Conformational solvatochromism: Spatial discrimination of nonpolar solvents by using a supramolecular box of a π-conjugated zinc bisporphyrin rotamer

Article abstract of DOI:10.1002/anie.200801332

(Graph Presented) Boxing clever: A cyclic tetramer of the dialkynylene-bridged zinc bisporphyrin rotamer [ZnP(≡)₂] shows a unique solvatochromic response to nonpolar solvents with equally low dielectric constants (2.23-2.57). This solvatochromism originates from the π conjugation of [ZnP (≡)₂]. The tetramer [{ZnP(≡) ₂}₄] can discriminate spectroscopically benzene from CCl₄ (see picture) and even the regioisomers of xylene.

Full text of DOI:10.1002/anie.200801332

Angewandte
Chemie
DOI: 10.1002/anie.200801332
Molecular Recognition
“Conformational” Solvatochromism: Spatial Discrimination of
Nonpolar Solvents by Using a Supramolecular Box of a p-Conjugated
Zinc Bisporphyrin Rotamer**
Junko Aimi, Yuka Nagamine, Akihiko Tsuda,* Atsuya Muranaka, Masanobu Uchiyama, and
Takuzo Aida*
“Solvatochromism” is the ability of chromophoric molecules
to change their absorption and/or luminescence colors as a
result of a change in solvent polarity.[1–5] In principle,
solvatochromic shifts are caused by a change in the energy
gap between a ground state and excited state of different
polarities,since a change in the solvent polarity leads to
differential stabilization of these electronic states. Therefore,
specific interactions between chromophore and solvent
molecules result in variations in the wavelength,intensity,
and shape of the absorption and/or luminescence spectra.
Recent examples of particular interest include conjugated
polymers that change their conformation and give rise to a
solvatochromic response.[4] From this general understanding
of solvatochromism,one may simply assume that the color of
a chromophoric molecule hardly depends on the chemical
structures of solvents when their polarities are,for example,
equally very low.[5] Here we report the interesting phenom-
enon that a dialkynylene-bridged zinc bisporphyrin rotamer
1H NMR,absorption,and emission spectroscopy,along with
size-exclusion chromatography (SEC; see FigureS1 in the
Supporting Information).[6] As reported in previous stud-
ꢀ
([ZnP( )2]) shows an explicit solvatochromic response to
nonpolar solvents with low dielectric constants (2.23–2.57). To
our surprise,it can discriminate the regioisomers of xylene.
ies,[7–9] the [ZnP( )1] monomer adopts exclusively a planar
ꢀ
ꢀ
conformation in its cyclic tetramer ([{ k ZnP( )1}4]),while
ꢀ
ꢀ
Zinc complexes [ZnP( )n] (n = 1,2,and 4) bearing two
longer alkynylene versions [{ZnP( )n}4] (n = 2,4) exist as a
meso-pyridyl (Py) groups are designed to form a box-shaped
cyclic tetramer in nonpolar solvents through Zn–Py coordi-
nation (Scheme 1). The assembling behavior of newly syn-
mixture of the two conformationally isomeric tetramers [{ ?
ꢀ
ꢀ
ZnP( )n}4] and [{ k ZnP( )n}4]. The cyclic tetramers composed
ꢀ
ꢀ
of [ ? ZnP( )n] are chiral,and that with n = 4 ([{ ? ZnP( )4}4])
has been demonstrated to sense the absolute structures of
asymmetric hydrocarbon solvents.[9] In the course of these
ꢀ
thesized [ZnP( )2] was unambiguously characterized by
ꢀ
[*] J. Aimi, Y. Nagamine, Dr. A. Tsuda, Prof. Dr. T. Aida
Department of Chemistry and Biotechnology
School of Engineering and Center for NanoBio Integration
The University of Tokyo
studies,we noticed that the colors of solutions of [ZnP( )2] in
benzene and CCl4 are quite different from one another
(Figure 1a),even though their dielectric constants are nearly
ꢀ
identical. The absorption spectrum of a solution of [ZnP( )2]
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7310
E-mail: tsuda@macro.t.u-tokyo.ac.jp
aida@macro.t.u-tokyo.ac.jp
in benzene showed Soret and Q bands at 447 (SI) and 626 nm
(QI),respectively (Figure 1a,green line). In contrast,the
absorption profile in CCl4 was considerably different,with the
Soret and Q bands appearing at 447 (SI)/483 (SII) and 686 nm
(QII),respectively (Figure 1a,red line). By reference to our
Dr. A. Tsuda
PRESTO, Japan Science Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama 332–0012 (Japan)
ꢀ
previous studies on a derivative of [ZnP( )2] with TMS-
[8]
ꢀ
Dr. A. Muranaka, Prof. Dr. M. Uchiyama
Advanced Elements Chemistry Laboratory
The Institute of Physical and Chemical Research, RIKEN
ethynyl groups at the meso positions and of [ZnP( )4],
together with the (TD)DFT-simulated absorption bands of a
ꢀ
dialkynylene-bridged zinc bisporphyrin reference [ZnP( )2*]
[**] This work was sponsored by a Grants-in-Aid for Scientific Research
(no. 17350044) and by an Encouragement of Young Scientists
(no. 18750113) from the Ministry of Education, Science, Sports, and
Culture, Japan. J.A. thanks the JSPS Research Fellowships for Young
Scientists. The calculations were performed on the RIKEN Super
Combined Cluster (RSCC).
with dihedral angles of 0 and 908 (B3LYP/6-31G*; see
Figure S2 in the Supporting Information),[6] the green and
orange-colored absorption spectra in Figure 1a can be
ꢀ
assigned to the conformational isomers [{ ? ZnP( )2}4] and
ꢀ
ꢀ
[{ k ZnP( )2}4],respectively. The box shape of [{ZnP( )2}4]
seems essential for the observed solvatochromic response,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801332.
ꢀ
since the absorption spectral profile of the free-base H2P( )2
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ꢀ
Scheme 1. Oligoalkynylene-bridged zinc bisporphyrin complexes [ZnP( )n] (n=1, 2, and 4) and their cyclic tetramers. Red and blue arrows
indicate the directions of the dipole moments.
ꢀ
In [D6]benzene at 208C,assembled [ZnP( )2] showed two
sets of upfield-shifted signals at d 2–3.5 and 5.5–6.5 ppm
which could be assigned to the ortho- and meta-H atoms of
the meso-pyridyl groups,respectively,upon coordination to
the zinc porphyrin moieties (Figure 2a). This feature most
ꢀ
likely originates from the assembly of [ZnP( )2] into the box-
ꢀ
shaped [{ ? ZnP( )2}4],where the inner and outer ortho- and
meta-H atoms of the coordinating pyridyl groups can be
discriminated from one another by NMR spectroscopy
(Scheme 1).[8] When this solution was evaporated to dryness,
and the residue was re-dissolved in CCl4,an extremely slow
ꢀ
ꢀ
transformation of [{ ? ZnP( )2}4] into [{ k ZnP( )2}4] took
place. As shown in Figure 2b,the eight characteristic
b pyrrole signals,initially observed at d 6.8–11 ppm,disap-
peared gradually with time,and a new set of eight b-pyrrole
signals appeared. This process was accompanied by a change
in the absorption spectra,as shown in Figure 1a (from the
green to the red curve),which finally subsided after 16 days.
An excellent correlation was found between the relative
ꢀ
Figure 1. Absorption spectra of [{ZnP( )2}4] at 208C in a) benzene
(green) and CCl4 (orange), and b) o- and p-xylene (red and black,
ꢀ
intensity of the QI/QII absorption bands and the [{ ? ZnP(
ꢀ
1
respectively). The inset shows photographs of solutions of [{ZnP( )2}4]
in benzene (left) and CCl4 (right).
ꢀ
)2}4]/[{kZnP( )2}4] ratio determined by H NMR spectroscopy.
This correlation allows convenient evaluation of the ratio of
the tetrameric conformers by absorption spectroscopy (see
below).
ꢀ
did not vary when the solvent was changed from benzene to
CCl4 (see Figure S3 in the Supporting Information).[6]
Cyclic tetramer [{ZnP( )2}4] can discriminate between
benzene and CCl4,and even the regioisomers of xylene
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Chemie
ꢀ
porphyrin units in [ZnP( )n] plays a critical role in the
observed solvatochromic response. One of the most impor-
tant factors in this respect is the p conjugation between the
zinc porphyrin units. As fully substantiated experimen-
tally,[14,15] bisporphyrins with
a
monoalkynylene bridge
always adopt a planar conformation,and this is also the
[8a]
ꢀ
ꢀ
case for [ZnP( )1] in its cyclic tetramer. Hence,[{ZnP( )1}4]
cannot show conformational solvatochromism. On the other
hand,the energy difference between the two conformers of
ꢀ
reference [ZnP( )4*] was estimated by means of DFT
calculations (B3LYP/6-31G*) to be negligibly small (0.09 kcal
molꢁ1; see Figure S5 in the Supporting Information).[6] This
small energy difference may be one of the main reasons why
ꢀ
[{ZnP( )4}4] does not show any solvatochromic responses. In
contrast,since the calculated energy gap of 0.57 kcalmol
ꢁ1
ꢀ
ꢀ
between [ k ZnP( )2*] and [ ? ZnP( )2*] (see Figure S5 in the
Supporting Information)[6,16] is not negligible,it can be
1
ꢀ
Figure 2. a) H NMR spectrum (d=2.0–11.0 ppm) of [{ZnP( )2}4] at
208C in [D6]benzene, and b) its time-dependent change (d=7.2–
10.2 ppm) after a change in the solvent to CCl4.
ꢀ
expected that [{ k ZnP( )2}4] will be preferred over [{ ?
ꢀ
ꢀ
? ZnP( )2}4]
ZnP( )2}4]. However,in some solvents,[{
ꢀ
rather than [{ k ZnP( )2}4] forms predominantly,which indi-
cates the presence of some competing factors (Figure 3). A
very likely candidate is a dipole–dipole interaction in the box-
shaped assembly,which cannot be ignored in nonpolar
solvents. As illustrated in Scheme 1,the dipole moments in
(Figure 1b). For convenience,“%stereoisomeric excess”
ꢀ
ꢀ
(%se) values,defined by ([{ k ZnP( )2}4]ꢁ[{?ZnP(
ꢀ
ꢀ
)2}4])/([{kZnP( )2}4]+[{?ZnP( )2}4]) ” 100,were employed
for illustrating the solvent-dependent conformational prefer-
ꢀ
ꢀ
ences of assembled [ZnP( )2] (Figure 3). As described above,
the %se values were estimated from the absorbances of the
[{ ? ZnP( )2}4] are more efficiently cancelled out than those
ꢀ
in [{ k ZnP( )2}4],thereby giving a certain advantage to [{ ?
ꢀ
ꢀ
QI and QII bands. [ZnP( )2] unit in the cyclic tetramer
ZnP( )2}4]. We presume that the balance between these
preferred to adopt a perpendicular conformation in p-
xylene (entry 2; se = ꢁ78%) but a planar conformation in
o-xylene (entry 7; se = 39%),while in m-xylene the mole ratio
competing factors,that is,the preferences arising from
p conjugation and dipole–dipole interaction,may vary
depending on the solvents,whilst their dipole moments are
equally low. More specifically,this change in balance may be
caused by particular electronic and/or steric effects of caged
ꢀ
ꢀ
of [ ? ZnP( )2] to [ k ZnP( )2] was nearly unity (entry 6; se =
2%). Since the dielectric constants of the regioisomers of
xylene are again nearly identical (2.27 and 2.57),the
ꢀ
solvent molecules within the cavity of the host [{ZnP( )2}4]
(see Figure S6 in the Supporting Information).[6,12] Again,for
ꢀ
solvatochromic response thus observed suggests that [ZnP(
ꢀ
)2] recognizes the shapes of solvent molecules when it forms
the cyclic tetramer.[10–13] Equally interesting,non-aromatic
hydrocarbons such as terpenes (entries 4 and 5; se = ꢁ25 and
ꢁ21%,respectively) were differentiated from terpinolene
(entry 9; se = 61%).
[{ZnP( )4}4],whose covalently linked zinc porphyrin units are
further apart from one another than those in [{ZnP( )2}4],the
issue of dipole–dipole interactions is considered much less
important. The large dimensions of the cavity and the
negligible p conjugation between the zinc porphyrin units
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
In contrast to [{ZnP( )2}4],[{ZnP( )1}4] and [{ZnP( )4}4],
which possess one and four triple bonds in the spacer,
respectively,were not solvatochromic in the nonpolar sol-
vents examined (see Figure S4 in the Supporting Informa-
tion),[6] which indicates that the spacer between the zinc
means that [{ZnP( )4}4] is intrinsically not capable of
conformational solvatochromism.
In conclusion,we have found a new principle of solvato-
chromism for nonpolar solvents with low dielectric constants
(2.23–2.57). This principle takes advantage of a confined
Figure 3. %Stereoisomeric excess (%se) values of [{ZnP( )2}4] in nonpolar solvents at 208C ([{ZnP( )2}]=5.9”10ꢁ6 m).
ꢀ
ꢀ
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www.angewandte.org
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Communications
R. Oitker,M. Yen,J. Zhao,L. Zang, J. Am. Chem. Soc. 2006, 128,
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chromophoric cavity,formed by the assembly of a p-
conjugated rotamer with various degrees of p conjugation.
In the assembly,the conformation of the rotamer is fixed as
either planar (conjugated) or perpendicular (nonconjugated),
which give rise to different absorption spectra. A solvato-
chromic response results because the ratio of the conformers
of the tetrameric assembly changes depending on the solvent.
This unique feature enables spectroscopic discrimination
between benzene (dielectric constant 2.27) and CCl4 (2.23)
and,to our surprise,even the regioisomers of xylene (2.27 and
2.57). The principle of these phenomena can be referred to as
“conformational” solvatochromism.
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