Exploration of Highly Efficient Blue-Violet Light Conversion Agents for an Agricultural Film Based on Structure Optimization of Triphenylacrylonitrile

Article abstract of DOI:10.1021/acs.jafc.8b05453

To obtain highly efficient blue-violet light conversion agents used for an agricultural film, six triarylacrylonitrile derivatives and their doping films were prepared. Further, the luminogens have the ability to convert ultraviolet light into blue-violet

Full text of DOI:10.1021/acs.jafc.8b05453

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Functional Structure/Activity Relationships
Exploration of highly efficient blue-violet light conversion agents
for agricultural film based on structure optimization of TPA
Yongtao Wang, Yu Yongjiang, Wenjing Liu, Litong Ren, and Gui-xian Ge
J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b05453 • Publication Date (Web): 26 Nov 2018
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Exploration of highly efficient blue-violet light conversion agents for agricultural
film based on structure optimization of TPA
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Yongtao Wang*ab, Yongjiang Yua, Wenjing Liua, Litong Rena and Guixian Ge*a
aSchool of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of
Chemical Engineering of Xinjiang Bingtuan, Shihezi University，Shihezi 832003, P. R. China；
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bShenyang University of Chemical Technology, Shenyang, P. R. China；
*Author for correspondence:wyt_shzu@163.com; geguixian@126.com.
Abstract
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To obtain highly efficient blue-violet light conversion agents used for agricultural film, six
triarylacrylonitrile derivatives and their doping films were prepared. Further, the luminogens have
the ability to convert UV light into blue-violet light, and exhibit aggregation dependent fluorescence
emission and high-contrast fluorescence quantum yields from 0.004 to 0.833. Based on X-ray single
crystal diffraction analysis and aggregation-induced emission activity tests, the variant fluorescence
quantum yields are attributed to intermolecular π-π stacking and phase transition between crystalline
state and amorphous state. In simulated greenhouse environment, the luminogens exhibit excellent
photostability. However, their fluorescence intensity drop to 17%-40% of prime intensity after
outdoor radiation for one month due to the ring-closing oxidation reaction (In summer). By
comprehensively considering the above photophysical properties and mechanical properties of the
doping film, Ph4TPA is a potential light conversion agent for agricultural film in winter. More
importantly, the results indicate the properties of the light conversion films are expected to be further
improved by molecular design inhibiting the ring-closing oxidation reaction and dispersion of
crystalline nanoparticles in the doping film.
Keywords: Light conversion agents ； Light conversion film ； Acrylonitrile derivatives;
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Photosynthesis；Fluorescence.
25 1. Introduction
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Light ecology studies have demonstrated the blue-violet light (400-480 nm) and red-orange light
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(600-700 nm) used as light fertilizer can enhance photosynthetic efficiency of crops1-3. The former
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Electronic supplementary information (ESI) available: H NMR, 13C NMR, fluorescence, absorption, transmittance
hydrogen bonding interaction and MALDI-TOF MS spectra. [Supplemental figures.]. See DOI: 10.1039/x0xx00000x
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is beneficial to the increase of protein content, while the latter can promote synthesis of carbohydrate.
Further, the light conversion shed film experiments show that growth cycle of crops, insect pests
and ultraviolet damage to polymer matrix are obviously reduced or shortened, meanwhile, quality
and yields of crops are significantly increased4-9. Therefore, the light conversion agents used for
agricultural film have received extensive attention and great progress have been made in the past
decades. However, the light conversion agents still face some problems such as spectra matching,
fluorescence quantum yield (PLQY) and weather resistance. How to design highly efficient light
conversion agents has become a very tricky thing.
To achieve perfect match with the absorption spectra of chlorophyll a, b and carotenoid in the blue-
violet region, the light conversion agents with short conjugate structure or weak intramolecular
charge transfer (ICT) should be adopted to realize a wide energy band gap10-11. As a matter of fact,
the shortage and lag of efficient deep-blue organic light-emitting materials indicate molecular design
of blue-violet light conversion agents will be full of frustrations for a given period of time. In
addition, aggregation caused quenching is another culprit restricting the effect of light conversion.
Fortunately, aggregation-induced emission luminogens (AIE) with twisted or propeller-like
molecular configuration are induced to emit efficiently in the aggregated state, but non-emissive in
the dissolved form12-15. In our previous research work, tetraphenylethylene (TPE) derivatives with
AIE activity, twisted diaryl ketones with thermal delayed fluorescent performance and
triphenylacrylonitrile (TPA) were chosen as light conversion materials. The results indicated that
TPE derivatives easily suffered from photo-oxidation leading to faint emission, while diaryl ketones
were ruled out because of weak fluorescence and their depressing mechanical properties.
Interestingly, TPA doped polyvinyl chloride (PVC) films exhibited deep-blue fluorescence emission
and excellent mechanical properties16. In comparison with TPE derivatives, TPA exhibits better
photostability due to electron-withdrawing effect of cyanide inducing reduction of the electron cloud
density of benzene ring17-19. Even so, TPA still cannot meet the request of practical application
because of low PLQY and unsatisfactory photostability.
To obtain highly efficient blue-violet light conversion agents and reveal the key factors affecting
the light conversion performance, in this paper, five novel triarylacrylonitrile derivatives named as
PhMe3TPA, PhMeO3TPA, Ph3TPA, Ph4TPA and PhMe4TPA were designed and synthesized (As
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shown in Scheme 1). The significantly enhanced PLQYs have been realized for crystal Ph4TPA and
PhMe3TPA compared with that of TPA. Based on X-ray single crystal diffraction analysis, the
marked difference of PLQY between Ph3TPA and PhMe3TPA are mainly attributed to different
molecular packing and arrangement. Unfortunately, the triarylacrylonitrile derivatives exhibit
inferior PLQY in PVC doped films. To make clear the latent mechanism of remarkable PLQY
reduction in PVC doped films, AIE activity test and the morphology regulation of nanoparticles are
completed by mixing THF/H2O and adding the cetyltrimethylammonium bromide (CTAB),
obviously, crystallization-induced emission enhancement (CIEE) active characteristics and
aggregation dependent fluorescence cause the deterioration of PLQY in PVC doped films. Further,
the inherent mechanism of photodegradation are analyzed and discussed by changing temperature,
light intensity, aggregation state and molecular structure. The results show the crystalline state is
not only conducive to maintaining high PLQY, but also improving the photostability of the light
conversion materials.
70 2. Experimental details
71 2.1 Measurements and characterizations
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The UV-vis spectra were determined on a Mapada UV-3200pcs spectrophotometer. Fluorescence
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measurements were taken on Agilent Cary Eclipse fluorescence spectrophotometer. H NMR and
13C NMR spectra were obtained with a Varian inova instrument at 400 MHz and 100 MHz using
tetramethylsiane (TMS) as the internal standard, and CDCl3 as the solvent in all cases.
MALDI/FIRMS were recorded on an Ultrafle Xtreme MALDI-TOF/TOF mass spectrometer
(Bruker, Germany). Fluorescence microscopy photos were obtained on OLYMPUS BX53.
Photodegradation experiments were carried out by a 300W xenon lamp equipped with a filter, and
radiation intensity of artificial sunlight is up to 129 Mw/cm2.
80 2.2 Materials and synthesis
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THF, DMF and toluene were dried according to standardized procedures previously described. All
the other chemicals and reagents used in this study were of analytical grade without further
purification. In general, all the intermediates and final compounds were purified by column
chromatography of silica gel (200-300 mesh), and the reaction process was monitored by thin-layer
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chromatography (TLC). The synthetic methods of intermediates and target compounds were shown
as Scheme 1. The synthetic method of TPA was carried out according to the reported literature17.
The light conversion films were prepared by referring to our previous method20, obtaining final
films with a 1% mass fraction.
CN
Br
CN
Br
Br
O
CN
CN
Br
Toluene, NaH
reflux
Toluene, NaH
reflux
(2)
(1)
Ar
1 -B(OH)
2
Pd(PPh
) 4
-B(OH)
Ar 2
3
2
3
)
Ar2
4
Pd(PPh
CN
CN
Ar1
Ph4TPA
PhMe3TPA
Ar1=
PhMeO3TPA
MeO
Ph3TPA
PhMe4TPA
Ar2=
CN
CHO
CN
40%NaOH
ethanol
light
H
CN
TPA
CN
tMePCN
PPACN
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Scheme 1. Synthetic routes of the luminogens.
2-(3-bromophenyl)-3, 3-diphenylacrylonitrile (1): In a three-necked round-bottom flask (250 mL)
equipped with a reflux condenser were placed benzophenone (4.00 g, 22 mmol), NaH (60%, 1 g, 25
mmol) and anhydrous toluene (50 mL) under nitrogen. The mixture was refluxed for 10 min. Then,
3-bromophenylacetonitrile (3.88 g, 20 mmol) in toluene (60 mL) was added dropwise over 1 h under
reflux condition. Evolved H2 was observed at the oil bubbler and the reaction was lasted for another
20 h. Upon cooling to room temperature, 100 mL of water was added to the mixture. The organic
layer was collected and washed with brine three times. Next, the organic phase was dried over
anhydrous Na2SO4 and concentrated under vacuum. Then, the crude product was purified by column
chromatography to give white solid. Yield: 83%.
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2-(4-bromophenyl)-3,3-diphenylacrylonitrile(2): The compound was synthesized by the same
procedure described for compound 1 using 4-bromophenylacetonitrile and benzophenone, the crude
product was purified by chromatography (silica gel, DCM: petroleum=1:10). White solid was
obtained. Yield: 76%.
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2-(2'-methyl-[1,1'-biphenyl]-3-yl)-3,3-diphenylacrylonitrile(PhMe3TPA): Into a three-necked
round-bottom flask (100 mL) equipped with a reflux condenser were placed compound 1 (1.80 g, 5
mmol), o-tolylboronic acid (0.82 g, 6 mmol), CH3COOK (1.17 g, 12 mmol), a mixed solution
(toluene: H2O=5:1, 20 mL) and Pd(PPh3)4 (0.17 g, 0.15 mmol) under argon atmosphere. And then,
the mixture was refluxed for 12 h under argon. After cooling to room temperature, the reaction
mixture was poured into water and extracted with ethyl acetate. The combined organic layers were
dried over anhydrous Na2SO4 and evaporated in vacuum. The crude product was purified by
chromatography (silica gel, ethyl acetate: petroleum=1:6). White solid was obtained. Yield: 81%.
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m.p. 115.3-117.0 °C. H NMR (400 MHz, Chloroform-d) δ 7.50-7.39 (m, J = 4.9 Hz, 7H), 7.38-
7.26 (m, 2H), 7.26-7.10 (m, 6H), 7.08-7.01 (m, 2H), 6.92 (d, J = 7.3 Hz, 1H), 1.98 (d, J = 1.4 Hz,
3H). (Fig. S6). 13C NMR (100 MHz, CDCl3) δ 158.06, 142.22, 140.93, 140.36, 139.20, 135.23,
134.79, 130.93, 130.88, 130.20, 129.93, 129.63, 129.20, 128.95, 128.48, 128.44, 128.38, 127.95,
127.46, 125.69, 120.14, 111.58, 20.20. (Fig. S7). HRMS (MALDI-TOF): m/z 371.1660,
calculated 371.1674. (Fig. S8).
2-([1,1'-biphenyl]-3-yl)-3,3-diphenylacrylonitrile(Ph3TPA): The compound was synthesized by
the same procedure described for PhMe3TPA using compound 1 (1.80 g, 5 mmol), phenylboronic
acid (0.732 g, 6 mmol), the crude product was purified by chromatography (silica gel, DCM:
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petroleum=1:10). White solid was obtained. Yield: 81%. m.p. 135.8-137.0 °C. H NMR (400
MHz, Chloroform-d) δ 7.53-7.42 (m, 7H), 7.39-7.33 (m, 2H), 7.33-7.25 (m, 7H), 7.24-7.21 (m, 1H),
7.12-7.04 (m, 2H). (Fig. S9). 13C NMR (100 MHz, CDCl3) δ 158.18, 141.47, 140.47, 140.38, 139.39,
135.27, 130.92, 130.08, 130.06, 129.16, 129.07, 128.86, 128.83, 128.62, 128.54, 128.53, 127.65,
127.24, 127.16, 120.20, 111.68. (Fig. S10). HRMS (MALDI-TOF): m/z 357.1506, calculated
357.1517. (Fig. S11).
