Synthesis and properties of highly fluorescent indolizino[3,4,5-ab] isoindoles

Article abstract of DOI:10.1021/ja049214x

We report here the synthesis, X-ray structures, optical and electrochemical properties, fabrication of light-emitting devices, and density functional calculations for indolizino[3,4,5-ab]isoindole (INI) derivatives. Strongly luminescent heterocycles based

Full text of DOI:10.1021/ja049214x

Published on Web 12/04/2004
Synthesis and Properties of Highly Fluorescent
Indolizino[3,4,5-ab]isoindoles
Teruyuki Mitsumori,† Michael Bendikov,† Olivier Dautel,† Fred Wudl,*,†
Takeshi Shioya,‡ Hideki Sato,‡ and Yoshiharu Sato‡
Contribution from the Department of Chemistry and Biochemistry, Exotic Materials Institute,
UniVersity of California, Los Angeles, California 90095-1569, and Science & Technology
Research Center, Mitsubishi Chemical Corporation, 1000 Kamoshida Aoba,
Yokohama 227, Japan
Received February 12, 2004; E-mail: wudl@chem.ucla.edu
Abstract: We report here the synthesis, X-ray structures, optical and electrochemical properties, fabrication
of light-emitting devices, and density functional calculations for indolizino[3,4,5-ab]isoindole (INI) derivatives.
Strongly luminescent heterocycles based on the INI unit were synthesized by 1,3-dipolar cycloaddition
reactions between pyrido[2,1-a]isoindole (PIS) and acetylene or ethylene derivatives. They are indolizino-
[3,4,5-ab]isoindoles 2-9 and 14-15, benzo[1′,2′-1,2]indolizino[3,4,5-ab]isoindoles 10, pyridazino[4′,5′:1,2]-
indolizino[3,4,5-ab]isoindoles 12-13, and 2,3-hydropyridazino[4′,5′:1,2]indolizino[3,4,5-ab]isoindole-1,4-
dione 11. The relative luminescence quantum yield can be as high as 90%. Their reduction and oxidation
potentials and high luminescence can make these heterocycles possible alternatives to tris(8-hydroxy-
quinolinato)aluminum (Alq3). The brightness of the light-emitting device reached as high as 104 cd/m2 and
indolizino[3,4,5-ab]isoindole 3 emits beautifully blue light. The X-ray crystal structures of INI derivatives
were obtained for the first time. The geometries obtained from X-ray data and density functional theory
calculations shed more light on an interesting formally antiaromatic 16π system, which is divided into 10π
and 6π aromatic systems. We also report a relatively easy protonation of INI, which occurs at a carbon,
rather than nitrogen atom.
Introduction
stability problems, and rules governing the correlation between
structure and fluorescence efficiency are still in their infancy.3
Light-emitting organic compounds (organic luminophors)
continue to arouse strong interests because of their fascinating
functions as electroluminescent materials, sensors, lasers, and
other semiconductor devices.1 Among these applications, organic
light-emitting diodes (OLEDs) have been most widely inves-
tigated.2 Two types of organic materials, small molecules and
polymers, are commonly used for OLEDs. Small molecules have
the advantages of facile emission color control and ease of
fabrication of multilayer devices. They, however, still have
While many chromophores have been designed and synthe-
sized for potential OLEDs,4,5 it is essential to find molecules
which exhibit high fluorescence, little self-quenching, ideal
HOMO and LUMO energy levels, pure red-green-blue (RGB)
color, and high stability. Among the chromophores, tris(8-
hydroxyquinolinato)aluminum (Alq3) is most commonly used
and known to be a good emitting and an electron transport
material. However, its electron mobility (on the order of 10-5
cm2/V‚s)6 and hydrolytic stability could be improved; also its
ligands have been suspected to be toxic.7
† University of California, Los Angeles.
‡ Mitsubishi Chemical Corp.
Recently our group has reported a series of highly blue
fluorescent pyrrolopyridazines, which are expected to be good
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J. AM. CHEM. SOC. 2004, 126, 16793-16803
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A R T I C L E S
Mitsumori et al.
Figure 1. Synthesized heterocyclic molecules used in this paper.
chromophores for highly efficient OLEDs.8 During our studies,
we found a series of indolizino[3,4,5-ab]isoindole (INI) with
quite interesting luminescence characteristics. Herein we report
the synthesis and the properties of 14 INIs9 which were
synthesized from pyrido[2,1-a]isoindole (PIS). This is the first
paper where the excellent luminescence and excellent LED
device performance, X-ray crystal structures, protonation, and
electronic properties of INIs, as well as theoretical studies and
applicability of INIs as an active semiconductor in field effect
transistors are reported. These heterocycles have excellent
luminescence (quantum efficiencies are as high as 90%) and
different emission colors (from blue to green), and their HOMO
and LUMO levels are easily tunable to afford compounds for
device fabrication. The structural data, combined with density
functional calculations, allowed for a better understanding of
the INI chromophore.
according to a modified literature procedure10 and used freshly
due to its sensitivity to oxygen.11 Cycloadditions occurred
smoothly when acetylene or ethylene derivatives were added.
For compounds 6 and 7, vinyl pyridine and vinylquinoline were
not reactive enough so that acetylene derivatives were used.
Oxidative dehydrogenations were performed using chloranil and
sulfur (except compound 10,9f which had been oxidized by air).
Adducts 6a and 7a are relatively stable toward oxidation, so
they needed stronger condition for aromatization (sulfur, around
130 °C).
Starting from compound 2 nine heterocylcles were synthe-
sized. They are indolizino-isoindoles (4, 5, 8, 9, 14), furan-
fused heterocycles (15), and pyridazine-fused heterocycles (11-
13). First, compound 2 was hydrolyzed quantitatively to obtain
compound 4 that was consequently decarboxylated by copper
chromite at high temperature (210 °C).9c The resulting hetero-
cycle 5 is unusually nucleophilic toward electrophilic substitu-
tion. From the results of NMR, mass spectra, and calculations
(see below), the substitution is expected to occur at position 2.
Compound 5 was easily brominated at moderately low temper-
ature (0 °C) without any catalyst to afford bromide 8 and was
also easily dimerized with benzaldehyde with a catalytic amount
of acid to form compound 9. Compound 2 was allowed to react
with phenyllithium to obtain a tetraphenyl adduct 14, which
subsequently, upon acid treatment, formed 15. Treatment of 2
with hydrazine12 formed hydrazide 11, whose fused ring was
subsequently aromatized with PCl5 to form pyridazine-fused
heterocycle 12. The chlorides in 12 were easily displaced by
phenoxide13 to afford 13 in good yield.
Results and Discussion
Synthesis. Compounds reported in this paper are presented
in Figure 1. Synthetic strategies for the INIs are depicted in
Scheme 1.
As shown in Scheme 1, the preparation of INIs were initially
carried out by cycloaddition with PIS (1), which was prepared
(8) (a)Cheng, Y.; Ma, B.; Wudl, F. J. Mater. Chem. 1999, 9, 2183. (b)
Mitsumori, T.; Bendikov, M.; Sedo´, J.; Wudl, F. Chem. Mater. 2003, 15,
3759.
(9) Synthesis and properties of some INIs (2,a-c 3,a,b 4,c 5,b-e 10 f-k) have
been previously reported. (a) Kajigaesh, S.; Mori, S.; Fujisaki, S.; Kanemasa,
S. Bull. Chem. Soc. Jpn. 1985, 58, 3547. (b) Matsumoto, K.; Uchida, T.;
Kato, T.; Toda, M.; Aoyama, K.; Konishi, H. Heterocycles 1990, 31, 593.
(c) Tominaga, Y.; Shiroshita, Y.; Gotou, H.; Matsuda, Y. Heterocycles
1986, 24, 3071. (d) Castle, L. W.; Tominaga, Y.; Castle, R. N. J. Heterocycl.
Chem. 1995, 32, 1033. (e) Tominaga, Y.; Shigematsu, Y.; Saeki, K. J.
Heterocycl. Chem. 2002, 39, 571. (f) Yamauchi, J.; Uchida, T.; Matsumoto,
K. Heterocycles 1990, 31, 1785. (g) Matsumoto, K.; Yamauchi, J. Uchida,
T. Heterocycles 1985, 23, 2773. (h) Uchida, T.; Aoyama, K.; Nishikawa,
M.; Kuroda, T.; Okamoto, T. J. Heterocycl. Chem. 1988, 25, 1793. (i)
Matsumoto, K.; Uchida, T.; Sugi, T.; Yagi, Y. Chem. Lett. 1982, 869. (j)
Matsumoto, K.; Katsura, H.; Uchida, T.; Aoyama, K.; Machiguchi, T. J.
