Blue-Emitting Arylalkynyl Naphthalene Derivatives via a Hexadehydro-Diels-Alder Cascade Reaction

Article abstract of DOI:10.1021/jacs.6b07647

We describe here three alkynyl substituted naphthalenes that display promising luminescence characteristics. Each compound is easily and efficiently synthesized in three steps by capitalizing on the hexadehydro-Diels-Alder (HDDA) cycloisomerization reacti

Full text of DOI:10.1021/jacs.6b07647

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Communication
Blue-emitting Arylalkynyl Naphthalene Derivatives via
a Hexadehydro-Diels-Alder (HDDA) Cascade Reaction
Feng Xu, Kyle W. Hershey, Russell J. Holmes, and Thomas R. Hoye
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b07647 • Publication Date (Web): 14 Sep 2016
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Blue-emitting Arylalkynyl Naphthalene Derivatives via a
Hexadehydro-Diels-Alder (HDDA) Cascade Reaction
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FengXu,†KyleW.Hershey,‡ RussellJ.Holmes,‡ ThomasR.Hoye*†
†
‡
Department of Chemistry and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis,
Minnesota 55455, USA
SupportingInformationPlaceholder
ABSTRACT: Wedescribeherethreealkynylsubstitutednaphthalenesthatdisplaypromisingluminescencecharacteristics.Eachcompoundis
easily and efficiently synthesized in three steps by capitalizing on the hexadehydro-Diels–Alder (HDDA) cycloisomerization reaction in
which an intermediate benzyne is captured by tetraphenylcyclopentadienone, a classical trap for benzyne itself. These compounds luminesce
in the deep blue when stimulated either optically (i.e., photoluminescence in both solution and solid films) or electrically [in a light-emitting
device (LED)]. The photophysical properties are relatively insensitive to the electronic nature of the substituents (H, OMe, CO2Me) that
define these otherwise identical compounds. Overall, our observations suggest that the twisted nature of the five adjacent aryl groups serves to
minimize the intermolecular interaction between core naphthalene units in different sample morphologies. These compounds represent
promising leads for the identification of others of value as the emissive component of organic LEDs (OLEDs).
o-Benzyne (or 1,2-dehydrobenzene, 2) is the parent member of
a
one of the most versatile classes of reactive intermediates in all of
chemistry. Among the myriad modes of reaction into which ben-
zynes enter is the Diels-Alder (DA) [4+2] cycloaddition reaction.
a
O
nor-
–CO
borna-
dien-
one
Ph

Ph
X
Y
1
2
The strained alkyne within the benzyne (the 2 component) en-
(cf.12)
Ph4
Ph
Y
Ph
gages a wide variety of 1,3-dienes (the 4component). Incorpora-
tion of structural complexity into the benzyne moiety using con-
ventional methods of benzyne generation1 becomes increasingly
challenging because these all involve producing the benzyne by
4
–XY
3
b
Y
Z
A
Z
Y
A
B
Z
removal of two adjacent atoms or groups from an aromatic precur-
sor (1 to 2, Figure 1a). In contrast the hexadehydro-Diels-Alder
(HDDA) reaction,2 which capitalizes on the simple thermal cycloi-
somerization of a 1,3,-diyne with a tethered alkyne diynophile (5 to
6, Figure 1b),3 results in de novo construction of the six-membered,
carbocyclic benzyne moiety. The HDDA reaction is highly amena-
ble to the incorporation of substituents and structural variants (A-
C, Y, and Z in Figure 1b) into the precursors, intermediate ben-
zynes, and trapped products.4 The HDDA reaction readily accom-
C
B
6
[4+2],
A
C
X
B

-X or
[O]
C
5
7
8
Figure 1. (a) Traditional benzyne generation1 involves removal of a
pair of ortho substituents from a benzenoid precursor ; Diels-Alder
reaction of with tetraphenylcyclopentadienone produces naphtha-
lene derivatives, following thermal extrusion of carbon monoxide. (b)
1
2
modates conjugated, -bond-rich linker units and substituents.
With an eye toward the rapid synthesis of compounds having
2 3
,
The HDDA thermal cycloisomerization
(5 to 6) produces benzynes
that are more complex and diverse in structure; trapping with myriad
dienes like 7 is envisioned to produce novel highly conjugated adducts
like 8.
new structural motifs, novel chromophores, and potentially valua-
ble photophysical properties, we have begun exploring HDDA
cascades [i.e., the sequential (and rate determining) generation
followed by rapid5 in situ trapping of an HDDA-generated ben-
zyne] in which the trapping step is an independent, second DA
reaction. For example, we envision that trapping of 6 by the diene
unit (blue) in one of a multitude of trapping agents having the gen-
eralized structure 7 will lead to many diverse, chromophoric, poly-
cyclic derivatives like 8.
