Synthesis and Atropisomerism of Cascaded Tetraphenylporphyrin-[60]Fullerene Hybrids

Article abstract of DOI:10.1002/chem.201501254

Flexible, linked dendritic tetraphenylporphyrin (TPP)-fullerene hybrids were synthesized. They were designed to gain insight into and mimic the primary events in the natural photosynthetic reaction center. These multiporphyrin moieties are based on a light-harvesting concept. Moreover, they incorporate multiple redox components aligned along a redox gradient. Newkome-type dendrons were added to these TPP-fullerene hybrids. In principle they can mediate pH-dependent water solubility, which, however, could not be observed in this case. A protecting-group strategy using tert-butyldiphenylsilyl groups allows convergent synthesis of the dendritic compounds. The dendritic multiporphyrins were synthesized separately and can be used as individual building blocks. Atropisomerism was observed in the dendritic compounds, and single atropisomers could be assigned to the corresponding peaks of a characteristic pattern in the NMR spectra. Deprotection of the Newkome-type dendrons was shown to be feasible under mild conditions that leave the redox gradient intact. Redox gradient: Flexible dendritic porphyrin-fullerene hybrids were synthesized. They comprise a large dendritic redox gradient Fc₉-(ZnP)₃-H₂P-C₆₀ (Fc: ferrocene, ZnP: zinc porphyrin, H₂P: free-base porphyrin, and C₆₀: fullerene), in which multiple porphyrins are embedded for light harvesting (see figure). Porphyrin-fullerene compounds with attached Newkome-type dendrons were also synthesized. They could be deprotected in a mild manner to give dendritic products with free carboxyl groups.

Full text of DOI:10.1002/chem.201501254

DOI: 10.1002/chem.201501254
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&
Porphyrinoids
Synthesis and Atropisomerism of Cascaded
Tetraphenylporphyrin–[60]Fullerene Hybrids
Sebastian Schlundt, Walter Bauer, and Andreas Hirsch*[a]
Abstract: Flexible, linked dendritic tetraphenylporphyrin
(TPP)–fullerene hybrids were synthesized. They were de-
signed to gain insight into and mimic the primary events in
the natural photosynthetic reaction center. These multipor-
phyrin moieties are based on a light-harvesting concept.
Moreover, they incorporate multiple redox components
aligned along a redox gradient. Newkome-type dendrons
were added to these TPP–fullerene hybrids. In principle they
can mediate pH-dependent water solubility, which, however,
could not be observed in this case. A protecting-group strat-
egy using tert-butyldiphenylsilyl groups allows convergent
synthesis of the dendritic compounds. The dendritic multi-
porphyrins were synthesized separately and can be used as
individual building blocks. Atropisomerism was observed in
the dendritic compounds, and single atropisomers could be
assigned to the corresponding peaks of a characteristic pat-
tern in the NMR spectra. Deprotection of the Newkome-type
dendrons was shown to be feasible under mild conditions
that leave the redox gradient intact.
Introduction
Porphyrin–fullerene conjugates are capable of photoinduced
electron transfer and thus can act as synthetic analogues of
natural photosynthesis. The conjugates can be incorporated in
an extended linear redox gradient to enhance their ability for
photoinduced electron transfer. Extended linear redox gradi-
ents were used by Imahori and co-workers[1–6] and Guldi and
co-workers.[7] The conjugates can also be part of a dendritic
molecule to improve light harvesting due to multiple chromo-
phores, which was shown by Gust and co-workers[8] and by Ito
and coworkers.[9] We have contributed porphyrin–fullerene
dyads,[10,11] water-soluble dyads,[12–14] and dendritic TPP–fuller-
ene conjugates (Figure 1).[15] Herein, we report the extension of
our previously reported dendritic TPP–[60]fullerene hybrid 1[15]
(Figure 1) by ferrocene, which resulted in conjugate
2
(Figure 2). Compared to compound 1, conjugate 2 has poten-
tially improved ability for photoinduced electron transfer due
to its larger dendritic gradient (C60-H2P-ZnP-Fc; H2P=free-base
porphyrin, ZnP=zinc porphyrin, Fc=ferrocene). Here we em-
phasize the synthesis and characterization of our compounds.
