Modulated emission from dark triplet excitons in aza-acene compounds: fluorescence versus phosphorescence

Article abstract of DOI:10.1039/c6nj02747d

In the field of organic light-emitting diodes (OLEDs), research interests focus on making the optically dark triplet excitons shine in order to increase the electro-optic conversion efficiency of devices. In this work, two kinds of phenazine compounds, i.e. dibenzo[a,c]phenazine (DBP) and tribenzo[a,c,i]phenazine (TBP), were synthesized and used as model compounds to regulate the emission efficiency of the dark triplet excitons by chemical modification. Charge-transfer induced ultrafast intersystem crossing (CT-ISC) with a time constant of ～1 ps was observed for these two phenazine derivatives upon photoexcitation with a high triplet yield of 77.1% for DBP and 58.7% for TBP. The triplet excited states of DBP can produce ultra-long phosphorescence with lifetime as long as 318 ms at 77 K. The quantum yield for phosphorescence (Φ_P) is determined to be 8.45%. In sharp contrast, the triplet-excited ³TBP* undergoes an efficient reverse intersystem crossing (RISC) process, resulting in bright delayed fluorescence emission with negligible phosphorescence. A controllable luminescence behavior from the triplet states between fluorescence and phosphorescence in phenazine derivatives is demonstrated. Theoretical calculations reveal that the structure-dependent triplet evolution is due to the charge-transfer induced energy level alignment within these compounds. Our results may have potential applications in the design of OLEDs and high triplet yield pure organic materials.

Full text of DOI:10.1039/c6nj02747d

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Modulated Emission from Dark Triplet Excitons in Aza-Acene
Compounds: Fluorescence Versus Phosphorescence
Jianwei Chen, ab┴ Yi Chen, c┴ Yishi Wu *a, Xuedong Wang, ab Zhenyi Yu, ab Lu Xiao, ab Yanping Liu, ab
He Tian*c , Jiannian Yao a and Hongbing Fu *ad
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The interest in the field of organic light-emitting diodes (OLEDs) is how to make the optically dark triplet
excitons shine in order to increase the electro-optic conversion efficiency of the devices. In this work, two
kinds of phenazine compounds, i.e. dibenzo[a,c]phenazine (DBP) and tribenzo[a,c,i]phenazine (TBP), were
synthesized and used as model compounds to regulate the emission efficiency of the dark triplet excitons by
chemical modification. Charge-transfer induced ultrafast intersystem crossing (CT-ISC) with time constant of
~ 1 ps was observed for these two phenazine derivatives upon photoexcitation with high triplet yield of
77.1% for DBP and 58.7% for TBP. The triplet excited states of DBP can produce ultra-long phosphorescence
with lifetime as long as 318 ms at 77 K. The quantum yield for phosphorescence (ΦP) is determined to be
8.45%. In sharp contrast, the triplet-excited 3TBP* undergoes efficient reverse intersystem crossing (RISC)
process, resulting in bright delayed fluorescence emission with negligible phosphorescence. A controllable
luminescence behavior from the triplet states between fluorescence and phosphorescence in phenazine
derivatives is demonstrated. Theoretical calculations reveal that the structure-dependent triplet evolution is
due to the charge-transfer induced energy level alignment within these compounds. Our results may have
potential applications for the designing of OLEDs and high triplet yields pure organic materials.
www.rsc.org/
is certified to be another promising approach for high performance
luminescence devices.11,19 Thermal activated delayed fluorescence
(TADF) has been achieved to enhance the luminescence of the
OLEDs, giving rise to a quantum yield of more than 90%.11
1. Introduction
The organic luminescent materials have witnessed significant
advance owing to their wide applications including latent
fingerprints,1 fluorescent probes,2 biological imaging,1-3
temperature sensor,4-5 organic solid state lasers6-9 and especially
Another kind of RISC process, which start from high-energy triplet
states (Tn) and is called as the hot excition path in the previous
publication,20 should be drawn enough attention. This hot excition
path RISC process has been found in several system such as
anthracene derivatives,21 organic dyes22 and excited-state
intramolecular proton transfer (ESIPT) materials.23 Recently, Ma
and co-workers successfully applied this process in the OLEDs
device and harvested efficiency as high as 93%,24 which strongly
suggest that the hot excition path RISC process is an efficient way to
transform the non-radiative triplet states to the radiative singlet
states.
organic light-emitting diodes (OLEDs)10-11
. Statistically, spin-
independent injection of charge carriers forms about 25% singlet
excitons and 75% triplet excitons in OLED devices. The utilization of
triplet excitons to generate light has received considerable
attention to exceed the predicted limitation for high performance
OLED devices during the past decades.12-17 One viable way is to use
organic phosphorescent semiconducting materials. Highly efficient
OLED devices have been developed using transition metal-
containing organic complex systems such as Alq3, Ir(ppy)3, etc.13, 18
Turning the non-radiative triplet states to the radiative singlet
states via promoting the reverse intersystem crossing (RISC) process
We here choose two benzophenazine(BP) derivatives,
dibenzo[a,c]phenazine (DBP) and tribenzo[a,c,i]phenazine (TBP) (Fig.
