A highly efficient silole-containing dithienylethene with excellent thermal stability and fatigue resistance: A promising candidate for optical memory storage materials

Article abstract of DOI:10.1021/ja5101855

Diarylethene compounds are potential candidates for applications in optical memory storage systems and photoswitchable molecular devices; however, they usually show low photocycloreversion quantum yields, which result in ineffective erasure processes. Here, we present the first highly efficient photochromic silole-containing dithienylethene with excellent thermal stability and fatigue resistance. The photochemical quantum yields for photocyclization and photocycloreversion of the compound are found to be high and comparable to each other; the latter of which is rarely found in diarylethene compounds. These would give rise to highly efficient photoswitchable material with effective writing and erasure processes. Incorporation of the silole moiety as a photochromic dithienylethene backbone also was demonstrated to enhance the thermal stability of the closed form, in which the thermal backward reaction to the open form was found to be negligible even at 100 °C, which leads to a promising candidate for use as photoswitchable materials and optical memory storage.

Full text of DOI:10.1021/ja5101855

Communication
pubs.acs.org/JACS
A Highly Efficient Silole-Containing Dithienylethene with Excellent
Thermal Stability and Fatigue Resistance: A Promising Candidate for
Optical Memory Storage Materials
Jacky Chi-Hung Chan, Wai Han Lam, and Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China
S
* Supporting Information
switchable molecular devices, the photochromic materials should
ABSTRACT: Diarylethene compounds are potential
candidates for applications in optical memory storage
systems and photoswitchable molecular devices; however,
they usually show low photocycloreversion quantum
yields, which result in ineffective erasure processes. Here,
we present the first highly efficient photochromic silole-
containing dithienylethene with excellent thermal stability
and fatigue resistance. The photochemical quantum yields
for photocyclization and photocycloreversion of the
compound are found to be high and comparable to each
other; the latter of which is rarely found in diarylethene
compounds. These would give rise to highly efficient
photoswitchable material with effective writing and erasure
processes. Incorporation of the silole moiety as a
photochromic dithienylethene backbone also was demon-
strated to enhance the thermal stability of the closed form,
in which the thermal backward reaction to the open form
was found to be negligible even at 100 °C, which leads to a
promising candidate for use as photoswitchable materials
and optical memory storage.
have high and comparable photocyclization and photocyclor-
eversion quantum yields with excellent thermal stability and
fatigue resistance. While there have been many efforts to
derivatize diarylethenes, especially dithienylperfluorocyclo-
pentenes, to tune their photophysical and photochromic
behaviors, studies on the design and synthesis of different
types of diarylethene backbones with high photoswitching
efficiency and high thermal stabilities are relatively less explored.
Most works mainly focus on modifications at the substituted aryl
groups of diarylethenes.1−3
Various researchers have shown that incorporation of
heterocycles into the bridging part of the diarylethene backbone,
instead of derivatizing the pendants of the dithienylperfluoro-
cyclopentene core, can enrich the photochromic and photo-
physical behaviors,5−10 such as enhancement of efficiency of the
photochromic system by stabilizing the photoactive antiparallel
conformation for the photochromic reaction.6 The most
commonly studied heterocycles include pyrroles, thiophenes,
indoles, thiazoles, imidazoles, and others.1−3,5−10 Recently,
various versatile photochromic compounds, such as imidazolium
salts and ionic liquids,3d,7e,8b N-heterocyclic carbenes,7e,8b,c
pyridylimidazoles,7d β-diketonates,8a,9c and phospholes,9e were
prepared to demonstrate the possibility of tunable photo-
chromism. However, incorporation of these heterocycles usually
shows less thermally stable closed forms, and the resistance to
photofatigue is not as good as the dithienylperfluoro-
cyclopentene derivatives, possibly due to the relatively higher
aromaticity of these heterocycles.1−3,5−9 Among the hetero-
cycles, siloles have been less extensively studied but have
attracted increasing interest due to their unique low-lying lowest
unoccupied molecular orbital (LUMO) level associated with the
σ*−π* conjugation between the σ* orbital of the silylene moiety
and the π* orbital of the endocyclic 1,3-butadiene.11 Apart from
the low-lying LUMO level, the π-electrons of siloles are mainly
localized on the CC bonds of the butadiene unit with only a
slightly higher aromaticity than cyclopentadiene; exploitation of
this interesting property has gained much less attention.12 We
believe that by utilizing this unique characteristic of siloles, the
designed photochromic diarylethenes may readily give rise to
some excellent photochromic properties as well as excellent
thermal stability and fatigue resistance.13 To the best of our
hotoswitchable materials have been widely used in photo-
P
switching, optical recording, and sensing devices.1,2 High
stability and efficiency are two major prerequisites for smart
materials.3 Among photoswitchable materials, photochromic
materials are the most widely explored. Of these, spiropyran is
most commonly used; however, common problems with
spiropyran-based smart materials include the presence of
efficient thermal reversible pathways and the loss of reversibility
brought about by photodegradation or photooxidation.4 Mean-
while, research on photochromic diarylethene compounds has
received much attention because of the distinguishable
absorption spectra, changes in emission intensity, and other
differences in physicochemical properties between the open and
closed forms. These properties have made diarylethene
compounds potential candidates for applications in optical
memory storage systems and photoswitchable molecular
devices.1,2 Although some of the photochromic diarylethenes
showed excellent thermal stability and fatigue resistance,3 they
usually show low photocycloreversion quantum yields1 that
result in inefficient photoinduced ring-opening processes and
slow photoresponses, which limit their potential use and
efficiency in photoswitchable devices. To be a promising
candidate of optical memory storage systems and photo-
Received: October 9, 2014
© XXXX American Chemical Society
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knowledge, although silole-containing materials have attracted
growing interest and have been studied by many groups, use of
the silole moiety as the heterocyclic backbone for photochromic
dithienylethenes has not been reported.14 Herein, we report the
syntheses of a silole-containing dithienylethene. Rich and
promising photochromic properties of the compound were
observed and investigated in detail. Insights into the electronic
structure of the compound and the nature of its excited states
have also been provided by computational studies.
shown in Figure 1. The significant bathochromic shift in the
closed form relative to its open form is mainly due to the increase
By modification of a synthetic pathway for the synthesis of
siloles reported in the literature,15 the target compound was
prepared in one single step by the intermolecular coupling
reaction of 2-silylaryl bromide with 1,2-bis(2,5-dimethyl-
thiophen-3-yl)ethyne (BDTE) associated with the C(sp3)−Si
bond cleavage in the trimethylsilane group, as shown in Scheme
1. BDTE was prepared by the iodination of 2,5-dimethylth-
Scheme 1. Synthesis of BzThSiMe2-DTE
Figure 1. UV−vis absorption spectral changes of BzThSiMe2-DTE in
benzene upon irradiation at 370 nm. Inset shows the plot of emission
change upon irradiation at 370 nm; asterisk represents an instrumental
artifact.
in planarity and extended π-conjugation across the 8a,8b-
dimethyl-1,8-thia-as-indacene moiety (Figure S2).7−9 The
thermal stability of the closed form is excellent, and the thermal
back-reaction into the open form is found to be negligible (<2%)
at 100 °C for 2000 min (Figure 2) and comparable to the
iophene, followed by Sonogashira coupling with ethynyl-
trimethylsilane and subsequent deprotection of the trimethylsilyl
group. Finally, Sonogashira coupling between 3-iodo-2,5-
dimethylthiophene and 3-ethynyl-2,5-dimethylthiophene pro-
duced the target compound in good yield. With BDTE, the target
compound could be prepared in one step via a Pd-mediated
coupling reaction. By utilizing this synthetic pathway and BDTE,
silole-containing dithienylethenes can be readily synthesized and
functionalized without the need for tedious synthesis and
modification of the dithienylethene framework.
BzThSiMe2-DTE was characterized by 1H NMR spectrosco-
py, mass spectrometry, and elemental analysis and was
structurally determined. The 1H NMR spectrum of the
compound revealed that the two dimethylthienyl moieties can
freely rotate at room temperature in solution medium, which
probably results from the lack of steric bulk on the thiophene
rings. Distances between the reactive carbon atoms on the
dithienylethenes are 3.571 and 3.609 Å, which indicate that the
compound in the crystal form is able to undergo photochemical
cyclization in the solid state.16
BzThSiMe2-DTE is found to dissolve in benzene to give a pale
yellow solution with two intense absorption bands at ∼332 and
371 nm (Figure S1; Table S1). The intense absorption bands are
tentatively assigned as intraligand π−π* transitions of the
benzo[b]silole moiety mixed with π−π* transitions of the
peripheral dimethylthienyl moieties. Upon photoexcitation at
room temperature, the compound displays intense greenish-
yellow luminescence at ∼491 nm, with a fluorescence quantum
yield of ∼0.14 in degassed benzene solution.
Upon photoexcitation at the π−π* absorption of BzThSiMe2-
DTE in degassed benzene, BzThSiMe2-DTE undergoes photo-
cyclization, and the solution turns to orange color with the
emergence of a low-energy absorption band at ∼474 nm. Two
well-defined isosbestic points at ∼370 and 415 nm are observed,
which indicate a clean conversion from the open to closed
form.1,7−9 The UV−vis absorption spectral changes upon
photoirradiation of BzThSiMe2-DTE in degassed benzene are
Figure 2. A plot of ln(A/Ao) versus time for the absorbance decay of
BzThSiMe2-DTE at 474 nm at 25 and 100 °C in argon-flushed 1,2-
dichlorobenzene solution. A denotes absorbance at time t; Ao denotes
initial absorbance; solid lines represent theoretical linear fits. Inset shows
the expanded plot of ln(A/Ao) versus time for the absorbance decay of
BzThSiMe2-DTE at 474 nm at 100 °C in argon-flushed 1,2-
dichlorobenzene solution.
thermally stable dithienylperfluorocyclopentenes.1 The decay
rate constant for BzThSiMe2-DTE is estimated to be 4.75 × 10−6
min−1, and the calculated half-life is found to be around 101 days
at 100 °C. To the best of our knowledge, this is one of the best
thermally stable photochromic compounds.1−3,7−10 It should be
noted that the closed form can return completely back to the
open form via photoirradiation at the lowest energy band of the
closed form with no signs of decomposition during the thermal
conversion measurement. As a result, the open form can only be
regenerated by photoexcitation.
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The quantum yields for both photocyclization and photcyclor-
eversion processes also were determined. A high photo-
cyclization quantum yield of ∼0.48 and a photocycloreversion
quantum yield of ∼0.42, which are comparable to each other and
seldom observed in photochromic dithienylperfluorocyclo-
pentene systems, were obtained.1−3,7−9 Interestingly, the
photocyclization quantum yield is close to the maximum value
of 0.5 that can be achieved for a 1:1 mixture of antiparallel and
parallel conformers in photochromic diarylethene systems, while
the photocycloreversion quantum yield is much higher than that
commonly observed in other photochromic diarylethene
systems, which is usually <0.1.1−3,7−10 With high photo-
cycloreversion quantum yield and excellent thermal stability,
the compound can undergo effective photoinduced ring-opening
processes. Large quantum yields in both the photocyclization
and photocycloreversion represent an important property to
increase the efficiency of photochromic-based molecular
memory devices.17 In addition, the conversion at the photosta-
tionary state of BzThSiMe2-DTE is determined to be ∼66%.
Besides excellent thermal stability, fatigue resistance represents
another important parameter commonly used to evaluate the
performance of photochromic materials.1 Photochromic materi-
als could lose their photochromic reactivities through side
reactions of the closed form; thus, photochromic compounds
should have a high fatigue resistance to be employed for different
practical applications. BzThSiMe2-DTE shows high fatigue
resistance as demonstrated in Figure 3 and Figure S3.
Figure 4. Color change of BzThSiMe2-DTE in benzene and in PMMA
film upon photoirradiation at 298 K.
attributed to the generation of the non-emissive closed form
and the depletion of the open form.7−9 Apart from conversion to
the non-emissive closed form, the absorption band of the closed
form occurs at the same region as the emission band of the open
form, which leads to possible reabsorption as well as energy
transfer quenching.1,7−9
Density functional theory (DFT) and time-dependent DFT
(TDDFT) calculations were performed to gain further insight
into the electronic structures and origins of the electronic
transitions of the open and closed forms. The thermal stability of
the closed form is governed by the ground-state energy difference
between the two forms. The smaller the energy difference, the
larger is the activation energy for cycloreversion. The energy
difference between the open and closed form for BzThSiMe2-
DTE is very small, in which the open form is more stable than the
closed form by 0.3 kcal mol−1. Such a small energy difference will
result in BzThSiMe2-DTE having excellent thermal stability.
On the basis of the TDDFT calculation (Table S2), the low-
energy absorption band for the open form is attributed to the
HOMO → LUMO excitation, which can be assigned as mainly
π−π* transition of the benzo[b]silole moiety mixed with the
π−π* transition of the peripheral dimethylthiophenes. It is noted
that a contribution from the σ* orbital of the exocyclic Si−C
bonds to the LUMO is found (Figure 5). However, the lower
energy absorption band for the closed form is attributed to the
π−π* transition predominantly localized on the condensed
thiophene moiety.
In conclusion, a highly efficient photochromic silole-
containing dithienylethene derivative was synthesized and
characterized. The photophysical, photochromic, and electro-
chemical properties were studied and found to be strongly
influenced by the silole backbone. Robust fatigue resistance was
observed in the present system, which suggests that incorpo-
ration of silole moiety into the diarylethene backbone can
effectively stabilize the closed form of the compounds to
eliminate photochemical side reactions during the ring-opening
and ring-closing processes. Most interestingly, the similar
aromaticity and energy level of the open form and closed form
of the photochromic siloles give rise to negligible thermal
cycloreversion and high photocycloreversion observed in this
novel compound. Together with these attractive features, it is
believed that this silole-containing photochromic compound
would act as excellent alternatives for dithienylperfluorocyclo-
pentenes in optical memory storage and photoswitching
material, and the present work would provide further insights
into future design of new thermally stable and fatigue-resistant
photochromic compounds.
Figure 3. UV−vis absorbance changes of BzThSiMe2-DTE at 474 nm
on alternate excitation at the isosbestic wavelength of 370 nm and the
absorption maximum of the closed form at 474 nm over five cycles in
degassed benzene solution at 298 K.
BzThSiMe2-DTE can undergo fully reversible photochemical
ring-opening and ring-closing processes without photochemical
side reactions in more than five cycles under both degassed and
nondegassed conditions. It should be mentioned that the ring-
closing process of the cycle is monitored upon irradiation at the
isosbestic wavelength until the photostationary state is reached,
which demonstrates the excellent fatigue resistance of the
compound.
In addition, a thin poly(methyl methacrylate) (PMMA) film of
BzThSiMe2-DTE was spin-coated on a quartz plate to study the
photochromic properties in rigid medium. The quartz plate was
covered with a patterned mask and exposed to light; the
uncovered area undergoes color change, as shown in Figure 4.
Moreover, the emission intensity was shown to be quenched
during the photoirradiation process (Figure 1), which is
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section; crystal structure determination, character-
ization, and photophysical data; computational details; synthetic
procedures; selected singlet−singlet transition and structural
parameters; electronic absorption spectra; perspective view; and
Cartesian coordinates of optimized structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
wwyam@hku.hk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.W.-W.Y. acknowledges support from The University of Hong
Kong under the URC Strategic Research Theme on New
Materials. This work was supported by the University Grants
Committee Areas of Excellence Scheme (AoE/P-03/08) and a
General Research Fund (GRF) Grant from the Research Grants
Council of the Hong Kong Special Administrative Region, P. R.
China (HKU 17305614). J.C.-H.C. acknowledges the receipt of a
postgraduate studentship from The University of Hong Kong.
The authors thank Dr. L. Szeto for the assistance in X-ray crystal
structure data collection and determination and the Information
Technology Services of The University of Hong Kong for
providing computational resources.
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