Intramolecular Charge Transfer Controls Switching Between Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence

Article abstract of DOI:10.1002/anie.201809945

Chemical modification of phenothiazine-benzophenone derivatives tunes the emission behavior from triplet states by selecting the geometry of the intramolecular charge transfer (ICT) state. A fundamental principle of planar ICT (PICT) and twisted ICT (TICT

Full text of DOI:10.1002/anie.201809945

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Accepted Article
Title: Intramolecular Charge Transfer Controls Switching Between
Room Temperature Phosphorescence and Thermally Activated
Delayed Fluorescence
Authors: Chengjian Chen, Ronjguan Huang, Andrei Batsanov, Piotr
Pander, Yu-Ting Hsu, Zhenguo Chi, Fernando Dias, and
Martin Robert Bryce
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809945
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Angewandte Chemie International Edition
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COMMUNICATION
Intramolecular Charge Transfer Controls Switching Between
Room Temperature Phosphorescence and Thermally Activated
Delayed Fluorescence**
†
†
Chengjian Chen, Rongjuan Huang, Andrei S. Batsanov, Piotr Pander, Yu-Ting Hsu, Zhenguo Chi,
Fernando B. Dias, and Martin R. Bryce*
[
2c,4,5]
Abstract: Chemical modification of phenothiazine–benzophenone
derivatives tunes the emission behavior from triplet states by
selecting the geometry of the intramolecular charge transfer (ICT)
state. A fundamental principle of planar ICT (PICT) and twisted ICT
strong SOC and hence efficient ISC for RTP.
Another way to obtain 100% IQE, without heavy metals, is
TADF, which is based on converting triplet states to fluorescent
singlet states by thermally activated reverse ISC (RISC). To
increase the RISC rate necessitates maximizing the SOC and
simultaneously minimizing the exchange energy gap between
[
1c]
(
TICT) is demonstrated to obtain selectively either room temperature
phosphorescence (RTP) or thermally activated delayed fluorescence
TADF), respectively. Time-resolved spectroscopy and time-
(
the lowest singlet and triplet states, ΔE(S1,
1
T ). Indeed,
dependent density functional theory (TD-DFT) investigations on
polymorphic single crystals demonstrate the roles of PICT and TICT
states in the underlying photophysics. This has resulted in a RTP
molecule OPM, where the triplet states contribute with 89% of the
luminescence, and an isomeric TADF molecule OMP, where the
triplet states contribute with 95% of the luminescence.
minimizing the energy gaps between charge-transfer (CT) and
3
3
3
1
local-excited (LE) triplet states, including LE- CT and LE- CT,
is critical for efficient TADF, since hyperfine coupling between
1
3
[6]
CT and CT is usually inactive. Recently, three regimes for
TADF were identified for a D–A–D emitter by changing the
[
6a]
polarity of the host environment.
This arises because ICT
states often differ markedly in their electronic structure and
molecular geometry, and are thus very sensitive to the
Organic luminescent materials based on donor–acceptor ICT
emitters (D–A or D–A–D) can efficiently harvest triplet excited
states. For non-planar donors quantum chemistry calculations
suggest that the D–A bridges should have optimal geometries to
molecules’ environment. Accordingly,
a strategy that can
stabilize molecular geometry with the desirable nearly-
perpendicular D–A units in the TICT state is important. Enforcing
rigidity on D–A molecules by chemical functionalization is
[1]
facilitate different ICT processes.
For example, different
[7]
exploited in new ways in the present work.
conformers of the D unit can show markedly different triplet
harvesting efficiencies. To promote the most efficient conformer
[
1f,g]
for TADF or RTP is a challenge.
TADF and RTP materials are of great interest due to their
[
2]
potential applications in optoelectronic and biological areas.
Phosphorescent materials can achieve up to 100% internal
quantum efficiency (IQE) by utilizing singlet-to-triplet intersystem
crossing (ISC). Fast ISC is usually promoted by incorporating a
heavy atom or an ICT state into the molecular structure to
Figure 1. Structures of OP, OPM and OMP studied in this work
[
3]
facilitate strong spin-orbit coupling (SOC).
