chiral sensors.8 However, all of these applications require
as a first step the synthesis of a chiral material with high
OR and transparency, a task difficult to fulfill due to the
KramersꢀKronig (KK) transforms.9 In the case of chiral
sensors, the spectral window of interest is the visible region
because most targeted chiral analytes (natural compounds,
drugs, etc.) only have significant optical activity in this part
of the electromagnetic spectrum.
functionalization and by the nature of the bridge. While
few structure vsoptical rotation studies are described in the
literature,15,18 this work affords new elements in this
direction.
The [6]-helicene with [R]D = 3640 and an absorption
tail-off wavelength in the UV at ∼370 nm would be a good
target if its synthesis were efficient. Additionally, the [R]D
of conjugated [n]-helicenes increases with the number of
rings (n),10 but as a consequence of KK relations, the
absorption wavelength is red-shifted, making them unsui-
table for use in the manufacture of visible-region chirode-
vices. More generally for material design, and for which-
ever the spectral window targeted, helicenes have two
unworkable drawbacks: the difficulties of their synthesis
and their low racemization barrier.11
Figure 1. Schematic view of the molecules showing the main
chiral interactions between chromophores (arrows) and the
adjustable structural parameters (red) involved in the chiropti-
cal properties.
Many original strategies for the synthesis of helical
molecules are reported in the literature: photochemical,12
Friedel and Craft,13 DielsꢀAlder,14 metal-catalyzed cou-
pling, or cycloaddition,15,16 reactions. Unfortunately they
have rarely been scaled up to gram production.17 To tackle
this issue, we have designed a simple reaction pathway
based on the use of easily accessible units featuring strong
chromophores, and their coupling as a helicene-like struc-
ture via two key steps known for their efficiency: the
arylꢀaryl homocoupling and the bridging between phe-
nolic functions. Finally, the resulting schematicframework
(Figure 1) features four strong interacting chromophores,
two phenyls (Ph) and two quinolyls (Qui). These chromo-
phores display strong electronic transitions in the UV part
and none in the visible part of the spectrum. Moreover,
associating the chromophores closely in space via the
helical skeleton induces strong resonant exciton coupling
and thus gives rise to a striking chiroptical response. The
inclusion of these moieties permits the modulation of the
chiroptical properties by modification of the phenyl unit
The synthetic pathway (Scheme 1) starts with the
€
Friedlander cyclization between 6-bromo-7-methoxy tet-
ralone 119 and the hydrochloride of aminobenzophenone
or aminoacetophenone. It gives the 1-bromo-2-methoxy-
5,6- dihydrobenzo[c]acridine unit 2 substituted at position
7 with an excellent yield (90%) if R = Ph (2b) and a lower
one (35%) when R = Me (2a), due to side reactions
resulting from the presence of R-hydrogens belonging to
the carbonyl in the latter case. The Ullmann’s homo-
coupling of 2 provides the expected biaryl structure 3. At
this stage, we chose to continue with the most easily accessible
Ph derivative (3b R = Ph (88%) vs 3a R = Me (42%)).
Demethoxylation of 3b is quantitatively achieved upon
treatment with tetramethylsilyl iodide and affords the
protonated biphenol derivative 4b. Finally, the standard
procedure reported for binol chemistry afforded a methy-
lene bridged compound 5b with a 94% yield. As it relies
on two very powerful and easy to handle reactions, the
Friedlander reaction20 for the monomer construction and
€
the Ullmann coupling for the dimerization,21 the racemic
synthesis of this dibenzo[c]acridine compound 5b is easy
and efficient (75% overall yield in four steps). Notably, it
can yield up to 5 g of the R = Ph derivative.
Racemic 3, 4, and 5 all crystallize in centrosymmetric
space groups, exhibiting no spontaneous resolution beha-
vior unlike that which is frequently observed in helicene
frameworks (see Table 1 and the Supporting Information
(SI)). One highlights that the targeted compounds of this
study have an important tendency to fold into an helical
shape in the solid state even if the structure is not enforced
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