Synthesis and properties of 2-phenylbenzoxazole-based luminophores for in situ photopolymerized liquid-crystal films

Article abstract of DOI:10.1002/hlca.200690035

We report the synthesis of a series of blue-emitting 2-phenylbenzoxazoles (PBOs) substituted at either the 5- or 6-position of the benzoxazole ring and the para-position of the phenyl substituent. The thermal and optical properties of the materials can be rationalized in terms of the position of the substituent at the benzoxazole moiety and the electron-withdrawing or electron-donating character of the substituents. From the results, we conclude that the combination of an electron-donating substituent at the benzoxazole fragment and an electron-withdrawing one at the phenyl fragment has a more marked effect on the electronic properties of the aromatic PBO core than other possibilities. This particular combination gives luminophores that are suitable for optical applications on the basis of their high emission efficiency and photostability. In view of that, oriented films were prepared by in situ polymerization of a mixture of a liquid crystalline direactive matrix containing 5% (w/w) of the luminophore. The films exhibit linearly polarized emission.

Full text of DOI:10.1002/hlca.200690035

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Helvetica Chimica Acta – Vol. 89 (2006)
Synthesis and Properties of 2-Phenylbenzoxazole-Based Luminophores for
in situ Photopolymerized Liquid-Crystal Films
by Raquel Giméneza), Luis Oriola), Milagros Piñolb), José Luis Serrano*a),
Ana I. Viñualesa), Thomas Fisherc), and Joachim Stumpec)
a) Departamento de Química Orgánica, Facultad de Ciencias-ICMA, Universidad de Zaragoza-CSIC,
E-50009-Zaragoza (phone and fax: +34976761209; e-mail: joseluis@unizar.es)
b) Departamento de Química Orgánica, Escuela Politécnica Superior de Huesca-ICMA, Universidad de
Zaragoza-CSIC, E-22071 Huesca
c) Fraunhofer-Institute of Applied Polymer Research, P.O. Box 600651, D-14476 Golm
We report the synthesis of a series of blue-emitting 2-phenylbenzoxazoles (PBOs) substituted at
either the 5- or 6-position of the benzoxazole ring and the para-position of the phenyl substituent. The
thermal and optical properties of the materials can be rationalized in terms of the position of the substitu-
ent at the benzoxazole moiety and the electron-withdrawing or electron-donating character of the sub-
stituents. From the results, we conclude that the combination of an electron-donating substituent at the
benzoxazole fragment and an electron-withdrawing one at the phenyl fragment has a more marked effect
on the electronic properties of the aromatic PBO core than other possibilities. This particular combina-
tion gives luminophores that are suitable for optical applications on the basis of their high emission effi-
ciency and photostability. In view of that, oriented films were prepared by in situ polymerization of a mix-
ture of a liquid crystalline direactive matrix containing 5% (w/w) of the luminophore. The films exhibit
linearly polarized emission.
1. Introduction. – In the last few years, there has been increasing interest in the field of
polarized-light-emitting-layers because these systems can provide the linearly polar-
ized emission that is needed for liquid-crystal displays (LCDs), thus simplifying their
manufacture and reducing costs [1][2]. Linearly polarized light emission can be
obtained from thin films with optically anisotropic emitters oriented in a specific direc-
tion [3]. From the point of view of structural order, self-organizing liquid crystals offer
excellent control over molecular orientation through the use of appropriate processing
techniques. A feasible approach to prepare a film that actively emits polarized light is
based on the incorporation of a luminophore into a liquid-crystal matrix, in which it
aligns as a result of the guest-host effect. Molecular orientation needs to be preserved,
and in situ photopolymerization of oriented reactive liquid-crystal monomers provides
an excellent and fast way to produce permanently cross-linked thin anisotropic poly-
meric films [4]. This strategy has already been successfully applied to the production
of oriented luminescent films in which the luminophore has been dispersed in, or cova-
lently bound to, the matrix [5–8]. A number of factors control the potential applica-
tions of a practical luminophore, and these include its compatibility with the liquid-
crystal matrix and its thermal and photochemical stability. Therefore, for practical
applications, the design and synthesis of suitable organic luminophores is still required.
© 2006 Verlag Helvetica Chimica Acta AG, Zürich

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In the search for new luminophores for use in anisotropic emissive thin films, we
have given consideration to the benzoxazole core because of its attractive optical prop-
erties and potential mesogenicity. Benzoxazoles are a group of heterocyclic compounds
that have attracted considerable interest because they are known as photostable and
highly efficient luminophores used as organic brightening agents, laser dyes, organic-
plastic scintillators, and optical-fiber sensors [9][10]. The fact that the luminescence
of this system is greatly influenced by substituents has also found applications, and pol-
ymers with pendant 2-phenylbenzoxazole (PBO) systems have been used in the gener-
ation of fluorescent images by photolithographic methods or as chemosensors for metal
ions [11][12]. Furthermore, several 2-phenylbenzoxazoles have been studied as blue
emitters in electroluminescence devices [13][14] and second-order NLO-active chro-
mophores (NLO=nonlinear optics) [15–19].
In this paper, we present the synthesis and properties of a series of blue-emitting
doubly substituted PBOs (Fig. 1). We focused our attention on PBOs with substituents
at both the benzoxazole moiety and the phenyl ring, and structural modifications were
aimed at tailoring the thermal and optical properties to obtain effective liquid-crystal-
line-compatible luminophores. The incorporation of suitable substituents at the PBO
core is a basic characteristic to obtain liquid-crystal phases and, for this reason, an ini-
tial screening of potential substituents was performed to identify their ability to pro-
mote mesophase formation. However, the ultimate choice of substituents was made
by considering their effect on the optical and photophysical properties of the PBO
derivative. Therefore, electron-donating and electron-withdrawing substituents at the
5- or 6-positions of the benzoxazole moiety and at the para-position of the phenyl
ring were combined to investigate their effect on the absorption/emission properties
and photostability of these systems. The target compounds are represented as nX-
PBO-Y where PBO represents the 2-phenylbenzoxazole core, X is the substituent at
either the 5- or 6-position (n=5 or 6, resp.) of the benzoxazole moiety, and Y is the sub-
stituent at the phenyl ring.
Fig 1. Chemical Structure and Nomenclature of the Synthesized Benzoxazoles
2. Results and Discussion. – 2.1. Synthesis and Liquid-Crystalline Properties of the
Benzoxazoles. The synthesis of the benzoxazoles was approached by ring closure of
the intermediate imines under oxidative conditions. The general procedure outlined
in Scheme 1 proved to be quite versatile. Condensation of a 2-aminophenol 2, carrying
the substituent X at the 4- or 5-position, with a benzaldehyde 3, carrying the substituent
Y at the 4-position, yielded the imine precursor 4 under acidic conditions. Initially, the

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Scheme 1. General Synthetic Procedure for the Preparation of the Benzoxazoles
oxidative cyclization of the imine was undertaken with lead(IV) acetate (Method A,
Scheme 1). However, yields were significantly higher on using DDQ (4,5-dichloro-
3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile) in toluene (Method B, Scheme 1).
The main problem encountered in the specific application of this strategy is the
availability of the starting 2-nitrophenol 1. For instance, when 5-(hexyloxy)-substituted
benzoxazoles (5HexO-PBO-Y) were desired, attempts were made to prepare the
required 4-(hexyloxy)-2-nitrophenol by nitration of 4-(hexyloxy)phenol [20]. However,
numerous by-products were obtained, and these proved difficult to separate and led to
a low yield (46%) of the desired product. The alternative route outlined in Scheme 2
was used to obtain from 5 the benzoxazoles 6 with a benzoyl-protected OH group.
After deprotection to 7, subsequent alkylation under Williamson conditions furnished
the desired POBs [21]. Nitration of 4-hydroxyphenyl benzoate to yield the 2-nitrophe-
nol intermediate 5 proved to be an efficient process and gave the required product in
83% yield.
The thermal behavior of the benzoxazoles was investigated by optical microscopy
(OM) and differential scanning calorimetry (DSC). The results are summarized in
Table 1. The PBO moiety substituted at the 5- or 6-position is a planar aromatic system,
at least formally, structurally related to the well known p,p’-disubstituted benzylidenea-
niline mesogens [22] in which the exocyclic C=N bond is constricted into a new hetero-
cycle by an O-bridge. Fixation of the structure improves the photoluminescence stabil-
ity but disturbs the desirable molecular linearity required for the formation of calamitic
mesophases. Nevertheless, the potential of the PBO moiety to promote liquid crystal-
linity has been proven in both low- and high-molecular-mass compounds [20]
[21][23–25].
The results collected in Table 1 show that substitution at the 5-position is more
favorable to the induction of mesogenicity than substitution at the 6-position, a trend
that has previously been pointed out by Pavluchenko et al. [23]. To support this exper-
imental observation, we performed geometrical calculations on the PBO structure pre-
viously optimized by molecular mechanics (MM2) and semi-empirical (MOPAC-AM1)

Helvetica Chimica Acta – Vol. 89 (2006)
307
Scheme 2. Synthesis of the 5-(Hexyloxy)benzoxazoles
Table 1. Thermal and Thermodynamic Data for the Benzoxazoles
Transition temperatures
[8]a)
Transition temperatures
[8]a)
5HexO-PBO-OHex
5HexO-PBO-CN
5HexO-PBO-COOHex Cr 82 I
5NC-PBO-OHex Cr 142 I
Cr 59 N 78 Ib)
5HexOOC-PBO-OHex Cr 97 (SmA 78) Ic)
6HexO-PBO-OHex
6HexO-PBO-CN
Cr 102 (N 95)d) I
Cr 45 (N 42) Ie)
Cr 98 I
6HexOOC-PBO-OHex Cr 82 I
a) Cr=crystal phase, N=nematic phase, SmA=smectic A phase, I=isotropic liquid; monotropic transi-
tion in parenthesis. b)DHN-I is 0.76 kJ molꢀ1. c) DHSmA-I is ꢀ5.42 kJ molꢀ1. d) Temperature determined by
optical microscopy. e) DHN-I is ꢀ0.48 kJ molꢀ1
.

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calculations, and it was found that deviation from linearity depends on the disubstitu-
tion pattern at the benzoxazole moiety (i.e., 2,5 vs. 2,6). Geometrical calculations indi-
cate that the 2,5-substitution pattern has a deviation from linearity of 248, while the 2,6-
substitution pattern gives rise to a deviation of 35.58 (Fig. 2).
Fig. 2. Linearity deviation of the 2,5-substitution pattern and 2,6-substitution pattern in PBO calculated
for the structure optimized by molecular mechanics (MM2) and semiempirical (MOPAC-AM1) calcu-
lations
Table 2. Optical Properties of Investigated Benzoxazoles in THF
ꢀ1
Substitution X/Ya)
labs [nm] (log e [M cmꢀ1])
lem [nm]
fem
PBO
299 (4.42)
324 (4.38)
336 (4.22)
332 (4.26)
312 (4.56)
312 (4.44)
320 (4.56)
340 (4.48)
320 (4.66)
338, 353
370, 382
438
0.49
0.41
0.70
0.67
0.50
0.51
0.47
0.90
0.50
5HexO-PBO-OHex
5HexO-PBO-CN
5HexO-PBO-COOHex
5NC-PBO-OHex
5HexOOC-PBO-OHex
6HexO-PBO-OHex
6HexO-PBO-CN
6HexOOC-PBO-OHex
D/D
D/A
D/A
A/D
A/D
D/D
D/A
A/D
430
355, 368 (sh)
350, 365
365, 382
417
368, 379 (sh)
a) Electronic character of the substituents X and Y: A=electron-accepting and D=electron-donating.
The mesomorphic properties are also affected by swapping the substituents at the
benzoxazole and phenyl fragments. For instance, 5HexO-PBO-CN shows a monotropic
nematic mesophase at 958 but its isomer 5NC-PBO-OHex has a higher melting point
and does not show mesomorphism. For the esters 5HexO-PBO-COOHex and
5HexOOC-PBO-OHex, only the incorporation of the ester group at the benzoxazole
moiety gives rise to a mesophase, with a monotropic smectic A mesophase observed
at 788.
2.2. Optical Properties and Photostability. The absorption and emission spectra of
the 2-phenylbenzoxazoles were recorded in THF solution (see Table 2). The UV/VIS
spectrum of PBO, taken as a reference, presents a distinct structured absorption
band centered at ca. 300 nm along with a structured emission band in the UV region
(354 nm). The fluorescence quantum yield is 0.49, which is comparable to that of qui-
nine sulfate.
The absorption and emission properties of PBO and some of its derivatives are well
documented [10][26–30]. Publications on this conjugated aromatic system mostly con-

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309
cern the incorporation of substituents at the phenyl ring and conclude that the photo-
physical behavior can be dramatically affected. For example, a strong red-shifted fluo-
rescence enhancement is observed when electron-donating substituents are located at
the para-position of the phenyl ring because of an intramolecular charge transfer in the
excited state from the phenyl ring to the benzoxazole electron-withdrawing moiety.
This phenomenon is the result of a partial C=C bond between the benzoxazole and
phenyl fragments, which restricts rotation of the two aromatic subunits of 2-phenylben-
zoxazole.
In this study, we prepared doubly substituted 2-phenylbenzoxazoles and, depending
on the electron-accepting (A) or electron-donating (D) ability of the substituents, sev-
eral combinations were investigated; these systems are denoted in Table 2 as X/Y=
D/D, D/A, or A/D. Substitution gives a systematic bathochromic shift of the absorption
that depends on the electronic nature of the substituents. Benzoxazoles with a D/A sub-
stitution present the largest bathochromic shift with a broad and featureless absorption
band. However, benzoxazoles with an A/D-type substitution pattern show the smallest
bathochromic shift and an absorption band with a vibrational structure similar to that
of PBO. Benzoxazoles with a D/D pattern show a behavior that is intermediate
between the two systems outlined above.
As far as the emission is concerned, comparison of the different systems reveals a
similar trend in terms of the position and shape of the emission bands as found for
the absorbance. The D/A combination gives the largest bathochromic shift in the emis-
sion, with broad and featureless bands located above 415 nm in the violet-blue region of
the spectrum. These systems are also the most efficient luminophores, with quantum
yields ranging from 0.65 to 0.90. In contrast, benzoxazoles of the A/D-type show emis-
sion bands in the range 355–370 nm, which is close to that of PBO. These bands also
have a weak vibronic structure similar to PBO. Once again, D/D-type benzoxazoles
show an intermediate type of behavior.
In relation to the position of the substituent at the benzoxazole moiety, benzoxa-
zoles substituted at the 6-position show higher molar absorption coefficients than the
related 5-substituted isomers. This is certainly due to the fact that the substituent at
position 6 of the benzoxazole is in a resonance position with respect to the N-atom
of the heterocycle.
The optical properties of the materials in cyclohexane were also examined. The low
solvent polarity leads to almost identical absorption wavelengths and molar absorption
coefficients but the bands are, in general, more structured in cyclohexane than in THF.
However, the emission spectra show substantial differences. In general, emission bands
appear at shorter wavelengths in cyclohexane than in THF and have more pronounced
vibronic features. The strongest solvatochromism is observed for D/A benzoxazoles. It
is known that polar excited states are stabilized by polar solvents and give broad emis-
sion bands, a fact that suggests that the excited states of D/A benzoxazoles are more
polar than the others.
The results show that optical behavior depends on the substituents and, in this
respect, A/D and D/D benzoxazoles resemble PBO more closely than D/A benzoxa-
zoles, which are the most suitable luminophores in terms of their efficiency and wave-
length emission.

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Evaluation of the photochemical stability is a critical issue from the point of view of
potential applications. For this reason, the photostability of the benzoxazoles was deter-
mined in CHCl3 solution and in PMMA (poly(methyl methylacrylate)) films. The sam-
ples were irradiated with 325-nm light, and changes in the UV/VIS spectra were
recorded. The photostability was assessed on the basis of the energy required to
decrease the absorbance to 90% of its original value at 325 nm, and this value is refer-
red to as E90. The relevant data are collected in Table 3.
Table 3. Photostability Data for the Benzoxazoles in CHCl3 Solution and in a PMMA Matrix
E90 [J cmꢀ2
]
E90 [J cmꢀ2
]
CHCl3
PMMA film
CHCl3
PMMA film
solution
solution
5HexO-PBO-OHex
5HexO-PBO-CN
5HexO-PBO-COOHex
5NC-PBO-OHex
3
15
36
68
27
5HexOOC-PBO-OHex
6HexO-PBO-OHex
6HexO-PBO-CN
9
0.1
95
21
2
70
25
a
)
)
a
39
6HexOOC-PBO-OHex 84
a) The photodegradation was not significant up to absorbed energy values of 100 J cmꢀ2
.
The values obtained show that the photostability is also dependent on the electronic
character and position of the substituents. It is interesting to point out the remarkable
stability of the 5D-PBO-A type benzoxazoles in solution. Up to an irradiation dose of
100 J cmꢀ2, degradation of the chromophores was almost insignificant. In contrast, the
fastest degradation was observed for D/D systems regardless of whether it was substi-
tuted at the 5- or 6-position of the benzoxazole moiety. The spectral changes under irra-
diation at 325 nm are shown in Fig. 3 for 5HexOOC-PBO-OHex and 5HexO-PBO-
OHex as representative examples. In the first case, a gradual decrease in the maximum
absorbance takes place with an increase of the absorbance above 330 nm, and an isos-
bestic point is observed at 335 nm. For 5HexO-PBO-OHex, the spectral changes are
more marked, and the absence of isosbestic points indicates the simultaneous occur-
rence of more than one photoreaction.
PBOs immobilized in a PMMA matrix were found to have a different photostability
from that of the corresponding system in solution (Fig. 4). The absorbance maxima
were recorded at the same wavelength as in solution, i.e., disregarding the formation
of aggregates, but E90 values are manifestly different in most cases. The stability
under illumination follows the same trend as in solution, with the D/A system being
the most photostable and D/D the least stable. Nevertheless, in this case, the photosta-
bility of 5D-PBO-A is comparable to the rest of the PBO systems. In general, PBOs of
the D/A and A/D type are more stable in solution, but the D/D systems are up to one
order of magnitude more stable in PMMA films.
2.3. Anisotropic Emissive Films. Studies published by Catalán et al. on PBOs have
established that the ground state and the excited state are almost coplanar, and that
the p–p* transition is polarized along the longitudinal axis of the molecule [27]. The
fact that these systems exhibit direction-dependent absorption of light is of basic inter-

Helvetica Chimica Acta – Vol. 89 (2006)
311
Fig. 3. UV/VIS Spectra (CHCl3) at different irradiation times for a) 5HexOOC-PBO-OHex and b)
5HexO-PBO-OHex
est for studies on oriented materials, and they are attractive candidates as emissive
guests in oriented liquid-crystalline networks.
Therefore, to evaluate the potential of these systems for polarized luminescence,
oriented networks were prepared by in situ photopolymerization of a direactive non-
luminescent liquid crystal, C6M, doped with 5% (weight) of the luminophore. It has
been reported that highly cross-linked films show lower dichroic ratios than films

