Tuning Electron-Withdrawing Strength on Phenothiazine Derivatives: Achieving 100 % Photoluminescence Quantum Yield by NO2 Substitution

Article abstract of DOI:10.1002/chem.202000754

The weak fluorescence (quantum yield <1 % in cyclohexane) of phenothiazine (PTZ) impedes its further application. In addition, the nitro group (NO₂) is a well-known fluorescence quencher. Interestingly, we obtained a highly fluorescent chromophore by combining these two moieties, forming 3-nitrophenothiazine (PTZ-NO₂). For comparison, a series of PTZ derivatives bearing electron-withdrawing groups (EWGs; CN and CHO) or electron-donating groups (EDGs; OMe) at the 3-position have been designed and synthesized. The phenothiazines bearing EWGs exhibited enhanced emission compared with the parent PTZ or EDG derivatives. Computational approaches unveiled that for PTZ and PTZ-OMe, the transitions are from HOMOs dominated by π orbitals to LUMOs of mixed sulfur nonbonding–π* orbitals, and hence are partially forbidden. In contrast, the EWGs lower the energy level of the lone-pair electrons on the sulfur atom, thereby suppressing the mixing of the nonbonding orbital with the π* orbital in the LUMO, such that the allowed ππ* transition becomes dominant. This work thus demonstrates a judicious chemical design to fine-tune the transition character in PTZ analogues, with PTZ-NO₂ attaining 100 % emission quantum yields in nonpolar solvent.

Full text of DOI:10.1002/chem.202000754

A Journal of
Accepted Article
Title: Tuning Electron-withdrawing Strength on Phenothiazine
Derivatives: Achieving 100% Photoluminescence Quantum Yield
by -NO2 substitution
Authors: Meng-Chi Chen, Yao-Lin Lee, Zhi-Xuan Huang, Deng-Gao
Chen, and Pi-Tai Chou
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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.202000754
Link to VoR: http://dx.doi.org/10.1002/chem.202000754
Supported by

Chemistry - A European Journal
10.1002/chem.202000754
Tuning Electron-withdrawing Strength on Phenothiazine
Derivatives: Achieving 100% Photoluminescence Quantum Yield
by -NO substitution
2
[
a]
[a]
[a]
[a]
[a]
Meng-Chi Chen, Yao-Lin Lee, Zhi-Xuan Huang, Deng-Gao Chen, and Pi-Tai Chou,*
M.-C. Chen and Y.-L. Lee made equal contributions.
[
a]
M.-C. Chen, Y.-L. Lee, Z.-X. Huang, D.-G. Chen, Prof P.-T. Chou
Department of Chemistry
National Taiwan University
No. 1, Section 4, Roosevelt Road, Taipei 10617 (Taiwan)
E-mail: chop@ntu.edu.tw
Supporting information for this article is given via a link at the end of the document.
Abstract: Weak fluorescence (quantum yields < 1% in cyclohexane)
of phenothiazine (PTZ) impedes its further utilization. Besides, the
and solids of parent PTZ (PLQY = 0.16% in cyclohexane) and its
derivatives hamper the pratical applications.[ In sharp contrast,
replacing sulphur in PTZ by the oxygen atom, forming PXZ,
8]
2
nitro group (-NO ) is a well-known fluorescence quencher.
Interestingly, we obtained a highly fluorescent chromophore by
renders moderate emission yield (PLQY
=
2.7% in
cyclohexane).[ The result manifests the role of sulphur atom in
harnessing the emission yields.[ Accordingly, the relatively soft
sulphur atom may lift the energy of lone pair electrons.Then, the
n* state is in close proximity to the * state in parent PTZ,
leading to the state mixing that reduces the transition moment.
This, together with the nonplanar of the PTZ and hence the
twisting motions that enhance the non-radiative decay pathways,
may account for the low emission yields for PTZ (vide infra).
In this study we report the design and syntheses of PTZ
derivatives in an aim to harvest the emission in terms of intensity
and energy by harnessing the substituent at the PTZ moiety.
