Chiral 1,2-subnaphthalocyanines

Article abstract of DOI:10.1021/ja2052667

Following the first suggestion of inherent molecular chirality in asymmetrically substituted subphthalocyanines by Torres and co-workers in 2000, elucidation of the relationship between structure and chirality has become an important issue. However, separation of the enantiomers has been prevented by the low solubility of the molecules synthesized to date, and it has not been possible to link the CD signs and intensities to their absolute structures. Recently, we observed that 1,2-subnaphthalocyanines possess two diastereomers with respect to the arrangement of the naphthalene moieties and that these novel chiral molecules exhibit moderate solubility in common organic solvents. This has enabled us to separate all of the diastereomers and enantiomers. The two diastereomers have been completely characterized by NMR spectroscopy and X-ray diffraction analysis. The absorption and magnetic circular dichroism spectra, together with theoretical calculation, reveal a small variation in the frontier molecular orbitals of the 1,2-subnaphthalocyanines compared with conventional subphthalocyanines, except for destabilization of the HOMO-3, which results in a characteristic absorption in the Soret band region. The chirality of 1,2-subnaphthalcyanines, including the CD signs and intensities, is discussed in detail for the first time with enantiomerically pure molecules whose absolute structures have been elucidated by single-crystal X-ray diffraction analysis.

Full text of DOI:10.1021/ja2052667

ARTICLE
pubs.acs.org/JACS
Chiral 1,2-Subnaphthalocyanines
Soji Shimizu,† Akito Miura,† Samson Khene,‡ Tebello Nyokong,*,‡ and Nagao Kobayashi*,†
†Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
‡Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
S Supporting Information
b
ABSTRACT: Following the first suggestion of inherent molecular chirality in
asymmetrically substituted subphthalocyanines by Torres and co-workers in
2000, elucidation of the relationship between structure and chirality has become
an important issue. However, separation of the enantiomers has been prevented
by the low solubility of the molecules synthesized to date, and it has not been
possible to link the CD signs and intensities to their absolute structures. Recently,
we observed that 1,2-subnaphthalocyanines possess two diastereomers with
respect to the arrangement of the naphthalene moieties and that these novel
chiral molecules exhibit moderate solubility in common organic solvents. This
has enabled us to separate all of the diastereomers and enantiomers. The two
diastereomers have been completely characterized by NMR spectroscopy and X-ray diffraction analysis. The absorption and
magnetic circular dichroism spectra, together with theoretical calculation, reveal a small variation in the frontier molecular orbitals of
the 1,2-subnaphthalocyanines compared with conventional subphthalocyanines, except for destabilization of the HOMOꢀ3, which
results in a characteristic absorption in the Soret band region. The chirality of 1,2-subnaphthalcyanines, including the CD signs and
intensities, is discussed in detail for the first time with enantiomerically pure molecules whose absolute structures have been
elucidated by single-crystal X-ray diffraction analysis.
’ INTRODUCTION
any mirror plane in their structures. This inherent chirality of the
asymmetrically substituted SubPc can be regarded as a type of
“bowl chirality”,14 which has been of intense interest in the fields
of fullerenes15 and carbon nanotubes,16 since molecules containing
three-dimensional chiral π-surfaces can provide a space for
molecular recognition and asymmetric catalytic reactions.
Although the use of chiral SubPcs for these applications has been
strongly anticipated, its chemistry is still at an early stage, with
little general information being available on the relationship
between the absolute molecular structures and the signs and
intensities of their circular dichroism (CD) spectra. Quantum
chemical calculation is a useful tool for predicting the structureꢀ
chirality relationship to a certain extent,17 but in a general sense,
elucidation of an absolute structure by single-crystal X-ray
diffraction analysis based on the Bijvoet method18 is much more
straightforward. However, in the previous studies, the low
solubility and small structural difference of the diastereomers
has prevented complete separation and characterization of all of
the diastereomers and enantiomers.12,13 For further study of the
structureꢀchirality relationship, SubPc molecules with moderate
solubility in common organic solvents and with small variations
in optical properties are required. 1,2-Naphthalocyanines19
synthesized from 1,2-naphthalenedicarbonitrile are known to
exist as four structural isomers having C4h, C2v, Cs, and D2h
symmetries with respect to the arrangement of the peripheral
Subphthalocyanine (SubPc) is a ring-contracted congener of
phthalocyanine (Pc), comprising three isoindole units linked by
three nitrogen atoms.1 As a result of its C3v-symmetric bowl-
shaped structure2 with a boron atom at the center and its 14π-
electron aromatic conjugation system, SubPc exhibits unique
optical properties, such as strong absorption in the UV/vis
region, intense fluorescence with moderate quantum yields,
and unique nonlinear optical properties.3,4 The central boron
atom plays a crucial role in maintaining the bowl-shaped
structure, as well as enabling further functionalization by linking
other functional units at the axial position.5 Due to the proximity
of their LUMO energy levels, hybrid materials of SubPc and C60
have been the subject of increasing interest in the fields of organic
solar cells6 and organic transistor devices,7 and the photoinduced
energy- and electron-transfer processes of SubPcꢀC60 conju-
gates have been intensively investigated.8,9 The bowl-shaped
molecular structure of SubPc is also beneficial for constructing
supramolecular architectures with fullerenes, both in solution10 and
in the solid state.11 Another unique property arising from the
structural features is an inherent molecular chirality when a
SubPc molecule is asymmetrically substituted, which was first
suggested by Torres and Claessens.12 From reactions of asym-
metrically substituted phthalonitriles, such as 4-iodophthalo-
nitrile12a and 3-nitro-5-tert-butylphthalonitrile,13 two structural
isomers having C3 and C1 molecular symmetries with respect to
the arrangement of the substituents were obtained, with each
structural isomer being inherently chiral due to the absence of
Received: June 7, 2011
Published: October 07, 2011
r
2011 American Chemical Society
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Journal of the American Chemical Society
ARTICLE
benzene rings, but the changes in positions and shapes of the
Q-band absorption from those of Pcs are small, except for the D2h
isomer.19b This prompted us to synthesize novel chiral SubPc
molecules by using a 1,2-naphthalene unit as a key building block.
The aim of this study was to separate all of the diastereomers and
enantiomers of 1,2-subnaphthalocyanines (1,2-SubNcs) and
determine their absolute structures by X-ray analysis, which has
enabled a comprehensive understanding of the structureꢀchirality
relationship of inherently chiral SubPcs.
Scheme 1. Synthesis of 1,2-Subnaphthalocyaninesa
’ EXPERIMENTAL SECTION
a Reaction conditions: (a) phenol, 120 °C, 2 h; (b) para-bromophenol,
120 °C, 2 h.
General Procedure. Electronic absorption spectra were recorded
on a JASCO V-570 spectrophotometer. CD and magnetic circular
dichroism (MCD) spectra were recorded on a JASCO J-725 spectro-
polarimeter equipped with a JASCO electromagnet, which produces
magnetic fields of up to 1.09 T (1 T = 1 tesla) with both parallel and
antiparallel fields. The magnitudes were expressed in terms of molar
ellipticity ([θ]/deg dm3 molꢀ1 cmꢀ1) and molar ellipticity per tesla
8.91 (d, J = 8.50 Hz, 3H), 8.27 (d, J = 8.50 Hz, 3H), 8.13 (d, J = 7.96 Hz,
3H), 8.00 (m, 3H), 7.77 (m, 3H), 6.76 (m, 2H; m-phenoxy), 6.62 (t,
J = 7.30 Hz, 1H; p-phenoxy), and 5.47 ppm (d, J = 8.62 Hz, 2H;
o-phenoxy); UV/vis (CHCl3): λmax [nm] (ε) = 291 (62700), 346
(17 300), 388 (20000), and 575 (91 200 Mꢀ1 cmꢀ1); HR-ESI-TOF-
MS: m/z (%): 661.1914 (100); Calcd for C42H23N6O1B1Na1 [M+ + Na],
661.1919.
([θ]M/deg dm3 molꢀ1 cmꢀ1 ꢀ1), respectively. Fluorescence spectra
T
were measured on a Hitachi F-4500 spectrofluorimeter. 1H NMR
spectra were recorded on a JEOL ECA-600 spectrometer (operating
as 594.17 MHz for 1H) and a Bruker AVANCE 400 spectrometer
(operating at 400.33 MHz) using the residual solvent as the internal
Axially Phenoxy Substituted 1,2-Subnaphthalocyanine
2c. H NMR (600 MHz, CDCl3, 298 K): δ = 10.19 (d, J = 8.20 Hz,
1
1
1H), 10.18 (d, J = 8.50 Hz, 1H), 10.13 (d, J = 8.62 Hz, 1H), 8.90 (d, J =
8.50 Hz, 1H), 8.87 (d, J = 8.50 Hz, 1H), 8.86 (d, J = 8.50 Hz, 1H), 8.26
(d, J = 8.32 Hz, 1H), 8.25 (d, J = 8.50 Hz, 2H), 8.12 (d, J = 7.61 Hz, 1H),
8.11 (d, J = 7.96 Hz, 2H), 8.03 (m, 2H), 7.99 (m, 1H), 7.76 (m, 3H), 6.76
(m, 2H; m-phenoxy), 6.62 (t, J = 7.31 Hz, 1H; p-phenoxy), and 5.47 ppm
(d, J = 8.60 Hz, 2H; o-phenoxy); UV/vis (CHCl3): λmax [nm] (ε) =
290 (77 600), 342 (19 800), 390 (20 600), and 575 (96 600 Mꢀ1 cmꢀ1);
HR-ESI-TOF-MS: m/z (%): 661.1915 (100) [M+]; Calcd for C42H23-
N6O1B1Na1 [M+ + Na], 661.1919.
Axially p-Bromophenoxy Substituted 1,2-Subnaphthalo-
cyanine 1d. 1H NMR (400 MHz, CDCl3, 298 K): δ = 10.13 (d, J =
8.05 Hz, 3 H), 8.91 (d, J = 8.55 Hz, 3 H), 8.28 (d, J = 8.30 Hz, 3 H), 8.13
(d, J = 8.05 Hz, 3 H), 8.00 (t, J = 7.05 Hz, 3 H), 7.77 (t, J = 8.05 Hz, 3 H),
6.85 (d, J = 9.06 Hz, 2 H; m-phenoxy), 5.35 ppm (d, J = 8.81 Hz, 2 H; o-
phenoxy); UV/vis (CHCl3): λmax [nm] (ε) = 291 (69 500), 347
(18 700), 389 (22 100), 534 (27 900), and 576 (97 000 Mꢀ1 cmꢀ1);
HR-ESI-TOF-MS: m/z (%): 739.1020 (100); Calcd for C42H22-
BBrN6NaO+ [M++ Na], 739.1024.
Axially p-Bromophenoxy Substituted 1,2-Subnaphthalo-
cyanine 2d. 1H NMR (400 MHz, CDCl3, 298 K): δ = 10.18ꢀ10.11
(m, 3 H), 8.91ꢀ8.85 (m, 3 H), 8.32ꢀ8.25 (m, 3 H), 8.18ꢀ8.10 (m, 3 H),
8.06ꢀ7.97 (m, 3 H), 7.85ꢀ7.74 (m, 3 H), 6.85 (d, J = 8.80 Hz, 2 H;
m-phenoxy), 5.35 ppm (d, J = 8.56 Hz, 2 H; o-phenoxy); UV/vis
(CHCl3): λmax [nm] (ε) = 291 (79 600), 343 (19 900), 391 (21 000),
and 575 (96 900 Mꢀ1 cmꢀ1); HR-ESI-TOF-MS: m/z (%): 739.1019
(100); Calcd for C42H22BBrN6NaO+ [M+ + Na], 739.1024.
reference for H (δ = 7.260 ppm for CDCl3). High resolution mass
spectra were recorded on a Bruker Daltonics Apex-III spectrometer.
Preparative separations were performed by silica gel column chroma-
tography (Merck Kieselegel 60H for thin layer chromatography).
Separation of all the enantiomers was carried out by high-performance
liquid chromatography (HPLC) with a preparative CHIRALPAK IA
column by monitoring absorbance at 520 nm.
Crystallographic Data Collection and Structure Refine-
ment. Data collection was carried out at ꢀ100 °C on a Rigaku Saturn
CCD spectrometer with Mo Kα radiation (λ = 0.71070 Å) for 1c and 2c
and at ꢀ173 °C on a Bruker APEXII CCD diffractometer with Mo Kα
radiation (λ = 0.71073 Å) for 1dFr1. The structures were solved by a
direct method (SHLEXS-97)20 and refined using a full-matrix least-
squares technique (SHELXL-97).20 CCDC-828217, -828218, and -828219
contain the supplementary crystallographic data for 1c, 2c, and 1dFr1,
respectively. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Molecular Orbital Calculations. The Gaussian 0321 software
package was used to carry out DFT and TDDFT calculations using the
B3LYP functional with 6-31G(d) basis sets. Structural optimization was
performed on 1b and 2b as model compounds for simplicity.
