Circularly Polarized Fluorescent Helicene-Boranils: Synthesis, Photophysical and Chiroptical Properties

Article abstract of DOI:10.1002/chem.202100356

Mono- and di-boranil-substituted helicenes were prepared by BF₂-borylation of the corresponding anils, readily synthesized by condensation of 2-amino- and 2,15-diamino-helicenes with 4-(diethylamino)salicylaldehyde. After enantiomeric resolutio

Full text of DOI:10.1002/chem.202100356

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Circularly Polarized Fluorescent Helicene-Boranils:
Synthesis, Photophysical and Chiroptical Properties
Aurélie Macé,[a] Khaoula Hamrouni,[a, b] Etienne S. Gauthier,[a] Marion Jean,[c]
Nicolas Vanthuyne,[c] Lucas Frédéric,[d] Grégory Pieters,[d] Elsa Caytan,[a] Thierry Roisnel,[a]
Faouzi Aloui,[b] Monika Srebro-Hooper,*[e] Bertrand Carboni,[a] Fabienne Berrée,*[a] and
Jeanne Crassous*[a]
In memory of François Diederich
Abstract: Mono- and di-boranil-substituted helicenes were
prepared by BF2-borylation of the corresponding anils, readily
synthesized by condensation of 2-amino- and 2,15-diamino-
helicenes with 4-(diethylamino)salicylaldehyde. After enantio-
meric resolution using HPLC, their chiroptical properties
including circularly polarized fluorescence in solution and in
PMMA films were investigated and rationalized with the help
of NMR, X-ray and quantum-chemical calculations.
Introduction
tions thanks to their tunable photophysical properties along
with easy processing and integration into optoelectronic
devices.[3] In comparison, chiral lanthanide complexes are
known to show glum values that can reach more than 1 thanks
to their f!f electronic transitions, formally forbidden by Laporte
selection rules, which lead to very small IL +IR.[4] However,
emission quantum yields reported for such compounds are
usually low.
Helicenes are chiral, screw-like shaped, polyaromatic hydro-
carbons (PAHs) displaying strong chiroptical activity.[1] Recently,
a particular focus has been put on their circularly polarized
emission (CPL)[2] for future potential applications in cryptog-
raphy, circularly polarized organic light-emitting diodes (CP-
OLEDs) technology, bioimaging or photocatalysis.[3] At the
molecular level, chiral organic molecules display CPL activity
with luminescence dissymmetry factors (glum =2(ILÀ IR)/(IL +IR)) of
10À 4–10À 2, due to underlying electric dipole-allowed electronic
transitions, resulting in not necessarily small ILÀ IR, but quite
large IL +IR, thus diminishing glum values. Despite this, such
systems have emerged as valuable candidates for CPL applica-
Among all the CPL-active organic small molecules, helicenes
have been shown to display rather strong CPL activity with glum
values as high as 10À 2.[5] Recently, the possibility to incorporate
heteroatoms (Si, S, B, N, P) into helicene frameworks using
various synthetic strategies has attracted a special attention
with the aim of creating/expanding structural diversity and
tuning the photophysical properties of such systems.[6] In this
regard, incorporating boron atoms into helicenes can be
considered as a valuable strategy to generate novel chiral
materials with tailored properties. For instance, due to the
electron-accepting and Lewis acidic characters of boron,
introducing one or several B atoms into a helicenic scaffold
generally results in strongly blue-emitting chiral fluorophores,
such as in CPL-active azaborahelicenes H1#[7] and H2#[8] (Fig-
ure 1). The three- or four-coordinate nature of the boron atom
enables to construct not only monohelicenic, but also multi-
helicenic units, such as oxabora-bishelicenic system H3#.[9] BF2
fragments can also be introduced within azahelicene units and
yield efficient CPL-active orange or red emitters, such as H4#[10a]
or H5#,[11] thus showing that the luminescence can be readily
tuned by choosing appropriate boron-doped chromophore. The
CPL emission can be further modulated by post-functionalizing
the same chiral system with, for instance, a chiral binaphthol
substituent on the boron atom in place of the two fluorines.[10b]
Finally, instead of incorporating aza- or oxa-boracycles into the
fused helicenic scaffold, boron-based fragments can be grafted
onto a helical core, as shown in the configurationally stable
[a] Dr. A. Macé, K. Hamrouni, Dr. E. S. Gauthier, Dr. E. Caytan, Dr. T. Roisnel,
Dr. B. Carboni, Dr. F. Berrée, Dr. J. Crassous
Univ Rennes, CNRS, ISCR-UMR 6226
Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex (France)
E-mail: fabienne.berree@univ-rennes1.fr
jeanne.crassous@univ-rennes1.fr
[b] K. Hamrouni, Dr. F. Aloui
University of Monastir, Faculty of Sciences, Laboratory of Asymmetric
Synthesis and Molecular Engineering of Materials for Organic Electronics
(LR18ES19), Avenue of Environment 5019 Monastir (Tunisia)
[c] M. Jean, Dr. N. Vanthuyne
Aix Marseille University, CNRS, Centrale Marseille, iSm2, Marseille (France)
[d] Dr. L. Frédéric, Dr. G. Pieters
Université Paris-Saclay, CEA, Département Médicaments et Technologies
pour la Santé (DMTS), SCBM, Gif-sur-Yvette, 91191 (France)
[e] Dr. M. Srebro-Hooper
Faculty of Chemistry, Jagiellonian University
Gronostajowa 2, 30-387 Krakow (Poland)
E-mail: srebro@chemia.uj.edu.pl
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100356
This article belongs to a Joint Special Collection dedicated to François
Diederich.
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and circularly polarized fluorescence, were examined and
rationalized with the help of first-principles calculations.
Results and Discussion
Synthesis and structural characterization of helicene-boranils
H1-H3
Helicene-boranils H1-H2 were first prepared following the
general synthetic route depicted in Scheme 1. Namely, heli-
cene-boronate derivatives 1a–b were first converted into
helicene-amines 2a–b,[18] which, in turn, gave imines 3a–b by
treatment with 4-(diethylamino)salicylaldehyde. A final com-
plexation with BF3·OEt2 in the presence of N,N-diisopropylethyl-
amine (DIEA), in refluxing 1,2-dichloroethane (1,2-DCE) for 24–
42 hours yielded the targeted compounds.
Figure 1. Chemical structures of known boron-based helicene derivatives.
pentahelicenic structure H6#, which is functionalized with a
BMes2 moiety (Mes=mesityl).[12] In this system, the presence of
electron-donating NMe2 unit allowed to tune the emission to
more red-shifted wavelengths thanks to a strong charge trans-
fer. Furthermore, the vacant pz orbital of boron enabled to use
H6# as a fluoride anion sensor with CPL signal as a read-out
response. Note that along with the boron atom, N or O atoms
are often introduced via BÀ N or BÀ O bonds. Indeed, the
interaction between the N/O lone pair and the unoccupied pz B
orbital enhances the chemical stability of such molecules.
Otherwise, sterically hindered substituents, such as mesityls in
H6#, need to be introduced to protect the boron atom.
Following the same approach, [6]helicene-2,15-bis-pinacol-
boronate 1c was first prepared according to Scheme 2, i.e. in
Boranils, another class of boron heterocycles derived from
salicylaldanilinimines, possess an NÀ BÀ O connectivity and are
particularly promising species due to their attractive optical
features.[13] Following the pioneering work of Hohaus et al. in
1973,[14] Ulrich, Ziessel et al. reported in 2011 the facile two-
steps preparation of boranils bearing donor/acceptor substitu-
ents, showing higher stability compared to their unsubstituted
congeners and good luminescence properties in solution and
solid states.[15] In addition, such compounds have the major
advantage of being easily synthesized, even on a large-scale,
from a variety of anilines or amines, including chiral ones.
Although a few chiral boranils[16] or related BF2 compounds[17]
with salicylaldehyde-based Schiff-base NbO-containing ligands
have been addressed so far, only few of them are CPL-active.
Herein, we report the preparation and detailed character-
ization of novel boron-containing helicenes H1-H3, which
combine carbo[4]- or carbo[6]-helicene moieties and one or two
boranil units (Figure 2). In particular, the chiroptical properties
of enantiopures H2 and H3, together with their nonpolarized
Scheme 1. Synthesis of helicene-boranils H1-H2 from helicene-boronates
1a–1b. i) NaN3, CuSO4·5H2O, MeOH, reflux, 19 hrs, then H2, 10% Pd/C, EtOAc
rt, 24 hrs, 2a: 65%, 2b: 75%; ii) 4-(diethylamino)salicylaldehyde, MeOH,
°
reflux, 3a: 18 hrs, 91%, 3b: 31 hrs, 93%; iii) BF3·OEt2, DIEA, 1,2-DCE, 85 C, H1:
42 hrs, 75%, (rac)-H2: 24 hrs, 75%.
Scheme 2. Synthesis of racemic boranil H3. i) n-BuLi, THF, Ar, rt, 3 hrs, 58%;
ii) hν, I2, propylene oxide, toluene, Ar, 18 hrs, 57%; iii) NaN3, CuSO4·5H2O,
°
MeOH, 40 C, 27 hrs, then H2, 10% Pd/C, EtOAc rt, 24 hrs, 67%; iv) 4-
(diethylamino)salicylaldehyde, MeOH, reflux, 22 hrs, 98%, then BF3·OEt2,
Figure 2. Chemical structures of new helicene-boranils presented in this
work.
°
DIEA, 1,2-DCE, 85 C, 23 hrs, 91%.
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two steps involving a Wittig reaction between 4-formylphenyl-
boronic acid pinacol ester 4 and (2,7-naphthalenedimethylene)-
bis[triphenylphosphonium-bromide][19a] 5 (58% yield based on
recovered phosphonium salt, see Supporting Information),
followed by an oxidative photocyclization reaction. Note that
1c has also been recently synthesized using another strategy
from 2,15-bis-bromo-carbo[6]helicene.[19b] The helicenic aniline
2c was then prepared in a 67% overall yield by copper-
catalyzed azidation using copper sulfate in refluxing methanol
and hydrogenation with H2 in the presence of Pd/C. Refluxing
2c with 4-(diethylamino)salicylaldehyde in methanol for
22 hours afforded the anil, which was converted to H3 (91%
yield) as previously described for 3a–b.
The resulting novel helicenic boranils H1-H3 were then fully
characterized by NMR and mass spectrometry (see Supporting
Information). While the 1H and 13C NMR spectra display the
typical, characteristic signals coming from both the helicene
and boranil units, these compounds show very informative 11B
NMR and 19F NMR (see Table 1 and Supporting Information).
Indeed, H1 displays two chemically and magnetically equivalent
fluorine atoms that resonate at À 136.5 ppm and demonstrate
similar BÀ F coupling constants (1JB–F =17.3 Hz). The 11B NMR
spectra show a triplet at δ=1.1 ppm in agreement with a four-
coordinate boron atom. By contrast, chiral H2 displays two
distinct 19F NMR signals that are doublets of 1:1:1:1 quad-
ruplets, at À 129.7 (dq, 2JF–F =90 Hz, 1JB–F =23 Hz) and
À 143.4 ppm (2JF–F =90 Hz, 1JB–F =11.1 Hz). The corresponding
11B NMR signal demonstrates a doublet of doublets (dd) at
0.6 ppm with different coupling constants with the two fluorine
atoms. Carbo[6]helicene-2,15-bis-boranil H3 displays very sim-
ilar features, i.e. dq signals for each fluorine and dd for the
boron, with the two BF2 groups being equivalent indicating C2-
symmetry of the molecule.[20]
Enantiomeric resolution of (rac)-H2 and (rac)-H3 was
performed using HPLC over chiral stationary phases, yielding
(P)-(+) and (M)-(À ) enantiomers of H2 and H3 with ee values
higher than 99% (see Supporting Information). Single crystals
of (+)-H2 were grown by slow diffusion of pentane vapors into
a CH2Cl2 solution and the structure was further ascertained by
X-ray diffraction crystallography. Compound (+)-H2 crystallized
in the non-centro-symmetric P212121 space group (with the
presence of P enantiomers only, see Figures 3a,b). The molec-
ular structure shows a C1 symmetry with a helicity (dihedral
°
angle between terminal rings of the helicene) of 52.34 , a
typical value for a carbo[6]helicene.[1] The OÀ B and the BÀ N1
bond-lengths are 1.445 and 1.582 Å, respectively, which are in
the range of similar 4-coordinate boron(III) derivatives. Interest-
ingly, the helicenic and the boranil parts are linked through a
CÀ N single bond (with a length of 1.433 Å), which displays axial
Table 1. 11B and 19F NMR data recorded in CDCl3 at rt for compounds H1-
H3. See also Supporting Information.
11B
19F
δ (ppm)
(multiplicity)
JB–F (Hz)
δ (ppm)
(multiplicity)
JF–F
(Hz)
Compound
H1
H2
1.1 (t)
0.6 (dd)
17.3
23.0 and
11.1
22.7 and
10.3
À 136.5 (q)
–
90
°
chirality due to a BÀ N1À CÀ C’ dihedral angle of À 50.51 , thus
À 129.7 (dq) and
À 143.4 (dq)
representing the (Ra) configuration associated with the (P)
stereochemistry of the carbo[6]helicene fragment. There is
therefore an interesting chiral induction from the helicenic part
H3
0.5 (dd)
À 129.7 (dq) and
À 142.7 (dq)
89.8
Figure 3. a) X-ray diffraction structure of (P)-(+)-H2 with (Ra) axial chirality highlighted along with b) its supramolecular arrangement in the crystal. c) Selected
optimized (BP/SV(P) with the continuum solvent model for CH2Cl2) structures of H2 (left) and H3 (right). Compare with Table 2 and see Supporting Information
for a full set of data. Values listed are the corresponding relative energies ΔE (in kcal/mol) calculated with BP-D3/TZVP and the BÀ N1À CÀ C’ dihedral angles
(provided in parentheses). Deposition number 2047289 contains the supplementary crystallographic data for this paper. These data are provided free of
charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
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to the axial chirality in the solid state. This can be explained by
the position of the more sterically hindered BF2 group away
from the helix and the existence of a CH-π interaction between
the proton of the CH=N imine and the external terminal ring of
the carbo[6]helicene (Figure 3a, H-centroid distance: 2.771 Å)
that ‘locks’ the structure into a closed form in the crystal. Finally,
intermolecular π-π stacking interactions (Figure 3b) are ob-
served in the molecular packing of homochiral dimers. Note
that the molecular geometry of H2 observed in its crystal
structure may not necessarily be dominant in solution, as a
careful analysis of the NOESY spectrum of this compound
showed that the proton H20 (of the CH=N imine) correlates with
H13 and H14 (other extremity of the helicene) and with both H1
and H3 (see Figures S1.16a–b), thus suggesting the fast rotation
around the C2À N (CÀ N1 in Figure 3a) bound (in the NMR
timescale). Similar conclusion holds also for H3, for which the
analogous correlation of protons in the NOESY spectrum is seen
(H12 with H1, H3 and H4, see Figures S1.37a–b).
Stereochemical and structural preferences for helicene-
boranils H1, H2, and H3 in solution were also examined with
quantum-chemical calculations. DFT geometry optimizations
(BP/SV(P) with the continuum solvent model for CH2Cl2,
followed by BP-D3/TZVP electronic energy evaluation; for a
complete description of the computational details, see Support-
ing Information) considered different possible diastereoisomers
(based on (P) helical stereochemistry associated with either (Ra)
or (Sa) axial conformational chirality) with different rotamers for
the relative orientation of the boranil fragment(s) and the
helicene. The results (see Figure 3c and Table 2, and Figur-
es S2.2-S2.3 and Table S2.1 in SI) show profound energetic
preferences for the H2 and H3 structures demonstrating a
combination of the (P) and (Ra) configurations stabilized by the
intramolecular CH-π boranil-helicene interaction, labelled in the
following as (P,Ra)-H2-I and (P,Ra,Ra)-H3-I. Coexistence of other
diastereoisomeric structures in solution mixture, such as for
example (P,Sa)-H2-II or (of mixed axial chirality) (P,Ra,Sa)-H3-II, is
however quite likely as their energy values were found to be
relatively close to those computed for the corresponding
structures I. An energetic barrier for a rotation around the
boranil-helicene bond is expected to be low, resulting in the
overall high molecular flexibility of the systems. All this correctly
reflects the results of the experimental (X-ray diffraction
crystallography and NMR) studies presented above. For the
[4]helicene-boranil H1 a roughly equal mixture of possible
diastereoisomers is expected based on the calculated data (see
Supporting Information).
Photophysical properties: UV-vis spectroscopy
The UV-vis spectra of the [n]helicene-boranils H1-H3 were
measured in CH2Cl2 solution at concentrations ca. 10À 5 M and
they are presented in Figure 4a. The [4]helicene-boranil H1
system displays two main wide and strong bands comprising: i)
two signals at 270 and 284 nm (ɛ~30000; please note that all ɛ
in this work are given in units of MÀ 1 cmÀ 1) accompanied with
broad structuration and ii) an even stronger signal at 415 nm
(ɛ~38000). Reflecting an increase in the π-electron system, the
[6]helicene-mono-boranil H2 and bis-boranil H3 exhibit stron-
ger UV-vis responses as compared to H1, with three main sets
of bands that include: i) a set of two signals at 246 and 255 nm
(ɛ~53000) for H2, which are weaker in H3 (243–255 nm, ɛ~
33000), ii) a set of three signals at 305, 333 and 351 nm (ɛ=
22000–29100) for H2, which appear at 305, 343 and 356 nm
(ɛ=19000–27500) for H3, and iii) a strong signal at 409 nm (ɛ=
50000) with shoulders at 390 and 423 nm for H2 that
corresponds to a strongly enhanced one at 412 nm (ɛ=70000)
with shoulders at 393 and 433 nm for H3.
To shed some light on the electronic origin of these bands,
time-dependent DFT (TD-DFT) calculations were then per-
formed. See Supporting Information for all computational
details and a full set of computed data. The simulated UV-vis
spectra of H1, H2, and H3 do not differ much for different
diastereomeric structures and visibly fail to reproduce exper-
imental relative intensities of the lowest-energy band for H1 vs.
H2-H3 (overestimating the intensity for H1) and of the two
bands around 330 nm and 410 nm for H2 and H3 (over-
estimating the intensity for the higher-energy – shorter-wave-
length – band); see the discussion below. However, other
important spectral features observed in the experiments (such
as relative and absolute energetic positions of the bands along
with a substantial increase in the low-energy absorption
intensity when going from H2 to H3) appear to be overall well-
described by theory (PBE0/SV(P) with a continuum solvent
model for CH2Cl2, see Figure S2.7), enabling assignment of the
particular bands. Namely, analysis of molecular orbital (MO) pair
contributions to selected excitations for H1, H2, and H3 shows
that the high-energy intense band of these systems observed
experimentally around 275 nm for H1 and 250 nm for H2 and
H3 originates from predominatly helicene-centered π-π* ex-
citations with contributions from helicene!boranil and bor-
anil!helicene charge-transfer (CT) transitions. The UV-vis
intensity around 330 nm can be, on the other hand, assigned to
a combination of mainly boranil!helicene and helicene!
boranil CTs mixed with π–π* transitions within helicene frag-
ment. Such assignments account for strong enhancement in
absorption observed experimentally in this spectral range for
Table 2. Summary of computed (BP/SV(P) with the continuum solvent
model for CH2Cl2) rotamer structures for helicene-boranils H2 and H3:
°
BÀ N1À CÀ C’ dihedral angles as defined in Figure 3 (ff, in ) along with
relative energies (ΔE, in kcal/mol) calculated with BP-D3/TZVP and the
°
corresponding Boltzmann populations at 25 C (nB, in %). For structures
visualization, see Figure 3 and Supporting Information.
Structure
ff
ΔE
nB
P-H2
(P,Ra)-I
(P,Sa)-II
(P,Ra)-III
(P,Sa)-IV
(P,Ra,Ra)-I
(P,Ra,Sa)-II
(P,Sa,Ra)-III
(P,Ra,Ra)-IV
(P,Ra,Ra)-V
(P,Sa,Sa)-VI
(P,Sa,Sa)-VII
À 38.64
0.00
1.92
2.40
3.94
0.00
2.13
2.20
2.28
4.83
5.26
7.14
94.6
3.7
1.6
0.1
93.2
2.6
2.2
2.0
0.0
0.0
0.0
40.84
À 136.69
136.75
P-H3
À 39.34/À 39.34
À 41.83/41.23
42.70/À 131.51
À 38.01/À 136.01
À 136.88/À 136.88
39.59/39.59
138.20/142.38
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Figure 4. Experimental a) UV-vis of H1-H3 and b) ECD spectra of (P) and (M) enantiomers of H2 and H3 (CH2Cl2, C~10À 5 M). c) Isosurfaces (�0.04 au) of
frontier MOs of the lowest-energy computed H2 and H3 structures. d) Simulated ECD spectra for representative (P) diastereoisomers of H2 and H3 with
selected calculated excitation energies and rotatory strengths (for structures I) indicated as “stick” spectra. See Supporting Information for a full set of data.
the more π-extended [6]helicene-based H2 and H3 compared
to the [4]helicene derivative H1. Finally, the computations
indicate that the lowest-energy band of all three systems,
centered around 410 nm, is due to excitations involving highest
occupied (HO) and lowest unoccupied (LU) MOs (HOMO/LUMO
for H1, H2; HOMO-1, HOMO/LUMO, LUMO+1 for H3; see
Figure 4c and Supporting Information) and accordingly corre-
sponding to π-π* transitions within the boranil fragment(s) π-
conjugated with the adjacent helicene rings. As the orbitals are
centered in slightly different parts of the π-electron system, the
excitations demonstrate also some CT signature.
Note that HOMO-1, HOMO, LUMO, and LUMO+1 of H3
have electronic density extended over both boranil substituents
and partially also over the helicene, but in-phase and out-of-
phase linear combinations may be taken where electronic
density localizes on one boranil (and adjacent helicene rings)
fragment. While in the case of H1 and H2, one intense
Chiroptical properties: Electronic circular dichroism, optical
rotation
The electronic circular dichroism (ECD) spectra and optical
rotation (OR) values of the [6]helicenic derivatives H2 and H3
were also examined (Figure 4, Table S1.1, Supporting Informa-
tion; please note that all Δɛ in this work are given in units of
MÀ 1 cmÀ 1, while specific rotations are given in degree
[dmgcmÀ 3]À 1). Both systems display strong specific rotation
values (½a�2D5 = +3900 and +3550 for (P)-H2 and (P)-H3,
respectively) that are similar to other carbo[6]helicenes.[1] As
depicted in Figure 4b, the (M) and (P) enantiomers display the
expected mirror-image ECD spectra. The three sets of bands
observed in UV-vis spectra are ECD-active in both H2 and H3.
For instance, (P)-H2 displays: i) a strong negative band at
245 nm (Δɛ=À 244) accompanied with two weaker ones at 265
(À 79) and 290 nm (À 29), ii) two positive bands of moderate
intensity at 319 (+57) and 329 nm (+51), and iii) two strong
positive bands at 407 (+98) and 417 nm (+103). Overall the
ECD spectral envelope of this [6]helicene-based system is
strikingly different from those measured for the classical
helicenic compounds such as nonsubstituted carbo[n]
helicenes.[1] This is also the case for the helicene-bis-boranil H3,
whose (P) enantiomer exhibits overall the same characteristic
bands in its ECD spectrum as (P)-H2, but with different relative
intensities or signs and a clear red-shift of the lower-energy
bands, i.e. i) a weaker negative band at 244 (À 143) accom-
panied with other ones at 277 (À 48), 284 (À 41), and 307 nm
excitation contributes to this band (no.
1 calculated at
respectively ca. 400 nm and 410 nm), for H3 an intense pair of
excitations was computed in this spectral region (no. 1 at ca.
420 nm and no. 2 at ca. 400 nm) that may indicate exciton
coupling between the two boranil-helicene π–π* transitions,
similar to what we previously showed for push-pull helicenic
diketopyrrolopyrrole systems.[21a] This can rationalize a red-shift
of the lowest-energy absorption band and an increase in its
intensity observed experimentally for H3 as compared to H2.
