Thermally Activated Delayed Fluorescent Donor-Acceptor-Donor-Acceptor π-Conjugated Macrocycle for Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/jacs.9b11578

A new class of thermally activated delayed fluorescent donor-acceptor-donor-acceptor (D-A-D-A) π-conjugated macrocycle comprised of two U-shaped electron-acceptors (dibenzo[a,j]phenazine) and two electron-donors (N,N′-diphenyl-p-phenyelendiamine) has been

Full text of DOI:10.1021/jacs.9b11578

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Article
Thermally Activated Delayed Fluorescent Donor–Acceptor–Donor–
Acceptor #-Conjugated Macrocycle for Organic Light-Emitting Diodes
Saika Izumi, Heather F. Higginbotham, Aleksandra Nyga, Patrycja Stachelek, Norimitsu
Tohnai, Piotr de Silva, Przemyslaw Data, Youhei Takeda, and Satoshi Minakata
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Jan 2020
Downloaded from pubs.acs.org on January 2, 2020
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Thermally Activated Delayed Fluorescent Donor–Acceptor–Donor–
p
Acceptor -Conjugated Macrocycle for Organic Light-Emitting Di-
odes
Saika Izumi,† Heather F. Higginbotham,‡ Aleksandra Nyga,∥ Patrycja Stachelek,§ Norimitsu Tohnai,#
⊥
∇
Piotr de Silva, Przemyslaw Data,*,§, , Youhei Takeda,*,† and Satoshi Minakata*,†
∥
†Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-
0871, Japan
‡School of Chemistry, Monash University, Clayton Victoria 3800, Australia
∥Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland
§Durham University, Physics Department, South Road, Durham DH1 3LE, United Kingdom
#Department of Material and Life Science, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka
565-0871, Japan
⊥Department of Energy Conversion and Storage, Technical University of Denmark, Anker Engelunds Vej 301, 2800
Kongens Lyngby, Denmark
∇Center of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland
ABSTRACT: A new class of thermally activated delayed fluorescent donor–acceptor–donor–acceptor (D–A–D–A) p-conjugated
macrocycle comprised of two U-shaped electron-acceptors (dibenzo[a,j]phenazine) and two electron-donors (N,N’-diphenyl-p-
phenyelendiamine) has been rationally designed and successfully synthesized. The macrocyclic compound displayed polymorphs-
dependent conformations and emission properties. Comparative studies on physicochemical properties of the macrocycle with a
linear surrogate has revealed significant effects of the structural cyclization of D–A-repeating structure, including more efficient
thermally activated delayed fluorescence (TADF). Furthermore, an organic light-emitting diode (OLED) device fabricated with the
macrocycle compound as the emitter has achieved a high external quantum efficiency (EQE) up to 11.6%, far exceeding the theo-
retical maximum (5%) of conventional fluorescent emitters and that with linear analog (6.9%).
In sharp contrast, p-conjugated macrocycles comprised of
p-electronic donors (D) and acceptors (A) (i.e., D–A-embedded
INTRODUCTION
p-conjugated macrocycles) have been less developed, and there-
fore their optoelectronic properties have been less investigated, 8
although the incorporation of D and A moieties in p-conjugated
main frameworks is a powerful strategy for tuning photophysi-
cal and redox properties of linear p-conjugated oligomers and
polymers in material science. In 2012, Jäkle developed ambipo-
lar B–p–N macrocycles, uncovering unique Lewis-base-
responsive photoluminescent behavior.9 In 2015, Jasti10a and
Itami10b independently developed an electron-deficient arene-
incorportated cycloparaphenylenes (CPPs), which are featured
with the narrower HOMO/LUMO band gaps than those with the
same ring-size all-carbon CPPs. The Nuckolls group has inten-
sively developed D–A-repeating carbon nanohoops comprised
of multiple D–A units (D = bithiophene derivatives; A =
perylene diimides).11 Notably, the validity of the D–A-
repeating p-conjugated macrocycles as functional materials has
Organic p-conjugated oligomers and polymers play crucial roles
in materials science, finding a number of applications such as
chemical sensors,1 bio-imaging,2 organic field-effect transistors
(OFETs),3 organic photovoltaics (OPVs),4 and organic light-
emitting diodes (OLEDs).5 Their photophysical and redox prop-
erties are finely tunable through exquisite modifications of
structure, electronic bias, and conjugation length of p-
conjugated systems, in accordance with the intended function.
