5,5-Dioxoyphenothiazine-based D-A-D type AIE molecules enabling persistent room temperature phosphorescence, white light emission and dual-mode mechanochromism

Article abstract of DOI:10.1016/j.dyepig.2021.109193

Novel heavy atom free 5,5-dioxoyphenothiazine-based “D–A–D” emitters with aggregation-induced emission (AIE), room-temperature phosphorescence (RTP), white light emission and dual-mode mechanochromic luminescence (MCL) characteristics have been designed and synthesized. Impressively, DCzCs and DCzCs-Et showed persistent RTP (pRTP) in the crystalline state with lifetimes of 528 ms and 137 ms in air, respectively. DCzCs-Ac exhibited white light emission with a CIE coordinate of (0.25, 0.26), due to the dual emission of fluorescence and phosphorescence. Detailed studies have shown that through simply functional group modification on the acceptor skeleton to modulate the molecular structure and intermolecular interactions, we fine-tuned the optical properties of the luminogens and realized white light emission. The dense crystal packing was proved to be closely related to the phosphorescence of the system and the special dual-mode MCL behavior was attributed to the conversion of crystalline-amorphous state under external force.

Full text of DOI:10.1016/j.dyepig.2021.109193

Dyes and Pigments 188 (2021) 109193
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
5,5-Dioxoyphenothiazine-based D-A-D type AIE molecules enabling
persistent room temperature phosphorescence, white light emission and
dual-mode mechanochromism
Xiaolu Zhoua, Songlin Xua, Liqiang Liua, Yiping Suna, Jinxi Chenga, Xingyu Duana,
,
,**
Lishan Zhoub *, Hongmei Qua
a Key Laboratory of Systems Bioengineering, Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, China
b CNOOC Tianjin Chemical Research & Design Institute Co. Ltd, Tianjin, 300131, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Novel heavy atom free 5,5-dioxoyphenothiazine-based “D–A–D” emitters with aggregation-induced emission
(AIE), room-temperature phosphorescence (RTP), white light emission and dual-mode mechanochromic lumi-
nescence (MCL) characteristics have been designed and synthesized. Impressively, DCzCs and DCzCs-Et showed
persistent RTP (pRTP) in the crystalline state with lifetimes of 528 ms and 137 ms in air, respectively. DCzCs-Ac
exhibited white light emission with a CIE coordinate of (0.25, 0.26), due to the dual emission of fluorescence and
phosphorescence. Detailed studies have shown that through simply functional group modification on the
acceptor skeleton to modulate the molecular structure and intermolecular interactions, we fine-tuned the optical
properties of the luminogens and realized white light emission. The dense crystal packing was proved to be
closely related to the phosphorescence of the system and the special dual-mode MCL behavior was attributed to
the conversion of crystalline-amorphous state under external force.
D-A-D system
Aggregation-induced emission
Room-temperature phosphorescence
White light emission
Dual-mode mechanochromism
1. Introduction
Furthermore, in most cases, long-lifetime phosphorescent emission of
pure organic phosphors can only be observed in specific conditions (for
Organic luminescent materials have received considerable attention
in the field of fundamental research and application in recent years,
owing to their multifarious luminescent properties [1–3].
Aggregation-induced emission (AIE) is a unique photophysical phe-
nomenon in which the formation of agglomerates results in the effective
emission of weak emitting chromophores [4,5]. This phenomenon was
first proposed in 2001 [6] and has been rapidly developed in many
luminescent systems, including room-temperature phosphorescence
(RTP) [3,7], thermally activated delayed fluorescence (TADF) [8,9],
mechanochromic luminescence (MCL) [10,11], etc. Given the low
toxicity, low cost and long lifetime characteristics, pure organic RTP
materials have received increasing interests [12–16] and have been
widely used in information security [17], OLEDs [18,19], bioimaging
[20] and so on. However, pure organic RTP materials generally need the
assistance of heavy halogen atoms to strengthen spin-orbit coupling
(SOC) and promote effective inter-system crossing (ISC) [21–24].
instance, at a low temperature of 77 K) because of the high sensitivity of
triplet excitons to oxygen and temperature [25,26]. As a result, current
pure organic RTP materials without heavy atoms, especially those that
exhibit persistent (τ > 100 ms) room temperature phosphorescence
(pRTP), white-light emission and stimuli-responsiveness are extremely
scarce and need to be further explored.
