Reversibly Switchable Phase-Dependent Emission of Quinoline and Phenothiazine Derivatives towards Applications in Optical Sensing and Information Multicoding

Article abstract of DOI:10.1002/chem.202004657

Three new quinoline and di-tert-butyl phenothiazine isomeric derivatives were synthesized and characterized towards applications for oxygen sensing and optical information multicoding. The compounds with phenylene linker showed outstanding phase-dependent reversibility between ON/OFF states (low and high emission intensity, drastic shifting of emission colors, short- and long-lived fluorescence) in systematic grinding/fuming cycles, as required for multichannel memory devices based on optical information multicoding. The conformational diversity of the phenothiazine unit resulted in dual emission of the doped films implemented by the different luminescence mechanisms with peaks located at 414/530, 416/540, and 440/582 nm. The presence of a phenylene linker and thus two rotational degrees of freedom resulted in quenching of the delayed fluorescence of quasi-equatorial conformers in the solid state. The compound containing no phenylene bridge was characterized by two different driving photoluminescence mechanisms of the doped films: short fluorescence of the quasi-axial conformer and thermally activated delayed fluorescence of the quasi-equatorial form. This compound showed oxygen sensitivity with a Stern–Volmer constant of 7.5×10^?4 ppm^?1.

Full text of DOI:10.1002/chem.202004657

Accepted Article
Accepted Article
Title: Reversibly Switchable Phase-Dependent Emission of Quinoline
and Phenothiazine Derivatives Towards Applications in Optical
Sensing and Information Multi-Coding
Authors: Galyna Sych, Ramin Pashazadeh, Yan Danyliv, Oleksandr
Bezvikonnyi, Dmytro Volyniuk, Algirdas Lazauskas, and
Juozas Vidas Grazulevicius
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202004657
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0
1/2020

Chemistry - A European Journal
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FULL PAPER
Reversibly Switchable Phase-Dependent Emission of Quinoline
and Phenothiazine Derivatives Towards Applications in Optical
Sensing and Information Multi-Coding
[
a]
[a]
[a]
[a]
[a]
Galyna Sych, Ramin Pashazadeh, Yan Danyliv, Oleksandr Bezvikonnyi, Dmytro Volyniuk,
[
b]
[a]
Algirdas Lazauskas and Juozas V. Grazulevicius*
[
a]
Dr. G. Sych, Dr. R. Pashazadeh, Dr. Y. Danyliv, Dr. Oleksandr Bezvikonnyi, Dr. D. Volyniuk, Dr. A. Lazauskas, Prof. J.V. Grazulevicius
Department of Polymer Chemistry and Technology
Kaunas University of Technology
Radvilenu pl. 19, LT-50254, Kaunas, Lithuania
E-mail: juozas.grazulevicius@ktu.lt
Dr. A. Lazauskas
[
b]
Institute of Materials Science
Kaunas University of Technology
K. Baršausko St. 59, LT51423, Kaunas, Lithuania
Supporting information for this article is given via a link at the end of the document.
Abstract: Three new quinoline and di-tert-butyl phenothiazine
hold an important place in organic electronics due to their
applications in memory storage, mechanosensors and secure
inks.[14,15] Moreover, materials exhibiting mechanically included
delayed fluorescence are of great interest as they can be applied
isomeric derivatives were synthesized and characterized towards
applications for oxygen sensing and optical information multi-coding.
The compounds with phenylene linker showed outstanding phase-
dependent reversibility between ON/OFF stages (low and high
emission intensity, drastic shifting of emission colors, short- and long-
lived fluorescence) under systematic grinding-fuming cycles required
for multichannel memory devices based on optical information multi-
coding. The conformational diversity of phenothiazine unit endowed
dual emission of the doped films implemented by the different
luminescent mechanisms with peaks located at 414/530 nm, 416/540
nm and 440/582 nm. The presence of phenylene linker and thus two
rotational degrees of freedom resulted in quenching of delayed
fluorescence of quasi-equatorial conformers in the solid state. The
compound containing no phenylene bridge was characterized by two
different driving photoluminescence mechanisms of the doped films:
short fluorescence of quasi-axial conformer and TADF of quasi-
equatorial form. This compound showed oxygen sensitivity with the
[16]
in oxygen and flexible pressure sensors,
for luminescence
biological imaging[
17]
and data storage. To the best of our
knowledge, there are only few reports on mechanoluminescent
(ML) materials with mechanically stimulated TADF and “turn-
on/off TADF”.[18–20] Materials exhibiting on/off transitions of
emission are potential candidates for memory devices (data
storage) with multi-coding properties (Figure 1a). Three channels
of coding/decoding of optical information can be realized using
the synthesized compounds. Channel 1: Coding/decoding by
emission intensity. The differences of 2-3 times in emission
intensities were observed for ON/OFF stages (“0”/”1”
coding/recoding). Channel 2: Coding/decoding by emission
wavelengths. The emission colors of the compounds were
reversibly switchable from blue/green (478/506 nm) of as-
prepared crystalline solids (stage OFF) towards yellow after
mechanical grinding (538/564 nm) and melting (540/569 nm)
-
4
-1
Stern–Volmer constant of 7.5·10 pmm .
