = 1.51 (s, 18 H, tBu), 1.77 (m, 12 H,
1
H, meso) ppm. HRMS (FAB) calcd. for [C70H68N4Zn]+: 1028.4735, -CH2CH3), 2.44 (s, 6 H, pyrrole-CH3), 2.53 (s, 6 H, pyrrole-CH3),
3
found: 1028.4734.
4.01 (m, 8 H, -CH2CH3), 7.44 (m, 3 H, ArH), 7.72 (dd, J = 8 Hz,
4J = 2 Hz, 2 H, ArH), 7.82 (t, J = 2 Hz, 1 H, ArH), 7.94 (m, 4 H,
4
ZnP-4B-H2P: Prepared from H2P-1B-I (36 mg, 0.042 mmol) and
ZnP-3B-ethynyl (33 mg, 0.032 mmol) as described for ZnP-2B-H2P
and gave ZnP-4B-H2P as a red solid (19 mg, 0.011 mmol, 34%);
1H NMR (CDCl3); δ = –2.43 ppm (br. s, 2 H, NH), 1.51 (s, 18 H,
tBu), 1.52 (s, 18 H, tBu), 1.79 (m, 24 H, -CH2CH3), 2.45 (s, 6 H,
pyrrole-CH3), 2.47 (s, 6 H, pyrrole-CH3), 2.55 (s, 6 H, pyrrole-
CH3), 2.57 (s, 6 H, pyrrole-CH3), 4.03 (m, 16 H, -CH2CH3), 7.67
(m, 4 H, ArH), 7.74 (m, 4 H, ArH), 7.82 (m, 2 H, ArH), 7.92 (d,
ArH), 8.10 (d, 3J = 8 Hz, 2 H, ArH), 10.19 (s, 2 H, meso) ppm.
HRMS (FAB) calcd. for [C60H64N4Zn]+: 904.4422; found:
904.4428.
ZnP-4B: Prepared from ZnP-1B-I (20 mg, 0.022 mmol) and eth-
ynyl-3B (9 mg, 0.030 mmol) as described for ZnP-3B-ethynyl-TIPS
and gave ZnP-4B as a red solid (8 mg, 0.007 mmol, 34%); 1H NMR
(CDCl3): δ = 1.51 (s, 18 H, tBu), 1.78 (m, 12 H, -CH2CH3), 2.44
(s, 6 H, pyrrole-CH3), 2.53 (s, 6 H, pyrrole-CH3), 4.02 (m, 8 H,
4
4J = 2 Hz, 2 H, ArH), 7.94 (d, J = 2 Hz, 2 H, ArH), 7.97 (m, 4
3
-CH2CH3), 7.37 (m, 3 H, ArH), 7.56 (m, 6 H, ArH), 7.62 (d, J =
H, ArH), 8.13 (m, 4 H, ArH), 10.22 (s, 2 H, meso), 10.25 (s, 2 H,
meso) ppm. HRMS (FAB) calcd. for [C122H128N8Zn]+: 1768.9553,
found: 1768.4747.
8 Hz, 2 H, ArH), 7.70 (d, 3J = 8 Hz, 2 H, ArH), 7.82 (t, 4J = 2 Hz,
1 H, ArH), 7.94 (m, 4 H, ArH), 8.11 (d, 3J = 8 Hz, 2 H, ArH),
10.20 (s, 2 H, meso) ppm. HRMS (FAB) calcd. for [C76H72N4-
Zn]+: 1104.5048; found: 1104.5050.
H2P-4B-ethynyl-TIPS: Prepared from H2P-1B-I (100 mg,
0.116 mmol) and TIPS-ethynyl-3B-ethynyl (69 mg, 0.144 mmol) as
described for ZnP-2B-H2P and gave H2P-4B-ethynyl-TIPS as a red
solid (99 mg, 0.081 mmol, 70%); 1H NMR (CDCl3): δ = –2.41 ppm
(br. s, 2 H, NH), 1.14 (m, 21 H, Si(iPr)), 1.51 (s, 18 H, tBu), 1.78
(m, 12 H, -CH2CH3), 2.46 (s, 6 H, pyrrole-CH3), 2.56 (s, 6 H, pyr-
role-CH3), 4.03 (m, 8 H, -CH2CH3), 7.48 (m, 4 H, ArH), 7.54 (m,
ZnP-5B: Prepared from ZnP-1B-I (100 mg, 0.107 mmol) and eth-
ynyl-4B (80 mg, 0.200 mmol) as described for ZnP-ethynyl-3B-
TIPS and gave ZnP-5B as a red solid (18 mg, 0.007 mmol, 14%);
1H NMR (CDCl3): δ = 1.50 (s, 18 H, tBu), 1.82 (m, 12 H,
-CH2CH3), 2.64 (m, 12 H, pyrrole-CH3), 4.04 (m, 8 H, -CH2CH3),
3
3
4 H, ArH), 7.61 (d, 3J = 8 Hz, 2 H, ArH), 7.69 (d, J = 8 Hz, 2 H,
7.42–7.52 (m, 13 H, ArH), 7.56 (d, J = 8 Hz, 2 H, ArH), 7.64 (d,
3J = 8 Hz, 2 H, ArH), 7.95 (m, 3 H, ArH), 8.04 (m, 4 H, ArH),
10.35 (s, 2 H, meso) ppm. HRMS (FAB) calcd. for [C84H76N4-
Zn]+: 1204.5361; found: 1204.5375.