In fact, hetero atoms (N, S, etc.) and carbonyl groups can also provide
lone pairs of electrons to enhance SOC and increase the efficiency of the
ISC process according to the El-Sayed rule [27,28]. Huang et al. [29]
–
have successfully elucidated the introduction of d-pπ bond (S O, etc.)
–
could reduce the (n, π*) proportion of the lowest triplet excited state (T1)
and inhibit radiative decay rates for phosphorescence. Moreover,
twisted donor-acceptor organic compounds with charge transfer (CT)
emission have been increasingly applied in organic RTP phosphors for
the following reasons [30–32]. The intramolecular charge transfer (ICT)
process could reduce the overlap of the HOMO and LUMO and
* Corresponding author.
** Corresponding author.
E-mail addresses: zhoulishan@tsinghua.org.cn (L. Zhou), ququhongmei@126.com (H. Qu).
https://doi.org/10.1016/j.dyepig.2021.109193
Received 17 November 2020; Received in revised form 21 January 2021; Accepted 26 January 2021
Available online 12 February 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

X. Zhou et al.
Dyes and Pigments 188 (2021) 109193
Scheme 1. The synthetic routes of DCzCs, DCzCs-Et and DCzCs-Ac.
contribute to obtaining a small singlet-triplet splitting energy (ΔEST),
thus promoting efficient ISC [33,34]. Compared with “flat” molecules,
2500 fluorescence spectrometer, respectively. Photoluminescence (PL)
spectra were recorded on the Jobin Yvon Fluorolog 3-21 spectropho-
tometer and F-7000 fluorescence spectrometer. X-ray powder diffraction
(PXRD) spectra were measured using an X-ray diffractometer (D/
the twisted molecular effectively avoids excessive
hibits luminescence quenching.
π-π stacking and in-
In this work, we designed and synthesized three twisted heavy atom-
free D-A-D luminogens (DCzCs, DCzCs-Et and DCzCs-Ac) with intra-
molecular charge transfer (ICT) characteristics and studied the effect of
functional units on photophysical properties. The luminescence mech-
anism of these phosphors was further elaborated, verifying the decisive
effect of crystal packing on phosphorescence emission. It turned out that
all phosphors were typical AIE-RTP molecules and displayed dual-mode
MCL (fluorescence red/blue-shift and RTP on-off switching) character-
istics which were caused by the transition from crystal state to amor-
phous phase. In particular, compound DCzCs exhibited longer
phosphorescence lifetime than DCzCs-Ac, probably due to the unique
molecular stacking modes that effectively prompted ISC processes and
inhibited the non-radiative transitions. Interestingly, compound DCzCs-
Ac showed white light emission with a CIE coordinate of (0.25, 0.26),
attributed to the dual emission of fluorescence and phosphorescence.
Through simply substituent variation on the acceptor skeleton to
modulate the intramolecular and intermolecular interactions, we suc-
cessfully regulated the room temperature phosphorescence emission
and realized white light emission. To our knowledge, this work is the
first report on the synthesis and characterization of D-A-D type phos-
phors based on 5,5-dioxoyphenothiazine.
MAX2500, Japan) with Cu Kα radiation operating at 40 kV and 40 mA
and scanning from 5◦ to 50◦ at a rate of 8◦/min. The time-correlated
decay profiles, temperature dependence experiments and absolute PL
quantum yield were processed on an Edinburgh Instruments FLS1000
spectrometer equipped with Oxford Instruments nitrogen cryostat
(Optistat DN) for temperature control. All measurements were carried
out at room temperature (RT) except for specifying otherwise. From the
emission spectral value, the Commission International del’Eclairage
(CIE) values were calculated by using MATLAB software.
