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ChemComm
50 molecules reported here display dual emission and
unusual
emissive properties upon fluoride binding, which are explained
on the basis of molecular structures and conformation dependent
energy transfer. Detailed studies on the structures, conformation
and photophysical properties of a series of borane-BODIPY
55 dyads are under investigation.
PT thanks, the Department of Science and Technology (DST)
New Delhi: CSIR New Delhi, and the ISRO-IISc Space
Technology Cell for the financial support. Mr. CAS thanks IISc
for SRF and SM thanks CSIR for SPM fellowship. We sincerely
60 thank Prof. N. Periyasamy for his valuable suggestions. We also
thank the anonymous reviewers for their helpful suggestions.
Fig 4 Spectral changes in the emission spectrum of 2 (left) and 3 (right) in
DCM (λex = 350 nm) upon addition of TBAF. Insets: pictures of the
solution under UV lamp before and after TBAF addition.
5
It is well known that fluoride ion binds strongly to boron.1,3 In
the case of triarylboranes (Ar3B), fluoride binding leads to a new
entity, namely, Ar3BF-. The energy levels, absorption and
emission properties of the Ar3BF- may be entirely different from
Notes and references
Department of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore 560012, India.Fax: 0091-80-23601552; Tel: 0091-
those of Ar3B. In the case of 2 and 3, fluoride binding would not 65 80-22933353; E-mail: thilagar@ipc.iisc.ernet.in
† Electronic Supplementary Information (ESI) available: [Synthetic
10 only affect the electronic structure of the boryl unit but also the
conformation and relative orientation of boryl and BODIPY
units, thereby potentially modifying the energy transfer
efficiency. Thus, we used fluoride (F-) as a probe to get further
insight into the dependance of the energy transfer efficiency of 2
15 and 3 on their molecular conformations. Experiments showed that
addition of TBAF (0-1.5 equiv) to dichloromethane solutions of
2, resulted in a gradual enhancement of both the bands of the dual
emission upon excitation of the boryl dominated band at 330 nm
giving rise to brighter green emission (Fig 4). The absorption
20 spectrum of the fluoride-bound 2 was similar to that of 2 (except
5 nm blue-shift in the lower energy band (see ESI)). This
indicated that the quantum yield of the boryl emission is higher
for the fluoride-bound 2 (QF: 49.5% and 74.3% for 2 and 2•F-
respectively) and the energy transfer efficiency remained
25 unchanged (the relative intensity of the emission peaks remain
constant). As expected, compound 3 behaves in the opposite
way. In the case of 3, upon addition of TBAF to the DCM
solutions and excitation of the boryl absorption dominated band
at 330 nm, it was observed that the intensity of the boryl band at
30 405 nm increased slightly but the BODIPY band at 515 nm
disappeared almost completely at ∼1 eq of fluoride (Fig.4) (QF:
42.51% and 18.13% for 3 and 3•F- respectively). Independently,
it was verified that TBAF does not quench BODIPY emission
(λex = 500 nm) in 3 or the model compound 6. The addition of F-
35 quenches both the absorption and emission bands (λex = 330 nm)
in 4.12 Thus, the selective disappearance of BODIPY emission in
fluoride-bound 3 when boryl unit is excited is interpreted as
indication of drastic decrease in energy transfer efficiency of
PhBMes2 unit in 3. A comparison of computational results
40 obtained for 2, 3, 2•F- and 3•F- (See ESI) support the above
inferences. Dual emissive BODIPYs are scarce in literature11, and
to our knowledge, compounds 2 and 3 are the first examples that
display such an unusual dual emission among known borane-
BODIPY dyads. The change of emission intensity upon addition
45 of TBAF to 2 or 3 is linear upto 1 equivalent of F- and is
negligible beyond 1 equiv. This confirms the formation of 1: 1
complex of 2•F- and 3•F-.
Methods and characterization data for all compounds, DFT results, X-ray
crystallographic data and Photophysical data]. For ESI see
DOI: 10.1039/b000000x/
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(12) Based on the PL characteristics of 4•F, its hard to explain the high
QF in 2•F. This needs detailed time resolved fluorescence and other
studies which is under progress.
In summary, we have developed a facile synthetic procedure for
simple and the most compact borane-BODIPY dyads. The two
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