Organogelator based on long alkyl chain attached excimer precursor: Two channels of TICT, highly efficient and switchable luminescence

Article abstract of DOI:10.1016/j.dyepig.2020.108433

A low molecular mass organic gelator (LMOG) T16 was obtained by introducing a long alkyl chain into an excimer precursor α–CN–TPA. Immobile organogels of T16 exhibiting both thermochromic and mechanochromic fluorescence switching between blue and green co

Full text of DOI:10.1016/j.dyepig.2020.108433

Journal Pre-proof
Organogelator based on long alkyl chain attached excimer precursor: Two channels
of TICT, highly efficient and switchable luminescence
Mi Ouyang, Chaozheng Zhuo, Feng Cao, Guoxiang Pan, Chunyan Lv, Shengchen
Yang, Chengjian Li, Cheng Zhang, Jingwei Sun, Yujian Zhang
PII:
S0143-7208(20)30574-X
DOI:
https://doi.org/10.1016/j.dyepig.2020.108433
DYPI 108433
Reference:
To appear in: Dyes and Pigments
Received Date: 5 March 2020
Revised Date: 2 April 2020
Accepted Date: 5 April 2020
Please cite this article as: Ouyang M, Zhuo C, Cao F, Pan G, Lv C, Yang S, Li C, Zhang C, Sun J,
Zhang Y, Organogelator based on long alkyl chain attached excimer precursor: Two channels of TICT,
highly efficient and switchable luminescence, Dyes and Pigments (2020), doi: https://doi.org/10.1016/
j.dyepig.2020.108433.
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2020 Published by Elsevier Ltd.

Author Statement：
Jingwei Sun: Conceptualization, Methodology, Writing- Original draft preparation.
Mi Ouyang: Data curation.
Chaozheng Zhuo: Visualization, Investigation.
Feng Cao: Software.
Guoxiang Pan, Chunyan Lv, Shengchan Yang, Chengjian Li: Investigation.
Cheng Zhang: Writing- Reviewing and Editing.
Yujian Zhang: Writing- Reviewing.

Graphical abstract:

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Organogelator Based on Long Alkyl Chain Attached Excimer Precursor: Two
Channels of TICT, Highly Efficient and Switchable Luminescence
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Mi Ouyang, Chaozheng Zhuo,
Feng Cao, Guoxiang Pan, Chunyan Lv,
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a,b
Shengchen Yang, Chengjian Li, Cheng Zhang, Jingwei Sun, * and Yujian
a,b
Zhang
a
College of Chemical Engineering, Zhejiang University of Technology, Chaowang
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Road No.18 Hangzhou 310014, P. R. China
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Department of Materials Chemistry, Huzhou University, East 2nd Ring Road. No.759,
Huzhou 313000, P. R. China
Abstract:
A low molecular mass organic gelator (LMOG) T16 was obtained by introducing a
long alkyl chain into an excimer precursor α-CN-TPA. Immobile organogels of T16
exhibiting both thermochromic and mechanochromic fluorescence switching between
blue and green could be formed. Far more than expected, the long alkyl chain endows
T16 with unique two channels of twisted intramolecular charge transfer (TICT)
emission in solution and highly luminescent properties in both a gel and solid state
(crystal). The fluorescence distribution ratio of the two channels is dependent on
irradiation wavelength and solvent polarity, leading to distinct effects of aggregation
induced enhanced emission (AIEE) and gelation induced enhanced emission (GIEE),
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respectively. The fluorescent quantum efficiency of T16 is increased more than 380
times in xerogel and almost 500 times in the crystal compared with that of α-CN-TPA
crystal, though both molecules have the same conjugated unit. The radiant transition
is found to be transformed from a forbidden state in α-CN-TPA to the allowed state in
T16, owing to the incorporation of a long alkyl chain which influences the
self-assembly structure and intermolecular interactions. Antiparallel molecular
H-stacking and multi-layered self-assembly are constructed in T16 under the driving
force of π−π, electrostatic and long alkyl-alkyl chain interactions. These findings offer
a simple and interesting platform for manipulating the fluorescence radiation
characteristics.
Keywords: long alkyl chain; twisted intramolecular charge transfer; gelation induced
enhanced emission; mechanochromism; self-assembly.
