Photocontrolled intramolecular charge/energy transfer and fluorescence switching of tetraphenylethene-dithienylethene-perylenemonoimide triad with donor-bridge-acceptor structure

Article abstract of DOI:10.1002/asia.201301071

Photochromic 1,2-dithienylethene (DTE) derivatives with a high thermal stability and fatigue resistance are appealing for optical switching of fluorescence. Here, we introduce a donor-photochromic bridge-acceptor tetraphenylethene-dithienylethene-perylenemonoimide (TPE-DTE-PMI) triad, in which TPE acts as the electron donor, PMI as the electron acceptor, and DTE as the photochromic bridge. In this system, the localized and intramolecular charge transfer emission of TPE-DTE-PMI with various Stokes shifts have been observed due to the photoinduced intramolecular charge transfer in different solvents. Upon UV irradiation, the fluorescence quenching resulting from photochromic fluorescence resonance energy transfer in TPE-DTE-PMI has been demonstrated in solution and in solid films. The fluorescence on/off switching ratio in polymethylacrylate film exceeds 100, a value much higher than in polymethylmethacrylate film, thus indicating that the fluorescence switching is dependent on matrices.

Full text of DOI:10.1002/asia.201301071

COMMUNICATION
DOI: 10.1002/asia.201301071
Photocontrolled Intramolecular Charge/Energy Transfer and Fluorescence
Switching of Tetraphenylethene-Dithienylethene-Perylenemonoimide Triad
with Donor–Bridge–Acceptor Structure
Chong Li,[a] Hui Yan,[a] Guo-Feng Zhang,[a] Wen-Liang Gong,[a] Tao Chen,[a] Rui Hu,[b]
Matthew P. Aldred,*[a] and Ming-Qiang Zhu*[a]
Abstract: Photochromic 1,2-dithienylethene (DTE) deriva-
tives with a high thermal stability and fatigue resistance are
appealing for optical switching of fluorescence. Here, we in-
troduce a donor–photochromic bridge–acceptor tetraphenyl-
spectroscopy provides an alternative to UV/Vis absorption
measurements to characterize the properties of molecular
photoswitches. This requires that molecular photoswitches
display both photochromism and fluorescent switching. Flu-
orescent on–off switching enables fluorescent imaging[4] and
sensing[5] using molecular photoswitches as probes. A great
deal of efforts have been made to develop novel photo-
switchable fluorophores with high fluorescent quantum
yields, on/off ratios, and switching speeds.[6–8] In recent years
perylenediimides (PDI) have been extensively investigated
because of their excellent photostability, high fluorescence
quantum yields in organic solvents, and electron-accepting
abilities.[9] In addition, PDI and/or photochromic triads have
also gained much attention because of extended p-conjuga-
tion and enhanced electronic communication between fluo-
rophores.[10] However, in most cases, PDIs exhibit strong flu-
orescent quenching in polar solvents, in aggregates, or in the
solid state, as it is common for most planar organic mole-
cules.[11] Compared with PDIs, perylenemonoimides (PMIs)
are highly luminescent both in solution and in the solid
state, and thus they are more promising for many optical de-
vices and sensing applications.[12] Taking advantage of stable
photoswitching of DTE and the high fluorescence of PMI, it
would be desirable to design DTE-PMI conjugates that pos-
sess fluorescence photoswitching behavior both in solution
and in the solid state. In addition, tetraphenylethene (TPE)
is attracting attention as a typical aggregation-induced emis-
sion (AIE) activator;[13] the AIE mechanism of TPE has al-
ready been well-studied since Tang et al. first found the AIE
phenomenon.[14] It was found in previous studies that TPE
integrates the donating properties and the AIE effect.[15] To
the best of our knowledge, there are few reports that TPE
can act as a moderate donor when linked to strong accept-
ors.[15] Here, we report the synthesis and optical properties
of a donor–photochromic bridge–acceptor (D–bridge–A)
tetraphenylethene–dithienylethene–perylenemonoimide
(TPE-DTE-PMI) triad. By covalently coupling TPE and
PMI using DTE, we investigate the absorption, emission,
photochromism, and fluorescent photoswitching behavior of
the donor–photochromic bridge–acceptor structure in solu-
tion, in aggregates, and in the solid state. Remarkably, the
conjugate emits strongly both in solution and in the solid
state, and thus it seems that the AIE effect of TPE has been
A
(TPE-DTE-
PMI) triad, in which TPE acts as the electron donor, PMI as
the electron acceptor, and DTE as the photochromic bridge.
In this system, the localized and intramolecular charge
transfer emission of TPE-DTE-PMI with various Stokes
shifts have been observed due to the photoinduced intramo-
lecular charge transfer in different solvents. Upon UV irra-
diation, the fluorescence quenching resulting from photo-
chromic fluorescence resonance energy transfer in TPE-
DTE-PMI has been demonstrated in solution and in solid
films. The fluorescence on/off switching ratio in polymethy-
lacrylate film exceeds 100, a value much higher than in poly-
methylmethacrylate film, thus indicating that the fluores-
cence switching is dependent on matrices.
As representative of molecular photoswitches, 1,2-dithie-
nylethene (DTE) derivatives that can undergo reversible
photocyclization reactions, which are thermally irreversible
and fatigue resistant, are appealing for molecular optical
memory and optical switching in photonic devices.[1–3] Com-
pared with absorbance measurements, which are usually
used for tracking changes in color by monitoring the absorb-
ance at the maximum absorption wavelength, fluorescent
[a] C. Li, H. Yan, G.-F. Zhang, W.-L. Gong, T. Chen, Dr. M. P. Aldred,
Prof. M.-Q. Zhu
Wuhan National Laboratory for Optoelectronics
School of Optical and Electronic Information
Huazhong University of Science and Technology
Wuhan, Hubei, 430074 (China)
Fax : (+86)027-8779341
E-mail: mqzhu@hust.edu.cn
mpaldred@hotmail.com
[b] Dr. R. Hu
Analytical and Testing Center
Institute of Hydrobiology
Chinese Academy of Sciences
Wuhan, Hubei, 430074 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301071.
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inhibited. Therefore, TPE mostly exhibits the electron-do-
nating property so that the fluorescence photoswitching
could be investigated both in solution and in the solid state.
The photochromic switching behavior of TPE-DTEO-PMI
(1O) and two control compounds, C8H17OPh-DTEO-PMI
(2O) and PMI-DTEO-PMI (3O), is shown in Scheme 1. The
trinsic absorption of TPE and the peak at about 514 nm is
ascribed to the absorption of PMI. The open forms (1O–
3O) did not show any noticeable absorption in the visible
region above 400 nm except for the intrinsic absorption of
PMI. Only small differences in the absorption of PMI in
1O, 2O, and 3O could be observed. It is possible that the
group on the other terminus
does not communicate well
with PMI when the DTE is
present in the open form
(DTEO). After irradiation of
1O with UV light for 1–3 min,
a new peak around 612 nm ap-
peared that is assigned to DTE
in its closed (DTEC). Similarly,
2O and 3O underwent obvious
photochromic reactions in or-
ganic solvents upon irradiation
at 302 nm. The absorption
peaks of DTE in the closed
form in 2C and 3C are located
at 606 nm and 600 nm, respec-
tively. TPE itself is a large con-
jugated structure and here, as
a donating group, it increases
the
conjugation
slightly
throughout the whole molecule
of 1C. Thus, the absorption of
1C is more red-shifted than
that of 2C containing the
Scheme 1. Photochromism and molecular structures of DTE-PMI derivatives used in this study.
detailed synthesis and characterization data are provided in
the Supporting Information. The absorption spectra of 1O–
3O are analyzed successively in order to follow the mainte-
nance of the photochromic bridge and the interaction be-
tween intramolecular subunits. Figure 1a and Figure S2 in
the Supporting Information show the absorption spectra of
1O, 2O, and 3O upon irradiation with 302 nm light
(0.85 mWcmÀ2) in toluene, THF, and DMF. The electronic
spectra of 1O–3O display an intense absorption band at
about 510 nm with a considerable extinction coefficient
(Table S1, Supporting Information), which is assigned to the
PMI subunit that the three molecules have in common.
Table 1 shows the spectroscopic properties of 1O. In the ab-
sorption spectra, the peak at 340 nm corresponds to the in-
C8H17OPh- moiety. Comparably, 3O showed a much poorer
photochromic behavior with a small absorption band at
600 nm upon irradiation at 302 nm, probably because of an
easier photoisomerization in the D–bridge–A than A–
bridge–A structure.
We have first investigated the AIE effect of 1O. This
compound emits strongly both in solution and in the solid
state. It is surprising that the fluorescence of TPE-DTE-
PMI at high water content in THF/water is much lower than
that in THF, thus indicating that its AIE effect is inhibited
(see Figure S3, Supporting Information). This means that
TPE-DTE-PMI does not exhibit the typical AIE properties.
Instead, TPE only exhibits the electron-donating property;
this conclusion contributes to a better understanding of the
effect of the TPE group on the
fluorescence properties and
photoswitching process in solu-
tion.[15] The fluorescence emis-
sion spectrum of 1O (Fig-
ure 1b) is structured in toluene,
and the maximum appears at
560 nm, which can be assigned
to the locally excited (LE) state
emission of PMI. However,
with increasing solvent polarity,
the emission peak exhibits a sig-
Figure 1. Absorption and photoluminescence spectra of 1O and 1C (1 mm) in solvents of different polarity.
nificant red shift and becomes
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Table 1. Optical properties of TPE-DTE-PMI (1O) in different matrices.
est among the three compounds owing to the electron-with-
drawing PMI group, while both 1O and 2O contain elec-
tron-donating groups. After irradiation with UV light, the
conjugation is extended by the closed-form DTE. Therefore,
photochromic fluorescence resonance energy transfer from
PMI to the ring-closed isomer of DTE as an efficient
quencher is induced by the photoisomerization.
[b]
[c]
Solvent Df[a]
lmax (abs.) lmax (PL) Stokes Shift FFÀO
FFÀPSS
[nm]
[nm]
[nm]
[%]
[%]
Toluene 0.014 516.5
564.0
583.0
623.0
589.0
596.0
47.5
71.5
107
78.5
87.5
80
65
22
–
18
19
11
THF
0.21
0.27
–
511.5
516.0
510.5
508.5
DMF
PMMA
PMA
–
–
Optimized structures of both 1O and 1C obtained by
density functional theory (DFT) calculations are shown in
Figure S1 in the Supporting Information. In the open form,
1O, the HOMO is localized around TPE and the adjacent
thiophene subunits with significant orbital density, while the
LUMO in 1O is restricted to the strong electron-withdraw-
ing PMI subunit. By comparison, the HOMO in 1C is delo-
calized with orbital density transfer from TPE through the
thiophene subunit due to the extended conjugation. The
LUMOs in 1C are localized around PMI and the pentafluor-
ocyclopentene. The HOMOs and LUMOs of 1O and 1C in-
dicated that photoswitchable intramolecular charge and
energy transfer occurs in the donor–photochromic bridge–
acceptor triad upon alternating irradiation with UV and visi-
ble light. Thus, it is demonstrated experimentally that, by
a primary externally photoswitching process (photoisomeri-
zation of DTE), two secondary externally initiated processes
(photoinduced intramolecular charge transfer in the ring-
opened isomer and fluorescence resonance energy transfer
in the ring-closed isomer) can be switched reversibly by UV/
visible light.
The initial ring-opened isomer 1O exhibits one absorption
peak at around 510 nm, which was observed in solution and
in solid film. Upon irradiation with UV light (302 nm), the
resulting UV/Vis absorption spectra (Figure 2a) shows the
evolution of two peaks around 510 nm and 620 nm, in which
the former peak is attributed to the PMI moiety and the
latter one to the closed form in DTE. Upon UV irradiation
for 120 s at around 0.85 mWcmÀ2, TPE-DTE-PMI in tolu-
ene attained 95% of the absorbance at the photostationary
state (PSS) under 302 nm irradiation. The photoisomeriza-
tion attains the stationary state at about 3 min. The absorb-
ance of 1O and 1C at 620 nm increased and decreased re-
versibly in solution with alternating irradiation with UV and
visible light, thus demonstrating the highly reversible and bi-
stable photochromism between 1O and 1C (Figure 2b).
The initial open-form compound 1O exhibits a strong
emission from PMI at around 560–620 nm in solution and in
the solid film, depending on the polarity of the solvent or
polymer matrices. The resulting fluorescence spectra (Fig-
ure 3a and Figure S4, Supporting Information) of TPE-
DTE-PMI in solution upon UV irradiation (302 nm) show
a gradual decrease in the emission intensity at 560–600 nm
because of the photochromic fluorescence resonance energy
transfer of PMI to the ring-closed isomer of DTE in 1C.
The reversibility of this process can be demonstrated by cy-
cling forward and backward between the open and closed
form. When a solution of 1O in toluene was excited at
514 nm, a strong emission at 567 nm was observed and, after
UV irradiation, fluorescence quenching >80% was detect-
[a] Df=the orientational polarisability. lmax (abs) is the lowest energy ab-
sorption peak at 1 mm of 1O; lmax (PL) was measured at lex =514 nm and
1 mm of 1O. [b] Quantum yield of the open form. [c] Quantum yield of
the photostationary state (302 nm, PSS). FF values were estimated
against rhodamine B (quantum yield of 70% in ethanol), lex =514 nm
(lem =520–800 nm). PMMA=polymethyl methacrylate. PMA=polyme-
thacrylate.
structureless (Table 1). The large solvatochromic shift of the
fluorescence (620 nm in DMF compared to 560 nm in tolu-
ene) in polar solvents indicates a strong photo-induced in-
tramolecular charge transfer (ICT) character of the fluores-
cent state. As predicted, upon irradiation with UV light
(302 nm), there is a significant energy overlap between the
emission band of PMI and the absorption band of 1C. Thus,
the resulting fluorescence of 1O decreases upon UV irradia-
tion, thereby indicating a photochromic fluorescence reso-
nance energy transfer from PMI to the ring-closed isomer of
DTE as an efficient quencher, which originates from photoi-
somerization of 1O to 1C (Figure 1b). The FRET quench-
ing efficiency in TPE-DTE-PMI after UV irradiation was
calculated from the observed quenching of the donor inten-
sity. For the open form of DTE (DTEO) the conjugation is
markedly restricted, and thus only charge transfer is allowed
within the TPE-DTE-PMI triad. On the contrary, for the
closed form of DTE (DTEC) for which the conjugation is
obviously improved, the emission of PMI overlaps well with
the absorbance of DTEC. Therefore, energy transfer domi-
nates in the closed form of TPE-DTE-PMI triad.[10a]
Compounds 1O, 2O, and 3O possess the same DTE-PMI
subunits. The difference in their molecular structure is that
the three fluorophores contain different substituted groups
at the other terminus of the triads. Therefore, the compari-
son of 1O with 2O and 3O could lead to insights into the
optical properties influenced by structural differences. The
absorbance and fluorescence of the three fluorophores
change similarly with an increasing polarity of the solvent.
The quantum yields of 1O, 2O, and 3O are very close in tol-
uene (~80%) and THF (65%). The three compounds also
have the similar emission spectra in DMF. It is worth noting
that 1O has the highest quantum yield in DMF (22%),
while it is 14% for 2O and 11% for 3O (Table S2, Support-
ing Information). The ICT properties in the open-form
triads are the same, while an electron-donating group such
as TPE and C8H17OPh could improve the quantum yields of
the triads in polar solvents. The fact that TPE-DTE-PMI
has the strongest emission in a good solvent argues against
the AIE effect, as TPE usually quenches emission in a good
solvent. It seems that here the AIE effect of TPE is too
weak to be observed. The photochromism of 3O is the poor-
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UV and visible-light irradiation
(Figure S5, Supporting Informa-
tion). The fluorescence quench-
ing is efficient with a higher
fluorescence on/off switching
ratio in PMMA film than in so-
lution (Figure 4c). However, in
PMA film, 1O even shows
a much faster photochromic flu-
orescent quenching with an on/
off ratio that is higher than 100
(Figure 4d). The reversible
photoswitchable fluorescence of
1O in PMMA and PMA films
was demonstrated by cycling
forward and backward, from
the open to the closed form and
reverse (Figure 4e and 4 f).
When TPE-DTE-PMI was ex-
cited at 514 nm in PMA film,
a strong emissions at 600 nm
was observed after visible-light
illumination for 1C, and the
emission of 1O after UV irradi-
ation was nearly completely
quenched.
Figure 2. (a) Absorption spectra of 1O (1 mm) in toluene upon irradiation with UV light (302 nm). The photo-
stationary state was determined by irradiating a solution of 1O with UV light until no changes were observed
in the spectrum. Inset: Photos of 1O in toluene taken at daylight after irradiation with visible light and UV
light, respectively. (b) Reversible absorption switching for 1O (1 mm) in toluene measured at 620 nm upon al-
ternating irradiation with UV light (302 nm, 3 min) and visible light (>495 nm, 20 min).
Optical switches usually work
in solid-state devices despite
many reports on fluorescent
molecular switches in solution.
