Bis-4-aza[6]helicene: A Bis-helicenic 2,2′-Bipyridine with Chemically Triggered Chiroptical Switching Activity

Article abstract of DOI:10.1021/acs.joc.9b00389

A novel enantiopure bis-helicenic 2,2′-bipyridine system was prepared using a Negishi coupling. Thanks to the bipyridine unit, the coordination with Zn^II and protonation processes were studied, revealing efficient tuning of photophysical (UV/visible and emission) and chiroptical properties (electronic circular dichroism and circularly polarized emission) of the system. The coordination/decoordination and protonation/deprotonation processes appeared reversible, thus constituting novel chiroptical switches.

Full text of DOI:10.1021/acs.joc.9b00389

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Article
Bis-4-aza[6]helicene: a bis-helicenic 2,2'-bipyridine
with chemically-triggered chiroptical switching activity
Helena Isla, Nidal Saleh, Jiang-Kun Ou-Yang, Kais Dhbaibi, Marion
Jean, Magdalena Dziurka, Ludovic Favereau, Nicolas Vanthuyne, Loic
Toupet, Bassem Jamoussi, Monika Srebro-Hooper, and Jeanne Crassous
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00389 • Publication Date (Web): 29 Mar 2019
Downloaded from http://pubs.acs.org on March 29, 2019
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chirality can provide unidirectionality, an important aspect in
Bis-4-aza[6]helicene: a bis-helicenic 2,2'-bipyridine
with chemically-triggered chiroptical switching
activity
rotatory motors such as in the biological case of ATP-synthase
and other original kinds of motions.2,3 Among chiroptical
switching materials, helicenes play an increasing role due to
their unprecedented inherent helical chirality accompanied by a
-conjugated electronic structure.6 The most appealing
attributes of helicene derivatives are their large-magnitude
chiroptical properties, i.e. optical rotation (OR), electronic
circular dichroism (ECD), vibrational circular dichroism
(VCD), and circularly polarized luminescence (CPL).7
Despite remarkable progress that has been accomplished in
helicene chemistry in the last decade, the development of
straightforward and efficient strategies to diversify helicenes
structure still remains a challenge for synthetic chemists. An
attractive alternative to a stepwise organic approach to a
construction of complex helicene-based architectures is
coordination chemistry.8,9 Indeed, metals are powerful
templates for assembling -conjugated ligands into well-
defined molecular structures.10 Furthermore, as the nature of the
metal can strongly impact the chiroptical properties of the
helical molecules, such connections give rise to chiral
molecular systems combining novel topological features with
enhanced photophysical and optical properties. In particular,
one can take benefit from the inherent properties of a ligand
and a metal ion to easily access novel helicene-based
organometallic chiroptical switches6e,f through the action of
Helena Isla,† Nidal Saleh,† Jiang-Kun Ou-Yang,† Kais
Dhbaibi,†,& Marion Jean,‡ Magdalena Dziurka,# Ludovic
Favereau,† Nicolas Vanthuyne,‡ Loïc Toupet,† Bassem
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#
Jamoussi,&,¶ Monika Srebro-Hooper,*, Jeanne Crassous*,†
†
Univ Rennes, Institut des Sciences Chimiques de Rennes,
UMR CNRS 6226 Campus de Beaulieu, 35042, Rennes Cedex,
&
France. University of Gabès, Faculty of Science of Gabès,
‡
Zrig, 6072 Gabès, Tunisia. Aix Marseille Université, CNRS,
¶
Centrale Marseille, iSm2, Marseille, France.
Université
Virtuelle de Tunis, UR17ES01 Didactique des Sciences
Expérimentales et de Chimie Supramoléculaire, Tunis, Tunisia.
#
Faculty of Chemistry, Jagiellonian University, Gronostajowa
2, 30-387 Krakow, Poland.
RECEIVED DATE (will be automatically inserted after
manuscript is accepted).
different stimuli such as
a
redox potential10a-f or
chemicals.10g,h,11 In addition, the coordination process can
induce strong conformational changes and intramolecular
motions based on which new chiral switches and molecular
motors can be envisioned.11
Among the helicenes bearing heteroatoms, azahelicenes12 are
especially interesting due to possibilities that nitrogen atoms
offer. The involvement of the N lone-pair in coordination or
protonation processes can induce a peculiar impact on the
chiroptical properties of the helicene molecule. The bipyridine
core, with the most widely used 2,2’-bipyridine unit13 (Scheme
1), has received much attention due to its remarkable stability
and exceptional coordination chemistry. Functionalized helical
bipyridine systems10d,h,13,14 exhibit an appealing set of
properties: chirality, an extended -electron system, rich and
high-affinity coordination chemistry with metals, good
acid/base and alkylation reactivity, and redox activity.
Complexation of such systems with a metallic ion (e.g. ZnII,
RuII, OsII, ReI) may enhance their properties even further, based
on strong optical activity, emission properties, and redox
activity furnished by the metal. All this makes helicenic-
bipyridines promising candidates for unprecedented chiroptical
switches.
ABSTRACT. A novel enantiopure bis-helicenic 2,2'-bipyridine
system was prepared using a Negishi coupling. Thanks to the
bipyridine unit, the coordination with ZnII and protonation
processes were studied revealing efficient tuning of
photophysical (UV/Visible and emission) and chiroptical
properties (electronic circular dichroism and circularly
polarized
emission)
of
the
system.
The
coordination/decoordination and protonation/deprotonation
processes appeared reversible thus constituting novel
chiroptical switches.
INTRODUCTION
In this article, we present the synthesis of a bis-azahelicenic
system in which bipyridine core is integrated in the
sophisticated aza[6]helicene structure (1, Scheme 1), and we
address its chiroptical switching activity triggered chemically
Chirality, ubiquitous in life and in chemical substances, has
growing implications in diverse domains of materials science.1
Indeed, there is a great demand in the development of novel
chiral molecules and architectures with tunable electronic and
optical properties, which may be used in molecular functional
devices. In particular, contributions to the field of chiroptical
switches2 are stimulated by their potential applications as
molecular memory elements or logic operators, in which data
storage and processing are the ultimate goals.3 Chiroptical
switches have also found applications in sensing, especially in
the detection of a multitude of different chiral analytes with a
high level of sensitivity,4 in asymmetric catalysis to offer
switchable stereoselectivity,5 and in molecular motors since
by
(i)
ZnII
coordination/decoordination,
and
(ii)
protonation/deprotonation processes. Read-out signals are
observed using UV/Vis, ECD, OR, and luminescence
techniques, and interpreted on the basis of first-principle
calculations. As complexation/decomplexation process induces
large conformational changes and therefore molecular motions
around the zinc center, reversibly interconverting transoid and
cisoid forms of 1 with different relative orientations of
helicenes moieties, we also explore computationally
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unidirectionality of the motion and a possible application of 1
5
as a molecular hinge.
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The bis-helicenic bipyridine system 1 was prepared by
generating in situ the zinc bromide derivative 6 from 5
followed by coupling 5 and 6 together under the regular
Negishi coupling conditions. Finally, decoordination of the Zn
using an excess of N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-
1,2-diamine (TPEN) was necessary to obtain pure 1 with 57%
yield. Under these conditions and starting from racemic (±)-5, a
mixture of stereoisomers, namely chiral (P,P)- and (M,M)-1
and achiral meso compound (P,M)-1 was expected. This was
indeed evidenced by chiral HPLC analysis, which showed three
peaks in 34:34:24 respective proportions (see SI).
i)
1.
(P,P)- Zn(OAc)2
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Zn(OAc)2
1.
(M,M)- Zn(OAc)2
N
N
TPEN
Zn
CH2Cl2
N
AcO OAc
N
1.
+
-
Triflic acid
H
N
(P,P)- 2H , 2OTf
1
(P,P)-
1.
+
-
(M,M)- 2H , 2OTf
N
H
NEt3
1
(M,M)-
CH2Cl2
N
, 2OTf-
Br
ii)
N
Scheme 1. Chemical structure of the bis-helicenic 2,2'-bipyridine 1 and
reversible (i) coordination/decoordination process using
Zn(OAc)2/TPEN chemical stimuli and (ii) protonation/deprotonation
N
with triflic acid/NEt3 as acid/base agents in dichloromethane.
(P,P)-1
P-5
RESULTS AND DISCUSSION
Figure 1. X-ray crystallographic structures of compounds 1 and 5
(only one stereoisomer is shown).
Synthesis and characterization of bis-helicene-bipyridine
1.
Single crystals of (±)-1 were grown from pentane diffusion
into a dichloromethane solution. X-ray crystallography revealed
a mixture of (P,P)- and (M,M)-1 (Pccn orthorhombic
centrosymmetric space group, see Figure 1 and SI). The
structure showed i) the C1 symmetry of the chiral compound 1,
ii) the two nitrogen atoms placed trans to one another as
common for 2,2'-bipyridines13 with a dihedral angle between
the different pyridine rings lower than 20°, and iii) typical
helicity value of the azahelicene moieties (59.83°).12
Compound (P*,P*)-117 was also characterized by NMR
spectroscopy. For example, its 1H NMR spectrum displayed the
two typical unresolved triplets around 6.9 and 7.4 ppm
corresponding to protons H15 and H14, respectively. Note that
the low quantities isolated and solubility issues did not allow to
characterize the meso-1 compound by NMR spectroscopy.
However, sufficient amounts of enantiopure (P,P)- and (M,M)-
1 compounds were directly prepared under similar Negishi
coupling conditions from enantiopure P- and M-5 samples.
The synthetic strategy chosen for the construction of the
helical architectures 1 is based on the Negishi coupling15
between the bromoaza[6]helicene 5 with the zinc bromide
derivative 6 (see Scheme 2). The 3-bromo-4-aza[6]helicene 5
was first prepared in two steps involving a Wittig reaction
between 2-methyl-naphthalene-phosphonium bromide 216 and
commercially available 2-bromo-pyridine-6-carboxaldehyde 3,
yielding olefin 4 as a mixture of cis and trans isomers, followed
by a photocyclization reaction with a 150 W mercury lamp with
2 equiv. of iodine and an excess of propylene oxide in toluene
for 15 hrs. It is worth noting that the photo-irradiation
conditions were compatible with the bromo-substituent
functionality. Compound 5 was obtained in the racemic form
and fully characterized by NMR spectroscopy and mass
spectrometry (see Supporting Information, SI). For example, it
displayed two unresolved triplets around 6.8 and 7.3 ppm that
are typical of protons H15 and H14, respectively.11b Its structure
was further ascertained by X-ray crystallographic analysis (see
Figure 1 and SI). Racemic 5 crystallized in the P-1 space group.
It displayed a helicity (dihedral angle between the two terminal
rings) of 49.71° and a C-Br bond length of 1.917 Å which both
correspond to regular values. Compound 5 can further be
obtained as enantiopure P-(+)- and M-(-) samples by HPLC
separation over a chiral stationary phase (see SI).
Photophysical and chiroptical properties of (P,P)- and
(M,M)-1
Photophysical and chiroptical properties of 1 were then
examined and compared to those of the precursor 5. As seen in
Figure 2a, the UV/Visible spectrum of (P*,P*)-117 shows a
very strong absorption band centered around 275 nm (
 7.6  104 M–1 cm–1) and
a set of bands of gradually
Br
diminishing intensity that extends into the visible region with
the longest absorption wavelength appearing at 420 nm ( ~
104 M–1 cm–1). In comparison, the compound 5 displays overall
less intense UV/Vis bands with the stronger one at 262 nm (
 5.7  104 M–1 cm–1) and the weaker one at the longest
wavelength of 405 nm ( ~ 103 M–1 cm–1). Furthermore, while 5
is not emissive, likely due to fluorescence quenching via
internal heavy atom (Br) effect,18 (P*,P*)-117 displays rather
intense blue fluorescence in CH2Cl2 at rt (max = 420 nm,  =
22%, Table S1.1 in the SI, and Figure 4c). Its structured
emission spectrum is characterized by a vibronic progression of
~ 1350 cm–1 with the maximum intensity being in the 0-0 band
14
16
N
i)
ii)
PPh3Br
15
2
+
1
Br
X
O
N
4
N
2
3
iii)
X = Br
5
( )
X = ZnBr
6
( )
iv,v)
5
6
+
1
(P,P)- , (M,M)- and (P,M)-
Scheme 2. Synthesis of 1 as a mixture of (P,P), (M,M), and (P,M)
stereoisomers. i) n-BuLi, THF, rt, Ar, 5 hrs, 90%; ii) h, I2 (2 equiv.),
propylene oxide (49 equivs), toluene, rt, Ar, 15 hrs, 45%; iii) n-BuLi,
ZnCl2, THF, rt, Ar, 5 hrs, 90%; iv) Pd(PPh3)4, THF, reflux, Ar, 15 hrs;
v) TPEN, CH2Cl2, 57% (two steps).
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that is indicative of a rigid aromatic fluorophore. Note that
5
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emission behavior of 1 is almost identical to that previously
observed for helicenic bipyridine and terpyridine systems.10h,11
Linear-response time-dependent density functional theory
(TDDFT) calculations,19 following the computational protocol
that was established in Ref. 11 for a bis-helicenic terpyridine
ligand and a corresponding Zn-based complex, reproduce
experimental data well (see Figure 2a, inset, and Table S2.9 in
the SI). Details regarding all the calculations presented in this
article along with additional results not discussed herein in
detail are provided in the SI. As expected, enhanced UV/Vis
properties of 1 as compared to 5 can be linked to its extended
-conjugation involving both azahelicene fragments. Indeed,
the additional low-energy (> 350 nm) absorption observed for 1
originates from excitations (nos. 1 calculated at 372 nm and 3
at 359 nm) that may be predominantly considered as -*
transitions across the whole molecule with some charge transfer
(CT) components from helicene moieties to bipyridine
fragment due to a partial spatial separation of the engaged
MOs. For example, both excitations demonstrate high HOMO-
LUMO contributions, with HOMO almost evenly extended
over -electron system within the whole molecule and LUMO
localized predominantly onto bipyridine fragment and its
adjacent helicene rings (see Figure 3). The same was found for
the frontier MOs at the optimized S1 excited-state geometry that
supports a predominantly bipyridine-centered LUMO-HOMO
emission for 1.
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Intense mirror-image ECD spectra were then recorded for
(M,M)- and (P,P)-1 enantiomers (Figure 2b), showing a strong
negative band around 273 nm ( = -333 M-1 cm-1) and strong
positive one at 332 nm ( = +309 M-1 cm-1) for the (P,P)-1.
Weaker positive ECD bands are also present at longer
wavelengths between 380 and 420 nm, i.e. at 382 ( = +70 M-
1 cm-1), 397 ( = +26 M-1 cm-1) and 417 nm ( = +23 M-1 cm-
1). For comparison, the P-5 displays a weaker ECD spectrum
than (P,P)-1 and similar to those observed for other
carbo[6]helicene derivatives, with a strong negative band
around 258 nm ( = -200 M-1 cm-1), a strong positive one at
326 ( = +173 M-1 cm-1), and no ECD-active band above 380
nm. Note that some vibronic structure is roughly seen in both
compounds 1 and 5. As seen in Figure 2b, inset, the calculated
ECD spectral features of 1 and 5 agree well with the
experiment. The MO analysis of dominant excitations
calculated in the low- and medium-energy regions of the
simulated ECD spectra of (P,P)-1 confirms that its main highly
intense positive band originates predominantly from -*
transitions within helicene moieties accompanied by
helicene→bipyridine and bipyridine→helicene CTs. See for
example, excitations nos. 2, 5, and 6 with sizable rotatory
strengths calculated at respectively 368, 331, and 322 nm and
their MO assignment in the SI. This clearly highlights the effect
of elongated -electron system due to the presence of two
azahelicene moieties in 1, resulting in a strong enhancement of
its chiroptical properties as compared to 5. In line with the ECD
spectra, high specific and molar optical rotation values were
thus obtained for 1 with its molar rotation 1.7 times higher than
Figure 2. Experimental UV/Vis spectra of 5 and (P*,P*)-117 (panel a)
and ECD spectra of enantiopure M- and P-5 and (M,M)- and (P,P)-1
(panel b) in CH2Cl2 (C ~ 3  10-5 M). Insets: Corresponding TDDFT
(LC-PBE0*/SV(P) with a continuum solvent model for CH2Cl2)
calculated data. No spectral shift has been applied. Numbered
excitations (with oscillator strength f, and rotatory strength R, in 10-40
cgs, values given in parentheses) are those analyzed in detail. See also
SI.
Coordination-induced chiroptical switching activity and
molecular motion in bis-helicenic bipyridine 1
The bipyridine system is a good scaffold for coordination to
a variety of transition-metal centers yielding complexes with
diversity of photophysical properties that may be appealing for
device applications.13 For example, Rebek, Jr. et al. have
integrated crown ether functionalities within the 2,2'-bipyridine
platform to study allosteric effects.20 Furthermore, the
molecular engineering of 2,2'-bipyridines with chiral units,
introducing controlled steric congestion and chirality, enables
to construct fascinating helicate structures,21a and molecular
motors displaying unidirectionality. Indeed, regarding the latter
property, Haberhauer has designed a molecular hinge based on
a 2,2'-bipyridine derivative grafted with a chiral peptidic clamp
that shows
a
unidirectional open-close motion upon
coordination to a CuII chemical stimulus.21b,c Yashima and co-
workers have developed unprecedented molecular springs
consisting of enantiomeric double-stranded spiroborate
helicates bearing 2,2'-bipyridine and its N,N-dioxide linkages
that demonstrate a unique allosterically regulated reversible
extension-contraction motions coupled with unidirectional and
helicity conservative twisting upon binding or release of
protons and/or metal ions.21d
23
[휙]2퐷3
2
-1
for 5 ((P,P)-1:
퐷 = +2600° cm g ,
= +17300° cm2
[훼]
23
dmol-1 (±5%, CH2Cl2, 4.2  10-5 M) vs. P-5:
퐷 = +2500°
[훼]
23
cm2 g-1,
M).
퐷 = +10200° cm dmol (±5%, CH2Cl2, 6  10-6
2
-1
[휙]
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These examples open up the possibility for further design spectroscopy (Figure 4a-c). Several changes were observed in
and development of molecular machines, both simpler or even the UV/Vis spectrum of (P*,P*)-117 upon ZnII binding, as
more sophisticated ones. Based on this idea, we then explored depicted in Figure 4a, with the presence of several isosbestic
how ZnII binding to a bis-helicenic 2,2'-bipyridine system could points, showing that the two species are in exchange. The main
induce chemical switching of the UV/Vis absorption, ECD, and modification of the spectrum appears between 420 and 500 nm
luminescence spectra via addition of i) Zn(OAc)2 for where an additional band emerges upon Zn complexation.
coordination and ii) TPEN for decoordination (see Scheme 1).
Accordingly, a colour of the solution turned from pale yellow
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Aliquots of Zn(OAc)2·2H2O (0.2 equiv. until saturation at to dark yellow. Substantial modifications were also observed
1.0-1.2 equivs) were thus added to 1 and the complexation for chiroptical properties of (M,M)-1 after
process was followed by UV/Vis, ECD, and emission
Figure 3. Left: Isosurfaces (±0.03 au) of frontier MOs of the bis-helicenic bipyridine ligand 1, of its corresponding Zn complex 1·Zn(OAc)2, and
of its doubly protonated species 1·2H+. Right: Isosurface (±0.0015 au) of dominant ETS-NOCV deformation density channel depicting formation
of the 1–Zn(OAc)2 bonding. Only the most populated (lowest-energy) conformers’ results have been shown; see SI for a full set of data.
a)
c)
80000
70000
60000
50000
40000
30000
20000
10000
0
40
35
30
25
20
15
10
5

0
300 350 400 450 500 550
400 450 500 550 600 650
  nm
400
300
200
100
0
 / nm
b)
d)
(M,M)-1
274 nm
454 nm
(M,M)-1.Zn(OAc)2
400
350
300
250
200
60
50
40
30
20
10
0

-100


-200
-300
-400
-10
0
2
4
6
8
0
2
4
6
8
cycles
cycles
250 300 350 400 450 500 550
 / nm
Figure 4. Evolution of UV/Vis spectrum of (P*,P*)-117 (panel a) and ECD spectrum of (M,M)-1 (panel b) upon addition of Zn(OAc)2∙2H2O
aliquots (0.2 equiv. until saturation at 1.0-1.2 equivs; C 3  10-5 M, CH2Cl2, rt). Panel c: Evolution of fluorescence of (P*,P*)-117 (ex = 350 nm, C
2.2  10-5 M, CH2Cl2, rt). Panel d: Reversible ECD274nm and ECD454nm switching process.
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Figure 5. Comparison of the simulated UV/Vis (panel a) and ECD (panel b) spectra of the bis-helicenic bipyridine ligand 1 and of its
corresponding Zn complex 1·Zn(OAc)2 (Boltzmann-averaged spectrum at 25 ºC based on the most populated conformers found). No spectral shift
has been applied. Calculated excitation energies along with the corresponding oscillator and rotatory strengths (for the lowest-energy conformer in
the case of 1·Zn(OAc)2) indicated as ‘stick’ spectra. Numbered excitations correspond to those analyzed in detail. See SI for a full set of data.
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Zn(OAc)2·2H2O addition (see Figure 4b). For example, the
strong positive ECD-active band at 273 nm decreases from
+413 to +298 M-1 cm-1, the strong negative band around 332
nm ( = -338 M-1 cm-1) evolves into a new one centered at 342
nm ( = -202 M-1 cm-1), and an additional negative band of
medium intensity appears at 454 nm ( = -45 M-1 cm-1).
Furthermore, magnitude of the molar rotation significantly
increases for 1·Zn(OAc)2 comparing to 1 (e.g. (M,M)-
23
23
2
-1
[휙]
[휙]
1·Zn(OAc)2:
퐷 = -25000° cm dmol , (M,M)-1:
퐷 = -
16700° cm2 dmol-1 (±5%, CH2Cl2, 1.910-4 M), see also SI).
Finally, as presented in Figure 4c, ZnII complexation led also to
significant changes in the luminescence profile of 1, with the
appearance of a structureless intense fluorescence at 520 nm (
= 44%) accompanied by a structured emission at 421 and 446
nm that resembles that from the ligand itself in line with the
presence of equilibrium between 1 and 1·Zn(OAc)2. This was
followed by a striking change in the emission colour from blue
to green (see Figure 8d). Note that in order to obtain full
transformation of the emission signal corresponding to full
conversion to Zn complex, it was necessary to add an excess of
Zn(OAc)2. Finally, having in hands chiral species displaying
efficient fluorescence, it was appealing to examine their
corresponding circularly polarized luminescence (CPL).22,23
The CPL responses for (P,P)-/(M,M)-1 and (P,P)-/(M,M)-
1·Zn(OAc)2 were thus measured in CH2Cl2 at rt and the
resulting spectra are depicted in Figure 6, showing maximum
glum = 2I/I values of -3.7 and +4.8  10-3 for (M,M)-1 and
(P,P)-1 at 425 nm, respectively, and of -1.5 and +1.8  10-3 for
(M,M)-1·Zn(OAc)2 and (P,P)-1·Zn(OAc)2 at 495 nm,
respectively, which appear quite similar to values found for the
previously examined helicene-bipyridine and bis-helicenic
terpyridine systems (i.e. 3.4  10-3 and 7  10-3 for the
bipyridine and terpyridine ligand, respectively, and 1  10-3 for
the terpyridine Zn complex, P enantiomers).10h,11 Accordingly,
based on the fact that 1 and its Zn complex show clearly
distinct and good CPL responses, one may envision to use
CPL-activity at one chosen fixed wavelength to read-out the
coordination/decoordination switching process for 1.
Figure 6. CPL spectra of (P,P)-1 (solid black), its Zn complex (P,P)-
1·Zn(OAc)2 (solid red), and its fully protonated species (P,P)-
1·2H+, 2OTf- along with the corresponding spectra of their mirror-
images (dash lines) measured in CH2Cl2 at rt (C ~1  10-5 M).
Excitation wavelengths: λex at 350 nm for 1 and 1·Zn(OAc)2, and 460
nm for 1·2H+, 2OTf.
Calculations (TDDFT LC-PBE0*/SV(P) with a continuum
solvent model for CH2Cl2) performed for bis-helicenic
bipyridine complex of ZnII correctly reproduce the spectral
trends observed experimentally upon binding of the metal
fragment to 1. Due to a lack of X-ray crystal data for the
1·Zn(OAc)2 compound, various conformations, differing in the
spatial arrangement of acetate ligands around ZnII, were
considered in geometry optimizations of this system. The
lowest-energy structures with non-negligible populations, all
comprising acetate ion(s) coordinated to the metal center in a
bidentate fashion (see SI), reveal very similar overall UV/Vis
and ECD spectral envelopes, and, as can be seen from Figure 5,
their Boltzmann-averaged spectra agree well with the
experiment. Contrary to the previously reported bis-helicenic
terpyridine chiroptical switching system,11 the simulated
UV/Vis and ECD spectra of the ligand 1 in the cis
conformation of the bipyridine fragment clearly differ from
those calculated for 1·Zn(OAc)2 (see SI). This may indicate that
the spectral modifications accompanying 1→1·Zn(OAc)2
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transition originate predominantly from electronic rather than that transform an ‘open’ non-complexed form (nitrogen atoms
geometric changes occurring in 1 upon complexation. Indeed, in trans configuration, NCC’N’ dihedral angle of ~180°) to a
although no contributions of Zn d orbitals to the helicenic - ‘closed’ complexed one (nitrogen atoms in cis configuration,
electron system can be noticed, as expected for closed-shell d10 NCC’N’ dihedral angle of ~0°) and vice versa. The second one
metal center, electronic interaction between Zn and 1 induces (i.e. a control of the orientation of the movement between
polarization of electron density and has a stabilizing effect on ‘open’ and ‘close’ states) is, however, more difficult to achieve
the orbital energies in 1·Zn(OAc)2 vs. 1 (see Figure 3 and SI) as a rotation around the C-C’ bipyridine bond is usually
that may rationalize differences in the spectral envelopes of 1 possible in both directions. The unidirectionality of the
and 1·Zn(OAc)2.11,10h In particular, a shift of the electronic movement can be enforced for example by introducing a meta-
density towards terminal rings of helicenes’ moieties in HOMO bridge within the bipyridine part.21a,b However, one could
and towards bipyridine fragment in LUMO, along with a envision a similar effect in systems with spatially extended
decrease in the HOMO-LUMO gap due to a strong energetic substituents attached to the bipyridine core (such as for
stabilization of the LUMO level, observed for 1·Zn(OAc)2 as example helicenes’ moieties in 1) since such fragments,
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compared
to
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activate
intense
low-energy interacting with each other during a rotation around the C-C’
helicenebipyridine CT excitation within the ligand -system axis, could make one of ‘open’↔‘closed’ transition pathways
(see excitation no. 1 calculated at ca. 390 nm of predominantly preferable over the other providing a control over a movement’
HOMO-LUMO character, Figure 5 and SI). The excitation is direction. Accordingly, the energy profile for the full rotation
responsible for the appearance of the additional low-energy around the C-C’ axis in 1 was calculated both at DFT and DFT-
intensity in the UV/Vis and ECD spectra upon complexation. D3 (i.e. with inclusion of van der Waals (dispersive) forces via
Furthermore, the calculations link the pronounced modification semi-empirical approach, see SI) level of theory and the
(a red-shift and a decrease in intensity) of the main band in the corresponding results are shown in Figure 7. As can be seen,
ECD spectrum for 1·Zn(OAc)2 vs. 1 to the presence of two additional stabilizing dispersion interactions between helicenes’
excitations with strong rotatory strength values of opposite moieties located close to each other in the structure II (see also
signs (nos. 5 and 6 calculated at ca. 340 and 330 nm, Figure S2.18 in the SI for overlays of the corresponding DFT-
respectively) which involve mainly helicenebipyridine CT and DFT-D3-optimized geometries) lower its energy compared
transitions rather than classical helicene-centered -* ones to the structure IV in which helicenes’ fragments are located
(vide supra). The aforementioned activation of low-energy CT apart (compare 7.0 kcal/mol vs. 8.9 kcal/mol for II and IV,
transitions in 1 due to ZnII coordination rationalizes also the respectively, relative to the lowest-energy structure I). This
experimentally observed emission switching process which is leads to asymmetry of the rotational profile for 1, which
quite well reproduced by the computations: from 409 nm S1-S0 indicates that ‘open’: I → ‘close’: III transition may preferably
fluorescence calculated for 1 to 440-470 nm (depending on the occur via the pathway proceeding through structure II.
conformer) for 1·Zn(OAc)2. Note that strong interaction However, even if we disregard a known tendency of DFT-D3
between ZnII and 1 in 1·Zn(OAc)2 was indeed confirmed using dispersion corrections to overestimate interactions between the
the ETS-NOCV24 charge and bonding-energy analysis, which aromatic rings of the helicene moieties,25 this asymmetry seems
showed one main NOCV-based contribution to the total to be too small to guarantee the preferential direction of rotation
deformation density corresponding to a formation of the ZnII-N in 1. Accordingly, it is rather unlikely that this type of systems
-bonding associated with a charge transfer from lone-pair (LP) can act as molecular ‘hinges’.
of bipyridine nitrogen atoms to the metal center (Figure 3).
The reversibility of the ZnII binding process was examined
Chiroptical switching activity of
protonation/deprotonation process
1
induced via
by using N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine
(TPEN) as a competitive ligand. Indeed, upon addition of
TPEN, the system returned to its original state, i.e. chiral (P,P)-
and (M,M)-1 non-coordinated species. Up to 9-10 steps could
be performed upon successive additions of 1 equiv. of
Zn(OAc)2 and TPEN, following the ECD tuning at the selected
wavelengths at which the changes are clearly visible (for
example, 274 nm:  = 150 M-1 cm-1, see Figure 4d).
Accordingly, the bis-helicenic bipyridine system 1 constitutes
another example of a chemically-triggered chiroptical switch
with multi-output read-out including UV/Vis, ECD, and
luminescence, for which the switching process is accompanied
by nanomechanical molecular movements interconverting a
trans conformation in the free ligand to a cis one in the Zn-
complex.11
Finally, the acid-base switching for 1 was studied by using i)
triflic acid and ii) triethylamine for protonation and
deprotonation step, respectively (see Scheme 1). Upon adding
aliquots of triflic acid (0.2 equiv.) to (M,M)-1 in CH2Cl2 until
saturation at 8 equivs, a full protonation of (M,M)-1 is expected
with a formation of doubly protonated species (M,M)-
1·2H+, 2OTf-. The process was followed by UV/Vis, ECD, and
emission measurements, and the corresponding responses are
presented in Figure 8. As can be seen, the UV/Vis spectrum of
the protonated species (M,M)-1·2H+, 2OTf- significantly differs
from that of the non-protonated system (M,M)-1 with 1) a slight
increase in intensity of the band centered at 275 nm
accompanied by its 5 nm bathochromic shift, 2) an intensity
decrease of the bands at 325 and 383 nm, and 3) an appearance
of strong broad band at longer wavelengths (420-550 nm, λmax
= 475 nm, ε = 15000 M-1 cm-1, Figure 8a). Similarly, the ECD
spectrum of (M,M)-1 was substantially modified upon
protonation showing a less intense positive band at 275 nm (
= +330 M-1 cm-1), a less intense negative band at 335 nm ( =
-243 M-1 cm-1), and an additional moderate negative band at
460 nm ( = -32 M-1 cm-1) for the (M,M)-1·2H+, 2OTf-
enantiomer (Figure 8b). Significant changes can also be
observed for the luminescence profile of 1 for which, upon
Having established that the ZnII coordination/decoordination
effectively and reversibly interconverts transoid and cisoid
forms of 1, the question arises as to whether 1 can act as a
molecular hinge. Molecular hinges are systems that
demonstrate a high relative amplitude of motion (> 90°)
accompanied by controllable unidirectional ‘open-close’
movements. The first of these requirements can be in general
easily fulfilled for 2,2’-bipyridine-based systems thanks to
efficient metal-ion complexation / decomplexation processes
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adding triflic acid,
a
structureless intense orange-red +2.5  10-3 for (M,M) and (P,P) enantiomer, respectively. Note
fluorescence emission appeared around 600 nm ( = 28%), that very similar glum values were also obtained for protonated
while the structured blue emission at 421 and 446 nm, coming species of helicenic bipyridine system, although introduction of
from the non-protonated system itself (vide supra), almost two helicene moieties onto the bipyridine core leads to much
completely disappeared (Figures 8c and 8d). This clearly more red-shifted CPL signal (compare 643 nm for 1 vs. 590 nm
confirms that under saturation conditions only protonated with glum = 3.2  10-3 (P enantiomer) for helicene-bipyridine).10h
species are present in the solution. Finally, the CPL responses Summarizing, thanks to reversibility of the process (upon
of (M,M)- and (P,P)-1·2H+, 2OTf- were also measured in addition of aliquots of NEt3 the UV/Vis and ECD, emission and
CH2Cl2 at rt (see Figure 6) and showed distinct signals centered CPL activity spectra of non-protonated species were
around 605 nm with glum maxima of the same order of recovered), the presented here bis-helicenic bipyridine system 1
magnitude as found for 1 and 1·Zn(OAc)2, namely -2.0 and can also act as an efficient pH-triggered (chir)optical switch.26
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Figure 7. Energy profiles for the rotation around CC’ bond in bis-helicenic bipyridine 1 (panel a) along with molecular structures corresponding to
their characteristic points (panel b). BP and BP-D3 SV(P) calculations with continuum solvent model for CH2Cl2. Values listed in panel b are
relative energies ΔE (in kcal/mol) and NCC’N’ dihedral angles (in deg) obtained with BP / BP-D3.
40
a)
c)
80000
70000
60000
50000
40000
30000
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10000
0
(M,M)-1
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5
(M,M)-1.2H+, 2OTf-

0
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 / nm
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 / nm
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600
b)
d)
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100
0
M,M-2
M,M-2+acid (2 eq.)

-100
-200
-300
-400
1
1.Zn 2+ 1.2H+
300
400
500
600
 / nm
Figure 8. Evolution of UV/Vis spectrum (panel a), ECD spectrum (panel b), and fluorescence spectrum (panel c) of (M,M)-1 upon addition of
TfOH aliquots (0.2 equiv. until saturation at 8 equivs; C 2.2  10-5 M, CH2Cl2, rt, ex = 350 nm). Panel d: Picture of emissive solutions of 1, its ZnII
complex, and its protonated species irradiated with 365 nm lamp.
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To provide some insight into experimentally observed
singly protonated species are present in the solution, while
above it doubly protonated species clearly predominate.
tuning of photophysical and chiroptical properties in 1 upon
its protonation, (TD)DFT calculations were then performed
for both singly (n = 1) and doubly (n = 2) protonated 1·nH+
species, as they all are likely to be present in the reaction
mixture below saturation limit. As expected,27 the mono-
protonated system preferentially adopts a conformation with
nitrogen atoms within bipyridine core placed in cis
configuration (see structure 1·H+-I in Figure S2.9 in the SI),
while for di-protonated species transoid arrangement is
favoured (1·2H+-I and 1·2H+-II). Although the
corresponding TDDFT calculations provide less satisfactory
(than in the case of the neutral species) agreement with the
experimental spectral envelopes for 1·2H+, 2OTf-, as a low-
energy band computed for 1·2H+ appears too red-shifted,
which is likely due to solvent and counter-ion(s) effects,10h,28
the most important changes in photophysical and chiroptical
properties of 1 observed experimentally upon its protonation
are reproduced correctly. In particular, in line with the
experiment, the computations for the protonated species
demonstrate i) appearance of additional absorption and ECD
intensity above 420 nm, ii) a significantly reduced intensity
of the ECD band around 330 nm, and iii) a large red-shift (>
100 nm) of the emission signal. All these features can be
linked to a strong impact of protonation on the -electronic
system of 1 that manifests itself through a clear spatial
separation of the frontier MOs of 1·nH+, with HOMO
predominantly localized on the central and terminal parts of
helicenic moieties and LUMO limited to the bipyridine core
and the adjacent rings, accompanied by the strong
stabilization of their energy levels (see Figure 3 and SI),
especially pronounced for LUMO, resulting in a significant
reduction of the HOMO-LUMO gap. Accordingly,
photophysical and chiroptical properties of 1 upon its
protonation appears to be dominated by low-energy CT-type
transitions rather than classical helicene-centered -* ones
(see SI).10h For example, the new lowest-energy UV/Vis and
ECD bands in 1·2H+ correspond to a clear CT electronic
transition between helicene -system and bipyridine core of
the predominant HOMO-LUMO composition that further
amplifies its CT character. Decreased (increased)
contributions from helicene-centered -* (CT-type)
transitions to the excitations calculated in the medium-
energy part of the ECD spectra are also responsible for a
decrease in intensity of the ECD band around 330 nm.
Similarly, very pronounced red-shift of the fluorescence
signal for 1 observed upon protonation can be traced back to
the predominant CT character of the corresponding S1
excited state and the stabilization of the LUMO level. Note
also that the calculations enable to shed some light on
evolution of the emission spectra of 1 upon adding aliquots
of triflic acid that involves i) a gradual diminishing of the
signal centered at 420 and 440 nm accompanied by a
gradual rise of emission intensity around 520 nm observed
before reaching saturation, and ii) disappearance of the
signal centered at 420 and 440 nm and appearance of
separate intense band around 600 nm seen after saturation
conditions are met. Based on the computed S1-S0
fluorescence for the neutral system 1 (409 nm, see SI) and
for its protonated species 1·nH+ (495 nm for n = 1, and 660-
690 nm (depending on the conformer) for n = 2), it appears
therefore that below saturation limit neutral and mainly
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CONCLUSIONS
In conclusion, a novel chiral 2,2'-bipyridine-based system
endowed with two helicenic units has been synthesized in
enantiopure forms from
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a
3-bromo-substituted 4-
displays strong
aza[6]helicene. This compound
photophysical and chiroptical properties including intense
UV/Vis, ECD and emission spectra, and good CPL activity.
The 2,2'-bipyridine chelating ability and basic properties
were studied through the coordination and protonation
processes with Zn(OAc)2 and triflic acid, respectively.
These reversible processes, as shown by quantum-chemical
calculations, activate tuning between -*-type and charge-
transfer-type excitations that leads to efficient changes of
the spectral properties of 1. Accordingly, a new example of
versatile molecular chiroptical switch emerges, behaving
like a chiral dye. Finally, the 1 / 1·2H+, 2OTf switching
system emits a circularly polarized luminescence in the red
region, which may be of interest in the field of bioimaging.
EXPERIMENTAL SECTION
All experiments were performed under an atmosphere of
dry argon using standard Schlenk techniques. Commercially
available reagents were used as received without further
purification. All solvents were dried according to standard
procedures. Preparative separations were carried out by
gravity column chromatography over silica gel (Merck
Geduran 60, 0.063-0.200 mm) in 3.5-20 cm columns.
Analytical thin layer chromatographies (TLC) were
performed using aluminium-coated Merck Kieselgel 60
F254 plates. NMR spectra were recorded on a Bruker 300
Avance (1H: 300 MHz; 13C: 75 MHz), a Bruker Avance 400
or a Bruker Avance 500 (1H: 500 MHz; 13C: 125 MHz)
spectrometers at rt, unless stated otherwise, using partially
deuterated solvents as internal standards. Coupling constants
(J) are given in Hz and chemical shifts () in ppm.
Multiplicities are denoted as follows: s = singlet, d =
doublet, t = triplet, m = multiplet, while b = broad.
Assignment of hydrogen atoms is based on 1H-1H COSY
experiment. Assignment of carbon atoms is based on
HMBC, HMQC and DEPT-135 experiments. Mass
spectrometry was performed by the CRMPO, University of
[훼]20
Rennes 1. Specific rotations (
measured in a 1 dm thermostated quartz cell on a
PerkinElmer Model 341 polarimeter. Molar rotations
are given in deg cm2 dmol-1. Circular dichroism (in M-1 cm-
1) was measured on a Jasco J-815 Circular Dichroism
Spectrometer (IFR140 facility - Biosit - University of
Rennes 1). For details on UV/Vis and non-polarised
luminescence measurements see SI. Photoirradiation
in deg cm2 g-1) were
퐷
[휙]2퐷0
experiments were performed on
immersion Hg lamp (150 W).
a Heraeus TQ718
3-Bromo-4-aza[6]helicene 5. Helicene derivative 5 was
prepared in two steps, i.e. by a Wittig reaction followed by a
photocyclization. 1) First, benzo[g]phenanthren-3-ylmethyl
triphenylphosphonium bromide 2 was prepared according to
the literature.16 A solution of 2 (400 mg, 0.686 mmol) in dry
THF (16 mL) was cooled to -78º C and then n-BuLi (0.34
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mL, 2.2 M in hexanes, 0.748 mmol) was added dropwise.
(M,M)-1: 1H NMR (400 MHz, CD2Cl2)  6.86 (2H, t, J =
The suspension turned orange and was left to stir at -78º C
for 20 minutes, then the reaction mixture was warmed up to
room temperature and kept stirring for 20 minutes more
before cooling again (dark-red color). A solution of 6-
bromo-2-pyridinecarboxaldehyde 3 (116 mg, 0.624 mmol)
in dry THF (5 mL) was added at -78º C. The reaction
mixture was warmed up to room temperature and stirred for
5 hours. The orange suspension was filtered over Celite®
and washed with THF. The solvent was stripped off and the
olefin 4 was obtained as an E and Z isomers (230 mg, 90 %)
which was used as such in the next step. 2) In a flask well-
adapted to the immersion Hg lamp, a solution of the olefinic
precursor 4 (100 mg, 0.243 mmol) in degassed toluene (700
mL) was prepared. Iodine (123 mg, 0.486 mmol) and
propylene oxide (0.85 mL, 12 mmol) were then added. The
solution was strongly stirred overnight for 15 hours under
the 150 W Hg lamp. The crude mixture was evaporated
under vacuum and the product was purified by column
chromatography (SiO2, heptane:Et3N 0.5% for packing, and
the heptane:ethyl acetate 5-15%). The reaction was repeated
five times (500 mg of precursor) to prepare (±)-5 as a
yellow solid (222 mg, 45%).
7.6 Hz, H15), 7.37 (2H, t, J = 7.6 Hz, H14), 7.73 (2H, d, J =
8.5 Hz), 7.98 (2H, d, J = 7.9 Hz), 8.02 (8H, dd, J = 9.0, 3.0
Hz), 8.05-8.08 (7H, m), 8.08-8.16 (3H, m), 8.20 (2H, d, J =
8.9 Hz) ppm.. 13C{1H} NMR (400 MHz, CD2Cl2) : 154.7,
147.7, 136.1, 133.9, 132.8, 132.3, 132.0, 130.9, 130.2,
130.0, 128.9, 128.6, 128.3, 127.8, 127.6, 126.8, 126.4,
126.1, 126.0, 124.5, 117.4 ppm. HR-MS (QTOF), ESI, m/z:
calcd. for C50H28N2Na [M+Na]+ 679.2145 found (pos.)
679.2146. Mp = 170-175 °C.
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1·Zn(OAc)2 complex. The complexation was carried out
by simple solvation of the ligand 1 (5 mg, 7.6 mmol) and
Zn(OAc)2∙2H2O (1.8 mg, 8.4 mmol, 1.1 equivalents) in 5
mL of dichloromethane. One minute of ultrasounds bath
was used for total solvation of the complex. The colour
intensity increased upon complexation, and once evaporated
1
the solvent, only one set of signals was visible in H NMR
spectra.
The same procedure was used to prepare enantiopure
(M,M)- and (P,P)-1·Zn(OAc)2 by starting from (M,M)- or
(P,P)-1, respectively.
(M,M)-1·Zn(OAc)2: 1H NMR (400 MHz, CD2Cl2)  2.07
– 2.00 (6H, s), 6.72 (2H, t, J = 7.6 Hz, H15), 7.10 (2H, t, J =
7.6 Hz, H14), 7.28 (2H, d, J = 9.0 Hz), 7.58 (2H, d, J = 8.5
Hz, H16), 7.79 (2H, d, J = 8.0 Hz, H13), 7.91-8.02 (4H, m),
8.11 (4H, bs), 8.14 (2H, d, J = 8.1 Hz), 8.24 (2H, d, J = 8.1
Hz), 8.34 (2H, d, J = 9.0 Hz) 8.52 (2H, d, J = 9.0 Hz), 8.68
(2H, d, J = 9.0 Hz) ppm. 13C: After experiment overnight in
500 MHz the signals were not large enough to give an
accurate set of peaks. Yellowish solid; Mp = 162-167 °C.
Enantiopure M- and P-5 samples were obtained by HPLC
separation over a chiral stationary phase (see SI).
1H NMR (300 MHz, CD2Cl2) : 6.67 (1H, d, J = 9.0 Hz),
6.81 (1H, bt, J = 7.6 Hz), 7.32 (1H, bt, J = 7.6 Hz), 7.54
(1H, d, J = 8.6 Hz), 7.67 (1H, d, J = 9.0 Hz), 7.89 (1H, bd, J
= 8.0 Hz), 7.96 (2H, bs), 8.00-8.11 (5H, m), 8.22 (1H, d, J =
9.0 Hz) ppm. 13C{1H} NMR (75 MHz, CD2Cl2) δ: 123.8,
124.2, 124.9, 126.0, 126.6, 126.7, 127.65, 127.7, 127.8,
127.8, 128.2, 128.5, 128.6, 128.9, 129.7, 131.7, 131.9,
132.3, 132.8, 134.0, 137.8, 140.8, 148.3 ppm. HR-MS
(QTOF), ESI, m/z: calcd. for C25H15N79Br [M+H]+:
408.0382; found: 408.0383. Mp = 165-169 °C.
Acid-base switching of (M,M)-1. To a solution of (M,M)-1
(2.2  10-5 M in CH2Cl2) were added aliquots of a solution
of CF3SO3H (1.14  10-3 M in CH2Cl2) until saturation of
the UV/Vis and ECD signals (5 L added). Upon addition
of aliquots of NEt3 (1.4  10-3 M in CH2Cl2), the UV/Vis
and ECD spectra of (M,M)-1 were recovered.
3,3'-Bis-4-aza[6]helicene 1. Helicene derivative 1 was
synthesized by means of a Negishi coupling of the
enantiopure 3-bromo-4-aza[6]helicene 5. To a solution of n-
BuLi (100 L, 0.168 mmol) in dry THF (0.4 mL) at -78º C,
a solution of 3-bromo-4-aza[6]helicene 5 (36 mg, 0.088
mmol) in dry THF (0,4 mL) was added dropwise. A color of
reaction mixture changed from colorless to wine red at the
end of the addition. The solution was kept stirring for 30
minutes at -78º C and then ZnCl2 (225 L, 1.0 M in diethyl
ether) was added. The mixture changed from a red-wine
solution to a dark-yellow suspension. The temperature was
warmed up to 0º C for 1 hour and then to room temperature
for 2 hours more. The reaction mixture was cooled again to
0º C and the 3-bromo-4-aza[6]helicene 5 (33 mg, 0.080
mmol) and catalyst Pd(PPh3)4 (3 mg) were then added in
solution in dry THF (0.3 mL). After 1 hour at 0º C, the
reaction mixture was warmed up to reflux. After stirring and
refluxing for 15 hours, a bright-yellow precipitate was
obtained. Dichloromethane (15 mL) and TPEN (36 mg,
0.0849 mmol) were then added and the mixture sonicated
for 5 minutes. Final purification by column chromatography
(SiO2, heptane:ethyl acetate 15%) yielded 1 (30 mg, 57%)
as a bright-yellow solid. Note that around 10% of 4-
aza[6]helicene was obtained as a side product.
ASSOCIATED CONTENT
Supporting Information
1H, 13C, and COSY spectra of compounds 5, (M,M)-1, and
(M,M)-1·Zn(OAc)2. X-ray diffraction data of 1 and 5. HPLC
conditions for 1 and 5. Photophysical and chiroptical
properties of (M,M)-1, (M,M)-1·Zn(OAc)2, and (M,M)-
1·2H+, 2OTf-. Computational details. Additional calculated
results. This material is available free of charge on the ACS
Publications website at DOI:
AUTHOR INFORMATION
Corresponding Authors
* jeanne.crassous@univ-rennes1.fr
* srebro@chemia.uj.edu.pl
ORCID
Jeanne Crassous: 0000-0002-4037-6067X
Monika Srebro-Hooper: 0000-0003-4211-325X
The same procedure was used to prepare enantiopure
(M,M)- and (P,P)-1 by starting from M- or P-5, respectively.
The meso-1 compound was not characterized due to low
solubility and small mass quantity available.
Notes
The authors declare no competing financial interest.
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vinylhelicenes: chiroptical properties and redox switching. Chem. Eur. J.
ACKNOWLEDGMENTS
2015, 21, 17100-17115; c) Shen, C.; Loas, G.; Srebro-Hooper, M.;
Vanthuyne, N.; Toupet, L.; Cador, O.; Paul, F.; López Navarrete, J. T.;
Ramírez, F. J.; Nieto-Ortega, B.; Casado, J.; Autschbach, J.; Vallet, M.;
Crassous, J. Iron-alkynyl-helicenes: redox-triggered chiroptical tuning in the
vibrational and telecommunication domain. Angew. Chem. Int. Ed. 2016, 55,
8062-8066; d) Saleh, N.; Vanthuyne, N.; Bonvoisin, J.; Autschbach, J.;
Srebro-Hooper, M.; Crassous, J. Redox-triggered chiroptical switching
We acknowledge the Ministère de l’Education Nationale,
de la Recherche et de la Technologie, the Centre National de
la Recherche Scientifique (CNRS), the CNRS (Chirafun
network), and the ANR (12-BS07-0004-METALHEL-01),
for financial support. H. I. thanks the Région Bretagne for
financial support. K. D. thanks The University of Gabès, the
University of Rennes 1 and Campus France for financial
support. M.S.-H. acknowledges the young researchers’ T-
subsidy from the Ministry of Science and Higher Education
in Poland.
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activity of ruthenium(III)-bis-(-diketonato) complexes bearing
a
bipyridine-helicene ligand. Chirality 2018, 30, 592-601; e) Shen, C.; Srebro-
Hooper, M.; Weymuth, T.; Krausbeck, F.; López Navarrete, J. T.; Ramírez,
F. J.; Nieto-Ortega, B.; Casado, J.; Reiher, M.; Autschbach, J.; Crassous, J.
Redox-active Chiroptical Switching in Mono- and Bis-Iron-Ethynyl-
Carbo[6]Helicenes Studied by Electronic and Vibrational Circular
Dichroism and Resonance Raman Optical Activity. Chem. Eur. J. 2018, 24,
15067-15079; f) Shen, C.; He, X.; Toupet, L.; Norel, L.; Rigaut, S.;
Crassous, J. Dual redox and optical control of chiroptical activity in
photochromic dithienylethenes decorated with hexahelicene and bis-ethynyl-
ruthenium units. Organometallics 2018, 37, 697-705; g) Anger, E.; Srebro,
M.; Vanthuyne, N.; Roussel, C.; Toupet, L.; Autschbach, J.; Réau, R.;
Crassous, J. Helicene-grafted vinyl- and carbene-osmium complexes: an
example of acid-base chiroptical switching. Chem. Comm. 2014, 50, 2854-
2856; h) Saleh, N.; Moore, II, B.; Srebro, M.; Vanthuyne, N.; Toupet, L.;
Williams, J. A. G.; Roussel, C.; Deol, K. K.; Muller, G.; Autschbach, J.;
Crassous, J. Acid-base triggered switching of circularly polarized
luminescence and electronic circular dichroism in organic and
organometallic helicenes. Chem. Eur. J. 2015, 21, 1673-1681.
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