1,2,3,4-tetraphenylnaphthalene (5), via thermal extrusion of CO
from the initial [4+2] DA adduct 4.6 (ii) Alkynyl substituents on
polycyclic aromatic rings often impart interesting or valuable excit-
ed state emission properties.7 Accordingly, one of the early reac-
tions we explored was that between the tetrayne 9a and TPCP (3,
Figure 2).8 The symmetry of this HDDA substrate make it attrac-
9
tive from a preparative perspective. It is made in two simple steps.
This HDDA cascade, using 1.5 equiv of 3, proceeded cleanly at 80
°C over the course of 12 hours and produced a near quantitative
Two facts guided the thinking that initiated the studies whose re-
sults we report here. (i) The DA reaction of
o-benzyne (2) with
tetraphenylcyclopentadienone( ,TPCP)efficientlyproduces
3
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Page 2 of 6
yield (82% following three recrystallizations) of the naphthalene
adduct 10a. An added advantage of using a tetrayne substrate is that
the intermediate benzyne, here 11, and, consequently, the naphtha-
lene formed following CO extrusion from 12, bears an arylethynyl
substituent. The product 10a showed, qualitatively, strong emissive
properties that were blue to the eye. This was true for the material
whether in a solid film, adsorbed on silica gel (tlc analysis), or dis-
solvedinsolution.
Given the promising solution PL efficiencies as well as the quali-
1
2
3
4
5
6
7
8
tative emission seen from solid-state samples (third vial in Figure
3b), the emissive properties of all three compounds were investi-
gated in thin films to explore their possible utility as emissive spe-
cies in blue organic light-emitting devices (OLEDs). OLEDs are
attractive for both display and white lighting applications due to
their (i) tunable spectral characteristics, (ii) relatively high lumi-
nescence efficiency, and (iii) compatibility with high throughput,
large-area processing.12 The development of novel light-emitting
R
R
molecules for efficient, deep-blue fluorescence remains an im-
portant area of research for the realization of high performance,
low-cost OLED-based displays.13
9
R
R
MeO2C
MeO2C
a
b
H
10
11
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OMe
c CO2Me
9a-c
R
b
a
+
80 o
C
80000
60000
40000
20000
0
O
MeO2C
MeO2C
1.0
10a
10b
10c
Ph
Ph
Ph
Ph
12 h
0.8
0.6
0.4
0.2
0.0
yield (%)
Ph
Ph
Ph
100% 10%
THF aq THF state
solid
Ph
10a (82%)
10b (85%)
10c (83%)
3
10a-c
HDDA
– CO
Ar
Ar
250 300 350 400 450 500 550
Wavelength (nm)
10b
MeO2C
MeO2C
Ar
O
3
Ar
Ph
Ph
MeO2C
MeO2C
Figure 3.
ized photoluminescence (PL) emission (at 10-6 M, solid lines) spectra
inTHF.( )Emissionseenfrom10binvariousmedia/states.
(a
) Solution absorption (at 10-5 M, dashed lines) and normal-
[4+2]
Ph
b
Ph
11
12
In order to probe thin film PL, each compound was examined
both as a pure film (100%) as well as in a host-guest arrangement
with the wide energy gap material 1,4-bis(triphenylsilyl)benzene
Figure 2. HDDA cascade reaction between tetraynes 9 and 3, proceed-
ing via benzynes 11 and their Diels-Alder adducts 12, which ejects
carbon monoxide to produce the fluorophores 10 in excellent yield.
(
UGH2) serving as host. Thin films were deposited on cleaned
quartz substrates by high vacuum thermal evaporation (<10-7
Torr). It is notable that following deposition of each of the naph-
thalenes 10a-c, there was virtually no residue remaining in the heat-
ing crucible, suggesting little if any thermal degradation of these
compounds. In fact, this point could be preliminarily established
for samples of 10a-c by observing (i) their sublimation in common
laboratory apparatus with essentially no decomposition and (ii)
their robust nature when held open in the air at a temperature of
≥300 °C, where they showed slight coloration but no sign of any
significant decomposition. Finally, examination by differential
scanning calorimetry again gave no indication of decomposition
(exotherm) below 300 °C (for 10a and 10b) or 310 °C (for 10c),
and thermogravimetric analysis showed mass loss of greater than
5% only at temperatures >336-357 °C (see SI).
In view of these promising properties, we decided to probe the
effect of the electronic character of the aryl substituents. Hence, we
synthesized two additional analogs in which the phenyl ring was
modified by the presence of a pair of electron-donating (10b) as
well as electron-withdrawing (10c) groups. These analogs were
readily prepared by an entirely parallel and equally efficient se-
quence9 of reactions.
Figure 3a shows the solution absorption (10-5 M) and photolu-
minescence (PL, 10-6 M) spectra of 10a-10c in tetrahydrofuran
solution. The PL quantum yields for each compound are
11.0±2.6%, 4.8±1.3%, and 55.7±8%, for 10a, 10b, and 10c, respec-
tively. There was nearly no change (≤2 nm) in the max of emission
for 10a and 10b; a red shift of 10 nm in that max was seen for 10c.
Surprisingly, these compounds also showed a behavior typical of
aggregation-induced emission (AIE).10 Namely, as the strong sol-
vent THF was exchanged for increasing amounts of the non-
solvent water, the efficiency of blue emission grew (see SI). As an
example, compare the left and middle vials in Figure 3b, which
contain the same amount of 10b in pure THF vs. 10% THF in wa-
ter, respectively. However, given the array of five orthogonal aryl
substituents, one would not expect that two (or more) molecules of
10 would be able to reside close enough to show substantial ex-
cimeric behavior. Perhaps the planar (arylethynyl)naphthalene
portion of the molecule provides enough of a footprint for close
association of molecular orbitals from two molecules. We do note
Films composed of host and guest (UGH2 and 10) in the ratios
of 96:4, 80:20, and 0:100 (i.e., 4%, 20%, and 100% loading, respec-
tively) were studied. The thin film PL spectra of the 20% samples of
each of 10a-c are shown in Figure 4a. The solution and film emis-
sion maxima (Figures 3a vs. 4a) are similar, suggesting that there
may not be intimate contact in the film samples. The twisted nature
of the five aryl substituents on the central naphthalene ring as well
as the quasi-orthogonal orientation of the pair of malonate car-
bomethoxy groups may be responsible for preventing close associa-
tion between molecules of 10 in films. These features can be seen in
the single crystal X-ray crystallographic structure shown in Figure
4b. The PL emission spectra of all three compounds showed a red
shift of between 10-20 nm across the extreme concentrations of 4%
vs. 100% content of 10. This could be a result of at least some de-
gree of aggregation with increasing concentration in the film, a
response to a change in bulk dielectric properties or to differing
that
a topologically related compound, 1-methyl-1,2,3,4,5-
pentaphenylsilole, also shows significantly enhanced emission
when placed under aggregate-inducing conditions.11
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Journal of the American Chemical Society
intermolecular restraints that change the geometry of the excited
states. DFT calculations (M06-2X/6-31G(d)) of the optimized
lowestenergyconformerforeachof10a-cshowedboththetwisted
a
b
SIPh3
Al (100 nm) on LiF (0.5 nm)
3TPYMB (30 nm)
1
2
3
4
5
6
7
8
Emissive Layer:
N
the ethynyl linker, quite similar to those features seen for the single
crystal (see SI).
SIPh3
UGH2
10 + UGH2 (20 nm)
3
3TPYMB
(p-Tol)2N
TAPC (30 nm)
PEDOT:PSS (40 nm)
ITO (150 nm)
N(p-Tol)2
a
b
1.0
0.8
0.6
0.4
0.2
0.0
9
10a
10b
10c
Glass
10
11
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TAPC
c
d
400
450
500
550
10b
Wavelength (nm)
Figure 4. (a) Thin film (20 nm) normalized PL spectra of 20:80 mix-
tures of 10a-c co-deposited with the wide-gap host UGH2 under exci-
tation at a wavelength of 300 nm. (b) Single crystal X-ray structure of
10b showing the relative orientation in the solid state of all aryl substit-
uents with respect to the core naphthalene ring.
Figure 5. OLEDs (a) Device architecture. (b) Compounds used as: the
electron transport layer (3TPYMB), the host in the emissive layer
(UGH2), and the hole transport layer (TAPC). (c) Photograph of a
typical device. (d) Electroluminescence intensity (normalized) from
devices using 20 wt% of emitter 10 doped in UGH2.
OLEDs using each of 10a-c as the emissive layer were con-
structed by high vacuum thermal evaporation. Devices (Figures 5a-
c) were deposited on glass substrates, pre-patterned with a semi-
transparent, hole-injecting anode of indium-tin-oxide. These sub-
strates were initially spin-coated with a 40-nm-thick planarizing
layer of the ionomeric mixture of poly(3,4-ethylenedioxythio-
phene) and polystyrene sulfonate (PEDOT:PSS) prior to vacuum
deposition. The device architecture consisted of a 30-nm-thick hole
measurements, the external quantum efficiency (EQE) was calcu-
lated. The EQE is a direct measure of photons emitted from the
device in the forward viewing direction per electron injected. De-
vice performance for all emitter species and concentrations are
shown in Table 1. The highest performance device overall was
found to be 10a doped into UGH2 at 20% with an EQE of 3%. It is
worth noting that for simple fluorescent emitters, a maximum theo-
transport layer (HTL) of 4,4′-cyclohexylidenebis[N,N-bis(4-
methylphenyl)benzenamine] (TAPC), a 20-nm-thick emissive
layer of 10a, 10b, or 10c doped into the wide energy gap host
UGH2 at a concentration of 4, 20, or 100% of the emitter, and a 30-
15 Peak EQEs for all three com-
retical efficiency of ~5% is expected.
pounds are realized for devices having an emissive layer composi-
tion of 20% of 10. This does not correlate directly with the PL
nm-thick electron transport layer (ETL) of tris(2,4,6-trimethyl-3-
(pyridin-3-yl)phenyl)borane (3TPYMB). All devices were capped
with an electron-injecting cathode comprising a 0.5-nm-thick layer
of LiF and a 100-nm-thick layer of Al. Electroluminescence (EL)
intensity spectra collected for devices containing 20% loading are
shown in Figure 5d. Data for devices containing a neat emissive
layer of 10c (see SI) show a significant red shift compared to the PL
spectrum. We speculate that this emission may originate from an
interfacial exciplex in this particular device. Upon doping with the
UGH2 host, this effect is eliminated for the devices containing the
20%(and 4%, see SI)films.
Table 1. Summary of device performance
CIEe
c
d
Conca
(%)
VTO
(V)
b
EQE
Emitter
Compounds

PL

(%)
(%)
x
y
4
1.4
93.3
42.9
3.6
3.5
0.18
0.15
0.10
0.09
20
3.0
10a
10b
10c
100
4
1.6
0.9
34.8
51.1
3.3
3.8
0.15
0.18
0.13
0.11
To evaluate spectral compatibility with display applications, the
coordinate system describing the color gamut developed by the
International Commission on Illumination (CIE) is used. Deep
blue emission is required to fully reproduce the visible color spec-
trum for displays as well as to achieve true white colors. For applica-
tion in television displays, desirable deep blue emission has CIE
coordinates of x = 0.131, y = 0.04614a or x = 0.14, y = 0.08.14b The
coordinates observed for the above devices based on 10a-c are
reported in Table 1 (and graphically in the SI). All but the last
(100% 10c) emit in the deep blue.
20
100
4
1.9
1.9
0.7
0.9
0.4
46.2
43.0
73.9
56.6
30.2
3.5
3.1
4.6
3.6
3.1
0.15
0.15
0.18
0.15
0.23
0.08
0.10
0.14
0.17
0.43
20
100
aWeight% of emitter molecule 10 in UGH2 host in the active layer of
b
the device. External quantum efficiency (EQE) of champion devices.
cThin film photoluminescence efficiency (PL), measured for each
d
emissive layer on a quartz substrate. VTO: voltage to realize a current
density of 0.1 mA/cm2. eInternational Commission on Illumination
(CIE) coordinates calculated from device electroluminescence at a
current density of 2 mA/cm2.
Current density-voltage and brightness-voltage characteristics
were also measured for devices based on each emitter. From these
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2. (a) Hoye, T. R.; Baire, B.; Niu, D. W.; Willoughby, P. H.; Woods, B.
efficiency PL, which increases monotonically with dilution, likely
reflecting a role played by the emitter in charge transport. The wide
energy gap of UGH2 (HOMO = 7.2 eV; LUMO = 2.8 eV)16 forces
charge transport to occur in part via the emissive guest molecule,
especially for holes. This is supported by the observed increase in
P. Nature 2012, 490, 208-212. (b) Baire, B.; Niu, D.; Willoughby, P. H.;
Woods, B. P.; Hoye, T. R. Nature Protocols 2013, 8, 501–508.
3. (a) Bradley, A. Z.; Johnson, R. P. J. Am. Chem. Soc. 1997, 119,
9917-9918. (b) Miyawaki, K.; Suzuki, R.; Kawano, T.; Ueda, I.
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5746–5749. (b) Chen, J.; Palani, V.; Hoye, T. R. J. Am. Chem. Soc. 2016,
138, 4318–4321 (and references therein).
5. For a recent study of benzyne electrophilicity in which the relative
reactivities of various trapping agents were exploited, see: Fine Nathel, N.
F.; Morrill, L. A.; Mayr, H.; Garg, N. K. J. Am. Chem. Soc. 2016, 138,
10402–10405.
6. (a) Wittig, G.; Knauss, E. Chem. Ber. 1958, 91, 895–907. (b) This
reaction has been performed by countless numbers of students in introduc-
tory organic chemistry laboratories for decades: (i) Fieser, L. F. Organic
Experiments. Heath and Co. 1964. (ii) Fieser, L. F.; Haddadin, M. J. Can.
J. Chem. 1965, 1599–1606. (c) Reactions of TPCP with benzyne deriva-
tives have been described in dozens of publications. For the most recent,
see: Pozo, I.; Cobas, A.; Peña, D.; Guitián, E.; Pérez, D. Chem. Commun.
2016, 52, 5534–5537.
7. (a) Cacioppa, G.; Carlotti, B.; Elisei, F.; Gentili, P. L.; Marrocchi,
A.; Spalletti, A. Phys. Chem. Chem. Phys. 2016, 18, 285–294 (and refer-
ences therein). (b) Giguere, J.-B.; Sariciftci, N.-S.; Morin, J.-F. J. Mater.
Chem. C 2015, 3, 601-606. (c) Zhang, C.-H.; Wang, L.-P.; Tan, W.-Y.;
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C.-Y.; Peng, J.; Cao, Y. J. Mater. Chem. C 2016, 4, 3757–3764. (d) Brog-
gi, A.; Tomasi, I.; Bianchi, L.; Marrocchi, A.; Vaccaro, L. ChemPlusChem
2014, 79, 486–507 (and references therein).
1
2
3
4
5
6
7
8
turn-on voltage (푉!") with decreasing concentration. These devic-
es could likely be further optimized by using a charge transporting
host and a lower guest concentration, capitalizing on the improved
PL efficiency observed for more dilute films.
9
In conclusion we have shown that the arylalkynyl naphthalenes
10a-c, which differ in the electronic character of the substituents on
two of their aromatic rings, can be efficiently prepared by an
HDDA-enabled strategy. This involves just three chemical reac-
tions from commercially available materials. The PL of each com-
pound was measured in solution as well as in thin films produced
by vapor deposition. Films having varying concentrations of 10
were produced by co-deposition with UGH2. The PL of these films
showed only a small red shift with increasing concentration, largely
a reflection, we presume, of the twisted nature of the five aryl sub-
stituents that serve to enshroud the central naphthalene chromo-
phore. Devices were constructed using 10a-c as the emissive spe-
cies. Compound 10a shows the highest external quantum efficien-
cy. Overall there was not a strong impact of the varying electronic
character of the substituents (H vs. OMe vs. CO2Me) on the chro-
mophores of either the absorption or emission spectra. These re-
sults suggest that other novel chromophoric structures that can be
accessed by the powerful HDDA cascade approach may also be
deserving of consideration for applications in various organic elec-
tronics settings.
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8. As our work was progressing, the use 9a and 9b as HDDA substrates
was reported; the intermediate benzyne was efficiently trapped by various
cyclic dienes (furan, cyclopentadiene, pyrrole, and thiophene derivatives).
Zhang, M.-X.; Shan, W.; Chen, Z.; Yin, J.; Yu, G.-A.; Liu, S. H. Tetrahe-
dron Lett. 2015, 56, 6833–6838.
9. Bromination of dimethyl dipropargylmalonate (i) gives ii, which is
then cross-coupled with an arylethyne (iii) of choice.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
Details for the preparation of new compounds; line listings of structural
characterization data for new compounds; copies of H and 13C NMR
spectra; and computed (DFT) geometries of 10a-c and the energies of
their HOMOs and LUMOs; CIE diagram and EL data; current-
voltage, brightness-voltage, and device EQE plots (PDF).
10. Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B.
Z. Chem. Rev. 2015, 115, 11718–11940.
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AUTHOR INFORMATION
Corresponding Author
* hoye@umn.edu
Notes
FX and TRH are inventors on a provisional patent application. RJH is a
member of The Dow Chemical Company Technical Advisory Board.
ACKNOWLEDGMENT
This research was carried out with support from the National Institute
of General Medical Sciences of the National Institutes of Health,
U.S.A. (GM65597, TRH) and The Dow Chemical Company (RJH).
NMR spectral data were obtained using instrumentation purchased
with funds awarded through the NIH Shared Instrumentation Grant
program (S10OD011952).
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Thompson, M. E. Chem. Mater. 2004, 16, 4743–4747.
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