The strategy to improve thermal and optical properties of diphenylfluoranthene based on silicon-cored derivatives

Article abstract of DOI:10.1039/c3ra47751g

A series of novel silicon-cored diphenylfluoranthene derivatives was synthesized in this paper to realize efficient solid-state emission. These silicone-cored diphenylfluoranthene derivatives show better fluorescent properties in the solid state than diph

Full text of DOI:10.1039/c3ra47751g

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The strategy to improve thermal and optical
properties of diphenylfluoranthene based on
silicon-cored derivatives†
Cite this: RSC Adv., 2014, 4, 17171
a
b
a
a
a
a
*
Hua Wang, Yan Liang, Yike Wang, Huanling Xie, Linglong Feng, Haifeng Lu
a
*
and Shengyu Feng
A series of novel silicon-cored diphenylfluoranthene derivatives was synthesized in this paper to realize
efficient solid-state emission. These silicone-cored diphenylfluoranthene derivatives show better
fluorescent properties in the solid state than diphenylfluoranthene because these silicone-cored
derivatives exhibit weaker p–p interactions among molecules. What is more, these silicone-cored
derivatives exhibit very high thermal stabilities and exceptionally high glass transition temperatures.
Interestingly, one silicon-cored diphenylfluoranthene derivative exhibited similar fluorescence emission
spectra in both solution and solid state, and may be the most obvious candidate for an efficient solid-
state emitter.
Received 18th December 2013
Accepted 19th March 2014
DOI: 10.1039/c3ra47751g
www.rsc.org/advances
been reported to exhibit more advantages than other materials
in optical applications because of their high brightness,
Introduction
thermal stability, amorphous lm-forming capability, and so
on.6,9–17 Surprisingly, no systematic structural analysis and
comparison of properties between silicon-cored materials and
common structural materials have been reported.
Luminescent materials have attracted a surge of interest
recently because of their great potential applications in many
aspect of modern life, such as at panel and full color range
displays.1–3 Flat aromatic molecules and linear p-conjugated
systems are highly uorescent and exhibit other impressive
properties such as thermal stability. However, an enfeebling
and long-standing problem with such systems is the molecular
aggregation via p–p stacking. This kind of aggregation usually
leads to the formation of p-aggregates/excimers and causes
bathochromic shis and low solid-state uorescence quantum
yields.4 Many publications and patents have been reported to
produce an efficient, cheap, and robust blue-light emitter.5 It is,
of course, not an easy task to nd a small molecule that
possesses not only a very large band gap but also thermal and
chemical stabilities and a very large quantum yield in the solid
state.6 Nonplanarity is an important solution to overcome the
aggregation, and nonplanar conguration can be easily ach-
ieved with the use of a silicon-cored molecular structure.7,8 In all
organic luminescent materials, silicon-cored compounds have
The chemistry of uoranthene and studies of the optical
properties developed rapidly aer the elucidation of its struc-
ture at the turn of the 20th century.18,19 Fluoranthene and many
of its derivatives have been reported to exhibit excellent optical
properties and thermal properties.20–28 However, the utility of
uoranthene and its derivatives as emitting materials has been
greatly restricted due to aggregation between the planar uo-
rophore, like other planar molecules.29,30 To meet the practical
demand for highly efficient solid-state emitters in optical elds,
much effort has been devoted to eliminate the undesirable
effect of aggregation-caused quenching of planar molecules.31
The uoranthene derivatives laterally substituted with phenyl
groups have been reported to reduce facial contacts that lead to
excimer quenching and bathochromic shis.21 However,
because every chromophore has an inherent tendency to
aggregate in the solid phase, the resistance against the intrinsic
aggregation nature is not good enough to make use of it.
Recently, aggregation-induced emission has been reported to
modify the uoranthene molecule and obtain an efficient
emitter in the solid state.25,28 However, although the uores-
cence in the solid state of this derivative was strong, the
maximum emission wavelength also changed to 526 nm from
the 451 nm of uoranthene. For the application of devices, the
optical band gap, energy level of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of luminescent materials should correlate with those of
aKey Laboratory of Special Functional Aggregated Materials, Ministry of Education,
School of Chemistry and Chemical Engineering, Shandong University, Jinan,
250100, People's Republic of China. E-mail: fsy@sdu.edu.cn; Fax: +86-531-
88564464; Tel: +86-531-88364866
bCollege of Food and Bioengineering, Qilu University of Technology, Jinan, Shandong
250353, People's Republic of China
† Electronic supplementary information (ESI) available: The 1H NMR, 13C NMR
and 29Si NMR spectra of 3, 7, 8 and 9. CCDC 971315–971318 contain the
supplementary crystallographic data for compounds 3, 7, 8 and 9. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ra47751g
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the host molecules for efficient energy or charge transfer using only ethanol (EtOH) and (optionally) xylenes for solvents;
processes. Therefore, a new method to weaken the intermolec- all are inexpensive and readily available (Scheme 1).
ular interactions and keep the uorescent properties of uo-
7,9-Diphenyl-cyclopenta[a]acenaphthylen-8-one (1). 5.45 g
ranthene molecules was necessary in these applications. From acenaphthenequinone (0.03 mol) and 6.30 g 1,3-diphenylace-
this viewpoint, the characteristics of silicon-cored structures tone (0.03 mol) were dissolved in 60 mL ethanol, and the
ꢀ
may serve the role.
mixture was heated to 78 C. A solution of 0.8 g KOH in 8 mL
In this paper, we report the synthesis of diphenyl- ethanol was added slowly to the mixture. The reaction mixture
uoranthene and new silicon-cored diphenyluoranthene was stirred at 78 ꢀC for 15 min, and then cooled to room
derivatives. Systematic comparison of the properties and temperature. Solid product was ltrated and washed three
structural analysis of common structural materials and silicon- times with ethanol. Aer drying, a bright black solid was
ꢀ
cored materials are discussed in particular. By introducing obtained. The total yield was 90% (mp. 218 C).
silicon-cored structures into uoranthene, we hope to take
(7,10-Diphenyluoranthen-8-yl)trimethylsilane (2). 0.74 g
advantage of the rigid and planar structure of this compound (7.5 mmol) ethynyltrimethylsilane and 1.71 g (5 mmol) 1 were
and turn it into an efficient solid-state emitter. In contrast to dissolved in 30 mL xylene in a sealed stainless steel reactor, and
diphenyluoranthene, the silicon-cored compounds exhibit the reaction was run for 20 h at 230 ꢀC under a nitrogen
high thermal stability and exceptionally high glass transition atmosphere. Then xylene was removed under reduced pressure.
temperatures. Particularly, due to the steric hindrance of the The product was precipitated in methanol and puried with
silicon-cored structure, all the silicon-cored derivatives exhibit silica gel column chromatography. A yellow solid powder was
1
weaker p–p molecular interactions and less bathochromic shi obtained with yield 57%. H NMR (DMSO, 400 MHz, ppm): d
of uorescence emission in the solid state than does diphe- 0.02 (s, 9H), 6.30 (d, 1H), 7.35 (t, 1H), 7.46–7.68 (m, 13H), 7.82–
nyluoranthene. Moreover, other groups at the Si atom exhibit a 7.89 (m, 2H). 13C NMR (CDCl3, 100 MHz, ppm): d 1.07 (Si–CH3),
great inuence on the molecular structures and result in 123.19, 123.31, 127.30, 127.76, 128.28, 128.51, 128.65, 129.13,
different optical properties. We hope the study in this paper will 129.27, 129.35, 129.94, 130.06, 132.427, 135.53, 135.63, 136.37,
provide some guidance on the molecular design of silicon-cored 136.64, 137.03, 137.24, 138.72, 140.83, 141.35.
planar materials in optical application.
7,10-Diphenyluoranthene (3). 1.00 g 2 (2.3 mmol) and 2 mL
concentrated hydrochloric acid was dissolved in 30 mL THF.
The mixture was reuxed for 10 h with stirring, and then the
solvent was distilled off. The product was precipitated in
methanol and puried with silica gel column chromatography.
A yellow crystal was obtained. The yield was 91%. 1H NMR
(CDCl3, 400 MHz, ppm): d 7.24 (d, 2H), 7.29 (s, 2H), 7.37 (t, 2H),
7.52–7.60 (m, 6H), 7.67 (m, 4H), 7.78 (d, 2H). 13C NMR (CDCl3,
100 MHz, ppm): d 123.0, 126.7, 127.6, 127.8, 128.7, 129.1, 129.8,
132.8, 136.3, 136.8, 138.0, 141.0. Anal. calcd for C28H18: C
94.88%, H 5.12%; found: C 94.26%, H 5.07%.
Experimental section
Material
Acenaphthenequinone and 1,3-diphenylacetone were obtained
from the Shanghai Chemical Reagent Company. Chlorosilanes
were obtained from the Zibo Yuxing Chemical Engineering
Company and used aer fractionating.
Synthesis
Diethynylsilanes (4–6). Diethynyldimethylsilane (4), dieth-
ynylmethylphenylsilane (5) and diethynyldiphenylsilane (6)
were prepared according to the published method.32,33
4.8 g (0.2 mol) magnesium powder and 50 mL dry THF were
mixed into a 250 mL three-necked, round-bottomed ask under
an argon atmosphere with stirring. 22.0 g (0.2 mol) bromo-
ethane in 50 mL dry THF were slowly dropped into the mixture.
Aer the magnesium powder disappeared, the mixture was kept
boiling for 2 h. Aer cooling to room temperature, the bromo-
ethane Grignard reagent was obtained.
The silicon-cored uoranthene derivatives were synthesized via
the Knoevenagel/Diels–Alder method from commercial starting
materials, such as acenaphthenequinone and diphenylacetone,
Acetylene, which was cooled at À78 ꢀC, was bubbled into the
dry 150 mL THF for 1 h to obtain the acetylene saturated solu-
tion. Then the bromoethane Grignard reagent was dropwise
added into the acetylene saturated solution. In this process the
acetylene was always kept bubbling into the mixture. The
reaction was kept for 3 h aer the addition of the bromoethane
Grignard reagent was completed, and then the ethynylmagne-
sium chloride was obtained.
A 1 L three-necked, round-bottomed ask was equipped with
a reux condenser and a Teon-covered magnetic stirring bar.
The ask was charged with the ethynylmagnesium chloride
obtained at the last step under an argon atmosphere.
Scheme 1 The synthesis of diphenylfluoranthene and the silicon-
cored derivatives.
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Dimethyldichlorosilane (7.80 g, 60 mmol) and THF (50 mL)
were placed into the addition funnel and added dropwise to the
well-stirred reaction mixture over 1 h. It was quenched by
addition of saturated aqueous ammonium chloride (10 mL),
and THF (30 mL) was added. The organic layer was washed with
water, dried with anhydrous MgSO4, ltered, and concentrated
at reduced pressure. The residue was then fractionally distilled
to give 4 (colorless liquid with yield 34%); bp 86.5 C; H NMR
(CDCl3, 400 MHz, ppm): d 2.48 (s, 2H, –C^CH), 0.35 (s, 6H,
–SiCH3).
5 was prepared in a similar manner to that for the synthesis
of 4. The methylphenyldichlorosilane (11.5 g, 60.0 mmol) was
used to obtain colorless liquid with yield 70%. 1H NMR (400
MHz, CDCl3, ppm): d 7.76 (m, 2H, ArH), 7.28–7.46 (m, 3H, ArH),
2.75 (s, 2H, –C^CH), 0.64 (s, 3H, –SiCH3).
Results and discussion
Thermal properties of diphenyluoranthene and its silicon-
cored derivatives
The thermal properties of diphenyluoranthene (3) and its
silicon-cored derivatives (7, 8 and 9) were evaluated by ther-
mogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) under nitrogen conditions.
1
ꢀ
TGA was measured with a heating rate of 10 ꢀC minÀ1, and is
displayed in Fig. 1 and Table 1. Corresponding to 5% weight
loss, decomposition temperatures (Td) are 298 ꢀC, 390 ꢀC,
430 ꢀC and 444 ꢀC for 3, 7, 8 and 9, respectively. All these
compounds possess excellent thermal stability. Moreover, all
the silicon-cored diphenyluoranthene derivatives exhibited
higher thermal stability than diphenyluoranthene. The reason
could be attributed to the activity change of the H in the C8
position. This H in molecules of 3 was reactive. Aer the H was
replaced by the Si atom, the thermal stabilities of the
compounds were enhanced. Furthermore, the Td of the silicon-
cored compounds increased in proportion to the size of
substitution groups at the Si atom. The data of DSC are listed in
Table 1. The melting temperatures (Tm) were identied for 3, 7,
8 and 9 at 165 ꢀC, 260 ꢀC, 286 ꢀC and 350 ꢀC. The glass transition
temperatures (Tg) were obtained for 3, 7, 8 and 9 at 60 ꢀC,
150 ꢀC, 220 ꢀC and 250 ꢀC, respectively. Similar to that of TGA,
all the silicon-cored diphenyluoranthene derivatives exhibited
higher glass transition temperatures than diphenyl-
uoranthene. Compared to the blue luminescent materials,
such as the uorene and anthracene derivatives, the silicon-
cored compounds in this paper showed higher thermal stabil-
ities.6,34 The high glass transition temperatures of the
6 was prepared in a similar manner to that used for the
synthesis of 4. The diphenyldichlorosilane (15.2 g, 60.0 mmol)
1
was used to obtain the straw-yellow crystal with yield 70%. H
NMR (400 MHz, CDCl3, ppm): d 7.75 (m, 4H, ArH), 7.26–7.50 (m,
6H, ArH), 2.75 (s, 2H, –C^CH).
Bis(7,10-diphenyluoranthene)dimethylsilane (7). 7.12 g 1
(0.02 mol) and 1.08 g diethynyldimethylsilane (0.01 mol) were
dissolved in 30 mL xylene in a sealed stainless steel reactor, and
the reaction was run for 20 h at 230 ꢀC under a nitrogen
atmosphere. Then xylene was removed under reduced pressure.
The product was puried with silica gel column chromatog-
1
raphy. The yield was 46%. H NMR (DMSO, 400 MHz, ppm): d
0.08 (s, 6H), 6.17 (s, 2H), 7.19 (t, 6H), 7.26 (t, 2H), 7.37 (t, 2H),
7.46 (t, 6H), 7.56–7.63 (m, 10H), 7.81–7.88 (m, 6H). 13C NMR
(CDCl3, 100 MHz, ppm): d À1.10 (Si–CH3), 120.99, 121.07,
124.64, 125.17, 125.76, 125.84, 125.99, 126.29, 126.85, 126.96,
127.65, 127.90, 130.44, 133.72, 133.94, 134.40, 134.69, 135.03,
135.66, 138.56, 138.83. 29Si NMR (CDCl3, 80 MHz, ppm): d
À7.56. Anal. calcd for C58H40Si: C 91.06%, H 5.27%; found: C
90.47%, H 5.05%.
Bis(7,10-diphenyluoranthene)methylphenylsilane (8). This
compound was prepared by a procedure similar to that for 7.
The yield was 41%. 1H NMR (DMSO, 400 MHz, ppm): d 0.35 (s,
3H), 6.34 (d, 2H), 7.17 (t, 4H), 7.23–7.32 (m, 12H), 7.38–7.49 (m,
13H), 7.58 (d, 4H), 7.70 (d, 2H), 7.77 (d, 2H). 13C NMR (CDCl3,
100 MHz, ppm): d À0.85 (Si–CH3), 123.19, 123.29, 126.33,
126.88, 127.49, 127.55, 127.57, 127.61, 127.68, 128.16, 128.18,
128.53, 128.81, 129.19, 129.71, 129.98, 130.17, 132.89, 135.43,
136.14, 136.19, 136.80, 137.42, 137.60, 137.98, 138.62, 141.11,
143.77. 29Si NMR (CDCl3, 80 MHz, ppm): d À8.42. Anal. calcd for
C
63H42Si: C 91.49%, H 5.12%; found: C 91.36%, H 5.18%.
Bis(7,10-diphenyluoranthene)diphenylsilane (9).
Fig. 1 TGA thermograms of compounds 3, 7, 8 and 9.
The
compound was synthesized using a similar process. The yield
was 35%. 1H NMR (DMSO, 400 MHz, ppm): d 6.95 (d, 2H), 6.94
(d, 4H), 7.03 (t, 4H), 7.16–7.26 (m, 16H), 7.41 (s, 2H), 7.47–7.60
(m, 12H), 7.80 (d, 2H), 7.89 (d, 2H). 13C NMR (CDCl3, 100 MHz,
ppm): d 123.19, 123.42, 126.27, 126.96, 127.14, 127.30, 127.41,
127.61, 127.69, 127.96, 128.28, 128.56, 129.26, 129.66, 130.06,
132.87, 133.88, 135.96, 135.99, 136.45, 136.57, 136.85, 137.55,
138.39, 139.00, 140.56, 141.13, 144.43. 29Si NMR (CDCl3, 80
MHz, ppm): d À14.71. Anal. calcd for C68H44Si: C 91.85%, H
4.99%; found: C 91.28%, H 4.83%.
Table 1 Thermal properties of diphenylfluoranthene and silicon-
cored derivatives. Tg: glass transition; Tm: melting temperature; Td:
decomposition temperature
Compound
Tg/ꢀC
Tm/ꢀC
Td/ꢀC
3
7
8
9
60
150
220
250
165
260
286
350
298
390
420
444
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compounds were expected to give a high operational lifetime result, the molecular orbital analysis clearly indicates that the
and stability for optical devices. groups at the silicon atom do not inuence the blue emission
Above all, the Td, Tm and Tg of the aromatic chromophores nature of the diphenyluoranthene unit at all.
could be enhanced aer they were transformed to silicon-cored
derivatives. Moreover, these silicon-based compounds have
great potential for optical devices.
Optical properties of diphenyluoranthene and its silicon-
cored derivatives in solution
Fig. 3(a) shows the absorption spectra of diphenyluoranthene
(3) and its silicon-cored derivatives (7, 8 and 9) in THF solution.
A summary of photophysical data of the four compounds is also
Density functional theory calculations of
diphenyluoranthene and its silicon-cored derivatives
given in Table 2. In solution, diphenyluoranthene exhibited
Density functional theory (DFT) calculations have also been
three absorption peaks centered at 284, 326 and 371 nm,
performed to characterize the three-dimensional geometries
respectively, originating from the p–p* transitions. It is inter-
and the frontier molecular orbital energy levels of 3, 7, 8 and 9 at
esting to note that the data are not signicantly different from
the B3LYP/6-31G* level by using the Gaussian 03 program. The
those observed for the basic unit uoranthene (289 and 360
calculated geometries of 3, 7, 8 and 9 are shown in Fig. 2. The
nm). This suggests that the outer phenyl rings do not extend the
diphenyluoranthene showed two phenyl groups twisting out of
conjugation of the core dramatically. This is possibly due to the
the plane of the aromatic p-system and do not extend the
twisting of the phenyl groups out of the plane of the aromatic
conjugation of the core dramatically. For the silicon-cored
p-system which inhibits an effective p-conjugation with the
derivatives, the two 7,10-diphenyluoranthene units are
uoranthene core. All the silicon-cored diphenyluoranthene
signicantly twisted against each other because of the tetrahe-
derivatives exhibited similar peak shapes to that of diphenyl-
dral environment of the central silicon core, resulting in a non-
uoranthene with a very slight red shi. This could be attrib-
coplanar structure in each molecule. These geometrical char-
uted to a very weak dp–pp effect of the Si atom and
acteristics can effectively prevent intermolecular interactions
diphenyluoranthene. It indicated that the silicon-cored deriv-
between p-systems and suppress molecular recrystallization,
atives did not extend the conjugation of diphenyluoranthene.
