Utilizing d–pπ Bonds for Ultralong Organic Phosphorescence

Article abstract of DOI:10.1002/anie.201901546

Developing pure organic materials with ultralong lifetimes is attractive but challenging. Here we report a concise chemical approach to regulate the electronic configuration for phosphorescence enhancement. After the introduction of d–pπ bonds into a phenothiazine model system, a phosphorescence lifetime enhancement of up to 19 times was observed for DOPPMO, compared to the reference PPMO. A record phosphorescence lifetime of up to 876 ms was obtained in phosphorescent phenothiazine. Theoretical calculations and single-crystal analysis reveal that the d–pπ bond not only reduces the (n, π*) proportion of the T₁ state, but also endows the rigid molecular environment with multiple intermolecular interactions, thus enabling long-lived phosphorescence. This finding makes a valuable contribution to the prolongation of phosphorescence lifetimes and the extension of the scope of phosphorescent materials.

Full text of DOI:10.1002/anie.201901546

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Accepted Article
Title: Utilizing d-pπ Bond for Ultralong Organic Phosphorescence
Authors: Shuai Tian, Huili Ma, Jie Li, Xuan Wang, Anqi Lv, Huifang Shi,
Yun Geng, Fushun Liang, Zhong-Min Su, Zhongfu An, and
Wei Huang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901546
Angew. Chem. 10.1002/ange.201901546
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Utilizing d-p Bond for Ultralong Organic Phosphorescence
Shuai Tian+, Huili Ma+, Xuan Wang, Anqi Lv, Huifang Shi, Yun Geng, Jie Li, Fushun Liang,* Zhong-Min
Su,* Zhongfu An* and Wei Huang
Abstract: Developing pure organic materials with ultralong
lifetimes is attractive but challenging. Here we report a concise
chemical approach to regulate the electronic configuration for
phosphorescent enhancement. After introduction of d-p bonds
in phenothiazine model, up to 19 times phosphorescence
lifetime enhancement was observed for DOPPMO, compared
with the reference PPMO. A record phosphorescence lifetime of
up to 876 ms was obtained in phenothiazine phosphors.
Theoretical calculations and single crystal analysis reveal that
the d-p bond not only reduces the (n, *) proportion of T1, but
also frames rigid molecular environment with multiple
intermolecular
interactions,
thus
enabling
long-lived
phosphorescence. This finding will take a giant step forward in
prolonging phosphorescence lifetime and extending the scope of
phosphorescent materials.
Phosphorescence has attracted intense attention in diverse
applications, such as anti-counterfeiting,[1] molecular sensing[2]
and display[3] due to their long-lived emission lifetimes.[4] For
Figure 1. Schematic illustration of the phosphorescence lifetime enhancement
by the introduction of d-p bond (top). Chemical structures investigated in this
work (bottom).
instance,
phosphorescence can eliminate background
fluorescence interference via the time-gate technique in the
biological field.[5] Till now, most luminescent materials with
phosphorescence feature are confined to inorganic and precious
metal complexes with distinct shortcomings, such as high cost,
biological toxicity, and instability in aqueous solutions.[6]
Comparatively, pure organic phosphors are inexpensive,
relatively safe to the environment, and the intrinsic structures are
easy to modulate.[7] However, how to develop efficient and
ultralong organic phosphorescence (UOP) still remains a great
challenge. The key issues involve the inefficient intersystem
crossing (ISC) caused by weak spin-orbit coupling (SOC) and
the rapid non-radiative decay due to molecular motions and
oxygen quenching, etc.[8] In this regard, developing organic
phosphors with efficient UOP needs to satisfy at least the
following two factors: i) incorporating functional groups such as
carbonyls, heavy atoms as well as heteroatoms (N, S, P, etc.)
to promote ISC through SOC enhancement;[9] ii) constructing a
rigid environment to suppress the non-radiative decay of triplet
excitons by series of feasible approaches, such as
crystallization,[10] hydrogen bonding,[11] halogen bonding,[12] self-
assembly,[13] H-aggregation,[14] hydrogen-bonded organic
aromatic frameworks (HOAFs)[15] etc. Through the tremendous
efforts of scientists, enumerable work of organic phosphors with
ultralong lifetime have been developed, along with the proposed
mechanism.[16] Among them, most work focus on creating a
more rigid external environment to suppress the non-radiative
pathways for prolonging the phosphorescence lifetime, while
less attention was paid to the intrinsic molecular structure
engineering. Hence, a concise yet efficient chemical strategy to
prolong lifetime is attractive and practical.
d-p bond is widely distributed in transition metal complexes
and oxacids, but it is very rare in common organic compounds.[17]
It mainly exists in S=O, P=O types of molecules. Different from
the normal  bond arised from the shoulder to shoulder overlap
of two p orbitals (such as C=C, C=O), d-p bond is formed by
the conjugation between a d orbital and a p orbital (dxz-pz).[18] As
examplified in Figure 1, top, the energy level of 3d orbital is
much higher than that of the 2p orbital, so it is not conducive to
delocalize electrons on oxygen atoms. Meanwhile, the d-p
bond leads to the nonplanar conformation between sulfonyl
group and phenyl ring. From the viewpoint of electronic and
steric effects, the communication between the lone electrons on
the oxygen and adjacent ingredient will be weakened, which
may regulate the n orbital composition of the lowest triplet
excited state (T1).
[a] S. Tian, J. Li, Prof. F. Liang
Institute of Organic Luminescent Materials (IOLM), College of Chemistry,
Liaoning University, 66 Chongshan Mid. Road, Shenyang 110036, China
E-mail: fsliang@lnu.edu.cn
[b] Dr. H. Ma, X. Wang, A. Lv, Dr. H. Shi, Prof. Z. An, Prof. W. Huang
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced
Materials (IAM)
Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing
211816, China
E-mail: iamzfan@njtech.edu.cn
[c] Prof. F. Liang, Prof. Z.-M. Su
School of Chemistry and Environmental Engineering, Changchun
University of Science and Technology
7089 Weixing Road, Changchun 130022, China
E-mail: zmsu@nenu.edu.cn
[d] Dr. Y. Geng, Prof. Z.-M. Su
Based on the above understanding, we speculated that some
organic phosphors with d-p bond could prolong the organic
phosphorescence lifetime in crystal state under ambient
conditions. To prove this hypothesis, organic compounds
containing the phenothiazine units were chosen as models,
Department of Chemistry, Northeast Normal University,
5268 Renmin St., Changchun 130024, China
[e] Prof. W. Huang
Institute of Flexible Electronics (IFE), Northwestern Polytechnical
University (NPU), 127 West Youyi Road, Xi’an 710072, China
+These authors contributed equally.
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Figure 2. Photophysical properties of phenothiazine derivatives in crystal state under ambient conditions. a) Steady-state photoluminescence (black solid lines)
and phosphorescence spectra (red sold lines) of the phosphors in crystal at 298 K. Inserts show the photographs of the phosphors under 365 nm UV light on and
removal of light source. b) Phosphorescence lifetimes of emission bands at 539 nm for PPMO, 490 nm for DOPPMO, 521 nm for PEO, and 514 nm for DOPEO in
crystal state at 298 K. c) Excitation-phosphorescence mapping of DOPEO phosphor.
because phenothiazine with a unique non-planar “butterfly”
structure could hinder the possible intermolecular intensive -
stacking, beneficial for inhibiting excited triplet-triplet annihilation
(Schemes S1, Table S1).[19] We herein synthesized two N-acyl
phenothiazine derivatives, namely, (10H-phenothiazin-10-yl)
(phenyl)methanone (PPMO) and 1-(10H-phenothiazin-10-
yl)ethan-1-one) (PEO). Through subtle chemical modification,
i.e., one-step sulfur oxidation, a d-p bond was introduced into
the luminogens (Figure 1, bottom). As expected, all the
compounds show phosphorescence feature under ambient
diffraction (Figures S1−8). PPMO, DOPPMO and DOPEO show
good thermal stability and high melting points (Figure S9), while
PEO was found to be decomposed before melting.
The photophysical properties of the phenothiazine derivatives
were systematically investigated in both solution and crystalline-
state. In dilute THF solution (5  10-5 M), both PPMO and
DOPPMO have the same -* absorption bands at around 230
nm, but different n-* absorption bands with peaks at 273 and
303 nm, respectively. For PEO and DOPEO, the absorption
peaks corresponding to n-* transition also show a 30 nm blue
shift (Figure S10). They all have fluorescence peaks generated
by the excited state of phenothiazine at around 360 nm, which
are slightly affected by the polarity of the solvents (Figure S11).
We also investigated the emission behaviors of single molecule
at 77 K. When excited at 290 nm, PPMO and PEO show similar
profiles with peaks at 442 nm owing to their identical and
effective chromophoric components. However, a significant blue
shift was observed for DOPPMO and DOPEO, indicating that
sulfonyl group has a great impact on the excited states of
luminogens in single-molecular level (Figure S12a). Furthermore,
photoluminescence (PL) and phosphorescence spectra of these
compounds in crystals at 298 K were conducted in Figure 2.
When irradiated at 330 nm, a large overlap can be observed in
PL and phosphorescent spectra for PPMO and PEO, implying
sufficient ISC in S1-Tn transition. DOPPMO exhibits an intense
fluorescence peak centered at 353 nm and relatively low
intensity phosphorescence peak around 400-510 nm due to its
low quantum yield. We found that phosphorescent peaks located
conditions.
A remarkable increasement was observed in
phosphorescence lifetime when d-p bond was introducd (up to
19 times). Particularly, DOPEO exhibits balanced long
phosphorescence lifetime of 876 ms and phosphorescence
quantum yield of 8.2%, which represent the highest values
among phenothiazine-based phosphors. Furthermore, crystal
analysis and theoretical calculations reveal that d-p bond plays
a significant role in manipulating the electronic configuration and
molecular stacking of these compounds. A nearly pure (, *)
configuration of T1 and more rigid molecular environment in the
crystal state were attained, which give rise to a substantial
increasement of phosphorescence lifetime.
The synthetic routes for PPMO, DOPPMO, PEO and DOPEO
were outlined in Schemes S2. Luminogens of DOPPMO and
DOPEO were obtained in overall 77% and 86% yields,
respectively, by two-step reactions from phenothiazine and the
corresponding acyl chloride. All compounds were fully
characterized by 1H, 13C NMR and the single-crystal X-ray
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Figure 3 a) Single-crystal structure and molecular packing of PPMO and DOPPMO. b) - interactions of PPMO and DOPPMO. c) Natural transition orbitals of
T1 for PPMO and DOPPMO molecules in crystals. d) (n, *) configuration proportions, f, kr and knr of T1 for compounds PPMO and DOPPMO.
at 435 and 490 nm with different lifetimes (234 ms for the former
and 455 ms for the latter), suggesting that they are likely to be
produced by different excited states. In addition, we investigated
the emission spectra of DOPMMO at 77 K both in dilute solution
and in 5% PPMA (Figure S12b). Similar emission behavior was
also observed, which was attributed to single molecule.[16c]
DOPEO exhibits dual emissions with almost equal intensity. The
prompt PL spectrum shows fluorescence signals at 380−450 nm
and phosphorescent emission at 460−600 nm, respectively. The
phosphorescence spectra of four compounds change when
states. Typically, the lifetime of phosphor DOPPMO shows a
huge increasement by 19 times compared to that of PPMO.
Among
them,
DOPEO
exhibits
a
balanced
long
phosphorescence lifetime of 876 ms and a phosphorescence
quantum yield of 8.2%. The phosphorescence spectra of
DOPEO in solid state remain at the same emission position with
the excitation wavelength varies from 300 to 400 nm, and the
phosphorescence intensity reaches maximum when excited at
352 nm (Figure 2c).
