Azabora[5]helicene Charge-Transfer Dyes Show Efficient and Spectrally Variable Circularly Polarized Luminescence

Article abstract of DOI:10.1002/chem.201801908

Three helicenes based on a borylated arylisoquinoline skeleton have been prepared in their enantiopure forms and characterized with respect to their photophysical properties, including the use of chiroptical spectroscopies. The dyes show varying charge-tr

Full text of DOI:10.1002/chem.201801908

A Journal of
Accepted Article
Title: Azabora[5]helicene Charge-Transfer Dyes Show Efficient and
Spectrally Variable Circularly Polarized Luminescence
Authors: Zoe Domínguez, Rocío López-Rodríguez, Eleuterio Álvarez,
Sergio Abbate, Giovanna Longhi, Uwe Pischel, and Abel Ros
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To be cited as: Chem. Eur. J. 10.1002/chem.201801908
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Azabora[5]helicene Charge-Transfer Dyes Show Efficient and
Spectrally Variable Circularly Polarized Luminescence
Zoe Domínguez,[a] Rocío López-Rodríguez,[b,c] Eleuterio Álvarez,[b] Sergio Abbate,[d] Giovanna
Longhi,*[d] Uwe Pischel,*[a] and Abel Ros*[b,c]
Dedicated to Professor Rosario Fernández on the occasion of her 60th birthday
Abstract: Three helicenes based on a borylated arylisoquinoline
skeleton have been prepared in their enantiopure forms and
characterized with respect to their photophysical properties,
including the use of chiroptical spectroscopies. The dyes show
varying charge-transfer character and efficient emission (quantum
yields between 0.13 and 0.30, in toluene), which is governed by the
electron-donor substitution (p-MeO-phenyl, p-Me2N-phenyl) at the
helicene. Marked differences in the emission wavelength and Stokes
shift are observed, with the dimethylamino-substituted derivative
emitting most red-shifted (maximum at ca. 590 nm) and displaying
charge-transfer fluorophores with application potential in
10, 28]
bioimaging.[9,
With the idea to further enrich the
photophysics of this fascinating platform we have built on their
biaryl structure, which provides a means to arrive at dyes with
helical chirality and the corresponding observation of interesting
chiroptical properties, including the detection of circularly
polarized luminescence (CPL).[29-32] The phenomenon is well
established for supramolecular assemblies or nanostructures.[29,
33-40]
However, the observation of CPL from small organic
molecules (SOMs), albeit generally showing smaller
dissymmetry factors (g), has recently created considerable
interest in the dye chemistry community.[30] Potential applications
are chiral sensing or the preparation of chiroptical systems with
innovative photophysical design.[29, 30, 41-46] Helicenes belong to
1
the highest Stokes shift (ca. 6000 cm ) in toluene. The helicenes
show electronic circular dichroism (ECD) and significant circularly
polarized luminescence (CPL) with dissymmetry factors of up to 3.5
3
× 10 . The sign of the ECD band corresponding to the first transition
and the CPL spectrum depend sensibly on the electron-donor
substitution.
the most frequently reported CPL-SOM architectures, among
47-51]
them carbo[n]helicenes,[33,
helicenes that incorporate
54]
heteroatoms
(azahelicenes,[52-55]
oxahelicenes,[34,
thiahelicenes[56]), and metallahelicenes.[57, 58]
Introduction
Four-coordinate organoboron chelates[1-3] are attractive targets
for the design of photostable fluorescent dyes for the most
varied applications, including bioimaging,[4-12] sensing,[13-17] and
organic light-emitting diodes.[18] Much effort has been dedicated
to the development of synthetically flexible platforms which
enable the photophysical fine-tuning of the dyes.[3, 9-11, 19-27] As
part of our own research program we have developed borylated
arylisoquinoline (BAI) dyes, which in their N,C-chelate variant
are strongly emitting, solvatochromic, and two-photon-absorbing
Figure 1. Structures of the two enantiomers of the helicenes 1012.
