Insights into energy transfer pathways between the exciplex host and fluorescent guest: attaining highly efficient 710 nm electroluminescence

Article abstract of DOI:10.1039/d0tc00986e

Energy transfer between the exciplex host and fluorescent guest is a demanding process for attaining high-performance organic light-emitting diodes (OLEDs), particularly in the near-infrared (NIR) region, and insight into the dynamics of energy transfer h

Full text of DOI:10.1039/d0tc00986e

View Article Online
View Journal
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Huang, S. Ho,
C. Lai, C. Ko, Y. Wei, J. Lin, D. Chen, T. Ko, K. Wong, Z. Zhang, W. Hung and P. Chou, J. Mater. Chem. C,
2020, DOI: 10.1039/D0TC00986E.
Volume
Number
6
5
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
7
February 2018
Pages 911-1266
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
rsc.li/materials-c
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
a multi-layered
single-walled carbon nanotubes (LACNTs) in
ceramic/metal electrode system (MLES)
rsc.li/materials-c

Page 1 of 12
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC00986E
Insight into the energy transfer pathways between the exciplex host and NIR
fluorescence chromophores.

Please do not adjust margins
Journal of Materials Chemistry C
Page 2 of 12
View Article Online
DOI: 10.1039/D0TC00986E
ARTICLE
Insights into Energy Transfer Pathways between Exciplex Host and
Fluorescent Guest: Attaining Highly Efficient 710 nm
Electroluminescence.
Received 00th January 20xx,
Accepted 00th January 20xx
Chun-Ying Huang,1,‡ Ssu-Yu Ho,1,‡ Chien-Hsun Lai,2 Chang-Lun Ko,2 Yu-Chen Wei,1 Jia-An Lin,1 Deng-
Gao Chen,1 Tzu-Yu Ko,1 Ken-Tsung Wong,1 Zhiyun Zhang,3,* Wen-Yi Hung,2,4,*and Pi-Tai Chou1,*
DOI: 10.1039/x0xx00000x
Energy transfer between exciplex host and fluorescent guest becomes one of the demanding processes to attain high
performance of the organic light emitting diodes (OLEDs), especially in the near infrared (NIR) region. However, the insight
into the dynamics of energy transfer remains elusive. In this study, new deep red/NIR chromophores NOz-TPA and NOz-t-
TPA where NOz and TPA denote naphthobisoxadiazole and triphenylamine, respectively, are developed with an electron
donor-acceptor-donor (D-A-D) configuration. The optimized 1 wt% doped films for NOz-TPA and NOz-t-TPA blended with
Tris-PCz:CN-T2T (1:1 in molar ratio) exciplex host show similar deep red/NIR emission with photoluminescence quantum
yields (PLQY) of 42 (680 nm) and 28%, (709 nm), respectively. Comprehensive time resolved measurements and dynamics
analyses reveal significant differences in energy transfer pathways, i.e. Förster versus Dexter type energy transfer, between
exciplex host and the fluorescent guest, in which the introduction of bulky tert-butyl groups in the NOz-t-TPA doped film
greatly suppresses Dexter type energy transfer pathway. Despite lower PLQY, the analytical simulation predicts NOz-t-TPA
to be a better candidate to realize a highly efficient electroluminescence. The confirmation is then given by the performance
of NOz-t-TPA doped OLED, showing an external quantum efficiency (EQE) of 6.6% with peak wavelength at 710 nm, which
is among the best records for the metal-free NIR OLEDs around 710 nm. The insight of energy transfer pathways thus plays
a pivotal role to achieve the high performance OLEDs that incorporate exciplex host and fluorescent guest.
First, the pure metal-free fluorescent devices have the
advantage of lower cost compared to that of the heavy-metal
based devices. Second, the small concentration used for the
Introduction
Since the first demonstration by Tang and VanSlyke in 1987,
organic light-emitting diodes (OLEDs) have attracted worldwide
attention.1 With decades of effort, researches and
developments of OLEDs have gained significant improvement in
the field of display panels and light sources.2-4 Recently, it
becomes recognized that energy transfer between exciplex host
and regular fluorescent guest provides a facile means to
promote OLED performance.5-7 In this approach, the thermally
activated delay fluorescence (TADF) exciplex material serves as
a host to harvest both the singlet and triplet population,
followed by Főrster type energy transfer (FRET) to the
fluorescent guest. As such, the upper-limit on external quantum
efficiency (EQE) of conventionally fluorescent device can be
overcome. This strategy provides three manifolds in benefit.
fluorescent dopants may minimize the serious decrease in EQE
under high current density, i.e. a roll off process, while this
phenomenon is commonly observed among phosphorescence
devices due to triplet-triplet and/or triplet-polaron
annihilation.8 Third, the fine tuning of spectral range of device
can be easily accessed by varying the suitable combination of
exciplex host and fluorescence dopants. In this aspect, the use
of exciplex for the energy transfer materials also features its
facility for synthesis and versatility in wide tuning the emission
region compared with those of intramolecular TADF
molecules.The promotion of OLEDs’ performance at different
spectrum window can thus be achieved with straightforward
design.6 Nevertheless, it should be addressed that Dexter type
energy transfer (DET), which transfers the energy from the
triplet state of host TADF material to the triplet state of
fluorescent guest, should be avoided as much as possible.9 In
this aspect, it is surprising that the detailed discussion on energy
transfer remains elusive.
a. Department of Chemistry, National Taiwan University, Taipei, 10617, Taiwan,
R.O.C.
b. Department of Optoelectronics and Materials Technology, National Taiwan
Ocean University, Keelung 202, Taiwan, R.O.C.
c. Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East
China University of Science & Technology, Shanghai 200237, P.R. China.
d. Center of Excellence for Ocean Engineering, National Taiwan Ocean University,
Keelung, 202, Taiwan, R.O.C.
In yet another approach, near-infrared (NIR) OLEDs show
great perspective in optical communication,10 phototherapy,11
night-vision technologies,12 and sensors.13-15 Many research
teams have pursued boosting the performance of deep-red/NIR
OLEDs,16-25 of which the efficiency is generally low as the deep-
red/NIR emission intensities are limited by the energy gap
† Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
‡
These authors contribute equally.
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 3 of 12
ARTICLE
Journal Name
View Article Online
DOI: 10.1039/D0TC00986E
Scheme 1 Synthetic pathways for (a) NOz-TPA and (b) NOz-t-TPA.
law.26-27 The heavy-metal based devices such as Pt-based NIR region.37 None of the D-A-D emitters based on NOz as the
porphyrin complexes generally showed remarkable EQE within acceptor, so far, have been developed.
NIR region,28 but serious roll-off (vide supra). On the other hand,
In this study we not only strived to attain high performance
a number of organic molecules possessing thermally activated NIR OLEDs based on the energy transfer methodology between
delay fluorescence (TADF) in the red and NIR region were exciplex host and NOz-TPA (or NOz-t-TPA) fluorescent guest;
reported to have highly efficient NIR OLEDs.17-18, 29-31 However, more importantly, we made comprehensive time resolved
the design and synthesis of NIR emitters having TADF properties measurements and dynamics analyses in an aim to gain insights
is nontrivial and a great challenge, which may hamper their into the energy transfer pathways. Despite the structure
further advance. The quest of highly efficient NIR OLEDs with similarity between NOz-TPA and NOz-t-TPA they surprisingly
straightforward design is thus in urgency. In this regard, the reveal significant difference in energy transfer pathways, i.e.
aforementioned energy-transfer strategy from exciplex host to Förster versus Dexter type energy transfer. Albeit lower
NIR fluorescent dopants provides an alternative opportunity. In photoluminescence quantum yield (PLQY) for NOz-t-TPA doped
this approach, design of deep red/NIR organic dyes may be in a host matrix, analytical simulation based on relaxation
accessed via extending planar -conjugation, so that the dynamics predicts its better electroluminescence yield due to
delocalization of -electron can narrow the energy gap, while the much suppressed DET. Affirmation is then given by the high
the rigid planar structures can suppress the rotational disorder performance of NOz-t-TPA doped OLED, showing an external
and hence decrease the non-radiative relaxation process.32 quantum efficiency (EQE) of 6.6% with peak wavelength at 710
Combined with exciplex material possessing green-yellow nm. The agreement between analytical approach and the
emission, the NIR emission can be reached by a variety of performance of fabricated OLEDs sheds light on the mechanism
organic fluorescence materials endowed with electron donor- to ascertain high performance NIR OLEDs via energy transfer.
acceptor-donor (D-A-D) architectures through FRET.17, 33-36
Herein, we present two new deep red/NIR chromophores
NOz-TPA and NOz-t-TPA (Scheme 1) where NOz and TPA
denote naphthobisoxadiazole and triphenylamine, respectively,
configured with an electron donor-acceptor-donor (D-A-D)
Detail of results and discussion is elaborated below.
Results and Discussion
Synthesis and characterization
structure.
NOz
belongs
to
the
category
of
The synthetic pathways for NOz-TPA and NOz-t-TPA are
depicted in Scheme 1. In brief, 5,10-dibromonaphtho[1,2-c:5,6-
c]bis([1,2,5]oxadiazole (DiBr-NOz) was synthesized following
the procedure described in the literature,37 which was then
used to prepare the designated compound NOz-TPA via Suzuki
coupling and Buchwald-Hartwig reaction. Compound 3, which is
one of the starting materials of NOz-t-TPA, was synthesized
from the commercially available reagent bis(4-tert-
butylphenyl)amine (1). 1 was then reacted with 1,4-
dibromobenzene to yield 2 through Buchwald-Hartwig reaction.
naphthobischalcogenadiazole (NXz). NXz is a tetracyclic
heteroaromatic ring and has been utilized in the field of solar
cell as the acceptor unit in semiconducting polymer,37-40
affording low lying lowest unoccupied molecular orbital (LUMO)
and rigid planar structure. In 2014, Michiaki et al. synthesized
naphthobisthiadiazole (NTz)-based highly emissive red dyes.41
Comparing NTz, naphthobisoxadiazole (NOz), in which the
sulfur atom of NTz is substituted by oxygen, is expected to reach
even lower LUMO than that of NTz due to the stronger
withdrawing-ability of oxygen, narrowing the energy gap to the
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Journal of Materials Chemistry C
Please do not adjust margins
Page 4 of 12
Journal Name
ARTICLE
Table 1. Photophysical properties of NOz-TPA and NOz-t-TPA
View Article Online
DOI: 10.1039/D0TC00986E
max abs (nm)a
maxem (nm)b
 (%)
neat film
 (ns)
neat film
kr (107s-1)c
compound
NOz-TPA
TOL
neat film
TOL
neat film
TOL
40
TOL
TOL
neat film
0.43(30%)
3.44(70%)
0.16(71%)
0.82(29%)
548
570
570
653
730
22
4.54
8.81
8.67
NOz-t-TPA
586
675
750
35
6.8
4.59
7.62
19.3
Notes that TOL represents toluene. aThe wavelength was recorded from the maximum value of the first band. bThe wavelength was recorded from the maximum
value of the emission spectra. cRadiative decay rate constant (kr) was calculated according to the equation kr=Φ/τ.
extinction coefficients () of > 104 M-1cm-1 at peak wavelength
and the computational approach based on the time-dependent
density functional theory (TD-DFT, cam-B3LYP hybrid function,
see ESI for computational details).42 As shown in Table S2, the
highest occupied molecular orbital (HOMO) and LUMO for title
compounds are all mainly located on the donors (TPA, t-TPA)
and acceptor (NOz), respectively, manifesting an intramolecular
charge transfer (ICT) from donors (t-TPA, TPA) to acceptor
(NOz). In addition to the ICT event, the central acceptor shows
part of transition character with obvious overlap between
Figure 1. Packing structure of NOz-TPA in its single crystal. The thermal ellipsoid is shown
at the 30% probability level with hydrogen atoms omitted for clarity.
Borylation of 2 with bis(pinacolato)diboron gave 3 which was
used to prepare the designated compound NOz-t-TPA via
Suzuki coupling by reacting with DiBr-NOz that has been
elaborated in previous steps (vide supra). Details of synthetic
routes are provided in Electronic Supplementary Information
(ESI).
All synthesized compounds were characterized by 1H and 13
C
NMR spectroscopy. The corresponding crystal and packing
structures obtained from X-ray structural analysis are shown in
Figure 1. The packing mode of NOz-TPA exhibits relatively close
intermolecular distance, which are around 3.4Å measured from
the centroid of acceptor to the plane made by the neighboring
acceptor. This relatively close intermolecular distance may lead
to slight intermolecular interaction and result in spectral shift
from solution to the solid phase (vide infra). Attempts to grow
single crystal of NOz-t-TPA unfortunately were not successful,
due perhaps to the involvement of bulky donor moiety.
Nevertheless, it may be safely to expect that NOz-t-TPA has
similar but less ordered architecture than that of NOz-TPA
because of bulky t-TPA group. Thermogravimetric analysis
(TGA) was recorded on a TA Q500 under nitrogen atmosphere
at a heating rate of 5 °C/min. The temperature of 5% weight loss
(Td) was at 362 °C for both NOz-TPA and NOz-t-TPA. The high
thermal decomposition temperatures reveal their good thermal
stability.
Photophysical properties
The UV/Vis absorption and emission spectra of title compounds
in various solvents are recorded in Figure 2(a), and Table 1
(detailed data are listed in Table S1). The lowest lying
Figure 2. (a) Absorption spectra (lines with circles) and emission spectra of NOz-TPA and
absorption band of NOz-TPA and NOz-t-TPA with absorption NOz-t-TPA in various solvent. Note that CHX, TOL, DCM denote cyclohexane, toluene,
and dichloromethane, respectively. (b) Normalized absorption and emission spectra of
NOz-TPA and NOz-t-TPA in neat film.
peak at 548 and 570 nm, respectively, are ascribed to the
 transition mixed with donor-to-acceptor charge transfer
(CT) character. This statement is supportive by their high
HOMO and LUMO on the six-membered aromatic rings.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 5 of 12
ARTICLE
Journal Name
Therefore, the introduction of such acceptors provides strong lifetime of title compounds in the selected solveVnietws Aartnicdle Onnelianet
electron-withdrawing character and thus the elongation of the films (Figure S1-2) with pertinent data listDeOdIi:n10T.a10b3l9e/1D0aTnCd0T09a8b6leE
-electron delocalization, enhancing optical transition S1. In all cases, the population lifetime of title compounds can
probability. In terms of NOz-t-TPA, replacement of TPA with t- be fitted well with exponential decay in the nanosecond range,
TPA further enhances the electron-donating strength compared implying their fluorescence nature. These, together with the
to NOz-TPA, leading to the lift of HOMO energy and hence the measured data of QY, lead us to deduce the corresponding
decrease of the absorption energy gap. The emission spectra of radiative rates (kr), which all exhibit a fast radiative rate around
title compounds are red shifted as increasing the polarity of ~108 s-1 (Table 1), clearly suggesting an efficient optical
solvents, showing positive solvatochromism. For example, in transition between lowest singlet excited state (S1) and ground
dichloromethane (DCM), the emission peak wavelength of both state (S0).
title compounds shifts to NIR region, with the emission peak
In light of the high transition moment and excellent
wavelength of NOz-t-TPA (800 nm) > NOz-TPA (746 nm). fluorescence yield in NIR for title compounds, we have
Remarkably, QY of both NOz-TPA and NOz-t-TPA were investigated their PL performance under host-guest system. In
measured to be 40% and 35%, respectively, for 653 nm and 675 this aspect, exciplex is chosen as the cohost in an aim to realize
nm emission in toluene.
their high efficiency OLEDs via energy transfer.43 Exciplex is
In the solid neat film, shown in Figure 2(b), both absorption formed with charge-transfer characters between electron-
and emission bands of NOz-TPA and NOz-t-TPA are red shifted donating and electron-accepting molecules, which shows
compared with those observed in toluene. Due to the lack of intrinsically small EST due to the suppression of electron
fast environment relaxation in solid, such a red shift manifests correlation energy and allows the triplet excitons up-converting
the influence of static intermolecular interaction, in part, in to their singlet state by reverse intersystem crossing (RISC)
solid state. Importantly, the emission bands for NOz-TPA and process. Hence, the singlet excitons of exciplex cohost can
NOz-t-TPA in solid neat film have reached the NIR range with transfer directly to singlet state of the fluorescent dopants via
peak wavelength at 730 and 750 nm, and retaining good QY of FRET pathway.5 Additionally, to suppress the possibility of
22% and 6.8%, respectively. We then measured the population undesired Dexter process that depletes the triplet state, the
Figure 3. (a) Chemical structures of Tris-PCz and CN-T2T. (b) Overlap of photoluminescence (PL) spectrum (gray area) of the exciplex Tris-PCz:CN-T2T (1:1) and absorption
spectrum (line with dots) of the dopants NOz-TPA and NOz-t-TPA in dichloromethane. (c,d) Absorption (line with dots, left side) and emission spectra (right side) of Tris-PCz:CN-
T2T:(c) NOz-TPA and (d) NOz-t-TPA in various doping concentration. λex = 310 nm for (c,d).
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Journal of Materials Chemistry C
Please do not adjust margins
Page 6 of 12
Journal Name
ARTICLE
View Article Online
DOI: 10.1039/D0TC00986E
Table 2. Photophysical properties of Tris-PCz:CN-T2T:dopant (x wt%) blend film.
Dopant
NOz-TPA
x wt%a
1
λpeak (nm)b
QY (%)
42
obs/ns (pre-exponential factor)
680
0.692 (-0.391), 10.8 (0.605), 131 (0.003)
4.54 (0.552) 8.61 (0.446), 43.1 (0.003)
3.30 (0.450), 7.82 (0.545), 31.5 (0.004)
0.893 (-0.412), 9.30 (0.584), 252 (0.004)
4.43 (0.418), 10.4 (0.019), 99.1 (0.000)
3.34 (0.967), 9.43 (0.033)
5
10
1
697
698
709
710
715
29
29
28
26
20
NOz-t-TPA
5
10
a Doping concentration represented by percentage in weight. bPeak wavelength of emission band from dopants.
concentration of dopants needs in a suitably low level to keep
an effective long-range singlet-singlet energy transfer in the
system.
Here, we selected the highly efficient delayed fluorescent
exciplex of 9,9′,9″-triphenyl-9H,9′H,9″H-3,3′:6′,3″-tercarbazole
(Tris-PCz)44:
3′,3′′′,3′′′′′-(1,3,5-triazine-2,4,6-triyl)tris(([1,1′-
biphenyl]-3-carbonitrile)) (CN-T2T)45 by 1:1 weight ratio as
cohost (Figure 3(a)). The Tris-PCz:CN-T2T (1:1) exciplex emission
shows a high PL of 0.53 and a small EST = 0.59 kcal/mole,
which leads to a highly efficient green exciplex OLED with EQE
of 11.9%.6 To realize an effective FRET, spectrum overlap
between the emission of the host (energy donor) and the
absorption of the dopant (energy acceptor) is required.46 Figure
3(b) displays the region of overlap between the absorption
spectra of dopants and the PL spectrum of Tris-PCz:CN-T2T,
which allows efficient energy transfer from the exciplex to the
dopants. Figure 3(c) shows the absorption and emission spectra
of the doped films in various doping concentration represented
by percentage in weight (wt%) versus the host. For the doping
concentration of 1 wt%, the first absorption band (locating
around 550 ~ 600 nm) as well as the highly energetic absorption
band (300~400 nm) of dopants (NOz-TPA and NOz-t-TPA)
couldn’t be measured (absorbance  0.005). Therefore, the
corresponding emission spectra shown in Figure 3(c,d) excited
at 310 nm unambiguously result from the energy transfer from
the exciplex host. Comparing that of the NOz-TPA 1 wt% doped
film (Figure 3c), the NOz-t-TPA 1 wt% doped film has obvious
residue emission from exciplex at ~500 nm (Figure 3d),
suggesting its less efficiency in energy transfer. This observation
can be rationalized, in a qualitative manner, by the less
spectrum overlap between PL spectrum of exciplex and
absorption spectrum of NOz-t-TPA (Figure 3(b)). The
photophysical properties of doped films are collected in Table
2. PLQY of 1 wt% doped films (Tris-PCz:CN-T2T:dopant (1 wt%))
were measured to be 42 and 28% for NOz-TPA (680 nm
emission) and NOz-t-TPA (709 nm emission), respectively.
To acquire the preliminary information of parameters that
corresponds to the energy transfer inside the doping system,
we conducted the lifetime measurements with time correlated
single photon counting (TCSPC) system incorporating a
0.82MHz femtosecond laser (180 fs pulse duration) and
multichannel plate as pump pulse and detecting system,
respectively, so that a temporal resolution of as short as 20 ps
Figure 4. Observed population decay of Tris-PCz:CN-T2T:1 wt% NOz-TPA / NOz-t-TPA
doped film monitored at 670 / 690 nm with excitation at 357 nm. (a) TADF character of
the emission from the corresponding doped film. (b) Rise trajectory of the emission from
the corresponding doped film.
can be obtained. The population decay of the emission of NOz-
t-TPA and NOz-TPA 1 wt% doped film (Figure 4(a), Table 2), of
which the corresponding decay curves can be well fitted by two
single exponential components, which are 9.30 and 252 ns for
NOz-t-TPA (monitored at 690 nm), 10.8 and 131 ns for NOz-TPA
(monitored at 670 nm), respectively, denoted as prompt and
delayed fluorescence decay. These results suggest the TADF
characters for both emissions from NOz-TPA and NOz-t-TPA
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 7 of 12
ARTICLE
Journal Name
due to the energy transfer from TADF of exciplex in the doped of exciplex is much faster than all other kinetic parameters, the
View Article Online
DOI: 10.1039/D0TC00986E
film. To gain more insight, the early fast energy transfer exciplex singlet CT state (S*(CT)) can be assumed to be
dynamic was also probed and the results of rise component populated instantaneously at t = 0 prior to any energy transfer
shown in Figure 4(b) clearly indicate that NOz-t-TPA and NOz- processes. Standing on this basis, we tentatively propose an
TPA emission consists of a rise time of 893 ps and 692 ps, energy transfer mechanism depicted in Figure 5(b), which
respectively. The time-resolved measurements for other doping incorporates transfer of energy from TADF exciplex S*(CT) state
concentration were also conducted with data shown in Figure and triplet T*(CT) state to the fluorescent dopant in the singlet
S3 and Table 2. Two remarks can be promptly pointed out (DS*) and triplet (DT*) manifolds. For the convenience of kinetic
according to Figure 4(a) and Figure 4(b). Firstly, under the derivation, we simplify S*(CT) and T*(CT) to S* and T*,
identical experimental condition, as shown in Figure 4(a), the respectively, in the following text. This proposed energy
delayed component of NOz-TPA doped film is nearly two folds transfer mechanism is commonly adopted the but never being
shorter than that of NOz-t-TPA doped film. Secondly, the rigorously validated in a quantitative manner, especially the
slightly faster rise time for the emission of NOz-TPA (cf. NOz-t- competitive S* → DS* and T* → DT* processes, which are
TPA) suggests faster energy transfer rate between exciplex host interplayed due to the thermally accessible process between
and NOz-TPA. Both observations significantly influence their and S* and T* reversibly because of their origin from TADF host.
performance on OLED. Detailed quantitative analyses are Mechanism depicted Figure 5(b) is utilized to fit the
elaborated below.
experimental data obtained in this study. Prior to the following
fitting process, we have fully deconvoluted the measured
population lifetime with instrumental response function (IRF),
Analysis in energy transfer dynamics
For clarity, we first probed the dynamics of the formation of eliminating the broadening effect. For the detailed
exciplex between Tris-PCz and CN-T2T for host. The result deconvolution process, please see Simulative Approach in ESI.
shown in Figure S4(a) indicates that upon 357 nm excitation We first attempted to fit the population evolution of pure TADF
(repetition rate of 82kHz and pulse duration time of 180 fs) and host (Tris-PCz:CN-T2T) with the following equations:
dS*
monitoring at 500 nm exciplex emission the rise time is
irresolvable (< 20 ps). Since the time constant for the formation
  k  k S* t  krisc T* (t)
 
,
(1)

isc 
f
dt
Figure 5. The proposed kinetic model for energy transfer process between exciplex host and fluorescent dopants in (a) Tris-PCz:CN-T2T and (b) Tris-PCz:CN-T2T:1 wt% dopant
(Blue words for NOz-TPA and red words for NOz-t-TPA) solid thin film. Parameters kCT is the rate of the formation of singlet CT state S*(CT) from localized excited state S*(LE); kf
and kp are the population decay rate constants for the S*(CT) and triplet T*(CT) states of the exciplex (Tris-PCz:CN-T2T); kisc and krisc are intersystem crossing and reverse
intersystem crossing rate constants, respectively. kFE is FRET rate constant from exciplex S*(CT) to singlet state of dopant DS*, and kDE is DET rate constant from exciplex T*(CT)
to triplet states of dopant DT*. kdf and kdp are the population decay rate constants of DS* and DT*, respectively.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Journal of Materials Chemistry C
Please do not adjust margins
Page 8 of 12
Journal Name
ARTICLE
dT*
are then applied as the initial guessed values for the
View Article Online
  k  k
T* (t)  kisc S* (t)

risc 
photoluminescence data of Tris-PCz:CDNO-TI:21T0:.1103w9/tD%0TCd0o0p9a86nEt
(NOz-TPA, NOz-t-TPA) solid thin film incorporating energy
transfer model shown in Figure 5(b). Under the assumption of
low doping concentration (1 wt% doped films), the direct
excitation of dopants can be neglected (vide supra). Hence, the
decay process of each excited species can be expressed as
follows:
p
dt
.
(2)
Here, kf and kp denote the observed decay rate of fluorescence
and phosphorescence, respectively. kisc and krisc represent the
intersystem crossing and reverse intersystem crossing rate
constant, respectively, of exciplex. We initially guessed kisc, krsic
,
kf, and kp to be (21.9 ns)-1, (357 ns)-1, (1 s)-1, and (10 s)-1,
respectively, according to the previous work that assumed the
pre-equilibrium between the exciplex CT S1 and T1 states.6
These parameters were fitted iteratively with one parameter
free to fit. As a result, we obtained the best fit shown in Figure
S5(a) after several loops of adjustment for the parameters. kisc,
krisc, kp, and kf were fitted to be (20.1 ns)-1, (333 ns)-1, (4.98 μs)-
1, and (327 ns)-1, respectively. We then plugged in all parameter
to eqs. (1-2) to calculate the prompt and delayed decay rates.
The results of (17.9 ns)-1 and (2.82 s)-1 for prompt and delayed
fluorescence, respectively (Figure S5(b) and eq. (3) below)
highly reproduce the experimental value of (20.8 ns)-1 and (2.69
dS*
 (kf  kisc  kFE )S* (t)  krisc T* (t)
,
(4)
(5)
(6)
(7)
dt
dT*
dt
 (kp  krisc  kDE )T* (t)  kisc S* (t)
,
dDS*
 kdf  DS* (t)  kFE S* (t)
,
dt
dDT*
dt
 kdp  DT* (t)  kDE T* (t)
.
The parameters kdf and kdp denote the observed decay rate of
dopant’s fluorescence and phosphorescence, respectively; kFE
and kDE represent the rate constant of FRET and DET,
respectively. Due to the * character and hence large EST
between DS* and DT* it is reasonable to assume the trivial
efficiency of intersystem crossing and that the decay of DT*
doesn’t influence the population evolution of DS*. Therefore,
kdt can be fixed as a small constant, which is reasonable set to
be e.g. (10 μs)-1.
s)-1 (see Figure S4 (b)).
S* (t)  0.95et
For the convenience of discussion, all fitted parameters are
shown in Figure 5(a). These deduced TADF kinetic parameters
are then applied as the initial guessed values for the
photoluminescence data of Tris-PCz:CN-T2T:1 wt% dopant
For the convenience of discussion, all fitted parameters are
shown in Figure 5(a). These deduced TADF kinetic parameters
5
2.82s
17.9ns
 0.05e
(3)
Figure 6. Model fitting of the population decay curve of (a, c) Tris-PCz:CN-T2T:1 wt% NOz-t-TPA and (b, d) Tris-PCz:CN-T2T:1 wt% NOz-TPA with rise component (a, b) and prompt-
delayed component (c, d). Block circles are experimental data of two doped films acquired from TCSPC measurements under room temperature. Red lines are the fitting results
for the population curves.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 9 of 12
ARTICLE
We initially guessed the TADF associated parameters kisc, krisc
Journal Name
,
View Article Online
DOI: 10.1039/D0TC00986E
kf, and kp to be (20.1 ns)-1, (333 ns)-1, (327 ns)-1 and (4.98 μs)-1,
respectively, for the following fit. The dopant related
parameters, kdf and kFE of 1 wt% NOz-t-TPA were initially
guessed to be (9.30 ns)-1, and (893 ps)-1, respectively, from its
experimental prompt and rise lifetime; while for NOz-TPA, kdf
and kFE were initially guessed to be (10.8 ns)-1 and (692 ps)-1,
respectively. As for kDE of both emitters, we initially guessed
them to be (1 μs)-1 according to the literature.47 The fitting
process is separated into two parts, one for the rise component
and the other for the prompt-delay component with the same
parameters. Through the iterative fitting process, the best fit to
the time-evolution of DS* for both NOz-t-TPA and NOz-TPA 1
wt% doped in the Tris-PCz:CN-T2T film is depicted in Figure
6(a,c) and Figure 6(b,d), respectively. Detailed fitting
parameters for all rate constants are tabulated in Table S3. For
the system with NOz-t-TPA as dopants, the associated rate
constants for kFE (FRET) and kDE (DET) are thus determined to be
(1.05 ns)-1 and (26.0 s)-1, respectively, while they are (0.77 ns)-1
and (0.43 μs)-1, respectively, for NOz-TPA as the dopant. The
resulted faster rate of kFE (FRET) for NOz-TPA is consistent with
the more spectral overlap between PL spectrum of exciplex and
absorption spectrum of NOz-TPA (vide supra, see Figure 3(b)),
supporting the validity of fitted kFE parameters in terms of
physical meaning. The analyses also indicate that the rate of
Dexter type energy transfer kDE (DET) for NOz-t-TPA is ~60 times
slower than that for NOz-TPA doped in the Tris-PCz:CN-T2T film.
This apparently results in the longer delayed component for
NOz-t-TPA (cf. NOz-TPA, see Figure 4(a)). From the viewpoint of
chemical structure, the hindrance in DET in NOz-t-TPA can be
rationalized by the far separated distance between exciplexes
and the core of dopant chromophore, i.e., NOz-t-TPA, because
of four peripheral bulky tert-butyl groups anchored on the TPA
moiety.
Figure 7. Simulated population decay of Tris-PCz:CN-T2T:dopant (1 wt%) under (a) PL and
(b) EL.
behavior with that in toluene. Hence, we choose kr measured in
toluene as parameters, which are 8.81  107 and 7.62  107 s-1
for NOz-TPA and NOz-t-TPA, respectively. As a result, the PL
intensity (I) ratio (INOz-t-TPA / INOz-TPA) is deduced to be 0.79,
consistent with the lower QY (28%) for NOz-t-TPA (cf. 42% for
NOz-TPA).
Taking all fitted parameters in Table S3, the analytical
solutions of DS*(t) for both NOz-TPA and NOz-t-TPA are shown
in eqs. (8-9) and their simulated curves are shown in Figure 7(a)
where DS*(t=0) is normalized to be unity. Note that in this
simulation a fourth exponential term originating from the decay
of triplet state, DT* is omitted because of the forbidden
transition. The simulated rise, prompt, and delayed lifetimes
match the experimental fitted parameters shown in Table 2.
However, these parameters are now endowed with physical
Based on the simulation protocol we then made attempts to
estimate the relative electroluminescence (EL) intensity. In this
approach, the initial condition S*(0) and T*(0) is reset to be
statistically 1 and 3, respectively, in equations (4) and (5) to
generate the population curves for NOz-t-TPA and NOz-TPA 1
wt% doped in the Tris-PCz:CN-T2T film under electronic
excitation. The results are shown in Figure 7(b). Upon
integrating the area and multiplying the fluorescence radiative
decay rate constant kr for NOz-t-TPA (or NOz-TPA), the ratio of
EL intensity for INOz-t-TPA versus INOz-TPA is estimated to be 1.04.
Despite less intensity in PL, the simulation predicts better
performance on EL of NOz-t-TPA. Although the real device
performance also depends on other parameters such as device
configuration and balance between electron and hole, etc., this
methodology of using PL measurement serves as a paradigm to
preliminarily estimate EL efficiency in OLEDs based on energy
transfer between exciplex host and fluorescent guest. Its
validity is further proved by the following practical EL
measurement.
meaning.
DSNOzTPA (t)  1.00et
1.00e
t
*
717 ps
10.79ns
3.29103 et
148ns
,
(8)
(9)
DSNOztTPA (t) 1.01et
1.00e
t
*
954 ps
9.71ns
3.73103 et
228ns
.
To compare the relatively steady state PL intensity between
the doped films of NOz-TPA and NOz-t-TPA, we then integrate
the simulated population curves from t = 0 to 1 s, followed by
the multiplication with the corresponding kr to simulate the
fluorescence intensity. Here, we assume that the dopants in
dilute concentration of 1 wt% should possess similar radiative
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Journal of Materials Chemistry C
Please do not adjust margins
Page 10 of 12
Journal Name
ARTICLE
View Article Online
DOI: 10.1039/D0TC00986E
Figure 8. (a)Schematic energy level diagram of the device. (b) Schematic of the emission mechanism in exciplex cohost system doped with fluorophores. (c) Current density-voltage-
radiance (J-V-R) characteristics. (d) External quantum efficiencies (EQE) and power conversion efficiencies (PCE) as a function of radiance. (e) EL spectra and (f) Comparison between
NIR OLEDs in this work and other published metal-free NIR OLEDs.
Table 3. Electroluminescence Data of NIR OLED Devices.
dopant
X
[%]
Vona
[V]
bR/J at 7V
[mW sr-1m-2/mA cm-2]
EQE/PCEmax
[%/%]
λmax
[nm]
1
2.2
14094/878
6.0/4.6
670
NOz-TPA
5
10
1
2.2
2.2
2.0
5960/839
5837/1012
60150/2160
5.0/3.5
4.3/3.0
7.4/5.3
689
700
683
NOz-t-TPA
5
2.0
2.0
21310/1455
12510/1160
6.9/4.9
6.6/4.4
697
710
10
aR is radiance and PCE is power conversion efficiency. bTurn-on voltage at which emission became detectable
Electroluminescence
Tris-PCz (15 nm)/ Tris-PCz:CN-T2T: 1, 5, 10 wt% dopant (20
nm)/CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm) according to
previous report.6 Figure 8(a) shows the schematic energy level
diagram of the device. According to the energy alignment the
holes and electrons are able to be injected from Tris-PCz and
CN-T2T to the emissive layer (EML) without any energy barrier,
respectively. Figure 8(c)-(e) depicts the EL characteristics of the
Tris-PCz:CN-T2T doped 10% NOz-TPA and NOz-t-TPA devices,
and the key data are summarized in Table 3. We also show the
EL characteristics with different doping concentration in Figure
S8-S9. The device performance parameters are summarized in
Table 3.
The 1 wt% NOz-TPA doping device exhibited maximum EQE
and power conversion efficiency (PCE) of 6.0% and 4.6%,
respectively, with an EL peak at 670 nm. However, at such low
doping concentrations, the EL spectrum shows residual
emission of exciplex. The 5 wt% NOz-TPA doping device
displayed completely energy transfer. The EL displays a peak at
Prior to the EL measurement, it is necessary to experimentally
determine the energy level of HOMO and LUMO for title compounds
in the solid thin film. To achieve this aim, photoelectron
spectroscopy (PES) was conducted and the corresponding data
are shown in Figure S6 and Table S3. Note that in the PES
approach, the LUMO energy level was calculated from the
corresponding orbital by adding the absorption energy gap in
the solid neat film to the acquired HOMO level. For comparison,
cyclic voltammetry measurement was also conducted in
solution for acceptors (NOz) with corresponding data shown in
Figure S7. Compared to other reported deep-red/NIR molecules
with D-A-D systems, title compounds show relatively lower lying
LUMO of -3.56 eV,48-50 which suggest that NOz should be among
the best acceptors for NIR D-A molecules
EL properties of OLEDs were investigated by fabricating
devicewith architectures of ITO/ 4wt% ReO3: Tris-PCz (60 nm)/
.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 11 of 12
ARTICLE
Journal Name
689 nm and lower maximum EQE (5.0 %). When the doped measurements suggest quiet different kinetic trace and hence
View Article Online
concentration increases to 10%, the maximum EQE drops energy transfer pathways between host DanOdI: 1e0a.1c0h39fl/uDo0TreCs0c0e9n8c6eE
to4.3% as well as the EL peak redshift to 700 nm, respectively. emitter. Detailed analyses conclude much different Dexter type
As the doping concentration increases, the distance between of triplet-triplet energy transfer. Hindered by bulky tert-butyl
guest molecules decreases, which increases the local substitution groups on the para sites of TPA, NOz-t-TPA doped
polarization field and the increased local polarization field tends film possesses ~60 times slower DET rate. Importantly, the
to cause the spectral redshift.7 The lower efficiency might analog analyses derived from the energy transfer mechanism
originate from the concentration quenching effects and the (Figure 5) predict the relative PL and EL ratio for NOz-t-TPA
increase in DET from the triplet state (T1) of exciplex to T1 of versus NOz-TPA in the doped exciplex film. Despite the higher
dopant as Figure 8(b) shown. To eliminate the DET process from PL intensity in the NOz-TPA doped film, due to the original 75%
T1 of the exciplex cohost to T1 of the fluorescent emitters as well population at the triplet state for which the energy dissipation
as to inhibit the concentration quenching process, a low doping is dominated by Dexter type energy transfer, NOz-t-TPA doped
concentration of the fluorescent emitter is thus required.
film predicts better OLED performance regarding e.g. EQE and
Notably, by adding tert-butyl groups on the para sites of PCE. This is further firmly proved by the real OLED operation
phenyl rings in TPA, forming NOz-t-TPA, the NIR OLED devices (Figure 8 and Table 3). Correlated well with the analytical
have further high EQE and redshifted emission. When EML approach, the device of NOz-t-TPA displays a NIR emission peak
consists of low wt% such as 1% NOz-t-TPA to avoid the charge at 710 nm with EQE up to 6.6%. This methodology incorporating
trapping process, the device displays maximum EQE of 7.4% and comprehensive kinetic analyses proves to be powerful for
the EL spectrum shows clearly weak exciplex emission at 510 exciplex host/fluorescent guest type of OLED, which may play a
nm. Next, upon increasing the doping level of NOz-t-TPA to 5 crucial role in guiding further NIR OLED development.
wt%, the device displayed a radiance (R) of 21310 mW sr-1m-2 at
7.0 V (1455 mA cm-2) with a peak at 697 nm, which achieved
maximum EQE of 6.9% and PCE of 4.9%. Eventually, 10 wt%
Conflicts of interest
NOz-t-TPA as dopant, the device shows 71% of such emission
falls in the NIR (> 700 nm) with an EL peaked at 710 nm. The
remarkable point is that 10 wt% NOz-t-TPA device still
maintains a maximum EQE of 6.6 %, which decreases a little
compared to device with 5 wt% dopant (EQE of 6.9%). This
suggests that the growth of DET accompanying the increase of
doping level is not serious in NOz-t-TPA system (cf. NOz-TPA).
The result is consistent with the early interpretation of PL
results based on far separated distance between host material
and guest molecules induced by the bulky tert-butyl substituent
in NOz-t-TPA (vide supra), reaffirming the above kinetic
analyses.
In regard to the current progress on the NIR OLEDs we then
show a plot of the EL efficiency for the reported metal-free NIR
OLEDs between 700 and 715nm in Figure 8(f).29-31, 51-53 Clearly,
this work presents an outstanding progress among the metal-
free NIR OLED through the conventional organic fluorescent
dyes pumped by exciplex host via energy transfer. And
importantly, this study sheds light on the overall energy transfer
mechanism that is of fundamental importance to the relevant
research fields.
There are no conflicts to declare.
Acknowledgements
This work was supported by the funding from Ministry of
Science and Technology (MOST), featured areas research
program within the framework of the Higher Education Sprout
Project administrated by Ministry of Education (MOE) of
Taiwan. We are grateful to the National Center for the High-
performance Computing (NCHC) of Taiwan for the valuable
computer time and facilities. We are thankful for the TGA data
from Thermal Analysis System of Instrumentation Center,
National Taiwan University. Z. Z. thanks the Program for
Professor of Special Appointment (East-ern Scholar) at Shanghai
Institutions of Higher Learning and the Start-up Grant of the
East China University of Science and Technology (SP1901J010).
Notes and references
‡ CCDC number 1984380 contains the X-ray crystallographic
coordinates for the compound NOz-TPA reported in this article.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.
Conclusions
1
2
3
4
C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913-
915.
M. A. Baldo, D. F.O 'Brien, Y. You, A. Shoustikov, S. Sibley, M.
E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi,
Nature, 2012, 492, 234.
J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.
Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature,
1990, 347, 539.
X.-K. Liu, Z. Chen, C.-J. Zheng, M.C hen, W. Liu, X.-H. Zhang and
C.-S. Lee, Adv. Mater., 2015, 27, 2025-2030.
In summary, exploiting two newly designed D-A-D type
fluorescence NIR emitters, NOz-TPA and NOz-t-TPA, we have
gained fundamental insights into energy transfer pathways
between exciplex host and fluorescent guest. Both
chromophores are applied as the guest emitter to fabricate thin
film with the exciplex host Tris-PCz:CN-T2T exiplex where the
host acts as an energy donor. The PLQY of NOz-TPA and NOz-t-
TPA NIR 680 nm and 709 nm emission are obtained to be 42 %
and 28%, respectively, when optimized at 1 wt% blended in the
5
Tris-PCz:CN-T2T
film.
Comprehensive
time-resolved
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Journal of Materials Chemistry C
Please do not adjust margins
Page 12 of 12
Journal Name
ARTICLE
6
W.-Y. Hung, P.-Y. Chiang, S.-W. Lin, W.-C. Tang, Y.-T. Chen, S.-
H. Liu, P.-T. Chou, Y.-T. Hung and K.-T. Wong, ACS Appl. Mater.
Interfaces, 2016, 8, 4811-4818.
B. Zhao, T. Zhang, B. Chu, W. Li, Z. Su, H. Wu, X. Yan, F. Jin, Y.
Gao and C. Liu, Sci. Rep., 2015, 5, 10697.
N. C. Giebink and S. R. Forrest, Phys. Rev. B., 2008, 77, 235215.
D. L. Dexter, J. Chem. Phys., 1953, 21, 836-850.
Janssen, E. W. Meijer, P. Herwig and D. M. de Leeuw, Nature,
View Article Online
1999, 401, 685.
39 K. Kawashima, Y. Tamai, H. Ohkita, I. DOOsIa:k1a0.1a0n3d9/DK.0TTCa0k0im98iy6Ea
Nat. Commun., 2015, 6, 10085.
40 K. Kawashima, I. Osaka and K. Takimiya, Chem. Mater., 2015,
27, 6558-6570.
41 T. Ishi-i, R. Hashimoto and M. Ogawa, Asian. J. Org. Chem.,
2014, 3, 1074-1082.
42 T. Yanai, D. P. Tew and N. C. Handy, Chem. Phys. Lett., 2004,
393, 51-57.
43 Will W. H. Lee, Z. Zhao, Y. Cai, Z. Xu, Y. Yu, Y. Xiong, R. T. K.
Kwok, Y. Chen, N. L. C. Leung, D. Ma, J. W. Y. Lam, A. Qin and
B. Z. Tang, Chem. Sci., 2018, 9, 6118-6125.
44 P. Murto, A. Minotto, A. Zampetti, X. F. Xu, M. R. Andersson,
F. Cacialli and E. G. Wang, Adv. Opt. Mater., 2016, 4, 2068-
2076.
45 T. X. Liu, L. P. Zhu, C. Zhong, G. H. Xie, S. L. Gong, J. F. Fang, D.
G. Ma and C. L. Yang, Adv. Funct. Mater., 2017, 27, 1606384.
46 M. Sarma and K.-T. Wong, ACS Appl. Mater. Interfaces, 2018,
10, 19279-19304.
47 T. Lazarides, D. Sykes, S. Faulkner, A. Barbieri and M. D. Ward
Chem: Eur. J., 2008, 14, 9389-9399.
7
8
9
10 J.-C. G. Bünzli and S. V. Eliseeva, J. Rare Earths, 2010, 28, 824-
842.
11 C.-L. Ho, H. Li and W.-Y. Wong, J. Organomet. Chem., 2014,
751, 261-285.
12 G. Qian and Z. Y. Wang, Chem. Asian. J., 2010, 5, 1006-1029.
13 E. L. Williams, J. Li and G. E. Jabbour, Appl. Phys. Lett., 2006,
89, 083506.
14 M. Sauer, Angew. Chem. Int. Ed., 2003, 42, 1790-1793.
15 G. S. He, G. C. Xu, P. N. Prasad, B. A. Reinhardt, J. C. Bhatt and
A. G. Dillard, Opt. Lett., 1995, 20, 435-437.
16 R. Yang, R. Tian, J. Yan, Y. Zhang, J. Yang, Q. Hou, W. Yang, C.
Zhang and Y. Cao, Macromolecules, 2005, 38, 244-253.
17 S. Wang, X. Yan, Z. Cheng, H. Zhang, Y. Liu and Y. Wang,
Angew. Chem. Int. Ed., 2015, 54, 13068-13072.
18 C. Li, R. Duan, B. Liang, G. Han, S. Wang, K. Ye, Y. Liu, Y. Yi and
Y. Wang, Angew. Chem. Int. Ed., 2017, 56, 11525-11529.
19 L. H. Slooff, A. Polman, F. Cacialli, R. H. Friend, G. A. Hebbink,
48 W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou and
K.-T. Wong, J. Mater. Chem., 2010, 20, 10113-10119.
F. C. J. M. van Veggel and D. N. Reinhoudt, Appl. Phys. Lett., 49 H.-F. Chen, T.-C. Wang, S.-W. Lin, W.-Y. Hung, H.-C. Dai, H.-C.
2001, 78, 2122-2124.
Chiu, K.-T. Wong, M.-H. Ho, T.-Y. Cho, C.-W. Chen and C.-C.
Lee, J. Mater. Chem., 2012, 22, 15620-15627.
50 D. Li, Y. Hu and L.-S. Liao, J. Mater. Chem. C, 2019, 7, 977-985.
20 N. Tessler, V. Medvedev, M. Kazes, S. Kan and U. Banin,
Science, 2002, 295, 1506.
21 R. J. Curry and W. P. Gillin, Appl. Phys. Lett., 1999, 75, 1380- 51 X. B. Du, J. Qi, Z. Q. Zhang, D. G. Ma and Z. Y. Wang, Chem.
1382.
Mater., 2012, 24, 2178-2185.
22 J. Kido and Y. Okamoto, Chem. Rev., 2002, 102, 2357-2368.
23 M. Cocchi, D. Virgili, V. Fattori, J. A. G. Williams and J.
Kalinowski, Appl. Phys. Lett., 2007, 90, 023506.
24 K. Tuong Ly, R.-W. Chen-Cheng, H.-W. Lin, Y.-J. Shiau, S.-H. Liu,
P.-T. Chou, C.-S. Tsao, Y.-C. Huang and Y. Chi, Nat. Photonics,
2016, 11, 63.
52 S. P. Wang, Y. Miao, X. J. Yan, K. Q. Ye and Y. Wang, J. Mater.
Chem. C, 2018, 6, 6698-6704.
53 A. D'Aleo, M. H. Sazzad, D. H. Kim, E. Y. Choi, J. W. Wu, G.
Canard, F. Fages, J. C. Ribierre and C. Adachi, Chem. Commun.,
2017, 53, 7003-7006.
25 X. Ai, E. W. Evans, S. Dong, A. J. Gillett, H. Guo, Y. Chen, T. J. H.
Hele, R. H. Friend and F. Li, Nature, 2018, 563, 536-540.
26 M.B ixon, J. Jortner, J. Cortes, H. Heitele and M. E. Michel-
Beyerle, J. Phys. Chem., 1994, 98, 7289-7299.
27 R.E nglman and J. Jortner, Mol. Phys., 1970, 18, 145-164.
28 C. Borek, K. Hanson, P. I. Djurovich, M. E. Thompson, K.
Aznavour, R. Bau, Y. Sun, S. R. Forrest, J. Brooks, L. Michalski
and J. Brown, Angew. Chem. Int. Ed., 2007, 46, 1109-1112.
29 J. Xue, Q. Liang, R. Wang, J. Hou, W. Li, Q. Peng, Z. Shuai and
J. Qiao, Adv. Mater. 2019, 31, e1808242.
30 D. H. Kim, A. D'Aleo, X. K. Chen, A. D. S. Sandanayaka, D. D.
Yao, L. Zhao, T. Komino, E. Zaborova, G. Canard, Y. Tsuchiya,
E. Choi, J. W. Wu, F. Fages, J. L. Bredas, J. C. Ribierre and C.
Adachi, Nat. Photonics, 2018, 12, 98.
31 Y. Hu, Y. Yuan, Y. L. Shi, D. Li, Z. Q. Jiang and L. S. Liao, Adv.
Funct. Mater., 2018, 28, 1802597.
32 H. Lu, Y. Zheng, X. Zhao, L. Wang, S. Ma, X. Han, B. Xu, W. Tian
and H. Gao, Angew. Chem. Int. Ed., 2016, 55, 155-159.
33 Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim
and H. Yan, Adv. Mater., 2003, 15, 353-389.
34 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev.,
2003, 103, 3899-4032.
35 K. R. JustinꢀThomas, J. T. Lin, M. Velusamy, Y. T. Tao and C. H.
Chuen, Adv. Funct. Mater., 2004, 14, 83-90.
36 M. S. A. Abdel-Mottaleb, R. O. Loutfy and R. Lapouyade, J.
Photochem. Photobio. A, 1989, 48, 87-93.
37 Z. Zhang, F. Lin, H.-C. Chen, H.-C. Wu, C.-L. Chung, C. Lu, S.-H.
Liu, S.-H. Tung, W.-C. Chen, K.-T. Wong and P.-T. Chou, Energy
Environ. Sci., 2015, 8, 552-557.
38 H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K.
Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
Please do not adjust margins