Chem. Soc., Perkin Trans. 1 1996, 21, 2599. (k) Matsumoto, K.; Katsura,
H.; Yamauchi, J.; Uchida, T.; Aoyama, K.; Machiguchi, T. Bull. Soc. Chim.
Fr. 1996, 133, 891.
For benzannulated compound 10, benzyne was used as an
acetylene derivative. Compound 1 was allowed to react with
(10) Fozard, A.; Bradsher, C. K. J. Org. Chem. 1967, 32, 2966.
(11) Bradsher C. K.; Voigt C. F. J. Org. Chem. 1971, 36, 1603.
(12) Clifford, M. C. Synth. Commun. 1989, 19, 1981.
(13) Becker, H.; King, S. B.; Tanigushi, M.; Vanhessche, K.; Sharpless, K. B.
J. Org. Chem. 1995, 60, 3940.
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16794 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Highly Fluorescent Indolizino[3,4,5-ab]isoindoles
Scheme 1. Synthetic Route to 2-15a
A R T I C L E S
a (i) Dimethyl maleate, CH2Cl2, 0 °C, 1 h; (ii) NaOH(aq), THF, reflux, 5 h; (iii) 2CuO‚Cr2O3, quinoline, 210 °C, 4 h; (iv) 2,2,6,6-tetramethylpiperidine,
n-BuLi, bromobenzene, THF, -78 °C, 1 h; (v) methyl acrylate, CH2Cl2, 0 °C, 1 h; (vi) 2-ethynylpyridine, dichloroethane, 0 °C, 1 h; (vii) 2-ethynylquinoline,
dichloroethane, 0 °C, 1 h; (viii) sulfur, chlorobenzene, reflux, 10 h; (ix) sulfur, chlorobenzene, reflux, 10 h; (x) phenyllithium, THF, -75 °C, 4 h; (xi) acetic
acid, THF, 80 °C, 4 h; (xii) hydrazine, EtOH, chlorobenzene, reflux, 5 days; (xiii) PCl5, POCl3, 130 °C, 4 h; (xiv) phenol, K2CO3, DMF, reflux, 3 days; (xv)
bromine, CH2Cl2, -10 °C, 1 h; (xvi) benzaldehyde, p-toluenesulfonic acid, THF, 50 °C, 3 h.
Table 1. Quantum Efficiency (%) of Compounds 2-15 in Different
benzyne, which was generated from bromobenzene, to afford
compound 10.
Solvents
MeOH
CH2Cl2
DMSO
hexane
THF
AcOH
Optical Properties. The absorption and fluorescence spectra
were recorded in dimethyl sulfoxide (DMSO), methanol,
methylene chloride, tetrahydrofuran (THF), hexane, and acetic
acid at room temperature.14,15 As shown in Table 1 and Figure
2, the INIs are highly luminescent and emit light from blue to
green.16 While the quantum efficiencies (QE) are solvent-
dependent, most compounds reach efficiencies of 80-90%,
except 7 and 10. The brominated compound 7 has lower QE,
probably due to heavy atom effect.17 Interestingly most Stokes
shifts were quite small (except that of compound 2),18 less than
20 nm and even less than 10 nm in a nonpolar solvent, such as
hexane. Apparently the INIs are so rigid that there is very little
2
3
4
5
6
7
8
9
10
11
12
13
14
15
82
80
78
55
77
92
9
74
29
26
47
78
58
51
68
57
40
45
74
81
9
83
26
-
71
79
62
79
87
78
8
79
40
25
44
82
74
76
55
68
-
32
63
89
7
34
18
-
69
69
71
46
80
87
8
64
31
37
51
74
82
64
81
88
78
79
3
<0.1
8
58
33
82
19
62
83
88
45
67
62
63
-
53
42
-
difference in energy between the Frank-Condon state and the
(14) Sharma, A.; Schulman, S. G. Fluorescence Spectroscopy; John Wiley &
Sons: New York, 1999.
(15) Relative quantum yields were determined by using perylene in degassed
cyclohexane (Φ ) 0.94) as reference; see: Berlman, I. Handbook of
Fluorescence Spectra of Aromatic Molecules; Academic Press: New York,
1965.
(16) Absorption and fluorescence spectra of all INIs synthesized as well as λmax
of the absorption spectra, λmax of the fluorescence spectra, and the Stokes
shift are given in the Supporting Information.
(17) Becker, R. S. Theory and Interpretations of Fluorescence and Phospho-
rescence; John Wiley & Sons: New York, 1969.
equilibrium state (especially in nonpolar solvents).17
(18) The Stokes shift of compound 2 was bigger because the longest wavelength
vibrational band became smaller than the shorter wavelength one (this
depends on the difference of the relative location of the potential energy
minima between the excited state and the ground state),17 and the Stokes
shift was calculated between the strongest emission and absorption bands.
Thus, it is difficult to compare the Stokes shift in compound 2 with those
of compounds 3-5.
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A R T I C L E S
Mitsumori et al.
Figure 2. Absorption and fluorescence spectra of compounds 5 and 10.
Figure 3. Solvent dependence of fluorescence spectra of compound 11.
The QEs of compounds 2-5 are high in polar solvents,
especially in acetic acid.19 Interestingly, the fluorescence of
compounds 2-5, 11, 14, and 15 was enhanced by acid, while
that of compounds 6, 7, and 12 was quenched (because the
nitrogen of pyridine or quinoline could be protonated), especially
compound 11, which behaved as a pKa sensor (Figure 3). The
quantum efficiency changed from 82% (in acetic acid) to 11%
(in triethylamine). We will discuss these phenomena later.
Due to their expanded π conjugation, compounds 6 and 7
showed red-shifted spectra (around 20 nm)16 compared to
compound 5. Interestingly the first absorption and fluorescence
bands of 6 and 7 are almost identical, which indicates the
perturbations to the INI by pyridine and quinoline are essentially
identical. Compounds 10, 12, and 13, which have their conjuga-
tion extended by ring fusion, showed more red-shifted spectra
than 5-7. Among them, 10 shows the most red-shifted spectra.
This might be because benzannulated 10 has a more delocalized
π-conjugation system than 12 or 13, which have a fused
pyridazine.
Crystal Structure. Single crystals of compounds 2, 6, and
7 were grown from CH2Cl2/hexane solution and characterized
using X-ray crystallography. To the best our knowledge, this is
the first X-ray crystal structure of the INI ring. Interestingly,
and in accord with their aromaticity, all molecules have planar
geometries (Figure 4). All bond lengths and angles have the
expected values for aromatic compounds (aromatic Csp2-Csp2
bond length around 1.40 Å), except for, for example, in
compound 6, where C(5)-C(6) and C(11)-C(12) are relatively
long (1.452 Å, 1.442 Å, respectively).
Figure 4. ORTEP diagram of compounds 2, 6, and 7: N(1)-C(12), 1.3677-
(17); N(1)-C(5), 1.3707(17); N(1)-C(1), 1.3736(17); C(1)-C(2), 1.403-
(2); C(1)-C(14), 1.429(2); C(2)-C(3), 1.386(2); C(3)-C(4), 1.415(2);
C(4)-C(5), 1.376(2); C(5)-C(6), 1.452(2); C(6)-C(7), 1.393(2); C(6)-
C(11), 1.4342(19); C(7)-C(8), 1.384(2); C(8)-C(9), 1.404(2); C(9)-C(10),
1.381(2); C(10)-C(11), 1.396(2); C(11)-C(12), 1.4421(19); C(12)-C(13),
1.4006(19); C(13)-C(14), 1.4137(19).
3.46 Å in 7. These values are only slightly larger than that of
the interplanar distance in graphite (3.35 Å).20 Remarkably, the
distances between columns are also quite small in compounds
6 and 7. The closest intermolecular aromatic ring contacts (C-
C, C-N, N-N) between columns are only 3.40 Å along the c
axis and 3.67 Å along the b axis in 6 and 3.69 Å along the a
axis and 3.72 Å along the c axis in 7. This π-stacking and
contact system are strong enough so that the heterocylcle is
expected to be useful for applications that need high mobility,
such as FET devices.21
The molecules stack along the a axis in compounds 2 and 6
and the b axis in compound 7 to form columnar structures
(Figure 5). The crystal has a strong π stacking system, and the
distance between planes is only 3.42 Å in 2, 3.44 Å in 6, and
(19) According to 1H NMR of 5 in acetic acid, a proton bonded to C1 is
exchanged with a proton of acetic acid, but 5 remains mostly unprotonated,
as evident by the similarity of 1H NMR before and after addition of acetic
acid.
(20) Chung, D. D. L. J. Mater. Sci. 2002, 37, 1475.
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Highly Fluorescent Indolizino[3,4,5-ab]isoindoles
A R T I C L E S
Table 2. Electrochemical Properties of 2-3, 5-7, and 9-15 (in
CH3CN)
a
b
compound
Eox
Ered
2
3
5
6
7
1.24/1.20c
1.11/1.01c
0.81/0.77c
0.80/0.70
0.87/0.73c
0.67/0.61c
0.69/0.50
1.26/-d
-1.65/-1.77
-2.03/-2.25
<-2.30e
-2.11/-2.38
-2.18/-d
9
<-2.40e
10
11
12
13
14
15
<-2.40e
-1.15/-d
1.22/1.14
-1.50/-1.62
-1.80/-2.01
<-2.30e
1.34/-d
0.81/0.71
0.93/0.78
<-2.40e
b
a Eox.pa/Eox.pc (vs/SCE). Ered.pa/Ered.pc (vs/SCE). c Partially reversible.
d Irreversible. e Undetectable.
Figure 6. Cyclic voltammetry of compounds 2, 3, and 13.
The reduction potentials of these compounds were also revers-
ible, so we could expect better stability than Alq3 in reduced
states. Alq3 is known to be unstable in electrochemical studies.
As shown in Table 1, compounds 2 and 13 also have a high
fluorescence, suggesting that they could be good candidates for
alternatives to Alq3.23
Oxidation and reduction potentials became smaller as electron-
withdrawing ester groups were removed from 2 to 3 and from
3 to 5. The oxidation potential of parent compound 5 is around
0.8 V, which is a desirable energy level for a hole transport
material24 in usual organic semiconductor devices, such as
OLED. As a result of substitution, when 2 is compared with 3,
the reduction potentials change more than oxidation potentials.
This indicates the carbonyl at position 1 affects the LUMO level,
but the HOMO level is affected to a much lesser extent. Frontier
orbital theory25 provides an explanation for such behaviors. The
LUMO of the parent INI has a large coefficient at position 1
(11%, Figure 7) so that the substituent effect in this position
Figure 5. π stacking diagram of compounds 2, 6, and 7.
Electrochemical Studies. Electrochemical data for com-
pounds 2, 3, 5-7, and 9-15 are presented in Table 2.
Compounds 2, 12, and 13 showed similar potentials (Figure 6)
as the conventional electron transport material22 Alq3 (oxidation
potential, 1.2 to 1.4 V; reduction potential, -1.6 to -1.8 V).
(22) (a) Posch, P.; Thelakkat, M.; Schmidt, H. W. Synth. Met. 1999, 102, 1110.
(b) Kim, J.; Kim, E.; Choi, J. J. Appl. Phys. 2002, 91, 1944. (c) Lee, S. T.;
Hou, X. Y.; Mason, M. G.; Tang, C. W. J. Appl. Phys. 1998, 72, 1593. (d)
Vestweber, H.; Bassler, H. J. Appl. Phys. 2001, 89, 3711.
(21) (a) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. DeV. 2001, 45, 11.
(b) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99.
(c) Bredas, J. L.; Calbert, J. P.; Filho, D.A. S.; Cornil, J. Proc. Natl. Acad.
Sci. U.S.A. 2002, 99, 5804. (d) Afzali, A.; Dimitrakopoulos, C. D.; Breen,
T. L. J. Am. Chem. Soc. 2002, 124, 8812. (e) Anthony, J. E.; Broks, J. S.;
Eaton, D. L.; Parkin, S. R. J. Am. Chem. Soc. 2001, 123, 9482. (f) Bredas,
J. L.; Beljonne, D.; Cornil, J.; Calbert, J. P.; Shuai, A.; Silbey, R. Synth.
Met. 2002, 125, 107. (g) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org.
Lett. 2002, 4, 15. (h) Gruhn, N. E.; Filho, D. A. S.; Bill, T. G.; Malagoli,
M.; Coropceanu, V.; Kahn, A.; Bre´das, J.-L. J. Am. Chem. Soc. 2002, 124,
7918.
(23) Additionally, compounds 2 and 13 also have high luminescence in a solid
state, which indicates those have little self-quenching in condensed states.
(24) (a) Thelakkat, M.; Shmitz, C.; Hohle, C.; Strohriegl, P.; Shmidt, H. W.;
Hofmann, U.; Schloter, S.; Haaser, D. Phys. Chem. Chem. Phys. 1999, 1,
1693. (b) Hu, N.; Xie, S.; Popovic, Z. D.; Ong, B.; Hor, A. M. Synth. Met.
2000, 111, 421.
(25) Fleming, I. Frontier Orbitals and Organic Chemical Reactions; John Wiley
& Sons: New York, 1976.
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A R T I C L E S
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Figure 7. HOMO and LUMO of compound 5 at the MP2/6-31G(d)//B3LYP/6-31G(d) level of theory.
on the reduction potential is large. On the contrary, the HOMO
of INI has a small coefficient at position 1 (1%) so that the
substituent effect on the reduction potential is small.
The oxidation potential did not change from compound 5 to
6 or 7 as a result of a pyridine or a quinoline substitution, but
each compound became more reducible due to the enhanced
electronegativity of the attached rings. Substituent effects of
pyridine and quinoline on redox potentials appear to be similar
to the effect on optical spectra (see above).
Because the two INI rings in 9 interact through a methylene
bridge, the oxidation potential of 9 became smaller (ca. 0.15
eV) compared with 5, while that of 14 and 15 remained almost
the same. A similar effect was observed in optical spectra (red
shift about 20 nm) in compound 9, compared with compounds
5, 14, and 15. The optical spectra of 14 and 15 were almost the
same as observed for 5.
Figure 8. Optimized geometry (B3LYP/6-31G(d)) of 5.
Compounds 10,9f,g 12, and 13 have interesting 20π conjuga-
tion systems compared with 2 (16π). The oxidation potential
of compound 10 9f was lowered by about 0.6 eV due to the
fused phenyl ring. On the other hand, the oxidation and reduction
potential of compounds 12 and 13 was increased because of a
fused electronegative pyridazine ring. From these values,
compound 10 is expected to be a good donor or a good hole
transport material, and compounds 12 and 13 could be electron
transport materials as well as good emitters.
Computational Study of INI. According to the Hu¨ckel [4n
+ 2] rule,26 INI has 16π electrons and is formally antiaromatic.
However, this is a stable planar compound which has NMR
signals characteristic for aromatic compounds. To obtain insight
into the electronic structure of INIs, we have performed density
functional calculations at the B3LYP/6-31G(d) level of theory.
Compound 5 was shown to be planar by frequency analysis.
Examination of calculated geometry helped to solve this
dichotomy. In 5, and in accord with X-ray data, C5a-C5b and
C9a-C9b bond lengths are long (1.453 and 1.443 Å, respec-
tively, Figure 8), while all other bonds are normal aromatic
bonds. Thus, the π-electrons of INI should be considered as
two independent aromatic systems, benzene and indolizine, or,
in other words, INI is an aromatic system consisting of 6π +
10π electrons. Similar structural effects are observed in perylene,
which is formally a 20π electron system.27
The calculated vertical ionization potential (IP) of 5 is
relatively low, 7.0 eV at B3LYP/6-311+G(2df,p)//B3LYP/6-
31G(d), only by 0.4 eV higher than the IP of pentacene (6.59
eV) and comparable with that of tetracene (6.97 eV).28 The
predicted reorganization energy (calculated as the difference
between vertical and adiabatic ionization energies at the B3LYP/
6-31G(d)//B3LYP/6-31G(d) level)29 is only 0.092 eV. Both of
these values, together with the planar structure of INI, make it
an attractive candidate for possible applications in field effect
transistors.21
Protonation Study. As described in the optical properties
section, these heterocycles are expected to be basic. Normally
protonation of a nitrogen heterocycle occurs at nitrogen, rather
than carbon. But heteroaromatic systems in which the nitrogen
has three single bonds such as indolizino isoindoles are likely
to protonate to carbon.30 For example, pyrrole,31 indole,32
(27) Nather, C.; Bock, H.; Havlas, Z.; Hauck, T. Organometallics 1998, 17,
4707.
(28) Gruhn, N. E.; da Silva Filho, D. A.; Bill, T. G.; Mlagoli, M.; Coropceanu,
V.; Kahn, A.; Bre´das, J.-L. J. Am. Chem. Soc. 2002, 124, 7918.