2.2. Theoretical calculations
All quantum-chemical calculations were performed by Gaussian 09
program packages at the B3LYP/6–31 G (d, p) level of theory [35] and
the initial geometries for the dimers were obtained from the single
crystal data. The visualization and distribution of molecular orbital were
analyzed by GaussView and VMD Program [36].
2.3. Synthesis
The synthetic routes of DCzCs, DCzCs-Et and DCzCs-Ac are showed
in Scheme 1. Detailed information on the synthesis of certain in-
termediates is given in the Electronic Supporting Information (ESI).
Synthesis of DCzCs: 1-(3,7-dibromo-5,5-dioxido-10H-phenothiazin-
10-yl)ethan-1-one (3a) (0.44 g, 1 mmol) and carbazole (0.35 g, 2.1
mmol) were dissolved in anhydrous DMF (15 mL) and bubbled with
nitrogen for 15min. Then copper iodide (0.1 g, 0.5 mmol), 1,10-phenan-
throline (0.36 g, 1.99 mmol) and potassium carbonate (0.6 g, 4.5 mmol)
were added to the solution and degassing continued for an additional 15
2. Experimental section
2.1. Materials and measurements
All materials were purchased from commercialized companies and
used as received without further purification except for specifying
otherwise. 1H NMR and 13C NMR were performed with a Bruker Avance
III (400 MHz) in d-dimethyl sulfoxide (DMSO‑d6). High-resolution mass
spectrometry (HRMS) data was measured by Bruker miorOTOF-Q II
mass spectrometer. UV–vis absorption spectra and fluorescence spectra
of the solution were recorded on a UV-4802 spectrophotometer and F-
◦
min. The reaction mixture was heated at 145 C under a nitrogen at-
mosphere for 48h. Upon cooling to room temperature, water (100 mL)
was added to the reaction mixture, and a brown solid precipitated. Then
the obtained solid was filtered, washed with water (3 × 35 mL), and
2

X. Zhou et al.
Dyes and Pigments 188 (2021) 109193
Fig. 1. (a) Normalized absorption spectra of
DCzCs, DCzCs-Et and DCzCs-Ac in THF (10ꢀ 5 M)
and normalized emission spectra of DCzCs, DCzCs-
Et and DCzCs-Ac in THF (10ꢀ 4 M); Fluorescence
spectra of DCzCs (b) and DCzCs-Ac (c) in different
water/THF (v/v) mixtures at a concentration of
10ꢀ 5 M. Insets in figure (b) and (c): emission images
of DCzCs and DCzCs-Ac in water/THF mixture so-
lution with various water contents. (d) CIE co-
ordinates of DCzCs-Ac in different THF/water (v/v)
mixtures.
dried in vacuo. The crude product was purified by silica gel chroma-
tography using petroleum ether: ethyl acetate (8:1, V/V) as eluent. 3,7-
di (carbazol-9-yl)-10H-phenothiazine 5,5-dioxide (DCzCs) was obtained
as white powder in 40% yield (0.275 g). 1H NMR (400 MHz, DMSO‑d6) δ
11.65 (s, 1H), 8.30 (d, J = 7.7 Hz, 4H), 8.15 (d, J = 2.4 Hz, 2H), 8.01 (dd,
J = 8.8, 2.4 Hz, 2H), 7.77 (d, J = 8.8 Hz, 2H), 7.50 (ddd, J = 8.3, 7.0, 1.2
Hz, 4H), 7.41 (d, J = 8.2 Hz, 4H), 7.38–7.33 (m, 4H). 13C NMR (101
MHz, DMSO‑d6) δ 140.86, 137.96, 133.15, 131.04, 126.92, 123.31,
121.57, 121.15, 120.88, 120.81, 120.19, 109.85. HR-MS (APCI): m/z
calcd for C36H23N3O2S [M + H]+ 562.1589, found 562.1583.
