Synthesis and characteristics of novel fluorescence dyes based on chromeno[4,3,2-de][1,6]naphthyridine framework

Article abstract of DOI:10.1016/j.saa.2012.10.075

A series of entirely new framework chromeno[4,3,2-de][1,6]naphthyridine derivatives containing carbazole groups have been carefully designed and prepared. The relationship of photoluminescence property and structure of these compounds was systematically investigated via UV-vis, fluorescence and electrochemical analyzer. The HOMO and LUMO distributions of these compounds were calculated by density functional theory (DFT) (B3LYP; 6-31G ^*) method. These compounds exhibited high fluorescence quantum yields, desirable HOMO levels and high thermal stability, indicating that the combination of chromeno[4,3,2-de][1,6]naphthyridine and carbazole could be an efficient means to enhance hole-transporting ability and fluorescent quantum yield.

Full text of DOI:10.1016/j.saa.2012.10.075

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 62–67
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and characteristics of novel fluorescence dyes based
on chromeno[4,3,2-de][1,6]naphthyridine framework
⇑
Haiying Wang, Jingjing Shi, Chao Wang, Xiaoxiao Zhang, Lingling Zhao, Yu Wan, Hui Wu
School of Chemistry and Chemical Engineering, Jiangsu Normal University, Jiangsu Key Laboratory of Green Synthetic for Functional Materials, Xuzhou 221116, China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Entirely new framework chromeno[4,3,2-de][1,6]naphthyridine derivatives containing carbazole groups
were synthesized and the optical, electrochemical properties were also investigated. Quantum chemical
calculations were used to obtain optimized ground-state geometry, spatial distributions of the HOMO,
LUMO levels of the compounds.
"
Entirely new framework
fluorescence dyes based on
naphthyridine were synthesized.
Compounds exhibited efficient green
emission with high quantum yields.
HOMO energy level were improved
by the introduction of the carbazole
units.
"
"
"
Compounds might application in
OLEDs as hole-transporting
materials.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2012
Received in revised form 31 October 2012
Accepted 31 October 2012
Available online 7 November 2012
A series of entirely new framework chromeno[4,3,2-de][1,6]naphthyridine derivatives containing carba-
zole groups have been carefully designed and prepared. The relationship of photoluminescence property
and structure of these compounds was systematically investigated via UV–vis, fluorescence and electro-
chemical analyzer. The HOMO and LUMO distributions of these compounds were calculated by density
functional theory (DFT) (B3LYP; 6-31G⁄) method. These compounds exhibited high fluorescence quantum
yields, desirable HOMO levels and high thermal stability, indicating that the combination of
chromeno[4,3,2-de][1,6]naphthyridine and carbazole could be an efficient means to enhance hole-trans-
porting ability and fluorescent quantum yield.
Keywords:
Chromene
Naphthyridine
Carbazole
Ó 2012 Elsevier B.V. All rights reserved.
Chemical calculation
Electrochemical
Fluorescent
Introduction
fabrication of the biological labels, photovoltaic cells, light emitting
diodes (LEDs), and optical sensors [1–4]. The nature of fluorescence
In the last decade, efforts have been devoted to the develop-
ment of novel organic fluorescent compounds useful for the
depends largely on molecular structure and molecular assembly.
Therefore, it is important to clarify the structure–property relation-
ship of fluorescence because such a relationship would allow us to
design more useful fluorescent reagents and probes which might
be applied in the fields of analytical, biological chemistry and
OLEDs in the future.
⇑
Corresponding author. Tel./fax: +86 516 83403163.
E-mail address: wuhui72@yahoo.com.cn (H. Wu).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.10.075

H. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 62–67
63
Carbazole and its derivatives have been widely used as a
functional building block in the fabrication of the organic photo-
conductors, nonlinear optical materials, and photorefractive mate-
rials [5–8] due to their specific optical and electrochemical
properties. In the field of OLED technology, carbazole derivatives
are usually used as promising blue light-emitting materials
[9–14]. Meanwhile, a great many of carbazole homopolymers or
oligomers as well as carbazole-containing polymers or small
molecules, such as widely used PVK (polyvinylcarbazole) and CBP
(N,N0-dicarbazolyl-4,40-biphenyl), are of excellent hole-transport
ability, due to the electron-donating capabilities [15–19].
Naphthyridine derivatives possesses a satisfactorily planarity
framework. Since planarity is commonly regarded as a positive
structural factor in enhancing the molecular fluorescent properties,
these compounds were used not only as luminescence materials in
molecular recognition [20–22], but also as new drug leaders and
anticancer active screening agents in new drug discovery [23,24].
Therefore, our continuing interests in suitable fluorescent mate-
rials [25–27] for analytical and biological chemistry lead to an
introduction of the carbazole units to chromeno[4,3,2-
de][1,6]naphthyridine framework in order to improve the hole-
transporting ability and fluorescent quantum yield. Although the
introduction of the carbazole units reduce the planarity of the
chromeno[4,3,2-de][1,6]naphthyridine framework to a certain ex-
tent, it indeed enhance the hole-transporting ability and the emis-
sion color of these compounds can be easily tuned from blue to
green by changing the number of carbazole moieties as expected.
Particularly, these compounds exhibit high fluorescence quantum
yields and high HOMO energy level (ꢀ5.25 to ꢀ5.49 eV) due to
the presence of the electron-rich amine moieties and increased
conjugation lengths, giving rise to more balanced charge-transport
characteristics, all of which lead to promising applications in
OLEDs.
5-Amino-2-(4-bromophenyl)-chromeno[4,3,2-de][1,6]naphthyri-
dine-4-carbonitrile (4a).
Yield (0.63 g) 76%, Yellow crysta, Melting points (m.p.) >300 °C.
1H NMR (400 MHz, DMSO-d6): d 8.59 (d, J = 7.2 Hz, 1H), 8.46 (d,
J = 8.8 Hz, 1H), 8.37 (s, 1H), 7.82 (d, J = 12.0 Hz, 1H), 7.73–7.66
(m, 3H), 7.53–7.47 (q, 4H).
HRMS (ESI): m/z calcd. for C21H11N4BrO, M, 414.0116; found,
415.0105 (M + H)+.
5-Amino-10-bromo-2-phenylchromeno[4,3,2-de][1,6]naphthyri-
dine-4-carbonitrile (4b).
Yield 71%, Yellow solid, m.p. >300 °C. 1H NMR (400 MHz, DMSO-
d6): d 8.61–8.59 (d, 2H, J = 8.0), 8.40–8.36 (m, 2H), 7.83–7.81 (d, 2H,
J = 8.0), 7.79–7.75 (m, 1H), 7.62–7.57 (m, 4H) ppm. HRMS: m/z
calcd. for C21H11N4BrO, M, 414.0116; found: 415.0115 (M + H)+.
5-Amino-2-(4-bromophenyl)-10-bromochromeno[4,3,2-
de][1,6]naphthyridine-4-carbonitrile (4c).
Yield (0.59 g) 60%, Yellow solid, m.p. >300 °C. 1H NMR
(400 MHz, DMSO-d6): d 8.64 (d, J = 8.0 Hz, 1H), 8.40–8.371 (m,
3H), 7.82–7.80 (m, 2H), 7.71 (s, 1H), 7.58–7.51 (m, 3H).
HRMS m/z calcd. for
C21H10N4Br2O: M, 493.9201; found:
493.9203 (M+).
3-Bromo-9-butylcarbazole (6)
In a flask, covered with aluminum foil, a stirred solution of 9-
butylcarbazole 5 (4.46 g, 20.0 mmol) in CHCl3 (100 mL) was cooled
to 0 °C. N-bromosuccinimide (NBS) (3.56 g, 20.0 mmol) was added
in small portions. The mixture was allowed to warm to room tem-
perature overnight. CHCl3 was evaporated and the crude product
was purified by extraction with diethylether and water. After same
work up as above, final product was obtained as colorless oil
(4.23 g, 70%). 1H NMR (400 MHz, CDCl3): d 8.19 (s, 1H), 8.02 (d,
J = 7.2 Hz, 1H), 7.36–7.53 (m, 3H), 7.23–7.26 (m, 2H), 4.23 (t,
J = 6.8 Hz, 2H), 1.77–1.87 (m, 2H), 1.28–1.39 (m, 2H), 0.86 (t,
J = 7.2 Hz, 3H) ppm.