Note that this approach is much distinct from the recent studies
9]
combining these two moieties, forming 3-nitrophenothiazine (PTZ-
10]
2
NO ). For comparison, a series of PTZ derivatives with electron-
withdrawing groups (EWG) (cyano -CN and formyl -CHO) and
electron-donating group (EDG) (methoxy -OMe) group at C3-
position were designed and synthesized. The EWGs exhibit
intensified emission compared with parent PTZ or EDG derivatives.
Computational approaches unveil that for PTZ and PTZ-OMe, the
transitions are from HOMOs dominated by  orbitals to LUMOs of
sulphur nonbonding-mixed * orbitals, and hence are partially
forbidden. On the contrary, the EWGs lower the energy level of lone
pair electrons on the sulphur atom, suppressing the mixing of
nonbonding orbital with * in LUMO such that the allowed *
transition becomes dominant. This work thus demonstrates a
judicious chemical design to fine-tune the transition character in PTZ
in which the whole PTZ moiety is used as an electron donating
group to construct a D-bridge-A TADF materials.[
3a-e]
Realizing
analogues, making PTZ-NO
2
attains unity emission quantum yield in
that the twisting sulphur lone pair electron along the sulphur-
carbon bond play a key role; on the one hand, we intend to
anchor various electron-withdrawing groups (EWG), including
nonpolar solvents.
2
formyl (-CHO), cyano (-CN) and even nitro (-NO ) functional
groups at the meta- position with respect to the sulphur atom to
lower the lone pair electron energy via the inductive effect. On
the other hand, these EWG are intentionally added in the para-
position of the nitrogen atom on PTZ, rendering a potential for
charge transfer (CT) character. We thus anticipate that the CT
effect may lean the PTZ moiety toward a planar configuration to
suppress the non-radiative twisting effect. For a fair comparison
we also synthesized methoxy (-OMe) substituted PTZ to
represent the electron-donating group (EDG). As a result, the
introduction of EDG and EWG revealed large effect on the
luminescence properties, which were also supported by the
corresponding theoretical approaches (vide infra). Remarkably,
exploiting the nitro group that is conventionally thought to be a
Introduction
Phenothiazine (PTZ) based derivatives have drawn great
attention in a variety of aspects such as pharmaceutical
chemistry,[1] dye-sensitized solar cells (DSSCs),[2] molecular
designs in the thermally activated delayed fluorescence (TADF)
molecules,[3] organic light-emitting diode,[4] and supramolecular
host-guest systems.[5] PTZ is known as an anthracene-shaped
heterocyclic compound with electron-rich nitrogen and sulphur
atom in the middle six-member ring (see Scheme 1). Hence, its
electron density is more abundant than that of other heterocyclic
congeners such as phenoxazine (PXZ), acridine and
carbazole.[3f, 6] Moreover, PTZ could be transformed to various
states such as neutral, cation radical and oxygenated sulfoxide
forms, providing synthetic perspective especially in
2
good emission quencher, the PTZ-NO achieve 100% PLQY in
nonpolar solvents. In other words, we also demonstrate how
nitro group can act as a fluorescence quencher, and how it
cannot.[11] Detail of results and discussion are elaborated below.
[
7]
pharmaceutical fields. Such bio-synthetic versatility intrigues
us to mull the potential of applying PTZ derivatives in
luminescence based bio-sensing. However, the low
photoluminescence quantum yields (PLQY) in both solutions
1
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Chemistry - A European Journal
10.1002/chem.202000754
Scheme 1. Chemical structures of PTZ and its analogues modified at C3-
position.
Photophysical properties
The steady-state absorption and emission spectra of the titled
compounds in three solvents, cyclohexane (CYH), toluene (TOL)
and dichloromethane (DCM), are depicted in Figure 1, and the
selected photophysical properties are summarized in Table 1.
The corresponding spectra using ethanol (EtOH) and acetonitrile
(
ACN) as solvent are provided in Figure S1. Detailed
photophysical properties of titled compounds are tabulated in
Tables S2-3. As shown in Figure 1, the absorption peak of the
EDG substituted PTZ-OMe around 320 nm with absorption
extinction coefficient being lower than 103 M-1 cm-1 can be
ascribed to a * mixed with certain partially forbidden (vide
infra) transition of the PTZ chromophore. Further, the emission
peaks of PTZ-OMe are not varied by employing different polarity
solvents and show mirror-image emission with respect to the
lowest lying absorption band, affirming the locally excited (LE)
emissive character with vibronic profile. Similar results can be
observed in PTZ with absorption maximum at 320 nm and
vibronic progressive emission maximized at 440 nm in all three
solvents.