The topological aspect also significantly matters in their func-
tions. Organic p-conjugated macrocycles, which are regarded as
terminal-less counterparts of linear p-conjugated oligomers and
polymers, have emerged as unique organic functional materi-
als.6,7 They can display not only unique photophysical and redox
properties but also highly-ordered 2D- and 3D-aligned molecu-
lar assemblies otherwise difficult to achieve with linear p-
conjugate systems, arising from their specific structural motifs
such as bent, curved, and/or twisted p-systems as well as well-
defined cavities.6 To date, a great number of structurally well-
defined p-conjugated macrocycles, particularly, hydrocarbon-
based p-conjugated macrocycles assembled from (het-
ero)arylenes, ethynylenes, and vinylenes, have been developed,
and their optoelectronic properties have been intensively studied
in the context of partial motifs of carbon materials.7
been demonstrated by applying the macrocycles to OFETs,11e
a
bulk heterojunction OPVs,11b and organic capsule transistors for
sensing chemicals.11d Therefore, the development of novel D–A
p-conjugated macrocycles and the exploration of their potency
as optoelectronic materials would offer significant opportunities
for tailoring optoelectronic properties by fluctuating three-
dimensional shapes (e.g., bent, curved, and twisted confor-
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mations) and by utilizing exterior/interior p-p interactions
and/or host-guest interactions otherwise difficult to realize with
flat and fully-p-conjugated systems. However, it should be stat-
ed that the synthesis of new p-conjugated macrocycles still re-
mains a challenge in itself, mainly due to limited synthetic strat-
egies and building blocks suitable for p-conjugated macrocycles.
The rational design of D–A p-conjugated macrocycles that sim-
ultaneously satisfy requirements for desired functions and syn-
thetic feasibility is to be devised.
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Thermally activated delay fluorescence (TADF) is a
unique photophysical phenomena and promising for enhancing
external quantum efficiency (EQE) of organic light-emitting
diodes (OLEDs), because TADF-active organic emitters can
theoretically harvest 100% excitons generated in the emitting
layer by electrical excitation and convert into light through effi-
cient reverse intersystem crossing (rISC).12 The key issues in
designing efficient TADF emitters include how to accelerate
spin-forbidden and endothermic rISC process (T1→S1). D–A p-
conjugated systems with a large D–A dihedral angle and with a
vibrational motion can provide a solution, minimizing singlet-
triplet energy gap (DEST) to lower activation energy for rISC
and mixing excited CT and LE states to allow spin-flip electron-
ic transitions.12c Therefore, a myriad of TADF organic materials
have been developed based on linear or branched D–A scaf-
folds.12 In sharp contrast, macrocyclic TADF materials have
been rarely explored.13–15 In 2009, Adachi realized the first
TADF-OLEDs using Sn4+-porphyrin complexes as the emitter,
which also represents the first example of macrocyclic organic
TADF material.13 However, the emitters contain environmental-
ly unbenign metal ion, and the TADF emission is very faint. In
2014, Kanbara revealed the delayed fluorescence behavior of
azacalix[n](2,6)pyridines (n = 3, 4), which represents the first
example of D–A-repeating organic TADF macrocycles.14 Yet,
the rather large DEST (calc.) values (>600 meV) of the azaca-
lixpyridines suggested that a harsh thermal reverse internal con-
version (RIC) from T1 to Tn is required for rISC process. More
recently, the Su and Huang developed a pure organic deep-blue
TADF emitting compound comprising triarylamines bridged
with electron-withdrawing SO2 group.15 In addition to the scar-
city of macrocyclic TADF-active compounds, to the best of our
knowledge, the EQEs of OLEDs fabricated with any macrocy-
clic TADF emitters have never been clarified. Furthermore, the
influence of the macrocyclization of linear D–A-repeating p-
conjugated systems on their physicochemical properties, espe-
cially TADF behavior, has remained an open question.
Figure 1. Designed (a) macrocycle 1 and (b) linear analog 2.
RESULTS and DISCUSSION
Design and Synthesis. We designed D–A–D–A macrocycle 1,
comprising two p-phenylenediamine derivative as the donors
and two dibenzo[a,j]phenazine (DBPHZ)17 as the acceptors, as
TADF p-conjugated macrocyclic compound (Figure 1a). As our
research program of developing multi-photofunctional organic
materials, we have demonstrated that DBPHZ-cored twisted
donor–acceptor–donor (D–A–D) scaffold holds great promise
for realizing efficient thermally activated delayed fluorescence
(TADF),18 multi-color-changing mechanochromic luminescence
(MCL),18b,d and room temperature phosphorescence (RTP).18c,d
In particular, it is noted that the DBPHZ core (Figure 1a) plays
an important role in TADF and RTP functions: the lowest triplet
excited state (T1) of the D–A–D family is exclusively localized
on the DBPHZ acceptor unit (3LEA), which allows narrowing
1
the DEST of the molecules by adjusting the CT energy levels
though fluctuation of electronic bias and D–A dihedral angles.
