Photochromic switching of excited-state intramolecular proton-transfer (ESIPT) fluorescence: A unique route to high-contrast memory switching and nondestructive readout

Article abstract of DOI:10.1021/ja0637604

Aiming at the high-contrast photochromic switching of fluorescence emission and its perfect nondestructive readout, a polymer film highly loaded with a specific photochromic compound, 1,2-bis(2′-methyl-5′-phenyl- 3′-thienyl)perfluorocyclopentene (BP-BTE), and an excited-state intramolecular protontransfer (ESIPT)-active compound, 2,5-bis(5′-tert- butyl-benzooxazol-2′-yl)hydroquinone (DHBO), was employed in this work. The special class of photochrome, BP-BTE, has negligible absorbance at 415 nm both in the open form and in the 365 nm photostationary state (PSS), and the ESIPT fluorophore, DHBO, emits large Stokes' shifted (175 nm; λ_max^abs = 415 nm, λ_max ^em = 590 nm) and enhanced fluorescence (Φ_F ^powder = 10%, Φ_F^soln = 2%). Bistability, high-contrast switching (on/off fluorescence switching ratio >290), nondestructive readout (over 125000 shots), and erasability were all together accomplished in this novel recording medium.

Full text of DOI:10.1021/ja0637604

Published on Web 10/19/2006
Photochromic Switching of Excited-State Intramolecular
Proton-Transfer (ESIPT) Fluorescence: A Unique Route to
High-Contrast Memory Switching and Nondestructive Readout
Seon-Jeong Lim, Jangwon Seo, and Soo Young Park*
Contribution from the School of Materials Science and Engineering ENG445, Seoul National
UniVersity, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, Korea
Received May 29, 2006; E-mail: parksy@snu.ac.kr
Abstract: Aiming at the high-contrast photochromic switching of fluorescence emission and its perfect
nondestructive readout, a polymer film highly loaded with a specific photochromic compound, 1,2-bis(2′-
methyl-5′-phenyl-3′-thienyl)perfluorocyclopentene (BP-BTE), and an excited-state intramolecular proton-
transfer (ESIPT)-active compound, 2,5-bis(5′-tert-butyl-benzooxazol-2′-yl)hydroquinone (DHBO), was
employed in this work. The special class of photochrome, BP-BTE, has negligible absorbance at 415 nm
both in the open form and in the 365 nm photostationary state (PSS), and the ESIPT fluorophore, DHBO,
powder
emits large Stokes’ shifted (175 nm; λmaxabs ) 415 nm, λmaxem ) 590 nm) and enhanced fluorescence (ΦF
) 10%, ΦF
soln
) 2%). Bistability, high-contrast switching (on/off fluorescence switching ratio >290),
nondestructive readout (over 125000 shots), and erasability were all together accomplished in this novel
recording medium.
Introduction
rial properties of the BTEs, however, practical applications of
these BTE switches to the fluorescence-based erasable optical
Photoswitchable organic molecules are drawing great attention
for their viable application to the ultrahigh-density erasable
optical memory.1 Among the various photon-mode molecular
memory systems, bistable photoswitching of fluorescence emis-
sion has been considered to be a promising signaling mode
because the fluorescence signals can be readily and sensitively
recognized while the weak and color-tuned excitation light
scarcely ever erases the recorded signal during the fluorescence
readout process.2-14 Aiming at the bistable photochromic
switching of the fluorescence emission, highly efficient gated
energy transfer from the fluorophore to the photochromic 1,2-
bis(thienyl)ethene derivatives (BTEs) has been extensively
investigated owing to the high thermal stability, excellent
photofatigue resistance, and characteristic bistability of the
BTEs.2-12,15-19 Despite these encouraging principles and mate-
memory are still limited by the insufficient performances of
high-contrast fluorescence recording and complete nondestruc-
tive readout of the recorded signals.
As for the former issue of high-contrast fluorescence record-
ing, many different classes of “fluorescent BTEs” (i.e., fluo-
rescent molecules with BTE switch), such as a tungsten complex,2
anthracene derivatives,3,6 2,4,5-triphenylimidazole,4 diporphy-
rins,5 p-phenylenevinylene moieties,7 and a 3a,4a-diaza-s-inda-
cene derivative,8 have been developed to demonstrate the higher
on/off fluorescence signaling ratio (mostly in solution) by
enhancing the value of fluorescence quantum yield (ΦF) in the
open-form BTE state. Bistable photochromic switching in these
fluorescent BTEs are well implemented by the efficient intra-
molecular energy transfer from the fluorophores to the BTE
units in an isolated molecular state either in solutions or in poly-
mer matrices. On the other hand, switching performances in
the highly loaded polymer films, which are inevitably needed
for their practical application to the ultrahigh-density erasable
optical memory, are more complicated and less investigated
because of the additional challenging problems, such as “concen-
tration quenching” of the fluorescence emission,20 irreversible
photochromism of the BTE moieties, and segregation (phase
separation) of the molecules in the highly loaded polymer films.
For the realization of high-performance polymer films with the
(1) Irie, M. PhotoreactiVe Materials for Ultrahigh-Density Optical Memory;
Elsevier: Amsterdam, 1994.
(2) Fernandez-Acebes, A.; Lehn, J.-M. AdV. Mater. 1998, 10, 1519.
(3) Kawai, T.; Sasaki, T.; Irie, M. Chem. Commun. 2001, 711.
(4) Yagi, K.; Soong, C. F.; Irie, M. J. Org. Chem. 2001, 66, 5419.
(5) Norsten, T. B.; Branda, N. R. J. Am. Chem. Soc. 2001, 123, 1784.
(6) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature (London)
2002, 420, 759.
(7) Cho, H.; Kim, E. Macromolecules 2002, 35, 8684.
(8) Golovkova, T. A.; Kozlov, D. V.; Neckers, D. C. J. Org. Chem. 2005, 70,
5545.
(9) Murase, S.; Teramoto, M.; Furukawa, H.; Miyashita, Y.; Horie, K.
Macromolecules 2003, 36, 964.
(10) Lim, S.-J.; An, B.-K.; Jung, S. D.; Chung, M.-A.; Park, S. Y. Angew. Chem.,
Int. Ed. 2004, 43, 6346.
(11) Lim, S.-J.; An, B.-K.; Park, S. Y. Macromolecules 2005, 38, 6236.
(12) Jiang, G.; Wang, S.; Yuan, W.; Jiang, L.; Song, Y.; Tian, H.; Zhu, D.
Chem. Mater. 2006, 18, 235.
(13) Ferri, V.; Scoponi, M.; Bignozzi, C. A.; Tyson, D. S.; Castellano, F. N.;
Doyle, H.; Redmond, G. Nano Lett. 2004, 4, 835.
(14) Tomasulo, M.; Giordani, S.; Raymo, F. M. AdV. Funct. Mater. 2005, 15,
787.
(15) Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136.
(16) Nakayama, Y.; Hayashi, K.; Irie, M. J. Org. Chem. 1990, 55, 2592.
(17) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and Thermochromic
Compounds; Plenum: New York, 1999; Vol. 1.
(18) Irie, M. Chem. ReV. 2000, 100, 1685.
(19) Tian, H.; Yang, S. Chem. Soc. ReV. 2004, 33, 85.
(20) Birks, J. B. Photophysics of aromatic molecules; Wiely: London, 1970.
9
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J. AM. CHEM. SOC. 2006, 128, 14542-14547
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High-Contrast Memory Switching and Nondestructive Readout
A R T I C L E S
Figure 1. (a) Chemical structure and photochromic reaction of BP-BTE; (b, c) Chemical structures of the model fluorophores, coumarin 480D and rhodamine
B, respectively; (d) four-level photochemical and photophysical excited-state intramolecular proton-transfer (ESIPT) process of DHBO.
high-contrast on/off fluorescence recording capability, conse-
quently, these problems should be answered all together.9-14
Very recently, one encouraging achievement was made by using
a special class of fluorophores showing a gradually enhanced
fluorescence emission with increasing concentration, a so-called
“aggregation-induced enhanced emission (AIEE)”.10,11
readout capability by employing a polymer film highly loaded
with a special class of fluorescent BTE incorporating a large
Stokes’ shifted excited-state intramolecular proton-transfer
(ESIPT) fluorescence.
Results and Discussion
As for the latter issue of nondestructive readout of the
fluorescence signal, it is demanded that the readout light for
the fluorescence excitation should not induce any photoisomer-
ization reactions during the fluorescence readout cycles. This
means that the photochromic units or the BTEs (both open-
form and closed-form) in the highly loaded polymer films should
have zero absorbance at the fluorescence excitation wavelength
ideally to generate no photochromic reactions during the
fluorescence readout process for their practical applications.9,11
Despite these simple strategic principles for the nondestructive
readout, no suitable material systems have been found so far
due to the inherent limitation of the Stokes’ shift in conventional
fluorescent molecules (vide infra).
A special class of BTE compound, 1,2-bis(2′-methyl-5′-
phenyl-3′-thienyl)perfluorocyclopentene (BP-BTE),21 was adopted
as a photochromic switch in this work not only because the
BP-BTE shows a highly efficient bistable photochromism (see
Figure 1a for its chemical structure and photochromic reaction)
but also because it has negligible absorbance at around 415 nm
both in the open form and in the 365 nm (hand-held lamp, 1.2
mW cm-2) photostationary state (PSS).15-19 The extinction
coefficient (ꢀ) at 415 nm of the closed-form BP-BTE dissolved
in hexane was estimated to be only 450 (M-1 cm-1) while that
in the poly(methyl methacrylate) (PMMA; Mw ≈ 120 000)
matrix was noted to be even less as shown in Figure 2a and c.
In this work, we demonstrate a high-contrast photochromic
switching of fluorescence emission and its perfect nondestructive
(21) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc. 2000, 122,
4871.
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A R T I C L E S
Lim et al.
Figure 2. (a) UV-visible absorption spectra of the poly(methyl methacrylate) (PMMA) films loaded with BP-BTE and BP-BTE/coumarin 480D in the
open-form state (black lines) and in the 365 nm photostationary state (PSS) (blue lines). UV-visible absorption and photoluminescence (PL) spectra of the
PMMA film loaded with coumarin 480D (green lines); (b) nondestructive readout capabilities of the BP-BTE/coumarin 480D-loaded PMMA film in the
open-form state (black lines, λex ) 415 nm, 200 µW cm-2) and in the 365 nm PSS (blue lines, λex ) 415 nm, 200 µW cm-2); (c) UV-visible absorption
spectra of the PMMA films containing BP-BTE and BP-BTE/rhodamine B in the open-form state (black lines) and in the 365 nm PSS (blue lines). UV-
visible absorption and PL spectra of the PMMA film containing rhodamine B (red lines); (d) PL spectra showing destructive readout trails of the BP-
BTE/rhodamine B-loaded PMMA film in the 365 nm PSS (blue lines, λex ) 560 nm, 150 µW cm-2) and PL spectrum of the film in the open-form state
(black line, λex ) 560 nm, 150 µW cm-2).
Considering that the ring-opening quantum yield of the closed-
their chemical structures) were used as the model fluorophores.
First, PMMA film containing a high level (17 wt %) of BP-
BTE/coumarin 480D was prepared at the molar ratio of 1.6 to
examine the nondestructive readout capability of the fluorescent
BP-BTE mixture. Inset in Figure 2a shows the UV-visible
absorption spectra of the BP-BTE/coumarin 480D-loaded
PMMA film in the open form and in the 365 nm PSS, in which
the new absorption band in the visible region (λmax ≈ 590 nm)
went up and down repeatedly with alternate UV and visible
light irradiation, respectively. When the film was irradiated with
the relatively high-intensity 415 nm light (200 µW cm-2) for
the fluorescence excitation, the initial “on” state was continu-
ously preserved regardless of the excitation numbers as shown
in Figure 2b. It is considered that such a nondestructive readout
capability with the 415 nm light is attributed to the absolutely
negligible absorbance of the open-form BP-BTE at 415 nm.
Furthermore, the quenched “off” state by the closed-form BP-
BTE in the 365 nm PSS did not increase with the irradiation of
the 415 nm light as well, as shown in Figure 2b again due to
the totally small absorbance of the closed-form BP-BTE at 415
nm. Despite these encouraging nondestructive readout capabili-
CfO
form BP-BTE was rather low (ΦPC
) 0.013 in hexane), it
was readily expected that the relatively weak and narrow range
of fluorescence emission in the visible region of the fluorophore
might induce no ring-opening reaction of the closed-form BP-
BTE during the fluorescence readout cycles. In addition,
photochromic conversion of the BP-BTE was significant to be
employed as a switch unit; by a simple calculation from the ꢀ
value at 280 nm of the open-form BP-BTE (35 600 M-1 cm-1
in hexane) and that at 575 nm of the closed-form BP-BTE
(15 600 M-1 cm-1 in hexane) as well as from the UV-visible
absorption spectra in the open form and in the 365 nm PSS, it
was readily estimated that at least 76% of the open-form BP-
OfC
BTE was converted into the closed form (ΦPC
) 0.59 in
hexane) under the 365 nm PSS.
To understand the fundamental principles and limitations of
high-contrast fluorescence recording and nondestructive readout,
spin-coated PMMA films containing a high level of conventional
fluorophores and BP-BTE photochrome were prepared. Com-
mercially available coumarin 480D (Aldrich Chemical Co.) and
rhodamine B (Aldrich Chemical Co.) (see Figure 1b and c for
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High-Contrast Memory Switching and Nondestructive Readout
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ties in the BP-BTE/coumarin 480D-loaded PMMA film, the
integrated fluorescence intensity of the film in the 365 nm PSS
was reduced down to only a 15% level of the initial open-form
state (on/off fluorescence switching ratio < 7). It was considered
that such a rather low on/off contrast ratio resulted from the
inefficient intermolecular energy transfer between the coumarin
480D and the closed-form BP-BTE in the 365 nm PSS (i.e.
only partial overlap between the emission band of the coumarin
480D and the absorption band of the closed-form BP-BTE,
see Figure 2a).12 It was also noted that the fluorescence intensity
of the coumarin 480D in the open-form BP-BTE state (“on”
state) itself was pretty low owing to the “concentration quench-
ing” effect in such a highly loaded polymer film.10,20
shifted keto emission.22-30 Because of these unique and
beneficial photophysical properties, ESIPT fluorophores have
been applied to the photostabilizers,25 photopatterning media,26
chemosensors,27 white-light-emitting diodes,28 and proton-
transfer lasers.29,30 Among various ESIPT fluorophores, 2,5-
bis(5′-tert-butyl-benzooxazol-2′-yl)hydroquinone (DHBO) was
selected as an ideal fluorophore in this work because it emits a
em
large Stokes’ shifted fluorescence (λmaxabs ) 415 nm, λmax
)
590 nm; see Figure 3a) as well as an enhanced fluorescence in
powder
the solid state than in the isolated molecular state (ΦF
10%, ΦF
)
soln
) 2% in chloroform; see the Experimental
Section).31 Aiming at the high-contrast on/off recording and
nondestructive readout, a highly fluorescent and optically
transparent PMMA film containing a very high level (20 wt
%) of BP-BTE/DHBO was prepared at an optimized molar ratio
of 1.3 (see the Experimental Section for details). UV-visible
absorption spectra of the BP-BTE/DHBO-loaded PMMA film
in the open form and in the 365 nm PSS are shown in Figure
3b. The new absorption band in the visible region (λmax ≈ 590
nm) went up and down repeatedly with alternate UV and visible
light irradiation indicating that the highly efficient bistable
photochromism of the BP-BTE worked very well in this memory
medium. When the film was irradiated with a 415 nm readout
light (200 µW cm-2) for the fluorescence excitation of the
To improve the on/off fluorescence contrast ratio, PMMA
film containing the same content (17 wt %) of BP-BTE/
rhodamine B which shows a larger spectral overlap was prepared
at a molar ratio of 1.0. The inset in Figure 2c shows the UV-
visible absorption spectra of the BP-BTE/rhodamine B-loaded
PMMA film in the open form as well as in the 365 nm PSS.
Absorbance change due to the closed-form BP-BTE is noted
over the 600-750 nm region, while the larger absorption band
at the 450-600 nm region is mostly due to the rhodamine B
absorption. As expected from the favorable spectral overlap, it
was observed that the integrated fluorescence intensity of the
film in the 365 nm PSS was reduced down to the 4% level of
the initial open-form BP-BTE state (on/off fluorescence switch-
ing ratio >25). It is obvious that this high-contrast fluorescence
switching is attributed to the highly efficient intermolecular
energy transfer between the rhodamine B and the closed-form
BP-BTE (i.e., large spectral overlap between the emission band
of the rhodamine B and the absorption band of the closed-form
BP-BTE; see Figure 2c). When the film was irradiated with
the 560 nm readout light (150 µW cm-2) for the fluorescence
excitation, however, the extinguished red fluorescence by the
closed-form BP-BTE in the 365 nm PSS gradually increased
up to reach the initial fluorescence intensity before the 365 nm
UV light irradiation as shown in Figure 2d. This means that
the 560 nm readout light for the fluorescence excitation induces
the ring-opening reaction of the closed-form BP-BTE bringing
about the destructive readout.12
DHBO, strong orange fluorescence in the open-form BP-BTE
flm
state (ΦF
) 10%) as well as the remarkably quenched
fluorescence by the closed-form BP-BTE in the 365 nm PSS
film
(ΦF
) 0.03%, or on/off switching ratio >290) could be
bistably and nondestructively preserved under repeated excita-
tion processes as shown in Figure 3c and d. Through a simple
calculation from the Figure 3c and d results, it was estimated
that the quenched fluorescence in the 365 nm PSS (ΦF ) 0.03%)
would increase up to reach only the 0.05% ΦF level after a
125 000 shots of excitation with the relatively high-intensity
415 nm readout light (200 µW cm-2) (see the Experimental
Section for details). This is a remarkable number considering
that the clear fluorescence readout is possible with the 415 nm
light whose intensity is as low as 10 µW cm-2. It is considered
that such a characteristically strong nondestructive readout
capability in this BP-BTE/DHBO-loaded PMMA film in the
365 nm PSS is attributed to the very low ring-opening quantum
yield of the closed-form BP-BTE (ΦPCCfO ) 0.013) as well as
the high ꢀ value at 415 nm of the DHBO (31900 M-1 cm-1 in
chloroform) in addition to the large Stokes’ shift requirement
described earlier. Although the relatively weak and narrow range
of fluorescence emission of the DHBO may induce the excitation
of the closed-form BP-BTE together with the fluorescence
quenching, it is reasonably thought that the number of closed-
form BP-BTEs for the fluorescence quenching can be suf-
Considering the BP-BTE/coumarin 480D and BP-BTE/
rhodamine B systems together, it is deduced that the ideal
fluorophore for the BP-BTE switch should have the maximum
absorption wavelength at around 415 nm for the nondestructive
readout capability as well as the maximum emission wavelength
at around 590 nm (Stokes’ shift ) 175 nm) to maximize its
energy transfer efficiency with the closed-form BP-BTE in the
365 nm PSS. Such a large Stokes’ shift is very unusual in a
conventional fluorophore but can be achieved with an ESIPT
fluorophore. As an additional requirement, the ideal fluorophore
should emit more enhanced fluorescence in the condensed solid
state than in the isolated molecular state to overcome the
“concentration quenching” problem in the highly loaded polymer
films.10,11
(22) Abou-Zied, O. K.; Jimenez, R.; Thompson, E. H. Z.; Millar, D. P.;
Romesberg, F. E. J. Phys. Chem. A 2002, 106, 3665.
(23) Liang, F.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Appl. Phys. Lett. 2002,
81, 4.
(24) Seo, J.; Kim, S.; Park, S. Y. J. Am. Chem. Soc. 2004, 126, 11154.
(25) Keck, J.; Kramer, H. E. A.; Port, H.; Hirsch, T.; Fischer, P.; Rytz, G. J.
Phys. Chem. 1996, 100, 14468.
(26) Kim, S.; Park, S. Y. AdV. Mater. 2003, 15, 1341.
(27) Tong, H.; Zhou, G.; Wang, L.; Jing, X.; Wang, F.; Zhang, J. Tetrahedron
Lett. 2003, 44, 131.
Fluorescent molecules with the ESIPT process have a
characteristic four-level photophysical scheme incorporating the
ground and excited states of two different tautomers as shown
in Figure 1d. Different absorbing (E f E*) and emitting (K*
f K) molecular species in this ESIPT cycle normally result in
the total exclusion of self-absorption and the large Stokes’
(28) Kim, S.; Seo, J.; Jung, H. K.; Kim, J.-J.; Park, S. Y. AdV. Mater. 2005, 17,
2077.
(29) Acun˜a, A. U.; Costela, A.; Mun˜oz, J. M. J. Phys. Chem. 1986, 90, 2807.
(30) Park, S.; Kwon, O.-H.; Kim, S.; Park, S.; Choi, M.-G.; Cha, M.; Park, S.
Y.; Jang, D.-J. J. Am. Chem. Soc. 2005, 127, 10070.
(31) Mello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997, 9, 230.
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Lim et al.
Figure 3. (a) UV-visible absorption spectra of the PMMA film loaded with BP-BTE in the open-form state (black line) and in the 365 nm PSS (blue line).
UV-visible absorption and PL spectra of the PMMA film loaded with DHBO (red lines); (b) UV-visible absorption spectra of the BP-BTE/DHBO-loaded
PMMA film in the open-form state (black line) and in the 365 nm PSS (blue line); (c, d) nondestructive readout capabilities of the BP-BTE/DHBO-loaded
PMMA film in the open-form state (black lines, λex ) 415 nm, 200 µW cm-2) and in the 365 nm PSS (blue lines, λex ) 415 nm, 200 µW cm-2). Inset graph
in (d) shows the photochromic modulation of the ESIPT fluorescence in the BP-BTE/DHBO-loaded PMMA film.
Figure 4. Microsized erasable ESIPT-fluorescence photoimaging on a spin-coated BP-BTE/DHBO-loaded PMMA film and its nondestructive readout
capability; (a) initial open-form state; (b) writing; (c) erasing; (d) rewriting; and (e) continuous nondestructive reading under irradiation with the relatively
high-intensity 415 nm light (200 µW cm-2) for 30 min. The dark region represents the irradiated area with the 365 nm UV light.
ficiently preserved because the ring-opening quantum yield of
the closed-form BP-BTE is low enough. It is also worth
mentioning that most of the 415 nm readout photons interact
favorably with the DHBO because the ꢀ value (at 415 nm) of
the DHBO is over 70 times larger than that of the closed-form
BP-BTE.
When the BP-BTE/DHBO-loaded PMMA film stabilized in
the “off” state was irradiated with visible light (>450 nm, xenon
arc lamp, 300 W, 5 min), it was reversibly and completely
recovered to the initial strongly fluorescent “on” state. The inset
graph in Figure 3d shows the highly bistable photochromic
modulation of the strong ESIPT fluorescence of the DHBO with
alternate UV (i.e., 365 nm, 5 min) and visible light (i.e., >450
nm, 5 min) irradiation.
nondestructive readout in the BP-BTE/DHBO-loaded PMMA
film suggests its viable application to the practically available
erasable optical memory medium. By employing the patterned
illumination through the photomasks, we could successfully
demonstrate that the different microsized images could be
effectively recorded, erased, and read out on a polymer film as
shown in Figure 4. Particularly, all the recorded images in Figure
4 could be nondestructively and repeatedly monitored even with
the relatively high-intensity (200 µW cm-2) readout light of a
different wavelength (i.e., 415 nm) from the writing (i.e., 365
nm) and the erasing (i.e., >450 nm) wavelengths inducing the
photochromic reaction of the BP-BTE. It must be noted that
the recorded fluorescence images could be clearly read out even
with the much lower intensity irradiation for the fluorescence
excitation (e.g., 415 nm, 10 µW cm-2).
Such a strong ESIPT fluorescence emission coupled with the
bistable and high-contrast photochromic switching and perfect
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source through a color filter (Newport, FS-C). The fluorescence images
were acquired on a photomicroscope (Axiophot, ZEISS).
Conclusion
In summary, we have demonstrated a novel and unique route
to the highly efficient erasable optical memory that utilizes the
photochromic switching of ESIPT fluorescence. Specific pho-
tochromic compound BP-BTE, which has negligible absorbance
at 415 nm both in the open form and in the 365 nm PSS, and
an ESIPT-active compound, DHBO, which emits large Stokes’
shifted (175 nm) and enhanced fluorescence, were employed
as the main components of an optical recording medium.
Bistability, high-contrast switching (on/off fluorescence switch-
ing ratio >290), nondestructive readout (over 125000 shots),
and erasability were all together demonstrated in these novel
recording mediums.
Fluorescence Quantum Yields (ΦF) Measurement: The fluores-
cence quantum yields (ΦF) were determined using a 6 in. (15.24 cm)
integrating sphere (Labsphere, 3P-GPS-060-SF) equipped with a 325
nm continuous wave (CW) HeCd laser (OmNichrome, Series 56) and
a photomultiplier tube (PMT) detector (Hamamatsu, PD471) attached
to a monochromator (Acton Research, Spectrapro-300i) (see ref 31 for
more details).
Optimized Molar Ratio of BP-BTE/DHBO in PMMA: A highly
fluorescent and optically transparent PMMA film containing a very
high level (20 wt %) of BP-BTE/DHBO was prepared in this work at
an optimized molar ratio of 1.3. On the other hand, when the molar
ratio of BP-BTE/DHBO was lower than 1.3, the strong ESIPT
fluorescence of the DHBO was not perfectly quenched in the 365 nm
PSS. Besides, when the BP-BTE was more added to the optimized
PMMA film containing 20 wt % of BP-BTE/DHBO (molar ratio of
1.3), some segregation (or phase-separation) of the active molecules
in the highly loaded polymer films took place urging the amount of
DHBO to be lowered in these PMMA films. In the case of these PMMA
films containing higher than 20 wt % of BP-BTE/DHBO, considerable
light scattering at the visible region was clearly observed in their UV-
visible absorption spectra. Consequently, the high-density inclusion of
the active fluorophore, DHBO, into the PMMA matrix and high-contrast
fluorescence recording capability could be all together accomplished
in this optimized PMMA film containing 20 wt % of BP-BTE/DHBO
(molar ratio of 1.3).
Experimental Section
Preparation and Structural Analytical Data of 1,2-Bis(2′-methyl-
5′-phenyl-3′-thienyl)perfluorocyclopentene (BP-BTE): BP-BTE was
synthesized according to the literature (see ref 21 for details). Analytical
1
data of BP-BTE: white colored amorphous powder; H NMR (300
MHz, CDCl3): δ 7.6-7.2 (m, 10H), 7.28 (s, 2H), 1.97 (s, 6H); MS
m/z 520 (M+); Elemental analysis, calculated for C27H18F6S2: C, 62.30;
H, 3.49; S, 12.32%. Found: C, 62.56; H, 3.59; S, 12.35%.
Preparation and Structural Analytical Data of 2,5-Bis(5′-tert-
butyl-benzooxazol-2′-yl)hydroquinone (DHBO): DHBO was syn-
thesized according to the literature (see ref 32 for details). Analytical
data of DHBO: yellow colored amorphous powder, 1H NMR (300
MHz, CDCl3) δ 11.10 (s, 2H), 7.77 (m, 4H), 7.56 (d, 2H, J ) 8.6 Hz),
7.48 (d, 2H, J ) 8.6 Hz), 1.41 (s, 18H); MS m/z 456 (M+); Elemental
analysis, calculated for C28H28N2O4: C, 73.66; H, 6.18; N, 6.14%,
Found: C, 73.76; H, 6.17; N, 6.00%.
Instrumentation: 1H NMR spectra were obtained on a JEOL JNM-
LA300 (300 MHz) using CDCl3 as the solvent. Mass spectra were
measured on a JMS AX505WA by EI mode, and elemental analysis
was carried out on an EA1110 (CE Instrument). UV-visible absorption
and fluorescence emission spectra were measured on a Shimadzu UV-
1650PC and Shimadzu RF-500 spectrophotometer, respectively. A UV
hand-held lamp (365 nm, 1.2 mW cm-2) was used as a UV light source,
and a xenon arc lamp (>450 nm, 300 W) was used as a visible light
Nondestructive Readout Time Calculation: The integrated fluo-
rescence intensity of the BP-BTE/DHBO-loaded PMMA film in the
365 nm PSS increased up to reach ((1 + 3.2) × 10-4) times of initial
value (ΦF ) 0.03%) after 60 shots of excitation. If it were assumed
that the ring-opening reaction of the closed-form BP-BTE occurred
linearly for a simple calculation, although it actually happened
logarithmically as shown in Figure 2d, the ΦF of 0.03% would increase
up to reach the 0.05% ΦF level after 120 500 shots of excitation.
Acknowledgment. This work was supported by the Ministry
of Science and Technology of Korea through the National
Research Laboratory (NRL) program awarded to Prof. Dr. Soo
Young Park.
(32) Stefani, V.; Souto, A. A.; Acun˜a, A. U.; Amat-Guerri, F. Dyes and Pigments
1992, 20, 97.
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