From 2-phenylbenzoxazole to diphenyl-bibenzoxazole derivatives: Comparative advantages of mono- and bis-chromophores for solution and solid-state fluorescence

Article abstract of DOI:10.1016/j.dyepig.2015.10.027

The optical properties of three bis-benzoxazole derivatives, namely 2,2'-diphenyl-6,6'-bibenzoxazole and its derivatives bearing methyl and tert-butyl groups at the para position, were investigated and compared to those of the corresponding subunits. As e

Full text of DOI:10.1016/j.dyepig.2015.10.027

Dyes and Pigments 125 (2016) 282e291
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
From 2-phenylbenzoxazole to diphenyl-bibenzoxazole derivatives:
Comparative advantages of mono- and bis-chromophores for solution
and solid-state fluorescence
, b, c,
Abdelhamid Ghodbane a
d, Patrice Bordat e, Nathalie Saffon f, Sylvie Blanc e,
, h, *
Suzanne Fery-Forgues g
a CNRS, ITAV-USR 3505, F31106 Toulouse, France
b
ꢀ
Universite de Toulouse, ITAV-USR 3505, F31106 Toulouse, France
c CNRS, IMRCP-UMR 5623, F31062 Toulouse, France
d
ꢀ
Universite de Toulouse, IMRCP-UMR 5623, F31062 Toulouse, France
Universite de Pau et des Pays de l'Adour, IPREM-UMR 5254, F64053 Pau Cedex 9, France
e
ꢀ
f
ꢀ
Service Commun RX, Institut de Chimie de Toulouse, ICT- FR2599, Universite de Toulouse III Paul Sabatier 31062 Toulouse Cedex 9, France
g CNRS, SPCMIB-UMR 5068, F31062 Toulouse, France
h
ꢀ
Universite de Toulouse, SPCMIB-UMR 5068, F31062 Toulouse, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The optical properties of three bis-benzoxazole derivatives, namely 2,2’-diphenyl-6,6’-bibenzoxazole and
its derivatives bearing methyl and tert-butyl groups at the para position, were investigated and compared
Received 24 June 2015
Received in revised form
5 October 2015
Accepted 23 October 2015
Available online 31 October 2015
to those of the corresponding subunits. As expected from the increase of the
p-electron conjugated
system, the absorption and fluorescence spectra were red-shifted when passing from mono-to bis-
chromophores. The three bis-benzoxazoles showed superior fluorescence quantum yields in solution, but
they were only weakly emissive in the solid state, the unsubstituted derivative even being virtually not
photoluminescent. The solid-state behaviour was explained by the fact that bis-chromophoric molecules
were locked in a non-planar conformation, as discussed on the basis of DFT calculations and X-ray data.
© 2015 Elsevier Ltd. All rights reserved.
Keywords:
Crystal structure determination
Fluorescent probes
Photoluminescence
Optical properties
DFT calculations
Non-doped emitter
1. Introduction
strong interactions that take place in these arrangements may
induce non-radiative deactivation processes that compete with
Among the broad range of fluorescent organic dyes, very few
emit efficiently in the solid state. However, identifying the right
candidate is an essential step to access new applications, in
particular the very promising development of pure-dye nano-
particles for optics [1,2] optoelectronics [3,4] and biology [5,6].
For the vast majority of dyes, the collapse of fluorescence effi-
ciency that is observed in the solid state is due to aggregation-
caused quenching (ACQ) [7,8]. The ubiquity of this phenomenon
can easily be understood by considering that organic dye molecules
consist of several aromatic or heteroaromatic rings, often situated
in the same plane. Dye molecules are thus prone to stacking. The
fluorescence. In particular, face-to-face arrangement is known to be
very detrimental to fluorescence emission. Various strategies may
be used to bring dye molecules to emit in the solid state [9]. The
most popular approach is to choose fluorophores that exhibit the
desired characteristics in solution and to prevent their aggregation.
To do so, bulky substituents may be introduced in the molecules
[10,11]. Dye molecules can also be included in cyclodextrins, zeo-
lites and dendrimers. Alternatively, the chemical structures may be
modified with the hope that molecules will adopt an arrangement
that promotes fluorescence emission, for example a brick-in-the-
wall or a crossed dipole arrangement. Our approach is slightly
different because it consists in selecting a family of dyes that
spontaneously adopt a fluorescence-compatible arrangement, and
progressively introducing chemical modifications until the desired
spectroscopic characteristics are reached.
ꢀ
* Corresponding author. Laboratoire SPCMIB, CNRS UMR 5068, Universite de
Toulouse III (Paul Sabatier), 118 route de Narbonne, 31062 Toulouse Cedex 9, France.
E-mail address: sff@chimie.ups-tlse.fr (S. Fery-Forgues).
http://dx.doi.org/10.1016/j.dyepig.2015.10.027
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

A. Ghodbane et al. / Dyes and Pigments 125 (2016) 282e291
283
This strategy was implemented in the 2-phenylbenzoxazole
2. Experimental
series, based on the empirical evidence that the simplest com-
pound of this family exhibited very good luminescence properties
in the crystal state upon illumination by a hand-held UV lamp.
