Macromolecules 2005, 38, 7207-7208
7207
Scheme 1
A Photoinduced Refractive Index Increase in
Poly(methyl methacrylate) Film Doped with
N-Acetyl-r-dehydroarylalanine Naphthyl
Esters
Kenta Tanaka,*,† Haruki Nakajima,‡
Tetsutaro Igarashi,‡ and Tadamitsu Sakurai*,‡
High-Tech Research Center, Kanagawa University,
Kanagawa-ku, Yokohama 221-8686, Japan, and
Department of Applied Chemistry, Faculty of Engineering,
Kanagawa University, Kanagawa-ku, Yokohama 221-8686,
Japan
N-acetyl-R-dehydrophenylalanine 1-naphthyl ester (1a)
or N-acetyl-R-dehydro(1-naphthyl)alanine 1-naphthyl
ester (1b) increases the refractive index of this polymer
film greatly, and then our refractive index photocontrol
system enables application to new optical materials.
The starting (Z)-isomers of 1a and 1b were prepared
by the ring-opening reactions of (Z)-2-methyl-4-(ben-
zylidene)-5(4H)-oxazolone (2a) and (Z)-2-methyl-4-(1-
naphthylmethylene)-5(4H)-oxazolone (2b) with 1-naph-
thol in dry chloroform containing triethylamine, respec-
tively.12,13 In Figure 1 are typically shown UV absorp-
tion spectral changes caused by the irradiation (λ > 280
nm, 450 W high-pressure Hg lamp) of a nitrogen-
saturated acetonitrile solution of 1a at room tempera-
ture. In both cases when the photoreaction proceeds to
a certain extent, the strong UV absorptions of the
starting 1a (290 nm) and 1b (314 nm) were decreased
with appearance of the 328 nm (1a) and 367 nm (1b)
absorptions, while there were isosbestic points at 300
nm (1a) and 280 and 325 nm (1b). Because the absorp-
tion band appeared on the long wavelength side is very
similar to that of the oxazolone derivative 2, we were
led to conclude that 1a and 1b undergo the almost
exclusive heterolysis of the ester C(dO)-O bond to
afford 2a and 2b, respectively, as already demonstrated
in the previous study.11 If we consider the irradiation
time at which the UV absorption spectrum of 1 stops to
change, the photoheterolysis of (Z)-1b in solution turns
out to be apparently slower than that of (Z)-1a by about
1 order of magnitude. This may be due to either a
decrease in rate for the heterolytic cleavage of the ester
bond in the singlet excited-state 1b or an increase in
rate for the recombination of a 1b-derived ion pair
intermediate.
Our attention is now directed to the UV absorption
spectral and refractive index changes of the PMMA film
containing the arylalanine naphthyl ester 1, caused by
irradiation. PMMA films doped with 1a and 1b were
made on silica glasses (for UV spectral measurements)
and on silicon wafers (for refractive index measure-
ments) by spin-coating of 2-methoxyethyl acetate solu-
tions followed by vacuum-drying at 40 °C. A comparison
of UV absorption spectral changes in solution (Figure
1) and in polymer film (Figure 2) revealed that the
photoheterolysis of (Z)-1 in the latter state proceeds
more rapidly as compared to that in the former state to
give 2, possibly, along with 1-naphthol (irradiation
conditions: λ > 280 nm, 450 W high-pressure Hg lamp).
It is very likely that the heterolytic ester bond cleavage
is a unimolecular process, and hence, its rate in solution
and film states is not much different from each other.
Thus, we were led to propose that the PMMA polymer
medium greatly suppresses the recombination of an
Received May 31, 2005
Revised Manuscript Received July 14, 2005
As a result of great demand for advanced optical
materials such as plastic optical fiber,1 the photochemi-
cal decrease in refractive index for polymer films has
attracted considerable attention for the past decade.2-4
We succeeded in lowering the refractive index of poly-
(methyl methacrylate) (PMMA) film (doped with hy-
droxy-substituted diarylnitrones) by as much as 0.014
(-∆n) by their quantitative photochemical conversions
into diarylformamides showing a much weaker π-con-
jugation.3 On the other hand, there are only a few
studies directed toward the photochemical increase in
refractive index for a given polymer film.6-10 Langer et
al.6 reported that the photoirradiation of polymers
bearing thiocyanate pendants enhances the polymer
refractive index by 0.031; an additional increase in
refractive index is observed upon treating the polymer
with hydrazine (∆n ) +0.035). Murase et al.7 also
showed that the 3 h photochemical and 3 h thermal
treatments of PMMA film doped with 30 wt % of phenyl
azide raise the index of this film by 0.0161. While the
refractive index changes described above seem to be
sufficient magnitude for utilizing these polymers as
optical materials, it is necessary to increase the stability
of polymer refractive index and also to speed up the
photochemical transformation being responsible for the
refractive index change of a given polymer film.
In the course of our systematic study regarding the
excited-state reactivities of N-acyl-R-dehydroarylala-
nines, it was found that (Z)-N-acetyl-R-dehydrophenyl-
alanine aryl ester derivatives in the singlet excited state
undergo the heterolytic cleavage of the ester C(dO)-O
bond to eventually form the corresponding oxazolones
and aryl alcohols by way of the cyclization of acylinium
intermediates (Scheme 1).11 In contrast to the photoi-
somerization of diarylnitrone derivatives, the photo-
heterolysis of dehydrophenylalanine aryl esters gave
oxazolones as one of the major products, the absorption
of which appeared on the much longer wavelength side
than that of the aryl esters. Thus, the novel photochemi-
cal transformation of these esters into oxazolones results
in an enhancement in the π-conjugation, namely, the
linear polarizability of given molecules, which allows us
to propose that this photoreaction is accompanied by an
increase in refractive index. In this Communication we
present the results which demonstrate that the short
period of time irradiation of PMMA film doped with
† High-Tech Research Center.
‡ Department of Applied Chemistry.
10.1021/ma051116o CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/26/2005
Tanaka, Kenta
