4-Azafluorenone and α-Carboline fluorophores with green and violet/blue emission

Article abstract of DOI:10.3390/molecules24132378

The emission properties of three 4-azafluorenone and five new α-carboline fluorophores in both solution and thin solid films were investigated. Fluorescence of the azafluorenone is clearly enhanced in thin solid films due to the presence of phenyl/biphenyl rotors, and these derivatives can be classified as green Aggregation-Induced Emission luminogens (AIEgens) with a non-emissive heteroaromatic core structure. Compared to azafluorenones, emission of α-carbolines is hypsochromically shifted to the blue region of the electromagnetic spectrum, and most of these derivatives exhibit strong violet-blue fluorescence in both solution and thin solid film layers. Further, the effective mobility and electroluminescence of new α-carbolines were investigated in prepared organic field-effect transistors and organic light-emitting diodes, respectively.

Full text of DOI:10.3390/molecules24132378

molecules
Article
4-Azafluorenone and α-Carboline Fluorophores with
Green and Violet/Blue Emission
1,
2
2
2
2
ˇ
Marek Cigánˇ
*
, Peter Danko , Henrich Brath , Matúš Cakurda , Roman Fišera ,
Jana Donovalová 1, Juraj Filo 1 , Martin Weis 3 , Ján Jakabovicˇ , Miroslav Novota 3 and
Anton Gáplovský 1
3
1
Faculty of Natural Sciences, Institute of Chemistry, Comenius University, Ilkovicˇova 6, Mlynsk
SK-842 15 Bratislava, Slovakia
SYNKOLA, Ltd., Ilkovicˇova 6, 842 15, Bratislava, Slovakia
Institute of Electronics and Photonics, Slovak University of Technology, Ilkovicˇova 3,
SK-81219 Bratislava, Slovakia
á
dolina CH-2,
2
3
*
Correspondence: marek.cigan@uniba.sk
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 5 June 2019; Accepted: 26 June 2019; Published: 27 June 2019
Abstract: The emission properties of three 4-azafluorenone and five new α-carboline fluorophores
in both solution and thin solid films were investigated. Fluorescence of the azafluorenone is
clearly enhanced in thin solid films due to the presence of phenyl/biphenyl rotors, and these
derivatives can be classified as green Aggregation-Induced Emission luminogens (AIEgens) with a
non-emissive heteroaromatic core structure. Compared to azafluorenones, emission of α-carbolines
is hypsochromically shifted to the blue region of the electromagnetic spectrum, and most of these
derivatives exhibit strong violet-blue fluorescence in both solution and thin solid film layers. Further,
the effective mobility and electroluminescence of new
α-carbolines were investigated in prepared
organic field-effect transistors and organic light-emitting diodes, respectively.
Keywords: 4-azafluorenone; α-carboline; fluorescence; aggregation-induced emission; organic thin
films and thin film devices
1. Introduction
The development of luminescent materials allowed gaining enormous knowledge that has
undoubtedly promoted high-tech innovations and benefited the whole world [ ]. For the vast majority
1
of practical applications, luminophores are used as films and aggregates. Due to intense intermolecular
π−π stacking interactions in the aggregate state, luminophores with aromatic rings often suffer from
the phenomenon called aggregation-caused quenching (ACQ), particularly those with a disc or rod-like
shape, and their emission is significantly reduced compared to the solution.
Fortunately, aggregation-induced emission (AIE) is another photophysical phenomenon associated
with chromophore aggregation [1–3]. Whereas the emissive chromophores as molecular species are
called “luminophores”, those non-emissive as molecules but emissive as aggregates are named
“luminogens”. Upon aggregate formation, the emission of luminogens without independent double
bonds is induced or rejuvenated by restricted intramolecular rotation (RIR) or vibratory motions
(RIV; in-plane/out-of-plane bending, flapping, stretching, scissoring, wagging, twisting, rocking,
etc., vibrations) or the synergistic effects of both motions (RIM; restricted intramolecular motion).
As revealed by the Thomson Reuters Essential Science Indicators, AIE has been ranked No. 3 among
the Top 100 Research Frontiers in the field of Chemistry and Materials Science in 2013, reflecting the
increasing global interest towards AIE research [1]. The exploitation of aggregation-induced emission
luminogens (AIEgens) is both of great academic and practical importance and can stimulate rapid
Molecules 2019, 24, 2378; doi:10.3390/molecules24132378
www.mdpi.com/journal/molecules

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development in the areas of optoelectronics, bio-imaging, nanoscience, etc. [
2]. Only recently, Jeffrey S.
Moore et al. demonstrated a simple, robust, and sensitive fluorescence-based approach for autonomous
detection of damage in polymeric materials and composites enabled by AIE [4].
Compared with the pure hydrocarbon aromatic rings, heterocycle rings usually have lone pair
electrons or empty orbits [3]. The AIE phenomenon of several heterocycle-based AIE systems with
high stability due to their whole aromatic conjugated structure, such as phenyl-substituted siloles,
pyrazines, pyrroles, and oxazoliums, has been investigated to date, although most of them are difficult
to synthesize and purify [2]. Moreover, their emissions are mainly centered in the blue light region,
which greatly hampers their practical applications in the field of biological imaging. This makes the
development of new heterocycle-based AIEgens very promising, but challenging. Houk and coworkers
predicted that N-heteroacenes could act as n-type semiconducting or electron-accepting materials
through theoretical calculation [5]. Moreover, N-substituted acenes are believed to be less sensitive
to degradation through oxidation or dimerization [6–8
]. Actually, the number, position, and valence
states of N atoms in backbones of azacenes have a strong effect on their properties [9–11].
Efficient blue organic light-emitting devices (OLEDs) play an important role in both full-color
display and solid-state lighting [12
energy-transfer donor for a low energy dopant to generate green, red, or white light, but also increase
the color gamut and reduce power consumption [15 18]. Fluorene has been intensively studied as
–14]. Especially deep blue emitting materials not only act as the
–
an attractive building block due to its high photoluminescence efficiency, high carrier mobility, and
easy modification. Many fluorene-based blue emitters and their derivatives have been reported and
exhibited impressive device performance [18,19]. Similarly, the fluorenone unit as a strong electron
acceptor was recently used in thermally-activated delayed fluorescence (TADF) materials [20]. TADF
material is a kind of noble metal-free fluorescent material able to transform triplet excitons into singlet
excitons through reverse intersystem crossing (RISC) to achieve 100% internal quantum efficiency in
modern blue OLEDs [20–23]. Efficient blue OLEDs play an important role in both full-color display
and solid-state lighting [18]. Although azafluorenone was discovered decades ago and its derivatives
have received widespread attention in various research fields, the application of azafluorenone and its
derivatives in OLEDs is rarely reported. Only recent results showed the high potential of azafluorenone
and its analogues in the building of efficient host materials [24].
Phosphorescent organic light-emitting diodes (PhOLEDs) have attracted considerable attention
due to high-efficiency device performance in flat-panel display applications. However, the development
of high-efficiency blue PhOLEDs still remains a challenge because a larger energy bandgap and higher
triplet energy are required for carrier transport materials and blue dopants, when compared to red-
and green-emitting layer materials [25]. Among high triplet energy host materials, carbazole has been
widely used as the high triplet energy moiety because of high triplet energy, easy synthesis, and facile
functionalization. However, it was difficult to balance holes and electrons in the emitting layer just by
using carbazole-based host materials due to the strong hole transport character of carbazole. Therefore,
carboline was developed as the high triplet energy unit to compensate for the poor electron transport
properties of carbazole [26]. However, further development of the high triplet energy host materials
derived from carboline is required due to insufficient device performances.
Herein, we report the results of a comprehensive investigation of the emission properties of three
(one new) 4-azafluorenone and five new
thin solid films. Furthermore, the effective mobility and electroluminescence of new
prepared organic field-effect transistors and organic light-emitting diodes were also investigated.
α
-carboline fluorophores (Scheme 1) in both solution and
α
-carbolines in