1), as model triplet generating materials to explore the hot excition
path RISC process for their charge-transfer induced high triplet
efficiency and simple synthesis route. By appropriate molecular
modification, the energy level structure of singlet and triplet
manifold can be reorganized. As a result of the modulated energy
level, different photophysical processes is demonstrated. DBP acts
as a good phosphorescence emitter with emission lifetime as long
as 318 ms at 77 K whereas TBP is shown to exhibit bright delayed
fluorescence due to reverse intersystem crossing (RISC) from high-
a.Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. E-mail:
hongbing.fu@iccas.ac.cn; yswu@iccas.ac.cn.
b.University of Chinese Academy of Sciences, Beijing, 100039, China.
c.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.Department of Chemistry, Capital Normal University, Beijing, 100048, China.
┴These authors contributed equally.
† Electronic Supplementary Informaꢀon (ESI) available. See
DOI: 10.1039/x0xx00000x
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energy triplet states (Tn) to lowest lying singlet state (S1). These Details for this system have been published previously.26 The
findings are meaningful for the development of high performance nanosecond flash photolysis signals were detected on the
OLED devices and high triplet yields pure organic materials.
Edinburgh LP920 and record on the Tektronix TDS 3012B
oscilloscope and computer.
2. Experimental
2.4 Quantum chemical calculations
2.1 Synthesis
All theoretical calculations were executed by the Gaussian 09
software package.27 The ground states of the BP compounds were
calculated by the density functional theory (DFT) method and the
excited states were calculated by the time-dependent density
functional theory (TD-DFT) method. For all atoms, the geometries of
molecules were optimized at the B3LYP/6-311++G** level in the gas
phase. The isosurface upon the S0→Sn and S0→Tn transitions were
carried out using the Multiwfn software.28
The BP compounds were prepared by amination of phenanthrene-9,
10-dione via one pot reaction with yield of more than 80%
according to the previous literature.25 Details of the synthetic route
were elaborated in the Electronic Supplementary Information (ESI).
Both DBP and TBP were fully characterized with 1HNMR, 13CNMR
and mass spectrometry (MS). All the chemical reagents and solvents
used were purchased commercial sources and were of analytical
grade.
3. Results and discussion
2.2 Spectral measurements
3.1. Basic spectral characterization
The absorption spectra were recorded on a SHIMADZU UV-3600
spectrophotometer. The room-temperature emission spectra were
recorded on a HORIBA Fluoromax-4NIR spectrophotometer. The
luminescence spectra, phosphorescence spectra and the decay
lifetime of phosphorescence at 77 K were recorded on a HITACHI F-
7000 spectrophotometer with the low temperature attachment.
The luminescence spectra contain not only the fluorescence spectra,
but also the phosphorescence spectra. A chopper system, which
includes two choppers and the rotating speed of these two
The phenazine derivatives, which belong to the N-heteroacenes
family, are considered as potential n-type semiconducting materials
to be good alternatives to acenes for their electrondeficient nature,
small band gaps and high thermal stabilities.25, 29 In addition, they
play an important role in fields such as photoinitiators,30 chemical
and biological Probe,31-32 phosphorescent materials ligands33 and
organic crystalline materials.34 The BP compounds were synthesized
based on the cyclocondensation reactions between their respective
1,2-di aminoarenes precursors and phenanthrene-9,10-dione mixed
with acetic acid and ethyl alcohol refluxing for 3h according to the
previous literature (Scheme S1 in Electronic supplementary
information).25
choppers can be controlled by
a computer, was used to
differentiate fluorescence and phosphorescence. By regulating the
rotating speed of chopper, short-lived photons were shielded by
the leaf of the chopper while long-lived photons were allowed to
pass through. The luminescence spectra were recorded at the
normal mode, which allows all the radiant photons including short-
lived fluorescence photons and long-lasting phosphorescence
photons to pass through. The phosphorescence spectra were
recorded at the chopper mode with the chopper rotating at the
setting speed which only allows long-life photons to pass through. A
streak camera system with a Ti:sapphire femtosecond laser system
generating the pump pulses (800 nm, 130 fs, 1 kHz), the accuracy of
which is 20 ps, was used to record the time-resolved fluorescence
decay transient spectra. This system was also used to provide the
pump pulses (413.8 uj/cm2) and the probe pulses for the
femtosecond ultrafast transient absorption system. The emission
spectra of singlet oxygen were record on a Princeton Instruments
IsoPlane 160 spectrophotometer with NIRvana ST 640 as the CCD.
Details for the measurement of singlet oxygen yields were
illustrated in the Result and discussion part.
Fig. 1 Chemical structures (up) and Calculated charge distribution
(bottom) by Gaussian 09 program for the ground states of
dibenzo[a,c]phenazine (DBP) and tribenzo[a,c,i]phenazine (TBP).
