Modulating the singlet oxygen generation property of meso-β directly linked BODIPY dimers

Article abstract of DOI:10.1039/c2cc30915g

We report the synthesis, X-ray analysis and singlet oxygen generation properties of a set of meso-β directly linked BODIPYs with the meso-aryl group (φ₁) and meso-BODIPY component (φ₂) free to rotate or constrained.

Full text of DOI:10.1039/c2cc30915g

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COMMUNICATION
Modulating the singlet oxygen generation property of meso–b directly
linked BODIPY dimersw
Weidong Pang,a Xian-Fu Zhang,*b Jinyuan Zhou,a Changjiang Yu,a Erhong Haoa and
Lijuan Jiao*a
Received 8th February 2012, Accepted 16th April 2012
DOI: 10.1039/c2cc30915g
We report the synthesis, X-ray analysis and singlet oxygen generation
properties of a set of meso–b directly linked BODIPYs with the
meso–aryl group (u1) and meso–BODIPY component (u2) free
to rotate or constrained.
the b–b linked BODIPY dimers C reported by Ziessel,5a
Shinokubo5b and Bard,5c the meso–meso linked BODIPY
dimers D developed by Thompson6 and Akkaya,7 and the
first and only meso–b linked BODIPY dimer E reported by
Akkaya and coworkers7 while we were working on this paper.
The relative rotation of the meso–aryl group to the mean
plane of the dipyrrin (defined as the dihedral angle j) in
meso–aryl BODIPYs A has been known to be able to greatly
affect the excited-state dynamics of the dyes.8 For example,
Lindsey8a and Samuel8b have separately demonstrated its
important effect on the photophysical properties of the excited
state and the non-radiative decay of the dyes.
BODIPYs (such as A in Fig. 1) known as the ‘‘little-sister’’ of
porphyrin, have attracted interest in a diversity of research
fields. They have been utilized as labeling reagents, laser dyes,
fluorescent switches, light harvesters, sensors and sensitizers
due to their tunable photophysical properties, versatility and
rich chemistry.1 Oligomeric, including some bridge-linked non-
orthogonal dimeric BODIPYs have been reported.2,3 Unlike the
well-developed porphyrin dimers, only several types of directly
linked BODIPY dimers have been synthesized:4–7 the a–a linked
Recently, we have developed an efficient synthesis of
b–formylBODIPYs9 that can be easily applied for the further
functionalization of BODIPY chromophore.10 Herein, using
these b–formylBODIPYs as a platform, a set of meso–b
directly linked BODIPY dimers 2a–d (Scheme 1) were synthe-
sized and characterized via X-ray diffraction. Carefully con-
trolled orthogonalities associated with their dihedral angles
(Fig. 1, j1 and j2) in BODIPY dimers 2b and 2d were achieved
through convenient substitutions at the meso-aryl and/or
pyrrolic positions in the BODIPY subunits, which restricted
the free rotation of the meso-aryl group and meso–BODIPY
component in these two BODIPY dimers. We rationalized
that a comparative study of these two orthogonal BODIPY
dimers 2b and 2d with their non-orthogonal analogue dimers
2a and 2c would be instructive.
BODIPY dimers B reported by Broring and co-workers,4
¨
BODIPY dimers 2a–d were straightforwardly synthesized
in good yields from the acid-catalyzed condensation of
b–formylBODIPYs 1 with pyrrole or 2,4-dimethylpyrrole,
Fig. 1 Chemical structures of some representative BODIPYs (A),
dimeric BODIPYs (B, C, D and E) and our BODIPY dimers 2. The
alkyl groups on the meso–aryl group and dipyrrin unit hinder the
rotation of the meso–aryl ring in BODIPYs A.
a Laboratory of Functional Molecular Solids, Ministry of Education,
Anhui Laboratory of Molecule-Based Materials, School of
Chemistry and Materials Science, Anhui Normal University, Wuhu,
China 241000. E-mail: jiao421@mail.ahnu.edu.cn
b Chemistry Department, Hebei Normal University of Science &
Technology, Qinhuangdao, Hebei, China 066004.
E-mail: zhangxianfu@tsinghua.org.cn; Fax: (+86) 1062770304
w Electronic supplementary information (ESI) available: Synthetic
procedures, photophysical and crystallographic data. CCDC
867740–867743. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc30915g
Scheme 1 The synthesis of dimeric BODIPYs 2a–d.
Chem. Commun., 2012, 48, 5437–5439 5437
c
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The pyrrolic substituents, especially the 1,7-positional sub-
stituents on the newly formed BODIPY core greatly affected
the intramolecular interactions between the two BODIPY
subunits. Dihedral angles close to 901 were observed between
the two C9BN2 BODIPY subunits in 2b (j2 = 841) and 2d
(j2 = 891). By contrast, 2a and 2c lacking pyrrolic substi-
tuents on the newly formed BODIPY subunit showed smaller
dihedral angles between the two BODIPY subunits (j2 = 341
and 391 for 2a and 2c, respectively). This difference may be
attributed to the presence of pyrrolic alkyl substituents in 2b
and 2d, which restrict the relative rotation between the two
BODIPY subunits.
The starting conformations in these dimeric BODIPYs also
affected their photophysical properties, as summarized in
Table 1. As shown in Fig. 3, BODIPY dimers 2a–d showed
characteristic absorption spectra similar to that of their
corresponding starting BODIPY monomers 19b and other
BODIPY chromophores. BODIPY dimers 2a and 2c with
smaller dihedral angles between the two BODIPY subunits
showed a very broad absorption band with an around 30 nm
red-shift of the absorption maximum and the appearance of an
additional band in comparison to those of monomers 1a and 1b.
This broadened and red-shifted absorption band may be
attributed to the formation of a ground-state charge-transfer
complex through excitonic coupling, as described in the
literature.6,11,12 In comparison with 2a and 2c, less exciton
couplings were observed for BODIPY dimers 2b and 2d. We
attributed it to the perpendicular dihedral angles (j2) between
the two BODIPY subunits in these two dimers, which reduced
the efficiency in the exciton coupling between them.6,7
Fig. 2 Top view (top) and side view (bottom) of the X-ray structures
of dimeric BODIPYs 2a (a), 2b (b), 2c (c) and 2d (d). Dihedral angles
(j1: C5-C6-C7-C8; j2: C1-C2-C3-C4) between the two BODIPY units
for 2a are j1 = 381, j2 = 341; for 2b j1 = 541, j2 = 841; for 2c j1 = 841,
j2 = 391 and for 2d j1 = 871, j2 = 891, respectively. C, light gray;
H, gray; N, blue; B, dark yellow; F, light green and Cl, violet.
followed by DDQ oxidation and the subsequent BF3 com-
plexation reactions (Scheme 1), and were characterized by
NMR, HRMS and X-ray analysis.
Single crystals suitable for X-ray diffraction studies were
obtained by slow evaporation of the dichloromethane solutions of
these compounds. The two different C9BN2 BODIPY subunits in
these dimeric BODIPYs remain essentially unperturbed, as shown
in Fig. 2, although the planarity of these two is less pronounced
than that in their corresponding monomers.9b Other bond
lengths, angles and distances within each of these two C9BN2
BODIPY subunits are very similar to that of their corresponding
monomers (Table S1 in ESIw). The C–C and C–N bond lengths
within the two C9BN2 BODIPY subunits showed no clear
distinction between single and double bonds, indicating the strong
delocalization within each of the BODIPY subunits. The B–N
and B–F bond lengths are typical (1.51–1.56 and 1.36–1.39 A,
respectively) in these BODIPY dimers and the angles around the
boron atom are close to the tetrahedral angle of 109.61.
In comparison to those of the corresponding BODIPY
monomers, much weaker fluorescence emission was observed
in organic solutions for the orange-colored BODIPY dimers
2a–d under ambient or hand-held UV lamp irradiation,
indicating competing excited states in these dimers. These
results are similar to that of the recently reported b–b5b and
Table 1 Comparative spectroscopic properties of dimeric BODIPYs
a
a
BODIPYs
labs (nm)
lems (nm)
jfa,b
t(ns)c
w2c
2a
2b
2c
2d
521
507
538
507
563
540
568
542
0.007
0.002
0.002
0.002
2.84
1.11
1.06
1.08
1.11
2.41, 5.60
1.37, 4.63
1.08, 3.85
a
b
In dichloromethane. The fluorescence quantum yields were calculated
These four dimeric BODIPY dyes differ by virtue of
the substitution patterns at the meso–aryl or pyrrole units.
Meso–aryl substituents greatly affected the dihedral angles
(j1) between the meso-aryl group and the mean plane of the
C9BN2 BODIPY subunits, similar to that described for most
monomeric BODIPYs. BODIPY dimers 2a and 2b without
meso–aryl substituents gave smaller dihedral angles between
the meso–aryl group and the mean plane of one BODIPY
subunit (j1 = 381 and 541 for 2a and 2b, respectively), while
BODIPY dimers 2c and 2d with two chlorine atoms at
2,6-positions of the meso–aryl group in one BODIPY subunit
gave a ‘‘near-perpendicular’’ dihedral angles (j1=841 and 871
for 2c and 2d, respectively). This may be attributed to the
steric hindrance effect from two chlorine atoms in 2c and 2d,
which restrict the rotation of meso–aryl group relative to the
mean plane of the BODIPY core.
using fluorescein in 0.1 M NaOH aqueous solution (j = 0.90) as the
standard. In toluene.
c
Fig. 3 Overlapped absorption spectra of BODIPYs 1a (green), 1b
(dark cyan) and BODIPY dimers 2a (black), 2b (red), 2c (magenta)
and 2d (blue) in dichloromethane.
c
5438 Chem. Commun., 2012, 48, 5437–5439
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molecule (Fig. 5 and Fig. S7–10w). Consistent with the singlet-
oxygen phosphorescence emission, the highest efficiency was
also observed for orthogonal meso–b linked dimer 2b under
comparable conditions and the second most efficient one
was 2d. In contrast, BODIPY dimers 2a and 2c showed much
lower efficiency in the singlet oxygen generation. Our results
confirmed the calculation by Akkaya regarding the great effect
of orthogonality of the BODIPY dimer on their efficiencies in
the singlet oxygen generation.7
This work is supported by the National Natural Science
Foundation of China (Grants 20802002, 20902004 and 21072005),
Anhui Province (Grants 090416221 and KJ2009A130) and
Ministry of Education of China (Grant 20093424120001).
Fig. 4 Singlet oxygen phosphorescence with sensitization from dimeric
BODIPYs 2b (solid) and 2d (dashed) in toluene at equal absorbances at
the wavelength of 509 nm with InGaAs as the detector. Plots were based
on the average of 10 times collection.
meso–b7 directly linked BODIPY dimers. The fluorescence
lifetime was measured by a time-correlated single photon
counting method excited by a CdS portable diode laser (50 ps
pulse width) and the emission was monitored at the maximum
emission wavelength. The time evolution of the luminescence is
described by either a single or double exponential decay for
these BODIPY dimers under an experimental resolution of
100 ps.13 The excimer formation shortens the lifetime of
monomer moieties, for which the value is 2.84, 2.41, 1.37
and 1.08 ns for 2a–d, respectively. The excimer lifetime is 5.60,
4.63 and 3.85 ns for 2b–d, respectively.
Notes and references
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To test a possible use of these dyes as singlet oxygen
photosensitizers, we studied their ability to generate singlet
oxygen.4c,7 The singlet oxygen phosphorescence for BODIPY
dimers 2a–d (Fig. 4 and Fig. S6w) was obtained in toluene,
excited at a wavelength of 509 nm and detected with a near-
IR-sensitive detector. When excited at equal absorptivity
concentrations for all compounds, BODIPY dimer 2b gave
the strongest singlet-oxygen phosphorescence emission at the
signature wavelengths (with a maximum at around 1260 nm).
A relatively weaker singlet-oxygen phosphorescence emission
centred at around 1274 nm was observed for BODIPY dimer
2d. By contrast, weak singlet oxygen phosphorescence emis-
sions were observed for 2a and 2c (Fig. S6w). In addition, they
were entrapped in the emissions of excited-state charge-
transfer species12 in these two BODIPY dimers, due to the
lack of pyrrolic substituents in the newly formed BODIPY
component to stabilize the excited-state charge-transfer species
in 2a and 2c.
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Fig. 5 Comparative DPBF (initial concentration at 5 Â 10À5 M)
degradation profiles in toluene by BODIPY dimers 2a (’), 2b ( ),
2c ( ) and 2d ( ) (1 Â 10À6 M). Filtered light > 455 nm used.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5437–5439 5439