Blue emitting C2-symmetrical dibenzothiazolyl substituted pyrrole, furan and thiophene

Article abstract of DOI:10.1039/c3tc00760j

The synthesis and photophysical properties of pyrrole, furan and thiophene substituted in their 2 and 5 positions by 2′-benzothiazolyl moieties have been investigated. The three species show deep-blue fluorescence with maxima in the 444-450 nm region, originating from excited states of π-π* character. The photoluminescence quantum yields were found to be higher for the pyrrole and furan compounds, 0.42 and 0.47 respectively, compared to the quantum yield of the thiophene species, 0.21. Light Emitting Devices were fabricated using the pyrrole species as 4% dopant in a 4,4′,4′′-tri(9- carbazoyl)triphenylamine emissive layer, or the same pyrrole species as a neat film. In low concentration of dopant, the device is characterised by deep-blue emission with CIE coordinates of x = 0.182, y = 0.185; on the other hand, a neat film of the dopant produces almost white light emission with CIE x = 0.381, y = 0.400 but at a cost of quantum efficiency due to self-quenching effects.

Full text of DOI:10.1039/c3tc00760j

Journal of
Materials Chemistry C
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PAPER
Blue emitting C2-symmetrical dibenzothiazolyl
substituted pyrrole, furan and thiophene†
Cite this: J. Mater. Chem. C, 2013, 1,
2209
Brodie L. Reid,a Steven B. Briggs,ab Louise E. Karagiannidis,ab Sara Muzzioli,c
a
b
c
Paolo Raiteri, Mark E. Light, Stefano Stagni, Pierpaolo Brulatti,d Philip A. Gale,b
*
a
a
*
Mark I. Ogden and Massimiliano Massi
The synthesis and photophysical properties of pyrrole, furan and thiophene substituted in their 2 and 5
positions by 20-benzothiazolyl moieties have been investigated. The three species show deep-blue
fluorescence with maxima in the 444–450 nm region, originating from excited states of p–p* character.
The photoluminescence quantum yields were found to be higher for the pyrrole and furan compounds,
0.42 and 0.47 respectively, compared to the quantum yield of the thiophene species, 0.21. Light
Emitting Devices were fabricated using the pyrrole species as 4% dopant in a 4,40,400-tri(9-carbazoyl)
triphenylamine emissive layer, or the same pyrrole species as a neat film. In low concentration of
dopant, the device is characterised by deep-blue emission with CIE coordinates of x ¼ 0.182, y ¼ 0.185;
on the other hand, a neat film of the dopant produces almost white light emission with CIE x ¼ 0.381,
y ¼ 0.400 but at a cost of quantum efficiency due to self-quenching effects.
Received 2nd December 2012
Accepted 27th January 2013
DOI: 10.1039/c3tc00760j
www.rsc.org/MaterialsC
These systems are analogous to the previously reported donor–
acceptor species, with the difference that in our case the molec-
Introduction
Benzothiazole and benzothiazolyl-containing species have attracted
signicant attention due to the optical properties associated with
their electronic excited states. These species have been investigated
in a variety of elds for many potential applications, including
strongly uorescent molecules with emission in the entire visible
spectrum,1–6 ligands for phosphorescent coordination complexes
including transition metals7–10 and as antennae for lanthanoid
elements,11–13 non-linear optics,14,15 photochromic sensors16 as well
as electron-transporting groups and uorescent dopants in the
fabrication of organic light emitting devices (OLEDs).17–19
ular species are symmetrical due to the equivalent substitution of
the pyrrole ring in positions 2 and 5 by two identical benzothia-
zole moieties. In a preliminary investigation, we reported the
steady state photophysical properties of 1, which revealed blue
uorescent emissionupon excitation at lex ¼ 385 nm.22 Prompted
by these ndings, we have continued this investigation with the
synthesis of the 2,5-(dibenzothiazol-2-yl)furan and thiophene
analogues 2 and 3 (Fig. 1) and we have examined their photo-
physicalproperties indetail. Moreover, compound1 was tested as
an emissive dopant for the fabrication of OLEDs.
The attachment of benzothiazolyl substituents to ve-
membered heterocycles including pyrrole, furan and thiophene
has been reported in detail by de Melo and Raposo et al.3,5,14,15,20,21
These studies were mainly focused on the emissive and non-
linear optical properties of non-symmetric donor–acceptor
molecular species, with the benzothiazole moiety serving as the
electron acceptor substituent. In our studies, we have previously
reported the synthesis of 2,5-(dibenzothiazol-2-yl) substituted
pyrrole 1 (Fig. 1) as a ligand for cobalt and copper complexes.22
Experimental section
General details
Solvents and reagents were purchased from Sigma Aldrich
and used as received without any further purication.
aDepartment of Chemistry, Curtin University, Kent Street, Bentley, 6102 WA, Australia.
E-mail: m.massi@curtin.edu.au
bChemistry, University of Southampton, Southampton, UK, SO17 1BJ
cDepartment of Industrial Chemistry Toso Montanari, University of Bologna, viale del
Risorgimento 4, 40126, Bologna, Italy. E-mail: stefano.stagni@unibo.it
dConsorzio MIST E-R, via P. Gobetti 101, 40129 Bologna, Italy
† Electronic supplementary information (ESI) available: Selected bond lengths
and angles for 2 and 3; calculated absorption proles, list of transitions, and
frontier orbital contours for 1, 2, and 3. See DOI: 10.1039/c3tc00760j
Fig. 1 Representation of the 2,5-(dibenzothiazol-2-yl) substituted pyrrole, furan,
and thiophene species prepared in this work.
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2,5-Bis(N-phenyl thioamide)-3,4-diphenylfuran,
Paper
2,5-bis(N- stirred at reux for 12 hours. The solvents were then removed
phenyl thioamide)thiophene, and 1 were prepared according to under reduced pressure and the residual solids were dissolved
previously published procedures.22–24 The purication of in 20 ml of ethyl acetate. The organic phase was then washed
compounds 2 and 3 was performed via column chromatography with water (3 ꢀ 25 ml) and dried over MgSO4. Aer removal of
using silica gel as the stationary phase (Grace, LC60A 40– the solvent under reduced pressure, the obtained solid was
63 mm). Proton and 13C NMR spectra were measured using a puried via column chromatography using a mixture of 5%
Bruker Avance 400 spectrometer (400.1 MHz for 1H, 100.0 MHz toluene in CH2Cl2 as eluent, yielding 3 as a pale yellow solid.
for 13C) at room temperature. Chemical shis were referenced Single crystals suitable for X-ray diffraction were grown by
to residual solvent resonances. IR spectra were obtained from vapour diffusion of hexanes into a chloroform solution of 3.
solid-state samples, on an attenuated total reectance Perkin Yield ¼ 25 mg (13%). Melting point 208.4–209.1 ꢁC. Anal. calcd
Elmer Spectrum 100 FT-IR with a diamond stage. Band inten- for 3$(CH2Cl2)1/6: C 59.84, H 2.86, N 7.68; found: C 60.27, H 2.75,
sities are reported as strong (s), medium (m), and weak (w). N 7.82. IR: n ¼ 3065 m, 1546 m, 1505 m, 1429 s, 1237 s cmꢂ1. 1H
Elemental analyses were performed by CMAS (Melbourne, NMR (CDCl3): d ¼ 8.06 (2H, d, J ¼ 8.0 Hz, benzothiazole-H), 7.88
Australia). The values were corrected for the presence of (2H, d, J ¼ 8.0 Hz, benzothiazole-H), 7.68 (2H, br s, thiophene-
residual solvent (dichloromethane) from the chromatographic
H
3,4), 7.51 (2H, dd, J ¼ 8.0 and 7.2 Hz, benzothiazole-H), 7.40
purication. Melting points were determined using a BI (2H, dd, J ¼ 8.0 and 7.2 Hz, benzothiazole-H) ppm. 13C NMR
Barnsted Electrothermal 9100 apparatus. X-ray diffraction data (CDCl3): d ¼ 160.4, 153.9, 140.4, 135.1, 128.9, 126.8, 125.8,
were collected on a Nonius KappaCCD. Following analytical 123.5, 121.7 ppm.
absorption corrections and solution by direct methods, the
structures were rened against F2 with full-matrix least-squares
using the program SHELXL-97.25 All hydrogen atoms were
Photophysical measurements
placed in idealised positions and rened using a riding model.
Absorption spectra were measured on a Varian Cary 5000 double-
beam UV-Vis-NIR spectrometer and baseline corrected. Steady-
state emission spectra were recorded on an Edinburgh FLSP920
spectrouorimeter equipped with a 450 W Xenon arc lamp,
double excitation and single emission monochromators, and a
peltier cooled (253.15 K) Hamamatsu R928P photomultiplier
tube (185–850 nm). The excitation spectra were recorded by
monitoring the maximum of the corresponding emission peaks.
Emission and excitation spectra were corrected for source
intensity (lamp and grating) and emission spectral response
(detector and grating) by calibration curve supplied with the
instrument. According to the approach described by Demas and
Crosby,26 luminescence quantum yields (Fem) were measured in
optically dilute solutions (O.D. < 0.1 at excitation wavelength)
obtained from spectra on a wavelength scale [nm] and compared
to the reference emitter by the following equation:
Synthesis of 2
A solution was prepared by dissolving 65 mg (1.63 mmol) of
NaOH in a mixture of 30 ml of EtOH and 4 ml of H2O. 100 mg
(0.20 mmol) of 2,5-bis(N-phenyl thioamide)-3,4-diphenylfuran
were then added to this solution. The obtained suspension was
dropwise added to a solution obtained by dissolving 257 mg (0.78
mmol) of K3[Fe(CN)6] in 8 ml of H2O. The mixture was vigorously
stirred at reux for 12 hours. The solvents were then removed
under reduced pressure and the residual solids were dissolved in
30 ml CH2Cl2. The organic phase was then washed with water (3
ꢀ 25 ml) and dried over MgSO4. Aer removal of the solvent
under reduced pressure, the obtained solid was puried via
column chromatography using a mixture of 5% toluene in
CH2Cl2 as eluent, yielding 2 as a pale yellow solid. Single crystals
suitable for X-ray diffraction were grown by slow evaporation of
a concentrated ethyl acetate solution of 2 in a couple of days.
Yield ¼ 33 mg (31%). Melting point 230.6–231.5 ꢁC. Anal. calcd
for 2$(CH2Cl2)1/5: C 72.03, H 3.68, N 5.56; found: C 72.05, H 4.14,
N 5.52. IR: n ¼ 3063 m, 1546 m, 1505 m, 1463 m, 1429 s, 1312 s,
1262 s, 1235 s cmꢂ1. 1H NMR (CDCl3): d ¼ 8.08 (2H, d, J ¼ 8.0 Hz,
benzothiazole-H), 7.89 (2H, d, J ¼ 8.0 Hz, benzothiazole-H), 7.48
(2H, dd, J ¼ 8.0 and 7.2 Hz, benzothiazole-H), 7.37–7.05 (12H, m
br, benzothiazole-H and phenyl-H) ppm. 13C NMR (CDCl3): d ¼
156.0, 153.5, 145.4, 135.0, 131.2, 130.3, 130.9, 128.9, 128.7, 126.6,
125.8, 124.0, 121.5 ppm.
ꢀ
ꢁꢀ
ꢁꢀ ꢁꢀ
ꢁ
2
ArðlrÞ IrðlrÞ nx Dx
Fx ¼ Fr
2
AxðlxÞ IxðlxÞ nr
Dr
where A is the absorbance at the excitation wavelength (l), I is
the intensity of the excitation light at the excitation wavelength
(l), n is the refractive index of the solvent, D is the integrated
intensity of the luminescence and F is the quantum yield. The
subscripts r and x refer to the reference and the sample,
respectively. All quantum yields were performed at identical
excitation wavelength for the sample and the reference,
cancelling the I(lr)/I(lx) term in the equation. All the
compounds were measured against Rhodamine 101 in ethanol
as ref. 27. Emission lifetimes were determined on the same
Edinburgh instrument with the single photon counting tech-
Synthesis of 3
A solution was prepared by dissolving 32 mg (0.80 mmol) of nique (TCSPC) using pulsed picosecond LEDs (EPLED 295 or
NaOH in a mixture of 30 ml of EtOH and 4 ml of H2O. 200 mg EPLED 360, FHWM < 800 ps, repetition rates between 10 kHz
(0.56 mmol) of 2,5-bis(N-phenyl thioamide)thiophene were then and 1 MHz) as the excitation source; in each case, the above-
added to this solution. The obtained suspension was dropwise mentioned R928P was used as detector. The goodness of t was
added to a solution obtained by dissolving 760 mg (3.19 mmol) assessed by minimizing the reduced chi squared function (c2)
of K3[Fe(CN)6] in 8 ml of H2O. The mixture was vigorously and visual inspection of the weighted residuals. All solvents
2210 | J. Mater. Chem. C, 2013, 1, 2209–2216
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were of spectrometric grade and all solutions were ltered substituted furan and thiophene with K3[Fe(CN)6] in an alkaline
through a 0.2 mm syringe lter before measurement. Experi- EtOH–H2O mixture, as exemplied in Fig. 2, followed by puri-
mental uncertainties are estimated to be ꢃ8% for lifetime cation via column chromatography. The extent of reaction
determinations, ꢃ20% for emission quantum yields, ꢃ2 nm could be easily monitored with the use of thin layer chroma-
and ꢃ5 nm for absorption and emission peaks, respectively.
tography, where the appearance of a blue-emitting spot under
The sample in polystyrene for the investigation of the solid- UV light reveals the formation of the benzothiazole moieties.
state photophysical properties of compound 1 was prepared by The formation of the expected products 2 and 3 was also
drop casting a chloroform solution of 1 with a weight ratio of conrmed with the use of spectroscopic techniques. The
0.1% of 1-to-polystyrene (PS). The thickness of the lm was about disappearance in the 1H NMR of the secondary amide H peaks
500 mm and its maximum in absorption was 0.2–0.3. Absorption at 9.23 ppm (in CDCl3) and 10.33 ppm (in d6-DMSO) for 2 and 3
and emission spectra were recorded with the use of a Perkin respectively, as well as the disappearance of one of the ortho-H
Elmer Lambda 950 UV/Vis spectrophotometer and an Edinburgh signals in the aromatic region, indicated the successful cycli-
FLS920 spectrouorimeter, respectively. The photoluminescence sation and formation of the benzothiazole heterocycles. Also, in
was measured by a custom-made integrating sphere system the IR spectra the formation of the product is conrmed by the
attached to the same spectrouorimeter, using the method of disappearance of the sharp NH peak around 3200 cmꢂ1 for both
Friend.28 Excited state lifetimes were obtained with an IBH500F initial substrates. Elemental analyses were also satisfactory for
time-correlated single-photon counting device.
the purity of the compounds.
OLED fabrication
X-ray crystal structures of 2 and 3
The structural details of compound 1 were reported elsewhere.22
A summary of the crystal data and structure renement for the
other two compounds is reported in Table 1. Compound 2
crystallises in the monoclinic C2/c space group and its structure
is shown in Fig. 3. One of the phenyl rings and one of the
benzothiazole groups shows rotational disorder; these were
modelled as split orientations using thermal parameter and
geometrical restraints/constraints. The two benzothiazole units
are oriented with the two N atoms on the same side of the
molecule with respect to the furan O atom. Both benzothiazole
units lie slightly distorted with respect the furan plane, with
twisting angles ranging from 14ꢁ to 24ꢁ. Both phenyl rings are
distorted with angles of approximately 80ꢁ with respect to the
furan plane, in a similar fashion to the previously reported
structure of 1. There is very limited evidence of p-stacking
throughout the lattice, whose formation seems to be somewhat
hindered by the presence of the bulky phenyl rings in the 3 and
4 positions of the furan heterocycle.