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Scheme 1. Molecular structure of the studied 4-azafluorenones 1a–1c and α-carbolines 2a–2e.
2. Results and Discussion
2.1. Emission Characteristics
Azafluorenone derivatives 1a
–
1c exhibited light absorption in the UV-A region of electromagnetic
spectrum and only poor emission in solution (low fluorescent quantum yields; Table 1). This behavior
was very similar to 9-fluorenone with a near 100% triplet quantum yield of fluorenone in the non-polar
solvent due to the efficient ISC (intersystem crossing) processes in the S1 state, particularly in the
non-polar solvents [27
,28]. Contrary to parent 4-azafluorenone, the increased amount of water
as a co-solvent in 1a 1c THF solutions led to a significant increase in emission intensity, and the
–
4-azafluorenones thus exhibited the typical feature of aggregation-induced emission luminogens
(AIEgens; Figure 1 and Figure S1). Because the AIE appeared only in the presence of freely-rotatable
phenyl or biphenyl substituents, we assumed that this behavior resulted particularly from the restriction
of intramolecular rotations (RIR) and not the restriction of intramolecular vibrations (RIV). The presence
of the freely-rotatable phenyl and biphenyl substituent was reflected particularly in almost equal values
of solution emission maxima (λE) of disubstituted 1a
, 1b, and monosubstituted 1c derivative (Table 1).
At a water content above 70–80%, the compounds started to precipitate from the solution. The emission

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of 1a–1c was clearly enhanced also in thin solid film layers, and the studied azafluorenones therefore
can be classified as new green AIEgens with a non-emissive heteroaromatic core structure (Figure 2).
Particularly the azafluorenone 1b with a biphenyl rotor has a really high emission enhancement factor
(αAIE), exceeding a value of 100 (Table 1).
Table 1. Photophysical properties of the studied azafluorenone derivatives 1a–1c.
λA (nm)
λE (nm)
ΦF (%)
αAIE
Cpd
Soln
Soln
Soln
Film
1a
1b
1c
386
388
376
475
477
476
1.0
0.5
1.0
11.2
75.0
26.1
11.2
150.0
26.1
Cpd, compound; λA, absorption maximum; λE, emission maximum; ΦF, fluorescent quantum yield; αAIE, emission
enhancement factor (αAIE = ΦF(Film)/ΦF(Solution)); Soln, THF solution.
Figure 1. Fluorescence spectra of 1a
(left) and 1b (right) in THF/water mixtures with different water
fractions. AIE, aggregation-induced emission.
Figure 2. Absorption and emission spectrum of 1b in the thin solid film layer on the quartz substrate
λEXC λA) (left) and micrograph of synthesized azafluorenone 1b under visible (white) light and UV
light (365 nm). The micrograph illustrates the crystals and fluorescence of 1b in thin solid film (right).
(
=
The photophysical characteristics of new
and 3. Due to the non-planar geometry of the ground state, which hampered effective
throughout the molecule (Table S1, Figures S2–S6), absorption of -carbolines 2a 2c and 2e fell again
into the near UV-A region of the electromagnetic spectrum. Compared to azafluorenone derivatives
1a 1c, the fluorescence of 2a 2e was hypsochromically shifted to the blue region of the electromagnetic
spectrum in both the solution and thin solid film layers.
α
-carboline derivatives 2a
–
2e are shown in Tables 2
π
-conjugation
α
–
–
–

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Table 2. Photophysical (absorption and luminescent) properties of the studied carboline derivatives
2a–2e.
λA (nm)
λF (nm)
Cpd
CHCl3
MeOH
CHCl3
MeOH
Film
2a
2b
2c
2d
2e
368
340
376
403
285
365
342
372
399
294
425/415 *
407
422/433 *
437
408/420 *
407
427/413 *
432
450
455
460
450
420
401
414
λA, long-wavelength absorption maximum; λF, fluorescence maximum; *, peak with a slightly smaller intensity due
to the vibrational structure of the emission band.
Table 3. Fluorescence characteristics of the studied carboline derivatives 2a–2e.
τ(CHCl3)
τ(CH3OH)
Cpd
2a
ΦF CHCl3 ΦF CH3OH
ΦF ss
τ(ss) (ns)
(ns)
(ns)
τ1 = 1.58 (23.0%)
τ2 = 4.07 (77.0%)
χ2 = 1.194
τ1 = 0.64 (65.1%)
τ2 = 2.58 (29.0%)
τ3 = 13.1 (5.9%)
χ2 = 1.124
τ1 = 1.30 (48.7%)
τ2 = 3.86 (51.3%)
χ2 = 1.113
τ1 = 0.17 (24.5%)
τ2 = 2.80 (43.4%)
τ3 = 23.9 (32.1%)
χ2 = 1.150
τ = 3.52
τ = 3.47
0.58
0.48
0.94
0.68
0.07
0.29
0.53
0.70
0.74
0.07
0.49
χ2 = 1.181
χ2 = 1.161
τ = 2.91
τ = 5.09
2b
2c
0.41
0.23
1.00
0.52
χ2 = 1.149
χ2 = 1.163
τ = 5.05
τ = 5.11
χ2 = 1.172
χ2 = 1.159
τ = 2.95
τ = 3.49
2d
2e
χ2 = 1.155
χ2 = 1.135
τ1 = 1.89 (15.1%)
τ2 = 8.60 (84.9%)
χ2 = 1.201
Low signal
intensity
Low signal
intensity
ΦF, fluorescent quantum yield;
fitting) %, relative fluorescence intensity of the corresponding component; ss, solid state.
τ, fluorescence lifetime; χ
2, reduced chi-squared (quality of fluorescence lifetime
Luminophores 2a and 2b preserved their solution emission efficiency also in thin solid films, and
only carboline 2e behaved as a violet-blue AIEgen with an αAIE factor value of 7.4 (Figure 3).
Figure 3. Absorption and emission spectrum of 2e in the thin solid film layer on the quartz substrate
(
λEXC
= λA) (left) and the micrograph of synthesized carboline 2e under visible (white) light and UV
light (365 nm). The micrograph illustrates the crystals and fluorescence of 2e in thin solid film (right).

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Carboline 2c exhibited the aggregation-caused quenching (ACQ) phenomenon, and its fluorescent
quantum yield significantly decreased from approximately 0.8 in solution to the value of 0.2 in the
thin solid film (Table 3). As expected, luminophore 2d with a rigid phenylethynyl spacer absorbed
light in the violet region of the electromagnetic spectrum due to increased conjugation of anthracene
with the phenylethynyl linker and exhibited really strong emission in the solid state with almost
100% efficiency (ΦF ~ 1; Table 3). Interestingly, when irradiated with visible (Vis) light of a 405-nm
wavelength, luminophore 2d in thin solid film underwent a photochemically-allowed [4
π + 4π ]
s s
dimerization through anthracene subunits, with the typical decrease of the long-wavelength absorption
band in the Vis region and the appearance of a new strong light anthracene dimer absorption around
250 nm (Figure 4) [29]; contrary to the Diels–Alder dimer of 9-phenylethynylanthracene with the
typical vibrational structure of the anthracene absorption band between 350 and 400 nm [30]. This Vis
light-driven dimerization phenomenon in the solid state and its back thermally-driven reaction could be
important for reversible optical writing and data storage applications [31–33]; however, the switching
system needs deeper investigation, which is out of the scope of this paper. Bi- and tri-exponential
fluorescence decay behavior in the solid films versus mono-exponential fluorescence decay in both
polar and non-polar solutions, together with evident bathochromic shift of the emission maximum,
clearly indicated aggregation of the carbolines 2a–2e in the solid state (Table 3, Figure 5).
Figure 4. Evolution of the absorption spectrum of carboline 2e in the thin solid film layer on the
quartz substrate during its irradiation with Vis light of a 405-nm wavelength (four epoxy-encased
LED diodes; LED405E from Thorlabs) (left) and the proposed structure of the anthracene dimer due
to photochemically-allowed [4πs+ 4πs] dimerization through anthracene subunits during the 2e thin
solid film irradiation with Vis light (λirr = 405 nm) (right).

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Figure 5. Fluorescence decay profiles of the carbolines 2e (A) and 2d (B)in thin solid films obtained
using an excitation wavelength of 340 nm and 402.8 nm, respectively (monitored at emission maximum;
IRF, instrument response function).
Although none of the studied carbolines exhibited the feature of delayed fluorescence (long
fluorescence lifetime component due to the back ISC from the triplet to singlet excited state), the
carboline luminophores 2a–2e were further investigated as emissive layers in organic light-emitting
diodes (OLEDs) and also as transport layers in organic field-effect transistors (OFETs).
2.2. Surface Potential Measurement
The surface potential evaluation by the Kelvin probe is recognized as a well-established method
for estimation of charge transport energy levels. In principle, the surface potential depicts the energy
difference between the work function of the metal substrate and the energy level of an organic
semiconductor. Since the work function, 5.2 eV, of the selected metal (Pd) is close to the calculated
HOMO level, the surface potential reflects hole transport through the metal/organic interface. Table 4
illustrates the HOMO energy levels of synthesized organic semiconductors. Subsequently, the optical
energy gap was estimated using the Tauc plot from the absorbance spectra of the organic film deposited
on the quartz substrate. Hence, the energies of LUMO levels, summarized in Table 4, were evaluated
by the combination of the surface potential measurement and optical absorbance.
Table 4. Comparison of optical energy gaps and electroluminescence peaks, together with the energy
of frontier orbitals. EL, electroluminescence.
Cpd
Eg,opt (eV)
EL (eV)
HOMO (eV)
LUMO (eV)
2a
2b
2c
2d
2e
3.08
3.26
3.05
2.82
3.08
3.25; 2.45
2.55
2.85; 2.60
2.30
−5.7
−5.7
−5.0
−4.5
−4.7
−2.6
−2.4
−2.0
−1.7
−1.6
2.95; 2.60; 2.00
HOMO energy, estimated using the surface potential measurement by means of the Kelvin probe method; LUMO
energy, calculated from the estimated HOMO energy and optical energy gap of the organic film deposited on the
quartz substrate.
2.3. Organic Field-Effect Transistors
The output and transfer characteristics have been recorded for voltage ranging from 40 V to 40 V;
−
however, the output current was significantly lower than 1 nA, which represents effective mobility
on the level of 10−6 or lower. It should be noted that the same result provided a combination of low
mobility and high contact resistance of the fabricated devices. Nevertheless, device optimization is
beyond the scope of the present work.

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Thermal stability is one of the key parameters for molecules used in organic electronics. As shown
in Figures S7–S14, the studied 4-azafluorenone and -carboline derivatives exhibited sufficient thermal
stability, which ranged from 284 ◦C to 392 ◦C for 4-azafluorenones 1a 1c and from 366 ◦C to 456 ◦C for
-carbolines 2a 2e, respectively (the thermal stability was determined from the extrapolated onset
temperature To).
α
–
α
–
2.4. Organic Light-Emitting Diodes
Figure 6 illustrates the energy band diagrams of the designed OLED devices. It should be noted
here that since the deviation of HOMO for Compounds 2a 2e varied from 4.5 to 5.7 eV, the design
should be adjusted to the specific energy levels of material used as an emissive one.
–
−
−
Figure 6. Energy band diagrams of OLED devices using (a) 2a, (b) 2b, (c) 2c, (d) 2d, and (e) 2e as the
emissive layer.
The electroluminescence (EL) spectra of fabricated OLED devices with carboline 2a–2e emissive
layers are depicted in Figure 7. It should be noted here that the EL spectra were independent of
the applied voltage, and only intensity increased proportional to the current density. Hence, all
plotted spectra were normalized for better comparison. Even though the devices were not stable
for long-period measurement, the devices exhibited reproducible characteristics, and spectra were
identical for the group of fabricated devices (eight devices for each derivative).
Figure 7. Normalized electroluminescence spectra of (a) 2a, (b) 2b, (c) 2c, (d) 2d, and (e) 2e.