(29) The reorganization energy (λ) for self-exchange corresponds to the sum of
the geometry relaxation energies upon going from the neutral-state geometry
to the charged-state geometry and vice versa. These two portions of λ are
typically nearly identical, see: Malagoli, M.; Bre´das, J.-L. Chem. Phys.
Lett. 2000, 327, 13.
(30) Fraser, M. J. Org. Chem. 1971, 36, 3087.
(31) Chiang, Y.; Whipple, E. J. Am. Chem. Soc. 1963, 85, 2763.
(32) Hinman, R.; Whipple, E. J. Am. Chem. Soc. 1962, 84, 2534.
(26) Huckel, E. Z. Phys. 1932, 76, 628.
9
16798 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Highly Fluorescent Indolizino[3,4,5-ab]isoindoles
A R T I C L E S
Table 3. Electroluminescence Data for the Compounds 3, 6, 7,
and 13
compound 3 compound 6 compound 7 compound 13
Vth (V)a
max luminance (cd/m2) 5674
6.0
7.0
7009
1.08
7.0
11390
1.24
6.2
8328
1.57
power efficiency,b
100 (lm/W)
voltageb 100 (V)
L/J c (cd/A)
peak (FWHMd) (nm)
CIEx
0.94
9.0
2.70
450(59)
0.165
0.188
9.8
3.37
486(51)
0.169
0.463
9.8
3.90
486(53)
0.177
0.449
9.2
4.59
488(40)
0.175
0.439
CIEy
a Turn on voltage. b Taken at 100 cd/m2. c Current efficiency at 100 cd/
m2. d Full width half-maximum.
Figure 9. 1H NMR spectra of (a) compound 5 and (b) protonated compound
5 in CD2Cl2. (a) The sample was measured in CD2Cl2. (b) One drop of the
solution (trifluoroacetic acid/trifluoroacetic anhydride ) 10/1) was added
to sample a. We used anhydride to get rid of water because the protonated
INI(5) is sensitive to water.
Figure 11. Comparison of the EL spectra of compounds 3, 6, 7, and 13.
Figure 10. Calculated (at B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) +
ZPVE level of theory) proton affinities of 5.
a high HOMO coefficient of 17% and has the highest proton
affinity, leading to an aromatic pyridinium cation.
From these results we can assign the structure of the
protonated compound and chemical shifts δ (ppm) of 1H NMR
as shown in the following diagram.38
isoindole,33 pyrrole thiazole,34 and indolizine30 were all reported
to be protonated at carbon.
Figure 9 shows a 1H NMR study of the protonation of
compound 5 by trifluoroacetic acid. After a trace of acid was
added, the NMR spectrum changed from (a) to (b) and a typical
singlet peak30-34 for protonation was found at 4.77 ppm for
two protons.19 From these NMR spectra35 the protonation site
can be deduced as the position 1 or 2. Gas-phase protonation
of 5 was studied computationally in order to determine the
protonation site.
In agreement with experimental results, 5 is predicted to
protonate easily, the calculated proton affinity is 227.1 kcal/
mol (in gas phase, at B3LYP/6-311+G(2df,p)//B3LYP/6-31G-
(d) + ZPVE). For comparison, gas-phase proton affinity of
tetracene is 216.4 kcal/mol,36 of pentacene it is 225.7 kcal/mol,37
and that of pyrrole is 222 kcal/mol.36 Calculations also reveal
the most preferred protonation place (Figure 10). In agreement
with the aromatic character of INI, protonation of 5 on nitrogen
is unfavorable (16c, 183.2 kcal/mol). Additionally, the HOMO
of 5 is delocalized over the aromatic system and the coefficient
on nitrogen is only 2% (Figure 7). Expectedly, position 2 has
Characteristics of Light-Emitting Devices. Using the
phosphors discussed above, we prepared efficient OLEDs (from
heterocycles 3, 6, 7, and 13), which emit from blue to green.
The organic EL devices were fabricated according to the
procedure previously reported.39 The OLEDs comprised four
organic layers between ITO and cathode: ITO/CuPc (copper
phthalocyanine, 10 nm)/CPB (4,4′-bis(carbazol-9-yl)biphenyl,
60 nm)/INIs (30 nm)/SAlq (bis(2-methyl-8-quinolinolato-N1,-
O8)(triphenylsilanolato)aluminum, 10 nm)/Alq3 (35 nm)/MgF2.
The most important parameters are compiled in Table 3 and
the voltage, current density, luminance characteristics and EL
spectra are displayed in Figures 11-13. The observed EL
spectra were almost identical with PL spectra, which means the
luminescences originated from the heterocycles and holes were
efficiently blocked by Salq to prevent Alq3 from emitting by
itself.
(33) Bender, C. O.; Bonnett, R. Chem. Commun. 1966, 7, 198.
(34) Malloy, B.; Reid, D. J. Chem. Soc. 1965, 4369.
(35) NMR of protonated compound 5. 1H NMR (500 MHz, CD2Cl2): δ 8.60
(dd, J ) 8.0, 7.8 Hz, 1H), 8.38 (d, J ) 8.0 Hz, 1H), 8.29 (ddd, J ) 7.6,
1.6, 0.9 Hz, 1H), 8.27 (ddd, J ) 7.6, 1.6, 0.9 Hz, 1H), 8.13 (d, J ) 7.8 Hz,
1H), 7.93 (td, J ) 7.6, 1.6 Hz, 1H), 7.91 (td, J ) 7.6, 1.6 Hz, 1H), 7.56 (s,
1H), 4.77 (s, 2H). 13C NMR (125 MHz, CD2Cl2): δ 143.2, 133.3, 132.1,
124.7, 124.4, 124.3, 120.4, 118.2, 44.8.
(36) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.;
Mallard, W. G. In Ion Energetics Data; Mallard, W. G., Linstrom, P. J.,
Eds.; NIST Chemistry WebBook, NIST Standard Reference Database No.
69, July 2001; National Institute of Standards and Technology: Gaithers-
burg, MD, 2001 (http://webbook.nist.gov).
(38) The pairs of (1,3-H), (6, 9-H), and (7,8-H) in protonated compound 5 are
changeable.
(39) Sato, Y.; Ogata, T.; Ichinosawa, S.; Murata, Y. Synth. Met. 1997, 91, 103.
(37) Notario, R.; Abboud, J.-L. M. J. Phys. Chem. A 1998, 102, 5290.
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16799

A R T I C L E S
Mitsumori et al.
the solid states. As for electronic properties, we could interpret
the interesting 16π ) 10π + 6π conjugation system from X-ray
structure and DFT calculations. An NMR-based protonation
study revealed the indolizino-isoindole was protonated at the
2-carbon.
Further research, including incorporation as transition metal
chelates, is in progress in our laboratory.
Experimental Section
Synthesis of Pyrido[2,1-a]isoindole (1). This compound was
prepared according to a modified literature procedure.10,11 First 2-bro-
mopyridine was allowed to react with benzyl bromide to afford
1-benzyl-2-bromopyridinium bromide.10 Then this salt (in 2% of
hydrobromic acid) was irradiated with a halogen lamp to afford 6H-
pyrido[2,1-a]isoindolylium bromide. The resulting bromide was aro-
matized by aqueous Na2CO3 to afford pyrido[2,1-a]isoindole (1)11
quantitatively, which was used freshly for the next steps.
Figure 12. Applied field vs current density characteristics of compounds
3, 6, 7, and 13.
Indolizino[3,4,5-ab]isoindole-1,2-dicarboxylic Acid Dimethyl Es-
ter (2). This compound was prepared according to the literature
method.9a Recrystallization from CH2Cl2/hexane afforded a fine yellow
solid in 65% yield. mp 182.0-183.0 °C (lit. 184-185 °C,9a lit. 175
1
°C,9c lit. 190 °C 9b). H NMR (CDCl3): δ 8.46 (d, J ) 8.6 Hz, 1H),
8.43 (d, J ) 9.0 Hz, 1H), 8.35 (d, J ) 8.0 Hz, 1H), 8.07 (d, J ) 8.0
Hz, 1H), 8.21 (dd, J ) 9.0, 8.6 Hz, 1H), 7.74 (t, J ) 8.0 Hz, 1H), 7.62
(t, J ) 8.0 Hz, 1H), 4.17 (s, 3H), 4.04 (s, 3H). 13C NMR (CDCl3): δ
165.0, 164.4, 130.7, 130.1, 129.8, 129.6, 129.3, 125.6, 125.0, 124.7,
122.4, 122.1, 118.5, 118.3, 110.0, 109.0, 52.4, 51.6. MS (EI+): m/z
307.1 (M+). Anal. Calcd. for C18H13NO4: C, 70.35; H, 4.26; N, 4.56.
Found: C, 70.50; H, 4.30; N, 4.60. IR (DRIFT): ν (CdO) 1731, 1701
-1
cm
.
Figure 13. Applied field vs EL intensity characteristics of compounds 3,
6, 7, and 13.
Indolizino[3,4,5-ab]isoindole-2-carboxylic Acid Methyl Ester (3).
This adduct was prepared according to a procedure similar to that for
compound 2, using methyl acrylate instead of methyl maleate. In 50%
yield. mp 118.0-119.0 °C (lit. 119-120 °C 9b). 1H NMR (CDCl3): δ
8.48 (d, J ) 8.4 Hz, 1H), 8.38 (d, J ) 8.0 Hz, 1H), 8.19 (d, J ) 8.0
Hz, 1H), 8.11 (s, 1H), 8.09 (d, J ) 7.4 Hz, 1H), 7.88 (dd, J ) 8.4, 7.4
Hz, 1H), 7.82 (t, J ) 8.0 Hz, 1H), 7.71 (t, J ) 8.0 Hz, 1H), 4.03 (s,
3H). 13C NMR (CDCl3): δ. 165.8, 130.3, 130.2, 129.7, 129.5, 128.9,
124.3, 123.9, 122.7, 122.5, 120.2, 116.8, 114.4, 110.5, 108.9, 51.3.
MS (EI+): m/z 249.1 (M+). Anal. Calcd for C16H11NO2: C, 77.10; H,
4.45; N, 5.62. Found: C, 77.37; H, 4.43; N, 5.38. IR (DRIFT): ν (Cd
O) 1693 cm-1
Indolizino[3,4,5-ab]isoindole-1,2-dicarboxylic Acid (4). This com-
pound was prepared on the basis of a literature method.9c Indolizino-
[3,4,5-ab]isoindole-1,2-dicarboxylic acid dimethyl ester (2; 0.25 g, 0.82
mmol) was dissolved in THF (5 mL) and aqueous NaOH (10%; 10
mL) was added. The mixture was refluxed for 5 h. After the mixture
was cooled to room temperature, aqueous HCl (20%) was added until
pH < 3. A yellow precipitate was collected and washed with water (2
× 5 mL) and dried in vacuo to obtain a yellow solid (0.19 g, 0.68
mmol) in 84% yield. mp 293 °C (decomp). 1H NMR (CDCl3): δ 8.75
(d, J ) 8.1 Hz, 1H), 8.58 (d, J ) 8.1 Hz, 1H), 8.49 (d, J ) 8.5 Hz,
1H), 8.48 (d, J ) 7.4 Hz, 1H), 8.08 (dd, J ) 8.5, 7.4 Hz, 1H), 7.82 (t,
J ) 8.1 Hz, 1H), 7.71 (t, J ) 8.1 Hz, 1H). 13C NMR (CDCl3): δ
169.8, 165.7, 131.6, 130.6, 130.4, 130.3, 129.6, 127.9, 127.2, 123.9,
123.8, 119.6, 112.5, 107.3. MS (EI+): m/z 279.1 (M+). Anal. Calcd
for C16H9NO4: C, 68.82; H, 3.25; N, 5.02. Found: C, 68.40; H, 3.29;
N, 5.20. IR (DRIFT): ν (CdO) 1695 cm -1, ν (OH) 2300-3600 cm-1
Indolizino[3,4,5-ab]isoindole (5). This compound was prepared on
the basis of a literature method.9c Quinoline (30 mL), indolizino[3,4,5-
ab]isoindole-1,2-dicarboxylic acid (4; 0.30 g, 1.25 mmol), and copper
chromite (0.67 g, 3.75 mmol) were charged in a round-bottom flask,
and the mixture was stirred for 4 h at 210 °C under N2. After the mixture
was cooled to room temperature, CH2Cl2 (50 mL) was added and a
black inorganic solid was removed by filtration. The solvents were
It is interesting to note that the device based on 3 emits
beautiful blue light, based on its CIE (Commission Internationale
d’Eclairage) chromaticity coordinates (x, y: 0.165, 0.188) and
the spectra for all devices are relatively sharp, which suggests
the emitting colors are pure. The brightness of the device reached
as high as 104 cd/m2. The stability (color and brightness) under
the highest voltage was similar to that of Coumarin 30. The
performance (especially in efficiency) for all compounds appears
to be promising when compared to the blue-emitting devices
reported recently.40
Conclusion
On the basis of the indolizino-isoindole unit, 14 heterocycles
have been prepared, and their optical, electronic properties and
crystal structures have been studied. The heterocycles have blue
or green emission, and their quantum efficiency was found to
be as high as 90%. The brightness of the light-emitting device
reached as high as 104 cd/m2, and 3 emits “perfect” blue light.
We also found some heterocycles could become alternatives to
Alq3 because they have almost the same energy potential levels
and have high luminescence, not only in solution but also in
(40) (a) Hosokawa, C.; Higashi, H.; Nakamura, H.; Kusumoto, T. Appl. Phys.
Lett. 1995, 67, 3853. (b) Gao, Z.; Lee, C. S.; Bello, I.; Lee, S. T.; Chen,
R. M.; Luh, T. Y.; Shi, J.; Wang, C. W. Appl. Phys. Lett. 1999, 74, 865.
(c) Balasubramananiam, E.; Tao, Y. T.; Danel, A.; Tomasik, P. Chem.
Mater. 2000, 12, 2788. (d) Leusng, L.; Lo, W. Y.; So, S. K.; Lee, K. M.;
Choi, W. K. J. Am. Chem. Soc. 2000, 122, 5640. (e) Tao, Y. T.;
Balasubramananiam, E.; Danel, A.; Jarosz, B.; Tomasik, P. Chem. Mater.
2001, 13, 1207. (f) Ko, C. W.; Tao, Y. T.; Danel, A.; Kreminska, L.;
Tomasik, P. Chem. Mater. 2001, 13, 2441. (g) Thomas, K. R.; Lin, J. T.;
Tao, Y. T.; Ko, C. W. Chem. Mater. 2002, 14, 1354.
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16800 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Highly Fluorescent Indolizino[3,4,5-ab]isoindoles
A R T I C L E S
removed under vacuum, and the residue was chromatographed on silica
gel with CH2Cl2/hexane ) 1/1. Recrystallization from CH2Cl2/hexane
afforded a fine yellow solid (0.13 g, 0.68 mmol) in 54% yield. mp
instead of 6a. In 42% yield. mp 192.0-193.0 °C. 1H NMR (CD2Cl2):
δ 9.17 (d, J ) 8.3 Hz, 1H), 8.47 (dd, J ) 8.0, 0.9 Hz, 1H), 8.27 (d, J
) 8.9 Hz, 1H), 8.26 (s, 1H), 8.21 (d, J ) 8.7 Hz, 1H), 8.20 (d, J ) 7.3
Hz, 1H), 8.16 (d, J ) 8.4 Hz, 1H), 8.08, (d, J ) 8.7 Hz, 1H), 7.93 (dd,
J ) 8.4, 7.3 Hz, 1H), 7.82 (dd, J ) 8.0, 1.2 Hz, 1H), 7.76 (ddd, J )
8.0, 7.1, 1.0 Hz, 1H), 7.72 (ddd, J ) 8.3, 6.9, 1.2 Hz, 1H), 7.57 (ddd,
J ) 8.9, 7.1, 0.9 Hz, 1H), 7.47 (ddd, J ) 8.0, 6.9, 1.2 Hz, 1H). 13C
NMR (CD2Cl2): δ 155.4, 148.4, 135.7, 129.5, 129.2, 129.1, 128.7,
128.5, 128.0, 127.2, 126.1, 124.7, 123.1, 122.3, 122.2, 121.9, 120.7,
120.7, 119.4, 118.6, 118.5, 110.5, 108.9. MS (EI+): m/z 318.1 (M+).
Anal. Calcd for C23H14N2: C, 86.77; H, 4.43; N, 8.80. Found: C, 85.84;
H, 4.13; N, 8.79.
1
85.0-86.0 °C (lit. 75 °C 9c). H NMR (CDCl3): δ 8.45 (dt, J ) 8.0,
1.0 Hz, 1H), 8.21 (dt, J ) 8.1, 1.0 Hz, 1H), 8.12 (d, J ) 7.4 Hz, 1H),
8.08 (d, J ) 8.1 Hz, 1H), 7.68 (dd, J ) 8.1, 7.4 Hz, 1H), 7.67 (td, J
) 8.0, 1.0 Hz, 1H), 7.66 (d, J ) 4.1 Hz, 1H), 7.52 (td, J ) 8.0, 1.0
Hz, 1H), 7.18 (d, J ) 4.1 Hz, 1H). 13C NMR (CDCl3): δ 129.6, 128.8,
128.3, 127.9, 127.8, 122.4, 122.3, 121.9, 119.2, 119.1, 115.3, 112.6,
107.9, 105.5 MS (EI+): m/z 191.1 (M+). Anal. Calcd for C14H9N: C,
87.93; H, 4.74; N, 7.32. Found: C, 87.82; H, 4.57; N, 7.22.
2-Pyridin-2-yl-2a,3-dihydroindolizino[3,4,5-ab]isoindole (6a). Py-
rido[2,1-a]isoindole (1) (0.80 g, 4.8 mmol) was dissolved in degassed
ClCH2CH2Cl (50 mL). To this solution, 2-ethynylpyridine (0.59 g, 5.7
mmol) in ClCH2CH2Cl (10 mL) was added dropwise and the mixture
was refluxed for 6 h. The mixture was diluted with CH2Cl2 (20 mL),
washed with H2O (50 mL), and dried over Na2SO4. The solvent was
evaporated and the residue was chromatographed on silica gel with
hexane/CH2Cl2 ) 4/1. Recrystallization from CH2Cl2/hexane afforded
green crystals (0.70 g, 2.6 mmol) in 54% yield. mp 146.0-147.5 °C.
1H NMR (CDCl3): δ 8.52 (dd, J ) 6.5, 1.1 Hz, 1H), 7.63 (td, J ) 7.5,
1.9 Hz, 1H), 7.56 (dd, J ) 7.5, 1.0 Hz, 1H), 7.51 (dd, J ) 7.5, 1.1 Hz,
1H), 7.43 (dd, J ) 7.5, 0.9 Hz, 1H), 7.37 (td, J ) 7.5, 0.9 Hz, 1H),
7.31 (dd, J ) 9.0, 3.0 Hz, 1H), 7.21 (td, J ) 7.5, 1.1 Hz, 1H), 7.01
(ddd, J ) 7.5, 6.5, 1.0 Hz, 1H), 6.71 (s, 1H), 5.89 (ddd, J ) 9.0, 6.4,
2.5 Hz, 1H), 4.89 (dd, J ) 15.7, 6.4 Hz, 1H), 2.86 (dt, J ) 16.4, 6.4
Hz, 1H), 2.38 (dddd, J ) 15.7, 16.4, 3.0, 2.5 Hz, 1H). 13C NMR
(CDCl3): δ 155.7, 149.3, 145.3, 136.1, 134.8, 134.7, 128.3, 125.2,
124.3, 124.1, 123.1, 122.2, 120.6, 119.8, 119.5, 119.4, 98.8, 52.9, 27.3.
MS (EI+): m/z 270.1 (M+). Anal. Calcd for C19H14N2: C, 84.42; H,
5.22; N, 10.36. Found: C, 84.56; H, 5.03; N, 10.58.
2-Bromoindolizino[3,4,5-ab]isoindole (8). Indolizino[3,4,5-ab]isoin-
dole (5; 0.25 g, 1.3 mmol) was dissolved in distlled CH2Cl2 (10 mL).
To this solution Br2 (0.21 g, 1.3 mmol) in CH2Cl2 (5 mL) was added
dropwise at -10 °C, and the mixture was stirred for 1 h at 0 °C. The
solution was diluted with CH2Cl2 (50 mL), and aqueous NaHCO3 (50
mL) were added. The organic layer was extracted, washed with H2O
(2 × 50 mL), and dried over Na2SO4. The solvents were evaporated,
and the residue was chromatographed on silica gel with hexane.
Recrystallization from CH2Cl2/hexane afforded a yellow solid (0.20 g,
1
0.96 mmol) in 74% yield. mp 88.0-89.5 °C. H NMR (CD2Cl2): δ
8.45 (dd, J ) 8.1, 1.0 Hz, 1H), 8.18 (dd, J ) 8.1, 1.0 Hz, 1H), 8.14 (d,
J ) 7.3 Hz, 1H), 8.03 (d, J ) 8.4 Hz, 1H), 7.77 (dd, J ) 8.4, 7.3 Hz,
1H), 7.73 (td, J ) 8.1, 1.0 Hz, 1H), 7.69 (s, 1H), 7.54 (td, J ) 8.1, 1.0
Hz, 1H). 13C NMR (CD2Cl2): δ 129.3, 128.4, 128.3, 126.5, 123.2,
122.6, 121.2, 128.7, 120.2, 119.4, 114.1, 113.8, 108.8, 91.0. MS
(EI+): m/z 269.0 (M+). Anal. Calcd for C14H8BrN: C, 62.25; H, 2.99;
N, 5.19, Br, 29.58. Found: C, 62.01; H, 3.15; N, 4.81, Br, 4.81.
Methylene Bis(indolizino[3,4,5-ab]isoindol-2-yl)Benzene (9). Dry
THF (5 mL), indolizino[3,4,5-ab]isoindole (5; 0.17 g, 1.0 mmol),
benzaldehyde (75 mg, 0.75 mmol), and p-toluenesulfonic acid (5 mg)
was charged in a round-bottom flask, and the solution was stirred at
50 °C for 3 h. The mixture was diluted with Et2O (30 mL), and aqueous
NaHCO3 (30 mL) was added. The organic layer was extracted, washed
with H2O (2 × 50 mL), and dried over Na2SO4. The solvents were
evaporated, and the residue was chromatographed on alumina with
hexane. Recrystallization from CH2Cl2/hexane afforded a yellow solid
0.20 g (0.96 mmol) in 22% yield. mp 158 °C (decomp). 1H NMR (CD2-
Cl2): δ 8.45 (d, J ) 8.0 Hz, 2H), 8.12 (d, J ) 8.0 Hz, 2H), 8.11 (d, J
) 7.5 Hz, 2H), 7.67 (t, J ) 8.0 Hz, 2H), 7.66 (d, J ) 8.2 Hz, 2H),
7.59 (d, J ) 7.3 Hz, 2H), 7.56 (s, 1H), 7.51 (dd, J ) 8.2, 7.3 Hz, 2H),
7.50 (dd, J ) 7.5, 7.3 Hz, 2H), 7.36 (t, J ) 8.0 Hz, 2H), 7.30 (t, J )
7.3 Hz, 2H), 6.79 (s, 1H). MS (EI+): m/z 470.2 (M+). Anal. Calcd
for C35H22N2: C, 89.33; H, 4.71; N, 5.95. Found: C, 89.30; H, 4.32;
N, 5.20.
2-Pyridin-2-ylindolizino[3,4,5-ab]isoindole (6). Chlorobenzene (30
mL), 2-pyridin-2-yl-2a,3-dihydroindolizino[3,4,5-ab]isoindole (6a; 0.30
g, 1.1 mmol) and sulfur (0.50 g, 16 mmol) were charged in a round-
bottom flask, and the mixture was refluxed for 10 h. The solvent was
evaporated, and the residue was chromatographed on silica gel with
hexane/ethyl acetate ) 5/1. Recrystallization from CH2Cl2/hexane
afforded a fine yellow solid (0.17 g, 0.66 mmol) in 59% yield. mp
111.5-112.5 °C. 1H NMR (CD2Cl2): δ 8.84 (d, J ) 8.3 Hz, 1H), 8.68
(d, J ) 4.1 Hz, 1H), 8.44 (d, J ) 8.0 Hz, 1H), 8.24 (d, J ) 8.0 Hz,
1H), 8.14 (d, J ) 8.0 Hz, 1H), 8.14 (s, 1H), 7.89 (d, J ) 8.0 Hz, 1H),
7.84 (dd, J ) 8.0, 8.3 Hz, 1H), 7.75 (dd, J ) 8.0, 7.2 Hz, 1H), 7.73
(dd, J ) 8.0, 7.3 Hz, 1H), 7.55 (dd, J ) 8.0, 7.2 Hz, 1H), 7.11 (dd, J
) 7.2, 4.1 Hz, 1H). 13C NMR (CDCl3): δ 156.2, 149.9, 136.8, 130.2,
129.8, 129.2, 128.6, 127.7, 123.8, 123.7, 122.9, 122.0, 121.2, 120.2,
120.0, 119.9, 118.5, 110.5, 109.3. MS (EI+): m/z 268.1 (M+). Anal.
Calcd for C19H12N2: C, 85.05; H, 4.51; N, 10.44. Found: C, 85.00; H,
4.25; N, 10.43.
Benzo[1′,2′-1,2]indolizino[3,4,5-ab]isoindole (10). 2,2,6,6-Tetra-
methylpiperidine (1.41 g, 10 mmol) was dissolved in dry THF (10 mL),
and 2.5 M n-butyllithium solution in hexane (4 mL, 20 mmol) was
added dropwise at 0 °C. The solution turned yellow. The mixture was
stirred for 30 min at 0 °C (solution A). Dry THF (60 mL), pyrido-
[2,1-a]isoindole (1; 1.25 g, 7.5 mmol), and bromobenzene (0.63 g, 4.0
mmol) were charged in another round-bottom flask, and solution A
was added dropwise at -78 °C under N2 to give a black solution. After
stirring 1 h at -78 °C, the solution was diluted with Et2O (20 mL),
washed with aqueous NH4Cl (20 mL), washed with H2O (20 mL), and
dried over Na2SO4. The solvent was evaporated, and the black residue
was chromatographed on silica gel with hexane/CH2Cl2 ) 4/1.
Recrystallization from CH2Cl2/hexane afforded an orange solid (60 mg,
0.25 mmol) in 6.6% yield. mp 169.0-171.0 °C (lit. 175-176 °C 9i).
1H NMR (CD2Cl2): δ 8.57 (dt, 8.2, 1 Hz, 2H), 8.56 (d, 7.8 Hz, 2H),
8.42 (dt, 8.2, 1.0 Hz, 2H), 7.90 (t, 7.8 Hz, 1H), 7.76 (ddd, 8.1, 7.0, 1.0
Hz, 2H), 7.50 (ddd, 8.1, 7.0, 1.0 Hz, 2H). MS (EI+): m/z 241.1 (M+).
Anal. Calcd for C18H11N: C, 89.60; H, 4.60; N, 5.81. Found: C, 89.83;
H, 4.87; N, 5.85.
2-Quinolin-2-yl-2a,3-dihydroindolizino[3,4,5-ab]isoindole (7a).
This compound was prepared in a procedure similar to that of compound
6a, using 2-ethynylquinoline instead of 2-ethybylpyridine (prepared
according to the literature method41). In 57% yield. mp 148.0-150.0
°C. 1H NMR (CDCl3): δ 8.20 (d, J ) 8.2 Hz, 1H), 7.99 (dd, J ) 8.0,
1.0 Hz, 1H), 7.76 (dd, J ) 8.0, 1.0 Hz, 1H), 7.75 (d, J ) 8.4 Hz, 1H),
7.71 (d, J ) 8.2 Hz, 1H), 7.66 (dd, J ) 8.4, 7.6 Hz, 1H), 7.60 (d, J )
7.6 Hz, 1H), 7.56 (dd, J ) 8.9, 3.0 Hz, 1H), 7.45 (td, J ) 8.0, 1.0 Hz,
2H), 7.39 (td, 7.6, 1.3 Hz, 1H), 6.87 (s, 1H), 5.98 (ddd, J ) 8.9, 6.5,
2.4 Hz, 1H), 4.95 (dd, J ) 15.8, 6.5 Hz, 1H), 2.90 (dt, J ) 16.3, 6.5
Hz, 1H), 2.43 (dddd, J ) 16.3, 15.8, 3.1, 2.4 Hz, 1H). MS (EI+): m/z
320.1 (M+). Anal. Calcd for C23H16N2: C, 86.22; H, 5.03; N, 8.74.
Found: C, 86.04; H, 4.84; N, 8.40.
2-Quinolin-2-ylindolizino[3,4,5-ab]isoindole (7). This compound
was prepared in a procedure similar to that of compound 6, using 7a
(41) Sakamoto, T.; Shirakawa, M.; Kondo, Y.; Yamanaka, H. Synthesis 1983,
4, 313.
9
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A R T I C L E S
Mitsumori et al.
2,3-Hydropyridazino[4′,5′:1,2]indolizino[3,4,5-ab]isoindole-1,4-di-
one (11). Ethanol (20 mL), chlorobenzene (20 mL), indolizino[3,4,5-
ab]isoindole-1,2-dicarboxylic acid dimethyl ester (2; 0.31 g, 1.0 mmol),
and hydrazine (1 mL) were charged in a round-bottom flask, and the
mixture was refluxed for 5 days. The yellow precipitate was collected
and washed with CH2Cl2 (10 mL), followed by ethanol (10 mL), and
dried in vacuo to obtain a yellow solid (0.24 g, 0.87 mmol) in 87%
yield. mp 346 °C (decomp). 1H NMR (DMSO- d6): δ 8.80 (d, J ) 7.4
Hz, 1H), 8.75 (dd, J ) 8.0, 0.9 Hz, 1H), 8.66 (d, J ) 8.2 Hz, 1H),
8.54 (dd, J ) 8.0, 0.9 Hz, 1H), 8.18 (td, J ) 8.0, 0.9 Hz, 1H), 7.89 (td,
J ) 8.0, 0.9 Hz, 1H), 7.71 (td, J ) 8.0, 0.9 Hz, 1H) 6.5-5.5 (br, 2H).
13C NMR (DMSO- d6): δ 147.8, 129.7, 129.4, 129.1, 125.1, 124.3,
123.9, 121.5, 117.6, 113.8. HRMS. Calcd for C16H9N3O2: 275.0695.
Found: 275.0690. IR: ν (NH) 2300-3500 cm-1, ν (CdO) 1651, 1660
cm-1
Na2SO4. The solvent was evaporated, and the residue was chromato-
graphed on alumina with CH2Cl2:hexane ) 1:1. Recrystallization from
CH2Cl2/hexane afforded a fine yellow solid (160 mg, 0.30 mmol) in
55% yield. mp 317 °C (decomp). Tg 300 °C.42 1H NMR (CD2Cl2): δ
8.48 (d, J ) 8.0 Hz, 1H), 8.20 (d, J ) 7.3 Hz, 1H), 8.06 (d, J ) 8.3
Hz, 1H), 7.99(d, J ) 8.1 Hz), 7.77 (dd, J ) 8.3, 7.3 Hz), 7.67 (t, J )
8.1 Hz 1H), 7.45-7.55 (m, 9H), 7.16-7.26 (m, 12H). MS (EI+): m/z
537.2 (M+). Anal. Calcd for C40H27NO: C, 89.36; H, 5.06; N, 2.61.
Found: C, 89.00; H, 5.45; N, 2.49.
Cyclic Voltammetry Measurement. Cyclic voltammetry (CV) was
performed with a three-electrode cell in an acetonitrile solution of 0.1
M tetrabutylammonium hexafluorophosphate (Bu4NPF6) at a scan rate
of 100 mV/s. A Pt wire was used as a counter electrode, and glassy
carbon was used as working electrode and a Ag/AgNO3 (0.1 M)
electrode was used as a reference electrode. Its potential was corrected
to the saturated calomel electrode (SCE) by measuring the ferrocene/
ferrocenium couple in this system (0.44 V versus SCE).43
Computational Methods. The GAUSSIAN 9844 series of programs
was used for all calculations. All molecules were fully optimized using
the hybrid density functional45 B3LYP level46 of theory with the 6-31G-
(d) basis set. All optimized structures were found to be planar by
frequency analysis. Proton affinities were evaluated at B3LYP/6-
311+G(2d,p) level corrected for unscaled zero point vibrational energies
(ZPVE) at B3LYP/6-31G(d) level using the B3LYP/6-31G(d) optimized
geometries. Vertical ionization potential was calculated at B3LYP/6-
311+G(2d,p) level using the B3LYP/6-31G(d) optimized geometry.
Reorganization energy was calculated as an energy difference between
cation radical, keeping geometry frozen to neutral molecule, and relaxed
cation radical. Orbitals and normalized squares of the orbital coefficients
(C2) were calculated at MP2/6-31G(d) density using B3LYP/6-31G(d)
geometries.
X-ray Crystal Structure. Diffraction data were collected at low
temperature (100 K) with graphite-monochromatized Mo KR radiation
(λ ) 0.710 73 Å). The cell parameters were obtained from the least-
squares refinement of the spots using the SMART program. The
structure was solved by direct method using SHELXS-97, which
revealed the positions of all non-hydrogen atoms. This was followed
by several cycles of full-matrix least-squares refinement. Absorption
corrections were applied by using SADABS.47 Hydrogen atoms were
included as fixed contributors to the final refinement cycles. The
positions of hydrogen atoms were calculated on the basis of idealized
geometry. In the final refinement, all non-hydrogen atoms were refined
with anisotropic thermal coefficients.
