Synthesis of highly twisted and fully π-conjugated porphyrinic oligomers

Article abstract of DOI:10.1021/ja511905f

Highly twisted π-conjugated molecules have been attractive but challenging targets. We report here an efficient synthesis of highly twisted diporphyrins with 126° and 136° twist angles that involves an oxidative fusion reaction of planar aminoporphyrin precursors at room temperature. Repeated amination-oxidative fusion sequences provide a unidirectionally twisted tetramer. The twisting angle of the tetramer is 298°.

Full text of DOI:10.1021/ja511905f

Communication
pubs.acs.org/JACS
Synthesis of Highly Twisted and Fully π‑Conjugated Porphyrinic
Oligomers
Satoru Ito,† Satoru Hiroto,*,† Sangsu Lee,‡ Minjung Son,‡ Ichiro Hisaki,§ Takuya Yoshida,⊥
Dongho Kim,*,‡ Nagao Kobayashi,*,⊥ and Hiroshi Shinokubo*,†
†Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
‡Department of Chemistry, Yonsei University, Seoul 120-749, Korea
§Department of Material and Life Science, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan
⊥Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
S
* Supporting Information
We attempted oxidative fusion of aminoporphyrin 3a with
ABSTRACT: Highly twisted π-conjugated molecules
four mesityl groups, which was prepared through nitration and
have been attractive but challenging targets. We report
reduction of 1a. Oxidation of 3a with DDQ (2.0 equiv) in
here an efficient synthesis of highly twisted diporphyrins
CHCl3 at room temperature afforded fused dimer 4a in 4%
with 126° and 136° twist angles that involves an oxidative
yield. After extensive optimization, we found that the oxidation
fusion reaction of planar aminoporphyrin precursors at
of 3a in α,α,α-trifluorotoluene/trifluoroacetic acid (TFA, 20
equiv) afforded 4a in 97% yield (Scheme 1). None of the
desired product was obtained without TFA. This protocol was
also applied to 3b with sterically more demanding substituents
to furnish 4b in 67% yield. Notably, this procedure enables the
synthesis of highly congested diporphyrins 4a within 10 min
even at room temperature.
room temperature. Repeated amination−oxidative fusion
sequences provide a unidirectionally twisted tetramer. The
twisting angle of the tetramer is 298°.
hree-dimensional (3D) π-conjugated molecules have
Treceived extensive attention in many chemical research
areas due to their interesting structure and unique character-
istics.1−3 Since the discovery of buckminsterfullerenes, synthetic
chemists have devoted considerable effort to develop bottom-
up approaches to 3D π-conjugated molecules. Construction of
3D structures generally requires harsh conditions, such as laser
irradiation, flash vacuum pyrolysis, and photochemical
processes, to overcome their high strain energy.4 However,
these protocols could not apply to π-conjugated functional
molecules such as porphyrins, which undergo decomposition
on heating and light irradiation.
Scheme 1. Oxidative Dimerization of Nickel(II) 3-
a
Aminoporphyrins 3 to Diporphyrins 4
Aniline oxidation is a classic methodology for linking
aromatic amines. However, the synthetic usefulness of this
method has been overshadowed because of uncontrollable
reactions to form a mixture of fused oligomers.5 In contrast, we
have found that the oxidation of 2-amino-5,10,15-triarylpor-
phyrins proceeded selectively to provide planar pyrazine-fused
diporphyrins in high yields in one step.6 Herein, we report the
synthesis of highly twisted diporphyrins and tetraporphyrin
with very large twisting angles (136° and 298°) from planar 2-
aminoporphyrin precursors with more bulky substituents.
Highly twisted molecules, particularly π-systems with 3D
character, remain attractive but challenging targets.7 Twisting π-
systems lead to the emergence of intriguing characteristics such
as chiroptical properties and anisotropic crystal packing. The
twisting approach to 3D π-systems has been investigated in the
synthesis of oligoacenes, referred to as “twistacenes”. The
maximum edge-to-edge twisting angle of the longest twistacene
is 144°.7j However, additional systems with greater twist angles
are needed if the specific features that are induced by molecular
twisting are to be explored in detail.
a
(a) LiNO3 (2 equiv), CHCl3/AcOH/Ac2O, 75 °C, 15 h for 1a;
LiNO3 (4 equiv), CHCl3/AcOH/Ac2O, 55 °C, 4 h for 1b. (b) Sn
(excess), conc. HCl, CHCl3, 60 °C, 1 h for 2a; Sn (excess), conc. HCl,
CHCl3, 60 °C, 3 h for 2b. (c) DDQ (2 equiv), TFA (20 equiv),
PhCF3, RT, 10 min for 3a; DDQ (2 equiv), TFA (20 equiv), PhCF3,
RT, 2 h for 3b.
