Readily accessible rhodamine B-based photoresponsive material

Article abstract of DOI:10.1007/s11426-013-5033-3

A photochromic rhodamine B-based material containing Cd(II) as bridge was facilely prepared. The 4-methoxylsalicylaldehyde rhodamine B hydrazone Cd(II) complex displayed unusual ring-open response upon 365 nm UV irradiation, exhibiting long photochromic lifetime and good fatigue resistance. The UV-induced ring-open of the complex led to a distinct color and fluorescence change in acetonitrile. A new mechanism was put forward: salicylaldehyde part in the complex underwent UV-promoted isomerization from enol-form to keto-form, enhancing the chelation of Cd(II) and yielding the ring-opening rhodamine B part. Compared to other reported photochromic systems, this new photochromic material offered attractive new insights into the development of low cost photochromic materials with good performance.

Full text of DOI:10.1007/s11426-013-5033-3

SCIENCE CHINA
Chemistry
February 2014 Vol.57 No.2: 248–251
doi: 10.1007/s11426-013-5033-3
• COMMUNICATIONS •
· SPECIAL ISSUE · The Frontiers of Chemical Biology and Synthesis
Readily accessible rhodamine B-based photoresponsive material
LI Kai1, XIANG Yu1, TONG AiJun1* & TANG Ben Zhong2*
1Beijing Key Laboratory Microanalytical Methods and Instrumentation; Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical
Biology, Ministry of Education; Department of Chemistry, Tsinghua University, Beijing 100084, China
2Department of Chemistry, Institute of Molecular Functional Materials, State Key Laboratory of Molecular Neuroscience,
The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
Received November 2, 2013; accepted November 4, 2013; published online November 25, 2013
A photochromic rhodamine B-based material containing Cd(II) as bridge was facilely prepared. The 4-methoxylsalicylalde-
hyde rhodamine B hydrazone Cd(II) complex displayed unusual ring-open response upon 365 nm UV irradiation, exhibiting
long photochromic lifetime and good fatigue resistance. The UV-induced ring-open of the complex led to a distinct color and
fluorescence change in acetonitrile. A new mechanism was put forward: salicylaldehyde part in the complex underwent
UV-promoted isomerization from enol-form to keto-form, enhancing the chelation of Cd(II) and yielding the ring-opening
rhodamine B part. Compared to other reported photochromic systems, this new photochromic material offered attractive new
insights into the development of low cost photochromic materials with good performance.
photochromism, coordination compound, rhodamine B, salicylaldehyde hydrazone, cadmium, UV-light
and good properties are of great interest and challenge [8,
1316].
Rhodamine B is a classic fluorescent dye with long wav-
1 Introduction
Photoresponsive materials have been widely used in molec-
ular machines [1, 2], molecular logic gates [3], photo-
electronic devices [4], photoresponsive liquid crystals [5, 6]
and photoinduced nanoparticle devices [7]. The classic
photoresponsive materials include diarylethene, azobenzene,
spiropyran, spirooxazine, anil, fulgide, polycyclic quinone,
and so on. Although they have demonstrated excellent pho-
tochromic properties [8, 9], these materials still possess
some limitations. For example, diarylethene has excellent
fatigue resistance and thermally irreversible properties, but
the production of this material requires complicated multi-
ple-step synthesis, resulting in high cost [10]. On the other
hand, azobenzene and spiropyran are easy to prepare, but
their fatigue resistance is moderate, which limits their prac-
tical applications [11, 12]. Therefore, the design and devel-
opment of photochromic systems with convenient synthesis
elength absorption/emission, high absorption coefficient/
quantum efficiency, and excellent photostability [17]. Its
derivatives have been widely applied in analytical applica-
tions. Recently, rhodamine-based fluorescent chemosensors
were developed in our group for the detection of a series of
analytes [1821]. Taking advantage of rhodamine B’s pho-
tophysical properties, new photochromic molecules could
be developed to overcome the challenges.
In this work, 4-methoxylsalicylaldehyde rhodamine B
hydrazone (1) and its Cd(II) complex (1+Cd(II)) were syn-
thesized. The complex could be facilely prepared with low
cost, and showed excellent photochromic properties in ace-
tonitrile, as well as good fatigue resistance and long photo-
chromic lifetime. Its photochromic properties, including
absorption/fluorescence spectral changes upon 365 nm UV
irradiation and photochromic kinetics were investigated in
details.
*Corresponding authors (email: tongaj@tsinghua.edu.cn; tangbenz@ust.hk)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com link.springer.com

