Association-mediated chromism of amphiphilic triphenyl-6-oxoverdazyl

Article abstract of DOI:10.1039/b811207j

We synthesized a novel amphiphilic compound, 1,5-diphenyl-3-(4-(8-(pyridin- 1-ium-1-yl)octoxy)phenyl)-6-oxoverdazyl bromide (PyC8TOV) thorough modification of 1,3,5-triphenyl-6-oxoverdazyl by introducing a pyridinium head group. This amphiphilic radical dissolved in water at concentrations lower than 0.1 mM but formed aggregates at higher concentrations. Depending on its concentration in water, PyC8TOV afforded crystals, or gels. The solvent- and concentration- dependences of UV and ESR spectra of PyC8TOV are discussed in terms of the environmentally sensitive electronic structure of this amphiphilic radical. The nonlinear dependence of the relaxivity of an aqueous solution of PyC8TOV strongly suggests the formation of aggregates in concentrated solution. PyC8TOV exhibited not only solvatochromism but also association-mediated chromism. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b811207j

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Association-mediated chromism of amphiphilic triphenyl-6-oxoverdazylw
Kentaro Suzuki,ab Michio M. Matsushita,b Hiroyuki Hayashi,c Noboru Kogac
and Tadashi Sugawara*ab
Received (in Montpellier, France) 2nd July 2008, Accepted 11th August 2008
First published as an Advance Article on the web 20th September 2008
DOI: 10.1039/b811207j
We synthesized a novel amphiphilic compound, 1,5-diphenyl-3-(4-(8-(pyridin-1-ium-1-yl)-
octoxy)phenyl)-6-oxoverdazyl bromide (PyC8TOV) thorough modification of 1,3,5-triphenyl-6-
oxoverdazyl by introducing a pyridinium head group. This amphiphilic radical dissolved in water
at concentrations lower than 0.1 mM but formed aggregates at higher concentrations. Depending
on its concentration in water, PyC8TOV afforded crystals, or gels. The solvent- and
concentration-dependences of UV and ESR spectra of PyC8TOV are discussed in terms of the
environmentally sensitive electronic structure of this amphiphilic radical. The nonlinear
dependence of the relaxivity of an aqueous solution of PyC8TOV strongly suggests the formation
of aggregates in concentrated solution. PyC8TOV exhibited not only solvatochromism but also
association-mediated chromism.
concentration of TPV in water is ca. 10À5 M),14 amphiphilic
TPV must be more firmly held by membranes. Mayr et al.
reported the pioneering synthesis of the water-soluble TPV
Introduction
Considerable interest has been focused recently on molecular
(WS-TPV) radical.15 However, the hydrophilic moiety of this
self-assemblies constructed by amphiphilic molecules in water,
such as. micelles, bimolecular membranes and vesicles.1 Many
investigations of such self-assemblies have been concerned
with local phase separation among multiple amphiphiles in
radical is a carboxylic group that permits the WS-TPV to
dissolve only in alkaline water (pH 4 10). Furthermore, the
concentration dependence of UV and ESR spectra for
the assembly, with the elasticity of the self-assembly as a
WS-TPV is small.
whole.2 To detect such subtle changes in the local environment
of these self-assemblies by spectroscopic means, use of water-
soluble amphiphilic spin-probes is a powerful, and non-
invasive technique.3 Amphiphilic radicals employed in this
manner are useful for biological and medical applications.4–7
For example, nitroxides are often used as spin-probes to detect
local concentrations of superoxides at the surface or within the
membrane of blood vessels.5 These spin-probes can also be
used as contrast media in ESRI (ESR imaging; e.g. PEDRI/
OMNI)6 or MRI measurements.4,7
We have prepared a novel amphiphilic radical based on
The novel amphiphilic TPV-based radical discussed in this
paper is endowed with two chemical modifications. First, a
long alkyl chain bearing a pyridinium ion at the end is
introduced to increase its solubility and self-aggregation
ability in water over a wide pH range.1,2 Second, a 1,3,5-
triphenyl-6-oxoverdazyl (TOV) unit, which is a carboxo-
derivative of TPV, is used in lieu of TPV to contact with water
locally as a radical unit. When the methylene group at the
6-position of TPV is replaced with a carboxo group to form
TOV, the spin-densities of TOV become inequivalent.9,10
Accordingly, the electronic structure of TOV becomes sensi-
tive to the presence of water due to the contribution of its
zwitterionic form (Scheme 1).
