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Fig. 3 (a) TBAOH-promoted stepwise conformational switching involving 7 and its computationally determined most stable intermediate anionic isomers 7c, 7ac, 7abc
and 7abcd by ab initio calculations at the B3LYP/6-31G level in THF. In 7abc and 7abcd, the interior methoxy methyl groups and exterior aromatic protons were removed
for clarity of view. Both 7 and 7c are fluorescent and all the anionic 7ac, 7abc and 7abcd likely fluoresce very weakly. (b) Fluorescence spectra of 7 at 10 mM in THF in the
presence of up to four equivalents of strong organic base TBAOH for deprotonating hydroxyl groups.
having a fluorescence curve almost identical to that of 6 and/or with the replacement of methoxy groups by other
(Fig. 2a), also possesses a conformation and backbone curva- bulkier or chiral groups at the two ends.
ture almost identical to those of 6. This fact further suggests
Financial support for this work was granted to H.Z. by NUS
that mono-anionic 7a and 7b, having their conformations and AcRF Tier 1 grants (R-143-000-534-112) and Singapore-Peking-
backbone curvatures, respectively, similar to those of 5 and 4 Oxford Research Enterprise (COY-15-EWI-RCFSA/N197-1).
should also display a similarity in molecular fluorescence. The
Notes and references
fluorescence properties of 7 and its anionic versions are there-
fore expected to follow the decreasing order 7 > 7a E 7b E 7c >
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7ab E 7bc E 7ac E 7abc E 7abcd where mono-anionic oligomers
are moderately fluorescent and di-/tri-/tetra-anionic oligomers
are essentially non-fluorescent.
In light of the above experimental and computational
results, the amine-induced folding and fluorescence quenching
shown in Fig. 2b possibly involve sequential generation of
mono-anionic intermediates such as 7c or 7b by the first two
equivalents of 1-octylamine, and dianionic 7ac or 7bc by the
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5307--5309 5309