Therefore, the effects of solid
matrices on the photoswitcha-
ble absorption and emission
properties of fluorophores are
Figure 3. (a) Emission spectra change of 1O (1 mm) in toluene upon irradiation with UV light (302 nm). Inset:
Photos of 1O in toluene taken at 365 nm UV light after irradiation with visible light and UV light, respective-
ly. (b) Reversible fluorescence switching for 1O (1 mm) in toluene measured at 567 nm (lex =514 nm) upon al-
ternating irradiation with UV light (302 nm, 3 min) and visible light (>495 nm, 20 min).
ed. The cycling behavior at alternating UV and visible-light
irradiation shows decent reversibility and good fatigue re-
sistance (Figure 3b).
indispensable. Here we found that both electron-withdraw-
ing and electron-donating substituted groups determine the
photoswitching properties. The fluorescent switching is de-
pendent on the matrices such as solvents with different po-
larities or polymer matrices with different glass transition
temperatures. The design of new molecules combined with
their optical characterization in different matrices will be
helpful for the development of novel photoswitchable fluo-
rophores.
In conclusion, we have demonstrated a simple design of
a donor–photochromic bridge–acceptor triad TPE-DTE-
PMI, in which TPE is used as the electron donor and PMI
as the electron acceptor with DTE as the photochromic
bridge. In this system, the LE state and ICT emission of 1O
with various Stokes shifts have been observed due to the
photoinduced intramolecular charge transfer in different sol-
vents. Upon UV irradiation, the fluorescence quenching re-
sulting from photochromic fluorescence resonance energy
transfer of 1C has been demonstrated in solution and solid
films. Both secondary photoinduced processes at 514 nm ex-
citation are reversibly controlled by primary externally pho-
toswitching stimuli with UV/visible-light irradiation. Thus,
we conclude that DTE acts as a reversible adjustor for intra-
It is found that 1O not only does not show evident fluo-
rescent quenching in the solid state due to aggregation-
caused quenching but also no strong fluorescent quenching
in solution due to the AIE effect induced by TPE groups. In
the solid state, 1O shows a different behavior with regard to
the photoswitching speed, fluorescent intensity, and the on/
off ratio, depending on the surrounding matrices. In poly-
methyl methacrylate (PMMA) with a glass transition tem-
perature of 1048C, the absorption band of 1O at 620 nm in-
creases slowly upon 302 nm irradiation, thus indicating the
slow photochromic transformation of 1O to 1C (Figure 4a).
In comparison, the absorption band of 1O increases much
faster in polymethacrylate (PMA), which has a much lower
glass transition temperature of 08C as compared to PMMA
film (Figure 4b). This is attributed to the different capability
of molecular motion in sticky or soft matrices. In soft PMA
with a lower glass transition temperature, the photochromic
speed is much faster than that in sticky PMMA. The highly
reversible and bistable photochromism between 1O and 1C
in PMMA and PMA films was verified by using alternating
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Acknowledgements
This work was supported by the NSFC (20874025, 21174045), National
Basic Research Program of China (Grant No. 2013CB922104). M.P.A. ac-
knowledges the NSFC Research Fellowship for International Young Sci-
entists (21150110141, 212111128) and the Special Fellowship of China
Post-doctoral Science Foundation (2012T50642). We also thank the Ana-
lytical and Testing Center of Huazhong University of Science and Tech-
nology.
Keywords: dithienylethene
fluorescence photochromism
·
donor–acceptor systems
perylene monoimide
·
·
·
·
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Received: August 10, 2013
Published online: && &&, 0000
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Chem. Asian J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

COMMUNICATION
The matrix reloaded: We report the
synthesis and photophysical characteri-
zation of a donor–photochromic
bridge–acceptor tetraphenylethene-
dithienylethene-perylenemonoimide
(TPE-DTE-PMI) triad that simultane-
ously exhibits photocontrolled intra-
molecular charge/energy transfer and
enhanced reversible fluorescence
switching, depending on the surround-
ing matrices.
Photochromism
Chong Li, Hui Yan, Guo-Feng Zhang,
Wen-Liang Gong, Tao Chen, Rui Hu,
Matthew P. Aldred,*
Ming-Qiang Zhu*
&&&& — &&&&
Photocontrolled Intramolecular
Charge/Energy Transfer and Fluores-
cence Switching of Tetraphenylethene-
Dithienylethene-Perylenemonoimide
Triad with Donor–Bridge–Acceptor
Structure
7
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&
Chem. Asian J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