Disentangling Multiple Effects on Excited-State Intramolecular Charge Transfer among Asymmetrical Tripartite PPI-TPA/PCz Triads

Article abstract of DOI:10.1002/chem.202002862

By utilizing the bipolarity of 1,2-diphenylphenanthroimidazole (PPI), two types of asymmetrical tripartite triads (PPI-TPA and PPI-PCz) were designed with triphenylamine (TPA) and 9-phenylcarbazole (PCz). These triads are deep-blue luminescent materials with a high fluorescence quantum yield of nearly 100 %. To trace the photophysical behaviors of these triads, their excited-state evolution channels and interchromophoric interactions were investigated by ultrafast time-resolved transient absorption and excited-state theoretical calculations. The results suggest that the electronic nature, asymmetrical tripartite structure, and electron–hole distance of these triads, as well as solvent polarity, determine the lifetime of intramolecular charge transfer (ICT). Interestingly, PPI-PCz triads show anti-Kasha ICT, and the charge-transfer direction among the triads is adjustable. For the PPI-TPA triad, the electron is transferred from TPA to PPI, whereas for the PPI-PCz triad the electron is pushed from PPI to PCz. Exploration of the excited-state ICT in these triads may pave the way to design better luminescent materials in the future.

Full text of DOI:10.1002/chem.202002862

Accepted Article
Accepted Article
Title: Multiple Disentangling Effects on Excited State Intramolecular
Charge Transfer Among Asymmetrical Tripartite PPI-TPA/PCz
Triads
Authors: Ming-De Li, Sirui Yang, Chen Cao, Amjad Islam, Shanshan
Sun, Ziqi Deng, Jiayu Li, Shaofei Ni, and Qingxiao Tong
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002862
Link to VoR: https://doi.org/10.1002/chem.202002862
01/2020

10.1002/chem.202002862
Chemistry - A European Journal
FULL PAPER
Multiple Disentangling Effects on Excited State Intramolecular
Charge Transfer Among Asymmetrical Tripartite PPI-TPA/PCz
Triads
Sirui Yang,[a] Chen Cao, [a] ,[b] Amjad Islam,[a] Shanshan Sun,[a] Ziqi Deng,[a] Jiayu Li,[a] Shaofei Ni,[a] Qing-Xiao
Tong,*[a] and Ming-De Li*[a]
Dedication ((optional))
[a]
[b]
Sirui Yang, Chen Cao, Dr. Amjad Islam, Shanshan Sun, Ziqi Deng, Jiayu Li, Dr. Shaofei Ni, Prof. Qing-Xiao Tong, Prof. Ming-De Li
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structure Materials of Guangdong Providence, Shantou
University, Shantou 515063, China
E-mail: qxtong@stu.edu.cn, mdli@stu.edu.cn
Chen Cao
Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR 999077, P. R. China
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract:
By
utilizing
the
bipolarity
of
1,2-
However, it is easy to crystallize rather than to form the
amorphous film. Therefore, pure anthracene is not suitable for
OLED devices. Thereupon, materials chemists modified
anthracene to synthesize multiform anthracene compounds to
improve its photoelectric properties.15-19 For the display
applications, blue emission plays a crucial role in organic
electroluminescence (EL), it not only acts as a part of Red-Green-
Blue primary colors but also can integrate low-energy-
phosphorescence emitters to compose the hybrid white organic
lighting-emitting devices (WOLEDs).20-22 Recently, plenty of work
has been done using phenanthroimidazole (PI) unit which
demonstrates that PI group is conducive to be chosen as the
central moiety of deep-blue emitters.23-24 When N1 and C2
positions of PI were modified with a benzene ring, one of the most
diphenylphenanthroimidazole (PPI), two types of asymmetrical
tripartite triads (PPI-TPA and PPI-PCz) have been designed with
triphenylamine (TPA) and 9-phenyl-carbazole (PCz), respectively.
These triads are the deep-blue luminescent materials with a high
fluorescence quantum yield of nearly 100%. To trace the
photophysical behaviors of these novel triads, the excited states
evolution channels and interchromophoric interactions of these triads
were investigated by ultrafast time-resolved transient absorption and
excited states theoretical calculations. The results suggest that the
electronic nature of moieties, asymmetrical tripartite structure and the
distance of electron-hole of these triads as well as the polarity of
solvent determine the lifetime of intramolecular charge transfer (ICT).
Interestingly, PPI-PCz triads demonstrate an anti-Kasha’s ICT, the
charge transfer direction among the triads is adjustable. For PPI-TPA
triad, the electron is transferred from TPA to PPI, while for PPI-PCz
triads, the electron is pushed from PPI into PCz moiety. The
exploration on the excited state ICT among these triads may pave the
way to design a better luminescent material in the future.
common
and
typical
derivatives
1,2-diphenyl-
phenanthroimidazole (PPI) (Scheme 1) was obtained.25-26 The
PPI group is a nitrogen-containing heterocyclic conjugate group
with a rigid plane conjugated structure possessing excellent
properties such as high fluorescence quantum yield and wide light
absorption, which makes it very attractive to be employed as a
building block for the preparation of blue-emitters.27-30 PPI can
effectively reduce the energy loss of non-radiation transition and
exhibits good thermal stability.31 Two distinct nitrogen atoms in
the imidazole ring render PPI to have a bipolar in nature, which
indicates that PPI can not only be used as an electron acceptor
when combined with a strong electron donor group but also has
the ability of donating electron when coupled with an electron-
deficient group or a weak electron donor group.32-34
Introduction
The organic lighting-emitting device (OLED) technology was first
developed in 1987.1 OLED material have diverse prevailing
advantages, including; high power efficiency,2-3 mechanical
applicability with flexible devices,4 lower cost and environmental
protection.5 Therefore, OLED is in a dominant position among
different types of technologies, such as; smart home devices, flat
displays and fluorescent materials over a couple of decades.6-7 To
date, abundant typical tricolor molecular materials have been
synthesized to fabricate full-color OLED displays,8-9 while some
issues related to the applicability remain to be resolved and the
most significant one is the blue device.10-11 In the recent decades,
development in blue electroluminescent materials have made a
great progress,12 especially in terms of both the color purity and
the device efficiency.13 One of the most typical and the earliest
organic electroluminescence phenomena was discovered in the
single-crystal anthracene by Pope and coworkers in 1963.14
Owing to their good hole transport properties performance and
high energy band-gap, aromatic amines are widely used in the
design of OLED materials. Among them, triphenylamine (TPA)
(Scheme 1)35-36 and carbazole are the most representative
integrants. Carbazole shows good optical and electronic
properties, and possesses high triplet energy level inducing there
is a small energy gap between triplet state and singlet state which
is beneficial for intersystem crossing (ISC).37 The lone pair of
electrons present on the N-atom of carbazole makes it to be a
good electron donor. However, after modified with a phenyl, for
example, 9-phenyl-carbazole (PCz), the ability as an electron
1
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10.1002/chem.202002862
Chemistry - A European Journal
FULL PAPER
donor will be weakened.
nm. Moreover, the main broad absorption bands of TPA for triad-
1 have been observed to red-shift from 300 nm to 360 nm. These
changes are attributed to the extensive conjugated system after
the construction of the tripartite structure instead of CT in the
ground state. The detail information will be discussed in the fs-TA
part.
Figure 1c shows the photoluminescence (PL) spectra of PPI, PCz
and TPA. PCz and TPA exhibit an ultraviolet emission peak at 365
nm after the irradiation of 300 nm whereas the emission peak of
PPI lies at 390 nm. However, when they are combined into the
asymmetrical tripartite triads’ structures, the emission peaks have
red-shifted to the deep blue region (Figure 1d). The maximum
fluorescence emission peak of PPI-PCz triads lies at 420 nm
which is red-shifted for 55 nm compared with that of PCz and for
30 nm compared with that of PPI, respectively. The triad-1’s (PPI-
TPA) emission peak locates at 466 nm which is redshifted for
nearly 100 nm compared with that of TPA and for 76 nm
compared with that of PPI. The fluorescence intensity of TPA is
weaker than that of PPI and PCz by an order of magnitude in the
same concentration, but the fluorescence intensity of triad-1 is
excellent and its fluorescence quantum yield is close to 100%,
which is same as that of the triads of PPI-PCz (Table S1). This
reveals that when these asymmetrical tripartite triads structures
are constructed successfully, their excited states’ evolution
channels have wide differences with the photo-behavior of the
individual PCz, TPA and PPI. Therefore, it is worth to uncovering
the intrinsic mechanism and the various effects which induce
those differences between the triads and individual moieties.
Scheme 1. The chemical structures of PPI (a1) , TPA(a2) , PCz (a3) and
modifying PPI with TPA (triad-1), PCz (triad-2), phenyl group of PCz (triad-3)
respectively at the site of C3 and with phenyl group of PCz (triad-4) and PCz at
the site of C3’(triad-5).
Previously, a lot of fluorescent molecules based on PPI, TPA and
PCz were developed as the emitters for OLED devices, and most
of the work has been focused on the redox potential, the highest
quantum efficiency (EQE)38 and other electroluminescence
performances of the OLED device. However, the excited state
transitions and its dynamics were scarcely deeply understood for
the photo-functional molecules consisting of PPI, TPA and PCz
moieties. Here, a PPI couples with two TPA or PCz moieties
forming two kinds of asymmetrical ternary structures, which are
designed and synthesized to explore the nature of charge transfer
(CT) in a series of innovative asymmetrical tripartite PPI-TPA/PCz
triads (Scheme 1). Ultrafast time-resolved transient absorption
spectroscopy has become a powerful technique to explore a
range of excited states and CT dynamics of molecules.39-41 By the
combination of femtosecond/nanosecond transient absorption (fs-
TA/ns-TA) spectroscopies and excited state theoretical
calculations, the completely photophysical and photochemical
arches of PPI-TPA/PCz triads were depicted.