The architecture of 1 and 2 is summarized as Prototype I
(Figure 3).
Figure 1. (ZnP)3-H2P-C60 1,[15] affiliated with Prototype I.
We also report extension of our dendritic conjugate 1 by
Newkome-type dendrons, which results in compounds 3 and 4
(Figure 4). Conjugate 3 can be deprotected to generate free
carboxyl groups (compound 4), which are in principle able to
mediate water solubility. Compounds 3 and 4 are symbolized
as Prototype II (Figure 5). During the course of the synthesis of
our new TPP moieties we noted interesting atropisomerism
phenomena using NMR spectroscopy. Herein, we report in
[a] S. Schlundt, W. Bauer, Prof. Dr. A. Hirsch
Department of Chemistry and Pharmacy &
Interdisciplinary Center of Molecular Materials (ICMM)
Friedrich-Alexander-Universität Erlangen-Nürnberg
Henkestrasse 42, 91054 Erlangen (Germany)
Fax: (+49)9131-852-6864
E-mail: andreas.hirsch@fau.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501254.
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Figure 4. (ZnP)3-H2P-A2G-C60 3 and deprotected compound 4.
Figure 2. Fc9-(ZnP)3-H2P-C60 2, affiliated with Prototype I.
Figure 5. Prototype II (dendronized TPP–fullerene hybrids).
Figure 3. Prototype I (TPP–fullerene hybrids).
detail on these observations, in particular with respect to pos-
sible up–down permutations in such species.
Scheme 1. Synthesis of aldehyde 6. i) Ferrocenecarboxylic acid, DCC, CH2Cl2.
Results and Discussion
ethoxy)benzaldehyde (5) gave 2-(3-formylphenoxy)ethyl ferro-
cenecarboxylate (6; Scheme 1).
Synthesis of Prototype I compounds
Firstly, we present the synthesis of ferrocene-containing Proto-
type I compound 2. It has a large dendritic redox gradient,
which starts at the nine ferrocene moieties (best donor), spans
over the three zinc porphyrins and the free-base porphyrin,
and ends at C60 (best acceptor). The nine ferrocene groups of
compound 2 were introduced in the form of an aldehyde. A
coupling reaction of ferrocenecarboxylic acid and 3-(2-hydroxy-
The ferrocene aldehyde was used to synthesize the peripher-
al ferrocene porphyrins of compound 2. Aldehyde 5 and alde-
hyde 6 were subjected to a Lindsey-type[16–21] condensation re-
action with pyrrole, followed by metalation of the resulting
porphyrin (Scheme 2). An alternative synthesis of porphyrin 7
can be found in the Supporting Information. Zinc porphyrin
Fc3-ZnP-OH 8 and dyad (HOOC)3-H2P-C60 9[15] were coupled in
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The product Fc9-(ZnP)3-H2P-C60 2 was obtained in 66% yield
by using well-established coupling reagents. Good solvents for
DCC coupling reactions in our experience are THF and DMF.
Reactions were monitored by TLC (EtOAc/CH2Cl2 1/20). Three
spots were observed on the TLC plate, corresponding to the
product 2, zinc porphyrin 8, and dyad 9 at the baseline. Addi-
[22]
tion of 1 vol% NEt3
to the mixture of eluting solvents
helped only slightly to suppress peak tailing. According to ex-
perience, the best mode of operation for column chromatogra-
phy of our compounds is gravity mode, although the dendritic
TPP–fullerene hybrid Fc9-(ZnP)3-H2P-C60 2 can be separated
from the precursor porphyrin 8 and dyad 9 by column chro-
matography. All dendritic compounds were obtained in chro-
matographically and spectroscopically pure form and were
fully characterized (1H NMR, 13C NMR, IR, UV/Vis, elemental
analysis, MALDI, and ESI-QIT and ESI-TOF MS). Even though ag-
gregation and atropisomerism complicated the interpretation
Scheme 2. Synthesis of ferrocene porphyrin 8. i) Pyrrole, PPh4Cl, EtOH,
BF3·Et2O, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), CH2Cl2;
ii) Zn(OAc)2·2H2O, THF.