ArH), 7.81 (t, 4J = 2 Hz, 1 H, ArH), 7.92 (d, 4J = 2 Hz, 2 H, ArH),
7.94 (d, 3J = 8 Hz, 2 H, ArH), 8.11 (d, 3J = 8 Hz, 2 H, ArH), 10.24
(s, 2 H, meso) ppm. HRMS (FAB) calcd. for [C87H94N4SiZn]+:
1222.7248, found: 1222.7242.
Supporting Information (see also the footnote on the first page of
this article): 13C NMR spectra and high-resolution mass spectrom-
etry data for all new compounds are available. The observed data
are all in good agreement with what is to be expected.
H2P-4B-ethynyl: Prepared by TIPS deprotection of H2P-4B-eth-
ynyl-TIPS (96 mg, 0.078 mmol) as described for ZnP-3B-ethynyl
and gave H2P-4B-ethynyl as a red solid (83 mg, 0.078 mmol,
100%). The product was used directly in the synthesis of ZnP-5B-
H2P without further purification or other characterization than
FAB-MS due to its instability and exceedingly poor solubility in all
organic solvents. HRMS (FAB) calcd. for [C78H74N4]+: 1066.5913,
found: 1066.5923.
Acknowledgments
This work was supported by grants from the Swedish Research
Council.
ZnP-5B-H2P: Prepared from ZnP-1B-I (79 mg, 0.085 mmol) and
H2P-4B-ethynyl (83 mg, 0.078 mmol) as described for ZnP-2B-H2P
and gave ZnP-5B-H2P as a red solid (61 mg, 0.033 mmol, 45%);
1H NMR ([D5]chlorobenzene): δ = –1.84 ppm (br. s, 1 H, NH),
–1.79 (br. s, 1 H, NH), 1.49 (s, 18 H, tBu), 1.51 (s, 18 H, tBu), 1.81
(m, 24 H, -CH2CH3), 2.63 (m, 12 H, pyrrole-CH3), 2.64 (s, 6 H,
pyrrole-CH3), 2.65 (s, 6 H, pyrrole-CH3), 4.04 (m, 16 H, -CH2CH3),
7.50–7.66 (m, 12 H, ArH), 7.94 (m, 10 H, ArH), 8.05 (m, 4 H,
ArH), 10.36 (s, 2 H, meso), 10.42 (s, 2 H, meso) ppm. HRMS (FAB)
calcd. for [C130H132N8Zn]+: 1868.9866, found: 1868.9881.
[1] T. Förster, Naturwiss. 1946, 33, 166–175.
[2] D. L. Dexter, J. Chem. Phys. 1953, 21, 836–850.
[3] H. M. McConnell, J. Chem. Phys. 1961, 35, 508–515.
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55–69.
H2P-2B: Prepared from H2P-1B-I (50 mg, 0.058 mmol) and phen-
ylacetylene (59 mg, 0.580 mmol) as described for ZnP-2B-H2P and
gave H2P-2B as a red solid (37 mg, 0.044 mmol, 76%); H NMR
[7] A. Osuka, N. Tanabe, S. Kawabata, I. Yamazaki, Y. Nishimura,
J. Org. Chem. 1995, 60, 7177–7185.
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Am. Chem. Soc. 1999, 121, 4207–4214.
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[11] A. Harriman, R. Ziessel, Chem. Commun. 1996, 1707–1716.
[12] A. Harriman, A. Khatyr, R. Ziessel, A. C. Benniston, Angew.
Chem. Int. Ed. 2000, 39, 4287–4290.
1
(CDCl3): δ = –2.43 ppm (br. s, 2 H, NH), 1.51 (s, 18 H, tBu), 1.78
(m, 12 H, -CH2CH3), 2.46 (s, 6 H, pyrrole-CH3), 2.56 (s, 6 H, pyr-
role-CH3), 4.03 (m, 8 H, -CH2CH3), 7.45 (m, 3 H, ArH), 7.71 (m,
4
4
2 H, ArH), 7.81 (t, J = 2 Hz, 1 H, ArH), 7.91 (d, J = 2 Hz, 2 H,
ArH), 7.94 (d, 3J = 8 Hz, 2 H, ArH), 8.10 (d, 3J = 8 Hz, 2 H, ArH),
10.24 (s, 2 H, meso) ppm. HRMS (FAB) calcd. for [C60H66N4]+:
842.5287; found: 842.5270.
ZnP-2B: The free-base porphyrin H2P-2B (13 mg, 0.015 mmol) was
dissolved in 10 mL CHCl3 and Zn(OAc)2·2H2O (10 mg,
0.045 mmol) dissolved in MeOH (1 mL) was added. The reaction
was stirred at room temperature for 2 h. The reaction mixture was
washed twice with 10% NaHCO3 and twice with water. Evapora-
[13] J. Kroon, A. M. Oliver, M. N. Paddon-Row, J. W. Verhoeven,
J. Am. Chem. Soc. 1990, 112, 4868–4873.
[14] K. Kilså, J. Kajanus, J. Mårtensson, B. Albinsson, J. Phys.
Chem. B 1999, 103, 7329–7339.
Eur. J. Org. Chem. 2006, 3087–3096
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T. Ljungdahl, K. Pettersson, B. Albinsson, J. Mårtensson
FULL PAPER
[15] Previously reported to be 25.3 Å but more refined methods
[24] K. Pettersson, A. Kyrychenko, E. Rönnow, T. Ljungdahl, J.
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Received: March 19, 2006
Published Online: May 12, 2006
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