1. Introduction:
Stimuli-responsive low molecular mass organic gelators (LMOGs) [1] have
received increasing attention owing to their potential applications in drug delivery [2],
chemosensors [3], imaging [4], adaptive materials [5] and nano/micro devices [6].
The LMOGs self-assemble into porous network structures under the driving force of
non-covalent interactions, leading to rapid responses towards external stimuli
accompanied with the change of some properties [7]. Among the varieties available,
fluorescent switchable LMOGs are of bright promise on account of their superiority
of sensitivity, visualization, simplicity and real-time detection [8]. There are some
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reports recently concerning the fluorescent switchable LMOGs upon exposure to
stimuli [9]. For example, Park et al. reported a π-conjugated LMOG based on
salicylidene-aniline derivative with terminal CF , which showed fluorescence on-off
3
switching under UV or thermal heating [10]. Ma and coworkers synthesized a LMOG
with the structural character of ALS (aromatic group A, linker L, steroidal group S),
which exhibited convertible luminescence upon ultrasound and thermal stimuli [6].
However, the design strategies for fluorescent switchable LMOGs are still limited,
due to the strict requirements on the balance of intermolecular interactions such as
hydrogen-bonds, van der Waals forces, π−π and dipole-dipole interactions, etc [11].
Strong π−π interactions are essential for bringing about the self-assembly of
conjugated molecules into ordered stacking [12-14], however if the interactions are
too intense, they can become detrimental to the gelation and luminescent efficiency
[15]. Therefore both appropriate choice of the conjugated chromophore and fine
control of the self-assembly are required for the designing of fluorescent switchable
LMOGs. Excimer precursors are prone to form excimers upon light excitation due to
their close π−π stacking, however, the fluorescence of which can be varied distinctly
when the distance or overlap between π planes change slightly [16, 17]. Besides, long
alkyl chains, which are found to be simple and excellent gelators of various organic
fluids [18], could provide a competitive yet balanced intermolecular interactions
against the π−π force. Accordingly, we propose it might be an effective method to
achieve fluorescent switchable LMOGs by attaching excimer precursors with long
alkyl chains at the terminal. Moreover, the effects of alkyl chains on the luminescence
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[19-21] or the gelation behavior [22] have been intensely investigated, respectively.
While for the fluorescent switchable LMOGs, related research is still rare, especially
the effects of long alkyl chains on regulating the channels of fluorescence radiation.
In this work, α-CN-TPA is adopted as the chromophore in view of its excellent
ability to undergo efficient π-stacking [23]. Long alkyl chain is introduced to obtain
the target compound T16 (Scheme 1). The gel behavior, optical properties and
self-assembly structure of T16 are studied by means of steady-state and transient
fluorescence spectroscopy, scanning electron microscope (SEM), transmission
electron microscope (TEM), x-ray diffraction (XRD) and quantum chemical
calculations. Noteworthy, though α-CN-TPA shows non-fluorescence, the introduction
of long alkyl chain induces two channels of twisted intramolecular charge transfer
(TICT) emission, aggregation induced enhanced emission (AIEE), gelation induced
enhance emission (GIEE) and switchable fluorescence between blue and green in T16.
Such highly efficient and diversely switchable luminescence of T16 pave the way for
its versatile applications in rewritable optical media, information storage, sensors and
display devices.
2. Experimental section
2.1 Materials
All the chemicals were purchased from commercial sources and used without further
purification,
including
p-hydroxybenzaldehyde,
hexadecane
bromide,
4-(diphenylamino)phenylboronic acid, p-bromophenylacetonitrile, potassium iodide
(KI), potassium carbonate (K CO ), tetrakis (triphenylphosphine) palladium
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Pd(PPh ) . Tetrahydrofuran (THF) was distilled over metallic sodium and
3 4
dimethylformamide (DMF) over calcium hydride before use. Ethanol is
chromatographic grade. All the other solvents are commercially available and used as
received, unless otherwise claimed.
2.2 Measurements
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The H and C NMR spectra were recorded on a Bruker AVANCE III 500−MHz
instrument (Bruker, Switzerland) using the chloroform−d (CDCl ) as the solvent.
