A Simple Molecular Design Strategy for Delayed Fluorescence toward 1000 nm

Article abstract of DOI:10.1021/jacs.9b09323

Harnessing the near-infrared (NIR) region of the electromagnetic spectrum is exceedingly important for photovoltaics, telecommunications, and the biomedical sciences. While thermally activated delayed fluorescent (TADF) materials have attracted much inter

Full text of DOI:10.1021/jacs.9b09323

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Communication
A simple molecular design strategy for delayed fluorescence towards 1000 nm
Daniel G. Congrave, Bluebell H Drummond, Patrick J. Conaghan, Haydn Francis, Saul
T. E. Jones, Clare P. Grey, Neil C. Greenham, Dan Credgington, and Hugo Bronstein
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Oct 2019
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A simple molecular design strategy for delayed fluorescence towards
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Daniel G. Congrave,*,§,a Bluebell H. Drummond,§,b Patrick J. Conaghanb, Haydn Francisa, Saul T. E.
Jonesb, Clare P. Greya, Neil C. Greenhamb, Dan Credgingtonb and Hugo Bronsteina,b,*
a
Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, U.K.
b Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, U.K.
Supporting Information Placeholder
emitter reported to-date exhibits a maximum peak emission
ABSTRACT: Harnessing the near-infrared (NIR) region of the
electromagnetic spectrum is exceedingly important for photovolta-
wavelength (λmax) of ca. 800 nm,8 marginally outside the visible
spectrum, and considerably below the values of ≥ 1000 nm that are
well established for 1st generation fluorescent emitters.5,7,9,10
Consequently, shifting the emission from TADF materials to
comparably low energies remains an immediate challenge if
potential applications in telecommunications, night vision, organic
photovoltaic devices (OPVs), organic lasers, photodetectors and
NIR bioimaging are to be realized.5,6,11
ics, telecommunications and the biomedical sciences. While ther-
mally activated delayed fluorescent (TADF) materials have at-
tracted much interest due to their intense luminescence and narrow
exchange energies (ΔEST), they are still greatly inferior to conven-
tional fluorescent dyes in the NIR, which precludes their applica-
tion. This is because securing a sufficiently strong donor–acceptor
(D–A) interaction for NIR emission alongside the narrow ΔEST re-
quired for TADF is highly challenging. Here, we demonstrate that
by abandoning the common polydonor model in favor of a D–A
dyad structure, a sufficiently strong D–A interaction can be ob-
tained to realize a TADF emitter capable of photoluminescence
(PL) close to 1000 nm. Electroluminescence (EL) at a peak wave-
length of 904 nm is also reported. This strategy is both conceptually
and synthetically simple and offers a new approach to the develop-
ment of future NIR TADF materials.
All-organic TADF emitters are typically assembled from
electron-rich donor (D) and electron-poor acceptor (A)
heterocycles, with twisted linkages to ensure a small exchange
energy (ΔEST). Emission occurs from an intramolecular charge
transfer (ICT) state. Since the seminal structures reported by
Adachi et al. (e.g. 4CzIPN),12,13 there has been an overwhelming
trend towards acceptor cores functionalized with multiple
peripheral donors i.e. polydonor Dn–Astructures where n > 1.2 This
also applies to the vast majority of deep-red and NIR TADF
candidates published to-date.6,14–21 For example, Jiang, Liao and
co-workers reported a D2–A acenaphthene-based emitter (APDC-
DTPA) (Figure 1) which exhibits emission at λmax = 687 nm and a
photoluminescence quantum yield (PLQY) of 63% when doped
into TBPi at 10% wt. (λmax = 756 nm, PLQY = 17% in neat film).15
Adachi et al. also studied two NIR TADF dyes based on
unconventional boron curcuminoid acceptors.8,16 The redder D4–A2
analogue (Figure 1) emits at λmax = 801 nm with a PLQY of 4% at
All-organic materials exhibiting thermally activated delayed
fluorescence (TADF) have emerged as next generation dopants in
organic light emitting devices (OLEDs).1,2 Efficient emission has
been reported throughout the visible spectrum, facilitated by
modular synthesis and rational design.3 Despite this, it has not yet
been possible to realize efficient emission in the near-infrared
(NIR) region (ca. 700–1000 nm).4–7 The longest wavelength TADF
Figure 1. Molecular structures of CAT-1 and state-of-the-art NIR TADF materials and their lowest energy reported photoluminescence λmax values.
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40 wt.% in CBP. These examples constitute the current state-of-
the-art NIR TADF emitters.
substantially stabilizes ICT to obtain unprecedentedly red-shifted
PL from a NIR TADF material.
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CAT-1 was designed using DFT/ TD-DFT (B3LYP/ 6-31G*)
(Figure 2, Figure S4, Tables S2–S8). In the optimized geometry the
phenyl spacer–A dihedral angle of 61° ensures a narrow calculated
ΔEST of 0.11 eV. Nevertheless, an oscillator strength (ƒ) of 0.1052
for the S0 → S1 excitation is maintained (Table S3). This compares
favourably with other NIR emitters that have narrow calculated
ΔEST values14,19,30 (e.g. APDC-DTPA (ΔEST = 0.10 eV, ƒ = 0.0266
– Table S4).15 We note that TD-PBE0/def2-SVP, TDA-PBE0/def2-
SVP and TDA-PBE0/def2-TZVP all also predict narrow ΔEST
values ≤ 0.15 eV (Tables S6–S8).34,35 Cyclic voltammetry (Figure
S2, Table S1) indicates that the additional CN affords a
deep14,15,28,29 LUMO energy of −4.11 eV (HOMO = −5.64 eV) and
a very narrow electrochemical bandgap (1.53 eV), highlighting the
high strength of the new electron acceptor unit.
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The photophysical properties of CAT–1 were studied in dilute
solution, doped evaporated films (with CBP as host) and in neat
films prepared by both thermal evaporation and drop-casting. λmax
and PLQY data are summarized in Table 1 (further data are
recorded in Table S10).
HOMO
LUMO
Figure 2. Numbering scheme, parameters calculated by TD-B3LYP/6-
31G* and HOMO and LUMO distributions for CAT-1.
Table 1. Summary of the PL data for CAT-1.
Sample
λmax PL /nm
PLQY[a] /%
An exceptionally stabilized ICT state is required for NIR TADF.
This is intuitively achieved through ensuring the D–A interaction
is as strong as possible, which is compromized in common
polydonor designs. This is because the multiple peripheral donors
that are typically employed in TADF materials are not directly
electronically coupled, reducing their additive effect.12,22,23 An
increase in the number of donors, while enhancing the net donor
strength, also suppresses the strength of the acceptor through
functionalizing it with an increasing number of discrete electron
donating groups, compromising any red-shift. For many red TADF
materials,19,22–25 any bathochromic shift in emission afforded by
additional donors is incremental at best compared to single-donor
D–A dyads, which now comprize a noteworthy portion of the most
efficient red examples.26,27 It was also recently demonstrated that
converting D2–A APDC-DTPA into its D–A analogue TPAAP
(Figure 1) red-shifts the PL by 21 nm to 777 nm in a neat film
(PLQY = 20%).28 Herein, we demonstrate a simple concept to
afford unprecedented progress into the NIR through adopting an
uncomplicated D–A dyad structure.
preparation
Toluene solution
770
763
820
887
950
3.9 ± 0.4
8.8 ± 0.2
1.98 ± 0.04
0.18 ± 0.04
≤ 0.18
10 wt.% in CBP
40 wt.% in CBP
Neat Evaporated
Neat Drop-Cast
[a] Absolute PLQY measured using an integrating sphere.
The absorption spectrum in toluene consists of three major bands
(Figure 3). The high energy region (≤ 380 nm) is ascribed to local
π–π* transitions, the broad low energy band centred at 560 nm is
assigned to D–A ICT, and the shoulder around 450 nm is mixed
character. This assignment is supported by TD-DFT (Figure S7).
The photoluminescence (PL) is broad and unstructured (toluene
λmax = 770 nm) and exhibits positive solvatochromism (Figure S10
and S11, Table 9), indicating emission from an ICT state. A large
red-shift in solution PL compared to APDC-DTPA15 (130 nm)
highlights the intrinsic efficacy of the design strategy in the absence
of aggregation.
The structure of the new NIR TADF emitter, CAT-1, is shown
in Figures 1 and 2. It can be easily synthesized (Scheme S1) on a
multi-gram scale for thermal evaporation (Td = 409 °C, Tg = 128
°C) (Figure S3) with minimal chromatography. The removal of an
electron donor in APDC-DTPA to afford TPAAP facilitates an
incremental 21 nm red-shift in neat film emission.28 In CAT-1 we
take greater advantage of the D–A dyad structure by replacing the
second electron donor with a more beneficial electron withdrawing
CN group.29–33 The additional CN group is both proximal to the
single electron donor and directly conjugated to it via the pyrazine
moiety, which may contribute to the greatly increased acceptor
When doped into CBP at 10 wt.%, CAT-1 exhibits low-energy
emission with λmax = 763 nm and a PLQY of 8.8 ± 0.2% (Figure 3).
As well as prompt fluorescence (τ < 5 ns), a delayed component (τ
= 80 μs, 13.5% contribution) to the PL is also observed (Figure 3,
Figure S13). The delayed PL is spectrally similar to the prompt
(Figure S14) and is thus assigned to delayed fluorescence from the
same state.
strength (Figure S1).
