Exploration of high-performance light-conversion agents based on cyanostilbene and phenanthrenecarbonitrile backbones:E/Zand position isomerism, high-contrast Michael addition reaction activity and intramolecular photocyclization

Article abstract of DOI:10.1039/d1tc02405a

As light-conversion agents, traditional organic fluorescent dyes easily suffer from solid-state fluorescence quenching and photooxidation, leading to the deterioration of their application performance. To enhance the fluorescence emission and photostability of light-conversion agents in a doped film, in this paper, some luminogens with a cyanostilbene or a phenanthrenecarbonitrile backbone were designed and synthesized based on aggregation-induced emission (AIE) theory,E/Zisomerization and our previous research results, and were characterizedviaNMR, HR-MS and single-crystal X-ray diffraction. The experimental results indicate that bothZ-oCaandZ-pCaexhibit high-contrast Michael addition reaction activity and intramolecular photocyclization, as well as fluorescence quantum yields in the doped film, but similarE/Zphotoisomerization and AIE activity. Besides,E-oCashows stable configuration in solution, but not forZ-pCa, meanwhile,E-oCaandZ-oCadisplay different AIE activities. More importantly,C-Cawith a twisted molecular configuration succeeded in avoiding fluorescence quenching induced by intramolecular photocyclization, emitting strong fluorescence in the solid state and the PABT-doped film (PLQY = 0.65). After an intensified UV radiation of 20 h, 94% of the initial intensity is reserved, and emission peaks exhibit tiny shifts. Based on matching excitation and emission maxima (374 nm and 433 nm), high PLQY, excellent photostability and light-conversion quality in the doped film,C-Cais a potential high-performance light-conversion agent. Finally, light-conversion properties and the intrinsic mechanism are discussed in detail using the UV-vis absorption spectra, fluorescence emission spectra, crystal analysis, the photosynthetic photon flux density, the percentages of different wavelength bands, theoretical calculations and XRD patterns. This work is the first report on a highly efficient light-conversion agent with the phenanthrenecarbonitrile backbone; meanwhile, it contributes considerably to the molecular design of photostable luminogens and basic photochemistry.

Full text of DOI:10.1039/d1tc02405a

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Exploration of high-performance light conversion agent based on
cyanostilbene and phenanthrenecarbonitrile backbone: E/Z and
position isomerism, high-contrast Michael addition reaction activity and
intramolecular photocyclization
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Yongtao Wang, *a Zhean Qian,a Xueming Li a, Anjun Qin, Yanjun Guoa and Benzhong Tanga
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7
As light conversion agent, traditional organic fluorescent dyes easily suffer from solid-state fluorescence
quenching and photooxidation, leading to deterioration of application performance. To enhance fluorescence
emission and photostability of light conversion agents in doping film, in this paper, some luminogens with
cyanostilbene or phenanthrenecarbonitrile backbone were designed and synthesized based on aggregation-
induced emission (AIE) theory, E/Z isomerization and our previous research results, and characterized by NMR,
HR-MS and X ray single crystal diffraction. The experimental results indicate that both Z-oCa and Z-pCa exhibit
the high-contrast Michael addition and intramolecular photocyclization reaction activity, as well fluorescence
quantum yield in doping film, but similar E/Z photoisomerization and AIE activity. Furthermore, obviously distinct
AIE activity and configuration stability can be observed between E-oCa and Z-oCa, and E-pCa. More importantly,
C-Ca with twisted molecular configuration succeeded in avoiding fluorescence quenching induced by
intramolecular photocyclization, emitting strong fluorescence in solid state and PABT doped film (PLQY = 0.65).
After intensified UV radiation of 20h, 94% of the initial intensity is reserved, and emission peaks show tiny shifts.
Based on matching excitation and emission maxima (374nm and 433 nm), high PLQY, excellent photostability and
light conversion quality in the doping film, C-Ca is a potential high-performance light conversion agent. Finally,
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a Guangxi Key Laboratory of Electrochemical and Magneto-chemical Function Materia, College of Chemistry and
Bioengineering, Guilin University of Technology, Guilin 541004, China.
b School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, PR China
E-mail: wyt_shzu@163.com; msqinaj@scut.edu.cn; tangbenz@ust.hk.

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† Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and MALDI-TOF MS spectra
[supplemental figures]. CCDC 2055106, 2055103, 2055099 and 2070567. For crystallographic data in CIF or the
other electronic format see DOI:10.1039/c9qm00293f.
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intrinsic mechanism are discussed in detail by UV-Vis absorption spectra, fluorescence emission spectra, crystal
analysis, photosynthetic photon flux density, percentages of different wavelength bands, theoretical calculation
and XRD. The work is the first report of highly efficient light conversion agent with phenanthrenecarbonitrile
backbone, meanwhile, it contributes considerably to molecular design of photostable luminogens and basic
photochemistry.
34 Introduction
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Agricultural shed film experiments indicated light conversion agents [1-4] can improve quality and yields of crops by
transforming ultraviolet (280–380 nm) and yellow-green light (510–580 nm) into blue (400–480 nm) and red light
(600–700 nm). As light conversion agents used for agricultural film, some requirements such as matching emission
spectra with crop absorption spectra, excellent photostability, high fluorescence quantum yield (PLQY) and good
compatibility with polymer matrix, should be guaranteed. Currently, light conversion agents mainly can be divided
into three types [4-11]: rare earth sulfides, organic rare earth complexes (Eu3+, Sm3+), organic fluorescent dyes.
Among of them, rare earth sulfides show excellent photostability, but poor deliquescence resistance and dispersion,
moreover, their preparation need high temperature sintering and high purity rare earth oxide. Compared with rare
earth sulfides, organic rare earth complexes have stronger fluorescence emission and better compatibility with
polymer matrix, but it is difficult to convert the yellow green light into blue or red light. Organic fluorescent dyes
have good resin compatibility and adjustable fluorescence emission, and easily to achieve the specificity of
agricultural films according to the absorption spectra of different crops. Especially, fluorescent dyes can avoid using
high purity rare earth oxide and promote the sustainable development of agricultural economy.
Presently, anthrone, rhodamine, triphendioxazine, pyrazine, pyrimidine and perylene bisimide derivatives had
been reported to be used as light conversion agents [12-13]. Furthermore, the organic fluorescent dyes show good

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fluorescence emission in single molecular state due to large conjugated and rigid planar structure,DwOhI:e1n0.1t0h3e9/D1TC02405A
fluorescent dyes are doped in polymer matrix, they easily suffer from intermolecular π-π stacking, leading to
fluorescence quenching or reduction. Besides, organic fluorescent dyes are easy to oxidize and decompose after
long-term solar radiation, which shortens the service life of the light conversion agents and increases the
application cost. To exploit high-efficient light conversion agents, carbazolyl benzophenone, tetraphenylethene
(TPE) and its derivatives, triphenylacrylonitrile (TPA) were designed and synthesized based on aggregation induced
emission (AIE) effect, thermal delayed fluorescence and theory of plant photosynthesis[14], furthermore, the tests
of UV-Vis absorption and fluorescence emission spectra, photostability and mechanical properties showed TPA has
the best light conversion properties, moreover, it's worth mentioning that electron-withdrawing cyano group can
increase the photo stability of TPA by reducing the density of electron cloud on benzene ring. However, the PLQY
for TPA in doping film is only 0.14, which cannot meet the request of high-performance light conversion agent.
Next, a series of triarylacrylonitrile derivatives were obtained by structural optimization of TPA [15], most notably,
PLQY of one luminogen is up to 0.833 in crystalline state, but it dropped sharply to 0.107 in doping film due to
morphology dependent fluorescence emission. In addition, it had been confirmed that strong light radiation could
cause intramolecular photocyclization and the resulting aggregation-caused fluorescence quenching for TPE and
TPA [14-15]. Therefore, enhancing PLQY and photostability of light conversion agents in doping film have become
two urgent tasks to improve the light conversion performance.
Cyanostilbene derivatives show twisted molecular conformation ascribed to intramolecular repulsion between
cyano group and ortho hydrogen atoms in solution, leading to torsion-induced nonradiative deactivation and the
resulting of faint fluorescence [16], but the luminogens often present aggregation-induced enhanced emission (AIEE)
effect due to more coplanar conformation and specific molecular packing mediated by intermolecular hydrogen
bonds and weak interactions [17]. Taking full advantage of the solid-state AIEE characteristics, cyanostilbene

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backbone has been become one of the most attractive platforms for the construction of various phDoOtoI:e1l0e.c1t0r3ic9/D1TC02405A
materials [18-27]. However, few attentions have been paid to cyanostilbene backbone used as light conversion agent.
As we know that both triarylacrylonitrile and cyanostilbene backbones have similar molecular structure, but
preparation and reaction condition of the latter are facile and mild compared with that of the former, moreover,
Z-isomer is often the main product in the process of preparation of cyanostilbene derivatives due to better stability
for Z- than E-. For Z-isomer of cyanostilbene derivatives, the two aromatic rings are located on the opposite side
of the double bond, obviously, it is impossible for Z-isomer to directly generate intramolecular photocyclization
product before configuration conversion. On the one hand, light conversion agents are doped into polymer matrix
at low concentrations, which may inhibit rotation of molecular bonds, thereby intramolecular photocyclization of
cyanostilbene derivatives is not expected to occur. On the other hand, we conjecture that the photocyclization
may be inhibited by increasing the steric hindrance of E/Z isomerization, furthermore, provided intramolecular
photocyclization is inevitable for cyanostilbene backbone, molecular design is a feasible way to avoid or reduce the
fluorescence quenching in the aggregated state by maintaining the twisted molecular configuration after
intramolecular photocyclization. Based on the above-mentioned, cyanostilbene backbone is introduced into the 9-
position of the carbazole unit, maintaining the suitable fluorescence emission wavelength and twisted molecular
configuration before/after photocyclization. Moreover, isomers have been an important tool exploring the internal
mechanism of mechanochromism, polymorph, thermal delayed fluorescence and room temperature
phosphorescence [28-38]. With the help of the tiny structure discrepancy and high-contrast photochemical
properties, the key factors affecting intramolecular photocyclization are expected to be elucidated. In this paper,
five luminogens were obtained (Scheme 1) and characterized by 1H-NMR, 13C-NMR and HR-MS, which contains E/Z
and position isomers (Z-oCa, E-oCa and Z-pCa), DCN coming from Michael addition of Z-oCa and phenylacetonitrile,
and C-Ca resulting from intramolecular photocyclization product of Z-pCa. Besides, DCN, Z-oCa and E-oCa were

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confirmed by X-ray crystallography. Furthermore, E/Z isomerism, Michael addition reaction, intramolecular
photocyclization and internal mechanism were discussed in detail by NMR, crystal analysis, theoretical calculation
and optical performance testing. More importantly, C-Ca can deal with intramolecular photocyclization-induced
fluorescence quenching and morphology dependent fluorescence emission very well, emitting strong fluorescence
in solution (Tol, THF, CH2Cl2), solid-state and doping film. By a comprehensive consideration on excitation and
emission spectra, PLQY and photostability in the doping film, C-Ca can be used as an excellent light conversion
agent for agricultural film.
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101 1. Results and discussion
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2.1. Synthesis and characterization
CN
40% aqueous NaOH
+
CN
N
N
+
CHO
Z-oCa
CN
I2 hv
CN
CN
40% aqueous NaOH
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CN
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DCN
E-oCa
NC
CN
40% aqueous NaOH
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CN
N
N
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Z-pCa
I2 hv
CHO
N
N
N
CN
CN
CN
CN
E-pCa
x
C-Ca
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Scheme 1. Synthetic routes of Z-oCa, E-oCa, DCN, Z-pCa and C-Ca.
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Under polar solvent and the catalysis of inorganic or organic bases, cyanostilbene derivatives are usually prepared
by a condensation reaction of aromatic aldehydes with arylacetonitriles. Actually, five carbazole-based
cyanostilbene derivatives with prominent mechanochromic properties had been obtained in our previous study.
To improve the conversion of aldehyde and simplify the post-processing process, the feed ratio of 1:1.5 is chosen
between 4-carbazol-9-ylbenzaldehyde (4Ca) or 2-carbazol-9-ylbenzaldehyde (2Ca) and phenylacetonitrile (PAN).

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As expected, Z-pCa is obtained with high yield (95%) after purification by column chromatography. However, no Z-
oCa but DCN is separated under the same reaction conditions as above, furthermore, Michael addition product
DCN is characterized by NMR, HR-MS and single crystal diffraction (Fig. S18). When the ratio of 2Ca and PAN is
reduced to 0.8: 1, Z-oCa is obtained with satisfactory yield (92% according to feed quantity of 2Ca), from here we
see that excessive 2Ca can inhibit the formation of DCN (CCDC 2055106). According to the reaction mechanism,
Michael addition reaction of Z-oCa and PAN should depend on the electron density and steric hindrance of carbon-
carbon double bond on Z-oCa. Guillot et al [39] investigated solvent-free condensation reaction of PAN with various
aldehydes in detail, but their focus is on yield and selectivity Z/E rather than Michael addition reaction.
Nevertheless, the above experimental results show that aldehydes with electron donating group (p-OMe) and steric
hindrance (O-(CH3)2) generate Michael addition products with lower yield compared with aldehydes with electron
withdrawing group (-CCF3), especially for aldehydes with steric hindrance. By comparison, Z-oCa needs to
overcome larger steric hindrance than Z-pCa in Michael addition reaction due to the ortho substitution effect of
carbazole unit (Ca), however, the steric hindrance effect is not enough to suppress the Michael addition between
Z-oCa and PAN due to rotation of Ca substitution around C-N bond, thereby it can be inferred that electron effects
played a major role. That is to say, para-position Ca (pCa) on cyanostilbene backbone has stronger electron
donating effect on vinyl of Z-pCa than Z-oCa. More interesting, steric hindrance of ortho-position carbazole (oCa)
cannot inhibit E/Z isomerization in THF solution under UV light irradiation and catalysis of I2, but intramolecular
photocyclization does not occur. H-NMR spectra of Z-oCa and E-oCa provide some obvious evidences for E/Z
isomerization, for example, the former is more dispersed, while the latter became more clustered due to
deshielding effect and similar H-NMR shifts of two adjacent benzene rings (Fig.1). In addition, H-NMR signals at
6.97ppm and 6.88ppm should be ascribed to the vinyl hydrogen atom (C=CH) on Z-oCa and E-oCa respectively,
furthermore, which can be confirmed in H-NMR spectra of DCN with the disappearance of single peak at 6.97ppm
(Fig.S8). Before/after UV light radiation, the H-NMR shift of C=CH from 6.97ppm to 6.88ppm (in CDCl3) was
consistent with the related reported data [10], which is due to the different electron withdrawing effect of cyano
group in Z/E configurations. For Z-pCa, H-NMR signal of C=CH was located at 8.21 and 7.64 ppm [29] respectively in
DMSO and CDCl3 due to different solvent effects, indicating conspicuous disparity between E-pCa and Z-oCa, E-
oCa. Theoretically, electron acceptor will make H-NMR signals move to low field and vice versa, thus low-field shift
of H-NMR signal (C=CH) should be reasonable for Z-pCa relative to Z-oCa and E-oCa according to electron donating
ability of Ca unit at ortho- and para-position, which is consistent with the simulation results (Chemdraw 14.0), but
it is contradictory with the above test results. In order to further clarify this contradiction and characterize Z-oCa
and E-oCa, their crystal structures are determined by X-ray single crystal diffraction, which indicates the vinyl
hydrogen atoms of the two cyanostilbene derivatives are located directly above Ca unit thereby leading to shielding

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effect and high-field shift (Fig. 5A and 5D). As far as we know, separation of E/Z isomer is usually difficult, luckily,
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it is very easy to separate isomers Z-oCa and E-oCa by column chromatography, which is attribute to obviously
distinct polarity and excellent stability for E-oCa. Different from Z-oCa, both E-pCa and C-Ca are obtained
simultaneously when Z-pCa is exposed to 150w UV radiation for 6 hours, from this we can infer that the reaction
rate control step of intramolecular photocyclization is not E/Z isomerism. As shown in Fig. 1, the H-NMR signal of
C-Ca (C=CH) further moves to low-field compared with that of Z-pCa, which is attributed to the better aromaticity
for 9-phenanthronitrile. Judging from thin layer chromatography (TLC) of E-oCa, E-pCa, Z-oCa and Z-pCa, polarity
of E-configuration is greater than the corresponding Z-configuration, and discrepancy between Rf values is obvious,
in theory separation and purification of E-pCa and Z-pCa should be feasible by column chromatography. However,
E-pCa separated always contains Z-pCa after repeated column chromatography, which may be due to configuration
transformation from unstable E-pCa to stable Z-pCa. Compared with E-pCa, sterically crowded conformation
contributes to a more stable E-configuration for E-oCa. In the previous study, triphenylacrylonitrile (TPA) exhibits
better photostability than tetraphenylethylene (TPE), which is attributed to electron withdrawing of CN reduce the
[40]
electronic cloud of the benzene rings [14]. Sekar et al
analyzed the photostability of the dyes by using
electrophilicity index, which is inversely proportional to the photostability of dye. Therefore, it is plausible to
associate the photostability with the electron-donating ability of carbazole unit although fully understanding the
exact mechanism remains a challenge. The electron donating ability of Ca unit is better for Z-pCa than for Z-oCa,
which leads to photocyclization for Z-pCa rather than Z-oCa.
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Fig. 1 The H-NMR spectra of Z-oCa, E-oCa, Z-pCa and C-Ca.
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2.2. Photophysical properties and theoretical calculations
To investigate the influences of intramolecular photocyclization, position and E/Z isomers on UV-vis
absorption and fluorescence emission, various dilute solutions of Z-oCa, E-oCa, Z-pCa and C-Ca were prepared
(1 × 10-5 mol L-1). As shown in Fig.2 and Table S1, four luminogens give intense absorption and multiple
absorption peaks in the region of 200-400nm, indicating excellent UV absorption ability. Moreover,
absorption maxima of the luminogens exhibit tiny shift ascribed to the small ground state dipole moment in

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various solvents. Moreover, bathochromic-shifts of absorption maxima are not always consistent with solvent
polarity, which may be due to the different interactions between solute and solvent, especially for Z-oCa in
DMSO, as well Z-pCa and C-Ca in n-Hexane and Toluene. Based on the initial absorption edge, optical band
gaps of Z-oCa (3.53), E-oCa (3.57), Z-pCa (3.01) and C-Ca (3.18) are calculated in toluene respectively, by
contrast, Z-pCa and C-Ca possess the smaller optical band gap compared with Z-oCa and E-oCa, which is
attributed to longer conjugation of the former than the latter, but the optical band gaps are almost the same
between E/Z isomers, hinting the configuration transition have weak influence on the molecular conjugation,
furthermore, intramolecular photocyclizations leads to obviously reduced molecular conjugation from Z-pCa
to C-Ca. Compared with absorption spectra, fluorescence emission spectra of the luminogens show the
significant stokes shifts in various solvents, which can be understood as the enhanced dipole moments for
excited states than ground states. Moreover, the maximum fluorescence emission wavelengths of Z-oCa and
E-oCa exhibit 22-60nm red-shift compared with that of Z-pCa in the same solvents, which may be due to the
more twisted molecular configuration that reduces the overlap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), producing the stronger ICT effect for Z-
oCa and E-oCa than Z-pCa. Moreover, the relative fluorescence quantum yields (PLQYs)of the luminogens are
determined in various solvents by using quinine sulfate as a reference (Table S1). The results show that PLQYs
of the luminogens in n-hexane are lower than that of other solvents due to different excited state, and the
next is DMSO, whose decreased emission are attributed to significantly increased solvent polarity, lastly, the
abnormal PLQYs are obtained for Z-oCa (8.60%) and Z-pCa (6.92%) in Tol and E-oCa in DCM (4.44%), which
may be due to the different interactions between solute and solvent. Most notably, C-Ca shows the higher
PLQYs than Z-pCa in the above test solvents, therefore, we speculated that photocyclization can reduce non-
radiative energy loss by inhibiting intramolecular rotation and rotation. In order to further prove the existence
of charge transfer transition and make a better comparison, density functional theory calculations on a single
molecule were conducted by extracting data from crystals Z-oCa, E-oCa and Z-pCa, but the ground-state
geometries of C-Ca are calculated and optimized by employing the Gaussian 09W program package in
B3LYP/TZVP level, and the energy levels and frontier molecular orbitals are being plotted in Fig. 3 employing
Multiwfn program [41]. As shown in Fig. 3, the luminogens exhibit twisted molecular conformations, and the
highest occupied molecular orbital (HOMO) is mainly located on carbazole moiety and the lowest unoccupied
molecular orbital (LUMO) is mainly distributed on acrylonitrile units, outlining the obvious shift of electron
cloud density. Notably, it is reasonable that the smaller the energy gap, the more red-shifted crystal emission
for Z-oCa, E-oCa and Z-pCa, but it's an exception between C-Ca and E-oCa due to the neglected intermolecular
interactions for C-Ca.

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Fig. 2 Normalized UV-vis absorption spectra of (A) Z-oCa, (C) E-oCa (E) Z-pCa (G)C-Ca in various solvents.
Normalized fluorescence spectra of (B) Z-oCa, (D) E-oCa (F) Z-pCa (H)C-Ca in various solvents (solution
concentration: 10 mM, excitation wavelength: 315nm for Z-oCa and E-oCa, 350nm for Z-pCa and C-Ca).
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Fig. 3. The optimized geometry, HOMO and LUMO energy level diagrams for Z-oCa, E-oCa, Z-pCa and C-Ca.
[42-43]
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The reported literatures
indicate that molecular arrangement and stacking modes affect fluorescence
emission of luminogens, which is often tuned by morphology, polymorph and phase transition. However, as light
conversion agent for agricultural film, emission spectra and maxima of the cyanostilbene derivatives need remain
steady or a narrow variation range in the different solid states. For these reasons, we investigate fluorescence
emission of the luminogens in different solid-state forms by crystallization, grinding, heating, solvent fumigation
and doping (Fig. 4 and Table S2). Firstly, crystal Z-oCa, E-oCa and Z-pCa are obtained by solvent diffusion method
from n-hexane/CH2Cl2 solution, and the doping films are prepared according to weight ratio of 1:100 between

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luminogens and polyvinyl chloride (PVC), meanwhile, the blank PVC film is prepared following as control group.
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Compared with crystalline state, emission maixma of Z-oCa, E-oCa and Z-pCa show blue shift of 1-14 nm after
grinding, heating and CH2Cl2 fumigation, furthermore, Z-oCa (2 nm) and Z-pCa (18 nm) show blue-shifted
fluorescence in PVC-doped films, while E-oCa shows red-shift of 5 nm. In general, the solid-state emission peaks of
the three luminogens are positioned within a narrow range, especially for Z-oCa and Z-pCa due to more rigid
molecular skeleton, which are desirable as light conversion agents, but not good for use as mechanochromic
[32]
materials. Gayathri et al
designed and synthesized Z-pCa and its analogues by halogen-substituted positional
isomers, and explored their solid-state fluorescence and mechanochromism, where Z-pCa shows enhanced
fluorescence with a blue shift emission by heating, which is consistent with our experimental results, while grinding
leads to red-shift of fluorescence emission rather than blue-shift mentioned above. By contrast, crystal Z-pCa is
[32]
grown from methanol/CH2Cl2
instead of n-hexane/CH2Cl2 solution, and the different solvents lead to
polymorphism emitting distinct fluorescence for crystal and ground Z-pCa. By the means of solvent diffusion and
slow evaporation, crystal C-Ca is not obtained, thereby its fluorescence emission spectra are investigated in pristine
solid-sate and doping film. Fig. 3C shows that emission maxima of pristine C-Ca are located at 433 nm, which is the
shortest fluorescence emission wavelength among of the luminogens, whose emission maxima is redshifted to 449
nm in the doping film. Considering absorption maxima of chlorophyll a (430 nm) and b (450 nm), emission maxima
of Z-oCa, E-oCa, Z-pCa and C-Ca are well matched with absorption of crops in the doping films. Furthermore, the
excitation spectra of the four luminogens cover the area of 250-425 nm, containing three main excitation bands at
wavelengths of about 292 nm, 325 nm and 370 nm for Z-oCa and E-oCa, 262 nm, 290 nm and 375 nm for C-Ca, 262
nm, 290 nm and 395 nm for Z-pCa, which confirmed the UV light conversion ability of the luminogens once again.
For comparison, all the luminogens are excited at the same wavelength, the absolute PLQYs of the luminogens are
obtained in the PVC film by adopting an integrating sphere method (excitation wavelength, Ex = 320 nm). The
results indicate that Z-pCa (0.50) and C-Ca (0.47) exhibit higher PLQYs than Z-oCa (0.15) and E-oCa (0.18), and
probably the more distorted conformation deteriorates the delocalization of carrier for Z-oCa and E-oCa, notably,
PLQYs of the luminogens are better than that of the best triarylacrylonitrile (0.11) reported in our previous study
[15]. To further explore the intrinsic mechanism of fluorescence emission in different solid-state forms, XRD of Z-
oCa, E-oCa and Z-pCa were investigated (Fig. S1). The main diffraction peaks are almost identical for samples Z-
oCa and E-oCa before/after heating, fuming and grinding, which indicate crystalline characteristics, therefore the
emission shifts should be due to different crystal size or surface morphology. After grinding, strong and sharp
diffraction peaks nearly disappeared for Z-pCa, revealing phase transition from crystalline state to amorphous state,
which leads to blue shift of fluorescence emission. By heating and fuming, ground Z-pCa returns to the crystalline
state accompanied by varied diffraction peaks, thereby the shift of emission peaks may be due to different

