Metal-organic approach to binary optical memory

Article abstract of DOI:10.1021/ja0255608

Two new Ru(II) diimine chromophores, each containing a single photochromic dianthryl unit, have been prepared and characterized. The photoluminescence from the Ru(II) complexes is modulated by the photochromic action of the dianthryl species, which serves as a triplet energy transfer quencher in one photochromic state. The coupling of the dianthryl photochromic action to the Ru(II) complex emission permits nondestructive photoluminescence readout of binary information photochemically recorded on the molecular level. Luminescent images stored on polystyrene films that contain these molecules maintained their integrity for periods of months with no apparent degradation or variation in the image resolution, suggesting their durability for long-term storage in read-only memory applications. Copyright

Full text of DOI:10.1021/ja0255608

Published on Web 04/06/2002
Metal-Organic Approach to Binary Optical Memory
Daniel S. Tyson,§ Carlo A. Bignozzi,‡ and Felix N. Castellano*,§
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43404 and Dipartimento di Chimica, UniVersita` di Ferrara, 44100 Ferrara, Italy
Received January 10, 2002
Photonic devices that incorporate photoswitchable molecules
represent the future of digital optical data systems where the
recording of information is accomplished with light.1,2 Given the
present demand for high storage densities and fast data-processing
rates, future molecular memory systems will be required to operate
exclusively on photon-mode effects, where binary data can be
written, nondestructively read, and erased using photons of differ-
ent colors.3 To this end, great success has been realized in a variety
of protein-based memory materials.4 However, in terms of fluo-
rescence-based devices using organic photochromes as the active
medium,2,3,5,6 readout instability remains a problem, and practical
applications require new molecules and design strategies that avoid
destructive readout. At the same time chromophores used in
molecular storage technology should meet the requirements of
photochemical bistability, fast writing and reading capability, and
high storage capacity in two and three dimensions.
The present study suggests an alternative molecular design that
Figure 1. Emission spectra of 1a (solid line) and 2a (dashed line) in CH3CN
avoids destructive readout in potential photon-mode binary storage
devices. In our strategy a Ru(II) chromophore with luminescent
metal-to-ligand charge transfer (MLCT) excited states is covalently
attached to a photochromic dianthryl molecule that serves as a triplet
energy-transfer quencher in one of its photochromic states.7 This
design incorporates several important principles that are vital to
the success of molecular optical storage technologies. First, the write
and read cycles are independently addressed with photons of
different wavelengths. Second, the photochromic quenching pro-
cesses that attenuate the MLCT emission are not accessed through
population of the MLCT excited states; that is, visible excitation
does not write new or erase stored information. In addition, the
polystyrene matrix utilized in this study serves as a filter of deep
UV light, suppressing the photochemical erase cycle. These
properties suggest that long-term binary data storage in metal-
organic materials is indeed feasible, the current system representing
a functional luminescence-based read-only memory (ROM).
Scheme 1 presents the structures and optical switching function
of two new Ru(II) diimine complexes (1a, 1b) each containing a
dianthryl species that have been synthesized and structurally
measured with 460-nm excitation. The spectrum of 2a was obtained after
390-nm photolysis (15 min) of 1a. (Inset) Absorption spectra of 1a and 2a
from the same experiment.
Scheme 1. Photoswitching Behavior of 1
1
characterized with H NMR and mass spectrometry. Our ligand
designs were inspired by a related photochromic structure originally
developed by Belser and co-workers.8 In the present work, the
well-established singlet state cycloaddition photochemistry of the
dianthryl unit is proposed as a means to store binary information
on the molecular level.8-10 In CH3CN solutions, the charge-transfer
emission in 1 is nearly quantitatively quenched when excited with
visible wavelengths, Figure 1. Nanosecond laser flash photolysis
confirms the prompt formation of the triplet-to-triplet absorption
spectrum of anthracene (τ ) 67 µs) immediately following a 532-
nm excitation pulse. These data suggest that the quenching of the
MLCT-based emission in both compounds results from rapid and
efficient triplet energy transfer to anthracene. The MLCT-based
emission can be “turned-on” through excitation into the anthracene
singlet absorption bands at 390 nm, producing 2. To avoid potential
photochemistry with O2, these solution-based experiments were
performed under an inert Ar atmosphere.8 The cycloaddition
photochemistry is accompanied by the loss of the long- and short-
wavelength anthracene absorption bands, Figure 1, inset. The
* To whom correspondence should be addressed. E-mail: castell@bgnet.bgsu.edu.
§ Bowling Green State University.
‡ Universita` di Ferrara.
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J. AM. CHEM. SOC. 2002, 124, 4562-4563
10.1021/ja0255608 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
cyclized photoproduct (2) no longer quenches the MLCT excited
states through energy transfer, and removal of this pathway affords
strong MLCT-based emission in the red, Figure 1. The excited-
state lifetime of this species independently measured with pulsed
luminescence and transient absorption was ∼1.2 µs and displayed
single-exponential decay kinetics. The 2 f 1 phototransformation
in CH3CN was accomplished using 230-nm excitation for 40 min;
however, complete reversion to the starting point was never realized.
Although switching occurs, changes in the absorption and emission
extremes are observed with repeated cycling (see Supporting
Information). This behavior obviously precludes the use of 1 in
rewritable memories. Attempts to heat samples of 2 for 60 min
at 70 °C resulted in less than 10% reversion to 1, illustrating
modest thermal stability for 2. Importantly, the cycloaddition
photochemistry is not induced with visible excitation (>445 nm),
and after irradiation at 390 nm, continuous exposure to 450 nm
does not degrade the emission signal that emerges from the sample.
These solution studies suggested that 1 might potentially serve as
the active component in a binary ROM.
To test this hypothesis, 1a or 1b (0.05 wt %) was dispersed in
a polystyrene (PS) matrix. Slow evaporation of these CH2Cl2
mixtures resulted in optically transparent films with 5-20 µm
thickness. Similar to that observed in solution, the MLCT based-
emission in both samples was nearly quantitatively quenched when
these solid-state samples were initially exposed to visible light. This
emission could be “turned-on” or written with a variety of excitation
sources that included a 450-W Xe lamp/monochromator (390 nm),
a Nd:YAG laser (355 nm), a UV-LED (390 nm), a N2-pumped
dye laser (390 nm), and a frequency-doubled Ti:sapphire laser (400
nm). Analogous to the solution phase, visible photolysis has no
effect on the observed emission intensity in the solid samples. After
completion of the “write” cycle, 2a and 2b display strong
photoluminescence centered near 650 nm when excited in the
visible. This photoluminescent state is believed to persist indefinitely
as the reverse photochromic reaction is largely suppressed in the
PS matrix. We believe this lack of reversibility is attributed to the
strong UV absorption of the PS matrix, advantageous for ROM
applications.
The PS materials doped with 1a and 1b were used for imaging
µm-sized objects. This was accomplished by 390-nm photolysis
performed through transmission electron microscopy (TEM) grids
with features in the µm-size regime. In these experiments, TEM
grids of varying dimensions were used as contact masks similar to
that used in photolithography.11 After the UV photolysis, the masks
were removed, and the formed images were observed in a
conventional fluorescence microscope (100-W Hg lamp excitation
source) using appropriate filters for blue-light excitation (450 nm).
Figure 2 illustrates one of the images generated from this experi-
ment. These samples were re-imaged 6 months later with no
apparent degradation or variation in the image resolution, providing
evidence of the durability of these materials for long-term memory
applications. Features on the µm level can be easily distinguished
in these samples, suggesting that with near-field techniques,
emission in these materials could be imaged on the nanometer scale,
greatly improving two-dimensional data storage density. As far as
writing speed is concerned, measurable microscope images have
been formed in these films resulting from 1-s exposure to 400-nm
light from a frequency doubled Ti:sapphire laser (80 MHz, 150 fs
fwhm).
Figure 2. Microscope image of a TEM grid in the bright field (top) and
fluorescence microscope image (bottom) generated from 390-nm photolysis
(not collimated) of 1a (0.05% in a 20-µm PS film) through a TEM grid
contact mask. The red regions indicate luminescence and the dark regions
are nonluminescent.
Supporting Information Available: Preparations, experimental
details, and selected spectroscopic data (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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