In order to elucidate the relationship between the molecular
structure and phosphorescent property, single crystals were
cultivated by slow evaporation from the mixture of acetone and
petroleum ether. Compounds PPMO and DOPPMO belong to
monoclinic systems and they have similar layered packing
(Figure S18). As shown in Figure 3a, abundant intermolecular
hydrogen bonds could be observed in the crystals. However,
additional C-H∙∙∙O (2.510 Å, 2.615 Å) hydrogen bonds were
found in DOPPMO crystals. A rigid 2D network structure is
established via molecular interlocking connected by C-H∙∙∙O
interactions in the same layer and - staking in different layers.
It is worth noting that the distance of the intermolecular -
staking was shortened (3.779 Å for PPMO vs. 3.654 Å for
DOPPMO, Figure 3b). This can be reasoned that the electron
density on the benzene rings of both sides of phenothiazine is
reduced greatly, upon oxidation of sulfur (electron donating
group) to sulfonyl (electron-withdrawing group). As a result, the
- repulsive force is reduced. The denser packing with highly
measured from vacuum to oxygen,
indicating the
phosphorescence nature of emission (Figure S13). Inset
pictures show the corresponding photographs taken before and
after the excitation source was turned off. We can see there is
an obvious emission color change after oxidation, indicating that
sulfonyl group may vary the energy gap between T1 and S0. The
Commission Internationale de l’Éclair-age (CIE) coordinate
corresponding to the prompt emission of PEO, DOPEO, PPMO
and DOPPMO was shown in Figure S16. Interestingly, when the
excitation light resource was switched off, a bright bluish white
color for DOPEO was observed by the naked eye, which can
last for more than 5 seconds (Videos SV1, ESI†). Figure 2b
shows the phosphorescence lifetimes of four compounds in
crystal under ambient conditions. Obviously, greatly enhanced
lifetime was observed when sulfur was oxidized (Figure S14 and
S15). These results indicate that the d-p bond has a significant
influence on prolonging the phosphorescence in the crystalline
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rigidified molecular conformations may greatly suppress the non-
radiative pathways of excited molecules, leading to pronounced
UOP. The denser packing model is further verified by their
crystal density of 1.3707 and 1.3975 g/cm-3 for PPMO and
DOPPMO, respectively. Compounds PEO and DOPEO also
belong to monoclinic systems. Compared to PEO, DOPEO has
abundant C-H∙∙∙, C-H∙∙∙O intermolecular interactions in crystals
(Figure S19). The multiple intermolecular interactions are helpful
to suppress molecular motions to reduce the possible energy
loss via non-radiative relaxation channels. On the basis of the
measured fluorescence and phosphorescence efficiencies and
lifetimes of the molecules, the radiative and non-radiative decay
rates were calculated following the standard methods.[20] We
found that the fluorescence radiation rate of DOPPMO is
increased compared to PPMO (from 1.6  107 to 2.7  107 s-1),
and the fluorescence emission is blue-shifted, indicating that the
intermolecular conjugation decreases and the S1 energy level
rises. Among the four examined compounds, the values of kISC
are relatively low, but they are still good enough to guarantee
sufficient ISC.[21] As shown in Table S4, both radiative and non-
radiative decay rates of DOPPMO are lower than that of PPMO
(from 2.95 to 0.08 s-1 for kr, from 38.55 to 2.12 s-1 for knr).
According to the equation of  = 1/(kr + knr), low kr and knr values
of DOPPMO are essentially accountable for its remarkable
phosphorescence lifetime (Figure 3c) .
pattern with two butterflies and one small sapling was fabricated
as shown in Figure 4. A blue sapling (made by DOPEO) and two
green butterflies (made by PEO) are clearly visualized under
365 nm UV-lamp irradiation. When switching off the UV-lamp,
the complete pattern can be seen in the first 1 s. As time goes
on, only the small sapling is visible. A persistent sapling shape
was observed lasting for around 3 s, thus affording means to
identify the truth from the fraud.
Figure 4. Demonstration of the security protection application of PEO and
DOPEO and photographs taken under 365 nm UV light (left) and after ceasing
the UV irradiation (right).
In summary, a d-p strategy was proposed to prolong the
phosphorescence lifetimes of organic phosphors. The protocol
was very simple and easy to realize by chemical modification.
Among them, the ultralong lifetime of DOPPMO increases by 19
times to 455 ms due to the introduction of d-p bond. DOPEO
with a record phosphorescent lifetime of up to 876 ms and
phosphorescent efficiency of 8.2% was attained under ambient
conditions. Combining single crystal analysis with theoretical
simulations, we concluded that the d-p bond not only reduces
the (n, *) proportion of T1, but also produces closer - stacking
and stronger intra/intermolecular interactions, leading to reduced
radiative and non-radiative decay rates for ultralong
phosphorescence. This result not only demonstrates the
potential of d-p bond for prolonging organic phosphorescence
lifetime, but also extends the scope of the metal-free organic
phosphors.
To gain deep insight into the origin of DOPPMO and DOPEO
with ultralong phosphorescence lifetime, time-dependent density
functional theory (TDDFT) calculations were carried out. The
calculated energy gap and SOC constants (ξ) between the
involved singlet and triplet states of PPMO and DOPPMO
compounds were shown in Figure S20. According to the El-
Sayed's rule, small energy gap and large SOC are favorable for
the ISC process. For DOPPMO molecule, there is a large ΔEST
(0.07 eV) and a small SOC between S1 and Tn, So the ISC of
DOPPMO is not so efficient compared to PPMO. On the other
hand, natural transition orbitals (NTOs) of T1 were also
calculated to qualitatively describe the electronic configuration
(Figure 3c). Notably, the (n, *) components of the T1 were
dropped from 10.68% in PPMO to 1.46% in DOPPMO,
indicating that the DOPPMO has a slower radiative decay rate
(kr) than that of PPMO, since the triplet excited state (T1) with a
pure (, *) configuration can show an extremely slow decay
rate (100 s-1). This was further confirmed by the hugely reduced
oscillator strength (f) (15 times) from DOPPMO to PPMO
crystals, owing the fact that the kr is proportional to f value. It is
thus speculated that the introduction of d-p bond to
phenothiazine system may significantly suppressed radiative
decay rate in the crystals, leading to a significant increase in
phosphorescence lifetime. To sum up, the introduction of d-p
bond leads to nearly pure (*) configuration of T1 with an
extremely slow decay rate and may contribute to the UOP. The
theoretical calculations and the experimental data agree well
with we envisioned.
Acknowledgements
Financial support from the National Natural Science Foundation of China
(grant No. 21372039, 21875104 and 51673095), National Basic
Research Program of China (973 Program, No. 2015CB932200), Natural
Science Fund for Distinguished Young Scholars of Jiangsu Province
(BK20180037) and Scientific Research Fund of Liaoning Province
(LT2017010) is greatly acknowledged. We are grateful to the High
Performance Computing Center of Nanjing Tech University for supporting
the computational resources.
Keywords: crystal engineering • organic phosphorescence • d-p
bond • phenothiazines • proportion of the excited states
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COMMUNICATION
COMMUNICATION
Shuai Tian+, Huili Ma+, Xuan Wang, Anqi
Lu, Huifang Shi, Yun Geng, Fushun
Liang,* Zhong-Min Su,* Zhongfu An,*
and Wei Huang
Page No. – Page No.
Utilizing d-pBond for Ultralong
Organic Phosphorescence
d-p bond was proposed and utilized to prolong organic phosphorescence lifetimes. Luminogen of N-acetyl phenothiazine S,S-
dioxide (DOPEO) exhibits an ultralong lifetime of 867 ms with a phosphorescent quantum yield of 8.2%, which are the highest values
among phenothiazine-based organic phosphors.
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