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intermolecular spacing.
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Fig. 4 Normalized fluorescence spectra of (A) Z-oCa, (B) E-oCa, (C) Z-pCa in different solid states and PVC-doped
films, and C-Ca in PVC-doped film (doping mass fraction: 1%, and excitation wavelength: 325 nm), (D) Normalized
excitation spectra of Z-oCa, E-oCa, Z-pCa and C-Ca in PVC-doped films. (doping mass fraction: 1%, and emission
wavelength: emission maxima).
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2.3. Aggregation-induced emission
Traditional dyes usually have strong fluorescence emission in solution, but aggregation often causes fluorescence
to weaken or quench and fail to use in the field of organic electroluminescent devices and biological imaging. Tang
et al found an abnormal phenomenon named as aggregation-induced emission (AIE). Generally, AIE-active
luminogens have twisted molecular conformation, which can inhibit π-π stacking ascribed to a critical factor of
aggregation-caused quenching. In consideration of the cyanostilbene derivatives with rotatable C-C/C-N single
bonds and twisted molecular conformations, potential AIE properties should be observed by choosing THF and
water as benign and poor solvents respectively. With the gradual addition of water, the initial fluorescence
emission intensity of Z-oCa and E-oCa continually decrease, accompanying with red-shift of emission maxima. The

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above phenomenon should be attributed to the increase of ICT effect and solvent polarity with theDcOoIn: t1i0n.1u0e3d9/D1TC02405A
addition of water, which leads to bathochromic emission and the resulting reduction in emission intensity
according to the energy rules. When volume ratio of water/THF increases to 80%, fluorescence emission of Z-oCa
and E-oCa began to increase, moreover, emission maxima move towards short wavelength. Furthermore,
enhanced emission of Z-oCa continues to water fraction of 95% from 80%, while E-oCa showed a slight decline at
water fraction of 95%. Obviously, aggregation plays a major role in this process, which reduced the polarity around
the luminogens resulting in hypsochromic shift of emission wavelength, while molecular rotation and vibration is
confined in aggregated state, which induces enhancement of radiative transitions. As previously reported, particle
radius and morphology of aggregation affect emission intensity, in general the tight intermolecular stacking is
beneficial to the enhancement of fluorescence emission and vice versa [44], thereby emission intensity of Z-oCa and
E-oCa present the opposite trend at water fraction of 95%. For comparison, initial emission intensity of Z-pCa is
lower than Z-oCa and E-oCa duo to different structural rigidity, obviously, ortho substitution has better inhibition
effect on molecular motion than para substitution in THF. Different from Z-oCa and E-oCa, emission intensity of Z-
pCa is almost a horizontal line at water fraction from 0% to 80%, indicating more active molecular motion for Z-
pCa. When water fraction increases to 95%, Z-pCa shows the most striking AIE activity among of four dyes, which
is due to Z-pCa has a more flexible structure than other molecules. Thus, the fluorescence intensity contrast is
more obvious before/after the formation of aggregated species. Among of the luminogens, C-Ca exhibits the most
complex fluorescence fluctuation. Firstly, reduced emission is observed with the increase of water fraction from 0%
to 20%, which is attributed to solvent polarity. Then, emission intensity reaches the maximum at water fraction of
30% due to aggregation. Next, emission intensity continued to decrease until water fraction is 95%. Furthermore,
emission wavelength continuously redshifts during the whole AIE test process, illustrating intermolecular J-type
stacking rather than H-type stacking in the aggregated state. More importantly, C-Ca always maintains strong
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fluorescence emission from solution to aggregated state although the twisted acrylonitrile unit is transformed into
planar phenanthrene, as far as our knowledge is concerned, there should be a larger dihedral angle between
carbazole and phenanthrene, thereby the donor-acceptor type molecule with twisted molecular configuration can
avoid serious π-π stacking. In general, Z-oCa and Z-pCa show significant AIE activity, while AIE activities of isomers
Z-oCa and E-oCa are obviously different, more importantly, C-Ca with multi-state emission can not only be used
in solution, but also in aggregated-state and solid-state.

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Fig. 5 Fluorescence emission spectra of (A) Z-oCa, (C) E-oCa (E) Z-pCa (G)C-Ca in the THF/water mixtures with
different fractions of water. Solution concentration: 10 μM. Chart of relationship between FL peak intensity and
water fraction for (B) Z-oCa, (D) E-oCa (F) Z-pCa (H) C-Ca. Solution concentration: 10 μM. (Inset: Photographs in
THF/water mixtures with different fractions of water under 365 nm UV illumination).
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Fig. 6 Single crystal structure of (A) Z-oCa, (D) E-oCa and (G) Z-pCa, molecular stacking mode of (B) Z-oCa, (E) E-
oCa and (H) Z-pCa and intermolecular hydrogen bonds and weak interactions (C) Z-oCa, (F) E-oCa and (I) Z-pCa.
1.4. X-ray crystal structure
To further understand the optical properties of E/Z and position isomers, the single crystal of Z-oCa (CCDC 2055103),
E-oCa (CCDC 2055099) and Z-pCa (CCDC 2070567) were obtained from a mixed solution of dichloromethane/n-
hexane by gradually evaporating the solvents at room temperature. As shown in Fig 6, the crystal Z-oCa is a triclinic
system with a space group P-1 [a = 7.8122(3) Å, b = 9.0769(3) Å, and c = 14.8557(7) Å; α = 73.323(1)°, β = 89.014(2)°,
and γ = 73.741(1)°], and the dihedral angles between ring A-B, A-C and B-C are 64.25o, 66.84o and 61.75o in turn,
showing twisted molecular conformations. While the crystal E-oCa is a monoclinic system with a space groups P1

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21/c 1 [a = 10.9957(5) Å, b = 16.7406(7) Å, and c = 11.2443(7) Å; α =90°, β = 107.548(5)°, and γ = 90°], whose
dihedral angles between ring A-B and A-C are 59.85o and 53.10o respectively, indicating the reduction of the twisted
degree for molecular conformation compared with that of Z-oCa, which leads to hypochromic shift (6 nm) of
fluorescence emission maxima for E-oCa than Z-oCa. In a unit cell, there are two Z-oCa molecules adopting
antiparallel arrangement, but a unit cell of E-oCa contains four molecules arranged like a windmill and linked
together by four intermolecular C-H···C hydrogen bonds (2.865 Å and 2.867 Å), moreover, the interconnected
carbazole units adopt T-shape arrangement. For a single-molecule, Z-oCa forms thirteen intermolecular hydrogen
bonds and weak interactions with its surrounding molecules (2.178, 2.789, 2.813, 2.841, 2.868, 2.859 and 2.885
Å), but the number of intermolecular interactions is reduced to eight (2.800, 2.865, 2.867, 3.368 and 2.800 Å) for
E-oCa, meanwhile, crystal Z-oCa (1.273 g cm-3) has a much greater density than E-oCa (1.247 g cm-3), more
importantly, no π-π stacking is found in the two crystals. The stronger and more intermolecular interactions
combined with higher density can effectively reduce the energy loss caused by molecular motion, thereby
endowing crystal Z-oCa the higher PLQY than crystal E-oCa. However, E-oCa exhibits slightly higher PLQY compared
with Z-oCa in PVC-doped films, which may be due to the amorphous rather than the crystalline state for Z-oCa and
E-oCa. Furthermore, the twisted conformation and intermolecular arrangement modes of E-oCa and Z-oCa
contribute to strong solid-state emission, and limit the extension of conjugation length, leading to blue-shift of
emission maxima in crystalline state compared with solution. Compared with Z-oCa and E-oCa, the molecular
conformation of Z-pCa tends to be planar, especially between ring B-C, resulting in red- shifted crystalline state
emission. Meanwhile, Z-pCa presents windmill-like intermolecular arrangement, suppressing intermolecular π-π
stacking, which, combined with more intermolecular weak interactions and higher density (1.272 g cm-3), indicates
strong emission.
2.5. Photostability
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Under the present of iodine and argon atmosphere, photostability of Z-oCa and Z-pCa have been investigated by
150 W high-pressure mercury lamp emitting a near-UV radiation of 365 nm wavelength in anhydrous THF solution.
Different from the above-mentioned, light conversion agents are doped in polymer matrix by melting and blown
film, or directly coating on them in industry. Moreover, the real application environment is atmospheric
environment rather than oxygen-free, and iodine catalysis is not involved. In order to simulate the real application
environment and accelerate photodegradation, the PVC-doped films are directly attached to the quartz
photoreactor with 150 W high-pressure mercury lamp for 6h, as a result, the doping films of Z-oCa, E-oCa and Z-