3
Mass spectroscopy was recorded with a Thermo LCQ Fleet MS spectrometer. The
UV−vis absorption spectrum was recorded on a Perkin Elmer Lambda 35
spectrophotometer. Fluorescent measurements were recorded on a Per-kin-Elmer
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LS−55 luminescence spectrophotometer. Fluorescence quantum yield φ was
F
determined using a calibrated integrating sphere. Fluorescence lifetime experiments
were performed by the time-correlated single-photon counting (PicoHarp 300 TCSPC)
system under right-angle sample geometry. A picosecond diode laser (PDL 800−B)
was used to excite the samples. The minimum pulse width is 50 ps. The detection
system uses a photomultiplier tube from Picoquant (R3809U−50) with an instrument
response time less than 250 ps. Powder X-ray diffraction (XRD) patterns were
performed on X’ Pert PRO, PANalytical, with Cu−Kα radiation operating at 40 kV
and 40 mA. Field-emission scanning electron microscopy (SEM) measurements were
taken by using a Hitachi S−4800 scanning electron microscopy (Hitachi, Japan).
Transmission electron microscopy (TEM) images were taken from a Tecnai G2 F30
S−Twin microscope at an accelerating voltage of 200 kV.
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Quantum chemical calculations were conducted using density functional theory
(DFT) calculations at the B3LYP/6−31G* level. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were obtained
on the basis of optimazed geometry of T16 in the ground state.
2.3 Gelation Test.
The gelation behavior of T16 in various organic solvents were tested through the
“stable to inversion of the test tube” method. 10 mg of T16 powder was mixed with a
series of solvents (1 mL) in a capped vial, respectively. The powder was found to be
initially insoluble in solvents at room temperature, but was completely dissolved after
heating above 70°C. Upon cooling and standing for a while at room temperature, the
vials were inverted to record the gelation ability of T16. If the sample kept
homogeneous and no flow was observed, a stable gel was formed which was noted as
gelation (G). If T16 was not completely dissolved when heated, it was noted as
insoluble (I). If the sample remained as a clear solution after cooling, it was noted as
soluble (S). If solid powder precipitated after cooling and the sample returned back to
the state before heating, it was noted as precipitate (P). The xerogel samples used for
XRD, SEM, PL, and fluorescence lifetime tests were prepared by naturally drying
method to remove the solvent.
2.4 Synthesis
4-(hexadecyloxy)benzaldehyde (1)
K CO (2.76 g, 20 mmol) and KI (catalytic amount) were added to a solution of
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4-hydroxybenzaldehyde (1.22 g, 10 mmol) in dry N, N-Dimethylformamide (DMF, 20
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mL), and 1-bromohexadecane (3.66 g, 12 mmol) was slowly dropped into the mixture.
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The mixture was stirred at 80 C overnight. After cooling to room temperature, the
mixture was poured into brine and extracted with dichloromethane. The organic phase
was dried over MgSO . The resulting filtrate was concentrated under vacuum, and the
4
crude product was purified by silica gel chromatography with petroleum ether to
afford 3.19 g of compound 1 (white powder) with a yield of 92.1%. FTIR (KBr): υ =
−
1
−1
−1
−1
−1
−1
-1
1
2946cm , 2857cm , 1725cm , 1597 cm , 1525 cm , 1492cm , 1072cm . H
NMR (500 MHz, CDCl ): δ (ppm) 9.89 (1H, s, H in aldehyde), 7.84 (2H, d, J = 8.5
3
Hz, H in phenyl), 7.00 (2H, d, J = 9.0Hz, H in phenyl), 4.05 (2H, t, J = 7.0Hz, H in
alkoxy), 1.85-1.79 (2H, m, H in alkoxy), 1.50-1.44 (2H, m, H in alkoxy), 1.37-1.1.27
1
3
(24H, m, H in alkoxy), 0.89 (3H, t, J = 7.0Hz, H in alkoxy). C NMR (125 MHz,
CDCl ): δ (ppm) 190.76, 164.29, 131.98, 129.80, 114.77, 68.46, 31.94, 29.70, 29.67,
3
29.59, 29.55, 29.36, 29.35, 29.07, 25.97, 22.70, 14.11. HRMS m/z: Found 347.2942
+
+
[M+H] ; molecular formula C H O requires 347.2999 [M+H] .
