Orientation effects on C2(5)-C2?(5?) linked bioxazole isomers synthesized via regioselective and sequential C[sbnd]H arylation

Article abstract of DOI:10.1016/j.cclet.2020.05.010

Bis(4-fluorophenyl) substituted oxazole (2,5-Oxz) and C2(5)-C2?(5?) linked bioxazole isomers (C2-C2?_BOxz, C2-C5?_BOxz and C5-C5?_BOxz) were concisely synthesized via palladium-catalyzed regioselective and sequential C[sbnd]H arylation in 1–3 reaction steps along with 20%–83% of total yields from oxazole and 4-bromofluorobenzene. The linking orientation plays a key role in the packing geometry and photophysical properties of C2-C2'_BOxz, C2-C5'_BOxz and C5-C5'_BOxz. These bioxazole isomers in solid state showed significant differences in photoluminescence quantum yields (PLQY) (0.33, 0.25 and 0.04, respectively), delayed fluorescence properties and powder X-ray diffraction (PXRD) patterns, suggesting the divergence in intermolecular interactions. The theoretically calculated gradient isosurfaces and complexation energies indicate the existence of intense π-π interactions between molecular layers, which are in good agreement with the variation trend of optical properties.

Full text of DOI:10.1016/j.cclet.2020.05.010

Journal Pre-proof
Orientation effects on C2(5)-C2null(5null) linked bioxazole isomers
synthesized via regioselective and sequential CnullH arylation
Qiang Guo, Li Tao, Chuanqi Liu, Xiaoyun Zhao, Danyang Wan,
Jincheng Zhang, Jianping Ai, Jie Li
PII:
S1001-8417(20)30287-4
https://doi.org/10.1016/j.cclet.2020.05.010
CCLET 5648
DOI:
Reference:
To appear in:
Chinese Chemical Letters
Received Date:
Revised Date:
Accepted Date:
5 April 2020
29 April 2020
9 May 2020
Please cite this article as: Guo Q, Tao L, Liu C, Zhao X, Wan D, Zhang J, Ai J, Li J, Orientation
effects on C2(5)-C2x2b9;(5x2b9;) linked bioxazole isomers synthesized via regioselective and
sequential Cx2012;H arylation, Chinese Chemical Letters (2020),
doi: https://doi.org/10.1016/j.cclet.2020.05.010
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Chinese Chemical Letters
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Communication
Orientation effects on C2(5)-C2ʹ(5ʹ) linked bioxazole isomers synthesized via
regioselective and sequential C‒H arylation
a
a
a
a
b
a
a
Qiang Guo , Li Tao , Chuanqi Liu , Xiaoyun Zhao , Danyang Wan , Jincheng Zhang , Jianping Ai , Jie
Li 
a,c,

Corresponding author.
E-mail address: lijie@cuit.edu.cn
a
College of Optoelectronic Engineering, Chengdu University of Information Technology, Chengdu 610225, China
Optical and Electrical Material Center, Xi’an Modern Chemistry Research Institute, Xi’an 710065, China
National Engineering Research Center for Domestic and Building Ceramics, Jingdezhen Ceramic Institute, Jingdezhen 333403, China
b
c
Graphical Abstract
Bis(4-fluorophenyl) substituted oxazole and C2(5)-C2ʹ(5ʹ) linked bioxazole isomers were concisely synthesized via palladium-
catalyzed regioselective and sequential C‒H arylation. Their photophysical properties as well as orientation effects were
investigated by experimental and computational analysis.
—
——

2
ARTICLE INFO
ABSTRACT
Article history:
Bis(4-fluorophenyl) substituted oxazole (2,5-Oxz) and C2(5)-C2ʹ(5ʹ) linked bioxazole isomers
(C2-C2ʹ_BOxz, C2-C5ʹ_BOxz and C5-C5ʹ_BOxz) were concisely synthesized via palladium-
catalyzed regioselective and sequential C‒H arylation in 1-3 reaction steps along with 20%-83%
of total yields from oxazole and 4-bromofluorobenzene. The linking orientation plays a key role
in the packing geometry and photophysical properties of C2-C2'_BOxz, C2-C5'_BOxz and C5-
C5'_BOxz. These bioxazole isomers in solid state showed significant differences in
photoluminescence quantum yields (PLQY) (0.33, 0.25 and 0.04, respectively), delayed
fluorescence properties and powder X-ray diffraction (PXRD) patterns, suggesting the divergence
in intermolecular interactions. The theoretically calculated gradient isosurfaces and complexation
energies indicate the existence of intense π-π interactions between molecular layers, which are in
good agreement with the variation trend of optical properties.
Received 5 April 2020
Received in revised form 29 April 2020
Accepted 3 May 2020
Available online
Keywords:
Bioxazole isomers
Palladium-catalyzed
C‒H arylation
Orientation effects
Photophysical properties
Theoretical calculation
Organic semiconductor materials have been widely applied in
various fields, such as organic light-emitting diodes (OLEDs),
organic field-effect transistors (OFETs), organic photovoltaic
lengthy synthesis steps and limited range of substrate scope,
which would entail barriers for further research and applications
of bioxazole derivatives.
(
OPV) devices and sensors [1-4]. They will be important
In recent years, transition-metal-catalyzed C‒H direct
arylations of non-prefunctionalized (hetero)arenes have been
efficient and concise routes to construct π-conjugated small
molecules, oligomers and polymers [14,15]. Through the C‒H
direct arylation using specific C–H bonds of (hetero)arenes as
functional groups, the requirement of preparing organometallic
reagents and formation of organometallic salt as by-product in
classical coupling methods can be avoided. Additionally, owing
to the electron-withdrawing nature of C=N and electron-donating
nature of heteroatom (O, S or N) in azoles, the C‒H bonds on C2,
C4 and C5 positions show diverse chemical activation. As a
consequence, the site-selective arylation of azoles at C2 or C5
position are able to be controlled depending on the reaction
conditions [16,17]. Considering the potential applications of
bioxazole derivatives and the advantages of C‒H arylation, we
herein presented a straightforward and concise way to synthesize
2,5-biarylated oxazole and bioxazole isomers (C2‒C2ʹ, C2‒C5ʹ
and C5‒C5ʹ linked orientation) via palladium-catalyzed
regioselective and sequential C‒H arylation. Under appropriate
reaction conditions, the 4-fluorophenyl substituted (bi)oxazole
isomers can be obtained in only 1-3 reaction steps with 20%-
83% of total yields. And the orientation effects on these
isomers were also investigated by experimental and
computational analysis.
components for the next-generation displays and wearable
electronic devices owing to their distinct advantages, such as
lightweight, flexibility and the modifiable molecular structures
along with tunable optical and electronic properties. 1,3-Azoles-
based materials (oxazole, thiazole and imidazole) have been
applied to high-performance organic electronic devices up to now
[
5-7]. Although having similar geometry as thiophene, furan and
pyrrole, 1,3-azoles are widely introduced into π-conjugated
semiconductors aiming at altering molecular orbital energy levels
and controlling film state morphology, owing to the weak
electron-withdrawing behavior, good ambient stability and large
polarity [7]. Biazoles, consisting of two asymmetric azoles, have
different site linked orientation (mainly C2‒C2ʹ, C2‒C5ʹ and C5‒
C5ʹ) in the molecular backbone, which could cause distinctly
different photoelectric properties [8-10]. Therefore, further
research about orientation effects on biazoles will promote
extensive applications of azoles. However, as far as we know, few
published studies about the orientation effects on bioxazole
isomers (C2‒C2ʹ, C2‒C5ʹ and C5‒C5ʹ) have been reported,
mainly due to the challenges in synthesizing biazole-based
building blocks [9], especially for synthesizing the bioxazole
isomers in concise methods. Conventionally, the bioxazole-based
derivatives were synthesized via the cyclization of amides or
coupling of organometallic reagents [11-13], with drawbacks of

Scheme 1. Synthetic routes to biarylated oxazole and bioxazole isomers via regioselective and sequential C‒H arylation. Condition A: 4-
Bromofluorobenzene (4.0 mmol), oxazole (2.0 equiv.), Pd(OAc) (5 mol%), butyldi-1-adamantylphosphine (10 mol%), K CO (3.0 equiv.),
PivOH (0.4 equiv.) in DMA (15 mL) at 110 C for 24 h. Condition B: 4-Bromofluorobenzene (4.0 mmol), oxazole (2.0 equiv.), Pd(OAc) (5
2
2
3
o
2
o
mol%), RuPhos (10 mol%), K
2
CO
3
(3.0 equiv.), PivOH (0.4 equiv.) in xylene (15 mL) at 110 C for 24 h. Condition C: 4-Bromofluorobenzene
o
(
4.0 mmol.), oxazole (2.0 equiv.), Pd(acac)
2
(2.5 mol%), Cs
(10 mol%), CuCl (20 mol%), Cu(OAc)
(10 mol%), Cs
(10 mol%), KOAc (2.0 equiv.) in DMA (4 mL) at 110 C for 24 h.
2
CO
3
(3.0 equiv.), in DMA (15 mL) at 100 C for 16 h. Condition D: 1a (1.0 mmol),
o
Pd(OAc)
2
2
(0.75 equiv.) in 1,4-dioxane/DMSO (v:v = 10:1, 4.4 mL), at 120 C for 1 h. Condition E: 1a
o
(1.0 mmol), 1c (1.0 mmol), Pd(acac)
2
2 3
CO (3.0 equiv.) in DMA (4 mL) at 110 C for 24 h. Condition F: 1b (1.0 mmol), 1c (1.0
o
mmol), Pd(OAc)
2
Fluorinated organic materials have gained great interest in the
area of organic electronics, because of the strong electronegativity
of fluorine atom, weak C–F∙∙∙π interaction [18] and further
derivatization via C–F bond activation [19]. Therefore, 4-
fluorophenyl bioxazole isomers were chosen as our research
objects. During the past decade, many efforts have already been
done to overcome the challenges in C–H arylation of oxazoles [20-
palladium-catalyzed concerted metalation-deprotonation (CMD)
pathway [16]. To increase the ratio of 1b:2,5-Oxz and suppress
C5-arylation of 1b, the reaction condition was modified slightly.
After reducing catalyst Pd(acac)
2
loading to 2.5 mol%, lowering
reaction temperature to 100 C and shorting reaction time to 16 h
̊
(condition C), 1b was successfully obtained in 37% of isolated
yield.
2
5]. Among them, Strotman et al. [24] and Shi et al. [25] have
With C2- and C5-monoarylated oxazole isomers (1a and 1b) in
hand, we began to explore methods to construct bioxazole isomers.
The 2,2'-bioxazole isomer (C2-C2'_BOxz), was synthesized via a
straightforward and atom-economic way, oxidative C2–H
successfully realized the regiodivergent arylation (C2- vs. C5-
arylations) of unsubstituted oxazole under their optimized
palladium-catalyzed reaction conditions with or without
phosphine ligands, respectively. As shown in Scheme 1,
employing 4-bromofluorobenzene (1.0 equiv.) and oxazole (2.0
equiv.) as sources, C5-monoarylated oxazole (1a, 5-(4-
fluorophenyl)-oxazole) was obtained in moderate yield (51%) in
the presence of Pd(OAc)
phosphine (10 mol%), K CO
N,N-dimethylacetamide (DMA, 15 mL) at 110 C for 24 h,
homocoupling of 1a. A mixture of 1a (1.0 equiv.), Pd(OAc)
2
(10
mol%), CuCl (20 mol%), Cu(OAc) (0.75 equiv.) dissolved in 1,4-
2
o
dioxane/DMSO (10:1, v/v) was heating at 120 C for 1 h (Scheme
1, condition D) [26], and the reaction run smoothly to form C2-
C2'_BOxz in good yield (86%). After bromination of 1b at C5
position, brominated product 1c was undergoing C–H/C–Br type
direct arylation with 1a and 1b to afford C2-C5'_BOxz (yield =
60%) and C5-C5'_BOxz (67% yield) under condition E and
condition F, respectively (Scheme 1). Under our synthetic strategy,
2,5-biarylated oxazole (2,5-Oxz) and bioxazole isomers (C2-
C2'_BOxz, C2-C5'_BOxz and C5-C5'_BOxz) can be successfully
obtained via palladium-catalyzed sequential C–H arylation in only
1-3 reaction steps along with total yields of 20%-83% from
commercially available sources. It is worth noting that the
substituent groups or coupling position of (bi)oxazoles are able to
be programed owing to the feasibility of regioselective arylation
of oxazoles.
2
(5 mol%), butyldi(1-adamantyl)-
2
3
(3.0 equiv.), PivOH (0.4 equiv.) in
o
(
(
Scheme 1, condition A). Unhappily, only 2,5-diarylated oxazole
2,5-bis(4-fluorophenyl)-oxazole, abbreviated as 2,5-Oxz) was
obtained in good yield (83%) instead of the desired C2-
monoarylated product under condition B. When choosing a new
catalytic system, 5 mol% of Pd(acac)
2
, 3 equiv. of Cs
DMA at 110 C for 24 h, a mixture of C2-monoarylated oxazole
1b, 2-(4-fluorophenyl)-oxazole) and 2,5-diarylated oxazole (2,5-
2 3
CO , in
o
(
Oxz) was detected. Obviously, the C5-arylation of formed 2-(4-
fluorophenyl)-oxazole and C2-arylation of oxazole are
competitive reactions under condition B. Because the activation
energy of C‒H bond at C5-position of oxazole is lower than that
at C2-positon, the arylation of C5‒H bond is always preferred in
Subsequently, as summarized in Table 1 and Fig. 1, optical
properties of 2,5-biarylated oxazole (2,5-Oxz) and bioxazole

isomers (C2-C2'_BOxz, C2-C5'_BOxz and C5-C5'_BOxz) were
characterized. All four compounds show very similar absorption
in the near-ultraviolet (UV) region (λmax,abs = 303, 334, 332, 328
nm in toluene and 305, 335, 334, 328 nm in doped films,
respectively) and various photoluminescence in the near-UV-blue
region (λmax,PL = 363, 399, 399, 395 nm in toluene and 362, 422,
in doped films exhibit two decay components, suggesting the
existence of intermolecular interaction [28]. Compared with C2-
C2'_BOxz and C2-C5'_BOxz, C5-C5'_BOxz possesses the most
distinctive delayed feature with emission lifetime about 9.4 μs,
which indicates that C5-C5'_BOxz has the strongest
intermolecular interaction in doped film. Moreover, all the PXRD
patterns presented clearly different diffraction peaks, indicating
their difference in molecular packing (Fig. S4 in Supporting
information) [29]. And the obvious and maximum 2θ peak
position at 33.2° of C5-C5'_BOxz also illustrates the existence of
a tightest d-spacing.
4
39, 447 nm in doped films, respectively). Meanwhile, the
photoluminescence quantum yields (PLQYs) of them in doped
films drastically decrease as compared to that in toluene. In
contrast with that of bioxazole isomers, mono-oxazole derivative
(2,5-Oxz) shows obvious blue-shift in absorption and
photoluminescence spectra, owing to its less π-conjugation. The
close resemblance of absorption spectra of the three bioxazole
isomers both in toluene solution and in thin films states indicate
that they have similar ground states. On the contrary, the
photoluminescence spectra of bioxazole isomers in film states are
different from each other. The maximum emission wavelengths of
C5-C5'_BOxz (λmax,PL = 447 nm) and C2-C5'_BOxz (λmax,PL = 439
nm) in film state show obvious red-shift comparing with that of
C2-C2'_BOxz (λmax,PL = 422 nm). We speculate that C5-C5'_BOxz
and C2-C5'_BOxz show stronger intermolecular interaction than
C2-C2'_BOxz in solid state, because strong intermolecular
interaction usually results in a spectral red-shift [27]. Meanwhile,
the PLQY of C5-C5'_BOxz in film state (0.04) is much lower than
that of the other two isomers (0.25 for C2-C5'_BOxz and 0.33 for
C2-C2'_BOxz). In order to clarify the intermolecular interactions
more clearly, transient PL decay characteristics and powder X-ray
diffraction (PXRD) of these compounds were measured (Figs. S1-
S4 and Table S1 in Supporting information). All the bioxazole
isomers in solution show only one prompt component with
emission lifetimes about 1.20-1.38 ns. On the contrary, all of them
Fig. 1. Normalized UV-vis absorption (a for solutions and c for films)
and photoluminescence (b for solutions and d for films) spectra of 2,5-
Oxz, C2-C2'_BOxz, C2-C5'_BOxz and C5-C5'_BOxz.
Table 1
Experimental and calculated optical properties of 2,5-biarylated oxazole and bioxazole isomers.
e
Experimental data
Solution
Calculated data
Solution
a
b
Compound
,5-Oxz
C2-C2'_BOxz 334
C2-C5'_BOxz 332
C5-C5'_BOxz 328
Film
λ
c
d
c
d
-1
-1
λmax,abs (nm)
λmax,PL (nm) PLQY
max,abs (nm)
λ
max,PL (nm) PLQY
λmax,abs (nm)
λmax,PL (nm)
k
r
(s )
k
nr (s )
8
4
4
4
5
2
303
363
399
399
395
0.69
0.77
0.72
0.75
305
335
334
328
362
422
439
447
0.11
0.33
0.25
0.04
302
327
326
327
358
396
393
394
4.94×108 3.76×10
5.50×10
5.42×10
5.37×10
4.58×10
3.11×10
8.80×10
8
8
a
b
c
d
e
-5
In dilute toluene solution (2×10 mol/L), abs = absorption, PL = photoluminescence.
0 wt% (bis)oxazole : polystyrene (PS) doped film.
1
Excited at the corresponding maximum absorption wavelength.
Absolute quantum yield.
Calculated by Gaussian 16 package and MOMAP program.
To further understand the orientation effects on these oxazole
derivatives, theoretical calculations were carried out by using
Gaussian 16 package [30]. Ground state geometries (S ) of
0
structures were fully optimized by using M06-2X hybrid
functional [31] of density functional theory (DFT) with 6-311G(d)
1
basis set, and their lowest singlet excited state (S ) geometries
were also optimized by using time-dependent density functional
theory (TD-DFT) at M06-2X/6-311G(d) level. Additionally, the
optimized geometries were further characterized by vibrational
frequencies analysis to make sure all stationary points are minima
three to four orders of magnitude higher than their corresponding
4
-5 -1
nonradiative internal conversion rates (knr = 10 s ) (Table 1),
suggesting that these oxazole derivatives would be good
chromophores for pure blue emitters with high PLQY.
The highest occupied molecular orbital (HOMO), the lowest
unoccupied molecular orbital (LUMO) and molecular
electrostatic potential (ESP) were calculated with the optimized
0
S geometries (Figs. S6 and S7 in Supporting information). All
four fluorophenyl oxazole derivatives have highly coplanar
geometries, and the electron density of HOMO, LUMO and ESP
were delocalized over the entire backbones without any
conjugation breakages. Through the calculated data, all of the
(zero imaginary frequencies). TD-DFT was then employed to
simulate the absorption and emission properties. The absorption,
emission spectra and (non)radiative transition rates of oxazoles in
toluene were performed by MOMAP program [32]. As shown in
Fig. S5 (Supporting information), the calculated optical spectra
are in good agreement with their experimental spectra, indicating
that the M06-2X/6-311G(d) optimized geometries are reasonable
to estimate the ground state and excited state geometries. The
*
electronic transitions of oxazoles are of the π-π type transition for
both absorption and emission with main transition configuration
of HOMO → LUMO along with giant oscillator strength (all over
1.0) (Fig. S6 and Table S2 in Supporting information). The
molecular ESP of these four oxazole compounds can be seen in
Fig. S7. The most negative (VS,min) and positive (VS,max) values of
electrostatic potential on molecular surfaces [33-35], as well as
8
-1
r 1 0
calculated radiative transition rates (k = 10 s ) from S to S are

the extreme values at the aromatic ring regions are significantly
different, which would cause different strength of noncovalent
interaction and solid morphology [36]. In addition, the
complexation energies of dimers of bioxazole isomers in gas
phase were also calculated to estimate the strength of noncovalent
intermolecular interactions with functional and basis set of M06-
strong interaction of C5-C5'_BOxz suggest its potential
applications for OFET devices. This work provides a valuable
strategy for synthesizing C2(5)-C2ʹ(5ʹ) linked bioxazole
isomers as well as investigating the orientation effects on their
optical properties and further applications.
2
(
X-D3/6-311+G(d,p) [37]. The basis set superposition error
BSSE) corrected complexation energies of C2-C2'_BOxz, C2-
C5'_BOxz and C5-C5'_BOxz antiparallel displaced dimers are -
9.84, -21.68 and -21.81 kcal/mol, respectively. This increasing
Declaration of interests
1
The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to influence
the work reported in this paper.
trend (absolute value) of complexation energies of bioxazole
dimers indicated the increasing strength of intermolecular
interaction (π-π stacking interaction, shown in Fig. 2) [38], and
might be the reason for the red-shifts in the photoluminescence
spectra of bioxazole isomers in film state. Similar results, stronger
intermolecular interaction of 5,5'-biazoles than that of 2,2'-
biazoles, can also be observed at other literature and would be
beneficial for high-performance OFET [8].
Acknowledgments
We gratefully thank the financial support from the National
Natural Science Foundation of China (Nos. 21801028, 11704050,
6
1505015), Scientific Research Project of the Department of
Education of Sichuan Province (Nos. 2018Z099) and Project of
Sichuan Provincial Science and Technology Department (Nos.
2
017FZ0085). We would like to thank Dr. Danyang Wan for the
quantum chemical calculations in this paper.
Fig. 2. Gradient isosurfaces (s = 0.5 au) for antiparallel displaced
dimer of a) 2,5-Oxz, b) C2-C2'_BOxz, c) C2-C5'_BOxz and d) C5-
C5'_BOxz. ΔE means BSSE corrected complexation energies of
dimers; the color-mapped reduced density gradient (RDG) isosurface
between dimer means π-π interaction.
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