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(À 11), ii) three negative bands of moderate intensity at 343
(À 44), 355 (À 39) and 389 nm (À 35), and iii) one strong positive
band at 435 nm (+197).
electronic π-conjugation between both fragments that is
expected to affect intensity of excitations within these
systems.[25] This is indeed supported by a substantial modifica-
tion of the ECD intensity found for some BP-D3-optimized
structures in which the boranil moiety strongly (and likely
exaggeratedly) bent towards terminal rings of the helicene (see
Supporting Information). Finally, the ECD signal around 325 nm
appears to be due to several excitations of sizable calculated
rotatory strengths (R) differing in sign that demonstrate mainly
a mixed helicene-/boranil-centered π-π* and helicene!boranil/
boranil!helicene CT character (e.g. excitations nos. 9 (+), 11
(+), and 12 (À ) calculated at ca. 328, 314, and 307 nm,
respectively); magnitudes of R for these excitations are clearly
dependent on a molecular structure leading to a cancellation of
their ECD intensity in the simulated spectrum for (P,Ra)-H2-I or
an appearance of a positive intensity in the case of, for example,
(P,Sa)-H2-II.
As can be seen in Figure 4, the simulated ECD spectrum for
the calculated lowest-energy structure (P,Ra,Ra)-H3-I (see Fig-
ure 3c and Table 2), agrees quite well with the experimental
data. In particular, it correctly reproduces a red-shift and
increase in intensity of the lowest-energy positive band along
with appearance of more negative intensity between ca. 400
and 280 nm that were observed in the experimental ECD
spectra of (P)-H3 vs. (P)-H2. Similarly to the mono-boranil H2, a
visible dependence of the bands intensity in the simulated ECD
spectra on the molecular structure of H3 was also noted, with
(Sa) axial chirality giving its clear signature in form of a positive
band centered around 325 nm (see Figure S2.5 and Figure 4d).
Since in the experiment a weakly intense positive band was
indeed observed in this spectral region, coexistence of various
structures of both (Ra) and (Sa) configurations in solution might
also be postulated for H3. The assignment of the H3 ECD
spectrum is qualitatively similar to that of H2. Namely,
predominantly boranil- and helicene-centered π–π* transitions
were found to be responsible for respectively lowest-energy
positive (ca. 430 nm) and highest-energy negative (ca. 240 nm)
bands present in the experimental spectrum (see excitations
nos. 1 and 46 calculated at ca. 420 nm and 245 nm), while the
negative ECD intensity appearing between ca. 400 and 280 nm
origin from helicene!boranil and boranil!helicene CT excita-
tions mixed with π-π* transitions within boranils and helicene
fragments (see e.g. excitations nos. 5, 8, 9, and 14 calculated
between around 380 and 330 nm). These latter excitations,
demonstrating sizable negative rotatory strength values, en-
hanced compared to H2 reflecting the presence of the second
boranil chromophore, lead to a very intense negative band in
the simulated spectrum that is strongly overestimated com-
pared with the experimental one likely due to similar reasons as
for H2 (vide supra). As in the case of H2, a sign of ECD intensity
around 325 nm for H3 appears to stem from a cancellation
((P,Ra,Ra)-H3-I) or enhancement ((P,Ra,Sa)-H3-II and (P,Sa,Sa)-H3-
VI) of intensity of the underlying excitations representing a
combination of mainly boranil- and helicene-centered π–π*
transitions.
The TD-DFT-simulated[22] (PBE0/SV(P) with a continuum
solvent model for CH2Cl2) ECD spectra for selected H2 and H3
diastereomeric structures are presented in Figure 4d; see also
Supporting Information for additional calculated data. ECD
spectral envelope obtained for the energetically most preferred
conformer (P,Ra)-H2-I, as indicated by DFT, (see Figure 3c and
Table 2), resembles the experimental ones but with “too
negative” intensity between ca. 375 and 300 nm. The corre-
sponding (P,Sa)-H2-II structure (of relatively low energy) demon-
strates very similar ECD spectrum but with a decreased intensity
of the first low-energy positive band and, more importantly,
with appearance of a positive band of moderate intensity
around 325 nm that matches well the second positive band
observed for this compound in the experiment. It is worth
mentioning that the spectral data obtained for other optimized
H2 geometries clearly confirm a high sensitivity of the ECD
intensity in the low- and medium-energy range to the rotamer
structure and particularly to its axial chirality (see Figure S2.4
and Figure 4d), with the (P,Sa) diastereoisomers emerging as
predominantly responsible for the positive ECD signal measured
at ca. 325 nm. This further supports the coexistence of various
H2 structures of both (Ra) and (Sa) axial chirality in solution
mixture also suggested based on the NMR studies (vide supra).
An analysis of the dominant excitations of the simulated spectra
of (P,Ra)-H2-I and (P,Sa)-H2-II conformers assigns the intense
negative ECD band centered at ca. 250 nm to excitations
involving predominantly, as expected, π-π* transitions within
the helicene moiety (e.g. excitation no. 29 calculated at
247 nm), although with additional small CT contributions
mostly of the boranil!helicene origin. The lowest-energy
positive band measured around 410 nm corresponds to the
HOMO-to-LUMO π–π* transitions centered mainly at the boranil
chromophore with noticeable involvement of helicene π-
orbitals electronically coupled with the boranil π-electron
system (excitation no. 1 calculated at ca. 410 nm). The decrease
in positive ECD intensity experimentally observed at around
350 nm for the (P)-H2 enantiomer appears to originate from
excitations representing a combination of predominantly bor-
anil!helicene CTs and helicene-centered π–π* transitions of
CT-like signature mixed with additional small contributions
from helicene!boranil CTs and π–π* transitions within the
boranil fragment (e.g. excitations nos. 6 and 7 calculated at ca.
345 and 336 nm, respectively). The overestimation of this
intensity drop observed in the computations (leading to the
appearance of the negative band in the calculated spectra) is
likely due to neglecting vibrational effects and/or disregarding
contributions from other less-energetic (equilibrium) conform-
ers. In the former matter, not only vibronic contributions (note
that indeed both UV-vis and ECD experimental spectra for all
the systems studied show vibronic fine structure),[23] but also
nonequilibrium structure effects,[24] might be crucial here to
ensure a satisfactory agreement of the simulated spectra with
experiments. In particular, we speculate that vibrational bend-
ing of the boranil-helicene bond can reduce/break the
Despite the similarities between the mono-boranil H2 and
bis-boranil H3 systems, a striking difference can be noted in
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their low-energy ECD-active excitations. Namely, unlike for (P)-
H2, for which calculations showed one intense excitation
underlying the lowest-energy band in the spectrum (excitation
no. 1 computed at ca. 410 nm, vide supra), in the case of (P)-H3
an intense pair of excitations involving HOMO-1, HOMO, LUMO,
and LUMO+1 is present in this spectral region (excitations nos.
1 and 2 at around 420 and 400 nm, respectively) that both
reveal the same (and shared with H2) predominantly boranil-
centered π-π* character with a noticeable involvement of the
helicene π-orbitals but demonstrate opposite-sign rotatory
strengths ((+) and (À ) for the lower-energy and higher-energy
excitation, respectively). The electronic and MO-pair assign-
ment, large rotatory strengths and the opposite signs of these
first two excitations clearly indicate exciton coupling between
the electric transition dipoles of the boranil fragments grafted
onto the extremities of H3, with noticeable contributions from
the helicene π-system, similar to what was shown by us recently
for helicene-diketopyrrolopyrrole derivatives.[21a] It should be
however noted that unlike for the helicene-diketopyrrolopyrrole
systems, in the case of (P)-H3 higher-energy negative exciton
couplet component does not appear to contribute much to the
following negative band in the ECD spectrum as it seems to be
efficiently suppressed by the nearby excitation with large
positive rotatory strength value (excitation no. 3 calculated at
ca. 395 nm) corresponding to boranil-centered π–π* transition
mixed with helicene!boranil CT.
Luminescence properties
Boranil derivatives can display appealing emission properties,
notably also in the solid state, and may show Aggregation-
Induced Emission (AIE) or Aggregation-Induced Emission
Enhancement (AIEE) effects.[16,17,26] The luminescence properties
of compounds H1-H3 were thus recorded in CH2Cl2 at room
temperature (rt, 298 K) and in the solid state. The rt emission
spectrum of H1 is displayed in Figure S1.43, showing two
fluorescence signals, at 480 and 504 nm; a quantum yield of 4%
(λexc =350 nm) was measured. The [6]helicene derivatives H2
and H3 also display two main fluorescence signals, at
respectively 485–515 nm (Figure 5a) and 493–520 nm (Fig-
ure 5b), thus only slightly red-shifted compared to H1, but with
visibly enhanced quantum yields of 30% and 20%; these values
are quite high for [6]helicenes[1] and consistent with the results
for other boron derivatives.[9–12,16,17] Note that the emission
spectra were also measured in other solvents but no solvato-
chromism was observed. Finally, the luminescence properties of
H2 and H3 were examined in powders and quantum yields
were found to be 8% at 519 nm for the mono-boranil and
almost zero for the bis-boranil derivative. Note that AIE
enhancement was studied in water/THF mixtures but no effect
was observed for these systems.
Similar luminescence properties of H1-H3 along with the
similarity of their corresponding lowest-energy absorption may
indicate fundamentally identical nature of the emitting S1 state
for all three systems. This was indeed ascertained using TD-DFT
calculations (PBE0/SV(P) with a continuum solvent model for
Figure 5. Experimental emission and CPL spectra of (P) and (M) enantiomers of a) H2 and b) H3 in CH2Cl2 at C~10À 5 M and rt, and c) H2 in PMMA. Excitation
wavelengths are 390 and 385 nm for H2 and H3, respectively.
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Chemistry—A European Journal
CH2Cl2). The computations involving S1 excited-state geometry Conclusion
optimizations reproduced correctly experimental energetic
positions of the emission maxima and their overall slight red-
shift when going from H1 to H2 to H3 and confirmed that in
each case S1!S0 fluorescence transition is of the boranil-
centered π–π* character with some helicene!boranil CT
signature (see Table S2.5 and Figure S2.15). Overall small CT
component in the emission is in line with the aforementioned,
rather minor effect of a solvent polarity on the experimentally
observed fluorescence spectra of these compounds.
We have prepared a new family of boron-substituted helicene
derivatives that enriches both helicenes and boranils chemistry,
namely carbo[4]helicene-2-mono-boranil (H1) along with carbo
[6]helicene-2-mono- and -2,15-bis-boranils (H2 and H3, respec-
tively), which display fluorescence emission in solution. Further-
more, enantiopure H2 and H3 systems also show efficient
circularly polarized luminescence, with absolute glum values up
to 2.5×10À 3 demonstrated by the bis-boranil derivative thanks
to the exciton coupling effect. The X-ray structure analysis of
H2 revealed an interesting chiral induction from the helicenic
part to the axial chirality that arises due to the presence of a
CH-π interaction between a proton of the boranil unit and the
carbo[6]helicene core which ‘locks’ the structure into a closed
form of energetically preferred (P,Ra)/(M,Sa) stereochemistry in
the solid state. As a result, the system keeps its CPL activity in
PMMA films. In the solution, coexistence of various structures of
both (Ra) and (Sa) axial chirality is however expected based on
both experimental and computational results. These structural
aspects put forward novel features in the domain of helicenes
and might inspire further research aiming at preserving and
controlling specific axially chiral conformations also in solution.
Circularly polarized luminescence
Helicenes are known to exhibit efficient circularly polarized
fluorescence or phosphorescence activity, which makes them
appealing for incorporation into OLEDs as circularly polarized
electroluminescent materials.[2] The CPL spectra of (P) and (M)
enantiomers of H2 and H3 derivatives and of their racemic
samples were thus recorded in CH2Cl2 at rt. As expected, the
enantiomers revealed mirror-image CPL relationships, while the
racemic samples exhibited no CPL activity. Regarding the
dissymmetry factors, which reflect the percentage of circularly
polarized emitted light (glum =2(ILÀ IR)/(IL +IR)), we obtained glum
of À 1.3×10À 3/+1.2×10À 3 for H2 and À 2.5×10À 3/+2.5×10À 3
for H3, for (M)/(P) enantiomers, respectively. The fact that H3 Acknowledgements
displays higher glum values compared to H2 is in agreement
with our previous results on helicene-diketopyrrolopyrrole and
-naphthalimide derivatives.[21] In particular, a clear reminiscence
to the former systems can be noted here, with the bis-boranil
derivative H3 showing exciton coupling chirality based on a
typical low-energy substituent-centered excition couplet which
induces enhanced chiroptical activity and stronger CPL (vide
supra). The calculations correctly reproduced an increase
(roughly doubling) of the glum values for H3 vs. H2 and linked it
predominantly to an increase in rotatory strength of S1!S0
We acknowledge the Centre National de la Recherche Scientifi-
que (CNRS) and the University of Rennes. This work was
supported by the Agence Nationale de la Recherche (ANR-16-
CE07-0019 “Hel-NHC” grant). M.S.-H. thanks the PL-Grid Infra-
structure and the ACC Cyfronet AGH in Krakow, Poland for
providing computational resources. Dr. Ludovic Favereau is
warmly thanked for fruitful discussions.
fluorescence transition for the bis-boranil system. The analysis Conflict of Interest
showed that this enhancement can be in turn traced back to a
more beneficial orientation (an increased deviation of the angle
The authors declare no conflict of interest.
°
from 90 ) between the underlying electric and magnetic
transition dipole moments observed for H3 as compared to H2
(see Table S2.6 and Figure S2.16). This might indeed be a
reflection of the exciton coupling effect in the excited state of
H3 with the resulting lower-energy couplet’s component
representing the S1 state being the emitting one. Finally, it was
appealing to see whether these derivatives display CPL activity
in the solid state. Indeed, spin-coated films of (P)- and (M)-H2
(0.2% in weight in PMMA films) gave nice mirror-image CPL
signals with maximum emission at 490 nm and glum values
similar to the solution state (Figure 5c), i.e. around �10À 3.
Satisfyingly, these chiral films appeared fully isotropic (see
Supporting Information).
Keywords: boranil
luminescence · helicene · quantum chemistry
·
chirality
·
circularly
polarized
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Manuscript received: January 29, 2021
Accepted manuscript online: March 26, 2021
Version of record online: ■■■, ■■■■
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FULL PAPER
Helicene-mono- and bis-boranils
have been prepared in enantiopure
forms. The chiroptical (electronic
circular dichroism and optical
rotation) and photophysical proper-
ties (unpolarized and circularly
polarized luminescence) of these new
chiral emissive helicenes have been
studied both experimentally and the-
oretically and highlight their
Dr. A. Macé, K. Hamrouni, Dr. E. S.
Gauthier, M. Jean, Dr. N. Vanthuyne,
Dr. L. Frédéric, Dr. G. Pieters, Dr. E.
Caytan, Dr. T. Roisnel, Dr. F. Aloui,
Dr. M. Srebro-Hooper*, Dr. B. Carboni,
Dr. F. Berrée*, Dr. J. Crassous*
1 – 10
Circularly Polarized Fluorescent
Helicene-Boranils: Synthesis, Photo-
physical and Chiroptical Properties
combined helical and axial chirality
effects.