Effect of the substituent position on the electrochemical, optical and structural properties of donor-acceptor type acridone derivatives

Article abstract of DOI:10.1039/d0cp00521e

Three new donor-acceptor (D-A) compounds, positional isomers of phenoxazine-substituted acridone, namely 1-phenoxazine-N-hexylacridone ( o-A ), 2-phenoxazine-N-hexylacridone ( m-A ) and 3-phenoxazine-N-hexylacridone ( p-A ), were synthesized. The synthesized compounds showed interesting, isomerism-dependent electrochemistry. Their oxidation was reversible and their potential (givenvs.Fc/Fc⁺) changed from 0.21 V for o-A to 0.36 V for p-A . In contrast, their reduction was irreversible, isomerism-independent and occurred at rather low potentials (ca.?2.25 to ?2.28 V). The electrochemical results led to the following values of the ionization potentials (IPs) and electron affinities (EAs): 5.03 eV and ?2.14 eV, 5.15 eV and ?2.20 eV, and 5.20 eV and ?2.28 eV for o-A m-A and p-A , respectively. The experimentally obtained values were in very good agreement with those predicted by DFT calculations. All three isomers readily formed single crystals suitable for their structure determination. o-A and p-A crystallized inP1? andP2₁/nspace groups, respectively, with one molecule per asymmetric unit, while m-A crystallized in theP2₁/cspace group with two molecules in the asymmetric unit accompanied by disordered solvent molecules. The UV-vis spectra of the studied compounds were isomerism and solvent independent, yielding absorption maxima in the vicinity of 400 nm. Their photoluminescence spectra, in turn, strongly depended on isomerism and the used solvent showing smaller Stokes shifts for the emission bands registered in toluene as compared to the corresponding bands measured in dichloromethane. The photoluminescence quantum yields (?) were systematically higher for toluene solutions reaching the highest value of 20% for p-A . For all three isomers studied, stationary and time-resolved spectroscopic investigations carried out in toluene at different temperatures revealed spectral features indicating a contribution of thermally activated delayed fluorescence (TADF) to the observed spectroscopic behaviour. The measured photoluminescence quantum yields (?) were higher for solid state films of pure compounds and for their dispersions in solid matrices (zeonex) than those recorded for toluene and dichloromethane solutions of the studied phenoxazine-N-hexylacridone isomers. The obtained experimental spectroscopic and structural data were confronted with theoretical predictions based on DFT calculations.

Full text of DOI:10.1039/d0cp00521e

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Phys. Chem. Chem. Phys., 2020, DOI: 10.1039/D0CP00521E.
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Page 1 of 17
Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
View Article Online
DOI: 10.1039/D0CP00521E
ARTICLE
Effect of substituent position on electrochemical, optical and
structural properties of donor-acceptor type acridone derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
Irena Kulszewicz-Bajer,* Malgorzata Zagorska,* Marzena Banasiewicz, Piotr A. Guńka, Petr
c
b
d
a
Toman, Boleslaw Kozankiewicz, Gabriela Wiosna-Salyga and Adam Pron*
DOI: 10.1039/x0xx00000x
Three new donor-acceptor (D-A) compounds were synthesized - positional isomers of phenoxazine-substituted acridone,
namely 1-phenoxazine-N-hexylacridone (o-A), 2-phenoxazine-N-hexylacridone (m-A) and 3-phenoxazine-N-hexylacridone
(
p-A). The synthesized compounds showed interesting, isomerism-dependent electrochemistry. Their oxidation was
+
reversible and its potential (given vs Fc/Fc ) changed from 0.21 V for o-A to 0.36 V for p-A. To the contrary, their reduction
was irreversible, isomerism-independent and occurred at rather low potentials (ca.-2.25 to -2.28 V). The electrochemical
results led to the following values of the ionization potentials (IPs) and electron affinities (EAs): 5.03 eV and -2.14 eV, 5.15
eV and -2.20 eV, 5.20 eV and -2.28 eV for o-A, m-A and p-A, respectively. The experimentally obtained values were in a
very good agreement with those predicted by DFT calculations. All three isomers readily formed single crystals suitable for
their structure determination. o-A and p-A crystallized in P1 and P2
1
/n space groups, respectively, with one molecule per
asymmetric unit, while m-A crystallized in the P2 /c space group with two molecules in the asymmetric unit accompanied
1
by disordered solvent molecules. The UV-vis spectra of the studied compounds were isomerism and solvent independent,
yielding absorption maxima in the vicinity of 400 nm. Their photoluminescence spectra, in turn, strongly depended on
isomerism and the used solvent showing smaller Stokes shifts for the emission bands registered in toluene as compared to
the corresponding bands measured in dichloromethane. The photoluminescence quantum yields  were systematically
higher for toluene solutions reaching the highest value of 20% for p-A. For all three isomers studied, stationary and time-
resolved spectroscopic investigations carried in toluene at different temperatures revealed spectral features indicating a
contribution of thermally-activated delayed fluorescence (TADF) to the observed spectroscopic behaviour. The measured
photoluminescence quantum yields  were higher for solid state films of pure compounds and for their dispersions in
solid matrices (zeonex) than those recorded for toluene and dichloromethane solutions of the studied phenoxazine-N-
hexylacridone isomers. The obtained experimental spectroscopic and structural data were confronted with theoretical
predictions based on DFT calculations.
.
component ambipolar organic field effect transistors7-9 and
OLEDs emitting visible or near infrared radiation, including
those which exploit the temperature activated delayed
Introduction
Low and high molecular mass donor-acceptor compounds of
DA and DAD type deserve a special interest because they offer
interesting electrochemical and optical properties, mainly
associated with their high electron affinity (EA) and relatively
low ionization potential (IP) which give rise, among others, to
their low band gaps.1 As a result they are very well suited for
fluorescence (TADF) effect where a small energy gap between
1
0-12
the singlet and triplet states, ΔEST < 0.1 eV is required.
Electrochemical applications of DA compounds involve mainly
1
3,14
the elaboration of new electrochromic devices
and
electrochemical supercapacitors based on pseudocapacitance
-4
1
5,16
phenomena.
5
,6
the use as active components of organic solar cells, one
All physical and chemical properties of DA and DAD molecules
are very sensitive to isomerism since the molecules geometries
and by consequence their electronic structures strongly
depend on the way the D substituent is being attached to the
A core. The substitution mode can have strong effects on the
oxidation/reduction potentials of DA and DAD compounds,
their electrochemical reversibility, the absorption and emission
spectra and other properties. 2D and 3D supramolecular
organizations are also affected by positional and geometrical
a. Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664
Warsaw, Poland.
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/44, 02-668
b.
Warsaw, Poland.
c.
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech
Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic.