Received: November 20, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ja511905f
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
X-ray diffraction analysis of the products revealed that the
present oxidative fusion reaction transformed 2D planar
precursors into 3D twisted π-systems. The structures of the
fused diporphyrins 4a and 4b are shown in Figure 1 and Figure
Figure 1. X-ray crystal structure of dimer 4a (a) side and (b) front
views. Mesityl groups are omitted for clarity. The thermal ellipsoids are
scaled at 50% probability level.
S11, respectively. Interestingly, both 4a and 4b exhibit highly
twisted conformations in which the central pyrazine ring is also
twisted. The twist angles of the pyrazine rings in 4a
(13.01(9)°) and 4b (13.18(8)°) are nearly the same. While
1a is completely planar (Figure S10), the porphyrin units in 4a
and 4b adopt ruffled conformations. The two porphyrin units
are twisted in the same direction, and the edge-to-edge twisting
angles in 4a and 4b are 125.6(8)° and 136.3(7)°, respectively.
Meanwhile, 4a and 4b demonstrate unusually high solubility
even in hexane.8 This fact demonstrates that deformation of π-
surfaces leads to significant enhancement of solubility. This is
remarkable since porphyrin oligomers typically suffer from low
solubility.
Figure 2. Magnetic circular dichroism (MCD) (top) and UV−vis
absorption (bottom) spectra of 1a (black dash line), 1b (red dash
line), 4a (black solid line), 4b (red solid line), and 7 (blue solid line)
in CH2Cl2. The inset displays the enlarged absorption spectra.
showed oppositely signed Faraday B terms, which indicate the
presence of two differently polarized transitions. Considering
their optically forbidden character, these bands can be assigned
as split Q-bands directed to two different axes (long and short
axes, respectively). This type of separation of two Q-bands is
often observed for fused mulitiporphyrins with effective π-
conjugation.13
To gain further insight into their electronic transitions,
electrochemical analysis was performed. The results of cyclic
voltammetry for 4a and 4b are summarized in Table S1. Both
compounds showed three reversible oxidation and two
reversible reduction peaks. Both oxidation and reduction
potentials in 4a and 4b were split, which indicates the presence
of effective electronic communication between two porphyrin
units. Only the first oxidation potential of 4b (0.419 V) was
lowered when compared with that of 4a (0.438 V). This result
suggests that twisting affects only the HOMO level to decrease
the HOMO−LUMO gap. The HOMO levels of monomers 1a
and 1b are the same (0.50 V); therefore, the increase in the
HOMO energy in 4b is primarily caused by π-twisting rather
than electronic factors of the substituents. These results
demonstrate the ability to control HOMO−LUMO gaps by
molecular twisting.
Two-photon absorption (TPA) is a third-order nonlinear
optical phenomenon, and its probability of occurrence is closely
related to π-electron behavior. TPA cross-section values were
measured by the Z-scan technique at 1200 nm.14 The TPA
cross-section value of 4a (640 GM) was somewhat lower than
that of planar dimer 4c (850 GM) (Figure S9).15
We attempted to synthesize π-extended porphyrin tetramer
7. The nitration of 4a with LiNO3 afforded nitrodiporphyrin 5
in 49% yield, which was converted to aminodiporphyrin 6 in
93% yield by reduction with NaBH4 in the presence of Pd/C.16
Oxidation of 6 under the standard conditions furnished
tetraporphyrin 7 in 9% yield. The use of dichloroacetic acid
in place of TFA improved the yield of 7 to 24% (Scheme 2).17
In contrast to dimer 4a, six peaks of mesityl groups were
observed as relatively sharp signals, which indicate that twist
Different dynamic behaviors between 4a and 4b were
observed in 1H NMR analysis. At room temperature, 4a
exhibited two broad signals due to protons at the 3- and 5-
positions of the mesityl groups. On the other hand, four aryl
proton peaks were observed in 4b. This difference is likely due
to the rapid twist inversion of 4a compared with the relatively
stable twisting of 4b at room temperature. In principle, the aryl
proton signals should appear as four peaks in the twisted
conformation. However, these peaks are interconvertible and
merged into two peaks by twist inversion. This hypothesis was
confirmed by variable temperature NMR analysis; four aryl
proton signals in 4b became broader at higher temperatures
and coalesced at 100 °C (Figure S5). Because of the high
rotation barrier of mesityl groups, the signal broadening can be
attributed to the helicity inversion.9
1
In H NMR spectra, pyrrole proton signals of 4b were
upfield shifted as compared to 4a, while chemical shifts of
monomers 1a and 1b were almost the same. This indicates that
the π-twisting significantly influences the ring current of the
porphyrin unit. The lower NICS(0) values of 4a than 4b also
support substantially weakened aromaticity of the porphyrin
ring (Figure S15).
