Physicochemical and Electronic Properties of Cationic [6]Helicenes: from Chemical and Electrochemical Stabilities to Far-Red (Polarized) Luminescence

Article abstract of DOI:10.1002/chem.201603591

The physicochemical properties of cationic dioxa (1), azaoxa (2), and diaza (3) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pK_R+=20.4, (Formula presented.) =?0.72 V) compared to its azaoxa 2 (pK_R+=15.2, (Formula presented.) =?0.45 V) and dioxa 1 (pK_R+=8.8, (Formula presented.) =?0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far-red regions. From 1 to 3, a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post-functionalization reactions (12 examples, compounds 4–15). The electronic absorption is modulated from the orange to the far-red spectral range (560–731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first-principles calculations.

Full text of DOI:10.1002/chem.201603591

DOI: 10.1002/chem.201603591
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Chromophores |Hot Paper|
Physicochemical and Electronic Properties of Cationic
[6]Helicenes: from Chemical and Electrochemical Stabilities to
Far-Red (Polarized) Luminescence
Johann Bosson,[a] Geraldine M. Labrador,[a] Simon Pascal,[a] FranÅois-Alexandre Miannay,[b]
Oleksandr Yushchenko,[b] Haidong Li,[c] Laurent Bouffier,[c] Neso Sojic,[c] Roberto C. Tovar,[d]
Gilles Muller,[d] Denis Jacquemin,[e] Adꢀle D. Laurent,[e] Boris Le Guennic,[f] Eric Vauthey,*[b]
and Jꢁrꢂme Lacour*[a]
Abstract: The physicochemical properties of cationic dioxa
(1), azaoxa (2), and diaza (3) [6]helicenes demonstrate
The triplet quantum yields and circularly polarized lumines-
cence are also reported. Finally, fine tuning of the optical
properties of the diaza [6]helicene core is achieved through
selective and orthogonal post-functionalization reactions
(12 examples, compounds 4–15). The electronic absorption
is modulated from the orange to the far-red spectral range
(560–731 nm), and fluorescence is observed from 591 to
755 nm with enhanced quantum efficiency up to 70%
(619 nm). The influence of the peripheral auxochrome sub-
stituents is rationalized by first-principles calculations.
a much higher chemical stability of the diaza adduct 3 (pKR+
red
1=2
=20.4, E =ꢀ0.72 V) compared to its azaoxa 2 (pKR+ =15.2,
Er1e=d2 =ꢀ0.45 V) and dioxa 1 (pKR+ =8.8, Er1e=d2 =ꢀ0.12 V) ana-
logues. The fluorescence of these cationic chromophores is
established, and ranges from the orange to the far-red re-
gions. From 1 to 3, a bathochromic shift of the lowest
energy transitions (up to 614 nm in acetonitrile) and an en-
hancement of the fluorescence quantum yields and lifetimes
(up to 31% and 9.8 ns, respectively, at 658 nm) are observed.
Introduction
Previously, cationic dioxa, azaoxa, and diaza [6]helicenes (1–3)
have been reported (Figure 1, W,X,Y,Z=H).[1,2] Their preparation
involves five straightforward steps to reach a common late-
stage intermediate, and then divergent procedures to afford
[a] Dr. J. Bosson, G. M. Labrador, Dr. S. Pascal, Prof. J. Lacour
Department of Organic Chemistry, University of Geneva
Quai Ernest Ansermet, 30.1211 Geneva 4 (Switzerland)
E-mail: jerome.lacour@unige.ch
[b] Dr. F.-A. Miannay, O. Yushchenko, Prof. E. Vauthey
Department of Physical Chemistry, University of Geneva
Quai Ernest Ansermet, 30.1211 Geneva 4 (Switzerland)
E-mail: eric.vauthey@unige.ch
Figure 1. Cationic [6]helicenes (only P enantiomers shown): dioxa 1, azaoxa
2, and diaza 3–15 (W,X,Y,Z=H, halogens, NO2, CN, aryl, or amines; see
Figure 7 for details).
[c] H. Li, Dr. L. Bouffier, Prof. N. Sojic
ISM, UMR 5255, University of Bordeaux and CNRS, 33400 Talence (France)
[d] R. C. Tovar, Prof. G. Muller
Department of Chemistry, San Josꢀ State University
1 Washington Square, San Josꢀ, CA 95192-0101 (USA)
the final products.[3] Compounds 1–3 are effective chromo-
phores and can be isolated as single enantiomers (M or P con-
figurations). Pronounced chiroptical properties have been ob-
tained, for example, electronic circular dichroism (ECD) in both
UV and visible spectral windows.[4] Furthermore, regioselective
post-functionalization of diaza [6]helicene 3 is possible with
both electrophilic and nucleophilic reagents, enabling the in-
troduction of auxochromes in either the 8 and 10 positions or
5 and 13 positions, respectively (helicenes 4 to 15, see Fig-
ures 1 and 7).
[e] Prof. D. Jacquemin, Dr. A. D. Laurent
CEISAM, UMR CNRS 6230, Facultꢀ des Sciences et des Techniques
Universitꢀ de Nantes, 2, rue de la Houssiniꢁre 44322 Nantes (France)
Institut Universitaire de France, 1, rue Descartes
75005 Paris Cedex 05 (France)
[f] Dr. B. Le Guennic
Sciences Chimiques de Rennes UMR 6226, CNRS - Universitꢀ de Rennes 1
Campus de Beaulieu, 35042 Rennes (France)
Supporting information and ORCID number(s) for the author(s) of this arti-
cle are available under http://dx.doi.org/10.1002/chem.201603591.