saturated brine (3 × 15 mL). Finally, the solution was dried with MgSO4
and filtered, and the solvent was removed in vacuo to give the crude
product as a yellow oily liquid. The crude product was purified by silica
gel chromatography (petroleum ether: ethyl acetate = 2:1, V/V) gave 1-
(3,7-di (9H-carbazol-9-yl)-5,5-dioxido-10H-phenothiazin-10-yl)ethan-
1-one (4c) as a white crystal (0.03 g, 33.17%). 1H NMR (400 MHz,
DMSO‑d6) δ 8.36 (d, J = 8.7 Hz, 2H), 8.29 (d, J = 7.7 Hz, 4H), 8.24 (d, J
= 2.4 Hz, 2H), 8.14 (dd, J = 8.6, 2.5 Hz, 2H), 7.56–7.44 (m, 8H), 7.35
(ddd, J = 7.9, 6.7, 1.4 Hz, 4H), 2.55 (s, 3H). 13C NMR (101 MHz,
DMSO‑d6) δ 169.07, 140.38, 138.50, 136.50, 135.68, 132.20, 130.66,
127.03, 123.60, 121.62, 121.23, 121.15, 110.12, 24.24. HR-MS (ESI):
m/z calcd for C38H25N3NaO3S [M + Na]+ 626.1482, found 626.1509.
Synthesis of DCzCs-Et: Similar to the synthesis of compound DCzCs.
3,7-dibromo-10-ethyl-phenothiazine 5,5-dioxide (3b) (0.42 g, 1 mmol)
and carbazole (0.35 g, 2.1 mmol) were dissolved in anhydrous DMF (15
mL). Then, copper iodide (0.1 g, 0.5 mmol), 1,10-phenanthroline (0.36
g, 1.99 mmol) and potassium carbonate (0.6 g, 4.5 mmol) were added to
the solution and bubbled with nitrogen for 30 min. The reaction mixture
was heated at 145 ◦C under a nitrogen atmosphere for 48h and cooled at
room temperature. After that, water (100 mL) was added to the reaction
mixture, and an off-white solid precipitated. The obtained solid was
filtered, washed with water (3 × 35 mL), and dried in vacuo. The crude
product was purified by column chromatography (petroleum ether:
ethyl acetate = 10:1, V/V) to afford 3,7-di (carbazol-9-yl)-10-ethyl-
phenothiazine 5,5-dioxide (DCzCs-Et) as a pale pink solid (0.158 g, 28%
yield). 1H NMR (400 MHz, DMSO‑d6) δ 8.29 (d, J = 7.7 Hz, 4H), 8.20 (t,
J = 1.4 Hz, 2H), 8.10 (d, J = 1.6 Hz, 4H), 7.49 (ddd, J = 8.3, 7.0, 1.2 Hz,
4H), 7.41 (d, J = 8.2 Hz, 4H), 7.38–7.30 (m, 4H), 4.62 (d, J = 7.1 Hz,
2H), 1.62 (t, J = 6.9 Hz, 3H). 13C NMR (101 MHz, DMSO‑d6) δ 140.79,
139.44, 133.14, 131.47, 126.94, 124.10, 123.35, 121.15, 120.97,
120.86, 119.89, 109.83, 43.59, 12.71. HR-MS (APCI): m/z calcd for
3. Results and discussion
3.1. Synthesis and characterization
Based on metal-catalyzed Ullmann-type C–N coupling reaction, the
targeted molecules DCzCs and DCzCs-Et were synthesized by the reac-
tion of 3,7-dibromo-5,5-dioxoyphenothiazine derivatives with 9H-
carbazole in DMF under an inert atmosphere. Compound DCzCs was
further reacted with acetyl chloride to prepare DCzCs-Ac (Scheme 1).
The structure and state of phosphors were confirmed with 1H NMR, 13
C
NMR, HRMS and powder X-ray diffraction. Moreover, the single crystal
of DCzCs and DCzCs-Ac were obtained in ethyl acetate and dichloro-
methane/ethyl acetate, respectively, to further confirm the chemical
structure and explore their luminescence mechanism.
38H27N3O2S [M + H]+ 590.1902, found 590.1895.
3.2. Photophysical properties of solutions and aggregation-induced
emission (AIE)
C
Synthesis of DCzCs-Ac: 3,7-di (carbazol-9-yl)-10H-phenothiazine
5,5-dioxide (DCzCs) (0.084g, 0.15 mmol), DMAP (0.036 g, 0.3 mmol)
and triethylamine (0.1 mL) were dissolved in anhydrous THF (15 mL).