1,3,5-triphenylverdazyl (TPV) to be explored as a spin-probe
for the above purpose. Although TPV is a well-known
persistent radical8–13 and has been studied in respect of its
magnetic properties11,12 and reactivities,13 its behavior in
water has scarcely been explored yet. Since the hydrophobicity
of TPV is much higher than that of nitroxides (the saturation
a Department of Basic Science, Graduate School of Arts and Sciences,
The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902,
Japan. E-mail: suga@pentacle.c.u-tokyo.ac.jp;
Fax: +81-3-5454-6997; Tel: +81-3-5454-6742
b Research Centre for Life System as Complex Systems, Department
of Basic Science, Graduate School of Arts and Sciences, The
University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902,
Japan
We found that our novel amphiphilic TOV, 1,5-diphenyl-3-
(4-(8-(pyridin-1-ium-1-yl)octoxy)phenyl)-6-oxoverdazyl bro-
mide (PyC8TOV; Py = pyridyl bromide), dissolves in water
and forms a self-aggregate, a glue or crystals, depending on the
concentration of PyC8TOV in aqueous solution. Notably,
c Department of Chemo-Pharmaceutical Sciences, Graduate School of
Pharmaceutical Science, Kyushu University, 3-1-1 Maidashi,
Higashi, Fukuoka 812-8582, Japan
w CCDC reference number 698233. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b811207j
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Scheme 1
these forms of PyC8TOV exhibit not only solvatochromism
but also association-mediated chromism in aqueous solution,
presumably due to the high hydrophobicity of TOV and the
specific interaction between its caboxo-group and the sur-
rounding polar protic solvents.
Experimental
Measurements
UV-Vis spectra of organic and aqueous solutions of
PyC8TOV at ambient temperature were measured with a
Shimadzu UV-3150 UV-Vis-NIR Spectrophotometer. X-Band
ESR spectra of a polycrystalline sample of PyC8TOV, a
chloroform and an de-ionized water solution of PyC8TOV
and a benzene solution of BrC8TOV were recorded on a JEOL
JES-TE300 ESR spectrometer. The concentration dependence
of the T1-relaxation time of PyC8TOV in aqueous solution at
25 1C was measured with a JEOL JNM-MU25A pulse NMR
spectrometer. The particle size distribution of PyC8TOV
micellar aggregates in aqueous solution was measured by
means of dynamic light scattering in the range of 0.01–5 mm
with a Nikkiso Microtrac IPA-150 particle-size distribution
analyzer. Microscopic images were observed under an
Olympus IX71 phase contrast/fluorescence microscope
equipped with a Hamamatsu Photonics ORCA-ER cooled
CCD camera. Cryogenic scanning electron microscopy
(Cryo-SEM) images were obtained with a Hitachi S3000N
scanning electron microscope operating at liquid–nitrogen
temperature. The temperature dependence of the magnetic
susceptibility of a glue-like sample of PyC8TOV, polycrystals
of BrC8TOV and PyC8TOVÁ1.5H2O were measured indivi-
dually with a Quantum Design MPMS-5 SQUID magneto-
meter in a temperature range of 2–300 K, the applied mag-
netic field being 0.5 T. The susceptibility data
were corrected by subtracting the diamagnetic component
evaluated by extrapolating the magnetic data at higher
temperatures.
Scheme 2 Reagents and conditions: (i) phenylhydrazine; (ii) triphos-
gene, pyridine; (iii) phenylhydrazine, triethylamine; (iv) silver(I) oxide;
(v) pyridine.
38 mmol; 1) in ethanol–toluene (1 : 1 v/v), which was prepared
according to the literature procedure,16 and the mixture was
refluxed for 2 h. After cooling to room temperature, the
precipitated product was filtered and dried in vacuo to afford
4-(8-bromooctoxy)benzaldehyde phenylhydrazone, 2, as a
white powder (11 g, 70% yield). Under a nitrogen atmosphere,
2 (10 g, 2.4 mmol) and anhydrous pyridine (2.3 g, 29 mmol)
were dissolved in anhydrous toluene (200 mL), and the
mixture was cooled to 5 1C. A solution of triphosgene (2.8 g,
8.0 mmol) in anhydrous toluene (50 mL) was added dropwise
to the cooled solution, and the mixture was refluxed for 2 h.