This simple and rational addition
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Absorption
Photoluminescence
10 wt.% in CBP
40 wt.% in CBP
Neat film evaporated
Neat film drop cast
10 wt.% in CBP
40 wt.% in CBP
IRF from laser scatter
b)
c) 106
105
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Time (ns)
400
600
800
1000
600
700
800
900
1000
Wavelength (nm)
Wavelength (nm)
Figure 3. a) Normalized absorption and PL spectra for CAT-1 in toluene. λex = 520 nm. b) Normalized Steady state PL spectra for CAT-1 in doped and neat
films. λex = 520 nm. Spectra are reported to 1000 nm due to the low detector sensitivity beyond 1000 nm. c) Time-resolved PL intensity of CAT-1 doped into
CBP at different weight ratios recorded in 10-3 mbar vacuum. Instrument response function (IRF) plotted in blue. λex = 400 nm.
The intensity of the delayed fluorescence increases with
temperature between 10–292 K, confirming a TADF mechanism
(Figure S17). It is also significantly quenched in the presence of
oxygen (Figure S13), indicating a contribution from triplet states,
as expected. Furthermore, a narrow ΔEST of ca. 0.04 eV was
estimated experimentally from the onsets of the low temperature
fluorescence and phosphorescence spectra of CAT-1 in CBP at 10
wt.% (Figure S19).
In summary, strategic replacement of the second electron donor
of APDC-DTPA with a CN group in CAT-1 affords large batho-
chromic shifts in PL – 130 nm in toluene, 76 nm in 10 wt.% doped
film and 131 nm in evaporated film. This is considerably greater
than the evaporated film red-shift of 21 nm obtained upon sole re-
moval of an electron donor (TPAAP) (Figure S20).15,28 Notably,
when doped into CBP at 10 wt.% CAT-1 also emits with a longer
wavelength PL λmax (763 nm) than APDC-DTPA in a neat evapo-
rated film (756 nm). While the low PLQY precludes unambiguous
confirmation of TADF in the drop-cast film, the PL λmax of 950 nm
is red-shifted by 149 nm compared to the longest wavelength PL
λmax reported thus far for a TADF-capable emitter (curcuminoid,
Figure 1, Table S11).8 Furthermore, CAT-1 can display an
emission spectrum for which the entirety of the PL falls > 700 nm,
which is considered “real” NIR.7,11 In fact, the drop-cast film has
Increasing the doping ratio of CAT-1 in the evaporated films
leads to significant bathochromic shifts in emission (Figure 3,
Table 1, Table S10). This is common for highly polar TADF
materials and is attributed to the effects of solvatochromism and
aggregation.7,28,36 This is also accompanied by a sequential
decrease in PLQY,16,28 indicating a trade-off between PL
wavelength and quantum efficiency when exploiting aggregation
to red-shift emission. When CAT-1 is doped into CBP at 40 wt.%,
emission is recorded at λmax = 820 nm with a TADF component still
present (τ = 8 μs, 5.1% contribution) (Figures S13 and S18). This
is lower in energy than for the D4–A2 curcuminoid derivative
reported by Adachi et al. (λmax = 801 nm) (Figure 1),8 although our
PLQY is lower (2% vs. 4%). Unlike the curcuminoid the molecular
weight of CAT-1 is also sufficiently low for desirable thermal
evaporation.14,15,29 To the best of our knowledge this is the lowest
energy TADF confirmed by variable temperature time resolved
measurements to-date (Table S11).
1.0
0.8
0.6
0.4
0.2
0
In evaporated films of CAT-1 PL is recorded with λmax = 887 nm
(Figure 3). While the signal-to-noise ratio is poor due to the low
PLQY, the longer lifetime component of the PL decay also appears
to be quenched in the prescence of oxygen (Figures S15 and S16).
Emission from films drop-cast from chlorobenzene solution is
further reproducibly red-shifted to λmax = 950 nm with a sharpening
of the onset (Figure 3) and an associated red-shift of the absorption
spectrum (Figure S9). A similar PL red-shift upon solution
processing was recently reported for a red TADF emitter.21,37
Analysis of the drop-cast film by thin film X-ray diffraction
indicates a degree of solid state ordering consistent with enhanced
aggregation (Figures S26 and S27). The large PL red-shift in the
drop-cast films is attributed to this.28 No such crystallinity could be
observed for the evaporated film, although this may be related to
the low film thickness (Figure S28). While reabsorption in the
comparatively thick drop-cast films may also influence this red-
shift in PL, such a dependence of the emission color on the
processing method (e.g. thermal evaporation or solution
processing) is well recognized for TADF materials.38–41
700
800
900
1000
Wavelength (nm)
an impressive PL onset of ca. 800 nm, which is exceptional for a
material capable of TADF.11
Figure 4. EL spectrum for an undoped CAT-1 OLED at 5 V. λmax = 904
nm.