Department of Molecular Physics, Technical University of Lodz, Zeromskiego 116,
d.
9
0-924 Lodz, Poland.
†
Electronic Supplementary Information (ESI) available: [synthetic details,
characterization of compounds, electrochemical, spectroscopic, X-ray diffraction
studies, calculation details ]. See DOI: 10.1039/x0xx00000x
1
7,18
isomerism.
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Page 2 of 17
ARTICLE
Journal Name
Acridone seems to be a very promising acceptor in the mode). Phosphorescence decays were accumulated with a
View Article Online
DOI: 10.1039/D0CP00521E
development of electrochemically and optically active DA and Stanford
Research
SR-430
Multichannel
Scaler.
DAD compounds. It is not a strong acceptor but it shows Electrochemical measurements (CV and DPV) were performed
-3
excellent chemical and thermal stabilities. Surprisingly the using Autolab potentiostat (Eco Chemie). 1x10 M solution of
reports on acridone-based electroactive and luminescent studied compounds in 0.1M Bu NBF /CH Cl electrolyte was
4
4
2
2
compounds are rather scarce. Acridone derivatives were used placed in a three electrode, one-compartment cell with
in organic electronics as hole transporting materials1 and as platinum working and counter electrodes and an Ag/Ag
9
+
luminophores whose emission colors depended on the nature reference.
of D substituents.2
0,21
In particular, it was demonstrated that
diarylamine-disubstituted N-methylacridone, namely 2,7-
bis(anisylamino)-N-methylacridone, showed emission peaked
at 527 nm,22 whereas carbazole disubstituted N-hexylacridone
Quantum chemical calculations
Quantum chemical calculations were carried out with the goal
to theoretically support the obtained electrochemical and
spectroscopic data.
(
(2,7-bis(carbazole)-N-hexylacridone) revealed an emission
2
3
band at 469 nm.
Taking into account the above described
sensitivity of DA compounds to positional isomerism we
synthesized three new acridone derivatives in which a strong
donor (phenoxazine) was attached to this relatively weak
acceptor (N-hexylacridone) at three different positions: ortho,
meta and para, respectively. We carried out a detailed
The ground state molecular conformations of the neutral
molecules of the studied derivatives and their cation-radicals
and anion-radicals were optimized using hybrid Hartree–
24,25
Fock/Density Functional Theory (DFT) method B3LYP.
In
order to account for intramolecular dispersion forces between
conjugated molecular moieties B3LYP method was combined
with the Grimme empirical dispersion correction with the
characterization
of
these
compounds
combining
electrochemical, absorption and emission spectroscopic
experimental data with DFT calculations. We also determined
their crystal structures. Finally, for comparative reasons we
synthesized an acridone derivative disubstituted with
phenoxazine (2,7-bis(phenoxazine)-N-hexylacridone).
26
Becke-Johnson damping (GD3BJ). The 6-311++G(d,p) basis
set was used. The dispersion-corrected DFT methods usually
provides reliable ground-state molecular conformations and
other properties at moderate computational costs.
Furthermore, their application to the open-shell systems (ion-
radicals) does not suffer from high spin contamination, which
is common for wave-function based methods. They have been
successfully used for the conformation study of many different
Experimental
Synthesis
2
7-29
conjugated systems.
Molecular conformations were
Detailed descriptions of the preparation procedures, crystal
structure determination together with NMR data and
elemental analysis can be found in Electronic supplementary
information (ESI).
calculated for both the isolated molecules (in vacuum) and
molecules in a solution. Solvation effects were described by
means of the polarizable continuum model (PCM) using the
3
0
integral equation formalism variant.
The obtained
Characterization
equilibrium structures were checked by the normal mode
analysis (no imaginary frequency was found). Vertical and
adiabatic ionization potentials and electron affinities were
evaluated from differences of the total energies of the neutral
molecules and respective ion-radicals (so-called SCF method).
The vertical absorption spectra in toluene and
UV-vis absorption spectra of the synthesized compounds
dissolved in dichloromethane and toluene were recorded on a
Perkin-Elmer Lambda 35 spectrophotometer. Emission spectra
and fluorescence lifetimes at room temperature were
collected on a Horiba Fluorolog 3 fluorimeter. Excitation for
fluorescence lifetimes measurements was provided by Delta
Diode 336 nm. The solid state emission spectra were recorded
on a FLS980 (Edinburgh Instruments) spectrometer. Absolute
quantum yields of thin films were measured in an integrating
sphere, coated inside with BENFLEC®, provided by Edinburgh
Instruments. Optical, emission studies at low temperatures
were performed using a standard synchronous-chopper
dichloromethane solution were computed by the time
31-33
dependent version (TD-)
of the PCM-B3LYP-GD3BJ/6-
3
11++G(d,p) method at the ground-state molecular
geometries. All calculations of the electrochemical properties
and absorption spectra were performed using Gaussian 16
34
program package.
In view of the importance of theoretical calculations in the
elucidation of TADF phenomena we have included into
Electronic Supplementary Information details concerning
theoretical background of the thermally activated delayed
fluorescence modelling.
1
0
phosphorimetry technique.
The solution of acridone
derivative in liquid toluene was poured into a small cylinder-
shaped fused silica cuvette and quickly cooled in liquid
nitrogen (77 K) in order to prevent aggregation. Next, a
sample holder with the frozen sample was quickly inserted
into a cold cryostat which temperature was stabilized between
5
and 150 K. Sample was illuminated with the 405 nm line
Results and discussion
Synthesis
from a diode laser and the emission, dispersed with a
McPherson 207 monochromator, was detected with an
EMI9659 photomultiplier (operating in the photon counting
2
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
Chart 1. Structural formulae of the studied acridone derivatives.
The
hexylacridone,
studied
compounds
2-phenoxazine-N-hexylacridone,
i.e.
(1-phenoxazine-N-
3-
View Article Online
DOI: 10.1039/D0CP00521E
The synthetic pathway leading to the studied derivatives is
depicted in Scheme 1. N-3-bromophenyl-2-anthranilic acid 1
phenoxazine-N-hexylacridone and 2,7-bis(phenoxazine)-N-
hexylacridone) are depicted in Chart 1. In the subsequent text
they will be abbreviated as: o-A, m-A, p-A and di-m-A.
was prepared following a modification of the method
3
5
described in. Its cyclization in the presence of polyphosphoric
acid resulted in a mixture of 1-bromo- and 3-bromoacridones.
Both isomers were directly alkylated with n-hexyl bromide
yielding, after chromatographic separation, N-hexyl-1-
bromoacridone 2a and N-hexyl-3-bromoacridone 2b in a molar
ratio 3:2 (the overall yield was 73%). N-hexyl-2-bromoacridone
O
O
O
O
O
N
N
N
N
N
N
C6H13
C6H13
C6H13
O
2
c was obtained by bromination of N-hexylacridone with
bromine in acetic acid/sodium acetate medium (47% yield).
