Perylene-fused BODIPY dye with near-IR absorption/emission and high photostability

Article abstract of DOI:10.1021/ol102879g

A N-annulated perylene unit was successfully fused to the meso-and β-positions of a boron dipyrromethene (BODIPY) core. The newly synthesized BODIPY dye 1b exhibits intensified near-infrared (NIR) absorption and the longest emission maximum ever observed

Full text of DOI:10.1021/ol102879g

ORGANIC
LETTERS
2011
Vol. 13, No. 4
632–635
Perylene-Fused BODIPY Dye with Near-IR
Absorption/Emission and High
Photostability
Chongjun Jiao,† Kuo-Wei Huang,‡ and Jishan Wu*,†
Department of Chemistry, National University of Singapore, 3 Science Drive 3,117543,
Singapore, and KAUST Catalysis Center and Division of Chemical and Life Sciences and
Engineering, 4700 King Abdullah University of Science and Technology, Thuwal 23955-
6900, Kingdom of Saudi Arabia
chmwuj@nus.edu.sg
Received November 29, 2010
ABSTRACT
A N-annulated perylene unit was successfully fused to the meso- and β-positions of a boron dipyrromethene (BODIPY) core. The newly
synthesized BODIPY dye 1b exhibits intensified near-infrared (NIR) absorption and the longest emission maximum ever observed for all BODIPY
derivatives. In addition, this dye possesses excellent solubility and photostability, beneficial to practical applications.
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene, also known
as boron dipyrromethene (BODIPY, Figure 1), represents
an extraordinary class of fluorophore.1 Its unusual and
remarkable properties, such as high fluorescence high
quantum yield, large molar extinction coefficients, and
outstanding chemical, thermal, and photochemical
inertness, make it attractive for a variety of applications
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C. J. J. Am. Chem. Soc. 2010, 132, 1194–1195. (e) Atilgan, E.; Ozdemir, E.;
Akkaya, E. U. Org. Lett. 2010, 12, 4792–4795.
† National University of Singapore
‡ KAUST Catalysis Center and Division of Chemical and Life Sciences
and Engineering.
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r
10.1021/ol102879g
Published on Web 01/05/2011
2011 American Chemical Society

Scheme 1
Figure 1. Molecular appoaches toward BODIPY dyes with longer
absorption and emission.
including luminescent devices,2 chemical sensors,3 biologi-
cal labeling,4 and photovoltaic devices.5 BODIPY chro-
mophores generally possess visible absorption and
fluorescent emission located between 470 and 550 nm.
Considering the increasing interest in the preparation of
near-infrared (NIR) absorption as well as NIR emission
dyes,6 promotion of the absorption and emission of a BOD-
IPY dye to the far-red and even to the NIR spectral region by
structural modifications is crucial and necessary. As shown in
Figure 1, such modifications currently include (a) extension of
π-conjugation by fusing a rigid ring to the pyrrole unit,7 (b)
functionalization at the R- and/or meso-position to generate a
“push-pull” motif,8 and (c) replacement of the 8-carbon atom
with a nitrogen atom to form aza-BODIPY dyes.9
The fusion of polycyclic aromatic compounds to por-
phyrin cores has recently attracted considerable interest,10
and these fused hybrid molecules usually show intensified
NIR absorption and in some cases also exhibit moderate
NIR emission.10h BODIPY dye, porphyrin’s little sister,
provides a nice “zig-zag” geometry for fusion of an aromatic
unit to the meso- and β-positions (method (d), Figure 1).
Such a fusion is beneficial to the bathochromic shift of the
absorption and emission to a far-red and NIR spectral
region. Despite the structural similarities between BODIPY
and porphyrin, fusion of a polycyclic aromatic compound
onto the zig-zag edge (i.e., meso- and β-positions) of a
BODIPY core, to the best of our knowledge, has never been
reported. Herein, we report the first example of polycyclic
aromatic unit fused BODIPY 1b (Scheme 1) which exhibits
an intensified NIR absorption and acceptable NIR emission.
The N-annulated perylene-fused BODIPY 1b was
synthesized as shown in Scheme 1. Initially, we attempted
to prepare its analog 1a in which two ethyl groups are
attached to the R-positions of the BODIPY core. The
perylene aldehyde 3 was first prepared in 46% yield by
lithiation of monobrominated N-annulated perylene 211
followed by reaction with anhydrous DMF. Acid-cata-
lyzed condensation of the obtained aldehyde 3 with 2 equiv
of 2-ethyl pyrrole 4a led to the corresponding dipyrro-
methane derivative in good yield. Due to high reactivity,
this dipyrromethane was used for the next step without
further purification. Subsequent oxidation by 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) and complexation
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T.; Urano, Y. Tetrahedron Lett. 2001, 42, 6711–6713. (b) Shen, Z.; Rohr,
with BF3 OEt2 afforded the N-annulated perylene-BODIPY
3
H.; Rurack, K.; Uno, H.; Spieles, M.; Schulz, B.; Reck, G.; Ono, N. Chem.;
Eur. J. 2004, 10, 4853–4871. (c) Jiao, L.; Yu, C.; Liu, M.; Wu, Y.; Cong, K.;
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dyad 5a with an overall yield of 19% in three steps.
Iron(III) chloride, a well-known mild oxidant, has proven
to be effective to promote cyclodehydrogenation of many
branched oligophenylenes into polycyclic aromatic hydro-
carbons.12 Herein, intramolecular ring fusion of 5a by
using 2 equiv of FeCl3 as an oxidant yielded a mixture
with a longer absorption between 600 and 800 nm, indicat-
ing formation of the desired ring-fused product 1a. How-
ever, separation of this mixture turned out to be extremely
difficult due to strong aggregation of the products in both
the solid state and solution, despite the presence of bulky
4-tert-butylphenyl and branched aliphatic chains attached
to the N-annulated perylene unit. Such a troublesome
problem was also observed for aromatic ring-fused por-
phyrin systems, which can be somewhat alleviated by the
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Rohand, T.; Dehaen, W.; Boens, N. Org. Lett. 2005, 7, 4377–4380.
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1677–1679. (b) Zhao, W.; Carreira, E. M. Chem.;Eur. J. 2006, 12, 7254–
7263. (c) McDonnell, S. O.; O'Shea, D. F. Org. Lett. 2006, 8, 3493–3496.
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Org. Lett., Vol. 13, No. 4, 2011
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The absorption spectrum of precursor 5b in toluene
displays the characteristic bands of respective BODIPY
and N-annulated perylene while the fused compound 1b
demonstrates a significant bathochromic shift of the ab-
sorption maximum with respect to 5b (Figure 2a). Com-
pound 1b shows intensified absorption bands with the
absorption maximum at 670 nm (ε = 91 000 M-1 cm-1
)
together with a shoulder around 780 nm. The absorption
behavior of 1b is nearly independent of solvent polarity.
However, the photoluminescence (PL) spectrum of this
molecule exhibits solvent dependence (Figure S1 in the SI).
Dye 1b in different solvents displays two distinct emission
bands (band I located between 700 and 758 nm and band II
located between 790 and 860 nm). Upon increasing the
polarity of the solvent from hexane to toluene and to chloro-
form, the ratio of band I intensity to band II intensity
successively increases (Figure S1 and Table S1 in the SI).
Band I could be assigned to the original PL band whereas
band II at a longer wavelength is attributed to the forma-
tion of aggregates of the dye molecules in solution, which is
reflected by the fact that the ratios of the intensity of band
II to band I in emission spectra successively increase
upon increasing the concentration of dye 1b in toluene
(Figure S2 in the SI). Since many highly conjugated
π-systems prefer to form aggregates in a nonpolar solvent
via π-π interactions,14 such a solvent dependence of PL
spectra can be explained by the different degrees of aggregation
in different solvents. The excitation spectra of 1b at the two
main fluorescence peaks disclosed that even the absorption
spectrum of 1b is the superposition of the monomer and
aggregates (Figure S3 in the SI). Also noteworthy is that dye
1b in toluene exhibits emission with the maximum at 830 nm
(Figure 2a), which is the longest NIR emission maximum ever
observed for all BODIPY derivatives. Relatively low photo-
luminescence quantum yields up to 0.8% were observed for 1b
in toluene mainly due to the dye aggregation in solution. Of
course, other factors such as conformation change of the
excited molecules and weak photoinduced intramolecular
charge transfer from N-annulated perylene to the BODIPY
core can also decrease the fluorescence quantum yield.
Figure 2. (a) UV-vis-NIR absorption spectra of 1b and 5b in
toluene (1.0 ꢀ 10-5 M) and photoluminescence (PL) spectrum
(1 ꢀ 10-6 M) of compounds 1b in toluene (excitation wavelength
is 670 nm). (b) Cyclic voltammograms (CV) of 1b and 5b in DCM
with 0.1 M Bu4NPF6 as supporting electrolyte, AgCl/Ag as refer-
ence electrode, Au disk as working electrode, Pt wire as counter
electrode, and scan rate at 50 mV/s. The differential pulse voltam-
mogram (DPV) of 5b was shown to distinguish the oxidation
waves. Fc/Fcþ was used as external reference.
introduction of a bulky 3,5-di-tert-butylphenyl group
to the meso-positions of porphyrin.10f,g Inspired by this
strategy, we thus modified the structure of the target
compound by attaching two bulky 3,5-di-tert-butyl-
phenyl groups directly onto the R-positions of the
BODIPY core (1b). Accordingly, the 3,5-di-tert-butylphe-
nyl-substituted pyrrole 4b was prepared (see Supporting
Information (SI) for synthetic details) and used for the
acid-catalyzed condensation reaction with 3. Compound
5b was then obtained in an overall yield of 69% in three
steps, through the same synthetic route used for 5a.
Oxidative cyclization of precursor 5b was carried out
with FeCl3 in dichloromethane (DCM) to give the
desired product 1b in 23% yield. The final compound
1b has very good solubility, and it did not show strong
aggregation in solution and allowed us to separate and
characterize it more conveniently (see SI). In addition,
fusion of unmodified perylene to the BODIPY core was
also attempted by using the same strategy starting from
the reaction between the perylene aldehyde 613 and 4b. The
obtained perylene-BODIPY dyad 7 was submitted to
a similar cyclization with FeCl3. However, a strongly
aggregated mixture with a deep green color was obtained,
which cannot be further purified.
The electrochemical properties of 1b and 5b were in-
vestigatedbyvoltammetry in deoxygenatedDCM solution
containing 0.1 M tetra-n-butylammonium hexafluorophos-
phate as the supporting electrolyte. As shown in Figure 2b,
three reversible reduction waves with half-wave potentials
at -1.04, -1.31, -2.00 V (vs Fcþ/Fc) and three reversible
oxidation waves with half-wave potentials at 0.50, 0.64,
1.16 V were observed for 1b. A HOMO energy level
of -5.16 eV and a LUMO energy level of -3.83 eV were
estimated basedon the onset potential of the first oxidation
and the first reduction wave, respectively.15 An energy gap
(13) Skorobogatyi, M. V.; Pchelintseva, A. A.; Petrunina, A. L.;
Stepanova, I. A.; Andronova, V. L.; Galegov, G. A.; Malakhova, A. D.;
Korshun, V. A. Tetrahedron 2006, 62, 1279–1287.
€
(14) Wu, J.; Fechtenkotter, A.; Gauss, J.; Watson, M. D.; Kastler,
€
€
M.; Fechtenkotter, C.; Wagner, M.; Mullen, K. J. Am. Chem. Soc. 2004,
126, 11311–11321.
(15) The HOMO and LUMO energy levels were calculated from the
onset of the first oxidation and reduction waves according to equations
HOMO = -(4.8 þ Eoxonset) and LUMO = -(4.8 þ Eredonset).
634
Org. Lett., Vol. 13, No. 4, 2011

Figure 3. Optimized molecular structure, dipole moment (indicated
by arrow), and the frontier molecular orbital profiles of molecule 1b
(the branched aliphatic chain is replaced by an ethyl group during
the calculations).
of1.33eVwas thenobtainedwhichisinagreementwiththe
optical band gap (1.38 eV). In contrast, the precursor 5b
exhibits one reversible oxidation wave corresponding to a
perylene subunit with a half-wave potential at 0.47 V and
two oxidation waves corresponding to the BODIPY core
with half-wave potentials at 0.84 and 1.02 V and one
reversible reduction wave with a half-wave potential
at -1.35 V (Table S2 in the SI). Compared with 1b, a larger
energy gap (2.06 eV) was observed for 5b, and this is also
consistent with the optical band gap (2.02 eV).
To gain better insight into the geometric and electronic
structure, time-dependent density function theory (TDDFT
at B3LYP/6-31G**) calculations were performed and the
optimized molecular structure, dipole moment, and fron-
tier molecular orbital profile are shown in Figure 3. The
perylene moiety in 1b somewhat deviates from the BODOPY
plane due to the steric congestion between the protons in
the 7-position of BODIPY and the meta-proton of the
N-annulated perylene core. The asymmetry in 1b also
generates a large dipole moment, which is calculated as
9.80 D. The calculations also predict that compound 1b
will show major absorption bands at 705 and 594 nm
(Figure S5 and Table S4 in the SI), which are in good
agreement with the experimental data.
Figure 4. Change of optical density of 1b, 10, and 11 at the
absorption maximum wavelength with the irradiation time. The
original optical density before irradiation was normalized at
the absorption maximum. Solutions of compounds 1b, 10, and
11 in toluene were irradiated under 4 W UV light (emitting at
254 nm).
In summary, a successful fusion of a N-annulated per-
ylene to the meso- and β-positions of a BODIPY core was
achieved for the first time. Use of the bulky groups on both
N-annulated perylene and the BODIPY moiety is crucial
for the purpose of suppressing aggregation. The obtained
dye has a largely delocalized π-system and therefore
shows NIR absorption and emission. Noteworthy is
that the perylene-fused BODIPY dye exhibits excellent
photostability which is important for practical applica-
tions. Although the photoluminescence quantum yield
is relatively low in the case of 1b, our approach opens
opportunities to prepare a series of new active aromatic
unit-fused BODIPY dyes with tunable NIR absorption
and emission, and this work is currently underway in our
laboratories.
A solution of 1b in air-saturated toluene remains un-
changed over months at ambient conditions, indicating the
excellent photostability of 1b. To reveal the effect of
BODIPY moieties on the photostability, compound 1b
was compared with the N-annulated perylene-fused por-
phyrin dye 810h as well as the electron-rich bis-N-annulated
quarterrylene dye 9.11 Upon irradiation with UV light
(4 W) for 4000 min, the absorbance of dye 1b in air-
saturated toluene remained almost constant and only lost
Acknowledgment. J.W. acknowledges the financial sup-
port from the Singapore DSTA DIRP Project (DSTA-
NUS-DIRP/2008/03), NRF Competitive Research Pro-
gram (R-143-000-360-281), and NUS Young Investigator
Award (R-143-000-356-101). K.-W.H. acknowledges the
financial support from KAUST.
5% of the initial intensity. In contrast, a half-life time (t1/2
)
of around 244 min was measured in the case of compound
8 while dye 9 cannot even survive for longer than 10 min
under the same condition (Figure 4). This observation
clearly demonstrates that the fused BODIPY unit is the
most effective building block to stabilize the highly elec-
tron-rich N-annulated perylene. Moreover, the excellent
photostability of 1b is of great importance for practical
applications.
Supporting Information Available. Experimental de-
tails and characterization data of all new compounds, and
theoretical calculation data. This material is available free
of charge via the Internet at http://pubs.acs.org.
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