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pCa become yellow and brittle, accompanying the rapidly reduced emission intensity (Fig S2). TheDsOtrIo: 1n0g.10U3V9/D1TC02405A
radiation promotes aging and degradation of PVC, which affect light transmittance of light conversion film and
release hydrogen chloride, leading to accelerated degradation of light conversion agents and rapidly reduction of
emission intensity. Obviously, PVC without antiaging agents is not suitable for light conversion film. By evenly
spreading power Z-oCa, E-oCa, Z-pCa and C-Ca on the adhesive tape (polypropylene, PP), the doping films of four
luminogens were prepared and radiated by the above strong UV light. After irradiation for 6 hours, the light
conversion agents on the films were dissolved in CH2Cl2 and detected by TLC. Unexpectedly, not only
intramolecular photocyclization products were not found for Z-oCa and E-oCa, but also E/Z configurations
remained stable without mutual conversion. As Z-oCa and E-oCa, only C-Ca without E-pCa is detected for Z-pCa
film, while C-Ca exhibit the best photostability without new product, thereby it can be inferred that the
intramolecular photocyclization exists different reaction mechanism in solution and solid-state, and the addition
of iodine contributes to the Z- to E-configuration conversion. The photostability test results show that Z-oCa, E-
oCa and C-Ca have excellent photostability, which can be used as potential light conversion agents for agricultural
film. More importantly, transmittance and flexibility of adhesive tape remain intact after intense light exposure for
6h, therefore, the influence of the film material itself on the light conversion performance should not be ignored.
Considering the higher PLQY, Z-pCa or C-Ca and PBAT (polyurethane) are dissolved in DCM according to the weight
ratio of 1:100, and then the doping films are prepared by coating the above mixture on the adhesive tape. One
side of the adhesive tape is selected as the light receiving surface, under UV radiation (50 W), the fluorescence
emission intensity of the doping films is evaluated at different times. As shown in the Fig. 7, the initial emission
intensity of Z-pCa film displays slight decline after 20h, and emission peaks are unchanged. For C-Ca, 94% of the
initial intensity is reserved, but emission peaks show tiny shifts, which is attributed to slight deformation of light
conversion film during the test. Furthermore, emission maxima of C-Ca (433 nm) perfectly match with the
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absorption spectra of chlorophyll a (430 nm) in doping film, while emission maxima of Z-pCa (475 nm) pDroOdI:u1c0e.1r0e3d9/D1TC02405A
shift of 45 nm and 25 nm relative to chlorophyll a and b, respectively. Besides, the blue shifted excitation maxima
also contribute to the more efficient conversion of UV light for C-Ca (374 nm) relative to Z-pCa (397 nm). By a
comprehensive consideration on excitation and emission maxima, PLQY (0.65, Ex = 374 nm) and photostability in
the PBAT doped film, C-Ca is the best light conversion agent in the luminogens.
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Fig. 7 The excitation and emission spectra of luminogens in PBAT doped films under UV for different hours. (A)
excitation spectra (B) Z-pCa and (C) C-Ca.
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2.6. Light conversion quality
To further investigate light conversion performance of C-Ca film (the above-mentioned PBAT film), firstly, the
spectral intensities of blank and C-Ca film are measured from 380 to 780 nm by HiPoint HR-450. Fig. 8A shows the
intensity before 400 nm is obviously reduced for C-Ca film than blank film, while the intensity at 400-500 nm
increased significantly, which confirm that C-Ca film can convert UV light to blue light. Surprisedly, C-Ca film also
presents higher illuminance than blank film at 500-780 nm, which should be attributed to the twisted molecular
configuration of C-Ca enhancing transmittance of sunlight. Secondly, we investigate the photosynthetic photon
flux density (PFD) at different wavelengths, which are divided into five bands including ultraviolet (380-400 nm),
blue (400-500 nm), green (500-600 nm), red (600-700 nm) and infrared wavelengths (700-780 nm). As shown in
Fig. 8B and Table S3, except for UV band, PFD of C-Ca film is 1.35-1.5 times closer than that of blank film, which is
consistent with the results of Fig. 8A. Next, the spectral composition of the two films is analyzed. Fig. 8C and Table
S4 indicate that the percentages of UV and blue light among the whole waveband are 2.19% and 25.31% for the
blank film respectively, while that for C-Ca film are 0.93% and 27.76% in turn, indicating 1.26% reduction for UV
and 2.45% enhancement for blue light. That is to say, the blue composition is obviously improved by UV light
conversion. Compared with UV and blue light, composition of green, red and infrared wavelengths show small
variation. Beyond doubt, the results clearly present light transformation effect of C-Ca film. Besides, C-Ca film (2928
umol/(m2.s)) shows excellent light transmittance from 380 to 780 nm , whose PFD reaches 90% that of sunlight
(3253 umol/(m2.s)).

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Fig. 8 (A) Light quality measured under the bank and C-Ca film; (B) PFD-UV, visible and infrared light of the bank
and C-Ca film;(C) UV, visible and infrared light composition of the bank and C-Ca film.
414 2. Conclusions
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In summary, Z-oCa and Z-pCa with cyanostilbene backbone were designed and synthesized to explore high-
efficient light conversion agents for agricultural film, further, E-oCa, C-Ca and DCN were obtained by
photoisomerization, intramolecular photocyclization and Michael addition, and characterized by 1H-NMR, 13C-NMR,
HR-MS and X-ray crystallography. Z-oCa and Z-pCa show similar E/Z isomerism, but high contrast Michael addition
reaction activity and intramolecular photocyclization due to different electron-donating ability between -oCa and
-pCa. Z-oCa and E-oCa are easily separated by column chromatography, but E-pCa is difficult to be purified from
the mixture of E- and Z-configurations which are attributed to stronger molecular rigidity for E-oCa than E-pCa.
Furthermore, more distorted molecular configuration leads to excitation maxima of blue shift and reduced PLQY
for Z-oCa and E-oCa than Z-pCa and C-Ca in doping film, meanwhile, Z-oCa and E-oCa have close emission maxima
and PLQY in PVC doped films, showing unconspicuous E/Z isomerization effect. The strong UV radiation leads to
rapid aging and degradation of PVC, but adhesive tape remains excellent transmittance and flexibility after strong
UV radiation, obviously, photostability of film materials itself is an important factor affecting the light conversion
performance. By coating the mixture of Z-pCa or C-Ca and PBAT on adhesive tape, Z-pCa and C-Ca films show
excellent photostability, and more than 94% of the initial intensity is reserved for Z-pCa and C-Ca films after UV
radiation of 20h, becides, C-Ca films compared with the blank film can significantly improve transmittance of
sunlight. More importantly, C-Ca succeeded in avoiding fluorescence quenching induced by intramolecular
photocyclization, emitting strong fluorescence in solution, aggregated state, solid-state and doping film due to
maintaining twisted molecular configuration after intramolecular photocyclization. Considering high PLQY,
excellent photoability, suitable excitation maximum, matched fluorescence emission with chlorophyll a and b in
the doping film, easy preparation and low cost, C-Ca are best light conversion agents for agricultural film in the
luminogens. The last but not the least, this is the first report of highly efficient light conversion agent with
cyanostilbene backbone, which will contribute considerably to material design of stable E/Z isomers and high-
performance light conversion agent, internal mechanism of Michael addition reaction activity and intramolecular

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photocyclization, as well basic photochemistry.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 21766030 and 21566034),
Guangxi Natural Science Foundation under Grant (No. 2020GXNSFAA159147) and Guilin University of Technology
Research Fund (Grant No. GUTQDJJ2019038 and GUTQDJJ2018052).
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