Buchwald-Hartwig amination coupling of these
bromoderivatives, using phenoxazine and Pddba /t-Bu
catalyst, yielded the desired products: o-A (53% yield); p-A
87% yield) and m-A (92% yield). Disubstituted acridone
o-A
m-A
p-A
2
3
P
(
O
O
O
derivative was obtained via bromination of N-hexylacridone
core at 2,7 positions (in a process similar to monobromination
mentioned above) followed by the amination with
phenoxazine (82% yield).
N
N
N
C6H13
Detailed description of all synthetic procedures can be found
in ESI.
di-m-A
O
O
Br
HO2C
i
ii
+
Br
NH2
Br
NH
N
Br
N
iii
C6H13
C6H13
1
2a
2b
O
O
N
Br
iv
N
C6H13
C6H13
2c
O
O
N
v
2a
N
o-A
C6H13
O
O
N
N
v
v
2
c
m-A
p-A
C6H13
O
N
2b
N
O
C6H13
Scheme 1. Synthetic routes to phenoxazine-substituted N
polyphosphoric acid, 170 °C, iii) 1-bromohexane, Cs CO , NMP, 100 °C, iv) Br
0 °C.
-
hexylacridones i) 2-bromobenzoic acid, K
2
CO
3
, Cu/Cu
2
O, 2-ethoxyethanol, 130 °C, ii)
, t-Bu P, t-BuONa, toluene,
2
3
2
, NaOAc, acetic acid, 130 °C, v) phenoxazine, Pddba
2
3
9
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ARTICLE
Journal Name
View Article Online
DOI: 10.1039/D0CP00521E
Electrochemical investigations
Cyclic voltammetry investigations constitute an important
part of the characterization of new electroactive compounds
since they not only provide general information on their redox
properties but also allow for the calculations of their ionization
potential (IP) and electron affinity (EA) – two parameters of
predictive character as far as applications of these new
5
0
5
a)
-
-
10
compounds
in
electronics,
optoelectronics
and
electrochromism are envisioned.
-15
A representative CV voltammogram of o-A is shown in Figure 1
and compared with the corresponding data recorded for N-
hexylphenoxazine and N-hexylacridone i.e. compounds
mimicking, respectively, the donor and acceptor segments of
the studied DA compounds. CV and differential pulse (DP)
voltammograms of p-A, m-A and di-m-A can be found in ESI
-
-
20
25
-
2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
+
E [V] vs Fc/Fc
(Figure S1 and S2). All electrochemical data are collected in
Table 1. There are some common features in the
electrochemical behavior of the studied compounds. First,
based on the oxidation potentials values it can be postulated
that the three derivatives undergo reversible oxidation of their
phenoxazine part to a radical cation. The potential of this
oxidation, which is strongly dependent on the substitution
position, decreases in a series p-A>m-A>o-A (see Figure 2), in
the latter case it is even slightly lower than the corresponding
potential of N-hexylphenoxazine. Second, the reduction of the
N-hexylacridone to a radical anion is irreversible and much less
dependent on the substitution position. The irreversibility of
the reduction process is undoubtedly associated with low
stability of radical anions electrochemically generated at very
low potentials. It should be noted that in all three cases the
reduction potential, although low, is higher than that of N-
hexylacridone. The introduction of the second phenoxazine
substituent to yield symmetrical DAD compound (di-m-A
derivative) only slightly increases the reduction potential by 60
mV (see Table 1).
5
0
b)
-
5
O
N
-
-
-
10
15
20
C6H13
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
+
E [V] vs Fc/Fc
5
0
c)
For the calculations of IP and EA, the potentials of the
reduction and oxidation peaks onsets were taken because the
formal redox potential could not be determined for the
reduction process due to its irreversibility. The lowest
-
5
O
N
-10
“electrochemical” band gap was found for o-A, mainly because
-
-
-
15
20
25
of its significantly lower oxidation potential as compared to
the other compounds studied. The addition of the second
substituent of electro-donating properties lowered the band
gap by only 60 meV (compare m-A and di-m-A). These
intuitively rather unexpected results, ie. the lowest Eg for o-A
and the highest Eg for p-A, required a theoretical support. For
these reasons detailed DFT calculations were performed.
C H
6
13
-
2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
+
E [V] vs Fc/Fc
Fig. 1. Cyclic voltammograms registered for: a) o-A; b) N-hexylphenoxazine c) N-
hexylacridone. Electrolyte: 0.1M Bu NBF /CH Cl , scan rate: 50 mV/s, E vs Fc/Fc+.
4
4
2
2
4
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
Table 2. Excitation energies (E), wave lengths (), oscillator strengthsV(ife)w, aAnrdticdloemOinnlainnet
configurations of the three lowest excited states of the acridone derivatives calculated
DOI: 10.1039/D0CP00521E
by means of the PCM-B3LYP-GD3BJ/6-311++G(d,p) method in toluene and
dichloromethane.
o-A
N-hexPh
m-A
Excited State
E [eV]

f
Dominant
p-A
[
nm]
configurations
Toluene
o-A
S1
2.050
3.176
3.350
605
390
370
0.0013
0.0002
0.1574
HOMO→LUMO
(
99 %)
HOMO→LUMO+1
97 %)
HOMO-1→LUMO
95 %)
S2
S3
(
(
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
m-A
S1
+
E [V] vs Fc/Fc
2.402
3.370
3.373
516
368
368
0.0001
0.0016
0.1210
HOMO→LUMO
99 %)
HOMO→LUMO+1
(96 %)
(
Fig. 2. Differential pulse voltammograms registered for oxidation of o-A (red
line), m-A (green line), p-A (blue line) and N-hexylphenoxazine (N-hexPh) (black
line). Electrolyte: 0.1 M Bu NBF /CH Cl , E vs Fc/Fc+. Modulation time: 50 ms,
modulation amplitude: 10 mV, step potential: 5 mV.
S2
S3
4
4
2
2
HOMO-1→LUMO
(97 %)
+
p-A
Table 1. Oxidation and reduction potentials (vs Fc/Fc ), ionization potential (IP) and
electron affinities (EA) of acridone derivatives. IP and EA were calculated following a
procedure reported in ref.36, i.e. IP [eV] = ǀeǀ[1.15x Eox onset + 4.79] [eV], EA [eV] =
e[1.18xEredonset + 4.83] [eV].
S1
2.471
3.351
3.463
502
370
358
0.0029
0.1385
0.0026
HOMO→LUMO
(98 %)
HOMO-1→LUMO
S2
S3
(97 %)
compound
E0ox [V]
E0red [V]
IP [eV]
EA [eV]
Eg
HOMO→LUMO+1
o-A
m-A
p-A
0.212
0.317
0.360
0.332
1.020
0.223
-2.28
-2.23
-2.25
-2.16
-2.44
-
5.03
5.15
5.20
5.17
5.96
5.05
-2.14
-2.20
-2.18
-2.28
-1.95
-
2.89
2.95
3.02
2.89
4.01
(92 %)
Dichloromethane
o-A
S1
di-m-A
N-hexylacridone
N-hexylphenoxazine
2.177
3.319
569
374
0.0012
0.0309
HOMO→LUMO
(
99 %)
HOMO→LUMO+1
77 %)
HOMO-1→LUMO
19 %)
S2
S3
(
Quantum chemical calculations
(
Four principal goals of the performed DFT studies were
envisioned: i) elucidation of the molecules geometry; ii)
determination of the shape and distribution of the frontier
orbitals; iii) identification of dominant transitions and iv)
theoretical calculations of IP and EA values. The calculated
results are discussed in the perspective of the obtained
experimental findings.
The donor and acceptor moieties of all derivatives are almost
perpendicular to each other in both vacuum and a solution
see Figure S3 in ESI). Consequently, the HOMO and LUMO are
located on different parts of the molecule. Spatial separation
small overlap) of the frontier orbitals inhibits the S →S
3.325
373
0.1268
HOMO-1→LUMO
(78 %)
HOMO→LUMO+1 (19
%
)
m-A
S1
2.454
3.343
3.458
505
371
359
0.0001
0.1198
0.0014
HOMO→LUMO
99 %)
HOMO-1→LUMO
97 %)
HOMO→LUMO+1
97 %)
(
S2
S3
(
(
(
p-A
S1
(
0
1
2.468
3.322
3.487
502
373
356
0.0032
0.1368
0.0018
HOMO→LUMO
(98 %)
HOMO-1→LUMO
transition with a dominant HOMO→LUMO configuration (see
Table 2). Thus, the lowest energy peak exhibiting high
oscillator strength corresponds to a transition S →S , n>1, with
0 n
a dominant configuration HOMO-1→LUMO. In the same way,
S2
S3
(
97 %)
HOMO→LUMO+1
90 %)
(
if we consider the Kasha rule, small oscillator strength of the
S
1
→S
0
transition hinders the fluorescence (vide infra).
Figure 3 shows the distribution of the frontier orbitals in o-A,
m-A and p-A molecules. While HOMO is located on the
phenoxazine moiety independently of the isomer, HOMO-1
and LUMO are found on the acridone one. This distribution
strongly supports the electrochemical results implying that the
Therefore, emission in these molecules strongly depends on
the change of the torsion angle between two conjugated
moieties during the excited state relaxation.
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Page 6 of 17
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molecules are being oxidized at their phenoxazine parts and
reduced at the acridone ones.
View Article Online
DOI: 10.1039/D0CP00521E
compound
HOMO
HOMO-1
LUMO
LUMO+1
o-A
m-A
p-A
Fig. 3. HOMO, HOMO-1, LUMO, and LUMO+1 contours calculated using the PCM-B3LYP-GD3BJ/6-311++G(d,p) method in toluene
In Table 3 the results of the performed quantum chemical and vertical approaches (compare data in Tables 1 and 3).
calculations of the electronic properties are collected. High Significantly weaker effect of the phenoxazine group position
consistency of the theoretical IP and EA values with the on the calculated EA values was also evidenced by DFT
experimental ones should be pointed out. First, the calculated calculations, again consistent with the experimental findings,
IPs were very close to the electrochemically determined ones, although the experiments gave |EA| values slightly lower than
the latter being in between the values obtained using adiabatic those obtained theoretically using the vertical approach.
Table 3. HOMO, LUMO, IP and EA values of o-A, m-A and p-A from DFT calculations carried out for vacuum and DCM*.
compound conditions HOMO LUMO
E
g
IPadiabatic
IPvertical
EAadiabatic
EAvertical
o-A
o-A
m-A
m-A
p-A
p-A
vacuum
DCM
vacuum
DCM
vacuum
DCM
4.72
5.08
5.03
5.23
5.16
5.27
-2.15
-2.23
-2.17
-2.25
-2.17
-2.26
2.57
2.85
2.86
2.98
2.99
3.01
5.95
4.95
6.24
5.08
6.41
5.13
6.12
5.12
6.45
5.26
6.57
5.30
-0.95
-2.35
-0.94
-2.37
-0.96
-2.40
-0.82
-2.22
-0.82
-2.24
-0.82
-2.25
*All energies of HOMO, LUMO, ionization potential (IP), electron affinity (EA) given in eV and calculated by the method PCM-B3LYP-GD3BJ/6-311++G(d,p).
Easier oxidation (and by consequence lower IP) of o-A was not dihedral angles are compared with those obtained from X-ray
unequivocally reflected in its molecular geometry. We studies).
therefore analyzed the twist angle between the donor and
acceptor parts of the molecule since this angle might
Crystal structures
determine the redox, electronic and optical properties of the
3
7
All three isomers readily form single crystals suitable for
their structure determination. Detailed data collection as well
as crystal structure refinement details are given in ESI (Table
S2). o-A and p-A crystallize in P1 and P2
respectively, with one molecule per asymmetric unit, while m-
A crystallizes in the P2 /c space group with two molecules in
studied molecules, as known for other D-A compounds. In
both vacuum and DCM the highest twist angles around the
Cacridone-Nphenoxazine was found for m-A, (ca. 89°), whereas for o-
1
/n space groups,
A and p-A the corresponding angles were between 78 and 84°
see Table 4 in the subsequent section where the calculated
(
1
6
| J. Name., 2012, 00, 1-3
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
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the asymmetric unit accompanied by disordered solvent
molecules (see Figure 4 for their molecular structure). The
inspection of the structure with the MP program reveals that
the two m-A molecules are related by a pseudoinversion
center located at 0.75, 0.12, 0.24 with the largest
ARTICLE
View Article Online
DOI: 10.1039/D0CP00521E
‡, §
discrepancies exhibited by carbon atoms from aliphatic C
6
H
13
−
substituents.38 The interaction energy between the two
3
9,40
molecules, estimated by the method of Spackman et al.
amounts to ca. −45 kJ/mol and results from carbonyl∙∙∙carbonyl
and C−H∙∙∙π interactions (note that here and in the following
the intermolecular interaction energy is the total energy of
interaction between a pair of molecules and the designated
interactions are the strongest contributors to the energy for
the given pair of molecules, but they are not the only
contributors). The strongest intermolecular interactions for
both symmetry independent molecules occur between
molecules interacting via C−H∙∙∙O hydrogen bonds. These
hydrogen bonds contribute mostly to the electrostatic term of
the interaction energy. The total interaction energy is
dominated by dispersion, though, occurring probably mainly
between closely arranged hexyl chain and acridone moiety
(see Table S3 for details). The crystal structure of p-A exhibits
hierarchical supramolecular ordering. Molecules related by
inversion center form dimers with the interaction energy of
−56 kJ/mol. The dimers are in turn connected into double
chains running along the crystallographic Y axis via C−H∙∙∙π
interactions with intermolecular interaction energy of −48
kJ/mol. The double layers in the crystallographic (001) planes
are formed via C−H∙∙∙O hydrogen bonds with an intermolecular
interaction energy of −39 kJ/mol. The layers are linked into 3D
structure mainly by dispersion interactions with no close
contacts between the neighboring molecules (see Table S3-S5
for details of the intermolecular interactions). The
intermolecular interaction energies in o-A are less varied than
in the p-A, making the description of its crystal structure in
terms of the supramolecular hierarchy impossible. The
interactions in the crystal are dominated by π stacking and
C−H∙∙∙π interactions.
Fig. 4. Molecular structures of: a) o-A; b) m-A and c) p-A with atomic numbering
scheme. Thermal ellipsoids drawn at the 50% probability level and hydrogen
atoms omitted for clarity.
It is instructive to compare optimized geometries of neutral o-
A, m-A and p-A molecules obtained on the basis of DFT
calculations with their geometry in the solid state derived from
XRD measurements. This comparison is presented in Table 4
where the values of the dihedral angle between the planes of
acridone and phenoxazine are collected.
It could be noticed that the calculations of the geometry of an
isolated molecule led to a conformation of the phenoxazine
ring which was less planar than that determined in the solid
state by X-ray diffraction. Also the conformation of the n-hexyl
group of the para isomer was different in the crystal than that
computed in vacuum by DFT. These conformational
Table 4. Dihedral angles between donor and acceptor moieties of the neutral differences probably originate from the intermolecular
molecule determined by X-ray in the solid state and in the ground state
calculated by the (PCM-B3LYP-GD3BJ/6-311++G(d,p) method (in degrees).
interactions which promote a more efficient packing in the
solid state.
compound
X-ray
calculated
DCM
vacuum
toluene
o-A
m-A
80.42(4)
76.44(9),
82
89
83
89
83
89
7
9.83(10)
p-A
78.20(4)
79
84
84
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DOI: 10.1039/D0CP00521E
Spectroscopic studies in solution
Acridone derivatives, in addition to reversible electrochemistry
and ambipolarity, frequently exhibit interesting luminescent
properties2 . For this reason we have undertaken detailed
studies of the photophysics of o-A, m-A and p-A. Their
absorption, fluorescence and phosphorescence in toluene (T)
and dichloromethane (DCM) solutions were measured at room
temperature and at 5 K. The absorption spectra of all three
isomers showed two characteristic bands in the 375–425 nm
spectral range (see Figure 5 and Table 5), which were
essentially independent of the solvent. In the spectrum of o-A
an additional broad absorption band with a clear maximum at
a)
2, 23
4
73 nm in T and at 460 nm in DCM (for the latter see Figure S4
300
400
500
600
700
800
in ESI) was present which could be attributed to a CT
transition.
Wavelength [nm]
b)
Table 5. Photophysical parameters derived from the spectroscopic
measurements at RT of o-A, m-A and p-A at room temperature, where ɸ is the
fluorescence quantum yield, τ the fluorescence decay time and k
r
the radiative
decay rate.
compo
und
solvent

abs max/fl max
[nm]

[%]

[ns]
k
r
Stokes
shift
[cm ]
7
[10
-
1
-1
s ]
o-A
o-A
m-A
m-A
p-A
p-A
T
DCM
T
DCM
T
399,473 /618
403, 460/665
396/538
400/631
400/517
1.37
0.19
4.60
0.30
20
15
1.88
26
4.6
29
0.091
0.101
0.177
0.065
0.690
4961
6701
6665
9152
5657
300
400
500
600
700
800
Wavelength [nm]
DCM
402/615
3.45
16
0.216
8616
The fluorescence spectra, registered at RT in both solvents,
were broad and showed no vibrational structure. Contrary to
the absorption spectra, they were solvent sensitive, showing
the photoluminescence bands bathochromically shifted in
DCM as compared to those registered in toluene. In the case
of both solvents, the fluorescence quantum yields and
fluorescence decay times increased in the inversed series: p-
A> m-A> o-A, being however significantly higher for solutions
in T as compared to DCM (see Table 5).
c)
At 5 K prompt emission i.e. fluorescence was observed (Figure
6
), exhibiting a decay time of some nanoseconds. It was
accompanied by long-lived phosphorescence showing much
longer decay times (see Figure 6 for the exact values). Both
spectra nearly coincided and such observation could suggest
300
400
500
600
700
800
Wavelength [nm]
that the singlet and the triplet states origin was of the charge- Fig. 5. Absorption and fluorescence spectra of o-A (a), m-A (b) and p-A (c) in
toluene at RT, exc = 400 nm.
transfer character. It should be noted that the
phosphorescence decay time determined for o-A was over two
The fluorescence to phosphorescence intensity ratio at 5 K was
orders of magnitude shorter than in the case of m-A and p-A (2
dependent on the solvent and varied for the studied isomers.
ms vs 541 ms and 480 ms, respectively). Based on these
The most intense phosphorescence at 5 K was observed for p-
observations it can be concluded that the CT character of the
A in T. Therefore this sample was selected for the temperature
triplet state in o-A is higher than in the two other isomers,
dependence investigations in the temperature range inferior
consistent with the absorption spectroscopy data.
to the matrix melting point (178 K).
8
| J. Name., 2012, 00, 1-3
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
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ARTICLE
expressed by a single exponential relationship. It showed a fast
View Article Online
DOI: 10.1039/D0CP00521E
component contribution growing with temperature. Parallel to

= 2.0 ms
a)
shortening of the long-lived emission decay, a decrease of this
emission intensity could be observed together with a shift of
the spectrum towards shorter wavelengths (see Figure 7). The
prompt emission (in our phosphorimeter it composes of the
emission which decays faster than 0.1 ms) intensity was
approximately constant in the temperature range between 5
and 50 K, and started to increase at temperatures above 75 K.
All these temporal observations of spectral changes seem to
indicate that the thermal activation of delayed fluorescence is
involved in these processes, which shortens the triplet state
lifetime from millisecond to a microsecond time regime. In this
case the detected prompt emission would be composed of
fluorescence and delayed fluorescence.