2.3 Transient absorption spectroscopy
The femtosecond ultrafast transient absorption system is based on
a two probe beam system passing through two different parts of
the sample with one part of the sample excited by the pump beam.
Thus, we can calculate the absorbance difference for these two
probe beams to obtain the absorption spectra of the excited-states.
The donor-acceptor-donor (D-A-D) structure based organic
molecular have been shown to greatly enhance the ISC process
both from previous experimental and theoretical researches.35-38 As
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shown in Fig. 1, the introduction of two N atoms transforms the intensity at 539 nm with an absolute quantum efficiency of 8.45%
whole molecule skeleton to a D-A-D structure. As a result, the than fluorescence at 419 nm with yield of 1.13% (Fig. S1 and Table
excitation process features an obvious charge-transfer character, 1). In contrast, TBP demonstrates almost no phosphorescence
which additionally reinforces the ISC process (vide infra).Fig. 2a and emission at 77
K accompanied by considerable fluorescence
b depict the absorption spectra (black line), fluorescence spectra intensity at 492 nm with absolute efficiency of 7.07% (Fig. S1).
(red line) and phosphorescence spectra (blue line) of DBP and TBP However, singlet oxygen generation experiment (vide infra) using
in toluene (Tol) solution, respectively. Compared with DBP, spectra hypocrellin
A (HA) as a standard reference (Ф∆=0.84 in Tol
of TBP show obvious bathochromic shift due to the enlargement of solution)41 gives the high triplet yields of 77.1% and 58.7% for DBP
the conjugated regions. In addition, the absorption spectra of TBP and TBP as shown in Fig. 4 and Table 1, respectively. The
exhibit one more redundant weak peak at 473nm which discrepancy between the phosphorescence yields and triplet yields
demonstrates that the excitation progress of TBP is different from of DBP and TBP, both of which have high triplet yields but have
that of DBP.39 Considering the different electronic structures distinctly different phosphorescence yields, demonstrates
a
between DBP and TBP as shown in Fig. 1, the strong absorption possibly different fate for the CT-ISC produced triplet states
peak at 423 nm and the weak absorption peak at 473 nm for TBP between DBP and TBP. This may be related to the different kind of
may result from the strong charge-transfer from the naphthalene charge-transfer excited process between them.
group to the pyrazine group (short as N-P transfer) and the
negligible charge-transfer from the phenanthrene group to the
pyrazine group (short as P-P transfer), respectively. Because of the
a
DBP: τ1 = 0.07 ns (97.2 %)
τ2 = 0.63 ns (2.8 %)
TBP: τ = 0.73 ns
DBP
TBP
fitting
1.0
0.8
0.6
0.4
0.2
0.0
approximately equal electron donating ability of the benzene group
and the phenanthrene group in DBP, there is no abnormal
difference on the intensity of absorption peak. This distinctly
different charge-transfer induced excited states between DBP and
TBP lead to their discriminative features of emission spectra.
0.0
0.5
1.0
1.5
Time/ns
b
1.0
0.8
0.6
0.4
0.2
0.0
DBP
TBP
fitting
DBP: τ = 317.5 ms
TBP: τ = 5.2 ms
0.01
0.1
1 10
Time/ms
100
1000
Fig. 3 (a) Time-resolved fluorescence spectra decay transient of
DBP(red) and TBP (blue) in Tol dilute solution at room temperature
with excitation wavelength of 400nm. (b) Phosphorescence spectra
decay traces of DBP (red) and TBP (blue) in Tol dilute solution at 77
K with excitation wavelength of 400 nm. Note that the horizontal
axils for (a) and (b) are linear and logarithmic, respectively.
Fig. 2 (a) Normalized absorption spectra (black line), Fluorescence
spectra (red line) and phosphorescence spectra (blue line) of DBP
and (b) TBP in Toluene (Tol) dilute solution, respectively. The
phosphorescence spectra were record at 77 K. (c) the luminescence
spectra of DBP (black line) and TBP (red line) at 77 K.
3.2. Time-resolved luminescence kinetics
In order to investigate the low-lying excited states of singlet and
triplet excited states for the BP compounds, time-resolved
luminescence decay transient of DBP and TBP were measured in Tol
dilute solution as shown in Fig. 3. The Fluorescence of DBP decays
In most case, for the reason that the luminescence spectra directly
reflect the equilibrium excited state, luminescence spectra is
considered to be an important tool to obtain the information about
the electronic structure of the lowest excited state.40 To further
study the relationship between the CT characters and the emission
features, the luminescence spectra which contain the fluorescence
and phosphorescence information simultaneously for DBP (black
line) and TBP (red line) were collected at 77 K as shown in Fig. 2c.
