Diaminodicyanoquinones: Fluorescent Dyes with High Dipole Moments and Electron-Acceptor Properties

Article abstract of DOI:10.1002/anie.201903204

Fluorescent dyes are applied in various fields of research, including solar cells and light-emitting devices, and as reporters for assays and bioimaging studies. Fluorescent dyes with an added high dipole moment pave the way to nonlinear optics and polari

Full text of DOI:10.1002/anie.201903204

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Accepted Article
Title: Diaminodicyanoquinone – A Novel Class of Fluorescent Electron
Acceptor Dyes with High Dipole Moments
Authors: Philipp Rietsch, Felix Witte, Sebastian Sobottka, Gregor
Germer, Alexander Becker, Arne Güttler, Biprajit Sarkar,
Beate Paulus, Ute Resch-Genger, and Siegfried Eigler
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903204
Angew. Chem. 10.1002/ange.201903204
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10.1002/anie.201903204
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Diaminodicyanoquinone – A Novel Class of Fluorescent Electron
Acceptor Dyes with High Dipole Moments
Philipp Rietsch,[a],† Felix Witte,[a],† Sebastian Sobottka,[c] Gregor Germer,[a] Alexander Becker,[a] Arne
Güttler,[b] Biprajit Sarkar,[c] Beate Paulus,[a]* Ute Resch-Genger,[b]* and Siegfried Eigler[a]*
Dedicated to Professor Hans-Ulrich Reißig on the occasion of his 70th birthday
Abstract: Fluorescent dyes are applied in various fields of research,
including solar cells, light emitting devices and reporters for assays
and bioimaging studies. An additional high dipole moment paves the
road to nonlinear optics and polarity sensitivity. Redox activity allows
45%.[9] Consequently, DADQ derivatives are not considered as
fluorescent dyes. Conformational relaxations can have a high
impact on fluorescence,[10] as exploited e.g. for aggregation-
induced emission,[11] where blocking these pathways is used to
generate a signal amplification or used as sensing principle for
viscosity[12] and temperature responsive molecules.[10b, 13]
to
switch
the
molecule’s
photophysical
properties.
Diaminodicyanoquinone derivatives possess high dipole moments,
yet only low fluorescence quantum yields, and have therefore been
neglected as fluorescent dyes. Here we investigate the fluorescence
properties of diaminodicyanoquinones using a combined theoretical
and experimental approach and derive molecules with a fluorescence
quantum yield exceeding 90%. The diaminodicyanoquinone core
moiety provides chemical versatility and can be integrated into novel
molecular architectures with unique photophysical features.
Applications of fluorescent dyes range from light harvesting, in
solar cells,[1] light emitting devices,[2] reporters for the life
sciences,[3] molecular switches[4] and non-linear optics.[4-5]
7,7,8,8-Tetracyanoquinodimethane (TCNQ) is a strong electron
acceptor used in conductive donor-acceptor systems like TCNQ-
tetrathiafulvalene compounds.[6] An interesting sub-class are
diamino-substituted TCNQ derivatives, namely diamino-
dicyanoquinones (DADQs), that possess electron donating and
accepting moieties linked by a π-system resulting in high dipole
moments of 10–20 Debye.[7] DADQ derivatives have been
discussed as novel materials, e.g. for applications in non-linear
optics.[7a] However, their fluorescence quantum yields (QY) in
solutions are generally below 0.5% due to non-radiative
relaxations of the excited state on the ps timescale.[8] Only in the
solid state, DADQs are moderately emissive with QY of up to
[a]
M. Sc. Philipp Rietsch, M. Sc. Felix Witte, M. Sc. Gregor Germer, B.
Sc. Alexander Becker, Prof. Dr. Beate Paulus, Prof. Dr. Siegfried
Eigler
Institute of Chemistry and Biochemistry
Freie Universität Berlin
Figure 1: A) Synthetic route starting with the activation of TCNQ by reaction
with pyrrolidine and subsequent reaction with the respective diamine (shown
ethylene diamine) to yield compounds 1–7. B) Chemical structures of 1–4. C)
Photograph of 3 (left) and 4 (right) under illumination with 366 nm light with the
respective fluorescence quantum yields in DMSO. Right side: schematics of the
mesomeric forms of 4. The dihedral angles are referred to as Dα and Dβ.
Takustraße 3, 14195 Berlin, Germany.
E-mail: b.paulus@fu-berlin.de
E-mail: siegfried.eigler@fu-berlin.de
Here we demonstrate substitution pattern control of QY in DADQs,
with QY exceeding 90% (see Figure 1). Theoretical calculations
reveal that benzene functionalized DADQs show high QY as their
molecular structures remain close to the ground state
conformation after photoexcitation without being submitted to
internal conversion (IC) or intersystem-crossing (ISC).
Accordingly, DADQ derivatives may now be considered as
fluorescent dyes and thus applications, such as optical switches
or non-linear optics are coming into reach.
[b]
Dipl.-Ing.(FH) Arne Güttler, Dr. Ute Resch-Genger
Bundesanstalt für Materialforschung und –prüfung (BAM),
Department 1, Division Biophotonics
Richard Willstätter Straße 11, 12489 Berlin, Germany.
E-mail: ute.resch@bam.de
M. Sc. Sebastian Sobottka, Prof. Dr. Biprajit Sarkar
Institute of Chemistry and Biochemistry
Freie Universität Berlin
Fabeckstraße 34-36, 14195 Berlin, Germany
co-first authors contributed equally to the manuscript
[c]
[†]
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
deactivation of 1–3. By increasing the fraction of polyethylene
glycol (PEG) in ethanol (PEG-EtOH) from 0% to 75%, we observe
a 3 – 7-fold increase in fluorescence intensity for 1–3 (Figure 3
and Figure S7), resulting in an increase of QY of 2 from 0.4% to
2.9%. In contrast, the fluorescence of 4 is barely affected by an
increase in solvent viscosity (Figure S7D).
At first 7,7-diamino-8,8-dicyanoquinomethan (TCNQ) was
reacted with pyrrolidine to activate the substitution of the geminal
cyano groups of TCNQ in the 7,7 position.[14] This 7-pyrrolidino-
7,8,8-tricyano quinomethane (PTCNQ) reacts with the respective
amine to compounds 1–4 (Figure 1A) in 40–80% yields. The
compounds were characterized by 1H and 13C NMR, FTIR, EA
and MS. In addition, the absorption and fluorescence properties
were investigated in selected solvents.
Figure 3: Absorption spectrum and emission spectra of 3 in EtOH-PEG mixtures
ranging from 0 to 75% PEG in steps of 25%, depicted by the red arrow. Solid
0%, dash 25%, dot 50%, dash-dot 75%. With an increasing ratio of PEG and
hence increasing viscosity, the fluorescence intensity increases by a factor of
3.3.
Figure 2: Absorption (black) and fluorescence spectra (blue) of 3 (dashed) and
4 (solid) in acetonitrile (normalized with respect to the fluorescence of 4). The
vertical orange bars represent transitions of compound 3 (349 nm, oscillator
strength: 1.02 a.u.) and 4 (363 nm, oscillator strength: 1.30 a.u.) calculated
using TD-DFT at the CAM-B3LYP level. Insets: Difference densities between
ground state and excited state of 4 (left) and 3 (right) to visualize the electron
flow during the electronic excitation. Blue and red areas correspond to areas of
For compounds 1–4 the fluorescence intensity decreases by
more than 30% with increasing temperature in the temperature
range of 40 °C to 110 °C (Figure S9). Upon decreasing the
temperature (273 K–173 K), the fluorescence intensity increases
qualitatively for 1–4 (Figure S11). A closer look at the shape of
the fluorescence signals indicates that various fluorescent states
are populated (Figure S12). As viscosity effects and aggregation
upon these temperature changes are not thoroughly studied, the
observed trends are qualitative.
The experimental results indicate that rotations play a major role
to control the QY. In particular, the N-substituted derivatives 2 and
7 (Figure 1 and 4), twisted in their ground state structure at Dα
(Figure S17C), are non-fluorescent. In analogy to 4, compounds
5 (Br-derivative) and 6 (CN-derivative), both sharing the benzene
unit, are highly fluorescent (Figure 4 and Table 1).
electron enhancement and depletion, respectively. Isovalue = 0.005 Å−3
.
The molar absorption coefficients are solvent dependent and
reach values of up to 58,000 L mol-1 cm-1 in acetonitrile (ACN) as
depicted in Figure 2 for 4. While the unstructured absorption band
lies between 300 and 400 nm, the fluorescence is shifted
bathochromically by 40-90 nm (25x104-11.1x104 cm-1).
The difference densities between ground state and excited state
(inset Figure 2) illustrate the shift in electron density from the
dicyanomethane to the amine groups upon photoexcitation. This
transition leads to a notable reduction of the dipole moment in the
excited state from approximately 30 D to 22 D (Table S9), well
known for DADQs, and is the reason for the observed negative
solvatochromism (Figure S2 and Table S2).[15] In addition, the S0-
S1 transition in 4 is extended across the enlarged π-system,
including the phenyl ring. The, absolutely measured, QY values
are summarized in Table 1. Compound 4 shows very high QY in
all solvents, with values of 92% in dimethyl sulfoxide, (Figure 1C)
around 70% in tetrahydrofuran, dimethylformamide and
acetonitrile and 56% and 35% in ethanol and methanol,
respectively. In contrast, 1 and 3 show QY in the range of 0–12%
and 2 QY of < 1%. The fluorescence lifetimes of 1–4 and the
calculated radiative and non-radiative rate constants (Table
S3/S4) do not show a straightforward correlation with the QY data.
QY measurements in solvents of varying viscosity confirm the
assumption that rotations around the dihedral angles Dα and Dβ
Figure 4: Chemical structure of compounds 4–7 with symbolic pictures of their
fluorescence.
play
a significant role for the non-radiative excited state
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Table 1: Fluorescence quantum yields of compounds 1–7 in solvents of different polarity. Values for the normalized Dimroth-Reichardt Parameter ETN were taken
from Ref. [16]
1
2
3
4
5
6
7
N
ET
Solvent
THF
Փ [%]
7
Փ [%]
< 1
Փ [%]
7
Փ [%]
73
Փ [%]
34
Փ [%]
69
Փ [%]
< 1
3
0.207
0.386
0.444
0.460
0.654
0.762
DMF
7
< 1
10
72
62
81
DMSO
ACN
10
7
< 1
7
92
39
90
< 1
< 1
< 1
< 1
< 1
12
71
18
53
EtOH
MeOH
< 1
< 1
< 1
< 1
< 1
56
63
70
< 1
35
20
30
Theoretical Investigation
rotational barriers at Dα and Dβ do not critically influence each
other (Figure S17A/B), which is why we will treat Dα and Dβ
separately. In the S0, rotation around Dβ at the Franck-Condon
point is hindered by large barriers of around 60 kJ/mol for all
compounds, but a rotation around Dα is possible except for 2 and
7 where the N-substituents block the way (Figure S17C/S18A).
In contrast, in the S1 rotation around Dα is prevented with barriers
of 60–100 kJ/mol, but rotation around Dβ shows barriers of around
12 kJ/mol (1–3) to 25 kJ/mol (4, 7) (Figure S17D/Figure S18B).
Large barriers arise as a rotation around Dβ diminishes the basis
function overlap between the π-orbitals centered at the phenyl
ring and those at the nitrile groups in the HOMO, while a rotation
around Dα decreases the overlap between the phenyl ring’s π-
orbitals and the p-orbitals of the amine groups in the LUMO. Low
barriers are explained by nodes present in the HOMO at Dα and
in the LUMO at Dβ, where an intramolecular torsion would not
impede any orbital overlap (Figure 6).
To gain more insight into the optical properties of our DADQ
derivatives and to elucidate the remarkable QY of 4, we
performed time-dependent density functional theory (TD-DFT)
calculations at the CAM-B3LYP[17] level for 1–4 and 7 using ACN
as the solvent with CPCM (e = 37.5).[18]
A prerequisite for a high QY is a close similarity between the
relaxed structures of the excited state and the ground state
without IC or ISC events influencing the photoexcitation process.
The fluorescence deactivation of the DADQ species may be
controlled by two photoinduced intramolecular torsions at dihedral
angles Dα and Dβ (Figure 5).
Figure 5: Two kinds of intramolecular torsion angles (shown for 2) that affect
the fluorescence deactivation mechanism in DADQs: Dα (left) and Dβ (right).
To our surprise, the ground (S0) and excited state (S1) structures
of all compounds hardly differ (Figure S16) along with rather
constant oscillator strengths (Table S8). As this would imply all
compounds show similar QY, a more detailed look at the potential
energy surfaces (PES) of the intramolecular rotations is needed.
To account for the influence of multireference states we opted for
the DFT/MRCI method originally proposed by Grimme and
Waletzke[19] and revised by Marian et al.[20] with a Kohn-Sham
orbital basis generated at the CPCM/BHLYP/def2-TZVP[21] level
to obtain accurate energies. Test calculations showed that the
Figure 6: HOMO (left) and LUMO (right) obtained at the BHLYP/def2-TZVP
level. Rotations (indicated by arrows) are favored due to nodal planes in the
orbitals, while orbital overlap keeps the structures rigid at Dβ in the HOMO (left)
and at Dα in the LUMO (right), isovalue = 0.003 Å-3.
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SVP basis set and fixed solvent molecules to save computational
resources (Figure S21B). While the total barrier hardly changed
for 1, the barrier height for 4 increased by roughly 30%. Thus, the
solvent shell may play a significant role at least in the case of 4,
where it likely contributes to its large QY.
To further characterize 1–4, we performed cyclic voltammetry
(CV) experiments in a 0.1 M Bu4NPF6 solution of DMF (Figure
S14). The molecules show a complex redox behaviour, with
mostly reversible oxidations (for 4) and irreversible reductions
(quasi reversible for 2), which likely involve the proton of the
secondary amine and provide a hint for an electron transfer
followed by a chemical reaction (EC mechanism).[22] As expected,
the substitution of cyano moieties for secondary and tertiary
amines favours oxidation processes, which are not observed for
TCNQ (Figure S15). This also renders the molecules more
electron-rich and causes a cathodic shift of the reduction
potentials. (see Table S7 for more details). The HOMO-LUMO
gaps were calculated at the CPCM/BHLYP/MRCI level. The
trends between experiment and theory show overall agreement
(Table 2). The electrochemically measured gaps differ slightly in
absolute values, as the solvent model neglects bulk effects at the
electrode.[23]
Figure 7: PES scan of the S1 state around dihedral angle Dβ for 1–4 computed
at the CPCM/BHLYP/def2-TZVP/MRCI level. 7 is omitted as its fluorescence
deactivation mechanism is different.
The PES at Dα is less important for 2, 3, and 4, where an
electronic excitation somewhere along the S0 PES will just
recover the S0 minimum structure on the S1 PES through
relaxation. However, this torsion may account for fluorescence
quenching by IC or ISC in 7 and slightly in 1. For 1–4, QY loss is
expected to mainly occur due to the relatively small rotational
barrier in the S1 at Dβ (Figure 7). As expected, the respective
rotational barrier is highest for 4 and lowest for 1. This can be
attributed to the stabilization of the ground state geometry of 4
through the extended π-system. Using Arrhenius’ equation at T =
300 K, we define krot-1 (the reciprocal rate constant for the rotation
around Dβ, (Equation S5)) as a relative measure to compare
rotational barriers. Relating krot-1 for 1–4 illustrates that 4 is up to
480 times more likely to fluoresce after photoexcitation into the S1
than 1-3:
Table 2: Electrochemical, theoretical, and optical data of Compounds 1-4 with
the resulting orbital energies and HOMO–LUMO gaps (Eg) [eV].
Theoryc)
Experiment
CV data[a]
optical datab)
Eogpt [eV]
EHOMO
[eV]
ELUMO
[eV]
Eegl
[eV]
Etgheo [eV]
1
2
3
4
-2.37
-2.42
-2.27
-2.22
0.16
0.12
2.53
2.54
2.13
2.01
3.31
2.87
3.16
2.94
3.24
3.49
3.17
2.98
-0.14
-0.21
krot-1 (1) : krot-1 (2) : krot-1 (3) : krot-1 (4) = 1: 3 : 12 : 481.
ISC events between singlet and triplet surfaces contribute to
fluorescence quenching in all compounds. For 7 the most likely
deactivation mechanism is via an ISC of the S1 with T1 and T2
along Dα (Figure S19A). This mechanism may play a role for 1 as
[a] Orbital energies calculated from the first reduction or oxidation half-wave
potentials referenced to FcH/FcH+(-4.80 eV): ε = (-(E1/2 – 4.80) eV [b] The optical
HOMO-LUMO gap was determined from the onset at the red edge of the
absorption band [c] CPCM/BHLYP/def2-TZVP/MRCI.
well, however, it is assumed to have
a smaller impact
(Figure S19B). At Dβ all compounds show a singlet/triplet
separation energy of around 0.7 eV at the Franck-Condon point,
which should be just large enough to prevent ISC events.
However, as the S1 and the T0 eventually come very close at an
angle of around 90° (Figure S20) in all molecules, the rotational
barrier presented by krot-1 and the non-radiative relaxation through
ISC events are directly linked. Since 1–3 are much more likely to
overcome the rotational barrier at Dβ than 4, their QY is
significantly lower.
While this qualitatively accounts for the drastic differences in QY,
it does not fully explain the large QY of 4 by itself. As solvent
effects have only been included in an implicit way, an explicit
solvent cavity of ACN was set up (Figure S21A) and the rotational
barrier at Dβ was recalculated for 1 and 4 with the smaller def2-
Conclusion
We synthesized highly fluorescent diaminodicyanoquinones
showing quantum yields of over 90% when benzene substituted.
We attribute the high fluorescence quantum yields mainly to
restricted internal rotations while quenching occurs through the
intersection of singlet and triplet states. However, other effects,
such as molecular aggregation, may contribute as well. The
electrochemical characterization of these molecules reveals
complex redox activity. In summary, we could show that
diaminodicyanoquinones derivatives are
fluorescent dyes customizable for photophysical applications, the
potential of which needs to be studied in more detail.
a novel class of
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B.P.
and
F.W.
acknowledge
the
Deutsche
Forschungsgemeinschaft (DFG) for financial support within the
collaborative research area “Multivalenz als chemisches
Organisations- und Wirkprinzip: Neue Architekturen, Funktionen
und Anwendungen” SFB 765 and the High-Performance
[8]
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financial support. URG gratefully acknowledges financial support
by DFG (RE1203/23-1)
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Keywords: dipole moment • fluorescence • quantum yield •
quinone
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10.1002/anie.201903204
Angewandte Chemie International Edition
COMMUNICATION
Philipp Rietsch, Felix Witte, Sebastian
Sobottka, Gregor Germer, Alexander
Becker, Arne Güttler, Biprajit Sarkar,
Beate Paulus,* Ute Resch-Genger,* and
Siegfried Eigler*
Page No. – Page No.
Diaminodicyanoquinone – A Novel
Class of Fluorescent Electron
Acceptor Dyes with High Dipole
Moments
Controlling the internal rotation around dihedral angle Dβ in diaminodicyanoquinones in the excited
states, determines the fluorescence quantum yield exceeding 90%.
This article is protected by copyright. All rights reserved.