1,4-Dichloropyridazino[4′,5′:1,2]indolizino[3,4,5-ab]isoindoles (12).
Phosphorus oxychloride (3 mL), 2,3-hydropyridazino[4′,5′:1,2]in-
dolizino[3,4,5-ab]isoindole-1,4-dione (11) (0.20 g, 0.73 mmol), PCl5
(0.30 g, 1.45 mmol), and a drop of dimethylformamide (DMF) were
charged in a round-bottom flask. The solution was stirred at 130 °C
for 4 h, whereupon an orange solid precipitated. The mixture was cooled
to room temperature, and POCl3 was evaporated in vacuo. The residue
was washed with CH2Cl2 (20 mL), followed by EtOH (20 mL), and
dried in vacuo to obtain a brown solid (0.20 g, 0.63 mmol) in 86%
yield. mp 153 °C (decomp). 1H NMR (DMSO-d6): δ 9.09 (d, J ) 7.7
Hz, 1H), 9.01 (d, J ) 8.3 Hz, 1H), 8.79 (d, J ) 8.0 Hz, 1H), 8.71 (d,
J ) 8.0 Hz, 1H), 8.41 (t, J ) 8.0 Hz, 1H), 7.90 (dd, J ) 8.3, 7.7 Hz,
1H), 7.75 (t, J ) 8.0 Hz, 1H). HRMS. Calcd for C16H7Cl2N3: 311.0017.
Found: 311.0012.
1,4-Diphenoxypyridazino[4′,5′:1,2]indolizino[3,4,5-ab]isoin-
doles (13). Phenol (113 mg, 1.2 mmol), 1,4-dichloropyridazino[4′,5′:
1,2]indolizino[3,4,5-ab]isoindoles (12) (94 mg, 0.30 mmol), K2CO3 (300
mg), and DMF (5 mL) were charged in a round-bottom flask, and the
mixture was refluxed for 3 days. The mixture was diluted with Et2O
(200 mL), washed with H2O (40 mL), and dried over Na2SO4. The
solvent was evaporated, and the residue was chromatographed on
alumina with CH2Cl2. Recrystallization from CH2Cl2/hexane afforded
an orange solid (70 mg, 0.16 mmol) in 55% yield. mp 224.0-224.5
1
°C. H NMR (CDCl3): δ 8.78 (d, J ) 8.1 Hz, 1H), 8.64 (d, J ) 7.5
Hz, 1H), 8.63 (dd, J ) 8.0, 1.0 Hz, 1H), 8.60 (dd, J ) 8.0, 1.0 Hz,
1H), 8.16 (dd, J ) 8.0, 7.5 Hz, 1H), 7.83 (td, J ) 8.1, 1.0 Hz, 1H),
7.68 (td, J ) 8.1, 1.0 Hz, 1H), 7.41-7.53 (m, 8H), 7.24-7.31 (m,
2H). MS (EI+): m/z 427.1 (M+). Anal. Calcd for C28H17N3O2: C,
78.68; H, 4.01; N, 9.83. Found: C, 78.67; H, 4.09; N, 9.82.
Fabrication of Light-Emitting Devices. The ITO layer on glass
substrate was 120 nm thick with a sheet resistance of about 20 Ω/.
The stripe pattern with 2 mm wide of the ITO layer was etched by
means of conventional photolithographic technique. Prior to the organic
1,2-Bis(hydroxydiphenylmethyl)indolizino[3,4,5-ab]isoindole (14).
Indolizino[3,4,5-ab]isoindole-1,2-dicarboxylic acid dimethyl ester (2;
100 mg, 0.326 mmol) was dissolved in dry THF (20 mL). A 1.8 M
phenyllithium solution in cyclohexane (0.80 mL, 1.43 mmol) was added
dropwise at -75 °C under N2 and stirred for 4 h, whereupon the solution
turned yellow. The mixture was diluted with Et2O (100 mL), washed
with aqueous NH4Cl (40 mL), then H2O (40 mL), and dried over Na2-
SO4. The solvent was evaporated, and the residue was chromatographed
on alumina with hexane/ethyl acetate ) 1:1. Recrystallization from
CH2Cl2/hexane afforded a yellow solid (110 mg, 0.198 mmol) in 61%
(42) This glass transition temperature is considerably high, which means this
compound could have high thermal stability in a device. The ball-shape
structure could be a good methodology for high Tg compounds.
(43) Clerac, R.; Cotton, F. A.; Dunbar, K. R.; Lu, T.; Murillo, C. A.; Wang, X.
J. Am. Chem. Soc. 2000, 122, 2272.
(44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega, N.; Salvador,
P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B.
B.; Liu, G. Liashenko, A. Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M.
W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople,
J. A. Gaussian 98, Revision A.11.4; Gaussian, Inc.: Pittsburgh, PA, 2002.
(45) (a) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989. (b) Koch, W.;
Holthausen M. C. A Chemist’s Guide to Density Functional Theory; Wiley-
VCH: Weinheim, Germany, 2000.
1
yield. mp 284 °C (decomp). H NMR (CD2Cl2): δ 8.33 (d, J ) 7.5
Hz, 1H), 7.97 (d, J ) 7.3 Hz, 1H), 7.15-7.40 (m, 24H), 7.14 (dd, J )
7.2, 1.0 Hz, 1H), 6.01(t, J ) 8.4 Hz), 6.00 (t, J ) 8.4 Hz), 5.60 (s,
1H), 3.93 (s, 1H). MS (EI+): m/z 555.2 (M+), 538.2 (M+ - OH). Anal.
Calcd for C40H29NO2: C, 86.46; H, 5.26; N, 2.52. Found: C, 86.36;
H, 5.22; N, 2.49. IR (DRIFT): ν (OH) 2800-3400 cm-1
1,1,3,3-Tetraphenyl-1,3-dihydrofuro[3′,4′:1,2]indolizino[3,4,5-ab]-
isoindole (15). 1,2-Bis(hydroxydiphenylmethyl)indolizino[3,4,5-ab]-
isoindole (14; 300 mg, 0.540 mmol) was dissolved in THF (100 mL)
and acetic acid (10 mL) was added dropwise to this solution. The
mixture was stirred at 80 °C for 4 h. The resulting mixture was diluted
with CH2Cl2 (100 mL), washed with H2O (2 × 50 mL), and dried over
(46) (a) Lee, C.; Yang W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b) Becke,
A. D. J. Chem. Phys. 1993, 98, 5648.
(47) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
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16802 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Highly Fluorescent Indolizino[3,4,5-ab]isoindoles
A R T I C L E S
deposition, the patterned ITO glass was ultrasonically cleaned with
detergent, rinsed in water, and finished with the UV-ozone method.
The hole-blocking material (SAlq) was synthesized according to
literature procedures48 and sublimed twice prior to use. All the other
materials were purified by sublimation method prior to use. Evaporation
was carried out at a pressure of about 2 × 10-6 Torr. The deposition
rate of each organic layer was typically 0.1-0.3 nm/s. After the
deposition of the organic layers, the bilayer cathode was successively
deposited with a metal shadow mask, which defines a 2 mm stripe
cathode pattern perpendicular to the ITO anode stripe. The thickness
of the MgF2 insulating layer was 1.5 nm, capped with an aluminum
layer of 80 nm.
The luminance of the EL cell was measured with a luminance meter
(Minolta LS-110) and the I-V characteristics were measured by an
electrometer (HP 4140B). These measurements were all carried out in
an ambient atmosphere. The electroluminescence spectra of the devices
were measured by a multichannel photodiode array system (Otsuka
Electronics MCPD-2000).
Acknowledgment. We are grateful to Dr. S. I. Khan for X-ray
crystallographic analysis (Department of Chemistry and Bio-
chemistry, UCLA). We are grateful to Mitsubishi Chemical
Corp. for support of T.M.
Supporting Information Available: General experimental
procedure, crystal data, atomic coordinates, bond length, bond
angles, anisotropic displacement parameters, hydrogen coordi-
nates, isotropic displacement parameters, and torsion angles for
compounds 2, 6, and 7, absorption and fluorescence spectra of
all INIs synthesized as well as λmax of absorption spectra, λmax
of fluorescence spectra, and the Stokes shift, and absolute
energies and optimized geometries (in Cartesian coordinates)
for all calculated compounds at B3LYP/6-31G(d) (PDF, CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
(48) Moore, C. P.; Slyke, V.; Arland, S.; Gysling, H. J. Eur. Pat. Appl. No.
579151, 1996.
JA049214X
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