Tuning the solid-state luminescence of bodipy derivatives with bulky arylsilyl groups: Synthesis and spectroscopic properties

Article abstract of DOI:10.1002/chem.201200169

Boron dipyrromethenes (BODIPYs) with bulky triphenylsilylphenyl(ethynyl) and triphenylsilylphenyl substituents on pyrrole sites were prepared via Hagihara-Sonogashira and Suzuki-Miyaura cross-coupling with ethynyl-terminated tetraphenylsilane and boronic acid-terminated tetraphenylsilane. The chromophores are designed to prevent intermolecular p-p stacking interaction and enhance fluorescence in the solid state. Single crystals of 1a and 2b for X-ray structural analysis were obtained, and weak p-p stacking interactions of the neighboring BODIPY molecules were observed. Spectroscopic properties of all of the dyes in various solvents and in films were investigated. Triphenylsilylphenyl-substituted BODIPYs generally show more pronounced increases in solid-state emission than triphenylsilylphenyl(ethynyl)-substituted BODIPYs. Although the simple BODIPYs do not exhibit any fluorescence in the solid state (F=0), arylsilyl-substituted BODIPYs exhibit weak to moderate solid-state fluorescence with quantum yields of 0.03, 0.07, 0.10, and 0.25. The structure-property relationships were analyzed on the basis of X-ray crystallography, optical spectroscopy, cyclic voltammetry, and theoretical calculations.

Full text of DOI:10.1002/chem.201200169

FULL PAPER
DOI: 10.1002/chem.201200169
Tuning the Solid-State Luminescence of BODIPY Derivatives with Bulky
Arylsilyl Groups: Synthesis and Spectroscopic Properties
Hua Lu,[a, b] Qiuhong Wang,[a] Lizhi Gai,[a] Zhifang Li,*[a] Yuan Deng,[a] Xuqiong Xiao,[a]
Guoqiao Lai,[a] and Zhen Shen*[b]
Abstract:
(BODIPYs)
triphenylsilylphenyl
phenylsilylphenyl substituents on pyr-
role sites were prepared via Hagihara–
Boron
dipyrromethenes
with bulky
(ethynyl) and tri-
X-ray structural analysis were ob-
tained, and weak p–p stacking interac-
tions of the neighboring BODIPY mol-
ecules were observed. Spectroscopic
properties of all of the dyes in various
solvents and in films were investigated.
Triphenylsilylphenyl-substituted BODI-
PYs generally show more pronounced
increases in solid-state emission than
triphenylsilylphenyl(ethynyl)-substitut-
ACHTUNGTRENNUNG
G
ed BODIPYs. Although the simple
BODIPYs do not exhibit any fluores-
cence in the solid state (F=0), arylsil-
yl-substituted BODIPYs exhibit weak
to moderate solid-state fluorescence
with quantum yields of 0.03, 0.07, 0.10,
and 0.25. The structure–property rela-
tionships were analyzed on the basis of
X-ray crystallography, optical spectros-
copy, cyclic voltammetry, and theoreti-
cal calculations.
Sonogashira
and
Suzuki–Miyaura
cross-coupling with ethynyl-terminated
tetraphenylsilane and boronic acid-ter-
minated tetraphenylsilane. The chro-
mophores are designed to prevent in-
termolecular p–p stacking interaction
and enhance fluorescence in the solid
state. Single crystals of 1a and 2b for
Keywords: boron dipyrromethenes ·
dyes/pigments · fluorescence · pho-
tophysics · solid-state emission
Introduction
ces, and doped films of polymer matrix or tris(8-hydroxyqui-
nolinato)aluminum (Alq3) with a BODIPY gave organic
light-emitting diodes (OLEDs).[3j] However, BODIPY dyes
that exhibit prominent solid-state emission are rare, since
emission in the solid state is effectively suppressed due to
their small Stokes shifts, which leads to self-quenching
through energy transfer.[4] Moreover, due to their flat p-con-
jugated systems, tight fluorophore aggregation through p–p
interaction in the solid phase leads to significant quench-
ing.[5] A facile method to inhibit intermolecular aggregation
might be the introduction of bulky substituents at the pe-
riphery of the p-conjugated (indacene) plane.[6] Recently,
this strategy has been successful achieved by introducing
bulky substituents such as 4-tritylphenylethynyl, paracyclo-
phane, and tert-butylphenyl at the meso and 2,3-positions of
the BODIPY core.[7]
Efficient solid-state emission of organic materials is required
for optoelectronic devices such as organic light-emitting
diodes, organic solid-state lasers, and light-emitting electro-
chemical cells.[1] Therefore, the molecular design and devel-
opment of novel organic fluorescent materials with high
solid-state emission efficiency in the visible region have at-
tracted increasing attention as functional materials.[2]
As “porphyrinꢀs little sister”, the BODIPY family, first
developed as fluorescent tags and laser dyes, has shown ex-
cellent spectroscopic properties, high lasing efficiency, and
photostability, and plays increasingly important roles in new
applications.[3] For example, Ziessel et al. used films of
a pure BODIPY to produce stable electroluminescent devi-
In this study, we chose triphenylsilylphenyl and
triphenylsilylphenylACHTUNRGTNE(NUG ethynyl) as bulky substituents to in-
[a] Dr. H. Lu, Q. Wang, L. Gai, Prof. Z. Li, Y. Deng, Dr. X. Xiao,
Prof. G. Lai
crease the steric hindrance and suppress p–p interaction; in
addition the change in geometrical structure due to steric
hindrance and nonplanarity increase the Stokes shift and
therefore efficiently inhibit self-quenching on excitation.[8]
To investigate the role of bulky substituents in increasing
solid-state emission of BODIPYs, a series of BODIPY de-
rivatives bearing arylsilyl substituents at the 2-position
(single side) and 2,6-positions (both sides) of the BODIPY
core were synthesized and characterized. The photophysical
properties were investigated in various solvents and in the
solid state. Electrochemical properties and TDDFT calcula-
tions were carried out to explore the effect of the arylsilyl
substituent and structure–property correlations.