General Synthesis. To a 1,2,4-trichlorobenzene solution (8.0 mL)
of 1,2-naphthalenedicarbonitrile (357 mg, 2.0 mmol) was added a 1.0 M
xylene solution of boron trichloride (2.0 mL, 1.0 equiv) at room
temperature. The resultant mixture was gradually heated at 215 °C,
and the temperature was maintained for 3 h. After removal of the solvent,
an excess amount of phenol (>150 mg) was added, and the mixture was
heated at 120 °C for 2 h. The resultant solid was extracted with toluene
using a Soxhlet apparatus, to give a violet solid containing unreacted 1,
2-naphthalenedicarbonitrile and phenol, which were removed by sub-
limation at 140 °C under vacuum. The resultant mixture was chromato-
graphed on a flash silica gel (Silica gel 60H for thin layer chromato-
graphy, Merck), to purify two structural isomers, 1c (6.8 mg, 0.01 mmol,
1.6%) and 2c (24.3 mg, 0.04 mmol, 5.7%) by using a 1:1 (v/v) mixture of
CHCl3 and hexane as eluent. Axially p-bromophenoxy substituted 1,2-
subnaphthalocyanine was similarly synthesized and purified. The yields
of 1d and 2d were 1.7% (8.7 mg) and 12.6% (60.5 mg), respectively.
Axially Phenoxy Substituted 1,2-Subnaphthalocyanine 1c.
1H NMR (600 MHz, CDCl3, 298 K): δ = 10.13 (d, J = 8.50 Hz, 3H),
’ RESULTS AND DISCUSSION
Synthesis and Characterization. 1,2-SubNcs (1 and 2) were
synthesized from 1,2-naphthalenedicarbonitrile and boron trichl-
oride under conventional reaction conditions (Scheme 1).3
Separation of the diastereomers (1a and 2a) was prevented by
considerable tailing of axially hydroxy-substituted species (1b
and 2b), which were generated in the silica gel column. In order
to improve the column separation, an axial ligand exchange
reaction using phenol was performed prior to the separation
(Scheme 1). This enabled us to complete separation of 1c and 2c
in 1.6% and 5.7% yields, which were not poor considering that
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Figure 1. X-ray crystal structure of 1c, top view (left) and side view
(right). The thermal ellipsoids are scaled to the 50% probability level.
Figure 3. UV/vis absorption (bottom) and MCD (middle) spectra of
1c, and CD (top) spectra of 1cFr1 (blue) and 1cFr2 (red) in CHCl3.
Fluorescence spectrum in CHCl3 is shown as a dashed line (bottom).
Figure 2. X-ray crystal structure of 2c, top view (left) and side view
(right). The thermal ellipsoids are scaled to the 50% probability level.
approximately 70% of the starting 1,2-naphthalenedicarbonitrile
was recovered. Axially p-bromophenoxy-substituted 1,2-SubNcs,
1d and 2d, were similarly obtained in 1.7% and 12.6% yields,
respectively, using p-bromophenol in the last step of the reaction
(Scheme 1).
Compounds 1c and 2c were first characterized by high-
resolution electrospray ionization mass spectrometry (HR-ESI-
MS), with the observed molecular ion peaks at 661.1914 for 1c
and at 661.1915 for 2c (calcd for C42H23N6O1B1Na1 = 661.1919
[M+ + Na]). Successful replacement of the axial chlorine ligand
with p-bromophenoxy substituents was also confirmed by HR-
1
ESI-MS. The H NMR spectrum of 1c in CDCl3 exhibits
naphthalene proton peaks as a doublet at 10.13, 8.91, 8.27, and
8.13 ppm and as a triplet at 8.00 and 7.77 ppm due to the C3
1
symmetric structure, whereas the H NMR spectrum of 2c in
CDCl3 shows six pairs of either doublets or triplets at similar
chemical shifts to those of 1c because of the lower C1 symmetric
structure (Figure S1 in the Supporting Information (SI)). In
both cases, peaks of the axial phenoxy substituent are observed at
6.76, 6.62, and 5.47 ppm due to the diatropic ring current effects
Figure 4. UV/vis absorption (bottom) and MCD (middle) spectra
of 2c, and CD (top) spectra of 2cFr1 (blue) and 2cFr2 (red) in
CHCl3. Fluorescence spectrum in CHCl3 is shown as a dashed line
(bottom).
1
arising from the 14π-electron conjugation systems.3 The H
NMR measurements on 1d and 2d revealed similar signal
patterns to those of 1c and 2c (Figure S2 in the SI).
moieties of 1c are arranged in a clockwise manner, whereas
one of these points in the opposite direction in the case of 2c.
