Soluble Flavanthrone Derivatives: Synthesis, Characterization, and Application to Organic Light-Emitting Diodes

Article abstract of DOI:10.1002/chem.201600513

Simple modification of benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine-8,16-dione, an old and almost-forgotten vat dye, by reduction of its carbonyl groups and subsequent O-alkylation, yields solution-processable, electroactive, conjugated compounds of the periazaacene type, suitable for the use in organic electronics. Their electrochemically determined ionization potential and electron affinity of about 5.2 and -3.2 eV, respectively, are essentially independent of the length of the alkoxyl substituent and in good agreement with DFT calculations. The crystal structure of 8,16-dioctyloxybenzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine (FC-8), the most promising compound, was solved. It crystallizes in space group P1 and forms π-stacked columns held together in the 3D structure by dispersion forces, mainly between interdigitated alkyl chains. Molecules of FC-8 have a strong tendency to self-organize in monolayers deposited on a highly oriented pyrolytic graphite surface, as observed by STM. 8,16-Dialkoxybenzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridines are highly luminescent, and all have photoluminescence quantum yields of about 80 %. They show efficient electroluminescence, and can be used as guest molecules with a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl host in guest/host-type organic light-emitting diodes. The best fabricated diodes showed a luminance of about 1900 cd m^-12, a luminance efficiency of about 3 cd A^-1, and external quantum efficiencies exceeding 0.9 %. New life for an old dye: Simple modification of flavanthrone, an old, intractable, and almost-forgotten vat dye, by reduction of its carbonyl groups to phenolates followed by O-alkylation, gave a new group of solution-processable, electroactive, conjugated compounds (see figure). Their HOMO and LUMO energies, as well as their ionization potentials and electron affinities, make them suitable for application as components of organic light-emitting diodes.

Full text of DOI:10.1002/chem.201600513

DOI: 10.1002/chem.201600513
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&
Dyes/Pigments
Soluble Flavanthrone Derivatives: Synthesis, Characterization, and
Application to Organic Light-Emitting Diodes
Kamil Kotwica,[a] Piotr Bujak,*[a] Przemyslaw Data,[b, c] Wojciech Krzywiec,[a] Damian Wamil,[a]
Piotr A. Gunka,[a] Lukasz Skorka,[a] Tomasz Jaroch,[d] Robert Nowakowski,[d] Adam Pron,[a] and
Andrew Monkman*[b]
Abstract: Simple modification of benzo[h]benz[5,6]acridi-
no[2,1,9,8-klmna]acridine-8,16-dione, an old and almost-for-
gotten vat dye, by reduction of its carbonyl groups and sub-
sequent O-alkylation, yields solution-processable, electroac-
tive, conjugated compounds of the periazaacene type, suita-
ble for the use in organic electronics. Their electrochemically
determined ionization potential and electron affinity of
about 5.2 and À3.2 eV, respectively, are essentially independ-
ent of the length of the alkoxyl substituent and in good
agreement with DFT calculations. The crystal structure of
8,16-dioctyloxybenzo[h]benz[5,6]acridino[2,1,9,8-klmna]acri-
dine (FC-8), the most promising compound, was solved. It
held together in the 3D structure by dispersion forces,
mainly between interdigitated alkyl chains. Molecules of FC-
8 have a strong tendency to self-organize in monolayers de-
posited on a highly oriented pyrolytic graphite surface, as
observed by STM. 8,16-Dialkoxybenzo[h]benz[5,6]acridi-
no[2,1,9,8-klmna]acridines are highly luminescent, and all
have photoluminescence quantum yields of about 80%.
They show efficient electroluminescence, and can be used
as guest molecules with a 4,4’-bis(N-carbazolyl)-1,1’-biphenyl
host in guest/host-type organic light-emitting diodes. The
best fabricated diodes showed
a luminance of about
1900 cdmÀ12, a luminance efficiency of about 3 cdAÀ1, and
external quantum efficiencies exceeding 0.9%.
