Mechanochromism and electroluminescence in positional isomers of tetraphenylethylene substituted phenanthroimidazoles

Article abstract of DOI:10.1039/c7tc00950j

The study of aggregation-induced emission (AIE) luminogens has gained momentum due to their remarkable luminogenic properties and applications in mechano-sensors and organic light-emitting diodes (OLEDs). In this article we have studied three positional i

Full text of DOI:10.1039/c7tc00950j

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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Mechanochromism and electroluminescence in positional isomers of
tetraphenylethylene substituted phenanthroimidazoles
Thaksen Jadhav1, Jeong Min Choi2, Jivan Shinde1, Jun Yeob Lee2∗, Rajneesh Misra1∗
1Department of Chemistry, Indian Institute of Technology Indore, 452 020, India
2School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi, 446ꢀ740,
Korea
* Corresponding author
Email: rajneeshmisra@iiti.ac.in
Abstract
The study of aggregationꢀinduced emission (AIE) luminogens has gained momentum due
to its remarkable luminogenic properties and applications in mechanoꢀsensors and organic light
emitting diodes (OLEDs). In this article we have studied three positional isomers (ortho, meta,
and para) of phenanthroimidazoles 3a–3c and explored their AIE, mechanochromic and
electroluminescence behavior. The phenanthroimidazoles 3a–3c were synthesized by the Suzuki
crossꢀcoupling reaction of (2ꢀbromo/3ꢀbromo/4ꢀbromo)phenathroimidazoles 2a–2c with 4ꢀ
(1,2,2ꢀtriphenylvinyl)phenylboronic acid pinacol ester in good yields. The phenanthroimidazoles
3a–3c exhibit strong AIE. The mechanochromic study reveals reversible good color contrast
mechanochromism between blue and green color. The ortho (3a) and meta (3b) isomers exhibit
98 nm grinding induced spectral shift whereas paraꢀisomer (3c) shows 43 nm. Moreover, the 3a–
3c were explored as a nonꢀdoped blue emitters in efficient organic lightꢀemitting diodes. Among
the three emitters, 3c provided high quantum efficiency of 4.0% in a nonꢀdoped blue device.
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Introduction
Luminescent organic compounds are widely used in organic lightꢀemitting diodes
(OLEDs), bioꢀsensing, bioꢀimaging, chemoꢀsensors and mechanoꢀsensors.1ꢀ3 The
mechanochromic materials change their colors in response to external mechanical stimuli
(grinding, rubbing or crushing). The solid state emission is one of the important requirements for
mechanochromic and OLED materials.4 Recently mechanochromic materials have gained
significant importance due to their potential applications in the field of mechanoꢀsensors, optical
recording, security papers and optoelectronic devices.5ꢀ9 However, conventional organic
compounds suffer from notorious effect called aggregation caused quenching (ACQ) which
decreases their luminescence efficiency.10ꢀ12 The aggregation induced emission (AIE) is
promising solution of ACQ effect.13ꢀ16 The AIE molecules are highly emissive in the solid state
and successfully used in the OLEDs and mechanochromic material applications.17ꢀ19
The luminescence properties of compounds in the solidꢀstate are dependent on the
molecular arrangement as well as intermolecular interactions. The mode of the molecular
packing and/or conformation of the compound would affect the molecular stacking mode and
effective conjugation which alter the luminescent properties. Thus, controlling the mode of
molecular packing is significant in tuning the photophysical properties.20ꢀ22
The research on OLEDs have been continuously paying attention in both academia and
industry due of their potential applications in solid state lighting and flat panel display.23ꢀ25 The
phenanthroimidazole is an aromatic heterocycle, containing phenanthrene ring fused with
imidazole ring, is endowed with ambipolar nature.26ꢀ27 Phenanthroimidazole has been extensively
2

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used to construct efficient blue emitters with fine color purities.28ꢀ29 Efforts are made to increase
the efficiency of the OLED devices made from phenanthroimidazole derivatives.30
The tetraphenylethylene (TPE) is AIE active molecule and has been explored in
developing OLED and mechanochromic materials.17ꢀ19, 31ꢀ32 Our group is involved in the design
and synthesis of TPE based luminogens for mechanochromism and OLEDs.33ꢀ35 Recently, we
have reported TPE substituted phenanthroimidazoles as efficient mechanochromic materials and
luminogens for OLED.36ꢀ37 We were further interested to study the positional effect of TPE unit
by synthesizing positional isomers (ortho/meta/para) of TPE substituted phenanthroimidazole.
