Unprecedented radical-radical reaction of a [2.2]paracyclophane derivative containing an imidazolyl radical moiety

Article abstract of DOI:10.1021/ol1017933

Pseudogem-DPIM-DPI[2.2]PC dimer (3), with a C-N bond between the [2.2]paracyclophane ([2.2]PC) moiety and the imidazole ring, was synthesized. This is the first crystallographic observation of a highly sterically constrained 1-ene-2,5-cyclohexadiene structure for a [2.2]PC derivative. Compound 3 shows a photochromic behavior, exhibiting a color change from pale yellow to green upon either UV or visible light irradiation, both in the solid state and in solution at room temperature.

Full text of DOI:10.1021/ol1017933

ORGANIC
LETTERS
2010
Vol. 12, No. 18
4152-4155
Unprecedented Radical-Radical
Reaction of a [2.2]Paracyclophane
Derivative Containing an Imidazolyl
Radical Moiety
Sayaka Hatano,† Ken Sakai,‡ and Jiro Abe*,†
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin
UniVersity, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan, and
Department of Chemistry, Faculty of Science, Kyushu UniVersity, Hakozaki 6-10-1,
Higashi-ku, Fukuoka 812-8581, Japan
jiro_abe@chem.aoyama.ac.jp
Received August 1, 2010
ABSTRACT
Pseudogem-DPIM-DPI[2.2]PC dimer (3), with a C-N bond between the [2.2]paracyclophane ([2.2]PC) moiety and the imidazole ring, was
synthesized. This is the first crystallographic observation of a highly sterically constrained 1-ene-2,5-cyclohexadiene structure for a [2.2]PC
derivative. Compound 3 shows a photochromic behavior, exhibiting a color change from pale yellow to green upon either UV or visible light
irradiation, both in the solid state and in solution at room temperature.
The photochromism of a hexaarylbiimidazole (HABI),
formed by the radical-radical reaction of the oxidation
product of 2,4,5-triphenylimidazole (TPI), was first discov-
ered by Hayashi and Maeda.1a Since then, a large number
of HABI derivatives have been synthesized, and their
photochromic behaviors have been extensively investigated.1
In general, HABI derivatives are readily cleaved, both
thermally and photochemically, into a pair of triphenylimi-
dazolyl radicals (TPIRs) that can be recombined to afford
the parent HABI derivatives. The characteristic feature of
TPIR lies in their diversity of bond formation1f,g between
two TPIRs due to delocalization of an unpaired electron over
the whole molecule. We recently developed a photochromic
[2.2]PC-bridged imidazole dimer, with a [2.2]paracyclophane
([2.2]PC) moiety that couples two diphenylimidazole (DPI)
groups, that shows instantaneous coloration upon exposure
to UV light and rapid fading in the dark.2 The [2.2]PC-
bridged imidazole dimer shows photoinduced homolytic bond
cleavage of the C-N bond connecting the two imidazole
rings and successive fast intramolecular C-N bond forma-
tion, which can be well correlated with the behaviors
observed for HABI derivatives. In this study, we investigate
the radical-radical reaction of an analogous monoradical
containing a [2.2]PC moiety that sterically precludes the
intermolecular radical-radical reaction between the imida-
zolyl radicals.
† Aoyama Gakuin University.
‡ Kyushu University.
10.1021/ol1017933  2010 American Chemical Society
Published on Web 08/24/2010

