Effect of conjugation and aromaticity of 3,6 di-substituted carbazoles on triplet energy and the implication of triplet energy in multiple-cyclic aromatic compounds

Article abstract of DOI:10.1039/c8ra00674a

It is well-known that short conjugation is needed to obtain a high triplet energy. Carbazole has 3 fused rings and yet it has a high triplet energy. In order to illuminate the reason behind this, we synthesized a range of carbazole derivatives with substitution at the 3,6-positions. All carbazoles with phenyl moieties substituted at the 3,6-positions exhibit a lower triplet energy than that of carbazole itself. We also quantified the aromaticity of carbazole using the nucleus-independent chemical shift tensor. We discovered that the five-membered heterocyclic aromatic ring in carbazole has reduced aromaticity. This results in a reduced conjugation effect between the five-membered heterocyclic aromatic ring and the neighboring benzene rings. Inspired by this finding, the triplet energies of compounds with up to seven benzene units separated by heterocycles (furan, pyrrole, thiophene, silole, and phosphole) and cyclopentadiene were calculated using time-dependent density functional theory. A high triplet energy (>3 eV) can be obtained by alternating high aromaticity and reduced aromaticity in highly extended fused π systems containing furan and pyrrole. In tricyclic aromatic compounds (dibenzofuran, carbazole, fluorene, dibenzothiophene, 5H-benzo[b]phosphinedole and 9H-9-silafluorene) and their extended fused π systems that we have examined so far, the triplet energy is related to the electronegativity of the oxygen, nitrogen, carbon, sulfur, phosphorous and silicon atoms. These findings provide new intuitive insight related to the structures of molecules and the triplet energies, which could be useful in organic optoelectronics.

Full text of DOI:10.1039/c8ra00674a

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PAPER
Effect of conjugation and aromaticity of 3,6 di-
substituted carbazoles on triplet energy and the
implication of triplet energy in multiple-cyclic
aromatic compounds†
Cite this: RSC Adv., 2018, 8, 9850
a
b
b
c
*
*
Kai Lin Woon,
Azhar Ariffin, Kar Wei Ho
and Show-An Chen
It is well-known that short conjugation is needed to obtain a high triplet energy. Carbazole has 3 fused rings
and yet it has a high triplet energy. In order to illuminate the reason behind this, we synthesized a range of
carbazole derivatives with substitution at the 3,6-positions. All carbazoles with phenyl moieties substituted
at the 3,6-positions exhibit a lower triplet energy than that of carbazole itself. We also quantified the
aromaticity of carbazole using the nucleus-independent chemical shift tensor. We discovered that the
five-membered heterocyclic aromatic ring in carbazole has reduced aromaticity. This results in
a
reduced conjugation effect between the five-membered heterocyclic aromatic ring and the
neighboring benzene rings. Inspired by this finding, the triplet energies of compounds with up to seven
benzene units separated by heterocycles (furan, pyrrole, thiophene, silole, and phosphole) and
cyclopentadiene were calculated using time-dependent density functional theory. A high triplet energy
(>3 eV) can be obtained by alternating high aromaticity and reduced aromaticity in highly extended fused
p systems containing furan and pyrrole. In tricyclic aromatic compounds (dibenzofuran, carbazole,
fluorene, dibenzothiophene, 5H-benzo[b]phosphinedole and 9H-9-silafluorene) and their extended
fused p systems that we have examined so far, the triplet energy is related to the electronegativity of the
oxygen, nitrogen, carbon, sulfur, phosphorous and silicon atoms. These findings provide new intuitive
insight related to the structures of molecules and the triplet energies, which could be useful in organic
optoelectronics.
Received 23rd January 2018
Accepted 17th February 2018
DOI: 10.1039/c8ra00674a
rsc.li/rsc-advances
organic solar cells and dye sensitized solar cells, more extended
conjugation is highly desirable.2 For blue emitting materials,
short conjugation is important.3 Carbazole is a wide band-gap
Introduction
The manipulation of electron delocalization in an extended
system allows us to change the electronic and optical properties
of materials. By changing the degree of electron delocalization
in an organic molecule, the band-gap of the materials can be
adjusted. Such a conjugated system can be intuitively modeled
as an electron in a quantum box, where the energy difference
between two levels is inversely proportional to the length of the
box.1 This means that the more extended the electron delocal-
ization, the smaller the material band-gap.
material.4 However, the band-gap can be greatly reduced by
di-substitution at the 2,7-positions because of the reduced
exibility compared to di-substitution at the 3,6-positions.5
Carbazole and its derivatives are oen used as host materials for
guest/host systems in organic phosphorescent devices with
high efficiency.6 A wide band-gap and high triplet energy are
needed to prevent energy back-transfer from the phosphores-
cent dye.7 Thus, it is of great importance to design and
synthesize wide band-gap materials with high triplet energy. It
must follow that triplet energy can be reduced by increasing the
extent of electron delocalization.8 Since electron localization/
delocalization is central in determining various properties of
the materials, several methods of quantication have been
developed. Most of these descriptors reduce a molecule into
a number. These include the noncovalent interaction index, the
multicenter delocalization index, the uctuation index and the
harmonic oscillator model of aromaticity index, to name just
a few.9 This might be useful for small molecules, but it is hardly
useful for a more complex compound where substitution at
This intuitive picture of a quantum box has helped scientists
to devise strategies to synthesize a range of novel materials. For
aLow Dimensional Materials Research Center, Department of Physics, University of
Malaya, Malaysia. E-mail: ph7klw76@um.edu.my
bDepartment of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia.
