Preparation and photochromic properties of diarylethene nanoparticles

Article abstract of DOI:10.1016/j.cplett.2003.08.119

Nanoparticles of a 1,2-bis-(3-thienyl)perfluorocyclopentene derivative were first prepared by a reprecipitation method. The photochromism and kinetic behavior of the aggregates were investigated. As nanoparticles formed, the absorption maximum peaks of the closed-ring form were blue-shifted. The blue shift was explained by H-aggregates formation, which was consistent with the decreased fluorescence efficiency of the open-ring form. Spectrokinetic study showed that photochromic transformations were strongly affected by molecular aggregation. The decrease of photo-cyclization activity was mainly attributed to the intra- and intermolecular effects exerted by aggregation.

Full text of DOI:10.1016/j.cplett.2003.08.119

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Chemical Physics Letters 380 (2003) 206–212
www.elsevier.com/locate/cplett
Preparation and photochromic properties
of diarylethene nanoparticles
*
*
Fan Sun , Fushi Zhang , Fuqun Zhao, Xinhong Zhou, Shouzhi Pu
Department of Chemistry, Tsinghua University, Xinzhai Building, 824, Beijing 100084, PR China
Received 1 July 2003; in final form 14 August 2003
Published online: 26 September 2003
Abstract
Nanoparticles of a 1,2-bis-(3-thienyl)perfluorocyclopentene derivative were first prepared by a reprecipitation
method. The photochromism and kinetic behavior of the aggregates were investigated. As nanoparticles formed, the
absorption maximum peaks of the closed-ring form were blue-shifted. The blue shift was explained by H-aggregates
formation, which was consistent with the decreased fluorescence efficiency of the open-ring form. Spectrokinetic study
showed that photochromic transformations were strongly affected by molecular aggregation. The decrease of photo-
cyclization activity was mainly attributed to the intra- and intermolecular effects exerted by aggregation.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
tonic properties significantly different from those
of the free diarylethenes.
Photochromic diarylethenes, especially diary-
lethenes with a central 1,2-dithienylperfluoro-
cyclopentene unit, have attracted considerable
attention in the field of optical data storage and
switches due to their remarkably thermal stability
and photofatigue resistance [1–4]. So far, most
studies have focused on isolated diarylethene
molecules. Aggregates of these compounds have
rarely been studied. Nanoscaled particles com-
posed of diarylethenes are expected to have pho-
Molecular aggregation in polymer or LB film is
a common phenomenon observed in some photo-
chromic systems [5–10]. It was found that some
optical properties, such as absorption and emis-
sion spectra, photorefractive effect, NLO proper-
ties, and birefringence could be affected by the
aggregation. Upon irradiation with light, the ag-
gregation morphology and optical properties
could be changed reversible due to photo-induced
isomerization. The study of photochromic nano-
particles has scientific and applied interest attrib-
uted to their photochromic effects, aggregation
effects, and finite size effects. Therefore, it is in-
dispensable to extend the research on photochro-
mic diarylethenes from isolated molecules to
* Corresponding authors. Fax: +86-10-62770304.
E-mail addresses: sunfan00@mails.tsinghua.edu.cn (F. Sun),
zhangfs@mail.tsinghua.edu.cn (F. Zhang).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.08.119

