Photoswitchable organic nanoparticles and a polymer film employing multifunctional molecules with enhanced fluorescence emission and bistable photochromism

Article abstract of DOI:10.1002/anie.200461172

Get turned on: Fluorescent photochromic organic nanoparticles (FPONs) of 1 (see picture) show a strongly enhanced fluorescence emission with increasing concentration and bistable photochromism. High-contrast on/off fluorescence switching has been successfully implemented in size-tuned FPONs of 1 and also in a photo-rewritable polymer film highly loaded with 1. (Chemical equation presented).

Full text of DOI:10.1002/anie.200461172

Communications
Photochromism
Photoswitchable Organic Nanoparticles and a
Polymer Film Employing Multifunctional
Molecules with Enhanced Fluorescence Emission
and Bistable Photochromism**
Seon-Jeong Lim, Byeong-Kwan An, Sang Don Jung,
Myung-Ae Chung, and Soo Young Park*
Among the various photon-mode molecular memory systems,
bistable photoswitching of fluorescence emission is consid-
ered to be a promising signaling mode, not only because the
fluorescence signals can be readily and sensitively recognized,
but also because the small number of photons required for
their excitation induce few side effects to spoil the digitalized
[
1–9]
signals.
Multifunctional fluorescent (including phosphor-
escent) molecules combining bistable photochromism with
built-in 1,2-bisthienylethene (BTE) units have been success-
fully investigated with the aim of applications in ultrahigh-
density optical memory media, and they show, in principle,
[
10–15]
reversible and bistable photoswitching.
The important
challenge still remaining unsolved, however, is the general
problem of “concentration quenching” in the fluorescence
signal, which certainly restricts the application of these
multifunctional photochromic molecules to high-density
[
16]
optical memory systems.
Herein, we demonstrate an
innovative approach to this problem by employing a special
class of multifunctional photochromic molecule which shows
enhanced fluorescence emission with increasing concentra-
tion. It is readily expected that the high storage capability,
high sensitivity, and high-contrast on/off signaling ratio are
synergetically achieved with this molecule in neat nano-
particles or in a highly loaded polymer film.
Very recently, unconventional fluorescent molecules
showing aggregation-induced enhanced emission (AIEE)
[
17–19]
have been reported by several groups including ours.
Simple-structured 1-cyano-trans-1,2-bis-(4’-methylbiphenyl)-
ethylene (CN-MBE) is a typical AIEE molecule that
fluoresces strongly in the solid state even though it is virtually
[
19]
nonfluorescent in solution. The multifunctional fluorescent
molecule for photoswitchable memory media 1a was
designed and synthesized by replacing one of the end tolyl
groups of CN-MBE with the photochromic BTE moiety.
Compound 2a, the non-AIEE analogue of 1a which lacks
[
*] S.-J. Lim, B.-K. An, Prof. S. Y. Park
School of Materials Science and Engineering ENG445
Seoul National University
San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744 (Korea)
Fax: (+82)2-886-8331
E-mail: parksy@plaza.snu.ac.kr
Dr. S. D. Jung, Dr. M.-A. Chung
Basic Research Laboratory
Electronics and Telecommunications Research Institute (ETRI)
161 Gajeong-dong, Yuseong-gu, Daejeon 305-350 (Korea)
[**] This work was supported in part by CRM-KOSEF and ETRI.
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461172
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Angewandte
Chemie
only the CN group, was also synthesized to properly compare
the AIEE effect (see Figure 1 for the chemical structures and
the Supporting Information for the synthetic details). Com-
ure 1a). This observation contrasts strikingly with the behav-
ior of conventional analogue 2a, which shows significant
concentration quenching in the nanoparticle (ca. 30 times
reduction in fluorescence intensity, compare (VI) with (V) in
Figure 1c). Such a totally different fluorescence behavior
between 1a and 2a (AIEE versus quenching) is most
probably related to the different aggregation states in their
FPONs. In fact, the UV/Vis absorption spectra shown in
Figure 2a,c suggest J-type and H-type aggregations for 1a and
2a FPONs, respectively. The absorption spectra of 1a FPONs
were characteristically red-shifted (Figure 2a) and exhibited
Mie light scattering as a result of J-type aggregation and
nanoparticle formation, respectively. Notably, the absorption
maxima of FPONs of 1a showed a gradual red-shift with
1
pounds 1a and 2a were fully identified by H NMR spectros-
copy, FTIR spectroscopy, MALDI-TOF mass spectrometry,
and elemental analysis (analytical data are available in the
Supporting Information).
