'Latent' ultraviolet light absorbers

Article abstract of DOI:10.1039/b209394d

We have synthesized and characterized photo-labile and acid-labile esters of 2-hydroxybenzophenone and 2-hydroxybenzotriazoles, which, upon appropriate treatment, release classical UV absorbers. Two concepts of 'latent' UV absorbers are presented, namely,

Full text of DOI:10.1039/b209394d

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‘
Latent’ ultraviolet light absorbers
a
Christoph Kocher,* Christoph Weder and Paul Smith
b
a
a
Department of Materials, Eidgen o¨ ssische Technische Hochschule (ETH) Z u¨ rich,
Universit a¨ tsstrasse 41, UNO C14, CH-8092 Z u¨ rich, Switzerland.
E-mail: ckocher@ifp.mat.ethz.ch
b
Department of Macromolecular Science and Engineering, Case Western Reserve University,
2
100 Adelbert Road, Cleveland, OH 44106-7202, USA
Received 1st October 2002, Accepted 29th October 2002
First published as an Advance Article on the web 15th November 2002
We have synthesized and characterized photo-labile and acid-labile esters of 2-hydroxybenzophenone and
2
-hydroxybenzotriazoles, which, upon appropriate treatment, release classical UV absorbers. Two concepts of
latent’ UV absorbers are presented, namely, benzoyl-caged species which release their parent UV absorber
‘
upon short-wavelength UV irradiation, and acid-labile species caged with the tert-butoxycarbonyl group which
were employed in combination with a photoacid generator. The latent UV absorbers were melt processed into
thermoplastic polymers such as linear low density polyethylene and absorbing–non-absorbing patterns were
produced in the polymer blend films by spatially resolved conversion, illustrating their usefulness for optical
data storage applications.
Introduction
Results and discussion
Photochromic or, more generally, chromogenic molecules, in
which a photochemically induced change in the structure
causes a significant alteration of the absorption characteristics,
have proven useful in many applications, including, for
UV absorbers and derivatives
Amongst the vast array of available UV absorbers, we chose to
derivatize 2-hydroxybenzophenone (5), 2-(2-hydroxy-5-methyl-
phenyl)benzotriazole (6), and 2,4-di-tert-butyl-6-(5-chloro-2H-
benzotriazol-2-yl)phenol (7) (Fig. 1). The first two compounds
are structurally the most simple, commercially available repre-
sentatives of their absorber classes (benzophenones and
benzotriazoles, respectively), while the latter compound was
selected to demonstrate the limitations of the photo-lability of
the benzoyl caging group on the one hand, and the alternative
approach of t-Boc protection on the other. This chlorine-
substituted UV-absorber is characterized by a red-shifted
absorption spectrum with respect to non-derivatized benzo-
triazoles and is therefore particularly suited for applications
where UV light from the widely employed fluorescent tubes
1
example, in optical filters, as tracer molecules for rheology
2
3
studies, and in optical data storage media. Typically, the
wavelength range in which such chromogenic dyes function is
confined to the visible spectrum. In the design of new systems,
the UV range has long been neglected, despite the fact that in
applications such as optical data storage, performance can be
improved by using UV-sensitive species. Moreover, photo-
chromic dyes capable of developing a UV absorption band
would allow for additional applications such as patterned UV
filters, invisible markings, and selective UV screening in fluo-
rescent tracer experiments. Here, we present systems which,
upon appropriate treatment, release classical UV absorbers
and, thus, represent a potentially useful extension of the avai-
lable range of chromogenic dyes.
(
also known as ‘blacklight’) is selectively screened out.
Benzoyl chloride was used to produce the photo-labile
compounds 1–3, and di-tert-butyl dicarbonate to yield the acid-
labile species 4 (Fig. 1). As will be shown in detail below,
photocleavage of benzoic acid esters of the latent UV absorbers
has certain limitations. In systems in which the benzoyl group
represents the only substituent allowing for vibrational energy
dissipation, such as in the benzoyl derivatives (1 and 2) of
Classical UV absorbers are benzophenones and benzotri-
4
,5
azoles with ortho-hydroxy substituents. These molecules very
efficiently absorb incident UV radiation, typically in the range
3
00–400 nm. They dissipate the absorbed energy by a non-
radiative internal conversion process which involves the
reversible formation of a six-membered hydrogen-bonded
6
ring —a process which is commonly known as excited-state
2-hydroxybenzophenone (5) and 2-(2-hydroxy-5-methylphenyl)-
7
,8
benzotriazole (6), fast and exclusive restoration of the parent
UV absorber is observed upon irradiation with short wave-
length UV light. In contrast, photolytic ester cleavage is
strongly reduced in latent UV absorbers based on parents
comprising additional substituents that allow for vibrational
energy dissipation, such as 7. As a matter of fact, commercial
UV absorbers of the benzotriazole class often contain tert-
butyl, 1,1,2,2-tetramethylpropyl, and 1-methyl-1-phenylethyl
groups to increase the photostability. These groups not only
intramolecular proton transfer (ESIPT).
In the strategy
presented herein, we derivatized a number of classical UV
absorbers through esterification with benzoic acid and
produced esters lacking the ability to dissipate incident UV
radiation by internal conversion. Moreover, these caged UV
absorbers are characterized by absorption spectra that are
significantly shifted to shorter wavelengths when compared
to those of their parent hydroxy compounds; long wavelength
UV light is therefore transmitted.
9
13
The benzoyl group can be removed by exposure to short
wavelength UV light, restoring the parent UV absorber. Addi-
tionally, we employed the acid-labile tert-butoxycarbonyl
lock the molecule in its planar configuration, but also allow
1
4
for vibrational energy dissipation. In hydroxyphenylbenzo-
triazenes, another important class of UV absorbers, the three
phenyl groups have a similar effect. Consequently, photo-labile
latent UV absorbers bearing such additional substituents
are expected, and in fact were found, to have a reduced
1
t-Boc) group in combination with a so-called photoacid
0
(
generator (PAG).
1
1,12
Both concepts allow for photochemically
induced, spacially resolved formation of the parent UV absorber.
DOI: 10.1039/b209394d
J. Mater. Chem., 2003, 13, 9–15
9
This journal is # The Royal Society of Chemistry 2003

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Fig. 1 Structures of the photo-labile latent UV absorbers 1–3 and of the acid-labile latent UV absorber 4. Upon appropriate treatment (see text), the
corresponding UV absorbers (5–7) are released.
photosensitivity when compared to systems where the only
substitiuent is the photocleavable benzoyl group. In order to
also make use of such substituted UV absorbers in the context
of this work, the hydroxyl functionality was protected with an
acid-labile t-Boc group. Deprotection occurs in this case in the
presence of acid at elevated temperatures. Since the acid can be
photochemically released by employing a PAG, t-Boc-pro-
tected latent UV absorbers embedded in a polymer matrix offer
the substance virtually unchanged, while heating to tempera-
tures ranging from 170–250 uC leads to complete conversion to
7. This finding is in accordance with the commonly observed
1
6
thermal stability of t-Boc-protected compounds.
Incorporation of latent UV absorbers in LLDPE films
The conversion of the latent UV absorbers to their parent UV
absorbers was studied in linear low density polyethylene
(LLDPE) blend films containing 0.2% w/w of the compound.
In the case of the acid-labile latent UV absorber 4, the blend
film additionally contained 3% w/w of benzenesulfonic acid
1
5,16
similar opportunities for photolithographic patterning
as
9
their benzoyl-protected counterparts. Finally, the PAG can be
sublimed off after selective activation, which ultimately renders
patterned features insusceptible to unwanted conversion, e.g.
in daylight.
2
1
phenyl ester as the PAG. The films were produced in a
standard hot press and had a thickness of approximately
1
00 mm. The film thickness and chromophore contents
Synthesis of latent UV absorbers
employed allowed for experimentally convenient optical
extinctions and reaction kinetics.
The latent UV absorbers 1–4 were synthesized by reacting the
respective parent UV absorbers (ortho-hydroxy compounds)
5
–7 with either benzoyl chloride in the case of the photo-labile
Photophysical characteristics
systems 1–3, or with di-tert-butyl dicarbonate in the case of
the acid-labile version 4, in anhydrous pyridine as solvent.
The products were obtained in 77–96% yields after purifi-
cation. All latent absorbers prepared were of high purity, as
confirmed by H NMR spectroscopy and elemental analysis.
It should be noted that some of the compounds here described
have been prepared before, albeit for different applications.
UV-Vis spectrometry of the compounds in chloroform solu-
tions revealed that the spectra of all the latent UV absorbers
are characterized by a primary absorption maximum that is
shifted to considerably shorter wavelengths when compared
with the absorption of the parent hydroxy analog (Fig. 2).
This bathochromic shift may be as large as 42 nm (in the case
of 3). While the spectrum of hydroxybenzophenone (5) exhi-
bits absorption maxima at 340 and 265 nm, that of its
benzoyl ester (1) only exhibits a single absorption band cen-
tered at 244 nm. In the case of compound 6, two absorption
maxima are also observed, at 341 and 301 nm. The spectrum
of compound 2, on the other hand, exhibits a single absorp-
tion band centered at 303 nm. The absorber/latent absorber
pair 7/3 exhibit similar spectral properties to the pair 6/2, with
a slight bathochromic shift as a consequence of the chlorine
substituent on the benzotriazole moiety. The two absorption
maxima of 7 are located at 347 and 313 nm, while the spec-
trum of 3 exhibits a single absorption band centered at 305 nm.
The sole absorption band of the acid-labile t-Boc-protected
analog 4 is located at 309 nm. These key optical parameters
and the corresponding extinction coefficients are collected in
Table 1. When dispersed in LLDPE, the latent UV absorbers
exhibit virtually the same absorption characteristics as in
solution (see Table 1).
1
1
7
Compound 1 is an intermediate for, e.g., fungicides. Com-
pound 2 has been disclosed in a patent on UV absorbers for
1
the short wavelength UV regime and its thermal rearrange-
8
1
9
ment was studied. The preparation of compounds 2 and 3
2
0
has been described in a different patent.
Thermal stability
Regarding the envisioned applications in polymer blend films,
which are preferentially produced by common melt-processing
schemes, investigations concerning the thermal stability of the
latent UV absorbers were conducted using differential scanning
calorimetry (DSC). All benzoyl-caged latent UV absorbers
were found to be thermally stable up to a temperature of
2
50 uC, at least on the timescale of the DSC experiment. The
t-Boc-caged latent UV absorber 4, however, was found to be
thermolytically cleaved at temperatures of ca. 170 uC and
higher. The DSC data suggest that heating 4 to 160 uC leaves
1
0
J. Mater. Chem., 2003, 13, 9–15

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2
5
Fig. 2 Absorption spectra of the UV absorbers 5–7 and of the latent UV absorbers 1–3 (all solutions 0.5 6 10 M in CHCl
3
). See Table 1 for key
optical parameters.
Table 1 Absorption maxima (lmax) and extinction coefficients (e) of the parent UV absorbers and the corresponding latent UV absorbers in 0.5 6
2
0
5
1
M chloroform solutions. The absorption maxima of the compounds incorporated into LLDPE films are also indicated
2
)/L mol cm
1
21
Core
Compound
Caging group
3
lmax (CHCl )/nm
e (CHCl
3
lmax (LLDPE)/nm
Benzophenone
Benzotriazole
Substituted benzotriazole
5
1
6
2
7
3
4
—
Benzoyl
—
Benzoyl
—
340, 265
244
341, 301
303
347, 313
305
309
5100, 14 400
20 900
17 100, 14 600
19 300
16 800, 15 200
17 500
19 400
340
243
346
306
350
314
310
Benzoyl
t-Boc
All benzoyl-caged latent UV absorbers were found to be
virtually non-photoluminescent in solution, as well as when
incorporated in LLDPE films. At first, this finding seems
somewhat surprising considering the fact that these com-
pounds cannot undergo radiationless energy dissipation by
the ESIPT pathway, and especially in the view of the fact that
absorber 6 (cf. Fig. 3). The increase of the developing
absorption band appears to follow first order kinetics, as
can be concluded from Fig. 4. Importantly, apart from the
appearance of the respective parent UV absorbers’ absorption
bands, no change in the absorption characteristics (e.g.
yellowing) was observed.
1
3
22
both 6 in a non-polar configuration and the methyl ester
It should be noted that the concentration of latent UV absor-
ber (0.2% w/w), matrix material (commercial grade LLDPE
with some self-absorption at the activation wavelength) film
of 6, which both lack an intramolecular hydrogen bond, were
found to be photoluminescent in earlier studies. The obser-
vation that the benzoyl esters of the present UV absorbers are,
in contrast, non-photoluminescent may be attributed to a
2
2
thickness (100 mm), and optical power (340 mW cm ) were
chosen in order to conduct the photoactivation experiments
on an experimentally convenient timescale. With the optical
intensities usually employed for photolithograpy (e.g. laser
1
4
relaxation pathway that is referred to as the ‘loose bolt’ effect:
the benzoyl moiety provides multiple vibrational and rota-
tional degrees of freedom, which allow for a radiationless
15
systems), much shorter processing times can be expected.
As was reported by others, the acetate ester of 6 regenerates
1
4
transition from the excited to the ground state. We recently
reported on the same phenomenon observed for aromatic
esters of 2-(2-hydroxyphenyl)benzoxazole, which, consequently,
6 upon irradiation, and not, as might be expected, a photo-
22
Fries rearrangement product. However, in accordance with
9
form a novel class of caged photoluminescent dyes. Very
our previous findings concerning the cleavage of aliphatic
much in accordance with the results presented in that study,
the t-Boc-caged latent absorber 4 exhibits a fairly strong,
blue photoluminescence, both in chloroform solution and
when in a LLDPE matrix.
