Emission of boron dipyrromethene dyes through energy transfer to their S2 state from polysilane S1 state

Article abstract of DOI:10.1016/j.dyepig.2011.12.011

Irradiation of a mixture of a poly(methylphenyl)silane containing a Boron dipyrromethene dye was found to induce green color emission from the BODIPY dye via energy transfer from the polysilane S₁ state to the S₂ state of the BODIPY

Full text of DOI:10.1016/j.dyepig.2011.12.011

Dyes and Pigments 94 (2012) 183e186
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Dyes and Pigments
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Emission of boron dipyrromethene dyes through energy transfer to their S state
2
from polysilane S state
1
Qiuhong Wang, Hua Lu, Lizhi Gai, Weifeng Chen, Guoqiao Lai, Zhifang Li*
Key Laboratory of Organosilicon Chemistry and Material Technology, Ministry of Education, Hangzhou Normal University, Hangzhou 310012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 October 2011
Received in revised form
Irradiation of a mixture of a poly(methylphenyl)silane containing a Boron dipyrromethene dye was found
to induce green color emission from the BODIPY dye via energy transfer from the polysilane S
the S state of the BODIPY in the solid state, whereas in the solid the BODIPY dyes themselves are
nonfluorescent due to aggregation and self-quenching.
1
state to
2
21 December 2011
Accepted 23 December 2011
Available online 10 January 2012
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Polysilanes
BODIPY
Energy transfer
Solid-emission
Fuorescent dyes
Excited-state property
1
. Introduction
visible region through energy transfer to S
polysilane states [14,15]. Considering the high oscillator
strength and the conductivity by the -conjugated one-
1
level of dyes from
S
1
Polysilanes are well known as a new type of
s-conjugated
s
polymers having Si backbone with high oscillator strength and
conductivity [1,2]. They exhibit unique characteristics such as one-
dimensional semiconduction, photo conductivity with high hole
mobility, large non-linear optical effects and effective light emis-
sion in the near ultraviolet region due to one-dimensional exci-
tonic states [3e7]. The ultraviolet emission of polysilanes is
unsuitable for their application in electroluminescene and optical
devices. In previous studies, it has been demonstrated that the
luminescence wavelength could be controlled by introducing dyes
as the luminescence centers in linear polysilanes [8e11]. Shifting
the absorption and emission wavelengths of polysilanes to the
visible and even to the NIR region by structural modifications is
difficult, because of synthetic limitations [12,13]. Highly efficient
energy transfer could be realized by mixing dyes into the poly-
silane, the combination of polysilanes and dyes, Actually, perylene,
coumarin, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminos-
tyryl)-4H-pyran (DCM) and Zinc tetra-phenylporphyrin (ZnTPP)
have been reported to induce electroluminescene (EL) in the
dimensional configuration of polysilanes, design and synthesis of
new dye-polysilane systems for electoluminescene devices is
highly desirable [15].
As a new type dye for a variety of applications, boron dipyr-
romethene (BODIPY) is attractive, because they feature narro-
w.and intense absorption and fluorescence bands, high
fluorescencequantum yields, negligible triplet-state formation,
and high stability [16,17]. We notice that there is strong spectral
overlap of fluorescence bands of a polysilane with the higher
energy absorption (S
overlap with the BODIPY S
effectively occur from excited polysilanes to the S
0
/ S
2
) band of a BODIPY molecule, while no
/ S band. Energy transfer would
state of
0
1
2
BODIPY dyes. Spectral overlap is an essential requirements for
Foerster type energy transfer. However, less information is
available for the energy transfer to upper-lying excited states of
the acceptor, while several such cases have been described
including that from the pyrene S
[18,19].
1 2
state to the BODIPY S state
We report herein that when a mixture of BODIPY with a poly-
silane is irradiated in the solid state, intense green fluorescence of
the BODIPY is observed via efficient energy transfer from singlet-
*
Corresponding author. Tel.: þ86 571 28868529; fax: þ86 571 28865135.
E-mail address: zhifanglee@hznu.edu.cn (Z. Li).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.12.011

184
Q. Wang et al. / Dyes and Pigments 94 (2012) 183e186
2
excited state of the polysilane to the S state of the BODIPY. BOD-
R
IPYs are highly promising as an acceptor to modify the color of
polysilane-based electroluminescence and related optoelectronic
devices.
N
2
. Materials and methods
Si
Si
Si
N
N+
2
.1. Instrument and reagents
B-
n
n
n
F F
1H NMR spectra were recorded on a Bruker DPX400 spec-
PMPS PMTS PMDMAPS
R=OH, 1
trometer and referenced to the residual proton signals of the
solvent. Mass spectra were measured on Thermo Finnigan LCQ
Advantage Spectrometer. UVeVis absorption spectra were recor-
ded on a Shimadzu UV-2550 UVeVis Spectrophotometer. Fluo-
R=H, 2
Fig. 1. Structural formula of poly(methylphenyl)silane (PMPS), poly(methyl-4- tolyl)
silane (PMTS), poly[methyl-(4-dimethylaminophenyl)]silane (PMDMAPS), BODIPY 1
and 2.
rescence spectra were measured on
a Hitachi F-4600 FL
Spectrophotometer (the path-length of the quartz cell is 1 cm) with
a xenon arc lamp as the light source. All reagents were obtained
from commercial suppliers and used without further purification
unless otherwise indicated. All air and moisture-sensitive reactions
were carried out under nitrogen atmosphere. Dichloromethane
was distilled over calcium hydride. Triethylamine was obtained by
simple distillation. Toluene was dried over sodium metal and fresh
distilled before to use.
weight of the polymer was determined by gel permeation chro-
matography (GPC) calibrated by polystyrene standards with chlo-
1
roform as the eluent. PMPS: H NMR (400 MHz, CDCl
3
)
d
6.3e7.2
(
M
br, 4H), ꢁ0.9e0.1(br, 3H); UV
¼ 7,000, M ¼ 19,950, M /M ¼ 2.85.
Similarly, polysilanes PMTS and PMDMAPS shown in Fig. 1
l
max(log
3
¼
334 nm;
n
w
w
n
were obtained using dichloromethyl-4-tolylsilane and dichloro
methyl-(4-dimethylaminophenyl)silane as the substrates, the
corresponding PMTS and PMDMAPS were obtained in 35%
2
2
.2. Synthesis
1
and 30% isolated yields, respectively. PMTS: H NMR (400 MHz,
.2.1. Synthesis of BODIPY 1 and 2
CDCl
3
)
d
6.1e6.8 (br, 4H), 2.2 (br, 3H), ꢁ0.9e0.2(3H); UV
)(THF) ¼ 342 nm; M ¼ 5,100, M ¼ 10,965, M
¼ 2.15. PMDMAPS: H NMR (400 MHz, CDCl 6.7e7.7 (br,
max(log
) (THF) ¼ 350 nm,
4
-Hydroxybenzaldehyde (2 mmol) and 2,4-dimethylpyrrole
lmax(log
3
n
w
w
/
(
380 mg, 4 mmol) were dissolved in dry CH Cl (100 mL) under
2
2
1
M
n
3
) d
nitrogen. One drop of trifluoroacetic acid (TFA) was added, and the
solution was stirred for 4 h at ambient temperature in the dark. 2,3-
Dichloro-5,6-dicyanoquinone (DDQ, 442 mg, 2.0 mmol) was added
to the mixture and then stirred for additional 20 min. The reaction
mixture was then treated with triethylamine (3 mL) for 5 min and
boron trifluoride etherate (3.2 mL) for another 40 min. The dark
brown solution was washed with water (2 ꢀ 20 mL) and brine
4H), 2.9 (br, 6H), 0.1e0.6 (br, 3H); UV
l
3
M
n
¼ 1400, M ¼ 2,016, M /M ¼ 1.44.
w
w
n
3. Results and discussion
Poly(methylphenyl)silane (PMPS), poly(methyltolyl)silane
PMTS), and poly[methyl-(4-dimethylaminophenyl)]silane (PMD-
(
(
30 mL), dried over anhydrous magnesium sulfate, and concen-
MAPS) were prepared as suitable candidates of the energy donors,
by Wurtz-type condensation with sodium metal according to
the method of Miller and Michl [6]. BODIPY dyes (4,4-difluoro-
trated at reduced pressure. The crude product was purified by
silica-gel flash column chromatography (elution with 10% EtOAc/
petroleum ether) followed by recrystallization from CHCl
3
/hexane
to yield 1 as red crystal (yield 35%). H NMR (400 MHz, CDCl 7.13
d, J ¼ 8.4 Hz, 2H), 6.95 (d, J ¼ 8.4 Hz, 2H), 2.55 (s, 6H), 1.45 (s, 6H),
1
1,3,5,7-tetramethyl-4-bora-3a, 4a-diaza-s-indacene), 1 and 2,
3
) d
were prepared through TFA-catalyzed condensation of 2,4-
dimethylpyrrole with benzaldehyde and 4-hydroxybenzaldehyde,
respectively, in a oneepot reaction [20,21]. All compounds were
(
þ
þ
þ
ESI-MS: m/z 341.31 [M þ H] , 321.47 [M ꢁ F] ; HRMS-ESI: m/z:
þ
calcd for C19
H19BF
2
N
2
ONa : 363.1451 [M þ Na] , found: 363.1465;
1
ꢁ
1
identified by H NMR, MS and GPC.
IR (KBr pellet, cm ): 3419, 1543, 1509, 1307, 1197, 1157. A similar
The absorption spectra of 1 show a narrow spectral band at
497 nm with a shoulder at 475 nm, which is attributed to the strong
procedure using benzaldehyde instead of 4-hydroxybenzaldehyde
1
afforded 2 as red crystal (yield 33%). H NMR (400 MHz, CDCl
3
)
d
7.47 (m, 3H), 7.29 (d, 2H), 2.55 (s, 6H), 1.37 (s, 6H); ESI-MS: m/z
þ
þ
3
C
25.27 [M þ H] , 305.35 [M ꢁ F] , HRMS-ESI: m/z: calcd for
þ
þ
19
H19BF
2
N
2
Na : 347.1502 [M þ Na] , found: 347.1513; IR (KBr
ꢁ
1
pellet, cm ): 1543, 1509, 1305, 1183, 1155.
2.2.2. Synthesis of polysilanes
Dry toluene (50 mL) containing sodium metal (2.7 g,
117.4 mmol) was stirred at reflux in an argon atmosphere.
Dichloromethylphenylsilane (9.5 g, 50 mmol), was added dropwise
into the mixture. After the addition was complete, the reaction
mixture was stirred for 5 h. It was cooled down to room temper-
ature, and ethanol (10 mL) was added to remove the sodium metal
and to terminate the end groups. The mixture was washed with
water (2 ꢀ 20 mL) and brine (30 mL), dried over anhydrous
magnesium sulfate, and concentrated in vacuo to give the crude
product. Pure poly(methylphenyl)silane (PMPS, see Fig. 1) was
obtained as a white powder by reprecipitation using a toluene-
methanol system, in the yield of 4.8 g (60%). The molecular
Fig. 2. Fluorescence spectra of PMPS (black dash line) and 1 (red dash line), absorption
spectra of 1 (red line) in THF solution. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

Q. Wang et al. / Dyes and Pigments 94 (2012) 183e186
185
Fig. 3. Fluorescent spectra of films of polysilane-BODIPY mixed systems. (a) PMPS/1, (b) PMTS/1 and 2, (c) PMPS/2, and (d) PMDMAPS/1 and 2 systems.
S
0
eS
band in the 330e380 nm range assigned to the second lowest-
energy (S eS ) transitions [22] (Fig. 2). The absorption spectra of
is almost identical to that of 1, indicating the absence of inter-
1
transition, in addition to a broader and weaker absorption
emission was observed for pure BODIPY films as shown in Fig. 3
[25]. The fluorescent spectra of spin-coated PMPS and PMTS
without BODIPY show maxima at 358 nm and 360 nm with
a narrow and intense band, as expected. A film of a mixture of
a polysilane and a BODIPY was prepared typically as follows: a dye
is added to a THF solution of a polysilane and then spin-coated on
a quartz plate, which is utilized to measure the electronic spectra.
As shown in Fig. 3(a), a PMPS film doped with increasing amounts
0
2
2
actions between meso-position and indacene plane in the ground
state, which is obviously due to the sterically demanding methyl
groups that force the molecule into a twisted conformation [23]. As
reported, the fluorescent bands of polysilanes PMPS and PMTS
appear at 360 ꢂ 5 nm, which are attributed to a fluorescence from
of 1 shows a broad emission from the S
1
state of 1 (Fl
B
) at around
their lowest
s
es
* excited state [24]. The polysilane fluorescent
526 nm, in addition to a PMPS fluorescent band (Fl
S
) at 360 nm; the
bands overlap strongly with the absorption bands of the BODIPY-
based S state, but do not with the S state as shown in Fig. 2. In
contrast, the fluorescence of PMDMAPS occurs at around 385 nm
with weak spectra overlap with BODIPY-based S absorption,
because of the strong electron donor substituent effects of a p-
dimethylaminophenyl group on a Si atom.
1 band is a little broader than that in solution probably due to
significant intermolecular interaction in the solid state. Energy
2
1
transfer from the polysilane S
dye in the solid state is the responsible for the emission. The rela-
tive intensity of Fl to Fl depends on the concentration of 1. Thus,
the Fl /Fl value is consistent with the amount of BODIPY at lower
1 2
state to the S state of the BODIPY
2
B
S
B
S
Fluorescent spectra of solid BODIPY-polysilane mixed systems
are shown in Fig. 3. Usually, emission of BODIPYs is never observed
in the solid state, because the emission is completely quenched by
concentration of the dye than 3 mol % but decreases at the higher
concentration, probably due to the enhanced aggregation of 1. In
contrast, no emission of BODIPY dyes was observed for PMDMAPS
systems doped with 1 % 1 and 2, as shown in Fig. 3 (d). The
pep interaction in aggregation and self-quenching. As expected, no
Fig. 4. The color of polysilane powders with and without 1%wt 1 (excited at 365 nm using a high-pressure Hg arc lamp).

186
Q. Wang et al. / Dyes and Pigments 94 (2012) 183e186
characteristics are not only in good accord with the negligible
spectral overlap between PMDMAPS fluorescence with the BODIPY
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states of BODIPYs, it
to S is
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2
1
extremely fast (i.e., 100e250 fs) [27]. Because polysilanes are
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[
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2þ
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þ
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We have investigated the emission of BODIPY-doped polysilane
[
[
17] Tram K, Yan H, Jenkins HA, Vassiliev S, Bruce D. The synthesis and crystal
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systems for the first time. PMPS and PMTS films with less than
mol% of 1 or 2 in the solid state were found to show intense green
fluorescence of BODIPYs through efficient energy transfer from
polysilane S to the upper-lying singlet-excited state (S ) of the
3
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the s
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BODIPY acceptor, while no such energy transfer was observed for
energy miss-matching BODIPY-PMDMAPS systems. Because poly-
silanes are usually transparent in the visible region and their
fluorescence appears at near UV region, the color of photo-
luminescence of dye-doped systems is expected to be similar to
that of dye itself. BODIPY-doped polysilane systems are expected to
provide new types of polysilane-based electroluminescence and
related optoelectronic devices.
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transfer between dye compounds in micellar media. Dyes Pigm 2009;81:
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[
[
Acknowledgments
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transfer in amino-substituted boron dipyrromethene dyes and its inhibition
by cation complexation: a new design concept for highly sensitive fluorescent
probes. J Phys Chem A 1998;102:10211e20.
[24] Nakashima H, Fujiki M. Precise control of optical properties and global
conformations by marked substituent effects in poly(alkylarylsilane) homo-
and copolymers. Macromolecules 2001;34:7558e64.
We are thankful to the NSFC (nos. 21101049 and 20802014) and
the innovation teams for organosilicon chemistry (2009R50016) for
their financial support.
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substituted boradiazaindacene: pure red emission, relatively large stokes
shift and inhibition of self-quenching. Chem Commun 2008;39:4777e9.
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