Fluorescent
FPONs) of 1a were prepared by a reprecipitation method
to produce a highly concentrated photon-mode recording
photochromic
organic
nanoparticles
(
[
20]
medium. Quite uniform and size-tuned nanoparticles of 1a
see the scanning electron microscopy (SEM) photograph in
(
Figure 1e) were obtained; the diameters of FPONs of 1a
were 40 Æ 10, 125 Æ 25, 200 Æ 50, and 275 Æ 75 nm when
À5
soln
max
FPONs
prepared from THF/water concentrations of 2 ” 10 , 4 ”
increasing nanoparticle size; l = 360 nm, lmax = 371–
À5
À4
À4
1
0 , 1 ” 10 , and 2 ” 10 m, respectively (other images are
382 nm with increasing FPON size of 40–275 nm. Concom-
available in the Supporting Information). According to the
AIEE principle, FPONs of 1a showed strongly enhanced
fluorescence emission, although they were only weakly
fluorescent in a THF solution of the same concentration
itantly, FPONs of 1a showed strongly enhanced fluorescence
FPONs
FPONs
emission as shown in Figure 2b (FF
= 3.2–5.1%, lmax
=
485–493 nm) compared with the isolated 1a dissolved in THF
À4
soln:
soln
max
[21,22]
(2 ” 10 m, FF = 0.002%, l = 461 nm).
All of these
(
ca. 1700 times enhancement, compare (II) with (I) in Fig-
spectral changes corroborate that the J-type aggregation and
À4
Figure 1. a) Chemical structure of 1a and the fluorescence images of its THF solution (I, soln, 2”10 m) and the colloidal suspension (II) of fluo-
À4
rescent photochromic organic nanoparticles (FPONs; 2”10 m). b) Chemical structure of 1b and the fluorescence images of its THF solution
À4
À4
(
III, 2”10 m) and the colloidal suspension of FPONs (IV, 2”10 m) in the photostationary state (PSS). c) Chemical structure of 2a and the fluo-
À5
À5
rescence images of its THF solution (V, 2”10 m) and the colloidal suspension of FPONs (VI, 2”10 m). d) Chemical structure of 2b and the
fluorescence images of its THF solution (VII, 2”10 m) and the colloidal suspension of FPONs (VIII, 2”10 m) in the PSS. e) FE-SEM image of
À5
À5
À4
FPONs of 1a (275Æ75 nm) prepared from a THF/water mixture (2”10 m). f) Photo-rewritable fluorescence imaging on the polymer film loaded
À2
with 20 wt% of 1a by usingUV (365 nm, hand-held lamp, 1.2 mWcm ) and visible light (>500 nm). The dark regions represent the parts irradi-
ated with UV light; the real size of the photomasks is about 1 cm”1 cm.
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Communications
À4
À4
Figure 2. a) UV/Vis absorption spectra of 1a in THF (2”10 m) and the colloidal suspension of FPONs of 1a (2”10 m). b) PL spectra of
À4
FPONs of 1a (275Æ75 nm) in the PSS (bottom, blue line), 1a in THF (2”10 m, black line), and size-tuned FPONs of 1a of 40–275 nm (cyan–
green line, respectively). The inset graph shows a relative PL intensity modulation of 1a FPONs (275Æ75 nm). c),d) UV/Vis absorption and
À5
À5
PL spectra of 2a in THF (2”10 m) and the colloidal suspension of 2a FPONs (2”10 m). e) UV/Vis absorption spectra of 1a in THF
À4
(
2”10 m) and a 20 wt% 1a-loaded PMMA film. f) PL spectra of the 20 wt% 1a-loaded PMMA film. The inset graph shows the relative PL inten-
sity modulation of the film.
molecular planarization of 1a molecules were induced in their
Compounds 1a and 2a with open-ring BTE units were
converted into 1b and 2b with closed-ring BTE units when
irradiated with UV light (365 nm, 1.2 mWcm ). This process
[
19,23]
FPONs by specific intermolecular interactions.