Similar to conventional SubPcs, both 1c and 2c adopt bowl-
shaped conformations with a boron atom at the center coordi-
nated in a tetrahedral fashion. The bowl depths defined by the
deviation of the boron atom from the mean plane of the three
coordinating nitrogen atoms (3N-mean plane) are 0.60 Å for 1c
Crystal Structures of 1c and 2c. Crystals of 1c and 2c suitable
for single crystal X-ray analysis were grown by slow diffusion of
2-propanol into CHCl3 solutions of 1c and 2c. Since 1c and 2c
are racemic mixtures, both enantiomers are observed in the unit
cells (Figure S3 in the SI), and only clockwise isomers are
depicted in Figures 1 and 2. Three peripheral naphthalene
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Figure 5. Partial molecular orbital diagram of SubPc (left), 1 (middle), and 2 (right).
and 0.63 Å for 2c, both of which are in the range of those of
conventional SubPcs.3
the MCD signals in the Q-band region of 1c are theoretically
regarded as a Faraday A term, but in order to facilitate compar-
ison of the observed oscillator and rotational strengths between
1c and 2c, the Faraday A term is expressed with two closely lying
Faraday B terms (i.e., as a pseudo Faraday A term) in the
following discussion. In the 250ꢀ450 nm region, several MCD
signals are observed corresponding to the characteristic absorp-
tions of 1,2-SubNcs. A dispersion type MCD trough (399 nm)
and peak (364 nm) associated with the broad absorption at
388 nm for 1c, and a similar MCD peak and trough at 370 and
399 nm associated with the absorption peak at 390 nm for 2c,
indicate the presence of transitions to nearly degenerate excited
states. In addition to the variation in shape of the absorption
spectra from those of SubPcs in the higher energy region, these
MCD signals imply that the annulation of benzene rings at the
α- and β-positions of SubPc causes significant changes in the energy
levels of molecular orbitals (MOs) lying below the HOMO and
above the LUMO+1. Details of this are discussed in the following
section, based on theoretical calculations.
Both compounds 1c and 2c exhibit intense mirror-imaged
emissions at 585 and 586 nm, respectively, upon excitation at
495 nm (Figures 3 and 4). The Stokes shifts of 1c and 2c are 297
and 326 cmꢀ1, and the fluorescence quantum yields and lifetimes
are 0.14 and 2.1 ns for 1c and 0.11 and 2.1 ns for 2c.27 These
values are very similar to those of SubPcs reported to date.3
Electronic Structures and Theoretical Absorption Spectra.
Structural optimization using 1b and 2b as theoretical models
was performed based on the DFT method at the B3LYP/6-
31G(d) level, while time-dependent (TD) DFT calculation
was also carried out at the same level. As a reference, DFT and
TDDFT calculations were also performed on a model com-
pound of unsubstituted SubPc having an OH group as an
axial ligand. In the frontier MOs, 1 and 2 essentially exhibit a
Electronic Absorption, Magnetic Circular Dichroism, and
Emission Spectra. Apart from some differences in the shoulder-
like absorptions at ca. 530 and 560 nm, the absorption spectra of
1c and 2c have essentially similar shapes, including values of the
molar absorption coefficient as well as positions of the main
absorptions at 575, 388, 346, and 291 nm for 1c and at 575, 390,
342, and 290 nm for 2c (Figures 3 and 4). Compared to
unsubstituted SubPc,3 the Q-bands of 1c and 2c (575 nm) are
shifted slightly to the red, by 10 nm, although a significantly larger
red shift of 95 nm is seen for 2,3-subnaphthalocyanine (2,3-
SubNc).22 This slight change in the shape and position of the
Q-band absorption is similar to that of 1,2-naphthalocyanines
from normal Pcs.19 In contrast to the small change in the Q-band
region, a significant difference from conventional SubPcs is
observed in the region of 250ꢀ450 nm. The absorption spectra
of SubPc molecules generally comprises two main broad bands
in the Soret-band region,3,23 whereas 1c and 2c exhibit three
bands at 388, 346, and 291 nm, and at 390, 342, and 290 nm,
respectively.
