Synthesis, Structure and Properties of Fused π-Extended Acridone Derivatives

Article abstract of DOI:10.1002/ejoc.202000871

Benzene- and naphthalene-fused acridone derivatives with hexyl and phenyl groups at the amino position were synthesized and their properties were investigated experimentally and computationally. All the structures of these fused π-extended acridone derivatives were unambiguously confirmed by single-crystal X-ray analysis, which revealed the presence of face-to-face π–π stackings along the acridone moiety and the intermolecular hydrogen bond-directed molecular packings of the phenyl-substituted acridone derivatives in the crystals. Moreover, the dimerization of linearly fused acridone derivative was observed after storing the crystals over months. The benzene ring at the turning point of the angularly fused acridone derivatives contained relatively longer and shorter C–C bonds, which affected the molecular conjugation, as confirmed by the results of photophysical characterization and the study of the aromaticity. Mobility calculations based on the molecular packings in the single crystals showed that the linearly fused acridone derivatives bearing better electron and hole mobilities are good candidates of organic functional materials.

Full text of DOI:10.1002/ejoc.202000871

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Synthesis, Structure and Properties of Fused π-Extended
Acridone Derivatives
Authors: Hongshuai Gao and Gang Zhang
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000871
Link to VoR: https://doi.org/10.1002/ejoc.202000871
01/2020

10.1002/ejoc.202000871
European Journal of Organic Chemistry
FULL PAPER
Synthesis, Structure and Properties of Fused π-Extended
Acridone Derivatives
Hongshuai Gao,[a] and Gang Zhang*[a]
[a]
Hongshuai Gao, Prof. Dr. Gang Zhang
Jiangsu Co-Innovation Center for Efficient Processing and Utilization of Forest Products, College of Chemical Engineering
Nanjing Forestry University
Longpan Road 159, Nanjing, 210037, P. R. China
E-mail: gangzhang@njfu.edu.cn
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Benzene- and naphthalene-fused acridone derivatives with
anthraquinone to the amino position of acridone, the resulted
donor-acceptor system shows TADF feature.[7] When the
hexyl and phenyl groups at the amino position were synthesized and
their properties were investigated experimentally and computationally. rotatable triphenylamine and tetraphenylethene groups are linked
All the structures of these fused π-extended acridone derivatives were
unambiguously confirmed by single crystal X-ray analysis, which
revealed the presence of face-to-face π-π stackings along the
acridone moiety and the intermolecular hydrogen bond-directed
molecular packings of the phenyl substituted acridone derivatives in
the crystals. Moreover, the dimerization of linearly fused acridone
derivative was observed after storing the crystals over months. The
benzene ring at the turning point of the angularly fused acridone
derivatives contained relatively longer and shorter C-C bonds, which
affected the molecular conjugation, as confirmed by the results of
photophysical characterization and the study of the aromaticity.
Mobility calculations based on the molecular packings in the single
crystals showed that the linearly fused acridone derivatives bearing
better electron and hole mobilities are good candidates of organic
functional materials.
to the amino position of acridone, the obtained molecules
demonstrate interesting tunable aggregation induced emissions
and photoluminescence properties according to the external
stimuli.[8] Stronger electron-withdrawing malononitrile is attached
to the carbonyl position of acridone by the formation of C-C double
bond to adjust the energy levels of frontier molecular orbitals.[4a,9]
The condensation of cyanoacetic esters and acridone can form
molecular rotors with lower rotational barriers due to the
generation of donor-acceptor system.[10] After linking fluorene to
the carbonyl of acridone, the acquired molecules demonstrate
different kinds of chromisms with respect to the conformational
changes.[11]
All these aforementioned π-conjugated acridone derivatives
are synthesized by coupling the functional groups via simple C-C
or C-N bonds. But the reports on the π-extended acridone
derivatives in the fusion manner are still quite limited.[12] The π-
extension of acridone itself can generate a three dimensional
butterfly-shaped polycyclic arene with decreased fluorescence
intensity due to the twisted structure.[12a] Indole can be fused to
the benzene ring of acridone to provide an excited-state
intramolecular proton transfer (ESIPT) system with potential
applications in organic field-effect transistors.[12b] Two pieces of
quinolone are fused to the same benzene ring of acridone to form
intramolecular hydrogen bonds between the amine and the
neighbouring oxygen atom of carbonyl.[12c] This novel ESIPT
system shows efficient TADF property and has been applied in
the high perfomance OLED divices. Recently our group reported
the fusion of the pending benzene ring at the amino position of
acridone to form two adjacent pentagon rings can generate
interesting concave and boat-shaped polycyclic aromatic
hydrocarbons embedded with one or two nitrogen atoms.[12d,12e]
Although there have been other reports on the aromatic rings-
fused acridone derivatives, most of them are concerned on the
bioactivities and potential pharmaceutical applications without
regarding their structural information and optoelectronic
properties.[13]
Introduction
Organic π-conjugated polycyclic heteroaromatic compounds
have attracted remarkable attentions not only for the fundamental
studies of molecular properties, but also for their potential
applications in various electronic devices.[1] Among the diverse
types of heteroatom-doped polycyclic aromatic hydrocarbons, the
rigid and planar acridone has recently been realized as an
excellent unit in the construction of organic functional materials.
