An erasable photo-patterning material based on a specially designed 4-(1,2,2-triphenylvinyl)aniline salicylaldehyde hydrazone aggregation-induced emission (AIE) molecule

Article abstract of DOI:10.1039/c6tc03791g

Luminescent molecules with photochromic properties show strong potential in molecular switches, molecular logic gates, photo-controllable materials, bio-imaging, anti-counterfeiting, photo-patterning, etc. However, research is still scarce in such molecul

Full text of DOI:10.1039/c6tc03791g

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Erasable Photo-patterning Material Based on a Specially Designed
4-(1,2,2-triphenylvinyl)aniline Salicylaldehyde Hydrazone
Aggregation-Induced Emission (AIE) Molecule
Received 00th January 20xx,
Accepted 00th January 20xx
Lili Wang,a Yuanyuan Li,b Xuejiao You,a Kui Xu,a Qi Feng,a Jinmin Wang,a Yuanyuan Liu,a Kai Li,*a and
Hongwei Hou*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Luminescent molecules with photochromic properties show strong potential in molecular switches, molecular logic gates,
photo-controllable materials, bio-imagine, anti-counterfeiting, photo-patterning, etc. However, there is still rare of
research in such molecule exhibiting reversible photo-controllable color and fluorescence change in solid states, which due
to the aggregation-caused quenching (ACQ) effect of most luminogens. In this work, a reversible photochromic molecule
(1) with aggregation-induced emission (AIE) property was developed. Compound 1 exhibits typical AIE properties as a
result of restricted intramolecular rotations (RIR) and excited-state intramolecular proton transfer (ESIPT) processes. As a
photochromic molecule, compound 1 shows reversible color and fluorescence changes upon UV light irradiation with good
fatigue resistance. More importantly, conversion rate from its irradiated form to initial form is controllable by temperature
and long wavelength light irradiation, which make it suitable for photo-patterning material with erasable property.
fluorescence emission of the AIE group was partially or
completely quenched by the quencher but can be recovered
upon cleavage of the quencher under UV stimuli. A following
1 Introduction
Luminescent materials have attracted much attention due to
their diverse applications in chemistry, physic, material
science, and biology.1 However, most of their practical uses
are usually limited by the notorious aggregation-caused
quenching (ACQ) phenomenon: luminogens exhibit good
fluorescence in dilute solutions but weak or even no
fluorescence in concentrated solution or solid state as a result
of the self-quenching effect of adjacent molecules.2 In 2001, a
novel luminescence phenomenon of aggregation-induced
emission (AIE) was first observed by the group of Tang, which
is exactly opposite to the ACQ effect: the emission of
luminogens were very weak in solution but became intense
when aggregates formed.3 Since then, a variety of AIE systems
work of another photo-activatable AIE system based on caged
SA derivatives were reported by Xiang and Tong’s group.6
These novel photochromic systems show great potential of
application in photo-patterning, photo-activatable imaging and
anti-counterfeiting
related
areas.
However,
the
photochromism of these caged fluorophores are irreversible,
which limit their applications.7 To overcome this challenge,
some new strategies in the design of irreversible photo-
controllable AIE system have been introduced such as photo-
induced redox reactions and photo-induced cyclization
reactions. Nevertheless, the fatigue resistances of these
photochromic systems are still unsatisfactory.8
(tetraphenylethenes
(TPE),
hexaphenylsilole
(HPS),
cyanostilbenes and salicyladazine (SA), etc.) were reported in
succession, which have been widely applied in the field of
organic light-emitting diodes (OLEDs), chemical sensors and
multifunctional luminescent material.4
Scheme 1. Photochromic mechanism of salicylaldehyde hydrazone
upon UV irradiation.