[a]
Z. Domínguez, Dr. U. Pischel
CIQSO - Centre for Research in Sustainable Chemistry and
Department of Chemistry
University of Huelva
Campus de El Carmen s/n, E-21071 Huelva (Spain)
E-mail: uwe.pischel@diq.uhu.es
Dr. R. López-Rodríguez, Dr. E. Álvarez, Dr. A. Ros
Institute for Chemical Research (CSIC-US)
C/Américo Vespucio 49, E-41092 Seville (Spain)
E-mail: abel.ros@iiq.csic.es
Dr. R. López-Rodríguez, Dr. A. Ros
Department of Organic Chemistry and Innovation Centre in
Advanced Chemistry (ORFEO-CINQA)
University of Seville
C/Prof. García González 1, E-41012 Seville (Spain)
Prof. Dr. S. Abbate, Prof. Dr. G. Longhi
Department of Molecular and Translational Medicine
University of Brescia
Viale Europa 11, 25123 Brescia (Italy)
E-mail: giovanna.longhi@unibs.it
However, despite the fact that several Bodipy-based chiral
architectures were found to show significant CPL effects,[39, 46, 59-
64]
azabora[n]helicenes, that incorporate boron directly in the
helical architecture, were reported only in a single work so far.[65]
These systems build on borylated arylpyridine N,C chelates.
Their emission maximum can be fine-tuned by the helicene
length, but even for n = 10 the maximum stays ca. 20 nm below
500 nm. Using an isoquinoline fragment instead of a pyridine,
the charge-transfer properties of the dyes become more
pronounced,[9, 20, 24, 28] as we have shown for related BAI dyes. In
order to explore the possibility to combine the charge-transfer
features with the observation of spectrally-tunable CPL
emission,[54, 56] we have prepared new members of the BAI dye
platform that contain a shielding phenyl ring at the 8-position of
the naphthyl moiety (see Figure 1). This structural feature
introduces steric hindrance which enables the separation of the
helically chiral enantiomers of the dyes. Further, the additional
phenyl ring can be used to integrate electron-donating
[b]
[c]
[d]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201801908
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ARTICLE
10]
substituents such as -OMe or -NMe2 at its para position. Intense
CPL emission above 500 nm was observed especially for the
dyes 11 and 12. The stronger electron-donating -NMe2 shifts the
emission maximum even close to 600 nm. These new BAI dyes
usefully complement a previously investigated series of racemic
BAI dyes with demonstrated applications in bioimaging.[9,
Further, this work provides an attractive path for the design of
spectrally fine-tuned emission of CPL-SOM architectures.[30, 54, 56,
63]
Results and Discussion
Synthesis
6575%. The lithiation/borylation at the remaining bromo-
substituted position, followed by the one-pot Pd-catalyzed
Suzuki coupling reaction with 1-chloroisoquinoline afforded the
compounds 46 in yields of 7182%. Although these products
have axial chirality, the corresponding racemization barrier is
assumed to be low, which allows the free rotation around the
chiral axis at room temperature.
The synthesis of the desired helicene products 1012,
containing different aryl fragments at the 8-position of the
naphthyl moiety, started with the selective Pd-catalyzed Suzuki
coupling reaction of 1,8-dibromonaphthalene with a series of
boronic acids (Scheme 1). This provided the corresponding
monoarylated products 13 in moderate to good yields of
Scheme 1. Synthesis of arylisoquinolines 46.
With the arylisoquinolines at hand, we decided to
synthesize the axially chiral bromide 7 (see Scheme 2) as a
model substrate. Thus, following a previously developed CH
borylation/bromination methodology,[9, 66] the bromide ()-7 was
obtained from 4 in 65% yield (Scheme 2). The enantiomers were
separated by semipreparative chiral high-performance liquid
chromatography (see conditions in the Supporting Information);
aR-7 (retention time Rt = 7.11 min) and aS-7 (Rt = 9.05 min)
yielded 10 as enantiomeric M and P forms with moderate
3842% yields and an er > 99:1. The assignment of the absolute
configuration of the helicene was based on the crystal structure
of aS-7.
The helicenes 11 and 12 could be synthesized in principle
in the same way as above described for 10. However, in the
course of the preparation of 10 we noted in an individual
experiment that a small temperature rise to above 60 C during
enantiomers with an enantiomeric ratio (er) > 99:1 were obtained. the lithiation step of 7 caused already a partially racemized
The X-ray single-crystal study of enantiopure aS-7 allowed the
unambiguous assignment of the absolute configuration (see
Scheme 2 and Supporting Information). Finally, a lithiation-
borylation sequence[9] enabled the synthesis of the desired
azabora[5]helicene 10. In short, submitting the enantiopure
bromides aR-7 and aS-7 to a Br-Li exchange reaction at low
temperature (78 C) in tetrahydrofuran, followed by the reaction
of the lithiated intermediate with F-B(Mes)2 (78 C  25 ºC)
helicene product. Therefore we decided to reduce the
racemization risk by synthesizing the compounds 11 and 12 first
as racemic mixture and subsequently performing the
chromatographic separation of the enantiomers.[67] Using the
same CH borylation/bromination strategy as for the synthesis of
()-7 (see above), the axially chiral bromide ()-8 was obtained
as racemate in a yield of 54% (Scheme 3).
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Scheme 2. Preparation of the enantiopure azabora[5]helicene 10 in its M and P form; Mes: mesityl, pin: pinacolato.
Scheme 3. Preparation of the enantiopure azabora[5]helicenes 11 and 12 in their M and P forms. The inset shows the ORTEP diagram of ()-11. The thermal
ellipsoids are drawn at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles []: N2B2 1.666(3), C55B2 1.612(3),
N2C45C54C55 21.0, C46C45C54C63 30.0. Although only the P-11 enantiomer is displayed, the unit cell contains both enantiomers (see CIF file).
However, in the case of the preparation of the NMe2-
derivative ()-9 a partial demethylation reaction was observed,
and therefore, an optimization of the bromination conditions was
required (Scheme 3). Thus, reducing the amount of CuBr2 to 1
()-9 by a reductive amination using paraformaldehyde/NaBH4
and 2,2,2-trifluoroethanol (TFE) as the solvent (Scheme 3).[68]
The subsequent lithiation-borylation sequence of the bromides
()-8 and ()-9 afforded the racemic helicenes ()-11 and ()-12,
equiv. and limiting the reaction time (see Supporting Information), respectively, in moderate yields (49% and 54%, respectively).
the desired bromide ()-9 was obtained in 46% yield together
with the formation of the partially demethylated product ()-9' in
24% yield. The undesired ()-9' was selectively transformed into
Crystals of ()-11 of sufficient quality for single-crystal X-ray
analysis were obtained. The crystal structure (inset Scheme
3)[69] evidenced the helical chirality of the compound. A more
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distorted and bent structure (dihedral angles: N2-C45-C54-C55
21.0, C46-C45-C54-C63 30.0) than the previously crystallized
N,C-chelate[9] without substitution at the 8-position (dihedral
angles: N1-C1-C10-C11 16.8, C2-C1-C10-C19 27.7) was
notorious. The helicene racemates were resolved into the
enantiopure forms by semipreparative CSP-HPLC separation,
and the absolute configurations were first assigned by
band at  > 400 nm, that has been already seen for the
analogous dye that lacks the additional phenyl substitution at the
8-position of the naphthalene.[9] However, in the case of dye 12
this band is just observed as a shoulder and with a smaller
molar absorption coefficient. In both solvents a red-shift of this
band is noted with increasing electron-donor strength of the
phenyl substituent, i.e.,  = 19 nm for 12 versus 10. Even more
significant is the substituent effect on the fluorescence emission.
While a small red-shift of the emission maximum is observed for
the introduction of the OMe substituent, i.e.,  = 711 nm for 11
versus 10 in both solvents, the stronger electron-donating NMe2
substituent in dye 12 has a dramatic effect. For the latter dye a
further red-shifted (max = 586 nm) and considerably broadened
emission band was noted in toluene ( = 91 nm for 12 versus
10). In the more polar acetonitrile dye 12 is only very weakly
fluorescent, showing a very broad band with a maximum at ca.
680 nm. The Stokes shifts are elevated (ca. 4000 cm1 for 10
and 11 and ca. 60008500 cm1 for 12), corroborating the
involvement of charge-transfer phenomena in the photophysical
properties of the helicenes.
comparison of the optical rotation ([]20 96.8º for P-11 and
D
94.8º for M-11, c 0.05 in CHCl3; []20 72.7º for P-12 and
D
70.2º for M-12, c 0.1 in CHCl3) with that of the previously
assigned P-10 ([]20 106.4º; c 0.1 in CHCl3). This assignment
D
was further confirmed by the correspondence of experimental
and calculated ECD spectra; see Supporting Information.
Table 1. Photophysical properties of the dyes 1012 in aerated solutions.
abs,max
(nm)[a]
fluo,max

fluo
107×kr
107×knr
[d]
fluo
1
[c]
1
[f]
1 [g]
[
(nm)[b]
(cm
)
(ns)[e]
(s
)
(s
)
1
(M cm1)]
Toluene
414
[8200]
420
[7400]
433
[4000]
4.8
(89%)
7.7
(81%)
19.9
(86%)
10
11
12
495
502
586
4000
3900
6000
0.29
0.30
0.13
6.0
3.9
15
9.1
4.4
0.65
Acetonitrile
411
10
6.4
(85%)
9.9
(92%)
[i]
499
4300
4300
8500
0.28
4.4
2.9
11
[7600]
418
[7000]
11
510
0.29
7.2
430
[3400]
12
~680[h]
<0.005
[a] UV/vis absorption maximum of the long-wavelength band (>400 nm). In
square brackets the corresponding molar absorption coefficients are
provided. [b] Maximum of the fluorescence emission. [c] Stokes shift. [d]
Fluorescence quantum yield, error 20%. Measured by the relative method
against quinine sulfate as standard or using 4-amino-N-propyl-1,8-
naphthalimide as secondary reference (for 12); see Experimental Section. [e]
Fluorescence lifetime, error 5%. Only the main decay component is given
(weight in parenthesis). [f] Radiative decay rate constant; kr = fluo/fluo. [g]
Non-radiative decay rate constant; knr = (1fluo)/fluo. [h] The dye, that was
purified with the greatest care, showed a small fluorescence signal (at 500
nm) of a very minor impurity (not detectable in CSP-HPLC; see Supporting
Information). This signal was safely discarded based on its non-matching
excitation spectrum. Excitation at the red edge of the absorption spectrum
yields the undisturbed observation of a very broad emission at ca. 680 nm
(see Supporting information), having an excitation spectrum that matches the
absorption spectrum of 12. [i] Not determined due to very low fluorescence.
Figure 2. UV/vis-absorption and fluorescence spectra of the helicenes 10 (top),
11 (middle), and 12 (bottom) in aerated toluene solution. The dye
concentration was ca. 1030 M.
The fluorescence quantum yield is ca. 0.3 for the
azabora[5]helicene dyes 10 and 11 in toluene and in acetonitrile.
For dye 12 this value is somewhat lower in toluene (fluo = 0.13),
but still maintained at a significant level. However, such drop is
not unexpected for charge-transfer fluorophores such as 12,
resulting from a more efficient non-radiative deactivation (e.g.,
internal conversion, reverse charge transfer) of energetically
lower lying emissive states. This observation is often referred to
as energy-gap law.[9-11] Directly related is the observation that in
the polar acetonitrile the charge-transfer state seems
energetically stabilized to such extent that only a very weak and
further red-shifted emission is observed for 12. For the dyes 10
and 11 the fluorescence lifetimes were determined as ca.
4.87.7 ns in toluene and ca. 6.49.9 ns in acetonitrile, being
UV/vis-Absorption and Fluorescence Spectroscopy
The photophysical properties of the dyes were determined in
toluene and in acetonitrile solution, using UV/Vis-absorption
spectroscopy, fluorescence spectroscopy, and time-resolved
single-photon counting. This yielded insights into the electronic
structure of the investigated azaborahelicene dyes. The key data
are summarized in Table 1 and spectra are shown in Figure 2.
