An RGB color-tunable turn-on electrofluorochromic device and its potential for information encryption

Article abstract of DOI:10.1039/c7cc05938h

RGB color-tunable "turn-on" electrofluorochromic (EFC) devices with high color purity (457 nm for blue, 539 nm for green, and 641 nm for red), relatively quick response/fading speeds and remarkable fluorescence contrast ratios are successfully fabricated. They exhibit great potential for increasingly important multistage encrypted information storage and displays.

Full text of DOI:10.1039/c7cc05938h

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RGB Color-Tunable Turn-On Electrofluorochromic Device and its
Potential for Information Encryption
Xiaojun Wang,a, b Wen Li,a, b Wanru Li,b Chang Gu,b Hongzhi Zheng,b Yuyang Wang,a, b Yu-Mo
Zhang,*b Minjie Li,a, b and Sean Xiao-An Zhang*a, b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
RGB color-tunable "turn-on" electrofluorochromic (EFC) devices method is to separate the redox part and the fluorophore and
with high color purity (457 nm for blue, 539 nm for green, and 641 make use of proton transfer to control the emission. For
nm for red), relatively quick response/fading speed and example, photo-acid was used to switch the intermolecular
remarkable fluorescent contrast ratio are successfully fabricated. fluorescence in many excellent works.10,21,22 Electro-base,23,24
Their potential for increasingly important multistage encrypted the alkalinity could be controlled conveniently by redox,
information storage and display is exhibited.
provides a new way for developing desirable "turn-on" EFC
device. As shown in Scheme 1, when neutral p-benzoquinone
(p-BQ) as a redox active molecule is reduced, a radical anion
Electrofluorochromic EFC) devices as new smart luminescent
(
devices,1-3 in which the ON/OFF states of photoluminescence
are reversibly switched by electric field,4 have received great
attention5 due to their good potential application for optical
displays,6,7 information encryption8 and information
communications9,10. However, the color adjusting is difficult
for existing EFC materials. Therefore, development of desirable
RGB color-tunable EFC devices,11 has great significance for
accelerating practical optoelectronic applications.12-14
(
p-BQ•-) with strong alkalinity is generated. Thus, such p-BQ•-
will likely replace chemical base to switch some pH-sensitive
luminescent materials in a closed system. And such switched
luminescent materials could also be reversed back to their
original state when the newly protonated p-BQ•- is reoxidized
at an opposite voltage.
Hence, screening of suitable pH-sensitive luminescent
materials is the key to develop such RGB color-tunable EFC
devices with a "turn-on" mode. Based on the electro-base
mechanism, ideal pH-sensitive molecular materials need to
satisfy the followings: (1) Photoluminescence of the materials
can be changed from dark to brightness after stimulated by
base.
Since 2006, several EFC devices have been fabricated and
reported15, and those devices can be generally classified into
two categories.16 One is a "turn-off" mode6,17 in which
photoluminescence is switched from brightness to dark. Its
fluorophores can either be intrinsically electroactive18 or
combine an electroactive group via covalent linkage.12,19 The
other is a "turn-on" mode in which switching direction of
photoluminescence is from dark to brightness.20 Since this
mode is more suitable for modern information display,
development of this type of EFC material has great significance
to promote its application in optoelectronic device. However,
the fluorescence of existing EFC materials is always quenched
by radical cation or anion when materials are oxidized or
reduced.
To address these challenges and obtain desirable "turn-on"
EFC materials, the quenching of photoluminescence by the
radical anion/cation must be prevented. One ingenious
Scheme 1 The fluorescent switching mechanism induced by p-BQ as electro-
base.
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(2) Their pH sensitivity should match to the alkalinity of the p- that pH sensitivity of
Journal Name
B
molecule matches well with the
BQ•- for switching their photoluminescence. (3) Such materials alkalinity of p-BQ•- for switching the fluorescence on. Similar
are electrochemically stable at the voltage to reduce p-BQ
.
phenomena and the same consequences were also obtained
and molecules by the same experimental procedures as
Based on above discussion, choosing suitable pH-sensitive for
G
R
materials is the first step to fabricate the color-tunable EFC shown in Fig. 1 and supported by Fig. S2-S3.
devices. Fortunately, many pH-sensitive photoluminescent
molecules have been designed and synthesized by excellent
researches,25,26 so any color needed can possibly be found to
fulfill our goal. After careful screening, the
hydroxycoumarin), molecule (rhodol), and
(4-hydroxystyryl)-6-methyl-4H-pyran-4-
B
molecule (7-
G
R molecule (2-(2-
ylidene)malononitrile) were chosen as base-sensitive
materials with blue, green and red fluorescence respectively,
as shown in Scheme 1. The pH-sensitive fluorescent properties
of B, G and R molecules were studied systematically, as shown
in Fig. S1-S3. In order to obtain high fluorescent intensity and
high contrast ratio for EFC device, the relationship between
the concentration of B, G and R molecules and the intensity of
fluorescence was discussed in detail, as shown in Fig. S4-S7.
Excellent blue (454 nm), green (536 nm) and red (653 nm)
fluorescence can be generated after stimulated by sodium
tert-butoxide as chemical base.