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2
2-(4'-(diphenylamino)-[1, 1'-biphenyl]-4-yl)acetonitrile (2)
Under nitrogen atmosphere, a mixture of (4-(diphenylamino)phenyl)boronic acid
(1.74 g, 6 mmol), 2-(4-bromophenyl)acetonitrile (0.98 g, 5 mmol), Pd(PPh ) (0.065 g,
3
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0.06 mmol), Na CO (2.0 M, 3.0 mL), and toluene (50 mL)/THF (30 mL) was stirred
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at 90 °C for 24 h. After it was cooled to room temperature, CHCl (100 mL) was
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added to the mixture. The organic portion was separated and washed with brine before
dried over anhydrous MgSO . The solvent was evaporated off, and the solid residues
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were purified by column chromatography to afford 1.26 g of 2 (green powder) with a
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yield of 69.9%. FTIR (KBr): υ = 2949cm , 2230cm , 1595 cm , 1523 cm ,
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1490cm , 1080cm . H NMR (500 MHz, CDCl ): δ (ppm) 7.60 (2H, d, J=8.5 Hz, H
3
in phenyl), 7.47 (2H, d, J=8.5 Hz, H in phenyl), 7.40 (2H, d, J=8.0 Hz, H in phenyl),
7.30 (4H, t, J=7.5 Hz, H in phenyl), 7.16 (6H, d, J=8.5 Hz, H in phenyl), 7.07 (2H, t,
1
3
J=7.5 Hz, H in phenyl), 3.80 (2H, s, H in methylene). C NMR (125 MHz, CDCl ): δ
3
(ppm) 147.59, 140.59, 133.89, 129.32, 128.34, 128.25, 127.68, 127.27, 124.56,
+
123.71, 123.11, 117.82, 23.31. HRMS m/z: Found 361.1695 [M+H] ; molecular
+
formula C H N requires 361.1699 [M+H] .
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(Z)-2-(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)-3-(4-(hexadecyloxy)phenyl)
acrylonitrile (T16)
Compound 1 (2.08g, 6mmol), and 2 (1.80g, 5mmol) were dissolved in anhydrous
ethanol (20mL) in the presence of sodium methoxide (0.40g, 7.5mmol). The mixture
was stirred at room temperature for 6 hours. The resulting precipitate was filtered and
washed with methyl alcohol for several times to afford 2.86g of T16 (blue powder)
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−1
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with a yield of 83.0%. mp 132.91 C. FTIR (KBr): υ = 3030cm , 2948cm , 2856cm ,
−
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−1
−1
−1
−1
-1
1
2230cm , 1668cm , 1596cm , 1525 cm , 1490cm , 1076cm . H NMR (500
MHz, CDCl ): δ (ppm) 7.89 (2H, d, J=8.5 Hz, H in phenyl), 7.70 (2H, d, J=8.5 Hz, H
3
in phenyl), 7.63 (2H, d, J=9.0 Hz, H in phenyl), 7.50 (2H, d, J=7.0 Hz, H in phenyl),
7.49 (1H, s, H in vinyl), 7.28 (4H, t, J=8.5 Hz, H in phenyl), 7.14 (6H, d, J=8.0 Hz, H
in phenyl), 7.05 (2H, t, J=7.5 Hz, H in phenyl), 6.97 (2H, d, J=8.5 Hz, H in phenyl),
4.02 (2H, t, J=6.5 Hz, H in alkoxy), 1.82-1.78 (2H, m, H in alkoxy), 1.48-1.44 (2H, m,
1
3
H in alkoxy), 1.36-1.26 (24H, m, H in alkoxy), 0.88 (3H, t, J=7 Hz, H in alkoxy). C
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NMR (125 MHz, CDCl ): δ (ppm) 161.06, 147.56, 141.28, 140.94, 133.68, 133.27,
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131.18, 129.32, 127.62, 126.99, 126.37, 126.11, 124.61, 123.64, 123.15, 118.56,
114.91, 108.11, 68.26, 31.92, 29.69, 29.65, 29.59, 29.56, 29.36, 29.15, 26.00, 22.68,
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14.10. HRMS m/z: Found 711.4306 [M+Na] ; molecular formula C H N ONa
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2
+
requires 711.4285 [M+Na] .
3. Results and discussion
3.1 Synthesis
T16 was synthesized according to the routes outlined in Scheme 1. For comparison,
α-CN-TPA without the long alkyl chain was included, which was synthesized
according to the literature.[23] As reported, α-CN-TPA formed H-type stacking via
strong π−π interactions in crystalline state, which resulted in a quenched florescence
and mechanically responsive capacity. However, such strong π−π interactions are
favorable as the driving force in fabrication of organogel.[24] On the basis, long alkyl
chain, which was found to be excellent gelator of various organic fluids, was further
introduced to get T16. The long alkyl chain, on the one hand, could effectively
prevent neat crystallization of the π-backbone, on the other hand, facilitates gelation
via van der Waals interactions.[25] Therefore, T16 with the terminal alkyl chain is
expected to be a potential gelling agent. Moreover, the long alkyl chain was found to
have a great influence on the optical properties of T16, which induced unique two
channels of TICT emission in solution and highly luminescent self-assemblies with
H-stacking structure, significantly different from α-CN-TPA. Detailed results and
discussion are in the following sections.
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Scheme 1
3.2 Two channels of TICT fluorescence
The basic optical properties of T16 were investigated in solvents of varying
polarities. As shown in Fig. S1, T16 exhibits a remarkably red-shifted fluorescence
from blue to orange under 365 nm UV light when the solvent changes from hexane to
CH CN. Such continuously shifted fluorescence originates from TICT transition
3
according to the ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra in
Fig. S2 and Fig. 1, respectively. When irradiated at 370 nm, a narrow emission peak
with a vibrational structure was observed around 436 nm from T16 in nonpolar
hexane, which corresponds to a localized emission (Fig. 1a). With the increase of
solvent polarity, T16 shows a substantial red-shift of more than 150 nm and finally
moves to 587 nm in acetonitrile (Fig. 1a, b). At the meantime, the emission band
gradually broadens along with the disappearance of the vibronic resolution. Moreover,
T16 exhibits a wide absorption band at 365 nm, which changes little in low-polarity
solvents, while shifts to 380 nm in high-polarity solvents (Fig. S2). These results,
similar to those of most electron donor-accepter (D-A) molecules and α-CN-TPA,
indicate a clear TICT character in T16, which gets more significant in polar
environment due to the twist of molecular conformation.[26] The charge transfer
originates from the electron-rich group triphenylamine to the electron-withdrawing
unit –CN.
Fig. 1
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However, it is worth noting that a second TICT channel simultaneously emerges in
T16 when irradiated at a short wavelength, which has never been observed in
α-CN-TPA.[23] As shown in Fig. 1c, T16 exhibits two prominent emission bands
when excited at 330 nm. Such bimodal emission is dependent on the excitation
wavelength, as shown in Fig. S3. Besides the strong emission peaked at the same
position as the PL spectra excited at 370 nm, a weak peak arises in the short
wavelength range, which shows similar red-shifted and broadened change with the
increase of solvent polarity (Fig. 1c). These results indicate that the emission band in
the high energy range is also induced by the TICT transition, which could be excited
at a short wavelength. To better understand the two TICT emissions of T16, we
further studied the relationship of Stokes shift (ν -ν ) of the two peaks versus the
a
f
orientation polarizability(f) based on the Lippert–Mataga model.[27] The detailed
Stokes shifts and f(ε,n) are listed in Table S1 and S2. As shown in Fig. 1d, both TICT
fluorescence emissions display linear relations, and the slopes of individual fittings
are different. The first TICT emission excited at 330 nm has a relatively small dipole
moment compared with the second one, implying that the first TICT features a lower
degree of charge transfer which could only be excited by high energy stimulus.[28]
These results indicate that the first TICT may not originate from the
triphenylamine–cyano part, instead relate to the long alkyl chain. Therefore, T16 is
found to have two channels of TICT fluorescence with different extents of charge
transfer originated from different parts of molecule.
To gain an insight into the electronic structures of T16, the optimized geometry and
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spatial distributions of HOMO and LUMO in the ground-state were performed at the
DFT level with B3LYP functional and 6-31G* basis set. As shown in Fig. S4, highly
o
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twisted angles of 43.26 and 43.04 are observed between the terminal phenyls in
triphenylamine and the phenyl connected with cyanostilene. In addition, the HOMO is
mainly located at the triphenylamine and vinyl phenyl attached to it, while LUMO is
mainly distributed at the cyanostilbene core. Thus it is reasonable to assign the
fluorescence at low-energy region to the TICT from triphenylamine to cyano in T16.