The dyes 10 and 11 show a typical long-wavelength absorption
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10.1002/chem.201801908
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longest for dye 11 in both solvents (see Table 1). However, for
12 a remarkably long lifetime of 19.9 ns as main component of
the decay trace was found in toluene. For further insight the
radiative (kr) and non-radiative (knr) decay rate constants were
calculated (Table 1). As expected for a fluorescence quantum
yield much smaller than 1, the non-radiative decay has a
substantial contribution. The ratio knr/kr for 10 and 11 is
comparable (ca. 2.32.5), but dye 12 stands out with a much
long-wavelength band, corresponding to the first transition, is
inverted when comparing the ECD spectra of the corresponding
enantiomers of 10 versus 11 or 12. This switch of sign is
confirmed by time-dependent density-functional theory (TD-DFT)
calculations of the rotational strength, as will be discussed below.
more dominant non-radiative decay (knr/kr
=
6.8). The
thermodynamically more favorable charge-transfer in 12 lowers
the energy of the emissive state and the energy-gap law
promotes the excited-state decay via non-radiative channels.
Figure 4. CPL spectra of 10 (left), 11 (middle), and 12 (right) in aerated
toluene solution. The dye concentration was ca. 250400 M (see
Experimental Section). The red spectra correspond to the P-isomer and the
black spectra to the M-isomer. The spectra are normalized so that the I value
at the emission maximum reflects directly the |glum| value.
Drawing on the significant emission of the herein
investigated helicenes the measurement of circularly polarized
luminescence (CPL) was attempted; see Figure 4. Dye 10
showed nearly mirror-imaged CPL spectra for the two
enantiomers, peaking at ca. 480 nm. Besides, a small band at
ca. 540 nm was detected. While an artifact is safely excluded
(see measurement conditions in Experimental Section), the
origin of this band is not entirely clear. The dissymmetry factor
was determined as |glum| = 2.5 × 104. The OMe-substituted 11
showed a stronger CPL spectrum with a maximum at 500 nm
and a dissymmetry factor of |glum| = 9.5 × 104. In accordance
with the conventional emission spectroscopic measurements,
dye 12 showed strongly red-shifted CPL signals with a maximum
at about 590 nm. The dissymmetry factor |glum| reaches a value
of 3.5 × 103. Recently, it was shown for distorted helical Bodipy
architectures that an increase of charge-transfer character of the
Figure 3. ECD spectra of 10 (left), 11 (middle), and 12 (right) in aerated
toluene solution. The dye concentration was ca. 250400 M (see
Experimental Section). The red spectra correspond to the P-isomer and the
black spectra to the M-isomer.
Chiroptical Spectroscopy
The separated M- and P-enantiomers were submitted to
electronic circular dichroism (ECD) spectroscopy in toluene.
Both dyes show nearly perfect mirror-imaged Cotton effects for
the two enantiomeric forms (see Figure 3). Hence, the ECD
spectra will be discussed in the following only for one of the
enantiomers (M-form). M-10 has positive bands at 298 nm ( =
11 M1cm1) and at 430 nm ( = 5 M1cm1) and a negative
band at 317 nm ( = 8 M1cm1). This translates to a
dissymmetry factor of |gabs| = 7 × 104 for the longest wavelength
ECD band (430 nm). For helicene M-11 positive bands at 298
nm ( = 18 M1cm1) and at 347 nm ( = 8 M1cm1) and a
negative band at 320 nm ( = 8 M1cm1) were observed. At
452 nm a very weak negative band was seen ( ca. 1 M1cm1).
The |gabs| value for the ECD maximum at 347 nm is 1.1 × 103,
while for 452 nm the corresponding value is 5.6 × 104. Finally,
for M-12 positive bands at 303 nm ( = 28 M1cm1) and 392
nm ( = 13 M1cm1) as well as a negative band at 458 nm (
= 6 M1cm1) were observed. The dissymmetry factor for the
first transition is |gabs| = 2.0 × 103 at 458 nm. The sign of the
emitting state yields higher dissymmetry factors as
a
consequence of a reduced electric transition dipole moment.[64]
We find this theoretically expected relation nicely confirmed in
the herein investigated dye series, spanning |glum| values of
about one order of magnitude when comparing 10 and 12.