5
00
550
600
650
700
750
Wavelength [nm]

= 541 ms
a)
b)
1
5
1
1
1
2 K
0 K
00 K
25 K
50 K
4
50
500
550
600
4
50
500
550
600
650
700
Wavelength [nm]
Wavelength [nm]

  ms
c)
b)
1
5
1
1
2 K
0 K
25 K
50 K
4
50
500
550
600
650
4
50
500
550
600
Wavelength [nm]
Wavelength [nm]
Fig. 6. Fluorescence (black) and phosphorescence (red) spectra of o-A (a), m-A
b) and p-A (c) in toluene at 5 K, exc = 405 nm.
(
Fig. 7. Phosphorescence (a) and fluorescence (b) spectra of p-A in toluene at
temperatures between 12 and 150 K.
The phosphorescence intensity of p-A in toluene gradually
decreased with the temperature raising from 12 to 150 K Kinetic analysis of the triplet state thermal depopulation was
Figure 7) and then it disappeared at 175 K (close to the however difficult in the case of the studies of p-A in toluene
(
toluene melting point). Its decay curves could be well fitted because of the presence of an ensemble of different p-A
using a single exponential dependence, yielding decay times conformers, probably rotamers, frozen in the toluene matrix.
shortening from 516 to 461 ms upon the temperature raise Different rotamers could exhibit slightly different T -S energy
1 1
from 12 to 75 K. At temperatures > 100 K decays of the long- gaps, leading to non-exponential temporal activation. Thus,
lived emission became even shorter and clearly could not be simple and convincing analysis of the thermally activated
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Page 10 of 17
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delayed fluorescence (TADF), like that presented for a single calculations see Eq. (1) in ESI) at RT along with the energy
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1
0
DOI: 10.1039/D0CP00521E
averaged SOC matrix elements
crystal of the CT complex , is not possible in the present case. differences EST
,
At 175 K, i.e. close to the melting point of toluene, the long- (see Eq. (5), ESI), and reorganization energies
|
⟨
푆
1
│
퐻
푆푂
│ ⟩
|
푇
1
lived emission disappeared and the prompt emission . Direct intersystem crossing rates in o-A are about one order
underwent a bathochromic shift of the spectrum maximum of magnitude lower than in m-A and p-A. The reverse
(
from 470 to 537 nm). Such behavior can be considered as a intersystem crossing rate in o-A dissolved in toluene is very
5
6
manifestation of the strong dependence of the observed low, while all other rates are in the range from 9×10 to 9×10
-1
spectral features on the environment rigidity. These findings s .
seems to be confirmed by the results obtained for the solid
state emission of studied compounds (vide infra).
Table 6. Theoretical dihedral angles (in degrees) between the donor and acceptor
moieties of the acridone derivative dissolved in dichloromethane and toluene.
Calculation was performed by the COSMO-(TD)PBE0-GD3BJ/TZP method for the ground
Modeling of thermally activated delayed fluorescence (TADF)
state (S
0
) and lowest singlet (S
1 1
) and triplet (T ) excited states of the molecules.
In order to model the TADF process, equilibrium geometries of
relevant electronic states S , S , and T were calculated by
0
1
1
Compound
DCM
Toluene
means of the COSMO-(TD)PBE0-GD3BJ/TZP method for all
studied derivatives in toluene and dichloromethane. The
obtained dihedral angles between donor and acceptor
moieties are listed in Table 6. For the ground state, these
dihedral angles are in a very good agreement with ones
obtained at the PCM-B3LYP-GDBJ/6-311++G(d,p) level. For all
Ground state S
0
o-A
m-A
p-A
82
88
84
82
88
82
Excited state S
1
S
0
and S
range from 80° to 90°. On the other hand, dihedral angles
determined for the T geometry of the meta- and para-
1
state geometries, the dihedral angles are within the
o-A
m-A
p-A
85
87
86
87
90
88
1
derivatives are significantly lower. While dihedral angles in the
ground state are almost the same in both solvents, they are
lower in dichloromethane for the excited state geometries.
Then, a SOC calculation was performed for all optimized
structures by means of the COSMO-pSOC-TD PBE0-GD3BJ/TZP
method. Properties of SOC excitations corresponding to the
Excited state T
1
o-A
m-A
p-A
80
61
65
86
66
76
S
1
→S
Oscillator strengths of the T
because they are negligible (less than 10 ). It is obvious that It should be pointed out that the values of the intersystem
0
and T
1
→S
0
transitions are summarized in Table 7.
1
→S transitions are not shown,
0
-7
also S
1
→S
0
transitions at the S
1
4
geometries are very weak with crossing rates strongly depend on the reorganization energies
-
-3
oscillator strengths from 10 to 2×10 , that correspond to . Since computation of their external part ext is complicated,
-6
-5
radiative lifetimes from 3×10 to 4×10 s. However, this some authors suggest using some empirical values, e.g. 0.1 eV,
transition is much stronger at the T
geometries of the meta- either for ext, or for the whole , instead of its calculation
and para-derivatives with the oscillator strengths one to two according to Eq. (3) (see ESI).
1
4
1,42
Although incorporation of
order of magnitude greater (radiative lifetimes from 4×10 to such empirical reorganization energies would alter the
-8
-7
5
×10 s) than at the S
1
geometry. Table 8 shows direct (S
) intersystem crossing rates k (for insight.
1
1
→T
1
)
calculated intersystem crossing rates, it would not bring a new
and reverse (T →S
1
1
0 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0CP00521E
Table 7. Energies E, wave lengths , oscillator strengths f, and radiative lifetimes τ of the transitions from the lowest singlet (S
1
) and triplet (T
1
) excited states to the ground state in
toluene and dichloromethane calculated at the S or T equilibrium geometry. Triplet oscillator strengths are not shown, because they are negligible.
1
1
Toluene solution
) [nm] f(S
Derivative
Geometry
E(S
1
) [eV]
(S
1
1
)
τ [s]
E(T
1
) [eV]
1
(T ) [nm]
o-A
o-A
m-A
m-A
p-A
p-A
S
1
T
1
S
1
T
1
S
1
T
1
1.689
1.688
2.051
2.152
2.171
2.206
734
735
605
576
571
562
4.20E-04
4.76E-04
1.39E-04
9.82E-03
1.56E-04
3.36E-02
1.93E-05
1.70E-05
3.93E-05
5.07E-07
3.13E-05
1.41E-07
1.671
1.667
2.024
2.020
2.158
2.161
742
744
613
614
575
574
Dichloromethane solution
(S ) [nm] f(S
Derivative
Geometry
E(S
1
) [eV]
1
1
)
τ [s]
E(T
1
) [eV]
1
(T ) [nm]
o-A
o-A
m-A
m-A
p-A
p-A
S
1
T
1
S
1
T
1
S
1
T
1
1.838
1.844
2.109
2.216
2.164
2.233
675
672
588
559
573
555
7.54E-04
6.54E-03
7.24E-04
2.69E-02
1.84E-03
1.30E-01
9.05E-06
1.04E-06
7.15E-06
1.75E-07
2.68E-06
3.55E-08
1.818
1.816
2.077
2.008
2.150
2.139
682
683
597
617
577
580
Table 8. Intersystem crossing rates k, calculated at room temperature, energy differences between singlet and triplet states EST, and reorganization energies 
calculated for the direct (S
1
→T
1
) and reverse (T
1 1
→S ) intersystem crossing in toluene and dichloromethane solutions.