Molecules 2019, 24, 2378
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Table 4 summarizes the optical energy gap and the energy transitions corresponding to the observed
electroluminescence peaks. In most cases, the energy of emitted light was slightly lower than the energy
gap, which is a common phenomenon of emitting materials. Only the derivative 2a exhibited separated
EL with energy of 3.25 eV, which was higher than the energy gap. The origin of this emission should be
in exciplex formation on the interface of the 2a and 5-(4-biphenylyl)-3-(tert-butylphenyl)-1,2,4-triazole
(TAZ), which serves as an electron transport and hole blocking layer. Furthermore, the OLED device
with the 2d emissive layer showed the presence of exciplex on the 2d-4,7-diphenyl-1,10-phenanthroline
(BPhen) interface. The observed emission was in good agreement with the difference of the BPhen
LUMO level and 2d HOMO level. As a result, the EL emission apparently differed from the
photoluminescence emission due to the mutual interaction between the emissive layer and other layers
in the OLED device.
Micrographs depicted in Figure 8 illustrate crystals and photoluminescence. Obviously, the broad
emission spectra of some derivates led to the white-like emission even though the blue regions were
present there. Since the derivate 2d did not exhibit blue emission, the color appeared yellowish.
Figure 8. Micrographs of synthesized carboline derivatives 2a–2d under visible (white) light and UV
light (365 nm). The micrographs illustrate the crystals and photoluminescence of (a) 2a, (b) 2b, (c) 2c,
and (d) 2d.
3. Materials and Methods
3.1. Synthesis
All chemicals and solvents were purchased from Sigma-Aldrich (Merck KGaA, Darmstadt,
Germany), Fluorochem (Derbyshire, UK) and Merck (Darmstadt, Germany) chemical companies. FTIR
spectra were recorded on a Cary 630 FTIR (Agilent technologies, Santa Clara, CA, USA) using the
ATR technique. Elemental analyses were obtained on an Elementar vario MICRO cube (Elementar,
Langenselbold, Germany). Melting points were recorded on a Kofler apparatus Electrothermal IA-9200
(The Lab Warehouse, Grays, UK). NMR spectra were recorded in a 5-mm NMR tube on a Varian
1
VNMRS 600-MHz spectrometer (600 MHz for H and 150 MHz for 13C, Agilent, Santa Clara, CA,
USA) in DMSO-d6 or CDCl3, with tetramethylsilane (TMS) as an internal standard. High resolution
mass spectrometry (HRMS) analyses were performed on a Thermo Scientific Orbitrap Fusion Mass
Spectrometer (ThermoFisher Scientific, Waltham, MA, USA). Thermogravimetric (TGA) analysis was
carried out on a LINSEIS STA PT 1600 analyzer (Linseis, Selb, Germany).

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3.1.1. 4-Azafluorenones
Synthesis of 1a–1c, general scheme (Scheme 2):
Scheme 2. Synthesis of the studied 4-azafluorenones 1a–1c.
2,4-diphenyl-5H-indeno[1,2-b]pyridin-5-one (1a)
Acetophenone (1.18 mL,1.216 g, 10 mmol, 1 mol eq.), benzaldehyde (1.01 mL, 1.05 g, 10 mmol,
1 mol eq.), and ammonium acetate (1.54 g, 20 mmol,2 mol eq.) were added to a stirred solution of
1H-indene-1,3(2H)-dione (1.462 g, 10 mmol, 1 mol eq.) in anhydrous DMF (40 mL) under argon. The
mixture was warmed up to 100 ◦C and stirred five hours at this temperature. The reaction mixture was
cooled to RT and DMF removed under reduced pressure. Water (20 mL) and EtOAc (20 mL) were
added to the remaining wax and stirred for 30 min. The mixture was extracted with EtOAc (3
mL). The combined organic phases were dried over Na2SO4 and concentrated under reduced pressure.
×
20
The crude product was isolated by LCC (50 g SiO2; DCM in Hex 0/100 to 100/0) [34,35].
◦
2,4-diphenyl-5H-indeno[1,2-b]pyridin-5-one: yield 16.5%, yellow crystalline solid, m.p.: 167–169 C;
1H-NMR (600 MHz, DMSO-d6):
δ
(ppm) 8.31 (m, 2H), 7.96 (d, J = 7.3 Hz, 1H), 7.83 (s, 1H), 7.76 (m, 2H),
7.73 (dtr, J = 7.7, 7.3 Hz, 1H), 7.65 (d, J = 7 Hz, 1H), 7.49–7.57 (m, 7H); 13C-NMR (150 MHz, DMSO-d6):
(ppm) 190.6, 166.0, 160.5, 149.4, 142.6, 137.9, 135.8, 135.36, 135.32, 131.9, 130.8, 130.0, 129.8 (2C), 129.3
(2C), 128.5 (2C), 127.9 (2C), 124.0, 122.7, 121.5, 121.1; HRMS m/z calculated for C24H15NO [M + H]+:
334.1232, found: 334.1223; IR (neat): v = 1710, 1550, 746, 687 cm−1
δ
.
2-([1,10-biphenyl]-4-yl)-4-phenyl-5H-indeno[1,2-b]pyridin-5-one (1b)
Biphenyl-4-acetaldehyde (1.963 g, 10 mmol, 1 mol eq.), benzaldehyde (1.01 mL,10 mmol, 1
mol eq.), and ammonium acetate (1.54 g, 20 mmol, 2 mol eq.) were added to a stirred solution of
1H-indene-1,3(2H)-dione 1 (1.462 g, 10 mmol, 1 mol eq.) in anhydrous DMF (40 mL) under argon. The
mixture was warmed up to 110 ◦C and stirred five hours at this temperature. The reaction mixture was
cooled to RT and DMF removed under reduced pressure. Water (20 mL) and EtOAc (20 mL) were
added to the remaining wax and stirred 30 min. The mixture was extracted with EtOAc (5
The combined organic phases were dried over Na2SO4 and concentrated under reduced pressure. The
crude product was isolated by LCC (50 g SiO2; Hex to DCM).
×
10 mL).
2-([1,10-biphenyl]-4-yl)-4-phenyl-5H-indeno[1,2-b]pyridin-5-one: yield 17.6%, yellow crystalline solid, m.p.:
214–216 ◦C; 1H-NMR (600 MHz, DMSO-d6):
δ (ppm) 8.44 (d, J = 8.5 Hz, 2H), 8.00 (d, J = 7.3 Hz, 1H),
7.92 (s, 1H), 7.86 (d, J = 8.5 Hz, 1H), 7.80–7.76 (m, 4H), 7.74 (dtr, J = 7.6, 7.3 Hz, 1H), 7.76 (d, J = 7.3 Hz,
1H), 7.56 (dtr, J = 7.6, 7.3 Hz, 1H), 7.54–7.48 (m, 5H), 7.41 (tr, J = 7.6, 7.3 Hz, 1H); 13C-NMR (150 MHz,

Molecules 2019, 24, 2378
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DMSO-d6):
δ (ppm) 190.6, 166.1, 160.0, 149.5, 142.6, 142.3, 139.7, 136.9, 135.8, 135.4, 135.3, 132.0, 130.0,
129.9 (2C), 129.5 (2C), 128.5 (4C), 128.4, 127.5 (2C), 127.2 (2C), 124.0, 122.7, 121.4, 121.2; HRMS m/z
calculated for C30H19NO [M + H]+: 410.1544, found: 410.1537; IR (neat):
v .
= 1702, 1546, 746, 689 cm−1
2-phenyl-5H-indeno[1,2-b]pyridin-5-one (1c)
Dimethylamino-1-phenyl-2-propen-1-one (876 mg, 5 mmol, 1 mol eq.), ammonium acetate (611
mg, 10 mmol, 2 mol eq.), sodium iodide (150 mg, 1 mmol, 0.2 mol eq.), and cerium chloride (370 mg,
1.5 mmol 0.3 mol eq.) were added to a stirred solution of 1H-indene-1,3(2H)-dione (730 mg, 5 mmol,
1 mol eq.) in anhydrous isopropanol (25 mL) under argon. The mixture was refluxed 24 h. The reaction
mixture was cooled to RT, and isopropanol was removed under reduced pressure. Water (20 mL)
and EtOAc (20 mL) were added to the remaining wax and stirred 30 min. The mixture was extracted
with EtOAc (3
×
20 mL). The combined organic phases were dried overNa2SO4 and concentrated on a
rotavap. The crude product was purified by flash liquid chromatography (FLC) (50 g SiO2; Hex to
DCM) [36].
2-phenyl-5H-indeno[1,2-b]pyridin-5-one: yield 24%, yellow crystalline solid, m.p.: 148–150 ◦C, 1H-NMR
(600 MHz, DMSO-d6):
7.93 (d, J = 7.3 Hz, 1H), 7.74–7.70 (m, 2H), 7.57–7.50 (m, 4H); 13C-NMR (150 MHz, DMSO-d6):
191.3, 165.0, 161.1, 143.3, 138.0, 136.1, 135.3, 133.0, 131.9, 130.9, 129.4 (2C), 127.6 (2C), 126.7, 124.3, 121.2,
δ
(ppm) 8.22 (d, J = 7.9 Hz, 2H), 8.04 (d, J = 7.9 Hz, 1H), 7.95 (d, J = 7.9 Hz, 1H),
δ
(ppm)
120.5; HRMS: m/z calculated for C18H11NO [M + H]+: 258.0919, found: 258.0911; IR (neat):
v = 1708,
1568, 1408, 745, 687 cm−1
.
3.1.2. α-Carbolines
A. Synthesis of 2a–2c, general scheme (Scheme 3):
Scheme 3. Synthesis of the studied α-carbolines 2a–2c.

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Preparation of Intermediates A and B:
Preparation of 9-(4-iodophenyl)-9H-pyrido[2,3-b]indole (Intermediate A) and 9-(40-iodo-[1,10-
biphenyl]-4-yl)-9H-pyrido[2,3-b]indole (Intermediate B)
General Procedure A: 9-(40-iodo-[1,10-biphenyl]-4-yl)-9H-pyrido[2,3-b]indole (Intermediate B)
To a solution of 9H-pyrido[2,3-b]indole (1.00 g, 5.95 mmol, 1 mol eq.) in toluene/dry (60 mL) were
added 3.62 g of 4,40-diiodobiphenyl (8.93 mmol, 1.5 mol eq.), 6.32 g of K3PO4 (7.15 mmol, 5 mol eq.), 0.36
mL of trans-1,2-diaminocyklohexane (340 mg, 2.98 mmol, 0.5 mol eq.), and 225 mg of CuI (1.19 mmol.,
0.2 mol eq.). The resulting mixture was refluxed under argon for 18 h. After consumption of all starting
material (monitored by TLC), the reaction mixture was cooled to RT, filtered through Celite, and the
filtration cake was washed with DCM. The crude product was isolated by FLC (CombiFlash; 330 g
SiO2; 100 mL/min; Hex/EtOAc 3:1 v/v).
9-(40-iodo-[1,10-biphenyl]-4-yl)-9H-pyrido[2,3-b]indole: yield 1.06 g, 40%, white solid; m.p.: 210.1–212.3 ◦C
1
[EtOAc/Hex]; H-NMR (CDCl3, 600 MHz):
δ (ppm) 8.51 (dd, J = 4.8, 1.6 Hz, 1H), 8.41 (dd, J = 7.6,
1.6 Hz, 1H), 8.15 (d, J = 7.8 Hz, 1H), 7.82 (dm, J = 8.4 Hz, 2H), 7.79 (dm, J = 8.6 Hz, 2H), 7.74 (dm,
J = 8.6 Hz, 2H), 7.56 (d, J = 8.2 Hz, 1H), 7.50 (ddd, J = 8.2, 7.0, 1.0 Hz, 1H), 7.42 (dm, J = 8.4 Hz, 2H),
7.36 (ddd, J = 8.0, 7.0, 1.0 Hz, 1H), 7.27–7.25 (m, 1H); 13C-NMR (CDCl3, 150 MHz):
δ (ppm) 151.8, 146.3,
140.0, 139.9, 139.3, 138.0, 135.8, 129.0, 128.5, 128.2, 127.7, 127.1, 121.0, 120.93, 120.90, 116.5, 116.2, 110.4,
93.4; HRMS: m/z calculated for C23H15N2I [M + H]+; 447.0353, found: 447.0352.
9-(4-iodophenyl)-9H-pyrido[2,3-b]indole (Intermediate A)
Intermediate A was prepared according to General Procedure A.
9-(4-iodophenyl)-9H-pyrido[2,3-b]indole: yield 620 mg, 47%, white solid; m.p.: 145.0–146.7 ◦C [EtOAc/Hex];
1H-NMR (CDCl3, 600 MHz):
δ
(ppm) 8.47 (dd, J = 5.0, 1.6 Hz, 1H), 8.38 (dd, J = 7.7, 1.6 Hz, 1H),
8.12 (ddd, J = 8.6, 7.9, 0.9 Hz, 1H), 7.93 (dm, J = 8.6 Hz, 2H), 7.50–7.47 (m, 2H), 7.43 (dm, J = 8.6 Hz,
2H), 7.37–7.32 (m, 1H), 7.25 (q, J = 7.7 Hz, 1H); 13C-NMR (CDCl3, 150 MHz):
(ppm) 151.8, 146.6,
δ
139.8, 139.0, 136.2, 129.3, 128.7, 127.3, 121.3, 121.2, 116.8, 116.6, 110.4, 92.7; HRMS: m/z calculated for
C17H11N2I [M + H]+; 371.0040, found: 371.0039.
Reaction scheme (Scheme 4):
Scheme 4. Synthesis of the intermediates A and B.
Preparation of final products 2a–2c (Suzuki coupling reactions):
9-(40-(anthracen-9-yl)-[1,10-biphenyl]-4-yl)-9H-pyrido[2,3-b]indole (2a)
General Procedure B
In a 50-mL round-bottom flask, (intermediate B) (781 mg, 1.75 mmol, 1 mol eq.),
anthracene-9-boronic acid (583 mg, 2.63 mmol, 1.5 mol eq.), aqueous solution of K2CO3 (2.2 mL,
4.38 mmol, 2.5 mol eq.), EtOH (8 mL), and toluene (16 mL) were stirred under argon for 5 min. Then,