Experimental
9-Butylcarbazol-3-ylboronic acid (7)
Chemicals and instruments
A solution of 6 (3.3 g, 10.0 mmol) in anhydrous THF (50 mL) was
cooled to ꢀ78 °C. n-BuLi (2.5 mol Lꢀ1 in hexane, 4.8 mL,
12.0 mmol) was slowly added dropwise. After complete addition,
the reaction mixture was stirred for another 1 h. Then, triisopropyl
borate (3.5 mL, 15.0 mmol) was added at once. The mixture was al-
lowed to warm to room temperature for 15 h. The reaction was fi-
nally quenched with HCl (2.0 mol Lꢀ1, 40 mL) and the mixture was
poured into a large amount of water. After extraction with CH2Cl2
(3 ꢁ 20 mL), The organic layer was washed with brine, dried over
MgSO4, concentrated. Further purification by silica gel column
chromatography (petroleum ether/dichloromethane, 2/1, v/v)
afforded 7 as a white solid (1.44 g, 54%). m.p. 148–150 °C, 1H
NMR (300 MHz, DMSO-d6): d 8.59 (s, 1H), 8.11 (d, J = 7.5 Hz, 1H),
7.93 (s, 2H), 7.89 (s, 1H), 7.52–7.59 (m, 2H), 7.43 (t, J = 7.5 Hz,
1H), 7.20 (t, J = 7.5 Hz, 1H), 4.38 (t, J = 6.6 Hz, 2H), 1.71–1.76 (m,
2H), 1.25–1.32 (m, 2H), 0.87 (t, J = 7.5 Hz, 3H) ppm.
All solvents were carefully dried and freshly distilled according
to common laboratory techniques. All reactants were commer-
cially available and used without further purification. Melting
points were recorded on Electrothermal digital melting point appa-
ratus and were uncorrected. Nuclear Magnetic Resonance (NMR)
spectra were recorded at 295 K on a Bruker Advance DPX-
400 MHz spectrometer using DMSO-d6 as solvent and TMS as
internal standard. UV–vis spectra were recorded on a Shimadzu
UV-2501PC spectrometer. Fluorescence spectra were obtained on
a Hitachi FL-4500 spectrofluorometer. High Resolution Mass Spec-
troscopy (HRMS) data were measured using microTOF-Q(ESI)
instrument. Cyclic voltammetry was carried on a LK 1200A electro-
chemical analyzer with three-electrode cell (Platinum was used as
working electrode and as counter electrode, and SCE(saturated cal-
omel electrode) as reference electrode) in CH2Cl2 solution in the
presence of TBAHFP (tetrabutylammonium hexafluorophosphate)
(0.10 mol Lꢀ1) as supporting electrolyte.
General procedure for the synthesis of compounds (8)
Under a nitrogen atmosphere, a mixture of compounds (4a–c)
(1.0 mmol), Pd(PPh3)4 catalyst (0.04 mmol) and the corresponding
carbazole boronic acid was stirred in dry toluene (15 mL). Then,
2 mol Lꢀ1 K2CO3 (aq) solution (2 mL)was added via syringe. The
reaction mixture was heated to reflux for 72 h. After cooling, the
product was extracted with DCM, washed with water, dried over
MgSO4, filtered, concentrated and further purified by column chro-
matography (silica gel, hexane/dichloromethane, 10/1, v/v). The
pure compounds 8a–c were obtained.
General procedure for the synthesis of compounds 4
A mixture of substitution-2-hydroxyacetophenone (2.0 mmol),
aromatic aldehyde (2.0 mmol), malononitrile (4.0 mmol) and
0.03 g of silica gel was stirred in water (2 mL) at 80 °C. After 2 h
reaction, filtered, and then concentrated. The precipitate was col-
lected and purified by 95% EtOH-DMF (10:1). The analytical data
for represent compounds are shown below.

64
H. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 62–67
5-Amino-2-(4-(9-butyl-9H-carbazol-3-yl)phenyl)chromeno
[4,3,2-de][1,6]naphthyridine-4-carbonitrile (8a).
J = 6.6 Hz), 1.80–1.85 (m, 2H), 1.34–1.32 (m, 2H), 0.9 (t, 3H,
J = 7.6 Hz) ppm. IR (cmꢀ1): 3449, 3312, 2241, 1616, 1594, 1559,
1508, 1474, 1411, 1355, 1262, 775, 669.
Yield 85%, yellow solid, 1H NMR (400 MHz, DMSO-d6): d 8.87–
8.65 (m, 2H), 8.56 (d, 2H, J = 8.0 Hz), 8.45 (s, 1H), 8.40–8.38 (m,
1H), 8.30 (d, 2H, J = 8.0 Hz), 8.04 (d, 2H, J = 8.0 Hz), 7.94–7.91 (m,
1H), 7.83–7.81 (m, 1H), 7.74–7.72 (m, 2H), 7.55–7.49 (m, 5H),
7.27–7.23 (m, 1H), 4.48 (t, 2H, J = 6.6 Hz), 1.80–1.85 (m, 2H),
1.34–1.32 (m, 2H), 0.90 (t, 3H, J = 7.6 Hz) ppm. IR (cmꢀ1): 3448,
3301, 2214, 1630, 1619, 1598, 1560, 1265, 1073, 1007, 825.
HRMS m/z calcd. for C37H27N5O: M, 557.2216, Found: 556.2224
(M–H)+.
HRMS m/z calcd. for C37H27N5O: M, 557.2216, Found: 556.2245
(M–H)+.
5-Amino-10-(9-butyl-9H-carbazol-3-yl)-2-(4-(9-butyl-9H-car-
bazol-3-yl)phenyl)chromeno[4,3,2-de][1,6]naphthyridine-4-carbo-
nitrile (8c).
Yield 81%, yellow solid, 1H NMR (400 MHz, DMSO-d6): d 8.99 (s,
1H), 8.72 (s, 1H), 8.68–8.64 (m, 3H), 8.33 (m, 2H), 8.14 (d, 1H,
J = 8.8 Hz), 8.06 (d, 2H, J = 7.6 Hz), 8.02–8.00 (m, 1H), 7.95 (m,
4H), 7.78–7.73 (m, 2H), 7.65 (d, 3H, J = 8.0 Hz), 7.54–7.50 (m,
3H), 7.28–7.23 (m, 1H), 4.48 (t, 4H, J = 6.6 Hz), 1.80–1.85 (m, 4H),
1.36–1.30 (m, 4H), 0.9–0.89 (t, 6H, J = 7.6 Hz) ppm. IR (cmꢀ1):
3447, 3307, 2212, 1594, 1565, 1474, 1416, 1355, 1262, 799, 747.
HRMS m/z calcd. for C53H42N6O: M, 778.3420, Found: 777.3370
(M–H)+.
5-Amino-10-(9-butyl-9H-carbazol-3-yl)-2-phenylchromeno
[4,3,2-de][1,6]naphthyridine-4-carbonitrile (8b).
Yield 84%, yellow solid, 1H NMR (400 MHz, DMSO-d6): d 8.94 (s,
1H), 8.67 (s, 1H), 8.60 (s, 1H), 8.51 (m, 2H), 8.30 (d, 1H, J = 8.0 Hz),
8.12 (d, 1H, J = 8.0 Hz), 8.00–7.95 (m, 1H), 7.75 (d, 1H, J = 8.0 Hz),
7.62–7.60 (m, 5H), 7.53–7.49 (m, 2H), 7.25 (m, 1H), 4.47 (t, 2H,
R1
R2
O
OH
R1
O
CN
CN
H
a
2
CH3
+
+
R2
4a: R1 = H, R2 = Br
4b: R1 = Br, R2 = H
4c: R1 = R2 = Br
N
O
N
NC
1
3
2
NH2
4
HO
B
Br
OH
c
b
N
N
N
C4H9
5
C4H9
7
C4H9
6
Fig. 1. Synthetic routines for compound 4a–c and 7. Reagents and conditions: (a) silica gel, water, 80 °C; (b) carbazole:NBS = 1:1, CCl4, 80 °C; (c) n-BuLi (1.2 equiv), THF,
ꢀ78 °C, (CH3O)3B (1.5 equiv), H2O/HCl.
N
C4H9
d
4a-c +
7
N
O
N
NC
NH2
8a
N
C4H9
N
N
C4H9
C4H9
N
O
N
N
O
NC
N
NC
NH2
8b
NH2
8c
Fig. 2. Synthetic routines of 8. Reagents and conditions: (d) cat. Pd(PPh3)4, 2 mol Lꢀ1 K2CO3, toluene, 90 °C.

H. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 62–67
65
Table 1
Optical properties and electrochemical properties of compounds 8.
c
d
e
Compound
Abs. (nm)
Em (nm)
Ua
Band gapb
HOMO/LUMOb (eV)
Eg (eV)
Eox (V)
EHOMO/ELUMO (eV)
8a
8b
8c
422
422
417
502
503
526
0.29
0.20
0.22
3.11
3.33
3.15
ꢀ5.30/ꢀ2.19
ꢀ5.49/ꢀ2.16
ꢀ5.25/ꢀ2.10
2.77
2.77
2.75
0.86
0.89
0.52
ꢀ5.26/ꢀ2.49
ꢀ5.29/ꢀ2.52
ꢀ4.92/ꢀ2.15
a
b
c
The fluorescence quantum yields (U) were measured in THF using quinine sulfate (U = 0.55) as standard [17].
DFT/B3LYP calculated values.
Optical energy gaps calculated from the edge of the electronic absorption band.
Oxidation potential in CH2Cl2 (10ꢀ3 mol Lꢀ1) containing 0.1 mol Lꢀ1 (n-C4H9)4NPF6 with a scan rate of 100 mV sꢀ1
EHOMO was calculated by Eox + 4.4 V (vs NHE), and ELUMO = EHOMO ꢀ Eg.
d
e
.
Results and discussion
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Compounds 4a–c, 7 were synthesized according to methods de-
scribed in literature [25,28] (Fig. 1). For products 8a–c, the carba-
zole moiety was introduced via a Pd(0) catalyzed Suzuki C–C
coupling reaction [29] (Fig. 2). All of these new compounds were
characterized by MS spectrometry, 1H NMR spectroscopy.
8a
8b
8c
Absorption and fluorescence spectra
The absorption spectra of these compounds were measured in
CH2Cl2 solutions with a concentration of about 1.0 ꢁ 10ꢀ5 mol Lꢀ1
.
The optical properties of all new compounds are summarized in
Table 1. The absorption spectra of these compounds were compli-
cated due to multiple overlapping broad bands (Fig. 3). The maxi-
mum UV–vis absorptions of the compounds 8 are located in the
300
350
400
450
range of 417–422 nm, which is supposed to be ascribed to the
p–
Wavelength (nm)
p⁄ transition of the conjugated molecular backbone. The p⁄ en-
p–
Fig. 3. The absorption spectra of compounds 8 (1 ꢁ 10ꢀ5 mol Lꢀ1 in CH2Cl2).
ergy gaps (Eg) of these compounds were calculated from the UV–
vis absorption threshold (Table 1) [30–32].
Fig. 4 shows the fluorescence emission spectra of compounds 8
in the excitation of 350 nm. Compounds 8a, 8b and 8c present a
green fluorescence emission with the maximum emission peaks
at 502 nm, 503 nm and 526 nm, respectively. 8a–c have similar
fluorescence spectra because these compounds possess a similar
structure in CH2Cl2 solutions. The position of the maximum emis-
sion peaks was red-shifted at 526 nm. This is due to 8c possess
maxima conjugation length by introduced two carbazole moieties.
The fluorescence quantum yields (U) were measured in the THF
solution using quinine sulfate (U = 0.55) as a standard (Table 1)
[33]. The fluorescence quantum yields of the compounds are in
the range of 0.20–0.29. The emission efficiency in dilute solution
largely depends on the molecular structure. Moreover, this differ-
ence of the quantum yields may appear during the change of the
molecular size [34,35].