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Helvetica Chimica Acta – Vol. 89 (2006)
Fig. 4. Plot of the normalized absorbance at 325 nm vs. the absorbed energy for 5HexOOC-PBO-
OHex, 5HexO-PBO-COOHex, and 5HexO-PBO-OHex in CHCl3 solution and PMMA films
with low levels of cross-linking but their properties are almost temperature-indepen-
dent, and luminescence is thermally stable and photostable [6][7].
The reactive liquid-crystal mixtures were polymerized in the nematic phase
between 90 and 1008. Polymerization temperatures were selected according to the mes-
omorphic properties of these mixtures, which were investigated by OM with appropri-
ate light filters. The liquid-crystalline behavior of the mixtures was comparable to that
of C6M, the major component of the blend, which has a nematic phase in the range
86–1168. All the reactive mixtures developed a nematic phase above 788 and were com-
pletely miscible. Prior to the production of oriented films, the photopolymerization of
the blends was studied by photo-DSC to rule out any effect of the luminophore on the
isothermal photoinitiated polymerization of the reactive liquid-crystal matrix. C6M
does not absorb above 350 nm but some of the PBOs still have some residual absorp-
tion at 365 nm, the wavelength of the emission of the lamp, and might reduce the effi-
cacy of the photoinitiator I-651, which also absorbs at this wavelength. The samples
were heated to the selected temperature and irradiated with 365-nm light during 15
min. From the registered DSC scan, it was verified that photoinduced polymerization
proceeds rapidly during the first few seconds of exposure to light. Assuming a polymer-
ization enthalpy equal to 78 kJ molꢀ1, conversion degrees of 80–90% were calculated
from the integration of the polymerization exotherm.
Oriented thin films were obtained by carrying out the photopolymerization in com-
mercial glass cells with unidirectionally rubbed polyimide coatings for planar align-
ment. These cells were filled with the reactive mixtures by capillary action. In the

Helvetica Chimica Acta – Vol. 89 (2006)
313
course of polymerization, a transparent film of good optical quality and a single domain
structure with good planar orientation was obtained, as observed by optical microscopy.
Polarized UV/VIS spectroscopy was used to establish the orientation of the films; how-
ever, studies were complicated because the absorption of the glass cell is superimposed
to the absorption of the C6M matrix and, to some extent, to the luminophores. There-
fore, absorption was maximum along the rubbing in the cell and minimum along the
perpendicular to it. Dichroic ratios in absorption of ca. 3 were determined.
The emission properties determined for the different benzoxazoles are collected in
Table 4. It can be observed that there is good agreement between the emission wave-
lengths determined in the oriented films and those in solution and PMMA. In general,
broad and unresolved emission bands were registered for the solid films in comparison
to the spectra in solution, especially for A/D and D/D benzoxazoles, presumably
because of intermolecular interactions arising in the ordered solid phase. Accidental
photodegradation of the PBO luminophores during photopolymerization was not
apparent from the emission spectra.
Table 4. Emission Wavelength and Dichroism of the Anisotropic Polymeric Films
lem [nm]
Rem [I||/I^]
lem [nm]
Rem [I||/I^]
5HexO-PBO-OHex
5HexO-PBO-CN
5HexO-PBO-COOHex
5NC-PBO-OHex
388
431
427
377
3.4
3.4
3.3
4.0
5HexOOC-PBO-OHex
6HexO-PBO-OHex
6HexO-PBO-CN
372
375, 392
427
4.0
2.5
3.0
2.9
6HexOOC-PBO-OHex
392
Furthermore, films exhibited the maximum emission when the polarization axis of
the incident light was parallel to the preferential orientation direction in the cell (I||),
and minimum when it was perpendicular to it (I^) (Fig. 5). The dichroic ratios (Rem)
were all in the range 3–4. Because the principal electronic axis of PBOs is aligned
with the long molecular axis, the results show that there is a preferential orientation
of the luminophore within the film along the rubbing direction in the cell.
Therefore, it can be deduced that the PBOs are introduced in a ‘guest–host’ config-
uration in anisotropic films obtained by in situ photopolymerization of reactive liquid
crystal and that such a system actively produces linearly polarized light. Nevertheless,
6-substitution gives lower emission dichroism, which can be tentatively related to the
larger deviation of the molecular linearity for a 2,6-disubstitution pattern in the PBO
core (see Fig. 2).
3. Conclusions. – In this work, the mesomorphic behavior and the absorption/emis-
sion characteristics of several doubly substituted 2-phenylbenzoxazole (PBO) deriva-
tives were investigated. The PBOs were synthesized by oxidative ring closure of imines
under very mild reaction conditions, which make it possible to incorporate a diverse
range of substituents in the PBO core. The results show that tuning of the emission
is possible depending on the electronic nature and position of the substituents at the
PBO core. Amongst the various possibilities, an electron-donating substituent located
at the 5- or 6-position of the benzoxazole moiety and an electron-withdrawing in the
para-position of the phenyl ring is the best option in relation to the optical properties.

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Fig. 5. Polarized emission of the anisotropic film 5HexO-PBO-CNand C6M photopolymerized in a
cell for planar alignment at 908 in the nematic phase
Blue emission and quantum yields ranging from 0.60 to 0.90 are combined with suitable
photostability in the solid state. The 5-substitution pattern gives larger bathochromic
shifts but the 6-substitution produces a larger enhancement of the emission efficiency.
In relation to the mesomorphic properties, the extent of the unfavorable bend in the
molecular structure due to the presence of the benzoxazole ring depends on the substi-
tution pattern, and 5-substitution at the benzoxazole is structurally more acceptable
than the 6-substitution to promote mesomorphism in the PBO core.
In summary, D/A 5-substituted PBOs are suitable luminophores for obtaining
polarized luminescence as they combine the structural requirements compatible with
the formation of mesophases and appropriate emissive properties. These properties
are transferred to solid anisotropic films obtained by an easy and clean method, the
in situ photopolymerization of reactive low-molar-mass liquid crystals.
From the results we conclude that PBOs are very promising luminophores for the
generation of linearly polarized light by using the auto-organization capability of liq-
uid-crystalline media. These results have encouraged us to undertake further research
regarding the incorporation of these systems in photo-addressable liquid crystals.
This work was supported by the EEC Brite/Euram project BE97-4210, the CICYT-FEDER Spanish
projects MAT 2002-04118-C02-01 and MAT 2003-07806-C02-01, the ‘Ramón y Cajal’ program (MCYT-
MEC), and funding from the Government of Aragon.
Experimental Part
1. General. FC=Flash column chromatography. Optical microscopy (OM): Olympus-BH-2 polariz-
ing microscope equipped with a Linkam-THMS hot-stage central processor and a CS196 cooling system.
Differential scanning calorimetry (DSC): DSC 2910 from TA Instruments with samples sealed in alumi-

Helvetica Chimica Acta – Vol. 89 (2006)
315
nium pans; scanning rate of 108 minꢀ1; under N2; temp. were read at the onset of the peak after prior heat-
ing of the sample to the isotropic liquid and cooling to 08. UV/VIS Spectra: UV4-200 spectrophotometer
from ATI-Unicam; 10ꢀ5 M solns. in THF or cyclohexane. Emission spectra: Perkin-Elmer LS50B spectro-
fluorimeter; THF or cyclohexane solns. of ca. 0.01 absorbance (ca. 10ꢀ7 M) under excitation at the absorp-
tion maximum; emission quantum yields (fem) in THF relative to quinine sulfate in sulfuric acid as a ref-
erence (fem =0.546).
~
IR Spectra: ATI-Matsson Genesis-FTIR spectrophotometer; nujol mulls between NaCl disks; n in
cmꢀ1. 1H- and 13C-NMR Spectra: Varian Unity-300 spectrometer; d in ppm, J in Hz. Elemental analysis:
Perkin-Elmer 240C microanalyzer.
2. Syntheses. 2.1. 2-Nitrophenols 1. 4-Hydroxy-3-nitrobenzonitrile was purchased from Aldrich.
5-(Hexyloxy)-2-nitrophenol. A soln. of 3-(hexyloxy)phenol (prepared by a Williamson reaction
between resorcinol (=benzene-1,3-diol) and 1-bromohexane; 4.0 g, 0.02 mol) in CH2Cl2 (48 ml) and Et2O
(100 ml) was added to a stirred mixture of NaNO3 (1.7 g, 0.02 mol) in H2O (24 ml) and conc. HCl soln. (18
ml). Ac2O (2/3 drops) was added, and the mixture was stirred at r.t. for 4 h. The aq. layer was extracted
with Et2O, the combined org. extract washed with H2O, dried (MgSO4), and evaporated, and the crude oil
purified by FC (hexane/AcOEt 10 :1): 5-(hexyloxy)-2-nitrophenol (33%). Oil. 1H-NMR (300 MHz,
CDCl3): 11.02 (s, 1 H); 7.98 (d, J=10.3, 1 H); 6.49–6.45 (m, 2 H); 3.99 (t, J=6.5, 2 H); 1.82–1.72 (m,
2 H); 1.44–1.21 (m, 6 H); 0.90–0.85 (m, 3 H).
Hexyl 4-Hydroxy-3-nitrobenzoate. A mixture of 4-hydroxy-3-nitrobenzoic acid (7.4 g, 40.7 mmol)
and NaHCO3 (3.4 g, 40.7 mmol) in dry N,N-dimethylacetamide (50 ml) was heated under reflux until
the evolution of CO2 had ceased. Hexyl 4-methylbenzenesulfonate (8.06 g, 38.7 mmol; previously pre-
pared by reaction of hexanol (1 equiv.) with 4-methylbenzenesulfonyl chloride (1.2 equiv.) at 08 in dry
pyridine under Ar) was added, and the mixture was stirred and heated under reflux for 1 h. The mixture
was allowed to cool down to r.t. the resulting soln. poured into H2O, the mixture extracted with CH2Cl2
(3×50 ml), the extract washed with sat. aq. NaHCO3 soln. and H2O, the combined org. phase dried
(MgSO4) and evaporated, and the crude product purified by FC (silica gel, hexane/AcOEt 9 :1): Oil
(6.95 g, 67%). 1H-NMR (300 MHz, CDCl3). 10.85 (s, 1 H); 8.77 (d, J=2.1, 1 H); 8.20 (dd, J=8.8, 2.1, 1
H); 7.18 (d, J=8.8, 1 H); 4.30 (t, J=6.7, 2 H); 1.76–1.66 (m, 4 H); 1.42–1.20 (m, 4 H); 0.89–0.84 (m,
3 H).
Hexyl 3-Hydroxy-4-nitrobenzoate was prepared from 3-hydroxy-4-nitrobenzoic acid as described
above for hexyl 4-hydroxy-3-nitrobenzoate: 56% yield. 1H-NMR (300 MHz, CDCl3): 10.45 (s, 1 H);
8.10 (dd, J=8.8, 1.8, 1 H); 7.55 (m, 1 H); 4.31–4.26 (m, 2 H); 1.99–1.68 (m, 2 H); 1.43–1.23 (m, 6 H);
0.84 (m, 3 H).
2.2. 4-Substituted Benzaldehydes 3. 4-(Hexyloxy)benzaldehyde and 4-formylbenzonitrile were
obtained from Aldrich.
Methyl 4-Formylbenzoate. Conc. H2SO4 soln. (5.5 ml) was added to a soln. of 4-formylbenzoic acid
(4.0 g, 26.6 mmol) in MeOH (250 ml), and the mixture was heated under reflux for 24 h. The resulting
soln. was allowed to cool down to r.t., poured into 10% HCl soln. (150 ml), and stirred for 1 h. The solvent
was evaporated and the residue extracted with CH2Cl2. The combined org. extract was successively
washed with aq. NaHCO3 soln., H2O and brine, dried (MgSO4), and evaporated and the crude product
purified by recrystallization from 96% EtOH: crystalline white solid (2.4 g, 55%). 1H-NMR (300
MHz, CDCl3): 10.06 (s, 1 H); 8.15 (d, J=9.0, 2 H); 7.91 (d, J=9.0, 2 H); 3.92 (s, 3 H).
2.3. Intermediate Imines 4: General Procedure. A soln. of the appropriate 2-nitrophenol in abs. EtOH
and cyclohexene (ca. 45 ml/g of the nitro derivative) was heated under reflux with stirring. Then 20%
(w/w) Pd(OH)2/C (ca. 30% of the weight of the nitro derivative) was added under Ar. The mixture
was protected from light, and the progress of the reaction was followed by TLC. After consumption of
the starting material (ca. 4 h), the mixture was allowed to cool down to r.t. and filtered through a pad
of Celite®. The solvent was evaporated to give the 2-aminophenol 2 as an oil. This oil was dissolved in
abs. EtOH and added under Ar to a soln. of the appropriate benzaldehyde in abs. EtOH. The mixture
was stirred and heated for 12 h under reflux in the presence of a catalytic amount of AcOH. The resulting
soln. was then evaporated and the product recrystallized from abs. EtOH to give 4.
2.4. Benzoxazoles (nX-PBO-Y): General Procedure. Method A: Lead(IV) acetate (1.61 mmol) was
carefully added in several portions to a soln. of 4 (1.61 mmol) in boiling CHCl3 (40 ml; previously filtered

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Helvetica Chimica Acta – Vol. 89 (2006)
through activated neutral alumina). The mixture was stirred until a solid residue of lead(II) acetate was
formed (30 min). The solid was filtered off, the filtrate concentrated, and EtOH added to precipitate the
desired benzoxazole, which was further purified by FC.
Method B: DDQ (12.00 mmol) was added gradually to a soln. of 4 (6.00 mmol) in toluene (200 ml;
previously dried over 4 Å molecular sieves) under reflux. The mixture was heated under reflux for 2 h
and then allowed to cool down to r.t. Sat. aq. NaHCO3 soln. (200 ml) was added, the mixture extracted
with CH2Cl2, and the combined org. phase washed with H2O and dried (MgSO4). The soln. was concen-
trated, and a large volume of hexane was added to precipitate the desired compound, which was filtered
off and purified by FC (silica gel).
Hexyl-2-[4-(Hexyloxy)phenyl]benzoxazole-5-carboxylate (5HexOOC-PBO-OHex): By Method A:
48% yield. White solid. IR (nujol): 1706, 1610. 1H-NMR (300 MHz, CDCl3): 8.39 (s, 1 H); 8.16 (d,
J=8.8, 2 H); 8.05 (dd, J=8.4, 1.2, 1 H); 7.54 (d, J=8.6, 1 H); 6.99 (d, J=8.8, 2 H); 4.32 (t, J=6.6, 2
H); 4.02 (t, J=6.6, 2 H); 1.84–1.60 (m, 4 H); 1.46–1.31 (m, 12 H); 0.89 (t, J=6.6, 6 H). 13C-NMR (75
MHz, CDCl3): 166.4; 164.5; 162.3; 153.5; 142.4; 129.5; 127.2; 126.5; 121.4; 118.0; 114.0; 110.0; 68.0;
65.0; 31.0–14.0. Anal. calc. for C26H33NO4: C 73.73, H 7.8%, N 3.31; found: C 73.26, H 7.59, N 3.31.
2-[4-(Hexyloxy)phenyl]benzoxazole-5-carbonitrile (5NC-PBO-OHex): By Method B: 87% yield.
1
White solid. IR (nujol): 2221, 1615. H-NMR (300 MHz, CDCl3): 8.16 (d, J=9.2, 2 H); 7.99 (s, 1 H);
7.60 (m, 2 H); 7.01 (d, J=9, 2 H); 4.03 (t, J=6.5, 2 H); 1.83–1.75 (m, 2 H); 1.53–1.30 (m, 6 H); 0.89
(t, J=6.9, 3 H). 13C-NMR (75 MHz, CDCl3): 162.7; 153.1; 129.9; 128.6; 124.0; 118.9; 118.1; 115.0;
111.5; 108.3; 68.4; 31.5; 29.0; 25.6; 22.5; 14.0. Anal. calc. for C20H20N2O2: C 74.98, H 6.29, N 8.74;
found: C 74.64, H 6.24, N 8.59.
6-(Hexyloxy)-2-[4’-(hexyloxy)phenyl]benzoxazole (6HexO-PBO-OHex): By Method B: 81% yield.
1
White solid. IR (nujol): 1626. H-NMR (300 MHz, CDCl3): 8.10 (d, J=8.7, 2 H); 7.56 (d, J=8.8, 1 H);
7.06 (d, J=2.7, 1 H); 6.98 (d, J=8.8, 2 H); 6.90 (dd, J=8.7, 2.2, 1 H); 4.03–3.36 (m, 4 H); 1.87–1.75
(m, 4 H); 1.58–1.23 (m, 12 H); 0.92–0.87 (m, 6 H). 13C-NMR (75 MHz, CDCl3): 174.5; 157.4; 151.5;
135.9; 128.9; 119.7; 119.4; 114.7; 112.9; 96.1; 68.8; 68.2; 31.5; 29.2; 25.7; 22.6; 14.0. Anal. calc. for
C25H33NO3: C 75.91, H 8.41, N 3.54; found: C 75.98, H 8.45, N 3.41.
4-[6-(Hexyloxy)benzoxazol-2-yl]benzonitrile (6HexO-PBO-CN): By Method B: 54% yield. White
1
solid. IR (nujol): 2228, 1613. H-NMR (300 MHz, CDCl3): 8.28 (d, J=7.5, 2 H); 7.77 (d, J=7.5, 2 H);
7.64 (d, J=8.8, 1 H); 7.09 (d, J=2.2, 1 H); 6.97 (dd, J=8.2, 2.2, 1 H); 4.00 (t, J=6.5, 2 H); 1.86–1.76
(m, 2 H); 1.55–1.26 (m, 6 H); 0.90 (t, J=6.5, 3 H). 13C-NMR (75 MHz, CDCl3): 160.0; 158.6; 151.9;
135.5; 132.6; 131.3; 127.4; 120.5; 118.3; 114.2; 114.1; 95.9; 68.9; 31.5; 29.1; 25.7; 22.6; 14.0. Anal. calc.
for C20H20N2O2: C 74.98, H 6.29, N 8.74; found: C 74.84, H 6.27, N 8.65.
Hexyl-2-[4-(Hexyloxy)phenyl]benzoxazole-6-carboxylate (6HexOOC-PBO-OHex): By Method A:
40% yield. White solid. IR (nujol): 1706, 1612. 1H-NMR (300 MHz, CDCl3): 8.20 (s, 1 H); 8.17 (d,
J=8.7, 2 H); 8.05 (dd, J=8.3, 1.5, 1 H); 7.71 (d, J=8.4, 1 H); 7.00 (d, J=8.8, 2 H); 4.33 (t, J=6.6, 2
H); 4.02 (t, J=6.6, 2 H); 1.82–1.72 (m, 4 H); 1.47–1.27 (m, 12 H); 0.91–0.86 (m, 6 H). 13C-NMR (75
MHz, CDCl3): 166.3; 165.7; 162.5; 150.3; 146.2; 129.8; 126.9; 126.2; 118.9; 118.8; 114.9; 111.9; 68.3;
65.4; 31.5; 31.4; 29.1; 28.7; 25.7; 25.6; 22.6; 14.0. Anal. calc. for C26H33NO4: C 73.73, H 7.85, N 3.31;
found: C 72.47, H 9.10, N 3.82.
2.5. 2-(4-Phenyl)benzoxazol-5-ols 7. 4-Hydroxy-3-nitrophenyl Benzoate (5) was prepared by adapt-
ing methods described in [21]: Benzoyl chloride (7 ml) was added dropwise to a soln. of 4-(benzyloxy)-
phenol (10.0 g, 0.05 mol) and Et3N (6.1 g, 0.06 mol) in dry THF at r.t. The mixture was stirred for 30 min,
and the resulting precipitate was filtered off. The soln. was washed with sat. aq. Na2CO3 soln. (2×100 ml)
and H2O (3×100 ml), dried (MgSO4), and evaporated: 4-(benzyloxy)phenyl benzoate (13.4 g, 88%).
White solid.
Pd(OH)2/C (3.9 g) was gradually added under Ar to a boiling mixture of 4-(benzyloxy)phenyl ben-
zoate (13.0 g, 0.04 mol) in abs. EtOH (170 ml) and cyclohexene (85 ml). The mixture was stirred and
heated under reflux for 2 h, allowed to cool down to r.t., and filtered through a pad of Celite®. The solvent
was evaporated: 4-hydroxyphenyl benzoate (8.5 g, 99%). White solid.
A soln. of 4-hydroxyphenyl benzoate (8.5 g, 0.04 mmol) in CH2Cl2 (94 ml) and Et2O (189 ml) was
added to a stirred soln. of NaNO3 (3.4 g, 0.04 mol) in conc. HCl soln. (35.4 ml) and H2O (47 ml) at r.t.
Ac2O (0.6 ml) was added, and the mixture was stirred for a further 4 h. The aq. phase was extracted

