Supramolecular Gel Phase Controlled [4 + 2] Diels-Alder Photocycloaddition for Electroplex Mediated White Electroluminescence

Article abstract of DOI:10.1021/jacs.9b00955

Diels-Alder photocycloaddition of 9-phenylethynylanthracene results in multiple [4 + 2] and [4 + 4] cycloaddition products in solution, which can be controlled to form specific products under a restricted environment. We have exploited the gel phase of a

Full text of DOI:10.1021/jacs.9b00955

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Supramolecular Gel Phase Controlled [4+2] Diels-Alder
Photocycloaddition for Electroplex Mediated White Electroluminescence
Satyajit Das, Naoki Okamura, Shigeyuki Yagi, and Ayyappanpillai Ajayaghosh
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Mar 2019
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Journal of the American Chemical Society
Supramolecular Gel Phase Controlled [4+2] Diels-Alder Photocycloaddi-
tion for Electroplex Mediated White Electroluminescence
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Satyajit Das,†, ‡ Naoki Okamura, Shigeyuki Yagi, and Ayyappanpillai Ajayaghosh*
ǁ
ǁ
,†,‡
†Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR-National Institute for Interdisci-
plinary Science and Technology (CSIR-NIIST), Thiruvananthapuram 695019, India
‡Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
ǁDepartment of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-
ku, Sakai, Osaka 5999-8531, Japan
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Supporting Information Placeholder
devices. Though white light emission is a topic of current interest
ABSTRACT: Diels-Alder photocycloaddition of 9-phe-
1
2-14
in lighting applications,
cence is rarely observed.
single molecular white electrolumines-
Electromer and electroplex formation
nylethynylanthracene results in multiple [4+2] and [4+4] cycload-
dition products in solution, which can be controlled to form specific
products under restricted environment. We have exploited the gel
phase of a 9-phenylethynylanthracence derivative as a confined
medium to specifically yield the [4+2] cycloadduct in >90% yield.
The photocycloadduct (anti-form), exhibited a blue emission with
CIE chromaticity of x = 0.16/y = 0.16. Construction of organic light
emitting device with the photocycloadduct, using a carbazole based
hole transporting host, resulted in white light emission with CIE
chromaticity of x = 0.33/y = 0.32. This observation not only high-
lights the use of gel chemistry to achieve the otherwise difficult to
obtain photoproducts but also underlines their potential in optoe-
lectronic device fabrication.
1
4,15
by the emitting species and the hole transporting material is the rea-
1
4a,15
son for white electroluminescence.
Such electrically excited
complexes are akin to excimers and exciplexes observed in the case
of photoluminescence and has long wavelength mixed emission,
providing balanced color coordinates required for white light emis-
sion.
1
Confinement of molecular systems yielding regio- and stereo-
specific reaction products, has been reported in several cases. An
1
2
In this study, a 9-phenylethylanthracene derivative 1 was de-
example is the Diels-Alder photocycloaddition of anthracene and
16
3
signed which facilitates H-bonded self-assembly in methyl cyclo-
its 9-phenylethynyl derivative. While anthracene results in anti
hexane (MCH, 3.3 M, Figure S1a). Plots of the fraction of aggre-
gate (agg) against the temperature could be fitted to a cooperative
self-assembly pathway. The enthalpy of nucleation of the assembly
(Hnucl) was found to be -9 kJ/mol and the elongation temperature
was 303 K (Figure S1b and Table S1). The molecule 1 in hot MCH
and syn isomers of [4+4] photocycloadducts in the absence of oxy-
4,5
gen, 9-phenylethynylanthracene in principle should form four
different products (Scheme 1). Though these products are allowed
by selection rules, the unfavorable geometries enable only [4+2]-
anti (64%) and [4+4] anti (23%) addition. However, energeti-
3,6
(
1.19 w/v) upon sonication and cooling resulted in a reversible gel
cally unfavorable products were specifically obtained under con-
7,8
fined conditions. Majority of these reports have been limited to
the synthesis of selective products on confined surfaces, however,
practical application of these types of products are yet to be demon-
(Figure 1a,b) having a storage modulus (G) of one order of mag-
nitude greater than that of the loss modulus (G) (Figure 1d).
Atomic force microscopy (AFM) and transmission electron mi-
croscopy (TEM) analyses revealed tape like morphology (Figures
1e, S2 and S3). X-ray diffraction (XRD) of the xerogel exhibited
peaks corresponding to d-spacing of 47.9, 21.5 and 12.5 Å. First
three peaks followed the reciprocal relationship with a ratio of
9
strated.
Scheme 1. Photocycloaddition of 9-phenylethynylanthra-
6
,7
cene in solution.
1
:~2:~4, which correspond to multidimensional lamellar packing.
hn
hn
The peaks corresponding to 3.82 and 4.20 Å indicate -stacking
and long range ordered inter-molecular H-bonding, respectively.
The peaks corresponding to 8.69, 8.28, 7.49, 6.91, 6.31, 5.80, and
[
4+4]
[4+2]
5
.28 Å represent the interdigitated H-bonded assembly where the
anthracene moiety of the molecule is overlapped with the triple
bond (Figure 1 and S4). This is evident from temperature controlled
absorption changes in 230-260 nm region (Figure S5a), where the
1
band corresponding to B
b
transition dipole of anthracene, parallel
to the molecular axis is not being polarized via aggregation, and
[4+4]- syn
[4+4]- anti
[4+2]- anti
[4+2]- syn
17
remained nearly unsplit. The steady state fluorescence spectrum
18
(
Figure S5b) of the gel did not show excimer emission, as sup-
Supramolecular gels have been shown to be a suitable medium
ported by time correlated single photon counting experiment (Fig-
ure S5c). The fluorescence decay profile of the gel was fitted with
a bi-exponential decay having lifetime values of 1.95 and 4.00 ns.
1
0
for crystallization of molecules. H-Bonding and -stacking in the
gel state have advantage over solution state to obtain certain stereo-
11
specific products with high yield. Herein, we establish that mo-
lecular self-assembly and gel chemistry can be used to control
Diels-Alder photocycloaddition to achieve a specific product in
high yield, which can be used for fabricating white light emitting
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(b)
two relatively less intense bands at 393 and 373 nm and an intense
(
a)
(c)
hn (365 nm)
1
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9
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band at 299 nm. The gelator 1 exhibited two intense emission peaks
at 450 and 475 nm, whereas the photoadduct showed an emission
with maximum at 440 nm. The fluorescence quantum yields of 1
and the photoadduct were 0.79 and 0.57, respectively. The fluores-
cence lifetime analysis of 1 and its photoadduct in chloroform ex-
hibited a single exponential decay with lifetime values of 2.91 and
Cool
Heat
(d)
3
2
.0
.5
G' 1
G" 1
(e)
(f)
2.0
4
.28 ns, respectively (Figure 2c).
1
1
0
.5
.0
.5
(a)
(b)
1
(g)
(h)
1
Photoadduct
2
0
40
60
80
100
Photoadduct
-
1
0
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0
Angular Frequency(s
)
(
i)
37nm
2
m
2
50
300
350
400
450
400
500
600
1
Wavelength (nm)
Wavelength (nm)
(c)
4
(d)
10
10
10
1
hn (365 nm)
4+2]
Photoadduct 0.0
IRF
[
3
2
-
0.5
0
.0
0.2
1.1
0.4
1.2
0.6
0.8
0
Photoadduct
-
5
-
10
0
10
20
Time (ns)
30
1.0
1.3
1.4
Potential (V vs Ag/AgCl)
Figure 1. (a, b) Photographs showing reversible gelation of 1 in
MCH and (c) the gel after photoirradiation. (d) Rheological behav-
ior of the gel. (e, f) TEM images of the gel before and after pho-
toirradiation. (g) AFM image of the self-assembled 1 in MCH. (h)
The height profile. (i) Molecular assembly and structural change of
Figure 2. Comparative properties of 1 (red) and photoproduct
(blue). (a) Absorption, (b) emission, (c) fluorescence lifetime decay
profiles in chloroform and (d) cyclic voltammograms in dichloro-
methane.
1
before and after photoirradiation.
Cyclic voltamograms of 1 and the photoadduct (Figure 2d) sug-
gest that EHOMO of the photoadduct is (-5.54 eV) more stabilized
than that of the gelator 1 (-4.79 eV) due to loss of extended π-con-
jugation (Figures S12, S13). Similar observation has been made in
The absence of any emissive species with lifetime value of >10
ns indicates that no prominent overlap between two anthracene
cores are present in the molecular assembly. Thickness of a single
layered sheet evaluated by AFM analysis is 5.2 nm, corresponding
to a monolayer of the assembly (Figures 1g, h). The interdigitated
long-range arrangement of 1 is ideally suited for the specific [4+2]
Diels-Alder photoreaction (Figure 1i) as established by the irradia-
tion of a gel of the molecule 1, with 365 nm light. In the process,
the gel turned into a solution (Figure 1c), which could not be re-
versed to a gel by irradiation at shorter wavelength or by cooling.
E
LUMO of the photoadduct (- 2.59 eV) and 1 (-1.98 eV) as tabulated
in Table S2. The device characteristics of the gelator 1 was inves-
tigated by fabricating solution processed single layer organic light
emitting devices (Figure S14). The emitting layer (EML) com-
prised of the gelator 1, the carbazole derivatives PVCz or SB2
as the hole transporting materials (HTM) and the oxadiazole deriv-
ative PBD as the electron transporting material (ETM). PVCz was
a better HTM than SB2 as indicated by the higher Lmax value of the
devices with the former (Table S3). The intensity of the EL spectra
increased upon increasing the applied voltage, however, the struc-
ture of the spectra remained unchanged. The Lmax obtained for the
1
9a
19b
HPLC profile of the irradiated solution showed a major peak
corresponding to a 91% conversion of the starting compound (Fig-
ure S6a). The H NMR profile of photoirradiated gel suggested 90-
2% formation of photoadduct (Figure S6b), whereas, the xerogel
and the solution state photoreaction showed low conversion and
1
9
2
2
1
+PVCz was 1244 cd/m at 16.5 V, and 231 cd/m for 1+SB2 at
1
14.5 V. In both cases, blue EL with CIE coordinates of (0.16, 0.16)
and (0.16, 0.13), respectively were obtained and remained almost
constant at any applied voltages (Figure 3a,c,e,f).
less stereoselectivity, respectively (Figure S7). The H NMR and
NOESY spectra confirmed the anti-[4+2] cycloaddition (Figures
S8, S9). This geometry disturbs the assembly of the molecule re-
sulting in the dissolution of the gel. AFM and TEM analyses of the
photoadduct showed ill-defined particles as shown in Figure 1f and
S10b, justifying the destruction of the one-dimensional H-bonded
assembly of the gel phase. This observation is confirmed by the
XRD analysis of the gel after photoirradiation, which shows de-
crease in the number of diffraction peaks (Figure S10). Comparison
of the FT-IR spectra (Figure S11) before and after photoirradiation
revealed that H-bonded N-H stretching frequency of the gel (nN-H
Interestingly, the device fabricated using the photoirradiated
gelator with PVCz (or SB2) as EML and PBD as ETM exhibited
broad EL spectra between 400-750 nm, with maxima at 445 and
573 nm (Figure 3b,d), resulting in white electroluminescence. The
performance of the PVCz based device is better when compared to
that with SB2. The structure of the EL spectra remained unchanged
while the intensity was increased with increase in the voltage (Fig-
ures S15, S16). When SB2 was used as HTM, the CIE coordinate
of the white light emission was x = 0.33/y = 0.32, indicating high
color purity. On the other hand, the value of the CIE coordinates
was x = 0.28/y = 0.25 when PVCz was used as the HTM (Figure
-
1
-1
= 3261 cm ) was shifted to 3279 cm which is close to the mono-
meric state (3284 cm ), indicating the breakage of the H-bonding.
-
1
The absorption and emission features of the gelator 1 and its
photoadduct in chloroform (3.3 M) are shown in Figure 2a-c. The
absorption spectrum of 1 showed peaks at 266, 323, 366, 386, 406
and 429 nm. However, in the case of photoadduct, the absorption
bands at 429 and 406 nm were disappeared with the appearance of
3
e,f).
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
+
(
photoadduct + HTM ) formation is in analogy with previous re-
(
a)
200 (b)150
150
1
+SB2 Device
Photoadduct+PVCz Device
Photoadduct+SB2 Device
1
2
3
4
5
6
7
8
9
ports and should be responsible for the green-to-red emission along
with the blue emission of the photoadduct, leading to the white light
emission (Figure 4f).
between the emitter and the HTM, we fabricated devices by mixing
them together with an electron transporting layer ([1,1':3',1-ter-
phenyl]-4,4-diylbis(diphenylphosphine oxide), BPOBP) sepa-
rately coated in solution processed double layer devices (Figure
S18). As expected, the device with emitter 1 showed blue emission,
whereas the device made out of the photoirradiated gelator exhib-
ited white light emission, underlining the electroplex formation be-
tween the photoadduct and the carbazole based compounds (Figure
S19). The role of [4+2] photocycloadduct in the electroplex for-
mation is confirmed by the comparatively weak bluish-white elec-
troluminescence with a photoirradiated 9-phenylethynyleneanthra-
cene (9PEA) having a mixture of photoadducts (Figure S20).
1+PVCz Device
1200
3
00
1
5
0
00
1
5,20
To establish the electroplex formation
800
200
100
0
1
5
0
00
0
4
00
0
2
1
0
6
8
10
12
14
16
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8
10
12
14
16
Voltage(V)
Voltage(V)
(
c)
(d) 1.2
1.0
2
2
1
1
0
.5
.0
.5
.0
.5
0.20
0.15
0.10
Photoadduct + PVCz Device
Photoadduct+ SB2 Device
10
1 + SB2 Device
+ PVCz Device
1
8
6
4
2
0
0
.8
.6
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9
0
0
0.4
0.2
0.0
0
.05
0.0
0.00
300 400 500 600 700 800
Wavelength(nm)
400
600
800
Wavelength(nm)
(
i)
(ii)
(e)
(f)
12
10
8
2.5
2.0
1.5
1.0
0.5
0.0
(
a) 2.5
(b)
1.0
0.8
0.6
0.4
0.2
0.0
EL
PL
EL
PL
2.0
1.5
1.0
0.5
0.0
6
1
+PVCz
1+SB2
(iv)
4
(
iii)
(iv)
2
(iii)
(i)
0
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
(ii)
c) 0.25
0.20
(
1
2
10
8
(d)
1.0
1.0
0.8
0.6
0.4
0.2
0.0
EL
PL
Photoadduct+PVCz Photoadduct+SB2
EL
PL
0
.8
.6
0
0
0
0
.15
.10
.05
.00
0
Figure 3. Device characteristics (luminance and current density vs
voltage plot) of (a) 1 and (b) photoadduct. Electroluminescence
spectra of OLED made of (c) 1 and (d) photoadduct as emitter with
SB2 (red) and PVCz (blue) as hole-transporting materials. (e) Pho-
tographs of electroluminescent devices fabricated with the gelator
6
0.4
0.2
0.0
4
2
400
500
600
700
Wavelength (nm)
400
500
600
700
Wavelength (nm)
1
(i, ii) and photoadduct (iii, iv). (f) EL color coordinates with those
Photoadduct.+ Host+
Photoadduct.*
+ Host
(f)
(e)
device made of 1 and the photoadduct.
Photoadduct*
For a deeper understanding of the origin of the white electrolu-
minescence, the EL and the PL spectra of the devices fabricated
with the gelator 1 before and after photoirradiation using PVCz and
SB2 as the HTM were compared (Figure 4a-d). Both EL and PL
spectra of the gelator before photoirradiation are comparable,
which exhibited blue emission with maxima at 450 and 470 nm.
Similarly, the PL spectra of the devices fabricated using both PVCz
and SB2 as HTM showed blue emission with a maximum at 450
nm and a shoulder band at 475 nm. However, the EL spectra in both
cases exhibited a broad emission at 600 nm, in addition to the sharp
emission at 450 nm. Notably the intensity of the broad 600 nm
emission was less when PVCz was used as the HTM.
Electroplex
Exciplex
Photoadduct
Coordinate
Figure 4. Comparative PL and EL spectra of devices made of (a)
1+PVCz, (b) 1+SB2, (c) photoadduct+PVCz and (d) photoad-
duct+SB2 (e) Schematic representation of the electronic configura-
tion in molecular orbitals of the electroplex host/emitter. (f) Pro-
posed energy profile for the electroluminescence response of the
photoadduct, exciplex, and electroplex.
Since the PL emission of the photoirradiated gelator was blue,
we expected a similar emission characteristic for the EL spectrum
as well. Instead, we observed an electrogenerated white light emis-
sion, which may not be the property of the photoadduct alone. Fur-
thermore, film state absorption of EML did not show any new band
in the presence of the photoadduct (Figure S17), which is an indi-
cation of the absence of ground state charge transfer between the
emitter and the HTM. Therefore, the green and red emission re-
quired for the white emission must have generated through the in-
teraction between the photoadduct and the transporting materials
under the applied electric field. Probably, an electron transfer from
the HTM to the photoadduct could generate an excited complex
called electroplex, similar to a photoexcited exciplex (Figure 4e).
Such an emission by the electrically excited electroplex can be vis-
ible only by an EL spectrum and not by photoexcited PL spec-
In conclusion, the gel phase of the molecule 1 creates enough
confinement, selectively yielding the [4+2] photocycloadduct.
Light emitting devices fabricated with the gelator 1 exhibited a blue
emission, whereas the device fabricated with the photoirradiated
gelator showed a white electroluminescence. The generation of the
green and red emission required for white luminescence is ex-
plained on the basis of an electroplex formation between the pho-
tocycloadduct and the hole transporting material within the device
structure under an applied electric potential. This report highlights
the role of H-bonded supramolecular assembly and gelation of
functional molecules to achieve exclusive photoproducts for their
use in photonic device application.
1
5a
trum.
ASSSOCIATED CONTENT
The energy level of the electroplex is lower than that of the sin-
glet excited state of the photoadduct (Figure 4e). This electroplex
Supporting Information
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General methods, synthetic procedures, gelation, photoirradiation,
OLED fabrication and additional figures. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
ajayaghosh@niist.res.in
Notes
(12) Ying, L.; Ho, C.-L.; Wu, H.; Cao Y., Wong, W.-Y. White Polymer
Light-emitting Devices for Solid-state Lighting: Materials, Devices, and
Recent Progress. Adv. Mater. 2014, 26, 2459-2473.
The authors declare no competing financial interests.
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dred, M. White-light Emission from a Single Heavy Atom-free Molecule
with Room Temperature Phosphorescence, Mechanochromism and Ther-
mochromism. Chem. Sci., 2017, 8, 1909-1914.
ACKNOWLEDGMENT
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A.A. is grateful to Govt. of India, for a J. C. Bose National Fellow-
ship (SB/S2/JCB-11/2014). S.D. acknowledges DST for an
INSPIRE Fellowship. S.Y. and A.A. are grateful to DST and JSPS
for exchange program. Mr. Kiran Mohan is acknowledged for TEM
measurements. Dr. Narayanan Unni, Dr. Sreejith Shankar and Mr.
Ryota Kono are acknowledged for cross-checking the device char-
acteristics, CV data, and measurement of film-state absorption, re-
spectively.
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Table of Content Graphic
EL
PL
Gel phase
[4+2] hv
CIE (0.16,0.16)
400
500
600
700
Wavelength (nm)
EL
PL
Photoadduct
CIE (0.33,0.32)
4
00 500 600 700
Wavelength (nm)
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