Upon oxidation of pseudogem-DPIM-DPIH[2.2]PC (1)
with potassium ferricyanide under basic conditions, the color
of the solution changed from pale yellow to green. By
analogy to the oxidation product of TPI, the green-colored
species, ascribable to an imidazolyl radical, pseudogem-
DPIM-DPIR[2.2]PC (2) (Scheme 1), was formed. The
Scheme 1
.
Synthesis of the Pseudogem-DPIM-DPI[2.2]PC
Dimer
Figure 1. ORTEP representation of the molecular structure of 3
with thermal ellipsoids (50% probability), where nitrogen atoms
are highlighted in red. [2.2]Paracyclophane groups are disordered
over two sites, one of which is shown in this figure for clarity.
Hydrogen atoms and solvent molecules are omitted for clarity.
structure with a 1-ene-2,5-cyclohexadiene moiety but not the
tetraphenylethane structure.3 The most remarkable structural
feature of 3 is that the dimeric structure is given by a C-N
bond between the [2.2]PC moiety and the imidazole ring, in
which one of the benzene rings of the [2.2]PC moiety of a
monomer converted into a 1-ene-2,5-cyclohexadiene moiety.4
This is the first crystallographic observation of a highly
sterically constrained 1-ene-2,5-cyclohexadiene structure
for the [2.2]PC derivatives, even though the dimeric
structure involving a 1,4-cyclohexadiene moiety was
previously reported for the fluorinated 1,1,2,2,9,9,10,10-
octafluoro[2.2]paracyclophane.5 The C-N bond connecting the
[2.2]PC moiety and the imidazole ring in 3 (1.485(3) Å) has a
length quite consistent with that connecting the two imidazole
rings of HABI (1.482(2) Å).1j
3 shows a photochromic behavior, exhibiting a color
change from pale yellow to green upon either UV or visible
light irradiation, both in the solid state and in solution, at
room temperature. The vis-NIR absorption spectrum of 3 in
benzene is shown in Figure 2, along with that of the colored
species formed upon UV light irradiation (365 nm; a Keyence
UV-400 series UV-LED).
organic phase was separated from the mixture of benzene
and water, washed with water, and concentrated in vacuo
to afford a reddish orange powder. Recrystallization of
the compound from a mixture of dichloromethane and
acetonitrile with exclusion of light gave orange plates.
The X-ray crystallographic analysis revealed an unprec-
edented pseudogem-DPIM-DPI[2.2]PC dimer (3) (Figure
1) as a result of the intermolecular radical-radical reaction
of 2. This radical-radical reaction of 2 is analogous to that
of the triphenylmethyl radical. The structure of the dimer of
the triphenylmethyl radical was determined to be the quinoid
While both HABI derivatives and [2.2]PC-bridged imi-
dazole dimers have no absorption band at wavelengths longer
than 400 nm, 3 shows an absorption maximum at 420 nm,
which can be assigned to the π-π* transition localized at
the 2-(2,5-cyclohexadienylidene)-4,5-diphenyl-2H-imidazole
unit on the basis of the TD-DFT calculation (Figure S14,
Supporting Information), in addition to that at 360 nm. Thus,
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Org. Lett., Vol. 12, No. 18, 2010
4153