On one hand, the U-shaped structure of DBPHZ can be suitable
for the formation of macrocyclic structure. Therefore, we envis-
aged that a twisted D–A–D–A macrocycle composed of two
DBPHZs (A) and two bridging electron donors (D) should serve
as an efficient TADF p-conjugated macrocyclic material (Figure
1a). From the viewpoint of synthetic feasibility, appropriate
selection of bridging donors would be highly important. The
structural pre-organization required for macrocyclization is
highly dependent on the conformation and configuration of the
precursor/intermediate. Otherwise, undesired intermolecular
oligomerization/polymerization over desired intramolecular
cyclization can occur.19 Since triarylamines take propeller-like
geometries,20 we envisioned that the incorporation of N,N,N’,N’-
tetraphenylene-1,4-diamine (TPDA, Figure 1a) motif into mac-
rocyclic structure would allow for regulating conformational
flexibility of a precursor for macrocyclization (corresponding to
6 in Scheme 1) and facilitating the cyclization process. Also, the
bridging with a heteroatom (N) would allow for twisting of p-
conjugated panels and alleviating ring strain in forming macro-
cycle 1. To investigate the effect of macrocyclization of a D–A–
D–A scaffold on is physicochemical properties, a linear analog
2 (i.e., the open form of 1) was also designed (Figure 1b).
Herein we disclose the development of a purely organic
macrocyclic D–A–D–A p-conjugated macrocycle 1 (Figure 1a),
which displays efficient TADF emissions. From the comparison
of its physicochemical properties with those of linear analog 2
(Figure 1b), the macrocyclization effect on its physicochemical
properties was revealed. Most importantly, the OLEDs fabricat-
ed with the developed macrocyclic compound 1 as a TADF
emitter achieved as high EQE as 11.6%, exceeding the theoreti-
cal maximum (5%)16 of that utilizing conventional fluorescent
emitters and that with linear material 2 (6.8%).
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Scheme 1. Divergent synthetic routes to 1 and 2.
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Reagents and conditions: (a) Pd2(dba)3 (1 mol%), QPhos (4 mol%), NaOt-Bu (2.2 equiv), toluene, 60 °C, 12 h. (b) TFA (excess), CH2Cl2,
r.t., 40 min. (c) 4 (1.0 equiv), Pd2(dba)3 (5 mol%), QPhos (20 mol%), K2CO3 (2.2 equiv), 1,4-dioxane, 100 °C, 24 h. (d) TFA (8.0 equiv),
CH2Cl2, r.t., 30 min. (e) PhBr (1.0 equiv), Pd[P(t-Bu)3]2 (10 mol%), K2CO3 (3.0 equiv), 1,4-dioxane, 100 °C, 24 h. (f) TFA (excess),
CH2Cl2, r.t., 30 min. (g) dibenzo[a,j]phenazin-3-yl trifluoromethanesulfonate (1.5 equiv), Pd2(dba)3 (5 mol%), SPhos (15 mol%), K2CO3
(1.5 equiv), 1,4-dioxane, 100 °C, 24 h.
Macrocycle 1 and its linear analog 2 were successfully
synthesized in a divergent fashion by applying common inter-
mediate 5 (Scheme 1, for the detailed synthetic conditions and
procedures, see the Supporting Information). Starting from a
commercially available N-phenyl-p-phenylenediamine, donor 3
was prepared in 65% yield (in 4 steps), by modifying synthetic
methods for oligo(p-aniline) compounds (for the details, see the
SI).21 The donor 3 was attached to the DBPHZ acceptor through
a double Pd-catalyzed Buchwald-Hartwig amination with 3,11-
dibromodibenzophenazine 4 using a bulky phosphine ligand
(Qphos)22 to give the common intermediate 5 in a high yield.
The deprotection of the N-Boc groups of 5 with an excess
amount of trifluoroacetic acid (TFA) quantitatively afforded
macrocycle precursor 6. Notably, macrocyclization was success-
fully achieved through a Pd-catalyzed Buchwald-Hartwig dou-
ble amination of 4 with the D–A–D precursor 6 in 45%, which
represents a relatively high yield in cyclization of aromatic
components. On one hand, mono-deprotection of the N-Boc
groups of 5 with 8 equivalents of TFA gave intermediate 7,
which was then N-phenylated with bromobenzene to provide 8
in a good yield. The deprotection of the remaining N-Boc group
of 8 followed by the Buchwald-Hartwig amination with a
DBPHZ having an OTf group at the 3-position successfully
completed the synthesis of linear analog 2. All the newly syn-
thesized compounds were fully characterized by spectroscopic
and packing structures (Figure 2), which would give significant
influences on photophysical properties.23 The polymorph 1-O
crystallized in the triclinic space group P1 and exhibited weak
orange emission (lmax = 594 nm, FPL = 0.01, for the PL spectra,
see the Figure S7a). In the crystal, the macrocycle takes a highly
symmetric structure with the symmetric center (i) (Figure 2a).
As designed, the donor units take propeller-structure, with the
twisting angles between the phenylene and external N-phenyl
planes being 85.6° and 88.9° (Figure 2b). Also, the phenylene
planes of the donors are twisted against the DBPHZ acceptor
unit, with the dihedral angle between the phenylene plane and
the terminal benzene unit of the DBPHZ being 56.9° and 62.8°.