Key Laboratory of Organosilicon Chemistry
and Material Technology Ministry of Education
Hangzhou Normal University
Hangzhou, 310012 (P.R. China)
Fax : (+86)571-2886-8529
E-mail: zhifanglee@hznu.edu.cn
[b] Dr. H. Lu, Prof. Z. Shen
State Key Laboratory of Coordination Chemistry
Nanjing National Laboratory of Microstructures
School of Chemistry and Chemical Engineering
Nanjing University
Nanjing 210093 (P.R. China)
Fax : (+86)25-8331-4502
E-mail: zshen@nju.edu.cn
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Results and Discussion
products and used for investigating the heavy-atom effect
on optical properties. All products were characterized by
1H NMR and 13C NMR spectroscopy and MALDI-TOF
mass spectrometry.
Synthesis: As shown in Scheme 1, borylation of 4-bromo-
phenyltriphenyl silane (1), which was synthesized by the re-
action of chlorotriphenylsilane with monolithiated 1,4-dibro-
mobenzene, gave 4-triphenylsilylphenylboronic acid (2) in
40% yield according to reported procedures.[9] Hagihara–
Sonogashira cross-coupling of 1 with trimethysilylethyne af-
forded 3 in 79% yield, which was deprotected with K2CO3
in methanol to give activated 4 in high yield (Supporting In-
formation). Triphenylsilylphenyl and triphenylsilylphenyl-
Crystal structures: To understand the effect of substituents
on the structures and the spectroscopic properties of the
BODIPY derivatives in the solid state, it is important to
have X-ray structures, but only single crystals of 1a and 2b
suitable for X-ray structure analysis were obtained, by diffu-
sion of hexane into a solution of 1a and 2b in dichlorome-
thane. As shown in Figure 1, the boron and silicon atoms
are coordinated in a tetrahedral geometry by two nitrogen
and two fluorine atoms for boron and four carbon atoms for
silicon. The meso (or 8)-phenyl ring on the BODIPY core is
almost orthogonal to the indacene plane with torsion angles
of 808 for 1a and 71.88 for 2b. The indacene plane is highly
planar with average root-mean-square (rms) deviation of
0.0427 ꢂ for 1a and 0.0516 ꢂ
ACHTUNGTRENNUNG(ethynyl) BODIPY derivatives were synthesized from iodo-
BODIPY 6 or di-iodo-BODIPY 7 with ethynyl-terminated
tetraphenylsilane 4 and boronic acid-terminated tetraphe-
nylsilane 2, respectively, by reported Hagihara–Sonogashira
and Suzuki–Miyaura cross-coupling protocols.[10–11] 2-Iodo-6-
triphenylsilylphenyl
and
2-iodo-6-triphenylsilylphenyl-
ACHTUNGTRENNUNG(ethynyl) BODIPYs 1b and 2b were also isolated as by-
for 2b. The dihedral angles be-
tween the indacene plane and
the phenyl rings at the 2-posi-
tions of 43.38 for 1a and 12.28
for 2b imply partial electronic
coupling between the indacene
plane and phenyl ring at 2-posi-
tion and that the coupling inter-
action is larger in 2b. The bulky
triphenylsilylphenyl groups lead
to enhanced distortion of the
molecular structure and inhibit
p–p stacking. According to the
molecular packing diagram
(Figure 2), the distance between
neighboring head-to-tail over-
lapping indacene planes is
8.10 ꢂ, and the distance of par-
tially overlapping head-to-head
p–p stacking is approximately
4.33 ꢂ. For 2b partially over-
lapping head-to-tail p–p stack-
ing interactions are observed
with separations of approxi-
mately
4.04
and
4.66 ꢂ
(Figure 2). For a conventional
BODIPY dye (similar structure
to 5), the comparable distance
is less than 3.7 ꢂ with almost
full overlap.[12] This clearly indi-
cates that introduction of bulky
arylsilyl groups can effectively
suppress intermolecular p–p
stacking.
Scheme 1. Synthetic procedures and structures of tetraphenylsilyl- (1a–c) and tetraphenylsilylethynyl- (2a–c)
substituted BODIPYs. I) Et2O, nBuLi, Ph3SiCl, ꢀ788C; II) THF, nBuLi, ꢀ788C, B(OCH3)3; III) [PdCl2-
(PPh3)2], CuI, PPh3, trimethylsilylacetylene, NEt3/toluene, N2, reflux; IV) In methanol, K2CO3, RT; V) CH2Cl2,
NIS, RT.
Spectroscopic properties in so-
lution: The UV/Vis absorption
and emission spectra of 1a–
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
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Tuning the Solid-State Luminescence of BODIPY Derivatives
FULL PAPER
Figure 1. Front (top) and side (bottom) ORTEP views of the molecular structures of 1a and 2b with the thermal ellipsoids set at 30% probability.
Figure 2. View of the p–p stacking interactions of 1a and 2b along the b axis.
c and 2a–c were measured in hexane, dichloromethane, and
acetonitrile. The photophysical properties are summarized
in Table 1.
fwhm2a–c ꢁ1.5 fwhm1a–c with slight reduction in molar ab-
sorption coefficient, which suggests stronger vibrational cou-
pling of the S0–S1 transition for 2a–c. The absorption spectra
are only barely affected by the solvent polarity; the maxi-
mum is slightly hypsochromically shifted (ca. 8 nm) with in-
creasing solvent polarity for 1a–c and 2a–c (Table 1). The
As shown in Figure 3, all the BODIPY derivatives exhibit
a typical cyanine-type vibronic band in the visible region in
the range of 514–578 nm, which can be attributed to 0–0 vi-
brational band of a strong S0–S1 transition, and a shoulder
(more or less pronounced) at shorter wavelength is ascribed
to the 0–1 vibrational band of the same transition. A broad-
er, much weaker band around 400 nm can be attributed to
the second or third transition (Figure 3).[13] The absorption
spectra of 2a–c are broader than those of 1a–c, and the full
width at half-maximum (fwhmabs) follows the relationship
influence of the triphenylsilylphenylACTHNUTRGNEUNG(ethynyl) substituents
on the redshift of the absorption and emission spectra is
generally more pronounced than that of triphenylsilylphenyl
substituents in 1a–c; for example, the absorption maxima of
1c and 2c are bathochromically shifted by 22 and 65 nm in
MeCN, respectively, in comparison to that of 5.[13a] This is
not surprising, because the triphenylsilylphenylACHTUNGTRENNUNG(ethynyl)
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Z. Shen, Z. Li et al.
Table 1. Spectroscopic and photophysical properties of 1a–c and 2a–c in various solvents at 298 K.
Solvent
labs [nm]
lgemax
fwhmabs [nm]
lem [nm]
fwhmem [nm]
Dnemꢀabs [nm]
F
tf [ns]
kr [108 sꢀ1
]
knr [108 sꢀ1
]
1a
2a
hexane
CH2Cl2
CH3CN
hexane
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
hexane
CH2Cl2
CH3CN
hexane
CH2Cl2
CH3CN
518
518
514
540
536
531
534
556
533
531
527
578
575
570
4.74
4.71
4.72
4.79
4.78
4.80
4.77
4.70
4.81
4.85
4.83
4.79
4.83
4.83
28
29
29
51
48
48
34
50
35
36
36
54
58
59
537
538
534
563
566
564
554
579
556
558
554
600
600
595
31
34
35
27
35
38
20
38
31
32
33
32
36
39
19
20
20
23
30
33
20
23
23
27
27
22
25
25
0.90
0.89
0.92
0.52
0.32
0.29
0.12
0.05
0.75
0.67
0.69
0.17
0.15
0.16
3.15
3.42
3.36
3.49
2.78
2.40
0.46
0.60
2.99
3.24
3.17
2.92
2.83
2.48
2.86
2.60
2.74
1.49
1.15
1.21
2.61
0.83
2.51
2.07
2.18
0.58
0.53
0.65
0.32
0.32
0.24
1.38
2.45
2.96
19.1
15.8
0.84
1.02
0.98
2.84
3.00
3.39
1b
2b
1c
2c
is easier than that around the s bond in the 1 series, and ex-
cited-state conformational changes drive faster internal con-
version to the ground state in 2a and 2c, which quenches
the emission (higher knr values for 2 series than for 1 series,
Table 1).[14] Therefore, the fluorescence quantum yield of
the 2 series is lower than that of the corresponding 1 series.
The fluorescence spectra are mirror images of the lowest-
energy absorption spectra, and the band position of the
emission does not show any particular trend as a function of
solvent polarity. These features suggest that emission occurs
from the weakly polar, relaxed Franck–Condon excited state
of the fluorescent chromophores.[15] The Stokes shifts of the
arylsilyl-substituted BODIPYs (20–30 nm) are larger than
those of classical BODIPYs (ca. 10 nm for 5),[13,16] and this
indicates geometrical rearrangement on excitation. This is
also reflected in the increasing width of the fluorescence
emission spectra of arylsilyl-substituted dyes (fwhmem
>
35 nm in MeCN) compared to (fwhmem >23 nm in
5
MeCN), which can be explained by the crystal structure,
which shows a distorted structure with lower rotational
energy barrier around b–b linked bonds. Free rotation of
the arylsilyl moieties is indispensable for keeping the Stokes
shift large enough to suppress energy transfer from the ex-
cited state by the Fçrster mechanism.[2a]
Figure 3. Normalized absorption and fluorescence spectra of 1a–c and
2a–c in dichloromethane.
The fluorescence decay profiles of dyes 1a–c and 2a–
c could be described by a single-exponential fit (fluores-
cence lifetime in the range of 2.4–3.5 ns except for 1b and
2b, see Table 1 and Figure S4 in Supporting Information) in
all of the solvents investigated, similar to the lifetime data
of other BODIPY systems published in the literature.[13,15–17]
For 2a, the optical properties are slightly dependent on sol-
vent polarity; for example, the absorption band shows a hyp-
sochromic shift of approximately 11 nm and the Stokes shift
increases from 23 to 33 nm with increasing solvent polarity
from hexane to MeCN. The fluorescence quantum yields are
relative high and show a positive solvatokinetic behavior
from 0.52 in hexane to 0.29 in MeCN. The solvent depend-
ence of the optical properties could be attributable to the
unsymmetrical charge distribution, with the silyl group
showing some electronic donor ability due to s–p interac-
tion between silicon and the phenyl group, which increases
substituent displays larger electronic coupling with the
BODIPY chromophore and therefore significantly expands
the p-conjugated system. While the absorption probability is
enhanced by the extended conjugation, both the fluores-
cence quantum yield and the lifetime of 2a–c decrease with
respect to the corresponding 1 series, due to an increase in
nonradiative deactivation rate constant (Table 1). It is well
known that one of the main drawbacks of red-emitting dyes
is their lower fluorescence capacity, since the ground and ex-
cited state are energetically closer, and the probability of in-
ternal conversion is greatly enhanced due to the increased
likelihood of vibrational coupling between the electronic ex-
cited and ground states. Structurally, rotation of the triphe-
nylsilylphenyl group about the ethynyl bridge in the 2 series
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Tuning the Solid-State Luminescence of BODIPY Derivatives
FULL PAPER
the dipole moment of the ground state;[18] quantum chemical
calculations confirm this hypothesis.[19] The fluorescence
quantum yields of 1b and 2b are considerably lower than
those of 1a and 2a, and this trends is reflected by the rates
of nonradiative decay knr: iodo substitution leads to 60- and
fivefold increases for 1b and 2b, respectively (Table 1), due
to the heavy-atom effect of iodine atoms, which enhances
the rate of intersystem crossing of the dyes.[20]
c in the solid state, similar to the phenomena observed in so-
lution, that is, intermolecular vibronic coupling for 2a–2c is
stronger than for 1a–c.