The resulting mixture was filtered through a short silica gel
column to remove pyridine hydrochloride salt, and the filtrate
Synthesis of PyC8TOV
The amphiphilic TOV radical 1,5-diphenyl-3-(4-(8-(pyridin-1-
ium-1-yl)octoxy)phenyl)-6-oxoverdazyl bromide (PyC8TOV)
was synthesized by modifying the procedure reported by
Neugebauer et al.10 (Scheme 2). Chemicals were obtained
from commercial sources and used without further purifi-
cation. Phenyl hydrazine (3.8 mg, 35 mmol) was added
to a suspension of 4-(8-bromooctoxy)benzaldehyde (12 g,
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was evaporated. The crude product, 4-(8-bromooctoxy)-
benzaldehyde 2-chloroformyl-2-phenylhydrazone (3), was
dissolved in chloroform; phenylhydrazine (2.2 g, 20 mmol)
and triethylamine (2.2 g, 22 mmol) were added to this solution;
and the mixture was stirred for 12 h at room temperature. The
reaction mixture was washed with water followed by saturated
aqueous NaHCO3 (for removing triethylamine hydrochloride
salt), dried by sodium sulfate and evaporated. The crude
product was purified by recrystallization from dichloromethane–
methanol to afford 1,4,5,6-tetrahydro-2,4-diphenyl-6-(4-(8-bromo-
octoxyphenyl))-1,2,4,5-tetrazine-3(2H)-one, 4 (6 g, 38% yield)
as a colorless powder. To a solution of 4 (1 g, 1.9 mmol) in 10 mL
dichloromethane was added Ag2O (3 eq.), and the mixture
was stirred for 1 h at room temperature. After filtration to
remove excess Ag2O, the resulting solution was purified by
silica-gel column chromatography using CH2Cl2 as the
eluent, and the product was recrystallized in ethanol to
afford 1,5-diphenyl-3-(4-(8-bromooctoxy)phenyl)-6-oxoverda-
zyl (BrC8TOV) as a dark green powder (640 mg, 64% yield);
mp 81–83 1C (from ethanol); ESR (benzene): g = 2.0032,
a/mT 0.651 (N1,5), 0.448 (N2,4). A 50-mL solution of anhy-
drous pyridine containing BrC8TOV (400 mg, 0.74 mmol)
was refluxed for 30 h to afford PyC8TOV as a dark blue,
deliquescent glue-like compound (450 mg, quant.). The
glue-like product was recrystallized from water at 40 1C to
afford PyC8TOVÁ1.5H2O as dark green, needle-like crys-
tals; mp 133 1C (decomp., from water); lmax(CHCl3)/nm: 269
(e/dm3 molÀ1 cmÀ1: 25 200), 274 (25 300), 313 (11 900, sh), 330
(10 300, sh) 577 (1700); m/z (MALDI-TOF): 534.3 (M+ + H
À Br. C33H36N5O2 requires 534.29); ESR (CHCl3): g =
2.0033, a/mT 0.65 (N1,5), 0.45 (N2,4). Purities of BrC8TOV
and PyC8TOV were evaluated by determining their magnetic
susceptibilities (vide infra).
X-Ray crystal structural analysis of PyC8TOVÁ1.5H2O
X-Ray crystallographic analysis of
a single crystal of
PyC8TOVÁ1.5H2O was carried out at room temperature with
a Rigaku Mercury CCD X-ray diffractometer with graphite-
monochromatized Mo-Ka radiation (l = 0.71073 A; 50 kV,
250 mA). The crystal, sized 0.50 Â 0.10 Â 0.02 mm, was fixed
at the top of a thin glass capillary by Araldite glue.
The structure was solved by direct method (SHELXS-97)
and refined by full-matrix least squares (SHELXL-97).17
Hydrogen atoms were introduced at calculated positions
(riding method). The asymmetric unit contains two
PyC8TOV and three H2O molecules. One molecule shows
disorder at the octamethylene unit while the other con-
former does not. The ratio of the major and minor set
of the disorder were optimized to be 0.64 : 0.36 upon
refinement.
Fig. 1 Crystal structure of PyC8TOVÁ1.5H2O.17 (a) ORTEP drawing
(50% probability chosen for the ellipsoids). (b) Crystal structure,
viewed along the a*-axis. (c) Stacking of the two kinds of crystal-
lographically independent PyC8TOV molecules (red and blue). For
clarity, only the triphenyl-6-oxoverdazyl part is shown.
Crystallographic data for PyC8TOVÁ1.5H2O. Black plate,
size 0.50 Â 0.10 Â 0.02 mm, C33H38BrN5O3.5, Mr = 640.60,
ꢀ
triclinic, space group P1 (no. 2), l(Mo-Ka) = 0.71073 A, T =
293(2) K, a = 10.2900(5), b = 14.210(1), c = 22.722(2) A,
a = 78.55(1), b = 82.28 (1), g = 79.98(1)1, V = 3189.4(4) A3,
Z = 4, Dc = 1.334 g cmÀ3. Isotropic and anisotropic
least-squares refinement (763 parameters) on 13831 merged
reflections (Rint = 0.0464) converged at Rall = 0.1703 for all
data; RI 4 2s(I) = 0.0623 for 5970 observed data (I 4 2s(I)),
GOF = 0.960.
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C = 0.376 emu K molÀ1; Curie–Weiss temperature, y =
À7.2 K). The Curie constant (0.376 emu K molÀ1) of the
anhydrous glue-like PyC8TOV (Fig. 2, open circles)
was reproduced by 44% of the paramagnetic component
(y B 0 K) and 56% of the antiferromagnetic component
(y B À30 K), indicating that the amorphous PyC8TOV
sample contained both paramagnetic and the antiferro-
magnetic domains. This result suggests that the magnetic
interaction between the amphiphilic PyC8TOV radicals is
negligibly small when they are randomly arranged, whereas
approximately half of the radicals form a tight antiferro-
magnetic stacking. The temperature dependence of the
magnetic susceptibility of BrC8TOV (Fig. 2, triangles) was
analyzed by the antiferromagnetic dimer model18 with C =
0.375 emu K molÀ1, y = À12 K and an intradimer interaction,
2J/kB, of À37.5 K.
Results and discussion
Crystal structure of PyC8TOV
Each asymmetric unit contains two crystallographically inde-
pendent radical molecules, two bromide ions and three water
molecules (Fig. 1(a)). Two crystallographically independent
radicals are observed, which differ in the conformation of their
alkyl chains and in the dihedral angles of their aromatic
planes. Crystal water molecules are located around the
pyridinium ion site, not at the radical site, forming a
hydrogen-bonded cluster with counter-anions located between
the layers of radical molecules; PyC8TOV does not interact
with these hydrogen-bonded clusters as shown in Fig. 1(b).
Thus, the radical molecules and hydrogen-bonded counter-
anions form a layered structure along the c-axis: these two
types of conformers are shown in Fig. 1(c) as red and blue
molecules. Each conformer forms head-to-tail dimers with
inversion symmetry and the dimers stack along the a-axis,
forming two distinct columns. One column consists of an
almost regular stacking, although the intermolecular distance
is rather remote and no p–p overlapping is recognized. In
contrast, a dimeric structure is observed in the other column.
These two types of columns arrange alternately along the
b-axis to form a sheet-like structure within the ab plane
(Fig. 1(c)).
Concentration dependence of UV spectra in aqueous solution
PyC8TOV dissolved in de-ionized water or in standard phos-
phate buffered solutions with pH values of 10.1, 7.41, 6.86 or
4.01. Though a o1 mM aqueous solution of PyC8TOV was
red (absorption maximum of verdazyl radical, lmax
=
540 nm), solutions more concentrated than 1 mM exhibited
a blue color (lmax = 575 nm), as shown by the absorption
spectra in Fig. 3(a). The critical concentration for this
observed color change was estimated to be 0.1 mM by extra-
polating the concentration dependence of lmax as shown in
Fig. 3(b). Namely, the absorption maximum remained at
540 nm (red) at concentrations lower than 0.1 mM and red-
shifted linearly to 586 nm (blue) as the concentration in-
creased, though the absorption coefficient, e, was maintained
at nearly constant values throughout (Fig. 3(a)). In contrast, a
chloroform solution of PyC8TOV showed a blue color,
which was similar to the concentrated aqueous solution of
PyC8TOV, and the concentration dependence of lmax was
very small.
Magnetic properties of PyC8TOV and BrC8TOV
The temperature dependence of the magnetic susceptibilities of
microcrystalline PyC8TOVÁ1.5H2O, the glue-like sample of
PyC8TOV and microcrystalline BrC8TOV were measured by
a SQUID magnetometer (Fig. 2). The purities of all radicals
were confirmed by their Curie constants near room tempera-
ture. The antiferromagnetic behavior of microcrystal-
line PyC8TOVÁ1.5H2O (Fig. 2, filled circles) was interpreted
in terms of the Curie–Weiss law (Curie constant,
To evaluate the size distribution of PyC8TOV aggregates, a
1 mM aqueous solution of PyC8TOV was sonicated for 20 min
and then subjected to dynamic light scattering analysis 6 h
after sonication. Although the size distribution of the aggre-
gates was scattered immediately after sonication, it converged
within 6 h, with aggregate sizes ranging from 0.1 to 1.2 mm and
a maximum frequency at ca. 0.2 mm (Fig. 4). Moreover, a
concentrated (420 mM) aqueous solution of PyC8TOV
formed a gel (Fig. 5(a) with an inverse hexagonal (HII phase)
structure,19 containing ca. 5 mm channels as shown by Cryo-
SEM (Fig. 5(b)). The melting point of this gel was ca. 45 1C,
and the gel crystallized upon prolonged standing.
Furthermore,
a red aqueous solution of PyC8TOV
(0.25 mM) turned blue upon the addition of 410 mM sodium
chloride (Fig. 6). This color change was likely due to aggrega-
tion of PyC8TOV caused by the charge-screening (or ‘‘salting-
out’’) effect2 of the added sodium chloride. An aqueous
solution of PyC8TOV containing 10–100 mM sodium chloride
exhibited lmax of ca. 575 nm, which was almost the same as
that observed for the chloroform solution described above.
These results indicate that PyC8TOV exhibited association-
mediated chromism.
Fig. 2 Temperature dependence of magnetic susceptibilities of micro-
crystalline PyC8TOVÁ1.5H2O (K), glue-like, anhydrous PyC8TOV
(J) and microcrystalline BrC8TOV (m) as determined by SQUID
magnetometer. The solid lines are theoretical fits corresponding to
each magnetic structure (see text). Inset: magnified drawing of the low
temperature region.
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Fig. 5 Phase contrast microscopic image (a) and cryo-SEM image (b)
of a 20 mM aqueous solution of PyC8TOV. Scale bars: 100 mm.
Fig. 6 Observed color change of a 0.25 mM aqueous solution of
PyC8TOV (red, left) caused by the addition of 50 mM NaCl (blue,
right).