Fortunately, the 2-phenylbenzoxazole molecule lent itself well to
many possibilities of modifications. Variations in the nature of the
alkyl group borne by the phenyl ring were previously explored by
us [12], as well as the influence of substitution by a halogen atom
at this position [13]. All these compounds absorbed and emitted in
the UV region. Replacement of the benzoxazole heterocycle by a
naphthoxazole also led to strongly luminescent materials, with
the advantage that emission spanned the violet-blue region of the
visible spectrum [14]. In the present work, with the aim of pre-
paring compounds that can be used in the visible region, two 2-
phenylbenzoxazole units were assembled covalently to obtain
bis-chromophoric molecules (Fig. 1). It was decided to link the
subunits by their benzoxazole rings, so that the R group borne by
the phenyl rings at the two extremities of the molecule could be
allowed to vary. This is of special interest since we showed that, in
parent compounds, the R terminal group strongly influenced the
crystal packing mode, and thus the optical properties in the solid
state. The compounds obtained belonged to the series of 2,2’-
diphenyl-6,6’-bibenzoxazole. In the literature, compounds of this
family are the basis of polymers and co-polymers [15e22] that are
exceptionally stable upon exposure to radiations [15] and heat
[16e19]. Many patents report their incorporation in resins that
can be used in electronic components because of their excellent
electrical insulating properties [23,24] as well as their good me-
chanical and filmogenic properties. These compounds are also
precursors of photosensitive materials for data storage [25] and
electrophotography [26]. The spectroscopic behaviour of the first
member of the series (R ¼ H, compound 1, Fig. 1) has been studied
in solution by Roussilhe and Paillous [27]. The fluorescence
properties are astounding, with a quantum yield as high as 97% in
cyclohexane. To the best of our knowledge, the other members of
the series have not been investigated from a spectroscopic view-
point and the solid-state emission properties of all these com-
pounds are totally unknown. However, they could be valuable and
a significant shift of emission from UV to visible wavelengths can
be expected by comparison with simple 2-phenylbenzoxazole
derivatives.
2.1. Materials
Analytical grade absolute ethanol (Prolabo-VWR) and n-heptane
(SDS) were used as received. 3,30-Dihydroxybenzidine was from TCI
Europe. Benzoyl chloride (99%), p-toluoyl chloride (99%), 4-tert-
butyl-benzoyl chloride (97%) and N-methylpyrrolidinone were
purchased from Sigma Aldrich.
2.2. General procedure for synthesis
The 2-phenylbenzoxazole derivatives 4e6 were prepared as
reported elsewhere [12]. Bis-benzoxazoles were obtained using a
closely related procedure. 3,30-Dihydroxybenzidine (4.62 mmol,
1 g) was dissolved in 25 mL N-methylpyrrolidinone. The brown
solution was bubbled with nitrogen and cooled to 0 ꢀC. An aliquot of
acyl chloride (10 mmol), previously dissolved when necessary in a
minimum of N-methylpyrrolidinone, was added dropwise and the
mixture was stirred for 1h at 0 ꢀC. Pyridine (9.9 mmol, 0.8 mL) was
then added and the mixture was refluxed for 2 h. After cooling to
ambient temperature, 30 mL of a water/methanol (80:20, v/v)
mixture were added. The obtained suspension was cooled to 4 ꢀC,
the precipitate was filtered on a Buchner funnel, washed with
10 mL of chilled water/methanol (80:20, v/v) mixture, then with
10 mL ice-cold water. This solid was suspended in methanol and
suspension was refluxed for 30 min. After cooling to ambient
temperature, the solid was gathered by filtration and dried under
vacuum at 45 ꢀC. It was then sublimated under vacuum, yielding a
white powder. Compounds were characterized by usual methods.
2.2.1. 2,20-Diphenyl-6,60-bibenzoxazole (1)
Yield: 65%; m.p.: 248.2 ꢀC; 1H NMR (300 MHz, CDCl3):
0
0
d
ppm ¼ 8.29e8.35 (m, 4H, H11, H11 , H15 and H15 ), 7.89 (s, 2H, H7
3 4
0
0
and H7 ), 7.86 (s, 2H, H4 and H4 ), 7.68 (dd, 2H, JH-H ¼ 9 Hz, JH-
0
0
0
¼ 3 Hz, H5 and H5 ), 7.54e7.62 (m, 6H, H12, H12 , H13, H13 , H14 and
H
H
14 ); 13C NMR (75 MHz, CDCl3):
d
ppm ¼ 163.68 (Cq, C2 and C2 ),
0
0
0
0
151.47 (Cq, C8 and C8 ), 141.66 (Cq, C9 and C9 ), 138.61 (Cq, C6 and
0
0
0
0
C6 ), 131.70 (CH, C13 and C13 ),129.01 (CH, C12, C12 , C14 et C14 ),127.71
0
0
0
(CH, C11, C11 , C15 and C15 ), 127.06 (Cq, C10 and C10 ), 124.53 (CH, C5
and C5 ), 120.11 (CH, C7 and C7 ), 109.40 (CH, C4 and C4 ); IR (cmꢁ1):
¼ 1620 (C]N), 1592 (C]C), 1549 (N]CeO-); elemental analysis
calcd (%) for C26H16N2O2: C 80.40, H 4.15, N 7.21; found: C 79.52, H
0
0
0
To carry out this study, three derivatives of 2,2’-diphenyl-6,6’-
bibenzoxazole were considered. One of them was unsubstituted
compound 1 and the other two compounds (2 and 3) bore a methyl
and a tert-butyl group at the para position of the phenyl ring,
respectively (Fig. 1). Their spectroscopic properties were examined
both in solution and in the solid state. They were discussed on the
basis of DFT calculations and crystal data obtained by X-ray
diffraction analysis. A comparison was made with the basic chro-
mophores 4e6.
n
3.78,
N 7.91; HRMS (DCI-CH4) m/z calcd for C26H17N2O2:
389.1290 M þ Hþ; found: 389.1304.
2.2.2. 2,20-Di-p-tolyl-6,60-bibenzoxazole (2)
Yield: 72%; m.p.: 316.6 ꢀC; 1H NMR (300 MHz, CDCl3):
ppm ¼ 8.21 (d, 4H, 3JH-H ¼ 9 Hz, H11, H11 , H15 and H15 ), 7.85e7.88
0
3
0
d
(m, 4H, H4, H4 , H7 and H7 ), 7.68 (dd, 2H, JH-H ¼ 9 Hz, 4JH-H ¼ 3 Hz,
0
0
3
0
0
0
H5 and H5 ), 7.39 (d, 4H, JH-H ¼ 9 Hz, H12, H12 , H14 and H14 ), 2.50 (s,
6H, 2CH3); 13C NMR (75 MHz, CDCl3):
d
ppm ¼ 164.11 (Cq, C2 and
0
0
0
C2 ), 151.37 (Cq, C8 and C8 ), 142.44 (Cq, C13 and C13 ), 141.48 (Cq, C6
0
0
0
0
0
and C6 ), 138.50 (Cq, C9 and C9 ), 129.78 (CH, C12, C12 , C14 and C14 ),
0
0
0
127.74 (CH, C11, C11 , C15 and C15 ), 124.51 (CH, C5 and C5 ), 124.16 (Cq,
0
0
0
C
10 and C10 ), 119.87 (CH, C7 and C7 ), 109.35 (CH, C4 and C4 ), 21.73
(CH3 and CH3 ). IR (cmꢁ1):
n
¼ 1617 (C]N), 1599 (C]C), 1552 (N]
0
CeO-); elemental analysis calcd (%) for C28H20N2O2: C 80.75, H 4.84,
N 6.73; found: C 80.32, H 4.42, N 7.41; HRMS (DCI-CH4) m/z calcd for
C
28H21N2O2: 417.1603 M þ Hþ; found: 417.1620.
2.2.3. 2,20-Di-p-tert-butylphenyl-6,60-bibenzoxazole (3)
Yield: 75%; m.p.: 268.7 ꢀC; 1H NMR (300 MHz, CDCl3):
ppm ¼ 8.25 (d, 4H, 3JH-H ¼ 9 Hz, H11, H11 , H15 and H15 ), 7.89-7.86
0
0
Fig. 1. Chemical structure of the 2,20-diphenyl-6,60-bibenzoxazole derivatives 1e3 and
2-phenylbenzoxazole derivatives 4e6.
d
(m, 4H, H4, H4 , H7 and H7 ), 7.68 (dd, 2H, JH-H ¼ 9 Hz, 4JH-H ¼ 3 Hz,
3
0
0

284
A. Ghodbane et al. / Dyes and Pigments 125 (2016) 282e291
3
0
0
0
0
H5 and H5 ), 7.61 (d, 4H, JH-H ¼ 9 Hz, H12, H12 , H14 and H14 ), 1.43 (s,
and filtration on paper. For the measurement of molar extinction
coefficients, solutions of bis-benzoxazoles at around 7.5 ꢂ 10ꢁ4 M
in CCl4 were prepared from a precisely weighted amount of dye.
After sonication and gentle heating, 0.2 mL of these solutions was
mixed with n-heptane or ethanol until a total volume of 10 mL.
The obtained solutions contained 2% CCl4 and about 1.5 ꢂ 10ꢁ5 M
dye. Spectroscopic measurements were conducted at 20 ꢀC in
temperature-controlled cells, on air-equilibrated solutions.
UVeVis absorption spectra were recorded on a HewlettePackard
8452A diode array spectrophotometer with step of 2 nm and a
SAFAS Xenius spectrofluorometer with step of 1 nm. Corrected
steady state fluorescence spectra were recorded with a Photon
Technology International (PTI) Quanta Master 1 spectrofluorom-
eter. The fluorescence quantum yields (FF) of bis-benzoxazoles in
18H, 6CH3); 13C NMR (75 MHz, CDCl3):
C2 ), 155.33 (Cq, C8 and C8 ), 151.45 (Cq, C13 and C13 ), 141.79 (Cq, C6
and C6 ), 138.47 (Cq, C9 and C9 ), 127.54 (CH, C11, C11 , C15 and C15 ),
126.01 (CH, C12, C12 , C14 and C14 ), 124.42 (CH, C5 and C5 ), 124.29
d
ppm ¼ 163.86 (Cq, C2 and
0
0
0
0
0
0
0
0
0
0
0
0
0
(Cq, C1 and C1 ),119.96 (CH, C7 and C7 ),109.36 (CH, C4 and C4 ), 35.14
(Cq, C13 and C13 ), 31.19 (CH3 and CH3 ); IR (cmꢁ1):
n
¼ 1620 (C]N),
0
0
1596 (C]C), 1551 (N]CeO-); elemental analysis calcd (%) for
C
34H32N2O2: C 81.57, H 6.44, N 5.60; found: C 81.56, H 6.46, N 6.58.
HRMS (DCI-CH4) m/z calcd for C34H33N2O2: 501.2542 M þ Hþ;
found: 501.2563.
2.3. Crystallographic data
solutions were determined using the classical formula: FFx
¼
Crystal data (Table 1) were collected at a temperature of 193(2)K
on a Bruker-AXS APEX II diffractometer using a 30 W air-cooled
microfocus source (ImS) with focussing multilayer optics using
(As ꢂ Fx ꢂ n2x ꢂ Fs)/(Ax ꢂ Fs ꢂ n2s) where A is the absorbance at the
excitation wavelength, F the area under the fluorescence curve
and n the refraction index of the solvent used. Subscripts s and x
refer to the standard and to the sample of unknown quantum
yield, respectively. Quinine sulfate dihydrate in 1.0 M aqueous
MoK
a
radiation (wavelength ¼ 0.71073 Å). Phi- and omega-scans
were used. The structures were solved by direct methods and all
non-hydrogen atoms were refined anisotropically using the least-
square method on F2 [28].
H2SO4
(
F
¼ 0.546) was taken as the standard [29]. The fluores-
cence quantum yields were measured by exciting the samples
near their absorption maximum. The absorbance of the solutions
was equal or below 0.05 at the excitation wavelength. The error on
the quantum yield values was estimated to be about 10%.
These data can be obtained free of charge via www.ccdc.cam.
ac.uk/data_request/cif (or for the CCDC, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: þ441223 336033; e-mail: deposit@ccdc.
cam.ac.uk).
For solid state measurements, the diffuse reflectance spectra of
the powders were measured with a PerkineElmer 860 Spectro-
photometer equipped with a 15 cm diameter integrating sphere
bearing the holder in the bottom horizontal position. They were
recorded at room temperature in steps of 1 nm, in the range
250e400 nm with a bandwidth of 2 nm. The instrument was
calibrated with a certified Spectralon white standard (Labsphere,
North Sutton, USA) and spectra were acquired by inserting before
the detector a visible short-wave pass filter (LOT-Oriel 400FL07-50,
400 cut-off wavelength) in order to remove the fluorescence
component of the benzoxazole derivative. The KubelkaeMunk
model described light penetration in homogenous and optically
thick media with only two parameters: an absorption coefficient, K,
and anisotropic scattering coefficient, S (both in cmꢁ1) [30]. This
model allowed us to deduce that there was a simple relationship
between the reflectance at “infinite thickness” and the absorption
and scattering coefficients: F (R∞) ¼ (1eR∞)2/2R∞ ¼ K/S. For ma-
terials where the dye did not aggregate and did not fluoresce, the
law could be applied, so that K was the dye absorptivity (ε in
2.4. Apparatus and methods
The melting points were measured on a Stuart Automatic
SMP40 apparatus. The following characterizations were made in
the “Services Communs de l’ICT (Institut de Chimie de Toulouse)” at
ꢀ
Universite de Toulouse III. NMR spectra were recorded on a Bruker
AC 300 spectrometer operating at 300.13 MHz for 1H and
75.49 MHz for 13C. Attributions were made owing to 2D NMR data.
The IR spectra were measured on KBr pellets using a PerkineElmer
IR Ft 1600 spectrophotometer. Elemental microanalyses were ob-
tained with an EA1112 elemental analyzer from CE instruments.
High resolution mass spectra were obtained with a GCT Premier
spectrometer using the chemical ionization technique with CH4.
Most of dilute solutions were prepared by direct dissolution of
the dyes in n-heptane and ethanol, with sonication, gentle heating
Table 1
M
ꢁ1cmꢁ1) scaled by the effective concentration C (M). The K/S ratio
Crystal data of compounds 3 and 6.
was then equal to K/S ¼ (ε ꢂ C)/S. Solid state photoluminescence
quantum yields were recorded on a SAFAS Xenius spectrofluo-
rometer equipped with a BaSO4 integrating sphere and a Hama-
matsu R2658 detector. Solid samples were deposited on a metal
support and luminescence spectra were corrected. 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 fluorescence spectra
were recorded with the sample facing the source light (Ec) and out
from the 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
Compound
3
6
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C34H32N2O2.2CCl4
808.24
Triclinic
P1
C17H17NO
251.31
Monoclinic
P21/n
6.2147(14)
17.552(4)
17.629(4)
77.711(11)
88.463(12)
86.667(13)
1875.6(7)
2
0.50 ꢂ 0.06 ꢂ 0.02
29494/6323
Rint ¼ 0.2391
510/180
6.1197(2)
52.2480(16)
8.6784(3)
90
98.9716(17)
90
2740.90(16)
8
0.40 ꢂ 0.20 ꢂ 0.20
20861/6726
Rint ¼ 0.0346
413/90
b (Å)
c (Å)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
Volume (Å3)
Z
Crystal size (mm3)
Reflections
(FP) were determined by a method based on the one developed by
collected/independent
Parameters/Restraints
de Mello et al. [31] using the formula:
Final R1 indice I > 2
wR2 all data
s
(I)
0.0925
0.2774
0.666 and ꢁ0.388
0.0520
0.1345
0.155 and ꢁ0.164
FP ¼ Ec ꢁ ð1 ꢁ aÞEb=Laa
Largest diff. peak
and hole (e.Åꢁ3
)
with
a
¼ 1eLc/Lb. The error on the quantum yield values was esti-
CCDC
1058726
1058727
mated to be about 20%.

A. Ghodbane et al. / Dyes and Pigments 125 (2016) 282e291
285
2.5. Computational methods
exhibited fine vibrational structure. The unusual coexistence of a
diffuse absorption band and a vibrationally-structured emission
band was already encountered for naphthoxazole derivatives [14].
According to Roussilhe and Paillous [27], this effect can be attrib-
uted to the existence of a continuum of conformations in the
ground state deviating from the postulated planar structure, while
emission would only result from the planar conformation. In
ethanol, the emission spectra were very slightly shifted to the red
with respect to n-heptane. The fine vibrational structure was still
observable, although less distinct. This weak solvent effect was
characteristic of dyes that do not bear very polar groups and whose
dipolar moment hardly varies with passing from ground to excited
state. The fluorescence quantum yields were up to 0.93. These
values were lower than that reported for 1 in cyclohexane [27].
Despite the care brought to measurements, the quantum yield
values may be affected by the poor solubility of the compounds and
possible subsequent dissolution upon dilution.
For the sake of comparison, the spectroscopic data of related
monochromophoric molecules 4e6 were reported in Table 2 and
the spectra of compound 6 were displayed in Fig. 2b. The molar
extinction coefficients were measured in pure solvents. They were
in good agreement with those given in the literature for compound
4 [27,38e40]. It is noteworthy that they were less than half those of
bis-chromophores. They were found to be higher in n-heptane than
in ethanol, but the shape of the spectra was different, the absor-
bance maximum in n-heptane corresponding to a sharp vibrational
peak. The absorption and emission spectra of monochromophoric
dyes in solution were blue-shifted by 32 nm and 60 nm respec-
tively, with respect to bis-chromophores. The shape of the emission
spectra also differed by the relative intensity of the vibrational
bands. The fluorescence quantum yields in solution were signifi-
cantly lower for monochromophores than for bis-chromophores.
Quantum chemical (QM) calculations have been carried out
using Gaussian 09 [32]. Standard calculations for structural opti-
mization were performed at the B3LYP/6-31G** level. Both ab-
sorption and emission electronic transitions were calculated by
TDDFT (time-dependent density functional theory) at the same
level as optimizations, for consistency. Fluorescence emissions
were carried out by calculating the relaxation of the excited state
geometry followed by the emission state-specific solvation and by
the emission to the final ground state [33,34] as implemented in
Gaussian 09. The solvent was taken into account with the PCM
model implemented in Gaussian 09 [35]. Vibrational frequencies
were checked to ensure that optimized conformations correspond
to energy minima. Moreover, extra-calculations were performed
with CAM-B3LYP, wB97XD, M06, M06L and M062X DFT functionals
and extended bases as 6e311þþG(3df,3dp) and cc-pVTZ. These
extra-calculations confirmed the results found with the previous
modest strategy (B3LYP/6-31G**). So, in the following discussion,
interpretation was based on the results determined at the B3LYP/6-
31G** level.
3. Results
3.1. Synthesis
Many methods can be used for the synthesis of 2,20-diphenyl-
6,60-bibenzoxazole derivatives. 3,30-Dihydroxybenzidine can be
condensed with two molecules of phthalic anhydride [20] aldehyde
[24] carboxylic acid [23,36] or acyl chloride [37] to give an inter-
mediate that subsequently undergoes intramolecular cyclization. In
the present work, 3,30-dihydroxybenzidine was reacted with
appropriate acyl chlorides according to a variant of the procedure
used for the preparation of 2-phenylbenzoxazole derivatives 4e6
[12]. Compounds 1e3 were obtained with a satisfying yield
comprised between 65 and 75%. The first two compounds have
already been described in the literature, compound 3 was original.
3.3. Spectroscopic properties in the microcrystal state
Solid-state spectroscopic studies were performed on powder
compounds obtained by sublimation. Observation with a fluores-
cence microscope showed that the powders were made of small flat
microcrystals. The reflectance spectra of the powders were shifted
to long wavelengths with respect to solutions (Fig. 3), illustrating
the presence of specific interactions between the molecules in the
solid state. The spectrum of the tert-butyl derivative 3 was nar-
rower and more red-shifted than the other ones. It was observed
that all three microcrystalline powder compounds 1e3 emitted
blue light upon excitation with a hand-held UV lamp, although the
first one was much less emissive than the other two. The photo-
luminescence properties were studied with a spectrofluorometer
equipped with an integrating sphere (Fig. 4). The emission spectra
of the solids were significantly shifted to the red with respect to
solutions. They did not show clear vibrational resolution. The
maxima spanned from 412 to 424 nm depending on the compound.
The shape of the spectra was also different, the spectrum of the tert-
3.2. Spectroscopic properties in n-heptane and ethanol solutions
The absorption and fluorescence properties of bis-benzoxazole
derivatives 1e3 were first studied in solution in n-heptane and
ethanol. All data were collated in Table 2. Since the spectral char-
acteristics of the three compounds were very close, only the spectra
of 3 were given in Fig. 2a as an illustration.
The three bis-chromophores were very weakly soluble in com-
mon organic solvents, as already reported for 1 in the literature
[21]. However, it was noticed that they were fairly soluble in carbon
tetrachloride (CCl4). Consequently, the molar absorption co-
efficients were measured by preparing a concentrated solution in
CCl4 and then diluting this solution in n-heptane and ethanol. This
procedure allowed reliable measurements to be made, contrary to
direct dissolution of dyes in n-heptane and ethanol. The presence of
2% CCl4 in the dilute solutions induced no noticeable effect on the
absorption spectra. All absorption spectra peaked at around 330 nm
whatever the solvent and showed no vibrational structure. The
molar extinction coefficients (ε) were increased from 57100 to
65500 Mꢁ1cmꢁ1 with passing from 1 to 3 in ethanol. They were
slightly lower in n-heptane.
Fluorescence spectra were recorded at low concentration after
direct dissolution of the compounds in pure n-heptane and ethanol.
Excitation spectra were superimposable with absorption spectra
and independent from the emission wavelength. This observation
indicated the presence of only one emitting species in solution. In
n-heptane, emission spectra were centred at around 390 nm and
butyl derivative
3 being again the narrowest. The photo-
luminescence quantum yields of 2 and 3 (FP ¼ 0.14 for both com-
pounds) was 15 times higher than that of unsubstituted compound
1 (FP ¼ 0.009). It must be emphasized that these solid-state
quantum yield values only refer to the investigated microcrystal-
line powders. Most probably, different values would have been
found with single crystals as well as with powders prepared from
recrystallization or mechanical milling, since impurities and sur-
face defect play an essential role in solid-state quantum yield
determination [41].
By comparison, the monochromophoric compounds emitted in
the solid state at shorter wavelengths than the bis-chromophores
(Fig. 4, inset), the difference being comparable to that observed in

286
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Table 2
Spectroscopic properties of 2,2'-diphenyl-6,6'-bibenzoxazole derivatives 1e3 and 2-phenylbenzoxazole derivatives 4e6 dissolved in organic solvents and as microcrystalline
powders. Maximum absorption (labs) and emission wavelength (lem), molar absorption coefficient (ε), diffuse reflectance wavelength (lref), fluorescence and photo-
luminescence quantum yields (FF and FPP, respectively) are given. For fluorescence measurements, compounds 1e3 were excited at 330 nm both in solution and in the solid
state, compounds 4e6 were excited at 302 nm in solution and 310 nm in the solid state. a) Solvents containing 2% v/v CCl4; b) Data from ref. 12; c) Data from ref. 13. Maxima
underlined, s: shoulder, nd: not determined. The concentration of 2-phenylbenzoxazole derivatives in solutions was around 3 ꢂ 10e5 M for absorption measurements and
2 ꢂ 10e6 M for fluorescence. The bis-benzoxazole concentration in solutions was around 1.5 ꢂ 10e5 M for absorption measurements and around 8 ꢂ 10e7 M for fluorescence.
The error on fluorescence and photoluminescence quantum yield was estimated to be 10% and 20%, respectively.
