Cyclic 2,12-porphyrinylene nanorings as a porphyrin analogue of cycloparaphenylenes

Article abstract of DOI:10.1021/ja513102m

β-to-β Directly linked cyclic Ni(II) porphyrin trimer, tetramer, and pentamer ([3]CP, [4]CP, and [5]CP) have been synthesized by reaction of a 2,12-diborylated Ni(II) porphyrin with Pt(cod)Cl₂ followed by reductive elimination. The structures of these cyclic porphyrin arrays have been revealed by X-ray diffraction analysis. The strain energies of these cyclic oligomers are calculated to be 77, 57, and 47 kcal/mol for [3]CP, [4]CP, and [5]CP, respectively. Intramolecular excitation energy hopping was observed between the ³(d,d) states of the Ni(II) porphyrins with rates of 3.0, 4.4, and 4.6 ps for [3]CP, [4]CP, and [5]CP, respectively, reflecting the close proximity of the Ni(II) centers.

Full text of DOI:10.1021/ja513102m

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Communication
Cyclic 2,12-Porphyrinylene Nanorings As a
Porphyrin Analogue of Cycloparaphenylenes
Hua-Wei Jiang, Takayuki Tanaka, Hirotaka Mori, Kyu Hyung Park, Dongho Kim, and Atsuhiro Osuka
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja513102m • Publication Date (Web): 29 Jan 2015
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Cyclic 2,12-Porphyrinylene Nanorings As a Porphyrin
Analogue of Cycloparaphenylenes
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Hua-Wei Jiang,† Takayuki Tanaka,† Hirotaka Mori,† Kyu Hyung Park,‡ Dongho Kim,*,‡ and
Atsuhiro Osuka*,†
† Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
‡ Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University,
Seoul 120-749, Korea
realization of a meso-meso directly linked cyclic porphyrin
tetramer, hexamer and octamer by Ag(I)-promoted oxidative
coupling of 5,10-diaryl Zn(II) porphyrin unit.10 However, the-
se porphyrin rings are hardly regarded as porphyrin analogs
of [n]CPPs because of their orthogonal linking topologies,
which lead to disruption of the overall macrocyclic conjuga-
tion.
ABSTRACT: β-to-β Directly linked cyclic Ni(II) porphyrin
trimer, tetramer, and pentamer ([3]CP, [4]CP, and [5]CP)
have been synthesized by reaction of a 2,12-diborylated Ni(II)
porphyrin with Pt(cod)Cl2 followed by reductive elimination.
The structures of these cyclic porphyrin arrays have been
revealed by X-ray diffraction analysis. The strain energies of
these cyclic oligomers are calculated to be 77, 57, and 47
kcal/mol for [3]CP, [4]CP, and [5]CP, respectively. In-
tramolecular excitation energy hopping was observed be-
Chart 1. Structures of [n]CPPs, [4]cyclo-3,9-chrysenylene,
and [n]CPs.
3
tween the (d,d) states of the Ni(II) porphyrins with rates of
3.0, 4.4, and 4.6 ps for [3]CP, [4]CP, and [5]CP, respectively,
reflecting the close proximity of the Ni(II) centers.
π-Conjugated nanorings have been attracting increasing
attention in light of the curved conjugation along their pe-
ripheries, convex-concave host-guest interactions, synthetic
challenges associated with achieving their highly distorted
structures, and potential application in the bottom-up
growth of uniform carbon nanotubes.1,2 Recently, the inven-
tion of effective synthetic routes to cycloparaphenylenes
([n]CPPs), cyclic conjugated nanorings that consist of dis-
torted benzene rings linked at the para-positions, has spark-
ed a boom in this field (Chart 1).3 Through the successful and
independent explorations of [n]CPPs by Jasti, Itami, Yamago,
and Isobe,3-5 it has now been revealed that structural and
electronic properties of [n]CPPs and related analogues de-
pend on a delicate balance of strain and conjugation. The
smallest CPP so far synthesized is [5]CPP that displays a con-
siderably distorted structure.6 Tetrameric macrocycles have
been synthesized from larger polycyclic aromatic segments
such as pyrene and chrysene.7
Scheme 1. Synthesis of β-borylated porphyrins.
Cyclic porphyrin arrays have also been targets of exten-
sive studies because of their potentials as models of artificial
light-harvesting antenna complexes, hosts possessing con-
vergent inward-pointing coordinating sites, and scaffolds to
achieve efficient hole delocalization.8 While non-covalently
linked supramolecular cyclic porphyrins have been con-
structed in short steps with the aid of self-assembling strate-
gies, covalently linked cyclic arrays are more difficult to con-
struct,9 mostly due to the entropic disadvantage in the final
macrocyclization step. We described the successful synthetic
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aconc. HCl, CH2Cl2, rt, 4 h. bNi(acac)2, toluene, reflux, overnight.
acac = acetylacetonate. Ar = 3,5-di-tert-butylphenyl.
products. It is worth to note that none of [n]CPs were
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formed from either the zinc or freebase forms of 2,12-
diborylated porphyrin 3a or 3b under similar conditions,
suggesting that a Ni(II) porphyrin scaffold is of vital impor-
tance in the successful formation of [n]CPs.
Here we report the synthesis and characterizations of cy-
cloporphyrinylene nanorings, namely [n]CPs that consist of
Ni(II) porphyrins directly linked at the 2- and 12-positions.
As a macrocyclization strategy, we utilized a Pt(II)-mediated
homo-coupling strategy developed by Yamago and Isobe.4c,d
Along this strategy, we employed 2,12-diborylated Ni(II) por-
phyrin 3c as a building block, in which the two connection
points (2 and 12-positions, Scheme 1) are situated at the steri-
cally least hindered locations with a diagonal separation dis-
tance of ca. 8.5 Å. In addition, Ni(II) porphyrins can adopt a
ruffled or saddled conformation, which is favorable to reduce
the overall ring strain of the resulting [n]CPs.
Scheme 2. Synthesis of 2,12-porphyrinylene nanorings.
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Ir-catalyzed borylations of porphyrins via C–H bond acti-
vation proceed regioselectively at the β-positions next to an
unsubstituted meso-position, hence providing a reliable
means for the preparation of β-borylated porphyrins.11 2,12-
Diborylated Ni(II) porphyrin 3c, a key precursor of [n]CPs in
the present study, was formed in the borylation of 1c but was
very difficult to separate from an accompanying side product,
2,8-diborylated Ni(II) porphyrin 4c, due to almost identical
retention times on silica gel. Thus, 3c was prepared via an
indirect route. Borylation of 5,15-bis(3,5-di-tert-butylphenyl)
porphyrin
Zn(II)
complex
1a
with
equimolar
bis(pinacolato)diboron under Ir(I)-catalyzed conditions gave
a product mixture that contained β-borylated porphyrin
products (Scheme 1). Separation by silica gel column chro-
matography and recycling preparative gel permeation chro-
matography (GPC) gave a mixture of 3a and 4a in 50% yield
along with β-monoborylated porphyrin 2a (26%). Fortu-
nately, it was found that slow vapor diffusion of methanol
into a solution of the product mixture in chloroform caused
preferential recrystallization of 3a. Almost complete separa-
tion of 3a was possible via repeated recrystallization on large
scale. Then, Zn(II) porphyrin 3a was converted to Ni(II) por-
phyrin 3c via acid-catalyzed de-zincation followed by Ni(II)
insertion using Ni(acac)2.
By following the one-pot reaction procedure of CPP mac-
rocyclization,4d 2,12-diborylated Ni(II) porphyrin 3c was re-
acted with equimolar Pt(cod)Cl2 for 4 days at room tempera-
ture in the presence of cesium fluoride in THF. The resulting
reaction mixture containing platinum-bridged porphyrins
was refluxed in the presence of PPh3 in toluene to induce
reductive elimination (Scheme 2). After repeated separations,
cyclic oligomers [3]CP, [4]CP and [5]CP were obtained in 5.0,
4.4 and 2.4% yields, respectively, in a reproducible manner.
We attempted to optimize the reaction conditions by adjust-
ing variables such as concentration, temperature, and adding
extra equivalents of 1,5-cyclooctdiene, but we found that the
conditions mentioned above have provided the best result
with regard to the yields of [n]CPs with suppression of unde-
sirable linear polymeric side products. The parent ion peaks
for the [n]CPs were observed at m/z = 2220.90 (calcd. for
C144H150N12Ni3 = 2221.02, [M]+) for [3]CP, at m/z = 2961.48
(calcd. for C192H200N16Ni4 = 2961.36, [M]+) for [4]CP, and at
m/z = 3701.59 (calcd. for C240H250N20Ni5 = 3701.69, [M]+) for
[5]CP by MALDI-TOF MS. Although it was possible to detect
cyclic oligomers with n values greater than [5]CP by MALDI-
TOF MS, very poor yields made it extremely difficult to iso-
late them and so we abandoned full characterization of these
1
Figure 1. H NMR spectra of (a) [3]CP, (b) [4]CP, (c) [5]CP and (d) 1c
in the aromatic region in CDCl3 at room temperature.