Then acetyl chloride (0.5 mL, 7.15 mmol) was added dropwise slowly.
The optical properties of the synthesized phosphors were first
examined in THF (Fig. 1a). For all compounds, the absorption spectrums
◦
showed a strong absorption peak around 240 nm ascribed to the π-π*
The reaction mixture was then stirred at 50 C for 20h. The resulting
transition of the conjugated system. The weak absorption peaks around
290 nm–350nm were assigned to the ICT transitions in these molecules.
Compared with DCzCs, the absorption spectrum and emission spectrum
of DCzCs-Ac were red-shifted by 5 nm and 24 nm, respectively, due to
reaction was poured into water (50 mL) and extracted with CH2Cl2 (15
mL) three times. The organic extract was washed with water (3 × 15
mL), saturated aqueous sodium bicarbonate solution (3 × 15 mL) and
3

X. Zhou et al.
Dyes and Pigments 188 (2021) 109193
Fig. 2. (a) Steady-state PL and phosphorescence spectra of DCzCs (red line) (λex = 345 nm) and DCzCs-Et (blue line) (λex = 370 nm) under ambient conditions; (b)
Steady-state PL and phosphorescence spectra of DCzCs-Ac (λex = 365 nm). Inset: photographs of DCzCs-Ac under 365 nm UV light and CIE coordinates calculated
from the emission spectra; (c) Photographs of DCzCs under 365 nm UV light and the afterglow after turning off the UV light; Time-correlated decay curves of DCzCs
(d) at 568 nm (λex = 412 nm), DCzCs-Et (e) at 564 nm (λex = 345 nm) and DCzCs-Ac (f) at 507 nm (λex = 365 nm).
the enhancement of the conjugated chain. In the normalized emission
spectra in different solvents, the emission band displayed a continuous
red shift with the increase of solvent polarity (Fig. S1), owing to the
intramolecular charge transfer (ICT) transition, which was consistent
with theoretical calculations.
when fw = 70%. As shown in Fig. 1d, the fluorescent emission of
DCzCs-Ac realized the transition from blue to blue-green with the water
fractions increasing. Moreover, the PL intensity of DCzCs-Ac increased
sharply when the fw ≥ 70%, confirming the feature of AIE (Fig. 1c).
Considering the existence of highly twisted structures in these mol-
ecules, we studied the emission spectra of molecules in THF/water with
different water fractions (fw) to verify their AIE property. The results
indicated that compounds DCzCs, DCzCs-Et and DCzCs-Ac exhibited
typical AIE properties (Fig. 1 and Fig. S2). Compound DCzCs and
DCzCs-Et emitted blue fluorescence in pure THF solution around 400
nm. As the fw gradually increased from 0% to 60%, the PL intensity of
DCzCs and DCzCs-Et decreased and the emission wavelengths red-
shifted to 410–415 nm (Fig. 1b and Fig. S2). This phenomenon may
result from the ICT emission and stabilization of charge transfer on
increasing solvent polarity [37]. With further increasing fw from 70% to
90%, an abrupt increase in the emission intensity of DCzCs and
DCzCs-Et was observed, which indicated the occurrence of molecular
aggregates. Compound DCzCs-Ac showed emission at 430 nm in THF
and the emission peak red shifted to 480 nm ascribed to the ICT emission
3.3. Solid-state luminescence and mechanochromism
Although these three molecules showed similar photophysical
properties in solution, they exhibited dramatically different lumines-
cence properties in the aggregate state (Fig. 2). The XRD patterns of
these compounds showed multiple sharp peaks, implying the crystalline
structure in the original aggregates (Fig. S3). Interestingly, a long yellow
afterglow was observed from the crystalline powder of DCzCs and
DCzCs-Et on the removal of UV irradiation (Fig. 2c and Fig. S4). As
shown in Fig. 2a, the crystalline powder of DCzCs showed fluorescence
emission (black line) at 433 nm and phosphorescence emission (red line)
at 568 nm. Time-correlated decay profiles were further obtained at room
temperature. Its fluorescence lifetime was 0.4 ns (Fig. S5b) and phos-
phorescence lifetime was 528 ms (Fig. 2d). Similarly, DCzCs-Et pre-
sented fluorescence emission (black line) at 416 nm (τf = 0.24 ns) and
4

X. Zhou et al.
Dyes and Pigments 188 (2021) 109193
Fig. 3. (a) Steady-state emission spectra of the pristine (solid lines) and ground samples (dashed lines) of DCzCs (λex = 370 nm), the insets are the optical images of
samples with UV light turned on and off, respectively; (b) Steady-state emission spectra of the pristine and ground samples of DCzCs-Ac (λex = 365 nm), the inset is
the luminescence image of samples under UV light; XRD patterns of DCzCs (c) and DCzCs-Ac (d): original powder and ground sample.
phosphorescence emission (blue line) at 460 and 564 nm with lifetimes
Table 1
of 26 ms and 137 ms, respectively (Fig. 2 and Fig. S5). Impressively,
Summary photophysical data of DCzCs, DCzCs-Et and DCzCs-Ac.
different from DCzCs and DCzCs-Et, DCzCs-Ac emitted a charming
Compound
HOMO/
LUMOa
(eV)
Ebgap
(eV)
ES/EcT
(eV)
ΔEdST
τfe
τpf
ϕPg
white light under ultraviolet light, and there was no afterglow captured
when the ultraviolet light was turned off. Fig. 2b shows the steady-state
PL spectra of the crystalline powder of DCzCs-Ac, which exhibits dual
emission peaks at 402 and 525 nm. The CIE coordinate of DCzCs-Ac was
calculated to be (0.25, 0.26), confirming the white-light emitting
properties (Fig. 2b, inset). The phosphorescence spectra of DCzCs-Ac
showed a single band at 525 nm, revealing the emission bands at 402 nm
were short-lived emission. It was further confirmed by the time-
correlated decay curve, in which the fluorescence and phosphores-
cence emission lifetimes were 2.4 ns and 3 ms, respectively (Fig. S5d and
Fig. 2f).
(eV)
(ns)
(ms)
(%)
DCzCs
ꢀ 5.46/-
1.46
4.00
3.97
2.96/
2.34
0.62
0.59
0.4
528
1.33%
2.10%
DCzCs-Et
ꢀ 5.46/-
1.49
2.94/
2.35
0.24
137/
564
nm
26/
460
nm
3
DCzCs-Ac
ꢀ 5.88/-
3.79
3.30/
2.92
(T2)
0.38
2.4
6.04%
2.09
Subsequently, temperature-dependent emission spectra and low-
temperature lifetime measurements (80K) were performed to further
validate the existence of phosphorescence. As the temperature
decreased from 270 to 80 K, the PL intensity at long wavelengths of
DCzCs and DCzCs-Ac increased by three- and four-fold, respectively,
while the fluorescence peaks were almost unchanged (Fig. S6). More-
over, compared to room temperature, the lifetime at a low-temperature
of 80K was significantly increased (Fig. S7). The abovementioned results
were in line with the character of phosphorescence, clearly corrobo-
rating that the long-lived emission originated from phosphorescence.
Due to the high brightness of AIE materials in the solid state, the
mechanical response of MCL materials with AIE characteristics is visible
and has a high-contrast ratio, which makes them more reliable and
practical [38–40]. It’s exciting that all three compounds showed
intriguing dual-mode MCL properties (fluorescence red/blue-shift and
RTP on-off switching) in the solid state under UV light. Pristine powder
of DCzCs, DCzCs-Et and DCzCs-Ac exhibited purple emission and white
emission, respectively. After grinding, the emission color of all com-
pounds switched to blue (Fig. 3, inset). As shown in Fig. 3a, the fluo-
rescent emission band of DCzCs blue-shifted from 433 nm to 424 nm by
external shearing force to the original sample, and simultaneously, the
phosphorescent bands at 568 nm and 620 nm disappeared upon
/2.39
(T1)
ELUMO and EHOMO (a), ES and ET (c) performed with Gaussian 09 program
packages at the level of B3LYP/6-31 + G (d, p), Egap = ELUMO - EHOMO (b), EST
=
ES - ET (d); Lifetimes of fluorescence (e) and phosphorescence (f) measured at
room temperature; (g) Absolute photoluminescence quantum yield measured
with an integrating sphere at room temperature.