(a)2.0
(b)
1.4-biTPA-PPI
2.4-biPCz-PPI
3.4-bi(N-PCz)-PPI
4.3-bi(N-PCz)-PPI
5.3-biPCz-PPI
PCz
TPA
PPI
1.0
1.5
1.0
0.5
0.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
300 350 400 450 500
Wavelength (nm)
Wavelength (nm)
(c)
(d)
1.4-biTPA-PPI
2.4-biPCz-PPI
3.4-bi(N-PCz)-PPI
4.3-bi(N-PCz)-PPI
5.3-biPCz-PPI
PCz
TPA
PPI
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Results and Discussion
Absorption and emission studies on the fragments and triads.
Figure 1a shows the ground-state absorption spectra of the
molecular fragments like PPI, TPA and PCz dissolved in DMF with
the same concentration. PCz showed the maximum absorption
band at 280 nm and the shoulder absorption peaks at 325 nm and
340 nm. In contrast, TPA showed a broad absorption peak
centered at 300 nm. PPI has the absorption peaks at 312 nm and
340 nm which are overlapping with that of TPA and PCz, beyond
that while PPI has an extra absorption peak at 360 nm. Under the
same concentration, the absorption intensity of PPI above 320 nm
are higher than that of PCz and TPA, PPI’s molar absorption
coefficient (ε1=2.23×104 L·mol⁻¹·cm⁻¹) is higher than that of PCz
(ε2=1.44×104 L·mol⁻¹·cm⁻¹) and TPA(ε3=5.26×103 L·mol⁻¹·cm⁻¹).
The preparation methods of PPI-TPA/PCz molecular triads by
systematical synthesis methods are given in the Supporting
Information (Figure S1-S4), their absorption spectra are changed
a lot (Figure 1b). All the absorption bands are broadened, the
absorption peak’s edge of PPI-PCz triads is close to 400 nm. In
contrast, the absorption band’s edge of triad-1 has reached 425
350 400 450 500 550
Wavelength (nm)
400 450 500 550 600 650 700
Wavelength (nm)
Figure 1. (a) The UV-vis absorption spectra of PCz, TPA and PPI dissolved in
DMF with the same concentration of 5 mM. (b) Shown are the normalized
absorption spectra of the triads 1-5 dissolved in DMF. (c) The normalized
emission spectra of PCz, TPA and PPI dissolved in DMF with the same
concentration of 5 mM, the excitation wavelength is 300 nm. (d) The normalized
emission spectra of the triads 1-5 dissolved in DMF, the excitation wavelength
is 330 nm.
Femtosecond transient absorption studies on the evolution
channels of triads.
Fs-TA spectroscopy can detect and trace the excited states and
transient species after the excitation of the molecules in a specific
timescale.42-43 The studies of PPI-TPA/PCz triads are measured
in a strong polar organic solvent such as N, N-Dimethylformamide
(DMF) and a non-polar organic solvent such as Dichloromethane
(DCM), polar and non-polar solvent may aid to probe the solvent
polarity’s influence on the dipole-moment of excited states and
the rate of charge separation (CS) state.44,45 Fs-TA experiments
were performed to monitor the excited states
2
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10.1002/chem.202002862
Chemistry - A European Journal
FULL PAPER
(a)
(b)
(c)
DOD
0.28
6000
1.78 ps
2.31 ps
3.41 ps
5.50 ps
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13.0 ps
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(d)
(e)
(f)
0
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EAS 1
EAS 2
EAS 3
EAS 4
-0.1
-0.1
-2
400
500
600
700
400
500
600
700
400 450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Figure 2. The fs-TA spectra of triad-1 recorded in a DMF solution after 330 nm excitation. (a) The 3D fs-TA image of triad-1. The process of triad-1 (b) 0.19 ps to
1.78 ps, (c) 1.78 ps to 21.7 ps, (d) 21.7 ps to 118 ps, (e) 118 ps to 6.53 ns. (f) the global fitting dynamics (top) and the EADS (bottom) of triad-1.
evolution of the triad-1 in DMF solution under the excitation of 330
nm (Figure 2). Figure 2a indicates that a vivid 3D image can be
obtained, the fs-TA spectra shows that the triad-1’s main excited-
state absorption (ESA) peak locates at 623 nm, the ground state
bleaching (GSB) locates at 360nm and stimulated emission (SE)
band appears at 466 nm. Figure 2b-e displays four distinct
processes after the photoexcitation. Firstly, two positive peaks
centered at 520 nm and 621 nm are increased immediately, which
are attributed to the popularity of the excited states from a ground
state (S0) to a higher excited state (Sn (n ≥1)) under 330 nm
excitation in a sub-picosecond time scale from 0.19 ps to 1.78 ps.
After that, the transient spectra have changed significantly, the
transient absorption spectra are a red-shift from 400 nm to 411
nm and the SE move from 435 nm to 467 nm (Figure 2c), which
is associated by the electron transfer from the donor to the
acceptor. The observation of the apparent isosbestic points at 462
nm and 575 nm indicates that the complete conversion from the
singlet excited state to the CS state among triad-1 from 1.78 ps
to 21.7 ps. After reaching the maximum extent of ICT, the
transient absorption peaks do not show any change in the
intensity as well as wavelength shift along the train of delay time
from 21.7 ps to 118 ps (Figure 2d). This is probably due to the
strong stability of the excited CS state of triad-1, which indicates
that DMF solvent has little effect on its dipole moment. Ultimately,
the appearance of the isosbestic points at 400 nm, 420 nm and
515 nm suggests the excited CS state of triad-1 solely return to
the ground state by the charge recombination, which causes the
GSB at 364 nm and SE at 467 nm, and positive peaks at 415 nm
and 621 nm to drop out to zero gradually (Figure 2e). To clearly
describe the photophysical processes of the compounds upon
photoexcitation, the evolution associated difference spectra
(EADS) of triad-1 (Figure 2f, bottom) were obtained by the global
fitting, these results were outputted from Glotaran (a GUI software
for the “R” package, “TIMP”)51. Figure 2f (top) presents each
evolution associated spectra (EAS) in the EADS corresponding to
the lifetime values which were obtained by the global fitting.
To further explore the nature of ICT process, a non-polar solvent
DCM was used in the fs-TA experiments, the similar excited state
evolution channels of triad-1 were observed in DCM solvent
(Figure S5). But the maximum wavelength of transient absorption
peak is drifted, the SE peak has a blue-shift from 467 nm in DMF
to 452 nm in DCM. This also demonstrates that the second
process is the ICT because the polar solvent leads to the transient
absorption of ICT having a red-shift comparing to the results of
non-polar solvent of triad-1.
Like the fs-TA results of triad-1 obtained in a DMF solution, the
results of PPI-PCz triads are shown in Figure 3. It is obvious that
the transient absorption peaks of triad-2, triad-3, triad-4, and triad-
5 lie at 600 nm, 589 nm, 569 nm, and 640 nm, whereas their SE
peaks take the positions at 425 nm, 434 nm, 430 nm and 433 nm,
respectively. The detailed excited states evolutions of these triads
are presented in Figure S6-S9. The phenomena of triad-2, triad-
3, triad-4, and triad-5 are analogous to that of triad-1, in the same
way, there are four processes including the generation of the
excited states, ICT, CS state and charge recombination which are
obviously represented in their EADS. In Figure S6 and S10, the
isosbestic point of the triad-2 blueshifts from 467 nm in DMF to
450 nm in DCM solvent, which is caused by the energy level of
CS state being higher than that of polar solvent. Other triads also
exhibit different degrees of blue-shift in DCM solvent. Compared
with the results of these samples dissolved in the non-polar
organic DCM solvent (Figure S10-S13), all the SE peaks show
little blue- shift in contrast to the strong polar solvent DMF. In a
word, the results of fs-TA reveal that the solvent polarity will
evidently affect the property of CS state of the five PPI triads.
3
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10.1002/chem.202002862
Chemistry - A European Journal
FULL PAPER
DOD
(a)
4500
DOD
(b)
moieties, asymmetrical tripartite structure (combined positioned
of PCz) also has great contribution effects on the ICT process.
The above discussion confirms that these triads would transfer
the electron in the excited state rather than in the ground state,
the solvent polar, molecular structure and combined position of
PCz would affect the rate of ICT. It must be noted that PPI is a
bipolar molecule, PPI has two possible roles (donor or acceptor)
in the triads. Therefore, the next step will verify the direction of the
ICT process.
0.43
4500
0.42
0.32
0.22
0.12
0.02
-0.08
-0.18
0.34
0.25
3000
3000
1500
0
0.17
0.08
1500
0
-0.01
-0.10
-0.19
400
500
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700
DOD
400
500
600
700
DOD
0.42
Wavelength (nm)
Wavelength (nm)
(c)
0.48 (d)
4500
4500
0.32
0.22
0.12
0.02
-0.08
-0.18
0.38
0.28
0.18
0.08
-0.02
-0.12
Direction of electron transfer between PPI and TPA/PCz
triads.
3000
1500
0
3000
1500
0
Traditionally, carbazole is a kind of strong electron-donating
group because of the lone pair electrons in the N atom. However,
after substituting a benzene ring on the N atom, the lone pair
electrons will be delocalized to the benzene ring, which will visibly
diminish the ability of giving electrons of carbazole. When a novel
molecular structure was designed by the constitution between
PCz and bipolar PPI, the direction of ICT in the triads is vague to
be identified. Therefore, this part focuses on the ICT direction in
the PPI-PCz triads. Figure 1a suggests that 360 nm pump light
can excite the PPI individually. Fs-TA experiments pump by 360
nm were also performed for all the triads, the excited states
evolution of fs-TA pump by 360 nm are almost same as the results
under the irradiation of 330 nm whether the spectrum (Figure S14-
S17) or the lifetime (Table S1). Under the 330 nm and 360 nm
pump excitation, all the triads are expected to transfer the electron
from PPI to PCz. To further verify this speculation, ns-TA
experiments were carried out to monitor the excited states and
transient states of molecular fragments PPI and PCz.
Figure 4a displays the ns-TA results of PPI in a deoxygenated
DMF solvent. The major ESA peak locates at 473nm which is
distinct from the results of fs-TA for all the PPI triads. The kinetic
fitting at 473 nm shows that PPI has a short lifetime of τ1=7.6 ns
and a long lifetime of τ2=16.5 μs (Figure 4d). In order to confirm
that the long life-time transient state is a triple state of PPI, the
oxygen was purged into the solution. The long lifetime transient
state was quenched by the oxygen and the lifetime dropped down
to 0.16 μs (Figure S18). This demonstrates that the long lifetime
transient state of PPI has a triplet state nature. Therefore, ns-TA
quenched experiments for PPI demonstrates that PPI will
undergo the ISC to produce the triple state after the excitation by
330 nm. Given that the longest lifetime τ4 of PPI triads is around
1.5 ns for the results of fs-TA discussed previously, which is
unprecedentedly shorter than the long lifetime of ns-TA for PPI.
This demonstrates that PPI intends to transfer the electron to the
acceptor in the triads rather than undergoes the ISC to generate
the triple state in the triads. In Figure 4b, the 3D image of ns-TA
results for PCz in a deoxygenated DMF solvent show that the
major ESA peak locates at 408 nm which is also different to the
results of fs-TA of PPI triads. The kinetic fitting at 408 nm gains
the lifetimes including τ1=15 ns and τ2=2.47 μs (Figure 4e), which
is also longer than the lifetime τ4 of PPI triads, but the 2.47 μs is
much shorter compared to the long lifetime of PPI 16.5 μs. To
figure out the direction of electron transfer, the ns-TA experiments
were also carried out on the mixing solution of PPI and PCz. The
results show that a main ESA peak can be observed at 470 nm
which is characteristic feature peak of triplet state of PPI (Figure
4c).46 The ESA at 470 nm was quenched with the increase of the
concentration of PCz. Figure 4f displays that the lifetime of triplet
state of PPI gradually decreases (Figure 4f) while the lifetimes of
408 nm are getting longer and longer when the concentration of
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 3. The 3D images of fs-TA for the PPI triads in DMF solution (a) triad-2,
(b) triad-3, (c) triad-4 and (d) triad-5.
To trace the evolution of excited states and transient states of fs-
TA spectra for all the triads, the global fitting was used to fit the
dynamics of fs-TA spectra for all the triads, the lifetimes of various
processes are listed in Table 1. When we testify the global fitting,
four exponential function was found to gain the better dynamics
fitting for the fs-TA data. This reveals that there are four excited
state decay processes after the excitation for the triads, which is
consistent with the four processes being observed in Figure 2 and
Figure S5-S13. For all the triads, the first process is attributed to
the formation of their excited state, their lifetimes are within 1ps;
the second τ2 lifetime is accounted by the ICT as well as emitting
fluorescence for the triads; the third time period of τ3 is associated
by the CS state, in this case, the rearrangement with the
surrounding solvent may stabilize the dipole moment of the
excited CS states. And then finally, SE peak gradually disappears,
so the fourth τ4 lifetime is assigned to the charge recombination
for all the triads.
Table 1. Summary of the dynamics for all the triads measured by fs-TA in DMF
/ DCM solvent, respectively, pumped by 330 nm
triads
τ1 /ps
τ4 /ps
τ2 /ps
τ3 /ps
1
2
3
4
5
0.53/0.46
0.62/0.53
0.71/0.54
0.61/0.54
0.64/0.50
4.65/2.37
12.75/6.90
9.93/3.51
14.90/7.38
17.90/9.30
653/396
622/271
597/465
552/224
866/385
1723/1898
1667/1481
1797/1910
2052/1485
2416/1732
C
ompared to the dynamics processes obtained in DCM solvent
for all the triads, the data obtained in DMF solvent displays distinct
features compared with that recorded in DCM solvent for the
triads. The lifetimes of ICT (τ2) and solvent rearrangement (τ3) in
DMF solvent are much longer than that of in DCM solvent. This
reveals that the polar solvent makes the ICT to be more stable,
which costs more time to produce a steady CS state. In addition,
Table 1 also shows that the triad-5 spends the longest lifetime for
ICT, and the triad-1 costs the minimal lifetime no matter what kind
of solvent it dissolves. This indicates that the electronic nature of
4
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DOD×10-3
DOD×10-3
-3
(c)
15
(a)
(b)
DOD×10
5
4
3
2
1
0
-4
-4
-2
0
-5
15
-1
2
-2
0
12
9
12
9
5
2
3
6
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6
3
11
14
6
3
8
8
0
11
0
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Wavelength (nm)
400 450 500 550 600
Wavelength (nm)
400 450 500 550 600
Wavelength (nm)
(d)
(e)
(f)
6
1.0
0.8
0.6
0.4
0.2
0.0
Kinetic of PPI
Kinetic at 473 nm
Kinetic at 408 nm
Fit
6
Fit
Fit
Kinetic of 1:1
4
2
0
Fit
4
2
0
Kinectic of 1:2
Fit
Kinectic of 1:5
Fit
0
2
4
6
8
10
0
10 20 30 40 50 60
0
20
40
60
80
Time (ms)
Time (ms)
Time (ms)
Figure 4. The 3D images of ns-TA spectra of PPI (a) and PCz (b) with a concentration of 5 mM in deoxygenated DMF. (c) The 3D image of ns-TA spectra of the
mixed solution of PPI and PCz with the same concentration in DMF solution. The kinetic at 473 nm of PPI (d), at 408 nm of PCz (e) and at 473 nm of 5 mM PPI with
the increase of the concentration of PCz (f) in deoxygenated DMF.
