Two-photon fluorescent holographic rewritable micropatterning

Article abstract of DOI:10.1021/ja0735756

Monomeric amorphous systems involving an upper azo-based layer and a lower fluorophore layer have been carefully designed to enable holographic formation of micrometric emissive photopatterns through reversible photoinduced mass migration of azoderivatives. The fluorescent patterns evidenced by NIR biphotonic excitation match the micrometric surface relief. After successful erasure by using a circularly polarized visible light, new patterns can be written again. Copyright

Full text of DOI:10.1021/ja0735756

Published on Web 06/30/2007
Two-Photon Fluorescent Holographic Rewritable Micropatterning
Ele´na Ishow,* Arnaud Brosseau, Gilles Clavier, Keitaro Nakatani, Robert B. Pansu,
Jean-Jacques Vachon, Patrick Tauc,† Dominique Chauvat,‡ Cleber R. Mendonc¸a,§ and
Erick Piovesan§
Ecole Normale Supe´rieure de Cachan, PPSMsCNRS UMR 8531, 61 aV. Pdt Wilson, 94235 Cachan Cedex, France
Received May 18, 2007; E-mail: ishow@ppsm.ens-cachan.fr
Holographic data storage1 has recently attracted new interest with
the use of organic photopolymerizable matrices2 and photorefractive
Scheme 1. General Synthesis of the Glass-Forming Fluorescent 2
and Photochromic 4 Compoundsa
materials.3 However photopolymerization produces undesirable
matrix shrinkage while photorefractivity encounters progressive
orientational relaxation of the photo-oriented chromophores. An-
other competing recording route to increase data storage density
has been developed on the basis of the emission change induced
in fluorescent photochromic materials through confocal illumina-
tion.4 Unfortunately, the read-out process causes gradual erasure
of the illuminated areas because of photoinduced back reaction of
the bifunctional molecules even at very low irradiance.5 Recently
two-photon absorbing fluorescent materials have been used con-
jointly with photochromic dyes to reduce this back-reaction effect.6
Yet all these materials are light sensitive, and high-density recording
requires consuming pit-by-pit recording steps.
a Conditions: (i) 4-fluoro-1-nitrobenzene, K2CO3, DMSO, 140 °C, 12
h; (ii) 4-tert-butylphenylboronic acid, Pd(PPh3)4, toluene, 2 mol‚L-1
Na2CO3, 80 °C, 20 h; (iii) N2H4‚H2O, Pd/C 10%, reflux EtOH, 3 h; (iv)
4-nitrosobenzonitrile, room temp, 2 days.
such systems, the fluorescent and photochromic components must
comply with stringent material processing and spectral requirements.
First, these molecules must possess glass-forming properties to
yield multilayered amorphous monomeric materials with excellent
surface smoothness by successive vacuum thermal depositions.11
Second, only the fluorophore must show two-photon up-converted
fluorescence to avoid the erasure of the azo-SRG structure. Third,
the resulting fluorescence modulation must be caused by partial
reabsorption of the fluorophore’s emitted light by the azolayer. The
fluorophore emission must moderately overlap with the azolayer
absorption. Finally, a large modulation of the fluorophore’s emission
is easily achieved if the azo layer can be efficiently photostructured.
From our past expertise in azo-based monomeric materials, the best
candidate appeared to be 4-[bis(4′-tert-butylbiphenyl-4-yl)amino)]-
4′-cyanoazobenzene 4.12 Amid the various glass-forming fluorescent
polar molecules we synthesized, the compound bis(4′-tert-butyl-
biphenyl-4-yl)-4-nitrophenylamine 2 fulfills all structural and
spectral criteria.
We developed a novel synthetic strategy to generate the azo
derivative 412 directly from the nitro fluorophore 2 (Scheme 1).
The formation of the key amino intermediate 3 provides modular
access to a wide range of nonaggregating push-pull azo compounds
which are of high interest in the studies on single molecule
photoisomerization.13
Despite its large dipole moment (µcalcd ) 9.1 D), compound 2
forms stable glassy films when evaporated under vacuum. Glass-
transition temperature Tg was detected at 97 °C by differential
scanning calorimetry without evidence of a melting peak until
degradation occurred above 350 °C.
Surprisingly, the combination of both storage techniques, namely
holography involving two-photon fluorescent systems, has never
been envisaged to date even though it addresses all the performance
limitations cited above. Such an innovative all-optical approach
offers many attractive features. These features include rewritability,
high photosensitivity dynamics owing to easily detectable changes
in fluorescence, volumic recording, multiplexing possibilities, and
the use of distinct conditions (wavelength,5-7 polarization,8 inci-
dence angle9) to write and read-out the stored information.
We present herein bright fluorescent and stable 2D-photopatterns
obtained by holography from new azo-based photochromic and two-
photon fluorescent bilayer materials which provide nondestructive
two-photon read-out and all-optical rewritability. When exposed
to interferential illumination, azo derivatives undergo reversible
photoinduced mass migration, and form time- and light-stable
surface relief gratings (SRGs).10 All the current SRG-forming
systems require significant surface modulations to be locally
measured by atomic force microscopy (AFM), which makes the
read-out process tedious. On the contrary, introduction of fluoro-
phores enables easier optical detection of weak events such as small
peak-to-trough amplitudes.
As azoderivatives have proved to be strong emission quenchers
of the investigated fluorophores through their nπ* transition,
physical separation of both fluorophores and azoderivatives was
necessary. Thus, our system consists of a fluorescent lower layer
covered with a nonfluorescent azo-based photochromic layer. In
this way, the photostructurable azo-layer acts like a simple
rewritable photomask modulating emission of the fluorescent layer.
The read-out process, based on the fluorophore’s two-photon
excitation and emission monitoring, was subsequently performed
under distinct polarization, wavelength, and power excitation
conditions which ensures the azo photomask stability. To design
The strong similarities between the absorption and emission
properties of compound 2 in toluene solution and thin film indicate
the absence of intermolecular aggregations in the solid state, which
guarantees materials with high optical quality (Figure 1).
As required, the emission spectrum of 2, peaking at 601 nm in
thin film (574 nm in toluene) overlaps with the absorption spectrum
of the azo film 4 showing two maxima at 328 and 483 nm (see
† LBPA-UMR CNRS 8113, ENS Cachan, France.
‡ LPQM-UMR CNRS 8537, ENS Cachan, France.
§ Instituto de F´ısica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Brazil.
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J. AM. CHEM. SOC. 2007, 129, 8970-8971
10.1021/ja0735756 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
noise (<50 GM). Thus, use of a two-photon excitation at 820 nm
confines the excitation density within the fluorophore layer and
considerably improves the spatial resolution without recording
undesirable fluorescence or affecting the azo-pattern.
Detection of the emitted light above 580 nm revealed a regular
fluorescent pattern. Sizes of the fluorescent micrometric spots and
upper azo periodic 3D-structure correspond well with each other.
As expected, the black spots observed in one-photon transmission
at 488 nm and featuring the azo “hills” become optically the less
luminous ones (pink spots). After a 1-year storage under ambient
light exposure, the fluorescent gratings show no change in the
emission signal and prove to be perfectly stable.
Grating erasure has been successfully tested by exposing the
photopatterned bilayer film for 30 min under uniform circularly
polarized light of an argon-ion laser working at 488 nm (60
mW‚cm-2). Complete disappearance of the SRG structure has been
observed by AFM measurements, as well as its full recovery after
reiterated exposure to interferential illumination.
In conclusion, two-photon fluorescence micropatterning by using
holographic photochromism has been demonstrated for the first
time. The use of uncoupled photochromes and fluorophores,
holographic recording, and nondestructive near-infrared read-out
conditions has appeared as an effective strategy to generate stable
and rewritable photopatterns. These studies open up a new way of
recording optical information which is applicable to various organic
systems.
Figure 1. UV-vis absorption (s) and normalized emission spectra (-9-)
of thin evaporated film 2 and film 4 (200 nm thick).
Table 1. UV-Visible Spectroscopic Data of Compounds 2 and 4
λambasx (nm)a
λemmax (nm)a,b
Φf
τ
/nsa
a
2
4
408 (415), 324 (324)
478 (483), 329 (328)
574 (601)
0.12c (0.15)d
2.84e (∼2)
b
a Data measured in toluene solution (thin film). λexc ) 445 nm.
c Determined from coumarine 540A solution in EtOH as a fluorescence
d
standard (Φf ) 0.38). λexc ) 407 nm based on integration sphere
e
measurements. λexc ) 330 nm, ø2 ) 1.14.
Figure 2. Crossed SRGs recorded on a bilayer thin film 4/2: (a) AFM
3D-imaging; (b) one-photon transmission imaging at 488 nm with black
spots corresponding to azo hills; (c) TPF imaging at 820 nm excitation
(emission range 580-750 nm) with red areas featuring the brightest spots.
Acknowledgment. V. Simic and B. Geoffroy from CEA Saclay
and V. Gue´rineau from ICSN Gif-sur-Yvette are deeply acknowl-
edged for fluorescence quantum yield measurements in thin films
and mass spectrometry analyses, respectively.
Table 1). The large Stokes shift up to 170 nm and the strong
fluorescence exhibited by the nitro compound 2 at room temperature
have been assigned to the strongly twisted and consequently
decoupled dibiphenylamino group.14Bilayer thin films 4/2 were
fabricated by successive vacuum depositions of fluorophore 2 and
photochrome 4 onto precleaned 0.17 mm-thick glass cover-slips
(l.6 × 10-5 mbar). By adjusting the amount of material to be
evaporated and the final setpoint temperature, we obtained fluo-
rescent and photochromic layers with similar thicknesses of 200
nm.
The bicomposite film 4/2 was subsequently exposed to two
p-polarized interfering argon-ion laser beams (488 nm) of equal
intensity (15 mW‚cm-2) using a Lloyd interferometer setup. After
15 min, the film was rotated 90° and the previously exposed area
was subjected for a further 15 minute period to the same interference
fringe pattern. AFM measurements showed two series of gratings
crossed at a right angle with a 40 nm deep modulation and a spatial
period Λ equal to 1.82 µm in accordance with Bragg’s law λ )
2Λ sin θ (λ and θ represent the irradiation wavelength and the
beam incidence angle respectively: λ ) 488 nm, θ ) 7.5°) (Figure
2).
Supporting Information Available: Experimental procedures for
compounds 1-4, equipment description for thin film fabrication
photostructuration, and TPA measurements. This material is available
free of charge via the Internet at http://pubs.acs.org.
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