grinding, explaining the disappearance of the afterglow of the grinding
powder when the UV irradiation was removed. Similarly, under the
action of mechanical force, the fluorescent band of DCzCs-Et red-shifted
from 413 nm to 418 nm, and the ground powder turned dark instantly
when the UV light was turned off (Fig. S8a). The yellow phosphorescent
band of DCzCs-Ac disappeared and the blue fluorescent band red-shifted
20 nm after grinding (Fig. 3b). Besides, the XRD patterns of all pristine
powder showed multiple sharp peaks, whereas they became weak and
even disappeared after the mechanical stimulus, indicating the transi-
tion from the crystalline to the amorphous state (Fig. 3c and d and
Fig. S8b). The disappearance of the phosphorescence for grinding
powder proved that crystal packing played a pivotal role in achieving
phosphorescence emission. Therefore, it is believed that the MCL
5

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Dyes and Pigments 188 (2021) 109193
Fig. 4. (a) Optimized molecular configuration and calculated spatial distributions of HOMO and LUMO for DCzCs, DCzCs-Et and DCzCs-Ac. (b) Diagrams of the TD-
DFT calculated energy levels and possible ISC channels of a DCzCs monomer and its dimers at the singlet (S1) and triplet (Tn) states. The insets are molecular
geometries of the DCzCs monomer, dimer-1 and dimer-2 in the unit cell.
behavior of these compounds is related to the “crystalline-amorphous
transition”, which disrupts intermolecular interactions and opens
non-radiative decay channels to quench RTP [41].
The degree of absolute orbital overlap hints about the possible ICT
process and potentially enhances the ISC for these derivatives [43,44]. It
was further visualized by an electrostatic potential diagram in which the
positive region was mainly on the donor moiety and the negative one on
the acceptor moiety (Fig. S9). On account of the single crystal data, we
obtained two DCzCs dimers with strong intermolecular interactions and
one DCzCs-Ac dimer. Theoretically, for the isolated DCzCs monomer,
there was only one main transition (S1→T2) for intersystem crossing.
Whereas, for the coupled dimers, the number of energy transition
channels notably increased (for dimer-1, S1→T2, T4, T9; for dimer-2,
S1→T2, T6) and the intersystem crossing enhanced obviously (Fig. 4b).
Moreover, in the DCzCs dimer, many triplet states were extremely close
to the S1, promoting the possibility of inter-system crossing from S1 to Tn
[45]. In contrast, for DCzCs-Ac dimer, there was only one triple state
below S1 and fewer energy transition channels which both resulted in
less efficient intersystem crossing from singlet to triplet than DCzCs
(Fig. S10). These results confirmed that simply substituent variation on
the acceptor skeleton could effectively alter the interactions among
molecules and affect the possible ISC channels.
3.4. Theoretical calculation
Three compounds with only minor changes in the receptor skeleton
exhibited completely different photophysical properties especially the
phosphorescence, motivating us to explore the origin. The optimized
geometry, frontier molecular orbitals and energy levels of DCzCs,
DCzCs-Et and DCzCs-Ac were investigated by time-dependent density
functional theory (TD-DFT) and summarized in Table 1. Moreover, we
performed further investigations on single molecule and aggregated
structures in both singlet and triplet excited states (Fig. 4b, Fig. S10 and
Table S4), in which the channels from S1 to Tn were believed to share
part of the same transition orbital compositions and the energy levels of
the possible Tn states were considered to lie within the range of ES1±0.3
eV [42]. As shown in Fig. 4a, for all the molecules, LUMO was localized
over the electron-withdrawing 5,5-dioxoyphenothiazine moiety. HOMO
was mostly located on the carbazole and electron-rich nitrogen atom.