The electroluminescence (EL) performance of CAT-1 was also
evaluated in preliminary thermally evaporated OLED devices (Fig-
ures S21–S25). Crucially, we note that undoped devices exhibit
complete NIR (> 700 nm) emission with an EL λmax of 904 nm
(Figure 4). This red-shift in the undoped EL compared to the neat
evaporated film PL (904 nm vs. 887 nm) may be because excitons
are formed via a diffusional process in the device rather than direct
photo-excitation. Optical interference is another possible explana-
tion. While the maximum external quantum efficiency (ca.
0.019%) and radiance (ca. 1 × 103 mW sr−1 m−2) are low, the peak
EL λmax is much lower in energy than for any previously reported
TADF material (> 100 nm), while being competitive with conven-
tional fluorescent devices and red-shifted compared to almost all
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phosphorescent examples.11 These data importantly confirm that
the favourable long-wavelength PL of a neat film of CAT-1 can be
retained in an EL device.
Emitting Diodes with Doublet Emission. Nature 2018, 563, 536–
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Near-Infrared Electroluminescence and Low Threshold
In conclusion, this work provides a great advance into the NIR
from a delayed fluorescent material. At the high donor and acceptor
strengths required for NIR TADF, the incorporation of multiple do-
nors is essentially ineffectual at red-shifting emission.19,22–25,28 In
contrast we have shown that adopting an intrinsically simple D–A
system structurally liberates molecules for rational functionaliza-
tion to greatly stabilize the ICT state. We have succeeded in obtain-
ing a dramatic red-shift in PL λmax of over 100 nm from a material
capable of TADF compared to previous works (Figure 1). This fa-
cilitated fabrication of OLEDs displaying impressive EL λmax val-
ues of > 900 nm. The next step is clearly to improve the PLQY in
this newly accessed low energy region. This may be possible
through incorporation of a more rigid electron donor capable of
suppressing aggregation-induced quenching. Nevertheless, with re-
spect to emission wavelength, CAT-1 affords rapid and important
progress. These unprecedented results crucially stem from a design
strategy that is conceptually simple and easy to implement syntheti-
cally. It will be broadly applicable to other systems and heavily in-
fluence future NIR TADF design.
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Supporting Information
NMR spectra, electrochemical data, thermogravimetric analysis,
computational data, additional photophysical and device data, and
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thin
film
X-ray
diffraction
data
(PDF)
AUTHOR INFORMATION
Corresponding Authors
dc704@cam.ac.uk
hab60@cam.ac.uk
Author Contributions
§These authors contributed equally to the experimental work.
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Notes
The authors declare no competing financial interests.
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Yuan, Y.; Hu, Y.; Zhang, Y. X.; Lin, J. D.; Wang, Y. K.; Jiang,
Z. Q.; Liao, L. S.; Lee, S. T. Over 10% EQE Near-Infrared
Electroluminescence Based on a Thermally Activated Delayed
Fluorescence Emitter. Adv. Funct. Mater. 2017, 27, 1700986.
Kim, D.-H.; D’Aléo, A.; Chen, X.-K.; Sandanayaka, A. D. S.;
Yao, D.; Zhao, L.; Komino, T.; Zaborova, E.; Canard, G.;
Tsuchiya, Y.; Choi, E.; Wu, J. W.; Fages, F.; Brédas, J.-L.;
Ribierre, J.-C.; Adachi, C. High-Efficiency Electroluminescence
ACKNOWLEDGMENT
Prof. Martin R. Bryce is aknowledged for use of his potentiostat at
Durham University UK. This work was supported by the Engineer-
ing and Physical Sciences Research Council (grant no.
EP/M005143/1 and EP/S003126/1) and the European Research
Council (ERC). D.C. and S.T.E.J. acknowledge support from the
Royal Society (grant nos. UF130278 and RG140472). B.H.D.
acknowledges support from the EPSRC Cambridge NanoDTC
(grant no. EP/L015978/1).
and Amplified Spontaneous Emission from
a Thermally
Activated Delayed Fluorescent Near-Infrared Emitter. Nat.
Photonics 2018, 12, 98–104.
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Sun, K.; Chu, D.; Cui, Y.; Tian, W.; Sun, Y.; Jiang, W. Near-
Infrared Thermally Activated Delayed Fluorescent Dendrimers
for the Efficient Non-Doped Solution-Processed Organic Light-
Emitting Diodes. Org. Electron. physics, Mater. Appl. 2017, 48,
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Ni, F.; Wu, Z.; Zhu, Z.; Chen, T.; Wu, K.; Zhong, C.; An, K.;
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