In con-
À2
trast, the UV/Vis absorption spectra of 2a FPONs were
soln
max
FPONs
significantly blue-shifted (l = 360 nm, lmax = 329nm) as
was easily monitored by the appearance of new absorption
1
b
2b
shown in Figure 2c. Moreover, the fluorescence quantum
bands in the visible regions (l = 574 nm, l = 570 nm) as
max max
yield of FPONs of 2a was drastically reduced by 30 times
shown in Figure 2a,c (see the Supporting Information for
details). Accompanied by this photochromic ring closure, the
photoluminescence (PL) intensities of the 1a FPONs were
greatly reduced (compare (IV) with (II) in Figure 1a,b, and
see the Supporting Information for details). The fluorescence
quantum yields of the FPONs of 1a in the photostationary
À5
soln
from that of a solution of 2a in THF (2 ” 10 m, FF = 3.4%,
soln
max
FPONs
l
= 439nm, FF
= 0.1%) as indicated in Figure 2d. This
blue-shifted UVabsorption together with the salient concen-
tration quenching in FPONs of 2a is attributed to the
formation of H-type aggregation through strong cofacial p-
stacking interactions, which most likely provides a nonradia-
[
15]
FPONs
F
state (PSS) were reduced by 16–170 times (F
= 0.20,
[
24,25]
FPONs
tive decay route.
0.09, 0.04, and 0.02% in the PSS, FF
= 3.2, 5.1, 4.1, and
3
.4% in the 1a form with increasing FPON size of 40–275 nm,
6
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Chemie
[
21,22]
respectively).
However, only a small amount of fluores-
1a. The presence of molecular-scale aggregates is additionally
cence modulation (29% reduction, see the Supporting
evidenced by the strong effect of AIEE as the intense blue
film
film
max
soln
Information for details) was achieved in a solution of 2a
fluorescence (FF = 5.8%,
l
= 487 nm versus FF =
À5
soln
soln
soln
À4
(
2 ” 10 m, FF = 2.4% in the PSS, FF = 3.4% in the 2a
form).
It is known that there are two interconvertible conforma-
0.002%, l = 461 nm in 2 ” 10 m solution) from this
max
[
21,29,30]
polymer film (see Figure 2 f).
The AIEE fluorescence
from this PMMA/1a film was also photoswitched in a bistable
tions of BTE units: one is a parallel conformation and the
other is an antiparallel conformation, of which only the latter
manner by alternate UVand visible light irradiation with high
film
film
contrast (FF = 0.3% in the PSS, FF = 5.8% in the 1a form,
or on/off fluorescence intensity ratio > 19), as shown in the
inset graph in Figure 2 f. In addition, the practical capability
of rewritable fluorescence photoimaging on our AIEE
polymer film was investigated by patterned illumination
through photomasks. The emblem of Seoul National Univer-
sity was recorded as a first image (Figure 1 f), which was
subsequently erased and followed by the recording of a
second image, the abbreviation of the Molecular Photonics
Laboratory (MPL). This successful demonstration of rewrit-
able photoimaging on the polymer film suggests immediate
application of 1a molecules to ultrahigh-density rewritable
optical memory media or imaging processes.
In conclusion, we designed and synthesized a special class
of multifunctional molecule 1a, which shows a strongly
enhanced fluorescence emission as well as bistable photo-
chromism. High-contrast (> 10) on/off fluorescence switching
was successfully implemented in the size-tuned neat nano-
particles of 1a and also in a PMMA film highly loaded with
1a.
[
15]
allows an electrocyclic ring-closing reaction. Through the
1
À4
H NMR spectroscopic study (2 ” 10 m of CDCl solution,
3
2
28C, 300 MHz) it was determined that the parallel and
antiparallel conformations of 1a were equally populated, and
also that only about 35% of the 1a forms were photo-
isomerized to the 1b forms in the PSS under irradiation with
[
26,27]
3
65 nm light.
An even smaller extent of ring-closing
reaction in the PSS is implied for FPONs of 1a, because the
net absorbance of the 1b form at 574 nm is smaller in FPONs
than in solution (see Figure 2a). It is reasonably supposed,
therefore, that the photochromic interconversion between the
two conformations was suppressed within a FPON (which is
essentially a condensed solid state), or that the ring-closing
[
28]
reactions occurred only at the surfaces of the FPONs.
Given that the extent of the ring-closing reaction is less
than 35%, the experimentally observed extremely large
reduction in the fluorescence quantum yield FF (16–170-
fold decrease) of FPONs of 1a in the PSS must be achieved
not only by intramolecular energy transfer between the
fluorophore and the closed-ring form of BTE, although it is
very likely that the 29% reduction of the F value of the 2a
Received: July 3, 2004
F
solution occurs through this intramolecular energy trans-
[
2–5,7,8]
fer.
It is presumed that the condensed-state FPONs also
Keywords: aggregation · fluorescence · nanotechnology ·
photochromism · polymers
.
provide an additional quenching event, or intermolecular
energy transfer between the unconverted 1a forms and the
neighboring 1b forms, because of their proximity in FPONs.
Consequently, extremely high contrast in the on/off signaling
ratio is automatically implemented in the neat FPONs of 1a,
which is an additional advantage to the enhanced fluores-
cence and high storage capacity of the AIEE molecule.
Moreover, when the FPONs of 1a in the “off” state were
irradiated with visible light (> 500 nm), the PL intensities
were perfectly recovered to those of the initial “on” state as a
result of the reversible photochromic behavior of the BTE
unit. The inset graph in Figure 2b shows a reversible photo-
chromic modulation of relative fluorescence intensity in the
suspension of FPONs of 1a under alternating irradiation with
UV and visible light (note the on/off fluorescence intensity
ratio > 10).
[
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Angew. Chem. Int. Ed. 2004, 43, 6346 –6350
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Communications
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with distilled water. Volume fractions of THF and water were
adjusted to 20 and 80%, respectively. See ref. [19] for details.
21] Fluorescence quantum yields: The fluorescence quantum yields
2
[
[
[
1
2
[
(
F ) were relatively calculated using 9,10-diphenylanthracene
F
(
DPA) in benzene (see ref. [22]) and in PMMA (see refs. [29,30])
À3
as a standard reference (1 ” 10 m, F = 83%). Through this
F
method, the F values of FPONs of 1a were determined as 3.2,
F
FPONs
max em
FPONs
=
max abs
5
.1, 4.1, and 3.4% (l
= 485, 489, 491, and 493 nm, l
371, 374, 378, and 382 nm) in the 1a form, and as 0.20, 0.09, 0.04,
and 0.02% in the PSS with increasing FPON size of 40 Æ 10,
1
25 Æ 25, 200 Æ 50, and 275 Æ 75 nm, respectively. It is reasonably
considered that the slight reduction of the F values in 200 Æ 50
F
and 275 Æ 75 nm FPONs of 1a results from the highly increased
virtual absorbance values by increased light scattering. In fact,
the extent of the apparent absorbance of the FPONs of 1a at the
excitation wavelength, that is at 360 nm, was almost linearly
proportional to their suspension concentrations in spite of the
inverse proportional relationship between the mean radius and
the surface-to-volume ratio of the FPONs.
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À4
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6
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Angew. Chem. Int. Ed. 2004, 43, 6346 –6350