¯
crystallizes in space group P1 and forms p-stacked columns
Introduction
towards oxidative degradation should be noted. However,
their reduction potential is very low and, as a result, their
anion-radical forms are extremely unstable. This excludes the
use of acenes as potential n-type semiconductors or ambipolar
FETs.[1a,3]
Organic semiconductors of the acene and azaacene families
have been the subject of intensive research interest, mainly
due to their good electrical transport properties, which make
them interesting materials for applications in field-effect tran-
sistors (FETs) and other organic electronic devices.[1] Simple
acenes and their derivatives such as rubrene (5,6,11,12-tetra-
phenyltetracene) and picene (dibenzo[a,i]phenanthrene), for
example, exhibit very high charge-carrier mobilities compared
to other p-type organic semiconductors.[2] Thus, acenes are
good p-type semiconductors, although their limited stability
Introduction of nitrogen atoms into the fused aromatic
structure of acenes drastically changes their redox and elec-
tronic properties. Depending on the molecule geometry (linear
or nonlinear), the number of nitrogen atoms, and their distri-
bution within the conjugated core, their ionization potential
(IP) and electron affinity (EA) can be tuned over a wide
range.[4] In addition to their interesting electronic properties,
photo- and electroluminescence of acenes and azaacenes
become possible. These properties make them suitable candi-
dates for use as electroluminophores. The potential impor-
tance of these materials was highlighted last year by the deci-
sion of the United Nations proclaiming 2015 as the Internation-
al Year of Light and Light-based Technologies, with the princi-
pal goal of searching for new light sources free of toxic ele-
ments that, in addition, are less expensive in operation than
current ones. In this respect organic materials can easily com-
pete with inorganic counterparts such as nanocrystalline forms
of inorganic semiconductors.[5] Many acenes[6] and azaacenes[7]
exhibiting high photo- and electroluminescence quantum
yields have been reported as the active components of organic
light-emitting diodes (OLEDs).
[a] K. Kotwica, Dr. P. Bujak, W. Krzywiec, D. Wamil, Dr. P. A. Gunka, L. Skorka,
Prof. A. Pron
Faculty of Chemistry, Warsaw University of Technology
Noakowskiego 3, 00-664 Warsaw (Poland)
E-mail: piotrbujakchem@poczta.onet.pl
[b] Dr. P. Data, Prof. A. Monkman
Physics Department, University of Durham
South Road, Durham DH13LE (UK)
E-mail: a.p.monkman@durham.ac.uk
[c] Dr. P. Data
Faculty of Chemistry, Silesian University of Technology
M. Strzody 9, 44-100 Gliwice (Poland)
[d] Dr. T. Jaroch, Prof. R. Nowakowski
Institute of Physical Chemistry, Polish Academy of Science
Kasprzaka 44/52, 01-224 Warsaw (Poland)
In recent years, synthetic strategies leading to new organic
semiconductors in general, and to linear and nonlinear acenes
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600513.
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and azaacenes in particular, frequently involved modification
of old and sometimes almost-forgotten dyes.[7d,e,8] In these
cases the target azaacene core was first obtained in one step
and then functionalized with solubilizing side groups such as
triisopropylsilylethynyl by forming a ÀCꢀCÀ linkage with the
core in a second step.[1] Compounds in which other groups
6,15-dihydrodinaptho[2,3-a:2’,3’-h]phenazine-5,9,14,18-tet-
raone, commonly termed indanthrone, which was synthesized
by condensation of aminoanthraquinone at temperatures
below 2708C under basic conditions. Above 2708C benzo[h]-
benz[5,6]acridino[2,1,9,8-klmna]acridine-8,16-dione
(flavan-
throne), a yellow vat dye, could be obtained from the same
link the solubilizing side chain with the core, such as ÀCH2NHÀ substrate (see Scheme 1).
and ÀCH=NÀ, were also synthesized.[9] Recently, bis-tetracene
diketone, first reported in 1949, to which two triisopropylsilyle-
thynyl groups were attached, was used as a substrate in the
synthesis of several acenes, and gave good solubility of the
functionalized semiconductors in organic solvents.[10]
Previously, we transformed indanthrone into a solution-proc-
essable organic semiconductors, namely, 5,9,14,18-tetraalkoxy-
dinaphtho[2,3-a:2’,3’-h]phenazines, in a simple one-pot process
consisting of the reduction of the carbonyl group by sodium
dithionite followed by substitution with solubility-inducing
groups under phase-transfer catalysis conditions.[7d,e] The re-
sulting compounds showed high photoluminescence quantum
yields and could be applied as electroluminophores in guest/
host-type LEDs. Herein, we present a systematic study on the
synthesis and spectroscopic, structural (including self-assem-
bly), and electrochemical properties of new solution-processa-
ble, electroactive, conjugated compounds derived from flavan-
throne, and show that in many respects they are distinctly dif-
ferent from the indanthrone derivatives. We also describe
guest/host LEDs in which these new electroactive materials are
used as electroluminophores.
Several substrates known from the synthesis of dyes can
also be exploited in the preparation of azaacenes. These in-
clude derivatives of aminoanthraquinone or diketopyrrolopyr-
role. The synthetic pathway usually involves simple condensa-
tion reactions or oxidative CÀN coupling, and thus expensive
palladium-based catalyst systems can be dispensed with.[11]
In the early 1900s, extensive research was carried out on the
development of new dyes, one of which was the blue vat dye
Results and Discussion
The synthesis of flavanthrone has been known for over 100
years;[12] however, it had to be modified to meet the standards
of modern organic chemistry and to improve the reaction
yield. Flavanthrone is also commercially available, but requires
additional purification when used for the preparation of new
semiconductors.
Flavanthrone was obtained from 2-aminoanthraquinone (1),
which in the first step was converted to its monobromo deriv-
ative 2 by reaction with N-bromosuccinimide (NBS) in 1:1
molar ratio (Scheme 2). In the second step, the amino group
was protected to yield 3. Then, an Ullmann CÀC coupling reac-
Scheme 1. Indanthrone (6,15-dihydrodinaptho[2,3-a:2’,3’-h]phenazine-
5,9,14,18-tetraone) and flavanthrone (benzo[h]benz[5,6]acridino[2,1,9,8-
klmna]acridine-8,16-dione) from aminoanthraquinone.
Scheme 2. Synthetic route to dialkoxy derivatives of benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine-8,16-dione (FC-4–12)
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tion was carried out with high yield (95%).[13] In the last step
a spontaneous reaction of the carbonyl groups with the amino
groups took place on deprotection to yield the fused structure
of flavanthrone (5).
the diazapyrene core. The doublet at 10.30 ppm (J=8.2 Hz) is
attributed to the highly deshielded H(C) proton located in the
vicinity of the hexyloxy substituent, and the doublet at
8.77 ppm (J=8.2 Hz) is ascribed to the H(F) proton. These two
doublets, despite having the same coupling constant, do not
originate from the coupling of H(C) and H(F) protons, as is
clearly concluded from the COSY spectrum. The observed
chemical shifts of aromatic protons are in very good accord-
ance with the literature data for benzo[h]benz[5,6]acridi-
no[2,1,9,8-klmna]acridine-8,16-dione containing triisopropyl-
silylethynyl solubilizing substituents.[8b]
Flavanthrone was transformed into 8,16-dialkoxybenzo[h]-
benz[5,6]acridino[2,1,9,8-klmna]acridines in a one-pot process,
previously elaborated for indanthrone.[7d] In the first step the
carbonyl groups were reduced to phenolates by using Na2S2O4
in sodium hydroxide solution, and then, without the necessity
of intermediate product isolation, O-alkylation of phenolate
groups was performed with alkyl bromides. Application of
Na2S2O4 as a reducing agent also has a long history, since this
reagent was used in coloring fabrics in old procedures employ-
ing vat dyes. The reaction pathway is presented in Scheme 2
and the exact preparation procedures can be found the Sup-
porting Information. Five derivatives with increasing alkoxyl
chain length were synthesized: OC4H9 (FC-4), OC6H13 (FC-6),
OC8H17 (FC-8), OC10H21 (FC-10), and OC12H25 (FC-12).
Redox properties of the synthesized compounds were stud-
ied by cyclic voltammetry. The voltammograms were measured
on 5.0”10À4 m solutions in 0.1m Bu4NBF4/CH2Cl2 electrolyte
and were essentially independent of the length of the alkoxyl
substituent. Figure 2 shows a representative voltammogram
1
The H NMR spectrum of FC-8 is shown in Figure 1a. Since
the molecule is symmetrical, in the aliphatic part of the spec-
trum, at 4.43 ppm, only one triplet (J=6.6 Hz) is observed cor-
responding to two equivalent OCH2 groups in the alkoxyl sub-
stituents. In the aromatic part of the spectrum (Figure 1b) five
Figure 2. Cyclic voltammograms of FC-8 (electrolyte: 0.1m Bu4NBF4 in
CH2Cl2, scan rate: 50 mVsÀ1, potential vs. Ag/0.1m Ag+).
for FC-8. As reported for alkoxy-substituted dinaphthophena-
zines (indanthrone derivatives),[7e] FC-8 undergoes an irreversi-
ble oxidation at positive potentials (vs. Ag/0.1m Ag+). Howev-
er, its electrochemical behavior at negative potentials is dis-
tinctly different, since a quasireversible redox couple is found
for this compound, whereas the reduction of the previously
studied alkoxy-substituted dinaphthophenazines is irreversible.
From the onset potentials of the oxidation and reduction
peaks (+0.41 and À1.62 V vs. Fc/Fc+, respectively) the IP and
EA can be calculated by using Equations (1) and (2);[14] the re-
sults are listed in Table 1.
Figure 1. 1H NMR spectra of: a) FC-8 (whole range), b) FC-8 (aromatic part),
1
c) FC-6 (aromatic part), and d) FC-6 H–1H COSY.
À
Á
IP=eV ¼ jej EoxðonsetÞ þ 4:8
ð1Þ
ð2Þ
signals are observed, although six nonequivalent aromatic pro-
tons can be distinguished in the molecular core. This implies
that the spectral lines of two nonequivalent protons must
overlap. Spectral lines corresponding to six nonequivalent aro-
matic protons are, however, clearly resolved in the case of FC-
6 (Figure 1c), which greatly facilitates unequivocal assignment
À
Á
EA=eV ¼ Àjej EredðonsetÞ þ 4:8
According to the literature data, flavanthrone (Vat Yellow 1)
shows IP=6.3 eV and
jEAj =3.6 eV.[15] The IP of 5.21 eV obtained for FC-8 is lower
than that determined for a tetraoctyloxy derivative of dinaph-
thophenazine (5.34 eV),[7e] whereas jEAj is higher (3.18 and
2.94 eV, respectively). These differences can be considered to
1
of lines, especially as it is supported by the corresponding H–
1H COSY spectrum (Figure 1d). Two well-resolved doublets at
8.44 and 8.63 ppm (J=9.7 Hz) correspond to H(A) and H(B) of
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Table 1. HOMO and LUMO levels of FC-8 with corresponding Eg, IP, and EA calculated at the B3LYP/6-31G(d,p) level of theory. IP and jEAj values deter-
mined by cyclic voltammetry (CV) are shown for comparison. All values are given in electron volts.
HOMO (Au)
(vacuum)
HOMO (Au)
(vacuum)
HOMO (Au)
(vacuum)
HOMO (Au)
(vacuum)
HOMO (Au)
(vacuum)
HOMO (Au)
(vacuum)
HOMO (Au)
(vacuum)
HOMO (Au)
(vacuum)
À4.95
À2.42
5.13
2.61
5.03
2.70
5.21
3.18
be a manifestation of better conjugation within the more
fused 4,9-diazapyrene ring compared to the phenazine ring,
which facilitates donor–acceptor interactions between the sub-
stituent and the N-substituted rings. It is also interesting to
compare the IP of FC-8 with that determined electrochemically
for a compound with the same molecular core but different
solubilizing substituents, namely, triisopropylsilylethynyl
groups, reported in ref. [8b]. The latter shows a higher IP
(5.36 eV) than FC-8, again in accordance with stronger elec-
tron-donating properties of the alkoxyl group facilitating the
oxidation of FC-8. No electrochemical data for the reduction
process of this triisopropylsilylethynyl derivative are given in
ref. [8b].
cal ions show a similar behavior. The spin density of the radical
cation is mainly located in the proximity of the CO carbon
atom, with no contribution from the central part of the ring,
whereas in the radical anion it additionally occupies the space
near the nitrogen atoms.
DFT calculations also enable the determination of the IP and
EA values of isolated molecules and molecules in solution, in
the latter case by using the polarizable continuum model
(PCM). IP and EA values of FC-8 were calculated for a solution
in dichloromethane, that is, the solvent that was used in the
determination of the same parameters by cyclic voltammetry
(Table 1). Better agreement between the theoretical and exper-
imental values was obtained for the IP, and EA values differed
by 0.48 eV. However, compared to indanthrone, the overall
match between theory and experiment is far better.
In polyconjugated systems the first oxidation process leads
to the formation of a radical cation, and the first reduction pro-
cess to a radical anion. It is therefore helpful to analyze these
redox processes in connection with the plots of frontier orbi-
tals and spin-density distribution of the radical ions formed
(Figure 3). Both HOMO and LUMO occupy the central fused ar-
omatic core of the molecule, that is, the HOMO is located
mainly on the edge of this skeleton with the highest contribu-
tion from the carbon atom attached to the oxygen atom of
the alkoxyl substituent and with zero contribution from the
two central carbon atoms. The LUMO, on the other hand, is lo-
cated more towards the central part of the molecule and also
has strong contributions from the aforementioned CO carbon
and nitrogen atoms. The spin-density distributions in the radi-
Solution absorption and emission spectra of the various de-
rivatives are essentially independent of the length of the alkox-
yl substituent. Figure 4 shows the spectra of FC-8 as a repre-
sentative example, and those of the other derivatives are col-
lected in the Supporting Information. These spectra show fea-
tures typical of periacenes and periazaacenes,[8b,10,16] which
usually show a set of absorption bands originating from p!p*
and n!p* transitions, which are bathochromically shifted with
increasing size of the aromatic core.[4c,17] Pronounced vibronic
character of the lowest-energy band (at 515 nm for the 0–0
transition) is notable. The spectrum is consistent with the cal-
Figure 4. Absorption and emission spectra of FC-8 (chloroform as solvent).
The calculated positions and relative oscillator strengths of the electronic
transitions of this compound, obtained from TDDFT calculations, are depict-
ed as black bars for comparison.
Figure 3. Frontier molecular orbital plots for FC-8 (top: isosurface
value=0.03) and spin densities for radical cation and anion forms of this
compound (bottom: isosurface value=0.001).
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culated transitions (see Figure 4 and Table S2 in the Supporting
Information). The strongest band at 515 nm is almost purely
HOMO (H)!LUMO (L) in nature (98%). This transition is fa-
vored in centrosymmetric (Ci point group) molecules such as
FC-8, since the frontier orbitals (HOMO and LUMO) by symme-
try belong to two different irreducible representations Au and
Ag, respectively, and the H!L transition is highly allowed ac-
cording to the Laporte selection rule. Full analysis of the elec-
tronic transition can be found in the Supporting Information.
The optical bandgap Eg,opt of FC-8, determined from the
onset of the lowest-energy absorption band, is 2.33 eV, that is,
100 meV larger than that of isopropylsilylethynyl-functionalized
periazaacene with the same core.[8b] However, it is 70 meV
smaller than the that of 5,9,14,18-tetraoctyloxydinaphtho[2,3-
a:2’,3’-h]phenazine (the corresponding indanthrone deriva-
tive).[7e]
vidual molecules in the monolayer. Each stripe, clearly visible
in this image, is characterized by its inner structure, which con-
sists of two rows of well-resolved bright spots. It is expected
that this signal of higher tunneling current corresponds to the
conjugated cores of the investigated adsorbate. Microscopic
investigations performed with different bias potentials (not dis-
cussed here) enabled us to postulate the orientation of mole-
cules in the observed rows. This direction (708 with respect to
the direction of the row orientation) is marked schematically in
Figure 5b by an angle indicated in white. Each molecule is
therefore visible at submolecular resolution as a set of two
bright spots oriented along this axis. The 2D unit-cell parame-
ters determined from the STM images are: 0.91Æ0.05 nm/
2.30Æ0.1 nm/84Æ28. The corresponding model of the adsorp-
tion geometry is presented in Figure 5c. The dimension of the
unit cell along its shorter axis (0.91 nm) indicates that the
fused aromatic cores of adjacent molecules are tightly packed
in this direction. This correlation also shows that in the mono-
layer the conjugated cores of the molecules lie flat on the sub-
strate surface. The molecular rows formed in this way are sepa-
rated by a distance of 2.30 nm imposed by the interdigitation
of alkyl substituents.
All compounds of the FC-n series exhibit strong green pho-
toluminescence. Their emission spectra can be treated as
mirror images of the absorption spectra (see Figure 4, in which
the emission and absorption spectra of FC-8 are shown as an
example). This feature, together with a very small Stokes shift
of 9 nm, clearly indicates a very similar molecular geometry in
the ground and excited states, consistent with the rigid nature
of the core. The experimental emission wavelength corre-
sponds very well with that calculated by state-specific TDDFT
(lem =554 nm). The measured photoluminescence quantum
yield (PLQY) in all cases reaches 80%, a value which is signifi-
cantly higher than those measured for tetraalkoxydinaphtho-
phenazines, that is, the corresponding indanthrone derivatives
(ca. 60%).[7e]
We further investigated the self-organized 2D pattern to de-
termine how the observed monolayer structure of FC-8 is relat-
ed to its 3D organization in a single crystal. Crystal structure
analysis revealed that FC-8 crystallizes in triclinic space group
¯
P1. The molecule consists of a flat aromatic core and two n-oc-
tyloxy groups in the anti conformation (see Figure 6 for the
molecular structure of FC-8). The root mean square displace-
ment of atoms constituting the aromatic core from the mean
plane is 0.052 Š, and the C29-O1-O2-C37 torsion angle describ-
ing the mutual orientation of alkoxyl groups is 178.8(3)8.