Further investigation is in progress with the aid of theoretical calculations to explore
the origin of the bimodal emission irradiated at short-wavelength.
3.3 AIEE and GIEE properties
The AIEE property of T16 was studied in a mixture of THF/water with different
fractions of water (f ). As shown in Fig. S5, T16 displays similar absorption in
W
solutions with low f (< 60%). However, when f gets above 60%, the intensity of
W
W
the absorption band increases along with the tail raising up and extending into long
wavelength region. Such phenomenon results from the Mie scattering effect, which
indicates the formation of T16 aggregates in mixtures with f > 60%.[29] Fig. 2a, b
W
and Fig. S6 show PL spectra of T16 in mixture of THF/water at λ = 330nm and 370
ex
nm, respectively. With the increase of f from 0 to 50%, T16 shows a continuously
W
red-shifted PL peak from 516 to 552 nm with decreased intensity whether excited at
330 or 370 nm. This phenomenon, which is in accordance with the fluorescence
behaviors in solvents of increasing polarity, results from the TICT effect due to the
1
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high polarity of water.[30] However, the PL intensity abruptly increases when f gets
W
above 60%, and reaches the maximum at fW = 90%. Meanwhile, the maximum
emission wavelength blue shifts to 499 nm. These results indicate the AIEE property
of T16, is ascribed to the restricted intramolecular rotation and limited solvatochromic
effect in aggregated state.[31] Notably, when excited at 330 nm, the bimodal emission
is also observed in T16 solutions (f < 60%), however, the emission band in the short
W
wavelength region abruptly vanishes when T16 aggregates (f > 60%, Fig. 2b). This
W
phenomenon reveals that the aggregation of T16 forbids the TICT channel with higher
energy level, which might be caused by the blocked twist in some specific molecular
packing structure. As a result, a higher emission-enhancement ratio (ΔI/I ) is
0
achieved when excited at 330nm, compared to that at 370 nm (Fig. 2c).
Fig. 2
It is interesting to note that T16 could form immobile gels in various solvents
including hexane, petroleum ether, ethyl acetate, acetone, ethanol and DMSO. Among
them, the lowest critical gel concentration (CGC) is found in ethyl acetate (0.85
wt/vol%) with an aging time of 0.5 h. The detailed gelation behavior of T16 in
different solvents are summarized in Table S3. Noteworthy, all organogels fabricated
by T16 display the same blue fluorescence, which indicates the consistent molecular
self-assemblies in T16 gels regardless of the solvent. Besides, the gels have obviously
enhanced intensities in comparison to their solutions, which could be distinguished by
direct visual observation. Such GIEE property was characterized by PL spectra during
the aging process. As shown in Fig. 2d, the initial solution exhibits a green
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fluorescence at 518 nm. However, the emission peak blue shifts and the PL intensity
increases along with time prolonged, and finally a bright blue gel (482 nm) is
generated. The remarkably enhanced emission in the gels is ascribed to the formation
of self-assembly aggregates.[32]
Specially high fluorescence quantum efficiencies (φ ) of 49.3 and 38.3 % are
F
observed from T16 in crystalline and xerogel states, respectively, which are quite
larger than that of α-CN-TPA crystal (φ < 0.1%). The fluorescence lifetime (τ) was
F
further examined as shown in Table 1. Both T16 and α-CN-TPA, which have the same
conjugated backbone, show the same fluorescence decay profile with a long lifetime
of 20.8 ns in their crystals. These results reveal that T16 and α-CN-TPA may have the
same excited state and molecular packing mode.[33] However, the luminescence of
α-CN-TPA crystal is totally quenched while that of T16 is fairly radiate, as verified by
the fluorescent and non-fluorescent rate constants (k and k ). Such significantly
f
nf
increased φ of T16 is attributed to the introduction of long alkyl chain, which might
F
convert the forbidden transition to the allowed one. The highly luminescent property
is a prerequisite for the applications of T16 in optoelectronic devices, sensors and
bioprobes.[34]
Table 1
3.4 Mechanochromic and thermochromic properties
Similar to α-CN-TPA, mechanochromic [35-37] and thermochromic properties
were observed from T16. As shown in Fig. 3, both the crystalline T16 and its xerogel
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sample exhibit almost the same fluorescence, except that a shoulder presents at 464
nm in the spectra of xerogel. The shoulder peak appears in the drying process as
compared to the PL spectra of gel in Fig. 2d. After grinding, the luminescence is
switched from blue to green with the PL peak red shifted to 505 nm (Fig. 3). The
emission color can be recovered through fuming with various solvents such as
n-hexane, THF, ethyl acetate, acetone and triethylamine, indicating a reversible
mechanochromic property of T16. Besides, the melted and frozen T16, a completely
amorphous sample, shows a further red shifted PL that peaked at 521 nm. If the
melted T16 is cooled slowly, a crystalline solid emitting blue fluorescence is obtained,
whose φ reaches 50.27%. This indicates a reversible thermochromic property of T16.