Toluene solution
k(S
[
1
→T
s ]
1
)
k(T
[s ]
1
→S
1
)
EST
[eV]
|
⟨
푆1
│
퐻푆푂
│
푇1
-1
⟩
|
|
⟨
푆1
│
퐻푆푂
│
푇1
-1
⟩
|
(S
[eV]
1
→T
1
)
(T
[eV]
1
→S
1
)
Derivative
-
1
-1
1
(S →T
1
) [cm ]
1
(T →S
1
) [cm ]
o-A
m-A
p-A
2.3E+05
1.0E+06
8.5E+06
4.7E+03
9.2E+06
4.4E+06
0.020
0.051
0.019
0.015
0.029
0.058
0.021
0.300
0.083
0.002
0.024
0.007
0.001
0.085
0.033
Dichloromethane solution
k(S
[
1
→T
s ]
1
)
k(T
[s ]
1
→S
1
)
EST
[eV]
|
⟨
푆1
│
퐻푆푂
│
푇1
-1
⟩
|
|
⟨
푆1
│
퐻푆푂
│
푇1
-1
⟩
|
(S
[eV]
1
→T
1
)
1 1
(T →S
[eV]
)
Derivative
-
1
-1
1
(S →T
1
) [cm ]
1
(T →S
1
) [cm ]
o-A
m-A
p-A
2.1E+05
6.0E+06
5.5E+06
2.2E+06
1.3E+06
9.2E+05
0.021
0.106
0.041
0.020
0.085
0.061
0.047
0.344
0.082
0.001
0.074
0.027
0.011
0.120
0.088
On the basis of these results, we can describe the excited state the equilibrium geometry (see Table 7). Nevertheless, fluorescence
relaxation in the following way. Excitation to an absorbing state S is relatively weak, which is a necessary condition for the direct
n>1, is followed by the internal conversion to S state according to intersystem crossing S →T . Although SOC matrix elements are
the Kasha rule. Oscillator strengths f(S ) of the S →S transition, rather small, this process is facilitated by its exothermic character.
calculated at S geometry, are small due to almost perpendicular While there may be a significant competition between the prompt
mutual position of the donor and the acceptor moieties (see Table emission and the intersystem crossing in the ortho-derivative, the
). Thus, prompt emission in all derivatives should be very small. latter should be favored in others.
However, it should be pointed out that the potential energy surface
of the S state is rather flat in the vicinity of the S equilibrium
n
,
1
1
1
1
1
0
1
6
For o-A, dihedral angles between donor and acceptor moieties
remain close to 90° also during relaxation of the T state, especially
in toluene. Thus, also SOC matrix element remains relatively small.
Consequently, our model does not expect reverse intersystem
crossing T →S in the ortho-derivative in toluene. Although it should
1 1
be possible in dichloromethane, TADF is in this case limited by a
very small oscillator strength. Simultaneously, probability of
1
1
geometry. Therefore, the dihedral angle between the donor and
acceptor moieties can thermally fluctuate within a certain range ca.
from 75° to 105°, which induces an increase in the oscillator
strengths f(S
radiative decay rates k
1
1
). Consequently, values of the measured fluorescence
(see Table 5) are considerably greater than
r
1
those corresponding to the oscillator strengths f(S ) calculated at
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1 0
Therefore, some kind of phosphorescence T →S was found to be
negligible. non-radiative deexcitation should prevail in the case of
this monomer.
View Article Online
DOI: 10.1039/D0CP00521E
thin solid film
a)
b)
c)
2
5
% in zeonex
.5% in zeonex
For m-A and p-A, the dihedral angle between the donor and
acceptor planes significantly decreases during the T state
1
relaxation. This molecular deformation leads to an increase of
the SOC matrix elements, and consequently also the reverse
intersystem crossing rates, namely in the meta-derivative. On
1
the other hand, relaxation of the T state is also accompanied
an increase of the energy difference EST and the
reorganization energies , which have the opposite effect.
Consequently, the reverse intersystem crossing rates in
dichloromethane are lower than in toluene. Another
consequence of the reduced dihedral angle is greater overlap
4
00
500
600
700
800
Wavelength [nm]
between HOMO and LUMO, which promotes S
but oscillator strengths of the T →S phosphorescence remains
negligible. Therefore, fluorescence may occur, but only before
subsequent relaxation of the molecular conformation to the S
1 0
→S emission,
1
0
1
thin solid film
% in zeonex
.5% in zeonex
2
5
equilibrium geometry. If the molecule is not deexcited, it may
undergo repeated direct and reverse intersystem crossing
transitions.
We are aware of the fact that there exists a dynamic
competition between emission and non-radiative decay
processes, which were not included in our model. However,
we can assume that the non-radiative decay in all isomers may
occur with comparable rates due to their similar chemical
structure. Then, considering the radiative lifetimes shown in
Table 7 and intersystem crossing rates in Table 8, we can
expect more efficient TADF in m-A and p-A as compared to o-
A. Nevertheless, it should be remarked that our model
considers only molecules in their lowest-energy equilibrium
conformations, instead of a Boltzmann distribution of
conformations present in real solvents. Some non-equilibrium
400
500
600
700
800
Wavelength [nm]
1
conformations of o-A in the T state may possess significantly
lower dihedral angles between the donor and acceptor
moieties than the equilibrium conformation. For such
molecules, values of the intersystem crossing rates and
thin solid film
% in zeonex
.5% in zeonex
2
5
1 0
oscillator strength of the S →S emission may be comparable
with values obtained for m-A and p-A. Consequently, our
model cannot exclude possibility of an efficient TADF also in o-
A.
Spectroscopic studies in the solid state
If applications of the studied new derivatives are to be
considered, spectroscopic properties of their thin films have to
be determined. Drop casting and spin coating techniques were
used for the preparation of thin solid layers of pure o-A, m-A
and p-A yielding films of comparable quality. Additionally,
samples of these derivatives molecularly dispersed in a solid
matrix (2 wt% and 5.5 wt% of a given compound in zeonex)
were fabricated by spin coating. Zeonex is a convenient matrix
for dispersing luminophores since the resulting films are
usually uniform and show excellent optical quality.