which improves the morphological stability of thin lms of
Meanwhile, the three silicon-cored compounds exhibited the
these molecules. Calculated highest occupied molecular orbital
same absorption spectra in solution. This indicated that
(HOMO) and lowest unoccupied molecular orbital (LUMO)
besides the two diphenyluoranthene groups, the other
density maps of 3, 7, 8 and 9 are also included in Fig. 2. The
substitution groups at the Si atom did not have inuence on the
electron densities of the HOMOs and LUMOs are mostly local-
conjugation of the molecular structure.
ized on the diphenyluoranthene unit, implying that the
The uorescence emission spectra of diphenyluoranthene
absorption and emission processes can only be attributed to the
(3) and its silicon-cored derivatives (7, 8 and 9) in THF solution
p–p* transition centered at the diphenyluoranthene unit. As a
Fig.
2 Optimized geometries and calculated HOMO and LUMO Fig. 3 Absorption and fluorescence spectra of diphenylfluoranthene
density maps, respectively, of (a–c) 3, (d–f) 7, (g–i) 8 and (j–l) 9.
(3) and the silicon-cored derivatives (7, 8 and 9) in solution.
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Table 2 Optical properties of diphenylfluoranthene and silicon-cored
derivatives
levels are estimated from the optical band-gap energy, which is
obtained from the onset of the absorption edge. The optical
band-gap energies of 3, 7, 8 and 9 are 3.85, 3.88, 3.90 and 3.87
eV, respectively. The LUMO levels of 3, 7, 8 and 9 are 1.73, 1.71,
1.70 and 1.72 eV, respectively. The HOMO and LUMO energy
levels were nearly same as those of diphenyluoranthene (3)
and its silicon-cored derivatives (7, 8 and 9). Therefore, the
silicon-cored structure derivatives retained the optical band-gap
energy level of HOMO and LUMO of diphenyluoranthene.
The uorescence emission spectra of diphenyluoranthene
(3) and its silicon-cored derivatives (7, 8 and 9) in CH2Cl2 (DCM)
solution with different concentrations were shown in Fig. 5. For
3, as the concentration increased, the emission raised at rst.
When the concentration was higher than 2 Â 10À3 mol LÀ1, the
emission intensity began to fail. When the concentration was 2
 10À2 mol LÀ1, the emission spectra showed a red shi. The
similar silicon-cored derivatives 7 and 9 exhibited the similar
phenomenon. However, the silicon-cored derivative 8 showed
no obvious red shi at 10À2 mol LÀ1. The difference could be
attributed to the distinction of the aggregation states. A detailed
explanation could be found in the discussion of the optical
properties of diphenyluoranthene and its silicon-cored deriv-
atives in solid state. Specically, when the concentration of
diphenyluoranthene groups were all the same at 2 Â 10À2 mol
LÀ1 for 3, 7, 8 and 9, the emission intensities of the silicon-cored
derivatives were all higher than that for diphenyluoranthene.
In summary, compared to the diphenyluoranthene
aromatic compound, the absorption and emission spectra of its
silicon-cored derivatives exhibited no signicant difference
from those observed for the diphenyluoranthene aromatic
compounds. Besides the two diphenyluoranthene aromatic
groups, the other substitution groups at Si atom did not have
inuence in the conjugation of the molecular structure.
lmax(em)/nm
lmax(em)/nm
Compound
lmax(abs)/nm
(solution)
(solid)
3
7
8
9
371, 326, 284, 250
375, 328, 288, 251
376, 328, 287, 251
377, 328, 289, 251
443
445
445
446
509
480
453
479
are shown in Fig. 3(b). The emission peak of diphenyl-
uoranthene in solution appears at 443 nm. The emission
spectra of the silicon-cored derivatives exhibited similar
phenomena to the absorption spectra. All the silicon-cored
derivatives exhibited similar uorescence emission peaks to
that of diphenyluoranthene. A slight red shi was also
observed from all the silicon-cored compounds to diphenyl-
uoranthene. Similarly, all the three silicon-cored compounds
exhibited the same uorescence emission in solution. These
data also proved the existence of a weak dp–pp effect.
The uorescence emission spectra of diphenyluoranthene
(3) and its silicon-cored derivatives (7, 8 and 9) in different
solvents are shown in Fig. 4. As the polarities of solvent
increased, the maximum emission wavelength of diphenyl-
uoranthene (3) has no obvious variation and the emission
intensity increased. The silicon derivatives exhibited similar
properties. This indicated that the introduction of a silicon core
could retain the optical properties of diphenyluoranthene in
dilute solution.
The electrochemical behaviors of diphenyluoranthene (3)
and its silicon-cored derivatives (7, 8 and 9) were studied by
cyclic voltammetry (CV). The HOMO and LUMO levels of four
compounds were obtained according to the method in the
literature.22 The HOMO energy levels (EHOMO) are calculated
from the oxidation potential (Eox) using ferrocene as a standard
reference. The HOMO levels of 3, 7, 8 and 9 are 5.58, 5.59, 5.60
and 5.59 eV below the vacuum level, respectively. The LUMO
Optical properties of diphenyluoranthene and its silicon-
cored derivatives in solid state
The uorescence emission spectra of diphenyluoranthene (3)
and its silicon-cored derivatives (7, 8 and 9) in the solid state
Fig. 4 The fluorescence emission spectra of 3 (a), 7 (b), 8 (c) and 9 (d) Fig. 5 The fluorescence emission spectra of 3 (a), 7 (b), 8 (c) and 9 (d)
in different solvents.
in CH2Cl2 solutions with different concentrations.
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As shown in Fig. 7(a), in contrast to uoranthene molecules,
3 has two peripheral phenyl groups with torsional angles (50.2ꢀ
and 58.0ꢀ) to the main plane of the molecule, in which the
strong intermolecular p–p interaction generally observed in the
solid state of such planar molecules could be slightly weakened.
The p–p distance between two central planes of two adjacent
˚
molecules was 4.0 A and the C–H/p distance of two adjacent
˚
molecules was 2.67–3.19 A. The molecular cohesion in crystals
of 3 is dominated by special C–H/p interactions and weak p–p
interactions involving the central aromatic ring. In this packing
mode, the planes of adjacent molecules were parallel. As a
result, the uorescence of 3 in the solid state has a large red
shi in contrast to that in solution.
The structures of silicon-cored diphenyluoranthene deriv-
atives were further examined. Such silicon-cored compounds of
7, 8 and 9 have characteristic nonplanar toroidal topologies
which were quite different from the diphenyluoranthene
molecules. First, the torsional angles between the central plane
and peripheral phenyl groups become larger in silicone-cored
compounds. Two phenyl groups were almost perpendicular to
the main plane of the molecules. Therefore, the p–p distance
between two parallel central planes of two adjacent molecules
Fig. 6 Fluorescence spectra of diphenylfluoranthene (3) and the
silicon-cored derivatives (7, 8 and 9) in the solid state.
were also measured, and are shown in Fig. 6. The emission
spectra of diphenyluoranthene in the solid state exhibited a
signicant difference from that observed in solution. The
emission peak in the solid state shied to 509 nm. Compared to
that at 443 nm in solution, the emission spectra in the solid
state showed a signicantly red-shied and broad emission
band. This could be attributed to the extensive excimer forma-
tion caused by the close-packing and intermolecular interaction
of the molecules.
