Core-modified rubyrins containing dithienylethene moieties

Article abstract of DOI:10.1002/anie.201402711

Two stable core-modified rubyrins bearing one and two dithienylethene (DTE) units (1 and 2) have been synthesized. With one "closed-form" DTE unit, 1 shows aromaticity associated with its conjugated circuit of 26 π-electrons. In contrast, rubyrin 2 containing one "open-form" DTE unit has nonaromatic properties.

Full text of DOI:10.1002/anie.201402711

Angewandte
Chemie
DOI: 10.1002/anie.201402711
Heteroporphyrins
Core-Modified Rubyrins Containing Dithienylethene Moieties**
Zhikuan Zhou, Yi Chang, Soji Shimizu, John Mack, Christian Schꢀtt, Rainer Herges,
Zhen Shen,* and Nagao Kobayashi*
Abstract: Two stable core-modified rubyrins bearing one and
two dithienylethene (DTE) units (1 and 2) have been
synthesized. With one “closed-form” DTE unit, 1 shows
aromaticity associated with its conjugated circuit of 26 p-
electrons. In contrast, rubyrin 2 containing one “open-form”
DTE unit has nonaromatic properties.
(4N + 2) rule as 18 p-electron are delocalized along the inner
ligand perimeter. Porphyrins and their homologues, such as
corroles[4] and ring-contracted,[5] core-modified,[6] and ring-
expanded porphyrin analogues,[7] provide an ideal platform
for developing a deeper understanding of the aromaticity of
various compounds. Core-modified and ring-expanded por-
phyrins have been intensively studied because of their
suitability for various applications.[1d,8] The extent of macro-
cyclic aromaticity is closely related to the number of p-
electrons and the topology of the molecular structure. The
aromatic character can be fine-tuned through protonation,[9]
by modifying the solvent polarity[10] and coordination envi-
ronment,[11] and by incorporating novel heterocyclic moieties
as building blocks.[12]
Dithienylethene (DTE) can exhibit switchable properties
in response to certain external stimuli such as light, redox
potential, oxidation/reduction, and the presence of ions
(Scheme 1). This makes it one of the most interesting building
blocks for the fabrication of organic photoswitching and
photomemory devices.[13] In the open form of DTE, two
thiophene units are linked at their b-positions through
a cyclopentene moiety, and thus the conjugation pathway
(indicated by alternating single and double bonds) is not
planar. In the closed form, the two a-positions of the
thiophene units are linked directly, leading to the extension
of the p-conjugation system over the entire DTE moiety. The
unique structural flexibility inspired us to think about
introducing a DTE unit into a macrocycle. Core-modified
rubyrins were selected so that the aromaticity of porphyr-
inoids can be studied further. DTE has previously been used
as a building block for the formation of nonconjugated
macrocycles,[14] and as a bridging and fused moiety in
photochromic porphyrins[15] and phthalocyanines.[16] How-
ever, no example of a DTE moiety embedded in a conjugated
macrocycle has been reported to date.
I
n recent years there has been a growing focus on the
synthesis and characterization of ring-expanded porphyri-
noids with both heteroaromatic and nonaromatic p-systems.[1]
Aromaticity is a fundamental concept of chemical structure
and bonding, which describes the enhanced stability that is
derived from the delocalization of p-electrons in cyclic
conjugated macrocycles.[2] Although aromaticity cannot be
quantified directly by experiment, it can be analyzed by
studying the molecular and electronic structures, and the
external magnetic field induced diamagnetic ring current.
Theoretical calculations such as nucleus-independent chem-
ical shift (NICS) and anisotropy of the current-induced
density (ACID) can be used to determine the extent to
which a novel macrocycle is aromatic.[3] Typically, porphyrins
can be regarded as aromatic molecules following Hꢀckelꢁs
[*] Z. Zhou, Dr. Y. Chang, Prof. Z. Shen
State Key Laboratory of Coordination Chemistry
Nanjing National Laboratory of Microstructures
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing, 210093 (China)
E-mail: zshen@nju.edu.cn
Z. Zhou, Dr. S. Shimizu, Prof. N. Kobayashi
Department of Chemistry, Graduate School of Science
Tohoku University, Sendai, 980-8578 (Japan)
E-mail: nagaok@m.tohoku.ac.jp
Dr. J. Mack
Department of Chemistry, Rhodes University
Grahamstown (South Africa)
Herein, we report the first synthesis and characterization
of porphyrinoid macrocycles containing one (1) and two (2)
dithienylethene moieties (Scheme 1). The noteworthy find-
ings were that firstly, even under dark conditions, the
embedded DTE moiety of the compound with one DTE
moiety exists solely in the closed form, suggesting the
formation of a very stable aromatic macrocycle with a 26 p-
electron system. Secondly, the optical and redox properties of
1 differ markedly from those of conventional core-modified
rubyrins with a 26 p-electron system. Thirdly, due to the
constraints imposed by the geometry of the inner perimeter of
the macrocycle, the rubyrin with two embedded DTE units
(i.e. 2) contains one closed and one open moiety and retains
nonaromatic properties.
C. Schꢀtt, Prof. R. Herges
Otto-Diels-Institute for Organic Chemistry
University of Kiel (Germany)
[**] We acknowledge financial support provided by the Major State
Basic Research Development Program of China (2013CB922101
and 2011CB808704), the National Natural Science Foundation of
China (21371090) to Z.S. and the Japanese Ministry of Education,
Culture, Sports, Science, and Technology (MEXT) through Grant-in-
Aids for Scientific Research on Innovative Areas (25109502,
“Stimuli-Responsive Chemical Species”), Scientific Research (B)
(23350095), and Young Scientist (B) (24750031). R.H. and C.S. are
grateful for support from the Deutsche Forschungsgemeinschaft
SFB 677 “Function by Switching”. The theoretical calculations were
carried out at the Centre for High-Performance Computing in Cape
Town and at the Otto-Diels-Institute.
The key step in the short synthesis of 1 was the coupling of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402711.
diol 4 and modified tetrapyrrane 6[6,18] (Scheme 2, the
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torsionally distorted. The bithiophene
moiety forms a plane which bends over
the DTE plane (C5-C2-C2’-C5’) with
a dihedral angle of 34.38, thus the mole-
cule adopts a J-shaped conformation when
viewed from the side. There appears to be
a weak intermolecular hydrogen-bonding
interaction between a pyrrole b hydrogen
atom (H1) and an Fatom on a neighboring
ꢀ
molecule with a C H···F distance of
3.508 ꢂ and an angle of 145.78. As
a result, the N1 pyrrole ring has a larger
torsion angle (23.88, relative to the DTE
plane) than the N2 ring (6.38). The C1–C1’
distance (1.532(16) ꢂ) is consistent with
ꢀ
that of a typical C C single bond, which
indicates the presence of a “closed-form”
Scheme 1. Schematic representation of the cycloreversion of DTE (top) and structures of
rubyrins containing one (1) and two (2) DTE units (bottom).
DTE moiety in 1. It is noteworthy that the
experimental details are provided in the Supporting Informa-
tion). Diol 4 was prepared by the double Grignard reaction of
3, which was synthesized according to literature proce-
dures.[13] The other coupling component, compound 6, was
obtained by reacting 5[6b] with excess pyrrole in the presence
of trifluoroacetic acid (TFA). Finally, rubyrin 1 was prepared
Figure 1. The X-ray crystal structure of rubyrin 1. a) Top view, b) side
view (solvent molecules, hydrogen atoms, and peripheral groups are
omitted for clarity).
C4–C2, C3–C3’, and C2’–C4’ distances (1.336(5) ꢂ,
1.325(5) ꢂ, and 1.336(5) ꢂ, respectively) are consistent with
=
C C double bonds, which differs markedly from the com-
pounds containing “closed-form” DTE units that have been
reported previously.[13] It has been reported that the photo-
chromic quantum yields (FO!C) for DTE cyclization are 10–
100 times larger than those for cycloreversion (FC!O).[13] The
presence of only one DTE moiety makes it easy to carry out
the conversion from a nonaromatic “open-form” structure to
a more stable 26-p aromatic “closed-form” DTE rubyrin.
Therefore, we tried to incorporate two DTE moieties into
a single macrocycle using the synthetic conditions similar to
those used for 1 (Scheme 3).
The chemical structure of the product was found to be
consistent with that of 2 based on the HRMS data (m/z:
found: 1000.30098; calcd: 1000.30078 [M+] for 2). The
structure of 2 was the only possible structure in this context.
