Tetraaryl Cyclopentadienones: Experimental and Theoretical Insights into Negative Solvatochromism and Electrochemistry

Article abstract of DOI:10.1002/ejoc.202001091

The synthesis of a series of tetraaryl cyclopentadienones comprising different substitution patterns is reported. Their photophysical and electrochemical properties are investigated by UV/Vis spectroscopy and cyclic voltammetry as well as by supporting quantum chemical simulations and reveal a distinct effect of substituents on the redox behavior of the molecules as well as the absorption properties of this class of compounds. While electrochemical data display a shift in reduction potential of up to 200 mV between the differently substituted cyclopentadienones, their photophysical investigations in differently polar solvents suggest a negative solvatochromic effect, although protic solvents induce a bathochromic shift. Crystal structure analyses of some derivatives confirm similarity with related cyclopentadienones while providing insight into intermolecular C–H···O and C–H···π interactions in the solid state.

Full text of DOI:10.1002/ejoc.202001091

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Tetraaryl Cyclopentadienones
Tetraaryl Cyclopentadienones: Experimental and Theoretical
Insights into Negative Solvatochromism and Electrochemistry
Nicolas Meitinger,[a] Alexander K. Mengele,[a] Kamil Witas,[a] Stephan Kupfer,[b] Sven Rau,[a]
and Djawed Nauroozi*[a]
Abstract: The synthesis of a series of tetraaryl cyclopentadi- potential of up to 200 mV between the differently substituted
enones comprising different substitution patterns is reported. cyclopentadienones, their photophysical investigations in differ-
Their photophysical and electrochemical properties are investi- ently polar solvents suggest a negative solvatochromic effect,
gated by UV/Vis spectroscopy and cyclic voltammetry as well although protic solvents induce a bathochromic shift. Crystal
as by supporting quantum chemical simulations and reveal a structure analyses of some derivatives confirm similarity with
distinct effect of substituents on the redox behavior of the mo- related cyclopentadienones while providing insight into inter-
lecules as well as the absorption properties of this class of com- molecular C–H···O and C–H···π interactions in the solid state.
pounds. While electrochemical data display a shift in reduction
Introduction
The electronic nature of cyclopentadienones (CPDs) comprising
a polarized C=O bond and a reactive cyclopentadiene ring ren-
ders a rich chemistry, accessible for long known oxidation, re-
duction or halogenation reactions.[1] Yet, recent developments
Figure 1. Representation of the antiaromatic character of cyclopentadienone
(left) and the electro-reduction of the molecule to the corresponding stabi-
lized radical anion (right).
in functionalization of CPDs towards molecules suitable for
materials science applications or in catalysis[2,3] underline the
versatility and unique properties of this class of compounds.
Perhaps the most prominent transformation is the suitability of
CPDs for Diels-Alder reactions as dienophiles leading to polyar-
omatic hydrocarbons. Following a subsequent dehydrogenation
route, extended π-conjugated systems such as nanographenes
are obtained.[4–7] Apart from extensive synthetic efforts, the cy-
clopentadiene core and related heterocyclic fulvenes such as
CPDs have served as valuable molecules for theoretical investi-
gations on the concept of (anti-)aromaticity.[8–15] As such, re-
cent charge density studies have demonstrated the antiaro-
matic character of the cyclopentadienone core in both unsub-
stituted and tetraaryl-substituted CPDs with an unchanged di-
rection of polarity of the carbonyl group (Figure 1).[15]
In context of synthetic accessibility, cyclopentadienone itself
turned out to be highly reactive and tends to dimerize. How-
ever, many stable tetraaryl-substituted CPDs have been synthe-
sized via Knoevenagel reaction between 1,3-diarylacetones and
1,2-diketones allowing various substituents and a modular de-
sign. Transition metal-catalyzed reactions of differently substi-
tuted alkynes with suitable precursors have been used for the
efficient synthesis of CPDs as well.[1,16,17] Particular interest, pre-
sumably owing to their intense colors, has been devoted to the
photophysical properties of CPDs. The electrochemical proper-
ties though, have been reported in only a few works mainly
dealing with electro-oxidation or -reduction reactions of
CPDs.[18–21] While reduction of CPDs led to a fairly stable radical
anion species in absence of protons (Figure 1), electro-oxidation
has been shown to yield in α-pyrones.[21,22]
[a] N. Meitinger, A. K. Mengele, K. Witas, Prof. Dr. S. Rau, Dr. D. Nauroozi
Institute of Inorganic Chemistry I
Albert-Einstein-Allee 11, 89081 Ulm, Germany
E-mail: djawed.nauroozi@uni-ulm.de
https://www.uni-ulm.de/nawi/nawi-anorg1/
In the course of our efforts for the development of organic
molecules acting as suitable energy storage materials,[23] we
got interested in CPDs as a member of the carbonyl family,
which recently turned to a promising class of energy storage
compounds.[24–26] Herein, we present the synthesis of a series
of tetraaryl cyclopentadienones aiming to readdress phenyl and
heterocyclic substitution effects on both available electrochem-
ical window as well as photophysical properties. As such, we
examine the solvatochromic effects of these compounds with
combined experimental and detailed theoretical investigations
[b] Dr. S. Kupfer
Friedrich Schiller University Jena, Institute of Physical Chemistry
Helmholtzweg 4, 07743 Jena, Germany
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202001091.
© 2020 The Authors published by Wiley-VCH GmbH · This is an open access
article under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
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t
Scheme 1. i) NaH, toluene, Ar, 2 h reflux; 0 °C, HCl. ii) AcOH, HCl, H2O, 130 °C, 18 h. iii) R2 = Bu, EtOH, H2O, KCN, 4 h reflux. iv) Cu(OAc)2·H2O, NH4NO3,
80 % AcOH, 4 h reflux. v) R2 = OMe, Br: 1,2-dibromoethane, THF, Mg, 2 h reflux. vi) 1,4-dimethylpiperazine-2,3-dione, 2 h, 55 °C; HCl (1
reflux; CPDs 8 & 9: ethylene glycol, reflux, 30 min.
M), 0 °C. vii) EtOH, KOH,
on the nature of electronic excitations with implicit solvation bond lengths exhibit values of 1.357(2) Å, 1.519(2) Å, 1.359(1) Å
models using density functional theory (DFT) and time-depend- and 1.354(2) Å, 1.520(2) Å as well as 1.354(2) Å, respectively,
ent DFT (TDDFT) simulations.
which is in good agreement with similar tetra-aryl-substituted
cyclopentadienones.[15]
Results and Discussion
Synthesis
A modular strategy is used for the synthesis of CPDs allowing
the introduction of different substituents at the cyclopentadi-
ene core. Homocoupling of commercially available para-substi-
tuted alkyl 2-(phenyl)acetate derivatives via modification of
Romer's one-pot Claisen condensation[27] gave 1,3-diarylprop-
anones, while benzil derivatives were accessible either via
benzoin condensation and subsequent oxidation[28] or Grignard
reaction of arylmagnesium bromide with 1,4-dimethylpiper-
azine-2,3-dione.[29] Knoevenagel condensation of the precur-
sors in ethanolic KOH solutions gave CPDs 1–7 in good to mod-
erate yields (see SI).[29,30,39–41,31–38] For the pyridine containing
CPDs 8–9, commercially available 2,2′-pyridil was used in
Knoevenagel condensation reactions.[18] In contrast to the syn-
theses of CPDs 1–7 shown in Scheme 1, condensations gave
the monohydrated species bearing a hydroxyl group at the cen-
tral cyclopentadiene ring and needed harsh conditions for the
elimination of water to form the dienes. Utilization of POCl3 did
not give the desired products, while boiling the intermediates
in ethylene glycol under reflux was successful.[42] In total, nine
differently substituted CPDs were obtained, and investigated
both photophysically and electrochemically.
Figure 2. Solid-state structure of CPD3. Ellipsoids are drawn at the 50 % prob-
ability level and intermolecular short contacts (sum of vdW radii –0.1 Å, right)
are indicated by dashed lines.
Although the tert-butyl groups enhance the size of the C3
and C4 bound aryl rings, no increased tilt for these rings with
respect to the cyclopentadiene core compared to unsubstituted
CPD1 was observed. This might be explained by the para-loca-
tion of the tert-butyl-group; hence they do not come into close
proximity with the neighboring aryl rings. This is supported by
a recently published crystal structure bearing a C3 bound
phenyl ring with a nitro group in meta position. It was found
that this aryl ring exhibits a tilt of nearly 90° with respect to
the cyclopentadienone plane.[15] Additionally, the absence of a
pronounced effect of even very large para-located substituents
on this torsion angle has been found in various other structures
as well.[43,44] Similar to many other tetraaryl-substituted cyclo-
pentadienones, the torsion angle of the two phenyl rings
bound to the C2 and C5 is smaller than for the two rings bound
to C3 and C4.[15,43–48] This can be explained by the fact, that
the C2 and C5 substituents possess access to “open space”
around the C=O group, whereas the C3 and C4 substituents
experience steric pressure from two neighboring aryl rings.
It has been hypothesized that the relatively flat positioning
Crystal Structure Analysis
Crystals suitable for solid-state structure determination were
obtained by slow evaporation of CDCl3 from NMR samples of
CPDs 1 and 3 (see Figure 2). Both molecules exhibit very similar
bond lengths for the central cyclopentadienone core. The C1–
O1 bond lengths are 1.212(1) Å and 1.215(1) Å for compound of the C–O neighbored phenyl rings allows for stabilization of
CPD1 and CPD3, respectively. For the parent molecule and the the partially negatively charged O atom by weak intramolecular
tert-butyl-substituted version the C2–C3, C3–C4 and C4–C5 C–H···O interactions.[43] However for CPD1 and CPD3 no such
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short intramolecular contacts were found. Instead, several inter-
molecular C–H···O and C–H···π interactions were observed to
dominate the solid-state packing behavior of these compounds
(see Figure 2). Especially for the unsubstituted tetraphenyl
cyclopentadienone a pairwise stabilization of two molecules
sharing coplanar cyclopentadienone rings by a multitude of
these interactions was found. As these interactions are not sym-
metrically distributed over the molecule with respect to its cen-
tral mirror plane intersecting the C1–O1 and C3–C4 bond, for
every phenyl ring bound to the cyclopentadienone core a
different torsion angle was detected.
Finally, it should be noted that the introduction of N-atoms
into the aryl ring, resulting in CPD8, leads to similar solid-state
structures as for the above-mentioned species. However, a net-
work of both C–H···O as well as C–H···N short contacts contrib-
ute to the packing of CPD8 resulting in an anti-conformation
of the two pyridyl-rings (see Figure S67).