Interesting is the comparison between the herein observed
values of the dissymmetry factors with those of other helicenes
and helicenoids.[32] The gabs factors are well in the range of those
observed for small [n]helicenes (n = 46). The same can be
affirmed for the glum factors of 10 and 11, while the value for 12
is higher than usually observed for this class of CPL-SOM.
Recently, the correlation of both g factors for the most common
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CPL-SOM architectures was investigated.[32] The ratio glum/gabs
for 10 is 0.36. This is in accordance with the observation for
other [4]- and [5]-helicenes. The same ratio for 11 and 12 is 1.70
and 1.75, respectively, being clearly outliers in the gabs-glum plot
(see Supporting Information). In the first case (dye 11) the high
ratio is due to the very weak ECD band for the lowest energy
transition at 452 nm. For 12 a more accentuated change of
electronic structure upon excitation, being in line with the
comparably more pronounced charge-transfer character of the
dye, is held responsible for the observation.
In accordance with the observations made for the first
transition band in the ECD spectrum, the signs of the CPL
emission band of the corresponding enantiomers are inverted for
10 versus 11 or 12. Further, the sign of the long-wavelength
ECD band coincides with the sign of the CPL signal. This would
be expected for the case that the lowest-energy absorption
transition and emission are corresponding[70] and similar
observations were made for other helicenes.[48, 71] In the case of
charge-transfer fluorophores this relation is not ad hoc expected,
because the excited state that is populated by absorption and
the emissive state are significantly different (as expressed by the
rather large Stokes shift, see Table 1). However, for the herein
investigated dyes the chiroptical properties seem conserved in
going from the ground- to the excited-state, yielding
correspondence of the first-transition ECD and CPL signs.
Density-Functional Theory Calculations
Taking into account the rotational flexibility of the mesitylene
rings and in order to shine more light on the chiroptically active
conformations of the investigated azabora[5]helicene dyes,
density-functional theory (DFT) and time-dependent density-
functional theory (TD-DFT) calculations at the cam-
B3LYP/def2SVP level of theory were performed. For dye 10 two
different conformers were found, which differ in the relative
orientation of the mesitylene groups (see Figure 5 for M-10). The
energetically most stable conformation (10a, Figure 5) is the
main contributor (97%). Dye 11 presents additional
conformational freedom through the rotation of the OMe group.
However, the resulting conformers show only minor energetic
differentiation (0.2 kcal mol1). The main conformations of
helicene 11 (see 11a and 11b in the Supporting Information),
considering the relative orientation of the mesitylene groups,
contribute with a total of 97%. Noteworthy, the mesitylene
groups orient in a very similar way as for 10a. The same
dominant conformation (97%) is also present for compound 12
(see structure 12a in the Supporting Information). In
consequence,
the
main
conformers
present
nearly
superimposable structures for the three compounds (see
Supporting Information). However, it is noticeable that the ECD
spectra are quite different. In all cases, the calculated ECD
spectra, obtained by Boltzmann averaging the contributions of
the individual conformers, reproduce very well the
experimentally observed spectra of the three helicenes (see
Supporting Information).
Table 2. TD-DFT calculated absorption and emission energies (E), dipole
strength (D), rotational strength (R), and angle formed by electric (E) and
magnetic transition moments (M) of the principal conformers in their ground
state and in their first excited singlet state.
D
R
Conformation
[weight %][a]
Ecalc
Eexp
EM
40
44
(10
(10
(eV)
(eV)
angle (º)
esu2cm2)
esu2cm2)
Ground state
3.35
(S1←S0)
2.99
2.95
2.95
2.86
10a [97]
146045
133666
135995
91810
18.24
16.19
2.15
84
96
3.30
(S1←S0)
11a [58]
11b [39]
3.31
(S1←S0)
91
3.19
(S1←S0)
Figure 5. Conformations of helicene 10 (M-isomer). The conformer on the left
is the main contributor (97%). Note that the two conformers only differ in the
orientation of the two mesitylene units. Similar conformations with respect to
the mesitylene groups were observed for the helicenes 11 and 12 (see
Supporting Information).
12a [97]
Excited state
10a
114
50.32

2.69
(S1→S0)
2.50
2.47
2.47
2.12
183491
161906
168502
66611
13.89
87
97
2.63
(S1→S0)
11a
11b
12a
31.19
11.09
75.97
The frontier orbitals for the most stable conformations of
the dyes 1012 (see above) were calculated in the ground state
(see Figure 6 for the M-isomers). The HOMO orbital of the three
dyes presents some significant differences. While this orbital is
mainly localized on one of the mesitylene groups in the case of
10, it is partially delocalized from the mesitylene towards the
phenyl-OMe in 11 and dominantly located at the phenyl-NMe2
moiety in 12. Hence, the HOMO localization follows the electron-
donor capacity of the phenyl residue, clearly supporting the
charge-transfer character of the dyes, especially of 11 and 12.
However, the LUMO of all three dyes is always localized on the
2.65
(S1→S0)
92
2.60
(S1→S0)
129
[a] The weight of the main conformations in the ground state is given in
parentheses. See structures of conformations in Figure 5 (for helicene 10) and
in the Supporting Information (for 11 and 12). Similar conformations with
respect to the mesitylene groups were observed for all dyes. In the case of 11,
two conformers are given, differing by the orientation of the OMe group. All
calculations were done for the M-isomer.
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ARTICLE
helicene backbone. The calculated S1←S0 transition energy
follows nicely the trend of the absorption maxima of the
helicenes, see Table 2.
differences are observed concerning the HOMO orbitals: in
particular for 10 and 11 the HOMO orbital calculated in the
excited-state geometry is less delocalized on the mesitylene
groups and involves only the helicene moiety and the phenyl or
substituted phenyl group. Especially pronounced differences are
observed for compound 11 for which the (negative) rotational
strength is significantly enhanced in the excited state in
comparison to ground state; this is reflected also by the
observation that the angle between the electric and magnetic
transition moment is further deviated from 90° (Table 2). At the
bottom line this allows us to explain the intense CPL signal of 11,
despite the low rotational strength of the first transition, observed
in ECD spectroscopy.
To gain additional insight into the origin of the changes of
the chiroptical properties upon substitution, particularly
considering the change in sign of the first ECD transition of dye
10 versus dye 11 or 12, the dipole (D) and rotational strengths
(R) for the M-isomers of each dye were calculated (see Table 2).
The first transition for dye M-10 involves a positive rotational
strength, in agreement with the experimental observations made
by ECD spectroscopy. On the contrary, for dye M-11 the
rotational strength of the first transition adopts negative values,
which is again in strict agreement with the experiment. As noted
above, in case of dye M-11 the ECD band corresponding to the
first transition is very weak and calculations show that the sign of
the rotational strength is “non-robust”, i.e., different functionals
lead to a "toggling" of the sign. This is reflected also by the fact
that electric dipole (E) and magnetic dipole transition moments
(M) are nearly perpendicular one to another (see Table 2), a
situation similar to that observed for other simple helicenes.[48, 71]
Finally, dye M-12 presents a strong ECD band corresponding to
the first transition, which matches the calculated quite intense
negative rotational strength.
Conclusions
Conveniently modified four-coordinate organoboron N,C-
chelates with an arylisoquinoline backbone can be separated
into their enantiomeric azabora[5]helicenes. Some of these new
dyes show intense CPL emission with significant dissymmetry g
factors. The CPL can be spectrally fine tuned by drawing on the
charge-transfer character of the emissive state. Hence, the
variation of the electron-donating substituents enables the
observation of efficient CPL above 500 nm or even provides a
means to obtain CPL signals close to 600 nm in the case of
helicene 12. The different substituent effects on the sign of the
ECD and CPL spectra can be rationalized by TD-DFT
calculations. The fruitful combination of charge-transfer effects
and chiroptical properties provide an approach for the tailored
design of SOM-CPL architectures.
.
Experimental Section
Materials. The details of the synthetic procedures of the helicenes 1012
(see Schemes 13) and their precursors as well as the analytical
characterization data can be found in the Supporting Information.
