Novel multifunctional organic semiconductor materials based on 4,8-substituted 1,5-naphthyridine: Synthesis, single crystal structures, opto-electrical properties and quantum chemistry calculation

Article abstract of DOI:10.1039/c2ob25926e

A series of 4,8-substituted 1,5-naphthyridines (1a-1h) have been successfully synthesised by a Suzuki cross-coupling between 4,8-dibromo-1,5- naphthyridine (4) and the corresponding boronic acids (2a-2h) in the presence of catalytic palladium acetate in yields of 41.4-75.8% and have ben well characterized. They are thermally robust with high phase transition temperatures (above 186 °C). Compounds 1b, 1e and 1f crystallized in the monoclinic crystal system with the space groups P2₁/c, P2₁/c and P2₁/n, respectively. All of them show the lowest energy absorption bands (λ_max^Abs: 294-320 nm), revealing low optical band gaps (2.77-3.79 eV). These materials emit blue fluorescence with λ_max^Em ranging from 434-521 nm in dilute solution in dichloromethane and 400-501 nm in the solid state. 4,8-Substituted 1,5-naphthyridines 1a-1h have estimated electron affinities (EA) of (2.38-2.72 eV) suitable for electron-transport materials and ionization potentials (IP) of 4.85-5.04 eV facilitate excellent hole-injecting/hole-transport materials properties. Quantum chemical calculations using DFT B3LYP/6-31G* showed nearly identical the lowest unoccupied molecular orbitals (LUMO) of -2.39 to -2.19 eV and the highest occupied molecular orbitals (HOMO) of -5.33 to -6.84 eV. These results demonstrate the 4,8-substituted 1,5-naphthyridines 1a-1h with a simple architecture might be promising blue-emitting (or blue-green-emitting) materials, electron-transport materials and hole-injecting/hole-transport materials for applications for developing high-efficiency OLEDs.

Full text of DOI:10.1039/c2ob25926e

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PAPER
Novel multifunctional organic semiconductor materials based on
4,8-substituted 1,5-naphthyridine: synthesis, single crystal structures,
opto-electrical properties and quantum chemistry calculation†
Kun-Yan Wang,a Chen Chen,a Jin-Fang Liu,a Qin Wang,a Jin Chang,b Hong-Jun Zhu*a and Chong Lic
Received 14th May 2012, Accepted 20th June 2012
DOI: 10.1039/c2ob25926e
A series of 4,8-substituted 1,5-naphthyridines (1a–1h) have been successfully synthesised by a Suzuki
cross-coupling between 4,8-dibromo-1,5-naphthyridine (4) and the corresponding boronic acids (2a–2h)
in the presence of catalytic palladium acetate in yields of 41.4–75.8% and have ben well characterized.
They are thermally robust with high phase transition temperatures (above 186 °C). Compounds 1b, 1e and
1f crystallized in the monoclinic crystal system with the space groups P21/c, P21/c and P21/n,
respectively. All of them show the lowest energy absorption bands (λmaxAbs: 294–320 nm), revealing low
optical band gaps (2.77–3.79 eV). These materials emit blue fluorescence with λmaxEm ranging from
434–521 nm in dilute solution in dichloromethane and 400–501 nm in the solid state. 4,8-Substituted
1,5-naphthyridines 1a–1h have estimated electron affinities (EA) of (2.38–2.72 eV) suitable for
electron-transport materials and ionization potentials (IP) of 4.85–5.04 eV facilitate excellent
hole-injecting/hole-transport materials properties. Quantum chemical calculations using DFT B3LYP/
6-31G* showed nearly identical the lowest unoccupied molecular orbitals (LUMO) of −2.39 to −2.19 eV
and the highest occupied molecular orbitals (HOMO) of −5.33 to −6.84 eV. These results demonstrate the
4,8-substituted 1,5-naphthyridines 1a–1h with a simple architecture might be promising blue-emitting
(or blue-green-emitting) materials, electron-transport materials and hole-injecting/hole-transport materials
for applications for developing high-efficiency OLEDs.
potential advantages not only in lower synthesis costs, but most
importantly, organic semiconductor materials have been shown
Introduction
Organic light-emitting diodes (OLEDs) have attracted consider-
able attention in recent decades because of their promising appli-
cations for the next generation of flat panel displays1–3 and
general lighting4–6 with the advantages of low-driving-voltage,
high-efficiency, and the possibility of flexible and large-area
display applications.7–10 Organic small molecules11–13 have
to be amenable to large scale-up, high efficiencies, and simple
and inexpensive fabrication routes to realise these devices.14–18
Maintaining a balance between electron and hole currents plays
an important role in achieving high device efficiency in the
field.19 Therefore, the vast majority of synthetic effort and struc-
ture–property studies have to date been devoted to lots of
organic functional materials for OLEDs.20–23 Many nitrogen
heterocyclic fused ring compounds like conjugated quinoline
derivatives,22,24–27 quinoxaline derivatives28–33 and anthrazoline
derivatives34–37 have been investigated as building blocks for
increasing the electron affinity (EA) of current electron-transport
materials for OLEDs. In particular, polyquinolines and oligo-
quinolines exhibit outstanding electron-transport properties as a
result of their high EA values.38–40 For example, 1,5-naphthyri-
dine is electron deficient relative to quinoline, and should have
higher EA values than quinoline.41 Until recently, only a limited
number of 1,5-naphthyridine derivatives have been explored.