Figure 2
.
Vis-NIR absorption spectra of 3 in degassed benzene
Figure 3
.
ESR spectra of a UV-irradiated toluene solution (2.3 ×
(2.6 × 10-5 M, light-path length: 10 mm) measured at 298 K before
UV light irradiation (dashed line) and immediately after UV light
irradiation (solid line).
10-4 M) of 3 at 78 K. The inset is the detection of a forbidden
weak transition at half-field.
than 3. Though the presence of another nonphotochromic
dimer, given from the coupling of two molecules of 2, could
be considered, the details for such photoproducts of 3 are
under investigation.
The significant feature of the formation of 3 from two
molecules of 2 is in their unusual radical reaction on the
[2.2]PC moiety. To the best of our knowledge, no reports
have demonstrated such radical addition reactions at the
[2.2]PC moiety in the literature. The radical substituent on
the [2.2]PC moiety makes the radical-radical reaction
realized in this work possible. The resonance structure shown
in Figure 4a localizes the spin in the para-position to the
the photochromic color change of 3 can be realized under
visible light irradiation (at wavelengths shorter than 540 nm).
The light-induced color change gives rise to a sharp
absorption band at 400 nm and a broad absorption band
ranging from 500 to 900 nm. Obviously, the appearance of
these bands must be attributed to the photoinduced homolytic
cleavage of the C-N bond connecting the [2.2]PC moiety
and the imidazole ring, which was confirmed by the presence
of ESR-active species. The ESR signal intensities of the
toluene solution of 3 and the reference compound, DPPH
(diphenylpicrylhydrazyl), were recorded at 298 K to estimate
the degree of photodissociation in the photostationary state
upon UV light irradiation. By comparing the ESR signal
intensity of the sample solution with that of the DPPH
solution, the degree of photodissociation of 3 was estimated
to be 96%. The ESR spectra were also measured for the
toluene solution of 3 after light irradiation at 78 K. As shown
in Figure 3, a randomly oriented triplet pattern with an
overlapping doublet signal derived from a trace of free
radicals was observed for the UV-irradiated toluene solution.
The same spectrum was also observed under visible light
(450 nm) irradiation. The detection of a forbidden weak
transition (∆Ms ) 2) at half field shown in the inset of Figure
3 indicates the formation of a triplet-state species, which can
be assigned to the light-induced triplet radical pair. Therefore,
the colored species can be identified as a monoradical 2.
The back-reaction regenerating 3 is not accelerated by light
irradiation, and the thermal bleaching process takes place
over a day at 298 K in the dark because of the slow
intermolecular radical-radical reaction of 2 (Figure S12,
Supporting Information). Moreover, the back-reaction does
not proceed in a quantitative fashion, due to the fact that the
solution of 3 left to stand in the dark for a day after UV
light irradiation does not show a vis-NIR absorption spectrum
which entirely coincides with that observed for the solution
before UV light irradiation (Figures S11 and S13, Supporting
Information). This observation indicates the presence of a
side reaction leading to the formation of a byproduct other
Figure 4. (a) Resonance structures of 2. (b) The total spin-density
distribution of 2 calculated by the DFT UMPW1PW91/6-31G(d).
DPIR group, which will make a large contribution to the
radical-radical reaction on the [2.2]PC moiety. Indeed, the
spin-density distribution of 2 calculated by the DFT
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Org. Lett., Vol. 12, No. 18, 2010

UMPW1PW91/6-31G(d) method (Figure 4b) clearly shows
the delocalization of an unpaired electron over the [2.2]PC
moiety. Although the spin-density in TPIR also delocalizes
on the phenyl ring attached to the 2-position of the imidazole
ring, the radical addition on the phenyl ring does not lead to
formation of a bond between the imidazole ring and the
phenyl ring.1l This is due to the fact that the activation energy
for the bond formation between two imidazole rings is
different from that for the bond formation between the
imidazole and the phenyl rings. However, the bond formation
between the two imidazole rings of 2 seems unfavorable from
the steric viewpoints; there is a strong steric repulsion
between the DPI group and the methylated DPI group
(DPIM) that are facing each other.
In conclusion, based on the radical-radical reaction of a
[2.2]PC derivative containing an imidazolyl radical group,
we have synthesized a novel photochromic compound 3, with
a C-N bond formed between the [2.2]PC moiety and the
imidazole ring. This kind of bond formation is unusual and
quite unique in view of their photochromic behaviors. This
finding will lead to the development of a new class of
photochromic compounds based on imidazolyl radicals. We
also find for the first time that the stable 1-ene-2,5-
cyclohexadiene structure can be formed by such a radical
addition reaction by employing the [2.2]PC moiety.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research in a Priority Area “New
Frontiers in Photochromism (No. 471)” from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT), Japan, and by a High-Tech Research Center project
for private universities with the matching fund subsidy from
the MEXT.
Supporting Information Available: Synthesis of 3,
experimental details of spectroscopic measurement, crystal-
lographic data in CIF format, and details of DFT calculations.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL1017933
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