Fig. 2 (a) The fluorescent spectra and (b) the working electrodes’ pictures under
365 nm UV light of the mixture of p-BQ (1.0 × 10-3 M) and B molecule (1.0 × 10-5
M); p-BQ (1.0 × 10-3 M) and G molecule (1.0 × 10-5 M) in 0.1 M TBAPF6/CH3CN;
and p-BQ (1.0 × 10-3 M) and R molecule (5.0 × 10-5 M) in 0.1 M TBAPF6/CH2Cl2
stimulated by the voltage of -0.55 V. (c) The fluorescent spectra and (d) the
working electrodes’ pictures stimulated by the voltage of 0.8 V after stimulated
by -0.55 V for 60 s.
From the aforementioned experiments, we can confirm
that the electrochemical stability and pH sensitivity of such
three molecules fulfil the basic requirement for fabricating
desirable "turn-on" EFC devices. Next, in order to investigate
further the feasibility of their switchable emissions stimulated
by electric field in-situ, the fluorescent spectra of these three
molecules in solution mixed with p-BQ were measured by
spectrofluorimeter combined with electrochemical work
station in a spectro-electrochemical cell (Scheme S2). As
Fig. 1 (a) The cyclic voltammograms of B molecule, G molecule, R molecule
and p-BQ in 0.1 M TBAPF6/CH3CN (1.0 × 10-3 M) with scan rate at 100 mV/s.
(b) The fluorescent spectra and (c) the pictures under UV light of B molecule
(1.0 × 10-5 M) in CH3CN, G molecule (1.0 × 10-5 M) in CH3CN and R molecule
(1.0 × 10-5 M) in CH2Cl2 stimulated by p-BQ•- (1.0 × 10-3 M).
The appropriateness of these molecules was evaluated and
confirmed next by investigating their electrochemical stability
and pH sensitivity. Firstly, electrochemical properties of the B,
shown in Fig. 2a, no emission band of the molecule
B and p-BQ
mixture in solution can be observed without electrical field
stimuli. But, once a -0.55 V bias voltage was applied, an
emission peak appeared gradually at 454 nm along with an
obvious change of color emission from dark to blue (Fig. 2b).
The appeared emission band was exactly the same with the B-
ON form generated by chemical base (Fig. S8a). The intensity
of the emission band was dependent on simulation time, and
the maximum emission intensity can be obtained after 50 s
stimulation in cell. Meanwhile, the emission peak at 454 nm
and blue emission color disappeared reversibly at a reversed
voltage about 0.8 V as shown in Fig. 2c and 2d. However, the
G
and R molecules were measured by cyclic voltammetry. As
shown in Fig. 1, reduction potentials of these three molecules
were all above -0.55 V which is the voltage that the radical
anion p-BQ•- actually generates. This indicates that all of them
had desirable electrochemical stability when electric field was
applied to generate the p-BQ•-. And a new emission peak
appeared immediately at 454 nm when the p-BQ•- solution,
prepared in electrolytic cell (Scheme S1), was added to the
B
molecule solution, along with a distinctive fluorescent change
from dark to blue, as shown in Fig. 1b and 1c. More
interestingly, the emission peak of B molecule solution after
adding the p-BQ•- is exactly identical to the B-ON form
emission generating cannot be detected in solution of either
B
molecule or p-BQ alone under the same conditions, as shown
generated by chemical base (Fig. S1). These results indicate
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in Fig. S9a. Thus, the mixture solution of
B molecule and p-BQ quick response/fading time (Fig. 3d). For the blue device, only
exhibited a delightful EFC property, and its behavior was likely 970 ms response time was needed to obtain a delightful blue
engendered via the anticipated electro-base mechanism. The light (457 nm) with 255 contrast ratio. The bright blue light can
consequence was further proved by change of the absorption be maintained when a steady voltage was applied at -1.2 V,
spectra switched by electric field, as shown in Fig. S8b and S9b. and the blue light can be easily erased in 760 ms at + 0.3 V, as
Similar phenomena and the same consequences were also shown in Movie S1. Its contrast ratio is also extremely high and
obtained for
the same experimental procedure, as shown in Fig. 2 and voltage (-0.5 V) was needed to switch this device (Fig. S14).
G (536 nm) and R
(653 nm) molecules through satisfied for general information displays.11,27 Only a low
supported by Fig. S10-S13.
The other two EFC devices also exhibited excellent EFC
properties. For the green device, its response time was 720 ms
for bringing out the green light (539 nm) with 156 contrast
ratio, and 760 ms for erasing its green light. For the red device,
the response time was 2.05 s for bringing out the red light (641
nm) with 425 contrast ratio, and 3.36 s for erasing its red light.