In order to estimate the extent of the diamagnetic ring current
when an external magnetic field is applied to the molecule,
the 1H NMR spectra of 1 and 2 were recorded in CD2Cl2 for
comparison (Figure 2). In 1, the signal of the thiophene
methyl protons, which lie inside the conjugated macrocycle,
was detected at high field (d = ꢀ0.27 ppm), while the
Scheme 2. Synthesis of core-modified rubyrin 1.
by the reaction of compounds 4 and 6 under Lindsey reaction
conditions.[6a,17]
The structure of 1 was unambiguously elucidated by
single-crystal X-ray diffraction analysis (Figure 1).[19] Suitable
crystals were obtained by the slow diffusion of hexane into
a dichloromethane solution at room temperature. The two
sulfur atoms and methyl groups of the DTE moiety are
disordered. After the final refinement, the occupancies of the
two disordered parts were found to be 0.61 and 0.39,
respectively. Although the closed form of the DTE moiety
adopts a nearly planar conformation, the p-system of 1 is
2
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Figure 3. ACID plots of 1 (a) and 2 (b) at an isosurface value of 0.065.
Scheme 3. Synthesis of core-modified rubyrin 2.
The external magnetic field is applied orthogonal to the macrocycle
plane with its vector pointing towards the viewer. Current density
vectors (green lines and red conics) are plotted onto the ACID
isosurface.
thiophene protons of the DTE moiety appeared as a singlet at
d = 7.56 ppm. In 2, the chemical shifts of the thiophene methyl
protons (d = 2.01 (6H) and 2.18 (6H) ppm) and the thiophene
the 1H NMR measurements. The
p-system of 2 is clearly nonaro-
matic.
Molecular orbital (MO) and
time-dependent (TD)-DFT cal-
culations have been carried out
for 1 and 2 (Figures 4 and 5).
Although alternating single and
double bonds cannot be drawn
for the “open-form” structure of
2 (Scheme 1), when viewed in
the context of molecular orbital
theory, the spectroscopic proper-
ties can still be analyzed using
theoretical frameworks that
have been derived for conju-
gated macrocycles.[20] First, it is
noteworthy that the nodal pat-
terns of the HOMO and
HOMOꢀ1 and also the LUMO
and LUMO + 1 of 1 are consis-
tent with what would be antici-
pated for a heteroaromatic 26 p-
electron porphyrinoid (Support-
Figure 2. The 1H NMR spectra of a) 2 and b) 1 in CD2Cl2.
protons (d = 5.78 (2H) and 6.85 (2H) ppm) of the DTE
moiety are significantly different from those of 1, indicating
that there are two different types of methyl and DTE
thiophene protons, as shown in the structure of 2.
In order to disclose the conjugation pathways of 1 and 2,
the ACID was calculated at the B3LYP/6-31G* level of
theory (Figure 3, Figures S13–S16 in the Supporting Informa-
tion). There is a distinct diatropic ring current, which is
revealed through numerous current density vectors on the
isosurface, in the periphery of 1 (red line, clockwise). In
contrast, there are only local currents (in benzene, pyrrole,
and thiophene rings) in 2. At a higher isosurface value there is
a continuous isosurface in 1. In 2, however, the delocalization
is interrupted. When this is combined with the bond length
data for the 1 macrocycle (see Figure S12 in the Supporting
Information), it is clearly evident that 1 has an aromatic 26 p-
electron system. In contrast, there is no ring current on the
outer ring periphery of 2, as would be anticipated based on
Figure 4. The predicted energies of the frontier p-MOs based on the
X-ray structure of 1 and the optimized geometry of 2 at the B3LYP/6-
31G* level of theory. Black squares are used to denote occupied MOs.
Vertical arrows denote the one-electron transitions associated with the
L and B bands of Michl’s 4N+2 perimeter model[21] and the S, N1/2
,
and P1/2 bands of the 4N perimeter model.[22] The details of the TD-
DFT calculations and the angular nodal patterns of the main MOs are
provided in the Supporting Information.
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the MCD bands are an order of
magnitude weaker than those of 1,
since the induced excited-state mag-
netic moments are very weak due to
the absence of a heteroaromatic con-
jugation pathway.[23] The lowest
energy band is associated with the
sꢀ!s + transition, which is of an
intrashell nature. The absorption and
hence also the MCD intensity of the
S band is usually predicted to have
near-zero intensity. The weak tail of
intensity in the 700–850 nm region
can be assigned to this transition
(Figure 5). The other spin-allowed
electronic transitions within the
model are intershell, being derived
from
HOMO!SOMO
and
SOMO!LUMO one-electron tran-
sitions (Figure 4). Two weak (N1, N2)
and two strong (P1, P2) electric-
dipole transitions are predicted on
this basis, which can be readily
assigned based on the TD-DFT cal-
culation to the more intense coupled
pairs of Faraday b0 terms in the 450–
650 nm range.
