Monomeric and aggregation emissions of tetraphenylethene in a photo-switchable polymer controlled by cyclization of diarylethene and solvent conditions

Article abstract of DOI:10.1039/c7tc03071a

A novel photo-switchable polymer P-PHT containing diarylethene (DAE) and tetraphenylethene (TPE) moieties and a triazole linker in the repeating unit was synthesized to study the aggregation-induced emission (AIE) behaviour of TPE with both the open and closed forms of DAE in P-PHT with high water contents. The photo-switching phenomena of DAE (from open to closed forms under UV-irradiation) in P-PHT were prominent in organic solvent (THF), high water contents (at 90% H₂O) and acidic conditions. Upon UV-irradiation of P-PHT at 90% water content, the AIE of TPE was completely quenched via an energy transfer event from TPE to cyclized DAE. Interestingly, the rare monomeric emission of TPE was first discovered by the photo-cyclization of DAE in P-PHT compared with the AIE behaviour of TPE with the open form of DAE in P-PHT with AIE favourable conditions of high water content (90% H₂O) and acidic conditions.

Full text of DOI:10.1039/c7tc03071a

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DOI: 10.1039/C7TC03071A.
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PAPER
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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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ARTICLE
Monomeric and aggregation emissions of tetraphenylethene in a
photo-switchable polymer controlled by cyclization of
diarylethene and solvent conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Ravinder Singh,a Hsin-Yen Wu,a Atul Kumar Dwivedi,a Ashutosh Singh,a Chien-Min Lin,a Putikam
Raghunath,b Ming-Chang Lin,b Tung-Kung Wu,c Kung-Hwa Weia and Hong-Cheu Lin*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel photo-switchable polymer P-PHT containing diarylethene (DAE), tetraphenylethene (TPE) moieties and a triazole
linker in the repeating unit was synthesized to study the aggregation-induced emission (AIE) behaviour of TPE in both open
and close forms of DAE in P-PHT with high water contents. The photo-switching phenomena of DAE (from open to close
form under UV-irradiation) in P-PHT were prominent in organic solvent (THF), high water contents (at 90% H2O) and acidic
conditions. Upon UV-irradiation of P-PHT at 90% water content, the AIE of TPE was completely quenched via energy
transfer event from TPE to cyclized DAE. Interestingly, the rare monomeric emission of TPE was first discovered by the
photo-cyclization of DAE in P-PHT compared with the AIE behaviour of TPE in the open form of DAE in P-PHT with AIE
favourable conditions of high water contents (90% H2O) and acidic conditions.
Tang and co-workers in 2001.36,37 AIE is an excellent behaviour
of TPE that is caused by the restriction of intra-molecular
rotations (RIR) of connected periphery phenyl rings in
1. Introduction
Among various photo-switchable units, many researchers have
been attracted to diarylethenes (DAEs) due to their excellent
characteristics,1 such as thermal stability, fatigue resistance,
high sensitivity,2 and response in various states.3-6 The ease of
introducing organic functionalities to DAE units resulted in a
variety of fluorescent DAE derivatives by means of cyclization
for the applications of optical memory,7,8 intra-molecular
energy transfer,9-18 and electron transfer.19-21 The photo-
switching activities of DAEs could be facilitated by several
factors, including structural conformations of open-ring
isomers, electron donor/acceptor substituents, conjugation
lengths of hetero-aryl groups and other external stimuli (e.g.,
pH, cations, anions and biomolecules).22-33 Moreover, due to
better film qualities of polymers, photochromic DAE-based
polymers are preferred than small molecules for their practical
applications in the areas of optoelectronics, photonics and
linear and nonlinear optics.34,35
aggregates. The TPE unit also acts as an energy donor moiety
with the combination of some other fluorophores reported in
many literatures.38,39 Furthermore, with the fruitful advantages
of AIE effects for practical applications, a large variety of AIE-
based probes in the area of chemo- and bio-sensors,40,41
biomedical imaging processes,42 OLED devices,43 optical
waveguides,44 and liquid crystal displays with efficient solid-
state emissions have been widely explored and developed.45,46
Besides, many AIE-based fluorescent probes for pH detections
have been successfully created47 by taking accountable
principles of AIE-based pH sensing processes that are dis-
aggregations and aggregations of AIEgens in different pH
environments.
There are very few reports on photo-switching activities of
DAEs coupled with other fluorophores (or chromophores),48-50
i.e., tetraphenylethenes (TPEs), so it would be remarkable to
incorporate novel properties of energy/electron transfer from
TPEs to DAEs into a main-chain AIE-active polymer. Moreover,
TPE-based polymeric materials possess numerous advantages,
such as ease of processability and functionalization, structural
diversity and good thermal stability, which supply them with
great potentials in practical applications in contrast to AIE-
active small molecules.51-56 Therefore, constructing novel
polymers with the combination of DAE and TPE might be quite
interesting towards disclosing important associated properties.
Additionally, monomeric emissions of TPEs are rare and till
date only two reports presented such emissions in mono-
chromophoric systems.57,58
Nowadays, the aggregation-induced emission (AIE)
phenomenon of tetraphenylethene (TPE) fluorophore has
attracted much attention since it has been first introduced by
a.Department of Materials Science & Engineering, National Chiao Tung University,
Hsinchu 300, Taiwan *linhc@mail.nctu.edu.tw
b.Center for Interdisciplinary Molecular Science, Department of Applied Chemistry,
National Chiao Tung University Hsinchu 300, Taiwan
c. Department of Biological Science & Technology, National Chiao Tung University,
Hsinchu 300, Taiwan
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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As described in the literature, because the photo-switching chloride with analytical purity were used and dry solvents
DAE moiety attached to the fluorophore directly (or through a were obtained by distillation over appropriate drying agents.