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F. Sun et al. / Chemical Physics Letters 380 (2003) 206–212
207
F
F
F
F
F
F
F
F
UV
Vis
S
S
S
S
OH
HO
OH
HO
Scheme 1.
aggregates owing to their important optoelectronic
applications.
monochromator. The light intensity was deter-
mined by Aberchrome 540 actinometry. The
fluorescence quantum yields were determined with
a Luminescence spectrometer (LS45, Perkin–
Elmer). The sizes and shapes of the nanoparticles
were observed by means of a field emission scan-
ning electron microscope (JSM-6310F).
In this study, we prepared the nanoscaled ag-
gregates of 1,2-bis(2-methyl-5-(p-hydroxyphenyl)
thien-2-yl)perfluorocyclopentene (1a, Scheme 1) by
a reprecipitation method and investigated the ag-
gregation-induced effects on its photochromism.
Similar to some aggregates systems [11,12], the
molecular aggregation of 1a restricted molecular
mobility and thus influenced the photochemical
transformations. The absorption spectra, emission
efficiency, and photoisomerization rate showed
dramatic changes as nanoparticles formed. It is
possible to adjust the photochromic reactions and
optical properties of diarylethenes by controlling
the molecular aggregation.
3. Results and discussion
3.1. Preparation of diarylethene nanoparticles
The nanoscaled particles of diarylethene 1a
were prepared by a simple precipitation method
without surfactants. By this method, water was
used as a solvent for diarylethene in THF. The
suspension of nanoparticles was transparent with
no precipitates and was very stable after several
months. The stability is probably ascribed to the
formation of a particular structure around the
polar hydroxy group which induces the negative f
potential in nanoparticles [15,16]. Electrostatic
repulsion due to the negative f potential could
prevent flocculation/coalescence of nanoparticles
in water. The influence of diarylethene concen-
tration and the influence of temperature on the
size distribution of nanoparticles 1a were studied.
The experimental results showed that temperature
greatly influenced the process of nanoparticle
formation. The mean mass diameter of aggrega-
tion decreased with the increase of temperature.
The FESEM (field emission scanning electron
microscopy) images of the nanoparticles prepared
at 283 and 298 K were shown in Fig. 1, with a
particle size range from 60 to 100 and 170 to
230 nm, respectively. On the other hand, no ob-
vious influence of the concentration of 1a on the
2. Experimental
1,2-Bis(2-methyl-5-(p-hydroxy-phenyl)thien-2-
yl)perfluorocyclopentene (1a) was synthesized ac-
cording to literature method [13] and confirmed by
NMR. Nanoparticles were prepared as follows:
quantities of a diarylethene/THF solution were
regularly injected into distilled water with vigorous
stirring at certain temperature, and then THF was
removed by nitrogen gas. The necessary chemicals
were purchased from commercial sources. Solvents
were purified by standard methods.
The UV–Vis absorption spectra were measured
with a HP8452A diode array spectrophotometer.
A modified reaction cell was fitted in the spectro-
photometer to follow the photochromic reaction
during irradiation as described in literature [14].
Photo-irradiation was carried out using an UV
lamp (SHG-200) and a 150 W xenon arc lamp.
Light of appropriate wavelength was isolated by a

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208
F. Sun et al. / Chemical Physics Letters 380 (2003) 206–212
To investigate substituent effect, the nanoscaled
aggregates of 1,2-bis(2-methyl-5-phenythien-2-yl)
perfluorocyclopentene and 1,2-bis(2-methyl-5-(p-
methoxyphenyl)thien-2-yl)perfluorocyclopentene
were also tried to prepared by the same prepara-
tion procedure. However, instead of forming co-
herent nanoparticles, they aggregated to small
irregular crystal grain in water. It indicates that the
hydrogen bonding effect is indispensable for ag-
gregation. The nanoparticles of 1a are likely held
together by hydrogen bonding and hydrophobic
effects [21].
3.2. Photochromic properties of diarylethene nano-
particles
Compound 1a underwent photochromic reac-
tion in solid state. However, the aggregates of 1a
on glass substrate were found nonreactive due to
the restricted molecular mobility. On the other
hand, nanoparticles suspension showed photo-
chromic effects when exposed to UV light. Upon
irradiation with 297 nm light the colorless sus-
pension of N.1a (kmax, 292 nm) turned to blue. The
photogenerated blue closed-ring isomer N.1b had
a new absorption band appeared at 554 nm. The
blue color disappeared gradually upon irradiation
with visible light (k > 500 nm) and the absorption
band of the open-ring form was restored. For
Fig. 1. FESEM photographs of nanoparticles of N.1a prepared
at 298 K and N.2a prepared at 283 K.
size distribution of nanoparticles was observed
within a concentration range varied from 0.2775
to 22.20 mg/l. The mean mass diameter of nano-
particles changed little as the percentage of
aggregation increases. The influences of concen-
tration and temperature on nanoparticles forma-
tion can be explained by some classical nucleation
theories [17–20]. According to these theories, the
mechanism of diarylethene precipitation consists
of a nucleation phenomenon characterized by the
formation of big nuclei at a high constant rate.
Under the experimental conditions, the nucleation
was so rapid that almost all the precipitable
amount of diarylethene was consumed during the
nucleation period to create nuclei. As a result, the
size of final nanoparticles differed slightly from
that of nuclei. The dependence of obtained par-
ticle size distribution upon the temperature can
also be explained by the theories [20].
comparison, the absorption spectra of 1a (kmax,
290 nm) and 1b (kmax, 578 nm) in hexane were also
measured. The absorption Changes of 1a and N.1a
were displayed in Fig. 1. As expected, the UV–Vis
spectra of diarylethene nanoparticles were signifi-
cantly different compared to that of the corre-
sponding diarylethene solution. The peak width
was found to be broaden and the shape to be
asymmetric. Although the absorption spectra of
the open ring forms changed little, the absorption
maximum peaks of the closed-ring forms showed
great hypsochromic shift (>20 nm). The emission
properties were also investigated. The fluorescence
quantum yields of open ring forms of N.1a and 1a
were measured using phenol (/f ¼ 0:14, water) as
a reference [22]. The fluorescence quantum yields
were 0.0013 for N.1a and 0.0056 for 1a, respec-
tively. The results showed that the fluorescence
efficiency decreased as molecules aggregated.