Recently, Tang and co-workers have successfully expand the
application of AIE luminogens by developing a photo-
activatable AIE system based on a caged TPE derivative.5 The
According to the reports, salicylaldehyde hydrazone is a
reversible photochromic group with good fatigue resistance.9
Upon UV irradiation, there is a reversible tautomerism
between its enol-form and keto-forms (Scheme 1). Notably,
the enol-form of salicylaldehyde hydrazone is beneficial for
fluorescence emission due to an excited-state intramolecular
proton transfer (ESIPT) process.10 The keto-forms of
salicylaldehyde hydrazone, in contrast, is a typical fluorescence
quenching unit due to its strong electron-withdrawing
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property.11 These unique properties of salicylaldehyde photos and videos were carried with Nikon D5500 camera.
hydrazone suggest that it could be used as a photo- Laser of 405 nm used in Video 1 was produced by LD-T405F00
controllable quencher in the design of photo-controllable laser pointer (power output 20 mW). Visible light used in Video
luminescent molecules.
1 and light sources used in Table S1 and Figure 10 were
produced by a CEL-HXF300/CEL-HXUV300 xenon light source
with different optical filters.
Scheme 2. Proposed mechanism for the color change of 1 upon UV
irradiation.
In this work, a reversible photochromic AIE molecule was
developed by connecting salicylaldehyde hydrazone with a
typical AIE moiety of TPE (Scheme 2). This specially designed
AIE molecule of 4-(1,2,2-triphenylvinyl)aniline salicylaldehyde
hydrazone (
significant fluorescence change. Before UV irradiation,
yellow and exhibited strong fluorescence emission in
aggregative state. After UV irradiation, turned to red and the
fluorescence emission declined significantly. The good fatigue
resistance property of enables it to be applied in reversible
1
) exhibited reversible photochromic property with
1
was
1
1
Scheme 3. Synthetic routes for compound 1, 2 and 3.
photo-patterning and anti-counterfeiting related applications.
2.3. Synthesis.
Compounds 1-3 were synthesized according to the synthetic
2 Experimental
routes shown in Scheme 3. The new compounds were
characterized by NMR spectra and ESI-MS spectra. Detailed
synthetic procedures and characterization data are shown
below and in Supplementary Information.
2.1. Materials.
Salicylaldehyde,
aniline,
(2-bromoethene-1,1,2-
triyl)tribenzene, 4-aminobenzeneboronic acid hydrochloride,
o-anisaldehyde, tetrabutylammonium bromide (TBAB) were
purchased from J&K Chemical Co., Beijing, China. All the other
materials were purchased from Sinopharm Chemical Reagent
Beijing Co., Beijing, China. In these experiments, all the
materials of analytical grade were used without further
purification.
4-(1,2,2-triphenylvinyl)aniline salicylaldehyde hydrazone
(1). 4-(1,2,2-triphenylvinyl)aniline (1a) was prepared through a
reported procedure.12 Salicylaldehyde (0.73 g, 6 mmol) and
compound 1a (1.73 g, 5 mmol) was dissolved in 30 mL
absolute ethanol in a 50 mL flask. The stirred mixture was
heated to 90 oC for 2 h. Then the solvent was concentrated to
10 mL under reduced pressure and cooled to 4 oC. After that,
1
2.2. Characterizations.
1H and 13C NMR spectra were recorded on a Bruker 400
(2.08 g, 4.6 mmol) was obtained as a yellow precipitate with a
yield of 92%. 1H NMR (400 MHz, CDCl3), δ (ppm): 6.90 (m, 1H),
7.07 (m, 20H), 7.35 (m, 2H), 8.58 (s, 1H). 13C NMR (400 MHz,
CDCl3), δ (ppm): 117.34, 119.31, 120.50, 127.70, 131.34,
131.37, 131.40, 140.14, 141.50, 142.90, 143.54, 143.59, 143.63,
161.92. ESI-MS spectrometry: m/z calcd. for [M+H]+: 452.2;
found: 452.2. Elemental analysis, calcd. For C33H25NO: C, 87.77;
H, 5.58; N, 3.10, found: C, 88.24; H, 5.59; N, 3.10.
Avance NMR spectrometer operated at 400 MHz. UV-Vis
absorption spectra were recorded on
a JASCO V-750
spectrophotometer. UV-Vis diffuse reflectance spectra were
measured by Agilent Cary 5000 UV-Vis-NIR Spectrometer,
BaSO4 was used as reference. Fluorescence spectra were
obtained on a Hitachi F-4600 spectrometer. Fluorescence
lifetimes and quantum yields were recorded on Edinburgh FlS-
980 fluorescence spectrometer. Dynamic light scattering (DLS)
experiments were performed by a NanoPlus-3 dynamic light
scattering particle size/zeta potential analyzer at room
temperature. ESI-MS spectra were obtained on Agilent
Technologies 6420 triple quadrupole LC/MS without using the
LC part. Elemental analysis experiments were carried out on a
Flash EA 1112 automatic element analyzer. Single-crystal X-ray
diffraction intensity data were collected on a Rigaku Saturn
Aniline salicylaldehyde Schiff-base (2). Compound
prepared through a reported procedure.13
2 was
4-(1,2,2-triphenylvinyl)aniline-2-methoxybenzaldehyde
hydrazone (3). 2-methoxybenzaldehyde (0.20 g, 1.5 mmol) and
compound 1a (0.35 g, 1 mmol) was dissolved in 20 mL
absolute ethanol in a 50 mL flask. The stirred mixture was
heated to 90 oC for 2 h. Then the solvent was concentrated to
10 mL under reduced pressure and cooled to 4 oC. After that,
3
(0.35 g, 0.75 mmol) was obtained as a yellow precipitate with a
1
yield of 75%. H NMR (400 MHz, CDCl3), δ (ppm): 3.92 (s, 3H),
724 CCD diffractometer with Mo Kα radiation (λ= 0.71073 Å)
at room temperature. X-ray Powder Diffraction (XRD) patterns
6.97 (d, 1H), 7.12 (m, 20H), 7.46 (t, 1H), 8.16 (d, 1H), 8.94 (s,
1H). 13C NMR (400 MHz, CDCl3), δ (ppm): 55.60, 111.16, 120.66,
were obtained by a PANalytical X'Pert PRO diffractometer. The
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120.96, 126.46, 126.53, 127.57, 127.74, 127.88, 131.45, 132.23, different water fraction were measured (Figure 3B). In pure
140.67, 140.92, 141.48, 143.89, 150.71, 155.86, 159.53. ESI-MS ethanol, an absorption peak of 1 at 353 nm was observed.
spectrometry: m/z calcd. for [M+H]+: 466.2; found: Along with the increase of fw, the fine structures of absorption
466.2.Elemental analysis, calcd. for C34H27NO: C, 87.71; H, 5.85; spectra disappeared gradually, and level-off tails in the visible
N, 3.01, found: C, 88.57; H, 5.88; N, 3.00.
region could be clearly observed when fw was higher than 50%.
The level-off tails in the absorption spectra is believed to be
due to the light scattering of aggregates suspensions.14 The
3 Results and discussion
aggregation of
1 in poor solvent of water was further proved
by the dynamic light scattering (DLS) measurement (Figure 3C).
As shown, particles around 100 nanometer sizes were
detected in aqueous solution of 99% H2O/EtOH (v/v). On the
3.1 The AIE characteristics of 1.
The fluorescence characteristics of
in H2O/EtOH (H2O is poor solvent for
solvent for , water fraction (fw) from 0% to 99%, v/v) buffered
by 10 mmol/L PBS at pH 7.0. As shown in Figure 1, the
fluorescence emission of at 539 nm was rather weak in
1
was firstly investigated
1
while EtOH is good
contrary, no particle could be observed for 1 in ethanol.
1
1
ethanol and barely increased before the fw reached 50%.
When fw reached to 60%, the fluorescence emission suddenly
enhanced, which was almost 5-fold greater than that in
ethanol solution and even larger when fw varied from 70% to
99%. The enhanced fluorescence emission could be
contributed to the poor solubility of
EtOH, which indicated that
characteristics. The fluorescence spectra of
similar to that of in poor solvent (fw = 99%), as can be seen in
1
in water than that in
exhibited typical AIE
in solid state is
1
1
1
Figure 2, which further supported the AIE character.
Figure 3. A) photograph and B) absorption spectra of 1 in H2O/EtOH
mixtures with different fw. C) DLS result of 1 in aqueous solution of
99% H2O/EtOH (v/v), the polydispersity index is 0.138. Conditions:
The concentration of 1 is 50 μmol/L.
