Synthesis and photophysical properties of 1,2-diphenylindolizine derivatives: Fluorescent blue-emitting materials for organic light-emitting device

Article abstract of DOI:10.1002/bio.2968

New blue-emitting materials based on 1,2-diphenylindolizine were designed and synthesized through a microwave-assisted Suzuki coupling reaction. The photophysical, electrochemical, and thermal properties of the 1,2-diphenylindolizine derivatives were inve

Full text of DOI:10.1002/bio.2968

Research article
Received: 13 May 2015,
Revised: 8 June 2015,
Accepted: 12 June 2015
Published online in Wiley Online Library: 20 July 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2968
Synthesis and photophysical properties of
1,2-diphenylindolizine derivatives: fluorescent
blue-emitting materials for organic
light-emitting device
Yeong Ran Song,a Choon Woo Limb* and Tae Woo Kima*
ABSTRACT: New blue-emitting materials based on 1,2-diphenylindolizine were designed and synthesized through a microwave-
assisted Suzuki coupling reaction. The photophysical, electrochemical, and thermal properties of the 1,2-diphenylindolizine
derivatives were investigated using UV–visible and fluorescence spectroscopy, cyclic voltammetry, thermogravimetric analysis,
and differential scanning calorimetry. The 1,2-diphenylindolizine derivatives had band gaps of 3.1–3.4 eV and indicated proper
emission of around 450 nm without significant difference between in solution and thin solid film. The indolizine derivatives
show an enhanced thermal stability (ΔTm > 100 °C), compared with 1,2-diphenylindolizine. These results suggest the 1,2-
diphenylindolizine derivatives are suitable for blue-emitting materials in organic light-emitting devices. Copyright © 2015
John Wiley & Sons, Ltd.
Keywords: indolizine derivatives; fluorescent blue-emitting material; microwave-assisted Suzuki coupling reaction; OLED
Introduction
Experimental
Organic light-emitting diodes (OLEDs) are attracting scientific and
industrial interests related to their promising applications in large-
area, full colour, flexible display (1–4). Although OLED technology
has been commercialized for small- to medium-sized displays,
the development of emitting materials is still demanded for full
colour display applications. In particular, blue-emitting material is
considered as a key material for low power consumption and long
lifetime, as purity of blue colour is essential for the control of white
colour coordinates in a full colour display and the improvement of
blue-emitting materials has a crucial effect on device perfor-
mances such as current efficiency, operating voltage and lifetime
(3,5–10).
Various fluorescent blue-emitting materials including anthra-
cene (11–17), fluorene (18–27), pyrene (28–30), quinoline (31–33),
indenopyrazine (34), and styrene derivatives (35,36) have been
recently reported in order to improve device performances such
as colour purity.
Recently, indolizines, 10 π-electron aromatic N-heterocyclic
compounds, have been reported for use as luminescent materials
such as for optoelectronic devices (37,38), dyes (39–41), sensors
(42) and probes (43–45), Indolizines are highly fluorescent and
regarded as possible candidates for emitting material in organic
electronic devices. Although several applications are based on
the optical properties of indolizines, only in a few studies have in-
vestigated the synthesis and optical properties of indolizine deriv-
atives (46). In this viewpoint, we have designed and synthesized
new fluorescent materials based on their indolizine moiety, which
are probable for emitting blue colour in a device performance.
Their photophysical and thermal properties are also fully
investigated.
Materials and methods
All reagents were used as received from commercial sources with-
1
out further purification. H and 13C NMR spectra of the samples
were measured on a JNM-AL300 ( JEOL) spectrometer. Chemical
shifts were reported as units parts per million (ppm), and J-values
were in Hz. Mass spectra were obtained by fast atom bombard-
ment mass spectrometry (FAB-MS) using a JMS-700 ( JEOL).
Synthesis
1,2-Diphenylindolizine (1). 2-Benzylpyridine (1.69 g, 10.0mmol)
and 2-bromoacetophenone (1.99 g, 10.0mmol) in acetone
(30mL) were mixed well. The mixture was refluxed at 80°C for
* Correspondence to: Choon Woo Lim, Department of Chemistry, College of Life
Science and Nanotechnology, Hannam University, Daejeon 305–811, Republic
of Korea. E-mail: cwlim@hnu.kr
*
Tae Woo Kim, Graduate School of East-West Medical Science, Kyung Hee Uni-
versity, Gyeonggi-do 449–701, Republic of Korea. E-mail: tw1275@khu.ac.kr
a
Graduate School of East-West Medical Science, Kyung Hee University,
Gyeonggi-do, 449-701, Republic of Korea
b
Department of Chemistry, College of Life Science and Nanotechnology,
Hannam University, Daejeon 305-811, Republic of Korea
Abbreviations: CV, Cyclic voltammetry; DFT, density functional theory; DSC,
differential scanning calorimetry; HOMO, highest occupied molecular orbital;
LUMO, lowest unoccupied molecular orbital; OLED, Organic light-emitting
diodes; TGA, thermal gravimetric analysis.
Luminescence 2016; 31: 364–371
Copyright © 2015 John Wiley & Sons, Ltd.

Fluorescent blue emitting material
4 h with stirring. The reaction mixture was then cooled to room
temperature and filtered. The filter cake was dissolved in 80 °C wa-
ter (50 mL). Potassium carbonate (2.10 g, 15.2 mmol) was added to
the solution. The reaction mixture was refluxed at 80 °C for 8 h with
stirring. After filtering, the filter cake was dissolved in hot ethanol
and recrystallized. The recrystallization gave the desired product
microwave-irradiated for 60 min (temperature: 170 °C, initial set-
ting power: 50 W) in opened vessel mode at CEM Discover®. After
cooling, the mixture was poured into water and extracted with
dichloromethane. The organic phase was separated and dried
over anhydrous MgSO4. The solvent was removed by rotary
evaporation and from the triturate in methanol/THF, we obtained
1
1
as a yellow needle-shaped solid (0.624 g, yield: 23.1%). H NMR
the desired product (1.75 g, 55.0%). H NMR (CDCl3, 300 MHz): δ
(CDCl3, 300MHz): δ 7.87 (d, J =7.0 Hz, 1H), 7.47 (m, 2H), 7.33 (m,
7H), 7.22 (m, 3H), 6.78 (ddd, J = 9.1, 9.1, 1.1 Hz, 1H), 6.46 (td,
J = 6.8, 1.0 Hz, 1H); 13C NMR (CDCl3, 75MHz): δ 135.2, 135.1, 131.0,
130.2, 129.1, 128.3, 128.2, 128.2, 126.3, 125.6, 125.0, 118.0, 117.9,
112.4, 111.3, 111.0; LR-MS (FAB, positive): calculated 269 for
C20H15N, observed 269 for [M]+.
7.89 (d, J = 7.00 Hz, 1H), 7.79 (t, J = 8.4, 8.07 Hz, 2H), 7.64–7.59
(m, 2H), 7.49–7.25 (m, 15H) ,7.23–7.20 (m, 7H), 6.67 (ddd, J = 9.0,
6.4, 1.0 Hz, 1H), 6.48 (td, J = 6.8, 1.0 Hz, 1H); 13C NMR (CDCl3,
75 MHz): δ 151.8, 151.4, 145.9, 140.3, 139.8, 139.3, 139.0, 135.1,
134.2, 131.2, 130.3, 129.3, 128.3, 128.2, 128.1, 127.6, 127.5,
126.9, 126.6, 126.4, 126.2, 125.7, 124.9, 124.6, 120.4, 120.16,
117.89, 112.41, 111.25, 111.05; HR-MS (FAB, positive): calculated
585.2456 for C45H31N, observed 585.2457 for [M ]+.
2-(4-Bromophenyl)-1-phenyl-indolizine (1a). 2-Benzylpyridine
(6.32 g, 37.4 mmol) and 2,4′-dibromoacetophenone (10.0 g,
36.3 mmol) in acetone were mixed well in a reaction vessel of
CEM Discover®. The mixture was microwave-irradiated for
60 min (temperature: 65 °C, initial setting power: 70 W) in
opened-vessel mode. After cooling, the mixture was filtered with
acetone. The filter cake was dissolved in 80 °C water and potas-
sium carbonate was added to a solution. The mixture was mixed
well in a CEM Discover® reaction vessel. The mixture was
microwave-irradiated for 60 min (temperature: 110 °C, initial set-
ting power: 180 W) in opened-vessel mode. After filtering, the
mixture was recrystallized with ethanol. The recrystallization
gave the desired product as a yellow needle-shaped solid
(5.92 g, yield: 47.2%). 1H NMR (CDCl3, 300 MHz): δ 7.82 (d,
J = 7.0 Hz, 1H), 7.42–7.24 (m, 7H), 7.22–7.08 (m, 4H), 6.61 (dd,
J = 9.0, 6.8 Hz, 1H), 6.43 (td, J = 6.6, 1.0 Hz, 1H); 13C NMR (CDCl3,
75 MHz): δ 134.8, 134.2, 131.4, 131.2, 130.6, 130.2, 128.4, 126.9,
125.9, 125.0, 120.4, 118.1, 118.0, 112.3, 111.2, 111.1.
Diphenyl-[4′-(1-phenyl-indolizin-2-yl)-biphenyl-4-yl]-amine (4). To a
solution of 2-(4-bromophenyl)-1-phenyl-indolizine (3.00 g,
8.63 mmol) and 4-(diphenylamino)phenylboronic acid(3.61 g,
12.5 mmol) in 1,4-dioxane (17.5mL), cesium carbonate (17 mL,
1.5 M solution in water, 26.0mmol) and tetrakis(triphenyl-phos-
phine)palladium (0.397g, 0.344 mmol) were added. The mixture
was microwave-irradiated for 60 min (temperature: 110 °C, initial
setting power: 80 W) in opened-vessel mode in a CEM Discover®.
After cooling, to the mixture was added to dichloromethane and
water. The organic phase was washed with saturated NaHCO3
and brine. The organic layer was dried using anhydrous MgSO4. Af-
ter removal of the solvent, the residual was purified over silica gel
using hexane/dichloromethane (v/v, 20:1 to 5:1) to yield as a yel-
low solid (2.79 g, 63.2%). 1H NMR (CDCl3, 300 MHz): δ 7.88 (d,
J = 7.00 Hz, 1H), 7.48–7.45 (m, 6H), 7.36–7.30 (m, 6H), 7.27 (s, 1H)
,7.22 (d, J = 1.50 Hz, 3H), 7.12–7.09 (m, 6H), 7.00 (t, J = 7.32,
6.96 Hz, 2H), 6.65 (dd, J = 9.0, 6.4Hz, 1H), 6.47 (td, J = 6.8, 0.9Hz,
1H); 13C NMR (CDCl3, 75MHz): δ 147.7, 147.0, 138.3, 135.2, 134.8,
133.7, 131.2, 130.3, 129.3, 129.2, 128.4, 127.8, 127.5, 126.4, 125.7,
125.0, 124.3, 124.0, 122.8, 118.0, 117.9, 112.4, 111.3, 111.0; HR-MS
(FAB, positive): calculated 512.2252 for C38H28N2, observed
512.2255 for [M]+.
2-[4-(9,9-Dimethyl-9H-fluoren-2-yl)-phenyl]-1-phenyl-indolizine (2). To
a solution of 2-(4-bromophenyl)-1-phenyl-indolizine (1.92 g,
5.52 mmol) and 9,9-dimethylfluorene-2-boronic acid pinacol es-
ter (2.23 g, 6.97 mmol) in 1,4-dioxane (30 mL), cesium carbonate
(5.6 mL, 2 M solution in water, 11.1 mmol) and tetrakis(triphenyl-
phosphine)palladium (0.322 g, 0.279 mmol) were added. The
mixture was microwave-irradiated for 60 min (temperature:
110 °C, initial setting power: 50 W) in opened-vessel mode in a
CEM Discover®. After cooling, the mixture was poured into water
and extracted with dichloromethane. The organic phase was
separated and dried over anhydrous MgSO4. The solvent was
removed by rotary evaporation, and the trituration in
methanol/THF gave the desired product (2.00 g, 78.6%). 1H
NMR (CDCl3, 300 MHz): δ 7.83 (d, J = 7.00 Hz, 1H), 7.66 (t, J = 8.04,
7.62 Hz, 2H), 7.58 (d, J = 1.5 Hz, 1H), 7.51 (d, J = 8.00 Hz, 3H),
7.43–7.17 (m, 12H), 6.59 (dd, J = 7.9, 7.5 Hz, 1H), 6.41 (td, J = 6.6,
1.0 Hz, 1H), 1.44 (s, 6H); 13C NMR (CDCl3, 75 MHz): δ 154.2, 153.8,
140.0, 139.3, 138.9, 138.3, 135.2, 134.1, 131.2, 130.3, 129.38,
128.4, 127.7, 127.2, 127.0, 127.0, 125.9, 125.8, 125.0, 122.9,
121.1, 120.2, 120.0, 118.0, 117.9, 112.5, 111.3, 111.0, 46.9, 27.2;
HR-MS (FAB, positive): calculated 461.2143 for C35H27N, observed
461.2149 for [M]+.
9-Phenyl-3-[4-(1-phenyl-indolizin-2-yl)-phenyl]-9H-carbazole (5). To a
solution of 2-(4-bromophenyl)-1-phenyl-indolizine (0.812 g,
2.33 mmol) and 9-phenyl-3-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-
2-yl)-9H-carbazole (1.13 g, 3.06 mmol) in N-methyl-2-pyrrolidone
(20 mL), cesium carbonate (5.5 mL, 2 M solution in water,
11.0 mmol) and tetrakis (triphenyl-phosphine) palladium
(0.142 g, 0.122 mmol) were added. The mixture was microwave-
irradiated for 60 min (temperature: 170 °C, initial setting power:
50 W) in opened-vessel mode in a CEM Discover®. After cooling,
the mixture was poured into water and extracted with dichloro-
methane. The organic phase was separated and dried over anhy-
drous MgSO4. The solvent was removed by rotary evaporation.
From the triturates in methanol/THF, we obtained the desired
product (0.828 g, 69.5%). 1H NMR (CDCl3, 300 MHz): δ 8.29 (d,
J = 1.8 Hz, 1H), 8.10 (d, J = 7.70 Hz, 1H), 7.84 (d, J = 7.00 Hz, 1H),
7.59–7.49 (m, 7H), 7.43–7.27 (m, 13H), 7.24 (m, 2H), 6.60 (dd,
J = 9.0, 6.4 Hz, 1H), 6.42 (t, J = 6.4 Hz, 1H); 13C NMR (CDCl3,
75 MHz): δ 141.3, 140.3, 139.7, 137.6, 135.3, 133.5, 133.1, 131.2,
130.3, 129.9, 129.4, 128.4, 127.9, 127.5, 127.0, 127.0, 126.1,
125.7, 125.3, 125.0, 123.4, 123.5, 120.3, 120.0, 118.5, 118.0,
117.9, 111.3., 111.0, 110.0, 109.9; HR-MS (FAB, positive): calculated
510.2095 for C38H26N2, observed 510.2100 for [M]+.
2-[4-(9,9-Diphenyl-9H-fluoren-2-yl)-phenyl]-1-phenyl-indolizine (3). To
a solution of 2-(4-bromophenyl)-1-phenyl-indolizine (1.89 g,
5.42 mmol) and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9,9-diphenyl-9H-fluorene (3.15 g, 7.08 mmol) in N-methyl-2-
pyrrolidone (30 mL), cesium carbonate (5.5 mL, 2 M solution in
water, 11.0 mmol) and tetrakis(triphenyl-phosphine)palladium
(0.313 g, 0.271 mmol) were added. The mixture were
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Y. R. Song et al.
Scheme 1. Structures of 1–5 and their synthetic routes; (a) acetone, reflux, 4 h; (b) K2CO3, H2O, reflux, 8 h; (c) Pd(PPh3)4, Cs2CO3, NMP (2,3), MW, 1 h, 170 °C or 1,4-dioxane (4,5),
MW, 1 h, 110 °C . MW = microwave, NMP = N-methyl-2-pyrrolidone.
Figure 1. The absorption spectra and fluorescence spectra of (a) compound 1, (b) compound 2, (c) compound 3, (d) compound 4, (e) compound 5. (f) Photograph shows the
fluorescence of compounds (1–5) in dichloromethane taken under a laboratory UV lamp (λex = 365 nm) irradiation.
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Luminescence 2016; 31: 364–371

Fluorescent blue emitting material
UV–visible light and photoluminescence (PL) measurement
where ST and X denote the standard and the sample, respectively,
Φ is the fluorescence quantum yield and η is the refractive index of
the solvent. 9,10-Diphenylanthracene was used as the reference
standard (ΦST = 0.90 in dichloromethane) with an excitation wave-
length at 375 nm (49).
UV–visible spectra were measured on a JASCO V-560 UV-vis.
spectrophotometer at ambient temperature. A cuvette with a
1-cm path length (Quarts SUPRASIL 105B-QS) was used for mea-
suring samples that were prepared from serial dilution of a sam-
ple stock solution (1.0× 10–3 mol/L). The concentration of dilution
solution was 1.0× 10–5 mol/L. PL spectra were measured on an
RF5301PC (Shimadzu) at ambient temperature. A cell with a
1-cm path length was used for measuring the sample, which
was prepared from serial dilution of sample stock solution
(1.0 × 10–3 mol/L). The final concentration of dilution solution
was 1.0× 10–7 mol/L.
Cyclic voltammetry (CV)
Highest occupied molecular orbital (HOMO) levels of com-
pounds were obtained from the CV measurement. The redox
potentials of compounds were measured on a E2P (BASi) with
conventional three-electrode configuration, consisting of a
glassy carbon working electrodes of 3 mm diameter, a plati-
num wire counter electrode and a non-aqueous Ag/AgNO3
reference electrode at room temperature. The CV was carried
out in deoxygenated acetonitrile containing 1 mM compound
and 0.1 M tetrabutylammonium perchlorate as the supporting
electrolyte and the scan rate was 100 mV/s. The E1/2 values
were determined by (Epa + Epc)/2, where Epa and Epc are the an-
odic and cathodic peak potential, respectively. The followed
equation,
Film photoluminescence measurement
Film PL spectra were measured on a JASCO JP/FP-6500 at ambient
temperature. A film sample on quartz glass was prepared from
spin-coating with ACF-200 (DONG AH Trade Corp). The sample
was dissolved in chlorobenzene. The stock solution (400μL,
4–5 × 10–3 mol/L) was used for preparing the spin-coated sample.
À
Á
HOMO ðeVÞ
¼
ꢀ4:8 ꢀ E1=2
ꢀE1=2 ðferroceneÞ
Quantum yield
redox potential
The quantum yield was determined according to the HORIBA
Jobin Yvon guidelines (47,48). The absorbance for the standard
and the samples at the excitation wavelengths and the fluores-
cence spectra of the solutions were measured respectively. The
areas of the integrated fluorescence intensity were plotted against
the absorbance to obtain a linear plot with a gradient (Grad) that
could be used to calculate the quantum yield according to follow-
ing equation:
where 4.8 eV is the energy level of the ferrocene reference
(below the vacuum level) (50). The lowest unoccupied molec-
ular orbital (LUMO) levels were calculated from the HOMO
level and energy band gap, which was obtained from the
edge of absorption spectra.
Thermal analysis
À
Á
The glass-transition temperatures (Tg), crystallization temperatures
(Tc), melting temperatures (Tm) and degradation temperatures (Td)
ΦX ¼ ΦST ðGradX=GradST
Þ
η2x=η2ST
Table 1. UV–visible absorption wavelength of compounds (1–5) in various solutions and film
Compd λabs / nm
λ
em (fwhm)/ nm
MC
2-MeTHF
THF
MC
ACN
2-MeTHF
THF
ACN
Film
1
2
3
4
5
361
318
324
343
299
362
319
324
343
300
363
319
323
345
300
359
318
322
341
298
454 (81)
451 (80)
452 (80)
452 (80)
452 (81)
455 (81)
454 (79)
454 (78)
453 (81)
456 (80)
459 (82)
456 (81)
455 (80)
453 (85)
455 (82)
457 (83)
453 (80)
455 (84)
454 (81)
454 (81)
457 (86)
456 (82)
461 (89)
457 (81)
460 (87)
Table 2. Photophysical and electrochemical properties of compounds (1–5)
Compd
Φfa (%)
Eoxb (eV)
Egopt(cal), c (eV)
HOMO/LUMOopt(cal) (eV)
1
2
3
4
5
0.41
0.21
0.23
0.32
0.15
0.30
0.30
0.34
0.29
0.27
3.1(4.10)
3.4(3.82)
3.4(3.76)
3.2(3.87)
3.4(4.02)
5.0(4.92)/1.9 (0.82)
5.0(4.95)/1.6 (1.13)
5.1(4.94)/1.7 (1.18)
5.0(4.85)/1.8 (0.98)
5.0(4.90)/1.6 (0.88)
aMeasurement in CH2Cl2 solution; λex = each λmabasx. Quantum yields (Φf) were measured using 9, 10-diphenylanthracene as a reference
at room temperature.
bThe concentration of the complexes used in these experiments was 1 mM and the scan rate was 100 mVsec–1.
cEogpt, optical band gap obtained from the onset wavelength of optical absorption in CH2Cl2 solution (Eg = hc/ λonset).
Luminescence 2016; 31: 364–371
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Y. R. Song et al.
of the compounds were measured by carrying out differential
scanning calorimetry (DSC) under a nitrogen atmosphere at
heating/cooling rate of 10°C/min using a Q-1000 (TA instruments)
and thermogravimetric analysis (TGA) under a nitrogen atmo-
sphere at a heating rate of 10 °C/min using a Q 5000 IR (TA
instruments).
Results and discussion
Synthesis
Scheme 1. The synthetic routes for compounds 1–5 are outlined in
Scheme 1. The design idea to these compounds was straightfor-
ward: 1,2-diphenylindolizine backbone (45,51) was derived with
various modifying groups and the derivatization can be easily real-
ized by microwave-assisted Suzuki coupling reaction. The synthe-
sis could be generalized into three steps. Firstly, the reaction of
2-benzylpyridine with 2-bromoacetophenone led to the expected
pyridinium halide. Then, an intramolecular condensation by potas-
sium carbonate followed by crystallization afforded compound 1a
with a yield of 47%. Secondly, the brominated triphenylamine,
phenylcarbazole, dimethylfluorene, and diphenylfluorene were
changed into boronic acid or boronic acid pinacol ester (6). Finally,
the product were synthesized through a microwave-assisted
Suzuki cross-coupling reaction (52).
Theoretical calculation
Three-dimensional geometries and frontier molecular orbital en-
ergy levels of compounds (1–5) were calculated using density
functional theory (DFT) at the B3LYP/6–31G (d) basis set in vac-
uum. All calculations were performed with GAUSSIAN 09′ rev. 03
Photophysical property
Photophysical properties of the compounds 1–5 were investigated
by measuring the absorption spectra and photoluminescence
spectra in various solvents (2-methyltetrahydrofuran, tetrahydrofu-
ran, dichloromethane and acetonitrile) as well as in solid-state thin
film, which was prepared by spin-coating a chlorobenzene solu-
tion on to quartz plates. As can be seen in Fig. 1, the absorption
wavelength ranges from 300 nm to 365nm. The emission wave-
length of all compounds appears around 450 nm in various sol-
vents and solid-state thin film. The absorption and the PL spectra
of the synthesized materials in different solution and thin film
are almost identical, except for the thin film spectra, which are
red-shifted 5 ~ 10 nm. The small differences of spectra between
Figure 2. Cyclic voltammetry spectra (V versus Ag/AgNO3) of compounds (1–5) in
deoxygenated acetonitrile at scan rate 100 mV/sec.
Figure 3. The spatial distributions of HOMO and LUMO of compounds (1–5) from the DFT calculation (B3LYP/6–31G).
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Fluorescent blue emitting material
in solution and thin film suggest the absence of strong intermolec-
ular interactions in thin films (Table 1). It also indicates that the
bulky substituent and non-coplanar end-capped groups of these
compounds have restrained an expanded conjugation in the
molecules. In other words, the twisted substituent and the attach-
ment of bulky side-groups to the indolizine core can suppress π–π*
stacking interactions of the molecules in the solid state and reduce
self-quenching effects. Fluorescence quantum yields (Φf) of the
1,2-diphenylindolizine derivatives in dilute dichloromethane
solution were measured by using 9,10-diphenylanthracene
(Φf = 0.90 in dichloromethane) as a standards The quantum yields
of compounds 1–5 are shown in Table 2.
Table 3. Thermal properties (Tg, Tm, and Td) of compounds (1–5)
Compd
Tga (°C)
Tma (°C)
Tdb (°C)
1
2
3
4
5
–
114.02
274.75
225.45
196.44
237.45
213
299
358
359
350
102
106
109
116
Electrochemical property
Energy levels (HOMO and LUMO) of the compounds 1–5 were
studied using a CV and UV-visible spectrophotometer. The HOMO
levels were determined using CV in acetonitrile solution and are
shown in Fig. 2. HOMO levels of compounds 1–5 are –5.0, –5.0,
–5.1, –5.0 and –5.0eV, respectively. The CV curves of these
compounds with an irreversible two-electron oxidation might be
attributable to indolizine unit and substituted groups.
aTg ( glass-transition temperature) and Tm (melting temperature)
were obtained from DSC measurements under a N2 atmosphere
at a heating rate of 10 °C min–1.
bTd (decomposition temperature) was obtained from TGA
measurements and corresponds to 5% weight loss.
Figure 4. (a) TGA curves of compounds 1–5. DSC curve of (b) compound 1, (c) compound 2, (d) compound 3, (e) compound 4, and (f) compound 5.
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Y. R. Song et al.
The LUMO levels were estimated from the HOMO levels and the
optical bandgaps (Eg), which were derived by the onset wave-
lengths obtained from UV–visible absorption spectra of com-
pounds 1–5 (53). Herein, the onset wavelengths of UV–visible
absorption spectra of compounds 1–5 are 400, 365, 365, 388 and
365 nm, respectively. The LUMO levels of all materials were
obtained from the HOMO and the optical band gaps were –1.9,
–1.6, –1.7, –1.8 and –1.6eV, respectively. The values of HOMOs,
LUMOs and band gaps are summarized in Table 2.
Triphenylamine, phenylcarbazole, dimethylfluorene, and diphenyl-
fluorene were coupled to 2-(4-bromophenyl)-1-phenyl-indolizine
using a palladium-catalyzed Suzuki-Miyaura reaction. Theoretical
molecular orbital calculations have shown that all compounds
have a non-coplanar structure. Compounds 2–5 have a blue-
emitting wavelength of around 450 nm and the emission wave-
length of compounds 2–5 was rarely affected by solvent polarity
and by the liquid/solid state. The electrochemical properties were
verified through cyclic voltammetry and thermal stabilities were
enhanced by attaching substituents to a core moiety. The
photophysical, electrochemical, and thermal properties indicated
that 1,2-diphenylindolizine derivatives would be a promising can-
didate as blue fluorescent emitting materials in OLEDs.
Theoretical calculation
The corresponding HOMO of these 1,2-diphenylindolizine deriva-
tives in S0 state were calculated in the gas phase to approximate
the solution state instead of in the condensed phase and were
compared with each other in Fig. 3. Because electronic excitation
from the HOMO to the LUMO produces the excited state (S1), the
orbital features provide important clues towards understanding
the nature of the optically accessible excited state. In addition,
the structural features offer further explanation for the disruption
of the conjugation in the molecules. The calculation values of the
HOMO and LUMO levels were summarized in Table 2. Figure 3
shows the spatial distributions of the compounds 1–5. The elec-
tron density of the HOMO for most compounds is localized on
the indolizine unit, while that of compound 4 is mainly distributed
over triphenyl amine moiety. All molecules have non-coplanar
twisted conformation due to the steric hindrances between
indolizine moiety and the substituted phenyl groups. Such a
unique structural feature is beneficial in OLEDs application by
preventing the excessive intermolecular interaction against fluo-
rescence quenching and also by suppressing the crystallization
problem to establish the uniform amorphous morphology in the
film state of OLED devices (6).
Acknowledgements
This research was supported by the Basic Science Research Pro-
gram through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (NRF-2011–0024970 and
NRF-2013R1A1A2065643).
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