1
THF by using N,N’-dicyclohexylcarbodiimide (DCC), 1-hydroxy-
benzotriazole (HOBT), and 4-dimethylaminopyridine (DMAP;
Scheme 3).
of the H NMR and 13C NMR spectra, high-resolution ESI mass
spectra with fully resolved isotopic patterns support our struc-
tural assignments. MALDI-TOF-MS revealed [MH]+ (m/z=6326).
ESI-QIT-MS showed [M]3+ (m/z=2107.4), [M]4+ (1581.1), [M]5+
(1265.2), and [M]6+ (1054.5). ESI-TOF-HRMS gave evidence for
[M+2Na]2+ (m/z=3184.95901), [MH+Na]2+ (3173.96517), and
[MH2]2+ (3162.47224). ESI-TOF-HRMS revealed peaks for [M]3+
(m/z=2107.9754), [M]4+ (1581.2315), [M]5+ (1264.7844), and
[M]6+ (1054.1538). The dendritic reference compound 10
(Figure 6) was synthesized similarly to compound 2.
Figure 6. Fc9-(ZnP)3-H2P 10.
Synthesis of Prototype II compounds
In addition to the TPP cascade of the Prototype I compounds,
the Prototype II compounds contain a Newkome-type den-
dron, covalently attached by a short linker. In its protected
form (tert-butyl carboxylate end caps), the Newkome-type den-
dron enhances the solubility of the compounds in organic sol-
vents. In its deprotected form (free carboxyl end caps), it can
mediate pH-dependent water solubility. Our Prototype II com-
Scheme 3. Synthesis of Fc9-(ZnP)3-H2P-C60 2. i) DCC, DMAP, 1-HOBt, THF.
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pounds can be divided into
three parts, namely, the TPP cas-
cade, the Newkome-type den-
dron, and the fullerene.
Firstly, we present the synthe-
sis of the TPP cascade of the
Prototype II compounds. The
cascade contains one central
free-base porphyrin and three
peripheral zinc porphyrins. First,
the aldehydes for the central
porphyrin were synthesized.
Treatment of 3-hydroxybenzal-
dehyde with benzyl 2-bromoace-
tate afforded aldehyde 11
Scheme 6. Synthesis of central porphyrin (HOOC)3-H2P-OTBDPS 14. i) Pyrrole, PPh4Cl, EtOH, BF3·Et2O, DDQ, CH2Cl2;
ii) K2CO3, THF/H2O.
(Scheme 4). Compound 5 was transformed into its tert-butyldi-
phenylsilyl (TBDPS)-protected derivative 12 (Scheme 5).
Consecutively, the Newkome-type compound for our Proto-
type II moieties was synthesized. Coupling of spacer 18[26] and
Newkome-type dendron 19[27–29] was mediated by DCC and 1-
HOBt in dry DMF to give derivative Bn-sp-A2G 20 (Bn=benzyl,
sp=spacer, A2G=second-generation amine). The benzyl
group of 20 was subsequently removed by catalytic hydroge-
nation (Pd/C) in EtOH to give 21 (Scheme 8). A malonyl group,
which is needed for cyclopropanation of C60, was introduced
into compound 21 by malonic acid (Scheme 9). Polar EtOAc/
MeOH mixtures were necessary in the column-chromatograph-
ic workup of free carboxylic acid 22. Subsequent reactions of
Newkome-type compound 22 can be used to build dendron-
ized moieties. Coupling of 17 and 22 in dry DMF with DCC,
DMAP, and 1-HOBt gave (ZnP)3-H2P-A2G 23 (Scheme 10). The
yield of 24% is probably due to the sterically demanding enti-
ties. Cyclopropanation of (ZnP)3-H2P-A2G 23 was successful in
toluene with C60, iodine, and 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU; Scheme 11).
Scheme 4. Synthesis of aldehyde 11. i) Benzyl 2-bromoacetate, tetra-n-butyl-
ammonium bromide (TBAB), NaOH, H2O/CH2Cl2.
Scheme 5. Synthesis of aldehyde 12. i) TBDPSCl, pyridine, CH2Cl2, DMAP.
Mild and reliable methods are required for the deprotection
of (ZnP)3-H2P-A2G-C60 3. Acidic standard methods cannot be
applied, because under these conditions the zinc cation can
migrate from a ZnP to an H2P. Deprotected fullerene adduct
(ZnP)3-H2P-A2G-C60 4 was gained from its precursor (ZnP)3-H2P-
A2G-C60 3 by the use of Me3SiOTf and 2,6-lutidine in dry CH2Cl2
followed by hydrolysis in H2O (Scheme 12).[25,30–32] ESI-TOF-
HRMS supported the structural assignments of 3 and 4. The
Aldehydes 11 and 12 were subjected to a Lindsey-type con-
densation reaction with pyrrole, which gave porphyrin 13. The
latter was deprotected to give central porphyrin 14
(Scheme 6). An alternative synthesis of porphyrin 14 is provid-
ed in the Supporting Information. Focal porphyrin 14 was
treated with TPP 15[12] to obtain dendritic porphyrin 16, which
was deprotected to give TPP cascade 17 (Scheme 7).[23–25]
Scheme 7. Synthesis of dendritic TPP (ZnP)3-H2P-OH 17. i) [O-(7-Azabenzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium-hexafluorphosphate] (HATU), NEt3, DMF;
ii) TBAF, THF.
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[MÀ3H+Na]2À,
2430.3
[MÀ2H]2À, 1627.8 [MÀ4H+
Na]3À, and 1620.1 [MÀ3H]3À.
Similarly to compound 4, ref-
erence compound 24 was syn-
thesized (Scheme 10). However,
the deprotected compounds 4
and 24 were not soluble in
water. This is due to the size of
the Newkome-type dendron
with its nine carboxyl groups. A
higher-generation
dendron
should be used to achieve water
solubility. A slight uptake of sub-
stance after intense sonication
(20–60 min) was observed. This
was judged by a slight color
change of the solution, even
though a solid residue remained.
Because of the long sonication
time, it must be assumed that
decomposition of the porphyrins
and the fullerene occurred. This
assumption is supported by new
bands in the UV region below
300 nm. It is known that ful-
lerenes can be oxidized due to
hydroxylation[33] and that malon-
ic esters can be cleaved at
pH 10.
Scheme 8. Synthesis of Bn-sp-A2G 20 (A2G=second-generation amine) and HO-sp-A2G 21. i) DCC, 1-HOBt, DMF;
ii) H2/Pd/C, EtOH.
Scheme 9. Synthesis of 22. i) DCC, CH2Cl2.
The next synthetic targets
former gives rise to an isotopic pattern at m/z=2707.87052,
which was assigned to [M+2Na]2+. The latter required treat-
ment with NaOMe in MeOH to obtain a significant mass spec-
trum with peaks at m/z=2452.3 [MÀ4H+2Na]2À, 2442.3
were Prototype II compounds extended by ferrocene. Ferro-
cene porphyrin Fc3-ZnP-OH 8 was treated with porphyrin 14 in
a DCC coupling reaction in THF. Deprotection of the masked
OH group of Fc9-(ZnP)3-H2P-OTBDPS 25 could be achieved with
Scheme 10. Synthesis and deprotection of dendritic TPP (ZnP)3-H2P-A2G 23. i) DCC, DMAP, 1-HOBt, DMF; ii) Me3SiOTf, 2,6-lutidine, dry CH2Cl2.
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Scheme 13. Synthesis of dendritic ferrocene porphyrin 26. i) DCC, DMAP, 1-
HOBt, THF; ii) TBAF, THF.
