Res. Chem. lntermed., Vol. 22, No. 2, pp. 115-128 (1996) VSP 1996

ANTHRACENE MONOMER-DIMER : HIGH DENSITY 3D OPTICAL STORAGE MEMORY

A. S. DVORNIKOV and P. M. RENTZEPIS Department of Chemistry, University of California, Irvine. lrvine, CA 92 717-2025 USA

Received 11 October 1995; accepted 30 October 1995

Abstract--Ultrafast spectra and kinetics of all spectroscopic states and intermediate species in the photodimerization process of substituted have been identified and measured by means of ultrafast spectroscopy. The two forms of this process, monomer and dimer, have been used to construct a 3D optical storage memory device capable of 10 t3 bits/cm3 density and very large bandwidth. Storageand accessing 3D information is based on non-linear absorption and the different structures of the monomer and dimer. This rather novel 3D memory device and its operation is described in detail.

INTRODUCTION

Advances in computer technology have created the necessity to store, retrieve and process huge volumes of data at extremely high rates [1]. Advances in silicon technology have improved computer processing and usage to a high level making the memory capacity and I/O speed as the limiting factors in performance. The need therefore for a quantum jump improvement in information density storage and access speed becomes rather mandatory. Because of the huge data storage requirements these memories impose, we must find means for parallel execution of tasks which suggest need for a compact, very high capacity and density, low cost memory device probably of different form than the normal disks available today. We believe that three dimensional storage provides a means for satisfying these needs. Research efforts which may lead to 3D storage memory devices include persistent hole burning [2], phase holograms [3] and two photon optical 3D memories which utilize organic materials [4-6], and biomolecules such as bacteriorhodorpsin [7-10]. In a previous papers we have presented the basic theory of two-photon excitation and the means for utilizing this nonlinear process to write, read and erase information in 3D volume devices [4,5]. A two- photon absorption process as shown in Figure 1 causes a in the unwritten, ground state form, to be excited to a higher electronic state by the simultaneous absorption of two photons. The energy required to reach the excited state is greater than the energy of either photon alone therefore each beam propagates throughout the memory volume without being 116 A.S. Dvornikov and P.~'s Rentzepis

$1 S 1 v I Write v3 ] Read V4 ~Fluorescence v 2 So So

"Write" Form ~ "Read" Form

Figure I. Energy level diagram for two-photon processes.

absorbed. However, when the sum of the energies of these photons is equal to or greater than the energy gap between ground and first electronic excited states of the molecule (see Figure 1), then at the point of beam intersection, the two photons may be absorbed simultaneously by a molecule resulting in excitation of this molecule to a higher electronic state. The excited state decays subsequently into a different molecular ground state which becomes the written form of the molecule. These written absorb at longer wavelength than the original molecules and can be read by excitation with two photons of a lower energy, than those absorbed by the unwritten form. The excited written molecule fluoresces and detection of the induced fluorescence is the means for accessing the stored information. It is possible to access any place within the volume for either storing or reading information by simply intersecting the two beams at that place, Figure 2. The density of information which can be stored is limited by the memory volume divided by the optical spatial resolution, V/X 3. For k = 200 nm the upper bound in data storage density is as high as 1012 bit/cm 3. An important property of this type of 3D process is the capability of writing and reading complete 2D pages with one flash and thus become truly amiable to parallel processing. In order to achieve a high density and be suitable for use in 3D memory devices the photochromic materials should have a high nonlinear absorption cross-section, be stable in both write and read states, be photo erasable and the written form have a high fluorescent quantum efficiency. Most of the research, previously utilized for the 3D memory storage was based on spiropyran molecules dispersed in a host [4,5]. In this paper we will present our studies of new materials and systems for 3D storage memory devices by means of two-photon absorption. Namely photo induced reversible dimerization of substituted anthracenes which provides a means for writing and reading 3D information and in contrast to the spiropyrans, which exhibit rather high rate of thermal decay of the written form back to the original form, the molecules which we describe in this paper are stable at room temperature and their spectroscopic properties to be described make them suitable for 3D writing and reading. The class of materials which we describe are aromatic monomers and dimers. The dinaer species absorb in the ultraviolet and are used Monomer-Dimer Photochemistry 1 17

Frequency Shift (Raman Shifter, SHG, Image Dye Laser) ~, plate WT"-- I. , "~------~ _ ~.,~ ~ ~ I L ~o~o~ I Nd/YAGLaser I---t ~ ~ ~ beam

Cylindrical a) ( A l. lense NI I/ V ~lane addressing*- l'~matenal Delay Stage beam [ Cor~p~t~r] ~ CCD

Figure 2. a) Experimental system used for storing and retrieving information in 3D; b) Beam intersection within memory cube.

for writing, corresponding to "zero" in the digital format and upon excitation revert to the monomer which absorbs in the X < 400 nm range. When the written form molecules, monomers, are excited they fluoresce. The molecules are only a few Angstroms in size, therefore, they are not the limiting factor with regard to information density/unit volume as is the case in other computer memory devices. For application to practical devices it is important that the absorption and emission spectra are separated by a large Stokes shift in order to avoid crosstalk between writing and reading. Storage and retrieval of 3D information within the memory volume is achieved by the excitation of dimer molecules and monomer molecules, respectively, by two photons. 118 A.S. Dvornikov and P.l~ Rentzepis

EXPERIMENTAL SECTION

A. Store and Access of Information in 3D The experimental system used for the 3D memory writing and accessing is described briefly and shown schematically in Figure 2. An active-passive Nd/YAG mode-locked picosecond laser emitting 30 ps pulses at a repetition rate of 20 Hz was the laser light source. Wavelength tunability was achieved by using electrooptic crystals such as KDP and BBO, a dye laser and stimulated Raman scattering generated by , methane and other gases. Excitation of the written form, by two-photon absorption, results in the population of the first allowed excited state which decays within -5 ns accompanied with light emission. The fluorescence emitted by the written form of the sample was collected and focused onto the charge coupled device, CCD camera, coupled to a computer. The experimental system shown in Figure 2a allows for storing and accessing information in the form of 2D planes, disks, at once inside the 3D volume, instead of the normal disk bit by bit writing. The procedure for storing a 2D plane, equivalent to a several megabits disk, is shown in Figure 2b. The information to be stored is carried into the cube by plane front infrared, 1064 nm, beam. This beam first passes through a plate onto which the information to be stored is imprinted. After exiting this plate this beam carries within it the image of the information to be stored. Subsequently it propagates through the 3D storage device, cube. At the same time the 532 nm, addressing beam enters the cube propagating orthogonal to the information beam with a shape of a 1 cm x 1 cm x 150 m~ thick plane. These two beams intersect at a preselected area within the cube and because the sum of their frequencies is sufficient to populate the first allowed excited state, photochemistry takes place which induces the storage of the information. By moving the plane of beam intersection, see Figure 2, by -100 m/~ further inside the cube another disk is stored. By such means a large number of 2D planes, disks, may be stored in a l cm 3 cube. Accessing the stored 2D images was carried out by two photon or one photon induced fluorescence of the written form. This was achieved when a plane beam with the appropriate wavelength to be absorbed by the "stored" molecules was passed through the stored image plane. The fluorescent recorded image was focused onto a CCD camera and displayed on the computer monitor. Any selected bit of information may be read by means of two photon excitation, while one photon excitation allows for a complete stored page to be read at once. This one photon readout significantly reduces the system complexity and also is in effect suitable for parallel processing.