Bathochromic shifts in absorption spectra caused by the
formation of J-aggregates have been well studied.20 Such shifts
are derived from the coupling between transition dipoles of
chromophores that interact intensely upon arrangement into
an aggregate structure. Additionally, an intense increase in
absorbance typically accompanies such bathochromic shifts.
However, no enhanced absorbance upon the formation of
aggregates was observed in the current experiment (Fig. 3(b)).
Accordingly, the concentration dependence of the
observed color change cannot be ascribed to the formation
of J-aggregates.
Fig. 3 (a) UV-Vis spectra of PyC8TOV dissolved in water at con-
centrations of 2.0 (—) and 0.125 (--) mM. (b) Concentration depen-
dence of lmax of PyC8TOV dissolved in water (J) and in CHCl3 (K).
Concentration dependence of ESR spectra in aqueous solution
ESR spectroscopic measurement of PyC8TOV was performed
in aqueous and non-aqueous solutions and it was found that
the spectrum in a diluted aqueous solution is different from
those measured in other conditions (Table 1 and Fig. 7). The
g-values in a 1.0 mM aqueous solution, in a chloroform
solution, and of the polycrystalline PyC8TOVÁ1.5H2O were
ca. 2.003. Reference samples (BrC8TOV, TOV and TPV) also
showed similar g-values. On the other hand, the spectrum of a
0.5 mM aqueous solution21 showed a g-value of 2.0041, being
slightly larger than others.
The hyperfine structure constants (hfsc’s) of nitrogen atoms
in a verdazyl ring, a(N), at the 1,5- and 2,4-positions of 1,3,5-
triphenylverdazyl (TPV) are the same as aforementioned;
a(N1,5) = 0.6 mT and a(N2,4), = 0.6 mT.9 However, they
become inequivalent, a(N1,5) = 0.45 mT and a(N2,4) =
0.65 mT, in 1,3,5-triphenyl-6-oxoverdazyl (TOV).10 The dif-
ference between a(N1,5) and a(N2,4) values of all TOV deriva-
tives in Table 1 is 0.2 mT. However, the absolute value of
a(N1,5) and a(N2,4) of PyC8TOV in 0.5 mM aqueous solution
were appreciably larger than others. This discrepancy indicates
Fig. 4 PyC8TOV aggregate size distribution measurements obtained
by dynamic light scattering analysis of a 1 mM aqueous solution of
PyC8TOV.
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Table 1 ESR parameters of a microcrystal, aqueous and non-aqueous solutions of PyC8TOV, and reference verdazyl radicals
Radical
Solvent
c/mM
a(N1,5)/mT
a(N1,5)/mT
g
PyC8TOV
Water
Water
CHCl3
Polycrystal
Benzene
Benzene
Waterb
Toluene
0.5
1
0.5, 1
0.47
n.d.a
0.448
n.d.a
0.45
0.6
0.67
n.d.a
0.651
n.d.a
0.65
0.6
2.0041
2.0033
2.0033
2.0030
2.0032
2.0033
BrC8TOV
TPV9
0.5, 1
1
WS-TPV15
TOV10
0.6
0.450
0.6
0.645
2.0037
a
b
Not detected. With 1 mM g-cyclodextrin.
À1
Fig. 8 Concentration dependence of T1 for an aqueous solution
of PyC8TOV. The dashed line corresponds to a two-thirds law fit
(T1À1 = gca + const., a = 3/2). Inset: log–log plot (DT1À1 = T1
À1
À
À1
T1
|c=0.25 mM).
À1
Since the gradient of the concentration dependence of T1 is a
measure of the relaxivity, g of the paramagnetic species toward
protons in the solvent, the heavy deviation suggests that the
interaction between the radical site of the amphiphile and
the protons in water was suppressed due to the aggregation of
the amphiphilic radicals at high concentration. The correla-
Fig. 7 ESR spectra and their simulations for 0.5 mM PyC8TOV in
water (a) and for 0.5 mM BrC8TOV in benzene (b). The intensities of
central resonance lines for PyC8TOV could not be reproduced accu-
rately by simulation, possibly because of the contribution of the
aggregated species in the aqueous solution.
À1
tion between T1 and the concentration of the amphiphilic
that the larger spin densities reside on four nitrogen atoms of
PyC8TOV in a diluted aqueous solution.
radicals was linear at concentrations o0.5–1 mM, but at higher
concentrations in which the aggregates formed (40.5–1 mM), the
correlation followed a ca. 0.6 (B2/3) power law (Fig. 8
inset). Under the assumption that mainly spherical aggregates
were generated, these results suggest that the radicals at the surface
of the spherical aggregates effectively induced the relaxation of
water protons, because the total surface area of a spherical particle
is proportional to two-thirds (B0.67) of its total volume.
Incidentally, the ESR spectrum of a 1 mM aqueous solution
of PyC8TOV did not exhibit hfsc, but only a broad signal
(peak-to-peak line width, DHpp, B0.8 mT) was obtained. This
broadening of the resonance lines may be ascribed to the
aggregation of PyC8TOV in a concentrated aqueous solution
(vide infra).