1
All the H NMR spectra of [3]CP, [4]CP, and [5]CP feature
a distinct signature set of signals consisting of one singlet
associated with the meso-protons (Ha), one singlet (Hb) and
two doublets (Hc and Hd) corresponding to the β-protons,
indicating their highly symmetric structures (Figure 1). The
assignments of the signals have been fully performed
through 2D NOESY experiments. The protons at the porphy-
rin peripheries were shifted up field compared with the cor-
responding peaks of the monomer 1c, reflecting the influence
of the ring-currents of adjacent porphyrins.12 Such trend be-
comes more prominent in the order of [5]CP < [4]CP < 3[CP].
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rectly linked at the 5,10-positions,10 in line with the cyclic
structures.
The electrochemical properties of 1c, [3]CP, [4]CP, and
1
2
[5]CP were investigated by cyclic voltammetry and differen-
tial pulse voltammetry (SI). The monomer 1c shows two oxi-
dation potentials at 0.50 and 1.00 V and a reduction potential
at –1.78 V versus ferrocene/ferronenium ion couple, which
leads to an electrochemical HOMO–LUMO gap of 2.28 eV.
[3]CP exhibits split oxidation potentials at 0.36 and 0.60 V
and split reduction potentials at –1.71 and –1.91 V, reflecting
the electronic interactions among the Ni(II) porphyrins in
the ring. Similar but more complicated split potentials were
observed at 0.41, 0.58 and 0.64 V, and –1.64, –1.86, and –2.05
V for [4]CP, and 0.46, and 0.64 V, and –1.58, –1.65, and –1.80
V for [5]CP. Upon increase in the ring size, these potentials
are shifted to the anodic side.
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The excited state dynamics of [n]CPs exhibited ultrafast
decay dynamics that are characteristic of Ni(II) porphyrin
derivatives (SI).13 All [n]CPs exhibited broad excited-state
absorption bands (ESA) from 700 to 850 nm, which showed
very rapid decays. Simultaneous peak shifts in the ground-
state bleaching (GSB) around 600 nm and ESA near 500 nm
noticeably altered the shape of the initial transient absorp-
tion spectra. This initial decay with τ = 0.6 ps, common to all
[n]CPs, is associated with the transition from the lowest
1
3
energy (π,π*) state to Ni(II)-centered (d,d) state and is re-
sponsible for the TA spectral changes on sub-picosecond
3
timescale. Relaxed (d,d) state decayed to the ground state
with time constants of 50, 40, and 30 ps for [3]CP, [4]CP, and
3
[5]CP, respectively (SI). Decreasing lifetimes of (d,d) state
can be rationalized in terms of increasing nonradiative decay
rate with increasing molecular size.
Figure 2. X-ray crystal structures of [3]CP (a and b), [4]CP (c and d),
and [5]CP (e and f). tert-Butyl groups in (a) and (b), meso-Aryl
groups in (c), (d), (e) and (f), solvent molecules, and hydrogen atoms
are omitted for clarity. The ellipsoids are scaled to the 30% probabil-
ity. Disorders were found for [5]CP.
Interestingly, the transient absorption anisotropy for
[3]CP, [4]CP, and [5]CP decayed from the respective initial
anisotropy values of r0 = 0.05, 0.019, and 0.047 with time con-
stants of 1.0, 1.1, and 1.3 ps, respectively (SI). In contrast to the
high initial anisotropy value of 1c (r0 = 0.14), the observed
initial low anisotropy values of CP are caused as a conse-
quence of efficient excitation energy hopping along the por-
phyrin nanorings. While the anisotropy decay of 1c showed a
simple rotational diffusion with a time constant of 370 ps,
depolarization processes of CP contained ultrafast inter-
chromophoric excitonic process. Considering sub-picosecond
To our delight, crystals of [3]CP, [4]CP and [5]CP suitable
for single-crystal X-ray diffraction analysis were obtained by
slow solvent diffusion of a methanol/heptane mixture into
their respective solutions in chloroform. Definitive structural
determinations were then accomplished by revealing [n]CPP-
like hoop structures (Figure 2). In all cases, Ni(II) porphyrins
take severely saddled conformations, which is likely impor-
tant to accommodate large distortion arising from the hoop-
like cyclic architectures. The averaged Cβ—Cβ bond lengths
and diameters of the nanorings are 1.47 and 9.32 Å, and 1.45
and 11.95 Å, and 1.48 and 15.60 Å for [3]CP, [4]CP, and [5]CP,
respectively. Upon increasing the ring size, the Ni(II)—Ni(II)
center-to-center sepration distances are increased to be 7.36,
8.31, and 8.80 Å for [3]CP, [4]CP, and [5]CP, respectively.
These structural features are comparable to those of DFT
optimized structures (See SI).
The UV/Vis absorption spectra of [3]CP, [4]CP, and [5]CP
exhibit broader Soret-bands and red-shifted Q-like bands as
compared with those of monomer 3c (Figure 3); Soret-like
bands are observed at 409, 414, and 423 nm, and Q-like
bands are observed at 552 and 593 nm, 548 and 595 nm, and
535 and 595 nm, for [3]CP, [4]CP, and [5]CP, respectively.
Curiously, the peak positions in the absorption spectra of
[3]CP, [4]CP, and [5]CP are not so much changed, implying
similar exciton coupling interactions. These spectral features
are reminiscent of those of cyclic Zn(II) porphyrin rings di-
1
lifetimes of the (π,π*) state of the Ni(II) porphyrins, anisot-
ropy decays of CP can be interpreted in terms of EET be-
3
tween the metal-centered (d,d) states. Based on a polygon
model, the excitation energy hopping times have been
evaluated to be 3.0, 4.4, and 4.6 ps for [3]CP, [4]CP, and
[5]CP, respectively. Due to metal-localized orbital distribu-
tion of 3(d,d) states, the EET processes in [n]CPs are less effi-
cient than those in 1(π,π*) states of meso-meso directly linked
Zn(II) porphyrin rings that occur with time constants of 0.12,
0.34, and 0.24 ps for tetramer, hexamer, and octamer, respec-
tively.10 The small increase in energy hopping time with in-
creasing ring size is presumably attributed to increasing
Ni(II)—Ni(II) center-to-center separation distances between
the neighboring porphyrin chromophores.
MO calculations on these [n]CPs were performed using
the Gaussian 09 package at the B3LYP/6-31G*(for C, H, N)
+LANL2DZ (for Ni) level. While the HOMOs and LUMOs of
[3]CP and [5]CP are both degenerate, being similar to those
of monomeric porphyrins, the HOMO and LUMO of [4]CP
are both non-degenerate as a consequence of effective bond-
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ing interactions in the LUMO and anti-bonding interaction
in HOMO–1, phenomena observed only for even-
This work was supported by Grant-in-Aid from JSPS (No.
1
2
3
4
5
6
7
8
a
25220802 (Scientific Research (S) and No. 26810021 (Young
Scientist (B)). The work at Yonsei was supported by the
Global Research Laboratory (GRL) Program funded by the
membered nanorings as a characteristic feature of direct β-β
connectivity. This odd/even feature can also be seen in the
calculated MOs of [6]CP (SI). Finally, the ring strain energies
(ΔH) have been calculated according to the homodesmotic
reaction model14 to be 77.4, 57.7, and 47.4 kcal/mol for [3]CP,
[4]CP, and [5]CP, respectively. Therefore, it is possible to
infer that [3]CP, [4]CP, and [5]CP, are highly strained nanor-
ings comparable to [n]CPPs.15
Ministry
of
Science,
ICT
&
Future,
Korea
(2013K1A1A2A02050183). The authors thank Prof. Hiromitsu
Maeda and Dr. Yuya Bando (Ritsumeikan University) for
MALDI-TOF MS measurement.
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Figure 3. UV/Vis absorption spectra of [3]CP, [4]CP and [5]CP in
CH2Cl2.
In summary, cyclo-2,12-porphyrinylene nanorings [3]CP,
[4]CP, and [5]CP have been synthesized as the first porphy-
rin analogues of CPPs via Pt-mediated cyclization of a 2,12-
diborylated Ni(II) porphyrin followed by reductive elimina-
tion. Among these nanorings, [3]CP is the most strained sys-
tem with a calculated strain energy of 77.4 kcal/mol. The
conjugative interactions have been calculated to be effective
only for even-membered [n]CPs, which results in non-
degenerate HOMO and LUMO orbitals for [4]CP, while such
conjugative interactions are ineffective for [3]CP and [5]CP.
Exploration of further elaborate architectures utilizing alter-
native metal complexes and detailed photophysical analysis
of efficient energy transfer along these architectures is an
active area of research in our laboratory.
ASSOCIATED CONTENT
Supporting Information. Experimental procedure,
complete characterizations (NMR, UV/vis, MS, CV), DFT
calculations, excited-state dynamics, and X-ray crystal-
lographic data for 3a-c, [3]CP, [4]CP, and [5]CP. This materi-
al is available free of charge via the Internet at
http://pubs.acs.org.
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[4]CP, and 5[CP], respectively. These have been interpreted in terms
AUTHOR INFORMATION
Corresponding Author
E-mail: osuka@kuchem.kyoto-u.ac.jp
E-mail: dongho@yonsei.ac.kr
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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of different ring current effects of the neighboring porphyrin. The
averaged C—C bond distances of the direct connection are 1.47, 1.45,
and 1.48 Å.
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P.; Trachtman, M.; Bock, C. W.; Brett, A. M. Tetrahedron 1976, 32, 317.
(15) Strain associated with wheel formation is considered to be ac-
commodated within flexible Ni(II) porphyrin skeletons.
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