Synthesis, photophysical and electrochemical properties of 2,8-diaryl-dibenzothiophene derivatives for organic electronics

Article abstract of DOI:10.1007/s12039-010-0012-0

A series of 2,8-p-diaryldibenzothiophene derivatives were synthesized and characterized. These molecules have electron withdrawing or electron donating groups at the para phenyl position, which alters the electronic properties of these derivatives. The qu

Full text of DOI:10.1007/s12039-010-0012-0

J. Chem. Sci., Vol. 122, No. 2, March 2010, pp. 119–124. © Indian Academy of Sciences.
Synthesis, photophysical and electrochemical properties of
2,8-diaryl-dibenzothiophene derivatives for organic electronics
PABITRA K NAYAK, NEERAJ AGARWAL* and N PERIASAMY
Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road,
Colaba, Mumbai 400 005
e-mail: peri@tifr.res.in; nagarwal@tifr.res.in
MS received 16 February 2009; revised 30 June 2009; accepted 19 August 2009
Abstract. A series of 2,8-p-diaryldibenzothiophene derivatives were synthesized and characterized.
These molecules have electron withdrawing or electron donating groups at the para phenyl position,
which alters the electronic properties of these derivatives. The quantum yield, fluorescence lifetime,
singlet, triplet and EHOMO energy levels of these compounds were determined by fluorescence, phospho-
rescence and cyclic voltammetry. A plot of Hammett constants of the para substituents vs EHOMO
revealed a linear relationship. The usefulness of these molecules in organic light emitting diodes, OLEDs
is discussed vis-à-vis the energy levels and properties.
Keywords. Dibenzothiophene; Suzuki–Miyaura coupling; palladium catalyst; phosphorescence; triplet
OLED; hole blocker.
1. Introduction
result, the triplet energies of widely used systems
such as oligothiophenes and oligomers of the para-
5,12–14
The interest in new organic materials for electronic
applications has grown tremendously over the last
decade due to enormous market opportunities. The
synthetic versatility and favourable optical and elec-
tronic properties offered by thiophene containing
compounds make them materials of choice for non-
phenylene have been reported only recently.
The knowledge of triplet energy levels is important
in choosing the correct conjugated organic host/
guest materials in energy transfer processes. For
example, long lived electro-phosphorescence from
organometallic phosphor is efficient when the triplet
excited state of the host material is higher than that
of the dopant to prevent quenching of the dopant
linear optical, organic light-emitting, light-harvesting,
1–3
and transistor devices.
In addition to desirable
15
emission by back transfer. In OLEDs, optimum
macroscopic device characteristics, such as lumines-
cence efficiencies and low drive voltages for OLEDs
and organic field-effect transistor (OFETs), an in-
sight into the optical and electronic properties is also
required to elucidate structure–function relation-
alignment of triplet energy levels is required for
triplet energy transfer and hence to improve the per-
16–18
formance of doped
devices. In donor–bridge–
acceptor (DBA) systems, also, the alignment of the
triplet energy levels improve the electrolumines-
4
ships. The photophysical study of pyrrole derivatives,
5
oligo- and polythiophenes and other heterocycles
6
19
cence efficiency. Therefore, it is essential to
have been explored in detail. However, reports on
the optical properties of embedded thiophenes, par-
determine the triplet energies of conjugated organic
materials in addition to determination of HOMO,
LUMO, S1 energy levels and quantum yield for op-
7–9
ticularly dibenzothiophenes, are scarce.
20
An understanding of the photophysical properties
toelectronic devices.
of organic materials is fundamental to the under-
In this paper, we describe the synthesis of
diaryldibenzothiophene derivatives 1–4 and study
their physical properties. Specifically, we describe
the singlet and triplet state properties of 1–4 by
absorption, fluorescence, phosphorescence, time
resolved emission in the nanosecond time regime
and electrochemical studies. We discuss the effect of
5,10,11
standing of the functioning in many applications.
The triplet energy levels of many conjugated materials
have not been estimated due to difficulties in meas-
5
uring their low-intensity phosphorescence. As a
*For correspondence
119

120
Pabitra K Nayak et al
electron withdrawing and electron donating groups dibenzothiophene, 5 was obtained by selected bro-
21
on energy levels, emission and electrochemical mination at 2- and 8-positions of dibenzothiophene
properties of dibenzothiophene derivatives.
using bromine solution in chloroform at 0°C. Recys-
tallization of crude product with methanol gave 5 as
white solid in 70% yield. Dibenzothiophene deriva-
tives 1–4 were prepared by Suzuki–Miyaura cou-
2. Experimental
22
pling. 2,8-Dibromo-dibenzothiophene (0⋅58 mmol),
All the solvents were obtained from SD Fine Chemi-
cals (India). Aryl boronic acid(s), dibenzothiophene,
K2CO3, were purchased from Sigma-Aldrich and
were used as received. Pd(PPh3)4 was freshly pre-
pared using PdCl2 and PPh3. Reactions were carried
out under inert atmosphere of nitrogen in oven-dried
glasswares. Progress of reaction was monitored
using Silica Gel TLC plates and UV detection (254
and 365 nm). Silica gel (100-200 mesh) was used
for column chromatography.
arylboronic acid (1⋅26 mmol), Pd(PPh3)4 (0⋅03 g,
0⋅026 mmol) and K2CO3 (0⋅42 g, 2⋅9 mmol) were
placed under inert atmosphere in a reaction vessel.
1,2-Dimethoxyethane (6⋅0 mL) and water (6⋅0 mL)
were added while purging argon. The reaction mix-
ture was stirred at 85°C for approximately ~24 h.
After cooling the reaction mixture to room tempera-
ture, dichloromethane was added and organic layer
was separated from aqueous layer. Aqueous layer
was washed three times with dichloromethane. All
organic layers were combined and washed with brine
and dried over Na2SO4. The solvent was evaporated
to get the crude product. Silica gel column chroma-
tography using a mixture of hexanes and ethyl ace-
tate (9 : 1) as eluent afforded the pure compound in
good yields, 1 (75%), 2 (78%), 3 (83%) and 4 (71%)
as white solids. Compounds 1–4 were characterized
1
13
H and C NMR spectra were recorded using
Bruker spectrometer with working frequency of
1
13
500 MHz for H NMR and 125 MHz C NMR. The
chemical shifts were referenced to TMS (added) or
CHCl3 present as impurity in CDCl3. Mass spectra
were measured using MALDI-TOF mass spectrome-
ter. Photoluminescence were measured using SPEX
Fluorolog 1681. Quantum yield were calculated
using Anthracene standard. Photoluminescence
decays were measured using Time Correlated Single
Photon Counting (TCSPC) method. Cyclic voltam-
metry was done using CH Instruments 600°C. Oxi-
dation potentials were determined with respect to
ferrocene as internal standard.
1 13
by H NMR, C NMR, MALDI-TOF, FT IR etc. All
the compounds 1–4 were stable in air.
2.1a Characterization data for 2,8-diphenyl-diben-
1
zothiophene, 1: M.P. 147°C; H NMR (500 MHz,
CDCl3): 8⋅42 (s, 2H), 7⋅93 (d, 2H, J = 8⋅3 Hz), 7⋅73
(d, 6H, J = 7⋅2 Hz), 7⋅50 (t, 4H, J = 7⋅6 Hz), 7⋅39
13
2.1 Synthesis of 1–4
(t, 2H, J = 7⋅4 Hz); C NMR (125 MHz, CDCl3):
141⋅11, 139⋅00, 138⋅01, 136⋅10, 128⋅98, 128⋅77,
127⋅90, 127⋅29, 126⋅37, 125⋅75, 123⋅44, 122⋅77,
120⋅39, 120⋅17, 119⋅74; MALDI-TOF (m/z) for
C24H16S Calcd. 336⋅45, Obsd. 336⋅89.
Diaryldibenzothiophene derivatives, 1–4 were syn-
thesized as shown in scheme 1. 2,8-dibromo-
2.1b Characterization data for 2,8-di-(4-meth-
oxyphenyl)-dibenzothiophene, 2: M.P.: 184°C; FT
–1
1
IR: 2836 cm ; H NMR (500 MHz, CDCl3): 8⋅35
(s, 2H), 7⋅90 (d, 2H, J = 8⋅3 Hz), 7⋅65–7⋅68 (m, 6H),
13
7⋅05 (d, 4H, J = 8⋅6 Hz), 3⋅38 (s, 6H); C NMR
(125 MHz, CDCl3): 159⋅21, 138⋅38, 137⋅60, 136⋅16,
133⋅70, 128⋅89, 128⋅38, 128⋅34, 125⋅99, 123⋅34,
122⋅92, 120⋅07, 119⋅65, 119⋅05, 114⋅39, 53⋅36;
MALDI-TOF (m/z) for C26H20O2S Calcd. 396⋅50,
Obsd. 396⋅75.
2.1c Characterization data for 2,8-di-(4-cyano-
phenyl)-dibenzothiophene, 3: M.P. 286°C FT–IR:
–1
1
2223 cm ; H NMR (500 MHz, CDCl3): 8⋅41 (s,
2H), 8⋅00 (d, 2H, J = 8⋅3 Hz), 7⋅78–7⋅83 (d–d, 8H,
Scheme 1. Synthetic scheme for dibenzothiophene deri-
vatives 1–5.

Synthesis, photophysical and electrochemical properties of 2,8-diaryl-dibenzothiophene
121
J = 11⋅0 Hz and J = 8⋅3 Hz), 7⋅73 (d, 2H, J = charge transfer excited state with a large dipole
13
8⋅3 Hz); C NMR (125 MHz, CDCl3): 145⋅42, moment.
140⋅48, 136⋅11, 132⋅84, 127⋅80, 128⋅22, 125⋅78,
123⋅92, 120⋅23, 118⋅82, 111⋅09; MALDI-TOF (m/z) indicating absence of self quenching. Figure 3
for C26H14N2S Calcd. 386⋅47, Obsd. 386⋅86 shows fluorescence spectra of the molecules in thin
film. The fluorescence peaks in solid state showed
The molecules were fluorescent in the solid state
2.1d Characterization data for 2,8-di-(4-acetyl- little red shift with respect to the solution spectra for
phenyl)-dibenzothiophene, 4: M.P. 212°C FT IR: molecules 1, 2 and 4. For 3, the cyanophenyl deriva-
–1
1
1680 cm ; H NMR (500 MHz, CDCl3): 8⋅45 (s, tive, the fluorescence spectrum in the solid was red
2H), 8⋅10 (d, 4H, J = 8⋅4 Hz), 7⋅97 (d, 2H, shifted by 42 nm. Red shift of spectra in solid state
J = 8⋅3 Hz), 7⋅83 (d, 4H, J = 8⋅4 Hz), 7⋅76 (m, 2H), is common for most luminescent organic molecules.
2⋅67 (s, 6H); MALDI-TOF (m/z) for C28H20O2S While a slight red shift can be attributed to the fact
Calcd. 420⋅52, Obsd. 421⋅02.
that optical transition energy depends on the dielec-
3. Results and discussion
Absorption spectra of 1–4 were recorded in dichloro-
methane (figure 1) and the spectral data are summa-
rized in table 1. The UV-Vis spectra of 1–4 showed
two major absorption bands and a weak (logε < 4)
long wavelength band for 1 and 2. The substitution
of methoxy, cyano or acetyl at the p-phenyl position
causes a bathochromic shift of the intense absorp-
tion band at 286 nm for 1 to 303 nm for 4.
Steady state fluorescence spectra of 1–4 were re-
corded in dichloromethane, which are shown in fig-
ure 2. The fluorescence peak showed a large Stokes
shift (70–110 nm) with respect to the absorption
peak (286–303 nm) for all the molecules. The peaks
in excitation spectra were matching with those in
absorption spectra. The large red shift is attributable
to excited state relaxation presumably involving
Figure 2. Normalized fluorescence spectra of 1–4 in
dichloromethane.
Figure 1. Normalized absorption spectra of 1–4 in
Figure 3. Normalized thin film fluorescence spectra of
1–4.
dichloromethane.

122
Pabitra K Nayak et al
Table 1. Photophysical data of 1–4.
a
Absorbance
λfl
λfl
λph
S0 – S1
S0 – T1
a
(nm) (nm)
b
b
a
b
a
b
Compound
nm (logε)
(nm)
τ (ns)
0⋅90
τ (ns)
φ (%)
φ (%)
2⋅7
(eV)
(eV)
1
256 (4⋅52),
286 (4⋅25),
333sh (3⋅52)
356,
374
371
502
500
0⋅82
3
3⋅49
3⋅40
2⋅70
2⋅63
2
3
259 (4⋅81),
288 (4⋅63),
340sh (3⋅59)
367, 384 390
1⋅06
1⋅13
4
4
271 (4⋅85),
301 (4⋅75)
377
415
419 508, 544 0⋅90
419 511, 541 0⋅27
0⋅70
0⋅08
15
2
11⋅7
0⋅3
3⋅23
3⋅25
2⋅51
2⋅51
4
275 (4⋅51),
303 (4⋅54)
a
b
In dichloromethane, Thin film, λfl = fluorescence maximum, λph = phosphorescence maximum, φ = quantum yield,
τ = fluorescence life time
tric constant and refractive index which are different phosphorescence spectra. Phosphorescence emission
in the solid state. A large red shift, as is the case for spectra of 1–4 (figure 4) were recorded in powder
the cyanophenyl derivative, may suggest formation samples using flash lamp (λex = 300 nm for 1–2 and
of excimer-like species.
360 nm for 3–4) with a delay time of 100 μs and in-
Fluorescence lifetimes of 1–4 were measured in tegrated time of 2 s at 77 K. A red shift in phospho-
dichloromethane solution and in thin film by TCSPC rescence emission is observed for 1–4 (table 1).
23
technique. The fluorescence decays were multiex- S0 – T1 gap for 1–4 is in the range of 2⋅51–2⋅70 eV.
ponential (see Supporting Information) and the
Cyclic voltammetric oxidation and reduction poten-
average lifetimes are reported in the table 1. Quan- tials provide information on the HOMO and LUMO
24,25
tum yields were measured in solution
were low levels of the molecules, which in solid state are the
(2–4%) for 1, 2 and 4. The quantum yield in thin transport levels for the hole and electron, respectively.
films were calculated using lifetime ratios in solu- The linear correlation of electrochemical oxidation
tion and solid state, assuming constant radiative rate and reduction potential of organic molecules deter-
in the two media. The emission peaks, quantum mined in solvent like dichloromethane with the
yields, fluorescence lifetimes of 1–4 obtained in ionization potential and electron affinity of the
28
molecules in solid state is well established. Thus,
solution and in thin films are summarized in table 1.
28,29
The important energy levels of organic molecules EHOMO is calculated by using Epox,
EHOMO ≈ –
in solid state for organic electronics are exciton lev- Epox (vs Ferrocene) − 4⋅8 eV. Cyclic voltammetry of
els (singlet and triplet excited state), transport levels 1–4 in dichloromethane (conc. [1–4] ~ 1 mM,
(corresponding to cation and anion of the molecule) tetrabutyl ammonium hexafluorophosphate 0⋅1 M as
and exciton binding energy (excess energy required supporting electrolyte) showed irreversible oxida-
for exciton to give rise to a pair free cation/hole and tion indicating that cation radical is unstable in
26,27
anion/electron).
These energy levels are deter- dichlromethane. The peak potential was determined
versus ferrocene as internal standard. 1–4 were not
mined for 1–4 as follows.
Optical band gap (S0 – S1) of 1–4 is usually deter- reducible in the electrochemical range accessible in
mined from the transition energy at the intersection dichloromethane. In the absence of cyclic voltam-
of the normalized emission and excitation spectra in metric reduction, ELUMO cannot be calculated di-
thin films. This method is not suitable for 1–4 rectly. However, the transport gap, namely, the gap
because of the large Stokes shift. Thus, the transition between EHOMO and ELUMO is greater than S0 – S1 gap
energy at the rising edge (10% of the peak value) of by an amount equal to binding energy (BE) which is
30
the fluorescence spectra was used to calculate the nearly zero only for large conjugated molecules.
singlet energy (S0 – S1) level. A similar method was Table 2 shows the values of Epox, EHOMO and
used to determine triplet (S0 – T1) level using the (ELUMO – BE) for 1–4.