Results and Discussion
Synthesis Strategy and Molecular Structure
The chemical structures of the titled molecules are depicted in
Scheme 1. The brief synthetic routes and experimental
conditions are shown in Scheme 2 and addressed below (see
Supporting information (SI) for details).
In sharp contrast, as substituted by the EWG at the C3-
position, PTZ-CN exhibits a red-shifted absorption peak around
Firstly, PTZ was dissolved in THF and nitrated into PTZ-NO
2
with concentrated sodium nitrite aqueous solution in ice bath.[12]
PTZ-CHO was synthesized through a Duff reaction, i.e., the
hexamine aromatic formylation, by dissolving PTZ with
hexamine in an acetic acid solution and reflux overnight.[13]
Further, PTZ-CHO was converted to PTZ-CN via an improved
Schmidt conversion[14] by dissolving purified PTZ-CHO with
sodium azide in acetonitrile and addition of triflic acid during fast
stirring. The synthesis of PTZ-OMe was performed from a
mixture of 4-methoxyaniline and bromobenzene in dry toluene
through a palladium-catalysed coupling reaction to yield DPA-
OMe (see Scheme 2),[15] which then underwent a ring-closure
process by reacting with sulphur and iodine in 1,2-
dichlorobenzene overnight to yield PTZ-OMe.[12a, 16] All five titled
compounds were characterized by 1H and C NMR and
recorded in Figure S8 to S17. The appearance of five titled
compounds were recorded and shown in Figure S7.
3
40 nm (cf. PTZ). By varying the solvent polarity, the emission
peaks of PTZ-CN are observed at 450, 460 and 470 nm in
cyclohexane, toluene and dichloromethane solutions,
respectively, revealing solvatochromism. The result clearly
indicates that the lowest lying electronic transition of PTZ-CN is
no longer dominant by the LE transition but incorporates
noticeable CT transition. The observation is further supported by
introducing formyl and nitro EWGs. For PTZ-CHO, the
wavelength of the longest absorption tail (shoulder) is apparently
red shifted upon increasing solvent-polarity, showing a typical
character of intramolecular charge transfer (ICT) absorption
band. Furthermore, the emission peaks are recorded at 470, 510,
and 540 nm in cyclohexane, toluene, and dichloromethane
13
solutions, respectively. More impressively, PTZ-NO
strongest electron withdrawing -NO group, the emission band is
2
, having the
2
red shifted from 540 nm in cyclohexane to as far as 700 nm in
dichloromethane, showing the most prominent excited-state ICT
character among the titled compounds. Hence, the strength of
ICT effect of non-substituted and substituted PTZ analogies,
based on the result of solvatochromism, is increased in the order
Single crystals of PTZ-OMe, PTZ, PTZ-CHO, and PTZ-NO
2
had been successfully obtained and analysed with Xcalibur,
Atlas, Gemini ultra-diffractometer. Detailed crystallography data
were included in SI. The crystals of PTZ-OMe and PTZ are both
1 2
monolithic with a space group of P2 /c; the crystal of PTZ-NO
2
of PTZ-OMe ≈ PTZ < PTZ-CN < PTZ-CHO < PTZ-NO , which
and PTZ-CHO are both orthorhombic with a space group of
correlates well with the electron withdrawing strength of the
substituent. The transformation from LE to CT character is
further supported by the frontier orbital analyses elaborated in
the section of computational approaches (vide infra).
1
Pna2 . PTZ-NO
2
has a Herringbone packing,[ which indicates
17]
that there is a strong - interaction between different layers in
its solid state.
Scheme 2. Synthesis route of five titled molecules.
2
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Chemistry - A European Journal
10.1002/chem.202000754
(
0.29%) in the same solvent (see Table 1). Although there exist
CYH
TOL
DCM
1.0
0.8
0.6
some highly polarized or polarizable nitro chromophores which
could be quite fluorescent,[18] it is widely considered that -NO
is
4
3
2
1
00
00
00
00
0
2
0.4
a strong fluorescent quencher owing to its high energy level of
nonbonding orbital with lone pair electrons (vide supra). To gain
further insight, the fluorescence decay dynamics were measured
by time-correlated single photon counting (TCSPC). The
photoluminescence decays for all titled compounds in various
solvents are shown in Figure S2 to S6 with pertinent data listed
in Table 1. As a result, PLQY can be expressed as PLQY =
PTZ-OMe
0.2
0.0
1.0
0.8
0.6
0.4
4
3
2
1
00
00
00
00
0
PTZ
0.2
0.0
1.0
0.8
0.6
0.4
8
6
4
2
00
00
00
00
0
푘푟 푘표푏푠 = 푘푟 푘푟 + 푘푛푟 where 푘표푏푠 , 푘푟 and 푘푛푟 denote the
experimentally observed decay rate constant, the radiative
decay rate constant and the non-radiative decay rate constant,
respectively. Combining the acquired PLQYs and 푘표푏푠
experimentally, 푘 and 푘 can thus be deduced and listed in
⁄
⁄ (
)
PTZ-CN
PTZ-CHO
0.2
0.0
1
.0
.8
6
4
2
000
000
000
0
0
푟
푛푟
0.6
Table 1. As shown in Table 1, in a qualitative manner, it seems
that increase of the electron withdrawing strength of substituent
causes the increase of 푘 but decrease of 푘 . For example, 푘
0
.4
.2
0
0.0
푟
푛푟
푟
1
.0
.8
1
1
5000
0000
6
-1
0
of PTZ in cyclohexane solution is calculated to be 2.16 × 10 s ,
0.6
8
-1
which is ~60 times as small as that of PTZ-NO
2
(1.31 × 10 s ).
0
.4
.2
5
000
0
PTZ-NO2
This, together with the large 푘 of PTZ in cyclohexane (1.35 ×
0
푛푟
0.0
800
9
-1
1
0 s ), leads to its relatively much weaker emission compared
3
00
400
500
600
700
with the electronic withdrawing group substituted PTZ molecules.
Wavelength (nm)
6
-1
In fact, 푘 of 2.16 × 10 s in PTZ manifests its forbidden-like
푟
lowest lying transition. Moreover, 푘 increases as enhancing the
푟
Figure 1. Steady-state absorption spectra in extinction coefficient (M-1 cm-1)
dashed line) as well as photoluminescence (solid line) spectra of PTZ-OMe,
PTZ, PTZ-CN, PTZ-CHO and PTZ-NO in various solvents at room
temperature. The red, orange, and green lines represent cyclohexane (CYH),
toluene (TOL), and dichloromethane (DCM), respectively. See Table S1 for
the selected excitation wavelengths.
electron withdrawing strength (see Table 1), indicating the more
allowed lowest-lying transition. On the one hand, the results
imply increasing energy of the forbidden n* character, which
makes the sulphur lone pair electron involved upon increasing
the electron-withdrawing strength, proving the concept of lifting
the n* transition (vide supra) that is separated from the * state.
This viewpoint is also supported by the gradually increase of the
extinction coefficient for the lowest lying absorption band from
(
2
The above results, though intriguing, may not be surprising
in terms of interconversion between LE and CT states. What is
even more important, to our viewpoint, is that the corresponding
PLQY increases significantly from EDG to EWG substituted PTZ.
To our surprise, the PLQY of PTZ-NO is nearly 100% in the
2
nonpolar solvents such as cyclohexane, which is in sharp
EDG (e.g. -OMe, ~500 M cm-1 at peak in cyclohexane) to EWG
-1
4
-1
-1
(
2
e.g. -NO , > 10 M cm at peak) (Figure 1). On the other hand,
the decrease of 푘푛푟 may imply the enhancing planarization the
PTZ moiety upon increasing the CT property. Supports of these
viewpoints are elaborated in the section of theoretical approach.
contrast to the rather low PLQYs for PTZ (0.16%) or PTZ-OMe
Table 1. Experimental and calculated optical characteristics for the titled molecules a
c
Absorbance
→S
Emission
→S
Stokes shift
(cm )
Quantum yields / Lifetime (ns)
-
1
-1
-1
S
0
1
S
1
0
푘푟 (s ) / 푘푛푟 (s )
Compound
Exp. b /Cal. (nm)
ƒ)
Exp. b /Cal. (nm)
∆
E
exp, ∆Ecalc
CYH
TOL
DCM
(
(ƒ)
3
(
14 / 302
0.0041)
448 / 426
(0.0084)
0.29% / 0.38
7.56 × 10 / 2.60 × 10
0.42% / 0.75
5.63 × 10 / 1.33 × 10
0.56% / 2.33
2.40 × 10 / 4.26 × 10
PTZ-OMe
PTZ
9526, 9690
8352, 9211
8330, 8358
4762, 7703
5353, 7474
6
9
9
8
7
6
9
9
8
7
8
6
8
9
8
7
9
3
(
18 / 303
0.0007)
433 / 420
(0.0080)
0.16% / 0.74
0.24% / 0.86
0.22% / 0.81
6
6
6
2.16 × 10 / 1.35 × 10
2.80 × 10 / 1.16 × 10
2.73 × 10 / 1.24 × 10
3
(
31 / 323
0.0741)
457 / 443
(0.0559)
6.6% / 5.07
11% / 5.52
11% / 5.99
PTZ-CN
PTZ-CHO
7
7
7
1.31 × 10 / 1.84 × 10
2.13 × 10 / 1.60 × 10
1.81 × 10 / 1.49 × 10
3
(
76 / 340
0.1248)
458 / 461
(0.0988)
44% / 8.22
46% / 8.47
41% / 7.98
7
7
7
5.30 × 10 / 6.87 × 10
5.45 × 10 / 6.36 × 10
5.13 × 10 / 7.39 × 10
4
(
08 / 362
0.1873)
522 / 496
(0.1694)
100% / 7.63
29% / 2.76
0.54% / 0.24
PTZ-NO
2
8
8
7
1.31 × 10 /
-
1.06 × 10 / 2.57 × 10
2.29 × 10 / 4.22 × 10
a
Experiments were conducted under room temperature, and calculations were performed with m062x/6-31+g(d,p) in cyclohexane. Exp. denotes the lowest-lying
absorption or emission peak wavelengths from experimental results; Cal. denotes the computed lowest-lying absorption or emission peak wavelengths; ƒ
represents oscillator strengths; ∆Eexp stands for the Stokes shift calculated from experimental results; ∆Ecal stands for the Stokes shift calculated from
computational results. The experimental values are taken from the cyclohexane solutions. c Lifetime were measured by TCSPC with excitation wavelength at
b
360 nm for all five compounds.
3
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Chemistry - A European Journal
10.1002/chem.202000754
Figure 2. (a) Excited-state and ground-state optimized structure. (b) Excited-state optimized HOMO and LUMO orbitals of the titled molecules computed by m062x/6-
1+g(d,p) with cyclohexane as solvent. The bending angle () in (a) was defined as the angle intercepted by the two planes bisected from the six carbons on both
3
side of the six-membered ring of PTZ.
In sharp contrast to the nonpolar solvent, much decreases of
PLQYs for the electronic withdrawing group substituted PTZ
molecules were observed in polar solvent upon increasing the
2
being shifted from PTZ-OMe (-1.50 eV) to PTZ-NO (-2.84 eV)
seem to be more drastic than that of LUMOs, which plays a key
factor for in the excited-state mixing. Details are elaborated as
follows (vide infra).
2
solvent polarity (see Table 1). Especially, for PTZ-NO , PLQY
decreases from 100% in cyclohexane to 0.54
dichloromethane. Knowing that the emission of PTZ-NO
%
in
has
Computational approaches
2
To gain in-depth insight into the electronic transition character of
the titled molecules, computational approaches were conducted
using time-dependent density functional theory (TD-DFT) on the
basis of m062x functional associated with 6-31+G(d,p) basis sets
been red shifted to as far as 700 nm in polar solvent, another non-
radiative decay channel, i.e., the quenching of the S state by the
overlap with ground-state high-frequency or high-density
vibrational modes, is expected to be dominant. This so-called
quenching operated by the energy gap law increases with
1
(
see SI for details). Although b3lyp is also widely used for
computational approaches, the absorbance and emission
wavelengths calculated by m062x show less deviation from the
experimental result in these charge-transfer organics as tabulated
in Table S5. Thus, we provided the result of m062x in manuscript,
and attached the results of both m062x (see Table S5) and b3lyp
(see Table S6) in SI. The computed energy gaps and the
associated frontier orbitals are recorded in Table 1 and Figure 2.
decreasing energy gap, rendering the low PLQY in high polar
solvent.[19]
Cyclic voltammetry analysis
Cyclic voltammetry (CV) analysis was also conducted to verify
their intrinsic electronic properties, for which the corresponding
CV diagrams are provided in Figure S18 and the numeric data
are organized in Table 2. In the CV analysis, Ag/Ag+ (0.01M
0
After geometric optimization in their ground-states (S ), the titled
AgNO
3
) electrode was selected as a reference electrode. For
molecules all exhibit a bending type geometry owing to the
existence of the electron-rich sulphur atom as shown in Figure
2(a). However, as we defined the bending angle () as the angle
intercepted by the two planes bisected from the six carbons on
both side of the six-membered ring of PTZ, there exists a trend
toward planarization in ground-state optimized structures, for
which the angles are recorded in the order of PTZ-OMe > PTZ >
working electrode, the oxidation potentials and reduction potential
were measured using
dichloromethane and
dimethylformamide, respectively. Both experiments were
conducted with 0.1 M of NBu PF and used Pt wire as a counter
eletrode. The potentials were further referenced externally to the
a
platinum (Pt) electrode in
glassy carbon electrode in
a
4
6
+
ferrocenium/ferrocene (Fc /Fc) couple.
2
PTZ-CN ≈ PTZ-CHO > PTZ-NO . The EWG seems to reduce the
As tabulated in Table 2, the recorded HOMO are gradually
shifted to the more negative side in the order of PTZ-OMe (-4.8
eV) > PTZ (-4.9 eV) > PTZ-CN (-5.1 eV) ≈ PTZ-CHO (-5.1 eV) >
electron density on the sulphur atom, which decrease the size of
its lone pair, causing the decreased bending angle. Moreover, the
highest occupied molecular orbitals (HOMOs) of the titled
molecules are mainly localized at the middle of PTZ core as
shown in Figure S20.
With an aim to investigate their emissive properties, we then
conducted the geometric optimization in their lowest lying excited-
1
state (S ), and their relative structural characters as well as
2
PTZ-NO (-5.2 eV), which is consistent with the overall trend of
electron donor capability (EDG > non-substituted > EWG).
Moreover, as we computed the energy levels of LUMOs by adding
the energy gap of the absorption onset, we obtained the
associated values in the order of PTZ-OMe (-1.50 eV) > PTZ (-
1
.53 eV) > PTZ-CN (-2.30 eV) > PTZ-CHO (-2.48 eV) > PTZ-NO
2
frontier orbitals are shown in Figure 2 and Figure S19 to S20,
respectively.[20] As shown in Figure 2(b), while the lowest
unoccupied molecular orbitals (LUMOs) of the EDG substituted
PTZ-OMe and parent PTZ are still mainly localized on the PTZ
(
-2.84 eV) as shown in Table 2, which correlated well (in a reverse
manner) with the sequence of the HOMO and electron donating
capability. However, the modifications in energy levels of LUMOs,
4
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Chemistry - A European Journal
10.1002/chem.202000754
core, revealing a LE transition character, the contribution of the
non-bonding orbitals of the sulphur atom becomes non-negligible
in LUMOs. The results indicate that the lowest lying transition
contains mainly * character overlapping, in part, with certain
degrees of non-bonding character from sulphur atom.[21] This
forbidden character in transition is reflected in its small oscillator
strength calculated for PTZ-OMe (0.0041) and PTZ (0.0007) as
tabulated in Table 1), which is consistent with the experimental
angles of the titled molecules are PTZ-OMe (12.03°) > PTZ (4.45°)
≈ PTZ-CN (4.68°) > PTZ-CHO (0.07°) > PTZ-NO
2
(0.03°) in the
lowest lying excited-states (S ), which follows the similar trend,
1
but with a significantly smaller value. This is a general observation
[20]
in PTZ and other heterocyclic derivatives such as phenazine.
Accordingly, the more planar configuration upon increasing the
electron withdrawing ability (see Figure S19) gives less twisted
motion, and hence the decrease of non-radiative decay rate (푘푛푟),
result of smaller 푘 as well as low absorption extinction coefficient
2
rationalizing the ~100% PLQY for PTZ-NO in the nonpolar
푟
(
see Figure 1 and Table 1). In contrast, introducing EWGs on
solvent.
PTZ decreases the energy of the * orbital of PTZ, resulting in its
less mixing with the sulphur non-bonding orbital. From another
perspective, the EWGs make the substituted six-membered ring
mainly contributing to LUMOs, diminishing the mixing between
Table 2. Electrochemical properties and free energy of photoinduced electron
transfer of the three titled molecules with charge transfer. a

* and n* states. Accordingly, LUMOs of EWG substituted
퐷
퐴
HOMO b
LUMO b
∆G c
퐸표푥
퐸
푟푒푑
Compound
analogies (PTZ-CN, PTZ-CHO and PTZ-NO ) have negligible
2
V
( )
V
( )
(
eV
)
eV
( )
⁄
푘ꢑ푎푙 푚ꢒ푙
contribution from sulphur atom, and the net results reveal CT
character.
PTZ-OMe
PTZ
0.49
0.63
0.83
0.80
0.84
-2.12
-2.02
-2.12
-1.68
-1.00
-4.80
-4.94
-5.14
-5.12
-5.16
-1.85
-1.99
-2.19
-2.16
-2.20
2.01
3.16
Combining the above results, the rationalization of the
transformation from LE to CT character may be offered in a
quantitative manner based on the Marcus-Weller equation
expressed below (eq. 1) to deduce the associated free energy
PTZ-CN
PTZ-CHO
-8.77
∆G) of charge transfer in the excited-state.[22]. The equation is
-13.69
-21.22
(
listed below:
PTZ-NO
2
푒2
푒2
1
1
1
1
퐴
∆
퐺 = 퐸퐷 − 퐸 − 퐸00 − ꢀ4휋휀 푟푐ꢃ − ꢀ
ꢃ ꢄ푟 + 푟 ꢉ ꢀ휀
− 휀ꢂꢃ (eq. 1)
표푥
푟푒푑
ꢁ
휀
ꢂ
8휋휀
ꢁ
ꢅ
ꢆ
ꢇꢈ
ꢅ퐶푀
a
퐷
퐴
푟푒푑
퐸표푥 and 퐸
are anodic and cathodic peak potentials using Fc+/Fc as
b
ꢓ.ꢔ + 퐸퐷 − 퐸 ) and LUMO = HOMO + 퐸
퐹ꢐ
표푥
푃퐿
where 퐸표푥(ꢊ) and 퐸푟푒푑(ꢋ) are the oxidative and reductive
potential, respectively, measured by cyclic voltammetry; 퐸00 are
recorded from the 0-0 transition of the titled molecules where
reference. HOMO = −
(
표푥
, where
표푛푠푒푡
푃퐿
퐸
is the energy converted from onset of the locally excited emission from
표푛푠푒푡
PL spectra, which is 420 nm is our case. c Free energy, ∆G, is calculated by eq.
1
based on Marcus-Weller equation.
charge transfer process start to take place; ꢌ , ꢌ , and ꢌ are
퐷ꢍꢎ
0
푠
dielectric constants in vacuum, desired solvent and
dichloromethane, respectively; ꢏꢐ are recorded the distance
Conclusion
between donor and acceptor; ꢏ퐷
ꢆ
and ꢏ퐴
ꢈ
are the ionic radii of
, and
are visually shown in Figure S21. Though it is not common to
donor and acceptor, respectively. The definitions of ꢏ , ꢏ퐷
ꢆ
ꢐ
In conclusion, a series of PTZ analogies were strategically
designed and synthesized in an aim to boost the emission
intensity. PTZ and the EDG substituted PTZ-OMe exhibit partially
forbidden transitions because of the mixed * and n* (sulphur
nonbonding orbital) states. This, together with nonplanar structure
that activates the twisting non-radiative deactivation, rationalizes
their weak emission. Conversely, introducing EWGs lowers the
energy level of LUMO and diminishes the mixing of nonbonding
orbitals to facilitate the allowed -* transition. Also, the EWGs
substitution induces excited-state charge transfer, which
suppresses the twisting non-radiative deactivation to enhance the
ꢏ
ꢈ
퐴
adapt this equation that were originally designed for photoinduced
electron transfer (PET) to the charge transfer (CT) case, the
results are quiet convincing in a qualitative manner.
Employing all the recorded values to eq. 1, the corresponding
∆
G for undergoing charge transfer are recorded in Table 2.
Accordingly, ∆G for the titled molecules are in the order of PTZ
3.16 kcal/mol) > PTZ-OMe (2.01 kcal/mol) > PTZ-CN (-8.77
kcal/mol) > PTZ-CHO (-13.69 kcal/mol) > PTZ-NO (-21.22
(
2
kcal/mol). It is thus reasonable to expect that the exergonicity of
the titled molecules undergoing charge transfer follow the order of
emission. Among them,
quencher functional group (-NO
PTZ-NO exhibits 100% Q.Y. in nonpolar solvents (e.g.,
a
widely considered fluorescence
2
PTZ-OMe ≈ PTZ < PTZ-CN < PTZ-CHO < PTZ-NO . Additionally,
it is worth to note that the uncertainty of this assessment should
be appreciable in recording the emissive energy gap in solution
2
) was also employed in this work.
2
cyclohexane), which also shows significant solvatochromism in
polar solvents, covering the visible region from 500 to 700 nm.
and CV potentials. Also, the definition of ꢏ퐷
ꢆ
and ꢏ퐴
ꢈ
is
ambiguous, which is only localized at the donor and acceptor sites.
Nevertheless, PTZ-OMe and PTZ possess positive ∆G, which
indicates that the CT process is thermodynamically unfavourable,
consisting with their LE emission concluded experimentally. On Experimental Section
the contrary, the experimentally estimated ∆G of the EWG
substituted PTZ analogies are all negative, indicating the
accessibility of CT process and resulting the CT emission. The
trend of the exergonicity consists with the experimental results.
Last but not the least, it is worth to note that upon geometric
optimization in their excited states, PTZ core undergoes certain
Synthesis
The characterizations of all compounds are provided in Supporting
Information.
Absorbance
degrees of excited-state structural planarization to enhance its -
conjugation.[
3c, 23]
For example, as shown in Figure 2(a), the
Absorbance were measured with the double-beam spectrophotometer
bending angles () of the titled molecules in ground-state are in
the order of PTZ-OMe (33.97°) > PTZ (33.80°) > PTZ-CN (31.73°)
(U3310, manufactured by Hitachi). Two quartz cuvettes with dimensions
of 1×1×4.5 cm were cleaned with acetone, dried by heat gun, and whipped
with acetone-damped Kimwipe. In the sample compartment of, both
≈
PTZ-CHO (31.76°) > PTZ-NO (30.55°), whereas the bending
2
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5

Chemistry - A European Journal
10.1002/chem.202000754
reference slot and sample slot were placed with quartz cuvette, and filled
with desired solvent to half-full. Parameters, set in Method section of the
program, were configured to allow U3310 to scan from 900 nm to 300 nm
at a 600 nm/min speed. Then, the sample that was dissolved in desired
solvent in advanced was added into the cuvette in the sample slot. The
absorbances at intend excitation wavelength, usually around the first
absorption peak (longest in wavelength), was adjusted in a range of 0.10
to 0.11 for quantum yield analysis. For lifetime measurement and better
absorption and photoluminescence spectrum, the absorbance was
adjusted about 0.30.
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6
nm was 10 nm less than twice of the excitation wavelength to avoid second
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6

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10.1002/chem.202000754
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Chemistry - A European Journal
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Entry for the Table of Contents
A series of C3-substituted phenothiazine (PTZ) with different electron-donating and -withdrawing groups was investigated by
comprehensive computational approaches and photophysical measurements. Among them, a nitro group substituted PTZ possesses
1
00% quantum yields in nonpolar solvent. The results validate that the interplay of the nonbonding orbital of sulphur atom in PTZ plays
a crucial role in the electronic transition.
Institute and/or researcher Twitter usernames: @edchen1240
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8