Acridone is an anthracene-like tricyclic aromatic compound
containing a central pyridone ring with strong fluorescence
emission and good photostability.[2] The benzene rings, the
carbonyl and amino positions of acridone are good reaction sites
for further functionalization. For example, carbazole and its
dendron have been introduced to the benzene rings of acridone
to give compounds with thermally activated delayed fluorescence
(TADF) behaviour and potential applications in the organic light-
emitting diode (OLED) devices.[3] Diarylamines can couple with
the halgonated acridone to form the donor-acceptor system
serving as hole-transporting materials in optoelectronic devices.[4]
The conjugation of acridone can be further extended by the
formation of acridone oligomer and polymer.[5] The amino position
of acridone can be functionalized with aryl groups to furnish the
host materials for OLEDs.[6] Connecting the electron-withdrawing
The pyridone ring in acridone possesses weak aromaticity and
the intramolecular charge transfer from nitrogen atom to the
carbonyl group can supply the entire molecule with good
conjugation. Thus acridone is a promising unit in optoelectronic
materials, once its molecular skeleton is further π-extended to
1
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10.1002/ejoc.202000871
European Journal of Organic Chemistry
FULL PAPER
narrow the energy gap of the frontier molecular orbitals. We
envision that the linear fusion of anthracene-like acridone with
benzene and naphthalene may furnish molecules with similar
optoelectronic property to that of polycyclic acence.[14] To explore
the property of fused π-extended acridone derivatives and
investigate their structure-property relationships, here we report
the syntheses and properties of benzene- and naphthalene-fused
acridone derivatives with the N-substitution by phenyl and hexyl
groups.
The intramolecular electrophilic cyclization of compounds 2 is
regioselective and the reaction always occurs at the 1-position of
naphthalene and anthracene. The reason for the formation of
angular acridone derivatives rather than the linear ones was
studied by density functional theory (DFT) calculations.[16]
Compound 2BN was taken as an example to explain the reaction
pathway (Figure 1). Compared to that of the linear intermediate,
the formation of the angular intermediate needs to overcome
lower energy barrier and the generated angular intermediate
possesses lower potential energy. Thus the intramolecular
electrophilic cyclization of compounds 2 prefer to form angular
acridone derivatives via the thermodynamically more stable
angular intermediate.
Results and Discussion
Synthesis
The key step in the synthesis of fused π-extended acridone is the
construction of the pyridone ring, which can be achieved by
various reported methods.[15] A convenient way to synthesize
acridone is the acid-promoted cyclization of N-phenylanthranilic
acid.[15a] Based on this consideration, we carried out the synthetic
route starting with commercially available methyl 2-
bromobenzoate and lab-synthesized methyl 3-bromo-2-
naphthoate (Scheme 1). The palladium-catalyzed Buchwald-
Hartiwig amination of the bromide 1 and the arylamine (aniline, 2-
naphthylamine, 2-anthracylamine) with cesium carbonate as the
base afforded the intermediates 2 in 83-89% yields. Compound 2
was designed to generate the linear π-extended acridone
derivatives after the cyclization of carboxymethyl with the aryl
group connecting to the amine to form the pyridone moiety.
However, after the cyclization in methanesulfonic acid, the
obtained π-extended acridone derivatives 3 were all angular ones
with the exception of linear compound 3NB.
Figure 1. Potential energy diagram for the electrophilic cyclization of 2BN
calculated at the B3LYP/6-311+G (d,p) level of theory.
To improve the solubility of the fused acridone derivatives,
compounds 3 were further functionalized at the amino position
with either aliphatic hexyl group via nucleophilic substitution, or
aromatic phenyl group via copper-catalyzed Ullmann amination to
give the final products in low to moderate yields (Scheme 2).
Scheme 1. Synthetic route of the fused π-extended acridone derivatives with
phenyl and hexyl at the amine position.
Scheme 2. Structures and yields of the fused π-extended acridone derivatives.
2
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Figure 2. Crystal structures and bond lengths of fused π-extended acridone derivatives with hexyl groups at the amino position: crystal structure and top view of
the overlapped molecular π-π stacking in the crystal a, b) BN-C6; c, d) BA-C6; e, f) NB-C6; g, h) NN-C6; i, j) NA-C6; k) side view of the molecular packing of NA-
C6 formed by π-π interaction with different distances; l) crystal structure of the dimerized NB-C6.
Single crystals that were suitable for X-ray diffraction analysis
were obtained for all the target compounds.[17] The crystals were
Crystal structures and packings
grown by slow evaporation of the dichloromethane/hexane
solutions of BN-C6, BA-C6, NB-C6, NN-C6, BN-Ph, NB-Ph, NN-
3
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10.1002/ejoc.202000871
European Journal of Organic Chemistry
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Ph. For NA-C6, the crystals were obtained by slow evaporation of
its dichloromethane/methanol solution. The vapor diffusion of
methanol into the toluene solutions was used to get the crystals
of BA-Ph and NA-Ph. The crystallographic analysis supplied
detailed information of the molecular structures and packings in
the solid state. For the compounds with hexyl group at the amino
position of the π-extended acridone derivatives (Figure 2), the
bond lengths of carbonyl in the molecules are in the range of 1.24-
1.25 Å, which matches well with that of an acridone dimer with
hexyl groups,[5b] but longer than that of the C-O double bond. The
distances of C-C bonds (1.43-1.47 Å) between carbonyl and the
adjacent benzene rings and C-N bonds (1.38-1.40 Å) in the
molecules are shorter than the lengths of their corresponding
single bonds. The alternation of bond lengths in the pyridone
moiety clearly demonstrates the delocalization of electron from
nitrogen to carbonyl, which can be easily realized when the
resonance structures are considered.[7] The two terminal benzene
rings always contain C-C bonds with distances (1.33-1.38 Å) less
than that of benzene (1.39 Å), due to the influence of
intramolecular charge transfer. Moreover, the outer C-C bond at
the turning moiety of the angular acridone derivatives is always
short with the lengths in the scale of 1.34-1.35 Å, which are more
like the feature of olefin. Whereas the inner one is slightly longer
with the distances in the range of 1.44-1.45 Å, close to the length
of C-C single bond connecting two sp2 bonding carbon atoms.[18]
The long and short C-C bond lengths are similar to those of
phenanthrene and indicate somewhat different aromaticities in
these benzene rings.[19] The structures of these π-extended
acridone derivatives are nearly planar with small dihedral angles
of the two terminal benzene rings in the range of 1.7-6.2 °.
The rigid aromatic acridone derivatives favor the formation of
π-π stacking in the crystals with the distances in the range of 3.35-
3.58 Å. The two molecules prefer to be oriented with the hexyl
groups directing to the opposite direction in the packing due to the
steric hindrance, and overlapped at the linear acridone moiety for
compounds BN-C6, NB-C6, NN-C6 and NA-C6. However,
compound BA-C6 contains three rings at each side and the
molecules are packed by overlapping at the central benzene ring
with the acridone and anthracene moieties of the two molecules
in the anti form (Figure 2d). Although π-π stackings between two
neighboring molecules are observed, most of them are
intermittent and merely involve two molecules due to the
interference of flexible hexyl groups. Only compound NA-C6
demonstrates the consecutive molecular packing via π-π
interaction with the layer distances of 3.23 Å and 3.48 Å (Figure
2k).
Multiple C-H…π interactions between the hexyl group and the
aromatic rings are observed in all the crystals of the hexyl-
substituted acridone derivatives. The flexible hexyl chains
partially occupy the acridone surfaces of BN-C6, NB-C6 and NN-
C6, or lay over the anthracene moiety of BA-C6. Such C-H…π
interactions keep the adjacent molecules away from forming π-π
stackings. For NA-C6, C-H…π interactions are also present in the
crystal without occupying the aromatic surface, thus leading to a
continous π-π stacking.
It is worthwhile to note that two molecules of linear compound
NB-C6 form uniform π-π stacking with three of the four rings
overlapped each other in the crystalline state. In addition to the
aforementioned C-H…O and C-H…π interactions among hexyl
and acridone moieties, the attractive dispersion interaction
between hexyl chains is also involved to keep the π-π stacking
from ongoing growth. Furthermore, the X-ray structural analysis
of compound NB-C6 reveals a dimerized structure in the anti form
after storing the crystals over five months under the ambient
laboratory-lighting condition (Figure 2l). The molecules undergo
the dimerization process in the crystalline state, which is
reasonable considering the well overlapped molecular packing
(Figure 2f). To the best of our knowledge, this is the first example
of the dimerization of π-extended acridone derivative with the
structure confirmed by X-ray diffraction analysis. The dimerization
should be caused by the photo-induced cycloaddition, similar to
the photodimerization of anthracene and other acenes.[20]
According to the situation of the crystals of non-dimerized NB-C6,
the dimerization would not happen at least in one week, indicating
a very slow decomposition process.
For the phenyl-substituted acridone derivatives, the bond
lengths and molecular planarity are close to those of the hexyl-
substituted ones. The pending phenyl is nearly vertical to the
acridone plane with the dihedral angle in the range of 85-89°. The
intermolecular C-H…O hydrogen bond between carbonyl and the
hydrogen atoms of the pending benzene ring plays a dominant
role in directing the molecular packings, which are also found in
the molecular packing of other phenyl-substituted acridone
derivatives (Figure 3).[8c] With the assistant of intermolecular
hydrogen bonds, these molecules are well oriented along the
acridone moiety. However, the space between atoms of hydrogen
and oxygen in such hydrogen bond also makes the acridone
planes partially overlapped each other and leads to low degrees
of molecular stacking. The formations of intermittent π-π
stackings with the distances of 3.51 Å and 3.52 Å are observed in
the crystals of BN-Ph and BA-Ph due to the interference of C-
H…π interactions among the neighbouring molecules.
Compound NN-Ph shows a consecutive overlapped molecular
packing with the aid of intermolecular hydrogen bond. Besides the
formation of intermolecular C-H…O hydrogen bonds, π-π
interactions between molecules are observed in compounds NB-
Ph and NA-Ph, thus leading to the continuous molecular packings
along the acridone moiety. The distance between the acridone
planes in NA-Ph is 3.72 Å, which is slightly longer than the other
ones due to the C-H…π interactions between the pending
benzene ring and the adjacent anthracene moiety.
Intermolecular C-H…O hydrogen bonds of hydrogen atom of
the benzene ring at the turning point and oxygen of carbonyl are
observed in the crystals of BN-C6 and BA-C6 and the dihedral
angles of the acridone moieties are 77 ° and 85 °, respectively.
The hydrogen atom of the hexyl chain can also form C-H…O
intermolecular hydrogen bond with carbonyl in the crystal of NB-
C6. No hydrogen bonds are formed in the crystal of NN-C6. The
hydrogen atoms of the methylene linked to the amino position and
the terminal benzene ring of anthracene can generate
intermolecular C-H…O hydrogen bonds with carbonyl in NA-C6.
4
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10.1002/ejoc.202000871
European Journal of Organic Chemistry
FULL PAPER
Figure 3. Crystal structures of fused π-extended acridone derivatives with phenyl groups at the amino position: side view of molecular packings via intermolecular
hydrogen bond, a) BN-Ph; b) BA-Ph; side view of the molecular packing and top view of the overlapped molecular structure in the crystal c, d) NB-Ph; e, f) NN-Ph;
g, h) NA-Ph.
to the corresponding hexyl-substituted ones, owing to the p-π
conjugation of the pending phenyl group (see the supporting
information). The corresponding optical energy gaps are in the
range of 2.49-3.05 eV, estimated from the onsets of the
absorption spectra. The UV/Vis absorption of the fused π-
extended acridone derivatives was further studied by time-
dependent density functional theory (TDDFT) calculation at the
B3LYP/6-311G+(2p,d) level of theory in dichloromethane as
solvent. The energies of the first excited state are in the range of
2.59-3.33 eV with the HOMO to LUMO transition as the main
contribution to the fine structured absorption bands at the long
wavelength (see the supporting information).
Optical and electrochemical properties
The optical properties of the fused π-extended acridone
derivatives were characterized by UV/vis absorption and
fluorescence emission in dichloromethane. For the hexyl-
substituted acridone derivatives, the absorption maximum at the
longest wavelength is gradually red-shifted from 401 nm for
parent N-hexylacridone to 484 nm for NA-C6 due to the extended
conjugation (Figure 4, Table 1).[5b] Compared to that of the
angular acridone derivative BN-C6, the absorption maximum of
the linear isomer NB-C6 is red-shifted to 467 nm, attributable to
the enhanced degree of conjugation. However, even with
elongated conjugation, the absorption maximum at the longest
wavelength of the angularly fused acridone derivative NN-C6 is
blue-shifted to 461 nm in comparison to that of NB-C6. This is
possibly due to the steric hindrance caused non-planarity
resulting in a reduced conjugation in NN-C6. Moreover, the
reduced conjugation caused by the longer inner C-C bond at the
turning point is also responsible for the blue shift of absorption
spectrum. For the phenyl-substituted acridone derivatives, the
UV/Vis absorption spectra are slightly blue-shifted in comparison
5
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10.1002/ejoc.202000871
European Journal of Organic Chemistry
FULL PAPER
Figure 4. UV/vis absorption and fluorescence emission spectra of the fused π-
extended acridone derivatives with hexyl at the amino position measured in
dichloromethane at room temperature.
All these acridone derivatives are fluorescent in
dichloromethane. For the hexyl-substituted acridone derivatives,
compound BN-C6 shows a nearly same emission spectrum to
that of N-hexylacridone with the emission maximum at 404 nm.[5b]
However, the absolute quantum yield of BN-C6 (0.5%) is much
lower than that of N-hexylacridone (58%) in dichloromethane. The
benzene-fused acridone would extend the conjugation in the
molecule and should lead to a red-shift of the emission spectra.
Again, the unchanged emission spectrum with low intensity of BN-
C6 is related to the reduced conjugation by the steric hindrance
induced distortion and the relatively long inner C-C bond at the
turning moiety. This is also responsible for the even blue shift of
the emission spectrum of NN-C6 with declined intensity in
comparison to that of the linear NB-C6. The linear NB-C6
demonstrates bright green fluorescence with the emission
maximum at 485 nm, absolute quantum yield of 52% and
relatively long lifetime of 31 ns. Thus the linear extension of
acridone would maintain the molecular planarity with a red-shift in
both the UV/Vis absorption and emission spectra, as well as a
good fluorescence intensity. The phenyl-substituted acridone
derivatives share the same emission patterns and close
intensities to these of the hexyl-substituted ones with a little blue
shift in emission positions, which are similar to their UV/Vis
absorption spectra (see the supporting information). When the
solvent was changed to the nonpolar toluene and the polar DMF,
the UV/vis absorption and fluorescence emission spectra of each
compound are quite similar in patterns with slightly hypsochromic
or bathochromic shifts and small Stokes shifts (4-26 nm) (see the
supporting information), indicating the quite close structural and
electronic properties in the ground states and excited states.
Table 1. Summary of optical and electrochemical properties of the fused π-extended acridone derivatives.
[a]
[b]
[c]
[d]
[e]
[e]
[f]
[g]
λab
Eg,opt
(eV)
λem (λex)[c]
(nm)
ΦF
τ[c]
(ns)
λem (λex)[d]
(nm)
ΦF
τ[d]
(ns)
Eox1
Ered1
(V)
Eg,electro. EHOMO
(eV) (eV)
ELUMO
compd.
