Near-infrared light-controlled circularly polarized luminescence of self-organized emissive helical superstructures assisted by upconversion nanoparticles

Article abstract of DOI:10.1039/d0cc05910b

A near-infrared light-driven self-organized emissive helical superstructure was constructed by doping a new chiral fluorescent photoswitch and upconversion nanoparticles (UCNPs) into a nematic LC. The reversible switching of circularly polarized luminescence (CPL) can be achieved by modulating the power intensity of the 980 nm NIR excitation light.

Full text of DOI:10.1039/d0cc05910b

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Near-infrared light-controlled circularly polarized luminescence of
self-organized emissive helical superstructures assisted by
upconversion nanoparticles
Received 00th January 20xx,
Accepted 00th January 20xx
Ao Juan†, Hao Sun†, Jinghui Qiao† and Jinbao Guo*
DOI: 10.1039/x0xx00000x
A
near-infrared light-driven self-organized emissive helical
process of the chiral fluorescent photoswitches used in the systems.
In fact, using near-infrared (NIR) light in many fields is urgently
needed compared to the used UV or visible light due to its numerous
advantages, such as deep penetration, precise control with low
interference. Thus, it is highly desirable to develop NIR-light
responsive CLCs with tunable CPL signals.
superstructure was constructed by doping a new chiral fluorescent
photoswitch and upconversion nanoparticles (UCNPs) into a
nematic LC. The reversible switching of circularly polarized
luminescence (CPL) can be achieved by modulating the power
intensity of the 980 nm NIR excitation light.
In this work, we report the first example of 980 nm NIR-light-
triggered reversible tuning of CPL and photoluminescence (PL) in the
CLC with a self-organized emissive helical superstructure loaded with
upconversion nanoparticles (UCNPs). As is well known, UCNPs can
convert near-infrared light into UV light or visible light, thereby
triggering the photochemical reaction of the photoresponsive LC
materials.23 As shown in Fig. 1, the doped core-shell-shell NaYF4-
based UCNPs in the system can change the emitting light when the
power density of 980 nm NIR light is increased or decreased. On one
hand, the dominant 520 nm green light derived from the UCNPs at
low power densities can trigger trans to cis photoisomerization of the
novel chiral fluorescent photoswitch doped in the CLCs system,
yielding that a sharply decrease of both PL and CPL intensity of the
CLC shown in Fig. 1. On the other hand, the reverse process
happened when induced by the 365 nm UV and 450 nm blue
emissions upon irradiation by the 980 nm NIR laser at high power
density. This “remote-control” photoswitching using NIR light allows
us to achieve optically-rewritable PL pattern as well as CPL pattern
with a moderate resolution. This technology offers a highly
convenient method to effectively switch CPL and PL in an emissive
CLC using a single NIR-light source.
Chiroptical functional materials showing circularly polarized
luminescence (CPL) have attracted much attention because of their
attracting properties and promising potential for applications such as
3D displays, information encryption, optical sensors, and
photoelectric devices.1-4 The magnitude of circular polarization of
CPL materials in the excited state is generally quantified by the
luminescence dissymmetry factor (gem), which represents the ratio
of the difference in intensity divided by the average total
luminescence intensity：gem = 2 × (IL - IR)/(IL + IR) (1), where IL and IR
are the intensities of the left and right circularly polarized emissions,
respectively.3b To satisfy the requirement of practical applications,
the development of novel CPL materials with high gem values and
sensitive to external stimuli is highly desirable.5 Among the various
kind of the developed CPL materials,6-22 cholesteric liquid crystal
(CLC) with a self-organized helical superstructure is an ideal matrix
for developing strong CPL signals, high gem values and tunable CPL
responses.14-22 Tunable CPL properties in CLC media by external
stimuli such as temperature, light and electric field have been
reported recently.15c,16,20-22 As one of the most promising ones,
phototunable CPL responses in
a
self-organized helical
superstructure exhibit tunable optical characteristics and moderate
gem value, demonstrating great potentials for real applications. Very
recently, the switchable CPL intensity and helical handedness in the
CLCs media with luminescent helical superstructure have been
reported by our group.21,22 In these two cases, ultraviolet (UV) light
and visible light have been employed to trigger the photoreaction
A new chiral fluorescent switch 6 is used to induce the formation
of photoresponsive self-assembly helical superstructure shown in
Fig. 1. The switch 6 was synthesized following a procedure described
in our previous reports.21,24 The detailed description of the synthetic
procedures (Scheme S1) and structural characterizations are
provided in Supporting Information. The initial state and variation of
CD spectrum of final product switch 6 is shown in Fig. S1. The
optimized ground-state geometries of trans and cis isomers of switch
6 are calculated by Gaussian 09 using density functional theory (DFT)
at B3LYP/6-31G(d) level, in which trans isomers are 3.99 kcal/mol
lower than cis isomers (Fig. S2). A low energy gap between trans and
Key Laboratory of Carbon Fibers and Functional Polymers, Ministry of
Education, and College of Materials Science and Engineering, Beijing University
of Chemical Technology, Beijing 100029, China. E-mail: guojb@mail.buct.edu.cn
†These three authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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cis isomers is beneficial to inversion of C=C configuration, which (b) Schematic of reversible tuning of self-organized luminescent
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DOI: 10.1039/D0CC05910B
promotes the conversion of trans/cis photoisomerization of switch helical superstructures with switch 6 and UCNPs upon irradiation of
6. As shown in Fig. S3, the trans to cis photoisomerization of switch 980 nm NIR laser at different power densities.
6 is effectively triggered by 520 nm green light, and the reverse
isomerization occurs upon irradiation by 365 nm UV light or 450 nm
blue light. In fact, the experimental result is in well agreement with
the theoretical analysis (Table S1). PL spectra of switch 6 shows an
emission peak at around 580 nm shown in Fig. S4, which exhibits a
large intensity variation between initial state and photostationary
state of 520 nm (PSS520). And the PL intensity partly recovers at PSS450
and PSS365 upon 365 nm light or 450 nm light irradiations because of
the cis to trans isomerization. Furthermore, the photoisomerization
conversions could be verified based on the quantitative analysis of
The used NaYF4-based UCNPs with core−shell-shell nanostructures
was prepared using a three-step solvothermal method according to
the detailed description in the supporting information. To achieve a
better selective upconversion emission from NIR light to UV
light/blue light and green light in this study, β-NaYF4, 0.5 mol% Tm3+,
30 mol % Yb3+ cores were coated with a β-NaYF4, 2mol % Er3+, 15
mol % Yb3+ followed by an undoped β-NaYF4 shell. The nanoparticles
were easily verified to hexagonal-phase NaYF4 based on powder X-
ray diffraction data as shown in Fig. S8. Transmission electron
microscopy (TEM) images of the UCNPs (Fig. S9) shows that they
possess uniform hexagonal shapes. As shown in Fig. 2a, the 520 nm
green emission dominates the spectrum at low power density (0.15
W/mm2), while the 365 nm UV and 450 nm blue emission from the
UCNPs are greatly increased at high power density (3.0 W/mm2)
shown in Fig. 2b. As shown in Fig. S10, around 520 nm green
emissions (4H11/2→4I15/2 transition) generated from the Yb3+→Er3+
energy transfer process at low power density of 980 nm laser light
overlap with the absorption bands corresponding to trans form of
switch 6, in which this emission may be utilized to trigger the trans
to cis photoisomerization of the photoswitch. While 365 nm UV and
1
characteristic H-NMR peaks (Fig. S5 of supporting information). All
the above observations reveal that the photoswitchable reversible
trans/cis photoisomerization of switch 6 happens with moderate
conversions. In addition, the photo-responsive behavior of switch 6
in a nematic LC host was demonstrated by the Grandjean-Cano
method (Fig. S6 and Fig. S7), in which the weight ratio of switch 6 to
SLC1717 was 0.25:99.75. On the basis of calculated helical twisted
power (HTP) values in different photostationary states, it is clearly
observed that the HTP values obviously decreases from initial state
to PSS520 while it can partly recover separatedly irradiated by 450 nm
blue light or 365 nm UV light.
1
450 nm blue emissions (1D2→3H6 and D2→3F4 transitions) derived
from Yb3+→Tm3+ energy transfer process could be obtained in a
higher excitation power density of 980 nm laser light, thus making it
possible for the emitted UV and blue light of UCNPs to trigger its cis
to trans isomerization.
Fig. 2 Emission spectra of UCNPs (60 μg/mL) in dichloromethane at
room temperature upon irradiation with 980 nm NIR laser (a) at low
power density (0.15 W/mm2) and (b) at high power density (3.0
W/mm2).
Fig. 3a and 3b show the changes in the UV−vis spectra and PL
spectra of switch 6 (50 μg/mL) and UCNPs (2.0 mg/mL) in
dichloromethane respectively upon irradiation with 980 nm NIR
laser. When irradiated by 980 nm NIR laser at low power density
(0.15 W/mm2) for 30 min, the absorption band at around 490 nm
shifts to around 450 nm with decrease in intensity due to trans to cis
photoisomerization of switch 6. Upon 980 nm NIR laser at high power
density (3.0 W/mm2) for 10 min, the absorption band centered
around 450 nm, corresponding to π-π* transition of cis isomer, red
shifts with concomitant increase in its intensity till the PSS is reached
due to cis to trans isomerization. Meanwhile, a large PL intensity
variation could be clearly observed when trans to cis
photoisomerization of switch 6 happened. The PL intensity recovers
to an intermediate level upon cis to trans isomerization. All above
observations suggest that the photoisomerization properties of
Fig. 1 (a) Schematic diagram of the photoisomerization of switch 6,
which is triggered in a control process using 980 nm NIR laser light;
2 | J. Name., 2012, 00, 1-3
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switch 6 can be effectively modulated by 980 nm NIR laser light, and CPL-encryption information through changing the corresponding 980
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DOI: 10.1039/D0CC05910B
the reversible trans to cis and cis to trans photoisomerization process nm laser input signals was successfully established.
can be induced only by modulating the excitation power density of
the 980 nm laser light.
Fig. 3 Changes in the UV−vis spectra (a) and PL spectrum (b) of switch
6 (50 μg/mL) and UCNPs (2.0 mg/mL) in dichloromethane at room
temperature upon irradiation with 980 nm NIR laser at high power
density (3.0 W/mm2) and upon irradiation with 980 nm NIR laser at
low power density (0.15 W/mm2).
The phototunable PL and CPL behaviors of the CLC sample doped
with 1.0 wt% of switch 6 and 0.5 wt% UCNPs into SLC1717 were
Fig. 4 (a) PL spectra and (b) CPL spectra of photo-responsive CLC
doping 1.0 wt% of switch 6 and 0.5 wt% UCNPs into SLC1717 tuned
by 980 nm NIR laser at high power density (3.0 W/mm2) and at low
power density (0.15 W/mm2). (c) CD spectra of photo-responsive CLC
doping 0.2 wt% of switch 6 and 0.1 wt% UCNPs; Note that:
considering the testing range of CD spectrum, the doped switch 6 and
UCNPs were diluted by 5 times in the configured CLC mixture. (d)
Fatigue resistance test of variation of gem at 605 nm upon alternate
irradiations with 980 nm NIR laser at high power density and at low
power density.
recorded. As shown in Fig. 4a, the PL intensity of the CLC sample
drastically drops to a low level upon 980 nm NIR laser at low power
density (0.15 W/mm2) for 10 min. When exposed to 980 nm NIR laser
at high power density (3.0 W/mm2) for 1.0 min, the PL intensity
increased. We further investigated the changes of CPL spectra in the
CLC sample upon light irradiation shown in Fig. 4b and Fig. S11, the
phototunable CPL could also be observed. The CPL signal decreased
upon low-power 980 nm NIR light irradiation; while the intensity of
the CPL signal partly recovered when exposed to high-power 980 nm
NIR laser light. The tendency of this variation was further verified by
CD spectrum characterization shown in Fig. 4c. Notably, the gem
absolute value at the initial state could be close to 0.49, which is
ascribed to the self-organized helical structure of the CLCs.
Meanwhile, the gem absolute value also undergoes a decrease first
and then a slight increase during the light irradiations as shown in
Fig. 4d. And a dynamic reversible switching of the CPL signal and gem
value between PSS980-l and PSS980-h could be achieved upon 980 nm
laser light irradiations with alternate low power and high power. All
the above observations fully demonstrate that the PL as well as CPL
signal of the CLC could be reversibly switched upon the 980 nm NIR
laser at low power density and high power density.
A schematic of illuminating the PL/CPL dual-mode rewritable
optical storage device is shown in Fig. 5a, here a beam expander is
used in the system to modulate the emission intensity of 980 nm
laser light. The fluorescent dot array with modest contrast can be
written on the cell using a photomask upon 980 nm NIR laser at high
power density irradiation and the information can be erased by 980
nm NIR laser at low power density. In addition, the information or
data can be easily and rapidly written without physical contact and
photomask using the above equipment. As shown in Fig. 5b, a high-
contrast PL pattern could be written, erased and rewritten.
Meanwhile, due to the strong/weak switching of CPL signal, a CPL
pattern featuring S/W encrypted mode was also obtained as
demonstrated in Fig. 5a. Based on the spectacular feature of the
photo-responsive reversible dynamic tuning, a novel integrated
combinational dual-mode device capable of showing both PL and
Fig. 5 (a) Schematic diagram of illuminating the PL/CPL dual-mode
rewritable optical storage; (b) Real images of PL/CPL dual-mode
patterns (note that S: strong CPL signal; W: weak CPL signal).
In summary, we proposed a NIR-light-controlled CPL and PL in a
self-organized emissive helical superstructure fabricated by doping a
new chiral fluorescent photoswitch and UCNPs into a nematic LC
host. The consequent CPL as well as PL were found to be reversibly
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tuned simply by varying the excitation power density of the 980 nm 12 Q. Jiang, X. H. Xu, P.-A. Yin, K. Ma, Y. G. Zhen, P. F. Duan, Q. Peng,
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Conflicts of interest
There are no conflicts to declare.
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