Photochemical synthesis of naphthacene and its derivatives for irreversible photo-responsive fluorescent molecules

Article abstract of DOI:10.1016/j.tetlet.2013.01.014

Highly fluorescent naphthacene derivatives and their photoconvertible precursors were synthesized for irreversibly photo-responsive fluorescent molecules. The fluorescence quantum yields (Φ_f) of the precursors were less than 0.02, and the precu

Full text of DOI:10.1016/j.tetlet.2013.01.014

Tetrahedron Letters 54 (2013) 1790–1793
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photochemical synthesis of naphthacene and its derivatives for irreversible
photo-responsive fluorescent molecules
Tatsuya Aotake a, Yuko Yamashita b, Tetsuo Okujima b, Nobuhiko Shirasawa c, Yukari Jo c,
Shigeo Fujimori c, Hidemitsu Uno b, Noboru Ono b, Hiroko Yamada a,d,
⇑
a Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192, Japan
b Department of Chemistry and Biology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
c Electronic and Imaging Materials Research Laboratories, Toray Industries, Inc., 3-2-1 Sonoyama, Ohtsu 520-0842, Japan
d CREST, JST, Chiyoda-ku 102-0075, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly fluorescent naphthacene derivatives and their photoconvertible precursors were synthesized for
irreversibly photo-responsive fluorescent molecules. The fluorescence quantum yields (Uf) of the precur-
sors were less than 0.02, and the precursors can be converted to the highly fluorescent naphthacene
derivatives (Uf = 0.67–0.70) quantitatively by photo-irradiation.
Received 20 November 2012
Revised 28 December 2012
Accepted 7 January 2013
Available online 12 January 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Naphthacene
Photoconversion
Fluorescent
Photoprecursor
Organic semiconducting molecules are very attractive for new
device materials such as organic light emitting diodes (OLEDs), or-
ganic field-effect transistors (OFETs), and organic photovoltaic cells
(OPVCs). Acenes and their derivatives are classic fluorescent mate-
rials and can be used to fabricate OLEDs.1 In this context aryl-
substituted acenes have been reported to improve the fluorescence
quantum yields effectively.2 In many cases, however, these acenes
are insoluble in common organic solvents due to their high crystal-
linity. To solve such problems, the soluble precursors with ther-
mally or photochemically removable leaving groups have been
mally or photochemically reversible reactions. As a result, the
reacted molecules could be returned to the starting compounds.
For the application of bioimaging and read-only memories, an irre-
versible system is desirable. Herein we report the synthesis and
spectroscopic properties of highly fluorescent naphthacene deriva-
tives, 1b and 1c (Scheme 1), and their quantitative photochemical
generation from their non-fluorescent
a-diketone precursors (2b,
and 2c), proposing irreversibly photo-responsive fluorescent
molecules.
The synthesis of the substituted naphthacenes 1b and 1c is
shown in Scheme 2. 1,10-Biphenyl-4-yllithium, which was pre-
pared from 4-phenyl-bromobenzenene and n-BuLi, was reacted
synthesized.3–5 The photochemical conversion of an
a-diketone
precursor into anthracene with the production of two CO mole-
cules has been known for a long time.6 This reaction has recently
been applied to the photochemical synthesis of pentacene and lar-
ger acences,7,8 which had been difficult to synthesize due to their
air-instability and low solubility in common organic solvents.
The methodology was also applied to the solution process to make
OFET devices without using vacuum deposition.9
Now we found this reaction is useful for the synthesis of highly
fluorescent materials in situ from non-fluorescent compounds
upon photoirradiation. Some photochromic molecules have been
proposed for photo-responsive fluorescent molecules.10,11 How-
ever, in many cases these photochromic molecules undergo ther-
⇑
Corresponding author. Tel.: +81 743 72 6041; fax: +81 743 72 6042.
E-mail address: hyamada@ms.naist.jp (H. Yamada).
Scheme 1. Photochemical conversions of naphthacenediketones.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.014

T. Aotake et al. / Tetrahedron Letters 54 (2013) 1790–1793
1791
Scheme 2. Reagents and conditions: (a) 1,10-biphenyl-4-yllithium for 4b and 3,5-diphenylphenyllithium for 4c, dry-toluene, dry-Et2O, ꢀ45 °C, 5 h; 4b: 52%; 4c: 29%; (b)
SnCl2, concd HCl, reflux, 2 h; 1b: 95%; 1c: 87%; (c) vinylene carbonate, xylene, in autoclave, 180 °C, 3 days; 5a: 95%; 5b: 92%; 5c: 78%; (d) 4 M NaOH aq, 1,4-dioxane, reflux,
2 h; 6a: 95%; 6b: 83%; 6c: 72%; (e) dry-DMSO, TFAA, DIPEA, dry-CH2Cl2, ꢀ60 °C, 1.5 h; 2a: 69%; 2b: 85%; 2c; 82%.
rescence lifetimes (
tively, and are 2.5 times longer than that of 1a (
s
f) of 1b and 1c were 9.4 and 9.6 ns, respec-
f = 3.9 ns)
s
(Fig. S1). These phenomena are similar to the relationship between
naphthacene (1a), 5,12-diphenylnaphthacene, and 5,6,11,12-tetra-
phenylnaphthacene (ruburene). The Uf’s of 1a, diphenylnaphtha-
cene and ruburene were 0.12, 0.85, and 0.98, respectively and
their fluorescence lifetimes in benzene or toluene were reported
as 4.2, 15.2, and 16 ns, respectively.14 The fluorescence spectra of
1a–c were also measured as shown in Figure S2. The quantum
yields of 1a–c in solid state were <0.01, 0.03, and 0.11, respectively.
The bulky substituents seemed to disturb the stacking of the com-
pounds in solid state.
The electrochemical properties of naphthacenes were investi-
gated by cyclic voltammetry (CV) in anhydrous dichloromethane
(Fig. 2 and Table 1). Reversible oxidation peaks were observed
for 1a (0.51 V vs Fc/Fc+), 1b (0.44 V), and 1c (0.47 V), and the reduc-
tion potential observed for 1b (ꢀ2.10 V) and 1c (ꢀ2.09 V). The
Figure 1. Absorption spectra (solid lines) and fluorescence spectra (dotted lines) of
1a (black), 1b (blue), and 1c (red) in toluene.
with 5,12-naphthacenequinone (3) to afford 4b in 52% yield. The
compound 4b was reacted with SnCl2 and concd HCl in refluxing
THF to give naphthacene 1b in 95% yield. Similarly 4c was prepared
by the reaction of 3 and 3,5-diphenylphenyllithium, which was
prepared from 1-iodo-3,5-diphenylbenzene,12,13 in 29% yield, and
HOMO levels or ionization potentials (IP) were calculated by the
ox
known equation IP ¼ E
+4.80.15 The HOMO of 1b (5.17 eV)
onset
and 1c (5.20 eV) are lower than non-substituted naphthacene 1a
by 0.06 and 0.03, respectively. The calculated HOMO energy levels
relative to naphthacene 1a are also summarized in Table 1
(Fig. S3).16
then converted to 1c in 87% yield. The synthesis of a-diketone pre-
cursors 2a–c is also shown in Scheme 2. The Diels–Alder reaction
of naphthacenes 1a–c with vinylene carbonate quantitatively gave
5a–c, respectively. Their hydrolysis followed by Swern oxidation
The UV–vis absorption spectra of
and 2c in toluene are shown in Figure 3. The absorption spectra
show n–p⁄ absorption of
-diketone moiety at 468 nm.
a-diketone precursors 2a, 2b,
a
gave
a-diketone compounds 2a–c. The solubility of 1a, 1b, and
The absolute fluorescence quantum yield (Uf) of 2a was not
detected. The Uf of 2b and 2c were 0.016 and 0.017, respectively,
in the beginning of the measurement, but U0f s were increased grad-
ually at every measurement. These results imply that the precur-
sors immediately reacted to release CO molecules when they
were excited for the measurement of fluorescence quantum yield
(kex = 462 nm), and fluorescence from the generated acenes were
gradually observed depending on the irradiation period.
1c in toluene were 0.15, 0.81, and 5.2 mg/mL, respectively, and
the solubility of the precursors 2a, 2b, and 2c were much improved
to 17, 15, and >23 mg/mL, respectively.
Absorption and fluorescence spectra of naphthacenes 1a–c in
toluene are summarized in Figure 1 and Table 1. The absorption
maxima of 1b and 1c in toluene were red-shifted by 20 nm in com-
parison with those of 1a. Fluorescence peaks of 1b (511 nm) and 1c
(506 nm) are red-shifted by 30 and 25 nm from that of 1a
(481 nm). The Stokes shifts of 1a–c are 6, 16, and 12 nm, respec-
tively, as shown in Table 1. The absolute fluorescence quantum
yields of naphthacenes were 0.67 for 1b and 0.70 for 1c, which
are more than 5 times larger than that of 1a (Uf = 0.12). The fluo-
To investigate the photoreaction of the compounds 2a–c in de-
tail, the photoreactions were monitored by 1H NMR spectroscopy
(Fig. 4).
a-Diketone precursor 2b was placed in degassed CDCl3
and irradiated with blue LED (k = 470 nm, 25 W/m2) under nitro-
gen atmosphere. During the photoreaction, the singlet peak of
Table 1
Optical and electrochemical characterization of naphthacenes
a
a
ox
ox
onset
(V)b
red
kabs (nm) (log
e)
kem (nm)
Uf
sf
(ns)
HOMOexp HOMOcal LUMOexp LUMOcal Eg
E
E
E
1=2
1=2
(eV)c
(eV)d
(eV)e
(eV)d
(eV)f
(V)b
(V)b
1a 396 (3.22), 419 (3.52), 445 (3.81), 475 (3.85) 481, 512, 551 0.12 3.9
1b 411 (3.42), 436 (3.84), 463 (4.15), 495 (4.20) 511, 541, 583 0.67 9.4
0.51 0.43
0.44 0.37
0.47 0.40
—
ꢀ5.23
ꢀ4.86
ꢀ4.76
ꢀ4.77
ꢀ2.66f
ꢀ2.75
ꢀ2.77
ꢀ2.08
ꢀ2.04
ꢀ2.05
2.57
2.42
2.43
ꢀ2.10 ꢀ5.17
ꢀ2.09 ꢀ5.20
1c
410 (3.43), 436 (3.87), 464 (4.19), 494 (4.24) 506, 539, 589 0.70 9.6
a
b
c
d
e
f
Recorded in toluene at room temperature.
The values were obtained by cyclic voltammetry. V versus Fc/Fc+. See Supplementary data for experimental details.
The values were obtained by ionization potentials in toluene.
The values were calculated at the B3LYP/6-31G(d) levels.
The values were obtained from HOMO levels and Eg.
The values were obtained from the edge of the absorption spectra in toluene.

1792
T. Aotake et al. / Tetrahedron Letters 54 (2013) 1790–1793
(a)
(b)
Figure 2. Cyclic voltammograms of 1b (a) and 1c; (b) in 0.1 M TBAPF6 in
acetonitrile. Scan rate 0.1 V/s. [Naphthacene] = 0.1 mM. WE: glassy carbon, CE: Pt,
RE: Ag/AgNO3.
Figure 5. (a) The UV–vis absorption spectra change of 2b upon photoirradiation in
toluene (1.02 ꢁ 10ꢀ4 M) under argon atmosphere (kex = 468 nm); (b) the fluores-
cence spectra change of 2b upon photoirradiation in toluene (3.48 ꢁ 10ꢀ5 M) under
argon atmosphere (kex = 468 nm).
photoconversion of a-diketone precursor 2c to the naphthacene 1c
gave similar results (Fig. S4).
The change of UV–vis spectra during the photoreaction of 2b in
toluene under argon atmosphere is shown in Figure 5a. Before irra-
diation, only the broad n–p⁄ peak at 465 nm was observed. During
the irradiation, the new peaks at 436, 463, and 495 nm assigned to
1b increased. The absorbance of naphthacene became constant
after 60 min irradiation. Judging from the observation of the isos-
bestic points at 334 and 390 nm, the photoreaction proceeded di-
rectly. Similarly, a-diketone precursors 2a and 2c were converted
to 1a and 1c as shown in Figure S5. The change of the fluorescence
spectra of 2b during the photoirradiation is shown in Figure 5b.
The colorless fluorescence of the solution gradually changed to
light green. Fluorescence intensity of 2b increases significantly
with no change of the emission maxima position. After irradiation
for 40 min, the fluorescence intensity stayed constant and the reac-
tion seemed to be finished. The similar change was observed for 2c,
as shown in Figure S6.
Figure 3. Absorption spectra of a-diketone 2a (black line), 2b (gray line), 2c (dotted
line).
bridgehead protons at 5.2 ppm gradually decreased while singlet
peak due to peri-position of 1b increased at 8.4 ppm. These results
suggested the photoreaction form 2b to 1b proceeded clearly. The
Figure 4. 1H NMR spectra during the photoreaction of 2b in CDCl3 under nitrogen atmosphere.

T. Aotake et al. / Tetrahedron Letters 54 (2013) 1790–1793
1793
ject in NAIST sponsored by the Ministry of Education, Culture,
Sports, Science and Technology, MEXT, Japan, and Adaptable and
Seamless Technology Transfer Program through Target-driven
R&D (A-STEP) (No. AS2111283D to S.F. and H.Y.) sponsored by Ja-
pan Science and Technology Agency.
(a)
Supplementary data
Supplementary data (synthetic detail, characterization, fluores-
cence decay curves, and spectral change of absorption and fluores-
cence during the photolysis in solution) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2013.01.014.
(b)
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ene solution. The fluorescence of 1b in the PMMA film was also ob-
served which indicated the compound 2b was fully converted to 1b
and the obtained acene compounds are dispersed in the PMMA
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In conclusions, we have succeeded in preparing the highly fluo-
rescent naphthacenes 1b and 1c and their precursors 2b and 2c.
The non-fluorescent precursors can be easily converted to highly
fluorescent naphthacenes (Uf = 0.67–0.70) by photo irradiation in
solution. This conversion can also be possible in PMMA matrix.
These new materials can be applicable in the field of OLED and
memory media as printable fluorescent materials.
Acknowledgments
We thank Division of Synthesis and Analysis, Department of
Molecular Science, Integrated Center for Sciences (INCS), Ehime
University for its help in using mass and NMR spectrometer. We
also thank Venture Business Laboratory, Ehime University, for its
help in using TOF-MS. This work was partially supported by
Grants-in-Aid (No. 22350083 to H.Y.) and the Green Photonics Pro-