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Herein, the core properties of cationic helicenes 1 to 3 are
reported. Remarkable chemical stabilities are evidenced by
measurements of highly positive pKR+ values (up to +20.4)
and low one-electron reduction potentials. In-depth studies
also demonstrate low-energy emissions for 1–3 accompanied
by large fluorescence quantum yields and lifetimes at wave-
lengths that are unusual for purely organic helicenes (e.g., 3 at
ful dyes and the corresponding carbinols essentially colorless
[Eq. (3)].[1,4] In the case of the most reactive 1, measurements
were performed in 0.1m sodium borate buffered aqueous solu-
tions using NaOH as base (see Supporting Information).[7] How-
ever, with 2 and 3, equimolar concentrations of carbenium and
carbinol species, [R+]=[ROH], were never reached in aqueous
solutions, even at pH 14. For these dyes, the measurements
were therefore performed in mixtures of DMSO/water/Me4N+
OHꢀ, for which the DMSO/water ratio modulates the basicity
beyond pH 14.[8] For the calculations, it was then necessary to
use the C_ acidity function defined by Laursen and co-workers
instead of the regular pH scale.[9] For each carbenium ion, the
value of log([ROH]/[R+]) is plotted versus C_ (Figure 2 and Sup-
porting Information for details). The intercepts with the origin
658 nm: f =31% and t=9.8 ns in acetonitrile).[5] Transient ab-
fl
sorption measurements of 1–3 indicate that fluorescence and
internal conversion are the main decay processes of the lowest
singlet excited state. However, upon substitution with a heavy
atom such as iodine, intersystem crossing to the triplet state
becomes dominant. Circularly polarized luminescence (CPL) of
enantiopure samples of 1–3 is also reported at low energies
(e.g., 2 at ꢁ614 nm: glum values of ꢀ0.0021/+0.0020). Finally
and importantly, with modified helicenes 4–15, both absorp-
tion and emission properties can be adjusted by the nature of
the substituents introduced at the periphery of the helical
core. A maximum fluorescence quantum yield of 70% is ob-
tained for 14 (619 nm, W=NHPr; X,Y,Z=H), and the lowest-
energy emission is observed for 13 (731 nm, f 1%, W,Z=CN;
fl
X,Y=H). The influence of the different substitution patterns on
the electronic properties is further rationalized by in silico
modeling.
Results and Discussion
Core structures (1–3)
Derivatives 1–3 are [6]helicenes and triaryl methyl carbenium
moieties at the same time.[6] In principle, they can react with
nucleophiles in SN1-like addition processes. Their existence as
cations was consequently debatable under neutral and/or
basic or reductive conditions. Therefore, pKR+ values and one-
electron reduction potentials were measured to establish their
chemical stability.
Figure 2. Plots of log([ROH]/[R+]) versus acidity for dioxa 1 (red), azaoxa 2
(purple), and diaza[6]helicene 3 (blue).
of the linear fitted lines indicate the points at which equimolar
concentrations of [R+] and [ROH] are obtained, and hence, the
pKR+ values.[10] Finally, values of 8.8, 15.2, and 20.4 were deter-
mined for 1, 2, and 3, respectively.[11] As expected, compound
1 is clearly the most electrophilic. Replacement of the oxygen
atoms at bridge positions by more electron-donating nitrogen
leads to a substantial increase in chemical stability for carbeni-
um ions 2 and 3. An analogous trend has been observed previ-
ously for cationic [4]helicenes[8,12] as well as for related struc-
tures.[2k]
Carbenium stability
As electrophiles, triaryl methyl carbenium ions are in fact ex-
pected to react with water and form carbinol derivatives after
proton loss [Eq. (1)]. This reaction and the corresponding equi-
librium constant can be used to characterize the chemical sta-
bility of carbocationic structures [Eq. (2)]. For instance, highly
stable carbenium ions present a low reactivity, and hence,
highly positive pKR+ values [Eq. (3)].[2k]
Electrochemical properties
For further assessment of the stability of the cationic species,
the redox properties of compounds 1–3 were characterized.
The electrochemical behavior of these species was analyzed by
cyclic voltammetry (CV). The CV curves presented in Figure 3
were recorded in degassed acetonitrile solutions containing
1 mm of each of the [6]helicenes and n-tetrabutylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte
(data are gathered in Table 1).[13] Diaza [6]helicene 3 exhibits
a relatively simple redox behavior (Figure 3). This compound is
typically reduced at E1re=d2 =ꢀ0.72 V versus Fc/Fc+, with a peak-
to-peak separation (DEpeaks) of approximately 57 mV evidencing
Rþ þ H2O Ð ROH þ Hþ
ð1Þ
ð2Þ
½Rþꢂ
pKRþ ¼ C þ log
½ROHꢂ
Abs
pKRþ ¼ C þ log
1 ꢀ Abs
ð3Þ
Experimentally, the equilibrium can be monitored by UV/Vis
absorption spectroscopy, as cationic compounds 1–3 are color-
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versibility was observed upon increasing the scan rate (data
not shown). This indicates that the replacement of nitrogen
atoms by oxygen atoms irreversibly perturbs the oxidation
pathways, as the “substitution” clearly destabilizes the corre-
sponding dicationic structures. Notably, a similar structure–ac-
tivity relationship was observed previously for cationic triangu-
lene dyes.[14] Not surprisingly, the reduction behavior of 2
stands between those of compounds 1 and 3 as a one-to-one
comparison reveals cathodic shifts of 0.33 and 0.27 V, respec-
tively. As evidenced through the measurement of the pKR+
values of compounds 1–3 (vide supra) or observed in the case
of other cationic helicenes, the introduction of amino groups
in place of the O atoms in the core structures clearly stabilizes
the carbocations[2k,12b,d] Overall, the CV investigation reveals
a reversible monoelectronic reduction for all three cationic
[6]helicenes, whereas the oxidation could be either reversible
(diaza 3) or irreversible (dioxa 1 and azaoxa 2).
Figure 3. Cyclic voltammograms of 1 mm dioxa (1, red), azaoxa (2, purple)
and diaza (3, blue) [6]helicenes, recorded in degassed CH3CN solution con-
taining 0.1m TBAPF6 as supporting electrolyte (n=0.1 Vsꢀ1). The potential
was scanned first toward positive values and then toward negative ones.
From the recorded voltammograms of 1, 2, and 3, the corre-
sponding fundamental gaps were estimated in a first approxi-
mation (Table 1).[15] For the irreversible oxidation waves, the
peak potentials may shift depending on the scan rate, the con-
centration, and the kinetics of the subsequent chemical reac-
tions. Even if the peak potential could not be considered as
the standard potential for the oxidation of 1 and 2, we used
this value to estimate the fundamental gap and to compare
the resulting values with those from the optical experiments.
Taking into account this approximation, only a minor contribu-
tion of the N atom can be seen upon comparing dioxa 1 and
azaoxa 2. Diaza 3 stands apart, with a fundamental gap energy
substantially lower than those of the other two carbocationic
species. In an effort to gain more insights into the electronic
properties of compounds 1–3, we investigated their optical
properties and determined their optical gaps.
Table 1. Anodic and cathodic halfwave potentials (E1/2) values measured
by CV for 1–3 in acetonitrile (0.1m TBAPF6) at a Pt electrode (n=
0.1 Vsꢀ1), E versus Fc/Fc+. Fundamental gaps and optical gaps for 1–3.
red
1=2
ox1
1=2
Cpd
E
[V]
E
[V]
Fund. gap [eV][b]
Opt. gap [eV][c]
1
2
3
ꢀ0.12
ꢀ0.45
ꢀ0.72
2.14[a]
1.93[a]
1.40
2.26
2.38
2.12
2.15
2.11
1.95
[a] The anodic process is partially irreversible for 2 and fully irreversible
for 1, and therefore, the reported value corresponds to the oxidation
peak potential Eopx for these compounds. [b] Fundamental (Fund.) gap=
ox1
1=2
red
1=2
E
ꢀE . For the compounds characterized by irreversible oxidation, the
value of E is replaced by the peak potential Eopx in a first approximation.
ox1
1=2
[c] Optical (Opt.) gap=0–0 energy.
a reversible monoelectronic transfer generating a neutral spe-
cies from the cation. The first oxidation also shows a reversible
Absorption and fluorescence
behavior with E1o=x2 =1.40 V versus Fc/Fc+ and DEpeaks ꢁ62 mV. The electronic absorption and fluorescence properties of
The peak-to-peak separation deviates slightly from ideal, but [6]helicenes 1–3 were recorded in acetonitrile (Figure 4 and
still points to a reversible one-electron mechanism. The scan Table 2). The absorption spectrum of diaza [6]helicene 3 shows
rate was varied in the range 0.1–1 Vsꢀ1; the study of the cur- an intense (e=14700mꢀ1 cmꢀ1) lowest-energy transition cen-
rent peak shows a linear evolution with respect to the square tered at 614 nm, presenting a shoulder at higher energy, remi-
root of scan rate (data not shown), which is perfectly consis- niscent of its parent diaza [4]helicene cation.[5a] Compounds 1
tent with a diffusion-controlled process. A second oxidation is and 2 both exhibit a blueshifted absorption maximum at
observed at a more anodic potential of around 2 V, and the ir- 562 nm. The absorption profiles and molar extinction coeffi-
reversibility of this process can be assigned to an irreversible cients are comparable for 1 and 3, whereas noticeable broad-
oxidative decomposition of the [6]helicene core, which was ening and a hypochromic shift of the absorption peak are
not investigated further.
highlighted for the azaoxa derivative 2 (e=10700mꢀ1 cmꢀ1).
With azaoxa 2, the reductive behavior is apparently un- Optical gaps of 2.15, 2.11, and 1.95 eV were determined for 1,
changed with a single reversible one-electron reduction, albeit 2, and 3, respectively (Table 1). The difference in energy be-
occurring at a much less cathodic potential (Er1e=d2 =ꢀ0.45 V vs. tween the fundamental gap and the optical gap, that is, the
Fc/Fc+ and DEpeaks ꢁ58 mV). On the other hand, a partially irre- electron–hole pair binding energy, is rather low for 1 (0.11 eV)
versible oxidation process is observed, occurring at large compared with those for 2 and 3 (0.27 and 0.17 eV, respective-
anodic overpotentials (Epox =1.93 V vs. Fc/Fc+). Finally, the elec- ly).[15b,16]
trochemical characterization of dioxa [6]helicene 1 reveals an
In terms of emission, helicene 3 displays a fluorescence cov-
even easier reduction process (Er1e=d2 =ꢀ0.12 V vs. Fc/Fc+ with ering the whole orange–red range with a maximal peak inten-
DEpeaks ꢁ60 mV) but also a very large Eo1=x2 value (2.14 V vs. Fc/ sity at 658 nm. Dioxa 1 and azaoxa 2 present blueshifted emis-
Fc+) for the fully irreversible oxidation. No improvement of re- sion maxima centered at 595 and 614 nm, respectively.
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Table 2. Spectroscopic properties of 1–3 and substituted diaza[6]helicenes 4–15 in acetonitrile.
[a]
[b]
Cpd
Substituents: positions
labs [nm]
e [103 mꢀ1 cmꢀ1
]
lem [nm]
f [%]
fl
t [ns]
kR [106 sꢀ1
]
kNR [106 sꢀ1
]
1
2
3
4
5
6
7
8
–
–
–
562
562
614
560
630
622
620
642
657
650
731
724
682
582
598
15.5
10.7
14.7
18.2
14.4
10.5
13.4
9.8
8.9
8.3
1.9
4.6
595
614
658
591
690
678
675
711
755
740
–
12[c]
22[c]
31[c]
46[c]
13[c]
9[e]
19[e]
20[d]
50[e]
8[d]
4.0
6.8
9.8
10.9
6.1
3.0
1.0
2.0
0.3[f]
0.6[f]
–
30.0
32.3
31.6
42.2
31.3
21.7
26.0
8.0
6.7
5.0
–
–
220
115
70.4
49.5
143
312
974
492
3330
1660
–
NO2: 8,10
Cl: 8,10
Br: 8,10
I: 8
Ph: 8,10
p-anisyl: 8,10
PhCꢃC: 8,10
NH2: 8,10
NMe2: 8,10
CN: 5,13
NHPr: 5
NHPr: 5, Br: 8,10
6.5[c]
2.6[c]
4[d]
1.5[d]
1.6[d]
<1[d]
<1[d]
–
9
10
11
12
13
14
15
–
–
–
–
8.2
22.9
27.4
731
619
645
1.0[d]
70[c]
0.7[f]
12.3
0.9[f]
14.3
56.9
55.6
1410
24.4
1060
5.0[c]
5.0[d]
[a] kR =f/t. [b] kNR =(1ꢀf)/t. [c] Relative to cresyl violet (MeOH, f =54%). [d] Relative to oxazine 725 (EtOH, f =11%). [e] Relative to sulforhodamine 101
fl
fl
(EtOH, f =95%). [f] Fluorescence lifetimes measured by fluorescence upconversion technique. All the fluorescence decays were fitted as monoexponential
fl
functions. Lifetimes are given with an error of +/ꢀ0.1 ns for the TCSPC data and +/ꢀ0.3 ps for the upconversion data.
shift in the emission band and enhances the fluorescence
quantum yield. The corresponding lifetimes follow the same
trend, with relatively high values of 4.0, 6.8, and 9.8 ns for 1, 2,
and 3, respectively. As shown in Table 2, the radiative rate con-
stants calculated from the fluorescence quantum yields and
lifetimes are essentially the same for 1, 2, and 3, pointing to
differences in internal conversion and/or intersystem-crossing
efficiencies (vide infra). Interestingly, although 1 and 2 feature
similar absorption maxima, their fluorescence spectra are not
concomitant (excitation spectra are provided in Figures S9–
S11, Supporting Information).
The influence of different solvents on the optical properties
of 1–3 was also investigated (nine solvents, see Tables S1–S3
and Figures S12–S20, Supporting Information). Globally, the ab-
sorption and emission maxima of 1–3 exhibit only minor de-
pendences on the selected solvent and the profiles, that is, the
shape and intensity of the spectra did not change significantly.
However a non-monotonic evolution of the optical properties
is observed regarding the wide range of different polarities, re-
fractive indexes, and viscosities of the solvents tested. Com-
pounds 1–3 exhibit larger Stokes shifts in DMSO than in any
other solvents (946, 1653, and 1069 cmꢀ1 for 1, 2, and 3, re-
spectively). For 1, it was found that the helicene degraded
during the measurements in THF, methanol, and pentanol,
which is unsurprising considering its lower chemical stability as
indicated by the pKR+ value of 8.8 (vide supra). Note that 1–3
conserve a non-negligible fluorescence quantum yield in water
(from 7% for 2 up to 10% for 3), which is important for their
possible use as red bioimaging probes.[5b,17]
Figure 4. A) Absorption (plain lines) and normalized fluorescence spectra
(dashed lines) of dioxa (1, red), azaoxa (2, purple), and diaza [6]helicene (3,
blue) in acetonitrile (2.10ꢀ5 molLꢀ1). B) Intensity normalized fluorescence
time profiles.
Transient absorption and triplet quantum yields
Considering their luminescence located around or beyond
600 nm, 1, 2, and 3 possess relatively high fluorescence quan-
tum yields of 12, 22, and 31%, respectively (with cresyl violet
as reference, see Table 2). Note that the introduction of N
atom(s) within the cationic scaffold causes a progressive red-
Transient absorption measurements were performed in two
temporal windows: 0–2 ns with 150 fs instrument response
function[18] and 400 nm excitation (2 and 3, Figures S22 and
S24, Supporting Information), and 0–100 ms with 350 ps instru-
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Circularly polarized luminescence
As mentioned above, [6]helicenes 1–3 are chiral and can be
isolated as single enantiomers through ion-pairing strategies
(e.g., 3) or, more generally, through Chiral Stationary Phase
(CSP) HPLC resolution.[1,4] Strong chiroptical properties are re-
corded in the visible range of the light spectrum, as evidenced
by the intense Cotton effects observed in the electronic circu-
lar dichroism spectra.[4] As such, it was deemed interesting to
investigate their circularly polarized luminescence. The CPL
spectra presented in Figure 6 were recorded in dichlorome-
thane and show opposite signals for (+) versus (ꢀ) enantio-
mers of 1–3.
Figure 5. A) Transient absorption spectra measured at several time delays
after 355 nm excitation of 3 in acetonitrile and inverted stationary absorp-
tion spectrum (dashed). B) Species-associated difference absorption spectra
obtained from a global target analysis assuming a B!C! scheme (A!B
corresponding to the equilibration of the S1 state occurs on the picosecond
timescale (Figure S24, Supporting Information).
ment response function and 355 nm excitation (1–3, Figures 5,
S21, and S23).[19] The early transient spectra exhibit both nega-
tive and positive bands. The negative bands coincide with the
stationary electronic spectra, and thus, can be attributed to
both the bleach of the ground-state absorption and the stimu-
lated S1!S0 emission. The positive bands can be ascribed to
!
Figure 6. Circularly polarized luminescence (upper curve) and total lumines-
cence (lower curve) spectra of (+) and (ꢀ)-1 (left), -2 (middle), and -3 (right)
in 2 mm degassed dichloromethane solutions at 295 K, upon excitation at
435/435, 442/435, and 473/472 nm, respectively (black for (+) and red for
(ꢀ)).
Sn S1 absorption.
Small spectral changes that are probably caused by the dis-
sipation of the excess excitation energy can be observed
during the first few picoseconds (Figures S22 and S24). Subse-
quently, the intensity of all spectral features decreases on the
5–10 ns timescale. This results in a spectrum with very small
amplitude still containing the negative band owing to the
bleach of ground-state absorption, and decaying on the micro-
second timescale (Figures 5, S21, and S23). These transient
spectra were analyzed globally assuming a series of successive
exponential steps.[20] Three steps, that is, an A!B!C!
scheme, had to be used to reproduce the 0–2 ns and 0–100 ms
data. The first one, associated with a picosecond time con-
stant, can be ascribed to the equilibration of the lowest singlet
excited state. The time constant of the B!C step coincides
with the fluorescence lifetime measured by time-correlated
single photon counting (TCSPC), and consequently, B can be
interpreted as the S1 state. Finally, as the C!D step occurs on
the 10 ms timescale in N2-bubbled solutions and is faster in the
presence of air, species C is assigned to the triplet state.
Through comparison of the relative amplitude of the bleach in
the spectra associated with species B and C, the triplet yields
of 1–3 can be estimated to be smaller than 5%. We therefore
conclude that the main deactivation pathways of the S1 state
of 1–3 are fluorescence and internal conversion to the ground
state.
The glum values are estimated to be ꢀ0.00032/+0.00041,
ꢀ0.0021/+0.0020, and ꢀ0.0012/+0.0010 at the vicinity of the
maximum emission wavelength for (+)- and (ꢀ)-1, 2, and 3, re-
spectively (i.e., 595, 614, and 658 nm, respectively). These
values are of the same order of magnitude as those for other
examples of organic CPL-active helicenes.[5c,21] In a general
fashion, the CPL properties are influenced only marginally by
the replacement of O atom(s) by more electron-donating
amino group(s) in 1 to 3. This indicates that the chiral struc-
tures are more or less equivalent in terms of generating similar
CPL activities, and the replacement did not affect the chiral
properties at least from a CPL standpoint.
Tuning properties through chemical substitutions (4–15)
Absorption and fluorescence
Previously, the ability of cationic diaza [6]helicene 3 to react re-
gioselectively with both electrophilic and nucleophilic reagents
was evidenced.[1] Typically, in one step, substituents such as
NO2 (4), Cl (5), Br (6), and I (7) were introduced in positions 8
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(and 10), and CN functional groups were added to positions 5
and 13 (helicene 13). The dibromo derivative 6 was further en-
gaged in Pd-catalyzed cross-coupling reactions, yielding bis-
(phenyl) 8 and bis(ethynyl) 10 helicenes. Reduction of dinitro 4
provided the corresponding bis(amino) 11. Care was taken to
prepare three new functionalized derivatives, namely bis(p-
anisyl) 9, bis(dimethylamino) 12, and propylamino 14
(Figure 7).[22] The absorption and fluorescence properties of
these compounds carrying electron-donating (EDG), electron-
withdrawing (EWG), and possibly a combination of both types
of groups were investigated. Selected spectra are presented in
Figure 8 and the results are compiled in Table 2.
6–16 nm) in the order Cl > Br > I, and the extinction coeffi-
cients remain mostly unchanged, with the exception of the di-
bromo derivative 6 (e=10500mꢀ1 cmꢀ1). However, in this
series, going from 5 to 7, a decrease in both the fluorescence
quantum yields and lifetimes is observed, which is attributed
to heavy atom effects (vide infra).[23] The introduction of
phenyl or phenylethynyl moieties (8 and 10) induces a redshift
of the absorption and emission bands, along with a decrease
in the absorption molar extinction coefficient. The adjunction
of electron-donating methoxy groups on the aryl substituents
(9) further enhances this redshift for both absorption and emis-
sion. From the smaller kR values of 8–10 and the larger Stokes
shifts, it seems that the excited state has a substantial charge-
transfer (CT) character, which increases upon going from 8 to
9 and 10. In addition, an enhanced nonradiative de-excitation
through the rotation of the aryl groups can be considered.
The introduction of a strong EDG in the amino (11) and di-
methylamino (12) derivatives induces larger bathochromic and
hypochromic shifts of the absorption spectra, along with
broadening of the bands. However, these compounds are non-
fluorescent, probably because of an intramolecular charge
transfer along with a strong influence of the energy gap law,
which quench the fluorescence (see computational study
below).[23,24]
The influence of the substitution pattern in positions 8 and
10 was analyzed first (4–12). The presence of strong EWGs
such as nitro moieties induces significant blueshifts of both
the absorption and emission spectra of the whole series com-
pared with 3. Chromophore 4 possesses the lowest transition
energy (560 nm, e=18200mꢀ1 cmꢀ1) and is particularly fluores-
cent (f =46%) at 591 nm. The effect of halogen atoms on the
fl
absorption properties is less pronounced: the lowest-energy
absorption bands are slightly redshifted compared to 3 (Dl=
The effect of substitutions by EWGs and EDGs at positions 5
and/or 13 was also studied. The presence of EWGs in positions
5 and 13 induces a bathochromic shift of the optical proper-
ties. The bis(cyano) derivative 13 exhibits a lower-energy ab-
sorption band centered at 682 nm and weak fluorescence in
the far-red (lem =731 nm, with f <1%). Alternatively, an EDG
fl
such as the amino NHPr group in position 5 leads to hypso-
chromic and hyperchromic shifts in absorption, with 14 ab-
sorbing at 582 nm (e=22900mꢀ1 cmꢀ1) and emitting at
619 nm with a remarkable quantum efficiency of 70%. To the
best of our knowledge, this is the first time that such a strong
fluorescence has been measured for an organic helicene in the
orange–red region.[5] The reverse influence of EWGs and EDGs
in positions 8 and 10 or 5 and 13 is notable. Whereas EWGs in
positions 8 and 10 induce a blueshift of the optical properties,
EDGs in the same positions lead to a redshift. The opposite
trend is observed in positions 5 and 13. Finally, [6]helicene 15
was investigated; this compound bears both bromo EWG and
Figure 7. Substituted diaza [6]helicenes 4–15 carrying auxochrome substitu-
ents at positions 5, 8, 10, or 13. The BF4 counterion is omitted for clarity.
ꢀ
Figure 8. A) Selected absorption spectra, B) normalized fluorescence spectra, and C) normalized fluorescence decays of post-functionalized [6]helicenes in ace-
tonitrile.
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NHPr EDG substituents at positions 8, 10, and 5, respectively.
Comparison with 14 clearly indicates a moderate influence of
the bromine atoms in terms of absorption (labs =598 vs.
582 nm, respectively), but a drastic lowering of the emission
mination of the triplet quantum yield from the relative intensi-
ty of the bleach in the spectra associated with species B and C
is difficult. However, assuming similar absorptions of both S1
and T1 states in the overlapping region, the triplet quantum
yield can be estimated to be on the order of 80–90%.
properties (f =5% vs. 70%, respectively). The same trend is
fl
observed on comparison of unsubstituted 3 with dibromo 6.
Transient absorption measurements in the 0–2 ns time
window were performed with the iodo derivative 7 (Figure 9A,
B). The early spectra are dominated by a negative band at
Rationalization through computational analysis
To obtain further insights into the nature of the electronic ex-
cited states, we performed first-principles calculations. These
relied on a hybrid CC2/TD-DFT protocol that allows reliable 0–
0 energies to be obtained.[25] These energies can be compared
directly to the crossing point between the absorption and
emission spectra.[26] The results of this theoretical investigation
are listed in Table 3. The agreement between the CC2/TD-DFT
Table 3. Comparison between experimental and theoretical 0–0 energies
for the treated dyes. Note that during the theoretical calculations the
alkyl chains were substituted by methyl groups. All values are in nm.
Cpd
Exptl
Theory (CC2/TD-DFT)
Theory (TD-DFT)
1
2
3
4
11[a]
12[a]
13
14
578
589
635
577
731
724
708
599
574
592
634
569
738
748
730
595
493
507
535
490
634
610
600
513
[a] Experimental lmax and theoretical vertical absorption.
Figure 9. A,B) Transient absorption spectra recorded at various time delays
after 400 nm excitation of 7 in acetonitrile and inverted stationary absorp-
tion spectrum (dashed). C) Species-associated difference absorption spectra
obtained from a global target analysis assuming an A!B!C! scheme.
values and experimental results is very satisfying, with an aver-
age absolute deviation as small as 12 nm. The uncorrected TD-
DFT approach nicely restores the auxochromic trends but deliv-
ers too small wavelengths. Experimentally, both 11 and 12
were found to be nonfluorescent, which can be understood
from the calculations. Indeed, the computed CC2 emission en-
ergies for these two compounds are 1416 and 1446 nm, indi-
cating a very strong structural relaxation of the excited state.
Such wavelengths correspond to a gap lower than 1 eV, which
is clearly not favorable for the emission according to the
energy gap law.
650 nm and a positive band peaking at 460 nm. The negative
band is caused by the bleach of the ground-state absorption
and its red edge to stimulated emission. The positive band can
!
be assigned to Sn S1 absorption. The negative band is red-
!
shifted relative to the S1 S0 band, suggesting partial overlap
of the excited-state absorption. During the first picosecond,
the positive band changes its shape and acquires a shoulder at
around 600 nm, and the red side of the negative band increas-
es. Given the substantial amount of excess excitation energy,
these effects can be assigned to the equilibration of the S1
state. Subsequently, the transient spectra remain essentially
unchanged up to about 100 ps, and then evolve on a timescale
of hundreds of picoseconds into a long-lived spectrum in
which the 600 nm shoulder and the stimulated emission are
absent. These data could be reproduced assuming an A!B!
C scheme with the first and second steps associated with a 0.2
and 900 ps time constant, respectively (Figure 9C). The short
time constant agrees with the equilibration of the S1 state,
whereas the second one is similar to the fluorescence lifetime.
Given the small fluorescence quantum yield of 7 and the pres-
ence of the iodine substituent, the long-lived transient C is in-
terpreted as the triplet state. As the absorption of both the S1
and T1 states overlaps partially with the bleach, precise deter-
Experimentally, it was found that a strong EWG induces hyp-
sochromic/bathochromic shifts if located at positions 8,10/5,13,
whereas EDGs yield the opposite trend. As TD-DFT revealed
that the lowest-energy transitions present a strongly dominant
HOMO–LUMO contribution, we plotted these orbitals for com-
pound 3 (Figure 10). The HOMO presents significant density at
positions 8 and 10, meaning that the addition of an EWG
(EDG) stabilizes (destabilizes) its energy and hence increases
(decreases) the gap as the LUMO is almost unaffected. In con-
trast, the LUMO displays significant contributions at positions 5
and 13, so that the addition of an EWG (EDG) stabilizes (desta-
bilizes) its energy and hence decreases (increases) the gap.
Therefore, by playing with these positions, one can tune
(almost) independently the levels of the two frontier orbitals.
This fits perfectly with the measured evolutions.
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intense vibronic couplings related to the low-frequency vibra-
tions of the naphthalene subunits with scissoring/bending
(mode 3) and distortions involving CꢀC twisting and rocking
vibrations (modes 8 and 11). In addition, the small shoulder ob-
served in the spectra arises mainly from the contribution of
the high-frequency CꢀC asymmetric stretching (breathing)
mode of the entire structure (mode 91 in Table S4). Replacing
one oxygen atom by a NꢀCH3 moiety modifies the vibronic
contributions of the main band owing to the introduction of
a certain asymmetry of the molecule inducing three vibronic
contributions with equivalent intensities (0.34). Each of these
modes includes the rocking of the naphthalene located on the
NꢀCH3-substituted ring [Na(N)] with the rest of the conjugated
molecules undergoing a wagging (mode 1) or a twisting
(modes 5 and 14) vibration. Similarly to 1, a high-frequency
mode corresponding to the breathing of the structure is re-
sponsible for the shoulder. The vibronic modes of 3 are highly
similar to those of 2; more particularly, modes 1, 5, 40, and 105
of 3 correspond to modes 1, 5, 14, and 98 of 2 (see Supporting
Information). Nevertheless, two supplementary modes appear,
that is, the CH3 rotation (mode 12) and the asymmetric CꢀC
stretching of the conjugated moiety between the NꢀCH3 sub-
stituted rings (mode 119). Modes 105 and 119 combine to give
the shoulder.
Figure 10. HOMO (left) and LUMO (right) of 3.
The computed and convoluted absorption lineshapes are
represented in Figure 11 for compounds 1–3. As in the experi-
mental spectra, 1 does not exhibit a clear-cut broadening fea-
ture, whereas both 2 and 3 present a shoulder. Interestingly,
the relative intensities for 1 and 3 are highly similar, whereas
the intensity of 2 is 24% lower, as also observed in the experi-
mental data (decrease in the extinction coefficient of 27%).
Conclusions
The chemical stabilities and electrochemical and optical prop-
erties of a series of cationic hetero [6]helicenes have been
studied to reveal a drastic influence of 1) the heteroatoms
within the helical core and 2) auxochrome substituents intro-
duced at the periphery. An increasing chemical stability is evi-
denced for cationic dioxa 1, azaoxa 2, and diaza 3 [6]helicenes
through the measurement of (large) pKR+ values of 8.8, 15.2,
and 20.4, respectively. Clearly, the replacement of the O
atom(s) by more electron-donating amino group(s) going from
dioxa (1) to diaza (3) is beneficial for the chemical stability. The
same trend is observed through electrochemistry: the reduc-
tion and oxidation potentials of the cations are lowered sub-
stantially from 1 to 3 (i.e., from ꢀ0.12 to ꢀ0.72 V for Er1e=d2). In
terms of optical properties, the influence of the substitution of
internal atom(s) (O!N) is again prominent, inducing a batho-
chromic shift in absorption and emission on going from 1 to 3.
Interestingly, both quantum yields (12, 22, and 31% for 1, 2,
and 3, respectively) and fluorescence lifetimes (4.0, 6.8, and
9.8 ns for 1, 2, and 3, respectively) increase in this order. More-
over, those fluorophores exhibit only marginal solvatochrom-
ism (absorption and emission). Transient absorption measure-
ments indicate that the first singlet excited states of 1, 2, and
3 decay mostly through fluorescence and internal conversion
to the ground state, intersystem crossing being much less effi-
cient, with a triplet quantum yield smaller than 5%. However,
upon substitution with a heavy atom such as iodine, intersys-
tem crossing to the triplet state becomes the dominant decay
pathway. Isolated as single enantiomers, 1–3 emit circularly po-
larized light, the most efficient CPL emitter being the nonsym-
metrical azaoxa 2 (glum ꢀ0.0021/+0.0020). Further tuning of
Figure 11. Computed vibronic absorption spectra of 1 (red), 2 (purple), and
3 (blue). For their convolution, a HWHM of 0.06 eV was employed for 1 and
2, and 0.04 eV for 3. The inset represents the experimental data.
To gain further insights into the origins of specific band
shapes, we identified the key vibronic contributions for the
three dyes. Figure S25 (Supporting Information) shows the nor-
malized experimental and theoretical absorption and fluores-
cence lineshapes together with the stick vibronic contributions.
Overall, a good agreement between theory and experiment is
observed for both positions and bandshapes. Nevertheless, the
positions of the maximum bands in the absorption spectra are
redshifted for 2 and 3 but slightly blueshifted for 1. The com-
puted fluorescence spectra are systematically blueshifted. We
observe a significant difference in bandshapes caused by the
substitution of oxygen by NꢀCH3. Table S4 and Figure S26
(Supporting Information) list the key vibrational modes, ex-
plaining band broadening and the presence of shoulders. In 1,
the maximum band involves, beyond the 0–0 transition, three
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fl
far-red absorbers (lmax up to 730 nm) are now accessible. Inter-
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site to that in positions 5 and 13 (blueshift vs. redshift), and
vice versa for EDGs. This was rationalized through computa-
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ology of the frontier orbitals. Applications in several fields of
chemistry can be foreseen, from bioimaging to polarized emit-
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Acknowledgements
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We thank the University of Geneva, the Swiss National Science
Foundation, and the NCCR chemical biology funded by the
Swiss National Science Foundation. We also acknowledge the
contributions of the Sciences Mass Spectrometry (SMS) plat-
form at the Faculty of Sciences, University of Geneva. H.L. ac-
knowledges the China Scholarship Council for his PhD fellow-
ship. D.J. acknowledges the European Research Council (ERC)
and the Rꢁgion des Pays de la Loire for financial support in the
framework of a starting grant (Marches-278845) and the Lumo-
Mat project, respectively. This research used the resources of:
1) the GENCI-CINES/IDRIS, 2) CCIPL (Centre de Calcul Intensif
des Pays de Loire), and 3) a local Troy cluster. G.M. thanks the
NIH, Minority Biomedical Research Support (grant 1 SC3
GM089589-07) and the Henry Dreyfus Teacher–Scholar Award
for financial support, and R.C.T. thanks the SJSU RISE program
(NIH grant 5R25M71381) for a research fellowship.
[16] As already mentioned, the oxidation process is almost completely irre-
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comparison to 1 and 2 correlates with the optical gap (Table 1). Thus, it
validates the approximation used for the calculation of the fundamen-
tal gap.
Keywords: circularly polarized luminescence
functional calculations · electrochemistry · fluorescence
helicenes
·
density
·
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Received: July 28, 2016
Published online on && &&, 0000
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Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
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ꢃ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
In rainbows: The physicochemical prop-
erties of cationic dioxa, azaoxa, and
diaza [6]helicenes demonstrate remarka-
bly high chemical stabilities, fluores-
cence from orange to the far-red re-
gions with non-negligible quantum
yields and lifetimes, and circularly polar-
ized luminescence in the same ranges.
Selective post-functionalization of the
diaza [6]helicene introduces peripheral
auxochrome substituents, the influence
of which on the optical properties of
the compounds is rationalized by first-
principles calculations (see figure).
&
Chromophores
J. Bosson, G. M. Labrador, S. Pascal,
F.-A. Miannay, O. Yushchenko, H. Li,
L. Bouffier, N. Sojic, R. C. Tovar, G. Muller,
D. Jacquemin, A. D. Laurent,
B. Le Guennic, E. Vauthey,* J. Lacour*
&& – &&
Physicochemical and Electronic
Properties of Cationic [6]Helicenes:
from Chemical and Electrochemical
Stabilities to Far-Red (Polarized)
Luminescence
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
11
ꢃ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