The positional isomers (ortho/meta/para) may have different extent of conjugation and planarity
which will endow interesting effect on their photophysical and mechanochromic properties. In
orthoꢀisomer there is possibility of more twisting due to steric hindrance between TPE and
phenanthroimidazole, and in metaꢀisomer less conjugation due to metaꢀposition effect whereas
paraꢀisomer has possibility of high planarity and conjugation. The results obtained here support
our hypothesis and we are successful in improving mechanochromic properties of TPE
substituted phenanthroimidazoles. The phenanthroimidazoles 3a–3c show AIE, reversible
mechanochromism and highly efficient blue emission in nonꢀdoped OLED devices. The ortho
(3a) and meta (3b) isomers exhibit good mechanochromic spectral shift of 98 nm while para
(3c) isomer shows 43 nm. Additionally, the para isomer (3c) showed high quantum efficiency
(QE) of 4.0% in the nonꢀdoped blue device.
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Results and discussion
Synthesis
The TPE substituted phenanthroimidazoles 3a–3c were synthesized by the Suzuki crossꢀ
coupling reaction of 4ꢀ(1,2,2ꢀtriphenylvinyl)phenylboronic acid pinacol ester with corresponding
bromophenanthroimidazoles 2a–2c (ortho/meta/para) using Pd(PPh3)4 as catalyst which resulted
3a–3c in 51%, 65% and 67% yields, respectively (Scheme 1). The 9,10ꢀphenanthroquinone 1 on
condensation reaction with 4ꢀaminobenzonitrile and respective bromobenzaldehydes (2ꢀ
bromobenzaldehyde for 2a, 3ꢀbromobenzaldehyde for 2b, 4ꢀbromobenzaldehyde for 2c) resulted
2a–2c in 70%, 86% and 76% yields, respectively. The positional isomers of
phenanthroimidazoles 3a–3c were well characterized by 1H NMR, 13C NMR, and high resolution
mass spectrometry (HRMS).
Scheme 1: Synthetic route for the phenanthroimidazoles 3a–3c. (i) 4ꢀaminobenzonitrile, 2ꢀ
bromobenzaldehyde (for 2a)/ 3ꢀbromobenzaldehyde (for 2b)/ 4ꢀbromobenzaldehyde (for 2c),
acetic acid; (ii) 4ꢀ(1,2,2ꢀtriphenylvinyl)phenylboronic acid pinacol ester, K2CO3, Pd(PPh3)4,
Toluene:Ethanol:Water (10.0 mL:4.0 mL:0.5 mL).
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The thermal stability of organic materials is important for the device applications. To
check the thermal stability of the phenanthroimidazoles 3a–3c, the thermogravimetric analysis
(TGA) was performed (Fig. 1). The phenanthroimidazoles 3a–3c show excellent thermal
stability and the thermal decomposition temperatures (Td) corresponding to 5% weight loss under
a nitrogen atmosphere are 383, 433 and 446 °C for 3a, 3b and 3c respectively (Table S1). The
thermal stability of the phenanthroimidazoles 3a–3c follows the order 3c > 3b > 3a which
reveals paraꢀisomer has the highest thermal stability.
100
80
3a
60
40
20
0
3b
3c
100
200
300
400
500
600
Temperature °C
Fig. 1 Thermogravimetric analysis of phenanthroimidazoles 3a–3c measured at a heating rate of
10 °C/ min under nitrogen atmosphere.
Photophysical properties
The phenanthroimidazoles 3a–3c are highly soluble in common organic solvents such as
dichloromethane, tetrahydrofuran (THF), acetone, chloroform, etc., but they are insoluble in
water and methanol due to hydrophobic aromatic rings. The nonꢀrediative decay of the excited
state energy through molecular rotations of phenyl rings make phenanthroimidazoles 3a–3c
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weakly emissive in THF solution. The phenanthroimidazoles 3a–3c contains TPE unit and it is
known for AIE active nature therefore we have studied their AIE properties. The AIE behavior
was explored by preparing small aggregates of 3a–3c molecules and studied by fluorescence
spectroscopy. The aggregates were prepared in THF:water mixtures by gradual increase in the
water percentage. The emission spectra do not show distinct change in emission wavelength and
intensity, upto 80, 50 and 60% water fraction for 3a, 3b and 3c, respectively. However, above
80, 50 and 60% water fraction for 3a, 3b and 3c respectively, the enhancement in the emission
intensity with red shift was observed (Fig. 2 and Fig. S1). This enhancement in the emission
intensity may be due to suppressed free phenyl rings rotations due to the aggregate formation
which reduces the nonꢀradiative excited state energy loss. The weak fluorescence intensity of
phenanthroimidazoles 3a–3c in pure THF was enhanced in aggregated state (90% aqueous
mixture) by 8, 50 and 18 folds, respectively (Fig. 2).
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(A)50
(B)25
water (vol %)
water (vol %)
20
15
10
5
40
0
0
10
20
30
40
50
60
70
80
90
10
20
30
50
40
60
70
80
90
30
20
10
0
0
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
(C)
50
40
30
20
10
3a
3b
3c
0
0
20
40
80
100
Water fractio6n0 (vol %)
Fig. 2 (A) Fluorescence spectra of (A) 3a, and (B) 3b in THF–water mixtures with different
water fractions (10 ꢀM). (C) Plot of photoluminescence intensity (PL) vs. % of water fraction
(fw). Luminogen concentration: 10 ꢀM; intensity calculated at λmax
.
Mechanochromism
The high solid state emission and possibility of mechanochromic behavior encouraged us
to study the mechanochromic property. The mechanical stimuli responsive properties of
phenanthroimidazoles 3a–3c were studied by emission spectroscopy. The emission spectra of
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phenanthroimidazoles 3a–3c as synthesized, grinded and annealed solids are shown in Fig. 3 and
Fig. S2. The phenanthroimidazoles 3a–3c show good color contrast reversible mechanochromic
behavior between blue and green color. The pristine solids of phenanthroimidazoles 3a–3c show
blue color emission and emission maxima at 450 nm for 3c whereas 3a and 3b show dual
emissions at 370, 396 nm and 371, 394 nm respectively. Upon grinding using a mortar and
pestle, the blue emitting pristine solids were converted to a yellowish green emitting solids and
the emission peaks were redꢀshifted to 493, 494, and 493 nm for 3a, 3b and 3c respectively (Fig.
3 and Fig. S2). The mechanochromic effect of 3a, 3b and 3c shows spectral red shift of 98, 98
and 43 nm, respectively. The high spectral shift in 3a may be due to a highly twisted geometry
possibly created by the steric hindrance between TPE and phenanthroimidazole. However, in
case of metaꢀisomer (3b), less effective conjugation due to metaꢀposition effect and high twisting
results in high grinding induced spectral shift. The, paraꢀisomer (3c) possibly has a high
planarity and high effective conjugation which results in less grinding induced spectral shift. The
changes in emission intensity during mechanochromic behavior were evaluated by calculating
the fluorescence quantum yields in the solid state. The fluorescence quantum yields of pristine
solids 3a, 3b and 3c were 0.73, 0.54, and 0.73 respectively. The pristine solids decrease their
fluorescence quantum yields upon grinding and were observed 0.69, 0.57, and 0.46 for 3a, 3b
and 3c respectively. The grinded solids can be reverted to their pristine form by annealing or
fuming with solvent vapor. The grinded solid of 3c required annealing at 160 °C for 15 min
whereas 3a and 3b required 240 °C annealing for 15 min. The fumigation with dichloromethane
vapor for 2 min restored the original blue emission for all phenanthroimidazoles 3a–3c. The
annealed samples of 3a and 3b show emission tails which may be due to the incomplete
reversion of grinded solids to their pristine form.
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(B)
(A)
Synthesized
Grinded
Annealed
1.0
1.0
Synthesized
Grinded
Annealed
0.8
0.8
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 3 Emission spectra of (A) 3a, and (B) 3b as synthesized, grinded and annealed solids and
photograph taken under 365 nm UV illuminations.
The powder XRD study of pristine, ground and fumed form of phenanthroimidazoles 3a–
3c was performed for understanding the mechanochromic behavior. A sharp diffraction pattern
of the pristine samples suggested its crystalline nature (Fig. 4 and Fig. S3). However, the ground
samples exhibited a broad diffused pattern indicating its amorphous nature. The crystalline
nature of the solid could be restored by annealing or fumigation, giving sharp peaks in the
diffractogram as in pristine forms. The mechanochromic behavior of the phenanthroimidazole
solids can be confirmed from the PXRD studies and is associated to the reversible morphological
change from crystalline to amorphous form which further suggests the change in molecular
arrangement due to grinding.
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(B)
(A)
Annealed
Annealed
Grinded
Grinded
Pristine
Pristine
31.61 41.61
--
0
5
10
20 25
35 40
50 55
11.61221.61
θ
(degree)
2
15θ (de3g0ree) 45
Fig. 4 PXRD curves of (A) 3a, and (B) 3b as synthesized, grinded and annealed form.
Device performances
The phenanthroimidazoles 3a-3c were tested as nonꢀdoped emitters in the OLEDs based on
the AIE characteristics. A device structure of confining holes and electrons in the emitting layer
was adopted using 1,3ꢀbis(Nꢀcarbazolyl)benzene (mCP) and diphenylꢀ4ꢀtriphenylsilylphenylꢀ
phosphine oxide (TSPO1) carrier blocking layers to compare three emitters as nonꢀdoped
emitters. Current density (J) and luminance (L) of the three devices measured by voltage scan
method are presented in Fig. 5. The J of the 3a and 3c devices was quite similar, but the J of the
3b device was relatively lower than that of other devices. The low J of the 3b devices can be
correlated with the single carrier current density of the three emitters (Fig. 6). The electron
current density of 3a and 3c was much higher than that of 3b as shown in the single carrier
device data. As electrons are majority carriers in the devices by the electron deficient
phenanthroimidazole and benzonitrile moieties, the electron density dominated the current
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density of the devices for 3a–3c. Therefore, 3a and 3c showing high electron current density
increased the current density of the devices. Metaꢀ orientation of the two building blocks in 3b
hindered the hole and electron transport in the emitting layer.
40
35
30
25
20
15
10
5
10000
1000
100
10
3a
3b
3c
1
0
0.1
0
2
4
6
8
10
Voltage (V)
Fig. 5 Current densityꢀvoltageꢀluminance plot of nonꢀdoped 3a, 3b and 3c devices.
(a)
(b)
10
9
8
7
6
5
4
3
2
1
0
10
9
8
7
6
5
4
3
2
1
0
3a
3b
3c
3a
3b
3c
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage (V)
Fig. 6 Current densityꢀvoltage plots of hole only (a) and electron only (b) devices of nonꢀdoped
3a, 3b and 3c.
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QE of the nonꢀdoped devices was collected according to luminance in Fig. 7. The QE was
relatively high in the 3a device, but it was low in the 3b device. Two main parameters, PL
quantum yield and carrier balance, should be considered to interpret the QE data of the three
emitters. The PL quantum yields of 3a, 3b, and 3c compounds were 0.73, 0.54, and 0.73,
respectively, indicating that 3a and 3c are potentially better than 3b in terms of lightꢀemitting
efficiency of the emitters. As expected from the PL quantum yields, the maximum QE values of
3a, 3b and 3c were 4.0, 2.4, and 3.2%, respectively. The relatively high QE of 3a compared with
that of 3c is due to better carrier balance in the 3a device in spite of similar PL quantum yields.
In the case of 3a, both hole transport and electron transport were better than those of 3b and 3c,
which balanced carriers in the emitting layer and increased the QE. In the case of 3c, the hole
current density was too low, which decreased the QE of the 3c device because of poor charge
balance. Therefore, the order of QE of the nonꢀdoped device was 3a>3c>3b.
4.5
3a
4
3.5
3b
3c
3
2.5
2
1.5
1
0.5
0
1
10
100
1000
10000
Luminance (cd/m2)
Fig. 7 Quantum efficiencyꢀluminance plot of nonꢀdoped 3a, 3b and 3c devices.
The nonꢀdoped OLEDs were all skyꢀblue devices and exhibited skyꢀblue emission as
presented in Fig. 8. Peak wavelengths of the 3a, 3b, and 3c OLEDs were 480 nm, 483 nm, and
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482 nm, respectively, which corresponded to color coordinates of (0.199, 0.332), (0.212,0.372)
and (0.215,0.313), proving skyꢀblue emission color of the fabricated devices. There was no large
difference of the emission spectra because the same chromophore was used in the molecular
structure. The electroluminescence emission maxima are close to the emission maxima in the
amorphous solids which suggest the amorphous nature of the compounds in the OLED devices.
3a
3b
3c
380
480
580
680
780
Wavelength (nm)
Fig. 8 Electroluminescence spectra of nonꢀdoped 3a, 3b and 3c devices.
Conclusion
In conclusion, we have synthesized mechanochromic and electroluminescent positional
isomers of tetraphenylethylene substituted phenanthroimidazoles 3a–3c by Pdꢀcatalyzed Suzuki
crossꢀcoupling reaction. The phenanthroimidazoles 3a–3c show good thermal stability and paraꢁ
isomer exhibits highest thermal stability. The positional isomers 3a–3c in the aggregated state
show strong emission due to AIE compared to their emission in the solution state. The positional
isomers 3a–3c show reversible mechanochromism with good color contrast between blue and
green color. The use of ortho and meta isomers is found to be good strategy than para isomer for
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the development of mechanochromic materials and isomers 3a and 3b exhibit spectral shift of 98
nm whereas para isomer 3c exhibit spectral shift of 43 nm. Additionally, the 3a emitter show
high QE of 4.0% in the nonꢀdoped blue device.
Experimental details
General methods
Chemicals were used as received unless otherwise indicated. All oxygen or moisture sensitive
1
reactions were performed under argon atmosphere. H NMR (400 MHz), and 13C NMR (100
1
MHz) spectra were recorded on the Bruker Advance (III) instrument by using CDCl3. H NMR
chemical shifts are reported in parts per million (ppm) relative to the solvent residual peak
(CDCl3, 7.26 ppm). Multiplicities are given as: s (singlet), d (doublet), t (triplet), q (quartet), dd
(doublet of doublets), dt (doublet of triplets), m (multiplet), and the coupling constants, J, are
13
given in Hz. C NMR chemical shifts are reported relative to the solvent residual peak (CDCl3,
77.0 ppm. Thermogravimetric analyses (TGA) were performed on the Metler Toledo Thermal
Analysis system. The measurements were done at heating rate of 10 °C/minute and heated upto
800 °C. UV‐visible absorption spectra were recorded on a Carry‐100 Bio UV‐visible
Spectrophotometer. Emission spectra were taken in a fluoromaxꢀ4p fluorimeter from
HoribaYovin (model: FMꢀ100). The excitation and emission slits were 2/2 nm for the emission
measurements. All of the measurements were done at 25 °C. HRMS were recorded on Brucker‐
Daltonics, micrOTOF‐Q II mass spectrometer.
Synthesis:
Synthesis and characterization of phenanthroimidazoles 3a and 3b:
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General procedure for phenanthroimidazoles 3a and 3b: Pd(PPh3)4 (0.004 mmol) was added
to a well degassed solution of corresponding bromophenanthroimidazoles 2a and 2b (0.4 mmol),
4ꢀ(1,2,2ꢀtriphenylvinyl)phenylboronic acid pinacol ester (0.48 mmol), K2CO3 (1.2 mmol) in a
mixture of toluene (12 mL)/ ethanol (4.0 mL)/ H2O (1.0 mL). The resulting mixture was stirred
at 80 ºC for 24 h under argon atmosphere. After cooling, the mixture was evaporated to dryness
and the residue subjected to column chromatography on silica (hexaneꢀDCM 50:50 in vol.) to
yield the desired phenanthroimidazoles 3a and 3b as colorless powders. The compounds were
recrystallized from DCM:ethanol (8:2) mixtures.
3a: Yield: 51.0%. 1H NMR (400 MHz, CDCl3, 25 °C): δ 8.85 (d, 2H, J= 8 Hz), 8.77 (d, 1H, J=8
Hz), 7.68ꢀ7.79 (m, 3H), 7.58ꢀ7.62 (m, 1H), 7.41ꢀ7.47 (m, 2H), 7.31ꢀ7.38 (m, 3H), 7.22ꢀ7.25 (m,
1H), 7.00ꢀ7.07 (m, 7H), 6.88ꢀ6.96 (m, 6H), 6.80ꢀ6.83 (m, 2H) , 6.70ꢀ6.74 (m, 4H) , 6.48ꢀ6.65 (m,
3H) ppm; 13C NMR (100 MHz, CDCl3, 25 °C): δ 151.3, 143.6, 143.3, 143.1, 143.1, 141.4,
141.2, 140.8, 140.2, 137.8, 137.7, 132.6, 132.4, 131.2, 131.0, 130.9, 130.7, 130.4, 129.6, 129.4,
128.8, 128.4, 128.3, 127.8, 127.7, 127.6, 127.4, 127.3, 127.1, 126.6, 126.4, 126.2, 126.1, 125.8,
125.2, 124.3, 123.1, 122.9, 122.5, 120.7, 117.9, 112.3 ppm; HRMS (ESI): calcd. for C54H35N3: 7
48.2723 (M+Na)+, found 748.2702.
3b: Yield: 65.0%. 1H NMR (400 MHz, CDCl3, 25 °C): δ 8.86 (d, 1H, J= 8 Hz), 8.80 (d, 1H, J=8
Hz), 8.72 (d, 1H, J= 8 Hz), 7.88ꢀ7.91 (m, 2H), 7.66ꢀ7.76 (m, 5H), 7.53ꢀ7.58 (m, 2H), 7.28ꢀ7.34
(m, 3H), 7.22 (d, 1H, J= 8 Hz), 7.03ꢀ7.16 (m, 18H) ppm; 13C NMR (100 MHz, CDCl3, 25 °C): δ
149.9, 141.5, 137.6, 133.4, 132.8, 132.7, 131.9, 131.5, 129.5, 129.4, 128.4, 127.6, 127.1, 127.0,
126.7, 126.5, 126.0, 125.5, 124.7, 124.4, 123.2, 122.9, 122.3, 120.7, 117.7, 113.5 ppm; HRMS
(ESI): calcd. for C54H35N3: 7 48.2723 (M+Na)+, found 748.2708.
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Device fabrication and measurements
The device structure of the 3aꢀ3c devices was as follows.
Device fabrication and device characterization were already reported in our previous
work.38 All emitters were vacuum deposited using a vacuum thermal evaporation process.
Device structure of the nonꢀdoped devices was indium tin oxide (150 nm)/poly(3,4ꢀ
ethylenedioxythiophene):poly(styrenesulfonate) (60 nm)/4,4’ꢀcyclohexylidenebis[N,Nꢀbis(4ꢀ
methylphenyl)aniline] (20 nm)/mCP (10 nm)/emitter (25 nm)/TSPO1 (5 nm)/2,2′,2"ꢀ(1,3,5ꢀ
benzinetriyl)ꢀtris(1ꢀphenylꢀ1ꢀHꢀbenzimidazole) (30 nm)/LiF (1.5 nm)/Al (200 nm).
Supplementary data
Electronic supplementary information (ESI) available: NMR spectra and computational data for
3a and 3b and AIE and mechanochromic effect of 3c.
Acknowledgments
We acknowledge the support by DST, and CSIR Govt. of India, New Delhi. T J thanks UGC
for their fellowships. We acknowledge Sophisticated Instrumentation Centre (SIC) facility, IIT
Indore.
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