Li K, et al. Sci China Chem February (2014) Vol.57 No.2
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123.46, 124.37, 128.26, 129.38, 129.54, 131.67, 134.27,
149.04, 151.38, 151.90, 153.32, 159.85, 162.82, 163.76.
2 Experimental
2.1 Materials and methods
2.3 Preparation of 4-methoxylsalicylaldehyde rhoda-
mine B hydrazone Cd(II) complex (1+Cd(II))
Absorption spectra were performed on JASCO V-550
UV-Vis spectrophotometer. Fluorescence spectra were rec-
orded on JASCO FP-6500 spectrofluorimeter. The photos
were carried by Canon EOS 600D camera. NMR spectra
were recorded using a Joel JNM-ECA300 spectrometer op-
erated at 300 MHz. ESI mass spectra were obtained on HP
1100 LC-MS spectrometer. Rhodamine B, hydrazine hy-
drate, 4-methoxylsalicylaldehyde and cadmium nitratetet-
rahydrate were purchased from Alfa Aesar Co., Tianjin,
China. All the other materials were purchased from Si-
nopharm Chemical Reagent Beijing Co., Beijing, China.
1+Cd(II) were prepared by the addition of 10 μmol/L Cd(II)
to 10 μmol/L of 1 in acetonitrile at room temperature. ESI
mass spectrometry: m/z 703.2 ([M – H + Cd]+), calculated
703.2 (Figure S1).
3 Results and discussion
The complex 1+Cd(II) exhibited reversible photochromism
in acetonitrile between UV irradiation and dark. As shown
in Scheme 2, upon irradiation at 365 nm, the colorless solu-
tion gradually turned purple. When the UV light was re-
moved, the solution restored colorless in ten minutes. The
absorption spectra of 10 μmol/L 1+Cd(II) before UV irradi-
ation displayed no band above 450 nm. However, upon ir-
radiation at 365 nm, the solution became purple along with
an absorption enhancement of a new peak located at 555 nm
(Figure 1(a)). This photochromic process was also studied
by fluorescence spectra. When excited at 412 nm, fluores-
cence emission of 1+Cd(II) was observed at 522 nm before
UV irradiation. Subsequently, upon irradiation, the emission
band at 522 nm gradually decreased, with two new emission
bands at 517 and 579 nm emerged simultaneously (Figure
1(b)). The recovery reaction in dark was also investigated.
At room temperature, the time for 99.9% recovery was
about ten minutes (Figure 2).
2.2 Synthesis of 4-methoxylsalicylaldehyde rhodamine
B hydrazone (1)
The synthesis of 4-methoxylsalicylaldehyde rhodamine B
hydrazone was shown in Scheme 1. Rhodamine B
hydrazide was prepared through an early procedure [19].
0.91 g rhodamine B hydrazide (2 mmol) was dissolved in
40 mL absolute ethanol. 0.47 g 4-methoxylsalicylaldehyde
(3 mmol) was added, the mixture was refluxed overnight.
The reaction mixture was concentrated to about 15 mL and
allowed to stand at 4 °C overnight. The precipitate formed
was filtered and washed with 50 mL ethanol for 3 times.
After drying under reduced pressure, 0.66 g 1 was obtained
as a red-brown solid (61%). ESI mass spectrometry: m/z
591.3 (48% [M + H]+), 613.3 (52% [M + Na]+), M+ calcu-
1
lated 590.3. H NMR (300 MHz, DMSO-d6) δ (ppm): 1.06
(t, 12H), 3.29 (m, 8H), 3.70 (s, 3H), 6.39 (m, 8H), 7.10(d,
1H), 7.20 (d, 1H), 7.59 (m, 2H), 7.92 (d, 1H), 9.08 (s, 1H),
10.79 (s, 1H). 13C NMR (DMSO-d6) δ (ppm): 12.92, 44.19,
55.80, 66.05, 97.83, 101.69, 105.43, 107.01, 108.66, 112.49,
To get insight into the mechanism of the unique photo-
chromic property of 1+Cd(II), rhodamine B was used as a
control molecule. It is well known that rhodamine B is
Scheme 1 Preparation of 1+Cd(II).
Scheme 2 Proposed mechanism for photochromism of 1+Cd(II) upon 365 nm UV irradiation.