9
esters, this compound only regenerates traces of its parent
alcohol, even under the extremely harsh irradiation condition
Activation of photo-labile latent UV absorbers
Upon exposure to 254 nm UV light from a laboratory UV
lamp, chloroform solutions of the benzoyl-caged latent UV
absorbers were found to reproduce the absorption charac-
teristics of their parent hydroxy analogs. Importantly, resto-
ration of the absorption characteristics of the parent was also
observed when the latent absorbers were conatined in a
polymer matrix. Thus, we observed that the spectra of LLDPE
films containing 0.2% w/w of the 2-hydroxybenzophenone-
based, benzoic acid-caged latent UV absorber 1 developed an
absorption band centered around 340 nm upon UV irradiation.
This is indicative of the regeneration of 2-hydroxybenzophe-
none, which exhibits the same absorption characteristics. In
spectra of LLDPE films containing 0.2% w/w of the benzo-
triazole-based latent UV absorber 2, an absorption band
centered at 346 nm was observed on UV irradiation, in accor-
dance with the absorption band of its parent active UV
Fig. 3 Activation of 2 in a LLDPE film (0.2% w/w, thickness 100 mm)
upon irradiation at 254 nm. Absorption spectra prior to irradiation
(bold line) and after different photoactivation intervals.
J. Mater. Chem., 2003, 13, 9–15
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absorbers with reduced photosensitivity. In order to also make
the latter UV absorbers accessible from a latent precursor, we
employed the concept of protection of the hydroxy group with
1
0
the acid-labile t-Boc protective group (see below).
GC-MS studies
The change in the absorption characteristics of the photo-
labile, benzoyl-caged latent UV absorbers upon UV exposure
clearly suggests the release of the parent UV absorbers. In
order to obtain further evidence for the formation of the UV
absorbers and to identify possible additional by-products,
GC-MS studies were conducted. For that purpose, LLDPE
blend films containing the latent UV absorbers were exposed
to 254 nm UV light for 3 h and the reaction products were
extracted with chloroform (films containing 1 and 3) or toluene
(
film containing 2). In all solutions containing extracts from
Fig. 4 Development of the indicated UV absorption bands upon
conversion of the three benzoyl-caged latent UV absorbers 1 (',
the exposed latent UV absorber blend films, the corresponding
parent UV absorber could indeed be detected by GC-MS.
Moreover, in the case of 1, benzoic acid was identified as a
byproduct, which is in accordance with previously published
340 nm), 2 (&, 346 nm) and 3 ($, 350 nm) in 0.2% w/w LLDPE blend
films upon irradiation at 254 nm. Dotted lines are first order kinetic fits
calculated from the experimental data. For comparison, the absorption
data were scaled such that the absorbance of the peak in question in the
spectrum of the pristine film was 1 in all cases.
9
results. We thus conclude that the regeneration of the UV
1
4
absorbers occurs by homolytic a-cleavage of the respective
benzoyl esters, followed by hydrogen abstraction from the
environment.
2
2
employed in the study. The aromatic ester 2, in contrast, quite
efficiently releases 6. However, we found that 3 regenerated
the absorption characteristics of the parent active UV absor-
ber 7 only to some extent when exposed to short wavelength
UV light. As can be seen from Fig. 4, the conversion of 2
to 6 proceeds faster and more completely than the conversion
of 3 to 7. Note, that the absorption data plotted in Fig. 4
have been scaled such that the magnitudes of the primary
absorptions of the pristine samples are 1 in all cases; since
the extinction coefficients of the bands due to 6 and 7 are
very similar, it is obvious from Fig. 4 that the conversion of
Activation of acid-labile latent UV absorbers
The activation behavior of 3 in LLDPE blend films conatining
0
.2% w/w of the latent UV absorber and 3% w/w of benzene-
sulfonic acid phenyl ester as the PAG was studied. While the
sensitivity of this particular PAG (which is used in micro-
2
1
11
lithography) is usually too limited for most applications, it
allowes the acid-catalyzed conversion experiments described
here to be conducted on a suitable timescale, especially in the
1
5,16
rather low concentration
employed. Conveniently, the
3
to 7 is much less efficient than the conversion of 2 to 6.
absorption band of the PAG is located at a lower wavelength
than the absorption band of 4, so that the presence of 4 does
not considerably affect the photocleavage of the PAG.