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F. Sun et al. / Chemical Physics Letters 380 (2003) 206–212
209
It is well known that the arrangement of mol-
ecules in aggregates generally falls into two types,
ÔHÕ and ÔJÕ [23–26]. Formation of H-aggregates is
characterized by blue-shifted absorption bands
and relatively low fluorescence efficiency with re-
spect to those of isolated molecules. The broad
and blue-shifted bands of the closed-ring isomers
of nanoparticles in accord with the decreased
emission efficiency of the open-ring forms could
be explained by the H-aggregates formation re-
sulting from the strong intermolecular interaction.
And the aggregation morphology of the diary-
lethene molecules was not affected by photoiso-
merization.
2.0
1.5
1.0
0.5
0.0
Wavelength (nm)
(1)
The size effect on the optical properties of
nanoparticles was also measured. It was found
that the absorption spectra of N.1 and N.2 had no
evident change although they had different size
distribution while the emission spectra appeared
complicated owing to scattering and photochemi-
cal effects. Further study on size effects is currently
in progress.
To understand aggregation effects on photo-
chromism of 1a, the photochromic reactions of
both nanoparticles and isolated molecules were
investigated by spectrokinetic method using
monochromatic irradiation wavelength in the UV
and visible ranges.
0.3
0.2
0.1
0.0
Wavelength (nm)
Here is described the photokinetic mechanism
of photo-cyclization. The reaction spectra of 1a
and N.1a excited at 297 nm are shown in Fig. 2.
Since the reactions was found to yield good linear
AD-diagrams, they could be treated as uniform
ones and followed the mechanism proposed by
Mauser ands co-workers [27–29].
Fig. 2. Absorption changes of 1a in hexane (1) and N.1a in
water (2) upon irradiation at 297 nm.
And the PSS (photostationary state) is considered,
where dCb1=dt ¼ 0. Integration of the rate equa-
tion (1) yields
Z
t
E1k ꢀ EkðtÞ
Consider the reaction
ln
¼ ꢀ1000I0ðeaq1 þ ebq2Þ
F ðE0Þdt;
E1k ꢀ E0k
q1
0
A ꢀ B:
q2
ð4Þ
where I0 is the intensity of monochromatic radia-
tion and q1 and q2 are the A ! B and B ! A
quantum yields at the excitation wavelength. ea, eb
are the molar decadic absorption coefficients of A
The rate law is given by
dCb
dt
¼ 1000I0F ðE0Þðeaq1Ca ꢀ ebq2CbÞ:
ð1Þ
and B at the excitation wavelength. F ðE0Þ ¼
Also
0
ð1 ꢀ 10ꢀE Þ=E0 is the photokinetic factor with ab-
sorbance E0ðtÞ at the excitation wavelength at time
t. Ca and Cb are the concentrations of A and B at
time t. C0 is the total concentration of compounds
C0 ¼ Ca þ Cb;
ð2Þ
ð3Þ
Ek ¼ ekaCa þ ekbCb: