Phenothiazine Scope: Steric Strain Induced Planarization and Excimer Formation

Article abstract of DOI:10.1002/anie.201906083

Phenothiazine derivatives based on the 10-phenyl-10H-phenothiazine (NAS) chromophore, namely 7-phenyl-7H-benzo[c]phenothiazine (NAS-1) and 12-phenyl-12H-benzo[a]phenothiazine (NAS-2), were designed and synthesized. NAS-1 and NAS-2 are constitutional isomers with different steric strains imposed on the phenothiazine core moiety. In solution, the more-strained NAS-2 possesses a bent structure and undergoes photoinduced structural planarization (PISP). In the crystal, despite the absence of PISP, bent NAS-2 exhibits prominent excimer emission as well as emission mechanochromism, which is not observed in the planar-like NAS and NAS-1. This unconventional observation results from the bent core structure facilitating π–π stacking of the peripheral naphthalene moieties. Two-photon-coupled depth-dependent emission shows spectral differences between the surface and kernel of the NAS-2 crystal, and is believed to be a general phenomenon, at least in part, for materials exhibiting emission mechanochromism.

Full text of DOI:10.1002/anie.201906083

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Accepted Article
Title: A New Scope of Phenothiazines: The Steric Strain Induced
Planarization and Excimer Formation
Authors: Deng-Gao Chen, Yi Chen, Cheng-Ham Wu, Yi-An Chen,
Meng-Chi Chen, Jia-An Lin, Chun-Ying Huang, Jianhua Su,
He Tian, and Pi-Tai Chou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906083
Angew. Chem. 10.1002/ange.201906083
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A New Scope of Phenothiazines: The Steric Strain Induced
Planarization and Excimer Formation
Deng-Gao Chen,†,  Yi Chen,†, , ‡ Cheng-Ham Wu,† Yi-An Chen,† Meng-Chi Chen,† Jia-An
Lin,† Chun-Ying Huang,† Jianhua Su,‡ He Tian,*, ‡ and Pi-Tai Chou,*, †
Dedication ((optional))
Abstract: In this work, a series of phenothiazine derivatives based on
10-phenyl-10H-phenothiazine (NAS) chromophore, namely 7-phenyl-
To generalize as well as extend the scope of PISP, in this study,
new class of compounds bearing phenothiazine were
a
7H-benzo[c]phenothiazine
(NAS-1)
and
12-phenyl-12H-
synthesized, including 10-phenyl-10H-phenothiazine (NAS), 7-
phenyl-7H-benzo[c]phenothiazine (NAS-1) and 12-phenyl-12H-
benzo[a]phenothiazine (NAS-2) (Figure 1(a)), in which NAS-1
and NAS-2 are strategically designed to act as the constitutional
isomers but subject to different steric strain in the core
chromophore benzophenothiazine. The aim of this study is two
folds. Firstly, we intend to explore if heterocyclic systems other
than DAPCs are able to undergo PISP, broadening its horizon in
chemistry. Secondly, bearing the isomers with distinct steric strain,
the mechanism of structure-PISP relationship can be explored
from a chemistry viewpoint. Also, phenothiazines are emerging
materials, which have been recently reported to show persistently
long phosphorescence suitable for the potential applications of
bioimaging.[4]
As a result, NAS-2, which possesses a markedly bent structure,
undergoes prominent PISP, generalizing the mechanism of steric
hindrance governed PISP. Moreover, in solid, different from the
conventional wisdom of stacking planar structure induced
excimeric reaction[5], the bent NAS-2 reveals anomalous excimer
formation and mechanochromism.[6-7] The former is rationalized
by the bent core structure facilitating -* stacking of the
peripheral naphthalene moiety. As for the latter, comprehensive
spatially resolved two-photon luminescence spectroscopy draws
the caution on the conventionally dubbed mechanochromism,
which may, in part, originate from different surface-to-volume
ratios, depending on the way of preparation for solid. Detail of the
result and discussion is elaborated below.
Syntheses of NAS, NAS-1 and NAS-2 were achieved using the
Buchwald-Hartwig reaction.[7] Details of the synthetic route are
elaborated in Supporting information, and all studied
compounds were fully characterized. Moreover, the structures of
NAS,[8] NAS-1 and NAS-2 were confirmed by X-ray analysis
shown in Figure 1(b). Among them, the bending angle in the
phenothiazine core, defined as the dihedral angle ∠
C1−N1−S2−C2 (see Figure 1(a)), which also represents the
bending angle between planes 1 and 2 (a) as shown in Figure
1(b), is in the order of NAS (152.08) > NAS-1 (147.89) > NAS-
2 (131.12). For NAS and NAS-1, the appended N-substituted
phenyl rings show a perpendicular orientation with respect to the
phenothiazine chromophore, while NAS-2 inherits a structure
showing parallel orientation between the substituted phenyl ring
and the cored chromophore.[2] For comparison, the computational
approaches were also employed by using the density-function
theory (DFT) optimized structures for the titled molecules, and the
results are shown in Figure 3 (NAS-2) and Figure S6 (NAS and
benzo[a]phenothiazine (NAS-2), were strategically designed and
synthesized, in which NAS-1 and NAS-2 are the constitutional
isomers with different steric strain imposed on the phenothiazine core
moiety. In solution, the more strained NAS-2 possesses a much bent
structure and undergoes photoinduced structural planarization
(PISP); the result, from the chemical viewpoint, supports an emerging
topic regarding the steric hindrance induced PISP. In solid crystal,
despite the prohibition of PISP, the bent NAS-2 exhibits prominent
excimer emission as well as emission mechanochromism that is not
observed in the planar-like structure NAS and NAS-1. This
unconventional observation is rationalized by the bent core structure
facilitating - stacking of the peripheral naphthalene moiety. The two-
photon coupled depth-dependent emission shows spectral difference
between surface and kernel of the NAS-2 crystal, which is believed to
be a general phenomenon, at least in part, for materials claimed to
exhibit emission mechanochromism.
Photoinduced structural planarization (PISP) is a fascinating
and promising phenomenon that has received much attention.[1]
Recent examples are focused on the class of molecules bearing
N,N’‑disubstituted-dihydrodibenzo[a,c]phenazine chromophore
(DPAC, see Figure 1a).[2] Different from the early reported PISP
molecules that reveal an intrinsic nonplanar structure free from
the steric perturbation[2], the bent DPACs are purportedly due to
the steric hindrance imposed on the DPAC moiety, which, upon
optical-excitation, undergoes structural planarization, resulting in
a large Stokes shifted emission. Exploiting the resulted structure-
dependent emission properties, diverse applications of PISP have
been reported, including white light generation,[3] hydrolysis[2d]
and sensing for self-assembled metallacycles.[2a]
[†]
[‡]
[]
D.-G. Chen, Y. Chen, C.-H. Wu, M.-C. Chen, Y.-A. Chen, J.-A. Lin,
C.-Y. Huang, Prof. P.-T. Chou
Department of Chemistry, National Taiwan University, Taipei, 10617
Taiwan, R.O.C.
E-mail: chop@ntu.edu.tw
Y. Chen, J. Su, H. Tian
Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science & Technology
Shanghai 200237 (P. R. China)
E-mail: tianhe@ecust.edu.cn
D.-G. Chen and Y. Chen with equal contribution.
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. (a) Chemical structure of phenothiazine derivatives. (b) Single-crystal structures along the view of N-S axis, together with values for the bending angle
(a, b and c) of the phenothiazine skeleton. (c) Frontier molecular orbital contours of the titled molecules in optimized excited-state geometries. Steady-state
absorption in toluene solution (black dash line) and photoluminescence spectra (solid line) of (d) NAS, (e) NAS-1, (f) NAS-2 and (g) DPAC in various solvents at
room temperature. Notes that blue, green, yellow and red color represent cyclohexane, toluene, dichloromethane and acetonitrile solutions, respectively.
NAS-1). The computational results are consistent with crystal
structure in terms of trend of a, which is in the order of NAS
(143.51) > NAS-1 (142.94) > NAS-2 (133.05), indicating that
the bending structure resolved from x-ray structure is not mainly
caused by lattice packing. This leads us to safely conclude that
NAS-2 possesses a much bent angle a between planes 1 and 2
(see Figure 1(b)), intrinsically due to the steric hindrance
introduced by benzo[a]phenothiazine (cf. benzo[c]phenothiazine
in NAS-1).
The normalized absorption and emission spectra of the studied
phenothiazines are shown in Figure 1(d) to (f). For reference,
DPAC spectra are depicted in Figure 1(g). NAS and NAS-1
exhibit an onset of the lowest-lying absorption band around ~430
and 465 nm, respectively. In sharp contrast, the absorption onset
of NAS-2 is in much higher energy of ~400 nm. Comparing NAS-
1, the larger S0-S1 energy gap for NAS-2 manifests its more
bending structure that restricts the -electron delocalization. In
quantum yields (Q. Y.) recorded in cyclohexane solution,
respectively. Also, as shown by the purple circle that the onset of
the emission coincides with that of the absorption, the result
indicates a normal emission in nature. Although NAS-2 shows
similar Q.Y. (8.99%) in cyclohexane solution compared with NAS-
1, the onsets of absorption and emission for NAS-2, specified by
the blue and red circles (Figure 1(e)), are far separated, indicating
much different configurations between the originally prepared and
structurally relaxed S1 states. The result is essentially the same
as emissive character of DAPC where the far separation between
absorption (circled in blue) and emission (circled in red) onsets is
obvious (see Figure 1(g)). We thus conclude that similar to DAPC,
PISP takes place in NAS-2.[2]
To gain in-depth insight into PSIP of NAS-2, fluorescence up-
conversion techniques were conducted for NAS-2 in toluene. In
this approach 380 nm (~90 fs) laser was selected as the excitation
pulse, which is near the onset of the lowest transition. This is to
avoid the higher-lying vibronic relaxation processes that interfere
with the interpretation. The fluorescence decay dynamics were
then monitored at various wavelengths to trace the temporal
evolution. Upon monitoring at 460 nm in the region of the onset of
the emission band (see Figure 1(f)), the kinetic trace consists of
a system limited rise (< 120 fs) and a single exponential decay
component. The latter is fitted to be 0.7 ps, the fast decay of which
is attributed to slight structural relaxation. This can be supported
by monitoring at e.g. 520 nm, in which the 0.7 ps decay
component in 460 nm emission becomes the rise component,
accoz8mpanied by a long rise component that still increases its
intensity at the end of the acquisition window of 30 ps (Figure 2).
This long rise component is further fitted to be 28 ps (see Figure
S3). Upon monitoring at longer emission wavelength, such as 560
nm and 600 nm, the 0.7 ps rise component gradually disappear
fact, NAS-2 reveals similar absorption spectral profile with that of
[2]
DPAC
,
affirming
the
bending
feature
of
the
benzo[a]phenothiazine moiety. Further support is rendered by the
calculated frontier orbitals for the titled compounds (Figure 1(c)),
in which NAS and NAS-1 exhibit a more delocalized type *
transition whereas it is apparent that the lowest lying transition of
NAS-2 mainly possesses plane 1 (HOMO) to plane 2 (LUMO)
(see Figure 1(b) for definition) slightly charge transfer character
and higher calculated vertical transition energy (~375 nm) than
that (~410 nm) of NAS-1 (see Tables S6 and S8). The latter is
also consistent with the experimental results (vide supra).
Distinct emissive properties are also obtained for the studied
phenothiazines. Among them, both NAS (Figure 1(d)) and NAS-
1 (Figure 1(e)) show emission that is a mirror image with respect
to the lowest lying absorption band with 1.04% and 9.03%
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and the 28 ps rising curve becomes the major component (see
Figure 2), followed by a much longer population decay time that
cannot be resolved by the fluorescence up-conversion study. This
population decay is further measured by time correlated single
photon counting (TCSPC) technique to a lifetime of 12.22 ns (see
Figure S2 and Table S3).
I* shown in Figure 3, which reveal a rather small difference in a
and b between R* and I*. Then, a subsequent I*→P* process
takes place, for which the time constant 2 is as long as 28 ps
resolved from the rise component of the P* monitored at 600 nm.
The long 2 clearly indicates that the I*→P* process involves a
large-amplitude motion, which is also supported by the significant
difference in a between I* and P*(see Figure 3, vide infra). In
comparison, the planarization time constant of 28 ps (i.e. the
formation of the P* state) for NAS-2 as observed from 520 nm to
600 nm is significantly longer than that (c.f. 7.8 ps) of DPAC.[1b]
The result may manifest the chemical difference of core moieties
between phenothiazine (NAS-2) and phenazine (DPAC) where
sulfur and nitrogen, respectively, play a key role for the
planarization. The lone pair electron of sulfur atom in
phenothiazine is less susceptible to the -conjugation than that of
nitrogen in phenazine, giving less driving force for PISP in NAS-
2.
Further support of the structure-PISP relationship is provided
by the computational approaches, in which NAS-2 was fully
optimized in the ground state to exhibit a global energy minimum
R state and hence its lowest-lying excited state R* via vertical, i.e.,
Franck Condon excitation. I* and P* states were obtained by fully
optimized in S1 state to achieve its local and global energy
minimum, respectively. As shown in Figure 3, the computational
results indicated that the bending angle (a) changes from
133.05 to 130.27 in I* state with a more bent V-shaped
geometry. This can be considered as a local energy minimum,
from which the transformation starts to take place, resulting in the
nearly planarized (a = 173.87) global minimum geometry in the
P* state. Additionally, the other two bending angles in the middle
six-member ring of the phenothiazine, denoted as b and c, are
extended from 113.78 and 97.99 to 123.71 and 103.07 in R*
and P* states, respectively (see Figure 3). The expansion of the
two bending angles in the six-membered ring (i.e. b and c)
indicates more delocalization of the-electrons in the
phenothiazine skeleton, driven by the energy stabilization to gain
aromaticity. The net result is to reduce the * energy gap. The
calculated S1→S0 of 584 nm for P* (see Tables S8 to S10)
matches well the experimentally observed at ~550 emission
(Figure 1(f)). In stark contrast, the result of calculation assesses
a small change of a for NAS and NAS-1 after structural
relaxation in the S1 state (see Figure S4), supporting their normal
emission behavior.
Figure 2. Femtosecond fluorescence transients (open circles) of NAS-2 in
toluene at room temperature observed at various wavelengths as depicted.
The black solid lines in figure denote the instrument response functions (IRF)
and fitting curves, respectively. The fitted results are tabulated in Table S3.
ex: 380 nm.
In solid crystal, the imposed lattice energy forbids PISP;
therefore, one expects that NAS-2 should exhibit normal Stokes
shifted emission, similar to that of NAS and NAS-1 (see Figure
S1(a) and S1(b)). Unexpectedly, NAS-2 exhibits dual emission in
solid crystal maximized at 390 nm and 520 nm, denoted as F1 and
F2 bands, respectively (see Figure 4(a)). In comparison, DPAC
also shows a normal 386 nm emission band in solid crystal (see
Figure S1(d)). Therefore, the assignment of the F1 band to the
normal emission of the bent structure is unambiguous. It is
reasonable to conclude that the F2 band observed in the solid
crystal originates from the intermolecular-interaction induced
emission, most plausibly the excimer-like emission. This
viewpoint is supportive by the difference of molecular packing in
crystal between NAS-2 and NAS-1 shown in Figure 4(b) and (c),
respectively, in which NAS-2 shows a  stacking between
adjacent benzo[a]naphthalene moieties with a decent distance of
Figure 3. DFT-optimized structures of NAS-2 with recorded values for the
bending angle in phenothiazine chromophore (a) and the two bending angle
(b and c) in the six-membered ring in R*, I* and P* states, respectively.
The above resulted relaxation dynamics for NAS-2 can be
rationalized by a sequential kinetic pattern expressed as R∗
τ
1
τ
2
→ I∗ → P∗ , where R* specifies as the initial optically prepared
excited state, I* denotes the intermediate state and P* represents
the final planarized states. The first decay time constant (1) of 0.7
ps observed at 460 nm is reasonably assigned to a fast structure
relaxation pathway, forming the intermediate state (I*). This 0.7
ps time constant can also be recorded from 520 to 600 nm as a
rise component becasue these emission regions mainly
originates from I* that acts as a successor of R*. The small time
constant indicates a small change of the structure during R*→ I*
process. This is evidenced by the calculated structures of R* and
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Figure 4. (a) The emission spectra of the red dashed square region as shown in the merged imagines (d) recorded from surface to kernel (color solid line)
combining with the photoluminescence spectra of NAS-2 in ground solid crystal (black solid line). Insect: The view of the naked eye under 365 nm UV lamp
irradiation of crystal and the ground solid crystal of NAS-2. (b) NAS-2 and (c) NAS-1 with recorded values for distance between two proximal congeners. (e) The
transient photoluminescence characteristics of NAS-2 in crystal monitored at various wavelength as descripted. Notes that ex = 760 nm were employed for two-
photon studies, ex = 380 nm for transient measurements, and ex = 350 nm for PL measurement. Black dashed line in (a) denotes that the emission spectra by
two-photon studies are started from 420 nm due to the instrument limitation, and the IRF in (e) represents the instrumental response function. The results are all
recorded at room temperature.
3.34 Å (recorded from C8 to C9’ and C9 to C8’ as shown in Figure
4(b)) for intermolecular interaction. Figure 4(b) clearly shows that
the bent phenothiazine in NAS-2 avoids the steric hindrance
between the N-phenyl (molecule 1) and the adjacent
phenothiazine (molecule 2) moieties, affording a proximal 
interaction for the benzo[a]naphthalene moieties. On the contrary,
shown in Figure 4(c), the more planar NAS-1 has different
molecular packing where the benzo[c]naphthalene moieties are
arranged in opposite direction between adjacent NAS-1
molecules, avoiding any possible  stacking interaction.
Of equal importance is the observation of so called
“mechanochromism” in solid NAS-2 where the emission color,
from the view of naked eyes, changes from pale blue in crystal to
deep blue upon grinding the crystal as shown in Figure 4(a).[5] We
then employ two-photon excitation microscopic fluorescence
imaging to investigate NAS-2 in the solid crystal as a function of
the depth of crystal from its surface. Upon two-photon (380 nm)
excitation and probing the surface of the crystal, NAS-2 exhibits
mainly a 390 nm emission band, appearing as the purple-blue
emission hue (see Figure 4(d); the probing region is specified by
the dashed square area in red). Upon increasing the depth of the
excitation, the apparent 500 nm excimer emission increases
(Figure 4(a)), reaching a maximum at ~14 m that is nearly at the
kernel of the prepared crystal. In comparison, the emission of the
ground NAS-2 solid, as shown in the Figure 4(a), is dominated
by the 390 nm emission (see black solid line). Moreover, the
transient photoluminescence of NAS-2 in crystal solid (Figure
4(e)) also support the observations. By monitoring at normal
emission of 400 nm, the up-converted single shows a fast decay
component of 20.8 ps, accompanied by a long population decay
time of 1.0 ns that is further resolved by TCSPC (Figure S4(a)
and (b)). The 20.8 ps matches well with the rise component of
21.3 ps monitored at the excimer emission of 500 nm (see Figure
4(e)). Also, a population lifetime the excimer emission is
measured to be 1.5 ns (Figure S4(b) and (b)). Accordingly, it is
reasonable to assign the 1.0 ns decay component for the 400 nm
emission to the decay of normal emission on the crystal surface
where excimer formation is prohibited.
Having gathered the above observations, we are thus able to
assert the formation of the excimer in the kernel versus the cease
of the excimer formation on the surface due to the rupture of the
ordered stacking by the dangling NAS-2 molecules. The
observation of mechanochromism upon grinding in this study is
attributed to the increase of surface-to-volume ratio that
suppresses the excimer formation.[5] Note that the continuous
growth of the excimer emission from surface to kernel shown in
Figure 4(e) is due to the resolution in depth, which is limited to ~1
m in this study.