The conversion of 1 to 5 (Fig. 4) is also faster and more
complete than the conversion of 3 to 7, taking into account
the fact that the extinction of the absorbance due to 5 is only
about one third of that for 6 and 7. We attribute the fact
that the benzoic acid-caged latent UV absorbers 1 and 2 in
LLDPE films are readily converted to their active parent
hydroxyl analogs, hydroxybenzophenone and 6, respectively,
while conversion of the benzoic acid-caged latent UV absor-
ber 3 to 7 is less easy, to critical differences in the molecular
structures of these compounds. In 1 and 2, the only substi-
tuent having rotational and vibratory degrees of freedom is
the benzoyl moiety, while in 3, the two tert-butyl groups offer
additional degrees of freedom. When excited with UV light,
The acid-labile latent UV absorber 4 is converted to the
UV absorber 7 in the polyethylene film via a two-step process:
first, the strong sulfonic acid is released by irradiation of the
blend film; second, the acid catalytically cleaves the t-Boc
protective group of 4 in a post-exposure heat treatment
23
sometimes referred to as ‘post-exposure bake’, quantita-
tively restoring the UV absorber 7. LLDPE film samples
containing 0.2% w/w of 4 and 3% w/w of the PAG were
irradiated at 25 uC with 254 nm UV light for a pre-determined
time interval. After the irradiation step, the samples were
heated to 100 uC for 1 h. UV-Vis absorption spectra were
collected prior to and after this treatment. The progress of
the conversion as a function of the exposure time is repre-
sented in Fig. 5 in terms of the absorbance of the developing
absorption maximum at 350 nm. As can be seen from the
data in this figure, upon activation of the PAG at 25 uC, some
conversion of 3 to 7 can already be observed. In contrast, blend
films not containing the PAG did not exhibit similar absorp-
tion characteristics (spectra not shown). The same applies to
reference blend films containing 3% w/w of the PAG only, and
no 3. Direct photolysis of 4 or absorption of photo-generated
acid can therefore be excluded as causes of the observed
appearing absorption band. Upon heat treatment, the charac-
teristic absorption of 7 was restored. With the relatively thick
samples (100 mm) and the relatively low PAG concentration
(3% w/w) employed in this series, an irradiation time of 30 min
was required to obtain essentially complete conversion to 7
after treatment at 100 uC for 1 h. It is expected that this
processing time can dramatically be reduced by increasing the
PAG content, employing a more potent or more sensitive PAG,
and by using more powerful light sources.
1
ground state by vibrational relaxation due to the benzoyl
and 2 can dissipate the excess energy and return to the
1
4
moiety, the ‘loose bolt’ effect referred to above. Since the
benzoyl ester bond is the weakest bond in these molecules,
ester cleavage is likely to occur as a consequence of the
vibrational relaxation, yielding a biradical which quickly
converts to the hydroxyl-substituted UV absorber and ben-
zoic acid by hydrogen abstraction. Evidence supporting this
9
hypothesis can be found elsewhere. In the case of 3, the
benzoyl moiety as well as the two tert-butyl substituents allow
for vibrational energy dissipation, which consequently leads
to reduced sensitivity for 3 because energy dissipation does
not exclusively occur through vibration of the benzoyl moiety
and consequent ester cleavage.
This finding evidently has implications for the design of
other latent UV absorbers. Most commercial UV absorbers
comprise substituents that allow for vibrational energy dissi-
pation and, thus, enhance the photostability of the molecule.
As shown for 3, and for the reasons given above, derivati-
zation of such UV absorbers with benzoic acid yields latent UV
1
2
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Fig. 5 Conversion of a LLDPE film containing 0.2% w/w 4 and 3% w/w
PAG upon irradiation at 254 nm and subsequent post-exposure heat
treatment. The data points represent the scaled absorption at 350 nm
after irradiation (%) and after subsequent post-exposure heat
treatment ($). Dotted lines are first order kinetic fits calculated
from the experimental data.
Although the photo-labile systems presented here may at
first glance seem more convenient, the acid-labile systems
may have a number of advantages. As already mentioned
above, it appears that with the latter approach, many UV
absorbers that rely on internal conversion involving ESIPT
may be derivatized to yield a latent UV absorber. Since
only the photo-acid generator is UV sensitive, not the latent
absorber itself, these systems can be expected to be less sub-
ject to unwanted conversion, especially in applications where
long wavelength UV light is selectively blocked. Although
mainly sensitive to short wavelength UV light, the photo-labile
systems could suffer some unwanted conversion in these
applications. In acid-labile systems, only the PAG is light
sensitive and, in addition, a subsequent heat-treatment step is
required to cleave the latent UV absorber. Thus, even if some
PAG should involuntarily be generated during operation, no
significant quantity of additional UV absorber is released. As
mentioned above, the PAG may be removed after the desired
patterning is obtained, thus rendering the device ultimately
insensitive to further unwanted conversion.
Fig. 6 Demonstration object produced from two patterned LLDPE
films containing 0.2% w/w 4 and 3% w/w PAG. The patterned films are
laminated on either side of a photoluminescent LLDPE film (A). Under
normal illumination, the object appears essentially colorless and
transparent. Under UV irradiation, the latent image imprinted in the
layer facing the UV light source appears on the photoluminescent layer
(
B). Photographs were taken under UV irradiation at 365 nm.
transparent object that revealed either one or other of the latent
patterns as a photoluminescent image under UV illumination,
depending on the side of the film that was exposed. A series of
photographs of this object is presented in Fig. 6.
