Excellent blue fluorescent trispirobifluorenes: synthesis, optical properties and thermal behaviors

Article abstract of DOI:10.1016/j.tetlet.2007.10.137

Tetraiodotrispirobifluorene (1) was synthesized through cyclization of 2,7-diiodofluorene with pentaerythrityl tetrabromide under base condition. Subsequent treatment of 1 with arylboronic acid or arylacetylene under Pd-catalyzed coupling condition led to

Full text of DOI:10.1016/j.tetlet.2007.10.137

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 9112–9115
Excellent blue fluorescent trispirobifluorenes: synthesis,
optical properties and thermal behaviors
Shuqiang Yu, Haiyao Lin, Zujin Zhao, Zixing Wang and Ping Lu*
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China
Received 22 June 2007; revised 23 October 2007; accepted 26 October 2007
Available online 30 October 2007
Abstract—Tetraiodotrispirobifluorene (1) was synthesized through cyclization of 2,7-diiodofluorene with pentaerythrityl tetra-
bromide under base condition. Subsequent treatment of 1 with arylboronic acid or arylacetylene under Pd-catalyzed coupling
condition led to corresponding tetraaryl trispirobifluorenes (3–8). These trispirobifluorene derivatives exhibited bright-violet to blue
photoluminescence (PL) with excellent quantum efficiencies and showed high thermal stabilities. ‘Green-emission tail’ could not be
detected for those tetraaryl substituted trispirobifluorenes (3–8) annealed both in N2 and in air.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Spirobifluorene derivatives have attracted much atten-
tion due to their potential utilities in organic light emit-
ting devices (OLEDs),1 organic photovoltaic cells,2
optically pumped solid-state lasers,3 organic phototran-
sistors,4 nonlinear optics,5 photochromic materials,6 and
as asymmetric catalysts.7 Spiro-linked molecules com-
pared with the corresponding nonspiro-linked parent
compounds exhibit greater morphological stability and
more intense fluorescence.8 Furthermore, steric factors
can lead to an enhanced rigidity in the spiro center,
thereby preventing rotation of the adjacent aryl groups,
which reduce close packing and intermolecular inter-
action between chromophores in the solid-state.9 Thus,
the resulted stacking mode of the spiro-linked molecules
can efficiently suppress the appearance of ‘green-emis-
sion tail’ which is detrimental to OLEDs.10 However,
general synthesis of the spirobifluorene derivatives is
onerous.11 9,90-Spiro-bifluorene core could be prepared
in two steps. Nucleophilic addition of 2-biphenylmagne-
sium bromide to 9-fluorenone generated a tertiary
alcohol, which could be subsequently dehydrated in a
mixture of hydrochloride and acetic acid and led to
9,90-spirobifluorene. However, isolation of the bromi-
rene in four steps, the overall yield was 28%.13
Therefore, it is necessary to design new types of spirofluo-
renes and to find facile and efficient synthetic methods.
Ephritikhine’s group had ever reported the synthesis of
trispirobifluorene 2 from fluorenyl potassium salt, but
this family of compounds and their optical properties
had rarely been investigated by far.14 In this Letter, we
present a facile route to trispirobifluorene derivatives
and their electrochemical, optical properties and thermal
behaviors are investigated.
Tetraiodotrispirobifluorene (1) was synthesized by cycli-
zation of 2,7-diiodofluorene with pentaerythrityl tetra-
bromide in THF under base condition in 55% yield
and the detailed procedure is described in Supplemen-
tary data. In similar way, 2 was synthesized in 56% yield.
Arylboronic acids and arylacetylenes purchased from
Acros were used without further purification except that
9,90-diheptylfluoren-2-ylacetylene15 and 4-(3,6-di-tert-
butylcarbazol-9-yl)-phenyl boronic acid16 were synthe-
sized as reference methods. Subsequently, Pd-catalyzed
coupling reactions, such as Suzuki reaction or Sono-
gashira reaction, were employed to prepare trispirobifluo-
renes in moderate yields (70–90%) (see Scheme 1).
nated
9,90-spirobifluorenes
was
challengeable.12
Although Shu and Chen reported the improved syn-
thetic method for pure 2,20-dibromo-9,90-spirobifluo-
Figures 1 and 2 show the UV–vis absorption and
emission spectra of 2–8 in cyclohexane with a concentra-
tion of 1 · 10ꢀ6 M, respectively. A summary of their
optical, electrochemical, and thermal properties is listed
in Table 1. In cyclohexane solutions, all seven
Keywords: Trispirobifluorenes; Blue photoluminescence; Optical
stability; Thermal stability.
*
Corresponding author. E-mail: pinglu@zju.edu.cn
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.137

S. Yu et al. / Tetrahedron Letters 48 (2007) 9112–9115
9113
R
R
R
R
1 : R = I
55%
56%
C(CH2Br)4, THF
NaH, 50 oC, 24 h
R
R
2 : R = H
85%
83%
3 : Ar =
4 : Ar =
80 oC, 12 h
Ar
Ar
Ar
Ar
,
F
(CH3CH2)3N, CuI, PPh3
Pd(PPh3)2Cl2, N2
Ar
74%
5 : Ar =
C7H15
1
C7H15
90%
82%
6 : Ar =
7 : Ar =
8 : Ar =
Ar
Ar
Ar
HO
80 oC, 12 h
B
Ar
,
HO
S
Toluene, Ethanol
Na2CO3, Pd(PPh3)4, N2
Ar
70%
N
Scheme 1. Synthesis of compounds 1–8.
2
compounds exhibited strong absorptions with their
maximum absorption wavelengths in a range of 271–
375 nm. The maximum absorption wavelength of bare
compound 2 was found to be 271 nm. Compared with
2, the maximum absorption wavelengths of substituted
compounds 3–8 were all red-shifted (56–104 nm) as the
effective conjugation length increased. Compound 6
exhibited shorter maximum absorption wavelength
(327 nm) than that of 3 (340 nm) because of its twisted
biphenyl unit, which reduced the efficiency of conjuga-
tion.17 Compound 5 showed the longest absorption
wavelength (375 nm) with the most efficient conjugation
length and indicated the smallest HOMO–LUMO tran-
sition between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO).
3
4
5
6
7
8
1.0
0.8
0.6
0.4
0.2
0.0
250
300
350
400
Wavelength (nm)
Figure 1. Normalized UV–vis spectra of 2–8 in cyclohexane
(1 · 10ꢀ6 M).
The maximum emission wavelengths of 2–8 are in a
range of 308–399 nm with high PL quantum efficiencies
of 81–100% (3–8) in cyclohexane solutions referring to
9,10-diphenylanthracene standard. Emission spectra in
powders were all red-shifted (16–66 nm) resembling
those emissions in cyclohexane solutions as expected.
But a surprising emission band (537 nm) was observed
for 7 in the powder (Table 1 or Fig. S1). It could be
explained by the stacking mode that thiophene-substi-
tuted spirofluorene could form an effective p-stacking
in crystal alignment.17 This was also the reason why
there were significant bathochromic shifts of 7 in its
absorption and emission compared with those of 6
although they had similar efficient conjugation length.
2
3
4
5
6
7
8
1.0
0.8
0.6
0.4
0.2
0.0
HOMO energy levels were determined using cyclic
voltammetry, while LUMO energy levels were calcu-
lated based on the HOMO energy levels and the low-
est-energy absorption edges of the UV–vis absorption
spectra,18 respectively. The oxidative onset potentials
300
350
400
450
500
Wavelength (nm)
Figure 2. Normalized fluorescence spectra of 2–8 in cyclohexane
(1 · 10ꢀ6 M).

9114
S. Yu et al. / Tetrahedron Letters 48 (2007) 9112–9115
Table 1. Optical, electrochemical and thermal properties of compounds (2–8)
DEd (eV) HOMO/
LUMOd (eV)
Td (ꢁC)
Solutionf
Powder
kmax(FL) (nm)
a
b
e
ox
onset
Eg (eV)
E
(V) HOMO/
LUMOc (eV)
g
k
max(A) (nm) kmax (FL) (nm) UPL
2
3
4
5
6
7
8
3.99
3.28
3.30
3.05
3.45
3.22
3.20
1.69
1.36
1.41
1.27
1.34
1.19
1.03
ꢀ6.09/ꢀ2.10
ꢀ5.76/ꢀ2.48
ꢀ5.81/ꢀ2.51
ꢀ5.67/ꢀ2.62
ꢀ5.74/ꢀ2.29
ꢀ5.59/ꢀ2.37
ꢀ5.43/ꢀ2.23
8.48
7.39
7.33
7.18
7.72
7.42
7.17
ꢀ8.86/ꢀ0.39
ꢀ8.32/ꢀ0.94
ꢀ8.46/ꢀ1.13
ꢀ8.17/ꢀ1.00
ꢀ8.29/ꢀ0.57
ꢀ8.25/ꢀ0.83
ꢀ7.99/ꢀ0.82
277
459
310
394
353
322
429
271, 294, 306 308, 318
0.23
324, 343, 360
439
403
451
392
340, 353
339, 352
362, 375
316, 327
348
373, 393
371, 391
399, 423
361, 378
382, 403
390, 410
0.99
0.85
1.00
0.81
0.90
1.00
417, 437, 537
413, 424
297, 349
a Determined from UV–vis absorption spectra.
b Eooxnset: onset oxidation potential; potentials versus Ag/AgCl, working electrode Pt, 0.1 M Bu4NPF6–CH2Cl2, scan rate 100 mV/s.
þ 4:4;18 LUMO = HOMO ꢀ Eg.
c
ox
onset
HOMO ¼ E
d DE and HOMO/LUMO were calculated by semiempirical AM1 method.
e Td was defined as the temperature at which a 5% weight loss is recorded by the TGA analysis.
f Determined in cyclohexanes.
g Quantum yields (UPL) in cyclohexane were determined using 9,10-diphenylanthracene as the standard (UPL = 0.95 in cyclohexane); 2 was deter-
mined using p-terphenyl as the standard (UPL = 0.93 in cyclohexane).
of these compounds were determined in a range of 1.03–
1.69 V; correspondingly, their HOMO and LUMO lev-
els were at ca. 5.43–6.09 and 2.10–2.62 eV, respectively.
Among 3, 4, and 5, the highest oxidative onset potential
(1.41 V) and lowest HOMO energy (5.81 eV) of 4 could
be explained by fluorine atom with electron-withdraw-
ing nature.19 On the other hand, among 6, 7, and 8,
the lowest oxidative onset potential (1.03 V) and highest
HOMO energy (5.43 eV) of 8 could be ascribed to car-
bazole group with electron-donating nature. It could
be concluded that inductive effect of substituents would
efficiently adjust their electron-donating abilities and
thereafter influence the oxidative potentials. Theoretical
calculation was in good accordance with the experimen-
tal observation, which is listed in Table 1. Relative wide
energy gaps (Eg P 3.05 eV) indicated that 2–8 could be
used as efficient bright-violet to blue fluorescence emit-
ters. Compared with 2, tetraaryl substituted trispirofluo-
renes 3–8 had smaller optical energy gaps.
10 h, respectively. After annealing, samples were dis-
solved in cyclohexane and fluorescence spectra were
re-measured. No new emission bands were detected for
the annealed compounds (3–8) both in N2 and in air.
‘Green-emission tail’, which was assigned to the fluo-
renone defect and was the key issue in material applica-
tion, was not detected even for 5, which consisted of six
fluorene units. Figure 3 shows the emission spectra of 5
annealed both in N2 and in air, respectively. In a word,
tetrasubstituted trispirobifluorene could efficiently sup-
press the appearance of ‘green-emission tail’ and might
be used as optical materials.
Thermal stabilities of 2–8 were investigated by differen-
tial scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) as these behaviors played a critical role
on stability and lifetime of devices. TGA charts of 2–8
were shown in Figure 4. A heating rate of 10 ꢁC/min
was used to melt and decompose the compounds. DSC
analysis of 2–8 gave no melting peaks except 6.
Compounds 2–8 exhibited high thermal stabilities with
relative high decomposition temperatures (Td) being
recorded at 277–459 ꢁC, and no weight losses were
observed at lower temperature. Compound 3 presented
the highest decomposition temperature (Td = 459 ꢁC)
Optical stabilities of 3–8 were explored by annealing
powder samples both in N2 and in air at 150 ꢁC for
pristine
1.0
N2-annealed at 150 ºC
Air-annealed at 150 ºC
0.8
100
0.6
0.4
0.2
0.0
80
2
3
4
5
6
60
40
7
8
20
400
425
450
475
500
525
0
100
Wavelength (nm)
200
300
400
500
600
700
800
Temperature (ºC)
Figure 3. Photoluminescence spectra of
5
(in cyclohexane)
(1 · 10ꢀ6 M), pristine (square line), annealed in N2 (sphere line) and
Figure 4. TGA thermograms of 2–8 under nitrogen at a heating rate of
10 ꢁC/min.
in air (bar line).

S. Yu et al. / Tetrahedron Letters 48 (2007) 9112–9115
9115
Schmidt-Mende, L.; Zakeeruddin, S. M.; Gra¨tzel, M.
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and only 40% weight loss was detected when the sample
was heated to 800 ꢁC. The thermal stability of trispiro-
fluorene is comparable to those linker-free fluorene
derivatives. Taking 3 as an example, 20,70-diphenyl-
ethynyl-spiro(cyclopropane-1,90-fluorene),10 which had
identical substituents with 3, the decomposition temper-
ature was detected to be 302 ꢁC.
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In conclusion, we have described a facile and efficient
route to highly fluorescent trispirobifluorenes (1–8).
Compounds 2–8 exhibited bright violet to blue emission
with excellent quantum efficiencies in cyclohexane solu-
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and high thermal stabilities. These properties implied
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Ping Lu thanks National Nature Science Foundation of
China (20674070) and the Nature Science Foundation of
Zhejiang Province (R404109).
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.10.137.
´
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