Molecules 2019, 24, 2378
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Pd(PPh3)4 (61 mg, 0.053 mmol, 0.05 mol eq.) was added. The flask was evacuated and backfilled with
argon three times, and the resultant reaction mixture was stirred at reflux (oil bath) for 18 h. After
cooling to RT, the solvent was evaporated under reduced pressure. The solid material was dissolved in
DCM (30 mL), washed with water (3
of the solvent, the crude product was purified by FLC (CombiFlash, 120 g SiO2, 75 mL/min, Hex/DCM
1:1 v/v).
×
10 mL), and dried over Na2SO4. After filtration and evaporation
9-(40-(anthracen-9-yl)-[1,10-biphenyl]-4-yl)-9H-pyrido[2,3-b]indole: yield 575 mg, 66%, pale yellow solid,
m.p.: 312.4–316.8 ◦C [EtOAc/Hex]; 1H-NMR (CDCl3, 600 MHz):
δ (ppm) 8.55 (dd, J = 4.8, 1.6 Hz, 1H),
8.53 (s, 1H), 8.43 (dd, 7.7, 1.6 Hz, 1H), 8.18–8.15 (m, 1H), 8.08 (dm, J = 8.6 Hz, 2H), 7.99 (dm, J = 8.6 Hz,
2H), 7.89 (dm, J = 8.2 Hz, 2H), 7.82 (dm, J = 8.6 Hz, 2H), 7.78 (dd, J = 8.9, 0.5 Hz, 2H), 7.62 (d, J = 8.3
Hz, 1H), 7.56 (dm, J = 8.2 Hz, 2H), 7.53 (ddd, J = 8.3, 7.2, 1.1 Hz, 1H), 7.51–7.47 (m, 2H), 7.42–7.36 (m,
3H), 7.28 (q, J = 7.7 Hz, 1H); 13C-NMR (CDCl3, 150 MHz):
δ (ppm) 146.1, 140.3, 140.1, 139.5, 138.1,
136.6, 135.4, 131.8, 131.4, 130.2, 128.7, 128.5, 128.4, 127.7, 127.18, 127.15, 126.8, 125.4, 125.2, 121.03, 121.0,
120.9, 116.1, 110.6; HRMS: m/z calculated for C37H24N2 [M + H]+: 497.2012, found: 497.2012.
9-(40-(pyren-1-yl)-[1,10-biphenyl]-4-yl)-9H-pyrido[2,3-b]indole (2b)
Compound 2b was prepared according to General Procedure B.
9-(40-(pyren-1-yl)-[1,10-biphenyl]-4-yl)-9H-pyrido[2,3-b]indole: yield 308 mg, 53%, pale orange solid, m.p.:
235.6–237.4 ◦C [EtOAc/Hex]; 1H-NMR (CDCl3, 600 MHz):
δ (ppm) 8.55 (dd, J = 4.8, 1.6 Hz, 1H), 8.43
(dd, J = 7.6, 1.6 Hz, 1H), 8.3 (d, J = 9.3 Hz, 1H), 8.27 (d, J = 7.8 Hz, 1H), 8.22 (d, J = 7.7 Hz, 1H), 8.19 (d,
J = 7.4 Hz, 1H), 8.17 (d, J = 7.8 Hz, 1H), 8.12 (m, 1H), 8.08 (d, J = 9.3 Hz, 1H), 8.06 (d, J = 7.9 Hz, 1H),
8.04 (t, J = 7.6 Hz, 1H), 7.97 (dm, J = 8.2 Hz, 2H), 7.88 (dm, J = 8.2 Hz, 2H), 7.81 (dm, J = 8.2 Hz, 2H),
7.78 (dm, J = 8.2 Hz, 2H), 7.62 (d, J = 8.2 Hz, 1H), 7.53 (ddd, J = 8.2, 7.2, 1.0 Hz, 1H), 7.38 (ddd, J = 8.2,
7.2, 1.0 Hz, 1H), 7.28 (dd, J = 7.7 Hz, 1H); 13C-NMR (CDCl3, 150 MHz):
δ (ppm) 145.9, 140.5, 140.3,
140.1, 139.4, 137.2, 131.5, 131.2, 131.0, 130.7, 128.53, 128.51, 127.7, 127.61, 127.60, 127.5, 127.4, 127.3,
127.2, 126.0, 125.3, 125.2, 125.0, 124.93, 124.90, 124.7, 121.1, 120.8, 116.1, 110.6; HRMS: m/z calculated for
C39H24N2 [M + H]+: 521.2012, found: 521.2012.
9-(4-(10-phenylanthracen-9-yl)phenyl)-9H-pyrido[2,3-b]indole (2c)
Compound 2c was prepared according to General Procedure B.
9-(4-(10-phenylanthracen-9-yl)phenyl)-9H-pyrido[2,3-b]indole: yield 371 mg, 92%, pale yellow solid, m.p.:
265.5–268.0 ◦C [EtOAc/Hex]; 1H-NMR (CDCl3, 600 MHz):
δ (ppm) 8.61 (dd, J = 4.8, 1.5 Hz, 1H), 8.47
(dd, J = 7.7, 1.5 Hz, 1H), 8.2 (d, J = 7.7 Hz, 1H), 7.95 (dm, J = 8.2 Hz, 2H), 7.9 (dm, J = 8.3 Hz, 2H),
7.78–7.16 (m, 5H), 7.65–7.60 (m, 2H), 7.6–7.55 (m, 2H), 7.53–7.49 (m, 2H), 7.43–7.35 (m, 5H), 7.32 (q,
J = 7.7 Hz, 1H); 13C-NMR (CDCl3, 150 MHz):
δ (ppm) 146.3, 140.1, 139.0, 138.3, 137.5, 136.2, 135.6,
132.6, 131.3, 129.91, 128.6, 128.4, 127.5, 127.2, 127.1, 127.0, 125.2, 125.1, 121.1, 121.0, 116I.8, 116.2, 110.7;
HRMS: m/z calculated for C37H24N2 [M + H]+; 497.2012, found: 497.2011.
B. Synthesis of 2d and 2e, general scheme (Scheme 5):

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Scheme 5. Synthesis of the studied α-carbolines 2d and 2e.
9-(4-iodophenyl)-3-trifluoromethyl-9H-pyridino[2,3-b]indole (Intermediate C)
A portion of 3-trifluoromethyl-9H-pyridino[2,3-b]indole1 (826 mg, 3.5 mmol, 1 mol eq.),
1,4-diiodonenzene (1.24 g, 4.375 mmol, 1.25 mol eq.), K3PO4(3.715 g, 17.5 mmol, 5 mol eq.), CuI
(9.135 g, 0.7 mmol, 0.5 mol eq.), and trans-1,2-diaminocyclohexane (200 mg, 0.21 mL, 1.75 mmol, 0.5
mol eq.) were suspended in dry toluene (25 mL) under an argon atmosphere, and the mixture was
stirred at 110 ◦C for 18 h. After finishing the reaction, the mixture was filtered through Celite and
washed with DCM. The organic phase was concentrated on a rotavap. The crude product was purified
by FLC (CombiFlash, 120 g SiO2, 75 mL/min, Hex/DCM 1:1 v/v).
9-(4-iodophenyl)-3-trifluoromethyl-9H-pyridino[2,3-b]indole: yield 65.1%, white solid, m.p.: 151.4–153.2 ◦C;
1H-NMR (600 MHz, DMSO-d6):
δ
8.72 (d, J = 1.2 Hz, 1H), 8.61 (d, J = 1.8 Hz, 1H), 8.17 (d, J = 7.8 Hz,
1H), 7.97 (m, 2H), 7.56 (dtr, J = 8.4 Hz, 7.0 Hz, 1H), 7.49 (d, J = 8.4 Hz, 1H), 7.43–7.40 (m, 3H); 13C-NMR
(150 MHz, DMSO-d6): (ppm) 152.8, 143.68, 143.65, 140.5, 138.9, 135.3, 130.0, 128.9, 127.3, 126.7, 124.5,
δ
122.84, 122.35, 120.41, 118.76 (q), 115.93, 111.01, 94.32; HRMS: m/z calculated for C18H10F3IN2 [M + H]:
438.914, found: 438.9913.
Reaction scheme (Scheme 6):
Scheme 6. Synthesis of the intermediate C.