B. Spectroscopy and Kinetics Transient absorption spectra and ultra fast kinetics were measured by the laser system shown in Figure 3. A 355 nm pulse is generated by passing the 532 SHG nm and 1064 nm beams through a KDP crystal (THG). The picosecond continuum used to detect transient absorption spectra was generated by focusing part of the 532 nm or 1064 nm Anthracene Monomer-Dimer Photochemistry l 19 / SHG THG P % % [YAGLaser}---~ iI

532nm cm cell or ~ 355nm exc pulse 1064nm f [Continuum ,.~robe pulse /

HzO/Dz O C

Dela r stage Fo

Figure 3. Experimental system used for transient absorption spectroscopy.

pulse into a cell containing D20/H20 mixture. Self phase modulation and Raman scattering are responsible for generating a picosecond pulse with a bandwidth of several hundred nanometers. This continuum pulse, is absorbed by the ground state and transient state molecules, as its passes through the excited state volume in the sample cell. The computer subtracts the original ground state absorption leaving only the difference spectrum which identifies the transient photoinduced species. The spectra and kinetics of the transient species or transient states are measured by means of the pump-probe technique introduced in the mid 1960's. The arrival of the interrogating continuum pulse in the cell, is delayed by a preselected period of time i.e. 10 -13 - 10 -9 S, in reference to the pump, excitation, pulse. By arriving at a specific time after excitation, the continuum pulse records only the transient spectra present a that time. Changing the delay time a complete histogram of the spectra of all transient species and states maybe recorded. Photochromic spiropyran (Chromadye 5) was purchased from Chroma Chemicals Inc. All other chemicals were purchased from Aldrich. Thin polymer films (10-100 #) containing the photochromic molecules were prepared on glass slides by solvent casting from a dichlorethane solution. Solid blocks were also made by dissolving the photochromic materials in a monomer such as methylmethacrylate then adding a sufficient quantity of initiator to induce polymerization. The ground state absorption and fluorescence spectra of these samples were recorded by a double beam Shimadzu UV160U spectrophotometer and a Shimadzu RF 5000U spectrofluorophotometer respectively. 120 A.S. Dvornikov and P.M. Rentzepis

RESULTS AND DISCUSSION

The process of reversible photodimerization and photo dissociation of polycyclic aromatic hydrocarbons such as anthracene and its derivatives [11] may be used for developing photochromic materials for 3D optical memory devices. The photodimers are formed by excitation of the corresponding monomers, as shown in Scheme 1. It may be shown experimentally that dimers revert back to monomers when exposed to UV light.

R 355 nm

R=H;CH 3 266 nm

Scheme 1

The dissociation of a dimer results in the regeneration of a conjugated double bond system and the simultaneous red shift of the absorption band of the dimer. Figure 4 shows the absorption spectra of anthracene and anthracene dimer dispersed in PMMA. The monomer has its long wavelength absorption band in the 300-400 nm region, while the dimer is blue shifted and has practically no absorption at wavelengths longer than 300 nm. The

a C b ^ r

~o \ r.,O i I 0 O < I I [.r.., I I f

200 300 500 Wavelength. (rim)

Figure 4. Absorption spectra of a) dimer, b) monomer and c) fluorescence spectrum of anthracene monomer in PMMA. Anthracene Monomer-Dimer Photochemistry 121 monomer was found to emit with a fluorescence quantum efficiency of approximately 30%, while the dimers are practically void of any fluorescence. Figure 4c shows the fluorescence spectrum of anthracene dispersed in PMMA between 380-450 nm. The photodissociation and photodimerization processes, in the absence of , are very efficient and reversible, which suggests that the store and access processes can be repeated efficiently many times. The formation and photo dissociation mechanism of anthracene dimers and related compounds has been extensively investigated previously [12-18]. Anthracene and its derivatives may also form the corresponding dimers by excitation of the monomer to first singlet excited state [12] and subsequent interaction of two monomers. The quantum efficiency of this process is found to be independent of the excitation light wavelength. When the photodimers are excited to the singlet state they dissociate adiabatically via intermediate formation [13,14,17]. At low temperatures (77 K) the excited triplet state becomes the dominant channel for dissociation [17]. Studies of excimer formation in both photodimerization and photo dissociation processes of linked anthracenes in polar solvents have been reported previously [18]. However, for solutions of anthracene and its derivatives, no direct observation of excimer formation during photodimerization or photo dissociation processes has been reported previously. Our studies on anthracene and 9-methyl-anthracene solutions in dichloroethane by means of picosecond absorption spectroscopy show that after excitation with a 355 nm, 30 psec laser pulse, intermediates were formed which are characterized by the absorption spectra shown in Figure 5. Several new absorption bands appear immediately after

0.4

a

A b

I ! ! l / ~,,IL C / \ ! "\ t i , / "\ / t, t i fm- \. ~./1-- ~.. . .+ t i ,t X. \.

0 300 400 500 600 700 800 900

Wavelength (rim)

Figure 5. Transient absorption spectra of 9-methyl-anthracene monomer in dichloroethane: a) and b) 100 ps after excitation; c) 16 ns after excitation, a) probed 300-450 nm, b) probed 450-900 nm. 122 A.S. Dvornikov and P.M. Rentzepis excitation at 380, 560, 600 and 700 nm. The decay of all the absorption bands were found to follow first-order kinetics. The values of the decay rate constants were found to be the same at all wavelengths. This lead us to believe that all of these absorption bands belong to the same transient species. Comparison of this transient spectrum with a similar one depicted in the literature [19] strongly suggests that the transients may be the first excited singlet state of anthracene and 9-methyl-anthracene respectively. If this assumption is correct then the transient should decay with the same rate as the ground So and excited T~ states grow. To this effect we find that a band with a maximum at 430 nm (Figure 6) was

0 -4.5

-1 .3.5 O k = ~.1 x 108see-1 '8 v 5/-. .fi

-2 2.5 ,

-3 1.5 0 10 20 Time (ns)

Figure 6. Kinetics of 9-methyl-anthracene monomer in dichloroethane: a) Decay of transients at X = 380, 560, 600 and 700 nm and b) growth of transient at 430 nm as a function of time.

formed with the same rate as the decay of the singlet state. It is known [20] that the triplet-triplet absorption spectrum of anthracene has a band at 430 nm and it seems reasonable therefore to assume that the observed transient absorption at 430 nm is formed via intersystem crossing from S~ to T1 state. According to previously published data [20] the extinction coefficients, E, for triplet-triplet absorption of anthracene and 9-methyl- anthracene are 6.2 x 104 and 4.6 x 104 respectively and the quantum yield of intersystem crossing ~isc is -0.7 for both substances [21]. Using this data in conjunction with our measurements on the ratio of maximum intensities of S 1 --* S n and TI ---* Tn absorption bands at -100 ps, when no triplet states were observed, and at ~15 ns when the intersystem crossing process is practically completed, allows us to estimate the values of e for S~ ---, Sn transitions. The values of ~390 -3.2 x 104 and e400 -3.2 x 104 were calculated for Anthracene Monomer-Dimer Photochemistry 123 anthracene and 9-methyl-anthracene respectively. The extinction coefficient values for the ground state absorption of anthracene and 9-methyl-anthracene at 377 and 387 nm respectively are 6377 = 0.43 x 104 and 6387 = 0.66 x 104, which is 5-7 times less than the extinction coefficient, e, for the S 1 ---* S. absorption. Because of this difference in E, we could clearly observe the decay kinetics of the S l state at the wavelength region where both anthracene and 9-methyl-anthracene have strong absorption. We studied the photo dissociation of anthracene and 9-methyl-anthracene dimers in dichloroethane solutions under 266 rim, 30 ps laser excitation. The transient absorption spectra which may be seen in Figure 7 depict several broad bands with maxima at -370, 480, 780 and 860 rim. All the broad absorption bands in the visible region 480, 780 and 860 nm, were found to decay with the same lifetime of-3 ns. Simultaneously, with the decay of these bands, a new absorption with kmax at 430 nm was formed with the same rate constant. At the same time, the absorption band at -370 nm was found to undergo changes in its shape while a stable product was formed. This product has sharp absorption bands at 359 and 378 nm for anthracene and 369 and 389 nm for 9-methyl-anthracene dimers. These spectra (Figure 7) and the absorption spectra of monomers in the ground states are practically identical. This similarity in the spectra, plus the fact that photolysis of the dimers leads to monomer formation, lead us to believe that the stable product of the photolysis is the original anthracene and 9-methyl-anthracene monomers, respectively. Based on literature [13-18] data and our observations, we can now propose that the transient, with wide absorption bands at 480, 780 and 860 nm, is the excimer which is formed after 266 nm excitation of the dimer molecules. When the excimer relaxes it