400
500
600
700
800
Wavelength [nm]
Fig. 8. Emission spectra of solid thin film of pure compounds and their
dispersions in zeonex (2 wt % and 5.5 wt%): a) o-A b) m-A and c) p-A.
1
2 | J. Name., 2012, 00, 1-3
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
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ARTICLE
Table 9. Photophysical parameters derived from the spectroscopic measurements of o- Conclusions
A, m-A and p-A at room temperature, where ɸ is the fluorescence quantum yield.
View Article Online
DOI: 10.1039/D0CP00521E
To summarize, we have synthesized three positional
isomers of phenoxazine-substituted acridone, namely of 1-
compound
composition
2% in zeonex

abs max/fl max [nm]
 
o-A
396,480/575
397,477/590
366,407/575
393,550/500
393,565/530
389,411/555
393/477
13
9.5
13
10
10
4
30
33
16
phenoxazine-N-hexylacridone
(o-A),
2-phenoxazine-N-
5
.5% in zeonex
thin solid film
2% in zeonex
hexylacridone (m-A) and 3-phenoxazine-N-hexylacridone (p-
A). The investigated compounds show very interesting
electrochemistry. In particular, their electrochemical oxidation,
occurring on phenoxazine moiety at relatively low potentials is
reversible and isomerism-sensitive. The reduction of the
acridone moiety is irreversible, little dependent on isomerism
and starts at rather low potentials. The values of ionization
potentials (IP) and electron affinities (EA) calculated from the
electrochemical data are in a very good agreement with those
predicted by DFT calculations.
Stationary and time-resolved spectroscopic investigations
carried in toluene at different temperatures in all three cases
revealed features indicating a significant contribution of
thermally activated delayed fluorescence (TADF) to the
observed emission.
m-A
p-A
5
.5% in zeonex
thin solid film
2% in zeonex
5
.5% in zeonex
397/497
thin solid film
310,367,406/545
Absorption spectra of the studied molecules dispersed in
zeonex closely resemble the corresponding solution (T or
DCM) spectra. The positions of the dominant peak in the
spectra of all isomers are similar. However, they are slightly
hypsochromically shifted: by 3 nm for o-A, 4 nm for m-A and 7
nm for p-A with respect to the corresponding peaks registered
for solutions in T (compare data collected in Tables 5 and Table
A theoretical model of the TADF process in studied compounds
was proposed based on quantum chemical calculations. It
suggests an important role of the conformation relaxation in
9
) To the contrary, in the spectra of thin films of pure
compounds, bathochromic shifts are observed as compared to
those recorded in T by 8 nm (o-A), 15 nm (m-A) and 6 nm (p-
A).
the T
donor and acceptor moieties to the values around 70°, which
enables both the reverse intersystem crossing T →S and
subsequent S →S emission. These values of the dihedral angle
1
state, namely decrease of the dihedral angle between
The emission spectra of thin solid films of pure compounds are
more sensitive to isomerism (see Figs. 8 a, b. and c). The
wavelength of the emission band peak decreases in the
following order: o-A> m-A> p-A. In the case of m-A and p-A the
emission bands are substantially bathochromically shifted with
respect to the peaks recorded in T which can indicate strong
intermolecular interactions, less significant in o- isomer where
a hypsochromic shift is observed. All solid state emission
spectra are hypsochromically shifted with respect to the
corresponding solution spectra recorded in DCM.
1
1
1
0
are in a very good agreement with ones recently proposed by
Weissenseel et al. for an efficient TADF in 4,4′-(10H,10′H-9,9′-
4
4
spirobi[acridine]-10,10′-diyl)-dibenzonitrile (SBABz4).
Finally, the determined photoluminescence quantum yields ()
were systematically higher for solid state films of pure
compounds and for their dispersions in solid matrices (zeonex)
as compared to the  values measured for solutions. The effect
was rationalized by suppression of different vibration-induced
emission quenching mechanisms taking place in solutions and
nonexistent in the solid state due to intermolecular
interactions confinements.
In the case of molecules dispersed in zeonex hypsochromic
shifts, with respect to the corresponding bands in the solution
spectra (T and DCM), are observed for all compounds studied
(
compare Tables 5 and 9). These observations coincide very
well with fluorescence temperature dependence
measurements and confirm the sensitivity of molecules
emission to the rigidity of the environment which makes
relaxation of emissive states difficult.
Conflicts of interest
There are no conflicts to declare.
Significantly higher  values were measured for thin films of
pure compounds and films of these compounds dispersed in
zeonex, as compared to  values measured for solutions. The
Acknowledgement
most pronounced increase of  was observed for o-A where it This work was supported by the National Science Centre,
reached 13% both for pure o-A films and for 2 wt% dispersion Poland (NCN, Grant No. 2015/17/B/ST5/00179). M.B. and B.K.
of this isomer in zeonex, whereas in the case of its solution additionally acknowledge financial support from National
spectra (T) the values of  were one order of magnitude Science Centre, Poland under QuantERA programme, project
lower. Similar phenomenon was reported for a phenoxazine no 2017/25/Z/ST2/03038. P.A.G. thanks Warsaw University of
derivative of xanthen-9-on, a somehow similar compound to Technology for financial support. P.T. acknowledges financial
those described here. It was rationalized by suppression of support from the Czech Science Foundation, grant No. 17-
different vibration-induced emission quenching mechanisms, 02578S. The access to computing and storage facilities owned
existing in solution but nonexistent in the solid state due to by parties and projects contributing to the National Grid
4
3
intermolecular interactions confinements.
Infrastructure MetaCentrum provided under the programme
"
Projects of Large Research, Development, and Innovations
Infrastructures" (CESNET LM2015042), and to the Wroclaw
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 13
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1
‡
Molecular formula and formula weight for o-A, m-A and p-A:
C H N O
31 28 2 2
(M =460.55 g/mol). Crystal Data for o-A: triclinic,
space group 푃1 (no. 2), a = 9.3283(7) Å, b = 10.2674(8) Å, c =
1
2
1
1
3.0289(9) Å, α = 79.338(6)°, β = 87.728(6)°, γ = 73.749(7)°, V =
177.24(16) Å , Z = 2, T = 293(2) K, 16699 reflections
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4.5020(9) Å, c = 28.554(2) Å, β = 90.589(7)°, V = 5007.8(6) Å3,
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1
was 0.0512
(
2
1
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Physical Chemistry Chemical Physics
Page 16 of 17
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DOI: 10.1039/D0CP00521E
TOC
Electrochemical and fluorescent (TADF) properties of phenoxazine derivatives of acridone are
strongly isomerism-sensitive.

Page 17 of 17
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/D0CP00521E