8000
8a
7000
8b
8c
6000
5000
4000
3000
2000
1000
0
Electrochemical properties
450
500
550
600
650
Wavelength / nm
The electrochemical properties of compounds 8 are explored by
the cyclic voltammetry in the CH2Cl2 solutions in the presence of
tetrabutylammonium hexafluorophosphate (0.10 mol Lꢀ1) as the
supporting electrolyte (Table 1). All of compounds 8 have one irre-
versible oxidation peak which is an indication of a stable cation
radical (Fig. 5). The HOMO energy of these materials was calculated
to get a range of ꢀ4.92 to ꢀ5.26 eV [36,37], which is close to that of
the most widely used hole-transport material 4,40-bis(1-naphthyl-
phenylamino)biphenyl (NBP) (ꢀ5.20 eV, ꢀ2.4 eV) by the introduc-
tion of the carbazole moieties, it might be beneficial for the hole-
transport capacity [38]. Similarly, the optical edge was utilized to
deduce the band gap and the LUMO energies. As expected, these
compounds are of lower LUMO (ꢀ2.15 to ꢀ2.52 eV) energies,
which represent a small barrier for the electron injection from a
commonly used cathode such as barium, which has a work func-
Fig. 4. The emission spectra of compounds 8 (1 ꢁ 10ꢀ6 mol Lꢀ1 in CH2Cl2).
tion of ꢀ2.2 eV [39]. Therefore, these compounds might be very
useful as hole-transporting and electron-transporting materials in
applications for OLEDs [39].
Theoretical calculations
The electronic configurations were further examined using the
theoretical models implanted in the Gaussian 03 program [40].
The calculations based upon Density functional theory (DFT)
(B3LYP; 6-31G⁄) were carried out to obtain information about the
HOMO and LUMO distributions of the compounds 8 (Table 1).

66
H. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 62–67
Fig. 5. Cyclic voltammogram of compound 8a (1 ꢁ 10ꢀ3 mol Lꢀ1) as sample, in
Fig. 6. Optimized ground-state geometry of compounds 8c with B3LYP/6-31G⁄ in
gas phase.
CH2Cl2 containing 0.1 mol Lꢀ1 (n-C4H9)4NPF6, scan rate 100 mV/s.
Fig. 7. Calculated spatial distributions of the HOMO, LUMO levels of compound 8c as sample.
Due to an increase in the conjugation lengths, all of these com-
pounds possess a high HOMO energy level (ꢀ5.25 to ꢀ5.49 eV),
which could lead to their better hole-transport properties (Figs. 6
and 7) [38]. The low LUMO energy of these compounds (ꢀ2.10 to
ꢀ2.19 eV) is supposed to facilitate the acceptance of electrons from
the cathode. According to the DFT calculations and the experimen-
tal data from the UV-absorption spectra, the calculated band gaps
show a similar trend. The difference of HOMO/LUMO energy level
from calculation data and the experimental data may be related
to various effects from conformation and solvents, which have
not been taken into account here. Moreover, the electrochemistry
is complicated owing to the reversibility of one of the redox process
and the accuracy of the Eg value is relatively limited [41].
vel (ꢀ4.92 to ꢀ5.29 eV) due to the presence of the electron-rich
amine moieties and increased conjugation lengths, giving rise to
more balanced charge-transport characteristics, which have prom-
ising potential for application in OLEDs as a multifunctional
material.
Acknowledgements
We are grateful to Dr. Guo-Lan Dou in China University of Min-
ing and Technology for great computational assistances.
The work was partially supported by the foundation of the ‘‘Pri-
ority Academic Program Development of Jiangsu Higher Education
Institutions’’, Key Basic Research Project in Jiangsu University (No.
10KJA430050), Nature Science of Jiangsu Province for Higher Edu-
cation (No. 11KJB150017), Natural Science Foundation of Xuzhou
Normal University (No. 10XLR19).
Conclusions
In summary, a series of entirely new framework fluorescence
dyes based on chromeno[4,3,2-de][1,6]naphthyridine derivatives
have been prepared by a stepwise route involving a Pd(0) catalyzed
Suzuki coupling reaction in good yields. The optical properties
clearly indicate that the fluorescent emission properties of these
compounds rely largely on the molecular structure. As expected,
the optical band gaps decrease considerably as the number of car-
bazole moieties introduced into chromeno[4,3,2-de][1,6]naph-
thyridine framework increases. Moreover, compounds exhibited
high fluorescence quantum yields and excellent green lumines-
cence emission. These compounds possess a high HOMO energy le-
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