short linker) would induce opposite energy transfer rather By following standard procedures, all moisture sensitive
than cyclization,59-61 no photochromism could be observed. reactions were carried out under N2 gas atmosphere to avoid
This problem can be overwhelmed by introducing a flexible moisture and the product formations were monitored via
spacer between the DAE moiety and fluorophore, so we insert visualized the TLC plates under ultra-violet light (256 nm). The
the flexible spacer in our molecular design to explore the merck silica gel 60 (230-400 mesh) was used for column
energy transfer event. Herein, we report a novel polymer P- chromatography to purified the desired products and the
PHT consisting of DAE and TPE units with a protonable triazole reported yields are isolated yields. By using 1 cm quartz
linker via Cu(I) catalyzed click reaction and its synthetic route is cuvette the corresponding UV-Vis spectra and fluorescence
shown in Scheme 1, where the cyclization of DAE in both measurements were conducted in
organic solvent (THF) and 90% water were significant as spectrophotometer and HITACHI
a
Jasco UV-600
4000 series
monitored the energy transfer event from TPE to DAE. The spectrophotometer, respectively. All emission and excitation
Cu(I)-catalyzed azide-alkyne cycloaddition is popularly known spectra were corrected for the detector response and the
as the click reaction.62 Nowadays, click chemistry has found lamp output. The molecular weight of the polymer (P-PHT
)
widespread applications in various research areas, such as was measured by gel permeation chromatography (GPC) using
triazole-based and post- Waters 1515 (isocratic HPLC pump) and 2414 (reflective index
polymers,63 dendrimers64
funtionalized polymers.65 Since the click reaction adores the detector) instruments and eluting with tetrahydrofuran (THF).
benefits of mild reaction conditions, high efficiencies and The UV-irradiation was carried out using a 500 W Xenon arc
regioselectivities along with simple purification procedures, it lamp (Solar Electro-Optics Co., LTD) with an intensity of 1.17
has become a powerful synthetic tool with applicability in mW/cm2 as the light source. Mass spectra (HRMS) were
different research fields.66 In this study, we adopted such click obtained on the respective mass spectrometer. In order to
reaction to prepare our photo-switchable polymer. The photo- investigate the surface morphologies and particle size
switchable phenomena of
a designed DAE derivative distributions of P-PHT, thus the corresponding experiments
connected with tetraphenylethene (TPE) via a spacer are were performed by using a thermal field emission scanning
explored in various aqueous conditions and/or different pH electron microscope (SEM) (JEOL JSM-6500 F). SEM samples of
environments (to incorporate protonable moieties). During the P-PHT were prepared by drop-casting onto Si wafers followed
photo-switching of DAE (from open to close form under UV- by vacuum drying under room temperature. Furthermore, the
irradiation), the generation of monomeric emission with dynamic light scattering (DLS) analyses were studied by using a
simultaneous quench of aggregation-induced emission (AIE) of Malvern instruments-Zetasizer nano ZS90.
TPE occurred in high water content (90% H2O), which might be
NMR spectra were recorded on Bruker DRX-300 Avance
considered as the first example of a photo-switchable polymer. series (1H: 300 MHz; 13C: 300 MHz) at a constant temperature
of 298 K and chemical shifts were reported in parts per million
(ppm) from low to high field, referenced to residual solvents
(CDCl3 δ = 7.26 ppm and δ = 77.23 ppm; DMSO-d6 δ = 2.49
ppm and δ = 39.56 ppm, respectively) and tetramethylsilane
1
SiMe4 (TMS) was used as internal reference for the H and 13C-
NMR analyses. The coupling constant (J) were reported in
hertz (Hz). Standard abbreviations indicating multiplicity were
used as follows: s = singlet, d = doublet, dd = double doublet, t
= triplet, m = multiplet, br = broad.
Synthetic procedure of intermediates
Synthesis of 1,5-bis(5-chloro-2-methylthiophen-3-yl)pentane-
1,5-dione (1-1) and 1,2-bis(5-chloro-2-methylthiophen-3-
yl)cyclopent-1-ene (1-2). Synthesis of intermediates 1-1 and 1-
2
were performed by following the reported synthetic
procedure in the literature.61,62
Scheme 1. Synthetic route of P-PHT
.
ESI-HRMS of 1-1= Chemical Formula: C15H15Cl2O2S2, Measured
(M/Z): 360.9885. Calculated (M/Z) = 360.9887 [M+H]+.
Synthesis of intermediate 2-3. Intermediate 2-3 was
successfully synthesized in our previous report and carried
forward for this project.37
1H-NMR (300 MHz, DMSO-d6) δ (ppm): 7.07-7.15 (6H, m), 6.98
(4H, m), 6.88-6.93 (4H, m), 6.67-6.70 (4H, m), 3.95 (4H, t, J =
6.0 Hz, oxygen α-proton), 3.48 (4H, t, J = 9.0 Hz), 1.93 (4H, q, J
= 6.0 Hz, Br α-proton).
2. Experimental
Materials and characterization
All commercial available starting materials 2-chloro-5-
methylthiophene, 4-hydroxy benzophenone and glutaryl
2 | J. Name., 2012, 00, 1-3
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13C-NMR (300 MHz, DMSO-d6) δ (ppm): 157.2, 144.2, 139.7, 13C-NMR (300 MHz, CDCl3) δ (ppm): 136.2, 135.4, 134.5, 65.5,
136.2, 132.4, 131.2, 128.2, 126.7, 114.2, 64.8, 48.1, 28.5.
ESI-HRMS= Chemical Formula: C32H30N6NaO2, Measured (M/Z):
553.2324. Calculated (M/Z) = 553.2322 [M+Na]+
38.5, 23.0, 14.5.