The reason why the critical concentration of aggregate
formation estimated by the T1 relaxation time measurement
is larger than that determined by the UV spectral measurement
may be explained as follows. Although the aggregates are
formed at concentrations higher than 0.1 mM, the size of the
aggregates is small and most of the amphiphilic radicals
are located at the surface, interacting with water molecules.
A deviation from the linear correlation between the T1 values
and the concentration of the amphiphilic radical can not be
T1 relaxation time measurement in aqueous solution
To elucidate the extent of PyC8TOV aggregation in water, T1
relaxation times of aqueous PyC8TOV solutions were
measured by means of the inversion recovery method using
pulse 1H NMR (Fig. 8). The inverse of the longitudinal
relaxation time, T1À1, increased with increasing concentratioÀns1
of PyC8TOV in water only at concentrations o0.5 mM; T1
markedly deviated from this relationship at higher concentrations.
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observed until the size of the aggregate becomes large enough
to have an inner filled structure.
Association-mediated chromism
The nonlinear change of the concentration dependence of UV
and ESR spectra of PyC8TOV occurred at nearly the same
concentration (B0.1 mM) as the critical concentration for
aggregate formation. Accordingly, the color change observed
in an aqueous solution of PyC8TOV indicated the formation
of aggregates.
The inverse of absorption maxima (lmaxÀ1) of a solution of
amphiphilic radicals is proportional to the HOMO–LUMO
À1
energy gap (DE) of the solvent molecule, and DE (p lmax
)
changes sensitivelyÀw1 ith changes in the polarity of the sol-
vent.22 When lmax of PyC8TOV in polar protic solutions
were plotted against the permittivity (ep) of the solvents (C–F,
without A and B), a steep linear dependence was observed as
shown in Table 2 and Fig. 9. Additionally, the lmaxÀ1 value in
water (ep = 80) adhered to this linear dependence at concen-
À1
Fig. 9 Variation of lmax of PyC8TOV in aqueous solutions (A: c
o0.1 mM, B: c 42 mM), isopropanol (C), n-propanol (D), ethanol
(E), methanol (F), 1,4-dioxane (G), acetone (H), MeCN (I), DMF (J),
DMSO (K), CHCl3 (L), CH2Cl2 (M). The solid and dashed lines are
obtained by a least-square method for protic polar solvents (C–F), and
aprotic polar ones (G–H, J and K), excepting MeCN,23 respectively.
À1
trations lower than 0.1 mM (Fig. 9, point A). If lmax of a
concentrated aqueous solution (42 mM) of PyC8TOV is also
plotted in Fig. 9 as point B, assuming ep = 80, it is located fÀar1
down from the straight line. The extremely small lmax
suggests that the effective ep of the sample in the concentrated
aqueous solution must be very small. Plots of polar aprotic
solvents (points G–K), except acetonitrile (I), are also fitted
with a straight line, although its slope is much more gradual:
the upward deviation of I (acetonitrile) suggests a specific
interaction with PyC8TOV.23 In nonpolar solvents, such as
carboxo group at the 6-position of PyC8TOV and the protic
solvent. As clearly shown by the crystal structure of the
hydrated crystal of p-acetamidophenyl-2,4-dimethyl-6-oxover-
dazyl, the 6-carboxo group is hydrogen-bonded with water
molecules in the crystal.24
À1
chloroform (L) and dichloromethane (M), the values of lmax
remain almost constant, regardless both of the permittivity
Conclusion
and of the concentration.
Although a simÀila1 r tendency is observed for WS-TPV,15 the
difference in lmax of TPV between the dispersed state and
the aggregated state is one-fourth of the corresponding differ-
ence for PyC8TOV (points A and B). This observation in-
A novel amphiphilic radical, PyC8TOV, exhibited not only
solvatochromism but also association-mediated chromism.