More interestingly, both DBPHZ units take helically twisted
structure, with the dihedral angle between the terminal fused
benzene rings being 17.4° (Figure 2b), where the central phena-
zine unit is nearly flat (deviation angle: 5.9°). The twisting of
the DBPHZ would indicate a large strain accommodated into the
molecule by the macrocyclization. In the crystal polymorph 1-O,
the macrocycle molecules align along the c axis, and the highly
organized assemblies form porous columns with the cavity di-
ameter of 6.58 Å, where chloroform molecules are trapped with-
in the cavity (Figure 2c). On close inspection of the columns,
the benzo[f]quinoxaline moieties of the DBPHZ unit are contig-
uously stacked in a face-to-face manner with a very close inter-
plane distance (3.39 Å), indicating the operation of a strong
electronic interaction between the molecules through p-orbitals.
1
data (e.g., H & 13C NMR, IR, MS, and HRMS; for the details,
see the SI).
The polymorph 1-R formed a monoclinic system with the
space group P21/m and exhibited red emission (lmax = 654 nm,
FPL = 0.01, for the PL spectra, see the Figure S7a). Most im-
portantly, the macrocycle takes a saddle-shaped conformation
(Figure 2d and e). The phenylene donor units once again take
propeller shape (the twisting angles between the phenylene and
external N-phenyl planes range from 72.8° to 82.3°), and the
phenylene unit is more twisted against the acceptor than poly-
morph 1-O (the dihedral angle between the phenylene plane and
Polymorphism and Single Crystal X-ray Crystallographic
Analyses. The macrocycle 1 formed two polymorphs depending
on recrystallization conditions: Orange prism crystals (denoted
as ”polymorph 1-O”) were grown from a n-hexane/CHCl3 bi-
phasic solution through a liquid-liquid diffusion technique,
while deep-red prism crystals (denoted as ”polymorph 1-R”)
formed through slow evaporation of a CHCl3 solution of 1 over
2 weeks. Importantly, the X-ray crystallographic analyses of the
single crystals revealed the differences in molecular geometries
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the terminal benzene unit of the DBPHZ: 66.6°). In sharp con-
DBPHZ plane: 2.98–3.18 Å, Figure 2f), suggesting strong in-
termolecular charge-transfer interactions between the two mole-
cules. This would be in consistent with a drastic change in crys-
tal color and emission wavelength from polymorph 1-O. Single
crystals of linear compound 2 suitable for the X-ray single-
crystal analysis were not obtained regardless of a number of
attempts, implying more flexibility of the conformations than
cyclic compound 1. The solids of linear compound 2 displayed
an orange PL (lmax = 605 nm, FPL = 0.08, for the PL spectra,
see the Figure S7b in the SI) with a slightly higher PLQY than
those of polymorphs 1-R and 1-O. This would indicate that the
strong electronic interactions of macrocycle quench PL in the
crystals.
trast to polymorph 1-O, the DBPHZ units of the macrocycle in
polymorph 1-R is planar (Figure 2e). The less distortion of the
DBPHZ units indicated that the conformation in polymorph 1-R
is thermodynamically more stable than that in polymorph 1-O,
which was indeed supported by theoretical calculation by the
DFT method (see the following section and the SI). Each porous
column aligned along the a axis in polymorph 1-R are inde-
pendent from each other, with voids between each column being
filled with chloroform molecules (Figure 2f). Interestingly, each
column is formed by the assembly of dimeric pairs of the mac-
rocycle, where four D•••A pairs are stacked with a very close
distance (the interplane distances between the phenylene and the
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Figure 2. ORTEP drawings of 1 in (a)–(c) polymorph 1-O and (d)–(f) polymorph 1-R: (a), (d) molecular structures; (b), (e) side
views; (c), (f) packing structures (thermal ellipsoids are set at the 50% probability level). The hydrogen atom and partial crystal
solvent CHCl3 were omitted for clarity.
Steady-State Absorption and Photoluminescence Properties
in Solutions. To reveal the photophysical properties of 1 and 2
in solution, steady-state UV-vis absorption and photolumines-
cence spectra of their diluted solutions (cyclohexane, toluene,
THF, CH2Cl2 and CHCl3) were investigated (Figure 3, for the
summarized data, see the Table 1). As a whole, the line shapes,
the maximum absorption wavelengths (labs), and molar absorp-
tion coefficients (e) of the UV-vis absorption spectra of diluted
solutions of 1 were not significantly affected by the difference
in polarity of solvents used (Figure 3a). However, on close in-
spection of the absorption onsets of solutions of 1, a slight red-
shift as a function of solvent polarity was observed (Figure 3a).
In conjunction with a relatively large e value (ca. 50,000 M–
1cm–1) of the absorption peaked at around 470 nm and the ab-
sence of the corresponding peak in the absorption spectra of
DBPHZ17 and TPDA24 in the region (400–550 nm), the elec-
tronic transition observed at around 470 nm has the mixed char-
acter of intramolecular charge-transfer (ICT) and p-p* transition,
or hybrid CT (Figure 3). This was also supported by the TD-
DFT calculations (vide infra). The diluted solutions of linear
compound 2 also displayed similar absorption spectra with an
ICT peak (labs 477–485 nm) and the p-p* transition (labs 332–
337 nm) ascribed to the TPDA unit with a slightly smaller
e with a broader band than that of 1 (Figure 3b), which could
indicate the fluctuation of the conformers.
Upon the irradiation of UV light, the cyclohexane solution
of 1 displayed bright green emission from the singlet charge-
transfer excited state (1CT) state (lem 540 nm) with a moderate
photoluminescence quantum yield (FPL 0.31) (Figure 3a). In a
slightly polar solvent (toluene), the macrocycle exhibited orange
1
emission (lem 595 nm, FPL 0.28) from the more stabilized CT
state. In the case of more polar solvents (e.g. THF, CH2Cl2, and
CHCl3), no emission was observed, indicating strong ICT nature
in the excited states of the macrocycle.18 The cyclohexane solu-
tion of 2 displayed a very similar green emission (lem 534 nm)
with that of 1, but with a higher FPL (0.53) (Figure 3b and Table
1). Notably, the emission in toluene showed a red emission (lem
615 nm) with a lower FPL (0.20) (Figure 3b and Table 1). It
should be noted that the larger red-shift in emission (lem 2466
cm–1 for 2 vs 1712 cm–1 for 1) would indicate much more flexi-
bility of conformation of 2 in the excited states, probably due to
free D–A rotation that should lead to the dissipation of the ex-
cited energies through thermal processes.25
Since TADF is irradiated by way of the excited triplet
states, the intensity of the TADF is very sensitive toward the
presence of oxygen gas (O2), which can efficiently quench the
excited triplet states through non-irradiative pathways. To inves-
tigate how much TADF contributes to the PL of 1 and 2 in solu-
tion, the steady-state PL spectra of toluene solutions of 1 and 2
were measured in the presence and absence of O2 (Figure 3c and
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d). Upon degassing, a significant increase (66%) in emission
excursion to the triplet state, respectively, the number expressed
by the following equation {(DF/PF)–1}×100 (%) would indicate
the % of the harvested triplet in a form of DF. Given the DF/PF
ratio of 1 and 2, macrocyclic compound 1 is much more promis-
ing TADF material than 2, which was supported by the time-
resolved spectroscopic analyses and OLED performances (vide
infra).
intensity was observed for macrocycle 1, when compared to an
aerated condition (Figure 3c). In contrast, linear analog 2
showed only a 24% increase in emission intensity (Figure 3d).
Assuming that the outputs observed under aerated and degassed
conditions can be related to the prompt fluorescence (PF) and
the sum of the PF and the delayed fluorescence (DF) after an
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Table 1. Steady-state photophysical data of diluted solutions of 1 and 2.
solventa
b
compound
labs (nm)
l
em (nm)
FPL
1
cyclohexanec
toluene
THF
268, 338, 370, 476
283, 336, 367, 473
280, 335, 366, 474
281, 335, 368, 476
281, 339, 369, 478
286, 334, 475
540
595
ND
ND
ND
534
615
ND
ND
ND
0.31
0.28
<0.01
<0.01
<0.01
0.53
CH2Cl2
CHCl3
cyclohexanec
toluene
THF
2
289, 336, 479
0.20
286, 332, 477
<0.01
<0.01
<0.01
CH2Cl2
CHCl3
288, 334, 479
288, 337, 485
aConcentration: 10–5 M. bDetermined with an integrated sphere. cSaturated solution was used, due to the low solubility in cyclo-
hexane (concentration < 10–6 M).
vs Fc/Fc+) and a two-step oxidation process (ox1E1/2 = +0.27 V for
1; +0.22 V for 2 vs Fc/Fc+, ox2E1/2 = +0.69 V for 1; +0.63 V for 2
vs Fc/Fc+) (Figure 4), indicating their high electrochemical stabili-
ties suitable for carrier injection/transportation as optoelectronic
materials. The ionization potential (IP)/electron affinity (EA) of 1
and 2 determined by the CV experiment are 5.37 eV/3.22 eV and
5.32 eV/3.21 eV, respectively. The redE1/2 potentials (vs Fc/Fc+) of
1 and 2 are very close to that of DBPHZ (–1.88 V),17 while the
ox1
E
of 1 and 2 are slightly negatively shifted against that of
1/2
TPDA (+0.18 V).26 This negative shift of ox1E1/2 would be ration-
alized by the existence of electron-withdrawing aryl moieties
(DBPHZ) on the donor units.27 Therefore, these data would sug-
gest the localization of the HOMO and LUMO orbitals of both
compounds on the donors and acceptors, respectively, which is in
good agreement of the theoretical calculation (Figure 5, vide infra).
Figure 3. Steady-state UV-vis absorption (Abs) and photolu-
minescence (PL) spectra of dilute solutions (purple: cyclohex-
ane; sky blue: toluene; green: THF; orange: CH2Cl2; red:
CHCl3) of (a) macrocycle 1 and (b) linear analog 2 (lex = 340
nm, concentrations: 10–6–10–5 M). Absorption spectra of cy-
clohexane solutions of 1 and 2 are not shown, due to the uncer-
tainty of the exact concentration. PL spectra of the toluene
solutions (5 µM) of (c) 1 and (d) 2 with aerated (black lines)
and degassed (red lines) (lex = 470 nm).
Figure 4. Cyclic voltammograms of (a) macrocycle 1 (5×10–4
M) and (b) linear compound 2 (1×10–3 M) in CH2Cl2 contain-
ing 0.1 M Bu4NBF4. Electrodes: working (Pt), counter (Pt
wire), reference (Ag/AgCl). Scan rate: 50 mV s–1.
Electrochemical Properties. The electrochemical properties of
macrocycle 1 and linear analog 2 were investigated with cyclic
voltammetry (CV) (Figure 4). Both compounds exhibited a re-
versible one-step reduction (redE1/2 = –1.88 V for 1; –1.89 V for 2
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ters.28 The DEST values calculated using the optimized ground
state structures for helical and saddle conformers (Table S4 and
S5 in the SI) were found to be 0.55 eV and 0.50 eV, respectively.
These values are relatively large when compared to the experi-
mental gaps obtained with time-resolved emission spectroscopy
(vide infra), which suggests that the excited state relaxation plays
an important role in the emission process.
Theoretical Calculation. To obtain insights into the confor-
mations and electronic structure of the macrocycle and linear ana-
log, theoretical calculations using the DFT method were per-
formed (Figure 5; for the details, see the SI). The structural opti-
mization of macrocycle 1 was conducted with the geometries
obtained from the X-ray crystallographic analyses (for the details,
see the SI). The comparison of the energies of helical-type con-
former (denoted as “helical”) related to that found in the poly-
morph 1-O (Figure 2a–c) and saddle-type conformer (denoted as
“saddle”) related to that found in the polymorph 1-R (Figure 2d–
f), revealed that there is a large energy difference between these
conformers (helical has approximately 5 kcal/mol higher energy
than saddle. For detailed values, see Table S3 in the SI). This
suggests that the >99% of the macrocycle 1 takes the saddle-like
conformation as the single molecule. This thermodynamic prefer-
ence of the saddle conformer would rationally explain the exper-
imental observation that polymorph 1-O (helical conformer) was
obtained under kinetic crystallization conditions, while polymorph
1-R (saddle conformer) was formed under thermodynamic condi-
tions, although packing effect cannot be totally excluded. For both
conformers of the macrocycle, the HOMO and HOMO–1 orbitals
are delocalized over the entire ring, whereas the LUMO and
LUMO+1 orbitals are mostly on the acceptors (Figure 5a). The
localization of unoccupied frontier orbitals is more pronounced for
the saddle conformer. For the linear analogue 2, the conforma-
tional search found that the most stable conformer takes a twisted
geometry (Figure 5b). The occupied frontier orbitals are rather
delocalized; however, most of the amplitude is clearly localized
on the donor. The unoccupied frontier orbitals are mostly local-
ized on the acceptor units, similar as for the macrocycles.
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Figure 5. An illustrative summary of the theoretical calcula-
tions for (a) helical and saddle-shaped conformers of 1 and (b)
2 at the w*PBE/cc-pVDZ level. The first 3 vertical electronic
transitions are shown in red, green, and blue arrows.
Time-Resolved Spectroscopic Analysis in Host Matrices. To
investigate the TADF properties of compounds 1 and 2, time-
resolved luminescence spectroscopic analyses of both compounds
in a non-polar Zeonex® and 4,4’-bis(carbazole-9-yl)biphenyl
(CBP) host matrices were performed (Figure 6 and 7). In both host
matrices, both compounds displayed emissions in two distinct
time regions (Figure 7). The first component, decaying with a
lifetime in the nanosecond time regime in all the materials, is at-
tributed to prompt fluorescence (PF) from the singlet excited state
(S1), due to the independent emission profiles on temperatures
(Figure 6 and 7). In both cases (1 and 2), the spectra at the time
delay (TD) of 15 ns in both Zeonex® and CBP showed single
Gaussian-type spectra ascribed to charge transfer (1CT) emission
that decays over longer times (black lines, Figure 6). At a longer
delay time in the micro- and millisecond time regions, delayed
emissions were observed (Figure 7). Depending on the experi-
mental temperature, both the singlet state delayed emission and
triplet state emission were observed on similar millisecond time-
scales. Therefore, emission from each state is most easily eluci-
dated upon spectral inspection at different temperatures. At room
temperature, the delayed emission was observed with the same
spectral shape and onset energy as the prompt 1CT spectra (red
lines, Figure 6), which was identified as TADF. This assignment
as TADF was also confirmed with the linear power dependence of
the integrated emission intensity in the millisecond region of 1 and
2 on laser pulse fluence (Figure S10 and S11 in the SI).
The excited state calculations with time-dependent DFT
(TD-DFT) indicated that the lowest singlet excited state for both
conformers of 1 is dominated by the HOMO→LUMO transition
but is symmetrically-forbidden (red arrows in Figure 5a). The
second and third electronic transitions are allowed and dominated
by HOMO→LUMO+1 (green arrows) and HOMO–1→LUMO
(blue arrows) transitions, respectively (Figure 5a). The compari-
son of the calculated and experimental UV-vis spectra (Figure S8
in the SI) for both conformers of 1 in toluene indicates that the
saddle-type conformer is dominant in diluted solutions. This is
supported by the closer match of the lowest absorption peak as
well as the fact that only the calculated spectrum for the saddle
shows a double peak in the 300-400 nm range (Figure S8b). The
comparison of the spectra is also consistent with the thermody-
namic stability of both conformers. The TD-DFT calculation for
the energetically-lowest conformer of 2 suggested that the absorp-
tion peak at around 480 nm is composed of three nearly-lying
excitations which are dominated by the HOMO–1→LUMO,
HOMO→LUMO+1, and HOMO–1→LUMO+1 transitions, re-
spectively. The calculated spectrum also matches the experimental
spectrum rather well (Figure S9 in the SI). In particular, the pre-
dicted absorption peak is at 485 nm which is very close to the
experimental one at 479 nm. Considering that the experimental
lowest-lying absorption peaks of 1 and 2 are very close to each
other, the fact that the calculated peaks of saddle and the con-
former of 2 differ noticeably can be attributed to the importance of
vibronic effects. Indeed, the symmetry-forbidden transition to S1
in saddle is at 476 nm, almost on top of the experimental peak.
Therefore, mixing of electronic states due to vibrations would
make the transition to S1 allowed akin to the transitions in the
linear analogue. This would lead to the red shift of the calculated
band and increase of its intensity, thereby improving the agree-
ment with the experiment. An analogous mixing of states to make
forbidden transitions allowed has been recently proposed in the
four-state model of TADF to explain the possibility of simultane-
ous efficient rISC and fluorescence in donor-acceptor TADF emit-
On one hand, the emissions from the triplet state was ob-
served at low temperatures (blue lines, Figure 6), with the triplet
energy (ET1) for 1 being 2.19 eV in both Zeonex® and CBP hosts
(Figure 6a and c). Due to the polarity of CBP host, the 1CT energy
of macrocycle 1 in CBP (2.37 eV) is lower than that in Zeonex®
(2.43 eV), and therefore the ΔEST of the material was reduced
from 0.24 eV for Zeonex® to 0.18 eV for CBP, which indicates
moderate exchange energy. The behavior of linear compound 2
was slightly different from that of 1: the ET1 of 2 in Zeonex® (2.23
eV) is much higher than that in CBP host (2.11 eV) (Figure 6b and
d). This would suggest that the triplet excited states of more struc-
turally-flexible compound 2 is more influenced by the fluctuation
of structural conformation than cyclic compound 1. The polarity
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of host matrix once again gave an impact on the CT energy to
decrease, thereby the DEST of 2 was narrowed from 0.25 eV in
Zeonex® to 0.20 eV in CBP.
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The intensity decay of the emission at different time delays
were acquired for both compounds in Zeonex® and CBP hosts
(Figure 7). In all cases, the PF and phosphorescence (PH) were
observed at 80 K. Apparently, the decay curves of both com-
pounds are typical for CT-based organic TADF emitters,18a and
indeed, the DF process increased as the temperature elevated
(Figure 7c and d, Figure S10e and f, and Figure S11e and f).
However, with closer inspection, the temperature dependence of
the decay profiles of 1 and 2 were different from each other: The
DF process started at much lower temperature for macrocycle 1
(ca. 120 K) than for linear derivative 2 (ca. 200 K). This would
indicate that higher activation energy is required for the rISC pro-
cess in the case of 2. Moreover, the rise of DF component in the
case of 2 is much weaker than that of 1 (Figure 7b and d), indicat-
ing the less DF contribution to the PL of compound 2 in a host
matrix. These results are also consistent with the results obtained
with solutions (Figure 3c and d). Therefore, linear compound 2
exhibits lower device efficiency compared to macrocycle 1 (Fig-
ure 8, vide infra).
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Figure 7. Emission intensities of 1 and 2 in host matrices
against delay time measured at different temperature: (a) 1 and
(b) 2 in Zeonex® (1 wt% in host matrix); (c) 1 and (d) 2 in
CBP (10 wt% in host matrix).
Fabrication and Characteristics of OLED Devices. To investi-
gate the possibility of applying 1 and 2 as OLED emitters, ther-
mogravimetric analysis (TGA) of 1 and 2 were performed under
air and N2 (Figure S5 and S6 in the SI). High thermal decomposi-
tion temperatures [Td (5 wt% loss) 548–585 °C for 1 and 535–
578 °C for 2] indicated that these molecules would be applicable
to vacuum thermal deposition for purification and fabrication of
OLEDs. To evaluate the validity of the macrocycle 1 and linear
analog 2 as the TADF emitters, OLED devices were fabricated
with 1 (DEV1) and 2 (DEV2) using the co-evaporation technique.
The device structure fabricated was as follows: ITO/NPB (40
nm)/10 wt% TADF emitter (1 or 2) in CBP (30 nm)/TPBi (50
nm)/LiF (1 nm)/Al (100 nm). Both OLEDs fabricated with 1 and 2
showed a low turn-on voltage at 2.0 V and 2.5 V, respectively
(Figure 8a). Importantly, the device fabricated with macrocycle 1
as the emitter displayed a bright orange emission (lmax 589 nm)
and far exceeded the theoretical maximum EQE of that with con-
ventional fluorescent materials (5%), up to 11.6% (Figure 8b),
clearly presenting a proof-of-concept of twisted D–A–D–A p-
conjugated macrocyclic TADF emitter for OLEDs for the first
time. More importantly, the EQE of the OLEDs fabricated with 1
also surpassed the maximum value of OLEDs fabricated with 2
(up to 6.9%, Figure 8b). The luminance of device fabricated with
1 was quite higher (> 23,000 cd m–2) than that of 2 (ca. 12,500 cd
m–2), which suggested better charge recombination in the device
based on 1 (Figure 8c). Both devices showed low-to-moderate
efficiency roll-off (the roll-off of current efficiencies at 1,000 cd
m–2 were 3.98% for 1 and 16.04% for 2) (Figure 8d).
Figure 6. Normalized emission spectra of 1 and 2 in host ma-
trices at varying delay times at 300 K and 80 K: (a) 1 and (b) 2
in Zeonex® (1 wt% in host matrix); (c) 1 and (d) 2 in CBP (10
wt% in host matrix).
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*minakata@chem.eng.osaka-u.ac.jp
Notes
6
The authors declare no competing financial interest.
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8
ACKNOWLEDGMENT
9
This research was supported by the Grant-in-Aid for Scientific
Research on Innovative Areas “p-System Figuration: Control of
Electron and Structural Dynamism for Innovative Functions”
(JSPS KAKENHI Grant No. JP17H05155 to Y.T.), “Aquatic
Functional Materials: Creation of New Materials Science for En-
vironment-Friendly and Active Functions” (JSPS KAKENHI
Grant No. JP19H05716 to Y.T.), and the Continuation Grants for
Young Researchers from the Asahi Glass Foundation (to Y.T.).
P.D. acknowledges the kind support received from the First Team
program of the Foundation for Polish Science co-financed by the
European Union under the European Regional Development Fund
(project number: First TEAM 2017-4/32 POIR.04.04.00-00-
4668/17-00). Y.T. and P.D. acknowledge the EU’s Horizon 2020
for funding the OCTA project under grant agreement No. 778158.
We acknowledge the experimental supports of Professors Kei
Tsujimoto (Osaka University) and Hiroshi Uyama (Osaka Univer-
sity) for the DSC measurement.
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COCLUSION
We have succeeded in developing an efficient thermally activated
delayed fluorescent D–A–D–A p-conjugated macrocycle based on
rational molecular design, satisfying both of synthetic and func-
tional requirements. The developed macrocycle forms two poly-
morphs with different color and emission color, depending on
crystallization conditions. The X-ray crystallographic analyses
and DFT calculations revealed the importance of molecular con-
formations and packing structures in different colors. It is noted
that porous column structures assembled through intermolecular
D•••A interactions in red crystal would be a peculiar feature of the
D–A p-conjugated macrocycle. Comparative investigation of the
physicochemical properties of the macrocycle with its linear ana-
log has clearly demonstrated that the macrocyclization can be an
effective strategy for increasing TADF contribution through the
suppression of non-irradiative pathways. Most importantly, the
first OLED device fabricated with a purely organic macrocyclic
TADF emitter was realized, showing as high EQE as 11.6%. This
exceeds the theoretical maximum of those fabricated with conven-
tional fluorescent emitters (5%) and that with the linear analog
(6.9%). We believe that this research opens up an avenue for de-
signing new macrocyclic TADF materials. As a perspective, mac-
rocyclic TADF emitters can provide great opportunities for tuning
their photo- and redox properties through host-guest interactions.
ASSOCIATED CONTENT
Supporting Information. Supporting Information is available
free of charge via the Internet at http://pubs.acs.org.
Experimental procedures for the syntheses of materials, spectro-
scopic data of new compounds, theoretical details including xyz
files for coordinates, UV-Vis absorption and photoluminescence
spectra, thermogravimetric analysis (TGA) profiles, and the cop-
ies of 1H and 13C NMR spectra of new compounds.
AUTHOR INFORMATION
Corresponding Authors
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Seeing Polymers in a New Light. Angew. Chem., Int. Ed. 1998,
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*przemyslaw.data@polsl.pl
*takeda@chem.eng.osaka-u.ac.jp
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