Synthesis
of
1,2-bis(2-methyl-5-((prop-2-yn-1-
yloxy)methyl)thiophen-3-yl)cyclopent-1-ene (1-5). Compound
Synthesis
methylthiophene-2-carbaldehyde) (1-3). Compound 1,2-bis(5- diyl))dimethanol (1-4) (2.0 g, 1 eq.) was charged into a cleaned
chloro-2-methylthiophen-3-yl)cyclopent-1-ene 1-2 (5.5 g, dry 100 mL three necked round bottom flask at room
of
4,4'-(cyclopent-1-ene-1,2-diyl)bis(5- (4,4'-(cyclopent-1-ene-1,2-diyl)bis(5-methylthiophene-4,2-
(
)
1eq.) was charged into the cleaned dry 500 mL three necked temperature under nitrogen gas atmosphere followed by the
round bottom flask with the addition of dry ethyl ether (200 addition of dry DMF (100 mL) through syringe into reaction
mL) at room temperature under N2 gas atmosphere. The mixture and nitrogen gas was further purged for 15 min.
reaction mixture cooled to -30 to -40°C and stirred for 10 min. Strong base NaH (0.6 g, 4 eq.) was then added gradually to the
After that, n-BuLi (20 mL, 3eq.) was added into the above reaction mixture (over 10 minutes) followed by the addition of
reaction mixture under N2 gas atmosphere by syringe very imidazole (0.21 g, 0.5 eq.) with continuous stirring for 15-20
carefully and slowly about 20 to 30 min at -30 to -40°C and min until the reaction color turned to yellowish and left at
allowed to stir for 30 minute at the same temperature. Then, room temperature. Then, the propargyl bromide (2.36 mL, 5
the reaction mixture was allowed to stir at room temperature eq.) was added drop-wise into reaction mixture and allowed to
until the reaction mixture color became dark brown, followed stir for overnight. The reaction was monitored by TLC. On
by the addition of dry DMF (4 mL, 3eq.) into reaction mixture completion, reaction was quenched by crushed ice. Reaction
at room temperature very slowly within 10-15 minutes and mixture was then extracted with diethyl ether (200 mL). The
stirred for additionally 3hr. The progress of the reaction was Diethyl ether organic layer was separated out and washed with
monitored by TLC. After completion, the reaction mixture was H2O (2 x 100 mL) followed by utilizing brine solution (2 x 100
quenched by adding H2O (10 mL) and stirred for 20 min. The mL) and collected organic layer dried over MgSO4. Filtered
organic layer was separated, washed with H2O (200 mL x 2) organic layer was evaporated under reduced pressure and the
and brine solution (200 mL x 2) and dried over MgSO4, filtered crude residue obtained was subjected to column
and evaporated to dryness. The crude residue of desired chromatography (silica gel, Hexane/EtOAc: 9/1 for 1-5). Light
intermediate 1-3 was subjected to column chromatography brownish color liquid of desired intermediate 1-5 was
(silica gel, hexane/ethyl acetate: 8/2 for 1-3). Light greenish recovered (Desired yield = 1.5 g, 60% w/w).
color solid desired intermediate 1-3 was recovered (Desired 1H-NMR (300 MHz, CDCl3) δ (ppm): 6.67 (2H, s), 4.61 (4H, s),
yield = 2.5 g, 48% w/w)
4.09 (4H, d, J = 2.4 Hz), 2.73 (4H, t, J = 7.2 Hz), 2.44 (2H, s, J =
1H-NMR (300 MHz, CDCl3) δ (ppm): 9.74 (2H, s), 7.43 (2H, s), 2.4 Hz), 2.04–1.99 (2H, m), 1.92 (6H, s).
2.83 (4H, t, J = 7.5 Hz), 2.15–2.10 (2H, m), 2.05 (6H, s).
13C-NMR (300 MHz, CDCl3) δ (ppm): 182.3, 146.3, 140.2, 128.8, 79.5, 74.7, 65.6, 56.0, 38.3, 23.0,14.3.
13C-NMR (300 MHz, CDCl3) δ (ppm): 135.8, 135.3, 135.0, 134.6,
137.3, 137.0, 135.0, 38.4, 22.9,15.3.
ESI-HRMS= Chemical Formula: C23H24NaO2S2, Measured (M/Z):
ESI-HRMS= Chemical Formula: C17H17O2S2, Measured (M/Z): 419.1110. Calculated (M/Z) = 419.1112 [M+Na]+
317.0664. Calculated (M/Z) = 317.0668 [M+H]+.
Synthetic procedure of polymer (P-PHT)
Synthesis
of
4,4'-(cyclopent-1-ene-1,2-diyl)bis(5-
methylthiophene-4,2-diyl))dimethanol (1-4). The desired Compounds1,2-bis(2-methyl-5-((prop-2-yn-1-yloxy)methyl)
intermediate 4,4'-(cyclopent-1-ene-1,2-diyl)bis(5- thiophen-3-yl)cyclopent-1-ene (1-5) (0.5 g, 1eq.), 1,2-bis(4-(3-
methylthiophene-2-carbaldehyde) 1-3 (2.5 g, 1eq.) was azidopropoxy)phenyl)-1,2-diphenylethene (2-3) (0.66 g, 1 eq.)
(
)
charged into the cleaned dry 250 mL three necked round and Cu(PPh3)3Br (1.17 g, 1eq.) were charged into a cleaned dry
bottom flask with the addition of dry MeOH: DCM (75:75 mL, 100 mL three necked round bottom flask at room
1:1 ratio) at room temperature under N2 gas atmosphere. The temperature, followed by the addition of dry DMF (20 mL)
reaction mixture was cooled to 0°C and stirred for 10 minute. through syringe into reaction mixture and nitrogen gas was
After that, NaBH4 (0.747 g, 2.5 eq.) was added into the above further purged for 15 min. After that, the reaction mixture was
reaction mixture portion wise very slowly about 10 min at 0°C allowed to reflux at 80°C and stirred for 4 days. Then reaction
under N2 gas atmosphere and allowed to stir for 4 hr. The mixture was cooled to room temperature, diluted with DMF (5
progress of the reaction was monitored by TLC. After mL), added into the 500 mL THF/Hexane (1:20) and after 5 min
completion, the reaction mixture was quenched by adding H2O stirring precipitate out which collected by filtration, dried at
(5 mL) and stirred for 10 min. The organic layer was separated, 40°C to desired polymer P-PHT (Desired yield = 0.4 g, 42%).
washed with H2O (200 mL) and brine solution (200 mL) and GPC results: Mw = 11634 g/mol, Mn = 7489 g/mol, PDI = 1.53
dried over MgSO4, filtered and evaporated to dryness. The 1H-NMR (300 MHz, CDCl3) δ (ppm): 7.53 (2H, br), 7.05-6.99
crude residue was subjected to column chromatography (silica (14H, m), 6.90 (2H, br), 6.65-6.59 (4H, m), 4.58 (12H, br), 3.88
gel, hexane/ethyl acetate: 9/1 to 6/4). A light brown color (4H, br), 2.73 (4H, br), 2.32 (4H, br), 2.03-2.01 (2H, m), 1.92
liquid of desired intermediate 1-4 was recovered (Desired yield (6H, br).
