Facile preparation and investigation of the properties of single molecular POSS-based white-light-emitting hybrid materials using click chemistry

Article abstract of DOI:10.1039/c7nj03692b

Recently, organic white light emitting devices (OWLEDs) have attracted great interest in flexible displays and solid state lighting devices. Here we report a kind of novel POSS-based white-light-emitting single molecular nanohybrid (POSS-WLED), which was

Full text of DOI:10.1039/c7nj03692b

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Facile preparation and properties of single molecular POSS-based
white-light-emitting hybrid materials by click chemistry
a
a
b*
a
a*
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Gang Zhao, Yakun Zhu, Shanyi Guang, Fuyou Ke, Hongyao Xu
a
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State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College
of Material Science and Engineering, Donghua University, Shanghai 201620, China
School of Chemistry, Chemical and Bioengineering, Donghua University, Shanghai, 201620,
China
b
College of Material Science and Engineering & State Key Laboratory of Chemical Fibers and
Polymeric Materials modification, Donghua University, Shanghai 201620, China.
*Corresponding Author: Professor Hongyao Xu.
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Tel: +86ꢀ21ꢀ67792874, Eꢀmail: hongyaoxu@163.com
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ABSTRACT: Recently, Organic white light emitting devices (OWLEDs) have attracted great
interest in the flexible displays and solid state lighting devices. Here we report a kind of novel
POSSꢀbased whiteꢀlightꢀemitting single molecular nanohybrid (POSSꢀWLED), which was precisely
controllably prepared via click chemistry by simple controlling the feed ratio of blue (B) and yellow
(Y) emitting units. It was found that the selfꢀabsorption of emitting component and the
intramolecular energy transfer was well adjusted based on theoretical simulation and molecular
design. The incorporation of nanosized inorganic POSS effectively restrained intramolecular rotation
(RIR) and shows significantly decoupling effect of emitter and AIE (aggregation induced
enhancement) effect, which provided an important contribution to high emission efficiency of hybrid
molecules. The resultant nanometer organicꢀinorganic hybrid (W62) exhibited significantly enhanced
emission in the solid film (Φfilm=95%) because of significantly AIE effect which is attributed to the
incorporation of nanoꢀsized POSS moiety. Herein, this work will provide a novel strategy for design
and preparation of highly efficiency whiteꢀlightꢀemitting molecules with high emission efficiency,
thermal stability, and well film formability.
1. Introduction
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Nowadays, single molecular organic white light emitting materials, as a novel type luminescent
medium, have become increasingly popular in contemporary research owing to their potential for
applications in flexible displays and solid state lighting sources [1ꢀ8]. Compared with
multicomponent molecular emitters of whiteꢀlightꢀemitting devices (WLEDs), single molecular
organic white light emitters have exhibited some prominent advantages such as perfect color
reproducibility and stability [9ꢀ17]. In the previous literatures, whiteꢀlightꢀemitting single molecules
are mainly focused on polymers, which are composed of covalently linked different emitter
components on the backbone or in side groups. The white light emissions in these polymers are
generated via utilizing partial energy transfer from a wider bandꢀgap donor component to a narrower
bandꢀgap acceptor [18ꢀ22]. However, the inherent poor purity, low multiꢀpolydispersity, and low
thermal properties of polymers have greatly limited their luminous efficiency and practical
application. Hence, design and preparation of the organic whiteꢀlightꢀemitting macromolecule with
high thermal properties and well luminous efficiency will be a key issue for their application in the
flexible display. However, the inevitable energy transfer makes the molecular design difficult. As the
best knowledge of us, only Park and his coworkers [23,24] reported an excitedꢀstate intramolecular
proton transfer molecule composed of covalently linked blueꢀ and orangeꢀlightꢀemitting moieties,
which energy transfer was entirely frustrated and white luminescence was achieved. However, small
organic molecules often exhibited low thermal properties.
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Accordingly, it is a challenge to seek an alternation material system that combines the benefits of
organic small molecules (e.g., high purity), inorganic moiety (high thermal stability) and polymers
(e.g., good film forming ability and low processing cost). The discovery of polyhedral oligomeric
silsesquoixanes (POSS) with special structure provided an important chance to solve these problems.