4-Hydroxy-1,5-naphthyridine derivatives were synthesized as
chelating ligands and their metal chelates could be applied as a
highly efficient non-dopant deep blue emitter, good electron-
transport material and host material for yellow dopant.42
aDepartment of Applied Chemistry, College of Science, Nanjing
University of Technology, Jiangsu Key Laboratory of Industrial
water-Conservation & Emission Reduction, Nanjing 210009, China.
E-mail: zhuhjnjut@hotmail.com; Fax: +86 25 83587443;
Tel: +86 25 83172358
bSchool of Chemistry, Physics and Mechanical Engineering, Science
and Engineering Science and Engineering Faculty, Queensland
University of Technology, Brisbane 4000, Australia
cKey Laboratory for Organic Electronics & Information Displays and
Institute of Advanced Material, Nanjing University of Posts and
Telecommunications, Wenyuan Road 9, Nanjing 210046, China
†Electronic supplementary information (ESI) available: Experimental
details and characterization data of new compounds 4, 1a–1h; and mass
spectra of 4, 1a–1h and NMR spectra of 4, 1a–1h, calculation results of
1a–1h, single crystal structures for 1b, 1e and 1f. CCDC 866640,
881204 and 881205. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2ob25926e
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Chart 1 Structures of eight 4,8-substituted 1,5-naphthyridines 1a–1h.
4-Diphenylphosphino-1,5-naphthyridine derivatives were also
prepared as ligands for their copper(I) complexes.43 Ru(II) com-
plexes with 2,6-substituted and 4,8-substituted 1,5-naphthyri-
dines as bridging ligands were investigated for their
photophysical and electrochemical properties.44 However,
reports concerning the electrochemical and photophysical prop-
erties of organic small molecule materials based on 4,8-substi-
tuted 1,5-naphthyridines are limited.
(HOMO) close to the work function of indium tin oxide (ITO),
which could make them possible candidates for a class of
blue-emitting (or blue-green-emitting) materials, electron-trans-
port materials and hole-injecting/hole-transport materials in
OLEDs.
Results and discussion
Devices that have fewer layers and do not require doping are
much more desirable because they simplify the fabrication
process.45–49 In this work, moieties with hole-injection/
hole-transport properties, i.e., triphenylamine, fluorene and
carbazole were introduced into a 1,5-naphthyridine core. We aim
to develop as novel hybrid materials 1,5-naphthyridines with
simple architectures and to study further the underlying
structure–property relationships.
In this paper, we report the synthesis, detailed structural
characterization, and investigation of the electrochemical and
photophysical properties of a series of eight new multifunctional
materials (1a–1h) based on a 1,5-naphthyridine core structure
with various 4,8-bisaryl groups: 4,8-diphenyl-1,5-naphthyridine
(1a), 4,8-bis(4-(tert-butyl)phenyl)-1,5-naphthyridine(1b), 4,8-bis-
(4-(trifluoromethyl)phenyl)-1,5-naphthyridine (1c), 4,8-bis-
(4-methoxyphenyl)-1,5-naphthyridine (1d), 4,8-bis(4-phenoxy-
phenyl)-1,5-naphthyridine(1e), 4,8-bis(9,9-di-n-butyl-9H-fluoren-2-
yl)-1,5- naphthyridine(1f), 4,8-bis(4-(9H-carbazol-9-yl)phenyl)-
1,5-naphthyridine (1g), 4,8-bis-(4-(diphenylamino)phenyl)-1,5-
naphthyridine (1h) (Chart 1). These materials show superior
properties including high relatively fluorescence quantum
efficiency for 1g and 1h, excellent thermal stabilities, suitable
lowest unoccupied molecular orbital (LUMO) similar to oligo-
quinolines and proximate highest occupied molecular orbital
2.1. Synthesis and characterization
Scheme 1 outlines the synthesis of the series of blue fluorescent
materials 4,8-substituted 1,5-naphthyridines. The detailed pro-
cedures for the synthesis of the reaction intermediates and
final products are described in the ESI.† The synthesis pro-
cedures of molecules 1a–1h contain three parts: synthesis of the
arms (aryl boronic acids 2a–2h) (Scheme 2), synthesis of the
key intermediate 4,8-dibromo-1,5-naphthyridine 4 (Scheme 3)
and Pd-catalyzed Suzuki coupling reactions of compound 4
with 2a–2h. Suzuki coupling reactions in the synthetic route to
1a–1h were catalyzed by Pd(OAc)2 in mixed solvent (N,N-
dimethylformamide, DMF–H2O), giving the desired products
1a–1h, respectively, in 41.4–75.8% yields. Compounds 1a and
1c were obtained as white powders, and 1b was obtained as
small colorless crystals, 1d, 1e and 1f as light yellow needles, 1g
as a light yellow powder and 1h as a bright yellow powder.
All these newly synthesized 4,8-substituted 1,5-naphthyridines
(1a–1h) were purified by recrystallization with methanol and
THF twice and fully characterized by H and 13C NMR spec-
1
troscopy, IR spectroscopy and mass spectrometry (MS) (shown
in Fig. S21–S44 in ESI†). The purity of 1,5-naphthyridines
1a–1h was also confirmed by elemental analysis with satisfac-
tory results. All these spectra confirmed the proposed structures
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Scheme 1 Synthesis of 4,8-substituted 1,5-naphthyridines 1a–1h by a Suzuki coupling reaction. Reagents and conditions: (i) Pd(AcO)2, K2CO3,
DMF + H2O, 80 °C.
2.2. Thermal properties
of 1,5-naphthyridines 1a–1h. The solubility of the products is
good in many common solvents, e.g. THF, toluene, chloroform,
and dichloromethane (DCM), therefore they offer the possibility
of solution processing, such as ink jet printing and roll-to-roll
processing, a prerequisite for large area applications.50
Briefly, compound 4 (shown in Fig. S18–S20 in ESI†) was
prepared as a key intermediate in a yield of 79.3% by
bromination with POBr3 from 1,5-naphthyridine-4,8(1H,5H)-
dione (3), whose synthetic route was described in Scheme 3
according to Brown and Dewar.51 Besides, the aryl boronic acid
arms 2a–2h were also very important for the synthesis of
materials 1a–1h, which were prepared via Grignard reaction52 of
3a–3e and by lithiation53 of 3f–3h in good yield of 59.5%–
93.3%, followed by treatment with trimethyl borate and hydroly-
sis catalyzed by acid. The synthetic routes to intermediates
3e–3h are shown in Scheme 4 according to the corresponding
literature.54–57
The thermal properties of 4,8-substituted 1,5-naphthyridines
1a–1h were investigated by differential scanning calorimetry
(DSC) analyses, shown in Table 2 (see below). DSC was used to
investigate phase transitions (1a–1h are shown in Fig. S1 in
ESI†). No glass transitions or crystallization events were
observed by DSC scans of any of the molecules except for 1b
(glass transition temperature, Tg = 247 °C) and 1g (Tg
=
221 °C). These eight 1,5-naphthyridine-based materials are
thermally stable and lack any detectable phase transitions at low
temperatures, indicating they are suitable for OLED fabrication.
All the materials had melting transitions ranging from 186 °C for
1f to 365 °C for 1g. As might be expected, the compound 1f
containing hexyl group has the lowest melting temperature of
186 °C, and 1g has the highest melting temperature of 367 °C
because of the great planar rigidity of the 4-(9H-carbazol-9-yl)-
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Scheme 2 Synthesis route to aryl boronic acid arms (2a–2h). Reagents and conditions: (i) (1)Mg, THF, (2)B(OCH3)3, −30 °C, (3)H3O+; (ii) (1)n-BuLi,
THF, −78 °C, (2)B(OCH3)3, −78 °C, (3)H3O+.
Scheme 3 Synthesis route to the key intermediate 4,8-dibromo-1,5-naphthyridine (4). Reagents and conditions: (i) concentrated sulfuric acid, KNO3,
0 °C; (ii) NaOCH3, CH3OH, 0 °C; (iii) Fe, HCl, CH3OH–H2O; (iv) toluene, reflux; (v) diphenyl ether, reflux, 25 min; (vi) concentrated hydrobromic
acid, reflux; (vii) quinoline, reflux;51 (viii) POBr3, CH3CN.
2.3. X-ray crystal structures of compounds 1b, 1e and 1f
phenyl group. These data reveal that materials 1a–1h possess
excellent thermal stability. High thermal stability is advantageous
for OLED applications, as it enhances device stability and
lifetime.