Optical Spectroscopy. Conventional absorption and fluorescence
measurements were done on aerated acetonitrile or toluene solutions of
the compounds (1020 M, assuring that the absorbance at the
excitation wavelength did not exceed 0.2) in quartz cuvettes of 1 cm
optical pathlength. The UV/Vis-absorption spectra were registered with a
UVPC-1603 spectrometer (Shimadzu). Steady-state fluorescence
spectra were obtained with a standard fluorimeter (Varian Cary Eclipse)
with a pulsed Xe lamp. The samples were excited by irradiating into the
longest wavelength absorption band of the dyes. The fluorescence
Figure 6. Contour plots of the frontier orbitals, HOMO (bottom) and LUMO
(top), of the helicenes 10, 11, and 12. Note that the M-isomer is shown.
The three compounds have been optimized also in their
first excited state at the same level of theory as adopted for
ground state (cam-B3LYP/def2SVP). Structural information is
reported in Supporting Information also showing graphically how
the different structures superimpose. The excited-state geometry
differs from the ground state structure for both compounds 10
and 11, but it is on the contrary quite similar for compound 12.
The calculated rotational strengths reproduce the observed CPL
results, especially the experimentally observed switch in sign for
the corresponding enantiomers of 10 versus 11 or 12 (see Table
2). Regarding the frontier orbitals that are involved in the
emissive S1→S0 transition (see contour plots in the Supporting
Information), one can observe that the LUMO orbitals are very
similar to the orbitals calculated for the ground state. Some
quantum yields of 10 and 11 are referenced against quinine sulfate in
73]
0.05 M H2SO4 (fluo = 0.55) as standard,[72,
while dye 12 was
measured against 4-amino-N-propyl-1,8-naphthalimide (fluo = 0.48 in
acetonitrile).[11] The emission lifetimes were measured by means of time-
correlated single-photon-counting with a FS920 fluorescence equipment
from Edinburgh Instruments. A picosecond pulsed diode laser was used
employed as excitation source (EPL-445: exc= 442.2 nm, pulse width
78.3 ps) and the fitting of the kinetic decay traces was done by
deconvolution from the instrument response function.
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pathlength were used first for ECD and then for CPL measurements,
adopting concentrations of 0.25 mM, 0.3 mM, and 0.4 mM for 10, 11, and
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75]
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This work was financially supported by the Spanish Ministry of
Economy, Industry, and Competitiveness (CTQ2014-54729-C2-
1-P for U.P., CTQ2013-48164-C2-1-P, CTQ2013-48164-C2-2-P
for A.R., Ramon y Cajal contract RYC-2013-12585 for A.R., PhD
fellowship BES-2015-074458 for Z.D.), the ERDF, the Junta de
Andalucía (2012/FQM-2140 for U.P. and 2012/FQM-1078 for
A.R.), Fondazione Cariplo and Regione Lombardia (Big&Open
Data Innovation Laboratory, BODaI-Lab, University of Brescia).
We further acknowledge the use of computer and software
facilities at CINECA - Via Magnanelli 6/3 40033 - Casalecchio di
Reno (Bologna), Italy. A.R. thanks Prof. J. M. Lassaletta (CSIC,
Univ. Seville) and Prof. R. Fernández (Univ. Seville) for their
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Keywords: boron • helicene • push-pull chromophores • circular
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Entry for the Table of Contents
ARTICLE
Zoe Domínguez, Rocío López-
Rodríguez, Eleuterio Álvarez, Sergio
Abbate, Giovanna Longhi,* Uwe
Pischel,* and Abel Ros*
Page No. – Page No.
Azabora[5]helicene Charge-Transfer
Dyes Show Efficient and Spectrally
Variable Circularly Polarized
Luminescence
A new twist! The dye family of borylated arylisoquinolines (BAI dyes) has new
members. Enantiopure azabora[5]helicenes with varying charge-transfer character
were prepared and their photophysical properties were determined. Spectrally
variable and intense circularly polarized luminescence with high dissymmetry
factors was observed. Further, the charge-transfer character of the dye determines
the sign pattern of the chiroptical spectra.
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