The solvent and concentration dependences of UV and ESR
spectra of PyC8TOV are explained in terms of the environ-
mentally sensitive electronic structure of this amphiphilic
radical. Namely, red solutions of dispersed radicals in water
changed to blue solutions of aggregated radicals as the PyC8-
TOV concentration was increased. The nonlinear dependence
of the relaxivity of PyC8TOV was consistent with the above
À1
dicates that the slope of lmax vs. permittivity for WS-TPV
might be much smaller than that of PyC8TOV. This observa-
tion also strongly suggests that the steep slope observed for
PyC8TOV in polar protic solvents can be ascribed partly to
the specific hydrogen-bond-type interactions between the
Table 2 UV spectroscopic data of solutions of PyC8TOV and WS-TPV15 in several solvents
Radical
Solvent
Polarity
ep
c/mM
lmax/nm
lmaxÀ1/cmÀ1
Color
Symbola
PyC8TOV
Water
Water
IPA
NPA
EtOH
MeOH
1,4-Dioxane
Acetone
MeCN23
DMF
DMSO
CHCl3
DCM
Polar (protic)
Polar (protic)
Polar (protic)
Polar (protic)
Polar (protic)
Polar (protic)
Polar (aprotic)
Polar (aprotic)
Polar (aprotic)
Polar (aprotic)
Polar (aprotic)
Non-polar
80
80
18
20
24
33
o0.1
42
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
540b
586b
574
575
572
566
577
575
564
18 500
17 100
17 400
17 400
17 500
17 700
17 300
17 400
17 700
17 500
17 500
17 400–17 500
17 500
14 200
13 800
Red
Blue
A
B
C
D
E
F
G
H
I
Purple
Purple
Purple
Purple
Purple
Purple
Purple
Purple
Purple
Purple
Purple
Green
Green
2.3
20
37
38
47
4.8
9.1
80
80
572
573
J
K
L
M
0.05–1
572–576b
572
706
Non-polar
Polar (protic)
Polar (protic)
0.5
o0.1
ca. 10
WS-TPV
Water
Water
727
a
b
See Fig. 9. See Fig. 3.
ꢀc
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observations. The steeply linear correlation between the lmax
values and the permittivities of PyC8TOV in polar protic
solvents suggests that the carboxo group at the 6-position of
the verdazyl group specifically interacts with water (or polar
protic solvents), reflecting the contribution of the zwitterionic
structure (Scheme 1). Since PyC8TOV was soluble in water in
a wide pH range, it may be used in various media, including
(e) K. Golman, J. S. Petersson, J. H. Ardenkjaer-Larsen,
I. Leunbach, L. G. Wistrand, G. Ehnholm and K. Liu, J. Magn.
Reson. Imaging, 2002, 12, 929–938; (f) S. Subramanian,
K. Matsumoto, J. B. Mitchell and M. C. Krishna, NMR Biomed.,
2004, 17, 263–294.
7 (a) S. W. A. Blight, C. T. Harding, P. J. Sadler, R. A. Bulman,
G. M. Bydder, J. M. Pennock, J. D. Kelly, I. A. Latham and
J. A. Marriott, Magn. Reson. Med., 1991, 17, 516–532;
(b) A. Muhler and O. Nalcioglu, J. Magn. Reson. Imaging, 1999,
¨
biological systems, as
purposes.
a spin-probe for some specific
9, 177–186; (c) H. E. Daldrup-Link and R. C. Brasch, Eur. Radiol.,
2003, 13, 354–365.
8 B. D. Koivisto and R. G. Hicks, Coord. Chem. Rev., 2005, 249,
2612–2630.
9 (a) R. Kuhn and H. Trischmann, Monatsh. Chem., 1964, 95,
457–479; (b) F. A. Neugebauer, H. Brunner and K. H. Hausser,
Tetrahedron, 1971, 27, 3623–3628; (c) K. Mukai, T. Yamamoto,
M. Kohno, N. Azuma and K. Ishizu, Bull. Chem. Soc. Jpn., 1974,
47, 1797–1798.
Acknowledgements
This work was partly supported by KAKENHI (Grant-in-Aid
for Scientific Research) on Priority Areas ‘‘Application of
Molecular Spins’’ (Area No. 769, Proposal No. 15087101)
and on Priority Area ‘‘Soft Matter Physics’’ (Area No. 463).
10 F. A. Neugebauer, H. Fisher and C. Krieger, J. Chem. Soc., Perkin
Trans. 2, 1993, 535–544.
11 (a) K. Mukai, in Magnetic Properties of Organic Materials, ed.
P. M. Lahti, Marcel Dekker Inc., New York, 1999, ch. 21,
pp. 427–447; (b) K. Mukai, in Molecular Magnetism—New Mag-
netic Materials, eds. K. Itoh and M. Kinoshita, Gordon and
Breach Science Publishers, Netherlands, 2000, ch. 5.2,
pp. 215–223.
12 (a) K. Mukai, S. Kawasaki, J. B. Jamali and N. Achiwa, Chem.
Phys. Lett., 1995, 241, 618–622; (b) M. Mito, H. Nakano,
T. Kawae, M. Hitaka, S. Takaki, H. Deguchi, K. Suzuki,
K. Mukai and K. Takeda, J. Phys. Soc. Jpn., 1997, 66,
2147–2156; (c) K. Mukai, M. Nuwa, K. Suzuki, S. Nagaoka,
N. Achiwa and J. B. Jamali, J. Phys. Chem. B, 1998, 102,
782–787; (d) M. Fujiwara, M. Tamura, M. Hara, K. Suzuki and
K. Mukai, J. Chem. Phys. B, 2007, 111, 9492–9495.
13 G. F. Dvorko, E. A. Ponomareva and M. E. Ponomarev, J. Phys.
Org. Chem., 2004, 17, 825–836.
14 I. A. Bezvershenko and V. Kh. Premyslov, Chem. Heterocycl.
Compd., 1985, 21, 946.
