A dual emissive BODIPY dye and its use in functionalizing highly monodispersed PbS nanoparticles

Article abstract of DOI:10.1002/anie.201104690

Shiny particles: A newly synthesized BODIPY dye displays unusual dual emission and can be used to functionalize highly monodisperse PbS nanoparticles (see picture; B pink, F green, O red, C gray, H white). Electronic communication between the BODIPY dye and the PbS NPs can be exploited in a photovoltaic cell. Copyright

Full text of DOI:10.1002/anie.201104690

Communications
DOI: 10.1002/anie.201104690
Nanoparticles
A Dual Emissive BODIPY Dye and Its Use in Functionalizing Highly
Monodispersed PbS Nanoparticles**
Jia-sheng Lu, Huiying Fu, Yanguang Zhang, Zygmunt J. Jakubek, Ye Tao, and Suning Wang*
BODIPY (boron-dipyrromethene) dyes and derivatives are
well-known to be very effective in light-harvesting and
energy-transfer processes, owing to their high fluorescence
quantum yields, large molar absorption coefficients, relatively
long excited-state lifetimes, and excellent photochemical
stability.[1] Thus, they have frequently been used in light-
harvesting molecules, as dye sensitizers, and as probes and
labels for biomolecules.[1–2] One important class of materials
for various optoelectronic applications, including solar cells, is
the class of semiconductor nanoparticles (NPs). The optical
and energetic properties of these nanoparticles can be tuned
by varying their shape, size, or surface ligands.[3–4] Lead sulfide
NPs are particularly attractive among NPs owing to their
narrow band gap, large exciton Bohr radii, and their
absorption and emission in the near-IR region.[3e,5] Several
recent reports have shown that PbS NPs are very promising
materials for achieving high-performance photovoltaic devi-
ces.[4e,5,6] The most commonly used surface ligand for PbS NPs
is oleic acid, which can effectively protect the NPs from
oxidation and can facilitate their dispersion in organic
solvents.[3,5–7] However, because they lack any interesting
photophysical properties, oleic acid capping ligands do not
engage in any electronic communication or interactions with
the NPs and thus have little influence on the properties of the
NPs and insulate NPs from each other and the surrounding
medium. New surface ligands that can communicate elec-
tronically with PbS NPs and enhance their performance in
optoelectronic devices are therefore in demand. Based on this
consideration and the very attractive photophysical proper-
ties of BODIPY dyes, we initiated the investigation of new
BODIPY dyes as potential new surface ligands for PbS NPs.
Herein, we report the synthesis and photophysical properties
of a new BODIPY dye (BDY) and its use in PbS NPs
functionalization.
The procedure used to synthesize BDY is illustrated in
Scheme 1. The bromophenyl-BODIPY starting material 1
was synthesized by using a modified literature procedure.[8]
Scheme 1. Synthetic procedure for BDY.
[*] J.-s. Lu, Prof. Dr. S. Wang
The 3- and 5-methyl groups in 1 are acidic enough to undergo
Knoevenagel condensation,[9] which allowed the introduction
of two p-octyl styryl groups to the core to produce 2.
Department of Chemistry, Queen’s University
Kingston, Ontario, K7L 3N6 (Canada)
E-mail: wangs@chem.queensu.ca
The octyl groups are necessary to provide the desired
solubility of the functionalized NPs in organic solvents. The
conjugated terminal carboxylate group was connected to the
BODIPY core by using a standard Suzuki coupling method
between 2 and the corresponding boronic acid to produce 3.