Crystals were grown by slow evaporation of dichloromethane–
hexane, THF–hexane, and THF–methanol solutions for
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Scheme 4 Synthesis route to other intermediates. Reagents and conditions: (i) NBS, NH4NO3, CH3CN, 25 °C, 30 min;57 (ii) NBS, propylene car-
bonate, 60 °C; (iii) n-butyl bromide, KOH, KI, DMSO, 60 °C;75 (iv) CuI, DMF, K2CO3, reflux;55 (v) NBS, DMF, 25 °C.54
compounds 1b, 1e and 1f, respectively. The diffraction data were
collected on a Nonius CAD4 single crystal diffractometer
equipped with a graphite-monochromated MoKa radiation (λ =
0.71073 Å) by using an ω/2θ scan mode at 293 K. The crystal
structures were solved by direct methods and refined by the full-
matrix least squares procedure on F2 using SHELXL-97.58 All
nonhydrogen atoms were refined anisotropically, and the hydro-
gen atoms were introduced at calculated positions. Single crys-
tals from three of the eight 1,5-naphthyridines (1b, 1e and 1f)
were suitable for the determination of X-ray crystal structures.
1b was obtained as thin colorless prisms, and 1e was obtained as
yellow blocks. Colorless needles were obtained for 1f resulting
in excellent quality data. 1b and 1e were found to have primitive
monoclinic crystal systems. The unit cell parameters of a =
11.973(2) Å, b = 7.8020(16) Å, c = 12.534(3) Å and β =
102.95(3)° for 1b, a = 9.869(2) Å, b = 14.785(3) Å, c =
16.173(3) Å and β = 95.51(3)° for 1e, and a = 8.9420(18) Å,
b = 10.249(2) (16) Å, c = 21.626(4) Å and β = 96.87(3)° for 1f
are reported for the molecules. The space groups were P21/c,
P21/c and P21/n for 1b, 1e, and 1f, respectively. Structures were
refined to final residuals of R1 = 7.59% for 1b, R1 = 7.56% for
1e, and R1 = 7.66% for 1f. The detailed crystallographic data for
1b, 1e and 1f are collected in Table 1.
The single crystal structures and some packing diagrams of
1b, 1e and 1f are shown in Fig. 1–3 (other packing diagrams of
1b, 1e and 1f are shown in Fig. S45–S47 in ESI†). The core 1,5-
naphthyridine is almost coplanar for these three compounds. The
4-(tert-butyl)phenyl groups are twisted 46.7° from the plane of
the 1,5-naphthyridine unit in 1b. The crystal structure of 1e is a
double molecular structure. Its structural unit consists of two
similar 1e molecules. The two α-linked phenyl groups are
twisted 40.3° and 42.4° from the plane of the 1,5-naphthyridine
unit in 1e. The dihedral angles of the two phenyl moieties in the
4-phenoxyphenyl groups are 69.6° and 70.9° for 1e. The
α-linked phenyl group in 1f is twisted 40.7° from the core plane.
This suggests that the planarity of the molecules could increase
with longer 4,8-substituents and also demonstrates that a tert-
butyl substituent might be beneficial to achieve more bulky
molecules and alter the conjugation by means of its steric hin-
drance. Crystal 1b exhibits edge-to-face packing of the substi-
tuted phenyl rings to neighboring planar core units (1,5-
naphthyridines and the bis(α-linked phenyl) groups). In the mol-
ecular packing of 1e, offset face-to-face π-stacking interactions
between neighboring molecular 1,5-naphthyridine units are
observed and the intermolecular distance is 3.94 Å. Additionally
1e has edge-to-face packing of α-linked benzene rings in the
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4-phenoxyphenyls of neighboring molecules with distances of
2.92–2.97 Å. The other benzene rings of the 4-phenoxyphenyls
in neighboring molecules also exhibit edge-to-face packing with
distances of 2.94–3.02 Å. However, no π-stacking interaction is
observed in crystal 1f because the n-butyl groups could expand
the distance of neighboring molecules. This indicates that 1f has
very weak intermolecular interactions. As a result, the maximum
emissions of 1f are the same at 435 nm both in dichloromethane
and in the solid state.
substituted 1e show absorption maxima (λmaxAbs) ranging from
294 to 320 nm, which corresponds to n–π* absorption from their
parent 1,5-naphthyridine according to Singh.44 As expected, the
λmaxAbs of tert-butyl substituted 1b, methoxyl substituted 1d and
phenoxyl substituted 1e are red-shifted by 6 nm, 18 nm and
15 nm compared with unsubstituted 1a owing to the electron-
donating tert-butyl, methoxyl and phenoxyl moiety respectively.
Abs
The absorption λmax
of trifluoromethyl substituted 1c is blue
shifted by 8 nm compared with 1a as a result of the electron
attracting effect of the trifluoromethyl group. Fluorene contain-
ing 1f, 9H-carbazol-9-yl substituted 1g and diphenylamino sub-
2.4. Optical properties
Abs
stituted 1h have absorption λmax
that are red-shifted 41 nm,
40 nm and 97 nm respectively compared with 1a as these substi-
tuents could extend the conjugated backbone and increase elec-
tron density. The diphenylamine derivative (1h) possesses the
longest wavelength absorption while the carbazole derivative
(1g) shows a shorter wavelength absorption compared with 1h.