Scheme 11. Cyclopropanation of (ZnP)3-H2P-A2G 23. i) C60, I2, DBU, toluene.
one equivalent of tetra-n-butylammonium fluoride (TBAF) in
THF and furnished Fc9-(ZnP)3-H2P-OH 26 (Scheme 13).[23–25]
Coupling of Newkome-type derivative 22 and dendritic fer-
rocene porphyrin 26 and was unsuccessful. Because of the lim-
ited amount of substance, not enough synthetic trials could be
made. Thus, ferrocene-extended Prototype II compounds could
not be synthesized. In addition, recent studies suggest that fer-
rocene may hamper energy and electron transfer in multicom-
ponent scaffolds.[34]
Compounds 2 and 4 could be used as lead structures for
rigid derivatives and derivatives which carry higher generation
dendrons for mediation of water solubility.
Atropisomerism
Atropisomerism is well known in TPP chemistry, especially in
ortho-functionalized TPPs. For example, Gottwald and Ullman
1969,[35] Walker and Avery 1971,[36] Drexler et al. 1998,[37] and
Song et al. 1999[38] contributed to this topic. Collman and co-
workers used atropisomerism in porphyrins to build picket-
fence porphyrins to mimic heme-containing enzymes.[39,40] Por-
phyrins bearing bulky substituents in meta positions also ex-
hibit atropisomerism. This was demonstrated by Smith and co-
Scheme 12. Deprotection of fullerene adduct (ZnP)3-H2P-A2G-C60 3.
i) Me3SiOTf, 2,6-lutidine, dry CH2Cl2.
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workers in porphyrin–carborane systems,[41] by Nierengarten
and Urbani in C60 porphyrin bis-adducts,[42] and by us in trans-2
C60 porphyrin bis-adducts.[13,14] Atropisomerism was also ob-
served by NMR spectroscopy in linear bis([60]fullerene)–oligo-
porphyrin nanoarrays by Diederich and co-workers.[43,44] Herein,
we report atropisomerism in dendritic tetraphenyl-porphyrins,
which was observed by NMR spectroscopy. Analogous signals
were previously reported in the case of much simpler porphy-
rins.[35–38]
Figure 8. All six possible atropisomers emerging from an A3B porphyrin (re-
garding dendritic porphyrin 27; ellipse: H2P plane, blue: methyl malonyl sub-
stituent, red: ZnP substituent). This figure also represents the six different
chemical environments for one specific ligand in case of four identical li-
gands at the phenyl rings in TPPs.
TPPs are not flat like porphin (the porphyrin core), because
the four phenyl rings, which are attached in meso positions,
are oriented perpendicular to the porphyrin core. Consequent-
ly, there is only limited electronic communication between the
phenyl substituents and the porphyrin core.[45–48] The rotational
barrier for unsubstituted phenyl rings at room temperature is
about 73 kJmolÀ1. For ortho-substituted rings the rotational
barrier is even higher. The two facial sides of the porphyrin
core become distinguishable if one phenyl ring has different
substituents in ortho or meta position. Since the phenyl rings
are perpendicular to the porphin plane, the substituents can
be on one or the other side of this plane (a/b; Figure 7).
atoms, enables four different isomers. There are six different
chemical environments for the methyl group (Figure 8). This
leads to the statistical occurrence of the isomers of 1:1:2:2:1:1.
In fact, isomer III is a pair of enantiomers. Hence, it consists of
two isomers, which are formed in equal amount, with identical
physical properties and the same chemical shift. Consequently,
it gives an NMR signal with twice the intensity compared to
isomers I, II, V, and VI. The same is true for isomer IV, which is
also a pair of enantiomers. The six isomers cause the six
1
methyl signals observed in the H NMR spectrum of the ortho-
methyl-substituted Ni TPP. Drexler et al. (1998) synthesized
a TPP bearing methyl-protected catecholamido groups in the
ortho position and isolated all four atropisomers.[37] They iden-
tified the isomers with the help of NMR spectroscopy and X-
ray diffraction. Song et al. synthesized meso-tetrakis(3-sulfona-
tomesityl)porphyrin tetrasodium salt and characterized its
atropisomers in detail in a H NMR study.[38] They analyzed the
1
Figure 7. All possible TPP atropisomers bearing substituents above (b) and
below (a) the porphyrin plane, for the case of identical substituents.