3.2 The origination of the AIE characteristics of 1.
It is worth noting that the maximum fluorescence emission
wavelength of
monomer (
max = 453 nm),12 but similar with that of SA (λmax
542 nm).13 This result suggested that the AIE property of
1 is remarkably longer than that of TPE
λ
=
1
may
be not only originated from the TPE moiety but also from the
salicylaldehyde hydrazone moiety. To confirm this assumption,
anilinesalicylaldehyde
triphenylvinyl)aniline-2-methoxybenzaldehyde hydrazone (
were chosen as control compounds. As shown in Scheme 3,
compound was designed in lack of the TPE part as compared
to . As expected, exhibited no fluorescence in both EtOH
and aqueous solution (fw= 99%), which indicated that the TPE
hydrazone
(
2
)
and
4-(1,2,2-
3
)
Figure 1. A) fluorescence photograph and B) fluorescence emission
spectra of 1 in H2O/EtOH mixtures with different fw.C) is the
2
fluorescence emission at 539 nm of
1 as a function of fw.
1
2
Conditions: The photograph was taken under an irradiation of 365
nm UV light. The concentration of 1 was 50 μmol/L and the
excitation wavelength was 374 nm.
part in compound
S1).
1 is indispensable for its AIE property (Figure
It is well known that the AIE properties for TPE and its
analogues are come from the restricted intramolecular
rotations (RIR) process: In good solvents, the molecules in
single state and the conjugate plane of phenyl rings undergo
dynamic intramolecular rotations, resulting in excited state
non-radiative transition and fluorescence quenching. In poor
solvents or solid state, the molecules are aggregated and the
rotations are greatly restricted, which make the radiative
transition to be the primary pathway for the excited state
electrons back to ground state.14 To further verify that the RIR
Figure 2. Fluorescence spectra of 1 in solid state and in poor solvent
of 99% H2O/EtOH (v/v).
process contributed to the AIE property of 1, the fluorescence
To further verify that the emission enhancement was
emissions of
1
in solvent with high viscosity (ethanol/glycerin)
induced from aggregation, the absorption spectra of
1 in
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were investigated. As shown in Figure 4, the fluorescence
emission of enhanced gradually with the increase of glycerin
1
fraction (fg) because the high viscosity solvent restricted
intramolecular rotations. Meanwhile, no level-off tail could be
observed in their absorption spectra, which suggested there
was no aggregate of
the RIR process is one of the main causes for the AIE property
of . Moreover, the RIR process is also supported by the crystal
structure of Single crystal of was grown from
methanol/ethanol mixtures by slow solvent evaporation and
was characterized by X-ray crystallography. The crystal of
1 in glycerin. These results indicate that
1
1
.
1
1
was stable to moisture or atmosphere, and is soluble in
common organic solvents such as dichloromethane,
tetrahydrofuran, dimethylsulfoxide, and so on, but insoluble in
water. As shown in Figure 5A, all the carbon/nitrogen atoms in
the salicylaldimine moiety of
indicates possible intramolecular hydrogen. Meanwhile, the
intermolecular distance of
is 5.7 Å (far over 3.8 Å15), which
suggest that there is no face-to-face-stacking in the crystal
(Figure 5B). All of these structural features indicated that had
potential of AIE properties by RIR process in aggregate state.
1 are in the same plane, which
1
1
Figure 5. Crystal structure of 1. A) vertical direction, B) horizontal
direction, C) The dihedral angle between the two aromatic rings in
N-salicylideneaniline moiety of 1.
According to the reports, the AIE properties of SA and its
analogues originated from RIR and ESIPT process: On one hand,
the intramolecular hydrogen-bond in salicylaldimine moiety
stabilizes the conjugated plane structure, which lead to a
possible RIR process.13, 16On the other hand, a fast four-level
(E-c-E*-c-K*-K, Figure 6) cycle occurs immediately after photo
excitation of the SA molecule through the intramolecular
hydrogen-bond, resulting in an ESIPT fluorescence.10a, 17 Thus,
when the intramolecular hydrogen-bond was broken, the AIE
properties of SA and its analogues should disappear. Control
Figure 4. Fluorescence spectra of 1 in EtOH/glycerin mixtures
with glycerin volume fraction (fg) from 0% to 99%. Conditions:
The concentration of
wavelength is 370 nm.
1 is 50 μmol/L and the excitation
compound
3 containing a methoxy group instead of the
phenolic hydroxyl group was synthesized to demonstrate the
role of salicylaldehyde hydrazone moiety in the AIE property of
1
. As shown in Figure S1, no fluorescence could be detected
for both in good and poor solvents. This result clearly
demonstrated that the salicylaldehyde hydrazone moiety is
indispensable for the AIE property of . In addition,
characteristic large Stokes shift (176 nm) could be observed in
3
1
compound 1 (Figure 2), which is larger than that of the
reported TPE monomer (about 60 nm12), suggesting a
18
reasonable ESIPT process.10c,
Therefore, ESIPT process is
another main cause for the AIE effect of
process. Interestingly, compound exhibits unique stability in
its trans-keto form compared with conventional ESIPT
1 besides the RIR
1
19
molecules,17,
which make it a potential photochromic
material. The photochromic property of
discussed in the following pages.
1 will be further
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support this hypothesis, the photo-response performance of
control compounds
Figure S2, compound
property with
the UV light irradiation turned compound
trans-2-Keto 9
In contrast, compound
2
and
3
were measured. As shown in
2
exhibited similar photochromic
1
due to its salicylaldehyde hydrazone structure:
from 2-Enol into
exhibited no
2
.
3
photochromic property in lack of the salicylaldehyde
hydrazone moiety. Therefore, it could be deduced that the
salicylaldehyde hydrazone moiety of
its photochromic property.
1 was indispensable for
Interestingly, compound
bleaching rate. At room temperature without light, the
patterns recorded on the film of can stay for hours (Figure 8).
1 exhibits tunable thermal
Figure 6. Four-level energy diagram of ESIPT process of 1.
1
3.3 The reversible photochromic property of 1.
Elevated temperature would accelerate the conversion from
Compound
1 was supposed to exhibited reversible
trans-1-Keto back to 1-Enol (Figure 9). The light source to
photochromic property based on our design in Scheme 1,
which was characterized and supported by ultraviolet-visible
diffuse reflectance spectra (UV-DRS) and fluorescence spectra.
promote the photochromism of
1 was investigated, which is at
least in the range from 365 nm to 475 nm (Table S1). Light with
longer wavelength will not promote the photochromism
process but even accelerate the recovery rate. As shown in
As shown in Figure 7 and Video 1,
1 was yellow and there was
no absorption band above 550 nm before UV light irradiation.
It turned to red and exhibited a broad absorption band from
450 nm to 600 nm after UV light irradiation. When the UV light
Video 1, the letters were printed in a film of
1 by a household
laser point. After being irradiated by a visible light for about 10
s, the letters on the film could be erased completely. The
effect of light irradiation intensity and wavelength to the
recovery rate were investigated and the results were shown in
was removed, compound
gradually. In contrast, as can be seen in Figure 7B, the
fluorescence emission of around 545 nm was intense before
UV light irradiation (fluorescence quantum yields 4.88%),
while it decreased significantly after UV light irradiation
2.03%). The fluorescence of UV-irradiated also gradually
recovered in dark.
1 restored to its original state
1
Figure 10 and Table S1. The fluorescence intensity of
nm before and after excess UV light irradiation is 141 and 56,
respectively. After keeping in dark for 1 min, the intensity
1 at 545
ϕ
=
1
(ϕ
=
1
increased to 59. When the recovery experiments were taken
under visible light of 500, 550, 600 and 650 nm (light
irradiation intensity was about 2 mW/cm2), the intensity
reached to
87,
110,
62
and
61,
respectively.
(To ensure the veracity, all of the measurements were
repeated for 3 times). It can be seen that the visible light
around 550 nm will accelerate the recovery rate
most effectively. These results are consistent with the
reported ones.20
Figure 7. A) UV-DRS (left) and visible color (right) changes of 1
before and after UV light irradiation. B) Fluorescence spectra (left)
and luminescence (right) changes of 1 before and after UV light
irradiation. The photos in A) and B) are taken under daylight and UV
light, respectively.