We expected that the subtle effect of molecular twisting
could be detectable on the basis of change in interporphyrinic
interactions in the twisted diporphyrins.10 Figure 2 shows the
UV−vis absorption spectra of 1a, 1b, 4a, and 4b in CH2Cl2.
Both dimers 4a and 4b exhibit bathochromic shift and
broadening of the spectra compared with the corresponding
monomers 1a and 1b, which indicates effective expansion of π-
conjugation despite their largely twisted structures.11 Com-
pared with 4a, 4b shows slight bathochromic shift (51 cm−1) of
the lowest energy band.12 The spectrum of 4a was further
assigned by magnetic circular dichroism (MCD) spectral
analysis. MCD signals for two bands at 622 and 585 nm
B
DOI: 10.1021/ja511905f
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Journal of the American Chemical Society
Communication
This result demonstrates that π-twisting influences interchro-
mic electronic interactions.
Scheme 2. Synthesis of Pyrazine-Fused Tetraporphyrin 7
In conclusion, we have successfully synthesized a set of
highly twisted, but effectively π-conjugated, fused porphyrin
oligomers through oxidative coupling of aminoporphyrins
under very mild conditions. We have also demonstrated that
the twist angle and the extent of dynamic motion could be
controlled via the choice of substituents. Twisting of the π-
surface results in reduction of the HOMO−LUMO gap and an
enhancement of solubility. The present findings represent an
important advance toward the long-term goal of preparing π-
conjugated molecular wires, whose properties can be fine-tuned
through molecular twisting.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and spectral data for all new compounds.
Crystallographic data (CIF files) for 1a, 4a, 4b, and 7. This
material is available free of charge via the Internet at http://
pubs.acs.org.
inversion did not occur at room temperature. The helicity
inversion barrier in helical polymers is known to increase in
higher polymers.18 This is the first observation of a similar
phenomenon in twisted π-conjugated molecules. The unidir-
ectionality twisted structure of 7 was elucidated by X-ray
diffraction analysis (Figure 3). Three slightly different
AUTHOR INFORMATION
■
Corresponding Authors
hshino@apchem.nagoya-u.ac.jp
hiroto@apchem.nagoya-u.ac.jp
dongho@yonsei.ac.kr
nagaok@m.tohoku.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Crystallographic data collection was performed at the BL38B1
in the SPring-8 at the BL38B1 with approval of JASRI
(proposal No. 2014A1252). This work was supported by
Grants-in-Aid for Scientific Research on Innovative Areas “pi-
System Figuration” (26102003) and “Science of Atomic
Layers” (26107519) and the Program for Leading Graduate
Schools “Integrative Graduate Education and Research in
Green Natural Sciences”, MEXT, Japan. H.S. acknowledges the
Yamada Science Foundation for financial support. H.I.
acknowledges a Grant-in-Aid for Scientific Research (No.
24685026). This work was partly supported by a Grant-in-Aid
for Scientific Research on Innovative Areas “Stimuli-responsive
Chemical Species” (25109502) to N.K. N.K. also thanks Dr. T.
Furuyama for the help in MCD measurements. The work at
Yonsei University was financially supported by the Midcareer
Researcher program (2005-0093839) and the Global Research
Laboratory (2013K1A1A2A02050183) administered through
the National Research Foundation of Korea, funded by MEST,
Korea.
Figure 3. X-ray crystal structure of 7. (a) Front view and (b)
perspective view. The thermal ellipsoids are scaled at 50% probability
level. Mesityl groups are omitted for clarity.
conformations were observed in the crystal, with edge-to-edge
twisting angles of 298°, 281°, and 295° (Figure S13).
Calculated proton chemical shifts of 7 in the twisted structure
were obtained by the GIAO method at the B3LYP/6-31G(d)
level. A linear correlation between the calculated and observed
chemical shifts indicates that tetramer 7 also adopts a twisted
conformation in solution (Figure S14). The twisting angles of
each two porphyrin fragment were 141°, 161°, and 145°, all of
which are larger than that of 4a. The larger twisting is a reason
for higher inversion energy of 7. Tetramer 7 has remarkable
solubility in common organic solvents including hexane.
The UV−vis absorption spectrum of 7 in CH2Cl2 is shown in
Figure 2. The substantial bathochromic shift of the lowest
energy band suggests the existence of effective π-conjugation
over the entire molecule despite large twisting. The presence of
electronic communication was also confirmed by electro-
chemical analysis, which exhibited multiple splitting of
oxidation potentials (Figure S2). Interestingly, the average
splitting in 7 (0.14 V) was smaller than that in 4a (0.23 V).
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DOI: 10.1021/ja511905f
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