Synthesis and photostability of 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) structural isomers and their trimethylsilyl derivatives

Article abstract of DOI:10.1016/j.dyepig.2017.02.006

In this work, a versatile synthetic method for preparation of linear phenyloxazoles and their organosilicon derivatives under mild conditions via a combination of van Leusen and direct C[sbnd]H arylation reactions is reported. It was used for the synthesi

Full text of DOI:10.1016/j.dyepig.2017.02.006

Accepted Manuscript
Synthesis and photostability of 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP)
structural isomers and their trimethylsilyl derivatives
Maxim S. Skorotetcky, Oleg V. Borshchev, Nikolay M. Surin, Yaroslav Odarchenko,
Sergey A. Pisarev, Svetlana M. Peregudova, Karl W. Törnroos, Dmitry Chernyshov,
Dimitri A. Ivanov, Sergey A. Ponomarenko
PII:
S0143-7208(16)31379-1
10.1016/j.dyepig.2017.02.006
DYPI 5780
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 December 2016
Revised Date: 30 January 2017
Accepted Date: 2 February 2017
Please cite this article as: Skorotetcky MS, Borshchev OV, Surin NM, Odarchenko Y, Pisarev
SA, Peregudova SM, Törnroos KW, Chernyshov D, Ivanov DA, Ponomarenko SA, Synthesis and
photostability of 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) structural isomers and their trimethylsilyl
derivatives, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.02.006.
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Synthesis and photostability of 1,4-bis(5-phenyloxazol-2-yl)benzene
(POPOP) structural isomers and their trimethylsilyl derivatives
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Maxim S. Skorotetcky a, Oleg V. Borshchev a, Nikolay M. Surin a, Yaroslav Odarchenko bᶧ, Sergey
A. Pisarev a,c, Svetlana M. Peregudova d, Karl W. Törnroos e, Dmitry Chernyshov f, Dimitri A.
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Ivanov b, g, Sergey A. Ponomarenko a, h
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a Enikolopov Institute of Synthetic Polymeric Materials of Russian Academy of Sciences,
Profsoyuznaya St. 70, Moscow 117393, Russia
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b Institut de Sciences des Matériaux de Mulhouse-IS2M, CNRS UMR 7361, 15 rue Jean Starcky,
68057 Mulhouse, France
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c Institute of Physiologically Active Compounds of Russian Academy of Sciences,
Severny Pr. 1, Chernogolovka 142432, Russia
d Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences,
Vavilova St. 28, Moscow 119991, Russia
e Department of Chemistry, University of Bergen, Norway
f Swiss−Norwegian Beam Lines, ESRF BP-220, 38043 Grenoble, France
g Faculty of Fundamental Physical and Chemical Engineering, Moscow State University, GSP-1,
Leninskie Gory 1, 119991, Moscow, Russia
h Chemistry Department, Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia
*e-mail: ponomarenko@ispm.ru
ᶧCurrent address: Department of Chemistry, University College London, 20 Gordon Street,
London, WC1H 0AJ, UK

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In this work, a versatile synthetic method for preparation of linear phenyloxazoles and their
organosilicon derivatives under mild conditions via a combination of van Leusen and direct C-H
arylation reactions is reported. It was used for the synthesis of a novel trimethylsilyl (TMS)
derivative of blue laser dye 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) and its structural
isomer 1,4-bis(2-phenyloxazol-5-yl)benzene (isoPOPOP) with and without TMS substituents. All
of the compounds demonstrated high photoluminescence quantum yields and large molar
extinction coefficients, which further increased for their TMS derivatives. Measurements of
accelerated photodegradation of the dyes in diluted THF solutions revealed that POPOP-type
compounds are twice more stable than their isoPOPOP-type isomers, independent of the
presence or absence of TMS substituents. Cyclic voltammetry (CV) investigations revealed that
POPOP-type dyes are more electrochemically stable as compared to their isoPOPOP-type
isomers. Quantum chemical TD DFT calculations of the frontier energy levels of the dyes were
in a good agreement with the experimental data obtained by CV. Single-crystal X-ray diffraction
experiments revealed significant structural differences between crystal lattices of the dyes. Due
to facile synthesis and excellent optical properties as well as high photo- and thermo- stability
the novel luminescent dyes may find wide application in organic photonics.
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Keywords: phenyloxazole, direct C-H arylation, organosilicon luminophores, stability,
photodegradation, crystal structure
1. Introduction
Organic molecules emitting light in various spectral regions are widely used in many
fields of science and technology. Among them are aryl-substituted oxazoles, which are highly
efficient violet or blue emitting luminophores [1,2]. A burst of interest towards this class of
materials was triggered by their applicability as activators and spectral shifters in liquid and

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plastic scintillators detecting elementary particles and ionizing radiation [3,4]. Moreover, they
are widely used as dyes for organic lasers [5,6] and spectral shifters in photonics [7]. Synthetic
approaches for preparation of phenyloxazoles are well known: after E. Fisher had synthesized
2,5-diphenyloxazole in 1896, a wide variety of techniques for the preparation of this compound
and its derivatives were reported [8]. Additional interest of researchers is drawn to biological
activity of oxazole as a structural moiety of various alkaloids as well [9].
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In previous works we introduced a new class of highly efficient dyes – nanostructured
organosilicon luminophores (NOLs), which combine the best properties of quantum dots with
those of organic luminophores [10–13]. NOL molecule consists of two types of different organic
dyes covalently bonded to each other via silicon atoms, while one of the dyes has a wider and
the other – a lower bandgap relative to each other. Such structure ensures efficient
nonradiative intramolecular energy transfer (the so called “dendritic molecular antenna effect”
[14,15]), while the choice of organic dyes provides tunability of the optical properties of the
molecules such as the absorption and photoluminescence (PL) spectra lying within a desired
spectral region. One of the prospective organic dyes to be applied for NOLs architecting is a
blue-emitting luminophore 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP). It has a high
photoluminescence quantum yield (PLQY) along with a relatively large Stokes shift. The main
technique for its preparation is the Robinson-Gabriel synthesis using the corresponding ω–
amino methyl ketone and terephthaloyl chloride as the precursors, followed by dehydration in
concentrated sulfuric acid [16]. This method is yet not suitable for the preparation of POPOP
organosilicon derivatives as the latter are not stable in acidic conditions. At the same time, a
general approach for preparation of organosilicon derivatives of organic luminophores is based
on the reaction between organolithium precursors with corresponding chloro- or alkoxy-silanes
[17]. However, the oxazole ring undergoes decyclization in the presence of organolithium
reagents [18]. Therefore, the primary goal of this work was to develop a mild synthetic protocol
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for preparation of organosilicon derivatives of POPOP, which would allow overcoming the
contradiction between the low stabilities of arylsilanes and oxazole cycle at strong acidic and
strong basic conditions, respectively. While doing this work, it was found that preparation of
1,4-bis(2-phenyloxazol-5-yl)benzene iso-POPOP and its organosilicon derivatives under mild
conditions can be achieved easily as the POPOP itself. Therefore, the secondary goal of this
work was to compare the properties, in particular photostability, of both isomers and their
TMS-derivatives.
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2. Experimental section
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2.1. Materials
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Hexane solutions of 2.5 M n-butyllithium, toluenesulfonylmethyl isocyanide (TosMIC),
terephthaloyl aldehyde, anhydrous potassium carbonate, bromobenzene and 1,4-
dibromobenzene were obtained from Acros organics; tetrakis(triphenylphosphine)palladium (0)
Pd(PPh3)4, lithium tert-butoxide, 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) scintillation
grade were obtained from Sigma-Aldrich and used as received. 1-Bromo-4-
(trimethylsilyl)benzene (3) was obtained by the method described previously [19]. Synthesis of
1,4-bis(1,3-oxazol-5-yl)benzene (2), 1-bromo-4-(trimethylsilyl)benzene (3) and 4-(trimethylsilyl)
benzaldehyde (4) is described in the results and discussion section. THF, 1,4-dioxane and
methanol were dried and purified according to the well-known techniques and then used as the
solvents.
2.2. Characterization
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The H NMR spectra were recorded on a Bruker WP250 SY spectrometer (250.13 MHz)
using the residual signal of CDCl3 (δ 7.27 ppm) as the internal standard. The 13C NMR spectra
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were recorded on a Bruker Avance II 300 spectrometer at 75 MHz. In the case of H NMR
spectroscopy, the compounds to be analyzed were taken in the form of 1% solutions in CDCl3.
In the case of 13C NMR spectroscopy, the compounds to be analyzed were taken in the form of

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3-5% solutions in CDCl3. The spectra were then processed on the computer using the ACD Labs
software. Mass-spectra (MALDI) were registered on the Autoflex II Bruker (resolution FWHM
18000), equipped with nitrogen laser (work wavelength 337 nm) and time-of-flight mass-
detector working in the reflections mode. The resulting spectrum was the sum of 300 spectra
obtained at different points of the sample. 2,5-Dihydroxybenzoic acid (DHB) (Acros, 99%) and
α-cyano-4-hydroxycinnamic acid (HCCA) (Acros, 99%) were used as the matrices.
Elemental analysis of C, H, N elements was carried out using CHN automatic analyzer
CE1106 (Italy). Experimental error is 0.30−0.50 %. The burning was done in the Sheninger flask
using alkaline solution of hydrogen peroxide as an absorbent. Spectrophotometry technique
was used for the Si analysis.
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Thermogravimetric analysis was carried out in dynamic mode in 30-800 °C interval using
Mettler Toledo TG50 system equipped with M3 microbalance allowing measuring the weight of
the samples in 1-150 mg range with 1 µg precision. Heating/cooling rate was chosen to be 10
°C/min. Every compound was studied twice: in air and under nitrogen flow of 200 mL/min.
Polarization optical microscope Carl Zeiss Axioscop A40Pol with Linkam temperature controller
were used to determine the melting point and for the microphotographs of the crystals.
GPC analysis was performed on a Shimadzu instrument with a RID10AVP refractometer
and a SPD-M10AVP diode matrix as detectors using 7.8 × 300 mm2 Phenomenex columns (USA)
filled with the Phenogel sorbent with pore size of 500 Å and THF as the eluent. In the case of
column chromatography, silica gel 60 (“Merck”) was taken. For thin layer chromatography
(TLC), “Sorbfil” (Russia) plates were used. The solvents were removed under vacuum (1 mBar)
at 40 °C. All reactions, unless stated otherwise, were carried out in an inert atmosphere using
anhydrous solvents.
The absorption spectra were recorded on a Shimadzu UV-2501PC spectrophotometer in
the standard 10 mm photometric quartz cuvette using THF solutions with the concentrations of

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10-5 M. A scanning spectrofluorimeter ALS01M with registration in single photon counting
mode at successive time intervals and automatic adjustment of the intensity of the measured
emission was used for the measurements of PL spectra. Measurements were carried out for
several optical densities in the range from 0.06 to 0.12 absorbance units in 10 mm cuvette,
measurement geometry – 90°. The PLYQ was measured by comparing the integral PL intensity
of 10-6 M dilute solutions of luminophores in THF with the integral PL intensity of the standard
(POPOP) as described elsewhere [20].
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Cyclic voltammetry (CV) measurements were carried out on films rubbed on the
electrode in acetonitrile using 0.1 M Bu4NPF6 as the supporting electrolyte. The scan rate was
200 mV/s. The glassy carbon electrode was used as a working electrode. A platinum plate was
used as the counter electrode. The potentials were measured relative to a saturated calomel
electrode (SCE). The standard potentials for quasireversible reduction and irreversible oxidation
prosesses were estimated as φred = E1/2 = Ered + 0.06 V and φox = E1/2 = Eox - 0.06 V as
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red
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reported before [21]. CV curves of the luminophores investigated are shown in the Supporting
Information (Fig. S9).
2.3. Structure studies and molecular modeling
Quantum-mechanical calculations were performed using the Firefly software package
[22] with B3LYP5/6-31G[d] basis set and level theory [23]. Optimization of the singlet ground
state geometry was carried out without restriction on the symmetry. Adiabatic calculation of
excited states was carried out by TD DFT using Tamm-Dancoff approximation (TDA). In TD DFT
calculations 15 of the lower energy states were optimized.
Single-crystal X-ray diffraction was measured at the Swiss−Norwegian Beamlines of the
ESRF. The crystals for all compounds were formed during the recrystallization from toluene
solutions. The X-ray patterns were collected with a PILATUS2M pixel area detector. A
monochromatic beam at a wavelength of λ = 0.69411 Å was slit-collimated down to 100 × 100

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μm2. The sample-to-detector distance and parameters of the detector were calibrated using a
LaB6 NIST standard. The detector images were recorded by phi-scans in shutter-free mode with
a 0.1 deg angular step. The data were preprocessed by SNBL Tool Box [24] and then by CrysAlis
Pro [25]. The crystal structures were solved with SHELXS and refined with SHELX [26].
CCDC1493771 and CCDC1493772 contain the supplementary crystallographic data for this
paper. The data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/structures
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2.4. Synthesis of the phenyloxazoles
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5-(4-trimethylsilylphenyl)-1,3-oxazole (5). 4-(trimethylsilyl)benzaldehyde (4) (2.5 g, 14
mmol, 1 equiv.) and TosMIC (2.87, g 14.7 mmol, 1.05 equiv.) were dissolved in methanol (50
mL), followed by addition of anhydrous potassium carbonate (3.8 g, 28 mmol, 2 equiv.) [Note:
during addition of potassium carbonate the reaction mixture may produce significant amounts
of heat]. The reaction mixture was heated to reflux for 2 h. The progress of the reaction was
monitored by TLC and GPC analyses. Then the reaction mixture was cooled to room
temperature (23 °C) and poured into water (150 mL) and diethyl ether (150 mL). The organic
phase was separated, washed with water, dried over sodium sulfate and filtered. The solvent
was removed under reduced pressure on the Rotorvapor. The crude product was purified by
chromatography on silica gel using toluene as an eluent to give 2.71 g (yield 85.3%) of 96% pure
product 5 according to GPC analysis, which was used in the subsequent reactions without
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additional purification. H NMR (250 MHz, CDCl3): δ [ppm] 0.29 (s, 12H), 7.37 (s, 1H), 7.60 (dd,
4H, J1 = 17.39 Hz, J2 = 8.20 Hz), 7.92 (s, 1H). 13C NMR (75 MHz, CDCl3): δ [ppm] -1.23, 121.63,
123.51, 127.91, 133.84, 141.48, 150.47, 151.61. Calcd (%) for C12H15NOSi: C, 66.31; H, 6.96; N,
6.44; Si, 12.92. Found: C, 66.12; H, 7.07; N, 6.20; Si, 12.65.
1,4-bis(2-phenyloxazol-5-yl)benzene (isoPOPOP). Solutions of lithium tert-butoxide
(0.94 g, 12 mmol, 5 equiv.), 1,4-bis(1,3-oxazol-5-yl)benzene (2) (0.5 g, 2.4 mmol, 1 equiv.),

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bromobenzene (0.89 g, 5.6 mmol, 2.3 equiv.) and Pd(PPh3)4 (54 mg, 0.047 mmol, 2%) in
1,4-dioxane (60 mL) was heated to boiling. The reaction mixture was refluxed for 1 h. Progress
of the reaction was monitored by GPC analysis. After completeness of the reaction, the reaction
mixture was cooled to room temperature (23 °C) and poured into water (200 mL), filtered off
and thoroughly washed with water and acetone. The crude product was passed over a short
column of silica gel in toluene: ethyl acetate 10:1 mixture to remove the rest of the catalyst.
The product was purified by recrystallization from toluene to give a pure compound (686 mg,
yield 80%) as yellowish needle crystals, mp. 238-240 °C. 1H NMR (250 MHz, CDCl3): δ [ppm] 7.53
(8H, overlapping peaks), 7.82 (s, 4H), 8.15 (4H, overlapping peaks). 13C NMR (75 MHz, CDCl3): δ
[ppm] 124.12, 124.65, 126.38, 127.29, 127.78, 128.89, 130.53, 150.69, 161.45. Calcd (%) for
C24H16N2O2: C, 79.11; H, 4.43; N, 7.69. Found: C, 78.93; H, 4.53; N, 7.53. MALDI MS: found m/z
364.39; calculated for [M]+ 364.396.
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1,4-bis{2-[4-(trimethylsilyl)phenyl]-1,3-oxazol-5-yl}benzene (TMS-isoPOPOP-TMS). This
compound was obtained by the method described above for isoPOPOP using lithium tert-
butoxide (0.75 g, 9.4 mmol, 5 equiv.), 1,4-bis(1,3-oxazol-5-yl)benzene (2) (0.4 g, 1.9 mmol, 1
equiv.), 1-bromo-4-(trimethylsilyl)benzene (3) (1.3 g, 5.6 mmol, 3 equiv.) and Pd(PPh3)4 (43 mg,
0.04 mmol, 2 %) in 1,4-dioxane (50 mL). The product was purified by recrystallization from
toluene to give pure compound (709 mg, yield 74%) as yellowish plate-like crystals, mp. 267-
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269 °C. H NMR (250 MHz, CDCl3): δ [ppm] 0.33 (s, 18H), 7.52 (s, 2H), 7.66 (d, 4H, J = 8.55 Hz),
7.81 (s, 4H), 8.10 (d, 4H, J = 8.55 Hz). 13C NMR (75 MHz, CDCl3): δ [ppm] -1.27, 124.12, 124.60,
125.34, 127.35, 127.73, 133.73, 143.80, 150.61, 161.52. Calcd (%) for C30H32N2O2Si2: C, 70.82; H,
6.34; N, 5.51; Si, 11.04. Found: C, 71.07; H, 6.43; N, 5.39; Si, 11.09. MALDI MS: found m/z
508.75; calculated for [M]+ 508.758.
1,4-bis{5-[4-(trimethylsilyl)phenyl]-1,3-oxazol-2-yl}benzene (TMS-POPOP-TMS). This
compound was obtained by the method described above for isoPOPOP using lithium tert-

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butoxide (1.7 g, 21 mmol, 5 equiv.), 1,4-dibrombenzene (1 g, 4.2 mmol, 1 equiv.), 5-(4-
trimethylsilylphenyl)-1,3-oxazole (5) (2.21 g, 10 mmol, 2.4 equiv.) and Pd(PPh3)4 (98 mg, 0.085
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mmol, 2 %) in 1,4-dioxane (100 mL). The product was purified by recrystallization from toluene
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to give pure compound (1.78 g, yield 83%) as yellowish plate-like crystals, mp. 269-271 °C. H
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NMR (250 MHz, CDCl3): δ [ppm] 0.32 (s, 18H), 7.52 (s, 2H), 7.62 (d, 4H, J = 7.94), 7.73 (d, 4H, J =
7.94), 8.24 (s, 4H). 13C NMR (75 MHz, CDCl3): δ [ppm] -1.24, 123.40, 123.99, 126.61, 127.94,
128.71, 133.87, 141.51, 151.80, 160.43. Calcd (%) for C30H32N2O2Si2: C, 70.82; H, 6.34; N, 5.51;
Si, 11.04. Found: C, 70.56; H, 6.24; N, 5.51; Si, 10.84. MALDI MS: found m/z 508.75; calculated
for [M]+ 508.758.
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3. Results and discussion
3.1. Synthesis
The novel method developed for the synthesis of POPOP-like luminophores is described
in details below. It comprises a step-by-step upbuilding of a phenyloxazole fragment, leading to
the formation of POPOP luminophore structure. The method is based on the preparation of
5-substituted oxazoles starting from aldehydes under Van Leusen conditions [27] as well as on
the palladium complexes catalyzed direct C-H arylation [28], which has been widely used for the
synthesis of various conjugated oligomers and polymers with atomic precision [29]. The latter
method is especially important for oxazoles due to the lack of the need to synthesize reactive
organolithium, organomagnesium, organozinc, organoboron or organotin precursors leading to
a reduced number of reaction steps, and since the direct C-H arylation may lead to the products
of selective substitution at 2 or 5 positions in a given asymmetric heterocycle depending on the
reaction conditions and the catalyst used [30].
Two synthetic schemes were suggested based on the methods described above (Fig.1).
It was found that preparation of the POPOP isomer – 1,4-bis(2-phenyloxazol-5-yl)benzene (iso-
POPOP), which differs from the POPOP solely by the position of oxazole rings relative to the

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central 1,4-phenylene unit, comprises fewer reaction steps (method 1) as compared to the
synthesis of the exact POPOP structure (method 2). At the same time, method 1 is preferable
for further development of corresponding NOL preparation approach, since it allows direct
using of already available 4-bromophenylsilyl functional luminophore precursors [10]. However,
we were surprised to find out that almost no data is available in the literature regarding to this
iso-POPOP structure. The only description of this POPOP isomer preparation was reported in
1979 [31]. In this case 1,4-bis(2-phenyloxazol-5-yl)benzene was obtained via Lewis acid (boron
fluoride) catalyzed decomposition of the corresponding α-diazoketophenone in excess of
benzonitrile in 80% yield. However, this method is not applicable for the synthesis of
organosilicon derivatives due to sophisticated procedures of the starting materials preparation
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and high affinity of silicon to fluoride ion.
Method 1
R
Br
N
MeOH, TosMIC
K2CO3, reflux
H
O
O
H
R
N
N
O
O
Dioxane, t-BuOLi, Pd(PPh3)4
R
91%
O
O
N
1
2
R=H, isoPOPOP (yield 80%)
R=TMS, TMS-isoPOPOP-TMS ( yield 74%)
Method 2
Br
Br
CH3
CH3
H3C Si
CH3
O
CH3
H3C Si
CH3
O
1) THF, nBuLi, -78°C
MeOH, TosMIC
K2CO3, reflux
H3C Si
CH3
Br
N
Dioxane, t-BuOLi, Pd(PPh3)4
83%
2) DMF, HCl
93%
H
85%
5
4
3
H3C
Si
CH3
N
O
CH3
O
S
H3C
O
H3C
TosMIC =
Si
CH3
N+
C-
H3C
O
N
TMS-POPOP-TMS
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Fig. 1. Scheme of the synthesis of 1,4-bis(2-phenyloxazol-5-yl)benzene (isoPOPOP), 1,4-bis{2-[4-
(trimethylsilyl)phenyl]-1,3-oxazol-5yl}benzene (TMS-isoPOPOP-TMS) and 1,4-bis{5-[4-
(trimethylsilyl)phenyl]-1,3-oxazol-2-yl}benzene (TMS-POPOP-TMS).
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At the first stage of the isomeric luminophores synthesis a symmetric center 1,4-bis(1,3-
oxazol-5-yl)benzene (2) [32] was prepared from terephthaloyl aldehyde (1) and
tosylmethylisocyanate (TosMIC) under Van-Leusen conditions. The reaction was performed in
boiling methanol as a solvent. Unexpectedly, it was found that usage of ethanol instead of
methanol does not lead to the target product formation. Nevertheless, a mixture of methanol
and THF may be applied for aldehydes, which have poor solubility in methanol. IsoPOPOP and
its trimethylsilyl derivative, 1,4-bis{2-[4-(trimethylsilyl)phenyl]-1,3-oxazol-5yl}benzene (TMS-
isoPOPOP-TMS), were obtained at the following stages under direct C-H arylation conditions
starting from the corresponding bromides. This reaction was initially carried out under
microwave synthesis conditions (according to the original method [28] the reaction was
performed in sealed tubes above the boiling point of the solvent). However, it was found that
regular boiling leads to a rapid (30 min according to GPC analysis) reaction progress in a high
yield (74-83%) as well. Both compounds obtained were purified by means of recrystallization
from toluene.
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The second method demonstrates synthesis of the molecule identical to that of POPOP
luminophore bound to trimethylsilyl substituents - 1,4-bis{5-[4-(trimethylsilyl)phenyl]-1,3-
oxazol-2-yl}benzene (TMS-POPOP-TMS). In this case, 1-bromo-4-(trimethylsilyl)benzene (3) was
lithiated at the first stage at low temperature in THF to yield corresponding organolithium
derivative, which was reacted with DMF. The salt obtained formed the aldehyde via
acidification by the stoichiometric amount of 1 M HCl. 4-(Trimethylsilyl)benzaldehyde
synthesized (4) [33] was transformed into 5-substituted 4-(trimethylsilyl)phenyloxazole (5)
under Van-Leusen conditions. At the last stage the latter was subjected to direct C-H arylation
with 1,4-dibromobenzene. Although this method comprises more steps, all of them are
sufficiently selective and proceed in high yields (83-93%). Moreover, purification at some of the
stages is not mandatory as the impurities do not affect the following reactions, and their

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solubility differs significantly from that of the final product, which can thus be easily isolated by
means of recrystallization.
3
All the target compounds synthesized are yellowish crystals. It is worth to note that
POPOP and isoPOPOP form needle-shaped crystals, while in the case of trimethylsilyl
derivatives formation of plate-like crystals was observed (see Fig. S19 in ESI). Chemical
4
5
1
6
structure and purity of all intermediates and final compounds were confirmed by means of H
7
and 13C NMR as well as mass spectroscopy along with analytic GPC and elemental analysis (see
Experimental part and Fig. S1 – S12 in ESI).
8
9
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14
15
16
17
18
19
20
21
22
23
24
25
3.2. Thermal behaviour
Investigation of the thermal stability using thermogravimetric analysis revealed that
decomposition (5% weight loss) of POPOP, isoPOPOP, TMS-POPOP-TMS, TMS-isoPOPOP-TMS
oligomers in the air occurs at 294, 260, 317 and 318 °C respectively (see Fig. S13 in ESI). The
decomposition temperatures (5% weight loss) for POPOP and isoPOPOP under nitrogen flow
were found to be lower than those in the air and amount to 284 and 254 °С respectively. This
may be related to the partial sublimation under inert atmosphere. The thermal stability of TMS-
POPOP-TMS and TMS-isoPOPOP-TMS compounds under nitrogen flow and in the air were
almost the same. These results highlight the positive influence of trimethylsilyl substituents on
the thermostability of the phenyloxazoles. The residual weights do not exceed 2% at heating up
to 700 °C under nitrogen. Thus, isoPOPOP seems to be the most thermally unstable compound
among the oligomers investigated, while both TMS derivatives are the most and equally
thermostable.
3.2. X-ray measurements
To analyze the structure of the POPOP isomers and their TMS derivatives, single-crystal
X-ray diffraction experiments were carried out. Table 1 compares the unit cell parameters

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3
4
5
6
7
8
9
found for isoPOPOP and TMS-isoPOPOP-TMS with the cell parameters of the known POPOP
luminophore [34].
The single crystals exhibit preferential growing direction, which can be explained from
the X-ray data by stacking of the aromatic rings and formation of the layered structures similar
to the rod-like oligomers such as quinquethiophenes [35]. Despite some similarities, the
molecules in the successive layers in the unit cells of POPOP, isoPOPOP and TMS-isoPOPOP-
TMS crystals are packed differently.
Table 1. Summary of the unit cell parameters of the studied POPOP structural isomers.
Angle
between
layersc (deg)
82.12
Interplane
Compound
a (Å)
b (Å)
c (Å)
α
β
γ
sym
Zb
distanced
(Å)
POPOPa
isoPOPOP
9.14
5.08
6.46
5.25
19.20
90
90
90
91.81
97.93
94.96
90
90
90
P 21/c
P 21/n
P 21/c
2
2
2
3.410
3.312
3.224
10.54 17.32
8.28 25.26
52.01
TMS-isoPOPOP-TMS
45.86
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11
12
13
14
15
16
17
18
19
20
a Literature data [34]
b number of molecules per unit cell
c dihedral angle measured between the mean planes in the adjacent molecular layers
d distance measured between the mean planes in the adjacent molecular layers
Figure 2 allows appreciating important structural differences in the molecular packing.
Table 1 summarizes the values of the angles and distances between the adjacent molecular
planes in the layers. First of all, the molecular planes in the unit cell of POPOP are running
almost orthogonal to each other in the successive layers. The angle between the molecular
planes is approx. 82 deg (see Table 1). By contrast, in the unit cell of iso-POPOP, the molecules
are inclined to each other less strongly, with the angle between the molecular planes being only

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3
4
5
6
7
8
of about 52 deg. Surprisingly, the introduction of bulky TMS side groups almost does not affect
this angle. For TMS-isoPOPOP-TMS it equals ca. 46 deg. The distance between mean planes in
the adjacent molecular layers decreases with the angle from 3.41 to 3.22 Å. Also, an important
difference between the crystals consists in the positions of the nitrogen atoms. Thus, in the
POPOP crystals, all nitrogen atoms are located within one plane, which is 200. In the case of
isoPOPOP, the nitrogen atoms are split about the bc plane, being offset at a distance of 1.453
Å. For the lattice of TMS-isoPOPOP-TMS, this distance is increased to 2.957 Å, which could
explain different thermal stability of these materials.
isoPOPOP
TMS-isoPOPOP-TMS
POPOP
a)
c
b
b)
200
c
a
c)
9
10
Fig. 2. (a-b) ORTEP views along a (top) and b (middle) axis of the POPOP, isoPOPOP and TMS-isoPOPOP-
TMS unit cells. For POPOP all the nitrogen atoms (given in blue) are positioned within (200) plane. (c)
Projection showing the stacks of the molecular planes with angle between the molecular planes in the
successive layers reducing from 82.12 to 45.86 deg and interplane distance decreasing from 3.41 to 3.22
Å for POPOP and TMS-isoPOPOP-TMS, respectively. Hydrogens are omitted for clarity.
11
12
13
14

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4
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6
7
3.3. Optical properties
Optical properties of all four compounds were investigated in diluted THF solutions. The
corresponding absorption and PL spectra are shown in Fig. 3. The absorption and PL spectra of
the compounds synthesized are very similar to those of POPOP, but slightly shifted as
compared to the latter. Table 2 summarizes the absorption and emission maxima wavelength
positions (nm), 0-0 transition energy (eV), PLQY and molar extinction coefficients (E)
corresponding to the absorption maxima.
1,0
4A
0,8
3A
2A
4PL
1A
0,6
0,4
0,2
0,0
3PL
2PL
1PL
250
300
350
400
450
500
550
Wawelenght, nm
8
9
Fig. 3. Absorption (A) and photoluminescence (PL) spectra of the compounds
investigated in diluted THF solutions: 1A, 1PL – POPOP; 2A, 2PL – isoPOPOP; 3A, 3PL –
TMS-POPOР-TMS; 4A, 4PL – TMS-isoPOPOР-TMS.
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11
12
13
14
15
16
17
The isoPOPOP structure demonstrates a slight blue shift of the absorption (4 nm) and
emission (7 - 10 nm) maxima as compared to POPOP. Introduction of trimethylsilyl
substituents, on the contrary, leads to a slight red shift of the absorption (5 nm) and emission (6
- 9 nm) spectra as compared to the unsubstituted dyes. In the case of TMS-isoPOPOP-TMS the
two trends compensate each other, and as a result its absorption and emission spectra are
nearly identical to those of POPOP. It also worth to note that molar extinction coefficients of

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2
3
4
5
TMS-substituted dyes is ca. 10-20% higher as the corresponding values for the unsubstituted
dyes. All four dyes investigated show high PLQY in the range of 85-96%.
Table 2. Spectral characteristics of diluted THF solutions of the dyes investigated and their
cyclic voltammetry (CV) data.
opt
Abs
PL
Eg
PLQY,
%
ε,
103*M-1*
cm-1
CV
,
EC
eV
max, nm
λ
max, nm
φox(HOMO)
(V)/(eV)
φred(LUMO)
(V)/(eV)
-1.87/-2.53
Eg
λ
(eV)
POPOP
348,
393,
3.20
3.25
3.15
3.20
96
85
89
91
57.2
1.48/-5.88
3.35
361,379 416, 438
344, 385,
357, 375 406, 427
352, 399,
366, 385 422, 443
348, 393,
362, 381 415, 439
isoPOPOP
65.2
69.1
70.2
1.34/-5.74
1.49/-5.89
1.35/-5.75
-2.05/-2.35
-1.84/-2.56
n.d.
3.39
3.33
n.d.
TMS-POPOР-TMS
TMS-isoPOPOР-TMS
opt
6
7
Notes: Abs – absorption maximum, PL – photoluminescence maximum, Eg – optical energy
gap, PLQY – photoluminescence quantum yield, E – molar extinction coefficient, CV – cyclic
EC
8
voltammetry data, φox – standard oxidation potential, φred – standard reduction potential, Eg –
9
electrochemical energy gap. Excitation of the PL was made at the respective maximum of the
absorption band. Accuracy of the absolute value of PL reported is +/-7. Relative error of the PL
measurements is 0.5%.
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16
17
18
19
3.4. Photooxidaiton stability
In order to estimate photooxidation stability of the dyes in THF solution, accelerated
photostability tests with a help of 150W xenon lamp having a maximum emission at 300-400
nm wavelength range was performed. Fig. 4 shows plots of changes in the absorption spectra of
the dyes as a function of irradiation time. As can be seen from the data presented, under
continuous UV irradiation the long-wavelength absorption peaks at 300-400 nm dramatically
decrease, while absorption in the short-wavelength region (below 300 nm) simultaneously
increases. Such results indicate on destruction of the conjugated system of the phenyloxazoles

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investigated. Moreover, it was found that the compounds containing POPOP-like fragment are
more resistant to degradation than the corresponding isoPOPOP derivatives (Fig. 5). The
estimated half-life time of TMS-isoPOPOP-TMS and isoPOPOP dyes was found to be 19+/-4 and
21+/-4 min respectively. Whereas the half-life time of POPOP and TMS-POPOP-TMS dyes was
twice longer and reached 37+/-4 and 39+/-4 min, respectively. Thus, introduction of
organosilicon substituents does not have any significant influence on the photooxidation
stability, which is mainly defined by the conjugated core structure (POPOP or isoPOPOP). THF
was chosen as a solvent as one of the most aggressive for photooxidation stability
investigations that allows to reach the maximal photo bleaching of the dyes for a minimum
time.
3
4
5
6
7
8
9
10
a)
b)
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0 min
0 min
N
N
10 min
20 min
30 min
40 min
50 min
60 min
10 min
20 min
30 min
40 min
50 min
60 min
O
O
O
O
N
N
POPOP
isoPOPOP
250
300
350
400
250
300
350
400
11
c)
d)
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0 min
0 min
N
H
C
3
N
H
C
Si
3
O
10 min
20 min
30 min
40 min
50 min
60 min
10 min
20 min
30 min
40 min
50 min
60 min
Si
CH
H
C
CH
3
3
O
H
C
3
3
CH
O
H
C
CH
3
3
H
C
3
Si
3
O
Si
CH
3
N
CH
3
N
TMS-POPOP-TMS
TMS-isoPOPOP-TMS
250
300
350
400
250
300
350
400
12
13
Fig. 4. UV–vis absorption spectral changes of the dyes investigated in THF at 296 K as a function
14
of irradiation time upon exposure to light from 150 W xenon lamp in the presence of air.

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POPOP
isoPOPOP
1,0
TMS-POPOP-TMS
TMS-isoPOPOP-TMS
0,8
0,6
0,4
0,2
0,0
0
10
20
30
40
50
60
70
Irradiation time, min
1
2
Fig. 5. Changes in absorption (at their peak values) of the phenyloxazoles over irradiation time.
3.5. Electrochemical properties
3
4
Electrochemical properties of POPOP and isoPOPOP-based dyes synthesized were
studied using cyclic voltammetry (CV) technique (Table 2, Fig. S14 in ESI). For all the compounds
irreversible oxidation peaks were observed. POPOP was the only oligomer capable to undergo
further oxidation, but its CV curve demonstrates two irreversible anodic peaks. The data
presented in Table 2 shows that the first oxidation potentials of both isoPOPOP and TMS-
isoPOPOP-TMS isomers, as well as those of POPOP and TMS-POPOP-TMS are close to each
other and do not depend on the presence or absence of trimethylsilyl groups. At the same time,
the latter couple of compounds are oxidized as at higher potentials (140 mV) as compared to
their isomeric analogs.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
The reduction of POPOP and TMS-POPOP-TMS proceeds in one quasireversible step at
close potentials (-1.84 …-1.87 V), while isoPOPOP is irreversibly reduced at quite large negative
potentials (-2.05 V), and the CV of TMS-isoPOPOP-TMS did not demonstrate any reduction
peaks up to the background discharge potentials. Based on the CV oxidation and reduction
potentials, values of the highest occupied molecular orbitals (HOMO) and the lowest
unoccupied molecular orbitals (LUMO) were calculated using the equations 1 and 2 [36]:
E(HOMO)=-e(ϕox+4.40) (eV)
(1)

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2
E(LUMO)=-e(ϕred+4.40) (eV)
(2)
Thus, the electrochemical data evidence the ability of POPOP and TMS-POPOP-TMS
dyes to undergo reversible reduction and stable anion-radical formation as opposite to their
isomers isoPOPOP and TMS-isoPOPOP-TMS. Presence of the terminal trimethylsilyl groups
does not virtually affect the oxidation and reduction potential values and CV shapes.
3.5. Theoretical calculations
3
4
5
6
7
In order to confirm the trends revealed, computer modelling based on the method of
time-dependent density functional theory (TD DFT) using Tamm-Dancoff approximation (TDA),
was performed (Table 3). For all the four compounds investigated two structural shapes close in
full energy were found (Fig. S15-S18 in ESI). One of the shapes is polar (C2v symmetry group),
while the other has a zero dipole moment due to the C2h symmetry group. Dipole moments of
the main (μ0) and the first excited (μ1) states were calculated. Since the dipole moment in this
case is orientated along the main axis of symmetry, the table represents its projection up to the
sign. In isoPOPOP type structures dipole moment of the excited state of C2v symmetry is
opposite in direction to the dipole moment of the main state, while on POPOP-type structures
electronic density change under excitation is less significant.
8
9
10
11
12
13
14
15
16
17
Table 3. Calculated parameters for the phenyloxazoles using TD DFT computer modeling.
Δλ01,
nm
Symmetr
y type
μ0 (μ1),
Debye
E(HOMO),
eV
E(LUMO),
eV
EgDFT
,
ΔE01, eV
E0, Hatree
Oscillator
strength
2.014
eV
C2h
C2v
C2h
C2v
C2h
C2v
C2h
C2v
-1183.4278134
-1183.4276984
-1183.4280159
-1183.4280163
-2000.4718386
-2000.4716688
-2000.4716896
-2000.4717083
3.014
(3.788)
1.793
POPOP
isoPOPOP
3.31
3.32
3.22
3.23
375
373
385
383
-5.30
-5.25
-5.27
-5.22
-1.80
-1.75
-1.81
-1.76
3.5
3,5
1.796
2.071
(-0.255)
3.375
1.869
2.312
TMS-POPOP-TMS
3,46
3,46
(3.967)
2.192
1.993
2.355
TMS-isoPOPOP-
TMS
(-0.168)
2.055

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2
The calculated energies of S0 – S1 electronic transitions (Table 3) are approximately 0.1
eV larger as compared to the energies of 0-0 transitions evaluated from the THF solutions
spectra (Table 2). Increasing the values of the oscillator strengths calculated during the
transition to trimethylsilyl derivatives are in a good agreement with enlarging the molar
extinction coefficient data measured for THF solutions of these compounds. A general trend of
the reduction in energy of the molecular orbital levels for compounds of POPOP-type structure
as compared to their isomers and constriction of the energy gap for trimethylsilyl derivatives
are in a good agreement with the experimental data. On the other hand, similar energy gaps
calculated for the isomers well as their HOMO and LUMO energy levels, which are
overestimated by ca. 0.6 eV, does not correlate well with the experimental data. This
contradiction may be assigned to a low precision of the computer model applied, according to
which electronic properties of single molecules in vacuum were calculated.
3
4
5
6
7
8
9
10
11
12
13
14
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16
17
18
19
20
21
22
23
24
25
4. Conclusions
In this work two novel organosilicon derivatives of phenyloxazole luminophores were
synthesized under mild conditions using Van Leusen reactions and palladium complexes
catalyzed regioselective direct C-H arylation. All the reactions used were highly selective and led
to excellent yields. Moreover, they allow preparation of POPOP and its isomer iso-POPOP
derivatives, which are unstable under acidic conditions. Investigation of their spectral
luminescent properties revealed that all of them possess high PLQYs and high molar extinction
coefficients up to 70.2×103 M-1*cm-1, while their absorption and emission maxima vary within
357-366 nm and 406-422 nm, respectively. Cyclic voltammetry showed that the molecules with
the POPOP structure are more stable to oxidation as compared to their isomers, while presence
of the TMS substituents does not affect the values of the oxidation or reduction potentials.
Single-crystal X-ray diffraction experiments revealed important structural differences between

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2
the lattices of the isomers: the angle between the molecular planes and positions of the
nitrogen atoms is different for POPOP and isoPOPOP structures. Quantum mechanical
parameters of these oligomers calculated via TD DFT method (HOMO and LUMO energy values,
energy gap and oscillator strength) correspond well to the optical spectroscopy and cyclic
voltammetry experimental data. The novel trimethylsilyl derivatives of phenyloxazoles reported
in this work may find applications in organic photonics due to their high PLQY, large molar
extinction coefficients as well as high photo- and thermo- stability.
3
4
5
6
7
8
Acknowledgments
9
Synthesis and investigation of optical properties of the molecules was made under financial
support of Russian Science Foundation (grant 15-12-30031). Structural measurements we made
by Y.O. and D.A.I. who thank for support the European project Interreg IV Rhin-Solar (Grant
C25). The authors thank G.V. Cherkaev for NMR measurements and P.V. Dmitryakov for TGA
and DCS measurements.
10
11
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19
20
21
22
23
24
Supplementary material
1
Supplementary data contain H and 13C NMR spectra, MALDI MS spectra, GPC and CV curves,
TGA data, pictures of frontier molecular orbitals for the dyes calculated by DFT B3LYP/6-31G[d]
method, microphotographs of the crystals and single-crystal X-ray diffraction data (CCDC:
1493771-1493772). Supplementary data associated with this article can be found in the online
version at doi: 10.1016/
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A mild synthesis of phenyloxazoles and their trimethylsilyl derivatives is reported
All phenyloxazoles synthesized have high PLQYs and molar extinction coefficients
POPOP-type compounds are more stable than their structural isomers
X-ray diffraction revealed important structural differences between the lattices