Figure 8. Photographic images of thermal bleaching of 1 in dark at
room temperature.
These photochromic properties of
1 might be attributed to
the tautomerism of salicylaldehyde hydrazone from its enol-
form (1-Enol) to trans-keto-form (trans-1-Keto) (Scheme 2). To
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rings in 1 is 7.66o (Figure 5), which is much smaller than 25o. This
uncommon phenomenon can be attributed to the loose packing of
the TPE moiety, which enlarged the intermolecular distance
effectively. This packing is beneficial to the isomerization from cis-
keto to trans-keto form under light irradiation.
It is known that the carbonyl moiety in trans-1-Keto is an
efficient fluorescence quencher due to its strong electron-
withdrawing property.11 Therefore, the fluorescence intensity
of trans-1-Keto was much weaker than that of 1-Enol
.
Furthermore, the absorbance of trans-1-Keto at 450-600 nm is
significantly higher than that of 1-Enol (Figure 7A). Therefore,
the self-absorption of trans-1-Keto is another reason for its
weaker fluorescence intensity.
Figure 9. The thermal fading kinetics of 1 at different temperature.
Further information about the fluorescence change of
obtained from fluorescence dynamics measurement (Figure
11). The fluorescence lifetime ( ) of before UV light
irradiation is 0.83 ns, which is analogous to that of after UV
1 was
τ
1
1
light irradiation (0.75 ns). Radiative decay rate constant (kr)
and non-radiative decay rate constant (knr) were calculated
according to the definition of
τ = 1/(kr +knr) and ϕ = /(kr
kr
+knr).24 Before UV light irradiation, kr and knr of
1
were
5.88×107 s-1 and 1.15×109 s-1, respectively. While kr reduced to
2.57×107 s-1 and knr barely changed (1.23×109 s-1) after UV light
irradiation. The analogous non-radiative decay process of 1-
Enol and trans-1-Keto suggest their similar microstructure in
solid state. To confirm this assumption, X-ray powder
Figure 10. The dot lines are the fluorescence intensity of 1 at 545
nm before and after excess UV light irradiation. The scatterplot is
the fluorescence intensity of UV-irradiated 1 at 545 nm exposed in
light with different wavelength for 1 min.
diffractions (XRD) of
were carried out (Figure 12). It can be seen that the XRD
spectra of before and after UV light irradiation were almost
1 before and after UV light irradiation
According to the reports,9, 20a, 21 the transformation between
cis-keto form and trans-keto form is the major cause of the
high stability of UV-irradiated salicylaldehyde hydrazone
1
the same, which suggest their analogous crystal structures.
derivatives. To investigate the configuration of UV-irradiated
the photochromic properties of in different organic solvents
(dispersed state) were investigated first. As shown in Figure S3,
compound exhibited no photochromic property in its
dispersed state, which suggested that the aggregation of
molecule is indispensable to the stability of UV-irradiated
Since there is no steric hindrance in dispersed state, is more
1,
1
1
1:
1
inclined to exist as a lower energy state of cis-keto-form by
thermal motion, which can easily transform to its enol-form. In
aggregated state, however, the steric hindrance of contiguous
molecules will limit conformational change, which is beneficial
for the stability of trans-keto-form. Therefore, the UV-
Figure 11. Comparison of fluorescence lifetimes of 1 before and
after UV light irradiation.
irradiated
1 should be trans-form in aggregation state. The
trans-to-cis transformation is the rate determination step for
the recovery reaction. That is why the UV-irradiated
persist for hours at room temperature in dark (Figure 8).
1 can
The crystalline packing of molecules has an important effect on
their optical properties. According to the reports, salicylaldehyde
hydrazone can be classified into two categories, i.e., thermochromic
molecules and photochromic molecules.20b, 22 The thermochromic
molecules were usually observed as planar structures that are
closely packed in the crystal lattice. The dihedral angle between the
two aromatic rings is smaller than 25o. In contrast, the
photochromic molecules were usually found as twisted structures
Figure 12. Comparison of XRD results of 1 before and after UV light
irradiation.
with long intermolecular distances, and the dihedral angle between
23
the two aromatic rings is larger than 25o.22c,
Interestingly, 1
exhibited typical photochromic but not obvious thermochromic
properties, although the dihedral angle between the two aromatic
To gain further insight into the photochromic properties, the
calculations were performed on 1-Enol, cis-1-Keto and trans-1-
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Keto with Gaussian 09 program. The geometries were
optimized with RM062X hybrid density function and the basis
set used was 6-31+G(d,p). Theoretically, energy gap between
HOMO and LUMO (EHL) is correlated with the wavelength of
absorption: Smaller EHL enables the electron transit easier,
resulting in a red-shift of absorption wavelength.25 As shown in
Figure 13, the EHL of 1-Enol is 5.53 eV and the EHL of trans-1-
Keto is 4.97 eV. These calculated results agree well with the
red-shifted absorption wavelength obtained by experiments
(Figure 7A). Meanwhile, compared with 1-Enol, the HOMO and
LUMO of cis-1-Keto and trans-1-Keto distributed mainly in the
area near to the carbonyl moiety, which visually demonstrated
the electron-withdrawing property of carbonyl moiety. In
addition, the total energy (sum of electronic and zero-point
energies) of cis-1-Keto and trans-1-Keto was calculated to be -
38137.6774 eV and -38137.2176 eV, respectively. The higher
total energy of trans-1-Keto indicates that the trans-form is a
Figure 15. Fatigue resistance of 1 upon UV light irradiation and
standing in dark alternatively.
As a reversible photochromic material,
1 was capable of
serving as a material for photo-patterning with erasable
property. As shown in Figure 14, letters, 2-dimensional bar
code and other patterns could be recorded on the same film of
1
. The process was rather simple as discussed below. Pre-
organized pattern was firstly printed in transparencies, then
UV light was allowed to pass through it onto the film of and
metastable form of UV-irradiated 1. 1 is more inclined to exist
as a lower energy state of cis-1-keto in dispersed state, which
can easily transform to its 1-Enol. Thus, the photochromic
property is difficult to be observed in dispersed state.
Meanwhile, the total energy of cis-1-Keto and trans-1-Keto are
both higher than that of 1-Enol (-38137.9985eV), suggesting
that the compound have gained energy from UV light after
irradiation.
1
the pattern was subsequently recorded. Other patterns could
be recorded on the same film after the former pattern
disappeared gradually in dark or was erased quickly by long
wavelength light irradiation. The fatigue resistance of
1 was
investigated by toggled repeatedly between 1-Enol and trans-
1-Keto for 10 times. As shown in Figure 15, the fluorescence at
545 nm stayed almost constant without degradation,
indicating a good fatigue resistance of
controllable photochromic properties, as well as the good
fatigue resistance of indicate that it can be served as
1. The reversible,
1
promising solid material for erasable photo-patterning.
In conclusion, a specially designed AIE molecule of
1
connecting TPE and SA moieties has been developed.
Compound exhibits typical AIE properties due to RIR and
ESIPT process. Compare with TPE monomer, compound has
1
1
longer fluorescence emission wavelength of 545 nm and larger
Stokes shift of 176 nm. As a photochromic molecule,
Figure 13. Frontier molecular orbital amplitude plots and energy
levels of the HOMOs and the LUMOs of 1-Enol, cis-1-Keto and
trans-1-Keto. Value of contour envelopes is 0.02 au.
compound
1 shows reversible color and fluorescence changes
upon UV light irradiation. More importantly, recovery rate
from trans-1-Keto back to 1-Enol is controllable by
temperature and long wavelength light irradiation.
1 also
shows good fatigue resistance, which makes it an excellent
candidate for photo-patterning with erasable property.
Compared with the conventional caged photo-activatable
system, photo-induced redox system and photo-induced
cyclization system, this work provides a brand new strategy in
the design of reversible photo-controllable luminescent
system.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (No.21501150, 51502079, 21371155) and
Outstanding Young Talent Research Fund of Zhengzhou
University (1521316005). We are grateful for the helpful
advices in our experiments from Benzhong Tang’s lab of HKUST.
Figure 14. Generating different patterns on the same film of 1.
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