F
XRD patterns of both crystal and xerogel of T16 exhibit multiple sharp diffraction
peaks at the same positions, revealing that T16 adopts an ordered and consistent
molecular packing structure in the two states. However, the sharp diffraction peaks
become weaker and wider upon grinding, and even disappeared when melted and
frozen. These results indicate that both the grinding and melting treatment destroy the
ordered packing of molecules and produce amorphous phase, leading to the switch of
fluorescence to green. The decreased φ and τ further confirms the growing defects
F
and traps in the ground as well as melted and frozen T16 (Table 1).
Fig. 3
The fluorescence decay curves were tracked during the gel naturally drying process
to illustrate the different optical properties between crystal and xerogel of T16 (Fig.
3d). When the drying is prolonged to 5 and 7 days, the fluorescence decay curve
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gradually deviates from that of T16 crystal, along with the shortened τ. Finally the
xerogel shows a double exponential decay of fluorescence with the lowest τ (10.0 ns)
relative to that of the gel and crystal. A short-lived component (τ = 4.20 ns) is
1
generated in xerogel, which implies the presence of a newly-born excited state.
Besides, the k in xerogel gets more than twice as that of the crystal (Table 1). These
nf
results indicate that the same molecular packing structure and excited state exist in gel
and crystal of T16 at the beginning. However, during the xerogel formation, the
extraction of solvent reduces the order of self-assembled structure of gel, resulting in
a large numbers of structural defects, which causes partial fluorescence to decay in a
non-radiative manner.[38] Moreover, a distinct excited state might be generated
through the molecular slipping between each other since the xerogel shrinks after the
solvent evaporation. Therefore, the molecular self-assembling structures of T16 gel,
xerogel and crystal are essentially the same except different degrees of order. The
higher degree of order, the higher φ and τ of the aggregates.
F
3.5 Self-assembling structure in gel
To understand the self-assembling structure of T16, POM, SEM and TEM were
conducted. As shown in Fig. 4, spherulites with color birefringence are displayed
o
when the hot solution of T16 is cooled below 100 C, suggesting the formation of a
lamella structure in which the molecules are arranged perpendicular to the radial
direction. SEM of T16 gel in ethyl acetate also exhibits a three-dimensional network
constituted by the homogeneous micron ribbons, with width of about 1 µm and length
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of more than 10 µm. Such ribbons, which favor extending in one-dimension, could be
bended flexibly and tend to join together to form intercrossing networks. Besides, the
micro-ribbons are composed of pieces of nano-belt with a thickness of about 20 nm
stacking together, which is further confirmed by the TEM image. These results
indicate the multilayer assembly in T16 gel, which, together with the intercrossing
network, effectively captures solvent molecules by surface tension and solidifies them
to form a stable gel.
Fig. 4
There are two main phases in T16 solid: the ordered self-assembly showing blue
fluorescence and the amorphous phase showing green emission, which are defined as
T16-B and T16-G respectively. To study the molecular packing structures of T16 in
the two phases, small angle XRD were analyzed (Fig. 5). The diffraction pattern of
T16-B exhibits three reflection peaks, corresponding to the d values of 49.6, 24.5 and
9.72 Å, respectively. The ratio of d values follows the order of 1: 1/2: 1/5, indicating
that the molecules in T16-B are arranged into a layered structure with a layer spacing
of about 49.6 Å.[39] This result is consistent with the observation in SEM and TEM.
