2-Phenylbenzoxazole derivatives as solid-state fluorescence emitters: Influence of steric hindrance and hydrogen bonding on the optical properties

Article abstract of DOI:10.1016/j.saa.2019.117586

Six new 2-phenylbenzoxazole (PBO) derivatives, bearing either a methoxy or hydroxy group at position 5, were prepared via an efficient one-step synthesis that allowed both analogues to be obtained in equal proportions. These PBO derivatives also differed

Full text of DOI:10.1016/j.saa.2019.117586

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2-Phenylbenzoxazole derivatives as solid-state fluorescence emitters: Influence of
steric hindrance and hydrogen bonding on the optical properties
Emma Bremond, Nadine Leygue, Nathalie Saffon-Merceron, Suzanne Fery-Forgues
PII:
S1386-1425(19)30976-X
https://doi.org/10.1016/j.saa.2019.117586
SAA 117586
DOI:
Reference:
To appear in: Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
Received Date: 21 August 2019
Revised Date: 27 September 2019
Accepted Date: 30 September 2019
Please cite this article as: E. Bremond, N. Leygue, N. Saffon-Merceron, S. Fery-Forgues, 2-
Phenylbenzoxazole derivatives as solid-state fluorescence emitters: Influence of steric hindrance and
hydrogen bonding on the optical properties, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117586.
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GRAPHICAL ABSTRACT
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2-phenylbenzoxazole derivatives as solid-state fluorescence emitters: Influence of steric
hindrance and hydrogen bonding on the optical properties
Emma Bremond a, Nadine Leygue a, Nathalie Saffon-Merceron b and Suzanne Fery-Forgues a*
a
SPCMIB, UMR 5068, CNRS-Université de Toulouse III Paul Sabatier, 118 route de
Narbonne, Toulouse, 31062, France
b
Service commun RX, Institut de Chimie de Toulouse, ICT- FR2599, Université de Toulouse
III Paul Sabatier, 31062 Toulouse cedex 9, France.
* Corresponding author. E-mail address: sff@chimie.ups-tlse.fr (S. Fery-Forgues)
ABSTRACT
Six new 2-phenylbenzoxazole (PBO) derivatives, bearing either a methoxy or hydroxyl group
at position 5, were prepared via an efficient one-step synthesis that allowed both analogues to
be obtained in equal proportions. These PBO derivatives also differed by the presence or
absence of an alkyl substituent at the para-position of the phenyl group. In organic solutions,
all six compounds were strongly fluorescent in the near-UV range. In the solid state, the 5-
methoxy derivatives emitted bright light, ranging from violet to deep blue according to the
substitution of the phenyl group. The presence of a bulky tert-butyl group indeed resulted in
the separation of molecules, but also led to a deviation from molecular planarity. Remarkably,
the introduction of a methyl group had a far more beneficial effect on the optical properties.
With regard to the hydroxyl derivatives, none of them was photoluminescent, probably due to
strong intermolecular hydrogen bonding in the crystals. The 50:50 mixtures of methoxy and
hydroxyl analogues showed acceptable emission properties in the solid state. The substitution
pattern also influenced the crystal habit of the pure compounds and the crystallinity of the
mixtures. These cheap molecules could be adapted to suit a variety of applications in the field
of photoluminescent materials.
Keywords
2-Phenylbenzoxazole
Solid-state emission
Fluorescence
Photoluminescence
Crystal structure
Hydrogen bonding
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1. Introduction
Organic dyes that emit efficiently in the blue in the solid state are currently scarce and
much sought after, especially for applications in the field of optoelectronics [1,2]. Their
development is hampered by the difficulty to predict their photoluminescence properties, which
closely depend on the molecular arrangement [3,4]. In particular, fluorescence is generally
quenched by aggregation, due to π-π stacking of aromatic moieties. One of the strategies [5]
used to circumvent this problem is selecting one molecule with appropriate photoluminescence
properties, and studying in a systematic way the influence of small chemical modifications
upon the molecular arrangement and optical properties.
During the last decade, this strategy has been implemented in our group starting from 2-
phenylbenzoxazole (PBO), a low-molecular weight compound that displays outstanding
chemical, thermal and photochemical stability. As recently reviewed, many dyes of this family
show excellent fluorescence properties in the solid state, with emission ranging from deep-blue
to orange [6]. Some of them have been used as blue emitters in OLEDs [7,8]. Most of the time,
chemical modifications are introduced in the para position of the phenyl group for synthesis
conveniences, and the influence of other substitution patterns on the solid-state properties has
not been explored. In the present work, a methoxy group has been introduced in the 5-position
of PBO with the aim to shift the solid-sate emission spectrum from UV to the visible range
owing to the weak electron-donating character of this group. Some of our previous works have
also shown that the strong structuring effect of the methoxy group in the crystalline state is
associated to highly emissive materials [9,10]. A comparison was made with hydroxyl
analogues to highlight the effect of possible hydrogen bonds. Besides, the effect of steric
hindrance was also studied by comparing molecules whose phenyl group was either
unsubstituted, or substituted by a methyl or a tert-butyl group. It was expected that the latter
group, known to influence the crystal packing mode of closely related PBOs [11], would
separate molecules and favour light emission in the solid state [12-15]. Therefore, compounds
1 to 6 (Fig. 1) were prepared. Their spectroscopic properties were investigated in solution and
in the solid state, and compared with the previously studied compounds 7-9 that do not bear
oxygenated function at position 4 [11]. A relationship was made between the solid-state
properties of the new compounds and their crystallographic characteristics, allowing the effect
of the various substituents to be better understood. The influence of the substituents upon the
crystal shape was also briefly regarded.
22

Fig. 1. Chemical structure of the 2-phenylbenzoxazole derivatives.
2. Materials and methods
2.1. Materials and characterization
Absolute ethanol Chromasol quality (Sigma-Aldrich), as well as analytical grade
solvents from SDS were used as received. 2-Amino-4-methoxyphenol and the various acyl
chlorides were purchased from Aldrich. Reactions were monitored by analytical TLC using
silica gel on aluminium sheets (Kieselgel 60 F254, Merck) under UV light (254 nm).
Chromatographic purifications were made using silica gel (60-200 µm, porosity 60 Å)
purchased from Merck.
General synthesis procedure. To a solution of 2-amino-4-methoxyphenol (0.2520 g, 1.81
mmol) in 1 mL N-methylpyrrolidinone (NMP), acid chloride in stoichiometric proportion was
added dropwise under argon and left to stand under stirring at 0 °C for 1h. After addition of
pyridine (0.23 mL), the mixture was brought to reflux of NMP (bp: 202 °C) for 2h. After
cooling to room temperature, the crude was portioned between water (10 mL) and
dichloromethane. The organic layer was separated and the aqueous layer was extracted twice
with dichloromethane. The combined organic layers were dried with MgSO4, the solvent was
removed and the residue was purified by column chromatography on silica gel, yielding the
methoxy and hydroxyl analogues with yields close to 46% for each of them.
2-Phenyl-5-methoxybenzoxazole (1)
1
TLC (SiO2, petroleum ether/ethyl acetate 80:60) Rf = 0.40. Mp = 78°C. H NMR (300 MHz,
CD3OD): δ ppm = 8.22-8.17 (m, 2H, H2’ and H6’), 7.60-7.56 (m, 3H, H4’, H3’, H5’), 7.54 (dd, J
22

= 9, 0.6 Hz, 1H, H7), 7.22 (dd, J = 2.6, 0.6 Hz, 1H, H4), 7.00 (dd, J = 9.0, 2.6 Hz, 1H, H6), 3.86
(s, 3H, CH3). NMR 13C (75 MHz, CD3OD): δ ppm = 165.3 (Cq, C2), 159.3 (Cq, C5), 146.6 (Cq,
C9), 143.6 (Cq, C8), 133.0 (CH, C4’), 130.2 (CH, C3’ and C5’), 128.4 (CH, C2’ and C6’), 128.1
(Cq, C1’), 115.2 (CH, C6), 112.0 (CH, C7), 103.5 (CH, C4), 56.4 (CH3, OCH3). MS (ESI+): m/z
226.0 ([M+H]+, 100%), HRMS (ESI+): m/z calcd for C14H12NO2 [M+H]+ 226.0868; found
226.0872. IR: 1622 cm-1 (νC=N), 1553 cm-1 (ν–N=C-O-), 1426-1480 cm-1 (νC=C), 1111 cm-1 (νC-O).
2-Toluyl-5-methoxybenzoxazole (2)
1
TLC (SiO2, petroleum ether/ethyl acetate 80:20) Rf = 0.50. Mp = 116°C. H NMR (300 MHz,
CD3OD): δ ppm = 8.08-8.04 (m, 2H, H2’ and H6’), 7.52 (dd, J = 8.9, 0.7 Hz, 1H, H7), 7.39-
7.35 (m, 2H, H3’, H5’), 7,20 (dd, J = 2.6, 0.8 Hz, 1H, H4), 6.97 (dd, J =8.9, 2.6 Hz, 1H, H6),
3.86 (s, 3H, OCH3), 2.43 (s, 3H, CH3). 13C NMR (75 MHz, CD3OD): δ ppm = 165.5 (Cq, C2),
159.2 (Cq, C5), 146.5 (Cq, C9), 143.9 (Cq, C4’), 143.6 (Cq, C8), 130.8 (CH, C3’ and C5’), 128.4
(CH, C2’ and C6’), 125.3 (Cq, C1’), 114.8 (CH, C6), 111.9 (CH, C7), 103.3 (CH, C4), 56.4 (CH3,
OCH3), 21.6 (CH3). MS (ESI+): m/z 240.1 ([M+H]+, 100%), HRMS (ESI+): m/z calcd for
C15H14NO2 [M+H]+ 240.1025; found 240.1028. IR: 1610 cm-1 (νC=N), 1556 cm-1 (ν–N=C-O-),
1408-1479 cm-1 (νC=C), 1106 cm-1 (νC-O).
2-(4’-tert-Butylphenyl)-5-methoxybenzoxazole (3)
1
TLC (SiO2, petroleum ether/ethyl acetate 90:10) Rf = 0.45. Mp = 94°C. H NMR (300 MHz,
CD3OD): δ ppm = 8.13-8.09 (m, 2H, H2’ and H6’), 7.63-7.58 (m, 2H, H3’ and H5’), 7.52 (dd, J =
8.9, 0.6 Hz, 1H, H7), 7.21 (dd, J = 2,7, 1H, H4), 6.98 (dd, J = 8.9, 2.6 Hz, 1H, H6), 3.86 (s, 3H,
CH3), 1.37 (s, 9H, CH3). 13C NMR (75 MHz, CD3OD): δ ppm = 165.5 (Cq, C2), 159.2 (Cq, C5),
156.8 (Cq, C4’), 146.5 (Cq, C9), 143.6 (Cq, C8), 128.3 (CH, C3’ and C5’), 127.2 (CH, C2’ and
C6’), 125.2 (Cq, C1’), 114.9 (CH, C6), 111.9 (CH, C7), 103.4 (CH, C4), 56.4 (CH3, OCH3), 36.0
(Cq, tBu), 31.5 (CH3, tBu). MS (ESI+): m/z 282.1 ([M+H]+, 100%), HRMS (ESI+): m/z calcd
for C18H20NO2 [M+H]+ 282.1494; found 282.1491. IR : 1611 cm-1 (νC=N), 1553 cm-1 (ν–N=C-O-),
1412-1482 cm-1 (νC=C), 1115 cm-1 (νC-O).
2-Phenyl-5-hydroxybenzoxazole (4)
1
TLC (SiO2, dichloromethane/methanol 97:3) Rf = 0.30. Mp = 176°C. H NMR (300 MHz,
CD3OD): δ ppm = 8.22-8.16 (m, 2H, H2’ and H6’), 7.60-7.54 (m, 3H, H3’, H4’, H5’), 7.47 (dd, J
= 8.8, 0.8 Hz, 1H, H7), 7.09 (dd, J =2.5, 0.8 Hz, 1H, H4), 6.88 (dd, J = 8.9, 2.4 Hz, 1H, H6). 13C
22

NMR (75 MHz, CD3OD) : δ ppm = 165.1 (Cq, C2), 156.6 (Cq, C5), 146.0 (Cq, C9), 143.6 (Cq,
C8), 132.9 (CH, C4’), 130.2 (CH, C3’, C5’), 128.4 (CH, C2’, C6’), 128.1 (Cq, C1’), 115.2 (CH,
C6), 111.8 (CH, C7), 105.5 (CH, C4). MS (ESI+): m/z 212.0 ([M+H]+, 100%), HRMS (ESI+):
m/z calcd for C13H10NO2 [M+H]+ 212.0712, found 212.0711. IR: 3153 cm-1 (νO-H), 1621 cm-1
(νC=N), 1549 cm-1 (ν–N=C-O-), 1390-1472 cm-1 (νC=C).
2-(4’-Toluyl)-5-hydroxybenzoxazole (5)
1
TLC (SiO2, dichloromethane/methanol 97:3) Rf = 0.40. Mp = 201°C. H NMR (300 MHz,
CD3OD): δ ppm = 8.01-7.97 (m, 2H, H2’ and H6’), 7.40 (dd, 1H, J = 8.8, 0.6 Hz, H7), 7.31-7.27
(m, 2H, H3’ and H5’), 7.05 (dd, 1H, J = 2.5, 0.5 Hz, H4), 6.84 (dd, 1H, J = 8.8, 2.5 Hz, H6), 2.38
(s, 3H, CH3). 13C NMR (75 MHz, CD3OD): δ ppm = 165.3 (Cq, C2), 156.4 (Cq, C5), 145.8 (Cq,
C9), 143.7 (Cq, C4’), 143.5 (Cq, C8), 130.7 (CH, C3’ and C5’), 128.3 (CH, C2’ and C6’), 125.2
(Cq, C1’), 114.9 (CH, C6), 111.6 (CH, C7), 105.3 (CH, C4), 21.6 (CH3). MS (ESI+): m/z 226.0
([M+H]+, 100%), HRMS (ESI+): m/z calcd for C14H12NO2 [M+H]+ 226.0868; found 226.0866.
IR: 3192 cm-1 (νO-H), 1613 cm-1 (νC=N), 1552 cm-1 (ν–N=C-O-), 1419-1500 cm-1 (νC=C).
2-(4’-tert-Butylphenyl)-5-hydroxybenzoxazole (6)
1
TLC (SiO2, dichloromethane/ethylacetate 90:10) Rf = 0.3. Mp = 224°C. H NMR (300 MHz,
CD3OD): δ ppm = 8.12-8.07 (m, 2H, H2’ and H6’), 7.62-7.57 (m, 2H, H3’ and H5’), 7.45 (dd, J =
8.8, 0.6 Hz, 1H, H7), 7.08 (dd, J = 2.5, 0.6 Hz, 1H, H4), 6.86 (dd, J = 8.9, 2.4 Hz, 1H, H6), 1.37
13
(s, 9H, CH3). C NMR (75 MHz, CD3OD): δ ppm = 165.3 (Cq, C2), 156.7 (Cq, C4’), 156.5
(Cq, C5), 145.9 (Cq, C9), 143.5 (Cq, C8), 128.3 (CH, C3’ and C5’), 127.2 (CH, C2’ and C6’),
125.3 (Cq, C1’), 114.9 (CH, C6), 111.7 (CH, C7), 105.4 (CH, C4), 35.9 (Cq, tBu), 31.5 (CH3,
tBu). MS (ESI+): m/z 268.1 ([M+H]+, 100%), HRMS (ESI+): m/z calcd for C17H18NO2 [M+H]+
268.1338; found 268.1336. IR: 3122 cm-1 (νO-H), 1609 cm-1 (νC=N), 1550 cm-1 (ν–N=C-O-), 1363-
1480 cm-1 (νC=C).
2.2. Apparatus and methods
The melting points were measured on a Melting Point System MP50 Mettler Toledo
apparatus. Chemical characterizations were performed in the relevant services of Institut de
1
Chimie de Toulouse (ICT). The H NMR and 13C NMR spectra were recorded on a Bruker
AC300 spectrometer operating at 300 MHz and 75 MHz, respectively. Chemical shifts are
22

reported in ppm, with residual protonated solvents as the internal references. Protons and
carbon atoms were numbered according to Fig. 1. Electrospray (ES) mass spectra and High-
Resolution Mass Spectra (HRMS) were obtained on a Xevo G2 QTof Waters spectrometer.
Infrared spectra were obtained using a Nexus ThermoNicolet FTIR spectrophotometer
equipped with a diamond ATR.
Spectroscopic measurements in solution were conducted at 20 °C in a temperature-
controlled cell of 1 cm optical pathway. Both UV-visible absorption and fluorescence spectra
were recorded with a Xenius SAFAS spectrofluorimeter. All fluorescence spectra were
corrected. The relative fluorescence quantum yields (ΦF) of solutions were determined using
the classical formula:
2
ΦFx = (As × Fx × nx × ΦFs)/(Ax × Fs × ns2)
(1)
where A is the absorbance at the excitation wavelength, F is the area under the fluorescence
curve, and n is the refraction index. Subscripts “s” and “x” refer to the standard and to the
sample of unknown quantum yield, respectively. Quinine sulfate in 0.1N HClO4 (ΦF = 0.59)
was taken as the standard [16]. The absorbance of the solutions was equal or below 0.050 at the
excitation wavelength. The error on ΦFx values is estimated to be about 10%. For solid
samples, the photoluminescence was analyzed on the same apparatus using a BaSO4
integrating sphere. Solid samples were deposited on a metal support. The excitation source was
scanned in order to evaluate the reflected light for the empty sphere (La), the samples facing the
source light (Lc) and the sample out of the irradiation beam (Lb). The luminescence spectra
were recorded with the sample facing the source light (Ec) and out from direct irradiation (Eb).
The PM voltage was adapted to the measurement of reflected light and emission spectra,
respectively, and proper correction was applied to take into account the voltage difference. The
absolute photoluminescence quantum yield values (ΦPL) were calculated by a method based on
the one developed by de Mello et al. [17] using the formula:
ΦPL = [Ec-(1-α)Eb]/Laα
(2)
with α = 1-Lc/Lb. The error on the ΦPL value was estimated to be about 20%.
2.3. Crystallography
Crystal data of 2 were collected at 193(2)K using a Bruker-AXS APEX II
diffractometer equipped with a 30 W air-cooled microfocus source (ImS) with focusing
multilayer optics, with graphite-monochromated MoKα radiation (λ = 0.71073 Å). Data of 1, 3,
4, 5 and 6 were collected using a Bruker-AXS D8-Venture diffractometer equipped with dual
22

Cu/Mo source and a CMOS detector. Phi- and omega scans were used. The structures were
solved using an intrinsic phasing method (ShelXT) [18]. All non-hydrogen atoms were refined
anisotropically using the least-squares method on F2 [19]. Selected crystallographic data are
presented in Table 1. CCDC supplementary crystallographic data can be obtained free of
charge
from
The
Cambridge
Crystallographic
Data
Centre
via
https://www.ccdc.cam.ac.uk/structures.
Table 1
3. Results and discussion
3.1. Synthesis
The compounds were prepared by condensation of an acid chloride with 2-amino-4-
methoxyphenol in refluxing N-methylpyrrolidone, according to a one-pot procedure previously
used for the preparation of PBO derivatives [11]. Remarkably, the reaction mixture contained
an almost equal proportion of methoxy and hydroxyl analogues, each of them obtained with a
yield close to 46%, which corresponds to a very good overall condensation yield of 92%. Very
likely, demethylation was catalyzed by pyridine hydrochloride formed during the reaction.
According to literature, the mechanism starts with proton transfer from pyridinium ion to the
aryl methyl ether, a highly unfavorable step that may account for the high temperature required
[20]. Selecting another synthesis procedure from the many that have been reported in the
literature would certainly allow preparing each compound independently, with high yield.
However, obtaining in one batch both compounds in significant proportions was of interest for
the present work. After purification by silica chromatography, compounds 1-6 were
characterized by usual methods (1H and 13C NMR, HRMS and FTIR).
3.2. Spectroscopic study in solution
All spectroscopic data are gathered in Table 2. As an example, the spectra of
compounds 2 and 5 are shown in Fig. 2. The compounds were first studied in organic solutions,
where they exhibited rather close properties. For every compound in n-heptane, as well as for
22

the methoxy derivatives in ethanol, the absorption spectra showed two strong bands in the UV
region, at 272 nm and 316 nm. For the hydroxyl derivatives in ethanol, the two bands were
slightly shifted to the red and their relative intensity was reversed. The substituent in the para
position of the phenyl group played an insignificant role. The molar extinction coefficient of 2
and 5 in ethanol was measured to be 17200 M-1.cm-1 and 14400 M-1cm-1 at the maximum of the
long-wavelength band, respectively. By comparison, the spectra of compounds 7-9 display
only one band at around 300 nm [11].
10
8
0.6
0.5
0.4
0.3
0.2
0.1
0
6
4
2
0
220 260 300 340 380 420 460 500 540
Wavelength (nm)
Fig. 2. Normalized UV-Vis absorption spectra (left) and fluorescence emission spectra (right)
of methoxy derivative 2 (blue lines) and hydroxyl derivative 5 (red lines) dissolved in n-
heptane (solid lines) and ethanol (dotted lines). λex = 310 nm. Concentrations around 4 × 10-5 M
for absorption and 3 ×10-6 M for emission.
All compounds were strongly fluorescent in solution. The emission spectra, recorded in
dilute solutions, showed one band with weak vibrational resolution in n-heptane, and no
resolution in ethanol. For the methoxy derivatives, the maximum was at 374 nm in n-heptane,
slightly red-shifted in ethanol, and the quantum yield was rather high (up to 0.59) in both
solvents. The hydroxyl derivatives exhibited almost the same spectra as their methoxy
analogues in n-heptane. However, in ethanol, their emission maxima were shifted to the visible
range, and their fluorescence quantum yield was reduced by half. By comparison, the parent
compounds 7-9 emit in the UV range with a maximum around 316 nm in n-heptane and 322
nm in ethanol, and a high quantum yield of about 0.74 in both solvents [11].
22

Consequently, the substitution at position 5 by a methoxy or hydroxyl group induced
the emission spectra to shift significantly to long wavelengths, and generated a solvatochromic
behavior, with respect to the parent compounds. The fluorescence efficiency seems to be
related to the ability of the dye molecules to form hydrogen bonds with the solvent. Due to
their heteroatoms, methoxy-substituted PBO are H-bond acceptors, and the influence of these
bonds on the emission properties is quite moderate. In contrast, the hydroxyl derivatives may
be involved as donors in H-bonding interactions, which may account for the strong
solvatochromic behavior, and also be detrimental to the emission efficiency. Indeed, H-bond
assisted nonradiative deactivation of the excited states is a well-known fluorescence quenching
mechanism for various fluorophores [21]. For all our compounds in both solvents, the electron-
donating effect of the methyl and tert-butyl group seems to increase the emission efficiency.
Finally, it is noteworthy that no changes in the shape and position of the emission spectra were
detected with increasing concentration up to 1.4 × 10-3 M. In particular, no excimer emission
was observed in this concentration range.
3.3. Spectroscopic study in the solid state
In the solid state, the methoxy compounds emitted strong violet-blue light when
illuminated by a hand-held UV lamp (365 nm), while the hydroxyl derivatives appeared dark.
The emission spectra were an unresolved broad band. For 1 and 3, the spectra were centered at
the frontier between near UV and the visible range. Remarkably, the spectrum of 2 peaked in
the violet range at 426 nm (Fig. 3), red-shifted by 57 nm by comparison with the reference
compound 8. The photoluminescence quantum yields (PLQY) were 0.42 for 1 and 2,
significantly increased with respect to parent compounds, and 0.34 for 3.
For the hydroxyl derivatives, the solid-state emission spectra were in the same range of
wavelength as above, but their intensity was very low, so that no accurate measurement could
be performed using our apparatus. It cannot be excluded that emission arouse from traces of
methoxy analogues.
22

Fig. 3. Comparative solid-state emission spectra of the methoxy derivatives 1 (orange solid
line), 2 (blue solid line) and 3 (green solid line), and hydroxyl derivatives 4, 5 and 6 (red and
grey lines, superimposed). The height of the spectra is proportional to the PLQY. λex = 360 nm.
Besides, the solid-state spectroscopic properties of the 50:50 mixtures were studied.
Indeed, it is now well established that solid-state fluorescence arises either from single
molecules influenced by electrostatic interactions due to their bulk environment, or from
excimers formed between neighboring molecules [3,4]. In both cases, energy transfer processes
play an important role. This is the reason why any molecule acting as energy trap may totally
quench fluorescence emission in the solid state, even at the very low concentration of one
quencher molecule per 200 [22] or 420 fluorophores [23]. Therefore, it seemed instructive to
know whether or not the fluorescence of the methoxy derivatives is quenched by the presence
of some hydroxyl derivative. To this aim, solid samples containing an equal proportion of
methoxy and hydroxyl analogues were prepared by mixing dichloromethane solutions of the
pure compounds, followed by slow evaporation of the solvent. All the samples were
fluorescent under UV illumination. The position of the emission spectra was almost unchanged
and the quantum yields were approximately reduced by five with respect to pure methoxy
compounds.
Table 2
22

3.4. Crystallographic study
Single crystals were grown by slow evaporation of organic solutions of the six
compounds. The results of the X-ray analysis are illustrated in Fig. 4 to 6. Regarding molecular
geometry, a deviation from planarity was observed with passing from methoxy to hydroxyl
derivatives, and with increasing the bulkiness of the R2 substituent. For instance, the dihedral
angle between the benzoxazole heterocycle and the phenyl ring was below 1° for 1 and reached
27 ° for compound 6 (Fig. 4). Most likely, the driving force in packing was to maximize the
number and strength of interactions, and packing has taken place here at the expense of
planarity. This result is noticeable because the PBO framework is generally planar, whatever
the substituent in the para position of the phenyl group [11, 24-28].
Fig. 4. Molecular views of compounds 1 to 6.
The molecular arrangement was also closely linked to the nature of the R1 and R2
substituents. With regards to the methoxy derivatives (Fig. 5), for compound 1, a molecule and
the one situated immediately above point in the same direction and are laterally shifted, so that
their aromatic systems weakly overlapped. These dimers then adopt a herringbone pattern. For
compound 2, superimposed molecules also face the same direction, but they are longitudinally
shifted one with respect to the other, with significant overlap of their aromatic systems.
22

Besides, the molecules displayed crossed arrangement, as already reported for many PBO
derivatives [11, 24-28]. For compound 3, the steric hindrance due to the tert-butyl group
induces the stacked molecules to form anti-parallel dimers, with very little overlap. The dimers
then display a herringbone arrangement. For the three methoxy compounds, the plane-to-plane
intermolecular distance was slightly increased with increasing the bulkiness of the substituent
in the para-position of the phenyl group (i.e. 3.46 Å and 3.58 Å for 1 and 3, respectively).
Fig. 5. Left column: Crystal packing of the methoxy derivatives 1, 2 (view along a) and 3
(view along a). Right column: overlap of the aromatic moieties.
Considering now the three hydroxyl derivatives, the striking feature is that their
organization is totally structured by hydrogen bonds (Fig. 5) [29]. More precisely, H-bonds
take place between the nitrogen atom of one molecule and the hydroxyl group of the closest
molecule situated in another plane, resulting in a closely intertwined molecular network. For
compound 4, molecules are situated in two different planes, and stacked molecules are
22

displayed antiparallel. For compound 5, molecules occupy four different planes (not
represented), which are connected by pairs owing to H-bonds. Stacked molecules are displayed
parallel. For compound 6, the molecules displayed on two different planes formed crossed
arrangement. In the three cases, the aromatic systems of stacked molecules have little overlap
due to lateral and/or longitudinal shift of molecules with respect to each other. The spacing
between stacked molecules was increased from 3.42 Å for 4 to 4.04 Å for 6, showing the value
of the bulky ter-butyl group to separate the molecules in the crystal.
Fig. 6. Left column: Crystal packing of the hydroxyl derivatives 4 (view along a), 5 and 6
(view along c). Right column: overlap of the aromatic moieties. Hydrogen bonds are in blue
ink.
In summary, the molecular arrangement is different for each compound, but in every
case it should be favorable to light emission because π-stacking interactions are quite small, as
is the case for most of the members of the PBO family investigated to date. Remarkably,
among the three compounds that emit in the solid state, compound 2 exhibits the largest
22

overlap of aromatic moieties. This could explain the strong red-shift of its emission spectrum
compared to compounds 1 and 3. From a molecular viewpoint, emission in organic crystals is
often assumed to arise from high-energy excimers, but it may also come from single
fluorophores influenced by their bulk surrounding environment [30]. Calculations could allow
us to distinguish between both mechanisms. As expected, the bulky tert-butyl group separates
the molecules. However, these positive trends are counterbalanced by the deviation of the
molecular planarity, and this may explain why the introduction of the bulky tert-butyl group
did not lead to superior fluorescence properties. Moreover, strong H-bonds take place in
hydroxyl derivatives, and are probably responsible for the very weak fluorescence of these
compounds in the solid state. The effect is spectacular because the groups involved in H-bonds
are directly borne by the electron conjugated system, and not only used to control the distance
between and the relative orientation of electronically independent fluorophores [31].
3.5. Crystal habits
For use as materials, the homogeneity and crystallinity of the active layer are often very
important. Therefore, the solids obtained after drop casting and air drying of dichloromethane
solutions (1×10-2 M) were examined. Both techniques gave the same type of crystals, with the
difference that the crystals obtained by drop casting were much smaller and often arranged
around a nucleation center. Most of the compounds gave platelets. An example is given for 2 in
Fig. 7a. Generally speaking, the hydroxyl derivatives gave crystals with a more defined shape.
For instance, 4 provided diamond-shaped platelets, and 5 led to beautiful needles of very
regular gauge (Fig. 7b), a tendency already reported for methyl-PBO [11]. Interestingly, a
50:50 mixture of 2 and 5 resulted in rods terminated by microfibers, possibly due to the fact
that the less soluble compound, which crystallizes first, was used as a template by the second
one (Fig. 7c). In contrast, the mixture of 3 and 6 was amorphous.
22

Fig. 7. Observation by polarized microscopy of the crystals formed after slow evaporation of
dichloromethane solutions of compounds 2 (a), 5 (b) and an equimolecular mixture of 2 and 5
(c).
4. Conclusion
This work showed that the substitution pattern of our PBO derivatives strongly
influence their fluorescence properties. The presence of a weak electron donor group at
position 5 is enough to shift the emission spectra to the red with respect to parent compounds.
The key factor for optical properties is indeed the capacity of this substituent to be involved in
H-bonds. For instance, the methoxy derivatives could be used as fluorescent tracers in solvents
of various polarities and are efficient light emitters in the solid state. In contrast, the hydroxyl
derivatives have attractive properties in a nonpolar medium, but lose their appeal in polar
solvents due to detrimental H-bonds with solvent molecules. In the solid state, intermolecular
H-bonds become preponderant and result in almost total quenching of photoluminescence.
Remarkably, the role of the bulky tert-butyl group is more complex than expected. Combining
a methoxy group at position 5 with a methyl group at the para position of the phenyl ring, the
best solid-state emitter, i.e. compound 2, was obtained.
The dye mixtures, directly obtained from synthesis with very low production costs,
keep moderate photoluminescence efficiency. The elongated shape of the crystals obtained in
mixtures of 2 and 5 is an advantage for applications such as the authentication of precious
documents, while the amorphousness of other mixtures could be appreciated for applications in
optoelectronics. By selecting the proper substitution pattern, these cheap molecules could be
adapted to suit a variety of applications in the field of photoluminescent materials.
Acknowledgements
ANR is gratefully acknowledged for funding (SUPERFON project # ANR-17-CE07-0029-03).
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22

Table 1. Selected crystallographic data of compounds 1 to 6.
1
2
3
4
5
6
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C14H11NO2
225.24
Monoclinic
P21
C15H13NO2
239.26
C18H19NO2
281.34
Monoclinic
P21/c
C13H9NO2
211.21
Monoclinic
P21/n
C14H11NO2
225.24
C17H17NO2
267.31
Monoclinic
P21/c
Orthorhombic
Pca21
Orthorhombic
P212121
11.1155(5)
3.9025(2)
13.2862(5)
108.7336(14)
545.80(4)
2
12.6367(14)
5.0487(6)
38.014(5)
90
9.1097(3)
8.2557(2)
19.9688(7)
90.2561(15)
1501.78(8)
4
5.812(3)
19.261(8)
8.755(4)
93.743(14)
978.0(7)
4
4.0259(2)
10.7878(8)
25.0214(17)
90
12.7819(4)
8.6214(3)
13.2019(5)
104.9320(12)
1405.70(8)
4
b (Å)
c (Å)
β (°)
Volume (Å3)
2425.2(5)
8
1086.69(12)
4
Z
Crystal size (mm3)
0.350 × 0.200 ×
0.060
0.600 × 0.250
× 0.050
0.300 × 0.300
× 0.250
0.200 × 0.120 ×
0.080
0.440 × 0.100
× 0.080
0.400 × 0.150
× 0.100
Reflections
20789/3336
23116/4938
R(int) = 0.0922
329/1
48591/3080
R(int) = 0.0919
194/0
24972/2254
R(int) = 0.0798
149/0
24504/3138
R(int) = 0.0614
159/0
57891/4117
R(int) = 0.0470
188/0
collected/independent R(int) = 0.0324
Parameters/Restraints
Final R1 indices
I>2σ(I)
155/1
0.0383
0.0675
0.0481
0.0488
0.0479
0.0441
wR2 all data
0.0971
0.2053
0.1344
0.1038
0.1041
0.1257
Largest diff. peak and 0.151 and -0.213 0.287 and -0.261 0.333 and -0.327 0.208 and -0.289 0.185 and -0.202 0.307 and -0.226
-3
hole (e.Å )
CCDC
1944367
1944368
1944369
1944370
1944371
1944372
22

Table 2. Maximum absorption wavelength (λabs), maximum emission wavelength (λem.), fluorescence and photoluminescence quantum yields (ɸF
and ɸPL) for compounds 1 to 6. Dye concentration in solution around 4 × 10-5 M for absorption and 3 ×10-6 M for emission. Excitation at 310 nm
for solutions and at 360 nm for the solid state. Peaks of highest intensity underlined. [a] from ref. 11.
Compound
n-Heptane
Ethanol
Solid state
λabs.
λem.
ɸ
λabs.
λem.
ɸ
λem.
ɸ
272, 316
374
0.50
0.59
0.57
0.45
0.55
0.57
0.69
0.74
0.77
272, 316
394
0.49
0.54
0.56
0.18
0.25
0.27
0.80
0.75
0.83
398
0.42
0.42
0.34
< 0.01
< 0.01
< 0.01
0.26
0.34
0.33
1
272, 316
374
272, 316
386
426
2
272, 316
374
272, 316
386
398
3
272, 312
372
274, 318
412
418
4
272, 314
370
274, 319
408
424
5
272, 314
370
276, 320
408
424
6
7 a
8 a
9 a
292, 298, 312
294, 302, 314
294, 302, 316
316, 330, 346
318, 334, 350
318, 334, 352
292, 298, 310
292, 302, 314
292, 302, 314
322, 336, 350
322, 338, 354
324, 340, 354
360
369, 383
362, 374

HIGHLIGHTS
•
•
All the compounds in solution were fluorescent in the near-UV range
One of the 5-methoxy derivatives is a good solid-state emitter in the violet to deep-
blue range
•
The hydroxyl derivatives are virtually not photoluminescent, due to intermolecular H-
bonds
•
•
The bulky tert-butyl group plays a role more complex than expected.
The dye mixtures could be used as cheap fluorescent materials

Declaration of interests
☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: