NIR photodevices including NIR photoswitches, highly sensitive
biosensors, etc.
This research was financially supported by Grants-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
Notes and references
z The methoxy groups significantly improve solubility of 1,4-bis(1H-
pyrrol-2-yl)-2,5-dimethoxybenzene as a synthetic intermediate (see ESIw)
to organic solvents.
Fig. 3 Changes of absorbance at 645 nm (red line) and fluorescence
intensity at 755 nm (blue line) for three cycles of the folding–unfolding
process: (a) Cs+ (10 eq.); (b) 18C6 (20 eq.); (c) Cs+ (20 eq.); (d) 18C6
(20 eq.); (e) Cs+ (20 eq.); (f) 18C6 (20 eq.), [B3] = 2.0 mM.
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A decrease in the fluorescence at 755 nm and a concomitant
increase in the fluorescence at 708 nm were observed in B3 by
the addition of Cs+ (Fig. 2c). These blue shifts also suggested
the decrease of the effective p-conjugation length between the
BODIPY moieties upon cation binding. The fluorescence
quantum yield slightly decreased upon the addition of Cs+
(0.47 to 0.41). We can realize the guest binding by the naked
eye based on not only the absorption change, but also the
fluorescence change (Fig. 2e). On the other hand, monomeric
compound B1 showed a negligibly small spectral change even
in the presence of excess Cs+. Thus, this also supported the
fact that the Cs+ binding of B2 and B3 was caused by the
simultaneous chelation of at least two BODIPY units, indicative
of the formation of foldamers. This structural change was also
supported by 1H NMR and theoretical examinations (vide infra).
The Hartree–Fock calculation of the B3ꢀCs+ supported a
folded structure, in which the three BF2 moieties contact a
Cs+ ion. The 19F NMR spectral titration of B3 with Cs+
showed downfield shifts (Dd: ca. 3 ppm), suggesting the
B–Fꢀ ꢀ ꢀCs+ interactions.13 In addition, the ROESY spectrum
confirmed the cation-induced folding. The correlation peaks
between the aromatic proton Hb of the terminal phenyl group
and methoxy protons of the dimethoxyphenylene linker were
observed, while no correlation existed for the free B3 (see ESIw).
Finally, we successfully switched the NIR photophysical
properties of the oligomers by utilizing a reversible inter-
conversion between the folded and unfolded structures. The
Cs+ binding caused a color change in the B3 solution from
blue to pink. In contrast, upon the addition of 18-crown-6
(18C6) the pink solution immediately changed to blue due to the
removal of Cs+ from the complex. The electronic absorption and
fluorescence spectra above 400 nm indicated that these two
processes proceeded almost quantitatively and repeatedly (Fig. 3).
We have synthesized a series of new a-bridged linear
BODIPY oligomers, which exhibited NIR photophysical
properties such as a strong absorption and high fluorescence
efficiency. The oligomers can be converted to the novel NIR
emissive foldamers upon selective complexation with Cs+ as
an external stimulus. Reversible switching between the folded
and unfolded structures was achieved by the addition and
removal of Cs+ in order to regulate the NIR photophysical
properties. These frameworks based on the switchable NIR
fluorescent BODIPY oligomers would lead to controllable
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4820 Chem. Commun., 2012, 48, 4818–4820
This journal is The Royal Society of Chemistry 2012