Novel 10,13-disubstituted dipyrido[3,2-a:2′,3′-c]phenazines and their platinum(II) complexes: Highly luminescent ICT-type fluorophores based on D-A-D structures

Article abstract of DOI:10.1016/j.tetlet.2014.07.069

Novel donor-acceptor-donor (D-A-D) π-conjugated molecules based on a dipyrido[3,2-a:2′,3′-c]phenazine (dppz) skeleton were synthesized, and their luminescent properties were investigated. Introduction of various aryl substituents to the 10- and 13-positio

Full text of DOI:10.1016/j.tetlet.2014.07.069

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Novel 10,13-disubstituted dipyrido[3,2-a:20,30-c]phenazines
and their platinum(II) complexes: highly luminescent ICT-type
fluorophores based on D–A–D structures
a
a
b
Tatsuya Shigehiro a, Shigeyuki Yagi a, , Takeshi Maeda , Hiroyuki Nakazumi , Hideki Fujiwara ,
⇑
Yoshiaki Sakurai c
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b Department of Chemistry, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
c Textile and Polymer Section, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel donor–acceptor–donor (D–A–D)
p-conjugated molecules based on a
dipyrido[3,2-a:20,30-c]
Received 12 June 2014
Revised 14 July 2014
Accepted 18 July 2014
Available online xxxx
phenazine (dppz) skeleton were synthesized, and their luminescent properties were investigated. Intro-
duction of various aryl substituents to the 10- and 13-positions of dppz allowed us to tune the emission
properties through modulation of the intramolecular charge transfer (ICT) character on the D–A–D
chromophores. Coordination of platinum(II) to the diimine site of dppz also gave rise to facilitation of
the ICT to induce a significant red shift of the emission.
Keywords:
Ó 2014 Elsevier Ltd. All rights reserved.
Dipyridophenazine
Platinum complex
Chromophore
Intramolecular charge transfer
Fluorescence
Luminescent molecules with organic and organometallic frame-
works have so far been developed, aimed at applications in various
fields such as biochemical and medicinal analyses,1,2 chemosenso-
ry systems,3–5 molecular logic gates,6 organic light-emitting
diodes,7,8 and so on. As traditional fluorescent dyes, coumarines,9
rohdamines,10 cyanines,11 and pyrene derivatives12 are representa-
tive examples, and these dyes are still useful for practical applica-
naphthalene diimide, 2,1,3-benzothiadiazole, and N-containing
heterocycles such as pyrazine and quinoxaline.16,20–23
In the present Letter, we report the synthesis and photolumi-
nescent properties of novel fluorescent compounds based on a
dipyrido[3,2-a:2030-c]phenazine (dppz) skeleton appended with
electron-donating side-arms at its 10- and 13-positions, as shown
in Figure 1. As dppz is an electron-deficient polycyclic compound,
it should exhibit a strong electron-withdrawing character. In addi-
tion, the diimine site of dppz allows for coordination to various
transition metal ions to yield stable metal–organic complexes with
tions. More recently, various types of linearly
p-conjugated
compounds, including -conjugated polymers such as poly(pheny-
p
lene)s,13 poly(phenylene-vinylene)s,14 and poly(phenylene-ethyn-
ylene)s,15 have been eagerly developed especially as emitting
materials for organic electronics, where critical tuning of HOMO
and LUMO levels is necessary to achieve optimized device perfor-
mance, along with color tuning for their purposes. Intramolecular
charge transfer (ICT)-type compounds, consisting of donor (D)
and acceptor (A) units, have often been reported, which allows
us to tune emission colors by adjusting the ICT character.16–19 Lots
special electronic properties based on d-
p perturbation. Indeed,
10
13
N
N
Electron-deficient
N-heterocycles
Electron-donating
side-arms
of electron-donating p-building blocks have been used to tune the
ICT; fluorenes,16 carbazoles,17 and so on. On the other hand, few
numbers of electron-withdrawing counterparts that serve as gen-
eral acceptor building blocks have been established except for
N
N
site
Metal-chelating
⇑
Corresponding author. Tel.: +81 72 254 9324; fax: +81 72 254 9910.
E-mail address: yagi@chem.osakafu-u.ac.jp (S. Yagi).
Figure 1. Molecular design of dppz-based ICT fluorophores.
http://dx.doi.org/10.1016/j.tetlet.2014.07.069
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Shigehiro, T.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.07.069

2
T. Shigehiro et al. / Tetrahedron Letters xxx (2014) xxx–xxx
dppz derivatives have often been used as metallo-chelating ligands
for DNA intercalators.24,25 However, very few examples of dppz-
based luminescent materials have been reported.26 So we here
demonstrate the development of novel highly emissive dppz-based
fluorophores. We also report how the coordination of platinum(II)
to the dppz fluorophore affected the emission properties through
electronic perturbation in the ICT chromophoric system.
The synthesis of dppz derivatives 1a–g and platinum(II) com-
plexes Pt-1e–g is shown in Scheme 1. First, 4,7-dibromo-2,1,3-ben-
zothiadiazole was converted to 4,7-disubstituted derivatives 2a–g
in excellent yields more than 80% except for 2c (39%) by the
Suzuki–Miyaura or the Sonogashira coupling reaction, followed
by reduction with LiAlH4 to obtain the diamines 3a–g in 59–88%
yields. The target compounds 1a–g were prepared in 2–53% yields
by condensation of 3a–g with 1,10-phenanthroline-5,6-dione. The
platinum(II) complexes Pt-1e–g were prepared by the reaction of
1e–g with K2PtCl4 in 19–72% yields. The X-ray crystallographic
analysis of the model compound of Pt-1e (i.e., Pt-1e0), that pos-
sesses 9,9-dimethylfluorenyl side-arms, revealed that platinum(II)
coordinated to the diimine site of the dppz skeleton (Fig. 2).
In Figure 3 are shown electronic absorption spectra of 1a–g in
dichloromethane at rt, and the spectral data are listed in Table 1.
The dialkylated dppz 1a exhibited absorption bands at 250–
Figure 2. ORTEP view of the molecular structure for Pt-1e0 (See Supplementary
data, Table S1 and Fig. S1).
1a
1b
1c
1d
1e
1f
15
10
5
3
2
1
0
330 nm (molar absorption coefficient
287 nm) and 330–450 nm (
abs = 12600 Mꢀ1 cmꢀ1 at 363 and
381 nm), assignable to
p⁄ and weak ICT transitions at the dppz
e
abs; 47900 Mꢀ1 cmꢀ1 at
e
400 450 500 550
Wavelength (nm)
1g
p–
chromophore, respectively. On the other hand, when the butyl
groups were replaced by the phenyls (i.e., 1b), the absorption
shoulder emerged at 400–450 nm. Furthermore, when extensively
0
250
300
350
400
450
500
550
600
Wavelength (nm)
Figure 3. UV–vis absorption spectra (10 lM) of 1a–g in dichloromethane at rt.
Inset: enlarged view at 380–550 nm.
S
S
N
N
cat. "Pd" a
N
N
Br
Br
R
R
,
R-B
2c
(for ),
pin
R-B(OH)2
or ethynylbenzene
2d
(for
)
p-conjugated aromatic components such as biphenylyl, phenyleth-
2a−g (39−97%)
ynyl, fluorenyl, carbazolyl, and bifluorenyl groups were introduced
at the 10- and 13-positions of dppz, broad absorption bands with
relatively small eabs (6500–18200 Mꢀ1 cmꢀ1) were additionally
observed for 1c–g in the longer wavelength regions (400–
530 nm), that are assignable to the transition bands based on the
ICT from the electron donating side-arms to the dppz core. The
assignment of the absorption bands to the ICT transitions was sup-
ported by TD-DFT calculations. In Figure 4 are shown the HOMO
and LUMO profiles of 1e and 1f. The HOMO–LUMO transition of
1a mainly occurs on dppz (See Supplementary data, Fig. S2),
O
O
R
R
H2N
NH2
R
N
N
N
N
N
N
LiAlH
4/THF
R
NEt3/EtOH
b
3a−g
(59−88%)
1a−g (2−53%)c
R
N
R
whereas, in the case of 1b–g, the HOMO is delocalized on the
p-
conjugation system including both side-arms, and the LUMO is
localized on the dppz moiety. These results clearly show that the
attachment of electron-donating side-arms to dppz yielded D–A–
D-type ICT chromophores, and thus the onset of the absorption
spectrum was comparable to the electron-donating ability of the
side-arms: a couple of side-arms as stronger donors gave a more
bathochromically shifted onset, that is a narrower optical band
gap Eg. The electrochemical properties, obtained by cyclic voltam-
metry (See Supplementary data, Fig. S3 and Table S2), clearly indi-
cated that the reduction of Eg in accordance with the increase in
the electron-donating ability was caused by both destabilization
of HOMO and stabilization of LUMO.
N
K
2PtCl4
Pt-1e (72%)
Pt-1f (19%)
Pt-1g (48%)
-H
/CHCl3 2O
1e or 1f or 1g
N
N
Pt
Cl
Cl
a: n-
:
c:
:
d
Bu
b Ph
Ph
CH(C )C
2 2 5 4
CH H9
H
C6H13
C6H13
C
H13
C6
6H13
N
Photoluminescence (PL) spectra of 1a–g in dichloromethane at
rt are shown in Figure 5, and the PL quantum yields (UPL) and
emission lifetimes (sPL) are also summarized in Table 1. Weak blue
e:
:
:
g
f
H
2
and bluish green emissions (UPL 6 0.10) were observed for 1a and
1b, respectively, indicating that the dppz without strong donor
side-arms is not so emissive. On the other hand, 1c–g exhibited
enhanced PL (UPL = 0.49–0.91), the emission maxima kPL of which
Scheme 1. Synthesis of 1a–g and Pt-1e–g. aThe detailed conditions are shown in
Supplementary data. bAs 3c was poorly soluble, it was used in the next step without
purification. So, the yield of 3c was not obtained. cThe yield of 1c (2%) was
determined as the one from 2c.
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T. Shigehiro et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 1
UV–vis absorption (10
l
M) and PL (1
l
M) data of 1a–g and Pt-1e–g in CH2Cl2 at rt
abs/L molꢀ1 cmꢀ1
Compound
kabs/nm (
e
)
Ega/eV
kPL (kex)/nm
UPL
s
PL/ns (v2
NDb
)
1a
1b
287 (47900), 363 (12600), 381 (12600)
297 (47900), 382 (13200)
3.11
2.73
2.63
2.52
2.48
2.45
2.42
2.28
2.21
2.20
472 (301)
505 (383)
522 (420)
522 (451)
569 (451)
579 (450)
592 (452)
623 (450)
650 (469)
655 (470)
0.009
0.10
0.68
0.91
0.84
0.49
0.78
0.45
0.066
0.24
1.79 (1.07)
11.2 (1.08)
11.5 (1.09)
13.7 (1.08)
9.34 (1.02)
9.61 (1.00)
11.2 (1.00)
4.43 (1.06)
5.57 (1.02)
1c
1d
1e
1f
301 (66100), 384 (11200), 422 (6500)
311 (56000), 395 (12600), 440 (10200)
318 (69200), 387 (9300), 441 (7600)
315 (75900), 386 (12000), 443 (9300)
344 (166000), 448 (18200)
1g
Pt-1e
Pt-1f
Pt-1g
318 (79400), 407 (11200), 475 (7800)
314 (77600), 404 (12600), 475 (8300)
346 (147900), 493 (11200)
a
Eg values were estimated from the onset of UV–vis absorption spectra.
Not determined due to too weak emission.
b
Table 2
PL data (1
l
M) of 1e in various solvents
Solvent (ET(30))
kPL (nm)
UPL
kr
(l
sꢀ1
)
knr (l )
sꢀ1
Hexane (31.0)
Toluene (33.9)
Acetone (42.2)
DMSO (45.1)
524
538
572
590
0.74
0.99
0.99
0.99
79.2
104
68.3
72.8
27.8
1.05
0.690
0.735
the dppz-based chromophores. PL profiles of 1e–g in various sol-
vents are shown in Figure S4 and Table S3 (Supplementary data),
and the data for 1e are excerpted in Table 2 from Table S3. As
the polarity of the solvent increased, positive solvatochromism of
PL was observed for each of 1e–g. For example, 1e exhibited drastic
PL spectral changes when the solvent polarity was varied: in hex-
ane, 1e exhibited green emission at 524 nm, whereas orange emis-
sion was observed at 590 nm in DMSO. On the other hand, the
absorption spectra were almost unchanged (See Supplementary
data, Fig. S4), indicating that these compounds are less polarized
in the ground state. Thus, the positive solvatochromism observed
for PL indicates that the present ICT-type chromophores are highly
polarized in the excited states. Indeed, the differences in dipole
Figure 4. Electron distributions of the HOMO and LUMO for 1e and 1f calculated at
the B3LYP/6-31G⁄ level of theory (implemented in Gaussian 09 package27). The
calculations were performed on the model structures in which the alkyl substit-
uents are replaced by methyl groups.
moments between the excited states and the ground ones (
were investigated for 1e–g by employing the Lippert–Mataga
plot28,29 (See Supplementary data, Fig. S5), and the
s for 1e–g
Dl)
1a
1b
1c
1d
1e
1f
D
l
were estimated to be 12.3, 14.6, and 16.8 D, respectively. This
result clearly shows that the increase in the electron-donating abil-
ity of the side-arms made the excited state more polarized. In
terms of UPL, a strong emission (UPL P 0.74) was observed for
1e, regardless of solvent polarities. In general, the increase in sol-
vent polarities made ICT-type chromophores less emissive because
significant facilitation of nonradiative decay was caused by solvent
relaxation.30,31 For 1e, however, the increase in the nonradiative
decay rate knr was suppressed even in highly polar solvents such
as acetone and DMSO (Table 2). Thus, 1e was still highly emissive
in these solvents (UPL = 0.99). The excellent luminescent properties
of 1e make itself potentially applicable to a fluorescent probe in
polar media.32 In the case of 1f, the nonradiative decay was facili-
tated with the increase in solvent polarity, as shown in Table S3.
1g
400
500
600
700
800
Wavelength (nm)
Figure 5. PL spectra (1
l
M) of 1a–g in CH2Cl2 at rt.
range at 522–592 nm. The introduction of biphenylyl (1c) and
phenylethynyl (1d) groups gave rise to green emission showing
the kPLs at 522 nm. Upon comparison between 1c and 1e, suppress-
ing the bond rotation of the biphenyl by the methano bridge led to
Taking the
Dl value of 1f into consideration, one can see that
the deteriorated PL of 1f in polar solvents is attributed to the more
polarized excited state in comparison with that of 1e. Thus, 1f
should be susceptible to emission quenching by solvent relaxation.
On the other hand, 1g exhibited relatively intense emission even in
a red shift of ca. 50 nm due to
p-extension of the side-arms. As
shown in 1g, further extension of the side-arms by additional
fluorenyl groups gave rise to a red shift at 592 nm, emitting strong
orange PL (UPL = 0.78). Hence, as is expected from the optical
properties, the increase in electron-donating ability of the
side-arms yielded red-shifted emission. It is worthy to note that
polar solvents despite the large
Dl value. Although the detailed
mechanism is still unclear, we found that the fluorene-derived
side-arms are effective to obtain highly emissive dppz derivatives
in wide range of solvents.
Taking advantage of metal-chelation ability of the diimine site,
the platinum(II) complexes Pt-1e–g were also prepared. These
the remarkable ICT is essential to obtain an enlarged UPL
.
ICT-based luminescence is often affected by solvent polarity.30–32
So, the solvent effect on the emission behavior was investigated for
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T. Shigehiro et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Supplementary data
Supplementary data (synthetic procedures, characterization
data including crystal structure analysis of Pt-1e0, results of DFT
calculations, electrochemical data, and optical and PL data in
various solvents) associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.tetlet.2014.07.069.
The supplementary crystallographic data for Pt-1e0 have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publications CCDC 1007550. Copies of the data
can be obtained free of charge by application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (e-mail: deposit@ccdc.cam.ac.uk).
Figure 6. UV–vis absorption (10 lM) and PL (1 lM) spectra of Pt-1e–g in
dichloromethane.
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This work was supported by a Grants-in-Aid for Scientific
Research on Innovative Areas ‘New Polymeric Materials Based on
Element-Blocks (No. 2401)’ (25102537) of The Ministry of Educa-
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Please cite this article in press as: Shigehiro, T.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.07.069