1:1:2:2:1:1 pattern and assigned each of the six peaks to the
corresponding atropisomer.
We were able to observe a similar pattern in our NMR spec-
tra. Dendritic TPP 27[15] (Figure 9) is discussed as a representa-
tive example of our dendritic porphyrins and TPP–fullerene hy-
brids, which all show to some degree signs of atropisomerism.
The 1H NMR spectrum of dendritic porphyrin 27[15] at room
temperature in CDCl3 is shown in Figure 10. The building
blocks of the molecule (ZnP, H2P) can be recognized in the
If all phenyl rings carry the same substituents at the same
positions, four different isomers are possible. In the bbbb
isomer all substituents are on one side of the porphyrin. In the
abbb isomer, one substituent is on one side of the porphyrin
and the other three are on the other side of the porphyrin. In
the aabb isomer, two adjacent substituents are on one side of
the porphyrin and two adjacent substituents are on the other
side. In the abab isomer, the substituents alternately are on
one side and on the other side of the porphyrin.
It is helpful to discuss previous publications before de-
scribing the atropisomerism of our dendritic TPPs: Gottwald
and Ullman (1969),[35] Walker and Avery (1971),[36] Drexler et al.
(1998),[37] and Song et al. (1999).[38] In 1969 Gottwald and
Ullman reported an early example of atropisomerism in
TPPs.[35] They identified the four stereoisomers of an ortho-hy-
droxy substituted TPP. In 1971, Walker and Avery observed
atropisomerism in an ortho-methyl substituted Ni TPP by
1H NMR spectroscopy.[36] The atropisomerism in the investigat-
ed porphyrin was evidenced by a pattern of six signals with in-
tensity ratio of 1:1:2:2:1:1, which was assigned to the four
methyl groups. Each one of them is located in an ortho posi-
tion of one of the four phenyl groups of the TPP. The hindered
rotation of the phenyl groups, due to steric interaction be-
tween the ortho-methyl groups and the b-pyrrolic hydrogen
Figure 9. Dendritic TPP 27.
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Figure 11. Porphyrin 28.
on the same side of the plane of the free-base porphyrin. The
shielding originates from the faces of the zinc porphyrins due
to their aromatic ring current. The number and vicinity of zinc
porphyrins to the methyl malonyl substituent increase from
isomer VI to isomer I (Figure 8) in a corresponding way to the
increasing shielding effect and chemical shift from isomer VI to
isomer I. The single free-base porphyrin 28 can be used as
a reference.
The shielding effect of the zinc porphyrin can be quantified
and correlates very well with the distance of the methyl ma-
lonyl substituent to the zinc porphyrin in the individual atrop-
isomers. In fact it is additive, that is, the number of zinc por-
phyrins in direct proximity to the methyl malonyl substituent
is reflected by the chemical shift. In this way the patterns in
the two sets of six singlets (3.60, 3.58, 3.51, 3.49, 3.42,
3.40 ppm and 3.24, 3.19, 3.06, 3.02, 2.90, 2.86 ppm; Figure 11,
1
Figure 10. Top: H NMR spectrum of (ZnP)3-H2P 27 (400 MHz, CDCl3, RT).
Bottom: 1H NMR spectrum of (ZnP)3-H2P 27 (400 MHz, C2D2Cl4, 1008C).
1H NMR spectrum of dendritic porphyrin 27. Line broadening
and signal overlap indicate a flexible molecule and various
conformations. The most interesting feature are the resonan-
ces corresponding to the methyl malonic CH2 group and the
OCH3 group. They were assigned to the two sets of six peaks
between 2.8 and 3.7 ppm, each with an integral ratio of
1:1:2:2:1:1. Walker and Avery[36] described this pattern in 1971
for ortho-methyl substituted TPPs. Song et al.[38] (1999) as-
signed the single peaks in the 1:1:2:2:1:1 pattern of meso-tetra-
kis(3-sulfonatomesityl)porphyrin tetrasodium salt. We wit-
nessed coalescence of the 1:1:2:2:1:1 pattern when we record-
1
top) in the H NMR spectrum of dendritic TPP 27 at room tem-
perature in CDCl3 can be rationalized.
If a ZnP substituent is added directly adjacent to the methyl
malonyl substituent, for example, transformation of isomer VI
into isomer IV, V into III, IV into II, or III into I, it causes an addi-
tional shift of 0.09 ppm to the corresponding signal of the
methyl group. A second type of transformation is adding
a ZnP substituent on the opposite side of the ring relative to
the methyl malonyl substituent (isomer VI to isomer V, IV to III,
or II to I). This causes an additional shift of 0.02 ppm to the
corresponding signal of the methyl group. The corresponding
set of six signals, which is assigned to the OCCH2CO group in
the methyl malonyl substituent, can be treated in the same
way. However, different additional shifts, that is, 0.18 to
0.16 ppm for a directly adjacent ZnP and 0.05 to 0.04 ppm for
a ZnP on the opposite side of the ring, are observed.
1
ed a H NMR spectrum of (ZnP)3-H2P 27 at high temperature
(1008C) in a high-boiling solvent (C2D2Cl4; Figure 10, bottom).
Therefore, we conclude that atropisomerism occurs in (ZnP)3-
H2P 27, which is a meta-substituted TPP.
Atropisomerism is common for ortho-substituted TPPs, but
was also reported for meta-substituted TPPs with bulky sub-
stituents.[13,41,42] In our case, the three zinc porphyrins attached
to the central free-base porphyrin are bulky substituents.
Atropisomerism was not observed in the case of porphyrin 28
(Figure 11), which lacks the bulky ZnP substituents.
The individual building blocks of (ZnP)3-H2P 27 can be rec-
ognized in its 13C NMR spectrum (Figures 12 and 13). Com-
pared to the signals of precursor porphyrins 15 and 28, the
signals of the dendritic TPP 27 are ragged. The pattern dis-
For the assignment, we propose the following hypothesis.
Isomers VI to I (Figure 8) were assigned to the peaks of the
1:1:2:2:1:1 pattern (Figure 10). The most downfield peak corre-
sponds to isomer VI, the next peak upfield to isomer V and so
on up to the most upfield peak of the 1:1:2:2:1:1 pattern,
which was assigned to isomer I. This correlation is based on
the assumption of a shielding effect caused by the zinc por-
phyrins towards the methyl malonyl substituent if they are in
close proximity to each other. This is only possible if they are
1
cussed for the H NMR spectrum can also be recognized if the
13C NMR resonances corresponding to the two malonyl carbon-
yl carbon atoms of 27 are considered.
The signals that are assigned to the two malonyl carbonyl
carbon atoms can be observed around 166.3 ppm (166.56,
166.42, 166.30, 166.14, 166.04 ppm). The observed pattern in
the 13C NMR spectrum (Figure 12) is similar to the 1:1:2:2:1:1
1
pattern in the H NMR spectrum (Figure 11). However, they are
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Conclusion
Several flexible, linked dendritic TPPs and TPP–fullerene com-
pounds of Prototypes I and II were synthesized. These struc-
tures have a dendritic redox gradient along a porphyrin cas-
cade. Additionally, the Prototype II compounds bear a New-
kome-type dendron, which could be successfully transformed
into its free carboxyl form by a mild procedure. The 1:1:2:2:1:1
pattern observed in the NMR spectra of the dendritic TPPs has
been discussed and is ascribed to atropisomerism of the cen-
tral TPP (H2P) of the dendritic moieties.
Experimental Section
Figure 12. 13C NMR spectrum of dendritic TPP 27 (100.5 MHz, CDCl3, RT).
For experimental details, see Supporting Information.
Acknowledgements
Financial support by the German Science Foundation (DFG)
through SFB 583 is gratefully acknowledged.
Keywords: atropisomerism
metallocenes · porphyrinoids
·
dendrons
·
fullerenes
·
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Received: March 30, 2015
Published online on July 31, 2015
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