Helvetica Chimica Acta – Vol. 89 (2006)
317
with Et2O (2×60 ml), the combined org. phase dried (MgSO4) and evaporated, and the resulting solid
recrystallized from EtOH: 4-hydroxy-3-nitrophenyl benzoate (7.8 g, 76%). Spongy yellow solid. 1H-
NMR (300 MHz, CDCl3): 10.50 (s, 1 H); 8.17 (dd, J=8.4, 1.3, 2 H); 7.99 (d, J=2.7, 1 H); 7.65 (m,
1 H); 7.51 (dd, J=8.4, 7.5, 2 H); 7.47 (dd, J=9.0, 2.7, 1 H); 7.21 (d, J=9.0, 1 H).
2-[4-(Hexyloxy)phenyl]benzoxazol-5-yl Benzoate (6a): By Method A or Method B: 63 or 85% yield,
resp. White solid. 1H-NMR (300 MHz, CDCl3): 8.23 (d, J=7.6, 2 H); 8.16 (d, J=8.8, 2 H); 7.64 (m, 1 H);
7.49–7.56 (m, 4 H); 7.15 (dd, J=8.6, 2.1, 1 H); 7.01 (d, J=9.0, 2 H); 4.03 (t, J=6.4, 2 H); 1.87–1.74 (m,
2 H); 1.63–1.25 (m, 6 H); 0.90 (t, J=6.9, 3 H).
2-(4-Cyanophenyl)benzoxazol-5-yl Benzoate (6b): By Method A or Method B: 43 or 77% yield, resp.
White solid. 1H-NMR (300 MHz, CDCl3): 8.34 (d, J=8.7, 2 H); 8.22 (d, J=7.1, 2 H); 7.81 (d, J=8.7, 2 H);
7.65–7.61 (3 H); 7.52 (m, 2 H); 7.25 (dd, J=8.8, 2.4, 1 H).
2-[4-(Methoxycarbonyl)phenyl]benzoxazol-5-yl Benzoate (6c): By Method B: 89% yield. White
1
solid. H-NMR (300 MHz, CDCl3): 8.34–8.17 (m, 6 H); 7.65–7.61 (m, 5 H); 7.55–7.50 (m, 1 H); 3.95
(s, 3 H).
2-[4-(Hexyloxy)phenyl]benzoxazol-5-ol (7a): A soln. of KOH (202 mg) in H2O (5 ml) was added
dropwise to a soln. of 6a (1.5 g, 3.60 mmol) in THF (200 ml) and 96% EtOH (35 ml) at r.t. The mixture
was stirred for 2 h, and a large volume of H2O was added. The resulting mixture was neutralized with
AcOH. The THF was evaporated to precipitate the product, which was filtered off and dried at 458
1
for 48 h under vacuum: 7a (965 mg, 86%). H-NMR (300 MHz, CDCl3): 8.12 (d, J=8.6, 2 H); 7.35 (d,
J=8.7, 1 H); 7.16 (d, J=2.4, 1 H); 6.97 (d, J=9.1, 2 H); 6.82 (dd, J=8.7, 2.5, 1 H); 6.08–6.04 (m,
1 H); 4.01 (t, J=6.7, 2 H); 1.78 (m, 2 H); 1.55–1.23 (m, 6 H); 0.89 (t, J=6.8, 3 H).
1
4-(5-Hydroxybenzoxazol-2-yl)benzonitrile (7b). As described for 7a, from 6b: 7b (93%). H-NMR
(300 MHz, CDCl3): 8.31 (d, J=8.4, 2 H); 7.79 (d, J=8.0, 2 H); 7.45 (d, J=8.8, 1 H); 7.2 (d, J=2.6,
1 H); 6.92 (dd, J=9.3, 2.5, 1 H); 4.92 (s, 1 H).
4-(5-Hydroxybenzoxazol-2-yl)benzoic Acid (7c). A soln. of KOH (11.88 g) in H2O (40 ml) was added
to a stirred suspension of 6c (4.20 g, 11.28 mmol) in boiling EtOH (125 ml). After 5 min, H2O (450 ml)
was added and the soln. heated under reflux for 30 min and then allowed to cool down to r.t. The mixture
was neutralized with dil. HCl soln., and the solid was filtered off, washed with H2O and recrystallized
1
from EtOH: 7c (2.5 g, 87%). White solid. H-NMR (300 MHz, (D6)DMSO): 10.25–9.25 (s, 1 H); 8.24
(d, J=8.4, 2 H); 8.11 (d, J=8.4, 2 H); 7.57 (d, J=9.0, 1 H); 7.13 (d, J=2.4, 1 H); 6.89 (dd, J=9.0, 2.4,
1 H).
2.6. 5-(Hexyloxy)-2-(4-Y-phenyl)benzoxazoles (5HexO-PBO-Y): General Procedure. Hexyl bromide
(0.42 ml, 3.01 mmol) was added dropwise under Ar to a mixture of the corresponding 7 (2.73 mmol) and
K2CO3 (5.46 mmol) in dry dimethylformamide (75 ml). The mixture was heated under reflux for 1 h and
then allowed to cool down to r.t. H2O (120 ml) was added, the mixture extracted with hexane/AcOEt 1:1
(4×100 ml), and the combined org. extract washed with H2O (3×100 ml), dried (MgSO4), filtered
through a pad of silica gel, and evaporated. The crude product was recrystallized from abs. EtOH:
5HexO-PBO-Y, which in some cases was further purified by FC (silica gel, CH2Cl2).
5-(Hexyloxy)-2-[4-(hexyloxy)phenyl]benzoxazole (5HexO-PBO-OHex): 72% yield. White solid. IR
(KBr): 1606. 1H-NMR (300 MHz, CDCl3): 8.12 (d, J=8.8, 2 H); 7.38 (d, J=8.8, 1 H); 7.19 (d, J=2.6, 1
H); 6.98 (d, J=8.8, 2 H); 6.87 (dd, J=8.8, 2.6, 1 H); 4.00 (m, 4 H); 1.80–1.70 (m, 4 H); 1.50–1.21 (m,
12 H); 1.01–0.9 (m, 6 H). 13C-NMR (75 MHz, CDCl3): 163.0; 161.0; 156.0; 145.0; 143.0; 129.0; 119.0;
114.0; 113.0; 110.0; 103.0; 68.8; 68.2; 31.6; 31.5; 29.2; 29.1; 25.74; 25.66; 22.6; 22.5; 14.03; 14.01. Anal.
calc. for C25H33NO3: C 75.91, H 8.41, N 3.54; found: C 76.23, H 8.49, N 3.55.
4-[5-(Hexyloxy)benzoxazol-2-yl]benzonitrile (5HexO-PBO-CN): 170 mg (35%). White solid. IR-
1
(nujol): 2223, 1612. H-NMR (300 MHz, CDCl3): 8.29 (d, J=8.5, 2 H); 7.77 (d, J=8.7, 2 H); 7.44 (d,
J=8.9, 1 H); 7.22 (d, J=2.4, 1 H); 6.97 (dd, J=9.0, 2.5, 1 H); 3.98 (t, J=6.5, 2 H); 1.84–1.71 (m, 2 H);
1.49–1.2 (m, 6 H); 0.89 (t, J=7, 3 H). 13C-NMR (75 MHz, CDCl3): 161.5; 157.3; 142.7; 132.6; 131.2;
127.8; 118.2; 115.6; 114.5; 110.9; 103.7; 68.9; 31.6; 29.2; 25.7; 22.6; 14.0. Anal. calc. for C20H20N2O2:
C 74.98, H 6.29, N 8.74; found: C 74.76, H 6.38, N 8.59.
Hexyl 4-[5-(Hexyloxy)-2-yl]benzoatebenzoxazole (5HexO-PBO-COOHex): 76% yield. White solid.
IR (nujol): 1710, 1637. 1H-NMR (300 MHz, CDCl3): 8.27 (d, J=8.2, 2 H); 8.15 (d, J=8.2, 2 H); 7.44 (d,
J=8.8, 1 H); 7.23 (d, J=2.5, 1 H); 6.95 (dd, J=9.0, 2.4, 1 H); 4.33 (t, J=6.6, 2 H); 3.99 (t, J=6.7, 2 H);

318
Helvetica Chimica Acta – Vol. 89 (2006)
1.85–1.71 (m, 4 H); 1.5–1.3 (m, 10 H); 0.89 (t, J=7.0, 6 H). 13C-NMR (75 MHz, CDCl3): 165.0; 162.0;
157.0; 145.0; 142.0; 132.0; 131.0; 130.0; 127.0; 115.0; 110.0; 103.0; 68.0; 65.0; 31.0; 29.0; 28.0; 25.0;
22.0; 14.0; 13.0. Anal. calc. for C26H33NO4: C 73.73, H 7.85, N 3.31; found: C 73.28, H 8.03, N 3.73.
3. Photophysical and Photochemical Investigations. The photochemical stability of the benzoxazoles
was investigated with CHCl3 solns. and with dispersions in solid films of PMMA. Samples were irradiated
with a HeCd-laser (Kimmon Electronics, Ltd.), providing a power density of 12 mW cmꢀ2 for the
expanded beam at 325 nm, at different time intervals up to 90 min. Photodegradation of PBOs was fol-
lowed by UV/VIS spectroscopy and resulted in a decrease in absorbance (A). The absorbance at 325 nm
was measured before and after every illumination period, and the absorbed energy (E) was calculated
from the power density of the incident light P, the irradiation time t, and the mean absorbance Amean
(Eqn. 1).
À
Á
E ¼ t ꢁ P ꢁ 1 ꢀ 10ꢀA
(1)
mean
For comparative purposes, all the nonirradiated samples were prepared to have absorbance 1 at 325
nm to ensure that the same amount of energy was absorbed at the beginning of the irradiation. The pho-
tostability of the benzoxazoles was evaluated from the energy required to decrease the absorption at 325
nm to 90% of the initial value; this value is referred as E90 and is given in J cmꢀ2. The E90 parameter was
calculated by plotting the normalized absorbance (A/A0) vs. the absorbed energy (E).
Solns. of the benzoxazoles in CHCl3 were placed in a quartz cuvette and exposed to the laser beam.
The soln. was stirred with a magnetic bar to guarantee a well-defined dose of optical energy on the whole
soln.
Films of benzoxazoles in PMMAwere prepared containing 0.1 mol of the benzoxazole/kg of PMMA.
Films were deposited onto quartz plates by spin-coating at 2500 r.p.m. for 30 s with a spin-coater (CT 60
from the Karl Suess Company) from a soln. containing 10 mmol of the corresponding dye and 1 ml of
PMMA in CHCl3. A PMMA film was used as a reference in the photostability experiments.
4. Preparation of Oriented Networks and Determination of the Polarized Emission. Photopolymeriz-
able samples were prepared by dissolving the reactive liquid crystal 4-{{6-[(1-oxoprop-2-enyl)oxy]hexyl}-
oxy}benzoic acid 2-methyl-1,4-phenylene ester (C6M supplied by Philips), the corresponding 2-phenyl-
benzoxazole (95 :5 w/w), the UV photoinitiator Irgacure 651 (from Ciba Spec. Chem.; 1% weight), and
the thermal inhibitor 2,6-di-tert-butylphenol (200 ppm) in CH2Cl2. The solvent was evaporated at r.t., and
the residual solvent was removed under vacuum. Inspection of the blends by optical microscopy indicated
good miscibility of the components. Photopolymerization was carried out in cells for planar alignment (5-
mm thickness; from Linkam), and these were filled by capillary action. The oriented films were produced
at the nematic phase temp. (between 90 and 1008) under illumination with 365-nm light (PL-S 9W/10-
Philips lamp) for 15 min. The material C6M has a nematic phase between 86 and 1168, and shows a max-
imum absorbance at 268 nm without emission.
Polarized luminescence measurements were recorded by means of a Perkin-Elmer LS50B spectro-
fluorimeter with a sample holder designed to measure liquid-crystal cells [7]. The measurements were
taken with the excitation beam polarized in parallel to the rubbing direction of the cell and the detection
polarizer either parallel or perpendicular. Therefore, the emission intensity at the maximum of the emis-
sion band in the rubbing direction (I||) and perpendicular (I^) to it were determined and corrected for the
dichroic response of the equipment. The emission dichroic ratio (Rem) was calculated as the quotient of
these two values.
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Received October 18, 2005