POSS is a nanometerꢀsized cubeꢀlike molecule with inorganic core surrounded by eight organic
corner groups (active or inert), which has been demonstrated to be an excellent platform and building
block for architecture of novel 3D organicꢀinorganic molecular hybrid materials with a number of
desirable properties [25ꢀ31]. Rigid POSS cage enables them to significantly improve the thermal and
mechanical properties of the materials. In our previous work, many POSSs containing
organicꢀinorganic molecular hybrids were designed and prepared for investigation of preparation
methods of these hybrids and enhancement mechanisms of their thermal properties [29ꢀ31,32ꢀ33].
Moreover, some functional hybrids with high thermal stability and nonlinear optical properties were
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prepared and the enhancement mechanisms of optical properties were investigated [34,41,42]. It was
found that a suitable molecular design will effectively improve the thermal stability and optical
properties.
In this paper, based on theoretical simulation, three POSSꢀbased luminous single molecular
nanohybrids containing a yellow monochromatic emitter (Y) with a large Stokes shift and a blue
monochromatic emitter (B) were designed and prepared, where both emitters possess similar
absorption spectra to avoid the strong selfꢀabsorption between different emitter. The partial energy
transfer from B to Y is allowed for effectively adjusting the balance of the white light emission. It is
hopefully realized that the incorporation of nanosized inorganic POSS can inhibit molecular
aggregation and improve the luminous efficiency. In addition, emission mechanism of white light of
single nanohybrid molecule was also investigated in detail on the basis of experimental and
theoretical method.
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Scheme 1. The synthetic route of the blue monomer (B) and the yellow monomer (Y).
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Scheme 2. The synthetic route of whiteꢀlight hybrids.
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2. Experimental
2.1. Materials
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2 3 2
Unless otherwise noted, all commercial reagents were used as received. PdCl (PPh ) ,
9ꢀbromoanthracene, 1,2ꢀdimethoxyethaneand ethynyltrimethylsilane were purchased from J&K
Scientific LTD. NaN , 4ꢀbromobenzaldehyde and CuI were purchased from Shanghai Jiachen
3
Chemical Plant. Acetylacetone, NaOH, tetrabutylammonium bromide, DMSO, 1, 4ꢀdioxane, KF,
CuBr, PPh
3
and THF were purchased from Shanghai Chemical Reagent Company.
Octakis(azidomethyl)(dimethylsiloxy)octasilsequioxane (ODZMS) were obtained according to the
literature procedure [35]. The TEA and THF solution was distilled immediately prior to use.
2.2. Instruments
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FTIR spectra were recorded as KBr disk by a Nicolet NEXUS 8700 FTIR spectrometer. H NMR
spectra were collected on a Bruker DMXꢀ400 spectrometer using chloroform (CDCl
3
) as solvent.
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Si NMR measurements were obtained at room temperature using a Bruker DMXꢀ400 spectrometer
at a resonance frequency of 79.49 MHz. Matrix assisted laser desorption ionization time of flight
mass spectrometry (MALDIꢀTOF MS) was carried out using a VoyagerꢀDE RP in linear and
reflection modes. The optical properties of the samples were measured by Shimadzu UVꢀ265
spectrophotometer and FP6600 Fluorescence spectrometer with a 1 cm quartz cell in tetrahydrofuran
ꢀ5
(THF, 1×10 M). Elemental analysis (EA) was carried out by a CHNOS Elemental Analyzer, vario
EI Ⅲ. DSC (differential scanning calorimetry) was performed on a TA instruments DSC 9000
ꢀ1
equiped with a liquid nitrogen cooling accessory unit under a continuous purge (50 mL min ). The
o
ꢀ1
o
scan rate was carried out at a heating of 10 C min within the temperature range 20ꢀ300 C
thermogravimetric analysis (TGA) were obtained by a PerkinꢀElmer TGA under an nitrogen
o
ꢀ1
atmosphere at a heating rate of 10 C min . The atomic force microscope (AFM) images of films
were obtained by trapping mode on Agilent 5500 AFM.
Quantum Efficiency: Photoluminescence excitation (PLE) was defined through a modification of
relative approach to describe via Crosby and Demas using 1,10ꢀdiphenylanthracence as references.
The absorption at 365 nm was established for each materials at different concentrations (maximum
absorption of 20%). Then these samples were diluted through equal number so as to prevent excimer
formation and fluorimeter saturation, and the whole area of the emission spectrum was calculated.
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For each solvent, absorption and emission measurements were duplicated at least two times and
averaged. The slop of a plot of emission versus absorption was ascertained for each materials, and
2
according to the equation ΦPL(x)=(A
s
/A
x
)(F
x
/F
s
)(n
x
/n
s
) ΦPL(x) was calculated the relative quantum
efficiency.
Where x is the sample to evaluated, s the reference, A the absorption at the excitation wavelength,
F the entire integrated emission, n the refraction index of the solvent, and the quantum ΦPL yield.
The materials of PLQE (Φ) in film was collected through 9,10ꢀdiphenylanthrancene (scattered in the
ꢀ
3
PMMA film with a concentration lower than 1×10 M and a PLQE of 83%) as a standard.
2.3. Synthesis
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The synthesis of the monomers and hybrid materials is shown in Scheme 1 and Scheme 2,
respectively. The experimental details are described as follows:
2.3.1. Synthesis of 2ꢀmethylꢀ6ꢀtertbutylꢀ4Hꢀpyrone (2).
The compound 2 was obtained according to the literature procedure [36]. The isolated yield of the
1
product was 80 %. H NMR (400 MHz, 298 K, CDCl
3
): δ = 6.16 (s, 1H, J=2.17 Hz, PrꢀH), 6.07 (s,
ꢀ1
1H, PrꢀH), 2.43 (s, 3H, CH ), 1.35 (s, 9H, C(CH ) ). FTIR (KBr), ν (cm ): 3070 (w, C=CꢀH), 2972,
3
3 3
2932, 2871 (m, ꢀCH), 1655 (s, C=O).
2.3.2. Synthesis of 2ꢀ(2ꢀtertbutylꢀ6ꢀmethylꢀ4Hꢀ4ꢀpyranyl) malononitrile (3).
The compound 3 was prepared by between compound 2 and malononitrile according to literature
1
procedure [36]. H NMR (400 MHz, 298 K, CDCl
3
): δ = 6.49ꢀ6.50 (m, 2H, PrꢀH), 2.27 (s, 3H, CH
3
),
ꢀ1
1.23 (s, 9H, C(CH
3
)
3
). FTIR (KBr), ν (cm ): 2970, 2879 (m, ꢀCH ), 2212 (s, ꢀCN), 1652 (C=C),
3
1342, 1275, 1201 (m, CꢀOꢀC).
2.3.3. Synthesis of 4ꢀtrimethylsilyl ethynylbenzaldehyde (4).
1
The compound 3 was also prepared based on the reported literature in prior [37]. Yield: 85%. H
NMR (400 MHz, 298 K, CDCl ): δ = 9.93 (s, 1H, CHO), 7.74ꢀ7.76 (d, 2H, J=8.3 Hz, ArꢀH),
3
ꢀ1
7.52ꢀ7.55 (d, 2H, J=8.3 Hz, ArꢀH), 0.20 (s, 9H, CH ). FTIR (KBr), ν (cm ): 2961 (SiꢀCH), 2830,
3
2732 (HC=O), 2159 (C≡C), 1705 (C=C), 1251, 845 (SiꢀC).
2.3.4. Synthesis of 4ꢀethynylbenzaldehyde (5).
1
Compound 5 was synthesized using a modified literature procedure [38]. Yield 80%. H NMR
4
4
(400 MHz, 298 K, CDCl
3
): δ =9.95 (s, 1H, H ), 7.77ꢀ7.79 (d, 2H, J=8.2Hz, H ), 7.56ꢀ7.58 (d, 2H,
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J=8.2Hz, H ), 3.23 (s, 1H, H ). FTIR (KBr, cm ): 3293 (s, C≡CH), 2107 (C≡C), 1666 (HC=O).
2.3.5. Synthesis of 2ꢀ(2ꢀtert butylꢀ6ꢀ(4ꢀalkynyl styrene)ꢀ4Hꢀ4ꢀsub pyranyl) two propylene nitrile
(Y).