PCz increases (Table S2).
greater number (f=0.75), however, the value of oscillator strength
of the S1 is less, which means that triad-2 will reach the S2 state
for a great proportion after irradiation. The results of other triads
are displayed in the SI (Figure S20-S22). The calculated results
reveal that all the triads composed by PPI and PCz have a higher
probability to be excited to S2. As shown in Figure 5, when triad-
1 is excited to S1, the electron cloud density of the HOMO orbital
mainly locates in TPA part while the electron cloud density of the
LUMO orbital mainly locates in PPI part and benzene part. When
triad-2 is excited to S2, the electron cloud density of the HOMO
orbital mainly locates in PPI part while the electron cloud density
of the LUMO orbital mainly locates in PCz part and benzene part.
Moreover, other PPI-PCz triads also have the same tendency as
triad-2 (Figure S20-S22). Above results prove that PPI part plays
two different roles between triad-1 and PPI-PCz triads. PPI acts
as the acceptor when connected with TPA in triad-1 while it
becomes as the donor part when constructed with PCz moiety in
the triad-2 to triad-5.
Furthermore, the distribution of hole and electron in the triads is
necessary to expound the extent of ICT. The hole and electron
can be defined in different ways. Lu and co-workers47 gave a new
definition that the distribution of the hole signifies where the
electron leaves, and the distribution of the electron is where the
excited electron stays in the excited state. In the iso-surface of
hole & electron, the green part is the positive side, corresponding
to the increase in the electron density of the excited state relative
to the ground state, and blue part is the opposite side. According
to this rule, the electrons are more concentrated on the PPI side
while the TPA side is occupied by holes in the S1 state for triad-1.
However, the completely different phenomena are observed in
PPI-PCz triads because the PPI part plays an electron acceptor
role in triad-1 but an electron donor role in PPI-PCz triads. The
above results are consistent with the calculated distribution of
electron cloud density on HOMO and LUMO orbitals. The iso-
surface of hole & electron can be visualized as two ellipses named
To further confirm the ICT, we have calculated the ESA of the
cation or the anion of the molecular fragments like PPI, TPA and
PCz. The calculated results shown that the oscillator strength
value for the cation of PPI at 630 nm is 0.1838 and the oscillator
strength value for the anion of PPI at 662 nm is 0.1478 nm.
Therefore, the ESA peak of both the cation and the anion of PPI
located at 650 nm (Figure S19). After the ICT, the transient
absorption spectra at 623 nm (triad 1), 600 nm (triad 2), 589 nm
(triad 3), 569 nm (triad 4), and 640 nm (triad 5) gradually increase
(Figure 2a and Figure 3), this reveals that these transient
absorption peaks are associated by the generation of the anion of
PPI for triad 1 or the cation of PPI for triad 2-5 . In addition, we
also calculated the ESA of the cation of TPA (TPA+) and the anion
of PCz (PCz–), the results found that they have an overlapping
peak at around 550 nm (Figure 19S). For triad 1, the transient
absorption peak at 520 nm gradual redshift to 550 nm in Figure
2c indicates the produce of the TPA+. In addition, the transient
absorption at 560 nm of the mixing solution of PPI and PCz is
obviously higher than the individual PPI and PCz at 2 μs under
the same concentration (Figure 19S(f)), which suggests the
generation of the PCz– in the mixing solution after ICT. Therefore,
these evidences support that the electron will be transported from
PPI to PCz in the triads under the irradiation of 330 nm light.
TD-DFT calculations for electronic and hole density of
excited states of triads.
Figure 5 shows the electronic and hole density patterns, iso-
surface of Cele and Chole, Δr index, oscillator strengths, and the
energy level of the highest occupied molecular orbital
(HOMO)/lowest unoccupied molecular orbital (LUMO) of singlet
excited states for triad-1 and triad-2. The results convey that triad-
1 will be excited to the first singlet excited state (S1) with a high
probability for a larger oscillator strength (f=0.823) while the value
of oscillator strength of the second singlet excited state (S2) is very
small. In contrast, the oscillator strength of the S2 in triad-2 has a
5
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10.1002/chem.202002862
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3.15 Å
4.64 Å
3.40 Å
2.03 Å
Figure 5. The calculated data of the electronic and hole density patterns, iso-surface of Cele and Chole functions, Δr index, oscillator strengths, and energy level
and electron density of HOMO and LUMO orbital of the singlet excited states for triad-1 and triad-2.
Chole & Cele, while the distance between the center of the blue and
green ellipses (corresponding to the centroid position of the hole
and the electron) is the Δr index. Table 2 shows the Δr index and
oscillator strengths. The Δr of triad-1 in the S1 state is 3.15 Å,
obviously, this distance can be considered as CT excitation.48 All
the PPI-PCz triads have a larger value of oscillator strengths in S2
state, this reveals that these triads will be probably excited to S2.
Interestingly, given that the possibility of observing anti-Kasha
phenomenon in other fundamental reactions, the excited-state
intramolecular proton-transfer reaction once demonstrated anti-
Kasha behavior.49 The anti-Kasha reaction rate depends on the
interplay with vibrational relaxation, internal conversion and
radiative or nonradiative relaxation to the ground state. In this
work, the ICT process of PPI-PCz triads may take place in S2 state,
it may call as anti-Kasha’s ICT. The Δr values of S2 state of triad-
4 and triad-5 are maximum, in contrast, the Δr of S2 state of triad-
3 is minimum. Figure 6 shows that the calculated Δr values of S2
state for PPI-PCz triads are related with the lifetime of ICT
process (τ2) observed by the fs-TA experiments. Therefore, the Δr
index may also determine the lifetime of ICT in the triads.
Table 2. The summary of the Δr index and oscillator strengths (f) in the singlet
Figure 6. The correlation between Δr and τ2 lifetime in the triads 2-5.
excited states of PPI triads 1-5.
S1
S2
f
∆r
f
∆r
Conclusion
triad-1
triad-2
triad-3
triad-4
triad-5
0.823
0.48
0.14
0.086
0.12
3.15 Å
3.40 Å
4.84 Å
5.20 Å
6.0 Å
0.325
0.75
0.84
0.73
1.12
4.64 Å
2.63 Å
1.02 Å
3.23 Å
3.21 Å
The present study has explored the properties of excited states
and interchromophoric interactions of two kinds of tripartite PPI-
based bipolarity triads 1-5 by UV-vis absorption, fluorescence, fs-
TA/ns-TA and DFT calculations. The ICT does not occur in the
6
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10.1002/chem.202002862
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ground-state which is proved by no new redshift UV-vis
absorption peak and fluorescence peak detected in DMF solvent.
All the triads showed almost 100% absolute PL quantum yield at
blue range. Excited states studies determined that the ICT is
crucial process to account for the enhancement of the
fluorescence for all the triads. Fs-TA results show that there are
four processes (Scheme 2) including the popularity of the excited
states, ICT, CS state and charge recombination. Ns-TA studies
demonstrate that PPI is electron acceptor moiety in triad-1 when
connecting with TPA chromophore while PPI is electron donor
part in the triads 2-5 when building up with PCz chromophore. The
electronic nature of moieties, asymmetrical tripartite structure, the
polarity of solvent, excited ICT states and CS distance Δr index
will affect the direction of electron and lifetime of ICT of all the
triads. Interestingly, the solvent polarity does not change the
dipole moment and energy of the CS state because the transient
absorption of CS state is remained constant during the third
process (τ3). Eventually, the calculated results demonstrate that
PPI-PCz triads demonstrate an anti-Kasha’s ICT process and the
calculated Δr between Cele and Chole can match very well with the
lifetime of the ICT process (τ2) observed by fs-TA experiments.
This study is crucial for the design and application of these
luminescent materials for OLED devices in future.
Synthesis of 4-biTPA-PPI: Yield: 1.5 g (82%). 1H NMR (400 MHz, CD2Cl2)
δ 8.81 (d, J = 8.5 Hz, 1H), 8.76 (d, J = 8.3 Hz, 1H), 7.88 (d, J = 8.4 Hz, 2H),
7.76 (dt, J = 25.0, 8.6 Hz, 4H), 7.59 (ddd, J = 34.7, 23.4, 8.7 Hz, 10H), 7.43
– 7.23 (m, 11H), 7.23 – 7.02 (m, 15H).
Synthesis of 4-biPCz-PPI: Yield: 1.8 g (88%) 1H NMR (400 MHz, CD2Cl2)
δ 8.73 (d, J = 8.3 Hz, 1H), 8.68 (d, J = 8.3 Hz, 1H), 8.51 (d, J = 1.6 Hz, 1H),
8.33 (d, J = 1.5 Hz, 1H), 8.17 (d, J = 7.7 Hz, 1H), 8.09 (d, J = 7.8 Hz, 1H),
7.97 (d, J = 8.4 Hz, 2H), 7.82 – 7.15 (m, 31H).
Synthesis of 4-bi(N-PCz)-PPI: Yield: 1.5 g (83%) 1H NMR (400 MHz,
CD2Cl2) δ 8.84 (d, J = 8.4 Hz, 1H), 8.78 (d, J = 8.3 Hz, 1H), 8.21-8.13 (m,
4H), 8.08-8.02 (m, 4H), 7.91-7.65 (m, 15H), 7.62-7.26 (m, 16H).
Synthesis of 3-bi(N-PCz)-PPI: Yield: 1.4 g (82%) 1H NMR (400 MHz,
CD2Cl2) δ 8.84 (d, J = 8.4 Hz, 1H), 8.78 (d, J = 8.3 Hz, 1H), 8.20 (t, J = 8.0
Hz, 4H), 7.59 (dddd, J = 30.6, 24.2, 20.2, 16.0, 4.8 Hz, 33H).
Synthesis of 3-biPCz-PPI: Yield: 1.4 g (82%) 1H NMR (400 MHz, CD2Cl2)
δ 8.73 (d, J = 8.4 Hz, 1H), 8.67 (d, J = 8.2 Hz, 1H), 8.22 (d, J = 8.0 Hz, 1H),
8.17 – 8.03 (m, 3H), 7.88 (d, J = 8.1 Hz, 2H), 7.77 – 7.14 (m, 33H).
A. Femtosecond transient absorption (fs-TA) experiments: The fs-TA
measurements were performed with an apparatus and methods detailed
previously and details are given in references 42 and 43. Very briefly, fs-
TA spectra were obtained by employing the harmonic (330 and 360 nm
light) for the pump pulse and white-light continuum (350800 nm light) for
the probe pulse generated from the 800 nm output of a femtosecond
regenerative amplified Ti:sapphire laser system For the measurements
here, a solution of 40 mL was passed through a 2 mm thick cell, excited
by a 330 and 360 nm pump pulse and then interrogated by the white light
continuum pulse.
τ2
S2
τ2
B. Excited state calculations: To have a better understanding about the
direction of CT for all the triads and to quantify the extent of ICT, density
function theory (DFT) and time-dependent density function theory (TD-
DFT) calculations were used to optimize molecular structure and study
their excited states. The calculations were conducted by employing B3LYP
method and 6-311G** through the Gaussian 16 program package.50 After
that, the Multiwfn program47 was used to analyze and visualize hole-
electron distribution, transition density and calculate Δr index to measure
CT length.
S1
τ3
CS
τ3
CS
hv
τ4
τ4
τ1＜1ps
S0
Scheme 2. The visualization of the whole excited state evolution channels in
the triads.
Acknowledgements
Experimental Section
This project are financially supported by the National Natural
Science Foundation of China (grant numbers 21773151), STU
Scientific Research Foundation for Talents (130-760189),
Innovation Team Project (2019KCXTD007) of Educational
Commission of Guangdong Province of China, the Guangdong
Province Universities and Colleges Pearl River Scholar Funded
Scheme 2019 (GDUPS2019) for the financial support and
Guangdong-Hong Kong-Macau Greater Bay Area Joint Breeding
Laboratory (130/18219470)
Experimental and Computational Methods
Sample preparation.
Synthesis: A 100 mL, flame dried, two-necked round-bottomed flask was
equipped with a 2.4 cm Teflon-coated magnetic oval stir bar. The side-
neck was fitted with a rubber septum using an inlet needle connected to a
Schlenk line. The central neck was equipped with a water-jacked reflux
condenser. The assembled apparatus is charged with 2Br-PPI (1eq,
2mmol), Boric acid derivatives (2.5eq, 5mmol), saturated potassium
carbonate solution (10ml), EtOH (10ml), Toluene (25ml) with argon
bubbling 45 mins. After that, Pd (PPh3)4 (5% equal, 0.1mmol) was added
into the solution. The reaction was processed in 90 ℃with TLC plate
monitoring. After being cooled to room temperature, the solution was
washed with 40 mL water three times and extracted with dichloromethane;
then, collected organic solution was dried with anhydrous Mg2SO4 and
concentrated by rotary evaporation. Finally, the residue was purified by
column chromatography (petroleum ether: CH2Cl2 = 2: 1) to obtain the pure
white powder.
Keywords: Intramolecular charge transfer • OLED • Excited
states • Donor and acceptor • triads
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Entry for the Table of Contents
e-
UV
e-
PPI-PCz
TPA-PPI
e-
e-
ICT
ICT
τ =4.65 ps
τ =12.75 ps
QY≈100%
λem=425 nm
λem=466 nm
Two types of asymmetrical tripartite triads (PPI-TPA and PPI-PCz) have been designed for deep-blue luminescent materials with a
high fluorescence quantum yield of nearly 100%. For these novel triads, the electronic nature of moieties, asymmetrical tripartite
structure and the distance of electron-hole of these triads as well as the polarity of solvent determine the lifetimes of excited state
intramolecular charge transfer (ICT). Interestingly, the charge transfer direction among the triads is adjustable.
9
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