MCD measurements were performed on 1c and 2c (Figures 3
and 4) in order to obtain additional information on the electronic
structures. The MCD spectra of both 1c and 2c exhibit similar
spectral features over the entire region, where dispersion-type
signals are observed corresponding to the Q-bands, with negative
to positive envelopes at 578 and 566 nm for 1c and at 580 and
567 nm for 2c. Based on the molecular symmetries of C3 and C1,
these MCD signals can be assigned as a Faraday A term for 1c and
a pseudo Faraday A term for 2c. A pseudo Faraday A term is
generally observed when two Faraday B terms lie close in energy,
which occurs when Pc analogues with low symmetry accidentally
have nearly degenerate excited states.24ꢀ26 In the current case,
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Table 1. Selected Transition Energies and Wave Functions of 1, 2, and SubPc Calculated Based on the TDDFT (B3LYP/6-
31G(d)) Method
energy
compd
[nm]
f a
R(velocity)b
wave functiond
1
511
0.36
ꢀ51.6 (C)c
51.4 (AC)c
ꢀ49.5 (C)
49.7 (AC)
ꢀ52.5 (C)
53.1 (AC)
ꢀ48.9 (C)
48.7 (AC)
ꢀ11.3 (C)
11.3 (AC)
ꢀ20.3 (C)
20.3 (AC)
ꢀ19.5 (C)
19.5 (AC)
ꢀ17.7 (C)
17.7 (AC)
+ 0.621|146 r 145> ꢀ 0.114|147 r 138> + 0.167|147 r 142> + ...
508
399
395
515
505
403
397
0.35
0.24
0.24
0.28
0.42
0.20
0.20
+ 0.621|147 r 145> + 0.116|146 r 138> ꢀ 0.172|146 r 142> + ...
+ 0.636|146 r 142> ꢀ 0.149|147 r 143> ꢀ 0.128|146 r 144> + 0.112|147 r 145> + ...
+ 0.658|147 r 142> ꢀ 0.101|146 r 143> ꢀ 0.104|146 r 145> + ...
2
+ 0.622|146 r 145> ꢀ 0.125|147 r 138> ꢀ 0.150|147 r 142> + ...
+ 0.615|147 r 145> + 0.126|146 r 138> + 0.171|146 r 142> + ...
+ 0.591|146 r 142> ꢀ 0.104|146 r 138> + 0.125|147 r 142> ꢀ 0.250|146 r 144> ꢀ 0.118|147 r 145> + ...
+ 0.634|147 r 142> ꢀ 0.163|146 r 143> ꢀ 0.112|147 r 143> + ...
SubPc
489
486
299
0.31
0.31
0.26
+ 0.617|107 r 106> ꢀ 0.109|108 r 100> + 0.176|108 r 103> + ...
+ 0.618|108 r 106> + 0.111|107 r 100> ꢀ 0.177|107 r 103> + ...
+ 0.224|107 r 98> + 0.225|108 r 99> + 0.361|107 r 101> + 0.289|108 r 102> ꢀ 0.319|107 r 103> +
0.100|107 r 104> + 0.112|108 r 105> ꢀ 0.105|109 r 106> + ...
297
0.24
+ 0.225|108 r 98> ꢀ 0.196|107 r 99> + 0.163|108 r 100> + 0.435|108 r 101> ꢀ 0.123|107 r 102> +
0.353|108 r 103> + ...
a Oscillator strength. b Rotational strength in 10ꢀ40 cgs. c C and AC represent a clockwise and an anticlockwise isomer, respectively. d The wave functions
are based on the eigenvectors predicted by TDDFT. The |145> and |106> represent the HOMO of 1 and 2 and that of SubPc, respectively. Eigenvectors
greater than 0.10 are included.
type MCD signals. In the case of SubPcs, the bands next to the
Q-bands were calculated to lie at 299 and 297 nm, assigned to
transitions from MOs below the HOMO to the degenerate
LUMO and LUMO+1, whereas the corresponding bands of 1 at
399 and 395 nm and of 2 at 403 and 397 nm comprise transitions
mainly from the HOMOꢀ3 to the LUMO and LUMO+1
(Table 1). As a result of the small degree of mixing of bands in
this region, dispersion-type signals are clearly observed in the
MCD spectra, corresponding to the absorptions at 388 (1c) and
Figure 6. Diastereomers and enantiomers of 1,2-subnaphthalocyanines.
390 (2c) nm. These results indicate that the main change due to
the oblique annulation of benzene rings at the α- and β-positions
of SubPc is destabilization of the HOMOꢀ3. Thus, the two weak
bands at ca. 350ꢀ430 nm and the single intense band at ca.
290 nm in the observed absorption spectra are characteristic of
the electronic absorption spectra of 1,2-SubNcs, while conven-
tional SubPcs and 2,3-SubNcs only exhibit two broad bands in
the same region.3,22,23
Chiral Separation and Circular Dichroism Spectra. Optical
resolution of 1c and 2c was performed on an HPLC equipped
with a chiral column using a mixture of CH2Cl2 and hexane (1:1
(v/v) for 1c and 1:3 (v/v) for 2c), and two fractions were
obtained (1cFr1 and 1cFr2, and 2cFr1 and 2cFr2, Figure 6 and
Figure S5 in the SI). In the CD spectra, the first fractions, 1cFr1
and 2cFr1, show a negative sign between 275 and 600 nm and a
positive sign in the higher energy region, whereas the second
fractions, 1cFr2 and 2cFr2, show mirror-imaged positive and
negative signs in the corresponding regions (Figures 3 and 4).
Band deconvolution analysis28 was performed on the absorption,
similar electron density distribution pattern to that of SubPc,
and delocalization of electron density on the exterior benzene
rings is also observed for both 1 and 2 (Figure 5). The energy
levels of the HOMO and the first and second LUMOs are
slightly stabilized compared to those of SubPc. The HOMOꢀ
LUMO energy gaps of 1 and 2 are both 2.67 eV, which is smaller
than that of SubPc (2.73 eV). In-depth analyses on the MOs
above the LUMO+1 and below the HOMO reveal a slight
stabilization of the degenerate LUMO+2 and LUMO+3 and
significant destabilization of the HOMOꢀ1—HOMOꢀ3 com-
pared to SubPcs.
The TDDFT calculation estimated the Q bands to lie at 511
and 508 nm for 1 and at 515 and 505 nm for 2, which comprise
transitions from the HOMO to the nearly degenerate LUMO
and LUMO+1, and these values are shifted slightly to the red
compared to SubPcs (489 and 486 nm, Table 1). These results
are in good agreement with the observed small red shift of the
Q-band in the absorption spectra and the associated dispersion-
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Journal of the American Chemical Society
ARTICLE
The theoretical CD spectra based on TDDFT calculations
provide an initial clue for assigning absolute structures. Model
compounds with a clockwise arrangement of benzene rings
exhibit negative CD bands in the Q-band region, while, inversely,
those with an anticlockwise arrangement show positive CD
bands (Table 1 and Figures S8 and S9 in the SI). The theoretical
CD spectra also reproduce the intensity difference between
compounds 1 and 2.
Absolute Structure of 1d Elucidated by X-ray Crystallo-
graphic Analysis. Enantiomers of axially p-bromophenoxy-sub-
stituted species 1d and 2d, i.e. 1dFr1 and 1dFr2, and 2dFr1 and
2dFr2 (Figure 6), were similarly obtained by optical resolution
using the HPLC technique. These isomers exhibit similar CD
spectra to those of 1c and 2c (Figure S10 in the SI). Suitable
crystals of 1dFr1 were obtained by slow diffusion of methanol
into a toluene solution, and the crystal structure was unambigu-
ously elucidated by single crystal X-ray analysis (Figure 7 and
Figure S4 in the SI). The crystal structure of 1dFr1 indicates a
clockwise arrangement of the naphthalene moieties when viewed
from the axial position of the boron atom. This result is in good
agreement with the predicted molecular structure based on the
theoretical CD calculation. The negative CD signs in the Q-band
region are thus indicative of the clockwise isomer, whereas
the positive signs indicate an anticlockwise isomer (1dFr2 in
Figure 6). Since the CD signal is determined by the orientation of
the naphthalene moieties, the structureꢀchirality relationship
obtained for 1d appears also true for 2d. Moreover, it can be
reasonably claimed that this relationship will be applicable to 1c
and 2c, since the axial ligands have only a minor influence on the
absorption spectra and chirality of the molecules.
Figure 7. X-ray crystal structure of 1dFr1. The thermal ellipsoids are
scaled to the 50% probability level.
CD, and MCD spectra in the Q-band region in order to estimate
the rotational strengths from the observed spectra (Figures S6
and S7 in the SI). Eleven and twelve components, respectively,
were used to fit the region between 450 and 650 nm for 1cFr1
and 2cFr1. The experimental rotational strengths (Robsd) and
oscillator strengths (fobsd) of the Q 00-bands were estimated
using eqs 1 and 2,29,30 in which μ, h, m, ν, N, and c represent the
transition electric dipole moment, Planck’s constant, the electron
mass, the frequency at which absorption occurs, Avogadro’s
number, and the speed of light in a vacuum, respectively. The
experimental rotational strengths of 1cFr1 (ꢀ5.85 ꢁ 10ꢀ39 and
ꢀ5.19 ꢁ 10ꢀ39 cgs) are about three times larger than those of
2cFr1 (ꢀ2.08 ꢁ 10ꢀ39 and ꢀ2.06 ꢁ 10ꢀ39 cgs), whereas the
experimental oscillator strengths of 1c (0.13 and 0.12) are similar
to those of 2c (0.14 and 0.13).
’ CONCLUSIONS
In summary, the separation and characterization of all of the
diastereomers and enantiomers of 1,2-SubNcs was achieved
successfully, the first example in the chemistry of inherently
chiral SubPcs. Electronic absorption and MCD measurements, in
addition to theoretical calculation, reveal that the oblique annu-
lation of the benzene rings to SubPc causes minor alterations in
the energy levels of the HOMO, LUMO, and LUMO+1 but
major changes in the HOMOꢀ3, which leads to the appearance
of characteristic bands in the Soret band region. The CD spectra
of chiral 1,2-SubNcs exhibited similar shapes to those of
the absorption spectra apart from their signs, and the smaller
rotational strengths of enantiomers of 2c compared to those of 1c
were also observed, which can be ascribed to the reduction of
transition magnetic dipole moments of 2 by the one oppositely
arranged naphthalene moiety in 2. The absolute structure of
1dFr1 led us to a comprehensive understanding of the structureꢀ
chirality relationship of 1,2-SubNcs; i.e. the negative CD signs in
the Q-band region are indicative of a molecular structure where
the naphthalene moieties are arranged clockwise, while the positive
CD signs in that region indicate an anticlockwise arrangement.
The discussion in this article is useful for the analyses of the
structureꢀchirality relationship of the other chiral SubPcs and,
moreover, indispensable, considering the potential utility of this
three-dimensional chirality for future applications.
8π2mν
3he2
2
2
fobsd
¼
jμj ¼ 0:476 ꢁ 1030νjμj
ð1Þ
Z
Z
hc
48π2N
θðνÞ
θðνÞ
Robsd
¼
dν ≈ 0:696 ꢁ 10ꢀ42
dν ð2Þ
ν
ν
In principle, the CD intensity (rotational strength R) of chiral
molecules is expressed by the Rosenfeld equation.30
R ¼ Imðμ mÞ
ð3Þ
3
In this equation, μ and m are a transition electric dipole
moment and a transition magnetic dipole moment, respectively,
and Im denotes the imaginary part of μ m. Since oscillator
3
strengths (fobsd) are proportional to μ based on eq 1, 1c and
2c can be expected to have similar values of transition electric
dipole moments. Thus the smaller rotational strengths (Robsd) of
2c compared to those of 1c can be ascribed to the smaller
transition magnetic dipole moments of 2c as compared with
those of 1c. Considering that circular redistribution of charges in
transitions generates transition magnetic dipole moments, it
appears plausible that one oppositely arranged naphthalene
moiety in 2c reduces the net m values of 2c to one-third of
those of 1c.
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR, UV/vis absorption,
S
b
MCD, and CD spectra, crystal packing diagrams, chiral HPLC
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Journal of the American Chemical Society
ARTICLE
chromatograms, theoretical absorption and CD spectra, results
of the band deconvolution analyses, and complete ref 21. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(d) Iglesias, R. S.; Claessens, C. G.; Torres, T.; Rahman, G. M. A.;
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’ AUTHOR INFORMATION
(10) (a) Claessens, C. G.; Torres, T. J. Am. Chem. Soc. 2002,
124, 14522. (b) Claessens, C. G.; Torres, T. Chem. Commun. 2004, 1298.
(11) Shimizu, S.; Nakano, S.; Hosoya, T.; Kobayashi, N. Chem.
Commun. 2011, 47, 316.
Corresponding Author
nagaok@m.tohoku.ac.jp; t.nyokong@ru.ac.za
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’ ACKNOWLEDGMENT
This work was partly supported by a Grant-in-Aid for bilateral
program promoted by Japan Society for the Promotion of Science
(JSPS) and National Research Foundation (NRF) of South Africa,
a Grant-in-Aid for Scientific Research on Innovative Areas (No.
20108007, “pi-Space”) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT), and a Grant-in-Aid for
Scientific (C) (No. 23550040) from JSPS. We also thank NRF of
South Africa for funding (Grant number: 72195). The authors
thank Dr. Eunsang Kwon in Tohoku University for X-ray analysis,
and Prof. Takeaki Iwamono and Dr. Shintaro Ishida in Tohoku
University for X-ray measurement.
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