E-mail: azhar70@um.edu.my
cDepartment of Chemical Engineering and Frontier Research Center on Fundamental
and Applied Sciences of Matters, National Tsing-Hua University, 101, Section 2,
Kuang-Fu Road, 30041 Hsinchu, Taiwan, ROC
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra00674a
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different positions within the molecule results in totally
different behavior. Mapping of how electron delocalization
changes within a single molecule might yield useful informa-
tion. Such methods have not yet been applied in explaining
organic optoelectronic materials.
Carbazole is a tricyclic aromatic heterocyclic compound
consisting of two benzene rings fused on either side of a ve-
membered nitrogen ring. Its intriguing structure exhibits high
triplet energy. In order to examine how conjugation affects the
triplet energy, carbazoles with phenyl derivatives as substitu-
ents at the 3,6-positions were synthesized. The aromaticity of
a given compound is quantied using isosurface mapping of
the magnetic shielding tensor. We not only show that conju-
gation is important, but also that aromaticity within the
compounds must be taken into account in order to explain the
high triplet energy of the carbazole molecule.
Scheme 1 Synthesis routes to compound side chains 3–6. Reagents
and conditions: Pd(PPh3)4, 2 M K2CO3, and toluene, at 90 ꢁC overnight.
Results and discussion
Synthesis of the carbazole derivatives
Compounds P1–P3 were synthesized by reacting compound
1 with the side chains 3–5, respectively, in Ullman coupling
reactions as demonstrated in Scheme 2. All three compounds
were isolated in very low yields of 20%, 28%, and 17%,
respectively.
The synthesis procedures for compounds P4 and P5 were
more complicated than those for compounds P1–P3.
Compounds P4 and P5 were synthesized with the intention to
extend the molecular backbone by increasing the number of
carbazoles at the side chains.
In this study, different groups such as tert-butyl, anisole, uo-
rophenyl, and carbazole were introduced as side chains onto
the tetraphenylsilane-carbazole core. The structures of the
derivatives (P1–P5) based on the tetraphenylsilane-carbazole
core are shown in Fig. 1.
Compounds P1–P5 were obtained through Ullmann
coupling reactions of brominated tetraphenylsilane 1 with
carbazole side chains (3–5, 11 and 12). Compound 1 was ob-
tained from the reaction between dichlorodiphenylsilane and
lithiated 1,4-dibromobenzene at ꢀ78 ꢁC, as reported
previously.10
Firstly, the amine proton of the iodinated carbazole (2) was
protected using p-toluenesulfonyl chloride according to a liter-
ature procedure, to give compound 8 in a 62% isolated yield.12
Compound
8 was then reacted with compound 6 and
Side chains 3–6 were prepared as follows. Suzuki coupling
reactions of the iodinated carbazole 2 with boronic acid deriv-
atives afforded the side chains 3–6 (Scheme 1). In the Suzuki
coupling reactions, Pd(PPh3)4 was used to catalyze the reactions
in the presence of 2 M K2CO3 and toluene solution. Compound
7 was prepared by reacting carbazole with tert-butylchloride in
a Friedel–Cras alkylation reaction.11
compound 7 under Ullmann conditions to obtain 9 and 10 in
moderate yields (49% and 66%, respectively).12 The tosyl group
was then deprotected using base hydrolysis to obtain
compounds 11 and 12 in quantitative yields (Scheme 3).
The side chains 11 and 12 were then reacted with compound
1 in Ullman coupling reactions similar to the reactions in
Scheme 2, to produce compounds P4 (43%) and P5 (18%). All
newly synthesized products were fully characterized using 1H
NMR, 13C NMR and MALDI-TOF mass spectroscopy. The results
conrmed the proposed structures. The products are readily
Scheme 2 Synthesis of compounds P1–P3. Reaction conditions: (a)
copper powder, 18-Crown-6, K2CO3, and o-DCB, at 180 ꢁC for 24–
72 h.
Fig. 1 Tetraphenylsilane-carbazole derivatives, P1–P5.
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Fig. 3 shows the triplet energies for P1, P2, P3, P4 and P5,
which are 2.34 eV, 2.49 eV, 2.54 eV, 2.30 eV and 2.72 eV,
respectively. For P1, P2, P3 and P4, the triplet energies follow
the trends of the PL as shown in Fig. 3(a). The more red-shied
the PL, the lower the triplet energy. Compared with P1 and P3,
the uorine has an unusual effect in reducing the reduction of
the triplet energy of the molecules. Despite the fact that P3 has
the least red-shied photoluminescence, it has a lower triplet
energy than P5. One would argue that this could be the result of
the torsional angle between two aromatic rings, like for the
difference in the triplet energy observed between 4,40-bis(N-
carbazolyl)-1,10-biphenyl (CBP) and 4,40-bis(9-carbazolyl)-2,20-
dimethylbiphenyl (CDBP). The torsional angles and the ener-
gies between the carbazole and benzene with different substit-
uent groups, -(4-tert-butylphenyl) (for P1), -(3-methoxyphenyl)
(for P2) and -(4-uorophenyl) (for P3), are similar to those of P5
(see ESI Fig. S49†). In this case, the torsional angle shouldn’t
have a signicant effect on the difference in the triplet energy
observed between P1, P2, P3 and P5. Carbazole has 3 fused
rings, while the derivative groups such as P1, P2 and P3 have
only one ring. The intuitive picture of conjugation seems to fail.
The presence of uorine in P3 gives us a clue that the intuitive
picture of conjugation from the chemical structure might be
incorrect. Fluorine is a highly electron withdrawing group and
there is a very strong likelihood that within the molecule, the p-
delocalized electrons are not evenly distributed. It may be that
aromaticity gives a better quantication.
Aromaticity is not a directly measurable quantity. However,
magnetic properties are the most closely related to aromaticity,
as the induced ring currents are associated with cyclic electron
delocalization.14 These can be used to intuitively visualize the
aromaticity. Since ring currents due to the cyclic p electron
delocalization are induced primarily by the external magnetic
eld applied perpendicular to the ring (the z direction), the out-
of-plane component of the nucleus-independent chemical shi
(NICS) tensor should contain the information most relevant for
aromaticity evaluations.15 Using the NICS concept, the Nuclear
Magnetic Resonance (NMR) shieldings can be visualized as iso-
chemical-shielding surfaces (ICSSs).16
Scheme 3 Synthesis routes to compounds 11 and 12. Reagents and
conditions: (a) p-toluenesulfonyl chloride, NaH and DMF, at 0 ꢁC for
4 h; (b) compound 6 or 7, Cu2O and N,N-dimethylacetamide, reflux for
48 h; (c) (i) KOH, DMSO, and H2O, reflux for 4 h; (ii) 6 N HCl.
soluble in common solvents such as dichloromethane, chloro-
form, tetrahydrofuran and dimethylformamide, which indi-
cates the possibility for solution processing.13
Different substituents result in different degrees of red-shi,
as shown in the photoluminescence (PL) spectra in Fig. 2(a),
with P3 being the least red-shied followed by P1, then P2 and
lastly P4. The degree of red-shi is oen correlated with the
degree of conjugation, with P4 being the most conjugated. The
peaks of uorescence for P1, P2, P3 and P4 are at 393.2 nm,
402.5 nm, 388.4 nm and 410.6 nm, respectively. The difference
between P2 and P3 is clearly the result of the electron donating
ability of the substituent. It seems that the less electron with-
drawing the substituent, the more red-shied the spectrum.
Hence, –F is more electron withdrawing than the tert-butyl
group while the –OMe group is less electron withdrawing than
the tert-butyl group. Comparing P3, P4 and P5 as shown in
Fig. 2(b), attaching the 3-methoxyphenyl moiety at the 3,6-
positions of carbazole results in signicant red-shi compared
with functionalizing with a bulky side group at the 3,6-posi-
tions. In –OMe, the oxygen donates one of its lone pairs of
electrons via resonance into the benzene p system, which in
turn increases the electron density in the p system of carbazole.
Hence, it experiences the largest red-shi. For the tert-butyl
group, the electron-releasing effect occurs via the inductive
effect. This effect is weaker than the resonance effect as it
involves the strongly held s-bond electrons. The uorine atom
has 3 lone pairs of electrons, compared to the oxygen atom. The
inductive effect of uorine can occur but it is overwhelmed by
the strong electronegativity of the uorine atom and hence it is
the least red-shied.
Fig. 4(a) represents the isosurface mapping of the magnetic
˚
shielding tensor perpendicular to and 1 A above the plane of the
molecule. Here, we refer to it as ICSSZZ(1). The different colour
schemes are used to indicate different magnetic shielding
Fig. 2 (a) The normalized absorption and photoluminescence of P1, Fig. 3 (a) The normalized low temperature phosphorescence of P1,
P2, P3 and P4 in solutions of dichloromethane. (b) Comparison of the P2, P3 and P4 in solutions of dichloromethane. (b) Comparison of the
absorption and photoluminescence of P3, P4 and P5.
low temperature phosphorescence of P3 and P5.
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positions of carbazole will effectively extend the conjugation.
Hence, a signicant reduction of triplet energy occurs. This also
explains why naphthalene has such a low triplet energy
compared to carbazole, as two benzene rings are conjugated
together. However, for a high triplet energy, both short aromatic
rings and a high torsional angle between aromatic moieties are
important.21
Fluorene and dibenzofuran, as tricyclic aromatic
compounds, might share some of the characteristics of carba-
zole to various degrees. Inspired by this idea, we carried out
triplet energy calculations with up to seven benzene rings
separated by different types of 5-membered aromatic ring and
fused together. The general structure of the molecules is shown
in Fig. 5(a), with oxygen, nitrogen, carbon, sulfur, phosphorous
or silicon as the X atoms. Fig. 5(b) shows the ICSSZZ(1) mapping
of four benzene units separated by pyrrole rings. We can see
that there is alternating high aromaticity and reduced aroma-
ticity. Dibenzofuran and carbazole have the highest triplet
energy, followed by dibenzothiophene, uorene, then 5H-benzo
[b]phosphinedole and lastly 9H-9-silauorene, as shown in
Fig. 5(c). There is a reduction of triplet energy when more and
more repeated units are fused together. The degree of reduction
seems to follow the trend of how electronegative the X atom is,
as shown in Fig. 5(d). Using the Pauling scale, oxygen has an
electronegativity of 3.44 followed by nitrogen (3.04), sulfur
(2.58), carbon (2.55), phosphorus (2.19) and silicon (1.90).
However, carbon has a slightly smaller reduction than sulfur
despite the fact that sulfur is 0.04 higher in electronegativity on
the Pauling scale than carbon. It may be that the number of
electronic shells (size of the atom) is the underlying secondary
factor. For the seven benzene units separated by furan, the
triplet energy only reduced by 0.14 eV compared to the diben-
zofuran, while for the seven benzene rings separated by silole,
the triplet energy is reduced by as much as 0.32 eV compared to
9H-9-silauorene. These extended fused p-systems might
improve the charge mobility as the intermolecular p–p overlap
Fig. 4 (a) ICSSZZ(1) mapping of the carbazole molecule. Positive and
negative Z-components indicate shielding and de-shielding values
when a magnetic field along the z-direction is applied. (b) The labelling
of positions of the atoms in the carbazole molecule.
isosurface values in ppm. The orange-red region at the two
benzene rings of carbazole indicates that the z-direction
magnetic eld is largely shielded due to the induced ring
current originating from the delocalized p-electrons. This
observation implies the strong aromaticity of the two benzene
rings of carbazole. However, the ve membered heterocyclic
ring fused at the middle by the two benzene rings has signi-
cantly less aromaticity.
Carbazole has a lone pair of electrons in the pz orbital of the
nitrogen atom. However, there is a small orbital overlap
between the lone pair of electrons occupying the pz orbital with
the electrons in the pz orbitals of the neighboring carbon atoms
(8a and 9a as shown in Fig. 4(b)). This is probably due to the
high electronegativity of the nitrogen atom resulting in
contraction of the pz orbital and hence a smaller wavefunction
˚
overlap, despite the bond length being 1.39 A as indicated by
single crystal X-ray diffraction.17 This is supported by natural
bond orbital (NBO) analysis, where the occupancy of the lone
pair of electrons is 1.67 at the nitrogen atom. NBO analysis also
indicated that C8a–C4a and C9a–C4a are electron rich, as sup-
ported by the ICSSZZ(1) in Fig. 4(a). The bond length between
the carbon atoms at C4a–C4b is 1.47 A.16 The ICSSZZ(1) indicates
˚
a sigma bond character. This is in contrast with pyrrole, where
the carbon–carbon double bond shows a strong p-character.18
Carbazole is analogous to phenanthrene, where according to
Clar’s rule, the two side rings are expected to have a larger local
aromaticity than the central ring.19 Clar’s rule only applies to
aromatic p-sextets, however, carbazole is not truly an aromatic
p-sextet due to the presence of a ve membered ring at the
center. The large bond length between C4a and C4b could
signicantly reduce the overlapping of p-orbitals. This is in
contrast to molecular orbital (MO) theory where the highest
occupied molecular orbital is spread out between C4a and C4b,
suggesting the existence of extended p-electrons across the two
benzenes.20 This is probably the main weakness of MO theory,
where the electrons are treated such that they are spread out
under the inuence of the nuclei in the whole molecule. The
partial break of conjugation between C8a–N9 and N9–C9a
allows the nitrogen atom to be connected with another aromatic
ring with a small reduction of the triplet energy. This is clearly
indicated by the P5 molecule. The two benzene rings of carba-
zole are electron rich. Attaching the benzene rings at the 3,6-
Fig. 5 (a) The general molecular structure of alternating benzene and
heterocycle rings where X can be O, N, C, S, P or Si. (b) The ICSSZZ(1)
mapping of four benzene rings separated by pyrrole rings. (c) The
triplet energies of extended p systems with different numbers of
benzene units separated by different heterocycles. (d) The normalized
triplet energies with different numbers of repeating benzene units.
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can be higher between two extended fused p-systems.22 The 4- 9.49 mmol) were dissolved in 40 ml toluene in a round bottom
membered ring in the biphenylene molecule is well-known to ask under nitrogen. A solution of potassium carbonate (2 M,
exhibit an anti-aromatic character. However, such a molecule 10 ml) was added to the solution. Tetrakis(triphenyl-phosphine)
has a very low calculated triplet energy (1.16 eV). Due to the palladium(0) (0.47 g, 0.41 mmol) was added. The reaction
extended aromaticity between two benzene rings in naphtha- mixture was reuxed for 24 h under nitrogen. Aer cooling to
lene and three benzene rings in anthracene, their triplet ener- room temperature, the reaction mixture was evaporated to
gies are lower than that of carbazole, with naphthalene having remove solvent and extracted with chloroform and water. The
a triplet energy of 2.65 eV (ref. 23) and anthracene having organic layer was removed under reduced pressure, and the
a triplet energy of 1.85 eV.24
residue was subjected to column chromatography using
hexane/ethyl acetate as the eluent to obtain the known
compound as a white solid (yield 17.4%, 0.63 g). Mp: decom-
poses >200 ꢁC. 1H NMR (400 MHz, CDCl3) d [ppm]: 8.36 (d, J ¼
Conclusions
The decrease of aromaticity of the ve membered rings in 1.83 Hz, 2H), 8.17 (s, 1H), 7.72 (dd, J ¼ 8.24, 1.83 Hz, 2H), 7.53,
carbazole results in insensitivity of triplet energy change when (d, J ¼ 8.24 Hz, 2H), 7.38–7.48 (m, 2H), 7.31–7.37 (m, 2H), 7.24–
the 9-position of carbazole is functionalized with a benzene 7.31 (m, 2H), 6.87–7.00 (m, 2H), 3.94 (s, 6H). 13C NMR (100 MHz,
ring. A range of tricyclic aromatic compounds were examined. It CDCl3) d [ppm]: 160.07, 143.63, 139.58, 133.12, 129.83, 125.75,
seems that by alternating high aromaticity and reduced 124.05, 119.93, 119.04, 113.06, 112.09, 110.99, 55.44. MALDI-
aromaticity in an extended fused p-system, high triplet energy TOF MS (m/z): 378.868.
can be somewhat preserved, particularly when furan and
Compound 5
pyrrole are used. In future, extended fused p-systems with high
3,6-Bis(4-uorophenyl)-9H-carbazole. 4-Fluorophenylboronic
triplet energy can be synthesized and the charge mobility of acid (0.93 g, 6.7 mmol) and compound 2 (1.27 g, 3.03 mmol)
such materials tested.
were dissolved in 24 ml toluene in a round bottom ask under
nitrogen. A solution of potassium carbonate (2 M, 6 ml) was
added to the solution. Tetrakis(triphenyl-phosphine)
palladium(0) (0.13 g, 0.11 mmol) was added. The reaction
mixture was reuxed for 24 h under nitrogen. Aer cooling to
Experimental
General information
Proton and carbon nuclear magnetic resonance spectra (1H room temperature, the reaction mixture was evaporated to
NMR and 13C NMR spectra) were recorded on a Bruker AVANCE remove solvent and extracted with chloroform and water. The
III 400 MHz FT-NMR spectrometer using deuterated chloroform organic layer was removed under reduced pressure, and the
as the solvent with tetramethylsilane (TMS) as an internal residue was subjected to column chromatography using
standard. Matrix-assisted laser desorption/ionization time of hexane/ethyl acetate as the eluent to obtain the known
ight (MALDI-TOF) measurements were carried out using compound as a white solid (yield 30.4%, 0.33 g). Mp: decom-
a Bruker Microex series spectrometer in order to measure the poses >200 ꢁC. 1H NMR (400 MHz, CDCl3) d [ppm]: 8.29 (s, 2H),
molecular masses. 2,5-Dihydroxybenzoic acid (DHB) was used 8.17 (br s, 1H), 7.62–7.72 (m, 6H), 7.53 (d, J ¼ 8.44 Hz, 2H), 7.19
as a matrix.
Compound 3
3,6-Bis(4-(tert-butyl)phenyl)-9H-carbazole.
(t, J ¼ 8.68 Hz, 4H). 13C NMR (100 MHz, CDCl3) d [ppm]: 163.34,
160.90, 139.36, 138.15, 132.32, 128.80, 128.71, 125.58, 123.97,
4-Tert-butylphe- 118.79, 115.69, 115.48, 111.01. MALDI-TOF MS (m/z): 354.818.
nylboronic acid (1.1 g, 6.2 mmol) and compound 2 (1.25 g,
Compound 6
3 mmol) were dissolved in 25 ml toluene in a round bottom
3,6-Bis(4-methoxyphenyl)-9H-carbazole. 4-Methoxyphenylbor-
ask under nitrogen. A solution of potassium carbonate (2 M, onic acid (1.31 g, 8.6 mmol) and compound 2 (1.51 g,
1.7 ml) was added to the solution. Tetrakis(triphenyl- 3.6 mmol) were dissolved in 30 ml toluene in a round bottom
phosphine)palladium(0) (0.14 g, 0.12 mmol) was added. The ask under nitrogen. A solution of potassium carbonate (2 M,
reaction mixture was reuxed for 24 h under nitrogen. Aer 3.6 ml) was added to the solution. Tetrakis(triphenyl-
cooling to room temperature, the reaction mixture was evapo- phosphine)palladium(0) (0.17 g, 0.14 mmol) was added. The
rated to remove solvent and extracted with chloroform and reaction mixture was reuxed for 24 h under nitrogen. Aer
water. The organic layer was removed under reduced pressure, cooling to room temperature, the reaction mixture was evap-
and the residue was subjected to column chromatography using orated to remove solvent and extracted with chloroform and
hexane/ethyl acetate as the eluent to obtain the known water. The organic layer was removed under reduced pres-
compound as a white solid (yield 85.6%, 1.11 g). Mp: decom- sure, and the residue was subjected to column chromatog-
poses >200 ꢁC. 1H NMR (400 MHz, CDCl3) d [ppm]: 8.34 (d, J ¼ raphy using hexane/ethyl acetate as the eluent to obtain the
1.71 Hz, 2H), 8.13 (s, 1H), 7.66–7.73 (m, 6H), 7.49–7.56 (m, 6H), known compound as a white solid (yield 43.5%, 1.13 g). Mp:
1
1.42 (s, 18H). 13C NMR (100 MHz, CDCl3) d [ppm]: 149.46, 198–200 C. H NMR (600 MHz, CDCl3) d [ppm]: 8.30 (s, 2H),
ꢁ
139.30, 139.16, 132.99, 126.92, 125.72, 125.52, 124.04, 118.71, 8.10 (br s, 1H), 7.60–7.78 (m, 6H), 7.50 (d, J ¼ 8.25 Hz, 2H),
110.85, 34.51, 31.44. MALDI-TOF MS (m/z): 431.165.
6.97–7.09 (m, 4H), 3.90 (s, 6H). 13C NMR (100 MHz, CDCl3)
d [ppm]: 158.65, 139.10, 134.74, 132.82, 128.27, 125.32,
Compound 4
3,6-Bis(3-methoxyphenyl)-9H-carbazole. 3-Methoxyphenylbor- 124.05, 118.43, 114.24, 110.85, 55.40. MALDI-TOF MS (m/z):
onic acid (3.17 g, 20.9 mmol) and compound 2 (3.98 g, 379.511.
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8.80 Hz, 8H), 3.89 (s, 12H). 13C NMR (100 MHz, CDCl3) d [ppm]:
158.66, 146.60, 141.42, 139.33, 134.73, 130.06, 128.30, 126.11,
125.35, 123.82, 120.44, 119.70, 118.41, 114.25, 112.13, 110.03,
55.40. MALDI-TOF MS (m/z): 921.795.
3,300,6,600-Tetrakis(4-methoxyphenyl)-90-tosyl-90H-9,3’:60,900-ter-
carbazole. A solution of compound 8 (1.16 g, 2.03 mmol) and
compound 6 (1.68 g, 4.42 mmol) in N,N-dimethylacetamide
(10 ml) was prepared. Copper oxide (0.59 g, 4.1 mmol) was added
to the solution. The mixture was then reuxed for 48 h and
cooled to room temperature. Aer cooling to room temperature,
the reaction mixture was evaporated under reduced pressure to
remove solvent and extracted with chloroform and water. The
organic layer was evaporated under reduced pressure, and the
residue was subjected to column chromatography using hexane/
ethyl acetate as the eluent to obtain the new compound as
a white solid (yield 49.1%, 1.07 g). Mp: decomposes >300 ꢁC. 1H
NMR (400 MHz, CDCl3) d [ppm]: 8.66 (d, J ¼ 8.80 Hz, 2H), 8.36 (d,
J ¼ 1.35 Hz, 4H), 8.18 (d, J ¼ 1.83 Hz, 2H), 7.98 (d, J ¼ 8.44 Hz,
2H), 7.83 (dd, J ¼ 8.93, 2.08 Hz, 2H), 7.66 (d, J ¼ 8.80 Hz, 8H),
7.63 (dd, J ¼ 8.25, 1.83 Hz, 4H), 7.46 (d, J ¼ 8.56 Hz, 4H), 7.35 (d,
J ¼ 8.07 Hz, 2H), 7.04 (d, J ¼ 8.80 Hz, 8H), 3.90 (s, 12H), 2.44 (s,
3H). 13C NMR (100 MHz, CDCl3) d [ppm]: 158.75, 140.74, 137.71,
135.05, 134.51, 134.08, 133.51, 130.17, 128.35, 128.29, 127.06,
126.82, 125.49, 125.46, 124.09, 118.88, 118.50, 116.47, 114.28,
109.87, 55.41, 21.73. MALDI-TOF MS (m/z): 1075.875.
Compound 12
3,300,6,600-Tetra-tert-butyl-90H-9,3’:60,900-tercarbazole. To a solu-
tion of compound 10 (1.39 g, 1.59 mmol) in 10 ml tetrahydro-
furan, dimethyl sulfoxide (4.8 ml) and water (1.6 ml) were
added, and the mixture was stirred for 10 min. Subsequently,
potassium hydroxide (1.59 g, 0.03 mol) was added to this
mixture, and the reaction mixture was reuxed for 4 hours,
cooled to room temperature and diluted with water. Aer
neutralization with HCl solution, the crude product was ltered
and recrystallized from dichloromethane–hexane (1 : 1) to give
the known compound 12 as a white solid (yield 99.5%, 1.14 g).
Mp: decomposes >300 C. H NMR (600 MHz, CDCl3) d [ppm]:
8.52 (br s, 1H), 8.18 (d, J ¼ 1.65 Hz, 6H), 7.70 (d, J ¼ 8.16 Hz, 2H),
7.63 (dd, J ¼ 8.48, 1.88 Hz, 2H), 7.47 (dd, J ¼ 8.57, 1.65 Hz, 4H),
7.33 (d, J ¼ 8.07 Hz, 4H), 1.48 (s, 36H). 13C NMR (100 MHz,
CDCl3) d [ppm]: 142.50, 140.19, 139.07, 125.97, 124.11, 123.54,
123.07, 119.43, 116.18, 111.84, 109.11, 102.16, 34.73, 32.05.
MALDI-TOF MS (m/z): 721.654.
1
ꢁ
Compound P1. Compound 1 (0.25 g, 0.51 mmol), carbazole 3
(0.48 g, 1.12 mmol), copper powder (0.13 g, 2.05 mmol), 18-
Crown-6 (0.013 g, 0.049 mmol) and K2CO3 (0.55 g, 4.0 mmol)
were dissolved in 4 ml anhydrous o-dichlorobenzene under
a nitrogen atmosphere. The reaction mixture was reuxed for
Compound 10
3,300,6,600-Tetra-tert-butyl-90-tosyl-90H-9,3’:60,900-tercarbazole.
A
solution of compound 8 (1.44 g, 2.51 mmol) and compound 7
(1.54 g, 5.5 mmol) in N,N-dimethylacetamide (10 ml) was
prepared. Copper oxide (0.73 g, 5.1 mmol) was added to the
solution. The mixture was then reuxed for 48 h and cooled to
room temperature. Aer cooling to room temperature, the
reaction mixture was evaporated under reduced pressure to
remove solvent and extracted with chloroform and water. The
organic layer was evaporated under reduced pressure, and the
residue was subjected to column chromatography using
hexane/ethyl acetate as the eluent to obtain the known
compound as a white solid with a yield of 65.5% (1.44 g). Mp:
decomposes >300 ꢁC. 1H NMR (600 M Hz, CDCl3) d [ppm]: 8.59
(d, J ¼ 8.80 Hz, 2H), 8.16 (d, J ¼ 1.65 Hz, 4H), 8.07 (d, J ¼ 2.02 Hz,
2H), 7.94 (d, J ¼ 8.25 Hz, 2H), 7.75 (dd, J ¼ 8.80, 2.20 Hz, 2H),
7.47 (dd, J ¼ 8.62, 2.02 Hz, 4H), 7.35 (d, J ¼ 8.25 Hz, 4H), 7.32 (d,
J ¼ 8.25 Hz, 2H), 2.42 (s, 3H), 1.48 (s, 36H). 13C NMR (100 MHz,
CDCl3) d [ppm]: 145.49, 143.02, 139.51, 137.38, 135.02, 134.54,
130.07, 127.09, 126.77, 123.70, 123.38, 118.50, 116.32, 108.97,
34.74, 32.00, 21.69. MALDI-TOF MS (m/z): 875.756.
ꢁ
72 h at 180 C. Aer cooling to room temperature, the mixture
was diluted in dichloromethane and ltered through silica gel.
The ltrate was evaporated under reduced pressure to give
a crude product, which was puried using column chromatog-
raphy to give the new compound as a white powdery product
(yield 20.1%, 0.123 g). Mp: decomposes >300 ꢁC. 1H NMR
(400 MHz, CDCl3) d [ppm]: 8.41 (d, J ¼ 1.47 Hz, 4H), 7.95 (d, J ¼
8.31 Hz, 4H), 7.46–7.80 (m, 38H), 1.42 (s, 36H). 13C NMR
(100 MHz, CDCl3) d [ppm]: 149.63, 140.32, 139.26, 138.98,
137.99, 136.48, 133.71, 133.55, 133.23, 130.09, 128.24, 126.95,
126.11, 125.77, 125.61, 124.25, 118.71, 110.25, 34.52, 31.43.
MALDI-TOF MS (m/z): 1195.097.
Compound P2. Compound 1 (0.20 g, 0.404 mmol), carbazole
4 (0.33 g, 0.86 mmol), copper powder (0.10 g, 1.57 mmol), 18-
Crown-6 (0.011 g, 0.042 mmol) and K2CO3 (0.44 g, 3.2 mmol)
were dissolved in 4 ml anhydrous o-dichlorobenzene under
a nitrogen atmosphere. The reaction mixture was reuxed for
Compound 11
ꢁ
3,300,6,600-Tetrakis(4-methoxyphenyl)-90H-9,3’:60,900-tercarba-
zole. To a solution of compound 9 (1.04 g, 0.965 mmol) in 9 ml
tetrahydrofuran, dimethyl sulfoxide (2.45 ml) and water (1.0 ml)
were added, and the mixture was stirred for 10 min. Subse-
quently, potassium hydroxide (1.36 g, 0.024 mol) was added to
this mixture, and the reaction mixture was reuxed for 4 hours,
cooled to room temperature and diluted with water. Aer
neutralization with HCl solution, the crude product was ltered
and recrystallized from dichloromethane–hexane (1 : 1) to give
the new compound as a white solid (yield 99.4%, 0.884 g). Mp:
72 h at 180 C. Aer cooling to room temperature, the mixture
was diluted in dichloromethane and ltered through silica gel.
The ltrate was evaporated under reduced pressure to give
a crude product, which was puried using column chromatog-
raphy to give the new compound as a white powdery product
1
ꢁ
(yield 27.7%, 0.122 g). Mp: decomposes >300 C. H NMR (600
MHz, CDCl3) d [ppm]: 8.43 (d, J ¼ 1.47 Hz, 4H), 7.96 (d, J ¼
8.25 Hz, 4H), 7.79 (dd, J ¼ 7.98, 1.56 Hz, 4H), 7.75 (d, J ¼
8.25 Hz, 4H), 7.72 (dd, J ¼ 8.53, 1.47 Hz, 4H), 7.64 (d, J ¼ 8.44,
4H), 7.51–7.60 (m, 6H), 7.40–7.46 (m, 4H), 7.35 (d, J ¼ 8.07 Hz,
4H), 7.29–7.31 (m, 4H), 6.94 (dd, J ¼ 8.25, 1.83 Hz, 4H), 3.94 (s,
12H). 13C NMR (100 MHz, CDCl3) d [ppm]: 160.06, 143.37,
140.58, 139.15, 138.03, 136.47, 133.76, 133.48, 133.39, 130.13,
1
ꢁ
decomposes >300 C. H NMR (600 MHz, CDCl3) d [ppm]: 8.62
(br s, 1H), 8.25–8.44 (m, 6H), 7.40–7.82 (m, 20H), 7.04 (d, J ¼
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129.81, 128.26, 126.13, 125.75, 124.20, 119.88, 118.97, 113.12,
112.07, 110.32, 55.39. MALDI-TOF MS (m/z): 1091.070.
Photophysical properties
The UV-visible and photoluminescence emission spectra of the
compounds in solutions of dichloromethane (CH2Cl2) at room
temperature were obtained using a Shimadzu UV-Vis spectro-
photometer (UV-2600) and a uorescence spectrophotometer
(Cary, Eclipse), respectively. For triplet energy measurement,
thin lm samples were prepared via drop-casting onto a glass
substrate by heating at 60 ꢁC from chloroform solution con-
taining hosts at a concentration of 40 mg mlꢀ1. The samples
were attached to the sample holder in a nitrogen cryostat
(Janis). The samples were excited at 350 nm by a 150 fs pulsed
Ti:sapphire laser (Spectra-Physics Hurricane i) at a 1 kHz repe-
tition rate in conjunction with an ultrafast optical parametric
amplier (Quantronix TOPAS). The luminescence from the lm
was allowed to pass through a long pass lter (395 nm) and
a monochromator (Princeton Instruments Acton SP 2150) and
then to a gated intensied CCD camera (Andor ICCD 334T). The
intensied CCD was operated synchronously with a gate delay
of 100 ms for all samples and a detection window width of
850 ms. Measurements were carried out in a cryostat under
a vacuum of 10ꢀ7 Torr at 77 K for all samples.
Compound P3. Compound 1 (0.20 g, 0.40 mmol), carbazole 5
(0.31 g, 0.87 mmol), copper powder (0.10 g, 1.57 mmol), 18-
Crown-6 (0.011 g, 0.042 mmol) and K2CO3 (0.44 g, 3.2 mmol)
were dissolved in 4 ml anhydrous o-dichlorobenzene under
a nitrogen atmosphere. The reaction mixture was reuxed for
ꢁ
72 h at 180 C. Aer cooling to room temperature, the mixture
was diluted in dichloromethane and ltered through silica gel.
The ltrate was evaporated under reduced pressure to give
a crude product, which was puried using column chromatog-
raphy to give the new compound as a white powdery product
(yield 16.9%, 0.072 g). Mp: decomposes >300 ꢁC. 1H NMR
(600 MHz, CDCl3) d [ppm]: 8.33–8.38 (m, 4H), 7.97 (d, J ¼
8.25 Hz, 2H), 7.41–7.84 (m, 32H), 7.15–7.24 (m, 8H). 13C NMR
(100 MHz, CDCl3) d [ppm]: 163.03, 161.37, 140.81, 140.44,
139.11, 138.06, 137.95, 136.47, 133.02, 131.19, 130.02, 128.85,
128.81, 128.76, 128.28, 126.13, 125.65, 125.61, 124.19, 118.86,
118.81, 118.75, 115.72, 115.58, 115.55, 110.45, 110.41, 110.27.
MALDI-TOF MS (m/z): 1042.574.
Compound P4. Compound 1 (0.22 g, 0.45 mmol), carbazole
11 (0.89 g, 0.97 mmol), copper powder (0.11 g, 1.73 mmol), 18-
Crown-6 (0.012 g, 0.045 mmol) and K2CO3 (0.49 g, 3.5 mmol)
were dissolved in 4 ml anhydrous o-dichlorobenzene under
a nitrogen atmosphere. The reaction mixture was reuxed for
Computational calculations
Geometry optimizations were performed at the B3LYP/6-31G**
level of theory. Iso-chemical-shielding surfaces (ICSSs) were
easily constructed by employing the Nucleus-Independent
Chemical Shi (NICS) concept of Paul von Rague Schleyer. In
this process, a lattice of “ghost atoms” surrounding the p
electron system located in the center of a virtual cube were
calculated by employing the Gauge-Independent Atomic Orbital
(GIAO) perturbation method25 as implemented in Gaussian 09
using the same basis set. The resulting data sets were trans-
formed into a contour le of ICSSs using a wavefunction anal-
ysis program, Multiwfn.26 The GIAO perturbation was
performed using an Intel core i7 6700K system overclocked at
4.4 GHz with 16GB RAM. For triplet energy calculations, density
functional theory (DFT) as implemented in the Terachem 1.9
soware package27 was used. The Becke three parameter hybrid
exchange-correlation functional (B3LYP) with increasingly large
basis sets was used until the molecule was optimized at the 6-
311G** basis set. Unrestricted Hartree Fock theory was used to
obtain the triplet energy of the molecules, since spin coupling
in time-dependent DFT has not yet been implemented in Ter-
achem 1.9. The ground state and triplet energy calculations
were performed using a GPU server that had 64 GB RAM
installed to support eight Tesla K10 graphic cards.
ꢁ
72 h at 180 C. Aer cooling to room temperature, the mixture
was diluted in dichloromethane and ltered through silica gel.
The ltrate was evaporated under reduced pressure to give
a crude product, which was puried using column chromatog-
raphy to give the new compound as a white powdery product
(yield 43.3%, 0.41 g). Mp: decomposes >300 ꢁC. 1H NMR
(600 MHz, CDCl3) d [ppm]: 8.31–8.43 (m, 12H), 7.60–7.88 (m, 44H),
7.58–7.40 (m, 14H), 7.04 (d, J ¼ 8.62 Hz, 16H), 3.90 (s, 24H). 13C
NMR (100 MHz, CDCl3) d [ppm]: 158.67, 141.38, 141.35, 140.77,
136.45, 136.37, 134.73, 133.11, 131.39, 130.34, 130.31, 128.30,
128.25, 127.28, 126.14, 125.37, 123.98, 123.85, 119.59, 118.43,
114.25, 111.41, 110.03, 55.40. MALDI-TOF MS (m/z): 2175.097.
Compound P5. Compound 1 (0.28 g, 0.57 mmol), carbazole
10 (0.87 g, 1.20 mmol), copper powder (0.14 g, 2.20 mmol), 18-
Crown-6 (0.015 g, 0.057 mmol) and K2CO3 (0.61 g, 4.4 mmol)
were dissolved in 4.5 ml anhydrous o-dichlorobenzene under
a nitrogen atmosphere. The reaction mixture was reuxed for
ꢁ
72 h at 180 C. Aer cooling to room temperature, the mixture
was diluted in dichloromethane and ltered through silica gel.
The ltrate was evaporated under reduced pressure to give
a crude product, which was puried using column chromatog-
raphy to give the new compound as a white powdery product
(yield 17.9%, 0.178 g). Mp: decomposes >300 ꢁC. 1H NMR
(600 MHz, CDCl3) d [ppm]: 8.27 (d, J ¼ 1.83 Hz, 4H), 8.18 (d, J ¼
1.65 Hz, 8H), 8.06 (d, J ¼ 8.25 Hz, 4H), 7.88 (d, J ¼ 8.25 Hz, 4H),
7.82–7.84 (m, 4H), 7.79 (d, J ¼ 8.80 Hz, 4H), 7.65 (dd, J ¼ 8.71,
1.93 Hz, 4H), 7.59–7.60 (m, 2H), 7.58 (d, J ¼ 7.34 Hz, 4H), 7.48
(dd, J ¼ 8.62, 1.83 Hz, 8H), 7.36 (d, J ¼ 8.26 Hz, 8H), 1.48 (s,
24H). 13C NMR (100 MHz, CDCl3) d [ppm]: 142.59, 140.11,
Conflicts of interest
There are no conicts to declare.
Acknowledgements
138.87, 138.25, 136.49, 133.27, 131.11, 130.25, 128.34, 128.10, We acknowledge funding from the University of Malaya Chan-
126.42, 125.99, 124.20, 123.57, 123.13, 119.34, 116.23, 111.27, cellery High Impact Research Grant (UM.C/625/1/HIR/208) and
109.06, 34.73, 32.04. MALDI-TOF MS (m/z): 1776.242.
the Postgraduate Research Grant (PPP PG181-2014B). We thank
9856 | RSC Adv., 2018, 8, 9850–9857
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RSC Advances
ItraMAS Corporation. Sdn. Bhd for partial support of the
research. We would like to thank Dr Liew Chee Sun from the
Faculty of Computer Science and Information for the support
with the computing facility.
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This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 9850–9857 | 9857