In contrast, the corresponding d values in T16-G are 53.7 and 31.4 Å, respectively,
whose ratio follows the order of 1: 1/√3, revealing a hexagonal columnar arrangement
on mesoscopic scale in T16-G.[40]
Fig. 5
Combining the distance measured by small angle XRD with the optimized
molecular structure, a schematic representation of the possible self-assembly in T16 is
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proposed in Fig. 6. The calculated molecular length of the optimal configuration is
about 38.8 Å, which is quite smaller than the layer spacing of 49.6 Å, indicating
partial overlap between adjacent molecules.[41] Moreover, the blue fluorescence with
a long τ of 20.8 ns reveals an H-type stacking with strong π−π interactions between
molecules.[42] Owing to its high dipole moment, T16 molecules would be arranged
into an antiparallel H-stacking under the driving of electrostatic force.[43] Such
packing structure is similar to that of α-CN-TPA crystal (Fig. S7).[23] However, the
long alkyl chain in T16 could effectively inhibit the excimer formation as well as the
intramolecular rotation, both of which favors non-radiative decay.[44] Then the
columns assemble into a belt via the alkyl-alkyl chain interactions, which also causes
the multiple layers of belts stacking together.[18] Finally spherulites are constructed
by the bunched belts growing homogeneously along their respective long axises.
Fig. 6
4. Conclusions
In summary, a LMOG T16 was obtained by introducing a long alkyl chain into an
excimer precursor α-CN-TPA. It is found that T16 in solution has two channels of
TICT fluorescence depending largely on the irradiation wavelength and solvent
polarity. However, the channel at higher energy level is blocked when T16 aggregates,
leading to a more concentrated emission. Distinct to α-CN-TPA with forbidden radiant
transition due to the formation of excimers, T16 shows a high φ of 49.3 and 38.3 %
F
in crystal and gel, respectively, revealing its AIEE and GIEE properties. Besides, such
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highly luminescent gels of T16 exhibit both thermochromic and mechanochromic
fluorescence switching between blue and green as a result of transformation between
an ordered structure and an amorphous phase. Antiparallel molecular H-stacking and
multi-layered self-assembly are constructed in T16 under the driving force of π−π,
electrostatic and long alkyl-alkyl chain interactions. This work offers a simple method
to develop highly luminescent gels through attaching a long alkyl chain into a D-A
type excimer precursor.
Acknowledgment
This work was supported by National Natural Science Foundation of China
(51803056, 21875219); the Natural Science Foundation of Zhejiang Province
(LQ18E030002 and LY14B060006); the Department of Education of Zhejiang
Province (Y201635396).
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Figure capitals ordered in which they are cited:
Scheme 1. Synthetic routes and chemical structure of T16. The homologue
α-CN-TPA is included for comparison.
Fig. 1. PL spectra of T16 in different solvents (10 μM) upon excitation at 370 nm (a,
b) and 330 nm (c), respectively. The abbreviations IPE and DCM refer to isopropyl
ether and methylene chloride, respectively. (d) Linear correlation of orientation
polarization f(ε, n) of solvent media with the Stokes shift (υ -υ , a: absorption; f:
a
f
fluorescence) of PL peaks at the short (1st) and long (2nd) wavelength region,
respectively. See Table S1 for detailed data.
Fig. 2. (a, b) PL spectra of T16 in THF/water mixtures (10 μM) with different water
fractions (λ = 330 nm). (c) Water fractions (fw) dependent emission-enhancement
ex
ratio of T16 upon excitation at 330 and 370 nm, respectively. (d) PL spectra of T16
upon aging its ethyl acetate solution to the gel state (λ =330nm). Inset shows
ex
photographs of T16 in hot solution (left vial) and in gel state (right vial) in ethyl
acetate irradiated at 365nm.
Table 1 Optical properties of T16 and α-CN-TPA in different states.
Fig. 3. (a) Photographs of T16 in crystalline and xerogel states irradiated at 365 nm
and their reversible fluorescence variation upon grinding and heating, respectively. (b)
PL, (c) XRD and (d) fluorescence lifetime of T16 in different states.
Fig. 4. (a) POM images of T16 gel formed upon cooling its hot sol. (b) SEM and (c)
TEM images of T16 in xerogel state.
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Fig. 5. SAXD profiles of T16-B and T16-G phases, respectively. Inset summarizes the
detailed data of the peaks.
Fig. 6. Schematic illustration of the H-stacking and layered self-assembly in the
organogel of T16.
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