The cycle speeds of these devices are relatively quick
compared with reported EFC devices.11,17 For optoelectronic
display, the reversibility is another important property for
practical applications. As shown in Fig. 3e and S15, The devices
have very good durability with no degradation observed after
1000 test cycles for
B and G devices and 200 test cycles for R
device. A single display device (Fig. 3f) with the RGB letters and
colors was fabricated (Scheme S4). No letters and fluorescence
can be seen without applying external voltage, while
remarkable RGB letters appeared after -1.0 V voltage was
applied. Therefore, the “turn-on” RGB color-tunable EFC solid
devices have surprising behaviors for great color purity,
remarkable fluorescent contrast ratios, relatively quick
response/fading speed and highly durability. We strongly
believe that these new RGB color-tunable EFC devices will
stimulate and accelerate future development of the full-color
EFC display devices.
Fig. 3 (a) The structure of EFC device. (b) The fluorescent spectra of EFC
devices, respectively, excited by 380 nm, 480 nm and 570 nm. (c) The emission
of EFC devices in chromaticity diagram with chromaticity coordinates for R (x:
0.68, y: 0.32), G (x: 0.36, y: 0.63) and B (x: 0.14, y: 0.12). The pictures of EFC
devices before stimulated and after stimulated by the voltage at -1.2 V are
included. (d) The response and fading speeds of the R, G and B solid devices.
(e) The reversibility of the B solid device for 1000 cycles. (f) The single device
shows the RGB letters with RGB colors at the same time.
With the feasibility of these RGB color-tunable EFC
materials proved, solid EFC devices were then fabricated
(Scheme S3). As shown in Fig. 3a, a three-layer structure
containing EFC layer, ion conductive layer and ion storage layer
was used to investigate their performance. Fortunately, the
anticipated “turn-on” EFC properties were also obtained using
such solid EFC devices. And the emission peaks at 457 nm, 539
nm, and 641 nm can be reversibly switched by electric field, as
shown in Fig. 3b. The EFC devices also exhibited very high color
purity of blue, green, red colors shown in chromaticity diagram
(Fig. 3c). More importantly, these new EFC devices exhibited
excellent properties with high contrast ratios and relatively
Fig. 4 (a) The schematic diagram for a encrypted information storage and
display device. (b) The RGB color-tunable EFC demo device used for three-
letters encrypted information storage and display. (c) The EFC devices for nine-
numbers encryption digiboard.
For EFC devices, two signals (electricity and excitation light)
are needed to input simultaneously if the output signal
(fluorescence) can be read successfully. That is to say, EFC
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devices can be possibly used for encrypted information There are no conflicts of interest to declare.
storage and display meeting today’s needs of privacy and
security. Besides, multistage information can be attached if
Notes and references
color is endowed with cipher. The encrypted device which is
applied in confidential message storage and display was shown
in the schematic diagram (Fig. 4a). Based on such proposal, our
RGB color-tunable EFC devices were utilized as an interesting
multistage encrypted information storage and display. Due to
that fluorescent change is always coupled with color changing
which can disturb the application of security information
display, ion conductive layer and ion storage layer were mixed
with conductive carbon black which can eliminate the
interference of color change (Fig. S16-S18). The carbon black at
ion conductive layer and ion storage layer would not hinder
the emission of these devices (Fig. S19). As shown in Fig. 4b,
"SOS" letters were displayed in our RGB color-tunable EFC
demo device (Scheme S5) only when electricity (-1.2 V) and
excitation light (365 nm) were input simultaneously. However,
no letter can be seen if either electricity (-1.2 V) or excitation
light (365 nm) was input alone. More interestingly, for this
three-letter device, 27 color combinations can be chosen to
transmit one additional information when the device was
made. And 19683 color combinations can be chosen in nine-
number encryption digiboard to transmit one additional
information when the sequence of the number was stored in
the device. Only when electricity (-1.2 V) and excitation light
(365 nm) were input simultanuously, can the inputted
sequence of number and color combination be read as shown
in Fig. 4c. The so many choices (19683) show the great
importance of multicolor for the real applications of EFC
devices.
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In conclusion, RGB color-tunable EFC devices (red, green,
and blue) with a "turn-on" mode were newly fabricated based
on the same electro-base mechanism. Such "turn-on" color-
tunable EFC devices, containing pH-sensitive fluorescence
molecules and p-BQ as an electro-base, show high color purity
(457 nm for blue, 539 nm for green, and 641 nm for red),
relatively quick cycle speed (response/fading time: 970 ms/750
ms for blue, 720 ms/760 ms for green, and 2.05 s/3.36 s for 21. S. Swaminathan, M. Petriella, E. Deniz, J. Cusido, J. D. Baker, M.
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red) along with high fluorescent contrast ratio (255 for blue,
156 for green, and 425 for red), and excellent durability. The
potential application in encrypted information storage and
display was exhibited. In addition, the screening criterion
(electrochemical stability and pH sensitivity) of desirable pH-
sensitive fluorescence materials for ideal EFC devices using
electro-base
mechanism
was
systematically
and
comprehensively elaborated. Thus, this paper opens a new
way for developing different color EFC devices, and will surely
stimulate and accelerate more applications in future, such as
multi-functional electronic displays.
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This study was supported by the National Science
Foundation of China (Grant No. 21602075, 21572079) and the
program of Chang Jiang Scholars and Innovative Research
Team in the University (IRT101713018) for financial support.
Conflicts of interest
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