15 A. J. Mayr, M. P. Eastman, C. J. Hartzell, D. Dong and
C. McClellan, J. Magn. Reson., 1992, 99, 387–390.
16 Y. Okahata and T. Kunitake, J. Am. Chem. Soc., 1979, 101,
5231–5234.
17 G. M. Sheldick, SHELX97-Programs for Crystal Structure
Analysis (Release 97-2), 1998.
18 B. Bleaney and K. D. Bowers, Proc. R. Soc. London, Ser. A, 1952,
214, 451.
References
1 (a) M.-A. Guedeau-Boudeville, A.-L. Bernard, J.-C. Bradley,
A. Singh and L. Jullien, in Giant Vesicles, eds. P.-L. Luisi and
P. Walde, John Wiley & Sons, New York, 2000, ch. 26,
pp. 341–349; (b) K. Velonia, J. J. L. M. Cornelissen,
M. C. Feiters, A. E. Rowan and R. J. M. Nolte, in Nanoscale
Assembly: Chemical Techniques, ed. W. T. S. Huck, Springer
Science+Business Media, New York, 2005, ch. 7, pp. 119–185;
(c) T. Kunitake, Angew. Chem., Int. Ed. Engl., 1992, 39, 709–726.
2 (a) C. Taupin, in Soft Matter Physics, eds. M. Daoud and
C. E. Williams, Springer, Berlin, 1999, ch. 4, pp. 133–150;
(b) I. W. Hamley, Introduction to Soft Matter—Polymers, Colloids,
Amphiphiles and Liquid Crystals, John Wiley & Sons, New York,
2000.
3 (a) S. Ohnishi and H. M. McConnell, J. Am. Chem. Soc., 1965, 87,
2293; (b) W. L. Hubbell and H. M. McConnell, Proc. Natl. Acad.
Sci. USA., 1969, 63, 16–22; (c) W. L. Hubbell and
H. M. McConnell, Proc. Natl. Acad. Sci. USA, 1969, 64, 20–27;
(d) R. D. Kornberg and H. M. McConnell, Biochemistry, 1971, 10,
1111–1120; (e) P. Devaux and H. M. McConnell, J. Am. Chem.
Soc., 1972, 94, 4475–4481; (f) Spin Labeling, ed. L. J. Berliner,
Academic Press, New York, 1976; (g) Spin Labeling II, ed.
L. J. Berliner, Academic Press, New York, 1976.
19 J. M. Seddon, Biochim. Biophys. Acta, 1990, 1031, 1–69.
20 (a) G. Scheibe, Angew. Chem., 1936, 49, 563; (b) E. E. Jelly, Nature,
1936, 138, 1009–1010.
21 ESR measurements of PyC8TOV in lower concentrations than 0.5
mM is difficult due to the high dielectric constant of water, even
using a capillary tube.
4 L. J. Berliner, V. Khramtov, H. Fujii and T. L. Clanton, Free
Radical Biol. Med., 2001, 5, 489–499.
5 (a) K. Yamada, I. Yamamiya and H. Utsumi, Free Radical Biol.
Med., 2006, 40, 2040–2046; (b) H. Utsumi, K. Yasukawa,
T. Soeda, K. Yamada, R. Shigemi, T. Yao and M. Tsuneyoshi,
J. Pharmacol. Exp. Ther., 2006, 317, 228–235.
22 E. M. Kosower, An Introduction to Physical Organic Chemistry,
John Wiley & Sons, New York, 1968, ch. 2.6–2.8, pp. 293–342.
23 The inversed absorption maxima (lmaxÀ1) of acetonitrile–water
solutions of PyC8TOV increased proportionally with the ratio of
water. The intercept of lmaxÀ1 was almost the same as of lmaxÀ1 of
an acetonitrile solution which was doubly distilled over CaH2
(Fig. 9. point I).
6 (a) D. J. Lurie, D. M. Bussell and J. R. Mallard, J. Magn. Reson.,
1988, 76, 366–370; (b) D. Grucker, T. Guiberteau, B. Eclancher,
J. Chambron, R. Chiarelli, A. Rassat, G. Subra and B. Gallez,
J. Magn. Reson., Ser. B, 1995, 106, 101–109; (c) M. C. Krishna,
S. English, K. Yamada, J. Yoo, R. Murugesan, N. Devasahayam,
J. A. Cook, K. Golman, J. H. Ardenkjaer-Larsen, S. Subramanian
and J. B. Mitchell, Proc. Natl. Acad. Sci. USA, 2002, 99,
2216–2221; (d) D. J. Lurie, H. H. Li, S. Petryakov and S. J.
L. Zweier, Magn. Reson. Med., 2002, 47, 181–186;
24 M. J. Plater, S. Kemp, E. Coronado, C. J. Gomez-Garcıa,
´ ´
R. W. Harrington and W. Clegg, Polyhedron, 2006, 25, 2433.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
2208 | New J. Chem., 2008, 32, 2201–2208