Inversion-related molecules form dimers through p stacking
accompanied by CÀH···O interactions (Figure 7, d(C29···O2i)=
3.332(4) Š, d(H29B···O2i)=2.44 Š, C29-H29B···O2=1508, i=1Àx,
As with tetraalkoxydinaphthophenazines,[8b] compounds of
the FC-n series are capable of self-assembling into highly or-
dered 2D supramolecular structures in monolayers deposited
on suitable substrates such as highly oriented pyrolytic graph-
ite (HOPG). Figure 5a and b show representative STM images
Figure 5. a), b) STM images and c) proposed adsorption geometry of a mono-
layer of FC-8 on HOPG. Scanning area and parameters: a) 61”61 nm,
b) 9”9 nm2, Vbias =1 V, It =0.3 nA.
of a monolayer of FC-8 deposited on a HOPG substrate. The
adsorbate molecules are well-organized into large domains
with typical size of a few hundred nanometers, which differ in
their orientation (see upper part of the large-scale image in
Figure 5a). Each domain is observed by STM as a set of parallel
bright stripes corresponding to rows of molecules oriented in
one direction.
Figure 6. Molecular structure of FC-8 with atom numbering scheme. Ther-
mal ellipsoids drawn at 50% probability and hydrogen atoms omitted for
clarity.
Images obtained at higher magnification (Figure 5b) provide
more detailed information concerning the arrangement of indi-
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Figure 8. a) Device structure and energy diagram of the green LED. b) Elec-
troluminescence spectra of 1 wt% dispersion of FC-4, FC-8, and FC-12 in
CPB matrix. c) Luminance versus voltage characteristics for diodes with
active layers containing increasing amounts of FC-8: 1, 3, and 5%. d) Exter-
nal quantum efficiency versus luminance characteristics for OLEDs with FC-4
and FC-8 (1 wt%).
Figure 7. Unit cell of FC-8 (ball-and-stick model)viewed along the [010] di-
rection. CÀH···O hydrogen bonds are shown as dashed lines.
1Ày, 1Àz). The dimers in turn form p-stacked columns running
along the [100] direction (Figure S2 in the Supporting Informa-
tion). The distance between mean planes of the aromatic cores
in the dimers is 3.4398(11) Š with a plane shift of 1.7825(12) Š,
while the interplanar distance between molecules from
the adjacent dimers is 3.3030(15) Š, but the plane shift is
7.8503(14) Š (Figure S2 in the Supporting Information). Col-
umns are held together in the 3D structure by dispersion
forces, mainly between interdigitated alkyl chains. In this crys-
talline arrangement it is possible to identify almost flat and
densely packed slices of molecules with 2D ordering very simi-
lar to that found in the monolayer deposited on HOPG (Fig-
ure S3 in Supporting Information). The 2D unit-cell parameters
of the slice are 0.9365(4) nm/2.3929(4) nm/86.86(2)8, which
confirm the unit-cell parameters of the monolayer determined
from STM measurements (corresponding parameters: 0.91Æ
0.05 nm/2.30Æ0.1 nm/84Æ28).
transporting layer of poly(9-vinylcarbazole) (PVK) and a hole-
blocking layer of 2,2’,2“-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-
benzimidazole) (TPBi) by a hybrid solution/vacuum codeposi-
tion method. These two layers were separated from the elec-
trodes (indium tin oxide (ITO) and Al) by layers of poly(3,4-eth-
ylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and
LiF, respectively.
Figure 8b shows the electroluminescence spectra measured
for the emissive layer containing 1% of the FC-n luminophore.
These spectra resemble the photoluminescence spectra record-
ed from chloroform solutions of the studied luminophores,
with retention of the vibrational structure but the emission
peaks bathochromically shifted by 10–15 nm (cf. Figures 4 and
8b). Figure 8c shows plots of luminance versus voltage for in-
creasing contents of FC-8 luminophore in the CBP matrix.
Clearly, with increasing content of the electroluminophore the
luminance decreases with simultaneous increase of the turn-on
voltage. This may suggest nanoaggregation occurring at
higher electroluminophore contents, which favors nonradiative
quenching of the emitter. The parameters of all studied diodes
are listed in Table 2. The highest luminance values (approach-
ing 1900 cdmÀ12) were obtained for FC-4 and FC-8, whereas
the highest luminous efficiency (>3 cdAÀ1) was found for FC-
8. In the case of FC-4 and FC-8 the highest external quantum
efficiencies (EQEs), exceeding 0.9%, were measured for moder-
ate luminances, but they never dropped below 0.65% for the
whole range of luminances studied (see Figure 8d).
The triisopropylsilylethynyl-substituted analogue of the FC-n
series of compounds also readily forms single crystals, and
hence it was used in the fabrication of single-crystal FETs
showing reasonable hole mobility.[8b] Considering the excellent
luminescent properties of all FC-n derivatives (PLQY of 80%),
we explored their application as high-mobility electrolumino-
phores in OLEDs, especially in view of the fact that such appli-
cations are extremely rare in the case of azaacenes and peria-
zaacenes.[7d–f,18]
The device structure used for guest/host-type diodes was
ITO/PEDOT:PSS/PVK/(FC-(4,8,12)-CBP/TPBi/LiF/Al. The corre-
sponding energy-level scheme for this structure is shown in
Figure 8a. The emissive layer was varied over a range of emit-
ter concentrations of 1–15% of a given FC-n compound mo-
lecularly dispersed in a 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP)
matrix. This emissive layer was sandwiched between a hole-
Chem. Eur. J. 2016, 22, 7978 – 7986
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Computational Methods
Table 2. Electroluminescence data for diodes containing FC-4, FC-8, and FC-12 (1–
15 wt%) as electroluminophores.
DFT calculations were carried out with the Gaussian 09
Revision D.01[20] package by employing the B3LYP[21]
hybrid exchange correlation functional and 6-31G(d,p)
basis set. Ground- and excited-state geometries were
fully optimized until a stable local minimum was found,
which was confirmed by normal-mode analysis (no imag-
inary frequencies were present). Initial structures were
constrained to the Ci symmetry point group and then re-
laxed if a saddle point was found. Solvent–solute interac-
tions were taken into account with the aid of the PCM
model[22] and dichloromethane as solvent. The oscillator
strengths and energies of the vertical singlet excitations
were calculated by employing the time-dependent (TD)
c [wt%] FC-4
FC-8
FC-12
L
[a] [cdmÀ2
]
LE[b] [cdAÀ1
]
L [cdmÀ2
]
LE [cdAÀ1
]
L [cdmÀ2
]
LE [cdAÀ1
]
1
3
5
10
15
1885
988
1100
746
2.74
1.21
0.96
0.66
0.29
1860
1790
717
619
–
3.06
1.17
0.34
0.39
–
1460
1580
262
54
2.13
1.63
0.12
0.23
0.16
500
155
[a] Luminance. [b] Luminous efficiency.
Conclusion
version of DFT[23] at respective optimized geometries. Multiconfi-
gurational character of the most pronounced excitations was ana-
lyzed with Natural Transition Orbitals (NTO).[24] Excited-to-ground
state transitions were calculated by the state-specific solvation ap-
proach taking into account the Franck–Condon rule.[25] The neces-
sary data of DFT calculations were retrieved from output files by
using GaussSum 2.2.[26] All necessary initial geometries and final
graphics (molecular orbitals and spin densities) were generated in
GaussView 5.0.[27]
We have demonstrated that by simple modification of ben-
zo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine-8,16-dione (flavan-
throne), an old, intractable, and almost-forgotten vat dye,
through reduction of its carbonyl groups to phenolates fol-
lowed by their O-alkylation, it is possible to obtain a new
group of solution-processable, electroactive, conjugated com-
pounds, namely, 8,16-dialkoxybenzo[h]benz[5,6]acridino[2,1,9,8-
klmna]acridines. HOMO and LUMO energies of these com-
pounds as well as their IPs and EAs make them suitable candi-
dates for application as components of organic FETs and LEDs.
The latter possibility was explored, and green-emitting diodes
were obtained in a guest/host configuration with CBP host.
Self-assembly of these compounds resulted in formation of
monolayers on the surface of HOPG, in which highly ordered
2D supramolecular organization extends over a few hundred
nanometers.
Scanning tunneling microscopy
Monomolecular layers were prepared by drop casting from a solu-
tion of the investigated compound in hexane (ꢁ2 mgLÀ1) on
a freshly cleaved HOPG surface (SPI Supplies, USA). The layers were
dried under ambient conditions and then imaged in air at room
temperature with an STM system (University of Bonn, Germany).[28]
All images were recorded in constant-current mode with mechani-
cally cut Pt/Ir (80/20%) tips. The proposed real-space models of
the monomolecular layers were obtained by correlation of the
layer structure deduced from the STM images and the molecular
model of the investigated adsorbate determined by using the Hy-
perChem software package.
Experimental Section
Synthesis
X-ray diffraction
A list of all chemicals, detailed descriptions of the synthesis of all
investigated compounds, and spectroscopic characterization data
can be found in the Supporting Information.
A suitable single crystal of FC-8, grown from chlorobenzene, was
selected under a polarizing microscope, mounted in inert oil (Para-
tone N), and transferred to the cold gas stream of the diffractome-
ter. Diffraction data were collected with an Oxford Diffraction
k-CCD Gemini A Ultra diffractometer at 120(2) K with mirror-fo-
cused CuKa radiation. Cell refinement and data collection, reduc-
tion, and analysis were performed with the CrysAlisPRO software.[29]
The structures were solved by direct methods and subsequent
Fourier-difference synthesis with SHELXT and refined by full-matrix,
least-squares methodsagainst F2 with SHELXL-2014 within the
Olex2 program suite.[30] All non-hydrogen atoms were refined ani-
sotropically. Hydrogen atoms were introduced at calculated posi-
tions and refined as riding atoms. Data were analyzed with Olex2,
Platon, and Diamond.[30b,31] Crystal data and structure refinement
parameters are given in Table S3 of the Supporting Information.
Images were created using Olex2 and Diamond and were rendered
with POV-Ray.[30b,31b,32]
Spectroscopy
1H NMR and 13C NMR spectra were recorded with a Varian Unity
Inova 500 MHz spectrometer and referenced to TMS or solvents.
The elemental analyses were carried out with a Vario EL III (Elemen-
tar) CHN analyzer. HRMS spectra were recorded in methanol with
Mariner ESI-TOF (Applied Biosystems) mass spectrometer. Solution
UV/Vis/NIR spectra were recorded in chloroform with a Cary 5000
(Varian) spectrometer, and the emission spectra with an Edinburgh
FS 900 CDT fluorometer (Edinburgh Analytical Instruments). Photo-
luminescence quantum yields were determined by using quinine
sulfate in 0.05 moldmÀ3 H2SO4 (ffl =0.51) as standard.[19]
Cyclic Voltammetry
Cyclic voltammetry was performed in a three-electrode, one-com-
partment cell with a platinum working electrode (3 mm2), a plati-
num-wire counter electrode, and a Ag/0.1m AgNO3/CH3CN refer-
ence electrode. A given flavanthrone derivative was dissolved in
0.1m Bu4NBF4 in dichloromethane to yield a 5”10À4 m solution.
Electroluminescence measurements and LED fabrication
Guest/host-type electroluminescent diodes were fabricated by mo-
lecular dispersion of FC-4, FC-8, or FC-12 (1 to 15 wt%) in an one-
component matrix consisting of 4,4’-bis(N-carbazolyl)-1,1’-biphenyl
Chem. Eur. J. 2016, 22, 7978 – 7986
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7984
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´
c) M. K. We˛cławski, M. Tasior, T. Hammann, P. J. Cywinski, D. T. Gryko,
Chem. Commun. 2014, 50, 9105–9108.
(CBP). Poly(9-vinylcarbazole) (PVK) and 2,2’,2’’-(1,3,5-benzinetriyl)-
tris(1-phenyl-1-H-benzimidazole) (TPBi) were used as hole-trans-
porting layer and hole-blocking layer, respectively. The active layer
of about 50 nm was deposited on top of an ITO electrode precoat-
ed with a PEDOT:PSS layer of about 50 nm thickness and with
a PVK layer of about 10 nm thickness. In the subsequent step
a TPBi (50 nm) layer and an ultrathin (1 nm) LiF layer were evapo-
rated followed by deposition of an Al layer. The fabricated devices
were tested under ambient conditions. Characteristics of OLED de-
vices were measured in a ten-inch integrating sphere (Labsphere)
connected to a Source Meter Unit (and calibrated with NIST calibra-
tion lamp)
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Keywords: alkylation · dyes/pigments · luminescence · self-
assembly · semiconductors
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Published online on April 23, 2016
Chem. Eur. J. 2016, 22, 7978 – 7986
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7986
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim