Mechanism and fluorescence application of electrochromism in photochromic dithienylcyclopentene

Article abstract of DOI:10.1021/ol300604n

The kinetic process of key intermediates involved in the electrochemical ring opening of photochromic dithienylcyclopentenes (DTEs) has been observed for the first time, where the electronic nature of the DTEs is an important factor that determines the rate-determining step in the electrochromism. The dual chromic property has been implemented to a single molecular fluorescence memory.

Full text of DOI:10.1021/ol300604n

ORGANIC
LETTERS
2012
Vol. 14, No. 9
2238–2241
Mechanism and Fluorescence Application
of Electrochromism in Photochromic
Dithienylcyclopentene
Sumin Lee,† Youngmin You,*,† Kei Ohkubo,‡ Shunichi Fukuzumi,*,†,‡ and
Wonwoo Nam*,†
Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea,
and Department of Material and Life Science, Graduate School of Engineering, Osaka
University, ALCA, Japan Science and Technology Agency (JST), Suita,
Osaka 565-0871, Japan
odds2@ewha.ac.kr; fukuzumi@chem.eng.osaka-u.ac.jp; wwnam@ewha.ac.kr
Received March 10, 2012
ABSTRACT
The kinetic process of key intermediates involved in the electrochemical ring opening of photochromic dithienylcyclopentenes (DTEs) has been
observed for the first time, where the electronic nature of the DTEs is an important factor that determines the rate-determining step in the
electrochromism. The dual chromic property has been implemented to a single molecular fluorescence memory.
Dithienylcyclopentene (DTE) undergoes reversible in-
terconversion under photoirradiation between open and
closed form isomers with distinct optical changes. The
photochromic DTE derivatives gather enormous research
interests because they exhibit ideal properties for molecu-
lar switches, such as bistability, thermal irreversibility, and
ultrafast conversion.1ꢀ4 Recently, it has been found that
some of DTE molecules feature similar conversion in their
oxidized or reduced states.5ꢀ7 Such electrochromism of
photochromic DTEs is valuable, offering an additional
means to address switching states (i.e., open and closed
states).8ꢀ14 In fact, the unique dual chromism has been
exploited to control memory functions15ꢀ18 and photo-
polymerization.19
In DTE molecules, electrochromism has resemblance to
photochromism in that it provides a pathway to bypass a
huge ground-state thermodynamic barrier required for
isomerization.20 Previous studies performed by several
groups revealed that substituents of the thiophenes have
profound consequence to direction of electrochromism
† Ewha Womans University.
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Perkin Trans. 2 1993, 1181–1186.
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‡ Osaka University.
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J. 1995, 1, 285–293.
r
10.1021/ol300604n
Published on Web 04/26/2012
2012 American Chemical Society

(i.e., ring opening vs ring closing).21,22 In contrast to
well-established photochromism,1,23 however, key inter-
mediates in the electrochromic processes have yet to be
fully elucidated to date. For example, the Branda,24ꢀ27
Launay,28 and Irie22 groups identified radical cations as
being responsible for oxidative ring opening or closing
reactions, whereas the Feringa group proposed that the
reactions proceed via dicationic states.29 Furthermore, the
rate-determining step in the electrochromism involving
electron transfer, isomerization, and back electron transfer
has not been established. Since the dual chromic properties
hold great promise for key applications in multifunctional
molecular devices, knowledge of mechanistic details is of
central importance.
In this study, the kinetics of oxidative ring-opening
reactions (i.e., c f c•þ f o•þ f o; c and o stand for closed
and open forms, respectively) of a series of DTE com-
pounds has been investigated by performing stopped-flow
UVꢀvis absorption measurements. The direct spectro-
scopic studies establish that rates for isomerization of the
key intermediates (i.e., c•þ f o•þ) are increased by elec-
tron-rich groups, whereas overall reaction rates display an
opposite trend. The unique dual chromic functionality has
been further implemented to a single molecular switch,
demonstrating bistable fluorescence memory.
Figure 1 displays dual chromism of DTEs. In order to
investigate electronic influence in the electrochromic pro-
cesses, four DTE compounds were prepared by introdu-
cing pyridyl (PDTE),30,31 phenyl (PhDTE),32 4-methoxy-
phenyl (MDTE),29 and 3,4-dimethoxyphenyl (CDTE) at
the 2-position of thiophene rings. The compounds were
fully characterized with standard identification methods,
and the results fully agreed with the proposed structures
(Supporting Information, Materials and Experimental
Details). The DTE molecules undergo efficient photochro-
mic interconversion between a closed form (DTEc) and an
open form (DTEo) under alternating photoirradiation at 254
and >500 nm, respectively. Photophysical data including
photochromic conversion yields and photochromic quantum
yields are listed in Supporting Information, Table S1.
Addition of oxidants, such as Cu(ClO4)2 (Ep(ox) =
1.09 V vs SCE), [Fe(bpy)3](PF6)3 (E1/2(redox) = 1.06 V vs
SCE), and [Ru(bpy)3](ClO4)3 (E1/2(redox) = 1.26 V vs
SCE), toacetonitrile(CH3CN) solutions containing 50μM
closed form of DTE compounds bleached the character-
istic blue absorption color of the closed form (Figure 1 and
Supporting Information, Figure S1),27 indicating occur-
rence of ring-opening reactions (see Supporting Informa-
tion, Figure S2 for 1H NMR spectra). The ring opening is
due to oxidation of DTEc because electron transfer from
DTEc to the oxidants is thermodynamically favored as
judged by the oxidation potentials of DTEc (Table 1). The
oxidation of DTEs generated radical cations (DTEc•þ) as
supported by distinct ESR signals of CH3CN solutions
of PhDTE, MDTE, and CDTE in the presence of 1 equiv
of [Ru(bpy)3](PF6)3 (Supporting Information, Figure S3).
A second oxidation to a dication of the closed form
(DTEc2þ) was not observed in the cyclic voltammograms
until the end of the oxidation of the open form (Supporting
Information, Figure S4). The oxidative ring opening is
efficient with conversion yields larger than 88%, as deter-
mined by 1H NMR spectroscopy (Table 1).
Table 1. Electrochemical and Kinetic Data of DTE Compounds
Ep(ox) (V, vs
SCE)a
kobs (sꢀ1 b
open closed c•þfo•þ
)
kobs (sꢀ1 b
overall densityc yield (%)d
)
spin
opening
PDTE NDe 1.05
0.058
0.071
0.33
2.1
0.058
0.037
0.0063
0.0596
0.0529
0.0465
quant
90
Figure 1. Structures of the open and closed forms of DTE com-
pounds and UVꢀvis absorption spectra displaying the photo- and
electrochromism of MDTE. Inset photo: CH3CN solutions of
open (left) and closed (right) forms of 10 μM MDTE.
PhDTE 1.54 0.90
MDTE 1.24 0.72
CDTE 1.15 0.69
88
0.00014 0.0463
quant
a Pt disk working electrode and Pt wire counter electrode; Ag/
AgNO3 pseudo reference electrode; 0.1 M n-Bu4NPF6 and 1 mM DTE
compound in deaerated CH3CN; scan rate = 100 mV/s. b 50 μM DTE þ
4 equiv Cu(ClO4)2 at 10 °C. c Quantum chemically calculated spin-
density of the cleaved CꢀC bond. See Supporting Information for
details. d Determined by 1H NMR. e Not detected.
(19) Areephong, J.; Kudernac, T.; de Jong, J. J. D.; Carroll, G. T.;
Pantorott, D.; Hjelm, J.; Browne, W. R.; Feringa, B. L. J. Am. Chem.
Soc. 2008, 130, 12850–12851.
(20) Staykov, A.; Areephong, J.; Browne, W. R.; Feringa, B. L.;
Yoshizawa, K. ACS Nano 2011, 5, 1165–1178.
(21) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.;
Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.;Eur. J.
2005, 11, 6430–6441.
(22) Moriyama, Y.; Matsuda, K.; Tanifuji, N.; Irie, S.; Irie, M. Org.
Lett. 2005, 7, 3315–3318.
(23) Nakamura, S.; Yokojima, S.; Uchida, K.; Tsujioka, T.;
Goldberg, A.; Murakami, A.; Shinoda, K.; Mikami, M.; Kobayashi,
T.; Kobatake, S.; Matsuda, K.; Irie, M. J. Photochem. Photobiol., A
2008, 200, 10–18.
Org. Lett., Vol. 14, No. 9, 2012
2239

As summarized in Table 1, reaction rates for DTEc•þ
f
DTEo•þ heavily depend on the nature of the attached aryl
rings. Electron-rich CDTE showed the fastest conversion,
whereas electron-deficient PDTE featured a slow ring
opening. This tendency can be understood in the light of
the calculated spin density of the cleaved CꢀC bond
between two thiophene rings (Table 1). Since SOMO
(singly occupied molecular orbital) of DTEc•þ contains a
bonding orbital interaction of the CꢀC bond (Supporting
Information, Figure S9),20 a smaller spin density value
implies an increased probability for ring opening. The
effect becomes more pronounced by delocalization of the
unpaired electron by substituents such as the 3,4-di-
methoxyphenyl group, resulting in the faster ring opening
of CDTE. In contrast, the overall reaction (i.e., DTEc f
DTEc•þ f DTEo•þ f DTEo) of CDTE is the slowest
among the tested DTE compounds. The overall reaction
rate constant of CDTE was 1.4 ꢁ 10ꢀ4 sꢀ1, whereas PDTE
exhibited the fastest ring opening with a reaction rate
constant of 5.8 ꢁ 10ꢀ2 sꢀ1 (Table 1 and Supporting
Information, Figure S10). We consider that an equilibra-
tion back to DTEc•þ (i.e., DTEo•þ f DTEc•þ) becomes
dominant or a reductive back electron transfer (i.e.,
DTEo•þ f DTEo) is retarded in the case of electron-rich
DTEs.33 In fact, slower back electron transfer toCDTEo•þ
is predicted from the lowest oxidation potential (Table 1).
Therefore, there is a kinetic trade-off between the isomer-
ization and the rest of reaction processes including back
electron transfer or an equilibrium between radical inter-
mediates. Based on the calculation, spectroscopic, and
electrochemical results, a reaction mechanism is proposed
in Scheme 1.
Figure 2. Stopped-flow UVꢀvis absorption spectra of 50 μM
MDTEc (CH3CN) after the addition of 200 μM Cu(ClO4)2
(CH3CN) at 10 °C: black, before the addition of Cu(ClO4)2;
red, 50 s after the addition of Cu(ClO4)2. Inset graph is a decay
trace of MDTEc•þ observed at 750 nm.
Reactions of DTEc•þ were monitored by stopped-flow
UVꢀvis absorption spectroscopy employing a single-mix-
ing technique. As shown in Figure 2, a broad absorption
band around 750 nm appears immediately after the addi-
tion of 200 μM Cu(ClO4)2 to 50 μM MDTEc (CH3CN,
10 °C). The absorption band is bathochromically shifted
relative to the neutral form (MDTEc, 593 nm), which is
consistent with a predicted UVꢀvis absorption spectrum
for MDTEc•þ (TD-DFT, uB3LYP/6-311þG(d,p); Sup-
porting Information, Figure S5). The 750 nm band of
MDTEc•þ obeys a monoexponential decay with a con-
comitant rise of new absorption bands at 534 and 420 nm,
which originate from a radical cation of an open form
(MDTEo•þ). TD-DFT calculations also support MDTEo•þ
being responsible for these absorption bands. In addition,
nearly identical UVꢀvis absorption changes were observed
when the oxidative reaction was triggered by [Fe(bpy)3](PF6)3
or electrolysis at 0.89 V (Supporting Information, Figures S6
and S7). The presence of a clear isosbestic point at 627 nm
indicates the conversion of MDTEc•þ f MDTEo•þ without
further oxidations or byproduct formation. PDTE, PhDTE,
and CDTE display similar spectral behaviors (Supporting
Information, Figure S8), indicating the identical cyclorever-
sion in the cyclohexatriene core (i.e., DTEc•þ f DTEo•þ).
Scheme 1. Mechanism of the Electrocatalytic Ring Opening of
DTE Compounds
With the understanding of the electrochromism of
photochromic DTEs, utility of the dual chromism has
been extended to fluorescence memory (SM1). For this
purpose, red-fluorescent rhodamines were introduced to
MDTE via ester linkages (Figure 3, refer to Supporting
Information for synthetic details). In the closed state
of SM1 (SM1c), photoexcitation of rhodamine (λex =
495 nm) produced weak fluorescence emission due to non-
radiative fluorescence-resonance energy transfer (FRET)
to the MDTEc moiety. The UVꢀvis absorption spectrum
of SM1c is summation of the absorption spectra of MDTEc
and rhodamine, revealing the absence of any ground-state
electronic interaction (Figure 3). Oxidative ring opening by
(24) Gorodetsky, B.; Branda, N. R. Adv. Funct. Mater. 2007, 17, 786–
796.
(25) Gorodetsky, B.; Samachetty, H. D.; Donkers, R. L.; Workentin,
M. S.; Branda, N. R. Angew. Chem., Int. Ed. 2004, 43, 2812–2815.
(26) Peters, A.; Branda, N. R. Chem. Commun. 2003, 954–955.
(27) Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404–
3405.
(28) Guirado, G.; Coudret, C.; Hliwa, M.; Launay, J.-P. J. Phys.
Chem. B 2005, 109, 17445–17459.
(29) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.;
Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.;Eur. J.
2005, 11, 6414–6429.
(30) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. J. Chem. Soc., Chem.
Commun. 1993, 1439–1442.
(31) Samachetty, H. D.; Branda, N. R. Chem. Commun. 2005, 2840–
2842.
(32) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc.
2000, 122, 4871–4876.
(33) The electron-rich CDTE and MDTE display temperature de-
pendence in their overall reaction rate constants stronger than that of
PDTE, which suggests equilibrium favored to the radical cation of
closed forms in the case of electron-rich DTEs. Refer to Supporting
Information, Figure S16.
2240
Org. Lett., Vol. 14, No. 9, 2012

