10.1002/anie.202002517
Angewandte Chemie International Edition
CHCl3).[33] In addition, there is a 50 nm bathochromic shift
between the two fluorescence maxima of [4]C-diBu-F (515 nm)
and [5]C-diBu-F (466 nm), similar to that observed between
[8]CPP and [10]CPP (64 nm). If in the case of CPPs, some
explanations have been provided,[20] the origin of this feature
remains to be elucidated in the case of cyclofluorenes. The
fluorescence quantum yields of [5]C-diBu-F and [4]C-diBu-F
were respectively measured to be 0.61 and 0.21, very similar to
those of [10]CPP and [8]CPP (0.59 and 0.25, measured in
identical conditions), showing the small influence of the bridges
on the emission efficiency. The fluorescence decay curves of
[5]C-diBu-F provide two lifetimes, =1.4 and 4.3 ns, which are
around half those of [4]C-diBu-F, =3.0 and 8.4 ns (Table 1).
One can note that the longer lifetime (4.3 ns) of [5]C-diBu-F is
very similar to that measured for non-bridged analogue [10]CPP
(3.9 ns). Thus, the time-resolved emission properties seem to
be driven more by the number of constituting phenyl units than
by the presence of the bridges.
The specificity of the optical characteristics of [5]-cyclofluorenes
is confirmed by the study of [5]C-diHex-F. Indeed, the electronic
properties of [5]C-diHex-F appear very similar to those of
[5]C-diBu-F, Table 1. Of particular interest, a similar resolved
fluorescence spectrum with the same two bands (443, 466 nm,
Figure 4, Bottom-right) and a high quantum yield (0.61) was
obtained for [5]C-diHex-F. The lifetimes measured, 1.8 and 4.4
ns, are also almost identical to that of [5]C-diBu-F, indicating
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that
a similar molecular fragment is involved in these
deactivation processes.
To summarize, we have synthesized the first examples of
[5]-cyclofluorenes and have shown their singular structural and
electronic behaviour. In the cyclofluorenes family, as the size of
the nanoring increases from 8 to 10 constitutive phenyl units: (i)
the phenyl units are less bent (as for CPPs) and less twisted
(oppositely to CPPs), (ii) the HOMO and the LUMO are
decreased (as and oppositely to CPPs respectively) and (iii) the
fluorescence emission is improved and blue shifted (as in CPPs)
and resolved (oppositely to CPPs). Thus, the ring enlargement
confers [5]-cyclofluorenes specific electronic properties
compared
to
[4]-cyclofluorenes.
Furthermore,
the
cyclofluorenes display specific properties compared to CPPs
due to the rigidity induced by the bridges. If the properties of
CPPs are well known as of now, other families of nanorings
such as cyclofluorenes remain barely explored. For all
nanorings, defining the evolution of their structural and
electronic properties as a function of the ring size is a crucial
step in the understanding of these new generations of -
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example, it has been recently reported that the size does not
influence the emission characteristics of CPPs geodesic
phenine frameworks,[23] which is different from that highlighted
for CPPs[20, 36] or the present cyclofluorenes. This shows the
significant specificities of each family of nanorings and the
richness of this research field.
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Acknowledgements
This project has received funding from the European Union’s
Horizon 2020 research and innovation program under grant
agreement No 699648 (FRODO). We thank the CINES (Montpellier
N°2019-A0040805032), the ANR (n°14-CE05-0024), the Region
Bretagne (DIADEM), Rennes Metropole, the CDFIX and CRMPO
(Rennes). We also thank Dr. E. Caytan for fruitful discussions and
Dr J.F Bergamini for the graphical abstract design.
Keywords: cyclofluorene • nanorings • ring bridging •ring size effect
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