The absorption spectra of all eight 1,5-naphthyridines in dilute
dichloromethane solution (5 × 10−5 M) are shown in Fig. 4.
Photoluminescence spectra both in dilute dichloromethane
solution (10−6 M) (a) and in the solid state (b) are shown in
Fig. 5. Table 2 lists their optical properties.
The solution absorption spectra of unsubstituted 1a, alkyl sub-
stituted 1b and 1c, alkoxyl substituted 1d and phenoxyl
Table 1 Crystallographic data for 4,8-substituted 1,5-naphthyridines
1b, 1e and 1f
1b
1e
1f
Formula
C28H30N2
394.55
Monoclinic
Colorless
P21/c
11.973(2)
7.8020(16)
12.534(3)
102.95(3)
1141.1(4)
293(2)
C32H22N2O2
466.52
Monoclinic
Orange
P21/c
9.869(2)
14.785(3)
16.173(3)
95.51(3)
2348.9(8)
293(2)
4
C50H54N2
682.98
Monoclinic
Colorless
P21/n
8.9420(18)
10.249(2)
21.626(4)
96.87(3)
1967.7(7)
293(2)
4
Formula weight
Crystal system
Color of crystal
Space group
a [Å]
b [Å]
c [Å]
β (°)
Volume [Å3]
Temperature [K]
Z
4
F(000)
424
1.148
0.067
0.0861
2101
976
1.319
0.083
0.0424
4310
736
1.153
0.066
0.0396
3607
Dcalcd. (g cm−3
)
μ (Mo-Kα)(mm−1
Rint
No of reflection
No of parameters
R1, R1
)
124
325
235
0.0759, 0.1810 0.0756, 0.1729 0.0766, 0.1535
0.1069, 0.1340 0.1411, 0.1725 0.1626, 0.1974
1.002
wR2, wR2
GOF
1.004
1.005
Fig. 1 Single crystal structure of 1b (a), view down the b axis (b).
Table 2 Photophysical properties of 4,8-substituted 1,5-naphthyridines 1a–1h
Photophysical
ε × 104 (M−1 cm−1
)
λabsa (nm)
λedge (nm)
Egopt b (eV)
Egcal c (eV)
λem d,e (nm)
Φu
f
Compd.
Thermal Tg/Tm (°C)
1a
1b
1c
1d
1e
1f
—/270
247/300
—/268
—/253
—/210
—/186
221/365
—/280
2.37
2.23
2.62
2.39
302
308
294
320
333
347
327
378
367
391
397
448
3.72
3.57
3.79
3.28
3.38
3.17
3.12
2.77
4.22
4.08
4.32
3.89
3.91
3.63
3.36
3.09
448 (403)
441 (401)
443 (400)
434 (423)
439 (421)
435 (435)
479 (445)
521 (501)
0.06
0.03
<0.005
0.09
0.19
0.14
0.61
0.80
1.84
317
2.84, 2.75
5.27, 2.97
3.71, 2.43
275, 343
293, 342
302, 399
1g
1h
a Recorded in 5 × 10−5 M solutions of dichloromethane at r.t. b Optical band gaps determined from the absorption edge of the normalized absorption
spectra. c Optimized using density functional theory (DFT) methods at the B3LYP/6-31G* level. d Recorded in 10−6 M solutions of dichloromethane
at rt. λexcitation = λabs of the lower energy band (λexcitationsolid = 300 nm for 1a to 1g, 400 nm for 1h). e Values in parentheses correspond to fluorescence
in solid state. f Quantum yield (Φu) measured in dilute dichloromethane relative to that of 9,10-diphenyl anthracene (0.9 in cyclohexane). Excitation
wavelength = 320 nm for 1a to 1f, 365 nm for 1g and 1h.
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fluorene moiety, 9-phenyl-9H-carbazole moiety and triphenyl-
amine moiety by virtue of the large molar extinction coefficients
(ε > 28 400 M−1 cm−1), shown in Table 2. The lower energy
bands in the region of 342 nm to 399 nm probably originate
from the charge transfer transition between the electron rich sub-
stituent groups and electron accepting 1,5-naphthyridine
core.60–62 The mediocre intensity observed for the charge trans-
fer transition indicates a poor orbital overlap. Such weak charge
transfer transitions are also reported for indoloquinoxaline
derivative hybrid materials.60
The dilute solution photoluminescence (PL) spectra (Fig. 5a)
of 1a–1h show that compounds 1a–1g in dilute solution in
dichloromethane emit blue light with the emission maximum
ranging from 434 nm for 1d to 479 nm for 1g while compound
1h emits blue-green light at 521 nm because of its long conju-
gation length. The blue shift for 9H-carbazol-9-yl substituted 1g
compared with diphenylamine derivative 1h is also observed in
aryl-oxadiazole–fluorene triad molecules.63
The PL quantum yield (Φu) of compounds 1a–1h in dilute
solution in dichloromethane ranged from 0.03 for 1b to 0.80 for
1h (Table 2) whereas that of 1c is significantly lower (<0.005).
The PL quantum yields measured in dilute dichloromethane
solution using 9,10-diphenylanthracene (Φu = 0.9) as standard
increase with the increase of substituted aromatic ring electron
density. The lower emission quantum yield in 1c is due in large
part to the donor–acceptor–donor nature of this 1,5-naphthyri-
dine and weak intramolecular charge transfer for the substituted
4-(trifluoromethyl)phenyl group.64 On the contrary, carbazole
derivative 1g and diphenylamine derivative 1h had dramatically
high quantum yields, presumably due to significant intramolecu-
lar charge-transfer (ICT) attributed to the strongly electron-
donating carbazole and diphenylamine respectively.65
Compounds 1a–1g emit in the blue region (Fig. 5b) with the
emission peaks in the range of 400–445 nm in the solid state
while 1h emits in the blue-green region with an emission peak at
501 nm. The main absorption bands observed in solution are
also seen in the solid state. The solid state absorption bands are,
however, blue shifted in 1,5-naphthyridine compounds except
for 1f compared with their corresponding ones in solution. 1f
has the same maximum emission wavelength both in solution
and in solid state owing to the space effect of long n-butyl in
fluorene moiety. The order of λEm of the eight 1,5-naphthyridines
in the solid state agrees with their λAbs in solution.
Fig. 2 Single crystal structure of 1e (a), view down the a axis (b).
The extended conjugation present in the above derivatives may
lead to a red-shifted absorption. On the contrary, the blue-shifted
profile observed for 1g may be attributed to the retardation of
electronic delocalization caused by the twist between the carba-
zole and α-linked phenyl group (vide supra).59 Optical band
gaps (Egopt) determined from the absorption edge are given in
Table 2 and vary from 2.77 eV for 1h to 3.79 eV for 1c, slightly
bigger than for 4,8-diphenyl-1,5-anthrazolines.34,37 Compound
1h has the lowest bandgap of 2.77 eV, which is attributed to
better π-conjugation of the 4-(diphenylamino)phenyl substituent
over other substituted derivatives.
Each absorption spectrum of compound 1f, 1g and 1h shows
two main absorption peaks, in which the higher energy bands
are at 275 nm, 293 nm and 302 nm, respectively and the lower
energy bands at 343 nm, 342 nm and 399 nm. The higher
energy bands may be from the π–π* or n–π* transitions of the
The absorption and fluorescence emission spectra of 1g in
various solvents are shown in Fig. 6 while the absorption and
fluorescence emission spectra of the other compounds are shown
in the ESI (Fig. S2–S17†). The absorption and fluorescence
emission spectral data are listed in Table 3. All of the
1,5-naphthyridines studied in this work exhibited a slight shift in
the absorption peak when recorded in different polar solvents
such as toluene, dichloromethane, THF, acetonitrile and
dimethylformamide (shown in Fig. S2–S9†). This reveals that
the absorption maxima of 1,5-naphthyridines are almost inde-
pendent of solvents. However, in solution, compounds 1g and
1h show a strongly solvent-dependent emission band while the
shifts of the other compounds’ emission maxima with solvents
are not obvious. The fluorescence emission spectra of the two
compounds shift toward longer wavelength with increasing
polarity of the solvents. This might be due to the difference in
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Fig. 3 Single crystal structure of 1f (a), (b), view down the a axis (c).
2.5 Electrochemical properties
The electrochemical behavior of the eight materials 1a–1h in
dichloromethane–acetonitrile (7 : 3 v/v) solution has been
examined by the reduction cyclic voltammograms (CVs) using a
standard one-compartment three-electrode cell equipped with a
platinum disk as working electrode, a Ag/Ag+ electrode (Ag in
0.1 M AgNO3 solution, from CHI, Inc.) as the reference elec-
trode, a Pt wire as the auxiliary electrode, and tetrabutylammo-
nium perchlorate, TBAP (0.1 M), dissolved in dichloromethane–
acetonitrile (7 : 3 v/v) as an electrolyte solution.
The cyclic voltammograms (CVs) of materials 1a and 1h
using ferrocene as a standard are shown in Table 4 while the
CVs of the other compounds are shown in the ESI (Table S1†).
All eight compounds in solution clearly show oxidation peaks
(0.74 V to 0.89 V vs. SCE) and reduction peaks (−2.23 V to
−2.01 V vs. SCE). These suggest both hole and electron trans-
port leading to a single-layer electroluminescent device.
Fig. 4 UV-vis absorption spectra of 1a–1h in DCM solution (5 ×
10−5 mol L−1).
Table 5 shows the solution electrochemical data for 1a–1h.
Reduction potentials vary from −2.01 V for 1c to −2.23 V for 1f
(vs. SCE) and the onset reduction potentials of 1a–1h are in the
range of −1.68 to −2.02 V (vs. SCE). Electron affinities (EA)
the dipole moments of the molecules in the ground state and the
excited states and their stabilization by solvent molecules invol-
ving various specific interactions such as dipole–dipole, hydro-
gen bonding, and solvation.59 Compounds 1g and 1h are
confirmed to be bipolar molecules and therefore could have
potential applications as multifunctional organic semiconductor
materials.
were estimated from the onset reduction potentials in the solution
onset
CVs (EA = Ered
+ 4.4 eV) under the premise that the energy
level of SCE is 4.4 eV below the vacuum level.37 These com-
pounds have EA values of 2.38–2.72 eV (below vacuum), in
6700 | Org. Biomol. Chem., 2012, 10, 6693–6704
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Fig. 5 PL spectra of 1a–1h in DCM solution (∼10−6 mol L−1) (a) and
in the solid state (b).
which material 1g has the highest EA values of 2.72 eV. The EA
values of these 1,5-naphthyridines are similar to the EA values
of oligoquinolines66,67 and polyquinolines68–70 already found to
be good electron-transport materials for OLEDs. This suggests
that these 1,5-naphthyridines could be attractive because of their
excellent electron-transport materials properties.
Fig. 6 UV-vis absorption spectra (a) and fluorescence emission spectra
(b) of 1g in different solvents.
2.6 Theoretical calculation
The geometry of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
energy levels of 1a–1h were optimized using density functional
theory (DFT) method at the B3LYP/6-31G* level, as
implemented in the Gaussian 09 program. The core 1,5-
naphthyridine moiety shows a nearly coplanar structure, while
the arm substituents on the 1,5-naphthyridine unit (R group) are
twisted from the plane of the core. This geometry could lead to
excellent solution processability. From the density distribution, it
is clear that for the molecules the HOMO distributes in the
whole molecule for 1a–1f with slightly higher density on the
1,5-naphthyridine core units, which is very beneficial to hole
transport through the molecules when used as hole-injecting/
hole-transporting materials in OLEDs. The HOMO orbitals of
compounds 1g–1h are located mainly on the two arms of the
molecule. The HOMO energy values of 1a–1h were from −6.84
to −5.33 eV, respectively. The LUMO orbitals are predominantly
located on the 1,5-naphthyridine core units for materials 1a–1h
(see Table S2† in ESI). The LUMO energy values of 1a–1h
were from −2.39 to −2.19 eV; very similar to the EA values.
The HOMO–LUMO energy differences (energy band gaps, cal-
culated Egcal) are presented in Table 2. The results suggest that
Compounds 1a and 1c have one irreversible oxidation peak.
In the case of compounds 1b and 1d–1g, two or more irrevers-
ible oxidation peaks were seen in their CVs. Oxidation cyclic
voltammograms of compound 1h having two terminal triaryl-
amine moieties showed that starting from the second cycle an
additional peak developed at a potential lower than the oxidation
of the starting compounds (Fig. 7) also observed in CVs of
triphenylamine–oxadiazole–fluorene triad molecules.71 This
might be due to electropolymerisation of triarylamine derivative
1h and deposition of the polymer on the electrode surface. The
formal oxidation potential varies from 0.74 V for 1h to 0.89 V
for 1c (vs. SCE) and the onset oxidation potentials of 1a–1h are
in the range 0.49 to 0.64 V (vs. SCE). Ionization potentials (IP)
were estimated from the onset oxidation potentials in the solution
CVs (IP = Eoxonset + 4.4 eV) by using the ferrocene energy level
of 4.4 eV versus vacuum. These compounds had IP values of
4.89–5.04 eV (below vacuum) which are close to the work func-
tion of ITO and thus this allows efficient hole injection from the
ITO.72–74 Therefore 1,5-naphthyridines 1a–1h might be candi-
dates for excellent hole-injecting/hole-transport materials for
OLEDs.
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Table 3 Absorption spectral data and fluorescence emission spectral data for the 1,5-naphthyridines 1a–1h in different solvents
λabs (nm)
λem c (nm)
Compd.
Tola
CH2Cl2
THF
MeCNb
DMF
Tol
CH2Cl2
THF
MeCN
DMF
1a
1b
1c
1d
1e
1f
304
314
300
322
302
308
294
320
303
313
296
323
299
305
292
318
303
315
296
321
441
443
345
435
440
434
363, 432
464
448
441
349, 443
434
439
435
376, 479
522
444
439
371, 450
437
441
434
373, 459
506
450
440
346, 451
436
441
433
376, 515
541
441
433
344, 440
435
436
441
399, 518
534
319
317
318
315
318
282, 344
292, 342
301, 398
275, 343
293, 342
302, 399
274, 342
293, 341
297, 396
274, 336
292, 341
295, 389
275, 343
293, 342
298, 395
1g
1h
a Recorded in 5 × 10−5 M solutions of toluene at r.t. b Recorded in 5 × 10−5 M solutions of acetonitrile at r.t. c Recorded in ∼10−6 M in different
solvents at rt (λexcitation = λabs).
Table 4 Reduction cyclic voltammograms and oxidation cyclic voltammograms of compounds 1a and 1h (10−4 mol L−1) in CH2Cl2–acetonitrile
(7 : 3 v/v), TBAP (0.1 M), Scan rate = 100 mV s−1
Compd.
Reduction cyclic voltammograms
Oxidation cyclic voltammograms
1a
1h
cal
Eg values decrease as the length of the π-conjugated system
Conclusions
cal
increases. These predicted Eg values (3.09–4.32 eV) are larger
than Eg (2.77–3.79 eV) estimated from the onset of UV-vis
absorption. Material 1c has the highest Eg and Eg of 4.32
and 3.79 eV, respectively, whereas material 1h with the longest
conjugated length has the lowest Eg and Eg of 3.09 and
2.77 eV. There are factors which may be responsible for errors
because the orbital energy difference between HOMO and
LUMO is still an approximate estimation of the transition energy
since the transition energy also contains significant contributions
from some two-electron integrals. The real situation is that an
accurate description of the lowest singlet excited state requires a
linear combination of a number of excited configurations.
opt
We have synthesized and investigated the thermal stabilities,
electrochemical and photophysical properties of a series of eight
novel n-type organic optoelectronic materials 1a–1h based on a
1,5-naphthyridine core. The new materials have good solubility,
therefore they have the potential to be applied as solution-proces-
sable organic layers in OLEDs. Besides, the new 4,8-substituted
1,5-naphthyridines have robust thermal stability with high glass
transition temperatures. They emitted in the blue region from
400–501 nm in solid state. Additionally, the molecular design of
the new multifunctional materials 1,5-naphthyridines enabled
achievement of EA values (2.38–2.72 eV, below vacuum)
cal
opt
cal
opt
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Table 5 Electrochemical propertiesa and energy values theoretically calculated for molecules in gas phase of 1,5-naphthyridines 1a–1h
Eoxpeak (V)
Calc.c
onset
onset
Compd.
Eredpeak (V)
Ered
(V)
EAb (eV)
E2
E1
Eox
(V)
IPb (eV)
Egel (eV)
LUMO (eV)
HOMO (eV)
1a
1b
1c
1d
1e
1f
−2.10
−2.22
−2.01
−2.17
−2.14
−2.23
−2.03
−2.14
−1.96
−2.02
−1.76
−2.00
−1.95
−1.96
−1.68
−1.78
2.44
2.38
2.64
2.40
2.45
2.44
2.72
2.57
—
2.11
—
1.31
1.68
1.27
1.72
0.75
0.78
0.89
0.78
0.75
0.77
0.77
0.74
0.61
0.63
0.64
0.63
0.60
0.61
0.62
0.49
5.01
5.03
5.04
5.03
5.00
5.01
5.02
4.89
2.40
2.65
2.40
2.63
2.55
2.57
2.30
2.27
−2.29
−2.24
−2.52
−2.19
−2.25
−2.30
−2.39
−2.24
−6.51
−6.32
−6.84
−6.08
−6.16
−5.93
−5.75
−5.33
1g
1h
—
a Redox potential recorded in dichloromethane–acetonitrile (7 : 3 v/v) solution (10−4 M) with 0.1 M TBAP as supporting electrolyte; scan rate =
100 mV s−1, measured versus SCE. b Estimated from onset reduction/oxidation voltages with reference to SCE (4.4 eV vs. vacuum). c Optimized
using density functional theory (DFT) method at the B3LYP/6-31G* level.
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suitable for electron-transport materials and IP values
(4.89–5.04 eV, below vacuum) that could also facilitate excellent
hole-injecting/hole-transport materials properties. Quantum
chemical calculations using DFT B3LYP/6-31G* showed nearly
identical the lowest unoccupied molecular orbitals (LUMO) of
−2.39 to −2.19 eV and the highest occupied molecular orbitals
(HOMO) of −5.33 to −6.84 eV. These results suggest that the
4,8-substituted 1,5-naphthyridines 1a–1g might be promising
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applications in developing high-efficiency OLEDs with a simple
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