The PL spectra of the silicon-cored derivatives in the solid
state also exhibited a red-shi compared with those in solution
owing to the solid-state effect. Differently, the PL spectra of all
the silicon-cored derivatives showed shorter wavelengths of red
shi than that of diphenyluoranthene. This indicated that the
silicon-cored compounds had weaker intermolecular interac-
tions than the aromatic compounds. The tetrahedron structure
of silicon-based compounds could effectively prevent the close-
packing of constituent molecules and thus effectively shield the
intermolecular interactions.
For the silicon-cored derivatives with different substituting
groups at the Si atom, the emission peaks of 7 and 9 shied to
480 nm and 479 nm. Compared with the PL spectra observed in
solution, the uorescence emission spectra showed an obvious
red shi of about 30 nm in solid state. Interestingly, the PL
spectra of 8 exhibited no obvious red shi between the solid
state and the solution. The emission peak appeared at 453 nm,
which was very similar to that at 445 nm in solution. It could be
found that the substituting groups at the Si atom had a great
inuence in the uorescence emission spectra. These inter-
esting properties could be attributed to the inuence of the
different substituting groups at the Si atom. This could be
further investigated according to the single crystal structures of
diphenyluoranthene and its silicon-cored derivatives.
˚
was much larger than 4.0 A. This indicated that the tetrahedron
structure of silicon-cored compounds could effectively prevent
the p–p interactions which were generally observed in the solid
state. Secondly, the molecular cohesions in crystals of 7, 8 and 9
are dominated by weak van der Waals forces and special C–H/
p interactions involving the central aromatic ring. Therefore,
the uorescence of all silicon-cored derivatives in the solid state
has
a
smaller red shi in contrast to that of
diphenyluoranthene.
In three silicon-cored diphenyluoranthene derivatives, the
different substituent groups which were attached to Si atom
also have great inuence on the crystal structures. The molec-
ular structures of the silicon-cored diphenyluoranthene
derivatives in the crystal were extremely different from those of
theoretical calculations. Because the other two substituent
Crystal structures of diphenyluoranthene and its silicon-
cored derivatives
The crystal structures of diphenyluoranthene and its silicon-
cored derivatives were further investigated to explain their
optical properties.
Fig. 7 The single crystal structures of diphenylfluoranthene and the
silicon-cored derivatives (3, 7, 8 and 9).
17176 | RSC Adv., 2014, 4, 17171–17178
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groups attached to the Si atom were same, crystals of 7 and 9
exhibited symmetrical structures. As shown in Fig. 7(b) and (d),
the two uoranthene planes at the same Si atom showed a “V”
shape with large angles, 99ꢀ for 7 and 83ꢀ for 9. The torsional
angles of the central plane and peripheral phenyl groups
become larger in silicone-cored compounds. Interestingly,
crystals of 8 exhibited a great difference to those of 7 and 9. The
other two substituent groups attached to the Si atom were
different and the molecular structure showed a twisty “V” shape
in contrast to that of 7 and 9. As shown in Fig. 7(c), the diphe-
nyluoranthene group was approximately perpendicular to the
other diphenyluoranthene group in one molecule of 8. The
reason for the twisty structure formation could be attributed to
the intramolecular CH/p interactions. Because of this unique
structure, the intermolecular p–p interactions of 8 molecules in
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (no. 21274080 and 21204043), the 27 X. Ma, W. Wu, Q. Zhang, F. Guo, F. Meng and J. Hua, Dyes
Key Natural Science Foundation of Shandong Province of China
Pigm., 2009, 82, 353.
(no. ZR2011BZ001 and ZR2009BZ006) and the Doctor Station 28 Y. Liu, Y. Lv, X. Zhang, S. Chen, J. W. Y. Lam, P. Lu,
Foundation of Chinese Education Department (no. 200603086).
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17178 | RSC Adv., 2014, 4, 17171–17178
This journal is © The Royal Society of Chemistry 2014