Glass plates pre-coated with 150 nm-thick ITO (indium/tin oxide)
with a sheet resistance of 20 U per square were used as substrates
to grow a sequence of thin layers. All layers were deposited in
succession by thermal evaporation under vacuum (ꢄ10ꢂ6 hPa),
followed by the cathode layer consisting of 0.5 nm thick layer of
LiF and 100 nm of Al. The current–voltage characteristics were
measured with a Keithley Source-Measure unit, model 236, under
continuous operation mode, while the light output power was
measured with an EG&G power meter and the electrolumines-
cence spectra were acquired with a Stellarnet spectroradiometer.
All measurements were carried out at room temperature under
argon atmosphere and were reproduced for many runs, excluding
any irreversible chemical and morphological changes in the
devices. The performance of the emissive layer (EML) has been
optimised by locating it between exciton blocking layers of
4,40,400-tri(9-carbazoyl)triphenylamine (TCTA) and 1,3,5-tri(1-
phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBi), the latter acting
also as an electron-transporting and hole-blocking layer.
Compound 3, shown in Fig. 4, crystallises into the mono-
clinic P21/n space group. As in the case of 2, the two benzo-
thiazole units are arranged in order to localise the N atoms lying
in a cis conguration with respect to the S atom of the thio-
phene ring. One of the benzothiazole substituent lies essentially
coplanar with the central thiophene group, whereas the second
benzothiazole substituent is slightly twisted from planarity by a
torsion angle of ca. 12ꢁ. There is limited p-stacking throughout
the lattice involving adjacent benzothiazole units arranged to
form innite monodimensional layers of parallel molecules,
Computational calculations
The molecular structures and the absorption spectra were
calculated from rst principles with the program GAUSSIAN 09
(ref. 29) using the B3LYP hybrid functional and the 6-311++g**
basis set. The absorption spectra were calculated within the time
dependent density functional theory (TDDFT) formalism and the
presence of the solvent (dichloromethane) was mimicked with
the PCM implicit solvation model.30 The 25 lowest excitation
energies for each species were determined and the spectrum was
produced by Gaussian functions with height proportional to the
calculated intensities and a FWHM of 18 nm.
Results and discussion
Synthesis and characterisation
The synthesis of compound 1 was reported elsewhere.22 In an
analogous manner, compounds 2 and 3 were synthesised via
Fig.
2
General methodology for the preparation of the dibenzothiazole
oxidation of the corresponding 2,5-N-phenylthioamide- compounds 1, 2, and 3.
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Table 1 Crystal data and structure refinement details
2
3
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell
C
30H18N2OS2
C18H10N2S3
350.46
486.58
120(2) K
0.71073 A
Monoclinic
C2/c
120(2) K
0.71073 A
˚
˚
Fig. 4 X-ray crystal structure of 3 with ellipsoids represented at 35% probability
Monoclinic
P21/n
level.
˚
˚
a ¼ 26.3406(14) A
a ¼ 6.98500(10) A
˚
˚
dimensions
b ¼ 9.3058(5) A
b ¼ 20.4992(5) A
˚
˚
c ¼ 19.5502(9) A
c ¼ 10.7665(3) A
˚
with an interplanar distance of ca. 3.4 A. Compared to the cases
b ¼ 105.696(3)ꢁ
b ¼ 99.844(2)ꢁ
of 1 and 2, the p-stacking of 3 is facilitated by the absence of the
bulky phenyl substituents in the positions 3 and 4 of the thio-
phene ring. Within the layer, each unit is displaced so that only
the benzothiazole units take part in the p-stacking arrange-
ment. Neighbouring layers are arranged so that individual
molecules almost lie perpendicular with respect to each other,
as shown in Fig. 5.
3
˚
3
˚
Volume
Z
Density
(calculated)
Absorption
coefficient
F(000)
4613.5(4) A
1518.92(6) A
8
4
1.401 Mg mꢂ3
1.533 Mg mꢂ3
0.259 mmꢂ1
0.487 mmꢂ1
2016
720
Crystal
Plate; pale orange
Block; colourless
Crystal size
q range for data
collection
Index ranges
0.24 ꢀ 0.20 ꢀ 0.04 mm3 0.13 ꢀ 0.04 ꢀ 0.03 mm3
3.02–25.03ꢁ
3.12–25.02ꢁ
Photophysical investigation
A summary of the photophysical data is reported in Table 2. The
species 1, 2, and 3 display similar absorption and emission
spectra, as shown in Fig. 6, in diluted dichloromethane solu-
tions (ca. 10ꢂ5 M). In particular, all the absorption plots consist
ꢂ31 # h # 31,
ꢂ11 # k # 9,
ꢂ23 # l # 23
28 906
ꢂ8 # h # 8,
ꢂ24 # k # 24,
ꢂ12 # l # 12
15 141
Reections
collected
Independent
reections
4070 [Rint ¼ 0.1214]
2685 [Rint ¼ 0.0773]
Completeness to
99.8%
99.9%
q ¼ 25.03ꢁ
Absorption
correction
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Max. and min.
transmission
Renement
method
Data/restraints/
parameters
Goodness-of-t F2
Final R indices
[F2 > 2s(F2)]
0.9897 and 0.9405
0.9855 and 0.9394
Full-matrix least-
squares on F2
4070/26/213
Full-matrix least-
squares on F2
2685/0/208
1.074
1.125
R1 ¼ 0.1334,
R1 ¼ 0.0601,
wR2 ¼ 0.1108
R1 ¼ 0.0836,
wR2 ¼ 0.1224
wR2 ¼ 0.2508
R indices (all data) R1 ¼ 0.1960,
Fig. 5 Layering structure in the lattice of 3.
wR2 ¼ 0.2863
ꢂ3
ꢂ3
˚
˚
Largest diff. peak
and hole
1.065 and ꢂ0.606 e A
0.469 and ꢂ0.433 e A
Table
2
Summary of the photophysical data from diluted (ca. 10ꢂ5 M)
dichloromethane solutions
Absorption
Emission, 298 K
lmax [nm]
(1043 [Mꢂ1 cmꢂ1])
l [nm]
s [ns]
F
1
291 (1.7)
444
1.76
0.42
384 (3.1)
405 (2.9)
1a
2
443
448
2
2.75
0.37
0.47
261 (2.3)
366 (6.8)
383 (7.7)
254 (0.6)
374 (4.5)
394 (2.9)
3
450
1.80
0.21
Fig. 3 X-ray crystal structure of 2 with ellipsoids represented at 35% probability
a
From a polystyrene matrix doped with 1 (0.1%).
level and disordered atoms omitted for clarity.
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units. These can possibly be mixed by n–p* transitions of lower
intensity, although they cannot be clearly distinguished from
the spectra in solutions.
In all cases, the p–p* absorption band displays a distinct
vibronic progression, with a separation between adjacent
resolved peaks of the order of 1350–1200 cmꢂ1. Previously
reported electronic absorption proles for similar substrates,
possessing benzothiazole units connected to ve-membered
heterocyclic substituents, but lacking the C2 symmetry, showed
absorption bands almost devoid of vibrational structures.21 In
those cases, the structureless shape of the bands was attributed
to a relatively high degree of rotational freedom for the bond
connecting the benzothiazole units to the heterocycles, this
effect being observed even in low-polarity solvents such as
methylcyclohexane. The appearance of vibrational features in 1,
2, and 3 might be due to increased rigidity in the compounds
reported here. The blue-shied higher-energy band is ascribed
to a combination of p–p* transitions involving the two phenyl
rings attached to the central heterocycles. The quasi-perpen-
dicular arrangement of the phenyl substituents with respect to
the central heterocycles signicantly reduces the extent of
interannular conjugation and the degree of delocalization, thus
the electronic behaviour of the two systems are likely to be
almost independent of each other. This hypothesis is supported
by the fact that in the absorption prole of compound 3, which
does not possess the phenyl substituents in the 3 and 4 posi-
tions of the thiophene ring, this higher-energy band is almost
absent with respect to compounds 1 and 2. The absorption
maxima of the benzothiazole units in the three compounds do
not seem to vary signicantly upon changing the heterocyclic
core. The most notable shi with Dlmax ¼ 10 nm occurs on
comparing 1 and 2, which have almost identical maxima at
lmax(1) ¼ 384 nm and lmax(2) ¼ 383 nm, with respect to 3, for
which lmax(3) is blue-shied to 374 nm. While more
pronounced shis were previously reported on changing the
substituent on the benzothiazole unit from pyrrole to thio-
phene,21 in this case the heterocycle is linking two symmetri-
cally equivalent benzothiazole units and its electronic effect on
each benzothiazole is therefore likely to be reduced.
Upon excitation at 360 nm, compounds 1, 2 and 3 display
bright luminescence with blue-coloured emission maxima
centred in the range 444–459 nm (Fig. 6). In particular, all the
emission spectra show a resolved vibrational structure mirror-
ing with the absorption proles. All the compounds exhibit a
variable degree of overlap between the absorption and emission
proles, with the lowest Stokes shi visible for compound 1.
The photoluminescence of 1 was also trialled by using poly-
styrene (PS) as a host matrix with concentration of the dopant at
0.1%. The absorption and emission proles in the solid matrix
are very similar to the ones measured from dichloromethane
solutions (see ESI, Fig. S2†).
Fig. 6 Absorption (black) and emission (blue) plots of a diluted (10ꢂ5 M)
dichloromethane solution of 1, 2, and 3.
The excited state lifetimes of the three species are charac-
teristic of spin-allowed radiative decays (uorescence) with
of an intense band centered between 300 and 400 nm, together values between 1 and 3 ns. The values of the quantum yield for
with a weaker and blue-shied transition occurring between species 1 and 2, possessing the two phenyl rings attached to the
254 and 291 nm. The lower energy band is ascribed to p–p* central heterocycle, are 42% and 47% respectively. On the other
transitions, localised on the benzothiazole and heterocycle hand, compound 3 exhibits a reduced quantum yield of 21%.
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The lower quantum yield of 3 is tentatively ascribed to the species, and the quantum yield of 3 is inferior with respect to 1
presence of the extra S atom in the thiophene ring, which could and 2, we have decided to use 1 as a dopant for the fabrication of
favour some degree of intersystem crossing between the lowest blue OLEDs. The electroluminescence of compound 1 was
singlet and triplet excited states in competition with radiative employed in two multilayer OLEDs as uorescent emitter. In
decay. This effect has been previously reported for analogous both cases, the device architecture and composition adopted
porphyrinic systems.31
was ITO/MoO3 (2 nm)/TCTA (80 nm)/EML (30 nm)/TPBi
(30 nm)/LiF (0.5 nm)/Al (100 nm). In the rst device, the EML
was obtained by blending 1 in TCTA at a concentration equal to
4% w/w. For comparison, a second device was prepared with the
emitting layer being composed of 100% 1, as a neat lm. Fig. 8
displays the performances of both devices, the brightness as a
function of the driving voltage and the external electrolumi-
nescent quantum efficiency as function of the current density
(j). Maximum luminance reaches 2500 cd mꢂ2 and 650 cd mꢂ2
at about 100 mA cmꢂ2 for EML ¼ 4% of 1 in TCTA and EML ¼
100% of 1, respectively. The external quantum efficiency for the
device where EML ¼ 4% of 1 in TCTA is ꢄ2.5% at low current
density, which corresponds to the theoretical upper limit when
the light extraction factor is assumed to be 1/4 of the total
emission. In the case of the device with a neat lm as EML, the
external quantum efficiency is of the order of 0.1%, suggesting
that the emitting material undergoes self-quenching when
forming aggregate species. The higher roll-off effect of the
DFT calculations
To obtain more insight into the photophysical properties of the
compounds, DFT calculations were performed. In general, the
simulated absorption proles for 1, 2, and 3 (shown in ESI,
Fig. S1†) are in qualitative agreement with the experimental
data. The relaxed geometries for all the three compounds
evidence a planar arrangement, thus favouring extended delo-
calization of the p-electrons and enhancing the rigidity of the
species in solution. This result is in agreement with the resolved
vibrational structures observed in the experimental absorption
spectra. The calculated proles highlight an intense lower-
energy band followed by a less intense higher-energy band, the
latter more pronounced for 1, and 2. According to the data, the
major contributor to the lower-energy band is a HOMO–LUMO
transition, both of which lie localised between the central
heterocycle and the ve-membered units of both benzothiazole
substituents (Fig. 7). The contours of these orbitals are identical
is all three cases. For 1 and 2, the higher-energy transitions
originate from orbitals localised on the phenyl rings to p*
orbitals localised on the central heterocycle, suggesting transi-
tions with partial charge transfer character.
Electroluminescence and OLED fabrication using 1 as a blue-
emissive dopant
Given that compound 1 can be obtained in higher yields, the
steady-state emission proles are very similar for the three
Fig. 7 Molecular orbital contours highlighting the LUMO (top) and HOMO
(bottom) for species 1.
Fig. 8 Brightness vs. voltage plot (top); EL quantum efficiency vs. electric current
density.
2214 | J. Mater. Chem. C, 2013, 1, 2209–2216
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Journal of Materials Chemistry C
OLED with a low concentration of emitter, compared to the neat single molecule emission in the high-energy region of the band
lm as EML, as shown in Fig. 8, suggests a more efficient effect between 400 and 500 nm. The combination of these two emis-
of the excitonic dissociation by the electric eld on the molec- sion proles, from the single molecules of 1 and aggregates,
ular excited state than on aggregate species.32
allows coverage of most of the visible spectrum, thus the neat-
The electroluminescence spectra of both devices are repor- lm device yields white emission, with CIE coordinates of x ¼
ted in Fig. 9. No contribution to the electroluminescence 0.381 and y ¼ 0.400 (see Fig. 9) as well as CRI ¼ 86 (Colour
emission from the TCTA binder or from the TPBi electron- Rendering Index).33 Various strategies have been reported in the
transport layer is observed. The absence of the TCTA and TPBi literature in order to obtain white light in OLEDs using a single
emission indicates that the excitons are conned to the EML, molecule as dopant to simplify the fabrication process (instead
where they are localised on the uorescent molecule prior to of a balanced mixture of the blue, red, and green primary
radiative emission. The electroluminescence of 1 blended in colours).34 As evidenced by the electroluminescent data,
TCTA is of deep-blue colour and results in a slight red-shi compound 1 allows white emission from the formation of
(lmax ¼ 458 nm) in relation with both solution and dispersion aggregates, however this also results in a lower efficiency due to
into a polystyrene matrix (lmax ¼ 444 nm and 443 nm, respec- self-quenching effects.
tively). This red-shi effect could be attributed to a partial
charge transfer character of excited state, which would be
inuenced by the polarity of the surrounding medium.
Conclusions
The device with the neat lm of 1 as emitting layer displays
In conclusion, the photophysical properties of C2-symmetrical
emission characteristics consistent with the formation of
dibenzothiazolyl substituted pyrrole, furan, and thiophene have
aggregate species, arising from interaction of neighbouring
been investigated, revealing blue emission originating from
electronic p–p* excited states. The quantum yield of the emis-
molecules. The electroluminescent emission of this device is
broad, almost structureless and with a maximum at lower
sion was found to be higher in the case of pyrrole and furan with
energies (lmax ꢄ 600 nm) with respect to the other OLED. It is
respect to thiophene, which was tentatively attributed to the
presence of the S atom in the heterocycle. Due to its more effi-
possible to distinguish a small contribution as a shoulder of the
cient synthesis and good quantum yield, the pyrrole
1
compound was used as dopant in the fabrication of OLED
architectures. Using 4% of 1 blended with TCTA yielded devices
with deep-blue emission. On the other hand, using 1 as a neat
lm in the emissive layer yielded devices characterised by white
light emission as a consequence of aggregation, however this
also caused a drop in quantum efficiency originating from self-
quenching effects.
Acknowledgements
The work was supported by the Australian Research Council.
MM wishes to thank the ARC for the APD Fellowship
(DP0985481). PR thanks the ARC for the ARF fellowship (DP
0986999) and iVEC and the National Computational Infra-
structure for the provision of computer time. The OLED work
was supported by Consorzio MIST E-R (project FESR-Tecnopolo
AMBIMAT). Dr Massimo Cocchi (ISOF, Bologna, Italy) is kindly
acknowledged for useful discussion and help with the fabrica-
tion of the OLED. We thank the EPSRC for access to the crys-
tallographic facilities at the University of Southampton.35
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