accomplished by a combination of photochromism and
electrochromism. Specifically, a write process by oxidation
sets the “fluorescence-on” state. Read-out of the fluores-
cence information can be done by photoexcitation at 495
nm, which does not evoke photochromic ring opening of
SM1 (Supporting Information, Figure S15). Finally, ring
closing by photoirradiation at 365 nm is employed for an
erasure process to set the “fluorescence-off” state, completing
a full memory process, and this can be repeated many times
(Figure 4). The unique advantage of this system is the
additional addressability, which enables nondestructive
read-out because undesirable crosstalks between the read-
out and the write processes are inherently prohibited.
Figure 3. Fluorescence turn-on by the oxidative ring opening of
SM1. UVꢀvis absorption (dotted line) and fluorescence (solid
line) spectra of an open form (red), a closed form (blue), and a
closed form in the presence of 3 equiv of Cu(ClO4)2 (green) of 10 μM
SM1 (CH3CN) are shown. The UVꢀvis absorption spectrum of a
closed form of MDTE is included for comparison (gray).
Table 2. Spectroscopic Data of SM1a
conversion yield (%)e
λ
ems (nm)
τfl
PCQY
(%)d
Figure 4. Fluorescence switching of 10 μM SM1 (CH3CN) by
alternating photochromism (λirr = 365 nm þ 1 equiv of TPEN)
and electrochromism (oxidant = 1 equiv of Cu(ClO4)2).
(PLQY, %)b (ns)c
photo
electrof
open
591 (22)
590 (3.3)
1.57
1.39 2.0 (c f o) 48 (c f o)
33 (o f c) 60 (o f c) NRg (o f c)
85 (c f o)
closed
a 10 μMinCH3CN. b Photoluminescence quantum yield (λex = 495 nm).
c Fluorescence lifetime (λex = 450 nm; λobs = 590 nm). d Photochromic
quantum yield. e Determined by 1HNMR. f Oxidant = Cu(ClO4)2 (1equiv).
g No reaction.
In summary, the electrochemical ring opening step in
photochromic DTE compounds has been spectroscopi-
cally observed. The kinetic studies revealed that the elec-
tronic nature of the DTEs is the key factor that governs the
rate-determining step in the overall ring-opening reaction.
The dual chromic property was implemented to demon-
strate bistable fluorescence memory functions. Modula-
tionof intramolecular FRET between the DTE moietyand
fluorescent rhodamine was successfully accomplished by
the combination of electrochromism and photochromism.
Cu(ClO4)2 or photoirradiation at >500 nm eliminated the
characteristic absorption band of MDTEc, while the ab-
sorption band of rhodamine was intact. Conversion yields
for the photochromic and electrochromic ring opening are
48% and 85%, respectively (Table 2). The opening of SM1
turns on the fluorescence intensity with an on/off ratio of 7
due to decreased spectral overlap between the absorption
and the fluorescence emission bands. Suppression of the
nonradiative FRET in the open state of SM1 (SM1o) is
supported by anincrease in the photoluminescence lifetime
from 1.39 μs (SM1c) to 1.57 μs (SM1o). The fluorescence
modulation is reversible because photoirradiation at
365 nm regenerated SM1c with a conversion yield of
60% in the presence of a metal chelator such as N,N,N0,
N0-tetrakis(2-picolyl)ethylenediamine (TPEN).34 The fluor-
escence on/off memory function of SM1 can be therefore
Acknowledgment. This work was supported by CRI,
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.) a Global
COE program from the JSPS, Japan, and RP-Grant 2010
(Y.Y.) of Ewha Womans University.
Supporting Information Available. Synthesis and ex-
perimental details; Figures S1ꢀS39; Tables S1and S2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(34) Coordination of Cu(II) to TPEN induces a large negative shift of
the one-electron reduction potential when no cathodic peak was ob-
served within 0.4ꢀ1.2 V (vs SCE). In such a case, Cu(II) does not act as a
one-electron oxidant for DTEc.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 9, 2012
2241