4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene, also known
as boron dipyrromethene (BODIPY), is a class of highly
fluorescent organic fluorophore.8 Its extraordinary chemical
and physical properties, such as high quantum yield, large
extinction coefficient, and good thermal and photochemi-
cal stability, make it attractive for wide applications includ-
ing luminescent devices,9 biological imaging and labeling,10
sensors,11 and dye-sensitized solar cells (DSCs).12 The
BODIPY core usually exhibits visible absorption and
emission located between 470 to 550 nm. To further push
the absorption/emission wavelength to far-red and even
the NIR region, a variety of strategies have been used: (1)
extension of π-conjugation by fusing rigid ring to the
pyrrole unit (e.g., replacing pyrrole with isoindole),13 (2)
functionalization at the R- and/or meso-position to gen-
erate a “pushꢀpull” motif,14 and (3) replacement of the
8-carbon atom with a nitrogen atom to form aza-BODIPY
dyes.15 Very recently, our group developed a new strategy
by fusing an aromatic unit to the zigzag edge of a BODIPY
core via intramolecular oxidative cyclodehydrogenation
reaction.16 By this approach, perylene- and porphyrin-fused
BODIPY NIR dyes with long-wavelength absorption/
emission and high photostability have been successfully
prepared. The major challenge is the choice of an aromatic
unit with appropriate geometric and electronic structure,
both of which play important role on the cyclodehydro-
genation reaction. Anthracene, an electron-rich aromatic
hydrocarbon with zigzag edges that potentially match the
zigzag edge of a BODIPY core, is therefore tested in this
work with the objective of obtaining new anthracene-fused
BODIPY NIR dyes.
Scheme 1. Synthetic Route for Compounds 7a and 7b
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As shown in Scheme 1, three anthracene-BODIPY dyads
2a, 2b, and 6 were first prepared and then submitted to an
FeCl3-mediated oxidative cyclodehydrogenation reaction
to generate fused π-systems. The anthracene monoalde-
hyde 1a (R = H)17 and 1b (R = Br)18 condensed with
2-ethylpyrrole in the presence of trifluoroacetic acid (TFA)
followed by oxidative dehydrogenation with 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) and complexation
with BF3 Et2O to give the corresponding anthraceneꢀ
3
BODIPY dyads 2a and 2b in 31% and 23% yield, respec-
tively. However, treatment of 2a or 2b with excessive
anhydrous FeCl3 in nitromethane and dichloromethane
(DCM) did not afford the target products 3a or 3b.
Alternatively, inseparable BODIPY oligomers via inter-
molecular couplingtogetherwithotherdecomposed BOD-
IPY side products were obtained. This failure may be
ascribed to the relatively low reactivity at the 1,8-positions
of the anthracene unit in 1a and 1b. A similar problem was
also observed during the synthesis of anthracene-fused
porphyrins.19 To resolve this problem, two strongly
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