In summary,
a series of phenothiazine derivatives are
synthesized and characterized. For NAS-2, it is observed that the
replacement of one N-phenyl functionality in DPAC with sulfur
atom leads the molecule to undergo PISP and hence a large
Stokes-shifted emission. In comparison to NAS-1 isomer, the
steric hindrance imposed by the benzo[a]naphthalene moiety
plays a key role for the bent NAS-2 structure, which is also the
origin of PISP. The result generalizes the PISP mechanism for the
classes of benzophenazines and benzophenothiazines and is
believed to be applicable to other heterocyclic congeners. In solid
crystal, despite the prohibition of PISP, the bent NAS-2 exhibits
prominent excimer emission that is not observed in the planar-like
structure NAS and NAS-1. This unconventional observation is
rationalized by the bent core structure facilitating - stacking of
the peripheral naphthalene moiety. Finally, the depth-dependent
ratiometric intensity for normal versus excimer emission in the
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crystal solid leads us to conclude that the emission
mechanochromism for NAS-2 mainly originates from the
difference of surface-to-volume ratio between crystal and its
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ground solid powder. This is believed to be
a general
phenomenon, or at least in part, for other materials claimed to
exhibit emission mechanochromism. Using phenothiazines as a
prototype we thus establish the relationship among structure,
PISP, excimeric reaction and mechanchromism.
Associated Content
Supporting Information. The supporting information is available
free of charge via the Internet. Synthetic methods for the
compounds, NMR, and computational details are provided.
Y. A. Chen, W. C. Chao, J. H. Su, H. Tian, P. T. Chou, J. Am.
Chem. Soc. 2017, 139, 1636-1644. (e) N.Oldani, S. K. Doorn,
S. Tretiak, S. Fernandez-Alberti, Phys. Chem. Chem. Phys.
2017, 19, 30914-30924
Crystallographic data. The X-ray crystallographic coordinates
for the compounds reported in this article has been deposited at
the Cambridge Crystallographic Data Centre (CCDC) under
deposition number CCDC 1878210 (NAS-1), 1878211 (NAS-2).
Notes that NAS CCDC number is 1135756, which is reported in
previous research.[7] These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.
[3] (a) D.-G. Chen, T.-C. Lin, C.-L. Chen, Y.-T. Chen, Y.-A. Chen,
G.-H. Lee, P.-T. Chou, C.-W. Liao, P.-C. Chiu, C.-H. Chang,
Y.-J. Lien, Y. Chi, ACS Appl. Mater. Inter. 2018, 10, 12886-
12896; (b) D.-G. Chen, T.-C. Lin, Y.-A. Chen, Y.-H. Chen, T.-
C. Lin, Y.-T. Chen, P.-T. Chou, J. Phys. Chem. C 2018, 122,
12215-12221.
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Acknowledgements
P.-T. C. thanks for the financial support by Ministry of Science and
Technology (MOST), featured areas research program within the
framework of the Higher Education Sprout Project administrated
by Ministry of Education (MOE) of Taiwan. H. T. thanks financial
support by NSFC/China (21788102). We are grateful to the
National Center for the High-performance Computing (NCHC) of
Taiwan for the valuable computer time and facilities.
Author contributions
Photophysical analysis and DFT calculation, D.-G. C., C.-H. W.,
M.-C. C., Y.-A. C., J.-A. L. and C.-Y. H.; Synthesis and Structure
characterization, Y. C.; Writing - Original Draft, D.-G. C. and Y.
C.; Writing - Review & Editing, D.-G. C., Y. C., H. T. and P.-T. C.;
Supervision, J. H. S, H. T. and P.-T. C.; Funding Acquisition, H.
T. and P.-T. C.
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: Conformation analysis • Steric Hindrance • Excimer •
Charge transfer • Phenothiazine
References
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10.1002/anie.201906083
Angewandte Chemie International Edition
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COMMUNICATION
Photoinduced
Planarization
excimeric
achieved
phenothiazine analogies.
Structural
Deng-Gao Chen, Yi Chen,
(PISP)
and
were
the
Cheng-Ham Wu, Yi-An Chen,
Meng-Chi Chen, Jia-An Lin,
Chun-Ying Huang, Jianhua Su,
He Tian* and Pi-Tai Chou*
reaction
from
Page No. – Page No.
A New Scope of
Phenothiazines: The Steric
Strain Induced Planarization
and Excimer Formation
This article is protected by copyright. All rights reserved.