Clearly, with this approach, many different PL systems can
be patterned, including oriented systems displaying polarized
PL, which cannot be obtained in a patterned fashion by com-
25,26
mon printing techniques.
possibility of readily creating high information-content struc-
Thus, this technology offers the
27
tures which may readily be personalized by very simple means.
Conclusions
Practical application
We have synthesized and characterized a number of compounds
capable of releasing classical UV absorbers. Two classes of
such latent UV absorbers were produced, namely, photo-labile
and acid-labile systems. LLDPE blend films containing photo-
labile latent UV absorbers or the acid-labile variety in com-
bination with a photoacid generator allowed for spacially
resolved conversion and, thus, for the generation of absorbing–
non-absorbing patterns. The present systems convert upon UV
irradiaton at short wavelengths (250–350 nm), which may
make these compounds particularly interesting for optical data
storage systems with high storage densities. Moreover, the
liberated UV absorber exhibits a dramatically shifted absorp-
tion with respect to the latent UV absorber, which allows for a
highly wavelength-selective activation and easy information
read-out by simple and cost-effective means. We believe that
the present systems represent a potentially valuable extension
to the available chromogenic dyes working in the visible range
of the electromagnetic spectrum.
The latent UV absorbers presented above have the pro-
perty of being able to transmit UV light with wavelengths
longer than 350 nm, while the UV absorber they release
very effi- ciently absorbs UV light in the range 275–400 nm
(
Fig. 2). Since UV radiation of that latter wavelength regime is
commonly employed for excitation of photoluminescent
PL) materials, the latent UV absorbers can be used to selec-
(
tively screen out the excitation radiation and, therefore, block
photoluminescence of the underlying PL material. Accord-
ingly, PL patterns and images can be produced. Such images
are of interest in a number of practical applications, including
personalized, ready-made items. Similar PL images currently
2
4
are produced by lithography of a pigmented photoresist,
deprotection of caged PL dyes through photogenerated
15,16
acid catalysts,
9
selective photoactivation, or selective photo-
2
5
bleaching. The latent UV absorbers presented here provide
an alternative to these methodologies. In order to illustrate
this, a demonstration device was produced by selectively acti-
vating two pieces of LLDPE film containing 0.2% w/w of 4
and 3% w/w PAG by exposure to 254 nm UV light through
different photomasks and subsequent post-exposure heat treat-
ment. Lamination of the two films imprinted with different
latent images onto each side of a PL polyethylene sheet [cf.
exploded view in Fig. 6(A)] yielded an essentially colorless,
Experimental
Materials
All UV absorbers were purchased from Aldrich and used as
received. All solvents were of analytical grade. For spectroscopic
J. Mater. Chem., 2003, 13, 9–15
13

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experiments, chloroform (stabilized with 0.8% v/v ethanol) was
used. Pyridine was dried by refluxing over KOH and subse-
quently distillied onto molecular sieves. For the preparation of
polymer blend films, a commercial-grade LLDPE obtained
from Dow Chemicals was used (Dowlex BG 2340).
Benzenesulfonic acid phenyl ester. 990.4 mg (10.5 mmol) of
phenol were dissolved in 5 mL anhydrous pyridine. To the
stirred mixture, 1.6 mL (2.2 g, 12.5 mmol) of benzenesulfonyl
chloride were added. The mixture was stirred for 2 h at
room temperature, washed with water (3 6 100 mL) and
2 2
extracted with CH Cl (150 mL). The organic layer was dried
over MgSO , filtered, and the solvents were evaporated. The
4
Synthesis
residual colorless oil was subjected to column chromatography
2-Benzoylbenzophenone (1). 882.5 mg (4.452 mmol) of
-hydroxybenzophenone was dissolved in 5 mL anhydrous
(
a colorless liquid. IR (KBr, cm ): 3070w, 1589m, 1488s,
CH Cl , silica gel) to yield 2.15 g (87%) of the product as
2 2
2
21
pyridine. To the stirred mixture, 0.6 mL (0.7 g, 5.2 mmol) of
benzoyl chloride were added. The mixture was heated to reflux,
stirred for 2 h, and subsequently allowed to cool to room
temperature. The mixture was washed with water (3 6 100 mL)
1
1
6
2
8
449s, 1375s, 1312w, 1294w, 1200s, 1176s, 1147s, 1093s, 1071s,
024m, 1000m, 912m, 862s, 783s, 758m, 742s, 705m, 687s,
1
29w, 583s, 554s. H NMR (CDCl
3
, 300 MHz): d 7.83 (m,
H, ar), 7.66 (tr, JH,H ~ 7.6, 2.0 Hz, 1H, ar), 7.52 (tr, JH,H
.0 Hz, 2H, ar), 7.26 (m, 3H, ar), 6.99 (m, 2H, ar). Anal. calcd
S (234.28): C, 61.52; H, 4.30; O, 20.49; S, 13.69;
~
and extracted with CH
dried over MgSO , filtered, and the solvents were evaporated.
The resulting colorless oil was subjected to column chromato-
graphy (CH Cl , silica gel) to yield 1.35 g (96%) of 1 as a
colorless, highly viscous oil. Mp 17 uC. IR (KBr, cm ): 3062w,
2 2
Cl (150 mL). The organic layer was
4
10 3
for C12H O
found: C, 61.39; H, 4.48; O, 20.23; S, 13.78%.
2
2
2
1
Methods
1
9
740s, 1604s, 1601s, 1450s, 1263s, 1201s, 1104s, 1059s, 1023s,
1
28s, 848m, 800m, 761s, 744s, 699s, 634s, 514m. H NMR
Spectroscopy. Absorption spectra were recorded with a
2
5
(
(
7
(
CDCl
m, 1H, ar), 7.77 (m, 1H, ar), 7.61 (tr, 2H, ar), 7.52 (tr, JH,H
.5, 1.3 Hz, 1H, ar), 7.46 (tr, 1H, ar), 7.41 (d, 1H, ar) 7.39–7.36
m, 4H, ar), 7.32 (t, 1H, ar). Anal. calcd for C20 (302.33):
3
, 300 MHz): d 7.85 (m, 1H, ar), 7.83 (m, 1H, ar), 7.79
Perkin Elmer Lambda 900 instrument. 0.5 6 10 M solutions
of the chromophores in CHCl3 were employed. H NMR
spectra are expressed in ppm relative to internal standard and
were obtained from CDCl solutions with a Bruker DPX300
NMR spectrometer. Melting points and decomposition inter-
1
~
H
14
O
3
3
C, 79.46; H, 4.67; found: C, 79.28; H, 4.85%.
vals were derived from DSC data, collected under nitrogen flow
on a Netzsch DSC200 calorimeter in connection with a Netsch
TASC 414/3 instrument and a Netzsch CC200 temperature
controller at a heating rate of 5 uC min . IR spectra were
recorded from KBr pellets in a Bruker IFS 66v spectrometer.
Benzoic acid 2-benzotriazol-2-yl-4-methylphenyl ester (2). The
procedure employed for the synthesis of 1 was repeated,
using 2-(2-hydroxy-5-methylphenyl)benzotriazole instead of
2
1
2
(
-hydroxybenzophenone. Recrystallization from methanol
instead of column chromatography) yielded 2 as white crys-
2
1
talline needles (1.4 mg, 85%). Mp 117 uC. IR (KBr, cm ):
Preparation of polyethylene blend films. Films containing
.2% w/w of the latent UV absorbers in LLDPE (Dowlex BG
340, Dow Chemicals) were produced from a 1% w/w master
3
1
6
074w, 1733s, 1514s, 1451s, 1404m, 1292m, 1272s, 1222s,
200s, 1180s, 1062s, 1029s, 967s, 895m, 869s, 806s, 736s,
0
2
1
75m. H NMR (CDCl
3
, 300 MHz): d 8.10 (m, 2H, ar), 8.00
batch which was prepared by dissolving 5 mg of the absorber in
approx. 2 mL CH Cl and spreading that solution over 495 mg
(
tr, 1H, ar), 7.72 (m, JH,H ~ 9.7, 3.0 Hz, 2H, ar), 7.60 (m,
H,H ~ 7.4, 1.3 Hz , 1H, ar), 7.45 (tr, JH,H ~ 4.9 Hz, 2H,
ar), 7.35 (m, 2H, ar), 7.31 (m, 2H, ar), 2.50 (s, 3H). Anal.
calcd for C20 (329.36): C, 72.94; H, 4.58; N, 12.76;
2
2
J
of LLDPE pellets. After evaporation of the solvent at ambient
temperature, the coated pellets were pressed into a blend film
between aluminum sheets in a hot press (Carver 2518) at a
pressure of 870 kPa for 2 min. For the benzoyl-caged latent
UV absorbers, the processing temperature was 180 uC, for the
t-Boc-caged latent UV absorber 4, the processing temperature
was 140 uC, in order to prevent any unwanted conversion of
this somewhat heat-sensitive compound. The quenched and
solidified blend films thus obtained were cut into pieces, which
were subsequently mixed and again processed into a film, using
identical conditions. This process was repeated four times in
order to obtain a homogeneous distribution of the latent UV
absorber within the polymer matrix. The resulting materials
were finally used as master batches to produce 0.2% w/w
blend films, by processing 100 mg of the 1% w/w blend film
with 400 mg of LLDPE pellets according to the above method.
For blend films containing 0.2% w/w of the acid-labile latent
UV absorber 4 and 3% w/w PAG, 15 mg of the PAG dissolved
in approx. 2 mL CH Cl were additionally processed into the
15 3 2
H N O
found: C, 72.75; H, 4.79; N, 12.64%.
Benzoic acid 2,4-di-tert-butyl-6-(5-chlorobenzotriazol-2-yl)-
phenyl ester (3). The procedure employed for the synthesis of
2
was repeated, using 2,4-di-tert-butyl-6-(5-chlorobenzotriazol-
-yl)phenol instead of 2-(2-hydroxy-5-methylphenyl)benzotri-
2
azole, resulting in colorless, cubic crystals of 3 (468 mg, 77%).
Mp 161 uC. IR (KBr, cm ): 3067w, 2980m, 1761s, 1566s,
2
1
1
1
512s, 1454m, 1372s, 1350s, 1343s, 1300s, 1256s, 1229s, 1143s,
1
074s, 1044m, 969s, 905s, 873s, 828s, 809s, 774s, 753s, 732s. H
NMR (CDCl
.4 Hz, 1H, ar), 7.64 (m, 4H, ar), 7.45 (tr, JH,H ~ 7.9 Hz, 2H,
ar), 7.22 (dd, JH,H ~ 9.1, 1.9 Hz, 1H, ar), 1.45 (s, 9H), 1.42 (s,
H). Anal. calcd for C27 Cl (461.99): C, 70.20; H, 6.11;
3
, 300 MHz): d 8.04 (m, 2H, ar), 7.92 (d, JH,H ~
2
9
28 3 2
H N O
N, 9.10; found: C, 70.09; H, 6.17; N, 8.87%.
2
2
Carbonic acid tert-butyl ester 2,4-di-tert-butyl-6-(5-chloro-
benzotriazol-2-yl)phenyl ester (4). The procedure employed for
the synthesis of 3 was repeated, using di-tert-butyl dicarbo-
nate instead of 2,4-di-tert-butyl-6-(5-chlorobenzotriazol-2-
yl)phenol, yielding 4 as a white crystalline powder (652 mg,
blend film after evaporation of the solvent. All films obtained
were colorless, transparent, and of good optical quality, and
had a uniform thickness, typically 100 mm.
Photoactivation of LLDPE/latent UV absorber blend films.
In the case of photo-labile, benzoyl-caged UV absorbers, a film
sample containing 0.2% w/w of the absorber was mounted on a
supporting frame and exposed to UV light from a standard
UV lamp (Bioblock Scientific, center frequency 254 nm,
9
(
3.4%). Mp 147 uC, decomposition interval 170–240 uC. IR
KBr, cm ): 3114w, 2959s, 2871m, 1757s, 1616m, 1558s,
21
1
1
484s, 1459s, 1420m, 1395s, 1368s, 1274s, 1232s, 1152s, 1106s,
1
145s, 936s, 892s, 879m, 868m, 813s, 774s, 715m, 708w. H
22
NMR (CDCl
7
J
3
, 300 MHz): d 7.95 (m, 2H, ar), 7.92 (s, 1H, ar),
.89 (m, 1H, ar), 7.58 (d, JH,H ~ 2.4 Hz, 1H, ar), 7.36 (dd, 2H,
H,H ~ 1.9, 9.1 Hz, ar), 1.45 (s, 9H), 1.39 (s, 9H), 1.26 (s, 9H).
340 mW cm ). Prior to exposure and after every desired expo-
sure interval, the UV spectrum of the sample was recorded,
using a neat LLDPE film sample as a reference. In the case of
acid-labile, t-Boc-caged UV absorbers, a set of film samples
containing 0.2% w/w of the absorber and 3% w/w of the
Anal. calcd for C H ClN O (460.02): C, 65.28; H, 7.45; N,
2
5
34
3
3
9
.13; found: C, 65.72; H, 7.23; N, 9.11%.
1
4 J. Mater. Chem., 2003, 13, 9–15

View Article Online
PAG was produced. All samples were exposed to UV radia-
tion as described above, but in a thermostatically controlled
environment at 25 uC. For each of the samples, the absorp-
tion spectrum was recorded prior to exposure, after exposure
for the required time interval, and after post-exposure treat-
ment at 100 uC for 1 h. To account for slight variations in
optical density, the measured absorption in the developing
absorption maximum was scaled with the absorption in the
primary absorption maximum for each sample.
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Acknowledgements
The authors are indebted to Sharon Ohba and Prof. Hatsuo
Ishida (both of Case Western Reserve University) for the
extensive GC-MS studies and for providing access to the GC-
MS equipment, respectively.
27 C. Kocher (Landqart), Eur. Pat., 02 007 000.9, 2002.
J. Mater. Chem., 2003, 13, 9–15
15