We can see that DBP exhibits extremely stronger phosphorescence
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Table 1. Experimental and Calculated Optical Characteristics for the BP compounds.a
Absorption
λcal
Emission
Phosphorescence
τP
Фp
/ %b
λexp
λexp
λcal
τF
ФF
λexp
λcal
/ nmb / nm
Ф∆
f
f
/ msb
/ nm / nm
/ nm / nm
/ ns / %
/ %
0.56
0.07
0.63
DBP 394
362 0.2385 419
377 0.2325
539
722
537 318 77.1 8.45
823 5.2 58.7 <0.1
1.03b
0.67
423
TBP
312 0.8025
492
7.07b
416 0.7564 0.73
473
392 0.2854
a
The experimental data is extracted from the spectra in Tol solution at room-temperature if no special illustration. λexp= experimental
symmetry-allowed lowest-lying absorption or emission wavelengths, λcalc= calculated symmetry-allowed lowest-lying absorption or
emission wavelengths, f = oscillator strengths, ФF = the quantum yield of fluorescence, Фp = the quantum yield of phosphorescence, ФT =
triplet yield, τF =fluorescence lifetime, τP = phosphorescence lifetime. b measured at 77 K.
bi-exponentially which is dominated by short lifetime of 0.07 ns
(97.2 %) accompanied with ignorable percent long lifetime of 0.63
ns (2.8 %) at room temperature. The time-resolved fluorescence
spectra for DBP (Fig. S4) exhibits little difference between spectra
integrated over 0.0 – 0.1 ns and 0.1 – 1.0 ns, which demonstrates
that the two lifetime fluorescence result from the same excited
state but produced by different processes. Considering the relative
low phosphorescence yield for DBP compared with the high triplet
yield (77.1 %), the longer lifetime fluorescence is probably
attributed to the delayed fluorescence from the triplet state.11, 19, 42
At the same time, the fluorescence of TBP follows a mono-
exponential decay with lifetime of 0.73 ns, which is at the same
timescale as the longer lifetime of DBP. The mono-exponentially
decay of fluorescence, and the same excitation spectra for 490 nm
and 524 nm (Fig. S2) and the same emission peaks with excitation
wavelength of 420 nm and 470 nm (Fig. S3) demonstrate that the
fluorescence of TBP derived from the same lowest lying singlet
state. In addition, the high triplet yield (58.7 %), the almost
negligible phosphorescence yield at 77K and the much higher
fluorescence efficiency for TBP than DBP strongly suggest that
fluorescence of TBP is the delayed fluorescence.
It should be emphasized that the “missing” of the normal
fluorescence in TBP results from the high consumption of non-
radiative transition process. We then performed the time-resolved
fluorescence decay transient of DBP and TBP at 77 K (Fig. S5) to
identify the delayed fluorescence. The fluorescence lifetime of DBP
extents to 0.45 ns and 5.85 ns, respectively, with little percent
change for the short and long lifetime compared with room
temperature. However, the fluorescence of TBP follows bi-
Fig.4 The luminescence intensity for singlet oxygen at 1270 nm as a
exponentially decays with lifetime of 0.78 ns and 10.02 ns.
function of absorbance for (a) hypocrellin A (HA), (b) DBP and (c)
TBP in Tol solution at room temperature.
According to the lifetime of DBP at 77 K, lifetime of 0.78 ns is
corresponding to the normal fluorescence and lifetime of 10.02 ns
is corresponding to the delayed fluorescence. Compared with
lifetime at room temperature, because of the restriction of non-
-time fluorescence (which is attributed to the normal fluorescence)
radiative transition by temperature, the appearance of short life-
at 77 K further confirms the existence of delayed fluorescence. On
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the other hand, in contrast to fluorescence, lifetime for the
phosphorescence of DBP extends to as long as 317.5 ms and is
about two orders of magnitude longer than that of TBP with value
of 5.2 ms at 77 K as shown in Fig. 3b. The magnitude difference
between the lifetime of delayed fluorescence and phosphorescence
illustrates the delayed fluorescence is probably not derived from
the lowest triplet states T1 but from higher energy level triplet
states. In addition, the large ∆EST values (0.66 ev and 0.81 ev for
DBP and TBP, respectively) make it almost impossible to conduct
the reverse intersystem crossing process from the T1 state to the S1
state, which strongly support our opinion.
The singlet oxygen quantum yield (Ф∆) can be calculated using the
following equation:44
IS AS
KS
KR
ФS =ФR ⋅
= ФR ⋅
IR AR
Where Ф is the singlet oxygen quantum yield, I is the intensity of
the emission at 1270 nm, A is the corresponding absorbance, S and
R
refer to the sample and the reference, respectively. The
luminescence intensity for singlet oxygen at 1270 nm as a function
of the corresponding absorbance for (HA), DBP and TBP in Tol
solution are shown in Fig. 4. TheФ∆ values of DBP and TBP are
77.1 % and 58.7% , respectively, and are collected in Table 1. The
high singlet oxygen yields strongly suggest high ISC efficiency exists
in the BP compounds.
3.3. Singlet oxygen yield determination
The Ф∆ value was measured by the singlet oxygen luminescent
method using hypocrellin A (HA) as a standard reference (Ф∆=0.84
in Tol solution).41 Triplet state whose energy is higher than that of
triplet oxygen can transform the triplet oxygen to the singlet
oxygen. Then, the singlet oxygen gives the 1270 nm emission upon
returning to the triplet oxygen.42 As a result, the amounts of triplet
states are proportional to that of singlet oxygen and hence are
proportional to the intensity of emission at 1270 nm.43
3.4. Femtosecond transient absorption spectra and data analysis
Fig. 5 (a) The femtosecond ultrafast transient absorption spectra of DBP and (c) TBP in TOL solution. (b) The kinetics traces (colored points)
and fitting lines (black solid line) of DBP and (d) TBP (the kinetic of 565 nm was multiply by 1.5 to distinguish the kinetic of 555 nm) taken
from the corresponding transient absorption spectra monitored at the absorption maxima of the transients. The pump laser wavelength is
400 nm.
decay at ~ 470 nm for DBP and ~ 565 nm for TBP, respectively. We
attribute this fast decay species to the vertical excitation high
energy level singlet states Sn (vide infra). Meanwhile, analysis of ~
520 nm for DBP and ~ 510 nm for TBP both produce one
To further confirm the above proposed photophysical process of
the BP compounds, femtosecond transient absorption spectra were
recorded in Tol solution with 400nm excitation of DBP (Fig. 5a) and
comparable rise time constant of ~ 1 ps and one long lifetime decay.
TBP (Fig. 5c). Upon photoexcitation, both DBP and TBP exhibit
This long lifetime specie is considered to be the triplet state and is
abnormally fast excited-states dynamics characterized by a ~1 ps
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further confirmed by the nanosecond flash photolysis experiments ISC route from T7 to S1 gives the negligible delayed fluorescence in
(Fig. S6). Apparently, the similar decay and rise time constants DBP.
reveals a close correlation between the decay of the Sn state and
the formation of the triplet state.26 Combined the same charge-
transfer characters for the singlet state and the triplet state of
these BP compounds in the ISC process as shown in Fig. 6, the fast
timescale demonstrates a charge-transfer induced ultrafast ISC
process. This conclusion is in accordance with the high yield of
triplet states for DBP and TBP as shown in Table 1 and Fig. 4. For
DBP, the lowest lying singlet state S1 follows a bi-exponential decay
at ~ 490 nm with short lifetime of 75.8 ps and long lifetime of
nanosecond level. The long lifetime decay species belongs to the
triplet state on account of the big superposition bands between the
absorption spectra of the singlet state and the triplet state. Due to
this complicated superposition spectra and the negligible percent
observed in the time-resolved fluorescence decay data, decay for
the delayed fluorescence is not visible.
As to TBP, however, the lowest lying singlet state S1 decays bi-
exponentially with lifetime of 35.8 ps (22.7 %) and 675.6 ps (77.3 %)
accompanied by the same absorption peaks at ~ 465 nm and ~ 555
nm. A nanosecond level decay was also observed at the same band
for the triplet state as a result of the superposition absorption
spectra of the singlet state and the triplet state. Apparently, we
observe both the normal singlet state (35.8 ps) and the reverse ISC
process produced singlet state (675.6 ps). For the minority of the
normal singlet state and the high possibility of nonradiative
transition in TBP, only the delayed fluorescence can be
characterized by the time-resolved fluorescence experiment at
room temperature as shown in Fig. 3a. The appearance of short
lifetime decay for the fluorescence of TBP at 77 K confirms this
opinion further (Fig. S5).
3.5 Theoretical calculations
Finally, we performed theoretical computational studies using TD-
DFT methods at the B3LYP/6-311++G** level to gain more insight on
the photophysical mode of the BP compounds.27-28 The BP
molecular skeleton can be treated as a Donor-Acceptor-Donor (D-A-
Fig. 6 Schematic diagrams showing the TD-DFT-calculated energy
D) structure and has the C2v symmetry with A1 ground state.
levels, isosurface, symmetry and main orbital configurations of (a)
Therefore, the excitation process should be symmetry-preferential
DBP and (b) TBP at singlet (Sn) and triplet (Tn) states. Note that H
process.45 Fig. 6 reproduces the TD-DFT-calculated energy levels,
and L stand for the highest occupied molecular orbital and lowest
isosurface, symmetry and main orbital configurations of (a) DBP and
unoccupied molecular orbital, respectively. The green lines refer to
(b) TBP at singlet (Sn) and triplet (Tn) states. For DBP, the lowest
the energy level which is symmetry-allowed excited. The red lines
lying symmetry-allowed singlet excited state is the S3 state, which is
refer to the intersystem crossing processes probably occurring from
attributed to the charge transfer transition both from the
the Sn state to its higher- or lower-lying triplet states (Tn). It also
phenanthrene group and the benzene group to the pyrazine group
should be noted that the orange dashed line is not the ground state
(Fig. 5a from the yellow part to the blue part for S3) with oscillator
energy level.
strengths of f = 0.2385 (Table 1 and Table S1). Upon photoexcitation,
owing to the symmetry restriction, the DBP molecular was excited
to the S3 state. Then the S3 state undergoes the charge-transfer
induced ultrafast ISC process to the high energy level triplet state Tn
photophysical process. The different electron donating ability of the
(T2 and T7 as shown in Fig. 6a). The large difference between the
timescale of delayed fluorescence and phosphorescence illustrate
two distinct charge-transfer features for the lowest two symmetry-
the reverse ISC process is not from the T1 state but from a higher
energy level triplet state. The T7 state can either transfer into the S1
state is tagged by the naphthalene to phenazine (N-P) transfer with
state via the reverse ISC process (RISC) or converts to the T1 state
via the internal conversion (IC) process. However, the large energy
gap exists between the S1 state and the T2 state prevents the RISC
process from the T2 state. As a consequence, the only one reverse
By contrast, the TBP molecule conducts a more complicated
phenanthrene group and the naphthalene group in TBP results into
allowed singlet excited states (S4 and S6) as shown in Fig. 6b. The S4
oscillator strengths f = 0.2854 while the S6 state is marked by the
phenanthrene to phenazine (P-P) transfer with oscillator strengths f
= 0.8025 (Table 1 and Table S2). These two excited states and their
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oscillator strengths perfectly consistent with the weak absorption phenomenon can only be observed in few points among plenty of
peak at 473 nm and the strong absorption peak at 423 nm in Fig. 2b. DBP crystals upon photoexcitation. We attributed it to crystal
For the much stronger oscillator strengths of the S6 state than the S4 defect for the reason that DBP prefer to form twin crystal. Because
state, most of the TBP molecules were excited to the S6 state. of this, it is difficult for us to character the room temperature long-
Correspondingly, the Tn (T2, T7, T10, T14) state produced from this lasting phosphorescence of DBP, such as lifetime, spectra and
state via the CT-ISC process will be much higher in energy level. As a photograph. Therefore, further experiments are required to
result, the increased number of RISC channels from the Tn state (T7, completely realize the long-lasting room temperature
T10, T14) in TBP to the S1 state enhances the delayed fluorescence. phosphorescence in DBP crystal.
Automatically, only the delayed fluorescence can be observed at
room temperature with the high consumption of the S1 state by
nonradiative transition at room temperature. The high triplet yield
and the negligible phosphorescence at 77 K further verify the high
reverse ISC efficiency.
3.6 Kinetic model
Therefore, we propose the model for the tunable lifetime and yield
of fluorescence and phosphorescence in the BP compounds and
describe it in Fig. 6. For the restriction of symmetry, the BP
compounds were firstly excited to higher energy level singlet states
(Sn) instead of the S1 state. The charge-transfer character of the Sn
state enhances the ISC process and gives the high yields of high
energy level triplet states (Tn) other than the T1 state. Then the Tn
state can decays into either the S1 state via the RISC process and
hence gives the delayed fluorescence or the T1 state via the IC
Fig. 7 Proposed mechanism for the photophysical process of the BP
process. It should be noted that the more number of the Tn states
compounds.
produced by the CT-ISC process means the more channels of RISC
process to the S1 state. We can conclude directly from Fig. 6 that
the energy gap between the firstly excited state Sn and the S1 state
4 Conclusions
(short as ∆E) greatly affects the number of Tn. As a result, the large
∆E value in TBP leads to the high efficiency RISC process which
accounts for the long lifetime of fluorescence and the negligible
In conclusion, we have synthesized two aza-acenes of
phosphorescence. However, the small ∆E value in DBP vastly
dibenzo[a,c]phenazine (DBP) and tribenzo[a,c,i]phenazine (TBP)
prevents the RISC process and gives the much stronger
with C2v symmetry and demonstrated the mechanism for the
phosphorescence (8.45 %) than fluorescence (1.03%) for the high
tunable lifetime and yield of fluorescence and phosphorescence.
ISC efficiency. It is well known that the phosphorescence lifetime τP
Because of the symmetry for the benzophenazine compounds, the
can be expressed by the following equation:31-32
excited states transition process is symmetry-controlled. Therefore,
the BP compounds were firstly excited to the high energy level
singlet state Sn instead of the S1 state. Besides, the higher energy
level Sn state will lead to the increased numbers of the Tn state via
the intersystem crossing process, which in turn increase the
ФST
ФST ⋅ Kr
ФP
Kr
τP
=
=
=
Kr + Knr + KTS
K + K + KTS ⋅ K
(
)
r
nr
r
channels of the reverse intersystem crossing process. As
a
consequence, considerable longer fluorescence lifetime due to the
delayed fluorescence and negligible phosphorescence were
observed. This is the situation in TBP though with high triplet yield
produced by the charge-transfer induced ultrafast intersystem
crossing (CT-ISC) process. In DBP, however, the weaker electron
donating group results into the relative lower energy level Sn state
upon photo-excitation, which greatly reduce the channels for the
reverse intersystem crossing process. Accompanied with the high
yield of triplets (77.0 %) produced by the charge-transfer induced
ultrafast intersystem crossing (CT-ISC) process, DBP exhibits much
stronger long-lasting phosphorescence (8.45 % yields and 317.5 ms)
than fluorescence (1.03 %, 0.45 ns) at 77 K, which premise the
achievement of long-lasting room temperature phosphorescence in
the DBP crystal. This mechanism will be helpful in designing pure
organic high triplet yields materials and high performance OLEDs
materials.
Where Kr and Knr are the rate constants for radiative and
nonradiative transition from the T1 state, respectively, the KTS is the
rate constant for the RISC process from the triplet excitons to the
singlet excitons, and the ФST and ФP are the efficiency of the ISC
process and phosphorescence emission, respectively. Typically, the
Kr value is always 10-1 ~ 10-2 S-1 for hydrocarbon materials.48-49
Therefore, large ФST value and small KTS value will contribute to
forming the long-lasting phosphorescence. Consequently, it is
reasonable to achieve long-lasting phosphorescence when strictly
prevent the RISC process in high triplet yields material By this way,
the high phosphorescence yield (8.45 %) in DBP gives the long-
lasting phosphorescence (318 ms) than that of TBP (5.2 ms) with
negligible phosphorescence yield (< 0.1 %).
Based on the strong phosphorescence for DBP in toluene solution at
77 K, we successfully observed the room temperature long-lasting
phosphorescence in the crystal of DBP.
Unfortunatelythis
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Journal Name
22 R. Redmond, I. Kochevar, M. Krieg, G. Smith, W. McGimpsey,
J. Phys. Chem. A 1997, 101, 2773–2777.
Acknowledgements
23 K. Tokumura, M. Kurauchi, O. Oyama, J. Photochem.
Photobiol. A 1994, 81, 151–158.
24 W. Li, Y. Pan, L. Yao, H. Liu, S. Zhang, C. Wang, F. Shen, P. Lu,
This work was supported by the National Natural Science
Foundation of China (Nos. 21573251, 21521062, 21190034, and
21221002), project of the State Key Laboratory on Integrated
Optoelectronics of Jilin University (IOSKL2014KF16), project of
Construction of Innovative Teams and Teacher Career Development
for Universities and Colleges Under Beijing Municipality
(IDHT20140512), the National Basic Research Program of China (973)
2011CB808402, 2013CB933500, and the Chinese Academy of
Sciences.
B. Yang, Y. Ma, Adv. Optical Mater. 2014,
25 P. K. Sahoo, C. Giri, T. S. Haldar, R. Puttreddy, K. Rissanen, P.
Mal, Eur. J. Org. Chem. 2016, , 1283-1291.
2, 892-901.
7
26 Y. Wu, K. Liu, H. Liu, Y. Zhang, H. Zhang, J. Yao, H. Fu, J. Phys.
Chem. Lett. 2014, 5, 3451-3455.
27 G. W. T. M. J. Frisch, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A.
Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A.
F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi,
J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S.
Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.
Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J.
Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT,
Gaussian 09, Revision A.02, 2009.
Notes and References
1
2
3
4
5
6
7
8
9
L. Xu, Y Li, S. Li, R. Hu, A. Qin, B. Z. Tang, B Su, Analyst 2014,
139, 2332.
L. Yuan, W. Lin, K. Zheng, L. He, W Huang, Chem. Soc. Rev.
2013, 42, 622-661.
G. Zhang, G. M. Palmer, M. W. Dewhirst, C. L Fraser, Nat.
Mater. 2009, 8, 747-751.
X. D. Wang, O. S. Wolfbeis, R. J Meier, Chem. Soc. Rev. 2013,
42, 7834-7869.
P. R. N. Childs, J. R. Greenwood, C. A Long, Rev. Sci. Instrum.
2000 71, 2959.
28 T. Lu, Multiwfn - A Multifunctional Wavefunction Analyzer.
Multiwfn, Version 3.3.7 2015.
29 L. Qiu, X. Zhuang, N. Zhao, X. Wang, Z. An, Z. Lan, X. Wan,
Chem. Commun. 2014, 50, 3324–3327.
30 Z. Kucybała, A. Kosobucka, Polymer Bulletin 2006, 56, 321–
329.
31 D. Dey, A. Bose, D. Bhattacharyya, S. Basu, S. S. Maity, S.
Ghosh, J. Phys. Chem. A 2007, 42, 10500-10506.
32 W. Guo, B. J. Engelman, T. L. Haywood, N. B. Blok, D. S.
Beaudoin, S. O. Obare, Talanta 2011, 87, 276–283.
33 T. Bura, P. Retailleau, M. T. Indelli, R. Ziessel, Dalton Trans.
2013, 42, 4544.
X. Wang, Q. Liao, Q. Kong, Y. Zhang, Z. Xu, X. Lu, H Fu, Angew.
Chem. Int. Ed 2014, 53, 5863-5867.
X. Wang, H. Li, Y. Wu, Z. Xu, H. Fu, J. Am. Chem. Soc. 2014,
136, 16602-16608.
X. Wang, Q. Liao, H. Li, S. Bai, Y. Wu, X. Lu, H. Hu, Q. Shi, H.
Fu, J. Am. Chem. Soc. 2015, 137, 9289-9295.
X. Wang, Q. Liao, X. Lu, H. Li, Z. Xu, H. Fu, Sci. Rep. 2014, 4,
7011.
10 C. Murawski, K. Leo, M. C. Gather, Adv. Mater. 2013, 25,
6801-6827.
34 A. Putta, J. D. Mottishaw, Z. Wang, H. Sun, Cryst. Growth Des.
2014, 14, 350−356.
35 D. Veldman, S. Chopin, S. Meskers, R. Janssen, J. Phys. Chem.
A 2008, 112, 8617–8632.
11 H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature
2012, 492 , 234-238.
12 S. Mukherjee, P. Thilagar, Chem. Commun. 2015, 51, 10988.
13 Y. Tao, C. Yang, J. Qin, Chem. Soc. Rev. 2011, 40, 2943-2970.
14 R. Kabe, N. Notsuka, K. Yoshida, C. Adachi, Adv. Mater. 2016,
28, 655-60.
36 Z. Dance, S. Mickley, T. Wilson, A. Ricks, A. Scott, M. Ratner,
M. Wasielewski, J. Phys. Chem. A 2008, 112, 4194-4201.
37 I. Gould, J. Boiani, E. Gaillard, J. Goodman, S. Farid, J. Phys.
Chem. A 2003, 107, 3515-3524.
38 M. Moral, L. Muccioli, W. Son, Y. Olivier, J. Sancho-García, J.
Chem. Theory Comput. 2015, 11, 168−177.
15 S. Hoshino, H. Suzuki, Appl. Phys. Lett. 1996, 69, 224.
16 H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, V. Podzorov, Nat.
Mater. 2010, 9, 938-943.
39 M. Kobayashi, N. Hayakawa, T. Matsuo, B. Li, T. Fukunaga, D.
Hashizume, H. Fueno, K. Tanaka, K. Tamao, J. Am. Chem. Soc.
2016, 138, 758-761.
17 D. Chaudhuri, E. Sigmund, A. Meyer, L. Rock, P. Klemm, S.
Lautenschlager, A. Schmid, S. R. Yost, T. Van Voorhis, S.
Bange, S. Hoger, J. M. Lupton, Angew. Chem. Int. Ed. 2013,
52, 13449-13452.
40 N. Mataga, Y Murata, J Am. Chem. Soc. 1969, 91, 3144-3152.
41 Z. Diwu, J. Lownt, J. Photochem. Photobiol. A: Chem. 1992, 64,
273-287.
42 C. Sun, Q. Liao, T. Li, J. Li, J. Jiang, Z. Xu, X. Wang, R. Shen, D.
Bai, Q. Wang, S. Zhang, H. Fu, H. Zhang, Chem. Sci. 2015, 6,
761.
43 H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck, T. Fischer,
18 A. Montes Victor, P.-B. C., Agarwal Neeraj, Shinar Joseph, Jr.
Pavel Anzenbacher, J. Am. Chem. Soc. 2006, 128, 12436-
12438.
19 M. Segal, M. Singh, K. Rivoire, S. Difley, T. Van Voorhis, M. A.
Baldo, Nat. Mater. 2007, 6, 374-378.
Coordin. Chem. Rev. 2011, 255, 2622-2652.
44 Z. Yu, Y. Wu, Q. Peng,C. Sun, J. Chen, J. Yao and H. Fu, Chem.
Eur. J. 2016, 22, 4717-4722.
20 D. Hu,L. Yao,B. Yang,Y. Ma, Phil.Trans. R. Soc. A 2015, 373
20140318.
:
45 R. Han, Y. Wu, J. Feng, X. Ai, J. Zhang and L Skibsted
21 K. Kikuchi, H. Fukumra, H. Kokubun, Chem. Phys. Lett. 1986,
123, 226–228.
Photochem. Photobiol. 2004, 80, 326-333.
8 | J. Name., 2017, 00, 1-3
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46 Y. Gong, G. Chen, Q. Peng, W. Z. Yuan, Y. Xie, S. Li, Y. Zhang,;
B. Z. Tang, Adv. Mater. 2015, 27, 6195-6201.
47 M. S. Kwon, Y. Yu, C. Coburn, A. W. Phillips, K. Chung, A.
Shanker, J. Jung, G. Kim, K. Pipe, S. R. Forrest, J. H. Youk, J.
Gierschner, J. Kim, Nat. Commun. 2015, 6, 8947.
48 A. Köhler, H. Bässler, Sci. Eng. R 2009, 66, 71-109.
49 M. Gouterman, J. Chem. Phys. 1962, 36, 2846.
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The enhanced charge-transfer interaction from DBP to TBP leads to higher energy
triplet states, which reinforces the reverse intersystem crossing.
1