Fluorescence of a chiral pentaphene derivative derived from the hexabenzocoronene Motif

Article abstract of DOI:10.1039/c9cc05451k

A new fluorescent pentaphene derivative is presented that differs from hexabenzocoronene (HBC) by one carbon atom in the basal plane skeleton. A 500% increased fluorescence quantum yield is measured compared to the HBC derivative. The pentaphene compound,

Full text of DOI:10.1039/c9cc05451k

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Soyka, S. Brülls, J. Er, K. Hoffmann, J. Beerhues, B. Sarkar, U. Resch-Genger and S. Eigler, Chem.
Commun., 2019, DOI: 10.1039/C9CC05451K.
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DOI: 10.1039/C9CC05451K
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Fluorescence of a Chiral Pentaphene Derivative Derived from the
Hexabenzocoronene Motif
Philipp Rietsch,[a] Jan Soyka,[a] Steffen Brülls,[d] Jasmin Er,[a] Katrin Hoffmann,[b] Julia Beerhues,[c]
Received 00th January 20xx,
Accepted 00th January 20xx
Biprajit Sarkar,[c] Ute Resch-Genger,[b]* and Siegfried Eigler[a]*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new fluorescent pentaphene derivative is presented that differs similar vibronic fine structure at about 500 nm.[5, 7] Another dye
from hexabenzocoronene (HBC) by one carbon atom in the basal class related to HBCs are hexapyrrolohexaazacoronenes or
plane skeleton. A 500% increased fluorescence quantum yield is hybrids with HBC as reported by Müllen et al. in 2013.[20] In
measured compared to the HBC derivative. The pentaphene 2017, Jux et al. prepared a racemic [5]helicene, structurally
compound, obtained by a modified Scholl oxidation, is also closely related to HBCs, that contains one pyrrole moiety.[21]
emissive in the solid-state, due to the packing motif in the crystal.
Recently, high ꢀFl values exceeding 80% were found for
oxa[7]superhelicenes, realized by ether-bridging two
hexaphenylbenzene moieties, followed by oxidation. This
yielded a chiral fluorescent oxa[7]superhelicene. Another
strategy that can lead to a high fluorescence are push-pull
systems. This was as e.g. reported for HBCs substituted by B-, N-
Hexabenzocoronenes (HBCs), a subgroup of polyaromatic
hydrocarbons (PAHs), have 42 sp2-carbon atoms assembled by
linking seven benzene rings. The resulting properties make
them interesting for optical and electronic applications,[1-5] self-
6]
assembly,[2, surface functionalisation[7] and the bottom-up
containing
derivatives.[22-23]
Herein, we present the synthesis and optical-spectroscopic
characterization of an enantiomeric pentaphene derivative
with 41 sp2-carbon atoms. Compound
could be prepared by
implementing an unprecedented Scholl oxidation step (Fig.
).[24] First, a Diels-Alder reaction[25-26] between 1,4-bis(4-(tert-
butyl)phenyl)buta-1,3-diyne and was performed to yield
hexaarylbenzene 3. Under Scholl oxidation conditions
compound (conditions: CH3NO2, 12 eq. FeCl3, room
temperature (RT), 15 h) was transformed into compound in
moieties
and
diaminodicyanoquinone-
synthesis of graphene.[8-10] The HBC scaffold has D6 symmetry.
Through the variation and arrangement of the substituents, a
plethora of derivatives with a wide range of different chemical
properties and of well-defined symmetry (C3, C2 and C1) were
synthesized.[7, 11-12]
4,
4
Pentaphenes, another interesting subgroup of PAHs, have been
synthesized since the 1940s, mostly by cycloaddition reactions
and subsequent oxidation.[13-17] The fluorescence properties of
pentaphene derivatives, which emit light between 400 and 500
nm, were studied by Fetzer et al.[18-19] However, neither the
fluorescence quantum yields (ꢀFl) nor an emission in the solid-
state were reported.[18-19] The pentaphene motif can also be
found in HBCs, which show a red-shifted fluorescence with a
4
2
1
3
4
80% yield, due to an unprecedented ring- closing reaction. The
ring-closing reaction between the alkyne moiety and the
benzene ring (Fig. 1A step iii) proceeds in analogy to the
synthesis of, e.g. corannulenes, HBCs or naphthalenes.[27-30] The
analytical characterization of
4 including single-crystal X-ray
a. M. Sc. Philipp Rietsch, Prof. Dr. Siegfried Eigler; Institute of Chemistry and
Biochemistry
analysis, is given in the Supporting Information (SI). We tested
the reproducibility of this reaction by synthesizing the
phenanthrene derivative, 9-(4-methoxyphenyl)phenanthrene,
starting from 2-((4-methoxyphenyl)ethynyl)-1,1'-biphenyl (SI).
This reaction was performed in dry dichloromethane with
0.9 eq. FeCl3 with a yield of 69%. In addition, for our
Freie Universität Berlin Takustraße 3, 14195 Berlin, Germany.
E-mail: siegfried.eigler@fu-berlin.de
b. Dr. Ute Resch-Genger, Dr. Katrin Hoffmann; Bundesanstalt für Materialforschung
und -prüfung (BAM), Department 1, Division Biophotonics, Richard Willstätter
Straße 11, 12489 Berlin, Germany, E-Mail: ute.resch@bam.de
c. M. Sc. Julia Beerhues, Prof. Dr. Biprajit Sarkar
Institute of Chemistry and Biochemistry, Freie Universität Berlin, Fabeckstraße 34-
36, 14195 Berlin, Germany
photophysical property studies, HBC
5 was synthesized as
reference compound by a reported one pot reaction of
hexaphenylbenzene and tert-BuCl with FeCl3 acting as oxidant
and Lewis acid catalyst.[31]
d. Chamlers University of Technology SE-412 96 Gothenburg, Sweden
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Compared to the HBC core with 42 sp2-carbon atoms, the (EtOH; polar and protic). The absorption spectra and normalized
View Article Online
DOI: 10.1039/C9CC05451K
missing carbon atom in compound
4 induces chirality as shown fluorescence spectra of both compounds in CH are shown in
by the structural analysis. Moreover,
fluorescence, both in solution, as well as in the solid-state.
4
reveals a strong Fig. 2. Whereas the molar absorption coefficients reach values
of about 60,000 L mol-1 cm-1 at 360 nm for both compounds, the
ꢀFl of is about five times higher than that of compound
Table 1 and Fig. 2).
4
5
(
Fig. 2. Absorption spectra (black) and fluorescence spectra (blue, normalized and
multiplied by the correspondingly measured ꢀFl) of compound 4 (solid) and 5 (dashed)
in cyclohexane. The fluorescence quantum yield of 4 (ꢀFl = 11%) in cyclohexane is about
five times higher than that of HBC 5 (ꢀFl = 2 %) in this solvent. Both compounds were
measured at a concentration of 4x10-6 mol/L and excited at 375 nm.
In addition, we measured fluorescence maps (excitation
emission matrices (EEM); see Fig. S12 and SI, Fig. S11 and
Fig. S12). For both compounds, excitation at 370 nm leads to
the highest fluorescence (Fig. S12) with the fluorescence
maxima being located between 470 and 500 nm. The emission
spectrum of
structure compared to
4
reveals a more pronounced vibrational fine
and resembles, with its two main peaks
5
and shoulders, the emission spectra of the pentaphene
derivatives investigated by Fetzer et al..[18-19]
An overview of the absorption and emission maxima together
with the Stokes shift, which is between 100-130 nm for both
compounds, is given in Table S2 *
.
The ꢀFl and the mean
(derived from the multiexponential
in the three solvents used are
always exceeds that
, reaching a maximum of 16% in DCM compared to 3%
obtained for . The mean fluorescence lifetimes of are also
always longer than those of with values of 17.2 ns (CH) and
21.4 ns (DCM) compared to 11.0 ns (CH) and 14.2 ns (DCM),
respectively. Temperature-dependent studies of the decay
kinetics of both compounds in the temperature range of -100 °C
to 50 °C (see SI, Fig. S14) reveal an increase in mean
fluorescence lifetime by about a factor of 3.5 when comparing
fluorescence lifetimes
decay kinetics) of
τ
4
and 5
summarized in Table 1. Obviously, ꢀFl of
of
4
Fig. 1. A) Synthetic route to the pentaphene derivative 4 and hexabenzocoronene
5
(HBC) 5.
A
Diels-Alder reaction between
1
and dialkyne
2
leading to
5
4
hexaphenylbenzene 3 was used. Trimerization of alkyne 2’ leads to
hexaphenylbenzene 3’. Cyan: pentaphene moiety; green: pyrene moiety. i) 24 h, 250 °C
in diphenylether; ii) Co(CO)8, dioxane, 24 h, reflux; iii/iv) CH3NO2, 12 eq. FeCl3, 15 h, RT.
The inset shows solutions of the respective compound in dichloromethane under
irradiation at 366 nm. The respective ꢀFl is given below each compound. B) Single-crystal
X-ray structures of 4 and 5. Ellipsoids are drawn at 50% probability.
5
To gain first insights into the photophysics of these compounds,
mean lifetimes recorded at 50 °C (
4: 18.02 ns, 5: 7.93 ns)
we analyzed the photophysical characteristics of
4 in
and -100 °C ( : 63.23 ns, : 27.67 ns), respectively. An increase
4
5
comparison to in three solvents of varying polarity and
5
in fluorescence lifetime upon cooling has been observed for
many dye classes including rhodamines[32] and the recently
analyzed diaminodicyanoquinones.[22] It typically suggests that
proticity, here cyclohexane (CH; apolar and aprotic),
dichloromethane (DCM; medium polarity, aprotic) and ethanol
* As the absorption and emission spectra are in a similar
wavelength region, the Stokes shifts are provided on a
wavelength scale and not on an energy scale.
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at least one non-radiative decay channel involves a rotation that structure of compound
is slowed down or even completely hindered at very low structure with an inter-planar distance of 3.44 Ȧ.
temperatures.[33]
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5
is classified as a sandwich herringbone
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DOI: 10.1039/C9CC05451K
Table 1. ꢀFl and amplitude-weighted mean fluorescence lifetimes τ of 4 and 5 in
solvents of different polarity. Values for the normalized Dimroth-Reichardt
N
Parameter ET were taken from Ref. [34]
.
4
5
N
ET
Solvent
CH
DCM
EtOH
Solid
ՓFl [%]
11
16
11
11
τ [ns]
17.2
21.4
17.8
-
ՓFl [%]
τ [ns]
12.4
14.2
11.0
-
0.207
0.386
0.654
2
3
2
2
Surprisingly,
4
shows an intense solid-state fluorescence with
is only
2% (Table 1 and Figure S15). The solid-state fluorescence of is
ꢀFl=11% (Fig. 3 and Fig. S15-S17 in the SI), while ꢀFl of
5
4
bathochromically shifted by about 35 nm compared to the
emission in DCM although the spectral shape and vibronic fine
structure of the emission band is only slightly altered and
broadened.
Fig. 4. Arrangement of the molecules of 4 in the crystal. A) Tert-butyl groups in green and
the helical moieties in red. The interplanar distance is 9.24 Ȧ. B) Enantiomeric pair of 4.
Blue M(-) and turquoise P(+).
The π-π interactions as well as C-H-π interactions determine this
packing motif. This dense packing in the crystal might induce an
aggregation induced quenching of the fluorescence. In contrast,
the pentaphene
4 presents a β-system with graphitic planes
(
Fig. 4) separated by 9.24 Ȧ. The space between the π-planes is
filled by the bulky tert-butyl groups (Fig. 4). The pentaphene
derivatives published by Fetzer et al. have a distance of the π-
planes of about 3.52 Ȧ.[17] As the inter-planar distance in
4 is
induced by the helicene motif (Fig. 5A), which is absent for
we compared the angles between the planes A-D of with those
of [4]helicene (Fig. 5
Table 2).[39] The A-D angle of 32.46 ° of
compared to 26.68 ° ([4]helicene) indicates a stronger curvature
of , which is mainly induced by ring D, with 20% (A-D) up to
5,
Fig. 3. Comparison of the solution (blue, in DCM) and solid-state fluorescence (green) of
4. The absorption spectra in solution (DCM) is shown in black. The excitation wavelength
4
was 355 nm, respectively. The solid-state fluorescence, recorded with
a spectral
,
4
scanning Laser scanning microscope (CLSM), is bathochromically shifted by about 35 nm.
The insets show the fluorescence in solution (left, blue frame) and in the solid-state
(right, green frame).
4
almost 100% (C-D) increased angles compared to [4]helicene.
The latter is often observed for solid-state emission
(inhomogeneous broadening). The Stokes shift is thus increased
from 130 nm in solution to 165 nm in solid-state. Whether this
shift originates from polarity and/or crystal packing effects or a
combination of is only 4 nm. In addition, the solid-state
emission-band of
extended from 600 nm in solution to 700 nm (Fig. S15 and S17).
To correlate the solid-state fluorescence of (see Fig. 3) with
the molecular structure and the crystalline packing motif[35], we
studied the crystal structure of and . The pentaphene 4
5 is broadened by about 100 nm and thereby
4
4
5
ꢁ
crystallizes in a triclinic lattice with the space group P1 , similar
to the previously mentioned helicene-HBC hybrid.[21] As
Fig. 5. A) Schematic presentation of the 4[helicene] moiety in compound 4 with the
crucial benzene rings defined A-D. B) 4[Helicene]. C) Steric hindrance between hydrogen
atoms in an extract of compound 4.
discussed by Müllen et al. the π-system of 5 is slightly bent due
to the sterically demanding tert-butyl groups.[36] According to
the four basic packing types of aromatic systems,[37-38] the
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DOI: 10.1039/C9CC05451K
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(
Fig. 5A), and the three hydrogen atoms (Fig. 5C). In addition,
the helical chirality is visible in the crystal structure as
enantiomeric pairs (Fig. 4B). The enantiomers of [4]helicenes
cannot be separated at RT as the racemisation barrier is below
25 kcal/mol.[40-41] Nevertheless, we used density functional
theory (DFT) at the B3LYP/6-31G(d) level of theory to calculate
the racemisation barrier of 4 [42-43]
Fig. S18).
,
which is about 10 kcal/mol
(
Table 2. Angles between planes A-D (Fig. 5A) of 4 and [4]helicene[39] derived from
crystallographic data.
A-B
A-C
A-D
B-C
B-D
C-D
4
5.24°
16.76°
32.46°
11.86°
27.23°
16.54°
[4]
helicene
10.26°
18.12°
26.68°
7.87°
16.55°
8.83°
In conclusion, we reported a novel pentaphene derivative,
which is closely related to HBCs, however, with only 41 sp2-
carbon atoms, instead of 42. Consequently, the pentaphene 4 is
chiral and crystallizes as enantiomeric pair with a distance of the
π-planes of almost 1 nm. This substitution pattern results in an
enhanced fluorescence in solution and the solid state compared
to the HBC motif. The presented synthetic strategy to
pentaphenes could lead to more advanced derivatives with
improved photophysical properties.
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There are no conflicts to declare.
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