(
1
stage ON). Channel 3: TADF with long-living component (1.5–
.9 µs) of the compounds with phenyl linker was rapidly switched
on/off.
Introduction
Aromatic, π-conjugated organic compounds with donor-acceptor
framework are an object of intensive scientific interest during last
decades because of their effective utilization in the field of organic
electronics. Such compounds found application as hosts for
OLEDs,[
1,2]
emitters in heavy-metal-free OLEDs,
[3,4]
absorbing
materials in dye-sensitized solar cells,[5] and as hole-transporting
materials in perovskite solar cells.[6] In turn, multifunctional
materials showing not only efficient TADF, but aggregation
induced emission, polymorphism or mechanoluminescence are of
[
7–9]
great demand affording new design strategies
and
applications in non-doped OLEDs. [10,11] Since the first report in
013,[12] TADF emitters were characterized as effective
a)
b)
2
[
13]
Figure 1. (a) Schematic visualization of multi-coding/multi-recording on TADF
materials sensitive to external forces (b) Molecular design strategy and chemical
structures of compounds pPZPQ, mPZPQ and PZPQ.
radiometric sensors of oxygen.
Luminescent materials with the
ability to alternate emission wavelength under mechanical stimuli,
1
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Data encryption realized by the hardware encoding devices is
probably the most prominent and essential application of such
kind of materials with several condition-switching properties.
There are examples of application of inorganic-organic
composites for the multi-coding purposes.[21] However purely
organic materials also may be applied successfully in the similar
devices.[22] They assure specific advantages, such as tunability,
flexibility etc.
Hartwig cross-coupling reaction of halide-containing quinoline
derivative and di-tert-butyl-phenothiazine NH-donor to gain the
target compounds. The chemical structures of pPZPQ, mPZPQ
and PZPQ were proved by H and C NMR, mass spectrometries
and elemental analysis (see experimental section in SI).
1
13
In our study, we report on materials with reversibly switchable
emission, different in color, intensity and PL lifetimes towards
optical sensing and information multi-coding for memory devices
(
Figure 1b). With this propose, derivatives of phenothiazine and
quinoline with donor-acceptor (D-A) and donor-π-acceptor (D-π-
A) structures were synthesized and characterized. D-A type
compound (3,7-di-tert-butyl-10-(2,4-diphenylquinolin-6-yl)-10H-
phenothiazine (PZPQ) showing ML/constant TADF was applied
for the fabrication of oxygen sensing devices.
D-π-A type compounds (3,7-di-tert- butyl-10-(4-(4-phenylquinolin-
2
1
-yl)phenyl)-10H-phenothiazine (pPZPQ) and 3,7-di-tert-butyl-
0-(3-(4-phenylquinolin-2-yl)phenyl)-10H-phenothiazine
(
mPZPQ) (Figure 1b) demonstrated ML and switchable TADF.
Although, ML and constant TADF were previously shown for the
phenothiazine derivatives,[23,24] the ability of two presented here
compounds, mPZPQ and pPZPQ (Figure 1b) to show reversibly
switchable emission from purely singlet emission to TADF under
mechanical stimuli is a unique property which potentially can find
application in memory devices. This assumption is supported by
reversibility/stability tests showing stable transition between
singlet emission and TADF which can be systematically repeated
over several grinding-fuming cycles.
Scheme 1. Synthetic routes towards compounds pPZPQ, mPZPQ and PZPQ.
Thermal, electrochemical and photoelectrical properties
The thermal properties of the synthesized materials were
investigated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under nitrogen atmosphere. The
results are shown in Figure 2. Thermal characteristics are
summarized in Table 1. The 5%-weight loss temperatures were
found to be close for all three compounds and were situated in the
range of 348–385 °C. The highest 5% weight loss temperature
was observed for pPZPQ. It was by 25 °C higher than that of
meta- isomer. This difference can be attributed to the stronger
intermolecular interactions of pPZPQ, caused by denser packing
of pPZPQ molecules which is possible due to more symmetrical
structure of para-isomer.[26]
Results and Discussion
Synthesis and characterization
The synthesis of target compounds is depicted in Scheme 1.
Synthetic pathways towards quinoline-based compounds were
similar to the previously reported ones.[25] The first step was acid-
catalyzed Friedlander cyclocondensation to yield a halogenated
quinoline derivative. This reaction was followed by the Buchwald-
Figure 2. (a) TGA curves, (b) cyclic voltammograms and (c) photoelectron emission spectra of pPZPQ, mPZPQ and PZPQ.
2
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In TGA experiments complete weigh loss was observed for the
samples of mPZPQ and PZPQ, which is the evidence of
sublimation process (Figure 2a). The target compounds were
obtained after the synthesis and purification as crystalline
substances. Therefore, they showed endothermic melting signals
during the first DSC heating scans (Figure S1). Compound
pPZPQ exhibited slightly higher melting temperature of 244 °C in
comparison with that of the corresponding meta-isomer mPZPQ
which melted at 230 °C. This observation again can be explained
by the stronger intermolecular interaction in the crystals of para-
isomer. All three compounds were capable to form molecular
glasses during second heating scan. Due to the similar molecular
architectures, compounds exhibited close glass transition
temperature values, which were in the range of 114–116 °C.
strong dispersity was observed for all the compounds (Figure S2).
Electron and hole mobilities exceeded 10-4 cm /Vs in the layers of
compound pPZPQ at electric fields higher than 3×105 V/cm
(Figure S2). Considerably lower electron and hole mobilities were
detected for the layers of compounds mPZPQ and PZPQ at the
same electric fields. Superior charge-transporting properties of
pPZPQ are presumably related to the favorable molecular
packing with high number of close CH···π interaction sites in solid
state. For compounds mPZPQ and PZPQ, such interactions are
limited. The synthesized compounds are promising candidates for
the applications in organic optoelectronic devices due to their
balanced charge-transporting properties and abilities of triplet
harvesting.
2
Theoretical studies
To get deeper insight into the conformational diversity[ of the
structures of pPZPQ, mPZPQ and PZPQ, optimization of ground
state geometries of quasi-axial (q-ax) and quasi-equatorial (q-eq)
phenothiazine conformers was performed using B3LYP/6–31G (d,
p) basis set in vacuum. Also, comparative stability and possibility
of existence of different molecular conformations were estimated
by comparison of energies of the ground state of the conformers
28]
Table 1. Thermal and electrochemical properties of pPZPQ, mPZPQ and
PZPQ.
[
a]
[a]
[b]
[b]
[c]
[d]
d-5%,
[e]
[e]
m
Compound
E
V
ox
E
red
IPCV
eV
EACV
eV
IPPE
eV
T
T
g
T
V
°C
°C
°C
pPZPQ
mPZPQ
PZPQ
0.31 -1.95
0.35 -1.95
0.26 -1.98
5.11
5.15
5.06
2.85
5.43
5.40
5.33
385
360
348
115
116
114
244
230
243
(
Table S1). For all the molecules, q-ax forms were found to be
2.85
2.82
slightly more stable in comparison to the q-eq conformers (their
theoretical energies of the ground state are lower by ca. 1.5
kcal/mol). Minor energy differences of two conformers for each
compound suggest that both species with the different excited
states can exist. The time-dependent density functional theory
method (TD-DFT) with the 6-31G (d, p) basis set was used to
understand the nature of excited states of q-ax and q-eq
conformers of the compounds. The optimized geometries of the
[
+
a] Estimated from CV data; [b] Estimated using the approximation IPCV = |e|(4.8
ox); EACV = |e|(4.8 + Ered); [c] Estimated from photoelectron emission spectra;
E
Estimated from [d] TGA and [e] DSC.
Cyclic voltammetry (CV) was employed for the estimation of
ionization potentials (IPCV) and electron affinities (EACV) of the
compounds. CV curves are shown in Figure 2b. The respective
electrochemical data are collected in Table 1. Due to the similar
structure of the compounds, which contain electron-rich 3,7-di-
tert-butyl phenothiazine heterocycle and electron-deficient
phenyl-quinolin-yl aromatic system, their CV curves were found to
have analogous shapes. The compounds demonstrated both
reversible oxidation and reduction processes during the first CV
scans. Oxidation potential values were found to be 0.31 V for
pPZPQ, 0.35 V for mPZPQ and 0.26 V for PZPQ, while reduction
potentials were found to be -1.95 for pPZPQ and mPZPQ and -
1 1
ground and excited states (S , T ) of the compounds with the
different phenothiazine conformers are depicted in Figure 3. In the
ground state q-ax conformers of compounds pPZPQ, mPZPQ and
PZPQ adopted more planar conformations with dihedral angles
between di-tert-butyl phenothiazine and phenylquinoline units of
8°, 4° and 5°, respectively. Meanwhile, q-eq conformers of
pPZPQ, mPZPQ and PZPQ showed dihedral angles of 81°, 81°,
and 80° respectively (Figure 3).
To investigate the electronic transitions, theoretical UV-vis
spectra were obtained for each conformer with up to 40 calculated
excited states. For better visualization, experimental UV-vis
spectra of THF solutions are shown along with the theoretical
absorption spectra of the compounds in Figure S3. Experimental
spectra are in good agreement with the theoretical ones
resembling the UV-vis bands of both q-ax and q-eq conformers.
Upon vertical excitation, at excited state, q-ax conformer of para-
and meta- congeners of pPZPQ and mPZPQ adopt even more
planar conformation with dihedral angle between phenothiazine
and phenylquinoline planes of 1° (Figure 3). In the meantime, q-
eq conformers of pPZPQ and mPZPQ adopt flat phenothiazine
form with perpendicular geometry between phenothiazine and
1
.98 for PZPQ. The IPCV and EACV values were calculated using
the equations: IPCV = |e|(4.8 + E onset); EACV = |e|(4.8 + Eredonset).