Fig. 5. Intermolecular interactions of DCzCs and DCzCs-Ac in single crystal structures.
6

X. Zhou et al.
Dyes and Pigments 188 (2021) 109193
Fig. 6. Luminescence photographs of the “key” pattern on paper under 365 nm UV light and after removal of the UV light.
3.5. Crystal structure analysis
force was proved to be the cause of the dual-mode MCL behavior.
Theoretical calculations and crystal structure analysis indicated that the
unique molecular stacking modes endow DCzCs with a longer phos-
phorescence lifetime than others. Our research confirms that the sub-
stituents on the acceptor skeleton could flexibly tune the optical
properties of the luminogens by changing the molecular structure and
intermolecular interaction. Owing to their simple molecular structures
and multifunctionality, these molecules will be potentially applicable in
the fields of display systems, anti-counterfeiting materials and future
sensors.
To gain further insight into the distinct PL behaviors, X-ray crystal-
lographic analyses of DCzCs and DCzCs-Ac were performed (Fig. 5 and
Figs. S11–13). In the single crystal structure of DCzCs, the donor moiety
exhibited dihedral angles of 59.39 and 45.96 to the acceptor group on
both sides, while for DCzCs-Ac, both dihedral angles were determined to
be 59.24 (Fig. S11). It suggested that the molecular conformation of
DCzCs-Ac may be more twisted than that of DCzCs, resulting in the
relatively weak intermolecular interactions and the following results
provided the basis for it. As shown in Fig. 5, for DCzCs, the adjacent
dioxophenothiazine subunits showed enhanced π-π interactions (3.79
CRediT authorship contribution statement
Å), and the lone pair of sulfonyl and small steric hindrance induced
strong N–H⋯O hydrogen bonds with lengths of 2.35 Å and 3.02 Å.
Moreover, the adjacent carbazole subunits also exhibited enhanced
Xiaolu Zhou: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Data curation, Writing - original draft, Writing -
review & editing, Visualization. Songlin Xu: Conceptualization, Soft-
ware, Writing - review & editing, Project administration. Liqiang Liu:
Conceptualization, Investigation, Validation, Writing - original draft.
Yiping Sun: Validation, Investigation, Writing - review & editing. Jinxi
Cheng: Validation, Investigation, Writing - review & editing. Xingyu
Duan: Investigation, Writing - review & editing. Lishan Zhou:
Conceptualization, Resources, Supervision, Project administration.
Hongmei Qu: Conceptualization, Resources, Writing - review & editing,
Supervision, Project administration, Funding acquisition.
C–H⋅⋅⋅π interactions (2.77 Å). In contrast, the acetyl group in DCzCs-Ac
provided significant steric hindrance to isolate accepter groups of the
neighboring molecules which prevented the generation of hydrogen
bonds. As a result, the molecules of DCzCs-Ac were mainly interacting
via weak interaction, namely C–H⋅⋅⋅π interactions (3.09 Å). As a whole,
compared to DCzCs-Ac single crystal, DCzCs molecules had smaller
steric hindrance and molecular distortion, thus leading to more compact
molecular packing and stronger intermolecular interaction. The inter-
molecular interactions derived from the N–H⋯O hydrogen bond and π-π
stacking in DCzCs effectively prevented the vibration and rotation of
molecular fragments, thus inhibiting the radiation-free decay of the
triplet excitons and finally yielding longer phosphorescence lifetime
than DCzCs-Ac.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
The excellent pRTP character and distinct MCL character make these
compounds extremely promising in encryption, anti-counterfeiting and
bioimaging. As exampled in Fig. 6, when the crystal powder of DCzCs
was placed in a “key” pattern, purple fluorescence was clearly observed
under ultraviolet light. Once the excitation was removed, the pattern
color changed to yellow, which can be easily identified by naked eyes. In
addition, the single-molecule white light emitter DCzCs-Ac has great
potential applications in display systems and lighting equipment, thanks
to its distinctive advantages, such as minimal color-aging, good stability,
easy fabrication, etc.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109193.
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