The absolute quantum yields FF of the BODIPY dyes in
powder were measured by Edinburgh Photonics FLS920
fluorescence spectrometer. Dye 5 did not exhibit any fluo-
rescence in the solid state, similar to the behavior of other
BODIPYs that have been reported so far.[7,8a,21] Our de-
signed BODIPYs showed prominent solid-state fluores-
cence, and the quantum yields for 1a, 1c, 2a, and 2c of 0.25,
0.07, 0.10, and 0.03, respectively, suggest that introduction of
the bulky arylsilyl group is very effective in enhancing the
solid-state emission efficiency of BODIPY dyes. According
to the design principal, more intensive solid-state emission
of 1c and 2c compared to 1a and 2a is expected, but the ob-
served solid-state fluorescence contradicts this prediction.
To better understand this result and investigate the possible
effect of the arylsilyl group on fluorescence efficiency, we
evaluated the relative fluorescent intensity in doped film
(Figure S4, Supporting Information). As expected, the rela-
tive fluorescent intensity of 1c is higher than that of 1a due
to the enhanced steric hindrance of two bulky arylsilyl
groups in the BODIPY chro-
Solid-state photophysical properties: Films were prepared
by evaporation of the poly(methyl methacrylate) (PMMA)
containing 10 wt% dye in dichloromethane onto a quartz
plate, which was utilized to measure the electronic spectra.
Absorption and emission spectra of the dyes in film are
shown in Figure 4 and the main photophysical properties
are summarized in Table 2. The absorption band of dyes 1a,
1c, 2a, and 2c in doped films are almost unchanged, with
a slightly broader shape, slightly redshifted emission band,
and increased Stokes shift with respect to the spectra in so-
lution, which can be attributed to the weak intermolecular
interaction and inhomogeneous broading in the film. The
absorption spectra of 2a–c are broader than those of 1a–
mophore. The difference in
fluorescence behavior of 1a
and 1c in powders and in
doped film could be ascribed to
the change of aggregative state,
which increases the reabsorp-
tion of 1c and leads to fluores-
cent quenching in powders.
Film-doping dye 2c with two
triphenylsilylphenylACTHNUGRTENUNG(ethynyl)
substituents exhibits weaker rel-
ative fluorescent intensity than
monosubstituted 2a, most likely
due to the enhanced probability
of internal conversion caused
by electronic coupling between
triphenylsilylphenylACTHNUGRTENUNG(ethynyl)
and BODIPY. The result is in
consistent with the fluorescence
behavior in powder and in solu-
tion. Previously, Dreuw et al.
showed that the of molecular
properties of the individual
molecules are preserved in the
solid state.[25b–c]
Figure 4. Absorption (solid line) and fluorescence spectra (dotted line) of 1a, 1c and 2a, 2c in dichlorome-
~
&
thane ( ) and in doped PMMA film ( ).
Molecular orbitals and transi-
tion: To investigate the influ-
Table 2. Spectroscopic and photophysical properties of 1a, 1c and 2a, 2c in doped
PMMA film at 298 K.
ence of structural modification with bulky groups
on the spectroscopic and electronic properties of
the 2- and 2,6-substituted BODIPYs, ground-state
geometry optimizations were performed by DFT
with B3LYP functional and 6-31G(d) basis set
(LanL2DZ basis for I atom) by using the Gaussi-
[a]
labs [nm]
fwhmabs [nm]
lem [nm]
fwhmem [nm]
Dnemꢀabs [nm]
Fpowder
1a
2a
1c
2c
516
537
533
575
36
59
44
78
544
575
563
613
35
46
34
44
28
38
30
38
0.25
0.10
0.07
0.03
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Z. Shen, Z. Li et al.
an 03 software package.[22] UV/Vis spectra were simulated
by utilizing the first 20 excited states and the time-depen-
dent density functional theory (TDDFT) routine with the
same approach. For 1a, 1c and 2a, 2c, the calculated ab-
sorption spectra in the gas phase are blueshifted, similar to
reported BODIPY systems, and their sequence agrees
rather well with the measured absorption spectra in solu-
tion. Figure 5 shows the frontier molecular orbitals of 1a–
Figure 5. Frontier orbital energies of 5, 1a–c, and 2a–c calculated at the
B3LYP level with LanL2DZ basis set for I atoms and 6-31G(d) basis sets
for the other atoms. 5: R1 =R2 =H; 1a: R1 =TPS, R2 =H; 1b: R1 =TPS,
R2 =I; 1c: R1 =R2 =TPS; 2a: R1 =TPSE, R2 =H; 2b: R1 =TPSE, R2 =I;
2c:
triphenylsilylphenylACHTUNGTRENNUNG
R1 =R2 =TPSE.
(ethynyl).
TPS=triphenylsilylphenyl,
TPSE=
Figure 6. Nodal patterns of the frontier p MOs of 1a–c (top) and 2a–
c (bottom) at an isosurface value of 0.02.
c and 2a–c. Table 3 list the relevant lowest-energy transi-
tions. According to Table 3, the lowest-energy excitation of
dyes is mixed HOMO!LUMO and HOMOꢀ1!LUMO
transition with high oscillator strength. For 1a–c, HOMO
and LUMO are almost localized on the BODIPY core and
less on the phenyl (2-position) fragments, and only
HOMOꢀ1 is localized on the phenyl-BODIPY chromo-
phores. In contrast, HOMO and HOMOꢀ1 are almost fully
distribution of the frontier MOs explains the absorption
spectra well. The LUMO and HOMO of 1a and 1c are
slightly different from those of unsubstituted 5 due to the
weak electronic coupling between phenyl group and
BODIPY core. The triphenylsilylphenylACTHNUTRGNEUNG(ethynyl) substitu-
ents in 2a–c seem to stabilize the LUMO and destabilize
the HOMO, and thus effectively narrow the energy gap,
which is responsible for the more bathochromic shift when
compared with the spectral band positions of 1a–c and un-
substituted dye 5. The HOMO and LUMO are both stabi-
lized for I-substituted 1b and 2b compared to 1a and 2a, re-
spectively, which can be ex-
localized on the triphenylsilylphenylACHTNUTRGNEUNG(ethynyl) BODIPY
chromophores for 2a–c and the LUMO is localized mainly
on the BODIPY core (Figure 6). Therefore, the electronic
plained by significant participa-
Table 3. Calculated electronic excitation energies, oscillator strengths, and related wave functions.
Dye State[a] Energy [eV] l [nm] f[b] Orbitals (coefficient)[c]
tion of the I atom in HOMO
and HOMOꢀ1. The slightly
greater stabilization of the
LUMO compared to the
HOMO explains the small red-
shift of the absorption and
emission maxima. The lowest-
energy excitation of 2c is pre-
dicted to lie at 544 nm (f=
1.012) and to arise primarily
from the HOMO!LUMO and
HOMOꢀ2!LUMO transitions
(Table 3). The wavelengths of
the third lowest energy transi-
tions in the calculated spectrum
1a
1b
1c
S1
S2
S1
S2
S1
S2
S3
S1
S2
S1
S2
S1
S2
S3
2.85
3.28
2.34
2.93
2.73
3.24
3.26
2.48
3.17
2.28
2.91
2.28
2.70
3.16
434
377
530
424
455
382
381
500
3p91
543
425
544
460
392
0.610 H–L (0.552) Hꢀ1–L (ꢀ0.280)
0.257 H–L (0.215) Hꢀ1–L (0.604)
0.250 H–L (0.576) Hꢀ1–L (0.315)
0.555 H–L (0.569) Hꢀ1–L (ꢀ0.228)
0.883 H–L (0.568) Hꢀ1–L (ꢀ0.265)
0.044 Hꢀ1–L (0.139) Hꢀ2–L (0.658)
0.380 H–L (0.191) Hꢀ1–L (0.595) Hꢀ2–L (ꢀ0.145)
0.523 H–L (0.598) Hꢀ1–L (ꢀ0.252)
0.653 H–L (0.165) Hꢀ1–L (0.585) H–L+1 (ꢀ0.122)
0.259 H–L (0.606) Hꢀ1–L (0.267)
2a
2b
2c
0.611 H–L (ꢀ0.183) Hꢀ1–L (0.580)
1.005 H–L (0.619) Hꢀ2–L (0.193)
0.041 Hꢀ1–L (0.671)
1.011 Hꢀ2–L (0.601) H–L+1 (0.135) H–L (ꢀ0.108) Hꢀ1–L+2 (0.117)
[a] Excited state. [b] Oscillator strength. [c] MOs involved in the transitions.
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Tuning the Solid-State Luminescence of BODIPY Derivatives
FULL PAPER
closely match the experimental data around 400 nm. The
lowest-energy excitations of 1b and 2b are predicted to lie
at 530 and 543 nm with oscillator strengths of 0.250 and
0.259, respectively. Compared to calculated and experimen-
tal absorption spectra of 1a, 1c and 2a, 2c, the calculated S1
displays lower energy with less strong oscillator strength,
and the singlet has some triplet character. This mixing of
singlet and triplet character is the result of spin–orbit cou-
pling due to introduction of heavy iodine atoms (Figure S2,
Supporting Information).[23]
Analysis of oscillator strengths and fluorescent quantum
yields suggested that the sequence of fluorescent quantum
yields is 1a, 1c>2a, 2c>1b, 2b. The S0!S1 and S0!S2 (S3)
transitions of 2a, 2c have comparable oscillator strengths,
the S0!S1 transitions for 1a, 1c are stronger than the S0!S2
(S3) transitions, and the S0!S1 transitions for 1b, 2b are
weaker than the S0!S2 transitions. Invoking Kashaꢀs rule,
which states that fluorescence occurs in appreciable yield
only from the lowest excited state S1,[24] fluorescence
quenching is predicted to result from loss of energy in the S2
(S3) excited state through its decay to the S1 state. In fact,
for most dyes, if the energetically lowest excited state S1 is
a forbidden state (or has low oscillator strength), the exis-
tence of potential-energy surfaces of the excited state
should lead to fluorescence quenching.[25]
which indicates that the materials are suitable for applica-
tion in OLEDs, because these energy levels match well with
those of commonly used hole-transfer layer materials (e.g.,
NPB: ꢀ5.2 eV).[27]
Conclusion
The synthesis, characterization, and theoretical analysis of
a series of arylsilyl-substituted BODIPY dyes have been de-
scribed. The introduction of triphenylsilylphenyl and
triphenylsilylphenyl
promising for new organic materials with efficient solid-
state emission, but challenges remain. The
triphenylsilylphenyl(ethynyl) substituent was found to favor
ACHTUNGTRNE(NUNG ethynyl) as bulky groups is clearly
AHCTUNGTRENNUNG
redshifts of the spectra but decreases the fluorescence quan-
tum yield in solid state and solution; therefore, this group is
not a good candidate for organic emitting materials. Triphe-
nylsilylphenyl-substituted BODIPYs exhibit efficient solid-
state emission compared to triphenylsilylphenylACTHNUTRGENUG(N ethynyl)-
substituted BODIPYs with slightly redshifted spectra. The
heavy-atom effect of an iodo substituent leads to fluores-
cence quenching due to enhanced rate of intersystem cross-
ing of the dyes. According to our research, three factors
should be considered for the design of BODIPY dyes with
efficient solid-state emission: 1) large Stokes shifts, which
can be achieved either through internal charge transfer or
excitation energy transfer (EET) assembly of an excitation
energy donor and energy acceptor linked through either
a nonconjugated spacer (in the case of fluorescence reso-
nance energy transfer, FRET) or a conjugated spacer (in the
case of through-bond energy transfer, TBET).[28] 2) Weaker
intermolecular interaction, by introducing a bulky group to
inhibit intermolecular p–p interaction. 3) The inner struc-
ture factor: strong fluorescence in solvents with high oscilla-
tor strength (S0–S1) and weak oscillator coupling (with the
exception of aggregation-induced emission, as proposed by
Tang et al.).[29]
Electrochemistry: The electrochemical properties of the ar-
ylsilyl-BODIPYs were studied by cyclic voltammetry in di-
chloromethane with 0.1m tetra-n-butylammonium hexafluor-
ophosphate (TBFP) as supporting electrolyte (Table 4).
Compounds 1a and 1c display a reversible one-electron oxi-
dation wave and two reversible one-electron reduction
waves indicating formation of stable dianions, whereas 2a
and 2c exhibit a reversible one-electron oxidation wave and
a reversible one-electron reduction wave. Compared to the
conventional BODIPY dye 5 (Eo1=x2 =760, E1re=d2 =ꢀ1560 mV
vs. Fc+/Fc),[15a] the reduction potentials of 1a, 1c and 2a, 2c
are positively shifted and the oxidation potentials are slight-
ly lower, as the introduction of the arylsilyl group causes
slight stabilization of the LUMO and destabilization of the
HOMO, which lead to narrowing of the HOMO–LUMO
bandgap and hence result in redshifting of the lowest-energy
absorption bands. The ionization potentials (IP) or HOMO
levels of the dyes were estimated to lie in the range of
ꢀ5.10 to ꢀ5.15 by using the equation IP=Eo1=x2 +4.4,[26]
Experimental Section
Synthesis of 2-iodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazain-
dacene (6): At 08C, N-iodosuccinimide (NIS, 70.0 mg, 0.31 mmol) in dry
CH2Cl2 (10 mL) was added dropwise to
a solution of 5 (100 mg,
0.31 mmol) in dry CH2Cl2 (20 mL) over about 30 min. After addition, the
reaction mixture was allowed to stir at room temperature for
1 h. The reaction mixture was then concentrated under re-
duced pressure, and the crude product was purified by silica-
gel column chromatography (hexane/ethyl acetate, 5/1, v/v).
The second band was collected to give the product as a red
solid in 65% yield. 1H NMR (400 MHz, CDCl3): d=7.51–7.48
(m, 3H), 7.28–7.25 (m, 2H), 6.04 (s, 1H), 2.63 (s, 3H), 2.57 (s,
3H), 1.38 ppm (s, 6H); MALDI-TOF MS: m/z calcd for
[C19H18BF2IN2]+: 450.06; found: 450.3 [M]+, 431.3 [MꢀF]+.
Synthesis of 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluor-
oboradiazaindacene (7): An excess of NIS (837 mg, 3.72 mmol)
was added to a solution of 5 (300 mg, 0.93 mmol) in anhydrous
CH2Cl2 (40 mL) and the mixture stirred at room temperature
for about 30 min (monitored by TLC until complete consump-
Table 4. Redox potentials and electronic states of compounds 1a, 1c and 2a, 2c (in di-
chloromethane, 0.1m TBFP, scan rate 100 mVsꢀ1).[a]
[b]
ox
1=2
red
1=2
red
1=2
Eg
E
E
E
HOMO/LUMO[c]
HOMO/LUMO[d]
1st
2nd
1a
1c
2a
2c
2.35
2.28
2.24
2.11
750
730
720
700
ꢀ1320
ꢀ1350
ꢀ1420
ꢀ1390
ꢀ1590
ꢀ1630
n.d.
ꢀ5.15/ꢀ2.80
ꢀ5.13/ꢀ2.85
ꢀ5.12/ꢀ2.88
ꢀ5.10/ꢀ2.99
ꢀ5.30/ꢀ2.37
ꢀ5.28/ꢀ2.39
ꢀ5.22/ꢀ2.50
ꢀ5.20/ꢀ2.64
n.d.
[a] Versus Fc+/Fc. [b] Energy bandgap, determined from wavelength of intersection
point of UV/Vis and photoluminescence. [c] HOMO=Eo1=x2 +4.4 eV, LUMO=
HOMOꢀEg. [d] Energy levels estimated from theoretical calculation.
Chem. Eur. J. 2012, 00, 0 – 0
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Z. Shen, Z. Li et al.
tion of the starting material). The reaction mixture was concentrated
under vacuum, and the crude product was purified by silica-gel column
chromatography with hexane/CH2Cl2 (2/1, v/v) as eluent affording a red
solid (450 mg, yield 84.0%). 1H NMR (400 MHz, CDCl3): d=7.52 (m,
3H), 7.26 (m, 2H), 2.65 (s, 6H), 1.38 (s, 6H); MALDI-TOF MS: m/z
calcd for [C19H17BF2I2N2]+: 575.97; found: 575.8 [M]+, 556.8 [MꢀF]+.
Synthesis of 2-iodo-6-triphenylsilylphenyl-1,3,5,7-tetramethyl-8-phenyl-
4,4-difluoroboradiazaindacene (1b) and 2, 6-ditriphenylsilylphenyl-
1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (1c): Com-
Synthesis of 2-triphenylsilylphenylACTHNGUTER(NNUG ethynyl)-1,3,5,7-tetramethyl-8-phenyl-
4,4-difluoroboradiazaindacene (2a): Compound 2a was obtained as a red
solid in 65% yield by following a procedure similar to that of 2c.
1H NMR (400 MHz, CDCl3): d=7.57–7.54 (m, 6H), 7.52–7.50 (m, 4H),
7.46–7.43 (m, 6H), 7.40–7.36 (m, 6H), 7.30–7.29 (m, 2H), 6.04 (s, 1H),
2.71 (s, 3H), 2.59 (s, 3H), 1.50 (s, 3H), 1.41 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=157.7, 156.6, 144.7, 142.1, 136.3, 136.2, 134.6,
134.4, 133.8, 130.5, 129.7, 129.2, 129.2, 127.9, 127.8, 124.7, 122.1, 95.9,
83.1, 14.8, 14.5, 13.5, 13.1 ppm; UV/Vis (CH2Cl2): lmax (e)=536 nm
(61000 dm3 molꢀ1 cmꢀ1);
MALDI-TOF
MS:
m/z
calcd
for
pound
7 (150.0 mg, 0.26 mmol), 4-triphenylsilylphenylboronic acid
[C45H37BF2N2Si]+: 682.28; found: 682.5 [M]+, 663.5 [MꢀF]+.
(179 mg, 0.47 mmol), and tetrakis(triphenylphosphine)palladium(0)
(18.2 mg, 0.026 mmol) under argon atmosphere were added to THF
(20 mL). After addition of 3 mL of aqueous 2N sodium carbonate solu-
tion, the reaction mixture was heated at 808C for 24 h. The cooled crude
mixture was poured into water (50 mL), extracted with CH2Cl2 (3ꢃ
60 mL), and dried over anhydrous magnesium sulfate. Silica-gel column
chromatography (hexane/CH2Cl2 4/1) gave 1c as a red solid (142 mg,
55% yield) and byproduct 1b as an orange red solid (25 mg, 12% yield).
Compound 1c: 1H NMR (400 MHz, CDCl3): d=7.59–7.56 (m, 16H),
7.51–7.36 (m, 23H), 7.18–7.16 (m, 4H), 2.56 (s, 6H), 1.32 ppm (s, 6H);
13C NMR (100 MHz, CDCl3) d 154.3, 139.4, 136.4, 136.3, 135.4, 135.0,
134.2, 133.5, 132.8, 131.4, 129.7, 129.6, 129.3, 129.1, 128.1, 127.9, 13.5,
12.9 ppm; UV/Vis (CH2Cl2): lmax (e)=531 nm (71000 dm3 molꢀ1 cmꢀ1);
MALDI-TOF MS: m/z calcd for [C19H17BF2I2N2]+: 993.15; found: 993.1
[M]+, 973.9 [MꢀF]+. Compound 1b: 1H NMR (400 MHz, CDCl3): d=
7.59–7.55 (m, 8H), 7.50–7.49 (m, 3H), 7.45–7.36 (m, 9H), 7.30–7.25 (m,
2H), 7.15 (s, 1H), 7.13 (s, 1H), 2.66 (s, 3H), 2.55 (s, 3H), 1.39 (s, 3H),
1.31 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=156.4, 154.9, 143.5,
141.8, 140.9, 136.4, 135.1, 134.1, 133.2, 129.7, 129.5, 129.4, 127.9, 14.2,
Spectroscopic measurements: UV/Vis spectra were recorded on a Shimad-
zu UV-2550 spectrophotometer. Fluorescence spectra were measured on
a Hitachi F-2700 FL spectrophotometer with a xenon arc lamp as light
source. Samples for absorption and emission measurements were con-
tained in 1ꢃ1 cm quartz cuvettes. For all measurement, the temperature
was kept constant at 298ꢂ2 K. Dilute solutions with absorbance of less
than 0.05 at the excited wavelength were used for the measurement of
fluorescence quantum yields. The luminescence quantum yields in solu-
tion were measured by using rhodamine 6G with excitation wavelength
of 488 nm (FF =0.88 in ethanol) as reference.[30] The quantum yield F as
a function solvent polarity was calculated by using Equation (1)
Isample
Istd Asample nstd
nsample
2
Astd
Fsample ¼ Fstd
½
ꢃ½
ꢃ½
ꢃ
ð1Þ
where subscripts sample and std denote the sample and standard, respec-
tively, F is quantum yield, I the integrated emission intensity, A the ab-
sorbance, and n the refractive index.
14.1,
13.7,
13.0 ppm;
UV/Vis
MALDI-TOF
(CH2Cl2):
MS:
lmax
(e)=534 nm
calcd for
The fluorescence lifetimes of the samples were measured on a PS920
Lifespec-ps (Edinburgh) with picosecond pulsed diode laser (PDL800-B)
as excitation source. The goodness of fit of the single decays, as judged
by reduced chi squared (c2R) and autocorrelation function C(j) of the re-
siduals was c2R <1.1.
(59000 dm3 molꢀ1 cmꢀ1);
m/z
[C43H36BF2IN2Si]+: 784.56; found: 784.8 [M]+, 765.8 [MꢀF]+.
Synthesis of 2-iodo-6-triphenylsilylphenyl
phenyl-4,4-difluoroboradiazaindacene
ACHTUNGTRENNUNG
When the fluorescence decays were monoexponential, the rate constants
of radiative (kf) and nonradiative (knr) deactivation were calculated from
the measured fluorescence quantum yield Ff and fluorescence lifetime t
according to Equations (2) and (3).
ditriphenylsilylphenyl
boradiazaindacene (2c): Under argon atmosphere,
0.26 mmol), triphenylsilylphenylacetylene (169 mg, 0.47 mmol), [PdCl2-
ACHTUNGTRENNUNG(PPh3)2] (18.2 mg, 0.026 mmol), PPh3 (13.6 mg, 0.052 mmol), and CuI
ACHTUNGTRENNUNG
7
(5.4 mg, 0.026 mmol) were dissolved in triethylamine (3 mL) and toluene
(9 mL). The solution was stirred at 808C overnight. The solvent was re-
moved under reduced pressure, and the crude product was purified by
silica-gel column chromatography with CH2Cl2 as the eluent to obtain 2c
as a dark red solid (138 mg, 51% yield) and byproduct 2b as a purple-
red solid (30 mg, 14% yield). Compound 2c: 1H NMR (400 MHz,
CDCl3): d=7.56–7.51 (m, 20H), 7.46–7.42 (m, 11H), 7.39–7.35 (m, 12H),
2.72 (s, 6H), 1.52 ppm (s, 6H); 13C NMR (100 MHz, CDCl3) d 158.6,
144.2, 142.6, 136.4, 136.3, 134.7, 133.8, 130.5, 129.7, 129.4, 127.9, 127.8,
124.5, 96.6, 82.7, 13.4 ppm; UV/Vis (CH2Cl2): lmax (e)=575 nm (67000
dm3 molꢀ1 cmꢀ1); MALDI-TOF: m/z calcd for [C24H26BF2IN2Si]+:
1041.19; found: 1040.2 [M]+, 1021.0 [MꢀF]+. Compound 2b: 1H NMR
(400 MHz, CDCl3): d=7.56–7.51 (m, 12H), 7.45–7.41 (m, 5H), 7.38–7.35
(m, 6H), 2.70 (s, 3H), 2.66 (s, 3H), 1.49 (s, 3H), 1.40 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=156.4, 154.9, 143.5, 141.8, 141.0, 136.4,
135.1, 134.5, 134.1, 133.2, 131.5, 131.3, 129.7, 129.5, 129.4, 127.9 ppm; UV/
Vis (CH2Cl2): lmax (e)=556 nm (50000 dm3 molꢀ1 cmꢀ1); MALDI-TOF
MS: m/z calcd for [C45H36BF2IN2Si]+: 808.17; found: 808.7 [M]+, 789.7
[MꢀF]+.
kf ¼ Ff=t
ð2Þ
ð3Þ
knr ¼ ð1ꢀFfÞ=t
Computational details: The ground-state structures of BODIPYs were
computed by the DFT method with the hybrid-generalized gradient ap-
proximation (H-GGA) functional B3LYP.[31] 6-31G(d) basis set was as-
signed to the elements C, H, N, Si, B, and F, which guarantees a reasona-
ble balance of computational cost and the reliability of the results. For
iodo substituents, the effective core potential LanL2DZ basis set was em-
ployed to incorporate the relativistic corrections. The absorption proper-
ties were predicted by the TDDFT method with the same basis set. All
calculations were performed with the Gaussian 03 program package.
Acknowledgements
We are thankful to the NSFC (Nos. 21101049, 20903032, and 20971066),
the Chinese Ministry of Educationꢀs Program for New Century Excellent
Talents in Universities (no. NCET-08-0272), and the innovation teams for
organosilicon chemistry (2009R50016) for their financial supports.
Synthesis of 2-triphenylsilylphenyl-1,3,5,7-tetramethyl-8-phenyl-4,4-di-
fluoroboradiazaindacene (1a): Compound 1a was obtained as a red solid
in 61% yield by following a procedure similar to that of 1c. 1H NMR
(400 MHz, CDCl3): d=7.60–7.57 (m, 8H), 7.50–7.47 (m, 3H), 7.46–7.42
(m, 3H), 7.40–7.38 (m, 6H),7.34–7.32 (m, 2H), 7.17–7.15 (d, J=8 Hz,
2H), 6.02 (s, 1H), 2.59 (s, 3H), 2.56 (s, 3H), 1.40 (s, 3H), 1.33 ppm (s,
3H); 13C NMR (100 MHz, CDCl3): d=155.8, 153.9, 143.4, 141.9, 136.4,
136.2, 135.2, 135.0, 134.1, 132.7, 129.6, 129.5, 129.2, 129.0, 128.0, 127.9,
121.4, 14.6, 14.4, 13.4, 12.7 ppm; UV/Vis (CH2Cl2): lmax (e)=518 nm
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[C43H37BF2N2Si]+: 658.28; found: 658.4 [M]+, 639.4 [MꢀF]+.
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Tuning the Solid-State Luminescence of BODIPY Derivatives
FULL PAPER
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Received: January 16, 2012
Revised: March 16, 2012
Published online: && &&, 0000
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These are not the final page numbers!

Tuning the Solid-State Luminescence of BODIPY Derivatives
FULL PAPER
Enhanced solid-state emission: A
series of arylsilyl-substituted BODIPY
dyes, designed to prevent intermolecu-
lar p–p stacking interactions and
enhance emission in the solid state,
were synthesized and their spectro-
scopic properties investigated.
Although simple BODIPYs like 5 are
nonfluorescent in the solid state
(FF =0), arylsilyl-substituted BODI-
PYs, such as 1a and 2a, exhibit FF
values of up to 0.25 (see figure).
Dyes/Pigments
H. Lu, Q. Wang, L. Gai, Z. Li,*
Y. Deng, X. Xiao, G. Lai,
Z. Shen* ....................... &&&& — &&&&
Tuning the Solid-State Luminescence
of BODIPY Derivatives with Bulky
Arylsilyl Groups: Synthesis and Spec-
troscopic Properties
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
11
&
ÞÞ
These are not the final page numbers!