= 2.3 g, 90% w/w).
1H-NMR (300 MHz, CDCl3) δ (ppm): 6.66 (2H, s), 4.68 (4H, s), 135.8, 135.7, 135.3, 134.6, 132.6, 131.4, 127.8, 127.6, 126.3,
13C-NMR (300 MHz, CDCl3) δ (ppm): 156.2, 144.2, 139.7, 136.9,
2.76 (4H, t, J = 7.5 Hz), 2.07–2.04 (2H, m), 1.96 (6H, s).
113.7, 67.0, 64.0, 47.3, 38.4, 30.1, 23.0, 14.4.
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UV-irradiation exhibited monomeric emission band at λmax
=
3. Results and discussion
385 nm (with a quantum yield = 19% for P-PHT (close) in THF
solvent media by using a pyrene standard), which belonged to
the monomeric emission of TPE.57,58 The higher monomeric
emission of P-PHT in the close form rather than the open form
was observed in THF due to a more planar and rigid structure
of the cyclized DAE unit upon UV-irradiation,71 where the free
rotations of the ethene moiety and aryl groups were restricted
to induce the steric hindrance with larger separations and less
π-π interactions among TPE.
Design and synthesis
The designed photo-switchable polymer (P-PHT) is composed
of diarylethene (DAE) and tetraphenylethene (TPE) moieties
along with a protonable triazole linker as its repeating unit.
The cyclized DAE unit after UV-irradiation adopts a more
planar and rigid core but pushes the bulky methyl group out of
the core plane, which results in intermolecular steric hindrance
among cyclized DAE units in the whole molecular assembly.
Therefore, such events of intermolecular repulsions among
cyclized DAE units are supposed to be associated with the
tuning of AIE behaviour in P-PHT. Moreover, the protonable
triazoles in P-PHT with their cation characteristics under acidic
conditions might be useful to explore further insights into the
findings.
Chart 1. Structure of P-PHT
.
The synthetic route to obtain polymer P-PHT is shown in
Scheme 1. Commercially available starting compound (2-
chloro-5-methylthiophene) has been used to obtain the
intermediate 1-167,68 by employing Friedel-Crafts Acylation
reaction condition and intermediate 1-2 obtained through
Mcmurry reaction condition. After that, n-BuLi chemistry was
employed to get intermediate 1-3 with a new synthetic
method and the desirable yield, then the terminal ends of
alcohol intermediate 1-4 were generated via reducing agent
NaBH4 and alkyne terminated intermediate 1-5 was obtained
in the presence of NaH, Imidazole and propargyl bromide in
DMF at ambient temperature, which were further reacted with
azide terminated TPE-based intermediate 2-3 38
via Cu(I)
,
catalyzed click reaction to obtain P-PHT (open).69 Intermediate
2-3 was successfully synthesized in our previous report and
carried out in this study.
Photo-switching behavior of DAE in organic solvent media (THF)
The optical property of polymer P-PHT was first measured in
THF. As demonstrated in Fig. 1(a), upon UV-irradiation the
open-form of P-PHT was optically switched to its close-form,
i.e., from P-PHT (open) to P-PHT (close). As summarized in Fig.
1, the UV/Vis and photoluminescence (PL) spectral changes of
P-PHT (10 µM) after UV-irradiation (40 min and λ = 346 nm)
and the saturation time was monitored in Fig. S1. The UV-Vis
spectrum of P-PHT (open) exhibited absorption bands at 245
nm (n-π* transition of DAE) and 328 nm (π-π* transition of TPE
unit), see Fig. 1(b). After UV-irradiation, P-PHT (close)
illustrated an enhanced absorption band at 245 nm and a blue-
shifted absorption shoulder at 308 nm. Due to the formation
of cyclized conformation of DAE in P-PHT (close),70 we also
observed a broad and less intense absorption band at 544 nm,
which is shown in the inset of Fig. 1(b). Under similar
conditions in Fig. 1(c), the PL spectrum of P-PHT (close) after
Fig.1 P-PHT in THF before and after UV-irradiation, (a) Pictorial
diagram of P-PHT (open to close). (b) UV-Vis spectral changes of P-PHT
(open to close). (c) PL spectral changes of P-PHT (open to close). Inset:
Photoimages of P-PHT at 0 min and 40 min. (λex = 320 nm for PL exp.)
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Under similar experimental conditions (Fig. 1), we have of the further intensity increments or decrements of
performed new control experiments to study the accessibility monomeric emissions of TPE were observed. Therefore, our
of such monomeric emission band at λmax = 385 nm by taking experimental results revealed a quite higher chemical stability
into account of DAE intermediate, such as compound 1-5 and of P-PHT under acidic condition.
corresponding UV-Vis and PL spectral changes are monitored
Furthermore, we also explored the structural conformation
1
and summarized in Fig. S2, After UV-irradiation, a slight changes of DAE moiety in P-PHT via H-NMR spectroscopy in
increase of absorption band at 241 in compound 1-5 was Fig. 2. From the 1H-NMR spectra of P-PHT(open) and P-
observed. Due to the formation of cyclized conformation of PHT(close) in Fig. 2, the chemical shift of the methyl proton (–
DAE in compound 1-5 (close), we also observed a broad and CH3 group) on thiophene ring of DAE in P-PHT (open) is located
less intense absorption band at 455 nm in Fig S2(a). Under at 1.92 ppm and for photo-cyclized DAE in P-PHT the methyl
similar conditions, the PL spectrum of compound 1-5 (close) proton (–CH3 group) was slightly shifted to up-field at 1.90
1
after UV-irradiation illustrated none of the monomeric ppm. Generally, the H NMR of polymers are broaden but still
emission band at λmax = 385 nm was observed in Fig. S2(b). we can observed the noticeable changes in the close form of
Thus, DAE unit itself alone cannot generate the monomeric DAE in P-PHT (changed from 1.70 to 2.75 ppm), which further
emission in organic solvent media (THF) under UV-irradiation. confirmed the structural changes before and after UV-
Therefore, the emission peak at λmax = 385 nm is belongs to irradiation. Since the chemical shifts of methyl protons (–CH3
the monomeric emission of TPE in P-PHT upon UV-irradiation. group) on thiophene rings of DAEs vary according to the
However, the irradiation of visible light (λ > 400 nm) on P-PHT structural design, some reports demonstrate chemical shifts
(close) was irreversible towards its original open-form. Since up to 0.04 ppm in small molecular systems.80,81 However, CH3
DAEs in polymer backbone adopt anti-parallel conformation protons in our polymer P-PHT exhibited broad 1H-NMR pattern
and thus to enhance the efficiency of cyclizations on DAEs and thus to induce a less chemical shift (0.02 ppm up field).
(close) up to 100%.72 Therefore, the incorporation of DAEs Moreover, CH2 protons on cyclopentenes ring of DAE exhibited
without any extended conjugation in polymer backbone as the the chemical shift difference of 0.03 ppm (i.e., from 2.743 ppm
part of repeating unit is an interesting and unique case. to 2.713 ppm), where the integrated areas indicated a 100%
Furthermore, the cyclization events of DAEs to cause their conversion of P-PHT (open) to P-PHT (close).
bulkier alkyl substituents out of plane with respect to DAE rigid
cores might enforce polymer backbones to adopt more
favorable and stable conformations,71 which also resisted
cycloreversions of DAEs with intact and unique monomeric
emissions of TPEs. It demonstrates that the substituents
exerting a +M effect on the reactive carbon atoms, e.g.
methoxy groups73 or fluorine atoms,74 significantly stabilize the
ring-closed isomers resulting in marked decreases of quantum
yields for ring opening. On the contrary, the electron-
withdrawing cyano groups attached to reactive carbon atoms
on DAEs accelerate the photochemical cycloreversion with
high quantum yields.75 Moreover, the lack of any electron
withdrawing substituents (such as cyano groups and fluoro
atoms) on DAEs in our system induce irreversibility.70,76 Fig. 2 1H-NMR of P-PHT (6 mg in CDCl3) before and after UV-irradiation
Interestingly, the photo-irreversibility of DAEs is advantageous (at λ = 346 nm for 40 min).
and especially desired for the optical memory use, where
many-time readouts without erasing memory can be
AIE emissions in semi-aqueous media
achieved.77-79 With the presence of the protonable triazole As illustrated in Fig. 3(a), upon adding water to P-PHT (open) in
linker, we further explored the photo-switching behaviour by THF the AIE behaviour was introduced due to the restriction of
UV exposure of DAE in P-PHT (10 µM in THF) under an acidic intra-molecular rotation of phenyl ring and significant UV-Vis
condition (HCl 100 eq.). Similar to the previous photo- and PL spectral changes of P-PHT were observed and
switching behaviour of P-PHT without HCl, the PL spectrum in summarized in Figs. S5 and 3(b), respectively. As shown in Fig.
Fig. S3 showed a comparable pattern but illustrated a shorter 3(c), by increasing the water content in P-PHT (open), the AIE
UV-switching time (from open to close for m) of 20 min with emission at λmax = 480 nm was gradually enhanced at 20-80%
HCl in contrast to 40 min without HCl. Therefore, under the H2O due to the formation of self- assembly among TPE units.
acidic condition (HCl 100 eq.) the photo-switching behaviour of However, the AIE emission of P-PHT (open) was blue-shifted to
P-PHT with the protonable triazole was further activated in λmax = 466 nm over 90% H2O (quantum yield = 20% for P-PHT
THF. The irradiation of visible light (λ > 400 nm) on P-PHT (open) at 90% water content by using quinine hemisulfate
(close) was also irreversible even under the acidic condition. In standard and molar absorption coefficient (E) = 21,700 L mol-1
addition, we have performed the chemical stability experiment cm-1), due to the formation of higher aggregation of TPE.
of P-PHT (close) under acidic condition (HCl 100 eq. in THF, at However, as shown in Figs. 3(b) and 3(c) the AIE intensity was
different time intervals from 0 to 24 hr) in Fig. S4, where none suppressed with over-supply of water content up to 100% in
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contrast to 90% H2O. Therefore, the maximum AIE at 90% H2O where the monomeric emission (without AIE emission of TPE)
was utilized for the rest of experiments, where AIE behavior of P-PHT (close) was gradually quenched up to 99% H2O. As
will be examined for the energy transfer from TPE to DAE in P- illustrated in Fig. 4 (b), due to the Förster resonance energy
PHT. Moreover, the related absorption bands of DAE (245 nm) transfer (FRET) from TPE (donor) to the cyclized DAE unit
and TPE (328 nm) of P-PHT (open) at high water contents (over (acceptor) the AIE emission of TPE was totally prohibited even
80%) in Fig. S5 were red-shifted to 251 and 354 nm, with high water contents in P-PHT (close) in contrast to P-PHT
respectively, which further confirmed the aggregation of TPE.
(open) at 90% H2O in Fig. 3. Hence, the monomeric emission
(without AIE emission of TPE) of P-PHT (close) was gradually
quenched in Fig. 4 (c) owing to the π-π stacking of TPE by
increasing the water contents (0 to 99% H2O).
Since we have studied the AIE behaviour of P-PHT (open
and close) upon adding water in Fig. 3 and 4, we also
performed other comparative experiments of P-PHT at 90%
water content before and after UV-irradiation as illustrated in
Fig. 5(a).