Figure 5. The observed electronic absorption spectra of 1 and 2, and the calculated TD-DFT
(diamonds) oscillator strengths based on the X-ray structure of 1 and the optimized geometry of 2
at the B3LYP/6-31G* level of theory plotted against the primary and secondary vertical axes,
respectively. Black and light gray diamonds are used to highlight the L and B bands from Michl’s
4N+2 perimeter model[21] and the S, N1/2, and P1/2 bands from the 4N perimeter model.[22] The
details of the TD-DFT calculations and the angular nodal patterns of the main MOs are provided in
the Supporting Information.
In summary, a heteroaromatic
core-modified rubyrin bearing one
dithienylethene moiety (1) has been
ing Information), since six angular nodal planes are predicted
for the HOMO level and seven for the LUMO level due to an
ML = 0, ꢁ 1, ꢁ 2, ꢁ 3, ꢁ 4, ꢁ 5, ꢁ 6, ꢁ 7, ꢁ 8, ꢁ 9, ꢁ 10, ꢁ 11,
ꢁ 12, 13 sequence in terms of ascending energy for the MOs
derived from a parent C26H26 aromatic hydrocarbon perim-
eter with D26h symmetry. Michl and co-workers have demon-
strated that this is the pattern that would normally be
anticipated for a heteroaromatic 4N + 2 p-system with 26
atoms and 26 p-electrons.[21] Since only relatively minor
energy splittings (the DHOMO and DLUMO values to use
Michlꢁs terminology) are predicted for the four frontier p-
MOs derived from the HOMO and LUMO of the parent
perimeter, almost fully forbidden L bands and strongly
allowed B bands are expected to dominate the UV/Vis
absorption spectrum, with pairs of coupled Faraday b0 terms
of opposite signs anticipated in the magnetic circular dichro-
ism (MCD) spectrum in each case. The bands in the 450–600
and 650–850 nm ranges (Figure 5) can be assigned to the B
and L transitions based on the TD-DFT calculations and the
sign sequences observed in the MCD spectrum.
successfully synthesized and characterized for the first time,
along with a nonaromatic macrocycle containing both an
“open-form” and a “closed-form” dithienylethene moiety (2).
An analysis of the MCD spectral data and the TD-DFT and
ACID calculations clearly demonstrated that the p-systems of
1 and 2 are aromatic and nonaromatic, respectively. The use
of a dithienylethene moiety as a building block, therefore,
offers a new vista of opportunities for tuning the aromaticity
of ring-expanded core-modified porphyrins. Further research
on the incorporation of DTE moieties into larger conjugated
macrocycles to form stimuli-active molecules is currently in
progress.
Received: February 24, 2014
Published online: && &&, &&&&
Keywords: ACID plot · aromaticity · core-modified rubyrins ·
.
dithienylethene · macrocycles
Due to the presence of the “open-form” DTE unit, the
optical properties of the p-system of 2 can be readily
interpreted using Michlꢁs 4N perimeter model (Figure 4).[22]
Six frontier p-MOs derived from the HOMO, SOMO, and
LUMO of the parent hydrocarbon perimeter (referred to by
Michl as the hꢀ, h + , sꢀ, s + , lꢀ, and l + MOs) have six,
seven, and eight angular nodal planes (Figure S19 in the
Supporting Information), respectively, and play the key role
in the low-energy singlet electronic states. The intensities of
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Heteroporphyrins
Z. Zhou, Y. Chang, S. Shimizu, J. Mack,
C. Schꢁtt, R. Herges, Z. Shen,*
N. Kobayashi*
&&&& — &&&&
Core-Modified Rubyrins Containing
Dithienylethene Moieties
Tuning aromaticity: Two stable core-
modified rubyrins bearing one (1) and
two (2) dithienylethene (DTE) units have
been synthesized. Compound 1, with
a “closed-form” DTE unit, has a cyclic
conjugated system with 26 p-electrons. In
contrast, macrocycle 2 containing one
“open-form” DTE unit has nonaromatic
properties.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!