2-(4'-methoxy-[1,1'-biphenyl]-3-yl)-3,3-diphenylacrylonitrile(PhMeO3TPA): The compound
was synthesized by the same procedure described for PhMe3TPA using compound 1 and (4-
methoxyphenyl)boronic acid, the crude product was purified by chromatography (silica gel, DCM:
petroleum=1:10). White solid was obtained. Yield: 89%. m.p. 145.2-147.0 °C. 1H NMR (400 MHz,
Chloroform-d) δ 7.50-7.37 (m, 7H), 7.33-7.25 (m, 3H), 7.25-7.19 (m, 4H), 7.07 (dt, J = 8.5, 1.9 Hz,
2H), 6.92-6.86 (m, 2H), 3.82 (s, 3H). (Fig. S12). 13C NMR (100 MHz, CDCl3) δ 159.32, 157.93,
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140.92, 140.39, 139.32, 135.06, 132.78, 130.80, 129.94, 129.92, 128.99, 128.89, 128.48, 128.38,
128.27, 128.07, 127.82, 126.68, 120.10, 114.15, 111.67, 55.35. (Fig. S13). HRMS (MALDI-TOF):
m/z 387.1661, calculated 387.1623. (Fig. S14).
2-(2'-methyl-[1,1'-biphenyl]-4-yl)-3,3-diphenylacrylonitrile(PhMe4TPA): The compound was
synthesized by the same procedure described for PhMe3TPA using compound 2 and o-tolylboronic
acid, the crude product was purified by chromatography (silica gel, DCM: petroleum=1:10).
White solid was obtained. Yield: 79%. m.p. 162.8-164.0 °C. 1H NMR (400 MHz, Chloroform-
d) δ 7.49 (dt, J = 6.5, 1.9 Hz, 2H), 7.47-7.40 (m, 3H), 7.30 (ddt, J = 12.7, 9.2, 2.9 Hz, 4H), 7.26-
7.16 (m, 7H), 7.07 (dt, J = 8.5, 1.9 Hz, 2H), 2.24 (s, 3H). (Fig. S15). 13C NMR (100 MHz, CDCl3)
δ 158.14, 142.46, 141.38, 140.88, 139.59, 135.68, 133.73, 131.27, 130.87, 130.40, 130.34, 130.03,
129.87, 129.73, 129.49, 128.92, 128.69, 127.98, 126.27, 120.62, 111.84, 20.84. (Fig. S16). HRMS
(MALDI-TOF): m/z 371.1668, calculated 371.1674. (Fig. S17).
2-([1,1'-biphenyl]-4-yl)-3,3-diphenylacrylonitrile(Ph4TPA): The compound was synthesized by
the same procedure described for PhMe3TPA using compound 2 and phenylboronic acid, the crude
product was purified by chromatography (silica gel, DCM: petroleum=1:10). White solid was
obtained. Yield: 59%. m.p. 136.3-137.2 °C. 1H NMR (400 MHz, Chloroform-d) δ 7.58-7.53 (m,
2H), 7.49-7.38 (m, 9H), 7.36-7.31 (m, 3H), 7.29-7.25 (m, 1H), 7.24-7.18 (m, 2H), 7.08-7.04 (m,
2H). (Fig. S18). 13C NMR (100 MHz, CDCl3) δ 157.65, 140.98, 140.49, 140.03, 139.17, 133.74,
130.80, 130.14, 129.98, 129.91, 129.07, 128.48, 128.36, 127.71, 127.03, 126.98, 120.13, 111.29,
77.36, 77.04, 76.73. (Fig. S19). HRMS (MALDI-TOF): m/z 357.1513, calculated 357.1517.
(Fig. S20).
3-mesityl-2-phenylacrylonitrile (tMePCN): The compound was synthesized by the same method
described for MePCN. Yield: 89%. m.p. 96.8-98.5 °C. 1H NMR (400 MHz, Chloroform-d) δ 7.84-
7.57 (m, 3H), 7.53 -7.39 (m, 3H), 6.95 (s, 2H), 2.31 (s, 9H). (Fig. S21). 13C NMR (100 MHz, CDCl3)
δ 142.73, 138.35, 135.81, 133.17, 130.60, 129.19, 128.90, 128.51, 125.71, 118.46, 116.69, 20.94,
20.09. (Fig. S22). HRMS (MALDI-TOF) m/z: Mass 247.1361, Calc. Mass 247.1361. (Fig. S23).
1
10-phenylphenanthrene-9-carbonitrile (PPACN) m.p. 183.2-185.0 °C. H NMR (400 MHz,
Chloroform-d) δ8.81-8.73 (m, 2H), 8.44-8.36 (m, 1H), 7.84-7.75 (m, 3H), 7.71 (d, J = 8.3 Hz, 1H),
7.65-7.55 (m, 4H), 7.54-7.47 (m, 2H) (Fig. S24). 13C NMR (100 MHz, CDCl3) δ 147.40, 137.03,
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131.77, 130.18, 129.92, 129.62, 129.44, 128.93, 128.84, 128.73, 128.66, 128.33, 128.07, 127.41,
126.48, 122.90, 117.26, 109.62 (Fig. S25). HRMS (MALDI-TOF) m/z: Mass 279.1048, Calc. Mass
279.3420. (Fig. S26).
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165 3. Results and discussion
166 3.1 Photophysical properties of triarylacrylonitrile derivatives
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Fig. 1. The UV-visible absorption and fluorescence spectra of the luminogens. (a) UV-vis spectra
in THF (1x10-5 mol/L) (b) fluorescence spectra in THF (1x10-5 mol/L) (c) fluorescence spectra in
crystalline state (d) fluorescence spectra in PVC film.
To investigate photophysical properties of the dyes, the corresponding THF solutions (1×10-5 mol
L-1) and PVC doped films were prepared. As shown in Fig. 1, PhMeO3TPA showed different UV-
vis spectra compared to the other four compounds. PhMe3TPA, Ph3TPA, Ph4TPA and PhMe4TPA
have two obvious absorption bands located at 230-280nm and 280-400nm respectively, while
PhMeO3TPA shows four absorption bands without clear boundary. The differences should be
attributed to the stronger electron donating ability of -OCH3, which induces ICT effect and produces
new absorption bands. Compared with meta-substitution, para-substitution is beneficial to extension
of conjugate structure. Accordingly, the maximum absorption peaks of Ph4TPA and PhMe4TPA
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show obvious red-shifted than that of the other three dyes, and so do emission maxima. Contrary to
electron donating ability, the maximum emission peaks of PhMeO3TPA, PhMe3TPA and Ph3TPA
exhibit bathochromic shift in turn, and the twisted conformation induced by Me- and MeO- groups
is suggested to be a possible reason of abnormal shifts further discussed in the part of the crystal
analysis. Compared with THF solution, the emission maxima of luminogens show red shift of 40-
70nm at solid state and in PVC doped films, especially marked red shifts for solid state due to
molecular stacking and arrangement. In addition, PhMe4TPA exhibits two splitting emission peaks
in PVC doped films, indicating different electronic structures between PhMe4TPA and the other
luminogens. Based on UV absorption and blue-violet light emission spectra of the luminogens, they
are suitable sunlight conversion agents used for agricultural film.
Generally, the transmission curve is used to investigate the conversion efficiency of UV,
transmittance of visible light and the proper addition ratio of light conversion agents in polymer
matrix. As shown in Figure S1, the UV-vis transmittance curve of the PVC doped films indicate the
transmittance between 200nm and 400nm reduces significantly, which means most of ultraviolet
irradiation are absorbed and converted. Meanwhile, the visible light (400-700 nm) transmittance of
the doped films remains a tiny change, even a slight increase for PhMe3TPA, Ph3TPA and Ph4TPA,
which indicates the transmittance of visible light is not affected by doping of the luminogens.
According to the reported literature20, transmittance of light conversion films and PVC blank film
are calculated at 200-390nm and 400-700nm respectively. As shown in Fig 2, less than 7.5% UV
transmittance for all the doping films means 1% weight ratio of light conversion agents relative to
PVC is appropriate.
The decrease in UV transmittance means that UV is converted into desired blue-violet light, but it
may also be thermal radiation. Thus, the integral area of emission spectra should be the more proper
evaluation indicator than transmittance for the light conversion properties of the luminogens.
Considering absorption maxima of chlorophyll a (430 nm) and b (450 nm), integral area of blue-
violet light should be confined to 400-480nm. Under the excitation wavelength of 317 nm and 351
nm, Ph4TPA exhibits the largest the integral area among of the dyes via direct visual observation,
and excitation dependent fluorescence emission due to molecular tumbling (Fig. 3). Obviously,
choosing Ph4TPA as sunlight conversion agent should be the wisest decision purely from the light
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conversion efficiency of view.
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Fig. 2 Transmittance of the doping films at 200-390 nm and 400-700 nm.
3.2 Fluorescence quantum efficiency and x-ray single-crystal analysis
Fig. 3 The fluorescence spectra of the luminogens in PVC films under excitation
at 317 nm(a) and 351 nm(b).
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The luminogens with minor structural differences show high-contrast fluorescence emission. Based
on naked eye observation, the fluorescence of PhMe3TPA and TPA is faint compared to that of the
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others in THF solution, while PhMe3TPA, PhMe4TPA, Ph4TPA and TPA exhibit strong
fluorescence emission in crystalline state (Fig. 4). Obviously, PhMeO3TPA and Ph3TPA belong to
traditional fluorophores with the aggregation-caused quenching (ACQ) effect, while PhMe3TPA,
PhMe4TPA, Ph4TPA and TPA, especially for PhMe3TPA and Ph4TPA have excellent CIEE
characteristic. Further, the absolute PLQYs of the luminogens are obtained in crystalline state and
PVC film by adopting integrating sphere method, and the relative PLQYs in different solvents are
calculated by using quinine sulphate as a standard, which testify the above results (Fig. 4).
Fig. 4 PLQYs and pictures of the luminogens under UV-irradiation (a) in THF solution (1x10-
5mol/L) (b) in crystalline state (c) in PVC film (fluorescence microscopy photos).
However, the CIEE-active luminogens suffer from serious decreasing PLQY in doping film
compared with crystalline state. To make clear the internal mechanism, firstly, crystal culture and
analysis of Ph3TPA (CCDC 1870773) and PhMe3TPA (CCDC 1870774) with opposite
characteristic are completed. Secondly, AIE activity of PhMeO3TPA, Ph4TPA and PhMe4TPA are
tested in THF-water mixture. Thirdly, fluorescence changes of Ph4TPA are observed by adding
CTAB in mixed solution of THF/H2O. The crystal Ph3TPA was a triclinic system with the space
group P -1 (a = 8.9129(9) Å, b = 10.4465(12) Å, c = 11.1785(11) Å; α = 107.535(7)°, β = 93.797(7)°,
γ = 98.838(7)°). Two adjacent molecules with tail-to-tail arrangement linked together by two
intermolecular C-N...H hydrogen bonds (d = 2.722 Å) and terrible π-π stacking (d = 3.566 Å), and
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the strong intermolecular hydrogen bond and π-π stacking make molecules to form a dimer (Fig. 5),
leading to red-shifted and weak fluorescence emission. The crystal PhMe3TPA is a monoclinic
system with the space group P 21/c (a = 13.874(6) Å, b = 10.834(5) Å, c = 16.943(6) Å; α =90°, β
= 123.73(2)°, γ = 90°). Intermolecular hydrogen bonds play a major role in the molecular packing
and every one molecule is connected with adjacent six molecules by C-N...H (CH3), C-H...C and
N-C...C interactions, forming three-dimensional space structure, however, no π-π stacking was
observed due to steric effect of methyl leading to significantly increased the dihedral angle between
A ring and B ring (Fig. S2 and Fig. 5). Obviously, two compounds possess twisted molecular
configuration, but different hydrogen bonds, molecular arrangement and intermolecular π-π
stacking, triggering two different fluorescence emission. For typical AIE luminogens, the
fluorescence emission intensity is gradually increase with the addition of poor solvent. Figure S3
indicates that PhMeO3TPA, Ph4TPA and PhMe4TPA present different characteristics in THF and
THF/water mixtures with varying water fractions. Compared with THF solution, the fluorescence
intensity of PhMeO3TPA remains an obvious increase in the H2O/THF mixtures from 10% to 40%,
then drops fast. Similar to PhMeO3TPA, Ph4TPA also shows decreasing fluorescence after the first
increase. Different from PhMeO3TPA and Ph4TPA, continuous fluorescence reduction appeared
for PhMe4TPA. The complex fluorescence phenomenon should be attributed to solvent polarity and
formation of aggregation state. When the ratio of H2O/THF increases, the increased solvent polarity
is beneficial to stabilize the ground state and excited state of polar molecules, thereby reducing non
radiation and enhancing fluorescence emission. If the increased ratio leads to unordered and loose
nanoparticles, the following fluorescence quenching will occur. In addition, the maximum
fluorescence emission wavelength of three luminogens red shift continuously with the increase of
solvent polarity, which hinted the luminogens have ICT characteristic. Finally, the amorphous film
of PhMe3TPA was prepared by spin casting its dichloromethane (DCM) solution on a glass sheet
and rapidly drying under vacuum, and showed very weak emission under UV light. Based on the
above results, the inferior PLQY of doping film should be attributed to the aggregation dependent
emission, by the preparation and doping of crystalline nanoparticles, PLQY of PhMe3TPA and
Ph4TPA doping film should be sharply improved. Peng et al regulate the packing and arrangement
of molecules by surfactants and obtain self-assembled polymorphs21, based on this idea, CTAB is
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selected to optimize the fluorescence emission of TPA in aggregated state. Then, enhanced
fluorescence emission was observed, but its effect is not significant (Fig. S4).
3.3 Photothermal stability
Fig. 5 Single crystal structure and molecular stacking mode of crystal
Ph3TPA (a) and crystal PhMe3TPA (b).
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Under sunlight irradiation, free radicals are easily generated and lead to degradation of luminogens.
In addition, temperature is another important factor triggering photodegradation of organic dyes.
Combining aggregation dependent PLQYs of the luminogens, four variables including light
intensity, temperature, crystalline state and amorphous state were investigated to explore the critical
factor of photothermal stability. Silver-foiled crystal and amorphous TPA were putted into an oven
o
o
o
at 50 C, 60 C and 70 C for a night, but no new dot was found by thin layer chromatography,
illustrating TPA was stable and insensitive to temperature. Then, crystal (slow solvent evaporation)
and amorphous (rapid evaporation of CH2Cl2 solution on quartz sheet) TPA were irradiated under
8 W ultraviolet lamp at 365 nm for 24 h, and the radiation process did not cause an obvious increase
in temperature. As a result, there was still no new product to be detected. Next, crystal and
amorphous TPA was placed in a quartz bottle and radiated under sunlight for one month (in summer).
Crystal TPA exhibited excellent stability, but a new product was found in amorphous sample.
Further, the oxidation product induced by ring-closing reaction was separated, purified and
characterized by HR-MS and NMR. Thereby, it is inferred that amorphous state and strong light
radiation can accelerate the ring-closing reaction of TPA. In the crystalline state, weak bond
interactions and ordered arrangement are not destroyed, which is helpful to inhibit rotation of
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benzene rings and the resulting oxidation reaction. Besides, the strong light radiation usually results
in a rise in temperature, thereby activation of the rotation and vibration of bonds, and thus TPA is
instable under sunlight, but remains stable under the low power ultraviolet lamp. Fortunately, the
suitable temperature of crops growth is between 20 oC and 30oC. At this suitable temperature, the
luminogens show excellent stability after 100 minutes of irradiation (Fig. 6). However, the intensity
of fluorescence emission changed dramatically when the doping films were placed outdoor for 30
days in summer (Fig. S5), which may be due to ACQ characteristic of oxidation products induced
(Fig. S27). As a conclusion, Ph3TPA and PhMe3TPA are potential light conversion agents for
agricultural film in winter due to below 30oC inside the greenhouse. To inhibit the fore-mentioned
oxidation reaction, tMePCN are synthesized by simple condensation reaction of aryl aldehyde and
aryl acetonitrile. Because of occupation of active site by 2, 6-dimethy of tMePCN, photostability of
luminogens is expected to be improved. Luckily, occupation of active site should be an effective
method inhibiting the ring-closing oxidation reaction, and the new substances is not found in
tMePCN sample under strengthening solar radiation (129 mW/cm2).
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Fig. 6 The fluorescence spectra of Ph3TPA(a), Ph4TPA(b), PhMe3TPA(c), PhMe4TPA(d),
PhMeO3TPA(e) in PVC films under artificial sunlight irradiation at different times.
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3.4 Mechanical properties
Referring to our previously reported methods, the mechanical properties of doping films and PVC
blank film were studied (Table 1). By contrast, PhMe4TPA exhibits the worst elongation among
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these doping films, more interesting, elongation of Ph3TPA and Ph4TPA is better than that of
PhMe3TPA and PhMe4TPA respectively. Obviously, methyl substituent is detrimental to
elongation, and the reason may be methyl substituent causes twisted molecular configuration,
thereby reduces intermolecular interactions. Different from elongation, tensile stress of all the
doping films are improved to some degree compared with that of PVC blank film.
Table 1 Mechanical property of light conversion films
Elongation (%)
Transverse/Portrait
10.01/12.40
Tensile stress(MPa)
Transverse/Portrait
37.60/33.77
Thickness
(mm)
Entry
PhMeO3TPA
PhMe3TPA
Ph3TPA
0.051
0.042
0.038
0.039
0.040
0.053
15.68/20.01
37.63/48.50
18.56/34.75
35.60/49.67
Ph4TPA
13.28/25.41
39.38/47.98
PhMe4TPA
PVC
4.25/4.90
44.57/33.58
13.50/12.49
33.13/31.20
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In summary, we designed and synthesized six triarylacrylonitrile and one diarylacrylonitrile
derivatives to obtain highly efficient blue-violet light conversion agents for agricultural film.
Despite similar molecular structures, serious π-π stacking furnished PhMeO3TPA and Ph3TPA with
ACQ characteristic, while PhMe3TPA, PhMe4TPA, Ph4TPA and TPA exhibited CIEE
characteristic, showing strong fluorescence in the crystalline state. More importantly, the
luminogens exhibited aggregation dependent fluorescence leading to undesirable PLQY in doping
film. Even so, PLQY of Ph4TPA is up to 0.833 and 0.107 in solid state and doping film respectively,
which are almost 4 times and 5 times that of TPA in corresponding state. Furthermore, the
fluorescence emission intensity of the doping films displays slight changes for 100 min under a
simulated greenhouse environment, however, only 15%-40% of the original intensity was retained
due to the increase of light intensity after outdoor exposure to sunlight in summer for one month.
Photothermal stability test further indicated crystalline state can improve potostability of dyes
compared with amorphous state, and the ring-closing oxidation reaction is the main cause of the
decrease of fluorescence intensity, but the reaction can be effectively inhibited by occupying active
site. Last but not least, Ph4TPA has potential application value as blue-violet light conversion agent
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in winter, and preparation of crystalline nanoparticles and dispersion of Ph4TPA in polymer matrix
will be very important to further improve the efficiency of light conversion and photostability, while
occupation of active site will offer theoretical guidance and practical reference for next step
molecular design of light conversion agents for agricultural film.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant Nos.
21766030, 21566034).
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Exploration of highly efficient blue-violet light conversion agents for
agricultural film based on structure optimization of TPA
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High contrast PLQYS of luminogens with tiny structural differences in solution,
crystalline and doping films
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