The obtained values of IPCV and EACV are given in Table 1.
Considering minor differences in the structure of the compounds,
the IPCV and EACV values were found to be expectedly close. The
lowest IPCV value of 5.06 eV was demonstrated by PZPQ and can
be assigned to the direct, spacerless D-A linkage.[27] Ionization
potentials of the materials were also estimated for the solid
samples with utilization of photoelectron emission spectrometry
ox
(
Figure 2c). The obtained IPPE values were found to be higher
than the corresponding IPCV values, which is common observation, phenylquinoline moieties with dihedral angle of 89–90°. In excited
and apparently is related to the presence of intermolecular
interactions (e. g. π-π stacking) in the solid state. The IPPE values
obtained by photoelectron emission spectrometry varied from
state, q-ax and q-eq conformers of PZPQ maintained identical
perpendicular geometry between phenothiazine and quinoline
fragments. The molecular orbital profiles revealed that in the first
singlet excited state of q-ax conformers of pPZPQ, HONTO is
delocalized throughout the whole phenyl phenothiazine and
quinoline system. Meanwhile, HONTO of the q-ax conformer of
meta-substituted isomer mPZPQ spreads only over phenyl
phenothiazine moiety, apparently, due to the higher extent of π-
5.33 to 5.44 eV (Table 1).
To demonstrate the potential of compounds pPZPQ, mPZPQ and
PZPQ as light-emitting materials, charge-transporting properties
of their vacuum deposited layers were investigated by time of
flight (TOF) technique. Bipolar charge transport with relatively
3
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conjugation for the para-isomer pPZPQ in respect to its meta-
Photophysical properties
congener. HONTO of q-eq conformers, in the first excited state S
1
,
UV-vis absorption and fluorescence spectra of the solutions of the
compounds in the solvents of the different polarities are shown in
Figure S4 and Figure 4. Supported by theoretical calculations,
experimental absorption spectra of the compounds contain the
absorption bands of both phenothiazine conformers (Figure S3).
The lowest energy bands (LEBs) of the UV-vis spectra of
is localized mainly on the phenothiazine fragment. This
observation can be explained by the perpendicular geometries
between di-tert-butyl phenothiazine and phenyl quinoline planes
(
Figure 3).
compounds pPZPQ and mPZPQ correspond to the S
transition (H→L) of q-ax conformer. The LEB of UV-vis spectra of
mPZPQ corresponds to the S →S transitions of q-ax conformer,
while S →S1, 2 transition of q-ax and q-eq conformers are hidden.
0 1
→S
0
3
0
High-energy absorption bands at 250–274 nm and 300–347 nm
are related to the n–π* and π–π* electronic transitions of locally
excited di-tert-butyl phenothiazine and quinoline derivative
(
Figure S4).
Two conformations of di-tert-butyl phenothiazine resulted in
existence of two excited CT states. Consequently, compounds
exhibited dual emissions in solutions and in solid state (Figure 4).
Taking into account the results of theoretical calculations and
previously reported data,[ the peaks at high energy region can
be assigned to the emission of q-ax conformers, while the
intensive and dominated peak at low-energy region is attributed
to the emission of q-eq conformers. All the compounds showed
positive solvatochromism which resulted in bathochromic shifts of
photoluminescence (PL) spectra with increasing solvent polarity
from toluene (ε=2.3) going to the polar dichloromethane (ε=8.93)
28]
Figure 3. Optimized geometries of pPZPQ, mPZPQ and PZPQ (B3LYP/6-31G
Table S3).[29] Both q-ax and q-eq conformers were found to be
(
(
(
d, p) in the ground and excited states (S1, T1) along with molecular orbitals
MO) distribution at first singlet and triplet excited states.
sensitive to the increase of the polarity of the media confirming
the CT character of the excited states. However, q-eq conformers
were much more sensitive to the polarity of the media than the q-
ax conformers with more pronounced red-shifting of the emission
Therefore, HONTO of both q-ax and q-eq conformers of PZPQ in
excited state is primarily distributed on electron-donating di-tert-
butyl phenothiazine fragment. LUNTO of pPZPQ, mPZPQ and
(
100–130 nm) from toluene to DCM solutions (Table S3).
Therefore, the emission intensity of q-eq conformers dropped
significantly with increase of the solvent polarity in respect to that
of q-ax conformers of pPZPQ, mPZPQ and PZPQ. In the emission
spectrum of mPZPQ, the emission intensity of q-ax conformer
was much weaker than that of q-eq conformer and could be
PZPQ in S
deficient unit. The MO distribution is also different for the excited
triplet state (T ) of the conformers. The T for all the q-ax
1
is strictly localized on phenylquinoline electron-
1
1
conformers of each compound are of the local character where
the HONTO and LUNTO are mainly localized on phenylquinoline
fragment. Meanwhile, the first excited triplet states T of the q-eq
1
observed only for the solutions in highly polar CHCl
3
and DCM
(
Figure 4). Experimental data, in combination with the results of
theoretical calculations, confirm the stronger CT character of
excited state of q-eq conformers in comparison with that of q-ax
conformers. PL quantum yields (PLQY) of the solutions of the
compounds in toluene varied from 5% to 13%. D-A compound
without π-linker (PZPQ) exhibited the highest PLQY values of
both solution and solid sample of 13% and 3%, respectively.
