.
Angewandte
Communications
Table 3: Synthesis of TPA-bearing 2-(2-hydroxyphenyl)oxazoles.[a,b]
a significant KIE of 2.89 was obtained between 2a and [D1]-
À
2a with 1a. These observations revealed that the C H bond
breaking of benzoxazole might be involved in the rate-
limiting step.[8] Furthermore, addition of 2,2,6,6-tetramethyl-
1-piperidinyloxy (TEMPO) as a radical scavenger had
a negligible effect on the reaction between 1a and 2a, and
thus ruled out a radical pathway (Table S1, entry 18). Thus,
a tentative mechanism is proposed (see Figure S11). First, the
coordination of the amide nitrogen atom to RhIII and
À
subsequent carboxylate-assisted ortho-C H bond activation
of phenol produce a cyclorhodium intermediate,[5a,9] which
then reacts with benzoxazole to give the key benzoxazolyl–
RhIII–phenyl intermediate. This intermediate then undergoes
reductive elimination to a form RhI intermediate. Subse-
À
quently, intramolecular oxidative addition of the N O bond
to RhI generates a RhIII intermediate, which upon protonation
by PivOH, forms 3a along with the regeneration of the RhIII
catalyst.
White organic light-emitting diodes (WOLEDs) have
attracted much interest because of their great potential in flat
panel displays and illuminations.[10] Single-molecular white-
light-emitting materials have many advantages such as long-
term color balance, stability, and simple device fabrication
process. However, it remains a challenge to discover a single
chromophore that exhibits a broad emission covering the
whole visible range (l = 400–700 nm). Up to now, the
examples describing single-molecular white-light-emitting
materials are very rare.[11,12] In principle, 2-(2-hydroxy-
phenyl)azoles enable a dual-emission behavior involving an
enol-form emission with a relatively short wavelength and
a keto-form emission with a relatively long wavelength owing
to the excited-state intramolecular proton transfer
(ESIPT),[1f,g] and thus are regarded as ideal candidates to
design single-molecular white-light-emitting materials.
Unfortunately, the typical 2-(2-hydroxyphenyl)azoles, includ-
ing the coupled products 3a–r and 4a–n described herein,
exhibit solely the keto-form emission and lack the enol-form
emission in both the solid state and in nonpolar solvents, and
therefore hardly generates a broadband emission covering the
whole visible range. The protocol to attain white light is not
yet established in a single ESIPT system.[12] Given that
triphenylamine (TPA) possesses good electron-donating
properties, hole-transporting capability, and propeller-like
nonplanar geometry,[13] we envisioned that the incorporation
of TPA into 2-(2-hydroxyphenyl)azole skeletons could pre-
vent the molecular aggregation, and may thus lead to a strong
emission. More importantly, the excellent electron-donating
ability of TPA is capable of lowering the acidity of the
phenolic hydroxy group, and may be beneficial to the enol-
form species in the excited state, and thus lead to a dual
emission covering the entire visible range.
[a] Reaction conditions: 1 (0.2 mmol) and 2 (0.3 mmol) in DMF (1.0 mL)
under N2 atmosphere. [b] Yield of isolated product. [c–e] Absolute
quantum yields, absorption and emission maxima were measured in
toluene (5.010À5 m), in toluene (1.010À6 m) and in PS films
(0.2 wt%), respectively.
afforded the desired products in satisfactory yields (Table 3).
As expected, the dual emission is observed when TPA is
introduced to the phenol moiety of 2-(2-hydroxyphenyl)-
azoles (5a and 5b; Table 3 and Figure 1a). However, the
enol-form emissions are not strong enough to achieve white-
light generation. Surprisingly, when TPA is connected to the
azole moiety, the resulting molecules exhibit only the enol-
form emission (5c and 5d; Table 3 and Figure 1a). To our
delight, the ESIPTequilibrium between the enol-form and the
keto-form could be established through introducing TPA to
both the phenol and azole moieties (5e and 5 f; Table 3 and
Figures 1b and 2a). More importantly, the ESIPTequilibrium
is observed not only in solution but also in the solid state. As
shown in Figure 1b, the films of 5e and 5 f (0.2 wt%)
dispersed in polystyrene (PS) clearly display both the enol-
and keto-form emissions, which cover the whole visible range
extending from l = 400 to 700 nm. Both compounds exhibit
bright white-light emissions with CIE coordinates of 5e (0.32,
0.38) and 5 f (0.34, 0.39) in toluene, and 5e (0.30, 0.33) and 5 f
(0.29, 0.34) in PS films, respectively (Figure 2b,c). Notably, 5e
and 5 f exhibit high quantum yields in films (Table 3), and
predicts potential high-efficiency OLEDs.
Given that the competition from the highly electron-rich
TPA makes the selective halogenation of both the TPA-
bearing phenols and the TPA-bearing azoles more difficult,
common synthetic disconnections for the rapid construction
of TPA-bearing 2-(2-hydroxyphenyl)azoles are difficult. The
internal oxidative coupling strategy developed herein could
greatly streamline access to such scaffolds. No matter whether
TPA is located on the phenol or azole moiety, both substrates
Further investigation demonstrates that 5e and 5 f exhibit
large HOMO–LUMO energy gaps and low-lying HOMO
levels. The HOMO–LUMO optical energy gaps estimated
from the absorption edges are 2.95 eV for 5e and 2.92 eV for
5 f, and the HOMO levels defined by cyclic voltammetry are
À5.22 eV for 5e and À5.23 eV for 5 f (see Table S2 and
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 14008 –14012
Li, Bijin