Synthesis, photophysical and electrochemical properties of 2,8-diaryl-dibenzothiophene
Table 2. Electrochemical data and energy levels of 1–4.
Epox EHOMO ELUMO – BE
123
a
Compounds
(V)
(eV)
(eV)
1
2
3
4
1⋅14
0⋅94
1⋅34
1⋅25
–5⋅94
–5⋅74
–6⋅14
–6⋅05
–2⋅45
–2⋅34
–2⋅91
–2⋅80
a
vs ferrocene, EHOMO = –(Epox (vs ferrocene) + 4⋅8) eV,
ELUMO = EHOMO + ES0–S1 + BE
Figure 5. Correlation of EHOMO of 1–4 with Hammett
constant.
HOMO levels suggest that 1–4 will be suitable as
good hole blocking material if ITO/TPD anode is
used for hole injection. 1–4 are phosphorescent with
triplet energy in the range of 2⋅5–2⋅7 eV, and thus
suitable as host material for harvesting triplets in
33
OLEDs to increase the internal efficiency, even
100%, by using energy compatible, guest phosphor
dopants of high quantum yield.
4. Conclusions
Figure 4. Phosphorescence spectra of 1–4 in powder
sample at 77 K.
In conclusion, we have synthesized and characterized
a series of 2,8-diphenyldibenzothiophene derivatives
having different substituents in the phenyl group.
The photophysical properties of these compounds
showed that the substitution of phenyl groups at the
para position causes red shift in absorption, fluores-
cence and phosphorescence. HOMO, LUMO levels,
singlet and triplet energies were calculated from the
experimental data.
Theoretical study of 2,8-diphenyl-dibenzo-
thiophene, 1 was carried out by density functional
31
theory (DFT) calculations using Gaussian 03. This
calculation provided an insight on the distribution of
the HOMO densities at different positions in the
molecule (See Supporting Information). It shows
that ortho and para positions of phenyl group con-
tribute to the HOMO. Hence, the substitution at
ortho and para position of the phenyl group with
electron withdrawing/donating groups should tune
the HOMO energy level. Experimentally determined
Epox (EHOMO) vary substantially for 1–4. A linear
relationship EHOMO with Hammett constant of the
substituent in 1–4 (figure 5) is observed, which is in
agreement with the linear free energy relation of the
Acknowledgement
We are thankful to the National Facility for high field
NMR at TIFR for NMR spectra.
Supporting information
32
1
13
H, C NMR spectra, emission spectra in solution,
Hammett constant.
The usefulness of 1–4 in organic electronics is now
life time data in dichloromethane and in thin films,
discussed. As emission layer in OLED 3 may be cyclic voltammograms of 1–4 and Gaussian calcula-
useful because of the higher quantum yield (~12% in tion for dibenzothiophene are provided. See
thin film) and absence of self quenching. The deep
www.ias.ac.in/chemsci for supporting information.

124
Pabitra K Nayak et al
Stossel P and Brunner K 2004 J. Am. Chem. Soc. 126
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