Highly fluorescent free-standing films assembled from perylenediimide microcrystals for boosting aniline sensing

Article abstract of DOI:10.1039/c9tc05587h

Molecular assembly has emerged as a key protocol for designing functional materials, although building in task-specific applications remains challenging. Here, a simple solvent-diffusion fabrication of highly fluorescent free-standing films (FFSFs) obtained from perylenediimide (PDI) microcrystals is described. The high fluorescence intensity of the resulting FFSFs follows from the mode of solid-state packing of the PDI molecules. The porous, crystalline FFSFs provide increased surface area and enable unobstructed diffusion of guest molecules for boosting aniline sensing with low detection limit, high selectivity and reversibility. Density functional theory (DFT) calculations indicate that the fluorescence quenching is caused by photoinduced electron transfer (PET). The new FFSFs furnish amplified discrimination of analytes and represent a major step ahead toward the rational synthesis of assembled sensing materials.

Full text of DOI:10.1039/c9tc05587h

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Materials Chemistry C
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Highly fluorescent free-standing films assembled
from perylenediimide microcrystals for boosting
aniline sensing†
Cite this:DOI: 10.1039/c9tc05587h
a
a
a
b
c
Baozhong Lu,
¨
Meizhen Yin
Pengyu Li,
Pengfei Li, Yantu Zhang,* Klaus Mullen
and
¨
a
*
Molecular assembly has emerged as a key protocol for designing functional materials, although building
in task-specific applications remains challenging. Here, a simple solvent-diffusion fabrication of highly
fluorescent free-standing films (FFSFs) obtained from perylenediimide (PDI) microcrystals is described.
The high fluorescence intensity of the resulting FFSFs follows from the mode of solid-state packing
of the PDI molecules. The porous, crystalline FFSFs provide increased surface area and enable
unobstructed diffusion of guest molecules for boosting aniline sensing with low detection limit, high
selectivity and reversibility. Density functional theory (DFT) calculations indicate that the fluorescence
quenching is caused by photoinduced electron transfer (PET). The new FFSFs furnish amplified
discrimination of analytes and represent a major step ahead toward the rational synthesis of assembled
sensing materials.
Received 12th October 2019,
Accepted 6th December 2019
DOI: 10.1039/c9tc05587h
rsc.li/materials-c
fluorescence restricts the application of PDIs in practical optical
devices and fluorescent sensing in the solid state. To solve the
1
. Introduction
Solid-state fluorescent materials that can detect even trace problem, on the molecular side, PDIs were covalently connected
amounts of organic amine vapors mark one of the active fields with b-cyclodextrins to enhance binding of aniline, while on the
1
–6
of sensing research. Anilines play a key role as intermediates processing side, PDIs were always electrospun into nanofiber
1
13,15–18
of chemical, pharmaceutical and food industries. Detecting films.
The complicated preparation, low FQY (below
1
5
anilines with low detection limit, high selectivity and reversi- 15%), and the irreversible fluorescence quenching for amines
bility is essential for air pollution control, food monitoring and are the essential factors that limit the widespread applications
7
8,9
medical diagnosis. In spite of significant progress,
the of these methodologies. Thus, improving the solid-state
The use fluorescent properties and developing new-type preparation
1
0,11
sensing of gaseous anilines remains challenging.
of organic fluorescence for sensing purposes must meet two strategies for detective materials are the crucial challenges to
demands: strong solid-state emission, in spite of the known be solved.
tendency of organic fluorophores to undergo self-quenching,
and convenient manufacturing processes.
Through modification at bay- and imide-position with
appropriate groups, the H-aggregation of PDIs that leads to
1
2,13
1
5,16
Perylenediimides (PDIs) are widely employed as industrial fluorescence quenching could be effectively avoided.
Thus,
14
colorants and n-type organic semiconductors. PDIs with many PDI-based crystalline materials with intense fluorescence can
advantages, such as high fluorescence quantum yield (FQY) in be obtained and, further, exhibit ideal sensing ability. Taking
solution and outstanding stability, can exhibit excellent aniline advantage of the hierarchical assembly strategy, crystalline
14–16
sensing properties.
However, the aggregation-induced weak free-standing films entirely from small molecular PDIs can
greatly decrease the complexity of material preparation. Such
a
crystalline polymeric materials can be facilely fabricated and
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced
Innovation Center for Soft Matter Science and Engineering, Beijing Laboratory of
Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029,
P. R. China. E-mail: yinmz@mail.buct.edu.cn
recycled, and can exhibit robustness as well as advantageous
1
9–22
optoelectronic properties.
Herein, we introduce highly fluorescent free-standing films
FFSFs) formed from PDI microcrystals (Fig. 1a) that can be
b
c
College of Chemistry and Chemical Engineering, Yan’an University, Yan’an,
Shanxi Province 716000, P. R. China. E-mail: zhyt1969@163.com
(
employed in efficient fluorescence probing of gaseous anilines.
It is the appropriate chemical modification of the PDI core
Max Planck Institute for Polymer Research, Institute of Physical Chemistry,
Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc05587h as realized in compound P1 (Fig. 1b) which establishes a
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Fig. 1 (a) Illustration describing the film fabrication from fibrous PDI microcrystals. (b) Molecular structures of P1. (c) TEM image of P1 microcrystals.
d) SEM image of a representative dry P1 film. Inset: Free-standing P1 film with a diameter of 1.3 cm.
(
molecular packing favorable for the desired strong fluorescence. T20 instrument. Differential Scanning Calorimetry (DSC) was
Indeed, the fabricated FFSFs show high solid-state FQY (F = 39%) studied using DSC Q200 (TA Instruments), under N flow with a
heating rate of 10 1C min in aluminum T-Zero pans.
2
23,24
À1
which is an outstanding value amongst PDI crystals.
In addition, the porosity of FFSFs not only provides an increased
surface area for enhanced adsorption of gaseous molecules, but
2.2 Synthesis of P1–P6
also enables efficient diffusion of guest molecules across the film The detailed syntheses of P1–P6 are described in the ESI†
matrix. The underlying mechanism is photoinduced electron (Schemes S1–S3 and Fig. S1–S14).
transfer (PET). This is the first report on the simple fabrication
2
.3 Microcrystals and film self-assembly
of PDI-based FFSFs for the sensing of aniline and its derivatives
with low detection limit, high selectivity and reversibility.
A stock solution of P1–P6, respectively, in chloroform at a final
concentration of 2 mg mL was prepared. 1 mL stock solution
À1
was placed at the bottom of a 10 mL vial, a buffer layer of
2. Experimental section
0.5 mL of 1 : 1 chloroform/methanol mixture was carefully
2
.1 Materials and methods
placed on top of the stock solution, and then 2 mL of methanol
was added on top. The vial was capped and the solution was
allowed to age for 3–5 days to finally give belt-like microcrystals
(Fig. S15, ESI†). A controlled pressure setup was used for
depositing microcrystals from the solutions. An aged solution
was filtered through an organic microporous Teflon filter
membrane (0.45 mm) in a vacuum system (Beijing Synthware
Glass). The PDI films were detached from the supports after
drying for 2 h.
All chemical materials were purchased from commercial sources
and used without further purification unless otherwise noted.
1
1
H-NMR spectra were recorded on a Bruker 400 (400 MHz H)
spectrometer at room temperature. Matrix-assisted laser
desorption ionization time-of-flight mass spectrometry (MALDI
TOF MS) was performed on an AXIMA-CFR plus MALDI TOF
mass spectrometer. Powder X-ray diffraction (PXRD) patterns
of the powder were measured using a Rigaku 2500VB2 + PC
diffractometer using Cu K radiation (l = 1.541844 Å) at 40 kV
a
and 50 mA in the step-scanned mode at 0.041 (2y) per step
and at a count time of 10 s per step in the range from 51 to 401.