Fig. 3 (a) Pictorial diagram of P-PHT (open only) by increasing water
contents. (b) PL spectral changes of P-PHT (open) in THF by increasing
water contents. Inset: Photoimages of P-PHT (open) at 0 and 100%
water contents. (c) Maximum PL intensities of P-PHT (open) by
increasing water contents. (λex = 320 nm for PL exp.)
Photo-switching behavior of DAE in semi-aqueous media
Compared with P-PHT (open) by adding water in Fig. 3, the AIE
Fig. 4 (a) Pictorial diagram of UV-irradiated P-PHT (close) at different
water contents (0 and 99% H2O). (b) PL spectra of P-PHT (close) at 0
and 99% water contents. Inset: Photoimages of P-PHT (close) at
corresponding water contents. (c) Maximum PL intensities of P-PHT Fig. 5 P-PHT at 90% water content before and after UV-irradiation (at λ
(close) by increasing water contents. (λex = 320 nm for PL exp.)
= 346 nm), (a) Pictorial diagram of P-PHT (90% H2O) from open to close
form upon UV-irradiation. (b) and (c) PL spectral changes of P-PHT
behaviour of the close form in P-PHT (by UV-irradiation at λ = with HCl (100eq.), respectively. Insets: Photoimages of P-PHT (90%
346 nm for 40 min) was also studied in various aqueous H2O) at 0, 40 min (UV-irradiation) without HCl and at 0, 20 min (UV-
conditions (0 and 99% H2O) as shown in Fig. 4(a). The irradiation) with HCl, respectively. (λex = 320 nm for PL exp.)
corresponding PL spectral changes are summarized in Fig. 4(b),
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The corresponding PL spectral changes without and with HCl protonation of triazole to induce less π-π stacking and larger
(100eq.) are summarized in Figs. 5(b) and 5(c), respectively. In separation of TPE, which became comparable with the larger
addition, the gradual PL saturation emissions for the ring monomeric emission of TPE (at 385 nm) of P-PHT (close) in THF
closure of DAE after different UV-irradiation time intervals are as shown in Fig. 1(c). Simultaneously, the AIE of TPE with HCl in
displayed in Fig. S6.
Fig. 5(c) was completely quenched in a shorter UV-switching
Interestingly, upon UV-irradiation (at λ = 346 nm for 40 time (from open to close form) of 20 min with HCl in contrast
min) of P-PHT (open) at 90% water content, the AIE emission to 40 min without HCl in Fig. 5(b), which might be due to the
of TPE at λmax = 466 nm was gradually quenched and emission protonable triazole was further separated under the acidic
band at λmax = 385 nm were simultaneously enhanced in Fig. condition to induce larger seperation and easier photo-
5(b). Therefore, regardless of high water contents done after cyclization of DAE. Therefore, the photo-switching behaviour
or before UV-irradiation, the similar results of UV exposure to of DAE in P-PHT with the protonable triazole under the acidic
induce only monomeric emissions (without AIE) were observed condition was further stimulated with larger separations and
in Figs. 4 and 5, respectively. Thus, the AIE emission of P-PHT less π-π interactions among TPE at 90% water content.
(open) was quenched due to the efficient energy transfer from
Study of photo-switchable polymer (P-PHT) by time-resolved
TPE to the cyclized DAE unit, because an emitting
photoluminescence (TRPL)
chromophore could be quenched by the nearby cyclized DAE
unit (as an acceptor) to release energy via non-radiative The fluorescence lifetime values of polymer P-PHT before and
transition.70 In addition, to fulfil the requirement of FRET after UV-irradiation with/without HCl (100 eq.) at 90% water
between the TPE (donor) and photo-switchable unit DAE content are summarized in Fig. 6.
(acceptor) in P-PHT, the maximum overlap of the AIE emission
of TPE and the absorption of the cyclized DAE fully supported
the occurrence energy transfer event in P-PHT (close) of Fig.
S7. Interestingly, the monomeric emission of P-PHT (close) was
originated from TPE still existed in the AIE favorable condition
(even at 90% water content). Therefore, we believe that the
more planar and rigid structure of the cyclized DAE unit under
UV-irradiation induced the steric hindrance with larger
separations and less π-π interactions among TPE in P-PHT
(close). Moreover, the monomeric emission (at λmax = 385 nm)
of TPE of P-PHT (close) in 90% water content in Fig. 5(b) was
reduced by the larger aggregation of TPE in H2O in comparison
with the monomeric emission of TPE of P-PHT (close form in
THF) in Fig. 1(c). Under similar experimental conditions (i.e.,
Fig. 5(b)), we also performed another control experiment to
investigate the accessibility of monomeric emission (at λmax
=
385 nm) of TPE with intermediate 2-3 after UV-irradiation (40
min at λ = 346 nm) at 90% water content as illustrated in Fig.
S8. Upon UV-irradiation, no any further changes in the
intensity of AIE emission for TPE were observed and none of
the monomeric emission (at λmax = 385 nm) could be noticed in
intermediate 2-3 in Fig. S8. Therefore, TPE unit itself alone
cannot generate the monomeric emission in AIE favorable
conditions (at 90% water content) under UV-irradiation.
Similarly, we also explored the PL spectral changes in Fig.
5(c) and the AIE behaviour by UV-irradiation of P-PHT (90%
H2O) under an acidic condition (HCl 100 eq.) and the saturation Fig. 6 (a) Before UV-irradiation, time-resolved fluorescence of polymer
time is illustrated in Fig. S6(b), where the AIE emission (at λmax P-PHT (open) of AIE emissions (at λmax = 466 nm) with/without HCl
= 466 nm) of TPE (with a quantum yield = 15% for P-PHT (100eq.). (b) After UV-irradiation, time-resolved fluorescence of
(open) at 90% water content under acidic condition (HCl polymer P-PHT (close) of monomeric emissions (at λmax = 385 nm)
100eq.) by using a quinine hemisulfate standard) was slightly with/without HCl (100eq.).
reduced due to the presence of electrostatic repulsion among
protonated triazole in P-PHT (open). After UV-irradiation, the
AIE of TPE (at λmax = 466 nm) was diminished completely in Fig. emission (at λmax = 466 nm) in P-PHT (open) at 90% water
5(c) but obtained two-fold higher monomeric emission of TPE. content without HCl (100 eq.) was = 1.80 ns, whereas in the
Moreover, the two-fold monomeric emission of TPE (at λmax presence of HCl (100 eq.), the fluorescence lifetime of AIE
385 nm) of P-PHT (close) under the acidic condition in 90% emission was slightly decreased to = 1.54 ns in contrast to
water content as shown in Fig. 5(c) was improved by the without HCl in Fig. 6(a). Therefore, the decrease of the
Before UV-irradiation, the fluorescence lifetime of AIE
τ
=
τ
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fluorescence lifetime for AIE emission in P-PHT (open) under Furthermore, to understand more about the molecular
an acidic condition (HCl 100 eq.) is further validated our structure and associated properties of P-PHT, we performed
findings in Fig. 5(c). After UV-irradiation, the fluorescence theoretical studies, including density functional theory (DFT)
lifetime of monomeric emission (at λmax = 385 nm) in P-PHT and time-dependent DFT (TDDFT),82 by taking PH-T as a model
(close) at 90% water content without HCl (100 eq.) was
τ
1.16 ns, whereas in the presence of HCl (100 eq.), the a carbon distance less than 4.2 Å is responsible for the
=
compound in THF. It has been reported that the DAE unit with
fluorescence lifetime monomeric emission in P-PHT (close) was photochromism,83 so the carbon distance of the DAE unit in
slightly increased to τ = 1.29 ns in Fig. 6(b). Thus, the increase PH-T needs to be verified theoretically. Thus, the geometrical
in the fluorescence lifetime of monomeric emission P-PHT parameters of PH-T revealed the suitable bond lengths (<4.2 Å)
(close) under an acidic condition (HCl 100 eq.) supports our of separated C-atoms in both open and close forms of DAE to
results of the photo-switching behaviour for DAE in P-PHT with be 3.676 and 1.544 Å, respectively. The optimized open and
the protonable triazole under the acidic condition, which was close structures of PH-T (before and after UV-irradiation) are
further stimulated at 90% water content in Fig. 5(c).
demonstrated in Fig. 8(a) and their respective highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) levels are also illustrated in Figs. 8(b)
and 8(c).
Aggregation states of photo-switchable polymer (P-PHT) in semi-
aqueous media
The AIE behavior of TPE in P-PHT (open and close forms by UV-
irradiation at λ = 346 nm for 40 min) was further explored by
the scanning electron microscopy (SEM) in the absence and
presence of water (i.e., 0% and 90%) as shown in Fig. S9(a)-
S9(c). Before UV-irradiation, the morphological patterns of P-
PHT (open) in THF at 0% and 90% of water exhibited uniform
and nano-spherical aggregates in Figs. S9(a) and S9(b),
respectively and thus AIE of TPE in P-PHT (open) was verified.
After UV-irradiation, the morphological pattern of P-PHT
(close) in THF at 90% of water content caused diminished
nano-spherical aggregates, which further validated the energy
transfer event from TPE to DAE as shown in Fig. S9(c).
Under similar conditions, the aggregation behaviors of P-
PHT (open and close form under UV-irradiation) in THF
solution with various water contents were further verified by
dynamic light scattering (DLS) experiments and illustrated in
Figs. 7(a) and 7(b), respectively.
Fig. 8 (a) Geometries of optimized structures of PH-T (open and close
forms). Frontier molecular orbital distributions of HOMO, LUMO,
HOMO-1 and LUMO+1 iso-surfaces along with the energy eigen values
in (eV), UV-Vis studies (b) PH-T (open) (c) PH-T (close) calculated at
B3LYP/6-31G(d,p) level.
Fig. 7 DLS measurements of (a) P-PHT (open) in THF (i.e., 0%) and at
90% water content. Upon UV-Irradiation, (b) P-PHT (close) in THF (i.e.,
0%) and P-PHT (close) at 90% water content.
Before and after UV-irradiations of PH-T (open and close
forms), the HOMO and LUMO levels were exactly located on
the TPE in Fig. 8(b) and cyclized DAE unit in Fig. 8(c),
respectively. Moreover, due to UV-irradiations the locations of
HOMO and LUMO, alone with HOMO-1 and LUMO+1, were
completely reversed in Figs. 8(b) and 8(c). According to the
theoretical results, the HOMO-LUMO bandgap of PH-T (close)
was quite smaller as compared with HOMO-LUMO bandgap of
PH-T (open), which is mainly due to the occurrence of energy
transfer from TPE to DAE units. Therefore, the energy transfer
event from TPE to DAE in P-PHT (close) was theoretically
validated. Furthermore, the calculated transitions in UV-Vis
spectral details of PH-T (from open to close form) at their
minima and their main contributions to the first excitation are
Before UV-irradiation in Fig. 7(a), P-PHT (open) in THF with
0% and 90% of water revealed aggregates mean sizes of 164
and 458 nm, respectively. After UV-irradiation (at λ=346 nm
for 40 min), P-PHT (close) in THF revealed slightly larger
aggregates with the mean size of 255 nm in Fig. 7(b). Whereas,
upon similar UV-irradiation the aggregates of P-PHT (close) at
90% of water content were significantly reduced to the mean
size of 295 nm in Fig. 7(b). Therefore, the optical events and
respective morphological patterns are further verified by DLS
experiments.
Theoretical calculations of photo-switchable polymer (P-PHT)
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summarized in Table S10. The theoretically calculated Fig. 8(b), which might be due to the protonable triazole was
transitions in the UV-Vis of P-PHT (before and after UV- further separated under the acidic condition to induce less
irradiation) were almost quite similar as observed steric hindrance and easier photo-cyclization of DAE in solid
experimentally.
films. Nonetheless, due to the large π-π stacking and serious
aggregation of TPE in solid films the monomeric emission of
TPE (at λmax = 385 nm) in solid films of P-PHT (close) could not
Photo-switching behavior of DAE in solid films
In general, the isomerization from open to close forms of DAE be improved by the protonation of triazole under the acidic
in both solutions and solid films are quite common because of condition as that happened in 90% water content in Fig. 4(c).
relatively small structural changes.71 In addition, TPE-based
Likewise, we also performed the other comparative
materials exhibit enhanced emission intensities in solid films as experiments of P-PHT by employing the polymethyl
read out signals.46 Moreover, to study the photo-switching methacrylate (PMMA) matrix to further investigate the
behaviour of DAE with the combination of TPE unit into accessibility of monomeric emission in polymer blend film.
polymer P-PHT on solid films would be interesting. Therefore, Therefore, we prepared the solution of P-PHT:PMMA (10:90
both UV/Vis and PL spectral changes of P-PHT (before and wt. ratio) in DCM and drop coasted on glass plates. The
after UV-irradiation) on uniform solid films of glass plate corresponding UV-Vis and PL spectral changes of P-PHT
without and with HCl (100 eq.) are summarized in Fig. 9 and (before and after UV-irradiation) on PMMA matrix without and
S11, respectively.
with HCl (100 eq.) are summarized in Figs. S12 and S13,
respectively. After UV-irradiation in Fig. S12(b), the AIE
emission of TPE at λmax = 458 nm was completely quenched.
Related UV/Vis spectral changes of P-PHT/PMMA polymer
blend film by UV-irradiation, the broad absorption band at 358
nm was completely quenched due to the cyclization events of
DAE in P-PHT (close), a new and more intense absorption band
in Fig. S12(a) appeared at 315 nm. Additionally, the similar
experimental results were observed under acidic condition in a
shorter period of time in Figs. S13(a) and S13(b). However, the
monomeric emission of TPE (at λmax = 385 nm) in P-PHT (close)
could not be observed in PMMA matrix and neat solid films,
which was because the separation of TPE could not be
accessible in solid films as the monomeric emission of TPE
obtained in solution. Therefore, the phenomena of energy
transfer from TPE to DAE in the close form of DAE in P-PHT
(close) along with AIE of TPE in the open form of DAE in P-PHT
(open) occurred in both solutions and solid films. Significantly,
Fig. 9 P-PHT in solid films before and after UV-irradiation, (a) UV-Vis
spectral changes of P-PHT (open to close). (b) PL spectral changes of P-
PHT (open to close). Inset: Photoimages of P-PHT at 0 min and 40 min.
(λex = 320 nm for PL exp.)
Before UV-irradiation, UV-Vis spectra of P-PHT (open) in
neat solid films exhibited broad absorption band at 345 nm in
Fig. 9(a), due to the π-π stacking of TPE. After UV-irradiation
(at λ = 346 nm for 40 min), the absorption peak at λmax = 345
nm were completely quenched. Due to the cyclization events
of DAE in P-PHT (close), a new and more intense absorption
band in Fig. 9(a) appeared at 313 nm along with broad and less
intense absorption pattern from 400 to 550 nm. Therefore, we
believe that little UV-Vis spectral change of P-PHT might be
due to non-extended conjugation on DAEs into our systems.
Under similar condition, PL spectra of P-PHT (open) in neat
the disappearance of the monomeric emission of TPE (at λmax
=
385 nm) in solid films of P-PHT (close) was totally different
from the result happened in solution, so this observation fully
verify the occurrence of the monomeric emission of TPE (at
λmax = 385 nm) in the solution media of P-PHT (close) was due
to the separation of TPE in the solution state.
solid films exhibited prominent emission peaks at similar 4. Conclusions
position 458 nm in Fig. 9(b), respectively, due to the π-π
In conclusion, the photo-switching behaviour of DAE (from
stacking of TPE. After UV-irradiation (at λ = 346 nm for 40 min),
the emission peaks at λmax = 458 nm were completely
open to close form under UV-irradiation) in P-PHT were
prominent in organic solvent (THF), high water contents (at
90% H2O) and solid films. The AIE behaviour of TPE in P-PHT
(close) at 90% water content was quenched and special
monomeric emissions were induced by efficient energy
transfer from TPE to the cyclized DAE unit. Regardless of high
water contents done after or before UV-irradiation, the similar
results of UV exposure to induce only monomeric emissions
(without AIE) were observed. Furthermore, the photo-
switching behaviour of DAE in P-PHT with the protonable
triazole under the acidic condition was further stimulated in
both organic solvent (THF) and high water contents. Moreover,
the experimental findings were fully validated by theoretical
quenched. However, the monomeric emission of TPE (at λmax
=
385 nm) in P-PHT (close) could not be observed in neat solid
films, which was because the separation of TPE could not be
accessible in solid films as the monomeric emission of TPE
obtained in solution. Under similar conditions, we also
explored the PL spectral changes of the AIE behaviour by UV-
irradiation of P-PHT (solid films) under an acidic condition (HCl
100 eq.) as illustrated in Fig. S8. After UV-irradiation of P-PHT
in solid films, the AIE of TPE with HCl in Fig. S11(b) was
completely quenched in a shorter UV-switching time of 20 min
(from open to close form) in contrast to 40 min without HCl in
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20 M. Berberich, M. Natali, P. Spenst, C. Chiorboli, F. Scandola
and F. Wurthner, Chem. Eur. J., 2012, 18, 13651–13664.
studies. However, the monomeric emission of TPE in P-PHT
(close) could not be observed in solid films, because the
separation of TPE could not be accessible in solid films as the
monomeric emission of TPE obtained in solution. Hence, this
observation fully verifies the occurrence of the monomeric
emission of TPE in the solution media of P-PHT (close) was due
to the separation of TPE in the solution state. Therefore, such
photo-chromism events of P-PHT arouse significantly
decreased AIE emissions and increased monomer emissions,
which provided a new insight into the fluorescence of AIE
compounds for future applications.
21 D. E. Williams and N. B. Shustova, Chem. Eur. J., 2015, 21
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The financial supports of this project are provided by the
Ministry of Science and Technology (MOST) in Taiwan through
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