Bipolar luminescent materials containing pyrimidine terminals: Synthesis, photophysical properties and a theoretical study

Article abstract of DOI:10.1039/c3ra43631d

A new series of 4-monosubstituted pyrimidine bipolar materials containing a carbazole (PM1-PM2) or a triphenylamine (PM3-PM5) moiety as the electron donor have been synthesized by a ZnCl₂-catalyzed three-component coupling reaction. These 4-monosubstituted pyrimidine compounds were characterized by UV-vis and fluorescence spectroscopy, cyclic voltammetry, as well as density functional theory (DFT) calculations. It was intriguing to find that these 4-monosubstituted pyrimidine compounds exhibited bright fluorescence with excellent quantum yields (～0.53-0.93) in the blue region. PM1 and PM2 had a lower LUMO than that of CBP (4,4-di(9H-carbazol-9-yl)biphenyl), meaning that both of them had better electron injection and transfer abilities. The variation tendencies of energy levels and absorption spectra obtained from DFT calculations agreed well with those from experiment. Other electronic properties including ionization potentials (IP), electronic affinities (EA), and reorganization energies (λ_hole and λ _electron) have been theoretically studied as well. These electronic properties could be tuned by introducing a different number of pyrimdin-4-yl moieties onto to the donor fragments. The incorporation of pyrimdin-4-yl can significantly decrease the λ_electron and then improve the electron-accepting and hole-electron charge balance abilities. The combined theoretical and experimental studies offer insights into the nature of bipolar molecules containing 4-monosubstituted pyrimidine moieties, and provided a fertile ground for the design of appropriate ambipolar materials for optoelectronic applications. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43631d

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Bipolar luminescent materials containing pyrimidine
terminals: synthesis, photophysical properties and a
theoretical study†
Cite this: RSC Adv., 2013, 3, 21877
a
a
a
a
c
ab
*
*
Jiena Weng, Qunbo Mei, Quli Fan, Qidan Ling, Bihai Tong and Wei Huang
A new series of 4-monosubstituted pyrimidine bipolar materials containing a carbazole (PM1–PM2) or a
triphenylamine (PM3–PM5) moiety as the electron donor have been synthesized by a ZnCl2-catalyzed
three-component coupling reaction. These 4-monosubstituted pyrimidine compounds were
characterized by UV-vis and fluorescence spectroscopy, cyclic voltammetry, as well as density functional
theory (DFT) calculations. It was intriguing to find that these 4-monosubstituted pyrimidine compounds
exhibited bright fluorescence with excellent quantum yields (ꢀ0.53–0.93) in the blue region. PM1 and
PM2 had a lower LUMO than that of CBP (4,40-di(9H-carbazol-9-yl)biphenyl), meaning that both of
them had better electron injection and transfer abilities. The variation tendencies of energy levels and
absorption spectra obtained from DFT calculations agreed well with those from experiment. Other
electronic properties including ionization potentials (IP), electronic affinities (EA), and reorganization
energies (lhole and lelectron) have been theoretically studied as well. These electronic properties could be
tuned by introducing a different number of pyrimdin-4-yl moieties onto to the donor fragments. The
incorporation of pyrimdin-4-yl can significantly decrease the lelectron and then improve the electron-
accepting and hole–electron charge balance abilities. The combined theoretical and experimental
studies offer insights into the nature of bipolar molecules containing 4-monosubstituted pyrimidine
moieties, and provided a fertile ground for the design of appropriate ambipolar materials for
optoelectronic applications.
Received 15th July 2013
Accepted 20th August 2013
DOI: 10.1039/c3ra43631d
www.rsc.org/advances
character.16–19 In order to circumvent this issue, considerable
efforts have been placed into the development of bipolar
Introduction
Since the rst report of organic light-emitting diodes (OLEDs),1
many studies have focused on designing and synthesizing
highly efficient materials in order to improve the device effi-
ciency.2–7 One of the critical requirements for organic light
emitting material is its carrier transport property. A large
number of emitters and host materials have been developed
with building blocks of carbazole2,6–15 or triphenylamine
(TPA),2,3,15,16 well-known electron donors. Many of these mate-
rials favor injection/diffusion of holes over electrons and result
in unbalanced charge injection into an emitting material layer
materials with building blocks of both electron donors and
acceptors.2,6,20–24 Some common electron acceptors, including
benzimidazole,25–28 1,3,4-oxadiazole,22,29,30 triazine,31–33 pyri-
dine,14,34 pyrimidine,14 phosphine oxide,35–39 and so on, were
introduced into p-conjugated systems to develop novel mate-
rials with improved electron mobility.
Among these blocks of electron acceptors, the pyrimidine
unit has only very rarely been incorporated into blocks of elec-
tron donors to develop ambipolar materials, although the
pyrimidine group had a higher electron affinity than pyridine.40
The introduction of the pyrimidine moiety was expected to
improve the electron affinity of the material,41 which made the
pyrimidine moiety an attractive electron acceptor candidate to
construct ambipolar materials. For example, blue OLEDs
(EML), and present essentially
a
so-called unipolar
aKey Laboratory for Organic Electronics & Information Displays, Institute of Advanced
Materials, Nanjing University of Posts & Telecommunications, Nanjing 210046, China.
E-mail: iamqbmei@njupt.edu.cn; iamwhuang@njupt.edu.cn; Fax: +86 25 8586 6533;
Tel: +86 25 8586 6396
bJiangsu-Singapore Joint Research Center for Organic/Bio-Electronics & Information
Displays, Institute of Advanced Materials, Nanjing University of Technology, Nanjing
211816, China. Fax: +86 25 5813 9988; Tel: +86 25 5813 9001
cInstitute of Molecular Engineering and Applied Chemistry, College of Metallurgy and
Resources, Anhui University of Technology, Ma'anshan, Anhui 243002, P. R. China
† Electronic supplementary information (ESI) available: 1H NMR, 13C NMR, MS
spectra and additional data. See DOI: 10.1039/c3ra43631d
employing
spirobiuorene (TBPSF) as an emitter or the emitting host
exhibited high currents over 5000 mA cmꢁ2 42 High efficiencies
2,7-bis[2-(4-tert-butylphenyl)pyrimidine-5-yl]-9,90-
.
were achieved for red phosphorescent OLEDs based on 4,6-bis
[3-(carbazol-9-yl)phenyl]pyrimidine, which is an ambipolar host
material with
a
pyrimidine core.14 The pyrimidine core
improved the electron injection and carrier balance and thus
reduced triplet exciton–polaron quenching at high current
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density. Most recently, extremely low turn-on voltages for fac-
tris(2-phenylpyridine)iridium(III) (Ir(PPy)3)-based green phos-
phorescnet OLEDs were achieved by utilizing pyrimidine-con-
taining electron transport materials as an electron-transport
and hole/excition-block lay.43
The pyrimidine ring was attractive to us for its high electron
affinity.40,44 As far as we know, although efficient pyrimidine-
containing host materials14,45 and electron transport mate-
rials43,46 have been reported, almost all of these materials were
constructed with a pyrimidine core and there was no report on
the electronic properties of luminescent materials containing a
4-monosubstituted pyrimidine moiety. Herein, a series of
bipolar molecules (PM1–PM5, Scheme 1), containing the elec-
tron acceptor of pyrimidine and the electron donor of carbazole
or triphenylamine, were designed. Our current work focuses on
the relationships of molecular structure and photoelectric
property of the PM (including PM1–PM5) compounds. The
photophysical and electronic properties of the PM compounds
were systematically varied by chemical modication. In addi-
tion, density functional theory (DFT) and time-dependent
density functional theory (TD-DFT) calculations were also pre-
sented to predict the photoelectric properties of the PM
compounds.
Scheme 1 Synthetic routes of PM1–PM5. Conditions: (i) AlCl3, acetic anhydride,
DCM, 0 ꢂC to room temperature (RT); (ii) AlCl3, acetyl chloride, DCM, 0 ꢂC to RT; (iii)
AlCl3, acetyl chloride, DCM, 0–40 ꢂC; (iv) triethyl orthoformate, ammonium
acetate, ZnCl2, toluene, 100 ꢂC.
Results and discussion
Synthesis
Many methods, including the Leuckart reaction47 and the Bre-
dereck-type48,49 synthesis method, were available for the
synthesis of 4-arylpyrimidine compounds from aromatic
ketones. For example, Helland and Lejon50 reported the
synthesis of 4-phenylpyrimidine from condensation of aceto-
of the lowest energy band in the absorption spectra of PM1–
PM5 was due to an intramolecular p / p* charge transfer
(ICT)52,53 from the donor (carbazole or TPA)-based molecular
orbitals to the acceptor (pyrimidine)-based molecular orbitals
in the relevant dipolar compounds. The absorption band at
around 300 nm of PM1–PM3 could be assigned to the carbazole/
TPA-centered n / p* transition. For PM1–PM2, compared to
the carbazole-centered n / p* transition band, the absorbance
at the lowest energy band increased with the introduction of
more pyrimdin-4-yl moieties. Meanwhile, it was obviously
observed from Fig. 1(c) and (d) that the TPA-centered n / p*
ꢂ
phenone with formamide at 179 C for 17 h in the presence of
cuprous chloride as an oxidant. Another simple and single step
synthesis of 4-arylpyrimidine using a combination of ketones,
formamide and a source of ammonia under microwave irradi-
ation was also reported later.49 We had try to synthesise our
target products PM1–PM5 (Scheme 1) from the corresponding
ketones (A1–A5) using the two methods mentioned above.
Unfortunately, the formation of the corresponding formyl
amide, a major side product, resulted in a decrease in the
product yield and difficult purication. As shown in Scheme 1,
the third method we had adopted was a ZnCl2-catalyzed three-
component coupling reaction involving aromatic ketones,
triethyl orthoformate, and ammonium acetate, which led to the
production of 4-substituted pyrimidine derivatives in a single
step.51 Although the product yields (14.5% to 20%) were still
unsatisfying, the target PM products could be puried simply by
silica gel chromatography. The structures of the target products
were identied by using 1H NMR, 13C NMR, and GC-MS or
MALDI-TOF-MS.
Table 1 Optical properties of PM1–PM5
Solutiona
lPL (nm) FF
Solid lmb
labs (nm) lPL (nm)
Molecule labs (nm)
PM1
PM2
PM3
PM4
PM5
248, 300, 337 412
271, 308, 353 397
251, 300, 370 487
253, 351, 389 472
0.92 232, 302, 341 424.6
0.93 270, 305, 356 423.8
0.70 257, 302, 374 472
0.65 255, 353, 389 471
255, 384
464
0.53 258, 384
473.8
a
The values of absorption and photoluminescence spectra were
recorded in DCM. Excitation wavelength ¼ 360 nm for measurements
of photoluminescence spectra and FF. The FF values recorded are
Photophysical properties
The UV-vis absorption spectra of the PM compounds investi-
gated in this work were recorded in diluted solution and in the
solid state (see Table 1 and Fig. 1). As shown in Fig. 1, the origin
relative to that of 9,10-diphenylanthracene (0.90 in cyclohexane).
b
Excitation wavelength
¼
360 nm for measurements of
photoluminescence spectra.
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Fig. 1 Absorption and photoluminescence spectra (lex ¼ 360 nm) of PM1–PM5 in the DCM solution state (a), (c) and the thin film state (b), (d).
transition band red-shied from PM3 to PM4, and nally dis- emissions in DCM, respectively. In addition, broad emissions
appeared for PM5. This could be attributed to stronger ICT ranging from 330 nm to 650 nm were observed from the thin
formed between the central donor and the outer pyrimidine. lms of PM1 and PM2, which were considerably broader than
Absorption maxima were found to undergo a bathochromic those observed in solution. Compared with the thin lm
shi by ca. 12 nm and 19 nm for PM2 and PM4 as compared to emission spectra of PM1 and PM2, a distinct emission shoulder
PM1 and PM3, respectively, owing to an increased conjugation peak at around 500 nm came out with the introduction of two
in PM2 and PM4. However, with the number of pyrimdin-4-yl pyrimdin-4-yl moieties. This could be affirmed by comparing
moiety increasing in the TPA-based compound, the lowest the thin lm emission spectra of the TPA-based compounds,
energy band maxima of PM5 was slightly blue-shied relative to PM3–PM5. The long wavelength emission peaks could be
PM4. It was probably due to the symmetrical geometry and possibly attributed to the intermolecular interactions in the
relatively small dipole moment of PM5 when compared to PM4, excited state occurring in the solid state or by the formation of
which is demonstrated by DFT calculations below. UV-vis excimers.
absorption spectra of spin coating lms for the PM compounds
For each PM molecule, weak solvatochromism was
on quartz substrates measured at room temperature are shown observed in the ICT absorption band. As shown in Fig. 2(a), the
in Fig. 1(b) and (d), which were almost similar to those in dilute ICT absorption maximum of PM5 red-shied from 322 nm in
dichloromethane (DCM) solution. The nearly identical absorp- nonpolar n-hexane to 343 nm in highly polar methanol.
tion spectra in solution and in lm suggested that there was Similar data for the other PM compounds are shown in the
minimal intermolecular interaction in the ground state in the supporting information (Fig. S1†). The optical absorption
thin lm.
properties of ve PM molecules in solvents of varying polarity
Upon excitation at 360 nm, PM1–PM5 exhibited intense are summarized in Table S1.† While the effect of solvent
uorescence in DCM at room temperature. The uorescence polarity on the absorption spectra was minimal, the uores-
quantum yields of PM1–PM5 in DCM solution were measured to cence spectra of each PM molecule in polar solutions were
be ꢀ0.53–0.93 relative to 9,10-diphenylanthracene (Table 1). A signicantly broadened and red-shied when compared with
blue-shi of the emission spectra was observed with introduc- those obtained in n-hexane (Fig. 2(b) and S2†). Thus, the sol-
tion of more pyrimdin-4-yl moieties, which was not affected by vatochromic effect was related to a decrease in the energy of
the increased conjugation. The thin lm emission spectra were the singlet excited state because of the reorientation or relax-
considerably different from those recorded in dilute DCM ation of the solvent dipoles.54
solution. The thin lm emissions of PM1 and PM2 were red-
To study in more detail the solvatochromism, a Lippert–
shied by ca. 13 nm and 27 nm relative to the corresponding Magada plot (Fig. 3) was constructed, which described the
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Fig. 2 Normalized absorption (a) and photoluminescence ((b), lex ¼ 360 nm) spectra of PM5 recorded in different solvents.
dependence of Stokes shis (expressed in wavenumbers) on the linear relationship and largest slopes, indicating that the dipole
orientation polarizability Df derived from eqn (1).
moment variation of PM3 between ground-state and excited-
state was largest among these PM compounds. The enhanced
excited state dipole moment of the solute upon excitation
induced the local dipole–dipole interactions that forced the
reorientation of the polar solvent molecules around the excited
solute.55 This was accompanied by the molecular structural
relaxation and gave rise to inhomogeneous broadening of the
uorescence spectrum with emission occurring from a relaxed
state at lower energy.56 Therefore, the emission spectra of PM
compounds in ethanol and methanol were obviously
broadened.
ð3 ꢁ 1Þ
ðn2 ꢁ 1Þ
Df ¼
ꢁ
(1)
ð23 þ 1Þ ð2n2 þ 1Þ
According to the Lippert equation,54 Stokes shi is propor-
tional to the square of the change in dipole moments between
the ground and excited states:
À
Á
2
2 me ꢁ mg
nabs ꢁ nem
¼
Df þ C
-
hca3
where 3 is the solvent dielectric constant and n is the solvent
refractive index, ħ is the Planck constant, c is the speed of light,
nabs and nem are the wavenumbers of absorption and emission,
respectively (cmꢁ1), a is the Onsanger radius, me ꢁ mg is the
difference between dipole moment in the ground and excited
state, and C is a constant. For a typical solvatochromic
compound, the dependence of a Stokes shis on Df is linear in
the absence of solvent-specic interactions, where the slop of
the plot reected solvatochromic sensitivity of a uorophore.
The most sensitive solvatochromic compounds are those with
the steepest slope and therefore the largest change in the dipole
moment. The Lippert–Magada plots of PM3 showed the best
Electrochemical properties
Knowledge of HOMO and LUMO energy levels of organic elec-
tronic materials, especially OLED materials, is important
because they might affect the charge injection from adjacent
layers. To further evaluate the potential applicability of the
present PM compounds in electronic devices, cyclic voltam-
metry (CV) experiment was carried out to investigate their
electrochemical properties. In this work, we measured the
oxidation potentials of all PM compounds in DCM solution and
used these electrochemical values to estimate the HOMO energy
levels (EHOMO) versus ferrocene/ferrocenium (Fc/Fc+). The LUMO
energy levels (ELUMO) were calculated from the EHOMO and
optical band gap. The oxidation potentials Eox, EHOMO and
ELUMO values derived from CV were shown in Table 2.
One quasi-reversible oxidation peak was observed for each
PM compound, as shown in Fig. S3.† For carbazole based PM
compounds, PM1 and PM2, the nature of the acceptor terminal
had a small inuence on the oxidation potentials. Compared to
the carbazole-based compound CBP (4,40-di(9H-carbazol-9-yl)
biphenyl), which was a hole transporting (HT) emitter and
widely utilized as HT host materials, the EHOMO of PM1 and PM2
(ꢁ5.47 and ꢁ5.49 eV, respectively) were slightly higher than that
of CBP (ꢁ5.50 eV).57 Nevertheless, their ELUMO (ꢁ2.16 and ꢁ2.20
eV) obtained from the HOMO and optical band gap were much
lower than that of CBP (ꢁ2.0 eV).57 The lower LUMO of PM1 and
PM2 compared with that of CBP suggested that both of them
had better electron injection and transfer abilities than CBP,
Fig. 3 Lippert–Magada plots of PM1–PM5. The y and x in the equations of the
fitted lines represented Stokes shift and solvent orientation polarizability,
respectively. The correlation coefficients (R2) of the fitted lines were 0.49, 0.45,
0.93, 0.89 and 0.87 in turn.
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Table 2 Experimental and theoretical HOMO, LUMO and singlet band gap of PM1–PM5
From CV
B3LYP/6-31G(d)
Molecule Egopa (eV) Eoonx set/peak (V) EHOMO (eV) ELUMO (eV) Ecgalc (eV) EHOMO (eV) ELUMO (eV) Ecgald (eV) EHOMOd (eV) ELUMOd (eV)
b
c
c
PM1
PM2
PM3
PM4
PM5
3.31
3.29
2.98
2.91
2.93
0.78/1.16
0.80/1.20
0.64/0.86
0.65/0.90
0.75/0.98
ꢁ5.47
ꢁ5.49
ꢁ5.33
ꢁ5.34
ꢁ5.44
ꢁ2.16
ꢁ2.20
ꢁ2.35
ꢁ2.43
ꢁ2.51
4.16
4.08
3.66
3.52
3.52
ꢁ5.48
ꢁ5.63
ꢁ5.15
ꢁ5.31
ꢁ5.44
ꢁ1.32
ꢁ1.55
ꢁ1.49
ꢁ1.79
ꢁ1.92
4.06
3.97
3.59
3.45
3.48
ꢁ5.54
ꢁ5.64
ꢁ5.21
ꢁ5.30
ꢁ5.37
ꢁ1.48
ꢁ1.67
ꢁ1.62
ꢁ1.85
ꢁ1.89
Obtained from the optical gap (onset of absorption in DCM). b ELUMO ¼ EHOMO + Egop
. Theoretical values calculated by DFT at B3LYP/6-31G(d) in
a
c
vacuum. d Theoretical values calculated by DFT at B3LYP/6-31G(d) in solvent (DCM) with polarizable continuum model (PCM).
benecial for charge injection balance. For the TPA based PM the HOMOs of all PM compounds are orbitals of p character
compounds, the EHOMO of PM3 and PM4 were very close while delocalized over the entire molecules, suggesting that the
the EHOMO of PM5 decreased by 0.1 eV compared to that of PM4. HOMOs are determined by both carbazole/TPA and pyrimidine.
The more pyrimidin-4-yl substituents present resulted in larger The LUMOs of PM1–PM4 are predominately localized on the
decrease in EHOMO in the following order: PM3 > PM4 > PM5, terminal pyrimidine moieties and the aryl groups around them
probably because of the electronegative nature of the pyrimi- suggesting that the LUMOs are predominately determined by
dine moiety.
the pyrimidine. PM5 is composed of three pyrimidin-4-yl-phenyl
moieties with a central amine nitrogen atom. The central NC3
moiety is planar and forms an equilateral triangle. The pyr-
imidin-4-yl-phenyl moieties are symmetrically twisted from that
plane so the whole molecule can be visualized as a propeller
(Table 3). Although the optimized geometry of PM5 is
symmetrical, the LUMO diagram shows that the electron clouds
are p orbitals delocalized mainly on two of the three pyrimidin-
4-yl-phenyl moieties. It can be obviously seen that central
nitrogen does not contribute to the LUMO of PM5, however, the
pyrimidin-4-yl interacts with the phenyl ring.
The optoelectronic devices depend on the charge injection,
transfer, as well as balance. It was necessary to investigate the
ionization potentials (IPs), electronic affinity (EAs), and
reorganization energies (l), so that we could evaluate the
energy barrier for injection and transport rates of the holes
and electrons. The IPs and EAs, together with the hole
extraction potential (HEP) and electron extraction potential
(EEP) for PM1–PM5 were calculated by DFT and derived from
the computed results according to literature pubilica-
tions.61–65 The IP and EA values were used to evaluate the
energy barrier for the injection of holes and electrons.61 HEP
is the energy difference between the neutral molecule (M) to
cationic (M+) using the M+ geometric structure in the calcu-
lation, while EEP is the energy difference between the M to
anionic (Mꢁ) using the Mꢁ geometric structure.62,63,65 In
addition, the reorganization energy is the energy difference
between the two molecules which have gone through electron
transfer and provides a qualitative indication of the charge-
transport rate (the lower the l values, the bigger the charge-
transport rate).65
Theoretical studies
To elucidate the inuence of the structure on the electronic
properties of PM compounds, theoretical calculations were
performed using the Gaussian09 program package.58 The opti-
mizations of the molecular geometry and total energy calcula-
tions were carried out using density functional theory (DFT)
calculations at B3LYP/6-31G(d) level.59 All the optimized
ground-state geometries are shown in the ESI (Tables S2–S6†).
As listed in Table 2, the HOMO and LUMO level energies and
the energy gaps (Ecgal) both in vacuum and DCM were well pre-
dicted by DFT calculations with good correlation. Although the
theoretical energy gaps of PM1–PM5 were overestimated
compared to the experimental values, excellent correlations
could be found between the theoretical and experimental energy
gaps (Fig. 4), which suggested that the DFT calculation results
indeed provide good estimations of the energy gap trends.60
The frontier molecular orbital electron density distributions
of the PM compounds are shown in Fig. 5. It can be seen that,
As shown in Table 4, the IP values of PM2 and PM1 changed
slightly (ca. 0.011 eV), while the EA values increased signi-
cantly (ca. 0.520 eV) from PM1 to PM2. For the TPA based ones,
both IP and EA values gradually increased in the following
order: PM3 < PM4 < PM5, as more and more pyrimidin-4-yl
Fig. 4 The relationship between the theoretical and experimental energy gap. moieties are introduced. The increase of EA values was much
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Fig. 5 The optimized ground geometries, frontier molecular orbital energies and electron density distributions of PM1–PM5.
Table 4 Ionization potential (IP), electronic affinity (EA) and reorganization
energy (l) of PM1–PM5 by DFT at B3LYP/6-31G(d)a (in eV)
larger than that of IP values, which demonstrates that the
incorporation of pyrimidin-4-yl could benet the electron-
accepting ability. The negative calculated EA value of PM1 was
raised by the incomplete cancellation of electronic self-inter-
action energy due to the use of an inexact density functional and
a nite basis set.66,67
The calculated reorganization energies are also listed in
Table 4. The lhole values for PM1–PM5 were all smaller than
their respective lelectron values, indicating that the hole transfer
rate was higher than the electron transfer rate. As more and
more pyrimidin-4-yl moieties are introduced, the lelectron values
decrease sharply with large variations while the lhole values
hardly change. PM2 and PM5 have the smallest lelectron for the
PM compounds with a carbazole and a TPA unit, respectively,
Molecule IP (v) IP (a) HEP EA (v) EA (a) EEP lhole lelectron
PM1
PM2
PM3
PM4
PM5
6.935 6.880 6.822 ꢁ0.233 ꢁ0.070 0.104 0.113 0.336
6.946 6.890 6.837
6.483 6.424 6.368
6.540 6.483 6.429
6.586 6.532 6.480
0.287
0.036
0.608
0.822
0.412 0.525 0.110 0.238
0.301 0.550 0.114 0.513
0.747 0.882 0.110 0.273
0.925 1.025 0.105 0.203
a
The IPs and EAs can be either for vertical excitations (v, at the
geometry of the neutral molecule) or adiabatic excitations (a,
optimized structure for both the neutral and charged molecule).
which suggested that PM2 and PM5 have a higher electron (0.098 eV), implying that PM2 and PM5 have better hole- and
transfer rate. In addition, the difference between lhole and electron-transporting balance and could act as good ambipolar
lelectron gradually decreases in the following order: PM1 (0.223 materials. Therefore, the pyrimidin-4-yl group was a nice elec-
eV) < PM2 (0.128 eV) and PM3 (0.399 eV) < PM4 (0.163 eV) < PM5 tron-accepting candidate for designing ambipolar materials.
Table 3 The optimized ground geometry and relevant bond lengths, angles, and dihedrals of PM5 by DFT at B3LYP/6-31G(d)
C1–N1
C2–N1
1.41898
1.41896
˚
Bond length (A)
C3–N1
1.41941
Bond angle 4 (ꢂ)
C1–N1–C10
C2–N1–C20
C3–N1–C30
C10–C1–N1–C2
C20–C2–N1–C3
C30–C3–N1–C1
119.96168
120.10335
119.93472
ꢁ40.46637
ꢁ40.83949
ꢁ40.03956
Torsion angle q (ꢂ)
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In order to understand electronic transitions of PM following order: PM1 (344.5 nm) < PM2 (357.3 nm) and PM3
compounds, time-dependent density functional theory (TD- (393.7 nm) < PM4 (415.2 nm), which are consistent with those in
DFT) calculations on the absorption spectra in both vacuum vacuum. However, it was interesting to note that the maximum
and solvent were performed. The calculated wavelengths from absorption wavelength of PM5 in DCM was slightly smaller (ca.
the absorption spectra, the main transition congurations, and 3 nm) than that of PM4, which was probably due to the decrease
the oscillator strengths for the most relevant singlet excited of the dipole moment of PM5. In both vacuum and in polar
states of PM compounds were listed in Tables 5 and S7,† and solvent DCM, the dipole moment of PM4 was slightly shorter
the simulated absorption spectra are shown in Fig. S4.† All the than that of PM3, and their DlDCM–vacuum (the difference
electronic transitions of the PM compounds were of the p / p* between the maximum absorption wavelength simulated in
type for the absorption spectra, where the electron transitions DCM and that in vacuum) was very close (DlDCM–vacuum ¼ 18.4
were from the initial state (predominately contributed by the nm and 18.0 nm for PM3 and PM4, respectively). Compared to
HOMO) to the nal state (predominately contributed by the PM3 and PM4, the dipole moment of PM5 is much shorter,
LUMO). For carbazole based PM compounds, the maximum which is due to its symmetrical geometry. Therefore, the
absorption wavelengths in vacuum exhibited a red-shi (ca. 12 DlDCM–vacuum of PM5 (12.7 nm) is signicant smaller than those
nm) from PM1 to PM2. The HOMO and LUMO energy levels of of PM3 and PM4. In conjunction with these changes, the
PM1 are ꢁ5.48 and ꢁ1.32 eV, respectively. The calculated rst absorption spectrum of PM5 in DCM is slightly blue-shied as
excited state had an excitation energy of 3.76 eV (330.2 nm) with compared with that of PM4. Whereas the accuracy of the
an oscillator strength of 0.2954. The incorporation of one more calculated excitation energy is not high enough by the level of
electron-drawing pyrimidin-4-yl at the 6-position of the carba- our calculations, these calculated results evidently demonstrate
zole ring (PM2) led to a decrease both in the HOMO and LUMO the nature of the transition signicantly dependent on the
energy level by 0.15 and 0.23 eV, respectively. Consequently, the electronic effect of the pyrimidin-4-yl substituent and the
calculated rst excited energy of PM2 was reduced to 3.62 eV substitution number of the molecular structure. The introduc-
(342.5 nm). This change led to a notable increase in the oscil- tion of the electron-drawing pyrimdin-4-yl groups onto electron
lator strength of the rst excited state ( f ¼ 0.4677). For the TPA donor (carbazole/TPA) led to efficient intramolecular CT tran-
based PM compounds, the maximum absorption wavelengths sition for those D–A conjugated systems, which is consistent
in the vacuum gradually red-shied in the following order: PM3 with our experimental results.
(375.3 nm) < PM4 (397.2 nm) < PM5 (399.1 nm), which conrms
the prediction from the energy gaps listed in Table 2. When
three pyrimidin-4-yl groups were introduced onto 4,40,400-posi-
Conclusions
tions of the TPA framework, both the HOMO and LUMO energy
A series of bipolar luminescent materials, PM1–PM5, contain-
levels of PM5 were lowered compared to those of PM4, but the
ing 4-monosubstituted pyrimidine moieties as the electron
calculated ground state energy gap was very close to that of PM4. acceptor, were synthesized using a ZnCl2-catalyzed three-
The excitation energy of the rst excited state of PM5 was component coupling reaction and their optoelectronic proper-
decreased slightly by 0.01 eV compared to PM4.
ties were systematically studied. These PM compounds exhibi-
ted intense uorescence in DCM at room temperature with
uorescence quantum yields of ꢀ0.53–0.93. Introducing more
pyrimdin-4-yl moieties onto the carbazole or triphenylamine
fragment, the maximum emission peaks in DCM solution blue
shied and hardly shied in solid state. The CV data demon-
The inuence of solvent (DCM) on the absorption spectra of
the PM compounds was simulated by using the PCM model.61,64
The maximum absorption wavelengths in DCM were larger than
those in vacuum, indicating that the solvent effects stabilized
the excited state, which induced the red-shi of the absorption
spectra as compared with those in vacuum. The maximum strated that the HOMO level energies of these PM compounds
absorption wavelengths in DCM gradually red-shied in the slightly decreased when more pyrimdin-4-yl terminals were
Table 5 The absorption spectra calculated by DFT at B3LYP/6-31G(d) in vacuum and solvent (DCM) of PM1–PM5
Calculated
wavelength
(nm)
Dipole
moment
(Debye)
Electron
transition
Excitation
energy Es1
Main transition
conguration
Oscillator
strength f
Molecule
PM1
States
Gas-phase
DCM
Gas-phase
DCM
Gas-phase
DCM
Gas-phase
DCM
S0–S1
S0–S1
S0–S1
S0–S1
S0–S1
S0–S1
S0–S1
S0–S1
S0–S1
S0–S1
330.2
344.5
342.5
357.3
375.3
393.7
397.2
415.2
399.1
411.8
3.76
3.60
3.62
3.47
3.30
3.15
3.12
2.99
3.11
3.01
HOMO / LUMO (0.92)
HOMO / LUMO (0.96)
HOMO / LUMO (0.96)
HOMO / LUMO (0.98)
HOMO / LUMO (0.99)
HOMO / LUMO (0.99)
HOMO / LUMO (0.99)
HOMO / LUMO (0.99)
HOMO / LUMO (0.99)
HOMO / LUMO (0.99)
0.2954
0.4228
0.4677
0.6355
0.6056
0.7083
0.7848
0.9041
0.6520
0.7685
5.1315
6.5696
7.7232
10.1313
3.9140
4.8549
3.7588
4.7973
3.0592
4.2579
PM2
PM3
PM4
PM5
Gas-phase
DCM
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onset
ox
introduced. Both PM1 and PM2 had a lower LUMO than that of was for Fc vs. Ag/Ag+ and E
was the onset potential. The
CBP, suggesting both of them had better electron injection and uorescent quantum efficiency (F) was measured with 9,10-
transfer abilities. Theoretical calculations were also performed diphenylanthracene as a standard.69
to elucidate the inuence of the structure on the electronic
properties of PM compounds. The variation tendencies of
energy levels and absorption spectra obtained from DFT
calculation well agreed with the experiment data. The DFT study
also suggested that the EAs and lelectron of PM compounds could
be modied effectively. The narrow differences between lhole
and lelectron were found in PM2 and PM5, which implied that
PM2 and PM5 could act as excellent ambipolar materials.
Quantum yield measurement
Fluorescence quantum yield was determined using optically
matching solutions of 9,10-diphenylanthracene (Ff ¼ 0.9 in
cyclohexane) as standards at an excitation wavelength of 360
nm and the quantum yield was calculated using eqn (2).69
ꢀ
ꢁ
2
ðIunk=AunkÞ hunk
Funk ¼ Fstd
(2)
ðIstd=Astd
Þ
hstd
where Funk and Fstd are the uorescence quantum yields of the
sample and the standard, respectively. Iunk and Istd are the
integrated emission intensities of the sample and the standard,
respectively. Aunk and Astd are the absorbance of the sample and
the standard, respectively. And hunk and hstd are the refractive
indexes of the corresponding solutions.
Experimental section
Materials
All reagents were purchased from commercial sources and were
used without further purication, unless otherwise noted. In
synthetic preparations, dichloromethane (DCM) were distilled
from CaH2. Similarly, toluene was dried by distillation from
sodium/benzophenone. 9-Ethyl-9H-carbazole (ECz) was
prepared according to known literature procedures.68
Synthesis
General procedure of Friedel–Cras acylation reaction. The
general synthesis protocols used to produce the acetyl-
compounds were shown in Scheme 1. AlCl3 (1.5 equiv. to 5
Characterization
The molecular structures of all newly synthesized compounds equiv.) and DCM were placed into a two neck-ask with a drying
were unambiguously determined through NMR (1H and 13C) tube, acetyl chloride or acetic anhydride (1.2 equiv. to 3.6 equiv.)
and mass (GC-MS or MALDI-TOF-MS) spectroscopy. The NMR was added dropwise at 0 ꢂC and stirred until AlCl3 was
spectra were recorded with a Bruker Ultra Shield Plus 400 MHz completely dissolved. The mixture prepared above was removed
spectrometer, with the chemical shis reported in ppm suing to a dropping funnel and added dropwise to DCM solution
tetramethylsilane (TMS) as an internal standard. The concen- containing ECz or TPA (1.0 equiv.) at 0 ꢂC. The mixture was
trations of these compounds solution were adjusted to 10–12 stirred at 0 ꢂC for 1 h to 2 h. Aer warmed to room temperature
mg mLꢁ1 and 20–30 mg mLꢁ1 for 1H NMR and 13C NMR slowly, the mixture was stirred at room temperature or at 40 ꢂC
experiments, respectively. Mass spectrometric data were and monitored by TLC analysis until the reaction had been
obtained with a Shimadzu GCMS-QP 2010 Plus spectrometry or completed. The mixture was poured into a large mount of ice
Bruker Daltonics matrix-assisted laser desorption/ionization water slowly and stirred until the inorganic layer was clear. The
time-of-ight mass spectrometer. UV-vis spectra were obtained mixture was extracted several times with DCM, and the
using a Shimadzu UV-3600 UV-vis-NIR spectrophotometer using combined organic extracts were dried over Na2SO4, ltered, and
quartz cell. Room temperature uorescence measurements then concentrated under reduced pressure. The crude product
were performed using Shimadzu RF-5301PC uorescence was puried by silica gel chromatography (ethyl acetate–petro-
spectrophotometer. For the spectra recorded in solution, the leum ether) to produce acetyl-compound (A1–A5).
concentrations of all the materials were 1 ꢃ 10ꢁ5 M. For the
General procedure for the synthesis of pyrimidine
thin lm spectra, materials (1 mg) were rst dissolved in 1 mL of compounds. To a toluene solution of ZnCl2 (0.1 equiv. for an
DCM containing PMMA (20 mg mLꢁ1), and spin-coated at a acetyl) and triethyl orthoformate (3 equiv. for an acetyl) was
speed rate of 1500 rpm for 30 s onto the quartz slides. The added acetyl-compound (1 equiv.) (A1–A5) and ammonium
energy gaps (Eg) were calculated from the edge of the visible- acetate (2 equiv. for an acetyl). The mixture was heated at 100 ꢂC
absorption bands in the DCM spectra (Eg ¼ hc/lonset). The cyclic for 48 h. Aer cooled to room temperature, a saturated aqueous
voltammograms (CV) were performed using a CHI660C elec- solution of NaHCO3 was added to the mixture to quench the
trochemical workstation at room temperature. The measuring reaction. The mixture was extracted several times with DCM and
cell was composed of a glassy carbon as the working electrode, the combined organic extracts were dried over Na2SO4, ltered,
an Ag/AgNO3 electrode as the reference electrode and a plat- and then concentrated under reduced pressure. The crude
inum wire as the auxiliary electrode. The electrodes were product was puried by silica gel chromatography (ethyl
immersed in a DCM solution containing 0.1 M tetrabuty- acetate–petroleum ether) to produce pyrimidine compounds
lammonium perchlorate (n-Bu4NClO4) and 0.001 M PM1–PM5. (PM1–PM5).
The highest occupied molecular orbital (HOMO) energy levels of
1-(9-Ethyl-9H-carbazol-3-yl)-ethanone (A1). 70% yield; a
the materials were estimated based on the reference energy white solid; 1H NMR (400 Hz, CDCl3) d (ppm): 8.75 (d, J ¼ 1.4 Hz,
levels of ferrocene (Fc, ꢁ4.8 eV with respect to the vacuum level): 1H), 8.20–8.10 (m, 2H), 7.53 (t, J ¼ 7.5 Hz, 1H), 7.43 (dd, J ¼ 13.8,
onset
ox
HOMO ¼ ꢁ[(E
ꢁ 0.11) + 4.8] eV, where the value of 0.11 V 8.4 Hz, 2H), 7.31 (t, J ¼ 7.4 Hz, 1H), 4.40 (q, J ¼ 7.2 Hz, 2H), 2.73
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(s, 3H), 1.46 (t, J ¼ 7.2 Hz, 3H); 13C NMR (100 Hz, CDCl3) d (m, 7H); 13C NMR (100 MHz, CDCl3) d (ppm): 163.08, 159.09,
(ppm): 197.70, 142.70, 140.66, 128.77, 126.47, 126.44, 123.28, 157.28, 149.82, 146.35, 130.59, 129.78, 128.30, 126.13, 124.98,
122.71, 121.94, 120.70, 119.99, 109.01, 108.04, 37.84, 26.67, 123.50, 116.25; MALDI-TOF-MS: M, found 401.359. C22H17N3
13.82. GC-MS: M, found 237. C16H15NO requires 237.30.
requires 401.463.
1-(6-Acetyl-9-ethyl-9H-carbazol-3-yl)-ethanone (A2). 63%
Tris-(4-pyrimidin-4-yl-phenyl)-amine (PM5). 12% yield; a
yield; a white solid; 1H NMR (400 Hz, CDCl3) d (ppm): 8.75–8.71 yellow solid; 1H NMR (400 MHz, CDCl3) d (ppm): 9.23 (d, J ¼ 1.2
(m, 2H), 8.14 (dd, J ¼ 8.6, 1.7 Hz, 2H), 7.41 (dd, J ¼ 8.6, 0.4 Hz, Hz, 3H), 8.74 (d, J ¼ 5.4 Hz, 3H), 8.08–8.03 (m, 6H), 7.67 (dd, J ¼
2H), 4.37 (q, J ¼ 7.3 Hz, 2H), 2.71 (s, 6H), 1.45 (t, J ¼ 7.2 Hz, 3H); 5.4, 1.3 Hz, 3H), 7.32–7.26 (m, 6H); 13C NMR (100 MHz, CDCl3) d
13C NMR (100 Hz, CDCl3) d (ppm): 197.44, 143.35, 129.67, (ppm): 162.93, 159.13, 157.42, 149.18, 131.73, 128.52, 124.59,
127.00, 122.93, 121.97, 108.69, 38.14, 26.68, 13.85. GC-MS: M, 116.42; MALDI-TOF-MS: M, found 479.531. C22H17N3 requires
found 279. C18H17NO2 requires 279.33.
479.535.
1-(4-Diphenylamino-phenyl)-ethanone (A3). 75% yield; a
1
white solid; H NMR (400 MHz, CDCl3) d (ppm): 7.84–7.74 (m,
Theoretical and computational methodology
2H), 7.35–7.29 (m, 4H), 7.19–7.08 (m, 6H), 7.01–6.96 (m, 2H),
2.53 (s, 3H); 13C NMR (100 MHz, CDCl3) d (ppm): 196.50, 152.19,
146.52, 129.90, 129.81, 129.62, 125.97, 124.64, 119.69, 26.24.
GC-MS: M, found 287. C20H17NO requires 287.36.
1-{4-[(4-Acetyl-phenyl)-phenyl-amino]-phenyl}-ethanone (A4).
42% yield; a light yellow solid; 1H NMR (400 MHz, CDCl3) d
(ppm): 7.85 (d, J ¼ 8.7 Hz, 4H), 7.36 (t, J ¼ 7.8 Hz, 2H), 7.21 (t, J ¼
7.4 Hz, 1H), 7.14 (d, J ¼ 7.5 Hz, 2H), 7.12 (dd, J ¼ 17.4, 8.1 Hz,
4H), 2.55 (s, 6H); 13C NMR (100 MHz, CDCl3) d (ppm): 196.58,
151.05, 145.85, 131.67, 129.67, 126.73, 125.71, 122.40, 26.40.
GC-MS: M, found 329. C20H17NO requires 329.39.
All computations were done with Gaussian09 program package
with different parameters for structure optimizations and
vibrational analysis. The singlet ground-state geometries were
fully optimized by the Becke's three-parameter exchange func-
tional59 along with the Lee–Yang–Parr correlation functional
with the restricted (B3LYP) and the unrestricted formalism
(UB3LYP) for neutral and ion state molecules70 at the standard
split valence plus polarization function 6-31G(d) basis set. The
fully optimized stationary points were further characterized by
harmonic vibrational frequency analysis to ensure that real
local minima had been found without imaginary vibrational
frequency. The electronic absorption spectra in vacuum were
performed using TD-DFT method of B3LYP/6-31G(d) on the
basis of the optimized ground structures. The polarizable
continuum model (PCM) was adopted in TD-DFT calculation of
the absorption spectra in the solvent (DCM).
1-{4-[Bis-(4-acetyl-phenyl)-amino]-phenyl}-ethanone
(A5).
1
70% yield; a yellow solid; H NMR (400 MHz, CDCl3) d (ppm):
7.89 (d, J ¼ 8.6 Hz, 6H), 7.14 (d, J ¼ 8.6 Hz, 6H), 2.56 (s, 9H); 13
C
NMR (100 MHz, CDCl3) d (ppm): 196.56, 150.31, 132.88, 130.14,
123.93, 26.48. GC-MS: M, found 371. C20H17NO requires 371.43.
9-Ethyl-3-pyrimidin-4-yl-9H-carbazole (PM1). 20% yield; a
Generally, organic p-conjugated materials were assumed to
transport charge at room temperature via a thermally activated
hopping-type mechanism.71–73 The charge transport was the
intermolecular process in which the charge hoped between two
molecules, which can be summarized as follows:
1
white solid; H NMR (400 MHz, CDCl3) d (ppm): 9.27 (s, 1H),
8.90 (s, 1H), 8.72 (d, J ¼ 5.4 Hz, 1H), 8.20 (t, J ¼ 8.4 Hz, 2H), 7.80
(d, J ¼ 5.5 Hz, 1H), 7.48 (dq, J ¼ 24.6, 8.1 Hz, 3H), 7.30 (t, J ¼ 7.4
Hz, 1H), 4.39 (q, J ¼ 7.2 Hz, 2H), 1.46 (t, J ¼ 7.2 Hz, 3H); 13C NMR
(100 MHz, CDCl3) d (ppm): 164.57, 159.05, 157.07, 141.82,
140.59, 127.12, 126.35, 124.82, 123.57, 123.13, 120.76, 119.81,
119.70, 116.43, 108.90, 108.82, 37.80, 13.85. GC-MS: M, found
273. C18H15N3 requires 273.33.
M+ + M / M + M+
Mꢁ + M / M + Mꢁ
9-Ethyl-3, 6-di-pyrimidin-4-yl-9H-carbazole (PM2). 14.5%
yield; a white solid; 1H NMR (400 MHz, CDCl3) d (ppm): 9.27 (s,
2H), 8.97 (s, 2H), 8.75 (d, J ¼ 5.4 Hz, 2H), 8.27 (dd, J ¼ 8.6, 1.0
Hz, 2H), 7.82 (d, J ¼ 5.4 Hz, 2H), 7.51 (d, J ¼ 8.6 Hz, 2H), 4.42 (q,
J ¼ 7.2 Hz, 2H), 1.49 (t, J ¼ 7.2 Hz, 3H); 13C NMR (100 MHz,
CDCl3) d (ppm): 164.23, 159.09, 157.23, 142.42, 128.03, 125.45,
123.72, 120.05, 116.45, 109.30, 38.12, 13.92. GC-MS: M, found
351. C18H15N3 requires 351.40.
where M represented the neutral molecule interacting with
neighboring oxidized or reduced M+/ꢁ. The standard Macus–
Hush equation74,75 approximately described the hole/electron
transfer rate, assuming that hole/electron traps were
degenerate:
ꢀ
ꢁ
ꢀ
ꢁ
1=2
2
p
V
lhole=electron
hole=electron
-
Khole=electron
¼
exp
ꢁ
lhole=electronkT
h
4kT
Diphenyl-(4-pyrimidin-4-yl-phenyl)-amine (PM3). 19% yield;
a light yellow solid; 1H NMR (400 MHz, CDCl3) d (ppm): 9.19 (s, where T is the temperature, Vhole/electron is the electronic
1H), 8.68 (d, J ¼ 5.4 Hz, 1H), 7.96 (d, J ¼ 8.8 Hz, 2H), 7.61 (dd, J ¼ coupling matrix element between neighboring molecules in the
5.5, 1.2 Hz, 1H), 7.30 (t, J ¼ 7.9 Hz, 4H), 7.20–7.06 (m, 8H); 13
C
organic singlet crystal, k and ħ refer to the Boltzmann constant
NMR (100 MHz, CDCl3) d (ppm): 163.27, 159.04, 157.10, 150.71, and Planck constants, respectively. lhole/electron is the hole/
146.93, 129.52, 129.00, 128.10, 125.45, 124.07, 121.77, 116.01; electron reorganization energy due to geometric relaxation
GC-MS: M, found 323. C22H17N3 requires 323.39.
accompanying charge transfer, which was calculated by using
Phenyl-bis-(4-pyrimidin-4-yl-phenyl)-amine (PM4). 15% the adiabatic potential energy surfaces method.64 Experimen-
1
yield; a light yellow solid; H NMR (400 MHz, CDCl3) d (ppm): tally, Vhole/electron showed a rather narrow rang of values,71 and
9.21 (d, J ¼ 1.2 Hz, 2H), 8.71 (d, J ¼ 5.4 Hz, 2H), 8.05–7.95 (m, an even more limited rang of was expected due to the direct
4H), 7.65 (dd, J ¼ 5.4, 1.3 Hz, 2H), 7.38–7.31 (m, 2H), 7.24–7.15 contacts in amorphous solids. Therefore, the hole/electron
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mobility of PM compounds in our work was considered to be 22 Y. Tao, Q. Wang, C. Yang, Z. Zhang, T. Zou, J. Qin and D. Ma,
dominated by lhole/electron without Vhole/electron
.
Angew. Chem., Int. Ed., 2008, 47, 8104–8107.
23 X. Cai, A. B. Padmaperuma, L. S. Sapochak, P. A. Vecchi and
P. E. Burrows, Appl. Phys. Lett., 2008, 92, 83308.
24 S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, K. Y. Ko, J. Y. Park
and Y. G. Baek, Appl. Phys. Lett., 2008, 93, 63306.
Acknowledgements
This work was nancially supported by the National Basic
Research Program of China (973 Program, 2009CB930601), the 25 C. H. Chen, W. S. Huang, M. Y. Lai, W. C. Tsao, J. T. Lin,
National Natural Science Foundation of China (Project no.
50803027, no. 50903001, no. 20905038), and the Program for
Y. H. Wu, T. H. Ke, L. Y. Chen and C. C. Wu, Adv. Funct.
Mater., 2009, 19, 2661–2670.
Postgraduates Research Innovation of Colleges and Universities 26 Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike,
in Jiangsu Province (CXLX11_0421), the Research Fund for
Nanjing University of Posts and Telecommunications
H. Miyamoto, T. Kajita and M. Kakimoto, Chem. Mater.,
2008, 20, 2532–2537.
(NY212074), the Natural Science Foundation from Bureau of 27 S. Y. Takizawa, V. A. Montes and P. Anzenbacher, Jr, Chem.
Education of Anhui Province (KJ2013A108).
Mater., 2009, 21, 2452–2458.
28 Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike,
H. Miyamoto, T. Kajita and M. Kakimoto, Adv. Funct.
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