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Note: The corresponding starting
steps with a satisfactory yield.
α
-carboline was prepared according to the literature [37] in two
9-[4-(anthracene-90-ethynyl)phenyl]- 3-trifluoromethyl-9H-pyridino[2,3-b]indole (2d)
9-(4-iodophenyl)-3-trifluoromethyl-9H-pyridino[2,3-b]indole (391 mg, 1.0 mmol, 1 mol eq.),
9-ethynylanthracene (242 mg, 1.2 mmol, 1.2 mol eq.), and PdCl2(PPh3)2 (20 mg, 0.03 mmol, 3 mol%)
were dissolved in freshly-degassed triethylamine (10 mL) under argon atmosphere. After 5 min, copper
(I) iodide (11.8 mg, 0.025 mmol, 2.5 mol%) was added. The reaction mixture was stirred at 70 ◦C for 16
h. The crude product was purified by FLC (CombiFlash 50 g silica gel) and gave the title compound
(145 mg, 56.6%) as a yellow solid. The chromatographed product was crystallized from EtOAc (yellow
powder, 84 mg. 32.78%).
9-[4-(anthracene-90-ethynyl)phenyl]-3-trifluoromethyl-9H-pyridino[2,3-b]indole: yield: 32.78%, yellow
◦
1
powder, m.p.: 242.0–244.5 C, H-NMR (600 MHz, DMSO-d6):
δ 9.20 (d, J = 1 Hz, 1H), 8.86 (d,
J = 1 Hz, 1H), 8.76 (s, 1H), 8.68 (d, J = 8.4 Hz, 2H), 8.52 (d, J = 7.8 Hz, 2H), 8.22 (d, J = 8.4 Hz, 2H), 8.16
(d, J = 8.4 Hz, 2H), 7.90 (d, J = 8.4 Hz, 2H), 7.76 (dt, J = 7.2, 1.0 Hz, 2H), 7.67–7.63 (m, 4H), 7.50 (dt,
J = 7.2, 1.0 Hz, 1H); 13C-NMR (150 MHz, DMSO-d6):
δ (ppm) 153.2, 143.9 (t, J = 3.9 Hz), 140.8, 135.7,
133.2, 132.9, 131.4, 129.0, 128.4, 128.3, 127.5, 127.0, 126.9, 125.9, 125.9, 125.8, 125.8, 123.7, 122.1, 121.7,
119.8, 119.6, 117.1, 116.1, 11.1, 100.1, 87.7; HRMS: m/z calculated for C34H19F3N2 [M + H]+: 513.1573,
found: 513.1574.
4,40-Bis(3-trifluoromethyl-9H-pyridino[2,3-b]indole-9-yl)-1,10-biphenyl (2e)
A portion of 3-trifluoromethyl-9H-pyridino[2,3-b]indole (605 mg, 2.50 mmol, 2.5 mol eq.),
4,40-diiodobiphenyl (507 mg, 1.0 mmol, 1 mol eq.), K3PO4(5.31 g, 25 mmol, 20 mol eq.), CuI (80 mg,
0.5 mmol, 0.4 mol eq.), and trans-1,2-diaminocyclohexane (286 mg, 0.30 mL, 2.5 mmol, 2 mol eq.) were
suspended in dry toluene (36 mL) under an argon atmosphere, and the mixture was stirred at 110 ◦C
for 18 h. After finishing the reaction, the mixture was filtered through Celite and washed with DCM.
The crude product was purified by FLC (CombiFlash, 120 g SiO2, 75 mL/min, Hex/DCM 3:1 to 2:3 v/v).
4,40-Bis(3-tri◦fluoromethyl-9H-pyridino[2,3-b]indole-9-yl)-1,10-biphenyl: yield 420 mg, 67.46%, white solid,
◦
1
m.p.: >325 C (dec) C, H-NMR (600 MHz, DMSO-d6):
δ (ppm) 9.20 (d, J = 1.8 Hz, 1H), 8.85 (d,
J = 1.8 Hz, 1H), 8.51 (d, J = 7.8 Hz, 1H), 8.12 (dd, J = 8.4, 1.8 Hz, 2H), 7.88 (d, J = 8.4 Hz, 2H), 7.66–7.63
(m, 1H), 7.49 (t, J = 7.8 Hz, 1H); not sufficiently soluble for 13C-NMR in either DMSO-d6 or CDCl3;
HRMS: m/z calculated for C36H20F6N4 [M + H]+: 623.1665, found: 623.1665.
3.2. Spectroscopic Measurements
Electronic absorption spectra were obtained on an Agilent 8453 diode array spectrophotometer
(Hewlett Packard, Palo Alto, CA, USA). The solvents used were HPLC or UV spectroscopy grade
(MeOH, for spectroscopy, Acros Organics, Geel, B; CHCl3, Uvasol®, Merck, Darmstadt, Germany) and
were used without further purification. Solution fluorescence was measured in a 1-cm cuvette with an
FSP 920 (Edinburgh Instruments, Livingston, UK) spectrofluorometer in a right-angle arrangement.
Fluorescence of thin solid film layers on quartz substrate (1 cm
arrangement (FSP 920 spectrofluorometer; Edinburgh Instruments, UK). The fluorescent quantum yield
ΦF) of the studied compounds in solution or in thin solid film layer was determined by Equations (1)
×
3 cm) was measured in front-face
(
and (2) using an integrating sphere (Edinburgh Instruments):
LSam
ERef − ESam
ΦFX
=
(%)
(1)
(2)
corrected to re-absorption by:
ΦFX
ΦF
=
(%)
1 − a + aΦXF /100

Molecules 2019, 24, 2378
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where LSam is the area under the detected spectrum in the part of the spectrum where sample emission
occurs, ERef is the area under the reflection part of the detected spectrum using the pure solvent (or
uncovered quartz substrate) as the reference material (diffuse reflectance), ESam is the area under
the reflection part of the detected spectrum after absorption by the sample, and a is the reabsorbed
area. The time-resolved fluorescence measurements were performed on an FSP 920 (Edinburgh
Instruments, UK) spectrofluorometer with a time-correlated single-photon counting (TCSPC) module
and a red-sensitive high-speed photomultiplier in Peltier housing, featuring a Hamamatsu H5773-04
detector (R928P detector; Edinburgh Photonics, Livingston, UK). The excitation source was a 342.0-nm
picosecond pulsed LED (Model EPLED-340; pulse width: 950.3 ps; Edinburgh Photonics, UK) or a
402.8-nm picosecond pulsed diode laser (Model EPL-405; pulse width: 60.5 ps; Edinburgh Photonics,
UK). Fluorescence decay (lifetime) of thin solid film layers on the quartz substrate (1 cm
measured in front-face arrangement. Reconvolution fit analysis software (F900, Edinburgh Instruments)
was used for lifetime data analysis.
×
3 cm) was
3.3. Organic Thin Films and Thin Film Devices
Organic thin films and thin film devices were fabricated by thermal evaporation under a high
vacuum. All solid substrates such as quartz, glass covered by indium tin oxide (ITO), or silicon wafers,
were cleaned in an ultrasonic bath subsequently by acetone, isopropyl alcohol, 20 vol% ethanolamine
aqueous solution, and in deionized water prior to further processing. UV/ozone cleaning (UV-2 by
SAMCO, Santa Clara, CA, USA) was performed just prior to thin film deposition to remove residual
organic contamination. Then, a 100 nm-thick organic layer was thermally evaporated in a vacuum
with a pressure lower than 10−4 Pa with a fixed deposition rate of 3 nm/min (SPECTROS 100 by Kurt
J. Lesker, Jefferson Hills, PA, USA) controlled by a quartz crystal microbalance. The organic film
thickness was verified by a mechanical profilometer (Dektak 150 by Bruker, Billerica, MA, USA).
Copper phthalocyanine (CuPc), 2,20,200-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)
(TPBi), and 4,7-diphenyl-1,10-phenanthroline (BPhen) were supplied by Sigma-Aldrich, whereas
3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) and 4,7-diphenyl-1,10-
phenanthroline (BPhen) were provided by Ossila company. All materials were used without further
purification. The CuPc film of 5 nm was used as a hole injection layer only, whereas other layers were
designed as hole-transport layers or electron-transport and hole-blocking layers.
The optical and fluorescence microscopy images were taken by an optical microscope with a 1000
objective and large working distance under white light or UV (365 nm) illumination. Distance/size
calibration was done using a 1951 USAF resolution test chart.
×
The energy level of the highest occupied molecular orbital (HOMO) was estimated using the
surface potential measurement by means of the Kelvin probe method. A highly sensitive electrostatic
voltmeter 320C and Kelvin probe electrode (Model 3250 by TREK Inc., Lockport, NY, USA) that allowed
measuring of surface potential with an accuracy of 1 mV were used. The probe was situated in air
approximately 1 mm above the organic layer surface. To perform the surface potential measurement,
the palladium layer (100 nm) was deposited on the quartz substrate (1 cm
×
3 cm), whereas the
organic layer was subsequently deposited on half of the surface to record the surface potential profile
and energy difference between the Fermi energy of the metal and the HOMO level of the organic
semiconductor. We decided to use palladium due its environmental stability, as well as the well-defined
Fermi energy of polycrystalline film (5.22 eV).
The organic light-emitting diodes (OLEDs) were used for estimation of electroluminescence
spectra. The ITO was used as the transparent anode for hole injection, whereas the calcium protected by
a silver layer was applied as the metal cathode for electron injection. The hole-injection, hole-transport,
electron-transport, and electron-injection layers were selected in accordance with the specific values
of the HOMO and LUMO energy levels of the emissive layer. Electroluminescence spectra were
recorded in a nitrogen atmosphere by a spectrometer (AvaSpec-2048 FiberOptic Spectrometer by
Avantes, Apeldoorn, The Netherlands).

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The effective mobility was evaluated using organic field-effect transistor (OFET) devices with
top-contact bottom-gate geometry. The silicon wafers with a 110-nm thermally-grown silicon dioxide
(SiO2) layer as a substrate and gate electrode were used. Organic semiconductors were deposited
onto a bare and 1,1,1,3,3,3-hexamethyldisilazane-modified (HMDS, Sigma-Aldrich) SiO2 surface to
verify the film transport properties deposited on the hydrophilic, as well as the hydrophobic gate
insulator. After the deposition of the semiconducting layer, copper was thermally evaporated through
the shadow mask to form the source and drain electrodes. The OFET devices had channel lengths L
and a width W of 50~200 µm and 2.5 mm, respectively. All output and transfer characteristics were
measured in a nitrogen atmosphere using a semiconductor parameter analyzer (B1500A by Keysight,
Santa Rosa, CA, USA).
3.4. Quantum-Chemical Calculations
Ground state molecular geometry was optimized at the B3LYP/G-311G* level of theory using
Spartan ’10, Version 1.1.0 (Wavefunction, Inc., Irvine, CA, USA).
4. Conclusions
This paper investigated the emission properties of three 4-azafluorenone and five α-carboline
fluorophores in solution and thin solid film layers. Azafluorenone derivatives exhibited typical
features of AIEgens with the emission enhancement factor ranging from 10 to 150, whereas most
of the
solid films. Although the prepared
field-effect transistors and the OLED devices with the
α
-carboline fluorophores preserved their strong violet-blue solution emission also in thin
-carbolines had low effective mobility in prepared organic
-carbolines as emissive layers were not stable
α
α
for long-period measurement, both types of compounds could represent an important class of new
fluorophores/fluorogens in the blue/green emission region or host materials in blue phosphorescent
organic light-emitting diodes (PhOLEDs).
Supplementary Materials: The following are available online: Figure S1: Dependence of the fluorescence intensity
of 1a and 1b on the water fraction in THF, Table S1: Calculated dihedral angle values between aryl moieties
of 2a
Thermogravimetric curves of 1a
of 4-azafluorenones 1a
1c; Figures SX13–SX43: 1H-NMR, 13C-NMR, FTIR, and HRMS spectra of
2a–2e and Intermediates A, B, and C.
–
2e, Figures S2–S6: Molecular structure and ground state molecular geometry of 2a
1c and 2a
2e, Figures SX1–SX12: 1H-NMR, 13C-NMR, FTIR, and HRMS spectra
-carbolines
–2e, Figures S7–S14:
–
–
–
α
ˇ
Author Contributions: M.C. wrote the article and designed the experiments; P.D. and M.C. synthesized and
characterized carbolines 2a 2e; H.B. synthesized and characterized azafluorenones 1a 1c; R.F. designed the
–
–
synthesis; J.D. carried out all UV-Vis and fluorescence experiments and determined the basic spectral characteristics;
J.F. carried out the quantum-chemical calculations and performed all NMR experiments; M.W. designed the
mobility and electroluminescence experiments and wrote the article part related to these experiments; J.J. and
M.N. carried out and interpreted the experiments related to OFETs and OLEDs; A.G. designed the whole project
and experiments.
Funding: This work was financially supported by the Slovak Research and Development Agency (Grant
APVV-15-0087).
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; nor in the
decision to publish the results.
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Sample Availability: Samples of the compounds are not available from the authors.
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