Synthesis and photoluminescence properties of π-extended fluorene derivatives: the first example of a fluorescent solvatochromic nitro-group-containing dye with a high fluorescence quantum yield

Article abstract of DOI:10.1016/j.tetlet.2009.10.118

Functional π-extended fluorene derivatives, 2,7-di(p-substituted-phenyl)fluorenes containing different functional groups such as hydrogen, trimethylsilyl (TMS), methoxycarbonyl, cyano, and nitro groups, were synthesized. Except for the nitro group, the resulting compounds exhibited extremely high fluorescence quantum yields (Φ_F >0.85 in chloroform). The diphenylfluorene containing nitro groups have higher fluorescence quantum yield (Φ_F = 0.31 in N,N'-dimethyl-formamide) than other nitro-group-containing fluorophores which were previously reported (Φ_F <0.1). Furthermore, this compound exhibited large Stokes' shift with green to orange emission and unique on-off behavior of the emission by solvents.

Full text of DOI:10.1016/j.tetlet.2009.10.118

Tetrahedron Letters 51 (2010) 181–184
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and photoluminescence properties of -extended fluorene
p
derivatives: the first example of a fluorescent solvatochromic
nitro-group-containing dye with a high fluorescence quantum yield
b
Hidehiro Kotaka a, Gen-ichi Konishi a, , Kazuhiko Mizuno
*
a Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
b Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Functional
p-extended fluorene derivatives, 2,7-di(p-substituted-phenyl)fluorenes containing different
Received 25 September 2009
Revised 23 October 2009
Accepted 26 October 2009
Available online 30 October 2009
functional groups such as hydrogen, trimethylsilyl (TMS), methoxycarbonyl, cyano, and nitro groups,
were synthesized. Except for the nitro group, the resulting compounds exhibited extremely high fluores-
cence quantum yields (UF >0.85 in chloroform). The diphenylfluorene containing nitro groups have
higher fluorescence quantum yield (UF = 0.31 in N,N’-dimethyl-formamide) than other nitro-group-con-
taining fluorophores which were previously reported (UF <0.1). Furthermore, this compound exhibited
large Stokes’ shift with green to orange emission and unique on–off behavior of the emission by solvents.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Fluorene
Nitro group
Solvatochromism
Fluorescence quantum yield
Polyfluorene (PF) and its derivatives possess exceptional electro
optical characteristics and are thus essential components of organ-
ic light-emitting diodes (OLEDs).1–4 PF and its derivatives have
high solubility, good film-forming capability, and high lumines-
cence quantum yields in the blue region. However, PF generally
shows excimer emission (the formation of dimerized units in the
excited state that emit at lower energies) in the solid state; this af-
fects the bathochromic shift of the fluorescence wavelength and
the lifetime of light-emitting devices.5–7 Bifluorene, in contrast,
exhibits the same blue photoluminescence as that emitted by a
PF chromophore, in solution as well as in the solid state, although
it consists of only two fluorene moieties linked to each other.8–10
Therefore, in the case of bifluorene, high luminescence quantum
groups, remain relatively unknown. Moreover, to the best of our
knowledge, there have been no reports on nitro-group-containing
p-extended fluorophores exhibiting high fluorescence quantum
yields. Nitro-group-containing polycyclic hydrocarbons such as
fluorescein derivatives exhibit a maximum fluorescence quantum
yield of 0.07.16 In general, the fluorescence quantum yield of a fluo-
rophore containing nitro groups is extremely low because of the
decrease in the radiative rate and the increase in the internal con-
version rate of an excited state fluorophore.15–18 We observed that
attaching a nitro group directly to the fluorophore skeleton re-
sulted in a low fluorescence quantum yield. Therefore, fluorescent
solvatochromism in the case of a nitro-group-containing dye has
not been achieved. In order to obtain a fluorene derivative with
high fluorescence quantum yield, we attempted to introduce a ni-
tro group with a p-substituted phenyl group into the fluorophore
core.
yields can be obtained by a slight extension of
p-conjugation.
One method for obtaining fluorene derivatives with extended
p
-
conjugation involves the introduction of two benzene rings into
the 2,7-positions of the fluorene core. This is an extremely conve-
nient method for obtaining p-extended fluorene derivatives. p-Ex-
In this Letter, we report the synthesis of 2,7-diarylfluorenes
containing different functional groups, such as methoxycarbonyl,
tended fluorene derivatives containing different p-functionalized
phenyl groups11–15 have already been synthesized according to
this method. If carboxyl groups are present at the p-positions,11
a
p
-extended fluorene system can be used to synthesize polyester
or polyamide. However, the details of the photochemical behavior
of -extended fluorene derivatives, especially silyl and nitro
p
* Corresponding author. Tel.: +81 3 5734 2321; fax: +81 3 5734 2888.
E-mail address: konishi.g.aa@m.titech.ac.jp (G. Konishi).
Chart 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.10.118

182
H. Kotaka et al. / Tetrahedron Letters 51 (2010) 181–184
trimethylsilyl, cyano, and nitro groups (Chart 1). Our objective was
to investigate how the photoluminescence properties of a -ex-
p
tended fluorene system are affected by the substituents on the flu-
orene core. Moreover, we report the first example of a fluorescent
solvatochromic nitro-group-containing dye with high fluorescence
quantum yield.
The p-extended fluorenes 3a–e were successfully prepared by a
palladium-catalyzed Suzuki–Miyaura reaction19 of fluorene bis
boronic ester and iodobenzene derivatives (Scheme 1).20–23 The
p
-extended fluorenes were obtained in 64–98% yields. The struc-
tures of 3a–e were confirmed by 1H NMR spectra, FT-IR spectra,
mass spectrometry, and elemental analysis.
Scheme 1. Synthesis of p-extended fluorene derivatives (3a–e).
The UV–vis absorption, fluorescence spectra, and absolute
24
quantum yields (UF
)
of the p-extended fluorenes were obtained
25
in deaerated chloroform solution. The fluorescence lifetimes (sF
)
Table 1
Spectroscopic parameters of fluorene derivatives at room temperature
of 3a–e were measured in deaerated N,N-dimethylformamide
(DMF) because the highest fluorescence intensity of 3e was ob-
tained. The spectroscopic parameters of the fluorene derivatives
3a–e are shown in Table 1. Figure 1 shows the UV–vis absorption
and fluorescence spectra of compounds 3a–e. As it can be observed
in Figure 1a, the UV–vis absorption maxima of compounds 3a and
3b were almost the same. The UV–vis absorption maxima of 3c and
3d, which have electron-withdrawing groups, were red-shifted by
ca. 10 nm relative to that of 3a and the enormous red shift of 46 nm
eb
kF
UF
sF
a
c
d
e
Entry
Functional
group
kA
(nm)
(MÀ1 cmÀ1
)
(nm)
(ns)
3a
3b
3c
3d
3e
H
327
330
341
341
373
76,100
80,400
68,600
63,400
49,300
361, 377
368, 385
388, 405
385, 403
538
0.85
0.89
0.94
0.95
0.11
1.40
1.01
1.48
0.95
1.92
TMS
COOCH3
CN
NO2
a
Absorption wavelengths taken at the maximum of the absorption band in
chloroform.
was observed in 3e. The molar absorption coefficients (e) at the
b
Absorption coefficient at the maximum of the absorption band in chloroform.
Fluorescence wavelengths in chloroform.
Absolute quantum yields of 3a–e were measured in chloroform.
Fluorescence lifetime in N,N-dimethyl formamide.
maxima of the molar absorption bands of compounds 3a–d were
large. However, the molar absorption coefficient of 3e was the low-
est among the 2,7-diphenylfluorene derivatives. It is likely that the
UV–vis absorption spectrum of 3e was affected by spin-forbidden
charge transfer. The fluorescence spectra of 3a, excited at absorp-
tion maximum, showed two peaks at 361 and 371 nm (Fig. 1b).
The fluorescence maxima of 3c were red-shifted by ca. 20 nm rel-
ative to those of 3a. Therefore, 3c and 3d fluoresced with a bright
blue color in chloroform. These results suggest that the excited
state energies of 3c and 3d, which have more polar structures than
3a, might be lower than that of 3a in a slightly polar solvent such
as chloroform. The fluorescence quantum yield of 3a was 0.85;
therefore, a highly fluorescent fluorene chromophore can be de-
signed by introducing two benzene rings into the fluorene core,
as described in an earlier paper.12 The fluorescence quantum yield
c
d
e
of 3a is higher than that of the other known
(
p-conjugated PFs
UF = 0.55–0.79).26,27 The fluorescence quantum yield of 3b con-
taining TMS groups was high (UF = 0.89), which is corresponded
to the fact that the TMS-group-containing fluorophore shows high
fluorescence quantum yield.28 Furthermore, the absolute fluores-
cence quantum yields of 3c and 3d were 0.94 and 0.95, respec-
tively. Derivatives 3c12 and 3d, which have electron-withdrawing
groups, exhibited high fluorescence quantum yields. This implies
that most of the photons absorbed by compounds 3c and 3d were
emitted exclusively as fluorescence. The fluorescence maximum of
3e was red-shifted by more than 100 nm relative to the fluores-
cence maxima of the other p-extended fluorenes, and the emission
band of 3e in the long-wavelength region was barely discernible as
Table 2
Spectroscopic parameters of 3e in various solvents at room temperature
Solvent
kA
(nm)
e
kF
(nm)
UF
sF
(ns)
Dielectric
constanta
(MÀ1 cmÀ1
)
Cyclohexane 357
50,000
48,200
49,300
45,700
45,100
42,100
N.D.b
N.D.b
538
N.D.b N.D.b 2.0
Toluene
Chloroform
EtOH
365
373
368
380
382
N.D.b N.D.b 2.4
0.11
1.18
4.8
N.D.b
569
N.D.b N.D.b 24.5
DMF
0.31
0.28
1.92
1.82
36.7
46.7
DMSO
589
Figure 1. (a) UV–vis absorption and (b) fluorescence spectra of 3a–e ([3a–
e] = 1.0 Â 10À5 M) in deaerated chloroform. Inset: detail of the longer-wavelength
region.
a
10
Data from Ref.
.
b
N.D.: Not determined because of no emission.

H. Kotaka et al. / Tetrahedron Letters 51 (2010) 181–184
183
compared to the emission bands of 3a–d. The fluorescence life-
times of these fluorene derivatives were approximately 1 ns; the
short lifetime is responsible for the high fluorescence quantum
yields in these molecules.12,28 Here, it should be noted that the
fluorescence quantum yield of 3e (0.11) is lower than the yields
were measured in several polar and non-polar media such as cyclo-
hexane, toluene, chloroform, ethanol (EtOH), DMF, and dimethyl
sulfoxide (DMSO). The photophysical properties of 3e are listed
in Table 2. Figure 2 shows the UV–vis absorption and fluorescence
spectra of 3e in various solvents. The UV–vis absorption intensities
of 3e in various solvents were almost the same. However, the UV–
vis absorption maxima were distinctly red-shifted when solvents
with high dielectric constants were used.29 The UV–vis absorption
maximum of compound 3e in DMSO was red-shifted by 25 nm rel-
ative to that of 3e in cyclohexane. These drastic shifts of the UV–vis
absorption wavelengths might be affected due to the charge trans-
fer molecule whose energy in grand state changes by solvents.
Fluorescence emission of 3e was observed in chloroform, DMF,
and DMSO. However, no emission was observed in cyclohexane,
toluene, and EtOH. The fluorescence maximum of 3e in DMSO
was red-shifted by 51 nm relative to that of 3e in chloroform.
The emission wavelengths for this molecule range from yellow to
orange region (Fig. 2b). The fluorescence quantum yield of 3e in
DMF (0.31) was the highest in three solvents, which is an extre-
mely high value for a nitro-group-containing fluorophore. This
high value is most certainly a result of the introduction of a phenyl
group between the fluorescence core (fluorene skeleton) and the
nitro groups. The fluorescence lifetimes of 3e in three different sol-
vents were correlated with its fluorescence quantum yields in each
solvent. Thus, it can be inferred that the absorption and fluores-
cence maxima are controlled by the solvent. Further, the on–off
behavior of the emission is controlled by the solvent. The on–off
behavior of the emission of 3e might be attributed to the reorgani-
zation of the fluorene and p-functionalized phenyl units into
of the other
p-extended fluorenes. However, this fluorescence
quantum yield of 3e is exceptionally higher than the yields of other
general nitro-group-containing fluorophores (UF <0.01).17,18
In order to examine the solvent sensitivity of the unique UV–vis
absorption and emission properties of nitro-group-containing
p-
extended fluorene, the UV–vis absorption and fluorescence spectra
a
highly twisted charge transfer excited state conformation
(Chart 2), analogous to a twisted intramolecular charge transfer
(TICT) model.30,31 Long-wavelength emission, which might be
assigned to the emission from the charge transfer state, was
observed in a polar solvent, as suggested by the TICT model. The
detailed assignment of the on–off emission and the changes of
fluorescence intensities or fluorescence lifetimes are now in progress.
In conclusion, p-extended fluorenes containing different functional
groups were successfully synthesized by means of the Suzuki–
Miyaura cross-coupling reaction. Except for the nitro-group-
containing
p-extended fluorene system, all other derivatives
showed intense blue light emission. Fluorene derivatives contain-
ing cyano and methoxycarbonyl groups had high fluorescence
quantum yields.
p-Extended fluorene systems containing nitro
groups exhibited unique on–off emission that was dependent on
the solvent. Furthermore, the absolute fluorescence quantum yield
of compound 3e is extremely higher than that of the other well-
known nitro-group-containing fluorophore. Owing to their excel-
lent emission properties,
can be used as luminescent sensor materials.
p-conjugated donor–acceptor molecules
Figure 2. (a) UV–vis absorption and (b) fluorescence spectra of 3e
([3e] = 1.0 Â 10À5 M) in various solvents.
Chart 2.

184
H. Kotaka et al. / Tetrahedron Letters 51 (2010) 181–184
21. Compound 3b (–Si(CH3)3) yield 64%; pale yellow oil; 1H NMR (400 MHz, CDCl3)
References and notes
d 7.77 (d, J = 7.8 Hz, 2H), 7.68–7.56 (m, 12H), 2.04–2.00 (m, 4H), 1.19–1.05 (m,
20H), 0.77 (t, J = 7.1 Hz, 6H), 0.74–0.72 (m, 4H), 0.32 (s, 18H) ppm; 13C NMR
(100 MHz, CDCl3) d 151.7, 142.1, 140.2, 140.0, 139.0, 133.8, 126.5, 126.0, 121.6,
120.0, 55.2, 40.4, 31.8, 30.0, 29.1, 23.8, 22.6, 14.1, À1.1 ppm; FT-IR (NaCl) 2926,
2854, 1598, 1465 cmÀ1; MS (EI) calcd for C47H66Si2 686.4703, found 686.4696
(M+).
1. Fukuda, M.; Sawada, K.; Yoshino, K. Jpn. J. Appl. Phys. 1989, 28, 1433–1435.
2. Leclerc, M. J. Polym. Sci., Polym. Chem. Ed. 2001, 39, 2867–2873.
3. Jo, J.; Chi, C.; Höger, S.; Wegner, G.; Yoon, D. Y. Chem. Eur. J. 2004, 10, 2681–
2688.
4. Morisaki, Y.; Chujo, Y. Bull. Chem. Soc. Jpn. 2005, 78, 288–293.
5. Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 118, 7416–7417.
6. Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran, M.; Wu, W. W.; Woo,
E. P. Appl. Phys. Lett. 1998, 73, 629–631.
7. Sainova, D.; Miteva, T.; Nothofer, H. G.; Scherf, U.; Glowacki, I.; Ulanski, J.;
Fujikawa, H.; Neher, D. Appl. Phys. Lett. 2000, 76, 1810–1812.
8. Grisorio, R.; Dell’Aquila, A.; Romanazzi, G.; Suranna, G. P.; Mastrorilli, P.;
Cosma, P.; Acierno, D.; Amendola, E.; Ciccarella, G.; Nobile, C. F. Tetrahedron
2006, 62, 627–634.
9. Grisorio, R.; Piliego, C.; Fini, P.; Cosma, P.; Mastrorilli, P.; Gigli, G.; Suranna, G.
P.; Nobile, C. F. J. Phys. Chem. C 2008, 112, 7005–7014.
10. Wang, I.; Botzung-Appert, E.; Stéphan, O.; Ibanez, A.; Baldeck, P. L. J. Opt. A: Pure
Appl. Opt. 2002, 4, 258–260.
11. Ergen, E.; Weber, M.; Jacob, J.; Herrmann, A.; Müllen, K. Chem. Eur. J. 2006, 12,
3707.
12. Belletete, M.; Morin, J. F.; Beaupre, S.; Leclerc, M.; Durocher, G. Synth. Metall.
2002, 126, 43–51.
22. Compound 3d (–CN) yield 87%; colorless crystal; mp 106.9–107.2 °C; 1H NMR
(400 MHz, CDCl3)
d 7.83 (d, J = 11.5 Hz, 2H), 7.78–7.76 (m, 8H), 7.60 (d,
J = 11.5 Hz, 2H), 7.56 (s, 2H), 2.08–2.02 (m, 4H), 1.18–1.06 (m, 20H), 0.78 (t,
J = 10.0 Hz, 6H), 0.71–0.68 (m, 4H) ppm; 13C NMR (100 MHz, CDCl3) d 152.1,
145.9, 140.9, 138.4, 132.6, 127.7, 126.4, 121.5, 120.6, 119.0, 110.7, 55.5, 40.3,
31.7, 29.9, 29.1, 23.3, 22.6, 14.0 ppm; FT-IR (KBr) 2925, 2851, 2225, 1603, 1509,
1465 cmÀ1.; MS (FAB) calcd for C43H48N2 593.3896, found 593.3855 (M+); Anal.
Calcd for C43H48N2: C, 87.11; H, 8.16; N, 4.73. Found: C, 87.17; H, 8.41; N, 4.69.
23. Compound 3e (–NO2) yield 98%; yellow crystal; mp 135.0–135.4 °C; 1H NMR
(400 MHz, CDCl3)
d 8.34 (d, J = 12.9 Hz, 4H), 7.87–7.80 (m, 6H), 7.65 (d,
J = 11.7 Hz, 2H), 7.60 (s, 2H), 2.10–2.04 (m, 4H), 1.20–1.07 (m, 20H), 0.78 (t,
J = 9.9 Hz, 6H), 0.74–0.65 (m, 4H) ppm; 13C NMR (100 MHz, CDCl3) d 152.2,
147.9, 147.0, 141.2, 138.1, 127.8, 126.7, 124.1, 121.7, 120.7, 55.6, 40.3, 31.7, 29.9,
29.1, 23.8, 22.5, 14.0 ppm; FT-IR (KBr) 2923, 2851, 1594, 1464, 1515, 1344 cmÀ1
;
MS (FAB) calcd for C41H48N2O4 633.3701, found: 633.3692 (M+); Anal. Calcd for
C41H48N2O4: C, 77.82; H, 7.65; N, 4.43. Found: C, 77.73; H, 7.81; N, 4.39.
24. Absolute fluorescence quantum yields were determined by the
photoluminescence (PL) method using a Hamamatsu DynaSpect C 9920-02
absolute PL quantum yield measurement system.
13. Wu, R.; Schumm, J. S.; Pearson, D. L.; Tour, J. M. J. Org. Chem. 1996, 61, 6906.
14. Destri, S.; Pasini, M.; Botta, C.; Porzio, W.; Bertini, F.; Marchiò, L. J. Mater. Chem.
2002, 12, 924.
15. Neeraj, A.; Pabitra, K. N.; Periasamy, N. J. Chem. Sci. 2008, 120, 355.
16. Rtishchev, N. I.; Samoilov, D. V.; Martynova, V. P.; El’tsov, A. V. Russ. J. Gen.
Chem. 2001, 71, 1467–1478.
25. Time-resolved emission was measured on a Hamamatsu OB920 by time-
correlated single-photon counting using a hydrogen flash lamp. Fluorescence
lifetimes were determined by a single exponential curve fit.
26. Chou, C.-H.; Hsu, S.-L.; Dinakaran, K.; Chiu, M.-Y.; Wei, K.-H. Macromolecules
2005, 38, 745–751.
27. Burrows, H. D.; Melo, J. S.; Serpa, C.; Arnaut, L. G.; Monkman, A. P.; Hamblett, I.;
Navaratnam, S. J. Chem. Phys. 2001, 115, 9601–9606.
28. Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K.; Shimizu, H.; Inouye, M. Chem.
Eur. J. 2006, 12, 824–831.
17. Samori, S.; Tojo, S.; Fujitsuka, M.; Spitler, E. L.; Haley, M. M.; Majima, T. J. Org.
Chem. 2007, 72, 2785–2793.
18. Beinhoff, M.; Weigel, W.; Jurczok, M.; Rettig, W.; Modrakowski, C.; Brüdgam, I.;
Hartl, H.; Schlüter, A. D. Eur. J. Org. Chem. 2001, 20, 3819–3829.
19. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.
20. Typical procedure: 3a (–H): The toluene solution of 9,9-dioctylfluorene-2,7-
29. Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of Photochemistry,
3rd ed.; CRC Press, 2006.
bis(trimethyleneborate)
1 (0.56 g, 1.00 mmol), iodobenzene 2a (0.45 g,
2.20 mmol), K3PO4ÁnH2O (1.92 g, 6.69 mmol), and Pd(PPh3)4 (0.10 g,
0.089 mmol) was heated under argon atmosphere for 24 h. After cooling to
room temperature, the resulting mixtures were concentrated under reduced
pressure. The residue was purified by column chromatography on silica gel
using toluene as an eluent: yield 66%; colorless oil.
30. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003, 103, 3899–4031.
31. Dias, F. B.; Pollock, S.; Hedley, G.; Pålsson, L.-O.; Monkman, A.; Perepichka, I. I.;
Perepichka, I. F.; Tavasli, M.; Bryce, M. R. J. Phys. Chem. B 2006, 110, 19329–
19339.