Photoelectrocatalysis to improve cycloreversion quantum yields of photochromic dithienylethene compounds

Article abstract of DOI:10.1002/anie.201206256

Photochromic cis-1,2-dithienylethene (DTE) compounds exhibit a reversible interconversion between the open and closed forms under alternating photoirradiation. DTEs could be potentially used for molecular photonic applications because the photochromism of

Full text of DOI:10.1002/anie.201206256

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Angewandte
Communications
DOI: 10.1002/anie.201206256
Photochromism
Photoelectrocatalysis to Improve Cycloreversion Quantum Yields of
Photochromic Dithienylethene Compounds**
Sumin Lee, Youngmin You,* Kei Ohkubo, Shunichi Fukuzumi,* and Wonwoo Nam*
Photochromic cis-1,2-dithienylethene (DTE) compounds
exhibit a reversible interconversion between the open and
closed forms under alternating photoirradiation. DTEs could
be potentially used for molecular photonic applications
because the photochromism of DTEs has advantageous
characteristics, such as fatigue resistance, short response
time, and reversibility.[1] In particular, the photochromism is
thermally irreversible, enabling high fidelity recording of
information based on a variety of signals, including fluores-
cence,[2] conductivity,[3] and chiroptical responses.[4] This
thermal irreversibility is attributable to the huge ground-
state thermodynamic barrier between the open and closed
forms of DTEs; this barrier is due to the symmetry constraint
as dictated by the Woodward–Hoffmann rule. For instance,
the barrier height of cis-1,2-di(3-thienyl)ethene is as high as
46 kcalmolꢀ1.[5] The barrier height is significantly lower in the
Figure 1. Schematic representation of cycloreversion in the photo-
chromism (a) and the electrochromism (b) of DTE compounds.
photoexcited state (i.e., DE* < DE0 in Figure 1a), thus
enabling chromic interconversion.[5,6] In this context, photo-
chromism can be referred to a process in which photo-
excitation provides an excited-state reaction path, thus
allowing a smaller energy barrier.
forms in the adiabatic potential energy surface (broken
arrows in Figure 1a).[5,6,9] There have been several
approaches to improve the FC!O. Irie and co-workers
accomplished a FC!O as high as 0.5 ꢁ 0.1 by multiphoton
excitation.[10] Alternatively, Scandola and co-workers
employed triplet sensitization to achieve a FC!O value of
0.51.[11] These methods take advantage of the higher excited
state or the triplet state having smaller activation barriers
(DE*). Despite of these achievements, however, the applica-
tion of these methods are limited because of the requirement
of sophisticated laser sources or judicious ordering among
electronic states of DTEs and the triplet sensitizer. Further-
more, because the direction and efficiency of the photo-
chromic reaction depends strongly on the electronic structure
of DTEs,[8,12] synthetic attempts to increase multiphoton
absorption cross sections or to conjugate triplet sensitizers
might offset the overall efficiency.
In contrast to photochromic cyclization,[7] photochromic
cycloreversion of DTEs is very inefficient. Typically, for
cycloreversion the photochromic quantum yields (FC!O) are
10ꢀ2–10ꢀ1-fold smaller than those for cyclization,[1b,8] and
high-power irradiation sources or long irradiation time is
required. This inefficiency arises because there is an activa-
tion energy to access the conical intersections from the closed
form (i.e., DE* in Figure 1a) and also because there is
branching at the conical intersections to both open and closed
[*] S. Lee, Dr. Y. You,[+] Prof. W. Nam
Department of Bioinspired Science, Ewha Womans University
Seoul 120-750 (Korea)
E-mail: odds2@kist.re.kr
It has been recently reported that photochromic DTEs
undergo cycloreversion in their oxidized states.[13] The elec-
trochromism features high conversion yields and enables dual
chromism because the oxidation reaction is independent of
the excited-state electronic structure of DTEs.[14] Recent
mechanistic studies suggest that the electrochromic cyclo-
reversion involves a radical cation of the closed form as
a reactive intermediate.[13a,e,f] Spontaneous cycloreversion of
the intermediate produces a radical cation of the open form;
this radical cation is neutralized by the neutral closed form.
This electrochromism is catalytic because the last step
regenerates the reactive intermediate.[13b]
wwnam@ewha.ac.kr
Homepage: http://cbs.ewha.ac.kr
Dr. K. Ohkubo, Prof. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering, Osaka University
ALCA, Japan Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[+] Current address: Korea Institute of Science and Technology (Korea)
[**] This work was supported by GRL (2010-00353) and WCU program
(R31-2008-000-10010-0) (W.N.) from the NRF, Korea, Grants-in-Aid
(nos 20108010 and 23750014) (S.F. and K.O.) of the Global COE
program from the JSPS (Japan), and RP-Grant 2010 (Y.Y.) of Ewha
Womans University.
The electrochromism involves radical intermediates pos-
+
sessing significantly lower barrier energies (i.e., DEC < DE0 in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206256.
Figure 1b),[14a] an effect similar to photoexcitation in the
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photochromism. This similarity stimulated us to develop
a photoelectrocatalytic strategy to improve the efficiency of
the cycloreversion. The idea is to employ a photoredox
catalyst that triggers catalytic electrochromism (Scheme 1).
Photoexcitation of 9-mesityl-10-methylacridinium ion (Acr+-
Mes) transiently generates the long-lived electron-transfer
Spectroscopic and electrochemical data of the DTE com-
pounds are listed in the Supporting Information, Table S1.
Figure 2a shows UV/Vis absorption spectra of acetonitrile
solutions of 2,7-dimethyl-9-mesityl-10-methylacridinium
(Me2Acr+-Mes) perchlorate[18] and the open (PhDTEo) and
the closed (PhDTEc) forms of PhDTE. The spectrum of
Me2Acr+-Mes featured vibronic absorption bands at 405–
450 nm, where both PhDTEc and
+
(eT) state (AcrC-MesC ) having high oxidation power.[15] An
PhDTEo had negligible absorption. Mon-
ochromatized 410 nm photoirradiation of
a CH3CN solution containing 1 mm of
Me2Acr+-Mes and 1 mm of PhDTEc led to
complete ring opening of PhDTEc, as
determined from the UV/Vis absorption
1
and H NMR spectra (Figure 2b and the
Supporting Information, Figure S2). The
absorption bands of Me2Acr+-Mes were
intact during the cycloreversion of
PhDTEc, thus indicating the catalytic
role of Me2Acr+-Mes. Indeed, full cyclo-
reversion was achieved in the presence of
catalytic amounts of Me2Acr+-Mes (0.1–
1 equiv; see the Supporting Information,
Figure S3). FC!O values, determined by
standard ferrioxalate actinometry (see the
Supporting Information), increased in
proportion with the concentration of
PhDTEC (the Supporting Information,
Figure S4). The photoaction spectrum
plotting the ratio of FC!O in the presence
of 1 mm of Me2Acr+-Mes to
FC!O in the absence of 1 mm of Me2Acr+-
Mes (i.e., FC!O(Me2Acr+-Mes)/FC!O(no
catalyst)) as a function of the photoirra-
diation wavelength is shown in Figure 2a.
This spectrum overlaps with the high-
Scheme 1. Mechanism of the photoelectrocatalytic cycloreversion of DTE compounds.
+
+
exoergic eT from DTE to the MesC moiety of AcrC-MesC
order vibronic absorption bands of Me2Acr+-Mes, thus
implying that cycloreversion of PhDTEc was due to photo-
excitation of Me2Acr+-Mes.
initiates the electrochromic ring opening of DTE in competi-
tion with the back eT (BeT) from the AcrC moiety to the
radical cation of the closed form (termination 1). The eT from
a closed form of neutral DTE to the open-form radical cation
completes cycloreversion to regenerate the closed-form
radical cation; this is the propagation step of the electro-
catalytic chain mechanism in Scheme 1.[16] The chain process
is terminated by BeT from the AcrC moiety to the open-form
radical cation (termination 2). This strategy benefits from the
catalytic nature of the electrochromism; in contrast to the
photon-stoichiometric photochromism, the photon economy
gains a leverage effect, thus leading to a greatly improved
FC!O. In addition, decoupling the photoexcitation compo-
nent (photoredox catalyst) from the cycloreversion compo-
nent (DTE) evokes excellent fatigue resistance (see the
Supporting Information, Figure S1). Herein, we describe the
proof-of-concept experiment for the photoelectrocatalytic
cycloreversion of DTEs. A FC!O value as high as 0.54 was
accomplished.
Under the optimized conditions (lex = 410 nm and
CH3CN solutions containing 1 mm of photoredox catalyst
and 1 mm of DTE), we determined the photoelectrocatalytic
FC!O of PDTE, PhDTE, MDTE, and CDTE. Two different
9-mesityl-10-methylacridinium ion derivatives, Me2Acr+-Mes
and Acr+-Mes, were employed as photoredox catalysts. The
photoelectrocatalytic FC!O values are summarized in Table 1,
which reveals that there is one order of magnitude enhance-
ment of FC!O relative to the conventional photochromic
FC!O (control). The largest FC!O value is 0.54, which is
comparable to the values obtained by multiphoton excita-
tion[10] and triplet sensitization.[11] It should also be empha-
sized that our method does not require expensive laser
sources and sophisticated energy alignments.
Based on the reaction paths depicted in Scheme 1, the
photoelectrocatalytic FC!O can be expressed as FC!O
=
a·((kini kp)/(kt1 kt2))(k1/(k1 + kꢀ1)), where kini, k1, kꢀ1, kt1, kt2,
and kp are rate constants for the initiation through oxidative
eT (kini), ring opening (k1) and ring closing (kꢀ1) between the
radical intermediates, two termination processes through
Four DTE compounds (PDTE, PhDTE, MDTE, and
CDTE; Scheme 1),[17] each having different terminal aryl
rings were used for the photoelectrocatalytic ring opening.
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Figure 2. a) UV/Vis absorption spectra of 1 mm of Me2Acr+-Mes and
the open (PhDTEo, 1 mm) and closed (PhDTEc, 1 mm) forms of
PhDTE (CH3CN, room temperature). The broken line with filled
Figure 3. a) Transient absorption spectra of 100 mm of Me2Acr+-Mes
(deaerated CH3CN) in the presence of 20 mm MDTEc after nanosecond
photoexcitation at lex =430 nm. b) Decay traces of the 480 nm absorp-
tion band at different concentrations of MDTEc (0–40 mm). Inset graph
is a plot of kobs versus [MDTEc], where kobs is the decay rate of the
480 nm band.
~
triangles ( ) is a photoaction spectrum plotting ratios of FC!O of
PhDTEc in the presence of 1 mm of Me2Acr+-Mes to FC!O in the
absence of 1 mm of Me2Acr+-Mes (FC!O(Me2Acr+-Mes)/FC!O(no
catalyst). b) Photoelectrocatalytic cycloreversion of 1 mm of PhDTEc
under 410 nm photoexcitation of Me2Acr+-Mes (1 mm; deaerated
CH3CN).
1070 nm, which correspond to the radical cation of the mesityl
Table 1: Cycloreversion quantum yields of DTE compounds.[a]
+
group (MesC ), and a dimer of the eT state and the neutral
PDTE
PhDTE
MDTE
CDTE
species (i.e., [(Me2AcrC-MesC+)·(Me2Acr+-Mes)]), respec-
tively.[19] The decay rate of the 480 nm absorption band
increased in the presence of increasing amounts of MDTEc
(Figure 3b). This increase is due to eT from MDTEc to the
Me2Acr+-Mes
Acr+-Mes
0.54
0.29
0.062
0.29
0.22
0.016
0.17
0.11
0.048
0.17
0.14
0.023
no catalyst[b]
Mes+ moiety with a large positive driving force of ꢀDGeT
=
[a] Determined by standard ferrioxalate actinometry (6.0 mm K3[Fe-
(C2O4)3], F=1.1 at 410 nm). Conditions: lex =410 nm
e[E*red(Me2Acr+-Mes)ꢀEox(MDTEc)] = 1.34 eV. The elec-
tron transfer rate (keT) was determined to be 1.0 ꢀ
1010 mꢀ1 sꢀ1 (Figure 3b), which is close to that of the diffusion
limited regime. Because keT values for the other combinations
of DTEs and photoredox catalysts were in the range of 0.26–
1.0 ꢀ 1010 mꢀ1 sꢀ1 (see the Supporting Information, Figure S5
and Table S2), it is inferred that keT, which corresponds to kini,
(8.3ꢀ10ꢀ10 einsteinsꢀ1), 1 mm of photoredox catalyst and 1 mm of DTE
in deaerated CH3CN. [b] Quantum yields in the absence of the photo-
redox catalyst. Conditions: lex =410 nm (8.3ꢀ10ꢀ10 einsteinsꢀ1) and
1 mm of DTE in deaerated CH3CN.
reductive BeT (kt1 and kt2), and propagation (kp). a is
a constant that accounts for molar absorbance and preserves
the dimensionless nature of FC!O. To determine kini, we
acquired transient absorption spectra of Me2Acr+-Mes after
nanosecond photoexcitation at lex = 430 nm (Figure 3a). The
transient absorption spectra comprised two bands at 480 and
is not a limiting parameter of FC!O
.
Once the radical cation of the closed form is generated by
the photoinduced eT, it undergoes thermodynamically
allowed ring opening. We previously determined the ring-
opening rates (k1) of the radical intermediates and found that
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Keywords: electron transfer · photochemistry · photochromism
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step competes with the termination 1 step. The FC!O value
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