Table 1. Absorption maxima and molar absorption coefficients for the elec-
tronic spectra of CPDs 1–9 in MeCN or benzonitrile solutions.[a]
max
max
max
Compound
λ3
λ2
λ1
ελ3
ελ2
ελ1
CPD1
CPD2
CPD3
CPD4
CPD5[b]
CPD6
CPD7[b]
CPD8
CPD9
259
(22600)
264
(26380)
261
(21075)
265
329
(5635)
327
(5460)
344
(7740)
343
(8120)
370
(9835)
337[c]
(6650)
340
(7838)
320
(6300)
320
497
(1140)
509
(1620)
497
(930)
512
(1460)
554
(1458)
537
(1850)
528
(1680)
491
(24670)
–
273
(22950)
–
255
(26875)
260
(1460)
504
(26430)
(5600)
(1850)
Photophysical Characterization
[a] Absorption maxima are reported in nm, molar absorption coefficients in
L mol–1 cm–1. [b] Measurements performed in benzonitrile. [c] Shoulder with
λmax of 384 nm.
Absorption spectra of tetraaryl cyclopentadienones usually
feature a pronounced high energetic transition at around
260 nm, herein referred to as λ3, a broad transition between
320–350 nm, λ2, and a less pronounced broad transition peak-
ing mostly between 490–540 nm, λ1. It has already been shown,
that para-substitution of the two chemically different phenyl
rings within symmetrical tetracyclones strongly impacts the
photophysical properties of the resulting derivatives,[49,50] while
pairwise symmetrical substitution has been shown to result in
increased effects compared to the monosubstituted ana-
logues.[51] Thus, the absorption properties of CPDs 1–9 offering
electron donating or electron accepting characteristics were in-
vestigated in acetonitrile or benzonitrile (Figure 3). The perti-
nent data are summarized in Table 1. It is noteworthy, that all
CPDs were non-emissive in acetonitrile solutions at ambient
conditions.
effects, seem to have no effect on peak positions here[50] (CPDs
6–7), while electron withdrawing heterocycles led to hypso-
chromic shifts of λ2, making them shoulders of the high-energy
λ3 transition. These show the least impact upon substitution
with bathochromic shifts up to 14 nm (0.25 eV) for CPD6.
The polarized nature of cyclopentadienones suggests solva-
tochromic effects, which were analyzed via absorption spectro-
scopy in solvents of different polarity (see SI). Quantitative ab-
CPD1 without any substitution pattern at the periphery ab-
sorbs in a range between 250 and 650 nm, with maxima at 259,
329 and 497 nm, respectively. Substitution of the 2,5-phenyl
rings with inductively electron-donating tert-butyl groups re-
sulted in comparable bathochromic shifts of λ1 of 12–15 nm
(0.06–0.07 eV) for CPDs 2, 4 and 7, whereas the stronger donat-
ing methoxy groups upon their mesomeric effect gave batho-
chromic shifts of λ1 of up to 41 nm (0.19 eV) in CPDs 5 and 6.
As depicted via the photophysical investigation of CPD3, substi-
tution of 3,4-phenyl rings did not affect λ1. However, the intro-
duction of electron withdrawing heterocycles in these positions
in CPDs 8–9 led to 6 nm (0.03 eV) hypsochromic shifts com-
pared to their phenyl analogues.
The second-lowest energetic transition λ2 in the electronic
spectra, in terms, was not quite affected by substitution of the
2,5-phenyl rings in CPD2, while for electron-donating substitu-
ents on 3,4-phenyl rings again bathochromic shifts of λ2 were
observed.
These were in the same way stronger for methoxy groups
(41 nm, 0.42 eV) compared to tert-butyl groups (15 nm,
0.16 eV). Bromine substituents, given the balance of –I and +M
Figure 3. A UV/Vis absorption spectra of CPDs 1–4 in acetonitrile. B UV/Vis
absorption spectra of CPDs 1, 6, 8 and 9 in acetonitrile. C UV/Vis absorption
spectra of CPDs 1, 5 and 7 in benzonitrile due to low solubility in MeCN. The
max
insets show the visible region of the spectra with λ1
.
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sorption spectra were recorded at analogous concentrations in THF and MeCN. To account for a balanced description of local
the range of 0.04–0.01 m
form and THF solutions.
M
in acetonitrile/benzonitrile, chloro- as well as of charge transfer excitations, we applied the long-
range corrected CAM-B3LYP[52] functional with the def2-SVP ba-
sis set.[53,54] Implicit solvent effects on equilibrium structures
and excited state properties were considered by a polarizable
continuum model[55,56] (THF and MeCN), see SI for more details
regarding the computational setup.
Upon changing the solvent from acetonitrile to less polar
THF and CHCl3 bathochromic shifts, mainly of λ1 and λ2 absorp-
tion maxima were observed for all CPDs. In case of CPD6, the
detected redshift of the λ1 absorption maximum was 12 nm in
THF and 17 nm in chloroform, respectively (Figure 4). Moreover,
for CPD4 these solvatochromic shifts were most pronounced
with 23 nm for λ1 and 15 nm for λ2 in chloroform.
The performed TDDFT simulations allowed to provide an un-
ambiguous assignment of the electronic excitations underlying
the measured absorption bands. This is shown exemplarily for
the calculated UV/Vis spectrum of CPD6, see Figure 5A, while
the electronic nature of the respective excitations is visualized
based on the charge density differences (CDDs; Figure 5B).
The quantum chemical results clearly reveal the charge
transfer character of the lowest excited singlet state, S1, while
electron density is shifted upon excitation from the anisyl moie-
ties in 2 and 5 positions to the central cyclopentadienone core.
In very good agreement with the experimental results, this state
is predicted by TDDFT in MeCN at 539 nm (2.30 eV), while a
slight redshift is observed in THF (543 nm, 2.28 eV), see Table
S5 for details. This rather unsuspected stabilization of a charge
transfer state by the nonpolar THF vs. the polar MeCN can be
explained by a comparison of the equilibrium structures in both
solvents: As predicted by previous simulations, the energy lev-
els of the frontier orbitals, i.e. of the π-system, considerably
depends on the planarity of the substituted cyclopentadi-
Figure 4. Solvatochromic effects in the UV/Vis absorption spectra of CPD6 in
max
MeCN, THF and CHCl3, the inset showing the visible region with λ2
.
CPD6 shows a slightly stronger λ1 absorption in THF, while
it is comparable in MeCN and CHCl3. However, absorption in λ2
and potentially λ3 was stronger in both THF and CHCl3 in about
the same manner, giving a hint rather on changes in torsion enones.[51a,51b] Ultimately, the size of the π-system is tuned by
angles of the phenyl rings towards the central cyclopentadiene variation of the dihedral angles between the central five-mem-
as compared to the situation in MeCN than stabilization of the bered ring and the substituents in positions 2–5. In case of
excited state by the respective solvent. The observed shifts of CPD6, dihedral angles of 39.8 and 38.2° are predicted in MeCN,
absorption maxima would suggest a negative solvatochromic while with 38.7 and 36.9° slightly smaller angles are observed
effect leading to hypsochromic shifts of absorption maxima
with increasing solvent polarity. In connection with electronic
absorption measurements in different solvents, it is noteworthy,
that some CPDs revealed small bands between 425–475 nm,
which give rise to detailed investigations.
within THF. This marginal partial planarization from the equilib-
rium structure in MeCN to the structure in THF enhances the
overlap of the π-system of the fragments, which leads – in con-
sequence – to a reduced HOMO-LUMO gap (Figure 5C) as well
as to a stabilization of the S1 state. Furthermore, the quantum
In order to elucidate the nature of the observed solvatochro- chemical simulations allow to assign the observed absorption
mic effect in detail, we performed DFT and TDDFT simulations feature between 425–475 nm to a (semi-)dark nCOπ*-state, S2,
for three CPDs, i.e. CPD5, CPD6 and CPD8, within gas phase, at 398 and 402 nm (3.11 and 3.08 eV) in MeCN and THF, respec-
Figure 5. A Simulated UV/Vis absorption spectrum of CPD6 obtained at the TDDFT level of theory (CAM-B3LYP/def2-SVP) within gas phase (reference, in
black), acetonitrile (MeCN, in red) and tetrahydrofuran (THF, in blue); implicit solvent effects were considered by a polarizable continuum model. B Charge
density differences (CDDs) of selected singlet excitations contributing to the electronic absorption spectrum in gas phase are provided exemplarily; charge
transfer occurs from red to blue. C Energy levels of the frontier orbitals, i.e. HOMO-1, HOMO and LUMO, of CPD6 in gas phase, MeCN and THF, all values are
shown relative to the HOMO energy of the gas phase reference system.
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tively, see Figure 5B. The partial optical accessibility of this state
is a consequence of the mixing with ππ* transitions; for details
see Tables S5 and S6. Similar effects have been observed for
the charge transfer state S3 – corresponding to the λ2 absorp-
tion band – as well as for higher lying excited states contribut-
ing to λ3. Further details on the computational results address-
ing ground and excited state properties of CPD6 as well as of
CPD5 and CPD8 are collected in the supplementary informa-
tion.
It is noteworthy that qualitative measurements in the most
polar methanol revealed slight bathochromic shifts, predomi-
nantly for λ1. This finding would not support negative solvato-
chromism but might be explained due to hydrogen bonding
interactions of the protic solvent with the negatively charged
carbonyl oxygen in the excited state. Unfortunately, the per-
formed (TD)DFT simulations, incorporating merely implicit sol-
vent effects, are not suitable to analyze the interaction with
protic solvents, such as methanol.
Table 2. Cyclic voltammetry data of cyclopentadienones 1–9.
1/2[a]
[b]
[c]
Compound
Eox [V]
Ered
[V]
Epc – Epa [mV]
ipa/ipc
CPD1
CPD2
CPD3
CPD4
CPD5
CPD6
CPD7
CPD8
CPD9
1.11
0.97
1.06
0.93
0.71
0.75
1.14
1.05
1.05
–1.28
–1.32
–1.32
–1.35
–1.41
–1.27
–1.21
–1.23
–1.24
87
88
83
95
90
78
115
75
129
0.99
0.99
0.99
0.97
0.92
0.91
0.99
–
0.93
[a] Half-wave potential of the redox processes. [b] Difference of reductive
peak potentials. [c] Ratio of the anodic and cathodic peak currents.
signed as quasi- or irreversible under the chosen conditions.
However, performing cyclic voltammetry measurements in di-
chloromethane led to reversible processes for oxidation of most
CPDs (see SI).
In order to evaluate the kinetics of the reduction processes,
measurements at different scan rates were employed, and elec-
trochemical parameters were calculated to determine the re-
versibility of the events. Figure 7 shows exemplary the recorded
voltammograms for CPD4, and the data for all CPDs are sum-
marized in Table 2. The separation between the oxidation and
reduction peaks of all CPDs are between 78–95 mV. Considering
the theoretical value of 59 mV for a Nernstian, one-electron
transfer couple, all measured data point towards a quasi-reversi-
Electrochemical Investigations
The synthesized series of cyclopentadienones further allowed
the investigation of the impact of different substituents on the
CPDs′ electronic properties. Thus, all compounds have been in-
vestigated electrochemically by means of cyclic voltammetry in
acetonitrile solutions of each compound containing nBu4NPF6
as supporting electrolyte. The voltammograms are shown in
Figure 6 and the pertinent data are summarized in Table 2 (ref-
erenced against Fc+/Fc).
Each cyclopentadienone shows two redox events, both an
oxidation and a reduction wave. The reduction event in all cases
is a reversible process, while the oxidation is quasi- or irreversi-
ble, respectively. CPD1 shows a reduction wave at –1.28 V vs.
Fc+/Fc. CPD2 and CPD3 are reduced at lower negative poten-
tials regardless of the tert-butyl phenyl groups being at the 2,5-
or 3,4-positions of the cyclopentadienone core. This is intuitive
as electron donating groups minimize the tendency of the mo-
lecule for taking up another electron. This trend is emphasized
marginally (30 mV) by substitution of all phenyl rings with tert-
butyl groups in CPD4. Substitution of all phenyl rings with
methoxy groups underlines this trend as the reduction poten-
tial is further shifted to lower values by 60 mV for CPD5
(–1.41 V vs. Fc+/Fc). Keeping the substitution pattern of the 2,5-
positions constant (tert-butyl or methoxy) while introducing
bromine functionalities at 3,4-positions seems to affect the elec-
tronic properties dramatically. As a result, CPD6 is reduced eas-
ier than CPD2 by 50 mV, the same applies for CPD7 and CPD5,
as well, where the substitution of an electron-donating meth-
oxy group with a -withdrawing bromine atom resulted in easier
reducibility by 200 mV. Apparently, introduction of pyridine
moieties instead of phenyl lead to lower reduction potentials
for the molecule, which is intuitive as pyridines are electron
deficient and take up electrons easier upon electrochemical
charging. In total, variation of substitution pattern at cyclo-
pentadienone moiety allows for shifting the reduction potential
up to 200 mV within the studied series of CPDs.
Figure 6. Cyclic voltammograms of CPDs 1–9 (except 7) in the order of their
reduction potentials [highest (bottom) and lowest (top)] value for reduction
potential. Measurements were performed in 0.02 m
M MeCN solutions of each
compound containing 0.1 nBu4NPF6 at a glassy carbon as working elec-
M
While the reduction events of the CPDs suggest a rather re-
versible behavior, the oxidation events in all CPDs can be as-
trode, Pt wire as counter electrode and Ag wire as reference electrode. Scan
rate: 200 mV s–1. All data referenced against Fc+/Fc.
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ble event, although with our setup a peak separation of 77 mV electrochemical investigations, where the total electron density
was measured for the ferrocene couple as reference during of the systems determined the redox behavior. Investigations
these studies. However, the ratio of cathodic to anodic peak on stabilization of the excited states by solvents indicated nega-
currents is in all cases ≈ 1 and the current density follows a tive solvatochromism when using differently polar solvents. This
linear fashion when correlated to the square root of the scan is surprising as the dipole moment in the excited state is usually
rate (Figure 7 and Figure 8).
increased due to its charge transfer nature and a bathochromic
shift of the absorption spectrum with increasing solvent polar-
ity is expected. However, a partial planarization of the ground
state equilibrium structure in nonpolar solvents, i.e. THF, leads
to an enhanced overlap of the π-system and, thus to a reduced
HOMO-LUMO gap and a stabilization of the S1 state as pre-
dicted by (TD)DFT simulations.
Given the different substitutions within the investigated cy-
clopentadienone series, electrochemical analyses displayed a
rational access for manipulating the redox behavior of the mo-
lecules based on the electronic nature of the substituents. As
such, electron withdrawing groups lower the reduction poten-
tial of a molecule while electron donating groups require higher
potentials towards reduction. Following this design, molecules
with a difference of reduction potential up to 200 mV were
accessed. Owing to the modular synthetic accessibility and the
observed features, we will pursue the design of further func-
tionalized cyclopentadienones and their possible use as energy
storage materials.[41–43] More detailed investigations into the in-
triguing correlations between molecular structure and photo-
physical properties are underway.
Figure 7. Cyclic voltammograms of CPD4 at different scan rates exemplary
for all CPD derivatives (see SI). Peak potentials are unchanged upon different
scan rates (dotted lines shall guide the eye).
Experimental Section
1,3-diphenylpropan-2-one, benzil, 4,4′-dibromobenzil and 2,2′-
pyridil were purchased and used as delivered from Sigma-Aldrich.
Reagents were of commercial grade and used without further purifi-
cation. Solvents were received from commercial sources and dis-
tilled prior to use or purified and deaerated using a MBraun SPS 800
solvent purification system. NMR spectra were recorded on a Bruker
Avance III HD 400 at 293 K and processed with MestReNova software
(Version 12.0.0). The chemical shifts δ are reported in parts per
million (ppm). 1H- and 13C-NMR shifts are referenced according to
the used deuterated solvent as internal standard. Constants J are
presented as absolute values in Hz. For the characterization of
the NMR signals the following abbreviations are used: s = singlet,
d = doublet, t = triplet, q = quartet, m = multiplet and dd = doublet
of doublets. Elemental analysis (CHN) were measured in duplicates
using a Elementar Vario MICRO cube. MALDI-TOF mass spectra were
measured on a Bruker Daltonics REFLEX III equipped with a pulsed
nitrogen laser (λ = 337 nm). Data were processes with Bruker Dalto-
nik Compass DataAnalysisViewer software (Version 5.0). Values are
given in fractions m/z. For determining molar absorption coeffi-
Figure 8. Plot of peak current density as a function of (scan rate)1/2 for CPD4
showing a linear dependency.
cients, two 20 mL stock solutions (4 × 10–4
before measurements and diluted directly into 1 cm quartz cuvettes
to 4 × 10–5 , 3 × 10–5 , 2 × 10–5 and/or 1 × 10–5
respectively,
M) were freshly prepared
Conclusion
M
M
M
M
A series of nine tetraaryl cyclopentadienones comprising differ-
ent substituents at the periphery of the aryls were successfully
synthesized. The results obtained from UV/Vis spectroscopy,
quantum chemical simulations and electrochemical analyses of
the compounds revealed the effect of para-functionalization of
the phenyl rings or introduction of heteroatoms on the elec-
tronic effects, especially stabilizing or destabilizing polarized
excited states. Whereas the position of substituents played a
using suitable Hamilton microliter syringes. Each compound was
measured at least twice at three different concentrations showing
linear absorption behavior. No emission was found for the CPD de-
rivatives in MeCN under ambient conditions.
Literature-known compounds CPDs 1–8 were synthesized following
standard procedures given in the SI.
1,3-Bis(4-methoxyphenyl)propan-2-one (1c): Synthesis was car-
pivotal role in absorption maxima shifts, this was negligible in ried out upon a modified literature procedure.[27] Sodium hydride
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(2.77 g, 115 mmol) was dispersed in dry toluene (11 mL) in a 50 mL 159.49, 153.72, 131.91, 131.56, 126.32, 124.65, 124.42, 114.04,
Schlenk flask and methyl-2-(methoxy-phenyl)acetate (11.35 g, 113.90, 55.72. EA [%] calcd. C 78.55, H 5.59; found C 75.33, H 5.15.
10 mL, 63 mmol) was added dropwise (ca. 1 h), while a constant
stream of Argon was remained. Upon complete addition, the mix-
ture was refluxed for 2 h (75 °C). After cooling in an ice bath, it was
slowly added (ca. 2 h) to an ice cold HCl (15 mL + 5 mL of H2O)
under vigorous stirring. Toluene was removed under reduced pres-
sure and glacial acetic acid, HCl and water (8:4:4 mL) added to the
residue of the organic phase. The mixture was refluxed overnight
at 130 °C. The hot mixture was filled into a pre-heated separating
funnel, leading to rapid precipitation of the product, and extracted
five times into 200 mL of hot n-hexane. Crystallization gave color-
less needles. The solvent of the filtrates was removed and the resi-
due recrystallized from ethyl acetate to increase yields to 47 %
(4.09 g). 1H-NMR (400 MHz, CDCl3, 25 °C) δ = 7.07 (d, J = 8.7 Hz, 4H),
6.86 (d, J = 8.7 Hz, 4H), 3.80 (s, 6H), 3.65 (s, 4H) ppm. 13C-NMR
(101 MHz, CDCl3, 25 °C) δ = 206.54, 158.77, 130.60, 126.22, 114.24,
110.07, 55.34, 48.14 ppm.
HRMS: expected: 504.19367, found 504.19310.
2,5-Bis(4-methoxyphenyl)-3,4-bis(4-bromophenyl)cyclopenta-
1
dienone[40] (CPD6): 39 % isolated yield. H-NMR (400 MHz, CDCl3,
25 °C) δ = 7.34 (d, J = 8.4 Hz, 4H), 7.15 (d, J = 8.8 Hz, 4H), 6.84–6.73
(m, 8H) 3.79 (s, 6H) ppm. 1H-NMR (400 MHz, CD2Cl2, 25 °C) δ =
7.35 (d, J = 8.5 Hz, 4H), 7.15 (d, J = 8.9 Hz, 4H), 6.87–6.75 (m, 8H),
3.78 (s, 6H) ppm. 13C-NMR (101 MHz, CDCl3, 25 °C) δ = 200.73,
159.35, 151.26, 132.23, 131.62, 131.50, 131.09, 125.15, 122.94,
122.83, 113.94, 55.73 ppm. 13C-NMR (101 MHz, CD2Cl2, 25 °C) δ =
201.16, 159.87, 152.03, 132.85, 131.95, 131.91, 131.56, 125.65,
123.37, 123.17, 114.21, 55.73 ppm. EA [%] calcd. C 61.82, H 3.68;
found C 58.60, H 3.36. HRMS: expected: 601.99152, found
601.99095.
2,5-Bis(4-tert-butylphenyl)-3,4-bis(4-bromophenyl)cyclopenta-
dienone[36,41] (CPD7): 42 % isolated yield. 1H-NMR (400 MHz, CDCl3,
25 °C) δ = 7.35 (d, J = 8.5 Hz, 4H), 7.28 (d, J = 8.5 Hz, 4H), 7.14 (d,
J = 8.6 Hz, 4H), 6.82 (d, J = 8.6 Hz, 4H), 1.30 (s, 18H) ppm.
1H-NMR (400 MHz, CD2Cl2, 25 °C) δ = 7.36 (d, J = 8.5 Hz, 4H), 7.31
(d, J = 8.5 Hz, 4H), 7.15 (d, J = 8.5 Hz, 4H), 6.85 (d, J = 8.5 Hz, 4H),
1.31 (s, 18H) ppm. 13C-NMR (101 MHz, CDCl3, 25 °C) δ = 200.46,
152.10, 150.94, 132.15, 131.59, 131.11, 129.79, 127.34, 125.60,
125.36, 123.01, 34.77, 31.36 ppm. 13C-NMR (101 MHz, CD2Cl2, 25 °C)
δ = 200.81, 152.89, 151.49, 132.75, 131.94, 131.55, 130.25, 128.04,
126.18, 125.69, 123.27, 35.09, 31.52 ppm. EA [%] calcd. C 67.90,
H 5.24; found C 63.31, H 4.67. HRMS: expected: 654.09559, found
654.09499.
2,3,4,5-Tetraphenylcyclopentadienone (CPD1): 50 % isolated
yield. 1H-NMR (400 MHz, CDCl3, 25 °C) δ = 7.25–7.20 (m, 12H), 7.18–
7.13 (m, 4H), 6.93–6.90 (m, 4H), ppm. 1H-NMR (400 MHz, CD2Cl2,
25 °C) δ = 7.31–7.16 (m, 16H), 7.01–6.93 (m, 4H) ppm. 13C-NMR
(101 MHz, CD2Cl2, 25 °C) δ = 200.84, 155.32, 133.76, 131.54, 130.
69, 129.81, 129.01, 128.52, 128.50, 128.01, 126.00 ppm. EA [%] calcd.
C 90.60, H 5.24; found C 90.61, H 5.23. HRMS: expected: 384.15142,
found 384.15080.
2,5-Bis(4-tert-butylphenyl)-3,4-diphenylcyclopentadienone[30,31]
(CPD2): 74 % isolated yield. 1H-NMR (400 MHz, CD2Cl2, 25 °C) δ =
7.30–7.14 (m, 14H), 7.00–6.95 (m, 4H), 1.29 (s, 18H) ppm. 13C-NMR
(101 MHz, CD2Cl2, 25 °C) δ = 201.50, 154.83, 151.09, 134.12, 130.25,
129.75, 128.78, 128.55, 128.47, 125.61, 125.49, 35.02, 31.51 ppm. EA
[%] calcd. C 89.47, H 7.31; found C 89.47, H 5.88. HRMS: expected:
496.27662, found 496.27600.
2,5-Diphenyl-3,4-bis(2-pyridyl)cyclopenta-3-ol-1-one[18] (CPO):
1.00 g (4.72 mmol) 2,2′-pyridil and 1.32 g (6.28 mmol) 1,3-diphenyl-
acetone were stirred under reflux in 40 mL of EtOH, then 80 mg
(1.43 mmol) KOH dissolved in 10 mL of EtOH were added to the
reaction mixture and stirred for another hour under reflux. The re-
sulting white precipitate was filtered off and thoroughly washed
with EtOH to get 1.20 g (3.0 mmol, 64 %) of the product as a white
2,5-Diphenyl-3,4-bis(4-tert-butylphenyl)cyclopentadien-
1
one[29,32–34] (CPD3): 67 % isolated yield. H-NMR (400 MHz, CD2Cl2,
1
25 °C) δ = 7.29–7.17 (m, 14H), 6.87–6.83 (m, 4H), 1.27 (s, 18H) ppm.
solid. H-NMR (400 MHz, CDCl3, 25 °C) δ = 8.41 (d, J = 4.8 Hz, 1H),
13C-NMR (101 MHz, CD2Cl2, 25 °C) δ = 201.05, 155.56, 152.38,
8.35 (d, J = 4.8 Hz, 1H), 7.67–7.59 (m, 2H), 7.44–7.40 (m, 6H), 7.36–
131.91, 130.74, 130.68, 129.58, 128.45, 127.79, 125.61, 125.23, 35.10, 7.30 (m, 3H), 7.18 (d, J = 8.0 Hz, 1H), 7.09–7.03 (m, 4H), 6.64 (s, 1H),
31.46 ppm. EA [%] calcd. C 89.47, H 7.31; found C 88.97, H 6.05.
HRMS: expected: 496.27662, found 496.27601.
4.35 (s, 1H) ppm. 13C-NMR (500 MHz, CDCl3, 25 °C) δ = 205.88,
164.25, 163.69, 154.18, 148.97, 148.47, 141.71, 136.52, 136.37,
134.75, 130.76, 130.57, 130.01, 128.88, 128.61, 128.13, 127.39,
125.91, 123.50, 121.89, 121. 05, 83.63, 65.44 ppm. HRMS: expected:
404.15248, found 405.15969 [M + H].
2,3,4,5-Tetrakis(4-tert-butylphenyl)cyclopentadienone[29,32,34–37]
(CPD4): 81 % isolated yield. 1H-NMR (400 MHz, CDCl3, 25 °C) δ =
7.28–7.21 (m, 8H), 7.18–7.14 (m, 4H), 6.87–6.83 (m, 4H), 1.30 (s, 18H),
1
1.29 (s, 18H) ppm. H-NMR (400 MHz, CD2Cl2, 25 °C) δ = 7.35–7.29 2,5-Diphenyl-3,4-bis(2-pyridyl)cyclopentadienone[18,42] (CPD8):
(m, 4H), 7.26–7.19 (m, 8H), 6.94–6.87 (m, 4H), 1.34 (s, 18H), 1.31 (s,
18H) ppm. 13C-NMR (101 MHz, CDCl3, 25 °C) δ = 201.54, 154.39,
151.48, 150.15, 130.64, 129.79, 129.21, 128.26, 125.04, 124.70,
124.41, 34.78, 34.70, 31.41, 31.38 ppm. 13C-NMR (101 MHz, CD2Cl2,
25 °C) δ = 201.78, 155.18, 152.14, 150.89, 131.19, 130.30, 129.54,
0.275 g (0.68 mmol) 2,5-Diphenyl-3,4-bis(2-pyridyl)cyclopenta-3-ol-
1-one (CPO) were stirred under reflux for 20 min in 10 mL of ethyl-
ene glycol. Upon cooling to room temperature, the formed precipi-
tate was filtered off and recrystallized from EtOH to yield 0.132 g
(0.34 mmol, 50 %) as a red solid. 1H-NMR (400 MHz, CDCl3, 25 °C)
128.97, 125.48, 125.24, 125.21, 35.13, 35.06, 31.61, 31.55 ppm. EA δ = 8.35 (m, 2H), 7.52 (m, 2H), 7.26 (m, 12H, overlay with solvent),
[%] calcd. C 88.76, H 8.61; found C 88.70, H 8.65. HRMS: expected:
608.40182, found 608.40125.
7.07 (m, 2H) ppm. 13C-NMR (500 MHz, CDCl3, 25 °C) δ = 200.73,
153.22, 149.02, 135.92, 130.20, 130.03, 128.18, 128.01, 126.67,
124.98, 122.41 ppm. HRMS: expected 386.14191, found 386.14140.
2,3,4,5-Tetrakis(4-methoxyphenyl)cyclopentadienone[38–40]
(CPD5): 79 % isolated yield. 1H-NMR (400 MHz, CDCl3, 25 °C) δ =
7.19 (d, J = 8.8 Hz, 4H), 6.86 (d, J = 8.6 Hz, 4H), 6.79 (d, J = 8.6 Hz,
2,5-Bis(4-tert-butylphenyl)-3,4-bis(2-pyridyl)cyclopentadienone
(CPD9): 0.100 g (0.47 mmol) 2,2′-pyridil and 0.172 g (0.53 mmol)
1,3-bis(4-tert-butylphenyl)propan-2-one were refluxed in 20 mL of
EtOH and a solution of 8 mg (0.14 mmol) KOH in 5 mL of EtOH was
added. After 1.5 h, the solvent was removed and the residue was
taken up into DCM, washed with water, dried with sodium sulfate
and DCM was removed. 8 mL of ethylene glycol were added and
the mixture was stirred for 0.5 h under reflux. Upon cooling to room
1
4H), 6.72 (d, J = 8.8 Hz, 4H), 3.79 (s, 12H) ppm. H-NMR (400 MHz,
CD2Cl2, 25 °C) δ = 7.16 (d, J = 8.9 Hz, 4H), 6.86 (d, J = 8.7 Hz, 4H),
6.79 (d, J = 8.7 Hz, 4H), 6.72 (d, J = 8.8 Hz, 4H), 3.78 (s, 6H), 3.77 (s,
6H) ppm. 13C-NMR (101 MHz, CDCl3, 25 °C) δ = 201.39, 159.73,
158.92, 152.91, 131.51, 125.85, 124.09, 123.90, 113.75, 113.55,
55.31 ppm. 13C-NMR (500 MHz, CD2Cl2, 25 °C) d 201.76, 160.31,
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water and recrystallized from EtOH to yield 50 mg (0.1 mmol, 29 %)
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Received: August 10, 2020
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Tetraaryl Cyclopentadienones
N. Meitinger, A. K. Mengele,
K. Witas, S. Kupfer, S. Rau,
D. Nauroozi* ..................................... 1–10
Tetraaryl Cyclopentadienones: Ex-
perimental and Theoretical Insights
into Negative Solvatochromism and
Electrochemistry
Substitution at tetraaryl cyclopentadi- substituted tetraaryl cyclopentadi-
enones is known to affect the photo- enones are studied upon so far un-
physical properties of the molecules. precedented negative solvatochromic
In a joint experimental and theoretical effect and, the impact of substituents
investigation, a series of differently on the electrochemical behavior.
doi.org/10.1002/ejoc.202001091
Eur. J. Org. Chem. 0000, 0–0
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9
© 2020 The Authors published by Wiley-VCH GmbH