Photochromism of a diarylethene charge-transfer complex: Photochemical control of intermolecular charge-transfer interaction

Article abstract of DOI:10.1039/b604388g

A diarylethene derivative bearing a phenylenediamine group formed radical ions with an electron acceptor molecule in solution, and the concentration of the radical ions was modulated by the photochromic reaction of the diarylethene, reflecting the difference in the electron-donating character between the open- and closed-ring isomers. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604388g

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Photochromism of a diarylethene charge-transfer complex:
photochemical control of intermolecular charge-transfer interaction{
Masakazu Morimoto,a Seiya Kobatakeb and Masahiro Irie*a
Received (in Cambridge, UK) 27th March 2006, Accepted 25th April 2006
First published as an Advance Article on the web 22nd May 2006
DOI: 10.1039/b604388g
As a first step toward the photochemical control of solid-state
physical properties of EDA complexes, we report on an EDA
A diarylethene derivative bearing a phenylenediamine group
formed radical ions with an electron acceptor molecule in
solution, and the concentration of the radical ions was
modulated by the photochromic reaction of the diarylethene,
reflecting the difference in the electron-donating character
between the open- and closed-ring isomers.
complex that contains
a photochromic diarylethene unit.
Diarylethene 1 bearing a N,N,N9,N9-tetramethyl-p-phenylenedi-
amine group, which exhibits strong electron-donating character,
was synthesized as a photochromic electron donor (Scheme 1;
synthetic procedures and analytical data of 1 are described in the
supplementary information{). 2,3,5,6-Tetrafluoro-7,7,8,8-tetra-
cyano-p-quinodimethane (TCNQF4) was selected as an electron
acceptor. The formation and photochromic behavior of the EDA
complex, which consists of 1 and TCNQF4, in the solution phase
were studied.
Photochromic compounds have attracted great attention owing to
their potential application to opto-electronic devices, such as re-
writable optical memory media and optical switches.1,2 Among the
photochromic compounds reported so far, diarylethene derivatives
are the most promising candidates for the application because of
their thermal stability, fatigue resistance, and high reactivity in the
crystalline state.3 The diarylethenes undergo reversible photo-
isomerization between two isomers, open- and closed-ring forms.
The two isomers are different from each other not only in
absorption spectra but also in other physical and chemical
properties, such as geometrical structures,4 refractive indices,5
and oxidation/reduction potentials.6
Diarylethene 1 underwent photochromism in acetonitrile. The
acetonitrile solution of open-ring isomer 1a was colourless. Upon
irradiation with ultraviolet (UV) light, the colourless solution
turned blue. The colour change is due to the formation of closed-
ring isomer 1b by UV-irradiation. 1b has an absorption maximum
at 600 nm in acetonitrile. 1b was thermally stable in the dark
at room temperature. Upon irradiation with visible light (l >
480 nm), the blue solution of 1b was completely bleached, and the
absorption spectrum returned to that of 1a.
Electron donor–acceptor (EDA) complexes have been exten-
sively studied in the widespread fields from biological to materials
science.7 The association behavior of the complexes and the
absorption property of intermolecular charge-transfer (CT)
transition are dependent on the ionization potential of the donor,
the electron affinity of the acceptor, and environmental conditions.
In addition, the EDA complexes have attracted interest as organic
conductors because they exhibit unique conducting behavior in the
solid state.8 The conductivity is controlled by molecular stacking
structures and CT interactions between the donor and the acceptor
The electrochemical property of 1 was studied by cyclic
voltammetry.{ Both 1a and 1b show two oxidation waves (1a:
20.01 V and 0.31 V, 1b: 0.02 V and 0.36 V (vs. ferrocene/
ferrocene+, in acetonitrile)). N,N,N9,N9-Tetramethyl-p-phenylene-
diamine (TMPD) also shows two oxidation waves at 20.28 V and
0.30 V. The oxidation waves of 1a and 1b correspond to two-step
one-electron oxidations of the phenylenediamine moieties in the
molecules.12 The oxidation of the phenylenediamine moiety of 1a
molecules. If
a photochromic diarylethene unit could be
introduced into the EDA complex, photoinduced reversible
changes of oxidation or reduction potential of the diarylethene
would lead to the modulation of the properties of the complex,
such as the association constant and absorption spectrum of
the CT complex. Furthermore, if the reaction would proceed in
the crystalline phase, solid-state physical properties of EDA
complexes, such as electrical conductivity,9 magnetic properties,10
and non-linear optical properties,11 could be photochemically
controlled.
aDepartment of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, Hakozaki 6-10-1, Higashi-ku,
Fukuoka, 812-8581, Japan. E-mail: irie@cstf.kyushu-u.ac.jp;
Fax: +81-92-642-3568; Tel: +81-92-642-3556
bDepartment of Applied and Bioapplied Chemistry, Graduate School of
Engineering, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku,
Osaka, 558-8585, Japan. E-mail: kobatake@a-chem.eng.osaka-cu.ac.jp;
Fax: +81-6-6605-2797; Tel: +81-6-6605-2797
{ Electronic supplementary information (ESI) available: Experimental
section and supplementary data. See DOI: 10.1039/b604388g
Scheme 1 Photochromic electron donor 1 (1a: open-ring isomer, 1b:
closed-ring isomer) and electron acceptor TCNQF4.
2656 | Chem. Commun., 2006, 2656–2658
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occurs at more negative potentials than that of 1b. This indicates
that 1a has stronger electron-donating character than 1b. TCNQF4
shows reduction waves at 0.18 V and 20.36 V.{
observed in the absorption spectra of 1a under the bulk electrolysis
at the potential where the first oxidation of 1a takes place.{ It is
also reported in previous literature that a radical mono-cation of
TMPD shows an electronic absorption band around 600 nm.15 In
addition to the absorption bands due to the radical ions, the mixed
solution shows a weak absorption at longer wavelength than
1100 nm (Fig. 1c, inset). This absorption band extends to 2000 nm.
The weak absorption band can be ascribed to intermolecular CT
transition between donor 1a and acceptor TCNQF4. A complex
composed of TMPD and TCNQF4 has a similar CT band in the
near-infrared region.16 These results indicate that, in the mixed
In order to know the details of the electrochemical response of
the diarylethene, molecular orbital calculation was carried out
(Hartree–Fock (HF) functional, 6-31G* basis set{), although it
shows just a trend not the absolute value in the electrochemistry.
According to Koopman’s theorem, the negative of the eigenvalue
of HOMO calculated at the HF level correlates with the vertical
ionization potential, which correlates with the oxidation poten-
tial.13 The eigenvalues of the HOMOs which localize on the
phenylenediamine moieties are 27.5307 eV and 27.6540 eV for 1a
and 1b, respectively. This indicates that 1a can be oxidized at lower
potential than 1b. This trend agrees with the electrochemical
response.
solution of donor 1a and acceptor TCNQF4, both the CT complex
2
(1a?TCNQF4) and the radical ions (1a+ and TCNQF4 ?) are in
?
equilibrium. Because of strong electron-donating and accepting
characters of 1a and TCNQF4, the CT complex can dissociate into
the radical ions.
The EDA complex was prepared by mixing 1a and TCNQF4 in
a mixed solvent of chloroform and acetonitrile. The solutions of 1a
and TCNQF4 are colourless and yellow, respectively, as shown in
Fig. 1a and 1b. Upon mixing the compounds in the molar ratio of
1 : 1, the solution turned bluish green, as shown in Fig. 1c. The
absorption spectrum of the mixed solution has electronic
absorption bands in the visible and near-infrared regions, which
are not observed in the pure compounds (Fig. 1c). The sharp
absorption bands at 863 nm and 760 nm are ascribed to a radical
Photochromic behavior of the EDA complex of 1 and TCNQF4
was examined. Fig. 2 shows a photoinduced absorption spectral
change in the mixed solvent of chloroform and acetonitrile. A 1 : 1
(molar ratio) mixture of 1a and TCNQF4 exhibited bluish green,
and has absorption bands due to the radical ions and the CT
complex. Upon irradiation with 305 nm light, the absorption
intensity around 600 nm increased, indicating that the diarylethene
underwent photocyclization reactions to yield the closed-ring
isomer. Accompanying this photochromic reaction, the absorption
2
?
mono-anion of TCNQF4 (TCNQF4 ). A potassium salt of
2
2
intensity of TCNQF4 ? decreased, and the absorption intensity of
TCNQF4 (K+TCNQF4 ?) shows the same absorption spectrum.{14
The broad band around 600 nm can be assigned to a radical
the CT complex increased. Upon irradiation with 578 nm light, the
absorption intensity around 600 nm decreased along with the
+
?
mono-cation of 1a (1a ), in which the phenylenediamine moiety is
2
?
increase of the absorption due to TCNQF4 and the decrease of
the absorption due to the CT complex. The spectral change
indicates that dissociation of the CT complex to the radical ions
in the mixed solution is controlled by the photochromic reactions
of 1.
one-electron oxidized. The similar absorption around 600 nm is
In order to interpret the above absorption spectral change,
absorption spectra of mixtures of 1a and TCNQF4 and 1b and
TCNQF4 were measured. Fig. 3a shows absorption spectra of the
mixtures. Both the mixtures have sharp absorption bands due to
2
TCNQF4 ? and weak ones due to the CT complexes. Closed-ring
isomer 1b also formed a CT complex and radical ions with
TCNQF4. Although the absorption band due to a radical cation of
+
?
1b (1b ) should be observed around 600 nm, it overlaps with the
Fig. 1 Absorption spectra and photographs of (a) 1a (1.0 6 1025 M),
(b) TCNQF4 (1.0 6 1025 M), and (c) a 1 : 1 (molar ratio) mixture of 1a
and TCNQF4 ([1a] = [TCNQF4] = 7.5 6 1025 M) in a mixed solvent of
chloroform and acetonitrile (v/v 10 : 1). In c, the inset shows an absorption
spectrum of high-concentration solution ([1a] = [TCNQF4] = 2.5 6
1024 mol dm23) from 900 to 2200 nm.
Fig. 2 Photoinduced absorption spectral change of a 1 : 1 (molar ratio)
mixture of 1 and TCNQF4 ([1] = [TCNQF4] = 4.0 6 1025 M) in a mixed
solvent of chloroform and acetonitrile (v/v 10 : 1).
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in comparison with 1b. The difference is ascribed to the difference
in the electron-donating ability between the open- and closed-ring
isomers. The cyclic voltammetry indicates that 1a shows oxidation
waves at more negative potentials than 1b, that is, 1a is more
electron-donative than 1b. Such differences in the equilibrium
between the open- and closed-ring isomers leads to the
photoinduced change in the absorption intensities of the radical
ions accompanying the photochromic reactions of the diaryl-
ethene, as shown in Fig. 2.
In conclusion, we have demonstrated that the formation of
radical ions can be modulated upon photoirradiation using a
complex of a diarylethene bearing an electron-donative phenyle-
nediamine unit and TCNQF4. The absorption intensities of the CT
complex and the radical ions were controlled by photochromic
reactions of the diarylethene. This behavior is ascribed to the
difference in the electron-donating character between the open-
and closed-ring isomers of the diarylethene. To the best of our
knowledge, this is the first example of the photochemical control of
intermolecular CT interaction between electron donor and
acceptor molecules by the photochromic reaction.
Fig. 3 (a) Absorption spectra of 1 : 1 (molar ratio) mixtures of 1a and
TCNQF4 (dotted line) and 1b and TCNQF4 (solid line) ([1a] = [1b] =
[TCNQF4] = 5.0 6 1025 M) in a mixed solvent of chloroform and
acetonitrile (v/v 10 : 1). The inset shows an absorption spectrum of high-
concentration solution ([1a] = [1b] = [TCNQF4] = 2.5 6 1024 M) from 900
to 2200 nm. (b) Job plots of the absorbance for 1a and TCNQF4 (open
circles) and 1b and TCNQF4 (closed circles) at 550, 863, and 1300 nm. x is
molar fractions of donor (x = [1a]/([1a] + [TCNQF4]) or [1b]/([1b] +
[TCNQF4])).
This work was partly supported by Grant-in-Aids for Scientific
Research (S) (No. 15105006), Scientific Research on Priority Area
(432) (No. 16072214), and the 21st century COE program from the
Ministry of Education, Culture, Sports, Science, and Technology
of Japan. M. M. wishes to thank Research Fellowships of the
Japan Society for the Promotion of Science for Young Scientists.
Notes and references
absorption band due to the diarylethene moiety in the closed-ring
form. The stoichiometry of the CT complexes and the radical ions
was determined by Job-plot analysis of the corresponding
absorption bands.{ The Job plots shown in Fig. 3b indicate that
the donor and acceptor molecules interact in a 1 : 1 ratio.
Therefore, the equilibrium in the mixed solutions can be expressed
as shown in Scheme 2. It should be noted that the absorption
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2
intensities of TCNQF4 ? and the CT complexes are dependent on
2
the donors 1a and 1b. The absorption intensity of TCNQF4 is
?
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+
2
?
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complexes and the radical ions are in equilibrium and the
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determined to be 1.01 ¡ 0.06 and 0.32 ¡ 0.02 for 1a and 1b,
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Scheme 2 Equilibrium in mixtures of 1 (1a or 1b) and TCNQF4.
2658 | Chem. Commun., 2006, 2656–2658
This journal is ß The Royal Society of Chemistry 2006