250
Li K, et al. Sci China Chem February (2014) Vol.57 No.2
Figure 1 Absorption (a), fluorescence excitation (b, left) and emission
spectra (b, right) of 10 μmol/L 1+Cd(II) in acetonitrile upon 365 nm UV
irradiation.
Figure 3 Absorption (a), fluorescence excitation (b, left) and emission
spectra (b, right) of 10 μmol/L 1+Cd(II) in acetonitrile after 365 nm UV
irradiation and 10 μmol/L rhodamine B in acetonitrile with 100 μmol/L
HCl.
previous reports, salicylaldehyde hydrazones are sensitive to
UV irradiation [11, 22]. UV light can promote the isomeri-
zation of salicylaldehyde hydrazone from enol-form to keto-
form. Based on the known facts, we proposed a mechanism
for the color change of 1+Cd(II) upon the 365 nm UV irra-
diation. UV light promoted an isomerization of salicylalde-
hyde part in 1 from enol-form to keto-form, and such
change of chelating group to Cd(II) induced the ring open-
ing of rhodamine B part, yielding a purple product (as
shown in Scheme 2). Although the product has a larger
π-conjugation, which is supposed to produce stronger fluo-
rescence [23, 24], the product (1+Cd(II) after UV irradiation)
shows a weaker fluorescence than its precursor (Figure
1(b)). The fluorescence decrease might be due to the for-
mation of the keto-form that can serve as a strong fluores-
cence quenching unit. Interestingly, the bare compound 1
without coordination with Cd(II) displayed no photochromic
properties (Figure 4), suggesting Cd(II) chelation was essen-
tial for the photochromism.
Figure 2 The thermal fading kinetics of 10 μmol/L 1+Cd(II) in acetoni-
trile at room temperature. λ = 555 nm.
photo stable upon UV irradiation, and fluorescent in acidic
solutions rather than basic ones. The absorption and fluo-
rescence spectra of 1+Cd(II), after 365 nm UV irradiation,
were compared with those of rhodamine B in acidic ace-
tonitrile. As shown in Figure 3, the position and shape of
the peaks in 1+Cd(II)’s spectra after 365 nm UV irradiation
were similar to those of rhodamine B, indicating that
1+Cd(II) underwent a ring-open reaction to yield a
ring-open product with color. In addition, according to the
The fatigue resistance of 1+Cd(II) in acetonitrile was in-
vestigated to evaluate its advantage as a potential photo-
chromic material. As shown in Figure 5, 1+Cd(II) in ace-
tonitrile was toggled repeatedly between the purple state
(UV irradiation) and colorless state (dark) for 10 times.

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4 Conclusions
In summary, we have designed and synthesized a new pho-
toresponsive material based on salicylaldehyde rhodamine
B hydrazone Cd(II) complex. The system showed absorp-
tion “turn-on” and fluorescence “turn-off” response upon
365 nm UV irradiation in acetonitrile. The photochromism
was suggested as the result of UV light-promoted isomeri-
zation of salicylaldehyde part from the enol-form to the
keto-form, which changed the chelation with Cd(II) and
induced the ring opening of rhodamine B part. The photo-
chromic system exhibited good fatigue resistance and long
photochromic lifetime at room temperature. This study pro-
vided a new approach to design photoresponsive system
based on coordination compounds. Currently, more pho-
toresponsive molecules derived from such a rhodamine
metal complex are under further investigation in our labor-
atory.
This work was financially supported by the National Natural Science
Foundation of China (21175079 and 21375074).