Dearomatizing [4+1] Spiroannulation of Naphthols: Discovery of Thermally Activated Delayed Fluorescent Materials

Article abstract of DOI:10.1002/anie.202012842

Disclosed here is a palladium-catalyzed direct [4+1] spiroannulation of ortho-C?H bonds of naphthols with cyclic diaryliodonium salts to construct spirofluorenyl naphthalenones (SFNP) under mild reaction conditions. This spiroannulation directly transforms the hydroxy group into a carbonyl group, and also tolerates reactive functional groups such as the halo groups, which provide an opportunity to rapidly assemble structurally new thermally activated delayed fluorescent (TADF) materials that feature a carbonyl group with an adjacent spirofluorenyl unit as the acceptor. As an illustrated example, the OLED device utilizing the assembled DMAC-SFNP as the host material exhibits a low turn-on voltage of 2.5 V and an ultra-high external quantum efficiency of 32.2 %. This work provides inspiration for structurally new TADF materials, and also displays the potential of C?H activation as a synthetic strategy for the innovation of optoelectronic materials.

Full text of DOI:10.1002/anie.202012842

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Dearomatizing [4+1] Spiroannulation of Naphthols: Discovery
of Structurally Non-Traditional Thermally Activated Delayed
Fluorescent Materials
Authors: Wenbo Liang, Yudong Yang, Mufan Yang, Min Zhang,
Chengming Li, You Ran, Jingbo Lan, Zhengyang Bin, and
Jingsong You
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202012842
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Dearomatizing [4+1] Spiroannulation of Naphthols: Discovery of
Structurally Non-Traditional Thermally Activated Delayed
Fluorescent Materials
Wenbo Liang, Yudong Yang, Mufan Yang, Min Zhang, Chengming Li, You Ran, Jingbo Lan,
Zhengyang Bin,* and Jingsong You*
[*]
W. Liang, Prof. Dr. Y. Yang, M. Yang, M. Zhang, Dr. C. Li, Y. Ran, Prof. Dr. J. Lan, Prof. Dr. Z. Bin, Prof. Dr. J. You
Key Laboratory of Green Chemistry and Technology of Ministry of Education
College of Chemistry, Sichuan University
29 Wangjiang Road, Chengdu 610064, People's Republic of China
E-mail: binzhengyang@scu.edu.cn, jsyou@scu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: The development of streamlined synthetic routes to
spirofluorene derivatives, in particular those with a donor-acceptor
(D-A) system, is an appealing task in the field of organic functional
materials. In this work, we disclose a palladium-catalyzed direct
[4+1] spiroannulation of ortho-C−H bonds of naphthols with cyclic
diaryliodonium salts to construct spirofluorenyl naphthalenones
(SFNP) under mild reaction conditions. This spiroannulation not only
directly transforms the hydroxyl group into the carbonyl group, but
also tolerates reactive functional groups such as the halo groups,
which provide an opportunity to rapidly assemble structurally non-
traditional thermally activated delayed fluorescent (TADF) materials
that feature a carbonyl group with an adjacent spirofluorenyl unit as
the acceptor. As an illustrated example, the OLED device utilizing
the assembled DMAC-SFNP as the host material exhibits a low turn-
on voltage of 2.5 V and an ultra-high external quantum efficiency of
32.2%. This work not only gives us an inspiration for designing
structurally non-traditional TADF materials, but also displays the
charm of C−H activation as a revolutionary synthetic strategy in the
innovation of optoelectronic materials.
In recent years, transition metal-catalyzed dearomatization via
C−H activation has emerged as a powerful approach to
spirocycles.[6] In this regard, the dearomatizing spiroannulation
of phenol derivatives is particularly intriguing because it allows
the incorporation of
a carbonyl group into the resulting
spirocyclic products. It is worthy of note that the carbonyl group
is usually employed to form an acceptor unit in organic
optoelectronic molecules.[7] Recently, the dearomatizing [3+2]
spiroannulation of naphthol-based biaryls with alkynes, and
[2+2+1] spiroannulation of bromonaphthols with aryl iodides and
alkynes have been established to access spiroindenyl
naphthalenones (Scheme 1b).[6b,6e,8] Inspired by these elegant
works, we reasoned that spirofluorenyl naphthalenones could be
assembled by [4+1] spiroannulation of naphthols with aryl
equivalents instead of alkynes (Scheme 1c). Cyclic
diaryliodonium salts have been employed as biarylation
reagents in the transition metal-catalyzed [4+2] annulation
reactions for the construction of π-fused aromatic materials.[9]
Herein we present
a
palladium-catalyzed direct [4+1]
spiroannulation of ortho-C−H bonds of naphthols with cyclic
diaryliodonium salts, which enables the streamlined synthesis of
spirofluorenyl naphthalenones and rapid assembly of structurally
non-traditional TADF materials.
C−H bond activation is referred to the “holy grail” of organic
synthesis. The direct transformation of ubiquitous C−H bonds
not only provides unusual strategic disconnections for the
construction of hard-to-form bonds but also enables the rapid
assembly of complex structures from simple substrates with
excellent functional group tolerance.[1] Consequently, this
revolutionary synthetic strategy is believed to fuel the innovation
of organic functional materials and pharmaceuticals.[2]
Spirocyclic aromatic compounds have drawn tremendous
attentions for their broad applications in organic optoelectronic
materials, such as organic light emitting diodes (OLEDs) and
organic solar cells (OSCs).[3] In particular, the spirofluorene
derivatives featuring a donor-acceptor (D-A) system are one of
the most anticipated structures for OLED host materials,
because of their balanced carrier transporting property, high
triplet energy level and good thermal stability.[4] However,
conventional synthetic approaches to the spirofluorenyl
structures typically require multiple steps and the use of highly
reactive organometallic reagents (e.g., Grignard reagent and n-
butyllithium), which restrict the diversity of molecular skeletons
and further exploration of novel materials (Scheme 1a).[5] Thus,
the development of general and concise methods to overcome
the above limitations and enable rapid diversification of
spirofluorenes is highly desired.
Scheme 1. Synthesis of spirocyclic derivatives.
1
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The dearomatizing spiroannulation of 2-naphthol with cyclic
diphenyliodonium salt was investigated through palladium
catalysis (Table 1). After screening of a variety of parameters,
including palladium sources, bases, solvents and temperature,
the reaction of 1a with 2a afforded the desired spirofluorene 3aa
in 91% isolated yield at 60 oC (Table 1, entry 1). Control
experiments demonstrated that palladium catalyst and base
were indispensable (Table 1, entries 2 and 3). The replacement
of Pd2(dba)3 with Pd(PPh3)4, [Pd(C3H5)Cl]2 or Pd(OAc)2 resulted
in inferior efficiencies (Table 1, entries 4 and 5). Other solvents
such as toluene, DCE, 1,4-dioxane, HFIP and DMF were less
effective than TFE (Table 1, entries 6 and 7). Other bases (e.g.,
Cs2CO3 and K2CO3) gave slightly low yields (Table 1, entry 8). In
addition, the C−O coupling product was not detected in this
transformation although diaryliodonium salts are easily attacked
by nucleophilic reagents.[9a] The replacement of 2a with 2,2'-
diiodo-1,1'-biphenyl did not yet give the desired product.
could react with 2-naphthol to afford the dearomatized products
(4ab−4aj, and 4kg). The reactions with nonsymmetrical cyclic
diaryliodonium salts enabled the synthesis of unsymmetrical
spirofluorene derivatives (4ak−4am). The sterically hindered 2m
gave a relatively low yield of 41% with accompany of the
recovery of 1a in 52% yield. The excessive cyclic diaryliodonium
salt 2m was decomposed completely after the reaction. Besides,
the structures of 3fa and 4am were demonstrated by the X-ray
single crystal analysis (Part V, SI).[10]
Table 1. Optimization of the reaction conditions.[a]
Entry
Variation from the standard conditions
Yield [%][b]
1
2
3
4
5
6
7
8
none
96 (91)[c]
N.R.
without Pd2(dba)3
without K3PO4
N.R.
Pd(PPh3)4 instead of Pd2(dba)3
[Pd(C3H5)Cl]2 or Pd(OAc)2 instead of Pd2(dba)3
toluene, DCE, dioxane instead of TFE
HFIP or DMF instead of TFE
Cs2CO3, K2CO3 instead of K3PO4
68
88, 67
45, 41, 28
10, trace
89, 87
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Pd2(dba)3 (5.0 mol%),
K3PO4 (2.0 equiv) and TFE (1.0 mL) at 60 oC for 24 h under N2. [b] NMR yield
using dibromethane as an internal standard. [c] Isolated yield in parentheses.
DCE = 1,2-dichloroethane, HFIP = 1,1,1,3,3,3-hexafluoroisopropanol, DMF =
N,N-dimethylformamide, TFE = 2,2,2-trifluoroethanol.
With the optimized reaction conditions in hand, the substrate
scope with regard to 2-naphthols 1 was examined. As shown in
Scheme 2, a range of 2-naphthols smoothly underwent the
spiroannulation reaction with 2a to deliver spirofluorenyl
naphthalenones 3 in moderate to excellent yields (3aa-3pa). In
comparison with the conventional synthetic approaches to
spirofluorenes from fluorenones and organometallic reagents,[5]
this protocol tolerated a broad scope of functional groups,
including methyl, phenyl, 1-naphthyl, benzyloxy, methoxy, 2-
thienyl, and even ester, formyl, cyano, bromo and chloro (3ba-
3na, 3pa). The excellent functional group compatibility of this
reaction is highly important for further construction of functional
molecules. To demonstrate the synthetic applicability of this
reaction, a 1.0 mmol scale reaction was conducted, delivering
3la in 80% yield. Moreover, 9-phenanthrol and 6-
chlorophenanthren-9-ol also underwent this transformation to
provide the desired products in moderate yields (3oa and 3pa).
Next, the scope of cyclic diaryliodonium salts 2 was explored
(Scheme 2). The symmetrical cyclic diaryliodonium salts with
various substituents, such as methyl, t-butyl, phenyl, methoxy,
trifluoromethyl, fluoro and chloro, at either para or meta position
Scheme 2. Scope of substrates. Reaction conditions: 1 (0.2 mmol), 2 (0.4
mmol), Pd2(dba)3 (5.0 mol%), K3PO4 (2.0 equiv) and TFE (1.0 mL) at 60 oC for
24 h under N2. Isolated Yields. [a] 80 oC. [b] 1.0 mmol scale. [c] 40 oC.
Recently, the TADF materials featuring a donor−acceptor
structure have attracted much attention in the field of OLEDs
because they can theoretically realize 100% internal quantum
efficiency.[11] With a library of spirofluorenyl naphthalenones
(SFNP) in hand, we tried to assemble the TADF molecule
(DMAC-SFNP)
using
widely
used
9,9-dimethyl-9,10-
dihydroacridine (DMAC) as the donor (Figure 1a). The
spiroannulation developed herein is tolerant of the reactive
bromo group, which provides a streamlined route to the bromo-
2
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containing precursor 3la for the subsequent Buchwald–Hartwig
amination with DMAC (for details, see Part IX, SI). The
theoretical calculations demonstrate that the lowest unoccupied
molecular orbitals (LUMO) of DMAC-SFNP are mainly localized
on the electron-deficient naphthalenone plane, while the highest
occupied molecular orbitals (HOMO) are mostly distributed over
the electron-rich DMAC donor (Figure 1b). Therefore, small
overlap of the frontier orbitals (FMOs) leads to a tiny singlet-
triplet energy gap (ΔEST) of 0.013 eV, indicating the potential of
spirofluorenyl naphthalenones for designing TADF materials
(Figure 1c). To investigate the effect of π-π stacking between
the fluorene units on OLED performance, DMAC-tBuSFNP with
two sterically hindered tert-butyl groups on the fluorene unit is
designed for comparison (Figure 1a). Because the spiro
quaternary carbon center disrupts the π-conjugation between
the fluorene and napthalenone units, the LUMO distributions of
the compounds are not delocalized to the fluorene unit (Figure
1b). Therefore, we proposed that the introduction of electron-
donating t-butyl group on the fluorene unit would slightly
influence the electrochemical and photophysical properties of
molecules, which could be demonstrated by the same FMOs
energy levels of DMAC-SFNP and DMAC-tBuSFNP (-5.10 eV
for HOMO, and -2.38 eV for LUMO) (Figure S3) and the similar
emission peaks in the fluorescence spectra (527 nm for DMAC-
SFNP, and 524 nm for DMAC-tBuSFNP) (Figure S4).
SFNP and DMAC-tBuSFNP are shown in Figure 2.[10] The two
compounds show similar structures with large dihedral angels of
86.55° and 89.98° between the fluorene and naphthalene units,
and 67.78° and 80.85° between the DMAC donor and the SFNP
acceptor. However, as predicted, their packing patterns are quite
different. In the crystal of DMAC-SFNP, π-π interactions with
distances of 3.57 Å between the fluorene units are found. The
DMAC donors are also well-packed with the distance of 3.80 Å
via intermolecular π-π stacking interaction (Figure 2c). This
compact and ordered face-to-face packing mode would provide
a favourable condition for effective carrier transport between the
neighbouring molecules and potentially enable DMAC-SFNP a
superior candidate for TADF host material. In contrast, the
significant steric hindrance of t-butyl groups prevents the close
interaction between the neighbouring spiro fluorene units in the
crystal of DMAC-tBuSFNP, and thus changes the packing
structure to an unordered tail-to-tail mode with the interaction
among t-butyl groups (Figure 2d), which is unfavoured for carrier
transport.
Figure 2. Crystal structures of a) DMAC-SFNP and b) DMAC-tBuSFNP.
Packing structures of c) DMAC-SFNP and d) DMAC-tBuSFNP.
OLED devices were fabricated using DMAC-SFNP and
DMAC-tBuSFNP as the host materials and (Ir(mphmq)2(tmd)) as
the emitter in the emitting layers. The widely used diaryl ketone
TADF host material DMAC-BP was used for comparison (Figure
3).[12] All the devices show similar emission spectra with
emission peak at 605 nm, indicating the sufficient energy
transfer from host material to Ir(mphmq)2(tmd) emitter. Although
Figure 1. a) Molecular structures, b) theoretical LUMO (blue)/HOMO (red)
distributions and c) calculated energy levels of DMAC-SFNP (top) and DMAC-
tBuSFNP (bottom).
DMAC-SFNP
and
DMAC-tBuSFNP
possess
similar
photophysical and electrochemical properties, the device
performances are considerably different. For example, the turn-
on voltage (Von) of DMAC-SFNP-based device is only 2.5 V and
the maximum external quantum efficiency (EQEmax) is as high as
32.2%, which are significantly superior to those of DMAC-
tBuSFNP-based devices (4.1 V and 18.5%). Compared with the
highly efficient DMAC-BP host, DMAC-SFNP-based device
could even possess a better device performance with a lower
turn-on voltage (2.7 V for DMAC-BP) and a largely enhanced
efficiency (21.2% for DMAC-BP). Moreover, the current density
of DAMC-SFNP-based device is significantly higher than that of
DMAC-tBuSFNP-based devices, which further demonstrates
that the planar conformation of fluorene unit is quite important to
ensure the compact and ordered face-to-face packing mode,
which enhances device performances.
To examine the TADF characteristics of DMAC-SFNP and
DMAC-tBuSFNP, the UV-vis absorption and photoluminescence
(PL) spectra were measured (Part XII, SI). In the absorption
spectra (Figure S4), a broad absorption band appears in the
region of 350-450 nm for DMAC-SFNP and DMAC-tBuSFNP,
which is attributed to charge-transfer absorption from the DMAC
donor to the SFNP acceptor. The fluorescence spectra were
measured in 4,4’-bis(N-carbazolyl)biphenyl (CBP) host at room
temperature, and the phosphorescence spectra were measured
at 77 K. The singlet energies (ES1) and triplet energies (ET1)
were calculated to be 2.62 eV and 2.35 eV for DMAC-SFNP,
and 2.63 eV and 2.31 eV for DMAC-tBuSFNP, exhibiting small
ΔEST values of 0.27 eV and 0.32 eV, respectively. The transient
PL spectra also displayed obvious delayed PL characteristics
with average lifetimes of 88.6 µs and 53.7 µs for DMAC-SFNP
and DMAC-tBuSFNP, respectively, indicating the TADF
behaviour (Figure S5 and Table S3).
In conclusion, we have developed a dearomatizing [4+1] C−H
spiroannulation of naphthols with cyclic diaryliodonium salts for
the rapid assembly of spirofluorenyl naphthalenones, which are
The crystal X-ray structures and packing patterns of DMAC-
3
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further used as the acceptor to construct structurally non-
traditional TADF materials. The merging of fluorene units leads
to effective face-to-face packing, thus achieving high-
performance OLEDs. This work demonstrates the superiority of
spirofluorenyl
naphthalenone
bifunctional
skeleton
for
constructing efficient TADF materials.
Figure 3. a) Device structure and energy-level diagram of OLED devices. Optimized device structure: ITO/HATCN (5 nm)/NPB (30 nm)/TCTA (10
nm)/Ir(mphmq)2(tmd):TADF host (3%, 25 nm)/Tmpypb (50 nm)/LiF (0.8 nm)/Al (100 nm); b) Molecular structures used in OLED devices; c) EL spectra at the
luminance of 1000 cd/m2; d) Luminance and current density versus voltage curves; and e) EQE and power efficiency versus luminance curves of OLED devices.
Liao, Adv. Funct. Mater. 2018, 28, 1706228; c) C. Han, Z. Zhang, D.
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We acknowledge financial support from the National NSF of
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Keywords: C−H activation • spiroannulation • dearomatization •
thermally activated delayed fluorescence
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4
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Entry for the Table of Contents
Synthetic strategy unlocks novel optoelectronic materials: A dearomatizing [4+1] C−H spiroannulation of naphthols with cyclic
diaryliodonium salts has been developed to rapidly deliver spirofluorenyl naphthalenones, which unlocks an opportunity to discover
structurally non-traditional TADF materials.
5
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