Compound
Solution in n-heptane
Solution in ethanol
Solid state
labs (nm)
ε (Mꢁ1cmꢁ1
)
lem (nm)
FF
labs (nm)
ε (Mꢁ1cmꢁ1
)
lem (nm)
FF
lref (nm)
lem (nm)
FP
1
2
3
4
5
6
330
331
56000a
58300a
63400a
23700
27500
30600
368, 388, 408
370, 390, 410
372, 392, 412
330b
0.90
0.93
330
333
57100a
59200a
65500a
21700
24200
28100
376, 394, 412s
376, 394, 412s
376, 394, 412s
336b
0.82
0.89
360
366
377
345
nd
nd
408s, 422
410s, 424
412, 438s
360b
0.009
0.14
332
0.90
334
0.87
0.14
298b
302b
302b
0.69b
0.74b
0.77b
298b
302b
302b
0.73b
0.72b
0.75b
0.26b
0.34b
0.33b
c
334b
338b
369b
334b
340b
362b
1.2
1.0
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
500
Wavelength (nm)
Fig. 3. Remission function of the UVevisible diffuse reflectance spectra of powder
compounds 1 (brown line), 2 (blue line), 3 (red line). (For interpretation of the refer-
ences to colour in this figure legend, the reader is referred to the web version of this
article.)
100
100
80
Fig. 2. a) UVevis absorption spectra (dotted lines), normalized fluorescence excitation
60
(
lem ¼ 400 nm) and emission (lex ¼ 330 nm) spectra (solid lines) of bis-benzoxazole 3
80
in n-heptane (purple lines) and ethanol (red lines). Dye concentration 1.18 ꢂ 10e5
M
40
for absorption and 7.9 ꢂ 10e7 M for fluorescence. b) UVevis absorption spectra (dotted
lines) and normalized fluorescence emission (lex ¼ 302 nm) spectra (solid lines) of the
monochromophoric compound 6 in n-heptane (purple lines) and ethanol (red lines);
normalized fluorescence excitation (lem ¼ 360 nm) spectrum in n-heptane (purple
solid lines). Dye concentration 2.45 ꢂ 10e5 M for absorption and 1.7 ꢂ 10e6 M for
fluorescence. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
20
60
0
300 350 400 450 500
Wavelength (nm)
40
20
solution. The photoluminescence spectrum of the tert-butyl de-
rivative 6 was now broader and better resolved than the other
spectra, contrary to what was observed for the bis-chromophores.
The spectrum shape and broadness might be related to peculiar-
ities in the crystal arrangement, which were obviously different in
both series of compounds. The photoluminescence quantum yields
were much higher for the monochromophoric compounds than for
bis-chromophores. It is noteworthy that the unsubstituted deriva-
tive 4 was less emissive than the methyl and tert-butyl analogues 5
and 6, but the difference was not very spectacular, in contrast to
what happened in the bis-chromophoric series (Table 2).
0
350
400
450
500
Wavelength (nm)
550
600
Fig. 4. Normalized photoluminescence spectra of bis-benzoxazole 1 (brown line), 2
(blue line) and (red line) in the microcrystalline state lex 330 nm). Inset:
Normalized photoluminescence spectra of monochromophores 4 (brown line), 5 (blue
line) and 6 (red line) in the microcrystalline state (lex ¼ 310 nm). (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version
of this article.)
3
(
¼
3.3.1. Crystal structure
structure-property relationship. However, to the best of our
knowledge, no such data was currently available in the literature.
Several attempts were made, therefore, to grow monocrystals of
our compounds in order to undertake X-ray crystallographic
It is now well established that the solid-state emission proper-
ties closely depend on the molecular arrangement [9,42e44]. The
crystal structure of the bis-benzoxazoles was necessary to study the

A. Ghodbane et al. / Dyes and Pigments 125 (2016) 282e291
287
analysis, but they were limited by the lack of appropriate solvents.
The best results were obtained by slow evaporation of tetra-
chloromethane solutions. Crystals of 1 and 2 were not usable, but
those obtained with 3 were particularly big and seemed suitable for
X-ray analysis. Against all odds, they only diffracted weakly, but the
structure could be solved. Each solvate crystal cell contained two
organic molecules and four molecules of CCl4. All molecules had
their long axis oriented in the same direction (Fig. 5a), and were
organized into a laminar structure. The interplanar distance was
3.4 Å. However, overlapping molecules were mutually offset by one
another in the longitudinal and lateral extent, so that the overlay of
the aromatic systems was very weak (Fig. 5b and c). Most inter-
estingly, the molecules of 3 were not planar. They were twisted by
about 40ꢀ around the carbonecarbon single bond that links the two
moieties of 2-(p-tert-butylphenyl)-benzoxazole (Fig. 5b). Each
molecule was involved in 16 short contacts measuring between 2.3
and 3.4 Å with neighbouring organic molecules and 3 short con-
tacts with CCl4 molecules. The tert-butyl groups were involved in
many short contacts and thus seemed to play a structuring role.
They brought steric hindrance that contributed to reducing the
molecular overlap.
Crystals of the corresponding monochromophore 6 were easily
grown in methanol and analysed for the sake of comparison. Each
crystal cell contained eight molecules displaying various orienta-
tions one with respect to the other. Molecules could be divided into
two types (I and II) that displayed between them an angle close to
80ꢀ. For type II molecules, close contacts only involved the tert-
butyl group of neighbouring molecules. No overlap of aromatic
systems between neighbouring molecules was observed. For type I
molecules, a reduce overlap was found. It must be noticed that a
similar crossed arrangement was previously observed for 2-
phenylbenzoxazole derivatives bearing a methyl group [12] and
an iodine atom [13] at the para position, as well as for naphthox-
azole derivatives [14]. It is obvious for 6 that the tert-butyl group
brought steric hindrance and a structuring ability, making the
molecular arrangement much more complex than for small
analogues.
3.3.2. DFT calculations
In light of these results, it seemed instructive to know whether
the bis-chromophoric molecules are twisted in solution as they are
in the crystal, and if a difference of conformation exists between the
ground state and the excited state. DFT calculations were therefore
undertaken for the six molecules in a vacuum, n-heptane and
ethanol. As expected, monochromophores 4, 5 and 6 were planar in
both the ground and excited state. In contrast, in the ground state,
bis-chromophores exhibited a twist between the two benzoxazole
subunits (Fig. 6a). Fig. 6b and c shows the highest occupied (HOMO)
and lowest unoccupied (LUMO) molecular orbitals of 1, respec-
tively. The delocalization of both molecular orbitals combined to a
strong overlap was in line with high molar extinction coefficient
and near-UV absorption, taking into account the size of the
molecules.
From the quantum chemical optimizations for both S0 and S1
electronic states of compounds 1, 2 and 3 in a vacuum as well as
in solvents (n-heptane and ethanol), the value of the inter-
benzoxazole dihedral angle was obtained (Table 3). While bis-
chromophores exhibited a dihedral angle between 36.2ꢀ and
38.2ꢀ in the S0 state, very close to the dihedral angle of 3 deter-
mined in the crystal, they flattened in the first singlet excited state
in a vacuum and in the investigated solvents, reaching a dihedral
angle between 11.5ꢀ and 18.8ꢀ.
Fig. 5. a) Crystal cell of 2,20-di-(p-tert-butylphenyl)-6,6'-bibenzoxazole (3); b and c) arrangement of stacked molecules. d) Crystal cell of p-tert-butylphenyl-benzoxazole (6); e)
Arrangement of type I and II molecules. f) Arrangement of stacked type I molecules. Short contacts are in blue ink. The pink color indicates the overlap of the aromatic systems. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

288
A. Ghodbane et al. / Dyes and Pigments 125 (2016) 282e291
Fig. 6. a) Optimized conformation of 1 in the ground state. b) HOMO and c) LUMO
snapshots of 1.
Table 3
Calculated dihedral angle (in degree) between the two 2-phenylbenzoxazole sub-
units of the bis-chromophoric molecules in a vacuum and organic solvents for both
S
0 and S1 states.
Compound
1
State
Vacuum
n-Heptane
Ethanol
S0
S1
S0
S1
S0
S1
38.20
18.57
38.04
18.78
37.57
18.21
37.83
16.75
37.62
16.98
37.15
16.38
36.88
11.92
36.75
12.78
36.22
11.54
Fig. 7. Energy profile of the inter-benzoxazole dihedral angle of 1 in (a) ground elec-
tronic state S0, (b) first singlet excited state S1, in a vacuum (black), n-heptane (red) and
ethanol (blue). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
2
3
vibrational mode with a frequency of 74 cmꢁ1 giving a flip-flop
oscillation period of 450 fs).
The energy profile of the inter-benzoxazole torsion of 1 in both
S0 and S1 electronic states was determined (Fig. 7). The reference
energy set to zero was considered as the energy of the optimized
structure of 1 in a given electronic state (S0 or S1) in a specific
environment (vacuum, n-heptane or ethanol). We clearly observed
a different energy profile between the S0 and S1 states of 1, what-
ever the environment. Indeed, in S0 state, the equilibrium was
found at around 38ꢀ as mentioned previously with barrier heights
near 2 kcal/mol and 3 kcal/mol at 0ꢀ and 90ꢀ, respectively. The
influence of the solvent was weak, although the solvent tended to
decrease the barrier height towards 0ꢀ and to increase the barrier
height towards 90ꢀ. In S1 state, the equilibrium of 1 was found to be
between 12ꢀ and 18.6ꢀ depending on the environment. Now, the
energy profile of the torsion exhibited a very small barrier at 0ꢀ that
was equal to 0.31 kcal/mol in a vacuum, 0.29 kcal/mol in n-heptane
and only 0.11 kcal/mol in ethanol. In contrast, the barrier height at
90ꢀ was huge, at least 15 kcal/mol in a vacuum and even more in
solvents. Once again, ethanol tended to decrease the barrier height
at 0ꢀ and to increase it at 90ꢀ.
The wavelengths of the S0 / S1 transition were determined
from quantum chemical calculations and reported in Table 4. The
maximum absorption of monochromophores 4, 5 and 6 in n-hep-
tane and ethanol was found to be between 296.0 nm and 300.6 nm,
in very good agreement with the experimental values that were
between 298 and 302 nm. Moreover, the strong oscillator strengths
found from calculations were well correlated with the strong molar
extinction coefficients measured experimentally. Both values were
higher for bis-chromophores than for basic chromophores. This
observation allows us to postulate that the B3LYP/6-31G** level of
quantum chemical calculations gave reasonably good results. In
contrast, for bis-chromophores 1, 2 and 3 in solution, calculations
led to absorption maxima comprised between 356.2 nm and
360.3 nm, while experimental values were significantly lower
(between 330 nm and 334 nm) suggesting an overestimation of the
conjugated delocalization in the bis-benzoxazoles from the
calculations.
Regarding emission, for the monochromophores, calculations
indicated that the maxima in n-heptane and ethanol were between
347.7 nm and 376.9 nm, while these compounds emitted experi-
mentally between 330 nm and 340 nm. Calculations were therefore
not very precise, but this is commonly the case. For the bis-
chromophores in solution, the discrepancy between calculated
Assuming that compounds 2 and 3 display the same behaviour
than 1, we can unambiguously state at this stage that bis-
benzoxazoles (in a vacuum or in solution) are non-planar in S0
state, while in S1 state they are instantaneously non-planar, but
planar on average (knowing that the inter-benzoxazole torsion is a

A. Ghodbane et al. / Dyes and Pigments 125 (2016) 282e291
289
Table 4
Calculated maximum absorption and emission wavelengths (in nm). Oscillator strengths are in brackets.
Compounds
Absorption
Vacuum
Emission
Vacuum
n-Heptane
Ethanol
n-Heptane
Ethanol
1
2
3
4
5
6
349.8 (1.515)
352.5 (1.693)
354.1 (1.847)
289.3 (0.728)
292.2 (0.851)
293.4 (0.934)
356.6 (1.723)
359.2 (1.905)
360.3 (2.030)
296.4 (0.876)
299.6 (1.008)
300.6 (1.086)
356.2 (1.717)
358.9 (1.908)
360.2 (2.035)
296.0 (0.867)
299.4 (1.004)
300.6 (1.081)
414.1
417.8
419.1
333.0
337.4
338.7
431.8
434.9
436.0
347.7
351.7
352.1
459.6
461.4
462.2
372.0
376.2
376.9
and experimental values was even more pronounced with calcu-
lated wavelengths between 431.8 nm and 462.2 nm. The poor
agreement between experimental and calculated wavelengths for
emission probably reflects the difficulty to determine accurately
the first singlet excited state of bis-chromophores by quantum
chemical calculations. The strong red shift observed for calculated
emission wavelengths compared to experimental values is partly
due to the too planar optimized geometries of bis-chromophores in
S1 state. Consequently, the energy profile of the inter-benzoxazole
torsion of 1 in S1 state appears probably underestimated near
0ꢀ and overestimated near 90ꢀ. More sophisticated quantum
chemical methods such as Coupled-Cluster, multiconfigurational
SCF (CASSCF) with dynamical electron correlation treated with
multi-reference CI or second order perturbation theory (MS-
CASPT2) would probably provide a better description of the elec-
tronic state. However, those very time-costly calculations are out of
the scope of the present paper. The TDDFT strategy used here
already highlights the flattening of bis-chromophores in S1 state
relative to S0 state, in solutions.
Moreover, it should be noted that the emission transition of bis-
benzoxazoles was calculated to be shifted by around 26 nm with
passing from n-heptane to ethanol, whereas such a strong red shift
was not observed experimentally. This discrepancy points out the
limitation of the simple PCM model. A more accurate description of
the solvent effect would consists in modelling explicitly the first
solvation shell in a first approximation, using for example the
ONIOM method [45e47].
biphenyl derivatives is attained when both phenyl rings are
coplanar [50]. Fluorescence efficiency can therefore be very high
[48,51]. The molecules that cannot assume a planar conformation
due to steric hindrance are weakly emissive [50] because the
probability of intersystem crossing increases with the value of the
dihedral angle [52,53]. In the present case, time-resolved exper-
iments would be necessary to confirm the photophysical behav-
iour of our bis-chromophores in solution.
In the solid state, our bis-chromophores were much less emis-
sive than expected given their properties in solution. Unsubstituted
derivative 1 was even virtually not emissive. The origin of this weak
photoluminescence efficiency can be sought in the molecular
conformation. The crystal structure of compound 3 showed that the
two benzoxazole moieties were situated in different planes. It can
be noticed that the dihedral angle issued from X-ray data (42ꢀ) was
in good agreement with that found by DFT calculations. The
molecule was thus locked in this twisted conformation and was
unable to become planar in the excited state, as could be the case in
solution. As stated above, non-planar conformations promote non-
radiative deactivation in biphenyl derivatives and this might
explain the moderate photoluminescence efficiency observed for
the bis-chromophores.
A comparative examination of the crystal structures provides
further information. The crystal packing mode of the 2-
phenylbenzoxazole derivative
6 closely resembled that of
analogue 5 [12] with the slight difference that the tert-butyl group
of 6 led to a longitudinal offset of stacked molecules, thus
decreasing the overlap of aromatic systems. Molecules displayed a
characteristic crossed arrangement with little overlap of their
conjugated electron systems. This arrangement promoted solid-
state emission and accounted for the very good quantum yields
observed in these series. It must be noticed that a similar packing
mode was also encountered in the naphthoxazole derivatives [14].
In this series, the offset between molecules was increased and the
overlap of p-electron systems was decreased in the presence of an
alkyl group on the phenyl ring, the effect being all the more pro-
nounced as the alkyl group was bulky.
In contrast, the crystal packing mode of 3 showed that all
molecules were oriented in the same direction. At first glance, this
arrangement seemed to be detrimental to light emission. However,
the molecules were offset in relation to one another and, as a result,
4. Discussion
In solution, the absorption and emission spectra of 2,20-
diphenyl-6,60-bibenzoxazole derivatives were red-shifted by 32 nm
and 60 nm, respectively, with respect to the corresponding 2-
phenylbenzoxazoles. This shift is in line with the extension of the
conjugated electron system, although it is not stronger than that
obtained by replacing the benzoxazole heterocycle by a naph-
thoxazole. Most interestingly, the fluorescence quantum yields
were significantly higher for the bis-chromophores than for the
monochrophores. This effect can also be attributed to the increase
of the conjugation length, as recently reported by Yamaguchi et al.
for various p-conjugated hydrocarbons [48].
Bis-benzoxazoles are characterized by the presence of a CeC
single bond that, theoretically, allows free rotation of the ben-
zoxazole moieties in solution. DFT calculations confirmed that
these molecules were indeed twisted in solution in the ground
state with a dihedral angle close to 37ꢀ. In this respect, they are
reminiscent of biphenyl-based chromophores, the dihedral angle
of which is generally between 15ꢀ and 40ꢀ in the ground state [49].
In the S1 excited state, 1 was calculated to be almost planar.
Here again, our bis-chromophores resemble biphenyl derivatives,
the photophysical behaviour of which has been extensively
investigated. In fact, in the excited state, optimal conformation of
the overlap of the p-conjugated systems was reduced, making the
arrangement quite favourable to light emission. This offset was
probably promoted by the presence of the tert-butyl group that
plays a structuring role in the molecular arrangement, as observed
here for the 2-phenylbenzoxazole series and previously reported
for the 2-phenyl-naphthoxazole series [14]. It is possible that, in the
present case, molecules of unsubstituted compound 1 were ar-
ranged closer to front-face parallel than the substituted analogues.
This hypothesis could explain the bad performance of 1 and why
the nature of the alkyl substituent had such a strong influence on
the photoluminescence quantum yield. It would deserve to be

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A. Ghodbane et al. / Dyes and Pigments 125 (2016) 282e291
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