Meso-alkynyl BODIPYs: Structure, photoproperties, π-extension, and manipulation of frontier orbitals

Article abstract of DOI:10.1002/asia.201201176

Bodipy building: meso-Alkynyl BODIPY monomers and dimers were synthesized and their crystal structures, photochemical properties, and electronic structures disclosed. The meso-arylethynyl group is coplanar with the BODIPY core. The resulting efficient π-c

Full text of DOI:10.1002/asia.201201176

COMMUNICATION
DOI: 10.1002/asia.201201176
meso-Alkynyl BODIPYs: Structure, Photoproperties, p-Extension, and
Manipulation of Frontier Orbitals
Shinpei Kusaka,[a] Ryota Sakamoto,*[a] Yasutaka Kitagawa,[b] Mitsutaka Okumura,[b] and
Hiroshi Nishihara*[a]
The importance of 4,4-difluoro-4-bora-3a,4a-diaza-s-inda-
cene (BODIPY,[1] Scheme 1) has increased at an accelerated
pace: In addition to existing applications (fluorescent bio-
tions have been reported to expand the p-conjugation effec-
tively.[8] Most of the modifications have been undertaken at
the 1–3 and 5–7 positions (Scheme 1); however, the 8 posi-
tion (meso position) has not been exploited fully in the p-
extension of BODIPYs. For example, an aryl group on the
meso position cannot contribute to the p-extension owing to
an orthogonal configuration. Other types of meso-modifica-
tions include a terminal ethynyl group,[9a] alkenyl group,[9b]
formyl group,[9c] and oxidative annulation between an an-
thracenyl group at the meso position[9d] and the 1 and 7 posi-
tions. However, the first type of modification resulted in low
thermal stability, while the rest led to significant quenching
of fluorescence.
ACHTUNGTRENNUNG
probes[2] and laser dyes[3]), new practical approaches in areas
In this communication, we report the synthesis of
BODIPY derivative 1 (Scheme 1) with an arylethynyl group
at the meso position. In addition, a new type of BODIPY
dimer 2 featuring a p-diethynylaryl linker was created. We
investigated how p-conjugation between the BODIPY core
and arylethynyl group works from various aspects, such as
single-crystal X-ray structure analysis, photochemical mea-
Scheme 1. Chemical structures of meso-alkynyl BODIPY monomers
1 and dimers 2, and meso-aryl BODIPY monomers 3 and dimers 4. The
numbering of the carbons in an unsubstituted BODIPY is also shown.
AHCTUNGTREGsNNUN urements, cyclic voltammetry, and density functional theory
(DFT) calculations. In this context, comparison with conven-
tional meso-aryl-substituted BODIPY 3 (Scheme 1) assists
us greatly to highlight the p-conjugation effect in 1 and 2.
We note that a meso-arylethynyl BODIPY similar to 1 has
been synthesized independently very recently.[10]
such as photodynamic therapy (PDT)[4] and solar cells[5]
have been explored recently using BODIPY-based com-
pounds. The increased efforts have led to numerous
BODIPY derivatives. The main purpose of modifications is
to redshift and broaden the absorption and fluorescence in
order to facilitate BODIPYs ability to use solar radiation
and tissue-penetrating light effectively, and to obtain color
variations. The simplest modification reported thus far in-
cludes the introduction of p-conjugating substituents such as
ethynyl[6] and vinyl[7] groups. Additionally, annulation reac-
The synthetic strategy for meso-alkynyl BODIPY mono-
mer 1 is shown in Scheme 2. Sonogashira coupling of tert-
butyliodobenzene with propargyl alcohol gave 5, which was
converted into precursor aldehyde 6 by means of oxidation
with pyridinium chlorochromate. Condensation between
pyrrole derivatives and aldehyde in the presence of a catalyt-
ic amount of trifluoroacetic acid (TFA), successive oxidation
by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and
complexation with BF3·OEt2 gave BODIPY 1 (1a: 5%, 1b:
17%). BODIPY dimer 2 was also obtained (2a: 24%, 2b:
25%) by using a method similar to that for 1. BODIPY
dimer 2 was sufficiently soluble in typical organic solvents.
To reveal the structure of meso-alkynyl BODIPYs, single-
crystal X-ray diffraction analysis was conducted for 1a and
2a:[11] Their thermal ellipsoid plots and packing structures
are shown in Figure 1, and the crystallographic data are
summarized in Table S1 in the Supporting Information. This
is the first crystallographic report on meso-alkynyl BODI-
PYs. The dipyrrin moieties and arylethynyl groups are ap-
[a] S. Kusaka, Dr. R. Sakamoto, Prof. Dr. H. Nishihara
Department of Chemistry, Graduate School of Science
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5841-8063
E-mail: sakamoto@chem.s.u-tokyo.ac.jp
[b] Dr. Y. Kitagawa, Prof. Dr. M. Okumura
Department of Chemistry
Graduate School of Science
Osaka University
1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201201176.
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Ryota Sakamoto, Hiroshi Nishihara et al.
overlaps so that there is no p–
p interaction among the layers.
The electronic spectra and
fluorescence spectra of meso-
alkynyl BODIPYs 1a and 2a
and the reference meso-aryl
BODIPY 3a in toluene are
shown in Figure 2. Their nu-
merical data are collected in
Table 1. Compound 1a showed
an intense 1p–p* absorption
and fluorescence with maxima
at 557 nm and 570 nm, respec-
tively, which were redshifted
by 44 nm and 46 nm from
those of 3a. This result indi-
cates that effective p-extension
can be achieved by the intro-
duction of the arylethynyl
group at the meso-position.
Also noteworthy is the fluores-
cence quantum yield (fF) of
1a. The value (fF =0.86) is as
high as for ordinary BODIPYs,
and importantly, is much great-
Scheme 2. Syntheses of 1a and 2a. Hex denotes n-C6H13. PCC=Pyridinium chlorochromate.
er than for any other p-expanded BODIPYs at the meso po-
sition, all of which suffer from significant quenching of fluo-
rescence (e.g., meso-styryl BODIPYs show negligible fluo-
rescence (fF <0.01)[9b]). As for arylethynyl BODIPYs, sub-
stitution at either 2 position or 3 position was reported.[6]
Table 1 also lists the optical properties of such examples,
Figure 1. ORTEP drawings (50% probability level) and packing struc-
tures of a) 1a and b) 2a. Pink: B, gray: C, blue: N, and green: F. Hydro-
gen atoms are omitted for clarity.
Figure 2. UV/Vis (solid lines) and fluorescence spectra (dashed lines) of
1a–3a in toluene.
Table 1. Photochemical parameters of 1a–3a, 9, and 10 in toluene.
proximately coplanar in 1a. Similarly, the p-diethynylaryl
linker and dipyrrin ligand lie on the same plane in 2a. These
structural features suggest that 1 and 2 should enjoy effec-
tive p-conjugation. The packing structure of 1a adopts a her-
ringbone-like alignment with short CH–p contacts between
10ꢀ4 emax [Mꢀ1 cmꢀ1
]
labs [nm] lem [nm] Stokes shift [cmꢀ1
]
fF
1a
2a
3a
9[a]
6.1
7.9
8.9
557
568
513
566
526
570
592
524
577
575
409
714
409
337
1620
0.86
0.59
0.95
0.91
0.47
[c]
–
10[b] 7.2
ꢀ
the aryl C H bond and the pyrrole ring. On the other hand,
the structure of 2a features layer-by-layer piles. The inter-
[a] From ref. [6e]. [b] From ref. [6f], in dichloromethane. [c] No data re-
ported.
layer distance is 3.6 ꢁ, yet the molecules avoid effective
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Ryota Sakamoto, Hiroshi Nishihara et al.
9[6e] and 10[6f] (Figure 3). This comparison suggests that
meso-arylethynyl BODIPY 1a undergoes p-extension com-
parable with those of 9 and 10.
Figure 3. Chemical structures of previously reported arylethynyl BODI-
PYs 9 and 10.
meso-Alkynyl BODIPY dimer 2a has absorption and
fluorescence maxima at 568 nm and 592 nm. These values
are greater than those of 1a by 11 nm and 22 nm. Further-
more, the absorption band of 2a is broadened significantly.
These facts indicate that 2a enjoys more intense p-conjuga-
tion than 1a upon dimerization. The high fF of 2a (0.59) is
also distinctive. BODIPY oligomers tend to have reduced fF
values; for example, a BODIPY dimer tethered at the meso
position with a phenylene linker, 4a (Scheme 1), possessed
a fF of merely 0.095 in the same medium, toluene.[12] There-
fore, we can conclude that arylethynyl substituents have
a good affinity with BODIPYs in terms of meso-substitu-
tion: p-extension and fluorescent ability are compatible.
To gain insight into the frontier orbital energy of the
meso-alkynyl BODIPYs, cyclic voltammetry was conducted.
The electrochemical reversibility of 1a and 2a was poor. On
the other hand, 1b and 2b (Scheme 1) with additional alkyl
groups on the BODIPY core had good electrochemical re-
versibility (Figure 4). Thus, hereafter, 1b and 2b are dis-
cussed with regard to referential meso-aryl BODIPY 3b.
The formal potentials (E0’) of the BODIPYs are given in
Table 2. meso-Alkynyl BODIPY 1a showed reversible one-
0
00
red
electron oxidation and reduction at E0ox =0.63 V and E
=
ꢀ1.46 V, respectively. On the other hand, those of meso-aryl
0
0
BODIPY 3b emerged at E0ox =0.58 V and Er0ed =ꢀ1.84 V.
The oxidation is ascribable to electron subtraction from the
HOMO, the highest p orbital of the BODIPY core, whereas
the reduction can be ascribed to electron donation to the
LUMO, the lowest p* orbital of the BODIPY core. The
small change in the oxidation potential upon introduction of
the arylethynyl group instead of the aryl group indicates
that the HOMO level is not affected by the modification. In
sharp contrast, the reduction potential underwent a signifi-
cant positive shift of 0.38 V, indicative of substantial lower-
ing of the LUMO by the introduction of the arylethynyl
group in 1b. The oxidation and reduction of 2b appeared at
Figure 4. Cyclic voltammograms of a) 1b, b) 2b, and c) 3b (0.5 mm each)
in dichloromethane containing 0.1m tetrabutylammonium perchlorate.
Table 2. Formal potentials of 1b–3b.
00
00
red
E
[V]
E
[V]
ox
1b
2b
3b
0.63[a]
0.66[b]
0.58[a]
ꢀ1.46[a]
ꢀ1.38[b]
ꢀ1.84[a]
[a] A one-electron process. [b] A two-electron process.
LUMO level while the HOMO level remained unchanged.
To elucidate the electronic structure of the meso-alkynyl
BODIPYs, DFT calculations were conducted. To reduce the
cost of the calculations, compounds 1c–3c that did not con-
tain alkyl groups on the pyrrole rings were used (Figure 5
and Figure S1 in the Supporting Information). Figure 5 also
shows the energy diagram for the frontier orbitals of meso-
alkynyl 1c and meso-aryl 3c. The HOMO and LUMO of 3c
are p and p* orbitals of the BODIPY core: The aryl group
0
0
E0ox =0.66 V and Er0ed =ꢀ1.38 V. Judging from greater peak
currents of 2b as compared to those of 1b, the oxidation
and reduction of 2b are attributed to one-step two-electron
processes. We can recognize a positive shift of the reduction
potential in 2b compared to 1b.
The photochemical investigation implied that meso-alky-
nylation afforded successfully p-extension, whereas the elec-
trochemical measurement suggested that it lowered the
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Ryota Sakamoto, Hiroshi Nishihara et al.
tion. As a result, a reduction of the LUMO and LUMO+
1 level takes place.
In conclusion, BODIPY monomers 1 and dimers 2 bear-
ing arylethynyl substituents at the meso position were syn-
thesized. Both absorption and fluorescence were redshifted
upon meso-alkynylation, thus indicating the existence of ef-
fective p-conjugation between the arylethynyl groups and
BODIPY cores. Unlike other substitutions at the meso posi-
tion, high fluorescence quantum yields were retained. Cyclic
voltammetry revealed that the reduction potentials of 1b
and 2b were shifted substantially in the positive direction,
whereas their oxidation potentials were unchanged. DFT
calculations rationalized the shift of the redox potentials.
The HOMO of the BODIPY core had a node on the meso
carbon, which prevented it from p-conjugation with the aryl-
AHCTUNGTREGUNeNN thynyl group. On the other hand, the LUMO of the
BODIPY core included an orbital contribution from the
meso carbon, which led to effective p-conjugation with the
arylethynyl group, and resultant reduction of the LUMO
level. meso-Alkynylation thus allows for flexible molecular
orbital engineering of BODIPY derivatives, which leads to
a tactical molecular design for applications such as dye-sen-
sitized solar cells (DSSCs), organic light-emitting diodes
(OLEDs), and artificial photosynthesis.
Figure 5. Energy diagram of the frontier orbitals of 1c and 3c estimated
by DFT calculations. The black and gray solid lines indicate occupied
and unoccupied orbitals, respectively.
Acknowledgements
does not contribute to the HOMO and LUMO, thus reflect-
ing the orthogonality with the BODIPY core. Also notewor-
thy is a node of the HOMO at the meso carbon. The other
carbon atoms of the BODIPY core have substantial orbital
contributions, and the LUMO has no node on the carbons.
Considering the node on the meso carbon, the phenylethyn-
yl group of 1c does not contribute to the HOMO. On the
other hand, the LUMO of 1c is delocalized over the whole
molecule, which indicates the existence of effective p-conju-
gation between the phenylethynyl group and the BODIPY
core. The p-conjugation is reflected appreciably in the mo-
lecular orbital energy. The energy level of the HOMO is
almost the same in 1c and 3c, whereas significant stabiliza-
tion is found in the LUMO of 1c. This result is consistent
with the electrochemical measurement, in which only the re-
duction potential undergoes a significant positive shift upon
meso-alkynylation (Table 2). In other words, the meso-alky-
nylation can manipulate the LUMO level and maintain the
HOMO level. This is hardly attained by modifications at the
1–3 and 5–7 positions because these carbons contribute sub-
stantially to both HOMO and LUMO. The smaller HOMO–
LUMO gap in 1c gives rise to the redshift of absorption and
fluorescence.
The authors acknowledge Grants-in-Aid from MEXT of Japan (Nos.
24750054 and 21108002, area 2107, Coordination Programming) and
a JSPS Research Fellowship for Young Scientists. R.S. is grateful to the
Iketani Science and Technology Foundation, the Murata Science Foun-
dation, and the Japan Prize Foundation for financial support.
Keywords: alkynylation
·
BODIPY
·
conjugation
·
electrochemistry · fluorescence
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Received: December 12, 2012
Revised: December 25, 2012
Published online: && &&, 0000
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These are not the final page numbers! ÞÞ

COMMUNICATION
Fluorescent Dyes
Bodipy building: meso-Alkynyl
BODIPY monomers and dimers were
synthesized and their crystal structures,
photochemical properties, and elec-
tronic structures disclosed. The meso-
arylethynyl group is coplanar with the
BODIPY core. The resulting efficient
p-conjugation red-shifts the intense
absorption and bright fluorescence by
about 40–70 nm. Cyclic voltammetry
and DFT calculations revealed that the
LUMO level is stabilized by meso-
alkynylation, while the HOMO level
remains constant.
Shinpei Kusaka, Ryota Sakamoto,*
Yasutaka Kitagawa,
Mitsutaka Okumura,
Hiroshi Nishihara*
&&&& — &&&&
meso-Alkynyl BODIPYs: Structure,
Photoproperties, p-Extension, and
Manipulation of Frontier Orbitals
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ÝÝ These are not the final page numbers!