The Journal of Organic Chemistry
Note
acid (2) and the respective aryl halides (ArHal; Figure 2a). We
hypothesized that this methodology can be especially attractive
because starting materials are commercially available or can be
easily synthesized. As for example, 2 was easily obtained via the
(Table S4). For the D2-based device, the maximum external
quantum efficiency equaled to 2.82%. Additionally, this device
exhibited turn-on voltage at the level of 3.4 V and featured the
−
2
maximum luminance of 5515 cd·m at 8.9 V together with
as-fabricated OLED devices are characterized by the
satisfactory or comparable values in comparison with similar
aromatic dendrimers for OLED applications reported in the
23
acid-catalyzed trimerization of 4-cyanophenylboronic acid,
while 1-bromo-3,5-diphenylbenzene (3) was fabricated via
Suzuki−Miyaura cross-coupling reaction starting from 1,3,5-
24
tribromobenzene.
To our delight, compound D1 was successfully synthesized
Optimization experiments (Table S1) revealed that the highest
yield (50%) was archived with the use of tetrakis-
11−15
Thus, D1 and D2 might be
considered as new examples of aromatic dendrimers exhibiting
good light emission properties.
(
triphenylphosphine)palladium(0) as a catalyst and K CO
Encouraged by the efficient light-emission properties
exhibited by D1 and D2, we also investigated a fully innovative
application of these aromatic dendrimers as the photocatalysts
in the oxidation of benzylamine to N-benzylidene benzylamine.
Finding new ways of selective oxidative coupling of amines
yielding the respective imines, which serve as important
building blocks in organic synthesis and industry, is of a
highest importance. The reported catalytic systems commonly
include toxic metal complexes together with harsh reaction
2
3
as a base in toluene/ethanol/water solvent system under reflux
conditions. The formation of pure D1 was confirmed
1
1
1
13
spectroscopically ( H NMR, H− H COSY NMR,
C
Information for the full compound characterization data). It is
1
noteworthy that H NMR analysis revealed the presence of six
sets of signals for D1, which means that the D1 is indeed
symmetric.
27−29
conditions, such as high temperature or high pressure.
The successful synthesis of D2, which comprises four 1,3,5-
triazine skeletons, was achieved with the treatment of 2 with
the excess of 2-chloro-4,6-diphenyl-1,3,5-triazine (4) (Figure
Thus, there is a continuous interest in the design of new,
sustainable, and efficient approaches for aerobic oxidation of
30−38
amines to respective imines.
In the recent years there has
2
c). The reaction has been investigated by changing various
been a great progress in designing novel photocatalysts
dedicated to such oxidation processes. These studies include,
reaction temperature, and the type of a solvent (Table S2).
Under optimized reaction conditions that employed [1,1′-
bis(diphenylphosphino)ferrocene]dichloropalladium(II) as a
catalyst, K CO as a base in DMF at 80 °C, D2 was
3
9,40
41,42
e.g., the use of novel BODIPY,
phenoxazine,
or
43
salicylic acid derivatives as effective photocatalysts, also for
3
9,40
the aerobic oxidation processes.
We hypothesized that newly synthesized D1 or D2 might
work as an efficient catalyst in this photocatalytic process. First,
there are literature reports on the use of 1,3,5-triazine-
containing polymers or materials in photocatalytic oxidation
2
3
synthesized with 41% yield. Dendrimer D2 was subjected to
1
1
1
13
NMR analyses ( H NMR, H− H COSY NMR, C NMR,
for the full compound characterization data). Three sets of
37−44
of primary amines.
We supposed that D2 might exhibit
better catalytic performance than D1 because of above-
1
discussed enhanced light-emission properties arising from the
3
8
The profiles of UV−vis spectra of D1 and D2 (Figure S25)
were similar. Both D1 and D2 exhibited two strong absorption
maxima (λmax) located at ca. 250 nm and ca. 325 nm. Slight red
shift (ca. 5 nm) and higher molar absorption coefficient value
for λmax = 323 nm than for λmax = 254 nm was found for D2.
The observed absorption maxima were ascribed to the π−π*
transitions originating from the presence of benzene and 1,3,5-
higher content of pyridinic nitrogen atoms. Second,
45
Tauc plot method (3.5−3.6 eV; see Section S3) suggest that
oxidation as well as electron transfer processes in the studied
reaction should be feasible. The literature points that the
3
7,46
mechanism
would include the following key trans-
formations: (i) generation of an excited state of D1/D2
9
,16,17
triazine moieties.
(D1*/D2*) that reduces O to its active species: superoxide
2
radical (O2• ) and singlet oxygen ( O ), (ii) oxidation of
−
1
intensity for D1 and D2 were observed (Figure S25). Both D1
and D2 exhibited higher emission intensity values for
excitation wavelength (λ ) of 315 nm in comparison to λ
2
benzylamine to cationic intermediate by D1*/D2* that
mechanism, see Section S3). To enable such reaction pathway,
the LUMO level of a photocatalyst should be higher than the
respective value for oxygen (−3.8 eV), while the HOMO level
should be higher than HOMO of benzylamine (−5.9 eV).
Relatively high values of HOMO−LUMO band gaps for D1
The performance of D1 and D2 as photocatalysts in the
selective oxidation of benzylamine (5) to N-benzylidene
benzylamine (6) is summarized in Table S3. The reactions
were carried out on air at room temperature under irradiation
with UV-LED as a light source (wavelength of light of 365
nm). D1 and D2 exhibited the photocatalytic performance. D2
provided more satisfactory reaction results in comparison to
ex
ex
=
250 nm. D2 featured ca. 3-fold higher emission intensity
value in comparison to D1. Emission maximum (λ ) for D2
em
was also red-shifted (ca. 40 nm) in comparison to D1. These
changes were ascribed to the higher content of 1,3,5-triazine
skeletons in D2 (four units) in comparison to D1 (one unit)
5
,17,25
and the expansion of a π-conjugation system.
Only slight
quantum yields (Φ ) (λ = 315 nm) estimated by the relative
F
ex
2
6
method were 0.32 and 0.78, for D1 and D2, respectively.
Considering these satisfactory emission properties, prelimi-
nary OLED application trials with D1 and D2 were
1
3
are listed in Section S6. Device consisting of D2 exhibited
better performance than the respective device composed of D1
6
857
J. Org. Chem. 2021, 86, 6855−6862
Cyniak, Jakub S.