Replacement of the ester group by a carboxylic acid converts
3 to the final product BDY, which was fully characterized by
1H NMR spectroscopy, HRMS, and elemental analysis. BDY
has a moderate solubility in most organic solvents, including
toluene and hexanes. Although we have not been able to
obtain single crystals of BDY, the structure of its ester (3) was
determined successfully by X-ray diffraction analysis.[10]
Compound 3 forms needlelike crystals that aggregate into
black balls of spikes. The crystal structure of 3 is shown in
Figure 1. In the crystal lattice, molecules of 3 arrange in a
head-to-head and tail-to-tail fashion, with extensive intermo-
lecular interactions involving the styryl groups, the biphenyl
H. Fu, Y. Zhang, Y. Tao
Institute for Microstructural Science
National Research Council
Ottawa, 1200 Montreal Road, K1A 0R6 (Canada)
Dr. Z. J. Jakubek
Steacie Institute for Molecular Science
National Research Council
100 Sussex Drive, Ottawa, K1A 0R6 (Canada)
[**] This work is supported financially by the Natural Sciences and
Engineering Research Council of Canada, Business Development
Bank of Canada, the National Research Council of Canada. We thank
Dr. Jian-ping Lu for his assistance in PbS NPs synthesis.
Supporting information (including synthetic procedure and char-
acterization data of BDY, PbS nanoparticle synthesis, crystal-
structure data, TD-DFT computational details, photophysical prop-
erty characterization, and electrochemical data) for this article is
available on the WWW under http://dx.doi.org/10.1002/anie.
201104690.
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Angew. Chem. Int. Ed. 2011, 50, 11658 –11662

Figure 1. Top: Crystal structure of 3 and a photo showing the “spike-
ball”-like crystals of 3 in hexanes. Bottom: a diagram showing
intermolecular interactions of 3; distances in ꢀngstrom.
group, and the BODIPY core (Figure 1; see also the
Supporting Information).
The BDY molecule has an intense dark purple-blue color
in solution and the solid state. Its absorption spectrum in
toluene has two intense absorption bands at 580 and 636 nm
and a shoulder band at approximately 520 nm (Figure 2). It is
fluorescent with two emission peaks, one in the orange region
lmax ꢀ 585 nm (t = 3.75(16) ns) and one in the red region, lmax
ꢀ 655 nm (t = 4.13(10) ns) in toluene. The fluorescence
quantum efficiency of BDY was determined to be approx-
imately 0.24, with lex = 560 or 510 nm, using Rhodamine B as
the standard. In the solid state, BDY is a dark black solid and
not emissive at all. Both absorption and emission spectra of
BDY experience a hypsochromic shift with increasing solvent
polarity, thus indicating that the dye has a much greater dipole
in the ground state than in the excited state. Although the
absorption spectrum and the fluorescence spectrum of BDY
bear some resemblance to those of several reported phenyl-
vinyl functionalized BODIPY molecules,[11] the two absorp-
tion and emission peaks at 580–590 nm and 640–660 nm
cannot be simply attributed to intramolecular electron
transitions.
Figure 2. Absorption (top) and fluorescence (bottom) spectral changes
of BDY with increasing concentration in toluene. Inset: the relative
absorbance change of the peak at 640 nm vs. that at 590 nm plotted
against concentration of BDY.
shows a clear red shift and a steady decrease of intensity with
increasing concentration. Thus, the low-energy peak in both
the absorption and the emission spectrum most likely has an
intermolecular origin. Further evidence comes from the
excitation-energy dependence of BDY dual emission. As
shown in Figure 3, the emission peak at approximately
655 nm does not change intensity significantly with excitation
energy, while the peak near 590 nm is very sensitive to
excitation energy: its intensity is highest for excitation at
570 nm and becomes weaker as excitation energy increases.
The distinct red or orange emission color of BDY can be
observed visually with the appropriate choice of the excita-
tion source (Figure 3). This finding strongly suggests that the
two emission peaks are from two different species. This
conclusion is further supported by fluorescence anisotropy
measurements, which revealed that the two emission peaks
have distinct anisotropies (0.0606(28) for the peak at 585 nm
and 0.0249(22) for the peak at 655 nm). The anisotropic
parameters indicate that the two species have either a very
different shape, size, or molecular weight.[12] NMR spectra did
The first evidence for intermolecular electronic transi-
tions is the concentration-dependent behavior of both
absorption and emission spectra of BDY. In the absorption
mode, in the low concentration range (5.0 ꢀ 10ꢁ7 m to 5.0 ꢀ
10ꢁ6 m), when the concentration of BDY increases, the
intensity of the peak at 640 nm increases much faster than
that of the peak at 590 nm (Figure 2). In the emission mode,
the peak near 655 nm also experiences a much greater
increase of intensity with concentration than the peak at
590 nm in the relatively low concentration range (5.0 ꢀ 10ꢁ7
to 1.0 ꢀ 10ꢁ5 m). At higher concentrations, the 655 nm peak
m
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intermolecular interactions are indeed possible for BDY, as
revealed by the crystal structure of 3 in Figure 1. We thus
suggest that the low-energy absorption/emission band at
approximately 640 nm is caused by molecular aggregation
and intermolecular interactions. The same intermolecular
interactions are likely responsible for the lack of emission of
BDY in the solid state. Although many BODIPY derivatives
are known in the literature,[1–2,11] and intramolecular excimer
emission involving a BODIPY dye is also known,[11e] BDY is
the first example, to our knowledge, that displays such
unusual dual emission among this family of compounds.
Functionalization of PbS NPs with BDY was achieved
successfully by simply mixing BDY with oleic acid capped PbS
NPs[5c,d] in toluene under nitrogen, and subsequent precip-
itation of the BDY-capped PbS NPs (BDY-PbS NPs) using
acetone. The ligand-exchange process was monitored by
1H NMR spectroscopy (see the Supporting Information). The
best procedure is to use a tenfold excess of BDY ligand to
exchange with the oleic acid on the PbS NPs at room
temperature for 10 min. The aromatic-proton chemical shifts
of the bound BDY in BDY-PbS NPs appear as a broad peak in
the NMR spectrum that covers the entire aromatic region,
attributable to the low mobility of the BDY ligand on the
surface of the PbS NPs. The signals from oleic acid and
unbound BDYare completely absent in the NMR spectrum of
isolated BDY-PbS NPs.
The formation of new BDY-PbS NPs was further verified
by TEM imaging, which shows that the BDY-PbS NPs are
highly uniform with an average size of approximately 3 nm,
similar to the original oleic acid capped PbS NPs (Figure 4).
Figure 3. Top: the dependence of BDY fluorescence on excitation
energy (1.0ꢁ10ꢁ5 m in toluene) with photos showing the orange
emission color of the BDY solution upon excitation by a green laser
(left) and the red emission color (right) upon irradiation by a UV lamp
at 365 nm. Bottom: the dependence of BDY-PbS NP fluorescence on
excitation energy in toluene, inset: the NIR emission of oleic acid
capped PbS NPs and BDY-PbS NPs (lex =578 nm, in toluene).
Figure 4. TEM images of BDY-PbS NPs (left) and oleic acid capped
PbS NPs (right).
not show any detectable impurities or an olefin trans–cis
isomerization process that may be responsible for the dual
emission phenomenon. Hydrogen-bonded dimers are also
ruled out, because the ester molecule 3 displays similar dual
absorption and emission bands (see the Supporting Informa-
tion). Time-dependent density functional theory (TD-DFT)
computational results for the discrete BDY molecule showed
only one absorption band (lmax = 571 nm) that matches well
with the experimental absorption band at 580 nm, thereby
supporting the conclusion that the peak at 636 nm does not
have an intramolecular origin. Furthermore, TD-DFT estab-
lished that the lowest energy transition is a p!p* transition
localized on the BODIPY–bis(styryl) portion of the molecule
(see the Supporting Information). Hence, intermolecular
interactions involving this part of the molecule will likely
have an impact on the fluorescence of the molecule. Such
However, the separation distance between the NPs is much
longer (ca. 7 nm) for BDY-PbS NPs than for oleic acid PbS
NPs (ca. 5 nm). This result is in excellent agreement with the
fact that BDY is a much longer and bigger ligand (ca. 3 nm in
length) than oleic acid (ca. 2 nm). The retention of a high
uniformity and of the particle size of the BDY-PbS NPs is
quite remarkable, because increased NP sizes and poor
uniformity were widely reported after ligand exchange of
NPs.[4,13] These results support that BDY is a highly effective
ligand for PbS NPs.
The BDY-PbS NPs have a distinct turquoise color in
solution (Figure 5). Upon irradiation by a UV lamp at
365 nm, the BDY-PbS NPs emit red light, which is much
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of the NIR emission band, as confirmed by fluorescence
titration experiments (see Figure 3 and the Supporting
Information). Since BDY has no absorption in the NIR
region, the NIR emission quenching is most likely caused by a
charge-transfer process between BDY and the PbS NP core.
To illustrate the impact of BDY functionalization on the
performance of PbS NPs in photovoltaic (PV) devices, we
fabricated a simple PV device with the structure of indium tin
oxide (ITO)/BDY-PbS NPs (100 nm)/LiF (1 nm)/Al
(100 nm). The BDY-PbS NPs were deposited onto the ITO
substrate by spin-coating a corresponding chloroform solu-
tion. Photocurrent production upon simulated solar irradi-
ation was observed from this simple device with Voc = 0.37 V,
Jsc = 0.10 mAcmꢁ2, and a fill factor (FF) of 0.31. Although this
simple PV device is not efficient at all, it is much better than
the control PV device with the oleic acid capped PbS NPs as
active layer; this control device did not display any detectable
photocurrent at all. The improved PV performance of the
BDY-PbS NPs device can be attributed to the improved
electronic communications between the surface ligand and
the NP core. Efforts are being taken to further understand the
electronic properties of the BDY-PbS NPs and to explore
their applications in PV and other optoelectronic devices.
In summary, a new BODIPY molecule BDY that pos-
sesses unusual dual-emissive properties has been synthesized.
Its use as a highly effective surface ligand for achieving highly
uniform and monodispersed PbS NPs has been demonstrated.
The presence of electronic communication between BDY and
the PbS core has been established. The enhancement in PV
performance of PbS NPs by the BDY surface ligand,
compared to oleic acid, has been verified by a simple PV
device.
Figure 5. Absorption spectra of BDY (red), oleic acid capped PbS NPs
(black), and BDY-PbS NPs (blue dashed) in toluene. Inset: The
absorption spectra in the NIR region and photos showing the colors of
the three samples under ambient light (top) and under UV (365 nm)
irradiation (bottom).
weaker than that of free BDY in toluene. Interestingly, the
BDY-PbS NPs display similar dual emission peaks in the 500–
700 nm region as BDY, except that the emission peak at
645 nm is blue-shifted about 10 nm compared to free BDY at
a similar concentration in toluene. Like BDY, the emission
spectrum of BDY-PbS NPs is also highly sensitive to
excitation energy (Figure 3), thus supporting the conclusion
that interligand interactions similar to those of BDY are also
present in the NPs. However, while for BDY the low energy
emission band at 655 nm does not change intensity with
excitation energy, the 645 nm emission band of the BDY-PbS
NPs does lose intensity as the excitation energy increases; this
observation is an indication of either energy transfer or
charge transfer from the ligand to the NPs. Fluorescence
titration experiments of BDY with oleic acid capped PbS NPs
showed approximately 70% quenching of BDY emission by
the NPs. This quenching may be caused by energy transfer,
because the PbS NPs have considerable absorption in the
600–700 nm region. On the other hand, cyclic voltammetry
measurements indicate that the LUMO (lowest unoccupied
molecular orbital) level of BDY is at approximately ꢁ3.4 eV,
significantly above the electron affinity energy (ca. ꢁ4.2 eV)
of the PbS NP core, while the HOMO (highest occupied
molecular orbital) level of BDY is similar to the ionization
potential energy of PbS NPs (ca. ꢁ5.2 eV).[14] It is therefore
conceivable that excited-state charge transfer from BDY to
the NP core may occur, which can also act as an effective
pathway for quenching the emission of BDY. The PbS NPs
have a distinct NIR exciton peak at 1024 nm and a NIR
emission band at approximately 1100 nm. The replacement of
oleic acid surface ligands by BDY did not change the NIR
absorption and emission energy at all, thus further supporting
the conclusion that there is no surface-state change or etching
of the NPs during the ligand exchange.[4] The ligand exchange,
however, did lead to approximately 50% intensity reduction
Received: July 7, 2011
Published online: October 13, 2011
Keywords: dual emission · dyes/pigments · fluorescence ·
.
nanoparticles · synthesic methods
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