0.4

b ./ e~ I / t~ f~"N. e~ \. J" \. j J" I

0 300 400 500 600 700 800 900 Wavelength (rtm)

Figure 7. Transient absorption spectra of 9-methyl-anthracene dimer in dichloroethane: a) and b) 100 ps after excitation; c) 16 ns after excitation, a) probed 300-450 nrn, b) probed 450-900 nm. 124 A.S. Dvornikov and P.M Rentzepis dissociates into two monomer molecules; one in the excited triplet state with the characteristic absorption band at 430 nm and the other in the ground state. The fact that the absorption band at 370 nm does not suffer any significant change in intensity with time, but only in shape, suggests that the excimer also has an absorption in this region and its extinction coefficient is close to that of the monomer in the ground state. It was shovvaa previously [13-17], that the dissociation reaction of the dimer at room temperature proceeds from the S~ excited state. It was also observed, by means of fluorescence [13], that the monomer molecules were formed in the S t state during the photo dissociation of the dimer. Based on all available data we can now propose a complete mechanism for the photo dissociation reaction of these dimers:

0.5x10 8 s q M + fluorescence hv , 3x10 8 s-1 I - M 2~1M 2 ~ "(MM) ~IM + M

[-----)~ 3M ----.-.1~ M lxlO8s q

The photodimerization and photo dissociation reactions for linked anthracenes proceed [18] via the formation of common intermediates which are referred to as . In our experiments on the photochemistry of anthracene and 9-methyl-anthracene monomers in solution with concentrations -6 x 102 M we did not observe any intermediates which may be interpreted as excimers. We can assume therefore, that in the case of anthracene and 9- methyl-anthracene the photodimerization reaction proceeds also via excimer formation, but owing to the fact that this is a diffusion controlled bimolecular reaction, the concentration of the corresponding intermediates may be too low to be detected in our kinetic studies.

3D Storage and Accessing." To demonstrate the writing process we stored several 2D pages, megabit disks, simultaneously. The fundamental 1064 nm information beam and SHG 532 nm addressing beam intersected at a thin plane inside the memory cube, which contains the photochromic material, such as spiropyran or anthracene monomer/dimer system, dispersed in poly(methyl methacrylate). The information is stored by the means described and shown schematically in Figure 2. By these means and employment of special light modulators several gigabits of information may be stored into sequence of thin planes within the cube. Accessing is achieved by the same optical system except that the wavelength of one or both beams is longer. In the case where the stored information in a whole page is to be accessed simultaneously, rather than in a single bit by bit mode, a low intensity 532 nm plane beam alone may be used to illuminate the written 2D page and induce fluorescence by one photon process. The 532 nm beam has the dimensions of the stored 2D plane and intersects only the plane which contains the information (Figure 2b). Therefore, it illuminates only one Anthracene Monomer-Dimer Photochemistry 125

2D plane without disturbing the neighbouring stored planes of information. The fluorescence is detected and recorded by a CCD which transmits the signal in digital form directly to the processor. Several 2D planes were stored inside the 1 cm 3 cube. Figure 8a shows the position of eight such 2D disks inside the cube. In addition, Figure 8b, shows the information retrieved from one of these 2D planes. The pattern shown was written simultaneously by intersecting inside the cube containing spiropyran in PMMA. The 1064 nm information carrying pulse, see Figure 2, and the 532 nm addressing beam shaped to the form of a thin, -20 ~m, plane. Accessing any 2D pattern within the memory volume is simply achieved by illuminating this area with a thin plane of light of the appropriate wavelength to induce fluorescence.

a) b)

Figure 8. a) Cross-section of eight 2D plane disks stored inside the 1 cm3 memory cube; b) the pattern stored on one 2D plane.