Fluorescence spectroscopic studies were performed on a
3
. Results and discussion
3.1 Fabrication of free-standing films
fluorescence spectrophotometer (Horiba JobinYvon FluoroMax-4 Chromophore molecule P1 carries four phenoxy groups in the
NIR, NJ, USA). UV-Vis spectra were obtained on a spectrometer bay positions and linear alkyl chains ensuring good solubility
(UV-2600). The contribution of each lifetime and fluorescence in common solvents (such as chloroform) and processability.
quantum yield was recorded using an Edinburgh Instruments’ The self-assembly of P1 into microcrystals was accomplished
FLS 980 fluorescence spectrometer. The morphology was by a slow solvent-diffusion process within a closed chamber
investigated by field emission scanning electron microscopy (Fig. S15, ESI†). Flexible microcrystals with high-aspect ratios
(HITACHI S-4700, Japan). Transmission electron micro- were obtained, as revealed by TEM and SEM imaging (Fig. 1c).
scopy investigations were carried out on a FEI Tecnai G2 The microcrystals were typically 1–5 mm in width and hundreds
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of micrometers in length. To highlight the special design of P1, powders after drying. Differential scanning calorimetry (DSC) of
the molecules P2–P6 were made for comparison (Scheme S1, films made from P1 also revealed a high thermal robustness
ESI†). P2 and P3, having a similar structure as P1, can self- (Fig. S20, ESI†).
assemble into elongated microcrystals as well (Fig. S16 and S17,
ESI†), while P4 without modification in the bay position has
3.2 Molecular packing and highly fluorescent films
poor solubility in common solvents. The singly crystalline UV-Vis and fluorescence spectra can elucidate the interaction
nature of the P1 assemblies was evidenced from multipoint between the aromatic cores (Fig. 2a). P1 FFSF exhibits mainly
electron diffraction measurements obtained at different three PDI absorptions in the 450–650 nm range, which are
2
8
locations along the long axis of the microcrystal (Fig. S18, similar to the absorptions of alkyl-substituted PDI thin films.
2
5
ESI†). Identical electron diffraction patterns were observed P1 FFSF displays an obvious bathochromic shift in the region of
throughout the whole microcrystal. Further, polarizing optical 600–650 nm, and this shift is known to result from a combi-
microscopy (POM) was utilized to confirm the anisotropies nation of excitonic coupling and charge transfer interactions
of microcrystals (Fig. S19, ESI†). The optical intensity of between the p-stacked chromophores (Fig. 2a). There are two
microcrystals changed significantly after a deflection of 451, fluorescence peaks of P1 in chloroform solution (10 M): one
2
9
À5
indicating a high degree of molecular anisotropy within the is a monomer emission peak at around 607 nm and the other
2
6
solid packing.
is a broad excimer emission peak at around 656 nm from the
3
0
Film fabrication was achieved by filtering the aged micro- PDI moieties. Once P1 forms the microcrystals, the monomer
crystals of P1 over a filter membrane with suction. This is an emission disappears and the excimer emission emerges at
economic protocol since only 2 mg of compound P1 appeared 665 nm (Fig. 2a), which suggests a dominant excimer emission
to be sufficient for film formation. The film could be tailored of P1 microcrystals.
into appropriate shapes using a home-made mould (inset Powder X-ray diffraction (PXRD) of P1 FFSF was measured to
1
4,30
in Fig. 1d), and after drying could be detached manually. obtain insight into the molecular packing of the microcrystals
Typically, free-standing macroscopic films had a diameter of (Fig. 2b). The clear diffraction peaks indicate the high-ordered
several centimeters (inset in Fig. 1d). SEM images of P1 films molecular packing. The sharp peak at 2y = 5.21 (d = 1.7 nm)
displayed a porous network of interwoven microcrystalline corroborates the end-to-end distance between phenoxy groups
belts (Fig. 1d). Elongated microcrystals were expected to connected to the PDI-core, reflecting fiber growth along this
2
7
triaxially weave as depicted in Fig. 1a. The self-assembly of direction with a d-spacing of 1.7 nm (Fig. 2c). Furthermore,
P5 with cyclohexyl and P6 with chloro-substituents in the peaks at 2y = 26.41 (d = 0.34 nm), 2y = 13.81 (d = 0.65 nm) and
bay positions was also studied, but elongated microcrystals 2y = 6.71 (d = 1.34 nm) can be assigned to face-to-face p–p
could not be obtained, and the formed aggregates gave rise to packing. The interplanar distance between PDI moieties is
Fig. 2 (a) UV-Vis and fluorescence spectra of P1 FFSF and in chloroform, (b) PXRD pattern of P1 FFSF, and (c) proposed model showing the
self-assembly of P1.
