Synthesis and luminescent properties and theoretical investigation on electronic structure of nitrile-based 2-pyridone molecules

Article abstract of DOI:10.1016/j.saa.2011.05.092

3-Cyano-4,6-dimethyl-2-pyridone and 3-cyano-4-methyl-6-phenyl-2-pyridone were synthesized effectively by the reaction of readily available 1,3-diketone and malononitrile directly and in good yield. Upon photoexcitation, 3-cyano-4-methyl-6-phenyl-2-pyridone in ethanol shows strong blue emission. The ground- and excited-state geometries, charge distributions, and excitation energies of 2-pyridone derivatives were evaluated by ab initio calculations. Organic light-emitting diodes (OLED) made using 3-cyano-4-methyl-6-phenyl-2- pyridone as dopant showed blue light emission with a maximum electroluminescence (EL) emission at around 456 nm.

Full text of DOI:10.1016/j.saa.2011.05.092

Spectrochimica Acta Part A 79 (2011) 1926–1930
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and luminescent properties and theoretical investigation on electronic
structure of nitrile-based 2-pyridone molecules
Liuqing Chena,b, Xuguang Liua,c,∗, Bingshe Xua,b, Chunyan Suna,c, Peng Taoa,b
a Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, Taiyuan 030024, China
b The College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
c The College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
3-Cyano-4,6-dimethyl-2-pyridone and 3-cyano-4-methyl-6-phenyl-2-pyridone were synthesized effec-
tively by the reaction of readily available 1,3-diketone and malononitrile directly and in good yield.
Upon photoexcitation, 3-cyano-4-methyl-6-phenyl-2-pyridone in ethanol shows strong blue emission.
The ground- and excited-state geometries, charge distributions, and excitation energies of 2-pyridone
derivatives were evaluated by ab initio calculations. Organic light-emitting diodes (OLED) made using
3-cyano-4-methyl-6-phenyl-2-pyridone as dopant showed blue light emission with a maximum elec-
troluminescence (EL) emission at around 456 nm.
Received 8 February 2011
Received in revised form 22 May 2011
Accepted 25 May 2011
Keywords:
Pyridine
Synthesis
Electronic structure
OLED
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
2-pyridones involves Guareschi–Thorpe condensation of cyanoace-
toamide and 1,3-diketone compounds with basic catalysts, and
Organic fluorescent heterocyclic chromophores have a wide
range of applications in, for example, traditional textile coloration,
molecular probes [1], organic light-emitting diodes (OLED) [2], pho-
tovoltaic cells [3], and mass coloration of polymers [4]. 2-Pyridone
is such an organic N-heterocyclic compound, its derivatives are
also the key intermediates in the synthesis of the correspond-
ing pyridine, quinoline, quinolizidine and indolizidine alkaloids
[5]. Milrinone and their analogues which have 2-pyridone moi-
ety are used as cardiotonic agents for the treatment of heart
failure [6]. Owing to their luminescent and immense biological
properties, substituted 2-pyridone derivatives are catching much
interest in the development of simple and convenient method-
ologies for the synthesis from easily available starting materials
[7]. Many synthesis methods of 2-pyrone have been reported to
date [8], e.g. oxidation of a N-substituted pyridinium salt [9],
and Knovenagel-type reactions [10] such as cyclocondensation of
ketene dithioacetals with compounds containing active methylene
group like cyanoacetamide. Most of earlier reported methods for
the synthesis of 2-pyridones involving the condensation of 1,3-
binucleophiles with cyanoacetamide need strong bases like sodium
hydride, sodium hydroxide or alkoxides [11]. Mathews et al. used
ammonium acetate/acetic acid medium to synthesize 2-pyridones
[12]. One of the most important methods for the synthesis of
scope of such reactions was well established [13]. Despite the
large number of existing synthesis methods, new procedures for
2-pyridones are continuously being developed [14].
2. Experimental
2.1. Characterization methods
1H NMR (300 MHz) spectra were obtained using a Brucker spec-
trometer. Chemical shift ı was calibrated against TMS as an internal
standard. FT-IR data were collected on a Brucker spectrometer.
Differential scanning calorimetry (DSC) was carried out using a
PerkinElmer thermogravimeter under a dry nitrogen gas flow at a
heating rate of 10 ◦C/min. Crystal data were obtained with graphine
˚
monochromated Cu K␣ radiation (ꢀ = 0.71073 A) on a Smart charge-
coupled device (CCD) X-ray diffractometer. The structure was
solved by direct method with SHELXL 97 program and refined
with full-matrix least-squares methods. The absorption and flu-
orescence spectra were recorded using a Lambda Bio 40 UV–vis
spectrophotometer and an LS-50B fluorescence spectrophotome-
ter, respectively. Elemental analyses were carried out with a
VarioIII analyzer. Oxidation potential (Eoox) and reduction potential
(Eored) were measured directly by cyclic voltammetry (Autolab/PG
STAT302). To understand the ground- and excited-state electronic
properties of chromophore 1 and 2, density functional theory (DFT)
as well as time-dependent DFT (TD-DFT) calculations were per-
formed with the B3LYP functional and 6–31g** basis set, using
∗
Corresponding author. Tel.: +86 351 6014138; fax: +86 351 6010311.
E-mail address: liuxuguang@tyut.edu.cn (X. Liu).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.092

L. Chen et al. / Spectrochimica Acta Part A 79 (2011) 1926–1930
1927
R1
Gaussian(R) 03 program. Ground state and excited state were cal-
culated for HOMO(LUMO) and absorption spectra, respectively.
For compound 1, 30 states have been calculated, while 10 states
for 2. The organic EL devices were fabricated on a glass substrate
coated with ITO (30 ꢁ/square) in a vacuum of 2 × 10−4 Pa, and the
ITO surface was ultrasonicated in a detergent solution followed by
deionzed water rinse and dip into acetone. The evaporating rates
were kept at 0.1–0.2 nm/s for organic layers, 0.1 nm/s for LiF and
1 nm/s for Al in the same vacuum run.
O
HN
O
O
CN
CN
HMDS
CH2
+
R2
C
N
CH3COOH
R1
R2
R1=R2=CH3
R1=CH3, R2=Ph 2
1
2.2. Reagents and solvents
Scheme 1. Reaction of 1,3-diketone with malononitrile.
All reagents and solvents were commercially available and used
without further purification.
that are potential candidate for application in organic elec-
tronics under mild reaction conditions. Nitrile-based 2-pyridone
molecules 3-cyano-4,6-dimethyl-2-pyridine (1) and 3-cyano-4-
methyl-6-phenyl-2-pyridone (2) were synthesized in moderate to
good yields (75–90%) by a one-step condensation of acetylacetone
or benzoylacetone with malononitrile in refluxing acetic acid in the
presence of weak base hexamethyldisilazane (Scheme 1). The IR
absorptions of the C O stretching bands were observed at around
1656 and 1634 cm−1 for 1 and 2, respectively. These red-shifted
values relative to free carbonyl stretching frequency may reflect
the effect of the hydrogen bond. The NMR signal of the NH pro-
tons at a low field of ı = 12.30 and 12.55 ppm, observable for 1 and
2, also indicates the presence of hydrogen bonds. Some structural
characteristics of 1 and 2 were compared in Table 1.
2.3. Preparation of compounds
2.3.1. Preparation of 3-cyano-4,6-dimethyl-2-pyridone (1)
Hexamethyldisilazane 1.84 g (0.012 mol) was added to acetic
acid (14 ml) and acetylacetone 1.20 g (0.012 mol) at an appropri-
ate rate to maintain the internal temperature at 70 ◦C. The HMDS
and acetic acid mixture was added to a solution of acetylacetone
1.20 g (0.012 mol) and malononitrile 3.17 g (0.048 mol) in acetic
acid (14 ml). After 3 h, the mixture was cooled to ambient tem-
perature and washed four times with water. And the residue was
isolated by crystallization in CH3CN. Yield 85%. Compound 1 was
characterized by 1H NMR and 13C NMR, IR spectroscopy and ele-
mental analysis. Assignment of NMR spectra was possible using
1H–13C COSY. 1H NMR (300 MHz, DMSO-d6), ı (ppm): 12.30(s,1H,
3.2. X-ray structures
–NH), 6.15(s, 1H, –CH), 2.50 (s, 3H, 6-CH3), 2.21(d, 3H, 3-CH3). 13
C
NMR (75 MHz, DMSO-d6), ı (ppm): 162.5(C1), 161.9(C3), 152.7(C5),
117.6(C6), 108.9(C4), 100.6(C2), 22.1(C8), 20.4(C7). Anal. calcd. for
C8H8N2O (%): C, 64.85; H, 5.44; N, 18.91. Found: C, 64.86; H, 5.41;
N, 18.91. Infrared spectra of 1: FTIR (KBr, cm−1): 2894, 3017, 2210,
1677, 1472, 1376, 1335, 1212, 1130, 925, 829, 775, 720, 624, 542,
474.
Their structures were also confirmed by single-crystal X-ray
analysis (Fig. 1 for 1 and Fig. 2 for 2). In the crystal structure of
1 and 2, two molecules are arranged in close proximity to each
other, so that intermolecular hydrogen bonds of the N H· · ·O type
are formed between the pyridines, which seem to be typical of 2-
pyridones. A summary of the refinement details and the resulting
factors is given in Table 2. The N H· · ·O hydrogen bond distances
2.3.2. 3-Cyano-4-methyl-6-phenyl-2-pyridone (2)
˚
˚
are 1.886(9) A and 1.851(15) A for 1 and 2, respectively, and the
corresponding N H· · ·O angles are approximately 173.1(12)◦ and
174.8◦, respectively (see Table 1).
Hexamethyldisilazane 0.92 g (0.006 mol) was added to acetic
acid (7 ml) and benzoylacetone 0.97 g (0.006 mol) at an appropri-
ate rate to maintain the internal temperature at 70 ◦C. The HMDS
and acetic acid mixture was added to a solution of benzoylace-
tone 0.97 g (0.006 mol) and malononitrile 1.58 g (0.024 mol) in
acetic acid (7 ml). After 3 h, the mixture was cooled to ambient
temperature and washed four times with water. And the residue
was subjected to crystallization in CH3CN. Yield 75.7%. Compound
2 was characterized by 1H NMR and 13C NMR, IR spectroscopy
and elemental analysis. Assignment of NMR spectra was possi-
ble using 1H–13C COSY. 1H NMR (300 MHz, DMSO-d6), ı (ppm):
12.55(s, 1H, –NH), 7.78–7.54(m, 5H, H–Ph), 6.74(s, CH), 2.42(s,
3H, –CH3). 13C NMR (75 MHz, DMSO-d6), ı (ppm): 162.9(C11),
161.8(C9), 152.0(C7), 133.7(C6), 132.6(C3), 130.5(C1, C5), 129.0 (C2,
C4), 117.5 (C12), 108.6 (C8), 102.3 (C10), 22.4 (C13). Anal. calcd. for
3.3. Spectroscopy
The UV–vis spectrum of the chromophore 1 recorded in ethanol
shows strong absorption bands at around 327 nm and between 200
and 300 nm at the concentration of 1 × 10−5 mol/L. Absorption band
of 1 between 200 and 300 nm is assigned to pyridine ring ␲–␲* elec-
tron transition, while 327 nm is assigned to carbonyl n–␲* electron
transition. With excitation of 291 nm, 1 shows blue fluorescence
with emission maximum at 375 nm and a high quantum yield of
70.9% (using quinine sulphate as standard [15]) while the Stokes
shift amounts to 7698 cm−1 (Stokes shift = 1/ꢀex − 1/ꢀem). Further-
more, the fluorescence–excitation spectrum (measured at 375 nm
emission wavelength) agrees well with the corresponding absorp-
tion profile. On the other hand, chromophore 2 absorbs at 250 nm
and 357 nm at the concentration of 1 × 10−5 mol/L, as shown
C
13H10N2O (%): C, 74.27; H, 4.79; N, 13.33. Found: C, 73.59; H, 4.83;
N, 13.20. Infrared spectra of 2: FTIR (KBr, cm−1): 2969, 2929, 2275,
1634, 1392, 1320, 1215, 992, 940, 888, 711, 574.
3. Results and discussion
Table 1
Some structural characteristics of 1 and 2.
3.1. General considerations
Compound 1
Compound 2
The reaction of malonitrile with 1,3-diketones to afford 2-
pyridones is well known [15], and very mild reaction conditions
are also known. But in past reports, pyridone and methoxypyridine
are formed in the presence of Et3N and MeOH. We present here a
facile way to prepare novel nitrile-based 2-pyridones (Scheme 1)
˚
˚
N
H· · ·O
Bond distance
Bond angles
1.886(9) A
1.851(15) A
173.1(12)◦
1656 cm−1
12.30 ppm
174.8◦
IR(C
1634 cm−1
12.55 ppm
O)
1H NMR(NH)
Boiling point
290 ◦
C
313 ◦
C

1928
L. Chen et al. / Spectrochimica Acta Part A 79 (2011) 1926–1930
˚
Fig. 1. X-ray crystal structure and the overall packing diagram of 1 (ORTEP, thermal ellipsoids set at the 50% probability level). Selected bond lengths [A]: O(1)–C(1) 1.2495(15),
N(1)–C(5) 1.3619(16), N(1)–C(1) 1.3779(15), N(1)–H(1) 0.995(16), N(2)–C(6) 1.1497(15), C(4)–C(5) 1.3691(17), C(2)–C(3) 1.3865(18), C(3)–C(7) 1.5003(16).
˚
Fig. 2. X-ray crystal structure and the overall packing diagram of 2 (ORTEP, thermal ellipsoids set at the 50% probability level). Selected bond lengths [A]: O(1)–C(11)
1.2450(13), N(1)–C(7) 1.3678(15), N(1)–C(11) 1.3806(14), N(1)–H(1A) 0.921(9), C(1)–C(2) 1.3835(16), C(6)–C(7) 1.4800(16), C(7)–C(8) 1.3686(16), C(12)–N(2) 1.1492(14).
Fig. 3. Electronic absorption together with the calculated oscillator strength (a) and fluorescence (b) spectra of 1 (ꢀex = 291 nm, ꢀem = 378 nm); electronic absorption together
with the calculated oscillator strength (c) and fluorescence (d) spectra of 2 (ꢀex = 391 nm, ꢀem = 430 nm) in EtOH solution.

L. Chen et al. / Spectrochimica Acta Part A 79 (2011) 1926–1930
1929
Fig. 4. Molecular orbitals of 1 and 2 that are involved in the S0–S1 and S0–S2 transitions.
Table 2
(4.06 eV), as shown with the stick spectrum in Fig. 3(a). Both
the transition energy and intensity are in very good agreement
with the longest wavelength absorption (327 nm), for which the
281–366 nm energy-integrated absorption also yields an oscillator
strength of f = 0.19. The S0–S2 excitation is predicted to be a ␲–␲*
transition at 223 nm, with an oscillator strength of f = 0.023. For 2,
the S0–S1 excitation to be an intense n–␲* transition (f = 0.36) at
341 nm (3.65 eV), as shown with the stick spectrum in Fig. 3(c).
Both the transition energy and intensity are in very good agree-
ment with the longest wavelength absorption (357 nm), for which
the 284–414 nm energy-integrated absorption yields an oscillator
strength of f = 0.48. The S0–S2 excitation is predicted to be a ␲–␲*
transition at 247 nm, with an oscillator strength of f = 0.20. This is in
excellent agreement with the second UV absorption band observed
at 250 nm with intensity similar to the S0–S1 band.
According to the TD-DFT calculation, the S0–S1 electronic exci-
tation of 1 is dominated by the HOMO–LUMO contribution (80%). As
Fig. 4 shows, this corresponds partially to a n-electron flow from the
carbonyl O atom toward the pyridine ring, where electron density
spreads toward the methyl group. The S0–S2 transition is mainly
dominated by the one-electron excitation from HOMO-2 to the
LUMO (52%). On the other hand, the S0–S1 electronic excitation of
2 is dominated by the HOMO–LUMO contribution (82%). As Fig. 4
shows, this corresponds partially to a n-electron flow from the car-
bonyl O atom toward the benzene and pyridine ring, where electron
density spreads toward the methyl group. The S0–S2 transition is
mainly dominated by the one-electron excitation from HOMO-3 to
the LUMO (72%). This transition moves ␲-electron density from the
Crystallographic data of 1 and 2.
1
2
Empirical formula
fw
Cryst syst
C8H8N2O
148.16
Triclinic
P-1
3.9049(8)
7.3288(15)
12.694(3)
77.09(3), 89.16(3),
89.40(3)
354.06(12)
2
156
1.390
0.71073
143(2)
1.65–25.01
R1 = 0.0324,
wR2 = 0.0903
R1 = 0.0413,
wR2 = 0.0942
1.003
C13H10N2O
210.23
Monoclinic
P2[1]/n
12.8513(3)
7.0586(14)
13.042(6)
˛ = ꢂ = 90.00,
120.31(2)
1021.3(6)
4
440
1.367
0.71073
113(2)
3.41–25.02
R1 = 0.0351,
wR2 = 0.0960
R1 = 0.0410,
wR2 = 0.0998
1.062
Space group
˚
a, A
˚
b, A
˚
c, A
˛, ꢂ, ˇ, (◦)
3
˚
V, A
Z
F (0 0 0)
D (calcd.), mg cm−3
˚
ꢀ, A
Temperature, K
2ꢃ range (◦)
Final R indices
[I > 2ꢄ(I)]
R indices (all data)
Goodness-of-fit on F2
in Fig. 3. Upon UV excitation at 391 nm, chromophore 2 shows
an intensive blue fluorescence with emission peak maximum at
430 nm and a high quantum yield of 88.3% (using quinine sul-
phate as standard). Compared with chromophore 1, chromophore
2 shows a shifted emission of about 52 nm to longer wavelengths.
Optical properties of 1 and 2 were list in Table 3. Such a behavior
indicates that the incorporation of benzene segment to pyri-
done frame changed the optical properties of chromophore 2 and
increased the effective conjugation length.
C
O bonding ␲ orbital into the benzene ring and carbonitrile.
3.5. Form-filming ability
Forming homogeneous and stable amorphous film by thermal
evaporation is a basic requirement for the application in OLEDs as
electronic material. Fig. 5 shows two dimensional AFM micrographs
3.4. Computational studies
For 1, the vertical TD-DFT calculation predicts the S0–S1
excitation to be carbonyl n–␲* transition (f = 0.19) at 306 nm
˚
taken for 2 films (about 500 A thickness) deposited on quartz sub-
strates by thermal evaporation in vacuum chamber at 5 × 10−4 Pa.
The root-mean-square roughness of 2 obtained by conducting sec-
tion analyses was about 1.877 nm. Such quantitative values indicate
that 2 formed a more uniform and flatter molecular film, which is
required for OLED materials.
Table 3
Optical properties of 1 and 2.
Materials
Units
1
2
Abs (EtOH)a
Em (EtOH)b
Em (SF)c
ꢀmax (nm)
223
375
395
357
430
547
Optical
properties
3.6. Energy gap
d
ϕf
%
70.9
88.3
a
Absorption in EtOH.
Emission in EtOH.
Emission in solid film.
The emission color of the organic materials is a function of
HOMO–LUMO energy gap (Eg) of luminescent molecule. The fea-
tures of the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) of 2 are presented
b
c
d
Fluorescence quantum efficiency, relative to quinine sulphate in H2SO4
(ϕf = 55%).

1930
L. Chen et al. / Spectrochimica Acta Part A 79 (2011) 1926–1930
CBP: 2 (6%) as an emitting layer, ITO as anode, and LiF/Al as cathode
to form a structure of ITO/NPB(50 nm)/CBP: 2 (6%) (20 nm)/Bphen
(20 nm)/LiF (1 nm)/Al (100 nm). PL of powder and EL spectra of
device of 2 were measured. PL and EL spectra can be seen in Fig. 6.
PL emission maximum is located at 546 nm and maximum value
of EL emission is 456 nm, EL spectrum has a very big blue shift of
90 nm. The CIE coordinates for the device based on 2 are X = 0.2071,
Y = 0.2061.
4. Conclusion
In summary, a convenient and brief synthesis route for poly-
substituted pyridine, highly functionalized with 4,6-disubstituted
group flanked by one carbonitrile substituent has been proposed.
Remarkably, this efficient one-pot procedure provides a new and
straightforward entry to the regioselective formation of poly-
substituted pyridine. The formation of 1 and 2 is explained by
the Knoevenagel condensation reaction of ketone followed by
cyclization reaction. Moreover, it is noteworthy that 1 and 2
have promising topology and functionality as useful substrates for
conjugate additions, thereby further generating molecular diver-
sity and functionality. Apart from this, the device designed as
ITO/NPB(50 nm)/CBP: 2 (6%) (20 nm)/Bphen (20 nm)/LiF (1 nm)/Al
(100 nm) emits blue light emission with a maximum EL emission
at around 456 nm.
Fig. 5. Two-dimensional AFM images for the 4.0 ␮m × 4.0 ␮m 2 thin films.
1.0
0.8
0.6
0.4
0.2
0.0
EL
PL
Acknowledgements
The authors acknowledge financial support from Program for
Changjiang Scholar and Innovative Research Team in Univer-
sity (IRT0972), National Natural Science Foundation of China
(20971094, 20671068, 60976018) and Research Fund for Returned
Scholars (2008-31), Natural Science Foundation of Shanxi Province
(2010021023-2, 2008011008). The authors thank professor Hao
Yuying, College of science of Taiyuan University of Technology, for
GAUSSIAN 03 calculations.
300
400
500
600
700
800
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in Fig. 4. The HOMO density is mostly located on the pyridine
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.
EHOMO (or ELUMO) = −4.74 − eEoox (or Eored
)
(1)
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In this study, an EL device was fabricated using NPB as hole-
transporting layers, and BPhen as electron-transporting layers,