Metal-free
phosphors are relatively rare and generally suffer from extremely
weak phosphorescence under ambient conditions due to the
spin forbidden nature of the triplet to singlet transition.
Therefore, to achieve pure organic RTP, the SOC strength
needs to be enhanced, and the nonradiative dissipations should
be suppressed. Mixing the singlet-triplet energies of different
molecular orbitals (El-Sayed’s rule) by incorporating, for
example, aromatic carbonyls and ICT interactions, can promote
The quasi-axial (ax) and quasi-equatorial (eq) conformers of
phenothiazine derivatives were computationally predicted in
[2-4]
[1f,8]
2001.
Recently, the two conformers were shown
[
9]
experimentally to co-exist in a Dax–A–Deq molecule. We now
report three D–A phenothiazine–benzophenone derivatives (OP,
OPM, and OMP, Figure 1 and Scheme S1) wherein carbonyl
[2c,4]
units can provide orbitals to promote ISC from S
1 n
to T states.
OPM and OMP have a methyl substituent on the D and A unit,
[
10]
respectively, to provide steric constraints. Since the ax and eq
conformers of phenothiazine derivatives have not previously
[
8,9,10a]
[*]
Dr. C. Chen, Dr. A. S. Batsanov, Dr. Y.-T. Hsu, Prof. M. R. Bryce
Department of Chemistry, Durham University, Durham, DH1 3LE,
UK. E-mail: m.r.bryce@durham.ac.uk
been separated,
we applied two very different conditions
to grow single crystals of OP, i.e. a dilute solution with slow
evaporation, and a saturated solution without evaporation. The
ax and eq phenothiazine conformers were exclusively obtained
in the single crystals, α-OP and β-OP, respectively. Figures 2A
and S1 (in Supporting Information) show the X-ray molecular
structures of α-OP and β-OP.
R. Huang, P. Pander, Dr. F. B. Dias
Department of Physics, Durham University, Durham, DH1 3LE, UK.
Dr. C. Chen, Prof. Z. Chi
PCFM Lab, GD HPPC Lab, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, P. R. China
Dr. C. Chen and R. Huang contributed equally.
†
[
[
]
**]
This work was financially supported by EPSRC grant number grant
number EP/L02621X/1. P. P. acknowledges the EU’s Horizon 2020
for funding the EXCILIGHT project under grant agreement No
674990. The authors thank Prof. A. Beeby, Prof. S. J. Clark and Dr.
J. S. Ward for helpful discussions.
Supporting information can be found via a link at the end.
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Angewandte Chemie International Edition
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COMMUNICATION
typical TICT model, whereas, in the PICT model, a relatively
small energy gap ΔE(S
process, rather than the coplanarity of the D–A moieties.
ΔE(S ,S ) of α-OP was calculated to be 0.09 eV (Figure 3) which
is a negligible gap that favours the vibronic coupling between S
and S states. Figure S4 shows that α-OP and β-OP have
1 2
,S ) plays a more critical role in the ICT
[
11a]
The
1
2
1
2
almost equal stable energies; thus any attempt to explain results
based on one selected conformation in solution or matrix is
[
8b]
unsatisfactory.
-(N,N-dimethylamino)benzonitrile
derivatives have been widely studied to elucidate the roles of
4
(DMABN)
and
its
[
11a]
different ICT states.
According to their design principles, a
methyl group was introduced at specific positions of OP, i.e.
molecules OPM and OMP, to rigidify the conformers and
stabilize PICT and TICT states, respectively. Two similar ax
conformers were observed in the single crystals of OPM. Three
different polymorphs of OMP (α, β and γ) were obtained: all
three show similar eq conformers (Figures 2A and S7).
To investigate the PICT and TICT states, time-resolved
spectroscopy was employed from 80 K to 290 K to measure
luminescence from crystals (Figures 2B-I). In Figures 2B and 2D,
0
0
0
0
0
0
0
.012
.010
.008
.006
.004
.002
.000
0.012
0.010
0.008
0.006
0.004
0.002
B 443 nm
-OP-80K
524 nm -OP-80 K
C
536 nm
1.1 ns
1.1 ns
5.4 ns
5.61 s
1.99 ms
35.5 ms
43.5 ns
70.8 s
56.2 ms
the dual emission of α-OP was identified as the LE and CT
[11]
0.000
processes in accordance with the PICT model.
The spectra
4
00 450 500 550 600 650 700
400 450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
had the profile and dynamic behavior of a CT band undergoing a
0
0
0
0
0
0
0
.012
.010
.008
.006
.004
.002
.000
0.014
0.012
[
12]
D 450 nm 540 nm
-OP-290K
520 nm
-OP-290 K
transient Stokes shift. Since dual emission is very common in
E
1
5
4
1
.1 ns
.4 ns
3.5 ns
.24 s
1.1 ns
the PICT model, α-OP emits blue at ~450 nm and yellow at
0
.010
.008
1.10 s
158.5 s
~540 nm, which provides a design for white light emission from a
0
112 s
0.1 ms
[2b,8c,13]
single organic molecule.
The white emission from OPM
5
0.006
0.004
crystals (Figure S2) demonstrates this application of the PICT
model.
0.002
0.000
α-OP shows dual emission on the microsecond time-scale
(up to 70.8 µs, Figure 2B, and 1.24 µs Figure 2D), giving clear
indication of the RISC from triplet to singlet states. Moreover, α-
OP shows phosphorescence at ~540 nm at 290 K, and at ~536
nm at 80 K. Time dependent density functional theory (TD-DFT)
calculations (see below) show that the α-OP geometry favors
the n-π* transition of the carbonyl group and hence facilitates
4
00 450 500 550 600 650 700
Wavelength (nm)
4
00 450 500 550 600 650 700
Wavelength (nm)
0
.012
.010
.008
.006
.004
.002
.000
0.012
0.010
0.008
0.006
0.004
445 nm
OPM-80 K
525 nm
541 nm
OMP-80 K
F
G
535 nm
0
1.1 ns
1.1 ns
4
.1 ns
121.7 ns
77.8 s
1.12 ms
0.1 ms
15.8 ns
0
0
0
0
0
25.1 s
1
2.24 ms
56.2 ms
5
[
4,5]
0.002
ISC according to El-Sayed’s rule.
and π-π* orbital configurations in α-OP activate a radiative
transition from T to S leading to phosphorescence after being
Meanwhile, the hybrid n-π*
4
00 450 500 550 600 650 700
0.000
4
00 450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
1
0
0
0
0
0
0
0
0
.012
.010
.008
.006
.004
.002
.000
0.012
0.010
0.008
0.006
0.004
OPM-290 K
444 nm
525 nm
OMP-290 K
rigidified in the crystalline state.
Figures 2F and 2H show that for OPM the blue emission at
H
I
1.1 ns
.7 ns
1.76 s
12.2 s
1.26 ms
0.1 ms
1.1 ns
543 nm
2
69.5 ns
141.2 s
~445 nm has LE character with well-resolved spectra at both 80
1
K and 290 K. Well-resolved LE emission is generally observed
only under frozen conditions (e.g. 80 K) or in a restricted
structure. The 290 K data imply that the methyl group in OPM
locks the conformation into the PICT geometry, which
simultaneously minimizes non-radiative deactivation processes
of triplet excitons. OPM shows obvious RTP (Figure 2H). To
further enhance the rigidity, two methyl groups were attached to
the phenothiazine to give OP2M (Figure S3). LE emission is
observed in OP2M in both toluene solution and Zeonex film
compared to OPM (Figures S6 and S17).
5
0.002
4
00 450 500 550 600 650 700
0.000
4
00 450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
Figure 2. X-ray molecular structures of α-OP, β-OP, OPM, and OMP(α-OMP)
(
A). Time resolved area normalized emission spectra of crystals: α-OP at 80 K
B), 290 K (D), β-OP at 80 K (C), 290 K (E), OPM at 80 K (F), 290 K (H), and
(
OMP(α-OMP) at 80 K (G), 290 K (I). All spectra were excited at 355 nm.
In contrast to OPM, both β-OP and OMP show perfect TADF
spectra, i.e. the delayed and prompt spectra are identical
(Figures 2E and 2I). The OMP phosphorescence (from LE) is at
In the ax α-OP the three N–C bonds are almost coplanar with
the benzene ring of the benzophenone. In the eq β-OP the
corresponding geometry is nearly perpendicular (Figures 1A and
S1). A mutually perpendicular D–A geometry is required for the
3
~541 nm at 2.24 ms and 56.2 ms (Figure 2G), and is crucial for
[
6]
efficient TADF in the vibronic coupling RISC model.
Both β-
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COMMUNICATION
1
09
107
109
107
OMP and γ-OMP (Figure S7) show the eq conformers, thus
TADF properties were obtained (Figures S13 and S14).

-OP
-OP
80 K
120 K
150 K
290 K
80 K
120 K
DF
05
03
105
103
101
DF
1
1
1
180 K
290 K
Phos.
01
A
00
B
1 -
0
1
-1
100
10
102
10
4
106
108
102
4
106
108
1
10
Time (ns)
Time (ns)
08
OMP
1
1
1
OPM
08
1
1
8
0 K
80 K
06
04
145 K
200 K
06
DF
145 K
220 K
290 K
State
Transition configuration
104
290 K
α-OP
S
S
1
/3.60 eV
/3.69 eV
H-4→L 7.7%, H-2→L 76.1%, H→L 7.9%.
H-2→L 7.9%, H→L 84.8%.
102
Phos.
102
2
T
T
1
/2.80 eV
/3.09 eV
/3.60 eV
/3.74 eV
/2.86 eV
/3.15 eV
H-2→L 5.4%, H→L 75.7%.
H-4→L 2.9%, H-2→L 79.4%, H→L 5.8%.
H-2→L 76.7%, H→L 8.5%.
H-2→L 7.3%, H→L 68.6%, H→L+1 21.1%.
H-2→L 15.8%, H-1→L 11.7%, H→L 59.3%.
H-2→L, 72.3%, H→L, 14.2%.
00
100
1
2
C
00
D
-2
1
0-2
10
OPM
S
S
1
102
104
Time (ns)
106
108
100
102
4
106
108
1
10
Time (ns)
2
T
T
1
2
Figure 4. Temperature dependence of emission decays of α-OP (A), β-OP (B),
OPM (C) and OMP(α-OMP) (D) crystals from 290 K to 80 K.
Figure 3. Frontier molecular orbitals of α-OP and OPM, and electronic
transitions at B3LYP/6-31G* (H = HOMO; L = LUMO). The same components
1
of S are in red text.
Since the methyl substituent in OPM suppresses non-
radiative pathways by rigidifying the structure, triplet excitons are
stabilized. Consequently, Figure 4C shows a gradual decay of
To probe the mechanism underlying the observed spectra,
TD-DFT investigations were performed on the singlet and triplet
4
phosphorescence and reveals RTP. Accordingly, we propose
that hybridizing n-π* and π-π* transitions in the triplet states of
[
2a,14]
excited states.
The frontier molecular orbitals (H = highest
[2c]
α-OP and OPM is the key to activating phosphorescence.
occupied molecular orbital, HOMO; L = lowest unoccupied
molecular orbital, LUMO) and electronic transitions are shown in
Figures 3, S8-S10 and Tables S3-S7 using B3LYP/6-31G*.
Similar orbital distributions were obtained using M062X/6-31G*
Since α-OP and OPM are PICT systems, with two singlet states
(S and S ) that are close in energy, and with different transition
1 2
characters to enhance ISC, we propose the PICT model is a
[7,11]
new design strategy for RTP after rigidifying the geometry.
and B3LYP/6-31G* functionals (Figures S11 and S12). S
and T of α-OP and OPM are mixed electronic states with n-π*
H-2→L) and π-π* (H→L) transitions, which involve the oxygen
lone pairs (carbonyl group) and the conjugated structure,
1 2 1
, S , T
Figure 4B shows a gradual DF decay component of β-OP with a
very sensitive temperature response, i.e. this TICT molecule
presents efficient TADF with a small ΔE(S ,T ) of 0.05 eV (Table
1 1
S4). Analogously, OMP(α-OMP) and β-OMP have similar
geometries to β-OP and their efficient TADF properties are seen
in Figures 4D and S13. Interestingly, γ-OMP retains nearly
perpendicular geometry, despite showing a different conformer
2
(
[
2b]
respectively (Figure 3). Meanwhile, S
by an n-π* transition, while S and T
transition character. Therefore, SOC occurs from (n-π*) to (π-
1
2
and T are dominated
2
1
possess more π-π*
1
3
1
3
π*), and from (π-π*) to (n-π*) because of effective orbital
(
Figure S7). Therefore, γ-OMP has a very sensitive temperature
[2c]
overlap according to El-Sayed’s rule.
and T are hybridized local and charge-transfer (HLCT) states,
since H-2→L is assigned as an LE transition and H→L is
1 2 1
In addition, S , S , T
response in the DF decay component (Figure S14C).
Consequently, the TICT model is strictly controlled with the
methyl group, and hence the TADF properties are effectively
retained. To further elucidate the development of triplet states,
the steady state emission of the molecules in Zeonex matrix was
measured in vacuum and in air at RT. Figures S17B and S17C
2
[
15]
assigned as a CT transition.
molecules, RISC from upper triplet to singlet levels probably
occurs in α-OP and OPM as ΔE(S ,T ) is large (0.80 eV for α-OP
and 0.74 eV for OPM). Interestingly, the LE emission (2.24
and 56.2 ms, Figure 2G) of OMP is ascribed to the contributions
from the phenothiazine group (Figure S15), according to the
calculated triplet states which show a H→L+1 transition (Table
According to reported HLCT
1
1
[15]
3
show that the triplet states contribute 89% and 95% of the OPM
[8c]
and OMP luminescence, respectively.
To further investigate
the PICT and TICT models, organic light emitting diodes
(
(
OLEDs) were fabricated using OPM and OMP as the emitters
Figure S18 and Table S8). Device 1 based on OMP had an
S4 and Figure S9). Furthermore, both S
transitions (H→L), thus, the efficient RISC model of CT- LE- CT
is demonstrated by OMP(α-OMP) with a small ΔE(S ,T ) of 0.03
1 1
and T show CT
3
3
1
emission peak at 558 nm with a maximum external quantum
efficiency (EQE) of 10.2%, which was attributed to a TADF
mechanism. As expected, dual emission was observed from
OPM based Device 2, involving RTP with a maximum EQE of
only 0.6%.
In summary, the ax and eq conformers α-OP and β-OP are
ideal PICT and TICT model systems, respectively. Based on TD-
DFT calculations and photoluminescence spectra the PICT
1
1
[
6a,16]
eV (Table S5).
The time-resolved temperature dependence of the emission
decays were plotted from 290 K to 80 K (Figure 4). α-OP at 80 K
has three components, ascribed to prompt fluorescence (PF),
delayed fluorescence (DF) and phosphorescence (Phos.) based
[
15]
on their different temperature effects (Figure 4A).
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Angewandte Chemie International Edition
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model is proposed as a new strategy for obtaining RTP. We
have established that selectively attaching a methyl group to the
OP molecule can stabilize either the PICT geometry in OPM, or
the TICT geometry in the isomer OMP. White light emission from
OPM is a result of dual emissive processes according to the
PICT mechanism. This rational conformational control has led to
efficient utilization of triplet states for RTP in OPM and for TADF
in OMP. Related luminescent D–A molecules can be developed
using these guidelines.
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Angewandte Chemie International Edition
10.1002/anie.201809945
COMMUNICATION
COMMUNICATION
†
†
Intramolecular Charge Transfer
Controls Switching Between Room
Temperature Phosphorescence and
Chengjian Chen, Rongjuan Huang,
Andrei S. Batsanov, Piotr Pander, Yu-
Ting Hsu, Zhenguo Chi, Fernando B.
Dias, and Martin R. Bryce*
Thermally
Activated
Delayed
Fluorescence: Rapid and efficient
utilization of triplet states to generate
room temperature phosphorescence
Page No. – Page No.
Intramolecular Charge Transfer
Controls Switching Between Room
Temperature Phosphorescence and
Thermally Activated Delayed
Fluorescence
(
RTP) or highly efficient thermally
activated delayed fluorescence
TADF) is achieved by structural
(
modification to give a planar or twisted
intramolecular charge transfer (PICT
or TICT) geometry, respectively.
This article is protected by copyright. All rights reserved.