(nm)
(%)
(%)
(V)
(eV)
[h]
409,433 (383)
402,426 (377)
454,482 (419)
448,475 (414)
485,514 (442)
481,507 (435)
471,502 (435)
464,494 (429)
493,527 (454)
486,519 (448)
0.5
BN-C6
BN-Ph
BA-C6
BA-Ph
NB-C6
NB-Ph
NN-C6
NN-Ph
NA-C6
NA-Ph
404
2.99
0.63
0.45
4.5
1.2
31
440,487,519 (383)
441,486,510 (377)
491,516,545 (419)
504 (414)
4.8
24
0.98
-
-
-5.68
-2.69[i]
1.4
1.4
1.1
52
66
13
14
6
397
445
439
467
461
461
454
484
476
3.05
2.72
2.74
2.54
2.57
2.61
2.64
2.49
2.52
7.1
0.9
0.9
4.9
6.5
6.1
0.3
4.9
0.9
3.0
42
1.03
0.74
0.80
0.79
0.93
0.80
0.97
0.68
0.72
-2.38
-2.20
-2.17
-2.08
-2.08
-2.08
-2.06
-2.04
-2.04
3.06
2.76
2.78
2.68
2.73
2.68
2.75
2.51
2.55
-5.67
-5.44
-5.49
-5.52
-5.55
-5.52
-5.58
-5.36
-5.43
-2.61
-2.68
-2.71
-2.84
-2.82
-2.84
-2.83
-2.85
-2.88
1.8
31
569 (442)
14
548 (435)
16
3.0
3.3
2.4
2.9
525 (435)
3.6
4.0
0.6
1.0
523 (429)
550 (454)
8.4
539,576(448)
[a] Absorption at the longest wavelength in dichloromethane at room temperature. [b] Estimated from absorption onset. [c] Measured in dichloromethane at room
temperature. [d] Measured in solid state at room temperature. [e] From the DPV measurements in 0.1 M nBu4NPF6 dichloromethane solution at room temperature
with ferrocene as internal reference. [f] EHOMO = -(4.8 + Eox1,onset) eV. [g] ELUMO = -(4.8 + Ered1,onset) eV. [h] Not observed. [i] ELUMO = EHOMO + Eg.opt
.
6
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European Journal of Organic Chemistry
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The fluorescence spectra of the acridone derivatives were
measured in the solid state (see the supporting information). The
emission maxima are red-shifted in comparison to those
measured in the dichloromethane solution due to the close
contacts caused excimer coupling.[21] The absolute quantum
yields of the acridone derivatives in the solid state are in the range
of 0.3%-7.1%. The fluorescence intensities of BN-C6 and BN-Ph
are better than those in the solutions due to the restricted
vibrations at the fjord of carbonyl and naphthalene in the solid
state.[22] All the other compounds show decreased quantum yields
in the solid state for the excited states of the aggregates decay or
relax to the ground state in the nonradiative fashion.[23]
angular acridone derivative, the linear isomer possesses a
relatively high HOMO energy level and a low LUMO energy level,
thus resulting in a decline in the energy gap. For example, the
HOMO and LUMO energy levels of the angular BN-C6 are -5.78
eV and -2.02 eV, respectively; the corresponding energy levels
are changed to -5.64 eV and -2.26 eV for the linear NB-C6. Thus,
the energy levels of the fused π-extended acridone derivatives
can be tuned by changing the manner of fusion and the distance
of extension.
The redox properties of these acridone derivatives are
characterized by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) measurements in dichloromethane. The CV
measurements show that all the oxidative waves are irreversible.
Irreversible reductive waves are observed for the acridone
derivatives starting with methyl 2-bromobenzoate, while the rest
ones demonstrate reversible reductive waves. The first oxidative
potentials of these acridone derivatives are in the range of 0.68-
1.03 V, estimated from the DPV measurement. The HOMO
energy level calculated from the onset of the first oxidative wave
is -5.68 eV for BN-C6, which is raised to -5.36 eV for NA-C6. The
potential of the first reductive wave is -2.38 V for BN-Ph and
increases to -2.04 V for NA-Ph. The estimated LUMO energy
levels ranges from -2.61 eV to -2.88 eV. The corresponding
energy gaps from the electrochemical measurements are in the
scale of 2.51-3.06 eV, which are in good agreements with their
optical energy gaps.
Table 2. NICS(1)zz values of the fused acridone derivatives.[a]
Compd.
BB-C6
BB-Ph
BN-C6
BN-Ph
BA-C6
BA-Ph
NB-C6
NB-Ph
NN-C6
NN-Ph
NA-C6
NA-Ph
A
B
C
D
E
F
-
-
-24.1
-25.6
-25.9
-26.1
-25.9
-26.1
-27.1
-27.6
-26.9
-28.4
-26.9
-28.4
-4.9
-5.6
-4.6
-5.5
-4.8
-5.5
-1.3
-1.9
-2.9
-2.8
-2.9
-2.8
-24.1
-25.6
-20.7
-20.7
-16.4
-16.3
-24.3
-24.6
-18.9
-19.8
-14.8
-15.7
-
-
-
-
-
-28.9
-29.1
-32.7
-32.8
-
-
-
-26.5
-
-26.6
-25.9
-26.0
-25.2
-26.1
-25.5
-26.2
-
Theoretical calculations
-
-
-28.1
-28.6
-31.4
-32.3
-
-
-26.2
-26.3
[a] Calculated at the GIAO/DFT/B3LYP/6-311+G(d) level of theory. For
comparison, the NICS(1)zz values of N-hexylacridone (BB-C6) and N-
phenylacridone (BB-Ph) are also calculated.
Since the crystal structural analysis reveals the presence of
relatively longer and shorter C-C bonds in the benzene ring at the
turning points of the angularly fused acridone derivatives, their
aromaticities were studied by the nucleus-independent chemical
shift (NICS) calculations (Table 2).[24] All the benzene rings in the
acridone derivatives are aromatic according to the NICS(1)zz
values. However, the aromaticity of benzene ring D at the turning
point of the angularly fused acridone rerivatives is weakened
when more benzene rings are fused. This is consistent with the
crystal structural analysis. The pyridone ring C possesses a weak
aromatic character, which is probably caused by the
intramolecular charge transfer induced electron delocalization
over the pyridone ring and can be easily recognized when its
corresponding resonance structure of acridine is considered, as
supposed in the crystal structure analysis.
Figure 5. DFT calculated frontier molecular orbitals and energy levels of the
fused π-extended acridone derivatives with hexyl at the amino position
calculated at the B3LYP/6-311+G (d,p) level of theory.
To investigate the influences of π-extension of acridone on the
electronic properties, DFT calculations were carried out at
B3LYP/6-311G+(d,p) level of theory in vacuo (Figure 5 and Figure
S91). The HOMO and LUMO orbitals are distributed over the
whole molecular skeleton of the π-extended acridone. For the N-
hexylacridone, the HOMO and LUMO energy levels are -5.87 eV
and -1.95 eV, respectively, and the corresponding energy gap is
[5c]
The π-extended acridone derivatives possess the similar
structures to polyacences and have potential applications as
photoelectronic materials.[14] Considering the presence of face-to-
face π-π stackings in the crystals, the charge transport property
3.92 eV.
With the increase of π-extension of the acridone
derivatives, the frontier molecular orbital energy levels are
convergent to -5.40 eV and -2.39 eV for NA-C6, which reduces
the corresponding energy gap to 3.01 eV. Compared to the
7
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10.1002/ejoc.202000871
European Journal of Organic Chemistry
FULL PAPER
was preliminarily evaluated via the equation based on the Marcus
theory and hopping mechanism.[25] According to the previously
investigations, such theoretical calculation is reliable in studying
the charge transport property.[12b,26]
level of theory and both the hole and electron transports were
considered. The calculated results are listed in Table 3.
The calculations reveal that all the fused acridone derivatives
possess comparable and even larger hole reorganization energy
in comparison to that of N-phenylacridone BB-Ph. But for the
electron reorganization energy, the acridone derivatives that are
synthesized starting from benzoate ester give relatively higher
reorganization energies than that of BB-Ph, whereas the other
ones have lower reorganization energies. Moreover, the
recoganization energies for hole transports are lower than those
for electron transports, making these acridone derivatives more
suitable as hole transports materials. When the transfer integral
is concerned, the linearly fused acridone derivatives NB-C6 and
NB-Ph demonstrate the largest values of electronic coupling,
131.6 meV and 146.3 meV, respectively. These values are much
higher than those of their congener rubrene (th = 100 meV and te
= 53 meV).[29]
1
⁄
푒휋푑2푡2
휋
휆
휇 =
(
)
2 푒푥푝 (−
)
푘퐵푇ℎ 휆푘퐵푇
4푘퐵푇
Here, e, kB, h and T are the electron charge, Boltzmann
constant, Planck constant and temperature, respectively. d is the
center-of-mass distance in the charge transfer direction and can
be obtained from the molecular π-π stackings in the crystal. λ is
the reorganization energy and is defined as the energy required
to reorganize the molecular geometry from initial to final
coordinates.[25b,27] t is the transfer integral and is sensitive to the
mode of packing.[25b,28] Thus the two molecules of π-π stacking in
the crystal were used directly without the optimization of geometry.
All the calculations were carried out at the B3LYP/6-311G+(d,p)
Table 3. Summary of the calculated hole and electron mobilities of acridone derivatives at 300 K.[a]
Compd.
BN-C6
λh (meV)
137.8
126.7
172.0
174.3
101.1
113.3
113.3
89.8
th (meV)
35.8
63.3
33.3
84.5
131.6
15.1
75.0
3.7
λe (meV)
271.8
250.9
255.4
251.7
193.6
195.3
195.3
171.0
169.8
169.8
176.9
176.9
175.8
175.8
240.8
te (meV)
32.4
d (Å)
4.94
4.09
4.09
3.38
4.62
3.84
5.43
3.61
3.73
3.78
3.73
6.32
3.59
5.21
3.82
μh (cm2V−1s−1
0.72
)
μe (cm2V−1s−1
0.12
)
BN-Ph
93.3
1.79
0.83
BA-C6
77.8
0.27
0.55
BA-Ph
125.6
89.7
1.17
1.02
NB-C6
14.2
1.95
NB-Ph-a
NB-Ph-b
NN-C6
128.3
146.3
114.4
105.0
84.5
0.11
2.69
5.33
7.00
0.008
0.16
2.56
NN-Ph-a
NN-Ph-b
NA-C6-a
NA-C6-b
NA-Ph-a
NA-Ph-b
BB-Ph
98.5
16.9
24.5
10.6
5.2
2.33
98.5
0.34
1.55
125.8
125.8
128.6
128.6
91.8
55.0
0.04
0.59
19.5
0.03
0.21
12.2
24.8
23.5
58.1
0.05
0.61
73.5
0.43
2.07
2.6
0.35
0.0006
[a] All the calculations were carried out at the B3LYP/6-311+G(d,p) level of theory.
Based on the reorganization energy, transfer integral and the
center-of-mass distance, the hole and electron mobilities of
acridone derivatives are calculated by setting the temperature at
300 K. The hole and electron mobilities of the parent N-
phenylacridone BB-Ph are estimated to be 0.35 cm2V−1s−1 and
0.0006 cm2V−1s−1, respectively, according to its crystal
packings.[30] When more benzene rings are fused to acridone, the
mobility varies according to the molecular shape and N-
substituents. The acridone derivatives with one side containing
benzene ring demonstrate better hole mobilities than those with
one side bearing linearly fused naphthalene. However, the latter
ones show the relatively better electron mobilities than those of
the former ones. The N-substitution with phenyl lead to better
mobilities than those with hexyl, except the electron mobility of
NN-Ph, which is slightly lower than that of NN-C6. The linear
compounds NB-C6 and NB-Ph exhibit good transfer integrals,
thus resulting the best hole and electron mobilities of 14.2
cm2V−1s−1 and 7.00 cm2V−1s−1, respectively, which are much
higher than those of BB-Ph, even with relatively farther center-of-
mass distances. However, it is worthwhile to note that the π-π
stackings of NB-C6 are not continuous in the crystal, which will
affect the intermolecular electronic coupling. For compounds NB-
Ph, NN-Ph, NA-C6 and NA-Ph with consecutive π-stackings in
the crystals, their charge transports are determined by the two
8
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10.1002/ejoc.202000871
European Journal of Organic Chemistry
FULL PAPER
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We are grateful for the financial support from the Jiangsu
Specially Appointed Professor Plan.
Keywords: acridone • aromaticity • conjugation • nitrogen
heterocycles • polycyclic aromatic hydrocarbons
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Entry for the Table of Contents
Aromatic systems
The angularly and linearly fused acridone derivatives were synthesized and their properties were shape-depended. The angularly
fused acridone derivatives showed weakened aromaticity of the benzene ring at the turning point, whereas the linearly fused ones
demonstrated stronger fluorescence intensity and better electronic couplings with potential applications in organic electronics.
11
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