Synthesis, characterisation and electroluminescence behaviour of conjugated imidazole-isoquinoline derivatives

Article abstract of DOI:10.1016/j.dyepig.2013.11.002

A series of highly fluorescent, isoquinoline conjugated imidazole derivatives have been synthesized and fully characterized. Their photophysical, electrochemical and thermal properties have also been discussed. The interplay of amorphous and crystalline nature of the compounds in thin film and fine powder have been analysed by using X-ray diffraction and atomic force microscopic (AFM) techniques. An organic light emitting diode consisting of 4-(4-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenyl)isoquinoline as the emitting layer doped with 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CPB) showed ideal blue emission (CIE: 0.16, 0.08). Pure white light (CIE: 0.34, 0.32) comprising of a blue light from excimers and an orange light from electromers at a high voltage has been achieved by using the compounds 2-(4-(isoquinolin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole & 2-(4-(isoquinolin-4-yl)phenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d] imidazole as single-emitting components. To elucidate the structural and optical properties, computational calculations have been performed. Computed results reveal all the electronic transitions are of type and energy of the S ₀S₁ follows same variation trend with the band gap.

Full text of DOI:10.1016/j.dyepig.2013.11.002

Dyes and Pigments 102 (2014) 180e188
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, characterisation and electroluminescence behaviour
of
p-conjugated imidazoleeisoquinoline derivatives
,
N. Nagarajan a, Asit Prakash b, G. Velmurugan a, Nanda Shakti b, Monica Katiyar b
**
,
,
a,1,
*
P. Venuvanalingam a , R. Renganathan
*
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
b Department of Materials Science & Engineering and Samtel Centre for Display Technologies, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of highly fluorescent, isoquinoline
p-conjugated imidazole derivatives have been synthesized
Received 6 September 2013
Received in revised form
31 October 2013
Accepted 3 November 2013
Available online 13 November 2013
and fully characterized. Their photophysical, electrochemical and thermal properties have also been
discussed. The interplay of amorphous and crystalline nature of the compounds in thin film and fine
powder have been analysed by using X-ray diffraction and atomic force microscopic (AFM) techniques.
An organic light emitting diode consisting of 4-(4-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-
yl)phenyl)isoquinoline as the emitting layer doped with 4,40-Bis(N-carbazolyl)-1,10-biphenyl (CPB)
showed ideal blue emission (CIE: 0.16, 0.08). “Pure” white light (CIE: 0.34, 0.32) comprising of a blue light
from excimers and an orange light from electromers at a high voltage has been achieved by using the
compounds 2-(4-(isoquinolin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole & 2-(4-(isoquinolin-4-yl)
phenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole as single-emitting components. To
elucidate the structural and optical properties, computational calculations have been performed.
Keywords:
Imidazole
Isoquinoline
Blue light
Pure white light
Electromer
TD-DFT
Computed results reveal all the electronic transitions are of
follows same variation trend with the band gap.
p
/
p
* type and energy of the S0 / S1
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
compounds. But most of them lack amorphous nature in thin films
and OLEDs usually require formation of very thin organic layers
with amorphous morphologies that provide pinhole free thin films.
Compared to the multi-emitting-component WOLEDs, a single-
emitting-component WOLEDs could show many advantages, such
as better stability, better reproducibility, and a simpler fabrication
process. However, few materials are known to show white-light
emission as a single-emitting component but synthesized by
multistep process, among them very few have been reported to
emit “pure” white light [10e12]. Therefore, the search for new
organic light-emitting materials with new structures for use in
single-emitting-component WOLEDs is of obvious interest and
importance. Excluding the utilisation of mixture of three organic
components emitting three colours (blue, green, and red) to get
white light [13.14], a widely employed approach in recent years is
the use of single chromophore that emits white light resulted from
the combination of its monomeric (blue) and excimeric or elec-
tromeric (orange) emission [15e18]. The latter emission results
Since Tang and co-workers [1] report of an archetypical example
of a modern Organic light emitting diodes (OLEDs), it has been a
topic of great interest for many researchers due to their applica-
tions in lighting and flat panel displays. A wide variety of im-
provements to OLEDs have been made using novel materials and
device structures to achieve improved efficiency at simple syn-
thetic routes, easy device fabrication and low driving voltages [2e
5]. The hunt for efficient blue and white electroluminescence is of
particular interest because it is an essential component to realise
OLEDs in display as well as lighting applications. Many research
groups have successfully prepared efficient blue [6,7] and white
fluorophores [8,9] for OLEDs, but efficient one, which can match the
National Television System Committee (NSTC) and Commission
Internationale d’Énclairage standard blue CIE (x,y) coordinates of
(0.14, 0.08) and white (0.33, 0.33) has been achieved by very few
from a stacking of molecules that is usually induced by
interactions.
Materials containing aromatic heterocyclic segments, e.g.,
quinoline [19,20], and oxidazole [21], had been reported to exhibit
better electron injection and transport ability, but these materials
pep
* Corresponding authors. Tel.: þ91 431 2407053; fax: þ91 431 2407045.
** Corresponding author.
E-mail address: rrengas@gmail.com (R. Renganathan).
1
Tel.: þ91 431 2407053; fax: þ91 431 2407045.
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.11.002

N. Nagarajan et al. / Dyes and Pigments 102 (2014) 180e188
181
were found to be undesirable as emitting layers in OLEDs due to
2.2.2. Synthesis of compounds 1e4
their low luminance [22]. The typical heterocyclic molecule, imid-
azole with N1, C2, C4 and C5 positions substituted with different
groups are used extensively in OLEDs for efficient blue [23e25] and
white light applications [26]. Herein, we report the synthesis,
crystal structure, film forming behaviour, photophysical, photo and
electroluminescence properties of imidazole substituted iso-
A
mixture of corresponding diketone (1 mmol), 4-(iso-
quinolin-4-yl)benzaldehyde (1 mmol), p-anisidine (4 mmol, in
case of 2 & 4), ammonium acetate (4 mmol) and glacial acetic acid
(8 mL) was refluxed for 4 h under nitrogen atmosphere. After
cooling to room temperature, the reaction mixture was poured
into distilled water with stirring. The separated solid was filtered
off, washed with water and dried to give the expected product in
good yields.
quinoline derivatives. These new
p-conjugated compounds, in
particular, the phenanthrene based compounds, can serve as a
single-emitting component for WOLEDs, emitting almost “pure”
white light with stable CIE coordinates under different driving
voltages.
2.2.2.1. 4-(4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl)isoquinoline
(1). 1H NMR (DMSO-d6): 7.24 (1H, d, 7.2 Hz), 7.31 (2H, t, 7.6 Hz),
7.39 (1H, d, 7.2 Hz), 7.45 (2H, t, 7.6 Hz), 7.53 (2H, d, 7.2 Hz), 7.58
(2H, d, 7.6 Hz), 7.67 (2H, d, 8.4 Hz), 7.75 (1H, m), 7.82 (1H, m),
7.93 (1H, d, 8 Hz), 8.23 (1H, d, 7.6 Hz), 8.28 (2H, m), 8.52 (1H, s),
9.36 (1H, s), 12.84 (1H, s); 13C NMR: 123.9, 125.4, 126.5, 127.1,
127.5, 127.7, 128.0, 128.1, 128.2, 128.4, 128.6, 129.9, 130.2, 130.9,
131.1, 131.3, 132.0, 133.0, 135.1, 136.0, 137.3, 142.4, 145.1, 152.0;
HRMS (ESI) calcd for C30H21N3 (M þ H)þ 424.1813, found
424.1816.
2. Experimental section
2.1. General methods
The 1H and 13C NMR spectra were measured on a Bruker Avance
400 (400 MHz) NMR spectrometer. Mass spectra were obtained on an
FDMS, VG Instruments ZAB-2 mass spectrometer. Steady state spec-
troscopic measurements were conducted both in solution and thin
films prepared by vacuum (2 ꢀ 10ꢁ6 mbar) deposition on a quartz
plate. The thickness of films was measured using Alpha Step profil-
ometer (KLA Tencor). Absorption spectra of solution and thin film
were obtained using UVevis spectrophotometers (JASCO V360).
Photoluminescence emission spectra of solution and thin film were
obtained using fluorescence spectrophotometer (PerkinElmer e LS55
& Horiba Jobin Yvon FluoroLog 3 Spectrofluorometer). Photo-
luminescence quantum efficiencies (FPL values) for solutions were
obtained using 9,10ꢁdiphenylanthracene as reference [27]. The FPL
values of thin films on quartz plates were measured using a 6 inch
integrating sphere (Labsphere) attached with Horiba Jobin Yvon Flu-
oroLog 3 Spectrofluorometer through optical fibre and a PMT detector
(Hamamatsu). Thermogravimetric analyses (TGA) and differential
scanning calorimetry (DSC) were performed under nitrogen atmo-
sphere at heating rate of 10 ꢂC/min. Melting points were determined
by the open capillary tube method using a Toshniwal melting point
apparatus and are uncorrected. The topography of the thin films was
2.2.2.2. 4-(4-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)
phenyl)isoquinoline (2). 1H NMR (CDCl3): 3.79 (3H, s), 6.85 (2H, d,
8.8 Hz), 7.07 (2H, d, 8.8 Hz), 7.19 (3H, m), 7.25 (6H, m), 7.42 (2H, d,
7.2 Hz), 7.63 (5H, m), 7.91 (1H, d, 8 Hz), 8.01 (1H, d, 7.6 Hz), 8.46 (1H,
s), 9.24 (1H, s); 13C NMR: 55.4, 114.3, 124.7, 126.6, 127.2, 127.3, 127.9,
128.0, 128.2, 128.4, 128.6, 128.9, 129.5, 129.8, 129.9, 130.4, 130.6,
130.7, 131.1, 131.4, 132.7, 134.0, 134.5, 136.7, 138.4, 142.8, 146.5, 152.1,
159.3; HRMS (ESI) calcd for C37H27N3O (M þ H)þ 530.2232, found
530.2235.
2.2.2.3. 2-(4-(isoquinolin-4-yl)phenyl)-1H-phenanthro[9,10-d]imid-
azole (3). 1H NMR (DMSO-d6): 7.65 (2H, m), 7.81 (6H, m), 7.98 (1H,
d, 8.8 Hz), 8.26 (1H, d, 8 Hz), 8.51 (2H, d, 8.4 Hz), 8.57 (2H, m), 8.63
(1H, d, 7.2 Hz), 8.87 (2H, m), 9.39 (1H, s), 13.60 (1H, s); 13C NMR:
121.9, 123.9, 1254.3, 126.4, 127.1, 127.5, 128.0, 128.1, 129.9, 130.4,
131.2, 131.9, 133.0, 137.1, 142.4, 148.7, 152.2; HRMS (ESI) calcd for
C
30H19N3 (M þ H)þ 422.1657, found 422.1655.
analysed by Atomic Force Microscopy imaging (5 ꢀ 5
mm and
2 ꢀ 2 m) using acoustic AC mode with a silicon nitride tip (resonance
m
2.2.2.4. 2-(4-(isoquinolin-4-yl)phenyl)-1-(4-methoxyphenyl)-1H-
phenanthro[9,10-d]imidazole (4). 1H NMR (CDCl3): 3.96 (3H, s), 7.14
(2H, d, 8.8 Hz), 7.27 (2H, m), 7.46 (5H, m), 7.65 (3H, m), 7.68 (1H,m),
7.78 (2H, m), 7.90 (1H, d, 8.4 Hz), 8.04 (1H, d, 7.6 Hz), 8.47 (1H, s),
8.71 (1H, d, 8.0 Hz), 8.78 (1H, d, 8.4 Hz), 8.90 (1H, d, 8.0 Hz), 9.25
(1H, s); 13C NMR: 55.7, 115.3, 120.9, 122.7, 123.1, 124.1, 124.6, 124.9,
125.6, 126.3, 127.2, 127.9, 128.3, 128.4, 128.5, 129.3, 129.4, 130.0,
130.1,130.3,130.7, 131.2,132.7,134.0,137.4, 142.6,150.6,152.1,160.4;
HRMS (ESI) calcd for C37H25N3O (M þ H)þ 528.2075, found
528.2074.
frequency of 295 kHz), in order to evaluate the effect of thermal stress
on the thin film morphology. Fluorescence lifetime measurements
were carried out in a picosecond time correlated single photon
counting (TCSPC) spectrometer. The excitation source is the tunable
Tiesapphire laser (Tsunami, Spectra Physics, USA). The fluorescence
decay was analysed by using the software provided by IBH (DAS-6).
2.2. Syntheses
2.2.1. Synthesis of 4-(isoquinolin-4-yl)benzaldehyde (2)
4-bromoisoquinoline (1) (1 g, 4.7 mmol) was treated with 4-
formylphenylboronic acid (0.77 g, 5.17 mmol), Pd[PPh3]4 (0.27 g,
0.235 mmol), K2CO3 (1.94 g, 14.1 mmol) in THF (50 ml) at 70 ꢂC
under nitrogen atmosphere for 12 h and progress of the reaction
was monitored by TLC. The reaction mixture was quenched with
water, extracted with CH2Cl2 and washed with brine (2 ꢀ 10 mL).
The organic layer was separated, dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The crude product was pu-
rified by column chromatography (EtOAc/hexane) to afford pure 4-
(isoquinolin-4-yl)benzaldehyde. The yields and important spectral
data are given below.
2.3. Computational details
All calculations on the synthesized molecules have been
performed using Gaussian 09 [28]. The ground-state geometries
of the studied molecules were fully optimized at the DFT level
using B3LYP [29e32] functional with 6-31G(d,p) basis set and
the excited state geometries were optimized by the ab initio
configuration interaction singles method (CIS) [33]. The vibra-
tional frequency analysis of the optimized geometries confirms
that all the optimized geometries are found to be of minimum
energy on the potential energy surface by exhibiting all real
frequencies. The electronic absorption and emission spectra,
both in vacuum and in solvent, were systematically investigated
by the time-dependent density functional theory (TD-DFT) [34e
37] method at PBE0 [38-39]/6-31þG(d,p) level. The solvent
1H NMR (CDCl3): 7.66 (4H, m), 7.83 (1H, d, 8.4 Hz), 8.02 (3H, m),
8.47 (1H, s), 9.26 (1H, s), 10.09 (1H, s); 13C NMR: 53.5, 76.9, 77.3,
77.6, 124.1, 127.4, 128.0, 128.3, 129.3, 130.7, 131.0, 131.9, 133.5, 135.7,
142.7, 143.2, 152.7, 191.7.

182
N. Nagarajan et al. / Dyes and Pigments 102 (2014) 180e188
effect has been included by the polarized continuum model
(PCM) [40].
2.4. OLED fabrication
Prior to device fabrication, ITO on glass was patterned as 6 mm
wide stripes with a resistivity of 15e20
U
cmꢁ2 and then pixelized
using photolithography technique of 4 mm ꢀ 4 mm to reduce the
leakage current. The substrates were cleaned by sonication in soap
solution, rinsed with deionized water, acetone and isopropyl alcohol
for 10 min in each solvent and dried with nitrogen. Finally, the sub-
stratesweretreatedwithUVozonefor10min.Theaqueoussolutionof
PEDOT:PSS [poly(3,4ꢁethylenedioxythiophene) poly (styrenesulfo-
nate)] was spin coated at 1500 rpm for 1 min to form a 40 nm hole
injecting layer (HIL). Organic layers were deposited sequentially by
thermal evaporation from resistively heated tungsten boats and cru-
Scheme 1. Structures of the synthesized compounds.
cible onto the substrate at a rate of 1 A sꢁ1. The base pressure at room
ꢀ
temperature was 3e4 ꢀ 10ꢁ6 mbar. The deposition rate and the
thickness were measured with a quartz control monitor located near
the substrate holder. After organic film deposition, the substrate was
transferred to glove box and the chamber was vented to load the
cathode sample. A cathode consisting of 0.5 nm LiF or 16 nm calcium
3. Results and discussion
The chemical structure of the synthesized compounds (Scheme
1) and synthetic route are shown in Scheme 2. 4-
bromoisoquinoline and 4-formylphenyl boronic acid were
commercially obtained and compound A can be easily prepared by
using one step Suzuki coupling reaction and it was isolated and
purified by column chromatography on silica gel with good yields
(85%) and high purity. The compound A was then involved in
multicomponent cyclization with respective diketone and amine in
presence of ammonium acetate and acetic acid to give the target
emitters 1e4. The molecular structure of the synthesized com-
pounds were confirmed with NMR, Mass spectrometry and single
crystal X-ray diffraction (for compound 2 & 4) analysis.
ꢁ1
ꢀ
followed by aluminium was deposited at a rate of 0.1 A s for LiF,
ꢁ1
ꢁ1
ꢀ
ꢀ
0.3 A s for calcium and 2 A s for aluminium. The devices were
tested in air within 10 h of fabrication. Only light emitting from the
front face of the OLEDs was collected and used in subsequent effi-
ciencycalculations.Theopticalpropertiesoffabricateddevicessuchas
currentevoltageeluminance (IeVeL) characteristics, electrolumi-
nescence spectra (EL) and CIE colour coordinates have been done
using CS-1000 spectro-radiometer (Konica Minolta) and Keithley
2400 source metre interfaced with computer under dark room.
2.5. Crystallographic measurements
3.1. X-ray crystallography
Crystals of 2 and 4 were grown from CH2Cl2/n-hexane at room
temperature. The preliminary examination and data collection were
performed using a Bruker kappa Apex II CCD detector system single-
Single crystal X-ray diffraction studies were used to further
confirm the structure of 2 (Fig. S1) and 4 (Fig. S2). Crystals 2 and 4
crystallized in a triclinic and monoclinic crystal system in P21/n and
P-1 space group respectively. The important bond lengths, bond
angles and dihedral angles are listed in Table S1 and structural
refinement data are listed in Table S2. The twist angle between the
imidazole ring and 4-methoxy phenyl ring for 2 and 4 are 90.3(5)
and 72.9(2) respectively. The free rotation of phenyl groups
attached in C4 and C5 position in 2 leads to high twist angles about
83.4(5) and 24.2(5) but which is restricted in 4 leads to planner
structure of phenanthrene moiety. The restricted free rotation also
crystal X-ray diffractometer equipped with a fine-focus sealed X-ray
ꢀ
tube usinggraphite-monochromated MoK
a
radiation (
l
¼ 0.71073 A).
Structure solution and refinement were carried out using the X Shell
and SHELXLꢁ97 software package. The unit-cell parameters were
obtained by least-squares fit to the automatically centred settings for
reflections. All calculations were performed using the WINGX soft-
ware package and SHELX programme. Structure solutionwas done by
direct method and refined bya full-matrixleast-squares method on F2
(F: structure factor).
Scheme 2. Synthetic pathway used for the preparation of compounds 1e4.

N. Nagarajan et al. / Dyes and Pigments 102 (2014) 180e188
183
Table 1
Optical, electrochemical and thermal properties of the compounds.
labs (nm)a soln
lem (nm)a soln
labs (nm) film
lem (nm) film
ε ꢀ 103 (Mꢁ1 cmꢁ1
)
Tg/Tm/Td5 (ꢂC)
EoxOnset (V)
Eg (eV)
HOMO/LUMOc (eV)
b
1
2
3
4
333
328
370
365
442
432
435
429
334
329
355
349
443
421
448, 494
450
35.5
28.5
12.8
32.3
112/254/338
102/220/334
167/280/362
108/252/343
1.14
1.32
1.24
1.33
3.18
3.12
3.24
3.17
ꢁ5.49/ꢁ2.31
ꢁ5.67/ꢁ2.55
ꢁ5.59/ꢁ2.35
ꢁ5.68/ꢁ2.57
a
b
c
Absorption and emission spectra were measured in THF solution (<10ꢁ5 M) lem
Estimated from onset of the absorption spectra Eg ¼ (1240/lonset).
¼
lexc.
The HOMO and LUMO energies were determined from cyclic voltammetry and the absorption onset. Ferrocene (4.8 eV) was used as the internal standard in each
experiment. The ferrocene oxidation potential was located at þ450 mV, relative to the Ag/AgNO3 non-aqueous reference electrode.
reduces the twist angle between the imidazole ring and 4-methoxy
phenyl ring in 4. Further details regarding the crystal structure can
be found from the CCDC 917839 for 2 and CCDC 917481 for 4.
position of imidazole group, which can further increase the
interaction in 3.
From the absorption spectra (Fig. S4), it can be seen that all the
compounds show similar absorption bands at a shorter wavelength of
pep
approximately270nm, whichcanbeattributedtothe
of their common benzene ring and the longer wavelength absorption
bands in the range of 325e374nm, whichcanoriginatefromthe
transition of the substituent’s on the C2-position of the imidazolering.
The fluorescence quantum yields (Ffl) were measured sequentially in
solution as well as in solid films to be as high as 0.54 to 0.93 for so-
lutions and 0.27 to 0.69 for solid films (Table 2). From Fig. S5, the
pep*transition
3.2. Optical properties
pep*
Photophysical properties of the four compounds were investi-
gated by measuring the PL spectra in both THF solutions as well as
solid-state thin films on quartz substrates. Key data are summa-
rized in Table 1. As can be seen in Fig.1, all compounds exhibit broad
structureless emission peaks ranging from 418 to 487 nm. It should
be noted that the compounds 1 and 2 show negligibly small peak
shifts in their absorption and PL spectra from THF solutions to solid
states. This indicates that the highly twisted substituent on the 1H-
imidazole position as well as the non-coplanar phenyl moiety at C4
fluorescence decay profiles of all the compounds in THF (
1.12 ns) could be satisfactorily fitted well in monoexponential decay
with a
2 values of nearly unity, indicating that the emission from a
sfl z 0.99e
c
single excited state in each case. Thus, the possibility of the imidazole
and isoquinoline moieties behaving as two separate chromophoric
units can be ruled out. The calculated radiative (kr) and nonradiative
(knr) decayconstants (Table 2)suggestthattheenergyofelectronically
excited state is mainly dissipated by radiative transition pathway.
With these overall photophysical properties of the compounds, we
suggested that these compounds can be utilised as better candidates
for OLED applications.
and C5 position can effectively suppress the intermolecular
pep
stacking in the solid state. On the other hand, it is worth noting that
the absorption and PL spectrum of 3 and 4 is broadened and more
red-shifted in solid thin films. The observed high FWHM of 138 nm
and 104 nm for 3 and 4 may be attributed to the intermolecular
stacking in the solid state. It can be evidenced from the crystal
packing arrangement of 4, in which we noticed strong edge-to-face
interaction between the C22 & C23 carbon of the phenyl group
attached to the isoquinoline moiety with one of the centre core ring
of phenanthrene moiety. The distance between these stacking
pe
p
pep
3.3. Thermal properties
pep
ꢀ
interaction is around 3.5e3.8 A which are shown in Fig. S3. Thus,
the molecular architecture in which conjugated units is stacked to
one another in such a way that strong electronic interactions can
occur over the entire moiety. Notably, 3 shows more bathochromic
shift and two distinct peaks viz., a shoulder at 448 nm may be
ascribed to the normal emission and major emission at 494 nm may
be ascribed to the influence of stacking interaction in solid state
[41,42]. This observed effect is more prominent in 3 and not in 4
possibly due to the absence of 4-methoxy phenyl moiety at 1H-
Thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) were employed to investigate the thermal properties of
the compounds; the related data are listed in Table 1. As shown in
Fig. S6(A), decomposition temperatures (Td), defined as the temper-
ature at which the materials show a 5% weight loss, were measured to
be 338, 334, 362 and 343 ꢂC for 1e4 respectively. From the DSC
measurement (Fig. S6(B)), it is important to note that all the com-
pounds have high glass transition temperature (Tg) of 112 ꢂC, 102 ꢂC,
167 ꢂC and 108 ꢂC for 1e4 respectively. Such high Tg values imply that
they could form morphologically stable amorphous films upon ther-
mal evaporation, which is highly important for application in OLEDs.
1 soln
2 soln
3 soln
1.0
3.4. Electrochemical properties
4 soln
1 film
2 film
3 film
4 film
0.8
0.6
0.4
0.2
0.0
To probe the electrochemical properties of the new compounds,
cyclic voltammetry (CV) measurements have been performed in a
Table 2
Quantum yield (Ff) in both thin film and solution, lifetime (
nonradiative (knr) decay constant values of the compounds.
sf), radiative (kr) and
a
Ffl film
Fflb soln
sfl (ns)
kr (108 sꢁ1
)
knr (108 sꢁ1
)
1
2
3
4
0.46
0.64
0.27
0.69
0.60
0.76
0.54
0.93
0.99
1.06
1.12
1.10
6.07
7.17
4.85
8.41
4.03
2.25
4.07
0.61
400
450
500
550
600
650
Wavelength (nm)
a
Calculated by using integrating sphere method.
Calculated by using 9, 10-diphenylanthracene as a reference in ethanol
b
Fig. 1. Photoluminescence spectra of compounds in THF solution (thin line) and thin
films (thick line).
(
Fr ¼ 0.95) solution (<10ꢁ5 M).

184
N. Nagarajan et al. / Dyes and Pigments 102 (2014) 180e188
Table 3
three-electrode cell setup with 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as the supporting electrolyte and
ferrocene (Fc) as the internal standard. Representative cyclic vol-
tammograms are shown in Fig. S7. The details of the electro-
chemical properties of these compounds are listed in Table 1.
During anodic scans between 0 and 2 V in DMF, only one oxidation
peak is observed at 1.34, 1.45, 1.49 and 1.55 V for 1, 2, 3 and 4
respectively. A one-electron irreversible oxidation wave observed
for all compounds is attributed to the oxidation of the imidazole
based conjugated spacer and electron-rich methoxy segments
which increase the oxidation ability of the compounds (2 and 4)
and they show more anodically shifted redox potentials. This is
consistent with results reported in the literature for the electro-
chemical behaviour of imidazole and its derivatives [43,44]. The
energy levels of the HOMOs of these materials were calculated with
reference to ferrocene (4.8 eV) and the values ranges from ꢁ5.4
to ꢁ5.7 eV. These together with absorption spectra were then used
to obtain the LUMO energy levels (Table 1).
Electroluminescence performance of devices IeV for the compounds 1e4.
b (Cd mꢁ2
)
hcc (Cd Aꢁ1
)
CIE (x,y)
a
Device Emitters EL (nm) Von (V)
L
I
II
III
I
III
IV
III
III
V
1
1
1
2
2
2
3
4
4
612
600
497
473
450
438
8.5
<1
<1
221
5
37.5
591
7.9
e
0.46, 0.37
0.44, 0.41
0.29, 0.42
0.24, 0.29
0.20, 0.20
0.16, 0.08
0.34, 0.32
0.36, 0.35
0.37, 0.37
12.5
8.5 V
12
8.5 V
5.5
e
0.99
e
0.20
e
452, 600 15 V
499, 614 17
512, 650 21
0.19
e
<1
<1
e
Device I: ITO/PEDOT:PSS(40 nm)/NPB (30 nm)/EML (40 nm)/LiF (0.5 nm)/Al
(100 nm).
Device II: ITO/PEDOT:PSS(40 nm)/NPB (30 nm)/EML (40 nm)/Ca (16 nm)/Al
(100 nm).
Device III: ITO/NPB (60 nm)/EML (40 nm)/BCP (15 nm)/Alq3 (20 nm)/LiF (0.5 nm)/
Al(100 nm).
Device IV: ITO/NPB (60 nm)/CBP: 10% EML (40 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al
(100 nm).
Device V: ITO/EML (130 nm)/LiF (0.5 nm)/Al (100 nm).
a
Von: turn-on voltage.
L: Luminance.
hc: current efficiency.
3.5. Morphology
b
c
Furthermore, to investigate the morphology of the films, thin
film of the compounds have been prepared on quartz plate sub-
strates by vacuum deposition and imaged by atomic force micro-
scopy (AFM). In order to evaluate the stability of the thin films, the
surface has been exposed to air under thermal stress conditions
from room temperature to 100 ꢂC. The non-heated film surface
presents a regular and smooth morphology (Fig. S8(a) and (b)) and
surface roughness (Ra) of the film appears to be very low, around
1e3 nm. Interestingly, the roughness is kept almost unchanged of
annealing at 100 ꢂC which is highlighting the very good quality of
the film surface (Fig. S8(c) and (d)) and the high stability of the
compounds upon exposure to stress circumstances. In addition, the
powder X-ray diffraction (XRD) measurement has been carried out
to envisage the nature of thin films. As shown in Fig. S9(A), the thin
films of 1 and 2 both show relatively broad and random scattering
peaks clearly demonstrating the non-crystalline amorphous nature
of compounds. In contrast, the powder form of the compounds 1
and 2 shows notable sharp peaks (Fig. S9(B)) in XRD measurement
implies that the compounds have crystalline nature in powder state
which turns into the amorphous nature on thin films. In this
context, amorphous films are most appropriate for practical ap-
plications, since in crystalline films the existence of number of
inter-crystalline boundaries that limit the charge mobility and give
rise to morphological inhomogeneities [45]. The excellent
morphological stability under very harsh conditions and non-
crystalline amorphous nature of films is a key point for any
possible OLED applications [46].
phenanthroline] as the hole blocking layer (HBL) and Alq3 as the
electron transporting layer (ETL). The fabricated device configura-
tion is [ITO/NPB (60 nm)/EML (40 nm)/BCP (15 nm)/Alq3 (20 nm)/
LiF (0.5 nm)/Al(100 nm)]. Herein, the enhanced device performance
has been registered for 1 and 2 compared with previous one
(Table 3). The combined PL and EL spectra of 1 and 2 in different
device configuration (II, III and IV) are shown in Fig. 2.
For 1, the red shifted EL spectrum at 497 nm with maximum
luminance of 221 Cd mꢁ2 (CIE: 0.29, 0.42) with turn on voltage of
8.5 V and maximum efficiency of 0.99 Cd Aꢁ1 has been achieved.
The observed EL spectrum and CIE coordinates correspond with
yellowish green colour. In turn, compound 2 shows the EL spectrum
at 450 nm with turn on voltage of 8.5 V and maximum luminance of
37.5 Cd mꢁ2 (CIE: 0.20, 0.20). As this device has the EL behaviour
related to blue colour, we further modified the device configuration
to achieve our primary interest of getting pure colour. For that, we
doped the compound 2 with CPB [4,40-Bis(N-carbazolyl)-1,10-
biphenyl] because CPB can act as a host as well as hole transporting
layer which can help for efficient exciton recombination in the
emissive layer. The device configuration is [ITO/NPB (60 nm)/CBP:
10% EML (40 nm)/TPBi (20 nm)/LiF (0.5 nm)/Al(100 nm)], where
TPBi [1,3,5-Tri(1-phenyl-1H-benzo[d] imidazol-2-yl)phenyl] was
employed as the hole blocking layer and electron transport layer.
1-Device III
1.0
2-Device III
3.6. OLED device performance
2-Device I
0.8
To investigate the EL properties of the compounds 1e4 as an
emitting layer (EML), a series of devices (IeV) with different device
configuration have been fabricated and the results are shown in
Table 3. Initially, simple devices consisting of [ITO/
PEDOT:PSS(40 nm)/NPB (30 nm)/EML (40 nm)/LiF (0.5 nm) or Ca
(16 nm)/Al(100 nm)] have been fabricated. Where PEDOT:PSS was
the hole injecting layer (HIL), NPB [N,N0-Di(naphthalen-1-yl)-N,N0-
diphenyl-benzidine] was employed as the hole-transporting layer
(HTL) and LiF/Al or Ca/Al was employed as the cathode. From these
device configurations, we observed poor device performance for 1
and 2. This can be ascribed to the unfavourable alignment of energy
levels of different layers to recombine the generated exciton within
the emissive layer (Fig. S10). Hence, we modified the device
configuration by introducing BCP [2,9-Dimethyl-4,7-diphenyl-1,10-
2-Device IV
1 - PL
0.6
0.4
0.2
0.0
2 - PL
400
440
480
520
560
600
640
680
Wavelength (nm)
Fig. 2. PL (thin line) and EL (thick line) spectra of
configurations.
1
and 2 in various device

N. Nagarajan et al. / Dyes and Pigments 102 (2014) 180e188
185
Device V at 21V
Device III at 17V
Device III at 19V
Device III at 21 V
From this device, we observed better device performance with
brightness of 591 Cd mꢁ2 at 18 V and turn on voltage of 5.5 V and
achieved almost pure blue light (CIE: 0.16, 0.08) and EL spectrum at
438 nm. The observed CIE coordinates are well matched with the
standard values suggested by NSTC and CIE for pure blue light (0.14,
0.08). It implies that the exciton recombination is well confined
within the CBP molecular layer which is hosting emitter 2.
1.0
0.8
0.6
0.4
0.2
Interestingly, the emitters 3 and 4 in device III configuration
exhibited white EL with the spectra covering the whole visible
range as a result of the combination of the blue and orange emis-
sion. Devices based on 3 and 4 showed pure white light with CIE
coordinates of 0.34, 0.32 (Fig. 3) and 0.36, 0.35 at 17 V (Fig. 4), with
the brightness values of 8 and 0.25 Cd mꢁ2 respectively. Even
though these preliminary results are inferior to those of recently
reported white OLEDs, efficient single-emitting component white
OLEDs are expected after optimizing the device configuration.
The EL spectra of the devices differ significantly from their PL
spectra. The emission in the blue region resembles with the
respective PL of each compound and it evidently originates from
the singlet-excited state of an individual molecule. The emission in
the orange region with a peak at about 620e650 nm for 3 and 4
does not appear in the PL spectrum either in the solution or in the
film, indicating that the orange emission is a peculiar characteristic
of the EL. This interesting feature normally observed due to the
aggregates or electromer or electroplex. To check the possibility of
electroplex formation [excited state complex formed between two
different compounds in the device configuration (Xþ/Yꢁ)*] in the EL
spectrum, the single layer devices with 130 nm of compound 4
sandwiched between ITO and LiF/Al electrodes are fabricated. The
EL spectra of the single layer device are shown in Fig. 4.
Obviously, the emissive features of the single layer device are
quite similar to the EL spectra of multilayer one. This result in-
dicates that the orange emission is an inherent property for the EL
of the molecules and the electroplex emission as the source of the
long wavelength emission band is cast away. It is thus plausible to
assign the additional orange emission band to electromer produced
only by electric excitation. We infer that the orange emission in our
single layer andþmultilayer devices can arise from the electromer
(3þ/3ꢁ)* and (4 /4ꢁ)*. The energy of such a holeeelectron pair
trapped by two molecules would be lower than that of the mono-
mer exciton. As a result, direct cross recombination of the electron
and hole could result in emission at longer wavelengths compared
to the PL of 3 and 4. It is hypothesised that the plausible driving
force for the formation of electromer on electric excitation is the
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 4. EL characteristics of 4 in device III and V.
the electronic interaction within the molecules which can facilitate
the formation of trapped level of holeeelectron pair.
3.7. Geometry optimization and Frontier molecular orbitals
The optimized geometries of these compounds are depicted in
Fig. S11. The selected important geometric parameters (bond
lengths, bond angles and dihedral angles) of these compounds are
in accordance with the X-ray data which is listed in Table S1. It will
be useful to examine the highest occupied molecular orbitals
(HOMOs), lowest unoccupied molecular orbitals (LUMOs) and band
gaps (HOMOeLUMO gap) of the synthesized compounds since the
relative ordering of the molecular orbitals provides qualitative
indication of the excitation properties.
Therefore HOMOs and LUMOs of compounds 1e4 are investi-
gated here, the plots of frontier molecular orbitals (from HOMOꢁ3
to LUMOþ3) and the corresponding energy level diagram are
shown in Fig. 5. As observed in Fig. 6, it is interesting to note that
the electronic cloud distribution of 1e4 of HOMO orbitals is spread
over the imidazole core where as the LUMO is mainly centralized
on the isoquinoline and phenyl part. Actually, the first dipole-
allowed transitions correspond exclusively to the promotion of an
electron from the HOMO / LUMO as described in the absorption
and emission section. From Fig. 5, it can be seen that the energy
gaps of compounds 1e4 were found to be 3.66, 3.73, 3.63 and
3.65 eV respectively, which matches well with the electrochemi-
cally determined HOMO and LUMO potentials. From Fig. 5 and
Table 4, it can be seen that, the calculated HOMO and LUMO en-
ergies and the excited energies (Eg) have the same variation with
edge to face
pe
p
interaction which would be expected to enhance
1.0
0.8
0.6
0.4
0.2
17V (0.34, 0.32)
19V (0.36, 0.39)
21V (0.37, 0.38)
0.0
400
500
600
700
Wavelength (nm)
Fig. 3. EL spectra of compound 3 for device III in various applied voltages. Inset is
photograph of device III for compound 3 at a driving voltage of 17 V.
Fig. 5. Frontier molecular orbital levels of compounds 1e4.

186
N. Nagarajan et al. / Dyes and Pigments 102 (2014) 180e188
Fig. 6. Plots of HOMO and LUMO of 1e4 by DFT//B3LYP/6-31G(d,p).
the experimental data. The value of HOMO levels of 1e4
are ꢁ5.24, ꢁ5.19, ꢁ5.29 and 5.18 eV respectively and the LUMO
levels are found to be ꢁ1.58, ꢁ1.45, ꢁ1.66 and ꢁ1.52 eV respectively.
It is essential to identify and understand the nature of various
segments of the molecule and their individual contributions to-
wards HOMOs and LUMOs for synthesizing newer molecules.
Hence the contributions of various fragments of the molecules have
been computed using QMForge [47]program. The whole molecule
has been segmented into two fragments, namely imidazole and
isoquinoline part, their corresponding percentage contributions are
summarized in Table S3. As evident from the molecular orbital di-
agrams, in all the compounds the contribution of the imidazole part
is about 82% towards the HOMO level and interestingly LUMO is
majorly stabilized by isoquinoline part with 90%. This indicates that
these parts in 1e4 are responsible for the
p / p* transition upon
excitation.
3.8. Computed absorption and emission spectra
Table 4
Computed absorption data obtained by TD-DFT (lmax) for compounds 1e4 at the
B3LYP/6-31G(d,p) optimized geometry.
To understand electronic transitions of these compounds, TD-
DFT calculations on the absorption and emission spectra in both
vacuum and solvent (THF) were performed. The lowest excited
states were calculated by means of TD-DFT method and the effort to
rationalize the nature of electronic transitions, the contributing
configurations to the transitions and charge transfer probability.
The calculated wavelengths from absorption and emission spectra,
excitation energies, main transition configurations and oscillator
strengths for the most relevant singlet excited states of each
compounds are summarized in Tables 4 and 5 respectively.
States
Electron Expt Cal.
Oscillator
E
Major contribution
transition lmax lmax strength f (eV)
(nm) (nm)
1
2
3
4
Gas-phase
THF
Gas-phase
THF
Gas-phase
THF
Gas-phase
THF
S
S
S
S
S
S
S
S
0 / S1
0 / S1 333 364.7 0.7950
0 / S1 354.3 0.4350
0 / S1 328 353.5 0.6019
0 / S1 366.4 0.8041
0 / S1 370 368.2 1.0268
0 / S1 362.7 0.6067
0 / S1 365 361.4 0.8553
362.3 0.6384
3.42 HOMO / LUMO (96%)
3.40 HOMO / LUMO (94%)
3.50 HOMO / LUMO (95%)
3.51 HOMO / LUMO (93%)
3.38 HOMO / LUMO (95%)
3.37 HOMO / LUMO (93%)
3.42 HOMO / LUMO (94%)
3.43 HOMO / LUMO (91%)
As shown in Table 4, all the electronic transitions are of the
p
/ p* type, and excitation to S1 state corresponds to almost

N. Nagarajan et al. / Dyes and Pigments 102 (2014) 180e188
187
Table 5
processes and fully characterized. These compounds exhibited
good thermal and morphological stabilities over very harsh con-
ditions. Blue emission in solution and slightly red shifted emission
in thin films with high PL quantum yields (0.27e0.93) were
observed for these compounds. To elucidate the structural and
optical properties of these compounds the DFT and TD-DFT calcu-
lations have been performed. A close agreement was found be-
tween the band gaps determined electrochemically (Eg) and those
determined from the computational methods. The electronic
Computed emission data obtained by TD-DFT for compounds 1e4 at the CIS/
6-31G(d) geometry.
States
Electron Expt Cal.
transition lmax lmax strength f (eV)
Oscillator
E
Major contribution
(nm) (nm)
1
2
3
4
Gas-phase
THF
Gas-phase
THF
Gas-phase
THF
Gas-phase
THF
S
S
S
S
S
S
S
S
1 / S0
1 / S0 442 416.4 0.9865
1 / S0 415.4 0.9453
1 / S0 432 424.9 1.1314
1 / S0 411.5 1.2074
1 / S0 435 420.8 1.4229
1 / S0 413.6 1.1182
1 / S0 429 421.9 1.3648
408.2 0.8511
3.04 HOMO / LUMO(99%)
2.98 HOMO / LUMO(98%)
2.98 HOMO / LUMO(99%)
2.92 HOMO / LUMO(98%)
3.01 HOMO / LUMO(99%)
2.95 HOMO / LUMO(98%)
3.00 HOMO / LUMO(99%)
2.94 HOMO / LUMO(98%)
transitions are of the
p / p* type, and excitation to S1 state cor-
responds to almost exclusively to the promotion of an electron from
HOMO / LUMO in all the compounds. Accordingly, the energy of
the S0 / S1 electronic transition follows the same variation trend
with the HOMOeLUMO energy gap of the molecule. The TD-DFT
calculations lead to a good accordance with experimental absorp-
tion and emission spectra. Molecular structure dependant elec-
troluminescence behaviour of the compounds was noticed in
different device configurations. Compound 2 showed ideal blue
colour (CIE: 0.16, 0.08) when doped in CBP as a host. The possibility
of energy transfer from CBP to compound 2 was evidenced from PL
spectra of CBP and absorption spectra of compound 2. The emitters
3 & 4 as a single-emitting component in multilayer device was used
to achieve “pure” white light (CIE: 0.34, 0.32) and electromer for-
mation were also confirmed by fabricating single layer device. This
white light was considered as a combination of a blue emission
band from excimers and an orange emission band from electromers
at a high voltage. Our further research will focus on the optimiza-
tion of device structures to improve performances.
exclusively to the promotion of an electron from HOMO / LUMO.
The oscillator strengths (f) of the lowest S0 / S1 electronic tran-
sition are the largest in these compounds. The excitation energies
calculated for compounds 1e4 are 3.40, 3.52, 3.37 and 3.48 eV,
respectively, which corresponds to the absorption wavelength of
364.7, 353.5, 368.2 and 361.4 nm, respectively. For the emission
spectra, the emission peaks in THF solvent, with the largest oscil-
lator strength for the compounds 1e4 are all assigned to
p / p*
character, arising from the S1, HOMO / LUMO transition (99%). The
calculated values of the fluorescence wavelength in THF for 1e4 are
located at 416.4, 424.9, 420.8 and 421.9 nm respectively. The
calculated absorption and emission bands are in good agreement
with the experimental results. The highest oscillator strengths of
the S1 / S0 transition for 1e4 imply that they have large fluores-
cent intensity and that they are useful as fluorescent OLED mate-
rials. Overall, the DFT and TD-DFT calculations reveal deeper
insights into the electronic structures, optical properties and the
nature of transition of the synthesized molecules.
Acknowledgements
N. N thank DSTꢁINSPIRE program for the financial assistant as a
JRF (IF10495). R. R and N. N thank DST, India, for their financial
support in the form of a research grant (Ref. No. SR/S1/PCꢁ12/2011,
dt. 20/09/2011). G. V thanks University Grants Commission (UGC),
India for the financial support in the form of Science Meritorious
Student Fellowship. P. V thanks Department of Science & Technol-
ogy for Major Research Project (Ref. No: SR/S1/PCꢁ52/2012). Au-
thors also thank UGC/DSTꢁFIST for UVꢁVIS and Fluorescence
facilities in the School of Chemistry, Bharathidasan University. We
are thankful to Prof. P. Ramamurthy, NCUFP, University of Madras,
Chennai for providing TCSPC and Laser flash photolysis instru-
mental facilities.
3.9. Ionization potential, electron affinity and reorganization
energy
The charge-injection/transport and their balance are crucial for
optoelectronic compounds; therefore, it is important to investigate
their ionization potentials (IPs), electronic affinities (EAs) and
reorganization energies (l) to evaluate the energy barrier for in-
jection and transport rates of the holes and electrons. We calculated
IPs and EAs, together with the hole extraction potential (HEP),
which is the energy difference from Mþ (cationic) to M (neutral
molecule) using the Mþ geometric structure, and the electron
extraction potential (EEP), which is the energy difference from Mꢁ
(anionic) to M using the Mꢁ geometric structure. The IPs and EAs
can be either for vertical excitations (v, at the geometry of the
neutral molecule) or adiabatic excitations (a, at the optimized
structures for both the neutral and charged molecule).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.11.002.
The calculated reorganization energies are listed in Table S4. To
be an emitting layer material, it needs to achieve the balance be-
tween hole injection and electron acceptance. By theoretical
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