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found to be B0.34 nm, which is similar to that in other PDI Table 1 Photophysical properties of P1–P5 in the solid state
2
4
crystals. Based on these results, it is proposed that p–p
a
b
c
d
À1
d
À1
Sample
l
em (nm)
F (%)
t (ns)
K
r
(ns
)
K
nr (ns
)
packing plays an important role in the formation of ordered
one-dimensional microcrystals.
P1 FFSF demonstrates strong red emission as depicted in
the fluorescence microscopy image (Fig. 3a). P1 FFSF with P4
3
1
P1
P2
P3
658
655
660
683
685
39
30
28
5
6.10
9.99
3.50
6.98
8.49
0.06
0.03
0.08
0.01
0.02
0.10
0.07
0.21
0.14
0.10
P5
13
strong red emission can clearly be observed even with the
a
b
naked eye as shown in the inset of Fig. 3a. The fluorescence
of P1 amorphous powders was also measured (Fig. 3b) and
the fluorescence intensity is remarkably decreased compared
with that of P1 microcrystals. The different fluorescence
lex = 450 nm. Fluorescence quantum yield was determined by the
c d
area integral method. Fluorescence lifetime. The rates of radiative
and non-radiative decay were estimated by K
= F/t and Knr = (1 À F)/t.
r
1
5
performance indicates that the ordered p–p packing of P1 with high FQY. The rate constants for radiative decay (K
) and
non-radiative decay (Knr) were also calculated for P1–P5
The photophysical properties of solid films from P1–P5 (Table 1). It is noteworthy that the K of P4 and P5 is signifi-
Table 1) document the key influence of the molecular structure. cantly lower than those of P1–P3, which can be ascribed as due
r
3
2,33
microcrystals leads to highly fluorescent materials.
r
(
3
8
The FQYs of P1–P3 microcrystals were measured to be 39%, to the decrease in oscillator strength and emission. The Knr of
0%, and 28%, respectively. The FQY of P1 microcrystals is P1 is lower than those of P2–P5, and the FQY value of P1 is
much higher than that of most other reported PDI assembled higher than those of P2–P5, and thus P1 is the best solid-state
3
2
3,24
materials.
P1–P3 with various linear side chains and appro- fluorescent material among P1–P5.
priate substituents in the bay positions showed more intense
solid emission than P4 without bay substituents and P5 with
3.3 Solid-state fluorescence probing for amine vapors
non-linear alkyl chains. This is because the non-planar struc- As shown in Fig. 4a, upon exposure to saturated vapor of aniline
ture of P1–P3 with minimal steric hindrance (usually referring in a sealed bottle, the fluorescence of P1 FFSF was almost
to a small or linear side chain) prefers a distorted p-scaffold completely quenched in about 10 s. Furthermore, the quenched
to afford increased fluorescence by enhancing the low-energy fluorescence can be recovered after heating P1 FFSF. As shown
1
0,34–37
excitonic transition.
Using an appropriate solvent-diffu- in Fig. 4a, after heating to 120 1C for 10 min, the fluorescence
sion assembly method, P1–P3 were the assembled materials intensity of P1 FFSF was restored. This is because thermal
treatment leads to desorption of aniline molecules. Thus, the
fluorescence detection can be easily repeated several times
without obvious fatigue (Fig. S21, ESI†). Previous PDI-based
15,16
fluorescence sensory materials have rarely operated reversibly.
Therefore, our reversible fluorescence sensing system appears to
be highly economical and environmentally friendly. As a compar-
ison, P1 amorphous powders were used for aniline sensing
(Fig. S22, ESI†). They gave an inferior response with a quenching
efficiency (1 À I/I ) of 79% in 240 s, while P1 FFSF showed a fast
0
response to aniline with a quenching efficiency of 97% in 40 s.
From the result it can be concluded that the microcrystal-based
FFSF sensor provided continuous channels formed by weaved
microcrystals, enabling convenient diffusion of aniline molecules
throughout the FFSF, and thus fast response for the sensing.
To explore the detection limit for aniline, the quenching
process was also examined for a more diluted aniline vapor.
Fig. 4b presents the fluorescence quenching efficiency of P1 FFSF
measured at different vapor pressures of aniline diluted from the
saturated vapor (880 ppm) at room temperature. It is obvious
that the quenching efficiency remains almost unchanged at a
concentration of aniline vapor above 220 ppm because of the
15
supersaturated adsorption of aniline vapor. When the concen-
tration of aniline vapor is below 110 ppm, the quenching
efficiency is well-fitted to the Langmuir equation (quenching
efficiency is proportional to the adsorption of aniline). From the
fitted plot, the detection limit of P1 FFSF can be estimated as low
as B0.1 ppm (Fig. S23, ESI†).
Fig. 3 (a) Fluorescence microscopy image of dispersive P1 FFSF; the inset
shows a P1 fluorescent film observed by the naked eye. (b) Fluorescence
contrast of P1 microcrystals and amorphous powders.
Selectivity in response to anilines is an important factor for a
sensor in practical applications. To evaluate the selectivity, P1
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Fig. 4 (a) Fluorescence quenching and recovery of P1 FFSF upon exposure to saturated aniline vapor (880 ppm) for 10 s. (b) Fluorescence quenching
efficiency as a function of aniline vapor pressure. (c) Fluorescence quenching using different vapors (for 10 s) at room temperature. (1) Aniline,
(
(
2) N-methylaniline, (3) triethylamine, (4) ammonium hydroxide, (5) hydrazine, (6) chloroform, (7) methanol, (8) acetone, (9) nitromethane, (10) phenol,
11) pyridine, (12) diethylamine, (13) acetonitrile, (14) cyclohexylamine, (15) trimethylamine.
FFSF was treated with various amine vapors and common
organic solvent vapors. As shown in Fig. 4c, P1 FFSF provides
high fluorescence quenching in response to aniline vapor,
while inferior quenching in response to other alicyclic amine
vapors and common organic solvent vapors under the same
conditions. Furthermore, N-methylaniline and five other aniline
derivatives were detected and all exhibited different degrees of
quenching efficiency (Fig. S24, ESI†), implying that P1 FFSF has
superior detection performance for anilines. Compared with the
previously reported aniline sensing materials, the P1 FFSF in this
work exhibits low detection limit, high reversibility and selectivity
at the same time, as presented in Table S1 (ESI†). The results
support the superior role of P1 FFSF for the practical detection of
aniline vapors.
Fig. 5 Molecular orbital energy diagrams which show the frontier orbitals
of the fluorophore and the amine. HOMO and LUMO energies are
calculated using the B3LYP/6-31G(d) basis set.
The frontier orbitals of the fluorophore and analyte play an
3
7
important role in sensing. To explore the causes of selectivity,
density functional theory (DFT) calculations for compound P1
and relevant amines were made (Fig. 5). DFT calculations
demonstrated that the HOMO energy level of aniline is a little
higher than that of P1. This implies that after excitation, the
4. Conclusions
positive charge of P1 could be filled by an electron from the In summary, we present a method of fabricating robust FFSFs
3
9–42
HOMO of aniline
giving rise to a typical photoinduced entirely from small molecular PDIs by solvent diffusion self-
electron transfer (PET) process. Thus, the excited electron assembly. Compared with other aniline sensing materials,
cannot return to the ground state through radiative transition the woven crystalline FFSFs achieved increased surface area
and fluorescence quenching occurs. The reason why a P1 film is for significantly improved aniline sensing. DFT calculations
insensitive to alicyclic amine vapors such as triethylamine is indicate that the selective fluorescence quenching is caused
that the HOMO level of triethylamine is lower than that of P1. by a PET mechanism. The presented FFSFs may offer great
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prospects for practical environmental sensing and a wide range 19 T. Aida, E. W. Meijer and S. I. Stupp, Science, 2012, 335,
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2
There are no conflicts to declare.
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