(1.07 g, 5 mmol) 3, (0.68 g 5.2 mmol) 5 and 0.2 mL pyridine was dissolved in the anhydrous 50
mL acetonitrile and refluxed for 24 h. After cooling the room temperature, Y of the resultant
precipitation was filtered and rinsed with enough acetonitrile. The crude was recrystallizated by
1
using the mixture solution of THF and ethanol give to yellow crystal Y in yield 70%. H NMR (400
2
ꢀ6
7
8,9
3
MHz, 298 K, CDCl ): δ = 7.43ꢀ7.49 (m, 5H, H ), 7.28ꢀ7.32 (d, 1H, H ), 6.64ꢀ6.64 (m, 2H, H ),
1
10
ꢀ1
3.23 (s, 1H, H ), 1.31 (s, 1H, H ). FTIR (KBr), ν (cm ): 3243 (s, C≡CH), 2970, 2926, 2855 (w,
1
3
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2
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2
3
CH
), 2103 (C≡C), 1650 (HC=O), 1508 (C=C). C NMR (100 MHz, 298 K, CDCl
): δ =32.54,
3 3
35.51, 70.12, 81.43, 42.38, 90.83, 112.54, 115.58, 116.63, 121.97, 127.74, 132.26, 134.41, 164.57,
+
+
178.43. MALDIꢀTOF MS [C H N O+2H] : Calcd (M+2H) /z, 326.1403; found M/z), 326.1675
2
2
18
2
2.3.6. Synthesis of 9ꢀtrimethylsilylacetylene anthracene (6).
Compound 6 was prepared in analogy to a literature procedure [39]. Yield 85%. H NMR (400
1
1
,5
2
MHz, 298 K, CDCl ): δ = 8.94ꢀ9.01 (m, 3H, H ), 8.51ꢀ8.53 (d, 2H, J=8.3Hz, H ), 8.00ꢀ8.14 (m, 4H,
3
3
,4
6
ꢀ1
H ), 0.20 (s, 9H, H ). FTIR (KBr), ν (cm ): 3047 (m, ArꢀH), 2956 (m; ꢀCH
884 (m, Ar), 1259, 843, 728 (s, Si (CH ).
2.3.7. Synthesis of 9ꢀethynyl anthracene (B).
3
), 2144 (w, C≡C), 1623,
3 3
)
1
Compound B was obtained using a modified literature procedure [39]. Yield 78%. H NMR (400
2
,5
6
3,4
MHz, 298 K, CDCl
3
): δ = 8.44ꢀ8.59 (m, 4H, H ), 7.99ꢀ8.03 (m, 1H, H ), 7.46ꢀ7.63 (m, 4H, H ),
1
ꢀ1
3.99 (s, 1H, H ). FTIR (KBr), ν (cm ): 3261 (s, C≡CH), 3049 (m, ArH), 2956 (m, ꢀCH
3
), 2089 (w,
C≡C), 1620, 884 (m, Ar).
2.3.8. Preparation of (W71).
To a 100 mL round bottom flask equipped with a magnetic stir bar was added into 6 (0.19g 0.7
mmol), Y (0.03g, 0.1 mmol), ODZMS (0.15g, 0.1 mmol), KF (0.23mg 4 mmol), and Cu(PPh ) Br
3
3
o
(0.01g, 0.08 mmol) in DMF (50 mL), which refluxed at 110 C for 24 h. The reaction mixture was
poured into a saturated solution of EDTA and the product was obtained by filtration. The crude
product was purified by column chromatography on silica gel (eluent: dichloromethane) to afford
1
fuchsia crystal W71.The isolated yield of the product was 95%. H NMR (400 MHz, 298 K, CDCl
3
):
δ = 8.54 (s, 7H, H), 8.19 (s, 8H, H), 8.06 (m, 28H, H), 7.16ꢀ7.46 (m, 32H, H), 6.46 (s, 2H, H), 5.29
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(s, 1H, H), 5.15 (S, 1H, H), 1.33 (m, 16H, H), 0.78ꢀ0.81(m, 9H, H), 0.01ꢀ0.02 (m, 48H, H). Si
ꢀ1
NMR (400 MHz, solid, δ): ꢀ65.3 (ArꢀCH –Si), ꢀ79.8 (anthraceneꢀCH –Si). FTIR (KBr), ν (cm ):
2
2
3054 (w, ArꢀH), 2962, 2929, 2857 (m, ꢀCH
3
), 1630 (m, trizole), 1095 (s, SiꢀOꢀSi). MALDIꢀTOF MS
+
+
152 26
[C158H N O21Si16+2H] : Calcd (M+2H) /z, 3196.7950; found M/z), 3196.7860. Anal. Calcd for
C H N O Si : C, 59.29; H, 4.79; N, 11.38. Found: C, 60.03; H, 4.737; N, 11.13.
158 152 26 21 16
2.3.9. Preparation of (W62).
1
This was prepared as above for W62. The product was yellow powder, yield 97%. H NMR (400
MHz, 298 K, CDCl ): δ = 8.54 (s, 7H, H), 8.05ꢀ8.07 (m, 6H, H), 8.13ꢀ8.15 (s, 8H, H), 8.05ꢀ8.07 (m,
24H, H), 7.38ꢀ7.49 (m, 32H, H), 6.46 (s, 4H, H), 5.29 (S, 2H, H), 5.15 (S, 2H, H), 1.33 (m, 16H, H),
3
2
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3
2
0.54ꢀ0.56 (m, 18H, H), 0.01ꢀ0.02 (m, 48H, H). Si NMR (400 MHz, solid, δ): ꢀ64.8 (ArꢀCH –Si),
ꢀ1
ꢀ79.1 (anthraceneꢀCH
2
ꢀSi). FTIR (KBr), ν (cm ): 3053 (w; ArꢀH), 2964, 2928, 2857 (m, ꢀCH
3
), 1650
+
+
(m, trizole), 1093 (s, SiꢀOꢀSi). MALDIꢀTOF MS [C H N O Si +2H] : Calcd (M+2H) /z,
164 160 28 22 16
3320.8601; found M/z, 3320.8590. Anal. Calcd for C164
Found: C, 59.03; H, 4.658; N, 12.04.
H
160
N
28
O22Si16: C, 59.25; H, 4.85; N, 11.80.
2.3.10. Preparation of (W ).
5
3
1
This was prepared as above for W53. The product was purple powder, yield 92%. H NMR (400
MHz, 298 K, CDCl
3
): δ =8.54 (m, 5H, H), 8.12ꢀ8.19 (m, 8H, H), 8.06 (m, 20H, H), 7.44ꢀ7.58 (m,
32H, H), 6.46 (s, 6H, H), 5.29 (S, 3H, H), 5.15 (S, 3H, H), 1.33 (m, 16H, H), 0.78ꢀ0.83 (m, 27H, H),
2
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0.01ꢀ0.02 (m, 48H, H).
(anthraceneꢀCH –Si). FTIR (KBr), ν (cm ): 3052(w; ArH), 2963, 2927, 2859 (m, ꢀCH
trizole), 1099 (s, SiꢀOꢀSi). MALDIꢀTOF MS [C170
found 3444.9190. Anal. Calcd for C H N O Si : C, 59.21; H, 4.91; N, 12.18. Found: C, 58.73;
2
Si NMR (400 MHz, solid, δ): ꢀ64.6 (ArꢀCH –Si), ꢀ80.5
ꢀ1
2
3
), 1635 (m,
+
+
H
168 30
N O23Si16+H] : Calcd (M+H) /z, 3444.9257;
1
70 168 30 23 16
H, 5.031; N, 11.97.
2
2
4
5
3. Results and discussion
3.1. Theoretical calculation
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To validate rationality of structural design of chromophores, a theoretical calculation was
conducted using the Gaussian09 software suite with a TDꢀDFT/6ꢀ31G+ level of theory, which was
used to simulate the absorption and emission spectra of candidates B and Y in THF[40,41]. The
calculation results are shown in Figure 1. Here, chromophore B and Y with similar absorption
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spectra and different stokes shift in their emission spectra were selected. It can be concluded from
Figure 1, B and Y show similar absorption spectra, so the same excited light source can be used.
Meanwhile, the emission spectrum of Y displays very large stokes shift. This not only make emission
spectra of B and Y cover the whole visible range of white light, but inhibits the intramolecular
selfꢀabsorption. Based on the high efficient “Click chemistry” and covalently incorporation of
nanoꢀsized inorganic POSS, the novel hybrid white emission materials with high efficiency is
successfully achieved by precisely adjusting the component of B and Y component.
B-abs
1.0
0.8
0.6
0.4
0.2
0.0
Y-abs
B-em
Y-em
200
300
400
500
600
700
800
Wavelength (nm)
8
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Figure 1. The normalized calculated absorption and emission spectra of B and Y in THF.
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3.2. Synthesis and Characterization
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Single molecule organic white light emitters (SMOLEDS) have exhibited some excellent
advantages such as perfect color reproducibility, stability and easy fabrication. SMOLEDS mainly
concentrated on a series of polymers. However, the inherent poor purity, multiꢀpolydispersity and
low thermal properties of polymers have greatly limited their luminous efficiency and practical
application. Hence, design and preparation of the hybrid whiteꢀlightꢀemitting macromolecule with
high thermal properties and well luminous efficiency will be a key issue for their application in the
flexible display. Based on the calculation of the density theory, three POSSꢀbased luminous single
molecular nanohybrid W71, W62 and W53 containing a yellow monochromatic emitter (Y) with a
large Stokes shift and a blue monochromatic emitter (B), in which the front value in the subscript is
defined as number of B group, the latter value is the number of Y group, were designed and
prepared.
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Y and B were synthesized by Knovenagel condensation and Sonogashira reaction (supporting
information). At first, 4ꢀethynylbenzaldehyde, a reactant of Y, was synthesized from
2ꢀmethylꢀ3ꢀbutynꢀ2ꢀol. However, intramolecular and intermolecular aldol condensation happened
during the hydrolysis reaction and the ideal compound was not obtained. Fortunately, after changing
2ꢀmethylꢀ3ꢀbutynꢀ2ꢀol
to
trimethylsilylacetylene,
the
problem
was
solved
and
4ꢀethynylbenzaldehyde was prepared successfully. 2ꢀ(2ꢀtertꢀButylꢀ6ꢀmethylpyranꢀ4ꢀylidene)
malononitrile was prepared following by reference [32]. All purified products gave the characteristic
1
spectroscopic data of FTIR, H NMR (see supporting information for details) and elemental analysis
corresponding to their expected structures.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
In the synthetic process of hybrids, oneꢀstep synthesis strategy was employed by highly efficient
click chemistry methods. The purification is easily achieved by simply dissolving
the precipitation process. Similar oneꢀstep strategies were reported in literature [33].
The structure of hybrids W71, W62 and W53 were determined by means of standard spectral
methods, elemental analysis and mass spectrum. Figure 2a shows the FTIR spectra of W71, W62 and
ꢀ1
W . As shown in Figure 2a, the characteristic absorptions at ~3100, 1625cm responding to triazole
5
3
stretching vibration absorption and the characteristic SiꢀOꢀSi stretching vibration of POSS core at
ꢀ1
~1100 cm appear in spectra of the resulting hybrids, and the characteristic absorptions of –N
3
and
ꢀ1
C≡C at ~2100 cm disappear, indicating that all the eight azide groups of POSSꢀN
3
completely
reacted with alkynyl groups of B and Y to form the resultant hybrids at almost stoichiometric pattern.
1
In H NMR spectra (Figure 2b), it is also found that the characteristic alkyne proton absorption
located at 3.13 ppm in the spectrum of B and Y completely disappears after click reactions and new
1
vibration bands at 1.33 ppm (ꢀSiCH ) and 0.29 ppm (ꢀSiCH ) turn up in the H NMR spectrum of
2
3
ODZMS, further supporting that objective molecule has been obtained. The characteristic spectral
peaks at ~8.54 ppm, assigned to the H proton of triazole, were observed in the spectrum of the
resultant hybrids, which further confirm that the click reactions were successfully made. Moreover,
the results from C, H, N elemental analyses almost completely corresponding to the theoretical
calculated values of objective compounds, further supporting that objective molecules were obtained.
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Figure 2. The FTIR and H NMR spectra of W71, W62 and W53
.
3
3.3. Photophysical Properties
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0
1
2
3
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0
1
All the resultants molecules W71, W62 and W53 are soluble in common organic solvents, such as
dichloromethane (DCM), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), etc. The solution
ꢀ5
ꢀbased emission and absorption measurements were performed at very low concentrations (10 M)
in an interꢀmediate dielectric constant solvent (THF, ε=7.6) to remove the effect of molecular
aggregation and differences in molecular dipole moments [53]. The results shown that Y’s TLC plate
showed a dark point at first, and then exhibited a yellow light point. This may be due to the
restriction of intramolecular rotation (RIR) induced emission. Accordingly, the emission spectra of Y
in THF/water mixture solvent were tested (Figure 3), where THF is a very good solvent and water is
a poor solvent. It is found that, in good solvent THF, Y shows a weak emission at ~470 nm; but after
1
1
1
1
1
1
1
1
1
1
2
2
2
the addition of poor solvent H O (>40% v/v), Y exhibits a strong yellow light emission with ca. 15
folds enhancement and two the red shift enhanced emission peaks at ~530 nm and 570 nm [28,35,53].
Interestingly, at low water content, the emitting spectrum shows significantly blue shift, then,
demonstrated shows a red shift with increase of water content owing to the aggregation enhanced
emitting effect. Simultaneously, the emitting intensity enhanced with increase of water content (20
fold at 90 (v/v)% water content) while Y’s Stokes shift don't further change after water content more
than 40 (v/v)%. As shown in Figure 3a, it emits a bright yellow light in the solid state under 365 nm
UV light. These results further indicated that a yellow emitter with large Stokes shift was obtained
successfully. Moreover, different from Y, the emission spectra of B was quickly quenched after the
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poor solvent was added and it fainted in the solid state. These shifts may be in virtue of the
dipoleꢀdipole intramolecular interactions between organic moieties and POSS cages to enlarge the
torsion angle between the fluorine ring and the triazole ring and low the effective conjugation length
of the molecules. Simultaneously, the W62, W53, W71 show higher quantum yield of 95%, 83%, 81%,
respectively, which are higher than 70% of B and 75% of Y.
6
7
8
Figure 3. The images of Y in THF/Water mixture solvent and solid state (a) and emission spectra of
ꢀ5
ꢀ1
9
Y in THF/Water mixture solvent (b) (10 mol L , λex=365 nm). Inset: changes in the PL peak
1
1
0
1
intensities (I) and Stocks shift (nm) of corresponding molecules with different water fractions in the
2
THF/H O mixtures.
1
1
1
2
3
4
To evaluate the rationality of our design strategy, the absorption and emission spectra of
monochromatic emitters B and Y in THF and THF/H O mixture solvent was compared. Figure 4 is
2
the normalized absorption and emission spectra of B and Y. As shown in Figure 4, B exhibits a
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strong absorption band from 315 to 475 nm, and Y shows a boarder absorption bond from 295 to 475
nm. They have an overlap between their absorption spectra at UV bands, which make sure that they
can be excited at the same band light, such as 365 nm light from normal UV laps. Meanwhile, the
absorption spectrum of Y and the emission spectrum of B weakly overlap between 390 ~ 475 nm,
which effectively limit the intramolecular selfꢀabsorption of the resultant hybrids. In addition, the
emission spectrum of B and Y fully cover light band from 400 to 700 nm, which make the realization
of white light emission possible.
B inTHF
Y in THF
Y in THF(10)/Water(90)
1
.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
300 350 400 450 500 550 600 650 700 750
Wavelength (nm)
8
9
Figure 4. The absorption and emission spectra of B and Y in THF and Y in THF/water (v/v=10: 90)
ꢀ
5
ꢀ1
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
5
6
7
8
9
0
1
mixture solvent (10 mol L , λex=365 nm).
To further investigate the intramolecular selfꢀabsorption of the resultant hybrids, the emission
spectra of three hybrids in THF were measured. In good solvent (THF), the emission of Y unit is not
strong enough to influence the emission spectrum of B unit, and change of emission spectra of
hybrids can be attributed to the intramolecular selfꢀabsorption from B to Y. As shown in Figure 5,
the emission spectrum of W71 shows three peaks at 385, 403 and 431 nm, respectively. With increase
of the ratio of yellow unit in hybrids, the emission spectra of the resultant hybrids such as W and
6
2
W
53 exhibit two peaks at 410 and 440 nm, respectively. The short wave band UV emission peak at
385 nm disappeared due to intramolecular selfꢀabsorption, while the long blue wave band is well
kept. These results reveal that the adjusted intramolecular selfꢀabsorption occurs in the resultant
hybrids: in one hand, the intramolecular selfꢀabsorption at blue band was prohibited and the blue
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lightꢀemitting component was well protected; on other hand, the UVꢀband emission of B was
selfꢀabsorbed by Y, which enhance the yellow lightꢀemitting component and further ensure the
equilibrious white light emission.
4
5
ꢀ5
ꢀ1
6
Figure 5. The emission spectra of W71, W62 and W53 in THF (10 mol L , λex=365 nm).
7
8
9
However, in mixed solvent, these resultant hybrids show a quite different spectral characteristics
compared to their emission spectra in good solvent THF (Figure 6). After the introduction of POSS,
the emission of hybrids with blue band does not quench with the increase of water content due to the
disaggregation effect of POSS cage. On the contrary, the emitting intensity shows a little increase,
implying that the POSS cage does not prohibit the interactions between Y groups and solvent
molecules, and the hybrids show an enhanced yellow band emission with the increase of water
content owing to the solvent effect. After adjustment of the ratio of B and Y in hybrids, W71 in the
film shows lightꢀblue due to the relatively weak yellow light, whereas W53 exhibits lightꢀgreen
emission due to the relatively strong yellow light (Figure S 22 and Figure S 23). We successfully
achieved white light emission from W62 (Figure 6) in THF/Water (10:90 v/v) and film (Figure 7).
And its Commission International d’Eclairage (CIE) coordinates is (0.29, 0.34) belonging to white
light range.
1
1
1
1
1
1
1
1
1
1
0
1
2
3
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ꢀ5
ꢀ1
3
4
Figure 6. The images (a) and emission spectra (b) of W in THF/water mixture solvent (10 mol L ,
62
λex=365 nm).
5
6
7
8
9
0
1
2
Figure 7 shows the emission properties of W62 in solid state. The film of W62 was prepared by
spinꢀcoating the hybrid solutions (ca. 10 mg/mL) on quartz plates at 1000 rpm for 30 s. To ensure the
accuracy of measurement, three samples were all prepared at the same condition. The AFM image of
W
62 film show that its surface is quite smooth and no cracks and aggregation are observed (Figure
7a), implying that the resultant organic hybrid has good film formability and structural homogeneity.
And the emission spectrum of W62 film was shown in Figure 7c. As shown in Figure 7c, the emission
spectrum of W62 film is similar to the emission spectrum of W62 in poor solvent and exhibits a
whiteꢀlight emission over the whole visible range from 400 to 700 nm (Figure 7b, c).
1
1
1
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Figure 7. (a) Atomic force microscope images of 5×5 ꢁm tapping mode for W62 film; (b) the
images of W62 film under 365 nm UV; (c) the emission spectra of W62 in film and THF/Water
(v/v=10:90) mixture solvent (λex=365 nm).
6
3.4 Thermal Properties
7
8
9
The thermal properties of white light emitting hybrid W62 were measured by TGA and DSC
(Figure 8). Usually, the thermal decomposition behavior of POSS macromeres exhibit two step
decompositions: the decomposition of organic component and inorganic POSS cage. However, the
TGA curve of W62 only shows the oneꢀstep decomposition. Meanwhile, Td5% value of W62 is at ca.
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
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7
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0
1
o
o
o
273 C, which is much higher than 192 C of Y and 205 C of B (Figure 8a). Moreover, the char
yield of the thermally cured POSSAF was 37.4%, which is not only much higher than that of B and
Y (almost no residual), but also higher than its theoretical rate of residual (28.9%). The thermal
stability enhancement and oneꢀstep decomposition behaviors are attributed to the uniform
incorporation of POSS cage and the jacket effect of the chromophore pendants [35,44ꢀ48]: that is,
in which the POSS core may have been surrounded by a rigid jacket formed through the strong intraꢀ
and interꢀ molecular electronic interactions of the organic arms, shielding the molecules from the
thermal attack. Simultaneously, the triazole ring also enhances the thermal stability of the hybrid
nanocomposite. Furthermore, as organic is oxidized away from the surface, the remaining residue
becomes enriched in silica and provides a barrier to further oxidation [36,49ꢀ52]. Especially, the
o
DSC curves of B and Y both show a large melting point at ~85 C, and Y exhibits a crystallization
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peak at ~ 120 C. However, the DSC curve of W62 is quite smooth and there is no melting or
crystallization point found (Figure 8b). These results all reveal that the material has excellent
amorphous state high thermal stability of W62 [53].
4
5
Figure 8. The (a) TGA and (b) DSC curves of B, Y and W62
.
6
4. Conclusions
7
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0
1
2
3
4
5
6
7
8
In conclusions, the novel nanoꢀsized hybrid with white light emission were realized by simple
control of the feed ratio of blue and yellow monochromatic moiety in hybrids via high efficient
“Click chemistry”. Based on the theory simulation and molecular design, the optimized molecular
structure and composition were obtained (component ratio of B: Y at 6:2). The investigation of
emitting mechanism shows that the allowable transfer in UV band improves the yellow light
emission intensity and the restraint of energy transfer in blue band ensures the blue light emission
intensity and the balance of resultant white light emission based on large Stokes shift of the yellow
monochromatic emitter. The incorporation of nanoꢀsized POSS not only shows significant AIE effect,
but also exhibits high thermal stability, which efficiently enhance the efficiency (Φfilm=95%) of the
resultant hybrids (W62) in solid state the deaggregation effect of POSS. This work provided a novel
strategy for design and preparation of whiteꢀlightꢀemitting molecules with high thermal stability,
high emitting efficiency and well file formability.
1
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1
1
1
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Acknowledgement
2
2
0
1
This research was financially supported by the National Natural Science Fund of China (Grant
Nos. 21671037, 21471030, 21271040).
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