To estimate singlet and triplet energy levels of the
compounds, PL and phosphorescence spectra of their solutions
in THF were recorded at 77 K (Figure S5). Two well-resolved
peaks were observed in PL spectra of the solutions of compounds
pPZPQ and PZPQ. However, only single peak was detected in PL
spectrum of the solution of mPZPQ. The peaks in high energy
region of PL spectra at 77 K of the solutions of pPZPQ and PZPQ
can be assigned to the emission of q-ax conformer, whereas the
second red-shifted peak can be attributed to the emission of q-eq
conformer. Similarly like in PL spectra of the solutions of mPZPQ
recorded at room temperature (Figure 4), emission intensity of q-
ax conformer in PL spectrum of the solution of mPZPQ recorded
at 77 K is negligible.
conformers of pPZPQ, mPZPQ and PZPQ are of the charge
transfer character where HONTO and LUNTO are localized on
electron-rich di-tert-butyl phenothiazine and electron-deficient
phenylquinoline fragments, respectively.
The energies of S1, ax of q-ax conformers are significantly higher
and above 3 eV (Table S2). Meanwhile, the S1, eq values of the
corresponding q-eq conformers are lower by ca. 0.65 eV. All the
1
conformers are characterized by the similar T energies ranging
from 2.35 eV to 2.56 eV, which probably stems from the locally
exited state of electron donor or acceptor. Therefore, the
calculated values of singlet-triplet energy splitting (ΔES-T) vary
significantly along the conformers. Hence, the ΔES-T values of q-
ax conformers are considerably higher (0.73–0.84 eV) in
comparison with those of the corresponding q-eq conformer with
the values of 0.01–0.02 eV.
4
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Figure 4. Normalized PL spectra of the solutions of (10-5 M) and of neat films of pPZPQ, mPZPQ and PZPQ (λexc=350 nm).
The peak in blue region of the spectrum was not detected.
Accordingly, the energies of singlet states (S1, exp) of pPZPQ and
PZPQ were found to be 3.21 and 3.04, respectively. Due to the
observation of single emission peak in the emission spectrum of
THF solution of mPZPQ recorded at 77 K, the band is attributed
to the singlet emission of q-eq conformer (2.58 eV). In agreement
with the results of theoretical investigations, the experimental
triplet state energies (T1, exp) determined from the onset of the
phosphorescence spectra (77 K) ranged from 2.56 eV to 2.59 eV
PZPQ were doped in 1,3-Bis(N-carbazolyl)benzene (mCP) host
in 1:10 ratio. mCP was chosen as the host material due to high
energy of the excited triplet state[32] as well as convenable overlap
between the absorption spectra of the compounds and PL
spectrum of mCP (Figure S8). Similarly to the PL of the solutions,
dual emission was observed for the doped films (Figure S6a).
Unlike the neat films, molecules in doped films experienced higher
intermolecular distances which reduced the possibility of energy
transfer between the two conformers. Time-resolved PL
measurements were carried out to estimate if the compounds
exhibit delayed fluorescence. The examination of PL decay
curves of the doped films recorded at the wavelengths of emission
of each conformer showed that both q-ax and q-eq conformers of
para-isomer (pPZPQ) and meta-isomer (mPZPQ) exhibited short-
living component in the range of nanoseconds (Figure S6b, Table
S4). Apparently, due to the large ΔES-T value[33], q-ax conformer
(λPL=440 nm) of compound PZPQ with direct D-A linkage did not
(
Table 2). The local excited triplet energies of the phenyl-quinoline
electron-acceptor and di-tert-butyl phenothiazine electron-donor
were similar to those of compounds with the calculated energies
of 2.59 eV and 2.64 eV, respectively (Figure S5).
The solid samples of pPZPQ, mPZPQ and PZPQ exhibited single
emission peaks at 515, 550 and 554 nm, respectively (Figure S6a,
Table 2). Appearance of single peaks at low-energy region of PL
spectra of neat films can be attributed to close intermolecular
distances which caused the enhanced energy transfer from q-ax
conformers with higher excited state levels to the q-eq conformers
with lower excited state energies.[30,31] Close packing of the
molecular conformers in neat films also resulted in concentration
quenching which led to the low PLQY of 2–3% and short PL
lifetimes in the range of 2.30–38.01 ns (Figure S7, Table S4).
1 2
exhibit delayed fluorescence either (τ =0.68 ns, τ =3.47 ns)
(Table S4). PL decay curve of q-eq conformer of PZPQ (λPL=582
nm) differed and was characterized by double exponential fit with
short- and long-lived components. Consequently, short lived
component with the lifetime of 23 ns is attributed to the prompt
fluorescence, while long-lived component with lifetime of 5.8 µs is
ascribed to the delayed fluorescence (Figure 5 (a, b), Table S4).
In contrast to the results of the theoretical calculations, the q-eq
conformers of pPZPQ and mPZPQ were not characterized by
delayed fluorescence. Apparently, the presence of phenyl linker
extends conjugation as well as increases quantity of the rotational
degrees of freedom between electron-donating di-tert-butyl
Table 2. Photophysical data for compounds pPZPQ, mPZPQ and PZPQ.
[
a]
[b]
PL
[c]
PLQY[a] PLQY[b]
Compound
pPZPQ
λ
PL
λ
PLQY
%
S
eV
1,exp
T
1,exp
eV
nm
nm
%
%
515 411/525
5
7
2
15
3.21
2.59 phenothiazine moiety and electron-accepting quinoline
heterocycle. PZPQ containing no linker possesses only one
rotational degree of freedom, i.e. the angle between
phenothiazine and quinoline units, while pPZPQ and mPZPQ
both possess two rotational axes, i.e. those between quinoline
unit and phenylene linker and between phenylene linker and
mPZPQ 550 415/571
2
3
4
2.58
3.04
2.57
PZPQ
554 440/582
13
24
2.56
phenothiazine unit. This extension of conjugation between
electron-donor and acceptor led to the possible molecular
configuration with weaker CT state which was enough to quench
delayed fluorescence.
To elucidate the contribution of triplet states to the total
emission of PZPQ, PL spectra of air equilibrated and
deoxygenated toluene solutions were recorded. Enhancement of
the emission intensity by the factor of 2.37 of the peak at 580 nm
λ
PL is the PL maxima; PLQY measured for [a] neat films; [b] films of 10% of
-
5
compounds doped in mCP; [c] toluene solutions (10 M); S1, exp/ T1, exp are the
energies of first excited singlet/triplet states calculated from the onset of PL and
phosphorescence spectra of THF solutions (77 K); λexc=330 nm;
To reduce concentration quenching and to investigate the nature
of each conformer in solid state, compounds pPZPQ, mPZPQ and
5
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was observed after purging the toluene solution of PZPQ with
nitrogen (Figure S9a). The intensity of the peak at 440 nm
practically did not change (Figure S9 (insert)).
complete restoration from the meta-stable ground sample to the
stable initial one. Surprisingly, upon grinding and melting of as-
prepared forms, along with the bathochromic shifts of the
emission spectra, long-lifetime components appeared in PL
decay curves of pPZPQ and mPZPQ (Table 3). Usually, ground
and melted samples of compounds experience distortion of the
molecular packing accompanied with longer PL lifetimes.[18] To
our knowledge, these compounds are among the first reported
examples of materials exhibiting morphology-dependent
mechanically-induced delayed fluorescence. Ground and melted
samples of compounds pPZPQ and mPZPQ showed slightly
higher PLQY of 3–8% relative to the as-prepared and fumed
forms (1–4%) (Table S5).
To examine the nature of delayed fluorescence, the decay
kinetics as a function of temperature for PZPQ molecularly
dispersed in mCP were studied (Figure S9b). Decrease of the
delayed fluorescence lifetime with the increase of the temperature
can be observed, which is assigned to the facilitated rISC. The
shape and position of PL spectra for the sample at different
temperatures remained the same (Figure S9b (insert)). The
dependence of intensity of delayed fluorescence as a function of
laser pulse energy was investigated to estimate the mechanism
of delayed fluorescence (Figure 5c). The slope of c.a. 1, indicates
the domination of TADF for q-eq conformer of PZPQ.[34]
Figure 5. (a, b) PL decay curves of PZPQ recorded at the emission wavelengths
of conformers (c) integrated delayed fluorescence area as a function of the
excitation dose (λexc=350 nm)
Mechanochromic properties
The studied compounds showed external force (heating, solvent
treatment, mechanical force) responsive photoluminescence (PL).
Reversible color variation upon application of external stimuli
(
grinding, fuming and melting) was observed (Figure 6), Table S5).
The as-prepared solids (obtained by recrystallization from
isopropanol) of pPZPQ, mPZPQ and PZPQ showed blue and
green emission with peaks located at 478, 506 and 519 nm,
respectively. Under grinding/melting of the as-prepared samples
the emission spectra red-shifted for ca. 30–60 nm and peaked at
538/540 (pPZPQ), 564/569 (mPZPQ) and 569/582 (PZPQ) nm. In
all cases, the luminescence color of the meta-stable ground states
recovered to that of the stable as-prepared forms by the treatment
of ground powders with DCM vapors for 2 min. Interestingly, the
emission maxima of single crystals was identical with the
emission of as-prepared powders (Table S5). This observation
shows that phenothiazine unit of as-prepared powders adopt q-
eq conformation, confirmed by single crystal X-ray analysis. To
get deeper insights into the ML properties and the nature of
emissive species, PL decay dynamics and photoluminescence
quantum yields (PLQY) of the different forms of compounds
pPZPQ, mPZPQ and PZPQ were studied (Figure S10, Table 3,
S5). PL decay curves of as-prepared and fumed stable states as
well as of single crystals of pPZPQ and mPZPQ were adequately
represented by single exponential fits with the lifetimes of 2.2 and
Figure 6. (a) PL spectra and images of different states of compounds (as-
prepared (i), ground (g), fumed with CH2Cl2 (f), melted (m) and crystallized (c))
(λexc=350 nm) (b) The images of the samples before and after applying external
forces taken under excitation at 365 nm.
PLQY enhancement can be attributed to the activated delayed
fluorescence which allows to harvest both singlet and triplet
excitons. Unlike compounds pPZPQ and mPZPQ containing
phenylene linkage, the as-prepared, fumed and crystalline forms
of PZPQ in their PL decay curves showed both prompt (7.1 ns,
9.9 ns and 10.2 ns, respectively) and long-living components (1.7
µs, 1.9 µs and 1.6 µs, respectively). PL decay curves of ground
and melted forms of PZPQ also exhibited prompt and delayed
components with the lifetimes of 25–26 ns and 1.3–1.6 µs,
respectively (Figure S10).
10.4 ns, respectively. The fumed samples of para- and meta-
isomers showed identical to their as-prepared samples blue-
green emission and the similar PL lifetimes in the range of
nanoseconds (2.2/10.6 ns). This observation confirmed the
6
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Table 3. PL decay characteristics of the samples of pPZPQ, mPZPQ and PZPQ (1st cycle).
pPZPQ
mPZPQ
PZPQ
Sample
τ
1
, ns
τ
2
, ns
τ
1
, ns
τ
2
, ns
τ
1
, ns
2
τ , ns
TADF on
TADF on
Ground
Melted
10.1 (29%)
9.3 (31%)
1579.3 (71%)
1747.7 (69%)
33.3 (33%)
30.3 (27%)
1524.1(67%)
1946.1 (73%)
25.9 (44%)
26.4 (57%)
1648.1 (56%)
1317.5 (43%)
TADF off
As-prepared
Fumed
2.2 (100%)
2.2 (100%)
2.2 (100%)
-
-
-
10.4 (100%)
10.6 (100%)
10.9 (100%)
-
-
-
7.1 (38%)
9.9 (20%)
10.2 (34%)
1703.9 (62%)
1922.8 (80%)
1651.5 (66%)
Crystalized
1/ 2
τ τ are the fluorescence lifetimes; The PL decay curves were fitted with residual values of 1.006-1.300
Interestingly, the compounds exhibited stable emission
reversibility of as-prepared powders after several grinding-fuming
cycles (Figure 7, S11a) without degradation. pPZPQ and mPZPQ
demonstrated intriguing stable switching between prompt
fluorescence of as-prepared and fumed samples to delayed
Likewise, PL of pPZPQ and mPZPQ, the emission of D-A
compound PZPQ recovered to the initial one after several
grinding-fuming cycles. However, PZPQ showed delayed PL
component in all the states obtained after grinding-fuming cycles.
Estimation of delayed fluorescence intensity as a function of laser
power disclosed the nature of delayed fluorescence of pPZPQ,
mPZPQ and PZPQ. The linear plot with a slope of 1 revealed the
monomolecular origin of delayed fluorescence, which
corresponds to TADF (Figure S12).
fluorescence
observed
under
grinding.
Stable
mechanoresponsive delayed fluorescence on/off was observed
during four grinding-fuming cycles (Figure 7b, S11, Table S6).
The unique property of mechanically switchable TADF can be
explained after careful examination of the single crystal and
powder X-ray analysis (Figure 8, S13). Single crystal X-ray
diffraction analysis revealed the predominance of q-eq
phenothiazine conformers of the compounds in the crystalline
forms. The torsion angles between phenothiazine and
phenylene/quinoline units for pPZPQ, mPZPQ and PZPQ were
found to be 85°, 88° and 79°, respectively. The angles between
quinoline unit and phenyl linker in pPZPQ and mPZPQ were found
to be of 2° and 3° for respectively. Thus, PZPQ containing no
phenyl linker can be characterized with only one rotational degree
of freedom, i.e. the angle between phenothiazine and quinoline
planes (Figure 1b). Meanwhile, pPZPQ and mPZPQ experience
more rotational flexibility between donor and acceptor moieties
due to the presence of two rotational axes, i.e. those between
quinoline unit and phenylene linker and between phenylene linker
and phenothiazine unit. Apparently, this extension led to the less
efficient CT which was enough to quench delayed fluorescence.
For all the different states of the compounds, upon applying of
external stresses the color variation of emission occurred due to
the changes of morphology between the crystalline and
amorphous phases. The as-prepared crystalline samples of the
compounds exhibited sharp and intensive diffraction peaks in
their powder X-ray diffractograms (PXRD). (Figure S13). The
peaks on PXRD graphs disappeared after grinding and melting.
This observation indicates the transition to amorphous phase.
Fuming of ground powders with DCM vapors resulted in
restoration of the as-prepared crystalline phase. The single
crystal XRD analysis showed that all the compounds were
characterized by CH···π interactions between adjacent molecules
in their crystalline lattice (Figure 8) which results in restrictions of
the internal rotations. Para- isomer (pPZPQ) experienced slightly
Figure 7. Reversibility of (a) wavelengths of PL maxima and (b) PL lifetimes of
compounds pPZPQ and mPZPQ after five grinding/fuming cycles (λexc=350
nm).
7
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Chemistry - A European Journal
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stronger CH···π interactions with the distances of 2.66 – 2.91 Å
than mPZPQ and PZPQ. As a result, the emission of crystalline
sample of PZPQ was hypsochromically shifted in comparison to
those of mPZPQ and PZPQ.[35] Due to the twisted conformations,
compounds pPZPQ and PZPQ did not experienced π–π stacking
interactions in their crystal cells. Interestingly, meta-isomer
displayed several π–π interactions between phenyl quinoline
fragments of neighboring molecules (Figure 8) which led to the
red-shifted emission band in respect to that of para- isomer. Due
to the restricted intramolecular rotations caused by CH···π
interactions, phenylene linker in pPZPQ and mPZPQ molecules
is located in the same plane with quinoline unit.[36]
probe. The 10 wt% solid dispersion of PZPQ in Zeonex® was
prepared for testing oxygen sensitivity of the compound. Zeonex®
was used as a rigid matrix to suppress intermolecular interactions
[37–39]
leaving the energy transfer to the molecules of oxygen to be
the main path of non-radiative deactivation of excitons and
excluding energy transfer between conformers. PL spectra were
recorded consecutively under conditions of different
nitrogen/oxygen ratio kept for 15 minutes each for the stabilization
of intensity. As it is shown in Figure 9a, emission of PZPQ is
strongly quenched by oxygen which intercepts the triplet
electronic excitation energy. The emission band related to the q-
eq conformer is considerably more suppressed than that of q-ax
conformer even at low portions of oxygen revealing its more
expressed TADF character. More specifically, TADF intensity of
the q-eq conformer of PZPQ derived from the measurement in a
nitrogen environment was found to be by ca. 22.5 times higher
than intensity of its prompt fluorescence taken under oxygen flow.
At the same time, only two-fold increase was observed for the
band related to the q-ax conformer caused by suppressing of the
non-radiative exciton deactivation. The respective Stern–Volmer
plot[40] reflecting suppression of emission with the increase of
0
oxygen concentration in the medium is given in Figure 9b. I is the
intensity of emission recorded under nitrogen atmosphere and I
denote the intensity at specific concentration of oxygen. The initial
linear dependence caused by a dynamic quenching is followed by
a downward curvature started at ~10000 ppm attributed to the
partial heterogeneity of the doped film. The so-called Stern–
/I-1 was estimated to be 7.5·10-4 pmm .
-1
Volmer constant KSV=I
0
Additionally, the reversibility of oxygen sensitivity of the system
was examined taking into account the relatively low adjusted R2
of 0.91 of the Stern-Volmer plot. After the measurement in air two
successive measurements after 35 minutes of exposure of the
film under purged oxygen and nitrogen atmosphere, respectively,
showed the range of intensities as presented in Figure 9c. All the
following cycles had duration of 10 minutes each.
Figure 8. Single crystal molecular structures and molecular packing of (a)
pPZPQ, (b) mPZPQ and (c) PZPQ obtained from single crystal X-ray analysis.
Such localization of phenylene ring, apparently, is responsible for
the extended conjugation and as a result higher ΔES-T value. Such
a molecular geometry presumably results in the decrease RISC
efficiency and consequently “turns-off” TADF of pPZPQ and
mPZPQ in crystalline phase. After mechanical or thermal
treatment (e.g. grinding, melting) the CH···π interactions are
eliminated and molecules pack in more condensed matter thus
experiencing stronger π–π stacking interactions. Molecular
fragments in amorphous powders are able to change their torsion
angles, which results in the decrease of conjugation followed by
1
reduction of S energy and, thus, decrease of ΔES-T value. This
can induce rISC and “turn-on” TADF and make the compounds
potential candidates for memory devices with multi-
coding/recoding abilities (Figure 1b).
Figure. 9. (a) PL spectra, (b) Stern-Volmer plot, (c) PL intensity of 10% solid
solution of PZPQ in Zeonex® recorded at room temperature and different ratio
of nitrogen and oxygen (λexc=350 nm).
Oxygen-sensing properties
While potential application of compounds pPZPQ and mPZPQ in
memory devices (data storage) with multi-coding properties was
demonstrated, compound PZPQ containing no phenylene linker
and characterized by constant TADF was used as oxygen sensing
The reference of 1.5 kppm of oxygen was taken for the test of
reproducibility of the same level of intensity at exact portion of
oxygen proving the great capacity of oxygen sensing of PZPQ.
8
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Table of Contents
New phenothiazine and quinoline derivatives exhibiting outstanding phase-dependant reversibility between ON/OFF stages (low and
high emission intensity, drastic shifting of emission colors, short- and long-lived fluorescence) under systematic grinding-fuming cycles
required for multichannel memory devices based on optical information multi-coding.
1
1
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