One advantage of dimer-monomer based 3D storage systems is that unlike the spiropyrans, where the written form decays spontaneously, at room temperature, into the original write form and effectively erasing the written information, both dimer and monomer forms are stable at room temperature. In addition, the high absorption cross- section and high quantum efficiency for both dissociation of dimers and fluorescence of monomers suggest that this photochromic system is potentially attractive for utilization in 3D memory and other optical devices. To store information patterns within the volume of the memory device, which consists of dimer molecules dispersed in a PMMA matrix, we used two 532 nm picosecond laser beams counter propagating with respect to each other or intersecting each other orthogonal, see Figure 2. Upon intersection of two 532 mn beams excitation into the first excited electronic state of the dimer occurs as a result of two-photon absorption, which is equivalent 126 A.S. Dvornikov and P.M. Rentzepis to the absorption of a single 266 nm photon. The excited dimer photo dissociates and forms the monomer, which is the written form, of the 3D memory. The number of monomer molecules generated during the "write" process was monitored by measuring the monomer fluorescence intensity. Figure 9 shows the increase in monomer concentration as a function of irradiation time of the dinaer by two 532 nm photons. The insert of

Figure 9 is a log-log plot of the relative monomer accumulation rate versus excitation beam intensity and the slope of 2 indicates that the process proceeds via two-photon absorption.

1- m

r 4 Slope = z /

~t 0.5-

l~o m t.n(i~.4 1.6 0

_ 300 4OO 5OO 6OO Wavelength (nm)

Figure 9. Increase in fluorescence intensity during the "write" process vs. excitation time interval: a) 0; b) 1; c) 2; d) 3 and e) 5 min. Insert: dependence of storing information rate on laser beam intensity.

Erasing of the stored information in such 3D memory devices may be achieved by irradiation of the written areas with the light of the wavelength required to excite the monomer, and induce photodimeriztion. It must be pointed out that when the monomer is excited the energy dicipation takes place either by fluorescence, resulting in the read process or conversion to the dimer causing erasing of the information. The erasing process is by an order of magnitude less than the read process, therefore the information can be read for many cycles. The utilization of this photochromic system, which is based on the reversible photodimerization/photo dissociation of polyacene compounds, for 3D information storage and access devices has several advantages over the previously used materials: 1) both the write and read forms are stable at room temperature, 2) high absorption quantum efficiency for write and read processes, 3) high fluorescence quantum yield (accessing information), Anthracene Monomer-Dimer Photochemistry 127

4) easily available materials. The high density of 3D storage devices -I0 ~2 bit/cm3, their inherent suitability for parallel processing and absence of moving parts makes this device rather attractive. At the present time we have been able to store and access more than 30 2D planes at the same time without crosstalk inside a 1 cm 3 cube. Other molecular systems and operations are currently studied which provide a large improvement over the materials and procedures used now.

CONCLUSIONS

In this paper we have presented experimental studies which made it possible to unravel the photochemistry, spectra and kinetics of ultrafast transients of photodimer molecules. We also showed that knowledge of the photophysics made possible the use of these materials for storing and accessing information in 3D space. We also have described the system which was used for the operation of these large bandwidth 3D memory devices. The molecules presented in this paper are only an example of the large molecular systems which may be used as 3D memory materials.

Acknowledgment This work was supported in part by the United States Air Force, Rome Laboratory under contract F-30602-93-0231.

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